Base catalysts are generally more active than acids in transesterification, and hence are particularly suitable for high purity oils with low FFA con­tent. Biodiesel synthesis using a solid base catalyst in continuous flow, packed bed arrangement would facilitate both catalyst separation and co­production of high purity glycerol, thereby reducing production costs and enabling catalyst re-use. Diverse solid base catalysts are known, notably alkali or alkaline earth oxides, supported alkali metals, basic zeolites and clays such as hydrotalcites and immobilised organic bases [104]. Basicity in alkaline earth oxides is believed to arise from M2+-O2- ion pairs pres­ent in different coordination environments [105]. 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 transesterification [106] and there are numer­ous reports on commercial and microcrystalline CaO applied to rapeseed, sunflower or vegetable oil transesterification with methanol [107, 108]. Promising results have been obtained, with 97 % oil conversion achieved at 75 °C [108], however, concern remains over Ca2+ leaching under reac­tion conditions and associated homogeneous catalytic contributions [109], a common problem encountered in metal catalysed biodiesel production which hampers commercialisation [110].

Alkali-doped CaO and MgO have also been investigated for TAG transesterification [111-113], with their enhanced basicity attributed to the genesis of O_ centres following the replacement of M+ for M2+ and associ­ated charge imbalance and concomitant defect generation. Optimum ac­tivity for Li-doped CaO occurs when a saturated Li+ monolayer is formed (Fig. 6) [113], although leaching of the alkali promoter remains problem­atic [114].

It is widely accepted that the catalytic activity of alkaline earth oxide catalysts is very sensitive to their preparation, and corresponding surface morphology and/or defect density. For example, Parvulescu and Richards demonstrated the impact of the different MgO crystal facets upon the transesterification of sunflower oil by comparing nanoparticles [115] ver­sus (111) terminated nanosheets [116]. Chemical titration reveals that both morphologies possess two types of base sites, with the nanosheets exhib­iting well-defined, medium-strong basicity consistent with their uniform exposed facets and which confer higher FAMe yields during sunflower oil transesterification. Subsequent synthesis, screening and spectroscopic characterisation of a family of size-/shape-controlled MgO nanoparticles prepared via a hydrothermal synthesis revealed small (<8 nm) particles terminate in high coordination (100) facets, and exhibit both weak po — larisability and poor activity in tributyrin transesterification with metha­nol [117]. Calcination drives restructuring and sintering to expose lower coordination stepped (111) and (110) surface planes, which are more po — larisable and exhibit much higher transesterification activities under mild conditions. A direct correlation was therefore observed between the sur­face electronic structure and associated catalytic activity, revealing a pro­nounced structural preference for (110) and (111) facets (Fig. 7).

Hydrotalcites are another class of solid base catalysts that have at­tracted recent attention because of their high activity and robustness in the presence of water and FFA [118, 119]. Hydrotalcites ([M(II)1 _ x M(III)x (OH)2]x+(Anx/n-) mH2O) adopt a layered double hydroxide structure with brucite-like Mg(OH)2) hydroxide sheets containing octahedrally coordi­nated M2+ and M3+ cations and An — anions between layers to balance the overall charge [120], 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 for TAG transesterifica­tion of poor and high quality oil feeds [121] such as refined and acidic cot­tonseed oil (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 parallel ill-defined ho­mogeneous contributions to catalysis arising from leached Na or K [122, 123]. This problem has been overcome by the development of alkali-free precipitation routes using NH3OH and NH3CO3, offering well-defined thermally activated and rehydrated Mg-Al hydrotalcites with composi­tions spanning x = 0.25 — 0.55 [118]. Spectroscopic measurements reveal that increasing the Mg:Al ratio enables the surface charge and accompa­nying base strength to be systematically enhanced, with a concomitant increase in the rate of tributyrin transesterification under mild reaction conditions (Fig. 8).

In spite of their promise for biodiesel production, conventionally prepared hydrotalcites are microporous, and hence poorly suited to ap­plication in the transesterification of bulky C16-C18 TAG components of bio-oils. This problem was recently tackled by adopting the same hard templating method utilising polystyrene nanospheres described in Scheme 4 to incorporate macroporosity, and thus create a hierarchical macropo­rous-microporous hydrotalcite solid base catalyst [124]. The introduc­tion of macropores as ‘superhighways’ to rapidly transport heavy TAG oil components to active base sites present at (high aspect ratio) hydrotalcite nanocrystallites, dramatically enhanced turnover frequencies for triolein transesterification compared with that achievable over an analogous Mg — Al microporous hydrotalcite (Fig. 9), reflecting superior mass transport through the hierarchical catalyst.

In terms of sustainability, it is important to find low cost routes to the synthesis of solid base catalysts that employ earth abundant elements. Do — lomitic rock, comprising alternating Mg(CO3)-Ca(CO3) layers, is structur­ally very similar to calcite (CaCO3), with a high natural abundance and low toxicity, and in the UK is sourced from quarries working Permian dolomites in Durham, South Yorkshire and Derbyshire [125]. In addition to uses in ag­riculture and construction, dolomite finds industrial applications in iron and steel production, glass manufacturing and as fillers in plastics, paints, rub­bers, adhesives and sealants. Catalytic applications for powdered, dolomitic rock offer the potential to further valorise this readily available waste min­eral, and indeed dolomite has shown promise in biomass gasification [126] as a cheap, disposable and naturally occurring material that significantly reduces the tar content of gaseous products from gasifiers. Dolomite has also been investigated as a solid base catalyst in biodiesel synthesis [127], wherein fresh dolomitic rock comprised approximately 77 % dolomite and 23 % magnesian calcite. High temperature calcination induced Mg surface segregation, resulting in MgO nanocrystals dispersed over CaO/(OH)2 par­ticles, while the attendant loss of CO2 increases both the surface area and basicity. The resulting calcined dolomite proved an effective catalyst for the transesterification of C4, C8 and TAGs with methanol and longer chain C16-18 components present within olive oil, with TOFs for tributyrin conversion to methyl butanoate the highest reported for any solid base (Fig. 10). The slower transesterification rates for bulkier TAGs were attributed to diffusion limitations in their access to base sites. Calcined dolomite has also shown promise in the transesterification of canola oil with methanol, achieving 92 % FAME after 3 h reaction with 3 wt% catalyst [128].

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 oils feedstocks is required to eliminate additional es­terification pre-treatments. To date, only a handful of biodiesel production processes employing heterogeneous catalysts have been commercialised, notably the Esterfip-H process developed by Axens and IFP which utilises a mixture of ZnO and alumina and is operated on a 200 kton per annum scale with parallel production of high quality glycerine [129].