Monomolecular Cracking Rates of Light Alkanes over Zeolites Determined by IR Operando Spectroscopy

The coverage of H-MFI zeolite acid sites by light alkanes (C-3-C-7) at monomolecular cracking reaction conditions was determined using infrared operando spectroscopy. Under such conditions, alkane adsorption through H-bonding leads to a fully reversible perturbation of the zeolite nu OH band at 3600 cm(-1). This was used to assess the coverage at various temperatures and pressures, allowing for the determination of the adsorption thermodynamic parameters at reaction conditions. The simultaneous determination of apparent monomolecular cracking rate constants allowed for the direct determination of the intrinsic cracking rate constants, activation energies, and activation entropies. These results show that while the coverage of the active sites increases with the alkane size, the differences tend to decrease at high temperature because of entropic effects. The intrinsic activation energy was constant for all alkanes investigated in this study (similar to 190 kJ mol(-1)), lying in the lower range of the values usually derived from alkane adsorption heats and apparent activation energies. The magnitude of the activation entropies obtained in the present study was also lower than those derived from low temperature adsorption measurements, indicating that temperature could increase the entropy of the adsorbed state. However, this decrease was much less dramatic than that predicted by recent state of art simulations. In any case, this operando study confirms that the activation entropy chiefly determines the variations of apparent protolytic cracking rates.