XRD patterns of the MAL trimetallic oxide catalysts with different molar ratios are shown in Figure 1. All the three XRD patterns show major reflections related to individual metal oxides such as MgO, Al2O3 and La2O3. The peaks at 23.5, 34.6, 44.3 and 56.5° correspond to Al2O3. Cubic form of MgO is responsible for the peaks at 36.8, 43.03 and 62.3º. Peaks at 22.1, 31.3 and 38.7° confirm the presence of La2O3 in MAL catalysts. Along with these peaks the XRD patterns also show peaks related to bimetallic mixed oxides and trimetallic mixed oxides [21, 22].
XRD patterns of MAL catalysts show peaks at 18.9, 31.1 and 47.5° corresponding to cubic form of MgAl2O4 mixed metal oxide and prominent peaks at 27.2, and 48.8° correspond to La2MgOx mixed metal oxide. Also, there is a peak around 32.0° which is attributed to trimetallic mixed oxide La4Al2MgO10 (JCPDS No. 43-0922).
FT-IR spectra of the MAL catalysts are shown in Figure 2. All the bands are similar to bands exhibited by hydrotalcite-like phase with CO3-2 as the counter anion [23]. Typical of this spectrum is the strong and broad absorption band at 3800–2700 cm-1 associated with the superposition of the hydroxyl stretching band arising from metal hydroxyl groups and hydrogen bonded interlayer water molecules [24]. Another absorption band corresponding to H2O deformation mode is recorded at 1650 cm-1. The band observed at 1450 cm-1 is due to the v3(asymmetric stretching) of the CO3-2 ion in the interlayer. The bands observed in the low-frequency 400-1000 cm-1 region of the spectra can be interpreted as the lattice vibration modes attributed to M-O and M-OH vibrations [23]. This lowering of symmetry is probably due to the mono and bidentate CO3-2 anions located in the interlayer region [25, 26].
CO2-TPD measurements were carried out for Mg-Al-La catalysts to know their total basic strength. The complex desorption profiles shown in Figure 3 clearly reflect the large heterogeneity of the sites. Therefore, the CO2 desorption profile is divided, after de-convolution, into sites of different basic strengths: a low temperature peak, with its maximum desorption at temperature between 120-250 ºC attributed to interaction with sites having weak basic strength present in the catalyst. Peaks appearing between 250-500 ºC, are related to desorption of CO2 from sites having medium basic strength, which are also reported for hydrotalcites from CO2-TPD measurements [27]. Desorption area covering the temperature range from 550-800 ºC, is attributed to CO2 desorbed from sites with strong basic strength.
Densities of different types of basic sites in MAL catalysts with different molar ratios of metals are shown in Table 1. From the data shown in Table 1 total number of basic sites in catalyst MAL-3:1:1 is almost double to the basic sites in catalyst MAL-2:1:1. So it shows very good activity towards base catalysed transesterification of glycerol with DMC. Catalyst MAL-1:1:1 has less number of strong basic sites as it has a small peak around 550 ºC. Thus, among the three catalysts MAL-3:1:1 is the best catalyst.
Table 1. Densities of basic sites in MAL catalysts.
S.No.
|
Catalyst
|
Density of basic sites (mmol/g)
|
Wb
|
Mb
|
Sb
|
Tb
|
1
|
MAL-1:1:1
|
0.00
|
0.00
|
0.119
|
0.119
|
2
|
MAL-2:1:1
|
0.00
|
0.126
|
0.254
|
0.380
|
3
|
MAL-3:1:1
|
0.00
|
0.245
|
0.450
|
0.695
|
Wb = Density of weak basic sites, Mb = Density of medium basic sites,
Sb = Density of strong basic sites, Tb = Density of total basic sites.
|
BET surface areas of MAL catalysts with different molar rations are shown in Table 2. Surface area of the MAL catalyst gradually decreases with increasing Mg content in the catalysts. When compared with the surface area of the monometallic oxide Al2O3 (130 m2/g) the surface area of MgO (9 m2/g) is considerably low. MAL-3:1:1) catalyst has greater Mg composition than MAL-1:1:1. As the percentage of Al decreases from MAL-3:1:1 to MAL-1:1:1 the surface area of MAL catalysts is seen to decrease. MAZ-3:1:1 shows good activity among the three catalysts even though it has lower surface area. These results suggest that there is no direct relation between surface area and activity of the catalyst, as reported in the case of other catalysts.
Table 2. BET Surface area values of MAL catalysts with different molar ratio
S.No.
|
Catalyst
|
BET surface area (m2/g)
|
1
|
MAL-1:1:1
|
10
|
2
|
MAL-2:1:1
|
8
|
3
|
MAL-3:1:1
|
5
|
Catalytic activity
Glycerol carbonate can be prepared by solid base catalyst transesterification reaction in which dimethyl carbonate and glycerol are used. Here, we employed dimethyl carbonate as the solvent as well as the substrate for the development of solvent-free system. To evaluate the feasibility of dimethyl carbonate as the solvent, various MA (3:1), ML (3:1) and MAL (l:1:1, 2:1:1, 3:1:1) were tested for their abilities to catalysts the synthesis of glycerol, carbonate from glycerol in dimethyl carbonate. Among the tested catalysts, MAL (3:1:1) exhibited the highest conversion, while other catalysts had little activity (Table 3).
Table 3. Glycerol carbonate yield using different catalysts.
S. No
|
Catalyst
|
Glycerol carbonate yield (%)
|
1
|
Mg/Al/La (1:1:1)
|
52
|
2
|
Mg/Al/La (2:1:1)
|
87
|
3
|
Mg/Al/La (3:1:1)
|
96.91
|
4
|
Mg/Al (3:1)
|
10.85
|
5
|
Mg/ La (3:1)
|
14.05
|
6
|
Blank
|
0.71
|
2 g (0.0217 mol) of Glycerol, 9.76 g (0.1085 mol) of Dimethyl carbonates reaction time 90 min.
The high conversion within the short reaction time is due to the presence of a large number of medium and strong basic sites in the MAL-3 catalyst. Generally, the process of reactions is affected by the mode of the reaction condition, molar ratio, reaction time, amount of catalyst and temperature.
Effect of calcination temperature on the catalyst activity
Among the MAL catalysts, the catalyst with metal molar ratio MAL (3:1:1) showed the highest activity for the synthesis of GC. This catalyst was further studied to know more about the surface-structural properties and their relation with catalyst activity for transesterification of glycerol. MAL (3:1:1) catalyst was treated at different calcination temperatures in the range of 450 to 750 ºC because it is known that the acidic and basic properties of the mixed metal oxide vary with the change in calcination temperature, apart from its composition [28]. MAL (3:1:1) catalyst calcined at different temperatures was studied for the transesterification of glycerol and the results are presented in Table 4. As can be seen the catalytic activity increases with increase in calcination temperature up to 650 ºC and further increase in temperature results in low activity. Catalyst calcined at 650 ºC shows the highest activity among all the catalysts. In order to understand the variation in activity with change in calcination temperature these catalysts were further characterized in detail.
Table 4. Glycerol carbonate yield using different calcination temperature MAL-3:1:1 catalyst.
S. No
|
Catalyst
|
Calcination temperature (oC)
|
Glycerol carbonate yield (%)
|
1
|
Mg/Al/La (3:1:1)
|
Uncalcined
|
12.07
|
2
|
Mg/Al/La (3:1:1)
|
450
|
58
|
3
|
Mg/Al/La (3:1:1)
|
550
|
87
|
4
|
Mg/Al/La (3:1:1)
|
650
|
96.91
|
5
|
Mg/Al/La (3:1:1)
|
750
|
48
|
2 g (0.0217 mol) of Glycerol, 9.76 g (0.1085 mol) of Dimethyl carbonates reaction time 90 min.
XRD patterns of the MAL-3:1:1 catalyst calcined at different temperatures are shown in Figure 4. XRD patterns suggest well crystalline nature of the catalysts. XRD patterns mainly show the presence of MgO, Al2O3, La2O3, MgAl2O4, La2MgOx and La4Al2MgO10 crystalline phases. Catalyst calcined at 750 ºC show the presence of more number of mixed metal oxides as it has more peaks which are not in catalysts calcined at 450, 550 and 650 ºC. This is responsible for decreasing catalytic activity of catalyst calcined at 750 ºC.
CO2-TPD patterns of the MAL-3:1:1 Catalysts calcined at different temperatures are shown in Figure 5 and the densities of different types basic sites are shown Table 5. Data in table suggest that the catalyst calcined at 450 ºC has fewer number of medium basic sites and some amount of strong basic sites. The catalyst calcined at 550 ºC has all the three types of basic sites with total density of 0.334 mmol/g. Catalyst calcined at 650 ºC shows a broad peak in the temperature range 550-700 ºC which corresponds to strong basic sites. It also has a peak around 400 ºC which is related to medium basic sites. The amount of total basicity is more for the catalyst calcined at 650 ºC. It shows very good catalytic activity. Catalyst calcined at 750 ºC shows different TPD pattern as it exhibits a small desorption peak at 350 ºC. This peak can be attributed to the moderate basic sites present in the catalyst and it has least number of basic sites, hence this catalyst showed poor activity.
Table 5. Densities of basic sites of MAL catalyst calcined at different temperatures.
S.No.
|
Catalyst
|
Calcination Temperature (°C)
|
Density of basic sites (mmol/g)
|
Wb
|
Mb
|
Sb
|
Tb
|
1
|
MAL-3:1:1
|
450
|
0.00
|
0.010
|
0.230
|
0.240
|
2
|
MAL-3:1:1
|
550
|
0.127
|
0.113
|
0.094
|
0.334
|
3
|
MAL-3:1:1
|
650
|
0.00
|
0.245
|
0.450
|
0.695
|
4
|
MAL-3:1:1
|
750
|
0.00
|
0.115
|
0.015
|
0.130
|
Wb = Density of weak basic sites; Mb = Density of medium basic sites;
Sb = density of strong basic sites; Tb = Density of total basic sites
|
Effect of the reaction temperature
The influence of reaction temperature was studied and the results are shown in Figure 6. The Transesterification reactions were carried out at 55, 65, 75 and 85 ºC. The initial formation rate of glycerol carbonate yield was determined to be 38%, 64%, 96.91% and 98.5%, respectively. Although the catalytic activity of MAL-3 catalyst increased with increase in reaction temperature above 75 ºC no much increase in yield with further increase in temperature. The catalyst showed maximum activity at a reaction temperature of 75 ºC. The high activity of the present catalyst even at low temperature suggests that the MAL-3 catalysts are highly basic and more active.
Effect of the molar ratio of glycerol/dimethyl carbonate
Mole ratio of glycerol to DMC has a significant influence on the production of GC. As shown in Figure 7, when equimolar amounts (1:1) of glycerol and DMC are used low of GC yield (42%) is obtained. However, as the moles of DMC increases yield also increases and reaches 97 % when the mole ratio is 1:5. After that the yield has decreased with increasing moles of DMC. DMC can act as solvent in this transesterification reaction. Excess of solvent causes decrease in glycerol carbonate yield. Thus, the optimum mole ratio is 1:5.
Effect of catalyst loading
The effect of catalyst loading on the transesterification reactions has also been studied using different amount of MAL-3(0.1, 0.2, 0.3 and 0.4g). The results displayed in Figure 8 show that the maximum glycerol conversion (96.9%) has been achieved in presence of 0.2 g MAL-3 whereas in presence of 0.1 g catalyst the conversion is up to 86%. When the catalyst amount increased from 0.1 to 0.2 g, maximum glycerol conversion was observed. Thus, it is clear that the increase in catalyst loading above 0.2 g did not help to improve the initial rate of the reaction. The results suggest that a small amount catalyst is sufficient to attain maximum conversion.
Effect of reaction time
Synthesis of GC from glycerol and DMC by transesterification reaction over MAL-3:1:1 catalyst was carried out with different reaction times and the change in the GC yield with reaction time are presented in Figure 9. These results suggest that the GC yield increases with increasing time initially and it reaches a maximum after 90 min. Beyond 90 min there is a small increase in GC yield. The reaction was carried out from 30 min to 120 min with 30 min intervals. At 90 min 97 % GC yield is obtained after that the increase is only about 1 % upto 150 min. So there no considerable change in yield beyond 90 min. Hence 90 min is the optimum reaction time for transesterification reaction over MAL-3:1:1 catalyst.
catalyst recycles
GC synthesis using the MAL-3:1:1 catalyst was performed to establish its reusable property. The efficacy of the catalyst was tested upto five cycles and the results are shown the Figure 10. The procedure adopted for this study was: filtration of the reaction mixture and separation of catalyst; washing with 100 ml of methanol and drying at 120 º C temperature over night. These results reveal that the activity of the catalyst gradually decreased. For the second cycle the yield dropped by 20 % (from 97 % to 77 %). Fourth time the yield was only about 50 %. Catalyst lost its activity in recycling.