In situ SAXS/WAXS of Zeolite Microwave Hydrothermal Synthesis
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0900-O12-06.1
In situ SAXS/WAXS of Zeolite Microwave Hydrothermal Synthesis Geoffrey A. Tompsett1, Bernard Panzarella1, W. Curt Conner1 and Keith W. Jones2 1 Department of Chemical Engineering, University of Massachusetts, 159 Goessmann Lab, Amherst, MA 01003, USA 2 Environmental Sciences Department, Brookhaven National Laboratory, Building 901A, Upton, NY 11973-5000, USA. ABSTRACT The microwave hydrothermal synthesis of silicalite zeolite was monitored in situ using small and wide angle X-ray scattering (SAXS and WAXS). A custom-built waveguide reactor provided the microwave heating at 2.45 GHz frequency. Microwave power up to 300 W was supplied by a SAIREM generator via a coax connection. A sample port in the waveguide section allowed a thin-walled glass sample vessel to be held in the microwave field. The sample vial was sealed with a fiber optic temperature probe used for monitoring the temperature. Silicalite zeolite precursor solutions were heated temperatures between 100 and 130°C for up to 120 minutes. The SAXS or WAXS patterns were simultaneously scanned during the reaction under microwaves via X-ray beam through slots in the side of the waveguide. The progress of silicalite zeolite formation was followed and the analysis of SAXS patterns showed the primary particles of 2 nm shifted to aggregates of 5.8 nm in diameter, in 120 minutes at 130°C. INTRODUCTION Silicalite (MFI) zeolite is an important separation and catalytic material. Much work has been undertaken to understand the formation mechanism and kinetics in conventional hydrothermal synthesis. Microwave synthesis provides a fast efficient method of synthesizing many zeolites, including silicalite. Few studies of microwave synthesis have been carried out in situ. Typically ex situ samples are taken at varying conditions and times to ascertain the formation mechanism. However, for a better mechanistic and kinetic study on the formation of silicalite, in situ methods are preferred. Microwave synthesis provides unique difficulties in coupling with in situ analysis techniques due to the field containment and interaction issues. Whittaker et al.[1] developed an apparatus for studying the microwave hydrothermal formation of particles in solution using and small angle neutron scattering. These workers studied the formation of iron oxide heated to 95°C and showed that particle formation began in the first 30 minutes. SAXS measurements under conventional heating confirmed this observation and showed that particle growth to 10 nm takes place followed by agglomeration. Further, Harrison et al.[2] studied crystal microwave heating of crystals and powders using in situ neutron diffraction. Robb et al.[3] investigated the time-resolved, in situ X-ray diffraction of silver iodide under microwave radiation. The lowering of the AgI phase transition temperature was attributed to a microwave effect, namely the interaction of microwaves with the low-lying transverse optic modes of AgI. The conventional hydrothermal synthesis of silicalite zeolite has been investigated in
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