Synthesis of hydrotalcite-type mixed oxide catalysts from waste steel slag for transesterification of glycerol and dimethyl carbonate

The mixed metal oxides S-CaMgAl MO prepared by acidolysis, coprecipitation and calcination under different temperatures from S95 steel slag of Shanghai Baosteel Co., Ltd. were used to catalyze the transesterification of dimethyl carbonate (DMC) and glycerol for synthesizing glycerol carbonate (GC). The catalysts were characterized by EDS, XRD, FT-IR, SEM, CO2-TPD and nitrogen adsorption–desorption isotherms. S-CaMgAl MO calcined at 600 °C had excellent catalytic performance due to the large pore size and proper alkalinity. The effects of reaction temperature, reaction time and the amount of catalyst on transesterification were investigated to obtain the optimal reaction conditions. The glycerol carbonate yield reached 96.2% and the glycerol conversion was 98.3% under the condition of 3 wt% catalyst, 1:3 molar ratio of glycerol and DMC, 75 °C reaction temperature and 90 min reaction time. In addition, the GC yield and glycerol conversion still achieved above 90% after five cycles of S-CaMgAl MO.

Scientific RepoRtS | (2020) 10:10273 | https://doi.org/10.1038/s41598-020-67357-z www.nature.com/scientificreports/ utilization ways and low utilization rate 18 . A large number of unused steel slag has not only occupied valuable land resources, but also caused environmental pollution. The synthesis of novel multi-functional materials from the rich metal resources in steel slag can provide a new idea for the full use of steel slag and greatly reduce the preparation cost of corresponding materials 19 .
Hydrotalcite-like compounds, also known as layered double hydroxides (LDHs), belongs to anionic layered compounds. [M II 1-x M III x (OH) 2 ] x+ [A] x/n ·mH 2 O can be used to describe LDHs. The closer radius of M II and M III in hydrotalcites is generally more favorable to the formation of stable laminate structures 20,21 . LDHs are widely used in environmental remediation, catalysis, pharmaceutical and biological preparation due to the exchangeability of interlayer anions [22][23][24][25] . Kuwahara 26 prepared Ca-Al-Cl hydrotalcite from blast furnace slag and used it to adsorb phosphate in water. Its adsorption capacity was more than three times that of Mg-Al hydrotalcite. The prepared hydrotalcites were also used to trap CO 2 27 . The mixed oxide prepared by calcination of hydrotalcites was often used as catalysts due to the high basicity. Algoufi 28 synthesized the Sr-Al mixed oxide and investigated the catalytic effect of the two elements ratio on the esterification. Parameswaram 29 reported transesterification of glycerol with dimethyl carbonate for the synthesis of glycerol carbonate over Mg-Zr-Sr mixed oxide catalysts. Liu 30 synthesized glycerol carbonate by transesterification of glycerol with dimethyl carbonate over Mg-Al mixed oxide catalysts.
The materials prepared from steel slag are rich in calcium. The catalyst with high calcium content still has serious stability problems. In this paper, calcium-based catalysts were synthesized by calcining hydrotalcites from steel slag acidolysis to achieve the high-efficient conversion of glycerol as the high value-added by-product of biodiesel production. The catalytic performance and the stability of hydrotalcite-type mixed oxide in the transesterification reaction of glycerol and DMC to prepare GC was investigated. experimental Materials. The steel slag used in this study was Type S95 steel slag from Baosteel Co., Ltd. in Shanghai. The raw steel slag was white powder with small particle size. Its composition was analyzed by Energy Dispersive Spectrometry. Table 1 demonstrated that the raw material contained rich metal elements. The mass fraction of CaO, MgO and Al 2 O 3 was 53.87%, 6.09% and 12.13% respectively. Glycerol (AR), dimethyl carbonate (AR), NaOH (AR), Na 2 CO 3 (AR) and HNO 3 (68 wt%) were purchased from Aladdin Reagent Co. Ltd (China).
Catalyst preparation. The hydrotalcites were prepared by coprecipitation method after acidolysis of steel slag and calcined at different temperatures to obtain various mixed oxide catalysts. The metal elements in the steel slag can be extracted into acidolysis solution while SiO 2 remained in the filter residue due to its insolubility in acidolysis solution. 50.0 g Type S95 steel slag was weighed and added to 400 mL beaker. 200 mL nitric acid solution (7.6 mol/L) was slowly poured into it in order to ensure that the ratio of nitric acid and steel slag was 30 mol/kg. The steel slag was kept acidolysis for 25 min at 25 °C with continuous stirring. The acidolysis solution was obtained by filtration using Buchner funnel.
The hydrotalcites based on steel slag was prepared by coprecipitation. NaOH and Na 2 CO 3 at the molar ratio of 16:1 were dissolved in deionized water to prepare the alkaline solution. About 50 mL deionized water was added into a 250 mL three-neck flask and remained 70 °C by heating with stirring evenly. The acidolysis solution and the alkali solution were mixed by the double-titration coprecipitation method. The pH value of the solution was kept 10-11 during the titration. The white turbid liquid was stirred for 1 h at 70 °C after titration and then aged in the thermostatic waterbath of 70 °C for 12 h. The steel slag based hydrotalcites S-CaMgAl-CO 3 LDH can be obtained after filtering, washed and drying in the vacuum oven at 70 °C for 12 h. The steel slag based mixed oxides S-CaMgAl MO were prepared via calcination of hydrotalcite samples in muffle furnace at different temperatures for 6 h. Characterization methods. The crystallite sizes and morphologies of solid catalyst were characterized by X-ray diffraction (XRD) on Rigaku D/max2550VB/PC with CuKα radiation (λ = 0.154 nm, 100 mA, 45 kV). The scanning range was 0°-80° (2θ) with a rate of 0.02°/s. Surface elements were measured by Edax Falcon energy dispersive spectrometer (EDS). The pore size distribution and surface area of catalysts were obtained by Brunauer-Emmett-Teller (BET) method on Micromeritrics ASAP-2400. The infrared absorption spectra of the samples were determined by Fourier transform infrared spectroscopy (FT-IR) by KBr pellet. S-3400N scanning electron microscope (SEM) was used to observe the surface structure and shape of samples. The basicity

Results and discussion
Effects of calcination temperature on catalyst basicity and transesterification. S-CaMgAl MO prepared by S95 steel slag can be used as solid base catalysts since the steel slag contained a large number of metal elements. The esterification reaction of dimethyl carbonate and glycerol was catalyzed by S-CaMgAl MO calcined at 100 °C 350 °C, 600 °C and 850 °C respectively when the molar ratio of DMC and glycerol was 3:1, the catalyst dosage was 3 wt%, the reaction temperature was 75 °C and the reaction time was 90 min. The results were shown in Table 2. The basic strength and amount of S-CaMgAl MO were also measured in order to analyze the relationship between the catalytic effect and the basicity of catalysts. The basic amount of S-CaMgAl MO calcined at 100 °C, 350 °C, 600 °C and 850 °C was 0.87 mmol/g, 0.94 mmol/g, 0.99 mmol/g and 1.07 mmol/g respectively. The catalyst basicity and the conversion of glycerol increased with the increase of calcination temperature. However, the yield of glycerol carbonate decreased from 96.2 to 93.3% and TON reduced from 351 to 317 when the calcination temperature of catalysts changed from 600 to 850 °C due to the further formation of glycidol in the strong alkaline environment. The enhanced basicity was favorable to the conjugation of carboxylic acid ions. The gained stability of COO − led to the decarboxylation of glycerol carbonate. The metal cations were filled in the appropriate gaps of O 2− densely packed according to their ion radius to form the special structures of mixed oxides, which was obviously different from the mixture of metal oxides. The preparation of mixed oxides was related to the calcination temperature closely. 600 °C was the optimal temperature for the catalyst synthesis in this experiment and its catalytic performance was also the best. Therefore, S-CaMgAl MO calcined at 600 °C was chosen for further experiments. Figure 1 showed the X-ray diffraction patterns of S-CaMgAl MO samples calcined at different temperatures. The hydrotalcites characteristic symmetrical reflection of (003), FT-IR analysis. The FT-IR spectra of S-CaMgAl MO samples calcined at different temperatures were displayed in Fig. 2. The stretching vibration peaks near 700 cm −1 were caused by Ca-O, Mg-O and Al-O. The peaks observed at low wavenumbers (between 600 and 400 cm −1 ) were corresponded to metal hydroxides Ca(OH) 2 , Mg(OH) 2 and Al(OH) 3 when the calcination temperature was 100 °C. The characteristic peak of carbonates was at 1,440 cm −1 . The absorption peak at 3,470 cm −1 was ascribed to O-H group. The stretching vibration peaks of  3 2− in the layers of hydrotalcites by calcination. The absorption peaks of hydroxyl and carbonate ions disappeared completely when the calcination temperature was 600 °C and 850 °C, which indicated that the hydrotalcite material was completely transformed into mixed metal oxides at this point. Figure 4 showed the CO 2 -TPD profile of S-CaMgAl MO samples. Generally, the desorption peak of CO 2 at lower than 200 °C, 300 °C to 450 °C and higher than 450 °C match weak, moderate and strong basic sites respectively 32   www.nature.com/scientificreports/ bond. This peak was broad since there were several metal oxides in the catalysts. The CO 2 desorption peak around 600 °C was ascribed to the strong alkaline Center O 2− . The desorption peak shifted towards high temperature gradually with the increase of calcination temperature in Fig. 3, indicating that the basicity of S-CaMgAl MO was also enhanced. Metallicity is the ability of releasing electrons for an atom in chemical reactions. The strong metallicity means the strong electron-loss ability and the strong alkalinity of corresponding alkali. The metallicity of Ca is stronger than magnesium, thus the alkalinity of mixed oxides can be improved by partially substituting calcium for magnesium.

CO 2 -TPD analysis.
The formation of O 2− can be attributed to the special structures of hydrotalcites and the mixed oxides. The metal elements and oxygen elements were closely packed in the oxide lattices of general oxides regularly. However, part of Mg 2+ were replaced by Al 3+ in the preparation process of mixed oxides. In other words, the original space of three Mg 2+ was occupied by two Al 3+ in the oxide frame in order to achieve the charge conservation. The lattice disorder due to the dislocation of cations resulted in serious Schottky Defects. Mg 2+ on the catalyst surface moved inside to fill the inevitably generated cation vacancy of in lattices under the thermal movement and Coulomb force, which led to the partial unsaturated adjacent oxygen elements on the surface and the strong alkaline O 2− further 33 .
Element analysis. The surface elements of S-CaMgAl-CO 3 LDH and S-CaMgAl MO were analyzed in Table 3. The mass fraction of Ca, Mg, Al and O in S-CaMgAl-CO 3 LDH was 39.38%, 11.86%, 13.51% and 32.61% respectively. The molar ratio of divalent metal and trivalent metal was between 2 and 4, indicating that the prepared S-CaMgAl-CO 3 LDH was stable 34,35 . Moreover, the proportion of divalent metal ions and trivalent metal ions on S-CaMgAl MO was similar to S-CaMgAl-CO 3 LDH, which meant there was little loss of metal elements in the synthesis of the mixed oxides and the obtained catalyst met the expectation.
SEM analysis. The morphology of hydrotalcite and mixed oxide calcined at different temperatures were analyzed by Scanning Electronic Microscopy (SEM). As shown in Fig. 4, The hydrotalcite sample S-CaMgAl-CO 3 LDH displayed obvious characteristics of hollow "petals" layered structures. The layered structure of hydrotalcites disappeared gradually after calcination and the surface of corresponding mixed oxide S-CaMgAl MO presented abundant pore structures.   Table 4 and Fig. 5. The specific surface area of S-CaMgAl MO decreased due to the destruction of original layered structures in the hydrotalcite material S-CaMgAl-CO 3 LDH and the removal of OH − and CO 3 2− anions in the interlayer during the calcination process. However, the mesoporous mixed oxide samples presented larger pore size than hydrotalcites. In addition, S-CaMgAl-CO 3 LDH exhibited a Type IV isotherm with H3 hysteresis loop due to the typical layered structures of hydrotalcites while the N 2 adsorption-desorption isotherms of S-CaMgAl MO belonged to Type IV isotherm with H2 hysteresis loop which was characteristic isotherm of mesoporous materials with irregular pore structure 36 .

Effects of reaction conditions on transesterification between DMC and glycerol.
In the transesterification of glycerol and DMC, the basic active sites especially the highly alkaline O 2− and Ca-O in the mixed metal oxides split the hydrogen and oxygen of hydroxyl on the glycerol molecules to form the nucleophilic intermediate alkoxy. Then the nucleophilic alkoxy group attacked the carbonyl carbon of DMC thus resulted in the formation of hydroxyalkyl carbonate and methoxy anion. Further molecular reactions generated the target product GC and two methanol molecules as by-products. Finally, GC was separated from the metal sites of the catalyst then the catalyst was used in the next reaction cycle. The optimal reaction conditions were investigated in order to achieve the highest GC yield in this section.
Effects of catalyst dosage on glycerol conversion and GC yield. The influence of catalyst dosage on the transesterification was investigation under the conditions that the molar ratio of DMC and glycerol was 3:1, the reac-   Fig. 6. The number of basic active sites in the reaction system which was necessary for the glycerol deprotonation and subsequent reaction with DMC to produce glycerol carbonate increased with the increase of catalyst dosage. Thus the conversion of glycerol varied from 76.9 to 98.3% and the GC yield increased from 73.7 to 96.2% when the catalyst dosage rose from 1 to 3%. The conversion of glycerol increased not obviously when the catalyst dosage mounted up continuously because the particle agglomeration caused by excessive catalyst hindered the mass transfer from catalyst body to the active site and prevented the further conversion of glycerol. Moreover, the extra catalytic active sites provided places for the generation of glycidol via decarbonylation of GC, thus the yield of GC was reduced. Figure 7 showed the effect of DMC-toglycerol molar ratio on glycerol conversion and GC yield when the catalyst dosage, reaction temperature and reaction time was constant. The conversion rate of glycerol and the yield of GC are both 41.0% at 1:1 molar ratio of DMC to glycerol. The excessive DMC helped the chemical equilibrium shift to the direction of GC synthesis since the transesterification of glycerol and DMC was reversible. The conversion of glycerol and the yield of GC were 98.3% and 96.2% respectively when the mole ratio of DMC and glycerol rised to 3:1. However, there was no significant change in the glycerol conversion rate and the GC yield when the molar ratio of DMC and glycerol continued to mounted up. Thus the optimal molar ratio of DMC and glycerol was 3:1 considering the conversion of glycerol, the yield of GC and the cost of the reaction.

Effects of reactant molar ratio on glycerol conversion and GC yield.
Effects of reaction temperature on glycerol conversion and GC yield. The effect of reaction temperature ranging from 60 to 80 °C on glycerol conversion and GC yield was displayed in Fig. 8 when the molar ratio of DMC and glycerol was 3:1, the catalyst dosage was 3 wt% and the reaction time was 90 min. The conversion of glycerol increased from 53.3 to 98.3% and the yield of GC rose from 51.4 to 96.2% with the change of reaction temperature from 60 to 75 °C. The molecular velocity and the number of effective collisions between reactant and catalyst increased with the increase of reaction temperature. Thus the glycerol conversion and GC yield increased. There www.nature.com/scientificreports/ was no significant change in glycerol conversion and GC yield when the reaction temperature continued to rise above 75 °C because the transesterification reached equilibrium basically around this temperature. The viscosity of reactants was no longer the main factor that hindered the shift of chemical equilibrium to the direction of GC synthesis when the reaction temperature was above 75 °C 37 . Meanwhile, side reactions such as the decarbonylation of GC occurred with the rise of reaction temperature. Therefore, 75 °C was chosen as the optimal reaction temperature.
Effects of reaction time on glycerol conversion and GC yield. Figure 9 presented the influence of reaction time on glycerol conversion and GC yield with 3 wt% catalyst dosage under 75 °C at 3:1 molar ratio of DMC and glycerol. The conversion rate of glycerol and the yield of GC were both at a low level at 30 min. The glycerol was com- www.nature.com/scientificreports/ pletely converted to GC during this period. The glycerol conversion and GC yield increased significantly from 30 to 90 min. The maximum glycerol conversion rate and GC yield were 98.3% and 96.2% respectively at 90 min. The conversion of glycerol increased while the GC yield decreased slightly when the reaction time mounted up continuously. The optimal reaction condition for the transesterification of glycerol and DMC catalyzed by mixed oxide based on steel slag was 75 °C, 90 min and 3 wt% catalyst dosage from the above experimental results. The catalytic performance of different catalysts was listed in Table 5. The product mixture obtained from the catalytic reaction was analyzed by ICP-OES to investigated the leaching of metal ions in S-CaMgAl MO. Meanwhile, CaO was used to catalyze the transesterification under the same conditions for comparison. The experimental results showed that when the catalyst was CaO, the concentration of calcium ion in the product mixture was 4.6 mg/L while no metal ion was detected when the reaction was catalyzed by S-CaMgAl MO in Table 6.

Reusability and stability test of S-CaMgAl
The reusability and regeneration experiment of S-CaMgAl MO was carried out under the condition of 3 wt% catalyst dosage, 3:1 molar ratio of DMC to glycerol, 75 °C reaction temperature and 90 min reaction time. The catalyst was washed with methanol and dried after each reaction cycle. The five times reused catalyst was calcined at 600 °C for 6 h and then used in the transesterification to test the regenerability of S-CaMgAl MO. Figure 10 showed that S-CaMgAl MO remained 91.9% glycerol conversion and 90.5% GC yield in the fifth cycle. The regeneration catalyst retained 96.9% glycerol conversion and 95.0% GC yield. All indicated excellent reused and regeneration performance of S-CaMgAl MO.
The XRD patterns of regeneration catalyst and fresh catalyst were contrasted as shown in Fig. 11 to investigate the stability of the catalyst. The results demonstrated that the strength of characteristic peaks corresponded to CaO, MgO and Al 2 O 3 in the XRD pattern of regeneration catalyst was not weakened significantly compared with the fresh catalyst, which proved that the catalyst had high resistance to metal ion leaching in this reaction system. The strong stability of the catalyst was attributed to the intact microcrystalline structure derived from S-CaMgAl-CO 3 LDH. The FT-IR spectra of fresh catalyst and five times reused catalyst were also compared in Fig. 12. The wide absorption bands near 3,500 cm −1 and 1,500 cm −1 caused by hydroxyl and ester group were observed in the spectra of five times reused catalyst, which confirmed the adhesion of organics on the surface and pores of five times reused catalyst.
The surface area of catalyst before and after use was displayed in Table 7. The adhesion of organics resulted in the decreased specific surface area and the worse catalytic activity of S-CaMgAl MO. However, the specific surface area of regeneration catalyst changed little from that of fresh catalyst and the catalytic performance became better after calcination. Hence the deactivated catalyst caused by organics attachment can be restored via calcining at a suitable temperature.
The surface elements analysis of S-CaMgAl MO under different treatment conditions was analyzed. Table 8 showed the carbon element of the catalyst increased by about 5.44 wt% and the Ca element decreased by about 5.50 wt% after five times reuse compared with the fresh catalyst, while there was no striking difference of element contents between the regeneration catalyst and the fresh catalyst. The results indicated that the fall of calcium content was ascribed to the organics adhered to the surface of catalyst after repeated experiments but not the leaching of active sites.
CaO with fine catalytic activity would generate little active Ca(OH) 2 and inactive CaCO 3 in case of contacting with air. Pure CaO would also be destroyed and converted to ionic state if there was water and acid in the reaction system, which not only reduced the catalyst activity, but also increased the difficulty of catalyst recovery. The novel catalyst combined CaO with MgO and Al 2 O 3 was prepared in order to prevent the loss of active sites in this paper. The calcium-based catalysts would get deteriorated inevitably after prolonged exposure to the air because of the reaction with water vapor and carbon dioxide. However, the mixed oxide after absorbing certain water or carbon dioxide can be calcined under a suitable temperature to restore the original crystal structures, which was also known as the memory effect of hydrotalcites. Hence the mixed oxide was considered to have resistance to  Table 6. ICP-OES analysis of the product mixture.  Intensity(a.u.)

Fresh catalyst
Regeneration catalyst 2θ(°) Figure 11. X-ray diffraction patterns of S-CaMgAl MO before and after five times reuse.
Scientific RepoRtS | (2020) 10:10273 | https://doi.org/10.1038/s41598-020-67357-z www.nature.com/scientificreports/ elements. The results showed that the highest yield of GC reached 96.2% and the conversion of glycerol was 98.3% under the condition of 3 wt% catalyst, 3:1 molar ratio of DMC to glycerol, 75 °C reaction temperature and 90 min reaction time. Meanwhile, the crystal structure derived from S-CaMgAl-CO 3 LDH enhanced the anti-leaching ability of active sites in the catalyst. S-CaMgAl MO achieved 91.9% glycerol conversion and 90.5% GC yield in the fifth reaction cycles in spite of inevitable deactivation. The conversion rate of glycerol and GC yield still reached 96.9% and 95.0% respectively using the regeneration catalyst. Therefore, S-CaMgAl MO has significant industrial application potential in GC synthesis due to the high catalytic activity, good stability, mild reaction conditions and low production cost.