- Email: [email protected]
Variation of the pore properties of mesoporous silica after washing by water and ethanol-water solutions
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved
165
Variation of the pore properties of mesoporous silica after washing by water and ethanol-water solutions L. Pasqua^, F. Testa'', R. Aiello^, F. Di Renzo^ and F. Fajula'^ ^Universita di Napoli Federico II, DIMP Chimica Applicata, 80125 Napoli, Italy ^Dipartimento di Ingegneria Chimica e dei Materiali, Universita degli Studi della Calabria, Via Pietro Bucci, 87030 Rende, Italy. FAX +39-0984492058. E-mail: [email protected] '^Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS, ENSCM, 8 rue de I'Ecole Normale, 34296 Montpellier Cedex 5, France. Pure-silica and iron-containing mesoporous materials are obtained by different synthesis procedures in the presence of cetyltrimethylammonium cation at neutral or slightly acidic pH. Several salts or hydrochloric acid are used as condensing agents. The property of the samples are characterized as a function of the washing procedure. Washing by water alone does not extract the template and does not affect the stability of the solid. Washing by ethanolic solution is an effective way to extract the template, and severely affect the stability and the porosity of the samples. 1. INTRODUCTION The acid synthesis pathway of mesoporous materials is based on the interaction between neutral or positively charged silica species and micelles of cationic surfactant whose positive charge is compensated by inorganic anions [1]. The polarisation of silica and the nature of the charge-compensating anion influence the properties of the materials [2-4]. Samples obtained from syntheses near the isoelectric point of silica usually undergo an important shrinking upon extraction of the template, due to their thin pore walls [4]. In this way, microporous materials can be obtained from precursor mesophases with 4 nm micelles [5]. Any method of extraction of the template, washing or calcination, brings about a pore shrinking and a corresponding increase of wall thickness. These effects proceed until a threshold of wall thickness is reached, at which the silica surface area is decreased enough to minimize the excess energy of the system [6]. Secondary treatment of the synthesis systems by water has been shown to positively affect the properties of several samples [7, 8]. Nevertheless, degradation or rearrangement of pore system can also occur through thermally induced hydrolysis of silicate upon hydrothermal treatment. Bagshaw et al submitted some assynthesised mesoporous materials to hydrothermal treatment at 373 K and materials with thicker pore walls resulted [9]. In this communication, the effect of washing on mesoporous silicas and iron silicates prepared in the presence of different salts is dealt with. 2. EXPERIMENTAL Molar compositions of the synthesis batches were nFe(NO3)3/MX/0.21 CTMABr/TEOS/146 H2O, with n in the range 0-0.05 and MX a salt or acid among NH4NO3,
166
NaNOs, NH4CI, NaCl, NH4F, HCl. The detailed preparation of samples and preliminary investigations on their stability were presented in a previous report [6]. For each composition, the products of five parallel preparations were washed according to one of five different procedures: (a) filtration without washing, filtration and washing with (b) 300 cm^ water, (c) 900 cm^ water, (d) 50 cm^ ethanol in 300 cm^ of water, (e) 150 cm ethanol in 300 cm of water. The amount of TEOS used in each synthesis batch corresponded to 4 g Si02. The washed samples were characterized by X-ray diffraction, thermogravimetry and N2 sorption on the solid calcined at 823 K in air flow. 3. RESULTS AND DISCUSSION In Figure 1, the evolution of the a parameter of the hexagonal cell is reported as a function of the washing procedure. Washing with water alone does not seem to affect the cell parameter in a significant way, while washing by ethanolic solution brings about a decrease of the cell parameter for all samples.
50
100
150
200
0
washing H2O / cm3 g-i
10
20
30
40
50
washing EtOH / cm3 g-i
Fig. 1. Hexagonal cell parameter of non calcined samples as a function of washing procedure. Amounts of solvent normalised on the total silica present in the system. Samples prepared with NaCl (round dots), HCl (lozenges), Fe and HCl (squares), Fe and NH4F (triangles). I.5O1
0.00
50
100
150
200
washing H2O / cm3 g-i
3
10
20
30
40
50
washing EtOH / cm3 g-i
Fig. 2. Cetyltrimethylammonium content of non calcined samples as a function of washing procedure. Samples as per Figure 1.
167
The variation of the cell parameter can be related to the change of composition of the samples, reported in Figure 2. Washing with water alone is not an effective method to extract the surfactant, while ethanolic solution is able to extract most of the surfactant from the solid. It can be observed that the cell shrinking presents a not linear dependence on the amount of surfactant extracted [10]. The (3c parameter of the hexagonal cell for the calcined materials is reported in Figure 3. Its dependence on the washing procedure closely parallels the effects observed for the cell parameter of the non-calcined materials.
50
100
150
200
0
washing H2O / cm3 g-i
10
20
30
40
50
washing EtOH / cm3
Fig. 3. Hexagonal cell parameter of calcined samples as a function of washing procedure. Samples as per Figure 1. 1.25[ 1.001 0.7^ 0.50 0.25 0.00
50
100
150
200
washing H2O / cm3 g-i
0
10
20
30
40
50
washing EtOH / cm3
Fig. 4. Structural mesopore volume of calcined samples as a function of washing procedure. Samples as per Figure 1. The structural pore volume of the calcined materials are reported in Figure 4. When water alone is used for washing, the pore volume of non-washed solids is somewhat lower than the pore volume of washed solids for all samples except the silica sample prepared in the presence of HCl. It looks like some retention of salts for samples prepared at pH between 2 and 7 can negatively affect the calcination. The evolution of pore volume with ethanolic washing follows the trends of the surfactant content reported in Figure 2, albeit in a nonquantitative way. The decrease of pore volume after washing is lower than the decrease of
168
template content. Very likely, the surfactant left in the solid after ethanolic washing does not occupy the whole available volume. In Figure 5, typical isotherms for a ferrisilicate sample prepared in the presence of HCl are presented. The positive effect of water washing is evident, as well the negative effect of washing by ethanolic solution.
0.4 0.6 Relative pressure P/P^
0.8
1.0
Fig. 5. Nitrogen adsorption-dcsorption isotherms of a ferrisilicate sample prepared in the presence of HCl (a) non-washed, (b) washed with water, (c) washed with ethanolic solution. 4. CONCLUSIONS It can be concluded that, for samples synthesized at neutral or moderately acidic pH, the extraction of template by solvent at room temperature implies a degradation of the structure far exceeding the usual shrinking observed during calcination. REFERENCES 1. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schuth and G.D. Stucky, Nature 368 1994, 317 2. H.P. Lin, S. Cheng and C.Y. Mou, Microporous Mater. 10 1997, 111. 3. H. Yang, G. Vovk, N. Coombs, I. Sokolov and G.A. Ozin, J. Mater. Chem. 8 1998, 743. 4. F. Di Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galarneau, D. Plee and F. Fajula, Microporous Mesoporous Mater. 28 1999, 437. 5. A.C. Voegtlin, A. Matijasic, J. Patarin, C. Sauerland, Y. Grillet and L. Huve, Microporous Mater. 10 1997, 137. 6. L.Pasqua, F. Testa, R. Aiello, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal. 135 2001, 06-P-28. 1. Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 1996, 1147. 8. L. Chen, T. Horiuchi, T. Mori, K. Maeda, J. Phys. Chem. B 103 1999, 1216. 9. S. A. Bagshaw. Stud. Surf. Sci. Catal., 117 1998, 381. 10. F. Di Renzo, D. Desplantier, A. Galarneau and F. Fajula, Catal. Today 66 2001, 75.
165
Variation of the pore properties of mesoporous silica after washing by water and ethanol-water solutions L. Pasqua^, F. Testa'', R. Aiello^, F. Di Renzo^ and F. Fajula'^ ^Universita di Napoli Federico II, DIMP Chimica Applicata, 80125 Napoli, Italy ^Dipartimento di Ingegneria Chimica e dei Materiali, Universita degli Studi della Calabria, Via Pietro Bucci, 87030 Rende, Italy. FAX +39-0984492058. E-mail: [email protected] '^Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS, ENSCM, 8 rue de I'Ecole Normale, 34296 Montpellier Cedex 5, France. Pure-silica and iron-containing mesoporous materials are obtained by different synthesis procedures in the presence of cetyltrimethylammonium cation at neutral or slightly acidic pH. Several salts or hydrochloric acid are used as condensing agents. The property of the samples are characterized as a function of the washing procedure. Washing by water alone does not extract the template and does not affect the stability of the solid. Washing by ethanolic solution is an effective way to extract the template, and severely affect the stability and the porosity of the samples. 1. INTRODUCTION The acid synthesis pathway of mesoporous materials is based on the interaction between neutral or positively charged silica species and micelles of cationic surfactant whose positive charge is compensated by inorganic anions [1]. The polarisation of silica and the nature of the charge-compensating anion influence the properties of the materials [2-4]. Samples obtained from syntheses near the isoelectric point of silica usually undergo an important shrinking upon extraction of the template, due to their thin pore walls [4]. In this way, microporous materials can be obtained from precursor mesophases with 4 nm micelles [5]. Any method of extraction of the template, washing or calcination, brings about a pore shrinking and a corresponding increase of wall thickness. These effects proceed until a threshold of wall thickness is reached, at which the silica surface area is decreased enough to minimize the excess energy of the system [6]. Secondary treatment of the synthesis systems by water has been shown to positively affect the properties of several samples [7, 8]. Nevertheless, degradation or rearrangement of pore system can also occur through thermally induced hydrolysis of silicate upon hydrothermal treatment. Bagshaw et al submitted some assynthesised mesoporous materials to hydrothermal treatment at 373 K and materials with thicker pore walls resulted [9]. In this communication, the effect of washing on mesoporous silicas and iron silicates prepared in the presence of different salts is dealt with. 2. EXPERIMENTAL Molar compositions of the synthesis batches were nFe(NO3)3/MX/0.21 CTMABr/TEOS/146 H2O, with n in the range 0-0.05 and MX a salt or acid among NH4NO3,
166
NaNOs, NH4CI, NaCl, NH4F, HCl. The detailed preparation of samples and preliminary investigations on their stability were presented in a previous report [6]. For each composition, the products of five parallel preparations were washed according to one of five different procedures: (a) filtration without washing, filtration and washing with (b) 300 cm^ water, (c) 900 cm^ water, (d) 50 cm^ ethanol in 300 cm^ of water, (e) 150 cm ethanol in 300 cm of water. The amount of TEOS used in each synthesis batch corresponded to 4 g Si02. The washed samples were characterized by X-ray diffraction, thermogravimetry and N2 sorption on the solid calcined at 823 K in air flow. 3. RESULTS AND DISCUSSION In Figure 1, the evolution of the a parameter of the hexagonal cell is reported as a function of the washing procedure. Washing with water alone does not seem to affect the cell parameter in a significant way, while washing by ethanolic solution brings about a decrease of the cell parameter for all samples.
50
100
150
200
0
washing H2O / cm3 g-i
10
20
30
40
50
washing EtOH / cm3 g-i
Fig. 1. Hexagonal cell parameter of non calcined samples as a function of washing procedure. Amounts of solvent normalised on the total silica present in the system. Samples prepared with NaCl (round dots), HCl (lozenges), Fe and HCl (squares), Fe and NH4F (triangles). I.5O1
0.00
50
100
150
200
washing H2O / cm3 g-i
3
10
20
30
40
50
washing EtOH / cm3 g-i
Fig. 2. Cetyltrimethylammonium content of non calcined samples as a function of washing procedure. Samples as per Figure 1.
167
The variation of the cell parameter can be related to the change of composition of the samples, reported in Figure 2. Washing with water alone is not an effective method to extract the surfactant, while ethanolic solution is able to extract most of the surfactant from the solid. It can be observed that the cell shrinking presents a not linear dependence on the amount of surfactant extracted [10]. The (3c parameter of the hexagonal cell for the calcined materials is reported in Figure 3. Its dependence on the washing procedure closely parallels the effects observed for the cell parameter of the non-calcined materials.
50
100
150
200
0
washing H2O / cm3 g-i
10
20
30
40
50
washing EtOH / cm3
Fig. 3. Hexagonal cell parameter of calcined samples as a function of washing procedure. Samples as per Figure 1. 1.25[ 1.001 0.7^ 0.50 0.25 0.00
50
100
150
200
washing H2O / cm3 g-i
0
10
20
30
40
50
washing EtOH / cm3
Fig. 4. Structural mesopore volume of calcined samples as a function of washing procedure. Samples as per Figure 1. The structural pore volume of the calcined materials are reported in Figure 4. When water alone is used for washing, the pore volume of non-washed solids is somewhat lower than the pore volume of washed solids for all samples except the silica sample prepared in the presence of HCl. It looks like some retention of salts for samples prepared at pH between 2 and 7 can negatively affect the calcination. The evolution of pore volume with ethanolic washing follows the trends of the surfactant content reported in Figure 2, albeit in a nonquantitative way. The decrease of pore volume after washing is lower than the decrease of
168
template content. Very likely, the surfactant left in the solid after ethanolic washing does not occupy the whole available volume. In Figure 5, typical isotherms for a ferrisilicate sample prepared in the presence of HCl are presented. The positive effect of water washing is evident, as well the negative effect of washing by ethanolic solution.
0.4 0.6 Relative pressure P/P^
0.8
1.0
Fig. 5. Nitrogen adsorption-dcsorption isotherms of a ferrisilicate sample prepared in the presence of HCl (a) non-washed, (b) washed with water, (c) washed with ethanolic solution. 4. CONCLUSIONS It can be concluded that, for samples synthesized at neutral or moderately acidic pH, the extraction of template by solvent at room temperature implies a degradation of the structure far exceeding the usual shrinking observed during calcination. REFERENCES 1. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schuth and G.D. Stucky, Nature 368 1994, 317 2. H.P. Lin, S. Cheng and C.Y. Mou, Microporous Mater. 10 1997, 111. 3. H. Yang, G. Vovk, N. Coombs, I. Sokolov and G.A. Ozin, J. Mater. Chem. 8 1998, 743. 4. F. Di Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galarneau, D. Plee and F. Fajula, Microporous Mesoporous Mater. 28 1999, 437. 5. A.C. Voegtlin, A. Matijasic, J. Patarin, C. Sauerland, Y. Grillet and L. Huve, Microporous Mater. 10 1997, 137. 6. L.Pasqua, F. Testa, R. Aiello, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal. 135 2001, 06-P-28. 1. Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 1996, 1147. 8. L. Chen, T. Horiuchi, T. Mori, K. Maeda, J. Phys. Chem. B 103 1999, 1216. 9. S. A. Bagshaw. Stud. Surf. Sci. Catal., 117 1998, 381. 10. F. Di Renzo, D. Desplantier, A. Galarneau and F. Fajula, Catal. Today 66 2001, 75.