Abstract:
A process of abstracting sulfur from H 2  S and generating hydrogen is disclosed comprising dissolving Pd 2  X 2  (μ-dppm) 2  in a solvent and then introducing H 2  S. The palladium complex abstracts sulfur, forming hydrogen and a (μ-S) complex. The (μ-S) complex is readily oxidizable to a (μ-SO 2 ) adduct which spontaneously loses SO 2  and regenerates the palladium complex.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the interaction of transition metal complexes, more specifically, palladium complexes with H 2  S. More particularly, this invention discloses a process for removing H 2  S from a feedstock such as natural gas by means of a palladium metal-metal bonded dimer that abstracts the sulfur and generates hydrogen. 
     Field natural gas typically contains undesirable constituents such as H 2  S which must be removed because of its corrosive and noxious nature. H 2  S is now removed by treatment with aqueous ethanolamine in a countercurrent cycle followed by regeneration in a stripper column. Amines are often used and typical amines used are diisopropylamine or methyldiethanol amine. B,B&#39;-hydroxyaminoethyl ether known as diglycolamine is also sometimes used. 
     The preferred alkanolamines typically have a hydroxyl group to lower the vapor pressure and to provide water solubility. The alkaline amine group absorbs acidic contaminant gases. 
     The alkanolamines, however, suffer several drawbacks--a problematic and costly one being the formation of irreversible reaction products with some contaminants such as COS and CS 2 . This results in an economic loss from loss of alkanolamine if these contaminants are present in the well. Other drawbacks of alkanolamines include relatively high corrosivity, vaporization losses due to their relatively high vapor pressure, and the need often to keep water content below 5% which necessitates high reboiler temperatures. 
     A sweetening process for soured natural gas, able to effectively remove H 2  S but without many of the above attendant drawbacks, would be an advance in the art. 
     2. Description of Related Art 
     The palladium dimer complexes Pd 2  Cl 2  (μ-dppm) 2  have been reported by A. L. Balch, L. S. Benner, and M. M. Olmstead, Inorg. Chem., 1979, 18, 2996 and C. L. Lee, B. R. James, D. A. Nelson, and R. T. Hallen, Organometallics, 1984, 3, 1360. 
     The present invention discloses a new and useful process utilizing these complexes. 
     SUMMARY OF THE INVENTION 
     This invention discloses a process for the removal of H 2  S from a gas feedstock and the conversion of the H 2  S to hydrogen and organosulfur compounds. 
     The invention is based on the discovery of the reaction: ##STR1## where X=Cl, Br, or I. 
     The palladium complex (1a) is [Pd 2  X 2  (μ-dppm) 2  ], where dppm is bis(diphenylphosphino)methane and X is Cl, Br, or I. The complex (1a) is a bridged dppm dimer, [Pd 2  X 2  (μ-dppm) 2  ], but is written for convenience as Pd 2  X 2  (dppm) 2 . All of the above are to be understood herein and are defined herein as equivalent representations of the same palladium complex. 
     This reaction can be carried out in solution under ambient conditions. Therefore, Pd 2  Cl 2  (dppm) 2  can be used to remove H 2  S from soured natural gases. 
     It has been found that H 2  S can be removed from natural gases by bubbling the gas through a CH 2  Cl 2  solution of Pd 2  Cl 2  (dppm) 2  at 10 -1  -10 -2  M. The palladium complex abstracts sulfur with the concomitant release of hydrogen. The sulfur-bearing palladium complex then can be oxidized with a mild oxidant to an SO 2  containing product, Pd 2  Cl 2  (μ-dppm) 2  (μ-SO 2 ), that spontaneously releases the SO 2  with regeneration of the palladium complex. 
    
    
     DETAILED DESCRIPTION 
     When Pd 2  Cl 2  (dppm) 2  is dissolved in a solvent such as dichloromethane and exposed to H 2  S, Pd 2  Cl 2  (dppm) 2  (μ-S) is formed together with the evolution of hydrogen gas. Essentially, hydrogen is split from the H 2  S molecule and sulfur is incorporated between the two palladium atoms. The reaction can be written as 
     Pd 2  Cl 2  (dppm) 2  +H 2  S→Pd 2  Cl 2  (dppm) 2  (μ-S)+H 2   
     or in general as: ##STR2## wherein P   P is bis(diphenylphosphino)methane, and X is selected from Cl, Br or I. 
     Quantitative measurements show that 97% of the theoretical amount of hydrogen is obtained. 
     The relative reactivities toward H 2  S of the differing halogen substituents are: 
     Pd 2  Cl 2  (dppm) 2  &gt;Pd 2  Br 2  (dppm) 2  &gt;Pd 2  I 2  (dppm) 2 . 
     Pd 2  Br 2  (dppm) 2  reacts considerably slower than Pd 2  Cl 2  (dppm) 2 , and the iodo substituted complex slower still. 
     The commercial significance of the Pd 2  Cl 2  (dppm) 2  route to hydrogen generation becomes apparent upon the recognition that Pd 2  Cl 2  (dppm) 2  can be regenerated from Pd 2  Cl 2  (dppm) 2  (μ-S) by oxidizing the sulfur to SO 2 . The complex thus becomes valuable for the quantitative recovery of hydrogen from H 2  S. 
     A one step hydrogen separation and sulfur abstraction from gas mixtures with compositions similar to that of oxygen blown coal gas is made possible by the invention. Quite generally it can be stated that Pd 2  X 2  (dppm) 2  (μ-SO 2 ) can be generated from Pd 2  X 2  (dppm) 2  (μ-S) by oxidation wherein X=Cl or Br or I. Oxidation can be carried out using oxidants such as H 2  O 2 , pyridinium chlorochromate, pyridinium dichromate, and m-chloroperbenzoic acid. The SO 2  adduct is formed in high yields when the oxidation is performed at -20° C. with two equivalents of oxidant. Oxidation of Pd 2  Cl 2  (dppm) 2  (μ-S) with a slight excess of m-chloroperbenzoic acid at -20° C. followed by addition of hydrazine to element excess oxidant resulted in the formation of Pd 2  Cl 2  (dppm) 2  (μ-SO 2 ) in 76% isolated yield. 
     Though the SO 2  adduct tends to spontaneously lose SO 2  thus regenerating the Pd 2  X 2  (dppm) 2  catalyst, the rate and extent of regeneration of the Pd 2  X 2  (dppm) 2  catalyst can be maximized or enhanced by using means removing SO 2  from the SO 2  adduct. The removal means to liberate SO 2  can be selected from N 2  gas introduction, application of heat, or application of reduced pressure. 
     The violet Pd 2  X 2  (dppm) 2  (μ-SO 2 ) (X=Cl, Br, I) complexes lose SO 2  when N 2  is bubbled through the solution or if subjected to heat or vacuum. The rate of SO 2  loss decreases in the order to I&gt;Br&gt;Cl as judged by color change in samples of similar concentration. 
     
                       TABLE 1______________________________________PHYSICAL PROPERTIES OF SOLVENTS FORDISSOLUTION OF Pd.sub.2 Cl.sub.2 (dppm).sub.2        Solubility                  Vapor Pressure                              BoilingSolvent      Parameter at 25° C. (torr)                              Point (°C.)______________________________________dichloromethane        9.80      424         39.7dimethylacetamide        10.80     ˜0    165diphenylether        10.10     0.10        2581,2,3,4-tetrahydro-        9.50      0.38        205naphthalene(tetralin)dibutylphthalate        9.85      ˜0    3401,1,2-trichloroethane        9.88      22.49       113.81,2,3-trichloropropane        10.09     3.33        156______________________________________ 
    
     Pd 2  Cl 2  (dppm) 2  has a solubility of 0.15M in trichloroethane and 0.027M in trichloropropane. 
     Solvent choice appears to have an effect on the lifetime of the complex in solution. 
     Pd 2  Cl 2  (dppm) 2  is soluble in dichloromethane to form a 0.05M solution. The boiling point (39.7° C.) and vapor pressure (424 torr at 25° C.) of dichloromethane however do not make it the easiest solvent to work with. 
     Solvents more suitable for dissolution of Pd 2  Cl 2  (dppm) 2  can be determined by the concept of solubility parameters as illustrated by Hildebrand, J., and Scott, R. L. The Solubility of Nonelectrolytes, 3rd Ed., Reinhold Publishing Corp., New York, 1950, and Hoy, K. L. J. Paint Technol., 42, 1, 1970. 
     The solubility parameter (δ) is related to the internal pressure or cohesive energy density 
     δ=(ΔE/V) 
     where 
     ΔE=energy of vaporization and 
     V=molar volume. 
     The solubility parameter of Pd 2  Cl 2  (dppm) 2  is estimated as approximately 10 based on its solubility in dichloromethane. 
     Possible process solvents can be tetralin [though its UV/VIS spectrum overlaps somewhat with that of Pd 2  Cl 2  (dppm) 2  ], dibutylphthalate, or diphenylether. Table 1 lists several possible solvents. Based on costs, 1,1,2-trichloroethane was preferred for Pd 2  X 2  (dppm) 2 , where X=Cl or Br. 
     The projected economics for hydrogen separation by the process of the invention are dependent on the lifetime of the palladium catalyst in solution. Complex lifetime appears influenced by the solvent selected. Laboratory experience with trichloroethane as solvent seemed to suggest that the complex longevity in trichloroethane might be less than optimum in some commercial operations. More favorable complex lifetimes were found with dichloromethane solvent. 
     The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited solely to the specific structure and formulas disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes can be made by those skilled in the art without departing from the spirit and scope of the invention.