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.

Experimental measurements of the conversion of m-cresol over Pt and Ru/SiO 2 catalysts show very different product distributions, even when the reaction is conducted at similarly low conversions and the same operating conditions (300 °C, 1 atm). That is, although ring hydrogenation to 3methylcyclohexanone is dominant over Pt, deoxygenation to toluene and C−C cleavage to C 1 −C 5 hydrocarbons prevail over Ru. For understanding the differences in reaction mechanisms responsible for this contrasting behavior, the conversion of mcresol over the Pt(111) and Ru(0001) surfaces has been analyzed using density functional theory (DFT) methods. The DFT results show that the direct dehydroxylation of m-cresol is unfavorable over the Pt(111) surface with an energy barrier of 242 kJ/mol. In turn, the calculations suggest that the reaction could proceed through a keto tautomer intermediate, which undergoes hydrogenation of the carbonyl group followed by dehydration to form toluene and water. At the same time, a low energy barrier for the ring hydrogenation path toward 3-methylcyclohexanone compared to the energy barrier for the deoxygenation path toward toluene over the Pt(111) surface is in agreement with the experimental observations, which show that 3methylcyclohexanone is the dominant product over Pt/SiO 2 at low conversions. By contrast, the direct dehydroxylation of mcresol becomes more favorable than the tautomerization route over the more oxophilic Ru(0001) surface. In this case, the deoxygenation path exhibits an energy barrier lower than that for the ring hydrogenation, which is also in agreement with experimental results that show higher selectivity to the deoxygenation product toluene. Finally, it is proposed that a partially unsaturated hydrocarbon surface species C 7 H 7 * is formed during the direct dehydroxylation of m-cresol over Ru(0001), becoming the crucial intermediate for the C−C bond breaking products C 1 −C 5 hydrocarbons, which are observed experimentally over the Ru/SiO 2 catalyst.

A combined experimental and theoretical comparative study of the hydrodeoxygenation (HDO) of anisole was conducted over Pt, Ru, and Fe metals. In the experimental part, an inert silica support was used to directly compare the catalytic activity and selectivity of the three metals at 375 ºC under H 2 flow at atmospheric pressure. In parallel, for density functional theory (DFT) calculations the close-packed Pt(111), Ru(0001), and Fe(110) surfaces were employed to compare the possible mechanisms on these metals. It was observed that over Pt/SiO 2 and Ru/SiO 2 catalysts, both phenol and benzene were the major products in a phenol/benzene ratio that decreased with the level of conversion. By contrast, over the Fe/SiO 2 catalyst, no phenol formation was detected, even at low conversions. The DFT results show that over all the three metal surfaces the dehydrogenation at the-CH 3 side group occurs before the CO bond breaking. This removal of H atoms from the-CH 3 group facilitates the activation of the aliphatic C alkyl-O bond. Therefore, it can be concluded that a common intermediate for the three metals is a surface phenoxy and the significant differences between the three metals is related to the reactivity of this surface phenoxy. That is, over Pt(111) and Ru(0001) the phenoxy intermediate is hydrogenated to phenol, which in turn, can undergo further HDO to form benzene. This result is in agreement with the experiments over Pt/SiO 2 and Ru/SiO 2 catalysts. Over these catalysts, both phenol and benzene are major products, with the selectivity to benzene increasing with conversion at the expense of phenol. In contrast, over the Fe(110) surface, the strong metal oxophilicity makes the direct cleavage of the CO bond in the surface phenoxy easier than

A new type of catalyst has been designed to adjust the basicity and level of molecular confinement of KNaX faujasites by controlled incorporation of Mg through ion exchange and precipitation of extraframework MgO clusters at varying loadings. The catalytic performance of these catalysts was compared in the conversion of C2 and C4 aldehydes to value-added products. The product distribution depends on both the level of acetaldehyde conversion and the fraction of magnesium as extraframework species. These species form rather uniform and highly dispersed nanostructures that resemble nanopetals. Specifically, the sample containing Mg only in the form of exchangeable Mg(2+) ions has much lower activity than those in which a significant fraction of Mg exists as extraframework MgO. Both the (C6+C8)/C4 and C8/C6 ratios increase with additional extraframework Mg at high acetaldehyde conversion levels. These differences in product distribution can be attributed to 1) higher basicity density on the samples with extraframework species, and 2) enhanced confinement inside the zeolite cages in the presence of these species. Additionally, the formation of linear or aromatic C8 aldehyde compounds depends on the position on the crotonaldehyde molecule from which abstraction of a proton occurs. In addition, catalysts with different confinement effects result in different C8 products.

The conversion of furfural has been investigated in vapor and liquidphases over a series of supported monometallic Pd and bimetallic Pd-Fe catalysts. Over the monometallicPd/SiO 2 catalyst, the decarbonylation reaction dominates, yieldingfuran as the main product. By contrast, over the bimetallic Pd-Fe/SiO 2 catalyst a high yield of 2-methylfuran is obtained with much lower yield to furan. Interestingly, changing the catalyst support affects the product distribution. For instance, using-Al 2 O 3 instead of SiO 2 as support of the bimetallic catalyst changed the dominant product from 2-methylfuran to furan. That is, Pd-Fe/-Al 2 O 3 behaves more like monometallic Pd/SiO 2 than bimetallic Pd-Fe/SiO 2. A detailed characterization of the catalysts via XPS, XRD, and TEM indicatedthat a Pd-Fe alloy is formed on the SiO 2 support but not on the-Al 2 O 3 support.Theoreticaldensity functional theory calculations suggest that on the Pd-Fe alloy binding of the furan ring to the surface is weakened compared to on pure Pd. This weakening disfavorsthe ring hydrogenation and decarbonylation paths, while the oxophilic nature of Fe atoms enhances the interaction of the C=O and theOH groups with the metal surface, whichfavors the C=O hydrogenation and CO bond cleavage paths. The presence of the solvent has a less pronounced effect, but clearly has a stronger inhibition on CC bond cleavage (decarbonylation to furan) than on CO bond cleavage (hydrogenolysis to methyl furan). *FOL = furfuryl alcohol, THF= tetrahydrofuran, MF= 2-methylfuran, THFOL= tetrahydrofurfuryl alcohol, MTHF= 2-methyltetrahydrofuran, CPON =cyclopentanone.

A novel linear-hyperbranched multiblock polyethylene was prepared from ethylene monomer alone via chain walking and chain shuttling polymerization in the presence of chain transfer agent (ZnEt 2 ). In the binary catalyst system, R-diimine nickel(II) bromide complex (CatA) produced chain segments with hyperbranched architecture via chain walking, and ansa-ethylenebis(1-η 5 -indenyl)zirconium dichloride (CatB) yielded linear chain segments, while ZnEt 2 facilitated chain exchanges between two active metal centers. The resultant multiblock polyethylene featured a narrow weight distribution and useful combining properties of linear and branched polyethylene. Further, polymers were expediently prepared in an effective and economical polymerization process from ethylene monomer alone.

No abstract

Aldol condensation is a key CC coupling reaction for upgrading of biomass-derived oxygenates to fuels and chemicals. Here, we investigate the effects of added water on the condensation of cyclopentanone (CPO) on hydrophobized MgO catalysts. We have found that the role of water strongly depends on the degree of hydrophobic functionalization of the MgO surface. That is, on a nonfunctionalized (hydrophilic) high-surface-area MgO catalyst, the rate decreases with the addition of water, mostly due to active site blockage. By contrast, on MgO hydrophobized via silylation with octadecyltrichlorosilane (OTS), the rate actually increases with added water. A concomitant change in kinetics is observed from the pristine (hydrophilic) MgO to the hydrophobized sample. Specifically, on the hydrophilic sample, the reaction is first-order, as expected if the rate-limiting step is the formation of an enolate intermediate via α-H abstraction at a basic site, as widely reported in previous literature. By contrast, on the hydrophobized sample, the reaction becomes second-order, indicating a shift in ratelimiting step to the bimolecular CC coupling. On pristine MgO, acid−base pairs are fully available on the surface, with the acid site polarizing the CO group in the second cyclopentanone (electrophile) and making the attack by the enolate very favorable. It is proposed here that grafted OTS molecules interfere between active sites, making the adsorbate−adsorbate interaction on the surface less likely and reducing the rate of CC coupling. Both isotope effect experiments and ab initio molecular dynamics simulations of cyclopentanone adsorption at the MgO/OTS interface further support this argument. Therefore, the promotional role of water seems to be the assistance of the CC bond-forming step. It is proposed that, at low concentrations, water can help the second molecule (electrophile) be polarized from a remote Mg 2+ site through a chain of Hbonded molecules.