The role of crystal diversity in understanding mass transfer in nanoporous materials

Abstract

Nanoporous materials find widespread applications in our society: from drug delivery to environmentally friendly catalysis and separation technologies. The efficient design of these processes depends crucially on understanding the mass transfer mechanism. This is conventionally determined by uptake or release experiments, carried out with assemblages of nanoporous crystals, assuming all crystals to be identical. Using micro-imaging techniques, we now show that even apparently identical crystals (that is, crystals of similar size and shape) from the same batch may exhibit very different uptake rates. The relative contribution of the surface resistance to the overall transport resistance varied with both the crystal and the guest molecule. As a consequence of this crystal diversity, the conventional approach may not distinguish correctly between the different mass transfer mechanisms. Detection of this diversity adds an important new piece of evidence in the search for the origin of the surface barrier phenomenon. Our investigations were carried out with the zeolite SAPO-34, a key material in the methanol-to-olefins (MTO) process, propane-propene separation and adsorptive heat transformation.

Interplay between hydrophilicity and surface barriers on water transport in zeolite membranes

“…Recently, it has been reported that additional transport barriers (arising from contamination, pore blockages or pore collapse) occurring at the surface of zeolite crystals could potentially explain the substantial difference between effective (apparent) and intracrystalline diffusivity of hydrocarbons in zeolites212257. When the fluid diffuses through surfaces (‘barriers’) of dramatically decreased permeability, the overall measured mass transport properties in the porous material may significantly change.…”

“…In other words, the surface of nanoporous particles can be seen as a generally impermeable layer showing rare and homogeneously dispersed pore openings2160. In case of zeolites, the blockage or the narrowing of pores could arise from a variety of surface defects, mainly either the presence of large amorphous silica surface patches or local surface terminations that block pores’ entrance, although no direct structural evidence has been presented so far57.…”

“…By measuring guest molecule diffusion within individual crystallites (microimaging techniques) rather than crystallite assemblages (‘macroscopic’ methods), the intracrystalline diffusion resistance was found to be only a part of a more complex series of transport resistances, including barriers in the nanoporous particle and on its surface1921. In general, the transport diffusivities measured by ‘macroscopic’ methods are orders of magnitude lower than predictions from molecular simulations22.…”

Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal

“…We speculate that the interparticle differences arise due to structural heterogeneities, such as amorphous deposition or framework distortions that may strongly influence local surface mass transport and reactivity. 69 …”

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

“…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].…”