Article pubs.acs.org/JPCC
AlPO4‑54−AlPO4‑8 Structural Phase Transition and Amorphization under High Pressure Frederico G. Alabarse,*,†,§ Jean-Blaise Brubach,§ Pascale Roy,§ Abel Haidoux,† Claire Levelut,∥ Jean-Louis Bantignies,∥ Olivier Cambon,† and Julien Haines*,† †
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS, Equipe C2M, Université de Montpellier, 34095 Montpellier Cedex 5, France § Synchrotron Soleil, L’Orme les Merisiers, Saint AubinBP48, 91192 Gif sur Yvette, France ∥ Laboratoire Charles Coulomb, UMR 5221 CNRS-UM2, Département CVN, Université de Montpellier, 34095 Montpellier Cedex 5, France ABSTRACT: Microporous AlPO4-54, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction (XRD), mid- and far-infrared (IR) spectroscopy in diamond anvil cells. The material undergoes a phase transition beginning around 0.8 GPa. The amount of AlPO4-8 gradually increases with pressure and the phase transition is complete between 2 and 3 GPa. The closure of the (POAl) angle destabilizes the structure of AlPO4-54, which drives the transition to AlPO4-8. The pressure-induced phase transformation of AlPO4-54 to AlPO4-8 is associated with a symmetry reduction from hexagonal to orthorhombic and with a change in the unidirectional ring channel parallel to the c-axes from 18 to 14 AlO4 and PO4 tetrahedra. An abrupt decrease along the b direction is linked to the formation of 4 new rings of 6 tetrahedra with significant structural reorganization. The transition is followed by irreversible amorphization beginning around 3.5 GPa due to the collapse of the pores. The amorphization of AlPO4-8 was detected from the disappearance of the XRD lines, abrupt shifts, and strong broadening of the mid-IR modes, and by changes in the pressure dependence of the mid- and far-IR modes, indicating a lower compressibility for the more dense amorphous form. Far-IR spectra indicate that the new amorphous form retains the local structure of AlPO4-8.
1. INTRODUCTION Zeolites have been widely used as ion-exchangers, molecular sieves, sorbents, and catalysts1 and studies of their phase stability are thus of great interest. Pressure is an under-explored variable in the physical-chemistry of microporous materials and the knowledge of pressure-induced phase transformations, especially in zeolites, is limited compared to the large number of studies as a function of temperature.2,3 The response of open frameworks to pressure is a growing scientific domain that requires more studies in order to design new advanced materials.4 Recently, the synthesis of new zeolite structures via a pressure-induced phase transition process has been reported,5 in which, compared to high temperature synthesis, the high pressure process decreases bond distances and atomic mobility, gives new insight and possibilities to prepare new functional materials or highly dense materials.5,6 Under high pressure conditions, some zeolites were found to exhibit structural distortions due to tilting of the rigid TO4 tetrahedra around bridging oxygen atoms and by the shortening of TO distances.7,8 Reconstructive phase-transitions with a change in topology, are rare in open-frameworks in response to applied pressure at ambient temperature. Instead, changes toward energetically costly structural configurations with TOT angles ≤120° generally occur. These retain the symmetry of the © 2015 American Chemical Society
open-framework topology of the zeolites up to the onset of the amorphization processes, indicating the influence of composition on the structural configuration of the tetrahedral framework and on their flexibility.8 In dehydrated analcime, phase transitions are associated with deformations of the four-membered rings (as the minimum size secondary building units) by rotation of rigid TO4 units.9 Results in the analcime-like feldspathoids, a group of microporous minerals with the ANA framework topology, have shown that displacive structural phase transitions to lowsymmetry forms occur at relatively low pressures (0.5−3.2 GPa),10,11 in which these phase transitions were controlled by the “flexibility window” of the framework topology. Pressureinduced phase transformations were also observed for the zeolite RHO from a large-volume centric to a small-volume acentric structure, which appears to be analogous to those driven by dehydration in this material.12 Instead of these pressure-induced transitions retaining the initial topology, some zeolites undergo reconstructive phaseinduced transitions as an alternative prior pressure-induced Received: January 12, 2015 Revised: February 23, 2015 Published: March 23, 2015 7771
DOI: 10.1021/acs.jpcc.5b00318 J. Phys. Chem. C 2015, 119, 7771−7779
Article
The Journal of Physical Chemistry C amorphization (PIA). Jorda and co-workers5 obtained the new zeolite ITQ-50 from the pure silica ITQ-29 (LTA topology) using nonpenetrating pressure transmitting media at pressures of 1.2 GPa, with a second phase transition occurring at 3.2 GPa, which is irreversible upon pressure release. This shows how new materials can be obtained using high-pressure conditions that retain their new structures after returning to ambient pressure and may be used in technological applications. Microporous zeolites, may also transform upon compression to amorphous states. In zeolites, PIA is typically an irreversible process that depends on the pressure transmitting media (PTM) and of the guest molecules used, for which different results have been obtained at high pressure.13−19 The behavior of hydrated and dehydrated zeolites has been compared. In zeolite NaA, depending on the water content, different results were obtained after compression up to 4.5 GPa: hydrated NaA zeolite exhibited a 25% of reduction of intensity in XRD patterns in contrast to the superhydrated sample for which the crystalline phase was conserved, but the completely amorphous form was obtained in the dehydrated sample.16 Raman studies on the PIA of dehydrated leucite have shown that this material undergoes a reversible phase transition at 2.3 GPa20 and that characteristic peaks of the crystalline phase were observed up to 16 GPa in a 1.3 day experiment.13 This result was not expected, as hydrated zeolites of the natrolite group usually became amorphous at 8−9 GPa.21 This high stability was related to the absence of water in the channels and to the difference in the framework topology.22 Differences in the PIA effect observed for as-made and calcined ZSM-5 zeolite (silicalite-1) showed the influence of template cations (TPA+) in the amorphization process.23 The pressure range at which the amorphization in the calcined zeolite begins was much lower than that of the asmade zeolite, 1.5 compared to 3 GPa, respectively. This difference was related to template molecules that reinforce the 10-MR channels of the structure.23 Amorphization can be deactivated up to pressures of at least 25 GPa by the insertion of guest atoms or molecules.15 AlPO4-54 exhibits the largest pores among zeolites and aluminophosphates and possesses unidirectional channels parallel to the c-axis, therefore the study of this microporous solid is of great interest and can give insights for designing new materials. The channels have an internal free diameter of about 12.7 Å corresponding to rings of 18 AlO 4 and PO 4 tetrahedra,24,25 which can be expected to collapse at high pressure. In the dehydrated form (hexagonal VFI structure), all Al and P cations are in 4-fold coordination.26 In the hydrated form of AlPO4-54, amorphization was found to begin near 2 GPa using either a nonpenetrating PTM silicone oil or no PTM by in situ X-ray powder diffraction and Raman spectroscopy.27 AlPO4-54·xH2O starts to become amorphous at even lower pressures when H2O is used as a PTM (0.9 GPa) with superhydration effects observed by the increase of the unit cell volume prior to the beginning of PIA due to insertion of the H2O molecules in the pores. The local structure of the recovered pressure-amorphized microporous AlPO4-54·xH2O, by nuclear magnetic resonance and by X-ray absorption spectroscopy indicated that, upon increasing pressure, the structure of the material is destabilized due two water molecules entering in the coordination sphere of tetrahedrally coordinated IVAl.27 The goal of the present study is to investigate the effect of pressure on the stability of the dehydrated microporous aluminophosphate AlPO4-54 with empty pores.
2. EXPERIMENTAL SECTION AlPO4-54·xH2O was synthesized from pseudoboehmite alumina (Pural, 76.1%, SB Sasol), phosphoric acid (85% w/w, Panreac), n-Ethyl-n-butylamine (Aldrich) as a template and distilled water by a sol−gel procedure followed by hydrothermal treatment based on the optimization of methods already described.25,27,28 The sol−gel procedure was performed using a mixture with the following molar composition 1.0 Al2O3: 1.0 P2O5: 1.0 Template: 40 H2O. The gel was transferred into PTFE-lined autoclaves and heated at 130 °C for 4.5 h.25,28−31 AlPO4-54·xH2O with the hexagonal VFI structure (space group P63 )27,32,33 was dehydrated (AlPO4 -54) without significant structural change using the following time/temperature regime under vacuum (∼10−2 mbar): 72 h at room temperature (∼20 °C), 24 h at 50 °C, 24 h at 80 °C, 8 h at 100 °C, 16 h at 120 °C, 8 h at 150 °C, 16 h at 180 °C, 8 h at 200 °C, and 72 h at room temperature. It has been reported previously34 that the dehydration of AlPO4-54 under vacuum at room temperature produces a structure, which is stable up to 600 °C. Drying AlPO4-54 with minimal structural change requires removing as much water as possible at as low a temperature as possible, which is in agreement with previous results.35 The dehydrated AlPO4-54, still under vacuum, was transferred in a closed glass tube to a glovebox under controlled atmosphere (pH2O < 4 ppm), where it was loaded and sealed in a 0.3 mm diameter glass capillary. The sample was then characterized at ambient pressure by X-ray powder diffraction (XRD) using a PANalytical X’Pert diffractometer equipped with an X’Celerator detector in transmission geometry using Cu−Kα1 radiation. The presence of the dehydrated phase was confirmed based on the lattice parameters found by a Le Bailtype fit obtained using the XRD patterns. The program Fullprof36 was used to refine the unit cell parameters. Angle dispersive X-ray diffraction under high pressure was performed in a diamond anvil cell (DAC). The sample and pressure transmitting media (PTM) were placed in a tungsten gasket with a hole size of about 160 × 70 μm (diameter × thickness) and were loaded in the DAC inside a glovebox (pH2O < 4 ppm). Silicone oil and quartz were used as PTM and calibrant, respectively. X-ray diffraction patterns were obtained on an image plate placed at a distance of 143.61 mm from the sample using zirconium filtered molybdenum Kα radiation (λ = 0.71073 Å) from a microfocus tube (800 W). An X-ray capillary was used to provide a 100 μm diameter incident beam. Exposure times were typically between 48 and 60 h. Lanthanum hexaboride (LaB6) was used to calibrate the XRD instrument. The intensities were integrated as a function of the diffraction angle 2θ to obtain a conventional one-dimensional diffraction pattern using the software Fit2d.37 The pressure was determined using the third order Birch−Murnaghan equation of state (EOS)38 of Quartz (B0 = 37.1 GPa, B′0 = 6.0).39 The intensity of the reflections of the pressure calibrant (101) was used to correct for the reduction in sample thickness at high pressure in order to normalize the intensity of the diffraction lines of the sample. No significant broadening of the quartz reflections was observed except at the highest pressure reached, indicating that below 6.8 GPa, pressure gradients were not significant. Mid- and far-Infrared (IR) spectra under high pressure were obtained with the synchrotron source at the Advanced Infrared Line Exploited for Spectroscopy beamline (AILES, Synchrotron 7772
DOI: 10.1021/acs.jpcc.5b00318 J. Phys. Chem. C 2015, 119, 7771−7779
Article
The Journal of Physical Chemistry C SOLEIL) equipped with a Bruker Instruments IFS 125 FT-IR spectrometer modified to operate with the synchrotron source.40 The use of the synchrotron, with its much higher brilliance in the far- and mid-infrared, provides a better signalto-noise ratio than a laboratory source41 and can be ideally coupled to a DAC allowing measurements in the complete IR range on very small samples.42 Mid-IR spectra (4000-400 cm−1) were obtained using a Fourier Transform Infrared Spectrometer (FT-IR) equipped with a MCT (5000-600 cm−1) detector and a KBr beamsplitter. The. Far-IR domain was investigated using a 6 μm Mylar beamsplitter and a 4.2 K Sibolometer detector (700−10 cm−1). Even though small features from the sample material may appear at the limit of the spectral range of the detectors, the higher level of noise in these regions prevents these data from being analyzed. The spectra were recorded with a resolution of 2 cm−1 with 400 and 800 scans in the far- and mid-IR, respectively. The gasket thickness was 5060 μm with a hole diameter of 250 μm. This limited the far-IR range to 50 cm−1 in the diamond anvil cell. The sample and PTM were loaded in the DAC inside a glovebox (pH2O < 4 ppm). The PTM used was KBr for the mid-IR and polyethylene (PE) for the far-IR. KBr was dehydrated at 120 °C under 10−6 mbar and PE at ambient temperature under 10−6 mbar and both were transferred to the glovebox. Ruby was used as a pressure calibrant and the pressure was determined based on the displacement of the R1 and R2 ruby fluorescence lines.43 The ruby fluorescence lines remained sharp up to 5 GPa indicating that the absence of noticeable nonhydrostatic stress up to this pressure. The AILES beamline provides a setup for measuring rapidly and reproducibly the mid- and far-IR spectra of samples under high pressure,44 which consists of a box with central dimensions of approximately 24 × 40 × 32 cm3, pumped down to a vacuum of ∼10−5 mbar. In this set up, the synchrotron IR light is directed by 12 gold coated mirrors through a double condenser system (Cassegrain objectives, N.A. 0.28) and a DAC in confocal configuration. Both Cassegrain objectives are adjustable using step motorized systems and the DAC position is controlled via a xyz step motorized system. This computer controlled system permitted an optimized alignment of these three components under vacuum. This improved set up combines both IR absorption and ruby fluorescence measurements without breaking the vacuum, just sliding the DAC from the IR measurement position to in front of the fluorescence optic system, using a motorized rail for which the exact same position is used for both the sample and the ruby.
Figure 1. In situ powder X-ray diffraction patterns of AlPO4-54 in silicone oil at selected pressures [(Δ) peaks corresponding to AlPO454, (Θ) AlPO4-8, (*) quartz pressure calibrant, and (+) the metal gasket. (F) Pressure release].
GPa are a = 31.053, b = 14.482, and c = 7.946 Å. At room pressure, the following values were obtained by Richardson and Vogt:45 a = 33.0894, b = 14.6832, and c = 8.3630 Å. An abrupt decrease in the intensity of the diffraction lines was observed for the new phase with further increases in pressure. The pressure-induced phase transition of AlPO4-54 to AlPO4-8 starts at 0.8 GPa. The most intense reflection of AlPO4-54, located at small angles (at ∼2.5°, Figure 1) and corresponding to the 100 reflection, becomes progressively weaker. In contrast, the 110 reflection of AlPO4-8 located at small angles (at ∼3.0°, Figure 1) becomes progressively stronger with a maximum intensity at 3 GPa. The 310 reflection of AlPO4-54 (at ∼5.0°, Figure 1) was found to disappear at 2 GPa. These results indicate that the transition is a gradual and first-order with large cell parameter and volume changes (Figures 2 and 3) and slow kinetics. There was no evidence of any intermediate phase as no additional unindexed diffraction lines were observed. As pressure is further increased, the reflections of AlPO4-8 broaden very slightly. Above 3 GPa, the two most intense reflections of AlPO4-8 (at ∼2.5° and ∼3.0°, Figure 1) corresponding to the 200 and 110 reflections, become progressively weaker and broader. Between 4 and 7 GPa, all the reflections clearly disappear corresponding to pressureinduced amorphization (PIA). Amorphization is irreversible upon pressure release, only the quartz reflections remain. Cell parameters were obtained under compression for both AlPO4-54 and AlPO4-8, Figure 2. The lattice parameter a of AlPO4-8 was compared to the equivalent orthorhombic lattice parameter a for AlPO4-54 (orthohexagonal cell). Both phases exhibited anisotropic compression. The lattice parameter c of AlPO4-54 is more compressible than the lattice parameter a (Ka = 0.016(1) GPa−1, Kc = 0.030(4) GPa−1); however, at the transition the compression along c direction lies between the compressibilities along the a and b directions (Ka = 0.017(3) GPa−1, Kb = 0.012(1) GPa−1, Kc = 0.014(1) GPa−1). At the
3. IN SITU X-RAY POWDER DIFFRACTION AT HIGH PRESSURE In situ X-ray diffraction measurements (XRD) on powder samples of AlPO4-54 were carried out at high pressure using diamond anvil cells (DAC). The refined cell parameters of the dehydrated starting material (Figure 1), space group P63/mcm, a and c of 18.5457(3) Å and 8.3992(1) Å, respectively, are very similar to corresponding values of 18.549 and 8.404 Å reported by Richardson and co-workers.26 Figure 1 shows a series of X-ray diffraction patterns of AlPO4-54 recorded from ambient pressure up to 6.8 GPa. With increasing pressure, the reflections of AlPO4-54 are replaced by new diffraction peaks, which can be readily indexed based on the AlPO4-8 structure (orthorhombic with AET zeolite type topology and Cmc21 space group).45 The lattice parameters at 2 7773
DOI: 10.1021/acs.jpcc.5b00318 J. Phys. Chem. C 2015, 119, 7771−7779
Article
The Journal of Physical Chemistry C
3.1. Phase Transition from AlPO4-54 to AlPO4-8. The pressure-induced phase transformation of AlPO4-54 to AlPO4-8 is associated with a symmetry reduction from hexagonal to orthorhombic and with a change in the unidirectional ring channel parallel to the c-axes from 18 to 14 AlO4 and PO4 tetrahedra. The Cmc21 space group of AlPO4-8 is a sub group of that of AlPO4-54 and is consistent with the softening of the zone center mode Γ−5 . This transition occurs due to the higher density of the resulting phase (at 1.6 GPa the difference of relative volume between AlPO4-54 to AlPO4-8 is about 17%, Figure 3) and by the reduction in the diameter of the pores. At the transition, the b cell parameter (Figure 2) decreases significantly, indicating the direction in which the major changes occur. At the structural level (black ellipses, Figure 4), four of the six adjacent rings of 4 tetrahedra (AlPO4-54,
Figure 2. Lattice parameters as a function of pressure for AlPO4-54 (in ■ a54 and c54) and AlPO4-8 (in ● a8, b8, c8, and room pressure values from the literature: a8*, b8*, and c8*).45
Figure 4. Top: the framework structures of AlPO4-54 (left) and AlPO4-8 (right). The polyhedra are the alternating AlO4 and PO4 units of AlPO4. Bottom: zoom showing the environment changing from rings of 4 tetrahedra to rings of 6 tetrahedra (A) and the resulting new ring of 4 tetrahedra (B). Figure 3. Relative volume as a function of pressure to AlPO4-54 and AlPO4-8 (room pressure value from the literature: V8*).45
Figure 4 top and bottom A) are converted to rings of 6 tetrahedra (AlPO4-8, Figure 4 top and bottom A, B) with a structural reorganization creating new rings of 4 tetrahedra (Figure 4B). In the process, the alternation of Al and P on tetrahedral nodes is retained, AlPO4-8 thus has 4 new rings of 6 tetrahedra.45 It is well established that a similar solid-state transformation can occur by calcination,45,47 in which the respective hydrated phase, AlPO4-54·xH2O, transforms to AlPO4-8 and the unidirectional ring channels change from 18 AlO6 octahedra, AlO4 and PO4 tetrahedra to 14 AlO4 and PO4 tetrahedra. In contrast to the pressure-induced transition, during the calcination process, the phase transition mechanism is a chemical reaction linked to the loss of two water molecules from the 6-fold coordinated Al octahedra. The relationships between the AlPO4-54 and AlPO4-8 structure types have been described based on neutron26,45 and X-ray48,49 powder diffraction under ambient conditions. Due to the complexity of the two structures and the number of crystallographically distinct atoms and the absence of single crystals of the anhydrous forms, there remains significant uncertainly in the interatomic distances and angles in the two structures.
transition, a significant decrease occurs along b indicating that major changes occur along this direction at the transformation and structural rearrangement to form AlPO4-8. Figure 3 shows the evolution of the relative volume (V/V0) as a function of pressure for both AlPO4-54 and AlPO4-8. The bulk moduli at ambient temperature were determined by fitting the pressure−volume data below 1.6 GPa to a second order Birch−Murnaghan equation of state (EOS) (i.e., B′0 = 4),38 for AlPO4-54 (B0 = 12.4(2) GPa) and below 3.4 GPa for AlPO4-8 (B0 = 12.4(4) GPa). The values of the bulk modulus for both phases are both very close to 13 GPa determined for the hydrophobic siliceous zeolite silicalite-1-F with empty pores in silicone oil as a PTM.14 The similarity between both bulk moduli (B0−54 = B0−8) is due to the empty unidirectional ring channel (pores) parallel to the c-axes present in both phases. The B0 values observed for AlPO4-54 and AlPO4-8 are lower compared to the corresponding hydrated phase (AlPO4-54· xH2O, B0 = 24 GPa)27 and to nonporous α-quartz-type berlinite (AlPO4, B0 = 36 GPa).46 Due to the presence of empty pores, AlPO4-54 and AlPO4-8 are 2 and 3 times more compressible with respect to AlPO4-54·xH2O and berlinite, respectively. 7774
DOI: 10.1021/acs.jpcc.5b00318 J. Phys. Chem. C 2015, 119, 7771−7779
Article
The Journal of Physical Chemistry C In the case of AlPO4-54 at high pressure, the phase transition to AlPO4-8 is followed by amorphization. All aluminum atoms in AlPO4-54 are 4-fold coordinated and no 6-fold coordinated aluminum is present. In contrast in AlPO4-54·xH2O, 1/3 of aluminum atoms are in 6-fold coordination33 and no transformation to AlPO4-8 is observed at high pressure, Instead, on compression, the adsorbed water from the pores enters in the coordination sphere of IVAl, changing the coordination from 4to 6-fold which is the stable state of Al at high pressure27 resulting in the amorphization of the structure. The presence of H2O, which increases the amount of higher 6-fold coordinated Al, thus prevents the phase transition from AlPO4-54·xH2O to AlPO4-8 at high pressure. 3.2. Amorphization of AlPO4-8. The maximum relative amount of AlPO4-8 was observed at 3 GPa (110 reflection at ∼3.0°, Figure 1). At 2 GPa, the 310 reflection of AlPO4-54 (at ∼5.0°, Figure 1) disappears. However, until the complete pressure-induced phase transition, the X-ray diffraction lines of AlPO4-54 may contribute to the intensity of the AlPO4-8 reflections due to overlap. In the present study, AlPO4-8 became amorphous at high pressure and thus in our experiment we could not verify that the quantity of AlPO4-8 formed at 3 GPa could be fully recoverable upon pressure release. Upon increasing pressure, the intensity of the 200 reflection of AlPO4-8 (at ∼2.5° in 2θ, Figure 1) decreases slowly (Figures 1 and 5), to about 67% of its initial value, at 3.4 GPa. A strong
Figure 6. Mid- (a) and far- (b) IR spectra in the region of the AlPO454 framework modes at selected pressures on compression and on decompression (*).
600−300 cm−1, characteristic of aluminophosphate molecular sieves.50,51 The bands in the 1400−900 cm−1 region arises from the asymmetric stretching vibrations (νa POAl), Figure 6a (modes νa2 and νa1). The symmetric stretching mode (νs P OAl) is observed around 800−650 cm−1 (Figure 6a, modes νs2 and νs1). The band at ∼477 cm−1 arises from POAl bending and is linked to four T atom rings in molecular sieve structures (Figure 6b, mode δ3). The 650−300 cm−1 region contains many components (Figure 6b, modes δ5, δ4, δ2, and δ1) and the observed bands can be assigned to ring bending (δ POAl) vibrations.25,27,50,51 Band assignments are given in Table 1. The spectral profile of the AlPO4-54 framework lattice modes can be very satisfactorily described by the superposition of several Gaussian components after removal of a slowly rising linear background.27,42,52 In the case of asymmetric stretching vibrations, for example, a two-component analysis, although crude, provides extremely good fits to the data, with a minimum number of adjustable parameters (Figure 6a, modes νa2 and νa1). As a first step of the fitting procedure, the background was evaluated by as a straight line between two regions where no modes are observed. This linear background determined at the lowest pressure was then subtracted prior to the Gaussian fit. This background function was fixed and used for spectra at all pressures. The structure of the framework bands is well fitted by a sum of four Gaussians in the mid-IR (Figure 6a), while five Gaussians are used in the far-IR (Figure 6b). For clarity, we only present the framework band region prior to the background removal together with the fitted Gaussians and their assignment (see Figure 6). Note that although the present Gaussian and background fit is not
Figure 5. Normalized intensity of the 200 reflection of AlPO4-8 as a function of pressure.
decrease in intensity of the diffraction lines is observed at 4 GPa corresponding to pressure-induced amorphization (PIA). Amorphization was found to be irreversible. PIA has been observed in other zeolites with a decrease in intensity followed by the complete disappearance of XRD lines, as in the case of the hydrated form of AlPO4-54.27 In these materials, amorphization is associated with the collapse of the pores.18
4. IN SITU MID- AND FAR-INFRARED SPECTROSCOPY AT HIGH PRESSURE AlPO4-54 was studied under high pressure by mid- and far-IR spectroscopy (Figures 6 and 7). AlPO4 molecular sieves display several bands in between 1400 and 225 cm−1 corresponding to framework lattice modes. The IR spectrum of AlPO4-54 exhibits three broad features at 1400−900, 800−650, and 7775
DOI: 10.1021/acs.jpcc.5b00318 J. Phys. Chem. C 2015, 119, 7771−7779
Article
The Journal of Physical Chemistry C
1.8 GPa compared to the room pressure. The decrease in amplitude with the appearance of a small shoulder near νs1 in the 1−2 GPa pressure range can be related to the formation of AlPO4-8.50,53 The gradual decrease in amplitude observed for the νs POAl modes may be due to the amorphization of the material (Figure 6a, modes νs2 and νs1). At higher pressures, the lower frequency band located in the antisymmetric stretching vibration (νa POAl) region (Figure 6a, νa1), is more affected and exhibits a strong decrease in amplitude to 27% (Figure 7a, νa1) of its initial value with a 105 and 40% increase of the fwhm of the higher frequency (νa P−O−Al) and lower frequency (νs POAl) bands νa2 and νs1, respectively. The modes of the far-IR region (Figure 6b) are similar for both materials, AlPO4-54 and AlPO4-8. Notwithstanding, all these modes were strongly affected, losing amplitude together with strong shifts, during the phase transformation (Figures 6b and 7b, modes δ5, δ4, δ3, δ2, and δ1). Note that the saturation observed for the strongest band of the far-infrared region at low pressure does not affect significantly the intensity and band position deduced from the best fit. The high absorbance of the framework modes (absorbance level of the AlPO4-54 in the farIR was higher than 3) required an ultimate signal to noise level to extract intensity and frequency for all modes. Up to 1.7 GPa the most affected modes, δ5, δ4, δ3, and δ2, show a strong reduction of amplitude to 62, 68, 54, and 70%, respectively, compared to their initial values (Figure 6b and represented as the symbols size in Figure 7b), together with an increase of the fwhm of about 13% for δ3 mode. The 650−300 cm−1 region is dominated by the δ3 band, linked to four T atom rings. The strong reduction of amplitude of the δ3 mode up to 1.7 GPa is in agreement with the mid-IR and XRD results showing that a large amount of the material has changed phase in this pressure range. This is thus a clear evidence for the transformation from AlPO4-54 to AlPO4-8. In the FIR, the strong reduction in amplitude of the δ3 band correspond to a structure reduction of 4 T-rings, due to the transformation to AlPO4-8. After this phase transition, the amplitude of all modes exhibits a constant decrease. Between 3.8 and 6.9 GPa, the modes δ5, δ4, δ3, δ2, and δ1, exhibit a reduction in amplitude of about 34, 44, 34, 53, and 89%, respectively, compared to their initial values, which can be linked to the amorphization of the material. Figure 7 shows the pressure-induced frequency shift of selected modes of AlPO4-54. In the asymmetric stretching (νa POAl) region, the feature above 1200 cm−1 is common in molecular sieves and the frequency depends on the ring size of the unidirectional channels.50 Located initially at around 1265 cm−1 in AlPO4-54, upon increasing the pressure, this shoulder of (νa POAl) does not shift until 1.8 GPa (Figure 7a, νa2), above this pressure this mode starts to shift to lower
Figure 7. Pressure dependence of the IR modes of the AlPO4-54 framework in the mid- (a) and far-IR (b). Open and solid symbols correspond to points obtained on compression and decompression, respectively. Symbol size corresponds to the amplitude of the IR modes with diameter being proportional to the amplitude of the band at a given pressure.
unique, such fit provides a sound tool to quantitatively account for the changes of framework lattice modes. The AlPO4-54 framework modes behave differently with increasing pressure. In the mid-IR region (Figure 6a), upon compression at room temperature, the bands of the crystalline phase between 1400 and 900 cm−1 and between 800 and 650 cm−l become broader. A major change of amplitude is observed at 1.8 GPa in the region of asymmetric stretching vibrations and symmetric stretching mode, showing that a large amount of material is transformed. At this pressure, the amplitude of the bands νa2, νa1, νs2, and νs1 (Figure 6a) exhibit a strong decrease to 65, 59, 78 and 63% of their initial values, respectively. Their amplitude is represented by the symbols size in Figure 7a. A significant increase of the full width at half-maximum (fwhm) of the bands νa2 and νs1 by 31 and 7% respectively, is observed at Table 1. Band Assignments for AlPO4-54.25,27,50,51 wavenumber (cm−1)
assignment
label
1265 1113 763 718 567 526 477 431 374
antisymmetric stretching vibrations (νa POAl) antisymmetric stretching vibrations (νa POAl) symmetric stretching mode (νs POAl) symmetric stretching mode (νs P−O−Al) ring bending (δ POAl) ring bending (δ POAl) four T atom ring bending (δ POAl) ring bending (δ POAl) ring bending (δ POAl)
νa2 νa1 νs2 νs1 δ5 δ4 δ3 δ2 δ1
7776
DOI: 10.1021/acs.jpcc.5b00318 J. Phys. Chem. C 2015, 119, 7771−7779
Article
The Journal of Physical Chemistry C
phase, except for the δ3 mode which exhibits lower values than those observed at high pressures. 4.1. IR Spectra of the New Amorphous Form. The amorphization of AlPO4-8 was detected by abrupt shifts of the antisymmetric stretching vibrations to lower frequencies in the mid-IR range (νa2 and νa1, Figure 7a), and by shifts to high frequencies and a change in the evolution of the pressure dependence of the lower wavenumber bending modes (δ2 and δ1, Figure 7b) in the far-IR. In the mid-IR strong broad bands were observed for the amorphous form. In both mid- and farIR, the lower slope for most modes is an indication of the lower compressibility of the amorphous form due to its higher density. On pressure release, the bands of the amorphous form were found at similar wavenumbers as those of the νa PO Al modes at high pressures. Although a detailed interpretation of the spectra can be very difficult because of the influence of the local interactions55 the wavenumber values for the amorphous form on pressure release are generally similar to those of the α-AlPO4 (Berlinite).56 Moreover, the νa POAl mode located at lower frequency at high pressures (νa1 at 7 GPa) is observed at similar wavenumbers as those found in the high pressures Raman study on berlinite.57,58 The similar pressure dependence of the far-IR modes observed for both the AlPO4-8 modes and its amorphous form is an indication that both phases present similar local structure. The very small shifts observed for the δ3 mode, together with the increase in amplitude during the pressure release, are related to the presence of four T atom rings in the new amorphous form. Since the modes in the 650−300 cm−1 region can be assigned to ring bending (δ POAl) vibrations, the spectra of the farIR region on decompression is representative of a local structure involving both 4 and 6 T atoms rings. This is another indication that during the pressure-induced amorphization up to 7 GPa, the degree of crystallinity of the material decrease, but the local structure is preserved, as has been observed in the case of PIA in other zeolites.14,42
frequencies, which can be related to the reduction in the ring channel size. Although dominated by stretching vibrations, the νa PO Al and νs POAl modes in the mid-IR involve some bending of bridging oxygens atoms. As the POAl angle narrows, the bending character increases for the νa POAl mode causing a decrease in frequency while similarly, increasing of the stretching character in the νs POAl mode results in an increase in frequency. The shift of νas (POAl) to lower frequencies and of the νs (POAl) to higher frequencies is correlated to the closure of the (POAl) angle [νas−νs = f (
