Lawrence Berkeley National Laboratory

An amorphous nickel and boron composite (NBC) was synthesized from nickel chloride hexahydrate (NiCl2·6H2O) and sodium borohydride (NaBH4) in absolute ethanol, both in bulk and supported on mesoporous aluminosilicate nanoparticles (MASN). Comparatively, NBC-MASN demonstrated better catalytic activity for the selective reduction of the nitro group on a variety of polysubstituted nitroarenes, using hydrazine hydrate (N2H4·H2O) as the reducing agent at 25 °C. Reuse and regeneration of NBC-MASN for the reduction of p-nitrotoluene to p-toluidine were studied with NaBH4 acting as a regeneration agent. Good catalytic activity was sustained through nine reuse cycles when equimolar NaBH4 was present in situ with N2H4·H2O (99%-67% isolated aniline yield). The structure and composition of NBC and NBC-MASN were examined by electron microscopy, energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and inductively coupled plasma optical emission spectroscopy (ICP-OES). The results for NBC-MASN show that a thin (<10 nm) amorphous coating forms over the MASN surface, consisting of a mixture of metallic and oxidized nickel (9 wt % Ni), and various species of boron (atomic ratio of Ni:B = 2). For unsupported NBC, metallic nickel nanocrystals (1-3 nm) were discovered imbedded within an amorphous matrix of a similar composition. Upon calcination at 550 °C in a N2 atmosphere, partial conversion of unsupported NBC to crystalline Ni3B was observed, whereas only crystalline metallic Ni was observed for NBC-MASN. To explain these differences, further evidence is given to suggest the presence of residual boron hydrides encapsulated in the bulk unsupported NBC, suggesting Ni3B was an artifact of processing rather than an initial product. ©