Sonochemical-assisted magnesium borate synthesis from different boron sources

Abstract In this study, sonochemical-assisted magnesium borate synthesis is studied from different boron sources. Various reaction parameters are successfully applied by a simple and green method. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Raman spectroscopies are used to characterize the synthesized magnesium borates on the other hand surface morphologies are investigated by using scanning electron microscope (SEM). The XRD analyses showed that the products were admontite [MgO(B2O3)3 · 7(H2O)] with JCPDS (Joint Committee on Powder Diffraction Standards) no. of 01-076-0540 and mcallisterite [Mg2(B6O7(OH)6)2 · 9(H2O)] with JCPDS no. of 01-070-1902. The results that found in the spectroscopic studies were in a good agreement with characteristic magnesium borate bands in both regions of infra-red and visible. According to SEM results, obtained borates were in micro and sub-micro scales. By the use of ultrasonication, reaction yields were found between 84.2 and 97.9%. As a result, it is concluded that the sonochemical approach is a practicable synthesis method to get high efficiency and high crystallinity in the synthesis magnesium borate compounds.


INTRODUCTION
In recent years, the green chemistry approach has been become even more important because it aims to increase selectivity of chemical reactions and energy effi ciency, to modify the reaction parameters and to minimize the hazardous wastes. Sonochemical synthesis method, which has a conformity with the green chemistry principles, ensures shorter reaction times, enhances reaction yield and provides energy saving 1-3 .
In sonochemical-assisted chemical reactions, the interaction of reagents and mass transfer are accelerated with the help of acoustic cavitation which increases surface area for reaction with producing shock waves and causes a strong connection between reactant particles and solution medium 4-9 . In literature, there are many studies that benefi t from these advantages of sonochemical synthesis, for producing organic and inorganic compounds such as various nanoparticles (TiO 2 , ZnO, Fe 2 O 3 etc.), catalysts and biomaterials 10- 16 .
Being a sub-group of borate compounds, magnesium borates have attractive physical, mechanical and chemical properties that provide them to use in a wide range of industrial applications. Magnesium borates are not only show high heat and corrosion resistance but also have excellent strength, insulation and elasticity specialties. Due to these characteristic features, different types of magnesium borates can be used in various areas, including the thermo-luminescent phosphors, catalysts for hydrocarbon conversions, additives in tunable laser applications, reinforcement in the electronic ceramics, wide band gap semiconductors, reinforcing composites in plastics/aluminum matrixes 17 BO 3 , nevertheless, the synthesized product is a mixture of admontite and mcallisterite types of magnesium borates. However, in this study pure admontite production is aimed. Despite the studies about application sonochemistry on the synthesis of different borate compounds 32-35 are accessible in literature, using the sonochemical procedure in the magnesium borate synthesis which presents moderate reaction conditions with short reaction periods has not been studied thoroughly.
In this study, a green, effi cient and simple synthesis procedure was applied to obtain magnesium borates from different boron sources under ultrasonic irradiation. Besides different boron sources, effects of various reaction times (5-20 min) and reaction temperatures (60-100 o C) were investigated. In addition to reaction yield and boron oxide (B 2 O 3 ) calculations; chemical, morphological and spectral properties of produced magnesium borates were compared to fi nd out the optimum reaction conditions for sonochemical-assisted magnesium borate production.

Sonochemical-assisted synthesis
Sonochemical-assisted synthesis of magnesium borate compounds was carried out by the hydrothermal procedure by direct-immersion of Bandelin Sonopuls HD 2070 ultrasonic homogenizers (Bandelin electronic GmbH & Co. KG, Berlin, Germany) that use the operation frequency of 20 kHz. In the experiments, 1:6 magnesium to boron (Mg:B) molar ratio was used, which was determined from preliminary synthesis 29 , magnesium and boron sources were reacted with ultrasonic irradiation in a batch type reactor fi lled with distilled water obtained from GFL 2004 (Gesellschaft für Labortechnik, Burgwedel, Germany) water purifi cation system.
In To investigate the infl uence of reaction temperature and sonifi cation time on the products, the reaction temperature was varied between 60-100 o C and sonifi cation time was varied between 5-20 min. At the end of determined sonifi cation time, the reaction slurry was placed in an Ecocell LSIS-B2V/EC55 model incubator (MMM Medcenter Einrichtungen GmbH, Planegg, Germany) maintained at 40 o C. In the incubator the magnesium borates were crystallized and the excess water was evaporated. In the synthesis, the usage of sodium borates leads to form sodium chloride (NaCl) as by-product, so on the purpose of removing that byproduct and excess or unreacted amount of H 3 BO 3 , the products were washed with ethanol (96%) obtained from Merck chemicals (Merck KgaA, Darmstadt, Germany) (CAS No: 64-17-5) and fi ltered through chm F2044 grade (Ashless, slow fi lter speed) 90 mm blue ribbon fi lter paper (Chmlab, Barcelona, Spain). Then the fi lter cake was dried in the incubator again at 40 o C. The reaction scheme of the entire production is given in Figure 1.

Characterization studies of synthesized magnesium borates
XRD, Fourier transform infrared spectroscopy (FT--IR), Raman spectroscopy and scanning electron microscopy (SEM) were applied to characterize the synthesized products. Phase composition of products was determined by XRD in the pattern range of 7-60 o . The infrared absorption spectra of the products in the wavenumber range of 1800-650 cm -1 were obtained by PerkinElmer Spectrum One FT-IR (PerkinElmer, MA, USA). Raman spectra were obtained between the ranges of 1800-250 cm -1 by using Perkin Elmer Raman Station 400F (PerkinElmer, CT, USA). The spectroscopic ranges were determined by through previous studies that the characteristic peaks of borate compounds, which were observed in the range of 500-1500 cm -1 18, 36 .
The morphological properties and particle sizes of magnesium borates were investigated by CamScan Apollo 300 Field-Emission SEM (CamScan, Oxford, United Kingdom) working at 20 kV. Backscattering electron detector (BEI) was used in the morphological studies. The SEM images of products were magnifi ed as 10 000 times.
In terms of economy, the commercial value of borate compounds is assessed depending on their B 2 O 3 content. Therefore, B 2 O 3 percentage of synthesized compounds were calculated by the method given by the studies in literature 30, 31, 36 .
Yield analysis was conducted with choosing MgCl 2 . 6H 2 O as a key component of the procedure of given by Derun et al. 37 . The experiments were carried out three repetitions and the average yield and standard deviation values were calculated. For biphasic products, Figure 1. Reaction scheme the yield calculations were applied according to the phase which's molecular weight is higher.
The overall yields, Y D , is calculated by dividing the number of moles of product at the fi nal stage, N D , by the number of consuming moles of the key reactant, A. The consuming numbers of moles of reactant, A, is calculated by using the initial (N A0 ) and the fi nal (N A ) numbers of moles reactant with the Eq. 1 37, 38 . (1)

Results of the raw material characterization
According to the XRD results of the raw materials, the boron sources were identifi ed as sassolite (H 3

XRD results of the synthesized magnesium borates
XRD scores of synthesized magnesium borates obtained from different sources of boron compounds are given in Table 1. From the results, in all of the four sets, the major component was found as "admontite" In the set of "Mc-Bx-B", the highest and the lowest admontite XRD scores were observed at the conditions of 80 o C -5 min and 80 o C -20 min, respectively. Since the XRD scores were close to each other it can be said that the admontite crystal formation was not affected by the reaction temperature and reaction time much. In the set of "Mc-Bx-H" the highest and the lowest admontite XRD scores were observed at the at the condition of 60 o C -20 min and 100 o C -15 min, respectively. In this set the XRD scores were lower than the set of "Mc-Bx-B".
In general, higher XRD scores were obtained at the reaction temperature of 60 o C and were decreased with increasing reaction temperature. In the set of "Mc-T-B", the highest and the lowest admontite XRD scores were observed at the at the condition of 100 o C -10 min and 60°C -5 min (also 80 o C -10 min), respectively. In this set of experiments, higher XRD scores were obtained that the sets of "Mc-Bx-B" and "Mc-Bx-H" and these scores were increased with increasing reaction temperature. In Comparison with the literature, the ultrasonication ensures more crystalline magnesium borate powders produced in previous studies even for 5 minutes of reaction at 60 o C 29, 31, 36 .

FT-IR and Raman spectroscopy of the synthesized magnesium borates
FT-IR and Raman spectra of the selected magnesium borate minerals are presented in Figure 3 and Figure 4, respectively. According to FT-IR spectra, two absorption peaks were observed at 1420 cm -1 and 1345 cm -1 . The peaks were arisen because of the asymmetric stretching of the three-coordinate boron to oxygen bands [ν as (B (3 Both FT-IR and Raman spectra of products showed characteristic absorption and scattering peaks of magnesium borates, respectively 29, 31, 37 .

Surface morphology and particle size of the synthesized magnesium borate compounds
To fi nd out the effect of different reactants, reaction temperatures, and reaction times on the surface morphology of the produced magnesium borates, SEM analyses were applied to the products. The surface morphologies and particle size ranges of optimum products are shown in Figure 5. FT-IR spectra of optimum magnesium borates multangular layer with smaller pieces. Also irregularities in particle texture with sharp edges could be observed in Set 1. In the reactions that borax and boron oxide used as boron source (Set 2), the products showed non-According to SEM results, the particle size of synthesized minerals is in micro and sub-micro scale. For Set 1, the particle size of products changed between 1.18 μm -228.87 nm and the acquired particles had smooth,  . SEM morphologies of the selected magnesium borates uniform particle size distribution which varied between 1.50 μm -218.66 nm. For Set 3 and Set 4, in which tincalconite was used, the particle size changed between 1.39 μm -243.75 nm and 1.44 μm -639.98 nm. Also it was seen that when tincalconite used as a boron source, the particle sizes were increased because of the increase of the agglomeration. For Set 3, conglomerated structures could be noticed which was because of summing up of small particles.
Produced magnesium borate powders had smaller average particle size contrary to previous studies 29, 31, 37 since the ultrasonic probe leads more homogeneous stirring in the reaction medium.

Yield calculation of synthesized magnesium borates
Reaction yields of magnesium borates are presented in Figure 6a for Set 1 and Set 2, in Figure 6b for Set 3 and Set 4. According to the results, for all reaction sets the highest reaction yields were obtained at increasing reaction time and reaction temperature. When each reaction set was scrutinized in detail, it is seen that the yield of reactions in which H 3 BO 3 was used is found higher.
The calculated reaction yields were between 85.8-94.6%, 87.2-97.9%, 84.2-94.1%, 86.5-96.1% for Set 1, Set 2, Set 3 and Set 4, respectively. The highest yield was observed in the experiment of Set 2 which was taking place at 100 o C during 20 min of reaction time.

CONCLUSIONS
In this study, a novel and green method was developed to synthesis magnesium borate compounds with high crystallinity from different boron sources by ultrasonication. XRD results showed that admontite [MgO(B 2 O 3 ) 3 · 7(H 2 O)] and mcallisterite [Mg 2 (B 6 O 7 (OH) 6 ) 2 · 9(H 2 O)] were produced during study where the main phase was admontite in the most of the experiments. The reaction yields of experiments changed between 84.2-97.9%.
The study indicates that even at 60 o C and 5 min of reaction, high crystalline magnesium borate compounds can be produced. Therefore, in comparison with our previous study 29 , the usage ultrasonic synthesis method provides lower reaction times and reaction temperatures, which ensures energy saving, simple and green obtainment of desired compounds.