1.2.1 SOLID ACID CATALYSED BIODIESEL SYNTHESIS

A wide range of inorganic and polymeric solid acids are commercially available, however, their application for the transesterification of oils into biodiesel has only been recently explored, in part reflecting their lower activity compared with base-catalysed routes [27], in turn necessitating higher reaction temperatures to deliver suitable conversions. While their activities are generally low, solid acids have the advantage that they are less sensitive to FFA contaminants than their solid base analogues, and hence can operate with unrefined feedstocks containing 3-6 wt% FFAs [27]. In contrast to solid bases which require feedstock pretreatment to re­move fatty acid impurities, solid acids are able to esterify FFAs through to FAME in parallel with transesterification major TAG components without soap formation and thus reduce the number of processing steps to bio­diesel [70-72].

Mesoporous silicas from the SBA family [73] have been examined for biodiesel synthesis, and include materials grafted with sulfonic acid groups [74, 75] or SO4/ZrO2 surface coatings [76]. Phenyl and propyl sulfonic acid SBA-15 catalysts are particularly attractive materials with activities com­parable to Nafion and Amberlyst resins in palmitic acid esterification [77].

Phenylsulfonic acid functionalised silica is reportedly more active than their corresponding propyl analogues, in line with their respective acid strengths but is more difficult to prepare. Unfortunately, conventionally syn­thesised sulfonic acid functionalised SBA-15 silicas with pore sizes below ~6 nm possess long, isolated parallel channels and suffer correspondingly slow in-pore diffusion and catalytic turnover in FFA esterification. How­ever, poragens such as trimethylbenzene [78] triethylbenzene or triisopro­pylbenzene [79] can induce swelling of the Pluronic P123 micelles used to produce SBA-15, enabling ordered mesoporous silicas with diameters spanning 5-30 nm, and indeed ultra-large-pores with a BJH pore diameter as much as 34 nm [79]. This methodology was recently applied to prepare a range of large pore SBA-15 materials employing trimethylbenzene as the poragen, resulting in the formation of highly ordered periodic meso — structures with pore diameters of ~6, 8 and 14 nm [80]. These silicas were subsequently functionalised by mercaptopropyl trimethoxysilane (MPTS) and oxidised with H2O2 to yield expanded PrSO3-SBA-15 catalysts which were effective in both palmitic acid esterification with methanol and trica — prylin and triolein transesterification with methanol under mild conditions.

FIGURE 2: Superior performance of interconnected, mesoporous propylsulfonic acid KIT-6 catalysts for biodiesel synthesis via FFA esterification with methanol versus non-interconnected mesoporous SBA-15 analogue. Adapted from reference [82]. Copyright 2012 American Chemical Society

For both reactions, turnover frequencies dramatically increased with pore diameter, and all sulfonic acid heterogeneous catalysts significantly out­performed a commercial Amberlyst resin (Fig. 1). These rate enhance­ments are attributed to superior mass transport of the bulky FFA and tri­glycerides within the expanded PrSO3-SBA-15. Similar observations have been made over Poly(styrenesulfonic acid)-functionalised ultra-large pore SBA-15 in the esterification of oleic acid with butanol [81].

Improving pore interconnectivity, for example through swapping the p6 mm architecture of SBA-15 for the Ia3d of KIT-6 was subsequently explored as an alternative means to enhance in-pore active site accessi­bility (Scheme 1) for FFA esterification [82]. KIT-6 mesoporous materi­als exhibit improved characteristics for biomolecule immobilisation [83] reflecting superior diffusion within the interconnected cubic structure. A family of pore-expanded propylsulfonic acid KIT-6 analogues were pre­pared via MPTS grafting and oxidation and screened for FFA esterification with methanol as a function of alkyl chain length under mild conditions. As-synthesised PrSO3H-KIT-6 exhibited respective 40 and 70 % TOF en­hancements toward propanoic and hexanoic acid esterification compared with a PrSO3H-SBA-15 analogue of comparable (5 nm) pore diameter as a consequence of the improved mesopore interconnectivity. However, pore accessibility remained rate-limiting for esterification of the longer chain lauric and palmitic acids. Hydrothermal aging protocols facilitated expan­sion of the KIT-6 mesopore up to 7 nm, with consequent doubling of TOFs for lauric and palmitic acid esterification versus PrSO3H-SBA-15 (Fig. 2).

While numerous solid acids have been applied for biodiesel synthesis [27, 32, 84], most materials exhibit micro — and/or mesoporosity which, as illustrated above, are not optimal for accommodating bulky C16-C18 TAGs of FFAs. For example, incorporation of a secondary mesoporosity into a microporous H-p-zeolite to create a hierarchical solid acid significantly increased catalytic activity by lowering diffusion barriers [85]. Templated mesporous materials are widely used as catalyst supports [86, 87], with SBA-15 silicas popular candidates for reactions pertinent to biodiesel syn­thesis as previously discussed [75, 77, 88]. However, such surfactant-tem — plated supports possessing long, isolated parallel and narrow channels are ill-suited to efficient in-pore diffusion of bio-oil feedstocks affording poor catalytic turnover. Further improvements in pore architecture are hence required to optimise mass transport of heavier bulky TAGs and FFAs com­monly found in plant and algal oils. Simulations demonstrate that in the Knudsen diffusion regime [89], where reactants/products are able to dif­fuse enter/exit mesopores but experience moderate diffusion limitations, hierarchical pore structures may significantly improve catalyst activity. Materials with interpenetrating, bimodal meso-macropore networks have been prepared using microemulsion [90] or co-surfactant [91] templating routes and are particularly attractive for liquid phase, flow reactors where­in rapid pore diffusion is required. Liquid crystalline (soft) and colloidal polystyrene nanospheres (hard) templating methods have been combined to create highly organised, macro-mesoporous aluminas [92] and ‘SBA — 15 like’ silicas [93] (Scheme 4), in which both macro- and mesopore di­ameters can be independently tuned over the range 200-500 and 5-20 nm, respectively. The resulting hierarchical pore network of a propylsulfonic acid functionalised macro-mesoporous SBA-15 is shown in Fig. 3, where­in macropore incorporation confers a striking enhancement in the rates of tricaprylin transesterification and palmitic acid esterification with metha­nol, attributed to the macropores acting as transport conduits for reactants to rapidly access PrSO3H active sites located within the mesopores.

The hydrophilic nature of polar silica surfaces hinders their application for reactions involving apolar organic molecules. This is problematic for TAG transesterification (or FFA esterification) due to preferential in-pore diffusion and adsorption of alcohol versus fatty acid components. Surface hydroxyl groups also favour H2O adsorption, which if formed during FFA esterification can favour the reverse hydrolysis reaction and consequent low FAME yields. Surface modification via the incorporation of organic functionality into polar oxide surfaces, or dehydroxylation, can lower their polarity and thereby increase initial rates of acid catalysed transformations of liquid phase organic molecules [94]. Surface polarity can also be tuned by incorporating alkyl/aromatic groups directly into the silica framework, for example polysilsesquioxanes can be prepared via the co-condensation of 1,4-bis(triethoxysilyl)benzene (BTEB), or 1,2-bis(trimethoxysilyl)- ethane (BTME), with TEOS and MPTS in the sol-gel process [95, 96] which enhances small molecule esterification [97] and etherification [98]. The incorporation of organic spectator groups (e. g. phenyl, methyl or propyl) during the sol-gel syntheses of SBA-15 [99] and MCM-41 [100]

sulphonic acid silicas is achievable via co-grafting or simple addition of the respective alkyl or aryltrimethoxysilane during co-condensation pro­tocols. An experimental and computational study of sulphonic acid func­tionalised MCM-41 materials was undertaken to evaluate the effect of acid site density and surface hydrophobicity on catalyst acidity and associated performance [101]. MCM-41 was an excellent candidate due to the avail­ability of accurate models for the pore structure from kinetic Monte Carlo simulations [102], and was modified with surface groups to enable dynam­ic simulation of sulphonic acid and octyl groups co-attached within the MCM-41 pores. In parallel experiments, two catalyst series were inves­tigated towards acetic acid esterification with butanol (Scheme 5). In one series, the propylsulphonic acid coverage was varied between 0 (RSO3H) = 0-100 % ML over the bare silica (MCM-SO3H). For the second oc­tyl co-grafted series, both sulfonic acid and octyl coverages were tuned (MCM-Oc-SO3H). These materials allow the effect of lateral interactions between acid head groups and the role of hydrophobic octyl modifiers upon acid strength and activity to be separately probed.

SCHEME 4: Liquid crystal and polystyrene nanosphere dual surfactant/physical templating route to hierarchical macroporous-mesoporous silicas

FIGURE 3: (Left) SEM (a) and low and high magnification TEM (b, c) micrographs of a hierarchical macro-mesoporous Pr-SO3H-SBA-15; (right) corresponding catalytic performance in palmitic acid esterification and tricaprylin transesterification with methanol as a function of macropore density versus a purely mesoporous Pr-SO3H-SBA-15. Adapted from reference [93] with permission from The Royal Society of Chemistry

To avoid diffusion limitations, butanol esterification with acetic acid was selected as a model reaction (Fig. 4). Ammonia calorimetry revealed that the acid strength of polar MCM-SO3H materials increases from 87 to 118 kJ mol-1 with sulphonic acid loading. Co-grafted octyl groups dra­matically enhance the acid strength of MCM-Oc-SO3H for submonolayer SO3H coverages, with _AHads(NH3) rising to 103 kJ mol-1. The per site activity of the MCM-SO3H series in butanol esterification with acetic acid mirrors their acidity, increasing with SO3H content. Octyl surface func­tionalisation promotes esterification for all MCM-Oc-SO3H catalysts, doubling the turnover frequency of the lowest loading SO3H material. Molecular dynamic simulations indicate that the interaction of isolated sulphonic acid moieties with surface silanol groups is the primary cause of the lower acidity and activity of submonolayer samples within the MCM-

Advanced Biofuels: Using Catalytic Routes for Conversion

SO3H series. Lateral interactions with octyl groups help to re-orient sul — phonic acid headgroups into the pore interior, thereby enhancing acid strength and associated esterification activity.

In summary, recent developments in tailoring the structure and sur­face functionality of sulfonic acid silicas have led to a new generation of tunable solid acid catalysts well-suited to the esterification of short and long chain FFAs, and transesterification of diverse TAGs, with methanol under mild reaction conditions. A remaining challenge is to extend the dimensions and types of pore-interconnectivities present within the host silica frameworks, and to find alternative low cost soft and hard templates to facilitate synthetic scale-up of these catalysts for multi-kg production. Surfactant template extraction is typically achieved via energy-intensive solvent reflux, which results in significant volumes of contaminated waste and long processing times, while colloidal templates often require high temperature calcination which prevents template recovery/re-use and re­leases carbon dioxide. Preliminary steps towards the former have been recently taken, employing room temperature ultrasonication in a small sol­vent volume to deliver effective extraction of the P123 Pluronic surfactant used in the preparation of SBA-15 in only 5 min, with a 99.9 % energy saving and 90 % solvent reduction over reflux methods, and without com­promising textural, acidic or catalytic properties of the resultant Pr-SO3H — SBA-15 in hexanoic acid esterification (Fig. 5) [103].