Conversion of renewable biomaterials and their intermediates as well as end products has great importance to deal with the possibility of sustainable synthesis of chemicals and fuels. Manufacturing of carboxylate esters from lignocellulosic biomass extracted materials is one of the leading researches for renewable supply chain of energy and manufacturing of fine chemicals having value addition to the present system.
SA is one of the top (bio) platform molecules and is available from the bioconversion of glucose at concentrations as high as approximately 6 wt%. Esterification of SA with methnol/ethanol/2-Propanol produces Dimethyl succinate (DMS)/Diethyl Succinate (DES)/Diisopropyl Succinate (DIPS), one of the most useful transformations for organic acids,especially for a dicarboxylic acid as the diester can be used as an intermediate in the manufacture of polymers, fine chemicals, perfumes, plasticizers and solvents. Many acid catalysts have been reported in these reactions, although only a few authors have dealt with the esterification of either SA (Budarin et al.2007). DMS/DES/DIPS is a promising alternative to petrochemical dibasic esters with direct applications as solvent and polymer additive (Camilo et al.2014). SAN, a major byproduct is also being used for productin of DMS/DES/DIPS, which produces many valuable products such asγ-Butyrolactone (GBL),tetrahydrofuran (THF) and 1,4-Butanediol (BDO) which are truly a building block for chemical synthesis.
Esterification of succinic acid and acetic acid was performed by fermenting biomass and carbohydrates with ethanol in continuous reactive distillation unit using Katapak-SP11 structured packing and Amberlyst 70 as catalyst. 100% conversion was achieved with 98% diethyl succinate as bottom product and ethyl acetate as top product (Orjuela et al.2012). In another study they performed liquid phase esterification of the same acid mixture using Amberlyst 70 as catalyst varying experimental parameters like ethanol:acid molar ratios, temperatures and catalyst loadings. NRTL activity model were used to evaluate esterification reaction kinetics (Orjuela et al.2011). A membrane process which consist of a nanofiltration and vapor permeation was evaluated as purification process for succinic acid and the esterification of succinic acid with ethanol. It was found that yield of diethyl ester was the function of initial reactant ratio and also temperature played crucial role in productivity (Lubsungneon et al. 2014). Candida antarctica lipase B was immobilized on acrylic resin to esterify SA with ethanol at 313 and 323 K and experiments showed that Apparent equilibrium constant (Km) depends on molality of water, succinic acid and also on the temperature (Altuntepe et al. 2017). Succinic acid was extracted using aqueous two-phase system (ATPS) composed of ethanol and salts, from fermentation broth and then esterification reaction was performed with ethanol. It was found that ATPS having high extractability but conversion and yield of succinic acid was very low because of large amount of co-extracted water (Matsumoto and Tatsumi 2018). H+-Zeoliteβ was found to be effective solid acid catalyst in the esterification reaction of succinic acid and phenol which gave yield of 96% of diphenyl succinate. Catalyst activity was recycled upto 5 times without any noticeble change in catalytic activity (Le et al. 2019). Glucose and Benzyl chloride are thermally first carbonized and then sulfonized to design a sulphated rich carbonaceous catalyst with higher acidic strength and good ratio of acidic groups which then applied to the reaction of esterification of SA and also for fructose dehydration to form HMF.Carbonaceous solid acid showed higher catalytic activity and stability than Amberlyst-15 catalyst (Liu et al.2021). Recently esterification reaction of succinic acid with ethanol is evaluated in presence of ZSM-5 and HZSM-5 catalyst which resulted 79% and 94% conversion respectively under 348 K, 1 to 3 molar ratio of succinic acid and methanol and using1g of both catalyst (Parmar et al.2021).
The aim of the present study is to investigate catalytic activity of D-Hβ zeolite in the esterification of carboxylic esters using microwave as a source of energy. In our previous study we found that D-Hβ catalyst is an effective catalyst for estertrification of carboxylic acids as it shows moderate Bronsted acidity required for esertification reaction. Due to desilication number of Si atom decreases, so interaction of oxygen with the nearest Al cation will be stronger which increases Bronsted acidity and enhances the catalytic activity (Umrigar et al.2018).
The microwave (MW) assisted chemical reactions are much more greener and ecofriendly to the environment than conventional reactions as it provides shorter reaction times, clean and improved product yields and less waste generation (Kappe 2004). The MW dielectric heating effect uses the ability of some liquids and solids to transform electromagnetic energy into heat and thereby drive chemical reactions. Heck reaction,Ullmann condensation reaction, and transition metal catalyzed carbonylation reactions etc are the various examples which were carried using microwave energy (Verma and Namboodiri 2001).
Thus, in light of the literature reported, the present work is to study the behavior of several variables using minimum quantity of SA, alcohol and D-Hβ. Temperature, power, dosing of zeolite D-Hβ were optimized for esterification of SA with methanol to maximize the conversion and the selectivity of mono, di-ester of succinate (Monomehtyl Succinates (MMS), Dimethyl Succinates (DMS)) and to minimise unwanted product Succinic anhydride (SAN). For optimization of process parameters Box-Behnken design was performed using Design-Expert Version10.0 (Stat-Ease, Inc. Minneapolis). SA esterification with ethanol and 2-propanol were also carried out at this optimum condition.
Several solvent extraction and separation steps had been carried out to get pure form of products and were analyzed using Gas Chromatography-Mass Spectrometry (GCMS) Agilent 5975 GC/MSD with 7890A GC system having HP-5 capillary column of 60 m length and 250 micrometer diameter with a programmed oven temperature from 50 to 280°C, at 1 mL/ min flow rate of He as carrier gas and ion source at 230°C.
2.Materials, Methods and mechanism:
2.1Materials: Succinic Acid, Methanol, Ethanol, 2-Propanol and Hβ with a quoted purity of 0.99,1.0, 0.99, 0.995 and 0.99 respectively were obtained from Merck, India. All analytical reagents like diethylether, sodium bicarbonate for neutralization and separation and diethylether for GCMS (Gas chromatography Mass Spectrophotometer) were also obtained from Merck, India.
2.2 Method:Pure Succinic Acid, Methanol, Ethanol, 2-Propanol and D-Hβ were used for the above esterification reactions.Main products like mono esters like Monomethyl Succinate (MMS), and diesters like Dimethyl Succinate (DMS), Diethyl Succinate (DES), Diisopropyl Succinate (DIPS), Di-n-Propyl Succinate (D-n-PS) (Scheme 1, 2 &3). Succinaic Anhydrides (SAN)i.e. Dihydrofuran-2,5-dione (DHF) is also produced due to variation in concentration and temperature during the reactions (Scheme 4).
Reactions were carried out in a thermo-stated microwave (MILESTONE, India) assisted glass reactor equipped with a magnetic stirrer with a reflux condenser attachment. For each run, succinic acid and alcohols (excess) were used and D-Hβ catalysts were mixed to prepare reaction mixture. Reaction was carried out at different temperatures, catalyst (D-Hβ) amount and microwave power. Esterification reaction (total volume 60 ml for each run) was taken and temperature was varied from 60-80ºC. Samples of about 10ml were withdrawn from the reactor at different intervals of time and analyzed.
2.4 Microwave Assisted Esterification of succinic acid:
As esterification is an equilibrium reaction (Williamson 1994; Khosravi and Shinde 2014), several methods are available in order to shift the reaction towards formation of the desired product such as continuous water removal from the reaction mixture and using excess of alcohols. From our previous study it was found that nano-crystal of D-Hβ catalyst possesses large number of active sites on the external surface which promotes rate of reaction and product selectivity. For preparation of D-Hβ by desilication of Hβ was mentioned in our previous study (Umrigar et al.2018).
From Fig. 1, it was observed that DMS production was maximum at its’ boiling point i.e. 70ºC. So temperature was maintained at corresponding b.pt of alcohols i.e. MeOH (70ºC), Ethanol (80 ºC) and 2-PrOH (85-90ºC).To optimize different parameters, reactions were carried out varying reaction time (10-20 min), catalyst amount (0.2-1.0 g) and microwave power (200-400W). Samples of about 10ml were withdrawn from the reactor at different intervals of time and analyzed.
Fig:1 Effect of temperature on the production of DMS