Mechanism of Catalytic Transfer Hydrogenation for Furfural Using Single Ni Atom Catalysts Anchored to Nitrogen-Doped Graphene Sheets

Abstract

Catalytic transfer hydrogenation (CTH) of α,β-unsaturated aldehydes using single metal atom catalysts supported on nitrogen-incorporated graphene sheet (M–N x -Gr) materials has attracted increasing attention recently, yet the reaction mechanism remains to be explored. Compared to the Ni–N4-Gr model in which the dissociation of isopropanol is highly unfavorable as a result of steric hindrance and inertness of the Ni–N4 site embedded in graphene, the Ni–N3 site in Ni–N3-Gr is more active and facilitates the formation of *H with isopropanol as the H donor, where the dissociation of H from isopropanol with an energy barrier of 0.83 eV is the rate-determining step. An alternative reaction path starts from the coadsorption of isopropanol and furfural molecules at the Ni–N3 site, followed by a direct hydrogen transfer between the two molecules; however, the rate-determining step has a much higher energy barrier of 1.32 eV. Our calculations suggest that the hydrogenation of the aldehyde group is kinetically more favorable than the CC hydrogenation, revealing the high chemoselectivity of furfural to furfuryl alcohol. Our investigations reveal that the CTH mechanism using the Ni–N3-Gr catalyst is different from that on traditional metal oxides, where the former has only one single active site, while two active sites are required for the latter. The proposed reaction mechanism of CTH for furfural in this study should be helpful to guide the design of single metal atom catalysts with appropriate N coordination for application in chemoselective hydrogenation reactions.

Hybrid Nanocomposite Comprising Mg–Al Hydrotalcite Nanocrystals on ZSM-5 Zeolite for Production of Renewable Fuel Additives from Furfural

“…Possible reaction mechanisms for hydrogenation of furfural over metallic sites in the presence of molecular hydrogen as well as hydrogen sacrificial agent, e.g., ethanol, have been proposed earlier. ,− Similarly, the possible reaction mechanism for the conversion of FA to EL in liquid ethanol is also reported and few reaction pathways through formation of various intermediates were suggested. , Recently, the role of hydrotalcite originated basic sites in the adsorption and activation of the carbonyl group of furfural is reported. The surface basic sites of hydrotalcite-derived mixed oxide in close proximity with active metal sites are capable of having interaction with the π* acceptor orbital of carbonyl group, which consequently leads to the enhanced reactivity of the carbonyl group .…”

Selective hydrogenation of lignocellulosic biomass over single-atom catalysts

“…Afterward, a computational study on the reaction mechanisms of furfural hydrogenation into FAL on nitrogen-doped graphene sheets-supported Ni SACs (Ni–N x –Gr) has been reported. 45 Ni–N 3 –Gr configuration was advantageous over the Ni–N 4 sites for the formation of *H with isopropanol. Two competitive pathways involving the subsequential adsorption or desorption of the isopropanol and furfural were considered and calculated.…”

Single-Atom Metal Catalysts for Catalytic Chemical Conversion of Biomass to Chemicals and Fuels

“…To investigate the reaction mechanism of furfural transfer hydrogenation to furfuryl alcohol, Wang et al calculated the catalytic process for furfural hydrogenation over Ni 1 /N x -Gr catalyst using isopropanol as the H donor. 85 They found that the configurations and electronic structures of isopropanol and furfural on Ni 1 /N 3 -Gr with Ni−N 3 sites were active, while Ni 1 / N 4 -Gr with Ni−N 4 sites were inert. The difference was attributed to the lower steric hindrance and inertness of the Ni−N 3 sites.…”