Effect of reaction substrates on membrane-assisted transesterification reactions

Highlights

• Conversion increased in all tested membrane-assisted transesterification reactions.

• Long-alkyl-chain methyl esters and alcohols afford high vapor methanol concentration.

• Methanol removal increased with temperature and vapor-phase methanol concentration.

Abstract

Zeolite membranes are promising for the assistance of transesterification reactions, but the effects of the reaction conditions have not been fully clarified. In this study, the membrane-assisted transesterification of several methyl esters and alcohols was examined to investigate the influence of the substrates on the conversion and methanol permeation. The conversion of methyl hexanoate was 55.0% without a membrane when reacted with 1-hexanol at 353 K. When the methanol byproduct was removed using an FAU-type zeolite membrane, the conversion increased to 79.6%. A large increase in conversion was also obtained using substrates with long linear alkyl chains. The highest conversion among the 18 transesterification reactions was obtained when methyl hexanoate was reacted with 1-hexanol. These results suggest that the reaction substrates influence the boiling point of the solution and the vapor-phase methanol concentration. Additionally, the effect of temperature and feed concentration on the methanol permeation performance was evaluated for the FAU-type zeolite membrane by pervaporation to discuss their influence on the conversion. Higher reaction temperatures were favorable for membrane-assisted transesterification because more methanol was removed by the membrane. This suggests that controlling the vapor-phase methanol concentration is significant for membrane-assisted transesterification reactions.

Introduction

Inorganic membranes can be used for separation processes at high temperatures and in organic solvents because of their high thermal and chemical stability. In particular, zeolite membranes with their excellent separation performances for water [1], [2], [3], [4], [5], carbon dioxide [6], [7], [8], and hydrocarbons [9,10] have been developed since they can separate mixtures based on molecular size and affinity corresponding to microstructure and composition. Therefore, commercialization and demonstration studies using zeolite membranes have been reported for alcohol dehydration and carbon dioxide/methane separation [3,7].

Many studies on the combination of zeolite membranes with chemical reactions have been reported toward increasing the conversion, such as esterification reactions [11], [12], [13], [14], [15], the synthesis of carbonates [16] and methanol [17,18] for carbon dioxide utilization, and methanol to olefin reactions [19], [20], [21], [22]. In particular, esterification reactions using dehydration zeolite membranes are approaching commercialization. Esters are important functional chemicals and intermediates for pharmaceuticals, foods, and cosmetics. Approximately 10% of chemicals are synthesized by esterification and transesterifications. Since these are equilibrium reactions, the conversion can be increased by adding an excess amount of alcohol [23].

Membrane-assisted esterification is a promising technique for reducing the amount of added alcohol. Many studies combining esterification with zeolite membranes have been reported [11], [12], [13], [14], [15]. Tanaka et al. examined the esterification of acetic acid with ethanol using T-type zeolite membranes. The conversion of acetic acid was increased to 98% by the membrane [11] compared with 76% without. Moreover, the conversion could be predicted based on a reaction kinetic equation containing the reaction rate constant and water flux. Zhu et al. increased the conversion of acetic acid to 98% with sulfuric acid as a catalyst using a MOR-type zeolite membrane [12], which could be used repeatedly because of its high acid stability. Besides ethyl acetate, zeolite membranes have also been applied to other esterification reactions with target products such as ethyl lactate [13], ethyl oleate [14], and diisopropyl adipate [15]. For example, a CHA-type zeolite membrane was applied to produce diisopropyl adipate by esterification at an increased yield from 56% at equilibrium to 98% [15]. In particular, the water permeation behavior through the membrane was monitored continuously during the reaction, which indicated that the yield of the target ester was increased by the equilibrium shift caused by the removal of water as a byproduct by the CHA-type zeolite membrane. These previous studies have indicated that the use of zeolite membranes in esterification reactions requires a high water flux and good stability under the selected reaction conditions.

Recently, membrane reactors for transesterification have focused on the methanol removal performance of zeolite membranes [24,25]. For these transesterification reactions, methyl esters and alcohols are used as reaction substrates to produce the target ester products and methanol as a byproduct:

The selective removal of the methanol byproduct would theoretically increase the transesterification conversion. Kumakiri et al. applied FAU-type zeolite membranes to the transesterification of methyl acetate and 1-butanol at 323 K with Amberlyst 15 as a catalyst [24]. The methanol flux of the FAU-type zeolite membrane was 12.5 mol m⁻² h⁻¹ for a 9.7wt% methanol/butanol mixture at 323 K, and the methanol concentration on the permeate side was 97.6%. The conversion without the membrane reached 43% at equilibrium. The membrane was then soaked in the reaction solution, and removal of the methanol byproduct was attempted. However, the conversion could not be increased. They concluded that this was attributable to a reduction in the methanol flux due to the influence of the acid and byproducts. Recently, we reported the membrane-assisted transesterification of methyl acetate with 2-propanol using a FAU-type zeolite membrane [25]. The conversion was increased from 38% at equilibrium to 57% by the selective removal of methanol from the vapor phase. The membrane performance could be maintained by positioning the membrane in the vapor phase to avoid contact with the reaction solution. Since only a few studies on transesterification with zeolite membranes have been reported, there are many unknowns, such as the required membrane flux and separation performance, the effect of the reaction substrates, and the operating conditions.

In this study, the membrane-assisted transesterification of several methyl esters and alcohols was examined to investigate the influence of the reaction substrates. Moreover, the permeation properties of methanol through FAU-type zeolite membranes and the effect of reaction temperature were determined to discuss the influence of the operating conditions.