Solventless Glycerol Etherification to Di- and Tri-Glycerol over Mg-La Mixed Oxides Derived from Layered Double Hydroxides

Abstract

Mg–La mixed metal oxides derived from layered double hydroxide (LDH) precursors were synthesized via coprecipitation and evaluated as heterogeneous catalysts for solventless glycerol etherification to short-chain polyglycerols. The influence of Mg/La molar ratio on the structural, textural, and catalytic properties of the catalysts was systematically investigated using XRD, BET, SEM-EDX, FTIR, TPD-CO₂, TPD-NH₃ and ICP-OES analyses. XRD confirmed the formation of La₂O₂CO₃ phases, while CO₂-TPD analysis revealed the presence of abundant medium-to-strong basic sites. Among the synthesized catalysts, Mg_0.25La_0.75O₂ exhibited the highest basic site concentration (6830 µmol g⁻¹) and superior catalytic performance due to the possible cooperative interaction between Mg- and La-derived sites. Under optimum reaction conditions of 220 °C, 8 h, and 2 wt% catalyst loading, the catalyst achieved 90% glycerol conversion with 70% diglycerol selectivity, 23% triglycerol selectivity, and 84% combined diglycerol and triglycerol yield. Reaction temperature, catalyst loading, and reaction duration significantly influenced oligomer distribution and catalyst performance. Reusability studies demonstrated acceptable catalyst stability for up to four cycles before gradual deactivation caused by oligomer deposition and metal leaching. The results highlight Mg–La mixed oxides as promising catalysts for sustainable solvent-free glycerol valorization, while demonstrating a scalable and environmentally benign strategy for maximizing lower-degree polyglycerol production within shorter reaction durations and reduced processing cost.

1. Introduction

The swift advancement of economies and the rise in population have led to a significant uptick in global energy production and consumption. Fossil fuels are more cost-effective to produce compared to renewable sources, positioning them as a significant feedstock for energy infrastructure. Nonetheless, their depletion may adversely affect the global economy, leading to increased petrochemical prices and contributing to climate change. The global market is showing a growing interest in renewable alternative resources like biofuels. Biodiesel is often suggested as a viable alternative due to its renewable nature, environmental friendliness, biodegradability, and cost-effectiveness for sustained production [1,2,3]. Glycerol, a notable byproduct of biodiesel synthesis, presents considerable prospective precursor to acquire valuable chemicals such as glycerol ethers, 1,2-propanediol, 1,3-propanediol, and glyceric acid [4,5]. The conversion of unprocessed triglycerides into biodiesel results in the production of glycerol as a by-product, with a yield of approximately 100 kg of glycerol per ton of biodiesel produced [4,6,7]. Glycerol has reached an excess due to an increase in production spurred by the growth of oleochemicals and biodiesel. The surplus supply resulted in a decline in glycerol prices, attributed to advancements in biodiesel production, thereby establishing it as a viable chemical for the synthesis of other value-added compounds [8]. Recent reports indicate that crude glycerol is typically valued at approximately USD 280–320 t⁻¹, whereas refined glycerol can command prices exceeding USD 1400–2800 t⁻¹ depending on purity and application [9]. In contrast, polyglycerols are considerably higher-value specialty chemicals, with reported market prices of approximately USD 4–5 kg⁻¹ and increasing demand in food, cosmetic, pharmaceutical, surfactant, and polymer industries. The global glycerol market was valued at approximately USD 4.5 billion in 2025 and is projected to continue growing due to increasing demand from personal care, pharmaceutical, and chemical sectors [10]. Therefore, the selective conversion of low-value glycerol into di- and triglycerol represents an economically attractive route for producing value-added bio-based chemicals while enhancing the sustainability of biodiesel production [11].

Etherification [12], esterification [13], oxidation [14], dehydration [15] and reforming [16] are several examples of catalytic processes used to convert glycerol. The etherification process has attracted considerable attention because of the potential of glycerol tert-butyl ethers, especially di-ethers and tri-ethers, as biofuel additives that improve combustion efficiency. When added to biodiesel, these ethers lower its viscosity and decrease emissions of hydrocarbons, aldehydes, particulate matter, and carbon monoxide [17]. Glycerol serves as an important chemical compound in the synthesis of biodiesel, as it can be transformed into di-, tri-, tetra-, or polyglycerols, thereby enhancing yield. During etherification, glycerol molecules undergo condensation to form di- and tri-glycerol, exhibiting various structural configurations such as linear, branched, or cyclic forms. The condensation occurring at primary or secondary hydroxyls plays a crucial role in determining the final shape, while intramolecular condensation may influence the structure of the product [6,17]. Continuous investigation is enhancing the yields of lower-degree polyglycerol. The development of a catalyst exhibiting exceptional characteristics for achieving high glycerol conversion and producing greater quantities of the desired lower degree polyglycerols in a solventless environment continues to pose a significant scientific challenge. Recent advancements in glycerol etherification have concentrated on enhancing yield and conversion rates of lower degree polyglycerols and glycerol ethers using solventless techniques supported by catalysts. This was brought on by the numerous drawbacks of solvent-based etherification, including the creation of unwanted byproducts [18,19], a significant amount of solvent needed to reach the proper degree of glycerol conversion [20,21] and higher investment and operating costs as a result of water molecules forming during the reaction [22]. Application of heterogeneous base catalysts [12], acid catalysts [23] and heteropolyacids [24] catalyzed reactions is commonly employed in the solventless etherification process. The processes of etherification that are catalyzed by acid lead to faster reactions; however, they suffer from a lack of selectivity. This is attributed to increased oligomerization with secondary products, which can lead to an unwanted side reaction that involves the dehydration of glycerol, ultimately producing acrolein [25,26]. The base-catalyzed catalytic etherification process leads to improved glycerol conversion and more selectivity for preferred polyglycerol molecules [6,12,27,28].

Charles et al. demonstrated that a 2 wt% Na₂CO₃ catalyst resulted in 96% glycerol conversion over a period of 8 h at a temperature of 260 °C, achieving 81% selectivity for di-, tri-, and tetraglycerols [29]. The utilization of a Li/zeolite Y microporous catalyst demonstrated selectivity of 21% for di-glycerol and 32% for tri-glycerol at a temperature of 240 °C over an 8-h period, with a catalyst loading of 2 wt%, resulting in a maximum glycerol conversion of 98% [30]. The study by Park et al. revealed that X-type zeolite heterogeneous catalysts containing alkali metal ions such as Li⁺, Na⁺, and K⁺ improve glycerol conversion, while concurrently diminishing the selectivity for diglycerol and triglycerol owing to enhanced oligomerization. The catalytic activity for glycerol conversion ranked as follows: XZ-K > XZ-Li > XZ-Na [31]. The Ca_1.6Al_0.4La_0.6O₃ mixed metal oxide catalyst demonstrated a conversion rate of 96% and a selectivity of 52% for diglycerol at 250 °C over a duration of 8 h within a mesoporous structure; however, the selectivity for triglycerol was not reported [32]. In addition to a diglycerol selectivity of 29% and a triglycerol selectivity of 20%, the hydrotalcite catalyst, which exhibited a macroporous structure and a 2:1 Ca/Al ratio, attained the utmost glycerol conversion at 84%. A total yield of 59% was attained with a catalyst dosage of 2 wt% at a temperature of 235 °C for a 24-h period [33].

Mixed metal oxides obtained from layered double hydroxides (LDHs) show great potential for practical applications, owing to their straightforward synthesis and ease of handling. The solids are composed of brucite-type layers in which M²⁺ ions are partly substituted by M³⁺ ions, resulting in a net positive charge. Anions in the interlayer region and water molecules provide compensation for this. The basicity, interlayer distance, and crystallite morphology of these solids are susceptible to modifications by altering the type of cations, the metal ratio in the brucite-like layers, and the interlayer anionic species. Hydrotalcite-like materials’ fundamental characteristics and the uniform distribution of mixed oxides during thermal decomposition are essential components of their effective utilization in catalysis [34,35]. The overall expression for these solids is [M²⁺_1−x M³⁺_x(OH)₂]A_n^x/_n·mH₂O, where M²⁺ denotes the divalent metal cations, such as Mg²⁺, Ca²⁺, Ni²⁺, and Cu²⁺, while M³⁺ indicates the trivalent metal cations, including La³⁺, Al³⁺, Cr³⁺, and Ga³⁺. The molar fraction, x, is calculated using the formula [M³⁺]/([M²⁺] + [M³⁺]), where Aⁿ⁻ represents the interlayer anion, such as OH⁻, NO₃⁻, CO₃²⁻, or SO₄²⁻. The synthesized hydrotalcite undergoes thermal decomposition, resulting in a mixed metal oxide characterized by robust basic sites, [M²⁺_1−xM³⁺_x]O₂ [36,37].

The plausible etherification of glycerol over heterogeneous mixed-metal oxide catalysts is proposed to proceed via an S_N2-type pathway [38,39], as illustrated in Figure 1. The surface M–O–M′ acid–base pair (where M represents a divalent metal cation, M²⁺, and M′ represents a trivalent metal cation, M³⁺) facilitates glycerol activation through synergistic interactions with the hydroxyl groups. The basic lattice oxygen abstracts a proton from a terminal hydroxyl group of glycerol, generating a surface-stabilized glycerolate species with enhanced nucleophilic character. The resulting alkoxide oxygen subsequently attacks the less sterically hindered terminal carbon atom of a second glycerol molecule in an S_N2-like nucleophilic substitution step. This leads to the formation of a diglycerol intermediate, followed by proton transfer and dehydration, yielding diglycerol as the primary product together with a molecule of water [40,41,42]. The synergistic interaction between the acidic metal centers and basic lattice oxygen sites is therefore essential for glycerol activation and subsequent C–O–C bond formation.

Hydrotalcite, where La³⁺ ions partially replace Mg²⁺, has been studied across multiple disciplines. Pd/MgLa mixed oxide hydrotalcite catalysts were effectively utilized and reused in catalytic transfer hydrogenation reactions within ionic liquids, leading to outstanding hydrogenation of α,β-unsaturated carboxylic acid derivatives, and the catalysts were regenerated with minimal activity loss [43]. The MgLa mixed oxide served as a catalyst in the oxidation of dibenzothiophene at 60 °C using hydrogen peroxide. Enhanced reaction rates and elevated activity were observed following calcination and rehydration, attributed to the increased basic strength of the mixed oxide [44]. Investigations conducted by Desmartin et al. on the fundamental characteristics of mixed oxide catalysts observed an improved distribution of basic strength attributed to the increased basicity of the MgLa mixed oxides. The use of MgLa mixed oxide catalysts as a robust solid base and effective agent for the transesterification process involving soybean oil and methanol at 65 °C yields a reaction rate similar to that documented for anchored guanidines [45]. The integration of Mg and La to create hydrotalcite precursors across various fields has shown significant promise, particularly because of their enhanced basicity, superior activity, and regeneration ability with minimal activity loss. In terms of novelty, the objective of this study is to evaluate the catalytic performance of MgLa-mixed metal oxides that are derived from hydrotalcite precursors and used in the solventless glycerol etherification process, bearing the objective of achieving a higher yield of lower-degree polyglycerols at a shorter reaction duration. The effects of the MgLa molar ratio on the physical and chemical characteristics, reactivity, catalytic efficacy, and reusability of the catalysts were examined to show that these catalysts exhibit high activity and selectivity in producing lower degree polyglycerol in a solventless environment.

2. Results

2.1. Characterization of Catalysts

The XRD patterns of Mg_1−xLa_xO₂ displayed distinct and pronounced diffraction peaks at 2θ = 25.8°, 30.3°, and 44.4°, regardless of the molar ratio, as shown in Figure 2. The observed peaks correspond to lanthanum oxide carbonate, La₂O₂CO₃, which exists as hexagonal structure (JCPDS File: 00-037-0804) and monoclinic structure (JCPDS File: 00-048-1113) [46,47]. The hexagonal configuration has been reported to be more advantageous in the catalytic process owing to its stability [48]. The formation of this phase also indicates a reconstruction of a layered structure during the coprecipitation and thermal treatment [49]. The peaks exhibited greater crystallinity with an increase in the lanthanum molar ratio. The distinctness of the peaks results from an increase in the crystallite size, as shown in

Table 1. Physical, chemical and reactivity analyses of Mg_1−xLa_xO₂.

Catalysts	Crystallite Size (nm) 1	SBET (m2 g−1) 2	Vp (cm3 g−1) 2	Elemental Composition (Mg:La) 3	Strong Basic Site Concentration (µmol g−1) 4	Weak Acid Site Concentration (µmol g−1) 5	Moderate Acid Site Concentration (µmol g−1) 5
Mg0.2La0.8O2	64.49	71.12	0.415	17:83	3805	52.90	31.63
Mg0.25La0.75O2	64.37	74.17	0.698	24:76	6830	44.39	34.77
Mg0.4La0.6O2	63.01	78.63	0.489	39:61	4184	42.65	28.42
Mg0.6La0.4O2	61.68	81.21	0.378	62:38	3597	39.71	24.40
Mg0.8La0.2O2	60.67	87.29	0.273	81:19	2922	36.83	20.30

¹ Crystallite size was assessed through XRD analysis employing Scherrer’s equation. ² Surface area and pore volume as determined by BET analysis. ³ EDX analysis. ⁴ Determined from TPD-CO₂. ⁵ Determined from TPD-NH₃.

. The observed increase in crystallite size corresponding to the molar concentration of La³⁺ supports these findings [50].

The calcination process conducted at 450 °C resulted in solids exhibiting specific surface areas that varied from 74 m² g⁻¹ to 87 m² g⁻¹, as detailed in Table 1. The findings indicated that the surface area diminished as the La³⁺ molar ratio increased, aligning with the XRD results that showed a rise in crystallite size. High concentrations of La³⁺ lead to the formation of monoclinic La₂O₂CO₃ phases, which are linked to a decrease in surface area [47]. The enhancement of the catalyst’s pore volume with increased La³⁺ ion concentration illustrates that La³⁺ contributes to the porosity of the synthesized catalyst [51]. Abundant La³⁺ might have caused partial collapse of the catalyst microstructure, resulting in a decreased pore volume for Mg_0.2La_0.8O₂. This suggests a delicate balance is required when optimizing La³⁺ ion concentration to achieve the desired porosity without compromising the structural integrity of the catalyst. EDX analyses revealed that the elemental composition of the Mg:La ratio closely aligns with the molar ratio of the synthesized catalysts, indicating a uniform distribution of the elements on the surface of the catalysts. Lower Mg ratio was detected in the Mg_0.2La_0.8O₂ as compared to the other synthesized catalysts, which could be attributed to the excessive La³⁺ ions predominating the outer layer of the catalysts, while the Mg²⁺ located in the inner particle sites.

The morphology of catalysts with different metal loadings was examined using Scanning Electron Microscopy (SEM) analysis. The SEM micrographs presented in Figure 3 illustrate that the synthesized catalysts consist of irregularly shaped platelets [49] that aggregated to form a cluster of particles lacking a distinct shape [37]. When the loading of the La³⁺ ion exceeded that of the Mg²⁺ ion, the aggregation of irregularly shaped platelets was reduced, leading to a well-exposed platelet with enhanced accessibility. The increased catalysts’ pore volumes, as indicated in Table 1, supports this observation. Fourier transform infrared spectra depicted in Figure 4 illustrate the MgLa mixed metal oxide catalysts. Two distinct bands were detected at roughly 1450 cm⁻¹ and 860 cm⁻¹. The band at 1450 cm⁻¹ coincides with those of hexagonal La₂O₂CO₃ [52], which further validates the XRD findings. The band observed at 860 cm⁻¹ can be attributed to the stretching vibration of the surface CO₃²⁻ groups, suggesting that a small quantity of CO₂ has been absorbed onto the catalyst surface [53]. Bands below 1000 cm⁻¹ are associated with the M–O–M′ vibration mode (where M = M²⁺ and M′ = M³⁺), thus, in this case, it represents the Mg–O–La vibration. This highlighted the effectiveness of the coprecipitation method in integrating Mg²⁺ and La³⁺, as well as the significance of this bond in CO₂ absorption [54].

TPD-CO₂ analysis reveals the presence of basic sites with varying strengths on the synthesized catalysts, corresponding to weak (50–200 °C), medium (200–350 °C), and strong (>350 °C) basic sites. These categories correspond to the release of CO₂ from metal-hydroxide, metal-oxygen ion pairs, and unsaturated O²⁻ anions, respectively [12,55]. The analysis showed that all synthesized catalysts exhibited predominantly strong basic sites, as indicated in Figure 5 and the concentration of strong basic sites in Table 1. This suggests that the surface basicity of the Mg–La mixed oxides is mainly associated with strongly basic O²⁻ species, which can facilitate glycerol activation through deprotonation of the hydroxyl group to form glycerolate intermediates [40,41,42]. Furthermore, this indicates that a mixed metal oxide catalyst featuring robust basic sites, [M²⁺_1−xM³⁺_x]O₂, was successfully synthesized through the thermal treatment of hydrotalcite, allowing for desorption [36,37]. A variety of scholarly articles have indicated that the formation of carbonate species occurs due to the absorption of CO₂ on strong basic sites [12,36,37,56]. Additionally, the asymmetry of the peak concentrated close to 700 °C suggests the formation of various carbonate species, such as bridging, chelating, unidentate, and bidentate carbonates [55,57,58,59,60]. When the molar concentration of La³⁺ surpassed that of Mg²⁺, the catalysts displayed two pronounced desorption peaks, leading to an increased removal of CO₂. Research conducted by Garbarino et al. indicates that the addition of La³⁺ enhances the catalyst’s basicity through specific interactions and coupling with the unsaturated O²⁻ ion, facilitating the adsorption of CO₂ [61]. The addition of La³⁺ to the system must be carefully controlled, as an excess can result in particle aggregation on the surface of the catalytic systems under investigation [62].

Temperature-programmed desorption of ammonia (TPD-NH₃) is a well-established method for probing the surface acidity of solid catalysts, providing both the distribution and relative strength of acid sites [63]. On the synthesized catalysts, the NH₃ desorption profile in Figure 6 reveals acid sites spanning a continuum of strengths, with temperature ranges that closely parallel those observed in the TPD-CO₂ analysis. The desorption curve can be deconvoluted into three regions [12,36,49,64] where weak acid sites (50–200 °C) of NH₃ desorbing in this low-temperature window correspond to surface hydroxyl groups that retain ammonia through hydrogen-bonding interactions. These sites are the most abundant but least energetic [65]. Moderate acid sites (200–350 °C) where the intermediate desorption range is associated with metal–oxygen pairs (M–O–M’) in the mixed oxide lattice. This assignment aligns with the medium-strength acid site window (200–350 °C). The intimate mixing of metal components in the oxide framework generates Lewis acid centers of appreciable strength through the polarization of M–O–M’ bridges [65]. Strong acid sites (>350 °C) are attributed to exposed M²⁺ or M³⁺ cations functioning as strong Lewis acid centers, with the highest-strength sites residing at edges or corners of crystallites where coordinative unsaturation is greatest. Such coordinatively unsaturated metal centers are described as potent Lewis acid sites owing to their enhanced electron-accepting character [66]. The TPD-NH₃ profiles reveal that the distribution of acid sites is strongly governed by the Mg²⁺/La³⁺ ratio. The low-temperature desorption region, assigned to weak acid sites, decreases progressively with increasing Mg content, indicating a gradual reduction in weakly bound NH₃ species. In contrast, the intermediate-temperature desorption region, attributed to moderate acid sites, exhibits a non-monotonic dependence on composition [63,65]. This behavior suggests that limited Mg incorporation modifies the local surface environment in a manner that favors the generation or exposure of acid sites of intermediate strength, potentially associated with coordinatively unsaturated metal cations and mixed M–O–M’ surface ensembles [66,67]. At higher Mg contents, however, the abundance of these acidic surface species appears to diminish, resulting in a progressive decrease in both weak and moderate acidity. Overall, these results demonstrate that the surface acidity of Mg–La mixed oxides can be effectively tuned by adjusting the Mg²⁺/La³⁺ ratio.

2.2. Catalytic Activity

2.2.1. Performance of Synthesized Catalysts

The etherification of glycerol to polyglycerol with heterogeneous catalysts is becoming increasingly favored because of advantages like easy catalyst separation and reuse. However, it still faces challenges, including insufficient catalytic activity, the need for harsher reaction conditions, and the potential leaching of the catalyst into the reaction medium [36]. Figure 7 illustrates the catalytic outcomes achieved with Mg_1−xLa_xO₂ catalysts over a reaction period of 8 h at 220 °C, utilizing a catalyst loading of 2 wt%. The conversion and selectivity of di- and triglycerol improved with reaction time, reaching a peak at 8 h. The catalyst Mg_0.25La_0.75O₂ attained the highest conversion rate of 90%, with selectivity values of 70% for diglycerol and 23% for triglycerol. Theoretical investigations of alkaline earth metal oxides have highlighted the critical role of Lewis acidic metal centers and surface basic oxygen sites in governing glycerol activation and etherification pathways, indicating that the balance between acidity and basicity is a key determinant of catalytic performance [68]. Lewis acid sites consist of coordinated unsaturated metal cations on metal oxides, whereas Lewis base sites are constituted by oxygen anions [69,70]. Metal cations that lack electrons exhibit acidity as they accept electrons, whereas oxygen anions that are rich in electrons serve as basic donors of electrons [70]. The arrangement of unsaturated surface metal ions has been demonstrated in numerous investigations to promote the hydroxyl departure in the glycerol etherification reaction, thereby triggering the oligomerization of glycerol [32,71,72]. The catalyst that achieved the ideal equilibrium between basicity and Lewis acidity demonstrated the highest level of activity, highlighting the significance of both sites. The coexistence of acidic and basic functionalities boosts the molecular interactions and activation of glycerol on the catalyst surface [32,72,73]. It can be suggested that the unsaturated metal cations (Mg²⁺ and La³⁺) function as Lewis acid sites, which are counterbalanced by the formation of hexagonal La₂O₂CO₃ as the molar concentration of La³⁺ increases. Additionally, the presence of the labile O²⁻ anion enhances the catalytic activity of the synthesized catalyst.

2.2.2. Impact of Reaction Time

The impact of etherification duration on glycerol conversion, selectivity, and product yield is shown in Figure 8. As the reaction time increased, glycerol conversion also increased. The Mg_0.25La_0.75O₂ catalyst showed rapid conversion up to approximately 8 h, after which the rate of increase became slightly lower. This behavior may be attributed to the gradual approach toward reaction equilibrium, the reduced glycerol concentration [37], the accumulation of higher oligomers on active sites [56] and mild diffusion limitations caused by the increasing viscosity of the reaction mixture [49]. The continued increase in conversion beyond 8 h suggests that equilibrium had not yet been reached under the investigated conditions [38,74]. In parallel, the decrease in diglycerol selectivity and increase in triglycerol selectivity are consistent with the consecutive etherification pathway of glycerol [74,75]. Diglycerol is initially formed as the major product and then undergoes further etherification with glycerol or oligomeric intermediates to produce triglycerol and higher polyglycerols [74,76,77]. Therefore, diglycerol selectivity decreases over time as it is progressively consumed to form higher oligomers such as triglycerol and tetraglycerols [56,74,78]. The more pronounced decline observed for Mg_0.25La_0.75O₂ suggests that the catalyst possessed higher catalytic activity and more accessible active basic sites [49], thereby facilitating further oligomerization reactions [56]. Triglycerol selectivity increased with reaction time due to the sequential oligomerization pathway of glycerol etherification, where diglycerol acted as the intermediate before further conversion into triglycerol and higher oligomers. Triglycerol formation accelerated after approximately 4–5 h as sufficient diglycerol accumulated to promote secondary etherification reactions. Mg_0.25La_0.75O₂ exhibited the highest triglycerol selectivity due to its optimal balance of accessible medium-strength basic sites, favorable Mg–La interaction, and enhanced oligomerization efficiency [56,79]. Lower-activity catalysts showed delayed triglycerol formation because slower glycerol conversion reduced diglycerol accumulation and suppressed secondary oligomerization [31,80]. Near 24 h, triglycerol selectivity plateaued slightly because triglycerol underwent further etherification into tetraglycerol, while increasing viscosity and oligomer accumulation introduced mild diffusion limitations that reduced the net triglycerol formation rate [76]. The diglycerol + triglycerol yield profile shown in the graph is scientifically consistent with the catalytic behavior expected for solventless glycerol etherification over Mg–La mixed oxide catalysts. Initially, product yield increased slowly because glycerol conversion and oligomer formation were still limited at the early reaction stage. As the reaction progressed, yield increased more rapidly due to accelerated glycerol conversion and enhanced formation of diglycerol and triglycerol intermediates. Mg_0.25La_0.75O₂ exhibited the highest yield throughout the reaction due to its optimal balance of accessible medium-strength basic sites, favorable Mg–La interaction, and improved active-site accessibility, which enhanced oligomerization efficiency. Similar observations have been reported for heterogeneous mixed oxide catalysts where optimal acid–base properties and mesoporosity promoted higher oligomer yields [37].

2.2.3. Impact of Reaction Temperature

The results in Figure 9 demonstrate that reaction temperature significantly influenced glycerol conversion, oligomer selectivity, and diglycerol + triglycerol yield during solventless glycerol etherification over Mg_0.25La_0.75O₂ catalyst. Increasing the reaction temperature from 200 °C to 240 °C progressively enhanced glycerol conversion from 85% to 98%, indicating that higher temperature accelerated glycerol activation and oligomerization kinetics. Similar temperature-dependent conversion behavior has been widely reported for heterogeneous glycerol etherification over basic mixed oxide catalysts [37,56,75]. Given that multiple studies have demonstrated that reaction temperatures exceeding 250 °C considerably diminish product selectivity, no exploration into increasing the reaction temperature further was conducted [31,37,78,81,82]. At lower temperature (200 °C), diglycerol selectivity remained high (83%) because the reaction favored partial etherification while suppressing secondary oligomerization into higher oligomers. As temperature increased to 220 °C and 240 °C, diglycerol selectivity decreased substantially while triglycerol and tetraglycerol selectivity increased. This behavior suggests that elevated temperature promoted sequential oligomerization reactions in which diglycerol underwent further etherification into triglycerol and tetraglycerol. Similar observations have been reported for glycerol oligomerization over solid base catalysts, where higher temperatures enhanced secondary condensation reactions and favored higher oligomer formation [49]. The catalyst activity showed a notable decline when the reaction temperature was lowered to 200 °C, indicating that the rate of oligomerization is quite minimal at these reduced temperatures, aligning with previously documented results [29,32,83,84]. The highest overall diglycerol + triglycerol yield was obtained at 220 °C, indicating that this temperature provided the optimum balance between glycerol conversion and selective oligomer formation. Although 240 °C produced the highest conversion, excessive secondary oligomerization likely reduced the yield of desirable oligomers due to formation of heavier products and possible side reactions. Similar optimum-temperature behavior has been reported in glycerol etherification studies involving heterogeneous mixed oxide catalysts [49].

2.2.4. Impact of Catalyst Loading

The results shown in Figure 10 indicate that catalyst loading significantly influenced glycerol conversion, oligomer selectivity, and overall diglycerol + triglycerol yield during solventless glycerol etherification over Mg_0.25La_0.75O₂ catalyst. Increasing catalyst loading from 1 wt% to 3 wt% progressively increased glycerol conversion from 85% to 98% due to the higher availability of accessible active basic sites that enhanced glycerol activation and etherification reactions more effectively [37,81,82]. A decrease in catalyst loading often leads to lower glycerol conversion, as there is reduced catalyst availability to facilitate the process. An increase in catalyst concentration frequently leads to higher glycerol conversion rates and enhanced selectivity for di- and triglycerol. The observed outcome was correlated with the desiccation process of glycerol molecules. Interaction between these oxide catalysts with O-H bonds of glycerol may have strengthened the hydroxyl oxygen’s nucleophilic nature, facilitating the oligomerisation of glycerol [75]. The hydroxyl group of one glycerol molecule may have attacked a polarised glycerol molecule, potentially leading to the cleavage of a water molecule and the formation of lower degree polyglycerols [75,83]. Conversely, there have been observed declining trends in diglycerol selectivity that may be elucidated by the phenomenon where diglycerol reverts to glycerol [75]. Additionally, the reaction could have experienced further inhibition due to mass transfer occurring during the process [83]. At lower catalyst loading (1 wt%), diglycerol selectivity remained comparatively high because the limited number of active sites favored partial etherification while suppressing secondary oligomerization into higher oligomers. As catalyst loading increased, diglycerol selectivity decreased substantially, whereas triglycerol and tetraglycerol selectivity increased. This behavior suggests that higher catalyst loading enhanced secondary etherification reactions by increasing the density and accessibility of medium-strength basic sites, thereby promoting further oligomerization of diglycerol into triglycerol and tetraglycerol [31,56]. The highest diglycerol + triglycerol yield was obtained at 2 wt% catalyst loading, indicating that this loading provided the optimum balance between glycerol conversion and selective oligomer formation. Although 3 wt% catalyst loading produced the highest conversion, excessive secondary oligomerization likely reduced diglycerol selectivity and promoted formation of heavier oligomers, thereby lowering the overall desirable oligomer yield. In addition, excessive catalyst loading may increase oligomer accumulation on catalyst surfaces, causing mild diffusion limitations and reduced selectivity toward desirable oligomer products [49,75,78].

2.2.5. Reusability Studies

The ability to reuse the catalyst is a crucial factor in catalytic reactions. This study investigated the endurance of the catalytic efficacy of a mixed oxide Mg_0.25La_0.75O₂ catalyst through its recycling in five successive batch runs under optimal glycerol etherification conditions. After each catalytic experiment, the catalyst underwent filtration and was rinsed with methanol (Merck, 99.9%), followed by drying in the oven at 80 °C for 8 h. Figure 11 illustrates the transformation of glycerol and the corresponding output over five consecutive cycles. The leaching of active metals during reactions diminishes catalytic activity, as the metals that are lost can dissolve into the reaction media. Leached metals contribute to the homogeneity of the solution. This happens given that specific species, including starting materials, products, by-products, or intermediates, establish stronger interactions alongside the metal intricate compared to the ligand ability on the solid substrate. Modifications applied to the gathered catalyst, such as filtration, washing, and drying, can affect catalytic activity in subsequent tests [32,77,85]. The catalyst reusability results demonstrate that Mg_0.25La_0.75O₂ maintained relatively high catalytic activity during repeated glycerol etherification cycles, although gradual deactivation became noticeable after prolonged reuse. Glycerol conversion decreased slightly from 90% in the first cycle to 88–89% during the second to fourth cycles, indicating that the catalyst initially retained good structural stability and active-site accessibility. However, conversion decreased more significantly to 75% during the fifth cycle, suggesting progressive catalyst deactivation. Similar reusability behavior has been widely reported for heterogeneous glycerol etherification catalysts and is commonly associated with oligomer deposition, partial blockage of active basic sites, and gradual loss of accessible surface area [32,75]. Diglycerol selectivity gradually decreased from 70% to 65% with increasing reaction cycles, whereas triglycerol and tetraglycerol selectivity increased progressively. This behavior suggests that repeated catalyst usage promoted secondary oligomerization reactions and accumulation of heavier oligomers on catalyst surfaces. The increasing triglycerol and tetraglycerol selectivity indicates that residual oligomer species adsorbed on active sites may have facilitated further condensation reactions during reuse cycles [56,71,78,86]. The diglycerol + triglycerol yield remained relatively stable during the first four cycles before decreasing substantially during the fifth cycle, indicating that Mg_0.25La_0.75O₂ possessed acceptable catalyst durability and structural stability under solventless etherification conditions. The gradual decline in catalytic performance at prolonged reuse cycles was likely caused by deposition of higher oligomers, partial hydroxylation of surface basic sites, and mild pore blockage that reduced active-site accessibility and diffusion efficiency [49,82].

The dissolution of metal species from the fluid phase was evaluated using ICP-OES, in the solution of the reaction mixture and presented in

Table 2. ICP-OES analysis on the reaction mixture.

Catalysts/Mg0.25La0.75O2	Mg2+ (ppm) 1	La3+ (ppm) 1	SBET (m2 g−1) 2
Reaction cycle 1	1.2	0.10	-
Reaction cycle 2	1.8	0.12	-
Reaction cycle 3	2.4	0.15	-
Reaction cycle 4	3.2	0.18	-
Reaction cycle 5	5.5	0.30	39.27

¹ ICP-OES. ² BET surface area.

. This indicates that Mg and La undergo partial dissolution in the liquid phase throughout the etherification process. The slight decrease in catalytic activity after repeated reuse cycles may be attributed to partial blockage of active basic sites by higher oligomer deposition and minor Mg leaching from the mixed oxide structure [87]. The gradual increase in Mg concentration detected by ICP-OES with increasing reaction cycles indicates minor deterioration of accessible Mg-based active sites during prolonged catalyst reuse [88]. The leaching of La from the utilized catalyst also contributes to the catalyst’s diminished activity and selectivity. The findings are corroborated by the research conducted by Seshu Babu et al., which indicates that Mg/La catalysts serve as reusable catalysts. Incorporation of La enhances the catalyst’s basicity, attributed to the presence of strong basic sites. The leaching of La diminishes the catalyst’s basicity [89], resulting in the formation of more acidic catalysts. The investigation into the surface characteristics of the utilized catalyst after the fifth cycle revealed a notable decrease in surface area, from 74.17 m² g⁻¹ to 39.27 m² g⁻¹. The reduction in the catalyst’s surface area could be attributed to the products partially obstructing it during the reaction process. The surface of the catalyst was obscured by the oligomers produced during the reaction, potentially diminishing the availability of active sites [32].

A similar pattern may be seen in the catalyst’s XRD analyses before and after the four etherification cycles (Figure 12). The primary peaks, 2θ = 25.8°, 30.3° and 44.4°, which are attributed to the La₂O₂CO₃ species, are clearly visible. After the fifth cycle, the strength of these conspicuous peaks diminished. This modest reduction, which is comparable to the fresh mixed oxide Mg_0.25La_0.75O₂ catalyst sample, suggests a decent retention of the structure. The lack of these peaks or their reduced intensities, conversely, indicates that leaching is taking place throughout the reaction; there is a decline in the quantity of metals present within the catalyst framework. The intensity of the La₂O₂CO₃ component peaks was notably reduced after the fifth cycle in comparison to the fresh catalyst. The reduction in the basicity of the catalysts, as La leaches from the mixed metal oxide catalysts, results in a significant decline in catalytic performance. To enhance the reusability of mixed metal oxides, one could consider strategies such as improving their stability [90,91], developing effective regeneration techniques [92] and investigating innovative synthesis methods that regulate their structure and composition [93,94,95].

Although the Mg₀.₂₅La₀.₇₅O₂ catalyst demonstrated satisfactory reusability, further improvements in catalyst durability remain desirable for long-term industrial implementation. Future studies could focus on optimizing catalyst composition as well as synthesis and calcination conditions, since previous studies on LDH-derived and related mixed oxides for glycerol etherification show that these variables strongly influence catalyst acid-base properties and catalytic behavior [36,56,96,97]. Such optimization may help maintain catalytic performance over extended use. In addition, these catalyst systems have been widely investigated for glycerol etherification to short-chain polyglycerols, so preserving high activity and selectivity toward di- and triglycerol remains an important objective for future catalyst development [36,56,96,98].

2.2.6. Comparison of Catalytic Performance of Mg_0.25La_0.75O₂ with Other Catalysts

The Mg_0.25La_0.75O₂ catalyst demonstrated superior catalytic efficiency for solventless glycerol etherification when benchmarked against previously reported heterogeneous catalysts, as outlined in

Table 3. Comparison of catalytic performance between LDH-derived heterogeneous catalysts for solventless glycerol etherification to short-chain polyglycerols.

Catalysts	Catalyst Loading (wt%)	Reaction Temperature (°C)	Reaction Duration (h)	Conversion (%)	Diglycerol Selectivity (%)	Triglycerol Selectivity (%)	Di- and Triglycerol Yield (%)	Reference
Mg0.25La0.75O2	2	220	8	90	70	23	84	Present work
Mg0.25La0.75O2	2	220	24	100	50	28	78	Present work
MgFeO4	2	220	24	41	91	9	41	[36]
MgAl_Na	2	220	24	50.7	84.8	15.2	50.7	[37]
cHC4/1073	2	220	24	84	29	20	41.2	[56]
Mg1Al1 LDH/CaCO3	3	220	24	52.1	92	8	52.1	[12]
7.5%Ca-MgAl MMO	3	220	24	40.4	78.3	11.7	36.4	[49]

. Among the LDH-derived catalysts reported for solventless glycerol etherification, Mg_0.25La_0.75O₂ demonstrated the most balanced catalytic performance under comparatively moderate reaction conditions. The catalyst achieved 90% glycerol conversion together with high diglycerol selectivity (70%), triglycerol selectivity (23%), and overall oligomer yield (84%) within only 8 h of reaction. In contrast, most previously reported LDH-based catalysts required significantly longer reaction durations (typically 24 h) while producing lower conversion or oligomer yield [12,36,37,49,56]. The superior catalytic activity of Mg_0.25La_0.75O₂ may be attributed to the possible cooperative interaction between Mg- and La-derived sites, which enhanced medium-strength basicity, Lewis acidity, and active-site accessibility. These properties promoted efficient glycerol activation and controlled oligomerization toward desirable short-chain polyglycerols under solvent-free conditions. These findings indicate that Mg_0.25La_0.75O₂ is a promising heterogeneous catalyst for efficient and selective production of short-chain polyglycerols from glycerol under industrially relevant solvent-free conditions.

3. Materials and Methods

3.1. Catalyst Synthesis

A coprecipitation method at a constant pH was selected to synthesize a series of MgLa mixed metal oxides by varying the molar ratios of Mg and La, while maintaining a total cation concentration of 1 M. In 50 mL of distilled water, 10.260 g of Mg(NO₃)₂•6H₂O (Merck, 99.0%) was dissolved to create 0.8 M Mg²⁺. Using the same volume, 4.330 g of La(NO₃)₃•6H₂O (Merck, Petaling Jaya, Malaysia, 99.9%) was dissolved to yield 0.2 M of La³⁺. Both solutions were placed into a flask and vigorously stirred at room temperature. A precipitant solution of NaOH (Merck, 97.0%) and Na₂CO₃ (Merck, 99.5%) at concentrations of 2 M and 0.125 M, respectively, was used to maintain the pH at 10. After a 24-h ageing period for the precursor, the solids underwent centrifugation and were subsequently dried at 80 °C. A series of catalysts was synthesized by repeating a similar procedure, with modifications made to the molar concentration of both cations. The precursors were transformed into the appropriate mixed metal oxides by activating the hydrotalcites in a tube furnace at 450 °C for 15 h, utilizing nitrogen gas flowing at a rate of 40 mL min⁻¹ [36].

3.2. Catalyst Characterization

Powdered XRD patterns were collected at ambient temperature utilizing Cu Kα radiation with a Shimadzu model XRD-6000 Diffractometer (from Shimadzu Corporation, Kyoto, Japan) across a 2θ range of 5° to 80°. Crystallite sizes were determined from X-ray peak broadening utilizing the Debye-Scherrer equation, as illustrated in Equation (1).

$$t={\displaystyle \frac{0.89\lambda }{{\beta }_{hkl}{\theta }_{hkl}}}$$

(1)

where t = crystallite size, nm

λ = Wavelength of X-ray for Cu Kα radiation

β_hkl = Full width at half maximum (FWHM) of the hkl plane

θ_hkl = diffraction angle of hkl plane

Overall surface area of these catalysts was evaluated through the Brunauer–Emmett–Teller (BET) method utilizing a Micromeritics 3Flex Adsorption Analyzer (from Micromeritics Instrument Corporation, Norcross, GA, USA). Prior to nitrogen adsorption, the samples underwent degassing at 450 °C (10 °C min⁻¹) for a duration of 18 h. The Nicolet™ iS™ 10 FTIR Spectrometer (from Thermo Fisher Scientific, Shah Alam, Malaysia) was employed to analyze Fourier transform infrared (FTIR) spectra. The spectra were gathered within the wave number range from 4000 to 500 cm⁻¹ at atmospheric pressure and room temperature. The Hitachi S-3400N (from Hitachi High-Tech Corporation, Kuala Lumpur, Malaysia), in conjunction with the EDAX-Ametex-Apollo X59 (from AMETEK Inc., Mahwah, NJ, USA), was utilized for scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. The basicity of the catalysts was investigated using temperature-programmed CO₂ desorption on a Thermo Electron TPDRO 1100 (from Thermo Fisher Scientific, Shah Alam, Malaysia). Approximately 100 mg of the material underwent pretreatment for 60 min at a temperature of 450 °C while being exposed to helium flow at a rate of 50 mL min⁻¹, with a ramping rate of 10 °C min⁻¹. A pure CO₂ stream (50 mL min⁻¹) was introduced for a duration of 60 min following the reduction of the reaction temperature to 100 °C. A thermal conductivity detector (TCD) was employed to quantify the CO₂ produced during the TPD-CO₂ reaction, conducted in helium flow at temperatures ranging from 100 °C to 1000 °C (30 mL min⁻¹ with a ramp rate of 10 °C min⁻¹). The total acidity of the catalysts was analyzed using temperature-programmed NH₃ desorption on a Micrometrics Autochem II 2920 equipped with TCD (from Micromeritics Instrument Corporation, Norcross, GA, USA). After cleaning 100 mg of the material at 450 °C in helium flow rate of 50 mL min⁻¹ with a ramping rate of 10 °C min⁻¹, the reaction temperature was reduced to 100 °C, followed by the introduction of a pure stream of NH₃ at 50 mL min⁻¹ for 60 min. The TPD-NH₃ was carried out between 100 °C and 550 °C (30 mL min⁻¹ with a ramp rate of 10 °C min⁻¹). Metal leaching from the catalyst was evaluated by PerkinElmer Optima 7000 DV Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) (from Perkin Elmer Sdn. Bhd., Petaling Jaya, Malaysia). Analysis of the liquid phase collected after the solventless glycerol etherification reaction. After each reaction cycle, the reaction mixture was cooled to room temperature and the solid catalyst was separated by centrifugation, followed by filtration. Due to the high viscosity of the solventless glycerol etherification mixture, the liquid sample was diluted with deionized water at a 1:9 ratio. The diluted sample was then acidified with nitric acid to obtain approximately 2 vol% HNO₃, ensuring complete stabilization of dissolved Mg and La species in solution. The measured concentrations were corrected according to the dilution factor.

3.3. Catalyst Reaction and Analysis

As illustrated in Figure 13, a 250 mL glass batch reactor featuring three necks and a circular bottom was employed to assess the solventless etherification of glycerol. The reactor featured a water-cooled condenser along with a Dean Stark apparatus designed exclusively for the elimination of surplus water. 50 g of glycerol (Merck, 99.5%) were mixed with 2 wt% of fresh calcined catalyst. The reactor’s atmosphere was maintained inert through a nitrogen gas flow of 10 mL min⁻¹ to prevent glycerol oxidation. The injector and detector temperature were set at 100 °C and 220 °C, respectively. The reaction was conducted at 1000 rpm using a magnetic stirring heating mantle, with temperature regulation achieved through a thermocouple linked to a proportional-integral-derivative regulator. After the procedure, the resulting mélange underwent rapid cooling in an ice bath. The catalysts underwent separation through filtration, followed by washing with water and subsequent drying. The influence of the temperature of the reaction mixture (200 °C, 220 °C, and 240 °C) and catalyst loading (1 wt%, 2 wt% and 3 wt%) was further assessed. To evaluate the study’s repeatability, the catalysts were filtered, washed with distilled water, dried at 80 °C, and subjected to solventless etherification until a significant decrease in catalyst efficacy was observed.

The reaction products were analyzed utilizing a Perkin Elmer Clarus 500 gas chromatograph (GC) (from Perkin Elmer Sdn. Bhd., Petaling Jaya, Malaysia), featuring a flame ionization detector. The system consisted of a Zebron GC column, ZB-1, that measured 30 m in length, featuring an internal diameter of 0.25 mm along with a film thickness of 0.25 µm. Following silylation [36,82], gas chromatography analysis was conducted on the collected product samples. A 0.05 mL aliquot of the reaction mixture was dissolved in 1.5 mL of dried pyridine (Merck, 99.9%). 0.15 mL of this mixture was combined with 0.15 mL of N,O-Bis(trimethylsilyl)trifluoroacetamide (Merck, 99.0%), and subsequently, 0.75 mL of dry pyridine was added. The mixture underwent ageing in an oven at 60 °C for a duration of 60 min, followed by analysis using gas chromatography. The analysis revealed the presence of unreacted glycerol, diglycerol, and triglycerol as the identified products through GC. The investigation focused on the reusability of the optimized mixed metal oxide catalysts to determine the operational cycle count. The formulas employed to calculate glycerol conversion (%), diglycerol selectivity (%), triglycerol selectivity (%), and diglycerol + triglycerol yield (%) are outlined below:

$$\mathrm{Glycerol} \mathrm{conversion} (\%)={\displaystyle \frac{\mathrm{Glycerol} \mathrm{transformed} \left(\mathrm{moles}\right)}{\mathrm{Initial} \mathrm{glycerol} \left(\mathrm{moles}\right)}}\times 100$$

(2)

$$\mathrm{Product} \mathrm{selectivity} (\%)={\displaystyle \frac{\mathrm{Diglycerol}/\mathrm{triglycerol} \mathrm{produced} \left(\mathrm{moles}\right)}{\mathrm{Total} \mathrm{product} \left(\mathrm{moles}\right)}}\times 100$$

(3)

$$\mathrm{Product} \mathrm{yield} (\%)={\displaystyle \frac{\mathrm{Diglycerol}/\mathrm{triglycerol} \mathrm{produced} \left(\mathrm{moles}\right)}{\mathrm{Initial} \mathrm{glycerol} \left(\mathrm{moles}\right)}}\times 100$$

(4)

4. Conclusions

In conclusion, this study demonstrates that LDH-derived Mg–La mixed oxides are not merely strongly basic catalysts, but composition-tunable bifunctional surfaces in which the Mg²⁺/La³⁺ ratio controls crystallinity, porosity, and the balance between basic and acidic surface sites. Among the synthesized materials, Mg_0.25La_0.75O₂ exhibited the most favorable catalytic behavior because it combined the highest strong basic-site concentration with enhanced Mg–La interfacial interaction and improved active-site accessibility, delivering 90% glycerol conversion, 70% diglycerol selectivity, 23% triglycerol selectivity, and an 84% combined yield under the optimized conditions of 220 °C, 8 h, and 2 wt% catalyst loading. These findings indicate that selective etherification is governed not by maximizing a single surface property, but by achieving an optimal acid–base synergy that promotes glycerol deprotonation and controlled sequential oligomerization. The performance trends further reveal that excessive Mg or La enrichment is detrimental because it perturbs the surface architecture and reduces the population of accessible active sites, thereby weakening catalytic efficiency and selectivity. Although the catalyst retained acceptable activity over the first four reuse cycles, the decline observed upon prolonged recycling suggests that partial leaching of Mg/La species, as well as possibly oligomer deposition and loss of accessible basic sites, remain the principal causes of deactivation. Overall, this work establishes compositional tuning of Mg–La mixed oxides as an effective strategy for directing solventless glycerol etherification toward short-chain polyglycerols, while also highlighting the need for improved structural stabilization to extend catalyst lifetime and industrial relevance.