1,934 research outputs found
An overview of recent advances of the catalytic selective oxidation of ethane to oxygenates
The selective partial oxidation of short chain alkanes is a key challenge within catalysis research. Direct ethane oxidation to oxygenates is a difficult aim, but potentially rewarding, and it could lead to a paradigm shift in the supply chain of several bulk chemicals. Unfortunately, low CāH bond reactivity and kinetically labile products are just some reasons affecting the development and commercialisation of such processes. Research into direct ethane oxidation is therefore disparate, with approaches ranging from oxidation in the gas phase at high temperatures to enzyme catalysed hydroxylation under ambient conditions. Furthermore, in overcoming the barrier posed by the chemically inert CāH bond a range of oxidants have been utilised. Despite years of research, this remains an intriguing topic from both academic and commercial perspectives. Herein we describe some recent developments within the field of catalytic ethane oxidation focusing on the formation of oxygenated products, whilst addressing the key challenges which are still to be overcome
The design of new catalysts for the partial oxidation of methane to methanol.
The direct partial oxidation of CH4 to CH30H would offer considerable economic
advantages over the current two stage process. It would also facilitate the utilisation
of natural gas reserves in remote locations.
To date, the catalytic partial oxidation of CH4 to CH30H has been extensively
studied, however, it has proved to be an extremely demanding reaction which has
met with little success.
This study has adopted a design approach for the identification of new catalysts by
considering the efficacy of single oxides for CH4 activation, CH30H oxidation and
02 isotope exchange activity.
On the basis of CH30H stability Sb203 was the best oxide, showing only 3 %
CH30H conversion at 500Ā°C. The majority of oxides totally combusted CH30H
below 400Ā°C. Mo03, Nb20S, Ta20S and W03 showed high selectivity to HCHO
and (CH3hO with low levels of COx throughout the range of conversion. These
oxides were not considered unsuitable from the perspective of CH30H stability as
the products HCHO and (CH3hO are not considered undesirable by products from
a CH4 partial oxidation process. A weak but significant correlation was observed
between the combustion activity of the oxides and the oxygen exchange rate.
Using CH4/D2 exchange as an indication of CH4 activation it has been shown that
Ga203 was a particularly good catalyst, followed by ZnO and Cr203. A relationship
between exchange activity and oxide basicity was established for the rare earth
sesquioxides, MgO and CaO. This relationship indicak:d that CH4 activation took
place by H+ abstraction to form a surface CH3- species.
From these results and literature studies of oxygen isotope exchange, dual
component oxides have been formulated as catalysts for CH4 partial oxidation. The
best catalysts was Ga203/Mo03, prepared by a physical mixing process. This
catalyst showed an increased yield of CH30H over the homogeneous gas phase
oxidation of CH4 in a quartz chips packed reactor. This increased yield has been
attributed to the development of a cooperative effect between the two component
oxides. Comparison of the catalytic data with the homogeneous reaction in the
empty reactor tube showed that the presence of a catalyst had a detrimental effect on
the CH30H yield
Nanoporous aluminosilicate-catalyzed telescoped acetalization-direct aldol reactions of acetals with 1,3-dicarbonyl compounds
Nanoporous aluminosilicate materials, synthesized by an evaporation-induced self-assembly process, catalyze the direct aldol reaction of acyclic acetals with a range of 1,3-dicarbonyl compounds to produce the corresponding aldol addition products in high yield, rather than the expected Knoevenagel elimination products. By carrying out the reaction in the presence of either dimethoxy propane or the corresponding orthoester, it is possible to capitalize on the ability of these aluminosilicate materials to catalyze the corresponding acetalization reaction leading to the development of novel telescoped, acetalization-direct aldol addition reaction protocols
Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements
The microwave cavity perturbation (MCP) technique is used to identify the transition from magnetite (Fe3O4) to the meta-stable form of maghemite (Ī³-Fe2O3). In this study Fe3O4 was annealed at temperatures from 60 to 300 Ā°C to vary the oxidation. Subsequent to annealing, the complex permittivity and magnetic permeability of the iron oxide powders were measured. The transition to Ī³-Fe2O3 was corroborated with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM). XRD, XPS and VSM implied that the starting powder was consistent with Fe3O4 and the powders annealed at more than 200 Ā°C were transitioning to Ī³-Fe2O3. The MCP measurements gave large differences in both complex permittivity and magnetic permeability of the two phases in the frequency range of 2.5ā10.2 GHz. Magnetic permeability decreased with annealing temperature, though magnetic losses showed frequency dependent behaviour. Complex permittivity measurements showed a large decrease in both dielectric constant and losses at all measurement frequencies, as well as a prominent loss peak centred around the phase transition temperatures. We interpret the loss peak as being a consequence of field effects due to an intermediate multi-phase mixture. Additionally, almost no frequency dependence was observed. The reduction in complex permittivity implies that the cations in the lattice provide a significant contribution to polarization at microwave frequencies and the effects of are nominal in comparison. The change in loss can be explained as a combination of the differences in the effective conductivity of the two phases (i.e. Fe3O4 exhibits electron-hopping conduction whereas the presence of vacancies in Ī³-Fe2O3 nullifies this). This shows that the non-invasive MCP measurements serve as a highly sensitive and versatile method for looking at this phase transition in iron and potentially the effects of oxidation states on the polarization in other iron oxides
The selective oxidation of n-butanol to butyraldehyde by oxygen using stable Pt-based nanoparticulate catalysts: an efficient route for upgrading aqueous biobutanol
Supported Pt nanoparticles are shown to be active and selective towards butyraldehyde in the base-free oxidation of n-butanol by O2 in an aqueous phase. The formation of butyric acid as a by-product promoted the leaching of Pt and consequently the activity of the catalysts decreased upon reuse. Characterisation showed that the degree to which Pt leached from the catalysts was related to both the metalāsupport interaction and metal particle size. A catalyst active and stable (<1% metal leaching) in the aqueous reaction medium was obtained when Pt nanoparticles were supported on activated carbon and prepared by a chemical vapour impregnation method. The presence of n-butanol in the aqueous medium is required to inhibit the over oxidation of butyraldehyde to butyric acid. Consequently, high selectivities towards butyraldehyde can only be obtained at intermediate n-butanol conversion
Structural characterization of Niobium Phosphate Catalysts used for the Oxidative Dehydrogenation of Ethane to Ethylene
Dehydrative etherification reactions of glycerol with alcohols catalyzed by recyclable nanoporous aluminosilicates: telescoped routes to glyceryl ethers
Catalytic
strategies for the efficient transformation of abundant
sustainable bioderived molecules, such as glycerol, into higher value
more useful products is an important research goal. In this study,
we demonstrate that atom efficient dehydrative etherification reactions
of glycerol with activated alcohols are effectively catalyzed by nanoporous
aluminosilicate materials in dimethylcarbonate (DMC) to produce the
corresponding 1-substituted glyceryl ethers in high yield. By carrying
out the reaction in acetone, it is possible to capitalize on the ability
of these materials to catalyze the corresponding acetalization reaction,
allowing for the development of novel, telescoped acetalization-dehydrative
etherification reaction sequences to selectively produce protected
solketal derivatives. These materials also catalyze the telescoped
reaction of glycerol with <i>tert</i>-butanol (TBA) in acetone
to produce the corresponding solketal mono <i>tert</i>-butyl
ether product in high yield, providing a potential route to convert
glycerol directly into a useful and sustainable fuel additive
One-step production of 1,3-butadiene from 2,3-butanediol dehydration
We report the direct production of 1,3-butadiene from the dehydration of 2,3-butandiol by using alumina as a catalyst. Under optimized kinetic reaction conditions, the production of methyl ethyl ketone and isobutyraldehyde, formed via the pinacol-pinacolone rearrangement, was markedly reduced and over 80% selectivity to 1,3-butadiene and 3-buten-2-ol could be achieved. The presence of water plays a critical role in the inhibition of oligomerization.The amphoteric nature of Ļ- Al2O3 was identified as important and this contributed to the improved catalytic selectivity when compared with other acidic catalysts
The role of copper speciation in the low temperature oxidative upgrading of short chain alkanes over Cu/ZSM-5 catalysts
Partial oxidative upgrading of C1āC3 alkanes over Cu/ZSMā5 catalysts prepared by chemical vapour impregnation (CVI) has been studied. The undoped ZSMā5 support is itself able to catalyse selective oxidations, for example, methane to methanol, using mild reaction conditions and the green oxidant H2O2. Addition of Cu suppresses secondary oxidation reactions, affording methanol selectivities of up to 97ā%. Characterisation studies attribute this ability to population of specific Cu sites below the level of total exchange (Cu/Al<0.5). These species also show activity for radicalābased methane oxidation, with productivities exceeding those of the parent zeolite supports. When tested for ethane and propane oxidation reactions, comparable trends are observe
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