Cite this article as: |
Fabiane Carvalho Ballotin, Mayra Nascimento, Sara Silveira Vieira, Alexandre Carvalho Bertoli, Ottávio Carmignano, Ana Paula de Carvalho Teixeira, and Rochel Montero Lago, Natural Mg silicates with different structures and morphologies: Reaction with K to produce K2MgSiO4 catalyst for biodiesel production, Int. J. Miner. Metall. Mater., 27(2020), No. 1, pp. 46-54. https://doi.org/10.1007/s12613-019-1891-9 |
In this work, different magnesium silicate mineral samples based on antigorite, lizardite, chrysotile (which have the same general formula Mg3Si2O5(OH)4), and talc (Mg3Si4O10(OH)2) were reacted with KOH to prepare catalysts for biodiesel production. Simple impregnation with 20wt% K and treatment at 700–900°C led to a solid-state reaction to mainly form the K2MgSiO4 phase in all samples. These results indicate that the K ion can diffuse into the different Mg silicate structures and textures, likely through intercalation in the interlayer space of the different mineral samples followed by dehydroxylation and K2MgSiO4 formation. All the materials showed catalytic activity for the transesterification of soybean oil (1:6 of oil : methanol molar ratio, 5wt% of catalyst, 60°C). However, the best results were obtained for the antigorite and chrysotile precursors, which are discussed in terms of mineral structure and the more efficient formation of the active phase K2MgSiO4.
Magnesium-based materials can be found in nature as silicates, for example, serpentinites (Mg3Si2O5(OH)4) and talc (Mg3Si4O10(OH)2). Serpentinite is a family of hydrated magnesium silicates having an octahedral magnesium layer linked with a tetrahedral silicon sheet [1]. They are constituted mainly by lizardite, antigorite, and chrysotile minerals, which have the same composition, Mg3Si2O5(OH)4, but different structures [2] due to the balance of the octahedral and tetrahedral layers [3].
Generally, the misfit of ions leads to a curved or flat structure. Lizardite has a flat crystal structure, which is favored by Al3+ and Fe3+ substitution with Mg and Si [3]. Chrysotile has Al3+ and Fe3+ deficiencies and a tubular/fibrous texture, which has been suggested to present some toxicity [4]. Antigorite has a curved structure and is composed of octahedral continuous layers linked with alternated tetrahedral layers [5] by strong covalent Si–O bonds [3]. Talc, Mg3Si4O10(OH)2, also common in nature, is constituted of three layers, whereby an Mg(OH)6 layer is located between two Si
The antigorite mineral has been demonstrated to be an excellent precursor to produce Mg silicates containing alkaline metals Li+ [7], Na+ [8], and K+ [9]. These alkaline metal Mg silicates have many potential technological applications. For example, in Ref. [7], simple solid-state reaction with Li+ led to the formation of the Li2MgSiO4 and Li4SiO4 phases, with excellent results for the CO2 capture. In Ref. [8], the reaction of antigorite with Na+ led to the formation of Na2Mg2Si2O7, showing promising results as catalysts for transesterification. Additionally, in Ref. [9], potassium reaction was also found to produce a new basic phase K2MgSiO4, which is a highly active catalyst for biodiesel production [9].
On the other hand, no information is available on the reaction of these alkaline metals with other serpentinites phases such as lizardite and chrysotile and also with other Mg silicates such as talc, which has a different composition and structure compared to serpentinites.
Therefore, the main objective of this work is to investigate the reaction of Mg silicates, mainly those based on antigorite, lizardite, chrysotile, and talc, with different structures and morphologies, with K ions to produce the catalytic active phase K2MgSiO4 for the transesterification of soybean oil with methanol.
The serpentinite samples used in this work were provided by Pedras Congonhas LTDA and extracted from a mine at Nova Lima, Minas Gerais/Brazil.
All the samples were impregnated with 20wt% of K, dried for 24 h at 90°C, and thermally treated at 700, 800, and 900°C for 3 h. The natural catalysts were called Atg/Liz, Chr, Talc1, Atg, and Talc2, and the materials with 20wt% of K and calcined at 700°C were named as Atg/Liz_20K700. The sample with 0% potassium was named as Atg/Liz_0K700.
Scanning electron microscopy (SEM) images were obtained in a Quanta FEG 3D FE equipment. The samples were dispersed in acetone, deposited onto a silicon plate, and metalized with gold. The structural characterization was performed on a Shimadzu diffractometer, model XRD-7000, with CuK radiation at a scan speed of 4°·min−1. Energy-dispersive X-ray spectroscopy (EDX) analyzes were carried out on a Shimadzu energy-dispersive X-ray fluorescence spectrophotometer, model XRD-720.
Biodiesel synthesis was performed using soybean oil, methanol, and the prepared catalysts. The reactions were conducted at 60°C in a two-necked glass reactor under reflux and mechanical stirrer for 180 min. The molar ratio of oil to methanol was 1:6, and 5wt% of catalyst was used.
Leaching tests were performed using soybean oil, methanol (molar ratio of 1:6, 5wt% catalyst, at 60°C). The catalyst was contacted with methanol for 2 h. Afterward, the catalyst was filtered using a pump vacuum. The methanol volume was calibrated, and soybean oil was added to the reaction based on the methanol volume. The mixture reacted for 2 h. Then, the product was quantified by 1H NMR.
To calculate the yield of the transesterification reaction for biodiesel synthesis, 1H NMR analysis was used [10]. Mixtures of pure biodiesel and soybean oil were used to build a calibration curve with mass concentrations of biodiesel ranging from 0 to 100%.
The biodiesel was separated and analyzed by 1H NMR on a Bruker DPX 200 spectrometer for conventional reactions and on a Bruker DPX 400 equipment for reuse reactions.
In this work, five natural magnesium silicate samples with different compositions/phases and textures were used to produce heterogeneous catalysts for biodiesel synthesis. The aspect of the samples, Atg/Liz, Chr, Talc1, Atg and Talc2, and SEM images can be seen in Fig. 1.
SEM images (Fig. 1) showed that sample Atg/Liz is a compact material with particle size of 10–50 μm, whereas Chr sample is composed of typical long aligned fibers [11]. The Talc1, Atg, and Talc2 showed textures featuring agglomerated particles and fibers of 300 µm, similar to the observation by Shakoor and Thomas (2014) [12]. The Mg and Si contents obtained by XRF for the different samples are presented in Table 1.
Geological classification | Name | Formula | MgO / wt% | SiO2 / wt% | Al2O3 / wt% |
Antigorite/lizardite | Atg/Liz | Mg3Si2O5(OH)4 | 30.2 | 40.9 | 1.2 |
Chrysotile | Chr | Mg3Si2O5(OH)4 | 37.4 | 34.6 | 1.5 |
Talc | Talc1 | Mg3Si4O11H2O | 24.2 | 59.9 | 1.0 |
Antigorite | Atg | Mg3Si2O5(OH)4 | 34.0 | 39.9 | 1.4 |
Talc | Talc2 | Mg3Si4O11H2O | 26.5 | 60.4 | 1.2 |
The Atg/Liz and Atg samples showed contents of 30.2wt% and 34wt% MgO and 40.9wt% and 39.9wt% SiO2, respectively, which agrees with Ref. [13]. The Chr sample showed the highest MgO content 37.4wt%. In addition, samples Talc1 and Talc2, rich in talc, were found to have similar MgO and SiO2 contents, 25wt% MgO, and 60wt% SiO2, which agree with the previously reported values [14]. In all samples, it was possible to observe only small amounts of Al2O3.
The X-ray diffraction analysis results of all the samples are shown in Fig. 2. The XRD result of sample Atg/Liz showed a mixture of phases antigorite Mg3Si2O5(OH)4 (JCDPS 44-1447) and lizardite Mg3Si2O5(OH)4 (JCDPS 9-444). The samples Atg and Chr were composed mainly of phases antigorite and chrysotile Mg3Si2O5(OH)4 (JCDPS 2-94), respectively. The diffractograms of samples Talc1 and Talc2 were very similar, with talc Mg3Si4O11H2O as the main phase (JCDPS 2-66). Furthermore, it is important to highlight that not all reflections could be assigned, which might be due to the presence of impurities.
The XRD result of sample Atg/Liz showed a mixture of phases antigorite Mg3Si2O5(OH)4 (JCDPS 44-1447) and lizardite Mg3Si2O5(OH)4 (JCDPS 9-444). The samples Atg and Chr were composed mainly of phases antigorite and chrysotile Mg3Si2O5(OH)4 (JCDPS 2-94), respectively. The diffractograms of samples Talc1 and Talc2 were very similar, with talc Mg3Si4O11H2O as the main phase (JCDPS 2-66). Furthermore, it is important to highlight that not all reflections could be assigned, which might be due to the presence of impurities.
Talc is hydrated magnesium silicate, constituting three layers in the proportion of 2:1, whereby one Mg(OH)6 is located between two Si
Potassium-containing materials were prepared with 20wt% K, defined according to the preliminary work [9, 11] in order to optimize the formation of the K2MgSiO4 phase.
The XRD analyzes of the prepared catalysts showed dehydrated phases of magnesium silicates (Fig. 3) and K2MgSiO4 formation (JCDPS 39-1426) for all samples, which is probably responsible for the basicity of the material [9].
For samples Atg/Liz_20K700 and Chr_20K700, which are based on antigorite/lizardite and chrysotile, the thermal treatment led to the formation of the main phase K2MgSiO4, forsterite Mg2SiO4 (JCDPS 4-768), and probably olivine (Mg,Fe)2SiO4 (JCDPS 2-1346). According to previous studies, serpentinite phases decomposition products are non-hydrated silicate magnesium phases [8–9, 11].
Talc1_20K700 and Talc2_20K700 diffractogram patterns showed that at 700°C, the K2MgSiO4 phase was formed. A previous study has shown that once the talc begins to decompose at 800°C, peaks related to the hydrated phase occur, and ensatite (MgSiO3) is formed [16]. In the present study, the presence of potassium at 700°C probably led to K2MgSiO4 formation without the formation of ensatite due to the relatively low temperatures.
The sample Atg_20K700 also showed that at 700°C, decomposition was partial with a small peak due to K2MgSiO4 formation. The partial decomposition of Atg_20K700 might be related to the amount of antigorite on the sample. According to the Ref. [17], antigorite decomposes on temperatures higher than 700°C, while lizardite decomposition occurs below 700°C. For the sample Talc2_20K700, the formation of the K2MgSiO4 phase with the presence of small amounts of the talc phase was observed.
All samples impregnated with potassium and treated at 700°C showed a significant decrease of fibrous structure (Fig. 4) and very low BET surface areas, i.e., between 5 and 10 m2·g−1 with no significant porosity. Simple HCl titration measurements of basic site concentration showed values of 7–11 mmol·g−1.
The obtained materials were tested as basic heterogeneous catalysts for the transesterification reactions using soybean oil and methanol at 60°C (Fig. 5). Conversions of 98%, 86%, 71%, 88%, and 76% were obtained for the catalysts Atg/Liz_20K700, Chr_20K700, Talc1_20K700, Atg_20K700, and Talc2_20K700, respectively. A previous study in which antigorite/K was used as a catalyst for transesterification reations showed a similar conversion, 98%, under the same reaction conditions [9].
These results showed that samples Talc1_20K700 and Talc2_20K700, based on talc, led to lower conversions (70%) than the other samples, Atg/Liz_20K700, Chr_20K700, and Atg_20K700. These lower conversions are likely related to the incomplete decomposition of hydrated phases to form the catalytic phase. Previous studies using crysotile [11] and antigorite/K+ [9] as basic catalyst showed conversions higher than 95% [9], which was attirbuted to the active phase K2MgSiO4 formation [9]. Maleki and coworkers [18] studied a catalyst based on talc/Ca2+ and obtained a conversion of 97% for the best material, using a ratio canola oil : methanol of 1:15, 5wt% of catalyst, and process temperature of 65°C.
Based on biodiesel reactions conversion and on the XRD patterns, the Chr_20K700 and Talc2_20K700 samples were chosen to be studied in more detail, while varying the calcination temperature, i.e., 700, 800, and 900°C.
The SEM images showed that when pure Chr was calcined at 700, 800, and 900°C, the fibrous morphology was maintained (Fig. 6).
On the other hand, K impregnation and subsequent calcination of samples Chr_20K700, Chr_20K800, and Chr_20K900 led to a complete change in texture and morphology, causing particle agglomeration and loss of the fibrous structure. Similar results were observed by Teixeira and coworkers [11].
Studies perfomed with chrysotile showed that these materials are toxic due to the fibrous structure [19–20], and several countries have banned them from the market. Therefore, the elimination of the fibrous structure is an important factor for safe applications.
When Talc2 sample with 20wt% Kwas treated at 700, 800, and 900°C, agglomeration took place with the destruction of the fibrous structure, especially at 900°C (Fig. 7). A previous study has shown that talc occurs in shreds, plates, and flakes [21]; however, the sample could be contaminated with other minerals, such as actinolite, which has a fibrous morphology [21].
The XRD results of Chr (Fig. 8) showed a pattern relative to chrysotile phase. Calcination at 700°C (Chr_0K700) promoted chrysotile dehydroxylation, generating forsterite, Mg2SiO4, and dispersed phases of MgO and SiO2 in addition to water, suggesting the following reaction:
$$ {\rm{M}}{{\rm{g}}_3}{\rm{S}}{{\rm{i}}_2}{{\rm{O}}_5}{\left( {{\rm{OH}}} \right)_4} \to {\rm{M}}{{\rm{g}}_2}{\rm{Si}}{{\rm{O}}_4} + {\rm{MgO}} + {\rm{Si}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} $$ | (1) |
Potassium addition and subsequent calcination led to the formation of K2MgSiO4 in addition to Mg2SiO4 and water.
$$ {\rm{M}}{{\rm{g}}_3}{\rm{S}}{{\rm{i}}_2}{{\rm{O}}_5}{\left( {{\rm{OH}}} \right)_4} + 2{\rm{KOH}} \to {\rm{M}}{{\rm{g}}_2}{\rm{Si}}{{\rm{O}}_4} + {{\rm{K}}_2}{\rm{MgSi}}{{\rm{O}}_4} + 3{{\rm{H}}_2}{\rm{O}} $$ | (2) |
The intensity of the signals related to the forsterite phase at 35.7° and 36.6° decreased, and the signal intensity at 32.4° relative to the K2MgSiO4 phase increased, as reported by Ballotin and coworkers [9], for the serpentinite sample rich in the antigorite mineral.
On the other hand, a previous study with chrysotile/K did not show the formation of the K2MgSiO4 phase as a decomposition product [11]. The difference might be in the sample composition since the minerals came from different mines. The chrysotile from the previous study was obtained from SAMA mining [11], while the sample used in this work came from Pedras Congonhas LTDA.
X-ray diffraction for samples Talc2 (Fig. 9) showed patterns related to talc phase.
The thermal treatment at 700°C did not lead to any significant change in the talc diffraction pattern. A previous study has shown that talc begins to decompose at 800°C [16] to form ensatite in addition to SiO2 and H2O [14].
$$ {\rm{M}}{{\rm{g}}_3}{\rm{S}}{{\rm{i}}_4}{{\rm{O}}_{11}}{{\rm{H}}_2}{\rm{O}} \to 3{\rm{MgSi}}{{\rm{O}}_3} + {\rm{Si}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}} $$ | (3) |
Potassium addition was followed by thermal treatment at 700°C; this however promoted a change in the diffractogram, which led to K2MgSiO4 formation. When the temperature was increased to 800°C and 900°C, a higher definition of the K2MgSiO4 phase peaks and the formation of the enstatite phase, MgSiO3 (JCDPS 2-546), were observed.
$$\begin{aligned} & {\rm{M}}{{\rm{g}}_3}{\rm{S}}{{\rm{i}}_4}{{\rm{O}}_{11}}{{\rm{H}}_2}{\rm{O}} + 2{\rm{KOH}} \to 2{\rm{MgSi}}{{\rm{O}}_3} + \\ & {{\rm{K}}_2}{\rm{MgSi}}{{\rm{O}}_4} + {\rm{Si}}{{\rm{O}}_2} + 2{{\rm{H}}_{\rm{2}}}{\rm{O}} \end{aligned}$$ | (4) |
Based on the reaction stoichiometry, K2MgSiO4 was formed in larger amount for chrysotile compared with the talc sample.
Transesterification reactions of soybean oil in the presence of the chrysotile and Talc2-based catalysts were performed. The Chr samples Chr_20K700, Chr_20K800, and Chr_20K900 led to conversions of 86%, 72%, and 74%, respectively (Fig. 10). The material treated at 700°C showed better results, probably due to the formation of dehydrated phases with the milder sintering.
On the other hand, the use of sample Talc2 impregnated with potassium and calcined at different temperatures showed smaller conversions, with values of 76%, 31%, and 44%, respectively, for materials Talc2_20K700, Talc2_20K800, and Talc2_20K900. Again, the most active catalyst was the one calcined at 700°C. In accordance with the literature, when the serpentinite-based samples were used as basic catalysts in transesterification reactions, the material treated at 700°C had the highest basicity [9], which is because at serpentinite decomposition at 900°C, the reaction between SiO2 and MgO occurred to form neutral magnesium silicates, while at 500°C the dehydroxylation was not complete.
The conversion was lower for the talc-based sample, probably because the talc reaction with KOH followed by thermal treatment led to a smaller quantity of K2MgSiO4, compared to the chrysotile sample. Moreover, the low conversion when the Talc2_20K800 and Talc2_20K900 materials were used may have occurred due to the formation of the MgSiO3 phase, which is probably not active to catalyze transesterification reactions.
Previous studies have shown that the serpentinite rock, composed of lizardite, antigorite, and chrysotile, is formed by a tetrahedral layer of Si
The material impregnation with potassium and treatment at temperatures up to 500°C likely took place by K ions intercalation in the interlamellar spaces. The distance between silicon and magnesium atoms and between oxygen atoms of different layers were 0.29–0.38 nm, and the radius of the K cation was 0.1 nm, allowing the K ions to be easily intercalated between the layers. The treatment at 700°C then led to the decomposition of the intercalated K intermediate/Mg3Si2O5(OH)5, forming the K2MgSiO4 phase (Fig. 12).
The presented results suggest that serpentinite precursor structure, i.e., antigorite, lizardite, talc, and chrysotile, affected the reaction with KOH to form the K2MgSiO4 phase, which is responsible for the high catalytic activity. The low catalytic activity obtained for Talc1 and Talc2 is probably related to the formation of important amounts of enstatite (MgSiO3) by talc dehydroxylation at high temperatures (≥700°C) (Fig. 13). It is interesting to observe that in the lizardite, chrysotile, and antigorite structures, Mg(OH)6 layers were connected to Si
On the other hand, K+ intercalation, followed by K2MgSiO4 formation apparently occurs easily on lizardite and antigorite-rich [9] samples. According to the structural data, the formed K2MgSiO4 phase crystallized in an orthorhombic system with space group Cmcm composed of [Si
Thus, despite the nature of the catalytic sites present in the K2MgSiO4 phase, it can be considered that the K+ in the K2MgSiO4 structure interacts with the Si
Different magnesium silicate minerals could be impregnated with KOH and thermally treated at 700°C to form the basic catalyst phase K2MgSiO4 for the transesterification of soybean oil with methanol. The silicate precursor structures had a significant effect on the K2MgSiO4 active phase formation, although they did not have a significant effect on the surface area and the concentration of basic sites. On serpentinite minerals, i.e., antigorite, lizardite, and chrysotile, K+ diffusion allowed a good contact with Mg and Si, and during the dihydroxylation at 700°C, K2MgSiO4 phase was formed more efficiently. On the other hand, when talc was used, enstatite phase MgSiO3 was also formed likely due to the isolation of Mg(OH)6 layer between two SiO4 layers, and a smaller quantity of the active phase K2MgSiO4 was formed.
Thus, after 3 h and at 60°C, the catalysts based on talc led to smaller conversions (70%) than those based on chrysotile, antigorite, and lizardite when 5wt% of catalyst and 1:6 soybean oil : methanol were used.
The authors acknowledge Pedras Congonhas LTDA for the samples, the UFMG microscopy center for the images and the support of CNPQ, INCT Midas, CAPES and FAPEMIG.
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Geological classification | Name | Formula | MgO / wt% | SiO2 / wt% | Al2O3 / wt% |
Antigorite/lizardite | Atg/Liz | Mg3Si2O5(OH)4 | 30.2 | 40.9 | 1.2 |
Chrysotile | Chr | Mg3Si2O5(OH)4 | 37.4 | 34.6 | 1.5 |
Talc | Talc1 | Mg3Si4O11H2O | 24.2 | 59.9 | 1.0 |
Antigorite | Atg | Mg3Si2O5(OH)4 | 34.0 | 39.9 | 1.4 |
Talc | Talc2 | Mg3Si4O11H2O | 26.5 | 60.4 | 1.2 |