Abstract
Experimentally determined activation energies of propane dehydrogenation catalyzed by ZSM-5 zeolites have been used to test the SET theory. The basis of this theory is that the catalyst system transfers vibrational energy via a resonance process to a specific vibration mode of the reacting molecule. Being excited up to a certain number of vibrational quanta the molecule is brought to reaction. By analyzing the above-mentioned activation energies we found the wave number of this “specific mode” to be 1065 cm−1. This is very close to the rocking vibration of propane (1053 cm−1). We suggest that the propane molecule reacts when excited so that the CH3 group has been forced towards a flat structure with a carbon atom hybridization that is more sp2 than sp3. Consequently there is no way for three H-atoms to bind to the carbon and one of them must leave. This is the starting point of the reaction. The isokinetic temperature of the system was found as Tiso = 727 ± 4 K. From the SET formula for Tiso when both energy-donating (ω) and energy-accepting (ν) vibrations have the same frequency, viz., Tiso = Nhcν/2R, we obtain ν = ω = 1011 ± 6 cm−1. This agrees rather well with the CH3 rocking mode (1053 cm−1) and also with asymmetric “TO4” stretching vibrations of the zeolite structure (ω).
Highlights
The dehydrogenation of propane over zeolite catalysts is of great industrial importance as the product, propene, is used for further synthesis
Many attempts have been made to explain the reaction mechanism from ab initio calculations [1,2] The measurement of the temperature dependence of the reaction rate over ZSM-5 zeolites [3], makes it possible to directly determine the nature of the reacting species from the model of Selective Energy Transfer (SET), developed by the present author [4,5]
The method is described in detail in a recent review [5], but we can state here that the more vibrational quanta needed to excite the molecule to such a structural form that it fits into the structure of the catalyst, the larger is the activation energy
Summary
The dehydrogenation of propane over zeolite catalysts is of great industrial importance as the product, propene, is used for further synthesis. The basic idea of this model of catalytic reactions is that energy is transferred via a resonance effect from a vibration within the catalyst system (ω) to one specific vibration of the reacting molecule (ν). The rate of such an energy transfer is treated in classical physics and we put the rate of the chemical reaction equal to the sum of such transfers from one energy level to the up to a number of vibrational levels, so that the molecule is so excited that it reacts.
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