C2 Oxygenates Research Articles

Source and major species of CHx(x = 1–3) in acetic acid synthesis from methane–syngas on Rh catalyst: a theoretical study

Density functional calculations have been carried out to investigate the source and major species of CHx (x = 1–3) involved in acetic acid synthesis from methane–syngas on the Rh(111) surface. All possible formation pathways of CHx (x = 1–3) from methane and syngas have been systematically investigated. For CHx formation from methane, our results show that CH is the most abundant species; for CHx formation from syngas, all CHx (x = 1–3) species form from CHO by CO hydrogenation, and the optimal formation routes of CHx show that CH and CH3 are the most abundant species rather than CH2 and CH3OH. On the other hand, CH formed by methane is more favourable than CH and CH3 formed by syngas; meanwhile, CO insertion into CHx species to form C2 oxygenates as acetic acid precursors is more favourable than CO hydrogenation to CH and CH3. As a result, in acetic acid synthesis from methane–syngas, CHx (x = 1–3) species come from methane rather than syngas, and the corresponding primary species is CH. In addition, the CO in syngas is predominantly responsible for insertion reactions that produce CHCO, which is a C2 oxygenate precursor leading to the formation of acetic acid. Furthermore, microkinetic modelling analysis shows that the major product of acetic acid synthesis from methane–syngas on the Rh(111) surface is CH3COOH, and that the production of CH3OH cannot compete with that of CH3COOH.

Ethanol Synthesis from Syngas on the Stepped Rh(211) Surface: Effect of Surface Structure and Composition

The mechanisms of ethanol synthesis from syngas including CHx (x = 1–3) species and C2 oxygenates on the stepped Rh(211) surface have been systematically investigated by using density functional theory together with a periodic slab model. Our results show that among all CHx (x = 1–3) species, CH3 is the most favorable monomer, which is more favorable than CH3OH formation; this result is quite different from that on the flat Rh(111). Beginning with CH3 species, CH3 + CHO → CH3CHO dominantly contributes to the formation of C2 oxygenates; subsequently, CH3CHO is successively hydrogenated to ethanol via CH3CH2O intermediate. Meanwhile, CH4 formation by CH3 hydrogenation is energetically compatible with CH3CHO formation; that is, the productivity and selectivity of ethanol is determined by the formations of CH3CHO and CH4. As a result, the strategy of improving the Rh-based catalyst with the help from the promoter Mn is employed to tune the relative activity of CH3CHO and CH4 formations that control the productivity and selectivity of ethanol from syngas. Our results show that the productivity and selectivity of ethanol from syngas can be improved by introducing promoter Mn into the Rh catalyst.

Insight into the Effect of Promoter Mn on Ethanol Formation from Syngas on a Mn-Promoted MnCu(211) Surface: A Comparison with a Cu(211) Surface

Density functional theory calculations have been employed to investigate the effect of promoter Mn on ethanol formation from syngas on a Mn-promoted MnCu(211) surface. Our results show that CO + 3H → CHO + 2H → CH2O + H → CH3O is an optimal pathway for the overall CO conversion. Starting with CH3O, CH3 is formed via CH3O → CH3 + O. Then, CHO insertion into CH3 can form CH3CHO, and further, CH3CHO is successively hydrogenated to ethanol via CH3CH2O intermediate. Meanwhile, CH3OH is formed via CH3O + H → CH3OH. Compared to the pure Cu(211) surface, CH3 formation is found to be energetically compatible with CH3OH formation on the MnCu(211) surface, which can lead to more CH3 sources and less CH3OH; thus, the productivity and selectivity of ethanol can be improved. On the other hand, starting from CH3, the MnCu(211) surface is more favorable for CHO insertion into CH3 to CH3CHO in comparison with CH3 hydrogenation, dissociation and coupling to CH4, CH2, and C2H6 due to their high activation barriers; namely, the MnCu(211) surface exhibits a better selectivity toward C2 oxygenates rather than hydrocarbons. As a result, we can show that, by introducing promoter Mn into Cu catalyst, the productivity and selectivity to ethanol from syngas can be effectively improved.

Partial Oxidation of Ethane to Oxygenates Using Fe- and Cu-Containing ZSM-5

Iron and copper containing ZSM-5 catalysts are effective for the partial oxidation of ethane with hydrogen peroxide giving combined oxygenate selectivities and productivities of up to 95.2% and 65 mol kgcat(-1) h(-1), respectively. High conversion of ethane (ca. 56%) to acetic acid (ca. 70% selectivity) can be observed. Detailed studies of this catalytic system reveal a complex reaction network in which the oxidation of ethane gives a range of C2 oxygenates, with sequential C-C bond cleavage generating C1 products. We demonstrate that ethene is also formed and can be subsequently oxidized. Ethanol can be directly produced from ethane, and does not originate from the decomposition of its corresponding alkylperoxy species, ethyl hydroperoxide. In contrast to our previously proposed mechanism for methane oxidation over similar zeolite catalysts, the mechanism of ethane oxidation involves carbon-based radicals, which lead to the high conversions we observe.

Insight into the preference mechanism for C[sbnd]C chain formation of C2 oxygenates and the effect of promoters in syngas conversion over Cu-based catalysts

Cu-based catalysts are often used for the production of C2 oxygenates or hydrocarbons from syngas, although numerous studies have been reported, the exact mechanism remains in debate, and presents a major challenge in catalysis. In this study, the preference mechanism for CC chain formation of C2 oxygenates and the role of promoters in syngas conversion on Cu(211) and metal-doped MCu(211) (M=Rh, Ni) surfaces, including CHx (x=1–3) hydrogenation, dissociation and coupling, as well as CO or CHO insertion into CHx (x=1–3), have been systematically investigated by using density functional theory method. Our results show that pure Cu(211) surface shows a better catalytic activity to C2H4 formation rather than C2 oxygenates. However, promoters Rh and Ni-doped Cu(211) surfaces show a better activity and selectivity to C2 oxygenates rather than hydrocarbons, in which CH2 species is responsible for C2 oxygenates formation by CHO insertion. In addition, the difference between CO and CHO insertion into CHx (x=1–3) to form C2 oxygenates is mainly attributed to the HOMO–LUMO gap of CHxCHO and CHxCO, the smaller HOMO–LUMO gap greatly facilitates the charge transfer and hybridization between adsorbed species and catalysts.

Carbon dioxide conversion to valuable chemical products over composite catalytic systems

This paper reports an experimental study on catalytic conversion of carbon dioxide to methanol, ethanol and acetic acid. Catalysts having different catalytic functions were synthesized and combined in different ways to enhance the selectivity to desired products. The combined catalyst system possessed the following functions: methanol synthesis, Fischer-Tropsch synthesis, water-gas-shift and hydrogenation. Results showed that the methods of integrating these catalytic functions played an important role in achieving the desired product selectivity. We speculate that if methanol synthesis sites were located adjacent to the C–C chain growth sites, the formation rate of C2 oxygenates would be enhanced. The advantage of using a high temperature methanol catalyst PdZnAl in the combined catalyst system was demonstrated. In the presence of PdZnAl catalyst, the combined catalyst system was stable at 380 °C. It was observed that, at high temperature, kinetics favored oxygenate formation. The results implied that the process can be intensified by operating at high temperature using Pd-based methanol synthesis catalyst. Steam reforming of the byproduct organics was demonstrated as a means to provide supplemental hydrogen. Preliminary process design, simulation, and economic analysis of the proposed CO2 conversion process were carried out. Economic analysis indicates how ethanol production cost was affected by the price of CO2 and hydrogen.

Insight into the Preference Mechanism of CHx (x = 1–3) and C–C Chain Formation Involved in C2 Oxygenate Formation from Syngas on the Cu(110) Surface

The possible formation pathways of CHx (x = 1–3) and C–C chain involved in C2 oxygenate formation from syngas on an open Cu(110) surface have been systematically investigated to identify the preference mechanism of CHx (x = 1–3) and C–C chain formation. Here, we present the main results obtained from periodic density functional calculations. Our results show that all CHx (x = 1–3) species formation starts with CHO hydrogenation; among them, CHx (x = 2, 3) are the most favored monomers, however, CH3OH is the main product from syngas on the Cu(110) surface, and the formation of CHx (x = 1–3) cannot compete with CH3OH formation. Further, on the basis of the favored monomer CHx (x = 2, 3), we probe into the C–C chain formation of C2 oxygenates by CO or CHO insertion into CHx (x = 2, 3), as well as the hydrogenation, dissociation, and coupling of CHx (x = 2, 3), suggesting that CO insertion into CH2 to form C2 oxygenates is the dominant reaction for CH2 on the Cu(110) surface with an activation barrier of 44.5...

Deep UV photocatalytic activation of ethane on silica surfaces

Deep UV photolysis (165 or 185nm) of surface silanol groups leads to the homolytic OH bond breaking, generating silyloxyl radicals and hydrogen atoms. Silyloxyl radicals are able to activate ethane through hydrogen abstraction, whereby ethyl radicals are formed. Coupling of these ethyl radicals with silyloxyl radicals forms surface bound ethoxysilane that eventually will form ethanol. The product distribution of this radical process depends on the absence or presence of oxygen and may lead to the formation of ethanol together with light alkanes (methane, propane, butane and hexane) accompanied by C2 (acetaldehyde and acetic acid) and C1 (methanol, formaldehyde and formic acid) oxygenates. The presence of oxygen enhances ethane conversion and quenches the formation of alkanes by trapping alkyl radicals. It was found that micro and mesoporous silicas behave qualitatively similar with some differences in the product distribution. The most efficient material (higher conversion and higher percentage of products in the solid) was found to be Al-MCM 41. The energy consumption estimated based on a conversion of 6% on commercial beta zeolite was 2.0Gcal per mol of ethane converted that is about 3.6 times smaller than the energy consumed form methane activation through an analogous process.

Density Functional Theory Study of Methane Oxidation and Reforming on Pt(111) and Pt(211)

The activation barriers of elementary-like reactions pertaining to the oxidation and reforming of methane on Pt(111) and Pt(211) surfaces have been calculated using periodic density functional theory (DFT) calculations. We have investigated the adsorption of CHx(x=1–3)OH and CHx(x=1–3)O, all the O and OH-assisted dehydrogenation reactions of CHx(x=1–4), all the C–O bond coupling reactions forming C1 oxygenates, and their subsequent dehydrogenation. It has been found that (i) COH and CHO are the most stable C1 oxygenates on Pt(111) and Pt(211), repectively; (ii) In the presence of O on Pt(211), oxidative dehydrogenation of CH by O is more kinetically favorable than the pyrolytic CH dehydrogenation; (iii) CO can be generated by oxidation of C with a low reaction barrier on Pt(211); (iv) The reactions involving OH and the dehydrogenation of CHx(x=1–3)OH and CHx(x=1–3)O appear to be secondary reaction pathways on Pt. Based on the activation barriers, we conclude that the major reaction pathways on Pt(111) and...

Synthesis of C2 oxygenates from syngas over monodispersed SiO2 supported Rh-based catalysts: Effect of calcination temperature of SiO2

The effect of calcination temperature of the monodispersed SiO2 synthesized by the Stöber method on the catalytic performance of RhMnLi/SiO2 for CO hydrogenation to C2 oxygenates was investigated. The means of N2 adsorption-desorption, in-situ FT-IR, H2-TPD, H2-TPR, CO-TPD, and TPSR were used to characterize the physico-chemical properties of SiO2 supports and the corresponding RhMnLi/SiO2 catalysts. The results showed that the amount of surface Si-OH groups on SiO2 affects the Rh dispersion and RhMn interaction. The increase in the number of SiOH groups on the SiO2 was favorable for the Rh dispersion and CO adsorption, further enhanced the activity of the catalyst. In addition, an appropriate amount of surface SiOH groups on the SiO2 calcined at 350°C can gain a moderate RhMn interaction. This interaction is conducive to achieve the right hydrogenation ability, which is favorable for the CO insertion into metal-CHx band, and ultimately increases the selectivity of C2 oxygenates.

Structure and Catalytic Mechanism of 3-Ketosteroid-Δ4-(5α)-dehydrogenase from Rhodococcus jostii RHA1 Genome

3-Ketosteroid Δ4-(5α)-dehydrogenases (Δ4-(5α)-KSTDs) are enzymes that introduce a double bond between the C4 and C5 atoms of 3-keto-(5α)-steroids. Here we show that the ro05698 gene from Rhodococcus jostii RHA1 codes for a flavoprotein with Δ4-(5α)-KSTD activity. The 1.6 Å resolution crystal structure of the enzyme revealed three conserved residues (Tyr-319, Tyr-466, and Ser-468) in a pocket near the isoalloxazine ring system of the FAD co-factor. Site-directed mutagenesis of these residues confirmed that they are absolutely essential for catalytic activity. A crystal structure with bound product 4-androstene-3,17-dione showed that Ser-468 is in a position in which it can serve as the base abstracting the 4β-proton from the C4 atom of the substrate. Ser-468 is assisted by Tyr-319, which possibly is involved in shuttling the proton to the solvent. Tyr-466 is at hydrogen bonding distance to the C3 oxygen atom of the substrate and can stabilize the keto-enol intermediate occurring during the reaction. Finally, the FAD N5 atom is in a position to be able to abstract the 5α-hydrogen of the substrate as a hydride ion. These features fully explain the reaction catalyzed by Δ4-(5α)-KSTDs.

Structure of the GcpE (IspG)–MEcPP complex from Thermus thermophilus

Isoprenoid precursor biosynthesis occurs through the mevalonate or the methylerythritol phosphate (MEP) pathway, used i.e., by humans and by many human pathogens, respectively. In the MEP pathway, 2-C-methyl-d-erythritol-2,4-cyclo-diphosphate (MEcPP) is converted to (E)-1-hydroxy-2-methyl-but-2-enyl-4-diphosphate (HMBPP) by the iron–sulfur cluster enzyme HMBPP synthase (GcpE). The presented X-ray structure of the GcpE–MEcPP complex from Thermus thermophilus at 1.55Å resolution provides valuable information about the catalytic mechanism and for rational inhibitor design. MEcPP binding inside the TIM-barrel funnel induces a 60° rotation of the [4Fe–4S] cluster containing domain onto the TIM-barrel entrance. The apical iron of the [4Fe–4S] cluster ligates with the C3 oxygen atom of MEcPP.

Language-oriented rule-based reaction network generation and analysis: Applications of RING

Applications of RING in the generation and analysis of complex thermochemical reaction networks are presented. Automated generation and topological network analysis features in RING allow for: (a) constructing reaction networks exhaustively in a rule-based manner, (b) identifying dominant pathways in networks using estimates of kinetic parameters, (c) hypothesis and testing of mechanisms by comparing pathway results from RING with experimental data, and (d) predicting atom-efficient synthetic routes to valuable chemicals from known chemistries and commonly available chemicals. Case studies involving three chemical systems are used to demonstrate these features in RING: (a) acid-catalyzed propane aromatization, (b) glycerol and acetone dehydration on acid catalysts, and (c) C4–C9 mono-alcohols synthesis from C2 and C3 oxygenates on acid, base, and metal catalyzed chemistries. Through these case studies, we demonstrate that RING can be used to postulate mechanisms and predict likely products for a given system, thereby guiding experimentation and computational analysis.

The effect of Fe on the catalytic performance of Rh–Mn–Li/SiO2 catalyst: A DRIFTS study

Different amounts of Fe promoted Rh–Mn–Li/SiO2 catalysts were prepared and investigated for the synthesis of C2 oxygenates from syngas. The results showed that the low amount of Fe (≤ 0.1 wt.%) improved the reactivity and yield of C2+ oxygenates, but an opposite effect appeared at high content of Fe (> 0.1 wt.%). Diffuse reflectance infrared Fourier transform spectroscopy was used to probe the effects of Fe on CO adsorption and hydrogenation, and two opposing effects were evidenced. Moreover, it is proposed that the facile transformation of dicarbonyl Rh+(CO)2 into H–Rh–CO is responsible for high selectivity of C2+ oxygenates.

Mesoporous Silica Nanoparticle‐Stabilized and Manganese‐Modified Rhodium Nanoparticles as Catalysts for Highly Selective Synthesis of Ethanol and Acetaldehyde from Syngas

AbstractWell‐defined and monodispersed rhodium nanoparticles as small as approximately 2 nm were encapsulated in situ and stabilized in a mesoporous silica nanoparticle (MSN) framework during the synthesis of the mesoporous material. Although both the activity and selectivity of MSN‐encapsulated rhodium nanoparticles in CO hydrogenation could be improved by the addition of manganese oxide as expected, the carbon selectivity for C2 oxygenates (including ethanol and acetaldehyde) was unprecedentedly high at 74.5 % with a very small amount of methanol produced if rhodium nanoparticles were modified by manganese oxide with very close interaction.

Enhanced C2 oxygenate synthesis by CO hydrogenation over Rh-based catalyst supported on a novel SiO2

Abstract A novel SiO 2 synthesized by the Stober method was studied as the support for Rh–Mn–Li catalyst, and better performance for C 2 oxygenate synthesis via CO hydrogenation was achieved compared with the counterpart using commercial SiO 2 as the support. The investigation based on the catalytic performance and characterizations of the catalysts suggests that the hydroxyl–metal interaction over the catalyst supported on SiO 2 prepared by the Stober method results in a change in the CO adsorption on Rh particles, which promotes the CO dissociation and CO insertion, leading to higher CO conversion and C 2 oxygenate selectivity.

Comparison of Reaction Pathways of Ethylene Glycol, Acetaldehyde, and Acetic Acid on Tungsten Carbide and Ni-Modified Tungsten Carbide Surfaces

Selectively converting biomass-derived oxygenates to H2 or syngas (H2 and CO) is critical in the utilization of biomass to replace fossil fuels. In previous studies, monolayer (ML) Ni on a Pt substrate showed enhanced conversion and selectivity for oxygenate conversion. In the current work, tungsten monocarbide (WC) is used to support monolayer Ni, with the aim of replacing ML Ni–Pt with ML Ni–WC. C2 oxygenates with different functional groups, ethylene glycol, acetaldehyde, and acetic acid, are studied on clean WC and Ni-modified WC surfaces. For each C2 oxygenate, density functional theory (DFT) calculations reveal different binding energies on WC and Ni–WC surfaces. Parallel experimental measurements using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) confirm the different reaction pathways on the two types of surfaces, with the dominant decomposition pathway being C–O bond scission on clean WC and C–C bond cleavage on Ni-modified WC surfaces. Fu...

Conversion of Methane into C1 Oxygenates by Deep‐UV Photolysis on Solid Surfaces: Influence of the Nature of the Solid and Optimization of Photolysis Conditions

Deep-UV photolysis (either 165 or 185 nm) of surface hydroxy groups leads to homolytic O-H bond-cleavage with the generation of oxyl radicals that can initiate the room-temperature radical-chain methane activation. Whilst in the absence of oxygen, radical coupling reactions to give low-molecular-weight alkanes are observed in the gas phase, the presence of some oxygen quenches these radicals and increases the selectivity towards C1 oxygenates (methanol, formaldehyde, and formic acid species). The nature of the solid influences the efficiency of the photochemical process and the distribution between products in the gas and solid phases. Using Beta-, delaminated ITQ2 and ITQ6, and medium-pore ZSM5 zeolites, mesoporous MCM41 silicates, and non-porous TiO(2), we observed that confinement and porosity increased the proportion of C1 oxygenates adsorbed onto the solid and reduced the contribution of the gas-phase products. In addition, the presence of aluminum in the zeolite framework, which is responsible for the generation of acid sites, increased overoxidation of methanol and methoxy groups into formaldehyde and formic acids. For a given amount of methane and unchanged photolysis conditions, the conversion increased with the amount of the solid used as photocatalyst. In this way, methane conversions of up to 7% were achieved for the 185 nm photolysis of methane for 1 h with a 76 MJ mol(-1) energy consumption.

The synthesis of hydrocarbons and oxygenates during CO hydrogenation on CoCuZnO catalysts: Analysis at the site level using multiproduct SSITKA

This paper addresses the effect of component interaction in CoCuZnO catalysts on oxygenate synthesis during CO hydrogenation. Formation of the various products was investigated for the first time using in-situ multiproduct SSITKA. CO hydrogenation was carried out in a fixed-bed differential reactor at 250°C and 1.8atm. Reaction results indicate that the activities for all products decrease with the combination of Co with Cu, possibly, based on SSITKA results, due to partial blockage of the Co surface by Cu. ZnO alone, on the other hand, seems to serve primarily as a support for Co but may increase slightly the intrinsic activities for higher oxygenates. The effects of Cu and ZnO on Co, however, were not additive. The Co–Cu–ZnO combination resulted in a synergy that greatly increased selectivities for higher oxygenates by significantly decreasing the ability for hydrocarbon formation. Interestingly, the rate of synthesis for C2 oxygenates on Co/CuZnO was identical to that on Co/Al2O3 (considered by most to be only a hydrocarbon synthesis catalyst)—but without the high production rate of hydrocarbons.

Selective, Room-Temperature Transformation of Methane to C1 Oxygenates by Deep UV Photolysis over Zeolites

Methane can directly be transformed into liquid C(1) oxygenated products with selectivities above 95% at 13% conversion by deep UV photocatalysis, in the presence of H(2)O and air. Pure silica zeolites, and more specifically, beta zeolite with a large number of internal silanol groups is active and selective, while amorphous silica with no micropores is much less efficient. Irradiation produces the homolytic cleavage of surface hydroxyl groups, leading to silyloxyl radicals that will generate methyl radicals from methane. The selectivity arises from the occurrence of the reaction in a confined space restricting the mobility of the radical intermediates that will be mostly attached to the solid surface. Energy consumption of the process is in the order of 7.2 Gcal × mol(-1) that compares very favorably with the energy required for transforming methane to synthesis gas (15.96 Gcal × mol(-1)).