Introduction The interest on the high-pressure (HP) behaviour of zeolites significantly increased during the last decade. The growing number of studies allowed the recognition of complex and variable responses to the applied pressure1,2: a wide compressibility range, a remarkable structural flexibility and the relevant influence played by the framework (topology, cation ordering etc.) and extraframework components on the HP behaviour. Beside the intrinsic properties of the open-framework compounds, a significant number of experiments have been devoted to the high-pressure interactions between zeolites and the P-transmitting medium,as the so-called P-induced over-hydration: i.e. the P-induced penetration of H2O molecules into the structural voids3. The P-driven penetration of molecules is not limited to H2O, several “pore-penetrating” P-transmitting media may be involved in this process. In this light, pressure may be applied not only to explore the elastic and structural properties of zeolites, but also as a tool to drive changes of these properties: e.g. improving the efficiency in the field of heterogeneous catalysis, favoring the access of reactants and products to/from the catalytically active sites; modifying the physical-chemical properties through irreversible phase transitions; or for the engineering of technological materials through irreversible molecules intrusion. In this study, we explore (by both single-crystal and powder synchrotron X-ray diffraction) the high-pressure behaviour and crystal-fluid interactions of a synthetic siliceous matrix with ferrierite topology [Si-FER: Si36O72], using a number of penetrating and non-penetrating pressure-transmitting media. Materials and experimental methods The single crystals and powders used have all been selected from the same starting sample of synthetic pure Si-ferrierite (SI36O72)4. The in-situ high-pressure single-crystal (SC) and powder X-ray diffraction experiments have been performed at the ID09A and BM01-Swiss-Norwegian beamlines at ESRF (Grenoble), respectively, using diamond anvil cells. Four different P-transmitting media have been used for both SC and powder experiments: non-penetrating silicon oil and potentially penetrating methanol:ethanol:H2O=16:3:1 mix (m.e.w.), ethylene glycol (egl) and 2methyl-2propen-1ol (mpo). Results and discussion The HP-study of Si-FER compressed in silicon oil evidenced the remarkable flexibility of this framework: a first displacive phase transition was observed from the orthorhombic Pmnn to the monoclinic P121/n1 space group at ~ 0.7 GPa. A second displacive phase transition, involving a significant unit-cell volume contraction, was observed at ~ 1.24 GPa from the P121/n1 to the P21/n11 space group (through an intermediate P-1 structure,“type-II” transition according to Christy5). The high-P P21/n11 polymorph was found to be stable at least up to 3.00(7) GPa, whereas - upon pressure release - the starting Pmnn structure was fully recovered. The three polymorphs were found to share a virtually identical bulk elastic behaviour, being their average volume compressibility βV: 0.051(4), 0.056(9) and 0.055(3) GPa-1, respectively. The structure deformation is governed by the tilting of the tetrahedra around the shared oxygen hinges. The bulk V-contraction is mainly accommodated, after the Pmnn-to-P121/n1 transition, by the compression and deformation of the 8- and 10-ring channels, running along b and c, respectively. The compression of Si-FER using penetrating P-transmitting media, studied by both SC and powder diffraction, showed different results with respect to the silicon oil experiment. Common features among the three experiments are the following: 1) the Pmnn-to-P121/n1 phase transition at ~ 0.7 GPa (except for m.e.w., where it occurs at ~ 1.5 GPa) and 2) the absence of the P21/n11 polymorph (previously observed in silicon oil), coupled with a less pronounced compressibility compared to that in silicon oil. For all the in-situ experiments, the orthorhombic symmetry was restored upon decompression. The different phase-transition paths and the different compressional patterns, observed using different P-transmitting media, suggest the occurrence of P-induced crystal-fluid interactions. The difference-Fourier maps of the electron density, calculated by the structure refinements at any P-point, showed weak and broadened peaks, which did not allow the location of the molecules within the FER-structural voids. However, the analysis of the P-induced framework deformation mechanisms revealed different deformational patterns of the 8- and 10-ring channels of Si-FER compressed in the different fluids. Although a direct evidence of the penetration of molecules could not be proved by the diffraction data, the different deformation mechanisms at the atomic scale are an indirect evidence of the P-induced intrusion of molecules from the fluid into the structural pores of the zeolite. The observed broad distribution of the electron density within channels and cages suggests the presence of numerous partially occupied extraframework sites, where P-transmitting media molecules can be hosted. Interestingly, a “fast compression” of Si-FER from room-P to ca. 2 GPa triggered the transition Pmnn-to-P21/n11 (with enhanced unit-cell volume contraction) both in egl and mpo, suggesting a strong kinetic control on the P-induced molecules penetration and crystal-fluid interactions. Acknowledgements The authors acknowledge the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACT- RBFR12CLQD”. References: 1 Gatta, G. D.; Lee, Y. Mineral. Mag. 2014, 78, 267-291. 2 Vezzalini, G; Arletti, R; Quartieri, S. Acta Cryst. 2014, B70, 444-451.3 Lee, Y.; Vogt, T.; Hriljac, J. A.; Parise, J. B.; Hanson, J. C.; Kim, S. J. Nature 2002, 420, 485-489. 4 Arletti, R.; Vezzalini, G.; Quartieri, S.; Di Renzo, F.; Dmitriev, V. Micropor. Mesopor. Mater. 2014, 191, 27-37. 5 Christy, A. G. ActaCryst. 1993, B49, 987-996.

Compressibility and crystal-fluid interactions in Si-ferrierite / P. Lotti, R. Arletti, G.D. Gatta, S. Quartieri, G. Vezzalini, M. Merlini. ((Intervento presentato al 12. convegno National Congress of Zeolites Science and Technology tenutosi a Amantea nel 2015.

Compressibility and crystal-fluid interactions in Si-ferrierite

P. Lotti
Primo
;
G.D. Gatta;M. Merlini
Ultimo
2015

Abstract

Introduction The interest on the high-pressure (HP) behaviour of zeolites significantly increased during the last decade. The growing number of studies allowed the recognition of complex and variable responses to the applied pressure1,2: a wide compressibility range, a remarkable structural flexibility and the relevant influence played by the framework (topology, cation ordering etc.) and extraframework components on the HP behaviour. Beside the intrinsic properties of the open-framework compounds, a significant number of experiments have been devoted to the high-pressure interactions between zeolites and the P-transmitting medium,as the so-called P-induced over-hydration: i.e. the P-induced penetration of H2O molecules into the structural voids3. The P-driven penetration of molecules is not limited to H2O, several “pore-penetrating” P-transmitting media may be involved in this process. In this light, pressure may be applied not only to explore the elastic and structural properties of zeolites, but also as a tool to drive changes of these properties: e.g. improving the efficiency in the field of heterogeneous catalysis, favoring the access of reactants and products to/from the catalytically active sites; modifying the physical-chemical properties through irreversible phase transitions; or for the engineering of technological materials through irreversible molecules intrusion. In this study, we explore (by both single-crystal and powder synchrotron X-ray diffraction) the high-pressure behaviour and crystal-fluid interactions of a synthetic siliceous matrix with ferrierite topology [Si-FER: Si36O72], using a number of penetrating and non-penetrating pressure-transmitting media. Materials and experimental methods The single crystals and powders used have all been selected from the same starting sample of synthetic pure Si-ferrierite (SI36O72)4. The in-situ high-pressure single-crystal (SC) and powder X-ray diffraction experiments have been performed at the ID09A and BM01-Swiss-Norwegian beamlines at ESRF (Grenoble), respectively, using diamond anvil cells. Four different P-transmitting media have been used for both SC and powder experiments: non-penetrating silicon oil and potentially penetrating methanol:ethanol:H2O=16:3:1 mix (m.e.w.), ethylene glycol (egl) and 2methyl-2propen-1ol (mpo). Results and discussion The HP-study of Si-FER compressed in silicon oil evidenced the remarkable flexibility of this framework: a first displacive phase transition was observed from the orthorhombic Pmnn to the monoclinic P121/n1 space group at ~ 0.7 GPa. A second displacive phase transition, involving a significant unit-cell volume contraction, was observed at ~ 1.24 GPa from the P121/n1 to the P21/n11 space group (through an intermediate P-1 structure,“type-II” transition according to Christy5). The high-P P21/n11 polymorph was found to be stable at least up to 3.00(7) GPa, whereas - upon pressure release - the starting Pmnn structure was fully recovered. The three polymorphs were found to share a virtually identical bulk elastic behaviour, being their average volume compressibility βV: 0.051(4), 0.056(9) and 0.055(3) GPa-1, respectively. The structure deformation is governed by the tilting of the tetrahedra around the shared oxygen hinges. The bulk V-contraction is mainly accommodated, after the Pmnn-to-P121/n1 transition, by the compression and deformation of the 8- and 10-ring channels, running along b and c, respectively. The compression of Si-FER using penetrating P-transmitting media, studied by both SC and powder diffraction, showed different results with respect to the silicon oil experiment. Common features among the three experiments are the following: 1) the Pmnn-to-P121/n1 phase transition at ~ 0.7 GPa (except for m.e.w., where it occurs at ~ 1.5 GPa) and 2) the absence of the P21/n11 polymorph (previously observed in silicon oil), coupled with a less pronounced compressibility compared to that in silicon oil. For all the in-situ experiments, the orthorhombic symmetry was restored upon decompression. The different phase-transition paths and the different compressional patterns, observed using different P-transmitting media, suggest the occurrence of P-induced crystal-fluid interactions. The difference-Fourier maps of the electron density, calculated by the structure refinements at any P-point, showed weak and broadened peaks, which did not allow the location of the molecules within the FER-structural voids. However, the analysis of the P-induced framework deformation mechanisms revealed different deformational patterns of the 8- and 10-ring channels of Si-FER compressed in the different fluids. Although a direct evidence of the penetration of molecules could not be proved by the diffraction data, the different deformation mechanisms at the atomic scale are an indirect evidence of the P-induced intrusion of molecules from the fluid into the structural pores of the zeolite. The observed broad distribution of the electron density within channels and cages suggests the presence of numerous partially occupied extraframework sites, where P-transmitting media molecules can be hosted. Interestingly, a “fast compression” of Si-FER from room-P to ca. 2 GPa triggered the transition Pmnn-to-P21/n11 (with enhanced unit-cell volume contraction) both in egl and mpo, suggesting a strong kinetic control on the P-induced molecules penetration and crystal-fluid interactions. Acknowledgements The authors acknowledge the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACT- RBFR12CLQD”. References: 1 Gatta, G. D.; Lee, Y. Mineral. Mag. 2014, 78, 267-291. 2 Vezzalini, G; Arletti, R; Quartieri, S. Acta Cryst. 2014, B70, 444-451.3 Lee, Y.; Vogt, T.; Hriljac, J. A.; Parise, J. B.; Hanson, J. C.; Kim, S. J. Nature 2002, 420, 485-489. 4 Arletti, R.; Vezzalini, G.; Quartieri, S.; Di Renzo, F.; Dmitriev, V. Micropor. Mesopor. Mater. 2014, 191, 27-37. 5 Christy, A. G. ActaCryst. 1993, B49, 987-996.
giu-2015
Ferrierite; Synchrotron XRD; High pressure; P-induced molecule intrusion; Displacive phase transition
Settore GEO/09 - Georisorse Miner.Appl.Mineral.-Petrogr.per l'amb.e i Beni Cul
Settore GEO/06 - Mineralogia
Associazione Italiana Zeoliti
Compressibility and crystal-fluid interactions in Si-ferrierite / P. Lotti, R. Arletti, G.D. Gatta, S. Quartieri, G. Vezzalini, M. Merlini. ((Intervento presentato al 12. convegno National Congress of Zeolites Science and Technology tenutosi a Amantea nel 2015.
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