The hydrogenation of acetylene

Among the early systemmatic studies of the metal-catalysed hydrogenation of acetylene were those of Sheridan et al. [158,168—170] who investigated the kinetics and product distributions over pumice-supported metals. Subsequently, the reaction has been extensively studied by Bond et al. [9,165,171—175] over pumice- and alumina-supported metals and metal powders. The reaction of acetylene with deuterium over nickel [91, 163] and alumina-supported noble Group VIII metals [164,165] has also been investigated. 

The overall order of reaction as determined from the pressure fall 

One of the characteristic features of the metal-catalysed reaction of acetylene with hydrogen is that, in addition to ethylene and ethane, hydrocarbons containing more than two carbon atoms are frequently observed in appreciable yields. The hydropolymerisation of acetylene over nickel—pumice catalysts was investigated in some detail by Sheridan [169] who found that, between 200 and 250°C, extensive polymerisation to yield predominantly C4 - and C6 -polymers occurred, although small amounts of all polymers up to Cn, where n 31, were also observed. It was also shown that the polymeric products were aliphatic hydrocarbons, although subsequent studies with nickel—alumina [176] revealed that, whilst the main products were aliphatic hydrocarbons, small amounts of cyclohexene, cyclohexane and aromatic hydrocarbons were also formed. The extent of polymerisation appears to be greater with the first row metals, iron, cobalt, nickel and copper, where up to 60% of the acetylene may polymerise, than with the second and third row noble Group VIII metals. With alumina-supported noble metals, the polymerisation prod- 

Initial rate kinetics and activation energies for acetylene hydrogenation 

The mechanism of the hydropolymerisation of acetylene is not too clear. It has been suggested [9,169] that in the hydrogenation of acetylene to ethylene, the half-hydrogenated state, an adsorbed vinyl species, may exist in either a normal or free radical form, viz. 

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Amines (7a,12a), especially pyridine (75), have also been used as solvents in the hydrogenation of acetylenes. Hydrogenation of 3 over 5% Pd-on-BaS04 in pyridine gave df-cis-jasmanate (4) quantitatively (40). The authors comment that this combination for reduction of acetylenes was superior to the Lindlar catalyst in all cases examined. (See also Refs. 12 and 24 for similar conclusions.)... 

The much more stable MIL-lOO(Cr) lattice can also be impregnated with Pd(acac)2 via incipient wetness impregnation the loaded catalyst is active for the hydrogenation of styrene and the hydrogenation of acetylene and acetylene-ethene mixtures to ethane [58]. MIL-lOl(Cr) has been loaded with Pd using a complex multistep procedure involving an addition of ethylene diamine on the open Cr sites of the framework. The Pd-loaded MIL-lOl(Cr) is an active heterogeneous Heck catalyst for the reaction of acrylic acid with iodobenzene [73]. 

The points for Ag and Pd-Ag alloys lie on the same straight line, a compensation effect, but the pure Pd point lies above the Pd-Ag line. In fact, the point for pure Pd lies on the line for Pd-Rh alloys, whereas the other pure metal in this series, i.e., rhodium is anomalous, falling well below the Pd-Rh line. Examination of the many compensation effect plots given in Bond s Catalysis by Metals (155) shows that often one or other of the pure metals in a series of catalysts consisting of two metals and their alloys falls off the plot. Examples include CO oxidation and formic acid decomposition over Pd-Au catalysts, parahydrogen conversion (Pt-Cu) and the hydrogenation of acetylene (Cu-Ni, Co-Ni), ethylene (Pt-Cu), and benzene (Cu-Ni). In some cases, where alloy catalysts containing only a small addition of the second component have been studied, then such catalysts are also found to be anomalous, like the pure metal which they approximate in composition. 

Problem 13 Write a balanced molecular reaction for the hydrogenation of acetylene to a saturated alkane. How many liters of hydrogen gas are needed to react completely with 100 liters of acetylene ... 

Significant improvement of the activity of selectivity of Pd on Si02 could be achieved in the hydrogenation of acetylene by adding Ti, Nd, or Ce oxides to the catalyst.395 The metal oxides modify both geometrically and electronically the Pd surface. They retard the sintering of the dispersed Pd particles, suppress the formation of multiply bound ethylene, and facilitate the desorption of ethylene. The beneficial effect of lead in the hydrogenation of 1,3-butadiene over a Pd-Pb-on-... 

One of the characteristics of many catalytic hydrogenation reactions is the ability of the catalyst to promote the formation of more than one reaction product. Thus, for example, in the hydrogenation of acetylene, ethylene may be formed as an intermediate in the production of ethane and may be the major product in the initial stages of the reaction... 

Fig. 26. Forms of pressure fall against time curves observed in the hydrogenation of acetylene over noble Group VIII metal catalysts.

The general theoretical approach to the selectivity observed in the hydrogenation of acetylene has been discussed in Sect. 2.3, where it was noted that the observed selectivity may be dependent upon both thermodynamic and mechanistic factors. A possible explanation of the operation of a mechanistic factor has been discussed in Sect. 4.3. The selectivity values, defined as S = Pc2h4/(Pc2h4 + Pc2h6) observed for various metal catalysts are shown in Table 15. Selectivities have been observed to... 

In selective poisoning or selective inhibition, a poison retards the rate of one catalysed reaction more than that of another or it may retard only one of the reactions. For example, there are poisons which retard the hydrogenation of olefins much more than the hydrogenation of acetylenes or dienes. Also, traces of sulphur compounds appear selectively to inhibit hydro-genolysis of hydrocarbons during catalytic reforming. 

The value of AH° for the hydrogenation of acetylene to ethane is equal to the sum of the two reactions just calculated ... 

Pd-BaS04 catalyst has often been used for selective hydrogenations such as the Rosenmund reduction (Chapter 13) and the hydrogenation of acetylenic to ethylenic compounds (see, e.g., Chapter 4, eqs. 4.8-4.10). 

Nickel catalysts have been employed successfully for the semihydrogenation of various acetylenic compounds.1 Dupont was the first to study the hydrogenation of acetylenes using Raney Ni. Little or no change in the rate of hydrogenation on the uptake of 1 equiv of hydrogen was observed with the monosubstituted acetylenes, 1-heptyne... 

The behavior of 1,3-cyclohexadiene in the presence of hydrogen at 170°C is very similar to that of benzene (Fig. 17), with a transient formation of acetylene. Similarly, in the presence of He, 1,3-cyclohexadiene is cracked into acetylene, and this reaction can be repeated for many successive doses of the reactant. Now, the behavior of the isomer, 1,4-cyclohexadiene, is different because in the presence of hydrogen as well as He this reactant is only cracked into acetylene (182,183). The explanation of this different evolution has been provided before 1,4-cyclohexadiene is indeed a poison for the hydrogenation of acetylene and the stepwise reaction of hydrogenolysis of 1,4-cyclohexadiene (in the presence of H2) stops with the production of acetylene. Finally, it should be mentioned that cyclohexene, either in H2 or in He, is not catalytically transformed on silica activated by hydrogen spillover. 

Table I summarizes the application of various low-valent titanium metallocenes as catalysts for olefin hydrogenation. Compounds 10 and 37 are very effective hydrogenation catalysts for C2H4 and cyclohexene. Since different researchers have used widely varying conditions, we can only estimate that the polystyrene-supported (7j-C8H8)2Ti (142) is comparable in activity to compounds 10 and 37. When one recalls that 37 was prepared by a formal oxidation of the Ti centers in 10, it is remarkable that 37 is as good a catalyst as 10. Solutions prepared by reaction of l-methyl-17-allylbiscyclopentadienyltitanium (54) with H2 do appear to be more active hydrogenation (126) catalysts than 10 and 37. The dicarbonyl complex, (17-CsH5)2Ti(CO)2 (39), has been shown to be a catalyst for the hydrogenation of acetylene at —50 atm of H2 (143). It does not catalyze the hydrogenation of simple olefins. However, Floriani and Fachinetti discovered that if...

Consider a catalyst particle in a fixed bed through which gas is flowing, and assume that the reaction takes place at the catalyst surface and, accordingly, that the net heat of reaction is released at this surface. An example of such a system is the hydrogenation of acetylene in an ethylene stream, according to... 

While the slow protonation of the hydride species leads to H2 evolution, the ligation of acetylene to the Co(II)-protoporphyrin results in intramolecular hydride transfer and the hydrogenation of acetylene to ethylene. The oxidized dye is reduced by EDTA and thus the photobiocatalyst functions as a cyclic photoen2yme. Photo-induced hydrogenation of acetylenedicarboxylic acid proceeds stereoselectively and yields the thermodynamically less stable cis product, maleic acid. 

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