Probing the Nature of Surface Barriers on ZSM‐5 by Surface Modification

Abstract

The nature of external surface barriers in the zeolite ZSM-5 was explored by comparing mass transfer of n-heptane in samples with different crystal sizes, surface structure, and composition. These different surface properties were achieved by HF acid etching and SiO 2 deposition on an as-synthesized ZSM-5 crystal sample. HF etching does not reduce surface barriers. SiO 2 deposition on the same sample reduces surface barriers significantly. These insights into the nature of surface barriers could be used to guide the rational design of zeolitic materials with better mass transport properties.

Understanding the Role of Internal Diffusion Barriers in Pt/Beta Zeolite Catalyzed Isomerization of n‐Heptane

“…[5] However,e ven when the diffusion path length is shortened to single-unit-cell thickness ( % 2nm), transport limitations could still persist, due to the presence of outer-surface and internal diffusion barriers. [6,7] Theo uter-surface diffusion barriers have been extensively probed by both experiments and simulations, [6][7][8][9][10][11][12][13][14][15][16] while the effects of internal diffusion barriers are much less well documented.Internal diffusion barriers were discovered when studying mass transfer in very large zeolite crystals (> 10 mm). [17][18][19][20][21] In these studies,abnormal mass transfer behavior was observed, e.g.,u nusual transient concentration profiles.T his behavior could only be explained when accounting for the presence of internal diffusion barriers.Such barriers exist on the internal interfaces between intergrowing components of zeolite crystals and originate from mismatches in structure and pore alignment.…”

“…[5] However,e ven when the diffusion path length is shortened to single-unit-cell thickness ( % 2nm), transport limitations could still persist, due to the presence of outer-surface and internal diffusion barriers. [6,7] Theo uter-surface diffusion barriers have been extensively probed by both experiments and simulations, [6][7][8][9][10][11][12][13][14][15][16] while the effects of internal diffusion barriers are much less well documented.…”

IR Microimaging of Direction‐Dependent Uptake in MFI‐Type Crystals

“…Inserting, for an order-of-magnitude estimate, L = 5 μm and D xy = 10 -12 m 2 s -1 (i.e., the values reported in [44] for diffusion perpendicular to longitudinal crystal extension) into Eq. While at the outer surface of the original crystals used here no surface barriers were evidenced for the uptake of isobutane [44], this finding might not hold for other batches, guest molecules or crystals of different size [18]. The chosen way of sample preparation turns out to be insufficient for their removal.…”

“…Such possibilities can be largely excluded by improving the fracturing procedure, e.g., by focused ion beam sectioning [67,68]. Even if this procedure did not always change the permeability of the outer surface ( [18] contains a systematic investigation and extended discussion on this subject), notable changes are expected when it is applied to the (template-free) fractured crystal face. Even if this procedure did not always change the permeability of the outer surface ( [18] contains a systematic investigation and extended discussion on this subject), notable changes are expected when it is applied to the (template-free) fractured crystal face.…”

“…Deviations of the host material from a regular crystal structure like the presence of stacking faults in the interior [14,15] or anomalies (surface barriers [16,17]) on their external surface are often found to be rate determining for mass transfer [18]. Deviations of the host material from a regular crystal structure like the presence of stacking faults in the interior [14,15] or anomalies (surface barriers [16,17]) on their external surface are often found to be rate determining for mass transfer [18].…”