Isomorphously substituted molecular sieve membranes
View Patent ↗Zeolite membranes that can be used to continuously separate components of mixtures are disclosed. The zeolite membranes are prepared by isomorphous substitution, which allows systematic modification of the zeolite surface and pore structure. Through proper selection of the basic zeolite framework structure and compensating cations, isomorphous substitution permits high separation selectivity without many of the problems associated with zeolite post-synthesis treatments. The inventive method for preparing zeolite membranes is alkali-free and is much simpler than prior methods for making acid hydrogen zeolite membranes, which can be used as catalysts in membrane reactors.
1. A method of making an isomorphously substituted zeolite membrane, the method comprising:
preparing a porous support:
contacting the porous support with an aqueous zeolite-forming gel, the gel being substantially free of alkali hydroxides and comprising silica, a quaternary organic ammonium template, and a source of ions, wherein the ions are Al +3 , Ge +4 , Fe +3 , Ga +3 or B +3 or combinations thereof;
heating the support and the gel to form a zeolite layer at least in part on the surface of the porous support; and
calcining the zeolite layer to remove the template wherein the contacting and heating steps are repeated until the zeolite layer is substantially impermeable to N 2 .
2. The method of claim 1 , wherein the zeolite layer has a framework composition represented by the formula:
└( y 1 T 1 ·y 2 T 2 ·y 3 T 3 . . . )O 2(y 1 +y 2 +y 3 + . . . )┘;
wherein T 1 is tetrahedrally coordinated Si,
T 2 is a tetrahedrally coordinated element and is B, Ge, Ga or Fe, or combinations thereof,
T 3 is tetrahedrally coordinated Al, y 1 , y 2 , and y 3 , are stoichiometric coefficients, and T 1 /T 2 is between about 12 and about 600.
3. The method of claim 1 , further comprising washing and drying the zeolite layer.
4. The method of claim 1 , wherein the porous support is a container having at least one opening and an inner surface.
5. The method of claim 4 , further comprising placing the gel in the container and capping the at least one opening of the container.
6. The method of claim 1 , wherein calcining includes maintaining the porous support and the gel at a temperature between about 403 K and about 469 K for at least about two hours.
7. The method of claim 1 , wherein an acid hydrogen form of the isomorphously substituted zeolite membrane is synthesized directly from the gel.
8. The method of claim 1 , wherein the quaternary organic ammonium template is tetrapropyl ammonium hydroxide, tetrapropyl ammonium bromide, tetrabutyl ammonium hydroxide, tetrabutyl ammonium bromide, tetraethyl ammonium hydroxide or tetraethyl ammonium bromide or combinations thereof.
9. The method of claim 1 wherein the source of ions comprises at least one of Al(i-C 3 H 7 O) 3 , Ge(C 2 H 5 O) 4 , Fe(NO 3 ) 3 , and B(OH) 3 .
10. The method of claim 1 wherein the ions are Al +3 .
11. The method of claim 1 wherein the ions are Ge +4 .
12. The method of claim 1 wherein the ions are Fe +3 .
13. The method of claim 1 wherein the ions are Ga +3 .
14. The method of claim 1 wherein the ions are B +3 .