Kinetic Relevance of Hydrogen Desorption Steps and Virtual Pressures on Catalytic Surfaces during Reactions of Light Alkanes

Hydrogen removal steps limit alkane dehydrogenation reactions on cation-exchanged H-ZSM5 and cause desorption bottlenecks and the formation of hydrogen-rich reaction intermediates and products. For this reaction, the catalytic surface acts for all kinetic purposes as if it were in equilibrium with a H2 pressure greater than in the prevalent gas phase. The hydrogen chemical potential within adsorbed intermediates is described rigorously by a virtual H2 pressure, defined as that required to achieve the prevalent surface hydrogen content if adsorption-desorption steps were equilibrated. These virtual pressures can be measured from the deuterium content in products formed from C3H8/D2 reactants. H2 virtual pressures are high during propane reactions on H-ZSM5, because recombinative desorption of hydrogen atoms formed in C-H activation steps is slow. H2 virtual pressures decrease as Zn cations replace protons in H-ZSM5, because cations catalyze hydrogen adsorption-desorption steps and provide a kinetic path for communication between H2 in the gas phase and propane-derived reaction intermediates. As a result, the addition of H2 to C3H8 reactants decreases propane reaction rates and aromatics selectivity on Zn/H-ZSM5, causing it to resemble kinetically H-ZSM5. The hydrogen chemical potential on these catalytic surfaces and the hydrogen content within reactive intermediates reflect the rate at which hydrogen is formed in either C-H bond activation steps or in the dissociative chemisorption of gas-phase H2. The reaction pathways for species derived from each of these two H-sources are kinetically indistinguishable. Both sources contribute hydrogen atoms or hydrogen-rich species to the prevalent pool of reactive intermediates, the hydrogen content of which determines the rate and selectivity of all surface chemical reactions