Solid State Reaction of Serpentinite Mg₃Si₂O₅(OH)₄ with NaOH to Produce a New Basic Catalytic Phase Na₂Mg₂Si₂O₇ for Biodiesel Production

Abstract

Solid state reaction of serpentinite Mg₃Si₂O₅(OH)₄ with NaOH was investigated to produce new NaMgSi oxide phases with basic properties for application in catalysis. X-ray powder diffraction (XRD) showed that NaOH impregnated on serpentinite, specially 20% sodium and treated at 700 ºC (20Na₇₀₀), led to the formation of a new crystalline phase Na₂Mg₂Si₂O₇. This solid state reaction is discussed in terms of a first step involving Na⁺ diffusion and intercalation in the interlayer space of the serpentinite structure followed by a dihydroxylation and formation of the NaMgSi oxides. Temperature programmed reaction and desorption experiments with 20Na₇₀₀ combined with XRD suggested a strong basicity with high CO₂ uptake likely due to diffusion and interaction with the basic sites in the material structure. These materials were tested as catalysts for the transesterification of soybean oil with methanol. The obtained results are discussed in terms of a basic site Si-O^-Na⁺ present in the in the Na₂Mg₂Si₂O₇ structure.

Keywords:
serpentinite; sodium silicate; biodiesel; basic catalyst

Introduction

Serpentinite is a mineral family of hydrated magnesium silicates with formula Mg₃Si₂O₅(OH)₄ with 32-38% MgO, 35-40% SiO₂ and 12-13% H₂O, in addition to small amounts of Fe, Al, Cr and Ni.¹ Serpentinite is arranged in layers of 1:1 type² consisting of octahedral sheets of MgO₂(OH)₄ bound to tetrahedral sheets of SiO₄.^3,4

A previous work showed that serpentinite can react with K⁺ to produce a new phase, K₂MgSiO₄, under relatively mild conditions.⁵ On the other hand, chrysotile with the same chemical composition Mg₃Si₂O₅(OH)₄, but with a different structure, did not produce this phase.⁶

The layered serpentinite structure (lizardite) has a very interesting feature, which is the possibility of cations diffusion into the interlayer space followed by a solid state reaction to produce different MgSi oxide phases in relatively mild conditions, e.g., ca. 500 ºC (Figure 1). On the other hand, the same phase K₂MgSiO₄ can also be prepared in a classical solid state reaction mixing K₂CO₃, SiO₂ and MgO, but at much higher temperatures, e.g., 1200 ºC.^7,8

Figure 1
Diffusion of metal cations into serpentinite (lizardite) interlayer space followed by a dehydration to form MSiMg oxides.

Therefore, the intercalation of metal cations in serpentinite is potentially versatile mild route to prepare different metal MgSi oxides for a variety of applications. For example, the K₂MgSiO₄ phase showed basic properties and promising results as heterogeneous catalysts for different reactions such as isomerization of hexoses⁹ and transesterification.^5,10 MSiMg oxides with Ce³⁺ or Tb³⁺ (M_xSr₂MgSi₂O₇),⁸ zinc magnesium silicate (Zn_xMg_2-xSiO₄),¹¹ Ni/MgSiO₃ doped with alkaline earth,¹² Li₂MgSiO₄,¹³ Ca₂MgSi₂O₇,¹⁴ and Ce³⁺ and Eu²⁺ activated Ca₇Mg(SiO₄)₄¹⁵ also showed very interesting properties such as ceramics, insulators, catalysts, adsorbents, luminescent and others.

Heterogeneous basic catalysts for biodiesel synthesis have been intensively investigated in the last years such as alkaline and earth alkaline oxides,^16-18 transition metal oxides and rare earths (Mg/La and Al/La,¹⁹ Zn/La,²⁰ Na₂ZrO₃),²¹ zeolites and mesoporous silicas (NaX,²² zeolites X and Y,²³ Na₂O/NaX,²⁴ SBA-15),²⁵ hydrotalcites^26,27 and some minerals (Ca₂MgSi₂O₇,¹⁴ combination of chrysotile with KOH⁶ and combinations of serpentinite with KOH).⁵ Hereon, it is investigated the use of serpentinite as an available and low cost precursor to produce the unique Na₂Mg₂Si₂O₇ phase used for the first time as basic catalyst for biodiesel production.

Experimental

The serpentinite used in this work was provided by Pedras Congonhas Ltda. The samples, retained in 200 mesh sieves, were impregnated with aqueous NaOH in proportions of 5, 10 and 20% by weight of sodium (Na⁺). The impregnation was done in a beaker, on a heating plate and magnetic stirring, at 80 ºC. The materials were oven dried for 24 h at 80 ºC to ensure complete drying and calcined at a heating rate of 10 ºC min^-1 in a horizontal tubular oven at 500, 700 or 900 ºC for 3 h under an atmosphere of air. The impregnation of serpentinite was repeated according to the procedure described by Ballotinet al.⁵ These samples are named hereon according to the Na⁺ content and temperature treatment, for instance 20Na₇₀₀ contains 20 wt.% Na⁺ treated at 700 ºC.

The structural characterization was performed by X-ray powder diffraction (XRD) on a Shimadzu diffractometer, model XRD-7000 with CuKα (1.5406 Å) and scanning speed of 4º min^-1. The chemical composition was determined by fluorescence spectroscopy (FRX) on a Shimadzu EDX-720 vacuum spectrometer.

Thermogravimetric analyzes were performed on a Shimadzu DTG 60H equipment with synthetic air flow (50 mL min^-1), temperature range of 30-1000 ºC and heating rate of 10 ºC min^-1. In order to determine the basic properties of the material 20Na₇₀₀, a simultaneous thermogravimetric analysis mass spectrometry (TG-MS) analysis was applied. The base peak (m/z 44) was selected to be monitored in a Netzsch TG/STA equipment coupled with Aelos spectrometer, model 7.0. The catalysts were previously treated in argon atmosphere (20 mL min^-1) at 500 ºC for 1 h, followed by treatment at 50 ºC under CO₂ flow (20 mL min^-1) for 1 h. Then, the material was heated to 1000 ºC in argon at a rate of 5 ºC min^-1. The measurements of Raman spectroscopy were performed on a Raman Senterra spectrometer from Bruker with a coupled optic microscope (Olympus BX51). The sample was excited using the laser at wavelength 633 nm, with a power of 0.2 mW. The number of settings was 10 and the integration time was 10 s.

Scanning electron microscopy (SEM) measurements were obtained on a Quanta 200-FEG 3D-FEI equipment. The specific surface areas (Brunauer-Emmett-Teller, BET) of the samples were analyzed by adsorption of N₂ at 77 K using the Autosorb1-MP Quantachrome equipment. Samples were degassed at 200 ºC for 24 h prior to analysis.

Biodiesel was synthesized using typical conditions found in the literature.⁵ The reactions were carried out in a glass batch reactor at 60 and 100 ºC under continuous stirring for 3 h with sample collection and analysis every 30 min. The molar ratios of oil:methanol used were 1:6, 1:9 and 1:12. The catalyst concentration in the reaction was 1, 5 and 10 wt.% in relation to oil. The reuse tests were carried out under the optimal reaction conditions established by experiments with fresh catalysts. After the reaction, the liquid phase was separated from the catalysts, and the recycling experiments were done by simple reuse of the catalysts without any treatment. Leaching was evaluated at the molar ratio of 1:9, using 5% catalyst. For the tests, the catalyst was transferred to the reaction medium containing only methanol. The system was maintained under constant stirring for 30 min at 60 or 100 ºC. After this period, methanol was removed and transferred to a vial containing only soybean oil, thus proceeding with the reaction.

The methyl ester content was analyzed by gas chromatography coupled with flame ionization detector (GC-FID) using a Shimadzu QP apparatus 2010, equipped with Rtx-Wax capillary column (30 m, 25 mm and internal diameter of 0.25 μm).

Results and Discussion

The serpentinite used in this work shows an approximate composition of 40% SiO₂ and 30% MgO, 10% Fe₂O₃ and small concentrations of Al, Ca, Ni and Mn. The thermogravimetric profile (Supplementary Information (SI) section) showed that serpentinite decomposes between 500-800 ºC with a main weight loss of 8-9% related to the dehydroxylation of Mg₃Si₂O₅(OH)₄.

The main phases produced in this decomposition are magnesium silicates, e.g., forsterite (Mg₂SiO₄ JCDPS 4-769) as observed by X-ray diffraction (Figure 2). Peaks at 2θ such as 9.5 and 28.6º also suggest the formation of another Mg silicate Mg₃Si₄O₁₀(OH)₂ (JCDPS 13-558). equation 1 represents a simplified serpentinite decomposition process to form forsterite.

(1) ${\mathrm{Mg}}_3{\mathrm{Si}}_2{O}_5{\left(\mathrm{OH}\right)}_4\to {\mathrm{Mg}}_2{\mathrm{SiO}}_{4\left(\mathrm{forsterite}\right)}/\mathrm{MgO}/{\mathrm{SiO}}_2+2H_{2}O$

Figure 2
X-ray diffraction patterns of serpentinite samples, Serp₇₀₀ and 5Na₇₀₀, 10Na₇₀₀ and 20Na₇₀₀.

Figure 3 shows the crystalline structures for both serpentinite and forsterite. The serpentinite crystallizes as a triclinic system with space group P₁ based on layers (100) of [SiO₄] tetrahedra and [MgO₆] octahedra layer connected by two [SiO₄] tetrahedra to form [Si₂O₇] (Figure 3a).

Figure 3
Structure of (a) serpentinite and (b) forsterite.

The forsterite structure resulted from the decomposition of the brucite (Mg(OH)₂) layer to form Mg-O-Si bonds. This structure crystallizes as orthorrombic system with space group P_nma where the (010) plan is composed of Mg-oxygen octahedra intercalated with [SiO₄]. In each tetrahedron [SiO₄] all the oxygen atoms are shared with Mg (Figure 3b). All structures were generated from the Crystallographic Information File (CIF) taken from the database, Crystallography Open Database²⁸ and generated from Vesta Visualization software for Electronic and Structure Analysis. ²⁹

The XRD (Figure 3) profiles of the samples Serp₇₀₀ and 5Na₇₀₀ were very similar. On the other hand, the samples 10Na₇₀₀ and 20Na₇₀₀ showed a peak at ca. 21º suggesting the formation of the structure Na₂Mg₂Si₂O₇ (JCDPS 53-0626). In addition, a small peak at 43º indicates the formation of small amounts of MgO (JCDPS 45-946). These results suggest a process described by the simplified equation 2:

(2) $2\mathrm{NaOH}+{\mathrm{Mg}}_3{\mathrm{Si}}_2{O}_5{\left(\mathrm{OH}\right)}_4\to {\mathrm{Na}}_2{\mathrm{Mg}}_2{\mathrm{Si}}_2{O}_7/\mathrm{MgO}+3H_{2}O$

XRD peaks related to forsterite gradually decreased in the presence of Na⁺. A simple analysis of peak intensities of Na₂Mg₂Si₂O₇ (2θ = 21º) and Mg₂SiO₄ (2θ = 32.5º) phases suggests that the increase of sodium caused the ratio I_{(Na2Mg2Si2O2)}/I_(Mg2SiO2) increase from 0.3 to 5.4 for the sample 20Na₇₀₀. These results clearly indicate that the presence of Na⁺ led to the formation of the phase Na₂Mg₂Si₂O₇. It is interesting to observe that the phase Na₂Mg₂Si₂O₇ has a Na:Mg:Si atomic ratio of 1:1:1, whereas the sample 20Na₇₀₀ has a slightly different Na:Mg:Si ratio (1:1.2:0.8). The small excess of magnesium is segregated as MgO as indicated by the XRD pattern for the sample 20Na₇₀₀.

The phase Na₂Mg₂Si₂O₇ crystallizes in a monoclinic system with space group Pc composed of sheets formed by [MgO₄] octahedra sheet connected by Si-O-Si bonds formed by Si₂O₇ units (Figure 4). The Na⁺ species are located in the interlayer spaces, as shown in Figure 4. Considering the similarity between the layer structure and the Mg and Si distribution of the serpentinite and the Na₂Mg₂Si₂O₇ structure, one may consider that the reaction pathway likely involves the diffusion/intercalation of Na⁺ in the interlayer space of serpentinite, followed by a thermal decomposition to produce the phase Na₂Mg₂Si₂O₇. If the reaction is carried out in a different sequence, first treatment of serpentinite at 700 ºC and only after this treatment impregnation with NaOH, then treatment again at 700 ºC, no formation of the Na₂Mg₂Si₂O₇ phase was observed.

Figure 4
Representation of the reactions of serpentinite leading to forsterite (Mg₂SiO₄) and intercalation with Na⁺ to produce the phase Na₂Mg₂Si₂O₇.

The effect of temperature on the sample 20Na was investigated at 500, 700 and 900 ºC (see XRD in SI section). Pure serpentinite decomposes only at temperatures higher than 700 ºC. However, in the presence of Na⁺, the decomposition of serpentinite takes place at much a lower temperature, 500 ºC, leading to the formation of the Na₂Mg₂Si₂O₇ and Na₂MgSiO₄ phase. As the treatment temperature increased to 700 and 900 ºC the peaks related to the phase Na₂Mg₂Si₂O₇ increased in intensity.

SEM images for the serpentinite precursor (Figure 5) showed needle shaped fragmented particles with size varying from 1-10 µm. After impregnation with Na⁺ and the thermal treatment a strong sintering takes place to form a solid with a compact surface. As a result of this sintering/compacting process, the surface decreased from 12 (Serp₇₀₀) to 8, 5 and 4 m² g^-1 for 20Na₅₀₀, 20Na₇₀₀ and 20Na₉₀₀, respectively.

Figure 5
SEM images of the samples serpentinite, 20Na₅₀₀, 20Na₇₀₀ and 20Na₉₀₀.

The CO₂ uptake by the sample 20Na₇₀₀ was investigated by temperature programmed reaction experiments (Figure 6). It can be observed that the sample 20Na₇₀₀ from ca. 50 up to 400 ºC absorbed a relatively large amount of CO₂, ca. 12 wt.%, which indicates by a simple calculation a ratio of one CO₂ molecule for two Na⁺ ions.

Figure 6
Temperature programmed reaction of 20Na₇₀₀ with CO₂ (detail: dimensions of the Na₂Mg₂Si₂O₇ structural cavity and a CO₂ molecule).

This result suggests a strong basicity likely due to the presence of basic Na⁺ species, which are available for the interaction with CO₂. The formation of carbonate species was investigated by Raman spectroscopy and XRD. Raman spectra of the obtained material did not show the typical absorption for CO₂ at 1381 cm^-1 (SI section).²⁸ XRD of the 20Na₇₀₀ sample after CO₂ reaction also did not show the presence of crystalline phases related to sodium carbonate and magnesium carbonate (SI section). It was only observed a general broadening and a shift of the peaks to lower 2q of the XRD peaks suggesting that the Na₂Mg₂Si₂O₇ structure was not destroyed by CO₂ up to 900 ºC. Although the exact physico-chemical process of CO₂ interaction with Na₂Mg₂Si₂O₇ is not clear, one can speculate that CO₂ molecules are diffusing into the solid structure Na₂Mg₂Si₂O₇ to interact with the basic sites. The Na₂Mg₂Si₂O₇ structure shows cavities with relatively large size, e.g., distances Si-Si 8.3 Å and Mg-Mg 6.4 Å, which could easily accommodate a CO₂ molecule with dimension of 2.5 Å. The detail in Figure 6 shows a schematic representation of a CO₂ molecule and the Na₂Mg₂Si₂O₇ cavity. However, more detailed studies are necessary to investigate the nature of the CO₂ interaction with Na₂Mg₂Si₂O₇.

Temperature programmed desorption/decomposition was performed for samples 20Na₇₀₀ in a TG-MS system (Figure 7). The sample 20Na₇₀₀ showed a small desorption peak at ca. 80-90 ºC, likely related to weak surface basic sites.⁵ Another desorption process was observed at much higher temperature, 500-700 ºC, related to the release of CO₂ molecules located in the Na₂Mg₂Si₂O₇ structure.

Figure 7
Temperature programmed CO₂ desorption of the sample 20Na₇₀₀ (after pretreatment at 500 ºC under argon and exposure to CO₂ at 50 ºC).

The different materials obtained by impregnation of 5, 10 and 20% sodium (Na⁺) and treated at 500, 700 and 900 ºC were tested as catalysts for the transesterification reaction of soybean oil in methanol (molar ratio of 1:9) using 5 wt.% catalyst at 60 and 100 ºC. Blank tests (without catalyst) at 60 ºC showed no reaction, whereas at 100 ºC conversions lower than 5% were obtained.

The original serpentinite before and after treatment at 500-900 ºC showed no significant activity for biodiesel production. On the other hand, materials impregnated with 5-20% Na⁺ treated at 500 ºC showed relatively low activities, i.e., up to 50% conversion. This result indicates that the Na⁺ present in the sample is not fully active for the transesterification reaction. However, as the treatment temperature increased to 700 ºC, the conversion improved significantly, reaching values higher than 90%. The sample treated at 900 ºC showed a decrease on the catalytic activity (Figure 8).

Figure 8
Transesterification results of soybean oil with methanol (molar ratio of 1:9, 5 wt.% catalyst at 100 ºC for 3 h).

The transesterification kinetic was also investigated for the catalyst 20Na₇₀₀ and the obtained results are shown in SI section. These reactions were conducted in the molar ratio of 1:9, using 5% catalyst at temperatures of 60 and 100 ºC. At 60 ºC, the reaction reached equilibrium at ca. 60 min with maximum conversion of 55%. On the other hand, at 100 ºC the conversion further increased to 95% reaching equilibrium at ca. 150 min.

The effects of the amount of catalyst as well as the different oil/alcohol molar ratios were evaluated and are presented in Figure 9. The tests were performed at 60 and 100 ºC. The reaction time was 3 h.

Figure 9
(a) Conversion (percentage) of 20Na₇₀₀ catalyst as a function of the amount of catalyst in the molar ratio of 1:9; (b) conversion of 20Na₇₀₀ catalyst as a function of different molar ratios using 5% catalyst.

Figure 9a data suggest the conversion of soybean oil to biodiesel improved when the amount of catalyst increased from 1 to 5 wt.%. On the other hand, when the catalyst amount increased to 10%, no significant change was observed, reaching 97% at 100 ºC. The increase in the molar ratio (oil/methanol) from 1:6 to 1:9 led to an increase in the conversion, especially at 100 ºC. However, the conversion did not vary significantly when the molar ration was further increased to 1:12 (Figure 9b).

The reuse of the catalyst was investigated using the 20Na₇₀₀ sample (Figure 10). After the first use, the conversion decreased from 92 to 63%. For the 3^rd, 4^th and 5^th reactions the conversion slowly decreased reaching values ca. 30%.

Figure 10
20Na₇₀₀ reuse tests. Detail: conversion obtained during the leaching study.

The presence of Na⁺ leaching and homogeneous reaction was investigated for the 20Na₇₀₀ catalyst mixed with methanol and heated to 60 or 100 ºC and after 30 min the mixture was filtered hot and the methanol was used for the reaction with the soybean oil. These results were compared with a normal reaction with the catalyst 20Na₇₀₀ to separate the homogeneous and heterogeneous contributions. The results (Figure 10, detail) showed that the major contribution is heterogeneous (ca. 63% conversion at 100 ºC), however, a significant leaching and homogeneous reaction is taking place (ca. 29% conversion at 100 ºC). The 2^nd use showed a significant deactivation compared to the initial conversion, from 92 to 60% at 100 ºC. After the 3^rd use it was observed only a slight decrease on the catalytic activity. Preliminary results indicated that thermal treatment of the deactivated catalyst in air at 500-700 ºC led to a partial recovery of the activity likely due to the elimination of organics from the catalyst surface. A more systematic work is necessary to understand the deactivation mechanism and possible reactivation processes.

Although the nature of the catalytic site is not clear, one can consider that in the Na₂Mg₂Si₂O₇ structure the Na⁺ ions are interacting with one of the oxygens of the Si₂O₇ unit forming a basic species Si-O^-Na⁺. One possible mechanism is the interaction of CH₃OH with the basic site Si-O^-Na⁺ to form methoxide (equation 3).

(3) $\mathrm{Si}-O^{-}{\mathrm{Na}}^{+}+{\mathrm{CH}}_3\mathrm{OH}\to \mathrm{Si}-\mathrm{OH}+{\mathrm{Na}}^{+}+{\mathrm{CH}}_3{O}^{-}$

The formed methoxide is able to react with soybean ester to form biodiesel and regenerate the basic species.²⁹ A simplified local structure of the basic site and the reaction with methanol is represented schematically in Figure 11.

Figure 11
Schematic representation of the interaction of methanol molecule with the structural basic site Si-O^-Na⁺.

Conclusions

The impregnation of serpentinite with NaOH and thermal treatment at temperatures 500-900 ºC led to the formation of the new phase Na₂Mg₂Si₂O₇. This reaction is discussed in terms of a diffusion and intercalation of Na⁺ ions in the interlayer space of the serpentinite structure followed by dehydration. This phase Na₂Mg₂Si₂O₇ presented basic properties as observed by CO₂ temperature programmed reaction and desorption. The obtained materials showed catalytic activity for transesterification of soybean oil with methanol to produce biodiesel and were tested at 60 and 100 ºC. The catalytic site is discussed in terms of a Na⁺ interacting with an Si₂O₇ moiety to form the basic species Si-O^-Na⁺.

Supplementary Information

Supplementary data (TG/DTG, XRD, Raman spectra and kinetic data of the 20Na₇₀₀ catalysts) are available free of charge at http://jbcs.sbq.org.br as PDF file.

Acknowledgments

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.

References

• ¹
  Yoo, K.; Kim, B.-S.; Kim, M.-S.; Lee, J.; Jeong, J.; Mater. Trans. 2009, 50, 1225.
• ²
  Dlugogorski, B. Z.; Balucan, R. D.; Renewable Sustainable Energy Rev. 2014, 31, 353.
• ³
  Hirth, G.; Guillot, S.; Elements 2013, 9, 107.
• ⁴
  Cao, C. Y.; Liang, C. H.; Yin, Y.; Du, L. Y.; J. Hazard. Mater. 2017, 329, 222.
• ⁵
  Ballotin, F. C.; Cibaka, T. E.; Ribeiro-Santos, T. A.; Santos, E. M.; Teixeira, A. P. C.; Lago, R. M.; J. Mol. Catal. A: Chem. 2016, 422, 258.
• ⁶
  Teixeira, A. P. C.; Santos, E. M.; Vieira, A. F. P.; Lago, R. M.; Chem. Eng. J. 2013, 232, 104.
• ⁷
  Dollase, W. A.; Powder Diffr. 1996, 11, 51.
• ⁸
  Xia, Y.; Chen, J.; Liu, Y.-G.; Mei, L.; Huang, Z.; Fang, M.; Mater. Express 2016, 6, 37.
• ⁹
  Shen, X.; Wang, Y.; Ahring, B. K.; Lei, H.; Gao, Q.; Liu, H.; RSC Adv. 2015, 5, 96990.
• ¹⁰
  Qian, K.; Shen, X.; Wang, Y.; Gao, Q.; Ding, H.; Energy 2015, 93, 2251.
• ¹¹
  Devi, K. B.; Lee, B.; Roy, A.; Kumta, P. N.; Roy, M.; Mater. Lett. 2017, 207, 100.
• ¹²
  Ghods, B.; Meshkani, F.; Rezaei, M.; Int. J. Hydrogen Energy 2016, 41, 22913.
• ¹³
  Ornar, A. A.; Ceram. Int. 1990, 16, 47.
• ¹⁴
  Wang, J.; Yang, L.; Luo, W.; Yang, G.; Miao, C.; Fu, J.; Xing, S.; Fan, P.; Lv, P.; Wang, Z.; Fuel 2017, 196, 306.
• ¹⁵
  Jia, Y.; Qiao, H.; Zheng, Y.; Guo, N.; You, H.; Phys. Chem. Chem. Phys. 2012, 14, 3537.
• ¹⁶
  Teo, S. H.; Rashid, U.; Thomas Choong, S. Y.; Taufiq-Yap, Y. H.; Energy Convers. Manage. 2017, 141, 20.
• ¹⁷
  Fan, M.; Liu, Y.; Zhang, P.; Jiang, P.; Fuel Process. Technol. 2016, 149, 163.
• ¹⁸
  Tubino, M.; Rocha, J. G.; Bauerfeldt, G. F.; Catal. Commun. 2016, 75, 6.
• ¹⁹
  Santório, R.; Veloso, C. O.; Henriques, C. A.; J. Mol. Catal. A: Chem. 2016, 422, 234.
• ²⁰
  Veiga, P. M.; Veloso, C. O.; Henriques, C. A.; Renewable Energy 2016, 99, 543.
• ²¹
  Santiago-Torres, N.; Romero-Ibarra, I. C.; Pfeiffer, H.; Fuel Process. Technol. 2014, 120, 34.
• ²²
  Manadee, S.; Sophiphun, O.; Osakoo, N.; Supamathanon, N.; Kidkhunthod, P.; Chanlek, N.; Wittayakun, J.; Prayoonpokarach, S.; Fuel Process. Technol. 2017, 156, 62.
• ²³
  Al-Ani, A.; Darton, R. J.; Sneddon, S.; Zholobenko, V.; ACSAppl. Nano Mater. 2018, 1, 310.
• ²⁴
  Martínez, S. L.; Romero, R.; Natividad, R.; González, J.; Catal. Today 2014, 220-222, 12.
• ²⁵
  Xu, J.; Chen, T.; Shang, J. K.; Long, K. Z.; Li, Y. X.; Microporous Mesoporous Mater. 2015, 211, 105.
• ²⁶
  Barakos, N.; Pasias, S.; Papayannakos, N.; Bioresour. Technol. 2008, 99, 5037.
• ²⁷
  Guzmàn-Vargas, A.; Santos-Gutérrez, T.; Lima, E.; Flores-Moreno, J. L.; Oliver-Tolentino, M. A.; Martínez-Ortiz, M. D. J.; J. Alloys Compd. 2015, 643, S159.
• ²⁸
  Chen, Y.; Wang, H.; Li, J.; Lockard, J. V.; J. Mater. Chem. A 2015, 3, 4945.
• ²⁹
  Cordeiro, C. S.; da Silva, F. R.; Wypych, F.; Ramos, L. P.; Quim. Nova 2011, 34, 477.

Publication Dates

• Publication in this collection
  Sept 2018

History

• Received
  14 Nov 2017
• Accepted
  21 Mar 2018