“Synthesis of Aromatic Dibenzyl Disulfide using H S and Solid

“Synthesis of Aromatic Dibenzyl Disulfide using H S and Solid
“Synthesis of Aromatic Dibenzyl Disulfide using H2S and Solid
Reusable Resin as Catalyst”
Thesis Submitted by
In partial fulfillment for the award of the Degree of
Under the guidance of
May, 2014
This is to certify that the thesis entitled “Synthesis of Aromatic Dibenzyl Disulfide using H2S and Solid
Reusable Resin as Catalyst”, submitted by Dorothy Chetia (212CH1496) to National Institute of Technology
Rourkela, in fulfillment of the requirements of the degree of Master of Technology in Chemical Engineering is a
bonafide record of the research work carried out by her, in the Department of Chemical Engineering, National Institute
of Technology, Rourkela under my supervision and guidance. Miss Dorothy has worked on this topic from July, 2013
until May, 2014 and the thesis, in my opinion, is worthy of consideration for the award of the degree of “Master of
Technology” in accordance with the regulations of this Institute. The results embodied in the thesis have not been
submitted to any other University or Institute for the award of any degree or diploma.
Date: 23 May 2014
Dr. Sujit Sen
Department of Chemical Engineering
National Institute of Technology
Rourkela, Orissa-769008
I wish to express my heartily gratitude to my guide Dr. Sujit Sen for his valuable guidance
with constant flow of existing new ideas and contagious enthusiasm through my work. My
association with my guide will remain a memorable part of my life. I want to thank sir for
constantly motivating me through him valuable counsel as well as excellent tips to build my
research and writing skills.
I want to acknowledge the support and encouragement given by Priya Nakade, Ujjal
Mondal and Gaurav Singh throughout the period of my work.
I also take this opportunity to express my sincere thanks to Dr. R. K. Singh, Head of
Department of Chemical Engineering for making available necessary facilities to complete this
research work. I am also grateful Dr. B. Munshi and all other faculty members of department for
their help whenever sought for.
I am obliged to all my friends for their friendships and encouragements. 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, for their unconditional love and constant
Date- 23/05/2014
Fig. No.
Page No.
Fig. 1.1
Structural formulas of different Alkanolamines
Fig. 1.2
Flow diagram of Amine treating process
Fig. 1.3
Process flow diagram of Sulfur Recovery unit
Fig. 1.4
Stark’s Mechanism
Fig. 1.5
Brandstorm Montanari Mechanism
Fig. 1.6
Schematic diagram of classification of PTC
Fig. 1.7
Two distinct mechanisms for solid liquid are
(a) Heterogeneous solublization, (b) Homogeneous solublization
Fig. 1.8
Activity of liquid-liquid-solid tri-phase catalysis
Fig. 1.9
Ion exchange resin
Fig. 3.1
Schematic diagram for the absorption of H2S in MEA solution
Fig. 3.2
Schematic diagram of Batch reactor
Fig. 3.3
A Gas Chromatography
Fig. 3.4 (i)
Chromatogram at 5 min
Fig. 3.4 (ii)
Chromatogram at 480 min
Fig. 4.1
Fig. 4.2
Effect of speed of agitation
Fig. 4.3
Effect of elemental sulfur loading
Fig. 4.4
Effect of Temperature
Fig. 4.5
Arrhenius plot for Activation energy
Fig. 4.6
Effect of catalyst loading
Fig. 4.7
Plot of natural logarithm of initial rate versus the natural logarithm
of catalyst concentration
Fig. 4.8
Effect of concentration of Benzyl Chloride
Fig. 4.9
Effect of sulfide concentration
Fig. 4.10
Mechanism of Triphase Catalysis
Fig. 4.11
Conversion of Benzyl Chloride
Table No.
Page No.
Table 1.1:
Advantages and Disadvantages of MEA
Table 1.2:
Advantages and Disadvantages of DEA
Table 1.3:
Advantages and Disadvantages of MDEA
Table 1.4:
Various commonly used PT catalyst
Phase Transfer Catalysis
Benzyl Chloride
Benzyl Mercaptan
Dibenzyl Sulfide
Dibenzyl Disulfide
Gas Liquid Chromatography
One variable at a time
Parts per million
Triethanolamine TPC Tri Phase Catalyst
During hydro desulfurization or hydro treatment of crude oil in petroleum refineries, one
or more gaseous by products are produced with hydrogen sulfide (H2S) infused into it. Since the
presence of H2S, corrodes the process equipment and is highly toxic to the environment, it is
eliminated from the gas stream and transformed into harmless forms. In general, H2S from gas
stream is removed using amine treating unit and is treated in the Claus unit for the production of
elemental sulfur. This elemental sulfur is used mostly to produce sulphuric acid and also in rubber
industries. But in Claus process valuable hydrogen energy is lost and requires exact process control
over ratio of oxygen to hydrogen sulfide feed. Therefore, an alternative process which could
transform H2S to valuable chemicals, is greatly welcome in the process industry in improving the
sulfur disposal problem. H2S laden gas can be used in a more beneficial way by absorbing into
aqueous alkanolamine like monoethanolamine (MEA) and then use this reagent to produce fine
chemicals like disulfides, thioether, mercaptans, , amines etc., which have high usability and good
market value. These organo-sulfide such as Dibenzyl disulfide (DBDS) find their application in
manufacturing corrosion inhibitors, fragrance compounds, high pressure lubricant additives and
other organic compounds. Since the alkanolamine phase and the organic phase are not miscible,
to carry out this biphasic reaction, phase-transfer-catalyst (PTC) has been tired out a few times and
often the application of PTC gives enhancement in reaction rate, more conversion for reactants
and better selectivity of desired products.
This work was carried out to study the synthesis of fine chemicals like Dibenzyl Disulfide
(DBDS) and using H2S-rich monoethanolamine (MEA) and benzyl chloride (BC) under LiquidLiquid-Solid phase transfer catalysis with a tri-phase catalyst. Owing to its solid state, availability,
cost and reusability, Amberlite IR 400 is selected as a PTC. A parametric study was carried out
which emphasized upon the impact of catalyst loading as one of the process variables on the
conversion of benzyl chloride and selectivity of DBDS and parametric study with these substances
revealed 100 % selectivity for desired product at ambient condition. The effects are utilized to
establish a suitable mechanism of the reaction to explain the course of the reaction. The overall
objective of this study is to maximize conversion of reactant BC and selectivity of DBDS.
Keywords: Hydrogen Sulfide; Alkanolamines; Dibenzyl Disulfide; Liquid-Liquid-Solid Phase
Transfer Catalysis; Mechanism; Selectivity; Conversion.
Title Page
Certificate by the Supervisor
List of Figures
List of Tables
List of Abbreviations
Chapter 1
Source of Hydrogen Sulfide
Reasons for removal of H2S
Industrial Process for removal and recovery of H2S
Ammonia based process
Alkanolamine process
Types of amines: Advantages & Disadvantages
Claus process of Sulfur recovery from H2S gas
Other ways to process sour gas
Wet Oxidation Lo-Cat process
Present Work
Phase Transfer Catalysis
Mechanism of PTC
Major classification of PTC
Types of Phase Transfer Catalyst
Choice of Immobilized PTC
Tri-phase transfer catalysis
Catalyst used in tri-phase catalysis
Chapter 2
Chapter 3
Chapter 4
Literature Survey
Use of aqueous alkanolamines for the removal of H2S
Liquid-Liquid Phase Transfer Catalysis
Synthesis of Dibenzyl-Disulfide
Use of Triphase catalyst
Experimental Setup
Preparation of H2S-rich aqueous alkanolamine
Experimental procedure
Method of Analysis
Determination of Sulfide concentration
Analysis of Organic Phase
Reaction of Benzyl Chloride with H2S-rich aqueous mono-etha
-nolamine under liquid-liquid-solid phase transfer Catalysis
Results and Discussion
Effect of Speed of Agitation
Effect of elemental sulfur loading
Effect of temperature
Effect of catalyst loading
Effect of concentration of benzyl sulfide
Effect of sulfide concentration
Chapter 5
Mechanistic Investigation
Mechanism of Triphase Catalysis
Recycle and reusability of catalyst
Summary and Conclusions
Future Work
5.2.1 Finding out Effect other parameters
Chapter 1
Chapter 1
About 90 % of the hydrogen sulfide gas available in the air originates from the natural
resources like volcanoes, hot springs and underwater thermal vents. Hydrogen sulfide is released
into the air as a result of the breakdown of dead plant and animals, particularly when this happens
in damp conditions with inadequate supply of oxygen, for example in swamps and sewers. This
process is known as anaerobic digestion. Maximum man made hydrogen sulfide is not produced
from industry, merely as a by-product of manufacture. Anthropogenic discharges of H2S into the
air is the outcome of industrial processes, particularly from the extraction and purification of oil
and mineral gas and from paper and pulp manufacturing, chemical manufacturing, however the
gas is also present in sewage treatment plants, tanneries, waste disposal and coke oven plants. The
principal source of anthropogenic hydrogen sulfide is as a by-product in the hydro-treatment
process of natural gas and refined form of crude petroleum. H2S can be liberated into air from
anywhere where elemental sulfur occurs in contact with organic material, mainly at high
temperatures. Atmospheric releases of hydrogen sulfide most significantly affect the public health
because of the geothermal energy industry. The importance of hydro-treatment is increasing day
by day in the refineries. This is imputable to the reason that the source of crude oil which are easy
and light to process is progressively turning down besides the people working in the refineries all
over the globe are enforced to hold down the sulfur and nitrogen concentration to that particular
level set by the environmental protection agency. In the course of processes like hydro treatment
and hydro-desulphurization of crude oil in coal and petroleum industries, one or more gaseous by
products are produced with H2S and NH3 infused into it. The coal gas contains 3.3% H2S and
about 1.1% NH3 as the primary non-hydrocarbon impurities. The variance in the composition of
crude natural gas can be seen broadly from field to field. The content of H 2S ranges between 0.1
ppm and 150,000 ppm.
Because of the environmental protection agency’s rules and ordinance, the refineries all
over the world are facing problems with the disposal of hazardous materials such as H2S a
satisfactory manner. The various lawsuits for the removal of H2S gas are described as follows:
(Occupational and safety health administration)
Hydrogen sulfide is a highly noxious and volatile in nature. It may move along the ground and
can also deposit in low lying, enclosed and poorly ventilated areas. With continuous low level
exposure or at high concentration, the victim remains unaware and loses its ability to smell
even though it still exists. Low concentrations affect the eyes, nose, throat and respiratory
system, whereas high concentration leads to shock, breathless, rapidly unconsciousness, coma
and death. If the level of H2S gas exceeds 100 ppm, it would immediately be risky to life and
health. For safety purpose, the material safety data sheet (MSDS) of H2S should be referred.
Hydrogen sulfide is a poisonous in very low concentrations, highly corrosive gas in the
presence of air and the monetary value of continuous care and replacement of pipelines, pipes
and other equipment makes the prospect of managing natural gas with high levels of H2S to
be breakeven at best. Gas stream must be made completely free from H2S before use and
preferably before transportation. Consequently, the amount of H2S is limited to less than
0.25gm/100ft3 of gas in the case of pipeline specification (Thomas, 1990).
Since hydrogen sulfide gas is a flammable gas and thus mixtures of gas and air can occur
explosion. This gas/air mixture may travel to ignition source and flash back. If the mixture
travels to the ignition source and as a result it gets ignited than the gas from the mixture burns
to produce toxic vapours and harmful gases, namely sulfur dioxide. The existence of H2S in
the refinery gas streams can hamper the posterior processes by equipment degradation,
increase in the process pressure requirements, increase in the gas compressor capacity,
deterioration or deactivation of catalyst, unwanted side reactions etc.
Many industrial processes have been developed for the removal and the recovery of H2S
from the fluid stream. Due to the acidic (weak acid) nature of H2S, it can be removed by using
some alkaline solution. Strong alkaline solution such as sodium hydroxide forms irreversible
chemical reaction products and as a result cannot be utilized for the elimination of hydrogen
sulphide from the gas streams. It becomes more unfavourable when the gas contains both
hydrogen sulphide and carbon dioxide and the CO2 concentration is more than 4% (Robin 1999).
So, weak alkaline solutions like ammonia and alkanolamines are used for the removal and
retrieval of the H2S.
1.3.1 Ammonia based processes:
This process is utilized normally for eliminating acidic gases from flue gases produced by
refineries, petrochemical industrial, and other helps. Removal of H2S using aqueous ammonia has
become a well-established process and well-practiced by Hamblin, 1973 and Harvey and
Makrides, 1980. As stated by this process, H2S and NH3 rich gas stream passes through a H2S
scrubber and NH3 scrubber placed in series. NH3 scrubber is provided with stripped water from
the top where ammonia is absorbed from the gas. The following ammonical solution is utilized as
absorbent in H2S scrubber. The solution which comes out from this unit carries ammonium
sulphide, which is decomposed to yield hydrogen sulphide and ammonical liquor in the
decandifier. The process is depicted in the figure below. The reactions are:
𝑁𝐻3 + 𝐻2 𝑂 → 𝑁𝐻4 𝑂𝐻
𝑁𝐻3 + 𝐻2 𝑂 → 𝑁𝐻4 𝐻𝑆
2NH3 + H2 O → [NH4 ]2 𝑆
The main advantages of using ammonia based process are:
Ammonia based process is well acceptable for gas streams having both H2S as well as
NH3. The elimination of ammonia and hydrogen is done in single step in ammonia process
while in alkanolamine it is done in two steps.
Ammonia is one of the most largely produced chemicals in the world for the reason that
it is a low-cost solvent, does not degrade in the presence of O2 and other species in the
flue gas, and also less corrosive, in comparison to other amines. The effects of ammonia
on environment and health are well considered and found more benevolent than amines.
Ammonia has high CO2 removal efficiency and low regeneration energy.
When a gas carrying both CO2 and H2S comes in contact with aqueous ammonia solution,
the H2S is absorbed at a high rate. The selective absorption of H2S and CO2is feasible in
liquid ammonia by changing the concentration of liquid ammonia. The selective
absorption of H2S can be done by using spray column and giving short time contact.
In spite of these advantages, the use of ammonical scrubbing has not been accepted all over the
world in the gas treating technique as the ideal method for removing H2S from a gas stream. This
is for the reason of a number of functioning problems related with its applications (Hamblin,
1973), such as:
High partial pressure of ammonia forces the scrubbing step to lead with relatively dilute
NH3 solutions or at relatively high pressures or a distinct water wash step after the NH3
scrubbing step in order to eliminate NH3 from the treated gas stream. Moreover, the
utilization of dilute scrubbing solutions normally increases considerably the regeneration
costs where the regeneration step is conducted at a significantly higher temperature than
the scrubbing step.
The regeneration of rich absorbent solution withdrawn from the scrubbing step consists of
the use of soluble catalysts, so sulphur products get contaminated in the bearing of the
1.3.2 Alkanolamine process:
In the past few years, alkanolamine process for the elimination of acid gases has achieved
full acceptance. This is due to its merits such as ease of retrieval and low vapour pressure. In case
of operational temperature, pressure and concentration of alkanaloamine, the low vapour
pressures gets the operation more flexible. In this process, alkanolamine is used at a high
concentration with the carelessness of a corrosion inhibitor. Triethanolamine(TEA) was the first
alkanolamine employed in early gas treating plants (Bottoms, 1930). However, this amine has
been displaced to a large extent by Diethanol-amine (DEA) and Monoethanol-amine (MEA)
which possess the advantage of lower molecular weights and are capable of effecting more
complete H2S removal. The most important amines that are used for purification of gas are
Diethanolamine (DEA), Monoethanolamine (MEA) and Methyl- diethanolamine (MDEA). These
amines replaced the Triethanol-amine due to its low capacity, its low reactivity and its relatively
poor stability. These various amine mixtures have got mixture of names with formulated amines
and Methyldiethanolamine based amines. Traditionally, MDEA has been known mainly due to
its ability of selective absorption of H2S from a gas and giving CO2 in the gaseous state. The
structures of amines are:
Monoethanolamine (MEA)
Diethanolamine (DEA)
Diisopropanolamine (DIPA)
Methayldiethanolamine (MDEA)
2-(2-aminoethoxy) Ethanol
Triethanolamine (TEA)
Fig 1.1: Structural Formulas of Different Alkanolamines
Each of the above alkanolamine has one hydroxyl as well as one amino group. The
hydroxyl group helps to decrease the vapor-pressure and increase the solubility in water. The
amino group offers the required alkalinity in water so that the H2S gets absorbed.
1.3.3 Types of amines: Their advantages and disadvantages
1. Monoethanolamine (MEA)
Table 1.1: Advantages and Disadvantages of MEA
The molecular weight of MEA is low and
It is not possible for MEA to
so it has high solution capability at
absorb H2S selectively from gas
moderate concentrations.
stream carrying both H2S and CO2.
MEA is a comparatively strong base
It is corrosive in nature.
having a fast reaction rate and yielding a
Heat of reaction with H2S and CO2
low CO2 concentration.
leads to high energy requirement
It is the most reactive in the reaction.
for stripping.
2. Diethanolamine (DEA)
Table 1.2: Advantages and Disadvantages of DEA
It is less corrosive than MEA.
Due to low vapour pressure of DEA, it is
suitable for operation.
distillation DEA undergoes numerous
The reclaiming of contaminated DEA
considerably less reactive with COS and
producing corrosive ruin products,
CS2 as compared to primary amine. So it
and therefore, it may not be the
is a better option for gas stream carrying
optimal option for handling gases
both COS and CS2.
carrying a high CO2 content.
3. Methyldiethanolamine (MDEA)
Table 1.3: Advantages and Disadvantages of MDEA
Selectively absorb H2S from gas streams
The cost of MDEA, which is higher
containing both H2S and CO2.
than the other amines, has prevented
Energy saving because of lower desorption
its use.
temperature and lower heat of reaction
compared to MEA and DEA.
It is less corrosive than MEA and DEA.
Since it has low vapor pressure, MDEA
can be utilized in concentration up to 60
weight% in aqueous solutions with less
evaporation losses.
1.3.4 Process:
The absorption reaction proceeds in absorber and then stripping of absorbed gases takes
place in stripping column. The concentrated H2S gas is then subjected to sulfur recovery. The basic
flow diagram of the amine-based acid gas removal process is depicted in figure 1.2. Treatment
with alkanolamine involves the circulation of the gas stream upward through the absorber, countercurrent to the flow of aqueous alkanolamine solution. This solution coming from the bottom end
of the absorber is heated with the help of heat exchanger with lean solution from the bottom of the
stripping column where the absorbed gases are stripped of from the alkanol amine solution. The
regenerated alkanolamine is then recycled to the absorber. The concentrated hydrogen sulfide gas
obtained from top of the stripping column is then subjected to S recovery or disposal.
Reaction with H2S:
2𝑅𝑁𝐻𝟐 + 𝐻2 𝑆 ↔ [𝑅𝑁𝐻3 ]2 𝑆
𝑅𝑁𝐻2 + 𝐻2 𝑆 ↔ 𝑅𝑁𝐻3 𝑆𝐻
Reaction with CO2
2𝑅𝑁𝐻2 + 𝐶𝑂2 + 𝐻2 𝑂 ↔ [𝑅𝑁𝐻3 ]2 𝐶𝑂3
𝑅𝑁𝐻2 + 𝐶𝑂2 + 𝐻2 𝑂 ↔ 𝑅𝑁𝐻3 𝐶𝑂3 𝐻
2𝑅𝑁𝐻2 + 𝐶𝑂2 ↔ 𝑅𝑁𝐻 − 𝐶𝑂 − 𝑂𝑁𝐻3 𝑅
The process diagram:
Fig 1.2: Flow Diagram of Amine Treating Process
Through this process, the toxic H2S gas is converted to non-toxic and useful elemental
sulphur. It is a standard process used in industries. The basic purpose is to recover sulfur from the
gaseous H2S originated in natural gas as well as from the by-product gases. The by-product gases
mostly originated from several gas treatment units found in various refineries, natural gas
processing units, gasification plants etc. Hydrocarbons, sulfur dioxide or ammonia and hydrogen
cyanide are present in these by-product gases. H2S gas, i.e. separated by stream of gas using amine
extraction, is provided to the Claus unit. In Claus unit, it is transformed in two steps i.e. the thermal
step and the catalytic step. The H2S rich gas in a reaction furnace reacts in a sub-stoichiometric
combustion in the presence of air at temperatures between 1000 - 1400 ºC in the thermal step.
Claus gases with no more flammable substances apart from H2S are burned in burner. This is a
strongly exothermic oxidation reaction. The ratio of air to the acid gas is controlled in such a way
that in total one third of all H2S is converted to SO2. This ensures a stoichiometric reaction in the
catalytic step. In the catalytic step, the reaction gases leaving the sulfur condenser are reheated to
200 -350ºC and fed to the series of catalytic converter and sulfur condenser where H2S react with
SO2 to produce elemental sulphur. The catalyst used in the catalytic converter is normally either
activated aluminium (III) or titanium (IV) oxide. At last, a little quantity of H2S remains in the tail
gas known as the residual quantity. This residual is mixed with other trace of sulphur compounds
to give overall sulfur recovery of about 99.8%. Sulfur is utilized for making sulfuric acid,
greasepaints, fertilizers, medicine, manures, rubber products and elemental sulfur.
Reaction in the thermal step:
2𝐻2 𝑆 + 2𝑂2 → 𝑆𝑂2 + 2𝐻2 𝑂 + 𝑆
Reaction in the catalytic step:
2𝐻2 𝑆 + 𝑆𝑂2 → 3𝑆 + 2𝐻2
Fig. 1.3: Process Flow diagram of Sulfur Recovery Unit
However, the process has got certain disadvantages (Plummer et a.; 1994; Beazley et al.;
1986; Plummer and Zimmerman, 1986) such as:
It operates at high temperatures.
It needs exact process control over the ratio of oxygen to H2S in the feed.
The valuable hydrogen energy is lost in this process.
It needs costly pre-treatment of the feed gas if CO2 is present in high concentrations.
The residue of Claus process i.e., the sulphur gas released to the atmosphere is usually too
high to meet strict environmental guidelines. To obey these guidelines, it is crucial to add
more Claus stages and/or employ a distinct tail gas clean-up process at great expense.
Amine extraction is not suitable for the treatment of H2Sgases which contains high
concentration of CO2. Therefore such streams go through processing by CrystaSulf, a “liquid
redox” process used for the treatment of natural gas.In such type of processes, a liquid solution
carrying oxidized iron is used in substitution of air. As an emblem of novelty, H2S Splitting Process
is also being developed to make hydrogen as well as sulfur from H2S.
1.5.1 CrystaSulf:
CrystaSulf is the name of a chemical process which has been developed specifically in
refineries to separate H2S from sour natural gas and gas streams (Deptt. of Energy Report).
CrystaSulf has reasonable operating costs and reasonable capital costs, which makes it a good fit
for several mid-size sulfur retrieval applications. It is quite flexible process and can be used in two
different cases i.e. direct removal of H2S from gas stream or clause tail gas treatment. The liquid
phase Claus reaction of H2S with SO2 is used to convert H2S into sulfur and the sulfur thus obtained
is then removed by filtration. In CrystaSulf process heavy liquid hydrocarbon gush out in a sudden
and forceful stream through an absorber where the liquid comes in contact with H2S rich gas
streams. In the following step, H2S gets absorbed from the gas stream and the resulting clean gas
stream then leaves absorber. The H2S existing the liquid reacts with the SO2 to form elemental
sulfur and water.
2𝐻2 𝑆 + 𝑆𝑂2 → 3𝑆 + 2𝐻2 𝑂
1.5.2 Wet Oxidation LO-CAT Process:
A liquid phase oxidation process utilizes solution that are not noxious but a little alkaline
which contains an iron chelate complex as a redox catalyst. This process involves the conversion
of H2S into elemental sulfur, taking into consideration the environmental issues. Gas stream
carrying H2S is contacted with aqueous LOCAT solution in a venture-scrubber. The H2S absorbed
in solution is instantly reacts with oxygen to form elemental sulfur. The reduced catalyst H2S
solution is regenerated by air in a vessel known as oxidizer. Sulfur is eliminated from the
circulating solution by going down in the underside of the oxidizer or in a free-standing nonaerated vessel. Sulfur can be retrieved from the sulfur slurry by melting or via centrifuge. The
process can take away to 99.9% of H2S from gas flow with any H2S concentration.
The current work was undertaken to develop low cost and environmentally benign
alternatives to the Claus process for better consumption of H2S present in several gas streams. The
current work deals with various byproduct gases to be utilized for the synthesis of fine value added
chemicals obtained from different chemical industries. In conformity with the present process,
value-added chemicals were produced from the H2S-rich aqueous ammonia or alkanolamine that
could be obtained from scrubbing step of the corresponding ammonia- or alkanolamine-based
process. In other word, the removal of H2S was expected to be performed by conventional process.
The current investigations are dedicated to:
Synthesis of value-added fine chemicals like dibenzyl disulfide and dibenzyl sulfide using
the H2S -rich aqueous alkanolamines under three phase (liquid-liquid-solid) conditions in
the presence of Amberlite IR 400 as PTC.
Examine the influence of process variables like stirring speed, elemental sulfur loading,
concentration of reactant, temperature, sulphide concentration, catalyst loading on the
conversion of organic reactants as well as on the selectivity of products.
Reusability of catalyst is also tested, which is the main advantage of using a solid catalyst
over a liquid catalyst.
Establishment of suitable mechanism utilizing the effects of several parameters on the rate
of reaction as well as on the conversion to explain the course of the reaction.
PTC is an exceptionally effective technology which improves environmental performance
over pollution prevention, green chemistry and pollution treatment. There are reactions which are
unable to take place since reactants are unapproachable to one another. The issue relating the
carrying an organic water insoluble electrophilic reagent and water soluble nucleophilic reagent
together has been arises. This issue was resolved conventionally by adding solvent that is both
hydrophilic and lipophilic. An effective process developed over a last twenty five years is
dissolving nucleophilic reagent into water and electrophilic reagent into organic solvent and then
employed on a catalyst amount which transfer active anion from solid/aqueous phase into organic
phase where the reaction takes place. This is Phase Transfer Catalysis (PTC) process. On the other
hand, it is likewise essential that the transferred active ion pair remains in an active form for
necessary phase transfer catalytic action, and that it is reclaimed during the reaction in the organic.
PTC is nowadays a broadly used process for organic synthesis due to its various merits such as its
selectivity, simplicity, mild operating conditions, inexpensive catalyst, easily available catalyst,
reduced consumption of organic solvent, raw materials, easily available bases for anion generation
and enhanced rate of reaction.
1.7.1 Mechanism of Phase Transfer Catalysis:
The mechanism of the PTC was best described by Stark et al.; (1970) where a quaternary
onium halide present in the aqueous phase (Q+X-) go through anion exchange with the anion (Y) of the reactant (MY) present in the aqueous phase. The ion-pair formed (Q+Y-) is able to pass
the liquid-liquid interface because of its lipophilic nature and goes from the interface into the
organic phase, this step is the phase transfer. Since the anion of the ion-pair in the organic phase
is nucleophilic, it go through a nucleophilic substitution reaction with the reagent (RX) forming
the product (RY).
Figure 1.4: Stark’s Mechanism
The catalyst then proceeds to the aqueous phase and in this way the cycle goes on. Fig.1.4 shows
actual mechanism of PTC. This mechanism is used only when catalyst is hydrophilic in nature.
Sometimes it may possible that PT catalyst is so lipophilic in nature that it can’t dissolve in aqueous
phase. In this case, anion exchange of nucleophile with catalyst takes place at or near the
interphase. This mechanism was given by Brandstorm Montanari is also known as interfacial
mechanism shown in fig. 1.5.
Figure 1.5: Bransdstorm Montanari Mechanism
1.7.2 Major classification of PTC:
PTC reactions are categorized into two categories: soluble and insoluble Phase Transfer
Catalysis. Each category is again further carved up into a number of classes. Depending upon the
physical form of the phases, PTC systems generally composed of, as shown in fig 1.6. To discuss
the whole major classification is quite lengthy. So briefly, let us discuss the classification tree of
PTC to one by one starting from soluble PTC.
The soluble phase transfer catalysis (Soluble PTC) is again classified as liquid-liquid phase
transfer catalysis (L-L PTC), solid-liquid phase transfer catalysis (S-L PTC) and gas-liquid phase
transfer catalysis (G-L PTC). The nucleophile (M+Y-) is dissolved in an aqueous phase in the case
of L-L PTC but in the case of S-L PTC, it is solid suspended in the organic phase. In L-L systems
more applications have been reported regarding PTC but sometimes S-L are also used in order to
escape undesirable side reactions like hydrolysis to increase selectivity of product. There are
basically two mechanisms, namely, 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 to low lipophilic catalysts and the mechanism has already been mentioned above. The
interfacial phenomenon allows the catalysts to remain in the organic phase only owing to their
high lipophilicity. In this phase, anions are also exchanged across the liquid-liquid interface.
Fig 1.6: Schematic diagram of classification of PTC
In S-L PTC, (Starks and Liotta, 1978, Yadhav and Sharma 1981), two mechanisms have
been proposed. One among the two is appropriate for conditions in which there is finite solubility
in the inorganic salt in the given solvent. There is no direct reaction between the catalyst and the
solid surface. And the second mechanism is functional in various situations where the inorganic
salt is either soluble or insoluble in negligible proportions in the organic solvent and there is direct
reaction between the catalyst and the solid surface. This whole mechanism is known as
homogeneous and heterogeneous solubilisation (Melville and Goddard). Furthermore, an increase
in rate of reactions due to less amount of water in solid-liquid phase is termed as omega phase.
The two distinct mechanisms are shown in the fig 1.7.
Figure 1.7: Mechanisms for Solid-Liquid are
(a) Heterogeneous solubilization (b) Homogeneous solubilization
G-L PTC involves the utilization of phase transfer catalyst in gas-liquid-solid systems. In
this system, the organic substrate that remains in a gaseous form, is allowed to pass over a bed
comprising of the inorganic reagents which are layered with a phase transfer catalyst in its melted
state. But extraction and regaining of the PT catalyst from the organic phase becomes very
expensive and difficult process because it gets almost dissolve in the organic phase and most of
the times it is treated as waste. If it becomes possible to inhibit the PT catalyst to a third insoluble
phase, whether it is liquid or solid, then the separation step can be carried away easily without any
difficulty. The commonly used immobilized phase transfer catalysis is tri-phase system and third
liquid PTC.
1.7.3 Types of phase transfer catalyst
Many varieties of PTC are available, such as phosphonium salts and quaternary ammonium
salts, crown ethers, cryptands, ionic liquid, polyethylene glycol, etc. Out of these, the quaternary
ammonium salts are widely used in the industry. PEG is the most economical while cryptands and
crown ethers are extremely costly among the usually used PT catalysts. PEG’S, cryptands, crown
ethers and ionic remains steady at higher temperatures (150-200ºC). One should keep in mind that
various applications of phase transfer catalysis require temperatures ranges between 50-120 ºC and
quaternary onium salts remains active, steady, and is broadly appropriate under these specified
conditions. But PEG’s, crown ether and cryptands have higher stability to basic conditions as
compared to quaternary onium salts but have some disadvantages.
Table 1.4: Several commonly used PT catalysts (Sanjeev D. Naik and L. K. Doraiswamy)
Stability and Activity
Use and Recovery of
Moderately stable under normal Broadly used & recovery
up to 100º C. Moderately active.
Phosphonium Expensive
compared to ammonium salts, but is difficult to some extent.
not much stable under basic
Crown Ethers Expensive
is comparatively difficult.
as Widely used but recovery
Stable and highly active both Frequently
under usual conditions and also at regaining of the catalyst is
higher temperature between 150 difficult
to 200 ºC.
environment due to its
toxic nature.
Stable and highly reactive, except Used sometimes despite
in the presence of strong acids.
of high cost and toxic
nature, due to
More stable than quaternary Often used. It can be in
lower use where larger amounts
of catalyst creates no
easy to
Moreover, the separation and recovery of catalyst are also important challenge. Solid phase
transfer catalyst,commonly recognised as reusable reagents have attracted growing attentions, by
reason of their specific advantages, such easily recovering and reusing of the catalyst.
1.7.4 Choice of Immobilized PTC (Tri-phase catalysis):
Over insoluble PTC, the conventional soluble PTC has one major drawback of being nonrecoverable and non-reusable from the liquid phase. Although several chemical separation
processes, e.g. distillation or extraction, are available. But it can be difficult to separate the catalyst
from the product through these processes and may considerably affect the monetary value as well
as product purity. To overwhelm this primary problem, immobilized phase transfer catalyst on a
solid support such as a polymeric resin or inorganic solid can be preferred and implement. This
method is widely known as tri-phase transfer catalysis.
1.7.5 Tri-phase transfer catalysis (TPC):
In TPC, the catalyst distribution in the reaction system is more restricted since the active
sites of the catalyst are immobilized on the solid support. Reactants present in both organic and
aqueous phases need to transfer from their corresponding phases to the surface of the catalyst to
contact the catalytic (active) sites. The activity of tri-phase catalyst, thus, can be summarized
stepwise as:
a. Mass transfer of reactants from bulk liquid phase to the surface of the catalyst.
b. Diffusion of the reactant molecules from the surface of the catalyst particle to the active
sites inside the porous particle.
c. Intrinsic reactivity of reaction at the active sites.
These external and intra-particle mass transfer requirements can significantly affect the rate of
reaction; so it is normally supposed that tri-phase catalysts possess lower reactivity than in biphasic
reaction systems. On the other hand, there are exceptional cases stated in the literature which shows
higher reactivity of the tri-phase catalysis. Tundo et al.; (1989) reported that catalysts made by
immobilization of onium salts on inorganic supports (silica and alumina) allow high nucleophilic
activity in bromide displacement on octylmethanesulfonate of which result in higher rate of
reaction than for the same reaction in a homogeneous phase.
Fig 1.8: Activity of liquid-liquid-solid tri-phase catalysis
1.7.6 Catalyst used in tri-phase catalysis:
Most commonly used catalyst in tri-phase catalysis is ion exchange resin or Amberlite IR400. An ion-exchange resin also known as ion-exchange polymer is an immiscible support
structure or matrix, generally in the form of small beads and in the range of 0.5 to 1 mm diameter.
The colour of the resin is white or yellowish and is fabricated with an organic polymer substrate.
High surface area of the catalyst beads are due to its porous nature which helps the reaction to
takes place effectively. The trapping of ions take place with releasing of other ions; therefore the
process is known as ion-exchange. Variety of ion-exchange resin are available in the market. Most
of the commercially used resins are usually of polystyrene sulfonate.
When the application is being concerned, ion-exchange resins are generally utilized in
various processes like separation, purification, and decontamination. Water softening and water
purification are among the most common examples. In various cases, ion-exchange resins were
found to be more flexible replacement for the use of natural or artificial zeolites. In the method of
biodiesel filtration, ion exchange resins are found to be extremely effective. Furthermost usual ionexchange resins depends on acrylic structure or cross linked polystyrene. To boot, in the case of
polystyrene, cross linking is introduced by means of copolymerization of styrene and a few percent
of divinylbenzene. The polymers which are non-cross linked are soluble in water. Cross linking
reduces ion-exchange capability of the resin and extends the time required to carry through the ion
exchange processes. The size of the particle too influence the resin parameters, smaller particles
possess a larger outer surface, but then again cause larger head loss in the column procedures.
Fig 1.9: Ion exchange resin
There are four main kinds of ion exchange resin which have differences based on their functional
(a) strongly acid nature , for example, sulfonic acid groups;
(b) strongly basic nature, for example, quaternary amino groups;
(c) weakly acidic nature, for example, carboxylic acid groups;
(d) weakly basic nature, for example, primary, secondary, and/or ternary amino groups.
Chapter 2
Aqueous alkanolamines has a broad application in removing H2S gas from gas streams
generated in various industries. The amines that are proved to be of principal importance for gas
purification are diethanolamine (DEA), monoethanolamine (MEA), and methyldiethanolamine
(MDEA). The first used alkanalomine in industry was tri-ethanolamine (Bottoms et al., 1930) but
due to certain disadvantages such as its low capacity, low reactivity and poor stability, it has been
displaced largely. Aqueous MEA has been broadly used due to various properties such as high
reactivity, ease of reclamation, low absorption of hydrocarbons, low solvent cost and low
molecular weight (which results high solution capacity at moderate concentrations). MDEA has
also been used because they are regenerable, they have high removal potential efficiencies and
their ability to remove either H2S or both CO2 and H2S is high (Zicarai et al., 2003) but its cost is
high as compared to MEA. Several researchers studied the equilibrium solubility of pure H 2S
(Lawson et al., 1976; Lee et al., 1976; Isaacs et al., 1980), mixture of acid gases (H2S and CO2)
(Lee et al., 1976; Lawson et al., 1976; Isaacs et al., 1980), and the mathematical portrayal of the
experimental solubility data for H2S, CO2 and mixture of CO2 and H2S (Austgen et al., 1989;
Weiland et al., 1993; Al-Baghli et al., 2001; Kaewsichan et al., 2001) using aqueous MEA.
Y. B. Jadhav and G. D. Yadav (2003) studied the reduction of para-chloronitrobenzene
using sodium sulphide under the various modes of Phase Transfer Catalysis such as Liquid-Liquid,
Liquid-Solid and Liquid-Liquid-Liquid PTC processes. The influence of co-catalyst in
intensification of rate of Liquid-Liquid PT catalysed reaction was also studied by them in reference
to produce p-chlorophenyl acetonitrile in presence of Tetrabutylammoniumbromideas a Phase
Transfer catalyst and KI as a co-catalyst. 100% selectivity of product was obtained due to cocatalyst. Kinetics of reaction between benzyl bromide and sodium benzoate in L-L system
catalyzed by alsiquat 336 at 70 °C for 3 h of reaction using chlorobenzene as a solvent was studied
by Yang H., Lin C. (2003). Product yield obtained was 98%. Reduction of nitro toluene using
(NH4)2S as a reducing agent was carried out in toluene as organic solvent under L-LPTC in
presence of Tetrabutylammoniumbromide. 100% selectivity of toluidine was found. (Pradhan
N.C., Maity S.K., Patwardhan A. V.; 2006) Wang M. (2007) studied kinetics of Phase Transfer
Catalysis etherification of 4, 4’-bis (chloromethyl)-1,1’-biphenyl with C6H6O i.e. phenol in
alkaline solution of KOH/organic solvent as a two phase medium. Kinetics and mechanism of
quaternary ammonium salt as a Phase Transfer Catalyst for thiophene oxidation was observed by
Zhao D, Ren H. (2007). Different catalyst with ultrasound was employed and quaternary
ammonium salt was found to be best with 94.67% desulfurization rate. The reactions of parachlorobenzyl chloride and benzyl chloride with sodium sulfide produces dibenzylsulfide and bis
(p-chlorobenzyl) sulfide, which are significant in the commercial field. These are called
diarylsulfides and finds several uses as supplementary for extreme pressure lubricants, as antiwear
additives for motor oils, as stabilizers for photographic emulsions, in refining and recovery of
precious metals, and in different anticorrosive formulations. (Pradhan and Sharma, 1990). Maity
S.K., Sen S., Pradhan N. C. (2007) used TBAB as phase transfer catalyst and produced DBS by
reacting BC and aqueous ammonium sulphide. Sen S., Pradhan N.C. and Patwardhan (2011) also
studied and examined the reaction of BC with H2S rich MEA solution under L-L PTC and found
that if the ratio of MEA/H2S is higher it favours the formation dibenzylsulfide and if the ratio is
lower it favours the formation of BM. TBAB was used as PTC.
Extensive research has been performed for the preparation of disulfide by various
researchers by the application of PTC. Depending on the reaction between sodium sulfide and
sulfur using didecyldimethylammonium bromide (DDAB), Sonavane et al., (2007) has developed
a simple and convenient method for the preparation of symmetrical as well as asymmetrical cyclic
disulfides using PT catalyst. Abbasi et al.; (2012) has introduced an effective and odourless
preparation of disulfides from alkyl halides by the use of elemental sulfur and thiourea with sodium
carbonate in wet PEG 200 at 400C. Further this procedure has also been extended for the synthesis
of disulfides from alkyl tosylates at 700C.
Alternative ways for the preparation of disulfide has been suggested by Tajbakhsh et al.;
(2004) where 2, 6-Dicarboxypyridinium chlorochromate has been found to be a methodical reagent
for the conversion sulfides to sulfoxides and thiols to disulfides under neutral and anhydrous
conditions in good to a worthy yields. At room temperature, the selective oxidation of thiols in the
presence of sulfides is also observed with this reagent.
On converting alkyl halides to the disulfides, Polshettiwar et al.; (2004) found Benzyltriethyl-ammonium tetra-cosathio-hepta-molybdate [(C6H5CH2N(Et)3)6Mo7S24] as a superior
sulfur transfer reagent with a worthy yields under normal reaction conditions. The application of
this reagent offers various advantages such as mild reaction conditions, economy process, easy
preparation of the reagent and a convenient workup procedure. Also, this method provides a
simple, versatile and general route for the preparation of a wide variety of disulfides due to its
Recently, on synthesizing the symmetrical disulfides starting from thiols, Thurow et al.;
(2011) have presented the results on the use of 1-n-butyl-3-methylimidazolium methylselenite,
[bmim] [SeO2(OCH3)]. This method is general for aromatic, aliphatic, and functionalized thiols
affording the disulfides in good to excellent yields after easy work up. The use of a microwave
accelerates the reaction and the [bmim] [SeO2(OCH3)] was reused for further oxidation reactions.
Way back, Sonavane et al.; (2007) developed a one-pot, rapid and general method for the synthesis
of symmetrical disulfides based on reaction of sulfur with sodium sulfide in the presence of
didecyldimethylammonium bromide (DDAB) as a phase transfer catalyst is reported. Reaction
with a variety of alkyl halides, at room temperature, afforded the disulfides in good to excellent
isolated yields in a short period.
Ali at el.; (2002), have developed a simple, efficient, and mild procedure method for
oxidizing thiols with molecular bromine on hydrated silica gel support for the conversion of thiols
to the disulfides and dithiols to cyclic disulfides. The procedure utilizes organic media and does
not require a base to neutralize HBr by-products to suppress acid promoted side reactions.
In the Liquid-Liquid PTC, the catalyst is generally not easy to recover so it is discarded
causing load to the surroundings. The catalysts in L-L PTC are soluble in both the phases where
product in pure form could not be obtained. Therefore, the catalyst is eliminated by treating the
organic phase of the system with large amount of water which is then sent to the effluent treatment
plant. But, water washing decreases the overall yield of the product which is undesirable.
Moreover, the separation of catalyst from the reaction mixture demands extraction, distillation and
adsorption which consume high energy. Therefore, tri-phase catalysis is the best alternative to LL-PTC where catalyst can be recover and reuse easily.
Pradhan et al.; (1992) have investigated the catalytic activity of some common tri-phase
catalyst namely alumina and Amberlyst A27(Cl-), on the reaction of benzyl chloride and parachlorobenzyl chloride with solid sodium sulfide. Dutta et al.; (1994) studied the reaction of phenol
in alkaline solution and benzyl chloride dissolved in toluene along with polymer supported tri-nbutylphosphonium ion as phase transfer catalyst in a slurry reactor and mass transfer effect have
been analysed with the standard theory of porous catalyst. The intrinsic reaction rate constant and
diffusion co-efficient within the porous polymer particle are estimated. Yadav et al. ;(1997) studied
the use of a novel catalyst based on heteropolyacid supported on clay, particularly
dodecatungstophosphoric acid (DTP) catalyst have been found as reusable and efficient at high
temperatures. Naik at el.; (2000), also prepared a clay supported phase transfer catalyst and used
it for the preparation of benzoic anhydride from benzoyl chloride and sodium benzoate using claysupported quaternary ammonium salts at 30 °C. The clay supported catalyst were more active than
polymer supported catalyst here and 100% selectivity has been obtained.
Desikan et al.; (2000) also investigated on the esterification of benzyl chloride with
aqueous sodium acetate with tributylmethylammonium chloride as catalyst and stated that the
faster rate of reaction have been observed with the polymer-supported catalyst than its soluble
analogue. Glatzer et al.; (2000) reported an assessment comparing the heterogeneous and
homogeneous PT catalysts with different categories of PTC systems. It was possible to find out
the conditions under which the polymer supported catalyst enhanced the reaction system distinctly
better than its soluble counterpart. Holger et al.; (2002) further reported the kinetic model for
triphase catalytic systems which is based on the traditional kinetic mechanisms of the LangmuirHinshelwood and Eley-Ridealtype’s modified to suit the special case of catalysis by solid
supported PT catalyst (i.e. TPC). The synthesis of octyl acetate from reaction between octyl
bromide and potassium acetate has been used to test the validity of this model. This model could
predict whether a triphase catalytic system is limited by the organic reaction step, ion-exchange
step, or a combination of both the steps.
Joshi et al.; (2001), reported the usability of solid acid catalyst for the synthesis of tert amyl
methylether (TAME) by reacting tert amyl alcohol and methanol. Yadav et al.; (2007) studied the
facts of formation of a third phase in biphasic reaction where Liquid-Liquid-Liuid Phase Transfer
Catalysis increases the conversion and selectivity. The confirmation have been made by the
reaction between phenol and benzyl chloride under Liquid-Liquid PTC. Maity et al.; (2008) have
reported the reduction of p-nitrotoluene by aqueous ammonium sulfide with serelite SRA400 as
catalyst using triphase catalysis reaction and established that reduction rate of of PNT is
proportional to the square of the concentration of sulfide and to the cube of the concentration of
PNT. Additionally, enhancement of the rate have also been observed with the once used catalyst
due to the presence of elemental sulfur on the surface of the catalyst.
Wang et al.; (2010), further reported the various type of quaternary salt type tri phase
catalyst on the esterification reaction of benzyl chloride with sodium acetate. Some important
observation have been reported as:
(1) Among quaternary phosphonium and quaternary ammoninum type TPC, ammonium
type have the higher activity than other type catalyst
(2) Triphase Catalyst which is highly lipophilic, substitution takes place at N atom and
their catalytic activity is high.
(3) Triphase Catalyst links the quaternary onium salt group to the matrix microsphere
because of its longer shaper arm and shows high catalytic activity
(4) The hydrophilic and hydrophobic property of the TPC is affected by the bonding
density of quaternary onium salt group and hence influences the catalytic activity.
Chapter 3
Chapter 3
Preparation of H2S rich mono ethanol amine
Sulphuric acid from Merck Pvt. Ltd., Mumbai (98%)
Iron Sulfide fused sticks for producing H2S from Merck Pvt. Ltd., Mumbai
Silicon high vacuum grease from RFCL Limited, New Delhi
Monoethanolamine from Loba Chem ie Pvt. Ltd., Mumbai (99%)
Estimation of sulphide content
Sodium thiosulphatepentahydrate (purified) from Merck Pvt. Ltd. (≥99%)
Potassium iodate GR from Merck Pvt. Ltd. (≥99.5%)
Sodium hydroxide pellets (purified) from Merck Pvt. Ltd. (≥97%)
Starch soluble GR from Merck Pvt. Ltd.
Potassium iodide GR from Merck Pvt. Ltd. (99.8%)
Preparation of Organic Phase
Toluene from RFCL Limited, New Delhi (≥99.5%)
Benzyl chloride from Merck (India) Ltd., Mumbai (≥99%)
Acetone from RFCL Limited, New Delhi (99.5%)
Amberlite 400IR
To prepare H2S-rich aqueous monoethanolamine (MEA), about 30- 35 weight% solution
of aqueous alkanolamine was developed by first adding a suitable standard of required
alkanolamine in distilled water. The H2S gas then produced in the Kipp’s apparatus as shown in
fig. 3.1, was bubbled through this aqueous alkanolamines solution in a gas bubbler of 250 mL
standard. In laboratory scale, H2S gas was prepared in Kipp’s apparatus by reacting FeS sticks
with H2SO4. 1 molar concentration of H2SO4 was taken and the reaction was carried out in Kipp’s
apparatus is as shown below.
Figure 3.1: Representation of diagram for the absorption of H2S in MEA solution.
The reaction of H2S with alkanolamine is exothermic in nature (Kohl and Nielsen, 1997), so the
gas bubbler holding aqueous alkanolamine was set engrossed in an ice water tub to avoid the
oxidation of sulfide and the formation of disulfide. The H2S gas coming from the first bubbler was
sent to another bubbler containing approximately 1M MEA solution. The outlet of this bubbler
was exposed to air. The bubbling of the gas was continued till the required sulfide concentration
was attained in the aqueous alkanolamines.
All the reactions were carried out in the batch reactor, in an entirely baffled
automatically agitated glass reactor of 250 cm3. At a height of 1.5 cm from the bottom of reactor
a six-bladed glass impeller of 2.0 cm-diameter is located. The provision of speed regulation is also
available. The impeller was provided in order to stir the reaction mixture. The reactor assembly
was held back in a constant temperature water bath whose temperature could be controlled within
±1°C. The diagram of the experimental arrangement is as shown below.
Figure 3.2: Representation of the diagram of the batch reactor
In the experiment, the three necked batch reactor was loaded with 50 ml of aqueous phase
with known sulfide concentration. The reactor is well stirred up to attain a steady state temperature.
After reaching that temperature, 50 ml of the organic phase containing a definite amount of organic
reactant (benzyl chloride) and phase-transfer catalyst (Amberlite IR400) is loaded and then the
reactor mixture is dissolved in toluene to make an organic solution. Toluene is used as the organic
solvent. Now, the organic solutions are mixed with the aqueous solution in a volumetric flask of
250-mL. In order to initiate the reaction, the flask was kept engrossed in an isothermal water bath.
At a constant speed, the reaction mixture was stirred. Now, after stopping the agitation of the
reaction mixture and letting to separate the phases, about 0.1 ml of the organic phase sample, of
the organic layer was taken out with the help of a pipette, at a fixed time interval of 5-480 min.
The pipette was then put into the test tubes. This sample i.e. 0.1 mL was taken out from the necked
batch reactor and then placed the sample into the 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.
Preparation of standard (0.025 M) KIO3solution:
5.3 gm of KIO3was weighed accurately and dissolved in distilled water and was built up to
1 L in a graduated volumetric flask.
Preparation of standard (0.1 M) sodium thiosulfate solution:
25 gm of Na2S2O3.5H2O crystals was weighed and dissolved in distilled water and made
up to 1 L in a graduated volumetric flask with distilled water. 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 days.
Standardization of sodium thiosulfate solution by standard potassium iodate solution:
25 mL of 0.025M KIO3solution was taken and 1 gm (excess) of potassium iodide (KI)
added to it followed by 3 mL of 1 M sulfuric acid. Thiosulfate solution was taken in the burette to
titrate the liberated iodine (I2).When the solution colour changes to pale yellow, it was diluted to
200 mL with distilled water. A few drops of starch solution were added, and the titration was
carried on until the colour transformed from blue to colourless. The chemical reaction involved in
this titration is given below.
𝑲𝑰𝑶𝟑 + 𝟓𝑲𝑰 + 𝟑𝑯𝟐 𝑺𝑶𝟒 ↔ 𝟑𝑰𝟐 + 𝟑𝑯𝟐 𝑶 + 𝟑𝑲𝟐 𝑺𝑶𝟒
𝟐𝑵𝒂𝟐 𝑺𝟐 𝟎𝟑 + 𝑰𝟑 ↔ 𝑵𝒂𝟐 𝑺𝟒 𝑶𝟔 + 𝟐𝑵𝒂𝑰
1 mole of KIO3≡ 3×2 mole of Na2S2O3
6∗𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝐾𝐼𝑂3 ∗𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐾𝐼𝑂3
Therefore, Strength of Thiosulfate solution=
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑖𝑜𝑠𝑢𝑙𝑓𝑎𝑡𝑒 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑
Estimation of sulfide concentration:
Hydrogen sulfide and soluble sulfides can be found by oxidation with potassium iodate in
an alkaline medium. 15 cm3 of standard (0.025M) potassium iodate solution was taken in a conical
flask. 10 cm3 of sulfide solution containing about 2.5 mg of sulfide was then added to it. After that
10 cm3of 10M sodium hydroxide solution was also added to it. The mixture was boiled mildly for
about 10 minutes, cooled, and 5 cm3of KI solution and 20 cm3of 4M sulphuric acid solution were
added to it. The liberated iodine was titrated, which was equivalent to the unused potassium iodate,
with a standard 0.1M sodium thiosulfate in the common way. The potassium iodate in the alkaline
medium oxidizes the sulfide to sulfate as given by the following reaction. For sulfide solution
having sufficiently high sulphide concentration, suitable dilution was prepared before the
estimation of sulfide by abovementioned procedure.
𝟒𝑰𝑶𝟑-+ 𝟔𝑶𝑯- + 𝟑𝑺2- = 𝟑𝑺𝑶𝟒2- + 𝟒𝑰- + 𝟔𝑯𝟐𝑶
4 mole of IO3-≡ 3 mole of 𝑺2H2S𝒄𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏 =
(𝑉thiosulfate ∗ 𝑆thiosulfate )
]*34*𝑁10 }
where, Siodate= Strength of KIO3
Vthiosulfate= Volume of thiosulfate
Sthiosulfate= Strength of thiosulfate
Nd= Number of times of dilution
3.5.2 Analysis of Organic Phase:
All organic samples were analyzed by the help of Gas Liquid Chromatography (GLC)
using a 30m long and 0.25mm in diameter capillary packed column. A Flame Ionization Detector
(FID) was used with nitrogen as the carrier gas. The nitrogen gas flow rate is kept at 25ml/min,
injection temperature was maintained at 250 ºC and detector temperature was 300 ºC. Oven
program is put in a way that oven is kept at 50 ºC for 2min and then it is heated up to 280 ºC at the
rate of 20 ºC/min and 300 ºC is maintained for 4 min.
Figure 3.3: A Gas Chromatography
The GC Spectra of present system at the beginning and end of synthesis is shown in figure
3.4 (i) and (ii) respectively. The figure shows that the components are leaving with different
retention time depending on their relative volatility with different area of chromatogram. Toluene
has constant area chromatogram as it is not participating in synthesis reaction but decrease in BC
chromatogram area is observed with the course of reaction and subsequently formation of DBDS
is notice with increase in its chromatogram area with increase in reaction time.
Figure: 3.4: (i) Chromatogram at 5min
Figure 3.4: (ii) Chromatogram at 480min
Chapter 4
Chapter 4
Value added products like DibenzylDisulfide (DBDS) and Dibenzylsulfide (DBS) are
formed in the present work by reacting benzyl chloride with H2S rich aqueous monoethanolamine
in the presence of sulfur powder and phase transfer catalyst. Disulfides are significant compounds
owning inimitable and varied properties in the biochemical as well as synthetic fields. Several
bioactive molecules predominantly contain large disulphide linked groups. Technologically,
disulfides find varied uses as vulcanizing agents for elastomers and rubbers which provides them
good tensile strength. The disulfide moieties forms in proteins and is set up in a mixture of small
naturally obtained products and pharmacologically active compounds. Additionally, cyclic and
acyclic disulfides is advantageous species in the production of biologically-active compounds in
many pharmaceutical as well as agro-chemical intermediates (Billard at el.; 1996, Nuzo et al.;
1983). Moreover, DibenzylDisulfide (DBDS) find their application in manufacturing corrosive
inhibitors, fragrance compounds, high pressure lubricant additives, organic compounds and many
Disulfide as well as polysulfide anions are effortlessly attained by mixing aqueous
solutions of S2- with sulfur. Numerous investigative procedures were recently established to assess
the polysulfide distribution in a number of sulfur/sulfide mixtures. In a general method, benzyl
chloride in an organic solvent was agitated with aqueous sodium sulfide and sulfur in the existence
of phase transfer catalysts at room temperature. In the present study, the reaction was held out in
batch mode under tri-phase conditions (liquid-liquid-solid) both with and without of phase transfer
catalyst (PTC) Amberlite IR 400 and in the presence of sulfur powder. A putative reaction
mechanism were proposed where, in the presence of sulfur, aqueous hydrogen sulfide forms
(RNH3)2S2.The latter reacts with the phase transfer catalyst to form the ion pair Q2S2, which is
transferred to the organic phase to react with the substrate to yield the dialkyl disulphide.
Figure. 4.1 Mechanism
Subsequently, the nature of the polysulfide anion in water (S2-x where x ¼ 2; 3; 4) can be
obtained by adjusting the sulphide to sulfur ratio and it is concluded that tri, tetra and higher
dialkylpolysulfides could be produced by means of the process described above.
DibenzylDisulfide (DBDS), Dibenzylsulfide DBS and trisulfide were detected as the products
from the reaction mixture by gas liquid chromatography (GLC). The selectivity of DBDS was
maximized by changing several parameters such as stirring speed, temperature, sulfur powder
loading, and aqueous sulfide concentration. Catalyst loading and concentration of benzyl chloride
as discussed below in the respective sections. From the thorough survey on the various parameters
of the reaction, an appropriate mechanism was built which could describe the progess of the
reaction. Parametric studies have been accomplished following one-variable-at-a-time (OVAT)
approach to take in the consequence of various parameters such as speed of agitation, temperature,
aqueous sulfide concentration, catalyst loading and BC concentration on BC conversion and
DBDS selectivity.
4.2.1 Effect of Speed of Agitation:
The mass transfer along with the chemical reaction is essential in effecting the conversion
on the rate of the three-phase reaction. In this study, to confirm the effects of mass transfer
resistance of the reactants on the reaction phase, effect of the agitation speed on the conversion of
benzyl chloride was varied in the range 1000-2000 rpm under the presence of PTC (Amberlite IR
400), as shown in Figure 4.2. It can be determined from the graph that there is a relatively less
conversion at 1000 rpm compared to that at 1500 rpm and 2000 rpm. Eventually, the reaction rate
at 1500 rpm is predominant while the mass transfer rate is negligible. Thus, the difference in
conversion of benzyl chloride with stirring speed is so less that the mass transfer resistance turn
out to be less significant and the reaction rate is held alone by the reaction kinetics. Consequently,
all further experiments were performed at a stirring speed of1500 rpm with the objective of
ensuring the absence of mass transfer resistance.
Figure 4.2: Effect of speed of agitation: Volume of Organic phase = 50ml; Volume of BC =
15ml; Volume of Aqueous Phase = 50ml; Catalyst loading= 5gm; Sulfide Concentration =
2.53kmol/m3; MEA/H2S mole ratio = 2.28; Temperature = 323 K; Sulfur loading= 3gm
4.2.2 Effect of elemental sulfur loading:
Elemental sulfur was loaded with H2S rich monoethanolamine as an aqueous reactant for
the synthesis of polysulfide. The colour of H2S rich aqueous MEA changes from green to reddish
brown. This colour change shows the formation of polysulfide ions (𝑆𝑥2− ) where x can be 2, 3, 4,
5 depending on sulphur loading. The effect of this sulfur loading in DBDS synthesis is investigated
by dissolving different concentration of sulfur powder in aqueous phase. At low concentration of
sulfur around1 to 2gm, low selectivity of DBDS was noticed and it is nearly negligible in absence
of sulphur powder as demonstrated in figure below.
It can be explained that at low sulfur concentration, hydrosulfide and sulfide ions are the
dominating sulfide ions, giving unwanted side product BM and DBS over DBDS. With further
increase in sulfur powder, polysulfide ions are formed in polysulfide synthesis, again giving low
selectivity of DBDS as shown in figure below at 4 to 5 gm of sulfur. 3gm of sulfur is seen as an
ideal reaction parameter giving 100% of DBDS.
Figure 4.3: Effect of Elemental Sulfur loading: Volume of Organic Phase = 50ml; Volume of
BC = 15ml; Volume of Aqueous Phase = 50ml; Catalyst loading= 5gm; Sulfide Concentration =
2.53kmol/m3; MEA/H2S mole ratio = 2.28; Stirring Speed =1500rpm; Sulfur loading=3gm;
Temperature = 323 K.
4.2.3 Effect of Temperature:
Herein, the work elucidates the reaction of BC in presence of Toluene catalysed by a new
resin catalyst Amberlite IR 400 in the presence of H2S rich aqueous monoethanolamine under
various reaction temperatures was studied under four different temperatures in the range 303333K. Fig. 4.4 shows the effect of temperature on conversion of benzyl chloride. As expected,
with the temperature increases as reaction rate of organic reaction increases as per the transitionstate theory. That's why, increase in temperature uphold slow organic phase reactions in PTC
system. It is obvious that the reactivity (conversion) of BC increases as the temperature increases.
In present synthesis, selectivity of desired product DBDS is always 100% at all temperature
conditions. Since, the rate of reaction is directly proportional to the temperature, as the temperature
increases, the reaction rate increases, thereby increasing the conversion of the reactant at higher
Figure 4.4: Effect of temperature: Volume of Organic Phase = 50ml; Volume of BC = 15ml;
Volume of Aqueous phase = 50ml; Catalyst loading= 5gm; Sulfide Concentration = 2.53kmol/m3;
MEA/H2S mole ratio = 2.28; Sulfur loading=3gm; Stirring Speed =1500rpm
Initially, reaction rate of benzyl chloride (BC) was found out at varied temperatures and an
Arrhenius plot of –ln (initial rate) versus 1/T was prepared as indicated in Fig. 4.5.By calculation,
the activation energy for the reaction of benzyl chloride was found from the gradient of the straight
line as 22.4 KJ mol-1. The final confirmation of the controlled reaction could be achieved from
the observed activation energy.
Figure 4.5: Arrhenius plot for Activation energy
4.2.4 Effect of catalyst loading:
The effect of catalyst loading was studied at five different catalyst concentrations in the
range of 0.0- 0.58kmol/m3as shown in Fig. 4.6. As the catalyst concentration increases, benzyl
chloride conversion and reaction rate increases. Benzyl chloride conversion of 100 % was attained
by increasing the catalyst concentration, whereas it was just about 45% in presence of catalyst
even after 480 minutes of reaction. This demonstrates the significance of PTC in increasing the
reaction rate.
The sulphide (S2-) and disulphide (S22− )ions forms a pair of ions [(QSQ and (𝑄2 𝑆22− )] with
quaternary cations [Q+], and then moves to the organic phase thereby reacting with benzyl chloride.
By increasing the concentration of the catalyst, more amount of (𝑄2 𝑆22− ) ion pair is formed and
moved to the organic phase to react with benzyl chloride to produce DBS and DBDS respectively.
In present reaction condition, selectivity was almost 100% due to the presence of adequate amount
of sulfur in aqueous phase.
Figure 4.6: Effect of catalyst loading: Volume of Organic Phase = 50ml; Volume of BC = 15ml;
Volume of Aqueous phase = 50ml; Sulfur loading= 3gm; Sulfide Concentration = 2.53kmol/m3;
MEA/H2S mole ratio = 2.28; Stirring Speed =1500rpm; Temperature = 323 K.
To find out order of reaction, the initial reaction rate at different Amberlite IR400
concentration is calculated and its natural logarithm is plotted with natural logarithm of catalyst
concentration as indicated in figure below.
Figure 4.7: Plot
of the natural logarithm of the initial rate vs the natural logarithm of the
catalyst conc.
Volume of Organic Phase = 50ml; Volume of BC = 15ml; Volume of Aqueous Phase = 50ml;
Sulfur loading= 3gm; Sulfide Concentration = 2.53kmol/m3; MEA/H2S mole ratio = 2.28; Stirring
Speed =1500rpm, Temperature = 323 K
4.2.5 Effect of concentration of benzyl chloride
Since the reaction has been carried out with pure BC, the effect of initial concentration of
BC on the conversion of BC was determined by changing the moles of BC at fixed sulfide
concentration i.e., by varying initial BC/sulfide mole ratio. It is understood from this figure that
with an increase in initial BC/sulfide mole ratio, BC conversion decreases because of the
inadequate amount of sulfide in the aqueous phase. With initial BC/sulfide mole ratio of 0.70,
100%BC conversion was mentioned, whereas it was around 85% at low initial BC/sulfide mole
ratio of 1.39 even after 480 min of reaction under otherwise identical experimental conditions as
observed from the Fig. 4.8.To get maximum conversion with maximum selectivity of DBDS,
lower range of initial BC/sulfide mole ratio is preferable.
Figure 4.8: Effect of concentration of benzyl chloride: Volume of Organic Phase = 50ml;
Volume of Aqueous Phase = 50ml; Sulfur loading=3gm; Catalyst loading= 5gm; Sulfide
concentration = 2.53kmol/m3; MEA/H2S mole ratio = 2.28; Stirring Speed =1500rpm;
Temperature = 323 K
4.2.6 Effect of sulfide concentration:
The effect of MEA to sulfide ratio was investigated by changing the amount of sulfide
concentration in the aqueous phase at constant 35 weight% of the aqueous MEA solution. For
fixed MEA concentration, with a reduction in initial sulfide concentration in the aqueous phase,
the conversion of BC reduces as the initial concentration of sulfide in the aqueous phase as shown
in Fig.4.9. At a fixed reaction condition, 100% conversion was reached after 480 min of run with
the sulfide concentration of 2.5kmol/m3.Continuous increase in sulfide concentration increases
conversion as well as selectivity. Increased selectivity was observed due to increase in disulfide
ions (𝑄2 𝑆22− ) compare to sulfide ions.
Figure 4.9: Effect of Sulphide Concentration: Volume of Organic Phase = 50ml; Volume of
Aqueous Phase = 50ml; Sulfur loading=3gm; Catalyst loading= 5gm; MEA/H2S mole ratio = 2.28;
Stirring Speed =1500rpm, Volume of BC=15ml; Temperature = 323 K.
The mechanism of DBDS is explained in the description given below:
4.3.1 Mechanism of Triphase Catalysis
In the mechanism given by Stark, the PT catalyst travels easily between the organic and
aqueous phases, yet in a TPC system, the catalyst movement is constrained and reagents of the
aqueous and organic phase need to be transported to the catalyst cation accordingly. The catalyst
active sites involved in the reaction are the one that are present on the interphase between the
Most of the inorganic reactants or reagents are immiscible in the organic phase. Therefore,
the reaction must be carried out at the aqueous-organic interface. Depending on the distribution of
the product, the reaction mechanism was represented by the reactions of scheme 4.10. The product
is obtained from the reaction contribution from both catalytic and non-catalytic pathway.
Non-catalytic Pathway: The dissolved sulfur and sulfide in aqueous phase forms an ion
equilibrium to produce hydrosulfide, sulfide and disulfide ions shown in the figure below. These
ion pairs react with aqueous MEA (RNH2) to give aqueous reactants ammonium sulfide (RNH3)2S
and disulfide ((NH4)2S2) via reaction (1)-(3). The aqueous reactant (RNH3)2S and (RNH3)2S2 react
with organic reactant RX at the interface to yield a byproduct DBS and desired product DBDS
respectively shown by reaction (7)-(9) in scheme below. The direct reaction of BC with (RNH3)2S
was assumed to proceed through an intermediate, RSNH3R, which further reacts with RX to
produce the byproduct DBS. Then (RNH3)2S2 reacts with the reactant to form the product DBDS.
The formed DBS and DBDS travels from interface to organic phase.
Catalytic Pathway: In the presence of catalyst, formed sulfide and disulfide ions from aqueous
phase form catalyst active intermediate QSQ and Q2S2 respectively as shown in reaction 4-6. These
active intermediate further reacts at interface with BC to give DBS and DBDS as shown by reaction
10-12. A Small quantity of sulfur in aqueous phase is responsible for HS- and S2- ion giving
undesirable Benzyl mercaptan and DBS. Excess quantity of sulfur is responsible for synthesis of
polysulfides. Therefore selectivity of product is increased by increasing the concentration of Q2S2
formed through maintaining sulfur concentration in aqueous. The formed product then travels from
interface to organic phase shown in scheme below.
Figure 4.10 Mechanism Triphase Catalysis
After the completing the kinetic run, reaction mixture was charged into a separating funnel
by pausing agitation and the phases was allowed to separate into three layers i.e. Liquid-Liquid
Solid. When the aqueous and organic phases were completely separated, the catalyst was filtered
from the solution by the use of filter paper. It was washed with acetone and water and was dried
at around 500 C to remove the adsorbed substance. Then the dry catalyst was again reused for four
times. The reusabality of catalyst was observed better till three uses where a reduction in
conversion was found during the fourth time since the quantity of third phase was very less as
compared with the previous run as shown in Fig 4.11. Every time the catalyst was lost with aqueous
and organic phase during catalyst regeneration. The catalyst obtained was reused and the
information obtained is shown in below figure.Fig.4.11 shows the conversion of benzyl chloride
with the cycle number. It can be seen that after 3 cycles, the activity of tri phase catalyst Amberlite
decreases. This tells that Amberlite has got excellent reuse property and high stability up to three
Figure 4.11 Conversion of Benzyl Chloride: Volume of Organic Phase = 50ml; Volume of
Aqueous Phase = 50ml; Sulfide Concentration=2.53kmol/m3, Sulfur loading=3gm; Catalyst
loading= 5gm; MEA/H2S mole ratio = 2.28; Volume of BC= 15ml, Temperature = 323 K; Stirring
speed =1500rpm
Chapter 5
Chapter 5
The present work carried out in this thesis contributes to the evolution of a modernistic
process to produce value-added chemicals utilizing the H2S present in various byproduct gas
streams. Since the removal of H2S by alkanolamines being well established and industrially
practiced by, the present work deals with a detailed written report of the production of value-added
chemicals utilizingH2S -rich aqueous alkanolamine in the batch mode.
The value-added chemicals produced in the present study are dibenzyldisulfide (DBDS),
dibenzylsulfide and trisulfide from benzyl chloride (BC). The reaction of BC with H2S-rich
aqueous alkanolamines was conducted batch-wise in mien of an organic solvent under LiquidLiquid-Solid mode with Amberlite IR 400 as PTC and sulfur powder. The impact of several
process variables such as stirring speed, catalyst loading, elemental sulfur loading, sulfide
concentration, reactant concentration and effect of temperature on the reaction rate, conversion,
and selectivity was studied. A suitable mechanism was built utilizing the effects of different
parameters on the rate of reaction and conversion to explain the course of the reaction. The two
active ion pairs (Q2S2 and QSQ) formed in the aqueous phase are first moved to the organic phase
where it reacts with BC to produce DBDS and DBS, respectively. Stirring speed has found to have
almost no effect on conversion of benzyl chloride between 1000-2000 RPM, signifying no masstransfer effect on reaction kinetics. The reaction rate was found to increase by changing
temperature and catalyst concentration. The reaction is kinetically controlled having an apparent
activation energy value of 22.4kJ/mol. Enhancement of production of dibenzyldisulfide was
observed to increase in initial sulfide concentration. Increases in the concentration of BC decreases
the conversion of BC and better selectivity of DBDS was achieved by adding adequate amount of
sulfur. The catalytic activity of the catalyst, amberlite IR 400 was observed to decrease to 68% of
the initial activity after 3 cycles. This reduction in catalytic activity is because of the loss of catalyst
with aqueous and organic phase during regeneration. This tells that amberlite IR 400 has got high
stability and exceptional reuse property up to three use.
In the reaction of BC with H2S rich aqueous MEA, almost complete sulfide utilization in
the aqueous phase was observed under certain experimental conditions. Therefore it needs no
further treatment before its reuse for the removal of H2S. Therefore, present process has enormous
potential to be considered as a feasible option to conventional process.
5.2.1 Finding out Effect of other Parameters
Effect of other anions as co-catalyst and the effect of the MEA concentration on addition of weak
base like NaOH that can act upon the response rate and selectivity of the desired product can also
be tested. In the present study, toluene has been utilized as a solvent in most of the cases because
it is inexpensive. The physical of organic solvent play an important part of the PTC reaction, not
only by acting upon the intrinsic organic reaction but also by affecting the transport properties of
PTC and active catalyst species (QSQ and QSH) (Yang et al., 2003). Other types of solvents like
chlorobenzene, n-heptane, and dichloromethane can be tried out. Other aqueous solution can be
utilized such as DEA and MDEA based on the different type of gas stream. Absorption using
MDEA will be best suited for selective removal of H2S, if the gas stream contains CO2 with H2S.
Tri sulfides and poly sulfides can also be prepared selectively by adjusting the amount of sulfur in
the aqueous phase.
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