Search

Butanal conversion has been studied over Zr‐BEA zeolites with different amount of partially hydrolyzed open Zr‐sites and fully condensed closed sites. The kinetic study pointed that the main reaction pathway of butanal conversion involves its self‐condensation into 2‐ethyl‐2‐hexenal (2‐EN), the selectivity to 2‐EN being within 80–85 %. Besides, small amounts of 1‐butanol and butanoic acid are formed via Cannizzaro disproportionation of butanal. Secondary reactions include butanoic acid ketonization to 4‐heptanone and esterification to butyl butanoate and tandem hydrogenation/dehydration of 2‐EN into octadienes. A number of techniques, including FTIR spectroscopy of adsorbed CO, pyridine, 2,6‐di‐tert‐butylpyridine (DTBPy), TPD of isopropylamine and selective poisoning with DTBPy were applied for characterization of Zr‐sites and elucidation of their activity. Open Zr‐sites were found to be more active with respect to closed Zr‐sites in butanal condensation: the TOF values estimated at 433 K being 1.4 s−1 for open Zr‐sites and 0.6 s−1 for closed sites. On the contrary, selectivity into 2‐EN was similar for open and closed sites. The results point that Zr‐BEA catalysts with enhanced content of open sites are perspective catalysts for the selective synthesis of 2‐EN. [ABSTRACT FROM AUTHOR]

The connection of active site structures and their catalytic chemistry during butanal deoxygenation on polyoxometalate clusters with varying H+ site densities and identity of central atoms [H xNa4− xSiW12O40 ( x=0-4) and H yNa3− yPW12O40 ( y=0-3)] was established with rate assessment, IR spectroscopic, and chemical titration methods. Butanal adsorbs on the H+ or Na+ ions on polyoxometalate clusters and forms RC=O⋅⋅⋅H+ or RC=O⋅⋅⋅Na+ complexes at 348 K. A portion of the adsorbed butanals on the H+ sites converts to surface acetates through their reactions with vicinal framework oxygen atoms, as confirmed from the detection of νas(OCO) band at approximately 1580 cm−1, and remains as the spectator species. Bimolecular reactions of butanals on the remaining H+ sites lead to 2-ethyl-2-hexenal as the predominant products, within which a small fraction undergoes sequential cyclization-dehydration to produce aromatics. A trace amount of butanal converts through minor, competitive pathways that form light olefins and dienes. These findings on the connection between active sites and their catalytic chemistry provide mechanistic insights useful for tuning the rates of the various concomitant paths and thus yields towards the different products during deoxygenation reactions. [ABSTRACT FROM AUTHOR]

Modified electrodes using immobilized alcohol dehydrogenase enzymes for the efficient electroreduction of butyraldehyde to butanol are presented as an important step for the utilization of CO2-reduction products. Alcohol dehydrogenase was immobilized, embedded in an alginate-silicate hybrid gel, on a carbon felt (CF) electrode. The application of this enzyme to the reduction of an aldehyde to an alcohol with the aid of the coenzyme nicotinamide adenine dinucleotide (NADH), in analogy to the final step in the natural reduction cascade of CO2 to alcohol, has been already reported. However, the use of such enzymatic reductions is limited because of the necessity of providing expensive NADH as a sacrificial electron and proton donor. Immobilization of such dehydrogenase enzymes on electrodes and direct pumping of electrons into the biocatalysts offers an easy and efficient way for the biochemical recycling of CO2 to valuable chemicals or alternative synthetic fuels. We report the direct electrochemical addressing of immobilized alcohol dehydrogenase for the reduction of butyraldehyde to butanol without consumption of NADH. The selective reduction of butyraldehyde to butanol occurs at room temperature, ambient pressure and neutral pH. Production of butanol was detected by using liquid-injection gas chromatography and was estimated to occur with Faradaic efficiencies of around 40 %. [ABSTRACT FROM AUTHOR]

A highly efficient solid-base organocatalyst for the gas-phase aldol self-condensation of n-butanal to 2-ethylhexenal was developed by grafting site-isolated amines on tailored silica surfaces. The catalytic activity depends largely on the nature of amine species, the surface concentration of amine and silanol groups, and the spatial separation between the silanol and amine groups. In situ FTIR measurements demonstrated that the formation of nucleophilic enamines leads to the enhanced catalytic activity of secondary amine catalysts, whereas the formation of imines (stable up to 473 K) leads to the low activity observed for silica-supported primary amines. Blocking the silanol groups on the silica support by silylation or cofeeding water into the reaction stream drastically decreased the reaction rates, demonstrating that weaker acidic silanol groups participate cooperatively with the amine groups to catalyze the condensation reaction. This work demonstrates that the spatial separation of the weakly acidic silanols and amines can be tuned by the controlled dehydration of the supporting silica and by varying the linker length of the amine organosilane precursor used to graft the amine to the support surface. A mechanism for aldol condensation was proposed and then analyzed by DFT calculations. DFT analysis of the reaction pathway suggested that the rate-limiting step in aldol condensation is carboncarbon bond formation, which is consistent with the observed kinetics. The calculated apparent activation barrier agrees reasonably with that measured experimentally. [ABSTRACT FROM AUTHOR]

The Cover shows butanal condensation reaction on polyoxometalate clusters (H4SiW12O40), grafted on mesoporous silica, as a catalytic route for upgrading the aldehydes derived from biomass feedstocks. The reaction creates an intermolecular C=C bond, ejects oxygen heteroatoms and produces larger, unsaturated aldehydes (e.g., 2‐ethyl‐2‐hexenal).In their Full Paper, Y. Yang et al. describe the catalytic fate of butanal during its deoxygenation sojourns on polyoxometalate clusters. The Brønsted acid sites (H+) of polyoxometalate clusters protonate the butanals and then catalyze predominantly the C=C bond formation through an aldol condensation mechanism. Thus, the rate of C=C bond formation is directly proportional to the H+ site density. A portion of the protonated butanals reacts with framework oxygen atoms, forming surface acetates as spectators that poison the H+ sites. This work establishes the connection among the active site function, identity of adsorbed species and rates. More information can be found in the Full Paper by Y. Yang et al. on page 287 in Issue 2, 2017 (DOI: 10.1002/cctc.201601042). [ABSTRACT FROM AUTHOR]