Unsaturated Ni^II Centers Mediated the Coordination Activation of Benzylamine for Enhancing Photocatalytic Activity over Ultrathin Ni MOF-74 Nanosheets

Abstract

Creating accessible unsaturated active sites in metal− organic frameworks (MOFs) holds great promise for developing highly efficient catalysts. Herein, ultrathin Ni MOF-74 nanosheets (NMNs) with high-density coordinatively unsaturated Ni II centers are prepared as a photocatalyst. The results of in situ ATR−IR, Raman, UV−vis DRS, and XPS suggest that abundant Ni II centers can act as the active sites for boosting benzylamine (BA) activation via forming −Ni−NH 2 − coordination intermediates. The generation of coordination intermediates assists the transfer of photo-generated holes to BA molecules for producing BA cation free radicals, better impelling the breaking of N−H bonds and the photooxidation of BA molecules. The photo-generated electrons further activate O 2 molecules to O 2 •− radicals for triggering the reaction. The experiments reveal that the coordination activation of BA molecules may be a rate-determining step on NMNs rather than the adsorption and activation of O 2 molecules. Moreover, NMNs possess a better ability for the separation of photo-generated carriers in comparison with bulk Ni MOF-74 (NMBs). As a result, NMNs achieve a kinetic rate constant of 0.538 h −1 for the photocatalytic oxidative coupling of BA under visible light, about 50 times higher than that of NMBs (0.0011 h −1 ). Finally, a probable synergetic catalytic mechanism with coordination activation and photocatalysis is discussed on a molecular level. This study not only highlights the importance of coordination activation for heterogeneous photocatalysis but also affords an inspiration for building ultrathin MOF nanosheets.

Photocatalytic Oxidative Coupling of Benzylamine to Schiff Base over 0D/2D CdS/CdIn2S4 Heterojunction

“…Subsequently, the •O 2 − species would oxidate PhCH 2 NH 2 •+ into phenylmethanimine (PheCH = NH) and then couple with other BA molecules to form N‐benzylidenebenzylamine product. [ 21 ]…”

Amino-functionalized MIL-125(Ti) for photodenitrification of pyridine in fuels via coordination activation by unsaturated Ti4+ centers

“…18 Wu and his co-workers also proved that the abundant NiII centers in MOF-74 can act as active sites for boosting benzylamine activation by forming –Ni–NH 2 – coordination intermediates. 19 It can be found that previous research studies on metal sites always focus on coordination activation but rarely combine this with coordination–photocatalysis synergism. Actually, MOFs are supposed to be credible platforms for researching the micro-mechanism of heterogeneous catalysis because of their explicit structure.…”

Tailoring defect-type and ligand-vacancies in Zr(iv) frameworks for CO2 photoreduction

“…[11][12][13] Much effort has been made to improve the efficiency of MOF photocatalysts including structural engineering, modication and functionalization. [14][15][16] Among these, defect engineering has been documented to be an effective approach to tailor the photocatalytic behavior. 17 Zr(IV)-based MOF UiO-66 has attracted extensive attention due to its ability to tolerate structural defects, discovered in 2011 by the discrepancy between the predicted and actual weight loss of thermogravimetric analysis disclosing a linker deciency in the Zr coordination shell.…”

“…11–13 Much effort has been made to improve the efficiency of MOF photocatalysts including structural engineering, modification and functionalization. 14–16 Among these, defect engineering has been documented to be an effective approach to tailor the photocatalytic behavior. 17…”