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Effect of trivalent elements on the thermal and hydrothermal stability of MCM-41 mesoporous molecular materials
PII: S0968-5677(98)00073-X
Supramolecular Science 5 (1998) 553—558 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00
Effect of trivalent elements on the thermal and hydrothermal stability of MCM-41 mesoporous molecular materials Nongyue He*, Zuhong Lu and Chunwei Yuan National Lab. of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, People’s Republic of China
Jianming Hong Center for Matertial Analysis, Nanjing University, Nanjing 210093, People’s Republic of China
Chun Yang Department of Chemistry, Nanjing Normal University, Nanjing 210096, People’s Republic of China
Shulin Bao and Qinhua Xu Department of Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China
Effect of trivalent elements on the thermal and hydrothermal stability of MCM-41 mesoporous molecular sieve materials has been investigated. Al(III) decreases the thermal and hydrothermal stability of MCM-41 materials, whereas La(III) and Fe(III), especially Fe(III), can improve the thermal and hydrothermal stability. Benzene adsorption and IR spectra suggested that thick channel wall and the fewer structural defect sites in MCM-41 would enhance the thermal and hydrothermal stability of MCM-41. 1998 Elsevier Science Limited. All rights reserved. (Keywords: thermal stability; hydrothermal stability; MCM-41)
INTRODUCTION In the past decade, the research for smaller, faster, more selective and efficient products and processes has greatly advanced. The ability to engineer well-defined spatial microarrangements of pure substances and composite materials has become pivotal in creating new molecularlevel electronic, optical and magnetic devices. One attractive method for assembling and maintaining controlled microstructures is to use solid host lattices that serve as templates within which a guest structure of nanometer architecture can be assembled. In this host—guest system, microporous zeolite materials, which possess nanometer dimension window, channel and cavity architecture, represent a ‘New Frontier’ of solid state chemistry with great opportunities for innovative research and development. Considerable effort has been directed at new use of zeolites as advanced materials in fields such as solar energy conversion, zeolite electrodes and electron relays, zeolite
*Corresponding author. Also affiliated to Department of Chemistry, Nanjing University
batteries, zeolite chemical sensors, zeolite imaging and data storage materials, zeolite lasers and displays, semiconductor clusters, molecular wires with tailored electronic and optical properties, enzyme mimicking organic complexes, zeolite permselective membranes or thin films. All of these have been reviewed extensively\. However, the pore sizes of all the microporous zeolite materials are smaller than 1 nm, and, therefore, their use as solid host is limited. For bulky molecular reaction and the assembling of nanometer materials with different sizes in zeolite molecular sieve materials, much zeolite molecular research was aimed at the synthesis of zeolite molecular sieve materials with pore sizes greater than 1 nm. Unfortunately, before the discovery of M41S in 1991 there are only three metallophosphate samples with pore size greater than 1 nm reported: VPI-5, cloverite and JDF-20. The cloverite is thermally stable up to 700°C, but the other two materials show poor thermal stability. Since the channel structure is uniform and the pore size can be tailored in the range of 2—10 nm, the synthesis of M41S mesoporous materials by researchers from Mobil company throws new light on the synthesis of molecular sieves. Among the M41S family, the MCM-41 hexagonal mesoporous
SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998 553
Stability of MCM-41 mesoporous molecular materials: N. He et al. molecular sieve is especially of potential application, as proved by the publication of so many papers and reviews . However, although this material shows attractive application prospect, it also displays poor hydrothermal stability and not very good thermal stability, which limits its application. Therefore, it is of significance to improve the stability, especially the hydrothermal stability, of MCM-41 mesoporous molecular sieve materials. It is well known that the incorporation of some metallic cations into zeolite molecular sieves will influence their stability. In the present paper, the incorporation of Al(III), Fe(III) and La(III) into MCM-41 was investigated. It was found that the stability of MCM-41 was increased by Fe(III) and La(III) incorporation, and decreased by Al(III).
EXPERIMENTAL Synthesis of materials The synthesis of pure siliceous and Al-containing MCM-41 mesoporous materials, named as SiMCM-41 and AlSiMCM-41, respectively, was performed following the procedures described in ref.. The synthesis of Fe and La-containing MCM-41 samples, designated as FeSiMCM-41 and LaSiMCM-41, has been described previously. Before further characterization, all the four as-synthesized samples were calcined at 813 K to remove the template (C H (CH ) NBr), these template removed samples are called template free samples. Characterization After calcination to remove template, the solid materials were characterized by low-angle X-ray diffraction (XRD) (Rigaku, D/max-cA) with Cu—Ka radiation, FT-IR (Nicolet, 5DX) with a resolution of 2 cm\, the adsorption of nitrogen at 77 K (Micrometrics, ASAP2000 instrument) and High Resolution Electron Microscopy (HREM) using a JEOL 200 cx instrument. The composition of samples was obtained on a Jarrell-Ash 1100 inductively coupled plasma quantometer. To compare the thermal stability, the above template free samples were calcined in a muffle stove in air once more at 1073 and 1153 K, respectively, followed by XRD detection. The hydrothermal stability of template free samples treated with 100% steam at 933 and 1043 K for 2 h was investigated by the conventional all-glass benzene adsorption technique. Before the measurement, the samples were evacuated at 673 K until a system pressure of 1.3;10\ Pa was reached.
RESULTS AND DISCUSSION Structural properties and composition of samples The X-ray diffraction patterns of the template free samples are shown in Figures 1a—4a. The observation of
Figure 1 The XRD patterns of SiMCM-41 after calcination at different temperatures. (a) Template free sample; (b) 1073 K, 2 h; (c) 1153 K, 2 h
four peaks of samples which can be indexed on a hexagonal lattice is typical of MCM-41 materials . From Table 1 we can find that all samples exhibit nanometer pore sizes of &3.3 nm (3.2 nm for SiMCM-41) and very large surface area (SA). The HREM pictures of hexagonal structure of the four template free samples are shown in Figure 5. All samples display hexagonal ordered pores, sizes of which are the same as those determined by N adsorption at low temperature (see Table 1). ¹hermal stability The XRD patterns of all template free samples after different calcination temperature are shown in Figures 1b—4c. Because other XRD peaks are much weaker and overlap each other, we investigate the thermal stability by comparing the intensity of the (1 0 0) peak. All the four samples show a very intensive (1 0 0) diffraction peak. However, after calcination at different temperature, the
554 SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998
Stability of MCM-41 mesoporous molecular materials: N. He et al.
Figure 2 The XRD patterns of AlSiMCM-41 after calcination at different temperatures. (a) Template free sample; (b) 1073 K, 2 h; (c) 1153 K, 2 h
intensity of (1 0 0) peaks of all samples changes, accompanied by a shift towards high 2h. After calcination at 1073 K, the (1 0 0) peak intensity for SiMCM-41 and LaSiMCM-41 changes little, whereas the (1 0 0) peak intensity for AlSiMCM-41 decreases significantly. From the literature about the Fe-containing microporous zeolite molecular sieve materials , it has been well known that the incorporation of Fe(III) into framework will destabilize microporous molecular sieves. However, interestingly here we find that the incorporation of Fe(III) into MCM-41 material does not decrease the (1 0 0) peak intensity, but increases the intensity. After calcination at 1153 K for 2 h, the (1 0 0) peak intensity of all the samples changes much more than that at 1073 K. Although having become much weaker, the (1 0 0) peak intensity of SiMCM-41, LaSiMCM-41 and FeSiMCM-41
Figure 3 The XRD patterns of LaSiMCM-41 after calcination at different temperatures. (a) Template free sample; (b) 1073 K, 2 h; (c) 1153 K, 2 h
still remains strong, while AlSiMCM-41 loses most of its structure. Among the former three samples, FeSiMCM-41 shows the greatest (1 0 0) peak intensity retention after calcination at 1153 K, whereas SiMCM-41 gives the weakest. Above results show that the sequence of the thermal stability for the investigated samples is FeSiMCM-41' LaSiMCM-41' SiMCM-41' AlSiMCM-41. Hydrothermal stability The hydrothermal stability of zeolite molecular sieve materials can be easily characterized by investigating the
SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998 555
Stability of MCM-41 mesoporous molecular materials: N. He et al. benzene adsorption behavior of the studied samples before and after steam treatment. If some components are removed from the framework of molecular sieves and agglomerated into a separate phase to occlude the channel or the framework collapses partially during steaming, the equilibrium benzene adsorption capacity after steaming will decrease. Listed in Table 2 are the equilibrium benzene adsorption capacity of all the four samples mentioned above at P/P "0.5 before and after the steam treatment (P and P are the equilibrium vapor pressure measured under the experimental conditions and the saturation
Figure 4 The XRD patterns of FeSiMCM-41 after calcination at different temperatures. (a) Template free sample; (b) 1073 K, 2 h; (c) 1153 K, 2 h
vapor pressure of benzene at the experimental temperature, respectively). It was reported that the framework of AlSiMCM-41 collapsed at 936 K in steam treatment. In our case, AlSiMCM-41 steamed at 933 K loses its XRD peaks almost completely, indicating its poor hydrothermal stability. All the other three samples still retain a strong (1 0 0) peak and a high adsorption capacity for benzene after steamed at 933 K. However, the values of c/c of FeSiMCM-41 and LaSiMCM-41 are greater than that for SiMCM-41. This indicates that the incorporation of Fe(III) and La(III) into MCM-41 gives rise to a better hydrothermal stability than the incorporation of Al(III) species. In order to differentiate the later three samples, the steam treatment was performed once more at higher temperature, 1043 K. The benzene adsorption capacities are also listed in Table 2. At this elevated temperature, only FeSiMCM-41 remains in a high equilibrium benzene adsorption capacity and this suggests that the introduction of Fe(III) into MCM-41 can enhance the hydrothermal stability than that of La(III) more significantly. Since the low hydrothermal stability in hot water and aqueous solution is a critical problem for many applications of MCM-41 with nanometer pore sizes, much attention has been paid to its improvement. Generally, the hydrothermal stability of MCM-41 increases with the channel wall thickness\. Salt effects during the crystallization process of MCM-41 was also used to improve the hydrothermal stability. Our results lead us to that the incorporation of Fe(III), even La(III) is another approach to improve the hydrothermal stability of MCM41. Generally, the incorporation of Fe(III) into framework of microporous zeolite molecular sieves will destabilize them . The enhanced hydrothermal stability of FeSiMCM-41 may be explained by using the concept of wall thickness mentioned above. From Table 2 we find that FeSiMCM-41 shows the thickest channel wall, which may be due to the transformation of most Fe(III) species from tetrahedral coordination condition to octahedrally coordinated state during the calcination to remove template. The transformation implies that the Fe(III) species would shift from the internal to the surface of the channel wall. The transformed Fe(III) species is supposed to highly disperse on the channel wall and thus makes the wall thicker.
Table 1 Structural properties and composition of samples Sample
Si/Me
d (nm)
a (nm)
SA (m/g)
Pore size (nm)
Wall thickness (nm)
SiMCM-41 AlSiMCM-41 LaSiMCM-41 FeSiMCM-41
700 29 42 31
4.03 4.19 4.08 4.46
4.65 4.84 4.71 5.15
1340 1084 947 998
3.2 3.3 3.3 3.3
1.45 1.54 1.41 1.85
Me"Al, Fe and La. XRD d -spacing. Unit cell parameter a "2 d /(3. Determined by N adsorption. Wall thickness"a -pore size.
556 SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998
Stability of MCM-41 mesoporous molecular materials: N. He et al.
Figure 5 The HREM pictures of template free samples. (a) SiMCM-41; (b) AlSiMCM-41; (c) LaSiMCM-41; (d) FeSiMCM-41
Table 2 Hydrothermal stability of samples Sample
Before steam treatment d (nm)
SiMCM-41 AlSiMCM-41 LaSiMCM-41 FeSiMCM-41
4.03 4.19 4.08 4.46
Wall thickness (nm)
1.45 1.54 1.41 1.85
After steam treatment (100% steam) c (g/g)
0.680 0.655 0.548 0.605
933 K, 2 h
1043 K, 2 h
d (nm)
c (g/g)
c/c (%)
c (g/g)
c /c (%)
3.72 — 3.72 4.12
0.381 0.278 0.348 0.408
56 42 64 67
0.184 0.079 0.110 0.303
27 12 20 50
c is the benzene adsorption of the template free samples at P/P "0.5; c and c are the benzene adsorption of template free samples at P/P "0.5 after given stream treatment. The structure collapsed almost completely.
The transformed Fe(III) species not only high-disperse on the surface of the wall, but also can fill the Al-depleted structural vacancies. The existence of structural defect in zeolite molecular sieve materials has been widely reported\. Generally, the molecular sieves with high framework Si/Al ratio or those templated by organic template, such as NON, DDR, MTW, AFI, MFI, VPI-5, AlPO -5, AlPO -8 and so on\, possess defects. All types of defect sites can exist in as-synthesized zeolites or in samples that have been calcined or exposed to various treatments, e.g., dealumination. Fejes et al.
reported a framework IR absorption band at 930 cm\ and ascribed it to the Al-depleted defect center. This band sifts towards high wave numbers with the increase of the Si/Al ratio of the tested samples\. From Figure 6 we can clearly find this band (at 959 cm\) also appears in the framework IR spectra for the SiMCM-41 (Figure 6a). However, this band becomes much weaker in the spectra for the other three samples. FeSiMCM-41 especially shows the weakest band, This suggests that there exist fewest Al-vacancies in the FeSiMCM-41. We suggest that, besides the thick channel wall, a more
SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998 557
Stability of MCM-41 mesoporous molecular materials: N. He et al. ACKNOWLEDGEMENT This project was supported by the China Postdoctoral Science Foundation, and the Natural Science Foundation of Human Province, China.
REFERENCES
Figure 6 The framework IR spectra of template free samples. (a) SiMCM-41; (b) AlSiMCM-41; (c) LaSiMCM-41; (d) FeSiMCM-41
1 Ozin, G.A., Kuperman, A. and Stein, A. Angew. Chem. Int. Ed. Engl. 1989, 28(3), 359 2 Srdanov, V.I., Blake, N.P., Markgraber, D., Metiu, H. and Stucky, G. D. Stud. Surf. Catal. Sci. 1994, 85, 115 3 Schulz-Ekloff, G. Stud. Surf. Catal. Sci. 1994, 85, 145 4 Rolison, D.R. Stud. Surf. Catal. Sci. 1994, 85, 543 5 Davis, M.E., Saldarriaga, C., Montes, C., Garces, L.M. and Crowder, C.A. Nature 1988, 331, 698 6 Estermann, M., McCusker, L.B., Baerlocher, Ch., Merrouche, A. and Kessler, H. Nature 1991, 352, 320 7 Huo, Q., Xu, R., Li, S., Ma, Z., Thomas, J.M., Jones, R.H. and Chippindale, A.M. J. Chem. Soc. Chem. Commun. 1992, 875 8 Kreesge, C.T., Leonowicz, M.E., Roth, W.J., Vartuli, J.C. and Beck, J.S. Nature 1992, 359, 710 9 Beck, J.S., Vartuli, J.C., Roth, W.J., Leonowicz, M.E., Kresge, C.T., Schmitt, K.D., Chu, C.T.-U., Olson, D.H., Sheppard, E.W., McCullen, S.B., Higgins, J.B. and Schlenker, J.L. J. Am. Chem. Soc. 1992, 114, 10834 10 Sayari, A. Stud. Surf. Catal. Sci. 1996, 145, 10,285, 145 11 Stucky, G.D., Huo, Q., Firouzi, A., Chmelka, B.F., Schacht, S., Voigt-Martin, I.G. and Schuth, F. Stud. Surf. Sci. Catal. 1997, 105, 3 12 He, N., Bao, S. and Xu, Q. Stud. Surf. Sci. Catal. 1997, 105, 85 13 He, N., Bao, S. and Xu, Q. Chinese J. of Chem., 1997, 15, 42 14 Ratnasamy, P. and Kumer, R. Catal. ¹oday, 1991, 2, 329 15 McCullen, S.B. and Vartuli, J.C. US5156829 1992 16 Luan, Z., Cheng, C.-F., He, H. and Klinowski, J. J. Phys. Chem., 1995, 99(26), 10,590 17 Ryoo, R. and Kim, J.M. J. Chem. Soc. Chem. Commun. 1995, 711 18 Coustel, N., Di Renzo, F. and Fajula, F. J. Chem. Soc. Chem. Commun. 1994, 967 19 Cheng, C.-F., Zhou, W., Park, D.H., Klinowski, J., Hargreaves, M. and Gladden L.F. 20 Ryoo, R. and Jun, S. J. Phys. Chem. B, 1997, 101, 317 21 Hong, S.B., Kim, S.J. and Uh, C.S. J. Chem., 1996, 100, 15923 22 Hong, S.B., Kim, S.J. and Uh, C.S. J. Am. Chem. Soc. 1996, 118, 8102 23 He, H., Barr, T.L. and Klinowski, J. J. Phys. Chem. 1994, 98, 8775 24 Koller, H., Lobo, R.F., Burkett, S.L. and Davis, M.E. J. Phys. Chem. 1995, 99, 12,588 25 Fejes, P., Hannus, I. and Kiricsi, I. Zeolites 1984, 4, 73 26 Liu, X., Lin, J., Liu, X. and Thomas, J.M. Zeolites 1992, 12, 936 27 Yang, C. and Xu, Q. J. Chem. Soc. Faraday ¹rans. 1997, 93(8), 1675
integrated structure would result in an additional enhancement effect on the hydrothermal stability of the FeSiMCM-41 sample.
558 SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998
Supramolecular Science 5 (1998) 553—558 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00
Effect of trivalent elements on the thermal and hydrothermal stability of MCM-41 mesoporous molecular materials Nongyue He*, Zuhong Lu and Chunwei Yuan National Lab. of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, People’s Republic of China
Jianming Hong Center for Matertial Analysis, Nanjing University, Nanjing 210093, People’s Republic of China
Chun Yang Department of Chemistry, Nanjing Normal University, Nanjing 210096, People’s Republic of China
Shulin Bao and Qinhua Xu Department of Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China
Effect of trivalent elements on the thermal and hydrothermal stability of MCM-41 mesoporous molecular sieve materials has been investigated. Al(III) decreases the thermal and hydrothermal stability of MCM-41 materials, whereas La(III) and Fe(III), especially Fe(III), can improve the thermal and hydrothermal stability. Benzene adsorption and IR spectra suggested that thick channel wall and the fewer structural defect sites in MCM-41 would enhance the thermal and hydrothermal stability of MCM-41. 1998 Elsevier Science Limited. All rights reserved. (Keywords: thermal stability; hydrothermal stability; MCM-41)
INTRODUCTION In the past decade, the research for smaller, faster, more selective and efficient products and processes has greatly advanced. The ability to engineer well-defined spatial microarrangements of pure substances and composite materials has become pivotal in creating new molecularlevel electronic, optical and magnetic devices. One attractive method for assembling and maintaining controlled microstructures is to use solid host lattices that serve as templates within which a guest structure of nanometer architecture can be assembled. In this host—guest system, microporous zeolite materials, which possess nanometer dimension window, channel and cavity architecture, represent a ‘New Frontier’ of solid state chemistry with great opportunities for innovative research and development. Considerable effort has been directed at new use of zeolites as advanced materials in fields such as solar energy conversion, zeolite electrodes and electron relays, zeolite
*Corresponding author. Also affiliated to Department of Chemistry, Nanjing University
batteries, zeolite chemical sensors, zeolite imaging and data storage materials, zeolite lasers and displays, semiconductor clusters, molecular wires with tailored electronic and optical properties, enzyme mimicking organic complexes, zeolite permselective membranes or thin films. All of these have been reviewed extensively\. However, the pore sizes of all the microporous zeolite materials are smaller than 1 nm, and, therefore, their use as solid host is limited. For bulky molecular reaction and the assembling of nanometer materials with different sizes in zeolite molecular sieve materials, much zeolite molecular research was aimed at the synthesis of zeolite molecular sieve materials with pore sizes greater than 1 nm. Unfortunately, before the discovery of M41S in 1991 there are only three metallophosphate samples with pore size greater than 1 nm reported: VPI-5, cloverite and JDF-20. The cloverite is thermally stable up to 700°C, but the other two materials show poor thermal stability. Since the channel structure is uniform and the pore size can be tailored in the range of 2—10 nm, the synthesis of M41S mesoporous materials by researchers from Mobil company throws new light on the synthesis of molecular sieves. Among the M41S family, the MCM-41 hexagonal mesoporous
SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998 553
Stability of MCM-41 mesoporous molecular materials: N. He et al. molecular sieve is especially of potential application, as proved by the publication of so many papers and reviews . However, although this material shows attractive application prospect, it also displays poor hydrothermal stability and not very good thermal stability, which limits its application. Therefore, it is of significance to improve the stability, especially the hydrothermal stability, of MCM-41 mesoporous molecular sieve materials. It is well known that the incorporation of some metallic cations into zeolite molecular sieves will influence their stability. In the present paper, the incorporation of Al(III), Fe(III) and La(III) into MCM-41 was investigated. It was found that the stability of MCM-41 was increased by Fe(III) and La(III) incorporation, and decreased by Al(III).
EXPERIMENTAL Synthesis of materials The synthesis of pure siliceous and Al-containing MCM-41 mesoporous materials, named as SiMCM-41 and AlSiMCM-41, respectively, was performed following the procedures described in ref.. The synthesis of Fe and La-containing MCM-41 samples, designated as FeSiMCM-41 and LaSiMCM-41, has been described previously. Before further characterization, all the four as-synthesized samples were calcined at 813 K to remove the template (C H (CH ) NBr), these template removed samples are called template free samples. Characterization After calcination to remove template, the solid materials were characterized by low-angle X-ray diffraction (XRD) (Rigaku, D/max-cA) with Cu—Ka radiation, FT-IR (Nicolet, 5DX) with a resolution of 2 cm\, the adsorption of nitrogen at 77 K (Micrometrics, ASAP2000 instrument) and High Resolution Electron Microscopy (HREM) using a JEOL 200 cx instrument. The composition of samples was obtained on a Jarrell-Ash 1100 inductively coupled plasma quantometer. To compare the thermal stability, the above template free samples were calcined in a muffle stove in air once more at 1073 and 1153 K, respectively, followed by XRD detection. The hydrothermal stability of template free samples treated with 100% steam at 933 and 1043 K for 2 h was investigated by the conventional all-glass benzene adsorption technique. Before the measurement, the samples were evacuated at 673 K until a system pressure of 1.3;10\ Pa was reached.
RESULTS AND DISCUSSION Structural properties and composition of samples The X-ray diffraction patterns of the template free samples are shown in Figures 1a—4a. The observation of
Figure 1 The XRD patterns of SiMCM-41 after calcination at different temperatures. (a) Template free sample; (b) 1073 K, 2 h; (c) 1153 K, 2 h
four peaks of samples which can be indexed on a hexagonal lattice is typical of MCM-41 materials . From Table 1 we can find that all samples exhibit nanometer pore sizes of &3.3 nm (3.2 nm for SiMCM-41) and very large surface area (SA). The HREM pictures of hexagonal structure of the four template free samples are shown in Figure 5. All samples display hexagonal ordered pores, sizes of which are the same as those determined by N adsorption at low temperature (see Table 1). ¹hermal stability The XRD patterns of all template free samples after different calcination temperature are shown in Figures 1b—4c. Because other XRD peaks are much weaker and overlap each other, we investigate the thermal stability by comparing the intensity of the (1 0 0) peak. All the four samples show a very intensive (1 0 0) diffraction peak. However, after calcination at different temperature, the
554 SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998
Stability of MCM-41 mesoporous molecular materials: N. He et al.
Figure 2 The XRD patterns of AlSiMCM-41 after calcination at different temperatures. (a) Template free sample; (b) 1073 K, 2 h; (c) 1153 K, 2 h
intensity of (1 0 0) peaks of all samples changes, accompanied by a shift towards high 2h. After calcination at 1073 K, the (1 0 0) peak intensity for SiMCM-41 and LaSiMCM-41 changes little, whereas the (1 0 0) peak intensity for AlSiMCM-41 decreases significantly. From the literature about the Fe-containing microporous zeolite molecular sieve materials , it has been well known that the incorporation of Fe(III) into framework will destabilize microporous molecular sieves. However, interestingly here we find that the incorporation of Fe(III) into MCM-41 material does not decrease the (1 0 0) peak intensity, but increases the intensity. After calcination at 1153 K for 2 h, the (1 0 0) peak intensity of all the samples changes much more than that at 1073 K. Although having become much weaker, the (1 0 0) peak intensity of SiMCM-41, LaSiMCM-41 and FeSiMCM-41
Figure 3 The XRD patterns of LaSiMCM-41 after calcination at different temperatures. (a) Template free sample; (b) 1073 K, 2 h; (c) 1153 K, 2 h
still remains strong, while AlSiMCM-41 loses most of its structure. Among the former three samples, FeSiMCM-41 shows the greatest (1 0 0) peak intensity retention after calcination at 1153 K, whereas SiMCM-41 gives the weakest. Above results show that the sequence of the thermal stability for the investigated samples is FeSiMCM-41' LaSiMCM-41' SiMCM-41' AlSiMCM-41. Hydrothermal stability The hydrothermal stability of zeolite molecular sieve materials can be easily characterized by investigating the
SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998 555
Stability of MCM-41 mesoporous molecular materials: N. He et al. benzene adsorption behavior of the studied samples before and after steam treatment. If some components are removed from the framework of molecular sieves and agglomerated into a separate phase to occlude the channel or the framework collapses partially during steaming, the equilibrium benzene adsorption capacity after steaming will decrease. Listed in Table 2 are the equilibrium benzene adsorption capacity of all the four samples mentioned above at P/P "0.5 before and after the steam treatment (P and P are the equilibrium vapor pressure measured under the experimental conditions and the saturation
Figure 4 The XRD patterns of FeSiMCM-41 after calcination at different temperatures. (a) Template free sample; (b) 1073 K, 2 h; (c) 1153 K, 2 h
vapor pressure of benzene at the experimental temperature, respectively). It was reported that the framework of AlSiMCM-41 collapsed at 936 K in steam treatment. In our case, AlSiMCM-41 steamed at 933 K loses its XRD peaks almost completely, indicating its poor hydrothermal stability. All the other three samples still retain a strong (1 0 0) peak and a high adsorption capacity for benzene after steamed at 933 K. However, the values of c/c of FeSiMCM-41 and LaSiMCM-41 are greater than that for SiMCM-41. This indicates that the incorporation of Fe(III) and La(III) into MCM-41 gives rise to a better hydrothermal stability than the incorporation of Al(III) species. In order to differentiate the later three samples, the steam treatment was performed once more at higher temperature, 1043 K. The benzene adsorption capacities are also listed in Table 2. At this elevated temperature, only FeSiMCM-41 remains in a high equilibrium benzene adsorption capacity and this suggests that the introduction of Fe(III) into MCM-41 can enhance the hydrothermal stability than that of La(III) more significantly. Since the low hydrothermal stability in hot water and aqueous solution is a critical problem for many applications of MCM-41 with nanometer pore sizes, much attention has been paid to its improvement. Generally, the hydrothermal stability of MCM-41 increases with the channel wall thickness\. Salt effects during the crystallization process of MCM-41 was also used to improve the hydrothermal stability. Our results lead us to that the incorporation of Fe(III), even La(III) is another approach to improve the hydrothermal stability of MCM41. Generally, the incorporation of Fe(III) into framework of microporous zeolite molecular sieves will destabilize them . The enhanced hydrothermal stability of FeSiMCM-41 may be explained by using the concept of wall thickness mentioned above. From Table 2 we find that FeSiMCM-41 shows the thickest channel wall, which may be due to the transformation of most Fe(III) species from tetrahedral coordination condition to octahedrally coordinated state during the calcination to remove template. The transformation implies that the Fe(III) species would shift from the internal to the surface of the channel wall. The transformed Fe(III) species is supposed to highly disperse on the channel wall and thus makes the wall thicker.
Table 1 Structural properties and composition of samples Sample
Si/Me
d (nm)
a (nm)
SA (m/g)
Pore size (nm)
Wall thickness (nm)
SiMCM-41 AlSiMCM-41 LaSiMCM-41 FeSiMCM-41
700 29 42 31
4.03 4.19 4.08 4.46
4.65 4.84 4.71 5.15
1340 1084 947 998
3.2 3.3 3.3 3.3
1.45 1.54 1.41 1.85
Me"Al, Fe and La. XRD d -spacing. Unit cell parameter a "2 d /(3. Determined by N adsorption. Wall thickness"a -pore size.
556 SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998
Stability of MCM-41 mesoporous molecular materials: N. He et al.
Figure 5 The HREM pictures of template free samples. (a) SiMCM-41; (b) AlSiMCM-41; (c) LaSiMCM-41; (d) FeSiMCM-41
Table 2 Hydrothermal stability of samples Sample
Before steam treatment d (nm)
SiMCM-41 AlSiMCM-41 LaSiMCM-41 FeSiMCM-41
4.03 4.19 4.08 4.46
Wall thickness (nm)
1.45 1.54 1.41 1.85
After steam treatment (100% steam) c (g/g)
0.680 0.655 0.548 0.605
933 K, 2 h
1043 K, 2 h
d (nm)
c (g/g)
c/c (%)
c (g/g)
c /c (%)
3.72 — 3.72 4.12
0.381 0.278 0.348 0.408
56 42 64 67
0.184 0.079 0.110 0.303
27 12 20 50
c is the benzene adsorption of the template free samples at P/P "0.5; c and c are the benzene adsorption of template free samples at P/P "0.5 after given stream treatment. The structure collapsed almost completely.
The transformed Fe(III) species not only high-disperse on the surface of the wall, but also can fill the Al-depleted structural vacancies. The existence of structural defect in zeolite molecular sieve materials has been widely reported\. Generally, the molecular sieves with high framework Si/Al ratio or those templated by organic template, such as NON, DDR, MTW, AFI, MFI, VPI-5, AlPO -5, AlPO -8 and so on\, possess defects. All types of defect sites can exist in as-synthesized zeolites or in samples that have been calcined or exposed to various treatments, e.g., dealumination. Fejes et al.
reported a framework IR absorption band at 930 cm\ and ascribed it to the Al-depleted defect center. This band sifts towards high wave numbers with the increase of the Si/Al ratio of the tested samples\. From Figure 6 we can clearly find this band (at 959 cm\) also appears in the framework IR spectra for the SiMCM-41 (Figure 6a). However, this band becomes much weaker in the spectra for the other three samples. FeSiMCM-41 especially shows the weakest band, This suggests that there exist fewest Al-vacancies in the FeSiMCM-41. We suggest that, besides the thick channel wall, a more
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Stability of MCM-41 mesoporous molecular materials: N. He et al. ACKNOWLEDGEMENT This project was supported by the China Postdoctoral Science Foundation, and the Natural Science Foundation of Human Province, China.
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Figure 6 The framework IR spectra of template free samples. (a) SiMCM-41; (b) AlSiMCM-41; (c) LaSiMCM-41; (d) FeSiMCM-41
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integrated structure would result in an additional enhancement effect on the hydrothermal stability of the FeSiMCM-41 sample.
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