The active sites for propane dehydrogenation in Ga/H-ZSM-5 with moderate concentrations of tetrahedral aluminum in the lattice were identified to be Lewis-Brønsted acid pairs. With increasing availability, Ga and Brønsted acid site concentrations changed inversely, as protons of Brønsted acid sites were exchanged with Ga. At a Ga/Al ratio of 1/2, the rate of propane dehydrogenation was 2 orders of magnitude higher than with the parent H-ZSM-5, highlighting the extraordinary activity of the Lewis-Brønsted acid pairs. Density functional theory calculations relate the high activity to a bifunctional mechanism that proceeds via heterolytic activation of the propane C-H bond followed by a monomolecular elimination of H and desorption of propene.
The
transport of p-xylene in purely siliceous
MFI was investigated in the presence of coadsorbed m-xylene by combining fast time-resolved IR spectroscopy and frequency
response methods. p-Xylene transport rates are dominated
by diffusion in the straight channels of MFI, which is 8 times faster
than transport in the sinusoidal channels. m-Xylene
adsorbed on the outer surface lowers the sorption rate of p-xylene significantly by blocking pore entrances, whereas m-xylene adsorbed in the MFI pores hardly influences the p-xylene transport rates. If the pore entrance is the rate-determining
step of the transport process, as for the MFI samples investigated, m-xylene sorption on the outer surface selectively impedes
faster transport through the straight pore openings
The
influence of Brønsted acidic sites at the pore entrance
of MFI type zeolites on the transport of p-xylene
was investigated by comparing the diffusion rates on samples with
and without Al. The pore entrance rate is directly proportional to
the concentration of molecules on the external surface. Brønsted
acid sites lead to a higher sorption equilibrium constant of xylene
because of stronger binding by about 3 kJ·mol–1, compared to a neutral surface. The stronger binding energy reduces
the apparent but not the true energy of activation for the pore entrance
step of p-xylene, leading to an enhanced pore entrance
rate.
The
effect of adsorbed m-xylene on the p-xylene transport rate in MFI type zeolites with and without
Brønsted acidic SiOHAl groups was investigated with and without
isomerization taking place. At 373 K, p-xylene is
hindered entering the pores by competitive adsorption of m-xylene, causing a low p-xylene surface coverage.
The long residence time of m-xylene at pore openings
compared to p-xylene statistically blocks the entrance
for p-xylene under these conditions. The higher heat
of m-xylene adsorption (58 kJ·mol–1) compared to the activation energy of the p-xylene
pore entrance (18 kJ·mol–1 on MFI-27) strongly
depletes the m-xylene coverage with increasing temperatures.
At 453 K, the desorption rate constant of m-xylene
and of the pore entrance of p-xylene is comparable
and the p-xylene transport is not inhibited by m-xylene.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.