Structural characteristics and catalytic activity of promoted Raney(RTM) nickel catalysts for selective hydrogenation of butyronitrile

Seqwana Nichole Thomas Pryor, Purdue University

Abstract

Catalytic hydrogenation of nitriles is an important industrial process in the production of the nylon monomer hexamethylenediamine from adiponitrile (ADN). An important potential catalyst for this process is Raney® nickel promoted with metals which improve the selectivity, activity, and stability. In this study the relationship between the catalyst structure and activity and the effects of promoters on selectivity have been addressed using butyronitrile (BN), chosen as the model reactant because its single nitrile group affords a simpler reaction mechanism compared to the more complex ADN hydrogenation reaction. A series of precursor alloys were prepared and analyzed for phase composition using optical microscopy and x-ray diffraction. The active catalysts, produced from the precursors by leaching in sodium hydroxide, have been characterized by measuring the BET total and metallic surface areas. The bulk composition of the catalysts was determined by atomic adsorption spectrometry; and the chemical state of each element and surface composition were determined by x-ray photoelectron spectroscopy (XPS). Reactions were carried out in methanol in a well-stirred autoclave batch reactor. A typical reaction mixture consisted of 300 ml methanol, 30 ml of butyronitrile, and 0.6–1.5 grams of catalysts using octanol as an internal standard. Rate constants have been measured at temperatures between 25°C–75°C and total hydrogen pressure of 100–200 psig over a two hour reaction time. XPS studies showed that Ni, Fe, and, surprisingly, Al were in the metallic form, while Cr was oxidized and enriched on the surface. Catalysts promoted with iron, chromium, or iron and chromium were found to have improved activity with the catalyst containing ca. 1% Fe in the precursor being the most active with a turnover frequency (TOF) of 0.35 sec−1 . The most significant effect of butyronitrile concentration on activity was observed for the unpromoted catalyst, causing a three fold decrease in activity as the initial concentration was increased from 1.0 mol/L to 5.6 mol/L. The addition of promoters had only a small effect on the selectivity and CO/H2 uptake, but substantially increased the BET areas. For the catalysts examined, the selectivity was approximately 75 ± 5%. There was no significant effect of temperature, pressure, or BN concentration on selectivity. The addition of sodium hydroxide to the reaction mixture, however, had a strong effect on the reaction behavior, causing a dramatic decrease in the amount of by-products formed. Kinetic modeling confirms that the effect of base is to decrease the rate constant for the condensation reaction.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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