Abstract:This article confirms the physical significance of a new method
for determining adsorption energy distributions
in porous materials. The premise of the model is that an
adsorption isotherm consists of several equilibrium
processes corresponding to adsorption into a distinct pore size regime.
A distribution type equilibrium constant,
K, involving the gas and adsorbate in a pore of capacity
n describes each process. In order to define the
n's
and K's for all the processes involved, isotherms are
collected at sev… Show more
“…These results are summarized in Table , and the ln K 1,ads vs 1/ T ( K ) plots for HZSM-5 are shown in Figure . The linearity of these and reported plots, as well as resulting enthalpies whose magnitudes are those expected for physisorption processes, support our contention that meaningful thermodynamic parameters result from the model. 3 ln K 1 vs 1/ T ( K ) plots for various adsorptives by HZSM-5.…”
Section: Resultssupporting
confidence: 78%
“…Equilibrium Analysis. The measured isotherms were analyzed using eq 3 by a procedure that was previously described. , In using this equation, the minimum number of processes required to obtain a good data fit are utilized. A detailed explanation of the criteria for selecting the correct number of processes is reported 4b…”
Section: Methodsmentioning
confidence: 99%
“…In the first two papers of this series, , it has been shown that, for porous carbonaceous materials, a simple one-process model does not adequately describe adsorption isotherms for gases above their critical temperature and up to 1 atm of external pressure. Several equilibria like that in eq 1 may be involved, and summation of i expressions like eq 2, to accommodate i multiple adsorption processes, leads to eq 3…”
Adsorption isotherms are reported for HZSM-5 and silica gel using
a series of gas adsorptives at
several temperatures above their critical temperature. The data
are analyzed with a new multiple equilibria
adsorption model producing equilibrium constants
(K
i
), capacities
(n
i
), and enthalpies
(−ΔH
i
) for each of
the
processes. Unlike the amorphous adsorbents studied earlier, which
contain a distribution of pore sizes, zeolitic
materials have uniform pore dimensions. This uniformity provides a
test of our interpretation of the number
of processes required in the isotherm fits of the multiple equilibrium
analysis, MEA. As expected from the
two pores in the structure of HZSM-5, most adsorbates require two
processes (K
1,ads and
K
2,ads) to fit the
adsorption isotherms. HZSM-5 is compared to amorphous carbonaceous
adsorbents, revealing fundamental
differences in their behavior. Small cylindrical channels in
HZSM-5 lead to an unfavorable entropic contribution
from restrictions imposed by adsorptive packing and interactions with
the channel walls. The pores of the
carbons studied (Drago, R. S.; Kassel, W. S.; Burns, D. S.; McGilvray,
J. M.; Lafrenz, T. J.; Showalter, S. K.
J.
Phys. Chem. B
1997, 101,
7548−7555) are slit-shaped, leading to less restriction and larger
equilibrium
constants. For several adsorbates, the greater enthalpic
interactions in the small HZSM-5 pores are accompanied
by lower equilibrium constants than in the larger pores because of
unfavorable entropic contributions. Finally,
the larger total micropore volumes of the carbons studied for these
adsorptives (Drago, R. S.; Kassel, W. S.;
Burns, D. S.; McGilvray, J. M.; Lafrenz, T. J.; Showalter, S. K.
J.
Phys. Chem. B
1997, 101,
7548−7555)
result in increased capacity compared to HZSM-5. The process
capacities from MEA (mol g-1) are
converted
to pore volumes using the molar volume. Surface areas are
calculated from molecular areas of the adsorbates.
Pore volumes and surface areas calculated from the process
capacities are compared to those from conventional
N2 porosimetry and are shown to provide a more detailed and
more accurate assessment of areas and volumes.
These results show that MEA has the potential of becoming a
standard characterization method for microporous
solids that will lead to an increased understanding of their behavior
in gas adsorption and catalysis.
“…These results are summarized in Table , and the ln K 1,ads vs 1/ T ( K ) plots for HZSM-5 are shown in Figure . The linearity of these and reported plots, as well as resulting enthalpies whose magnitudes are those expected for physisorption processes, support our contention that meaningful thermodynamic parameters result from the model. 3 ln K 1 vs 1/ T ( K ) plots for various adsorptives by HZSM-5.…”
Section: Resultssupporting
confidence: 78%
“…Equilibrium Analysis. The measured isotherms were analyzed using eq 3 by a procedure that was previously described. , In using this equation, the minimum number of processes required to obtain a good data fit are utilized. A detailed explanation of the criteria for selecting the correct number of processes is reported 4b…”
Section: Methodsmentioning
confidence: 99%
“…In the first two papers of this series, , it has been shown that, for porous carbonaceous materials, a simple one-process model does not adequately describe adsorption isotherms for gases above their critical temperature and up to 1 atm of external pressure. Several equilibria like that in eq 1 may be involved, and summation of i expressions like eq 2, to accommodate i multiple adsorption processes, leads to eq 3…”
Adsorption isotherms are reported for HZSM-5 and silica gel using
a series of gas adsorptives at
several temperatures above their critical temperature. The data
are analyzed with a new multiple equilibria
adsorption model producing equilibrium constants
(K
i
), capacities
(n
i
), and enthalpies
(−ΔH
i
) for each of
the
processes. Unlike the amorphous adsorbents studied earlier, which
contain a distribution of pore sizes, zeolitic
materials have uniform pore dimensions. This uniformity provides a
test of our interpretation of the number
of processes required in the isotherm fits of the multiple equilibrium
analysis, MEA. As expected from the
two pores in the structure of HZSM-5, most adsorbates require two
processes (K
1,ads and
K
2,ads) to fit the
adsorption isotherms. HZSM-5 is compared to amorphous carbonaceous
adsorbents, revealing fundamental
differences in their behavior. Small cylindrical channels in
HZSM-5 lead to an unfavorable entropic contribution
from restrictions imposed by adsorptive packing and interactions with
the channel walls. The pores of the
carbons studied (Drago, R. S.; Kassel, W. S.; Burns, D. S.; McGilvray,
J. M.; Lafrenz, T. J.; Showalter, S. K.
J.
Phys. Chem. B
1997, 101,
7548−7555) are slit-shaped, leading to less restriction and larger
equilibrium
constants. For several adsorbates, the greater enthalpic
interactions in the small HZSM-5 pores are accompanied
by lower equilibrium constants than in the larger pores because of
unfavorable entropic contributions. Finally,
the larger total micropore volumes of the carbons studied for these
adsorptives (Drago, R. S.; Kassel, W. S.;
Burns, D. S.; McGilvray, J. M.; Lafrenz, T. J.; Showalter, S. K.
J.
Phys. Chem. B
1997, 101,
7548−7555)
result in increased capacity compared to HZSM-5. The process
capacities from MEA (mol g-1) are
converted
to pore volumes using the molar volume. Surface areas are
calculated from molecular areas of the adsorbates.
Pore volumes and surface areas calculated from the process
capacities are compared to those from conventional
N2 porosimetry and are shown to provide a more detailed and
more accurate assessment of areas and volumes.
These results show that MEA has the potential of becoming a
standard characterization method for microporous
solids that will lead to an increased understanding of their behavior
in gas adsorption and catalysis.
“…b Handbook of Chemistry and Physics , 71st ed. ; CRC Press: Florida, 1991. c McClellan, A. L. Tables of Experimental Dipole Moments ; W. H. Freeman and Company: San Francisco, 1963; p 123. d Molar Volumes and cross-sectional areas calculated by ZINDO 15d …”
Section: Methodsmentioning
confidence: 99%
“…The adsorption data are fit to the MEA model (eq 1) where the total number of processes contributing to the adsorption isotherm is resolved 15a …”
This report investigates the causes of the unique properties and
reactivity of the titanium silicalite TS-1, a
commercial zeolite containing 1% Ti by weight. The combined
analysis of calorimetric and adsorption data,
cal−ad, for the reaction of TS-1 with pyridine shows the absence of
the strong acid site found in HZSM-5
and the presence of only 0.07 mmol g-1 of a
−15.1 kcal mol-1 hydrogen-bonding site.
Titrations with 2,6-lutidine indicate the same number of hydrogen-bonding sites with a lower
enthalpy (−9.8 kcal mol-1).
The
magnitude of these enthalpies indicate that the acceptor sites are
hydrogen-bonding sites that are comparable
in acidity to the strongest sites on silica gel. The small amounts
of these sites, 0.07 mmol g-1 compared
to
0.8 mmol g-1 in silica gel and 0.5 mmol
g-1 for the hydrogen-bonding sites of HZSM-5,
account for the
lower affinity of TS-1 for water. The absence of strong acidity
prevents epoxide ring opening and explains
the utility of TS-1 in epoxidation. Comparison of the results from
the multiple equilibrium analysis of gas-phase adsorption isotherms for CO and CH4 by TS-1, HZSM-5,
and silica gel indicates that the surface of
TS-1 is not more polarizable than the other solids. Thus, the
porosity and small number of hydrogen-bonding-sites enables TS-1 to adsorb hydrocarbons in the presence of
water.
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