CARBON ISOTOPE EXCHANGE IN THE SYSTEM CO2-CH4 AT ELEVATED TEMPERATURES

Abstract

Carbon isotope exchange was investigated for the system CO2-CH4 at 150 to 600°C in the presence of several potential catalysts by use of isotopically normal or 13C-enriched gases. Silica gel, graphite, molecular sieve Linde 4A, magnetite, and hematite oxidized small amounts of CH4 in starting CO2-CH4 mixtures to CO and CO2 but failed to enhance the net rate of carbon isotope exchange between CO2 and CH4, even after 169 to 1833 h at 400 to 500°C. In contrast, several commercial transition-metal catalysts (Ni, Pd, Rh, and Pt) promoted reactions significantly toward chemical and isotopic equilibrium. With the Ni catalyst, the attainment of carbon isotopic equilibrium between CO2 and CH4 was demonstrated for the first time at temperatures from 200 to 600°C by complete isotopic reversal from opposite directions. The experimentally determined carbon isotope fractionation factors between CO2 and CH4 (103lnα) were similar to, but slightly greater than (0.7-1.1%%, 0.89%% on average), those of statistical-mechanical calculations by Richet et al. (1977). The experimental results can be described by the following equation between 200 and 600°C only: 103lnα(CO2-CH4) = 26.70 - 49.137(103/T) + 40.828(106/T2) - 7.512(109/T3) (T = 473.15-873.15 K, 1σ = +/-0.14%%, n = 44). Alternatively, an equation generated by fitting Richet et al. (1977) data in the temperature range from 0 to 1300°C can be modified by adding +0.89%% to its constant; 103lnα(CO2-CH4) = 0.16 + 11.754(106/T2) - 2.3655(109/T3) + 0.2054(1012/T4) (T = 273-1573 K, 1σ = +/-0.21%%, n = 44). This and other recent experimental studies in the literature demonstrate that transition metals, which are widespread in many natural materials, can catalyze reactions among natural gases at relatively low temperatures (=<200°C). The role of natural catalysts, ''geocatalysts,'' in the abiogenic formation of methane, hydrocarbons, and simple organic compounds has important implications, ranging from the exploration of hydrocarbon resources to prebiotic organic synthesis.