“Parametric Studies on the Reduction of 4- Nitroacetophenone using Hydrogen Sulfide

“Parametric Studies on the Reduction of 4- Nitroacetophenone using Hydrogen Sulfide
M. Tech (Dual Degree) Thesis
“Parametric Studies on the Reduction of 4Nitroacetophenone using Hydrogen Sulfide
and Phase Transfer Catalyst.”
Submitted by
Mohamed Aslam Husein
Under the Supervision of
Prof. Sujit Sen
Department of Chemical Engineering
National Institute of Technology Rourkela
Rourkela – 769008, India
MAY 2015
This is to certify that the thesis entitled “Parametric Studies on the Reduction of 4Nitroacetophenone using Hydrogen Sulfide and Phase Transfer Catalyst.” submitted by
Mohamed Aslam Husein to National Institute of Technology Rourkela, India for the award
of degree of Master of Technology (Dual Degree) in engineering, is a bonafide record of
investigation carried out by him in the Department of Chemical Engineering, under the
guidance of Prof. Sujit Sen. The report is up to the standard of fulfilment of M. Tech (Dual
Degree) degree as prescribed by regulation of this institute.
Prof. Sujit Sen
Dept. of Chemical Engineering
This thesis would not have been possible without the support of many
individuals. I am certain to miss out on names, so please accept my apology in
Most importantly I wish to express my deep gratitude to Dr. Sujit Sen,
my supervisor, for his valuable guidance throughout my work. I would like to
thank him for constantly motivating me and instructing me wherever and
whenever necessary.
I also take this opportunity to express my sincere thanks to all the Faculty
of the Department of Chemical Engineering, NIT Rourkela, for their teachings
have often helped me in my work.
My deepest gratitude also extends to Mr. Ujjal Mondal, Mr. Gaurav
Singh, Miss Devipriya Gogoi, Miss Preeti Jha and Miss Ramya Shankar for
their knowledge and guidance.
I am obliged to all my friends for their help and encouragement. Finally,
the thesis would not have been completed without the support of the most
important people in my life − my family. I sincerely wish to thank my parents,
my wife and my siblings for their unconditional love and constant
Figure No
Figure caption
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Figure 1.1
Commonly used Alkanolamine
Figure 1.2
Flow diagram of Amine Treating Process
Figure 1.3
Flow diagram of Claus Process
Figure 1.4
Classification of Phase transfer catalysts
Figure 1.5
Stark’s Mechanism of PTC
Figure 1.6
Brandstorm Montanari Mechanism of PTC
Figure 1.7
(a) Heterogeneous (b) Homogeneous S-L PTC
Figure 1.8
Structure of TBPB
Figure 1.9
Commonly used effective Phase transfer catalyst
Figure 1.10
Properties of commonly used effective Phase transfer catalyst
Figure 1.11
(a) Aqueous Phase Reaction (b) Organic Phase
Reaction Mechanism of L-L-S PTC
Figure 1.12
Mechanism of L-L-L PTC
Figure 2.1
Stoichiometry of Zinin’s original reduction
Figure 3.1
Schematic diagram for absorption of H2S in MDEA solution
Figure 3.2
Batch Reactor Assembly
Figure 3.3
A Gas Chromatography assembly
Figure 3.4
Chromatogram of NAP Synthesis
Scheme 1
Ionic equilibria in H2S-MDEA-H2O system
Scheme 2
Proposed mechanism of reduction of NAP by H2S-laden MDEA
under L-L PTC
Figure 4.1
(a) Effect of stirring speed on conversion of NAP
(b) Effect of stirring speed on conversion of NAP with
and without catalyst
Figure 4.2
Effect of temperature on conversion of NAP
Figure 4.3
Arrhenius Plot for activation energy
Figure 4.4
Effect of catalyst loading on conversion of NAP
Figure 4.5
Plot of the natural logarithm of the initial rate vs the natural
logarithm of the catalyst conc.
Figure 4.6
Effect of sulfide concentration on conversion of NAP
Figure 4.7
Plot of the natural logarithm of the initial rate vs the natural
logarithm of the sulfide conc.
Figure 4.8
Effect of MDEA concentration on conversion of NAP
Figure 4.9
Effect of reactant concentration on conversion of NAP
Figure 4.10
Plot of the natural logarithm of the initial rate vs the natural
logarithm of the sulfide conc.
Figure 4.11
Effect of sulfur loading on conversion of NAP
Figure 4.12
Effect of different catalysts on conversion of NAP
Table No
Table caption
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Table 1.1
Health effects of H2S at various levels of exposure.
Table 1.2
H2S exposure limits.
Table 1.3
Advantages and Disadvantages of MEA.
Table 1.4
Advantages and Disadvantages of DEA.
Table 1.5
Advantages and Disadvantages of DGA.
Table 1.6
Advantages and Disadvantages of DIPA.
Table 1.7
Advantages and Disadvantages of MDEA.
Table 4.1
Initial reaction rate and enhancement factor.
Phase Transfer Catalyst
Quaternary Ammonium Cation
Aqueous Phase Reactant
Organic Product
Desired Product
Gas chromatography-Mass spectroscopy
Phase Transfer Catalysis
Tetrabutylphosphonium Bromide
Revolutions per minute
Tetrabutylammonium Bromide
Hydrogen Sulfide (H2S) is more often than not, found in the gas streams of Oil and Gas
Industry. This is due to the fact that crude oil has in its composition, a small percentage of
sulfur. Subduing the amount of sulfur in the crude oil is of paramount importance because it
can cause corrosion of equipments and pipelines and also leads to unwanted side reactions
leading to additional use of reactant and power. Conventionally, reduction in sulfur levels is
achieved by subjecting the gas stream to ammonia absorption followed by Claus Process, the
latter which converts H2S to elemental sulfur which is more productive (less poisonous) and
used in rubber vulcanization and sulfuric acid production. The problem associated with Claus
Process is that it is very expensive and the demand for elemental sulfur in the market is not as
high to redeem the cost. So alternatively, H2S is absorbed into an aqueous alkanolamine and
used in the reduction of Nitroarenes following the Zinin Reduction mechanism. Since the
alkanolamine and Nitroarene are in two different phases, a phase transfer catalyst (PTC) is
used to help increase the reaction rate, conversion of reactant and selectivity of products.
This work is focused on the reduction of 4-Nitroacetophenone (NAP) to 4Aminoacetophenone (AAP) using H2S rich N-Methyldiethanolamine (MDEA) and
Tetrabutylphosphonium bromide (TBPB) as the PTC under Liquid-Liquid Phase transfer
catalysis, and to study the process parameters involved to maximize the conversion of NAP
to AAP. The parameters studied are variation in Stirring speed, Catalyst concentration,
Temperature, Sulfide concentration, MDEA concentration etc.
This Method can be used as an alternative to Claus Process to produce value added fine
chemicals which have better use, in this case being Aminoacetophenone having applications
in clinical microbiology, in the synthesis of Pyrimidines, Coumarins, as a drug metabolite. It
also has antibacterial properties and used in the synthesis of HIV-1 growth inhibitors and
selective antagonists at human adenosine receptors.
Keywords: Hydrogen Sulfide, Liquid-Liquid Phase transfer catalysis, Aminoacetophenone,
Nitroacetophenone, N-methyldiethanolamine.
Title Page
List of Figures
List of Tables
List of Symbols and Abbreviations
Chapter 1
Hydrogen Sulfide: Its Sources
Need for treatment of H2S-rich Gas stream
Industrial processes for H2S recovery and reclamation
Ammonia based process
Alkanolamine process
Claus process to retrieve sulfur from H2S
Alternatives for processing H2S-rich Gas
Crystasulf process
Wet Oxidation LO-CAT process
Phase Transfer Catalysis (PTC)
Classification of PTC
17 L-L Phase transfer catalysis
17 S-L Phase transfer catalysis
18 G-L Phase transfer catalysis
Chapter 2
Chapter 3
Types of Phase transfer catalysts
22 Insoluble Phase transfer catalysis
22 L-L-S Phase transfer catalysis
22 L-L-L Phase transfer catalysis
Literature Survey
Use of Aqueous Alkanolamine for the removal of H2S
Synthesis of Aryl-amines using Ammonium Sulfide
Synthesis of Aryl-amines using Sodium sulfide
Nitroarene reduction
Experimental Work
Chemicals and Catalyst
Preparation of H2S-Rich Aqueous Alkanolamine
Experimental Procedure
Method of analysis
Determination of Sulfide conc.
Preparation of standard (0.025M) KIO3 solution
Preparation of standard (0.1M) Sodium thiosulfate solution 37
Preparation of standard (5% w/v) KI solution
Preparation of standard (1M) Sulfuric acid solution
Preparation of standard (10M) NaOH solution
Standardization of sodium thiosulfate solution by
standard potassium iodate solution
Estimation of sulfide conc.
Analysis of organic phase
Chapter 4
Chapter 5
Reduction of NAP with H2S-rich MDEA under L-L PTC
Zinin reduction
Proposed mechanism for reduction of NAP under L-L PTC
Present work
Parametric studies
Effect of Stirring Speed
Effect of Temperature
Effect of Catalyst (TBPB) loading
Effect of Sulfide Concentration
Effect of MDEA Concentration
Effect of Reactant variation
Effect of sulfur loading
Effect of Different Catalysts
Future work
Hydrogen sulfide is found naturally in crude petroleum and natural gas. It is also produced
through the bacterial breakdown of organic matter. Hydrogen sulfide can be produced by
decomposing human and animal waste, and is found in sewage treatment plants and livestock
areas. Hydrogen sulfide in the atmosphere is often from refineries, natural gas plants and food
processing plants. Automobiles emit hydrogen sulfide in their exhaust. About 11 % of the
global Hydrogen Sulfide (H2S) can be attributed to human practices. The natural breakdown
of organic matter and subsequent release of H2S occur mostly in deep sea vents, thermal
geysers, volcanoes, swamps etc, where the decomposition occurs in the absence of or
inadequate oxygen conditions. In general, H2S is produced when elemental sulfur is contacted
with organic matter at very high temperatures.
Apart from petroleum refineries, other industries involved in H2S emission includes coke
oven plants, paper and pulp industry, leather tannery, waste water treatment plants and textile
industry (Toxological Profile of hydrogen sulfide, 2006). However, the most prominent
contribution to the increasing global concentration of H2S rests in the oil and gas industry, in
the hydrotreatment of natural gas and processing for refined crude oil. There is growing
concern regarding the issue of public health with the rate of such increase in the atmospheric
H2S concentration given the dangers involved of prolonged exposure even at low levels.
Statistics explain that the reserves of easy to process crude oil is declining day by day and
refiners are now forced to work with heavy crude i.e. which has higher amount of sulfur and
nitrogen. To comply with the stringent environmental regulations, rigorous hydrotreatment of
the crude is followed leading to the removal of H2S and NH3 to the atmosphere in greater
proportions than before. On the other hand in the coal processing industry, it is generally seen
that H2S gas is given out as by-product when any kind of treatment is done to reduce the ash
content of coal. Coal gas has nearly about 0-3.5% of H2S as prominent non-hydrocarbon
H2S composition in natural gas in most cases is too high for it to be used as obtained
naturally. Although the composition varies from area to area, natural gas generally has H2S
concentration in the range of 0.5 ppm to 175000 ppm.
With the advent of global environmental establishments and activists, the chemical industries
are now compelled to dump, dispose and vent toxic substances in a very safe scale. H 2S,
which is a by-product of many industries, need to dispose it in a very controlled manner due
to the following reasons:
Hydrogen sulfide is a colourless, flammable, extremely hazardous gas with a “rotten
egg” smell. As it is denser than air, H2S has a tendency of accumulating at poorly
ventilated areas and voids. Although the smell is quite obvious in the beginning, the
victim is left unaware of its presence after some time (olfactory fatigue). Such
unaware prolonged low level exposure can cause inflammation of respiratory system
after affecting the eyes, nose and throat. The effects can be delayed for several hours,
or sometimes several days, when working in low-level concentrations. High level
exposure can lead to difficulty in breathing and in certain cases, coma and death.
Therefore, it is imperative to monitor the H2S levels at regular intervals and compare
it with the material safety data sheet of H2S.
From an industrial point of view, H2S can cause extensive damage due to its corrosive
nature leading to serious monetary repercussions. This is the reason it needs to be
removed as much as possible before passing it through pipelines. In many countries,
the H2S concentration is limited to 0.25g/100ft3 of gas when being transported through
Apart from corrosion of process equipment, H2S can also hamper the activity of
catalysts by its poisoning and deactivation.
Unwanted side reactions take place and it also increases the pressure sustenance of
process equipments.
Being a combustible gas and having an explosion limit of 4.3-46%, it also poses
threats of fire and explosion. With the help of a suitable ignition source or spark, H 2S
can burn with oxygen to produce noxious vapours and gases, like sulfur dioxide.
The people most prone to the dangerous effects of H2S include sewage treatment plant
workers, workers in manholes, tunnel workers, well diggers and chemical laboratory analysts.
Table 1.1: Health effects of H2S at various levels of exposure
The Hydrogen Sulfide exposure limits set by various safety establishments give us an idea of
the seriousness of the threat posed by the gas on human health. Globally, every 1 of 4 deaths
associated with chemical vapours is from H2S poisoning.
8hr TWA
Acceptable Max Peak above Ceiling for
an 8hr shift.
Max Duration
Federal OSHA
10 minutes once only if
no other measurable
exposure occurs during
TLV (2010)
Table 1.2: H2S exposure limits
H 2S
It is required by the industries to reduce the H2S concentration in the flue and by-product gas
stream in order to create a safe work environment for the workers and also for the overall
benefit of the global environment. Over the years, a number of processes have been designed
for the removal and retrieval of H2S from gas mixtures. Because of the acidic nature of H2S
(weak acid), most of the removal methods involve the use of alkaline solutions. However,
strong alkaline solutions are not preferred for the removal of H2S from gas streams because
they form irreversible products due to other chemical reactions. This tendency (of forming
irreversible products) is enhanced when the gas mixture contains both H2S and CO2 and the
concentration of CO2 is more than 4% (Robin, 1999). This is the main reason why weak
alkaline solutions such as ammonia and alkanolamines are used for the removal and
reclamation of H2S (Vago et al., 2011; Huertas et al., 2001).
This method of using ammonia for the removal of H2S from a gas mixture has been used
extensively since long (Hamblin, 1973; Harvey and Makrides, 1980). The procedure followed
in this process is as follows:
The gas mixture consisting of H2S and NH3 is allowed to pass through a H2S scrubber
and NH3 scrubber.
The NH3 scrubber was provided with a water stripper from the top to absorb the NH3
from the gas as ammonium hydroxide. This ammonium hydroxide solution acts as the
absorbent for the H2S scrubber.
Thus ammonium sulfide rich solution produced in the H2S scrubber is sent to a
deacidifier, where the ammonium sulfide is separated to obtain H2S rich vapour and
NH3 rich liquor. The reactions are :
𝑁𝐻3 + 𝐻2𝑂 → 𝑁𝐻4 𝑂𝐻
𝑁𝐻3 + 𝐻2S → 𝑁𝐻4𝐻𝑆
2NH3 + H2S → [NH4]2𝑆
The main advantages of NH3-based process are as follows:
This process is well suited for gas mixtures comprising of both H2S and NH3. The
removal of both H2S and NH3 is achieved in a single step whereas separate steps are
required for alkanolamines based process.
If a gas containing H2S and CO2 is contacted with the ammonium hydroxide solution
in the H2S scrubber, then it is found there is an extensive absorption of H2S. We can
selectively absorb H2S and CO2 by altering the concentration of the ammonium
hydroxide solution. Selective absorption of H2S can be achieved by using a spray
column and providing a short contact time.
Ammonia is one of the largest produced chemicals globally and hence a cheap
solvent. It also has the properties of not degrading or decomposing in the presence of
oxygen or any other component in the flue gas like carbonyl sulfide (COS), carbon
disulfide (CS2) and hydrogen cyanide (HCN). Ammonia is also found to have high
CO2 removal efficiency and low regeneration energy. It is less toxic to the
environment and human health than other amines.
Despite of these advantages, this process has lost popularity over the years and is now used
very remotely. It is not considered as a feasible way to remove H2S from a gas mixture
because of the following operational problems (Hamblin, 1973):
Because of the high partial pressure of ammonia (NH3), we are compelled to use a
dilute NH3 solution in the scrubbing step, or a relatively high pressured NH3 solution,
or introduce a separate wash with water step after the NH3 scrubbing in order to
separate all the NH3 from the gas mixture. Furthermore, using dilute NH3 solutions
will considerably elevate the regeneration costs as the regeneration step is operated at
a higher temperature than the scrubbing step.
The regeneration step involves the use of catalysts (like hydroquinone). The
ammonium sulfide poses a risk of poisoning and deactivation of the catalyst by
forming ammonium sulfate and thiosulfate. This will cause loss of both the catalyst
and the scrubbing NH3 solution (which is recycled).
The alkanolamine process gained acceptance over the past decade for the removal of acid gas
like H2S and CO2 from gas mixtures as compared to the ammonia process due to its
advantages of low vapour pressure (high b.p.) and ease of reclamation. Because of the low
vapour pressure of alkanolamines, the operation becomes flexible in terms of pressure,
temperature and concentration of alkanolamine. In the early days, Triethanolamine (TEA)
was considered for use in gas treating plants and it became the first commercially available
alkanolamine (R. R. Bottoms, 1930). Today, the amines that are of particular interest to gas
treatment are Monoethanolamine (MEA), Diethanolamine (DEA) and Methyldiethanolamine
(MDEA). Triethanolamine (TEA) was replaced because of its low reactivity, low capacity
and poor stability. MEA, DEA and MDEA have the advantages of larger capacity of
absorption and lower molecular mass. In our work MDEA has been used because of its
ability of selective absorption of H2S from a gas.
Figure 1.1: Commonly used Alkanolamine
If an amine group is attached to the alkanol, then the aqueous solution becomes basic in
nature in order to dissolve acid gases like H2S. On the contrary if a hydroxyl group is
attached to the amine, then vapour pressure is reduced and water solubility is increased. The
advantages and disadvantages of commonly used alkanolamine are shown in the Tables
1. Monoethanolamine (MEA)
Low cost.
Not selective in presence of CO2.
CO2, COS and CS2 can irreversibly
degrade the amine.
Products are very corrosive.
Used amine has lower acid gas
removal capacity.
High vaporization losses due to high
vapour pressure.
Table 1.3: Advantages and Disadvantages of MEA (Typical Concentration: 15-18 wt %)
2. Diethanolamine (DEA)
Moderate Cost.
Not selective in presence of CO2.
Selection less with CO2 and COS than
CO2, COS irreversibly damage the
Used products are less corrosive than
Used amine has lower acid gas
removal capacity.
Purification achieved by vacuum
regeneration than MEA.
Table 1.4: Advantages and Disadvantages of DEA (Typical Concentration: 25-30 wt %)
3. Diglycolamine (DGA)
Low specification achieved under
High Cost.
critical conditions.
Not selective in presence of CO2.
CO2, COS irreversibly damage the
Products are very corrosive.
High solubility of the aromatic,
olefins and heavy hydrocarbons in the
Extremely high energy required for
Table 1.5: Advantages and Disadvantages of DGA (Typical Concentration: 50wt %)
4. Di-isopropanolamine (DIPA)
Moderate selectivity in the presence
High Cost.
of CO2.
CO2, COS irreversibly damage the
Purification achieved by vacuum
required for regeneration.
Table 1.6: Advantages and Disadvantages of DIPA (Typical Concentration: 27wt %)
5. Methyl-diethanolamine (MDEA)
High selectivity to absorb H2S.
Higher acid gas removal capacity at
moderate concentration.
Higher cost than MEA, DEA and
Does not get damaged with CO2 or
More soluble than DEA in liquid
Non-corrosive at the concentration of
Table 1.7: Advantages and Disadvantages of MDEA (Typical Concentration: 35-50 wt %)
Figure 1.2: Flow Diagram of Amine Treating Process
The sour gas from the industries is fed from the bottom of the absorber where it is contacted
with lean aqueous alkanolamine in a counter-current manner. The gas mixture free from acid
gases is taken out from the top and the solution from the bottom of the absorber containing
H2S rich amine is heated along with the lean amine from the bottom of the regenerator
(stripping column) with the help of a heat exchanger. In the regenerator, the absorbed gases
(H2S) are stripped off from the alkanolamine solution and taken from the top. The
concentrated H2S gas is then sent for elemental sulfur recovery or disposal. The regenerated
lean amine is then sent back to the absorber as shown in Fig 1.2.
Reactions with H2S:
2RNH2 + H2 S ↔ [RNH3 ]2 S
Sulfide formation:
Hydrosulfide formation: RNH2 + H2 S ↔ RNH3 SH
Reactions with CO2:
Carbonate formation:
2RNH2 + CO2 + H2 O ↔ [RNH3 ]2 CO3
Bicarbonate formation:
RNH2 + CO2 + H2 O ↔ RNH3 CO3 H
Carbamate formation:
2RNH2 + CO2 ↔ RNH − CO − ONH3 R
More poisonous hydrogen sulfide gas is converted to less poisonous and more useful
elemental sulfur through the Claus process. This standard procedure is followed in industries
producing H2S as by-products since a long time. H2S is obtained as by-product mainly during
the treatment of natural gas in refineries, gasification plants etc. If any Hydrocarbons, sulfur
dioxide or ammonia and hydrogen cyanide are present in these by-product gases, then they
are separated from the H2S using amine extraction. Only H2S gas is supplied to the Claus
unit, the process of which comprises of two steps: Thermal and Catalytic.
In the Thermal step, the H2S gas is burned in a reaction furnace in a sub-stoichiometric
combustion process in the presence of air at temperatures between 1000 - 1400 ºC. Claus
gases (acid gas) with no further combustible contents apart from H2S are burned in burner.
The ratio of air to the acid gas, in this strongly exothermic oxidation reaction, is controlled in
such a way that one third of all H2S is converted to SO2. This facilitates the stoichiometric
reaction in the catalytic step.
Thermal step reaction:
2𝐻2𝑆 + 2𝑂2 → 𝑆𝑂2 + 2𝐻2𝑂 + 𝑆
In the Catalytic step, the gases leaving the thermal step are cooled in a sulfur condenser and
reheated to about 200-350oC before being fed to a series of catalytic converters and sulfur
condensers where H2S reacts with SO2 to produce elemental sulfur. Usually, the catalyst used
in the catalytic converter is either activated aluminium (III) or titanium (IV) oxide. A small
amount of H2S remains in the tail gas. This remaining quantity, together with other trace
sulfur compounds, is generally processed in a separate tail unit. It can give overall sulfur
recoveries of about 99.8%. Sulfur is used for manufacturing sulfuric acid, medicine,
cosmetics, fertilizers and rubber products. Elemental sulfur is used as fertilizer and pesticide.
Catalytic step reaction:
2𝐻2𝑆 + 𝑆𝑂2 → 3𝑆 + 2𝐻2𝑂
However, the Claus process has a number of disadvantages (Plummer, 1994; Plummer and
Beazley, 1986; Plummer and Zimmerman, 1986) namely:
It operates at high temperatures.
It requires exact process control over the ratio of oxygen to H2S in the feed.
The valuable hydrogen energy is lost in this process.
The feed would require expensive pre-treatment if it contains CO2 in high
concentration. In order to maintain the efficiency of the process, at least a portion of
the CO2 must be removed from the by-product gas by pre-treatment before oxidizing
the H2S.
The sulfur content of the tail gas in the Claus process is too high for it to be released
to the atmosphere. In order to comply with the stringent environmental regulations, a
lot more of the Claus stages would be needed to be added, thereby increasing the cost
of the process to a great extent.
Figure 1.3: Flow Diagram of Claus Process
Since it is not possible to treat gases containing H2S and high amounts of CO2 by amine
extraction, a separate process called „CrystaSulf‟ or „SulFerox‟ or „ARI-LO-CAT‟ is used. It
is a liquid reduction-oxidation process used for the treatment of natural gas. In this process,
instead of air, a liquid solution containing oxidised iron is used for the redox reactions. A new
technique called H2S splitting process is being developed to isolate both hydrogen and sulfur
from H2S. Hydrogen is used to store energy in fuel cells and also used in the oil industry and
for the production of ammonia and methanol.
A non-aqueous solution in which elemental sulfur has a good solubility is used for Crystasulf
Process. Due to the fact that elemental sulfur remains dissolved in the solution, the liquid
circulated to the absorber will have no solids. The advantage of CrystaSulf is that it avoids
the problems related to aqueous sulfur recovery systems and make them suitable for direct
treatment of high-pressure sour gas.
According to the process, a conventional tray absorber is used to remove H2S from the sour
gas. Dissolved elemental sulfur is produced from the reaction of H2S with sulfur dioxide SO2.
No solids are left in the absorber, flash tank, or solution lines, which reduces the chances of
plugging. From the absorber, the rich solution passes to a flash step in which the flash gas is
compressed and recycled to the inlet stream. After this, the solution is taken to a crystallizer
where the temperature is lowered to form solid elemental sulfur crystals. The only location
where sulfur solids exist within the process is in the crystallizer/filter area. The leaner liquid
is transferred back to the absorber by a pump when it overflows from the slurry tank to a
surge tank.
There are two different ways to add SO2. Liquid SO2 can be obtained and injected into the
lean solution line; this option is economically viable when SO2 is readily available for
purchase and sulfur throughputs are small. In the second method, a part of the product sulfur
is burned and the resulting SO2 absorbed into the solution through a separate, small
SO2 absorber.
In Crystasulf process, SO2 binds with species in the non-aqueous solution. Since this bond is
strong, it would be impossible to detect even small concentration of SO2 in the gas phase
anywhere within the system, including the sweetened gas. Operational flexibility is obtained
as a large excess of SO2 can exist within the solution, and this background concentration
causes a buffering effect. The CrystaSulf process provides the inherent capital cost benefit of
a single step approach and avoids the operating problems of the aqueous-based processes.
CrystaSulf is advantageous to operators who have avoided natural gas due to its H2S content,
or who are drilling gas wells in areas that tend to produce sour gas containing moderate
amounts of CO2.
Process Advantages
Elemental sulfur is dissolved in the solution and so, solids are absent (except in the
crystallizer/filter section).
There is no foaming in the solution because of the absence of surfactants or particles
There is no need for tail gas treating because the circulations rates are controlled and
there is complete removal of H2S.
Non-corrosive solution.
This is a liquid phase oxidation process that uses solution that is a little alkaline (but not
noxious) containing an iron chelate complex as a reduction-oxidation catalyst. This process
takes into account the environmental repercussions while converting H2S to elemental sulfur.
Gas mixture comprising H2S is treated with aqueous LOCAT solution in a venture-scrubber.
In the solution, the H2S is absorbed and instantaneously converted to elemental sulfur by
reacting with oxygen. Subsequently the iron ions are reduced from ferric to ferrous state,
which are then transported to an oxidizer from the absorber to regenerate the ferric ion by
contacting with atmospheric oxygen. These ions are then absorbed into the LOCAT solution
thereby replenishing the catalyst. Sulfur can be removed from the underside of the oxidizer
and thus from the circulating solution. Elemental sulfur can be obtained from the sulfur slurry
by centrifuge or by melting. The process has an efficiency of removing 99.9% of H2S from a
gas mixture comprising any concentration of H2S.
Many valuable reactions do not occur due to the constraint of insoluble nature of reactants in
one solvent. Conventionally we use a solvent that can dissolve all the reactants but in most
cases, these solvents are very expensive. Also, other factors like low rate of reaction (due to
excessive solvation of the nucleophile) and difficulty in separation of the product from the
reaction mixture adds to the disadvantages. In order to overcome this problem, we allow the
reactants to dissolve in their respective organic and aqueous phases and then, a particular type
of catalyst is added which facilitates the transfer of the reactant from the aqueous phase or
solid phase to the organic phase where the reaction occurs. Such type of catalysts are called
phase transfer (PT) catalyst and the phenomenon in named phase transfer catalysis (PTC).
PTC helps in the elevation of the environmental performance by improving pollution
prevention, green chemistry and pollution treatment. Because of its mild operating condition,
use of cheaper reagents, high selectivity of product in shorter time and suppression of
unwanted side reactions, PTC has proved to be better than the traditional synthesis method
(Weber and Gokel, 1977; Selvi et al., 2012). Effectively, what the phase transfer catalysis
involves is dissolving a nucleophilic reagent into water and electrophilic reagent into organic
solvent and then employing PTC on catalytic amounts to transfer active anion from solid or
aqueous phase into organic phase where the reaction takes place. It is necessary for the
transferred active ion pair to remain in an active form for necessary phase transfer catalytic
action, and to be reclaimed during the reaction in the organic phase.
PTC reactions are categorized into two major classes: soluble and insoluble Phase Transfer
Catalysis. Each category is then again divided into a number of classes. Soluble PTC is
further divided into liquid-liquid (L-LPTC), gas-liquid (G-LPTC) and solid-liquid (S-LPTC)
phase transfer catalysis according to the aqueous and organic phases present. Separation of
product and recovery of catalyst is difficult in case of soluble PTC. Hence, for increasing the
recovery and reuse of phase transfer catalyst, a catalyst rich layer is formed in between
aqueous and organic phase, and this kind of PTC is known as insoluble PTC.
In L-L PTC, the nucleophile (M+Y-) is dissolved in an aqueous phase whereas in S-L PTC, it
is a solid suspended in the organic phase. S-L PTC is mostly used to avoid undesirable side
reactions like hydrolysis and to increase selectivity of product.
Figure 1.4: Classification of PTC
1.6.2 MECHANISM: L-L Phase Transfer Catalysis:
There are basically two mechanisms: interfacial and extraction, which are used to describe
the L-L PTC depending on the lipophilicity of the quaternary cation. Stark‟s extraction
mechanism is applied in case of low lipophilic catalysts. In the interfacial mechanism, the
catalysts remain in the organic phase because of their high lipophilicity. In this phase, anions
are also exchanged across the liquid-liquid interface.
In the Stark‟s extraction mechanism, the quaternary onium halide (Q+X-) present in the
aqueous phase exchanges anion with the reactant (MY) present in the aqueous phase. The
ion-pair formed thus (Q+Y-) passes through the liquid-liquid interface because of its low
lipophilic nature and goes into the organic phase. Phase transfer occurs in this step. In the
organic phase the ion-pair (Q+Y-) undergoes nucleophilic substitution reaction with the
reagent (RX) forming the product (RY). Then the catalyst goes into the aqueous phase and
the process is repeated.
Figure 1.5: Stark‟s Mechanism of PTC
As mentioned before, the Stark‟s mechanism applies when the PT catalyst is hydrophilic in
nature whereas if it lipophilic in nature, then the PT catalyst cannot dissolve in the aqueous
phase. In such a case, the Brandstorm Montanari mechanism applies where the anion
exchange of the nucleophile with catalyst takes place at or near the interphase
Figure 1.6: Brandstorm Montanari Mechanism S-L Phase Transfer Catalysis:
We find that there are more industrial applications associated with L-L PTC but it has the
disadvantage of unwanted side reaction due to hydrolysis. In order to avoid this, the
nucleophile is taken in solid form suspended in organic phase, and it is called solid-liquid (SL) PTC. Thus, more yield and selectivity can be achieved through S-LPTC than L-LPTC.
During the reaction, the quaternary ion (Q+X-) first move to solid nucleophile for the
ion exchange reaction near the solid surface and active form of catalyst (Q+Y-) is formed.
Two mechanisms have been proposed for S-L PTC (Starks and Liotta, 1978, Yadhav and
Sharma 1981), one among which is fit for conditions when the inorganic salt has finite
solubility in the given solvent. This mechanism is called homogeneous solubilisation. The
other mechanism where the inorganic salt is insoluble in the given solvent is called
heterogeneous solubilisation. Therefore in homogeneous solubilisation, the quaternary ion
(Q+X-) does not come in direct contact with the solid surface of the nucleophile (KY) and the
ion exchange takes place with the dissolved KY at the interphase. In heterogeneous
solubilisation, the quaternary ion (Q+X-) has to go to surface of solid crystal lattice of the
nucleophile (KY) for ion exchange and then comes back to organic phase. The reaction of
QY with organic substrate RX takes place in bulk of organic phase (Melville and Goddard,
Figure 1.7: (a) Heterogeneous (b) Homogeneous S-L PTC G-L Phase Transfer Catalysis:
When phase transfer catalysis occurs in a gas-liquid-solid mode, it is called gas-liquid phase
transfer catalysis (G-L PTC). In this process, the organic substrate (RX) is in the gaseous
form and is allowed to pass over a bed of solid inorganic reagent (coated with PT catalyst) in
molten liquid form.
Due to constant flow of organic gaseous reactant over solid bed, G-L PTC has the advantage
of continuous mode of operation. PT catalyst can be easily replenished as it is directly loaded
on inorganic solid bed and unlike L-L PTC; there is increased selectivity as there are no
unwanted side reaction of hydrolysis.
A disadvantage related to G-L PTC is that in order to reclaim the organic substrate in the
gaseous form, the process needs to be carried out at high temperature. This may lead to the
fractional volatilization and thermal decomposition of the PT catalyst (Tundo et al., 1989).
There are many types of phase transfer catalysts such as quaternary onium salt (ammonium,
phosphonium, arsonium salts), crown ethers, cryptands, polyethylene glycol etc. Ionic liquids
have also been used in areas of phase transfer catalysis. Among several varieties of PTCs,
quaternary ammonium salts (Quats) are the most preferred for their better activity and
ease of availability. Among six different catalysts used to intensify the reaction of benzyl
chloride with solid sodium sulfide, Tetrabutylammonium bromide (TBAB) had been seen to
be the most active PTC (Pradhan and Sharma, 1990). Present study was carried out using
TBPB (Tetrabutylphosphonium bromide) as PTC as it is more stable at higher temperatures
and performs better than TBAB in adverse conditions also.
Figure 1.8: Structure of TBPB (Tetrabutylphosphonium bromide)
The qualities of a good PT catalyst are
Lipophilicity of the catalyst should be high for better transfer of nucleophilic anion to
organic phase from aqueous phase.
There must be a good partition coefficient between organic and aqueous phase.
For the cation to be easily available for anion exchange reaction in aqueous phase, the
catalyst should form loose cation-anion bonding.
The commonly used phase transfer catalysts are onium salts (ammonium and
phosphonium salts), crown ethers, cryptands and open chain polyethers like polyethylene
glycols (PEG) shown in Fig. 1.9 in the next page (Nathan, 2011).
Figure 1.9: Commonly used effective PT Catalyst
Phase transfer catalysis is an attractive technique for synthesis of organic chemicals from two
reactants existing in two or three different phases, which normally cannot react with each
other due to their low mutual solubility in other phase and their low interaction (Dehmlow
and Dehmlow, 1983; Starks et al., 1994).This technique is widely practised for large scale
production in industry as well as because of its additional advantages of milder operating
conditions, reduced consumption of solvents and raw materials, and enhanced selectivity over
the conventional process.
Ammonium Salt
Crown ethers
Very cheap
Stability and activity
Use and recovery of
Moderately active.
Commonly used but
difficult to recover.
Moderately active. Thermally more Commonly used but
stable than ammonium salt but difficult to recover.
decomposes under basic condition.
Highly active. Stable at both high Often used. Difficult
temperature and basic condition.
to recover due to
Highly active. Stable at both high Used sometimes due
temperature and basic condition.
Recovery is difficult
due to toxicity.
Lower activity but more stable than Rarely used where
onium salts.
high concentration of
catalyst does not affect
the synthesis reaction.
Easy to recover.
Figure 1.10: Properties of Commonly Used PT Catalyst (Naik and Doraiswamy, 1998)
PEG (Polyethylene glycol) is the cheapest among the usual PT catalysts unlike crown ethers
and cryptands which are very expensive. All of them are steady at high temperatures (150200ºC) and it should be kept in mind that various applications of phase transfer catalysis
require temperatures ranges between 50-120 ºC. As discussed before, quaternary onium salts
remains active and steady under these specified conditions. But PEG‟s, crown ether and
cryptands have higher stability to basic conditions as compared to quaternary onium. Insoluble Phase transfer catalysis:
The separation and recovery of catalyst is an important challenge. Conventional processes
like absorption, distillation and extraction are used for the separation of catalyst and product
from reaction mixture. If the relative volatility between solvent, product and catalyst are too
low, distillation becomes an energy consuming process. For extraction and absorption, more
solvent is required which is to be distilled off again (Yadav and Lande, 2005; Yadav and
Desai; 2005). Therefore, catalyst is generally treated as waste because of being in very less
quantity than the product (Jin et al., 2003). Also, complete separation of catalyst from the
product need not be possible leaving the product less pure.
These problems can be solved by converting biphasic PTC to tri-phasic system namely
liquid-liquid-solid (L-L-S) PTC and liquid-liquid-Liquid (L-L-L) PTC (Yadav and Motirale,
2010). Liquid-Liquid-Solid PTC:
The L-L-S PTC consists of an organic phase with the substrate, an aqueous phase with the
reagent and a solid supported PT catalyst. The mechanism involved is similar to that of
stark‟s extraction mechanism, which involves ion exchange in the aqueous phase and
synthesis reaction step in the organic phase, except for the fact that in L-LPTC, catalyst is
free to move between aqueous and organic phase while in L-L-S PTC, movement of catalyst
is restricted and aqueous and organic phase travels inside the catalyst to react with catalyst
cation (Satrio et al., 2000).The advantages of L-L-S PTC over normal PTC are:
Increase in reaction rates by orders of magnitude.
Easier catalyst recovery and reuse.
The reaction can be carried out in a continuous reactor by continuously separating the
Better selectivity.
Figure 1.11: (a) Aqueous Phase Reaction (b) Organic Phase Reaction Mechanism of L-L-S
Due to the limitations of diffusion and high cost, L-L-S PTC has not been extensively
industrialized. Also, the fact exists that there has been very little understanding of the
complex interactions between the three phases involved in L-L-S PTC.
The common examples of L-L-S PTC are Quaternary onium salts, crown ethers, cryptands,
and polyethylene glycol on various kinds of supports including polymers (most commonly
methylstyrene-co-styrene resin cross-linked with divinylbenzene), alumina, silica gel, clays
and zeolites. Liquid-Liquid-Liquid PTC:
Although the PT catalyst in L-L-S PTC can be easily separated from reaction mixture by
filtration and reused as it is bound on a solid matrix like polymer or inorganic support, the
rate of the reaction is slowed considerably because of intra-particle diffusion limitations.
Also, L-L-S PTC has a high cost of installation and catalyst preparation. In L-L-L PTC, a
middle catalyst rich phase (liquid) is formed in between aqueous and organic phase where the
reaction occurs.
There are several ways of creating this third liquid phase. It can be made by increasing
catalyst concentration above critical value or through saturation of aqueous phase. Studies
show that presence of such a third phase is accompanied by a sudden increase in reaction rate
with 100% selectivity of desired product in relatively less reaction time. Since the catalyst
rich phase is immiscible with the aqueous and organic phases, it can be easily separated and
reused. The high selectivity of product in L-L-L PTC is because of the fact that the organic
phase never comes in contact with the aqueous phase and so, unwanted side reactions of
hydrolysis are avoided.
The main advantages of L-L-L PTC are:
Even in mild operating conditions, it gives a high conversion.
The catalyst can be easily replenished to reuse.
As unwanted reactions are avoided, a high yield of the desired product is obtained.
The catalyst does not require any solid support.
L-L-L PTC requires high quantity of catalyst requirement making the initial cost of operation
very high but eventually, the catalyst can be separated and reused easily. Catalyst recovery
can be done by either reuse of only catalyst rich phase or reuse of catalyst rich phase along
with aqueous phase. There is always decrease in catalyst activity through each run due to loss
catalyst in aqueous and organic phase distribution (Yadav and Badure, 2007). However, this
method is not used for systems that require very high temperatures to carry out the reaction.
This is due to the fact that the stability of third liquid phase decreases as the temperature
Figure 1.12: Mechanism of L-L-L PTC
Presently, aqueous alkanolamines are being widely used in industries that produce H2S as byproduct for its removal. The amines mostly being used are Diethanolamine (DEA),
Monoethanolamine (MEA), and Methyldiethanolamine (MDEA). Using MEA has the
advantages of relatively low cost, high reactivity, ease of retrieval, low molecular weight
(which results high solution capacity at moderate concentrations) and low absorption of
hydrocarbons. MDEA is also used extensively because of regeneration ease, high selectivity
towards removal of just H2S or both H2S and CO2 (Zicarai et al., 2003).
Kohl and Nielsen (1997) studied the removal of H2S and its reclamation from gas mixtures
using ammonium hydroxide.
Hamblin (1973) established a method for the removal of hydrogen sulfide from gas mixtures
using ammonium hydroxide, producing ammonium hydrosulfide which was oxidised further
using a air stream to obtain ammonium polysulfide. This ammonium sulfide is then treated to
get elemental sulfur.
Asai et al. (1989) studied the rates of absorption of H2S and ammonia in water with a flat
interface in an agitated vessel.
Rumpf et al. (1999) studied the solubility of ammonia and hydrogen sulfide in water varying
the temperature from 313 to 393 K and pressure up to 0.7 MPa.
Such synthesis is done using three kinds of ammonium sulfide (i) aqueous ammonium sulfide
(ii) alcoholic ammonium sulfide (iii) ammonium sulfide. They are prepared by dissolving
equal amounts of ammonium chloride and crystalline sodium sulfide in alcohol or ammonium
Lucas and Scudder (1928) studied the reduction of 2-bromo-4-nitrotoluene to 2-bromo-4aminotoluene in a solution of ammonium sulfide dissolved in alcohol.
Idoux and Plain (1972) reduced 1-Substituted 2,4 Dinitrobenzenes using ammonium sulfide
or sodium hydrosulfide. It was found that the reduction took place at the position to which
electron donation is the least by 1- substituent.
Murray and Waters (1938) studied the reduction of p-Nitrobenzoic acid using ammonium
sulfide which was prepared by dissolving equal amounts of ammonium chloride and
crystalline sodium sulfide in alcohol or ammonium hydroxide.
Cline and Reid (1927) studied the reduction of 2,4-dinitroethylbenzene using alcoholic
ammonium sulfide. 50gm of 2,4-dinitroethylbenzene was taken in 150ml of ethyl alcohol and
reacted with 150ml of concentrated aqueous ammonia. The solution is then saturated with
H2S and boiled until there was found a 30gm increase in weight. The amine was separated by
pouring the solution over ice. The solution was then filtered off and dissolved in dilute HCl.
The acid solution was boiled with animal charcoal, filtered and cooled to separate the
hydrochloride. It was purified by recrystallizing several times with dilute acid, using animal
charcoal each time. The base was removed by NH3 and recrystallized from dilute alcohol,
which melts at 450C.
Meindl et al. (1984) studied the reduction of 3,5-dinitrobenzyl alcohol to 3-amino-5nitrobenzyl alcohol using ammonium sulfide solution. This ammonium sulfide solution was
prepared by reacting a solution of Na2S.9H20 (0.4 mol, 96g) and MeOH (250 ml) with a
solution of NH4Cl (1.6 mol, 85.6g) and MeOH (250 ml), and the NaCl is separated. The
solution so obtained was added to a solution of 3,5-dinitrobenzyl alcohol (0.2 mol, 39.6gm)
after 30 minutes in 700 ml of boiling MeOH, and the mixture was refluxed for 5 hours. The
mixture was allowed to cool at room temperature and the precipitate of sulfur was removed.
HCl (2N) was added and the solvent was distilled off. The aqueous solution was alkalized
and the product extracted with ether after the removal of starting material with ether. The
results found were : yield 62%; M.P. 91.50C.
Maity et al. (2006a, 2006b, 2008a, 2008b) have performed a lot of work in the reduction of
nitrotoluene and nitrochlorobenzenes using aqueous ammonium sulfide and TBAB as
catalyst. They also studied the reduction of p-nitrotoluene using aqueous ammonium sulfide,
and anion-exchange resin as the catalyst.
Hojo et al. (1960) worked on the kinetics of the reduction of nitrobenzene to aniline using
sodium disulfide in methanolic solution form. It was found that the rate of reaction was
proportional to the concentration of nitrobenzene and to the square of the concentration of
sodium disulfide.
Bhave and Sharma (1981) worked on the kinetics of three aromatic nitro compounds mchloronitrobenzene, m-dinitrobenzene and p-nitroaniline in two phase using aqueous
solutions of sodium disulfide and sodium monosulfide. The order of the reaction with respect
to the concentration of nitroaromatics and sulfide was found to be first order.
Pradhan and Sharma (1992b) studied the reduction of Chloronitrobenzenes to
Chloroanilines using sodium sulfide in the absence and presence of a Phase Transfer Catalyst
(PTC). The reactions of o-chloronitrobenzene and p-chloronitrobenzene gave 100%
conversion to chloroanilines in the absence of a PTC and 100% conversion to dinitrodiphenyl
sulfides in the presence of a PTC, in solid-liquid mode phase transfer catalysis whereas mchloronitrobenzene gave m-chloroaniline as the only product even in the presence of a PTC.
All the three reactants gave amine as the only product in the presence and absence of a PTC
in liquid-liquid mode phase transfer catalysis.
Pradhan (2000) studied the reduction of o-, m-, and p-nitrotoluenes to the corresponding
toluidines using sodium sulfide (as aqueous phase) in the liquid-liquid and solid-liquid
modes. TBAB acted as the Phase Transfer Catalyst. It was found that all the nitrotoluenes
were kinetically controlled in the liquid-liquid mode whereas in the solid-liquid mode, it was
found that o- and p-nitrotoluenes were kinetically controlled and m-nitrotoluene was mass
transfer controlled.
Yadav et al. (2003a) worked on the kinetics and mechanism of reduction of p-nitroanisole to
p-anisidine under liquid-liquid Phase transfer catalysis. The reaction rate was found to be
proportional to the concentration of sodium sulfide, p-nitroanisole(reactant) and TBAB
(catalyst). They have also reported on the detailed kinetics mechanism of the complex
liquid–liquid Phase Transfer Catalysis.
Yadav et al. (2003b) studied the reduction of p-chloronitrobenzene using sodium sulphide
under liquid-liquid, liquid-solid, and liquid-liquid-liquid forms of Phase transfer catalysis.
They found that There are two ways to get p-AP either we have to reduce p-NP by aqueous
Sodium Sulphide or first converting p-CNB to p-NP in situ under L-L PTC followed by
neutralization of aqueous phase and subsequent reduction with Sodium Sulphide under L-L
Rode et al. (1999) found a new method to prepare p-aminophenol by hydrogenation of
nitrobenzene (acid medium) in a single step when the conventional method involved a twostep iron-acid reduction of p-nitrophenol.
Jiang et al. (2001) studied a two phase (aqueous/organic) CO selective reduction of
nitroarenes catalyzed by Ru3(CO)9(PEO-DPPSA)3 {PEO-DPPSA- poly(ethylene oxide)substituted 4 (diphenylphosphino)benzenesulfonamide}, a thermoregulated phase-transfer
catalyst. It was found that there was good activity and selectivity towards nitro group when
carbonyl, cyano or halogen groups were present in the substrates.
Yadav et al. (2003) worked on the reduction of nitroaromatics and nitroanisoles to the
corresponding amines. They found that the reduction was affected by aqueous inorganic
sulfides, polysulfides and the rates of these biphasic reductions are responsive to
intensification under phase transfer catalysis.
Xiaozhi Liu, Shiwei Lu(2003) designed a new and efficient method for the production of
aromatic amines by catalytic reduction of aromatic nitro compounds. Selenium was used as
the catalyst. It was found that aromatic nitro compounds are reduced by H 2O/CO to form the
corresponding amines under atmospheric pressure. Irrespective of the functional groups
present on the aromatic ring, the reduction was found to be highly selective to the desired
Yadav et al. (2004) worked on the reduction of nitroaromatics to the corresponding amines.
The reduction was carried out in two-phase i.e. the nitroaromatics were first dissolved in
organic solvents and reduced using aqueous sodium sulfide and TBAB as the phase transfer
catalyst. This was liquid-liquid phase transfer catalysis and they compared it to liquid-liquidliquid phase transfer catalysis. It was found that the latter produced better selectivity and
higher rates of reaction.
Maity et al. (2006) worked on the reduction of all nitrotoluene isomers. The organic solvent
used was toluene and an aqueous ammonium sulfide acted as the reducing agent. TBAB was
the phase transfer catalyst and the experiments were done under liquid-liquid phase transfer
catalysis. The highest reaction rate was found for m-nitrotoluene, followed by p- and onitrotoluene.
Maity et al. (2007) worked on the reduction of nitrochlorobenzenes (NCBs) under liquid–
liquid mode using toluene as the organic solvent,TBAB as the phase transfer catalyst and
aqueous ammonium sulfide as the reducing agent.
Yadav at al. (2009) synthesised nitrophen from potassium 2,4-nitrophenolate and pnitrochlorobenzene under solid-liquid mode of phase transfer catalysis using PEG-400 as a
phase transfer catalyst. Xylene was used as the solvent and the experiment was done under
microwave irradiation. It was found that the participation of microwave activation and using
Solid-Liquid PTC (PEG 400) as the catalyst results in enhancement of reaction rate and
selectivity of nitrophen.
Farhadi et al.(2010) synthesised NiO nanoparticles by the thermal decomposition of
bis(dimethylglyoximato) nickel(II) complex, which was a reusable heterogeneous catalyst for
rapid and efficient microwave-assisted reduction of nitroarenes with ethanol.
Robert Kaplanek, Viktor Krchnak(2013) studied the parameters for a rapid and efficient
reduction of aromatic nitro compounds to hydrophobic polystyrene-based Wang and Ring
resins. They used sodium dithionite in dichloromethane–water under PTC conditions.
Tetrabutylammonium hydrogen sulfate (TBAHS) was the phase transfer catalyst found
effective for the reduction of nitro groups to amino groups under mild conditions with 100%
conversion. This method proved to be a better alternative to tin (II) chloride-based reduction.
Kiasat, Ali Reza (2011) devised an eco-friendly and simple method for the reduction of
Nitroarenes wherein Polyethylene glycol was easily grafted to silica gel and used as a phase
transfer catalyst under solid-liquid mode. This silica-grafted polyethylene glycol along with
zinc powder in water was found to be an efficient heterogeneous catalyst in reducing
nitroarenes to the corresponding aromatic amines. The reduction reaction did not affect other
sensitive functional groups and proved to have good chemoselectivity.
The methods used in the reduction of Nitroarenes are:
Bechamp reduction: Being the oldest method practiced industrially, it uses stoichiometric
amounts of finely divided iron metal (tin, zinc, and aluminium can also be used) and water in
the presence of small amount of acid. The main disadvantage associated with this process is
the formation of iron sludge, which is difficult to separate and dispose off in the environment.
Furthermore, this method cannot be used for substrates harmed by acid media or for the
reduction of a single nitro group in a polynitro compound.
Catalytic hydrogenation: In this method, metal hydrides like lithium aluminum hydrides are
used for reduction. Besides being expensive due to the requirement of expensive equipment
and facility to handle the hydrogen, it may also pose problems due to the risk of preparation
of catalyst, poisoning hazards due to catalyst, and the risk of reducing other groups too.
Moreover, the metal hydrides tend to convert the nitro compounds to a mixture of azoxy and
azo compounds.
Sulfide reduction: It enables chemoselective reduction of nitro compounds in the presence
of C=C, azo and other nitro compounds. The sulfide reduction of nitroarenes is commonly
carried out by sodium sulfide, disulfide, hydrosulfide, and ammonium sulfide.
Zinin reduction: The reduction reaction of nitroarenes by negative divalent sulfur (sulfide,
hydrosulphide and polysulphide) is called Zinin reduction.
The overall stoichiometry of the Zinin‟s original reduction of nitrobenzene by aqueous
ammonium sulfide is given below (Dauben, 1973). This stoichiometry is also applicable for
the reduction of nitroarenes by sodium sulfide (Bhave and Sharma, 1981; Pradhan and
Sharma, 1992; Pradhan, 2000; Yadav et al., 2003b, 2003c).
2R + 6 S + 7H2O
2R + 3 S2O3 + 6 HO
Figure 2.1: Stoichiometry of Zinin‟s original reduction.
Hydrogen sulfide (H2S)-rich alkanolamine was prepared in the laboratory using Kipp‟s
apparatus. The chemicals used for its preparation and estimation of sulfide content present in
it are as follows. Toluene (≥ 99.5 %) of analytical grade (used as solvent), NMethyldiethanolamine was acquired from Sigma-Aldrich (France) Ltd. The iron sulfide (FeS)
sticks were obtained from Thermo Fisher Scientific India Pvt., Ltd., Mumbai, India. Sodium
thiosulfate, Potassium iodide, Starch powder, Sulfuric acid (98 % pure) and Sodium
hydroxide pellets of analytical grade used for the iodometric titration for the estimation of
sulfide content in the H2S rich N-Methyldiethanolamine (MDEA) were purchased from
Rankem (India) Ltd., New Delhi, India. The reactant and PT catalyst used, 4Nitroacetophenone (NAP) and Tetra-n-butylphosphonium bromide (TBPB) respectively,
were acquired from Sigma-Aldrich (USA) Ltd. The water used here was distilled using a
distillation column.
H2S-rich aqueous N-methyldiethanolamine (MDEA) was prepared by firstly making a 3035%
methyldiethanolamine in distilled water. H2S gas is then produced in the Kipp‟s apparatus as
shown in Fig. 3.1 in the next page, by reacting FeS sticks with 1 molar H2SO4. The produced
gas is then taken from the apparatus through a tube to a 250 mL standard gas bubbler
containing the prepared N-methyldiethanolamine solution.
Such a reaction between H2S and N-methyldiethanolamine (MDEA) is exothermic (Kohl and
Nielsen, 1997) and so, the gas bubbler containing the N-methyldiethanolamine was kept on
top of an ice water bath. This helps to prevent the oxidation of sulfide and consequent
formation of disulfide. The outlet of this bubbler was open to the atmosphere. The H2S gas
was bubbled into the MDEA solution until the required concentration of sulfide content was
achieved, which was ensured by taking out samples at regular intervals of time and subjecting
them to iodometric analysis.
1M H2SO4
To Atmosphere
FeS Sticks
Ice Water
Figure 3.1: Schematic Diagram for Absorption of H2S in MDEA Solution
The reactions were carried out in a batch reactor (250 cm3), the temperature of which was
controlled using a thermostat. It also consisted of a mechanically agitated glass reactor that
was fully baffled and three-necked. The borosilicate glass beaker of the reactor was where the
solutions were contacted and the three-necked flask served purposes for agitating the
solution, inserting the thermometer, taking solution samples and feeding the solutions. In
order to agitate the solution mixture, there was provided a six bladed glass impeller of 2.0 cm
diameter at a height of 1.5 cm from the bottom of the glass reactor. Provision for speed
regulation is also included. Such an arrangement led to excellent solid-liquid mixing and high
mass transfer rates. Water was poured into the batch reactor around the glass reactor forming
a constant temperature water bath, the temperature of which could be controlled within ±1°C,
by mechanical stirring using an electric motor. The diagram of the experimental arrangement
is as shown in Fig.3.2 in the next page.
Motor with Digital Display
3-Necked glass reactor
Org. Phase
Aq. Phase
Temp. Controller
Figure 3.2: Batch Reactor Assembly
At first, 30 ml of aqueous phase with a known sulfide concentration was poured into the
three-necked glass reactor. The thermostat in the batch reactor was switched on with the
reactor well stirred to attain a steady state temperature. After attaining the required steady
state temperature, organic phase containing prescribed amounts of organic reactant (4Nitroacetophenone) and phase transfer catalyst (TBPB) dissolved in 30 ml toluene (organic
solvent) was transferred to the three-necked glass reactor. Since both phases were immiscible,
two layers were formed in the glass reactor, the organic phase above the aqueous phase. In
order to initiate the reaction, the glass reactor was kept in a constant temperature water bath
and mechanically agitated using the stirrer at constant speed. After regular known intervals of
time, the agitation was stopped and the phases were allowed to separate. About 0.1 ml of the
organic phase was pippeted out of the glass reactor. The samples were taken at intervals of 5,
10, 15, 30, 60, 120, 240, 360 and 480 minutes. Therefore the reaction extended for a total of 8
hours wherein 9 samples of the organic phase were taken out. The samples were transferred
into glass vials.
3.5.1 Determination of Sulfide Concentration
Determination of initial sulfide concentration was done by standard iodometric titration
method (Scott, 1966) as given below.
3.5.2 Preparation of standard (0.025 M) KIO3 Solution:
0.535 gm of KIO3 was weighed accurately and dissolved in 100 ml of distilled water and kept
in a graduated volumetric flask.
3.5.3 Preparation of standard (0.1 M) sodium thiosulfate Solution:
24.818 ~ 25 gm of Na2S2O3.5H2O crystals was weighed and dissolved in 1000 ml of distilled
water and kept in a graduated volumetric flask. Approximately, 0.1 g of sodium carbonate or
three drops of chloroform was added to this solution to retain the solution for more than a few
3.5.4 Preparation of standard (5% w/v) KI Solution:
5 gm of KI was weighed accurately and dissolved in 100 ml of distilled water and kept in a
graduated volumetric flask.
3.5.5 Preparation of standard (1 M) Sulfuric acid Solution:
53.26 ml of concentrated (98%) H2SO4 was taken and dissolved in distilled water to make up
1000 ml and kept in a graduated volumetric flask. Since this is an extremely exothermic
reaction, the Sulfuric acid must be added slowly to the distilled water under a cool water bath.
3.5.6 Preparation of standard (10M) NaOH Solution:
40 gm of NaOH pellets were weighed accurately and dissolved in 100 ml of distilled water
and kept in a graduated volumetric flask.
3.5.7 Standardization of Sodium Thiosulfate Solution by standard Potassium Iodate
25 ml of 0.025M KIO3 solution was taken. To it, 1 gm (excess) of potassium iodide (KI) was
added along with 3 ml of 1 M Sulfuric acid. According to the reaction below, the iodine was
liberated and titrated against thiosulfate solution (in the burette) until the colour of solution
changed from brown to pale yellow. Then the solution was made up to 200 ml using distilled
water and few drops of 0.2% iodine starch solution were added to it, changing the colour of
the solution to violet. After this, the titration was continued until colour changed from violet
to colourless. Readings of the burette during the colour changes were noted.
The starch solution was prepared by boiling 100 ml of distilled water and then adding 0.2 g
starch powder to the 100 ml boiled distilled water. The solution was again boiled for 2
minutes and then cooled and 2.5 gm of KI was added to make it 0.2% iodine starch solution.
The chemical reaction involved in this titration is given below.
KIO3 + 5KI + H2 SO4 ↔ 3I2 + 3H2 O + 3K 2 SO4
2Na2 S2 O3 + I2 ↔ Na2 S4 O6 + 2NaI
Therefore, 1 mole of KIO3 ≡ 3×2 mole of Na2S2O3.
∴ Strength of Thiosulfate Solution =
6 × Strength of KIO3 × Volume of KIO3
Volume of Thiosulfate Consumed
3.5.8 Estimation of Sulfide Concentration:
Hydrogen sulfide and soluble sulfides oxidise with potassium iodate in an alkaline medium.
This is used for sulfide estimation. 1 ml of the aqueous phase (H2S-rich MDEA) was taken
from the gas bubbler to a graduated volumetric flask and then made up to 100 ml using
distilled water to form the sulfide solution. A conical flask was taken and to it was added 10
ml of the diluted aqueous phase along with 15 ml of standard 0.025M potassium iodate
solution and 10 ml of 10M sodium hydroxide solution. The resulting mixture was then boiled
for about 10 minutes. After cooling, 20 ml of 4M Sulfuric acid solution and 5 ml of 5% KI
solution were added to the conical flask. The iodine liberated was titrated with 0.1 M Sodium
thiosulfate. When the colour in the conical flask turned pale yellow, it was diluted to 200 ml
using distilled water. This is followed by the addition of a few drops of 0.2% iodine starch
solution, changing the colour of the solution in the conical flask to violet. After adding starch
solution, the titration was continued until colour changes from violet to colourless. Readings
of the burette during the colour changes were noted.
The oxidation of sulfide to sulfate by potassium iodate in the alkaline medium is given by
reaction below.
4IO3− + 6OH − + S 2− → 4SO2−
4 + 4I + 6H2 O
4 moles of IO3− = 3 moles of S 2−
∴ H2 S Concentration = 15 × Siodate −
Where, 𝑆𝑖𝑜𝑑𝑎𝑡𝑒
Vthiosulfate × Sthiosulfate
= Strength of KIO3
𝑉𝑡ℎ𝑖𝑜𝑠𝑢𝑙𝑓𝑎𝑡𝑒 = Volume of thiosulfate
𝑆𝑡ℎ𝑖𝑜𝑠𝑢𝑙𝑓𝑎𝑡𝑒 = Strength of thiosulfate
𝑁𝑑 = Number of times of dilution
3.5.9 Analysis of organic phase:
The conversion of 4-Nitroacetophenone (NAP) to 4-Aminoacetophenone (AAP) in the
different sample vials were evaluated using gas liquid chromatography (GLC). GC is mainly
used as an analytical technique in industrial laboratories for the measurement and
identification of quantity of compounds present in a sample mixture. The working of a Gas
chromatography or GC is simple. It consists of a mobile phase and a stationary phase. The
mobile phase may be any inert gas like Helium or non-reactive gas like nitrogen. The
Stationary phase is essentially a very thin (microscopic) layer of liquid coated over the
In the process, firstly a pre-determined quantity of sample is injected with a microsyringe
onto the chromatographic column of the GC. The carrier gas carries the sample through the
stationary phase (coated column). Depending on the adsorption properties of different
components of the sample, they elute and have different retention time in the column. A
detector at the outlet of the column determines the retention time and amount of components
leaving the column.
Flame Ionization Detector (FID) is the most common detector for organic components. In
FID, we find electrodes at the end of the column along with burning fuel of air/hydrogen.
When the organic compounds exit the column, the flam pyrolizes the carbon compound and
with this, +ve and –ve ions are produced. This creates and electrical pulse between the
electrodes. The electrical pulse is instantaneous and appears in the form of peaks on the graph
(chromatogram) at the retention time of the component.
In present work, GC-MS from Agilent Technology of model 7890B was used with FID
Figure 3.3: A Gas Chromatography Assembly
Program for MS and FID was evaluated given below:
Injection Volume = 1μl
Heater = 2500C
Pressure = 14.306 psi
Total Flow = 84.6 ml/min
Purge Flow = 3 ml/min
Mode = Split
Split Ratio = 50:1
Agilent DB-5ms
Flow = 1.6 ml/min
Pressure = 14.306 psi
Holdup Time = 1.0793 min
Avg Velocity = 46.325 cm.sec
Oven Temperature = 600C
Maximum Oven Temperature = 3240C
Rate (0C/min)
Value (0C)
Hold Time
Run Time
Ramp 1
Ramp 2
Injection Volume = 1μl
Heater = 2500C
Pressure = 16.724 psi
Total Flow = 44.6 ml/min
Purge Flow = 3 ml/min
Split Ratio = 25:1
Agilent DB-5ms
Flow = 1.6 ml/min
Pressure = 16.724 psi
Holdup Time = 1.3436 min
Oven Temperature = 600C
Maximum Oven Temperature = 3240C
Rate (0C/min)
Value (0C)
Holdup Time
Retention Time
Ramp 1
Ramp 2
Heater = 3000C
Air Flow = 400 ml/min
H2 Flow = 30 ml/min
Make up Flow (N2) = 25 ml/min
Column Flow (N2) = 1.6 ml/min
The chromatogram of NAP system is shown in Fig.3.4. From the chromatogram, we learn
that toluene is not participating in the reduction reaction and act as a solvent.
Figure 3.4: Chromatogram of NAP Synthesis
In Scheme 1 given below, the equations (1) to (4) explains the ionic equilibrium between
sulphide ions (S 2−) and hydrosulphide ions (HS −) in H2 S-rich Alkanolamine solution.
During the reduction of NAP by H2 S-rich aqueous MDEA solution, (S 2−) and (HS −) ions
help in the formation of elemental sulfur or thiosulphate. The presence of both these ions
makes H2 S laden aqueous alkanolamine solution different than conventional reducing agents.
Scheme1. Ionic equilibria in H2S-MDEA-H2O system (Maity et al., 2007)
Eq. (5) explains the overall stoichiometry of the Zinin‟s original reduction of nitrobenzene by
aqueous ammonium sulphide, as proposed by Zinin in 1842. The same stoichiometry is
followed by reduction of nitroarenes by sodium sulphide [17,30,31,33].
It was seen from literatures that instead of thiosulphate, elemental sulfur can be
produced as a by-product if aqueous ammonium sulphide is used as a reducing agent when pnitrophenylacetic acid is reduced to p-aminophenylacetic acid as shown in Eq. (6) (H.Gilman,
From the above two reactions, it is clear that S 2−& HS − have reduced the reactant to
give elemental sulfur or thiosulphate as by-products. In the presence of a base, ammonia, the
dissociation equilibrium favours toward more ionization and the concentration of sulphide
ions (S 2−) relative to hydrosulphide (HS −) ions increases in the aqueous phase with the rise
in the ammonia concentration (Maity et al., 2006b).
The overall stoichiometry of the reduction reaction using sodium disulphide as the reducing
agent is as follows (H.Gilman, 1941).
Scheme 2. Proposed mechanism of reduction of NAP by H2S-laden MDEA under L-L PTC
Based on literature studies on reactions of reduction of nitroarenes by sodium sulphide, a
general reaction mechanism has been proposed (Scheme 2 in the previous page). Sulfur has
the ability to exist in various valency states from (-2) to (+6) making it favourable for the
formation of different anions ( HS −, HSO−, HSO−
2 , HSO3 ). These anions are capable of
pairing with quaternary cations in a rapid manner than other anions which require multiple
quaternary cations ( Q+
n X ). During reduction of nitroaromatic compounds by aqueous
sulphide solution, the nitro group present in the nitro aromatic compound is reduced by the
transfer of an electron from sulphide ion.
Present system is a liquid-liquid PTC system, consisting of an organic phase (4Nitroacetophenone dissolved in toluene solvent), an aqueous phase ( H2 S absorbed in
aqueous solution of MDEA) and quaternary phosphonium salt (TBPB), partitioned into both
the phases. According to Starks extraction mechanism, nucliophiles (anions) present in the
aqueous phase get attached to the catalyst cations and then are transported to the organic
phase for taking part in reaction with organic substrates.
In the aqueous phase, hydrosulphide ions (HS−) and sulphide (S 2−) anions are formed from
the reaction of H2S reacting with MDEA as shown in Scheme 1. As soon as the Quaternary
cation(Q+) comes into contact with the aqueous phase, it readily forms (Q+HS −) and
(Q+S 2−Q+) ion pair. This is followed by a series of reactions taking place in the organic
phase as shown in Scheme 2. Since the organic substrate has limited solubility in the aqueous
phase, reactions occur near the interphase between anions present in aqueous phase and
organic substrate on the organic phase. This is confirmed by the fact that products are formed
in the absence of catalyst also.
The contribution of several elementary reactions in the organic phase to the overall rate of the
reaction is elaborated by the above mechanism. In a series of complex elementary reactions,
4-Nitroacetophenone is converted to 4-Aminoacetophenone through the formation of
intermediates (4-nitrosoacetophenone and 4-hydroxylaminoacetophenone), both of which
haven‟t been detected by the GC-MS. Although the existence of these intermediates during
Zinin reduction have been established, the fact that they cannot be detected may be due to the
fact that of faster disapperance of the intermediates in the organic phase. When the catalyst
cations are pairing with the HS − anions, some water molecules transfer to the interphase and
they take part in the reaction (Eq.(19) to Eq. (21)). After a series of elementary reactions, we
can see from Scheme 2 that the ion pair 𝑄 +𝐻𝑆𝑂3− is formed in the organic phase and it is
transferred to the aqueous phase where it reacts with S 2− to regenerate 𝑄 +𝐻𝑆 − according to
Eq. (15). The regenerated quaternary cations are transferred to the organic phase again for the
reduction reaction and this completes a typical catalytic cycle.
As we can see from Scheme 2, the quaternary cations pair with different anions during
the reactions but majority of the catalyst cations remain in 𝑄 +𝐻𝑆 − form and the catalysis
cycle goes on. Nine reactions (Eq. (8) – Eq. (16)) took place in the aqueous phase and rest of
them (Eq. (17) – Eq. (22)) in the organic phase. 100% selectivity of 4-Aminoacetophenone is
obtained at the end of the reaction.
In the present work, more valuable chemicals like 4-Aminoacetophenone was synthesised
from 4-Nitroacetophenone dissolved in toluene using H2S-rich MDEA in a batch reactor
under L-L PTC in the presence of Tetrabutylphosphonium Bromide (TBPB). This process is
considered to be lower in cost and more environment friendly as compared to the Claus
process owing to the fact that H2S gas (noxious) is being utilised and the gas is produced in
plenty as byproduct gas in various industries.
Aminoacetophenone (AAP) is used used in clinical biology as a detector or marker for
Pseudomonas Aeruginosa in the cystic fibrostic lung. It is also a drug metabolite as it helps in
the biochemical modification of pharmaceutical substances by living organisms through
enzymes. AAP plays a major role in the determination of vitamin B6 and in the asymmetric
total synthesis of pactamycin. It was used as a starting agent during the synthesis of curcumin
mimics with substituted sulphonyl group. Also, it acts as a bifunctional coupling reagent
during the synthesis of pyrimidines. It has potential antibacterial properties and is used in the
synthesis of HIV-1 growth inhibitors and selective antagonists at human adenosine receptors.
AAP is also used as an intermediate in organic synthesis of pharmaceutical products like
Coumarins (benzopyrones).
The influence of process variables like stirring speed, elemental sulfur loading, concentration
of reactant, temperature, sulphide concentration, catalyst loading on the conversion NAP was
4.4.1 Effect of stirring speed:
The main objective of examining the effect of stirring speed on the reaction is to find the
effect of mass transfer resistance on the reaction kinetics. This is because along with the
reaction kinetics, the mass transfer is also important in influencing the conversion or rate of
the L-L PTC. The reaction was carried out at stirring speeds of 500, 1000, 1500 and 2000
revolutions/minute (rpm) under the exact same experimental conditions and in the presence
of TBPB as the phase transfer catalyst. From Fig. 4.1 (a), it is clear that the conversion of 4Nitroacetophenone is not influenced by the speed of agitation when it is above 1000 rpm.
This kind of observation was found to be quite unique as other reaction systems required a
high stirring speed to reach a more or less constant level of conversion.
Figure 4.1 (a): Effect of stirring speed on conversion of NAP.
Figure 4.1 (b): Effect of stirring speed on conversion of NAP with and without catalyst.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3 in org. phase, Concentration of toluene = 4.721 Kmol/m3 in
org. phase, Concentration of catalyst = 0.0442 Kmol/m3 in org. phase, Concentration of sulphide = 2.5
Kmol/m3, Concentration of MDEA = 3.04 Kmol/m3, Temperature = 323 K.
This is probably because the active intermediate of the catalyst (QSQ) is hydrophobic and
favours to stay in the organic phase, wherein the interfacial area is not important, whereas
other reaction systems require a larger interfacial area to increase the mass transfer rate. Thus
we can conclude from the above figures that the variation of conversion of NAP with the
speed of agitation is very small (in 1000-2000 rpm range) and so, the mass transfer resistance
becomes unimportant and we can say that the reaction is purely kinetically controlled. All
further experiments were carried out at a stirring speed of 1500 rpm to ensure there is no
mass transfer resistance.
4.4.2 Effect of Temperature:
Experiments were carried out with NAP in toluene catalyzed by TBPB in the presence of H 2S
rich MDEA under various reaction temperatures ranging from 400C to 700C. Rest of the
operating conditions were exactly the same for the experiments. The effect of temperature on
the conversion of NAP can be seen from the Fig. 4.2. From the transition state theory, we
know that rates of organic reactions increase with increase in temperature. This is why it can
be safely assumed that increasing temperature will elevate the rate of slow organic phase
reactions in the PTC system as the activation energy of molecules is overcome and more
molecules react to form the product when the temperature is increased. Furthermore, at
higher temperature, collision of the reactants is also increased. Hence, the reaction rate
undeniably increases with increase in temperature.
Figure 4.2: Effect of temperature on conversion of NAP.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3 in org. phase, Concentration of toluene = 4.721 Kmol/m3 in
org. phase, Concentration of catalyst = 0.0442 Kmol/m3 in org. phase, Concentration of sulphide = 2.5
Kmol/m3, Concentration of MDEA = 3.04 Kmol/m3, Stirring speed = 1500 rpm.
The initial rate of reaction of NAP was calculated for different temperatures and an Arrhenius
plot of ln (initial rate) vs. 1/T was made. The apparent activation energy for the kinetically
controlled reaction was calculated from the slope of best fitted the straight line as 9.645
Figure 4.3: Arrhenius plot for activation energy.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3 in org. phase, Concentration of toluene = 4.721 Kmol/m3 in
org. phase, Concentration of catalyst = 0.0442 Kmol/m3 in org. phase, Concentration of sulphide = 2.5
Kmol/m3, Concentration of MDEA = 3.04 Kmol/m3, Stirring speed = 1500 rpm.
4.4.3 Effect of catalyst (TBPB) loading:
The effect of catalyst (TBPB) loading on the conversion of NAP by H 2S- laden aqueous
MDEA was studied in the range of 0-0.0737 Kmol/m3 under otherwise identical experimental
conditions, as shown in Fig. 4.4. With the increase in catalyst quantity, the conversion of
NAP as well as reaction rate increases. Only by increasing the catalyst concentration, NAP
conversion of more than 80% was achieved whereas it was about 23% without catalyst after
480 minutes of reaction under otherwise identical conditions. This shows the importance of
PTC and our work.
The overall reaction rate of a L-L PTC (such as the present system) is controlled by the
transportation of anions (S2-, HS-, and S2-) from aqueous phase to organic phase. In presence
of PTC, the transportation of these anions is facilitated and the reaction becomes organicphase limited. In the aqueous phase, the anions readily form ion pairs [Q+HS-and Q+S2-Q+]
with the quaternary cations [Q+]. The ion pairs are then transported to the organic phase in
order to react with NAP as shown in Scheme 2. With increased catalyst concentration, more
amount of [Q+HS-and Q+S2-Q+] ion pairs are formed and transferred to the organic phase and
reacts 4-Nitroacetophenone (NAP) to form 4-Aminoacetophenone (AAP).
Figure 4.4: Effect of catalyst loading on conversion of NAP.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3 in org. phase, Concentration of toluene = 4.721 Kmol/m3 in
org. phase, Concentration of sulphide = 2.5 Kmol/m3, Concentration of MDEA = 3.04 Kmol/m3,
Stirring speed = 1500 rpm, Temperature = 323 K.
To find out order of reaction, the initial reaction rate at different TBPB concentration is
calculated and its natural logarithm is plotted with natural logarithm of catalyst concentration
as indicated in Fig. 4.5 below. From the slope of the linear fit line, the order of reaction was
determined. The order of the reaction with respect to TBPB concentration is found out to be
Figure 4.5: Plot of the natural logarithm of the initial rate vs the natural logarithm of the
catalyst conc.
Concentration of TBPB
(kmol/m3 org phase)
Initial reaction rate
Table 4.1: Initial reaction rate and enhancement factor.
4.4.4 Effect of sulfide concentration:
The effect of sulfide concentration in the aqueous phase on the conversion of NAP with
TBPB as PT catalyst in the presence of H2S rich MDEA was studied, by keeping the MDEA
concentration in the aqueous phase constant at 3.04 M. The sulfide concentration was varied
in the range of 1 Kmol/m3 to 2.5 Kmol/m3. It was found that for constant MDEA
concentration, the reaction rate increased with an increase in the sulfide concentration in the
aqueous phase, and a maximum of 47.5% conversion was achieved at the end of 480 minutes
with 2.5 Kmol/m3 of sulfide as shown in Fig.4.6.
Figure 4.6: Effect of sulfide concentration on conversion of NAP.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3 in org. phase, Concentration of TBPB = 0.0442 Kmol/m3 in
org. Phase, Concentration of toluene = 4.721 Kmol/m3 in org. phase, Concentration of MDEA = 3.04
Kmol/m3, Stirring speed = 1500 rpm, Temperature = 323 K.
This trend can be attributed to the fact that when more amount of sulfide is introduced, there
will be more ionization in the aqueous phase to produce more HS-, S2-, and eventually S22ions as shown in Scheme 2. This leads to greater formation of ion pairs [Q+HS-and Q+S2-Q+]
with the Quaternary cations [Q+]. These ion pairs are then transported to the organic phase to
cause reduction of NAP to the desired product. Therefore, more the sulfide concentration,
more the ion pairs which in turn increases the reduction and hence the reaction rate.
From the slope of the linear fit line, the order of reaction was determined. The order of the
reaction with respect to sulfide concentration is found out to be unity.
Figure 4.7: Plot of the natural logarithm of the initial rate vs the natural logarithm of the
Sulfide conc.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3 in org. phase, Concentration of TBPB = 0.0442 Kmol/m3 in
org. Phase, Concentration of toluene = 4.721 Kmol/m3 in org. phase, Concentration of MDEA = 3.04
Kmol/m3, Stirring speed = 1500 rpm, Temperature = 323 K.
4.4.5 Effect of MDEA concentration:
From literature, we find that MDEA does not have direct impact on reaction rate, but it does
affect the equilibrium among MDEA, H2S and water. As mentioned above in Scheme 1, in
the aqueous phase, sulphide (S2-) and hydrosulphide (HS −) active anions are formed which
are responsible for two different reactions (Eq. (5) & Eq. (6)). Basic nature of MDEA favours
more ionization and hence, sulphide ions (S2-) are more in number than hydrosulphide ions
(HS −) in the aqueous phase. The existence of two reactions can be proven only by varying
MDEA addition in the aqueous phase and fixing the sulphide concentration.
Various MDEA concentrations (sulfide concentration kept constant) were prepared by
taking 14.5cm3 of H2 S-laden aqueous MDEA (with known sulfide and MDEA
concentrations) solution and adding into it various proportions of pure MDEA and distilled
water in such a way that the total volume was made up to 30 cm3.
During the course of reaction colour of the aqueous solution was changed from
greenish yellow to orange and then to reddish brown which is useful in indicating the extent
of reaction. As reaction proceeds polysulphide formed which is reddish brown in colour.
Similar phenomenon was observed by Lucas and Scudder, 1928.
In this study, after 8 hrs of long run, 32.86% conversion of NAP was achieved with
maximum MDEA concentration of 6 Kmol/m3 while sulphide concentration in aqueous phase
was 1.20 Kmol/m3, as shown in the Fig. 4.8 in the next page. Conversion of NAP obtained is
much higher than we can get from Eq. (5) or Eq. (6). This leads to the conclusion that the Eq.
(7) is dominant over other two reactions as reaction progresses and polysulphide (formed
when elemental sulfur produced in Eq. (6) reacted with sulphide ions present in aqueous
phase) ions were formed during the course of reaction.
Formation of elemental sulfur was not reported for the reduction of Nitroarenes with
sodium sulphide (Pradhan and Sharma,1992; Yadav and Sengupta,2003b) and it can be
assumed that this reaction follows the stoichiometry of Eq. (5) by transfer of sulphide ions. It
was observed that initially with increase in MDEA concentration the conversion of NAP was
low up to a certain reaction time but then opposite trend was observed and finally higher
NAP conversion was achieved. This may be because at first, the reaction followed Eq.(17)
and Eq.(19) of Scheme 2. Eq.(19) was essential for the formation of elemental sulfur and
subsequent formation of disulfide and polysulfide ions. Increase in MDEA concentration
causes more ionisation of S2- ions rather than HS- ions. Since Eq.(17) is the main rate
determining step, lower HS- ions lowers the rate of reaction. But with time, disulfide ions are
formed which reduce Nitroarenes at a faster rate, thus making up for the increased overall
reaction rate. Therefore, for a fixed sulphide concentration in aqueous phase, the increase in
MDEA concentration results higher sulphide ions and that boost up the conversion of NAP.
Figure 4.8: Effect of MDEA concentration on conversion of NAP.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3 in org. phase, Concentration of TBPB = 0.0442 Kmol/m3 in
org. Phase, Concentration of sulfide = 1.20 Kmol/m3, Concentration of toluene = 4.721 Kmol/m3 in
org. phase, Stirring speed = 1500 rpm, Temperature = 323 K.
4.4.6 Effect of Reactant variation:
The effect of concentration of NAP on its conversion was studied at four different
concentrations in the range of 0.3028-1.2110 Kmol/m3 in the presence TBPB under otherwise
identical experimental conditions, as shown in Fig. 4.9. It is clear that with the increase of
concentration of NAP, its conversion is decreased. About 90% conversion of NAP was
obtained for a reactant concentration of 0.3028 Kmol/m3 and 45.6% conversion of NAP for
reactant concentration of 1.2110 Kmol/m3, after 480 minutes of reaction under otherwise
identical experimental conditions.
The increase in the reaction rate during the initial stage of reaction can be attributed to
increase in the concentration of the reactant NAP. Hence initially, when there is more number
of molecules of NAP reacting with more number of molecules of TBPB and H2S-rich
MDEA, it leads to more conversion. But the amount of sulphide in the aqueous phase
remained the same for all the experimental runs, which is why the conversion dropped
beyond a certain concentration of NAP in the reaction. For reduction of o-nitroanisole by H2S
rich Diethanolamine, similar observation was found (Maity and Pradhan, 2007). Conversion
and reaction rate were found to increase with decrease in NAP concentration at the end of 8
hours of reaction time.
Figure 4.9: Effect of reactant concentration on conversion of NAP.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of TBPB = 0.0442 Kmol/m3 in org. Phase, Concentration of sulfide = 2.5 Kmol/m3,
Concentration of toluene = 4.721 Kmol/m3 in org. phase, Concentration of MDEA = 3.04 Kmol/m3,
Stirring speed = 1500 rpm, Temperature = 323 K.
The order of reaction with respect to NAP concentration was obtained as unity. Hence the
reaction is first order with respect to the concentration of reactant. A similar observation was
found for reduction of Nitroarenes by aqueous sodium sulfide (Yadav and Sengupta, 2003b).
Figure 4.10: Plot of the natural logarithm of the initial rate vs the natural logarithm of the
NAP conc.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of TBPB = 0.0442 Kmol/m3 in org. Phase, Concentration of sulfide = 2.5 Kmol/m3,
Concentration of toluene = 4.721 Kmol/m3 in org. phase, Concentration of MDEA = 3.04 Kmol/m3,
Stirring speed = 1500 rpm, Temperature = 323 K.
4.4.7 Effect of sulfur loading:
The effect of elemental sulfur loading on conversion of NAP in the presence of H2S rich
MDEA and TBPB as the PT catalyst is shown in Fig. 4.11 in the next page. The elemental
sulfur was introduced into the aqueous phase. The dark greenish colour of H2 S-laden MDEA
solution became orange when elemental sulfur was added in it. As seen from Fig. 4.11,
initially the reaction rate increased with elemental sulfur addition but after certain time,
reaction rate slowed down. This can be attributed to the fact that with the elemental sulfur
introduced, it can combine with sulfide ions present in the aqueous phase to form disulfide
and polysulfide ion pairs. We know from literature that disulfide ion pairs can transfer to the
organic phase from the aqueous phase in quicker pace and they are capable of reducing NAP
at a faster rate than sulfide or hydrosulfide ion pairs. This is why initially; there is an increase
in the reaction rate when more elemental sulfur is introduced. But after a certain reaction
time, polysulfides increase in number having no effect on the conversion of NAP. Similar
explanation was given by Lucas and Scudder (1928). Also, the reason for decrease in reaction
rate may also be due to the reactants depleting with the elemental sulfur build up (which leads
to disulfide and polysulfide formation). Hence, it is found that conversion for zero elemental
sulfur addition is the highest at the end of 8 hour reaction time.
Figure 4.11: Effect of sulfur loading on conversion of NAP.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3, Concentration of TBPB = 0.0442 Kmol/m3 in org. Phase,
Concentration of sulfide = 2.5 Kmol/m3, Concentration of toluene = 4.721 Kmol/m3 in org. phase,
Concentration of MDEA = 3.04 Kmol/m3, Stirring speed = 1500 rpm, Temperature = 323 K.
4.4.8 Effect of Different catalysts:
The effect of different catalysts on the conversion of NAP was studied for 3 different
catalysts in otherwise identical conditions as shown in Fig. 4.12. TBAB and TBPB were
introduced at 0.0442 Kmol/m3 while Amberlite 400 was introduced at 0.29 Kmol/m3. It was
found that Tetrabutylammonium Bromide effected the highest conversion of 67%. This was
followed by Amberlite 400 having effected a conversion of 60%. TBPB which is known to be
more stable at adverse conditions effected a conversion of 44%.
Figure 4.12: Effect of Different catalysts on conversion of NAP.
Operating conditions: Volume of organic phase = 30 ml, Volume of aqueous phase = 30 ml,
Concentration of NAP = 0.9082 Kmol/m3, Concentration of sulfide = 2.5 Kmol/m3, Concentration of
toluene = 4.721 Kmol/m3 in org. phase, Concentration of MDEA = 3.04 Kmol/m3, Stirring speed =
1500 rpm, Temperature = 323 K.
The present work is a green process owing to the fact that it consumes H2S, which is a
commonly produced by-product gas, to produce value-added chemicals. With the declining
availability of easy-to-process crude oil, refineries are forced to use heavy oil containing high
sulfur content leading to the emission of dangerously high levels of H2S. The importance of
this work is directly reflected in this fact.
H2S is produced in Kipp‟s apparatus and then absorbed in aqueous MDEA. The sulfide
concentration in the MDEA after the Kipp‟s absorption was measured using Iodometric
technique. The samples drawn after the reaction was subjected to GC-MS.
4-Aminoacetophenone (AAP) was produced batch-wise by the reduction (Zinin reduction) of
4-Nitroacetophenone (NAP) using TBPB as phase transfer catalyst and H2S-rich MDEA as
the reducing agent. The effect of various process parameters like stirring speed, temperature,
catalyst loading, sulfide concentration, elemental sulfur loading, MDEA concentration and
reactant concentration were studied. Lastly Amberlite 400 and TBAB were used as phase
transfer catalysts in different experiments to compare the conversion results with that of the
ones with TBPB as the PTC.
A mechanism for the course of the reaction was determined from the results of the above
studies. It was found that the regeneration of the catalyst took place when [QHSO2] reacted
with the nitroso intermediate to give amine and [QHSO3] in the organic phase. This [QHSO3]
reacted with S2- in the aqueous phase to give [Q+HS-], which is used again in the process.
It was found that between 1000-2000 rpm, there was no significant effect in the conversion of
NAP, justifying the absence of any mass transfer effect on the reaction kinetics. As expected,
the reaction rate increased with increase in temperature and catalyst concentration. From the
studies of the effect of temperature on the conversion of NAP, the apparent activation energy
of the kinetically controlled reaction was found to be 9.645 KJ/mol. This owes to why the
reactions gave high conversion in short intervals of time. The enhancement factors were
found out from the catalytic studies and tabulated. Increase in sulfide concentration increased
the conversion of NAP. Although MDEA concentration did not have direct effects on the
conversion of NAP, it was found that experiments with higher MDEA concentration
eventually resulted in higher conversions (due to explained reasons in the appropriate
sections). As the reactant concentration was increased, a trend of decline in the conversion of
NAP was observed. Although increase in sulfur loading increased the reaction rates initially,
experiments with lower sulfur loading eventually gave higher conversions (as polysulfide
ions were produced).
In the present work, it was observed that there was complete utilization of sulfide in the
aqueous phase and therefore, no further treat was required for re-use. However, TBPB being
a soluble phase transfer catalyst, it becomes very difficult for its reclamation and reuse.
Nevertheless, there is a high probability for the present process to overthrow conventional
processes for disposal of H2S.
From section 4.2, we saw the mechanistic pathway for the reaction of NAP with H2S- rich
MDEA (Scheme 2). It was based on some earlier studies and observations from the present
study. The Aqueous phase can be analyzed to find out the different species present in it.
These species can be quantified in an MS to get a better understanding of the mechanistic
pathway of the aqueous phase in L-L system. If this practice is followed, then it will help in
getting a better insight into the reaction mechanism, and also to perform kinetic modelling.
Statistical modelling can also be done in the “Design Expert” software, where certain
additional experiments may be required to be performed at higher temperatures and catalytic
concentrations. Such a modelling could help us get simultaneous interactive effect of two
parameters on the NAP conversion.
Presently, Toluene is being used as the solvent for all the experiments as it is cheap and easily
available. The physical properties of the solvent have an important part to play in the Phase
transfer catalytic reactions. Not only do they carry out the reactions due to the presence of
HS-, S2- and S22-, they are also responsible for the transport of the phase transfer catalyst and
active catalyst species (QSQ and QSH). Many other solvents like chlorobenzene, n-heptane,
and dichloromethane can be tested for better conversion of NAP. Other co-catalyst and anion
can also be tested for better conversion.
Most of the effluent gases from industries contain both H2S and CO2 as major acid gas
impurities. Therefore we need to test H2S absorption efficiency with different aqueous
solutions. Although MDEA proved to be selective towards H2S, it is next to impossible to get
a 100% selective solvent. Since some CO2 always remains in the aqueous phase, we also
need to study the effect of CO2 on our system.
The present work needs to be scaled up and larger reactors should be utilized before the
process is taken up to an industrial scale. An efficient method for product separation and
purification should be designed. Although identification of the products are done through
GC-MS, other techniques such as nuclear magnetic resonance spectroscopy (NMR), Fourier
transform infrared spectroscopy (FTIR) can also be used.
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