Influence of Acid Sites on Xylene Transport in MFI Type Zeolites

Abstract

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.

Surface Effects Determining Transport in Binary Xylene Mixtures

“…The concentration of pore openings, determined from the external surface area and the crystallite geometry, is compiled in Table . , …”

“…Small-crystallite MFI type zeolites with a particle size of 150–200 nm and Si/Al ratios of 27 (MFI-27) and 34,000 (Si-MFI) were synthesized as described previously. , …”

“…The band at 1545 cm –1 (protonated pyridine) was integrated to determine the BAS concentration using a molar extinction coefficient of 0.73 cm 2 μmol –1 . The concentration of externally accessible BAS was quantified by mesitylene adsorption. , monitoring the decrease of the OH stretching vibration of SiOHAl groups (3608 cm –1 ).…”

“…Zeolite samples used for fast time-resolved IR spectroscopy studies were prepared by coating a CaF 2 infrared window with a suspension of the respective zeolite in isopropanol. Activation was done at pressures below 10 –6 mbar at 673 K for 1 h. , …”

“…It is, for example, unclear, how the adsorption strength, molecule diameter, diffusivity, and framework type influence multicomponent diffusion. The p -xylene pore entrance rate shows a first-order dependence on the concentration of externally adsorbed p -xylene for small crystallites where the rate determining step is the pore entrance . Under these conditions, diffusion is directly proportional to the concentration of pore openings at the external surface and the p -xylene surface concentration.…”