- Email: [email protected]
Amorphous microporous molecular sieves studied by laser-polarized 129Xe NMR spectroscopy
812
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
Amorphous microporous molecular sieves studied by laserpolarized 129Xe NMR spectroscopy M.-A. Springuel-Hueta, A. Vidal Moyab, M. J. Díaz-CabanҊasb, A. Cormab and A. Gédéona a
Laboratoire des Systèmes Interfaciaux à l'Echelle Nanométrique (SIEN), Université P. et M. Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France. Tel: 33 1 44 27 55 37, Facsimile: 33 1 44 27 55 36; Email: [email protected] b
Instituto de Tecnología Química, UPV-CSIC, Universidad Politécnica de Valencia, Avenida de los Naranjos sln, E-46022 Valencia, Spain ABSTRACT Crystalline zeolites, ZSM-12 and ITQ-21, and their amorphous precursors presenting the same porosity, have been studied by hyperpolarized 129Xe NMR spectroscopy of adsorbed Xe. The higher chemical shifts and the smaller chemical shift variations with Xe concentration observed for the amorphous compared to the crystalline materials show that the pores exhibit a surface roughness responsible for a stronger Xe-surface interaction that occurs at the expense of Xe-Xe interaction inside the micropores. 1. INTRODUCTION By analogy with the synthesis mechanism of mesoporous molecular sieves it has been recently shown that amorphous (from XRD point of view) zeolite precursors (ZP) can be obtained from self-assembling of organic structure-directing agents and silica [1]. These solids are thermally and hydrothermally stable upon calcination and their pore dimensions and topologies are very close to zeolites as shown by pore size distribution from Ar sorption isotherms [1]. They behave as shape selective catalysts and can be transformed into crystalline zeolites. Xe NMR spectroscopy is a valuable tool to investigate porous solids. It has been largely used for over 25 years to study zeolites and related materials [2], mesoporous silicas [3] and also clathrates [4], polymers [5], carbons [6] etc. The last development of hyperpolarization technique [7] that increases dramatically the sensitivity of detection of Xe NMR allows to study other solids such as low surface area oxides [8] or films [9]. It has been shown that the chemical shift of xenon adsorbed in porous system can be written as the sum of terms corresponding to each interaction undergone by the Xe atoms. In the absence of strong adsorption sites, of electric and magnetic fields created by cations, the expression is reduced to: G= Gs + G1 UXe [2]. The term, G1 UXe, due to Xe-Xe interactions (G1 characterizes the Xe-Xe interaction and UXe is the Xe concentration), is negligible for Xe adsorbed in mesoporous solids and the chemical shift is roughly constant with Xe pressure. It becomes significant for microporous systems due to the confinement of Xe atoms in small pores. A relationship
813 between the term Gs and the pore size has been established for zeolites [10]. The smaller the pore size the higher the chemical shift. The unexpectedly high chemical shift observed for mesoporous silicas, such as MTS materials, whose pores are larger than those of zeolites have been explained by a stronger Xe-surface interaction due to the surface roughness of amorphous materials [11]. Crystalline zeolites and their amorphous precursors give a unique opportunity to compare crystalline and amorphous materials with the same pore structure. We report here the study of ZSM-12 and ITQ-21 zeolites and their respective precursors using hyperpolarized 129Xe NMR. 2. EXPERIMENTAL 2.1. Materials The crystalline zeolites and their amorphous precursors were synthesized under hydrothermal conditions from gel composition: xGeO2 : (1-x)SiO2 : yAl2O3 : zOSDAOH : zHF : wH2O where OSDAOH is the organic structure directing agents (1,6bisquinuclidinium-hexane and N-methylsparteinium for ZSM-12 and ITQ-21 structures, respectively) during 2 days or 6 hours to obtain the zeolite (Z) or its precursor (P), respectively [1]. The final solids were recovered by filtration, washed with distilled water and dried at 373 K. They were calcined at 823 K in air for 3 h with a heating rate of 3 °C/mn. The pore structure of ZSM-12 consists of a one-dimensional 12-ring channel (0.56 × 0.60 nm in diameter) network [12]. The pore structure of ITQ-21 resembles to that of faujasite: spherical cavities (1.18 nm in diameter) connected to each other by six circular 12-ring windows (instead of 4 windows for faujasite) with an aperture of 0.74 nm [13]. 2.2. 129Xe NMR spectroscopy The materials were evacuated at 673 K overnight before Xe adsorption and NMR experiments. The Xe adsorption isotherms are measured at 295 K by manometry. The laser-polarized 129Xe NMR spectra were recorded on a Bruker AMX 300 spectrometer operating at 83.02 MHz under continuous gas flow using a home-built system [14]. The unusual long relaxation time T1, of the order of 50 s, justifies the need to use hyperpolarized Xe to obtain spectra within a reasonable time since the repetition time is then not chosen in function of T1. In hyperpolarized 129Xe experiments the signal usually reaches its maximum value with D1 of the order of a few seconds. The chemical shifts are referred to Xe gas phase line. The low-temperature experiments have been performed with thermallypolarized 129Xe using a NMR tube with small volume in order to keep the concentration of adsorbed Xe almost constant whatever the temperature that is not possible with hyperpolarized 129Xe. In this case the xenon is adsorbed at room temperature before NMR experiments. 3. RESULTS AND DISCUSSION The spectra of the loosely-packed samples show broad asymmetrical lines that move towards high chemical shifts and become narrow and symmetrical when the loose powder is compressed (Fig. 1). The lines become also narrow and symmetrical when the temperature decreases that is with the decrease of the Xe diffusion (spectra not shown). This behavior is characteristic of an exchange between adsorbed and interparticle gaseous Xe. It has been often observed in particular with mesoporous silica in which the Xe diffusion is very rapid [15]. The exchange depends on the particle size. The smaller the particles the more important
814 the exchange is. It is usually not observed with microporous solids where Xe diffuses less rapidly than in mesoporous systems. In this study the presence of an exchange proves that the particles are particularly small. The complex shape of the lines reflects the particle size distribution of the sample. To decrease the exchange and make it negligible, the samples were compressed under a pressure of 150 MPa. In the following all the data concern compressed powders. We checked by N2 adsorption at 77 K that the microporous structure is not destroyed after compression (results not shown). The chemical shift (G) variations versus Xe concentration, n, are linear in the pressure range studied (Fig. 2) for all the samples except for crystalline ITQ-21 which shows a slight increase of the slope with n. The chemical shift, extrapolated at zero coverage gives Gs= 57 ppm for ITQ-21 –Z. It is comparable to that observed for Y zeolite (55 to 60 ppm depending on the Si/Al ratio) which has a comparable pore structure [16]. The smaller slope of the G-n curve compared to that of NaY shows that the Xe-Xe interactions are less important in ITQ-1 –Z. This may be due the higher number of windows in the ITQ-1 –Z structure. For ZSM-12 –Z, the value of Gs (74 ppm) is in agreement with the results reported by Moudrakovski et al. [18] but the signal does not show any chemical shift anisotropy (CSA) as observed by these authors who attributed the CSA to the crystal anisotropy. The influence of the Xe exchange mentioned above, due to the presence of small particles, may be responsible to the absence of CSA. Indeed the CSA is only observed at 143 K when the Xe mobility is low (Fig. 3).
b a 240
200
160
120
80
40
0
(ppm) Fig. 1. Hyperpolarized 129Xe NMR spectra of loosely packed (a) and compressed under 150 MPa (b) powder of ITQ-21 –P. The line at 0 ppm is that of Xe gas phase. Xe pressure is 8u104 Pa.
The chemical shifts of the precursors, ZSM-12 –P and ITQ-21 –P, are much higher than that of the corresponding crystalline materials despite similar pore structures, but very close to each other despite different pore structures of ZSM-12 and ITQ-21 (Fig. 2). The chemical shifts of crystalline materials follow the relationship between GS and the pore size established for zeolites [10]. In contrary, the amorphous materials do not obey to this relationship as it was previously observed for mesoporous MCM-41 silica [18]. As in the latter case, the higher chemical shifts observed for amorphous materials, ZSM-12 –P and ITQ-21 –P, can be attributed to the stronger interaction between Xe and the pore surface. Indeed, in contrary to crystalline zeolites, the local curvature radius, at atomic scale, of the pore surface may be much smaller than the pore radius due to the surface roughness of the amorphous materials. This is confirmed by low-temperature experiments: the chemical shifts
815 are much higher for amorphous than for crystalline zeolites (Fig. 4). At low-temperature the residence time of Xe on the surface is long and the chemical shift essentially reflects the Xesurface interactions if the Xe-Xe interactions on the surface are negligible. 120
chemical shift (ppm)
110 100 90 80 70 60 50
0.E+00 1.E+20 2.E+20 3.E+20 4.E+20 5.E+20 6.E+20 7.E+20 8.E+20
Xe concentration (atoms/g)
Fig. 2. Chemical shift variation versus Xe concentration of ZSM-12 –Z (Ŷ), ZSM-12 –P (Ƒ), ITQ-21 – Z (Ÿ), ITQ-21 –P (ǻ), NaY (LZY-52 from UOP) (Ɣ) T (K) 143 183 243 298 160
120
80
(ppm)
40
0
Fig. 3. Thermally-polarized 129Xe NMR spectra of ZSM-12 –Z compressed under 150 MPa at different temperatures. Xe pressure is 15u103 Pa. The anisotropy parameters of the line at 143 K are Gcs = -249 ppm and Kcs= 0.1. 150 chemical shift (ppm)
140 130 120 110 100 90 80 70 60 50 120
140
160
180
200
220
240
260
280
300
320
T (K)
Fig. 4. Chemical shift variations of thermally-polarized Xe adsorbed at 15u103 Pa in ZSM-12 – Z (S) and ZSM-12 –P (Ŷ)
816 For ZSM-12 –Z, the chemical shift at 143 K is 95 ppm for a Xe pressure of 15u103 Pa at which the Xe-Xe interactions on the surface are assumed to be negligible. This value is in good agreement with the low-temperature value obtained for other zeolites, 97 ppm for ZSM5 and 93 ppm for NaY [19]. On the other hand the value obtained at 143 K for ZSM-12 –P (123 ppm) is comparable to that obtained with amorphous silicas at low temperature (around 110-140 ppm depending on the solids) [20]. Consequently, the microporous materials also show high chemical shifts, compared to the crystalline materials, when the pore walls are amorphous. The explanation proposed for mesoporous silicas, i.e., the influence of strong Xe-surface interactions due to the surface roughness of amorphous materials, is also valid for microporous solids. The smaller chemical shift variations with Xe concentration observed for the amorphous precursors show that the Xe-Xe interactions within the micropores of these solids are smaller than those occurring in the corresponding crystalline materials (Fig. 2). If the Xe atoms interact more strongly with the surface, their residence time on the surface is longer. Therefore, the time spent in the micropore volume is shorter and the Xe-Xe interactions are smaller. The higher Xe-surface interactions occur at the expense of the Xe-Xe interactions. This effect could not be observed with mesoporous silicas since the Xe-Xe interactions in mesopores are comparable to that of Xe gaseous phase and are not enough important in the pressure range studied to be measured by Xe NMR. 4. CONCLUSION 129
Xe NMR study of ZSM-12 and ITQ-21 zeolites and their amorphous analogues have shown that the chemical shifts observed for the amorphous materials are much higher than those of the corresponding crystalline zeolites. These results are consistent with what was observed with amorphous mesoporous silicas which present chemical shifts much higher than those expected, taking into account the relationship between chemical shifts and pore sizes established with zeolites. As for mesoporous silicas, the high chemical shifts have been attributed to the stronger Xe-surface interactions due to the surface roughness of the amorphous materials. In microporous systems these stronger Xe-surface interactions occur at the expense of the Xe-Xe interactions inside the pores. By combination of N2 and Ar adsorption and laser polarized 129Xe NMR it can then be concluded that the zeolite precursors studied are microporous materials with well-defined pores of dimensions very similar to the final zeolite, but with amorphous walls. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]
A. Corma and M. J. Díaz-CabaĖas, Microporous Mesoporous Mater., 89 (2006) 39. M. A. Springuel-Huet, J. L. Bonardet and J. Fraissard, Appl. Magn. Res., 8 (1995) 427 V. V. Terskikh, I. L. Mudrakovskii and V. M. Mastikhin, J. Chem. Soc., Faraday Trans., 89 (1993) 4239 J. A. Ripmeester, C. I. Ratcliffe and J. S. Tse, J. Chem. Soc., Faraday Trans. 184 (1988) 3731 W. S Veeman, and F. Junker, Polymer. Mater. Sci. and Eng., 82 (2000), 167 K. Saito, A. Kimura and H. Fujiwara,. Magn. Res. Imag, 21 (2003) 401 D. Raftery, H. Long, T. Meersmann, P. J. Grandinetti, L. Reven and A. Pines, Phys. Rev. Lett., 66 (1991) D. Raftery, L. Reven, H. Long, A. Pines, P. Tang and J. A. Reimer, J. Phys. Chem., 97 (1993) 1649
817 [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20]
E. Haddad, A. Nossov, F. Guenneau, M. Nader, D. Grosso, C. Sanchez, A. Gedeon, Stud. Surf. Sci. Catal., 154 (2004) 1464 M.-A. Springuel-Huet, J. Demarquay, T. Ito and J. Fraissard, Stud. Surf. Sci. Catal., 37 (1988) 183. M.-A. Springuel-Huet, K. Sun and J. Fraissard, Microporous Mesoporous Mater., 33 (1999) 89 R. B. LaPierre, A. C. Jr. Rohrman, J. L. Schlenker, J. D. Wood, M. K. Rubin, W. J. Rohrbaugh, Zeolites, 5 (1985) 346 A. Corma, M. J. Diaz-Cabanas, J. Martinez-Triguero, F. Rey and J. Rius, Nature, 417 (2002) 514 A. Nossov, F.Guenneau, M.-A. Springuel-Huet, E. Haddad, V. Montouillout, B. Knott, F. Engelke, A. Gédéon and C. Fernandez, Phys. Chem. Chem. Phys., 5 (2003) 4479. M.-A. Springuel-Huet, J. L. Bonardet, A. Gédéon, Y. Yue, V. N. Romannikov and J. Fraissard, Microporous Mesoporous Mater., 44-45 (2001) 775 T. Ito and J. Fraissard, Proc. Int. 5th Conf. Zeolites, Heyden Publi., London, 1980, 510 I. L. Moudrakovski, C.I. Ratcliffe and J. A. Ripmeester, Appl. Magn. Reson., 10 (1996) 559. M.A. Springuel-Huet J. Fraissard, R. Schimdt, M. Stöcker and W. C. Conner, in "Characterisation of Porous Solids IV", B. McEnaney et al. (Eds.), The Royal Society of Chemistry Publ., (1997) 452. Q. J. Chen and J. Fraissard, J. Phys. Chem., 96 (1992) 1809 W. C. Conner, E. L. Weist, T. Ito and J. Fraissard, J. Phys. Chem., 93 (1989) 4138
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
Amorphous microporous molecular sieves studied by laserpolarized 129Xe NMR spectroscopy M.-A. Springuel-Hueta, A. Vidal Moyab, M. J. Díaz-CabanҊasb, A. Cormab and A. Gédéona a
Laboratoire des Systèmes Interfaciaux à l'Echelle Nanométrique (SIEN), Université P. et M. Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France. Tel: 33 1 44 27 55 37, Facsimile: 33 1 44 27 55 36; Email: [email protected] b
Instituto de Tecnología Química, UPV-CSIC, Universidad Politécnica de Valencia, Avenida de los Naranjos sln, E-46022 Valencia, Spain ABSTRACT Crystalline zeolites, ZSM-12 and ITQ-21, and their amorphous precursors presenting the same porosity, have been studied by hyperpolarized 129Xe NMR spectroscopy of adsorbed Xe. The higher chemical shifts and the smaller chemical shift variations with Xe concentration observed for the amorphous compared to the crystalline materials show that the pores exhibit a surface roughness responsible for a stronger Xe-surface interaction that occurs at the expense of Xe-Xe interaction inside the micropores. 1. INTRODUCTION By analogy with the synthesis mechanism of mesoporous molecular sieves it has been recently shown that amorphous (from XRD point of view) zeolite precursors (ZP) can be obtained from self-assembling of organic structure-directing agents and silica [1]. These solids are thermally and hydrothermally stable upon calcination and their pore dimensions and topologies are very close to zeolites as shown by pore size distribution from Ar sorption isotherms [1]. They behave as shape selective catalysts and can be transformed into crystalline zeolites. Xe NMR spectroscopy is a valuable tool to investigate porous solids. It has been largely used for over 25 years to study zeolites and related materials [2], mesoporous silicas [3] and also clathrates [4], polymers [5], carbons [6] etc. The last development of hyperpolarization technique [7] that increases dramatically the sensitivity of detection of Xe NMR allows to study other solids such as low surface area oxides [8] or films [9]. It has been shown that the chemical shift of xenon adsorbed in porous system can be written as the sum of terms corresponding to each interaction undergone by the Xe atoms. In the absence of strong adsorption sites, of electric and magnetic fields created by cations, the expression is reduced to: G= Gs + G1 UXe [2]. The term, G1 UXe, due to Xe-Xe interactions (G1 characterizes the Xe-Xe interaction and UXe is the Xe concentration), is negligible for Xe adsorbed in mesoporous solids and the chemical shift is roughly constant with Xe pressure. It becomes significant for microporous systems due to the confinement of Xe atoms in small pores. A relationship
813 between the term Gs and the pore size has been established for zeolites [10]. The smaller the pore size the higher the chemical shift. The unexpectedly high chemical shift observed for mesoporous silicas, such as MTS materials, whose pores are larger than those of zeolites have been explained by a stronger Xe-surface interaction due to the surface roughness of amorphous materials [11]. Crystalline zeolites and their amorphous precursors give a unique opportunity to compare crystalline and amorphous materials with the same pore structure. We report here the study of ZSM-12 and ITQ-21 zeolites and their respective precursors using hyperpolarized 129Xe NMR. 2. EXPERIMENTAL 2.1. Materials The crystalline zeolites and their amorphous precursors were synthesized under hydrothermal conditions from gel composition: xGeO2 : (1-x)SiO2 : yAl2O3 : zOSDAOH : zHF : wH2O where OSDAOH is the organic structure directing agents (1,6bisquinuclidinium-hexane and N-methylsparteinium for ZSM-12 and ITQ-21 structures, respectively) during 2 days or 6 hours to obtain the zeolite (Z) or its precursor (P), respectively [1]. The final solids were recovered by filtration, washed with distilled water and dried at 373 K. They were calcined at 823 K in air for 3 h with a heating rate of 3 °C/mn. The pore structure of ZSM-12 consists of a one-dimensional 12-ring channel (0.56 × 0.60 nm in diameter) network [12]. The pore structure of ITQ-21 resembles to that of faujasite: spherical cavities (1.18 nm in diameter) connected to each other by six circular 12-ring windows (instead of 4 windows for faujasite) with an aperture of 0.74 nm [13]. 2.2. 129Xe NMR spectroscopy The materials were evacuated at 673 K overnight before Xe adsorption and NMR experiments. The Xe adsorption isotherms are measured at 295 K by manometry. The laser-polarized 129Xe NMR spectra were recorded on a Bruker AMX 300 spectrometer operating at 83.02 MHz under continuous gas flow using a home-built system [14]. The unusual long relaxation time T1, of the order of 50 s, justifies the need to use hyperpolarized Xe to obtain spectra within a reasonable time since the repetition time is then not chosen in function of T1. In hyperpolarized 129Xe experiments the signal usually reaches its maximum value with D1 of the order of a few seconds. The chemical shifts are referred to Xe gas phase line. The low-temperature experiments have been performed with thermallypolarized 129Xe using a NMR tube with small volume in order to keep the concentration of adsorbed Xe almost constant whatever the temperature that is not possible with hyperpolarized 129Xe. In this case the xenon is adsorbed at room temperature before NMR experiments. 3. RESULTS AND DISCUSSION The spectra of the loosely-packed samples show broad asymmetrical lines that move towards high chemical shifts and become narrow and symmetrical when the loose powder is compressed (Fig. 1). The lines become also narrow and symmetrical when the temperature decreases that is with the decrease of the Xe diffusion (spectra not shown). This behavior is characteristic of an exchange between adsorbed and interparticle gaseous Xe. It has been often observed in particular with mesoporous silica in which the Xe diffusion is very rapid [15]. The exchange depends on the particle size. The smaller the particles the more important
814 the exchange is. It is usually not observed with microporous solids where Xe diffuses less rapidly than in mesoporous systems. In this study the presence of an exchange proves that the particles are particularly small. The complex shape of the lines reflects the particle size distribution of the sample. To decrease the exchange and make it negligible, the samples were compressed under a pressure of 150 MPa. In the following all the data concern compressed powders. We checked by N2 adsorption at 77 K that the microporous structure is not destroyed after compression (results not shown). The chemical shift (G) variations versus Xe concentration, n, are linear in the pressure range studied (Fig. 2) for all the samples except for crystalline ITQ-21 which shows a slight increase of the slope with n. The chemical shift, extrapolated at zero coverage gives Gs= 57 ppm for ITQ-21 –Z. It is comparable to that observed for Y zeolite (55 to 60 ppm depending on the Si/Al ratio) which has a comparable pore structure [16]. The smaller slope of the G-n curve compared to that of NaY shows that the Xe-Xe interactions are less important in ITQ-1 –Z. This may be due the higher number of windows in the ITQ-1 –Z structure. For ZSM-12 –Z, the value of Gs (74 ppm) is in agreement with the results reported by Moudrakovski et al. [18] but the signal does not show any chemical shift anisotropy (CSA) as observed by these authors who attributed the CSA to the crystal anisotropy. The influence of the Xe exchange mentioned above, due to the presence of small particles, may be responsible to the absence of CSA. Indeed the CSA is only observed at 143 K when the Xe mobility is low (Fig. 3).
b a 240
200
160
120
80
40
0
(ppm) Fig. 1. Hyperpolarized 129Xe NMR spectra of loosely packed (a) and compressed under 150 MPa (b) powder of ITQ-21 –P. The line at 0 ppm is that of Xe gas phase. Xe pressure is 8u104 Pa.
The chemical shifts of the precursors, ZSM-12 –P and ITQ-21 –P, are much higher than that of the corresponding crystalline materials despite similar pore structures, but very close to each other despite different pore structures of ZSM-12 and ITQ-21 (Fig. 2). The chemical shifts of crystalline materials follow the relationship between GS and the pore size established for zeolites [10]. In contrary, the amorphous materials do not obey to this relationship as it was previously observed for mesoporous MCM-41 silica [18]. As in the latter case, the higher chemical shifts observed for amorphous materials, ZSM-12 –P and ITQ-21 –P, can be attributed to the stronger interaction between Xe and the pore surface. Indeed, in contrary to crystalline zeolites, the local curvature radius, at atomic scale, of the pore surface may be much smaller than the pore radius due to the surface roughness of the amorphous materials. This is confirmed by low-temperature experiments: the chemical shifts
815 are much higher for amorphous than for crystalline zeolites (Fig. 4). At low-temperature the residence time of Xe on the surface is long and the chemical shift essentially reflects the Xesurface interactions if the Xe-Xe interactions on the surface are negligible. 120
chemical shift (ppm)
110 100 90 80 70 60 50
0.E+00 1.E+20 2.E+20 3.E+20 4.E+20 5.E+20 6.E+20 7.E+20 8.E+20
Xe concentration (atoms/g)
Fig. 2. Chemical shift variation versus Xe concentration of ZSM-12 –Z (Ŷ), ZSM-12 –P (Ƒ), ITQ-21 – Z (Ÿ), ITQ-21 –P (ǻ), NaY (LZY-52 from UOP) (Ɣ) T (K) 143 183 243 298 160
120
80
(ppm)
40
0
Fig. 3. Thermally-polarized 129Xe NMR spectra of ZSM-12 –Z compressed under 150 MPa at different temperatures. Xe pressure is 15u103 Pa. The anisotropy parameters of the line at 143 K are Gcs = -249 ppm and Kcs= 0.1. 150 chemical shift (ppm)
140 130 120 110 100 90 80 70 60 50 120
140
160
180
200
220
240
260
280
300
320
T (K)
Fig. 4. Chemical shift variations of thermally-polarized Xe adsorbed at 15u103 Pa in ZSM-12 – Z (S) and ZSM-12 –P (Ŷ)
816 For ZSM-12 –Z, the chemical shift at 143 K is 95 ppm for a Xe pressure of 15u103 Pa at which the Xe-Xe interactions on the surface are assumed to be negligible. This value is in good agreement with the low-temperature value obtained for other zeolites, 97 ppm for ZSM5 and 93 ppm for NaY [19]. On the other hand the value obtained at 143 K for ZSM-12 –P (123 ppm) is comparable to that obtained with amorphous silicas at low temperature (around 110-140 ppm depending on the solids) [20]. Consequently, the microporous materials also show high chemical shifts, compared to the crystalline materials, when the pore walls are amorphous. The explanation proposed for mesoporous silicas, i.e., the influence of strong Xe-surface interactions due to the surface roughness of amorphous materials, is also valid for microporous solids. The smaller chemical shift variations with Xe concentration observed for the amorphous precursors show that the Xe-Xe interactions within the micropores of these solids are smaller than those occurring in the corresponding crystalline materials (Fig. 2). If the Xe atoms interact more strongly with the surface, their residence time on the surface is longer. Therefore, the time spent in the micropore volume is shorter and the Xe-Xe interactions are smaller. The higher Xe-surface interactions occur at the expense of the Xe-Xe interactions. This effect could not be observed with mesoporous silicas since the Xe-Xe interactions in mesopores are comparable to that of Xe gaseous phase and are not enough important in the pressure range studied to be measured by Xe NMR. 4. CONCLUSION 129
Xe NMR study of ZSM-12 and ITQ-21 zeolites and their amorphous analogues have shown that the chemical shifts observed for the amorphous materials are much higher than those of the corresponding crystalline zeolites. These results are consistent with what was observed with amorphous mesoporous silicas which present chemical shifts much higher than those expected, taking into account the relationship between chemical shifts and pore sizes established with zeolites. As for mesoporous silicas, the high chemical shifts have been attributed to the stronger Xe-surface interactions due to the surface roughness of the amorphous materials. In microporous systems these stronger Xe-surface interactions occur at the expense of the Xe-Xe interactions inside the pores. By combination of N2 and Ar adsorption and laser polarized 129Xe NMR it can then be concluded that the zeolite precursors studied are microporous materials with well-defined pores of dimensions very similar to the final zeolite, but with amorphous walls. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]
A. Corma and M. J. Díaz-CabaĖas, Microporous Mesoporous Mater., 89 (2006) 39. M. A. Springuel-Huet, J. L. Bonardet and J. Fraissard, Appl. Magn. Res., 8 (1995) 427 V. V. Terskikh, I. L. Mudrakovskii and V. M. Mastikhin, J. Chem. Soc., Faraday Trans., 89 (1993) 4239 J. A. Ripmeester, C. I. Ratcliffe and J. S. Tse, J. Chem. Soc., Faraday Trans. 184 (1988) 3731 W. S Veeman, and F. Junker, Polymer. Mater. Sci. and Eng., 82 (2000), 167 K. Saito, A. Kimura and H. Fujiwara,. Magn. Res. Imag, 21 (2003) 401 D. Raftery, H. Long, T. Meersmann, P. J. Grandinetti, L. Reven and A. Pines, Phys. Rev. Lett., 66 (1991) D. Raftery, L. Reven, H. Long, A. Pines, P. Tang and J. A. Reimer, J. Phys. Chem., 97 (1993) 1649
817 [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20]
E. Haddad, A. Nossov, F. Guenneau, M. Nader, D. Grosso, C. Sanchez, A. Gedeon, Stud. Surf. Sci. Catal., 154 (2004) 1464 M.-A. Springuel-Huet, J. Demarquay, T. Ito and J. Fraissard, Stud. Surf. Sci. Catal., 37 (1988) 183. M.-A. Springuel-Huet, K. Sun and J. Fraissard, Microporous Mesoporous Mater., 33 (1999) 89 R. B. LaPierre, A. C. Jr. Rohrman, J. L. Schlenker, J. D. Wood, M. K. Rubin, W. J. Rohrbaugh, Zeolites, 5 (1985) 346 A. Corma, M. J. Diaz-Cabanas, J. Martinez-Triguero, F. Rey and J. Rius, Nature, 417 (2002) 514 A. Nossov, F.Guenneau, M.-A. Springuel-Huet, E. Haddad, V. Montouillout, B. Knott, F. Engelke, A. Gédéon and C. Fernandez, Phys. Chem. Chem. Phys., 5 (2003) 4479. M.-A. Springuel-Huet, J. L. Bonardet, A. Gédéon, Y. Yue, V. N. Romannikov and J. Fraissard, Microporous Mesoporous Mater., 44-45 (2001) 775 T. Ito and J. Fraissard, Proc. Int. 5th Conf. Zeolites, Heyden Publi., London, 1980, 510 I. L. Moudrakovski, C.I. Ratcliffe and J. A. Ripmeester, Appl. Magn. Reson., 10 (1996) 559. M.A. Springuel-Huet J. Fraissard, R. Schimdt, M. Stöcker and W. C. Conner, in "Characterisation of Porous Solids IV", B. McEnaney et al. (Eds.), The Royal Society of Chemistry Publ., (1997) 452. Q. J. Chen and J. Fraissard, J. Phys. Chem., 96 (1992) 1809 W. C. Conner, E. L. Weist, T. Ito and J. Fraissard, J. Phys. Chem., 93 (1989) 4138