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Article

Impact of Macroporosity on the Transesterification of Triglycerides over MgO/SBA-15

by
Thomas A. Bryant
1,
Lois Damptey
2,
Mark A. Isaacs
3,4,5,
Christopher M. A. Parlett
6,7,
Lee J. Durndell
8,
Marta Granollers Mesa
1,
Georgios Kyriakou
9,
Karen Wilson
10,* and
Adam F. Lee
10,*
1
Energy and Bioproducts Research Institute, Aston University, Birmingham B4 7ET, UK
2
Department of Engineering and Innovation, The Open University, Milton Keynes MK7 6AA, UK
3
HarwellXPS, Research Complex at Harwell, Rutherford Appleton Labs, Harwell Campus, Didcot OX11 0FA, UK
4
Department of Chemistry, University College London, Gower Street, London WC1H 0AJ, UK
5
Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, UK
6
UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Labs, Oxford OX16 0FA, UK
7
Department of Chemical Engineering, University of Manchester, Manchester M13 9PL, UK
8
School of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth PL4 8AA, UK
9
Department of Chemical Engineering, University of Patras, Caratheodory 1 St., 26504 Patras, Greece
10
Centre for Catalysis and Clean Energy, Griffith University, Gold Coast, QLD 4222, Australia
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(11), 1054; https://doi.org/10.3390/catal15111054
Submission received: 3 October 2025 / Revised: 23 October 2025 / Accepted: 27 October 2025 / Published: 4 November 2025
(This article belongs to the Section Nanostructured Catalysts)
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Abstract

Biofuels are critical drop-in replacement energy sources to support the decarbonisation of hard-to-abate sectors such as aviation and marine shipping. Transesterification of non-edible oils is a well-established route to biodiesel as a versatile liquid transport fuel, but is challenging to scale using existing homogeneous liquid base catalysts. In this work, we report the synthesis, characterisation, and application of silica-supported MgO solid base catalysts for triglyceride transesterification with methanol and highlight the impact of silica pore structure on performance. True liquid crystal templating enables the one-pot synthesis of mesoporous MgO/SBA-15 catalysts with variable Mg content, or hierarchical macroporous–mesoporous MgO/SBA-15 analogues through the addition of polystyrene nanospheres. Both MgO/SBA-15 families exhibit highly ordered pore networks; however, ~280 nm macropores stabilise Mg-O-Si interfacial species even at high Mg loading, in contrast to the mesoporous support that permits sintering of ~14 nm MgO nanocrystals. Hierarchical porous MgO/SBA-15 catalysts exhibit higher specific activity and conversion of tributyrin to methyl butyrate than their mesoporous analogues (3 mmol⋅h−1⋅g−1 versus 2 mmol⋅h−1⋅g−1 at 60 °C and 11 wt% Mg). The magnitude of this rate enhancement increases with triglyceride chain length, being approximately three-fold for trilaurin (C12) transesterification at 90 °C, attributed to superior in-pore mass transport of bulky reactants through the hierarchical porous catalyst.

1. Introduction

Lowering global carbon emissions to meet Net-Zero 2050 commitments remains a global challenge whose success is closely linked to the transportation sector, which contributes to ~23% of energy-related CO2 emissions [1] through fossil fuel combustion. Modelling suggests that to limit global temperature rises to below 1.5 °C will require a 59% reduction (42–68% interquartile range) in transport-related CO2 emissions by 2050, relative to 2020 levels [1]. Achieving this goal will necessitate the phasing out of fossil liquid transportation fuels, and the rapid uptake of cleaner renewable alternatives, including the deployment of fuel cell and electric passenger vehicles that depend on the availability of raw materials for battery manufacturing [2]. Liquid biofuels, such as bioethanol and biodiesel, already make a contribution to mitigating carbon emissions when blended with petroleum fuel according to the U.S. Energy Information Administration, and will continue to mitigate emissions in this sector [3], but the expected increase in electrified passenger vehicles in Europe, China, and the US will shift their future application to hard-to-abate transportation sectors. Heavy-duty freight, maritime, and aviation sectors require liquid transportation fuels for the foreseeable future, with advanced biofuels, hydrogen, ammonia, and synthetic fuels critical to achieve the emissions reductions required to meet Net-Zero in the marine sector [4]. Recent upgrades in major ports such as Rotterdam and Singapore have seen bio-blended (B24 and B30) fuel sales increase from 300,000 tonnes in 2021 to >1.3 million tonnes in 2024, with projections showing a continued rise [5]. The transition away from fossil fuels in, e.g., Africa, Asia-Pacific, Latin America, the Caribbean, and the Middle East has been less rapid, and hence liquid biofuels will remain an important component of the energy mix across the global transportation sector [6].
Sustainable liquid biofuels should be derived from waste biomass, algae, oil producing yeast, or high-yielding biomass crops grown on non-arable land that avoid competition with food [7]. Biodiesel derived from oleaginous feedstocks is an attractive biofuel, typically comprising fatty acid methyl esters (FAMEs), and produced by the acid- or base-catalysed transesterification of triacylglycerides (TAGs) with methanol [8]. The base-catalysed route is favoured in industry due to faster reaction kinetics which enable lower temperature operation via a less sterically hindered alkoxy intermediate [9]. However, current industrial routes employ homogeneous alkali metal hydroxides or methoxides [10], which are problematic due to the vast quantities of corrosive aqueous waste generated when purifying the fuel. Solid base catalysts offer significant process advantages over homogeneous base counterparts, being easily separated from the reaction mixture and facilitating continuous operation, greatly reducing the cost of product purification and improving process efficiency [11].
A range of solid base catalysts have been explored for biodiesel production [12,13], including alkali earth metal oxides [14], alkali-doped metal oxides [15], hydrotalcites [16], basic zeolites [17], basic resins [18], and natural waste sources including dolomite [19,20]. Alkali earth metal oxides are amongst the most widely investigated solid base catalysts for biodiesel synthesis due to their strong basicity, earth abundance, and low toxicity [21,22,23,24]. While activated CaO has shown promise for the production of biodiesel from sunflower oil, the catalyst is found to be susceptible to CO2 poisoning as well as calcium dissolution during reaction [25], with leached calcium forming methoxide and diglyceroxide species [26,27]. Although strontium and barium oxide are stronger bases, and more active than CaO for transesterification, these are prone to rapid deactivation by carbonate formation [24] and also leach Ba2+ and Sr2+ in methanol [28], leading to the formation of soaps and emulsions via saponification [29]. Thus while the base strength and apparent activity of alkaline earth oxides decreases in the order BaO > SrO > CaO > MgO, lixiviation in methanol also decreases, and hence MgO is the more desirable alkali earth oxide for biodiesel synthesis [30,31]. The basicity of MgO can be tuned with nanoparticle size and exposed crystal facet, with catalytically active sites in more polar (110) and (111) surfaces conveying stronger basicity and transesterification activity [21,32,33]. Nanoparticulate MgO can be synthesised via solvothermal methods [33]; however, separation of <20 nm particles from reactions can be challenging at scale, and thus practical application requires nanoparticle immobilisation. Supported base catalysts have been explored for biodiesel synthesis [30,34,35,36], although most commercial supports exhibit micro- and/or mesoporosity, which are not optimal for accommodating bulky and viscous C16-C18 TAGs typical of oil feedstocks, resulting in diffusion-limited kinetics [37,38] Mass transport of TAGs can be improved using templated mesoporous materials as catalyst supports (e.g., SBA-15) [39,40,41,42,43], but long, non-interconnected mesopore channels are still prone to slow in-pore diffusion and poor active site accessibility. Pore expansion [44] or interconnected pores present in, e.g., KIT-6 mesoporous silicas [45] can improve mass transport, but tailoring catalyst porosity via the use of hierarchical networks is the most flexible approach to optimising mass transport of bulky/viscous C16-C18 TAGs [46,47,48]. The incorporation of macropores into a solid base hydrotalcite material increased the accessibility of base sites to large-chain TAGS, thereby increasing overall catalyst activity [49,50]. The introduction of MgO nanoparticles into hierarchical macro–mesoporous SBA-15 frameworks is therefore expected to promote the transesterification of bulky triglycerides for biodiesel production.
Synthesis of MgO/SBA-15 is typically performed by wet impregnation of the parent silica with nitrate or acetate salts [43], or one-pot approaches in which Mg2+ cations are first complexed with the P123 template [51,52]. Impregnation generally results in larger nanoparticles, as reported for CaO-MgO/SBA-15 catalysts containing 27–37 wt% MgO, wherein 37–45 nm MgO NPs were obtained that were bigger than the mesopores and exclusively decorated the external surface [53]. In contrast, in situ deposition of MgO from magnesium acetate during the sol–gel synthesis of SBA-15 is reported to produce highly dispersed, surface-grafted MgOx layers even for loadings reaching 30 wt% [54]. Comparison of MCM-41, KIT-6, and SBA-15 as supports for MgO by impregnation or in situ modification with Mg(CH3COO)2·4H2O or Mg(NO3)2·6H2O [43] reveal that the latter silica exhibits enhanced activity for vegetable oil transesterification. These one-pot approaches are believed to produce highly dispersed MgO species through the complexation of Mg2+ cations with the PEO groups of P123, which produces a [M(EO)x]Xn complex that increases MgO dispersion over SBA-15 [51,54,55,56]. However, in all cases these materials were synthesised by the conventional co-operative self-assembly method of Zhao et al. [57] that results in significant microporosity within the walls of the mesopore channels [58]. In contrast, true liquid crystal templating (TLCT) minimises micropore formation in SBA-15, and increases the structural order of the silica framework [59]. TLCT methods can also be used in conjunction with 150–700 nm polystyrene (PS) spheres as hard templates to produce highly ordered, hierarchical SBA-15 scaffolds with interconnected mesopore–macropore networks [48,60]. Sulfonation of these hierarchical porous silicas results in solid acid catalysts with high activity for biodiesel production due to efficient mass transport through the macropores [48]. However, to our knowledge the same methodology has not been exploited to produce hierarchical porous solid base catalysts by, e.g., the complexation of Mg2+ in the TLCT phase of a Pluronic surfactant permeating a PS colloidal crystal.
Herein, we explore the impact of macroporosity on the transesterification of triglycerides with methanol under mild conditions over silica-supported MgO solid base catalysts prepared by one-pot TLCT routes. Magnesium incorporation had little impact on the physicochemical properties of ordered mesoporous SBA-15 and hierarchical macroporous–mesoporous SBA-15 frameworks, even at 11 wt% loading. However, the hierarchical SBA-15 support promoted a strong interfacial interaction with Mg that improved dispersion of the alkaline earth and avoided the formation of large MgO crystallites observed on the external surface of mesoporous SBA-15 particles. Macropores (of ~280 nm diameter) promoted transesterification, with the magnitude of the promotion increasing with the triglyceride chain length, attributed to enhanced in-pore mass transport, and a higher density of MgO active sites.

2. Results and Discussion

2.1. Catalyst Characterisation

The successful synthesis of mesoporous and hierarchical macro–mesoporous MgO/SBA-15 with systematically varying Mg content was first demonstrated by porosimetry, low-angle XRD, and elemental analysis. Elemental analysis of mesoporous MgO/SBA-15 and hierarchical MgO/SBA-15 materials by ICP-OES evidenced an approximately linear correlation between the nominal and actual Mg loadings (Figure S1), which spanned ~2–10 wt%. The maximum Mg loading was selected to avoid disruption of the Pluronic hexagonal micellar phase and consequent loss of ordered mesoporosity [61,62]. Wide-angle powder XRD (Figure 1) of both families of materials exhibited a broad reflection around 22°, characteristic of the amorphous walls of templated silica materials that monotonically shifted to a higher angle with increasing Mg content, indicative of framework contraction [63]. Note that this apparent contraction cannot be attributed to isomorphic substitution of Mg2+ for Si4+ cations as the former is larger (72 pm versus 40 pm) [64], and would induce framework expansion. Reflections characteristic of periclase MgO crystallites were also observed in the MgO/SBA-15 diffractograms for Mg loadings ≥8.1 wt%, with volume-averaged sizes of ~14 nm for 10.9 wt% MgO/SBA-15; such crystallites are too large to incorporate within the mesopore channels (see subsequent porosimetry and electron microscopy) and hence must decorate the external surface of silica particles. The absence of any reflections for Mg-containing phases at lower loadings and in the hierarchical MgO/SBA-15 likely reflects their presence as highly dispersed nanoparticles with dimensions below the instrumental detection limit (~2 nm) or as an amorphous phase [65]. Corresponding low-angle diffractograms showed the (100), (110), and (200) reflections of p6mm SBA-15 for both families (Figure 2), arising from close-packed, ordered networks of two-dimensional, hexagonal mesopore channels [57]. Interpore distances calculated from the (100) reflections were independent of Mg loading and essentially identical for both families at 10.9 ± 0.5 nm (Figure S2).
Mesoporous and hierarchical MgO/SBA-15 materials exhibit type IV nitrogen porosimetry isotherms (Figure 3), characteristic of the ordered mesopore domains in SBA-15 silicas [66]. However, the hysteresis loops of both SBA-15 families switch from type H1 to H2 with increasing Mg loading [67]. H1 hysteresis is caused by the gradual evaporation of physisorbed liquid N2 from ordered, uniformly sized pore channels, whereas H2 hysteresis is characteristic of ink-bottle pores. Accumulation of MgO nanoparticles likely induces partial obstruction of mesopore channels, introducing bottlenecks that restrict N2 desorption and create a range of pore diameters. The type H2 hysteresis is more pronounced in the mesoporous MgO/SBA-15. Mesoporous materials exhibited a larger surface area than their hierarchical analogues due to their larger surface area/volume ratio and greater microporosity of the former (Table 1); note that synthesis of SBA-15 by a TLCT route typically yields reduced microporosity compared with conventional co-operative self-assembly [59,68]. The mean mesopore diameters of both SBA-15 families decrease with increasing Mg content (Figure S3). As the corresponding low-angle XRD diffractograms do not evidence any changes in interpore distance (Figure 2), this pore narrowing is attributed to partial blockage by MgO causing slight constriction of the pores that is reflected in their hysteresis loops.
Bulk and surface elemental analysis of the hierarchical MgO/SBA-15 materials (Figure S4) reveals a linear relationship between the total and surface Mg content consistent with a uniform distribution of the metal throughout the pore network and external surface. In contrast, the total and surface Mg contents of mesoporous MgO/SBA-15 materials deviate ≥4 wt% Mg, indicative of partial mesopore blockage and preferential decoration of the external surface of the silica, consistent with the emergence of (>14 nm) MgO crystallites by XRD (Figure 1). Sample morphology was examined by electron microscopy. The average diameter of the polystyrene nanospheres used to template macropores was 320 ± 5 nm from SEM (Figure S5a), whereas the diameter of macropores in the hierarchical SBA-15 support was 280 ± 30 nm (Figure S5b), suggesting that macropore contraction occurred during calcination as previously reported [69]. Macropores formed cubic close-packed arrays, interconnected by mesoporous windows of ~50 nm that can accelerate molecular diffusion through SBA-15 particles [48,60]. Corresponding HRTEM confirmed successful templating of mesopores and macropore–mesopore domains throughout the mesoporous (Figure 4a,b) and hierarchical (Figure 4c,d) MgO/SBA-15 materials, respectively.
Micron-long, ordered channels of ~5 nm diameter were observed for mesoporous MgO/SBA-15, in accordance with porosimetry, and ordered mesopore domains were visible between macropores for hierarchical MgO/SBA-15, as previously reported for propylsulfonic acid-functionalised hierarchical SBA-15 [46], and spatially orthogonal sulphated zirconia/MgO/SBA-15 [52] and Pd/Pt/SBA15 catalysts [60]. There was no evidence of MgO nanoparticles for low Mg loadings, consistent with the chelation of Mg2+ cations with the P123 surfactant resulting in the uniform dispersion of the metal throughout mesopore (and macropore) silica networks. However, MgO nanoparticles of ~21 ± 6 nm diameter were observed for the mesoporous 10.9 wt% MgO/SBA-15 sample (Figure 4b and Figure S6), presumably forming on the external surface of silica particles as they are larger than the mesopores. The size of these MgO nanoparticles is in reasonable agreement with the periclase crystallites observed by XRD (Figure 1 and Figure S7), and lattice fringes were indicative of (200) facets of periclase MgO (~2.1 Å) [70]. Such nanoparticles were not observed in the hierarchical MgO/SBA-15 even at high Mg loadings, despite the lower surface area of the hierarchical silica, suggesting that macropores hinder the diffusion of Mg2+ cations along mesopore channels during calcination, thus preventing MgO sintering.
Magnesium speciation within both catalyst families was investigated by XPS (Figure 5). The Mg 2p spectra of all mesoporous MgO/SBA-15 samples exhibited a spin–orbit split doublet (separation 0.28 eV) with a 2p3/2 peak at 51.9 eV binding energy, significantly higher than those reported for anthophyllite (Mg2[Mg5][Si8O22]OH2, 51.15 eV) [71] or cordierite ([MgO]2[Al2O3]2[SiO2]5, 51.4 eV) [72], implying strong charge transfer from Mg to Si across Mg-O-Si interfacial bonds in an initial state model. A second doublet emerged for Mg loadings ≥8.1 wt%, at 50.7 eV binding energy, in excellent agreement with that for our periclase reference, and consistent with the appearance of MgO nanoparticles by XRD and TEM (Figure 1a and Figure 4b). In contrast, hierarchical MgO/SBA-15 samples only exhibited Mg-O-Si chemical environments (Figure S8), indicative of a much higher dispersion of magnesium. Corresponding Si 2p spectra of both catalyst families exhibited three spin–orbit split doublets with 2p3/2 binding energies of 104.9 eV, 103.5 eV, and 102.5 eV (Figure S9). The intensity of the two higher binding energy doublets decreased with increasing Mg loading, accompanied by a concomitant increase in the intensity of the low binding energy doublet. These features are therefore assigned to Si-OH, Si-O-Si, and Si-O-Mg species, respectively, in accordance with the literature [73,74]. The relative intensity of the Si-O-Mg interfacial species was greater for hierarchical than mesoporous MgO/SBA-15 samples, as anticipated for the higher amount of magnesium dispersed in the former. Oxygen 1s spectra exhibited two peaks at 532.0 and 532.9 eV, assigned to Si-O-Mg and Si-O-Si species, respectively (Figure S10) [75,76,77]. A third peak at 530.1 eV emerged at high Mg loadings, notably for mesoporous MgO/SBA-15, and is assigned to Mg-O-Mg species within MgO nanoparticles.

2.2. Triglyceride Transesterification

Catalytic activities of mesoporous and hierarchical MgO/SBA-15 were first compared for the transesterification of tributyrin (a simple triacyl glyceride) with methanol at 60 °C and a methanol/triglyceride molar ratio of 30 (Figure 6). Calibration curves and a representative gas chromatogram are presented in Figure S11. Reaction conditions were chosen to avoid complete triglyceride conversion and associated bulk mass-transport limitations [78] that pervade the biodiesel literature [7]. Negligible conversion was observed for parent SBA-15 and macroporous SBA-15 supports. The initial rates of tributyrin conversion (Figure 6a), and 24 h conversion values (Figure 6b), were proportional to Mg loading for both catalyst families; however, the hierarchical MgO/SBA-15 catalysts exhibit ~60% higher initial activity and significantly higher final conversion than their mesoporous analogues. Tributyrin has a kinetic diameter estimated between 0.63 and 1.06 nm [79,80], and hence is expected to experience a small diffusion barriers in accessing active base sites within mesopores [81]. These trends were mirrored by the corresponding methyl butyrate yields (Figure S12). Specific activities for the highest loading 10.9 wt% MgO/SBA-15 catalysts far exceeded those of pure periclase and talc (Mg3Si4O10(OH)2) reference materials (Figure 7), with the hierarchical catalyst again outperforming its mesoporous analogue by ~45%. The higher specific activity of the hierarchical catalyst is attributed to faster in-pore mass transport and a higher surface area of active sites due to the greater dispersion of Mg at the interface with the silica support. Tributyrin conversion was a strong function of reaction temperature between 40 and 60 °C (Figure S13), increasing from ~5% to 35% across this range for the hierarchical 10.9 wt% MgO/SBA-15. The corresponding activation energy for tributyrin transesterification (determined from initial rates) was ~26 kJ·mol−1 (between 40 and 60 °C), slightly lower than the literature values for triglycerides over solid and liquid base catalysts (38–66 kJ·mol−1) [82,83,84].
Specific activity per mass of Mg of hierarchical 10.9 wt% MgO/SBA-15 is comparable to that of other MgO and Mg/ZrO2 catalysts (Table S1) but lower than values reported for hydrotalcites, which exhibit stronger basicity due to the formation of acid–base pairs [85].
Commercial biodiesel production using soluble alkali bases typically employs a methanol/oil molar ratio of ~10 to minimise solvent use and viscosity of the reaction mixture. The effect of the methanol/tributyrin molar ratio was therefore studied for the mesoporous and hierarchical 4.4 wt% MgO/SBA-15 catalysts, which in addition to their identical loadings were dominated by interfacial Mg-O-Si species and lacked crystalline MgO nanoparticles on the external surface of catalyst particles. Initial rates of tributyrin transesterification over both catalyst families were directly proportional to the methanol/oil molar ratio for values between 6 and 20 (Figure 8), likely reflecting tributyrin solubility and strong triglyceride adsorption that necessitate a high alcohol concentration to drive the rate-determining step according to the proposed Langmuir–Hinshelwood–Hougen–Watson mechanism [86,87]. Note that an Eley–Rideal mechanism is proposed for the transesterification of canola oil over calcined dolomite [88] and for ethyl acetate transesterification over commercial MgO [89]; however, most studies suggest this mechanism is only preferred over very strong (e.g., CaO or BaO) base catalysts. The benefits of higher methanol concentrations were more pronounced for hierarchical MgO/SBA-15, suggesting that tributyrin diffusion is rate-limiting at higher ratios (where its concentration is lower), with the hierarchical materials being dominated by molecular (Knudsen) diffusion and hence superior to the mesoporous MgO/SBA-15 which is dominated by effective pore diffusion.
Increasing the triglyceride chain length to C8 (tricaprylin) and C12 (trilaurin) significantly decreased transesterification rates and 24 h conversions over both 4.4 wt% MgO/SBA-15 catalysts (Figure 9 and Figure S14), as previously reported for Mg-Al hydrotalcite catalysts [49], attributed to poorer solubility and slower diffusion of the bulkier oils. However, the catalytic rate/conversion advantage of the hierarchical versus mesoporous MgO/SBA-15 increased with chain length (from ×1.9 for tributyrin to ×3.1 for trilaurin), associated with enhanced molecular mass transport through the macropores, as observed for triglyceride transesterification and fatty acid esterification over propylsulfonic acid analogues [46,48]. Additional rate enhancements may be achievable through hydrothermal treatments [90] or porogens to expand the mesopore diameter in the hierarchical catalysts, or to alter the morphology of mesoporous SBA-15 such as through the synthesis of platelet analogues [91]. The hierarchical 4.4 wt% MgO/SBA-15 catalysts showed minimal deactivation over four reaction cycles of tributyrin transesterification (Figure 10) with reactivation by either methanol washing (3 × 20 mL) and drying at room temperature for 12 h or 380 °C calcination in static air for 5 h to remove residual hydrocarbons. Calcination provided more effective regeneration, which is attributed to decomposition of reactively formed MgCO3 that is less basic than MgO.

3. Materials and Methods

3.1. Synthesis of MgO-Functionalised SBA-15

Magnesia-functionalised SBA-15 (MgO/SBA-15) was synthesised using our previously reported liquid crystal templating method. Briefly, 2 g Pluronic P123 (~5800 g·mol−1) was added to 2 mL deionised water and the pH lowered to 2 by addition of reagent grade (37%) HCl under stirring. Subsequently, magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) was added to the mixture in the desired amount (ranging between 0 and 2 g) during stirring at 40 °C until a uniform gel was formed, upon which 4.1 mL of tetramethyl orthosilicate (TMOS) was added. The resulting homogenised gel was then aged at 40 °C under 100 mbar vacuum without stirring for 24 h, and the obtained solid was then calcined at 550 °C (ramp rate 3 °C·min−1) for 5 h in air, cooled to room temperature, and stored in a vacuum desiccator to prevent carbonate formation.

3.2. Synthesis of Polystyrene Nanospheres

Polystyrene colloidal nanospheres were synthesised using the method of Sen et al. [92] Styrene (100 mL) was washed with 0.1 M NaOH solution (5 × 100 mL) to remove the polymerisation inhibitor, and further washed with deionised water (5 × 100 mL). The washed styrene was then added to 1 L degassed water and stirred at 300 rpm under flowing N2 at 80 °C for 1 h. Degassed potassium persulfate solution, (K2(SO4)2, 50 mL, 0.025 M), was then added dropwise over 15 min to the styrene solution and the mixture stirred at 300 rpm under flowing N2 overnight at 80 °C. Polystyrene nanospheres were isolated via centrifugation at 14,500× g rpm for 10 min, dried at room temperature, and stored under air.

3.3. Synthesis of Hierarchical MgO/SBA-15

A modified MgO/SBA-15 synthesis was employed wherein 6 g of polystyrene nanospheres was added to the magnesium containing Pluronic 123 gel immediately after TMOS addition. The resulting slurry was stirred to obtain a homogeneous mixture and then aged at 40 °C under 100 mbar vacuum for 24 h. The resulting solid was then calcined at 550 °C (ramp rate 3 °C·min−1) for 5 h in air, cooled to room temperature, and stored in a vacuum desiccator to prevent carbonate formation.

3.4. Material Characterisation

Powder X-ray diffraction (XRD) data were recorded using a Bruker D8 diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) employing a Cu Kα (1.54 Å) source fitted with a LYNXEYE high-speed strip detector. Crystallite size was estimated using the Scherrer equation, and crystalline phases were analysed by Rietveld refinement. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was performed using a Thermo iCAP 7000 (Thermo Fisher Scientific, Waltham, MA, USA) calibrated against standards spanning 1–100 ppm. Nitrogen physisorption was undertaken using a Quantachrome Nova 4000 porosimeter (Quantachrome Instruments, Boynton Beach, FL, USA) and NovaWin version 11.03 software: samples were outgassed at 120 °C for 2 h in vacuo, and multi-point surface areas calculated using the Brunauer–Emmett–Teller (BET) method over the relative pressure p/p0 = 0.02–0.2; pore diameters and volumes were calculated using the NLDFT method for mesoporous silica applied to the desorption branch. High-resolution scanning transmission electron microscopy (STEM) images were recorded on a JEOL 2100F FEG STEM (JEOL Ltd., Tokyo, Japan) operating at 200 keV and equipped with a spherical aberration probe corrector (CEOS GmbH) and a Bruker XFlash 5030 EDX. Samples were ultrasonicated in methanol and dropcast on holey carbon films supported on Cu grids. Base site densities were measured by CO2 pulse chemisorption, with samples outgassed at 400 °C under flowing helium (20 mL·min−1) for 1 h, and subsequent temperature-programmed desorption (TPD) on a Quantachrome ChemBET 3000 surface analyser (Quantachrome Instruments, Boynton Beach, FL, USA) coupled to an MKS Minilab quadrupole mass spectrometer (MKS Instruments, Andover, MA, USA). Scanning electron microscopy (SEM) images were recorded on a JSM-7800F Prime scanning electron microscope (JEOL Ltd., Tokyo, Japan) fitted with a field emission gun. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Supra XPS spectrometer (Kratos Analytical Ltd., Manchester, UK) fitted with a monochromated Al X-ray anode (1486.69 eV) and charge neutraliser, at a base pressure < 1 × 10−8 Torr. Survey spectra were recorded using a pass energy of 160 eV, and high-resolution spectra with a pass energy of 20 eV. Spectra were calibrated to adventitious carbon at 284.8 eV, and Shirley background subtracted and fitted using CasaXPS version 2.3.15 and appropriate instrument response factors. A MgO reference was prepared by calcining Mg(NO3)2.6H2O at 550 °C for 5 h in air and storing in vacuo prior to analysis.

3.5. Triglyceride Transesterification

A mixture of 10 mmol tributyrin, 50 mg catalyst, 300 mmol methanol (12 mL), and 1 mmol dihexyl ether as an internal standard was placed into a 3-necked round-bottom flask. The resulting mixture was heated to 60 °C in air and stirred at 700 rpm. Aliquots of 0.25 mL were periodically withdrawn over the course of 24 h, diluted with 1.75 mL dichloromethane, and analysed using a calibrated Shimazu GC-2010 gas chromatograph (Shimadzu Corporation, Kyoto, Japan) fitted with a CP-Sil-5 column (15 m × 0.25 mm × 0.25 µm). Tricaprylin and trilaurin transesterification were conducted at higher temperature to ensure complete miscibility using a 50 mL ACE round-bottom pressure flask fitted with a sampling dip-tube, and the same reactant, solvent, and internal standard concentrations but a 100 mg catalyst. Aliquots of 0.25 mL were periodically withdrawn, diluted with 1.75 mL dichloromethane, and analysed using a calibrated Varian GC-450 (Agilent Technologies, Inc., Santa Clara, CA, USA)fitted with a ZB-1HT column (15 m × 0.53 mm × 0.15 µm). Elemental analysis (ICP-OES) of both the catalyst and the reaction’s hot filtrate revealed negligible leaching of magnesium under any reaction conditions, indicating excellent catalyst stability.

4. Conclusions

Families of mesoporous and hierarchical macroporous–mesoporous MgO/SBA-15 materials were synthesised in a one-pot, true liquid crystal templating synthesis as solid base catalysts for the transesterification of triglycerides to fatty acid methyl esters. Mesoporous MgO/SBA-15 was prepared by addition of Mg(NO3)2·6H2O to the P123-based templating solution, and hierarchical porous analogues were obtained in the presence of polystyrene nanospheres. Pore networks in both families were highly ordered, with mesopores of ~4–5 nm diameter, and macropores of ~280 nm diameter in the hierarchical materials that significantly reduce the domain length of mesoporous channels. Low loadings of (~2–6 wt%) were highly dispersed throughout both mesoporous and hierarchical MgO/SBA-15 materials. However, higher loadings (up to 11 wt%) resulted in ~14 nm MgO nanoparticles (likely decorating the external surfaces) of mesoporous SBA-15 particles, whereas only Mg-O-Si species formed at the interface of the hierarchical SBA-15. Activity for tributyrin transesterification was explored as a function of MgO loading, MeOH/triglyceride ratio, and temperature. The benefits of hierarchical pore networks were demonstrated for 11 wt% MgO/SBA-15 catalysts, wherein the conversion and specific activity for tributyrin conversion to methyl butyrate were 3 mmol·h−1·g−1 compared with 2 mmol·h−1·g−1 for the mesoporous analogue. This catalytic advantage was enhanced for longer-chain triglycerides, evidencing that macropores improved mass transport and base site accessibility, with trilaurin (C12) transesterification at 90 °C being three times faster over the hierarchical versus mesoporous 4.4 wt% MgO/SBA-15 catalysts. A low temperature calcination was effective for maintaining activity over four reactions cycles. Future work will explore the impact of tuning macropore/mesopore dimensions on mass transport, and the integration of hierarchical MgO/SBA-15 catalysts into continuous flow reactors, including oscillatory baffled flow reactors, for biodiesel production from, e.g., waste cooking oil.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal15111054/s1: Figure S1: Actual versus nominal Mg loading determined by ICP-OES; Figure S2: Interpore distances of mesoporous and hierarchical materials, calculated using low angle X-ray diffraction reflections; Figure S3: Pore size distributions for (a) mesoporous and (b) hierarchical MgO/SBA-15 materials determined using N2 porosimetry; Figure S4: Surface versus bulk MgO loading determined by XPS and ICP-OES respectfully. A gradient of 1 (represented as a black line) would signify the surface Mg loading matching the bulk loading, representing even distribution throughout; Figure S5: Scanning electron micrographs of (a) polystyrene nanospheres and (b) hierarchical 10.9 wt% MgO/SBA-15, with inset particle and pore size distribution histograms, respectively; Figure S6: Transmission electron micrographs of mesoporous 10.9 wt% MgO/SBA-15; Figure S7: Wide angle X-ray diffractogram of periclase MgO; Figure S8: MgO:Mg-O-Si XP intensity ratio; Figure S9: Si 2p XP spectra of (a) mesoporous and (b) hierarchical MgO/SBA-15; Figure S10: O 1s XP spectra of (a) mesoporous and (b) hierarchical MgO/SBA-15; Figure S11: (top) Representative gas chromatogram for the transesterification of tributyrin with methanol over hierarchical 10.9 wt% MgO/SBA-15 catalyst, and (bottom) GC calibration curves for tributyrin and methyl tributyrate; Figure S12: Methylbutyrate yield versus time over mesoporous and hierarchical MgO/SBA-15 catalysts. Reaction conditions: 10 mmol tributyrin, 50 mg catalyst, 12 ml methanol, 1 mmol dihexyl ether, 60 °C; Figure S13: Tributyrin conversion versus temperature over hierarchical 10.9 wt% MgO/SBA-15 catalyst. Reaction conditions: 10 mmol tributyrin, 50 mg catalyst, 12 ml methanol, 1 mmol dihexyl ether; Figure S14: Triglyceride conversion over 4.4 wt% MgO/SBA-15 catalysts. Reaction conditions: 10 mmol tributyrin, 50 mg catalyst, 12 ml methanol, 1 mmol dihexyl ether, 90 °C; Table S1: Activity of selected Mg-containing catalysts for tributyrin transesterification with methanol. Supplementary references [32,49,93,94,95,96,97,98,99].

Author Contributions

Conceptualisation, A.F.L. and K.W.; methodology, T.A.B., L.D., M.A.I., L.J.D. and C.M.A.P.; formal analysis, T.A.B., A.F.L., and K.W.; investigation, T.A.B. and L.D.; resources, A.F.L. and K.W.; writing—original draft preparation, T.A.B., M.A.I., A.F.L., and K.W.; writing—review and editing, A.F.L., K.W., G.K., and M.G.M.; supervision, A.F.L., K.W., M.A.I., G.K., and M.G.M.; funding acquisition, A.F.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the EPSRC (EP/K036548/1 and EP/K014706/1) and Australian Research Council (DP200100204, DP200100313, and LE210100100). KW thanks the Royal Society for an Industry Fellowship. Support from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 604307 is also acknowledged.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Wide-angle X-ray diffraction patterns of (a) conventional and (b) hierarchical MgO/SBA-15. Reflections present at approximately 42° and 63° are characteristic of periclase magnesium oxide (JCPDS No-78-0430).
Figure 1. Wide-angle X-ray diffraction patterns of (a) conventional and (b) hierarchical MgO/SBA-15. Reflections present at approximately 42° and 63° are characteristic of periclase magnesium oxide (JCPDS No-78-0430).
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Figure 2. Low-angle X-ray diffraction patterns of (a) conventional and (b) hierarchical MgO/SBA-15: inset highlights (110) and (200) reflections.
Figure 2. Low-angle X-ray diffraction patterns of (a) conventional and (b) hierarchical MgO/SBA-15: inset highlights (110) and (200) reflections.
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Figure 3. Nitrogen porosimetry isotherms of (a) conventional and (b) hierarchical materials.
Figure 3. Nitrogen porosimetry isotherms of (a) conventional and (b) hierarchical materials.
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Figure 4. TEM images of magnesium oxide deposited on (a) 2.1 wt% mesoporous SBA-15, (b) 10.9 wt% mesoporous SBA-15, (c) 1.6 wt% hierarchical SBA-15, and (d) 10.9 wt% hierarchical SBA-15 materials.
Figure 4. TEM images of magnesium oxide deposited on (a) 2.1 wt% mesoporous SBA-15, (b) 10.9 wt% mesoporous SBA-15, (c) 1.6 wt% hierarchical SBA-15, and (d) 10.9 wt% hierarchical SBA-15 materials.
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Figure 5. Mg 2p X-ray photoelectron spectroscopy of (left) mesoporous and (right) hierarchical MgO/SBA-15. Black line—experimental data; red line—fitted envelope; grey lines—individual components.
Figure 5. Mg 2p X-ray photoelectron spectroscopy of (left) mesoporous and (right) hierarchical MgO/SBA-15. Black line—experimental data; red line—fitted envelope; grey lines—individual components.
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Figure 6. (a) Initial rate and (b) 24 h conversion over mesoporous and hierarchical MgO/SBA-15 catalysts for tributyrin transesterification. Reaction conditions: 10 mmol tributyrin, 50 mg catalyst, 12 mL methanol, 1 mmol dihexyl ether, 60 °C. Error bars represent mean of triplicate reactions.
Figure 6. (a) Initial rate and (b) 24 h conversion over mesoporous and hierarchical MgO/SBA-15 catalysts for tributyrin transesterification. Reaction conditions: 10 mmol tributyrin, 50 mg catalyst, 12 mL methanol, 1 mmol dihexyl ether, 60 °C. Error bars represent mean of triplicate reactions.
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Figure 7. Specific activity over mesoporous and hierarchical 10.9 wt% MgO/SBA-15 catalysts and reference materials for tributyrin transesterification, based on initial rates normalised to total Mg content. Reaction conditions: 10 mmol tributyrin, 50 mg catalyst, 12 mL methanol, 1 mmol dihexyl ether, 60 °C. Error bars represent mean of triplicate reactions.
Figure 7. Specific activity over mesoporous and hierarchical 10.9 wt% MgO/SBA-15 catalysts and reference materials for tributyrin transesterification, based on initial rates normalised to total Mg content. Reaction conditions: 10 mmol tributyrin, 50 mg catalyst, 12 mL methanol, 1 mmol dihexyl ether, 60 °C. Error bars represent mean of triplicate reactions.
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Figure 8. Initial rate of transesterification over mesoporous and hierarchical 4.4 wt% MgO/SBA-15 catalysts as a function of the methanol/tributyrin molar ratio. Reaction conditions: 12 mL methanol, 50 mg catalyst, 1 mmol dihexyl ether, 60 °C. Error bars represent mean of triplicate reactions.
Figure 8. Initial rate of transesterification over mesoporous and hierarchical 4.4 wt% MgO/SBA-15 catalysts as a function of the methanol/tributyrin molar ratio. Reaction conditions: 12 mL methanol, 50 mg catalyst, 1 mmol dihexyl ether, 60 °C. Error bars represent mean of triplicate reactions.
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Figure 9. Initial rate (a) and triglyceride conversion (b) of triglyceride transesterification over mesoporous and hierarchical 4.4 wt% MgO/SBA-15 catalysts as a function of triglyceride chain length. Reaction conditions: 10 mmol triglyceride, 50 mg catalyst, 12 mL methanol, 1 mmol dihexyl ether, 90 °C. Error bars represent mean of triplicate reactions.
Figure 9. Initial rate (a) and triglyceride conversion (b) of triglyceride transesterification over mesoporous and hierarchical 4.4 wt% MgO/SBA-15 catalysts as a function of triglyceride chain length. Reaction conditions: 10 mmol triglyceride, 50 mg catalyst, 12 mL methanol, 1 mmol dihexyl ether, 90 °C. Error bars represent mean of triplicate reactions.
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Figure 10. Methyl butyrate productivity for tributyrin transesterification over hierarchical 4.4 wt% MgO/SBA-15 as a function of reaction cycle and reactivation treatment. Reaction conditions: 10 mmol triglyceride, 50 mg catalyst, 12 mL methanol, 1 mmol dihexyl ether, 90 °C.
Figure 10. Methyl butyrate productivity for tributyrin transesterification over hierarchical 4.4 wt% MgO/SBA-15 as a function of reaction cycle and reactivation treatment. Reaction conditions: 10 mmol triglyceride, 50 mg catalyst, 12 mL methanol, 1 mmol dihexyl ether, 90 °C.
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Table 1. Textural properties of MgO/SBA-15 materials from N2 porosimetry.
Table 1. Textural properties of MgO/SBA-15 materials from N2 porosimetry.
Mesoporous MgO/SBA-15Hierarchical MgO/SBA-15
Mg Content a
/wt%		Surface Area b
/m2·g−1		Pore Size c
/nm		Mg Content a
/wt%		Surface Area b
/m2·g−1		Pore Size c
/nm
TotalMicroporeTotalMicropore
0.0370 ± 3772 ± 75.20.0265 ± 2705.0
2.1502 ± 50142 ± 14 5.21.6232 ± 2305.0
4.4525 ± 53126 ± 134.54.4322 ± 3214 ± 14.9
8.1604 ± 60130 ± 134.06.8327 ± 3304.2
10.9650 ± 65143 ± 143.910.9294 ± 2903.8
a ICP-OES. b N2 porosimetry: total area from BET method; micropore areas from t-plot method. c NLDFT method assuming slit-shaped pores.
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Bryant, T.A.; Damptey, L.; Isaacs, M.A.; Parlett, C.M.A.; Durndell, L.J.; Granollers Mesa, M.; Kyriakou, G.; Wilson, K.; Lee, A.F. Impact of Macroporosity on the Transesterification of Triglycerides over MgO/SBA-15. Catalysts 2025, 15, 1054. https://doi.org/10.3390/catal15111054

AMA Style

Bryant TA, Damptey L, Isaacs MA, Parlett CMA, Durndell LJ, Granollers Mesa M, Kyriakou G, Wilson K, Lee AF. Impact of Macroporosity on the Transesterification of Triglycerides over MgO/SBA-15. Catalysts. 2025; 15(11):1054. https://doi.org/10.3390/catal15111054

Chicago/Turabian Style

Bryant, Thomas A., Lois Damptey, Mark A. Isaacs, Christopher M. A. Parlett, Lee J. Durndell, Marta Granollers Mesa, Georgios Kyriakou, Karen Wilson, and Adam F. Lee. 2025. "Impact of Macroporosity on the Transesterification of Triglycerides over MgO/SBA-15" Catalysts 15, no. 11: 1054. https://doi.org/10.3390/catal15111054

APA Style

Bryant, T. A., Damptey, L., Isaacs, M. A., Parlett, C. M. A., Durndell, L. J., Granollers Mesa, M., Kyriakou, G., Wilson, K., & Lee, A. F. (2025). Impact of Macroporosity on the Transesterification of Triglycerides over MgO/SBA-15. Catalysts, 15(11), 1054. https://doi.org/10.3390/catal15111054

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