Abstract:
The present invention provides for methods and compositions for gas separation and purification utilizing a metallo-organic polymer adsorbent in processes for separating carbon dioxide, water, nitrogen oxides and hydrocarbons from gas streams. The metallo-organic polymer adsorbent composition has the formula: 
     
       
         [R(L) n ] m M n , 
       
     
     wherein R represents an organic spacer selected from the group consisting of an organic cyclic or acyclic compound; L represents a ligation group substituted on the organic spacer selected from the group consisting of carboxylate group, —C(═O)O − ; dithiocarboxylate group, —C(═S)S − ; and β-diketonate group, —C(═O)C(R′)═C(—O − )—, wherein R′═H, or an aliphatic or aromatic group; M represents a transition metal or a rare earth metal selected from the group consisting of I to VIIB and VIII metals; m is the oxidation state of transition metal; and n is the number of the ligation group substituted on the organic spacer.

Description:
This application claims priority from Provisional Patent Application No. 60/145,123 filed Jul. 22, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a new class of microporous adsorbents for gas separation and purification. For example, the novel adsorbents of the present invention may be used in pressure swing adsorption (PSA) or thermal swing adsorption (TSA) PPU (Pre-purification Unit) for removal of CO 2 , H 2 O, N 2 O, and oil vapor from air streams prior to cryogenic air distillation, hydrocarbons/CO 2  separations, and syngas separations. In particular, the present invention relates to novel microporous metallo-organic adsorbents having pore sizes and pore volumes that are appropriate for gas separation and purification. 
     BACKGROUND OF THE INVENTION 
     Presently, numerous microporous materials that are mainly based on zeolites, other zeo-type inorganic solids, various types of activated carbon, such as carbon molecular sieves and super-activated carbons, are being used as solid adsorbents for gas separation and purification. Zeolites are porous crystalline aluminosilicates, whose framework is constructed by SiO 4  and AIO 4  tetrahedra, which are joined together in various regular arrangements through shared oxygen atoms, to form an open crystal lattice containing uniform pores. Each aluminum atom within that lattice introduces one negative charge on zeolite framework which must be balanced by a positive charge of an exchangeable cation. After activation of zeolites, the exchangeable cations are located, in most cases, at preferred extra-framework sites within the voids formed by the lattice. These cations play a significant role in determining the adsorption properties of the particular zeolites. Moreover, zeolites have high chemical and thermal stability owing to their inorganic nature of the framework. Unlike zeolites, the structure of activated carbons mainly consists of elementary microcrystallites of graphite. These microcrystallites are stacked together in random orientation. The micropores are formed by the spaces between those microcrystallites, the diameter of which range from ˜3Å to ˜20Å. In addition, there may exist mesopores (20 to 500Å) and macropores (&gt;500Å), as well. Therefore, activated carbon adsorbents generally show very little selectivity over molecules with different sizes. However, due to the nonpolar surface of carbon, an activated carbon tends to be hydrophobic and organophilic. With respect to carbon molecular sieves, their structure is similar to that of activated carbon in most general terms, but they have a very narrow distribution of micropore sizes ranging from about 3 to 9Å, for the various types, and thus they behave as molecular sieves. 
     Clearly, all of these materials, zeolites, activated carbon, and carbon molecular sieves, combine microporosity with high chemical and thermal stability, which are essential for gas separation and purification. Although it is difficult to design and build the microporous structures of these sieve materials with specific pore sizes in a systematic way, a wealth of structures is known by now. Variation of chemical formulation and functionality, however, meets limits due to complexity of phenomena and costs involved. The present invention is directed to novel microporous adsorbents which can be designed with specific pore sizes in a systematic way. 
     SUMMARY OF THE INVENTION 
     It is the primary object of the present invention to provide a novel class of microporous adsorbents for gas separation and purification. 
     It is another object of the present invention to provide practical applications of the microporous adsorbents of the present invention in gas separation and purification processes. 
     To achieve the foregoing objects and advantages and in accordance with the purposes of the invention as embodied and broadly described herein, a new class of microporous adsorbents comprises a metallo-organic polymer, which is characterized by a formula set forth below: 
     
       
         [R(L) n ] m M n , 
       
     
     wherein 
     R is an organic spacer; 
     L is a ligation group substituted on an organic spacer, e.g., carboxylate group, —C(═O)O − ; dithiocarboxylate group, —C(═S)S − ; and β-diketonate group, —C(═O)C(R′)═C(—O − )—, R′═H; or an aliphatic or aromatic group; 
     n denotes the number of ligation group, n&gt;2; 
     M denotes transition metal or rare earth metal, excluding Co, Cu, Zn, Tb when the organic spacer is a benzene ring, the ligation group is carboxylate, and n equals 2 or 3; and 
     m denotes the oxidation state of transition metal. 
     Structural modification of the new class of microporous adsorbents can also be made to further enhance desirable functionalities. For example, the channel linings can be chemically functionalized using organic bases, or they can be doped with inorganic salts such as lithium, silver, copper(I/II) salts to create specific adsorption sites that are necessary for gas separation and purification. 
     Unlike zeolites, other microporous zeo-type materials and carbons, the new adsorbent materials provide unique surfaces which are comprised of carbon, hydrogen, oxygen and nitrogen atoms, and framework metal sites. Owing to their tunable organic and metallic parts of the microporous structures, they have advantages over presently utilized adsorbents in engineering of specific pore sizes and sorption sites in a systematic way. Therefore, along with zeolites, carbon molecular sieves and other carbonaceous adsorbents, this new class of adsorbents expand material selections for industrial gas separation applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph of adsorption equilibrium data for the use of an adsorbent of the present invention for CO 2  and CH 4 . 
     FIG. 2 is a graph of adsorption equilibrium data for the use of an adsorbent of the present invention for N 2 , O 2 , CO 2  and N 2 O. 
     FIG. 3 is a graph of adsorption equilibrium data for the use of an adsorbent of the present invention for N 2 O 2 , CH 4 , CO 2  and ethylene. 
     FIG. 4 is a graph of adsorption equilibrium data for the use of an adsorbent of the present invention for ethylene and ethane. 
     FIG. 5 is a graph of adsorption equilibrium data for the use of an adsorbent of the present invention for N 2 O. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention relates to a new class of microporous adsorbents for use in gas separation and purification. In particular, for use in PSA PPU and TSA PPU processes for CO 2 , H 2 O, N 2 O, and oil vapor removal from air, separation of hydrocarbon/CO 2  mixtures, and syngas. These microporous metallo-organic polymers can be produced by metal-ligand dative bonds from transition metal ions, and bi-, tri-, tetra-, or multicarboxylates, and the like, to form highly porous metal coordination polymers, which can be characterized by the following formula: 
     
       
         [R(L) n ] m M n , 
       
     
     wherein 
     R is an organic spacer; 
     L is a ligation group; 
     n denotes the number of ligation group, n&gt;2; 
     M denotes transition metal or rare earth metal; and 
     m denotes the oxidation state of transition metal. 
     The spacer, R, can be chosen from any organic cyclic or acyclic systems. For example, systems with double bonds, benzene rings, or macrocyclic rings are considered. L is a ligation group substituted on an organic spacer. It can be chosen from carboxylate group, —C(═O)O − ; dithiocarboxylate group, —C(═S)S − ; and β-diketonate group, —C(═O)C(R′)═C(—O − )—, wherein R′═H, or an aliphatic or aromatic group. M can be I-VIIB metals including rare earth metals, and VIII metals, where the Roman numbers refer to the Periodic Table of Elements. Typically, they are Zn, Cu, Co, Ru, Os, and rare earth metals. However, in the case where the organic spacer is a benzene ring, and the ligation group is a carboxylate (n=2, 3), M excludes Co, Cu, Zn, Tb. 
     The following examples are given to demonstrate novel microporous adsorbents of the present invention characterized by the formula [R(L) n ] m M n , where the ligation groups are dithiocarboxylate and β-diketonate groups, such as                           
     Table 1 below sets forth, for illustrative purposes, various multicarboxylic acids suitable for use in the formation of the novel microporous adsorbents of the present invention characterized by the formula [R(L) n ] m M n , 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Name 
                 Structure  
               
               
                   
               
             
             
               
                 oxalic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 acetylene dicarboxylic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 2-carboxycinnamic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 1,4-naphthalene-dicarboxylic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 2,6-naphthalene-dicarboxylic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 1,1′-ferrocene-dicarboxylic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 porphyrin-(CO 2 H) 4   
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 porphyrin-(CO 2 H) 8   
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
     The microporous metallo-organic polymers of the present invention are either X-ray crystalline, amorphous or at the borderline of such crystallinity. In general, their micropore topologies are mostly different from those of the inorganic zeo-type ones, but in a number of cases they may be analogous to the latter, in terms of structural patterns. Important features of such adsorbents were a very narrow (homogeneous) pore size distribution, a high pore volume and the opportunity of tailoring the pore size, intentionally. Unlike zeolites, the channel linings can be functionalized chemically. They can also be doped comparatively easily with inorganic salts such as lithium, silver, or copper(I) salts or mixtures thereof, to create specific adsorption sites for gas separation and purification. For example, copper(I/II) metal salts can be supported on these adsorbents for their use as CO-selective adsorbents. In order to do so, a cuprous compound or a cupric compound, such as CuCl or CuCl 2 , can be impregnated into a microporous metallo-organic framework such as that of copper (II) benzene-1,3,5-tricarboxylate, Cu-BTC, with the aid of solvents. The solvents include water, alcohol, acetone, ethyl acetate, acetonitrile. The most preferred solvents are water and alcohol. 
     It is envisioned that the novel microporous metallo-organic polymer adsorbents can be utilized in: 
     a) PSA PPU and TSA PPU for removal of CO 2 , H 2 O, N 2 O, and oil vapor from air streams; 
     b) Removal (trapping) of heavy hydrocarbons, in particular compressor oil vapor, in N 2  PSA and O 2  VSA/PSA processes; 
     c) PSA/TSA processes for bulk phase removal of CO 2 , H 2 O and high level of hydrocarbons (&gt;C 2 ) from CH 4 ; 
     d) PSA/TSA processes for separation of hydrocarbon/CO 2  mixtures such as ethylene/CO 2 ; 
     e) PSA/TSA processes for bulk phase separation and purification of CO from CO 2 , CH 4 , H 2 , N 2 , O 2  and Ar; 
     f) Removal of CO 2  from CO 2 /C 2 H 2  mixtures (bulk and trace); 
     g) Separation of gaseous olefin/paraffin mixtures of low molecular weight, e.g., ethylene-ethane mixtures; 
     h) Drying agent and moisture sensor utilization. 
     The following examples are set forth below for illustrative purposes. 
     EXAMPLE 1 
     Preparation of copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC) 
     Benzene-1,3,5-tricarboxylic acid (1 g, 4.76 mmol) was dissolved into ethanol (12 ml), and cupric nitrate hydrate (Cu(NO 3 ) 2 ·2.5H 2 O;2 g, 8.60 mmol) was dissolved into ethanol (12 ml). The two solutions were mixed at ambient temperature for 30 minutes. Then they were poured into an autoclave with a 45-ml teflon cup. The autoclave was heated at 110° C. under hydrothermal conditions for 17 hours. The reaction vessel was cooled to ambient temperature, and then the blue hexagonal crystals of Cu-BTC were isolated by filtration, and washed with water. The product was dried at 110° C. overnight. The yield was 87%. Analysis by TGA showed that a fully hydrated product contained up to 40 wt. % water, and it was stable up to ca. 340° C. 
     Adsorption Equilibrium Properties for Gas Separation Processes 
     The Cu-BTC adsorbent of Example 1 was tested for its adsorption equilibrium properties and suitability in a number of gas separation processes. The adsorption data were obtained using a piezometric (constant volume−variable pressure) method at 22° C. The results of these tests are set forth in FIGS. 1 to  5 . 
     FIG. 1 presents the adsorption equilibrium data for CO 2  and CH 4 . The data demonstrates that the adsorbent can be used in separation of gaseous mixtures containing CO 2  and CH 4 , such as syngas separations, due to preferential adsorption of CO 2 . 
     Adsorption equilibrium data for N 2 , O 2 , CO 2  and N 2 O on Cu-BTC sorbent are presented in FIG.  2 . The data demonstrates that the adsorbent can be used in PSA PPU processes for removal of CO 2  and N 2 O from air streams, due to preferential adsorption of CO 2  and N 2 O. 
     Adsorption equilibrium data for N 2 O 2 , CH 4 , CO 2  and ethylene on Cu-BTC sorbent are presented in FIG.  3 . The adsorption data demonstrates that the adsorbent preferentially adsorbing ethylene to other gases can be used in recovery of ethylene in ethylene partial oxidation processes. 
     Adsorption equilibrium data for ethylene and ethane on Cu-BTC adsorbent are presented in FIG.  4 . The adsorbent preferentially adsorbs ethylene as compared to ethane. It can also be used to separate other olefin/paraffin mixtures. 
     Adsorption equilibrium data for H 2 O on Cu-BTC sorbent are presented in FIG.  5 . The adsorbent provides a very large saturation capacity for water, and its color changes from dark purple when it is dry to light blue when water is adsorbed. Thus, the material can be used both as a drying agent and as moisture sensor, and also as an adsorbent for moisture removal using PSA/TSA processes, such as in PPU processes upfront to cryogenic air distillation. 
     EXAMPLE 2 
     Impregnation of CuCl 2  into Cu-BTC for CO-Selective Adsorbent 
     Cu-BTC was dried at 150° C. for 4 hours before use. CuCl 2 ·6H 2 O (1.22 g) were dissolved into absolute alcohol (20 ml). Cu-BTC (3 g) was then added to the solution. The mixture was stirred for 30 minutes using a magnetic stirrer before putting it in a mechanical shaker for 22 hours at ambient temperature. Next, all solvent was removed under a reduced pressure yielding a blue material. It was then dried at 110° .C under vacuum. The yield was 97%. 
     The adsorbent was heated at 190° C. under CO atmosphere for 3 hours and evacuated before the CO uptake test was performed on a microbalance. As a result, at 25° C., this adsorbent has a CO adsorption capacity of 1.08 mmol/g at a CO pressure of 984 mbar. 
     EXAMPLE 3 
     Oil Vapor Removal Test without Moisture 
     Sorbent beads of the metallo-organic polymer, made of Cu-BTC powder by shaping the latter at appropriate conditions, were evaluated for oil vapor removal from compressed air in an n-dodecane sorption breakthrough experiment. 
     About 94 gram of the adsorbent beads was packed in a 1″ adsorbent vessel. A feed consisting of dry N 2  and about 10 ppm n-dodecane vapor at total pressure of 80 psia, temperature, 25° C., and flowrate, 100 scfh, was used in an sorption breakthrough experiment to determine the sorption capacity of the novel sorbent for n-dodecane. The sorbent takes up ca. 68 wt. % of n-dodecane at n-dodecane partial pressure, 0.04 torr, by a physical sorption process, before the n-dodecane concentration in the effluent stream reaches 1 ppm. This experiment proves that the new sorbent has an extremely large sorption capacity for hydrocarbons. 
     EXAMPLE 4 
     Oil Vapor Removal Test with Moisture 
     Similar n-dodecane sorption breakthrough experiments as described in Example 3 were also carried out with a moisture-saturated N 2  stream and an n-dodecane concentration therein, that amounts to 100 ppm. A sorption capacity of 1.85 wt. % was measured at an ndodecane partial pressure of 0.4 torr before the n-dodecane concentration in the effluent stream reaches 0.2 ppm. Thus, it has proven that the novel sorbent can still sorb a significant amount of hydrocarbons under conditions of relatively high humidity. 
     EXAMPLE 5 
     PSA cyclic experiments of 30 minutes feed and 30 minutes N 2  purge were also performed to demonstrate the regeneration properties of the novel sorbent. The feed step was carried out at a total pressure, 80 psia, temperature, 25° C., with both dry and moisture-saturated gas streams containing 10 ppm n-dodecane at a flow rate of 100 scfh. House nitrogen at a flowrate, 50 scfh, was used to purge the adsorber at 25° C. and 15.0 psia. Cyclic experimental results with both dry and wet gas streams as feed have shown that the sorbed amount of n-dodecane can be desorbed, and the sorbent was gradually regenerated in the PSA cycles with a purge-to-feed ratio of ca. 2.6 for both cases. 
     While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.