Diffusion Measurements of Hydrocarbons in Zeolites with Pulse-Field Gradient Nuclear Magnetic Resonance Spectroscopy

Abstract

Pulse field gradient NMR technique was used to determine self-diffusivity of heptane and pentadecane at room temperature for microporous catalysts, used both as powders and shaped with a binder extrudates. The results showed that diffusivities increased with increasing specific surface area, micro-and mesopore volume of the studied catalysts. The presence of Bindzil binder together with H-Beta-25 decreased hydrocarbon diffusivities. Self-diffusivities of heptane and pentadecane were smaller for extrudates than for the powder catalysts. The detailed information about mass transfer limitations is needed to further process optimization since effective diffusivity is directly correlated with self-diffusion coefficients. The estimates of the ratio of porosity and tortuosity were also determined. The diffusion measurements with relatively long observation times Δ (20 up to 1000 ms) and fully immersed catalysts in pentadecane revealed that a small portion of sites exhibits very small diffusivities in H-Beta-25-Bindzil extrudates, which is correlated with a low ratio of mesopore to micropore volumes of this material.

Diffusion measurements of hydrocarbons in H-MCM-41 extrudates with pulsed-field gradient nuclear magnetic resonance spectroscopy

“…Although it has been observed that diffusivity apparently increases with the Lewis, Brønsted, and overall acidity of the powder catalyst, it can only be a reflection of the textural properties, which, like acidity, varied with the amount of a binder ( Table 1 ). This is in line with the previous observations 7,39 suggesting that strong interactions of acidic zeolites with the hydrocarbons can lead to lower diffusivities, 38 and, on the contrary, that heptane diffusivity increased with the increasing Lewis acidity. 7 No correlation with Lewis or Brønsted acidity was observed for pentadecane diffusion.…”

“…This is in line with the previous observations 7,39 suggesting that strong interactions of acidic zeolites with the hydrocarbons can lead to lower diffusivities, 38 and, on the contrary, that heptane diffusivity increased with the increasing Lewis acidity. 7 No correlation with Lewis or Brønsted acidity was observed for pentadecane diffusion. 7 …”

“…8a ) as was also observed for heptane and pentadecane diffusion in H-Beta-Bindzil catalysts. 7 Diffusivities also decrease with an increasing amount of the Bindzil binder for powder catalysts, while for extrudates diffusivity was constant. An increase of the effective diffusion coefficient was observed with an increase of the specific surface area ( Fig.…”

“…1 and ref. 7 , 25 ) while H-MCM-41 catalyst and its agglomerates with a binder ( Fig. 1 and 2 ) contained ca.…”

“…As H-MCM-41 porous materials have complex structure consisting of multitude of pore structure levels (extrudate → grain → particle → mesopores → micropores), the dependence of τ D on the properties of materials is complex. 19 Apart from few attempts to quantify the parameter τ D (or its inverse ξ = τ −1 D ) for extruded catalysts, 7 quantification of τ D for shaped catalysts, such as for extrudates or pellets, is lacking. Such materials contain not only micro- and mesopores, but also macropores by virtue of their preparation, enhancing thereby the transport properties in the longitudinal direction.…”