Preparation Of The Nano-sized Nickel Catalysts And Application In The Synthesis Of P-Aminophenol

Nano-sized nickel catalyst has received increasing attention as a novel, high-efficiency, and high-selectivity hydrogenation catalyst with tempting perspective in some application areas, such as petrochemical industry, fine chemical industry and fuel battery. Up to now, nano-sized nickel catalyst has been commercially produced, but the present production processes face some problems and limitations: the adoptive techniques of large-scale production are basically physical methods, and continuous process is little reported. At a certain extent these problems hamper further expandable production, and the high price of nano-sized nickel catalyst limits large-scale commercial applications. Therefore, it is very important to exploit a continuous production process of nano-sized nickel with low-cost and high-quality.The present study was aimed at improving conventional chemical reduction method and making some new explorations in the preparation, characterization, and continuous production of nano-sized nickel. Moreover, the catalytic properties of nano-sized nickel and Raney nickel were investigated and compared in the hydrogenation reaction of p-nitrophenol to p-aminophenol. These works would provide fundamental knowledge and theoretical foundation for the commercial applications of nano-sized nickel.Firstly, the conventional chemical reduction method was improved by introducing the reduction assistant SAT-1. Nano-sized nickel catalyst (denoted as IPNi) with high catalytic activity was directly prepared from aqueous solution at certain conditions by improved method. The textural properties of the as-prepared IPNi were systematically studied by XRD, SEM, HRTEM, and so on. The results showed that the average particle size of the as-prepared IPNi was 60nm and it consisted primarily of nickel (>99%). Compared with CVNi prepared by conventional method, IPNi had smaller particle size and higher density surface defects. CVNi catalyst had the core-shell structure of Ni/Ni+Ni(OH)2, while the surface of IPNi was mostly constituted with zero-valent nickel. Furthermore, the working mechanism of SAT-1 was brought forward: during the preparation process of nano-sized nickel, SAT-1 first reacted with the nickel salt to generate the nickelnucleus, which could catalyze the oxidation-reduction of hydrazine hydrate and the nickel salt, and as a result, the whole process could be carried out at lower concentration and temperature.Secondly, in the catalytic hydrogenation of p-nitrophenol to p-aminophenol, the effects of some preparation conditions (such as the SAT-1 amount, the feed concentration, the reaction temperature, and the residence time) on the catalytic activity of IPNi were studied. The optimal preparation conditions of IPNi were made. Meanwhile, the catalytic properties of IPNi, CVNi, and Raney nickel were compared. The catalytic activity and surface nature of the catalyst were preliminarily associated. Our study indicated that the catalytic activity of IPNi was about 4.3 times higher than that of CVNi, and 10 times than that of Raney nickel. The catalytic stability of IPNi was further superior to that of CVNi. We suggested that the reason for higher catalytic activity of nano-sized nickel catalyst (IPNi) was a combination effect of the smaller particle size, the higher density surface defects and the surface consisting mostly of Ni°.Thirdly, the effects of some nitrogen compounds on the catalytic activity of nano-sized nickel were examined, and the deactivation mechanisms were studied. The results showed that traces of ammonia could cause the deactivation of nano-sized nickel catalyst. There were two main reasons. First, ammonia might react with nickel catalyst and p-nitrophenol to form complex compounds. Second, nickel metal could be oxidized by p-nitrophenol to produce the nickel oxide, which would be dissolved by ammonia. The nano-sized nickel catalyst could also be poisoned in the presence of some organic amines. The primary cause could be the strong adsorption of the organic amines on the hydrogenation sites. The poisoning effect of organic amines was in association with their solution basicities (pKb). The toxic degree of the nano-sized nickel catalysts decreased along with the increase of the aqueous basicities of organic amines.Lastly, the pilot-scale amplification experiments of the nano-sized nickel were carried out. The effects of batch process and continuous process on the powders properties were respectively discussed, and the new production technology of nano-sized nickel by the continuous precipitation method was put forward. It was found that the continuous process fitted the amplification of this reaction system. In the continuous tubular reactor, the nucleation process and growth process were disparted each other at space and time, which would benefit the formation ofnano-sized nickel with uniform particle size and composition. In the kettle-tubular reactor, the backimixing degree of feeds could have an effect on the properties of catalyst. The catalytic activity was on the increase along with the decreasing kettle volume and the increasing feed flow. The results of mass production showed that this production technique was stable, simple, convenient and repeatable. The evaluation of economic effectiveness testified that this production process had low cost and was convenient for commercial processes. Furthermore, we designed a set of 10t/a continuous production unit of nano-sized nickel, which might lay the foundations for the follow-up commercial process exploitation.