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Preparation of guanidine bases immobilized on SBA-15 mesoporous material and their catalytic activity in knoevenagel condensation
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
505
Preparation of guanidine bases immobilized on SBA-15 mesoporous material and their catalytic activity in knoevenagel condensation Keun-Sik Kim, Jong Hun Song, Jong-Ho Kim and Gon Seo. Department of Chemical Technology & The Research Institute for Catalysis, Chonnam National University, Gwangju, 500-757, Korea. Guanidine was immobilized on SBA-15 mesoporous material by a consecutive addition reaction of precursors and a condensation reaction between presynthesized guanidinecontaining silane and hydroxyl groups of supports. Immobilized guanidine was thermally stable and showed the high activity in the Knoevenagel condensation between cyclohexanone and benzylcyanide. 1. INTRODUCTION Guanidine, non-ionic organic base, is widely employed as active base catalysts in various organic synthesis because of its strong basicity and high miscibility with organic reactants [1 ]. The difficulty in the separation of guanidine from products, however, reduces its economic feasibility by increasing separation expense. In addition, heating for distillation accelerates the formation of by-products, lowering the purity of desired products. Organic bases can be immobilized by the reactions between bases and chlorinated polystyrene supports [2]. Although immobilized bases show reasonable activity in basecatalyzed reactions, their low thermal stability and easy breaking of benzylic groups of polymer inhibit to achieve high activity and multiple use. Reaction of alkoxysilane with hydroxyl groups of solid silica supports provide an effective way to immobilize organic bases on them. Exceptional thermal stability of silica and strong Si-C chemical bond promise better performance of silica as catalyst support. In this study, three different kinds of immobilized guanidine base catalysts were prepared following the procedures shown in Scheme 1: through the stepwise reaction of 3-amino propyltriethoxysiliane (APTS) and N, N'-dicyclocarbodiimide (DCC) consecutively, and the reaction of presynthesized guanidine-containing silanes with hydroxyl groups of SBA-15 mesoporous material. The physico-chemical property and catalytic activity of guanidineimmobilized catalysts in Knoevenagel condensation were discussed relating to the basic character of immobilized guanidine. 2. EXPERIMENTAL SBA-15 mesoporous material was synthesized using an acidic reactant composing of tetraethoxysilane, polyalkylene oxide copolymer (Pluronic-123), trimethylbenzene (TMB), and hydrochloric acid [3]. Calcinated SBA-15 mesoporous material was used as a catalyst support in this study and guanidine was immobilized through the procedure described in Scheme 1.
506
O-NCNHQ -OH
-(gSi
Nil
•6
r-BuOH, reflux, 24 h
toluene, reflux, 12 h
[guan(step)/SBA]
OEt r, ^ . . ^ ^.. /-BuOH, reflux, 24 h
HN-(3
Art <-'Et
NH |
^IIP-OH" "-O"
toluene, reflux, 12 h
NH
6
0
[guan(mono)/SBA
Q-N:C:NHQ> OEt EtO-Si-'^^^-^^N'
m
0
r-OH ^—011 '—OH
toluene
Q
OEt Nil EtO-Si ^ ' ^ ^ - ^ N ^ ^ ^ N=< N=< y - ^
^ • ^ ^?2S'
iN=<
y N'
_,.HN-Q
0 [guanCdiVSBAI
N < )
0"
Scheme 1. Preparation of guanidine base catalysts immobilized on SBA-15 mesoporous material. Two types of Knoevenagel condensation were carried out over guanidine-immobilized catalysts: one was the condensation between benzaldehyde (BA) and ethylcyanoacetate (ECA. p/Ca=9), and the other was that between cyclohexanone (CH) and benzylcyanide (BC\ pKa=2\.9). Equal moles (10 mmol) of reactants were added to a nitrogen-purged 100 iiiC three neck flask accompanying with 0.1 g of base catalysts. Products were analyzed using HPLC equipped with a Capcell Pak CI8 column and an UV detector. Conversion was determined from the consumed amount of BA or CH as percentage. 3. RESULTS AND DISCUSSION TG curves of guanidine-immobilized catalysts show two weight decreasing regions: the first decrease below 100 "C is due to water desorption and the second one at 300-600 V is due to the combustion of immobilized guanidine. Immobilized amounts of guanidine on the SBA-15 support were estimated from weight loss in the TG curves and elemental analysis results. Those were 0.78, 0.33, 0.75 meq of guanidine per gram support on the guan(di)/SBA.
507
guan(step)/SBA and guan(mono)/SBA catalysts, respectively. Fig. 1 shows nitrogen adsorption isotherms of guanidine-immobilized catalysts. The SBA15 support synthesized using a mixed template of P-123 copolymer and TMB had large pores ranged from 130 to 150 A. A significant decrease in pore volume with guanidine immo bilization indicated that guanidine was mainly immobilized in mesopores of the support. IR spectra of guanidine-immobilized catalysts provide the immobilized state of guanidine (Fig. 2). Only a sharp absorption band at 3740 cm' attributed to external hydroxyl groups was observed on the evacuated support. By reacting APTS with hydroxyl groups lost methoxy groups as methanol and led to propylamine immobilization, disappearing 0-H band (3740 cm') and appearing N-H band (3300 cm') and C-H bands (2800-3000 cm' and 1610 cm'). Further reaction of DCC brought about a clear guanidine band at 1650 cm' on the guan(step)/SBA catalyst. Similar guanidine band was observed on the guan(di)/SBA catalyst, indicating that guanidine groups were immobilized on the SBA-15 support, regardless of preparation procedure. 1500 guan(step)/SBA
'SD
E
(J
guan(di)/SBA
1000
500
a 1500
2 'B
1000
o s
500
3
o
£
1500 1000
O
•o
500
3800
P/Po
Fig. 1. Adsorption-desorption isotherms of nitrogen on support and propylamine and guanidine-immobilized catalysts.
3300
2800
2300
1800
1300
Wavenumber (cm')
Fig. 2. IR spectra of the support and propylamine- and guanidine-immobilized catalysts after evacuation at 100 °C.
All prepared base catalysts were active in the Knoevenagel condensation of BA and HCA because of easy deprotonation of ECA. However, the deprotonation of methylene group of BC is more difficult than that of ECA, requiring strong base catalysts for the Knoevenagel condensation between CH and BC. Table 1 exhibits conversion and product composition over prepared base catalysts. The PA/SBA catalyst was not active in this Knoevenalgel condensation while both the guan(mono)/SBA and the guan(di)/SBA catalysts were active. Therefore, it was evident that guanidine-immobilized catalysts were more basic than propylamine-immobilized catalyst did. The yield of the dehydrated condensation product from CH and BC approached to 60% over the guan(di)/SBA catalyst at 150 °C. Although this value was low compared to that obtained on homogeneous l,5,7-triazabicyclo[4,4,0]dec-5-ene
508
(TBD) at 110 °C, the guan(di)/SBA was more feasible than TBD in the aspects of easy separation, repeated use, energy saving operation without using solvent, and the workability at elevated reaction temperature. Table 1. Knoevenagel condensation of CH and EC over immobilized base catalysts. CN
6 • "^a,CH
BC
C/ti. Dehydrated
Adduct
Catalyst PA/SBA guan(mono)/SBA guan(di)/SBA // PA/SBA guan(mono)/SBA guan(di)/SBA PA/SBA guan(mono)/SBA guan(di)/SBA TBD
CN
OH
-OCH3
Temp. (°C) 25
Solvent neat
Time (h) 48 12 48
t-BuOH
6
// 80
II II
150
neat
2 II II
110
toluene
6
Conv. (%) 0 5 20 30 0 62 59 0 65 61 83
Product Composition (%) Adduct Dehydrated
5 20 30
2
40 49
-
46
5
74
60
4. CONCLUSION Guanidine could be stably immobilized on SBA-I5 mesoporous material through the reaction between guanidine-containing silane and hydroxy 1 groups of catalyst support. Guanidine-immobilized catalysts showed high activity in the Knoevenagel condensation between CH and BC which was not activated by primary amine-immobilized catalysts. ACKNOWLEDGEMENT This work was supported by Korean Science and Engineering Foundation (2000-1-30700002-3). REFERENCES 1. G. Barcelo, D. Grenouillat, J.P. Senet and G. Sennyey, Tetrahedron, 46 (1990) 1839. 2. U. Schuchardt, R.M. Vargas and G. Gelhard, J. Mol. Catal. A: Chemical, 109 (1996) 37. 3. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc, 120 (1998) 6024.
505
Preparation of guanidine bases immobilized on SBA-15 mesoporous material and their catalytic activity in knoevenagel condensation Keun-Sik Kim, Jong Hun Song, Jong-Ho Kim and Gon Seo. Department of Chemical Technology & The Research Institute for Catalysis, Chonnam National University, Gwangju, 500-757, Korea. Guanidine was immobilized on SBA-15 mesoporous material by a consecutive addition reaction of precursors and a condensation reaction between presynthesized guanidinecontaining silane and hydroxyl groups of supports. Immobilized guanidine was thermally stable and showed the high activity in the Knoevenagel condensation between cyclohexanone and benzylcyanide. 1. INTRODUCTION Guanidine, non-ionic organic base, is widely employed as active base catalysts in various organic synthesis because of its strong basicity and high miscibility with organic reactants [1 ]. The difficulty in the separation of guanidine from products, however, reduces its economic feasibility by increasing separation expense. In addition, heating for distillation accelerates the formation of by-products, lowering the purity of desired products. Organic bases can be immobilized by the reactions between bases and chlorinated polystyrene supports [2]. Although immobilized bases show reasonable activity in basecatalyzed reactions, their low thermal stability and easy breaking of benzylic groups of polymer inhibit to achieve high activity and multiple use. Reaction of alkoxysilane with hydroxyl groups of solid silica supports provide an effective way to immobilize organic bases on them. Exceptional thermal stability of silica and strong Si-C chemical bond promise better performance of silica as catalyst support. In this study, three different kinds of immobilized guanidine base catalysts were prepared following the procedures shown in Scheme 1: through the stepwise reaction of 3-amino propyltriethoxysiliane (APTS) and N, N'-dicyclocarbodiimide (DCC) consecutively, and the reaction of presynthesized guanidine-containing silanes with hydroxyl groups of SBA-15 mesoporous material. The physico-chemical property and catalytic activity of guanidineimmobilized catalysts in Knoevenagel condensation were discussed relating to the basic character of immobilized guanidine. 2. EXPERIMENTAL SBA-15 mesoporous material was synthesized using an acidic reactant composing of tetraethoxysilane, polyalkylene oxide copolymer (Pluronic-123), trimethylbenzene (TMB), and hydrochloric acid [3]. Calcinated SBA-15 mesoporous material was used as a catalyst support in this study and guanidine was immobilized through the procedure described in Scheme 1.
506
O-NCNHQ -OH
-(gSi
Nil
•6
r-BuOH, reflux, 24 h
toluene, reflux, 12 h
[guan(step)/SBA]
OEt r, ^ . . ^ ^.. /-BuOH, reflux, 24 h
HN-(3
Art <-'Et
NH |
^IIP-OH" "-O"
toluene, reflux, 12 h
NH
6
0
[guan(mono)/SBA
Q-N:C:NHQ> OEt EtO-Si-'^^^-^^N'
m
0
r-OH ^—011 '—OH
toluene
Q
OEt Nil EtO-Si ^ ' ^ ^ - ^ N ^ ^ ^ N=< N=< y - ^
^ • ^ ^?2S'
iN=<
y N'
_,.HN-Q
0 [guanCdiVSBAI
N < )
0"
Scheme 1. Preparation of guanidine base catalysts immobilized on SBA-15 mesoporous material. Two types of Knoevenagel condensation were carried out over guanidine-immobilized catalysts: one was the condensation between benzaldehyde (BA) and ethylcyanoacetate (ECA. p/Ca=9), and the other was that between cyclohexanone (CH) and benzylcyanide (BC\ pKa=2\.9). Equal moles (10 mmol) of reactants were added to a nitrogen-purged 100 iiiC three neck flask accompanying with 0.1 g of base catalysts. Products were analyzed using HPLC equipped with a Capcell Pak CI8 column and an UV detector. Conversion was determined from the consumed amount of BA or CH as percentage. 3. RESULTS AND DISCUSSION TG curves of guanidine-immobilized catalysts show two weight decreasing regions: the first decrease below 100 "C is due to water desorption and the second one at 300-600 V is due to the combustion of immobilized guanidine. Immobilized amounts of guanidine on the SBA-15 support were estimated from weight loss in the TG curves and elemental analysis results. Those were 0.78, 0.33, 0.75 meq of guanidine per gram support on the guan(di)/SBA.
507
guan(step)/SBA and guan(mono)/SBA catalysts, respectively. Fig. 1 shows nitrogen adsorption isotherms of guanidine-immobilized catalysts. The SBA15 support synthesized using a mixed template of P-123 copolymer and TMB had large pores ranged from 130 to 150 A. A significant decrease in pore volume with guanidine immo bilization indicated that guanidine was mainly immobilized in mesopores of the support. IR spectra of guanidine-immobilized catalysts provide the immobilized state of guanidine (Fig. 2). Only a sharp absorption band at 3740 cm' attributed to external hydroxyl groups was observed on the evacuated support. By reacting APTS with hydroxyl groups lost methoxy groups as methanol and led to propylamine immobilization, disappearing 0-H band (3740 cm') and appearing N-H band (3300 cm') and C-H bands (2800-3000 cm' and 1610 cm'). Further reaction of DCC brought about a clear guanidine band at 1650 cm' on the guan(step)/SBA catalyst. Similar guanidine band was observed on the guan(di)/SBA catalyst, indicating that guanidine groups were immobilized on the SBA-15 support, regardless of preparation procedure. 1500 guan(step)/SBA
'SD
E
(J
guan(di)/SBA
1000
500
a 1500
2 'B
1000
o s
500
3
o
£
1500 1000
O
•o
500
3800
P/Po
Fig. 1. Adsorption-desorption isotherms of nitrogen on support and propylamine and guanidine-immobilized catalysts.
3300
2800
2300
1800
1300
Wavenumber (cm')
Fig. 2. IR spectra of the support and propylamine- and guanidine-immobilized catalysts after evacuation at 100 °C.
All prepared base catalysts were active in the Knoevenagel condensation of BA and HCA because of easy deprotonation of ECA. However, the deprotonation of methylene group of BC is more difficult than that of ECA, requiring strong base catalysts for the Knoevenagel condensation between CH and BC. Table 1 exhibits conversion and product composition over prepared base catalysts. The PA/SBA catalyst was not active in this Knoevenalgel condensation while both the guan(mono)/SBA and the guan(di)/SBA catalysts were active. Therefore, it was evident that guanidine-immobilized catalysts were more basic than propylamine-immobilized catalyst did. The yield of the dehydrated condensation product from CH and BC approached to 60% over the guan(di)/SBA catalyst at 150 °C. Although this value was low compared to that obtained on homogeneous l,5,7-triazabicyclo[4,4,0]dec-5-ene
508
(TBD) at 110 °C, the guan(di)/SBA was more feasible than TBD in the aspects of easy separation, repeated use, energy saving operation without using solvent, and the workability at elevated reaction temperature. Table 1. Knoevenagel condensation of CH and EC over immobilized base catalysts. CN
6 • "^a,CH
BC
C/ti. Dehydrated
Adduct
Catalyst PA/SBA guan(mono)/SBA guan(di)/SBA // PA/SBA guan(mono)/SBA guan(di)/SBA PA/SBA guan(mono)/SBA guan(di)/SBA TBD
CN
OH
-OCH3
Temp. (°C) 25
Solvent neat
Time (h) 48 12 48
t-BuOH
6
// 80
II II
150
neat
2 II II
110
toluene
6
Conv. (%) 0 5 20 30 0 62 59 0 65 61 83
Product Composition (%) Adduct Dehydrated
5 20 30
2
40 49
-
46
5
74
60
4. CONCLUSION Guanidine could be stably immobilized on SBA-I5 mesoporous material through the reaction between guanidine-containing silane and hydroxy 1 groups of catalyst support. Guanidine-immobilized catalysts showed high activity in the Knoevenagel condensation between CH and BC which was not activated by primary amine-immobilized catalysts. ACKNOWLEDGEMENT This work was supported by Korean Science and Engineering Foundation (2000-1-30700002-3). REFERENCES 1. G. Barcelo, D. Grenouillat, J.P. Senet and G. Sennyey, Tetrahedron, 46 (1990) 1839. 2. U. Schuchardt, R.M. Vargas and G. Gelhard, J. Mol. Catal. A: Chemical, 109 (1996) 37. 3. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc, 120 (1998) 6024.