Procedure for the preparation of microporous ceramic membranes for the separation of gas and liquid mixtures
This invention is concerned with the preparation of continuous microporous inorganic membranes of controlled pore size by electron beam evaporation. Such membranes are of great practical interest for the transport of gas and water because of the potential application to many separation problems in the liquid and the gas phase, where separation by molecular size differences is desirable or required. 
     In numerous processes individual gases have to be separated from mixtures like water in natural gas, hydrogen from carbon monoxide, hydrogen from methane or oxygen from nitrogen. This is usually achieved by distillation or by cryogenic procedures, both of which require a lot of energy. Concentration of solutions through the removal of small molecules like water from fruit juice or from biological fluids like milk or blood with retention of larger molecules like oligo-and polysaccharides, proteins, aroma compounds, vitamines and others are as much of general importance as are the removal of solvent from process solutions or the preparation of ultrapure water by pervaporation through inorganic membranes (Suzumura, Yanaka, JP 63134093 A 2, June 2, 1988). 
     The membranes used for gas separation are commonly polymer membranes, which do not separate by pore size but by the different solubility of the gases in the polymer matrix. So is the separation of nitrogen from natural gas improved upon by the use of polymer membranes (M. S. A. Baksh, A. Kapoor, R. T. Yang, Sep. Sci. Technol. 25 (1990) 845). A problem of polymer membranes is their inselectivity for the separation of inpolar small gases like hydrogen from argon or methane from other hydrocarbons. Another disadvantage of polymer membranes is their low thermal resistance, which usually forbids the separation at temperatures above 150° C. Alternative membranes are being prepared by SOL-GEL procedures, but the separation factors are commonly below the Knudsen limit (square root of the molar weight ratio of the gases (Klein, Gizpec Ceram. Bull. 69 (1990) 1821: Suzuki, Ononato, Kurokawa, J. Noncryst. Solids 94 (1987), 160). This indicates that the pores are still too large (&gt;1 nm) to observe a sieving effect. Smaller pores are approached by the preparation of zeolite membranes (Bein, Brown, Enzel, Brinker, Mter. Res. Soc. Symp. Proc 121 (1988), 761: Oayama, Masatsugu, JP 63291809-A 2, 1988); technical application of zeolite membranes is obstructed by the apparently unavoidable formation of pin-holes. Another method for the preparation of ceramic membranes is CVD (chemical vapor deposition) (Mori, Fujita, JP 61238303 A 2, October 1986;Lin, Fransen, DeVries, Burggraf, Proc. Electrochem. Soc. 90-12 (1990) 539), which produces pore size distributions of 1-50 nm, but due to the high production costs and low separation effectivity CVD is not competitive to other methods. Sintered ceramic membranes made of a wide range of metal oxides (U.S. Pat. No. 4,946,592) provide rather large pores (20-20,000 nm). In another procedure for the preparation of inorganic membranes with the aid of polymers, that are burned off after preparation (U.S. Pat. No. 4,980,062) give membranes with pore diameters of 100 to 500 nm, incorrectly claimed as microporous. α-alumina ceramic membranes with effective pore sizes of about 50 nm can be made from boehmite sols seeded with varying α-alumina particles (U.S. Pat. No. 4,968,426). Multilayered ceramic films with pore sizes in the upper mesopore range (10-20 nm) have been prepared from metal oxide particles (EP 0 426 546 A 2) without giving examples of separations. A SOL-GEL procedure for the preparation of metal oxide ceramics (EP 0 425 252 A 1) gives membranes with mean pore diameters, not exceeding 4 nm. Hydrogen selective inorganic membranes of unknown separation mechanism have been prepared by closing the pores of the support membrane by CVD (Tsapatsis, Kim, Nam, Gavalas, Ind. Eng. Chem. Res. 30 (1991), 2151). 
     No methods have been known to produce continuous inorganic membranes with confirmed pore diameters of less than 1.2 nm. We have found, however, that continuous inorganic membranes with pore diameters below 1 nm can be prepared by e-beam evaporation of inorganic oxides. (With continuous we mean a homogeneous uniform material even at the nm-scale.) According to this invention continuous microporous materials can be prepared by e-beam evaporation in a vacuum chamber by heating the starting material with an e-beam of at least 2 kW until it evaporates and condenses on a suitable support material. In this procedure the porosity of the material produced is not sensitive to the method of evaporation. Thin continuous films of any thickness with a better than nm precision can be prepared by this condensation method. Layer thickness can be controlled by a film thickness monitor. The pore size distribution of the membrane is determined by the nitrogen adsorption isotherm with many data points in the pressure range below 10 torr. The data treatment can be done with the t-plot method or by the method of Horvath and Kawazoe. 
    
    
     The invention will be further described with reference to the accompanying drawings, wherein: 
     FIG. 1 is a plot of the pore size versus frequency of a membrane in accordance with the invention; 
     FIG. 2 is a schematic view of an apparatus for use of a novel capillary for gas permeation; 
     FIG. 3 is a schematic view of an apparatus for use of a novel capillary for gas separation; and 
     FIG. 4 is a schematic view of another apparatus for use of a novel capillary for gas permeation. 
    
    
     Referring now more particularly to the drawings, FIG. 1 shows the size distribution of the pores of a membrane produced in accordance with the present invention determined by the nitrogen adsorption isotherm in an equipment with high sensitivity in the pressure range below 10 torr. The data were treated by the method of Dubinin (Dubinin Co. Interface Sci. 23 (1967) 478) and the t-plot-method (Mikhail, Brunauer, Bodor J. Coll. Int. Sci. 26 (1968) 45). Aerosil 200 was used as an unporous reference material. 
     Membranes suitable for gas separation can be prepared by condensation of the membrane on a mesoporous support membrane with symmetrical or asymmetrical pores of pore diameters larger than 5 nm, but smaller than 2 μm. Such support membranes, made of alumina or silica, are available commercially. Only flat membranes or capillary membranes qualify. The microporous membrane for the separation has to be deposited on the outside of the capillary and on the side of the bottle neck pores on asymmetrical flat membranes. Mesoporous capillary membranes have to be rotated stepwise or continuously during the condensation to achieve a homogeneous layer formation on the outer surface. 
     Such membranes can be used in commercial filtration units or capillary reactors for liquid or gas separation at temperatures up to 500° C. The separation efficiency increases with increasing pressure difference between permeate and feet. Best separation results are achieved by excess pressure on the feed and reduced pressure on the permeate side. 
     Suitable materials for the membranes are metal oxides of high thermal and chemical resistance like silica, alumina, titania, zirkonia and cer oxide. 
     The membranes can principally be used for the following applications: 
     Separation of ethane/ethene, propane/propene, methane/nitrogen, methane/water, oil/asphalt, ammonia/hydrogen/nitrogen, hydrogen/carbon monoxide. The membranes can be used to separate the water from fruit juices without loss of larger molecules like aroma compounds, fruit acids, vitamines and other important juice components. Similarly biological fluids can be concentrated by the removal of water while biologically important compounds like small peptides, hormones and others will be retained. The membranes can also be used for the preparation of ultrapure water. The removal of water from waste water, like the retention of the coal oil from the waste water from a coking plant should be possible with such a membrane. The membranes can also be used for the separation of solvents with retention of dissolved compounds like oligomers, polymers, unknown hazardous compounds or side products. If the membranes contain catalytically active centers like catalytically active metals, the membranes can be used for three phase reactions, where the reactant gas (hydrogen, oxygen, carbon monoxide or halogen) pass the membrane and react with the reactant liquid on the other side of the membrane. 
     EXAMPLE 1 
     A mesoporous BIORAN-glass capillary from the SCHOTT company (pore diameter 10 nm) was coated with silica at 10 -5  to 10 -4  torr e-beam evaporation of quarz in an evaporation chamber. The capillary was turned continuously 40 times/min during the deposition to assure a uniform deposition of the silica on the outer side of the capillary. The deposition at a rate of about 10 nm/min was controlled by a film thickness monitor and terminated at a layer thickness of 500 nm, which corresponds to a layer thickness of 160 nm on the capillary. The capillary was mounted in the apparatus shown in FIG. 2 and sealed by a silicon polymer. The gas permeation of several gases was determined at various pressures. The results, shown in the following table, demonstrate that at these conditions only the gas isobutane is strongly hindered in its diffusion, while all other gases can be characterized by Knudsen diffusion. This indicates a molecular sieving diameters of the size of molecules. 
     Table: Separation factors relative to hydrogen determined by gas separation through a glass capillary covered by a microporous silica membrane at various pressures. The permeation factors are given as the ratio of the space velocities V H .sbsb.2 /V gas . 
     
         ______________________________________                                    
Gas       0.5 bar 1 bar     2 bar theor. Value*                           
______________________________________                                    
CO        3.2     3.3       3.6   3.7                                     
N.sub.2   3.0     3.1       3.3   3.7                                     
i-butane  10.7    178.0     339.0 5.4                                     
Ar        3.6     3.8       4.4   4.4                                     
CH.sub.4  2.2     2.3       2.5   2.8                                     
He        1.8     1.6       1.6   1.4                                     
______________________________________                                    
 *(M 1/M 2).sup.0.5                                                       
 
    
     EXAMPLE 2 
     A mesaporous BIORAN-glass capillary (pore diameter 10 nm) was coated with a silica membrane as in Example 1. The capillary was mounted in an apparatus shown in FIG. 3, sealed by silicon. The separation of the membrane with several gas mixtures was determined at various pressure gradients with an on-line quadrupole. The composition was obtained through the calibrated ion pressure. All mixtures show separation factors better than Knudsen supporting the high separation efficiency of the small pores of these membranes (see Table). 
     
         ______________________________________                                    
P 2(Torr)                                                                 
        dp(Torr)  H.sub.2 /Ar                                             
                          H.sub.2 /CO                                     
                                 H.sub.2 /i-Bu                            
                                        H.sub.2 /N.sub.2                  
______________________________________                                    
760     310        2.8     3.9   1.1      3.5                             
50      930        6.4    15.3   1.3    3                                 
10      980       10.9    --     7.1    7                                 
 1      1010      28.2    29.6   8.5    --                                
______________________________________                                    
 
    
     EXAMPLE 3 
     A flat alumina membrane (diameter 48 mm, pore size 20 nm) from the company ANOTEC was coated with a silica membrane through e-beam evaporation of quarz in an evaporation chamber as described in Example 1. The evaporation at a rate of 15 nm/min was terminated at a layer thickness of 520 nm. The membrane on the alumina support was mounted in an apparatus as shown in FIG. 4. The gas permeation of helium and of hydrogen showed a separation factor of 1.7-1.9 at 0.2 bar excess pressure and 1.6-1.7 at 0.5 bar excess pressure, which is significantly better than the Knudsen limit of 1.4. 
     EXAMPLE 4 
     A Vycor glass membrane from Corning (diameter 47 mm, thickness 2 mm) was coated with a silica membrane, thickness 1000 nm, by e-beam evaporation of quarz as described in Example 1. This membrane was mounted in a stainless steel filtration unit (Sartorius) and apple juice with pH of 5.4 was filtered through this membrane at a hydrostatic pressure of 1 m. The permeate was clear colorless water of neutral pH and taste.