Hydrogenation Of Nitrobenzene Catalyzed By Metal-Organic Framework-Supported Ni Nanoparticles

Owing to the low cost, Ni-based catalyst is one of the widely used catalysts for hydrogenation. Hydrogenation of nitrobenzene is well known to be of significant importance in industrial process. The product, aniline, is a basic chemical raw material and an important intermediate for the medicine, pesticides, dyes, and rubber process. Owing to the large specific surface areas, high porosities, and adjustable pore size, metal-organic frameworks (MOFs) have recently been explored as new hosts of various metal nanoparticles and present unique characteristics and advantages. In most of the reports, MOFs were used as supports to incorporate noble metal nanoparticles as active sites. While, the catalysts Ni/MOFs, which using low-cost metal as active sites and MOFs as supports, were rarely reported.Three kinds of metal-organic frameworks, MIL-53(Al),MIL-96(Al) and MIL-12O(Al), have been used as supports to synthesize a series of catalysts Ni/MIL-n(n=53,96,120) by wet impregnation method. The samples were used as the catalysts in the hydrogenation of nitrobenzene to aniline and their catalytic performances were also compared with that of the industrial catalyst Ni/γ-Al2O3 to choose the one with the highest catalytic activity. It is found that the catalyst prepared with MIL-53(Al) as support possesses the intact framework of MOF and the moderate metal-support interaction; Ni/MIL-53(Al) presents highly dispersive Ni particles with the particle size concentrated at 6.2 nm; While, due to the strong metal-support interaction, the high temperature of calcination and reaction for Ni/γ-Al2O3 is inevitable. Ni particle size on support γ-Al2O3 concentrated at 15.9 nm, which is apparently larger than that on support MIL-53(Al), and large agglomerations of Ni particles are also observed. Ni/MIL-53(Al) presents the excellent catalytic performance with 100% conversion of nitrobenzene and 96.8% selectivity of aniline at 100℃,2.0 MPa Hb pressure in the hydrogenation of nitrobenzene. However, industrial catalyst Ni/γ-Al2O3 exhibits only 69.9% conversion of nitrobenzene with 93.3% selectivity of aniline.A series of Ni/MIL-53-M(M=NI, AC, EN) heterogeneous catalysts were prepared by using MIL-53(Al) as support and three different nickel precursors (nickel nitrate, nickel acetate and nickel ethanediamine). The catalytic performances of the catalysts were also studied in the hydrogenation of nitrobenzene to aniline. Characterization results based on powder X-ray diffraction, FT-IR spectra, inductively coupled plasma, N2 physical absorption-desorption, H2 temperature programmed reduction and transmission electron microscopy show that the nickel precursors seriously affect the metal-support interaction, size of Ni particles and their distribution. The catalyst using nickel ethanediamine as nickel precursor possesses the small size of nickel nanoparticles and high nickel distribution which is attributed to the restriction of migrations and agglomerations by the anchoring interaction from nickel ethanediamine. Both Ni/MIL-53-NI and Ni/MIL-53-EN present the superior catalytic activities with 100% conversion of nitrobenzene at 100℃,2.0 MPa H2 pressure in the hydrogenation of nitrobenzene. Furthermore, the catalytic performance of Ni/MIL-53:EN is more higher than that of Ni/MIL-53-NI at a lower temperature and H2 pressure. After five recycles, the Ni/MIL-53-EN catalyst retains the catalytic activity and presents the conversion of nitrobenzene at about 92%.