Direct enzymatic acylation of cellulose pretreated in BMIMCl ionic liquid

Abstract

Cellulose esters are an important class of functional biopolymers with great interest in the chemical industry. In this work the enzymatic acylation of Avicel cellulose with vinyl propionate, vinyl laurate and vinyl stearate, has been performed successfully in a solvent free reaction system. At first cellulose was putted into the ionic liquid BMIMCl (1-n-butyl-3-methylimidazolium chloride) in order to facilitate the unwrap of the structure of the polysaccharide molecule and make it accessible to the enzyme. Thus, after this pretreatment the enzymatic esterification reaction was performed using various hydrolases. The enzymes capable of catalyzing the acylation of cellulose were found to be the immobilized esterase from hog liver and the immobilized cutinase from Fusarium solani, while the lipases used did not show any catalytic activity. Cellulose esters of propionate, laurate and stearate were synthesized with a degree of esterification of 1.9%, 1.3% and 1.0%, respectively. It is the first successful direct enzymatic acylation of cellulose with long chain fatty acids.

Introduction

Cellulose is the most abundant polysaccharide in nature. It occurs in plants under the form of microfibrils, components of the structurally strong framework of the cell walls. Cellulose is also produced in a highly hydrated form by some bacteria (e.g. Acetobacter xylinum) but it is mostly prepared from wood pulp (Kadla and Gilbert, 2000). It is a linear polymer of β-(1,4)-d-glucopyranose units in C₁ conformation. The fully equatorial conformation of β-linked glucopyranose residues stabilizes the chair structure, minimizing its flexibility. Due to the extended intra- and intermolecular hydrogen bonding between the cellulose chains, this biopolymer has a high crystalline structure. As a result it is completely insoluble in water or in common solvents leading to difficulties in chemical manipulation (Kadla and Gilbert, 2000).

On the other hand the plenitude, the renewability and the low cost of cellulose make it an ideal feedstock for producing different materials. About a third of the world’s production of purified cellulose is used as the base substance for a number of derivatives with predesigned and wide-ranging properties depending on the groups involved and the degree of derivatization (Lee and Wang, 2006, Siro and Plackett, 2010, Takatani et al., 2008). Cellulose has great potential for the preparation of novel materials (e.g. thermoplastics), exhibiting a number of well-known advantages such as biocompatibility, high stiffness, good mechanical properties and biodegradability. In particular acyl esters of cellulose are considered to be an important class of polymers used in the production of fibers, plastics, films, cosmetics and drugs (Edgar, 2007, Gradwell, 2004, Wibowo et al., 2006).

To date the commercial synthesis of these compounds is achieved in heterogeneous reaction systems containing carboxylic acid anhydrides, tough solvents like N,N-dimethylacetamide and acid catalysts. This process is also limited to the production of short chain esters with a length of four carbon atoms or less. This is not advantageous because the cellulose ester is expected to have better thermoplastic properties when the chain of the ester contains more than six carbons. That happens because the longer chain acts as an internal plasticizer (Yin et al., 2007). In order to overcome the above situation and introduce biocatalysis into the field of cellulose esterification, we designed a system for the enzymatic acylation of this polysaccharide with long chain fatty acids. The realisation that enzymes can function in non-aqueous media has given to their synthetic potential a powerful boost for many processes (Ikeda and Klibanov, 1993, Klibanov, 1989, Patel et al., 1996, Zaks and Klibanov, 1985). Amongst the numerous enzymes studied in such systems, esterases and in particular lipases have been successfully utilized in a wide range of stereoselective and regioselective acylations of various target molecules including sugars (Kirk et al., 1995, Ljunger et al., 1994, Patel et al., 1996, Tsitsimpikou et al., 1998). The first attempt of the enzymatic esterification of cellulose was made a few years ago (Sereti et al., 1998, Sereti et al., 2001) by subjecting Avicel cellulose, the fatty acid as substrate and lipase into several organic solvents. The reaction was not successful due to the high crystallinity of cellulose. The macromolecule could not be dissolved and there was no accessibility by the enzyme. Only soluble cellulose derivatives, such as cellulose acetate and hydroxypropyl cellulose, were found to be acylated by lipases.

In the present work, this problem was tackled by introducing a preliminary step of cellulose treatment, using ionic liquids before the enzymatic reaction in order to make the cellulosic molecules as permeable as possible. Ionic liquids are organic salts whose ions do not pack well and remain liquid in ambient temperatures (Earle and Seddon, 2000). Some studies showed that cellulose can be dissolved in some hydrophilic ionic liquids such as 1-n-butyl-3-methylimidazolium chloride (BMIMCl) and 1-allyl-3-methylimidazolium chloride (AMIMCl) (Wu et al., 2004, Zhao et al., 2009, Zhu et al., 2006). Particularly BMIMCl can dissolve cellulose up to 25% (w/w). Cellulose in its BMIMCl solution can be easily precipitated by addition of water, ethanol or acetone. Moreover, the regenerated cellulose has almost the same degree of polymerization (DP) as the initial one (Zhao et al., 2009, Zhu et al., 2006). Furthermore, BMIMCl renders cellulose essentially amorphous, thus its structure may open and the macromolecule can be accessible by the corresponding enzyme (Dadi et al., 2006, Dadi et al., 2007). The only restriction is that ionic liquids that dissolve cellulose such as BMIMCl, cause the denaturation of the enzyme molecules; thus the treatment of cellulose with BMIMCl and the enzymatic acylation reaction has to be done separately. This process provides a new platform for the utilization of cellulose resources. Additional, advantages in the use of ionic liquids is the fact that they have low volatility, they are stable in a wide range of temperature, they are not flammable and also they can be easily recycled (Earle and Seddon, 2000, Park, 2003, Vanrantwijk et al., 2003). The above, render the ionic liquids solvents with unique properties, justifying the characterization ‘green solvents’.