On the Mechanism of Synthesis from Carbon Monoxide and Water Vapor
Abstract Generated abstract
This paper reconsiders the mechanism of organic compound synthesis from carbon monoxide and water vapor, questioning a scheme that treats the process mainly as carbon monoxide conversion followed by Fischer-Tropsch synthesis on an iron catalyst. Using macrokinetic considerations and earlier experimental observations on hydrodynamic effects, the authors propose a two-stage mechanism: initial formation and polymerization of methylene species on the catalyst surface, followed by steam conversion of the resulting hydrocarbons within the catalyst bed. The proposed scheme accounts for hydrogen evolution and for the formation of olefins, paraffins, alcohols, acids, aldehydes, and ketones, including the increased yield of oxygen-containing products relative to conventional Fischer-Tropsch synthesis. It also suggests that selectivity may be influenced by pressure, temperature, feed composition, hydrodynamic regime, and catalyst choice.
Full Text
ON THE MECHANISM OF SYNTHESIS FROM CARBON MONOXIDE AND WATER VAPOR
S. I. SMOL’YANINOV, V. M. MIRONOV
(Presented by the Organic Section of the Scientific Seminar of the Faculty of Chemical Technology)
At present there exists a view concerning the mechanism of the synthesis of organic compounds from carbon monoxide and water vapor, advanced by H. Kölbel and co-workers [1] and by I. F. Bogdanov [2]. The authors propose a mechanism following from formal stoichiometric relations:
\[ \mathrm{CO}+\mathrm{H_2O}=\mathrm{CO_2}+\mathrm{H_2} \tag{1} \]
\[
2\mathrm{CO}+\mathrm{H_2}=(-\mathrm{CH_2}-)+\mathrm{CO_2}
\tag{2}
\]
and, in summary,
\[ 3\mathrm{CO}+\mathrm{H_2O}=(-\mathrm{CH_2}-)+2\mathrm{CO_2}. \tag{3} \]
From the indicated equations it is evident that the synthesis described is most simply represented as the sum of consecutively occurring reactions: conversion of carbon monoxide by water vapor and Fischer–Tropsch synthesis on an iron catalyst. It is assumed that the rate of the conversion reaction (1) is many times lower than the rate of the Fischer–Tropsch reaction (2), and therefore the overall reaction rate is determined by the rate of stage (1). However, the relatively high yield of hydrogen during the synthesis process, especially at medium space velocities of the synthesis gas [1, 3, 4], is attributed by the authors of the cited works to the conversion reaction (1).
The formation of oxygen-containing compounds during synthesis from carbon monoxide and water vapor is assumed to proceed in parallel with polymerization of the methylene radical [2], or else as the initial stage of this polymerization [3].
On the basis of certain theoretical propositions of macrokinetics [5], and also on the basis of experimental data from a study of the influence of the hydrodynamic regime on the process described [4], one may assume the occurrence, during synthesis, of conversion of hydrocarbons by water vapor, described in the general case by the equation:
\[ \mathrm{C_nH_{2n}}+2n\mathrm{H_2O}=n\mathrm{CO_2}+3n\mathrm{H_2}. \tag{4} \]
Relying on this assumption and on certain indirect experimental data [4], the synthesis of organic compounds from carbon monoxide and water vapor may be represented by the following scheme:
Stage I. Formation of the methylene radical and its polymerization. The formation and polymerization of the methylene radical proceed directly on the active surface of the ca-
catalyst, in the first layers along the path of the synthesis gas. Depending on the reaction conditions, formation of the methylene radical can probably proceed both through formation of the oxymethylene radical HCOH and through formation of formic acid [6] according to the equation:
\[ \mathrm{CO}+\mathrm{H_2O}=\mathrm{HCOOH}. \tag{5} \]
Here, too, the conversion reaction of carbon monoxide with steam probably plays a certain role, but, undoubtedly, all the hydrogen formed by reaction (1) is immediately consumed by reaction (2).
The polymerization of the methylene radical according to the equation
\[ n(-\mathrm{CH_2}-)=\mathrm{C}_n\mathrm{H}_{2n} \tag{6} \]
with the formation predominantly of α-olefins has so far been studied poorly; however, in our opinion, of the many proposed schemes the most probable is that described by I. F. Bogdanov [2].
Stage II. Conversion of hydrocarbons by steam. This stage proceeds both in the first and in the subsequent layers of the catalyst, but it is especially significant in the middle of the catalyst bed, where steam is still present and there is already a considerable amount of synthesized hydrocarbons. Conversion of hydrocarbons by steam may proceed either with complete destruction of the hydrocarbon molecule or with partial destruction. In the latter case paraffinic hydrocarbons may be formed from olefins according to the equation:
\[ \mathrm{C}_n\mathrm{H}_{2n}+\mathrm{H_2O}=\mathrm{C}_{n-1}\mathrm{H}_{2n}+\mathrm{CO_2}. \tag{7} \]
At the same time, in these regions alcohols may be formed by the hydration reaction of olefins:
\[ \mathrm{C}_n\mathrm{H}_{2n}+\mathrm{H_2O}=\mathrm{C}_n\mathrm{H}_{2n+1}\mathrm{OH}. \tag{8} \]
The considerable amount of carboxylic acids in the synthesis products (10 times greater than under the same conditions in Fischer—Tropsch synthesis), as well as the formation of aldehydes and ketones, can be explained by incomplete conversion of hydrocarbons by steam according to the equations:
\[ \mathrm{C}_n\mathrm{H}_{2n}+\mathrm{H_2O}=\mathrm{C}_{n-1}\mathrm{H}_{2n-1}\mathrm{CHO}+\mathrm{H_2}, \tag{9} \]
\[
\mathrm{C}_n\mathrm{H}_{2n}+2\mathrm{H_2O}=\mathrm{C}_{n-1}\mathrm{H}_{2n-1}\mathrm{COOH}+2\mathrm{H_2}.
\tag{10}
\]
etc.
The hydrogen formed by reactions (4, 9 and 10) is either removed from the system or returned to the synthesis process.
On the basis of the foregoing, it may be assumed that increasing the pressure will lead to a relative increase in the yield of alcohols according to equation (8), which was also observed by H. Kölbel and co-workers [1].
Using the proposed mechanism for the synthesis of organic compounds from carbon monoxide and steam, it will be possible to approach selective conduct of the process, along with the selection of catalysts, by separate or combined variation of the reaction conditions (temperature, pressure, CO:H₂O ratio, hydrodynamic regime, etc.).
REFERENCES
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H. Kölbel, F. Engelhardt, H. Hammer. Synthesis of hydrocarbons from carbon monoxide and steam. Communication VII. Brennstoff-Chemie, 42, No. 3, pp. 65—72, 1961.
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