Bifunctional isomerization and β-scission of alkanes involve kinetic cascades mediated by alkenes that form at a (de)hydrogenation function that resides within diffusion distances from the acid function at which alkenes react. Such distances matter for reactivity and selectivity, especially within voids of molecular dimensions, because of gradients in reactant and product concentrations within acid domains. Dispersing Pt nanoparticles within zeolite crystals places the two functions within nanometer distances, instead of the larger distances prevalent when the Pt function resides outside zeolite crystals. Such intimacy leads to higher n-heptane conversion turnover rates (per H+) and shifts selectivities towards primary isoalkanes, most evidently on medium-pore zeolites (MFI), because confinement effects lead to slower diffusion and higher intrinsic H+ reactivity than in larger-pore zeolites, and for shorter Pt-H+ distances, which define the dimensions of the acid domain. Intracrystalline Pt nanoparticles do not affect the number or intrinsic properties of the metal or acid functions or introduce reaction channels mediated by more reactive (but less abundant) diene intermediates, as demonstrated here by reactive probes of metal (CO oxidation) and acid (CH3OH dehydration) functions, infrared spectra of OH groups and chemisorbed CO, kinetic effects of H2 pressures, and transition state energies from density functional theory. Metal-acid distances influence the requisite kinetic cascades instead through the formation and scavenging of reactants and intermediates along the isomerization-scission sequence in the presence of significant gradients in their concentrations. Reaction-transport treatments show that such gradients weaken with increasing intracrystalline densities of Pt nanoparticles (and consequent smaller acid domains), thus circumventing isoalkene equilibration and local thermodynamic bottlenecks through hydrogenation of the primary isoalkene products. Such scavenging inhibits the formation of dimethylpentenes, which act as required precursors to β-scission products, leading to higher selectivities to isoalkanes as Pt-H+ distances decrease. These data and reaction-transport formalisms resolve persistent controversies about the chemical or diffusional origins of rate and selectivity consequences of the location of a metal function. They demonstrate the essential, but often neglected, need to determine the precise location of the metal function and to develop reaction-transport treatments to describe rates and selectivity in bifunctional metal-acid catalysis.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry