Ind. Eng. Chem. Res. 2003, 42, 2773-2782
2773
KINETICS, CATALYSIS, AND REACTION ENGINEERING Hydrocracking of Heavy Vacuum Gas Oil with a Pt/H-beta-Al2O3 Catalyst: Effect of Zeolite Crystal Size in the Nanoscale Range Miron V. Landau,*,† Leonid Vradman,‡ Valentin Valtchev,§ Je´ roˆ me Lezervant,§ Elena Liubich,| and Michael Talianker| Blechner Center for Industrial Catalysis and Process Development, Department of Chemical Engineering, and Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel, Department of Chemical Engineering, Negev Academic College of Engineering, Beer-Sheva 84100, Israel, and Laboratoire de Materiax Mineraux, Universite de Haute Alsase, UMR-7016 CNRS, Ecole Nationale Superieure de Chimie Mulhouse, Mulhouse 68093 Cedex, France
Two 0.5 wt % Pt/(zeolite beta-alumina) catalysts with zeolite crystal sizes of 200-500 nm (designated as Z1) and 10-30 nm (designated as Z2) showed similar acidities (ammonia temperature-programmed desorption), surface areas, mesopore volumes (N2 adsorption), Ptcomponent hydrogenation activities, and coke contents after catalytic runs (temperatureprogrammed oxidation). The two zeolite components of these catalysts displayed the same framework structures (XRD, HRTEM, and FTIR spectroscopy). The nanocrystals of Z2 comprised individual single A (tetragonal) or B (monoclinic) polymorphs of zeolite beta, whereas the same polymorphs existed in the microcrystals of Z1 as domains of ∼20 nm that grow simultaneously, side by side, within the common grain. In the hydrocracking of heavy vacuum gas oil (HVGO) at 623 K and 5.5 MPa, the decrease in zeolite crystal size from the upper to the lower limit of the nanoscale range (i.e., from Z1 to Z2) gave an approximately 2-fold increase in both the rate constant and the effectiveness factor. The critical crystal size of zeolite beta beyond which the diffusion limitations became insignificant was 50 nm. The effective diffusion coefficient of HVGO inside the crystals of zeolite beta was 2 × 10-14 cm2/g. The kinetic measurements and Thiele modulus analysis provided evidence for the significant contribution of the internal surface of zeolite channels to the catalytic process. Introduction Zeolite catalysts are widely used in petroleum refining,1-3 petrochemistry,2,3 fine chemicals production,4.6 and environmental catalysis.7 Despite its long history, zeolite catalysis is still an active research field, with the contributions of the various properties of the zeolite microporous crystals to catalysis performance still to be definitively elucidated. One of the most important of these properties is crystal size, which influences the external surface area and the mesoporosity.8-10 Decreasing the crystal size in the nanoscale range to
