RESUMO
Catalytic transfer hydrogenation (CTH) of α,ß-unsaturated aldehydes using single metal atom catalysts supported on nitrogen-incorporated graphene sheet (M-Nx-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 CâC 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.
RESUMO
The design of all-solid heterogeneous catalysts with frustrated Lewis pairs (FLPs) has attracted much attention recently because of their appealing low dissociation energy for H2 molecules due to which a promotion of hydrogenation reaction is expected. The sterically encumbered Lewis acid (metal site) and base (nitrogen site) in the cavity of single transition metal atom-doped M/C2N sheets make them potential candidates for the design of catalysts with FLPs, while a comprehensive understanding of their intrinsic property and reactivity is still lacking. Calculations show that the complete dissociation of the H2 molecule into two H* states at the N sites requires two steps: heterolytic cleavage of the H2 molecule and the transfer of H* from the metal site to the N site, which are strongly related to the acidity of the metal site. Ni/C2N and Pd/C2N, which outperform the other eight transition metal atom (M) anchored M/C2N candidates, possess low energy barriers for the complete dissociation of H2 molecules, with values of only 0.30 and 0.20 eV, respectively. Furthermore, both Ni/C2N and Pd/C2N catalysts can achieve semi-hydrogenation of C2H2 into C2H4, with overall barriers of 0.81 and 0.75 eV, respectively, which are lower than those reported for many other catalysts. It is speculated that M/C2N catalysts with intrinsic FLPs may also find applications in other important hydrogenation reactions.
RESUMO
Improving the reaction selectivity and activity for challenging substrates such as nitroaromatics bearing two reducible functional groups is important in industry, yet remains a great challenge using traditional metal nanoparticle based catalysts. In this study, single metal atom doped M-C2N catalysts were theoretically screened for selective hydrogenation of 3-nitrostyrene to 3-vinylaniline with H2 as the H-source. Among 20 M-C2N catalysts, the non-noble Mn-C2N catalyst was found to have excellent reaction selectivity. Importantly, due to the solid frustrated Lewis pair sites in the pores of Mn-C2N, a low H2 activation energy is achieved on high-spin Mn-C2N and the rate-determining step for the hydrogenation reactions is the H diffusion from the metal site to the N site. The unraveled mechanism of the hydrogenation of 3-nitrostyrene using Mn-C2N enriches the applications of Mn based catalysts and demonstrates its excellent properties for catalyzing the challenging hydrogenation reaction of substrates with two reducible functional groups.