Recovery of glucose from an aqueous ionic liquid by adsorption onto a zeolite-based solid

Abstract

Batch adsorption was studied for d-glucose on different types of zeolite-based adsorbents from an aqueous solution of 1-ethyl-3-methylimidazolium acetate ([C₂mim][CH₃COO]). The goal of the studies is to establish a possible process for recovery of glucose and of ionic liquid for recycle in a continuous process for converting a wild grass (miscanthus) to a liquid biofuel. A significant increase in glucose uptake is obtained when ionic liquid is present in the aqueous glucose solution. Highest glucose uptake was obtained for Faujasite structures and for zeolites with a high Si/Al ratio. Adsorption on KX-zeolites can reach 45% glucose removal in one step. Desorption of glucose into water is complete in 50 min at 50 °C and in 10 min at 60 °C.

Introduction

The low cost of readily available lignocellulosic biomass has caused increasing interest in the bioconversion of this feedstock into liquid fuels and chemical products [1], [2], [3], [4], [5]. Cellulose and hemicellulose are hydrolyzed into sugar monomers that can be converted to liquid fuels [6], [7], [8], [9], [10]. Ionic liquids (ILs) are useful for dissolving or solvating biomass and subsequent separation of lignin from hemicellulose and cellulose [11], [12], [13], [14]. Although classical enzymatic hydrolysis is carried out with an excess of water, some aqueous ILs appear to allow enzymatic hydrolysis of dissolved cellulose without loss of enzyme stability [15]. When hydrolysis is carried out in an aqueous IL, glucose obtained from hydrolysis is also dissolved in that solution. This sugar must be removed prior to recycling the ionic liquid for dissolving biomass. Recent studies for a wild grass (miscanthus) showed good results for glucose production by enzymatic hydrolysis of cellulose in aqueous imidazolium ILs [12], [16], [17], [18], [19]. Ultimately, several authors reviewed the advantages on the use of acetate ILs over the classical use of the chloride ILs, including its lower melting point, lower viscosity, lower corrosive character and higher loadings [20]. In previous publications [21] it was reported that [C₂mim][CH₃COO] shows minimal inhibitory effects on enzymes compared to the ionic liquids containing the Cl⁻ or Br⁻ anions. The high solubility of glucose in these aqueous ionic liquids makes a challenging separation problem [22], [23], [24], [25] likely to encounter formidable problems in practice. In this work, we consider adsorption of glucose as a possible method for separation.

In addition to low operation cost, high selectivity and simplicity in implementation, essentially reversible physical adsorption becomes more attractive compared with other separation processes, because it is possible to regenerate the adsorbent. The adsorption of glucose and polysaccharides onto solid surfaces is important in several areas including soil chemistry and mineral processing. Previous studies [26], [27], [28], [29] report adsorption of sugars from water onto different adsorbents, e.g. mineral surfaces, ion-exchange resins, or zeolites. However, the goal of these previous studies, unlike ours, was the separation of different monomeric sugars for purification or their use in a flotation process to separate valuable ore from waste rock. In this work, we report results for adsorption of glucose from aqueous ionic liquid 1-ethyl-3-methylimidazolium acetate onto several solid zeolite-based adsorbents. In the earlier studies, the sugars were dissolved in water without ionic liquid.