The Relationship Between Adsorption and Solid Acidity of Heteropolyacids

Journal Article · Fri Jul 15 04:00:00 EDT 2005 · Catalysis Today, 105(1):134-143

DOI:https://doi.org/10.1016/j.cattod.2005.04.014· OSTI ID:921864

Janik, Michael J; Davis, Robert J; Neurock, Matthew

The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The adsorption energy of a base to a solid acid catalyst is often assumed to correlate with the acid strength of the catalyst. In this study, the influences of adsorbate type, binding configuration and solid acid composition on the adsorption energy are explored using quantum chemical methods. In particular, density functional theory is used to calculate the adsorption energies of functionalized hydrocarbons containing O, N, or S heteroatoms or C C to phosphotungstic (H₃PW₁₂O₄₀) and phosphomolybdic (H₃PMo₁₂O₄₀) acids. The adsorption energies of the different molecules bound to the same solid acid are not easily predicted by the proton affinity of the adsorbate because the stabilization of the protonated adsorbate also varies with composition. Bond order conservation helps to explain the relatively small variance in adsorption energies among reactants of widely varying base strength. The activation barriers to form carbenium-ion transition states from adsorbed olefins are also calculated over the two heteropolyacids. The stronger adsorption of propylene to phosphotungstic acid compared to phosphomolybdic acid results in a higher activation barrier to form the carbenium-ion transition state. These heteropolyacids are predicted to have higher activation barriers than zeolites for carbenium-ion formation, which is typically thought to be the rate controlling step in many hydrocarbon conversion processes. This contrasts with the ranking of acid strength based solely on the magnitude of the adsorption energy.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 921864
• Journal Information: Catalysis Today, 105(1):134-143, Journal Name: Catalysis Today, 105(1):134-143 Journal Issue: 1 Vol. 105; ISSN 0920-5861; ISSN CATTEA
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
ADSORPTION
AFFINITY
ALKENES
CATALYSTS
CONFIGURATION
Environmental Molecular Sciences Laboratory
FUNCTIONALS
HYDROCARBONS
MOLYBDOPHOSPHORIC ACID
PH VALUE
PROPYLENE
PROTONS
STABILIZATION
TUNGSTOPHOSPHORIC ACID
ZEOLITES