Studies On The Structure Of Melt-Quenched Ni-Al Alloys And The Performance Of Their Derived Raney-Ni Catalysts For Hydrogenation
Dr. Hao Lei (1999-2003)
Directed by Prof. Xinhe Bao
Abstract
Present address:
Dalian Institute of Chemical Physics, China

    Catalysts have a great influence on the product yields in many industrial processes. The catalytic performance is not only determined by the composition of catalysts but also closely related to the preparation method and the technological conditions. Hexanolactam is one of the most important raw materials for the petrochemical industry. Hydrofining is used to obtain high-quality hexanolactam with Raney-Ni as the preferred catalyst. Raney-Ni catalysts show high activity for hydrogenation reactions because of
their skeletal structure, and are usually obtained by leaching the Ni-Al alloy powder.

    Raney-Ni catalysts from domestic chemical plants demonstrate lower activity than imported ones. Meanwhile, traditional technologies for preparing Ni-Al alloys (e.g. smelting, casting, etc.) inevitably lead to the segregation of components and additives. The process of ball milling after the breaking of alloy ingots demands a narrow Ni-Al alloys composition (Ni wt% as 40~60wt%). Therefore, the Research Institute of Petroleum Processing in Beijing (RIPP) designed a novel procedure for the preparation of highly active Raney-Ni catalysts. Ni-Al ribbons containing transition metal additives were obtained by melt-quenching and then ball- milled to powder. After high temperature pretreatment, the powder was activated through the leaching of Al with NaOH. However, the engineering fundamentals of the melt-quenching technology, the effects of the transition metal additives, the relationship between the Ni-Al alloys and the corresponding catalysts, especially the effect of pretreatment, have never been systematically studied. In addition, when the catalysts dimensions are reduced to nano scales, they will show unique catalytic characteristics due to the high density of crystal lattice defects, the large surface area and the high surface energy accordingly. In the present work, we have carried a systematical study of the relationship between melt-quenched Ni-Al alloys and the corresponding catalysts, as well as the influence of technological conditions. Using high-energy ball milling and arc-discharge methods, we have also attempted to prepare Ni-Al alloys in ultrafine or nano scales. The results are as follows:

  1. By altering the rate of the copper roller or by varying the species or the amount of the transition metal additives, Ni-Al alloy ribbons as the precursors of the Raney-Ni catalysts are optimized through melt-quenching. Both the alloys and the final catalysts are characterized by XRD, XPS and metallographic microscopy. It is indicated that Raney-Ni catalysts derived from Ni-Al alloys with a rolling rate of 500 rpm and Fe, Cr or Mo additives, have high selectivity and catalytic activity for the hydrogenation of adiponitrile and cyclohe xanone. The relative content and domain of the Ni2Al3 phase play a crucial role in the subsequent leaching process, and the catalytic performance relates directly to the size of Ni crystalline as well as the Ni/Al and Ni/NiO ratios on the surface.
  2. From the results of metallographic microscopy and XPS depth analysis, the obvious differences are observed in the phase composition and the distribution between the commercial and melt-quenched Ni-Al alloys, especially after pretreatment, causing a different leaching mechanism. In the catalysts from pretreated Ni-Al alloys, there exists microcrystalline or amorphous Ni, the residual Ni2Al3 phase is a crucial factor for the enhancement of the stability, and the active Ni is distributed evenly on the catalyst framework with a large amount of Al oxide on the surface.
  3. Two additional parts are connected to the LHS-12 multifuctional surface analysis system: one is a high-pressure reactor, which can carry out in situ pretreatment for the catalysts . Followed by XPS experiments of the melt-quenched Ni-Al alloys, it is suggested that high-temperature pretreatment causes the segregation of Al component of Ni-Al alloys. The other is a device for the adsorption at low temperatures, with which the performance of adsorption and desorption can be investigated at liquid nitrogen temperature.
  4. The ultrafine Ni-Al alloys are obtained by high energy ball milling. The XRD results indicate that the NiAl3 phase content and the Ni2Al3 phase crystal size in the Ni-Al alloys decrease as the milling time increases, which leads to the increase of catalytic activity. The Ni-Al alloys in nano scales can be obtained by performing the arc-discharge equipment at various atmospheres.
        Unlike the Ni-Al alloys prepared under methane atmosphere, the alloys prepared under inert atmosphere are closer in composition to those obtained from the traditional method, suggesting an easier formation of active species in the subsequent leaching step. The morphology, the size of the as-prepared alloys and the segregation of Al on the surface of the alloys can also be controlled by changing the alloy composition or atmosphere.
The compositions of the Ni-Al alloys and their catalytic activities in the hydrogenation of cyclohexanone