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

Abstract

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 H 2 pressure greater than in the prevalent gas phase. The hydrogen chemical potential within adsorbed intermediates is described rigorously by a virtual H 2 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 C 3 H 8 /D 2 reactants. H 2 virtual pressures are high during propane reactions on H-ZSM5, because recombinative desorption of hydrogen atoms formed in C-H activation steps is slow. H 2 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 H 2 in the gas phase and propane-derived reaction intermediates. As a result, the addition of H 2 to C 3 H 8 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 H 2 . 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.

Regioselective H/D exchange of propane on Zn/H-MFI zeolite

“…Haouas et al have suggested that the mechanism presented in scheme 1 is initialized by hydride abstraction from propane with strong Lewis sites of sulfated zirconia [20]. For the case with Zncontaining H-MFI, the dehydrogenation of propane molecule to form carbenium ion can occur either on zinc cations or Lewis sites of subnanomeric custers of ZnO due to prevalence of these two Zn species in the zeolite [1,23,[26][27][28][29].…”

Isolation of Rhenium and ReOx Species within ZSM5 Channels and their Catalytic Function in the Activation of Alkanes and Alkanols

“…Exchanged cations in ZSM-5 have been shown to increase dehydrocyclization rates by providing sites for the recombinative desorption of H* surface species. [48] The conversion of CH 4 to benzene on ReÀZSM5 has also been reported. [4] Here, we report CH 4 reaction rates as a function of the Re/Al ratio and compare them with those on Mo À ZSM5 and W À ZSM5 catalysts by using similar reaction conditions.…”

“…C 3 H 8 cyclodimerization constants (k 2 ) reflect the density and reactivity of residual acid sites combined with the ability of these active sites to remove hydrogen formed in the dehydrogenation steps required to form arenes; [48] measured k 2 values are in the range of (100-180) 10…”

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)2Ta(V)(NH)(NH2)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D2

“…A classical way to start probing the reactivity of a new complex is the investigation of its H/D exchange reactions: such a longstanding approach in the study of C-H and H-H bond activation and reaction mechanisms [17][18][19][20][21][22][23][24] is being extended, given the renewed interest in catalytic C-H activation [25][26][27] and novel applications of deuterium tags in biocatalysis [28][29][30][31] and analytical chemistry [32,33], leading to the current renaissance of the topic [34,35].…”