3.8.1. Synthesis of 14-aryl-14H-dibenzoxanthene
The preparation of 14-aryl-14H-dibenzoxanthene occurred via the reaction of 2-napthol and benzaldehyde in presence of 50 mg of the activated catalyst at 120 oC for 90 min.
Reaction Conditions: Several effects are studied on the catalyst 20S-10W/NF-MCM-41-II for testing the best reaction conditions. The best molar ratio of 2-naphthol: benzaldehyde is found to be 2:1 as it yields 93.2% of the product. 1:1, 3:1 and 4:1 molar ratios give 60.4, 85.2, and 68.3% of the product, respectively. By changing the time of the reaction from 30, 60, 90 to 120 min, the %yield of the product changes from 60.5, 77.2, 93.2, and 92.9%, respectively. So, 90 min is selected as the best reaction time. Also, by testing the reaction using different weights of 20S-10W/NF-MCM-41-II catalyst as 30, 50, 70, and 100 mg. The best yield (93.2%) is obtained by the weight of 50 mg while 30, 70 and 100 mg give yields of 69.5, 93.0, and 92.9%, respectively.
Effect of sulfate content
As investigated in Table 1, By using MCM-41 as a catalyst for the formation of 14-aryl-14H-dibenzoxanthene, the yield is only about 6.7%. While by using NF- MCM-41 as a catalyst the yield reaches 40.7% due to the presence of Lewis acid sites. Loading 10wt.% PWA increases the yield to 49.9% due to the enhancement of both Lewis and Brønsted acid sites. By loading sulfate on 10W/NF-MCM-41 and increasing its content from 5 to 20 wt.%, the yield increases to 93.9% then decreases by increasing the sulfate content above 20wt.%. This is in good agreement with the results of B/L ratio, total acidity, and Ei as illustrated in Fig. 9A. The synthesis reaction of 14-Aryl-14H-dibenzo[a,j] xanthene is found to be catalyzed by Brønsted and/or Lewis acid sites.
Effect of calcination temperatures
The effect of calcination temperature on the synthesis of xanthene for 20S-10W/NF-MCM-41 catalyst is shown in Table 1 and Fig. 9B. By raising the calcination temperatures from 400 to 450 oC, the yield increases from 89.2–93.9%, respectively, then decreases again to 91.6% by raising the temperature to 550 oC in a good relationship with the Ei, the total number of acid sites and the B/L ratio.
Effect of different supports
By comparing the results in Table 1, we found that 20S/MCM-41-II catalyst yields 69.2% of xanthene. This percentage increases to 81.7, 88.5, and 93.9% in the case of using 20S/NF-MCM-41-II, 20S-10W/MCM-41-II, and 20S-10W/NF-MCM-41-II catalysts, respectively. This reflects the role of the support in enhancing the catalytic activity as well as the acidity.
The reaction Mechanism
The mechanism for the formation of 14-aryl-14H-dibenzoxanthene using 20S-10W/NF-MCM-41 MNP as shown in Scheme 2 can be explained via the activation of the benzaldehyde carbonyl group by the acid sites of the catalyst resulting in a fast nucleophilic attack of 2-naphthol and synthesis of a carbocation. Then the carbocation converts to an intermediate which finally converts to 14-aryl-14H-dibenzoxanthene via the cyclization and dehydration step [43].
Comparison studies
To reveal the good effects of S-W/NF-MCM-41 MNPs, the comparison of the xanthene yields obtained from our results are examined with previously reported using other catalysts and tabled in Table 2. According to the comparison, the prepared catalyst in our work reflects a good percent of the xanthene yield in comparison with other catalysts. Many advantages can distinguish our catalyst from the others as ease of separation from the reaction medium, low reaction time, reusing, and the high purity of the yield.
Table 1
Comparison between different catalysts in the formation of14-aryl-14H-dibenzoxanthene.
Catalyst
|
Yield
(%)
|
Time
(min)
|
Temperature
(◦C)
|
Reference
|
Polytungstozincate acid
Cellulose sulfuric acid
HO3S-SNPs
SO3H-functionalized
N-sulfonic acid PVP Cl
Silica sulfuric acid
S-W/NF-MCM-41
|
81
87
92
90
80
89
94
|
80
120
45
60
30
45
90
|
60
110
110
80
100
80
120
|
[44]
[45]
[46]
[47]
[48]
[49]
Our work
|
Reusability
After completing the reaction, the 20S-10W/NF-MCM-41-II catalyst was separated easily by a magnet from the reaction medium and washed well with ethyl alcohol, then dried in an oven at 100 oC for 2 h and reused in other reactions. It can be reused at least three runs without a high loss in the catalytic activity as seen in Fig. 10.
3.8.2. Synthesis of 7-hydroxy-4-methyl coumarin:
7-hydroxy-4-methyl coumarin can be formed easily from the reaction between 1 mmol resorcinol and 2 mmol ethyl acetoacetate in presence of 100 mg of the activated catalyst at 120 oC for 2 h.
The reaction conditions: The best reaction conditions are examined by studying the effects of some parameters on the reaction using the 20S-10W/NF-MCM-41-II catalyst. By studying the effect of different molar ratios of resorcinol and ethyl acetoacetate; 1:1, 1:1.5, and 1:2; the % yield is found to increase from 45.6 to 64.3, 78.3%, respectively, then decrease to 75.2% at 1:3 molar ratio due to blocking of the catalyst acid sites as a result of the surface saturation by ethyl acetoacetate [19]. So, we used resorcinol: ethyl acetoacetate with a 1:2 molar ratio in the synthesis of 7-hydroxy-4-methyl coumarin. In addition, the effect of different weights of the catalyst is studied and 100 mg is taken as the best catalyst weight for the reaction as it gives the best yield of 78.3% compared with 30, 50, and 70 mg which yield 46.2, 59.2 and 70.6%, respectively. On another hand by studying the effect of the reaction time on the reaction yield, it is found that the yield increases gradually with increasing the reaction time from 30 min to 120 min every 30 min then becomes constant after 120 min. So, 120 min is taken as the best reaction time for the coumarin synthesis. Finally, we studied the effect of different temperatures, 80, 100, 120, and 140 oC, on the reaction and 120 oC gives the best results so it is used as the temperature for the reaction.
Effect of sulfate content: By investigating Table 1, we can notice that no coumarin yielded by using MCM-41 or NF-MCM-41 but by using 10W/NF-MCM-41, the yield reaches 30.39%. This reflects the role of Brønsted acid sites of PWA in the coumarin synthesis and indicates that the mechanism of the reaction is dependent on the Brønsted acid sites and explains the absence of the yield in the case of using MCM-41 or NF-MCM-41 as catalysts because they have Lewis acid sites [39, 50]. By loading sulfate on 10W/NF-MCM-41 and increasing its content from 5 to 20 wt.%, the yield increases to 78.32% then decreases by increasing the sulfate content above 20wt.%. This is in good agreement with the results of the B/L ratio, total acidity, and Ei as illustrated in Table 1 and Fig. 11A.
Effect of calcination temperatures
The effect of calcination temperature on the coumarin synthesis for 20S-10W/NF-MCM-41 catalyst is illustrated in Fig. 11B and Table 1. By raising the calcination temperatures from 400 to 450 oC, the yield increases from 73.23–78.32%, respectively, then decreases again to 75.94% by raising the temperature to 550 oC in a good relationship with Ei, total number of acid sites and B/L ratio.
Effect of different supports
By comparing the results in Table 1, we found that 20S/MCM-41-II catalyst yields 60.47% of coumarin. This percentage increases to 64.67, 75.19, and 78.32% in the case of using 20S/NF-MCM-41-II, 20S-10W/MCM-41-II, and 20S-10W/NF-MCM-41-II catalysts, respectively. This reflects the role of the support in enhancing the catalytic activity as well as the acidity.
The reaction mechanism: The suggested mechanism of the synthesis of 7-hydroxy-4-methyl coumarin in presence of S-W/NF-MCM-41 MNPs can be summarized in three steps: 1) The protonation of the carbonyl group of EAA is activated by the H+ of Brønsted acid sites, 2) This enhances the nucleophilic-attack of the hydroxyl group of resorcinol causes a formation of an intermediate, 3) The intermediate is converted to the 7-hydroxy-4-methyl coumarin through cyclization via intermolecular-condensation as illustrated in Scheme 3.
The comparison studies
We compared our catalyst towards the formation of 7-hydroxy-4-methyl coumarin with that previously reported using other catalysts as revealed in Table 3. The prepared catalyst in our work reflects a good percent of the coumarin yield in comparison with other catalysts. Many advantages can distinguish our catalyst from the others as ease of separation from the reaction medium, low reaction time, reusing, and the high purity of the yield.
Table 3
Comparison between different catalysts in the formation of 7-hydroxy-4-methyl coumarin.
Catalyst
|
Time
(h)
|
Yield
(%)
|
Reference
|
Acidic ionic liquid
SA-MIL-101
PMA/ Cr-Mg-MOF
H-Beta zeolite
S-W/NF-MCM-41
|
2
2
2
4
2
|
75
73
69
72
78
|
[51]
[52]
[39]
[53]
This work
|
Reusability of catalyst
Recycling the catalyst is an important test for any industrial process. After completing the reaction, the catalyst was separated from the reaction mixture by a magnet and washed well with ethyl alcohol, then dried in an oven at 100 oC for 2 h and reused in other reactions. There is a slight loss in the activity of the 20S-10W/NF-MCM-41-II catalyst after three times as shown in Fig. 12.