PROCESS FOR CATALYTICALLY DEHYDROGENATING ANTHRAHYDROQUINONE

Abstract: A process for catalytically dehydrogenating an anthrahydroquinone to form anthraquinone and a hydrogen product. A catalyst which is a metal, a metal compound, or a metal or metal compound dispersed on a support or is a liquid is selected to have an electronegativity greater than about 2.30 so as to result in the substantially complete conversion of anthrahydroquinone to anthraquinone and hydrogen and thereby essentially eliminate formation of unwanted hydrogenolysis by-products, such as anthrones and/or anthranols.

Description: PROCESS FOR CATALYTICALLY DEHYDROGENATING ANTHRAHYDROQUINONE BACKGROUND OF THE INVENTION FIELD OF THE INVENTION: The present invention relates to a process for catalytically dehydrogenating an anthrahydroquinone to the corresponding anthraquinone and hydrogen, and more particularly, to such a process wherein the catalyst electronegativity is selected to increase anthraquinone and hydrogen selectivity to substantially one hundred percent and thereby essentially eliminate unwanted hydrogenolysis. DESCRIPTION OF RELATED ART: In many processes, solutions containing an anthrahydroquinone are thermally or catalytically regenerated thereby producing the initial anthraquinone form which is usually recycled for use in a continuous process. One such process involves contacting within a reactor a feed gas containing hydrogen sulfide with an anthraquinone which is dissolved in a polar organic solvent. The resulting reaction between hydrogen sulfide and the anthraquinone yields sulfur and the corresponding anthrahydroquinone. The sulfur precipitates from the solution and is recovered as a product while the remaining solution containing anthrahydroquinone is thermally or catalytically regenerated producing the initial anthraquinone form and releasing hydrogen gas. The anthraquinone is recycled back to the reactor and hydrogen gas is recovered as a product. Regeneration or dehydrogenation of an anthrahydroquinone often involves the use of supported metal catalysts. For example, metal oxide catalysts, e.g. chromium oxide, on alumina supports and platinum-ruthenium catalysts on alumina or silica-alumina supports have been employed in the catalytic dehydrogenation of anthrahydroquinones. However, the particular support employed or the metal oxide catalyst can cause hydrogenolysis during the catalytic dehydrogenation of anthrahydroquinones which results in the undesirable production of water and anthrones and/or anthranols. Thus, a need exists for such a process for the catalytic dehydrogenation of anthrahydroquinones wherein the anthrahydroquinone is entirely converted to 6/10005 PCMJS95/07650 anthraquinone and hydrogen products, and thus, unwanted hydrogenolysis by-products, such as anthrones and/or anthranols are eliminated. Accordingly, it is an object of the present invention to provide a process for catalytically dehydrogenating an anthrahydroquinone to the corresponding anthraquinone and hydrogen while eliminating substantial unwanted hydrogenolysis by-products. It is a further object of the present invention to provide a process for selecting a catalyst which results in complete selectivity to anthraquinone and hydrogen in the catalytic dehydrogenation of an anthrahydroquinone. SUMMARY OF THE INVENTION To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, one characterization of the present invention is a process for dehydrogenating an anthrahydroquinone to form anthraquinone and hydrogen. The process comprises selecting a catalyst having an electronegativity greater than about 2.30. The anthrahydroquinone which is dissolved in a polar organic solvent is reacted in the presence of the catalyst so as to substantially entirely convert anthrahydroquinone to anthraquinone and hydrogen. In another characterization of the present invention, a process is provided for converting hydrogen sulfide gas to sulfur and hydrogen. The process comprises contacting the hydrogen sulfide gas with a polar organic solvent having an anthraquinone dissolved therein. The hydrogen sulfide gas reacts with the anthraquinone to produce sulfur and an anthrahydroquinone in the solvent. The anthrahydroquinone is catalytically dehydrogenated to anthraquinone and hydrogen in the presence of a catalyst selected to have an electronegativity greater than about 2.30 so as to increase the selectivity of converting anthrahydroquinone to anthraquinone and hydrogen to about 100%. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawing, which is incorporated in and forms a part of the specification, illustrates the embodiments of the present invention and, together with the description, serves to explain the principles of the invention. In the drawing: FIG. 1 is a graph which depicts hydrogen and t-butyl anthraquinone (TBAQ) production selectivity during the dehydrogenation of t-butyl anthrahydroquinone (H 2 TBAQ) as a function of the catalyst metal electronegativity for two different metal supports with different electronegativities. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to the dehydrogenation of an anthrahydroquinone to the corresponding anthraquinone and hydrogen, as is employed in industrial processes, usually to regenerate anthraquinone for use in another process stage. For example, in one process the anthraquinone product from such dehydrogenation reaction which is dissolved in a polar organic solvent is used to contact a feed gas containing hydrogen sulfide (H 2 S) in a reactor. Should the feed gas contain large quantities of gases other than hydrogen sulfide which are inert to this process, such as nitrogen, carbon dioxide, methane or other low molecular weight hydrocarbons, the feed gas and polar organic solvent containing an anthraquinone may be initially contacted in an absorber. The feed gas may contain other sulfur compounds, such as COS, CS 2 and mercaptans, which are converted in the process to H 2 S, recycled to this reactor, and converted to sulfur. The solvent preferentially solubilizes hydrogen sulfide from the feed gas to form a reaction solution which is maintained in the reactor at a temperature of from about 0° C. to about 70° C. and at a H 2 S partial pressure of from about 0.05 to about 4.0 atmospheres and for a time which is sufficient to convert the hydrogen sulfide and anthraquinone to sulfur and anthrahydroquinone. The reaction solution is then removed from the H 2 S reactor and the insoluble sulfur, e.g. S β or other forms of polymerized sulfur, is separated therefrom by filtration, centrifugation, or any other means known in the art. The remaining reaction solution, comprising the polar organic solvent, dissolved anthrahydroquinone, and any unreacted anthraquinone, is then heated to from about 100° C. to about 150° C. at atmospheric pressure and fed to a flash tank where substantially all unreacted feed gas constituents, including H 2 S and C0 2 are removed from solution and recycled to the reactor. The solution is withdrawn from the flash tank and may be further heated to from about 150° C. to about 275° C. at a pressure sufficient to prevent solvent boiling. The heated solution is then fed to a dehydrogenation reactor where the anthrahydroquinone is catalytically converted to anthraquinone and hydrogen gas (H 2 ) under the temperature and pressure conditions stated above. In accordance with the present invention, Applicant has discovered that the selection of a catalyst having an electronegativity of greater than about 2.30 and preferably greater than about 2.35 unexpectedly results in hydrogen and anthraquinone product selectivity of substantially one hundred percent in the dehydrogenation of anthrahydroquinones, and thus, substantially eliminates unwanted hydrogenolysis by-products, such as anthrones and/or anthranols. The catalyst may be a solid or a liquid, and preferably is heterogenous or immisicible with the polar organic solvent at the dehydrogenation raction conditions. The catalyst may be a metal element, a metal compound, or such metal element and/or metal compound dispersed upon a solid support or the catalyst may be a liquid compound. Thus, in accordance with the present invention, the entire catalyst or a component of the catalyst is selected to have an electronegativity of greater than 2.30. If the catalyst is a metal element a metal compound or a liquid compound, then such metal element, metal compound or liquid compound should have an electronegativity greater than 2.30. If the catalyst is a metal element or metal compound dispersed on a support, then either the support or the metal element or compound should have an electronegativity greater than 2.30. Where the support has an electronegativity greater than about 2.30, then the metal element or the metal compound dispersed thereon should have an electronegativity greater than about 2.00. As utilized throughout this specification, the term "electronegativity" refers to the Pauling electronegativity as discussed at page 215, volume 17 (1961) of the "Journal Inorganic Nuclear Chemistry" by A.L. Allred. Electronegativities of several metal compounds and function groups are detailed in Structure and Bonding 66 Electronegativity, Editors K.D. Sen and C.K. Jorgensen, Springer- erlag, Berlin Heidelberg, 1987. While it is not exactly understood why the use of a catalyst electronegativity of greater than about 2.30 results in substantially complete conversion of anthrahydroquinone to anthraquinone and hydrogen product during dehydrogentation, it is believed that the need to remove an electron pair from the anthrahydroquinone during the conversion to anthraquinone and hydrogen is accomplished by increasing catalyst electronegativity. Suitable metal elements for use in accordance with the present invention are, for example, gold, selenium, tungsten, and mixtures thereof. These metals can be used as a catalyst in the process of the present invention or can be dispersed on supports which includes those supports having an electronegativity less than 2.30, such as silica, alumina, and mixtures thereof. Alternatively, these metals catalysts or other metal catalysts having electronegativities greater than 2.00, for example platinum (Pt), palladium (Pd), rhodium (Rh), molybdenum (Mo), titanium (Ti), rhenium (Re), or mixtures thereof, can be dispersed on supports which have e . tronegativities greater than 2.30, for example a polyperfluorosulfonic aciσ polymer, such as a Nation® polymer which is commercially available from E. I. Du Pont de Memours & Co. Other suitable supports w-.h electronegativities greater than 2.30 are carbon and hydrocarbons containing at least one of the group consisting of -BeF, -BF 2 , -NF 2 , -OF, -N(C 3 H 7 ) 2 , -OCH 3 , -COOH, -OS0 3 H, -CN, -NCS and -NCO. Also, metal compounds which have electronegativities greater than 2.30 can be used as a catalyst in accordance with the present invention, for example, WS 2 , SeO, and metals attached to at least one of the group consisting of -BeF, BF 2 , -NF 2 , -OF, -N(C 3 H 7 ) 2 , -OCH 3 , -COOH, -OS0 3 H, -CN, -NCS and -NCO. Suitable liquid compounds for use in accordance with the present invention are perfluoronated hydrocarbons, for example perfluorononane. After the dehydrogenation rea.oion, anthraquinone in its initial form is withdrawn from the dehydrogenation reactor dissolved in the polar organic solvent and is recycled to the H 2 S reactor, while the H 2 gas is recovered as a commercial product. Preferably, the process of the present invention is operated as a continuous process. Suitable polar organic solvents for use in such process include N- methyl-2-pyrrolidinone, N,N-dimethylacetamide, N.N-dimethylformamaide, sulfolane (tetrahydrothiophene-1 ,1 -dioxide), acetonitrile, 2-nitropropane, propylene carbonate and mixtures thereof. The most preferred solvent is N- methyl-2-pyrrolidinone (NMP). Useful anthraquinones are ethyl, t-butyl, t-amyl and s-amyl anthraquinones and mixtures thereof because of their relatively high solubilities in most polar organic solvents. The following examples demonstrate the practice and utility of the present invention, but are not to be construed as limiting the scope thereof. EXAMPLE 1 A mixture containing 18 wt% t-butyl anthrahydroquinone (H 2 TBAQ) and 7 wt% t-butyl anthraquinone (TBAQ) in n-methyl-2-pyrrolidinone (NMP) is dehydrogenated at 275° C and 72 psia using three catalysts in three separate runs. One catalyst composition is 2.75 wt% platinum (Pt) metal dispersed on a silica support (Grace Grade 57), the second catalyst composition is 2.75 wt% palladium (Pd) metal on a silica support (Grace Grade 57), and the third catalyst composition is 2.75 wt% tungsten (W) metal on the same support. The electronegativities of the Pd, Pt and W metals are 2.20, 2.28, and 2.36, respectively. The electronegativity of the support is estimated to be significantly below 2.30. The selectivity of converting the H 2 TBAQ to hydrogen and TBAQ is 68 mole% using the Pt catalyst, 25 mole% using the Pd catalyst, and 100% using the W catalyst. In each instance, the balance of the H 2 TBAQ is converted to the corresponding anthroπe or anthranol and water. These results, as correlated to the electronegativity of the metal catalysts, are illustrated in FIG. 1 , and demonstrate that a metal electronegativity greater than about 2.30 is sufficient to yield a selectivity to TBAQ and hydrogen of 100 mole % without production of unwanted anthrone or anthranol and water. EXAMPLE 2 A mixture of H 2 TBAQ and TBAQ which is identical to that used in Example 1 is dehydrogenated using a Nation® polymer from E.I. Du Pont de Memours & Co. as the support to which 0.8 meq of Pt/gm of Nation® polymer have been ion exchanged with the hydrogen proton of the sulfonic acid group. The electronegativity of this support is between 2.30 (the electronegativity of the -CF 2 - polymer backbone) and 2.91 (the electronegativity of the -CF 3 polymer terminal groups. The sulfonic acid group in this polymer has an electronegativity of 5.06. For comparison, a separate dehydrogenation run is conducted using Pt catalyst of Example 1. Dehydrogenation conditions are 190° C and 12 psia. The selectivity of converting H 2 TBAQ to TBAQ and hydrogen is 100 mole% for the Pt-Nafion® polymer catalyst and 7.7 mole% for the Pt-silica supported catalyst. This result illustrates the desirability of utilizing a catalyst support having an electronegativity of greater than 2.30 in the process of the present invention. EXAMPLE 3 An mixture of H 2 TBAQ and TBAQ which is identical to that used in Example 1 is dehydrogenated in separate runs at 190° C and 12 psia using a Nation® polymer from E.I. Du Pont de Memours & Co. as the support to which either 0.8 meq of Ag/gm or Sn/gm of Nation® polymer have been ion exchanged with the hydrogen proton of the sulfonic acid group. The electronegativity of Ag and Sn are 1.93 and 1.96, respectively. The electronegativity of this support is between 2.30 (the electronegativity of the -CF 2 - polymer backbone) and 2.91 (the electronegativity of the -CF 3 polymer terminal group. The sulfonic acid group in this polymer has an electronegativity of 5.06. The selectivity of converting the H 2 TBAQ to hydrogen and TBAQ is 59 mole% for the Ag catalyst and 75 mole% for the Sn catalyst, the balance of the H 2 TBAQ is converted to the corresponding anthrone or anthranol and water. This result is plotted versus metal electronegativity in FIG. 1. The results indicate that if a support with an electronegativity of greater than 2.30 is used , the dispersed metal electronegativity needs to be greater than about 2.00. EXAMPLE 4 A liquid mixture containing 11 .8 wt% t-butyl anthrahydroquinone (H 2 TBAQ) and 13.2 wt% t-butyl anthraquinone (TBAQ) in n-methyl-2- pyrolidinone (NMP) is dehydrogenated by contacting the mixture by means of intense mechanical mixing with liquid perfluorononane at 150°C to 180°C for 30 minutes. The NMP mixture and perfluorononane are immiscible liquids at these conditions. Upon conclusion of this dehydrogenation reaction, the H 2 TBAQ content in the NMP mixture is reduced to 0 wt% and the only products produced are TBAQ and hydrogen. Accordingly, undesirable by¬ products of hydrogenolysis, i.e. t-butyl anthrones (TBAN) and t-butyl anthranols (TBAL), are not produced. The two immiscible liquid phases, i.e. the NMP reaction mixture and perfluorononane, are then separated at room temperature and pressure. The electronegativity of perfluorononane is between 2.30 which is the electronegativity of the -CF 2 - backbone and 2.91 which is the electronegativity of the -CF3 terminal group. EXAMPLE 5 Two 25 gm liquid samples containing 1 1.64 wt% H 2 TBAQ and 12.74 wt% TBAQ in NMP solvent are placed into two separate reactors. 2.5 gms of liquid perfluorononane catalyst is also added to each of the two liquid samples in each reactor. The vapor space above the two immiscible liquids in each reactor is purged with hydrogen to remove all air. One reactor is placed in a hot oil bath at 150°C and the immiscible liquids are gently mixed for 10 minutes to contact the perfluorononane with the NMP solution. The pressure in the reactor increased from 0 to a stable value of 29 psig indicating the evolution of hydrogen. The reactor contents are then cooled to room temperature and the NMP solution is analyzed. The amount of H 2 TBAQ in the solution decreases to 8.80 wt% while the amount of TBAQ increases to 15.58 wt%. No TBAN or TBAL by-products are produce indicating that hydrogenolysis does not occur. The second reactor is placed in a hot oil bath at 200°C and the immiscible liquids are gently mixed for 16 minutes to contact the perfluorononane with the NMP solution. In the first 5 minutes of mixing.i.e. reaction, the pressure in the second reactor increased from 0 to 45 psig at which point the produced hydrogen is removed from the second reactor until a reactor pressure of 15 psig is reached. In a similar manner, the reactor pressure is reduced from 45 to 15 psig after 7 minutes of total reaction time and from 31 to 15 psig after 13 minutes of total reaction time. After 16 minutes, the reactor pressure increases to a stable pressure of 26 psig and the reaction was quenched at room temperature. The NMP solution is analyzed and determined to contain 5.34 wt% H 2 TBAQ and 19.04 wt% TBAQ. No TBAN or TBAL by-products are produce indicating that hydrogenolysis does not occur. Perfluorononane is easily removed from the NMP solution emanating from each reactor by conventional liquid phase separation techniques. Although selection of a catalyst having an electronegativity of greater than about 2.30 has been described above with respect to a process for removing hydrogen sulfide from a feed gas and recovering sulfur and hydrogen as products, the process of the present invention can be applied to any process where an anthrahydroquinone undergoes a dehydrogenation reaction to regenerate anthraquinone for further use. For example, a process for the conversion of hydrogen chloride gas to hydrogen and chlorine gases. While the foregoing preferred embodiments of the invention have been described and shown, it is understood that the alternatives and modifications, such as those suggested and others, may be made thereto and fall within the scope of the invention.

Claims: I claim: 1. A process for dehydrogenating an anthrahydroquinone to form anthraquinone and hydrogen, said process comprising: selecting a catalyst having an electronegativity greater than about 2.30; and dehydrogenating an anthrahydroquinone which is dissolved in a polar organic solvent in the presence of said catalyst thereby substantially entirely converting said anthrahydroquinone to anthraquinone and hydrogen. 2. The process of claim 1 wherein said catalyst is a solid. 3. The process of claim 2 wherein said catalyst is a metal element, a metal compound, or a metal element or a metal compound dispersed on a support. 4. The process of claim 3 wherein said metal element is gold, selenium, tungsten, or mixtures thereof. 5. The process of claim 3 wherein said metal compound is WS 2 , SeO, or metals attached to at least one of the group consisting of -BeF, -BF 2 , -NF 2 , -OF, -N(C 3 H 7 ) 2 , -OCH 3 , -COOH, -OS0 3 H, -CN, -NCS and -NCO. 6. The process of claim 3 wherein said support has an electronegativity less than 2.30. 7. The process of claim 6 wherein said support is silica, alumina, or mixtures thereof. 8. The process of claim 7 wherein the metal element or metal compound dispersed on said support has an electronegativity greater than 2.30. 9. The process of claim 3 wherein said support has an electronegativity greater than 2.30. 10. The process of claim 9 wherein said support is a polyperfluorosulfonic acid polymer, carbon, or hydrocarbons containing at least one of the group consisting of -BeF, -BF 2 , -NF 2 , -OF, -N(C 3 H ) 2 , -OCH 3 , -COOH, -OS0 3 H, -CN, -NCS and -NCO. 11. The process of claim 10 wherein the metal element or metal compound dispersed on said support has an electronegativity greater than 2.00. 12. The process of claim 1 wherein said catalyst is a liquid. 13. The process of claim 12 wherein said catalyst is a perfluorononated hydrocarbon. 14. The process of claim 13 wherein said catalyst is perfluorononane.

Inventors: PLUMMER MARK A, PLUMMER, MARK, A.
Applicants: MARATHON OIL CO, MARATHON OIL COMPANY
Application year: 1995
Main classification: C07C46
IPC codes: B01J27, B01J31, C01B3, C07B61, C07C46, C07C50