Can surface energy measurements predict the impact of catalyst hydrophobicity upon fatty acid esterification over sulfonic acid functionalised periodic mesoporous organosilicas?

Sulfonic acid functionalised periodic mesoporous organosilicas (PrSO3 H-PMOs) with tunable hydrophobicity were synthesised via a surfactant-templating route, and characterised by porosimetry, TEM, XRD, XPS, inverse gas chromatography (IGC) and ammonia pulse chemisorption. IGC reveals that incorporation of ethyl or benzyl moieties into a mesoporous SBA-15 silica framework significantly increases the non-specific dispersive surface energy of adsorption for alkane adsorption, while decreasing the free energy of adsorption of methanol, reflecting increased surface hydrophobicity. The non-specific dispersive surface energy of adsorption of PMO-SO3H materials is strongly correlated with their activity towards palmitic acid esterification with methanol, demonstrating the power of IGC as an analytical tool for identifying promising solid acid catalysts for the esterification of free fatty acids. A new parameter [-ΔGCNP-P], defined as the per carbon difference in Gibbs free energy of adsorption between alkane and polar probe molecules, provides a simple predictor of surface hydrophobicity and corresponding catalyst activity in fatty acid esterification. © 2014 Elsevier B.V.

Interdependent lateral interactions, hydrophobicity and acid strength and their influence on the catalytic activity of nanoporous sulfonic acid silicas

A series of propylsulfonic (MCM-SOH) and octyl co-functionalised propylsulfonic (MCM-Oc-SOH) catalysts have been prepared by post modification of MCM-41 with mercaptopropyltrimethoxysilane (MPTS) to achieve SOH surface coverages spanning the range 0.12-1 monolayer. Within the MCM-Oc-SOH series, samples with submonolayer MPTS coverages were further grafted with octyltrimethoxysilane to cap bare hydroxyl sites and tune the hydrophobicity of the support. For the MCM-SO H series NH calorimetry revealed acid strength increases as a function of sulfonic acid loading, with -ΔH(NH ) increasing from 87 to 118 kJ mol. In contrast, MCM-Oc-SOH exhibits a dramatic enhancement of acid strength for submonolayer SOH coverages, with -ΔH(NH ) found to increase to 103 kJ mol. In line with these acid strength measurements the per-site activity of the MCM-SOH series in the esterification of butanol with acetic acid was found to increase with SOH content. Incorporation of octyl groups further promotes esterification activity of all the samples within the MCM-Oc-SOH series, such that the turn over frequency of the sample with the lowest loading of SOH more than doubles. Molecular dynamic simulations indicate that the interaction of isolated sulfonic acid groups with the pore walls is the primary cause of the decrease in acid strength and activity of submonolayer samples within the MCM-SOH series. Incorporation of octyl groups results in a combination of increased hydrophobicity and lateral interactions between adjacent sulfonic acid head groups, resulting in a striking enhancement of acid strength and esterification activity. © 2010 The Royal Society of Chemistry.

Hierarchical macroporous-mesoporous SBA-15 sulfonic acid catalysts for biodiesel synthesis

Hierarchical macroporous-mesoporous SBA-15 silicas have been synthesised via dual-templating routes employing liquid crystalline surfactants and polystyrene beads. These offer high surface areas and well-defined, interconnecting macro- and mesopore networks with respective narrow size distributions around 300 nm and 3-5 nm for polystyrene:tetraethoxysilane ratios ≥2:1. Subsequent functionalisation with propylsulfonic acid yields the first organized, macro-mesoporous solid acid catalyst. The enhanced mass transport properties of these new bi-modal solid acid architectures confer significant rate enhancements in the transesterification of bulky glyceryl trioctanoate, and esterification of long chain palmitic acid, over pure mesoporous analogues. This paves the way to the wider application of hierarchical catalysts in biofuel synthesis and biomass conversion. © 2010 The Royal Society of Chemistry.

Tunable KIT-6 mesoporous sulfonic acid catalysts for fatty acid esterification

We report the first catalytic application of pore-expanded KIT-6 propylsulfonic acid (PrSO H) silicas, in fatty acid esterification with methanol under mild conditions. As-synthesized PrSO H-KIT-6 exhibits a 40 and 70% enhancement in turnover frequency (TOF) toward propanoic and hexanoic acid esterification, respectively, over a PrSO H-SBA-15 analogue of similar 5 nm pore diameter, reflecting the improved mesopore interconnectivity of KIT-6 over SBA-15. However, pore accessibility becomes rate-limiting in the esterification of longer chain lauric and palmitic acids over both solid acid catalysts. This problem can be overcome via hydrothermal aging protocols which permit expansion of the KIT-6 mesopore to 7 nm, thereby doubling the TOF for lauric and palmitic acid esterification over that achievable with PrSO H-SBA-15. © 2012 American Chemical Society.

Pore-expanded SBA-15 sulfonic acid silicas for biodiesel synthesis

Here we present the first application of pore-expanded SBA-15 in heterogeneous catalysis. Pore expansion over the range 6-14 nm confers a striking activity enhancement towards fatty acid methyl ester (FAME) synthesis from triglycerides (TAG), and free fatty acid (FFA), attributed to improved mass transport and acid site accessibility.

Optimising catalytic properties of supported sulfonic acid catalysts

Siliceous mesoporous molecular sieves (SBA-15) have been functionalised with propylsulfonic acid groups by both co-condensing 3-mercaptopropyltrimethoxysilane with the solid at the synthesis (sol-gel) stage and by grafting the same compound to pre-prepared SBA-15, followed, in both cases, by oxidation to sulfonic acid. The acidic and catalytic properties of the supported sulfonic acids prepared in the two ways have been compared, using ammonia adsorption calorimetry and the benzylation reaction between benzyl alcohol and toluene. Using a combination of X-ray photoelectron spectroscopy and other analytical techniques, the level of functionalisation and the extent of subsequent oxidation of tethered thiol to sulfonic acid, both in the bulk and close to the surface of SBA-15 particles, have been assessed. The research shows that the co-condensing route leads to higher levels of functionalisation than the grafting route. The extent of oxidation of added thiol to acid groups is similar using the two routes, about 70% near the surface and only 50% in the bulk. Comparison is made with polymer supported sulfonic acid catalysts, Amberlysts 15 and 35, and Nafion. Nafion shows the highest acid strength and the highest specific catalytic activity of all materials studied. Amongst the other materials, average acid strengths are broadly similar but there appears to be a relationship between the concentration of acid sites on the catalysts and their specific activity in the benzylation reaction. A model is proposed to explain this, in which clustering of sulfonic acid groups, even to a small extent, leads to disproportionately enhanced catalytic activity. © 2009 Elsevier B.V. All rights reserved.

Sol-gel sulfonic acid silicas as catalysts

Silica-supported sulfonic acids are a class of solid Brønsted acid catalysts that generally comprise organo-sulfonic acid groups tethered to silica surfaces. Methodologies to prepare organically modified silica have been widely developed in separation science and the techniques for their preparation are well documented. The application of this chemistry to prepare pure Brønsted sulfonic acid functionalized mesoporous silicas has stimulated significant research effort in this area, since these materials are interesting alternatives to commercially available sulfonated polymer resins, such as Amberlyst–15 and Nafion-H (sulfonated polystyrene and perfluorinated sulfonic acid resins respectively), which suffer from low surface areas and thermal stability. This chapter presents an overview of the preparation of mesostructured silica supported sulfonic acids, their catalytic applications and reviews the approaches taken to tune catalyst performance in organic transformations.

Hydrothermal saline promoted grafting:a route to sulfonic acid SBA-15 silica with ultra-high acid site loading for biodiesel synthesis

A simple grafting protocol is reported which affords a ten-fold enhancement in acid site density of mesoporous sulfonic acid silicas compared to conventional syntheses, offering improved process efficiency and new opportunities for tailored supported solid acids in sustainable chemistry. This journal is

Hydrothermal saline promoted grafting of periodic mesoporous organic sulfonic acid silicas for sustainable FAME production

Hydrothermal saline promoted grafting of sulfonic acid groups onto SBA-15 and periodic mesoporous organic silica analogues affords solid acid catalysts with high acid site loadings (>2.5 mmol g-1 H+), ordered mesoporosity and tunable hydrophobicity. The resulting catalysts show excellent activity for fatty acid esterification and tripalmitin transesterification to methyl palmitate, with framework phenyl groups promoting fatty acid methyl esters production. (Chemical Equation Presented)

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

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.

New insights in the deactivation of sulfonic modified SBA-15 catalysts for biodiesel production from low-grade oleaginous feedstock

Arenesulfonic-acid functionalized SBA-15 materials have been used in the production of biodiesel from low grade oleaginous feedstock. These materials display an outstanding catalytic activity, being able to promote the transformation of crude palm oil with methanol into fatty acid methyl esters with high yield (85%) under mild reaction conditions. However, high sensitivity of the catalyst against poisoning by different substances has also been detected. Thus, alkaline metal cations, such as sodium or potassium exert a negative influence on the catalytic activity of these materials, being necessary amounts around 500 ppm of sodium in the reaction media to decrease the catalytic activity of these materials to a half of its initial value in just two reaction runs. The deactivation of arenesulfonic acid functionalized SBA-15 materials seems to occur in this case by ion exchange of the acid protons at the sulfonic groups. Organic unsaponifiable compounds like lecithin or retinol also induce a negative influence in the catalytic activity of these sulfonic acid-based materials, though not so intense as in the case of alkaline metals. The deactivating mechanism associated to the influence of the organic compounds seems to be linked to the adsorption of such substances onto the catalytic acid sites as well as on the silica surface. The accumulation of lecithin in the surface of catalyst, observed by means of thermogravimetric analysis, suggest the creation of a strong interaction, probably by ion pair, between this compound and the sulfonic acid group.

Design and synthesis of proteoglycan analogues for tissue repair and regeneration

This thesis is concerned with the design and synthesis of a novel, injectable proteoglycan analogue for tissue repair. This is of particular relevance to the restoration of disc height to a degraded nucleus pulposus of the intervertebral disc. The focus is on the use of sulfonate monomers as proteoglycan analogues, in particular sodium 2-acrylamido-2-methylpropane sulfonic acid and the potassium salt of 3-sulfopropyl acrylate. For most biomedical applications, synthetic hydrogels need to show dimensional stability to changes in pH, osmolarity, and temperature. This is readily achieved by neutral structures however ionic sulfonate containing hydrogels are responsive to environmental change which renders them difficult to manage in most tissue replacement applications. In this case osmotic responsiveness rather than stability is desirable. Therefore sulfonate based materials possess advantageous properties. This is a result of the sulfonate becoming an ideal surrogate for the sulfate group present within the structure of natural proteoglycans. This thesis reports polymerisation studies based on the production of a redox initiated copolymer system capable of polymerising in situ within a timescale of circa. 5-7 minutes. The rheological properties, osmotic drive, and residual monomer content of successful compositions is analysed. Properties are adapted to mimic those of the target natural tissue. The adaptation of the material for use as an injectable intra-ocular lens, with hyaluronic acid as an interpenetrate is reported. The synthesis of a radiopaque macromer to allow visibility of the repair system once in situ is investigated and discussed. The results presented in this thesis describe a suitable proteoglycan tissue analogue which is injectable, biomimetic, osmotically responsive and mechanically stable in its desired application.

Structural effects in photopolymerized sodium AMPS hydrogels crosslinked with poly(ethylene glycol) diacrylate for use as burn dressings

Synthetic hydrogel polymers were prepared by free radical photopolymerization in aqueous solution of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid (Na-AMPS). Poly(ethylene glycol) diacrylate (PEGDA) and 4,4'-azo-bis(4-cyanopentanoic acid) were used as the crosslinker and UV-photoinitiator, respectively. The effects of varying the Na-AMPS monomer concentration within the range of 30-50% w/v and the crosslinker concentration within the range of 0.1-1.0% mol (relative to monomer) were studied in terms of their influence on water absorption properties. The hydrogel sheets exhibited extremely high swelling capacities in aqueous media which were dependent on monomer concentration, crosslink density, and the ionic strength and composition of the immersion medium. The effects of varying the number-average molecular weight of the PEGDA crosslinker from = 250 to 700 were also investigated. Interestingly, it was found that increasing the molecular weight and therefore the crosslink length at constant crosslink density decreased both the rate of water absorption and the equilibrium water content. Cytotoxicity testing by the direct contact method with mouse fibroblast L929 cells indicated that the synthesized hydrogels were nontoxic. On the basis of these results, it is considered that photopolymerized Na-AMPS hydrogels crosslinked with PEGDA show considerable potential for biomedical use as dressings for partial thickness burns. This paper describes some structural effects which are relevant to their design as biomaterials for this particular application. © 2013 Copyright Taylor and Francis Group, LLC.