Facile route to conformal hydrotalcite coatings over complex architectures: a hierarchically ordered nanoporous base catalyst for FAME production

An alkali-and nitrate-free hydrotalcite coating has been grafted onto the surface of a hierarchically ordered macroporous-mesoporous SBA-15 template via stepwise growth of conformal alumina adlayers and their subsequent reaction with magnesium methoxide. The resulting low dimensional hydrotalcite crystallites exhibit excellent per site activity for the base catalysed transesterification of glyceryl triolein with methanol for FAME production.

Rising global energy demand over the next 25 years, notably among emergent economies, 1 is driving the quest for sustainable routes to low cost, liquid transportation fuels from biomass feedstocks. 2 Around 9 % of transportation energy needs are predicted to be met via liquid biofuels by 2030. 3 The past decade has seen much criticism of first-generation biofuels derived from edible plant materials which are attributed to significant land use changes and deforestation in South East Asia. 4In order for advanced bio-fuels to be considered truly sustainable, they must be sourced from non-edible crop components, forestry waste, alternative non-food plants such as switchgrass, Miscanthus or Jatropha curcas 5 which require minimal cultivation and do not compete with traditional arable land or drive deforestation, algal sources or the lignocellulosic components of municipal waste such as packaging materials.
Although there is burgeoning interest in extracting bio-oils from aquatic biomass, which can yield 80-180 times the annual volume of oil per hectare than plants, 6 process scale-up and the availability of nutrient resources remains challenging. 7The biorefinery concept affords biomass the simplest and most popular approach to drop-in transportation fuels, 8 based upon carbohydrate pyrolysis and subsequent hydrodeoxygenation (HDO) 9 of the resulting bio-oils or their gasification and subsequent Fischer-Tropsch processing 10 to gasoline and diesel, 11 or lipid transesterification to biodiesel. 12Catalytic depolymerisation of lignin may also unlock opportunities for the co-production of phenolics and related aromatic compounds via biorefineries for fine chemical and pharmaceutical applications 13 improving their cost-effectiveness.
Biodiesel is a clean burning and biodegradable fuel 14 which remains popular for meeting transportation energy requirements in Europe, 15 Asia, 16 the Americas 17 and Africa. 18Commercial biodiesel is produced almost entirely via the liquid base catalysed transesterification of C 14 -C 20 triacylglyceride (TAG) components of lipids with C 1 -C 2 alcohols 19 into fatty acid methyl esters (FAMEs) which constitute biodiesel.Higher alcohols have also been exploited 20 as they offer a less corrosive FAME with improved physical characteristics. 21solation of the desired biodiesel product from homogeneous base catalysts (and unreacted mono-and di-alkyl glycerides and glycerol by-product) is necessary to circumvent saponification and emulsification side reactions and produce a high quality biofuel. 6Heterogeneous, solid acid [22][23][24] and base catalysts offer facile FAME separation, eliminating the requirement for quenching steps and permitting continuous biodiesel production, 25 and a purer glycerol by-product stream for use as a commodity chemical in the food and cosmetics industry.Among solid base catalysts, hydrotalcites, [26][27][28] alkaline earth oxides [29][30][31][32][33] and alkali-doped mesoporous silicas 34 are good potential candidates for biodiesel formation under mild conditions. Hydrotalcites ][40] We recently reported an alkali/nitrate-free route to tunable Mg-Al hydrotalcite coatings via the direct reaction of Mg(OCH 3 ) 2 with a conventional, bulk alumina support. 41While the resulting materials exhibited excellent Turnover Frequencies (TOFs) towards TAG transesterification, they suffer from a number of important drawbacks, namely: poor specific activity per unit mass towards bulky C 18 substrates (0.042 mmol.min - 1 .g - ), low surface areas, and restricted (and disordered) pore architectures available through the use of pure alumina templates.][44] Here we extend our previous methodology to create crystalline, catalytically active hydrotalcite coatings via a versatile two-step methodology, permitting (i) the first genesis of an ultrathin alumina adlayer over a complex (hierarchically ordered) template, and (ii) facilitating its subsequent reaction with Mg(OCH 3 ) 2 to form a stoichiometric HT/MM-SBA-15 hydrotalcite catalyst.This novel methodology opens the way to a new class of solid bases built upon the tunable interconnectivity and porosity afforded by underlying silica architectures.The resulting nanocomposite combines the high surface area and excellent mass-transport characteristics of the parent silica, and solid basicity and transesterification performance of a pure hydrotalcite.

Synthesis of macroporous-mesoporous SBA-15 (MM-SBA-15)
An hierarchical macroporous-meosporous SBA-15 silica was prepared following the method of Dhainaut et al. 22 Briefly polystyrene beads synthesised via the emulsion polymerisation approach of Vaudreuil and co-workers 45 were added to an acidified, aqueous solution of Pluronic® P123 surfactant prior to the addition of tetramethoxysilane.The resulting gel was hydrothermally aged without agitation, and the solid obtained filtered, washed and dried at room temperature before calcination at 550 °C for 6 h in air.

Alumina grafting onto MM-SBA-15 (Al-MM-SBA-15)
The Al-MM-SBA-15 hierarchical framework employed the method of Landau and co-workers developed for MCM-41. 46luminium-tri-sec-butoxide (14.5 g) was dissolved in anhydrous toluene (100 cm 3 ) at 85 °C with stirring.Triethylamine (2.1 cm 3 ) was added to this solution, followed by dried MM-SBA-15 (1 g).After 6 h stirring at 85 °C the solution was filtered under vacuum (~0.1 bar), with the recovered solid washed three times in toluene (100 cm 3 ).The alumina surface was then hydrolysed in ethanol (318 cm 3 ) containing water (1.6 cm 3 ) for 24 h at 25 °C with stirring, and the resulting solid recovered by vacuum filtration and washed with ethanol (300 cm 3 ) before drying at 80 ºC in a vacuum oven overnight.A three-step calcination sequence was utilised to form an alumina monolayer: the material was first heated to 250 °C for 1 h, then 400 °C for 1 h and finally 500 °C for 4 h (each ramp rate 1 °C.min - ).Consecutive grafting cycles were undertaken employing an identical protocol in order to progressively buildup alumina monolayers over the silica surface, adjusting the quantities to maintain the initial Al:Si stoichiometry.

Synthesis of hydrotalcite-coated MM-SBA-15 (HT/MM-SBA-15)
Magnesium methoxide solution (8-10 wt% in methanol) was added to Al-MM-SBA-15 (400 mg, dried for 1 h at 80 °C), at the minimum quantity to form a homogeneous paste on mixing.After stirring for 15 min, the mixture was dried under vacuum at 80 °C for 1 h to remove excess methanol.The surface Mg:Al atomic ratio was tuned by adjusting the volume of magnesium methoxide (10.8 cm 3 for the MM-SBA-15).The resulting material was calcined at 450 °C for 15 h under 20 cm 3 .min - O 2 (ramp rate 1 °C.min - ).After cooling to room temperature under N 2 (20 cm 3 .min - ), the powder was added to distilled water (50 cm 3 for every 300 mg of powder) in a 100 cm 3 roundbottomed pressure vessel and heated to 125 °C with stirring for 21 h.After cooling to room temperature, the final HT/MM-SBA-15 sample was filtered, washed with deionised water and dried in a vacuum oven overnight at 80 °C, before storage in a desiccator.This synthesis proved successful on the multi-gram scale.A conventional hydrotalcite reference material was prepared via our alkali-free, co-precipitation method from Mg(NO 3 ) 2 •6H 2 O and Al(NO 3 ) 3 •9H 2 O precursors, 26 with the Mg: Al atomic ratio tuned to match that of the MM-SBA-15.

Materials characterisation
Nitrogen porosimetry was undertaken on a Quantachrome Nova 1200 porosimeter.Multi-point BET surface areas were calculated over the relative pressure range 0.01-0.3.Pore diameters and volumes were calculated applying either the tplot or BJH methods to the desorption isotherm.Powder XRD patterns were recorded on a PANalytical X'pertPro diffractometer fitted with an X'celerator detector and Cu Kα source; the Scherrer equation was used to calculate HT crystallite sizes.XPS was performed on a Kratos Axis HSi Xray photoelectron spectrometer fitted with a charge neutraliser and magnetic focusing lens employing Al K α monochromated radiation (1486.7 eV).Spectral fitting was performed using CasaXPS version 2.3.15.Base site densities were measured via CO 2 pulse chemisorption and subsequent temperature programmed desorption (TPD) on a Quantachrome ChemBET 3000 system coupled to an MKS Minilab QMS.SEM analysis was carried out on a Carl Zeiss EVO SEM fitted with an Oxford Instruments energy dispersive X-ray (EDX) analyser employing Oxford Instruments Inca Software.TGA was performed using a Stanton Redcroft STA780 thermal analyser.

Transesterification
The HT/MM-SBA-15 and conventional HT materials were tested as catalysts in the transesterification of triolein to form methyl trioleate (FAME) using a Radleys Starfish parallel reactor.Briefly, 50 mg of catalyst was added to 10 mmol of triolein using a 30:14:1 methanol:butanol:oil ratio; butanol was added as a co-solvent to help solubilise the triglyceride.In light of the significant differences in HT content between our conventional and SBA-15 coated materials, a common total mass of catalyst (rather than mass of hydrotalcite) was employed to ensure identical mixing characteristics within the reaction vessel.Reactions were carried out at 90 °C in a modified ACE 50 cm 3 round bottom pressure flask, with aliquots removed periodically from the reaction mixture for analysis on a Varian 450 GC with 8400 autosampler (programmable on-column injection onto a Phenomenex ZB-1HT column (15 m x 0.53 mm x 0.15 µm film thickness).Initial rates were calculated from the linear portion of the reaction profile during the first 60 min of the reaction.Turnover frequencies (TOFs) were determined by normalising rates to the total base site density from CO 2 chemisorption.

Characterisation of Al-MM-SBA-15
Alumina grafted silica (Al-MM-SBA-15) was first prepared as support for subsequent conversion to a high area, hierarchically ordered hydrotalcite coating.The alumina grafting process was repeated four times to obtain a uniform multilayer interface, with textural properties characterised after each grafting in order to examine the evolution of the alumina-silica interface.Low angle XRD and TEM of the parent MM-SBA-15 and the sequentially alumina grafted variants (Figures S1-2) confirmed the presence of ordered mesopores indicative of SBA-15. 47haracteristic (100), ( 110) and (200) reflections were observed for all materials, indicative of the p6mm space group expected

Green Chemistry Accepted Manuscript
for hexagonally arranged mesoporous channels. 48Macropore incorporation shifted these reflections to higher angle relative to conventional mesoporous SBA-15, associated with a small contraction in the mesopore lattice parameter. 22This contraction is attributed to curvature of the mesopore channels as they coalesce around the polystyrene bead template due to strong electrostatic interactions between the beads, block copolymer and silica precursor.Long range, hexagonally ordered mesopores remained present for Al-MM-SBA-15 even following four consecutive grafting cycles.
Nitrogen porosimetry of the parent MM-SBA-15 and alumina grafted analogues confirmed that the mesoporosity intrinsic to the SBA-15 framework is maintained after each grafting cycle (Figures S3a-b).However, the BET surface area (and interconnecting micropore area from t-plot analysis), mean mesopore diameter, and total mesopore and micropore volumes decreased progressively with each grafting cycle (Table 1), consistent with an increasing thickness of conformal alumina overlayer uniformly distributed throughout the pore network.(derived from porosimetry and XPS) with that for a crystalline alumina phase such as α-Al 2 O 3 , which exhibits a rhombohedral (4.75 Å x 4.75 Å) surface unit mesh containing three Al atoms within the (006) plane as shown in Scheme 1. 50 With a total surface area of 473 m 2 .g - , a single α-Al 2 O 3 monolayer covering the entire silica pore network would contain 0.0009 mol Al, equating to an Al loading of 24.3 wt%.This is close to the observed value of 22.3 wt%, and indicates that an alumina film approximately 0.7 monolayers thick (~0.17 nm) is formed following four grafting cycles, which would constrict the mesopores by 0.34 nm relative to the parent MM-SBA-15, in excellent agreement with the observed pore diameter decrease of 0.3 nm seen in Table 1.

Characterisation of HT/MM-SBA-15
Powder XRD diffraction patterns for the methoxide functionalised Al-MM-SBA-15 material prepared via four alumina grafting cycles (HT/MM-SBA-15), and a reference bulk HT sample prepared by conventional alkali-free coprecipitation (ConvHT) are shown in Figure 2. The HT/MM-This journal is © The Royal Society of Chemistry 2012 SBA-15 sample shows a diffraction pattern characteristic of a pure HT phase, very similar to that observed for the ConvHT Scheme 1. Crystal structure of α-alumina with primitive cell highlighted.50   standard, but with broader reflections indicative of significantly smaller crystallite sizes (as anticipated in light of the highly dispersed alumina substrate, which by inference appears to undergo little restructuring during the crystallisation process) and turbostratic disorder. 51There was no evidence for brucite 52 and only a single weak reflection likely associated with trace MgO.This confirms the successful synthesis of a hydrotalcite phase through direct reaction of a pre-formed, ultrathin alumina film and magnesium methoxide from solution.Crystallite sizes determined using the Scherrer equation, interlayer spacings, lattice parameters and Mg:Al ratios determined using Vegard's law (Figure S4) are reported in Table 2.The composition, lattice parameter and interlayer spacings of the HT/MM-SBA-15 material were almost identical to that of the ConvHT, confirming that the hydrotalcite phase formed at the surface of the hierarchical silica support was essentially indistinguishable from that of obtained by traditional synthetic methods, but with a surface area around five times higher.However, the significant difference in microporous crystallites size is expected to hinder accessibility of reactatns to active sites within the ConvHT interlayers relative to the HT/MM-SBA-15 sample whose dimensions suggest a hydrotalcite bilayer wherein a far greater proportion of base sites reside on exposed surfaces.
Textural properties of the HT/MM-SBA-15 material are compared with those of the Al-MM-SBA-15 precursor in Figure 3. Nitrogen porosimetry evidences retention of mesopore and macropore character within the adsorption/desorption isotherms following HT crystallisation, although their demarcation is not as clear as for Al-MM-SBA-15 (Figure S3), while Table S1 shows virtually no change in either the mesopore volume or mean mesopore diameter upon reaction of the alumina adlayer with Mg(OCH 3 ) 2 .This suggests that either extremely thin HT crystallites are formed throughout the bimodal pore network (consistent with XRD), or that hydrotalcite formation is confined to the macropores.The latter would be expected to hinder accessibility of the mesopores (for which macropores serve as the principal conduits), and hence reduce both the mesopore volume and total surface area, in contrast to the observed values reported in Table S1.4) confirms the macropore network present within the parent MM-SBA-15 support is retained throughout the material after hydrothermal treatment, a measure of the excellent stability of silica frameworks towards high temperature water, and conditions that a comparable pure hierarchical alumina structure would be unlikely to survive.TEM shows macropores are decorated with high aspect ratio hydrotalcite nanocrystallites.Thermogravimetric analysis confirms the excellent thermal stability of the HT/MMSBA-15 (Figure S5), with only a small 10 % weight loss between 70 and 220 °C, associated with the desorption of physisorbed water from the HT surface and water from within the interlayers, 53 and a 5 % loss between 250 and 350 °C attributed to hydroxide anions in the brucite-like layers. 54EDX elemental analysis of the HT/MM-SBA-15 yields an overall Mg:Al atomic ratio of 2.2:1, in good agreement with that derived from Vegard's law in Table 2, and a total Mg content of 19.8 wt% i.e. a quarter that of a bulk hydrotalcite of comparable Mg:Al ratio, 26 consistent with the formation of hydrotalcite nanocrystals approximately 1 nm thick relative to silica walls around 4-5 nm thick in the MM-SBA-15 support. 49   it is important to recall that the HT/MM-SBA-15 only contains a thin hydrotalcite coating and the majority of this catalyst is composed of inert silica.A fairer comparison of the relative catalytic performance is obtained from their initial rates of triolein conversion and Turnover Frequencies (TOFs) normalised per base site utilising the CO 2 TPD measurements.This reveals a common initial rate of 1 mmol.gcatalyst -1 .min -1 , however one must recall that the HT/MM-SBA-15 material only contains one quarter of the amount of hydrotalcite present within the bulk ConvHT material, hence the rate normalised per mass of hydrotalcite is four times higher for HT/MM-SBA-15 catalyst.Since the base site density of the coated hydrotalcite is also ~34 % lower than that of its bulk counterpart, the rate enhancement per base site of the coated material is higher still, translating to TOFs of 7.6 min -1 for the bulk ConvHT standard versus 66 min -1 HT/MM-SBA-15.Hydrotalcites prepared via conventional co-precipitation are among the most widelyused catalysts for triglyceride transesterification to FAME, hence the nine-fold rate enhancement observed for our HT/MM-SBA-15 material provides a striking benchmark of its exceptional performance.While the magnitude of this enhancement does fall at longer reaction times, likely due to partial deactivation of the coating, the HT/MM-SBA-15 remains three times as active per base site as the bulk hydrotalcite, even after 1400 min reaction.
Since the intrinsic base strength of active sites within the conventional and hierarchical hydrotalcite catalysts is the same (common CO 2 desorption temperatures, Figure S6), we attribute this nine-fold rate enhancement of HT/MM-SBA-15 to superior mass-transport characteristics of the macroporousmesoporous architecture.Indeed, the magnitude of the HT/MM-SBA-15 enhancement with respect to the ConvHT standard is comparable to that previously reported for a macroporous pure HT material, 28 but affords a far more flexible and hydrothermally stable framework than the latter synthesis.

Conclusions
Sequential, wet-chemical surface modification of nanostructured silicas with Al and Mg precursors offers a versatile route to the preparation of high area, tailored solid base hydrotalcite catalysts.Stepwise grafting and thermal processing of aluminium-tri-sec-butoxide results in a uniform alumina monolayer throughout the bimodal macroporemesopore network.Subsequent reaction with Mg(OCH 3 ) 2 affords stoichiometric incorporation of aluminium from the alumina adlayer into ~1 nm Mg 2 Al hydrotalcite crystallites, which possess identical basicity as a co-precipitated, bulk hydrotalcite.In contrast to bulk (monomodal) alumina templates, the development of a silica based methodology results in HT/MM-SBA-15 catalyst exhibits similar specific mass activity in the transesterification of glyceryl triolein with methanol as a bulk hydrotalcite, despite containing only a small fraction of the number of active sites, indicating far greater active site accessibility to the bulky TAG reactant.The latter conclusion is supported by a nine-fold enhancement in the TOF per base site for the hierarchical hydrotalcite, indicating the majority of base sites in HT/MM-SBA-15 reside at the external surface of nanoscale crystallites within the meso-and macropores, rather than within the microporous interlayers of conventional hydrotalcites.Our methodology is readily extendable to diverse silica architectures and other metal oxides, opening up opportunities for the facile introduction of hydrotalcite solid basicity into complex two-or threedimensional materials, e.g.membranes and monoliths, for catalysis and sorption applications.

Figure 2 .
Figure 2. Powder XRD patterns for HT/MM-SBA-15 and a conventional HT standard (offset for clarity)

Figure 4 .Figure 5 .
Figure 4. SEM and TEM micrographs of HT/MM-SBA-15.Insets highlight macropore network and hydrotalcite crystallites decorating macropores.Surface base properties of the HT/MM-SBA-15 and ConvHT bulk reference materials were assessed by temperature-programmed desorption of CO 2 -saturated samples, presented in Figure S6.The desorption profile of HT/MM-SBA-15 mirrors that of the bulk hydrotalcite, both exhibiting a single desorption feature around 340 °C indicative of moderate strength base sites, albeit spanning a broader temperature range and hence distribution of basicity for the HT/MM-SBA-15.Total base site densities for the HT/MM-SBA-15 and ConvHT materials were 6.4x10 18 and 8.6x10 19 m -2 respectively, although one should recall that a large proportion of sites present within the interlayers of the bulk hydrotalcite structure may be inaccessible to sterically-demanding substrates.These results confirm that high aspect ratio hydrotalcite crystallites formed over the hierarchical silica support possess similar intrinsic basicity to conventional co-precipitated analogues.Surface analysis of HT/MM-SBA-15 yielded a Mg:Al atomic ratio of 2.21 and Mg content of 16.7 wt%, both very similar to values determined by EDX, evidencing uniform incorporation of Mg into the alumina film throughout the pore network of the Al-MM-SBA-15 precursor.Si 2p XP spectra This journal is © The Royal Society of Chemistry 2012

Table 2 .
Crystallographic properties of HT/MM-SBA-15 and ConvHT a Calculated using Scherrer equation; b Determined from lattice parameter using Vegard's Law.
chemical environment at 101.5 eV.The latter suggests that interfacial silicon atoms are now bound (through oxygen bridges) to a less polarising adlayer relative to alumina, consistent with the exchange of Al 3+ for Mg 2+ cations.Fig.5bshowsanalogous changes in the Al chemical environment, with attenuation of the pure (and interfacial) alumina adlayer, and emergence of a high energy Al state ~74 eV, consistent with the introduction of Mg 2+ cations into the grafted alumina film during hydrotalcite formation.The corresponding Mg 2s XP spectrum of HT/MM-SBA-15 presents a single chemical environment around 88.5 eV.In summary, over 75 % of the MM-SBA-15 silica surface is contacted with a hydrotalcite phase, and a similar proportion of the initially grafted alumina adlayer in Al-MM-SBA-15 is converted into hydrotalcite.Transesterification proceeded rapidly over both catalysts during the first hour of reaction before slowing dramatically, to give limiting conversions of 34 % and 64 % for HT/MM-SBA-15 and ConvHT respectively.While the absolute FAME productivity of the bulk hydrotalcite is clearly superior,