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MCM-41 type silicas as supports for immobilized catalysts
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 1995 Elsevier Science B.V.
173
M C M - 4 1 t y p e silicas as s u p p o r t s for i m m o b i l i z e d c a t a l y s t s Daniel Brunel*, Anne Cauvel, Francois Fajula and Francesco DiRenzo Laboratoire de Mat~riaux Catalytiques et Catalyse en Chimie Organique, CNRS-URA 418, Ecole Nationale Sup6rieure de Chimie, 8, rue de l'Ecole Normale, F-34053 - Montpellier C6dex 1 - Fax: + 33 - 67 14 43 49 Abstract MCM-41 type silicas were covalently grafted with various functional alkoxysilanes (RO)3Si(CH2)3 X with X = C1, NH(CH2)2NH2 and NHC(O)NSalpr. The functions of the first two organic moieties already attached were further transformed into respectively 2-(NHCH2)Pyr, 4-(NH(CH2)2NHCO)Pyr also known as organic ligands of transition metals. The grafted mesoporous silicas were c h a r a c t e r i z e d by IR, MAS-NMR s p e c t r o s c o p i e s , thermogravimetry, nitrogen adsorption and elemental analysis . The coupling reactions of organic ligands do not modify the covalent grafting bonds. Pore opening shrinks at increasing organic coverage. The diameter of the channels accessible to nitrogen varies from 33 to 13 /~. A total surface lining by concentrically oriented organic chains is suggested by the decrease in the nitrogen adsorption enthalpy as a function of the organic coverage. 1. INTRODUCTION MCM-41 molecular sieves are a new class of mesoporous aluminosilicates featuring cylindrical regular mesopores of monodispersed diameter w i t h potential applications in catalysis and adsorption [1,2]. They are obtained by precipitation of amorphous silica-alumina in the presence of cationic surfactants. The pores of the MCM-41 type materials are templated by the surfactant micelles. While the hydrocarbon chain length of the surfactant rules the pore diameter, the conditions of the hydrothermal synthesis, in particular the alkalinity of the parent hydrogel, influence the wall thickness [3]. An increase in the pore spacing leads to an improved thermal stability of MCM-41-type materials. Pure silica MCM-41s can also be prepared and they feature a better stability than their silica-alumina analogs. MCM-41-type silicas were obtained with a pore diameter of 33/~ and wall thickness higher than 10/~. MCM-41 type silicas are an ideal raw material for the design of new mesoporous solids having adjustable chemical and physicochemical properties [4] in view of applications in the field of adsorption and fine chemical catalysis. This objective can be reached by lining of the mesopore surface with covalently attached transition metal liganding moieties. The volume available inside the MCM-41 mesopores is much larger than in the channel volume of the usual
174 zeolite catalysts. The insertion and grafting of bulky, sterically-hindered functional molecules is hence possible, without a decrease of the accessibility of the catalytic groups. As the mesoporous surface of the MCM-41 type silicas presents the same surface silanol groups than the traditional amorphous silicas, it can be modified by applying usual methods of silica functionalization. They deal with the covalent linkage of organic groups to the silica surface by condensation of organotrialkoxysilanes with the silanol groups [5-22]. We report here the anchoring of organic groups to MCM-41 type silicas using functional propyltrialkoxysilanes. The organic moieties were f u r t h e r transformed by coupling reactions with suitable organic ligands of transition metals. Our goal in preparing these materials was to obtain mesoporous solidbound ligands that yield systems with potential for catalytic oxidation of organic substrates. As most of the monodentate ligands used to immobilize transition-metal catalysts suffer from the disadvantage of the metal complex leaching into the solution, there is considerable interest in covalently attaching multidentate chelating agent to a support surface. For this purpose, we selected the following l i g a n d s : (pyridino-2-methyl)amine, (2isonicotinamidoethyl)amine and pentadentate Schiff base as SalDPT [23,24]. In the case of the two first ligands, mild coupling organic reactions were carried out to perform the coupling of the liganding moieties while avoiding alteration of the covalent grafting bonds. The different modifications were controlled at each step using various characterization techniques. This study allowed the determination of the new adsorption properties of the modified mesopore surfaces. 2. E X P ~ A L 2.1. MCM-41 silica.
Pure MCM-41 mesoporous silica (MPS) was prepared in the presence of cetyltrimethylammonium hydroxyde according to the method described in the literature [1,2]. The MPS material consists in SiO2 containing 0.003 A1/(AI+Si) and 0.0014 Na/(AI+Si). X-ray powder diffraction pattern matches well with the patterns reported by J.S. Beck et al [2], with d loo = 40.0/~. The hexagonal lattice parameter is equal to 46.2 /~. The nitrogen sorption isotherm of MPS is a classical example of type IV isotherm [24], showing monolayer and multilayer adsorption on a mesoporous surface of very high area and a sharp, reversible step at P/P0 0.38, characteristic of capillary condensation within the mesopores (vide infra). No hysteresis loop is observed due to the low filling pressure of the small mesopores.
2.2. Functionalization procedure 3-chloropropylsilylated-MPS (C1-MPS), 3-(2-aminoethyl)aminopropylsilylated -MPS (NH2EtN-MPS) and 3-(bis-[3-(salicyliden-amino) propyl])carbamatopropyl silylated-MPS (SalDPT-amido-MPS). A suspension of freshly activated MCM-41 silica in toluene was refluxed and stirred for 1.5 hr. with the corresponding functional organoalkoxysilane under dry nitrogen atmosphere. After distillation in a Dean-Stark collector of a fraction of toluene containing volatile compounds, the mixture was again heated at toluene refluxing temperature for 1.5 hr. The distillation and heating sequences were repeated. After cooling, the solid was extracted in a soxhlet
175 apparatus overnight with ethyl ether and dichloromethane, then evacuated under vacuum at 200~ for 6 hr. Elemental analy~i~: C1-MPS: C 6.34%, C1 3,29%; NH2EtN-MPS" C 15,23%, N: 6.06%; SalDPT-amido-MPS: C 26.36%, N 5.17%.
2.3. Modification of the functional group 2.3.1.3-(2-pyridinomethyl)aminopropylsilylated-MPS (Pyr MeN-MPS) A suspension of activated C1-MPS material in toluene was refluxed and stirred in an excess of 2-(aminoethyl)pyridine for 6 hr. After separation, the modified solid was extracted according to the previous procedure, then dried under vacuum at 180~ overnight. Elemental analysis: C 14.62%, N 3.89% 2.3.2. 3-(2-isonicotinamidoethyl)aminopropylsilylated-MPS (IsoNicEtN-MPS) Activated NH2EtNH-MPS in suspension in dichloromethane was refluxed and stirred with an excess of a mixture (1/2) of isonicotinic acid and N,N'dicyclohexyl carbodiimine for 6 hr. The modified solid was then separated, washed in succession with water, ethanol, then treated according to the previous procedure. Elemental anolysi~: C 23.05%, N 6.98%. 3.3. Characterization Analyses of the modified solids were made using 13C MAS-NMR, Infrared and UV Spectroscopies, Thermogravimetry, Nitrogen Adsorption and Desorption Isotherm and Elemental Analyses. RESULTS AND DISCUSSION The grafting reactions on the MPS support were studied first by infrared and 13C MAS-NMR spectroscopies. Infrared spectra of the modified mesoporous silicas show a band at 2940 cm -1 characteristic of-CH2- stretching vibration whereas the band at 2970 cm-1 associated with the CH3 of the ethoxy group has a much lower intensity than in the grafting agent spectrum. Moreover the grafted MPS spectra exhibit lower silanol band intensity at 3741 cm -1 than the parent MPS. The release of ethanol, detected and characterized by GCMS during the process confirms the formation of the Si-O-Si linkage according to the following reaction scheme: ~-OH
,,, (EtO)3Si(CH2)3X
~
NN[_O\ S i / ~ / ~ X]---O/
X
+ 2 EtOH
X= CI, NH(CH2)2NH 2, NHC(O)SalDPT 13C MAS-NMR spectroscopy allows the determination of the structure of the grafted hydrocarbon chains. The assignments of the 13C NMR signals of the modified mesoporous silicas are reported in Table 1.
176 Table 1 13C CP-MAS NMR spectral features of the grafted mesoporous silicas Grafted species
Ca
C~
Cy
N(CH2)nN Aromatic C
-=SiCH2aCH2~CH2Y-CI
8.9
26
46
=_Si(CH2)3-NH(CH2)2-NH2
9.5 22.5
38
51
-Si(CH2)3-NHC(O)-SalDPT
9.5
22
40
45
118 131
161
-_-Si(CH2)3-NHCH2Pyr
10
19
38.5
52
125 138
149
=_Si(CH2)3-NH(CH2)2-NHC(O)Pyr 10
22
40
50
124
158
ppm
145
CO,CN
163
165
Thus, the eventual modification of the structure of the function can be monitored by the change in the 13C CP-MAS-NMR spectra resulting from the coupling reactions. This is examplified on Figure 1 showing the intensity of the signal assigned to the CH2-C1 group on the C1-MPS spectrum (Fig.la) at 47.2 ppm which is very low on the spectrum of the resulting Pyr MeNH-MPS (Fig. lb). Moreover this later spectrum exhibits other signals characteristic of the 2-pyridinomethylamino group. The disappearance of the residual ethyl arms during the coupling t r e a t m e n t probably results from nucleophilic assistance of siloxane coupling between adjacent chains.
=
S i ( O E t )
( C H 2 )
••-•• 3-CI
~.\ \ i ~ ~
-(CH1)3 - N I I - C I I .
1~o
.
_
,
.
.
1
-
,
6
--~
-
.
|
-
14o
.
.
,
- . -
,oo
,
-
.-
,
60
-
. - !
.
.
.
.
.
,
iO
PPM
Figure 1. 13C CP MAS-NMR of a)C1-MPS b) PyrMeN-MPS
PPM
Figure 2. a)13C CP MAS-NMR of SalDPT-amido-MPS. b) J-Mod. 13C NMR of SalDPT in CDC13 solution
On the other hand, Figure 2 illustrates the identification of the anchored
177 26,0 ~ SalDPT group attached with c a r b a m a t o p r o p y l silane chain (Fig 2a) by comparison with the s p e c t r u m of SalD PT material in CDC13 solution (Fig 2b). zo.( The diffuse r e f l e c t a n c e s p e c t r u m of SalD PT-amido-MPS presented on figure 3 is consistent with the s t r u c t u r e of the salen group linked by a carbamate function (~,= 254 14.( and 317 nm). The b a n d at ~,= 400 n m would K-M be c o n s i s t e n t w i t h excitonic t r a n s i t i o n r e s u l t i n g from a t i g h t ordered packing of 8. salen molecule. The i n f r a r e d s p e c t r u m of this solid is also in a good a g r e e m e n t with the proposed structure. This latter technique as well as 13C MAS-NMR spectroscopy also z. 200 300 ~0 500 indicate t h a t the coupling of organic ligands zi nm of t r a n s i t i o n m e t a l to the grafted molecules Figure 3. Diffuse reflectance spectrum does not a l t e r the covalent siloxane bonds of SalDPT-amido - MPS. between silica and organic moieties. Table 2 reports the organic content and coverage of the grafted mesoporous solids deduced from the elemental analysis and thermogravimetry.
Table 2. Organic coverage and content of modified MPS.
Materials Cl-MPS NH2EtNH-MPS PyrMN-MPS
Molar ratio C/Cl 6.1
chain nm 2. 0.6
10.3
-(ctl2) 3 -Nll-(Cll2)2-Ntt 2
C/N 2.7
1.3
23.5
"(C!]2)3 "NI]'CIt2 --~(,_))
C/N 4.4
0.9
26.7
C/N 3.8
1.0
33.5
C/N 5.9
0.6
40.7
Organic chain -(oil2)3 -ct
{or~) k SiO2
w
-(Cll2) 3 -NII-(CII2)2-Nlt
IsoNicEtN-MPS
SalDPT-amidoMPS
~,-~
oII -((;lt2)3 -Nn-C -N
llO
* surface area of the mineral support The organic coverage of the modified m a t e r i a l was c a l c u l a t e d from elemental analysis d a t a t a k i n g into account mass % chlorine for chloroalkyl
178 chain and mass % nitrogen for amino or amidoalkyl chain containing or not pyridino group. The organic content is obtained from thermogravimetric data as the mass loss at T > 200~ It is noticeable that the organic coverage is lower for C1-MPS than NH2EtNH-MPS. Probably the amino groups enhance the substitution reaction at the Si atom [25]. The low organic coverage of C1-MPS corresponds to a C/C1 ratio higher than the stoichometric value. It is likely that the C/C1 ratio is increased by the presence of EtO- groups linked to the Si atoms belonging to the grafted chain or to the surface. This hypothesis is consistent with the i.r. and 13C NMR results. It is noteworthy that C/N ratio confirms a complete coupling reaction whatever the previously grafted organic function. The nitrogen sorption isotherms of the functionalized MPS give informations on their texture and surface state. Figure 4 shows the isotherms of some materials grafted with chains of different lengths. Data derived from the sorption isotherms of all samples are reported in Table 3.
jf
8O0
n_
~=m600
j
v
w= 4 0 0 o
/
~
c 200 _1 o
.,,,..,.,.,,..~l~ ~ .
. -
450
400
300
l
200
100
0
,,~
o
~,
'
a.'2
]',, RS~..ATIV[
'
9
o'.6 PRESSURE
o'.~ ,
'
'
~
o
o
(J=/;=o)
.
0.2
.
.
o
u
Ri[LPTIV~
.
o
6
PRESSURE
o.~. ,
(P/Pro)
Fig.4. Nitrogen sorption isotherms of a) parent MPS b) (1) CI-, (2) PyrMN-, (3) SalDPTamido-, (4) IsoNicEtN-MPS. Table 3. Textural and thermodynamic parameters deduced from nitrogen sorption isotherms mesoporous mesoporous diameter BET Materials surface volume (cc/g) (A) parameter C (m2/g) MPS
920
0.76
33
100
C1-MPS
851
0.57
27
60
PyrMN-MPS
577
0.35
24
50
NH2EtNH-MPS
586
0.29
20
22
SalDPT-amido-MPS
608
0.31
20
19
isoNicEtN-MPS
436
0.22
13
28
179 The surface area and mesopore volume decrease with increasing length of the grafted chain, but the characteristic features of the MPS isotherms are essentially preserved. All isotherms are of type IV, indicating the preservation of the mesoporous system during the grafting reaction or the modification of the organic chain. However, the characteristic step of the sorption isotherm, corresponding to the Kelvin filling of the mesoporosity, becomes less sharp at increasing organic content, suggesting that the pore size distribution is widening. The pressure at which the step of the isotherm occurs decreases with increasing length of the organic molecules. The diameter of the organiclined mesopores can be evaluated by the ratio dmeso = 4V / S between mesopore volume and mesoporous surface area. The average pore diameter are reported in table 3. The pore opening shrinkage fits fairly well with the increase in the organic chain lengths. This trend is consistent with the variation of surface area and mesoporous volume. The application of the BET equation to the nitrogen sorption isotherms provides some useful information on the energy of the interaction between a probe molecule -nitrogen- and the surface. The BET parameter C is roughly connected to the nitrogen adsorption heat according to the following equation: C= exp[ (Eads- E1 ) /RT] where E1 is equal to liquefaction heat of nitrogen. Values of the parameter C for all samples are reported in Table 3. All organic-lined MPS feature much smaller values of the parameters C than the parent silica. The adsorption heat of nitrogen is indeed lower on organic surfaces than on hydroxyl-rich surfaces. As a consequence, the low C values for modified MPS can be considered as an indication of regular surface coverage by the organic groups. 4. CONCLUSION The MCM-41 type silicas provide suitable supports for anchoring organic moieties to the inner surface of a mesoporous system. The organic lined mesopores of regular, well-controlled diameter can be obtained by standard functionalization methods. The grafted function can be further transformed into well-defined transition metal liganding moities without loss of either the organic chain content or the regular mesoporous structure. Nitrogen sorption isotherms are useful tools to characterize the properties of such potentially hydrophobic materials. The strong decrease of the nitrogen adsorption heat with the organic coverage is a significant test of the modification of the surface properties. The decrease of the value of the C parameter of the BET equation indicates t h a t the mineral surface is no longer accessible to adsorbed molecules. These results confirm the unique properties of the composite materials prepared by grafting organic molecules to the inner surface of MCM-41 type silicas. Potential applications would be relevant to both specific oxidative catalysis induced by the anchored transition metal complexes and the hydrophobic property peculiar of the lined organic moieties. AI~OWLEDG~ The authors are grateful to ELF and CNRS for financial support. They thank Annie Finiels for her help in taking part of the 13C NMR spectra in
180 CDC13 solution. Anne Cauvel is indebted to ADEME (Agence de 1' Environnement et de la Maitrise de r Energie) for a doctoral grant. R~'ERENCI~ 1 J.C. Beck, C.T.-W. Chu, I.D. Johnson, C.T. Kresge, M.E. Leonowicz, W.J. Roth and J.C. Vartuli, to Mobil Oil Corporation, WO91/11390 (1991). 2 J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCuUen, J.B. Higgins and .L. Schlenker, J. Am. Chem. Soc., 114, 10834 (1992). 3 N. Coustel, F. Di Renzo and F. Fajula, J. Chem. Comm., Chem. Comm., 967 (1994). 4 A. Cauvel, D. Brunel, F. DiRenzo and F. Fajula, "Organic Coatings" 53rd International Meeting of Physical Chemistry, Paris, 2-6 jan. 1995. 5 W. Hertl, J. Phys. Chem., 72, 1248 (1968). 6 B. Arkles, Chemtech, 766 (1977). 7 D.R. Fruge, G.D. Fong and F.K. Fong, J. Am. Chem. Soc., i01, 3697 (1979). 8 L. Yu Fu, X. Yong-Xia, X. Don-Peng and L.J. Guang-Liang, J. Polym. Sci., 19, 3069 (1981). 9 D.W. Sindorf an G. Maciel, J. Am. Chem. Soc., 105, 3767 (1983). 10 E.J.R. SudhSlter, R. Huis, G.R. Hays and N.C.M. Alma, J. Coll. Interf. Sci., 103, 554 (1985). 11 R. Rosset, Bull. Soc. Chim. Fr., 1128 (1985). 12 W.H. Pirkle, T.C. Pochapsky, G.S. Mahler, D.E. Corey, D.S.Reno and D.M. Alessi, J. Org. Chem., 51, 4991 (1986). 13 J.W. De Haan, H.M. Van den Bogaert, J.J. Ponjed and L.J.M. Van de Ven, J. Coll. Interf. Sci., 110, 591 (1986). 14 U. Nagel and E. Kinsel, J. Chem. Soc. Chem. Comm., 1098 (1986). 15 K. Soai, M. Watanabe and A. Yamamoto, J. Org. Chem., 55. 4832 (1990). 16 E.I.S. Adreotti and Y. Gushikem, J. Coll. Interf. Sci., 142, 97 (1991). 17 H.U. Blaser, Tetrahedron Asymmetry, 2, 843 (1991). 18 P. Herman, C. del Pino and E. Ruitz-Hitsky, Chem. Mater., 4, 49 (1992). 19 B. Pugin and M. Miiller, in "Heterogeneous Catalysis and Fine Chemicals III", M.Guisnet et al. Eds., Elsevier Science Publishers, Stud. Surf. Sci. Catal., 78, 107 (1993). 20 Y.G. Akopyants, S.A. Borisenkova, O.L. Kalya, V.M. Derkacheva and E.A. Lukyanets, J. Mol. Catal., 83, 1 (1993). 21 M. McCann, E.M. Giolla and K. Maddock, J. Chem. Soc. Dalton Trans., 1489 (1994). 22 R.S. Drago, J. Gaul, A. Zombeck and D.K. Straub, J. Am. Chem. Soc., 102, 1033 (1980). 23 D.E. De Vos, F. Thibault-Starzyk and P.A. Jacobs, Angew. Chem. Int. Ed. Engl., 33, 431 (1994). 24 S. Brunauer, L.S. Deming, W.S. Deming and E. Teller, J. Am. Chem. Soc., 62, 1723 (1940). 25 A. Cauvel, D. Brunel, F. DiRenzo, P. Moreau and F. Fajula, in Proceeding of Zeocat'95, Stud. Surf. Sci. Catal. in press.
173
M C M - 4 1 t y p e silicas as s u p p o r t s for i m m o b i l i z e d c a t a l y s t s Daniel Brunel*, Anne Cauvel, Francois Fajula and Francesco DiRenzo Laboratoire de Mat~riaux Catalytiques et Catalyse en Chimie Organique, CNRS-URA 418, Ecole Nationale Sup6rieure de Chimie, 8, rue de l'Ecole Normale, F-34053 - Montpellier C6dex 1 - Fax: + 33 - 67 14 43 49 Abstract MCM-41 type silicas were covalently grafted with various functional alkoxysilanes (RO)3Si(CH2)3 X with X = C1, NH(CH2)2NH2 and NHC(O)NSalpr. The functions of the first two organic moieties already attached were further transformed into respectively 2-(NHCH2)Pyr, 4-(NH(CH2)2NHCO)Pyr also known as organic ligands of transition metals. The grafted mesoporous silicas were c h a r a c t e r i z e d by IR, MAS-NMR s p e c t r o s c o p i e s , thermogravimetry, nitrogen adsorption and elemental analysis . The coupling reactions of organic ligands do not modify the covalent grafting bonds. Pore opening shrinks at increasing organic coverage. The diameter of the channels accessible to nitrogen varies from 33 to 13 /~. A total surface lining by concentrically oriented organic chains is suggested by the decrease in the nitrogen adsorption enthalpy as a function of the organic coverage. 1. INTRODUCTION MCM-41 molecular sieves are a new class of mesoporous aluminosilicates featuring cylindrical regular mesopores of monodispersed diameter w i t h potential applications in catalysis and adsorption [1,2]. They are obtained by precipitation of amorphous silica-alumina in the presence of cationic surfactants. The pores of the MCM-41 type materials are templated by the surfactant micelles. While the hydrocarbon chain length of the surfactant rules the pore diameter, the conditions of the hydrothermal synthesis, in particular the alkalinity of the parent hydrogel, influence the wall thickness [3]. An increase in the pore spacing leads to an improved thermal stability of MCM-41-type materials. Pure silica MCM-41s can also be prepared and they feature a better stability than their silica-alumina analogs. MCM-41-type silicas were obtained with a pore diameter of 33/~ and wall thickness higher than 10/~. MCM-41 type silicas are an ideal raw material for the design of new mesoporous solids having adjustable chemical and physicochemical properties [4] in view of applications in the field of adsorption and fine chemical catalysis. This objective can be reached by lining of the mesopore surface with covalently attached transition metal liganding moieties. The volume available inside the MCM-41 mesopores is much larger than in the channel volume of the usual
174 zeolite catalysts. The insertion and grafting of bulky, sterically-hindered functional molecules is hence possible, without a decrease of the accessibility of the catalytic groups. As the mesoporous surface of the MCM-41 type silicas presents the same surface silanol groups than the traditional amorphous silicas, it can be modified by applying usual methods of silica functionalization. They deal with the covalent linkage of organic groups to the silica surface by condensation of organotrialkoxysilanes with the silanol groups [5-22]. We report here the anchoring of organic groups to MCM-41 type silicas using functional propyltrialkoxysilanes. The organic moieties were f u r t h e r transformed by coupling reactions with suitable organic ligands of transition metals. Our goal in preparing these materials was to obtain mesoporous solidbound ligands that yield systems with potential for catalytic oxidation of organic substrates. As most of the monodentate ligands used to immobilize transition-metal catalysts suffer from the disadvantage of the metal complex leaching into the solution, there is considerable interest in covalently attaching multidentate chelating agent to a support surface. For this purpose, we selected the following l i g a n d s : (pyridino-2-methyl)amine, (2isonicotinamidoethyl)amine and pentadentate Schiff base as SalDPT [23,24]. In the case of the two first ligands, mild coupling organic reactions were carried out to perform the coupling of the liganding moieties while avoiding alteration of the covalent grafting bonds. The different modifications were controlled at each step using various characterization techniques. This study allowed the determination of the new adsorption properties of the modified mesopore surfaces. 2. E X P ~ A L 2.1. MCM-41 silica.
Pure MCM-41 mesoporous silica (MPS) was prepared in the presence of cetyltrimethylammonium hydroxyde according to the method described in the literature [1,2]. The MPS material consists in SiO2 containing 0.003 A1/(AI+Si) and 0.0014 Na/(AI+Si). X-ray powder diffraction pattern matches well with the patterns reported by J.S. Beck et al [2], with d loo = 40.0/~. The hexagonal lattice parameter is equal to 46.2 /~. The nitrogen sorption isotherm of MPS is a classical example of type IV isotherm [24], showing monolayer and multilayer adsorption on a mesoporous surface of very high area and a sharp, reversible step at P/P0 0.38, characteristic of capillary condensation within the mesopores (vide infra). No hysteresis loop is observed due to the low filling pressure of the small mesopores.
2.2. Functionalization procedure 3-chloropropylsilylated-MPS (C1-MPS), 3-(2-aminoethyl)aminopropylsilylated -MPS (NH2EtN-MPS) and 3-(bis-[3-(salicyliden-amino) propyl])carbamatopropyl silylated-MPS (SalDPT-amido-MPS). A suspension of freshly activated MCM-41 silica in toluene was refluxed and stirred for 1.5 hr. with the corresponding functional organoalkoxysilane under dry nitrogen atmosphere. After distillation in a Dean-Stark collector of a fraction of toluene containing volatile compounds, the mixture was again heated at toluene refluxing temperature for 1.5 hr. The distillation and heating sequences were repeated. After cooling, the solid was extracted in a soxhlet
175 apparatus overnight with ethyl ether and dichloromethane, then evacuated under vacuum at 200~ for 6 hr. Elemental analy~i~: C1-MPS: C 6.34%, C1 3,29%; NH2EtN-MPS" C 15,23%, N: 6.06%; SalDPT-amido-MPS: C 26.36%, N 5.17%.
2.3. Modification of the functional group 2.3.1.3-(2-pyridinomethyl)aminopropylsilylated-MPS (Pyr MeN-MPS) A suspension of activated C1-MPS material in toluene was refluxed and stirred in an excess of 2-(aminoethyl)pyridine for 6 hr. After separation, the modified solid was extracted according to the previous procedure, then dried under vacuum at 180~ overnight. Elemental analysis: C 14.62%, N 3.89% 2.3.2. 3-(2-isonicotinamidoethyl)aminopropylsilylated-MPS (IsoNicEtN-MPS) Activated NH2EtNH-MPS in suspension in dichloromethane was refluxed and stirred with an excess of a mixture (1/2) of isonicotinic acid and N,N'dicyclohexyl carbodiimine for 6 hr. The modified solid was then separated, washed in succession with water, ethanol, then treated according to the previous procedure. Elemental anolysi~: C 23.05%, N 6.98%. 3.3. Characterization Analyses of the modified solids were made using 13C MAS-NMR, Infrared and UV Spectroscopies, Thermogravimetry, Nitrogen Adsorption and Desorption Isotherm and Elemental Analyses. RESULTS AND DISCUSSION The grafting reactions on the MPS support were studied first by infrared and 13C MAS-NMR spectroscopies. Infrared spectra of the modified mesoporous silicas show a band at 2940 cm -1 characteristic of-CH2- stretching vibration whereas the band at 2970 cm-1 associated with the CH3 of the ethoxy group has a much lower intensity than in the grafting agent spectrum. Moreover the grafted MPS spectra exhibit lower silanol band intensity at 3741 cm -1 than the parent MPS. The release of ethanol, detected and characterized by GCMS during the process confirms the formation of the Si-O-Si linkage according to the following reaction scheme: ~-OH
,,, (EtO)3Si(CH2)3X
~
NN[_O\ S i / ~ / ~ X]---O/
X
+ 2 EtOH
X= CI, NH(CH2)2NH 2, NHC(O)SalDPT 13C MAS-NMR spectroscopy allows the determination of the structure of the grafted hydrocarbon chains. The assignments of the 13C NMR signals of the modified mesoporous silicas are reported in Table 1.
176 Table 1 13C CP-MAS NMR spectral features of the grafted mesoporous silicas Grafted species
Ca
C~
Cy
N(CH2)nN Aromatic C
-=SiCH2aCH2~CH2Y-CI
8.9
26
46
=_Si(CH2)3-NH(CH2)2-NH2
9.5 22.5
38
51
-Si(CH2)3-NHC(O)-SalDPT
9.5
22
40
45
118 131
161
-_-Si(CH2)3-NHCH2Pyr
10
19
38.5
52
125 138
149
=_Si(CH2)3-NH(CH2)2-NHC(O)Pyr 10
22
40
50
124
158
ppm
145
CO,CN
163
165
Thus, the eventual modification of the structure of the function can be monitored by the change in the 13C CP-MAS-NMR spectra resulting from the coupling reactions. This is examplified on Figure 1 showing the intensity of the signal assigned to the CH2-C1 group on the C1-MPS spectrum (Fig.la) at 47.2 ppm which is very low on the spectrum of the resulting Pyr MeNH-MPS (Fig. lb). Moreover this later spectrum exhibits other signals characteristic of the 2-pyridinomethylamino group. The disappearance of the residual ethyl arms during the coupling t r e a t m e n t probably results from nucleophilic assistance of siloxane coupling between adjacent chains.
=
S i ( O E t )
( C H 2 )
••-•• 3-CI
~.\ \ i ~ ~
-(CH1)3 - N I I - C I I .
1~o
.
_
,
.
.
1
-
,
6
--~
-
.
|
-
14o
.
.
,
- . -
,oo
,
-
.-
,
60
-
. - !
.
.
.
.
.
,
iO
PPM
Figure 1. 13C CP MAS-NMR of a)C1-MPS b) PyrMeN-MPS
PPM
Figure 2. a)13C CP MAS-NMR of SalDPT-amido-MPS. b) J-Mod. 13C NMR of SalDPT in CDC13 solution
On the other hand, Figure 2 illustrates the identification of the anchored
177 26,0 ~ SalDPT group attached with c a r b a m a t o p r o p y l silane chain (Fig 2a) by comparison with the s p e c t r u m of SalD PT material in CDC13 solution (Fig 2b). zo.( The diffuse r e f l e c t a n c e s p e c t r u m of SalD PT-amido-MPS presented on figure 3 is consistent with the s t r u c t u r e of the salen group linked by a carbamate function (~,= 254 14.( and 317 nm). The b a n d at ~,= 400 n m would K-M be c o n s i s t e n t w i t h excitonic t r a n s i t i o n r e s u l t i n g from a t i g h t ordered packing of 8. salen molecule. The i n f r a r e d s p e c t r u m of this solid is also in a good a g r e e m e n t with the proposed structure. This latter technique as well as 13C MAS-NMR spectroscopy also z. 200 300 ~0 500 indicate t h a t the coupling of organic ligands zi nm of t r a n s i t i o n m e t a l to the grafted molecules Figure 3. Diffuse reflectance spectrum does not a l t e r the covalent siloxane bonds of SalDPT-amido - MPS. between silica and organic moieties. Table 2 reports the organic content and coverage of the grafted mesoporous solids deduced from the elemental analysis and thermogravimetry.
Table 2. Organic coverage and content of modified MPS.
Materials Cl-MPS NH2EtNH-MPS PyrMN-MPS
Molar ratio C/Cl 6.1
chain nm 2. 0.6
10.3
-(ctl2) 3 -Nll-(Cll2)2-Ntt 2
C/N 2.7
1.3
23.5
"(C!]2)3 "NI]'CIt2 --~(,_))
C/N 4.4
0.9
26.7
C/N 3.8
1.0
33.5
C/N 5.9
0.6
40.7
Organic chain -(oil2)3 -ct
{or~) k SiO2
w
-(Cll2) 3 -NII-(CII2)2-Nlt
IsoNicEtN-MPS
SalDPT-amidoMPS
~,-~
oII -((;lt2)3 -Nn-C -N
llO
* surface area of the mineral support The organic coverage of the modified m a t e r i a l was c a l c u l a t e d from elemental analysis d a t a t a k i n g into account mass % chlorine for chloroalkyl
178 chain and mass % nitrogen for amino or amidoalkyl chain containing or not pyridino group. The organic content is obtained from thermogravimetric data as the mass loss at T > 200~ It is noticeable that the organic coverage is lower for C1-MPS than NH2EtNH-MPS. Probably the amino groups enhance the substitution reaction at the Si atom [25]. The low organic coverage of C1-MPS corresponds to a C/C1 ratio higher than the stoichometric value. It is likely that the C/C1 ratio is increased by the presence of EtO- groups linked to the Si atoms belonging to the grafted chain or to the surface. This hypothesis is consistent with the i.r. and 13C NMR results. It is noteworthy that C/N ratio confirms a complete coupling reaction whatever the previously grafted organic function. The nitrogen sorption isotherms of the functionalized MPS give informations on their texture and surface state. Figure 4 shows the isotherms of some materials grafted with chains of different lengths. Data derived from the sorption isotherms of all samples are reported in Table 3.
jf
8O0
n_
~=m600
j
v
w= 4 0 0 o
/
~
c 200 _1 o
.,,,..,.,.,,..~l~ ~ .
. -
450
400
300
l
200
100
0
,,~
o
~,
'
a.'2
]',, RS~..ATIV[
'
9
o'.6 PRESSURE
o'.~ ,
'
'
~
o
o
(J=/;=o)
.
0.2
.
.
o
u
Ri[LPTIV~
.
o
6
PRESSURE
o.~. ,
(P/Pro)
Fig.4. Nitrogen sorption isotherms of a) parent MPS b) (1) CI-, (2) PyrMN-, (3) SalDPTamido-, (4) IsoNicEtN-MPS. Table 3. Textural and thermodynamic parameters deduced from nitrogen sorption isotherms mesoporous mesoporous diameter BET Materials surface volume (cc/g) (A) parameter C (m2/g) MPS
920
0.76
33
100
C1-MPS
851
0.57
27
60
PyrMN-MPS
577
0.35
24
50
NH2EtNH-MPS
586
0.29
20
22
SalDPT-amido-MPS
608
0.31
20
19
isoNicEtN-MPS
436
0.22
13
28
179 The surface area and mesopore volume decrease with increasing length of the grafted chain, but the characteristic features of the MPS isotherms are essentially preserved. All isotherms are of type IV, indicating the preservation of the mesoporous system during the grafting reaction or the modification of the organic chain. However, the characteristic step of the sorption isotherm, corresponding to the Kelvin filling of the mesoporosity, becomes less sharp at increasing organic content, suggesting that the pore size distribution is widening. The pressure at which the step of the isotherm occurs decreases with increasing length of the organic molecules. The diameter of the organiclined mesopores can be evaluated by the ratio dmeso = 4V / S between mesopore volume and mesoporous surface area. The average pore diameter are reported in table 3. The pore opening shrinkage fits fairly well with the increase in the organic chain lengths. This trend is consistent with the variation of surface area and mesoporous volume. The application of the BET equation to the nitrogen sorption isotherms provides some useful information on the energy of the interaction between a probe molecule -nitrogen- and the surface. The BET parameter C is roughly connected to the nitrogen adsorption heat according to the following equation: C= exp[ (Eads- E1 ) /RT] where E1 is equal to liquefaction heat of nitrogen. Values of the parameter C for all samples are reported in Table 3. All organic-lined MPS feature much smaller values of the parameters C than the parent silica. The adsorption heat of nitrogen is indeed lower on organic surfaces than on hydroxyl-rich surfaces. As a consequence, the low C values for modified MPS can be considered as an indication of regular surface coverage by the organic groups. 4. CONCLUSION The MCM-41 type silicas provide suitable supports for anchoring organic moieties to the inner surface of a mesoporous system. The organic lined mesopores of regular, well-controlled diameter can be obtained by standard functionalization methods. The grafted function can be further transformed into well-defined transition metal liganding moities without loss of either the organic chain content or the regular mesoporous structure. Nitrogen sorption isotherms are useful tools to characterize the properties of such potentially hydrophobic materials. The strong decrease of the nitrogen adsorption heat with the organic coverage is a significant test of the modification of the surface properties. The decrease of the value of the C parameter of the BET equation indicates t h a t the mineral surface is no longer accessible to adsorbed molecules. These results confirm the unique properties of the composite materials prepared by grafting organic molecules to the inner surface of MCM-41 type silicas. Potential applications would be relevant to both specific oxidative catalysis induced by the anchored transition metal complexes and the hydrophobic property peculiar of the lined organic moieties. AI~OWLEDG~ The authors are grateful to ELF and CNRS for financial support. They thank Annie Finiels for her help in taking part of the 13C NMR spectra in
180 CDC13 solution. Anne Cauvel is indebted to ADEME (Agence de 1' Environnement et de la Maitrise de r Energie) for a doctoral grant. R~'ERENCI~ 1 J.C. Beck, C.T.-W. Chu, I.D. Johnson, C.T. Kresge, M.E. Leonowicz, W.J. Roth and J.C. Vartuli, to Mobil Oil Corporation, WO91/11390 (1991). 2 J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCuUen, J.B. Higgins and .L. Schlenker, J. Am. Chem. Soc., 114, 10834 (1992). 3 N. Coustel, F. Di Renzo and F. Fajula, J. Chem. Comm., Chem. Comm., 967 (1994). 4 A. Cauvel, D. Brunel, F. DiRenzo and F. Fajula, "Organic Coatings" 53rd International Meeting of Physical Chemistry, Paris, 2-6 jan. 1995. 5 W. Hertl, J. Phys. Chem., 72, 1248 (1968). 6 B. Arkles, Chemtech, 766 (1977). 7 D.R. Fruge, G.D. Fong and F.K. Fong, J. Am. Chem. Soc., i01, 3697 (1979). 8 L. Yu Fu, X. Yong-Xia, X. Don-Peng and L.J. Guang-Liang, J. Polym. Sci., 19, 3069 (1981). 9 D.W. Sindorf an G. Maciel, J. Am. Chem. Soc., 105, 3767 (1983). 10 E.J.R. SudhSlter, R. Huis, G.R. Hays and N.C.M. Alma, J. Coll. Interf. Sci., 103, 554 (1985). 11 R. Rosset, Bull. Soc. Chim. Fr., 1128 (1985). 12 W.H. Pirkle, T.C. Pochapsky, G.S. Mahler, D.E. Corey, D.S.Reno and D.M. Alessi, J. Org. Chem., 51, 4991 (1986). 13 J.W. De Haan, H.M. Van den Bogaert, J.J. Ponjed and L.J.M. Van de Ven, J. Coll. Interf. Sci., 110, 591 (1986). 14 U. Nagel and E. Kinsel, J. Chem. Soc. Chem. Comm., 1098 (1986). 15 K. Soai, M. Watanabe and A. Yamamoto, J. Org. Chem., 55. 4832 (1990). 16 E.I.S. Adreotti and Y. Gushikem, J. Coll. Interf. Sci., 142, 97 (1991). 17 H.U. Blaser, Tetrahedron Asymmetry, 2, 843 (1991). 18 P. Herman, C. del Pino and E. Ruitz-Hitsky, Chem. Mater., 4, 49 (1992). 19 B. Pugin and M. Miiller, in "Heterogeneous Catalysis and Fine Chemicals III", M.Guisnet et al. Eds., Elsevier Science Publishers, Stud. Surf. Sci. Catal., 78, 107 (1993). 20 Y.G. Akopyants, S.A. Borisenkova, O.L. Kalya, V.M. Derkacheva and E.A. Lukyanets, J. Mol. Catal., 83, 1 (1993). 21 M. McCann, E.M. Giolla and K. Maddock, J. Chem. Soc. Dalton Trans., 1489 (1994). 22 R.S. Drago, J. Gaul, A. Zombeck and D.K. Straub, J. Am. Chem. Soc., 102, 1033 (1980). 23 D.E. De Vos, F. Thibault-Starzyk and P.A. Jacobs, Angew. Chem. Int. Ed. Engl., 33, 431 (1994). 24 S. Brunauer, L.S. Deming, W.S. Deming and E. Teller, J. Am. Chem. Soc., 62, 1723 (1940). 25 A. Cauvel, D. Brunel, F. DiRenzo, P. Moreau and F. Fajula, in Proceeding of Zeocat'95, Stud. Surf. Sci. Catal. in press.