Functionalized Periodic Mesoporous Organosilicas: Tunable Hydrophobic Solid Acids for Biomass Conversion

The catalytic deoxygenation of bio-based feedstocks to fuels and chemicals presents new challenges to the catalytic scientist, with many transformations either performed in or liberating water as a byproduct during reaction. The design of catalysts with tunable hydrophobicity to aid product and reactant adsorption or desorption, respectively, is vital for processes including (trans)esterification and condensation reactions employed in sustainable biodiesel production and bio-oil upgrading processes. Increasing surface hydrophobicity of catalyst materials offers a means to displace water from the catalyst active site, and minimizes potential deactivation or hydrolysis side reactions. Hybrid organic–inorganic porous solids offer exciting opportunities to tune surface polarity and hydrophobicity, as well as critical parameters in controlling adsorption, reactant activation, and product selectivity in liquid and vapor phase catalysis. Here, we review advances in the synthesis and application of sulfonic-acid-functionalized periodic mesoporous organosilicas (PMO) as tunable hydrophobic solid acid catalysts in reactions relevant to biorefining and biofuel production

Sol-gel synthesis of SBA-15:Impact of HCl on surface chemistry

Surface functionalisation of mesoporous silicas is critical to their application as sorbents and catalyst supports. Here we report the impact of chloride on the physicochemical properties of SBA-15, notably the surface density of reactive hydroxyl groups. Bulk and surface properties were characterised by N2 porosimetry, X-ray diffraction, SEM, TEM, FTIR spectroscopy, and Inverse gas chromatography (IGC). Increasing the HCl concentration from 0.1 → 2.0 M during the sol-gel preparation of SBA-15 increased the surface silanol coverage two-fold, and slightly widened mesopores from 4.2 to 4.9 nm. IGC reveals that the specific surface energy and corresponding surface polarity of SBA-15 correlate with surface silanol properties, and hence tuning the HCl concentration during SBA-15 synthesis offers a facile route to hydrophilic or hydrophobic silicas, and in turn a means to control their functionalisation and sorptive properties

Alkali-Free Zn–Al Layered Double Hydroxide Catalysts for Triglyceride Transesterification

Zn–Al layered double hydroxides (LDHs) of general formula [Zn2+(1−x)Al3+x(OH)2]x+(CO32−)x/2·yH2O are promising solid base catalysts for the transesterification of lipids to biofuels. However, conventional synthetic routes employ alkali hydroxide/carbonate precipitants which may contaminate the final LDH catalyst and biofuel. The use of (NH3)2CO3 and NH3OH as precipitants affords alkali-free Zn–Al-LDHs spanning a wide composition range. The hydrothermal reconstruction of calcined Zn–Al-LDHs offers superior solid basicity and catalytic activity for the transesterification of C4–C18 triglycerides with methanol, compared with cold liquid phase or vapour phase reconstruction. Hydrothermally activated Zn3.3–Al-LDH was stable towards leaching during transesterification

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

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

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

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

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

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

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

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