Synthesis of Sets of NiO Nanorods with Different Diameters for Applications as Heterogeneous Catalysts in Two-Component Organic Transformations

In this work, we report the synthesis of nickel oxide (NiO) nanorods (NiO_1 to NiO_3) via calcination of a microporous metal–organic framework precursor, {[Ni4(μ3–OH)2(D-2,4-cbs)2(H2O)4]·5H2O}n (Ni-CBS), at 600, 700, and 800 °C, respectively. The microscopic evaluation by field emission scanning electron microscopy and transmission electron microscopy of NiO_1 to NiO_3 shows a nanorod morphology with a systematic change in the diameter from 0.87, 0.33, and 0.29 μm, respectively, correlating well with an increase in the calcination temperature. This also indicates a slight decrease in the diameter for the variation of calcination temperature from 700 to 800 °C. Their phase purity and composition were confirmed by powder X-ray diffraction, energy dispersive X-ray analysis, elemental mapping, and X-ray photoelectron spectroscopy. The band gap values of NiO_1 to NiO_3 from the solid-state ultraviolet–visible diffuse reflectance spectroscopy range from 3.42 to 3.48 eV, indicative of their semiconductor nature with increasing calcination temperature. For comparison, when Ni(OAc)2·4H2O is used as the precursor, only pelletized NiO nanospheres are obtained at all three temperatures. Furthermore, a time-dependent formation of the nanorods from nanospheres has been followed by TEM to establish the growth mechanism. For an application, the synergistic effect of Lewis acid and Brønsted base possessed by these NiO is utilized as a heterogeneous catalyst for the C–C bond-forming Friedel–Crafts alkylation and Knoevenagel condensation reactions under ambient conditions. Among these NiO nanorods, NiO_1 displays the best catalytic activity. Additionally, parent Ni-CBS is also used for both reactions for comparison. These catalysts show good recyclability and stability up to three consecutive cycles. The mechanism of each reaction involving catalysts is also explained.