(491e) Simulation of the Diffusion-Limited Deposition of Diethylzinc in a Mesoporous Alumina Substrate

Synthesis of zeolitic imidazole framework-8 (ZIF-8) membranes with high propylene selectivity over propane and high propylene permeance has been accomplished through the conversion of impermeable zinc oxide (ZnO) deposits within a mesoporous membrane substrate [1]. In contrast to typical atomic layer deposition (ALD) processes in porous media, which achieve conformal coatings by using long precursor pulses, the ALD process for producing these membranes employs short precursor pulses to limit the precursor penetration depth, thereby minimizing the ZnO layer thickness and maximizing the resulting membrane permeance.

A reaction-diffusion model using finite differences was developed to describe the deposition of diethylzinc (DEZ) precursor in a mesoporous γ-Al₂O₃ substrate at a nominal DEZ pulse time of 15ms. A spatiotemporal distribution of DEZ in a commercial ALD reactor obtained by a computational fluid dynamics model was used as a boundary condition in the model. The reaction rate constants were fit such that the modelled penetration depth (~200 nm) matched that of a membrane cross-section determined using EDX [1]. The modelled concentration of Zn species as a function of depth agreed well with the experimentally measured concentration profile. The process was determined to be diffusion-limited due to the higher rate of adsorption of DEZ molecules to open hydroxyl sites relative to surface diffusion of DEZ within the substrate. The spatiotemporal distribution of DEZ during the pulse was decoupled from the reaction-diffusion process, as the DEZ flux into the substrate had a negligible effect on the concentration of DEZ within the reactor. The results of this model allow for improvements in the design of thin film membrane formation processes relying on ALD.

Reference

[1] X. Ma, P. Kumar, N. Mittal, A. Khlyustova, P. Daoutidis, K. A. Mkhoyan, M.Tsapatsis, Science (80-. ). 2018, 361, 1008 LP – 1011.