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In Situ Saxs/Waxs of Zeolite Synthesis with Microwave Heating

Bernard Panzarella, Chemical Engineering, University of Massachusetts Amherst, 159 Goessmann Lab, Amherst, MA 01003, Geoffrey A. Tompsett, Chemical Engineering, University of Massachusetts, 159 Goessmann Lab, Amherst, MA 01003, and William C. Conner, Department of Chemical Engineering, University of Massachusetts-Amherst, 105 Goessmann Lab, Amherst, MA 01003.

Microwave heating has proven to be an effective method for synthesizing zeolites.  Using microwave heating has been shown to have a number of advantages over the use of conventional heating techniques.  Among these are substantially faster product formation, more uniform product and improved selectivity (fewer impurities).  The synthesis rate observed with microwave heating is often an order of magnitude or more rapid compared to conventional hydrothermal synthesis.  The actual mechanism which causes these changes in reaction rate is not understood. Extensive work has been carried out to study zeolites formation under conventional heating conditions.  Much of this work has been carried out using X-ray powder diffraction.  These measurements are taken on product samples which have been washed of excess mother liquor and dried.  Such ex-situ characterization does not enable the direct measurement of zeolite nucleation and growth from precursor solutions or gels.  Furthermore, the sample may undergo a number of structural changes during product recovery.  Thus information pertaining to the formation of intermediate phases and their transformation to zeolite product is lost.  A few in-situ X-ray scattering studies have been carried out using synchrotron radiation to observe the crystallization of microporous materials and zeolites under hydrothermal heating conditions [1-14].  In situ studies such as these enable direct observation of the gel formation and dissolution, nucleation and growth processes.  To date, however, no work has been done to study the microwave synthesis of zeolites using in-situ X-ray scattering. A custom waveguide apparatus was constructed to study the microwave synthesis of zeolites by in situ small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS).  The waveguide was used to heat precursor solutions using microwaves at 2.45 GHz frequency.  The reaction vessels were designed to include sections of thin-walled glass which permitted X-rays to pass through the precursor solutions with minimal attenuation from the vessel.  Slots were machined into the waveguide to provide windows for X-ray energy to enter and scatter from solutions during microwave heating.  Synthesis of zeolites with conventional heating was also studied using X-ray scattering in the same waveguide reactor by heating air and flowing it into the cavity.  These studies were carried out using the X10A beam line at the NSLS, Brookhaven National Laboratory.  An X-ray wavelength (λ) = 1.0948Ǻ at an energy (E) = 11.325keV was used.  Spectra were collected with a Bruker 1500 CCD camera detector during the duration of the reaction to obtain a continuous series of measurements. NaY zeolite, beta zeolite, Si-MFI(TPA) (silicalite), and a mixture of NaA zeolite, NaX zeolite and sodalite were synthesized in the waveguide reactor.   SAXS studies showed that the crystallization of zeolites may be preceded by a reorganization of nano-sized particles (diameter between 2nm - 10.7 nm) in their precursor solutions or gels.  However, no precursor particles with in the range of our SAXS detector were observed for the synthesis of NaA, NaX and sodalite.  The evolution of these precursor particles during the nucleation and crystallization stages of zeolite formation depended on the properties of the precursor solution.  The Si-MFI(TPA) precursor was a clear solution containing tetrapropylammonium hydroxide as a template.  NaY and beta zeolite are synthesized from viscous aluminosilicate gels, the latter containing tetraethylammonium hydroxide as a template.  The NaA, NaX and sodalite mixture was synthesized from a dilute aluminosilicate gel.  SAXS measurements indicate the presence of particles with a diameter of 2.5 nm in the initial Si-MFI(TPA) precursor.  When the synthesis solution was heated, these particles initially grow by addition of soluble species in the solution phase.  These particles then combine to form larger particles in the synthesis solution.  In contrast the primary particles in NaY and beta zeolite synthesis gels do not appear to grow in the same manner as in the Si-MFI(TPA) synthesis.  In the beta zeolite precursor, primary particles were not initially detected.  After 28 minutes of heating, particles with diameters of 4.6 nm and 7.9 nm were detected.  The intensity of scattering from these particles increased during the first hour of the synthesis.  These remained present during the duration of the experiment, which probed the early stages of beta zeolite nucleation.  The NaY precursor solution contains two different populations of nano-sized particles (5.7nm and 9.3nm).  We observe the consumption of smaller particles and increase in scattering intensity due to large particles in NaY synthesis as the reaction proceeds.  Dissolution of the small primary particles does not occur, as smaller particles do not form during the reaction, nor is a decrease in particle size detected by shifts in the SAXS.  The growth of larger particles in these solutions is due to the addition of smaller particles with one another, or the addition of smaller particles to the surface of larger particles.  These processes occur prior to the detection of crystalline phases by WAXS.  Thus, nucleation of zeolites from either clear solution or gels occurs as a result of organization and growth of nano-sized particles. The formation of Si-MFI(TPA) under conventional heating, continuous microwave heating and pulsed microwave heating was also studied using SAXS and WAXS.  No differences were detected in the mechanism of precursor particle arrangement in the SAXS.  However, the rate of particle growth from SAXS and crystallization from WAXS differed for microwave and conventionally heated samples.  Silicalite nucleation and crystallization was 40 percent more rapid with microwave heating compared to conventional heating at 115°C.  Little difference was found in the rate of silicalite formation using continuous microwave heating compared to pulsed microwave heating. The results of WAXS studies on the synthesis of NaA, NaX and sodalite from a single zeolite precursor indicated that use of microwave heating led to a more rapid onset of product crystallization compared to conventional heating.  Microwave heating also shifted the selectivity of the reaction in favor of NaA and NaX over sodalite.  Nearly pure sodalite was formed conventionally.  The use of pulsed (on/off) microwave power delivery and continuous microwave heating were compared for this synthesis.  The resulting rate of formation of the zeolite products, and the relative amounts of the products determined from the WAXS spectra were similar for when either pulsed or continuous microwave heating was applied.  Our studies exhibit the advantage of using microwave heating to rapidly synthesize zeolites.  Additionally, they provide insight into the mechanism of zeolite formation under microwave heating and the influence of power delivery on microwave synthesis. 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Web Page: www.ecs.umass.edu/che/micro-synth/