We have developed force fields for the calculation of adsorption of NH3, CO2, and H2O on zeolite 4A by
performing Gibbs ensemble Monte Carlo simulations to fit experimental isotherms at 298 K. The calculated
NH3 and CO2 isotherms are in excellent agreement with experimental data over a wide range of temperatures
and several orders of magnitude in pressure. We have calculated isotherms for H2O in 4A using two different
models and have found that H2O saturates zeolite 4A even at pressures as low as 0.01 kPa for the range of
temperatures studied. We have studied the geometry of the adsorption sites and their dependence on loading.
At low pressures, CO2 molecules adsorb with their longitudinal axis pointing toward the center of the supercage,
whereas at higher pressures, the two oxygen atoms are equidistant from the Na atom in the binding site.
Infrared spectra have been recorded of solutions in liquefied
argon (85−115 K) containing both boron
trifluoride and carbon monoxide. The formation of complex species
was concluded from the appearance of bands
not attributable to the monomer molecules. Using concentration
variation, bands due to the 1:1 complex
BF3·CO
and to the 1:2 complex BF3·(CO)2 have been
identified. From temperature-dependant studies, the enthalpy of
formation
of the 1:1 complex was found to be −7.6 ± 0.3 and −14.5 ± 1.0
kJ mol-1 for the 1:2 complex. A very weak
band,
presumably due to a third complex, is tentatively assigned to the 1:1
complex BF3·OC in which CO is binding to the
boron atom via its oxygen atom. The solvent shifts of
ν2 and ν3 of BF3 and the
complexation shifts of these modes
are interpreted in terms of a simple induction model.
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