Abstract:
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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/177,542, filed Jan. 21, 2000, which is herein incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to membranes having molecular sieve properties and/or catalytic activity and to methods for producing and using the membranes, and more particularly, to isomorphously substituted zeolite membranes and their use in selective separations of molecules and in catalytic membrane reactors. 
     2. Discussion 
     Zeolites are crystalline aluminosilicates of Group 1 and Group 2 elements. Their basic structural framework can be viewed as a three-dimensional network of SiO 4  and [AlO 4 ] −  tetrahedra, which are linked by oxygen atoms. The structural framework encloses cavities and defines channels or pores that are substantially uniform in size within a specific zeolite. As discussed below, large ions (compensating cations, M) and water molecules occupy some of the cavities and have considerable freedom of movement within the zeolite lattice, which allows zeolites to perform ion exchange processes and reversible dehydration. 
     Because the zeolite pores are sized to accept molecules of certain dimensions for adsorption while rejecting molecules of larger dimensions, these molecules have come to be known as “molecular sieves.” Zeolites have been used commercially in ways that that take advantage of these properties, including adsorption separation processes and shape-selective catalytic processes. 
     Most commercial applications use zeolites in the form of granules or pellets. Zeolite granules exhibit high porosity and have a uniform pore size between about 0.3 and 1.2 nm that is dependent on the specific zeolite structure. Such granules are the catalysts of choice for the petrochemical industry. Shape-selective effects are possible because the catalytic sites are accessible only within the pores of a zeolite structure, and only those reactant molecules, transition states, intermediates, and/or product molecules with dimensions below a certain critical size can be adsorbed into this pore system. Shape-selective catalysis combines the molecular sieving effect with a catalyzed reaction. 
     Recently, zeolite membranes have been used to conduct molecular separations. Generally, a membrane can be defined as a semi-permeable barrier between two phases that is capable of restricting the movement of molecules across it in a very specific manner. The semi-permeable nature of the barrier is essential to obtaining an effective separation. A wide variety of molecular materials, mostly organic polymers, have been found to be suitable for use as membranes. However, organic polymer membranes have relatively short service lives because of their sensitivity to solvents and low stability at high temperatures. 
     Because of their superior thermal, chemical, and mechanical properties, zeolite membranes have substantial advantages over organic polymer membranes. The pore size is uniform within a specific zeolite material, and the pore size of a zeolite membrane can be synthetically tuned by choosing an appropriate zeolite structure and/or by exchanging compensating cations of different diameters. The hydrophilic/hydrophobic nature of a zeolite can be modified by changing the substituted metal (Me) in the framework and the Si/Me ratio. The basic/acidic nature of the zeolite can be modified by exchanging alkaline cations with protons. Moreover, zeolite membranes can be used for catalytic membrane reactors because they combine heterogeneous catalytic sites with membranes that allow only one component of a mixture to selectively permeate across the membrane. Zeolites can be considered as originating from a SiO 2  lattice in which Al 3+  is isomorphously substituted for a portion of tetrahedrally coordinated Si 4+ , and can be represented by the formula: 
       M   x/n ·[(AlO 2 ) x ·(SiO 2 ) y   ]·zH   2   O   I 
     where M represent a compensating cation with valence n, y is a number greater than or equal to x, and z is a number between about 10 and 10,000. In an isomorphous substitution, a second (different) element replaces some (or all) of an original element of the crystalline lattice. The second element has similar cation radius and coordination requirements as the original element so that the same basic crystalline structure is maintained. 
     Because aluminum is trivalent, every tetrahedral [AlO 4 ] unit carries a negative charge. Consequently, the substitution of aluminum for silicon generates an excess negative charge in the zeolite lattice that must be compensated by cations. These compensating cations may be exchangeable. Accordingly, the ion-exchange capacity of a zeolite is enhanced as the aluminum content is increased. Acid hydrogen forms of zeolites have protons that are loosely attached to their framework structure in lieu of inorganic compensating cations, and these proton sites function as Brönsted acids. Thus, the number of protons that may be attached to the zeolite framework is greater in zeolites having greater aluminum content. Consequently, increases in the aluminum content of a zeolite can result in additional Brönsted acid sites. Zeolites having additional catalytic sites exhibit greater activity in acid catalyzed reactions. Thus, the ion exchange and the catalytic properties of a specific zeolite depend on its chemical composition and, more particularly, on its Si/Al ratio. 
     Zeolites represented by formula I are often described in terms of their Si/Al ratio, because certain properties of zeolites appear to vary with Si/Al ratio. In an extreme case in which substantially all of the lattice ions are silicon, zeolites can have Si/Al ratios that approach infinity (e.g., silicalite-1). Such zeolites do not have a net negative framework charge and therefore do not contain compensating cations. As a consequence, these zeolites have no ion exchange capacity, cannot be acidic, and exhibit a high degree of hydrophobicity. These highly siliceous zeolites are organophilic and have been used for the selective adsorption of volatile organic compounds. Zeolites with Si/Al ratios as low as 0.5 have also been made (e.g. bicchulite). 
     With zeolite membranes, separation is thought to occur through at least three different, nonexclusive mechanisms, which are based on differences in component diffusion, on molecular sieving or size exclusion, and on preferential adsorption. Thus, two or more different types of molecules may access the pore system of the zeolite membrane, but their diffusion rates through the pores may vary because each type of molecule interacts differently with the zeolite surface and pore structure. Additionally, molecular sieving may occur when one type of molecule can access the zeolite membrane pore system, but a different type of molecule cannot because of its larger size. Finally, the pore system of the zeolite membrane may preferentially adsorb a first molecule, which blocks entry of a second, different molecule into the pore system. Because molecules with different sizes and shapes have different diffusivities, high separation selectivities have been reported for n-C 4 H 10 /i-C 4 H 10 , and n-C 6 H 14 /3-methyl pentane mixtures. Likewise, high separation selectivities based on molecular sieving were obtained for CH 4 /i-C 8 , n-C 6 /2,2 dimethylbutane, and p-/o-xylene mixtures. Selectivities have also been attributed to differences in adsorption properties. 
     It is important to recognize that adsorptive separation processes on granular molecular sieves are two-step batch processes involving successive adsorption and desorption of molecules. In contrast, membrane separations are continuous processes that are accomplished by applying a driving force across the membrane (e.g., pressure gradient, concentration gradient, or temperature gradient). Thus, membrane separations do not require regeneration of the active sites in the membrane by desorption. Instead, a vapor-phase feed stream is continuously applied to one side of the membrane while purified product is continuously removed from another (permeate) side of the membrane. Because zeolite membranes allow continuous separation of multi-component mixtures, they offer significant advantages over zeolite granules, including less capital expenditure for equipment and fewer processing steps. 
     Despite the perceived advantages of zeolite membranes, their use in separations and catalysis poses significant challenges. Because their ability to separate molecules depends on surface properties and pore structure, which can vary significantly among different types of zeolites, many zeolite membranes demonstrate limited selectivity for separating mixtures of molecular components. Previous attempts to improve membrane performance have met with limited success. For example, post-synthesis treatments such as CVD modification or coke deposition may block access to the zeolite pore system and/or reduce pore entrance diameters, thereby decreasing flux through the membrane. 
     Although the acid hydrogen form of zeolite membranes is useful for catalytic membrane reactors, synthesis of acidic zeolite membranes is a complex process. Conventional synthesis of acid zeolite membrane requires the use of alkali metal hydroxides. Subsequent steps involve acid treatment or ion exchange with an ammonium salt solution, followed by thermal decomposition of the ammonium ion to obtain the acid hydrogen form of zeolite membranes. 
     The present invention overcomes, or at least mitigates, one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     The present invention provides zeolite membranes that can be used to continuously separate components of mixtures. 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. 
     To achieve the foregoing and other objects, one aspect of the present invention provides a membrane comprising a layer of an isomorphously substituted zeolite. The isomorphously substituted zeolite membrane can be represented by the formula: 
     
       
           x   1   M   1   n1+   ·x   2   M   2   n2+ ·[( y   1   T   1   ·y   2   T   2   ·y   3   T   3  . . .)O 2(y     1     +y     2     +y     3     + . . .)   ]·z   1   A   1   ·z   2   A   2  . . . ;  II 
       
     
     wherein T 1  is tetrahedrally coordinated Si, T 2  is a tetrahedrally coordinated element and is B, Ge, Ga or Fe or combinations thereof In addition, T 3  is tetrahedrally coordinated Al, M 1  and M 2  are compensating cations having valences n1 and n2, respectively, A 1  and A 2  are adsorbed species located within the zeolite, and x 1 , X 2 , y 1 , Y 2 , Y 3 , Z 1 , and Z 2  are stoichiometric coefficients. The present invention also contemplates an acid hydrogen form of the isomorphously substituted membranes having protons attached to the zeolite framework in lieu of inorganic compensating cations. 
     The membranes of the present invention can be used in numerous processes, including component separations based on at least one molecular property selected from size, shape, and polarity. In particular, the claimed membranes are capable of separating non-condensable gaseous mixtures, condensable organic vapors, water from a mineral acid solution, and one or more components of aqueous organic mixtures. In addition, some of the claimed membranes can be used to catalyze chemical reactions. 
     The surface properties and the pore structure of the zeolite membranes can be altered by appropriate selection of membrane components, allowing superior separations for a wide variety of mixtures. In one embodiment, the zeolite membrane is substantially free of alkali metal hydroxides. In another embodiment, y 3  is substantially equal to zero, and the zeolite membrane is substantially free of aluminum. The ratio of T 1 /T 2  is generally between about 12 and about 600, and more typically, between about 12.5 and about 100. 
     Another aspect of the present invention provides an article of manufacture comprising a porous support and a membrane layer disposed on the porous support. The membrane layer comprises an isomorphously substituted zeolite having a composition that can be represented by formula II described above. The membrane may be substantially free of aluminum, with y 3  of formula II substantially equal to zero, and may be formed in-situ on and within the pores of the support. In one embodiment, the porous support has the form of a container, and the membrane is disposed on the interior surface of the container. Useful porous supports include tubes made of stainless steel, α-alumina, or β-alumina. 
     A further aspect of the present invention provides an apparatus for separating one or more components from a mixture. The apparatus includes at least one membrane unit, a device for introducing the multi-component mixture into the membrane unit, and a device for removing the components from the membrane unit. The membrane unit includes a porous support and a membrane layer disposed on the porous support. The membrane layer comprises an isomorphously substituted zeolite having a composition that can be represented by formula II. The apparatus may include a plurality of membrane units to enable rapid processing of large volumes of a multi-component feed. 
     Still another aspect of the present invention provides a method of making an isomorphously substituted zeolite membrane. The method includes preparing a porous support and contacting the porous support with an aqueous zeolite-forming gel. The gel is substantially free of alkali hydroxides and includes silica, a quaternary organic ammonium template, and a source of ions. Useful ions include Al +3 , Ge +4 , Fe +3 , Ga +3  or B +3  or combinations thereof. The method also includes heating the support and the gel to form (crystallize) a zeolite layer on the porous support, and calcining the zeolite layer to remove the template. The composition of the resulting zeolite layer can be represented by formula II. 
     In one embodiment of the method, the porous support is a container having at least one opening and an inner surface, and the gel is placed inside the container. During the heating step, additional gel may be placed in the container, and the container is sealed prior to heating. The heating step may be repeated one or more times to obtain a zeolite layer on the support that is substantially impermeable to nitrogen. Acidic ZSM-5 membranes can be obtained directly without additional steps involving ion exchange or acid treatment when the synthesis gel is substantially free of alkali metal hydroxides. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates a useful system for separating components of a mixture. 
     FIG. 2 shows a cross-sectional view of an embodiment of a membrane module. 
     FIG. 3 shows a partial plan view of a selectively permeable portion of a membrane assembly. 
     FIG. 4 shows a block diagram of a method of making a zeolite membrane layer through in-situ synthesis on an inner surface of a porous support. 
     FIG. 5 shows a schematic view of an apparatus that can be used to characterize zeolite membranes by measuring single-gas and/or multi-gas permeation rates at various temperatures. 
     FIG. 6 shows n-C 4 H 10 /i-C 4 H 10  separation selectivity as a function of temperature for three B-ZSM-5 membranes on stainless steel supports with different Si/B molar ratios as indicated and for a silicalite-1 membrane on a stainless steel support. 
     FIG. 7 shows n-C 4 H 10 /i-C 4 H 10  separation selectivity as a function of temperature for three alkali free B-ZSM-5 membranes on α-alumina supports with different Si/B molar ratios as indicated. 
     FIG. 8 shows n-C 4 H 10 /i-C 4 H 10  mixture permeance as a function of temperature for two alkali free B-ZSM-5 membranes on stainless steel and α-alumina supports; both membranes have Si/B molar ratios of 100. 
     FIG. 9 shows n-C 4 H 10 /i-C 4 H 10  mixture permeance as a function of temperature for two alkali free B-ZSM-5 membranes on stainless steel and α-alumina supports as indicated; each membrane has a Si/B molar ratio of 12.5. 
     FIG. 10 shows n-C 4 H 10 /i-C 4 H 10  mixture permeance as a function of temperature for three alkali free B-ZSM-5 membranes prepared under identical conditions on stainless steel supports; each membrane has a Si/B molar ratio of 100. 
     FIG. 11 shows n-C 4 H 10 /i-C 4 H 10  mixture permeance and separation selectivity as functions of time for an alkali free B-ZSM-5 membrane prepared on an α-alumina support and having a Si/B molar ratio of 12.5; measurements were taken at 473 K. 
     FIG. 12 shows n-C 4 H 10 /i-C 4 H 10  separation selectivity as a function of temperature for alkali free B-ZSM-5, Al-ZSM-5, and silicalite-1 membranes having various Si/Me molar ratios as indicated. 
     FIG. 13 shows n-C 4 H 10 /H 2  separation selectivity as a function of temperature for silcalite-1 and substituted ZSM-5 zeolite membranes prepared on stainless steel supports. 
     FIG. 14 shows H 2 /i-C 4 H 10  separation selectivity as a function temperature for silcalite-1 and substituted ZSM-5 zeolite membranes prepared on stainless steel supports. 
     FIG. 15 shows n-hexane/2,2-DMB permeance and separation selectivity as functions of temperature for B-ZSM-5 zeolite membranes prepared on alumina and stainless steel supports. 
     FIG. 16 shows n-hexane/2,2-DMB permeance and separation selectivity as functions of temperature for silicalite-1 and B-ZSM-5 zeolite membranes prepared on stainless steel supports. 
     FIG. 17 shows p-xylene and o-xylene steady state fluxes as functions of temperature for B-ZSM-5 zeolite membrane BZ1 and a feed partial pressure of 2.1 kPa per isomer. 
     FIG. 18 shows p-xylene/o-xylene steady state separation selectivity as a function of temperature for B-ZSM-5 zeolite membrane BZ1 and a feed partial pressure of 2.1 kPa per isomer. 
     FIG. 19 shows flux of p-xylene as a function of temperature for B-ZSM-5 zeolite membrane BZ2 and various feed partial pressures. 
     FIG. 20 shows flux of o-xylene as a function of temperature for B-ZSM-5 zeolite membrane BZ2 and various feed partial pressures. 
     FIG. 21 shows separation selectivity for p-xylene/o-xylene mixtures as a function of temperature for B-ZSM-5 zeolite membrane BZ2 membrane and various feed partial pressures. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 schematically illustrates a useful system  100  for separating one or more components of a condensed-phase mixture  102  using isomorphously substituted ZSM-5 zeolite membranes. Although the system  100  shown in FIG. 1 has been designed to separate components of liquid mixtures by pervaporation, it can be modified to separate mixtures comprised of vapor-phase components by vapor permeation as well (compare FIG.  5 ). 
     The system  100  includes a membrane module  104  having an inlet  106 , a first outlet  108 , and a second  110  outlet. A metering pump  112  drives the mixture  102  from a reservoir  114  to the membrane module  104  inlet  106  through a first conduit  116 . A section  118  of the conduit  116  upstream of the membrane module  104  inlet  106  is optionally wrapped in heating tape to preheat the mixture  102 . As described below, the membrane module  104  includes a zeolite membrane (not shown) that separates the feed stream  120  into a vapor-phase permeate stream  122 —the portion of the feed stream  120  that passes through the zeolite membrane—and a liquid-phase retentate stream  124 . The permeate  122  and the retentate  124  exit the module  104  through the first  108  and second  110  outlets, respectively. The retentate  124  returns to the reservoir  114  via the first conduit  116 . Component concentrations in the feed  120  and the permeate  122  streams can be measured by gas chromatography (GC), high-pressure liquid chromatography (HPLC), or by GC and HPLC. 
     As can be seen in FIG. 1, a vacuum pump  126  communicates with the first outlet  108  of the membrane module  104  via a second conduit  128  and provides a pressure drop, which drives the permeate  122  through the zeolite membrane. An electronic gauge  130  monitors the pressure in the second conduit  128 , which splits into a pair of conduits  132 ,  134  downstream of the pressure gauge  130 . Each of the conduits  132 ,  134  thermally contacts separate cold traps  136 ,  138 , which condense the permeate  122  flowing within the second conduit  128 . As depicted in FIG. 1, condensed-phase permeate  140  collects in the bottoms  142 ,  144  of U-shaped tubes  146 ,  148  that are immersed in liquid nitrogen baths  150 ,  152 . The U-shaped tubes  146 ,  148  comprise a portion of the permeate flow path between the first outlet  108  of the membrane module  104  and the vacuum pump  126 . The system  100  also includes numerous valves  154 ,  156 ,  158 ,  160 ,  162 ,  164 , which isolate the pressure gauge  130 , the cold traps  136 ,  138 , the vacuum pump  126 , and the reservoir  114 . 
     Prior to separation, the vacuum pump  126  evacuates the permeate  122  side of the membrane module  104 . Once the permeate side  122  of the membrane module  104  reaches a desired vacuum level, e.g., about 200 Pa absolute pressure, the valve  162  closes the fluid connection between the vacuum pump  126  and the first outlet  108  of the membrane module  104 . Because condensed-phase permeate  122  occupies little volume, the vacuum level, as indicated by the electronic pressure gauge  130 , ordinarily should change little—a few hundred Pa, say—during a pervaporative separation. In some cases, however, non-condensable gases (e.g., nitrogen, oxygen, etc.) may enter the permeate  122  stream via the feed  120  stream or through leaks in the system  100 . Over time, these gases may accumulate, reducing the vacuum level or increasing absolute pressure in the permeate side  122  of the membrane module  104 . In such cases, the vacuum pump  126  and valve  162  can be cycled to remove the non-condensable gases. 
     FIG. 2 shows a cross-sectional view of an embodiment of the membrane module  104 . The membrane module  104  includes a tubular membrane assembly  170 , having an elongated, selectively permeable portion  172 , which is connected at its ends  174 ,  176  to a pair of rigid, tubular end supports  178 ,  180 . The membrane assembly  170  is retained within a shell  182  made of brass, stainless steel, or other rigid and chemically resistant material. The shell  182  includes a body portion  184  and a pair of removable end caps  186 ,  188 . The end supports  178 ,  180  of the membrane assembly  170  are substantially impermeable to fluids. The end supports  178 ,  180  provide sealing surfaces  190 ,  192  for o-rings  194 ,  196  that are captured in grooves  198 ,  200  formed by opposing chamfered surfaces  202 ,  204 ,  206 ,  208  of the shell&#39;s  182  body portion  184  and end caps  186 ,  188 , respectively. The o-rings can be made of any inert material, including silicone-based polymers, and fluorinated elastomers such as polytetrafluoroethylene (PTFE), vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, and the like. Clamps, threaded fasteners, and the like (not shown) provide an axial compressive force sufficient to seal the o-rings  194 ,  196  against the shell  182  and the end supports  178 ,  180  of the membrane assembly  170 . 
     As indicated by an arrow  210  shown in FIG. 2, during pervaporation the feed stream  120  enters the membrane module  104  through the inlet  108 , and passes into a cavity  212  formed by an inner surface  214  of one of the end caps  186 . From the cavity  212 , the feed stream  120  enters an interior portion  216  of the membrane assembly  170 . There, one or more feed  120  components flow radially through the selectively permeable portion  172  of the membrane assembly  170 , as vapor, and collect in a cavity  218  formed by an inner surface  220  of the shell  182  and an outer surface  222  of the membrane assembly  170 . Components that cannot pass through the selectively permeable portion  172  of the membrane assembly  170  remain in the liquid phase, and flow axially into a cavity  224  formed by an inner surface  226  of the second end cap  188 . As shown by arrows  228 ,  230  the resulting permeate  122  and retentate  124  streams exit the membrane module  104  through the first  108  and second  110  outlets. 
     FIG. 3 shows a partial plan view of the selectively permeable portion  172  of the membrane assembly  170 . As can be seen in a cutaway  250  of the membrane assembly  170 , the selectively permeable portion  172  includes a zeolite membrane  252  layer or film disposed on an inner surface  254  of a porous support  256  layer. An outer surface  258  of the membrane layer defines the interior portion  216  of the tubular membrane assembly  170 . The porous support  256  should be able to carry the zeolite membrane  252  layer and should be able to resist chemical attack by the components of the feed stream  120 . As described below, the porous support  256  should also be able to withstand reaction conditions during preparation of the zeolite membrane  252  layer. The porous support  252  typically has an average pore size similar to or larger than the pore size of the zeolite membrane  252  layer. Useful porous supports  256  include stainless steel, α-Al 2 O 3 , γ-Al 2 O 3 , SiC, SiN 3 , SiO 2 , TiO 2 , ZrO 2  and other inorganic oxides. Alumina supports are commercially available from a variety of vendors. Stainless steel supports are mechanically robust and are particularly useful for separating acidic mixtures because they can withstand attack by concentrated acids. 
     Although the membrane assembly  170  shown in FIG.  2  and FIG. 3 is generally cylindrical, the membrane assembly  170  can assume any convenient shape. For example, the membrane assembly  170  can comprise one or more planar layers, or can have a cross-section normal to the retentate flow that is generally oval or polygonal. Furthermore, the membrane module  104  shown in FIG. 1 may include more than one tubular membrane assembly  170 , which can be connected in parallel to the feed  120 , permeate  122 , and retentate  124  streams. 
     The zeolite membrane  252  layer provides a semi-permeable barrier between the liquid-phase retentate  124  stream and the vapor-phase permeate  122  stream within the membrane assembly  170 . As a “semi-permeable barrier,” the zeolite membrane  252  layer is capable of selectively restricting the movement of molecules through the layer  252 , which is essential to obtain an effective separation of the components of the feed stream  120  by pervaporation. The zeolite membrane  252  layer&#39;s ability to separate components of the feed stream  120  depends, at least in part, on the particular zeolite&#39;s pore system, surface properties, and hence chemical structure. 
     Useful zeolites include silicalite-1, ZSM-5, and zeolite analogues having a SiO 2  crystalline lattice in which one or more elements other than aluminum have been isomorphously substituted for some of the tetrahedrally coordinated Si 4+ . These zeolite analogues can be represented by the formula: 
     
       
           x   1   M   1   n1+   ·x   2   M   2   n2+ ·[( y   1   T   1   ·y   2   T   2   ·y   3   T   3  . . .)O 2(y     1     +y     2     +y     3     + . . .)   ]·z   1   A   1   ·z   2   A   2  . . . ;  II 
       
     
     where the expression in brackets corresponds to the framework composition and other terms represent species that reside in the pores of the framework structure. In formula II, M 1  and M 2  are compensating cations with valences n 1  and n 2 , respectively; T 1 , T 2 , and T 3  are elements occupying the tetrahedral positions of the framework; A 1  and A 2  are adsorbed species located within the porous framework; and x 1 , x 2 , y 1 , Y 2 , z 1 , and z 2  are stoichiometric coefficients. In general, the bracketed quantity will have a negative charge and T 3  is nonzero. 
     Because a metal or metalloid species (Me) has been isomorphously substituted into tetrahedral positions of the zeolite framework, it is more useful to describe the zeolites represented by formula II using a Si/Me ratio rather than a Si/Al ratio. Alternatively, this ratio can be expressed in accordance with formula II, above, as a T 1 /T 2  ratio, where T 1  is Si and T 2  is B, Ge, Ga, Fe, or Al. Useful zeolites also include those having more than one isomorphously substituted element incorporated into the zeolite framework structure. Although zeolites traditionally have been defined to include only those aluminosilicates having an ordered, three-dimensional microporous structure, as used herein, the term “zeolite” also includes zeolite analogues having metals other than aluminum that are isomorphously substituted at the tetrahedral sites. 
     Isomorphous substitution has been shown to affect the surface properties and the pore structure of zeolites. For example, silicalite-1 and ZSM-5 have MFI structure, but silicalite-1 is composed of pure silica while ZSM-5 has aluminum substituted into a fraction of the silicon (tetrahedral) sites of the framework structure. It is known that silicalite-1 and ZSM-5 have different surface properties and pore structure due to changes in T—O—T bond angle and T—O bond length, where T represents Si or Al. Therefore, isomorphous substitution within the framework structure of silicalite-1 or ZSM-5 should also produce changes in T—O—T bond angle and T—O bond length, where T now represents Si, Al, Ge, B, Fe, or Ga. These changes should affect the surface properties and pore structure of the zeolite and the separation performance of the resulting zeolite membrane. Furthermore, with isomorphous substitution, the zeolite surface changes from hydrophobic (silicalite-1) to hydrophilic (ZSM-5) and from non-acidic (silicalite-1) to strongly acidic (ZSM-5). The Brönsted acid strength increases in the following order: silicalite-1, Ge-ZSM-5&lt;B-ZSM-5&lt;Fe-ZSM-5&lt;Ga-ZSM-5&lt;ZSM-5 (i.e., Al—ZSM-5). In acid-catalyzed reactions, the catalytic activity of isomorphously substituted zeolites should increase with Brönsted acid strength. Since reaction selectivity also depends on zeolite acid strength, the substituted zeolite membranes may be useful in catalytic membrane reactors. 
     Isomorphous substitutes of silicon must accept a tetrahedral coordination with oxygen. In addition to aluminum (cation radius of 0.051 nm), suitable substitutes (i.e., T 2 , T 3 , etc. in formula II) include boron (0.023 nm), iron (0.064 nm), germanium (0.053 nm), and gallium (0.062 nm). Among these elements, Ge 4+ has a diameter closest to those of Si 4+ (0.042 nm) and Al 3+ and thus substitutes more readily than other tetravalent ions. The B 3+  cation is much smaller than the other substituted cations, and it is less stable in the tetrahedral positions according to the Pauling Rule, which holds that cations are stable in the tetrahedral positions when the ratio of cation to oxygen radius is 0.225-0.425. Because of this instability, boron may be partially removed from the zeolite framework during preparation of the zeolite membrane layer  252  (i.e., during calcination). This extra-framework boron, which is located within the channels and on the external surface of zeolite, could affect membrane properties. Also, Fe 3+  may be difficult to incorporate into the zeolite framework because of its large diameter. Consequently, some extra-framework Fe 3+  may be present in the membrane as well. Other elements having similar cation radius and coordination requirements can also be isomorphously substituted into the zeolite structural framework. 
     Since Fe and B are trivalent, they create acid sites in the zeolite framework structure. In contrast, Ge is tetravalent and therefore does not create acid sites. Such differences in acidity may affect the permeability of the membrane. When boron is substituted into the silicalite-1 structure instead of aluminum, membranes can be prepared with Si/B ratios as low as about 12. In contrast, Al-ZSM-5 membranes are difficult to prepare with such low Si/Al ratios. 
     Referring once again to the drawings, FIG. 4 shows a block diagram of a method  280  of making the zeolite membrane layer  252  of FIG.  3  through in-situ synthesis on the inner surface  254  of the porous support  256 . The method  280  generally includes preparing  282  the porous support  256  to receive the membrane  252  layer. As described above, useful supports  256  include porous alumina and stainless steel tubes or containers. When using alumina supports  256 , the ends  174 ,  176  of the alumina tubes are glazed to provide end supports  178 ,  180  and sealing surfaces  190 ,  192  (see FIG.  2 ). Likewise, when using stainless steel supports  256 , non-porous stainless steel tubes are welded onto the ends  174 ,  176  of the porous stainless steel tubes to provide end supports  178 ,  180  and sealing surfaces  190 ,  192 . In either case, prior to use, the support  256  is cleaned by brushing the inner surface  254  of the support  256 , followed by immersing the support  256  in an ultrasonic bath of deionized water. The supports  256  are then boiled in distilled water and dried under vacuum with heating (at about 373 K for about 30 min). 
     As indicated in FIG. 4, the method  280  also includes contacting  284  an inner surface  254  of the support  256  with zeolite precursors. The zeolite precursors are provided as a synthesis gel comprised of silica, water, a source of metal ions (i.e., Al 3+ , B 3+ , Ge 4+ , Ga 3+ , Fe 3+ , etc.), and optionally, an organic template, such as tetrapropyl ammonium hydroxide (TPAOH), tetrapropyl ammonium bromide (TPABr), tetrabutyl ammonium hydroxide (TBAOH), tetrabutyl ammonium bromide (TBABr), tetraethyl ammonium hydroxide (TEAOH) or tetraethyl ammonium bromide (TEABr) or combinations thereof. Referring to FIG.  2  and to FIG. 3, the synthesis gel is placed in the interior portion  216  of the tubular membrane assembly  170 . The end supports  178 ,  180  are plugged with an inert material (e.g., PTFE) to form a container, and the gel is allowed to permeate the porous support  256 . Ordinarily, the gel will thoroughly permeate the porous support  256  in less than 24 hours when held at a temperature up to about 318 K. When possible, template-free synthesis is used because it costs less, does not use toxic amines, and does not require calcining, which may introduce cracks or other structural defects in the zeolite membrane  252  layer. 
     As shown in FIG. 4, the method  280  also includes crystallizing  286  the zeolite constituents to form a zeolite layer on the support  256  and, optionally, calcining  288  the resulting zeolite layer to remove any organic residues, including the organic template. The tubular membrane assembly  170  is placed in an autoclave and heated at a temperature sufficient to induce zeolite formation, which is generally between about 403 K and about 469 K. During heating, water within the synthesis gel is forced out of the interior portion  216  of the membrane assembly  170  through the pores of the support  256 , thereby forming a continuous zeolite layer on the inner surface  254  and within the pores of the support  256 . The organic template molecules provided in the synthesis gel are trapped within the zeolite pore system and may also block larger cavities in the zeolite membrane  252  layer. Thus, prior to calcining  288 , a zeolite membrane  252  layer without defects should be impermeable to gases, such as nitrogen, so that, as described below, vapor permeation measurements can be used to evaluate zeolite membrane  252  quality. Following crystallization  286 , the uncalcined zeolite membrane  252  layer is washed with deionized water and dried at 383 K for at least 12 hours. 
     The contacting  284  and the crystallizing  286  steps (hydrothermal synthesis) can be repeated one or more times to ensure that the zeolite membrane  252  layer, after drying and before calcination, has the requisite quality. Following crystallization  286 , the zeolite membrane  252  layer is calcined  288  to remove the organic template and any other residual organic material. The organic template must be removed from the zeolite pores to obtain open, micro-porous membranes. Calcining  288  generally comprises heating the zeolite membrane  252  layer at a prescribed rate until it reaches a desired temperature, e.g., about 750 K or higher. This temperature is maintained for a sufficient amount of time, e.g., about eight hours or more, to thermally decompose any organic material. Following thermal decomposition, the zeolite membrane  252  is cooled at a prescribed rate to minimize thermal stresses in the zeolite layer. Ideally, the temperature profile is carefully controlled to ensure uniform heating and cooling within the zeolite membrane  252  layer. Although uniform heating and cooling is generally best achieved using relatively low temperature ramping (˜1 K/min), the method  280  may employ higher heating rates as long as care is taken to minimize local overheating. Local overheating may result in partial degradation of the zeolite crystal structure and/or steam generation, which can cause siloxane bond hydrolysis and/or loss of aluminum from the zeolite framework. 
     The zeolite membranes  252  of FIG. 3 can be characterized using many different techniques. For example, the membranes  252  can be characterized by X-ray diffraction (XRD) analysis of zeolite powder residue sampled from the interior  216  of the membrane assembly  170  (FIG. 2) following hydrothermal synthesis. This technique avoids destroying the membranes  252 , and assumes that the zeolite membrane  252  layer and zeolite powder samples have the same crystal structure. A useful apparatus for performing XRD measurements includes a Scintag PAD-V diffractometer, which uses a diffracted beam monochromator and a line-source X-ray beam of Cu Kα radiation from a standard 2 kW sealed tube. The X-rays are counted using a standard scintillation detector. Individual samples are ground to a fine powder and dispersed on a glass slide or packed into a cavity mount. The scan range (2θ) is typically between about 2° and 50°, and phases are identified by comparing scattered intensity peaks with a library of known inorganic compounds. A useful library of approximately 20,000 inorganic compounds is available in a computer-readable format from the International Center of Diffraction Data. Peak intensities and angles may also be calculated from crystal structure data, if known. 
     The zeolite membranes  252  can also by characterized by pervaporating compounds of known sizes through the membrane  252  layer using the system  100  shown in FIG.  1 . Useful compounds include 2,2-dimethylbutane (DMB), which has a kinetic diameter (0.62 nm) that is larger than the XRD pore diameter of the MFI structure. Other useful compounds include o-xylene, p-xylene, benzene, tri-isopropyl benzene (TIPB), which have kinetic diameters of 0.685 nm, 0.585 nm, 0.585 nm, and 0.85 nm, respectively. Xylene isomers are challenging to separate because they have similar physical properties. 
     FIG. 5 shows a schematic view of an apparatus  310  that can be used to characterize zeolite membranes  252  by measuring single-gas permeation rates at various temperatures. As described below, with simple modification the apparatus  310  can also be used to measure multi-gas permeation rates. The vapor permeation apparatus  310  is similar to the pervaporation system  100  shown in FIG. 1, and includes a membrane module  104 ′ having an inlet  106 ′, a first outlet  108 ′, and a second  110  outlet′. A pressurized source  312  of gas  314  (H 2 , He, CO 2 , N 2 , n-C 4 H 10 , i-C 4 H 10 , etc.) enters the membrane module  104 ′ inlet  106 ′ through a first conduit  116 ′. A pressure regulator  316  and valve  318  located along the first conduit  116 ′, downstream of the source  312 , provide coarse control of the gas  314  flow rate. The first conduit  116 ′ is made of a material having good thermal conductivity (e.g., stainless steel), and a section  320  of the first conduit  116 ′ upstream of the membrane module  104 ′ inlet  106 ′ is coiled to increase heat transfer area. A temperature-controlled oven  322  encloses the membrane module  104 ′ and the coiled section  320  of the first conduit  116 ′, and provides substantially isothermal conditions within the module  104 ′ during permeation measurements. 
     As described above, the membrane module  104 ′ contains a tubular membrane assembly  170  (FIG.  2 ), which includes a zeolite membrane  252  layer and a porous support  256  layer (FIG. 3) that define a side wall portion of the assembly  170 . Generally, a portion of the gas  314  entering the membrane module  104 ′ passes radially through the zeolite membrane  252  and the porous support  256 , and exits the module  104 ′ through the first outlet  108 ′. The remainder of the gas  314  passes axially through the interior  216  of the membrane assembly  170 , and exits the module  104 ′ via the second outlet  110 ′. Second  128 ′ and third  324  conduits channel the resulting permeate  120 ′ and retentate  122 ′ streams, respectively, away from the membrane module  104 ′. The second  128 ′ and third  324  conduits converge at a two-way purge valve  326 , which allows venting of the permeate  120 ′ stream or the retentate  122 ′ stream through a common exhaust line  328  and a bubble flow meter  330 . 
     As shown in FIG. 5, the apparatus  310  may include temperature and pressure sensors. The embodiment shown in FIG. 5 includes first  332  and second  334  thermocouples that are located in the second outlet  110 ′ of the membrane module  104 ′ and in the common exhaust line  328 , respectively. A controller (not shown), which communicates with the first thermocouple  332 , compares the temperature in the membrane module  104 ′ with a desired set point, and adjusts the temperature of the oven  322  in response to any offset. The embodiment also includes first  336  and second  338  pressure sensors that are located, respectively, in a pressure line  340  that communicates with the first conduit  116 ′ and the second conduit  128 ′, and in the first conduit  116 ′ immediately down stream of the pressure regulator  316  and valve  318 . The first pressure sensor  336  is a differential pressure gauge that senses pressure differences between the gas entering the membrane module  104 ′ and the pressure of the gas in the permeate  120 ′ stream. To maintain a desired pressure drop across the membrane module  104 , the apparatus  310  includes a pressure regulator  342  (i.e., variable flow area valve) that communicates with the first pressure sensor  336 , and adjusts the flow rate of the gas permeate through second conduit  128 ′. 
     During a permeation measurement, gas  314  flow through second outlet  110 ′ is stopped at the two-way purge valve  326 , so that all of the gas  314  entering the membrane module  104 ′ passes through the zeolite membrane  252  layer and porous substrate  256  of the membrane assembly  170 . The resulting gas  314  flow rate through the zeolite membrane  252  layer is measured using the bubble flow meter  330 . In most of the gas permeation experiments, the pressure regulator  342  maintains a 138 kPa pressure drop across the membrane module  104 ′. Single gas permeation rates are usually measured at two or more temperatures, e.g., at 300 K and 473 K. 
     With simple modification, the vapor permeation apparatus  310  shown in FIG. 5 can also be used to measure multi-gas permeation rates. For example, the apparatus  310  may include one or more metering pumps (e.g., syringe pumps) that communicate with the first conduit  116 ′. During a measurement, the metering pump injects a liquid-phase mixture into a pre-heated carrier gas (e.g., He) flowing within the first conduit  116 ′. The liquid mixture vaporizes in the hot carrier gas, which transports the gas mixture into the membrane module  104 ′ inlet  106 ′. A portion of the gas mixture entering the membrane module  104 ′ passes radially through the zeolite membrane  252  (FIG. 3) and the porous support  256 , and exits the module  104 ′ through the first outlet  108 ′. The remainder of the gas  314  passes axially through the interior  216  of the membrane assembly  170 , and exits the module  104 ′ via the second outlet  110 ′. Second  128 ′ and third  324  conduits channel the permeate  120 ′ and retentate  122 ′ streams, respectively, away from the membrane module  104 ′. 
     In contrast to single-gas permeation measurements, the second  128 ′ and third  324  conduits do not converge at the two-way purge valve  326  of FIG. 5, but instead vent through separate exhaust lines and bubble flow meters. As described above, the pressure regulator  342  can be used to impose a desired pressure drop across the membrane module  104 ′, which drives diffusion through the selectively permeable portion  172  of the membrane assembly  170  (FIG.  2 ). Alternatively or additionally, the apparatus  310  may employ a sweep gas (e.g., He, Ar, etc.) to generate a concentration gradient across the selectively permeable portion  172  of the membrane assembly  170 . The sweep gas enters a cavity  218  formed by an inner surface  220  of the shell  182  and an outer surface  222  of the membrane assembly  170  through a port  360  in a body portion  184  of the shell  182 . Note that the port  360  shown in FIG. 2 is sealed with a removable plug  362 . 
     The disclosed isomorphously substituted zeolite membranes will find use in many different processes. For example, the membranes can be used to separate non-condensable gases. The thermal stability of the disclosed zeolite membranes makes them ideal for separating non-condensable gases, which are often available at high temperature. For example, the isomorphously substituted zeolite membranes of the invention could be used to separate H 2  from CO 2  in the water-gas shift reaction. 
     The membranes can also be used to separate condensable organic vapor mixtures. The separation of condensable organic vapors often involves separating isomers that have similar relative vapor pressures. Typically, these separations are carried out using multiple distillation columns, require hundreds of stages, and are energy intensive. Isomorphously substituted zeolite membranes provide a much simpler, and less energy intensive separation process. For instance, as described below in Example 4 and Example 7, the membranes of the invention have been used to separate n-C 4 H 10 /i-C 4 H 10  mixtures and mixtures of xylene isomers, respectively. 
     The removal of organic compounds or water from aqueous solutions is important for recovering valuable organic products from process streams, for recycling process water, and for treating wastewater. The disclosed hydrophobic or organophilic membranes can be used to separate such organic/water mixtures by pervaporation using the apparatus shown in FIG.  1 . The hydrophobic, isomorphously substituted Ge-ZSM-5 membranes possess a different pore structure than silicalite-1 membranes previously used for separating organic/water mixtures, and thus have different permeation and adsorption properties. These membranes may be able to separate organic compounds from water with greater selectivity than previously studied silicalite-1 membranes. 
     Because of their acid resistance, the disclosed isomorphously substituted zeolite membranes may also be used to separate mineral acids from water by pervaporation. Furthermore, the ability to vary the Brönsted acid strength of the disclosed membranes should prove useful in acid separations since water adsorption will likely vary with the number of acidic sites within the framework structure and on the membrane surface. 
     The disclosed isomorphously substituted zeolite membranes can also be used in catalytic membrane reactors. Zeolite membranes have many properties that make them particularly useful as catalysts. First, it is possible to introduce a large variety of cations, including protons, having different catalytic properties into the zeolite pore system. Second, zeolites exhibit a molecular sieving effect because of their ability to selectively adsorb molecules whose dimensions are below a certain critical size into their pore system. In shape-selective catalysis, the molecular pore structure and the presence of catalytically active sites is exploited to control reaction selectivity—i.e., to accelerate one of many potential reaction pathways. For example, ZSM-5 zeolites are used as shape selective catalysts in the conversion of methanol to gasoline and in the conversion of benzene and ethylene to ethyl benzene. Generally, the catalytic activity and selectivity of zeolites depend on Brönsted acid strength. Since it is possible to prepare membranes with different Brönsted acid strengths and with different numbers of acidic sites, it may be possible to tailor the catalytic activity and the selectivity of the disclosed membranes. Finally, the disclosed method of preparing zeolite membranes allows for direct synthesis of the acid hydrogen form of the zeolite, which is much simpler than known synthesis techniques. 
     EXAMPLES 
     The following examples are intended as illustrative and non-limiting and represent specific embodiments of the present invention. 
     Example 1 
     Isomorphously Substituted Zeolite Synthesis 
     Zeolite membranes are prepared by in situ crystallization from zeolite forming gels (zeolite precursors) on three types of porous support tubes. The support tubes (OD=1.0 cm) comprise α-alumina with an inner layer of γ-alumina having 5-nm diameter pores (0.70 cm ID, U.S. Filter), α-alumina with an inner layer of α-alumina having 200-nm diameter pores (0.70 cm ID, U.S. Filter), or porous stainless steel with an inner layer of stainless steel having 500-nm diameter pores (0.65 cm ID, Mott Metallurgical Co.). 
     Alkali-free zeolite forming gels are prepared using silica sol (Ludox AS40) as the silicon source. Other silicon sources such as tetraethyl-orthosilicate or fumed silica (Aeorsil-200) can also be used. A quaternary organic ammonium template, tetrapropyl ammonium hydroxide (TPAOH) is used to help stabilize and direct zeolite formation. Other quaternary ammonium compounds such as tetrapropyl ammonium bromide (TPABr), tetrabutyl ammonium hydroxide (TBAOH), tetrabutyl ammonium bromide (TBABr), tetraethyl ammonium hydroxide (TEAOH), tetraethyl ammonium bromide (TEABr) could also be used as templates. For isomorphously substituted zeolite gels, Al(i-C 3 H 7 O) 3 , Ge(C 2 H 5 O) 4 , Fe(NO 3 ) 3 , and/or B(OH) 3  are added synthesis solution, which is then stirred for at least five minutes. Other solutions containing ionic Al +3 , Ge +4 , Fe +3 , Ga +3 , or B +3  could also be used. In some cases, the zeolite forming gel also contains sodium hydroxide. 
     The isomorphously substituted zeolite membranes containing B, Fe, Ge, Ga, and Al are prepared by in-situ crystallization on porous supports. Tubular supports 2.8 cm in length are used because they are commercially available and because they are well adapted for growing continuous films. One end of each tube is plugged with a polytetrafluoroethylene (PTFE) cap to form a container. A zeolite forming gel comprising silica, water, TPAOH, and a source of boron, aluminum, germanium, and/or iron is placed inside the porous support container. The other end of the tube is plugged, and the container is left for periods up to 24 hours at room temperature. During this time, the porous support soaks up almost all of the synthesis gel. The container is again filled with gel, plugged, and placed in an autoclave to allow the gel to crystallize. The first crystallization is carried out hydrothermally at 458° K for 24 hours. When the capped container is heated, water within the gel is forced to permeate through the pores of the support container, thereby forming a continuous zeolite layer on the inner wall of the support container. 
     A synthesis using the same procedure but conducted at 453 K for 48 hours is repeated until an uncalcined membrane, after drying at 373 K, is impermeable to N 2  for a 138 kPa pressure drop at room temperature. After zeolite synthesis is complete, the membranes are washed, dried and calcined to remove the organic template molecules from the zeolite pores. A computer-controlled muffle furnace with heating and cooling rates of 0.6 and 1.1 K/minute, respectively, is used for calcining the membranes. The maximum calcination temperature is 753° K, and the membrane is held there for eight hours and then stored at room temperature under vacuum. 
     A series of isomorphously substituted zeolite membranes having Si/Me ratios of 100 was prepared using zeolite forming gels and synthesis conditions listed in Table 1. Each of the membranes was prepared on porous stainless steel supports. Because Fe 3+  can be difficult to incorporate into the zeolite framework because of its large diameter, some extra-framework Fe 3+  could be present within the membrane. However, the Fe-ZSM-5 membrane was prepared from a brown solution, but after synthesis, the membrane was white, and it remained white after calcination. This indicates that Fe 3+  cations were likely incorporated into the framework positions; membranes containing extra-framework Fe 3+  are expected to be brown. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Molar Compositions of Zeolite Precursors and Synthesis Conditions 
               
               
                 for Preparing Isomorphously Substituted Zeolite Membranes 
               
               
                 with Si/Me = 100 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Crystallization 
                   
               
               
                   
                   
                   
                   
                   
                 Time (h) @ 
               
               
                 Membrane 
                 TPAOH 
                 Metal 
                 SiO 2   
                 H 2 O 
                 Temp. (K.) 
                 # Layers 
               
               
                   
               
               
                 silicalite-1 
                 1.0 
                 0    
                 19.5 
                 438 
                 48 @ 458 
                 2 
               
               
                 Al-ZSM-5 
                 1.5 
                 0.195 
                 19.5 
                 438 
                 48 @ 458 
                 2 
               
               
                 Fe-ZSM-5 
                 1.5 
                 0.195 
                 19.5 
                 438 
                 48 @ 458 
                 2 
               
               
                 B-ZSM-5 
                 1.5 
                 0.195 
                 19.5 
                 438 
                 48 @ 458 
                 2 
               
               
                 Ge-ZSM-5 
                 1.0 
                 0.195 
                 19.5 
                 438 
                 24 @ 458 
                 4 
               
               
                   
               
             
          
         
       
     
     A second series of isomorphously substituted membranes having Si/Me ratios ranging between 12 and 600 was prepared from the synthesis gels listed in Table 2. The preparation conditions for these membranes are described in Table 3. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Molar Compositions of Synthesis gels 
               
               
                 for B-ZSM-5 and Al-ZSM-5 Zeolite Membranes 
               
             
          
           
               
                 Membrane 
                 TPAOH 
                 SiO 2   
                 B(OH) 3   
                 H 2 O 
                 NaOH 
                 Al 2 O 3   
                 Si/Me 
               
               
                   
               
               
                 M1 
                  2.0 
                 19.46 
                 0.39  
                 438 
                 2.0 
                 0.0   
                 50 
               
               
                 M2 
                  2.0 
                 19.46 
                 0.778 
                 500 
                 2.5 
                 0.0   
                 25 
               
               
                 M3 
                  2.0 
                 19.46 
                 1.62  
                 500 
                 3.0 
                 0.0   
                 12 
               
               
                 M4, M4a, 
                 1.55 
                 19.46 
                 0.195 
                 438 
                 0.0 
                 0.0   
                 100  
               
               
                 M4b, M7 
               
               
                 M5, M8 
                 2.22 
                 19.46 
                 0.778 
                 500 
                 0.0 
                 0.0   
                 25 
               
               
                 M6, M9 
                 4.44 
                 19.46 
                 1.55  
                 500 
                 0.0 
                 0.0   
                   12.5 
               
               
                 M10 
                  1.0 
                 19.46 
                 0.0  
                 438 
                 0.0 
                 0.0   
                 ∞ 
               
               
                 M11, M12 
                  1.0 
                 19.46 
                 0.0  
                 438 
                 0.0 
                 0.0162 
                 600  
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Membrane Preparation Conditions 
               
               
                 for Molar Compositions shown in Table 2. 
               
             
          
           
               
                   
                   
                 Crystallization Time (h) @ 
                   
               
               
                 Membrane 
                 Support 
                 Temperature (K.) 
                 # Layers 
               
               
                   
               
               
                 M1 
                 stainless 
                 48 @ 458 
                 4 
               
               
                 M2 
                 stainless 
                 24 @ 458 
                 4 
               
               
                 M3 
                 stainless 
                 24 @ 458 
                 5 
               
               
                 M4, M4a, M4b 
                 stainless 
                 48 @ 458 
                 2 
               
               
                 M5 
                 stainless 
                 24 @ 458 
                 4 
               
               
                 M6 
                 stainless 
                 24 @ 458 
                 5 
               
               
                 M7 
                 α-alumina 
                 48 @ 458 
                 2 
               
               
                 M8 
                 α-alumina 
                 24 @ 458 
                 4 
               
               
                 M9 
                 α-alumina 
                 25 @ 458 
                 5 
               
               
                 M10  
                 stainless 
                 48 @ 458 
                 2 
               
               
                 M11  
                 stainless 
                 48 @ 458 
                 2 
               
               
                 M12  
                 α-alumina 
                 48 @ 458 
                 2 
               
               
                   
               
             
          
         
       
     
     Example 2 
     ZSM-5 Zeolite Structural Confirmation 
     MFI-type zeolites, such as silicalite-1 (pure silica) and ZSM-5 (containing an isomorphously substituted element) have the same structure with XRD pore dimensions of 0.53 nm×0.56 nm. To confirm the MFI-structure of the membranes prepared in Example 1, XRD powder patterns were obtained for crystalline powders that were formed at the same time as the membranes. This procedure avoids destroying the membranes; the membranes and powders were assumed to have the same crystal structure. For all powders, the positions and the intensities of the diffraction peaks were identical to those reported for the MFI-structure. No additional peaks were observed, indicating that the powders had the pure MFI structure. 
     A Scintag PAD V automated powder diffraction unit using a diffracted beam monochromator and a line-source X-ray beam of Cu K-series radiation from a standard 2 kW sealed tube was used to characterize these crystals. The X-rays were counted using a standard scintillation detector. Each sample was ground to a fine powder and dispersed on a glass slide or packed into a cavity mount. For phase identification, the scan range was typically 2° to 50° 2θ. Phase identification was based on comparison of scattered intensity peaks with a standard file of approximately 20,000 known inorganic compounds. The standard file was provided by International Center of Diffraction Data (ICDD) (12 Campus Blvd, Newtown Square, Pa. 19073). Peak intensities and angles may also be calculated from crystal structure data, if known. 
     Additionally, the structure of membrane M 4  of Table 2 was broken and characterized by XRD using the Scintag PAD V automated powder diffraction unit. The sample tube was cut lengthwise and placed in a specially made sample holder so that the membrane was in the correct center position of the diffraction instrument. The spot-source beam was collimated so that only the portion of the tube in the correct position was exposed to the radiation. The positions and the intensities of all peaks in the XRD pattern for the boron-containing zeolite membrane M 4  were also in agreement with those reported for MFI zeolite. 
     Membranes prepared in accordance with the invention on α-alumina supports were also characterized by SEM. The SEM micrographs clearly show the presence of zeolite crystals on the alumina support. SEM photographs were obtained with an ISI-SX-30 scanning electron microscope. Cylindrical membranes were broken and fragments selected as samples. Photographs were taken of the cross section and inner surface to show the structure and morphology of the membrane. 
     Example 3 
     Inductively Coupled Plasma Experiments 
     The boron content of the B-ZSM-5 membranes prepared in Example 1 was verified by inductively coupled plasma after first dissolving the crystals in hydrofluoric acid. The Si/B ratios in the zeolite powders were determined to be similar to those in the zeolite forming gels. For example, zeolite powders formed from a gel having a Si/B molar ratio of 50 were found to have actual Si/B molar ratios of about 60. 
     Example 4 
     Single Gas and Mixture Permeation Experiments 
     Single-gas permeation rates of H 2 , N 2 , and CO 2  were measured over a range of temperatures for most of the B-ZSM-5 membranes of Example 1, as well as the Fe-ZSM-5, Ge-ZSM-5, Al-ZSM-5 and silcalite-1 membranes of Example 1. In addition, single-gas and mixture permeances of n-C 4 H 10  and i-C 4 H 10  were measured for all of the membranes shown in Table 1-Table 3 over the same temperature range. The single-gas permeation rates were measured by sealing the membrane in a stainless steel module with silicone o-rings in a dead end mode. The pressure drop across the membrane was 138 kPa, and the permeate side pressure was 83 kPa. The ratio of single-gas permeances is referred to as the ideal selectivity. 
     Mixture permeances were measured in a continuous-flow stainless module, using He as a sweep gas. A 50/50 mixture of n-C 4 H 10  and i-C 4 H 10 , with a total flow rate of 40 cm 3 /minute, flowed axially inside of the tube, and the permeate diffused radially outward. Silicone o-rings were used to seal the membrane inside the module. The module as wrapped in heating tape and insulation. A temperature controller maintained the desired temperature based on a thermocouple placed at the axial outlet of the membrane. The permeate stream and the retentate stream were analyzed using a HP 5890 gas chromatograph with a TC detector and a packed column (1% Alltech AT-1000 on Graph-GC). Each permeance was calculated from an average of four samples taken from the permeate and retentate streams. The calculated concentrations from the four samples at a given set of conditions typically varied less than 2%. The volumetric flow rates of retentate stream and the permeate stream were measured at room temperature and atmospheric pressure using soap-film flow meters. In Table 4, the n-C 4 H 10 /i-C 4 H 10  separation selectivities are the ratios of permeances, and the log-mean partial pressure was used for this calculation. 
     Results of the permeation experiments are shown in Table 4 and FIG.  6 -FIG.  12 . Table 4 shows single gas permeances and n-C 4 H 10  and i-C 4 H 10  selectivities for B-ZSM-5 membranes that were prepared under different conditions and that have various Si/B molar ratios. FIG.  6  and FIG. 7 demonstrate the influence of boron substitution and support composition on separation performance. FIG. 6 shows n-C 4 H 10 /i-C 4 H 10  separation selectivity as a function of temperature for three B-ZSM-5 membranes (M 4 -M 6 ) on stainless steel supports with different Si/B molar ratios (100, 25, 12.5) and for a silicalite-1 membrane on a stainless steel support. Similarly, FIG. 7 shows n-C 4 H 10 /i-C 4 H 10  separation selectivity as a function of temperature for three alkali free B-ZSM-5 membranes (M 7 -M 9 ) on α-alumina supports with different Si/B molar ratios (100, 25, 12.5). 
     FIG. 8 compares n-C 4 H 10 /i-C 4 H 10  mixture permeation rate as a function of temperature for two alkali free B-ZSM-5 membranes (M 4 , M 7 ) prepared on stainless steel (M 4 ) and α-alumina (M 7 ) supports. Both membranes have Si/B molar ratios of 100. To study membrane stability, n-C 4 H 10 /i-C 4 H 10  mixture permeation rates were measured twice for the M 4  membrane. Broken lines show the second set of permeation rate measurements, which were taken about 48 hours after the first set of permeation rate measurements. 
     FIG.  9  and FIG. 10 demonstrate the influence of support composition and batch to batch variability on separation performance. FIG. 9 shows n-C 4 H 10 /i-C 4 H 10  mixture permeance as a function of temperature for two alkali free B-ZSM-5 membranes on stainless steel (M 6 ) and α-alumina (M 9 ) supports. Each membrane has a Si/B molar ratio of 12.5. FIG. 10 shows n-C 4 H 10 /i-C 4 H 10  mixture permeance as a function of temperature for three alkali free B-ZSM-5 membranes (M 4   a , M 4   b , M 4   c ) prepared under identical conditions (Si/B molar ratio of 100) on stainless steel supports. 
     FIG. 11 demonstrates stability of an alkali free B-ZSM-5 membrane during an extended permeation experiment. FIG. 11 shows n-C 4 H 10 /i-C 4 H 10  mixture permeance and separation selectivity as functions of time for B-ZSM-5 membrane M 9 , which was prepared on an α-alumina support and had a Si/B molar ratio of 12.5. All of the measurements were made at 473 K. As shown in FIG. 11, the i-C 4 H 10  permeance decreased approximately 2% in 48 hours, whereas the n-C 4 H 10  permeance decreased by about 10% during the initial 24 hours of the experiment and was almost constant during the next 24 hours. The separation selectivity decreased from 59 to 56 after 42 hours. There was no evidence of butane decomposition. 
     FIG. 12 demonstrates the relative influence of boron and aluminum substitution in the MFI structural framework. FIG. 12 shows n-C 4 H 10 /i-C 4 H 10  separation selectivity as a function of temperature for alkali free B-ZSM-5 (M 4 , M 7 ) and Al-ZSM-5 (M 11 , M 12 ) membranes having various Si/Me molar ratios (100, 100, 600, 600). The boron and aluminum substituted membranes were prepared on stainless steel (M 4 , M 11 ) and α-alumina (M 7 , M 12 ). For comparison purposes, FIG. 12 also shows n-C 4 H 10 /i-C 4 H 10  separation selectivity as a function of temperature for an alkali free silcalite-1 membrane prepared on stainless steel. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Singe Gas Permeances and Selectivities at 473 K for Me-ZSM-5 Membranes 
               
             
          
           
               
                   
                 Permeance × 10 9   
                 n/i-C 4 H 10   
               
               
                   
                 (mol/m 2 /s/Pa 
                 Selectivity 
               
             
          
           
               
                 Membrane 
                 Si/Me 
                 H 2   
                 CO 2   
                 N 2   
                 n-C 4 H 10   
                 i-C 4 H 10   
                 Ideal 
                 Separation 
               
               
                   
               
             
          
           
               
                 M1 
                 50 
                  48 
                  40 
                  30 
                  30 
                 6.0 
                 5.0 
                 6.0 
               
               
                 M2 
                 25 
                 110 
                  89 
                  70 
                  68 
                 23 
                 3.0 
                 3.4 
               
               
                 M3 
                 12 
                 180 
                 100 
                  81 
                  80 
                 23 
                 3.5 
                 3.9 
               
               
                 M4 
                 100 
                  40 
                  32 
                  20 
                  20 
                 1.0 
                 20 
                 22 
               
               
                 M5 
                 25 
                  60 
                  51 
                  24 
                  25 
                 0.78 
                 32 
                 35 
               
               
                 M6 
                 12.5 
                  57 
                  53 
                  31 
                  37 
                 0.57 
                 65 
                 27 
               
               
                 M7 
                 100 
                  95 
                  88 
                  46 
                  59 
                 2.2 
                 27 
                 39 
               
               
                 M8 
                 25 
                 162 
                 162 
                  79 
                  60 
                 2.5 
                 24 
                 27 
               
               
                 M9 
                 12.5 
                 250 
                 250 
                 112 
                 162 
                 2.7 
                 60 
                 61 
               
               
                 M10 
                 ∞ 
                  77 
                  77 
                  48 
                  60 
                 11 
                 5.4 
                 5.0 
               
               
                 Fe-ZSM-5 
                 100 
                 100 
                  70 
                  40 
                  42 
                 6 
                 7 
                 8 
               
               
                 Ge-ZSM-5 
                 100 
                 140 
                 120 
                 100 
                 110 
                 7.5 
                 14.7 
                 14 
               
               
                   
               
             
          
         
       
     
     In addition to the observations noted above, it appeared that for all substituted ZSM-5 membranes, single gas permeances at 473 K showed a decreasing trend as the kinetic diameter of the molecule increased. All substituted membranes separated n-C 4 H 10 /i-C 4 H 10 , n-C 4 H 10 /H 2 , and H 2 /i-C 4 H 10  mixtures, and separation selectivity seemed to depend on the identity of the substituted metal or metalloid. However, no trend with acidity or hydrophobicity was observed. For most separations studied, the substituted membranes exhibited higher separation selectivity than a silicalite-1 membrane. The B-ZSM-5 membranes appeared to exhibit the highest separation selectivity; of these, membranes prepared from alkali-free gels exhibited the highest separation selectivity. The highest ideal selectivity at 473 K and 527 K was 60 and 24, respectively. For most B-ZSM-5 alkali-free membranes, n/i-C 4 H 10  ideal selectivity and separation selectivity increased with boron content, and membranes prepared on α-alumina supports appeared to exhibit higher permeance and separation selectivity than comparable membranes prepared on stainless steel supports. It appears that n-C 4 H 10 /i-C 4 H 10  separation is due to differences in diffusion rates and adsorption coverage. 
     Example 5 
     Separation of Binary Mixtures of Normal Butane and Hydrogen and Isobutane and Hydrogen 
     Three isomorphously substituted ZSM-5 membranes of Example 1 (M 4 , Fe-ZSM-5, Ge-ZSM-5) were used to separate binary mixtures of normal butane and hydrogen, and isobutane and hydrogen. For comparison purposes, a ZSM-5 membrane (M 4 ) and a silcalite-1 membrane (M 10 ) were also used to separate the n-C 4 H 10 /H 2  and i-C 4 H 10 /H 2  mixtures. Permeation rates at temperatures ranging from 300 K to 523 K were measured using a system similar to the apparatus described in Example 4. The single-gas permeation rates were measured by sealing the membrane in a stainless steel module with silicone o-rings in a dead end mode. The pressure drop across the membrane was 138 kPa, and the permeate side pressure was 83 kPa. The ratio of single-gas permeances is referred to as the ideal selectivity. 
     To measure mixture permeance, each of the binary mixtures was formed by evaporating n-C 4 H 10  or i-C 4 H 10 /H 2  into a helium stream flowing within the tubular membrane. The membrane was located in a stainless steel module that was heated by heating tapes. Each hydrocarbon mixture contained about 50/50 v/v mixture of n-C 4 H 10 /H 2  or i-C 4 H 10 /H 2 . During an experiment, both sides of the membrane were maintained at atmospheric pressure, and an argon sweep gas provided a driving force across the membrane by removing the permeating components. The permeate stream was analyzed using a gas chromatograph equipped with a flame ionization detector, as described in Example 4, and a log-mean pressure driving force was used to calculate permeance. Permselectivity was calculated as the ratio of the permeances. 
     FIGS. 13-14 show the results of the permeation experiments. FIG. 13 shows n-C 4 H 10  separation selectivity as a function temperature for the silcalite-1 and substituted ZSM-5 zeolite membranes. Because of preferential adsorption of normal butane, H 2  mixture permeance was significantly lower than its single gas permeance. As shown in FIG. 13, separation selectivity was highest at about room temperature, where the n-C 4 H 10  coverage (adsorption) was highest, and strongly decreased with increasing temperature. At higher temperatures, the n-C 4 H 10  coverage decreased and could not effectively inhibit H 2  transport, resulting in decreased n-C 4 H 10  separation selectivity at higher temperatures. This conclusion is supported by permeation measurements, which indicate that separation selectivity increases with increasing concentration of n-C 4 H 10  in the feed stream. 
     As shown in FIG. 13, n-C 4 H 10 /H 2  separation selectivity depended strongly on the substituted metal or metalloid, and at room temperature, increased in the following order: Fe-ZSM-5&lt;silicalite-1&lt;Ge-ZSM-5&lt;Al-ZSM-5&lt;B-ZSM-5. Based on permeance measurements, the higher selectivity was the result of lower H 2  permeance rather than higher n-C 4 H 10  permeance. Although large differences in selectivity were probably the result of different adsorption strengths of n-C 4 H 10  within different zeolites, the order of separation selectivity did not correlate with acid strength or hydrophobicity/hydrophilicity. 
     FIG. 14 shows H 2  separation selectivity for i-C 4 H 10 /H 2  mixtures as a function temperature for the silcalite-1 and substituted ZSM-5 zeolite membranes. In contrast to the n-C 4 H 10  /H 2  mixtures, H 2  permeated faster than i-C 4 H 10  for all membranes, even at low temperatures. At low temperatures, H 2  permeance in the mixture was lower than its single gas permeance, but was two to four times higher than H 2  permeance in the H 2 /n-C 4 H 10  mixture, indicating that i-C 4 H 10  blocked H 2  permeation, but less effectively than n-C 4 H 10 . Thus, H 2  permeated faster than i-C 4 H 10  in the mixture. Like the n-C 4 H 10 /H 2  mixtures, increasing i-C 4 H 10  concentration in the feed inhibited H 2  permeation and therefore decreased H 2 /i-C 4 H 10  separation selectivity. 
     As can be seen in FIG. 14, the H 2 /i-C 4 H 10  separation selectivity at 523 K increased in the following order: silicalite-1 &lt;Fe-ZSM-5&lt;Ge-ZSM-5&lt;Al-ZSM-5&lt;B-ZSM-5. The B-ZSM-5 membrane had the highest selectivity because it had the lowest i-C 4 H 10  permeance. 
     Example 6 
     Separation of N-Hexane From Binary Mixtures Containing 2,2 Dimethylbutane, Benzene or Cyclohexane 
     Two of the B-ZSM-5 membranes (M 6 , M 9 ) of Example 1 were used to separate n-hexane from binary mixtures containing 2,2 dimethylbutane, benzene or cyclohexane. Vapor permeation rates were measured using a system similar to system described in Example 4 at temperatures ranging from 373 K to 524 K. The hydrocarbon mixture was evaporated into a helium stream flowing within the tubular membrane. The membrane was located in a stainless steel module that was heated by heating tapes. Each of the binary hydrocarbon mixtures contained about 50 vol. % n-hexane, and the feed stream contained about 10 vol. % hydrocarbon and 90 vol. % helium. During an experiment, both sides of the membrane were maintained at atmospheric pressure, and a helium sweep gas provided a driving force across the membrane by removing the permeating components. The permeate stream was analyzed using a gas chromatograph equipped with a flame ionization detector, as described in Example 4, and a log-mean pressure driving force was used to calculate permeance. Permselectivity was calculated as the ratio of the permeances. 
     Results of the permeation experiments are shown in Table 5 and FIGS. 15-16. Table 5 lists mixture permeance and separation selectivity for both B-ZSM-5 membranes (M 6 , M 9 ). In addition to the binary mixtures disclosed in Table 5, the B-ZSM-5 membranes were used to separate a 50/25/25 mixture of n-hexane, cyclohexane, benzene mixture. The resulting separation selectivity ranged from 790-800 at 373 K, which is significantly higher than the separation selectivity for the binary mixtures shown in Table 5. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Mixture Permeance and Separation Selectivity of 50/50 v/v 
               
               
                 normal hexane/organic mixtures for B-ZSM-5 Zeolite Membranes. 
               
             
          
           
               
                   
                 α-Alumina Support 
                 Stainless Steel Support 
               
             
          
           
               
                   
                 Permeance 
                 Selectivity 
                 Permeance 
                 Selectivity 
               
             
          
           
               
                   
                 n- 
                   
                 n-C 6 H 14 / 
                 n- 
                   
                 n-C 6 H 14 / 
               
               
                 Organic 
                 C 6 H 14   
                 Organic 
                 organic 
                 C 6 H 14   
                 Organic 
                 organic 
               
               
                   
               
               
                 2,2-DMB 
                 1.5 
                  0.65 
                 2280  
                 1.5 
                  0.78 
                 1950  
               
               
                 Cyclo- 
                 2.4 
                 3.3 
                 720 
                 1.3 
                 2.2 
                 570 
               
               
                 hexane 
               
               
                 Benzene 
                 2.4 
                 5.5 
                 440 
                 1.0 
                 2.6 
                 370 
               
               
                 Benzene + 
                 2.2 
                 2.8 
                 790 
                 1.5 
                 1.8 
                 800 
               
               
                 Cyclo- 
               
               
                 hexane 
               
               
                 (50/50) 
               
               
                   
               
             
          
         
       
     
     FIG. 15 shows n-hexane/2,2-DMB permeance and separation selectivity as functions of temperature for both B-ZSM-5 zeolite membranes. Separation selectivity was highest (greater than 2000) at 373 K and decreased with increasing temperature; but even at 523 K, the B-ZMS-5 membrane supported on stainless steel (M 6 ) separated the n-hexane/2,2-DMB mixture with selectivity of 72. FIG. 16 shows n-hexane/2,2-DMB permeance and separation selectivity as functions of temperature for silicalite-1 (M 10 ) and one of the B-ZSM-5 zeolite membranes (M 6 ). The B-ZSM-5 membrane exhibited higher n-hexane permeance and lower 2,2-DMB permeance than the silicalite-1 membrane. The B-ZSM-5 membrane exhibited the highest n-C 6 H 14 /2,2-DMB separation selectivity, which was higher than the separation selectivity of the silcalite-1 membrane. 
     Example 7 
     Separation of P-xylene and O-xylene Mixtures 
     Two B-ZSM-5 zeolite membranes (BZ 1 , BZ 2 ) were used to separate binary mixtures of p-xylene and o-xylene. The membranes were prepared in a manner similar to membrane M 4  of Example 1, except membranes BZ 1  and BZ 2  had four synthesis layers instead of two. The single-gas permeation rates were measured by sealing the membrane in a stainless steel module with silicone o-rings in a dead end mode. The pressure drop across the membrane was 138 kPa, and the permeate side pressure was 83 kPa. 
     Permeation rates for the xylene mixtures at temperatures ranging from 373 K to 525 K were measured using a system similar to the apparatus described in Example 4. Para-xylene/o-xylene mixtures were evaporated into a helium stream flowing within the tubular membrane to measure mixture permeance. The membrane was located in a stainless steel module that was heated by heating tapes. In most of the separations, the binary p-xylene/o-xylene mixtures contained about 50/50 v/v mixture of the two isomers. The partial pressure of each of the isomers varied among separations (0.4 kPa, 0.9 kPa, 2.1 kPa, 2.5 kPa). During an experiment, both sides of the membrane were maintained at atmospheric pressure, and a helium sweep gas provided a driving force across the membrane by removing the permeating components. Helium flow rates for both the feed and the sweep gas were set at about 40 cm 3 /minute at STP using mass flow controllers. The permeate stream was analyzed using a gas chromatograph equipped with a flame ionization detector, as described in Example 4, and a log-mean pressure driving force was used to calculate permeance. Permselectivity was calculated as the ratio of the permeances. 
     Results of the permeation experiments are shown in FIG.  17 -FIG.  21 . FIG.  17  and FIG. 18 show, as functions of temperature, p-xylene/o-xylene steady state fluxes and separation selectivities, respectively, for B-ZSM-5 zeolite membrane BZ 1 . The partial pressure of each of the isomers in the feed was 2.1 kPa. FIG.  19  and FIG. 20 show, as functions of temperature, fluxes of p-xylene and o-xylene, respectively, for B-ZSM-5 zeolite membrane BZ 2  and various feed partial pressures. Finally, FIG. 21 shows the resulting separation selectivity for p-xylene/o-xylene mixtures as a function of temperature for B-ZSM-5 zeolite membrane BZ 2  membrane and various feed partial pressures. 
     The above description is intended to be illustrative and not restrictive. Many embodiments and many applications besides the examples provided would be apparent to those of skill in the art upon reading the above description. The scope of the invention should therefore be determined, not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications and publications, are incorporated by reference in their entirety for all purposes.