Structural and functional expansion
of metal–organic frameworks
(MOFs) is fundamentally important because it not only enriches the
structural chemistry of MOFs but also facilitates the full exploration
of their application potentials. In this work, by employing a dual-site
functionalization strategy to lock the ligand conformation, we designed
and synthesized a pair of biphenyl tricarboxylate ligands bearing
dimethyl and dimethoxy groups and fabricated their corresponding framework
compounds through coordination with copper(II) ions. Compared to the
monofunctionalized version, introduction of two side groups can significantly
fix the ligand conformation, and as a result, the dual-methoxy compound
exhibited a different network structure from the mono-methoxy counterpart.
Although only one almost orthogonal conformation was observed for
the two ligands, their coordination framework compounds displayed
distinct topological structures probably due to different solvothermal
conditions. Significantly, with a hierarchical cage-type structure
and good hydrostability, the dimethyl compound exhibited promising
practical application value for industrially important C2H2 separation and purification, which was comprehensively
demonstrated by equilibrium/dynamic adsorption measurements and the
corresponding Clausius–Clapeyron/IAST/DFT theoretical analyses.
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 CC 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.
Designing all-solid heterogeneous catalysts with frustrated Lewis pairs (FLPs) has aroused great attentions recently because of its appealing low dissociation energy for H2 molecule and thus 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 sheet makes it potential candidate with FLP, while a comprehensive understanding of its intrinsic property and reactivity is still required. Calculations show that the complete dissociation of H2 molecule into two H* at the N sites requires two steps, i.e., heterolytic cleavage of H2 molecule and the transfer of H* from metal site to N site, which are highly related to the acidity of the metal site. The Ni/C2N and Pd/C2N, which outperform over the other 8 transition metal atom (M) anchored M/C2N candidates, possess low energy barriers for the complete dissociation of H2 molecule, 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, lower than many reported catalysts. It is speculated that M/C2N catalysts with intrinsic FLPs may also find applications in other important hydrogenation reaction.
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