Solid Catalysts for the Upgrading of Renewable Sources

Abstract

The use of renewable resources as raw materials for the chemical industry is mandatory in the transition roadmap toward the Bioeconomy. However, this is a challenge for the setup of catalytic processes based on heterogeneous catalysts. First of all, when using biorenewables (particularly sugars) as starting materials, the process has to be designed in the condensed phase, as these kinds of molecules have little-to-no volatility and water is the solvent of choice with most bio-based systems. Moreover, many reactions designed to produce chemicals will also create water. This is the case for both etherification and esterification, which are widely used to produce fuel components and additives. For these reasons, the hydrothermal stability of the catalyst is one of the main problems when dealing with renewables. Another issue is due to the highly oxygenated nature of plant-derived raw materials and platform molecules. This makes oxygen removal reactions such as dehydration, hydrogenolysis, hydrogenation, decarbonylation, or decarboxylation almost ubiquitous in biomass valorization pathways. Therefore, there is a need for robust hydrogenation or hydrogen transfer catalysts and also water-resistant acidic catalysts, and possibly for bifunctional materials where different active sites are present. These challenges will be adressed in this special issue of Catalysts through several examples. A review article focused on the state of the art in the liquid phase depolymerization of lignin via catalytic transfer hydrogenolysis/hydrogenation reactions will open this interesting and current collection of papers [1]. Lignin is one of main structural components of lignocellulosic materials, and is widely available as a by-product in the pulp and paper industry and in the process of second generation bioethanol production. It could be a source of very valuable aromatic compounds if an effective method of depolymerization was available. It should be remembered that a shortage of aromatics, which are among the main building blocks in the chemical industry, is expected due to the shift from conventional fossil fuels to shale oil. This makes alternative routes to aromatics of particular interest. The review will also discuss the effect of lignin origin, as it is known that there are significant differences between hardwood, softwood, and straw lignins. The hydrogenolysis of dimethyl adipate to 1,6-hexandiol and the hydrogenolysis of xylitol in water to ethylene glycol, propylene glycol, and glycerol are the subjects of two other papers [2,3]. In particular, the hydrothermal stability of the catalyst used in the latter reaction was studied and improved by decreasing the amounts of aggressive by-products. Transfer hydrogenation is also one of the steps involved in the one-pot conversion of ethyl levulinate into gamma-valerolactone (GVL) [4]. In this reaction, a solid catalyst with both acidic and basic sites showing high thermal and chemical stability was successfully used. GVL is one of the most promising platform molecules we can obtain from biomass, as it can be upgraded to various chemicals and fuels, such as polymers, fuel additives, and jet fuel. A second review deals with a class of hybrid materials that can act as bifunctional catalysts in biomass conversion due to their particular structure, namely Metal Organic Framework (MOF) [5]. The structures of MOF show coordinatively unsaturated (open) sites, with Lewis acidity in inorganic