Formation of Oxygenated Products from Synthesis Gas

The oil crisis of 1973 brought home the fact, anticipated in the Report of the Club of Rome Project , that fossil fuels are non-renewable resources, and that natural gas and oil, which have been the twin pillars of the chemical industry for several decades, will be exhausted before coal. Moreover, the resources of oil are not equally distributed over the world and this makes formation of cartels easier (although it cannot of course guarantee their success). The free-world chemical industry is therefore quite vulnerable and can easily be a hostage to political pressure. Since the resources of coal are estimated to be four to six times larger than those of oil and are better spread over the world, coal utilization received a great impulse in 1973. A modest but important success of the efforts to find alternatives for oil and gas is already observable the lowest economic prices of alternative fuels, and of chemicals from alternative raw materials, now form the upper limit for prices dictated by the oil producing countries. 

Coal as a direct or indirect chemical feedstock can be utilized in a number of ways  

The last two reactions are usually performed simultaneously. The second reaction supplies then the heat necessary for the first, strongly endothermic reaction. There are very interesting catalytic problems in coal gasification, as well as challenging engineering problems in the gasifier-reactor design. 

Meadows, The Limits to Growth , A report for the Club of Rome Project, Universe Books, New York, 1972. 

However, these aspects of the production of oxygenates are beyond the scope of this review. 

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Formation of Oxygenated Products from Synthesis Gas and ligand-stabilized formyl compounds (3), (4) of Ta ... 

The formation of formaldehyde from synthesis gas is a thermodynamically unfavourable process. While this limitation precludes formaldehyde from being a significant reaction product, it does not forbid its presence tn kinettcally significant amounts, followed by its participation as a reaction intermediate. Oxygenated products can easily be deiived by this route (see glycol synthesis). 

Based on mechanistic and kinetic studies of the higher alcohol synthesis from synthesis gas, it has been shown that the ethanol in the mixed-oxygenate product is produced from intermediates derived from methanol, not CO [103,109]. Kinetic models of the synthesis have been developed that are able to explain the observed product distribution [110,111]. These models are based on a detailed understanding of the reaction mechanism in which two types of reactions dominate aldol condensation, which yields primarily 2-methyl branched alcohols, and Cl coupling reactions, which yield linear alcohols [106,111]. Estimates of the parameters of the kinetic models that quantitatively describe the oxygenate product distributions suggest that the rate of ethanol formation is about an order of magnitude lower than the rate of production of branched alcohols [111,112]. On the Cs/Cu/Zn catalysts, this results in a minimum in yield of ethanol compared with the yields of methanol, 1-propanol, and 2-methyl-1 propanol. Althou methanol conversion to ethanol has been confirmed as part of the hi er alcohol synthesis from synthesis gas, this synthesis does not offer a plausible route for the conversion of methanol to ethanol. Under the reaction conditions methanol rapidly decomposes, even at a pressme of 0.1 MPa [113], to yield an equilibrium mix of methanol, CO, and H2. Furthermore, as shown by the data in T able 7, the yield of ethanol remains low even with methanol in the feed. 

Violent explosions which occurred at —100 to —180°C in ammonia synthesis gas units were traced to the formation of explosive addition products of dienes and oxides of nitrogen, produced from interaction of nitrogen oxide and oxygen. Laboratory experiments showed that the addition products from 1,3-butadiene or cyclopentadiene formed rapidly at about — 150°C, and ignited or exploded on warming to —35 to — 15°C. The unconjugated propadiene, and alkenes or acetylene reacted slowly and the products did not ignite until +30 to +50°C [1], This type of derivative ( pseudo-nitrosite ) was formerly used (Wallach) to characterise terpene hydrocarbons. Further comments were made later [2],... 

The production of synthesis gas from methane oxidation was also studied overFe catalyst in fuel cell using solid electrolyte (YSZ) at 850-950°C at atmospheric pressure [8]. The anodic electrode was Fe and the cathode that was exposed to air was Pt. Reduced iron was more active than oxidized iron for synthesis gas formation. The maximum CO selectivity and yield were nearly 100% and 73%, respectively. Carbon deposition was reported at high methane to oxygen ration. 

Cracking/Reforming of the Volatile Matter. At somewhat higher temperatures (600°C or more) the volatile matter evolved by the pyrolysis reactions (step 1) reacts in the absence of oxygen to form a hydrocarbon rich synthesis gas. These gas phase reactions happen very rapidly (seconds or less) and can be manipulated to favor the formation of various hydrocarbons (such as ethylene). Rates and products of the cracking reactions for volatile matter derived from cellulose, lignin, and wood are now available in the literature (1, 3, 5, 6). 

The oxygenate catalyst volume is also related to the module of the gas from the synthesis gas production (Fig. 7). Mininum oxygenate catalyst volumes are obtained at high modules. The reason is simply that the reaction rate of MeOH formation depends on the partial pressure of hydrogen (ref. 7). 

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