“…473 K is assigned to the weak acid sites, whereas the second peak at ca. 653 K corresponds to strong acid sites . The amount of weak acid sites was determined to be much higher than the amount of strong acid sites.…”
Section: Resultsmentioning
confidence: 88%
“…653 K corresponds to strong acid sites. 35 The amount of weak acid sites was determined to be much higher than the amount of strong acid sites. Interestingly, higher amounts of strong acid sites were detected on larger-particle-size of Y zeolite.…”
Section: Effect Of Preparation Conditions On Particlementioning
The effect of particle size in the range of 0.16−2.01 μm on the hydrothermal stability of H−Y
zeolite under hydrothermal treatment conditions similar to those used in FCC processes was
investigated. The average particle size of 0.45 μm was found to be the optimum particle size of
H−Y zeolite to retain a high percent crystallinity upon hydrothermal treatment. A new
correlation was developed by plotting the relative crystallinity (C/C
0) against the reciprocal of
the square root of the particle size (d
0) of Y zeolite. The correlation accurately predicts the
hydrothermal stability of Y zeolite in small to medium particle sizes for a given temperature.
For the larger particle sizes of Y zeolite (>0.45 μm), no exact correlation between particle size
and hydrothermal stability was found, probably because of the presence of more structural defects
in the zeolite, as shown by higher Brønsted/Lewis acid ratios.
“…473 K is assigned to the weak acid sites, whereas the second peak at ca. 653 K corresponds to strong acid sites . The amount of weak acid sites was determined to be much higher than the amount of strong acid sites.…”
Section: Resultsmentioning
confidence: 88%
“…653 K corresponds to strong acid sites. 35 The amount of weak acid sites was determined to be much higher than the amount of strong acid sites. Interestingly, higher amounts of strong acid sites were detected on larger-particle-size of Y zeolite.…”
Section: Effect Of Preparation Conditions On Particlementioning
The effect of particle size in the range of 0.16−2.01 μm on the hydrothermal stability of H−Y
zeolite under hydrothermal treatment conditions similar to those used in FCC processes was
investigated. The average particle size of 0.45 μm was found to be the optimum particle size of
H−Y zeolite to retain a high percent crystallinity upon hydrothermal treatment. A new
correlation was developed by plotting the relative crystallinity (C/C
0) against the reciprocal of
the square root of the particle size (d
0) of Y zeolite. The correlation accurately predicts the
hydrothermal stability of Y zeolite in small to medium particle sizes for a given temperature.
For the larger particle sizes of Y zeolite (>0.45 μm), no exact correlation between particle size
and hydrothermal stability was found, probably because of the presence of more structural defects
in the zeolite, as shown by higher Brønsted/Lewis acid ratios.
“…The differential heats and uncertainty levels reported in Table were determined by taking the average and standard deviation of all the points below a coverage of 400 μmol/g for H-ZSM-5 and below 600 μmol/g in H-M. For many of these adsorbates, the binding energies are quite large. This strong interaction restricts the ability of adsorbate molecules to migrate to the “strongest” sites, even at 473 K. As a result, we believe that attempts to extract site distributions (if sites of different strength really exist) from the coverage-dependences of Figures −4 are not justifiable. 1b,, 1 Differential heats of adsorption of 2-methylpyridine (triangles), 4-methylpyridine (circles), and 3-methylpyridine (squares) on H-ZSM-5 at 470 K. 2 Differential heats of adsorption of 3-fluoropyridine (squares), 3-chloropyridine (circles), and 2-fluoropyridine (triangles, two runs) on H-ZSM-5 at 470 K. 3 Differential heats of adsorption of 2-fluoropyridine (circles), 2-methylpyridine (triangles), and 2,6-dimethylpyridine (squares) on H-M at 470 K. 4 Differential heats of adsorption of methylamine (squares), dimethylamine (closed triangles), trimethylamine (circles), and n -butylamine (open triangles) on H-M at 470 K.…”
Section: Resultsmentioning
confidence: 99%
“…However, there are several previous reports for the parent species, ammonia and pyridine, in both H-M and H-ZSM-5, and one report that compares the complete methylamine sequence in these two zeolites 1b, It seems likely that the variations result from differences in the samples themselves, their pretreatment, and the methods used to perform the calorimetry. The numbers that we report for ammonia and pyridine in Table lie near the mean of reported data and we believe that they are representative of binding at non-interacting Brønsted acid sites in carefully prepared crystallites of these zeolites.…”
Section: Resultsmentioning
confidence: 99%
“…Countless publications purport to use measurements of adsorption or desorption of these bases (e.g. TPD, or IR, or microcalorimetry) to demonstrate either acidity differences between zeolites of different crystal structures or acid strength distributions among available acid sites within a single zeolite . In this paper we demonstrate that the logic behind these approaches to the determination of solid acid strength is inherently flawed.…”
We have used microcalorimetry to measure the differential heats of
adsorption of both a series of alkylamines
and a series of substituted pyridines in H-ZSM-5 and H-Mordenite.
With few exceptions, the differential heats are
approximately constant to coverages close to the expected Brønsted
site concentration. Both zeolites show good
correlations between the average differential heats of adsorption and
the gas-phase proton affinities of the basic
adsorbates. Slopes of the correlation lines for the two zeolites
are similar; the intercepts differ by about 15 kJ/mol.
We use this data set to demonstrate that a self-consistent,
quantitative Brønsted acidity scale for solid acids
cannot
be obtained from heats of adsorption of ammonia or pyridine or any
other single reference base. However, the
correlation between heats of adsorption and gas-phase proton affinities
does provide a useful starting point for a
more complete description of the thermochemistry of proton transfer
reactions in zeolites. Deviations from the
correlation curves for specific zeolite/adsorbate pairs can be used to
infer how the strengths of Coulombic, hydrogen-bonding, or van der Waals interactions change with structure of either
the zeolite acid or the adsorbate base.
The sections in this article are
Introduction
Concepts of Acidity and Basicity of Zeolite and Oxide Catalysts
Analysis of Acidity and Basicity
Conclusion
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