The hydrogenation of CO 2 to methanol over copper-based catalysts has attracted considerable attention recently. Among all the proposed reaction mechanisms, a large number of experimental and theoretical studies have focused on the one that includes a HCOO intermediate due to the fact that high coverages of formate over catalyst surfaces were observed experimentally. To systematically understand the influence of formate species coverage on the reaction kinetics of methanol synthesis, the energetics of the CO 2 hydrogenation pathway over clean and one-or two-formate preadsorbed Cu(211) are obtained using density functional theory calculations, and these energetics are further employed for microkinetic modeling. We find that the adsorption energies of the intermediates and transition states involved in the reaction pathway are changed in the presence of spectating formate species, and consequently, the potential energy diagrams are varied. Microkinetic analysis shows that the turnover frequencies (TOFs) over different formate preadsorbed surfaces vary under the same reaction condition. In particular, the reaction rates obtained over clean Cu(211) are generally the lowest, while those over one-or twoformate preadsorbed surfaces depend on the reaction temperatures and pressures. Meanwhile, we find that only when the formate coverage effect is considered, some of the TOFs obtained from microkinetic modeling are in fair agreement with previous experimental results under similar conditions. After the degree of rate control analysis, it is found that the combination of HCOO and HCOOH hydrogenation steps can be treated as the "effective rate-determining step", which can be written as HCOO* + 2H* → H 2 COOH* + 2*. Therefore, the formation of methanol is mainly controlled by the surface coverage of formate and hydrogen at the steady state, as well as the free energy barriers of the effective rate-determining step, i.e., effective free energy barriers.

Water is able to promote many chemical reactions in an autocatalysis manner, and the essential role that water plays in the system is still worth discussing. In the process of methanol synthesis from CO2 hydrogenation on Cu, whether the promoting species is molecular water or water derived O/OH is controversial. To systematically understand the influence of the presence of O/OH on the reaction kinetics of CO2 hydrogenation to methanol, we here carry out density functional theory calculations to obtain the energetics over O/OH preadsorbed Cu(211) and further use them for microkinetic modeling in order to calculate the formation rate of methanol. The calculation results show that the free energy barriers of CO2 activation by molecular water through both HCOO and COOH routes are higher than those by the hydrogen atom on clean and OH or O preadsorbed Cu(211). The subsequent microkinetic modeling indicates that the formation rate of methanol over Cu(211) is improved in the presence of O/OH. Detailed analyses on the coverage and degree of rate control of surface species reveal that the presence of O/OH on the catalyst surface will destabilize the spectating formate and lower the energies of rate-controlling transition states. The formate coverage effect is further included in the microkinetic modeling, and we find that the reaction rate is further increased at lower temperatures. Our current work provides evidence that the surface adsorbed O and OH are able to promote the formation of methanol from CO2 hydrogenation and, more importantly, highlights the fact that the activity of methanol formation is sensitive to the surface adsorbates.

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Reaction pathways of methanol and carbon monoxide formation from CO2 hydrogenation over PdIn(110) and (211) with a combined density functional theory and microkinetic modeling approach.

Considerable attention has been paid to the development of new catalysts for methanol synthesis from CO 2 hydrogenation for a long period of time. The PdIn intermetallic catalyst was found recently to exhibit good stability and activity for methanol formation. We thus performed a theoretical study to understand the reaction mechanism of methanol synthesis on stepped PdIn(310), combining density functional theory (DFT) calculations and microkinetic analysis. On the basis of energetics obtained on clean PdIn(310), we found that the preferred reaction pathway for CH 3 OH generation proceeds through the COOH intermediate and further CO hydrogenation. However, microkinetic results suggested that the coverage of formate and carbon monoxide at the steady state is nearly one monolayer at the corresponding preferred adsorption site, that is, the In site for HCOO and the Pd site for CO, within the whole temperature range studied. Therefore, further studies were carried out to reveal the influence of coverage of preadsorbed formate and carbon monoxide on adsorption energies. It turned out that the differential adsorption energy of formate at the In site is comparable to that at the Pd site when the surface is covered by two formate at the In step-bridge site, indicating that it is possible for an additional formate to adsorb at either the Pd or In site under such condition. On this basis, we found with further DFT calculations and microkinetic analysis that the preferred reaction mechanism of methanol formation would change to the one including the HCOOH intermediate and the reaction prefers to happen at the Pd site, with two formate preadsorbing at the In step-bridge site at the same time. It was found that such changes can be attributed to the reduced barriers of elementary steps in this path introduced by the formate coverage effect. Therefore, it is imperative to carry out a theoretical study for surface reactions by combining DFT calculations and microkinetic analysis and to take the interactions between dominant surface species into account when identifying the mechanism under reaction conditions.

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Erythromycin, a medically important antibiotic, is produced by Saccharopolyspora erythraea. Unusually, the erythromycin biosynthetic gene cluster lacks a regulatory gene, and the regulation of its biosynthesis remains largely unknown. In this study, through gene deletion, complementation and overexpression experiments, we identified a novel TetR family transcriptional regulator SACE_3986 negatively regulating erythromycin biosynthesis in S. erythraea A226. When SACE_3986 was further inactivated in an industrial strain WB, erythromycin A yield of the mutant was increased by 54.2 % in average compared with that of its parent strain, displaying the universality of SACE_3986 as a repressor for erythromycin production in S. erythraea. qRT-PCR analysis indicated that SACE_3986 repressed the transcription of its adjacent gene SACE_3985 (which encodes a short-chain dehydrogenase/reductase), erythromycin biosynthetic gene eryAI and the resistance gene ermE. As determined by EMSA analysis, purified SACE_3986 protein specifically bound to the intergenic region between SACE_3985 and SACE_3986, whereas it did not bind to the promoter regions of eryAI and ermE. Furthermore, overexpression of SACE_3985 in A226 led to enhanced erythromycin A yield by at least 32.6 %. These findings indicate that SACE_3986 is a negative regulator of erythromycin biosynthesis, and the adjacent gene SACE_3985 is one of its target genes. The present study provides a basis to increase erythromycin production by engineering of SACE_3986 and SACE_3985 in S. erythraea.

Erythromycin A is a widely used antibiotic produced by Saccharopolyspora erythraea; however, its biosynthetic cluster lacks a regulatory gene, limiting the yield enhancement via regulation engineering of S. erythraea. Herein, six TetR family transcriptional regulators (TFRs) belonging to three genomic context types were individually inactivated in S. erythraea A226, and one of them, SACE_3446, was proved to play a negative role in regulating erythromycin biosynthesis. EMSA and qRT-PCR analysis revealed that SACE_3446 covering intact N-terminal DNA binding domain specifically bound to the promoter regions of erythromycin biosynthetic gene eryAI, the resistant gene ermE and the adjacent gene SACE_3447 (encoding a long-chain fatty-acid CoA ligase), and repressed their transcription. Furthermore, we explored the interaction relationships of SACE_3446 and previously identified TFRs (SACE_3986 and SACE_7301) associated with erythromycin production. Given demonstrated relatively independent regulation mode of SACE_3446 and SACE_3986 in erythromycin biosynthesis, we individually and concomitantly inactivated them in an industrial S. erythraea WB. Compared with WB, the WBΔ3446 and WBΔ3446Δ3986 mutants respectively displayed 36% and 65% yield enhancement of erythromycin A, following significantly elevated transcription of eryAI and ermE. When cultured in a 5 L fermentor, erythromycin A of WBΔ3446 and WBΔ3446Δ3986 successively reached 4095 mg/L and 4670 mg/L with 23% and 41% production improvement relative to WB. The strategy reported here will be useful to improve antibiotics production in other industrial actinomycete.