The importance of Brønsted acid sites on C O bond rupture selectivities during hydrogenation and hydrogenolysis of esters

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

“…, in the complete absence of Pd) primarily form acetic acid, propanol, and C 3 hydrocarbons by ester hydrolysis and alcohol dehydration (Figure S11b). Hydrolysis rates normalized by grams of the catalyst are similar on Nb 2 O 5 and Pd-Nb 2 O 5 , indicating that the Nb 2 O 5 support is responsible for hydrolysis (Figure S11b), which agrees with observations for reactions of esters with H 2 on Pd-WO 3 . C 5 H 10 O 2 consumption by hydrogenolysis is ∼50 times lower on Nb 2 O 5 -SEA than on samples with Pd (Figure S11b).…”

“…Mizugaki et al reported recently that solid Pt–Mo–ZrO 2 catalysts directly form ethers with high yields (60–90%) from reactions between H 2 and a broad spectrum of esters during batch reactions but did not examine the mechanism or site requirements closely . Contemporary work from our group demonstrated that secondary bimolecular dehydration is the dominant route for ether formation over Re-γ-Al 2 O 3 and that Pd nanoparticles deposited on metal oxides containing Brønsted acid sites, like WO 3 and Nb 2 O 5 , catalyze the direct reduction of esters to ethers using H 2 . , Yun et al showed that ratios of the rate for ether formation to hydrogenolysis increased with a nearly exponential dependence on the functional strength of Brønsted acid sites present on oxide supports, as assessed by measured values for the heat of adsorption for 2,6-di-tertbutyl pyridine . These reactions proceed in a catalytic cycle that involves both metal and acid active sites and diffusive communication between these functions, similar to the proposal of Beller et al…”

“…All of these routes have limitations including: (1) the use of strongly acidic or basic conditions that can lead to competing elimination reactions to give olefins; (2) limited options of biosourced raw materials due to lack of reactivity with the above reactions that limit the structural variety of products; (3) with alcohol dehydration only symmetric ethers are feasible; and (4) the use of toxic raw materials and the generation of waste streams from the manufacturing 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 .…”

“…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 .…”

“…Reactions of esters with H 2 over heterogeneous catalysts ( e.g. , Cu, − Pt, Pd, ,− Ru, and Co) typically produce alcohols by ester hydrogenolysis ( i.e. , C acyl –O cleavage).…”