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Reactivity of silica walls of mesoporous materials towards benzoyl chloride
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
497
Reactivity of silica walls of mesoporous materials towards benzoyl chloride L. Pasqua^ F. Testa^ R. Aiello^* ^Dipartimento di Ingegneria dei Materiali e della Produzione, Universita di Napoli Federico II, 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] Mesoporous materials synthesised using sodium silicate and an industrial neutral surfactant activated by calcination or solvent extraction are modified by reaction with benzoyl chloride to compare surface properties of both samples. FT-IR spectra of modified samples show a peak centred at 1702 cm"' assigned to the stretching vibration of carbonyl group of benzoic ester. Modified samples also show lower specific surface areas and pore volume compared to the corresponding values for starting materials. Amount of chemical modification in hydrolysis solutions is quantitatively evaluated by UV-spectrofotometry and TG analysis showing that the mesoporous sample activated by solvent extraction possess a much higher reactivity because of high hydroxyl groups population not affected by thermalinduced condensation. 1. INTRODUCTION Mesoporous ordered silicate can have important uses in catalysis, metal ion extraction and optical applications because they are characterized by an hexagonal array of mesopore whose diameters range, depending on synthesis parameters, between 15 and 100 A. On the other hand pore walls can be easily modified so that active sites can be designed to host as better as possible the substrate. Catalytically active materials have been prepared through introduction of inorganic heteroatoms by grafting metallocene complexes on mesoporous silica creating well-defined active sites [1] Silica sources containing a non-hydrolysable Si-C bond can be used to produce hybrid inorganic-organic materials by post-synthetic grafting or by simultaneous condensation [2-4]. In a recent work a functionalised hexagonal mesoporous SBA-15-type molecular sieves have been prepared using non-ionic block copolymers, was used for the immobilization of the enzyme trypsin [5]. MCM-41 was also studied as a drug delivery system. In particular inclusion and delivery mechanism, of Ibuprofen, an antiinflammatory drug, were investigated [6]. In this work mesoporous materials are synthesised using a simple synthesis procedure which employs sodium silicate and an industrial neutral surfactant. Activation of as-synthesised samples is performed according two alternative methods: calcination and solvent extraction by using ethanol-water mixtures. Pore surfaces of activated samples are modified by benzoyl chloride and the different reactivities compared. 2. EXPERIMENTAL The neutral surfactant polyoxyethilene(10)isononylphenylether (Nonfix 10, Condea) was used in the synthesis of sample A. The molar composition of the gel was: SiO2-0.6NaOH-0.064 Nonfix 10-0.8 HCl-58.1 H2O
498
Silica source was sodium silicate. The sample was calcined at 550°C in air with a heating rate of l°C/min. The surfactant was extracted from the as-made materials by three extraction cycles with water/ethanol 2/1 VV at 85 °C. Chemical modification was induced in tetrahydrofuran with benzoyl chloride (Sigma) at room temperature. UV spectra were acquired at 240 nm in corrispondence of the main absorption in the benzoic acid spectrum. A Shimadzu UV 160-A instrument was used. UV quantitative analysis was carried out on solutions (Buffer solution pH 1, Merck) used for hydrolysis of the modified sample. Hydrolysis was carried out at room temperature for 36 h. A 5 standards calibration curve was used whit the buffer solution as blank. Tg and DSC analyses were acquired on a Netzsch STA 409 instrument. 3. RESULTS AND DISCUSSION Table 1 shows SBET and pore volume of active (Acaic and Aextr) and modified samples (AcaicBz and AcxtrBz). Extracted sample shows higher values of specific surface area and pore volume. Table 1 SBET and pore volume of activated and modified samples SBET(niVg) Sample •^calc 791 Aextr 984 Acalc B Z 797 AcxtrBz 638
PV (cm7g) 0.48 0.75 0.46 0.41
Esterification of free silanols on pore walls was evidenced by FT-IR spectroscopy and evaluated by UV-spcctrophotomctry and thcrmogravimctrical analyses. Porosity of modified samples was evaluated by nitrogen adsorption-dcsorption at 77 K. FT-IR spectra of modified samples show a peak at 1702 cm" assigned to the stretching vibration of carbonyl group of benzoic ester. This peak, not present in the spectra of starting materials, disappears after hydrolysis of ester function.
4000
3500
3000 2500 2000 1500 Wavenumbcr (cm" )
1000
Fig. 1. FT-IR spectrum of sample Aextr after chemical modification.
500
499
360 H320 too
280
j1
B 240
^'•^^^X^^'^'—X-"^—
1
=sst^^^^
/
^ 200 —
^3
<
120 •] 0.0
DCS A^_^|^
— • — Ads A^^i^ — ^ — Des A^,^|^ Bz — • — Ads A . . Bz
160 1
0.2
0.8 0.4 0.6 Relative Pressure P/P,,
11
l.O
Fig. 2a. Nitrogen adsorption-desorption isotherms of sample Acaic and sample Acaic after chemical modification (Acaic Bz) 560 OH
H 480 H 00 toD
400 j^ 320
> 240
-e
160^
o C/5
<
0.4 0.6 0.8 Relative pressure P/P„
1.0
Fig. 2b. Nitrogen adsorption-desorption isotherms of sample Aextr and sample Aexi after chemical modification (Aextr Bz) Nitrogen adsorption-desorption isotherms of sample Acaic before and after chemical modification are shown in Fig. 2a. The two couple of curves are almost not distinguishable being just slightly in upper position the unmodified material curves. Presence of benzoyl groups does not affect pore structures of calcined samples. Fig. 2b shows nitrogen adsorptiondesorption isotherms of sample Aextr and sample Aextr after chemical modification (AextrBz). The presence of benzoic ester is able to shift to lower relative pressures the main nitrogen adsorption, indicating a reduction in average pore diameter and, moreover drastically reduces pore volume. UV quantitative measurements of benzoic acid concentration in hydrolysis solution show that, as expected, chemical modification on sample activated by solvent extraction is much larger in comparison with calcined sample. Benzoic acid/silica mass ratios derived from benzoic acid concentrations detected in hydrolysis solutions are 4.30»10''^ and 6.95»10'^ for
500
solutions coming from hydrolysis on calcined and extracted sample, respectively. Nevertheless benzoic acid is only partially released in hydrolysis solution. TG and DSC analysis were used to evaluate surface properties of both calcined and extracted materials and their modified derivatives. DSC analysis shows the exothermal peak assigned to the combustion of benzoyl group around 400°C. A blank thermogravimetric analysis was runned on template-free samples in order to compare the weight loss due to the high temperature condensation of vicinal and geminal hydroxyls in calcined and extracted samples. An OH group concentration of 0.23 and 0.56 mol in lOOg of dry Si02 in was calculated for calcined and extracted samples respectively. Benzoic group/dry composite sample mass ratio from thermogravimetric analysis of modified samples arc 4.12-10'^ and 2.30-10"' for materials obtained from calcined and extracted samples, respectively. Table2 Quantitative data on benzoyl and hydroxyl modification Sample Bz/(Si02 + Bz OH cone. Bz/(Si02 + Bz) Bz/Si02 + Bz (mass ratio)** (mol/lOOg) + ads. H2O) mol/lOOg (mass ratio) Dry Si02 0.042 4.12-10-^ 4.30-10-'^* 0.23 Acalc BZ 0.23 2.30-10-' 6.95-10-^* 0.56 AcxtrBz *Data obtained from UV quantitative measurements of hydrolysis solution **Data obtained from TG analyses
%ofOH reacted with Bz 18 41
0.042 and 0.23 moles of benzoyl group arc contained in 100 g of dry composite samples indicating that 18% and 41% of detected silanols, respectively, for calcined and extracted samples react with the carbonil group. Isolated silanols, not detectable, arc not considered in the total OH group concentration. Nevertheless they can react with benzoyl chloride. 4. CONCLUSIONS UV quantitative measurements show that the release of benzoic acid in hydrolysis solutions is not complete. Mcsoporous silica activated by solvent extraction exhibits a much higher surface reactivity compared to calcined samples. Hydroxyl group population remain quasi intact after solvent extraction of surfactant allowing massive reaction with benzoyl chloride. Calcination drastically reduces OH groups so decreasing the reactivity of pore walls.
REFERENCES 1. 2. 3. 4. 5. 6.
T. Maschmeier, F. Rey, G. Sankar and J.M. Thomas. Nature, 378 1995, 159. S.L. Burkett, S.D. Sims and S. Mann. Chem. Commun, 1996. 1367. M.H. Lim and A. Stein. Chem. Mater. 11 .1999, 3285. Brunei D. . Microp. and Mesop. Mater., 27.1999, 329. H.H.P. Yiu, P.A. Wright, N. P. Botting, J. Molee. Catal. B: Enzymatic 15, 2001, 81 M. Vallet-Regi, A Ramila, R:P. Del Real and J. Perez Pariente. Chem. Mater., 13 2001, 308.
497
Reactivity of silica walls of mesoporous materials towards benzoyl chloride L. Pasqua^ F. Testa^ R. Aiello^* ^Dipartimento di Ingegneria dei Materiali e della Produzione, Universita di Napoli Federico II, 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] Mesoporous materials synthesised using sodium silicate and an industrial neutral surfactant activated by calcination or solvent extraction are modified by reaction with benzoyl chloride to compare surface properties of both samples. FT-IR spectra of modified samples show a peak centred at 1702 cm"' assigned to the stretching vibration of carbonyl group of benzoic ester. Modified samples also show lower specific surface areas and pore volume compared to the corresponding values for starting materials. Amount of chemical modification in hydrolysis solutions is quantitatively evaluated by UV-spectrofotometry and TG analysis showing that the mesoporous sample activated by solvent extraction possess a much higher reactivity because of high hydroxyl groups population not affected by thermalinduced condensation. 1. INTRODUCTION Mesoporous ordered silicate can have important uses in catalysis, metal ion extraction and optical applications because they are characterized by an hexagonal array of mesopore whose diameters range, depending on synthesis parameters, between 15 and 100 A. On the other hand pore walls can be easily modified so that active sites can be designed to host as better as possible the substrate. Catalytically active materials have been prepared through introduction of inorganic heteroatoms by grafting metallocene complexes on mesoporous silica creating well-defined active sites [1] Silica sources containing a non-hydrolysable Si-C bond can be used to produce hybrid inorganic-organic materials by post-synthetic grafting or by simultaneous condensation [2-4]. In a recent work a functionalised hexagonal mesoporous SBA-15-type molecular sieves have been prepared using non-ionic block copolymers, was used for the immobilization of the enzyme trypsin [5]. MCM-41 was also studied as a drug delivery system. In particular inclusion and delivery mechanism, of Ibuprofen, an antiinflammatory drug, were investigated [6]. In this work mesoporous materials are synthesised using a simple synthesis procedure which employs sodium silicate and an industrial neutral surfactant. Activation of as-synthesised samples is performed according two alternative methods: calcination and solvent extraction by using ethanol-water mixtures. Pore surfaces of activated samples are modified by benzoyl chloride and the different reactivities compared. 2. EXPERIMENTAL The neutral surfactant polyoxyethilene(10)isononylphenylether (Nonfix 10, Condea) was used in the synthesis of sample A. The molar composition of the gel was: SiO2-0.6NaOH-0.064 Nonfix 10-0.8 HCl-58.1 H2O
498
Silica source was sodium silicate. The sample was calcined at 550°C in air with a heating rate of l°C/min. The surfactant was extracted from the as-made materials by three extraction cycles with water/ethanol 2/1 VV at 85 °C. Chemical modification was induced in tetrahydrofuran with benzoyl chloride (Sigma) at room temperature. UV spectra were acquired at 240 nm in corrispondence of the main absorption in the benzoic acid spectrum. A Shimadzu UV 160-A instrument was used. UV quantitative analysis was carried out on solutions (Buffer solution pH 1, Merck) used for hydrolysis of the modified sample. Hydrolysis was carried out at room temperature for 36 h. A 5 standards calibration curve was used whit the buffer solution as blank. Tg and DSC analyses were acquired on a Netzsch STA 409 instrument. 3. RESULTS AND DISCUSSION Table 1 shows SBET and pore volume of active (Acaic and Aextr) and modified samples (AcaicBz and AcxtrBz). Extracted sample shows higher values of specific surface area and pore volume. Table 1 SBET and pore volume of activated and modified samples SBET(niVg) Sample •^calc 791 Aextr 984 Acalc B Z 797 AcxtrBz 638
PV (cm7g) 0.48 0.75 0.46 0.41
Esterification of free silanols on pore walls was evidenced by FT-IR spectroscopy and evaluated by UV-spcctrophotomctry and thcrmogravimctrical analyses. Porosity of modified samples was evaluated by nitrogen adsorption-dcsorption at 77 K. FT-IR spectra of modified samples show a peak at 1702 cm" assigned to the stretching vibration of carbonyl group of benzoic ester. This peak, not present in the spectra of starting materials, disappears after hydrolysis of ester function.
4000
3500
3000 2500 2000 1500 Wavenumbcr (cm" )
1000
Fig. 1. FT-IR spectrum of sample Aextr after chemical modification.
500
499
360 H320 too
280
j1
B 240
^'•^^^X^^'^'—X-"^—
1
=sst^^^^
/
^ 200 —
^3
<
120 •] 0.0
DCS A^_^|^
— • — Ads A^^i^ — ^ — Des A^,^|^ Bz — • — Ads A . . Bz
160 1
0.2
0.8 0.4 0.6 Relative Pressure P/P,,
11
l.O
Fig. 2a. Nitrogen adsorption-desorption isotherms of sample Acaic and sample Acaic after chemical modification (Acaic Bz) 560 OH
H 480 H 00 toD
400 j^ 320
> 240
-e
160^
o C/5
<
0.4 0.6 0.8 Relative pressure P/P„
1.0
Fig. 2b. Nitrogen adsorption-desorption isotherms of sample Aextr and sample Aexi after chemical modification (Aextr Bz) Nitrogen adsorption-desorption isotherms of sample Acaic before and after chemical modification are shown in Fig. 2a. The two couple of curves are almost not distinguishable being just slightly in upper position the unmodified material curves. Presence of benzoyl groups does not affect pore structures of calcined samples. Fig. 2b shows nitrogen adsorptiondesorption isotherms of sample Aextr and sample Aextr after chemical modification (AextrBz). The presence of benzoic ester is able to shift to lower relative pressures the main nitrogen adsorption, indicating a reduction in average pore diameter and, moreover drastically reduces pore volume. UV quantitative measurements of benzoic acid concentration in hydrolysis solution show that, as expected, chemical modification on sample activated by solvent extraction is much larger in comparison with calcined sample. Benzoic acid/silica mass ratios derived from benzoic acid concentrations detected in hydrolysis solutions are 4.30»10''^ and 6.95»10'^ for
500
solutions coming from hydrolysis on calcined and extracted sample, respectively. Nevertheless benzoic acid is only partially released in hydrolysis solution. TG and DSC analysis were used to evaluate surface properties of both calcined and extracted materials and their modified derivatives. DSC analysis shows the exothermal peak assigned to the combustion of benzoyl group around 400°C. A blank thermogravimetric analysis was runned on template-free samples in order to compare the weight loss due to the high temperature condensation of vicinal and geminal hydroxyls in calcined and extracted samples. An OH group concentration of 0.23 and 0.56 mol in lOOg of dry Si02 in was calculated for calcined and extracted samples respectively. Benzoic group/dry composite sample mass ratio from thermogravimetric analysis of modified samples arc 4.12-10'^ and 2.30-10"' for materials obtained from calcined and extracted samples, respectively. Table2 Quantitative data on benzoyl and hydroxyl modification Sample Bz/(Si02 + Bz OH cone. Bz/(Si02 + Bz) Bz/Si02 + Bz (mass ratio)** (mol/lOOg) + ads. H2O) mol/lOOg (mass ratio) Dry Si02 0.042 4.12-10-^ 4.30-10-'^* 0.23 Acalc BZ 0.23 2.30-10-' 6.95-10-^* 0.56 AcxtrBz *Data obtained from UV quantitative measurements of hydrolysis solution **Data obtained from TG analyses
%ofOH reacted with Bz 18 41
0.042 and 0.23 moles of benzoyl group arc contained in 100 g of dry composite samples indicating that 18% and 41% of detected silanols, respectively, for calcined and extracted samples react with the carbonil group. Isolated silanols, not detectable, arc not considered in the total OH group concentration. Nevertheless they can react with benzoyl chloride. 4. CONCLUSIONS UV quantitative measurements show that the release of benzoic acid in hydrolysis solutions is not complete. Mcsoporous silica activated by solvent extraction exhibits a much higher surface reactivity compared to calcined samples. Hydroxyl group population remain quasi intact after solvent extraction of surfactant allowing massive reaction with benzoyl chloride. Calcination drastically reduces OH groups so decreasing the reactivity of pore walls.
REFERENCES 1. 2. 3. 4. 5. 6.
T. Maschmeier, F. Rey, G. Sankar and J.M. Thomas. Nature, 378 1995, 159. S.L. Burkett, S.D. Sims and S. Mann. Chem. Commun, 1996. 1367. M.H. Lim and A. Stein. Chem. Mater. 11 .1999, 3285. Brunei D. . Microp. and Mesop. Mater., 27.1999, 329. H.H.P. Yiu, P.A. Wright, N. P. Botting, J. Molee. Catal. B: Enzymatic 15, 2001, 81 M. Vallet-Regi, A Ramila, R:P. Del Real and J. Perez Pariente. Chem. Mater., 13 2001, 308.