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Mesoporous structure stability of zirconium-doped mesoporous silica at elevated temperature
Materials Letters 63 (2009) 2343–2345
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Mesoporous structure stability of zirconium-doped mesoporous silica at elevated temperature Zuowei Dong, Feng Ye ⁎, Haijiao Zhang School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 23 June 2009 Accepted 3 August 2009 Available online 8 August 2009 Keywords: Zr-MCM-41 Mesoporous structure stability Calcination treatment
a b s t r a c t In this work, the thermal stability of mesoporous structure of MCM-41 materials with different zirconium contents (Zr-MCM-41) was investigated. The results show that the obtained Zr-MCM-41 materials retain a relatively good long range order of mesopores up to 1000 °C. Increasing the calcining temperature to 1050 °C, the mesopore openings become anisotropic and a few new pores with dozens of nanometers are formed due to thermal stress effect. After calcination at 1100 °C, the mesopores structure of all the samples collapse completely and some tetragonal zirconia can be also detected in the sample with high zirconium content (15 ≤ Si/Zr ≤ 40). The synthesized Zr-MCM-41 materials with good thermal stability are expected to be used as the support of catalytic activators under process conditions. © 2009 Elsevier B.V. All rights reserved.
1. Introduction MCM-41 material, reported by researchers at the Mobil corporation [1], has been the most popular members of the M41s molecular sieve family widely investigated [2,3]. The MCM-41 material possesses a uniform hexagonal array of linear pores constructed with a silica matrix like a honeycomb, and is very promising due to its high pore volume (N0.7 cm3/g), surface area (N1000 m2/g) and large pore opening (2–10 nm) with narrow size distribution. Even though it is a promising candidate for the catalysis of organic transformations, pure silica-based MCM-41 shows limited application value due to the lack of catalytic acidic sites in the silica walls. Catalytic activity, acid or redox, fortunately, can be generated by modification of the siliceous framework by heteroelements, because of the significant improvement in acidic sites deriving from the incorporated metal ions in the mesoporous materials. Despite the noticeable success of microporous zirconium silicates in selective oxidation reactions [4–6], the synthesis of Zr-MCM-41 is still extremely important in the field of redox catalysis, because diffusion and conversion of bulky organic substances catalyzed are obviously restricted by the micropores of those materials. Apart from acting as a catalyst itself [7], Zr-MCM-41 is also a good support for various activators, such as nickel oxide [8], Pt/H3PW12O40 [9] and Ir/Pt [10]. The choice of this support instead of MCM-41 is due to the more favorable results attained concerning the dispersion degree of these active phases [11–13]. However, the previous reports on Zr-MCM-41only focused on the synthesis and catalytic application
⁎ Corresponding author. Tel.: +86 45186413921; fax: +86 45186413922. E-mail address: [email protected] (F. Ye). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.08.003
of activators supported. One key issue for the applicability of Zr-MCM41 support associates with the thermal stability of mesoporous structure under process conditions. To our knowledge, the thermal stability of its mesoporous structure has not been detailedly reported. In the study, Zr-MCM-41 samples with different Si/Zr molar ratio were synthesized by traditional hydrothermal method and their mesoporous structure stability in air up to 1200 °C was investigated. 2. Experimental In a typical synthesis procedure, the gel was prepared by adding tetraethyl orthosilicate (TEOS) and ZrOCl2·8H2O to an aqueous solution containing cetyltrimethylammonium bromide (C16TABr) and NH3·H2O. The molar composition of the initial gel mixture was 1.0: x: 0.15: 1.64: 130 for TEOS:ZrOCl2·8H2O:C16TABr:NH3·H2O:H2O, where x = 0.01333, 0.02, 0.025, 0.03333 and 0.06666, respectively. After stirring the mixture at 60 °C for 3 h, the gel was crystallized under static hydrothermal condition at 100 °C for 72 h. Then the solid product was filtered, washed with deionized water and finally dried in air at room temperature. The as-synthesized Zr-MCM-41 samples are called yZr-MCM-41, where y stands for the Si/Zr molar ratio of the starting mixtures, which is 75, 50, 40, 30 and 15 in this study. To clarify the thermal stability of as-synthesized Zr-MCM-41 materials, the obtained samples were heated to 700–1200 °C for 7 h at a rate of 1 °C/min. The powder XRD patterns were recorded using an X' Pert Philips diffraction instrument with Cu Kα radiation at 50 kV and 35 mA. The BET surface area and pore size distribution of the calcined samples were measured on a Quantachrome automated adsorption instrument using nitrogen as analytic gas. The microstructure was also investigated by TEM (Philips CM20, operated at 200 kV).
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3. Results and discussion Fig. 1 shows the XRD patterns of Zr-MCM-41 samples with different zirconium contents calcined in air at 700 °C for 7 h. With the increase in the zirconium content, the (100), (110), (200) and (210) diffraction peaks are less resolved, due to a less ordered solid. No peaks from the crystalline phases containing zirconium are detected in the higher 2θ region (results not shown). The structure parameters of these samples are summarized in Table 1. The pore volume (Vp) and surface area (SBET) decrease with the zirconium content, but pore size (Dp), framework wall thickness (ts) and unit cell parameter (a0) only slightly change. The changes of these parameters are similar to those of Co-MCM-41 [14]. The XRD patterns of 40Zr-MCM-41 sample calcined in air at various temperatures for 7 h are shown in Fig. 2a. With the increase in calcination temperature, the d100 spacing and long range order of 40Zr-MCM-41 decrease gradually, which is similar to the thermal stability research of MCM-41 [15–17]. At calcination temperature of 1050 °C, all the peaks at the low angles except (100) reflection disappear. Further increasing the calcining temperature N1100 °C, the peak due to (100) reflection is not resolved, implying that the pore structure of 40Zr-MCM-41completely collapses. For other samples, the four peaks indexed to the hexagonal unite cell also begin to disappear at 1100 °C (results not shown), indicating that the thermal stability of Zr-MCM-41 is independent of the zirconium content. Previous studies [18,19] have clearly demonstrated that the thermal stability of mesoporous structure of MCM-41-type materials is strongly related to the pore size, thickness and precursor of inorganic wall. In our present work Zr-MCM-41 materials were prepared by the same procedure and inorganic sources (TEOS and ZrOCl2·8H2O). These materials have close pore size and wall thickness (see Table 1). Therefore it should be reasonable that Zr-MCM-41 materials possess similar mesoporous structure stability. In the high 2θ region, the peaks ascribed to metastable tetragonal zirconia occur when the calcination temperature is over 1100 °C and the amount of crystalline zirconia increases with the calcination temperature (see inset Fig. 2a). One possible explanation is that the collapse of the mesopore promotes the nucleation and growth of tetragonal zirconia. Furthermore, the broad peak at 22° is assigned to amorphous silica and no evidence for crystalline cristobalite phase is found, which are not in agreement with the research of pure mesoporous silica [20]. Fig. 2b shows the XRD patterns of Zr-MCM-41 with different zirconium contents calcined at 1150 °C for 7 h. 75Zr-MCM-41 and 50Zr-
Fig. 1. XRD patterns of Zr-MCM-41 samples with different zirconium contents calcined in air at 700 °C for 7 h.
Table 1 Textural properties of the Zr-MCM-41 samples with different zirconium contents. Samples
Vp (cm3/g)
SBET (m2/g)
Dp (nm)a
a0 (nm)b
ts (nm)c
75Zr-MCM-41 50Zr-MCM-41 40Zr-MCM-41 30Zr-MCM-41 15Zr-MCM-41
0.81 0.76 0.70 0.63 0.56
1062 997 921 810 688
3.72 3.70 3.69 3.68 3.73
4.45 4.44 4.45 4.43 4.50
0.73 0.74 0.76 0.75 0.77
a b c
Mean pore diameter calculated by BJH method. Unit cell parameter value, a0 = 2d100/31/2. Framework wall thickness, ts = a0 − Dp.
MCM-41 only exhibit the broad amorphous silica peaks at 22°, and no tetragonal zirconia is found. It may be attributed to their low zirconium content. Theoretically, the relative amount of zirconia for 75Zr-MCM-41 and 50Zr-MCM-41 is 2.66 wt.% and 3.94 wt.%, respectively. Even if the crystalline zirconia occurs, its relative amount would be lower than the measured threshold of XRD instrument. When the Si/Zr molar ratio decreases from 50 to 40, the tetragonal zirconia can be detected. With the increase in zirconium content, the crystalline zirconia diffraction peaks are more resolved. Additionally, no peaks ascribed to cristobalite and zircon phases are clearly detected.
Fig. 2. XRD patterns of (a) 40Zr-MCM-41 calcined at various temperatures and (b) Zr-MCM41 with different zirconium amounts calcined at 1150 °C. T: tetragonal zirconia.
Z. Dong et al. / Materials Letters 63 (2009) 2343–2345
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4. Conclusion Zr-MCM-41 materials with different zirconium contents were synthesized by traditional hydrothermal method. The obtained Zr-MCM-4 materials possess excellent thermal stability. They can retain ordered mesoporous structure up to 1000 °C. Increasing the calcination temperature up to 1050 °C, the mesopore arrange tends to be disordered and a few new pores with dozens of nanometers are formed due to thermal stress effect. Higher temperature calcination results in the formation of tetragonal zirconia, and no evidence for the crystalline phase containing silicon is detected. The obtained Zr-MCM-41 samples have enough good thermal stability to act as the support of catalytic activators under process condition.
References Fig. 3. TEM image of 40Zr-MCM-41 calcined at 1050 °C for 7 h in air. The region earmarked by a white rectangle is enlarged as the lower left inset, and a few new pores with dozens of nanometers are formed due to the thermal stress effect.
The Zr-MCM-41 samples show at least three peaks of (100), (110) and (200) after calcination treatment up to 1000 °C, indicating that they still have relatively good long range order of mesopores. Further calcining up to 1050 °C, the array of mesopores becomes disordered in light of the occurrence of the only peak due to (100) reflection (Fig. 2a). The serious decrease in long range order of mesopores can also be confirmed by TEM analysis, as shown in Fig. 3. The mesopore openings are not isotropic but anisotropic (see inset Fig. 3), and the pore size decreases to about 2.5 nm. High calcination temperature of 1050 °C results in small pore size due to the shrinkage of mesopores. A few pores with dozens of nanometers are formed due to thermal stress effect. To our knowledge, the thermal stress-inducing pores during the calcination treatment are first reported in silica-based mesoporous materials. The obtained Zr-MCM-41 materials can possess a relatively good long range order of mesopores even after thermal treatment at 1000 °C. In contrast, the ordered mesoporous structure of MCM-41 is only maintained up to 650 °C [20]. Taking into account laboratory, technological or industrial applications, the Zr-MCM-41 materials with a higher thermal stability should be more suitable for the support of catalytic activators under process conditions.
[1] Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS. Nature 1992;359:710–2. [2] Bore MT, Mokhonoana MP, Ward TL, Coville NJ, Datye AK. Microporous Mesoporous Mater 2006;95:118–25. [3] Lee B, Ma Z, Zhang Z, Park C, Cai S. Microporous Mesoporous Mater 2009;122:160–7. [4] Ramaswamy V, Tripathi B, Srinivas D, Ramaswamy AV, Cattaneo R, Prins R. J Catal 2001;200:250–8. [5] Nie YT, Jaenicke S, Bekkum HV, Chuah GK. J Catal 2007;246:223–31. [6] Zhu YZ, Nie YT, Jaenicke S, Chuah GK. J Catal 2005;229:404–13. [7] Torri C, Lesci IG, Fabbri D. J Anal Appl Pyrolysis 2009:192–6. [8] Rodríguez-Castellón E, Díaz L, Braos-García P, Mérida-Robles J, Maireles-Torres P, Jiménez-López A, et al. Appl Catal A 2003;240:83–94. [9] Chen LF, Wang JA, Noreña LE, Aguilar J, Navarrete J, Salas P, et al. J Solid State Chem 2007;180:2958–72. [10] Mouli CK, Sundaramurthy V, Dalai AK, Ring Z. Appl Catal A 2007;321:17–26. [11] Rodríguez-Castellón E, Mérida-Robles J, Díaz L, Maireles-Torres P, Jones DJ, Rozière J, et al. Appl Catal A 2004;260:9–18. [12] Jiménez-López A, Rodríguez-Castellón E, Maireles-Torres P, Díaz L, Mérida-Robles J. Appl Catal A 2001;218:295–306. [13] Moreno-Tost R, Santamaría-González J, Maireles-Torres P, Rodríguez-Castellón E, Jiménez-López A. Appl Catal B 2002;38:51–60. [14] Todorova T, Pârvulescu V, Kadinov G, Tenchev K, Somacescu S, Su BL. Microporous Mesoporous Mater 2008;113:22–30. [15] Wan KS, Liu Q, Zhang CM. Mater Lett 2003;57:3839–42. [16] Chen HX, Wang YC. Ceram Int 2002;28:541–7. [17] Yu J, Shi JL, Wang LZ, Ruan ML, Yan DS. Mater Lett 2001;48:112–6. [18] Gaydhankar TR, Taralkar US, Jha RK, Joshi PN, Kumar R. Catal Commun 2005;6:361–6. [19] Robert M. J Phys Chem B 1999;103:10204–8. [20] Gu G, Ong PP, Chu C. J Phys Chem Solids 1999;60:943–7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Mesoporous structure stability of zirconium-doped mesoporous silica at elevated temperature Zuowei Dong, Feng Ye ⁎, Haijiao Zhang School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 23 June 2009 Accepted 3 August 2009 Available online 8 August 2009 Keywords: Zr-MCM-41 Mesoporous structure stability Calcination treatment
a b s t r a c t In this work, the thermal stability of mesoporous structure of MCM-41 materials with different zirconium contents (Zr-MCM-41) was investigated. The results show that the obtained Zr-MCM-41 materials retain a relatively good long range order of mesopores up to 1000 °C. Increasing the calcining temperature to 1050 °C, the mesopore openings become anisotropic and a few new pores with dozens of nanometers are formed due to thermal stress effect. After calcination at 1100 °C, the mesopores structure of all the samples collapse completely and some tetragonal zirconia can be also detected in the sample with high zirconium content (15 ≤ Si/Zr ≤ 40). The synthesized Zr-MCM-41 materials with good thermal stability are expected to be used as the support of catalytic activators under process conditions. © 2009 Elsevier B.V. All rights reserved.
1. Introduction MCM-41 material, reported by researchers at the Mobil corporation [1], has been the most popular members of the M41s molecular sieve family widely investigated [2,3]. The MCM-41 material possesses a uniform hexagonal array of linear pores constructed with a silica matrix like a honeycomb, and is very promising due to its high pore volume (N0.7 cm3/g), surface area (N1000 m2/g) and large pore opening (2–10 nm) with narrow size distribution. Even though it is a promising candidate for the catalysis of organic transformations, pure silica-based MCM-41 shows limited application value due to the lack of catalytic acidic sites in the silica walls. Catalytic activity, acid or redox, fortunately, can be generated by modification of the siliceous framework by heteroelements, because of the significant improvement in acidic sites deriving from the incorporated metal ions in the mesoporous materials. Despite the noticeable success of microporous zirconium silicates in selective oxidation reactions [4–6], the synthesis of Zr-MCM-41 is still extremely important in the field of redox catalysis, because diffusion and conversion of bulky organic substances catalyzed are obviously restricted by the micropores of those materials. Apart from acting as a catalyst itself [7], Zr-MCM-41 is also a good support for various activators, such as nickel oxide [8], Pt/H3PW12O40 [9] and Ir/Pt [10]. The choice of this support instead of MCM-41 is due to the more favorable results attained concerning the dispersion degree of these active phases [11–13]. However, the previous reports on Zr-MCM-41only focused on the synthesis and catalytic application
⁎ Corresponding author. Tel.: +86 45186413921; fax: +86 45186413922. E-mail address: [email protected] (F. Ye). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.08.003
of activators supported. One key issue for the applicability of Zr-MCM41 support associates with the thermal stability of mesoporous structure under process conditions. To our knowledge, the thermal stability of its mesoporous structure has not been detailedly reported. In the study, Zr-MCM-41 samples with different Si/Zr molar ratio were synthesized by traditional hydrothermal method and their mesoporous structure stability in air up to 1200 °C was investigated. 2. Experimental In a typical synthesis procedure, the gel was prepared by adding tetraethyl orthosilicate (TEOS) and ZrOCl2·8H2O to an aqueous solution containing cetyltrimethylammonium bromide (C16TABr) and NH3·H2O. The molar composition of the initial gel mixture was 1.0: x: 0.15: 1.64: 130 for TEOS:ZrOCl2·8H2O:C16TABr:NH3·H2O:H2O, where x = 0.01333, 0.02, 0.025, 0.03333 and 0.06666, respectively. After stirring the mixture at 60 °C for 3 h, the gel was crystallized under static hydrothermal condition at 100 °C for 72 h. Then the solid product was filtered, washed with deionized water and finally dried in air at room temperature. The as-synthesized Zr-MCM-41 samples are called yZr-MCM-41, where y stands for the Si/Zr molar ratio of the starting mixtures, which is 75, 50, 40, 30 and 15 in this study. To clarify the thermal stability of as-synthesized Zr-MCM-41 materials, the obtained samples were heated to 700–1200 °C for 7 h at a rate of 1 °C/min. The powder XRD patterns were recorded using an X' Pert Philips diffraction instrument with Cu Kα radiation at 50 kV and 35 mA. The BET surface area and pore size distribution of the calcined samples were measured on a Quantachrome automated adsorption instrument using nitrogen as analytic gas. The microstructure was also investigated by TEM (Philips CM20, operated at 200 kV).
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3. Results and discussion Fig. 1 shows the XRD patterns of Zr-MCM-41 samples with different zirconium contents calcined in air at 700 °C for 7 h. With the increase in the zirconium content, the (100), (110), (200) and (210) diffraction peaks are less resolved, due to a less ordered solid. No peaks from the crystalline phases containing zirconium are detected in the higher 2θ region (results not shown). The structure parameters of these samples are summarized in Table 1. The pore volume (Vp) and surface area (SBET) decrease with the zirconium content, but pore size (Dp), framework wall thickness (ts) and unit cell parameter (a0) only slightly change. The changes of these parameters are similar to those of Co-MCM-41 [14]. The XRD patterns of 40Zr-MCM-41 sample calcined in air at various temperatures for 7 h are shown in Fig. 2a. With the increase in calcination temperature, the d100 spacing and long range order of 40Zr-MCM-41 decrease gradually, which is similar to the thermal stability research of MCM-41 [15–17]. At calcination temperature of 1050 °C, all the peaks at the low angles except (100) reflection disappear. Further increasing the calcining temperature N1100 °C, the peak due to (100) reflection is not resolved, implying that the pore structure of 40Zr-MCM-41completely collapses. For other samples, the four peaks indexed to the hexagonal unite cell also begin to disappear at 1100 °C (results not shown), indicating that the thermal stability of Zr-MCM-41 is independent of the zirconium content. Previous studies [18,19] have clearly demonstrated that the thermal stability of mesoporous structure of MCM-41-type materials is strongly related to the pore size, thickness and precursor of inorganic wall. In our present work Zr-MCM-41 materials were prepared by the same procedure and inorganic sources (TEOS and ZrOCl2·8H2O). These materials have close pore size and wall thickness (see Table 1). Therefore it should be reasonable that Zr-MCM-41 materials possess similar mesoporous structure stability. In the high 2θ region, the peaks ascribed to metastable tetragonal zirconia occur when the calcination temperature is over 1100 °C and the amount of crystalline zirconia increases with the calcination temperature (see inset Fig. 2a). One possible explanation is that the collapse of the mesopore promotes the nucleation and growth of tetragonal zirconia. Furthermore, the broad peak at 22° is assigned to amorphous silica and no evidence for crystalline cristobalite phase is found, which are not in agreement with the research of pure mesoporous silica [20]. Fig. 2b shows the XRD patterns of Zr-MCM-41 with different zirconium contents calcined at 1150 °C for 7 h. 75Zr-MCM-41 and 50Zr-
Fig. 1. XRD patterns of Zr-MCM-41 samples with different zirconium contents calcined in air at 700 °C for 7 h.
Table 1 Textural properties of the Zr-MCM-41 samples with different zirconium contents. Samples
Vp (cm3/g)
SBET (m2/g)
Dp (nm)a
a0 (nm)b
ts (nm)c
75Zr-MCM-41 50Zr-MCM-41 40Zr-MCM-41 30Zr-MCM-41 15Zr-MCM-41
0.81 0.76 0.70 0.63 0.56
1062 997 921 810 688
3.72 3.70 3.69 3.68 3.73
4.45 4.44 4.45 4.43 4.50
0.73 0.74 0.76 0.75 0.77
a b c
Mean pore diameter calculated by BJH method. Unit cell parameter value, a0 = 2d100/31/2. Framework wall thickness, ts = a0 − Dp.
MCM-41 only exhibit the broad amorphous silica peaks at 22°, and no tetragonal zirconia is found. It may be attributed to their low zirconium content. Theoretically, the relative amount of zirconia for 75Zr-MCM-41 and 50Zr-MCM-41 is 2.66 wt.% and 3.94 wt.%, respectively. Even if the crystalline zirconia occurs, its relative amount would be lower than the measured threshold of XRD instrument. When the Si/Zr molar ratio decreases from 50 to 40, the tetragonal zirconia can be detected. With the increase in zirconium content, the crystalline zirconia diffraction peaks are more resolved. Additionally, no peaks ascribed to cristobalite and zircon phases are clearly detected.
Fig. 2. XRD patterns of (a) 40Zr-MCM-41 calcined at various temperatures and (b) Zr-MCM41 with different zirconium amounts calcined at 1150 °C. T: tetragonal zirconia.
Z. Dong et al. / Materials Letters 63 (2009) 2343–2345
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4. Conclusion Zr-MCM-41 materials with different zirconium contents were synthesized by traditional hydrothermal method. The obtained Zr-MCM-4 materials possess excellent thermal stability. They can retain ordered mesoporous structure up to 1000 °C. Increasing the calcination temperature up to 1050 °C, the mesopore arrange tends to be disordered and a few new pores with dozens of nanometers are formed due to thermal stress effect. Higher temperature calcination results in the formation of tetragonal zirconia, and no evidence for the crystalline phase containing silicon is detected. The obtained Zr-MCM-41 samples have enough good thermal stability to act as the support of catalytic activators under process condition.
References Fig. 3. TEM image of 40Zr-MCM-41 calcined at 1050 °C for 7 h in air. The region earmarked by a white rectangle is enlarged as the lower left inset, and a few new pores with dozens of nanometers are formed due to the thermal stress effect.
The Zr-MCM-41 samples show at least three peaks of (100), (110) and (200) after calcination treatment up to 1000 °C, indicating that they still have relatively good long range order of mesopores. Further calcining up to 1050 °C, the array of mesopores becomes disordered in light of the occurrence of the only peak due to (100) reflection (Fig. 2a). The serious decrease in long range order of mesopores can also be confirmed by TEM analysis, as shown in Fig. 3. The mesopore openings are not isotropic but anisotropic (see inset Fig. 3), and the pore size decreases to about 2.5 nm. High calcination temperature of 1050 °C results in small pore size due to the shrinkage of mesopores. A few pores with dozens of nanometers are formed due to thermal stress effect. To our knowledge, the thermal stress-inducing pores during the calcination treatment are first reported in silica-based mesoporous materials. The obtained Zr-MCM-41 materials can possess a relatively good long range order of mesopores even after thermal treatment at 1000 °C. In contrast, the ordered mesoporous structure of MCM-41 is only maintained up to 650 °C [20]. Taking into account laboratory, technological or industrial applications, the Zr-MCM-41 materials with a higher thermal stability should be more suitable for the support of catalytic activators under process conditions.
[1] Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS. Nature 1992;359:710–2. [2] Bore MT, Mokhonoana MP, Ward TL, Coville NJ, Datye AK. Microporous Mesoporous Mater 2006;95:118–25. [3] Lee B, Ma Z, Zhang Z, Park C, Cai S. Microporous Mesoporous Mater 2009;122:160–7. [4] Ramaswamy V, Tripathi B, Srinivas D, Ramaswamy AV, Cattaneo R, Prins R. J Catal 2001;200:250–8. [5] Nie YT, Jaenicke S, Bekkum HV, Chuah GK. J Catal 2007;246:223–31. [6] Zhu YZ, Nie YT, Jaenicke S, Chuah GK. J Catal 2005;229:404–13. [7] Torri C, Lesci IG, Fabbri D. J Anal Appl Pyrolysis 2009:192–6. [8] Rodríguez-Castellón E, Díaz L, Braos-García P, Mérida-Robles J, Maireles-Torres P, Jiménez-López A, et al. Appl Catal A 2003;240:83–94. [9] Chen LF, Wang JA, Noreña LE, Aguilar J, Navarrete J, Salas P, et al. J Solid State Chem 2007;180:2958–72. [10] Mouli CK, Sundaramurthy V, Dalai AK, Ring Z. Appl Catal A 2007;321:17–26. [11] Rodríguez-Castellón E, Mérida-Robles J, Díaz L, Maireles-Torres P, Jones DJ, Rozière J, et al. Appl Catal A 2004;260:9–18. [12] Jiménez-López A, Rodríguez-Castellón E, Maireles-Torres P, Díaz L, Mérida-Robles J. Appl Catal A 2001;218:295–306. [13] Moreno-Tost R, Santamaría-González J, Maireles-Torres P, Rodríguez-Castellón E, Jiménez-López A. Appl Catal B 2002;38:51–60. [14] Todorova T, Pârvulescu V, Kadinov G, Tenchev K, Somacescu S, Su BL. Microporous Mesoporous Mater 2008;113:22–30. [15] Wan KS, Liu Q, Zhang CM. Mater Lett 2003;57:3839–42. [16] Chen HX, Wang YC. Ceram Int 2002;28:541–7. [17] Yu J, Shi JL, Wang LZ, Ruan ML, Yan DS. Mater Lett 2001;48:112–6. [18] Gaydhankar TR, Taralkar US, Jha RK, Joshi PN, Kumar R. Catal Commun 2005;6:361–6. [19] Robert M. J Phys Chem B 1999;103:10204–8. [20] Gu G, Ong PP, Chu C. J Phys Chem Solids 1999;60:943–7.