Selective hydrogenation of
p-chloronitrobenzene was chosen as a model reaction to evaluate the catalyst performance (Fig.5). The main side reactions include the formation of intermediates and the dehalogenation of
p-chloroaniline (Fig.5(a)). The activity and selectivity of Ni@NCNs-T were strongly influenced by the pyrolysis temperature (Fig.5(b)). Nearly no products were observed for Ni-MOF precursor, excluding the catalysis of Ni
2+ and ligands. With Ni@NCNs-400 as catalyst, 18.0%
p-chloronitrobenzene conversion was achieved, while no
p-chloroaniline except for intermediates (e.g., azoxybenzene) were observed. As for Ni@NCNs-500,
p-chloronitrobenzene conversion increased remarkably to 93.8% and the
p-chloroaniline selectivity achieved 89.9%, attributing to the increased Ni
0 content in catalyst. When increasing pyrolysis temperature to 600 °C, the conversion and
p-chloroaniline selectivity further increased to 100% and 95.5%, respectively, in which no dehalogenated product (e.g., aniline) was detected. However, further increasing pyrolysis temperature to 700 °C decreased the conversion and
p-chloroaniline selectivity to 59.4% and 51.4%, respectively, ascribing to severe sintering of Ni nanoparticles and pore collapse of the catalyst. This phenomenon suggested that the dramatic decrease of nitrogen under 700 °C resulted in Ni aggregation and weaker metal-support interactions, decreasing the catalytic activity. Moreover, Ni@NCNs-600-static originating from static-crystallized Ni-MOF achieved 95.5% conversion and 81.0%
p-chloroaniline selectivity, highlighting the advantage of stirring-crystallization for the preparation of Ni-MOF and corresponding carbon-based catalysts (Fig.5(b)). When the substrate concentration was scaled up to 14.8 times (5 mmol), full conversion along with > 99%
p-chloroaniline selectivity was achieved at 60 min under the optimized conditions as referred to later (Fig.5(b)). The superior performance of Ni@NCNs-600 encouraged us to investigate the reason of high
p-chloroaniline selectivity. UV−vis spectra showed that the adsorption amounts of nitrobenzene and
p-chloronitrobenzene are much higher than that of chlorobenzene (Fig.5(c)), suggesting that the preferential adsorption of –NO
2 over –Cl can restrain the dechlorination of the desired product. Compared with other reported Ni-based catalysts (Fig.5(d), Table S4, cf. ESM) [
31,
37–
45], Ni@NCNs-600 presents extraordinary superiority with high catalytic productivity of 77.9 h
–1 and > 99%
p-chloroaniline selectivity under mild conditions (90 °C, 1.5 MPa H
2). According to previous characterizations, the superior performance of Ni@NCNs-600 can be ascribed to the following reasons: (1) higher content of Ni
0; (2) ultra-high dispersion of Ni nanoparticles; (3) uniform morphology of nanoneedles; (4) oriented adsorption of –NO
2.