Synthesis of Large ZSM-5 Crystals under High Pressure
- PDF / 1,228,294 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 93 Downloads / 207 Views
Synthesis of Large ZSM-5 Crystals under High Pressure Xiqu Wang and Allan J. Jacobson Department of Chemistry and Materials Research Science and Engineering Center, University of Houston, Houston, Texas 77204-5641 ABSTRACT The synthesis temperature for silicalite-I (MFI) can be raised to 300 ºC by applying high pressure to stabilize the structure-directing organic template. The elevated temperature and pressure favor the formation of crystals with improved quality. Prismatic silicalite-I crystals with a uniform size of about 0.7 x 0.2 x 0.2 mm have been obtained by heating a gel prepared from TMA-silicate solution, TPABr and sodium hexafluorosilicate at 250 ºC under a pressure of 80 MPa. The influence of synthesis conditions on the crystal sizes has been studied by systematically changing temperature, pressure and gel compositions. Under the specific conditions of 250 ºC and 80 MPa, a strong correlation was found between the crystal size and the F/Si mole ratio of the starting gel, which enables the preparation of uniform crystals of silicalite-I with preset dimensions. INTRODUCTION Large uniform zeolite crystals are highly desired for many uses that range from crystal structure analysis, adsorption and diffusion studies to zeolite functional materials. Synthetic zeolites obtained from hydrothermal reactions are usually in the form of microcrystalline powders. Considerable progress has been made in the growth of large zeolite crystals by carefully adjusting the solvents and starting materials. [1-4] We have been investigating the possibility of improving the size and crystal quality of microporous materials by using synthesis temperatures and pressures that are higher than those usually employed (220-400 °C, 50-200 MPa). By using this approach, we obtained crystals of the microporous titanosilicate ETS-10 suitable for single crystal X-ray structure analysis. [5] Similar experimental conditions were used to mimic the natural growth conditions of zeolite minerals. [6] It is a general rule that high pressures and high temperatures favor the formation of dense phases rather than microporous metastable phases such as zeolites. Many synthetic zeolites are stabilized by organic templates that act as space fillers in the crystal structure or as structuredirecting agents. Decomposition of the templates often determines the upper temperature limit of the zeolite synthesis. However, most organic templates can be stabilized to high temperatures by applying a high pressure. Therefore, under high pressure the synthesis temperatures of many zeolites may be substantially raised, which favors formation of large crystals because of higher crystal growth rates. We have chosen the silica form of the synthetic zeolite ZSM-5 (silicalite-I, MFI) to test this synthesis strategy because the synthesis of silicalite-I below 200 °C has been thoroughly GG8.1.1
investigated, and it is known that the presence of organic templates is crucial for its formation. [7,8] In addition, silicalite-I is one of the most important synthetic molecular sieve
Data Loading...
