(488f) Component Surface Concentrations of Gas Mixtures By Langmuir Theory and Ideal Adsorbed Solution Theory (IAST)

[Introduction]
Langmuir
theory is used in reaction kinetics to calculate the surface concentration or
coverage of each gas component on solid catalysts that are exposed to gas
mixtures. However, this theory is only consistent with thermodynamics when the
monolayer capacities are the same for all species [1]. In zeolites, there can
be spaces a small molecule can occupy but which are too constrained for a
bigger species or there are acidic sites where one species can adsorb but which
are too weak for a less basic species, which would cause the monolayer capacity
to differ for different species. Krishna and Baur [2] have shown using molecular simulation that the
different monolayer capacities of different hexane isomers in a MFI zeolite led
to erroneous reaction kinetics when Langmuir adsorption isotherms were used,
and that ideal adsorbed solution theory (IAST) should be used for surface
concentration calculations in place of Langmuir theory. In this study, we used
experiments with binary component mixtures to examine whether Langmuir theory
or IAST is better for surface concentration calculations. We compared the
estimated surface concentrations by Langmuir and IAS theories of binary
mixtures of methanol and dimethyl ether (DME) and C₃H₈ and
DME on a SAPO-34 catalyst with experimental measurements. Since the results are
of interest for the methanol-to-olefins (MTO) reaction, we also compared the
estimated surface concentrations by Langmuir and IAS theories of multicomponent
mixtures at 350°C.

[Experiment] Adsorption isotherms of pure gases and
binary component gas mixtures on a SAPO-34 zeolite at room temperature to 100°C were measured. Together with the adsorption isotherms, the differential heats of adsorption were also simultaneously measured with a heat flow microcalorimeter in
the volumetric manometry apparatus. The
total dead volume of the system was 40 ml, which allowed the composition of the
gas phase to be sampled by mass spectrometry (MS) after equilibrium was
achieved for each dosage. The MS-measured gas phase compositions were used to
experimentally verify the calculation of the individual surface concentrations
on the sample. The composition of the binary gas mixtures used were
methanol/DME = 63/37 [mole%] and C₃H₈/DME = 68/32 [mol%].
The sample used was a SAPO-34 catalyst powder synthesized using SiO₂/Al₂O₃
= 0.24, which had an acid site density of 1.0 mmol/g. The sample was first
pretreated at 400°C for 5 hours under a vacuum of 10^-5 Pa. After that, sample was kept at the adsorption temperature of 25, 60, 80 or 100°C.

[Results
and Discussion] First,
pure component adsorption isotherms of methanol, DME, C₃H₆,
and C₃H₈ were measured at 25, 60 and 100°C. The saturated surface concentration of methanol is quite different from those of the other adsorbates which differ slightly from one another. The saturation
coverage of methanol is approximately twice that of the other gases, which may
be due to the formation of methanol clusters stabilized by hydrogen-bonded
networks of the type reported for HZSM-5 [3]. These isotherms were fitted by
the single site Langmuir equation or, when it did not give a good fit, the dual
site Langmuir equation. The parameters obtained were used to calculate surface
concentrations of binary gas mixtures in the same temperature range using the
Langmuir and IAST theories to compare these with experimental measurements.
These parameters were also extrapolated to the MTO reaction temperature of 350°C to predict individual surface concentrations at 350°C by both Langmuir theory and IAST. In the methanol and DME adsorption experiments, some irreversible adsorption were observed, and a second measurement after
evacuation was performed to get an isotherm without irreversibly adsorbed
molecules for curve fitting. The component surface concentrations from the
methanol-DME mixture by the Langmuir theory and IAST showed significant
deviations, esp. at high pressures. The isotherm and differential adsorption
heat curve from methanol-DME binary mixture adsorption at 80°C are shown in Fig. 1, together with the corresponding curves for pure methanol and DME. The pure gas isotherms showed that the methanol saturation coverage is much larger than that
of DME. In the calorimetric curves, the initial heat of adsorption was 60
kJ/mole, which decreased to 45 kJ/mole when 0.5 mmole/g had been adsorbed for
all the gases used. After that, there was a plateau in the calorimetric curves,
which indicated the homogeneity of the adsorption sites in SAPO-34. With
methanol, DME and their mixture, falloffs in the heat of adsorption were then
observed at 6.5, 27 and 6.5 kPa, respectively, corresponding to surface
concentrations of 2.1, 1.0 and 1.5 mmole/g. The falloff in the heat of
adsorption indicated the saturation coverage at which regular chemisorption
sites were all occupied, and the remaining amount adsorbed after the falloff
began was attributed to adsorption on weak sites. This calorimetric trend
indicated that the dual site Langmuir equation should be used for fitting the
pure component isotherms. The fits are illustrated in Fig. 1a. Figure 2 shows the individual surface concentrations of methanol and DME measured in the mixture adsorption experiment, and the calculated amounts by Langmuir theory
and IAST. During adsorption from the gas mixture, there were constant increases
in methanol and DME surface concentrations up to 6.5 kPa where the calorimetric
curve began its falloff. At higher adsorption pressures, a DME surface
concentration decrease with increasing pressure was observed, i.e.,
there was desorption of previously adsorbed DME. A similar surface
concentration decrease with increasing adsorption pressure was simulated
by Krishna and Baur [2] who attributed it to entropy effects from
the adsorption of molecules with different saturation capacities. Fig. 2 shows
that IAST gave good calculated surface concentrations that also included the
desorption with increasing pressure, while there were significant deviations in
the calculated surface concentrations by Langmuir theory, esp. at higher
pressures where it failed to predict the desorption. This result may suggest
that Langmuir theory should not be used in the reaction kinetics, which was the
conclusion of Krishna and Baur. However, our results also showed that
deviations in the calculated surface concentrations by Langmuir theory were
significant only after the regular chemisorpion sites were saturated, that is,
the deviations appeared when adsorption on the weak adsorption sites became
significant. Further work is required to understand if these
sites are involved in the reactions.

[Conclusion] Component surface
concentrations from gas mixtures calculated using Langmuir theory and IAST that
were compared with experimental values showed that the calculated values by
Langmuir theory were wrong when the saturation coverages of the individual
gases were different and the total adsorbed amount from the mixture exceeded
the number of regular adsorption sites.

[References] (1) D.D. Do, Adsorption
Analysis: Equilibria and Kinetics, Imperial College Press, London, 1998. (2) R. Krishna, R. Baur, Chem. Eng. Sci. 60 (2005) 1155-1166. (3) C.-C. Lee, R. J. Gorte, W. E. Farneth, J. Phys. Chem.
B 101 (1997) 3811-3817.