Dynamic Properties of n-Alkyl and s-Alkyl Intermediates in Reactions of Simple Alkenes with Hydrogen on MoS2 Catalyst BY TOSHIO OKUHARA AND KEN-ICHI TANAKA" Research Institute for Catalysis, Hokkaido University, Sapporo, Japan Received 16th December, 1977 \ / / \ The hydrogen exchange between (Z)-[l-2Hl]propene, C=C , and [2H6]propene taking H H place on MoS2 in the presence of hydrogen yielded a variety of deuteropropene isomers in the relative proportions 7.5 % [l,l-2Hz], 4.5 % (Z)-[1,2-2H2], 34.0 % (E)-[l,2-2H2], 51.0 % (E)-[1-2H1], 0 % [2-2H1] and 3 % [2Ho]propene in the initial stage of the exchange reaction at room temperature. This distribution was in good agreement with calculations based on a relative contribution of 70 % n-propyl and 30 % isopropyl intermediates determined by a separate experiment, and the relative contributions of the n-alkyl and s-alkyl intermediates were found to be the same for the hydrogen exchange reaction of but-1-ene.The hydrogen addition and elimination processes in the isomeriza- tion reaction as well as in the hydrogen exchange of olefins are strictly controlled by a cis-stereo R \ / specificity of the site which has one coordination vacancy, Mo(S)4. The s-alkyl species formed on this site are restricted from free rotation around the coordination bond by steric interaction with the surrounding sulphur ions. Such dynamic properties of the s-alkyl intermediates give unexpectedly low values for [3-2Hl]propene : [l-2Hl]propene of c 1/15, and also cause two distinguishable confor- mations for the s-butyls, one from but-l-ene and the other from but-2-ene.depending on the orienta- R \ tion of the vacant coordination site. The alkyl species in the form of Mo(S)4 are intermediates for the isomerization and/or the hydrogen exchange but do not undergo consecutive hydrogenation, because dissociation of H2 requires two vacancies. / Reactions catalysed on solid surfaces may be classified phenomenologically into " structure sensitive and structure insensitive " reacti0ns.l Furthermore, some catalytic reactions such as ammonia synthesis (N2 + 3H2 3 2NH3) and methanation (CO + 3H2 + CH4 + H20) occur readily on heterogeneous catalysts but hardly proceed in a homogeneous catalyst system. This strongly suggests that structural requirements must be fulfilled and/or particular arrangements of sites must exist in order to promote smooth chemical reactions.Such structural prerequisites may depend on the type of chemical reaction. However, at present, in a few cases mechanistic bases have been established by which the structural requirement of the sites in heterogeneous catalysis can be deduced. we demonstrated that the alkyl intermediates in the hydrogen exchange of alkenes and/or their isomerization reactions on molybdenum disulphide and on sulphided nickel catalysts cannot be hydrogenated to alkanes, 7 In our previous papers,2*8 ALKENE REACTIONS WITH H ON MOS, CATALYST and the hydrogenation reaction on these catalysts occurs on different active sites. These discrete properties of the active sites for catalysis were explained by the different degrees of coordinative unsaturation of the sites,,.4* i.e., the active sites for hydrogenation require three degrees of coordinative vacancies such as are possessed by Wilkinson’s hydrogenation catalyst which exhibits activity through loss of one phosphine ligand.6 In contrast, while the alkyl species formed on sites with two degrees of coordinative unsaturation undergo isomerization and/or hydrogen exchange, they are not hydrogenated to alkanes, as shown in the following reaction scheme ; A3MH2- site) 1 (3M- site 1 SCHEME 1 .-Hydrogenation, S, & ./ Hz s’ I ‘s %M’H + ;c=‘c< - +h - s‘ I ‘s S (*MH-s i te 1 S SCHEME 2.-Isomerization and hydrogen exchange. where ,M and 3M denote the sites which originally have two or three degrees of coordinative vacancies, and which correspond to the B- and C-sites in our previous papers.If these sites are bonded to hydrogen atoms, they may be expressed as ,MH, 3MH and 3MHz, depending on the number of hydrogen atoms. The MoS, powder used here has a layer structure comprising a trigonal prismatic unit cell, the molybdenum ion being surrounded by six sulphur ions. However, the molybdenum ions exposed on the edge surface of a sandwich-like layer structure of MoS, are surrounded by fewer sulphur ions, and a hard sphere model suggests a probable structure where molybdenum ions are surrounded by four sulphur ions. Actually, the authors have succeeded in demonstrating that only the edge surface of MoS, single crystal wafers catalyses the isomerization and hydrogen exchange in olefins in the presence of hydr~gen.~.This has clarified the place and/or the sites on which the reactions occur, and our interest now focuses on the dynamic properties of the intermediates formed on these sites during catalysis correlated with the structure of the active sites. EXPERIMENTAL Reactions were carried out at room temperature in a conventional circulation system with a total volume of about 300 cm3, and the analysis of the olefins was performed with an on-line gas chromatograph. The material was obtained and purified as follows: [2H8]but-l-ene ; by deuteration of perdeuterobutadiene on a ZnO catalyst at room tempera- ture. [2H6]propene; by hydrogen exchange of propene with D2 on a MgO catalyst atT. OKUHARA AND K . - I . TANAKA 9 room temperature.(Z)-[l-2Hl]pr~pene ; [l-2Hllmethyl acetylene, obtained by shaking methyl acetylene in an alkaline deuterium oxide solution, was hydrogenated on a Pdlcarbon catalyst. All deutero-olefins obtained by the above methods were purified by gas chromato- graphic separation. The surface area of the MoS2 powder was 15 m2 g-l by the B.E.T. method with a nitrogen adsorbent, and X-ray diffraction indicated a hexagonal layer structure (2H-structure) composed of a trigonal prismatic unit cell. The impurities in the MoS2 analysed by atomic absorption analysis were ; Fe 0.02, Mg 0.0015, Ca 0.0077, Na 0.012, Mn 0.0003, Cr < 0.0001 and K < 0.1 %. The MoS2 powder was activated by evacuation at >400°C for such reactions as the hydrogenation, the equilibration of H2 and D2, the isomerization and/or the hydrogen exchange in olefins and the metathesis of olefins.8 A mass spectroscopic analysis was made using parent peaks obtained from 12 eV of the ionization voltage. A microwave spectroscopic analysis of the deutero-olefins was carried out at dry ice temperature using a spectrometer with 110 kHz sinusoidal Stark modula- t i ~ n , ~ ~ lo and multiple reflectance of microwaves in the absorption cell was prevented by setting a pair of ferrite isolators in the cell.RESULTS n-BUTYL AND S-BUTYL INTERMEDIATES In the absence of hydrogen, there was no appreciable double bond migration or cis-trans isomerization on MoS, catalyst; however, when hydrogen was added, these reactions accelerated greatly, as shown in fig. 1. This remarkable promoting effect of hydrogen seems to indicate the participation of an associative mechanism through butyl intermediates.The isomerization of but-1-ene in the presence of D2, however, gave > 90 % nondeuterated but-2-ene, and in contrast, the hydrogenation reaction taking place simultaneously predominantly yielded [1,2-2H2]butane.3 TABLE 1 .-GEOMETRICAL ISOMERS OF [2HJBUT-I-ENE AND C~S-[~H~]BUT-~-ENE FORMED DURING REACTION OF BUT-~-ENE WITH D2 (1 : 1) AT ROOM TEMPERATURE [ZH llbut- 1-ene ck+H ~Ibut-Z-ene runs (Z)-l-[ZH1] (E)-1-[2H1] 2-[2H1] 3-[2H1] 4-[2H1] 1-[2H1] 2-[2H11 - - run 1 a 29.6 % 34.3 36.1 0 0 run 2 b 28.0 % 31.0 35.0 6.0 0 88.0 % 12.0 a Conversions in hydrogenation and isomerization were 9.5 and 10.6 %, respectively and 12Hl]but-l-ene was NN 9.0 % of the total but-1-ene.Conversions in hydrogenation and isomerization were 42.0 and 34.0 %, respectively and the concentration of [2Hl]but-l-ene in the total but-1-ene was 32.3 % and of ~is-[~H~]but-2-ene was 29.8 % in the total cis-but-2-ene. To elucidate this phenomena, [2H,]but-l-ene and ci~-[~H,]but-2-ene which were formed as the minor products in the isomerization of but-1-ene in the presence of D2 were analysed by microwave spectroscopy. As shown in table 1, a deuterium atom H H \ / in [2H,]but-l-ene is equally distributed throughout the vinyl group, C=C , / \ H and a deuterium atom in ci~-[~H,]but-2-ene is located mainly on the outer carbon atoms. As will be discussed later, the formation of [2H,]but-l-ene by hydrogen exchange between but-1-ene and D2 is inevitably slow compared with the reshuffling of the vinyl hydrogens, which results in the random distribution of a deuterium atom in the vinyl group.10 ALKENE REACTIONS WITH H ON MoS, CATALYST TABLE 2.-HYDROGEN EXCHANGE BETWEEN [2Ho]BUT-I -ENE AND ['HslBUT-l-ENE IN THE PRESENCE OF AN Hz+D2 MIXTURE (1 : 1) AND IN THE ABSENCE OF HYDROGEN AT ROOM MENTAL RUNS.TEMPERATURE. RESULTS AT 3min AND 5min WERE OBTAINED FROM DIFFERENT EXPERI- (1) deuterium distribution of but-1-ene/ % time/ min 0 3 5 (120) 2Ho 2Hi 2Hz 2H3 2H4 2H5 2H6 2H7 2Hs H 2 HD D2 50.4 0.6 49.0 48.1 0.5 51.4 (50.4) 24.2 j 19.2 / 5.5 0.8 0.1 0.1 6.4 19.7 23.1 48.1 3.2 48.7 9.2 ! 19.5 15.4 4.9 0.6 5.0 16.3 20.0 9.1 41.9 19.2 39.0 (14.1) k21.8) _-----_---- i (10.7) (2.3) (-) (2.3) (11.9) (23.0) (14.0) - - - (0.5) (49.1) - (2) distribution of geometrical isomers of [2H1]but-l-ene/ % time/ [ZHllbut-1-ene isomers rnin (Z)-l-ZHl (E)-l-2H1 2-2H1 3-2H1 4-2H1 3 15.9 16.0 68.1 0 0 5 24.9 29.1 46.1 0 0 (120) (15.7) (18.5) (65.8) (0) (0) Results at 3 and 5 min were the separate experimental runs in the presence of hydrogen (H2/D2 = l), and the values in parentheses are the results in the absence of hydrogen.Simultaneous hydrogenation and double bond migration gave butane 1.8 % (4.9), cis-but-2-ene 2.7 % (5.6) and t-but-2-ene 1.5 % (3.7) at 3 min ( 5 rnin). In order to determine which is the primarily exchanged hydrogen in but-1-ene, intermolecular hydrogen exchange between [,H,]but-l-ene and [2H8]bUt-l-ene, and coisomerization of ~is-[~H,]but-2-ene and ~is-[~H,]but-2-ene were performed with the MoS, catalyst at room temperature.When a mixture of [2H,]but-l-ene (2 mmHg) and [,H,]but-l-ene (2 mmHg) was admitted with 3 mmHg of an H2 +D, mixture onto the MoS, catalyst, intermolecular hydrogen exchange occurred rapidly as shown in table 2. [2Hl]but-l-ene appearing in the initial stage of this exchange reaction should hold the deuterium atom at its originally exchanged position if its desorption is sufficiently rapid. The results indicate that the vinyl hydrogens in H H c=c \1 2/ / \ but-1-ene, , are particularly exchangeable on MoS, in the presence of H hydrogen, and that the relative ratio of hydrogen exchange on C-1 and C-2 is about 3/7 calculated from the ratio of [l-2Hl]but-l-ene/[2-2€€l]but-l-ene. This may indicate that the relative importance of the s-butyl and n-butyl intermediates in the inter- molecular hydrogen exchange is 3 to 7.Note that the geometrical distribution of [2Hl]but-l-ene appearing in the absence of hydrogen (run 120min in table 2) is similar to that in the presence of hydrogen. This suggests that the intermolecular hydrogen exchange reaction of but-1-ene taking place slowly in the absence of hydrogen might proceed uia the butyl intermediates over the small fraction of sites which contain a hydrogen atom which can produce the intermediates. As noted in our previous paper,l the coisomerization of ~is-[~H,]but-2-ene and ci~-[~H,]but-2-ene gave a value of 0.5 as the average number of exchanged hydrogens per isomerized molecule, trans-but-2-ene. This proves that a s-butyl intermediate from cis-but-Zene formed by picking up a hydrogen atom, H or D, changes to trans-T.OKUHARA AND K . - I . TANAKA 11 but-2-ene, if a hydrogen atom other than that picked up returns to the active site. That is, the cis-trans isomerization reaction on MoS, catalyst necessarily occurs with the hydrogen exchange. The presence of a s-butyl intermediate in the cis-trans isomerization is also supported by microwave spectroscopic analysis of cis-['H I]- but-2-ene which is nearly 100 % of the ci,~-[2-~H~]but-2-ene formed in the coisomeriza- tion. 10 x" 1 ri 2 0 g 5 a (4 0 50 I00 150 200 250 timelmin 0 50 100 I50 time/min FIG. 1.-Hydrogen promoting effects on isomerization reactions at room temperature. (a) But-1-ene : 0, 1-C4Hs; 0, C4HI0; A, t-2-C4Hs; 0, cis-2-C4H8. (b) cis-but-2ene: 0, 1-C4Hs; 0, C4 Hlo; A, C ~ ~ - Z C ~ H ~ ; C ) , ~ - Z C ~ H ~ .As described in scheme 2, the s-butyl species formed on the 2M-sites do not easily undergo isotopic exchange with D2, because the D2 molecule is hard to dissociate either on 2MH or s-butyl-2M which has only one coordinative vacancy. As a result, the isomerization of but-1-ene and/or cis-but-2-ene in the presence of D, gives nondeutero isomerized products ;3 the formation of ,MH-site is discussed in the following section.12 ALKENE REACTIONS WITH H ON MOS, CATALYST n-PROPYL AND ISOPROPYL INTERMEDIATES Similarly to the hydrogen exchange between but-1-ene and D2, the hydrogen exchange between propene and D2 is very slow although the intermolecular hydrogen exchange of propene is markedly enhanced by hydrogen.A microwave spectro- scopic analysis of [2H,]pr~pene slowly formed by hydrogen exchange between D2 and propene is shown in fig. 2. It is known that the vinyl hydrogens in propene are especially active in the exchange, as observed for but-1-ene in table 1; that is, the deuterium atom of [2Hl]propene is randomly distributed within the vinyl group, and [3-2H,]propene formation is unexpectedly slow provided that [ l-2Hl]propene is formed via the isopropyl intermediate. 6 i = 1 C idif6 FIG. 2.-Relative proportion of [2Hl]propene isomers formed in the reaction of propene with D2 at room temperature. + , CH2=CH-CH2D (3-[2H1]) ; 0, CHZ=CD--CH3 (2-[’H11) ; A, H CH3 D CH3 \ / \ / C=C ((E)-1-[2H111; A, C=C {(Z)-1-[2Hll 1.D ’ ‘H H ’ ‘H As hydrogen exchange between D2 and propene is slow, the initial exchange position of the deuterium atom is completely erased by a rapid reshuffle of the vinyl hydrogens through the intermolecular hydrogen exchange reaction. Consequently, to estimate the initial exchanged position, intermolecular hydrogen exchange between [2H,]propene and [2H6]propene was performed in the presence of hydrogen. As shown in table 3, the intermolecular hydrogen exchange proceeds stepwise. Accord- ingly, [2H,]propene appearing in the initial stage of the exchange reaction might not be reshuffled. Note that the ratio of [2-2H,]propene to [l-2H,]propene is about 713 in fig. 3, and that this ratio is approximately equal to that of [2-2H,]-but-l-ene to [l-2H,]but-l-ene in table 2.As mentioned above, if the hydrogen exchange of propene occurs via n- and/or isopropyl intermediates, the unusually low value of the ratio of [3-2H,]propene to [l-2H,]propene should be considered carefully. To confirm the contribution of theT . OKUHARA AND K . - I . TANAKA 13 alkyl intermediates in the hydrogen exchange and/or the isomerization reactions, hydrogen exchange between (Z)-[l-2Hl]propene and [2H6]propene, which is a method proposed by Kondo et aZ.,9, has been adopted. The (Z)-[l-2H,]propene used as a reactant gas in this experiment contained impurities, as shown in table 4(a), and hence is termed “crude (Z)-[l-2H,]propene” in this paper. A mixture of TABLE 3.-INTERMOLECULAR HYDROGEN EXCHANGE BETWEEN r2HO] PROPENE (3.9 m m g ) AND [’H6] PROPENE (3.1 mmHg) IN THE PRESENCE OF A MIXTURE OF H2 AND D2 (7 mmHg) ON MoS2 (0.lg) AT ROOM TEMPERATURE 3 X GHif i = O (6-i)zHi time/ 2Ho 2H 1 2H2 2H3 2H4 2H5 2H6 i = 4 min 0 55.2 3 50.2 6 47.5 12 41.8 24 35.0 36 30.3 48 26.9 130 16.2 - 2.8 5.3 0.3 - 0.7 7.2 (8.0)t 0.5 - 1.0 9.2 12.5 1.6 0.3 2.3 12.9 (16.3)T 3.5 0.9 4.7 15.6 19.3 5.3 2.0 6.8 17.2 20.9 6.7 2.5 8.1 18.4 (23.1)t 12.5 7.3 18.9 18.0 I - - t Subject to the microwave spectroscopic analysis shown in fig.concurrently occurring; 5.1 % at 55 min and 10.0 % at 130 min. 42.2 2.8 36.3 14.8 33.6 20.2 28.6 34.1 23.9 51 .O 19.2 66.2 16.6 76.4 8.7 116.4 eq. 190 5. Hydrogenation reaction 5 mmHg crude (Z)-[1-2Hl]propene and 5 mmHg [2H6]propene (96 %) containing 5 mmHg HD gas (98 %) was admitted to 0.1 g MoS, catalyst at room temperature.The primarily exchanged products of this exchange reaction are [2H2]propene and [ H,]propene. The [2Hl]isomers other than (Z)-[1-2Hl]propene are formed by a conformational change around the double bond of propene with no isotopic exchange. The results TABLE 4.-INTERMOLECULAR HYDROGEN EXCHANGE BETWEEN ( Z ) - [ 1 - 2 H l ] ~ ~ ~ ~ ~ ~ ~ (5 m H g ) AND [’HtiIPROPENE (5 mmHg) IN THE PRESENCE OF 5 m H g HD AT ROOM TEMPERATURE (a) composition of crude (Z)-[1-2Hl]propene deuterium distribution/ % geometrical isomers/ % [‘Ho]propene 3.6 [2Hl]propene 93.6 (2)-[1-’H1] 92 (E)-[l-2H1] 8 [’H2]propene 2.8 (2)-[1,2-’H,] 49 (E)-[l,2-2H2] 0 (l,l-2H2) 51 (b) deuterium distribution change in the reaction of (Z)-[1-2Hl]propene and E2H6]propene time/ min 2Ho 2H1 2H2 2H3 2H4 2H5 2H6 HZ HD D2 0 1.8 46.7 1.4 - - 2.0 48.1 1.9 98.1 - 2.5 2.0 44.1 4.0 - - 4.7 45.3 2.5 97.2 0.3 5.0 2.1 41.7 6.0 0.1 0.1 6.2 43.6 10 2.4 37.9 9.9 0.8 0.8 10.0 38.4 20 2.5 30.6 15.0 2.1 2.0 14.3 32.8 2.5 96.0 1.514 x \ 3 6 0 - .?= .- i3 8 40- 20 ALKENE REACTIONS WITH H ON MoS, CATALYST h " - Y 4 l o o k 80 0- 0 20 40 60 80 100 120 140 I( 3 6 i - 1 i = 4 C id+ C (6-i)di 3 FIG.3.-Relative proportions of [2Hl]propene isomers formed by the intermolecular hydrogen exchange between [2Ho]propene and [2H6]propene at room temperature. 0, (2-[2H11) ; 0 , (3-[2H~1> ; A, ((E)-1-[2H11} ; A, {(Z)-~-['HII}- time /min I H. X 40t ,c = C\ D H I 1 I I 5 10 15 20 time/min FJG. 4.-Intermolecular hydrogen exchange between (Z)-[1 -*Hl]propene and [2H,]propene at room temperature.(a) Relative proportion of [2Hl]propene isomers. (b) Relative proportion of [2H2]propene isomers.T . OKUHARA A N D K . - I . TANAKA 15 of the exchange reaction are summarized in table 4(6), and fig. 4(a) and (b) show the time dependence of concentration of the [2H,]propene and [2H,]propene isomers. The initial composition of the [2H2]propene isomers estimated by extrapolating the curves in fig. 4(b) is 71 % (E)-[1,2-2H,]propene, 16 % [1,1-2H,]propene and 13 % (2)-[ 1 ,2-2H,]propene. By making corrections for the impurities included in the starting (Z)-[l-2H,]propene, the [2H,]isomers formed directly from (Z)-{l-2H,]propene on MoS, are estimated as 81.6 % (E)-[1,2-2H,]propene and 18.4 % [l,l-2H,]propene. About 13 % (Z)-[1,2-2H2]propene observed in fig.4(b) in the initial stage is formed mainly from the (E)-[l-2H,]propene which was included in the starting materials as an impurity. The initial compositions of [2H,]propene and [2H,]propene are also estimated, as listed in table 5, by extrapolation. The values obtained by extrapolation for the [,H,]-, [,HI]- and [2H2]-isomers are in excellent agreement with the calculated values assuming an associative mechanism under strict cis-stereochemistry ; this is discussed in the next section. DISCUSSION A noticeable hydrogen promoting effect such as observed in fig. 1 has been accepted as evidence for formation of alkyl intermediates in the isomerization and/or the hydrogen exchange of olefins. In such a case, it has been tacitly assumed that the intermediates in the isomerization and/or the hydrogen exchange of olefins overlap with those of the hydrogenation reaction, that is, the reverse process of alkyl inter- mediate formation in the hydrogenation reaction causes the isomerization and/or the hydrogen exchange of olefins.12 Such an approach, however, is incompatible with the results on oxides and sulphide~.~-~* l3 A typical case is the reaction of but-1-ene with D2 on MoS, catalyst.By adding D2 to but-1-ene, both the isomerization and the hydrogenation of but-1-ene begin simultaneously, but the products are [1,2-,H2]- butane and [2H,]but-2-ene, respectively, even though both reactions could occur via half-hydrogenated butyl intermediate^.^. This indicates that a s-butyl intermediate if formed during the isomerization reaction, is not hydrogenated to butane.The hydrogen exchange reaction as well as the isomerization of olefins most probably proceeds via alkyl intermediates. Recent experiments in our laboratory on MoS, single crystal wafers have proved that the active sites are located on the edge surface of the MoS, cry~tal,~. upon which exposed molybdenum atoms may be in coordinative unsaturation. Accordingly, the hydrogenation reaction may proceed on sites with three degrees of coordinative unsaturation, while the isomerization occurs on sites with two degrees of coordinative unsaturation as described by scheme 1 and 2., The presence of the hydrogen H \ / promoting effect observed in fig. 1 suggests the formation of an 2MH-site, Mo(S),, on which adsorbed olefin is converted to alkyl species by picking up a hydrogen atom.The ,MH-site formation process remains undetermined but is assumed to be uia heterolytic dissociation of an H2 molecule onto an ,M-site (,MH) and sulphur atom (SH), and this dissociation seems to be irreversible at room temperature. Accordingly, if a mixture of D2 and but-1-ene is introduced onto the catalyst, ,MD-sites are formed first but are changed to ,MH-sites in forming alkyl intermediates during the isomeriza- tion reaction. That is, the sites used in the isomerization reaction are almost ,MH- instead of ,MD- during isomerization because the isotopic exchange of H-atoms on16 ALKENE REACTIONS WITH H ON MoS, CATALYST 2MH-sites with D2 is slow for the following reasons; (i) the 2MH-site formation process is irreversible and (ii) the reversible dissociation of D, cannot take place on 2MH-sites because the dissociation of D2 requires two coordinative vacancies.Therefore, the isomerization of but-1-ene in the presence of D2 yields >90 % C2H,]but-2-ene. Investigation of the coisomerization reaction of ci~-[~H,]but-2-ene and cis- [2H,]but-2-ene clarifies the processes of hydrogen addition and elimination taking place on the 2MH-sites,11 Taking the conformation of the s-butyl intermediate into account, the cis to trans rotation of but-2-ene is necessarily accompanied by hydrogen exchange as described in reaction scheme 3 on the 2MH-site. H3C, ,W3 S, 1 /D cis-elim. S' 1's cis-add. C=C, + Mo Mo H' H S SCHEME 3.--cis to trans rotation regulated bv cis-stereochemical hydrogen-transfer on the 2MH-site.This mechanism is realized by nearly 100 % ci~-[2-~H~]but-2-ene formation in the coisomerization reaction. Either the double bond migration or the cis-trans rotation reactions occur via s-butyl intermediates, while hydrogen exchange of but-1-ene may take place via both n-butyl and s-butyl intermediates, which is similar to the reactions of a-olefins. As described in scheme 4, hydrogen exchange of a-olefin via an n-alkyl intermediate gives [2-2Hl]-olefin, while exchange via a s-alkyl intermediate gives [1-*H,]-olefin. f 4 S, ! ,D S S'Y'S 7 /H HzC" 'D s, I ,' M i s' g s =i H, ,R / c = C\ H D H .R D H ;c =c, or S, i ,H s0 's + Mo S, I,H I- Mo s' A'S SCHEME 4.-Hydrogen exchange via n-alkyl and s-alkyl intermediates. The ratio of the two geometrical isomers accordingly represents the relative contribution of the two intermediates, n-alkyl and s-alkyl, in the hydrogen exchange reaction.As shown in table 2 and fig. 3, the ratios of [1-2H,]-olefin/[2-2H,I-defin are 3/7 for propene as well as for but-I-ene. This result may indicate that the a- olefins adsorbed on the 2MH-sites take the n-alkyl form more readily than the s-alkylT. OKUHARA AND K . - I . TANAKA 17 form, i.e., anti-Markovnikoff hydrogen addition predominates on the 'MH-sites. In contrast, the intermediates in the hydrogenation reactions which proceed on 3M-sites or on 3MH-sites, as shown in scheme 1, are s-alkyl~.~. l4 The fact that the n-alkyls predominate on the 2MH-sites which have tight spacing and the s-alkyls are more prevalent on the 3M-sites which have wider openings is comparable with a result of Wilke and coworkers,15 who observed that the size of the ligands of nickel complexes determines the proportion of n-propyl to isopropyl species in the dimerization of propene.If the contribution of the isopropyl intermediate is ~ 3 0 % total hydrogen exchange of the a-olefins over MoS, as estimated in this paper, the question arises as to why [3-2Hl]propene formation is so slow compared with [ l-2Hl]propene formation, as shown in fig. 2 and 3. If the isopropyl intermediate formed on the 2MH-site, (isopr o pyl) Mo(S),, would have an equal probability of losing a hydrogen atom from either of the two methyls, the ratio of [3-2Hl]propene/[l-2Hl]propene would be 3/2. If [l-2H,]propene is formed via an isopropyl intermediate, abnormally slow [3-2Hl]- propene formation indicates inequality of the two methyl groups in the isopropyl species formed on the 2MH-sites.In order to establish isopropyl intermediate formation and to estimate the contribution of dissociative type hydrogen exchange, hydrogen exchange between (Z)-[l-2H,]propene and [2H,]propene was performed. \ / ( I I Assotiative Mechanism cis-od_d. - CH3Dq: Mo (n-propyl) c:%fH M o l (iso-propyl) & 1, F H 3 cis-elim. Ic='\ D H SCHEME 5.-Hydrogen exchange by an associative and a dissociative mechanism. As shown in scheme 5, under the experimental condition of an equal population of ,MH-(Z = H) and ,MD-(Z = D) sites, the proportion of each geometrical isomer can be estimated for the two mechanisms.As the hydrogen addition and elimination processes on ,MH- or ,MD-sites are regulated by the cis-stereo chemistry, the reaction routes for the all isomers formed from (Z)-[l-2H,]propene can be described by scheme 6 ;18 ALKENE REACTIONS WITH H O N MOS, CATALYST ‘[HI--+ - D 2Ho SCHEME 6.-Reaction routes from (Z)-[l-2Hl]propene. where the loops indicate that the rotation of a methyl group is occurring necessarily in the hydrogen exchange reaction under cis-stereochemical regulation. n-propyl from (Z)-[l-2Hl]propene may have equal probability of yielding (E)-[1 ,2-2H2]propene and (E)-[l-2Hl]propene providing there is an equal population of 2MH- and 2MD- sites. Similarly, iso-propyl from (Z)-[l-2Hl]propene gives (E)-[l-2H,]propene, [2H,]propene and [l,l-2H2]propei~e in the ratio 2 : 1 : 1.If the relative contribution of the n-propyl and isopropyl intermediates is assumed to be 70 and 30 %, respect- ively, the [2H,]propene isomers formed from (Z)-[l-2H,]propene may be evaluated as 82.4 % (E)-[l,2-2H2]propene and 17.6 % [l,l-2H,]propene, which is in excellent agreement with the experimental values of 81.6 % (E)-[1,2-2H2]propene and 18.4 % [l,l-2H2]propene when a correction had been made for the contribution of (E)- [1-2H,]propene contained in the starting materials. The relative proportions of TABLE 5.-PROPORTION OF EXCHANGED PRODUCTS. EXCHANGE REACTION BETWEEN (2)- [ 1 -2H 1lPROPENE AND [2H 6IPROPENE ON MOSz AT ROOM TEMPERATURE time/ mm O f 2.5 5.0 10 20 calc. [2H~1, PH11 and [2Hzl~ro~ene/% 12H01 (E)-[l-’H11 [2-2H1I (E>-[1,2-2Hd (2)-[1,2-2H2] [l,l-2H2] 3.0 51 .O 0 34.0 4.5 7.5 3.8 46.5 0 32.1 7.7 9.8 3.0 50.6 0 27.5 8.5 10.4 3.5 42.2 4.4 26.1 10.6 13.1 2.9 36.0 5.6 26.0 16.0 13.3 7.5 50.0 0 35.0 0 7.5 t A graphical extrapolation to zero time was made.[2H,]propene isomers and [2H,]propene can be similarly estimated. As shown in table 5, on the calculation run the relative compositions calculated for all isomers are in excellent agreement with the experimental values extrapolated to zero time. These results prove that the hydrogen exchange reaction of propene proceeds entirely via n-propyl and isopropyl intermediates on MoS, catalyst. Accordingiy, the iso- propyl species coordinated to the 2M-sites is unsymmetrical in elimination of hydrogen from the two methyls. A slight inequality between the two methyls of the iso-propyl for hydrogen elimination has been reported on the EDA-complexes of phthalocyanine with alkali metals;I6 however, such large values (> 15) for the ratio ~f[l-~H,]propene/ [3-2H,]propene have not been observed in the past.The structure of the isopropyl isopropyl Mo(S),, suggests that one of \ intermediate coordinated to an 2M-site, /T. OKUHARA AND K . - I . TANAKA 19 the two methyls is closely adjacent to a vacant coordination site. Accordingly, if the isopropyl species bonded to the 2M-site is restricted from free rotation around the coordination bond, the two methyls undergo hydrogen atom elimination with different probabilities, as described in scheme 7 : , CH3-CH-f: H2 CHj-CH=CHD s, i ,D - H3C-CH=CH2 + Mo I ,,D, s’ I ‘s Mo’ [‘HJ prop-1-ene S Rotation 1 C H2 D-CH- CH, I ,-A - CHzD-CH=CHZ Mo‘ FHA prop-3- ene SCHEME 7.-Restricted rotation of the isopropyl intermediate formed on an *M-site.where the abstraction of a hydrogen atom from a methyl group occurs at a vacant coordination site which appears after donation of a hydrogen atom to adsorbed propene. If the rotational motion of the isopropyl species bonded to the 2M-sites is restricted either by the spacing and/or the configuration of sites, a similar restriction might be expected to hold for other s-alkyl species bonded to the sites. If the s-butyl is restricted from rotating around the coordination bond, two distinguishable conformations may be expected on the 2M-sites depending on the direction of the vacant site, as described in scheme 8.Rotation CHzD-CH-CH-CH3 s, i ,H - (sec- b u t y 1 -II 1 Mo 7 I -l!l Mo’. But-2-ene + s’ 1‘s S SCHEME 8.-Two distinguishable s-butyls formed on 2M-site. If the energy barrier for rotational motion of the s-butyl species is high, the vinyl hydrogen atoms of but-1-ene undergo rapid hydrogen exchange via s-butyl-I and n-butyl intermediates, and the cis-trans rotation of but-Zene also occurs rapidly via s-butyl-11. However the double bond migration reaction of but-1-ene to but-2-ene is expected to be slow because this isomerization reaction must pass over the rotational barrier from s-butyl-I to s-butyl-11. This is the case for the reactions of but-1-ene and but-2-ene on MoS, catalyst. Such slow rotation around a single bond in carbon-molybdenum may not be surprising, because it is known that some bulky groups have high rotational barriers around a single b0nd.l’ Thus it may be possible that the restricted rotation of the s-alkyl species coordinated to an Mo(S), site which has a narrow opening may be caused by the steric interaction of the s-alkyl species with the sulphur ions.These results remind us that allene and methylacetylene are hydrogenated to propene on \ /20 ALJSENE REACTIONS WITH H ON MoS, CATALYST MoS, catalyst via different intermediates, o-ally1 intermediate and n-propenyl intermediate respectively.’ They prove that the reactions over MoS, catalyst prefer intermediates taking lesser strain. As demonstrated here and in our previous work, the degree of coordinative unsaturation of active sites is a structural prerequisite for the catalytic hydrogenation reaction as well as for the isomerization and/or the hydrogen exchange reactions taking place via the alkyl intermediates.In this sense, these reactions should be structure sensitive. Recently, the isomerization of 2-methylbut-1-ene was found to proceed without hydrogen over MoS, catalyst although the isomerization of but-1-ene and of but-Zene requires the assistance of hydrogen.lg By using a single crystal of MoS,, it has been shown that the isomerization of 2-methylbut-1-ene takes place on the basal plane of the MoS, single crystal which is composed of a sulphur sheet, whereas the isomerization of but-1-ene or of but-Zene is promoted only on the edge surface of the crystal in the presence of hydr~gen.~ These facts indicate that the isomerization of 2-methylbut- 1-ene proceeds via a carbonium ion intermediate instead of the alkyl intermediates elucidated here.It is reasonable to deduce that isomerization via carbonium ion intermediates is virtually structure insensitive, because carbonium ion formation is entirely controlled by the proton activity of the surface. The coordinative unsaturation number is undoubtedly a structural prerequisite of the active sites upon which alkyl species are formed and consecutive hydrogenation of the alkyls requires one additional coordinative vacancy. It should be emphasized here that more complex catalytic reactions, such as ammonia synthesis and the methanation reaction of carbon monoxide, should have different structural prerequisites. Recently, Demitras and Muetterties” have shown an interesting example suggesting the existence of such complex structural requirements including ensemble operation of several sites using metal clusters.The authors thank Dr. T. Kondo of Sagami Chemical Research Centre for providing the microwave spectroscopic analysis, and they are also indebted to Prof. K. Miyahara of our Institute for his contribution to the mass spectrometry. M. Boudart, A. Aldag, J. E. Benson, N. A. Dougerty and C. G. Harkins, J. Catalysis, 1966 6, 92; D. W. Blakely and G. A. Somorjai, J. 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