- Email: [email protected]
Zn catalysts
Journal
o/Moiecuhr
Mechanistic
Catalysis,
Reffecticms
G. HENRICI-OLEVE Monsanto
Textiles
(Received
May
17 (1982)
on
89
89 - 92
the Methanol
Synthesis
with Cu/Zn
Catalysts
and S. OLIVE Cornpuny.
Pensacola.
FL 32575
(U.S.A.)
13, 1982)
The industrial low catalysts based on the Whereas Cr,O, or A1,Os copper and zinc oxide is
pressure (20 - 100 atm) methanol synthesis uses compositions Cu/ZnO/Cr203 or Cu/ZnO/Al,Os,. is considered non-essential, the presence of both viti for satisfactory catalysis, the combination of both being several orders of magnitxde more effect-we than either of the two components [I]. This mutual promotion has intrigued several authors (e.g. [I - 31). but no satisfactory mechanism has been found as yet. Based on a broad spectrum of detailed experimental results found in the literature, we would like to suggest a mechanism for this amazing case of synergism. The most useful pieces of information r.ome from the comprehensive study by Herman et aL [I] concern’mg catalysts of various compositions. While after calcination the mixed catalysts consist of hexagon& ZnO ant! tetragonal CuO, the actual active catalyst-, after reduction or after use. is made up essentially of metahic Cu and ZnO. However, there is spectra evidence for the presence of Cu(I) species which are visualized as ‘dissoIved’ in ZnO. From the known facts that Cu’ ions form carbonyls [4, 51 and that. ZnO activates molecular hydrogen by heterolytic splitting, giving r&e tc -2nH and -OH groups ]6], Herman et aL concluded that CO is coordinativeIy bonded to a &(I) center, and that Hz is heterolytictiy split by a nearby ZnO center, the proton attacking the carbon end and the hydride ion attacking the oxygen end of the CO molecule. (Since the dipole of CO has its negative side at the 0 and not at C, the contrary wouId have appeared more probable.) Herman et aL also assumed that the synthesis proceeds as a surface reaction, because no special pore distribution was found necessary for highly effective catalysis. However, it should be t&en into consideration that Cu(I) itself is well able to decompose hydrogen [7), and also to form hydrides [3, S] _ We therefore suggest that at least one of the major effects of the ZnO component is to stabilize the catalytically active CL@) state, for instance in the form of ‘end groups’ of ZnO chains which may formally be written as: Cu(I)-O-Zn(II)UZn(II).
.. 0 El-ier
Sequoia/Printed
in The NetterIanck
90
(Actually the structure is, of course, threedimensionai.) -4t the surface such Cu(L) species could easily coordinate to one or several CO molecules, under
the reaction conditions of the methanol synthesis. On the other hand, Goeden and Caulton [3] have shown that certain Cu-0 bonds (in their particular case Cu+oxy bonds) can react with Hz, involving a he’terolytic splitting of the hydrogen molecule. Transposing this finding to the present problem, we suggest the following formation of a surface Cu(I) carbonyl-hydride: H (CO),CuUZn-
= - f H,
+
(CO),&
+ HO-2r-r.
--
(1)
(Note that Goeden and CauIton [3] have also shown that Cu(I) hydrides are more stab!e than Zn(II) hydrides.) T’ne next step would be &and CO insertion into the Cu-H bond (or H migration to one of the coordinated CO molecules), with the formation of a for-my1 &and, much as has been suggested for the initiation step in the Fischer-Tropsch synthesis 19, lo] _ There is by now a large amount of evidence for the feasibility of this type of reaction. (For a recent compilation of references see [ll] _) H Ii (CO),&
0 f HO-Zn..-
+
(CO),_,
i
+ HO-Zn..-
(2)
Formyl ligands have been reported to transform to formaldehyde or methanol under the influence of proton donors 1121. The amphoteric character of zinc hydroxide invites the assumption that a proton donation to the coordinated formyl ligand may take place, given the ideal proximity of the surface HO-Zn group: H CO C u + HO-Zn...
+
H* Fi. ..(&-_O-Zn... 0
(3)
The reaction sequence suggested in eqns. (1) - (3), and invoIving the restoration of the original Cu-G-Zn arrangement, has a precedent in a soluble palladium hydrogenation catalyst [X3]. In that case, kinetic data and the pH dependence of the reaction had strongly suggested a heterolytic splitting of the hydrogen molecule, with the proton being taken up by one of the OH groups of t.he four-dentate &and SALEN*, and the H- by the metal, with a polar transition state:
I *SALEN:
fox-denkk
ligand
nC = ’
N =
CH
91
H-H
H
L
After the coordination of a substrate (oiefin) molecule at the free coordination site, hydrogenation takes place. The required steric proximity of the OH group is, in this case, guaranteed by the fact that the SALEN Ligand remains fixed to the metal center by thee of its four coordinating groups. The product molecule then Ieaves the complex, and the starting conf&sration is restured. Et should also 5e recalled that, in Nature, the action of hydrogenases appears to follow similar reaction patteEm (for a dklssion see [ 131). The further hydrogenation of the coordinated formaldehyde may be visualized along the same lines, involving heterolytic splitting of H, and formation of a methoxy tigznd, which then is hydrogenated to the product methanol: CH3 A yt!?z~
0
+
HO-Zn..-
+
L t
+HO-Zn--.
+
CH,OH f Cu-O-Zn.
--
(Note that Goeden and Caulton [3] provide solid experimental evidence that the reaction of Cu-H with formaldehyde leads to a methoxy, rather than to a hydroxymethyl, ligand as the Intermediate.) Poisoning of the catalyst by H2S and HCi may be expected to take place according to:
Cu-C-Zn Cu-O-Zn-.
- - - i I&S 2
CuSH f HO-Zn-
- + HCl ;;P’ CuCl + HO-Zn.
--*
in a manner very similar to that suggested by Herman ef aL [I j . It appears, then, that the synergistic effect of ZnO, rather than providing the activation of K, directly, has the dual purpose of maintaining the copper J&Ithe active Cu(1) state, and of providing a polar Cubond availabIe for the heterolytic splitting of the hydrogen moIecuIe. References 1 R. G. Herman,
(19?9)407. 2 K Shimomura,K
K. Kiter,
G.
IV. Simmons, B. P. Firm and 5. B. Bulko, J. Cat&.. 56
Ogawa,M.Oba
a&Y.
Kokra,J. GrfaZ..52 (iS78)191.
3 G. V. Goeden and K. G. Caulkm. J. Am. Chem Sot.. IO3 (1981) 7354. 4 F. A. Cotton and G. Wilkinson: Aduunced Inorganic Chemistry. 2nd Edn.. Interscience, London, 1966. p_ 898. 5 M. Pssqusli, C. Fioriani. A_ Gaetani-Mankedctti end C. GuastinI;d Am. Chcm Sot.. 103 (1981) 185, End references therein. 6 R. P. sixhens, W. A. PIiskin and M. J. D. Low, J. Cctal.. I (1962) 180. 7 See, e-g_, B. R. J-es, Homogeneou JIydrogencCion, W&q, New York, 1973. 8 B. Beguin, B. Dtnke and R. P. A. Sneeden, L UrgunometaL C-hem. 208 (1981) Cl8. 9 I-?_Pichler end H. SchuLz, Chem.-kg. Te&rz.. 42 (1970) 1162. 10 G. Henriei-Oliv6 and S. Ok& Angcw. Cirem. 88 (1976) 144; Artgew. Ckem. Int. Ed. En& 15 (3976) 136. 11 P. J. Fsgan, K_ G. Moloy and T. J. ,Marks, X Am Ckm. Sac.. 103 (1981) 6962. 12 J. P. Callman and S. R. Winter, J. Am. Ckem. Sot.. 95 (1973) COS9; C. P. Casey and S. M. Neumann, J. Am. Ckem. Sot. 98 (1976) 5335; ib;‘d.. IO0 (1978) 2544; J. A GLadysz and W_ Tam, J. Am. Ckem. Sot.. IOU (197s) 2545. 13 G. Eenrici-OlivL and S. Olive, J. fiiol. Cutal.. Z (1975/76) 121.
o/Moiecuhr
Mechanistic
Catalysis,
Reffecticms
G. HENRICI-OLEVE Monsanto
Textiles
(Received
May
17 (1982)
on
89
89 - 92
the Methanol
Synthesis
with Cu/Zn
Catalysts
and S. OLIVE Cornpuny.
Pensacola.
FL 32575
(U.S.A.)
13, 1982)
The industrial low catalysts based on the Whereas Cr,O, or A1,Os copper and zinc oxide is
pressure (20 - 100 atm) methanol synthesis uses compositions Cu/ZnO/Cr203 or Cu/ZnO/Al,Os,. is considered non-essential, the presence of both viti for satisfactory catalysis, the combination of both being several orders of magnitxde more effect-we than either of the two components [I]. This mutual promotion has intrigued several authors (e.g. [I - 31). but no satisfactory mechanism has been found as yet. Based on a broad spectrum of detailed experimental results found in the literature, we would like to suggest a mechanism for this amazing case of synergism. The most useful pieces of information r.ome from the comprehensive study by Herman et aL [I] concern’mg catalysts of various compositions. While after calcination the mixed catalysts consist of hexagon& ZnO ant! tetragonal CuO, the actual active catalyst-, after reduction or after use. is made up essentially of metahic Cu and ZnO. However, there is spectra evidence for the presence of Cu(I) species which are visualized as ‘dissoIved’ in ZnO. From the known facts that Cu’ ions form carbonyls [4, 51 and that. ZnO activates molecular hydrogen by heterolytic splitting, giving r&e tc -2nH and -OH groups ]6], Herman et aL concluded that CO is coordinativeIy bonded to a &(I) center, and that Hz is heterolytictiy split by a nearby ZnO center, the proton attacking the carbon end and the hydride ion attacking the oxygen end of the CO molecule. (Since the dipole of CO has its negative side at the 0 and not at C, the contrary wouId have appeared more probable.) Herman et aL also assumed that the synthesis proceeds as a surface reaction, because no special pore distribution was found necessary for highly effective catalysis. However, it should be t&en into consideration that Cu(I) itself is well able to decompose hydrogen [7), and also to form hydrides [3, S] _ We therefore suggest that at least one of the major effects of the ZnO component is to stabilize the catalytically active CL@) state, for instance in the form of ‘end groups’ of ZnO chains which may formally be written as: Cu(I)-O-Zn(II)UZn(II).
.. 0 El-ier
Sequoia/Printed
in The NetterIanck
90
(Actually the structure is, of course, threedimensionai.) -4t the surface such Cu(L) species could easily coordinate to one or several CO molecules, under
the reaction conditions of the methanol synthesis. On the other hand, Goeden and Caulton [3] have shown that certain Cu-0 bonds (in their particular case Cu+oxy bonds) can react with Hz, involving a he’terolytic splitting of the hydrogen molecule. Transposing this finding to the present problem, we suggest the following formation of a surface Cu(I) carbonyl-hydride: H (CO),CuUZn-
= - f H,
+
(CO),&
+ HO-2r-r.
--
(1)
(Note that Goeden and CauIton [3] have also shown that Cu(I) hydrides are more stab!e than Zn(II) hydrides.) T’ne next step would be &and CO insertion into the Cu-H bond (or H migration to one of the coordinated CO molecules), with the formation of a for-my1 &and, much as has been suggested for the initiation step in the Fischer-Tropsch synthesis 19, lo] _ There is by now a large amount of evidence for the feasibility of this type of reaction. (For a recent compilation of references see [ll] _) H Ii (CO),&
0 f HO-Zn..-
+
(CO),_,
i
+ HO-Zn..-
(2)
Formyl ligands have been reported to transform to formaldehyde or methanol under the influence of proton donors 1121. The amphoteric character of zinc hydroxide invites the assumption that a proton donation to the coordinated formyl ligand may take place, given the ideal proximity of the surface HO-Zn group: H CO C u + HO-Zn...
+
H* Fi. ..(&-_O-Zn... 0
(3)
The reaction sequence suggested in eqns. (1) - (3), and invoIving the restoration of the original Cu-G-Zn arrangement, has a precedent in a soluble palladium hydrogenation catalyst [X3]. In that case, kinetic data and the pH dependence of the reaction had strongly suggested a heterolytic splitting of the hydrogen molecule, with the proton being taken up by one of the OH groups of t.he four-dentate &and SALEN*, and the H- by the metal, with a polar transition state:
I *SALEN:
fox-denkk
ligand
nC = ’
N =
CH
91
H-H
H
L
After the coordination of a substrate (oiefin) molecule at the free coordination site, hydrogenation takes place. The required steric proximity of the OH group is, in this case, guaranteed by the fact that the SALEN Ligand remains fixed to the metal center by thee of its four coordinating groups. The product molecule then Ieaves the complex, and the starting conf&sration is restured. Et should also 5e recalled that, in Nature, the action of hydrogenases appears to follow similar reaction patteEm (for a dklssion see [ 131). The further hydrogenation of the coordinated formaldehyde may be visualized along the same lines, involving heterolytic splitting of H, and formation of a methoxy tigznd, which then is hydrogenated to the product methanol: CH3 A yt!?z~
0
+
HO-Zn..-
+
L t
+HO-Zn--.
+
CH,OH f Cu-O-Zn.
--
(Note that Goeden and Caulton [3] provide solid experimental evidence that the reaction of Cu-H with formaldehyde leads to a methoxy, rather than to a hydroxymethyl, ligand as the Intermediate.) Poisoning of the catalyst by H2S and HCi may be expected to take place according to:
Cu-C-Zn Cu-O-Zn-.
- - - i I&S 2
CuSH f HO-Zn-
- + HCl ;;P’ CuCl + HO-Zn.
--*
in a manner very similar to that suggested by Herman ef aL [I j . It appears, then, that the synergistic effect of ZnO, rather than providing the activation of K, directly, has the dual purpose of maintaining the copper J&Ithe active Cu(1) state, and of providing a polar Cubond availabIe for the heterolytic splitting of the hydrogen moIecuIe. References 1 R. G. Herman,
(19?9)407. 2 K Shimomura,K
K. Kiter,
G.
IV. Simmons, B. P. Firm and 5. B. Bulko, J. Cat&.. 56
Ogawa,M.Oba
a&Y.
Kokra,J. GrfaZ..52 (iS78)191.
3 G. V. Goeden and K. G. Caulkm. J. Am. Chem Sot.. IO3 (1981) 7354. 4 F. A. Cotton and G. Wilkinson: Aduunced Inorganic Chemistry. 2nd Edn.. Interscience, London, 1966. p_ 898. 5 M. Pssqusli, C. Fioriani. A_ Gaetani-Mankedctti end C. GuastinI;d Am. Chcm Sot.. 103 (1981) 185, End references therein. 6 R. P. sixhens, W. A. PIiskin and M. J. D. Low, J. Cctal.. I (1962) 180. 7 See, e-g_, B. R. J-es, Homogeneou JIydrogencCion, W&q, New York, 1973. 8 B. Beguin, B. Dtnke and R. P. A. Sneeden, L UrgunometaL C-hem. 208 (1981) Cl8. 9 I-?_Pichler end H. SchuLz, Chem.-kg. Te&rz.. 42 (1970) 1162. 10 G. Henriei-Oliv6 and S. Ok& Angcw. Cirem. 88 (1976) 144; Artgew. Ckem. Int. Ed. En& 15 (3976) 136. 11 P. J. Fsgan, K_ G. Moloy and T. J. ,Marks, X Am Ckm. Sac.. 103 (1981) 6962. 12 J. P. Callman and S. R. Winter, J. Am. Ckem. Sot.. 95 (1973) COS9; C. P. Casey and S. M. Neumann, J. Am. Ckem. Sot. 98 (1976) 5335; ib;‘d.. IO0 (1978) 2544; J. A GLadysz and W_ Tam, J. Am. Ckem. Sot.. IOU (197s) 2545. 13 G. Eenrici-OlivL and S. Olive, J. fiiol. Cutal.. Z (1975/76) 121.