Research interest in biomass conversion to fuels and chemicals has increased significantly in the last decade as the necessity for a renewable source of carbon has become more evident. Accordingly, many different reactions and processes to convert biomass into high-value products and fuels have been proposed in the literature. Special attention has been given to the conversion of lignocellulosic biomass, which does not compete with food sources and is widely available as a low cost feedstock. In this review, we start with a brief introduction on lignocellulose and the different chemical structures of its components: cellulose, hemicellulose, and lignin. These three components allow for the production of different chemicals after fractionation. After a brief overview of the main reactions involved in biomass conversion, we focus on those where bimetallic catalysts are playing an important role. Although the reactions are similar for cellulose and hemicellulose, which contain C6 and C5 sugars, respectively, different products are obtained, and therefore, they have been reviewed separately. The third major fraction of lignocellulose that we address is lignin, which has significant challenges to overcome, as its structure makes catalytic processing more challenging. Bimetallic catalysts offer the possibility of enabling lignocellulosic processing to become a larger part of the biofuels and renewable chemical industry. This review summarizes recent results published in the literature for biomass upgrading reactions using bimetallic catalysts.

Bimetallic catalysts for upgrading of biomass to fuels and chemicals

The synthesis of a large number of fine chemicals, particularly in the field of flavor and fragrance chemistry [1,2] and pharmaceuticals [3], involves the selective hydrogenation of unsaturated carbonyl intermediates as a critical step. The hydrogenation of α,β-unsaturated carbonyls into saturated carbonyls is comparatively easy to achieve because thermodynamics favor the hydro-genation of the C═C bonds; therefore, research efforts were more directed at improving the selectivity to unsaturated alcohols. When a substituent is present on the carbon atom of the carbonyl group (i.e. with ketones), there is no chance to hydrogenate selectively the C═O bond, and saturated ketones are obtained with a high yield. This review is thus mostly restricted to the hydrogenation of α, β-unsaturated aldehydes into the corresponding unsaturated alcohols.

Selective Hydrogenation of α,β-Unsaturated Aldehydes

An overview of dehydration, aldol-condensation and hydrogenation processes for production of liquid alkanes from biomass-derived carbohydrates

Advances in developing new heterogeneous catalysts which hydrogenate preferentially the C=O group of ά,β-unsaturated aldehydes to obtain the desired products, namely unsaturated alcohols, will be reviewed by considering the parameters which control the intramolecular selectivity: support material, nature and particle size of the individual metals, selective poisoning, presence of a second metal, preparation method or catalyst precursor, temperature of reduction, host-guest interactions in zeolites, substituents on the C=C bond and pressure effects. Recent attempts with bimetallic and silver catalysts to hydrogenate only the C=O group of other polyfunctional organic compounds in the gas phase without simultaneous hydrogenation of aromatic or furan rings will be shown.

Selective hydrogenation of ά,β-unsaturated aldehydes and other C=O and C=C bonds containing compounds

Promoters of metal catalysts play an essential role in the hydrogenation of unsaturated bonds, such as in carbonylic compounds. This promotion is a key to a high selectivity to unsaturated alcohols in hydrogenation of α,β-unsaturated aldehydes. Analysis of the information available in the literature on the adsorptions through the C—C and C—O groups, respectively, together with the information on the catalytic behaviour of the promoter-free and promoted catalysts, allows us to suggest a model of surfaces active in selective hydrogenation of unsaturated aldehydes.

On the role of promoters in hydrogenations on metals; α,β-unsaturated aldehydes and ketones

Activity and Selectivity in the Reactions of Substituted α,β-Unsaturated Aldehydes

A Study on the Selectivity in Acrolein Hydrogenation on Platinum Catalysts: A Model for Hydrogenation of α,β-Unsaturated Aldehydes

Hydrogenation of acrolein, as a model molecule representing the α,β-unsaturated aldehydes, has been studied with SiO2- and Al2O3-supported Pt and Cu catalysts. These catalysts were used to study the effects of various surface modifications. In addition to sulfurization by thiophene, the Pt/SiO2 catalysts were modified by acetylene, by inactive metal (Sn, Cu) admixture, and by gas modifiers (CO). Furthermore, the effect of modification of the surface by the reaction mixture in the initial stages of the reaction was followed. It appeared that the selectivity in the hydrogenation of the α,β-unsaturated aldehydes is influenced mainly by the presence of sites on the catalysts, which activate the aldehydic oxygen and allow an easy supply of hydrogen atoms. Effect of the ensemble size is much less important. Sulfurization as performed in this paper causes only a marginal effect on selectivity. Small selectivity effects can be also caused by gas modifiers like carbon monoxide.

T.B.L.W. Marinelli

Papers

Activity and Selectivity in the Reactions of Substituted α,β-Unsaturated Aldehydes

A Study on the Selectivity in Acrolein Hydrogenation on Platinum Catalysts: A Model for Hydrogenation of α,β-Unsaturated Aldehydes

A Study on the Selectivity in Acrolein Hydrogenation on Platinum Catalysts: A Model for Hydrogenation of α,β-Unsaturated Aldehydes.