L. Kh. Freidlin, Yu. Yu. Kaup

INVESTIGATION OF SELECTIVITY AND STEREOSPECIFICITY IN THE PROCESSES OF HYDROGENATION OF ACETYLENIC HYDROCARBONS ON METALLIC CATALYSTS

(Presented by Academician A. A. Balandin, June 5, 1963)

It was usually assumed that the selectivity of hydrogen addition to a triple bond is determined only by the relative adsorbability of the acetylenic hydrocarbon and of the olefin formed from it. Recently it has been shown that the selectivity of this process also depends on the hydrogenation mechanism \(\left(^{1-5}\right)\).

It was also believed that in the hydrogenation of disubstituted acetylenic hydrocarbons only the cis-olefin should be formed \(\left(^{6-10}\right)\). In fact, however, trans-isomers and products of double-bond migration are also obtained simultaneously. It has now been established \(\left(^{5}\right)\) that these processes too are connected with the mechanism of hydrogen addition to the triple bond.

Fig. 1. Kinetic curves of the hydrogenation of 2-pentyne (1) and 1-pentyne (2); a — on Co-, b — on Rh-, v — on Ni-, g — on Pt- and d — on Pd-catalysts.

In the present work we have summarized the results of our investigations of the dual nature of selectivity and stereospecificity in the processes of hydrogenation of acetylenic hydrocarbons (hexynes, pentynes, heptynes) on Ni-, Co-, Pd-, Pt- and Rh-catalysts. Hydrogenation was carried out in a glass thermostated vessel at temperatures close to room temperature, at atmospheric pressure and with vigorous shaking (up to 700–800 oscillations per minute). Skeletal catalysts were prepared by leaching 50% Ni—Al and Co—Al alloys with, respectively, 20% and 30% aqueous solutions of caustic soda. Pd-, Pt- and Rh-blacks were obtained by reduction of aqueous solutions of the chloride salts with formalin in an alkaline medium.

We hydrogenated \(0.01\) g/mole of hydrocarbon in 20 ml of solvent. The nickel (0.1 g) and cobalt (0.6 g) catalysts were used once. The amount of Pd-black was 0.02 g, and of Pt- and Rh-black, 0.04 g. The catalyzates obtained were analyzed by gas–liquid chromatography on columns with \(\beta,\beta'\)-oxidipropionitrile, tricresyl phosphate, and a solution of silver nitrate in ethylene glycol.

As is seen from Fig. 1, in the first stage of hydrogenation of monosubstituted acetylenic hydrocarbons hydrogen is absorbed at a constant rate. After completion of the hydrogenation of the acetylenic bond, the rate of hydrogen addition increases sharply. An exception is the cobalt ca-

catalyst: the rate practically does not change. On the contrary, disubstituted acetylenic hydrocarbons add the second mole of hydrogen at a considerably lower rate than the first.

From the data in Table 1 it is evident that, on Pd black, monosubstituted acetylenic hydrocarbons are hydrogenated highly selectively, whereas on Ni-, Co-, Pt-, and Rh-catalysts they are hydrogenated nonselectively. In this case the ratio limiting hydrocarbon:olefin remains constant until the acetylenic hydrocarbon has completely disappeared from the catalyst. With branching of the hydrocarbon radical, the selectivity of hydrogenation on the cobalt catalyst decreases. In the hydrogenation of monosubstituted acetylenic hydrocarbons on Ni-, Co-, and Rh-catalysts only α-olefins are formed, while on Pd- and Pt-catalysts olefins with another position of the double bond are also formed in small amounts (1 and 6%, respectively).

Table 1

Hydrogenation of monosubstituted acetylenic hydrocarbons

* Isoamylacetylene.
** Tertiary butylacetylene.

The main product of the hydrogenation reaction of disubstituted acetylenic hydrocarbons is the cis-olefin. At the same time, small amounts of trans-olefin, isomers with another position of the double bond, and the limiting hydrocarbon are formed. The ratios of the hydrogenation products formed remain constant until hydrogenation of the triple bond is complete. Table 2 gives the composition of the catalyst at the completion of hydrogenation of the triple bond of 2-pentyne.

After the triple bond has disappeared from the catalyst, the cis-olefin formed begins to be hydrogenated and isomerized (cis–trans conversion and migration of the double bond). In contrast, the α-olefin formed on Ni-, Co-, Pt-, and Rh-catalysts is hydrogenated but is not isomerized. Isomerization of the α-olefin proceeds only on Pd black, with formation of a mixture of isomers close in composition to the equilibrium mixture.

Table 2

Composition of the catalysts in mole percent

The ratio of the rates of hydrogenation and isomerization of cis-olefins is, on Ni and Co, ~1:2; on Pd, ~1:3; on Rh, ~3:1; and on Pt, ~5:1. The rate of isomerization of α-olefins on Pd black is almost twice as high as the rate of hydrogenation. Migration of the triple bond does not occur on any of the catalysts studied.

In the hydrogenation of acetylenic hydrocarbons, the limiting hydrocarbon may be formed by two mechanisms:

\[ \mathrm{R{-}C{\equiv}C{-}R} \ \xrightarrow{+H_2}\ \mathrm{R{-}CH{=}CH{-}R} \ \xrightarrow{+H_2}\ \mathrm{R{-}CH_2{-}CH_2{-}R} \tag{1} \]

\[ \mathrm{R{-}C{\equiv}C{-}R} \ \xrightarrow{2H_2}\ \mathrm{R{-}CH_2{-}CH_2{-}R} \tag{2} \]

Path (1) assumes that the olefin molecule formed is displaced from the catalyst surface and that the second molecule of hydrogen is added only after adsorption of the olefin on the catalyst. In this case the selectivity is due to adsorption displacement.

According to scheme (2), the acetylenic hydrocarbon adds two molecules of hydrogen without desorption of the intermediate product into the bulk, and the saturated hydrocarbon is formed already at the first stage of hydrogenation. In this case the selectivity is due to the mechanism by which hydrogen adds to the triple bond.

The routes of formation of the saturated hydrocarbon were studied by us by the method of hydrogenating acetylenic hydrocarbons on a nickel catalyst poisoned with pyridine ($^4$). Comparison of the kinetic data obtained with analytical data shows that, in the first stage of hydrogenation of monosubstituted acetylenic hydrocarbons, the saturated hydrocarbon is formed mainly according to scheme (2), and in the case of disubstituted hydrocarbons—entirely according to scheme (2).

For the purpose of quantitatively determining the selectivity and stereospecificity in processes of saturation of acetylenic hydrocarbons, the hydrogenation of their binary mixtures with olefins was studied ($^5$). The results obtained show that the mechanism of hydrogenation of acetylenic hydrocarbons can be expressed by the general scheme:

\[ \begin{array}{c} \begin{array}{ccccc} & +\,2H_2\,(a) & & & \\ R{-}C{\equiv}C{-}R' & \longrightarrow & R{-}CH_2{-}CH_2{-}R' & & \\ \Bigg\downarrow\!{+H_2\,(b)} & & \uparrow\!{+H_2\,(\gamma)} & & \\ & \longrightarrow & R{-}CH{=}CH{-}R'\;(\text{cis}) & \xrightarrow{\ +H_2\,(e)\ } & R{-}CH_2{-}CH_2{-}R' \\ & & \text{or} & & \\ & & \alpha\text{-olefin }(R'=H) & & \\ \Bigg\downarrow\!{+H_2\,(v)} & & \updownarrow\!{(d)} & & \\ & \longrightarrow & \text{isomeric olefins} & \left\{\begin{array}{l} \xrightarrow{\ (zh)\ }\ \text{cis-trans transformation}\\ \xleftarrow{\ }\ \text{and migration of the double bond} \end{array}\right. \end{array} \end{array} \]

where $R'$ is an alkyl group or H; (a), (b), (v), ($\gamma$), (d) are transformations in the first stage of hydrogenation; (e), (zh) are transformations in the second stage of hydrogenation.

These data make it possible to estimate approximately the quantities of products (in percent) formed by each of the indicated routes of scheme 3 (Table 3). From the data of Table 3 it is seen that the selectivity and stereospecificity of the hydrogenation of acetylenic hydrocarbons depend both on the mechanism of addi-

Table 3

* Isoamylacetylene.
** tert-Butylacetylene.

addition of hydrogen (formation of products according to (a), (b), and (c)), and on the adsorptive displacement (formation of products according to (d) and (e)) of the olefin by the acetylenic hydrocarbon.

From the data in Table 3, the degrees of selectivity \((S_{\mathrm{m}})\) and stereospecificity \((S_{\mathrm{m.\,isom}})\) according to the mechanism, and the degrees of selectivity \((S_{\mathrm{ads}})\) and stereospecificity \((S_{\mathrm{ads.\,isom}})\) due to adsorptive displacement, were calculated from the equation:

\[ S=\frac{A}{A+B}, \]

where \(A\) is the yield of olefin; \(B\) is the yield of the saturated hydrocarbon according to (a) in calculating \(S_{\mathrm{m}}\), and according to (d) in calculating \(S_{\mathrm{ads}}\).

In calculating \((S_{\mathrm{m.\,isom}})\), \(A\) is the yield of the cis-olefin, and \(B\) is the yield of the trans-olefin and of isomers with a different position of the double bond according to (c).

In calculating \((S_{\mathrm{ads.\,isom}})\), \(A\) is the yield of the cis-olefin, and \(B\) is the yield of the trans-olefin and of isomers with a different position of the double bond according to (e).

Table 4 gives the values of \(S_{\mathrm{m}}\), \(S_{\mathrm{ads}}\), \(S_{\mathrm{m.\,isom}}\), and \(S_{\mathrm{ads.\,isom}}\).

Table 4

* Isoamyleneacetylene.
** Tertiary butylacetylene.

As follows from the data of Table 4, in decreasing order of selectivity of action the catalysts are arranged in the following series:

in the hydrogenation reactions of monosubstituted acetylenic hydrocarbons:

\[ S_{\mathrm{m}} — \mathrm{Pd}>\mathrm{Pt}>\mathrm{Rh}>\mathrm{Ni}>\mathrm{Co}, \qquad S_{\mathrm{ads}} — \mathrm{Pd}>\mathrm{Ni}=\mathrm{Co}>\mathrm{Pt}>\mathrm{Rh}; \]

in the saturation processes of disubstituted acetylenic hydrocarbons:

\[ S_{\mathrm{m}} — \mathrm{Pd}>\mathrm{Ni}>\mathrm{Pt}>\mathrm{Co}>\mathrm{Rh}, \qquad S_{\mathrm{ads}} — \mathrm{Pd}=\mathrm{Ni}=\mathrm{Co}>\mathrm{Pt}>\mathrm{Rh}; \]

with respect to stereospecificity:

\[ S_{\mathrm{m.\,isom}} — \mathrm{Pd}>\mathrm{Ni}>\mathrm{Pt}>\mathrm{Rh}>\mathrm{Co}, \qquad S_{\mathrm{ads.\,isom}} — \mathrm{Pd}=\mathrm{Ni}=\mathrm{Co}>\mathrm{Pt}=\mathrm{Rh}. \]

The results of our work show that, both in selectivity and in stereospecificity due to adsorptive displacement, all the catalysts studied fall into one and the same series. Pd exhibits the highest overall selectivity and stereospecificity in the hydrogenation of acetylenic hydrocarbons. Ni and Co possess high selectivity and stereospecificity due to adsorptive displacement, and comparatively low values according to the mechanism.

Institute of Organic Chemistry named after N. D. Zelinsky
Academy of Sciences of the USSR

Received
24 V 1963

CITED LITERATURE

1. G. C. Bond, D. A. Dowden, N. Mackenzie, Trans. Farad. Soc., 54, 1537 (1958).
2. G. C. Bond, J. Newham, P. B. Wells, Actes Intern. Congr. Catalyse Paris, 1, 1177 (1961).
3. G. C. Bond, G. Webb, P. B. Wells, J. M. Winterbottom, J. of Catalysis, 1, No. 1, 74 (1962).
4. Л. Х. Фрейдлин, Ю. Ю. Кауп, Изв. АН СССР, ОХН, No. 9, 1660 (1962).
5. Л. Х. Фрейдлин, Ю. Ю. Кауп, Изв. АН СССР, ОХН, No. 1, 166 (1963).
6. A. Farkas, Trans. Farad. Soc., 33, 837 (1937).
7. K. Campbell, B. Campbell, Chem. Rev., 31, 77 (1942).
8. R. R. Burwell, Chem. Rev., 895 (1957).
9. W. M. Hamilton, R. R. Burwell, Actes Intern. Congr. Catalyse Paris, 1, 987 (1961).
10. R. R. Burwell, Rev. Inst. Franc. Petrol., 15, No. 1, 145 (1960).