Indirect hydrocarbon synthesis

Catalytic hydrogenation of CO2 to hydrocarbons is classified into two categories. The one is direct hydrogenation fix)m H2/CO2 to hydrocarbons. The other is indirect process which includes methanol sjmthesis fix>m H2/CO2, followed by in situ methanol conversion to hydrocarbons using sohd acid catalyst in H2/CO2 feed. Study on indirect hydrocarbon synthesis is now popular. 

Hydrocarbon synthesis, applying indirect processes and hybrid catalysts125,126 (first methanol is formed, then transformed to lower paraffins and olefins), either directly127,128 or using two-stage reactors126... 

It is not the object of this paper to summarize again all the literature. It would also be beyond the scope to discuss in detail industrial and engineering features of hydrocarbon synthesis, as well as questions which are only indirectly connected with the synthesis such as synthesis gas manufacture or conversion of primary synthesis products to other compounds. The scope of this paper inevitably limits the review to important steps of research work on the reactions of carbon monoxide and hydrogen, with special consideration regarding the behavior of the catalysts necessary to carry out the reactions. 

Basic research work. Emmett, Storch, Taylor, and co-workers, and other scientists carried out basic research work which directly or indirectly influenced the development work on catalysts for hydrocarbon synthesis. The studies of Emmett and Brunauer on adsorption of gases by different types of catalysts (75a) and the publications of Taylor and co-workers on activated adsorption (75b) are of particular importance. Emmett also published investigations on the reaction mechanism of hydrocarbon synthesis (Sec. III.4). 

Much recent research (7-5) has been devoted to converting methane to products that are more easily transported and more valuable. Such more valuable products include higher hydrocarbons and the partial oxidation products of methane which are formed by either direct routes such as oxidative coupling reactions or indirect methods via synthesis gas as an intermediate. The topic of syngas formation by oxidation of CH4 has been considered primarily from an engineering perspective (7-5). Most fundamental studies of the direct oxidation of CH4 have dealt with the CH4 + O2 reaction system in excess O2 and at lower temperatures (6-10). 

After World War II, direct liquefaction of coal became uneconomical as the use of lower-cost petroleum products became more widespread. However, the German process of indirect coal liquefaction, the Fischer-Tropsch process, continued to hold some interest. The Fischer-Tropsch process first involved production of a carbon monoxide and hydrogen-rich synthesis gas by the controlled gasification of coal followed by a catalytic reaction process to yield a valuable mixture of hydrocarbon products. Simplified Fischer-Tropsch reactions are shown by the following equations ... 

The composition of the synthesis gas, particularly the concentrations of hydrogen, carbon monoxide, and carbon dioxide, affects the atmosphere throughout the reactor directly, and also indirectly by its effect on the composition of the recycle gas. Synthesis gas, prepared by partial combustion of methane or some less hydrogen-rich carbonaceous material, lacks sufficient hydrogen for the conversion of all the carbon monoxide to hydrocarbons, and in this sense the synthesis gas is deficient in hydrogen. Stoichiometrically methane has sufficient hydrogen to convert all its carbon to olefins by the two-step process ... 

The development of chemical industry has provided us with the means to produce, on an ecologically significant scale, chemicals that interfere with the natural cycles of synthesis and breakdown either because they accelerate or slow down large-scale natural processes (e.g., the fluoro chloro hydrocarbons which accelerate the breakdown of ozone by sunlight) or, more commonly, because they resist breakdown themselves (e.g., certain synthetic polymers). This has become a matter of grave and widespread concern and has resulted in regulations and voluntary measures to restrict or prohibit the manufacture and use of materials that interfere with the natural cycles. This concern is particularly acute in those cases where this interference has direct or indirect adverse effects upon human health (as in the case of the fluoro chloro hydrocarbons), but it exists also where massive accumulation occurs without known health hazards (as in the case of the too-stable synthetic polymers). 

Fischer-Tropsch. The process most frequently considered for indirect coal liquefaction is the Fischer-Tropsch (F-T) synthesis, developed in 1925 by German chemists Franz Fischer and Hans Tropsch. In the F-T process, synthesis gas is reacted over a catalyst, typically iron or cobalt based, at 1-30 atm and 200-350°C to produce a wide range of mainly aliphatic hydrocarbons, including gas, LPG, gasoline, jet fuel, diesel oil, middle distillates, heavy oil, and waxes. Germany used this technology during World War II to produce nearly 15,000 barrels/day of military fuels. 

Indirect processes for converting natural gas to alcohols and higher hydrocarbons require the initial conversion of methane to synthesis gas (CO/H2). This is a difficult and expensive step normally carried out by steam reforming and partial oxidation (6). Subsequent synthesis gas conversion steps, such as FT synthesis and related processes (1,2), must occur with high selectivity to desired products in order to minimize extensive recycle of undesired products to the initial synthesis gas generation step. C5+ paraffins, low- and intermediate-molecular-weight olefins, and C20+ linear hydrocarbons provide useful feeds in downstream processes leading to fuels and petrochemicals. 

The indirect role of lithium alkyls in polyethylene production involves the use of sec-butyllithium (from eq 4.13) to manufacture "dibutylmagnesium" (DBM). Key synthesis reactions in aliphatic hydrocarbon solvents (e.g., heptane) are illustrated in eq 4.15-4.17 ... 

Fischer-Tropsch synthesis (FTS) has been widely studied during the past 80 years due to its significance in indirectly converting coal/natural gas to transportation fuels. However, one of big challenges is how to control hydrocarbon chain growth during the FTS reaction [1], It has been proposed that proper process conditions or some kinds of porous supports can be used to restrict chain propagation [1,2]. 

The other category of coal liqnefaction processes invokes the concept of the indirect liquefaction of coal. In these processes, the coal is not converted directly into liquid products but involves a two-stage conversion operation in which coal is first converted (by reaction with steam and oxygen) to prodnce a gaseons mixture that is composed primarily of carbon monoxide and hydrogen (syngas synthesis gas). The gas stream is snbseqnently pnrified (to remove snlfur, nitrogen, and any particnlate matter) after which it is catalytically converted to a mixtnre of liquid hydrocarbon prodncts. 

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