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Effects of isomorphous substitution of Si with Ti and Zr in mesoporous silicates with the MCM-41 structure
Applied Catalysis A: General 183 (1999) 231±239
Effects of isomorphous substitution of Si with Ti and Zr in mesoporous silicates with the MCM-41 structure M.L. Occellia,*, S. Biza, A. Aurouxb a
Zeolites and Clays Program, GTRI Georgia Institute of Technology, Atlanta, GA 30332, USA Institut de Recherches sur la Catalyse, CNRS 2 Av. A. Einstein, 69626 Villeurbanne, France
b
Received 6 December 1998; accepted 15 February 1999
Abstract Mesoporous silica and mesoporous silicates can be easily synthesized from the hydrothermal transformation at 1108C, of hydrogels with composition, M:25.95SiO2:5.2(C2H5)4NOH:7.5(CH3(CH2)15N(CH3)3)2O:790H2O, where M0, Zr(OC3H7)4 or Ti(OC4H9)4. These MCM-41 type crystals have Si/M molar ratio similar to the one in the parent hydrogel. After calcination at 6008C/12 h in air, the solids have surface area and pore volume in the 940±1740 m2/g range and 0.8±1.2 cm3/g range, respectively. The small mesopores have an average pore diameter (APD) of 2.8 nm and are separated by walls 1.30±1.65 nm thick. The isomorphous substitution of Si with Ti or Zr has little effects on pore size and on pore wall thickness. However, thermogravimetric analysis has indicated that charge compensating organic cations are located on different sorption sites and that site strength increases in the order: Si±OH
1. Introduction It has been estimated that heterogeneous catalysts generate fuel and chemicals contributing to half of the country GNP [1]. The worldwide catalyst sales now exceed $ 5.9 billion/year. The usefulness of a heterogeneous catalyst is controlled by its pore size. Accord-
*Corresponding author. Tel.: +1-404-303-7958; e-mail: [email protected]
ing to IUPAC [2], three types of pores can be present in a solid depending on d, the pore diameter: micropores (d<2.0 nm), mesopores (2.0 nm50 nm). However, most of the technologically important catalytic reactions have been performed with microporous materials such as zeolites with the faujasite structure. Microporous solids cannot process high molecular weight compounds of the type used in certain pharmaceutical and in homogeneous reactions. The demand for larger pore catalysts has triggered major
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M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
synthetic efforts in academic and industrial laboratories that culminated in 1990 when Yanagiswa et al. [3] ®rst reported the synthesis of mesoporous silica and workers at Mobil reported the synthesis of mesoporous aluminosilicates [4]. A recent review of the properties of these materials and of their possible applications can be found elsewhere [5,6]. Silicas are microporous solids used in a variety of commercially important applications that include dessicants, oligomerization catalysts, hydrogenation catalysts and catalysts for vinyl acetate (VA) preparation [7]. VA is an important chemical intermediate produced by the gas phase acetoxydation of ethylene over supported noble metals catalysts [7,8]. Although this process has been commercialized in 1968 it seems that all VA plants use the same type of catalyst consisting of Pd supported on a silica matrix having maybe, different promoters (such as Cd or Au) to stabilize Pd±silica interactions [9,10]. New silica supports could lead to new VA and hydrogenation catalysts. It is the purpose of this communication to report the synthesis and characterization of mesoporous silicas and silicates containing Zr or Ti that could be of interest as metals support in catalysts preparation. 2. Experimental 2.1. Synthesis Mesoporous materials were prepared by heating for three days at 1108C without stirring, hydrogels with molar composition: M : 26:0SiO2 : 5:2 C2 H5 4 NOH : 7:5 CH3 CH2 15 N CH3 3 2 O : 790H2 O where M0, Zr(OC3H7)4 or Ti(OC4H9)4. The hydrogels were prepared by adding to colloidal silica (Ludox AS-40, from Du Pont), a mixture obtained by reacting a 25% cetyltrimethyl ammonium chloride (C16H33(CH3)3NClÿ, from Aldrich ) solution with a mixture prepared by adding a 70% solution of zirconium propoxide (ZrPOX, from Aldrich) in butanol or Ti(IV) n-butoxide (TiBOX, from Aldrich), to 20% tetraethylammoniumhydroxide ((C2H5)4NOHÿ) from Sachem) in water. Hydrogels were then heated at 1108C/3d in Te¯on lined 750 ml Berghof autoclaves
equipped with temperature controllers and stirring mechanism. Crystals were separated from the mother liquor by ®ltration, washed with an excess of DI water at about 608C and dried in air at 1108C overnight. The oven temperature was then raised at a rate of 28C/min to 6008C and maintained at this value for 12 h. 2.2. Characterization X-ray diffractograms were obtained with a Scintag diffractometer using Cu K radiation at a speed of 1.08/min. Chemical analyses were performed by Galbraith Laboratories, Knoxville, TN. Surface area and pore volume were measured by nitrogen sorption at 77 K with an ASAP-2010 porosimeter from Micromeritics. Prior to nitrogen adsorption, the samples were degassed in vacuum at 3508C/12 h. Pore size distributions were obtained with the BJH method using the Harkins and Jura's thickness equation for multilayer thickness (ASAP 2010 built-on software, from Micromeritics). Thermogravimetric analyses (TGA and DTA) were obtained on a Seiko 210/310 equipped with 3.0 version software and a Perkin-Elmer 7.0 analyzer. Compressed nitrogen or air was introduced at a ¯ow rate of 80 ml/min. Samples were heated at a rate of 10.08/min up to 1008C. Heat of adsorption of NH3 was measured using a heat-¯ow microcalorimeter (of the Tian-Calvet type from Setaram) linked to a glass volumetric line. Successive doses of gas were sent onto the sample until a ®nal equilibrium pressure of 133 Pa was obtained. The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge from Datametrics. The adsorption temperature was maintained at 808C. Primary and secondary isotherms were collected at this temperature. All samples were degassed overnight under vacuum at 4008C before calorimetric measurements were undertaken. The accuracy of the data reported is 4 kJ/mol or 1 kcal/mol. 3. Results and discussion The isomorphous substitution of Si by Ti or by Zr can be accomplished by a variety of synthesis conditions based on the use of different Si/M (MTi, Zr)
M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
molar ratio, surfactants and silica sources [11±16]. Synthesis procedures to prepare these mesoporous materials have been reviewed elsewere [17]. 3.1. XRD results Physicochemical properties of the crystals are summarized in Table 1. The X-ray diffractogram of the materials (after calcination in air at 6008C/12 h) in addition to a sharp d100 re¯ection line near 22.58, has a broad peak in the 38±58 2 range attributed to broadening effects of higher re¯ection lines due to small particle sizes [18]. Before calcination, (Zr,Si)MCM and (Ti,Si)-MCM crystals (with Si/M24.7) have a d100 re¯ection at 4.11 and 4.33 nm. After calcination these d100 values shift to 3.83 and 3.77 nm, respectively. Irrespective of framework composition, removal of the organic component is accompanied by a small (<10%) contraction of the unit cell owing to the release on calcination, of strains imposed on the mesoporous framework by the bulky organic template and by the condensation of adjacent silanol groups. Results in Table 1 show that these synthesis procedures give reproducible results (samples 2 and 3, 8 and 9) and that stirring during the gel hydrothermal transformation to MCM-41 type crystals has little effects on material properties (samples 4 and 5, 7 and 8). Increasing Zr (or Ti) incorporation in the silicate framework decreases crystal's quality (samples 5, 6 and 11). In preparing (Si,Ti)-MCM-41 crystals, Ti(OC4H9)4 can be replaced by Ti(OC3H7)4
233
without affecting crystal's quality (sample 9). The dropwise addition of TiCl3 (in 1.9 M HCl) to the titaniumsilicate hydrogel (so that the Ti2O3/ Ti(OC4H9)4 molar ratio is 0.6), decreased the gel's pH to 10.2 from 12.4. After crystallization, this gel yields mesoporous materials characterized by high pore volume and Ti content (sample 11). Pore wall thickness (W) data in Table 1 has been estimated by subtracting the average pore diameter (APD) value p from a0 2d100 = 3, the hexagonal unit cell dimension. With the synthesis conditions used, the thickness of the condensed silicate phase that constitutes the pore wall is higher than in similarly prepared mesoporous aluminosilicate [17], and varies between 1.30 and 1.65 nm. In addition, results in Table 1 indicate that the isomorphous substitution of Si with Zr or Ti has little effects on the APD although the wall thickness of the calcined materials increased. Materials containing either Zr or Ti have comparable wall thickness. 3.2. Sorption isotherms Nitrogen sorption isotherms at liquid nitrogen temperature are presented in Fig. 1. At P/P00.5, the totality of accessible pores are ®lled with adsorbate and the isotherm reaches a plateau that remains fairly invariant as P/P0 approaches unity. Nitrogen sorption isotherms of the silicates with the MCM-41 structure, are type IV isotherms usually seen in mesoporous materials [19,20]. The total mesopore volume was
Table 1 Some properties of mesoporous metalsilicates; MZr, Ti (samples have been calcined at 600C/12 h in air; Sstirring and Rrepeat) Sample
Si/M
d100 (nm)
SA (m2/g)
PV (cm3/g)
APD (nm)
W (nm)
1. Silica gel 2. MLOS-94 C 3. MLOS-94 CR
± ± ±
± 3.60 3.60
639 1731 1720
0.35 1.20 1.20
2.20 2.80 2.80
± 1.32 1.32
4. MLOS-93BS 5. MLOS-93 B 6. MLOS-93 G
24.7 24.4 12.6
3.83 ± 3.97
1136 1160 796
0.99 0.98 0.78
2.80 ± ±
1.62 ± ±
7. MLOS-93AS 8. MLOS-93 A 9. MLOS-93 AR 10. MLOS-93 D 11. MLOS-93F
24.7 23.7 24.1 22.8 14.6
3.77 3.77 3.88 4.09 4.55
944 1198 1301 1138 570
0.67 0.70 0.95 0.92 0.92
2.82 ± ± ± ±
1.53 ± ± ± ±
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Fig. 1. Nitrogen sorption isotherms of (Si,Zr)-MCM-41 materials as a function of their Si/Zr molar ratios.
derived from the amount of vapor adsorbed at P/ P00.50 by assuming that all the mesopores were then ®lled with condensed liquid nitrogen in the normal liquid state, see Table 1. In Fig. 1, the deleterious effects of M(IV) metals insertion in the mesoporous silicate framework are well illustrated. Structural order decreases with Zr introduction and as the solid's SiO2/ZrO2 ratio reaches 3.2, type I isotherms (typical of microporous materials such as silica gel) are observed. Similar effects have been observed when Al was introduced into the silicate mesostructure structure [17]. 3.3. Thermal analysis Thermogravimetric analysis of the crystals show distinct weight losses that depend, in part, on framework composition [21±23]. For the parent Si-MCM-
41, the minor (2%) weight loss below 1508C corresponds to the desorption of physisorbed water (or ethanol) in the voids formed by crystals agglomeration. Above 2808C, breakage, decomposition and thermal desorption of organic fragments occur. The large (34%) weight change in the 150±3008C temperature region is attributed to losses of alkyl chains from micelle decomposition and to losses of water and ethanol molecules occluded at the organic/inorganic interface; Fig. 2 (A). Above 3008C, combustion of the residual organics occurs and a small (6%) but sudden weight loss can be seen together with a sharp exotherm near 3048C, see Fig. 2(A). The last 6% weight decrease above 3508C is attributed to water losses resulting from dehydroxylation reactions at the silicate surface. For Zr or Ti substituted Si-MCM-41 samples, total weight losses at 8008C remain in the 50±55% range.
M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
Fig. 2. Thermogravimetric profiles in air of: (A) Si-MCM-41 (B) (Si,Zr)-MCM-41 and (C) (Si,Ti)-MCM-41 materials.
235
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M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
Fig. 2. (Continued )
However, the distribution of successive weight losses depends on the framework composition; see Fig. 2(B) and (C). Between 2808C and 3408C, the oxidative decomposition of residual organic compounds in SiMCM-41 occurs in one step and after introducing Ti or Zr, these combustion reactions occur in two steps and are accompanied by two exotherms; Fig. 2(B) and (C). Similar results have been observed after introducing Al atoms in the silicate framework [17]. This suggests the existence of different sorption site strengths as a result of Ti or Zr incorporation in the silicate framework. Sorption site strengths seem to increase in the order: Si±OH
MCM-41 (sample 2, Table 1) and increases as Si is replaced with Zr(IV) or Ti(IV) atoms (samples 4 and 7, Table 1). The Si,Ti-MCM-41 sample with Si/ Ti14.6, although having the lowest surface area, can sorb more than twice the NH3 sorbed by the other mesoporous silicates. Not shown are secondary sorption isotherms, that is, sorption isotherms for samples after NH3 adsorption and degassing in vacuum at 1508C for silica gel and at 808C for the other samples. By subtracting the adsorbed volume of the secondary isotherms from that of the primary isotherms at the same equilibrium pressure (p0.2 Torr), it is possible to obtain Virr that is, the volume of irreversibly chemisorbed NH3. This value is believed to represent the total number of strong acid sites in the solids under study [24]. Initial heats, together with sites distribution and strength data in Table 2 clearly indicate that these mesoporous materials are weak solid acids and that introduction of heteroaroms such as Ti and Zr increases the solids' acidity. Acid sites density is the largest in (Si,Ti)MCM-41. Differential heat pro®les in Fig. 3(B) indicate that, for a given Si/M ratio, acidity increases in
M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
237
Fig. 3. (A) Ammonia sorption isotherms at 808C; and (B) differential heat profiles as a function of ammonia uptake.
the order: silica gel
mol range, absent in all the other materials examined, see Fig. 3(B). The acid sites density and strength in Table 2 is signi®cantly lower than the one measured in mesoporous aluminosilicates with the MCM-41 structure [17] and in amorphous aluminosilicates of the type used in hydrotreating catalyst preparations [26].
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Table 2 Ammonia chemisorption data at 808C (p0.2 Torr) (population of sites with given strength is in mmol NH3/g; MZr, Ti) Sample
1. 2. 3. 4. 5. 6.
Silica gel Si-MCM-41 (Si,Zr)-MCM-41 (Si,Zr)-MCM-41 (Si,Ti)-MCM-41 (Si,Ti)-MCM-41
Si/M
± ± 24.7 12.6 24.7 14.6
In. Heat (kJ/mol)
9 112 126 113 111 119
4. Summary and conclusions Silicon atoms in mesoporous silica with the MCM41 structure can be easily replaced by M(IV), atoms over a broad range of synthesis conditions, MTi or Zr. As the Si/M ratio decreases, structural order decreases and with it the solid's surface area and pore volume. Evidence of a mesoporous structure disappears at Si/M<5. These silicates have an APD similar to the one in the parent mesoporous silica prepared using the same surfactant but exhibit a pore wall thickness larger than the one in similarly prepared mesoporous aluminosilicates. Thermogravimetric analysis of these solids are essentially similar and all indicate the existence of different adsorption sites. The strength of these sites increases in the order: Si± OH
NH3 (mmol/g)
kJ/mol
VT
Virr
<90
>90
91 123 131 143 176 202
± 41 47 42 51 46
91 110 109 109 146 168
± 13 22 34 30 34
with initial materials preparation and XRD characterization. References [1] Federal Register, vol. 57, no. 13, January 1992. [2] IUPAC Manual of Symbols and Terminology, Pure Appl. Chem. 31 (1978) 578. [3] T. Yanagiswa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 988±992. [4] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [5] T.J. Pinnavaia, M.F. Thorpe, in: Access In Nanoporous Materials, Plenum Press, New York, 1995. [6] S. Biz, M.L. Occelli, Catal. Rev.-Sci. Eng. 40 (1998) 329± 407. [7] S. Nakamura, T. Yasui, J. Catal. 17 (1970) 336. [8] M. Dettmer, A. Renken, Ger. Chem. Eng. 6 (1983) 356. [9] Eur. Chem. News 11, 272, 40 (1967). [10] World Pet. Congress Proceeding, vol. 5, 1968, p. 41. [11] A. Corma, M.T. Navarro, J. Perez-Pariente, J. Chem. Soc., Chem. Commun. 147 (1994). [12] P.T. Tanev, M. Chbwe, T.J. Pinnavaia, Nature 368 (1994) 321. [13] S. Gontier, A. Tuel, Zeolites 15 (1995) 601. [14] J.S. Reddy, A. Dicko, A. Sayari, in: M.L. Occelli, H. Kessler (Eds.), Synthesis of Porous Materials: Zeolites Clays and Nanostructures, Marcel Dekker, New Work, 1997, p. 405. [15] A. Tuel, S. Gontier, R. Teissier, J. Chem. Soc., Chem. Commun. (1996) 651. [16] D.J. Jones, l. Jimenez-Jimenez, A. Jimenez-Lopez, P. Maireles-Torres, P. Olivera-Pastor, E. Rodriguez-Castellon, J. Roziere, Chem. Commun. 431 (1997). [17] M.L. Occelli, S. Biz, A. Auroux, G.J. Ray, Microporous and Macroporous Mater., in press. [18] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865±867. [19] P.J. Branton, P.G. Hall, K.S.W. Sing, J. Chem. Soc., Chem. Commun. (1993) 1257±1258. [20] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, New York, 1982.
M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239 [21] M. Busio, J. Janchen, J.H.C. van Hooff, Microporous Mater. (1995) 211±218. [22] Cong-Yan Chen, Hong-xin Li, M. Davis, Microporous Mater. 2 (1993) 17. [23] R. Schmidt, D. Akporiaye, M. Stocker, O. Ellestad, Stud. Surf. Sci. Catal. 84 (1994) 61±68.
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[24] A. Auroux, in: B. Imelik, J.C. Edrine (Eds.), Catalyst Characterization: Physical Techniques for Solid Materials, chapter 22, Plenum Press, New York, 1994. [25] A. Auroux, Topics Catal. 4 (1997) 71±89. [26] M.L. Occelli, S. Biz, A. Auroux, P.S. Iyer, Appl. Catal., in press.
Effects of isomorphous substitution of Si with Ti and Zr in mesoporous silicates with the MCM-41 structure M.L. Occellia,*, S. Biza, A. Aurouxb a
Zeolites and Clays Program, GTRI Georgia Institute of Technology, Atlanta, GA 30332, USA Institut de Recherches sur la Catalyse, CNRS 2 Av. A. Einstein, 69626 Villeurbanne, France
b
Received 6 December 1998; accepted 15 February 1999
Abstract Mesoporous silica and mesoporous silicates can be easily synthesized from the hydrothermal transformation at 1108C, of hydrogels with composition, M:25.95SiO2:5.2(C2H5)4NOH:7.5(CH3(CH2)15N(CH3)3)2O:790H2O, where M0, Zr(OC3H7)4 or Ti(OC4H9)4. These MCM-41 type crystals have Si/M molar ratio similar to the one in the parent hydrogel. After calcination at 6008C/12 h in air, the solids have surface area and pore volume in the 940±1740 m2/g range and 0.8±1.2 cm3/g range, respectively. The small mesopores have an average pore diameter (APD) of 2.8 nm and are separated by walls 1.30±1.65 nm thick. The isomorphous substitution of Si with Ti or Zr has little effects on pore size and on pore wall thickness. However, thermogravimetric analysis has indicated that charge compensating organic cations are located on different sorption sites and that site strength increases in the order: Si±OH
1. Introduction It has been estimated that heterogeneous catalysts generate fuel and chemicals contributing to half of the country GNP [1]. The worldwide catalyst sales now exceed $ 5.9 billion/year. The usefulness of a heterogeneous catalyst is controlled by its pore size. Accord-
*Corresponding author. Tel.: +1-404-303-7958; e-mail: [email protected]
ing to IUPAC [2], three types of pores can be present in a solid depending on d, the pore diameter: micropores (d<2.0 nm), mesopores (2.0 nm
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 0 5 9 - 9
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M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
synthetic efforts in academic and industrial laboratories that culminated in 1990 when Yanagiswa et al. [3] ®rst reported the synthesis of mesoporous silica and workers at Mobil reported the synthesis of mesoporous aluminosilicates [4]. A recent review of the properties of these materials and of their possible applications can be found elsewhere [5,6]. Silicas are microporous solids used in a variety of commercially important applications that include dessicants, oligomerization catalysts, hydrogenation catalysts and catalysts for vinyl acetate (VA) preparation [7]. VA is an important chemical intermediate produced by the gas phase acetoxydation of ethylene over supported noble metals catalysts [7,8]. Although this process has been commercialized in 1968 it seems that all VA plants use the same type of catalyst consisting of Pd supported on a silica matrix having maybe, different promoters (such as Cd or Au) to stabilize Pd±silica interactions [9,10]. New silica supports could lead to new VA and hydrogenation catalysts. It is the purpose of this communication to report the synthesis and characterization of mesoporous silicas and silicates containing Zr or Ti that could be of interest as metals support in catalysts preparation. 2. Experimental 2.1. Synthesis Mesoporous materials were prepared by heating for three days at 1108C without stirring, hydrogels with molar composition: M : 26:0SiO2 : 5:2 C2 H5 4 NOH : 7:5 CH3 CH2 15 N CH3 3 2 O : 790H2 O where M0, Zr(OC3H7)4 or Ti(OC4H9)4. The hydrogels were prepared by adding to colloidal silica (Ludox AS-40, from Du Pont), a mixture obtained by reacting a 25% cetyltrimethyl ammonium chloride (C16H33(CH3)3NClÿ, from Aldrich ) solution with a mixture prepared by adding a 70% solution of zirconium propoxide (ZrPOX, from Aldrich) in butanol or Ti(IV) n-butoxide (TiBOX, from Aldrich), to 20% tetraethylammoniumhydroxide ((C2H5)4NOHÿ) from Sachem) in water. Hydrogels were then heated at 1108C/3d in Te¯on lined 750 ml Berghof autoclaves
equipped with temperature controllers and stirring mechanism. Crystals were separated from the mother liquor by ®ltration, washed with an excess of DI water at about 608C and dried in air at 1108C overnight. The oven temperature was then raised at a rate of 28C/min to 6008C and maintained at this value for 12 h. 2.2. Characterization X-ray diffractograms were obtained with a Scintag diffractometer using Cu K radiation at a speed of 1.08/min. Chemical analyses were performed by Galbraith Laboratories, Knoxville, TN. Surface area and pore volume were measured by nitrogen sorption at 77 K with an ASAP-2010 porosimeter from Micromeritics. Prior to nitrogen adsorption, the samples were degassed in vacuum at 3508C/12 h. Pore size distributions were obtained with the BJH method using the Harkins and Jura's thickness equation for multilayer thickness (ASAP 2010 built-on software, from Micromeritics). Thermogravimetric analyses (TGA and DTA) were obtained on a Seiko 210/310 equipped with 3.0 version software and a Perkin-Elmer 7.0 analyzer. Compressed nitrogen or air was introduced at a ¯ow rate of 80 ml/min. Samples were heated at a rate of 10.08/min up to 1008C. Heat of adsorption of NH3 was measured using a heat-¯ow microcalorimeter (of the Tian-Calvet type from Setaram) linked to a glass volumetric line. Successive doses of gas were sent onto the sample until a ®nal equilibrium pressure of 133 Pa was obtained. The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge from Datametrics. The adsorption temperature was maintained at 808C. Primary and secondary isotherms were collected at this temperature. All samples were degassed overnight under vacuum at 4008C before calorimetric measurements were undertaken. The accuracy of the data reported is 4 kJ/mol or 1 kcal/mol. 3. Results and discussion The isomorphous substitution of Si by Ti or by Zr can be accomplished by a variety of synthesis conditions based on the use of different Si/M (MTi, Zr)
M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
molar ratio, surfactants and silica sources [11±16]. Synthesis procedures to prepare these mesoporous materials have been reviewed elsewere [17]. 3.1. XRD results Physicochemical properties of the crystals are summarized in Table 1. The X-ray diffractogram of the materials (after calcination in air at 6008C/12 h) in addition to a sharp d100 re¯ection line near 22.58, has a broad peak in the 38±58 2 range attributed to broadening effects of higher re¯ection lines due to small particle sizes [18]. Before calcination, (Zr,Si)MCM and (Ti,Si)-MCM crystals (with Si/M24.7) have a d100 re¯ection at 4.11 and 4.33 nm. After calcination these d100 values shift to 3.83 and 3.77 nm, respectively. Irrespective of framework composition, removal of the organic component is accompanied by a small (<10%) contraction of the unit cell owing to the release on calcination, of strains imposed on the mesoporous framework by the bulky organic template and by the condensation of adjacent silanol groups. Results in Table 1 show that these synthesis procedures give reproducible results (samples 2 and 3, 8 and 9) and that stirring during the gel hydrothermal transformation to MCM-41 type crystals has little effects on material properties (samples 4 and 5, 7 and 8). Increasing Zr (or Ti) incorporation in the silicate framework decreases crystal's quality (samples 5, 6 and 11). In preparing (Si,Ti)-MCM-41 crystals, Ti(OC4H9)4 can be replaced by Ti(OC3H7)4
233
without affecting crystal's quality (sample 9). The dropwise addition of TiCl3 (in 1.9 M HCl) to the titaniumsilicate hydrogel (so that the Ti2O3/ Ti(OC4H9)4 molar ratio is 0.6), decreased the gel's pH to 10.2 from 12.4. After crystallization, this gel yields mesoporous materials characterized by high pore volume and Ti content (sample 11). Pore wall thickness (W) data in Table 1 has been estimated by subtracting the average pore diameter (APD) value p from a0 2d100 = 3, the hexagonal unit cell dimension. With the synthesis conditions used, the thickness of the condensed silicate phase that constitutes the pore wall is higher than in similarly prepared mesoporous aluminosilicate [17], and varies between 1.30 and 1.65 nm. In addition, results in Table 1 indicate that the isomorphous substitution of Si with Zr or Ti has little effects on the APD although the wall thickness of the calcined materials increased. Materials containing either Zr or Ti have comparable wall thickness. 3.2. Sorption isotherms Nitrogen sorption isotherms at liquid nitrogen temperature are presented in Fig. 1. At P/P00.5, the totality of accessible pores are ®lled with adsorbate and the isotherm reaches a plateau that remains fairly invariant as P/P0 approaches unity. Nitrogen sorption isotherms of the silicates with the MCM-41 structure, are type IV isotherms usually seen in mesoporous materials [19,20]. The total mesopore volume was
Table 1 Some properties of mesoporous metalsilicates; MZr, Ti (samples have been calcined at 600C/12 h in air; Sstirring and Rrepeat) Sample
Si/M
d100 (nm)
SA (m2/g)
PV (cm3/g)
APD (nm)
W (nm)
1. Silica gel 2. MLOS-94 C 3. MLOS-94 CR
± ± ±
± 3.60 3.60
639 1731 1720
0.35 1.20 1.20
2.20 2.80 2.80
± 1.32 1.32
4. MLOS-93BS 5. MLOS-93 B 6. MLOS-93 G
24.7 24.4 12.6
3.83 ± 3.97
1136 1160 796
0.99 0.98 0.78
2.80 ± ±
1.62 ± ±
7. MLOS-93AS 8. MLOS-93 A 9. MLOS-93 AR 10. MLOS-93 D 11. MLOS-93F
24.7 23.7 24.1 22.8 14.6
3.77 3.77 3.88 4.09 4.55
944 1198 1301 1138 570
0.67 0.70 0.95 0.92 0.92
2.82 ± ± ± ±
1.53 ± ± ± ±
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M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
Fig. 1. Nitrogen sorption isotherms of (Si,Zr)-MCM-41 materials as a function of their Si/Zr molar ratios.
derived from the amount of vapor adsorbed at P/ P00.50 by assuming that all the mesopores were then ®lled with condensed liquid nitrogen in the normal liquid state, see Table 1. In Fig. 1, the deleterious effects of M(IV) metals insertion in the mesoporous silicate framework are well illustrated. Structural order decreases with Zr introduction and as the solid's SiO2/ZrO2 ratio reaches 3.2, type I isotherms (typical of microporous materials such as silica gel) are observed. Similar effects have been observed when Al was introduced into the silicate mesostructure structure [17]. 3.3. Thermal analysis Thermogravimetric analysis of the crystals show distinct weight losses that depend, in part, on framework composition [21±23]. For the parent Si-MCM-
41, the minor (2%) weight loss below 1508C corresponds to the desorption of physisorbed water (or ethanol) in the voids formed by crystals agglomeration. Above 2808C, breakage, decomposition and thermal desorption of organic fragments occur. The large (34%) weight change in the 150±3008C temperature region is attributed to losses of alkyl chains from micelle decomposition and to losses of water and ethanol molecules occluded at the organic/inorganic interface; Fig. 2 (A). Above 3008C, combustion of the residual organics occurs and a small (6%) but sudden weight loss can be seen together with a sharp exotherm near 3048C, see Fig. 2(A). The last 6% weight decrease above 3508C is attributed to water losses resulting from dehydroxylation reactions at the silicate surface. For Zr or Ti substituted Si-MCM-41 samples, total weight losses at 8008C remain in the 50±55% range.
M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
Fig. 2. Thermogravimetric profiles in air of: (A) Si-MCM-41 (B) (Si,Zr)-MCM-41 and (C) (Si,Ti)-MCM-41 materials.
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Fig. 2. (Continued )
However, the distribution of successive weight losses depends on the framework composition; see Fig. 2(B) and (C). Between 2808C and 3408C, the oxidative decomposition of residual organic compounds in SiMCM-41 occurs in one step and after introducing Ti or Zr, these combustion reactions occur in two steps and are accompanied by two exotherms; Fig. 2(B) and (C). Similar results have been observed after introducing Al atoms in the silicate framework [17]. This suggests the existence of different sorption site strengths as a result of Ti or Zr incorporation in the silicate framework. Sorption site strengths seem to increase in the order: Si±OH
MCM-41 (sample 2, Table 1) and increases as Si is replaced with Zr(IV) or Ti(IV) atoms (samples 4 and 7, Table 1). The Si,Ti-MCM-41 sample with Si/ Ti14.6, although having the lowest surface area, can sorb more than twice the NH3 sorbed by the other mesoporous silicates. Not shown are secondary sorption isotherms, that is, sorption isotherms for samples after NH3 adsorption and degassing in vacuum at 1508C for silica gel and at 808C for the other samples. By subtracting the adsorbed volume of the secondary isotherms from that of the primary isotherms at the same equilibrium pressure (p0.2 Torr), it is possible to obtain Virr that is, the volume of irreversibly chemisorbed NH3. This value is believed to represent the total number of strong acid sites in the solids under study [24]. Initial heats, together with sites distribution and strength data in Table 2 clearly indicate that these mesoporous materials are weak solid acids and that introduction of heteroaroms such as Ti and Zr increases the solids' acidity. Acid sites density is the largest in (Si,Ti)MCM-41. Differential heat pro®les in Fig. 3(B) indicate that, for a given Si/M ratio, acidity increases in
M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239
237
Fig. 3. (A) Ammonia sorption isotherms at 808C; and (B) differential heat profiles as a function of ammonia uptake.
the order: silica gel
mol range, absent in all the other materials examined, see Fig. 3(B). The acid sites density and strength in Table 2 is signi®cantly lower than the one measured in mesoporous aluminosilicates with the MCM-41 structure [17] and in amorphous aluminosilicates of the type used in hydrotreating catalyst preparations [26].
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Table 2 Ammonia chemisorption data at 808C (p0.2 Torr) (population of sites with given strength is in mmol NH3/g; MZr, Ti) Sample
1. 2. 3. 4. 5. 6.
Silica gel Si-MCM-41 (Si,Zr)-MCM-41 (Si,Zr)-MCM-41 (Si,Ti)-MCM-41 (Si,Ti)-MCM-41
Si/M
± ± 24.7 12.6 24.7 14.6
In. Heat (kJ/mol)
9 112 126 113 111 119
4. Summary and conclusions Silicon atoms in mesoporous silica with the MCM41 structure can be easily replaced by M(IV), atoms over a broad range of synthesis conditions, MTi or Zr. As the Si/M ratio decreases, structural order decreases and with it the solid's surface area and pore volume. Evidence of a mesoporous structure disappears at Si/M<5. These silicates have an APD similar to the one in the parent mesoporous silica prepared using the same surfactant but exhibit a pore wall thickness larger than the one in similarly prepared mesoporous aluminosilicates. Thermogravimetric analysis of these solids are essentially similar and all indicate the existence of different adsorption sites. The strength of these sites increases in the order: Si± OH
NH3 (mmol/g)
kJ/mol
VT
Virr
<90
>90
91 123 131 143 176 202
± 41 47 42 51 46
91 110 109 109 146 168
± 13 22 34 30 34
with initial materials preparation and XRD characterization. References [1] Federal Register, vol. 57, no. 13, January 1992. [2] IUPAC Manual of Symbols and Terminology, Pure Appl. Chem. 31 (1978) 578. [3] T. Yanagiswa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63 (1990) 988±992. [4] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [5] T.J. Pinnavaia, M.F. Thorpe, in: Access In Nanoporous Materials, Plenum Press, New York, 1995. [6] S. Biz, M.L. Occelli, Catal. Rev.-Sci. Eng. 40 (1998) 329± 407. [7] S. Nakamura, T. Yasui, J. Catal. 17 (1970) 336. [8] M. Dettmer, A. Renken, Ger. Chem. Eng. 6 (1983) 356. [9] Eur. Chem. News 11, 272, 40 (1967). [10] World Pet. Congress Proceeding, vol. 5, 1968, p. 41. [11] A. Corma, M.T. Navarro, J. Perez-Pariente, J. Chem. Soc., Chem. Commun. 147 (1994). [12] P.T. Tanev, M. Chbwe, T.J. Pinnavaia, Nature 368 (1994) 321. [13] S. Gontier, A. Tuel, Zeolites 15 (1995) 601. [14] J.S. Reddy, A. Dicko, A. Sayari, in: M.L. Occelli, H. Kessler (Eds.), Synthesis of Porous Materials: Zeolites Clays and Nanostructures, Marcel Dekker, New Work, 1997, p. 405. [15] A. Tuel, S. Gontier, R. Teissier, J. Chem. Soc., Chem. Commun. (1996) 651. [16] D.J. Jones, l. Jimenez-Jimenez, A. Jimenez-Lopez, P. Maireles-Torres, P. Olivera-Pastor, E. Rodriguez-Castellon, J. Roziere, Chem. Commun. 431 (1997). [17] M.L. Occelli, S. Biz, A. Auroux, G.J. Ray, Microporous and Macroporous Mater., in press. [18] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865±867. [19] P.J. Branton, P.G. Hall, K.S.W. Sing, J. Chem. Soc., Chem. Commun. (1993) 1257±1258. [20] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, New York, 1982.
M.L. Occelli et al. / Applied Catalysis A: General 183 (1999) 231±239 [21] M. Busio, J. Janchen, J.H.C. van Hooff, Microporous Mater. (1995) 211±218. [22] Cong-Yan Chen, Hong-xin Li, M. Davis, Microporous Mater. 2 (1993) 17. [23] R. Schmidt, D. Akporiaye, M. Stocker, O. Ellestad, Stud. Surf. Sci. Catal. 84 (1994) 61±68.
239
[24] A. Auroux, in: B. Imelik, J.C. Edrine (Eds.), Catalyst Characterization: Physical Techniques for Solid Materials, chapter 22, Plenum Press, New York, 1994. [25] A. Auroux, Topics Catal. 4 (1997) 71±89. [26] M.L. Occelli, S. Biz, A. Auroux, P.S. Iyer, Appl. Catal., in press.