Selective Hydrogenolysis of Polyols and Cyclic Ethers over Bifunctional Surface Sites on Rhodium–Rhenium Catalysts

Abstract

A ReO(x)-promoted Rh/C catalyst is shown to be selective in the hydrogenolysis of secondary C-O bonds for a broad range of cyclic ethers and polyols, these being important classes of compounds in biomass-derived feedstocks. Experimentally observed reactivity trends, NH(3) temperature-programmed desorption (TPD) profiles, and results from theoretical calculations based on density functional theory (DFT) are consistent with the hypothesis of a bifunctional catalyst that facilitates selective hydrogenolysis of C-O bonds by acid-catalyzed ring-opening and dehydration reactions coupled with metal-catalyzed hydrogenation. The presence of surface acid sites on 4 wt % Rh-ReO(x)/C (1:0.5) was confirmed by NH(3) TPD, and the estimated acid site density and standard enthalpy of NH(3) adsorption were 40 μmol g(-1) and -100 kJ mol(-1), respectively. Results from DFT calculations suggest that hydroxyl groups on rhenium atoms associated with rhodium are acidic, due to the strong binding of oxygen atoms by rhenium, and these groups are likely responsible for proton donation leading to the formation of carbenium ion transition states. Accordingly, the observed reactivity trends are consistent with the stabilization of resulting carbenium ion structures that form upon ring-opening or dehydration. The presence of hydroxyl groups that reside α to carbon in the C-O bond undergoing scission can form oxocarbenium ion intermediates that significantly stabilize the resulting transition states. The mechanistic insights from this work may be extended to provide a general description of a new class of bifunctional heterogeneous catalysts, based on the combination of a highly reducible metal with an oxophilic metal, for the selective C-O hydrogenolysis of biomass-derived feedstocks.

From 5-Hydroxymethylfurfural (HMF) to Polymer Precursors: Catalyst Screening Studies on the Conversion of 1,2,6-hexanetriol to 1,6-hexanediol

“…Similar observations were reported by Dumesic and co-workers [44] for the conversion of 5 using Rh-ReO x catalysts on carbon in the presence of 0.1 M NaOH (120°C, 34 bar H 2 , 4 h).…”

“…Surprisingly, the Rh-ReO x on carbon catalyst is inactive, an observation not in line with literature data [44,45]. Tomishige [45] reported 36 % conversion with 97 % selectivity to 1 (100°C, 80 bar, 24 h) whilst Dumesic [44] obtained 27 % conversion with 97 % selectivity to 1 (120°C, 34 bar, 4 h) using carbon supported catalysts. A possible explanation for these differences in catalytic performance is the use of a different catalyst preparation protocol.…”

“…There are also reports [46][47][48][49][50][51][52] on the use of homogeneous ruthenium catalysts in combination with triflic acid (HOTf) and sulfolane as the solvent for the selective hydrodeoxygenation of a secondary alcohol in the presence of a primary alcohol. For example, Schlaf et al [51] reported the production of 1-propanol from 6 using [{Cp*Ru(CO) 2 While preparing this manuscript, Dumesic et al [44] reported hydrodeoxygenation reactions of various diols and triols using bimetallic Rh-ReO x catalysts on carbon supports. When using 2 as the substrate, 1 was obtained in 99.9 % at 8 % conversion (120°C, 34 bar, 4 h).…”

Mechanistic Insights into Ring-Opening and Decarboxylation of 2-Pyrones in Liquid Water and Tetrahydrofuran

“…The structural relaxations were performed until the forces on each atom were below 0.05 eV/Å. 37 Converging the aqueous phase structures to higher tolerances was found to be considerably more difficult as a result of low energy modes in the water network. We were successful in converging some of the structures to within 0.025 eV/Å and found that the resulting reaction and activation energies were less than 2 kJ/mol than those from the 0.05 eV/Å convergence simulations.…”

Synthesis of Tetrahydropyran from Tetrahydrofurfuryl Alcohol over Cu–Zno/Al2O3 under a Gaseous-Phase Condition

“…There were intense studies carried out on the production of high value-added chemical intermediates and end products from HMF using heterogeneous catalysts [8][9][10][11]. Tetrahyrofurfuryl alcohol (THFA) can be obtained through hydrogenation of furfural at a very high yield [12], and from which various useful chemicals, such as 1,5-pentadiol (15PDO) [13][14][15][16][17][18][19][20][21][22][23][24][25], 3,4-2H-dihydropyran (DHP) [26][27][28], and 4-penten-1-ol can be produced [29,30]. 15PDO, via a ring closure pathway, can form tetrahydropyran (THP) through the participation of strong solid acid catalysts or high-temperature water [31].…”