3.2.1 Dissolution of Cellulose in Non-derivatizing Media

Cellulose is a highly valuable biopolymer of glucose from which chemical platforms, intermediates, ethanol and fuel additives can be then produced [4, 5]. All these processes initially imply the catalytic deconstruction of cellulose to glucose. The high crystallinity of cellulose is a serious bottleneck which is at the origin of the low accessibility of cellulose to (bio)catalysts. Hence, in many cases, harsh conditions of pressure and temperature are required for the deconstruction of cellulose making the control of the reaction selectivity very difficult. To overcome this issue, cellulose is generally subjected to a pre-treatment process prior to catalytic deconstruction. This pre-treatment aims at favoring a better accessibility of the cellulosic backbone to catalyst by reducing its crystallinity or particle size or degree of polymerization for instance. In this context, much effort has been recently devoted to the search of innovative media capable of dissolving and thus disrupting the supramolecular organization of cellulose. Dissolution of cellulose in a non-derivatizing solvent is an interesting approach that allows a change of the cellulose structure from a highly crystalline to a low crystalline structure, a key parameter in the subsequent catalytic hydrolysis of cellulose to glucose. After the dissolution process, cellulose is generally recovered, by precipitation, upon addition of an anti solvent such as ethanol or water. Historically, mixtures of DMSO/LiCl or DMA/LiCl (among other combinations) and more recently N-methylmorpholine-N-oxide (Lyocell process) have been used for the dissolution/decrystallization of cellulose [6]. Although these systems ensure a drastic decrease in the crystallinity index of cellulose, their recycling is difficult and rather expensive. Recently, ionic liquids (ILs) have received considerable attentions because of their ability to dissolve and thus to decrease the crystallinity index of cellulose (Scheme 3.2).[1]

Dissolution of cellulose in ILs has been firstly demonstrated at the beginning of the twentieth century in particular using ethyl ammonium nitrate [8, 9]. Nowadays, the use of ionic liquids for the dissolution/decrystallization of cellulose is now witnessing a sort of renaissance with the recent emergence of room temperature

ILs recycling

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Scheme 3.2 Decrystallization of the cellulosic fraction of lignocellulosic biomass by dissolution in ILs

ILs. To date, plenty of works have been recently reported in this field of chemistry and this topic is too large to be summarized here. Complementary information to this section can be found in excellent recent reviews.[2]

Analysis of the specialized literature reveals that two parameters mainly govern the dissolution of cellulose in ILs (1) the ability of ILs to disrupt the extensive hydrogen-bond network of cellulose and (2) hydrophobic interactions. In ILs, dissolution rate of cellulose closely depends on the temperature, time of heating and molecular weight of ILs.[3]

Anion of the ILs plays an important role in the dissolution process by inducing polar-interaction with the hydroxyl groups of cellulose thus weakening the hydro­gen bond network of cellulose. To date, chloride is one of the most efficient anion but its exact role is still subject to controversy in the current literature. Other anions such as acetate, formate or phosphate have been also proven to be effective. More generally, anion with a basic character seems to be more favorable for the disso­lution of cellulose. The cation composing the ILs plays also a major role in the dissolution process. Due to intra — and intermolecular hydrogen bonding, cellulose is composed of flat ribbons with sides that differ markedly in their polarity. Hence, amphiphilic cations are generally required in order to ensure an efficient dissolution of cellulose. The size of the cation is also a parameter that is taken into account in few literatures and an optimal size should be found in order to favor the diffusion of ILs within the cellulose microfibrils. In this context, the imidazolium moiety closely meets all these requirements.

Although elucidation of the exact mechanism governing the dissolution of cellulose in ILs is still not really clear, combination of an amphiphilic cation with a basic anion seems to be a good compromise. It is more or less accepted that the

cation has the role to slide open the cellulose fibrils and to transport the anion within the cellulose backbone where it interacts with the hydrogen bond network. To date, 1-butyl-3-methyl-imidazolium chloride and 1-ethyl-3-methyl-imidazolium acetate are considered as the best ILs for the dissolution of cellulose. Despite the remark­able ability of these ILs to dissolve cellulose, their industrial emergence is unfor­tunately hampered by their high cost, reactivity, toxicity and high viscosity. Additionally, RTILs are highly hygroscopic and presence of water, even in a trace amount, has a detrimental effect on the dissolution of cellulose. Hence, room temperature ILs are nowadays only regarded as excellent models to under­stand the mechanism governing the dissolution/decrystallization of cellulose.