Crystalline substituted stannosilicates and preparation and use thereof

Abstract: Crystalline substituted stannosilicates are disclosed of the formula xM a O·SnO₂·4±O.5(Si+T)O₂·yH₂O·zR, wherein M is a Group 1A metal and a=2, or M is a Group IIA metal and a=1, or M is a quaternary amine; T is germanium, aluminum, gallium, zinc or mixtures thereof; wherein x is 1.5 to 4, y is 4 to 15, R is an amine and z is 0 to 4; and wherein the framework species comprises tin in octahedral coordination, Si and T in tetrahedral coordination, and oxygen. Useful in H₂S contamination extraction from gaseous streams.

Description: This invention relates to novel class of highly crystalline substituted, stannosilicate materials and to their preparation and use. In one aspect the invention provides highly crystalline substituted, stannosilicates having the generic formula: xM 2 O.SnO 2 .4±0.5(Si+T)O 2 .yH 2 O.zR, wherein M may be any Group IA or, where M 2 O becomes MO, Group IIA metal, or a quaternary alkyl ammonium or alkylamine, e.g., tetraethylamine; and wherein T is germanium, gallium, aluminum, zinc or mixtures thereof; and x is 1.5 to 4 y is 4 to 15 R is an amine z is 0 to 4 and the ratio of (Al, Zn, Ga, and/or Ge)/Si is preferably from 0. 02 and 0.40. The silicon and the germanium, aluminum, gallium, or zinc are tetrahedrally coordinated through oxygen, and tin is octahedrally coordinated through oxygen and hydroxyl groups to form three dimensional framework structures within which the cations and alkylammonium molecules are located to charge balance the structure. The ionexchange properties of these materials, and their properties of reversibly sorbing water and other polar molecules, confirms their microporous nature in contrast to the non-sorbing dense structures of the prior art. The silicon and metal(s) substituted for a portion of the silicon are in tetrahedral coordination and tin is in five or six fold coordination. These novel materials are useful as catalysts, catalyst supports, sorbents, e.g., for the separation of hydrogen sulfides from gas streams containing hydrogen contaminated with hydrogen sulfides or oxysulfides. Thus, the framework substituted stannosilicates of this invention contain a backbone or framework comprising tin, silicon (and its substituents, Ga, Al, Ge, Zn or mixtures thereof), and oxygen in which the tin is octahedrally coordinated. The ratio of tin to tetrahedral elements (Si and its substituents) in the framework is about 1:3.5 to about 1:4.5 and the tin is not exchangeable and essentially all of the tin is in the framework. In accordance with another another aspect, the stanno-silicates may be prepared by reacting a base, that is, a Group IA or Group IIA metal (e.g., sodium, potassium cesium, rubidium, magnesium, calcium, strontium) hydroxide or mixtures thereof or quaternary amine with a water soluble tin salt, e.g., chlorides, nitrates, sulfates or salts derived from dissolving tin oxide in an acid or base, and silica or a source of silica and a source of soluble forms of germanium, zinc, aluminum, gallium, or mixtures thereof in an aqueous medium at conditions leading to the formation of these novel crystalline materials. Such soluble forms may be germanates, aluminates, gallates, zincates or acid salts of Ge Al, Zn and/or Ga, such as chlorides, nitrites, sulfates, etc. DESCRIPTION OF THE DRAWINGS Figure 1 shows four different kinds of structures comprising linked oxygen tetrahedra, octahedra or mixtures of both of these. In addition to corner sharing, edge and face shared octahedra are also common in natural and synthetic materials. Figure 1(a) is the chain of corner shared tetrahedra found in the zeolite mineral mordenite (Meier, "Molecular Sleves", Ed. R. M. Barrer, Soc. Chem. Ind. (London), p. 41 (1968)). Figure 1(b) is an illustration of sheets of corner shared tetrahedral linked to corner and edge shared octahedra in the mineral kaolinite, a linkage typical of all the clay minerals. Figure 1(c) is made up of edge and face shared octahedra, in this case characteristic of the mineral hollandite and the synthetic form α-MnO 2 , and typical of numerous oxides of W, Mn, Ti and Nb (see for a review, Wadsley, "Nonstoichiometric Compounds", Ed. L. Mandelcorn, Academic Press (London), p. 99 (1964)). Figure 1(d) comprises a three dimensional structure of Si 5 P 6 O 25 (Mayer, Monatsh. Chem., 105 , p. 46 (1974)) comprising Si and P tetrahedral linked to Si octahedra. Figure 2 comprises x-ray diffraction patterns (Cu K d radiation) for the novel substituted, stannosilicate phases A, B, and G of this invention. DESCRIPTION OF THE INVENTION Of the metal salts reacted with silica and germania, alumina, zinc, gallia, or mixtures thereof and the tin salt, the alkali metals: lithium, sodium, potassium, rubidium, cesium and mixtures thereof are preferred. Particularly preferred are sodium and potassium or mixtures thereof. In a preferred embodiment, the novel substituted, stannosilicates are formed using a mixed cation preparation, that is, lithium and sodium hydroxides, sodium and cesium hydroxides. Generally, mixtures provide substituted, stannosilicates of enhanced purity over preparations using only one alkali salt. The reaction generally occurs under autogenous pressure at temperatures above about 90°C and below about 250°C, preferably 150-225°C. The ratio of tin to silicon and substituents is no greater than about 1:20 and preferably about 1:2 to about 1:20, while the ratio of tin to germanium, aluminum, zinc, gallium or mixtures thereof is about 0 to 2.0. The reactants generally combine to form a gel-like substance and crystalllzation occurs at reaction temperatures for a period of a few days to several weeks, e.g., seven days to eight weeks. After the appropriate time period, the reaction is quenched, the crystalline powder is recovered by, for example, filtration, and thoroughly washed and dried. Drying can be accomplished by an overnight heating at about 100°C. Impurities can be removed by an acid treatment, e.g., 1N HCl. Generally the aluminum substituted forms crystallize in less time than the germanium and gallium forms. Often, it is preferred to age the reaction mixture at ambient conditions, i.e., below about 50°C for at least about 3 hours, preferably at least about 24 hours, to allow nucleation. Alternatlvely, nucleation can be effected by adding a nucleating compound such as a finely divided oxide, e.g., alumina, or a nucleation solution as used in zeolite synthesis and reported in U.S. patent numbers 3,808,326 and 4,178,352. Up to about 10 vol% nucleating compound can be employed. Thermal dehydration at, for example, 300°C will drive off water and the x-ray diffraction pattern will change slightly to show a contraction of the lattice, or movement of cations, or both. In adsorbent or catalytic processes the substituted, stannosilicate is usually dried or calcined to remove pore filling water cations or amine and in the generic formula shown above, y or z or both may then be zero. The following examples will further illustrate this invention. Reactant grade stannous chloride pentahydrate and anhydrous stannic chloride were used without further purification. Colloidal silica (Ludox HS-40) was obtained from DuPont and fumed silica (Cabosil) was obtained from Cabot Corp. Germanates, aluminates, zincates and gallates were made by dissolving the respective oxides or hydroxides in hot concentrated alkali solutions. All reactions were carried out hydrothermally in 75 ml stainless steel autoclaves under autogenous pressures. Powder x-ray diffraction measurements were conducted on a Siemens D500 powder diffractometer using a Siemens DIFFRAC-5 data system. Reactants were thoroughly mixed at room temperature according to the ratio 2-8 M 2 O:SnO 2 :2-20(Si, Al, Zn, Ga, Ge)O 2 :0-2:80-180 H 2 O where M = NH 4 , Na, R, Cs, Li, Rb, or alkyl (C 1 -C 4 ) quaternary ammonium mixtures of two or more cations. The resulting gel was then reacted at a temperature between 150°C and 250°C for period lasting between one week and two months. At the end of this time the autoclave was cooled to room temperature and the solid product separated by filtration. The crystalline powder was then washed repeatedly with water and dried at 100°C. The following are representatives of typical syntheses in the substituted sodium stannosilicate system. The phases are identified by their unique x-ray diffraction patterns. Example 1 A tin (IV) chloride pentahydrate (Aldrich) solution was prepared by addition of the solid to distilled water. A second solution was produced by combining sodium hydroxide (Fisher) and 40% colloidal silica (HS-40, DuPont) in distilled water followed by vigorous stirring; a source of alumina was next added to the second solution and sometimes heated to aid in dissolution. The two mixtures were combined at room temperature by slow addition of the alumina/silica solution to the aqueous tin chloride solution with rapid stirring (exothermic), producing a thick white gel. The gel was reacted in a polytetrafluoroethylene-lined autoclave (Parr) at 200°C to produce a white solid after the reactor was cooled to room temperature. This solid was washed repeatedly with distilled water, filtered, and dried to yield a fine white powder. Reactant ratios, sources, and conditions are given in Table 1. Chemical analyses of products are given in Table 2. CHEMICAL ANALYSES EXPERIMENT WT% Na WT% Sn WT% Si WT% Al WT% Al/Si 1 12.16 22.79 19.51 1.60 .09 2 10.93 23.66 19.07 2.22 .12 3 10.00 25.14 20.70 1.16 .06 4 10.17 21.76 19.77 2.09 .11 5 - - - - - Phase A was the only pure phase made in these experiments. The typical x-ray diffraction pattern for phase A is shown in Table 3. X-RAY DIFFRACTION PATTERN FOR PHASE A dÅ I/Io 6.33 ± 0.15 29 5.83 ± 0.15 100 5.56 ± 0.15 42 5.19 ± 0.10 12 4.73 ± 0.10 8 4.28 ± 0.10 18 3.30 ± 0.08 23 3.05 ± 0.08 73 2.91 ± 0.05 69 2.77 ± 0.05 27 2.64 ± 0.05 11 2.45 ± 0.05 15 2.35 ± 0.05 9 Example 2 A tin (IV) chloride pentahydrate (Aldrich) solution was prepared by addition of the solid to distilled water; 20% colloidal alumina (AL-20, PQ Corp.) was stirred into this solution and a white precipitate formed. A second solution was produced by dissolving base in distilled water and combining this solution with colloidal silica (HS-40, DuPont). This second mixture was slowly combined with the tin/alumina solution, with vigorous stirring, to produce a thick white gel (exothermic). The gel was reacted in a polytetrafluoroethylenelined autoclave (Parr) at 200°C to produce a white solid after the reactor was cooled to room temperature. This solid was washed repeatedly with distilled water, filtered, and dried to yield a fine white powder. Reactant ratios, sources, and conditions are given in Table 4 Chemical analyses of products are given in Table 5. In these experiments Phase A crystallized from the sodium experiments and Phase G from the potassium experiments. A typical x-ray diffraction pattern for Phase G is given in Table 6. Example 3 A tin (IV) chloride pentahydrate (Aldrich) solution was prepared by addition of the solid to distilled water; 20% colloidal alumina (AL-20, PQ Corp.) was stirred into this solution and a white precipitate formed. An aqueous potassium hydroxide (Fisher) solution was added to the tin chloride solution and stirred; colloidal silica (HS-40, DuPont) was added to the mixture (to give a final reactant ratio of 2K 2 O:3.8SiO 2 :SnO 2 :0.1Al 2 O 3 :80H 2 O) and the resulting gel stirred until homogeneous. The gel was reacted In a polytetrafluoroethylene-lined autoctave (Parr) at 200°C for 21 days to produce a white solid after the reactor was cooled to room temperature. This solid was washed repeatedly with distilled water, filtered, and dried to yield a fine white powder (Phase G). Chemical analysis indicated 13.67% K, 22.82% Sn, 18.69% Si, and 0.92% Al. REACTION CONDITIONS EXPERIMENT REACTANT RATIOS ALKALI SOURCE REACTION TIME PHASE 6 2Na 2 O:3.8SiO 2 :SnO 2 :0.1Al 2 O 3 :80H 2 O sodium hydroxide (Fisher) 21 days A 7 2Na 2 O:3.6SiO 2 :SnO 2 :0.2Al 2 O 3 :80H 2 O sodium hydroxide (Fisher) 14 days A 8 2Na 2 O:3.6SiO 2 :SnO 2 :0.2Al 2 O 3 :80H 2 O sodium hydroxide (Fisher) 21 days A 9 2K 2 O:3.6SiO 2 :SnO 2 :0.2Al 2 O 3 :80H 2 O potassium hydroxide (Fisher) 21 days G CHEMICAL ANALYSES EXPERIMENT WT% K WT% Na WT% Sn WT% Si WT% Al WT% Al/Si 6 - 10.61 24.71 20.71 1.06 .05 7 - 13.05 24.76 18.86 2.53 .14 8 - 11.99 23.99 18.49 5.55 .31 9 14.22 - 22.29 18.36 2.22 .13 X-RAY DIFFRACTION PATTERN FOR PHASE G dÅ I/Io 8.05 ± 0.20 17 6.58 ± 0.15 59 5.86 ± 0.15 76 5.35 ± 0.15 8 5.06 ± 0.10 18 4.84 ± 0.10 7 4.37 ± 0.10 40 4.02 ± 0.10 13 3.59 ± 0.08 18 3.51 ± 0.08 16 3.27 ± 0.08 17 3.12 ± 0.08 8 3.05 ± 0.08 19 2.98 ± 0.05 100 2.93 ± 0.05 38 2.86 ± 0.05 53 2.75 ± 0.05 12 2.67 ± 0.05 14 2.57 ± 0.05 10 2.50 ± 0.05 12 2.41 ± 0.05 9 2.14 ± 0.05 14 2.04 ± 0.05 12 1.93 ± 0.05 19 1.84 ± 0.05 10 Example 4 A tin (IV) chloride pentahydrate (Aldrich) solution was prepared by addition of the solid to distilled water. A second solution was produced by combining either soldium or potassium hydroxide (Fisher) and 40% colloidal silica (HS-40, DuPont) in distilled water followed by vigorous stirring; gallium oxide (Aldrich) was next added to the second solution and sometimes heated to aid in dissolution. The two mixtures were combined at room temperature by slow addition of the gallium oxide/silica solution to the aqueous tin chloride solution with rapid stirring (exothermic), producing a thick white gel. The gel was reacted In a polytetrafluoroethylene-lined autoclave (Parr) at 200°C to produce a white solid after the reactor was cooled to room temperature. This solid was washed repeatedly with distilled water, filtered, and dried to yield a fine white powder. Reactant ratios, sources, and conditions are given in Table 7. Chemical analyses of products are given in Table 8. A typical x-ray diffraction pattern for phase B is shown in Table 9. REACTION CONDITIONS EXPERIMENT REACTANT RATIOS ALKALI SOURCE REACTION TIME PHASE 10 2K 2 O:3.8SiO 2 :SnO 2 :0.1Ga 2 O 3 :80H 2 O potassium hydroxide (Fisher) 14 days G 11 2Na 2 O:3.6SiO 2 :SnO 2 :0.2Ga 2 O 3 :80H 2 O sodium hydroxide (Fisher) 14 days B CHEMICAL ANALYSES EXPERIMENT WT% K WT% Na WT% Sn WT% Si WT% Ga WT% Ga/Si 10 16.40 - 22.60 17.59 2.08 0.12 11 - 9.46 23.98 18.50 4.72 0.26 2 theta d obsv I/I o 5.577 15.8337 2.0 12.202 7.2472 5.5 13.254 6.6744 1.6 14.107 6.2728 72.1 15.802 5.6036 74.8 18.794 4.7175 10.7 20.061 4.4223 19.3 21.286 4.1706 2.4 23.584 3.7692 7.2 24.656 3.6076 8.5 25.861 3.4659 2.3 26.689 3.3373 44.4 28.568 3.1219 3.4 29.466 3.0288 100.0 31.203 2.8640 2.8 32.040 2.7910 7.3 32.872 2.7223 41.3 35.197 2.5476 6.5 36.701 2.4466 18.4 37.440 2.4000 5.2 38.484 2.3373 7.5 39.536 2.2774 5.4 41.558 2.1712 5.8 43.507 2.0783 1.4 44.128 2.0505 18.1 44.769 2.0226 2.1 46.011 1.9709 1.6 46.606 1.9471 14.5 47.205 1.9238 17.0 48.969 1.8585 3.7 49.562 1.8377 8.8 The substituted alkali-metal:tin:silica reaction system was found to yield an extended class of crystalline, microporous materials containing tin, silicon, and germanium, aluminum, zinc, gallium, or mixtures thereof and oxygen as framework species. The phases generated in this system are structure types, analogous to those found in the rim tin-silica system. They display reversible water loss, and are capable of ion exchange. The novel substituted stannosilicates of this Invention have a variety of uses, e.g., because the alkali metal can be exchanged as in a zeolite material, nuclear waste clean up can be effected when M in the generic formula is radioactive cesium or strontium. Substitution of silicon by aluminum, zinc and gallium enhance such exchange properties by increasing the strength of the OH - groups, and therefore, increasing their interaction with polar molecules; e.g., H 2 S, NH 3 , etc. A particular utility for the material is as a hydrogen sulfide sorbent. Catalytic reforming reactions, for example, processes utilizing a supported nobel metal (e.g., Platinum) catalyst to upgrade the octane level of naphtha, produce hydrogen which can be recycled to the reaction zone. Because naphtha feeds generally contain low sulfur levels which can build up during recycle processes and cause catalyst deterioration, the recycle hydrogen stream containing some hydrogen sulfide is passed through a sorbent to reduce the sulfide level of the recycle hydrogen. This prevents poisoning of the catalytic metal site by the sulfur compounds. The sorbent process can be conducted at reforming pressures, e.g., 125 psig to about 600 psig, to avoid recompression of the hydrogen and at temperatures ranging from about 50°C to 500°C. Hydrogen flow rates or space velocities, that is, volume of feed per hour per volume of sorbent, are easily determined based on the desired level of hydrogen sulfide removal, usually in excess of about 80%, preferably in excess of about 90%. Similar sorption processes are used to remove H 2 S from various hydrocarbon streams containing H 2 S such as sour natural gas streams, and streams resulting from petrochemical refining operations. The following example shows the ability of a form of the stannosilicate material to remove hydrogen sulfide from hydrogen streams and, more importantly, the regenerability of the stannosilicate so that it can be used and reused In multicycle fashion. In catalytic reforming operations, it is normal to have two or more beds of hydrogen sulfide sorbent so that the sulfide can be continuously adsorbed in one bed or another while the bed that is off stream is being regenerated. Such processes will be rather obvious to those skilled In the art. Regeneration of the substituted, stannosilicate is readily effected by passing a small amount of hydrogen through the sorbent substituted stannosilicate for several hours, e.g., one hour to 24 hours, while maintaining a positive pressure and at higher temperature than the adsorbing cycle but within the same general temperature range. Example 5 A stannosilicate prepared in the same manner as the alumino-substituted stannosilicate of Example 1 above was exchanged with NH 4 + and loaded into a thermogravimetric analyzer and heated overnight at 650°F in flowing hydrogen. 10.5% volatile matter, corresponding to 9.45 mg of the original 90.01 mg charge was lost. The results of several adsorption/desorption cycles are shown in Table 10 and demonstrate the ability of these highly crystalline materials to separate H 2 S from H 2 S containing streams. H 2 S Sorption Data For Phase A Cycle I Temp. Time Wt. Gain (Loss) sorb 10% H 2 S/H 2 66°C 6.5 hours 5.05 mg desorb H 2 316°C overnight (4.80) Cycle II sorb 0.2% H 2 S/H 2 66°C 3.35 hours .60 mg desorb H 2 316°C overnight (.55) Cycle III sorb 0.2% H 2 S/H 2 66°C 2.35 hours .36 mg desorb H 2 316°C overnight (.37) Cycle IV sorb 0.2 H 2 S/H 2 66°C 6.67 hours .67 mg desorb H 2 316°C overnight (.67) Cycle V sorb 0.2% H 2 S/H 2 66°C 6.67 hours 0.86 mg desorb H 2 -- -- -- Novel substituted stannosillcate phases were prepared having structures comprised of corner sharing tin oxide octahedra and silicon germanium, aluminum, zinc, gallium oxide or mixtures thereof, tetrahedra. These frameworks are generated hydrothermally from reaction gels containing base cations and a source of tin, silicon and germanium, aluminum, zinc, or gallium or mixtures thereof. The structure that results from a particular synthesis is highly dependent on the cation employed in the reaction. Cation mixtures were employed in several syntheses to generate new phases or to improve the purity of phases produced by single-cation systems.

Claims: A crystalline substituted stannosilicate of the formula xM a O.SnO 2 .4±0.5(Si+T)O 2 .yH 2 O.zR, wherein M is a Group 1A metal and a=2, or M is a Group IIA metal and a=1, or M is a quaternary amine; T is germanium, aluminum, gallium, zinc or mixtures thereof; wherein x is 1.5 to 4, y is 4 to 15, R is an amine and z is 0 to 4; and wherein the framework species comprises tin in octahedral coordination, Si and T in tetrahedral coordination, and oxygen. A stannosilicate according to claim 1, wherein T is aluminum or gallium. A stannosilicate according to claim 1 or claim 2, wherein M is sodium and/or potassium. A stannosilicate according to any preceding claim, wherein the ratio of T:Si is from 0.02 to 0.40. A stannosilicate according to claim 1, having the X-ray diffraction pattern of one of phases A, B or G, respectively, of the following tables: X-RAY DIFFRACTION PATTERN FOR PHASE A dÅ I/Io 6.33 ± 0.15 29 5.83 ± 0.15 100 5.56 ± 0.15 42 5.19 ± 0.10 12 4.73 ± 0.10 8 4.28 ± 0.10 18 3.30 ± 0.08 23 3.05 ± 0.08 73 2.91 ± 0.05 69 2.77 ± 0.05 27 2.64 ± 0.05 11 2.45 ± 0.05 15 2.35 ± 0.05 9 X-RAY DIFFRACTION PATTERN FOR PHASE B 2 theta d obsv I/I o 5.577 15.8337 2.0 12.202 7.2472 5.5 13.254 6.6744 1.6 14.107 6.2728 72.1 15.802 5.6036 74.8 18.794 4.7175 10.7 20.061 4.4223 19.3 21.286 4.1706 2.4 23.584 3.7692 7.2 24.656 3.6076 8.5 25.681 3.4659 2.3 26.689 3.3373 44.4 28.568 3.1219 3.4 29.466 3.0288 100.0 31.203 2.8640 2.8 32.040 2.7910 7.3 32.872 2.7223 41.3 35.197 2.5476 6.5 36.701 2.4466 18.4 37.440 2.4000 5.2 38.484 2.3373 7.5 39.536 2.2774 5.4 41.558 2.1712 5.8 43.507 2.0783 1.4 44.128 2.0505 18.1 44.769 2.0226 2.1 46.011 1.9709 1.6 46.606 1.9471 14.5 47.205 1.9238 17.0 48.969 1.8585 3.7 49.562 1.8377 8.8 X-RAY DIFFRACTION PATTERN FOR PHASE G dÅ I/Io 8.05 ± 0.20 17 6.58 ± 0.15 59 5.86 ± 0.15 76 5.35 ± 0.15 8 5.06 ± 0.10 18 4.84 ± 0.10 7 4.37 ± 0.10 40 4.02 ± 0.10 13 3.59 ± 0.08 18 3.51 ± 0.08 16 3.27 ± 0.08 17 3.12 ± 0.08 8 3.05 ± 0.08 19 2.98 ± 0.05 100 2.93 ± 0.05 38 2.86 ± 0.05 53 2.75 ± 0.05 12 2.67 ± 0.05 14 2.57 ± 0.05 10 2.50 ± 0.05 12 2.41 ± 0.05 9 2.14 ± 0.05 14 2.04 ± 0.05 12 1.93 ± 0.05 19 1.84 ± 0.05 10 A process for preparing crystalline substituted stannosilicates, wherein the framework comprises tin in octahedral coordination, Si and at least one of germanium, gallium, zinc or aluminum, in tetrahedral coordination, and oxygen; which process comprises reacting in an aqueous medium (i) a water soluble tin salt, silica or a source of silica, a water soluble source of germanium, gallium, zinc, or aluminum, wherein the ratio of tin:silicon and germanium, gallium, zinc, aluminum or mixtures thereof is no greater than 1:20 and the ratio of tin:germanium, gallium, zinc, aluminum or mixtures thereof is up to 1:2.0; and (ii) a Group IA or IIA metal hydroxide or mixtures thereof or amine or alkylammonium compound; and recovering the substituted stannosilicate. A process for removing H 2 S from a feed stream containing (i) H 2 S and (ii) hydrocarbons and/or hydrogen, comprising contacting the feed stream with a crystalline substituted stannosilicate claimed in any one of claims 1 to 5 or prepared according to claim 6 and recovering a stream depleted in H 2 S.

Inventors: CORCORAN EDWARD WILLIAM JR, VAUGHAN DAVID EVAN WILLIAM, CORCORAN, EDWARD WILLIAM, JR., VAUGHAN, DAVID EVAN WILLIAM
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1993
Main classification: C01B33
IPC codes: C01B3, C01B39, C01B33, B01J20, B01J29