J. Chem. SOC., Faraday Trans. I, 1988, 84(2), 601-608 Activation and Chain-carrying CH, Species in Terminal A1 kene Me tat hesis on Mol ybdena-Titania Catalysts Katsumi Tanaka" Research Institute for Catalysis, Hokkaido University, Kita-Ku, Sapporo 060, Japan Ken-ichi Tanaka The Institute for Solid State Physics, The University of Tokyo, 7-22-1 Roppongi, Kinato-Ku Tokyo 106, Japan The metathesis reaction of terminal alkenes other than isobutene took place on Moo,-JTiO,, whereas that of isobutene proceeded in the presence of ethylene and following treatment of the catalyst with SnMe, at room temperature. These results infer that chain-carrying CH, species are generated to only a small extent by the adsorption of isobutene on MOO,-,/ TiO,, although they are formed by the adsorption of alk-1-enes and SnMe,.The metathesis-inactive material, fully oxidized MoOJTiO,, was changed to a metathesis-active catalyst by treating it with SnMe,. This suggests that MoO,/TiO, has no ability to yield CH, species with terminal alkenes but that these species can be supplied with SnMe,. An analysis by X-ray photolectron spectroscopy infers that SnMe, adsorbed on MoO,/TiO, releases methyl groups concomitant with the oxidation of Sn. At the present time, a mechanism proceeding via metal-alkylidene and metalla- cyclobutane intermediates is accepted as an approved route for catalytic alkene metathesis reactions.' In fact, some non-Fischer-type metal-alkylidene complexes2 have been found to promote metathesis-like reactions, and metallacyclobutane derivatives of Ti3 and W4 undergo metathesis-like reactions with alkenes.It has also been shown by means of n.m.r. spectroscopy that the neopentylidene ligand in a tungsten complex is replaced by ethylidene and propylidene ligands on reaction with ~ent-2-ene.~ Homogeneous catalysts for alkene metathesis sometimes require a cocatalyst, such as the addition of EtAlCI, to WCl,,, and the role of Lewis acids or hard ligands in the cocatalysts is interpreted in terms of electronic effects due to the central Unlike the electronic effect produced by electron-deficient molecules, the activation of WCl, by ZnMe, may be caused by the formation of W=CH, species in the reaction with ZnMe,.8 Similar phenomena have been found using Cp,TaMe326 and C ~ T i c l , ~ treated with AlCl,.On the other hand, alkene metathesis on heterogeneous catalysts proceeds in general by contact with the alkenes. This fact implies that the key intermediates for alkene metathesis, metal alkylidenes, are produced automatically in the initial stages of the reaction by contact with the alkenes. If this is the case, whether or not a solid surface is active for alkene metathesis should be determined by the ability for the alkylidene formation process to take place. In this paper we will demonstrate that alkylidene formation is a significant step in heterogeneous alkene metathesis. Experimental Catalyst Preparation Molybdena-titania catalysts were prepared by immersing p-titanic acid, TiO, * H20, into an aqueous solution of ammonium paramolybdate, (NH,),Mo,O,, * 4H20.The solid was then dried at 120 "C in air for 12 h, and its MOO, content was found to be 6.7 wt '10. Details of the preparation of /?-titanic acid have been described previously." 60 1602 Alkene Metathesis on Molybdena-Titania Catalysts Freshly prepared catalyst was oxidized with 0, (ca. 200 Torr, 1 Torr = 101 325/760 Pa) at 500 "C for 1 h, which rzsulted in the fully oxidized MoO,TiO, catalyst. Partially reduced MOO,-,/TiO, (0.1 < x < 0.7) was prepared by reoxidizing a MoO,/TiO, catalyst (reduced with H,; 100-200 Torr, 500 "C, 1 h) with a 1 : 1 mixture of N,O and H, at ca. 300 Torr and 200 "C for 1 h.l0 Prior to the catalytic reaction or treatment with SnMe,, the catalyst was evacuated at 500 "C for 1 h. Reactions and Product Analysis Pretreatment of the catalyst and the reaction were performed in a closed glass circulation system with a volume of ca. 260 cm3.0.5 g of catalyst was treated with diluted SnMe, ( 5 Torr of SnMe, in ca. 60 Torr of He) for 30 min at room temperature, and this was followed by evacuation at the same temperature for 30 min." Metathesis was carried out at room temperature with an alkene pressure between 4 and 65 Torr. Analysis of ethylene, propene, but- 1 -ene, isobutene, but-2-ene, hex-3-ene and 2,3-dimethylbut-2-ene was performed using a gas-chromatograph with a 13 m length column comprising Sebaconitrile (25 %) on Uniport C (60-80 mesh), while for methane, ethane and ethylene the column comprised 2 m of Gaskuropack 54 (a copolymer of polystyrene and divinylbenzene, commercially available from Gasukuro Kogyo Co.). Deuterium atom distributions in the alkene products were calculated by mass-spectrometric analysis with an ionization voltage of 10-15 eV.[,H,]Ethylene, [2H,]propene (Merck Sharp & Dohme) and [l3C2]ethylene (Amersham International, 92 % 13C) were used without further purification. [,H,]But- 1 -ene was prepared by deuteration of commercially available E2H4]butadiene on ZnO at room temperature. [2H,]Isobutene was obtained by reacting isobutene with deuterium gas at room temperature on Mg (OH), evacuated at 450 "C. The prepared samples of [,H,]but- 1-ene and [2H,]isobutene were purified using gas- chromatographic separation. X.P.S. Analysis X-Ray photoelectron spectra of the catalyst were measured with a VG-ESCA 3 spectrometer.MoO,/TiO, powder was compressed into a disc and was mounted on a Ni holder with an internal standard Au wire. The catalyst was treated in the preparation chamber using the same procedure as performed in the circulation system. However, the adsorption of tetramethyl tin on the MoO,/TiO, disc was performed at liquid-nitrogen temperature (ca. - 190 "C). Binding energy (Eb) values were referenced to the 0 1s peak at 530 eV, and 2p,,, for Ti4+ was found to be at 458.7 eV, in good agreement with the value reported for Ti4+ on Ti0,.12 Results and Discussion Alkene Metathesis on Moo,-,/TiO, and Moo,-,/TiO,-SnMe, Table 1 summarises turnover frequencies for the metatheses of ethylene, propene, but- 1-ene and isobutene on Moo,-,/TiO, and MOO,-,/TiO,-SnMe, at room tempera- ture.The turnover frequencies are evaluated by assuming that all the Mo cations on TiO, participate in metathesis. Therefore the true turnover frequencies should be larger than those in table 1. It is of interest that ethylene, propene and but- 1 -ene undergo metath- esis, whereas for isobutene, metathesis was entirely prohibited and the polymerization of isobutene occurred on Moo,-,/TiO,. This polymerization may be cationic, via the tertiary butyl cation, or it may be an addition polymerization of a-alkenes via a Ziegler-Natta mechanism on reduced TiO,. Despite the fact that the metathesis of isobutene does not proceed on MoO,-,/TiO,, when a 1 : l : l mixture of isobutene,K . Tanaka and K-i. Tanaka 603 Table 1. Turnover frequencies of metathesis reactions of ethylene, propene, but- 1-ene and isobutene on MoO,-,/TiO, and Moo,-,/TiO,-SnMe, at room temperature metathesis reaction Mo03-,/Ti0, M00,-,~/Ti0,-SnMe, CH,=CH, + CD,=CD, - 2 CH,=CD, 3.2 x 10-5 1.9 x 10--3 2c=c-c - c=c + c-c=c-c 2.0 x 10-7" 5.4 x lo-J" 2c=c-c-c - c=c + c-c-c=c-c-c 7.9 x 2.0 x lo-" /c c, /c 'C c/ ' c 2c=c - c=c + c=c 0 3.3 x 10-3 a Ref.(1 1). Turnover frequencies (ethylene molecules per Molybdenum ration per second) were obtained by assuming that all Mo cations participate in metathesis. [l2C2]ethylene and [13C,]ethylene was added on Moo,-,/TiO, at room temperature, ['3C,]ethylene and [13C,]isobutene were formed by the following metathesis reaction (see fig. 1 ) : / c + C=13C ( l a ) '3c=c\c + 13C='3C /c 'C c=c C / C c\ / 2c=c - c=c + c=c 'C C ' 'c [13C,]ethylene is produced by the reactions (1 a ) and (1 b), and reaction (1 a ) yields equal amounts of [13C,]isobutene and [13Cc,] ethylene.Accordingly, reaction ( 1 b) is ca. 10 times faster than reaction (1 a). 2,3-Dimethyl but-2-ene was formed distinctly [reaction (1 c)] in the co-metathesis of isobutene and ethylene on Mo03-,/Ti0,; however, its rate was extremely slow. From these results it is concluded that the metathesis of isobutene proceeds on MoO,_,/TiO, in the presence of ethylene. Such a phenomenon can be interpreted as follows. Any metal alkylidene species required for alkene metathesis are not produced on MoO,-,/TiO, by the adsorption of isobutene ; however, the adsorption of ethylene gives a metal alkylidene species on the surface. As a result, the metathesis of isobutene can proceed in the presence of ethylene via the alkylidene species furnished from ethylene in the initiation steps. The fact that polymerization of isobutene is suppressed in the presence of ethylene while simultaneously metal alkylidene is supplied from ethylene under such conditions suggests that the metal alkylidene formation might proceed through a metal alkyl species formed by a reaction between ethylene and hydrogen species which yields the tertiary carbonium cation on the MoO,_,/TiO, catalyst .When MoO,_,/TiO, was treated with SnMe, at room temperature, it changed to a super-active alkene metathesis catalyst. The turnover frequency for the metathesis of propene was enhanced more than three orders of magnitude, and those of ethylene and but-1-ene were increased by factors of ca.10, and 3, respectively. Note that the604 Alkene Metathesis on Molybdena- Titania Catalysts > I 240 I 300 3( 0 t/min Fig. 1. Metathesis of a 1 : l : 1 mixture of isobutene, ethylene and [13C,]ethylene on MOO,-,/ TiO, at room temperature: 0, [l3C1]ethylene [reaction (1 a)]; 0, [13Cl]isobutene, x 10 [reactions (1 b) and (1 c)]: total pressure, 48 Torr; catalyst 0.5 g. metathesis of isobutene proceeded on MoO,-,/TiO,-SnMe,. This result clearly indicates that chain-carrying metal alkylidene species are formed on the Moo,-,/TiO, surface by the adsorption of SnMe,. When a 1 : 1 mixture of [,H,]- and [,H,]-ethylene, [,H,]- and [2H6]-propene or [,H,]- and [,H,]-but- 1 -ene was added on Moo,-,/TiO, at room temperature, neither the hydrogen- scrambling nor the double-bond shift reactions occurred during metathesis.In contrast, when a 1 : 1 mixture of [,H,]- and [2H,]isobutene was reacted on Moo,-,/TiO, at room temperature, hydrogen scrambling proceeded rapidly, concurrent with polymerization. This result may be due to a rapid equilibration between isobutene and the butyl carbonium cation, in which three equivalent methyl groups can participate in hydrogen exchange. When a 1 : 1 mixture of [,H,]- and [2H,]-isobutene was reacted on MOO,-, Ti0,-SnMe, at room temperature, metathesis occurred with little hydrogen mixing ; i.e. the productive metathesis of isobutene [reaction (2 a)] yielded ethylene composed of [,H,], ['H,] and ['H,] isomers and 2,3-dimethylbut-2-ene composed of [,H,] [,H6] and [2H12] isomers, and the degenerate metathesis [reaction (2 b)] gave [,H,]- and [2H6]isobutene : r 1 ethylene I 2,3-dimethylbut-2-ene + /CH3 CH,=C + 'CH, ,.D3 CD,=C \ CD, / \K.Tanaka and K-i. Tanaka 605 Table 2. Initial formation rates of methane, and ratios of ethane to methane formed in the early stages of the reaction of SnMe, (5 Torr) on molybdena-titanias (0.5 g of each) at room temperature CH, formation rate /(molecules Mo)-' s-I initial C,H,/CH, ratio catalyst MoO,/TiO, 6.3 x lo-, Moo,-,/TiO, 4.0 x M 00, /Ti0 2.5 x 10-5 4.0 0.5 0.0, From this result it is deduced that the activation of MOO,-,/TiO, with SnMe, is responsible for the introduction of chain-carrying CH, species on the surface and that the adsorption of SnMe, inhibits the formation of t-butyl carbonium ions on acidic sites.Note that the treatment of inactive MoO,/TiO, with SnMe, results in the formation of an active catalyst for the metathesis of isobutene, as well as of ethylene, propene and but-1 -ene at room temperature. In addition, productive and degenerate metathesis oc- curred selectively on the catalyst in a 1 : 1 mixture of [,H,]- and [2H,]-isobutene. Reaction of SnMe, on Molybdene Titania As discussed above, the role of SnMe, in activating molybdena-titania is undoubtedly to graft the CH, chain-carrying species required for alkene metathesis. When SnMe, was contacted with a molybdena-titania surface at room temperature, small amounts of methane, ethane and ethylene were evolved. Table 2 shows the rate of methane formation and the ratio of C,H, to CH, obtained in the initial stage of the reaction with SnMe, on MoO,/TiO,, Moo,-,/TiO, and MoOJTiO,.Not such a big difference is seen in the rate of methane formation on these catalysts, but the initial C,H,/CH, ratios strongly depend on the extent reduction of molybdenum oxide, i.e. ethane formation selectively proceeds on MoO,/TiO,. The turnover frequency of propene metathesis was 9.3 x on MoO,/TiO,-SnMe,. Consequently, CH, species may be produced on the MoO,/TiO, surface by the reaction of 2CH3+CH, + CH,. A reductive coupling of CH, giving ethane, 2CH,-+C2H6, may occur also on the MoO,/TiO, surface. So far, five different modes of alkyl metal cleavage have been proposed :lo (i) D-elimination, (ii) reductive coupling, (iii) a-elimination, (iv) hydrogen and alkyl transfer and (v) electrophilic attack.13 Cases (ii), (iii) and (iv) are relevant to the reaction of SnMe, on molybdenum oxides on TiO,.If the reductive coupling occurs on MoO,/TiO, in the reaction with SnMe,, the valence states of Sn and Mo should be of considerable interest. The X-ray photoelectron spectra of the Sn 3d region are shown in fig. 2. When MoOJTiO, was exposed to SnMe, with 60 [l L (1 langmuir) = 1 x lo-, Torr s] at liquid-nitrogen temperature, the peaks indicated as species I were observed at ca. 483 and 492 eV; these correspond to a 3d5/2 and 3d3,, doublet [fig. 2(a)] and were accompanied by weak peaks at ca. 486 and 495 eV indicated as species 11. The peaks were little influenced by continuing X-irradiation for 30 min [fig.2(b)]. However, if the MoO,/TiO,-SnMe, sample was left in vacuu overnight to reach room temperature, species I1 remained with the same intensity as in fig. 2(a) [see fig. 2(c)]. Species I corresponds to Sno;l4 however, the weak species I1 may be assigned to Sn2+ or Sn4+ because of their close binding energies.14* l5 These results imply that monolayer adsorption of SnMe, results in the oxidation of the Sn species, giving ethane and leaving the SnMe, overlayer in the Sno+ state. The oxidation of Sn may compensate the reduction of Mo6+ in MoO,/TiO,. The X.P.S. band for Mo 3d did not appreciably change on adsorption of SnMe,, perhaps because of the606 Alkene Metathesis on Molybdena-Titania Catalysts I I I t II I I I I I I I I I 1 I 480 484 488 492 496 EbIeV Fig.2. (-Ray photoelectron spectra of the Sn 3d region following addition of SnMe, to Lao,/ TiO,: (a) 60 L of SnMe, at ca. - 190 "C; (b) under X-irradiation for 30 min; (c) at room temperature following (b). Table 3. ,H distribution of methane, ethane and ethylene formed after 30 min of the reaction of SnMe, (10 Torr) on an MOO,/ TiO, catalyst (1 .O g) at room temperature" amount amount formed 2H distribution formed per total product /mol Mo ,H,, 2H, ,H, methane 1.16 x 2.5 x lo-, 81.5 18.5 0 ethane 2.78 x 6.0 x 100.0 0 0 ethylene 0.20 x 0.4 x 96.2 3.8 0 "The H atoms on the surface were replaced by ,H atoms. Amount of Mo in 1 g catalyst 4.66 x lov4 mol. amount of reduced Mo species. In the case of Re20,/y-A1203, reduction of the Re species by SnR, (where R = methyl, ethyl or butyl) is detected by e.s.r. spectroscopy.ls Note that the reduction of the MoO,/TiO, surface by SnMe, is not indispensable for activation: partially reduced MOO,-,/TiO, is not so active for metathesis but is changed into a superactive catalyst by treatment with SnMe, as shown in table 1.To clarify the role of SnMe,, surface hydrogen on MoO,/TiO, was exchanged with deuterium to 97 YO, and was subjected to reaction with SnMe,. The amount of methane, ethane and ethylene and their ratios to the total amount of Mo cation are listed in table 3. The amount of methane is ca. 0.25% of that of the total amount of Mo cations. Methane involves 18.7 YO CH,D, but there is no deuterium present in ethane. 85 % of the [,Holmethane in table 3 should be formed by an a-hydrogen abstraction between two CH, groups supplied from SnMe,, and [,H,]methane is formed by the reaction of the CH, group with deuterium atoms on on the surface: 2CH, -+ C,H, (reductive coupling) 2CH, + CH, + CH, CH, + OD + CH,D + [O] (a-hydrogen abstraction) (hydrogen and methyl transfer).K.Tanaka and K-i. Tanaka 607 The abstraction of hydrogen from the CH, group results in a grafting of CH, species onto the surface. A similar phenomenon was reported in the homogeneous metathesis system, WC1,-ZnMe,, in which methane formation is noted in the activation of WC1, by ZnMe,.8 The formation of ethylene shown in table 3 may reflect the formation of CH, species during the activation process, because ethylene is formed either by coupling of two CH, species or by the insertion of CH, into M-CH, (M=Mo and/or Sn). The fact that the amount of [2H,]ethylene (3.8%) is far lower than that of [,H,]methane (18.5 %) in fig.3 seems to support the following processes: CH, + CH, + C,H, (CH, coupling) M-CH, + CH, + M-CH, + CH, + M-H + C,H, (CH, insertion; @-hydrogen abstraction). In conclusion, the formation of highly active metathesis catalysts by treatment with SnMe, may be explained by the introduction of chain-carrying CH, species on their surfaces. Here a ligand effect of SnMe, is not excluded, because the activity in homogenous catalysis is known to be enhanced by the ligand effect, but the remarkable enhancement of metathesis activity observed here is interpreted by the grafting of chain- carrying CH, species onto the catalysts.This conclusion is quite feasible in the case of isobutene metathesis observed on Moo,-,/TiO, : Alk- 1 -enes can furnish alkylidene species by being adsorbed on Moo,-,/TiO,, but isobutene cannot furnish such a key species on the surface. (This may eventually aid the alkylidene formation process on solid metathesis catalysts, so that metathesis of isobutene is not catalysed by MOO,-,/ TiO,). However, if chain-carrying CH, species are supplied from alk-1-ene or from SnMe,, the propagation process via CH, species proceeds. According to this mechanism, hydrogen mixing in alkenes is a side reaction, and the metathesis of a 1 : 1 mixture of [,H,]- and [2H,]-isobutene also proceeds with no hydrogen mixing on MOO,-,/ Ti0,-SnMe, : initiation: M + propagation: on MoO,-,/TiO, /c \C c=c SnMe, c \ /c c’ ‘c c=c M o d ‘C / c ‘C c=c / c \C C608 References Alkene Metathesis on Molybdena-Titania Catalysts 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 I J.J. Rooney and A. Stewart, in Catalysis, ed. C. Kemball (Specialist Periodical Report. The Chemical Society, London, 1977), vol. 1, p. 277; (b) R. L. Banks, in Catalysis, ed. C. Kemball (Specialist Periodical Report, The Chemical Society, London, 1981), vol. 4, p. 100; (c) R. H. 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