Catalysis in Molten Salt Media George W. Parshall Contribution No. 1925 from the Central Research Department, Experimental Station, E. I . du Pont de Nemours and Company, Wilmington, Delaware 19898. Receiued M a y 30, 1972
Abstract: The low-melting tetraalkylammonium salts of the SnC13- and GeC13- anions are convenient solvents for some homogeneous catalytic reactions of olefins. These salts, when fused, dissolve up to 7 x PtC12to give deep red solutions which catalyze the hydrogenation, isomerization, hydroformylation, and carboalkoxylation of olefins. In the hydrogenation of 1,5,9-~yclododecatriene, considerable selectivity to cyclododecene is attainable at ca. 150" and 100 atm hydrogen pressure. The catalytic solutions of PtC12in [(C2H&N][SnC13] appear to contain SnC18- complexes of platinum including the known anions [Pt(SnCl3),I3-and [HPt(SnC13)II+. major problem in homogeneous catalysis is the separation of products from catalyst and solvent without decomposition of the catalyst. Recent approaches to solution of this problem have emphasized restraint of the catalytic complex by semipermeable membranes' or by polymeric ligands.? An approach that seems to have been under-utilized is the use of molten salts as stable, nonvolatile solvents3 from which organic products are readily separated by distillation. Although molten salts have been employed as solvents in a variety of catalytic oxidation r e a c t i ~ n s ,their ~ use has been limited by their physical properties. As pointed out by R ~ n y , who ~ j has carried out one of the few studies of homogeneous catalytic reactions of olefins in molten salt media, most conventional molten electrolytes are corrosive and present mass transfer problems. These problems are avoided by using tetraalkylammonium salts of GeC13- and SnC13- as solvents for catalytic reactions of olefins. These salts are stable and low melting and are good solvents for many olefins and complexes of the platinum metals. In addition, these salts act as ligands5 which stabilize the complexes against reduction to free metal and, as described below, confer good selectivity in many reactions.
A
Results Fused Salt Solutions. The tetraethylammonium salts of the trichlorogermanate( 1-)G and trichlorostannate( 1-)' ions melt at relatively low temperatures (68 and 78", respectively) to give clear, colorless liquids. The molten salts are stable in the absence of oxygen (1) M. T. Westaway and G. Walker, U. S. Patent 3,617,553 (1971). (2) (a) J. P. Collman, L. Hegedus, M. P. Cooke, J. R. Norton, G. Dolcetti, and D. N. Marquardt, J . Amer. Chem. Soc., 94, 1789 (1972); (b) R. H. Grubbs and L. H. Kroll, ibid., 93,3062 (1971); ( c ) M. Capka, P. Svoboda, M. Cerny, and J. Hetflejs, Tetrahedron Lett., 4787 (1971); (d) W. 0. Haag and D. D. Whitehurst, 2nd North American Meeting of the Catalysis Society, Houston, Tex., Feb 24-26, 1971 ; (e) A. J. MoRat, J . Catal., 19, 322 (1970); (f) F. W. Hoover and R. V. Lindsey, J. Org. Chem., 34, 5051 (1969). (3) J. E . Gordon, Tech. Methods Org. Organomerab Chem., 1, 51 (1969). (4) (a) H. F. Topsoe and A . Nielson, Trans. Danish Acad. Tech. Sci., 1, 18 (1948); (b) G. H. Tandy, J . Appl. Chem., 6 , 68 (1956); (c) A. R. Glueck and C. N. Kenney, Chem. Eng. Sci., 23, 1257 (1968); (d) D. M. Ruthven and C. N. Kenney, ibid., 23,981 (1968); (e) H. Nonnenmacher, et al., British Patent 941,293 (1963); (f) C. N. Satterfield and J. Loftus, A.E.Ch.E. J . , 11, 1103 (1965); (g) W. Friedrichsen and 0. Goehre, British Patent 1,082,326 (1967); (h) Shell Internationale Research Maatschappij N.F., British Patent 908,022 (1962); (i) P. R. Rony, Chem. Eng. Sci., 23, 1021 (1968); (j) P. R. Rony, A n n . N . Y . Acad. Sci., 172, 238 (1970); (k) P. R. Rony, Belgium Patent 711,042 (1968). (5) J. F. Young, Adcan. Znorg. Chem. Radiochem., 11, 9 1 (1968). (6) G. W. Parshall, Inorg. Syn., in press. (7) F. N. Jones, J . Orq. Chem., 32, 1667 (1967).
Journal of the American Chemical Society
for prolonged periods at 150-200" and are good solvents for olefins. For example, at 84" and ca. 1 atm, ethylene gives a 0.013 M solution in molten [(C2H5j4N][SnC13]. Chlorides of all the group VI11 metals dissolve to give deeply colored solutions8 which presumably contain complexes bearing the GeC13- and SnC13ligands. Platinum dichloride dissolves in [(C2H&N][SnCL] w/w) which disto give deep red solutions (up to play striking catalytic activity in hydrogenation, hydroformylation, and carboalkoxylation of olefins. By adjustment of reaction conditions, considerable selectivity for a particular reaction can be obtained as illustrated below in the hydrogenation of cyclododecatriene to cyclododecene. The PtCl? solutions seem to be considerably stabilized toward reduction by hydrogen since no metal deposition occurs at 160" and 100 atm pressure. Palladium chloride solutions are less stable, but are unchanged at 100" under 1 atm of hydrogen. Hydrogenation Reactions. Solutions of PtC12 in [R4N][SnC13], like the discrete complex [R4N]3[Pt(SnC13)s],g,'oare efficient catalysts for the hydrogenation of olefins. For example, ethylene is cleanly reduced to ethane in the presence of an equimolar amount of hydrogen. The deep red solution remains clear with no evidence of metal deposition. (The rate is much slower than that produced by an equivalent amount of platinum metal suspended in the molten salt.) The solutions can be used repeatedly without apparent loss of catalytic activity if oxidation by adventitious air is avoided. The rate of ethylene hydrogenation is quite sensitive to changes i n the medium. In matched experiments with 1.3 mmol each of ethylene and of hydrogen and 0.1 mmol of catalyst at loo", the following extents of reaction were attained in 5 hr: PtClz-Et4NGeCla, 20; PtC12-Et4NSnC13,50; cis-(Et3Asj2PtClz-Et~NSnC13, 27; ~is-(Ph~P)~PtCl~-Fit~NSnCl~, 20; cis-(EtgP)zPtCl?Et4NSnC13, 1 7 z . The rates attained with the phosphine and arsine complexes are significantly lower than that for the PtC1, solution even though these complexes, used in conjunction with SnCI2, are known to be olefin hydrogenation catalyts. l 1 Hydrogenation of ethylene
7z
(8) G. W. Parshall, U. S. Patent 3,565,823 (1971). (9) R. D. Cramer, E. L. Jenner, R. V. Lindsey, and U. G. Stolberg, J . Amer. Chem. Soc., 85, 1691 (1963). (10) R. D. Cramer, R. V. Lindsey, C. 7.Prewitt, and U. G. Stolberg, ibid., 87, 658 (1965). (11) R. W. Adams, G. E. Batley, a n d J. C. Bailar, ibid., 90, 6051 ( 19 68).
94:25 J December 13, I972
8717 Table I. Hydrogenation of 1,5,9-Cyclododecatriene by Fused Salt Solutions of PtC1.p Medium Et4NSnC13
Temp, "C
Pressure atm
Time, hr
100 140
100 100 100
6 8 8
85
500 30
6
5 7 0
160 Ph3MePSnBrr
140 140 150
100
6 6
7
Triene
2 2
Products, Diene Monoeneb 2 18 10 18 23
12 80
87 13 65 64
CizHza Trace Trace Trace 4 5 18
In each experiment 5.0 ml of cis,trans,trans-1,5,9-cyclododecatrieneand ca. 50 g of a 1 PtC12 solution were allowed to react under the conditions indicated in an 80-ml Hastelloy shaker tube. * The cyclododecene appeared to contain roughly equal amounts of the cis and trans isomers.
in molten tetra-n-butylammonium trichlorostannate Carbonylation Reactions. Platinum dichloride soluis slightly slower than in the tetraethylammonium salt, tions in molten [(C2HJ2N][SnC13]catalyze both hydrobut the lower melting point (59-60") permits operation formylation and carboalkoxylation of olefins. Reat lower temperatures. Hydrogenation activity was action of a 1 :2 :10 mixture of hydrogen, ethylene, and observed even at 50" in a solution of PtClz in a eutectic carbon monoxide at 90" and 400 atm gave a 65 % commixture of the two trichlorostannate salts. bined yield of propionaldehyde and its aldol dimer, Vinyl chloride is cleanly hydrogenated to ethane 2-methyl-2-pentenal. Carbomethoxylation of 1-hexene at 120" and 3 atm pressure in a PtCl2 solution in molten at 90" and 450 atm CO pressure gave a 4 :1 mixture of methyl heptanoate and the isomeric 2-methylhexano[(C2H&N][SnC13]. The reduction appears to proceed cia dechlorination to give ethylene which is subsequently ate. This activity in carbonylation of olefins is again hydrogenated since some ethylene is detectable when reminiscent of that observed with a mixture of H?PtCl6 the reaction is stopped after a short time. The apand SnClz.2H20in alcoholic media. l 4 pearance of ethane rather than butane or butenes as the product is surprising because vinyl chloride is readily Discussion coupled to butadiene by SnC13- salts in the presence The great similarity in catalytic activity between a of PtC12 in aqueous dimethylf~rmamide.~However, solution of PtClz in a molten trichlorostannate salt and when vinyl chloride is exposed to a PtC12 solution in that observed with preformed SnC13- complexes sug[(C?Hj)rN][SnC13]in the absence of hydrogen at 95", gests that the catalytic species may be similar or identithe olefin partially dissolves (0.045 M at 610 mm presIn the presence of a large excess cal in the two systems. sure) but no reaction is apparent. of SnC13ligand (75Sn/Pt for 1 % PtClz in EtrNSnC13), Platinum dichloride solutions in molten [(CZH&N]the predominant platinum species is expected to be rSnC13], like trichlorostannate complexes in more conthe known'O five-coordinate complex [Pt(SnC13)j]3-. ventional selectively hydrogenate dienes Indeed [(C2H,)4N]3[Pt(SnC13)6]was isolated by extracand trienes to monoolefins. A substantial advantage tion of a concentrated ( 7 % ) solution of PtCI2 in [(Czof the molten salt medium, however, is that the product Hj),lN][SnC13]. Fractional extraction of the salt memay be separated by decantation or simple distillation. dium recovered from one hydrogenation disclosed the This simplicity of isolation is especially useful in the presence of another previously characterizedL0 fivereduction of high-boiling polyenes such as the linoleate coordinate complex, [HPt( SnCl&] 3-. esters in soyabean oi1.12v13 Methyl linoleate is hydroThe identification of this hydrido-platinum species genated at 150" and 100 atm pressure in the presence suggests that the hydrogenation in this medium proof a 1 % solution of PtClz in [(CZH&N][SnCl3]to give ceeds by a conventional olefin insertion mechanism'j a mixture of esters containing 63 methyl oleate. (A similar mechanism has been as shown in Figure I . As shown in Table I, the hydrogenation of 1,5,9proposed for the isomerization of olefins by PtCLcyclododecatriene (cis,trans,trans) can be directed to SnClz solutions.16) In steps 3 and 4 the olefin comgive primarily cyclododecene under similar conditions. petes with excess SnC13- ligand for a coordination site In the range of 30-500 atm, hydrogen pressure does not in the hydridoplatinum complex. The large excess seem to be a critical variable but choice of time and of ligand in the molten salt probably accounts for the temperature is important in production of a maximum selective hydrogenation of polyenes to monolefins. yield of cyclododecene. A single experiment at 150" Conjugated dienes, such as those formed by isomerizawith the all-trans triene likewise gave 80+ % selection compete more successtivity to the monoolefin. When C ~ ~ - P ~ C ~ ~ [ A S ( C ~ Hof~ )1,5,9-cyclododecatriene, ~]~ fully for coordination sites than do monolefins by virtue was used as the catalyst, the reaction was selective to of their ability to form r-allyl complexes in step 5 . cyclododecene but much slower. After 10 hr at 150" The mechanism for the cleavage of the alkylor allylt a . 75 % triene remained but most of the cis,trans,trans platinum bond in this medium has not been established. isomer charged had been isomerized to the all-trans The oxidative addition of Hz (step 6 ) followed by elimand other isomers. Isomerization appears to be much ination of alkane (or alkene) seems plausible, but more rapid than hydrogenation in the PtCle-molten there is strong evidence for protolysis of the C-Pt bond salt solutions. Treatment of 1,5-cyclooctadiene with in hydroxylic media. l7 hydrogen at 1 atm and 100" gave over 90% 1,3 isomer but only traces of cyclooctene and no cyclooctane. (12) L. P. van't Hof and B. G. Linsen, J . Catal., 7, 295 (1967). (13) J. C. Bailar and H. Itatani, J . Amer. Oil Chem. SOC.,43, 337 (1966).
(14) E. L. Jenner and R. V. Lindsey, U. S. Patent 2,876,254 (1959). (15) C. A. Tolman, Chem. SOC.Reu., 1, 336 (1972). (16) I
