Carbocation Stability in H-ZSM5 at High Temperature.

Area of Science:

• Catalysis
• Materials Science
• Computational Chemistry

Background:

• Zeolites are vital industrial catalysts, particularly for alcohol dehydration to olefins.
• Proposed reaction mechanisms often involve carbocation intermediates, but their high-temperature stability in zeolites is debated.
• Previous studies suggested only tert-butyl cations are stable enough to form in zeolites.

Purpose of the Study:

• To investigate the stability and potential existence of carbocations in zeolites at high temperatures (∼500 °C).
• To compare the accuracy of ONIOM embedding methods versus periodic DFT for describing carbocation stability.
• To assess the relevance of carbocation intermediates in zeolite-catalyzed biomass upgrading.

Main Methods:

• Calculations using two ONIOM embedding methods (M06-2X and PBE-D3).
• Plane-wave density functional theory (DFT) with PBE functional, entropic, and van der Waals corrections.
• Comparison of relative energies between carbocations and alkoxides.

Main Results:

• ONIOM embedding methods are unreliable for primary carbocations but consistent with periodic DFT for secondary or higher carbocations.
• Significant discrepancies (0.23–14.30 kcal/mol) exist in relative energies between embedding and periodic DFT methods.
• At high temperatures, periodic DFT predicts significant populations (>10%) for tert-butyl and acetyl cations, while others are negligible.
• Mechanisms involving secondary/tertiary carbocations or those stabilized by adjacent oxygen/double bonds are possible in zeolites at high temperatures.

Conclusions:

• Periodic DFT is more reliable than tested ONIOM methods for studying carbocation stability in zeolites.
• Tert-butyl and acetyl carbocations can form and persist in zeolites at high temperatures relevant to biomass upgrading.
• Carbocation intermediates, though often minor species, can play a role in high-temperature zeolite-catalyzed reactions.