The understanding of the reaction mechanism of product formation in the oxidative coupling of methane (OCM) is the prerequisite for designing catalysts with improved selectivity to the desired products, i.e., C 2 H 6 and C 2 H 4 (C 2 hydrocarbons). One step in this direction is to understand the kind of oxygen species involved in the selective and nonselective reactions. Against this background, a series of kinetic and mechanistic tests of the OCM reaction with N 2 O (N 2 O−OCM) were carried out over the Mn−Na 2 WO 4 /SiO 2 system in the absence and the presence of cofed water. The usage of N 2 O instead of O 2 was proven to suppress the direct oxidation of CH 4 to CO 2 in favor of the formation of C 2 hydrocarbons. This inhibition was explained by a lower concentration of surface biatomic oxygen species participating in the formation of CO 2 . The formation of such species from O 2 was supported by oxygen isotopic exchange and electron paramagnetic resonance (EPR) tests. As proven by in situ UV−vis tests under various reaction conditions at 750 °C, N 2 O−OCM and O 2 −OCM mainly proceed through a Mars van Krevelen sequence. As MnO x is reduced faster than Na 2 WO 4 , it should be primarily involved in oxidant activation. In comparison with O 2 , N 2 O was proven to reoxidize reduced catalysts significantly slower. This results in reducing surface density (spatial separation) of lattice oxygen species in N 2 O−OCM that is unfavorable for the undesired oxidation reactions leading to carbon oxides. As a result, the selectivity to C 2 hydrocarbons increases. This knowledge is essential for catalyst design through controlling the rates of generation and consumption of oxygen species. A further aspect of this work is the positive effect of H 2 O on the rate of methane conversion and the selectivity to C 2 hydrocarbons in N 2 O−OCM. The strength of this effect on the activity is lower than in O 2 −OCM. The enhancing effect was related to H 2 O-mediated transformation of surface biatomic oxygen species into monatomic ones.
Ribosomal RNAs in all organisms are methylated. The functional role of the majority of modified nucleotides is unknown. We systematically questioned the influence of rRNA methylation in Escherichia coli on a number of characteristics of bacterial cells with the help of a set of rRNA methyltransferase (MT) gene knockout strains from the Keio collection. Analysis of ribosomal subunits sedimentation profiles of the knockout strains revealed a surprisingly small number of rRNA MT that significantly affected ribosome assembly. Accumulation of the assembly intermediates was observed only for the rlmE knockout strain whose growth was retarded most significantly among other rRNA MT knockout strains. Accumulation of the 17S rRNA precursor was observed for rsmA (ksgA) knockout cells as well as for cells devoid of functional rsmB and rlmC genes. Significant differences were found among the WT and the majority of rRNA MT knockout strains in their ability to sustain exogenous protein overexpression. While the majority of the rRNA MT knockout strains supported suboptimal reporter gene expression, the strain devoid of the rsmF gene demonstrated a moderate increase in the yield of ectopic gene expression. Comparative 2D protein gel analysis of rRNA MT knockout strains revealed only minor perturbations of the proteome.
Until now a great number of various materials have been tested for the oxidative coupling of methane (OCM). On the basis of previous statistical analysis of OCM-related literature data, we...
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