Separation of dimethyl carbonate/methanol/water mixtures by pervaporation using crosslinked chitosan membranes

Abstract

This study deals with the separation of dimethyl carbonate (DMC) by pervaporation using crosslinked chitosan membranes. DMC, an environmentally benign chemical, is commercially produced by oxidative carbonylation of methanol, and the separation and purification of DMC is critical to DMC manufacture due to azeotropic nature of the reaction mixtures. In this study, chitosan membranes crosslinked by dilute sulfuric acid were prepared for DMC separation by pervaporation. The membranes were tested for separation of binary DMC/water, DMC/methanol and methanol/water mixtures and ternary DMC/methanol/water mixtures in a temperature range of 25–55 °C. The experimental results showed that the membrane could be used effectively to break the DMC/methanol azeotrope and to remove a small amount of water from DMC. It was also shown that coupling effect was important in pervaporation separation of multi-component mixtures due to interactions among the permeating species.

Introduction

Dimethyl carbonate (DMC) is an environmentally benign chemical that has found wide applications in the past few years due to its low toxicity and quick biodegradation. It has become an important substitute for the toxic phosgene, dimethyl sulfate and methyl halides that have conventionally been used as methylating and carbonylating agents [1]. In addition, DMC has excellent characteristics as a fuel additive for internal combustion engines, which makes DMC especially important to the oil industry as an alternative to methyl tert-butyl ether (MTBE), the most widely used gasoline additive, due to environmental concerns associated with the use of MTBE [2]. It is predicted that a large scale up of current DMC production will be necessary if DMC is to be used as a fuel additive. DMC is primarily produced on an industrial scale by oxidative carbonylation of methanol, and a significant portion of the process costs is attributed to the separation of the reaction effluents because of the azeotropic nature of the DMC/methanol/water mixtures. Recently, synthesis of DMC from methanol and carbon dioxide has attracted a lot of attention as an alternative route partly because CO₂, a greenhouse gas, can be used as a C₁ feedstock [3], [4], [5], [6], [7]. As in oxidative carbonylation of methanol, ternary DMC/methanol/water mixtures are also going to be involved in the direct synthesis of DMC from methanol and CO₂. The difficulty in separation and purification of DMC has prompted research on various separation techniques, including pressure swing distillation, extractive distillation, low temperature crystallization, and liquid–liquid extraction [8].

It is well recognized that pervaporation separation, which is based on selective permeation through a membrane, is especially efficient for separation of azeotropic and close-boiling mixtures. Pervaporation appears to be a promising alternative process for DMC separation, at least as a complement to distillation that is being used in the commercial DMC manufacture. Landscheidt et al. disclosed a process scheme for the production of DMC based on catalytic reaction of carbon monoxide and methyl nitrite where pervaporation or vapor permeation is used in conjunction with distillation [9], and the simulation studies have shown that hybrid processes combining pervaporation and distillation are more favorable than the conventional distillation processes [10], [11].

In contrast to pervaporation separation of MTBE/methanol, which has been investigated extensively due to interest in using MTBE in reformulated fuels, there are few studies in the literature on DMC separation by pervaporation. In our previous work it was demonstrated that chitosan membranes are capable of separating DMC/methanol/water mixtures by pervaporation [12]. Chitosan is a polysaccharide polymer with reactive hydroxyl and amino groups, which can be used for various chemical modifications. Chitosan membranes for alcohol dehydration are well studied, and membrane modifications have been reported, including crosslinking [13], [14], [15], [16], [17], [18], [19], blending [20], chemical modification [21], [22], and multi-layer casting [23]. Among these, crosslinking is one of the most effective approaches for improving membrane stability.

In the present study, chitosan membranes crosslinked by sulfuric acid were prepared. They were evaluated for the separation of binary DMC/methanol, DMC/water and methanol/water mixtures as well as ternary DMC/methanol/water mixtures. Sulfuric acid is chosen as the crosslinking agent since it is one of the most effective crosslinking agents for chitosan membranes for alcohol/water mixtures [24].