Preparation of rhodium catalysts on laminar and zeolitic structures by anchoring of organometallic rhodium

Abstract

Rhodium catalysts supported on six different aluminosilicate structures were prepared by hydrogen reduction of a cationic organometallic rhodium complex anchored to the support. The precursor active phase was incorporated in acetone medium through ion exchange using [Rh(Me 2 CO) x (NBD)]-ClO 4 as the metal precursor species, in which NBD is 2,5-norbornadiene and (Me 2 CO) x is acetone. The effect of the structure and characteristics of the support on metal load and dispersion was studied in the heterogeneous catalysts thus prepared. The supports were characterized by X-ray diffraction, energy-dispersive X-ray analysis, volumetric adsorption and surface acidity. For the precursors and catalysts, the metal load was determined by UV±VIS spectra, the reduction temperature was determined by differential scanning calorimetry, and rhodium dispersion was measured by chemisorption. The structure of the materials used as supports had a great influence on the catalyst prepared. A higher metal content was achieved in the supports with laminar structures, whereas better dispersion was shown by the catalysts supported on zeolitic structures. Copyright # 2001 John Wiley & Sons, Ltd.KEYWORDS: organometallic rhodium; montmorillonite; zeolitic products; catalyst preparation Zeolites and zeolitic materials have an increasing role in heterogeneous catalysis, and are widely applied in largescale industrial processes. Developments in the synthesis and characterization of zeolites have favored the design of materials that efficiently accelerate the reaction and which, therefore, help to achieve favorable thermodynamics and rates, as well as controlling the selectivity of a chemical reaction.1 Activity, selectivity and durability are characteristics that favor the use of heterogeneous catalysts in a wide range of chemical reactions under different pressure and temperature conditions. In view of their characteristics, natural and synthetic aluminosilicates have been used as catalysts and heterogeneous catalysts for a great number of active phases. In addition, clays and zeolites have advantages as supports because they are chemically and physically robust, and inexpensive. Clays can easily be modified to improve their catalytic properties. For instance, in smectites, interlayer spacing can be adjusted by introducing substituents, by pillaring or solvent swelling, or even by transformation to new structures, and the acidic nature of the structure can also usefully be altered to improve their selectivity. 3 In any aluminosilicate, the porous structure provides a high surface area that enables it to receive high charges of a welldispersed active component. Therefore, the industrial efficacy of aluminosilicates results from a combination of porosity and mechanical resistance. Zeolites and zeolite-like products have regular pore and cage dimensions, which makes them different from other aluminosilicates. Most of the active sites are located in the molecular size pores and cages so that, during the reaction, the transforming molecules are subj...