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Article

From Waste to Catalyst: The Properties of Mixed Oxides Derived from Layered Double Hydroxide Mg/Al Synthesized from Aluminum Residues and Their Use in Transesterification

by
Tarsila Santos da Silva
1,
Laura Leticia Freitas Ferreira da Silva
1,
Evellyn Patricia Santos da Silva
1,
Rayssa Jossanea Brasileiro Motta
1,
Bruno José Barros da Silva
2,
Mario Roberto Meneghetti
1,
Lucas Meili
3 and
Simoni Margareti Plentz Meneghetti
1,*
1
Group of Catalysis and Chemical Reactivity (GCAR), Institute of Chemistry and Biotechnology, Federal University of Alagoas, Av. Lourival de Melo Mota, s/n°, Maceió 57072-970, AL, Brazil
2
Laboratory of Catalyst Synthesis (LSCat), Center of Technology, Federal University of Alagoas, Av. Lourival de Melo Mota, s/n°, Maceió 57072-970, AL, Brazil
3
Laboratory of Processes (LAPRO), Center of Technology, Federal University of Alagoas, Av. Lourival de Melo Mota, s/n°, Maceió 57072-970, AL, Brazil
*
Author to whom correspondence should be addressed.
Reactions 2025, 6(2), 33; https://doi.org/10.3390/reactions6020033 (registering DOI)
Submission received: 24 March 2025 / Revised: 23 April 2025 / Accepted: 7 May 2025 / Published: 10 May 2025
Browse Figures
Versions Notes

Abstract

:
Mixed oxides were obtained via calcination at 550 °C from layered double hydroxides (LDHs), which were synthesized in a previous study via co-precipitation and co-precipitation followed by hydrothermal treatment using aluminum residues as the source of this element. After characterization, these oxides (Mg-Al-LDH-CP and Mg-Al-LDH-H, named according to the synthesis methods of the precursor LDHs) were applied as heterogeneous catalysts in the methyl transesterification of ethyl acetate (EA). The formation of mixed oxides was confirmed by the absence of basal peaks associated with the layered LDH structure in the XRD analysis, due to calcination. Further characterization revealed that Mg-Al-LDH-CP exhibited the highest number of acidic sites, while Mg-Al-LDH-H had the highest number of basic sites. The transesterification activity was evaluated in the reaction between ethyl acetate (EA) and methanol (MeOH). The best result, obtained under a molar ratio of 1:5:0.005 (EA:MeOH:catalyst) at 120 °C, was a 63% conversion after 360 min of reaction for the Mg-Al-LDH-CP catalyst, which had a higher number of acidic sites and fewer basic sites. Additionally, the catalysts demonstrated robustness, maintaining catalytic activity over four cycles without a significant decrease in performance. These results indicate the feasibility of using mixed oxides derived from LDH, synthesized from aluminum residues, as heterogeneous catalysts in transesterification reactions, highlighting their potential for advancing more sustainable catalyst development.

1. Introduction

Aluminum is a highly versatile and widely used metal, prized for its lightweight nature, durability, and corrosion resistance. Its primary production involves extracting aluminum from bauxite ore through the Bayer and Hall–Héroult processes, which are energy-intensive and environmentally impactful, consuming approximately 14–16 MWh per ton of aluminum produced and generating significant greenhouse gas emissions and red mud waste. In contrast, secondary production, or recycling, entails re-melting aluminum scrap, requiring only about 5% of the energy used in primary production. This substantial energy saving translates to a 95% reduction in carbon emissions, making recycling a crucial component in reducing the environmental footprint of aluminum production and promoting a circular economy [1,2,3].
Most of the recycled aluminum in Brazil comes from beverage cans, such as those for soft drinks, beer, and juice. However, other aluminum products can also be recycled, including frames, windows, doors, household appliance components, and waste from aluminum utensil manufacturing. Developing cost-effective methods for reusing and recycling this waste is crucial for generating high-value products and reducing environmental liabilities, as many of these materials are improperly discarded [4].
Beyond traditional recycling, the chemical recovery of aluminum from industrial waste is gaining attention as a sustainable strategy. Instead of merely remelting scrap, aluminum can be chemically processed to produce value-added compounds, such as aluminum compounds and alumina-based materials. These compounds serve as precursors for synthesizing solid catalysts, including mixed oxides and supports with acidic or amphoteric properties, which are essential in various catalytic applications. For instance, studies have demonstrated the successful conversion of waste aluminum cans into γ-alumina using sol–gel methods, yielding materials suitable as catalysts or catalyst supports. Similarly, spent hydroprocessing catalysts containing alumina have been treated to recover aluminum and other valuable metals, contributing to waste reduction and the development of advanced materials [5,6,7]. This approach not only mitigates the environmental issues associated with aluminum waste, but also fosters innovation in material science and green chemistry.
In this context, aluminum hydroxide derived from aluminum waste emerges as an environmentally sustainable alternative with various applications, including the production of layered double hydroxides (LDHs). These compounds are obtained through the leaching of Al3+ ions in hydroxide form, followed by reaction with divalent cations [8,9], which are a class of two-dimensional anionic clays characterized by their tunable chemical composition, high surface area, and remarkable ion-exchange capabilities. These properties make LDHs highly versatile for applications in catalysis, environmental remediation, energy storage, and biomedical fields [10,11]. Recent studies have highlighted their efficacy in water purification processes, where LDH-based composites have been employed to remove organic and inorganic pollutants effectively [10]. In the realm of catalysis, LDHs serve as precursors to mixed metal oxides that exhibit enhanced catalytic activity for reactions such as transesterification and oxidation [11]. Furthermore, their biocompatibility and ability to intercalate various anions have opened avenues in drug delivery systems and other biomedical applications [12].
LDHs have attracted significant interest for both their environmental and economic advantages. Composed of abundant, non-toxic, recyclable, and low-cost materials, LDHs are easily handled and can be applied as catalysts in various reactions, including transesterification [13,14,15,16,17]. Moreover, LDHs can be readily separated from reaction mixtures, facilitating catalyst recovery and reuse, which are crucial for sustainable industrial applications. Recent studies have demonstrated the efficacy of LDH-based catalysts in biodiesel production, highlighting their potential in green chemistry applications [18,19,20].
The thermal treatment of LDHs at temperatures above 350 °C leads to the collapse of their layered structure, resulting in the formation of mixed oxides or hydroxides [21]. The mixed oxides obtained in this process exhibit remarkable properties, such as small crystallite sizes (around 10 nm), a high specific surface area, surface basicity, and a memory effect that enables the regeneration of the original structure [22].
Specifically, mixed metal oxides obtained through the controlled thermal decomposition of LDHs exhibit high surface areas (100–300 m2 g−1), acid–base properties, the homogeneous and thermally stable dispersion of metal ions, synergistic effects between elements, and the ability to reconstruct their structure under moderate conditions [23,24,25,26].
Given these characteristics, the production of Al2O3/MgO mixed oxides derived from LDHs becomes particularly relevant. This study aims to obtain Al2O3/MgO mixed oxides and evaluate their properties in relation to LDHs obtained via different synthesis methods and, consequently, their catalytic activity in the transesterification of ethyl acetate (EA) and methanol (MeOH). In addition to the innovative use of aluminum waste as a raw material, the intrinsic combination of acid–base properties in these LDH-derived materials offers a strategic advantage for catalytic applications. This dual functionality is particularly beneficial for reactions such as transesterification, where both acidic and basic sites can synergistically enhance the reaction pathway and improve overall efficiency.

2. Materials and Methods

2.1. Materials

The Al(OH)3 was obtained via the basic leaching of Al waste (see below), which was made available by an aluminum frame factory located in Maceio/AL, Brazil. Mg(NO3)2, NaOH, HCl, ethyl acetate (EA), and methanol (MeOH) were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received.

2.2. Basic Leaching of Aluminum Waste

The basic leaching process was used for the production of aluminum hydroxide (Al(OH)3) from aluminum residues. For that, 10 g of residue was used and dissolved in 100 mL of sodium hydroxide (NaOH) at 2.0 mol. L−1. Afterwards, the mixture was filtered and then HCl was added to the filtrate until pH = 7.5 was reached. The Al(OH)3 precipitate was washed with deionized water and dried at 100 °C for 24 h and, finally, the solid was macerated to obtain a fine powder [10].

2.3. Synthesis of LDH Mg/Al: LDH-CP and LDH-H

In a previous study [8], for the preparation of LDH Mg/Al using Al(OH)3 from aluminum waste as an input, the method of co-precipitation and hydrothermal treatment was used. For the co-precipitation method (LDH-CP), initially, 0.075 mol of magnesium nitrate was dissolved in 100 mL of deionized water; then, 0.025 mol of powdered aluminum hydroxide was added to the magnesium nitrate solution to form suspension A. In another container, 0.2 mol of sodium hydroxide and 0.05 mol of sodium carbonate were dissolved in 100 mL of deionized water to form solution B. Subsequently, solution B was carefully added to the suspension through vigorous stirring [27]. After 2 h, the resulting solid was washed with deionized water and centrifuged. Finally, the material was dried in an oven at 75 °C for 24 h, and then macerated to obtain a fine powder. For the synthesis of LDH from Mg/Al via the hydrothermal treatment (LDH-H), the same procedure as in the co-precipitation method was carried out, and the mixture was inserted in an autoclave and placed in an oven at 180 °C for 24 h. Subsequently, the material went through a process of washing with deionized water and centrifugation. Finally, the material was placed to dry in an oven at 75 °C for 24 h and, after drying, the solid was macerated to obtain a fine powder.

2.4. Mixed Oxides Mg-Al-OLDH-CP and Mg-Al-OLDH-H—Preparation and Characterization

To obtain the mixed oxides derived from LDH Mg/Al obtained via co-precipitation and co-precipitation followed by hydrothermal treatment, they were calcined at 550 °C on a ramp of 16 °C min−1, remaining at this temperature for 3 h. Subsequently, the materials were removed from the muffle while still being heated, at approximately 250 °C, and stored in a desiccator under an argon atmosphere. The calcination temperature was determined from the results of the thermogravimetric analysis of the LDH. The mixed oxides were named Mg-Al-OLDH-CP and Mg-Al-OLDH-H due to the methods of synthesis of the precursor LDHs, respectively, co-precipitation and co-precipitation/hydrothermal.
X-ray diffraction (XRD) measurements were performed using a Shimadzu diffractometer, model XRD-6000, and a Cu Kα radiation (1.5418 Å) source with a nickel filter to obtain the wide-angle diffraction patterns in the 2θ = 3–80° range. Nitrogen adsorption measurements were performed at 77.15 K using a gas adsorption analyzer (Micromeritics, model ASAP-2020, Micromeritics Instrument Corporation, Norcross, GA, USA) and the samples were pretreated under vacuum at 423.15 K for 24 h. The textural properties were determined from the N2 adsorption isotherms using the Brunauer–Emmett–Teller (BET) equation and the Barrett–Joyner–Halenda (BJH) method. Temperature-programmed desorption of carbon dioxide (CO2-TPD) measurements were performed using a Termolab multipurpose analytical system (SAMP3). For these measurements, approximately 100 mg of the sample was deposited in quartz wool, and the consumption of the gases was measured with a thermal conductivity detector (TCD). Fourier transform infrared (FTIR) spectra were obtained with a Shimadzu IR Prestige 21 infrared spectrophotometer using tablets of potassium bromide (KBr). Eighty scans were performed in transmittance mode in the spectral range of 4000–400 cm−1 at a resolution of 4.0 cm−1. Microscopic images of the samples were acquired using a Hitachi scanning electron microscope (SEM), model S-3400N, following gold sputter-coating of all samples. Transmission electron microscopy (TEM) imaging was performed using a FEI Tecnai G2 Spirit Twin microscope, operating at an accelerating voltage of 120 kV.

2.5. Catalytic Assays

The catalytic activity was evaluated through the transesterification of EA and MeOH, using a molar ratio of 1:5:0.005 (EA:MeOH:catalyst). The reactions were carried out at 120 °C for durations ranging from 15 to 360 min. The experiments were conducted in glass microreactors immersed in an oil bath, with magnetic stirring set at 1000 rpm. At the end of each reaction, the mixture was filtered to remove the catalyst.
The supernatants obtained from the transesterification reactions were analyzed via gas chromatography (GC) using a GC-2010 chromatograph (SHIMADZU) system equipped with a capillary injector operating at 150 °C and a split ratio of 100:1. A sample volume of 1 µL was injected. An Rtx-1 (Restek) capillary column was used, measuring 30 m in length and 0.32 mm in internal diameter, with a 3 µm film thickness. The column temperature was set at 50 °C with a hold time of 12 min. A flame ionization detector (FID) operating at 150 °C was employed. High-purity hydrogen (99.95%) was used as the carrier gas.
The reaction under study involves the conversion of EA into MA, with MeOH used as the alcoholysis agent. At the end of the reaction, only the reactants (MeOH and EA) and the products ethanol and MA were detected, indicating that the reaction proceeded selectively toward the desired product.
The conversion of EA to methyl acetate (MA) (XEA) was calculated according to Equation (1) [28], where AMA = peak area corresponding to MA, AEA = peak area corresponding to EA, MMMA = molar mass of MA, and MMEA = molar mass of EA.
X E A = ( A M A M M M A ) A E A M M E A + ( A M A M M M A )
The mathematical treatment of the data and statistical calculations were carried out by utilizing the Microsoft Excel 2019 software, and the slope of the lines was determined via linear regression, which calculates the fit of least squares for a line represented by Equation (2) (a is the slope and b is the point of intersection). The treatment of the data using the natural logarithm of the conversion of EA as a function of reaction time provides straight lines, and their slopes give the values for the apparent rate constants (kap), according to Equation (3).
Y = ax + b
ln EA % = kt + ln 100
For the recovery and reuse tests, the same conditions were employed (temperature of 120 °C and a molar ratio of 1:5:0.005 (EA:MeOH:catalyst)), but with a reaction time of 1 h. In each cycle, the solid was separated from the supernatant via filtration, placed in a desiccator, and vacuum-dried for 3 h before being reused. A total of four reaction cycles were performed.

3. Results and Discussion

Mixed oxides, containing Mg and Al in their composition, were obtained via the calcination of LDHs, giving rise to the materials Mg-Al-OLDH-CP and Mg-Al-OLDH-H, named in this way due to the methods of synthesis of the precursor LDHs (the co-precipitation and co-precipitation/hydrothermal method, respectively). It is worth noting that these precursors were synthesized and characterized in a previous study [8].
Some studies report the production of mixed Mg and Al oxides from LDHs [29]; however, it is important to highlight that one of the innovations of the present study lies in the fact that Al(OH)3, used as the Al precursor in this work, was obtained from aluminum residues, demonstrating the potential use of waste materials in a greener and more environmentally friendly synthesis of catalysts.
The detailed synthesis and characterization of the LDHs, using Al(OH)3 and Mg(NO3)2 as the starting reagents, were described in a recent study, as already mentioned [8]. However, to evaluate the thermal behavior of the LDH precursors and determine the appropriate calcination conditions for obtaining the corresponding mixed oxides, Figure 1A presents the thermogravimetric (TGA) and derivative thermogravimetric (DTG) curves of the LDH precursors. The observed mass loss profiles are characteristic of this type of material [30,31,32]. The first stage of thermal decomposition, observed below 120 °C, was attributed to the loss of adsorbed water and/or the removal of hydroxyl groups from the surface. The second stage, which starts from the previous stage and extends to the next inflection point (approximately 210 °C for LDH-CP and 230 °C for LDH-H), was associated with the elimination of interlamellar water. Dehydroxylation, which consists of the removal of the hydroxyl groups that make up the lamellae, and the decomposition of the interlamellar anion (carbonate), observed at around 370 °C and 420 °C, respectively, result in the structural collapse of LDH and the formation of oxides. Thus, a temperature higher than the latter was chosen to obtain the oxides derived from LDH-CP and LDH-H.
In this way, the calcination of the LDH precursors at 550 °C, the temperature employed in this study, ensures that the thermal decomposition produces a Mg–Al mixed oxide, theoretically exhibiting a larger area than the precursor, which presumably exposes Lewis and Brønsted acid and basic sites, whose properties depend on the ratio of Mg to Al content [29,33].
Figure 1B shows the TGA and DTG curves of the Mg-Al-OLDH-CP and Mg-Al-OLDH-H samples, where two main endothermic events associated with mass loss can be observed. The first event corresponds to the removal of physisorbed water molecules, while the second event, occurring at around 295 °C, is attributed to the release of adsorbed CO2 molecules [34].
The Al and Mg composition was determined for the LDH precursors [8], and since the oxides were synthesized via calcination, it was assumed that the Mg:Al ratio was maintained (2.3:1 and 3.1:1 for Mg-Al-OLDH-CP and Mg-Al-OLDH-H, respectively). When expressing these values in terms of Al/Al + Mg, the ratios are 0.30 and 0.23 for Mg-Al-OLDH-CP and Mg-Al-OLDH-H, respectively [26]. These values indicate that the Mg-Al-OLDH-CP material has a higher Al content in its composition.
It is important to mention that solid catalysts containing basic sites, such as those studied here, are susceptible to being poisoned by some components in the air such as CO2 and H2O, as they can interact with this type of site, altering their structure or decreasing their catalytic activity. Therefore, sample preparation for the various characterization techniques employed must be extremely rigorous to ensure that it represents the proposed material [27]. Thus, fresh materials were produced for both characterization and catalytic tests.
Figure 2 shows the X-ray diffractograms of the Mg-Al-OLDH-CP and Mg-Al-OLDH-H samples. The main diffraction peaks at 2θ = 36.95°, 42.91°, 62.31°, 74.68°, and 78.61° were observed, attributed to the crystallographic planes (111), (200), (220), (311), and (222) of the cubic structure with the space group Fm-3m (225) of magnesium oxide (MgO), (JCPDS No. 04-0829, International Centre for Diffraction Data (ICDD), Newtown Square, PA, USA). The X-ray diffractogram of the Mg-Al-OLDH-H sample suggests the formation of a slightly more crystalline material, evidenced by the presence of slightly narrower diffraction peaks compared to the Mg-Al-OLDH-CP sample. This change can be attributed to the different synthesis methods used and to the variations in the proportions of Al and Mg in the samples [29,34]. The absence of signals related to Al2O3 can be justified by the low aluminum content present in the composition of the materials [8,29]. Furthermore, both samples exhibited discrete diffraction signals at 2θ = 11.50°, 22.90°, 34.74°, 39.13°, and 55.66°, corresponding to the LDH precursor (JCPDS No. 00-014-0191, International Centre for Diffraction Data (ICDD), Newtown Square, PA, USA). However, these peaks showed a drastic decrease in diffracted intensity in relation to the characteristic peaks of the LDH precursors [8], indicating that the layers comprising the LDH are almost completely exfoliated. This indicates the loss of the laminar distribution, although a certain structure is still partially maintained in Mg-Al-OLDH-CP.
Furthermore, the reflections observed at approximately 2θ = 20°, 30°, and 65° (highlights with **) in the XRD patterns of the samples are most likely associated with a combination of structural transitions resulting from the thermal decomposition and partial reconstruction of the original LDH framework. This behavior is attributed to the so-called memory effect, whereby the rehydration of the calcined oxides in aqueous media promotes the partial reformation of the lamellar structure, as previously discussed. Nevertheless, the regenerated phase typically exhibits lower crystallinity compared to the pristine LDH, which manifests as broad and low-intensity peaks in the diffractogram, reflecting the coexistence of reconstructed lamellar domains and oxide phases [35,36,37].
It is worth noting that the data obtained via TGA (Figure 1) corroborate the results of XRD (Figure 2), confirming the oxide formation in conditions of calcination. In addition, the presence of diffraction peaks with discrete intensities attributed to the precursor LDH can be explained by the memory effect that allows for the regeneration of the original structure [23].
Figure 3 presents the absorption spectra in the infrared region (FTIR) for the mixed oxides, which indicate the presence of a broad and strong band at 3471 cm−1, probably related to the stretching vibration of –OH groups, indicating the presence of adsorbed H2O molecules or metal –OH groups. The absorption bands located at 1104 and 1373 cm−1 can be attributed to the stretching vibration of adsorbed CO2 molecules. The bands located at 856, 665, and 520 cm−1 can be attributed to the vibration modes of Al-O, Mg-O, and Mg(Al)O, respectively [38,39]. It is worth noting that the spectra in the mid-infrared region of both Mg-Al-OLDH-CP and Mg-Al-OLDH-H showed an absence of bands related to the vibrations of groups present in the starting LDHs, indicating in fact their conversion into the mixed oxides [8].
The adsorption–desorption isotherms of N2 and the textural properties calculated from them are analyzed in Figure 4 and Table 1. The isotherms are type IV according to the IUPAC classification, indicating that these materials are mesoporous. Hysteresis corresponds to an H3-type loop that is classically found in structures with non-rigid aggregates of plate-shaped particles [40,41]. For the samples, bimodal and wide pore distributions were observed in Figure 4, with pores concentrated at around 32 nm, confirming the mesoporosity of these materials.
The specific surface area values, 118.5 and 167.4 m2 g−1, respectively, for Mg-Al-OLDH-CP and Mg-Al-OLDH-H (Table 1), show that the use of the LDH precursor obtained via co-precipitation/hydrothermal treatment led to a larger area for the resulting oxide (Mg-Al-OLDH-H), in contrast to the other precursors (specific surface areas of 71.9 and 96.1 m2.g−1 for LDH-H and LDH-CP, respectively [8]). This behavior may be associated with hydrothermal treatment, which, while slightly reducing the surface area of the initial LDH, plays a crucial role in controlling the textural evolution during calcination. It likely promotes a more thermally stable and uniform precursor structure, which suppresses particle sintering and favors the development of a porous oxide network with enhanced accessibility. Moreover, the lower specific surface area of the Mg-Al-OLDH-CP-derived oxide may also be linked to its higher aluminum content. Increased Al3+ incorporation in LDHs has been reported to favor the formation of spinel-like or non-porous alumina phases upon calcination, potentially leading to densification, structural collapse, or pore blockage—all of which contribute to a reduced surface area. It should be noted that the observed area values are in line with those in other studies, in which mixed oxides with comparable proportions of Al and Mg were investigated [29,30].
The SEM images (Figure 5) confirm a morphology characterized by plate-like aggregates of varying sizes. In images (b) and (d), the presence of less dense and more dispersed plaques with different orientations can be observed, indicating structural disorder due to the complete exfoliation of the precursor LDHs. This observation aligns with the XRD and N2 physisorption data.
The transmission electron microscopy (TEM) results provide valuable insights into the structural, morphological, and compositional characteristics of Mg-Al-OLDH-CP (Figure 6). The plate-like aggregate structure is clearly observed (Figure 6a), while EDS mapping confirms the presence of Mg and Al, as expected (Figure 6b), demonstrating that these elements are uniformly distributed throughout the sample (Figure 7).
Since these materials can be considered acid–base bifunctional heterogeneous catalysts due to the presence of Mg and Al in their structure, infrared spectroscopy using pyridine as a probe molecule (FTIR-py) was employed to evaluate the nature of the acid sites. This technique allows for the identification of Lewis acid sites (SL) and Brønsted acid sites (SB) [42]. In this context, the SL/SB ratio, normalized by the specific surface area of each sample, was found to be 0.018 for Mg-Al-OLDH-CP and 0.012 for Mg-Al-OLDH-H. These results indicate a higher content of Lewis acid sites in Mg-Al-OLDH-CP, which is consistent with its higher aluminum content.
Additionally, the strength and total amount of basic sites were determined using the TPD-CO2 technique (Figure 8). The TPD results confirmed the presence of basic sites in the mixed oxide catalysts, with significant desorption peaks observed at approximately 250–500 °C and 100–250 °C, corresponding to moderate and weak basic sites, respectively. No peaks related to O2−, HCO3−, or CO32− species were detected [43,44,45]. The total basicity was measured at 147 μmol·g−1 for Mg-Al-OLDH-CP and 227 μmol·g−1 for Mg-Al-OLDH-H, indicating that Mg-Al-OLDH-H exhibits greater basicity.
The ability to modulate these characteristics in mixed oxides is essential for enhancing the system’s versatility across various biorefinery platforms. To evaluate their catalytic activity, the mixed oxides were tested in the transesterification of EA using methanol as the alcoholysis agent. Figure 9 presents the results obtained with Mg-Al-OLDH-CP and Mg-Al-OLDH-H, as well as those from the control experiment conducted without a catalyst. In addition, reactions were also performed using the precursors LDH-CP and LDH-H, to compare the performance of the materials.
Initially, the results demonstrate that the reaction only occured in the presence of a catalyst for the conditions studied. As expected, the two precursors exhibited catalytic activity, and the total conversion of EA was, at 360 min, 34% and 22% for the LDH-CP and LDH-H materials, respectively. The use of LDH in transesterification reactions has already been reported, and it has been assumed that due to the presence of metal oxides and the electropositivity of the cations present in their composition, LDH acquired active sites that are effective for transesterification reactions [46].
Nevertheless, the oxides produced in this work show better performance compared to the precursors, with a total conversion of EA of 63% and 43% for the Mg-Al-OLDH-CP and Mg-Al-OLDH-H catalysts, respectively. The apparent velocity constants (Kap) were estimated, considering transesterification as a pseudo-first-order reaction as a function of the excess MeOH used. They were calculated considering the kinetic control of the reaction between 0 and 45 min and confirmed the observed tendency, with values of 0.0098 (r2 = 0.9979) and 0.0034 (r2 = 0.9763) min−1 for Mg-Al-OLDH-CP and Mg-Al-OLDH-H, respectively.
The higher activity levels exhibited by the oxides in relation to the LDH precursors may be related to their larger surface area, in addition to the presence of both basic and acidic sites, as previously discussed. At rest, the higher catalytic activity observed for the Mg-Al-OLDH-CP system, when compared to Mg-Al-OLDH-H, despite its lower specific surface area (Table 1), can be attributed to its higher acidity and lower basicity. This indicates that acidity plays a key role in this type of reaction, outweighing the surface area effects in the evaluated conditions.
Additionally, the ability to reuse solid catalysts is crucial for maintaining their efficiency, stability, and longevity over multiple reaction cycles. Reusability tests are a key step in assessing their long-term performance and economic feasibility, while also contributing to a more sustainable chemical industry. Therefore, Mg-Al-OLDH-CP and Mg-Al-OLDH-H were subjected to reuse tests over four consecutive cycles to evaluate their reusability (Figure 10). The results indicate that the conversion remained practically constant across all four cycles under the tested reaction conditions, considering an experimental error of approximately 5.0%.

4. Conclusions

The results of X-ray diffractometry confirmed the formation of mixed oxides (Mg-Al-OLDH-CP and Mg-Al-OLDH-H) from the calcination of the synthesized LDHs. Among the catalysts tested, Mg-Al-OLDH-CP demonstrated the highest activity in the transesterification reaction, achieving a 63% conversion of EA within 360 min. This performance is attributed to its optimized acidity–basicity balance, where a higher acidity and lower basicity likely favor the transesterification reaction. The catalysts also demonstrated robustness, preserving their catalytic activity throughout four cycles without a significant decline in performance. As a forward-looking approach, studies are being conducted on the use of other types of oxides derived from waste materials. Utilizing waste to produce catalysts enhances the development of more sustainable processes, particularly within the framework of biorefineries.

Author Contributions

Conceptualization, T.S.d.S., L.M., and S.M.P.M.; methodology, T.S.d.S., L.L.F.F.d.S., E.P.S.d.S., R.J.B.M., B.J.B.d.S., M.R.M., L.M., and S.M.P.M.; investigation, T.S.d.S., L.L.F.F.d.S., E.P.S.d.S., R.J.B.M., B.J.B.d.S., M.R.M., L.M., and S.M.P.M.; writing—original draft preparation, T.S.d.S., L.L.F.F.d.S., E.P.S.d.S., R.J.B.M., M.R.M., L.M., and S.M.P.M.; writing—review and editing, T.S.d.S., L.M., and S.M.P.M.; supervision, M.R.M., L.M., and S.M.P.M.; validation, T.S.d.S., L.M., and S.M.P.M.; project administration, M.R.M., L.M., and S.M.P.M.; funding acquisition, M.R.M., L.M., and S.M.P.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Council for Scientific and Technological Development (CNPq), the Brazilian Federal Agency for the Improvement of Higher Education (CAPES)—Finance Code 001, the Brazilian Innovation Agency (FINEP), and the Alagoas Research Foundation (FAPEAL). T.S.d.S expresses appreciation for the fellowship granted by CAPES. S.M.P.M. and M.R.M. thank CNPq for the research fellowships.

Data Availability Statement

The original contributions presented in this study are included in the article.

Acknowledgments

The authors thank the GCAR-IQB and LSCAT-CTEC teams for their contributions and INCT Catalysis.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Reactions 06 00033 g001
Figure 1. Thermal profiles (TG/DTG) of LDH-CP and LDH-H (A); Mg-Al-OLDH-CP and Mg-Al-OLDH-H (B).
Figure 1. Thermal profiles (TG/DTG) of LDH-CP and LDH-H (A); Mg-Al-OLDH-CP and Mg-Al-OLDH-H (B).
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Figure 2. XRD patterns of Mg-Al-OLDH-CP and Mg-Al-OLDH-H.
Figure 2. XRD patterns of Mg-Al-OLDH-CP and Mg-Al-OLDH-H.
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Figure 3. FTIR spectra of Mg-Al-OLDH-CP and Mg-Al-OLDH-H.
Figure 3. FTIR spectra of Mg-Al-OLDH-CP and Mg-Al-OLDH-H.
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Figure 4. Nitrogen adsorption isotherms at 77 K (a) and the pore diameter distributions (b) of the materials Mg-Al-OLDH-CP e Mg-Al-OLDH-H.
Figure 4. Nitrogen adsorption isotherms at 77 K (a) and the pore diameter distributions (b) of the materials Mg-Al-OLDH-CP e Mg-Al-OLDH-H.
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Figure 5. Scanning electron microscopy (SEM) images: (a,b) Mg-Al-OLDH-CP and (c,d) Mg-Al-OLDH-H.
Figure 5. Scanning electron microscopy (SEM) images: (a,b) Mg-Al-OLDH-CP and (c,d) Mg-Al-OLDH-H.
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Figure 6. Transmission electron microscopy (TEM) images: (a) Mg-Al-OLDH-CP and (b) Mg-Al-OLDH-H.
Figure 6. Transmission electron microscopy (TEM) images: (a) Mg-Al-OLDH-CP and (b) Mg-Al-OLDH-H.
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Figure 7. Scanning transmission electron microscope (STEM) images of the Mg-Al-OLDH-CP: (a) without mapping; (b) Mg and Al mapping; (c) Mg mapping; and (d) Al mapping.
Figure 7. Scanning transmission electron microscope (STEM) images of the Mg-Al-OLDH-CP: (a) without mapping; (b) Mg and Al mapping; (c) Mg mapping; and (d) Al mapping.
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Figure 8. CO2-TPD of Mg-Al-OLDH-CP e Mg-Al-OLDH-H.
Figure 8. CO2-TPD of Mg-Al-OLDH-CP e Mg-Al-OLDH-H.
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Figure 9. Conversion of EA (%) in the presence of Mg-Al-OLDH-CP, Mg-Al-OLDH-H, LDH-CP, LDH-H, and without the use of a catalyst, at the molar ratio 1:5:0.005 (AE:MeOH:cat), for up to 360 min at 120 °C and 1000 rpm.
Figure 9. Conversion of EA (%) in the presence of Mg-Al-OLDH-CP, Mg-Al-OLDH-H, LDH-CP, LDH-H, and without the use of a catalyst, at the molar ratio 1:5:0.005 (AE:MeOH:cat), for up to 360 min at 120 °C and 1000 rpm.
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Figure 10. Reuse tests in conversion of EA (%) in the presence of Mg-Al-OLDH-CP and Mg-Al-OLDH-H at the molar ratio 1:5:0.005 (AE:MeOH:cat), for 1 h at 120 °C, and at 1000 rpm.
Figure 10. Reuse tests in conversion of EA (%) in the presence of Mg-Al-OLDH-CP and Mg-Al-OLDH-H at the molar ratio 1:5:0.005 (AE:MeOH:cat), for 1 h at 120 °C, and at 1000 rpm.
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Table 1. Textural and structural properties of the mixed oxides Mg-Al-OLDH-CP e Mg-Al-OLDH-H.
Table 1. Textural and structural properties of the mixed oxides Mg-Al-OLDH-CP e Mg-Al-OLDH-H.
SampleSBET a
(m2.g−1)		V b
(cm3.g−1)		DBJH c
(nm)
Mg-Al-OLDH-CP118.50.29032.7
Mg-Al-OLDH-H167.40.19232.6
a SBET, BET specific surface area; b V, pore volume; and c DBJH, pore diameter. The specific surface areas, total pore volumes, and pore diameters were calculated using the BET and BJH equations.
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da Silva, T.S.; da Silva, L.L.F.F.; da Silva, E.P.S.; Motta, R.J.B.; da Silva, B.J.B.; Meneghetti, M.R.; Meili, L.; Meneghetti, S.M.P. From Waste to Catalyst: The Properties of Mixed Oxides Derived from Layered Double Hydroxide Mg/Al Synthesized from Aluminum Residues and Their Use in Transesterification. Reactions 2025, 6, 33. https://doi.org/10.3390/reactions6020033

AMA Style

da Silva TS, da Silva LLFF, da Silva EPS, Motta RJB, da Silva BJB, Meneghetti MR, Meili L, Meneghetti SMP. From Waste to Catalyst: The Properties of Mixed Oxides Derived from Layered Double Hydroxide Mg/Al Synthesized from Aluminum Residues and Their Use in Transesterification. Reactions. 2025; 6(2):33. https://doi.org/10.3390/reactions6020033

Chicago/Turabian Style

da Silva, Tarsila Santos, Laura Leticia Freitas Ferreira da Silva, Evellyn Patricia Santos da Silva, Rayssa Jossanea Brasileiro Motta, Bruno José Barros da Silva, Mario Roberto Meneghetti, Lucas Meili, and Simoni Margareti Plentz Meneghetti. 2025. "From Waste to Catalyst: The Properties of Mixed Oxides Derived from Layered Double Hydroxide Mg/Al Synthesized from Aluminum Residues and Their Use in Transesterification" Reactions 6, no. 2: 33. https://doi.org/10.3390/reactions6020033

APA Style

da Silva, T. S., da Silva, L. L. F. F., da Silva, E. P. S., Motta, R. J. B., da Silva, B. J. B., Meneghetti, M. R., Meili, L., & Meneghetti, S. M. P. (2025). From Waste to Catalyst: The Properties of Mixed Oxides Derived from Layered Double Hydroxide Mg/Al Synthesized from Aluminum Residues and Their Use in Transesterification. Reactions, 6(2), 33. https://doi.org/10.3390/reactions6020033

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