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4.1.1. Equilibrium Shift by Water Removal As already discussed in Section 2.2. esterification is an equilibrium limited reaction, where water separation is required to increase conversion. Tanaka et al. [185,186] proposed zeolite T membranes for the pervaporation-aided esterification of acetic acid or lactic acid with ethanol, catalyzed by an exchange resin (Amberlyst 15, Organo) in batch reactor at 343 K. The membrane displayed good stability even being submerged in the acidic media and was able to selectively remove the produced water, thus exceeding the equilibrium limit. In such a way, nearly 100% conversion within 8 h of operation was reached. In this context, de la Iglesia et al. [168] employed modernite and zeolite A membranes in a continuous membrane reactor packed with AmberlystTM as catalyst and evaluated their joint performance during esterification of ethanol and acetic acid. The used membranes exceeded the equilibrium conversion towards the esterification products in less than 1 day of operation, however displayed different resistance to the acidic reaction conditions affecting their long term stability. The modernite membrane leveled at a conversion of nearly 90% for 5 days of experiment attributed to its stability under the conditions applied, whereas the instability of zeolite A caused tremendous conversion loss. Interestingly, in the case of the modernite membrane significant rise in the separation factors of H2O/Ethanol and H2O/acetic acid from 55 and 25, respectively, at the beginning of the experiment towards 200 and 95 at the third day was observed associated with the increasing production of water in the catalyst zone. On the contrary, the initial higher separation factors of H2O/Ethanol (315) and H2O/acetic acid (89) for the zeolite A membrane declined sharply to 32 and 18, respectively, leading to reactants loss and conversion decreasing from 72% at the beginning of the experiment to 53% after two days of operation. Beside esterification, further water releasing reactions can be enhanced using zeolite membranes for equilibrium shift. For instance, modernite or NaA zeolite membranes together with AmberlystTM were implemented in membrane reactors for the gas-phase synthesis of methyl-tert-butyl ether (MTBE) from tert-butanol (TBA) and methanol (MeOH) [187]. In the first step, the membranes were characterized by separation experiments of multicomponent mixtures containing water, MeOH, TBA, MTBE and i-butene (IB) displaying the following selectivity trend: SH2O/IB > SH2O/MTBE > SH2O/TBA > SH2O/MeOH. The selective permeation of water across the zeolite membranes was attributed to their high polarities resulting in preferential adsorption and pore blocking for the other components. Then, during the reaction experiments the water removal in the membrane reactor operation contributed to 67.5% MTBE yield showing 6.7% absolute increase in the MTBE yield compared to the conventional configuration without water removal. Very recently, NaA zeolite membranes were also used as water extractors in the methanol dehydration to dimethyl ether (DME) [169]. The γ-Al2O3 catalyst was located between two 21PDF Image | Zeolite Catalysis
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