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The influence of metal nature on the structure, adsorption and acidic properties of aluminophosphate with VFI structure
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
221
The influence of metal nature on the structure, adsorption and acidic properties of aluminophosphate with VFI structure Oleksiy V. Shvets, Pavel S. Yaremov, Vladimir G. Ilyin L.V. Pisarzhevsky Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Nauky Avenue, Kyiv 03028, Ukraine, phone + 380 44 525 41 96, fax + 380 44 525 62 16
Abstract Metaloaluminophosphate zeolites with VFI structure, containing incorporated metals cations, were synthesized. Essential deformation of VFI structure, correlating with the size of ionic radii of incorporated cation is found. The formation of strong acid centers was detected in the case of bivalent cation and Cr3+. It was established, that the stregth of the Brønsted acidic centers is sharply reduced in a sequence of substitution cations Mg >> Ca > Sr Ba and grows in a sequence Cu Ni Mn < Co < Zn < Cr < Mg, which may evidence for cumulative influence of electronegativity and the radius of metals cation on value of Me-O(H)-P angle and strength of the acidic centers. Keywords: Me-VFI, Brønsted acidic centers.
1. Introduction Since 1980 a family of aluminophosphate molecular sieves greatly expanded the structural diversity of crystalline microporous materials. Preparation of metal(II)containing aluminophosphates, containing negatively charged frameworks and Brønsted and Lewis acidic sites redox sites, opened new ways to utilize these molecular sieves as catalysts [1-8]. Transition metal containing aluminophosphates have great potential as heterogeneous oxidation catalysts for a variety of organic substrates [9]. The main aim of the present work was to investigate the effect of the nature of framework-forming elements on the structure and adsorption and acidic characteristics of metalloaluminophosphates of VPI-5 type (MeVPI-5) as well as to try to detect and analyze possible manifestations of isomorphism.
2. Experimental Samples of MeVPI-5 (Me = Mg2+, Ca2+, Sr2+, Ba2+, Ti4+, V4+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+) were synthesized according to a procedure similar to reported in [10]. Pseudoboehmite was added in small portions to a mixture of phosphoric acid and distilled water in 1:1:40 molar ratios. After ageing (2 h) 1 eq. of di-n-propylamine was added dropwise to the mixture, and obtained gel was stirred for 2 h. The source of the metal, 0.04 eq. of Mg(CH3COO)2·4H2O, Co(CH3COO)2·4H2O, Zn(CH3COO)2· 2H2O, Mn(CH3COO2)·4H2O, Ni(NO3)2·6H2O, CuCl2·6H2O, CaCl2 (anh), Sr(NO3)2 (anh), BaCl2·2H2O, FeCl3·6H2O, CrCl3·6H2O, VOSO4 (anh), or Ti(OC4H9)4, was dissolved (or dispersed) in 5 ml of distilled water and added to the gel, after which the mixture was stirred for 15 min. Obtained reaction mixture (RM) was transferred to Teflon beaker, placed in steel autoclave, heated to 150°C at 10 deg/min in pulsating oil thermostat, and
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kept at autogenic pressure for 3.5 h. The products were separated by decantation (when required with separation into coarse and fine crystalline fractions) and dried at RT. The structure of MeVPI-5 samples was monitored by X-ray diffractometry (DRON-3M instrument, CuK radiation). Infrared spectra were recorded on Specord IR-75 spectrophotometer. The spectra in UV and visible regions were recorded on Specord M40 (diffuse reflective regime) and Unico 4802 (absorption regime) spectrophotometers. N2 adsorption and gravimetric adsorption method (adsorbates methanol and water) was used to characterise porous structure, and the parameters of micropores were calculated by Dubinin–Radushkevich equation. Thermogravimetric analysis (TGA) of MeVPI-5 samples was performed on a Q-1000 derivatograph in Ar stream. The acidic characteristics of MeVPI-5 were studied by thermoprogrammed desorption of ammonia (TPDA) in the range of 20-650°C (heating rate 17 deg/min).
3. Results and discussion Analysis of XRD patterns of as-synthesed samples of MeVPI-5 shows that in the most cases well crystallized zeolites-like phosphates with the VFI structure are formed. In the case of the divalent metals I of the first reflection is significantly reduced, and a redistribution of I is observed for the reflections in the region of 20-24°. Analysis of observed changes of I allows to detect fairly pronounced effect of the size of the cations on the extent of deformation of the structure in the series Al3+ (0.59 Å) < Mg2+ (0.72 Å) < Ca2+ (1.01 Å) < Sr2+ (1.16 Å) < Ba2+ (1.36 Å). The distortions of the structure, due to incorporation of metal ions at crystallographic positions where Al3+ can change the tetrahedral environment to octahedral (these positions make a contribution to the intensity of the reflection from the 100 plane), increase in the following order: Ni2+ (0.70 Å) < Co2+ (0.73 Å) Cu2+ (0.75 Å) < Mn2+ (0.80 Ã) << Zn2+ (0.72 Å). Chemical analysis data show any differences in quantity of incorporated metals. Al/Me ratio for Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn-VPI-5 was 17.1, 21.6, 22.3, 56.8, 51.2, 52.4, 36.3, 12.4, 61.7, 17.3, 23.6, 27.9, 14.1 respectively. Analysis of IR spectra of obtained samples of MeVPI-5 does not reveal significant differences from reference sample (aluminophosphate VPI-5) or particular changes of I for the absorption bands, as well as there are no new bands, which may occur due to the formation of oxide fragments. In visible region of the spectrum the position of absorption bands, the number of components into which these bands are split, and splitting values indicate that Co2+ ions in the structure of as-synthesized CoVPI-5 zeolites are in tetrahedral coordination. In the NiVPI-5 sample Ni2+ ions are in two different positions—tetrahedral and octahedral. The results of thermogravimetric analysis (TGA) indicate a substantial decrease in general level of hydration of initial VPI-5 during introduction of divalent metals with cations of small radius (0.74-0.91 nm) in the framework, and their effect decreases in the order Co2+ > Zn2+ > Mn2+ > Mg2+. The occurrence of high-temperature maxima (450-580 °C) in this case indicates an increase in the energy of interaction between template molecules and the framework and, probably, acidic centers, in particular. An unexpectedly high template content in CrVPI-5 structure, indicating strong acidic centers formation, was observed in the Me(III,IV)VPI-5 series. The results of investigation of adsorption characteristics of MeVPI-5 samples indicate that metal's nature (valence, ionic radius, etc.) has appreciable effect on the volume of micropores. For the divalent metals a substantial decrease in overall volume of pores (minimum values 0.08 cm3/g for Co-, Mn-, Ca-, and SrVPI-5) are observed. The closeness of the values of pore volumes on water, methanol and nitrogen indicates the
The influence of metal nature on the structure, adsorption and acidic properties of aluminophosphate with VFI structure
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accessibility of adsorption space. On the isotherms of water adsorption by the VPI-5 and MeVPI-5 samples at small relative pressures [up to 0.015; 0.015-0.055; above 0.055 (Fig. 1)] there are three steps, which have already been mentioned in the literature. The presence of three steps on the isotherms may caused by the adsorption on high-energy P–OH defects and penta- and hexacoordinated hydration of aluminum atoms located on various sections of the AIP (MeAIP) framework respectively. For Me(II)VPI-5 stepwise form of initial sections of adsorption isotherms of water is leveled out appreciably, and the level of adsorption decreases with increase in the ionic radius of the metal. The volumes of the micropores (W01) for the initial sections of the isotherms of water adsorption correlate with the concentration of high-energy centers known from the literature. For majority of Me(II)VPI-5 samples the values of E01 at the first step of the isotherms appreciably increase, and maximum effect is observed in the series of alkaline-earth metals—for Mg-, Ca-, and SrVPI-5 E01 is 12.6, 18.8, and 21.0 kJ/mole respectively. At the second and third steps they decrease to E02 and E03 values in the range of 4.5-8.4 kJ/mole.
a
1 2 3 4 5 6 7 8 9 10 11
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W, cm /g 0.15
0.10
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I, a.u.
b
50
2 3
0.05 1 0
0.00 0.00
0.05
0.10 0.15 p/ps
0.20
5 4
100 200 300 o 400 500 600 T, C
Figure 1. The adsorption isotherms (20°C) of water by MeVPI-5 samples (a): 1) VPI-5; 2) MgVPI-5; 3) CoVPI-5; 4) ZnVPI-5; 5) FeVPI-5; 6) CrVPI-5; 7) VVPI-5; 8) TiVPI-5; 9) BaVPI-5; 10) CaVPI-5; 11) SrVPI-5, and the curves for the thermally programed desorption of ammonia by metal aluminophosphates with VFI structure (b): 1) VPI-5; 2) CoVPI-5; 3) MgVPI-5; 4) MnVPI-5; 5) ZnVPI-5. Analysis of the data obtained by TPDA method for Me(II)VPI-5 samples shows significant increase in the amount of retained NH3 and an increase of the temperature at which it is removed. The strength of coordination centers increases in the series of MeVPI-5 in the order Cu Ni Mn < Co < Zn < Mg and sharply decreases in the order Mg >> Ca > Sr Ba. High concentration and the strength of coordination centers for chromium-containing samples can be explained by a series of experimental data. Isomorphous incorporation
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of Cr3+ into the structure of a series of aluminophosphate zeolites containing octahedra has been confirmed by various experimental methods. During the calcination of Crcontaining VFI zeolites chromium(III) with distorted octahedral symmetry is oxidized to hexavalent state with formation of dioxochromium(VI) bonded to aluminophosphate lattice. Here, strongly acidic P–OH groups that maintain the balance of charges in zeolite lattice are formed during thermal degradation of template.
4. Conclusion It is thus clear that incorporation of metal ions in VFI structure is accompanied by deformation of labile aluminophosphate framework – appropriate “structural response” – and in particular by redistribution of the intensities of main reflections on XRD patterns in relation to the nature of metal. Incorporation of metal ions takes place mainly at the position of octahedral aluminum, and maximum structural deformation occurs with the change in the radius of divalent metal cation (an ionic substitution mechanism). An appreciable change in adsorption characteristics and in particular smoothing of three-step adsorption isotherm of water – and, accordingly, adsorption at high-energy P–OH defects, penta- and hexacoordinated hydration, confirmed that strong acidic centers were formed during the incorporation of divalent cations and Cr3+ as a result of P–OH groups formation during thermal treatment. The effect of the size and electronegativity of metal cation on the value of Me–O(H)–P bond angle, distribution of effective charges, and accordingly the strength of acid centers are clearly determining factors in these cases.
References [1] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M.Flanigen, J. Am. Chem. Soc., 106 (1984) 6092. [2] M.E. Davis, C. Montes, P.E. Hathaway, J.M. Garces, in P.A. Jacobs, R.A. van Santen (Eds.), Studies in Surfaces Sciences and Catalysis, Vol. 49, Part A, Elsevier, Amsterdam, (1989) 199 [3] P.S. Singh, R.A. Saikh, R. Bandyopadhyay, J. Chem. Soc. Chem. Commun., 22 (1995) 2255. [4] J.A. Martens, H. Geerts, L. Leplat, G. Vanbutsele, P.J. Grobet, P.A. Jacobs, Catal. Lett., 12 (1992) 367. [5] M. Hartmann, L. Kevan, J. Chem. Soc., Faraday Trans., 19 (1996) 3661. [6] S. Prasad, Micropor. Mater. 12 (1997) 123. [7} N.N. Tušar, N. Zabukovec Logar, I. Aron, F. Thibault-Starzyk, V. Kaui, Molecular Sieve Croat. Chem. Acta, 74 (2001) 837. [8] A. Risti, N.N. Tušar, G. Vlaic, I. Arcon, F. Thibault-Starzyk, N. Malicki, V. Kaui, , s, Microporous Mesoporous Mater., 76 (2004) 61. [9] M. Hartmann, L. Kevan, , Res. Chem. Intermed., 28 (2002) 625. [10] F.J. Luna, S.E. Ukava, M.Wallau, U. Schuchardt, J. Mol. Catal. A: Chem., 117 (1997) 405.
221
The influence of metal nature on the structure, adsorption and acidic properties of aluminophosphate with VFI structure Oleksiy V. Shvets, Pavel S. Yaremov, Vladimir G. Ilyin L.V. Pisarzhevsky Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Nauky Avenue, Kyiv 03028, Ukraine, phone + 380 44 525 41 96, fax + 380 44 525 62 16
Abstract Metaloaluminophosphate zeolites with VFI structure, containing incorporated metals cations, were synthesized. Essential deformation of VFI structure, correlating with the size of ionic radii of incorporated cation is found. The formation of strong acid centers was detected in the case of bivalent cation and Cr3+. It was established, that the stregth of the Brønsted acidic centers is sharply reduced in a sequence of substitution cations Mg >> Ca > Sr Ba and grows in a sequence Cu Ni Mn < Co < Zn < Cr < Mg, which may evidence for cumulative influence of electronegativity and the radius of metals cation on value of Me-O(H)-P angle and strength of the acidic centers. Keywords: Me-VFI, Brønsted acidic centers.
1. Introduction Since 1980 a family of aluminophosphate molecular sieves greatly expanded the structural diversity of crystalline microporous materials. Preparation of metal(II)containing aluminophosphates, containing negatively charged frameworks and Brønsted and Lewis acidic sites redox sites, opened new ways to utilize these molecular sieves as catalysts [1-8]. Transition metal containing aluminophosphates have great potential as heterogeneous oxidation catalysts for a variety of organic substrates [9]. The main aim of the present work was to investigate the effect of the nature of framework-forming elements on the structure and adsorption and acidic characteristics of metalloaluminophosphates of VPI-5 type (MeVPI-5) as well as to try to detect and analyze possible manifestations of isomorphism.
2. Experimental Samples of MeVPI-5 (Me = Mg2+, Ca2+, Sr2+, Ba2+, Ti4+, V4+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+) were synthesized according to a procedure similar to reported in [10]. Pseudoboehmite was added in small portions to a mixture of phosphoric acid and distilled water in 1:1:40 molar ratios. After ageing (2 h) 1 eq. of di-n-propylamine was added dropwise to the mixture, and obtained gel was stirred for 2 h. The source of the metal, 0.04 eq. of Mg(CH3COO)2·4H2O, Co(CH3COO)2·4H2O, Zn(CH3COO)2· 2H2O, Mn(CH3COO2)·4H2O, Ni(NO3)2·6H2O, CuCl2·6H2O, CaCl2 (anh), Sr(NO3)2 (anh), BaCl2·2H2O, FeCl3·6H2O, CrCl3·6H2O, VOSO4 (anh), or Ti(OC4H9)4, was dissolved (or dispersed) in 5 ml of distilled water and added to the gel, after which the mixture was stirred for 15 min. Obtained reaction mixture (RM) was transferred to Teflon beaker, placed in steel autoclave, heated to 150°C at 10 deg/min in pulsating oil thermostat, and
222
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kept at autogenic pressure for 3.5 h. The products were separated by decantation (when required with separation into coarse and fine crystalline fractions) and dried at RT. The structure of MeVPI-5 samples was monitored by X-ray diffractometry (DRON-3M instrument, CuK radiation). Infrared spectra were recorded on Specord IR-75 spectrophotometer. The spectra in UV and visible regions were recorded on Specord M40 (diffuse reflective regime) and Unico 4802 (absorption regime) spectrophotometers. N2 adsorption and gravimetric adsorption method (adsorbates methanol and water) was used to characterise porous structure, and the parameters of micropores were calculated by Dubinin–Radushkevich equation. Thermogravimetric analysis (TGA) of MeVPI-5 samples was performed on a Q-1000 derivatograph in Ar stream. The acidic characteristics of MeVPI-5 were studied by thermoprogrammed desorption of ammonia (TPDA) in the range of 20-650°C (heating rate 17 deg/min).
3. Results and discussion Analysis of XRD patterns of as-synthesed samples of MeVPI-5 shows that in the most cases well crystallized zeolites-like phosphates with the VFI structure are formed. In the case of the divalent metals I of the first reflection is significantly reduced, and a redistribution of I is observed for the reflections in the region of 20-24°. Analysis of observed changes of I allows to detect fairly pronounced effect of the size of the cations on the extent of deformation of the structure in the series Al3+ (0.59 Å) < Mg2+ (0.72 Å) < Ca2+ (1.01 Å) < Sr2+ (1.16 Å) < Ba2+ (1.36 Å). The distortions of the structure, due to incorporation of metal ions at crystallographic positions where Al3+ can change the tetrahedral environment to octahedral (these positions make a contribution to the intensity of the reflection from the 100 plane), increase in the following order: Ni2+ (0.70 Å) < Co2+ (0.73 Å) Cu2+ (0.75 Å) < Mn2+ (0.80 Ã) << Zn2+ (0.72 Å). Chemical analysis data show any differences in quantity of incorporated metals. Al/Me ratio for Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn-VPI-5 was 17.1, 21.6, 22.3, 56.8, 51.2, 52.4, 36.3, 12.4, 61.7, 17.3, 23.6, 27.9, 14.1 respectively. Analysis of IR spectra of obtained samples of MeVPI-5 does not reveal significant differences from reference sample (aluminophosphate VPI-5) or particular changes of I for the absorption bands, as well as there are no new bands, which may occur due to the formation of oxide fragments. In visible region of the spectrum the position of absorption bands, the number of components into which these bands are split, and splitting values indicate that Co2+ ions in the structure of as-synthesized CoVPI-5 zeolites are in tetrahedral coordination. In the NiVPI-5 sample Ni2+ ions are in two different positions—tetrahedral and octahedral. The results of thermogravimetric analysis (TGA) indicate a substantial decrease in general level of hydration of initial VPI-5 during introduction of divalent metals with cations of small radius (0.74-0.91 nm) in the framework, and their effect decreases in the order Co2+ > Zn2+ > Mn2+ > Mg2+. The occurrence of high-temperature maxima (450-580 °C) in this case indicates an increase in the energy of interaction between template molecules and the framework and, probably, acidic centers, in particular. An unexpectedly high template content in CrVPI-5 structure, indicating strong acidic centers formation, was observed in the Me(III,IV)VPI-5 series. The results of investigation of adsorption characteristics of MeVPI-5 samples indicate that metal's nature (valence, ionic radius, etc.) has appreciable effect on the volume of micropores. For the divalent metals a substantial decrease in overall volume of pores (minimum values 0.08 cm3/g for Co-, Mn-, Ca-, and SrVPI-5) are observed. The closeness of the values of pore volumes on water, methanol and nitrogen indicates the
The influence of metal nature on the structure, adsorption and acidic properties of aluminophosphate with VFI structure
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accessibility of adsorption space. On the isotherms of water adsorption by the VPI-5 and MeVPI-5 samples at small relative pressures [up to 0.015; 0.015-0.055; above 0.055 (Fig. 1)] there are three steps, which have already been mentioned in the literature. The presence of three steps on the isotherms may caused by the adsorption on high-energy P–OH defects and penta- and hexacoordinated hydration of aluminum atoms located on various sections of the AIP (MeAIP) framework respectively. For Me(II)VPI-5 stepwise form of initial sections of adsorption isotherms of water is leveled out appreciably, and the level of adsorption decreases with increase in the ionic radius of the metal. The volumes of the micropores (W01) for the initial sections of the isotherms of water adsorption correlate with the concentration of high-energy centers known from the literature. For majority of Me(II)VPI-5 samples the values of E01 at the first step of the isotherms appreciably increase, and maximum effect is observed in the series of alkaline-earth metals—for Mg-, Ca-, and SrVPI-5 E01 is 12.6, 18.8, and 21.0 kJ/mole respectively. At the second and third steps they decrease to E02 and E03 values in the range of 4.5-8.4 kJ/mole.
a
1 2 3 4 5 6 7 8 9 10 11
3
W, cm /g 0.15
0.10
100
I, a.u.
b
50
2 3
0.05 1 0
0.00 0.00
0.05
0.10 0.15 p/ps
0.20
5 4
100 200 300 o 400 500 600 T, C
Figure 1. The adsorption isotherms (20°C) of water by MeVPI-5 samples (a): 1) VPI-5; 2) MgVPI-5; 3) CoVPI-5; 4) ZnVPI-5; 5) FeVPI-5; 6) CrVPI-5; 7) VVPI-5; 8) TiVPI-5; 9) BaVPI-5; 10) CaVPI-5; 11) SrVPI-5, and the curves for the thermally programed desorption of ammonia by metal aluminophosphates with VFI structure (b): 1) VPI-5; 2) CoVPI-5; 3) MgVPI-5; 4) MnVPI-5; 5) ZnVPI-5. Analysis of the data obtained by TPDA method for Me(II)VPI-5 samples shows significant increase in the amount of retained NH3 and an increase of the temperature at which it is removed. The strength of coordination centers increases in the series of MeVPI-5 in the order Cu Ni Mn < Co < Zn < Mg and sharply decreases in the order Mg >> Ca > Sr Ba. High concentration and the strength of coordination centers for chromium-containing samples can be explained by a series of experimental data. Isomorphous incorporation
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of Cr3+ into the structure of a series of aluminophosphate zeolites containing octahedra has been confirmed by various experimental methods. During the calcination of Crcontaining VFI zeolites chromium(III) with distorted octahedral symmetry is oxidized to hexavalent state with formation of dioxochromium(VI) bonded to aluminophosphate lattice. Here, strongly acidic P–OH groups that maintain the balance of charges in zeolite lattice are formed during thermal degradation of template.
4. Conclusion It is thus clear that incorporation of metal ions in VFI structure is accompanied by deformation of labile aluminophosphate framework – appropriate “structural response” – and in particular by redistribution of the intensities of main reflections on XRD patterns in relation to the nature of metal. Incorporation of metal ions takes place mainly at the position of octahedral aluminum, and maximum structural deformation occurs with the change in the radius of divalent metal cation (an ionic substitution mechanism). An appreciable change in adsorption characteristics and in particular smoothing of three-step adsorption isotherm of water – and, accordingly, adsorption at high-energy P–OH defects, penta- and hexacoordinated hydration, confirmed that strong acidic centers were formed during the incorporation of divalent cations and Cr3+ as a result of P–OH groups formation during thermal treatment. The effect of the size and electronegativity of metal cation on the value of Me–O(H)–P bond angle, distribution of effective charges, and accordingly the strength of acid centers are clearly determining factors in these cases.
References [1] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M.Flanigen, J. Am. Chem. Soc., 106 (1984) 6092. [2] M.E. Davis, C. Montes, P.E. Hathaway, J.M. Garces, in P.A. Jacobs, R.A. van Santen (Eds.), Studies in Surfaces Sciences and Catalysis, Vol. 49, Part A, Elsevier, Amsterdam, (1989) 199 [3] P.S. Singh, R.A. Saikh, R. Bandyopadhyay, J. Chem. Soc. Chem. Commun., 22 (1995) 2255. [4] J.A. Martens, H. Geerts, L. Leplat, G. Vanbutsele, P.J. Grobet, P.A. Jacobs, Catal. Lett., 12 (1992) 367. [5] M. Hartmann, L. Kevan, J. Chem. Soc., Faraday Trans., 19 (1996) 3661. [6] S. Prasad, Micropor. Mater. 12 (1997) 123. [7} N.N. Tušar, N. Zabukovec Logar, I. Aron, F. Thibault-Starzyk, V. Kaui, Molecular Sieve Croat. Chem. Acta, 74 (2001) 837. [8] A. Risti, N.N. Tušar, G. Vlaic, I. Arcon, F. Thibault-Starzyk, N. Malicki, V. Kaui, , s, Microporous Mesoporous Mater., 76 (2004) 61. [9] M. Hartmann, L. Kevan, , Res. Chem. Intermed., 28 (2002) 625. [10] F.J. Luna, S.E. Ukava, M.Wallau, U. Schuchardt, J. Mol. Catal. A: Chem., 117 (1997) 405.