(707d) Plug Flow Membrane Reactor for Esterification of Acetic Acid with Mor-Type Zeolite Membrane

While esterification is important for chemical production, the attainable level of conversion is always thermodynamically limited. Thus, the product of esterification is a mixture of organic acid, alcohol, ester, and water. Dehydration membrane reactor has been drawn attention as a novel energy-saving process for esterification for recent decades. When a dehydration membrane is adopted to remove water from the reaction system, the attainable level of conversion will be improved beyond that limited by thermodynamics in conventional reactor. Consequently, one can expect that energy consumption for purification process after reaction is reduced as well as the improvement of one-path yield of products.

We previously reported MOR-type zeolite membrane for dehydration and pointed out that our MOR membrane had high chemical resistance for organic acid.¹ In this study, we prepared applied MOR membrane to a plug flow reactor for the esterification of acetic acid.

MOR membranes were synthesized by a seed-assisted crystallization method on the outer surface of porous tubular α-alumina support (Noritake, i.d. = 7 mm, o.d. = 10 mm) of which the average pore size of the top layer was ca. 150 nm. The outer surface of support was seeded by a dip-coating method using a colloidal suspension of MOR seed crystals.

The seeded support was placed vertically in a Teflon-lined stainless steel autoclave and filled with a reaction mixture. The reaction mixture had a molar composition of l0Na₂O:0.15Al₂O₃:36SiO₂:960H₂O and was prepared by mixing colloidal silica (ST-S, Nissan Chemical Ind. Ltd.), sodium aluminate (Kanto Chemical Co. Inc.), sodium hydroxide (>97 wt%, Kanto Chemical Co. Inc.), and deionized water. The reaction mixture was aged at 323 K for 4 h prior to use. Hydrothermal crystallization was carried out at 453 K for 7 h in an autoclave. After the crystallization, the autoclave was quenched with water. The synthesized membrane was washed in boiling water and dried over night at 383 K.

Dehydration performance of MOR membrane prepared was evaluated by the separation test for an equimolar mixture of water and acetic acid in a pervaporation mode at 343 K. The permeation side, the inner side of tubular membrane, was evacuated by using a vacuum pump. The permeate was condensed by a cold trap and was analysed for its composition by using GC-TCD.

The esterification of acetic acid was carried out in a plug flow membrane reactor as follows. The equimolar mixture of acetic acid and ethanol was fed to the outer surface of tubular membrane. Amberlyst-15 was packed on the outer side of membrane as catalyst. Temperature and pressure were adjusted at 403 K and 1.0 MPa, respectively. The permeate was analyzed for its composition. The yields of ethyl acetate produced in the plug flow reactor with and without MOR membrane were compared.

As a result of separation test for acetic acid and water, MOR membrane showed great dehydration performance. The separation factor for water/acetic acid was above 139 with its permeance of 1.65 x 10^-7 mol m^-2 s^-1 Pa^-1. Separation and permeation performance were stable at least up to 8 h, suggesting that MOR zeolite is one of the feasible membrane materials for dehydration under acidic conditions.

Whereas the yield of ethyl acetate in the plug flow membrane reactor without MOR membrane was thermodynamically limited at 66 %, its yield was improved up to 72 % with the membrane in the reactor tested. Selective removal of water by MOR membrane contributed to the enhancement of ethyl acetate yield.

(1) G. Li, E. Kikuchi, M. Matsukata, The control of phase and orientation in zeolite membranes by the secondary growth method, Microporous Mesoporous Mater. 62 (2003) 211-220

Acknowledgment

This work was partially supported by NEDO (New Energy and Industrial Technology Development Organization), Japan.