Membrane catalytic esterification

The catalytic esterification of ethanol and acetic acid to ethyl acetate and water has been taken as a representative example to emphasize the potential advantages of the application of membrane technology compared with conventional distillation [48], see Fig. 13.6. From the McCabe-Thiele diagram for the separation of ethanol-water mixtures it follows that pervaporation can reach high water selectivities at the azeotropic point in contrast to the distillation process. Considering the economic evaluation of membrane-assisted esterifications compared with the conventional distillation technique, a decrease of 75% in energy input and 50% lower investment and operation costs can be calculated. The characteristics of the membrane and the module design mainly determine the investment costs of membrane processes, whereas the operational costs are influenced by the hfetime of the membranes. 

Gao, Yue, and Li (1996) studied the same reaction using a zeolite A-PVA composite membrane (at temperatures ranging from 20 to 50 °C). In this work, together with the pervaporation-aided catalytic esterification of acetic acid with ethanol, the reaction between salicylic acid with methanol was also treated. Among other results, it showed that the continuous removal of water from the system displaced the equilibrium limit (79%), making possible a 95% conversion, when using PVA, PVA -I- KA, and PVA -I- CaA membranes for 20.0, 11.3, and 10.0 h, respectively. 

Pervaporation-assisted catalysis is a typical example of an operation eflide-ntly carried out in extractor-type catalytic membrane reactors. Esterification is by far the most studied reaction combined with pervaporation. " Esters are a class of compounds with wide industrial appUcation, from polymers to fragrance and flavour industries. Esterification, a reaction between a carboxylic acid and an alcohol with water as a by-product, is an equilibrium-limited reaction. So, this is a typical reaction that can be carried out advantageously in a extractor-type membrane reactor. By selectively removing the reaction product water, it is possible to achieve a conversion enhancement over the thermodynamic equilibrium value based on the feed conditions. 

Bagnell L, Cavell K J, Hodges A M, Mau A W H and Seen A J (1993), The use of catalytically active pervaporation membranes in esterification reactions to simultaneously increase product yield, membrane permselectivity and flux , J Membr Sci, 85,291-299. 

Zhu Y and Chen HF. Pervaporation separation and pervaporation-esterification coupling using crosslinked PVA composite catalytic membranes on porous ceramic plate. J Membr Sci 1998 138(1) 123-134. 

Their laboratory PVMR consisted of a reservoir in which the reactants were placed together with Nafion pellets, which acted as the catalyst. The liquid in the reservoir was continuously recirculated through the membrane tube, which was placed externally to the reactor. The membrane, itself, was also shown to be catalytic. A flow of inert gas (rather than vacuum) was used to remove the vapors and water from the membrane permeate. For the methanol esterification reaction the improvement in yield was modest (final conversion 77 % vs. 73 % corresponding to equilibrium), because the membrane was not very permselective towards the reaction products. Significant improvements, on the other hand, were observed with the butanol reaction (final conversion 95 % vs. 70 % corresponding to equilibrium), as the membrane is more permselective towards the products of this reaction. Exchanging the acidic protons in the Nafion membranes with cesium ions significantly improved the permselectivity, but also reduced membrane permeance. 

Peters et al. (2005a-d) developed a continuous composite catalytic PV man-brane. Composite catalytic membranes were prepared by applying a zeolite coating on top of a ceramic hf silica membrane. The performance of the composite catalytic membrane was examined in the esterification reaction between acetic acid and butanol (esterification coupling). In the PV-assisted esterification reaction, the catalytic membrane was able to couple catalytic activity and water removal. 

Zhu and Chen (1998) prepared cross-linked PVA composite catalytic membranes on porous ceramic plate for PV separation and PV-esterification coupling. The composite catalytic membrane was evaluated through the PV and a model system of n-butyl alcohol esterification coupling with the PV. The conversion of n-butyl alcohol reached 95% when a cross-linked PVA catalytic manbrane was used. The order of permeation fluxes was water > acid > alcohol > acetate and the total flux was greater than 0.5 kg/m h during the reaction time. The order of the separation selectivities of membranes was water-acetate > water-alcohol > water-acid. The parameters such as temperature, initial molar ratio of acid to alcohol, and catalyst concentration could be changed in order to attain the optimum of the PV-esterification coupling operation. 

The experimental results obtained with catalytic pervaporation membranes for the well-explored case of esterification reactions are generally not as good as those observed in integrated reaction-pervaporation processes with non-catalytic membranes. For the reasons mentioned above, although pervaporation catalytic membranes are potentially interesting, they require additional research in order to better analyse these phenomena and optimize the immobilization techniques of the catalyst in the membranes. 

Castanheiro J E, Ramos A M, Fonseca IM and Vital J (2006), Esterification of acetic add by isoamylic alcohol over catalytic membranes of poly(vinyl alcohol) containing sulfonic add groups , AppZ Catal A-Gen, 311,17-23. 

Figueiredo K C S, Salim V M M and Borges C P (2008), Synthesis and characterization of a catalytic membrane for pervaporation-assisted esterification reactors , Catal Today, 133-135,809-814. 

DABA moiety. At first, mono esterification took place between the -OH group of diol with a free -COOH group in the presence of catalytic amounts of p-toluene sulfonic acid. Thereafter, the monoester membranes were annealed at elevated temperatirres under vacuum to activate a solid-state cross-linking reaction (trans-esterification). A general reaction scheme of the two-step cross-linking reaction is shown in Scheme 3.16. 

With regard to C-PVMRs using bifunctional membranes, an interesting contribution was also made by Bernal et al. (2002). In particular, the H-ZSM-5 membrane used in this work had sufficient catalytic activity to perform the esterification reaction, and at the same time it was selective for water permeation. The conversion obtained at... 

Ma, Xu, Liu, and Sun (2010) used perfluorosulfonic acid-poly(vinyl alcohol)-Si02/ poly(vinyl alcohol)/polyacrylonitrile (PFSA-PVA-Si02/PVA/PAN) bifunctional hollow-fiber composite membranes. The catalytic and the selective layer of the membrane were independently optimized. These membranes were synthesized by dipcoating. The performance of these bifunctional membranes was evaluated by dehydrating the ternary azeotropic composed of a water, ethanol, and ethyl acetate system (top product of a reactive distillation process of esterification of acetic acid with ethanol), obtaining separation factors of water/ethanol up to 379. An extensive assessment on the esterification reaction of ethanol-acetic acid was later published (Lu, Xu, Ma, Cao, 2013). In this case, the reaction equilibrium was broken in less than 5 h, and a 90% conversion of acetic acid was achieved after 55 h. 

The esterification reaction can also be carried out in CMRs. The catalytic membranes are prepared by ion exchange modification. CMRs may outperform PVMRs in conversion with the same loading of catalyst dispersed in the liquid bulk [38]. 

One of them employs membrane-based separation processes connected to the esterification reaction. In this respect, vapor permeation and pervaporation process have been tested and dn-ee different layouts have been reported for ethyl lactate production. In one of them, membrane module is located outside the reactor unit and the retenate is recirculated to the reactor." " In another scheme, the membrane module is placed inside the reactor, but the membrane does not participate in the reaction directly and simply acts as a filter," " and in the third configuration, membrane itself participates in die reaction catalysis (catalytic membrane reactor)." Different hydrophilic membranes, such as polymeric, ceramic, zeolites and organic-inorganic hybrid membranes were tested. ... 

More recently, composite polymeric catalytic membranes consisting of a dense layer of a mixed-matrix of tiny particles of Amberlyst-35 dispersed in PVA cross-linked with maleic add cast over a commercial PVA membrane (PERVAP 1000), were effidently used in the pervaporation-assisted esterification of acetic acid and ethanol. After 8 h of reaction, a 60% increase in conversion was observed for the catalytic membrane configuration, compared to an inert membrane/fiuidized-bed configuration. 

The use of polymeric catalytic membranes in pervaporation-assisted catalysis applied to non-esterification readions has also been referred in recent reviews. ... 

The use of a non-pervaporative extractor-type catalytic polymeric membrane reactor has been reported for light alcohol/acetic acid esterifications. A cross-linked poly(styrene sulfonic acid) (PSA)/PVA blend flat membrane was assembled in the reactor in a vertical configuration, separating two chambers. One of the chambers was loaded with an aqueous solution of ethanol and acetic acid, while the other chamber was filled with chlorobenzene. The esterification equilibrium is displaced to the product s side by the continuous extraction of the formed ester. In the esterifications of methanol, ethanol and n-propanol with acetic acid, the reactivity through the PSA/PVA membrane was higher than that with HCl as catalyst. In that of n-butanol with acetic acid, however, it was viceversa. 

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