Sulfonic Acid-functionalized Silica Research Articles

Sulfonic acid-functionalized silica with controlled hydrophobicity as an effective catalyst for esterification of levulinic acid

As one of the essential additives in biodiesel and flavoring agents, solvents, and plasticizers in industry, ethyl levulinate (EL) has gained attention in the last two decades, and the global production of the compound is rapidly increased over the years. So far, esterification of biomass-derived levulinic acid (LA) with ethanol (EtOH) in the presence of highly corrosive mineral acid catalysts is the most common technique for the reaction. To avoid the harsh environment during the reaction and to ease the separation of the catalyst, the research has been recently focused on developing highly active solid acid catalysts. In this research, we have systematically investigated the catalytic activity of sulfonic acid-functionalized silica (SiO2-SO3H) with different hydrophobicity for esterification of LA with EtOH. The hydrophobicity of SiO2-SO3H was tuned by incorporating three different alkylsilanes, e.g., methyl (Me), octyl (Oct), and hexadecyl (HD) silanes on the surface of the material. We found that the longer the alkyl group, the lower the acidity and thus resulted in the lower the catalytic activity. On the other hand, the particle size and hydrophobicity increase with the length in the alkyl group. SiO2-SO3H modified with the methyl group (SiO2-SO3H/Me) showed the highest acidity (0.76 mmol g–1) and the highest catalytic performance (70.6% conversion). Moreover, while all alkyl-modified catalysts followed pseudo-first-order, the pseudo-first-order rate constant (k1p) for the reaction over SiO2-SO3H/Me was 5.4 × 10–3 min–1. The kinetic results of this study could be useful for reactor modeling and simulation in the future.

Composite proton conducting membranes from chitosan, poly(vinyl alcohol) and sulfonic acid-functionalized silica nanoparticles

Composite proton conducting membranes were successfully synthesized from chitosan, poly (vinyl alcohol) and sulfonic acid-functionalized silica nanoparticles. Sulfosuccinic acid (SSA) and glutaraldehyde were used as double crosslinking agents, where the effect of SSA content on membrane properties, including water vapor absorption, water uptake, ion exchange capacity, and proton conductivity was investigated and were found to increase as a function of SSA loading. The most promising membrane was then formed into a composite with either silica nanoparticles containing poly (2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS-Si) or poly (styrene sulfonic acid) (PSSA-Si). PAMPS-Si and PSSA-Si were characterized by thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The effects of nanoparticle loading and polymer shell on membrane properties were studied. Proton conductivity increased at higher nanoparticle loadings, and reached a maximum of 3.8–3.9 × 10−3 S/cm at 20% loading. The influence of polymer shell on membrane properties was not significantly observed.

Influence of casting substrate on bulk morphology and vanadium ion transport in ionomer nanocomposites

Perfluorosulfonic acid (PFSA) ionomer nanocomposites are a promising solution to address the poor ion selectivity of current membranes utilized in vanadium redox flow batteries. Herein, we investigate the impact of a casting substrate on the nanostructure and vanadium ion transport in bulk ionomer and ionomer nanocomposite membranes (i.e., films with thicknesses of ∼100 μm). Specifically, solution-cast ionomer nanocomposite membranes, containing either unfunctionalized (hydroxyl groups), amine-functionalized, or sulfonic acid-functionalized silica nanoparticles (SiNPs), were fabricated by casting on either a polished quartz or polytetrafluoroethylene (PTFE) substrates. Surprisingly, the choice of the casting substrate was seen to affect the bulk morphology of the PFSA ionomers, resulting in substrate-specific vanadium ion transport, where suppressed ion transport was observed for membranes cast on the polished quartz, when compared to their PTFE-cast counterparts. Additionally, the chemical composition of the substrate-adjacent surface was a function of both the substrate and the surface functionality of the SiNPs. Moreover, it was observed that both the chemical composition of the membrane surface and the substrate-induced changes to the bulk ionomer morphology governed vanadyl ion transport through the PFSA ionomers. Results from this work have direct implications for the design of next-generation ionomer nanocomposites, as the casting substrate used to fabricate these materials, and the orientation of these membranes inside the operating flow battery, can significantly influence transport of vanadium ions.

New fluorinated polymer- based nanocomposites via combination of sol -gel chemistry and reactive extrusion for polymer electrolyte membranes fuel cells (PEMFCs)

Proton exchange membranes designed via reactive extrusion from the in situ generation of functional silica-like particles in a new poly(vinylidene fluoride-co-hexafluoropropylene (poly(VDF-co-HFP)) copolymer are presented. These sulfonic acid-functionalized polysiloxane-based fillers were synthesized via sol-gel chemistry from 3-mercaptopropyltri(ethoxy)silane and polydimethoxysiloxane in the molten poly(VDF-co-HFP) copolymer. To process such nanocomposites materials, the reactive extrusion parameters were selected to (i) reach silanol condensation extents in the processing conditions of a conventional poly(VDF-co-HFP) copolymer and (ii) obtain a surface functionalization high enough with respect to reach a suitable protonic conductivity value of the nanocomposites films. Nevertheless, the enhancement of the proton conductivity being deeply associated with the morphologies of the nanocomposites materials (i.e. the dispersed functional silica phase needs to display the largest interface area with the polymer matrix), an interfacial agent was synthesized. This compatibilizer was either poly(VDF-co-α-trifluoromethacrylic acid) or poly(VDF-co-MAF) copolymers or a poly(VDF-co-HFP) copolymer grafted with maleic anhydride, denoted poly(VDF-co-HFP)-g-MA. In a second step, to obtain the required sulfonic acid-functionalized silica dispersed phase from the presence of the mercaptan functions, the resulting films were oxidized. Electrochemical properties were evaluated: for a theoretical ionic exchange capacity (IEC) of 2 meq.g−1, experimental IEC ranged between 1.0 and 1.3 meq.g−1. The proton conductivity was found to reach 78 mS cm−1 at room temperature for 100% of relative humidity. Interestingly, these values are higher than those of the same membranes processed without any compatibilizer (54 mS cm−1) and that of Nafion® NR112 (52 mS cm−1). This approach also allows limiting the swelling (or water uptake) of proton conductive membranes below 15 wt% while it reachs 30 wt% for Nafion® NR112 in similar conditions.

Fabrication of Hollow Silica Nanospheres with Ultra-High Acid Density for Efficient Heterogeneous Catalysis

Hollow silica nanospheres with ultra-high acid density were fabricated successfully via sulfonation of phenyl-functionalized hollow silica nanospheres, which were synthesized through a single micelle (F127 (EO106PO70EO106))-templated method, with phenyltrimethoxysilane and tetramethoxysilane (TMOS) as silane precursors under neutral conditions. The density of sulfonic acid reached as high as 1.97 mmol/g. The characterization results of 31P-NMR using triethylphosphine oxide as a probe molecule suggested that the acid strength of hybrid solid acids could be systematically tuned by tuning the content of sulfonic acid and higher acid density results in stronger acid strength. Attributed to the unique hollow structure and high-acid density, the sulfonic acid-functionalized hollow silica nanospheres exhibited good catalytic performance in the condensation reaction of benzaldehyde with ethylene glycol. Notably, this study found that the catalytic activity was significantly influenced by the acid density and the ultra-high acid loading was beneficial for the activity due to the enhanced acid strength. This novel solid-acid catalyst also showed good recyclability and could be reused for at least 11 runs.

A Combined Tamaru Allylation/Olefin Cross-Metathesis Approach for the Total Syntheses of (±)-Paniculidine B, (±)-Paniculidine C, and 2-Methylcarbazole

A concise approach to the total syntheses of racemic paniculidines B and C is described. The route features a combined Tamaru allylation/olefin cross-metathesis sequence for the regiocontrolled synthesis of prenylindole intermediates. In addition, we report a transformation of the prenylated indole into 2-methylcarbazole catalyzed by sulfonic acid-functionalized silica gel.

Efficient Flow Fischer Esterification of Carboxylic Acids with Alcohols Using Sulfonic Acid-Functionalized Silica as Supported Catalyst

Abstract Flow Fischer esterification of carboxylic acids using hydroxy-substituted sulfonic acid-functionalized silica (HO-SAS) packed into a stainless steel column reactor was investigated. HO-SAS well catalyzed flow esterification of long chain carboxylic acids with methanol within 3 min of residence time at 110 °C, and the methyl esters were quantitatively obtained. The flow esterification protocol was applied to the synthesis of a variety of esters (19 examples) and scalable synthesis was also successful.

Reductive amination of furfural over Me/SiO2–SO3H (Me: Pt, Ir, Au) catalysts

Gold, platinum and iridium colloids were supported on sulfonic acid-functionalized silica and were employed for the reductive amination of furfural. The catalysts were characterized by N2-physisorption at 77K, X-ray diffraction, H2 chemisorption and XPS. The results indicated that Ir, Au and Pt colloids were uniformly dispersed on the SiO2–SO3H support. XPS results showed the interaction of sulfonic acid groups and metal sites which favors the formation of imine and its subsequent hydrogenation to amine.

Synthesis of amorphous silica and sulfonic acid functionalized silica used as reinforced phase for polymer electrolyte membrane

In this work silica (SiO2) and sulfonic acid-functionalized silica (sul-SiO2) were synthesized by sol–gel method from tetraethoxysilane (TEOS) and 3-mercatopropyltrimethoxysilane (MPTMS) with various ratios between them. The synthesized materials were characterized by x-ray diffraction (XRD) for crystalline structure, Brunauer–Emmet–Teller (BET) specific surface area analysis, transmission electronic microscopy (TEM) and dynamic light scattering (DLS) for particle size analysis, and ion exchange capacity (IEC) for determining sulfur content in Sul-SiO2 materials. The initial results showed that the average particle size of amorphous SiO2 and Sul-SiO2 at different TEOS: MPTMS ratios are in narrow distribution with average diameter about 20–30 nm. The particle size of Sul-SiO2 is almost unaffected by the content of MPTMS while IEC depends strongly on it. Composite membranes of 60 μm thickness were successfully prepared from blending of poly(vinylidene fluoride) (PVDF) and synthesized amorphous SiO2. It was shown that the latter may be used as a reinforced phase for composite membrane electrolytes based on PVDF.

Use of sulfonic acid-functionalized silica as catalyst for esterification of free fatty acids (FFA) in acid oil for biodiesel production: an optimization study

This paper deals with esterification of free fatty acids (FFAs) in acid oil (a byproduct of oil refining) to obtain biodiesel. Sulfonic acid-functionalized silica (SiO2–Pr–SO3H) was used as promising solid-acid catalyst. The conditions affecting conversion to fatty acid methyl esters (FAME), for example reaction temperature, reaction time, catalyst concentration, and methanol-to-oil molar ratio, were investigated and optimized by use of the Taguchi method. The highest conversion obtained under the optimized conditions was 96.78 % after 8 h. Analysis of variance revealed that temperature was the most significant factor effecting conversion among the four conditions studied. The experimental results were found to fit a pseudo first-order kinetic law. SiO2–Pr–SO3H is a highly effective, reusable, and environmentally benign catalyst for biodiesel production from waste low-cost oil feedstock with a high FFA content.

Sulfonic acid-functionalized silica: a remarkably efficient heterogeneous reusable catalyst for the one-pot synthesis of 1,4-dihydropyridines

An efficient one-pot method for the synthesis of 1,4-dihydropyridines from \beta-dicarbonyl compounds, aldehyde, and ammonium acetate is reported using sulfonic acid-functionalized silica at 90 °C under solvent-free conditions with good to excellent yields. The catalyst is easily prepared, stable (up to 300 °C), reusable, and efficiently used under reaction conditions.

Direct synthesis of hydrogen peroxide on palladium catalyst supported on sulfonic acid-functionalized silica

The direct synthesis of H2O2 has been successfully achieved at high-concentration and high selectivity on Pd-loaded sulfonic acid-functionalized silica using non-acidic solutions and a reaction temperature of 313 K.

Sulfonic Acid Functionalized Silica: A Mild, Reusable and Efficient Heterogeneous Catalyst for the Highly Diastereoselective Synthesis of cis-Isoquinolonic Acids

Sulfonic acid functionalized silica (SAFS) is found to be a recyclable heterogeneous catalyst for the rapid and efficient synthesis of various cis-isoquinolonic acids from the three-component condensation reaction of homophthalic anhydride, an amine, and an aldehyde. This reaction was highly diastereoselective and only one diastereomer was obtained in all cases.

Sol–gel based sulfonic acid-functionalized silica proton conductive membrane

Sulfonic acid-functionalized glass membranes have been synthesized via sol–gel reactions for low-power direct methanol fuel cells (DMFCs). Minimizing the fuel loss due to methanol crossover is the most important issue for creating long-life, low-power DMFC sources. The inorganic glass membrane is of interest due to its low methanol permeability compared to polymer membranes. Three different alkoxy silane reactants were investigated in the sol–gel reaction: 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-mercaptopropyl trimethoxysilane (3MPS), and tetraethoxy orthosilicate (TEOS). The effect of oxidation time of the thiol group on the 3MPS, the mole fraction within the sol, and the water ratio in the reactant mixture were investigated. The ionic conductivity and methanol permeability has been characterized and optimized. The goal in this study was to find a balance between the ionic conductivity and methanol permeability, which determines the fuel conversion efficiency and device lifetime. The optimum glass membrane had a conductivity of 3.71 mS cm−1, and methanol permeability of 2.17 × 10−9 mol cm cm−2 day−1 Pa−1, which was significantly better than Nafion or other previously reported membranes for this application.

A novel and highly efficient method for the silylation of alcohols with hexamethyldisilazane (HMDS) catalyzed by recyclable sulfonic acid-functionalized ordered nanoporous silica

Silylation of alcohols with hexamethyldisilazane (HMDS) in dichloromethane provides the corresponding silyl ethers in almost quantitative yields at room temperature using 1–3 mol % of sulfonic acid-functionalized silica. Additionally, the catalyst displays a high activity and thermal stability (up to 240 °C) and it can be easily recovered and reused for at least 20 reaction cycles without loss of reactivity.

Acid-Functionalized Amorphous Silica by Chemical Grafting−Quantitative Oxidation of Thiol Groups

This work describes the synthesis and structural features of functionalized amorphous silica with sulfonic acid groups. The approach followed involved the condensation of (3-mercaptopropyl)trimethoxysilane or (3-mercaptopropyl)methyldimethoxysilane and/or phenyltrimethoxysilane on the hydroxyl groups of the silica substrate followed by oxidation with hydrogen peroxide. This oxidation reaction was effective, although some lixiviation of sulfur occurred, and the amount of S released depended on the hydrophobicity of the surface and the temperature of the treatment. The extent of oxidation of the thiol precursor, determined by photoelectron spectroscopy (XPS), was moderate at ambient temperature and much more effective at 333 K. The XPS technique proved to be extremely useful not only to discriminate between the −SH and −SO3H, whose chemical shifts differ by 5 eV, but also to quantify the proportion of these species and hence the conditions that make the oxidation process quantitative. The solid samples prepared according to this grafting−oxidation methodology exhibited strong acid sites, as revealed by their performance in the esterification reaction of acetic acid by methanol in liquid phase. Activity data indicated that the reaction is accelerated in the presence of the sulfonic acid-functionalized silica catalyst, being higher than that of the reference Nafion silica composite. Finally, this grafting procedure and the methodologies to quantify and test these acid-functionalized silica materials can be applied to other substrates.

Sulfonic acid-functionalized silica through quantitative oxidation of thiol groups.

A simple procedure for the preparation of amorphous silica containing thiol groups which quantitatively affords sulfonic acid groups has been developed, resulting in site densities and activity for the esterification of acetic acid with methanol greater than a commercial Nafion silica composite.

Porous PVDF asymmetric hollow fiber membranes prepared with the use of small molecular additives : Wang, Dongliang, Li, K., Teo, W.K., National University of Singapore, Singapore in journal of membrane science, vol 178, issue 1, 2000

Proton exchange membranes designed via reactive extrusion from the in situ generation of functional silica-like particles in a new poly(vinylidene fluoride-co-hexafluoropropylene (poly(VDF-co-HFP)) copolymer are presented. These sulfonic acid-functionalized polysiloxane-based fillers were synthesized via sol-gel chemistry from 3-mercaptopropyltri(ethoxy)silane and polydimethoxysiloxane in the molten poly(VDF-co-HFP) copolymer. To process such nanocomposites materials, the reactive extrusion parameters were selected to (i) reach silanol condensation extents in the processing conditions of a conventional poly(VDF-co-HFP) copolymer and (ii) obtain a surface functionalization high enough with respect to reach a suitable protonic conductivity value of the nanocomposites films. Nevertheless, the enhancement of the proton conductivity being deeply associated with the morphologies of the nanocomposites materials (i.e. the dispersed functional silica phase needs to display the largest interface area with the polymer matrix), an interfacial agent was synthesized. This compatibilizer was either poly(VDF-co-α-trifluoromethacrylic acid) or poly(VDF-co-MAF) copolymers or a poly(VDF-co-HFP) copolymer grafted with maleic anhydride, denoted poly(VDF-co-HFP)-g-MA. In a second step, to obtain the required sulfonic acid-functionalized silica dispersed phase from the presence of the mercaptan functions, the resulting films were oxidized. Electrochemical properties were evaluated: for a theoretical ionic exchange capacity (IEC) of 2 meq.g−1, experimental IEC ranged between 1.0 and 1.3 meq.g−1. The proton conductivity was found to reach 78 mS cm−1 at room temperature for 100% of relative humidity. Interestingly, these values are higher than those of the same membranes processed without any compatibilizer (54 mS cm−1) and that of Nafion® NR112 (52 mS cm−1). This approach also allows limiting the swelling (or water uptake) of proton conductive membranes below 15 wt% while it reachs 30 wt% for Nafion® NR112 in similar conditions.