CERTIFICATE
Scheme 4.9: Rate equation for the 4.22a catalyzed ethanol upgradations to n-butanol
4.3.3 Catalytic transformation of ethanol to biobutanol under microwave condition The studies were initiated by repeating the best results reported in the above discussion, where,
in the presence of 10 mol % NaOEt, 4.22a (0.025 mol %) catalyzed the Guerbet reaction under conventional heating at 140 C to yield an ethanol conversion of 58% (ca. 2100) at an initial rate of 710 TO/h (entry 1, Table 4.7 and 4.9). Lowering the temperature of conventional heating reactions gave poorer results (entry 2-4, Table 4.9). Considering the good microwave absorptivity of ethanol, it would be interesting to probe the reactivity of 4.22a towards Guerbet reaction under microwave heating at 110 C where conventional heating was ineffective. Under microwave conditions (75 W) at 110 C in the presence of 10 mol % of NaOEt, the 4.22a catalyzed Guerbet reaction proceeded at an initial rate of 1160 TO/h and resulted in about 50%
ethanol conversion in 2 h of which about 28% is n-butanol with a selectivity of 56% (entry 5, Table 4.9). At 110 C, the results obtained with microwave reactions are far superior (11-fold increase in the initial rate of n-butanol production) than the corresponding results obtained under conventional heating (compare entries 4 and 5, Table 4.9). Notably, while the productivity of microwave reaction after 2 h is comparable (entry 5 vs. entry l, Table 4.9) to the above results under conventional heating albeit at a much lower temperature (110 C), the initial rates obtained in the current microwave condition is about 1.6 folds higher. Poor results were obtained under microwave heating (75 W) at a lower temperature (100 C, entry 6, Table 4.9).
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Kanu Das, Ph.D Thesis, IIT Guwahati 153 Table 4.9: Upgradation of ethanol catalyzed by 4.22a under conventional and microwave heating.a
Entry T (C) Time (min)
Yield of 4.2 (%)b
Conversion of 4.1 (%)c
Selectivity of 4.2 (%)d
Total TONe
1 140
30 8 8.4 98 336
60 13 15 84 600
120 26 33 78 1320
4320 36 52 69 2099
2 130
30 3 3 99 120
60 4 4 99 176
120 6 6 99 260
4320 30 40 73 1614
3 120
30 2 2 99 80
60 2 2 99 88
120 6 6 99 240
4320 30 35 84 1416
4 110
30 1 1 99 37
60 1 1 99 45
120 3.7 3.8 99 152
4320 22 25 85 1014
5f 110
30 11 14 79 580
60 18 26 69 1045
120 28 50 56 2000
6f 100
30 0.7 0.7 100 30
60 0.9 0.9 100 40
120 2.0 2.0 100 80
7g 110
30 1 1 100 55
60 2 3 67 115
120 7 8 88 330
8h 110
30 6 8 75 335
60 13 17 76 675
120 20 30 67 1215
9i 110
30 12 14 86 580
60 18 24 75 945
120 29 40 73 1600
aReaction condition: ethanol (1 mL, 17.12 mmol), NaOEt (10 mol %) and 4.22a (0.025 mol %) at T C.
bYield was determined by GC analysis using toluene as an internal standard. cConversion was determined by GC analysis using toluene as an internal standard. dSelectivity = Yield of n- butanol/Conversion of 4.1. eTotal TON = Total yield of upgraded products/4.22a loading. fUnder 75 W microwave. gUnder 50 W microwave. hUnder 65 W microwave. iUnder 90 W microwave.
TH-3049_166122006
Kanu Das, Ph.D Thesis, IIT Guwahati 154 The initial rate and productivity of the n-butanol formation improved upon increasing the power of microwave irradiation and followed the trend 50 W < 65 W < 75 W (entry 5 vs. entries 7 and 8, Table 4.9). However, upon the further increase in the power to 90 W, there was hardly any improvement in the formation or n-butanol (entry 5 vs. entry 9, Table 4.9).
Further optimization was hence carried out using a microwave irradiation of 75 W at 110 C to arrive at the most efficient catalytic system. Various inorganic and organic bases were screened for the Guerbet reaction in the presence of (Bim2NNN)RuCl2(CO) (4.22a) (Table 4.10).
In comparison to NaOEt, under otherwise identical conditions, the productivity and initial rate in the presence of NaOtBu (10 mol %) was 3.6 folds and 72 folds lower respectively (entry 2 vs. l, Table 4.10).
The 4.22a catalyzed Guerbet reaction in the presence of KOtBu (10 mol %) proceeded about 1.65 times slower than the corresponding reaction with NaOEt but with comparable productivity (entry 3 vs. 1, Table 4.10). While both NEt3 and Na2CO3 were not reactive (entry 4 and 5, Table 4.10), poor activity (160 TON at an initial rate of 40 TO/h) were obtained upon the use of K2CO3 (entry 6, Table 4.10). The initial rate of the 4.22a catalyzed Guerbet reaction in the presence of NaOH was comparable to that performed in the presence of NaOEt albeit with 50% of the productivity (entry 7 vs. l, Table 4.10). In comparison, the results obtained from the corresponding reactions with KOH were modest (800 TON at an initial rate of 880 TO/h, entry 8, Table 4.10). Thus, the catalytic system containing NaOEt performed exceedingly well in comparison to other bases. Further optimization revealed that the best results were obtained at a loading of 10 mol % of NaOEt (entry 1 vs. entries 9-12, Table 4.10).
With the increase in 4.22a loading from 0.025 mol % to 1 mol % the initial-rate and productivity of n-butanol formation increased only up to about 1.5 times with an associated drastic decrease (28 folds) in turnovers (entry 1 vs. entries 2-6, Table 4.11). Hence, further optimization of various catalysts was carried out at a catalyst loading of 0.025 mol % (Table 4.12). However, it should be noted that with 1 mol % loading of 4.22a, 42% yield of n-butanol was obtained at an ethanol conversion of 72%. These unprecedented results eclipse the previous best obtained with these catalysts under conventional heating that too at considerably lower temperature (entry l, Table 4.9 vs. entry 6, Table 4.11). The performance of pincer-Ru catalysts based on bis(imino)pyridine ligands was not on-par in comparison with the corresponding catalysts based on 2,6-bis(benzimidazole-2yl) pyridine ligands (entries 1-4 and 7-10 vs. entries 5-6 and 11-12, Table 4.12). Furthermore, the pincer-Ru carbonyl complexes 4.22a and 4.22c
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Kanu Das, Ph.D Thesis, IIT Guwahati 155 were more productive towards n-butanol formation than their corresponding phosphine counterparts 4.21a and 4.21c (entries 5-6 vs. 11-12, Table 4.12).
Table 4.10: Upgradation of ethanol catalyzed by 4.22a in the presence of various bases.a
Entry Base (X mol %)
Time (min)
Yield of 4.2 (%)b
Conversion of 4.1 (%)c
Selectivity of 4.2 (%)d
Total TONe
1 NaOEt
10.0
30 11 14 79 580
120 28 50 56 2000
2 NaOtBu 10.0
30 2 2 100 80
120 10 14 71 560
3 KOtBu 10.0
30 8 8 100 350
120 34 46 74 1845
4 NEt3
10.0
30 0 0 -- --
120 0 0 -- --
5 Na2CO3
10.0
30 0 0 -- --
120 0 0 -- --
6 K2CO3
10.0
30 0.5 0.5 100 20
120 4 4 100 160
7 NaOH
10.0
30 10 13 77 520
120 22 29 76 1160
8 KOH
10.0
30 12 12 100 480
120 18 20 90 800
9 NaOEt
2.5
30 5 5 100 215
120 19 23 83 930
10 NaOEt 5.0
30 2 2 100 80
120 20 24 83 970
11 NaOEt 7.5
30 4 5 80 210
120 29 36 81 1455
12 NaOEt 20
30 2 3 67 120
120 8 9 89 380
aReaction condition: ethanol (1 mL, 17.12 mmol), base (X mol %) and 4.22a (0.025 mol %) at 110 C under 75 W microwave. bYield was determined by GC analysis using toluene as an internal standard. cConversion was determined by GC analysis using toluene as an internal standard. dSelectivity = Yield of n-butanol/Conversion of 4.1. eTotal TON = Total yield of upgraded products/4.22a loading.
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Kanu Das, Ph.D Thesis, IIT Guwahati 156 Table 4.11: Upgradation of ethanol at various loading of 4.22a.a
Entry 4.22a (Y mol %)
Time (min)
Yield of 4.2 (%)b
Conversion of 4.1 (%)c
Selectivity of 4.2 (%)d
Total TONe
1 0.025
30 11 14 79 580
60 18 26 69 1045
120 28 50 56 2000
2 0.050
30 14 16 88 320
60 25 32 78 640
120 34 51 67 1020
3 0.075
30 15 17 88 230
60 29 40 73 540
120 40 61 66 815
4 0.100
30 13 15 87 150
60 32 44 73 445
120 41 69 59 690
5 0.500
30 15 18 86 35
60 30 47 64 94
120 41 71 58 142
6 1.000
30 18 21 87 21
60 35 50 69 50
120 42 72 57 72
aReaction condition: ethanol (1 mL, 17.12 mmol), NaOEt (10 mol %) and 4.22a (Y mol %) at 110 C under 75 W microwave. bYield was determined by GC analysis using toluene as an internal standard. cConversion was determined by GC analysis using toluene as an internal standard. dSelectivity = Yield of n-butanol/Conversion of 4.1. eTotal TON = Total yield of upgraded products/4.22a loading.
Among the pincer-Ru carbonyl complexes based on 2,6-bis(benzimidazole-2-yl) pyridine ligands, while 4.22a demonstrated a better initial rate in ethanol to n-butanol upgradation, the catalyst 4.22c was not only more selective towards n-butanol formation but also more sustainable and hence more productive (entry 5 vs. 6, Table 4.12). The better n-butanol selectivity and productivity obtained with 4.22c offers great promise as its hydroxyl group could potentially be heterogenized on various solid supports to achieve recyclability.
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Kanu Das, Ph.D Thesis, IIT Guwahati 157 Table 4.12: Pincer-ruthenium catalyzed ethanol upgradation.a
Entry Catalyst Time (min)
Yield of 4.2 (%)b
Conversion of 4.1 (%)c
Selectivity of 4.2 (%)d
Total TONe
1 4.20a 30 4 4 100 160
120 4 4 100 180
2 4.20b 30 3 3 100 120
120 6 6 100 230
3 4.20c 30 3 3 100 105
120 4 4 100 180
4 4.20d 30 2 2 100 80
120 5 6 83 250
5 4.22a 30 11 14 79 580
120 28 50 56 2000
6 4.22c 30 8 8 100 310
120 45 61 74 2440
7 4.19a 30 3 3 100 125
120 5 5 100 205
8 4.19b 30 5 5 100 205
120 10 12 83 490
9 4.19c 30 5 5 100 190
120 6 6 100 250
10 4.19d 30 2 2 100 90
120 4 4 100 160
11 4.21a 30 8 9 89 375
120 27 36 75 1430
12 4.21c 30 7 9 78 380
120 27 31 87 1255
aReaction conditions: ethanol (1 mL, 17.12 mmol), NaOEt (10 mol %) 4.19a-d, 4.20a-d, 4.21a, 4.21c, 4.22a and 4.22c (0.025 mol %) at 110 C under 75 W microwave. bYield was determined by GC analysis using toluene as an internal standard. cConversion was determined by GC analysis using toluene as an internal standard.
dSelectivity = Yield of n-butanol/Conversion of 4.1. eTotal TON = Total yield of upgraded products/catalyst loading.
4.3.4 Mechanistic studies on the upgradation of ethanol catalyzed by pincer-ruthenium