Catalytic conversion of carbon dioxide into dimethyl carbonate using reduced copper-cerium oxide catalysts as low as 353 K and 1.3 MPa and the reaction mechanism.
ABSTRACT: Synthesis of dimethyl carbonate (DMC) from CO2 and methanol under milder reaction conditions was performed using reduced cerium oxide catalysts and reduced copper-promoted Ce oxide catalysts. Although the conversion of methanol was low (0.005-0.11%) for 2 h of reaction, DMC was synthesized as low as 353 K and at total pressure of as low as 1.3 MPa using reduced Cu-CeO2 catalyst (0.5 wt% of Cu). The apparent activation energy was 120 kJ mol(-1) and the DMC synthesis rates were proportional to the partial pressure of CO2. An optimum amount of Cu addition to CeO2 was 0.1 wt% for DMC synthesis under the conditions at 393 K and total pressure of 1.3 MPa for 2 h (conversion of methanol: 0.15%) due to the compromise of two effects of Cu: the activation of H2 during reduction prior to the kinetic tests and the block (cover) of the surface active site. The reduction effects in H2 were monitored through the reduction of Ce(4+) sites to Ce(3+) based on the shoulder peak intensity at 5727 eV in the Ce L3-edge X-ray absorption near-edge structure (XANES). The Ce(3+) content was 10% for reduced CeO2 catalyst whereas it increased to 15% for reduced Cu-CeO2 catalyst (0.5 wt% of Cu). Moreover, the content of reduced Ce(3+) sites (10%) associated with the surface O vacancy (defect sites) decreased to 5% under CO2 at 290 K for reduced Cu-CeO2 catalyst (0.1 wt% of Cu). The adsorption step of CO2 on the defect sites might be the key step in DMC synthesis and thus the DMC synthesis rate dependence on the partial pressure of CO2 was proportional. Subsequent H atom subtraction steps from methanol at the neighboring surface Lewis base sites should combine two methoxy species to the adsorbed CO2 to form DMC, water, and restore the surface O vacancy.
Project description:A new class of TiO2-doped CeO2 nanorods was synthesized via a modified hydrothermal method, and these nanorods were first used as catalysts for the direct synthesis of dimethyl carbonate (DMC) from CO2 and CH3OH in a fixed-bed reactor. The micromorphologies and physical-chemical properties of nanorods were characterized by transmission electron microscopy, X-ray diffraction, N2 adsorption, inductively coupled plasma atomic emission spectrometry, X-ray photoelectron spectroscopy, and temperature-programmed desorption of ammonia and carbon dioxide (NH3-TPD and CO2-TPD). The effects of the TiO2 doping ratio on the catalytic performances were fully investigated. By doping TiO2, the surface acid-base sites of CeO2 nanorods can be obviously promoted and the catalytic activity can be raised evidently. Ti0.04Ce0.96O2 nanorod catalysts exhibited remarkably high activity with a methanol conversion of 5.38% with DMC selectivity of 83.1%. Furthermore, kinetic and mechanistic investigations based on the initial rate method were conducted. Over the Ti0.04Ce0.96O2 nanorod catalyst, the apparent activation energy of the reaction was 46.3 kJ/mol. The reaction rate law was determined to be of positive first-order to the CO2 concentration and the catalyst loading amount. These results were practically identical with the prediction of the Langmuir-Hinshelwood mechanism in which the steps of CO2 adsorption and activation are considered as rate-determining steps.
Project description:A ceria (CeO2) promoted Cu-Ni bimetallic catalyst supported on SiO2 (Cu-Ni/CeO2-SiO2) was prepared and evaluated for catalytic hydrodeoxygenation (HDO) of vanillin. Silica supported monometallic Cu and Ni catalysts and bimetallic Cu-Ni catalyst (Cu/SiO2, Ni/SiO2, and Cu-Ni/SiO2), without a ceria promoter, were also synthesized and tested for the same application. The highest conversion of vanillin was achieved with the Cu-Ni/CeO2-SiO2 catalyst. Vanillyl alcohol was the sole product in the initial 2 h, followed by the formation of 2-methoxy-4-methylphenol, which was observed. Characterization of the synthesized catalysts revealed the presence of overlapping crystalline phases of CuO, NiO, and CeO2 on the Cu-Ni/CeO2-SiO2 surface. We extended our study to find out the results of using CeO2 as the support of the Cu-Ni bimetallic catalyst (Cu-Ni/CeO2). Partial incorporation of Cu and Ni cations into the ceria lattice took place, leading to the decrease of specific surface area and a concomitant compromise in the conversion. In the case of the Cu-Ni/CeO2-SiO2 catalyst, the higher conversion was accredited to the facile formation of Cu+ active centers by the synergistic interaction between Ce+4/Ce+3 and Cu+2/Cu+ redox couples and the incorporation of oxygen vacancies on the catalyst surface.
Project description:Graphene has attracted attention because of its interesting properties in catalyst applications including as a catalyst support; however, it is known that the graphene can be restacked, forming a graphite-like structure that leads to poor specific surface area. Hence, the high-porosity graphene aerogel was used as a Cu–Ni catalyst support to produce dimethyl carbonate (DMC) from carbon dioxide and methanol. In this work, we have introduced a new synthesis route, which can improve the dispersion of metal particles on the graphene aerogel support. Cu–Ni/graphene aerogel catalysts were synthesized by a two-step procedure: forming Cu–Ni/graphene aerogel catalysts via hydrothermal reduction and then Cu–Ni loading by incipient wetness impregnation. It is found that the catalyst prepared by the two-step procedure exhibits higher DMC yield (25%) and MeOH conversion (18.5%) than those of Cu–Ni loading only by an incipient wetness impregnation method. The results prove that this new synthesis route can improve the performance of Cu–Ni/graphene aerogel catalysts for DMC production.
Project description:Enhancing the intrinsic activity and space time yield of Cu based heterogeneous methanol synthesis catalysts through CO2 hydrogenation is one of the major topics in CO2 conversion into value-added liquid fuels and chemicals. Here we report inverse ZrO2/Cu catalysts with a tunable Zr/Cu ratio have been prepared via an oxalate co-precipitation method, showing excellent performance for CO2 hydrogenation to methanol. Under optimal condition, the catalyst composed by 10% of ZrO2 supported over 90% of Cu exhibits the highest mass-specific methanol formation rate of 524 gMeOHkgcat-1h-1 at 220?°C, 3.3 times higher than the activity of traditional Cu/ZrO2 catalysts (159 gMeOHkgcat-1h-1). In situ XRD-PDF, XAFS and AP-XPS structural studies reveal that the inverse ZrO2/Cu catalysts are composed of islands of partially reduced 1-2?nm amorphous ZrO2 supported over metallic Cu particles. The ZrO2 islands are highly active for the CO2 activation. Meanwhile, an intermediate of formate adsorbed on the Cu at 1350?cm-1 is discovered by the in situ DRIFTS. This formate intermediate exhibits fast hydrogenation conversion to methoxy. The activation of CO2 and hydrogenation of all the surface oxygenate intermediates are significantly accelerated over the inverse ZrO2/Cu configuration, accounting for the excellent methanol formation activity observed.
Project description:Silica-supported model copper catalysts were prepared by supporting bis(1,5-diphenyl-1,3,5-pentanetrionato)dicopper(II), Cu2(dba)2, on Cab-O-Sil by a batch impregnation technique. This metal complex showed a strong affinity for the silica support, developing monolayer coverages near the value predicted from a consideration of the size and shape of the planar metal complex (2.6 wt % Cu). The supported catalysts were subsequently activated by decomposing the organic ligands at 400 °C in air followed by reduction with 2% H2/He at 250 °C. One sample was prepared having a loading of 3.70 wt % Cu2(dba)2/silica catalyst, and it was examined for the methanol synthesis reaction under the following conditions: 250 °C with an equimolar gas mixture of CO and H2 in a high-pressure batch reactor. Kinetic data over the model catalyst were fit to a rate equation, second order in the limiting reactant (H2), with a pseudo-second-order rate constant k 2[CO]o[H2]o = 0.0957 [h-g total Cu]-1. A control experiment using a commercial catalyst, Cu/ZnO/Al2O3 with a copper loading of 41.20 wt %, showed a value of k 2[CO]o[H2]o = 0.793 [h-g total Cu]-1. A fresh sample of Cu2(dba)2/silica was examined for methanol decomposition reaction at 220 °C. The model catalyst shows a methanol decomposition first-order rate constant greater than that of the commercial Cu/ZnO/Al2O3catalyst: 1.59 × 10-1 [min-g total Cu]-1 versus 9.6 × 10-3 [min-g total Cu]-1. X-ray diffraction analyzes confirm the presence of CuO particles in both catalysts after calcinations. Copper metal particles were found in both catalysts (fractional Cu dispersions were 0.11 and 0.16 on commercial and model catalysts, respectively) after the reduced catalysts were used in both the methanol synthesis and decomposition reactions. Using the values of copper dispersion found in these samples, we recalculated the rate constants for the two reactions per unit surface copper. These refined rate constants showed the same trends as those reported per total amount of Cu. One role of the promoter(s) in the commercial catalyst is the inhibition of the methanol decomposition reaction, thus allowing higher MeOH synthesis reaction rates in those regimes not controlled by thermodynamics.
Project description:Monometallic cerium layered double hydroxides (Ce-LDH) supports were successfully synthesized by a homogeneous alkalization route driven by hexamethylenetetramine (HMT). The formation of the Ce-LDH was confirmed and its structural and compositional properties studied by XRD, SEM, XPS, iodometric analyses and TGA. HT-XRD, N2-sorption and XRF analyses revealed that by increasing the calcination temperature from 200 to 800 °C, the Ce-LDH material transforms to ceria (CeO2) in four distinct phases, i.e., the loss of intramolecular water, dehydroxylation, removal of nitrate groups and removal of sulfate groups. When loaded with 2.5 wt% palladium (Pd) and 2.5 wt% nickel (Ni) and calcined at 500 °C, the PdNi-Ce-LDH-derived catalysts strongly outperform the PdNi-CeO2 benchmark catalyst in terms of conversion as well as selectivity for the hydrogenolysis of benzyl phenyl ether (BPE), a model compound for the ?-O-4 ether linkage in lignin. The PdNi-Ce-LDH catalysts showed full selectivity towards phenol and toluene while the PdNi-CeO2 catalysts showed additional oxidation of toluene to benzoic acid. The highest BPE conversion was observed with the PdNi-Ce-LDH catalyst calcined at 600 °C, which could be related to an optimum in morphological and compositional characteristics of the support.
Project description:The effects of V and Ce concentrations (each varying in the 0-100% range) in vanadia-ceria multiphase systems are investigated for synthesis gas production via thermochemical redox cycles of CO2 and H2O splitting coupled to methane partial oxidation reactions. The oxidation of prepared oxygen carriers is performed by separate and sequential CO2 and H2O splitting reactions. Structural and chemical analyses of the mixed-metal oxides revealed important information about the Ce and V interactions affecting their crystal phases and redox characteristics. Pure CeO2 and pure V2O5 are found to offer the lowest and highest oxygen exchange capacities and syngas production performance, respectively. The mixed-oxide systems provide a balanced performance: their oxygen exchange capacity is up to 5 times higher than that of pure CeO2 while decreasing the extent of methane cracking. The addition of 25% V to CeO2 results in an optimum mixture of CeO2 and CeVO4 for enhanced CO2 and H2O splitting. At higher V concentrations, cyclic carbide formation and oxidation result in a syngas yield higher than that for pure CeO2.
Project description:Direct methane conversion (DMC) to high value-added products is of significant importance for the effective utilization of CH4 to combat the energy crisis. However, there are ongoing challenges in DMC associated with the selective C-H activation of CH4. The quest for high-efficiency catalysts for this process is limited by the current drawbacks including poor activity and low selectivity. Here we show a cerium dioxide (CeO2) nanowires supported rhodium (Rh) single-atom (SAs Rh-CeO2 NWs) that can serve as a high-efficiency catalyst for DMC to oxygenates (i.e., CH3OH and CH3OOH) under mild conditions. Compared to Rh/CeO2 nanowires (Rh clusters) prepared by a conventional wet-impregnation method, CeO2 nanowires supported Rh single-atom exhibits 6.5 times higher of the oxygenates yield (1231.7 vs. 189.4?mmol?gRh-1?h-1), which largely outperforms that of the reported catalysts in the same class. This work demonstrates a highly efficient DMC process and promotes the research on Rh single-atom catalysts in heterogeneous catalysis.
Project description:A facile way was developed to fabricate yolk-shell composites with tunable Cu cores encapsulated within hollow carbon spheres (Cu@C) with an average diameter about 210 nm and cavity size about 80 nm. During pyrolysis, the confined nanospace of hollow cavity ensures that the nucleation-and-growth process of Cu nanocrystals take place exclusively inside the cavities. The size of Cu cores can be easily tuned from 30 to 55 nm by varying the copper salt concentration. By deliberately creating shell porosity through KOH chemical activation, at an optimized KOH/HCS mass ratio of 1/4, the catalytic performance for the oxidative carbonylation of methanol to dimethyl carbonate (DMC) of the activated sample is enhanced remarkably with TOF up to 8.6 h-1 at methanol conversion of 17.1%. The activated yolk-shell catalyst shows promising catalytic properties involving the reusability with slight loss of catalytic activity and negligible leaching of activated components even after seven recycles, which is beneficial to the implementation of clean production for the eco-friendly chemical DMC thoroughly.
Project description:By integrating redox self-assembly and redox etching processes, we report a general one-pot strategy for the synthesis of Au@multi-M x O y (M = Co, Ce, Fe, and Sn) yolk@shell nanospheres. Without any additional protecting molecule or reductant, the whole reaction is a clean redox process that happens among the inorganic metal salts in an alkaline aqueous solution. By using this method, Au@Co3O4/CeO2 (Au@Co-Ce), Au@Co3O4/Fe2O3 (Au@Co-Fe), and Au@CeO2/SnO2 (Au@Ce-Sn) yolk@shell nanospheres with binary oxides as shells, Au@Co3O4/CeO2/Fe2O3 (Au@Co-Ce-Fe) yolk@shell nanospheres with ternary oxides as shells and Au@Co3O4/CeO2/Fe2O3/SnO2 (Au@Co-Ce-Fe-Sn) yolk@shell nanospheres with quaternary oxides as shells can be obtained. Subsequently, the catalytic CO oxidation was selected as the catalytic model, and the Au@Co-Ce system was chosen as the catalyst. It was found that the catalytic activity of Au@Co-Ce yolk@shell nanospheres can be optimized by altering the relative proportion of Co and Ce oxides.