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Adam Lee is Professor of Sustainable Chemistry and an EPSRC Leadership Fellow in the European Bioenergy Research Institute, Aston University. He holds a BA (Natural Sciences) and PhD from the University of Cambridge, and following postdoctoral research at Cambridge and Lecturer/Senior Lecturer roles at the Universities of Hull and York respectively, held Chair appointments at Cardiff, Warwick and Monash universities. His research addresses the rational design of nanoengineered materials for clean catalytic technologies, with particular focus on sustainable chemical processes and energy production, and the development of in situ methods to provide molecular insight into surface reactions, for which he was awarded the 2012 Beilby Medal and Prize by the Royal Society of Chemistry.
Dr James Andrew Bennett obtained his Master and PhD at the University of Leicester, where he investigated the use of perfluoroalkyl moieties to allow heterogenisation of homogeneous catalysts over zirconium phosphonate supports. He then worked at the University of Birmingham, researching biogenic heterogeneous catalysts composed of transition metal nanoparticles supported on bacterial biomass, using waste sources of metals and biomass to produce "green" catalyst materials. He is currently working with Professors Karen Wilson and Adam Lee at the European Bioenergy Research Institute at Aston University, developing environmentally sustainable catalysts derived from industrial waste for pyrolysis oil upgrading.
Dr Jinesh Manayil obtained his MSc in Chemistry from Mahatma Gandhi University in 2004, prior to a MTech in Industrial Catalysis from Cochin University of Science and Technology in 2007. He subsequently undertook postgraduate research in catalytic and ion-exchange applications of layered double hydroxides, receiving his PhD in 2012 from the Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), India under the supervision of Dr Kannan Srinivasan. He is currently a Research Associate with Professors Karen Wilson and Adam Lee at the European Bioenergy Research Institute at Aston University, where he is developing solid acid-base catalysts for biomass mass conversion.
Karen Wilson is Professor of Catalysis and Research Director of the European Bioenergy Research Institute at Aston University, where she holds a Royal Society Industry Fellowship. Her research interests lie in the design of heterogeneous catalysts for clean chemical synthesis, particularly the design of tunable porous materials for sustainable biofuels and chemicals production from renewable resources. She was educated at the Universities of Cambridge and Liverpool, and following postdoctoral research at Cambridge and the University of York, was appointed a Lecturer and subsequently Senior Lecturer at York, prior to appointment as a Reader in Physical Chemistry at Cardiff University.
Sustainability, in essence the development of methodologies to meet the needs of the present without compromising those of future generations, has become a watchword for modern society, with developed and developing nations and multinational corporations promoting international research programmes into sustainable food, energy, materials, and even city planning. In the context of energy, despite significant growth in proven and predicted fossil fuel reserves over the next two decades, notably heavy crude oil, tar sands, deepwater wells, and shale oil and gas, there are great uncertainties in the economics of their exploitation via current extraction methodologies, and crucially, an increasing proportion of such carbon resources (estimates vary between 65–80%1–3) cannot be burned without breaching the UNFCC targets for a 2 °C increase in mean global temperature relative to the pre-industrial level.4,5 There is clearly a tightrope to walk between meeting rising energy demands, predicted to climb 50% globally by 20406 and the requirement to mitigate current CO2 emissions and hence climate change. Similar considerations apply to ensuring a continued supply of organic materials for applications including polymers, plastics, pharmaceuticals, optoelectronics and pesticides, which underpin modern society, and for which significant future growth is anticipated, tracking the predicted four-fold rise in global GDP and associated requirements for advanced consumer products by 2050.7 The quest for sustainable resources to meet the demands of a rapidly rising world population represents one of this century's grand challenges.8,9 Heterogeneous catalysis has a rich history of facilitating energy efficient selective molecular transformations and contributes to 90% of chemical manufacturing processes and to more than 20% of all industrial products.10,11 In a post-petroleum era, catalysis will be central to overcoming the engineering and scientific barriers to economically feasible routes to alternative source of both energy and chemicals, notably bio-derived and solar-mediated via artificial photosynthesis (Scheme 1).
Scheme 1 Current and future roles for heterogeneous catalysis in the production of sustainable chemicals and fuels.
Scheme 2 Biorefinery routes for the co-production of chemicals and transportation fuels from biomass.
Biodiesel is a clean burning and biodegradable fuel which, when derived from non-food plant or algal oils or animal fats, is viewed as a viable alternative (or additive) to current petroleum-derived diesel.22 Commercial biodiesel is currently synthesised via liquid base catalysed transesterification of C14–C20 triacylglyceride (TAG) components of lipids with C1–C2 alcohols23–26 into fatty acid methyl esters (FAMEs) which constitute biodiesel as shown in Scheme 3, alongside glycerol as a potentially valuable by-product.27 While the use of higher (e.g. C4) alcohols is also possible,28 and advantageous in respect of producing a less polar and corrosive FAME29 with reduced cloud and pour points,30 the current high cost of longer chain alcohols, and difficulties associated with separating the heavier FAME product from unreacted alcohol and glycerol, remain problematic. Unfortunately, homogeneous acid and base catalysts can corrode reactors and engine manifolds, and their removal from the resulting biofuel is particularly problematic and energy intensive, requiring aqueous quench and neutralisation steps which result in the formation of stable emulsions and soaps.12,31,32 Such homogeneous approaches also yield the glycerine by-product, of significant potential value to the pharmaceutical and cosmetic industries, in a dilute aqueous phase contaminated by inorganic salts. The utility of solid base and acid catalysts for biodiesel production has been widely reported,15,25,33–41 wherein they offer improved process efficiency by eliminating the need for quenching steps, allowing continuous operation,42 and enhancing the purity of the glycerol by-product. Technical advances in catalyst and reactor design remain essential to utilise non-food based feedstocks, and thereby ensure that biodiesel remains a key player in the renewable energy sector for the 21st century. In this review, we highlight the contributions of tailored solid acid and base catalysts to catalytic biodiesel synthesis via TAG transesterification to FAMEs and free fatty acid (FFA) esterification.
Scheme 3 Biodiesel production cycle from renewable bio-oils via catalytic transesterification and esterification.
Basicity in alkaline earth oxides is believed to arise from M2+–O2− ion pairs present in different coordination environments.77 The strongest base sites occur at low coordination defect, corner and edge sites, or on high Miller index surfaces. Such classic heterogeneous base catalysts have been extensively tested for TAG transesterification78 and there are numerous reports on commercial and microcrystalline CaO applied to rapeseed, sunflower or vegetable oil transesterification with methanol.79,80 Promising results have been obtained, with 97% oil conversion achieved at 75 °C,80 however concern remains over Ca2+ leaching under reaction conditions and associated homogeneous catalytic contributions,81 a common problem encountered in metal catalysed biodiesel production which hampers commercialisation.82 While Ca and Mg are the more widely used alkaline earth metals in solid base catalysis, strontium oxides have also found application in biodiesel production. Pure strontium oxide possesses the highest base site density of the alkali earth oxides as determined by CO2 temperature programmed desorption (TPD),83 and a comparable base strength to that of BaO (26.5 < H−). Despite the lower surface area of SrO compared to Mg and Ca oxides (19, 14 and 3 m2 g−1 respectively), it showed the highest activity for hempseed oil transesterification, although it is questionable whether such low area/highly soluble materials could ever be commercially viable.
Fig. 1 Relationship between surface polarisability of MgO nanocrystals and their turnover frequency towards tributyrin transesterifcation. Adapted from ref. 90 with permission from The Royal Society of Chemistry.
Cesium doping via co-precipitation under supercritical conditions confers even greater activity towards tributyrin transesterification with methanol,85 due to the genesis of additional, and stronger, base sites associated with a new ordered mixed oxide phase which EXAFS analysis recently identified as Cs2Mg(CO3)2(H2O)4,92 resulting in superior performance compared with MgO and even homogeneous Cs2CO3 catalysts (Fig. 2). Unfortunately, surface carbon deposition and loss of this high activity Cs2Mg(CO3)2(H2O)4 phase due to partial Cs dissolution results in on-stream deactivation of Cs-doped MgO, although recalcination could help to regenerate activity.
Fig. 2 Formation of crystalline Cs2Mg(CO3)2(H2O)4 phase within co-precipitated Cs-doped MgO and resulting synergy in the transesterification of short and long chain TAGs with methanol compared with undoped nanocrystalline MgO. Adapted from ref. 85 with kind permission from Springer Science and Business Media and ref. 92 with permission from John Wiley and Sons.
Alkaline earth metal oxides may be incorporated into metal oxides to form composite oxides93 which are also suitable as solid base catalysts for biodiesel production. The activity of such composites is similar to that of the parent alkaline earth (typically CaO), but they exhibit greater stability and are less prone to dissolution, facilitating separation from the reaction media. Calcination temperature strongly influences the resulting catalytic activity towards transesterification. For example, a Ca–Al composite oxide containing Ca12Al14O33 and CaO thermally processed between 120 °C and 1000 °C showed maximal activity after a 600 °C treatment due to changes in specific surface area and crystallinity. CaO was only observed in samples prepared >600 °C, accompanied by the formation of crystalline Ca12Al14O33. Synergy between these two phases greatly improved the transesterification activity, however calcination at temperatures significantly above 600 °C induced crystallite sintering and concomitant loss of surface area and activity. Unfortunately the catalyst synthesis employed sodium precursors, hence alkali contamination of these catalysts cannot be discounted, and which in any event were employed at high loadings (6 wt%) and without recycle tests.
Calcium also forms a mixed oxide with MoO3.94 Supporting both oxides on SBA-15 mesoporous silica afforded a transesterification catalyst with improved stability relative to CaO due the presence of acidic MoO3 sites on the SBA-15. The impact of Ca : Mo ratio and calcination temperatures was explored, with a Ca : Mo ratio of 6 : 1 maximising activity for soybean oil conversion, boosting FAME yields from 48 to 83% over extremely long reaction times in excess of 50 h. Raising the calcination temperature from 350 °C to 550 °C induced CaO and MoO3 crystallisation, with a corresponding rise in activity; higher temperature calcination did not promote further crystallisation and was not beneficial for transesterification.
Alkaline earth oxides may be used to support acidic or amphoteric materials to form materials with mixed acid–base character. Transesterification of soybean oil over CaO supported SnO2 prepared via impregnation was highly dependent on calcination temperature and the Ca : Sn ratio.95 The interaction between acidic SnO2 and basic CaO resulted in a highly SnO2 phase and associated active sites. Calcination above 350 °C was required to initiate decomposition of the Ca precursor, with temperatures >650 °C driving complete conversion to Ca oxides. Optimal performance was obtained for high calcination temperatures, which maximised the CaO content. Further heating again led to particle sintering/agglomeration and decreased reactivity. Supported CuO can also produce biodiesel from hempseed oil,83 with 10 wt% CuO/SrO offering 20% higher FAME yields under optimised conditions than other alkaline earth oxides. The CuO could also undergo chemical reduction during transesterification to form an active catalyst for the selective hydrogenation of polyunsaturated hydrocarbons for further biodiesel upgrading. It should be noted that the catalyst loadings employed in this study of 4–12 wt% would likely prove prohibitive in any commercial process, and that small but significant (29 ppm) quantities of leached Ca may have contributed to the observed performance.
Composites of Sr and Al were prepared by Farzaneh et al. and evaluated for soybean oil transesterification with methanol.96 The dominant crystalline phase was Sr3Al2O6, giving rise to medium and high strength base sites with corresponding CO2 desorption peak maxima of 388 °C and 747 °C respectively. The Sr–Al oxide also possessed a higher density of base sites compared to solid bases such as CaO/Al2O3, reflected in an eight-fold higher CO2 adsorption capacity. These superior base properties enhanced the activity of the strontium composite for soybean transesterification to FAMEs, resulting in comparable conversions at a lower catalyst loading and shorter reaction time than for a MgAl hydrotalcite and CaO/Al2O3. While oil conversions fell noticeably with repeated re-use, there was no evidence of alkaline earth dissolution, and the resulting biodiesel fuel met ASTM and EN standards.
As shown in Fig. 1, lithium doped CaO can enhance tributyrin transesterification. Li doping has also been exploited over SiO2, wherein 800 °C calcination results in a lithium orthosilicate solid base catalyst, Li4SiO4.97 Although the basic strength of Li4SiO4, determined by Hammett indicators, was less than that of CaO, both materials exhibited similar initial activity towards soybean transesterification, with the lithium orthosilicate more stable and maintaining activity after prolonged exposure to air, in contrast to CaO. The superior stability of the Li4SiO4 catalyst was further demonstrated by its water and carbon dioxide tolerance, both of which poison conventional alkaline earth catalysts.
Sodium silicate, Na2SiO3, is also active for biodiesel production from rapeseed and jatropha oils under both conventional98 and microwave assisted conditions,99 with a 98% FAME yield after one hour reaction under mild conditions. Although this catalyst displayed good recyclability, TAG conversions fell steadily to <60% after four re-uses, attributed to water adsorption and Si–O–Si bond cleavage and sodium leaching.98 The same catalyst was evaluated using microwave heating for only five minutes at a range of powers between 100–500 W (Fig. 3).99 At low power only 18% rapeseed oil conversion was obtained. Higher powers heated the reaction mixture (to ∼175 C for 400 W) in turn boosting FAME yields from both oils to ∼90%, highlighting the use of microwave heating to accelerate biodiesel production. Recycle studies again showed slow in situ deactivation due to particle agglomeration, water adsorption of water, and associated loss of basicity due to sodium leaching into methanol during both transesterification and washing procedures between recycles. Despite some recent successes in the scale-up of microwave-assisted (homogeneously catalysed) biodiesel production (see Section 6),28,100 it remains unlikely that such heating solutions can deliver the high throughput demanded for commercial processes.
Fig. 3 Demonstration of the structural stability and catalytic activity of sodium silicate as a solid base for biodiesel production. Adapted from ref. 99. Copyright (2014), with permission from Elsevier.
Activated carbon can be used as an amphoteric support for basic alkaline metal salts such as K2CO3,101 which is known to be an active homogeneous catalyst for oil transesterification and biodiesel production.102 A study of K2CO3 supported over a range of support materials, such as MgO, activated carbon and SiO2, demonstrated that K2CO3 on basic carriers gave higher activity for rapeseed oil transesterification than when using acidic carriers (unsurprisingly due to self-neutralisation!).102 K2CO3/MgO was shown to be highly stable, with spent catalysts showing minimal loss of performance over six re-uses (though requiring 400 °C reactivation between cycles), and exhibiting negligible structural changes or potassium leaching. Kraft lignin is a low cost, renewable by-product of the Kraft wood pulping process, and possesses high carbon and low ash content and is therefore a popular precursor for activated carbons. Li et al. used K2CO3 in a one-pot method to prepare activated carbon and transform this into a solid base catalyst, namely K2CO3 on Kraft Lignin activated carbon (LKC), for biodiesel production.101 Thermal activation had a significant impact on the resulting catalytic activity, with higher calcination temperatures increasing the surface area and pore volume 100-fold and hence FAME production, however temperatures above 800 °C induced K2CO3 decomposition and poorer performance. Optimal reaction conditions of 65 °C, 3 wt% loading and a K/KLC ratio of 0.6, enabled a 98% FAME yield from rapeseed oil transesterification, which fell to 82% after four recycles as a result of progressive particle agglomeration and potassium leaching into the biodiesel. Wu et al. supported a range of potassium salts on mesoporous silicas for use as solid base biodiesel catalysts.103 A K2SiO3 impregnated catalyst proved superior to K2CO3 and KAc impregnated catalysts due to its higher base site density (1.94 versus 1.81 and 1.72 mmol g−1 respectively). Aluminium addition to the SBA-15 framework improved the morphology, increasing the surface area and pore volume, and CO2 desorption temperature indicative of a more strongly basic support; this observation is rather counter-intuitive, since Al-doping of SBA-15 is usually employed to promote the formation of Brönsted and Lewis acid sites of moderate acidity.104 A 30% K2SiO3/AlSBA-15 catalyst was used for the transesterification of Jatropha oil with MeOH at 60 °C, giving 95% conversion for a relatively low MeOH/oil molar ratio of 9 : 1. This catalyst was recycled five times with only a 6% drop in conversion, but the filtered catalyst required regenerative washing with a methanol–n-hexane mixture and re-calcination to avoid a significant drop in FAME yield to 47% after the fifth recycle. The magnitude of this activity loss indicates significant K leaching. In a related study, Xie et al. immobilised tetraalkylammonium hydroxides onto SBA-15 for soybean oil transesterification.105 The resulting SBA-15-pr-NR3OH catalyst gave 99% conversion to FAMEs under methanol reflux. Covalent linking of the tetraalkylammonium hydroxide to the silica surface prevented in situ leaching, resulting in only a 1% fall in FAME yield after five recycles and appears a promising methodology for biodiesel production at mild-moderate temperatures under which the covalently linked propyl backbone is thermally stable.
Solid bases usually afford higher rates of transesterification than solid acids, hence a range of transition metal oxides of varying Lewis base character have been explored in biodiesel production. MnO and TiO are mild bases with good activity for biodiesel production,111 and have been applied for the simultaneous transesterification of triglycerides and esterification of FFAs under continuous flow conditions using low grade feedstocks with high fatty acid contents (up to 15%). Soap formation, caused by leaching of metal from the catalyst surface under high FFA concentrations, was an order of magnitude less than that observed with conventional homogeneous base catalysts. Unfortunately, this study did not characterise the Mn or Ti oxidation state in either fresh or spent materials to confirm the nature of any catalytic centre. Zirconium has also been shown to activate and stabilise solid base catalysts for biodiesel production.101,112,113 Mixed oxides of CaO and ZrO2 prepared via co-precipitation showed increased surface area and stability with increasing Zr : Ca ratios (Fig. 4). However, the transesterification activity remained dependent upon the Ca content, decreasing at lower CaO loadings.112 Sodium zirconate, a potential CO2 adsorbent,84,114 has shown promise in biodiesel production,113 with 98% conversion of soybean oil to FAME after 3 h at 65 °C. Deactivation observed upon repeated decanting and recycling was attributed to surface poisoning, with methanol washing between cycles facilitating 84% conversion after five recycles. This material's affinity for carbon dioxide and large crystallite size/low surface area (∼1 m2 g−1) may render it air-sensitive and prone to further sintering. Zirconia was employed as a support for a range of sodium-containing bases, such as NaOH, NaH2PO4, C4H5O6Na (monosodium tartrate) and potassium sodium tartrate were doped on ZrO2 to prepare a series of catalysts with varying basic strength and total basicity for the microwave assisted transesterification of soybean oil with methanol.101 Catalytic activity was dependent upon basicity, increasing at higher Na : Zr ratios. The potassium sodium tartrate doped zirconia exhibited the strongest basicity and highest conversions, reaching 54% for Na : Zr = 1 and a 1 : 10 catalyst : soybean oil mass ratio at 60 °C under 600 W microwave power. Increasing the Na : Zr ratio to 2 improved conversion to 92%. Optimal conversions were obtained for catalysts calcined at 600 °C, possibly due to tartrate decomposition at higher temperatures, although this catalyst was recyclable via filtration and re-calcination.
Fig. 4 Effect of Zr-doping on CaO solid base catalysts for biodiesel production. Adapted from reference 112. Copyright (2012), with permission from Elsevier.
Porosity was introduced to a titania-based catalyst through the construction of sodium titanate nanotubes as solid base catalysts for soybean oil transesterification with methanol.115 The catalyst exhibited a range of active sites of varying basicity, however the high sodium content (10 wt%) is a cause for concern due to the high probability of leaching in situ and associated homogeneous chemistry. The pore distribution was bimodal, consisting of 3 nm wide tubular mesopores and ∼40 nm voids between the aggregated nanotubes. Biodiesel yields of >97% were obtained for 1–2 wt% of catalyst at 65 °C. However, a large excess of methanol to oil was required (40 : 1 molar ratio), and while this material could be re-used several times, it was less active than that of CaO and MgO lacking such a nanoporous architecture.
Hydrotalcites are another class of solid base catalysts that have attracted attention because of their high activity and robustness in the presence of water.116,117 Hydrotalcites ([M(II)1−xM(III)x(OH)2]x+(An−x/n)·mH2O) adopt a layered double hydroxide structure with brucite-like (Mg(OH)2) hydroxide sheets containing octahedrally coordinated M2+ and M3+ cations, separated by interlayer An− anions to balance the overall charge,118 and are conventionally synthesised via co-precipitation from their nitrates using alkalis as both pH regulators and a carbonate source. Mg–Al hydrotalcites have been applied to TAG transesterification of poor and high quality oil feeds,119 such as refined and acidic cottonseed oil (possessing 9.5 wt% FFA) and animal fat feed (45 wt% water), delivering 99% conversion within 3 h at 200 °C. It is important to note that many catalytic studies employing hydrotalcites for transesterification are suspect due to their use of Na or K hydroxide/carbonate solutions to precipitate the hydrotalcite phase. Complete removal of alkali residues from the resulting hydrotalcites is inherently difficult, resulting in ill-defined homogeneous contributions to catalysis arising from leached Na or K.120,121 This problem has been overcome by the development of alkali-free precipitation routes employing NH3OH and NH3CO3, which offer well-defined, thermally activated and rehydrated Mg–Al hydrotalcites with compositions spanning x = 0.25–0.55.116 Spectroscopic measurements reveal that increasing the Mg : Al ratio enables systematic enhancement of the surface charge and accompanying base strength, with a concomitant increase in the rate of tributyrin transesterification under mild reaction conditions (Fig. 5). Despite their high intrinsic activity, one limitation of co-precipitated pure hydrotalcites is their low surface areas, although delamination122,123 and grafting124 methodologies offer avenues to circumvent this.
Fig. 5 Impact of Mg:Al hydrotalcite surface basicity on their activity towards tributyrin transesterification. Adapted from ref. 117. Copyright (2005), with permission from Elsevier.
Fig. 6 Superior catalytic performance of a hierarchical macroporous–microporous Mg–Al hydrotalcite solid base catalyst for TAG transesterification to biodiesel versus a conventional microporous analogue. Adapted from ref. 128 with permission from The Royal Society of Chemistry.
Doping of (calcined) Malaysian dolomite with ZnO and SnO2 resulted in respective three- and four-fold increases in the catalyst surface area and active base density, and a concomitant rise in base strength.133 The SnO2 doped dolomite gave >99.9% conversion under optimised conditions with a low methanol : oil molar ratio and catalyst loading.
Other waste materials employed for biodiesel production include waste water scale (obtained from residential kitchens in China), which upon 1000 °C calcination yielded a solid base material mixture of CaO, MgO, Fe2O3, Al2O3, and SiO2 as a stable and active catalyst for soybean transesterification with methanol.134 This composition is similar to that of Red Mud mineral waste, recently shown to be an active ketonisation catalyst.135,136 This waste to resource approach of catalyst design is highly desirable in terms of green credentials and the biofuel ideology.
In summary, a host of inorganic solid base catalysts have been developed for the low temperature transesterification of triglyceride components of bio-oil feedstocks, offering activities far superior to those achieved via alternative solid acid catalysts to date. However, leaching of alkali and alkaline-earth elements and associated catalyst recycling remains a challenge, while improved resilience to water and fatty acid impurities in plant, algal and waste oil feedstocks is required in order to eliminate additional esterification pre-treatments.
The two dimensional, micron-length channels characteristic of the SBA-15 p6mm structure are known to hamper rapid molecular exchange with the bulk reaction media, and hence three dimensional interconnected channels associated with the Ia d structure of KIT-6 mesoporous silica offer one solution to improving the in-pore accessibility of sulfonic acid sites. Superior molecular transport within the interconnected cubic structure of KIT-6 has been shown to facilitate biomolecule immobilisation.149 This diversity of mesoporous silica architectures enabled the impact of pore connectivity upon FFA esterification to be quantified.150 A family of pore-expanded propylsulfonic acid KIT-6 analogues, PrSO3H-KIT-6, prepared via MPTS grafting and subsequent oxidation, have been screened for FFA esterification with methanol under mild conditions. Such a conventionally-prepared material exhibited 40 and 70% TOF enhancements for propanoic and hexanoic acid esterification respectively over an analogous PrSO3H-SBA-15 catalyst of comparable (5 nm) pore diameter, attributed to faster mesopore diffusion. However, pore accessibility remained rate-limiting for esterification of the longer chain lauric and palmitic acids. Pore expansion of the KIT-6 mesopores up to 7 nm via hydrothermal ageing doubled the resulting TOFs for lauric and palmitic acid esterification with respect to an unexpanded PrSO3H-SBA-15 (Fig. 7). It should be noted that the absolute conversions of FFAs over such tailored, inorganic solid acid catalysts remain significantly lower than those for commercial polymer alternatives which possess superior acid site densities (e.g. 4.7 mmol g−1 for Amberlyst-15151versus <1 mmol g−1 for PrSO3H-SBA-15 and PrSO3H-KIT-6150).
Fig. 7 Superior performance of interconnected, mesoporous propylsulfonic acid KIT-6 catalysts for biodiesel synthesis via free fatty acid esterification with methanol versus non-interconnected mesoporous SBA-15 analogue. Adapted from ref. 151. Copyright 2012 American Chemical Society.
Propylsulfonic acid functionalised SBA-15 (SBA-15-PrSO3H) has also been evaluated for oleic acid esterification with methanol,152 showing good stability in boiling water, with the mesopore structure allowing facile diffusion of the acid to active sites. This catalyst exhibited similar activity to phenylethylsulfonic acid functionalised silica gel, and was superior to dry Amberlyst-15, reflecting the higher surface area and pore volume of the SBA-15-PrSO3H relative to the more strongly acidic phenylethyl mesoporous silica. The SBA-15-PrSO3H could be recycled by simple ethanol washing and drying at 80 °C, and maintained an esterification rate of 2.2 mmol min−1 gcat−1. Simultaneous esterification and transesterification of vegetable oils with methanol has performed with Ti-doped SBA-15.153 A range of oils including soybean, rapeseed, crude palm, waste cooking oil and crude Jatropha oil (CJO), and palm fatty acid distillates were successfully converted to biodiesel by the Ti-SBA-15 catalyst at 200 °C. The mesoporous framework gave improved accessibility to the weakly Lewis acidic Ti4+ sites, affording higher activity than microporous titanosilicate and TiO2 supports. The Ti-SBA-15 was tolerant of common oil impurities, performing well in the presence of 5 wt% water or 30 wt% FFA. High catalyst loadings of 15 wt% relative to CJO permitted recycling without loss in conversion, although catalyst regeneration between recycles necessitated washing with acetone and subsequent 500 °C calcination.
Most solid acid catalysts employed in biodiesel synthesis are microporous or mesoporous,32,34,154 properties which the preceding sections highlights are not desirable for accommodating sterically-challenging C16–C18 TAGs or FFAs for biodiesel synthesis. Incorporation of secondary mesoporosity into a microporous H-β-zeolite to create a hierarchical solid acid significantly accelerated microalgae oil esterification with methanol by lowering diffusion barriers.155 Templated mesoporous solids are widely used as catalyst supports,156,157 with SBA-15 silica popular candidates for reactions pertinent to biodiesel synthesis as described above.142,144,158 However, such surfactant-templated supports possessing long, isolated parallel and narrow channels to not afford efficient in-pore diffusion of bio-oil feedstocks, with resultant poor catalytic turnover. Further improvements in pore architecture are hence required to optimise mass-transport of heavier, bulky TAGs and FFAs common in plant and algal oils. Simulations demonstrate that in the Knudsen diffusion regime,159 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 microemulsion160 or co-surfactant161 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 aluminas162 and ‘SBA-15 like’ silicas163 (Scheme 4), in which both macro- and mesopore diameters can be independently tuned over the range 200–500 nm and 5–20 nm respectively.
Scheme 4 Liquid crystal and polystyrene nanosphere dual surfactant/physical templating route to hierarchical macroporous–mesoporous silicas.
The resulting hierarchical pore network of a propylsulfonic acid functionalised macro-mesoporous SBA-15, illustrates how 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.
Fig. 8 Relationship between acid site density and catalytic performance in FFA esterification. Adapted from ref. 171. Copyright (2014), with permission from Elsevier.
Fig. 9 Surfactant template extraction via energy/atom efficient ultrasonication delivers a one-pot PrSO3H-SBA-15 solid acid catalyst with identical structure and reactivity to that obtained by conventional, inefficient reflux. Adapted from ref. 172 with permission from The Royal Society of Chemistry.
Heteropolyacids are another interesting class of well-defined acid catalysts, capable of exhibiting superacidity (pKH+ > 12) and possessing flexible structures.172 In their native form, heteropolyacids are unsuitable as heterogeneous catalysts for biodiesel applications due to their high solubility in polar media.173 Dispersing such polyoxometalate clusters over traditional high area oxide supports can modulate their acid site densities,174,175 but does little to improve their solubility during alcoholysis. Ion-exchanging larger cations into Keggin type phospho- and silicotungstic acids can increase their chemical stability. For example, Cs salts of phosphotungstic acid CsxH(3−x)PW12O40 and CsyH(4−y)SiW12O40 are virtually insoluble in water, with proton substitution accompanied by a dramatic increase in surface area of the resulting crystallites.137,176 As a consequence of these enhanced structural properties, albeit at the expense of losing acidic protons, both CsxH(3−x)PW12O40 and CsyH(y−x)SiW12O40 are active for palmitic acid esterification to methyl palmitate and tributyrin transesterification (Fig. 10). For CsxH(3−x)PW12O40, optimum esterification and transesterification activity was obtained for x = 2.1–2.4, a similar degree of Cs doping to that maximising palmitic acid esterification for CsyH(4−y)SiW12O40 catalysts (y = 2.8–3.4). These optimal compositions reflect a maximum in the density of accessible surface acid sites within the insoluble Cs-doped catalysts. For CsyH(4−y)SiW12O40, wherein C4 and C8 TAG transesterification were compared, the absolute reaction rates were faster for the shorter chain triglyceride, attributed to slow in-pore diffusion of the longer chain oil. Absolute TOFs for tributyrin transesterification over the optimised Cs-doped catalyst were greater than for the homogeneous H4SiW12O40 polyoxometalate clusters, a consequence of the greater hydrophobicity of the CsxSiW12O40 salts compared with the parent H4SiW12O40, which thus afford enhanced activity for the more lipophilic C8 TAG. Optimising the heterogeneous catalytic activity of CsyH4−ySiW12O40 requires a balance between the retention of acidic protons and generation of stable mesopores to facilitate molecular diffusion. Cs ion-exchange generates interparticle voids large enough to accommodate short-chain TAGs and longer saturated FFAs. Oil/fatty acid and biodiesel polarity and associated mass transport to/from active acid sites is obviously critical in regulating reactivity, and an area where improved materials design in conjunction with molecular dynamics simulations will offer further avenues for high-performance heteropolyacid catalysts.
Fig. 10 Impact of Cs ion-exchange into (left) both CsxH(3−x)PW12O40 for palmitic acid esterification and tributyrin transesterification with methanol; and (right) and CsyH(y−x)SiW12O40 for palmitic acid esterification, benchmarked against parent fully protonated, soluble clusters. Adapted from ref. 138 and 177. Copyright (2007 and 2009), with permission from Elsevier.
Duan et al. have prepared H3PW12O40 supported on magnetic iron oxide particles (MNP-HPA) via an acid–base interaction and tested them in palmitic acid esterification with methanol under mild conditions.177 The magnetic nanoparticles were first coated in a protective SiO2 layer and then functionalised with aminopropyl groups, with the heteropolyacid immobilised by reaction with the amine. Water tolerance was imbued by the addition of nonyl chains to the catalyst surface which lowered the acid loading but improved palmitic acid conversion to 90% at 65 °C. Magnetic separation enabled catalyst recycling without activity loss (Fig. 11), while the presence hydrophobic/oleophilic nonyl groups improved diffusion of the reagent to the active sites, enhancing TOFs compared to the parent MNP-HPA. However, the water tolerance of these materials was limited, with only 1 wt% water reducing FFA conversion to 34%.
Fig. 11 Preparation of water-tolerant heteropolyacid on magnetic nanoparticles for palmitic acid esterification. Reprinted from ref. 178 with permission from The Royal Society of Chemistry.
Mesostructured silicas have also been employed as supports for HPAs, for example 12-tungstophosphoric acid (TPA) dispersed over mesoporous MCM-48 is a promising solid acid catalyst for oleic acid esterification with methanol.178 This catalyst gave 95% conversion to biodiesel with modest alcohol : acid molar ratios, but very high catalyst loadings (30 wt% TPA). Leaching studies employing insensitive colorimetric tests, suggested good catalyst water stability, with minimal loss of W from MCM-48 detectable by atomic absorption (rather than more sensitive ICP), and retention of the majority of acid sites post-reaction (1.50 mmol g−1). No explanation was advanced for this extremely surprising water tolerance of TPA, which usually exhibits a high solubility in methanol; entrapment of primary Keggin units within the 3 nm diameter MCM-48 pores seems improbable, and any physical barrier to their dissolution would also likely hinder FFA and FAME access to TPA acid sites. The principal disadvantage of heteropolyacids for esterification and transesterification reactions in short-chain alcohols thus remains their limited water tolerance, which to date can only be overcome through advanced catalyst design and the sacrifice of their high acid strength and site density.
While inorganic frameworks such as SBA-15 or ZrO2 are popular supports for solid acid catalysis, their hydrophilic nature can hinder diffusion of organic reagents. This problem can be avoided by the use of hydrophobic and oleophilic supports, such as mesoporous organic polymers. Sulfonated mesoporous polydivinylbenzene (PDVB) is one such solid acid catalyst,179 which exhibits absorption capacities for sunflower oil and methanol three times those of H3PO40W12, sulfonated-ZrO2, SBA-15-SO3H or Amberlyst 15, and consequent superior performance in tripalmitin transesterification, giving an 80% yield of methyl palmitate after 12 h reaction. PDVB-SO3H proved easily recyclable, with only a modest drop in yield after three recycles, ascribed to a combination of its high surface area, large pore volume, high acid site density, and hydrophobic/oleophilic pore network. Liu et al. utilised an aminophosphonic acid resin based on a polystyrene backbone in the microwave-assisted esterification of stearic acid with EtOH.180 FAME yields of 90% were obtained after microwave heating to (notionally) 80 °C for 7 h at a catalyst loading of 9 wt%, with slower reaction and a lower limiting conversion of 88% resulting from conventional heating. Kinetic analysis suggested a pseudohomogeneous mechanism in which microwave radiation excited the polar reactants in the solution phase in addition to the solid catalyst. This resin was structurally stable as determined by XRD, TGA and SEM, and recyclable with 87% acid conversion after five uses (Fig. 12).
Fig. 12 Stability of a solid acid resin catalyst for stearic acid esterification. Adapted from ref. 181. Copyright (2013), with permission from Elsevier.
The acid exchange resin, Relite CFS, was tested under batch and continuous modes for the simultaneous esterification and transesterification of oleic acid and soybean oil with methanol,181 evidencing good activity with 80% FAME obtained after 150 min at 100 °C. Unfortunately this resin was deactivated via exchange with metals such as iron present in the feedstream causing catalyst discolouration of beads during continuous operation (Fig. 13); activity could be completely regenerated by suspending the resin in sulphuric acid for 24 h and a further lengthy washing and drying protocol. A copolymer of acidic ionic liquid oligomers and divinylbenzene (PIL) has also been utilised as a catalyst for simultaneous esterification and transesterification of FFA-containing triglyceride mixtures (waste cooking oil), possessing a high acid density of 4.4 mmol g−1, high pore volume and surface area of 323 m2 g−1, and 35 nm mean pore diameter.182 The latter and hydrophobic surface character permitted efficient substrate diffusion through the pore network. The PIL copolymer was more active than the acidic ionic liquid alone, giving >99% conversion of oleic acid with MeOH at only 1 wt% catalyst loading. PIL also achieved >99% yield in rapeseed transesterification with MeOH under the same reaction conditions, and proved able to convert high FFA content waste cooking oil into biodiesel with 99% yield in 12 h. The spent catalyst showed no structural changes or loss of acidic sulphur, and hence could be efficiently recycled with almost no loss in performance.
Fig. 13 Deactivation of an acid resin catalyst during continuous esterification/transesterification of FFA and oil mixtures. Adapted from ref. 182. Copyright (2010), with permission from Elsevier.
As discussed earlier in this review, many studies have investigated the development of carbon catalysts prepared from second generation biomass such as non-edible crop waste,2,106,107 algal residues108 and even waste products from biodiesel production.109 Sulfonated carbonaceous materials show promising activity for FFA esterification, generally affording higher rates of biodiesel production than commercial resins such Amberlyst with which they are often compared.
Residue of the non-edible seed Calophyllum inophyllum has been carbonised to make a biomass-derived solid acid catalyst via sulfonation.107 The resulting catalysts, comprising randomly oriented, amorphous aromatic sheets of low surface area (0.2 to 3.4 m2 g−1) and variable acid densities (0.6 to 4.2 mmol g−1 dependent on the S wt%), were tested in the simultaneous esterification and transesterification of Calophyllum inophyllum seed oil. Esterification activity was greatly proportional to the S loading, but also influenced by the balance of hydrophobic/hydrophilic sites on the carbon which affected diffusion and adsorption of oleo substrates. This balance, and related surface properties, varied with the carbonisation and sulphonation conditions employed; short carbonisation times lead to smaller sheets with higher SO3H densities and increased activity, but also increased S leaching and concomitant deactivation. Rice husk char was sulfonated with concentrated sulfonic acid under various conditions, and evaluated in the esterification of oleic acid with MeOH.2 All catalysts were amorphous, with a maximum SO3H density of 0.7 mmol g−1. High conversions were obtained at 110 °C in 2 h for a low alcohol : oil molar ratio of 4 : 1, with the catalyst recyclable and still delivering 84% methyl oleate after seven re-uses despite losing 23% of the initial S through leaching.
Peanut shells processed in a similar manner to that above also yield a strong Brönsted solid acid catalyst, with an acid strength superior to H-ZSM-5 (Si/Al = 75).183 This catalyst gave >90% conversion of cottonseed oil in methanol transesterification at a methanol : oil molar ratio of only 9 : 1. Recycling and re-use studies employed centrifugation to separate the catalyst, with subsequent acetone washes leading to a 50% reduction in acid site density, although regeneration was achievable by prolonged treatment with 1 M H2SO4 solution. Despite the environmental compatibility of waste biomass-derived solid acid catalysts, active site retention over prolonged use remains a critical challenge if they are to find implementation in continuous biodiesel production; leaching of sulphate or sulfonic acid groups into the product stream would both shorten catalyst lifetime and degrade fuel quality.
Microalgae are an exciting, potential feedstock for biodiesel production, but following extraction of algal oils, the residue is typically burned or discarded. Fu et al.108 has partially carbonised and sulfonated such residue to create a solid acid catalyst for the esterification of oleic acid and transesterification of triolein with methanol at 80 °C (Fig. 14). Although the resulting catalyst comprised disordered, non-porous aromatic carbon sheets with a very low surface area, the sulfonic acid density of 4.25 mmol g−1 afforded an active catalyst with a stable FFA conversion >98% over six sequential oleic acid esterification cycles. The corresponding FAME yield for triolein transesterification was only 22%, but likewise stable across numerous recycles. However, such catalysts were prone to deactivation by adsorbed methanol and hence required regenerative sulphuric acid and hot water washes between recycles. A similar approach was adopted for the waste glycerol by-product of biodiesel production, whereby the polyol was converted in situ by partial carbonisation and sulfonation into a solid acid catalyst.109 High catalyst loadings, reaction temperatures (160 °C) and MeOH : oil ratios (>45) were required to achieve 99% conversion of Karanja oil to FAME, with conversion dropping to only 5% after five recycles, although no analysis of the spent catalyst or leaching studies were reported. Leaching of acid sites was however addressed by Deshmane et al.,184 who investigated sulfonated carbon catalysts prepared from sugar and polyacrylic acid for oleic acid esterification. These catalysts were deactivated by the formation of irregularly-shaped, 1 μm colloidal carbon aggregates, comprised of sulfonated polycyclic hydrocarbons, during the hydrothermal, sulfonation or pulverisation preparative steps, which subsequently leaching into the esterification reaction mixture.
Fig. 14 Microalgae as a source of bio-oils/fatty acids for biodiesel production, and waste, biomass residue for the synthesis of solid acid catalysts to drive such biodiesel production.
The kinetics of palm oil fatty acid esterification with MeOH over carbonised, sulfonated microcrystalline cellulose (CSMC) have also been compared with those of homogeneous sulphuric acid catalysts,185 compensating for the phase equilibrium and reaction equilibrium to provide an accurate kinetic reaction model; this approach ensured the biphasic nature of the water–alcohol–oil reaction mixture was correctly represented instead of assuming a pseudo-homogeneous model. Methanol and FFA adsorption over the CSMC was believed a key step in the heterogeneous process, and hence adsorption equilibrium constants were calculated for these molecules along with water and FAME. Unsurprisingly, the free fatty acid was found to adsorb preferentially in the presence of low concentrations of the other molecules. At the start of the esterification reaction, FFA and alcohol were fully miscible, but water and FAME production led to the evolution of two phases; one comprising aqueous methanol and catalyst, and the other methyl ester and unreacted FFA. Mass-transport between these phases is essential, but likely the rate-limiting step. Kinetics of both homogeneously and heterogeneously catalysed biphasic systems were modelled with high conversions favoured by the limited solubility of water in the organic phase, and the use of hydrophobic catalysts which displace water from reaction sites.
A major drawback of the preceding sulfonated carbons is their low surface area, which can be alleviated through the use of carbon nanotubes. Poonjarernsilp and co-workers prepared solid acid catalysts by sulfonating single-walled carbon nanohorns (SWCNHs)186 which possessed surface areas of 210 m2 g−1 and could be further improved by high temperature calcination to open up micropores. The resulting oxidised nanohorns (ox-SWCNs) had surface areas of 1000 m2 g−1 and superior pore volumes. However the subsequent sulfonation step required to introduce surface acidity, somewhat lowered the final surface area and pore volume, and drastically altered the pore size distribution, eliminating all the meso- and macropores to leave a narrow range of 2–10 nm pores. Despite the improved morphology of the sulfonated ox-SWCNs relative to the SWCNs, the former had a lower acid site density and was consequently less active in palmitic acid the esterification with methanol; the best yield was obtained for SO3H-SWCNHs, which gave 93% methyl palmitate after 5 h with a catalyst : MeOH : FFA ratio of 0.15 : 0.15 : 5 g. Recycling tests showed a progressive decrease in methyl palmitate yield associated with a loss of acid sites.
A range of additional solid acids have also been investigated, including ferric hydrogen sulphate [Fe(HSO4)3],187 supported tungsten oxides (WO3/SnO2),188 supported partially substituted heteropolytungstates,189 and bifunctional catalysts, such as Mo-Mn/Al2O3-15 wt% MgO,190 designed to incorporate the benefits of both acid and base catalysis. The iron catalyst had a low surface area of 4–5 m2 g−1, and required higher operating temperatures than other solid acids to achieve good biodiesel yields (94% at 205 °C),187 but was easily recycled by simple washing and drying to remove adsorbed products, maintaining activity over 5 cycles with no evidence of metal leaching. WO3/SnO2 was water tolerant and showed good conversion of soybean oil to FAME at a lower reaction temperature (110 °C), but required high MeOH : oil ratios >30 to achieve a 78% yield,188 but was prone to on-stream deactivation upon recycling. Tungsten-containing HPAs supported on silica, alumina, and zirconia were also active in biodiesel production from 10 wt% oleic acid in soybean oil delivering FAME yields >75% at a high reaction temperature. Performance was unaffected by the presence of up to 25 wt% of the fatty acid blended with the oil. Cesium addition to the HPA suppressed leaching and thereby improved catalyst stability, resulting in only a 10% fall in biodiesel production after multiple recycles attributed to physical sample loss during product separation.
In an attempt to incorporate acid and base character in a single material, Farooq et al. prepared a Mo-Mn/γ-Al2O3-15 wt% MgO catalysts via wet impregnation of alumina with MgO, followed by impregnation of the γ-Al2O3-MgO with [(NH4)6Mo7O24]·4H2O and subsequently aqueous Mn(NO3)2.190 The resulting thermally processed catalyst possessed highly dispersed MoO3 and MnO acid sites, affording 75% biodiesel yield at 95 °C with a MeOH : oil molar ratio of 15. This bifunctional material could be repeatedly recycled with the yield falling by 20% after 10 uses, a modest deactivation that was attributed to poisoning by strongly adsorbed organics and leaching of the various active metals during transesterification.
Surface hydroxyl groups favour H2O adsorption, which if formed during FFA esterification can drive the reverse hydrolysis reaction and lowering 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.204 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 (BTSB), or 1,2-bis(trimethoxysilyl)-ethane (BTME), with TEOS and MPTS in the sol–gel process205,206 which enhances small molecule esterification207 and etherification.208 This approach has been adopted for the direct synthesis of Lewis acidic, zirconium-containing periodic mesoporous organosilicas (Zr-PMOs), in which zirconocene dichloride was employed as the zirconium source and BTEB was progressively substituted for TEOS.209 The resulting organosilanes were topologically similar to a purely inorganic Zr-SBA-15 material, but are strongly hydrophobic in nature. Although the one-pot metal doping protocol adopted resulted in relatively low densities of Zr incorporated into the final solid catalyst, hydrophobisation significantly enhanced the per acid site activity in the simultaneous esterification of FFAs and transesterification of TAGs in crude palm oil with methanol at 200 °C, with conversions approaching 90% after only 6 h (Fig. 15). As significant, the catalytic performance of the high organic content Zr-PMO materials was barely influenced by the addition of up to 20 wt% water to the feedstock, in contrast to the inorganic Zr-SBA-15 analogue which was completely poisoned by such water addition. The high water and fatty acid tolerance of these Zr-PMO catalysts renders them especially promising for biodiesel production from waste oil sources.
Fig. 15 (top) FAME yield and turnover frequency calculated for Zr-PMO materials in the methanolysis of crude palm oil highlighting the impact of catalyst hydrophobicity; and (bottom) FAME yield as a function of organic content for Zr-PMO materials in the presence of additional water in the crude palm oil reaction media evidencing superior water tolerance of hybrid solid acid catalysts. Reprinted from ref. 210. Copyright 2013 John Wiley and Sons.
The incorporation of organic spectator groups (e.g. phenyl, methyl or propyl) during the sol–gel syntheses of SBA-15210 and MCM-41211 sulphonic acid silicas is also 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 in order to evaluate the effect of acid site density and surface hydrophobicity on catalyst acidity and associated performance.212 MCM-41 was an excellent candidate due to the availability of accurate models for the pore structure from kinetic Monte Carlo simulations,213 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 θ(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 5 Protocol for the synthesis of sulfonic acid and octyl co-functionalised sulfonic acid MCM-41 catalysts. Adapted from ref. 213 with permission from The Royal Society of Chemistry.
To avoid diffusion limitations, butanol esterification with acetic acid was selected as a model reaction (Fig. 16). 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 ΔHads(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-SO3H series. Lateral interactions with octyl groups help to re-orient sulphonic acid headgroups into the pore interior, thereby enhancing acid strength and associated esterification activity.
Fig. 16 (left) Molecular dynamics simulations of MCM-SO3H and MCM-Oc-SO3H pore models highlighting the interaction between surface sulfonic acid and hydroxyl groups in the absence of co-grafted octyl chains; (right) influence of PrSO3H surface density and co-grafted octyl groups on catalytic performance in acetic acid esterification with butanol. Adapted from ref. 213 with permission from The Royal Society of Chemistry.
Fig. 17 Preferential dispersion of DMC in the nonpolar, organic phase, and SZ and Al-MCM-41 in the polar aqueous phase of (a) water–CCl4 and (b) water–toluene solvent mixtures. Reprinted with permission from ref. 202. Copyright 2010 American Chemical Society.
Cesium-doped dodecatungstophosphoric acid (CsPW) has shown promise as a water-tolerant solid acid catalyst for the hydrolysis of ethyl acetate,221 and found subsequent employ in the transesterification of Eruca sativa Gars (ESG) oil.202 The authors claimed that CsPW exhibited excellent water-tolerance towards ESG transesterification, despite oil conversions falling by ∼90% upon the addition of only 1% water. Zn containing HPAs display more impressive credentials for transforming challenging feedstocks, with zinc dodecatungstophosphate nanotubes possessing Lewis and Brönsted acid sites effective for the for the simultaneous esterification and transesterification of palmitic acid, and transesterification of waste cooking oils with 26% FFA and 1% water.
Periodic Mesoporous Organosilicas (PMOs) are a promising class of materials that can be used as catalyst supports for biodiesel production. PMOs are hybrid organic–inorganic materials with mesopore networks akin to SBA-15.236 Functionalisation of PMOs with catalytically active organic moieties is an emergent field of heterogeneous catalysis, and since the organic groups are dispersed throughout the framework (rather than confined to hydroxylated patches of the surface212), active sites and hydrophobic centres can be co-located in high concentrations. Methylpropyl sulfonic acid functionalised phenylene- and ethyl-bridged PMOs have been synthesised and tested for the transesterification of sunflower oil, canola oil, corn oil, refined olive oil and olive sludge.237 These functionalised PMOs gave comparable or better activity than SBA-15-PrSO3H under optimised conditions, with the ethyl-bridged PMO showing highest activity with a 98% yield. Water adsorption studies proved that the phenylene-bridged PMO was more hydrophobic than the ethyl-bridged variant, but less active, showing that a balance of hydrophobic versus hydrophilic mesostructural properties are necessary for optimum transesterification.
Heterogeneous catalysts with tunable hydrophobicity, acid/base character, and good thermal stability, whether based upon polymeric or inorganic frameworks, are hence promising new solutions to TAG transesterification and FFA esterification of high moisture content feedstocks.
Reactive distillation combines chemical conversion and separation steps in a single stage. This simplifies the process flow sheets, reduces production costs, and extends catalyst lifetimes through the continuous removal of water from the system. However, this technique is only applicable if the reaction is compatible with the temperatures and pressures required for the distillation. Kiss et al. demonstrated this approach for the esterification of dodecanoic acid with a range of alcohols catalysed by sulphated zirconia.245 Their reactive distillation was 100% selective, permitted shorter residence times than comparable flow systems, and did not require excess alcohol. The latter is a major advantage over the overwhelming majority of conventional biodiesel syntheses wherein, since reaction between the triglyceride and alcohol is reversible, large alcohol excesses are normally required to achieve full conversion (the excess alcohol must then be separated and re-used to ensure economic process viability).
Any continuous flow reactor must be designed appropriately to harness the full potential of the integrated heterogeneous catalyst; plug flow is a desirable characteristic since it permits tight control over the product composition, and hence minimises downstream separation processes, and associated capital investment and running costs. Conventional plug flow reactors are ill-suited to slow reactions such as FFA esterification and TAG transesterification, since they require very high length : diameter ratios to achieve good mixing, and in any event are problematic due to their large footprints and pumping duties, and control difficulties. Oscillatory Baffled Reactors (OBRs) circumvent these problems by oscillating the reaction fluid through orifice plate baffles to achieve efficient mixing and plug flow,249 thereby decoupling mixing from the net fluid flow in a scalable fashion, enabling long reaction times on an industrial scale, and have been applied to homogeneously catalysed biodiesel synthesis.250 Vortical mixing in the OBR also offers an effective, controllable method of uniformly suspending solid particles and was recently utilised to entrain a PrSO3H-SBA-15 mesoporous silica within a glass OBR under an oscillatory flow for the continuous esterification of propanoic, hexanoic, lauric and palmitic acid (Fig. 18).42 Excellent semi-quantitative agreement was obtained between the kinetics of hexanoic acid esterification within the OBR and a conventional stirred batch reactor, with fatty acid chain length identified as a key predictor of solid acid activity. Continuous esterification within the OBR improved ester yields compared with batch operation due to water by-product removal from the catalyst reaction zone, evidencing the versatility of the OBR for heterogeneous flow chemistry and potential role as a new clean catalytic technology.
Fig. 18 Schematic of reactor flow and mixing characteristics within an OBR, and associated optical images of a PrSO3H-SBA-15 solid acid powder without oscillation (undergoing sedimentation) or with a 4.5 Hz oscillation (entrained within baffles). Adapted from ref. 42 with permission from The Royal Society of Chemistry.
Fig. 19 Schematic of recirculating packed membrane reactors for continuous biodiesel production via (a) solid acid and (b) base catalysts. Reprinted from ref. 252 and 254. Copyright (2011 and 2014), with permission from Elsevier.
If sourced and produced in a sustainable fashion, biodiesel has the potential to play an important role in meeting renewable fuel targets. However, developments in materials design and construction are critical to achieve significant improvements in heterogeneously catalysed biodiesel production. Designer solid acid and base catalysts with tailored surface properties and pore networks offer process improvements over existing, commercial homogeneous catalysed production employing liquid bases, facilitating simple catalyst separation and fuel purification, coupled with continuous biodiesel synthesis. Tuning the surface hydrophobicity of heterogeneous catalysts can strongly influence oil transesterification and FFA esterification through the expulsion of water away from active catalytic centres, thus limiting undesired reverse hydrolysis processes, notably in high water content waste oils. Solid materials capable of simultaneous FFA esterification and TAG transesterification under mild conditions present a major challenge for catalytic scientists, although (insoluble) high area superacids represent a step in this direction. We predict that in the future, hierarchical solid acids may be employed to first hydrolyse non-edible oil feedstocks, and subsequently esterify the resulting FFAs to FAME. Synthesis of nanostructured (e.g. nanocrystalline) catalysts and the application of surface-initiated, controlled polymerisation to functionalise oxide surfaces with polymeric organic species to create hybrid organic–inorganic architectures with high active site loadings, will prove valuable in the quest for enhanced catalyst performance.
Despite concerns over long term biodiesel use in high performance engines, the implementation of FAME containing longer chain (>C18) esters in heavy-duty diesel engines should prove less problematic to on short timecales. However, the widespread uptake and development of next-generation biodiesel fuels requires progressive government policies and incentive schemes to place biodiesel on a comparative footing with (heavily subsidised) fossil-fuels. Blending of biodiesel with pyrolysis oil derived from lignocellulosic waste is an attractive route to power low-medium scale Combined Heat and Power (CHP) engines. Increasing use of waste or low grade oil sources remains a challenge for existing heterogeneous catalysts, since the high concentration of impurities (acid, moisture, heavy metals) induce rapid on-stream deactivation, and necessitate improved upstream oil purification, or more robust catalyst formulations tolerant to such components. Feedstock selection is dominated by regional availability, however the drive to use non-edible oil sources in areas where they cannot be readily sourced will require close attention to the entire supply chain and emissions/costs associated with new transportation networks, and may favour genetic modification of plant and algal strains to adapt to non-native climates.
The viscosity and attendant poor miscibility of many oil feedstocks with light alcohols continues to hamper the use of new heterogeneous catalysts for continuous biodiesel production, from both a materials and engineering perspective. Future process optimisation and growth in biodiesel supply and demand needs a concerted effort between catalyst chemists, chemical engineers and experts in molecular simulation in order to take advantage of innovative reactor designs and develop catalysts and reactors in tandem. Alternative reactor technologies and process intensification via e.g. reactive distillation and oscillatory flow reactors will facilitate distributed biodiesel production. It is essential that technical advances in both materials chemistry and reactor engineering are pursued if biodiesel is to remain a key player in the renewable energy sector during the 21st century.
A.F.L. thanks the EPSRC for the award of a Leadership Fellowship (EP/G007594/4). K.W. thanks the Royal Society for the award of an Industry Fellowship.
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