Biomass Promises: A Bumpy Road to a Renewable Economy

Biomass conversion strategies have been taking hold of the scientific community agenda in the evolution of renewable chemical and energy industries. Nonetheless, biomass conversion yet presents challenges due to its structural complexity, and high reactivity of the oxygen functionalities. This review aims to indicate the reader the main encountered difficulties in the conversion of plant-derived feedstocks and byproducts/waste, with a focus on the catalytic approaches taken so far and the efforts of our research group into contributing to a future bio-based economy. Introduction Since the industrial revolution of the 19 century, petroleum has been the major source of commodity chemicals and energy. The massive exploitation of this traditionally called burning water [1] not only has contributed to polluting each compartment of our planet (i.e. air, water, earth) [2-4], but also increased the occurrence of earthquakes in drilling areas [56]. Furthermore, world energy consumption is constantly increasing, while fossil resources are irreversibly being depleted, compelling the implementation of renewable sources. After the recent break of record of CO2 levels in the atmosphere [7], a closed carbon cycle and circular economy (i.e. take, make, recycle) are highly desirable for both materials and fuels purposes. Biomass transformation has taken a hold of the scientific community, as well as the different nations’ energy agenda (e.g. the European environmental research and innovation policy), thanks to its capacity to recycle CO2 during photosynthesis, and production of new bio-feedstocks (Figure 1) [8,9]. Furthermore, thanks to the existence of fast and non-edible growing plants, as well as the development of high-output agricultural technologies, a carbon-neutral cycle can be achieved in short periods of time [10]. A bio-based economy, in particular, becomes advantageous compared to other renewable energies (e.g. wind and solar) thanks to the limited seasonal/day fluctuations, and the possibility of using renewable sources and/or waste as feedstock in the chemical and materials industries. Figure 1. Closed carbon cycle for a bio-based economy. Biomass is a broader concept that includes various plant-based sources, but not solely. In detail, biomass comprises lignocelluloses, oilseed/sugar/starch crops, acquatic coltures (i.e. algae), and biowastes, such as agricultural/animal/anthropological wastes. Lignocelluloses in particular are not only the most abundant biomass, but also an optimal source of several of compounds: terpenes, carbohydrates, aromatics, and fatty esters. In fact, if possible to isolate and efficiently convert each plant component, every chemical market could be self-sustained with biomass. The overall structure of lignocellulosic biomass mainly comprises cellulose (35-50%), hemicellulose (2035%), and lignin (15-30%). Cellulose is the source of the plant tensile strength, being a crystalline and linear glucose polymer, thus being an optimal source of this hexose (sugar/carbohydrate). Hemicellulose further strengthens the plant structure by cross-linking with cellulose. Differently from the latter, hemicellulose is a branched random polymers combining a variety of pentose carbohydrates, along with hexoses and uronic acids sugar monomers, becoming an ideal source for sugars such as xylose, mannose, arabinose, galactose, to name a few. Lignin is a rather complex and recalcitrant polyaromatic macromolecule which confers the structural rigidity to the plant wall. Its irregular polymeric structure is comprised of phenylpropane type units, thus being a possible source of a plethora of aromatic molecules, most known being vanillin, eugenol, guaiacol, styrenes, and xylenes. Both fermentation processes and chemical transformations of lignocellulosic biomass can yield to valuable products which can substitute petroleum-platform molecules. In particular, microbial fermentation of cellulosic sugars (i.e. glucose) yields to biofuels (e.g. ethanol) or organic acids such as lactic and succinic, while acid-catalyzed dehydration of hemicelluloses-derived pentoses (i.e. xylose) or cellulose-derived hexoses (i.e. glucose, fructose, and mannose) yields to platform furanics, namely furfural and 5-hydroxymethylfurfural (HMF), respectively. From the hydrolysis of HMF is then obtained the linear and important platform chemical, levulinic acid, which in alcohol media becomes methyl levulinate [11] (Figure 2). Figure 2. Examples of products from the chemical and microbial transformations of biomass. The major issues related to the chemical transformation of non-edible biomass (i.e. lignocelluloses) feedstocks lies in its rather complex chemical structure (in particular, lignin) and the large presence of heteroatoms (in particular, oxygen), whose reactivity leads to low atom efficiency and undesired sideproducts, e.g. humins. In particular, humin by-products derive from the thermodynamically-favored random polymerization of the starting molecules (sugars) and products (furanics, levulinics), causing reactor fouling. Less humins can be formed in the presence of co-solvents such as toluene, GVL, and methyl-THF, although complete avoidance of humins is yet inevitable [12,13]. The use of larger volumes of solvents would increase the overall cost of the process, both in terms of reactor size and subsequent solvents/product separation (e.g. distillation), thus becoming uncompetitive with the current petroleum-based bulk and fine chemicals market. Thus, upgrading these side-products becomes crucial in order to achieve an economical bio-based market able to substitute traditional fossil feedstocks. The obstacles in biomass processing are particular relevant when traditional petrol-derived platform chemicals (i.e. aliphatics, olefins, aromatics) are the target product. A shift in the way we perceive platform molecules is indeed due. In order to influence this shift, a list of the top biomass-derived compounds has been first reported by the US Department of Energy in 2004 [14], later revisited by Bozell and Petersen [15]. The identified (group of) 10 molecules (Table 1) can be industrially produced with the current existing industries (drop-in technology), and represent promising building blocks for a variety of fine chemicals, specialty materials, and biofuels, to name a few. Table 1. The 10 promising biomass products identified by Bozell and Petersen [15]. Identified biobased platform chemicals Ethanol Succinic acid Furans (Furfural, HMF, FDCA) Hydroxypropionic acid/aldehyde Glycerol Levulinic acid Biohydrocarbons (Isoprene) Sorbitol Lactic acid Xylitol Up to date, mass production of some bio-compounds has already been achieved, as in the Biofine Process which produces levulinic acid, furfural, and formic acid [16]. Advances in the YXY® process developed by Avantium, now part of the joint venture with chemical company BASF, Synvina, aims to the production of an exclusively bio-derived plastic, PEF, based on 2,5-furandicarboxylicacid (FDCA). This plastic possesses superior mechanical and chemical properties, making it a great candidate in substituting petroleum-derived PET [17, 18]. However, both processes produce recalcitrant/tarry compounds (i.e. humin by-products) prone to reactor fouling. Furthermore, traditional petroleum catalysts might not be active in the processing conditions (i.e. aqueous media, lower operating temperature), thus calling for new catalytic materials. A bio-based market has the potential to be highly beneficial in environmental, economical, and social terms, although still presenting some adversities. This publication aims to be a compendium of the efforts of the NanoVal research group of Prof. Luque into solving the current challenges of biomass processing and upgrading. Other selected literature examples in biomass conversion technologies are also included for the completion of discussion. 1. Catalytic Materials for Biomass Conversion The limited solubility of monomeric bio-sugars (which often act as reactants/products) in organic solvents, as well as the predominance of decomposition/polymerization reactions at rather high temperatures, requires the use of water-stable catalytic materials possessing high activity at low operating conditions. In this regard, zeolites have been extensively investigated in the conversion of biobased feedstocks. The tunable acidity and shape selectivity of these tetrahedral oxides make these materials an attractive catalyst for biomass conversion. Several examples can be found in literature, such as in the aqueous/alcohol phase transformation of 1,3-dihydroxiacetone (DHA) to lactic acid/alkyl lactates [19], or in the conversion of bioderived sugars (e.g. cellulose, cellubiose, glucose, xylose) conversion to methyl levulinate[20,21] and furfural [22-26]. For more detailed information, the reader is kindly referred to recent excellent reviews on the use of zeolites in biomass processing [27,28]. In general, the state of the art on the matter evidences that zeolites traditionally employed in the petrochemical industry are not as effective in the conversion of biomass. In fact, contrary to fossil-feedstocks, lignocellulosic compounds often are bulky molecules presenting zeolite-poisoning elements such as Na, whose intermediates are highly oxygenated unstable molecules often prone to decomposition, polymerization to humins, or coke formation due to the presence of acidic sites on the zeolites themselves. In fact, a higher Si/Al ratio (>15) compared to typical oil cracking zeolites (Si/Al ratio of ca. 6[29]) has shown better activity in biomass cracking, thanks to the right balance of Brønsted and Lewis Acid sites. Furthermore, zeolites micropores limit the diffusion of the biomass bulky structures, thus decreasing the effective surface area. One approach advanced by Lima et al. [30] was to swell and ultrasonicate layered aluminosilicates to achieve single crystalline sheets, obtainin