17 research outputs found

    Intraparticle Diffusional Effects vs. Site Effects on Reaction Pathways in Liquid-Phase Cross Aldol Reactions

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    Chemo- and regioselectivity in a heterogeneously catalyzed cross aldol reaction were directed by tuning the nature of the sites, textural properties and reaction conditions. Catalysts included sulfonic-acid functionalized resins or SBA-15 with varying particle size or pore diameter, H-BEA zeolites, and Sn-BEA zeotype; conditions were 25 °C to 170 °C in organic media. Benzaldehyde and 2-butanone yielded branched (reaction at -CH2- of butanone) and linear (reaction at -CH3) addition and condensation products; and fission of the branched aldol led to β-methyl styrene and acetic acid. Strong acids promoted the dehydration step, and regioselectivity originated from preferred formation of the branched aldol. Both, resins and functionalized SBA-15 materials yielded predominantly the branched condensation product, unless particle morphology or temperature moved the reaction into the diffusion-limited regime, in which case more fission products were formed, corresponding to Wheeler Type II selectivity. For Hform zeolites, fission of the branched aldol competed with dehydration of the linear aldol, possibly because weaker acidity or steric restrictions prevented dehydration of the branched aldol

    Acceptorless Amine Dehydrogenation and Transamination Using Pd-Doped Hydrotalcites

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    The acceptorless dehydrogenation of acyclic secondary amines is a highly desirable but still elusive catalytic process. Here we report the synthesis, characterization, and activity of Pd-doped hydrotalcites (Pd-HTs) for acceptorless dehydrogenation of both primary and secondary amines (cyclic and acyclic). These multifunctional catalysts comprise Brønsted basic and Lewis acidic surface sites that stabilize Pd in 0, 2 + , and 4 + oxidation states. Pd speciation and corresponding catalytic performance is a strong function of metal loading. High activity is observed for the dehydrogenation of secondary aliphatic amines to imines, and N-heterocycles, such as indoline, 1,2,3,4-tetrahydroquinoline, and piperidine, to aromatic compounds. Oxidative transamination of primary amines is achieved using low Pd loading (0.5 mol %), without the need for oxidants. The relative yields of secondary imines afforded are consistent with trends for calculated free energy of reaction, while yields for transamination products correspond to the electrophilicity of primary imine intermediates. Reversible amine dehydrogenation and imine hydrogenation determine the relative selectivity for secondary imine/amine products. Poisoning tests evidence that Pd-HTs operate heterogeneously, with negligible metal leaching. Catalysts retain over 90% of activity over six reuse cycles, but do suffer some selectivity loss, attributed to changes of Pd phases

    Alkali-Free Hydrothermally Reconstructed NiAl Layered Double Hydroxides for Catalytic Transesterification

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    NiAl layered double hydroxides (LDHs) are promising bifunctional catalysts comprising tunable redox and Lewis acidic sites. However, most studies of NiAl LDH employ alkali hydroxide carbonate precipitants which may contaminate the final LDH catalyst and leach into reaction media. Here, we report an alkali-free route to prepare NixAl LDHs with a composition range x = 1.7 to 4.1 using (NH4)2CO3 and NH4OH as precipitants. Activation of LDHs by calcination−rehydration protocols reveal NixAl LDHs can be reconstructed under mild hydrothermal treatment (110 °C for 12 h), with the degree of reconstruction increasing with Ni content. Catalyst activity for tributyrin transesterification with methanol was found to increase with Ni content and corresponding base site loadings; TOFs also increased, suggesting that base sites in the reconstructed LDH are more effective for transesterification. Hydrothermally reconstructed Ni4.1Al LDH was active for the transesterification of C4−C12 triglycerides with methanol and was stable towards leaching during transesterification

    Beyond the simple Proximity Force Approximation: geometrical effects on the non-retarded Casimir interaction

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    We study the geometrical corrections to the simple Proximity Force Approximation for the non-retarded Casimir force. We present analytical results for the force between objects of various shapes and substrates, and between pairs of objects. We compare the results to those from more exact numerical calculations. We treat spheres, spheroids, cylinders, cubes, cones, and wings; the analytical PFA results together with the geometrical correction factors are summarized in a table.Comment: 18 pages, 19 figures, 1 tabl

    Enhancing nanofiltration in thin film nanocomposite membranes using Bi-Metal modified biochar nanofillers

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    The advancement in the development of nanofillers for thin-film nanocomposite (TFN) membranes, particularly those derived from eco-friendly sources, has gained increasing recognition. This is largely due to their potential to markedly improve both permeation and selectivity. However, the investigation of biochar (BC), a by-product of biomass pyrolysis, as a distinctive nanofiller remains limited. This study investigates the incorporation of porous iron/zinc (Fe/Zn) modified biochar (MBC) into a polyamide active layer for the purpose of fabricating TFN membranes on a polyethersulfone (PES) substrate via interfacial polymerisation (IP). Imaging confirmed the formation of metal nanoparticles dispersed uniformly throughout the porous BC substrate. Further crystallinity and surface analysis suggest strong interactions between metal and BC substrate, with a surface area of 117.99 m2/g and high nanofiller pore volume of 7.72 cm3/g. The effects of incorporating MBC into both the membrane substrate and polyamide (PA) layers on the physicochemical properties, permeation, and rejection of salts and dye were examined. Scanning Electron Microscopy (SEM) imaging has shown that the incorporation of MBC in both the substrate and PA layer results in the seamless formation of a finger-like structure spanning both layers. This incorporation also causes a minor increase in the surface roughness of the PA layer. Fourier transform Infra-Red (FT-IR) spectroscopy shows an enhancement in hydrophilic functional groups (–OH and –COOH) on the membrane surface, as evidenced by the reduced contact angle value of 55°. Permeation and rejection testing indicate that M5, where MBC was incorporated in both substrate and thin film structure, was the best performing membrane, with water permeance from the feeds of water, MO, MgSO4 and NaCl solutions of 46.55 ± 0.08, 44.49 ± 0.28, 37.43 ± 0.36, and 21.55 ± 0.03 Lm2h-1bar−1, respectively. Rejection of MO, MgSO4 and NaCl were recorded to be 99.53 ± 0.02, 99.25 ± 0.09 and 46.99 ± 0.69 %. This study provides a compelling perspective on the application of green-derived BC as a nanofiller in the fabrication of TFN membranes for desalination, resulting in enhanced water product quality

    Synthesis of amine functionalized mesoporous silicas templated by castor oil for transesterification

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    Mesoporous silicas were synthesized via a surfactant-templated sol-gel route using castor oil as the templating agent under acidic medium. The resulting silicas were subsequently amine functionalized with 3-aminopropyltriethoxysilane (NH2-MTS), [3-(2-aminoethylamino)-propyl]trimethoxysilane (NN-MTS), and [3-(diethylamino)propyl]trimethoxysilane(DN-MTS) to introduce surface basicity. Surface physicochemical properties were characterized by field emission gun scanning electron microscopy (FEGSEM), nitrogen porosimetry, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and diffuse reflectance infrared fourier transform spectroscopy (DRIFTS). As-synthesised materials exhibit type IV adsorption-desorption isotherms characteristic of mesoporous structures. Clusters of spherical shaped materials were observed by FEGSEM, suggesting growth of silica occurs within colloidal dispersions. High-resolution N 1s XP spectra and DRIFT spectra confirmed the presence of amine groups in the organo-amine functionalised mesoporous silicas. The amine functionalised mesoporous silicas were active for the transesterification of tributyrin with methanol, with conversion found to increase from NH2-MTS< NN-MTS< DN-MTS

    The effect of metal precursor on copper phase dispersion and nanoparticle formation for the catalytic transformations of furfural

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    The formation of copper-based catalysts ranging from nanoparticles to isolated and dimeric Cu species supported on nanophased alumina is reported and utilised for the catalytic liquid-phase hydrogenation of furfural. The materials were synthesised via wet impregnation using various copper precursors (nitrate, acetate and sulphate) at two different loadings. A high Cu loading (5.0 wt.%) led to the formation of well-defined nanoparticles, while a lower loading (1.0 wt.%) generated a highly dispersed phase consisting mostly of atomic and dimeric Cu species dispersed on Al 2O 3. The catalytic reaction was found to be structure sensitive, promoting decarbonylation reactions with low Cu loading. Copper sulphate derived catalysts were found to severely decrease furfuryl alcohol selectivity from 94.6% to 0.8%, promoting the formation of side reactions. The sulphur-free catalysts represent a greener and more sustainable alternative to the toxic catalysts currently used in industry, operating at milder conditions of 50 °C and 1.5 bar H 2

    Mesoporous sulfonic acid silicas for pyrolysis bio-oil upgrading via acetic acid esterification

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    Propylsulfonic acid derivatised SBA-15 catalysts have been prepared by post modification of SBA-15 with mercaptopropyltrimethoxysilane (MPTMS) for the upgrading of a model pyrolysis bio-oil via acetic acid esterification with benzyl alcohol in toluene. Acetic acid conversion and the rate of benzyl acetate production was proportional to the PrSO3H surface coverage, reaching a maximum for a saturation adlayer. Turnover frequencies for esterification increase with sulfonic acid surface density, suggesting a cooperative effect of adjacent PrSO3H groups. Maximal acetic acid conversion was attained under acid-rich conditions with aromatic alcohols, outperforming Amberlyst or USY zeolites, with additional excellent water tolerance

    Facile route to conformal hydrotalcite coatings over complex architectures:a hierarchically ordered nanoporous base catalyst for FAME production

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    An alkali- and nitrate-free hydrotalcite coating has been grafted onto the surface of a hierarchically ordered macroporous-mesoporous SBA-15 template via stepwise growth of conformal alumina adlayers and their subsequent reaction with magnesium methoxide. The resulting low dimensional hydrotalcite crystallites exhibit excellent per site activity for the base catalysed transesterification of glyceryl triolein with methanol for FAME production

    Influence of alkyl chain length on sulfated zirconia catalysed batch and continuous esterification of carboxylic acids by light alcohols

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    The impact of alkyl chain length on the esterification of C2–C16 organic acids with C1–C4 alcohols has been systematically investigated over bulk and SBA-15 supported sulfated zirconias (SZs). Rates of catalytic esterification for methanol with acetic acid are directly proportional to the sulfur content for both SZ and SZ/SBA-15, with the high dispersion of SZ achievable in conformal coatings over mesoporous SBA-15 confering significant rate-enhancements. Esterification over the most active 0.24 mmol gcat−1 bulk SZ and 0.29 mmol gcat−1 SZ/SBA-15 materials was inversely proportional to the alkyl chain length of alcohol and acid reactants; being most sensitive to changes from methanol to ethanol and acetic to hexanoic acids respectively. Kinetic analyses reveal that these alkyl chain dependencies are in excellent accord with the Taft relationship for polar and steric effects in aliphatic systems and the enthalpy of alcohol adsorption, implicating a Langmuir–Hinshelwood mechanism. The first continuous production of methyl propionate over a SZ fixed-bed is also demonstrated
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