Hydrogenation of cinnamaldehyde over Pt/RHCs in supercritical carbon dioxide – Influence of support pretreatment and phase behavior

The selective hydrogenation of cinnamaldehyde (CAL) was investigated using rice husk-based porous carbon (RHCs) supported platinum catalysts in supercritical carbon dioxide (scCO₂). The effects of surface chemistry treatment of the support and the reaction phase behavior have been examined. The Pt/H–RHCs (HNO₃-pretreated) was more active for CAL hydrogenation compared with Pt/NH₃–RHCs (NH₃ · H₂O-pretreated). The Pt/RHCs catalyst exhibited a higher selectivity to cinnamyl alcohol (COL) compared with commercial catalyst of Pt/C, which is relative to the micro–mesoporosity structure of the RHCs. Under the reaction conditions, the product distribution changes markedly with the variation of the phase behavior. A higher selectivity to hydrocinnamyl alcohol of 90% was obtained in two-phase reaction system of CO₂–riched gas phase (CO₂, H₂, dissolved cinnamaldehyde) and solid phase (catalyst); while, the main product changed to cinnamyl alcohol with a selectivity of 88% in three-phase reaction system of CO₂–riched gas phase (CO₂, H₂, dissolved cinnamaldehyde), cinnamaldehyde–riched liquid phase (cinnamaldehyde, dissolved CO₂ and H₂) and solid phase (catalyst). It is attributed to the influence of the molecule interactions and the substrate concentration effects.     

Catalytic Performance of Pt/ZrO2 and Pt/Ce-ZrO2 Catalysts on CO2 Reforming of CH4 Coupled with Steam Reforming or Under High Pressure

CO₂ reforming of methane was performed on Pt/ZrO₂ and Pt/Ce-ZrO₂ catalysts at 1073K under different reactions conditions: (i) atmospheric pressure and CH₄:CO₂ ratio of 1:1 and 2:1; (ii) in the presence of water and CH₄:CO₂ ratio of 2:1; (iii) under pressure (105 and 190 psig) and CH₄:CO₂ ratio of 2:1. The Pt supported on ceria-promoted ZrO₂ catalyst was more stable than the Pt/ZrO₂ catalyst under all reaction conditions. We ascribe this higher stability to the higher density of oxygen vacancies on the promoted support, which favors the cleaning mechanism of the metal particle. The increase of either the CH₄:CO₂ ratio or total pressure causes a decrease in activity for both catalysts, because under either case the rate of methane decomposition becomes higher than the rate of oxygen transfer. The Pt/Ce-ZrO₂ catalyst was always more stable than the Pt/ZrO₂ catalyst, demonstrating the important role of the support on this reaction.     

Catalyst Preparation Using Supercritical Carbon Dioxide: Preparation of Rh/FSM-16 Catalysts and Their Catalytic Performances in Butane Hydrogenolysis Reaction

Supported Rh catalysts on FSM-16 were prepared by treating FSM-16, impregnated with [Rh(OAc)₂]₂ in supercritical carbon dioxide at 398 K and 30.3 MPa, followed by calcination and hydrogen reduction. The resulting Rh/FSM-16 catalysts were characterized by CO chemisorption, XRD, TEM, FTIR and EXAFS, and catalytic performances of the Rh/FSM-16 were tested in butane hydrogenolysis reaction. It is demonstrated that highly dispersed Rh particles are obtained by the supercritical CO₂ treatment. In the hydrogenolysis reactions, the supercritical CO₂-treated catalyst showed higher conversions and ethane formation.     

Preferential CO oxidation over supported Pt catalysts

Preferential CO oxidation reaction has been carried out at a gas hourly space velocity of 46,129 h^?1 over supported Pt catalysts prepared by an incipient wetness impregnation method. Al2O3, MgO-Al₂O₃ (MgO=30 wt% and 70 wt%) and MgO were employed as supports for the target reaction. 1 wt% Pt/Al₂O₃ catalyst exhibited very high performance (X _CO >90% at 175 ^°C for 100 h) in the reformate gases containing CO₂ under severe conditions. This result is mainly due to the highest Pt dispersion, easier reducibility of PtO _x , and easier electron transfer of metallic Pt. In addition, 1 wt% Pt/Al₂O₃ catalyst was also tested in the reformate gases with both CO₂ and H₂O to evaluate under realistic condition.     

Performance evaluation under supercritical conditions of Fe–Mn–Cu–K composite catalysts synthesized by supercritical methods

A series of nanosized Fe–Mn–Cu–K composite catalysts was prepared by a supercritical combined technology. The nanosized catalysts were characterized by means of XRD, TEM and BET techniques, and tested for catalytic performance under Fischer–Tropsch synthesis (FTS) reaction conditions. The catalyst synthesized by the supercritical combined technology has some excellent properties. Additionally, the drying and crystallization of nanosize catalyst could be achieved simultaneously by this supercritical combined technology. The addition of Mn, Cu and K promoters can improve the catalytic performance properties of the catalyst, including lower CH₄ and CO₂ formation rates, and higher production rates of desired light-olefin. The optimal performance with a 95.7% CO conversion and a 46.5% light-olefin yield was obtained by using a catalyst component of Fe/Mn/Cu/K = 60:25:10:8.5. In summary, optimal catalytic performance was obtained by using the nanosized catalyst under supercritical reaction conditions, resulting in higher CO conversion, less byproduct CO₂ formation, and a higher yield of light-olefin.     

Ethanol decomposition and steam reforming of ethanol over CeZrO2 and Pt/CeZrO2 catalyst: Reaction mechanism and deactivation

The catalyst performance and the deactivation profiles of CeZrO₂ and Pt/CeZrO₂ were evaluated for the ethanol decomposition and steam reforming of ethanol reactions at 773 K. Unpromoted CeZrO₂ was quite stable whereas Pt/CeZrO₂ catalyst deactivated for all feed compositions studied. A reaction mechanism was proposed based on diffuse reflectance infrared spectroscopy analyses carried out under reaction conditions. The decomposition of the dehydrogenated and the acetate species is facilitated by the presence of the metal and resulted in high selectivity to hydrogen, methane, CO and CO₂ over Pt/CeZrO₂. Diffuse reflectance infrared spectroscopy, transmission electron microscopy and temperature programmed oxidation and desorption analyses indicate that the loss of the Pt-support synergy leads to a buildup of carbonaceous residue, which is the likely reason of the deactivation of Pt/CeZrO₂. In addition, once the interfacial boundary is lost, the demethanation of acetate becomes hindered, resulting in an increase in acetaldehyde selectivity with time onstream.     

Electrooxidation of acetaldehyde on carbon-supported Pt, PtRu and Pt3Sn and unsupported PtRu0.2 catalysts: A quantitative DEMS study

The oxidation of acetaldehyde on carbon supported Pt/Vulcan, PtRu/Vulcan and Pt₃Sn/Vulcan nanoparticle catalysts and, for comparison, on polycrystalline Pt and on an unsupported PtRu_0.2 catalyst, was investigated under continuous reaction and continuous electrolyte flow conditions, employing electrochemical and quantitative differential electrochemical mass spectroscopy (DEMS) measurements. Product distribution and the effects of reaction potential and reactant concentration were investigated by potentiodynamic and potentiostatic measurements. Reaction transients, following both the Faradaic current as well as the CO₂ related mass spectrometric intensity, revealed a very small current efficiency for CO₂ formation of a few percent for 0.1 m acetaldehyde bulk oxidation under steady-state conditions on all three catalysts, the dominant oxidation product being acetic acid. Pt alloy catalysts showed a higher activity than Pt/Vulcan at lower potential (0.51 V), but do not lead to a better selectivity for complete oxidation to CO₂. C–C bond breaking is rate limiting for complete oxidation at potentials with significant oxidation rates for all three catalysts. The data agree with a parallel pathway reaction mechanism, with formation and subsequent oxidation of CO_ad and CH _{x, ad} species in the one pathway and partial oxidation to acetic acid in the other pathway, with the latter pathway being, by far, dominant under present reaction conditions.     

Characteristics of adsorbed CO and CH3OH oxidation reactions for complex Pt/Ru catalyst systems

Pt/Ru powder catalysts of the same nominal Pt to Ru composition were prepared using a range of methods resulting in different catalyst properties. Two PtRu alloy catalysts were prepared, one of which has essentially the same surface and bulk Pt to Ru composition, while the second catalyst is surface enriched with Ru. Two powders consisting of non-alloyed Pt phases and surfaces enriched with Ru were also prepared. The oxidation state of the surface Ru of the latter two catalysts is mainly metallic Ru or Ru-oxides. The catalyst consisting of Ru-oxides was formed at 500 °C. Part of this catalyst was then reduced in a H₂ atmosphere under “mild” conditions, thus catalyst properties such as particle size are not changed, as they are locked in during previous high temperature treatment. The oxidation kinetics of adsorbed CO (CO_ads) and solution CH₃OH were studied and compared to the Ru ad-metal state and Pt to Ru site distribution of the as-prepared catalysts. The kinetics of the CO_ads oxidation reaction were observed to be slower for the catalyst containing Ru-oxides as opposed to mainly Ru metal. The CH₃OH oxidation activities measured per Pt surface area, i.e., the catalytic activities are better (by ca. seven times) for the alloy catalysts than the non-alloyed Pt/Ru catalysts. The latter two catalysts showed essentially the same catalytic CH₃OH oxidation activities, i.e., independent of the Ru ad-metal oxidation state of the as-prepared catalysts. Furthermore, it is shown that CO_ads oxidation experiments can be used to extract characteristics that allow the comparison of catalytic activities for the CO_ads oxidation reaction and Pt to Ru site distribution for complex catalyst systems.     

Hydrogenation of phenol in scCO2 over carbon nanofiber supported Rh catalyst

A catalyst of Rh nanoparticles supported on a carbon nanofiber, 5 wt.% Rh/CNF, with an average size of 2–3 nm has been prepared by a method of incipient wetness impregnation. The catalyst presented a high activity in the ring hydrogenation of phenol in a medium of supercritical CO₂ (scCO₂) at a low temperature of 323 K. The presence of compressed CO₂ retards hydrogenation of cyclohexanone to cyclohexanol under the reaction conditions used, and this is beneficial for the formation of cyclohexanone, increasing the selectivity to cyclohexanone. But the selectivity to cyclohexanone is very low at the completion of reaction in the absence of CO₂, at low CO₂ pressures, and in the presence of pressurized N₂ instead of CO₂. That is, high selectivity to cyclohexanone can be achieved with CO₂ species at higher pressures but not with the application of an inert hydrostatic pressure on the liquid substrate phase.     

Magnetically separable Pt catalyst for asymmetric hydrogenation

A magnetic Pt/SiO₂/Fe₃O₄ catalyst consisting of chirally modified platinum supported on silica coated magnetite nanoparticles was prepared using an easy synthetic route and successfully applied for the enantioselective hydrogenation of various activated ketones. The magnetic catalyst modified with cinchonidine showed a catalytic performance (activity, enantioselectivity) in the asymmetric hydrogenation of various activated ketones in toluene comparable to the best known Pt/alumina catalyst used for these reactions. The novel catalyst can be easily separated from the reaction solution by applying an external magnetic field and recycled several times with almost complete retention of activity and enantioselectivity.     

CO2 Hydrogenation Studies on Co and CoPt Bimetallic Nanoparticles Under Reaction Conditions Using TEM, XPS and NEXAFS

Cobalt and platinum?Ccobalt bimetallic alloy nanoparticles of uniform size distribution where prepared and supported on MCF-17 to produce a controlled and well-characterized model catalyst which was studied under reaction conditions during CO₂ hydrogenation. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to elucidate the oxidation state of the catalyst under reaction conditions while the effect of reducing H₂ gas on the composition and structure of the bimetallic PtCo nanoparticles was measured using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and environmental transmission electron microscopy (ETEM). NEXAFS indicates that Pt aids the reduction of Co to its metallic state under relevant reaction conditions, while AP-XPS and ETEM indicate that Pt is enriched at the surface by exchange with subsurface layers which become Pt deficient??in agreement with the ??Pt-like?? selectivity seen during catalytic testing of these materials.     

Role of the modifier in the enantioselective hydrogenation of ethyl pyruvate over Pt/Al2O3 catalyst

The time dependence of the conversion and enantioselectivity during the hydrogenation of ethyl pyruvate has been studied over an industrial Pt/Al₂O₃ catalyst. Various cinchona alkaloids were used as modifier and two different modes were applied for their introduction into the reaction system. The dependence of the enantioselectivity on conversion is strongly influenced by the mode of introduction and the structure of the modifier used. The conversion dependence of the enantioselectivity is attributed to the chemical transformations of the parent alkaloid observed under hydrogenation conditions. Experimental evidence is shown for the dynamic nature of the interaction between the modifier and the catalyst.On leave from the Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest.     

Ultra-rapid synthesis of syngas by the catalytic reforming of methane enhanced byin-situ heat supply through combustion

Development in highly active catalysts for the reforming of methane with CO₂ and partial oxidation of methane was conducted to produce hydrogen and carbon monoxide with high reaction rates. An Ni-based four-components catalyst, Ni-Ce₂O₃-Pt-Rh, supported on an alumina wash-coated ceramic fiber in a plate shape was suitable for the objective reaction. By combining the catalytic combustion of ethane or propane, methane conversion was markedly enhanced, and a high space-time yield of syngas, 25,000 mol/l·h was obtained at a catalyst temperature of 700 ‡C or furnace temperature of 500 ‡C. The extraordinary high space-time yield of syngas was also confirmed even under the very rapid flow rate conditions as a contact time of 3 m-sec by using a monolithic shape of catalyst bed without back pressure.     

Electrospray Ionization–Mass Spectrometry in the Enantioselective Hydrogenation of Ethyl Pyruvate Catalyzed by Dihydrocinchonidine Modified Pt/Al2O3 in Acetic Acid

The enantioselective hydrogenation of ethyl pyruvate (EtPy) on Pt–alumina (E 4759), Pt black, and Pt black+alumina (mixture) catalysts modified by dihydrocinchonidine (DHCD) in acetic acid was studied by electrospray ionization–mass spectrometry (ESI–MS). Application of the ESI–MS technique led to the recognition of a novel-type compound, O⁺[Al(OAc)₂]₃ (oxonium ions). The effect of the DHCD concentration, temperature, and oxonium cations on the reaction rate and the enantioselectivity has been studied. Using the Engelhard 4759 catalyst in acetic acid under mild experimental conditions (room temperature, hydrogen pressure 1 bar, DHCD concentration 0.01 mM/L) an optical yield of 92% can be achieved. The high enantioselectivity is accompanied by the following turnovers: EtPy/DHCD>43,000, EtPy/Pt_surface>1000, TOF=1–2 s⁻¹, and DHCD/Pt_surface ratio=0.0072. The enantioselectivity reducing factor is identified by ESI–MS as the gradual hydrogenation of the quinoline skeleton of DHCD that becomes more pronounced with increasing temperature and hydrogenation time. The discovery of oxonium cations, the extremely low DHCD/Pt_surface ratio, and the new data obtained by the Pt black + alumina mixture made possible an interpretation of the mechanism of the heterogeneous enantioselective hydrogenation of α-ketoesters.     

Improvement of Pt/ZrO2 by CeO2 for high pressure CH4/CO2 reforming

CH₄/CO₂ reforming over Pt/ZrO₂, Pt/CeO₂ and Pt/ZrO₂ with CeO₂ was investigated at 2 MPa. Pt/ZrO₂, which shows stable activity under 0.1 MPa, and Pt/CeO₂ showed gradual deactivation with time at the high pressure. The deactivation was suppressed drastically on Pt/ZrO₂ with CeO₂ prepared by different impregnation order (co-impregnation of Pt and CeO₂ on ZrO₂, and consecutive impregnation of Pt and CeO₂ on ZrO₂). The amount of coke deposition was found insignificant and similar among all the catalysts (including Pt/ZrO₂ and Pt/CeO₂). Catalytic activity after the reaction for 24 h was in agreement with Pt particle size after the reaction for same period, indicating that the difference of the catalytic stability is mainly dependent on the extent of Pt aggregation through catalyst preparation, H₂ reduction, and the CH₄/CO₂ reforming. Pt aggregation and the amount of coke deposition were least pronounced on (Pt–Ce)/ZrO₂ prepared by impregnation of CeO₂ on Pt/ZrO₂ and the catalyst showed highest stability.     

Heterogeneously Catalyzed Hydrogenation of Supercritical CO2 to Methanol

Interest in energy storage technologies is still increasing in times of excess of electricity generated by wind farms or solar plants. A key part of the energy storage technologies plays the efficient conversion of H₂ and CO₂ from renewable resources. Here, the process conditions for continuous catalytic hydrogenation of CO₂ to CH₃OH under supercritical conditions over lab‐synthesized Cu/ZnO/Al₂O₃ catalysts were investigated. A possible in situ phase separation of reaction products within the reactor due to the higher densities of the reaction mixture by the higher pressure could affect the kinetics and simplify downstream processing. The combination of thermodynamic studies and catalytic performance tests for CO₂ hydrogenation under supercritical conditions is discussed and a process concept is presented.     

Enantiodifferentiation in asymmetric sonochemical hydrogenations

The effect of sonochemical pretreatment on the enantioselectivity of Pt/Al₂O₃–cinchonidine-catalyzed ethyl pyruvate hydrogenation was studied at different hydrogen pressures in various solvents, mainly in acetic acid. The sonochemical pretreatment of a commercial Pt/Al₂O₃–cinchonidine catalytic system in acetic acid resulted in enhanced enantioselectivity providing excellent ee values (97% ee) under mild and widely varied experimental conditions. Moreover, the application of ultrasonics provides a possibility of the catalyst recycling without regeneration. The catalyst was tested by transmission electron microscopy to determine the effect of the sonication on the metal particle size morphology. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the contribution of X-ray absorption spectroscopy to explore structure and activity relations of Pt/ZrO2 catalysts for CO2/CH4 reforming

X-ray absorption spectroscopy (XAS) has proven to be a very useful technique in characterizing metal-based catalysts exposed to extreme operating conditions. The technique allows in situ evaluation of structural parameters (XAFS) and electronic properties (XANES). The elucidation of the nature and state of Pt-based catalysts in dry reforming of methane with carbon dioxide is presented as case study to show the contribution and potential of XAS to explore property/performance relationships for heterogeneous catalysts. Pt/ZrO₂ is an active and stable catalyst for the reaction between CH₄ and CO₂ to synthesis gas (H₂/CO). The activity and stability of the catalyst is strongly influenced by the catalyst pretreatment (calcination/reduction). The combination of hydrogen chemisorption, IR spectroscopy, XPS and XAS is shown to be suitable to track the changes of the state of the catalyst. In particular, it will be demonstrated, how XAFS helped to correctly attribute variations in the chemisorptive properties of Pt/ZrO₂ after severe temperature treatment to partial and reversible decoration of the small Pt particles with fragments of the oxide support. In situ tracking of the reduction of the catalysts by XANES additionally helped to semiquantitatively assess the partial reduction of the ZrO₂. Finally, XANES helped to demonstrate that CO₂ exposure under these severe conditions did not lead to detectable levels of surface oxidation of Pt. Based on XANES, IR spectroscopy and kinetic measurements it is concluded that in dry reforming activation of methane occurs on Pt, while CO₂ is activated on the support and the two entities react at the metal–support interface. This revised version was published online in June 2006 with corrections to the Cover Date.     

Significant Improvement in Activity and Stability of Pt/TiO2 Catalyst for Water Gas Shift Reaction Via Controlling the Amount of Na Addition

Na promoted Pt/TiO₂ catalysts have been studied under high severity, near equilibrium, conditions for use as a single stage WGS catalyst. Addition of 3 wt% Na to a 1 wt% Pt/TiO₂ catalyst has been found to improve water gas shift activity significantly compared to Pt/TiO₂, Pt/CeO₂, and Pt–Re/TiO₂ catalysts. This catalyst is stable when the reaction temperature is higher than 250 °C. Deactivation occurred when the reaction temperature was lower than 250 °C, however, returning the temperature to higher than 250 °C fully recovered activity. TEM observations revealed that addition of Na inhibited Pt particle sintering. These results suggest that Na promoted Pt/TiO₂ is a promising single stage water gas shift catalyst for small scale hydrogen production.     

Transformation of cinchonidine during the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate

The transformation of cinchonidine (CIN) during the enantioselective hydrogenation of ethyl pyruvate on Pt/Al₂O₃ was investigated. Under reaction conditions, cinchonidine was hydrogenated consecutively first at the quinuclidine part and then at the quinuline ring. However, the hydrogenation of the aromatic ring system, causing a drop in activity and enantioselectivity, did not begin until complete conversion of ethyl pyruvate occurred. By stopping the reaction at a conversion of approximately 70%, the activity and enantioselectivity of the catalyst could be maintained constant at high values for more than 10 repeated uses without the addition of fresh modifier at the beginning of each new hydrogenation cycle.