Cu2+ and Zn2+ ion-exchange locations in mordenite (MOR) were evaluated using infrared spectroscopy, pore-size distribution, and temperature-programmed reduction. Isolated copper ions were the most abundant ion-exchanged species, as detected by UV-vis spectroscopy, in addition to oxide nanoparticles, with no presence of binuclear species, which was assigned to a low copper loading of 0.3 Cu/Al. The characterization revealed that only zinc could exchange in 8-membered rings. Hartree-Fock modeling confirmed copper exchange into 12-membered rings involving at least one T1 atom, and zinc exchange in T4 sites and in 8-membered structures, including T3 sites. Copper ion exchange did not offer improvement in the dimethyl ether carbonylation rate or selectivity over acidic mordenite. Zinc ion exchange led to the selectivity and stability improvement with some loss of activity. This work contributes to the understanding of acid and metal site contribution to DME carbonylation and contributes to the understanding for Cu2+ and Zn2+ ion-exchange locations in MOR with a low metal/Al loading (<0.2). © 2018 Elsevier, reprinted with permission.
The study focuses on the determination of metal sites availability in silica-encapsulated Pd catalysts (Pd@SiO2). Existing synthetic methods are modified to achieve high metal loading (up to 6 wt.%) and porosity (surface area of 700 m2/g) while maintaining the original Pd nanoparticle size of 8 nm. Two synthesis schemes are used for encapsulating Pd NPs and the resulting catalysts are assessed in lean methane combustion at up to 823 K. Application of poly(vinylpyrrolidone) (PVP) as a Pd particle stabilizer and a potential porogen alone is concluded to be inadvisable as it results in catalysts with surface area of 70 m2/g that show extremely low activity due to Pd inaccessibility. The high-surface area materials (700 m2/g) prepared via a separate introduction of PVP and an additional porogen (cetyltrimethylammonium bromide, CTAB) are active and exhibit the same turnover frequencies as the traditional catalysts but require smaller reactor sizes because of the high metal loading. However, 2/3 of the Pd nanoparticle surface is blocked by the shell material even in the highly porous catalysts. The silica-encapsulated catalysts, thus, offer advantages of high mass-based activity and sintering resistance of the metal cores, but their high porosity must be ensured for efficient mass transfer by the addition of a porogen (such as CTAB) during the Stöber process. Above a certain limit, the increased amount of the porogen does not improve the metal accessibility and only leads to the precious metal loss during synthesis. © 2018 Reproduced by permission of the Royal Society of Chemistry.
The addition of platinum to palladium is known to provide bimetallic catalysts that are relatively active in lean methane combustion in the presence of water at temperatures below 773 K, which is of practical interest for exhaust treatment from natural gas vehicles. The study provides an insight into the wet lean methane combustion mechanism on the Pd-Pt/Al2O3 catalyst via in situ X-ray absorption spectroscopy studies at temperatures 473–873 K. The presence of water leads to the increased fraction of metallic Pd due to the lack of surface oxygen. The fraction of metallic Pd drops as the temperature increases. Oxygen deficit results in Pt atoms available for methane dissociation, which does not occur in the dry methane-lean feed in which oxygen poisons Pt. © 2017 Elsevier, reprinted with permission.
This computational study addresses oil sands separation under microwave irradiation. Electric field distribution and microwave energy dissipation were analyzed at the level of sand particles and bitumen. The simplified model does not include water or gas present in the oil sands. The microwave power dissipated in bitumen was found to be about 32 times that of the sand particle. In order to investigate the feasibility of heating the bitumen phase preferentially and sustaining a thermal gradient between the bitumen phase and the solid phase, thereby reducing the thermal losses to the solid phase, time scales were estimated using several different scenarios. The small size of the particles in the solid phase caused the time needed to reach thermal equilibrium between phases to be extremely short, in the order of 10 to 100 ms. To achieve separation times shorter than these, pressures up to 100 GPa would have to be sustained across the oil sands layer. Alternatively, buoyancy-driven separation by settling would require accelerations in the order of 10^(5) g. Therefore, the heating and separation of bitumen within the thermal conduction time scales seem to be theoretically possible but associated with high technological challenges in their implementation. © 2016 Canadian Society for Chemical Engineering.
Active site “spoiler”: Palladium–platinum bimetallic catalysts supported on alumina are studied in dry lean methane combustion at low temperature. Platinum addition decreases the catalyst activity by reducing active palladium oxide to inactive palladium(0):
Palladium–platinum bimetallic catalysts supported on alumina with palladium/platinum molar ratios ranging from 0.25 to 4 are studied in dry lean methane combustion in the temperature range of 200 to 500 °C. Platinum addition decreases the catalyst activity, which cannot be explained by the decrease in dispersion or the structure sensitivity of the reaction. In situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy measurements have been conducted for monometallic Pd, Pt, and 2:1 Pd−Pt catalysts. Monometallic palladium is fully oxidized in the full temperature range, whereas platinum addition promotes palladium reduction, even in a reactive oxidizing environment. The Pd/PdO weight ratio in bimetallic Pd−Pt 2:1 catalysts decreases from 98/2 to 10/90 in the 200–500 °C temperature range under the reaction conditions. Thus, platinum promotes the formation of the reduced palladium phase with a significantly lower activity than that of oxidized palladium. The study sheds light on the effect of platinum on the state of the active palladium surface under low-temperature dry lean methane combustion conditions, which is important for methane-emission control devices. © 2017 Wiley-VCH.
A catalyst for the carbonylation of dimethyl ether to methyl acetate. The catalyst comprises a zeolite, such as a mordenite zeolite, at least one Group IB metal, such as copper, and/or at least one Group VIII metal, such as iron, and at least one Group IIB metal, such as zinc. Such a catalyst with combined metals provides enhanced catalytic activity, improved stability, and improved selectivity to methyl acetate, and does not require a halogen promoter, as compared to a metal-free or copper only zeolite.
Bimetallic Pd-Y nanoparticles were synthesized by yttrium precursor reduction on the surface of colloidal Pd seeds catalyzed by palladium hydride. The addition of yttrium improved the thermal stability of the Pd nanoparticles to agglomeration. The nanoparticles before and/or after deposition on alumina were characterized by TEM, XPS, STEM with elemental mapping, temperature-programmed reduction, CO chemisorption and CO-DRIFT spectroscopy. The supported catalysts were evaluated in hydrodesulfurization (HDS) of 4,6-dimethydibenzothiophene at 350 °C and 1 MPa. Yttrium addition did not alter the overall HDS rate but increased the direct desulfurization selectivity from 71% to 84% and suppressed cracking twice as much as monometallic Pd catalyst did. The study demonstrates that PdY structures could be a promising candidate for low-pressure HDS of refractory sulfur compounds. © 2017 Elsevier, reprinted with permission.
Dimethyl ether carbonylation to methyl acetate was studied on a series of dealuminated mordenites with Si/Al ratios of 6.5 through 15.4. Dealumination was achieved via leaching with nitric acid. Selective coupled removal of T3 and T4 Al sites located in 4-membered rings in 8- and 12-membered channels, respectively, was suggested by a combination of DRIFTS, TPD of probe molecules, 27Al MAS NMR, surface area, pore distribution, and XRD analyses. The methyl acetate peak productivity was found to be equal when calculated per residual T3 sites in 8-membered rings, providing experimental evidence of the reaction site specificity. While dealumination led to the decrease in product peak rate because of active and selective T3 site removal, it doubled the catalyst lifetime and increased selectivity upon deactivation from 50% to 80%. The study decoupled the contribution of 4-membered (T3 and T4) and 5-membered (T1 and T2) acid sites to the products’ formation and catalyst deactivation. © 2017 Elsevier, reprinted with permission.
A plethora of previously unimaginable low-temperature C1 and C2 valorization reactions have become possible after the discovery of metal-exchanged solid-acid catalysts, with the most promising candidate being copper-mordenite. We show the dramatic effect of zinc addition to copper-exchanged mordenite in maintaining high Cu dispersion and reducing the catalyst poisoning as it relates to carbonylation of dimethyl ether to methyl acetate. Zinc maintains 90%+ selectivity even during deactivation versus 60% for the Cu-mordenite, which leads to 6-fold higher product yield. The concept of Zn addition is recommended for further exploration in the conversion of methane to methanol or acetic acid. © 2016 American Chemical Society, reprinted with permission.
Pd nanoparticle size sensitivity of 4,6-dimethyldibenzothiophene (4,6-DMDBT) hydrodesulfurization was investigated by using 4, 8, 13, and 87 nm particles and was compared with the sulfur-free and sulfur-inhibited hydrogenation of 3,3-dimethylbiphenyl, which is a product of direct desulfurization of 4,6-DMDBT. The smallest 4 nm particles provided unprecedented (for Pd at 5 MPa and 300 °C) direct desulfurization selectivity of 20 % at 40 % conversion because of the reduced contribution of the hydrogenation path. The 4 nm particles were poisoned by the adsorbed sulfur to the greatest extent. The optimal size, providing the highest Pd mass-based yield of the desulfurized products, was found to be 8 nm. The catalyst with 87 nm particles was based on Pd nanocubes with the lowest edge/terrace surface atom ratio and large terraces and this showed the lowest sulfur extraction from both 4,6-DMDBT and sulfurous intermediates as a result of the low availability of edge atoms for a perpendicular sigma-mode adsorption through the lone pair of the sulfur atom. © 2016 Wiley-VCH.
38% to 93% selectivity enhancement toward sulfur-free products was observed upon iridium addition to a palladium-only catalyst in the hydrodesulfurization of a refractory sulfur compound 4,6-dimethyldibenzothiophene (4,6-DMDBT) at the same 40% conversion from a 300 ppm, S-containing feed at 5 MPa and 300 °C. Pd promoted hydrogenation to sulfurous intermediates, while Ir catalyzed C-S hydrogenolysis and also improved Pd resistance to sintering. The selectivity in the direct desulfurization path for the Ir-containing catalysts increased up to 26% versus 5% for the Pd-only catalyst. The bimetallic catalyst allowed for a decrease in S from 300 ppm to 11 ppm. It can be used at reduced pressure (3 MPa) with only a 15% decrease in hydrodesulfurization conversion as compared to the operation at 5 MPa. © 2016 Elsevier, reprinted with permission.
Metal nanoparticle (NP) co-catalysts on metal oxide semiconductor supports are attracting attention as photocatalysts for a variety of chemical reactions. Related efforts seek to make and use Pt-free catalysts. In this regard, we report here enhanced CH4 formation rates of 25 and 60 μmol·g–1·h–1 by photocatalytic CO2 reduction using hitherto unused ZnPd NPs as well as Au and Ru NPs. The NPs are formed by colloidal synthesis and grafted onto short n-type anatase TiO2 nanotube arrays (TNAs), grown anodically on transparent glass substrates. The interfacial electric fields in the NP-grafted TiO2 nanotubes were probed by ultraviolet photoelectron spectroscopy (UPS). Au NP-grafted TiO2 nanotubes (Au-TNAs) showed no band bending, but a depletion region was detected in Ru NP-grafted TNAs (Ru-TNAs) and an accumulation layer was observed in ZnPd NP-grafted TNAs (ZnPd-TNAs). Temperature programmed desorption (TPD) experiments showed significantly greater CO2 adsorption on NP-grafted TNAs. TNAs with grafted NPs exhibit broader and more intense UV–visible absorption bands than bare TNAs. We found that CO2 photoreduction by nanoparticle-grafted TNAs was driven not only by ultraviolet photons with energies greater than the TiO2 band gap, but also by blue photons close to and below the anatase band edge. The enhanced rate of CO2 reduction is attributed to superior use of blue photons in the solar spectrum, excellent reactant adsorption, efficient charge transfer to adsorbates, and low recombination losses. © 2016, Tsinghua University Press and Springer, reprinted with permission.
The present invention is directed to a catalyst for hydrodesulfurization of thiohydrocarbons, said catalyst comprising highly dispersed nanoparticles of a transition metal selected from the group consisting of: iridium, palladium and iridium/palladium and methods of manufacture thereof.
Two types of bimetallic Pd-Ru catalysts with a 2:1 Ru:Pd molar ratio were prepared using a poly-(vinylpyrrolidone) stabilizer: one alloy structure with mixed-surface atoms and one core–shell structure with a Pd core and Ru shell, which were confirmed by a surface-probe reaction at mild conditions. In indan hydrogenolysis at 350 °C, inversion of the core–shell structure began with Pd atoms appearing on the surface of the particles. Both catalysts displayed distinctively different catalytic behavior and indicated the importance of structure control for this particular application within a studied time frame. For methane combustion over the 200–550 °C temperature range, both structures demonstrated identical activity, which was due to their structural evolution to one nanoparticle type with Pd-enriched shells, as evidenced by extended X-ray absorption fine structure. © 2015 Elsevier, reprinted with permission.
The major concern in estimating sonochemical yield and efficiency in ultrasound-assisted processes is in defining a “silent” control experiment, without cavitation effects. To estimate the potential benefit of the ultrasonic treatment as compared to conventional heating, we propose that the effects should be compared at the same power input, when the energy in a silent experiment is dissipated as heat. Our calculations of possible temperature increase under the silent conditions for oil sands extraction and upgrading showed necessity of such approach. © 2015 Elsevier, reprinted with permission.