J. Phys. Chem. B 2001, 105, 5647-5656
5647
Constrained Conformations of 2,2′-Bipyridine Occluded in Non Acidic-MFI Zeolites A. Moissette, I. Gener, and C. Bre´ mard* Laboratoire de Spectrochimie Infrarouge et Raman UMR-CNRS 8516, Centre d’Etudes et de Recherches Lasers et Applications, Baˆ t. C5 UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France ReceiVed: NoVember 14, 2000; In Final Form: February 2, 2001
The diffuse reflectance UV-visible absorption and Raman scattering spectrometry provide evidence of sorption of 2,2′-bipyridine (bpy, C10H8N2) as intact molecules into nonacidic MFI zeolites, silicalite-1, and Al-ZSM5. The effect of the aluminum content n, the nature of the charge-balancing cation Mn+ as well as bpy loading in Mm/n(AlO2)m(SiO2)96-m (Mm/nZSM-5; m ) 0, 3, 6; n )1, 2; M ) Na+, Zn2+) were examined. The Raman scattering as well as the X-ray powder diffraction experiments provide evidence of weak structural deformations of hosts upon bpy sorption. Clear evidence has emerged from Raman and UV-visible spectroscopic experiments to support the contention that sorption of bpy in dehydrated MFI type zeolites results in three different bpy conformations according to the aluminum content of the framework and the nature of extraframework cations. The modeling investigations using Monte Carlo simulations, molecular mechanics, and molecular dynamics calculations provide coherent structural support to the interpretation of spectroscopic results. It appears that the size and shape of straight channel itself of MFI type zeolites does not generate energetic and steric constraints to change the trans planar structure of free bpy molecule. Thus, the sorption of 2,2′-bipyridine into purely siliceous silicalite-1 retains the trans-planar conformation. The nonbonding interactions between the extraframework Na+ cation and occluded bpy in NanZSM-5 are the main driving forces that stabilize the cisoid nonplanar conformer, whereas the coordination bonding between Zn2+ and N atoms in Zn3ZSM-5 requires the cis-planar conformation. The spectroscopic and modeling results were found to be in agreement with previous quantum calculations concerning the conformational behavior of free 2,2′bipyridine relative to the inter-ring rotation.
Introduction 2,2′-Bipyridine (bpy) is a typical flexible bidentate chelating ligand for transition metal ions leading to complexes having great potential for solar energy conversion and catalytic chemical applications. Aluminosilicate zeolites have shown considerable promise for promoting stabilization of photochemical charge separation using occluded ruthenium complexes1,2 as well as for selective alkene oxidation through zeolite encapsulated manganese complexes.3 The encapsulated metallic complexes were synthesized within the void space by incorporation of bpy into transition metallic ion loaded zeolites. Despite the large interest, relatively few studies dealing with interactions between uncomplexed bpy and zeolites, clays, and oxide materials have appeared in recent literature.4 The incorporation of bpy in faujasitic zeolites within the large cavity network were studied previously using vibrational spectroscopy and the results provided valuable clues about the conformations.5 In the present article, we present the results of spectroscopic investigations of the sorption of bpy in silicalite-1, NanZSM-5 (n ) 3, 6) and Zn3ZSM-5 MFI-type zeolites with (SiO2)96, Nan(AlO2)n(SiO2)96-n and Zn3(AlO2)6(SiO2)90 formula per unit cell, respectively. ZSM-5 is a representative member of the class of MFI high-silica zeolites with medium pore size and contains only a small number of cations in its structure. The shape and size of the pore system as well as the low aluminum content are responsible for the extraordinary catalytic and adsorption * Corresponding author. Fax: 33 20 436755. E-mail: claude.bremard@ univ-lille1.fr.
properties of these materials. It is generally accepted that the elliptical section of the channels of MFI topology is responsible for its unique sorption activities with respect to flat-shaped molecules like aromatics. As the cations essentially represent the centers of interaction with incorporated guest molecules, the location of exchangeable cations is one of the fundamental characters of zeolite, and strongly affects the adsorptive and catalytic properties. Unfortunately, the location and movements of Na+ and Zn2+ cations of the dehydrated ZSM-5 zeolites have not been fully elucidated yet. The knowledge of the behaviors of bpy conformation and coordination to extraframework cations upon sorption is our main aim. The vibrational and electronic spectroscopies are well-known to be sensitive to the molecular conformation and to the chemical environment of the sorbate. This study seeks to correlate the observed spectra, the conformation adopted by the sorbate, and the nature of sorption sites within the channel structure of the MFI-type zeolites. The effect of the aluminum content, the complexing capability of the extraframework cations of the host, as well as the sorbate loading were examined. The prediction of the sorption energy, conformation, sorption site, and mobility of occluded bpy was carried out using Monte Carlo simulations (MC), molecular mechanics (MM), and molecular dynamics (MD) calculations. This modeling study was expected to support the structural interpretation of the spectroscopic results and particularly to provide information about the location and movements of the extraframework cations with respect to the framework and sorbed bpy in the absence of structural diffraction data.
10.1021/jp004188k CCC: $20.00 © 2001 American Chemical Society Published on Web 05/30/2001
5648 J. Phys. Chem. B, Vol. 105, No. 24, 2001 Experimental Section 1. Materials. The as-synthesized silicalite-1 sample was a gift from Dr. Patarin (ESA-CNRS 7015, Mulhouse, France). The starting highly siliceous MFI material (Si/Al ∼ 5000) has been synthesized in fluoride medium in the presence of organic template.6 The sodium-exchanged ZSM-5 samples (Si/Al ) 13, 25) were obtained from VAW aluminum (Schwandorf, Germany). The zinc-exchanged ZSM-5 sample (Si/Al ) 13) was prepared according to the process reported previously using Zn(NO3)2 aqueous solution.7 All the zeolite samples were used after calcination. The 2,2′-bipyridine (bpy) C10H8N2 (Merck) was used after dehydration. Pure and dry Ar gas was used. The unit cell compositions of the calcined and dehydrated silicalite1, Na3ZSM-5, Na6ZSM-5, and Zn3ZSM-5 samples were found to be Si96O192, Na3(AlO2)3(SiO2)93, Na6(AlO2)6(SiO2)90, and Zn3(AlO2)6(SiO2)90 from elemental analysis. The powder XRD patterns, 29Si, 27Al MAS NMR, IR absorption, and Raman spectra of bare MFI zeolites are welldocumented. However, to assist in interpreting the XRD, NMR, IR, and Raman data of the loaded samples, all the spectra of all the freshly dehydrated bare zeolites used in this present work were recorded in the same experimental conditions. The powder XRD patterns of the dehydrated samples correspond to the expected phases of these porous materials. Nevertheless, the 29Si NMR resolution is highly dependent on the synthesis conditions and postsynthesis treatments.8 So, the 29Si NMR spectrum of silicalite-1 exhibits well-resolved peaks and provide evidence of a low level of defects, whereas NanZSM-5 samples exhibit broader bands which are typical of defect groups. In addition, the 27Al NMR spectra provide evidence of small amount of extraframework hexacoordinated Al species in the hydrated Na6ZSM-5 sample. 2. Sorption of bpy. Weighed amounts (∼1.4 g) of powdered hydrated zeolite sample were introduced into an evacuable heatable silica cell. The sample was dried under vacuum (10-3 Pa) and heated stepwise to 500 °C under air. O2 was then admitted into the cell for 12 h at 500 °C. The thermal treatment removed completely the water content and the organic impurities. The crystallinity of the samples was checked by XRD and was not reduced by this treatment. Then, the sample was held under vacuum and then cooled to room temperature under dry argon. Weighed amounts of bisublimated bpy corresponding to 1, 2, or 3 bpy per unit cell were added to zeolite in a glovebox under dry argon, and the powder mixture was shaken. The sample was transferred under dry argon in a quartz glass Suprasil cuvette for FT-Raman and diffuse reflectance UV-visible experiments. The cuvette tubes were sealed under argon or under vacuum. All the spectra recorded with two types of sampling were found analogous. The cuvettes were kept in the dark in a closed oven at 40 °C. Bulk solid bpy stocked in these conditions exhibits no change in the Raman and UV-visible spectra over at least one year. The 29Si and 13C MAS NMR experiments as well as powder diffraction measurements were carried out as previously described.8 3. Instrumentation. The elemental analyses (C, H, Al, Si, Na, Zn) of the bare and loaded zeolites were obtained from the Service Central d’Analyse du Centre National de la Recherche Scientifique (Vernaison, France). The UV-visible diffuse reflectance spectra of the sample were recorded between 200 and 850 nm using a Cary 3 spectrometer equipped with an integrating sphere. The corresponding bare zeolite was used as the reference.9
Moissette et al. The data processing of the Raman spectra recorded during the course of the sorption was performed using the SIMPLISMA (SIMPle-to-use Interactive Self-modeling Mixture Analysis) approach.10,11 This method was applied to extract the characteristic Raman spectra of species generated through sorption from many spectral data. A Bruker IFS 88 instrument was used as a near-IR FT-Raman spectrometer with a cw Nd:YAG laser at 1064 nm as excitation source. A laser power of 100-200 mW was used. The spectra (3500-150 cm-1) were recorded at 4 cm-1 resolution using 400 scans. The Opus Bruker software was used for spectral acquisition, data treatment, and plotting. The 27Al MAS NMR spectra were recorded on Bruker MSL 300 and AS 400 Bruker spectrometers at a resonance of 78.2 and 104.3 MHz, respectively. The 13C CP-MAS NMR spectra were recorded on a Bruker AS 100 spectrometer at a resonance of 25.1 MHz. The 29Si CP-MAS NMR spectra were run on a AS 100 Bruker spectrometer at a resonance of 19.89 MHz. The diffraction data were collected at room temperature on a STOE-STADI-P X-ray diffractometer using Cu ΚRI radiation. Diffraction intensities were measured by step-scanning with a step size of 0.01°. The data were collected from 5 to 80° of 2θ. The calculations were performed on a Silicon Graphics workstation using Cerius2 (version 3.8) package from Molecular Simulation Incorporation. 4. The Model: Structure and Force Field. The atomic positions for the zeolite framework, silicalite-1, and ZSM-5 were obtained from previous X-ray determinations of the MFI structures. The framework corresponding to the MFI adopts polymorphic modifications according to the temperature and the host molecule loading. Dehydrated ZSM-5 zeolites with high Si/Al ratio (silicalite-1 and Al-ZSM-5 for Si/Al > 75) exhibit monoclinic symmetry (P21/n.1.1) and show a phase transition at about 350 K to an orthorhombic form with Pnma symmetry.12,13 Furthermore, this transition can also be brought about by the adsorption of various organic molecules.14 A second orthorhombic form (P212121) is observed in the case of high p-xylene loading.15 The structure of ZSM-5 framework is well established with Pnma space group; however, despite extensive studies, the crystallographic positions of extraframework cations in Al-ZSM-5 zeolite with low Si/Al ratio has not been fully elucidated. 16-21 In our study, we used the orthorhombic (Pnma) form of silicalite-1 to simulate structural and dynamic properties of occluded bpy in siliceous ZSM-5 at low and moderate coverage (# 3 bpy per unit cell (UC)). In addition, we used the X-ray data determination of Cs4ZSM-5 (Si/Al ) 26) to simulate the corresponding properties of occluded bpy in aluminum rich ZSM-5. Only one type of cation site was found in the Cs4(AlO2)4(SiO2)92 structure. The aluminum atoms are likely to be located in the four-membered ring in the vicinity of the cation location.16 The expected atomic positions of the Na+ cations in Na4(AlO2)4(SiO2)92 straight channel derived from the positions of Cs+ in Cs4(AlO2)4(SiO2)92 after an energy minimization procedure. It should be noted that after an annealed dynamic procedure, the energetically favored positions of Na+ cations are located inside cages consisting of two interconnected five-rings surrounded by two sinusoidal and two straight channels, the latter being accessible via six-ring windows. For computer calculations, the simulation box consists of two crystallographic cells superimposed along c direction. The resulting host systems consist of 192 Si, 384 O and 184 Si, 8 Al, 384 O, and 8 Na+ atoms for silicalite-1 and Na4ZSM-5, respectively. The partial atomic charges of the zeolite atoms were taken from previous works: silicalite-1: Si ) +1.48,
2,2′-Bipyridine Occluded in Non Acidic-MFI Zeolites
J. Phys. Chem. B, Vol. 105, No. 24, 2001 5649
TABLE 1: Lennard-Jones Parameters for Guest Molecule ZSM-5 Zeolite Short-Range Non Bonded Interactions atom paira
Aij (kJ mol-1Å12)
Bij (kJ mol-1Å6)
ref
Na-Na N-N Oz-Oz C-C H-H Oz-H Oz-C Na-H Na-C Na-Oz Na-N N-C N-Oz N-H C-H
38883 3929458 1150049 4900894 71959 250393 2265693 103356 1291019 738182 841507 4388380 2125811 531753 593855
246 3088 1781 2793 135 832 3116 232 953 6047 1874 2937 2347 644 615
[33] [34] [38] [34] [34] [32] [32] [32] [32] [33]
a
Oz represents the oxygen atoms of the zeolite framework.
O ) -0.74; Na4ZSM-5: Si ) +1.56, Al ) +1.232, O ) -0.794, Na ) +1.22 The geometrical parameters of bpy (C10H8N2) and the partial atomic charges are derived from ab initio calculations and X-ray determination.23-29 The potential energy E of the simulated zeolite-sorbate (ZS) system is calculated by the summation of four terms: The bonded interaction energy of Z, EZ; the bonded interaction energy of S, ES; the nonbonded interaction energy between Z and S, EZS; the nonbonded interaction energy between S and S, ESS. The force field values (bond, angle, torsion) to calculate the EZ term were derived from ab initio calculations of small model systems.30,31 The force field values to calculate the ES, EZS, and ESS terms were derived from previous works.32-38 The sum of a Coulomb potential and a Lennard-Jones (L-J) potential is used to describe the nonbonded interactions.
EZS + ESS )
∑ij ARβ/r 12 - BRβ/r 6 + qiqj/rij ij
ij
(1)
The L-J potential accounts for repulsive ARβ/rij12 and dispersive -BRβ/rij6 interactions, the parameters are listed in Table 1 as ARβ and BRβ. The short-range L-J interactions with Si and Al are ignored since they are well shielded by the O atoms of the framework. 5. Calculations. In the Monte Carlo simulations, the Si, Al, O atoms of the zeolite framework are fixed and Na+ cation positions are fixed in the straight channels. The bpy structure is assumed to be rigid. The MC simulations at fixed loading were carried out at 300 K using the conventional Metropolis algorithm based on the configurational energy change:
P ) min[1; exp(-∆E/kT)]
(2)
where P is the probability of the move being accepted, ∆E ) ∆(EZS + ESS) is the energy change between the new configuration and the previous configuration, k is the Boltzmann constant, and T the temperature of the simulation. To eliminate the effect of boundaries, we have used periodic boundary conditions with a period equal to two zeolite unit cells. A cutoff radius of 0.9 nm is applied to the Lennard-Jones interactions and the long-range electrostatic interactions are calculated using the Ewald summation technique. The simulation takes a number of steps to equilibrate from its original random position. For accurate statistical results, the steps made prior to equilibration should be excluded in the analysis. One typical MC run took 1500000 steps. From each sorption trajectory file, a histogram of energy distribution for each sorbate is plotted. In mass-
cloud analysis, the center of mass of each sorbate in each configuration is displayed as a dot in the model space. In the molecular mechanics (MM) and molecular dynamics (MD) simulations, the time-consuming Ewald summation is not performed, the electrostatic and (L-J) interaction cutoffs are defined by two parameters: the spline-on and the spline-off distances. Within these ranges the nonbond interaction energy is attenuated by the spline function. Beyond the spline-off distance, nonbond interactions are ignored. The spline-on and spline-off distances are taken to be 1.5 and 3 nm for both the electrostatic and (L-J) interactions. The energy minimization of E ) EZ + ES + EZS + ESS is performed using the conjugate gradient minimization procedure. The Si, Al, O atoms of the zeolite framework and the bpy structure were taken to be flexible and Na+ cations were taken to be mobile. MD simulations were performed at 300 K in the NVE ensemble for 500 ps (E ) EZ + ES + EZS + ESS) with fixed framework (Si, Al, O) mobile Na+ cations and flexible bpy. The equations of motions were integrated by using the velocity form of the Verlet algorithm with a time step of 1 fs. From the trajectory file, the mean-square displacements as well as radial distribution functions were calculated. Simulated annealing was performed from 300 to 800 K and back. This cycle was repeated 3 times. At the end of each temperature cycle, the lowest energy structure of each cycle was minimized Results and Discussion 1. Sorption of bpy into MFI Zeolites. The structure and conformation of bpy have been the subject of numerous experimental investigations. The crystal structure determined by X-ray diffraction39 shows a trans-planar conformation with mean values for the pyridyl C-N (0.136 nm) and C-C (0.139 nm) bonds typical of aromatic rings and an inter-ring C-C single bond (0.150 nm). In the gaseous state, electron diffraction data40 suggest that the potential energy curve relative to the interring rotation angle is rather flat with two probable shallow minima at the trans-planar geometry and for a cisoid twisted form analogous to the twisted equilibrium conformation of biphenyl. In solution, several experimental studies such as dipole moments,41-43 nmr,44,45 and electronic absorption46,47 provide concordant evidence for a planar or approximately planar conformation in all the solvents. The framework structure of MFI zeolites contains two types of intersecting channels,48 both formed by rings of 10 oxygen atoms, characterizing them as a medium-pore zeolite. One channel type is straight and has a nearly circular opening (0.53 × 0.56 nm), while the other one is sinusoidal and has an elliptical opening (0.51 × 0.55 nm). These channels are sufficiently wide to allow bpy molecules to pass through these types of channels and to diffuse into the void space. The location of Na+ and Zn2+ cations in straight and sinusoidal channels can strongly affect the adsorptive properties, but has not been fully elucidated yet in the bare NaZSM-5 and ZnZSM-5 zeolites. Solid bpy exhibits too weak vapor pressure at room temperature to carry out the sorption procedure through sublimation from a compartment containing the solid bpy to another compartment containing the freshly dehydrated zeolite sample even under gentle warming. The adsorption from bpy solution in organic solvent can be an alternative to avoid this problem. However, then it is difficult to eliminate the solvent from the void space without damaging the sample. The method used is the mixture under dry argon of weighted amounts of dehydrated
5650 J. Phys. Chem. B, Vol. 105, No. 24, 2001 MFI with crystal size in the 1-2 µm range and dry pure bpy in the same reactor. The cell was allowed to stand at 40 °C under argon or vacuum until the sorption of guest molecules into the void space of the zeolites went to completion. 1.1. bpy Sorption in Silicalite-1. The FT-Raman spectra recorded after the mixing and shaking of dry solid bpy and calcined silicalite-1 powdered samples remained typical of bulk solid bpy and empty silicalite-1 over one week. Progressively, the Raman features of bpy in the solid state evolved to the Raman characteristics of occluded bpy. Particularly, the two 236 and 224 cm-1 intense bands disappeared and concomitantly a weak band appeared at 228 cm-1. The data processing using the SIMPLISMA approach of all the spectra recorded in the mid-frequency region during the course of the sorption provided evidence of two independent spectra assigned to bulk solid bpy and occluded bpy. The typical spectra of occluded bpy were found to be identical for all the attempts in the experimental loading range (1-3 bpy/UC) and very analogous to the bpy solution spectrum. In addition, the program provided the respective concentration of solid and occluded bpy during the kinetic of the sorption. At a loading corresponding to 1 bpy per unit cell (1 bpy/UC), the sorption went to completion over one month at room temperature. At higher loading, typically 3 bpy/UC, the sorption takes place over 3 months. The UVvisible absorption spectra recorded during the course of the bpy sorption in silicalite-1 exhibits very weak changes. The broad absorption corresponding to a π* r π electronic transition is observed around 284 nm for bpy in the solid state and around 290 nm for occluded bpy. The UV-visible spectrometry does not provide an analytical mean to monitor the course of the sorption. The low-frequency Raman bands assigned to the deformation of the host framework around 300, 380 cm-1 do not change markedly upon bpy sorption; nevertheless the X-ray powder diffraction provides clear evidence of a monoclinicorthorhombic phase transition. 1.2. bpy Sorption in NanZSM-5 (n ) 3, 6). After the mixing and shaking under argon of calculated amounts of solid bpy and dehydrated zeolite samples, the Raman spectra recorded over one week were found to be typical of mixtures of the components. Over one month, marked changes were observed. The data processing using the SIMPLISMA program of all the Raman spectra recorded during each sorption process provided evidence of two independent spectra for all the attempts made: the bulk solid bpy spectrum and a spectrum representative of occluded bpy. This later spectrum was found to be analogous for all the NanZSM-5 samples used (n ) 3, 6) and all the loadings made (1, 2, or 3 bpy/UC). In contrast, the spectrum of bpy occluded in NanZSM-5 is found to be dramatically different than bpy occluded in silicalite-1. The respective concentrations of solid and occluded bpy as a function of time during the kinetic of the sorption, indicate the sorption went to completion over three months at room temperature at a loading corresponding to 1 bpy/UC. At higher loading, typically 3 bpy/UC, the sorption takes place over at least four months. The lowfrequency Raman bands assigned to the deformation of the host framework around 300, 380 cm-1 do not change markedly upon bpy sorption. Analogous results were deduced from the UV-visible absorption spectra recorded during the course of the sorption because a significant change was observed through the appearance of a characteristic shoulder at 315 nm. 1.3. bpy Sorption in Zn3ZSM-5. The FT-Raman spectra recorded after the mixing and shaking of calculated amounts of solid bpy and calcined exchanged Zn3ZSM-5 samples
Moissette et al. remained typical of the mixtures over two weeks at room temperature. Progressively, the Raman features of the bulk solid bpy evolved to the Raman characteristics of occluded bpy. The data processing using the SIMPLISMA program of all the Raman spectra recorded during each sorption process provided evidence of two independent spectra for all the attempts: the bulk solid bpy spectrum and a spectrum representative of occluded bpy. This later spectrum was found to be analogous for all the Zn3ZSM-5 samples used and all the loadings made (1-3 bpy/UC). In contrast, the typical Raman spectrum of bpy occluded in Zn3ZSM-5 is found to be markedly different from that of bpy occluded in NanZSM-5. The respective concentrations of solid and occluded bpy as a function of time during the kinetic of the sorption, indicates the sorption went to completion over two months at room temperature at a loading corresponding to 1 bpy/UC. At higher loading, typically 3 bpy/ UC, the sorption takes place over 12 months. The low-frequency Raman bands assigned to the deformation of the host framework around 300, 380 cm-1 do not change markedly upon bpy sorption. Analogous results were deduced from the UV-visible absorption spectra recorded during the course of the sorption. A significant change was observed through the shift of the absorption band from 290 to 310 nm. 2. Raman Scattering and UV-Visible Absorption of bpy Occluded in MFI Zeolites. The vibrational and Raman scattering properties of bpy molecule are well documented in solution as well as in the solid state.49-51 In solution, several experimental studies provide concordant evidence for a transplanar or approximately planar in all solvents. The 54 vibrational modes of trans-planar bpy (C2h symmetry) are shared among 19 Ag + 18 Bu + 8 Bg + 9 Au symmetry classes. Ag and Bg modes are Raman active while Au and Bu modes are infrared active. The Ag and Bu modes are in-plane vibrations. The Ag modes correspond to the inter-ring bond stretching and to the in-phase combinations (relative to the symmetry center) of the 17 pyridyl in-plane modes and of the inter-ring in-plane bending. The Bu modes correspond to the out-of-phase analogues of the 18 in-phase Ag modes. Au and Bg modes are out-of-plane motions. The Au modes correspond to the inter-ring torsion and to the out-of-phase components of the 7 ring out-of-plane modes and of the inter-ring out-of-plane bending. The Bg modes correspond to the in-phase analogues of the 8 out-of-phase Au modes. Except the region of the C-H stretches (4 Ag + 4 Bu) around 3000 cm-1, all the modes are observed in the 2001700 cm-1 region. The optically active frequencies of crystalline bpy with P21/c ) C52h and 2 molecules per unit cell were reported previously49-51 using polarized light. The molecular modes are nondegenerates and therefore do not split. The crystal symmetry involves two molecules that are weakly interacting, thus, vibrational frequencies corresponding to these modes show very similar values in both infrared and Raman spectra and were found analogous in frequency and relative intensity to the spectra recorded in solution. The Raman spectra of solid bpy and bpy solution are mainly representative of the internal vibration of the trans-planar conformation (C2h). However, some characteristic frequency shifts due to the crystal field effects are visible in the Raman spectra recorded at room temperature. Particularly, the bands at 1297 cm-1 (Ag), 440 (Ag, Bg), 236 (Bg), 224 cm-1 (Ag) are only characteristic of the solid bpy. The UV-visible absorption spectrum of bpy in solution in water exhibits two maxima at ca. 233 and 280 nm.46 These bands were assigned to π*r π electronic transitions. The UV-visible
2,2′-Bipyridine Occluded in Non Acidic-MFI Zeolites
J. Phys. Chem. B, Vol. 105, No. 24, 2001 5651 TABLE 2: UV-Visible Spectral Data and Conformation of 2,2′-Bipyridine 2,2′bipyridine
UV-visible absorption bands (nm)a
conformation
symmetry group
water solution solid state silicalite-1 Na3ZSM-5 Na6ZSM-5 Zn3ZSM-5 Zn(bpy)Cl2 complex
233, 280 238, 284 240, 290 245, 290, 300, 315 245, 290, 300, 315 242, 290 (sh), 300, 310 250 (w), 295, 323
trans planar46 trans planar39 transoid cisoid cisoid cis planar cis planar
C2h C2h C2 C2 C2 C2V C2V
a
sh: shoulder; w: weak.
Figure 1. Sorption of 2,2′-bipyridine in Na6ZSM-5 monitored by FTRaman spectrometry at room temperature in the low-frequency region (3 bpy per unit cell): (a) bpy in the solid state; (b) 1 day after the mixture of the powders; (c) 10 days; (d) 3 months; (e) 7 months; (f) empty Na6ZSM-5.
Figure 3. FT-Raman spectra at room temperature of 2,2′-bipyridine: (a) occluded in Na3ZSM-5 (1 bpy per unit cell); (b) occluded in Na6ZSM-5 (1 bpy per unit cell).
Figure 2. FT-Raman spectra at room temperature of 2,2′-bipyridine: (a) in chloroform solution, (b) occluded in silicalite-1 (1 bpy per unit cell).
absorption spectrum of bulk solid bpy also displays two bands at 238 and 284 nm. 2.1. bpy Occluded in Silicalite-1. The Raman features of bpy occluded in silicalite-1 at loading in the [1-3 bpy/UC] range were found to be analogous in terms of wavenumber and relative intensity to that exhibited in solution in the C-H stretching mode region as well as in the mid and low-frequency regions, Figure 2. No infrared active mode of bpy was detected in the Raman spectrum of occluded bpy. This finding provides evidence of the retention of C2h molecular symmetry and low electrostatic field in the vicinity of the sorption site. All the sorption sites appear analogous at low and high loading values and no intermolecular vibrational coupling was detected at high loading values. All the vibrational properties of occluded bpy are found to be analogous to that of bpy in solution and are relevant to
the trans-planar molecular structure. The bpy sorption in silicalite-1 induced some very weak frequency changes of the Raman active deformation modes of the framework around 380 and 300 cm-1. However, the X-ray powder diffraction pattern exhibits clearly the structural deformation of the channels upon bpy sorption induced by the tight fit channel of silicalite-1 to bpy size. The electronic transitions of bpy occluded in silicalite-1 observed at 290 and 240 nm were found to be analogous to that of bpy in the solid state and in solution (band shape, close position). This finding provides supplementary evidence of the retention of the trans-planar molecular structure of bpy upon sorption in silicalite-1 and weak electronic effect at the sorption site (Table 2). 2.2. bpy Occluded in NanZSM-5 (n ) 3, 6). The Raman features of bpy occluded in Na3ZSM-5 and Na6ZSM-5 were found to be very similar in terms of number of bands, frequency, and relative intensity, Figure 3. The Raman spectra do not depend on the loading values in the experimental range (1-3 bpy/UC). This finding provides evidence of one type of bpy molecular structure and one type of sorption site. The Raman spectrum representative of occluded bpy in NanZSM-5 is dramatically different from bpy solution. However, the spectrum is identical to a previously reported spectrum corresponding to bpy occluded in Na56FAU faujasitic zeolite5 and has typical resemblance in terms of relative intensity of the most prominent bands of Zn(bpy)Cl2 complex in term of frequency.5 This later analogy indicates clearly occluded bpy
5652 J. Phys. Chem. B, Vol. 105, No. 24, 2001 does not adopt the trans-planar conformation with C2h symmetry but it is not straightforward to assign a strict cis-planar conformation with C2V local symmetry to bpy as determined by X-ray diffraction in Zn(bpy)Cl2 complex.5 Some marked differences between both spectra can be observed in terms of number of bands in the 1350-1250 cm-1 range, 1256, 1283, and 1310 cm-1 for occluded bpy in NanZSM-5 and 1263 and 1312 cm-1 for the complex. In addition, there are some discrepancies in the frequency coincidence of the in-plane ring deformation and ring stretching modes observed at 1007 and 1063 cm-1 in occluded bpy and 1028 and 1062 cm-1 in the complex, respectively. In contrast, there is a strict analogy in band number, frequency and relative intensity of the Raman spectra of bpy occluded in NanZSM-5 and in Na56FAU. The observation of coincident Raman and IR absorption bands from bpy occluded in Na56FAU is consistent with the break down of the IR and Raman mutual exclusion rule of centrosymmetric molecular group C2h. This finding is consistent with a nearly planar cisoid conformation (C2 symmetry group) rather than a strict cis-planar conformation (C2V symmetry group). It appears clearly that the close proximity of extraframework Na+ cations and occluded bpy is the major feature of the conformation change of bpy upon sorption in these zeolites. These cations essentially represent the centers of interaction with the incorporated guest bpy. However, the Raman bands corresponding to the frustrated translational motions with respect to the framework and guest molecules occur at too low frequency and do not exhibit sufficient intensity to be detected in Raman spectra recorded with the FT-Raman as well as dispersive technique. The vibrational transitions of cations of bare Na6ZSM-5 were detected previously by far-infrared spectroscopy.18 Unfortunately, the attempts of in situ far-infrared spectroscopy do not provide any suitable information concerning the location and coordination of Na+ cations in the NanZSM-5 samples loaded with bpy, whereas, some Raman bands assigned to the framework deformations change weakly upon sorption. The deformations of the framework were more clearly demonstrated through the X-ray diffraction patterns. The π* r π electronic transitions of bpy occluded in NanZSM-5 (n)3, 6) were found to be observed at 245, 290, 300 and 315 nm and are markedly different of that exhibited in solution (Table 2). This finding provides supplementary evidence of the change of the molecular structure of bpy upon sorption in NanZSM-5 zeolites. 2.3. bpy Occluded in Zn3ZSM-5.The number of observed bands, frequency coincidence and relative intensity of the Raman spectrum representative of bpy occluded in Zn3ZSM-5 over the 1 to 3 bpy/UC loading range, closer resemble the spectrum of the Zn(bpy)Cl2 complex than bpy occluded in NanZSM-5, Figure 4. The Raman spectra are indicative of the C2V local symmetry of occluded bpy. This finding provides evidence of one type of bpy molecular structure, one type of sorption site and chelation of bpy through the extraframework Zn2+ cations in void space of zeolite. These facts imply the conversion of the trans-planar bpy structure to the cis-planar conformation through the constraint of the coordination of Zn2+ cation to the nitrogen atoms of bpy. The deformations of the framework upon sorption were more clearly demonstrated through the X-ray diffraction patterns than from Raman spectra. The π* r π electronic transitions of bpy occluded in Zn3ZSM-5 were found to be observed at ca. 242, 290 (shoulder), 300 and 310 nm and are markedly different to that exhibited in solution (233, 280 nm), whereas they have significant resem-
Moissette et al.
Figure 4. FT-Raman spectra at room temperature of 2,2′-bipyridine: (a) Zn(bpy)Cl2 complex in the solid state; (b) occluded in Zn3ZSM-5 (1 bpy per unit cell).
blances with bpy occluded in NanZSM-5 (240, 290, 300, and 315 nm). These features provide supplementary evidence of the change of the molecular structure of bpy upon sorption through complexation to Zn2+ of Zn3ZSM-5 zeolite (Table 2). The oxygen atoms of the framework probably complete the coordination sphere of intrazeolite Zn(bpy)2+ hemicomplex. However, the electronic effect appears somewhat different from that of chloro ligand in Zn(bpy)Cl2 through the electronic absorption spectra (250, 295, and 323 nm). 3. Monte Carlo Simulations and Molecular Mechanics Calculations. Numerous quantum calculation methods have been applied to analyze the conformational behavior of free bpy relative to the inter-ring rotation.51 Qualitatively similar rotational potential energy curves are predicted in all cases, with an absolute energy minimum at the trans-planar conformation, a secondary minimum for the cisoid nonplanar form and an absolute maximum at the cis-planar geometry. With most methods, the energy difference between the cis- and transplanar forms is relatively large (40 kJ mol-1) while the energy barrier from cisoid conformer to trans-planar conformer is low (8 kJ mol-1). Analogous qualitative results concerning the free bpy molecule were deduced from molecular mechanic calculations performed with the force field and set of charges used throughout this article. 3.1. bpy Occluded in Silicalite-1. The predictions of the energy and location of bpy sorption sites were carried out at 300 K and constant loading using the conventional Metropolis algorithm. The zeolite atoms were fixed and bpy is held rigid in the trans-planar conformation. The calculations took into account the non bonded atom-atom interactions according to eq 1 detailed in the Experimental Section, E ) EZS + ESS. The loading values used correspond to the values used in the spectroscopic measurements, namely 1, 2, or 3 bpy/UC. For 1 bpy/UC loading, the distribution of individual bpy energies exhibits a sharp maximum at -143 kJ mol-1 (Figure 5a) and the energy per mole does not increase as the loading is increased. No significant bpy-bpy interaction appears effective in this loading range. The distribution of positions occupied by bpy center of mass with energy close to -143 kJ mol-1 is not large, and indicates the net potential surface accessible to the molecule is maximum in the straight channel in the vicinity of zigzag
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Figure 5. Distribution of calculated energies of (a) trans 2,2′-bipyridine occluded in silicalite-1 zeolite at 300 K at 1 bpy per unit cell loading; (b) cis 2,2′-bipyridine occluded in Na4ZSM-5 zeolite at 300 K at 1 bpy per unit cell loading.
channel intersection. The bpy occupancy of the zigzag channel occurs at higher energy, -113 kJ mol-1. The energy minimization of the bpy/silicalite-1 system from the most probable location deduced from the MC simulations was carried out with all the structural parameters of host and sorbate allowed to be relaxed (E ) EZ + ES + EZS + ESS, see Experimental Section). The results are representative of the structures at absolute zero and predict the preferred conformation of bpy occluded in the porous void as well as the framework deformation. The energy minimization procedure for 1, 2, or 3 bpy/UC loading, predicts a nearly planar transoid bpy conformation and the dihedral angle value between both pyridyl rings was found to be 160°. The molecule resides in a straight channel in the vicinity of the intersection with the zigzag channel and the central C-C bond runs along the b direction, Figure 6, parts a and b. Some weak distortion of the silicalite-1 framework was observed in the vicinity of the guest. This deformation appears analogous to the structural changes determined previously through X-ray powder diffraction experiments during the sorption of naphthalene in silicalite-1 where the naphthalene sorption provokes an elliptical straight-channel deformation.52,53 3.2. bpy Occluded in Na4ZSM-5. The MC simulations of the energy and sorption sites of bpy occluded in Na4(AlO2)4(SiO2)92 could be considered to be representative of the behavior of bpy occluded in Nan(AlO2)n(SiO2)96-n (n ) 3, 6). These samples were studied using spectroscopic tools (see above). The structure of the host were derived from the previously reported X-ray diffraction study of Cs4(AlO2)4(SiO2)92 with Pnma space group.16 The cation sites were found to be located in the straight channels. The atomic positions of the Na+ cations in Na4(AlO2)4(SiO2)92 derived from the positions of Cs+ in Cs4(AlO2)4(SiO2)92 after energy minimization. The predictions of the energy and location of bpy sorption sites were carried out at 300 K and constant loading. The zeolite atoms were fixed and bpy is held rigid in the cis-planar conformation. The calculations took into account nonbonded atom-atom interactions according to eq 1. The loading values used correspond to the values used in the spectroscopic measurements, namely 1, 2, or 3 bpy/UC.
Figure 6. Predicted conformation and sorption site of 2,2′-bipyridine in straight channel of silicalite-1 zeolite at 1 bpy per unit cell loading. Black and shaded sticks represent the O and Si atoms of the (SiO2)96 framework, respectively. The white and shaded cylinders represent the H and C, N atoms of the C10H8N2 molecule, respectively. (a) View in the b, c plane; (b) view along the b axis.
For 1 bpy/UC loading, the distribution of individual bpy energies exhibits a maximum at -188 kJ mol-1 and does not increase significantly as the loading increased, Figure 5b. No significant bpy-bpy interaction appears effective in this loading range. The distribution of positions occupied by the center of mass of bpy with energy close to -188 kJ.mol-1 is not large, and indicates the net potential surface accessible to the molecule is maximum in the straight channel in the vicinity of the Na+ cations. The bpy occupancy of the zigzag channel occurs at higher energy, -134 kJ mol-1. The interactions of the N atoms with the Na+ appear as an important part of the energy sorption.
5654 J. Phys. Chem. B, Vol. 105, No. 24, 2001
Figure 7. Predicted conformation and sorption site of 2,2′-bipyridine in straight channel of Na4ZSM-5 zeolite at 1 bpy per unit cell loading. Black and shaded sticks represent the O and Si/Al atoms of the (AlO2)4(SiO2)92 framework, respectively. The white and shaded cylinders represent the H and C, N atoms of the C10H8N2 molecule, respectively. The shaded spheres represent the Na+ cations. (a) View in the b, c plane; (b) view along the b axis.
The energy minimization of the bpy/Na4ZSM-5 system from the most probable location deduced from the MC simulations was carried out with all the structural parameters of host and sorbate allowed being relaxed. The minimization confirms the most favorable sorption sites and points out the cisoid conformation of bpy in the void space. The molecule resides in straight channel in the vicinity of the intersection with the zigzag channel and the central C-C bond runs along the b direction, Figure 7, parts a and b. The dihedral angle value was found to be 20° and the Na+-N distances of occluded bpy were 0.27 nm. The closest Na+ cation is situated out of the mean plane of bpy with cisoid conformation. The positions of extraframework cations do not move markedly from the starting positions and some weak distortions of the framework were observed after the minimization procedure. It is obvious that the cisoid conformation of bpy is stabilized through the Na+-N contacts in the void space of NanZSM-5 zeolites, as well as probably in the Na56FAU faujasitic zeolites.5
Moissette et al.
Figure 8. Expected conformation and sorption site of 2,2′-bipyridine in straight channel of Zn2ZSM-5 zeolite at 1 bpy per unit cell loading. Black and shaded sticks represent the O and Si/Al atoms of the (AlO2)4(SiO2)92 framework, respectively. The white and shaded cylinders represent the H and C, N atoms of the C10H10N2 molecule, respectively. The shaded spheres represent the Zn2+ cations. (a) View in the b, c plane; (b) view along the b axis.
3.3. bpy Occluded in Zn2ZSM-5. No suitable MC simulations can be carried out taking into account the nonbonding interactions of eq 1 because of the expected bonding character of the interactions between Zn2+ and bpy. The close analogy observed between the corresponding spectroscopic features of bpy occluded in Zn3ZSM-5 and Zn(bpy)Cl2 complex (see above) indicates clearly a strict cis-planar structure of complexed bpy in the void space. To provide a reasonable picture of the occluded complex, the assumed Zn(bpy)2+ moiety was modeled in straight channel of empty ZSM-5 framework derived from Na4ZSM-5 structure as a rigid group through a minimization of the nonbonding interactions, Figure 8, parts a and b. The structural parameters of the moiety derive from the structure of Zn(bpy)Cl2.5
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The Zn(bpy)2+ moiety is stabilized through contacts between Zn atom and oxygen atoms of the framework in the straight channel and holds the bpy ligand in strict cis-planar conformation through the coordination of the N atoms with Zn2+. 4. Molecular Dynamics Calculations. The mean-square displacements, radial distribution functions, as well as velocity autocorrelation functions can be calculated from the trajectory file obtained from MD calculations. The bpy mean square center-of-mass displacements were not found sufficient to observe any bpy migration over 500 ps. So, the MD simulations concern bpy motions only in the vicinity of the sorption site. The Fourier transform of the velocity autocorrelation functions provides vibrational density of C, H, N atoms in the expected wavenumber ranges deduced from IR and Raman spectra. However, the calculated frequency shifts were found to be very weak according to the conformational changes. The calculations of polarizability in relation with Raman intensity fall beyond the scope of the present work. 4.1. bpy Occluded in Silicalite-1. MD simulations were performed at 300 K in the NVE ensemble for 500 ps (E ) ES + EZS + ESS) with fixed framework (Si, Al, O) and flexible bpy for 1 bpy/UC loading. The radial distribution function GAB(r) is defined as the probability that two centers A and B are separated by distance r with regard to a statistical distribution of both centers A and B:
GAB(r) ) NAB(r) V/(NANB - NAB) 4πr2dr
Figure 9. Radial distribution function G(r) at 300 K between the N atoms of 2,2′-bipyridine occluded in Na4ZSM-5 at 1 bpy per unit cell loading with respect to the Na+ cation.
(3)
with NA number of atoms in group A, NB number of atoms in group B, NAB number of atoms common to both groups A and B, V unit cell volume. The radial distributions G, representative of the bpy channel wall distances provide evidence of the persistence over the simulation time of H- - -O(zeolite) distances around the equilibrium values through prominent broad maxima at 0.3 nm. The distribution of the dihedral angle between the two pyridyl groups is in the (140-180°) range and exhibits a maximum at 160° and is representative of the rotational motion of the bpy transoid conformation. 4.2. bpy Occluded in Na4ZSM-5. MD simulations were performed at 300 K in the NVE ensemble with fixed framework (Si, Al, O) and mobile Na+ cations and flexible bpy for 1 bpy/ UC loading. The atomic positions of the Na+ cations in Na4(AlO2)4(SiO2)92 straight channel derive from positions of Cs+ in Cs4(AlO2)4(SiO2)92 after energy minimization procedure and correspond to local energy minima. It should be noted that after annealed dynamic procedure, the energetically favored positions of Na+ cations were located inside small cages consisting of two interconnected five-rings surrounded by two sinusoidal and two straight channels, the former being accessible via six-ring windows. This finding is in agreement with previous works concerning the cation siting in ZSM-5 zeolites.18 Annealed dynamics and subsequent minimization procedure of the model structure of 1 bpy/UC obtained from Monte Carlo simulations Section 3.2 provide analogous location and structure of bpy in contact with Na+. However, the remaining extraframework Na+ cations were found to be located inside small cages. The atomic positions of the latter structure was used as starting structure for MD calculations and the temperature was found to be stable around 300 K during the course of the calculations in the NVE ensemble. The radial distribution G representative of the distances between the nearest Na+ cation and N atoms of bpy exhibits a maximum around 0.27 nm whereas the distances between the C atom group and Na+ show a radial distribution maximum at 0.3 and 0.38 nm, Figures 9 and 10. These results provide
Figure 10. Radial distribution function G(r) at 300 K between the C atoms of 2,2′-bipyridine occluded in Na4ZSM-5 at 1 bpy per unit cell loading with respect to the Na+ cation.
evidence of the dynamics of the facial interaction between pyridyl groups and Na+ cation as well as between N atoms and Na+ cation. The distributions of the dihedral angle between the two pyridyl groups were found around 30°. These findings are representative of cisoid conformation as well as restricted internal rotation around the central C-C bond. From the present MD calculations no Na+ site to site jump was detected over the simulation time. In contrast, from the spectroscopic results, it appears clearly that the Na+ movements at large time scale involve migration from small cages to sinusoidal and straight channels. Previous works54 provide evidence of Na+ ion jumping between several unequivalent sites through the dielectric properties of ZSM-5 and other zeolites. The bpy occluded in the straight channel appears as a supplementary Na+ trapping site. 4.3. bpy Occluded in Zn2ZSM-5. No MD calculation was carried out because of the bonding nature of Zn2+-bpy interactions. The coordination of Zn2+ through N atom con-
5656 J. Phys. Chem. B, Vol. 105, No. 24, 2001 strains the bpy conformation to be cis-planar and the Zn(bpy)2+ moiety interacts with the wall of the channel through Zn2+-Oz interactions. Conclusions Clear evidence has emerged from the present Raman and UV-visible spectroscopic experiments to support that 2,2′bipyridine is sorbed as intact in dehydrated MFI type zeolites. According to the aluminum content of the framework and the nature of extraframework cations, three different 2,2′-bipyridine conformations were observed in the porous void space. The modeling investigations using Monte Carlo simulations, molecular mechanics, and molecular dynamics calculations provide a coherent structural support to the interpretation of spectroscopic results: (i) 2,2′-bipyridine is occluded in the straight channel of purely siliceous silicalite-1 with nearly trans-planar conformation; (ii) the sorption of 2,2′-bipyridine in the straight channel of NanZSM-5 (n ) 3, 6) stabilized the cisoid non planar conformation; (iii) the sorption of 2,2′-bipyridine within the straight channel of Zn3ZSM-5 constrains the conformation to be cis-planar. The conformation behavior of occluded 2,2′-bipyridine is found to be in agreement with quantum calculations applied previously to analyze the conformational behavior of free 2,2′bipyridine relative to the inter-ring rotation. Qualitatively similar rotational potential energy curves are predicted in all cases. With an absolute energy minimum at the trans-planar conformation, a secondary minimum for a cisoid nonplanar form and absolute maximum at the cis-planar geometry. The energy difference between the cis- and trans-planar forms is relatively large while the energy barrier from the cisoid conformer to the trans-planar conformer is low. It appears that the size and shape of straight channel of MFI type zeolites do not generate energetic and steric forces enough to change markedly the trans-planar structure of free 2,2′bipyridine molecule. The nonbonding interactions between the extraframework Na+ cation and occluded 2,2′-bipyridine are the main driving forces to stabilize the cisoid nonplanar conformer, whereas the coordination bonding between Zn2+ and N atoms holds the cis-planar conformation. Acknowledgment. The authors are very grateful to Dr. B. Sombret for assistance and advice while using FT-Raman spectrometry. The authors acknowledge Dr. J. Patarin for helpful discussions and advice. The Centre d’Etudes et de Recherches Lasers et Applications (CERLA) is supported by the Ministe`re charge´ de la recherche, the re´gion Nord/Pas de Calais, and the Fonds Europe´en de De´veloppement Economique des Re´gions. References and Notes (1) Kohle, O.; Gratzel, M.; Meyer, A. F.; Meyer, T. B. AdV. Mater.: (Weinheim, Ger.) 1997, 9 (11), 904-906. (2) Vitale, M.; Castagnola, N. B.; Ortins, N. J.; Brooke, J. A.; Vaidyalingam, A.; Dutta, P. K. J. Phys. Chem. B 1999, 103, 2408-2416. (3) Knops-Gerrits, P. P.; De Vos, D.; Thibault-Starzyk, F.; Jacobs, P. A. Nature 1994, 369 (6481), 543. (4) Bagshaw, S. A.; Cooney, R. P. J. Mater. Chem. 1994, 4 (4), 557. (5) Bartlett, J. R.; Cooney, R. P. Spectrochim. Acta 1987, 43A, 1543 and references therein. (6) Guth, J. L.; Kessler, H.; Wey, R. In Proceedings of the 7th International Zeolite Conference, Tokyo; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Elsevier: Amsterdam, 1986; p 121. (7) Bre´mard C.; Le Maire, M. J. Phys. Chem. 1993, 97, 9695. (8) Chezeau, J. M.; Delmotte, L.; Guth, J. L.; Gabelica, Z. Zeolites 1991, 11, 598. (9) Kubelka, P.; Munck, F. Z. Tech. Phys. 1931, 12, 593.
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