Selective catalytic conversion of cellulose fractions

Nowadays, there is a tremendous interest in greener alternatives to the petroleum-based production of fuels, chemicals and materials in classical refineries. The valorization of lignocellulosic biomass in so-called biorefineries could be an interesting alternative due to the renewable nature of biomass. Lignocellulose is an abundant biomaterial that can be found in cell walls of numerous plants like trees, grasses and energy crops and mainly consists of cellulose, hemicellulose and lignin, which each yield specific value-added products after downstream processing. Due to its rigid and complex nature, lignocellulosic biomass is typically first pretreated prior to its valorization in order to improve the accessibility and reactivity of cellulose and to isolate the different biopolymers and their derivatives. Although up till now research efforts mainly focused on the valorization of (hemi)cellulose, there is a recent interest in the valorization of lignin as well to improve process economics.

This doctoral research was performed within the EU project BIOCORE, in which the industrial feasibility of a biorefinery concept was investigated through an international collaboration of various companies, universities and research centers. The first main objective was to convert cellulose pulp, originating from the pilot plant-scale organosolv fractionation of wheat straw and forestry residues, into useful polyols like sorbitol, sorbitan and isosorbide using chemocatalytic conversion processes. These polyols can subsequently serve as important platform molecules for e.g. the food, cosmetic, pharmaceutical and chemical industries. The second main objective was to elucidate the role of biomass pretreatment in the chemocatalytic valorisation of cellulose, as biomass pretreatment is classically performed prior to enzymatic digestion to the make cellulose more accessible to the enzymes.

First, the role of pretreatment in the Ru/H-USY zeolite-catalyzed hydrolytic hydrogenation of cellulose to sorbitol, mannitol and sorbitan was investigated. The influence of different physical and chemical pretreatments was first investigated in the conversion of pure cellulosic substrates to identify the relative importance of key parameters determining cellulose reactivity, like degree of polymerization, crystallinity and particle size. The degree of polymerization appeared to be the most rate-determining parameter for a fast and selective conversion to hexitols, with crystallinity and particle size being more or less equally important for the conversion of microcrystalline cellulose substrates with a comparable low degree of polymerization. Next, the influence of organosolv pretreatment on the zeolite-catalyzed conversion of wheat straw and forestry residues was investigated. A thorough organosolv pretreatment and delignification appeared crucial to achieve acceptable hexitol yields from these substrates, although a complete purification of the cellulose component is not necessary to achieve hexitol yields comparable to pure cellulosic substrates. In this way, hexitol yields as high as 40% could be obtained.

Secondly, the role of organosolv pretreatment in the H4SiW12O40-Ru/C-catalyzed hydrolytic hydrogenation of cellulose to isosorbide was investigated. The bifunctional catalytic process was first optimized for microcrystalline cellulose and subsequently tested on wheat straw and forestry residues. Isosorbide yields as high as 50% were achieved from microcrystalline cellulose but where close to zero from the raw biomass substrates. However, isosorbide yields could be increased up to 60% after a thorough organosolv pretreatment and delignification, again emphasizing the important role of pretreatment in chemocatalytic biomass valorization processes.

However, upon recycling of the heterogeneous Ru/C catalyst, isosorbide yields dropped below 10% in subsequent runs. Subsequent research revealed a change in Ru/C hydrogenation activity due to the adsorption of H4SiW12O40 anions. After adsorption of heteropoly anions on Ru/C in hydrothermal conditions, sugar hydrogenation is substantially impeded, forcing formed sugars through a furan-based hydrodeoxygenation pathway which ultimately leads to the formation of liquid straight chain alkanes like n-hexane and n-pentane. Bio-derived n-hexane can be used as a technical solvent, building block for chemicals and, if sufficiently renewable and sustainable, as a precursor for greener transportation fuels. Further optimization of the adsorption conditions and reaction conditions enabled remarkable yields as 80% of n-decane soluble products, of which 50% n-hexane. This unexpected, yet innovative outcome of the doctoral research has attracted worldwide acclaim and resulted in a patent application.