Subnanoscale composite, N2-permselective membrane for the separation of volatile organic compounds

A self-terminating low pressure chemical vapor deposition process utilizing opposing reactant geometry, for microengineering the subnancoscale pore structure of a tubular composite membrane filter, comprising a mesoporous borosilicate glass substrate and deposited microporous film predominantly of silicon oxide and optionally silicon carbide or silicon nitride for the purpose of optimizing the stress and other mechanical, chemical, and thermal properties of said composite membrane filter, having high permeability for the filtrate passing therethrough, and a high selectivity for matter retained thereby, as well as having permeability for nitrogen and selectivity for volatile organic compounds which may be entrained with said nitrogen, comprising:

• (a) providing a tubular, borosilicate glass, mesoporous substrate substrate having pore diameters in the range of from about 30 .ANG. to about 3000 .ANG. as well pore and other surfaces;
• (b) maintaining said substrate as well as other surfaces, both inside and outside thereof, at a temperature in the range of from about 350.degree. C. to about 650.degree. C., and a pressure in the range of from about 400 mTorr to about 600 mTorr;
• (c) contacting said inside surfaces, including pore surfaces, of said tubular mesoporous substrate with a reactant gas stream comprising an oxide of nitrogen having a kinetic diameter intermediate in size between the kinetic diameter of said matter retained by said composite membrane filter and the kinetic diameter of said permeant gas passing therethrough, said oxide of nitrogen comprising one or more members selected from the group consisting of nitrous oxide, nitric oxide, nitrogen dioxide, and nitrogen trioxide;
• and optionally, ammonia or reactive nitrogen-containing gas;
• and at the same time
• (d) contacting said outside surfaces, including pore surfaces, of said tubular microporous substrate with a reactant gas stream comprising diethylsilane (DES), SiH.sub.2 (C.sub.2 H.sub.5).sub.2 or di-t-butylsilane (DTBS), SiH.sub.2 ›C(CH.sub.3).sub.3 !.sub.2 ;
• (e) causing said reactant gas streams to meet within said pores of said tubular microporous substrate, to react, and to deposit thereon a reaction product which by coating the surfaces, including pore surfaces of said mesoporous substrate, substantially reduces the pore diameters thereof, until the pore diameters approach the size of the kinetic diameter of said oxide of nitrogen, at which time said reactant gas streams cannot make contact within the pores, and the reduction of the pore diameters of said tubular mesoporous substrate self-terminates, resulting in a substantially uniform microporous film having pore diameters determined by the kinetic diameter of said oxide of nitrogen in the rang of from about 4 .ANG. to about 7 .ANG.;
• said reaction product comprising predominantly silicon oxide, --Si--O--, and optionally including for the purpose of optimizing the stress, and other mechanical, chemical and thermal properties of said film, minor amounts of one or more members, in any combination thereof, selected from the group consisting of polysilicon, --Si--Si--, silicon oxide, --Si--O--, silicon carbide, --Si--C--, and silicon nitride, --Si--N--.