Flexible Structure of a Thermally Stable Hybrid Aluminosilicate Built

Mar 14, 2014 - Eni s.p.a., Refining & Marketing Division, San Donato Milanese Research Center, via F. Maritano 26, I-20097 San Donato Milanese, Italy...
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Flexible Structure of a Thermally Stable Hybrid Aluminosilicate Built with Only the Three-Ring Unit Michela Bellettato,*,† Lucia Bonoldi,† Giuseppe Cruciani,‡ Cristina Flego,† Stefania Guidetti,† Roberto Millini,† Erica Montanari,† Wallace O’Neil Parker, Jr.,† and Stefano Zanardi*,† †

Eni s.p.a., Refining & Marketing Division, San Donato Milanese Research Center, via F. Maritano 26, I-20097 San Donato Milanese, Italy ‡ Department of Physics and Earth Sciences, University of Ferrara, Via G. Saragat 1, I-44100 Ferrara, Italy S Supporting Information *

ABSTRACT: Organic−inorganic aluminosilicate hybrids are an attractive new class of materials that add organic functionalities to conventional properties of solid inorganic catalysts. ECS-17, a novel crystalline hybrid, was synthesized using 1,4-bis-(triethoxysilyl)-benzene as the sole silicon source. Its structure was solved by direct methods starting from high-resolution synchrotron X-ray diffraction data and is composed of inorganic layers, characterized by 10 rings, held together by phenylene rings. ECS-17 is the first aluminosilicate built from only the three-ring secondary building unit. This new material shows intriguing reversible collapsibility upon dehydration/rehydration. Mild thermal treatment under vacuum causes its crystalline structure to collapse due to facile elimination of the water molecules around the cations. Successive exposure to ambient atmospheric moisture gives back the hydrated crystalline form. ECS-17 shows remarkably high thermal stability for a hybrid, being stable up to 450 °C under vacuum and breaking down at 350 °C in air. Structural, thermal, and optical properties were examined by X-ray powder diffraction, thermogravimetric analysis, nuclear magnetic resonance, and ultraviolet−visible-near-infrared reflectance and fluorescence spectroscopies.

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four rings composing the inorganic layers of ECS-3.4 ECS-14 resembles the AFI topology, and the SBU type-12 (or -6 or -4) constructs its inorganic part.5 This structural feature (i.e., alternation of organic and inorganic layers) finds similarity in the new hierarchical hybrid organic−inorganic materials obtained by post-synthesis treatment of conventional zeolites. In particular, it was shown that pillaring of the so-called 2-D zeolites (i.e., single layer of zeolite) with bridged silsesquioxanes produced very interesting materials. Two examples are reported so far. A hybrid material by pillaring MWW-type precursor with 1,4-bis-(triethoxysilyl)-benzene (BTEB) has been reported by Corma et al.6 Opanasenko et al. prepared a series of materials by pillaring layers of UTL-type zeolite with bridged silsesquioxanes of various complexity.7 The interest in hybrid materials composed of organic and inorganic units arises from the possibility of combining the advantages of both components, that is, mechanical, structural, and hydrothermal stability of the inorganic compound and the flexibility and functionality of the organic one, favorable for catalyzing consecutive or cascade reactions.

INTRODUCTION According to the updated web version of the database of zeolite framework types, the realm of zeolites covers 213 different framework topologies, all very fascinating and intriguing.1 Several framework topologies possess a crystal structure with high level of complexity; nevertheless, even the most complicated zeolite can be envisioned by the arrangement of finite or infinite building units. Finite units are secondary building units (SBUs) and composite building units (CBUs). Infinite units (or periodic building unit (PerBU)) can be layers or chains. Zeolite frameworks are built from one or more types of the 23 recognized SBUs. The 5−1 SBU is one of the most common. It is the sole unit in 19 framework topologies and a component in 25 topologies. The three-ring unit appears in only five framework topologies.1 Remarkably, only one of them (nitridophosphate-1, IZA code NPO) is made just from the three-ring unit.1,2 Zeolite SBUs have recently been recognized in crystalline hybrid aluminosilicate materials ECSs, a new family of crystalline materials synthesized using organically modified silanes as the sole silicon source.3 According to the structural determination of the different ECSs, their crystal structure can be described by the stacking of inorganic layers covalently bound to the organic moiety.3−5The inorganic layer of ECS-2 is made from the 1-4-1-type SBU.3 The 4=1 SBU is connected by © 2014 American Chemical Society

Received: January 16, 2014 Revised: March 13, 2014 Published: March 14, 2014 7458

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thermal treatment was detrimental for the scaffolding structure of ECSs. To overcome this problem, we took inspiration from the pioneering high-temperature ex situ X-ray diffraction experiments on zeolites by collecting data at room temperature on crystals that were dehydrated under vacuum, heated at a selected temperature, and subsequently sealed in glass capillaries.12,13 ECS-17 was then dehydrated under vacuum to remove water molecules at temperatures lower than those required at ambient pressure. To do this, borosilicate capillaries (0.5 mm i.d.) were filled with ECS-17 powder and treated under dynamic vacuum (1 × 10−3 mbar) overnight at 25, 70, 120, and 200 °C and at increasing temperatures up to 450 °C with 50 °C steps. After each vacuum-thermal treatment, the capillary was sealed with putty under a dry nitrogen atmosphere to prevent possible rehydration and examined. To verify the detrimental water effect on the ECS-17 crystal structure, capillaries were treated up to 350 °C in ambient with increasing temperature steps of 50 °C. X-ray diffraction data were collected using a PANalytical X’PERT PRO diffractometer equipped with a RTMS (real-time multiple strip) X-Celerator detector in Debye−Scherrer geometry. Patterns were collected in continuous mode over a 6 ≤ 2θ ≤ 90° angular region, with a scan speed of 0.008° s−1. Seven scans were carried out for each data collection to increase the data quality. At the end of the measurement, the diffraction data were rebinned with a step size of 0.0167° 2θ. Cu Kα radiation (λ = 1.54178 Å) was used. Rietveld refinements were carried out using the General Structure Analysis System (GSAS) package.14,15 The atomic coordinates obtained from the direct methods software EXPO200916 were used as a starting model for the crystal structure refinement of as-synthesized ECS-17. The refined structure obtained was, in turn, used as a starting model for refining the dehydrated structure. Geometric soft constraints were applied to tetrahedra [Si−O (1.61(2) Å), Al−O (1.70(2) Å), O−O (2.60(1), and 2.80(1) Å depending on the tetrahedrally centered atom, Si or Al, respectively)]. The constraint weighting factor was gradually decreased during the refinement to yield reasonable bond lengths. Atoms with the same structural role were constrained to have the same isotropic thermal displacement parameters, which were fixed during the refinements. A rigid body unit was used to model the aromatic ring allocation during refinement. Thermal analysis (TG-DTA-MS) was performed with a Seiko TG/DTA 6300 thermobalance, equipped with an alumina furnace. Data were collected from 30 to 950 °C, with a heating rate of 10 °C·min−1 with a steady flow of air (50 mL·min−1). The masses of the gases evolved were analyzed using an Agilent gas chromatograph (mod. 68501) and a mass analyzer (mod. 5975). SEM micrographs were collected on as-deposited powder with a field emission scanning electron microscope JEOL JSM7600F operating at 2 kV in gentle beam mode. Energy Dispersive Spectrometry analyses (SEM/EDS) were collected, instead, on embedded powder with the same microscope at 15 kV by the INCA EDS system of Oxford (Link Isis). An Agilent V-500 spectrometer was used to observe 1H (500 MHz), 13C (126 MHz), 23Na (132 MHz), and 27Al (130 MHz) of samples contained in 4 mm rotors spinning at 14 kHz. The rotor caps had holes that allowed mild dehydration under MAS (DUMAS). 13C spectra were collected with ramped-amplitude cross-polarization (2 ms contact time, 81.6 kHz 1H and 67.6 kHz 13C nutations), spinal decoupling, 1 s relaxation delay, and shifts referenced to adamantane (38.5 and 29.4 ppm). 27Al

The potential of hybrid materials as catalysts has recently been reviewed.8 In addition, hybrid organic−inorganic silicabased materials also show interesting optical properties.9 Aromatic ECS hybrids are optically active in absorption and emission5 and suitable for applications such as wavelength converters, gas-sensor devices, and antenna systems, where they could be employed as donors.10 While the presence of the organic component introduces interesting optical properties, it makes hybrid materials less thermally stable than their totally inorganic counterparts. This was clearly evidenced in our previous study of ECS-3, where structural breakdown began with one-sided Si−C bond rupture at 200 °C (in air) to form phenyl rings.11 Analogously to classical zeolites, the inorganic component is hydrophilic, and mild thermal treatment, well below the structural stability threshold, can cause dehydration with significant changes in the crystal structure of the material. These changes can deeply modify the framework, causing its collapse or even the breakdown of the crystalline structure. According to the most recent definition, collapsed zeolites retain some sorption properties and a recognizable X-ray diffraction pattern.12,13 This condition must be distinguished from structural breakdown, which causes complete amorphization or recrystallization.12,13 Obviously, while the collapse can be fully reversible (for example, after rehydration), structural breakdown is definitive. Our latest efforts to synthesize new ECSs have furnished a material referred to as ECS-17. Herein we present its synthesis, physicochemical characterization, and crystal structure, composed of only the three-ring SBU. In addition, a multidisciplinary study of its thermal behavior was performed, revealing changes in the structural and optical properties caused by dehydration. Surprisingly, dehydration under mild conditions collapsed the structure, giving a more compact one, which was then stable up to 450 °C under vacuum condition.

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EXPERIMENTAL METHODS Synthesis. ECS-17 synthesis involved the dissolution of 1.59 g of NaAlO2 and 1.73 g of KOH in 13.90 g of demineralized water at room temperature under vigorous stirring until the formation of a homogeneous solution. It was loaded into a stainless-steel autoclave, and 7.78 g of BTEB (JSI Silicone, purity >98%) were added. Hydrothermal treatment was performed at 100 °C for 7 days under autogenous pressure with rocking of the autoclave. After cooling to room temperature, a white solid was separated from the mother liquor by filtration, repeatedly washed with demineralized water, and dried overnight at 100 °C. Characterization. The high-resolution synchrotron X-ray powder diffraction data of as-synthesized ECS-17 were collected at room temperature on the BM1b beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble (F)). The beamline was set to deliver a wavelength of 0.50034 Å. The borosilicate glass capillary (1.0 mm i.d.), containing the ECS-17 sample, was spun during data collection to minimize preferred orientation phenomena. Data were collected in continuous mode over the range 1 ≤ 2θ ≤ 50° with increasing accumulation times for higher scattering angles and finally rebinned with a step size of 0.001° 2θ. Proton transfer from nearby water molecules to the phenylene bridging groups with the formation of Q3 Si and Si-phenyl groups was observed during our previous in situ hightemperature X-ray diffraction experiment on ECS-3.11 This brought us to conclude that the presence of water during the 7459

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spectra were collected with a 0.2 μs (12°) pulse, 1 s delay, and shifts referenced to aq. AlCl3 (0 ppm). A Bruker ASX-300 spectrometer was used to observe 29Si (59 MHz) spectra collected with a 3.8 μs (60°) pulse, 90 s delay, mlev16 1H decoupling, and shifts referenced to tetrakis(trimethylsilyl)silane (−9.8 and −135.2 ppm) for samples contained in 7 mm rotors spinning at 5 kHz and for 23Na spectra (79 MHz) collected with a 0.3 μs (10°) pulse, 1 s delay, and shifts referenced to aq. NaCl (0 ppm) for samples in airtight 4 mm rotors spinning at 12 kHz. Reflectance and fluorescence spectra were registered on thick layers (1 mm) of powdered samples in quartz cells provided with a stopcock for dehydration treatment under dynamic vacuum (10−3 mbar) at 20, 70, 120, 200, and 250 °C. UV−vis-NIR reflectance spectra were registered with a Perkin-Elmer Λ-750 S spectrophotometer equipped with a 60 mm diameter integrating sphere over the 200−2600 nm range with a resolution of 1 nm, slit of 1 nm, and scan speed of 120 nm/min. Fluorescence spectra in the UV−vis region were measured with a Perkin-Elmer LS50-B spectrofluorometer in front-face configuration at 30−60° excitation−emission geometry with a 10% transmission neutral filter between the sample and the detector to avoid detector saturation. Emission spectra were registered at the maximum excitation wavelength and corrected for the detector sensitivity using the correction file provided with the instrument. Excitation spectra were severely affected by self-absorption and were not considered.

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Figure 1. SEM micrographs showing the elongated prism crystals of ECS-17. Scale bar = 200 nm.

RESULTS AND DISCUSSION Crystal Structure of As-Synthesized ECS-17. The SEM micrograph of as-synthesized ECS-17 (Figure 1) showed crystals shaped as elongated prisms. A small fraction of poorly crystalline phase was also evident. EDS analysis of a single ECS17 crystal gave Si/Al and Na/K ratios equal to 2 and 3.6, respectively. The Si−C bond in ECS-17 was stable up to 300 °C by TGDTA-MS analysis under programmed heating in air (Figure S1 in the Supporting Information). Above this temperature, a 17.6% weight loss occurred with a maximum at 566 °C in the DTG curve. This feature is associated with structural breakdown due to combustion and removal of the phenylene moieties, as verified in the outlet gas phase by mass spectrometer. The atomic masses 18, 32, 44, and 78 amu were monitored (Figure S1 in the Supporting Information) and assigned to: (i) combustion of the organic moiety (amu 18 = H2O and 44 = CO2) with consumption of molecular oxygen (amu 32) and (ii) elimination of integral benzene molecules (amu 78). The crystal structure degradation of ECS-17 started around 330 °C (Figure S1 in the Supporting Information), when the amu 78 was initially detected. This agrees with our previously finding on the thermal behavior of ECS-3.11 The weight loss at higher temperature associated with detection of the amu 44 is most likely due to decomposition of previously formed Na/K carbonates. The total weight loss, including that associated with “zeolitic” water molecules (i.e., 12.1 wt % with maxima in the DTG at 83 and 125 °C), was equal to 29.8 wt %. The quality of the laboratory X-ray powder diffraction pattern (Figure 2) facilitated the indexing procedure, and a monoclinic unit cell with parameters a = 10.831, b = 10.493, c = 10.603 Å, and β = 109.9° was readily obtained. The spacegroup search routine implemented in EXPO200916 suggested P21/a as the most probable one. The amount of Na, K, Si, Al,

Figure 2. Comparison of powder diffraction patterns for ECS-17: assynthesized (black line) and after drying under vacuum at 25 (red line), 70 (blue line), and 450 °C (pink line). Re-equilibration under ambient humidity after drying at 450 °C under vacuum (orange line) restored the as-synthesized pattern. Structural breakdown occurred after heating in air at 350 °C (green line).

and water molecules was determined by EDS and TG analyses, while the C content was deduced considering the BTEB formula. The chemical formula of the unit cell obtained (Na0.86K3.14[Si8Al4O20(C6H4)4]·8.4H2O) was used into the direct methods software EXPO2009 input file.16 These data were introduced, along with the chemical composition (from EDS and TG analyses Na0.86K3.14[Si8Al4O20(C6H4)4]·8.4H2O) and the amount of carbon (calculated based on BTEB content), into the direct methods software EXPO2009 input file.16 Individual reflection 7460

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Table 1. Structural and Refinement Parameters for ECS-17 sample

as-synthesized

X-ray source λ (Å) crystal system space group a (Å) b (Å) c (Å) beta (deg) V (Å3) RF2 (%) Rp (%) Rwp (%) no. observations no. reflections no. parameters no. of geometric restrains

synchrotron (BM1B − ESRF) 0.550034

under vacuum room temperature

under vacuum 120 °C

PANalytical X’pert PRO 1.54178 monoclinic P21/a 10.7127(9) 10.0828(9) 10.7839(13) 110.451(6) 1091.4(2) 5.0 4.2 5.8 5049 2594 72 24

10.8310(3) 10.4782(4) 10.6059(4) 109.924(2) 1131.6(1) 9.5 6.5 8.2 9263 2298 88 30

intensities were extracted from the high-resolution synchrotron diffraction data. Several attempts with recycling of the Fourier maps were necessary to obtain the structural model of the inorganic layer (including the tetrahedral apical carbon). The phenylene group was completed using the proper tool implemented in EXPO2009.16 This structure was used as a trial model for the Rietveld refinement, which converged to satisfactory discrepancy factors (Table 1) and close agreement between experimental and computed powder patterns (Figure S2 in the Supporting Information). The ECS-17 framework structure is reported in Figure 3. Similarly to the other ECSs it can be described by the stacking of inorganic layers covalently linked by the organic moiety. In the case of ECS-17, the stacking vector is along the c axis, whereas the inorganic layer lays on the ab plane. It can be built starting from the three-ring SBU only and contains a large tenring with minimum and maximum dimensions of 3.2 and 7.2 Å, respectively (Figures 4 and Figure S3 in the Supporting Information). Importantly, all but one of the Si−O−T angles of the ECS17 framework are in the range 125.1−135.4° (Si1−O3−Al = 167.0°) with a mean value of 137.2°. (See also Tables S1 and S2 in the Supporting Information.) These angles are small compared with classical zeolites and ECS-2 (a hybrid crystalline material with similar Si/Al molar ratio and mean Si−O−T angle = 155.4°)3 and are found in zeolitic structures containing the three-ring SBU (e.g., ZSM-18).1 Two crystallographically independent silicon sites exist in the structure. Their coordination spheres involve O2, O3, O5, and C1 in the case of Si1 and O1, O2, O4, and C6 in the case of Si2. Several extra-framework sites were localized by analysis of the Fourier maps. Two of them (X1 and X2) were attributed to cations, but the broad range of bond distances with the framework oxygen did not allow a clear allocation of sodium and potassium atoms that most likely occupy statistically both sites (Table 2). The potassium scattering curve was used for the extra-framework sites occupancies for all Rietveld refinements. 68 electrons were then refined, in line with those calculated by EDS analysis (i.e., 69 e−). The oxygen scattering curve was used during the refinement of the water molecules occupancy sites, which converged to a value slightly higher (i.e., 9.0 water

10.1307(9) 9.1486(10) 10.6002(11) 106.53(1) 941.9(2) 5.4 4.5 6.1 4750 3300 53 24

Figure 3. Polyhedral representation of the as-synthesized ECS-17 scaffolding along the 010 direction (top) and 100 direction (bottom). AlO4 (blue) and SiO3C (green) tetrahedra are shown. Extraframework cations, water molecules, and hydrogen are not shown for clarity.

molecules per unit cell) than that obtained by TG analysis (8.4 water molecules per unit cell). Crystal Structure of Dehydrated ECS-17. Vacuum treatment at 25 °C modified the XRD pattern (Figure 2) without changing the unit cell symmetry and space group. However, a small contraction of the unit cell was observed by Rietveld refinement. This shrinkage was significant for the b unit-cell parameter, but because of the simultaneous expansion of parameter c, only a slight volume contraction (4%) was 7461

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decreased from 3.38(2) to 2.67(4) Å. This shift was more evident after drying under vacuum at higher temperatures. (See later.) More details of the Rietveld refinement are reported in Table 1 and Figure S4 in the Supporting Information. The XRD patterns of ECS-17 dehydrated under vacuum above 25 °C are totally equivalent (Figure 2); that is, the samples share the same crystal structure. Here we report only the results of the 120 °C refinement. (See also Table 1 and Figure S5 in the Supporting Information.) Although the 120 °C in-vacuum treatment caused dramatic changes in the XRD pattern of ECS-17, the resolution quality was high, witnessing largely preserved crystallinity (Figure 2). The unit-cell symmetry (monoclinic) and space group (i.e., P21/a) were unchanged. A significant contraction of the b parameter was evidenced by Rietveld refinement compared with the as-synthesized phase (Table 1), with a volume decrease of 16.8%. Again, the same refinement evidenced the complete loss of the water molecules. (NIR spectra will corroborate this; see the Optical and Vibrational Studies section.) As a result, cations interact more strongly with the framework oxygen atoms generating a dramatic rearrangement (collapse) of the inorganic part of the structure (Figures 4, Figures S3 and S6 in the Supporting Information). This leads to shrinkage of the ten-ring opening caused by inverse rotation of the three-ring SBU (Figure 4). In dehydrated ECS-17, the ten-ring-opening assumes an ellipsoidal shape with minimum and maximum dimensions 1.7 × 7.9 Å (Figures 4 and Figure S3 in the Supporting Information). The collapse of the ECS-17 structure is also reflected by the Si−O−T angles that are in the range 125−147°, with a mean value of 134.7°. In particular, the Si1−O3−Al angle dropped from 164.0 to 138.9° (Table S2 in the Supporting Information) after the inverse rotation of the three-ring SBU. Mild dehydration also varied the orientation of the phenylene rings. The aromatic rings changed their positions and orientation significantly, assuming a more coplanar conformation and decreasing their relative distance (Figure S7 in the Supporting Information). Although the uncertainty of the carbon position makes the allocation of the phenyl ring somewhat ambiguous, spectroscopic observations (see the Optical and Vibrational Studies section) also revealed variations in phenylene group orientations with dehydration. Concerning the extraframework cation sites, Rietveld refinement evidenced a dramatic decrease in X2 occupancy (to negligible value), while the X1 occupancy increased to nearly one (i.e., 76 electrons). This indicates a migration of the cations from X2 to X1 with dehydration. The shift of this site is probably due to the loss of coordinating H2O molecules and to the consequent migration to a position offering a better coordination with the framework oxygen atoms (Table 2 and Figure S3 in the Supporting Information). A reduction in cation

Figure 4. Top: Inorganic layer of as-synthesized ECS-17. Stick representation of the Si−T connection (red) evidences the three-ring SBU. Bottom: Inorganic layer of ECS-17 after collapse caused by drying at 120 °C under vacuum. Red arrows show the co-rotating movement (inversion) of the three-ring SBU causing shrinkage of the ten-ring.

observed (Table 1). After drying at 25 °C, the Si−O−T angles were roughly unchanged with a mean value of 137.4° (Table S2 in the Supporting Information). Although the framework was partially affected, the dehydration was extensive because the W5 site occupancy dropped to zero during the refinement. Again, compared with the completely hydrated structure, the occupancy of the remaining water molecules sites decreased, and 6.8 water molecules per unit cell were calculated by the refinement. The coordination sphere around the two cations changed sensibly (Table 2 and Figure S3 in the Supporting Information). In particular, compared with the as-synthesized sample, X2 was found shifted toward X1. In fact, the minimum distance between the two extra-framework cation sites Table 2. Selected Bond Distances for ECS-17 as-synthesized X1-O5 -O2 -O1 -W4 -W1 -W2 -W1

= = = = = = =

2.60(1) 2.93(1) 3.32(1) 2.70(3) 2.83(3) 3.08(5) 2.83(3)

under vacuum 120 °C

under vacuum room temperature

X2-O1 = 2.79(2) -O4 = 2.91(2) -W2 = 3.20(4)

X1-O1 -O2 -O5 -O1 -W1 7462

= = = = =

3.02(2) 2.83(2) 2.64(2) 3.30(2) 3.07(7)

X2-O1 -O3 -O5 -W4 -W3 -W3

= = = = = =

2.19(3) 2.73(3) 3.30(2) 2.29(8) 2.74(7) 3.12(6)

X1-O1 -O4 -O5 -O3

= = = =

2.789(13) 2.834(13) 2.865(14) 3.039(15)

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T3(2Al) silicon site that gave a signal at −73 ppm in the case of ECS-2.3 Because a T3(OAl) site in a noncrystalline hybrid gives a 29Si signal near −80 ppm and with a 5 ppm downfield shift for each OAl substitution (of OSi), we expect a shift near −70 ppm for a T3(2Al) silicon site, just as for ECS-2; see Table S1 in the Supporting Information.3 But the −65 ppm shift of ECS-17 is similar to the shifts of ECS-3 and ECS-14, which have T3(3Al) sites.3−5 This is because ECS-17, being exceptionally composed of only the three-ring SBU, has smaller Si−O−T angles than a “normal” crystalline aluminosilicate. It is possible to estimate how much lower the mean Si−O−T angle is for ECS-17. The 29Si signal of ECS-17 (−63 ppm) is 10 ppm downfield from the signal for ECS-2 (−73 ppm), both with Si/Al = 2. Using the simple relation of Thomas et al.,24 this corresponds to a 17° decrease in the mean T−O−T angle. The NMR estimate agrees well with the difference in mean Si− O−T angles calculated from the XRD structures, 137° for ECS17 and 155° for ECS-2 (Table S1 in the Supporting Information). Most of the aluminum sites (95%) in as-synthesized ECS-17 are tetrahedral (AlO4), in accord with the crystal structure. The 27 Al NMR signal at 60 ppm was symmetrical without spinning sidebands. A few days of dehydration under MAS (DUMAS), through the hole in the Kel-F cap, caused a reduction in AlO4 site symmetry. Spinning at 14 kHz (25 °C) causes an effective sample temperature of ∼44 °C due to frictional force; this was calculated for a bearing gas temperature of 28 °C using the equation reported by Grimmer et al.25 The 27Al signal became asymmetric with a large spinning sideband manifold (Figure 6).

coordination below four can be regarded as a trigger for structural collapse.12 This “dragging effect”, known to zeolite crystallographer, was recently reviewed.12 Similar behavior has also been reported for other microporous materials such as octahedral-pentahedral-tetrahedral (OPT) framework silicates.17−23 It is especially noteworthy that the collapse of ECS-17 is completely reversible. Exposure to ambient humidity recovered the pattern of the as-synthesized form (Figure 2). NMR Spectroscopy. Multinuclear NMR spectroscopy complimented XRD studies by providing information on the local chemical environments of Si, Al, C, and Na sites. The 29Si MAS NMR spectrum of as-synthesized ECS-17 showed mainly two signals at −63 and −67 ppm, confirming T sites (CSiO3) and the intactness of the Si−C bond from the precursor (Figure 5).

Figure 5. 29Si MAS NMR spectrum of as-synthesized ECS-17.

These two silicon sites correspond to those differentiated in the crystal structure (Si1 and Si2). An underlying broad signal (at −63 ppm) was included to obtain an optimal spectral deconvolution because the large peaks were not perfectly Gaussian-shaped (Figure S8 in the Supporting Information). The three weak signals at −80.5, −85.2, and −86.1 ppm (Table 3) are attributed to Q3 (3Al), Q3 (2Al), and Q4 (4Al), Table 3. 29Si MAS NMR Data for As-Synthesized ECS-17 Obtained by Deconvolution of the Spectrum in Figure S8 in the Supporting Information 29

Si peak 1 2 3 4 5 6

shift (ppm) −62.8 −62.8 −67.4 −80.5 −85.2 −86.1

% area 27 22 29 3 2 17

line width (Hz) 70 500 100 90 50 750

site 3

T (2Al) T3 (2Al) T3 (2Al) Q3 (3Al) Q3 (2Al) Q4 (4Al)

Figure 6. 27Al MAS NMR spectra of ECS-17 as-synthesized (bottom) and after 4 days of DUMAS (middle), followed by rehydration over water vapor for 15 h (top).

respectively. Considering the Si/Al molar ratio of ECS-17, we can reasonably suppose that: (i) the first and the latter silicon sites are related to the minor impurity phase observed by SEM and XRD and (ii) the second can be attributed to silanol sites located on the ECS-17 crystal surface. Nine days of DUMAS (5 kHz) did not significantly affect the 29 Si MAS spectrum, reflecting minor changes in the mean Si− O−T angles with dehydration. These 29Si shifts for a CSiO3 site near −65 ppm are unexpected based on the additive aluminum substituent effect known for aluminosilicates. The aluminum content (Si/Al = 2) and X-ray structure of ECS-17 require a

Progressive loss of physisorbed water with DUMAS was confirmed by 1H MAS NMR analysis (Figure S9 in the Supporting Information). This signal disappeared after 2 h of 14 kHz DUMAS. Rehydration of the open rotor to 50% humidity overnight completely reversed the reduction in AlO4 coordination symmetry (Figure 6). However, this rehydration did not restore the sharp 1H signal for physisorbed water near 4.8 ppm (Figure S9 in the Supporting Information). Additional 7463

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drying with most severe conditions (e.g., 70 °C DUMAS) caused no further changes in the 27Al spectrum (not shown). Reversible spectroscopic changes on dehydration−hydration were also found for 13C. As-synthesized ECS-17 exhibited two resonances located at 134 and 138 ppm (Figure 7). 13C cross-

caused both signals to broaden, indicating enhanced interaction with framework oxygens. Optical and Vibrational Studies. Absorption and emission features of ECS-17 are fundamentally due to the strong optical properties of the aromatic groups. Comparison of the spectra for ECS-17 (solid state) and the BTEB precursor (diluted in cyclohexane ≈ 10−5 M) reveals the characteristic (α, p, β) bands of the benzene ring (Figure 8).28

Figure 8. Optical spectra of ECS-17 (continuous line) and BTEB in diluted cyclohexane solution (dotted line). Blue: absorption, red: fluorescence emission. Absorption spectrum of BTEB is reported in Absorbance and that of ECS-17 in Kubelka−Munk function.

The lowest energy absorption band of the aromatic moiety occurs in the same position (268.5 nm) for the solid and diluted precursor, differing only in the vibrational structure due to restricted mobility. At higher energy, only one large band is present in the solid, with maximum at 227 nm, that is, redshifted with respect to the high-energy bands of the isolated species (peaked at 224 and 211 nm), pointing to different environment. In emission mode, the diluted cyclohexane solution of BTEB shows only the monomeric band around 292 nm, with two vibrational components at 289 and 294 nm. In ECS-17, despite dense ring packing, fluorescence is strong, as for ECS-2 and ECS-14 and other hybrids.5,10,9 In fact, a 10% transmission filter was employed during registration to avoid receiver saturation. The main emission component of ECS-17 is in the same position as that observed for the precursor in dilute solution, that is, isolated state. Only a weak shoulder is observed at ∼340 nm, in the emission wavelength region of excimers, as for ECS2, where rings are coupled in tight diads.5 In ECS-17, both the position and the intensity of the fluorescence indicate the lack of strong electronic interactions between aromatic rings despite their high spatial concentration, as described for ECS-14.5 Absorption spectra in the NIR region were collected to monitor changes in vibrational properties caused by the hydration state of ECS-17. These spectra are compared with the spectra for the pure BTEB precursor and a Na−Y zeolite (HSZ-320NAA Tosoh) in Figure 9. The vibrational absorption bands arise from: - silanols bonding water molecules, giving rise to a broad band near 6800 cm−1 (first overtone of OH groups) with two resolved components maxima at 6980 and 6740 cm−1, similarly to silanols in Y zeolite interacting with H2O or other OH groups - physisorbed water with a strong band centered at 5177 cm−1 - organic compounds giving a weak band at 5909 cm−1 due to the first overtone of C−H groups

Figure 7. 13C CP MAS NMR spectra of ECS-17 as-synthesized (bottom) and after 3 days of DUMAS (middle), followed by rehydration over water vapor for 15 h (top).

polarization dynamics revealed that the more downfield signal (138 ppm) is due to the carbon bound to the silicon atom. Overnight DUMAS shifted this carbon signal to 134 ppm, giving one signal. Equilibration over water vapor restored the 138 ppm signal. Two well-resolved 13C signals are observed for ECS-17 (Figure 7), as usual for phenylene ECS hybrids with aluminum,3,5 while purely siliceous (noncrystalline) phenylene hybrids (and BTEB in solution) show only one peak near 134 ppm. Without aluminum, the signal of the carbon bound to silicon is shifted upfield (shielded) and overlaps with the CH signal because of inductive electron donation from silicon. The electron donation is weak because the π system donates some electron density back to the vacant 3d orbitals of silicon.26 This “back-bonding” is very efficient in hydrated (or as-synthesized) ECS-17 and cancels the shielding effect from silicon giving two 13 C signals. Presumably a conformational change at the C−Si bond, by alignment with the nodal plane of the π system, enhances the hyperconjugative electron release. Dehydration changes this alignment and decreases back-bonding, allowing silicon to shield the attached carbon and give a 13C signal at 134 ppm (Figure 7). The 23Na MAS NMR spectrum of as-synthesized ECS-17 mainly showed (90%) a symmetrical peak near −11 ppm (Figure S10 in the Supporting Information), with a sharp signal near 0 ppm from the (amorphous) impurity phase. The major signal has the shift expected for a hydrated sodium cation in a zeolite and is symmetrical, revealing a weak interaction with framework oxygens.27 Dehydration at 70 °C under vacuum 7464

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Figure 10. Optical spectra of ECS-17 as-synthesized (blue), after dehydration overnight under vacuum at 25 °C (pink) and at 70 °C (red), and after re-equilibration under ambient humidity (purple). Continuous line = normalized absorption spectra reported in Kubelka−Munk function; dotted line = fluorescence emission at λexc = 270 nm.

Figure 9. Top: NIR spectra of ECS-17 as-synthesized (blue), after dehydration overnight under vacuum at 70 °C (red), and after reequilibration under ambient conditions (purple). Bottom: NIR spectra of the BTEB precursor (black, line-dot-dot) and Na−Y zeolite (black, dashed).

electronic absorption, and emission were restored as for the as-synthesized sample. ECS-17 Thermal Stability. Thermal stability is a critical issue for practical applications. ECS-17 treated under vacuum conditions demonstrated structural integrity and remarkable thermal stability up to 450 °C, maintaining a pattern typical of a crystalline material. Treatments from 120 to 300 °C under vacuum caused only minor variations in the NIR region with respect to the treatment at 70 °C. The absorption of isolated OH was slightly blue-shifted (maximum at 7236 cm−1 and one shoulder at 7283 cm−1), and the unidentified absorption around 5900 cm−1 was blue-shifted and more structured (Figure S11 in the Supporting Information). These changes are likely due to the elimination of water molecules bound to the cations and reveal two types of silanols, one being more polarized than the other. The thermal stability of ECS-17 strongly depends on the presence of water, similarly to ECS-3, where NMR studies revealed that structural breakdown begins with hydrolytic Si−C bond scission, rather than combustion of the organic moiety.11 In fact, the sample treated at 350 °C in air showed an XRD pattern with very poor quality that hardly resembles assynthesized ECS-17 (Figure 2), indicating the crystal structure breakdown. The 13C NMR analysis of the same sample revealed a signal at 129 ppm from a phenyl group, confirming Si−C bond cleavage (Figure S12 in the Supporting Information). Furthermore, while no variations were observed in optical emission on heating under vacuum (Figure 11), treatment in air (with humidity) for 2 h at 350 °C caused a significant increase in the excimeric component (the only one observed in noncrystalline hybrids7) and broadened the signals of both OH and benzene components (Figure S11 in the Supporting Information), in agreement with the decrease in crystallinity found by XRD.

- organic (with C−H and C−C combination) and inorganic (with O−H overtone) components in the complex 4200−4700 cm−1 region, with peaks at 4517 and 4416 cm−1 In Figure 9, the effect of dehydration/rehydration is reported as well, showing that treatment overnight under vacuum (10−3 mbar) at 70 °C effectively dehydrated the sample, removing the signal at 5177 cm−1 due to physisorbed water. The OH signals sharpened and shifted to higher energy (new maximum at 7193 cm−1) in the position attributed to isolated silanols in inorganic matrixes and in hydrophobic BTEB (i.e., silanols in BTEB and dehydrated ECS-17 experience similar environments). Note that under the same conditions Rietveld analysis found complete dehydration of the cation. The benzene absorptions around 5900 and 4500 cm−1 were not affected by dehydration. Concerning the electronic transitions, both absorption and emission are red-shifted with dehydration under vacuum over the 25−70 °C range. Absorption positions shift from 227 and 268.5 nm to 231 and 273.5 nm. In emission, the fluorescence intensity of monomeric species shifts from 292 nm to 298.7 and decreases in intensity, while the intensity of the shoulder at 340 nm due to excimers increases somewhat (Figure 10). These evidences points to an increased interaction among aromatic units in the dehydrated material. This could result from one or more factors: (i) changes in the ring’s chemical environment; (ii) reduced distance between rings; and (iii) increased coplanarity between rings. This finding is in agreement with the structural changes. (See the Crystal Structure of Dehydrated ECS-17 section and Figure S7 in the Supporting Information.) Optical and vibrational properties confirmed that physisorbed water is easily removed from ECS-17 in agreement with XRD and NMR data. The effects of rehydration, under ambient conditions, on the vibrational and electronic properties are reported in Figures 9 and 10. The OH absorption, physisorbed water signal,

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CONCLUSIONS ECS-17 is a new crystalline hybrid organic−inorganic aluminosilicate and the first one built from only three-ring SBU. Its inorganic layers are held together by organic moieties exhibiting a “flexible crystalline” structure. Reversible collapse of the ECS-17 framework with dehydration was evidenced by 7465

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the assistance during the under-vacuum dehydration experiments. Special thanks to the European Synchrotron Radiation Facility (ESRF) for providing access to BM01b under the public beamtime program.

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(1) Baerlocher, Ch.; McCusker, L. B. Database of Zeolite Structure. http://www.iza-structure.org/databases/. (2) Correll, S.; Oeckler, O.; Stock, N.; Schnick, W. LixH12−x−y +z[P12OyN24−y]ClzAn Oxonitridophosphate with a Zeolitelike Framework Structure Composed of 3-Rings. Angew. Chem., Int. Ed. 2003, 42, 3549−3552. (3) Bellussi, G.; Carati, A.; Di Paola, E.; Millini, R.; Parker, W. O., Jr.; Rizzo, C.; Zanardi, S. Crystalline Hybrid Organic−Inorganic AluminoSilicates. Microporous Mesoporous Mater. 2008, 113, 252−260. (4) Bellussi, G.; Montanari, E.; Di Paola, E.; Millini, R.; Carati, A.; Rizzo, C.; Parker, W. O., Jr.; Gemmi, M.; Mugnaioli, E.; Kolb, U.; et al. ECS-3: A Crystalline Hybrid Organic−Inorganic Aluminosilicate with Open Porosity. Angew. Chem., Int. Ed. 2012, 51, 690−693. (5) Bellussi, G.; Millini, R.; Montanari, E.; Carati, A.; Rizzo, C.; Parker, W. O., Jr.; Cruciani, G.; De Angelis, A.; Bonoldi, L.; Zanardi, S. A Highly Crystalline Microporous Hybrid Organic−Inorganic Aluminosilicate Resembling the AFI-type Zeolite. Chem. Commun. 2012, 48, 7356−7358. (6) Corma, A.; Dıaz, U.; Garcıa, T.; Sastre, G.; Velty, A. Multifunctional Hybrid Organic−Inorganic Catalytic Materials with a Hierarchical System of Well-Defined Micro- and Mesopores. J. Am. Chem. Soc. 2010, 132, 15011−15021. (7) Opanasenko, M.; Parker, W. O., Jr.; Shamzhy, M.; Montanari, E.; Bellettato, M.; Mazur, M.; Millini, R.; Č ejka, J. Hierarchical Hybrid Organic−Inorganic Materials with Tunable Textural Properties Obtained Using Zeolitic-Layered Precursor. J. Am. Chem. Soc. 2014, 136, 2511−2519. (8) Díaz, U.; Brunel, D.; Corma, A. Catalysis Using Multifunctional Organosiliceous Hybrid Materials. Chem. Soc. Rev. 2013, 42, 4083− 4097. (9) Tani, T.; Mizoshita, N.; Inagaki, S. Luminescent Periodic Mesoporous Organosilicas. J. Mater. Chem. 2009, 19, 4451−4456. (10) Bellussi, G.; Bonoldi, L.; Carati, A.; Montanari, L.; Rizzo, C.; Zanardi S.; Fusco, R. Wavelength Converter. Italian Patent MI2010A001926, October 21, 2010. (11) Zanardi, S.; Parker, W. O., Jr.; Carati, A.; Botti, G.; Montanari, E. On the Thermal Behaviour of the Crystalline Hybrid Organic− Inorganic Aluminosilicate ECS-3. Microporous Mesoporous Mater. 2013, 172, 200−205. (12) Cruciani, G. Zeolites Upon Heating: Factors Governing Their Thermal Stability and Structural Changes. J. Phys. Chem. Solids 2006, 67, 1973−1994. (13) Bish, D. L.; Carey, J. W. Thermal Properties of Natural Zeolites. In Reviews in Mineralogy and Geochemistry; Bish, D. L., Ming, D. W., Eds.; MSA: Washington D.C., 2001; Vol. 45, pp 403−452. (14) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS). Los Alamos National Laboratory Report LAUR 86− 748, 1994. (15) Toby, B. H. EXPGUI, a Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210−213. (16) Altomare, A.; Camalli, M.; Cuocci, C.; Giacovazzo, C.; Moliterni, A. G. G.; Rizzi, R. EXPO2009: Structure Solution by Powder Data in Direct and Reciprocal Space. J. Appl. Crystallogr. 2009, 42, 1197−1202. (17) Thorogood, G. J.; Kennedy, B. J.; Griffith, C. S.; Elcombe, M. M.; Avdeev, M.; Hanna, J. V.; Thorogood, S. K.; Luca, V. Structure and Phase Transformations in the Titanosilicate, Sitinakite. The Importance of Water. Chem. Mater. 2010, 22, 4222−4231. (18) Danisi, R. M.; Armbruster, T.; Lazic, B. In Situ Dehydration Behavior of Zeolite-Like Pentagonite: A Single-Crystal X-ray Study. J. Solid State Chem. 2013, 197, 508−516.

Figure 11. Emission spectra (normalized intensities) of ECS-17 assynthesized (red), after dehydration under vacuum (up to 250 °C), and in air at 350 °C (orange).

multidisciplinary analyses. Drying under vacuum, or simply dehydration with MAS, caused the structural collapse triggered by the loss of hydration water molecules around the cations. Rehydration completely reversed the collapse. Equating the hybrid structure of ECS-17 to an inorganic zeolite and following the classification schemes of the zeolite thermal behavior,12,13 we can conclude that the ECS-17 framework is “collapsible” following the Baur classification.29 Because the collapse is fully reversible, it is classified as category 2 (completely reversible dehydration accompanied by a large framework distortion and decrease in unit cell volume) according to Alberti and Vezzalini.30 The optical properties of “collapsed” ECS-17 differed from the as-synthesized form by red shifts in both absorption and emission spectra. Partial quenching of monomeric emission fluorescence agrees with the increased ring interaction for the dehydrated structure. Finally, the thermal stability of ECS-17 was evaluated in air (with ambient humidity) and under vacuum. In air, ECS-17 broke down structurally at 350 °C. Si−C bond cleavage initiated this breakdown, supporting a mechanism involving proton transfer from nearby water molecules to the phenylene bridging group with the formation of Q3 Si sites and Si-phenyl groups.11 ECS-17 possesses outstanding thermal stability when treated under vacuum. It remains crystalline up to 450 °C, a remarkable temperature considering its hybrid nature.

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ASSOCIATED CONTENT

S Supporting Information *

Tables on Si−O−T angles and 29Si MAS NMR sites assignments, thermal curves along with the individual mass patterns, final Rietveld plots, NMR and NIR spectra, pictures of the crystal structure of ECS-17 at different degrees of hydration, and crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*M.B.: Tel: +3952046640. E-mail: [email protected]. *S.Z.: Tel: +3952046519. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Massimo Nalli for the thermal analysis, Mr. Antonio Belloni for the synthesis, and Mr. Giuseppe Botti for 7466

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(19) Döbelin, N.; Armbruster, T. Microporous titanosilicate AM-2: Ion-exchange and Thermal Stability. Microporous Mesoporous Mater. 2007, 99, 279−287. (20) Döbelin, N.; Armbruster, T. Microporous Titanosilicate AM-2: Rb-exchange and Thermal Behaviour. Mater. Res. Bull. 2007, 42, 113− 125. (21) Zanardi, S.; Dalconi, M. C.; Gambaro, C.; Bellussi, G.; Millini, R.; Rizzo, C.; Carati, A. Investigation on the Hydrated and Dehydrated Forms of the Ion-exchanged Microporous Stannosilicate EMS-2. Microporous Mesoporous Mater. 2009, 117, 414−422. (22) Merlino, S.; Bonaccorsi, E.; Armbruster, T. The Real Structure of Tobermorite 11Å: Normal and Anomalous Forms, OD Character and Polytypic Modifications. Eur. J. Mineral. 2001, 13, 577−590. (23) Zanardi, S.; Bellussi, G.; Carati, A.; Cruciani, G.; Millini, R.; Rizzo, C. EMS-6, a Novel Microporous Gadoliniumsilicate With Monteregianite Structure: Synthesis, Crystal Structure and Thermal Behavior. Microporous Mesoporous Mater. 2010, 134, 115−123. (24) Thomas, J. M.; Klinowski, J.; Ramdas, S.; Hunter, B. K.; Tennakoon, D. T. B. The Evaluation of Non-equivalent Tetrahedral Sites From 29Si NMR Chemical Shifts in Zeolites and Related Aluminosilicates. Chem. Phys. Lett. 1983, 102, 158−162. (25) Grimmer, A. R.; Kretschmer, A.; Cajipe, V. B. Influence of Magic Angle Spinning on Sample Temperature. Magn. Reson. Chem. 1997, 35, 86−90. (26) Freeburger, M. E.; Spialter, L. Physical Organosilicon Chemistry. I. Nuclear Magnetic Resonance Studies of o-, m-, and psubstituted Phenyltrimethylsilanes. Evidence Bearing on the Existence of (p→d)π. “back-bonding” in Phenylsilanes. J. Am. Chem. Soc. 1971, 93, 1894−1898. (27) Klein, H.; Fuess, H.; Hunger, M. Cation Location and Migration in Lanthanum-Exchanged Zeolite NaY Studied by X-ray Powder Diffraction and MAS NMR Spectroscopy. J. Chem. Soc. Faraday Trans. 1995, 91, 1813−1824. (28) Suzuki, H. Electronic Absorption Spectra and Geometry of Organic Molecules; Academic: New York, 1967; pp 196−198. (29) Baur, W. H. Self-limiting Distortion by Antirotating Hinges is the Principle of Flexible but Noncollapsible Frameworks. J. Solid State Chem. 1992, 97, 243−247. (30) Alberti, A.; Vezzalini, G. Topological Changes in Dehydrated Zeolites: Breaking of T-O-T Oxygen Bridges. In Proceedings of the Sixth International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworth: Guildford, U.K., 1984; pp 834−841.

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