The acidity and its effects on reactivity of Keggin-type heteropolycompounds were examined by catalytic
probe reactions, microcalorimetry of ammonia sorption, and density functional quantum chemical calculations.
Phosphotungstic, phosphomolybdic, silicotungstic, and silicomolybdic acids were used as model compounds.
The specific rates of double-bond isomerization of both 1-butene and cis-2-butene were orders of magnitude
greater on the tungsten heteropolyacids than on molybdenum heteropolyacids, which suggests the tungsten-containing solids are stronger acids. The rate of double-bond isomerization over silicotungstic acid was similar
to that over phosphotungstic acid, indicating the minor role of the heteroatom. Results from ammonia sorption
microcalorimetry showed ΔH
sorp on tungsten-based heteropolyacids was approximately 40 kJ mol-1 higher
than the corresponding enthalpy obtained on molybdenum-based heteropolyacids. Residual waters of hydration
significantly affected both reaction rates and sorption enthalpies. Quantum chemical calculations revealed
the most energetically favorable site of the acidic proton to be a bridging oxygen atom in the anhydrous
heteropolyacid. Calculations on structurally optimized small metal oxide clusters, as well as the complete
Keggin unit, were used to determine the proton affinities by DFT methods. Regardless of cluster size, the
proton affinity of a tungsten cluster was always lower than that of an analogous molybdenum cluster by
about 20−40 kJ mol-1. The combination of results from experiments and quantum chemical calculations
provides a consistent ranking of acid strength for this important class of solid catalysts.
Visible-light-promoted
organic reactions can offer increased reactivity
and selectivity via unique reaction pathways to address a multitude
of practical synthetic problems, yet few practical solutions exist
to employ these reactions for multikilogram production. We have developed
a simple and versatile continuous stirred tank reactor (CSTR) equipped
with a high-intensity laser to drive photochemical reactions at unprecedented
rates in continuous flow, achieving kg/day throughput using a 100
mL reactor. Our approach to flow reactor design uses the Beer–Lambert
law as a guideline to optimize catalyst concentration and reactor
depth for maximum throughput. This laser CSTR platform coupled with
the rationale for design can be applied to a breadth of photochemical
reactions.
The basicity of alkali-metal-exchanged (Na, K, Cs) zeolites X and Y was probed by UV−vis diffuse reflectance
spectroscopy of adsorbed iodine. The observed blue shift in the visible absorption spectrum of adsorbed
iodine, compared to gaseous iodine, correlated well with the negative charge on the framework oxygen atoms
calculated from the Sanderson electronegativity equalization principle. The blue shifts associated with iodine
adsorbed on classical catalytic supports like silica, alumina, and magnesia suggest that the iodine adsorption
technique for probing basicity is applicable to a wide variety of solids. Iodine was also adsorbed on X and
Y zeolites containing occluded cesium oxide formed by decomposition of impregnated cesium acetate. However,
the iodine appeared to irreversibly react on these strongly basic samples, possibly forming an adsorbed triiodide
ion. As a complement to the adsorption studies, the activity of alkali-metal-containing zeolites for the base-catalyzed formation of ethylene carbonate from ethylene oxide and carbon dioxide was investigated. Among
the ion-exchanged zeolites, the cesium form of zeolite X exhibited the highest activity for ethylene carbonate
formation. The catalytic activity of a zeolite containing occluded cesium was even higher than that of a
cesium-exchanged zeolite. The presence of water adsorbed in zeolite pores promoted the rate of ethylene
carbonate formation for both cesium-exchanged and cesium-impregnated zeolite X.
A new technique for the measurement of 3D crystal morphology
and
identification of its polymorph using tomographic images is proposed.
Confocal microscopy is used for the first time to obtain tomographic
images of crystals that are coated with a suitable fluorescent dye.
A convex polyhedron is fitted through a stack of tomographic images
of a crystal to obtain the normal vectors of each facet and their
corresponding perpendicular distances from the center of the crystal.
The angular patterns are generated from the measured normal vectors
and are matched with the master angular patterns of each polymorph.
It is shown that the matching of the angular patterns is unique and
provides a simpler way to identify polymorphs. An image-analysis program
that can be integrated with conventional confocal microscopes was
created to sequentially perform image processing, morphology measurement,
and polymorph detection. This program was used to measure morphologies
and identify polymorphs of 2D and 3D acetaminophen crystals. Detailed
directions are provided to enable the application of the methodology
without the need for special-purpose software. The image-analysis
program is also suitable for repeated measurements to produce morphology
distributions. This technique will provide an effective platform for
measuring the 3D shapes of materials of interest to many applications.
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