DESC:FIELD OF THE INVENTION:

The present invention relates to a synthesis of cyclic carbonates from CO2 and alkylene oxide in presence of wood ash as catalyst and Triethanolamine as co-catalyst under gaseous state of CO2. Particularly this invention relates to a synergistic catalyst system having wood ash and Triethanolamine used in the synthesis of cyclic carbonates.

BACKGROUND OF THE INVENTION:

In recent years, much attention has been paid to utilization of CO2. The conversion of CO2 to valuable chemicals has received much attention because CO2 is a non-toxic, abundant and inexpensive C1 renewable resource. Recently, various methodologies have been intensively developed such as the reductive transformation of CO2 to methanol, formates and methane and the non-reductive transformation of CO2 to organic carbonates, carbamates, ureas, carboxylates, polyurethanes and polycarbonates. However, the scope of transformation methods of CO2 is limited because of its inert nature. Therefore, the development of effective catalytic methods for transformation of CO2 by low energy input is an important and urgent matter.

Considering that the stability is attributed to the most oxidative state of CO2, chemicals with high oxidation states will be desirable synthetic targets from the viewpoints of green chemistry and energy management. Therefore, the development of a non-reductive transformation of CO2 is desirable and organic carbonate is one of the most promising target chemicals in terms of green and sustainable chemistry due to its low toxicity, non-corrosiveness and biodegradability.

Though this may be less but presently more than 110 million tons of CO2 is being used to produce chemicals. This corresponds to only 1% of the net annual anthropogenic release (13 000 million tons) of CO2 to the atmosphere. The toxic carbon monoxide (CO) is the main competing feedstock to CO2 in many industrial processes because CO2 is less reactive than CO and a larger energy input is needed when using CO2 as raw material. According to 2010 report it is clear that around 105 million tons of CO2 are used for the production of urea. Similarly the production of salicylic acid (90,000 tons), cyclic carbonates (80,000 tons), and polypropylene carbonate (70,000 tons) are being produced from CO2 on industrial scale. Organic carbonates have been widely used as starting materials for production of polycarbonate resins, polyurethane resins, glycols, electrolytes for lithium ion batteries, alkylating and carbonylating reagents and inert solvents. Moreover, in the future, they are expected to be a fuel additive.

US Application No. 201408691908B2 discloses the synthesis of cyclic carbonates from CO2 and epoxide over a polymeric phosphonium salt grafted onto the carbon nanomaterial, whereas polymer has a number average molecular weight of 1,000-200,000. This invention claim the cyclic carbonates synthesis from CO2 and corresponding epoxide under 3-150 kg/cm2 pressures and below 90-200°C temperatures. The main disadvantage of this invention is use of costly raw materials required for the preparation of the catalyst. This catalyst contains poisonous phosphonium chemicals which is unsafe in the nature. Post reaction procedure requires solvents for the purification of cyclic carbonate which is not economical for scale-up.

Chunyan Liu et al. has published in Synthetic Communications, 43, 2985-2997 (2013) regarding utilization of KI in presence of triethanolamine for the synthesis of cyclic carbonates from alkylene epoxides and CO2. Though the reaction conditions are moderate, catalyst KI is homogeneous, expensive and corrosive in nature.

US Application No. 20050070724A1 has provided zeolite-based catalyst and Lewis base as co-catalyst for the preparation of cyclic carbonate from CO2 and epoxide. The heterogeneous catalyst together with Lewis base is able to convert an epoxide and CO2 into a cyclic carbonate in a non-polar solvent system. The reaction requires low carbon dioxide pressure and low temperature is the main advantage of this process.

US Application No.20107728164 provides a phosphonium bromide salt (tetraalkylphosphonium bromide) as a homogeneous catalyst, which catalyzes the synthesis reaction of a propylene carbonate in the presence of carbon dioxide. The phosphonium bromide salt catalyst also requires harsh reaction conditions for the transformation (more preferable conditions 50 bar & 180°C). This procedure also involves the usage of protogenic compounds such as water/alcohol (2% by weight) and also report formation of side product 1,2-propan-diol in minor quantity.

US. Pat. No. 2005/6,933,394 provides a method of using phosphonium iodine salt compounds for catalyzing the reaction of epoxides with carbon dioxide to produce cyclic carbonate. The reaction with the homogeneous catalyst requires higher gas pressure (more than 100 atm), and has a higher demand for production equipment, thus increasing production cost.

The reported catalysts like zeolite based, polymeric phosphonium salts and KI catalyst systems are facing drawbacks like costly chemicals are required for preparation of catalysts, homogeneous in reaction mixture and difficult in separation and high temperature and pressure conditions.

It has been disclosed in granted patent US9611210 B2 that the single pot synthesis of dialkyl carbonates using catalyst from natural source (wood ash). In this invention, the reaction has been claimed to be carried out under supercritical conditions of CO2 (100-180 °C, 70-90 bar). At supercritical phase of CO2, the catalyst wood ash is very active for facilitating reaction between CO2, propylene oxide and alcohol in single pot to produce corresponding dialkyl carbonates.

As per the above said literature, several homogeneous and heterogeneous catalysts have been reported for the synthesis of cyclic carbonates from CO2 and corresponding oxiranes, including alkali metal salts, organometallic complexes, organic bases, alkali halides/organic base, quaternary ammonium/phosphonium salts, N-heterocyclic carbenes, ionic liquids, modified molecular sieves, metal-oxides, and zeolites. However, use of homogeneous catalysts is not preferred in industry due to problems such as rigorous separation and purification of the products. Also the use of developed most of the heterogeneous catalysts is somewhat restricted due to low catalytic activity, harsh reaction conditions, poor stability and costly raw materials.

Therefore, it is of still great importance to explore heterogeneous catalysts with low cost, stable and excellent activity for CO2 insertion under mild reaction conditions.

OBJECTIVE OF THE INVENTION:

It is the primary objective of the invention is to provide a catalyst system prepared from wood ash wherein the process, by which the catalyst developed, involves use of novel, economical and eco-friendly catalyst from natural sources.

It is the further objective of the present invention that the utilization of naturally available, cost effective material from wood to be used as catalyst for the activation of CO2 and epoxide in presence of Triethanolamine as co-catalyst for the synthesis of industrially important cyclic carbonates from corresponding alkylene epoxides.

SUMMARY OF THE INVENTION:
Accordingly, the present invention provides a synergistic catalyst system for preparation of cyclic carbonates from alkylene epoxide and CO2, the catalyst system comprising:
(i) wood ash: and
(ii) trialkanolamine.

In an embodiment of the present invention, the concentration of wood ash in the catalyst system is in the range of 5% wt/wt of alkylene epoxide to 15% wt/wt of alkylene epoxide.

In yet another embodiment of the present invention, the concentration of triethanolamine in the catalyst system is 0.005 to 0.05 molar equivalents with respect to alkylene epoxide.

In an embodiment of the present invention, the wood ash is obtained by calcinating wood blocks at a temperature in the range of 500 to 900 °C. In one of the preferred embodiment, the wood ash is obtained by calcinating wood blocks in the range of temperature 600-800 °C.

In another embodiment of the present invention, the trialkanolamine is selected from the group of consisting of triethanolamine, triisopropanolamine, and triisobutanolamine. In one of the preferred embodiment, the trialkanolamine is triethanolamine.

The present invention also provides a method for the preparation of cyclic carbonates from CO2 using a synergistic catalyst system, said method comprising the steps of:
a) adding alkylene epoxide in a vessel;
b) mixing the synergistic catalyst system comprising wood ash and trialkanolamine with the alkylene epoxide;
c) adding CO2 gas in the mixture of step (b) and heating the same;
d) cooling the reaction mixture of step (c) to room temperature;
e) recycling the wood ash catalyst by filtering of reaction mixture of step (d) and washing with an alcoholic solvent followed by drying; and
f) separating trialkanolamine and product of step (e) to; and obtaining colour less pure form of cyclic carbonates.

In an embodiment of the present invention, the alkylene epoxide is selected from the group consisting of ethylene oxide, propylene oxide, 1,2-butyl epoxide, 1,2-pentyl epoxide, 1,2-hexyl epoxide, 1,2-heptyl epoxide, 1,2-octyl epoxide, 1,2-nonyl epoxide, and 1,2-decyl epoxide

In yet another embodiment of the present invention, the alcoholic solvent is selected from the group consisting of methanol, ethanol, propanol, and butanol. In one of the preferred embodiment, the alcoholic solvent is methanol.

In yet another embodiment of the present invention, the trialkanolamine is selected from the group consisting of triethanolamine, triisopropanolamine, triisobutanolamine. In one of the preferred embodiment, the trialkanolamine is triethanolamine.

In yet another embodiment of the present invention, the heating in step (c) is carried out at a temperature in the range of 60-140°C and maintaining CO2 gas pressure of 10-60 bars for a time interval of 5-20 hrs.

In yet another embodiment of the present invention, the drying in step (e) is carried at a temperature in the range of 80 to 120 °C for 12 to 24 hours. In one of the preferred embodiment, the drying in step (e) is carried at a temperature of 120°C for 24 hours.

In yet another embodiment of the present invention, the concentration of wood ash is in the range of 5% wt/wt of alkylene epoxide to 15% wt/wt of alkylene epoxide.

In yet another embodiment of the present invention, the concentration of triethanolamine is 0.005 to 0.05 molar equivalents with respect to alkylene epoxide.

In yet another embodiment of the present invention, the amount of cyclic carbonates are obtained in the range of 70% to 85%.

In yet another embodiment of the present invention, the concentration of alkylene epoxide is 100% and no solvent is used.

In yet another embodiment of the present invention, the cyclic carbonate obtained is alkylene carbonate.

DETAILED DESCRIPTION OF THE INVENTION:

While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the scope of the invention as defined by the appended claims.

According to the main embodiment, the present invention discloses a synergistic catalyst system for the preparation of cyclic carbonates from CO2 comprising of:
i. Wood ash
ii. Trialkanolamines like triethanolamine
Cyclic carbonates are prepared from alkylene epoxides selected from the group consisting of ethylene oxide, propylene oxide, 1,2-butyl epoxide, 1,2-pentyl epoxide, 1,2-hexyl epoxide, 1,2-heptyl epoxide, 1,2-octyl epoxide, 1,2-nonyl epoxide, 1,2-decyl epoxide. The cyclic carbonates of the present invention having following formula:

In detailed embodiment, the catalyst prepared from wood ash is novel, economical and eco-friendly. This invention is novel with high potential for its commercialization to produce industrially CO2 emissions to value added cyclic carbonates in presence of developed catalyst system from renewable sources.

The wood ash used in accordance with this invention can be obtained from biomass, including but not necessarily limited to, wood of trees such as Azadirachta indica, Acacia nilotica. The wood ash catalyst is basic in nature. Composition of wood ash catalyst is mixture of oxides of Ca and Mg along with sintered material calcium silica phosphates. Wood ash also contains potassium and small quantities of other metal derivatives. The typical combination of all these compounds in wood ash catalyst makes it a suitable catalyst for various organic transformations. The combination of wood ash catalyst and triethanolamine as co-catalyst was explored for the cyclic carbonates from CO2.

According to the other embodiment, the present invention covers the novel synthetic method for the preparation of cyclic carbonates from CO2 using synergistic catalyst system, said method comprising the steps of:
a) adding alkylene epoxide in a vessel;
b) mixing wood ash and Triethanolamine with alkylene epoxide;
c) adding CO2 gas in the mixture of step (b);
d) carrying out the reaction of step (c) at a temperature range of 60-140°C, maintaining CO2 gas pressure of 10-60 bars for a time interval of 5-20 hrs;
e) cooling the reaction mixture of step (d) to room temperature;
f) recycling the wood ash catalyst by filtering of reaction mixture of step (e) and washing with alcoholic solvent followed by drying at a temperature of 120°C for 24 hours.
g) carrying out vacuum distillation of filtrate of step (f) to separate triethanolamine and product; and obtaining colorless pure form of cyclic carbonates;

The alkylene epoxides used according to the present invention include ethylene oxide, propylene oxide, 1,2-butyl epoxide, 1,2-pentyl epoxide, 1,2-hexyl epoxide, 1,2-heptyl epoxide, 1,2-octyl epoxide, 1,2-nonyl epoxide, 1,2-decyl epoxide.

This reaction is neat and does not require any solvent. The co-catalyst triethanolamine was used in a catalytic amount to capture and activate maximum CO2 present in the reactor vessel. The CO2 insertion reaction into propylene oxide requires activation of epoxide and activation of CO2, which is the most crucial step. Synergistic catalyst system has combination of the above properties which can trap CO2 and activate both CO2 and epoxide simultaneously towards insertion reaction.

In accordance with the present invention, the complete characterization of wood ash is described in granted patent US9611210 B2.

The following non-limiting examples illustrate in details about the invention. However, they are, not intended to be limiting the scope of present invention in any way. More particularly, the following experiments establish the synergetic effect of wood ash and Triethanolamine catalyst system:

EXPERIMENT-1:

USE OF TRIETHANOLAMINE ONLY AS CATALYST:

To evaluate effect of Triethanolamine on synthesis of propylene carbonate from propylene oxide and CO2, experiment was carried out in an autoclave vessel fixed to stirrer and controlled heating system with using only Triethanolamine as catalyst. In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added Triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) only. The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and analyzed with GC analytical techniques. The results revealed that propylene carbonate was not formed.

EXPERIMENT-2:

USE OF WOOD ASH ONLY AS CATALYST

To evaluate effect of wood ash without Triethanolamine on synthesis of propylene carbonate from propylene oxide and CO2, experiment was carried out in an autoclave vessel fixed to stirrer and controlled heating system with wood ash alone. In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) only. The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 12 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 10.0 gm with yield 5.7%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-3:

WOOD ASH AND TRIETHANOLAMINE CATALYST SYSTEM

Synergetic catalyst system with wood ash and triethanolamine was evaluated for the effective CO2 insertion in to propylene epoxide for the synthesis of propylene carbonate. The reaction between propylene oxide and CO2 was carried out in a autoclave vessel fixed to stirrer and controlled heating system. In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01molar eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 149.5 gm with yield 85%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-4:

USE OF KI AS CATALYST:

To evaluate the effect of type of potassium salt on reaction, independent reactions were conducted with KI only and its activity was evaluated. Experimental observation of reaction with KI was absence of product propylene carbonate under standard conditions. Similar results were obtained with KCl also as a reagent in the synthesis of propylene carbonate from propylene oxide and CO2.

EXPERIMENT-5:

KCl/TRIETHANOLAMINE CATALYST SYSTEM

To evaluate effect of KCl and triethanolamine catalyst system on synthesis of propylene carbonate from propylene oxide and CO2, experiment was carried out in an autoclave vessel fixed to stirrer and controlled heating system. In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added triethanolmaine (2.58 gm, 0.017 mol, 0.01 eq.) and KCl (1.29 gm, 0.017 moles, 0.01 molar eq). The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and distilled under vacuum. The amount of propylene carbonate synthesized was 62.0 gm with yield 35% with respect to propylene oxide. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-6:

KI/TRIETHANOLAMINE CATALYST SYSTEM

To evaluate effect of KI and triethanolamine catalyst system on synthesis of propylene carbonate from propylene oxide and CO2, experiment was carried out in an autoclave vessel fixed to stirrer and controlled heating system with using triethanolamine alone as catalyst. In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added triethanolmaine (2.58 gm, 0.017 mol, 0.01 eq.) and KI (2.86 gm, 0.017 moles, 0.01 molar eq). The autoclave vessel was closed and pressurized with CO2 gas. At the optimized pressure of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and distilled under vacuum. The amount of propylene carbonate synthesized was 165.5 gm with yield 94.11% with respect to propylene oxide. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively. However KI is a homogeneous catalyst and catalyst leaching will takes place after every reaction cycle.

EXPERIMENT-7:

EFFECT OF MOLAR RATIO OF TRIETHANOLAMINE ON REACTION KINETICS:

To evaluate effect of molar ratio of triethanolamine on synthesis of propylene carbonate from propylene oxide and CO2 using wood ash as catalyst was carried out in the autoclave vessel with different dosages of triethanolamine. In a typical reaction process, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (1.54 gm, 0.010 mol, 0.005 molar eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure range of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 80.9 gm with yield 46%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-8:

In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01 eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 149.5 gm with yield 85%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-9:

In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and was added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (12.84 gm, 0.086 mol, 0.05 molar eq.) The autoclave vessel was closed and pressurised with CO2 gas. At the workable pressure range of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 140.5 gm with yield 79.6%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-10:

EFFECT OF AMOUNT OF WOOD ASH ON REACTION KINETICS

Synthesis of propylene carbonate from propylene oxide and CO2 was carried out in the autoclave vessel fixed to stirrer and controlled heating system. In a typical reaction process, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (5.0 gm, 5% wt/wt) along with organic base Triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure range of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 89.7 gm with yield 51%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-11:

In another experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01molar eq.) The autoclave vessel was closed and pressurised with CO2 gas. At the workable pressure range of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 149.5 gm with yield 85%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-12:

In an another experiment under stabilized operating conditions wood ash catalyst (15.0 gm, 15% wt/wt) was taken along with propylene oxide (100.0 gm, 1.724 mol) and organic base Triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) in an autoclave vessel. The autoclave vessel was closed and pressurized with CO2 gas. At the optimized pressure range of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 137.2 gm with yield 78%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-13:

EFFECT OF TEMPERATURE

The effect of temperature on the synthesis of propylene carbonate from propylene oxide and CO2 was carried out at different temperatures. In a typical reaction process, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and wood ash catalyst (10 gm, 10% wt/wt) was added along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) The autoclave vessel was closed and pressurised with CO2 gas. At the workable pressure range of 40 bar and temperature range of 60°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 91.5 gm with yield 52%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-14:

The reaction between propylene oxide and CO2 was carried out in a autoclave vessel fixed to stirrer and controlled heating system at different temperatures. In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 149.5 gm with yield 85%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-15:

The effect of temperature on the synthesis of propylene carbonate from propylene oxide and CO2 was carried out in the autoclave vessel fixed to stirrer and controlled heating system. The experiment was carried out with propylene oxide (100.0 gm, 1.724 mol) taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) The autoclave vessel was closed and pressurised with CO2 gas. At the workable pressure range of 40 bar and temperature range of 140°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 120 gm with yield 68.2%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT-16:

EFFECT OF PRESSURE - AT 10 BAR PRESSURE

In a typical reaction process, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and wood ash catalyst (10 gm, 10% wt/wt) was added along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the optimized range of 10 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurised and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colourless pure propylene carbonate of weight 128.3 gm with yield 73%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT 17:

AT 40 BAR PRESSURE

To evaluate effect of CO2 pressure on synthesis of propylene carbonate from propylene oxide and CO2 was carried out in the autoclave vessel fixed to stirrer and controlled heating system using CO2 at different pressures. In this experiment, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01molar eq.) The autoclave vessel was closed and pressurised with CO2 gas. At the workable pressure range of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 149.5 gm with yield 85%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT 18:

AT 60 BAR PRESSURE

To evaluate effect of CO2 pressure on synthesis of propylene carbonate from propylene oxide and CO2 was carried out in the autoclave vessel fixed to stirrer and controlled heating system using CO2 at different pressures. In this reaction process, propylene oxide (100.0 gm, 1.724 mol) was taken an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01molar eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure range of 60 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 142.3 gm with yield 82%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.
Effect of reaction time:

EXPERIMENT 19:

REACTION KINETICS:

Synthesis of propylene carbonate from propylene oxide and CO2 was carried out in the autoclave vessel fixed to stirrer and controlled heating system. In a typical reaction process, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure range of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly and reaction samples were collected at different time intervals (3 hrs, 5 hrs, 10 hrs, 12 hrs, 14 hrs). Each collected sample at different intervals was distilled independently and quantified the yields. The amount and yield of the propylene carbonate obtained at 3, 5, 10, 12 and 16 hrs intervals is 98.5 gm (56%), 108.5 (62%), 126.7 (72%), 149.5 (85%) and 149.6 (85.1%) respectively. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively.

EXPERIMENT 20:

RECYCLABILITY EFFICIENCY OF WOOD ASH CATALYST:

Recyclability of wood ash catalyst was established by carrying out reaction with used catalyst. In a typical reaction process, propylene oxide (100.0 gm, 1.724 mol) was taken in an autoclave vessel and added used wood ash catalyst (10 gm, 10% wt/wt) along with organic base triethanolamine (2.58 gm, 0.017 mol, 0.01 molar eq.) The autoclave vessel was closed and pressurized with CO2 gas. At the workable pressure range of 40 bar and temperature range of 120°C, the reaction mixture was stirred constantly for 12 hours. Then reaction mixture was cooled to room temperature, depressurized and filtered to remove the catalyst. The filtered catalyst was washed with methanol (50 ml) dried at 120°C for 24 hrs for reuse. The reaction mixture was subjected to vacuum distillation to get colorless pure propylene carbonate of weight 147.7 gm with yield 84%. The identification and purity of the product was confirmed by analytical techniques like FTIR, 1H NMR, 13C NMR and GC-MS respectively. The catalyst was reused for 4 cycles and the reaction yield was obtained in the range 80-84%.

ADVANTAGES OF THE PRESENT INVENTION:

The following are the merits of the present invention:

• Synergistic catalyst system from wood ash and Triethanolamine is novel, cost-effective and eco-friendly compared to catalysts reported in the literature heterogeneous catalysts like Zeolites, polymeric grafted phosphonium salts and homogeneous catalysts like KI.

• Regeneration of catalyst can be performed by low cost techniques like simple filtration and drying.

• Process is solvent free and no solvent is required for the purification process. Reaction temperature and pressures required are moderate (60-140°C and 10-60 bar).
,CLAIMS:We claim:
1. A synergistic catalyst system for preparation of cyclic carbonates from alkylene epoxide and CO2 comprising:
(i) wood ash: and
(ii) trialkanolamine.

2. The catalyst system as claimed in claim 1, wherein the concentration of wood ash is in the range of 5% wt/wt of alkylene epoxide to 15% wt/wt of alkylene epoxide.

3. The catalyst system as claimed in claim 1, wherein the concentration of triethanolamine is 0.005 to 0.05 molar equivalents with respect to alkylene epoxide.

4. The catalyst system as claimed in claim 1, wherein the wood ash is obtained by calcinating wood blocks at a temperature in the range of 500 to 900°C.

5. The catalyst system as claimed in claim 1, wherein the trialkanolamine is selected from the group consisting of triethanolamine, triisopropanolamine, and triisobutanolamine.

6. A method for the preparation of cyclic carbonates from CO2 using a synergistic catalyst system, said method comprising the steps of:
a) adding alkylene epoxide in a vessel;
b) mixing catalyst system comprising wood ash and trialkanolamine with the alkylene epoxide;
c) adding CO2 gas in the mixture of step (b) and heating the same;
d) cooling the reaction mixture of step (c) to room temperature;
e) recycling the wood ash catalyst by filtering of reaction mixture of step (d) and washing with an alcoholic solvent followed by drying; and
f) separating trialkanolamine and product of step (e) to; and obtaining colour less pure form of cyclic carbonates.

7. The method as claimed in claim 6, wherein the alkylene epoxide is selected from the group consisting of ethylene oxide, propylene oxide, 1,2-butyl epoxide, 1,2-pentyl epoxide, 1,2-hexyl epoxide, 1,2-heptyl epoxide, 1,2-octyl epoxide, 1,2-nonyl epoxide, and 1,2-decyl epoxide

8. The method as claimed in claim 6, wherein the alcoholic solvent is selected from the group consisting of methanol, ethanol, propanol, and butanol.

9. The method as claimed in claim 6, wherein the trialkanolamine is selected from the group consisting of triethanolamine, triisopropanolamine, and triisobutanolamine.

10. The method as claimed in claim 6, wherein the heating in step (c) is carried out at a temperature in the range of 60-140°C and maintaining CO2 gas pressure of 10-60 bars for a time interval of 5-20 hrs.

11. The method as claimed in claim 6, wherein the drying in step (e) is carried at a temperature in the range of 80 to 120°C for 12 to 24 hours.

12. The method as claimed in claim 6, wherein the concentration of wood ash is in the range of 5% wt/wt of alkylene epoxide to 15% wt/wt of alkylene epoxide.

13. The method as claimed in claim 6, wherein the concentration of triethanolamine is 0.005 to 0.05 molar equivalents with respect to alkylene epoxide.

14. The method as claimed in claim 6, wherein the amount of cyclic carbonates are obtained in the range of 70% to 85%.

15. The method as claimed in claim 6, wherein the concentration of alkylene epoxide is 100% and no solvent is used.

16. The method as claimed in claim 6, wherein the cyclic carbonate obtained is alkylene carbonate.