The use of synthetic zeolites in catalysis

This study reports on investigations into the use of synthetic zeolites in catalysis in an attempt to design catalyst systems which have size and shape selectivity. Zeolites have been widely used in petroleum refineries as acid catalysts. This has led to the field of study known as \shape selective catalysis\" since the product distribution is limited to those molecules which can pass through the zeolite channel. In this study zeolites have been used both as an acid catalyst for the cracking of hydrocarbons and as a catalyst support for a cationic palladium complex. n-Octane and 2-methylheptane were used in cracking reactions over zeolites HY HZSM-5 and their combinations. The two hydrocarbons were selected to discern the skeletal effect introduced by the structure of a molecule while the zeolites were chosen because of their common use in commercial catalytic cracking. Three ways of mixing the zeolites were employed by which the influence of each zeolite on a particular feedstock could be isolated:- i. Placing the bed of zeolite HY prior to that of zeolite HZSM-5. ii. Placing the bed of zeolite HZSM-5 prior to that of zeolite HY. iii. Mixing the two zeolites thoroughly. It was found that overall distributions of cracking products by carbon number from reaction of either paraffin on combinations of HY and HZSM-5 and from reaction of a mixture of the paraffins on either zeolite can be reasonably described by the addition of product distributions on individual catalysts and from individual paraffins weighted according to the relative amounts present. Ratios of branched to linear paraffins are much more strongly influenced by catalyst type and feedstock than the corresponding ratios for olefinic products. For reactions on catalyst mixtures distributions of the total products by carbon number correspond well to a summation o~ contributions on the individual catalysts. However a greater departure from prediction is seen for individual distributions of paraffins olefins and aromatics as well as for ratios of branched to linear paraffins showing that hydrogen transfer processes and isomerisation must occur. The addition of pentasil resulted in enrichment of the linear saturates at lower carbon number which is due to preferential cracking of linear paraffins over the branched isomers. The use of zeolite as a catalyst support was investigated by immobilising a cationic palladium complex [Pd(22'-bipy)i]2+ in NaY zeolite. Three immobilisation methods were investigated from which a model for location and distribution of the supported complex was proposed:- i. Prepared from [Pd(22'-bipy)i](N03)2 complex which was exchanged directly with the zeolite (method I). ii. Synthesised in-situ from Pd(22'-bipy)Cl2 complex which was exchanged with the zeolite in the presence of 22'-bipyridyl (method II). iii. Synthesised in-situ from [Pd(NH3)4]2+ which was exchanged with the zeolite. Free 22'-bipyridyl was then added to the cation-exchanged zeolite in solution to replace the ammine (method III). UV-visible atomic absorption and raman spectroscopies together with microprobe and elemental analyses were used to characterise the zeolite supported complexes. Attempts to directly ion-exchange [Pd(22'-bipy)i]2+ into the zeolite resulted in a maximum palladium loading of 0.62 wt% compared with a maximum loading of 8.97 wt% when [Pd(NH3)4]2+ was ion-exchanged into the zeolite. Efforts to obtain higher loadings of [Pd(22'-bipy)i]2+ by in-situ synthesis from Pd(22'-bipy)Cli and 22'-bipyridyl resulted in an increased maximum palladium loading to 1.15 wt%. Although a maximum Pd loading of 8.97 wt% could be obtained by ion-exchanging [Pd(NH3)4]2+ into the zeolite it was found that the [Pd(NH3)4]2+ could only be partially converted to [Pd(22'-bipy)i]2+. The inability to achieve 100% conversion using this method is probably due to the steric constraints within the smaller zeolite cages. The catalytic activity of the zeolite supported complexes was tested for dimerisation of ethene. It was found that the product distribution resulting from the supported complexes was similar to that obtained from the homogeneous system."