A spectroscopic investigation of adsorbate interactions with metal oxide and pillared clay surfaces
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. New approaches to the synthesis and characterisation of both established and novel Pillar Interlayered Clay catalysts are reported. Well known pillared clays such as alumina, zirconia and titania pillared montmorillonite were prepared, along with less established alumina and zirconia pillared rectorite. New pillared clays, titania pillared rectorite, alumina, zirconia and titania pillared synthetic mica-montmorillonite and gallium/alumina pillared rectorite have been prepared. The physico-chemical characteristics and thermal stabilities of these materials were studied using X-ray diffraction and surface area techniques. The results showed that gallium/alumina pillared rectorite was the most stable pillared clay, retaining a pillared structure after heating to 973 K in air. The pillared synthetic mica-montmorillonites exhibited little evidence of a pillared structure and the titania pillared clays appeared to be largely amorphous titania. The surface acid characteristics and Brönsted/Lewis acid site ratios of the various pillared clays were investigated using the infrared spectra of adsorbed pyridine. It was confirmed that the dominant surface acidity on pillared clays is Lewis type and the majority of the acid sites are found on the surfaces of the pillars. Considerable Brönsted acidity exists on alumina and gallium/alumina pillared clays, but generally fails to retain adsorbed pyridine above 673 K. The strength of the surface acidity depends less on the type of clay used than on the type and concentration of pillaring species present. The strength and abundance of both Lewis and Brönsted surface acidity can be augmented by increasing the concentration of pillars intercalated into the interlayer space. The adsorption of 2,2’-bipyridine on silica, ?-alumina, monoclinic zirconia and mixed anatase/rutile titania have been studied by infrared and Raman spectroscopic techniques. These oxides were chosen because they constitute pillars in various pillared interlayer clay (PILC) catalysts which were the subject of a separate 2,2'-bipyidine study. Four different types of adsorbed 2,2'-bipyndine molecule have been identified depending on the specific oxide. Physisorbed neutral molecules were common to all four oxides. Hydrogen-bonded neutral species were observed on silica, ?-alumina and titania, while neutral 2,2'-bipyridine, coordinatively bound in a bidentate manner to single Lewis sites were also detected on ?-alumina, zirconia and titania- An additional Lewis bound species was observed on ?-alumina where the adsorbate was bound to two adjacent Al3+ Lewis sites. At high temperature these adsorbed molecules were observed to dissociate into individual adsorbed pyridine molecules. Physisorbed bipyridine molecules exhibited trans symmetry, while H-bonded and chemisorbed bipyridine molecules possessed cis symmetry. The interaction of 4,4'-bipyridine adsorbed on silica, alumina, zirconia and titania has been studied using infrared spectroscopy. Neutral hydrogen-bonded molecules were observed on the silica surface, while both hydrogen bonded and coordinatively bound neutral species were detected on the remaining oxides. The strength of interaction between 4,4'-bipyridine and each oxide surface followed the order: alumina ? titania > zirconia > silica. Results are discussed with reference to those previously obtained for 2,2'-bipyridine adsorption, differences in basicities of the two adsorbates and changes in molecular symmetry. An explanation for the discrepancy between the intensities of the spectra of chemisorbed 4,4'- bipyridinc and 2,2'-bipyridine with respect to the spectra of the physisorbed adsorbates is proposed. Cleavage of the inter-ring bond of 4,4'-bipyridine adsorbed on alumina was observed at high temperature. 2,2'-bipyidine adsorption on alumina-, zirconia- and titania-pillared montmorillonite and rectorite was employed as a possible means of measuring the interpillar spacing and has been studied by infrared spectroscopy. The results are discussed with reference to earlier studies of 2,2'-bipyridine adsorption on metal oxide surfaces. It is proposed that 2,2'-bipyridine binds to single Lewis acid sites on the surfaces of single pillars via a bidentate C2?, interaction. Evidence was also found for the formation of mono-protonated 2,2'-bipyidinum ions also exhibiting cisoid symmetry. This is a result of the strong Brönsted acidity of Pillar Interlayered Clays. No evidence was obtained to allow assignment of a C2h or transoid Lewis-bipy bound to two separate pillars, neither was any evidence found that suggested dissociation of adsorbed bipy, as was seen on alumina and titania surfaces. The adsorption of 4,4'-bipyridine on alumina-, zirconia- and titania-pillared clays as a possible means of determing interpillar distances of Pillar Interlayered Clay catalysts, has been studied by infrared spectroscopy. The results are discussed with reference to 4,4'-bipyndine adsorption on metal oxide surfaces. Evidence was obtained to suggest that 4,4'-bipyridine adsorbed on alumina-pillared montmorillionite and zirconia pillared retorite, was bound to Lewis acid sites via both ends of the molecule between either the upper and lower clay layers of the pillared clay galleries, or between two adjacent pillars near the pore openings. These modes of interaction maintained the D2h symmetry of the free molecule and explained the low surface coverages observed. The titania-pillared rectorite surface consisted of excess amorphous titania, where 4,4-bipyndine was adsorbed in a mondentate mode, while no adsorption was detected on zirconia-pillared montmorillonite. Low concentrations of. 4,4'-bipyridinium ions were detected on alumina- and zirconia-pillared clays. The preparation, thermal stabilities and surface acid characteristics of new GaAl12-pillared rectorites arc reported. A range of these materials with different pillar concentrations were prepared, with the pillared clay containing the lowest pillar population (2.0 mmol or 14meq GaAl12 g-1 clay) exhibiting poor thermal stability (<673 K). GaAl12-pillared rectorite with the highest pillar concentration (6.0 mmol or 42 meq GaAl12 g-1 clay) exhibited outstanding thermal stability, maintaining a strongly ordered gallery height of 9.3 Å after heating in air at 973 K for 2 h. The three pillared clays were calcined at different temperatures and the surface acid characteristics investigated using IR spectra of adsorbed pyridine. These spectra demonstrated strong Lewis acid and weak Brönsted acid character with the highest concentration of pillars producing the strongest and most abundant acidity.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1994