Synthesis of MCM-22 zeolite membranes and vapor permeation of water/acetic acid mixtures

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Abstract

MCM-22 membranes were prepared on a porous α-alumina tube using a secondary growth technique consisting of the deposition of seed crystals on a substrate followed by crystal growth under hydrothermal conditions. In this study, two types of seed crystals, MCM-22 and ITQ-2, were used. The MCM-22 membrane prepared using ITQ-2 seed crystals (MCM-22 (I) membrane) was composed of MCM-22 polycrystals with a thickness of 5 μm, which is much thinner than that of the MCM-22 membrane prepared using MCM-22 seed crystals (MCM-22 (M) membrane) (15–20 μm). Vapor permeation of water/acetic acid mixtures through the MCM-22 membranes was studied. The MCM-22 (I) membranes calcined at 400 °C showed a high separation factor of 78 with a water permeance of 4 × 10−8 mol m−2 s−1 Pa−1 at 120 °C. Elevating the calcination temperature to 500 °C resulted in a lower separation factor. The MCM-22 (I) membrane calcined at 400 °C has a hydrophilic internal surface due to the presence of silanol groups within the framework that contribute to the high selectivity to water permeation.

Research highlights

► MCM-22 membranes were prepared using a secondary growth technique. ► Two types of seed crystals (MCM-22 and ITQ-2) were used. ► We studied the effect of the seed on the thickness and the vapor permeation results. ► We studied the effect of calcination temperature on the separation factor. ► The MCM-22 membranes showed a separation factor of 78 for water/acetic acid vapors.

Introduction

Acetic acid is an important chemical intermediate in the synthesis of vinyl acetate, terephthalic acid, cellulose ester, and other esters. The current processes for production of acetic acid include the carbonylation of methanol, the liquid-phase oxidation of hydrocarbons, and the oxidation of acetaldehyde [1]. The separation of water from acetic acid is an important part of these processes. Azeotropic distillation and extractive distillation have been developed to separate water and acetic acid, but distillation is energy-intensive due to the small differences in the volatilities of water and acetic acid in dilute aqueous solution [2], [3].

Vapor permeation and pervaporation using membranes are alternative energy-conserving separation techniques that are often used for the separation of azeotropic and/or close boiling point mixtures. A hybrid distillation-membrane process, in which the vapor mixture from the top of the distillation column is led to a membrane separator, has been proposed as an energy-efficient separation process [4]. Another potentially useful application of membranes is the pervaporation of liquid mixtures with close boiling points, such as aqueous solutions of acetic acid.

Zeolites have attracted considerable research attention as membrane materials due to their superior thermal, mechanical, and chemical properties compared to organic polymers. So far, most studies on zeolite membranes have focused on zeolite A [5], [6], [7], [8], [9], FAU-type zeolites (X and Y) [10], [11], mordenite [12], [13], MFI-type zeolite (ZSM-5 and silicalite) [14], [15], [16], [17], [18], [19] and SAPO-34 membranes [20], [21]. Zeolite NaA membranes have been extensively developed, and have been reported to show very good performances for vapor permeation and pervaporation of water/organic liquids [8]. They are currently commercially available and have been employed for large-scale dehydration of organic solvents [9].

However, long-term use of A-type zeolite membranes in aqueous solutions, especially in acidic media, remains problematic because of dealumination from the zeolite framework. The hydrophilicity of zeolites increases with increased Al content in the framework, whereas the acid resistance simultaneously decreases because strong acids leach Al from the zeolite, resulting in the breakdown of its framework structure [22], [23]. New types of membranes that show both hydrophilicity and stability in acidic media are therefore needed for dehydration of organic liquids containing an acid, such as water–acetic acid mixtures. Zeolite membranes with different pore structures and pore sizes, such as T-type [24], mordenite [25], ZSM-5 [26], and MER [27] membranes have been studied. However, the appropriate pore size, pore structure and Si/Al ratio for use in practical applications are still unclear from the standpoint of not only separation performance but also the feasibility of membrane preparation.

In this study, a new type of hydrophilic membrane, made of zeolite MCM-22, was prepared on a porous α-alumina support by hydrothermal synthesis. MCM-22 is predicted to have high acid-proof properties due to the high Si/Al ratio in its framework. The precursor for MCM-22 (MCM-22 (P)) is composed of layered aluminosilicate with a high content of silanol groups at the surfaces of the thin layers [28], [29], as shown in Fig. 1. The MCM-22 zeolite structure is formed by calcination of MCM-22 (P) through the condensation of silanol groups. However, we would expect to calcine MCM-22 at relatively low temperatures (300–450 °C) to retain sufficient silanol groups, which show hydrophilic properties, on its internal surfaces.

In this study, MCM-22 membranes were prepared on a porous α-alumina tube using a secondary growth technique consisting of the deposition of seed crystals on the substrate, followed by crystal growth under hydrothermal conditions. Here, two types of seed crystals (MCM-22 and ITQ-2) were used. The ITQ-2 zeolite is a monolayer of crystalline aluminosilicate that can be obtained by delamination of the layered precursor MCM-22 (P), as shown in Fig. 1. The effect of type of seed crystals on the formation of the MCM-22 membranes was studied, and the morphology of the MCM-22 membranes and their vapor permeation performance for water/acetic acid mixtures were elucidated.

Section snippets

MCM-22

The MCM-22 and ITQ-2 seed crystals were synthesized using the method described in the literature [28], [30]. A starting solution was prepared by mixing 0.106 g of sodium aluminate (Al/Na2O = 0.79, Wako Pure Chemical Industries, Ltd.), 0.416 g of sodium hydroxide solution (4 N, Wako Pure Chemical Industries, Ltd.), 1.26 of amorphous fumed silica (Aerosil 200, Evonik Industries), 1.04 g of hexamethyleneimine (HMI, Wako Pure Chemical Industries, Ltd.) and 16.3 g of deionized water. The molar ratios in

Acid resistance of MCM-22

Fig. 3 shows the time course of the mass remainder of MCM-22 after immersion in acetic acid at 120 °C. Data for NaA zeolite are shown in this figure for comparison. The mass of MCM-22 powder fell by 20% in the first 30 min, but then remained constant with elapse of time. On the other hand, the mass of NaA zeolite fell by 80% after 90 min. The relative peak intensity of NaA and MCM-22 after the acid treatments is plotted in Fig. 4. The (8 2 2) XRD peak intensity of NaA zeolite fell by 70% in the

Conclusions

The MCM-22 membranes were synthesized by secondary crystal growth using ITQ-2 and MCM-22 seed crystals. The surface of the alumina support was completely covered with the amorphous layer from the early stage of membrane formation. The amorphous layer was converted to the layered MCM-22 structure after 4 days. The MCM-22 (I) membrane showed a larger water permeance than the MCM (M) membrane, possibly because it has a more uniform and thinner layer (5 μm) than the MCM-22 (M) membrane. The MCM-22

Acknowledgements

This work is supported by NEDO's “Development of fundamental technologies for Green and Sustainable chemical processes, Green and sustainable chemistry/fundamental development of ordered nanoporous membranes for highly refined separation technology” (FY2009–FY2011). We gratefully thank the GHAS Laboratory at Osaka University for the SEM measurements.

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