Composite inorganic membranes for gas separations : chemical vapor deposition of hydrogen permselective oxide membranes and preparation of supported zeolite NaA films

Citation

Tsapatsis, Michael (1994) Composite inorganic membranes for gas separations : chemical vapor deposition of hydrogen permselective oxide membranes and preparation of supported zeolite NaA films. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/RZP1-G920. https://resolver.caltech.edu/CaltechETD:etd-11102005-083452

Abstract

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Hydrogen-perselective membranes were synthesized by chemical vapor deposition of SiO2, TiO2, Al2O3, and B2O3 layers within the pores of Vycor tubes. The deposition involved reaction of SiCl4, etc., with water at 100-800 [degrees]C depending on the chloride and the reaction geometry. The reactants were passed either through the bore of the support tube (one sided geometry) or separately through the bore and the outside surface of the support tube (opposing reactants geometry). Permselective SiO2 and B2O3 layers could be formed in either the one-sided or the two-sided (opposing reactants) geometry, while deposition of TiO2 and Al2O3 layers was achieved only in the two-sided geometry. The permeances at 450 [degrees]C were 0.3 and 0.1 [...]-min-atm for SiO2 membranes produced in the one-sided geometry and two-sided geometries respectively. The H2:N2 permeance ratios were 500-5000. The TiO2 and Al2O3 membranes had somewhat lower permeance and H2:N2 ratios. Annealing at high temperatures causes densification of the deposited material as evidenced by increased activation energy for H2 permeation and correspondingly reduced permeance. The presence of H2O vapor accelerates the densification process. The densified membranes had a H2 permeance as high as 0.1 [...](STP)/min-atm-[...] at 500 [degrees]C and a H2/N2 permeance ratio above 500.

Silica, titania and alumina layers deposited in an opposing reactants geometry were characterized by scanning electron microscopy (SEM) and electron microprobe analysis (EPMA). The layers are asymmetric, having a long tail towards the side of the chloride flow and a sharp boundary at the other side. The deposit thickness is several tenths of microns while the totally plugged region is of order of one micron. Silica deposit layers prepared in the one-sided geometry were examined by transmission electron microscopy (TEM), SEM, and EPMA. When the deposit was confined inside the pores of the Vycor substrate, the membranes were mechanically stable, but when it extended substantially outside of the porous matrix, the stresses induced by thermal cycling led to crack formation and propagation. Electron microscopy revealed that the SiO2 deposit density is maximum in a region about 0.5 µm thick adjacent to the bore surface and gradually declines to zero within a depth of about 10 µm from the surface. The thin region of maximum deposit density is responsible for permselectivity, for it essentially blocks the permeation of nitrogen and larger molecules while allowing substantial permeation of hydrogen. This region contains about 10% by volume trapped voids and as a result has relatively high permeability as suggested by percolation theory.

A mathematical model is developed for SiO2 deposition in porous Vycor using SiCl4 hydrolysis. The model describes reaction, diffusion and evolution of the pore structure due to accumulation of the solid product. The deposition reaction is described by transient heterogeneous kinetics in terms of the concentrations of silanol and chloride groups in the product layer as well as the concentrations of the gaseous reactants. Pore structure evolution was modeled by incorporating results of percolation theory in a continuum model. The model is capable of generating deposit profiles in good agreement with the experiment. It is shown that for typical deposition conditions the pseudosteady state approximation for surface species could lead to erroneous predictions. Pore connectivity interruption at a nonzero void fraction leads to thinner deposits and shorter deposition times for pore plugging compared to the corresponding ones for an infinitely connected medium.

The growth of zeolite NaA films on various inorganic substrates from a clear solution with composition 10 Na2O - 0.2 Al2O3 - 1SiO2 - 200 H2O was followed by XRD, SEM and TEM. Deposition geometries and conditions are identified for the development of uniform polycrystalline thin (<5µm) films consisting of intergrown crystals of zeolite NaA on porous and nonporous Al2O3 disks. The films exhibit high crystallinity good adhesion to the substrate and thermal stability. The deposition technique demonstrates potential for developing molecular sieve membranes.

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