DIRECT SYNTHESIS OF HIGH-SILICA MORDENITE WITH SMALL CRYSTAL SIZE

INTRODUCTION Mordenite is a zeolite with an ideal composition of Na8Al8Si40O96.nH2O and a structure refined in the Cmcm space group[1]. The micropore system of mordenite consists of two pore channels; an elliptical pore channel (6.77.0A) which runs parallel to the c-axis, and another pore channel which runs parallel to the b-axis (2.65.7A) [2]. Due to its good thermal stability and acid property, mordenite has been used as catalysts for many important reactions such as hydroisomerization, alkylation, reforming, etc[3,4]. High-silica MOR zeolite is preferred in these reactions, which is usually prepared through removal of aluminum from the zeolite framework by treatment with steam and/or acid. However, the dealuminated MOR zeolite is known to be less thermally stable than the directly synthesized high-silica MOR zeolite [5]. Smaller crystals of zeolites have larger surface areas and less diffusion limitations compared to zeolites with larger crystals [6]. In the present work, mordenite with high silica to alumina ratio(15-35) and small crystal size has been prepared directly by surface-wetting method. The catalytic performance is evaluated in isomerization of C5/C6 hydrocarbon. EXPERIMENTAL Synthesis mixtures were prepared with the following reagents: commercial solid silica gel, pseudoboehmite, sodium hydroxide, aqueous tetraethylammonium hydroxide (TEAOH), and distilled water. The gel was homogenized by stirring, and transferred to a stainless-steel autoclave for crystallization. A two-stage crystallization procedure was employed [7]. The crystalline phases of the solid products were analyzed by powder X-ray diffraction (XRD) using a D/MAX-11A diffractometer (CuK radiation). Crystal morphologies were observed by transmission electron microscopy (TEM) using a JEM-2000FXII microscope. The SiO2/Al2O3 ratio in the samples was determined by 29Si and 27Al MAS NMR spectra on a Bruker DRX-400 spectrometer. Surface area and total pore volume were obtained from low temperature N2 adsorption experiments using a Micromeritics ASAP2400 instrument. The catalytic tests were carried out in a microreactor system consisting of a stainless-steel tube reactor, equipped with a flow controller and heated by an electrical heater. The isomerization of C5/C6 hydrocarbon was then studied under the following conditions: temperature 533 K; pressure 1.6 MPa; WHSV=1.0 h-1; H2/hydrocarbon=2:1(mol). The gas flow rate was such as to exclude the effect of external diffusion. During each run, products were collected every 4 h until the reaction reached a pseudo-stable state. Products were analyzed by GC. RESULTS AND DISCUSSION The direct synthesis of high-silica MOR zeolite was carried out by different parameters, such as dissolution of solid silica, crystallization temperature and time, and SiO2/Al2O3 ratio (15-35). The influences of all these parameters on crystallinity and crystal size of mordenite were studied. The results show that the source of silica has strong influence on the crystallization rate of mordenite. If uncalcined silica was used as silica source, mordenite crystals with 0.750.45m in size were obtained when crystallization time reached 72h. When using calcined silica as silica source, crystallization time must be increased to 84h in order to get the same crystallinity, and larger crystals(1.50.75m) of mordenite were obtained. The dissolution of amorphous