Molecular sieve film catalysts

Abstract: In this study, well defined ZSM-5 films were prepared on monoliths, ceramic foams, alumina beads, glass beads and crushed quartz glass by further refinement of a method originally developed at the division of Chemical Technology, Luleå University of Technology. The supports were seeded with silicalite-1 seeds and hydrothermally treated, either at 75 °C or at 150 °C in a single or several steps. By adding sodium to the solution the aluminum concentration increased in the zeolite, which is beneficial for catalytic activity. Consequently, films with different Si/Al ratios could be prepared. The film thickness could be controlled from 110 nm to 9000 nm. Short hydrothermal treatments and use of multi-step synthesis was utilized to prevent excessive bulk crystallization and ultrasound treatment was beneficial in order to remove sedimented crystals on top of the zeolite films. The choice of support material and its influence on the performance of thin ZSM-5 film catalysts was examined by testing the reactivity of the zeolite- coated materials in two reactions; para-xylene isomerization and triisopropylbenzene cracking. ZSM-5 films with a thickness of 150, 350, 800 and 2300 nm, respectively, were prepared on alumina beads and quartz glass. Based upon the zeolite content, the films on quartz glass were much more active for para-xylene isomerization and for cracking of triisopropylbenzene, which is attributed to poisoning of the films on alumina due to impurities in the support. Model parameters were fitted to experimental results. The simulations indicated that thicker films contained a higher fraction of defects, which may be caused by open grain boundaries and cracks. These defects explain higher xylene diffusivities and higher triisopropylbenzene cracking activity for thicker films. As expected, thicker films possessed higher diffusion resistance than thin films despite the higher fraction of defects. The present work has given substantial and valuable fundamental understanding of the performance of thin molecular sieve film catalysts. These findings will be beneficial for development of materials that may be used in novel industrial applications.