Advances in Understanding the Selective Hydrogenolysis of Biomass Derivatives

Abstract

Biomass has attracted great attention as an alternative to petrochemicals for the production of valuable chemicals and fuels. Hydrogenolysis reactions over heterogeneous catalysts offer pathways to upgrade renewable compounds to a variety of building block molecules; however, similarities among the functional groups in these reactants frequently limit the selectivity of these reactions. Consequently, a fundamental understanding of how oxygenates adsorb, activate, and react upon solid surfaces is essential for catalyst design and reaction engineering. In this Perspective, we describe significant advances in knowledge of the reaction networks and mechanisms, key reactive intermediates, and active site motifs that allow for the selective production of desired compounds by hydrogenolysis over heterogeneous catalysts. These insights draw from the mechanistic understanding gained from comparisons between kinetic measurements, in situ characterization of catalysts and organic intermediates, and ab initio calculations. Drawing from detailed insight taken from a century of study into reactions of hydrogen with hydrocarbons, we discuss the molecular similarities between reactions that cleave C−O and C−C bonds during hydrogenolysis of biomass-derived compounds that range from simple (e.g., alcohols) to polyfunctional molecules (e.g., guaiacol) on monometallic, bimetallic, and nonmetallic catalysts. Finally, we describe current challenges and opportunities for future research into hydrogenolysis reactions.

Pathways for Reactions of Esters with H2 over Supported Pd Catalysts: Elementary Steps, Site Requirements, and Particle Size Effects

“…In addition, the minimal change in E A,app for hydrogenolysis, along with the modest decrease in hydrogenolysis rates with increasing K atom coverage, implies that a distinct set of active sites catalyze hydrogenolysis. Based upon prior work on hydrogenolysis of alkanes − and oxygenates, ,,, the C acyl –O bond likely cleaves upon metallic Pd sites. Lastly, Lewis acid sites not affected by K atom addition must facilitate the transesterification process.…”

“…More recently, processes and catalysts that mediate the direct reduction of esters to ethers via cleavage of the carbonyl bond have been reported. − Previous attempts to directly reduce esters to ethers include noncatalytic reactions such as hydrosilylation initiated by γ-ray or ultraviolet irradiation, thiocarbonylation followed by hydrogenation, and reduction using a combination of mineral acids with potent reducing agents such as sodium borohydride or silanes. − Experimental studies demonstrate the catalytic reduction of esters to ethers using homogeneous transition-metal-catalyzed reduction with silanes − or H 2 . ,,, These routes involve the use of stoichiometric reductants, strongly acidic reagents, homogeneous catalysts that are hard to recycle, and costly and toxic manufacturing operations. Processes that use heterogeneous catalysts together with H 2 as a reductant would be more economically feasible and less environmentally impactful . In addition, such chemistry would provide a pathway to produce nonsymmetric ethers from esters derived by esterification of carboxylic acids and alcohols from biomass. − Hence, ethers with desirable properties as solvents and surfactants may be created by reactions between bioderived esters and H 2 .…”

Ring-opening of furfuryl alcohol to pentanediol with extremely high selectivity over Cu/MFI catalysts with balanced Cu0–Cu+ and Brønsted acid sites

“…It proves that Cu 0 is used for hydrogen activation and the hydrogenation of the intermediate product. 48 The experiments validate that the conversion of FFA to PDO proceeds via the ring-opening of FFA (not that of THFA). Furthermore, the presence of unsaturated CC bonds on the furan ring could reduce the energy barrier of the ringopening process.…”

“…It proves that Cu 0 is used for hydrogen activation and the hydrogenation of the intermediate product. 48…”

“…Furfuryl alcohol was primarily adsorbed on the Cu + sites through the oxygen of the -CH 2 OH units to generate a stable alkoxide species. 47,48 The hydrogen protons (H + ) generated from the B acid sites attacked the C2 and C5 atoms, resulting in the protonation of the C2 and C5 sites. The corresponding carbocation intermediates were formed (a and c).…”