|Ph.D Student||Alon-Rafael Dolev|
|Subject||Synthesis Characterization and Catalytic Activity of Pt|
Supported on Non-Hydrolytic Amorphous Alumina
|Department||Department of Chemical Engineering||Supervisor||Full Professor Grader Gideon|
In the last decade a new Non-hydrolytic (NH) sol-gel route to obtain large surface amorphous alumina was developed. In this work the utilization of NH alumina as a catalytic support was studied. For naphtha reforming applications the most widespread catalyst is platinum supported on crystalline g-alumina (Pt/g-Al2O3). Therefore the synthesis and catalytic activity for n-pentane reforming of Pt/NH-Al2O3 were studied and compared to those of Pt/g-Al2O3 (commercial g-Al2O3) analogs.
Since the most common route for the introduction of active metals into the support is by impregnation using aqueous solutions, the effects of contact with water and several common organic solvents on the NH alumina were studied. It was found that contact with the organic solvents does not cause morphological changes in the NH alumina, whereas contact with water at room temperature (RT) causes dramatic morphological changes. After contact with water the crystallization temperature of the NH alumina to g-Al2O3 decreases from ~800oC to ~450oC which is a typical crystallization temperature of hydrolytic sol-gel derived aluminas. The water contact time scale required for significant low temperature crystallization is between several days and several weeks. The surface area (SA) of the alumina after subsequent calcination decreases if the water contact period is less than a week and increases if the water contact period exceeds a week. Increasing the water contact temperature accelerates the morphological changes. After less than 1hour contact with water at 70oC a complete phase transformation to g-Al2O3 at low temperature (~450oC) accompanied with increase in SA in obtained. These findings are explained by a dissolution re-precipitation process of the alumina. Upon contact with water the NH alumina dissolves and re-precipitates via hydrolytic route, which causes crystallization at low temperature.
Impregnation of NH alumina with aqueous solution of H2PtCl6 fully leached the Pt from the solution; however relatively low Pt dispersion (15-20%) was obtained and the SA of the final catalyst after calcination was lower than the SA of the starting NH alumina. The decrease in SA area and the low Pt dispersion are caused by surface hydroxyls condensation which blocks the pores, and a dissolution re-precipitation process of the alumina which buries some of the adsorbed Pt on the surface. Impregnation of g-Al2O3 with H2PtCl6 results in full loading of the Pt from the solution, well dispersed Pt catalyst (~40%) and retention of the initial SA. Impregnation of NH alumina with platinum acetylacetonate [Pt(acac)2] solution in toluene results in full uptake of the Pt onto the alumina, moderate to well dispersed Pt (30-50%) and retention of initial SA. During impregnation of g-Al2O3 samples with Pt(acac)2, only partial Pt uptake was obtained. However, well dispersed Pt (~40%) and retention of SA were obtained. Alternatively to the impregnation methods, in-situ introduction of Pt precursor during the NH alumina gelation using organic solutions of PtBr4 or (C6H5CN)2PtCl2 were developed in this work. These in-situ methods yielded catalysts with full Pt uptake and a moderate (~30%) Pt dispersion.
In-spite of the high Cl content (~2.6% wt.) of the NH based catalysts, the Pt/g-Al2O3 catalyst exhibited higher conversions of n-pentane and higher selectivity to iso-pentane, than their NH analogs. Therefore it is concluded that the intrinsic Cl in the NH alumina has minor contribution to the acidity of the alumina compared to the acidity of g-Al2O3. However, the higher selectivity towards cracking products suggests of higher average acidity of the acidic sites on NH alumina.