(528g) In Situ Saxs/Waxs of Zeolite Synthesis with Microwave Heating

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.

References:

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5.         Walton,
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6.         Dokter,
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10.       Grandjean,
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