Synthesis of palladium nanoparticles on carbon nanotubes and graphene for the chemoselective hydrogenation of para-chloronitrobenzene

We have studied the synthesis of palladium nanoparticles over carbon nanotubes (Pd/CNT) and graphene (Pd/G) and we have tested their catalytic performance in the liquid phase chemoselective hydrogenation of para-chloronitrobenzene at room temperature. The catalysts were characterized by N2 adsorption/desorption isotherms, TEM, X-ray diffraction, infrared and X-ray photoelectron spectroscopy and ICP-OES. The palladium particle size on Pd/G (3.4 nm) and Pd/CNT (2.8 nm) was similar though the deposition was higher on Pd/G. Pd/CNT was more active which can be ascribed to the different surface area and electronic properties of the Pd nanoparticles over CNT, while the selectivity was 100% to the corresponding haloaniline over both catalysts and they were quite stable upon recycling.

Enhancement of the hydrogenation activity of a Pd-tridecilamine (TDA) complex by confinement in carbon nanotubes

An active hydrogenation Pd complex has been immobilised by impregnation on CNTs submitted to several treatments that lead to important differences in their surface chemistry and in the proportion of tubes with both ends open. Most of the hybrid catalysts are more active than the complex in homogeneous phase, but the support properties have an important impact in the catalytic activity. In general, the more developed the surface chemistry, the lower the activity. However, when CNTs are open at both ends, the Pd complex can enter the tubular cavity and an important enhancement of the catalytic activity due to a confinement effect is observed.

Structured carbons as supports for hydrogenation hybrid catalysts prepared by the immobilization of a Rh diamine complex

A CNF-monolith sample (carbon nanofibres grown on a ceramic monolith), and a granular carbon xerogel have been used as supports for hybrid catalysts where the active species is an Rh diamine complex. The advantages of these supports are their open porous structure and their morphology, which make catalyst handling easier and avoid difficult separation processes. The obtained catalysts are noticeably more active than the homogeneous Rh complex and are stable against leaching. At first use, partial reduction of the Rh complex takes place and nanometer-sized Rh particles develop, which increases the catalyst activity. Despite the open porous structure, mass transport limitations are present, especially in the case of the carbon xerogel based catalyst. Differences in internal mass transfer limitations are essentially due to the different diffusional path lengths.

Nanoparticles of tungsten as low-cost monometallic catalyst for selective hydrogenation of 3-hexyne

Low-cost tungsten monometallic catalysts containing variable amounts of metal (4.5, 7.1 and 8.5%W) were prepared by impregnating alumina with ammonium metatungstate as an inexpensive precursor. The catalysts were characterized using ICP, XPS, XRD, TPR and hydrogen chemisorption. These techniques revealed mainly WO3-Al2O3 (W6+) species on the surface. The effects of the content of W nanoparticles and reaction temperature on activity and selectivity for the partial hydrogenation of 3-hexyne, a non-terminal alkyne, were assessed under moderate conditions of temperature and pressure. The monometallic catalysts prepared were found to be active and stereoselective for the production of (Z )-3-hexene, had the following order: 7.1WN/A > 8.5 WN/A ≥ 4.5 WN/A. Additionally, the performance of the synthesized xWN/A catalysts exhibited high sensitivity to temperature variation. In all cases, the maximum 3-hexyne total conversion and selectivity was achieved at 323 K. The performance of the catalysts was considered to be a consequence of two phenomena: a) the electronic effects, related to the high charge of W (+6), causing an intensive dipole moment in the hydrogen molecule (van der Waals forces) and leading to heterolytic bond rupture; the H+ and H- species generated approach a 3-hexyne adsorbate molecule and cause heterolytic rupture of the C≡C bond into C- = C+; and b) steric effects related to the high concentration of WO3 on 8.5WN/A that block the Al2O3 support. Catalyst deactivation was detected, starting at about 50 min of reaction time. Electrodeficient W6+ species are responsible for the formation of green oil at the surface level, blocking pores and active sites of the catalyst, particularly at low reaction temperatures (293 and 303 K). The resulting best catalyst, 7.1WN/A, has low fabrication cost and high selectivity for (Z )-3-hexene (94%) at 323 K. This selectivity is comparable to that of the classical and more expensive industrial Lindlar catalyst (5 wt% Pd). The alumina supported tungsten catalysts are low-cost potential replacements for the Lindlar industrial catalyst. These catalysts could also be used for preparing bimetallic W-Pd catalysts for selective hydrogenation of terminal and non-terminal alkynes.

Platinum(II) olefin hydroarylation catalysts: tuning selectivity for the anti-Markovnikov product Tandem catalytic ester hydrogenation and deoxygenation of polyethylene terephthalate

Thesis (Master's)--University of Washington, 2016-06

Recent advances in catalysts for methanol synthesis via hydrogenation of CO and CO2

Since the start of last century, methanol synthesis has attracted great interests because of its importance in chemical industries and its potential as an environmentally friendly energy carrier. The catalyst for the methanol synthesis has been a key area of research in order to optimize the reaction process. In the literature, the nature of the active site and the effects of the promoter and support have been extensively investigated. In this updated review, the recent progresses in the catalyst innovation, optimization of the reaction conditions, reaction mechanism, and catalyst performance in methanol synthesis are comprehensively discussed. Key issues of catalyst improvement are highlighted, and areas of priority in R&D are identified in the conclusions.

Tailoring the temperature and strain coefficients of Type I and Type IA dual grating sensors:the impact of hydrogenation conditions

We report experimental findings for tailoring the temperature and strain coefficients of Type I and Type IA fibre Bragg gratings by influencing the photosensitivity presensitization of the host optical fibre. It is shown that by controlling the level of hydrogen saturation, via hot and cold hydrogenation, it is possible to produce gratings with lower thermal coefficients. Furthermore, there is a larger difference between the Type I and Type IA thermal coefficients and a significant improvement in the matrix condition number, which impacts the ability to recover accurate temperature and strain data using the Type 1-1A dual grating sensor. © 2006 IOP Publishing Ltd.

Coupling of Heck and hydrogenation reactions in a continuous compact reactor

A continuous multi-step synthesis of 1,2-diphenylethane was performed sequentially in a structured compact reactor. This process involved a Heck C-C coupling reaction followed by the addition of hydrogen to perform reduction of the intermediate obtained in the first step. Both of the reactions were catalysed by microspherical carbon-supported Pd catalysts. Due to the integration of the micro-heat exchanger, the static mixer and the mesoscale packed-bed reaction channel, the compact reactor was proven to be an intensified tool for promoting the reactions. In comparison with the batch reactor, this flow process in the compact reactor was more efficient as: (i) the reaction time was significantly reduced (ca. 7 min versus several hours), (ii) no additional ligands were used and (iii) the reaction was run at lower operational pressure and temperature. Pd leached in the Heck reaction step was shown to be effectively recovered in the following hydrogenation reaction section and the catalytic activity of the system can be mostly retained by reverse flow operation. © 2009 Elsevier Inc. All rights reserved.

Selectivity control in Pt-catalyzed cinnamaldehyde hydrogenation

Chemoselectivity is a cornerstone of catalysis, permitting the targeted modification of specific functional groups within complex starting materials. Here we elucidate key structural and electronic factors controlling the liquid phase hydrogenation of cinnamaldehyde and related benzylic aldehydes over Pt nanoparticles. Mechanistic insight from kinetic mapping reveals cinnamaldehyde hydrogenation is structure-insensitive over metallic platinum, proceeding with a common Turnover Frequency independent of precursor, particle size or support architecture. In contrast, selectivity to the desired cinnamyl alcohol product is highly structure sensitive, with large nanoparticles and high hydrogen pressures favoring C=O over C=C hydrogenation, attributed to molecular surface crowding and suppression of sterically-demanding adsorption modes. In situ vibrational spectroscopies highlight the role of support polarity in enhancing C=O hydrogenation (through cinnamaldehyde reorientation), a general phenomenon extending to alkyl-substituted benzaldehydes. Tuning nanoparticle size and support polarity affords a flexible means to control the chemoselective hydrogenation of aromatic aldehydes.

Ag alloyed Pd single-atom catalysts for efficient selective hydrogenation of acetylene to ethylene in excess ethylene

Semihydrogenation of acetylene in an ethylene-rich stream is an industrially important process. Conventional supported monometallic Pd catalysts offer high acetylene conversion, but they suffer from very low selectivity to ethylene due to overhydrogenation and the formation of carbonaceous deposits. Herein, a series of Ag alloyed Pd single-atom catalysts, possessing only ppm levels of Pd, supported on silica gel were prepared by a simple incipient wetness coimpregnation method and applied to the selective hydrogenation of acetylene in an ethylene-rich stream under conditions close to the front-end employed by industry. High acetylene conversion and simultaneous selectivity to ethylene was attained over a wide temperature window, surpassing an analogous Au alloyed Pd single-atom system we previously reported. Restructuring of AgPd nanoparticles and electron transfer from Ag to Pd were evidenced by in situ FTIR and in situ XPS as a function of increasing reduction temperature. Microcalorimetry and XANES measurements support both geometric and electronic synergetic effects between the alloyed Pd and Ag. Kinetic studies provide valuable insight into the nature of the active sites within these AgPd/SiO2 catalysts, and hence, they provide evidence for the key factors underpinning the excellent performance of these bimetallic catalysts toward the selective hydrogenation of acetylene under ethylene-rich conditions while minimizing precious metal usage.

Highly selective hydrogenation of furfural over supported Pt nanoparticles under mild conditions

The selective liquid phase hydrogenation of furfural to furfuryl alcohol over Pt nanoparticles supported on SiO2, ZnO, γ-Al2O3, CeO2 is reported under extremely mild conditions. Ambient hydrogen pressure, and temperatures as low as 50 °C are shown sufficient to drive furfural hydrogenation with high conversion and >99% selectivity to furfuryl alcohol. Strong support and solvent dependencies are observed, with methanol and n-butanol proving excellent solvents for promoting high furfuryl alcohol yields over uniformly dispersed 4 nm Pt nanoparticles over MgO, CeO2 and γ-Al2O3. In contrast, non-polar solvents conferred poor furfural conversion, while ethanol favored acetal by-product formation. Furfural selective hydrogenation can be tuned through controlling the oxide support, reaction solvent and temperature.

Platinum-catalysed cinnamaldehyde hydrogenation in continuous flow

Platinum is one of the most widely used hydrogenation catalysts. Here we describe the translation of batch reactions to continuous flow, affording tunable C=O versus C=C hydrogenation over a Pt/SiO2 catalyst, resulting in high steady state activity and single-pass yields in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol under mild conditions. Negligible catalyst deactivation occurs under extended flow operation due to removal of reactively-formed poisons from the reaction zone. Process intensification imparts a four-fold enhancement in cinnamyl alcohol productivity.

Nanoengineering heterogeneous catalysts for the selective hydrogenation of key agrochemical derivatives

Rapidly rising world populations have sparked growing concerns over global food production to meet this increasing demand. Figures released by The World Bank suggest that a 50 % increase in worldwide cereal production is required by 2030. Primary amines are important intermediates in the synthesis of a wide variety of fine chemicals utilised within the agrochemical industry, and hence new 'greener' routes to their low cost manufacture from sustainable resources would permit significantly enhanced crop yields. Early synthetic pathways to primary amines employed stoichiometric (and often toxic) reagents via multi-step protocols, resulting in a large number of by-products and correspondingly high Environmental factors of 50-100 (compared with 1-5 for typical bulk chemicals syntheses). Alternative catalytic routes to primary amines have proven fruitful, however new issues relating to selectivity and deactivation have slowed commercialisation. The potential of heterogeneous catalysts for nitrile hydrogenation to amines has been demonstrated in a simplified reaction framework under benign conditions, but further work is required to improve the atom economy and energy efficiency through developing fundamental insight into nature of the active species and origin of on-stream deactivation. Supported palladium nanoparticles have been investigated for the hydrogenation of crotononitrile to butylamine (Figure 1) under favourable conditions, and the impact of reaction temperature, hydrogen pressure, support and loading upon activity and selectivity to C=C versus CºN activation assessed.

Acetylene hydrogenation over structured Au-Pd catalysts

Acknowledgements We thank the University of Aberdeen for financial support and Dr K. McManus (University of Aberdeen) for performing preliminary experiments with these samples. Electron microscopy and EDS were performed by RTB at the Electron Microscopy Facility, University of St Andrews.