Salvador Olivé

Transition Metal-Carbon Monoxide Interactions

In contrast with hydrogen, carbon monoxide interacts with metals predominantly as an intact molecule. Therefore, some of the properties of the isolated CO molecule, in particular its electronic structure, shall be discussed in this section.

Key Reactions in Catalysis

This Section reviews briefly those classes of reactions which play an important role in the catalytic hydrogenation of carbon monoxide. Some of the reactions have already been mentioned in earlier Chapters (e.g. oxidative addition of hydrogen in Sect. 2.1, ligand exchange in Section 4.2.2), but it is considered convenient to group the different types of reactions together here for easy reference in later Chapters. It should be noted that these “key reactions” are ubiquitous in catalysis with transition metals in general (certainly in homogeneous, and most probably also in heterogeneous catalysis), but only those aspects relevant to the hydrogenation of carbon monoxide will be discussed. For a broader treatment, the reader is referred to the specialized literature (e.g. Henrici-Olive and Olive, 1977; Collman and Hegedus, 1980; Cotton and Wilkinson, 1980).

Methanol from CO + H2

The industrial production of methanol from CO and H2 on metal oxide or metal catalysts is known since the early twenties (for a review see Falbe, 1977). Soon, it was recognized that iron should be eliminated from the catalyst formulations, since carburization led to desactivation of the catalyst, and to the formation of methane instead of methanol (cf. Chap. 7). The most important catalyst of the early times, and up to the late fifties, was ZnO/Cr2O3. The synthesis works at 250–350 atm and 300–400 °C. It had been noticed early that copper containing catalysts permit the synthesis to be carried out at considerably lower pressure and temperature, but these catalysts are extremely sensitive to sulfur impurities in the feed gas. Thus, 30 ppm H2S can be tolerated with Cr/Zn catalysts, whereas less than 1 ppm is required for Cu/Zn. The actual success of the latter catalysts is due to important improvements in the purification of the synthesis gas. Values of ≦0.1 ppm H2S can be achieved today, warranting a catalyst lifetime of over three years. The Cu/Zn catalyst requires some 2% of CO2 in the gas feed for optimum results (Klier et al., 1982). Copper (I) has been discerned as the active species (see Sect. 8.4). At CO2 concentrations < 2% the catalyst tends to become deactivated by over-reduction, and at higher concentrations strong adsorption of CO2 retards the synthesis. The low pressure Cu/Zn process (50–100 atm, 220–270 °C) appears to be the preferred one now, for evident reasons with regard to energy consumption. But the high pressure process is still important where sulfur (and other) impurities are a problem.

Fischer-Tropsch Synthesis

The Fischer-Tropsch synthesis is essentially a polymerization reaction — or perhaps better an oligomerization, since in most cases the average molecular weight of the product is not very high — where carbon-carbon bonds are formed between C atoms proceeding from carbon monoxide, under the influence of hydrogen and a metal catalyst, and with elimination of water. Without anticipating the detailed reaction mechanism, the main reaction of the Fischer-Tropsch (FT) synthesis may be formulated as:

Homogeneous CO Hydrogenation

n(CO + 2{H_2})\mathop \to \limits^{Cat} - {(C{H_2})_n} - + n{H_2}O

Transition Metal-Hydrogen Interactions

(9.1) Depending on catalyst and reaction conditions, the products are linear hydrocarbons, oxygenated derivatives thereof, or mixtures of both. Usually, a wide range of molecules with different chain length, and with a molecular weight distribution characteristic for polymerization reactions, are formed.

Catalysts and Supports

Methanol is synthesized from CO and H2 in large quantities at low price and with a high degree of purity (Danner, 1970; see also Sect. 8.4). This is the reason for the large amount of work towards syntheses starting with methanol, and aiming at the same or similar compounds obtainable from the direct hydrogenation of CO. In this Chapter, the most important of these methanol based processes will be discussed.

Non-Catalytic Interaction of CO with H2

Methanation (Chap. 7), methanol formation (Chap. 8), and Fischer-Tropsch synthesis (Chap. 9) are essentially domains of heterogeneous catalysis, although methanol is also produced, together with methyl formate and/or ethylene glycol with the homogeneous HCo(CO)4 system at high pressure, as described in Sect. 8.1.