Functionalization of HMS mesoporous molecular sieves and their base-catalytic performance

Studies in Surface Science and Catalysis 156 M. Jaroniec and A. Sayari (Editors) 9 2005 ElsevierB.V. All rights reserved

229

Functionalization of HMS mesoporous molecular sieves and their base-catalytic performance Chun Yang College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing, 210097, P. R. China

Organic NH2 group was grafted onto the surface of mesoporous molecular sieves by reacting 3-aminopropyltriethoxysilane or 3-ethyldiaminopropyltrimethoxysilane with calcined pure siliceous HMS with different pore sizes. Characterization of functionalized samples was made by means of XRD, FT-IR and N2 adsorption-desorption isotherms. These NH2-grafted HMS materials are highly active catalysts for Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate. The initial activity of catalyst depends not only on the density of NH2 groups, but also on the pore size and the size of organosilane molecule. In addition, the effect of reaction temperature and solvent on the activity, and the recycling of the catalyst are also investigated.

1. INTRODUCTION Linking organic functional groups onto the surface of mesoporous materials to endow the materials with functionalities for catalysis and other applications has become an attractive field in the study of mesoporous materials [ 1-5]. This technology greatly develops the area of surface modification of molecular sieves and realizes the heterogeneity of many homogeneous catalyses by supporting organic reagents onto mesoporous inorganic materials. The hybrid organic-inorganic materials acquired from this technology will provide new opportunities for the catalytic processes towards clean technology. In the methodologies for linking organic molecules to the surface of a silica-base support, the grafting of organosilane on the surface [1,3,4,6-10] and the co-condensation of a silica precursor (typically a tetraalkylorthosilicate) with an organosilane [2,11-16] are two approaches used usually. Amino groups have been linked onto the mesoporous materials by both approaches [4,6,12,13,16]. However, an investigation on broad range of loading amount of organosilane is lacking, and the effects of the pore size of mesoporous support, the size of organosilane and the linking approach on the properties and catalytic performance of functionalized samples are also not expatiated clearly. In the present paper, we modify hexagonal mesoporous silica, HMS, with two NH2-containing organosilanes,

230 3-aminopropyltriethoxysilane (AM) and 3-ethyldiaminopropyltrimethoxysilane (ED), by grafting method to obtain solid base catalysts. A series of samples with broad range of loading level are prepared, and their structures and pore characters are characterized. Then the base-catalytic performance of these materials is studied using the model Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate. The effect of structure and properties of the materials on the reactivity is investigated. 2. EXPERIMENTAL

2.1. Preparation of samples HMS were prepared by using tetraethylorthosilicate as silicon source, and dodecyl amine (DDA) and octadecyl amine (ODA) as templates, respectively, at a composition ratio: 1TEOS : 0.27DDA(ODA) : 6.5EtOH : 36H20. The template was removed by calcination in the air at 550~ The resultant samples are designated as DDA-HMS and ODA-HMS, which were found to possess a pore diameter of 2.35 nm and 3.04 nm, respectively. They were used as parent samples for further functionalization. The samples grafted with organosilane were prepared following the procedures: the parent sample was mixed with the given organosilane, AM or ED, in toluene, followed by refluxing under stirring for 3 h. Adjusting the dosage of organosilane in the solution facilitates the control of loading level. The resulted samples were filtered and the extra organosilane were extracted with CH2C12 in a Soxhlet apparatus twice. The obtained samples were dried and designated as x.xxAM(ED)-DDA-HMS and x.xxAM(ED)-ODA-HMS, respectively, where x.xx is the loading of organosilane in mmol g-l.

2.2. Characterization XRD patterns were recorded on the Rigaku D/max-yC X-ray diffractometer at 40 kV and 100 mA with Cu-Ka radiation. Element analyses for C and N were conducted on a Perkin-Elmer 2000 instrument. The loadings of organosilanes were evaluated from these analyses. N2 sorption measurements were performed on Micromeritics ASAP 2000 instrument after degassing the sample at 100~ and 0.67 Pa. Surface areas were evaluated by BET method and volumes of mesopore were calculated by BJH method (using desorption curve). IR spectra in hydroxyl vibration region were recorded on a Bruker IFS-48 FTIR instrument with a resolution of 2 cm -~. Before each IR measurement, self-supporting wafer (ca. 4.0 mg cm -2) was heated to 450~ under vacuum, followed by evacuation at 5.33x 10.2 Pa for 2 h, then cooled to ambient temperature to record the IR spectrum.

2.3. Catalytic performance The catalytic activities of samples for Knoevenagel condensation were investigated at 80~ in toluene solvent except for the experiments of varying temperature or solvent. Dosage of catalyst was 4.5% of the total weight of reactants (ca. 0.079g), and benzaldehyde and ethyl cyanoacetate were adopted 8 mmol each. The reaction mixtures were analyzed by gas chromatography (Varian 3400GC, SPB-5 capillary column).

231 3. RESULTS AND DISCUSSION 3.1. Grafting of organosilane and characterization of functionalized samples As expected, the amount of used organosilane in silylation influences the loading level (see Fig.l). When the dosage of organosilane is increased from 0 to 1 mmol per gram of parent sample, the loading of organosilane increases rapidly and all of organosilane is grafted onto the surface. No significant "T ~ 3 difference in loading is observed for both O E parent samples at this stage because the ~E 2 influence of pore size on diffusion of organosilane is inconsiderable at low loading O level. As the organosilane dosage further increases, the raise in loading in respective sample tends to reach a plateau. At higher 0 2 4 6 8 10 12 14 loading level, the loadings of organsilanes on ODA-HMS are greater than those on Dosage of organosilane / mmol g~ DDA-HMS, indicating that a larger pore size is favorable for grafting of organosilane Fig. 1. Effect of dosage of organosilane on within the channels of the parent HMS loading. (--~--)AM-DDA-HMS, ( ; ) material. AM-ODA-HMS, (--o---) ED-DDA-HMS, XRD patterns of the samples before and ( o )ED-ODA-HMS. after the functionalization (not shown here) show only a broad single peak (dl00), characteristic of low-ordered, hexagonal packing of channels [17], in range of 20 = 0.5~ ~ The similarity of patterns of the functionalized samples to those of parent samples suggests that the hexagonal mesophase remains practically intact after grafting of organosilane. For DDA-HMS, however, the intensity of XRD peak descends after the functionalization, meaning that the introduction of organosilane decreases the crystallinity of mesophase with smaller pore size. IR spectra of samples in the region of 2500-4000 cm ~ are shown in Figs.2 and 3. A clear O-H stretching band of silanol at 3740 cm -~ can be seen on parent samples after adsorbed water was removed under vacuum at 450~ (Figs.2a and 3a). After the functionalization, this band declines in intensity and even disappears as the loading of organosilane increases (Figs. 2b-d and 3b-e) due to the consumption of silanol in the reaction with organosilane. The bands at 3430, 2875 and 2934 cm "~ should be attributed to N-H stretching vibration of N-containing groups and to C-H stretching vibration of CH2 and CH3 groups in organic molecules, respectively. Our previous study [18] on the functionalization of mesoporous material SBA-3 showed that the intensity of 3740 cm l band, i.e., the population of silanols, didn't decrease with the increase of organosilane when the loading went up to a certain level, suggesting that some of silanols on the SBA-3 sample are difficult of access. However, IR spectra in Figs.2 and 3 reveal that all of the silanols on HMS samples are accessible and can react with organosilane. .

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4000

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2500

4000

3500

3000

2500

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3500 3000 Wavenumbers / c m -1

Fig.2. IR spectra of DDA-HMS(a), 0.86AM-DDA-HMS(b), 1.29AM-DDAHMS(c) and 1.94AM-DDA- HMS(d).

350 300 r~ 250 ~ 200

J ;>

Wavenumbers / cm -I Fig.3. IR spectra of ODA-HMS(a), 0.30ED-ODA-HMS(b), 1.33ED-ODAHMS(c), 1.91ED-ODA-HMS(d) and 2.31ED-ODA-HMS(e).

a_..~ Table 1. Pore parameters of samples from N2 adsorption-desorption experiments. SBET

Sample 100 50

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DDA-HMS 1.94AM-DDA-HMS 2.60AM-DDA-HMS

VM

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(m2g-l) (mL g-l) value 943 464 289 . . .

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0.69 0.14 0.10 . . . .

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33 68 100

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P/Po Fig.4. N2 adsorption-desorption isotherms of DDA-HMS(a), 1.94AM-DDA-HMS(b) and 2.60AM-DDA-HMS(c). This may be attributed to the existence of textural mesoporosity in HMS, which is contributive to the access to framework-confined mesopores and can improve the diffusion of molecules in samples, as reported by Tanev et al.[ 19]. Figure 4 shows the N2 adsorption-desorption isotherms of DDA-HMS before and after the functionalization with AM. For the parent sample, a step characteristic of framework mesopores with regular diameter occurs at p/p0-0.1-0.3. After the functionalization, the step disappears and the adsorbed volume decreases with increasing of the loading of organosilane, and the isotherm becomes similar to that of microporous solid. These features denote that the

233 functionalized samples possess less mesopores, as confirmed by pore parameters listed in Table 1. The simultaneous decrease of the surface area (SBET)and the mesopore volume (VM) implies the reduction in effective pore size, which is clearly caused by the filling of mesopore channels by the organosilane. The change of C value in BET equation after the functionalization (see Table 1) also suggests the enhancement of the interaction between adsorbate and adsorbent owing to the decrease of pore size [20]. 3.2. Base-catalytic performance for Knoevenagel condensation Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate was used to investigate the base-catalytic properties of the functionalized samples. Since no side-reaction occurs, the yield of condensation product, a ~, 13-unsaturated ester, can be considered as the activity of the catalyst. No notable yield is detected on the parent sample; but very high activities are observed after the functionalization (see Fig.5). The effect of temperature on the reaction is shown in Fig.6. Apparently, Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate can be performed efficiently at a mild temperature below 100~ by using a HMS catalyst grafted with NH2 groups, different from the result reported by Macquarrie et al.[16]. In their case, the reaction rate of benzaldehyde, which contains aromatic carbonyl with less electrophilicity, is surprisingly sluggish on a similar catalyst functionalized with aminopropyl groups by co-condensation method, suggesting that the catalysts prepared by different functionalization ways possess different structures and properties, which endow the sample with different catalytic performance. For the most of our catalyst samples, almost complete conversion is achieved after 2.5-~3 h of reaction time and the difference in activity appears only at initial stage of reaction. In order to compare the catalytic performance of various samples, we investigated the activities at 0.5 h of reaction time and show them in Figs.7 and 8 as a function of the loading of organosilane. It can be seen that the changes of initial yields with loadings are not monotone; a maximum exists for all types of samples. At lower loadings (i.e., before the yield reaches the maximum), 1O0

1O0

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20

20

0

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Fig.5. Activities change as a function of reaction time. (ODDA-HMS, ( 9 0.49AM-DDA-HMS, (II)0.95AM-DDA-HMS, (o) 1.80AM-DDA-HMS, (*) 2.60AM-DDA-HMS. Reaction conditions: 80~ toluene solvent.

0

1 2 3 Reaction time / h

Fig.6. Effect of reaction temperature on activities. (.) r.t., (m)50~ (A) 80~ (*) 100~ Reaction solvent:toluene. Catalyst: 0.86AM-DDA-HMS.

234 100

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Fig.7. Effect of loading on initial activity on AM-HMS. (o)AM-DDA-HMS, ( 9 AM-ODAHMS. Reaction conditions: 80~ toluene solvent.

20 0

l 2 3 -I Loadings / mmol g

Fig.8. Effect of loading on initial activity on ED-HMS. (o)ED-DDA-HMS, ( 9 ED-ODAHMS. Reaction conditions: 80~ toluene solvent.

similar activities are observed on both DDA-HMS and ODA-HMS with the same loading. But the activities drop more rapidly on DDA-HMS than on ODA-HMS at higher loading levels (i.e., after the yield reaches the maximum). These facts suggest that over-loading is unfavorable to the initial activities because the channels are filled and blocked partly by organosilane at the higher loading levels so that the diffusion of reactants and products is hindered and hence result in a decrease in the amount of accessible active sites. The smaller the pore size is, the more severe the blockage of channels is as the loading increases. However, the effect of diffusion doesn't occur at lower loadings and the activity varies only as a function of the density of accessible active sites. The effect of different organosilane on the initial activities can also be found from Figs.7 and 8. The optimum loadings, i.e., the loadings corresponding to the maximum initial activities, on ED-grafted samples are lower than those on AM-grafted samples, and at higher loadings, the yield on ED-grafted samples are lower than those on AM-grafted samples with the same loading. These observations mean that the size of organosilane molecule is also a factor contributing to over-loading. The employment of bulky organosilane molecules more easily induces the obstruction of diffusion. It is also found, however, that at lower loading levels, the yields on ED-grafted samples are higher than those on AM-grafted samples with the same loading, revealing ED molecule to be more active for Knoevenagel reaction when the effect of diffusion is ignored. This is because there are two amino groups in ED molecule and both of them can be accessed at lower loadings at which the ED molecules can lie on the surface instead of just standing on the surface (see scheme 1), as proposed by Feng et al.[ 1 ]. The effect of solvent on the activity is exhibited in Fig.9. Considering the different boiling points of various solvents, 50~ is chosen as the reaction temperature. It is found that the yields vary with solvents according to the following order: ethanol > toluene >n-hexane cyclohexane > chloroform, an obvious effect of solvent emerges. The solvent effect in

235

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molecules lying on the surface

molecules standing on the surface Scheme 1

Knoevenagel reaction has been investigated previously by Clark and Macquarrie et al.[5,16], and the relative polarity of solvent and catalyst was proposed to be responsible for the effect. The use of non-polar solvents will allow the reactant to preferentially adsorb onto the catalyst surface and, therefore, is favorable to the occurrence of reaction. They found that the yields on aminopropyl-grafled silica in various solvents were well corresponding to the polarities of those solvents. On aminopropyl-grafted HMS, however, the yield in toluene was higher than those in cyclohexane and n-hexane with less polarity [5,16], consistent with our above results. This seems to suggest that the solvent effect should be associated with more complex factors other than the relative polarity alone, which is supported by our experiment in which the highest activity is observed in ethanol. When the reaction reached to a steady state, the catalyst was taken out of the reactor by filtration and thrown into another reactant solution for the second run after a simple washing with the solvent. The same procedures were repeated 4 times and the activities in the recycling of AM-ODA-HMS are shown in Fig. 10. In the second run, the reaction rate reduces slightly but the yield can still reach 90% after a longer reaction time. However, the activity decreases greatly in the third run and almost loses in the fourth run. Since no notable change of structure was found from XRD pattern of used catalyst (not shown here), the reduction of activity may result from the coverage or the poisoning of active sites. 100 r

_

100

80

80

60

o 60 E

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tD o,..~

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20 0

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1 2 Reaction time / h

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Fig.9. Effect of solvents on activity. (*)ethanol, (9 (u)n-hexane, (o)cyclohexane, (A)chloroform. Reaction temp.: 50~ Catalyst: 1.43AM-DDA- HMS.

0

2

4 6 8 10 Reaction time / h

12

Fig.10. Recycling of catalyst. (e) first run, (o) second run, (A) third run, (D) fourth run. Reaction conditions: 80~ toluene solvent. Catalyst: 1.20AM-ODA-HMS.

236 4. CONCLUSION An organic basic group, NH2 group, can be grafted onto the surface of HMS mesoporous molecular sieves by silanization reaction of organosilane with surface silanols. All of the silanols on HMS are accessible and are consumed during the silylation treatment. The loading level of organosilane is influenced by the dosage of organosilane and the pore size of HMS. Although the hexagonal mesophase is retained mostly after the functionalization, the effective pore sizes of a considerable amount of mesopores decrease due to the occupation of space within channels by organosilane, and thus the surface area and the mesopore volume decrease with the loading of organosilane. NH2-grafted HMS materials are highly active catalysts for Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate. Almost complete conversion is achieved after 2.5-3 h of reaction time in toluene on the most of catalyst samples. The initial activity of catalyst depends not only on the density of NH2 groups but also on the pore size and the size of organosilane molecule. The smaller pores and the more bulky organosilane molecules readily cause the blockage of channels at higher loading level, resulting in the lower initial reactivity. In addition, temperature and solvent are also crucial factors for Knoevenagel condensation. REFERENCES [ I ] X. Feng, G E. Fryxell, L.-O. Wang, A. M. Kim, J. Liu, K. M. Kemner, Science, 276 (1997) 923. [2] I. Diaz, C. M~quez-Alvarez, F. Mohino, J. Prrez-Pariente, E. Sastre, J. Catal., 193 (2000) 283. [3] S. Jaenicke, G. K. Chuah, X. H. Lin, X. C. Hu, Microp. Mesop. Mater., 35-36 (2000) 143. [4] D. Brunel, Microp. Mesop. Mater., 27 (1999) 329. [5] J.H. Clark, D. J. Macquarrie, Chem. Commun., (1998) 853. [6] A. Cauvel, G. Renard, D. Brunel, J. Org. Chem., 62 (1997) 749. [7] A.M. Liu, K. Hidajat, S. Kawi, D. Y. Zhao, Chem. Commun., (2000) 1145. [8] K. Inumaru, J. Kiyoto, S. Yamanaka, Chem. Commun., (2000) 903. [9] R. Sercheli, R. M. Vargas, R. A. Sheldon, U. Schuchardt, J. Molecular Catal. A: Chemical, 148 (1999) 173. [ 10] I. Rodriguez, S. lborra, A. Corma, F. Rey, J. L. Jord~, Chem. Commun., (1999) 593. [ 11] J. Brown, R. Richer, L. Mercier, Microp. Mesop. Mater., 37 (2000) 41. [ 12] D.J. Macquarrie, Chem. Commun., (1996) 1961. [ 13] S.R. Hall, C. E. Fowler, B. Lebeau, S. Mann, Chem. Commun., (1999) 201. [14] R.J.P. Corriu, A. Mehdi, C. Reyr, C. Thieuleux, Chem Mater., 16 (2004) 159. [ 15] R.J.P. Corriu, L. Datas, Y. Guari, A. Mehdl, C. Reyr, C. Thieuleux, Chem. Commun., (2001) 763. [ 16] D.J. Macquarrie, D. B. Jackson, Chem. Commun., (1997) 1781. [17] P.T. Tanev, T. J. Pinnavaia, Science, 267 (1995) 865. [18] X.-P. Jia, C. Yang, Acta Chim. Sinica (in Chinese), 60 (2002) 1596. [ 19] P.T. Tanev, T. J. Pinnavaia, Chem Mater, 8 (1996) 2068. [20] S.J. Gregg, K. S. W. Sing, Adsorption, Surface Area and Porosity, 2nd Edition, Academic Press, London, 1982, p.211 and p.261.