On the thermal behaviour of the crystalline hybrid organic–inorganic aluminosilicate ECS-3

Microporous and Mesoporous Materials 172 (2013) 200–205

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On the thermal behaviour of the crystalline hybrid organic–inorganic aluminosilicate ECS-3 Stefano Zanardi ⇑, Wallace O’Neil Parker Jr., Angela Carati, Giuseppe Botti, Erica Montanari eni S.p.A. – Refining & Marketing Division, San Donato Milanese Research Center, Via F. Maritano 26, I-20097 San Donato Milanese, MI, Italy

a r t i c l e

i n f o

Article history: Received 23 October 2012 Received in revised form 22 January 2013 Accepted 23 January 2013 Available online 1 February 2013 Keywords: Hybrid materials ECS Thermal behaviour Proton transfer

a b s t r a c t The thermal behaviour of the crystalline hybrid organic–inorganic aluminosilicate ECS-3 was studied by in situ X-ray diffraction, thermal analysis, nitrogen adsorption and NMR spectroscopy. In situ X-ray diffraction demonstrated that ECS-3 is thermally stable up to 150 °C where a significant fraction of water molecules (37%) are still retained inside the pores. After heating at 200 °C in air, X-ray-diffraction data evidenced structural collapse, nitrogen adsorption was drastically reduced and 13C and 29Si MAS NMR studies confirmed single-sided Si–C bond rupture. Collective observations indicate that nearly 20% of the phenylene bridges were converted to phenyl groups while a corresponding amount of silicon T sites became Q. Structural collapse did not occur at 200 °C under vacuum conditions, supporting a mechanism where water promotes Si–C cleavage by initiating proton transfer to the phenylene. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction ECS (Eni Carbon Silicates) is a new class of crystalline hybrid organic–inorganic aluminosilicates recently synthesized at eni [1], whose properties are beginning to be investigated. The novelty of these materials concerns their structures, crystalline aluminosilicate scaffoldings with long-range 3D order, which distinguishes them from the previously reported amorphous and ‘‘crystallinelike’’ silica-based PMOs (periodic mesoporous organosilicas) [2]. Among the different materials belonging to the ECS family, ECS2, ECS-3 and ECS-14 were synthesized using 1,4-bis-(triethoxysilyl)-benzene (BTEB) as silica source and their crystal structures were determined by advanced techniques [1,3,4]. They have different microporosities: ECS-2 [1] has closed cages, ECS-3 has open sinusoidal channels with large lateral side pockets [3] and ECS14 has open straight 12-ring channels [4]. ECS-3 attracted our attention because of the N2 adsorption/ desorption type I isotherm, typical of microporous materials (specific surface area = 296 m2 g 1, specific pore volume = 0.13 cm3 g 1). The crystal structure of ECS-3 was recently solved by Automatic Diffraction Tomography [3]. It is monoclinic with space group Cc and unit cell parameters: a = 19.7678(5), b = 28.3299(6) and c = 9.7622(2) Å, b = 104.18(1)°. High resolution synchrotron X-ray powder diffraction data were required to complete and refine its crystal structure. The 3D scaffolding structure of ECS-3 can be described by the regular alternation of organic–inorganic layers stacked along the [1 0 0] crystallographic direction (Fig. 1). ⇑ Corresponding author. E-mail address: [email protected] (S. Zanardi). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.01.029

The inorganic layers, covalently connected by phenylene rings, are characterized by small eight-membered ring windows and can be built by a combination of SBU’s 4=1 along with 4 ring units [3]. The cation sites of ECS-3 are localized mainly close to the eightrings of the inorganic layer and showed a broad range of cationoxygen bond distances hampering the univocal allocation of Na and K cations [3]. A larger porosity exists along the [0 0 1] direction (Fig. 1), where elliptical rings are composed of 10 tetrahedra and 2 phenylene rings. This is the pore mouth of a sinusoidal channel that snakes in the [0 0 1] crystallographic direction. However, the accessibility of these channels to adsorbates is limited by the steric hindrance imposed by the phenylene rings. Obviously, the high organic content of ECS’s makes them less thermally stable than classical inorganic zeolites. As a consequence, analogously to other microporous materials such as zeolites [5–8], understanding the thermal behaviour of ECS’s structure is of crucial importance for applying them as catalysts or for producing thin films for applications in gas sensing and light harvesting [9–13]. In fact, the unstable organic moieties can lead to structural collapse or breakdown which compromises shape-selective catalytic processes. In addition, the low Si/Al molar ratios of ECS’s, with the required high amounts of extra-framework cations [1,3,4], could compromise the structural stability. A parallel can be made with zeolites, where a high Si/Al molar ratio affords greater thermally stable than their low-silica counterparts [5]. In this context, zeolite beta is one of the most representative examples. Commercial zeolite beta (Si/Al  13) is reportedly stable up to 750 °C with structural breakdown at 850 °C [14], while its monoclinicdominant natural counterpart tschernichite (Si/Al  2.5) under-

S. Zanardi et al. / Microporous and Mesoporous Materials 172 (2013) 200–205

Fig. 1. Polyhedral representation of the ECS-3 scaffolding along the 001 direction. AlO4 tetrahedra are in white whereas SiO3C are dark grey. Extra-framework cations, water molecules and hydrogen are not shown for clarity.

goes structural breakdown with complete amorphitization at 350 °C [15]. Previous thermogravimetric (TG) analyses of ECS-3 showed weight losses likely associated with breaking/burning of the organic groups above 350 °C [3]. This work focuses on verifying the temperature limits for ECS-3’s structural integrity, and if this limit would provide a fully dehydrated form useful for accurately defining cation positions. To this end, in situ variable temperature X-ray powder diffraction experiments were performed. The results of Rietveld analysis were used to evidence the structural modifications occurring. Thermal analysis and multi-nuclear NMR measurements provided molecular details regarding the structural changes. 2. Experimental 2.1. Synthesis ECS-3 was synthesized as follows: 2.97 g of NaAlO2 and 0.71 g of KOH were dissolved in 13.03 g of demineralized water at room temperature under vigorous stirring until a homogeneous gel was formed. The gel was charged into a stainless-steel autoclave and 7.29 g of 1,4-bis-(triethoxysilyl)-benzene (BTEB, JSI Silicone, purity >98%) were added. Hydrothermal treatment was performed at 100 °C for 4 days under autogeneous 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 demineralised water and dried overnight at 100 °C. 2.2. Characterization The in situ high-temperature X-ray powder diffraction (XRD) data collection were performed in air using a PANalytical X’PERT PRO diffractometer equipped with a RTMS (real-time multiple strip) X-Celerator detector. The sample was loaded in an alumina crucible, inserted in an Anton Paar HTK 1200 resistance heating chamber. Data were collected in continuous mode over 3° 6 2h 6 90° angular region, with a scan speed of 0.0163° 2h s 1. Four scans were carried out for each data collection in order to increase the data quality. At the end of each measurement, the diffraction data were re-binned with a step size of 0.0167° 2h. CuKa radiation (k = 1.54178 Å) was used. Seven XRD experiments were carried out at temperatures pre-selected from TG analysis (25, 50, 75, 100, 125, 150 and 200 °C). The heating rate was 2 °C min 1. Prior to each data collection, the sample was maintained at the selected temperature for 3 h to approach its thermodynamic equilibrium.

201

Rietveld refinements were carried out using the General Structure Analysis System (GSAS) package [16,17]. The previously reported ECS-3 atoms coordinates [3] were used as a starting model for the room temperature crystal structure refinement. The refined structure obtained was, in turn, used as a starting model for refining the structure obtained at 50 °C and so on, using each refined structure as a trial model for the subsequent higher temperature. Geometric soft constraints were applied to the 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 centred atom, Si or Al, respectively)]. The constraints weighting factor was gradually decreased during the refinement to yield reasonable bond lengths. Because of ECS-3’s complex structure, high correlation among parameters was expected. Thus, framework atoms of the same element were constrained to have the same isotropic thermal displacement parameter, which was fixed during the refinement. Throughout the refinement, rigid body units were used to model aromatic ring allocation. The previous study of ECS-3 evidenced that sodium and potassium allocations were hampered by the broad range of refined bond distances between extra-framework cations and framework oxygens [3]. Furthermore, it was possible to localize only four, fully occupied, cation sites. As a consequence, at least two (fully occupied) sites were missed. To account for this disordered distribution of cations, the following four assumption were made: (i) the scattering curve of sodium was used for the occupancies of the four extra-framework sites; (ii) sodium and potassium were assumed equally distributed; (iii) the extra-framework cations were allocated in six crystallographically (fully occupied) independent sites; (iv) their occupancies were set equal to 1.15 (using the Na scattering curve), according to the Na/K molar ratio obtained by the EDS analysis, and fixed during the refinements. TG-DTA-MS analysis was performed with a Seiko TG/DTA 6300 thermobalance, equipped with an alumina furnace capable of reaching 1300 °C. Data were collected from 25 to 900 °C, with a heating rate of 10 °C min 1. The measurement was carried out using ca. 17 mg of sample, housed in an alumina crucible placed in the centre of the furnace, with a steady flow of air (50 ml min 1). To detect the masses of the flue gas it was analysed using an Agilent gas chromatograph (mod. 68,501) and a mass analyser (mod. 5975). SEM micrographs were collected with a field emission scanning electron microscope JEOL JSM-7600 F operating at 2 kV. Energy dispersive spectrometry analyses (SEM/EDS) were collected with the SEM microscope JEOL 840 A working at 20 kV equipped with the EDS system of Oxford (Link Isis). MAS NMR spectra were collected using an Agilent V-500 for 13C (126 MHz, 3.8 ls = 90° pulse with DEPTH filter [18] consisting of a composite (90°–180°–180°) excitation pulse with 16-step phase cycling (onepuldpth pulse program) to suppress the background signal from the Kel-F spacers, 30 s delay, spinal 1H decoupling, shifts referenced to tetramethylsilane at 0 ppm using adamantane at 38.5 and 29.4 ppm) and 27Al (130 MHz, 0.35 ls = 12°, 1 s delay, shifts referenced to aq. AlCl3 at 0 ppm) for samples contained in 4 mm rotors spinning at 14 kHz. The 13C spectra were quantitative since the longitudinal relaxation times were 3–4 s. A Bruker ASX-300 was used to observe 29Si (59 MHz, 3.8 ls = 60° pulse, 90 s delay, mlev16 1 H decoupling, shifts referenced to tetramethylsilane at 0 ppm using tetrakis(trimethylsilyl)silane at 9.8 and 135.2 ppm) for samples contained in 7 mm rotors spinning at 5 kHz. N2 adsorption/desorption isotherms were obtained at 196 °C with an ASAP 2020.Three isotherm acquisitions were carried out: in the first two cases the samples (as-synthesized and heated at 200°C in air) were out-gassed overnight at 120 °C under vacuum, for the third acquisition the out-gassing procedure involved a thermal treatment at 120 °C overnight followed by another one at 200 °C, both under vacuum.

S. Zanardi et al. / Microporous and Mesoporous Materials 172 (2013) 200–205

100

100

3500

50 40

90

20 10

85

0

2700 2300

-10

80

Weight %

3100

30

90

DTA [uV]

Weight %

95

-20

85

75

-30 30

130

230

1900

330

Temperature [°C]

1500

80

1100

75

700 70

300

65

-100 30

130

230

330

430 530 630 Temperature [°C]

730

830

930

800 Abundance amu 18, 44, 78 [a.u.]

The present ECS-3 synthesis involved more potassium than the previous recipe [3]. This new synthesis route furnished the title compound in just 4 days of crystallization time, instead of 14 days in the original method [3]. The well crystallized material showed an ellipsoidal morphology resembling grains of rice (Fig. 2). EDS analysis gave a Na/K ratio of 3.7, that must be compared to 8.5 reported for the reference material [3]. This greater potassium content apparently affected the porosity, since a type I nitrogen adsorption/desorption isotherm was observed (Fig. 3) with the specific surface area of 246 m2/g being smaller than previously (i.e. 296 m2/g). TG-MS analysis of ECS-3 showed three different weight loss events (Fig. 4). In the DTA curve, the first weight loss (15.8 wt.%) was accompanied by a slightly endothermic very broad peak, associated with the elimination of water molecules from the ECS-3 porosity, as clearly indicated by the masses detected during heating (Fig. 4). With this, and the content of Na and K obtained by EDS analysis the ECS-3 unit cell composition was calculated as: Na18.9K5.1Si32Al24O96C96
45H2O. The second weight loss was accompanied by two exothermic peaks, near 360 and 430 °C, the latter being sharp and very strong. These features are associated with atomic masses of 18, 44 and 78 (Fig. 4 and Supplementary

95

DTG [uG/min] - DTA [uV]

3. Results and discussion

2000

700

32

1800

600 1600

500 400

1400

300 200 100

18

1200

44

1000

78

0 30

130

230

330

430

530

630

Abundance amu 32 [a.u.]

202

800 730

830

930

Temperature [°C] Fig. 4. Thermal curves (above) and individual mass patterns (below) for ECS-3. Above: TG (–), DTG (



) and DTA (
–
) curves. The inset shows the portion of the thermal curve between 30 and 330 °C evidencing the endothermic peak related to the water molecules escape from the ECS-3 porosity. Below: patterns for masses 18 (H2O), 32 (O2), 44 (CO2) and 78 (benzene) detected during the heating ramp. See also Supplementary Information Fig. S1.

Fig. 2. SEM micrograph of ECS-3.

Quantity adsorbed (cm3/g STP)

100

80

60

40

20

0 0

0.1

0.2

0.3

0.4 0.5 0.6 Relative Pressure (P/Po)

0.7

0.8

0.9

1

Fig. 3. Nitrogen adsorption/desorption isotherms obtained at 77 K for ECS-3 assynthesized (squares) and heated at 200 °C in air (triangles).

Information Fig. S1), indicating that structural degradation occurred by: (i) combustion of the organic moiety (18 = H2O, 44 = CO2), supported by consumption of molecular oxygen (amu 32) (Fig. 4); (ii) elimination of integral benzene molecules (amu 78). The third weight loss which occurred at high temperature (over 700 °C) likely involved the decomposition of compounds (e.g. coke, Na/K carbonates) formed during the earlier thermal event. In conclusion, thermal analysis guided the selection of temperatures for in situ XRD and revealed that most of the adsorbed water was lost at 250 °C. Consequently, considering the different experimental conditions (continuous heating for TGA vs. isotherm for XRD and heating rate of 10 °C/min for TGA vs. 2 °C/min for XRD), a maximum of 150 °C was initially chosen for the in situ XRD experiment. The starting ECS-3 structural model contains 32 water molecules, rather than the 45 estimated by TG [3]. As expected, the room temperature laboratory data did not allow localization of new extra-framework sites and allocated ca. 70% of the water molecules. The anomalous amount of water found during Rietveld refinement is due to the disordered situation affecting the extraframework positions, which prevented site assignment between water molecules and cations. Therefore, one crystallographic position can be randomly occupied by both species. This disorder is a typical feature of microporous materials with low Si/Al ratio and a large cavity: the poorly defined and broadly distributed electron density clearly affected the Rietveld refinement (Table 1). In situ high-temperature XRD analysis indicated that ECS-3 largely maintained its crystallinity up to 150 °C (Fig. 5), even if the quality of the Rietveld refinement was lower than that at 125 °C (see Table 1 and Supplementary Information Figs. S2–S7). As the temperature increased, the ECS-3 unit cell volume underwent an

203

S. Zanardi et al. / Microporous and Mesoporous Materials 172 (2013) 200–205 Table 1 Structural and refinement parameters for ECS-3. Temperature

Room temperature

Crystal system Space group

monoclinic Cc

a (Å) b (Å) c (Å) Beta (°) V (Å3) RF2 (%) Rp (%) Rwp (%) No. observations No. reflections No. parameters No. of geometric restrains

19.811 (2) 28.473 (3) 9.798 (1) 104.32 (2) 5355.0 (7) 12.4 7.8 10.3 5207 4121 181 158

50 °C

75 °C

19.774 (2) 28.278 (3) 9.758 (1) 104.16 (2) 5290.7 (7) 11.0 7.2 9.5

19.744 28.077 9.7260 104.14 5228.2 10.8 7.5 9.8

4240 181

4181 180

100 °C

(2) (3) (9) (1) (6)

19.738 27.976 9.7073 104.16 5197.4 9.5 7.1 9.3

125 °C

(2) (3) (8) (2) (6)

19.729 27.919 9.6865 104.17 5173.1 9.7 7.5 9.8

4163 177

150 °C

(2) (3) (9) (2) (6)

19.747 (5) 27.821 (6) 9.655 (2) 103.83 (3) 5149.3 (22) 10.0 9.2 11.5

4150 171

4138 165

0.99

22500

(var.)T / (var.)25°C

Intensity[A.U.]

1

40000

10000

a/a0 b/b0

0.98

c/c0 β/β0

2500 0.97

6

8

10

12

14

110

16

18

20

22

24

0.96

200

40000 22500

11-1

021

020

10000 2500 5

6

7

8

9

10

11

12

2Theta [°] Fig. 5. Above: comparison of ECS-3 powder diffraction patterns at (from bottom) room temperature, 50, 75, 100, 125, 150 and 200 °C. Below: portion of the pattern of ECS-3 at 125 (bottom), 150 (middle) and 200 °C (top) along with the indexes of the main reflections. The broad band between 6.5° and 8° 2h is due to the kapton window of the camera.

almost uniform contraction (Fig. 6). This was due to the continuous decrease in the unit cell parameters b and c. On the other hand, the unit cell parameter a underwent a contraction up to 125 °C and then, rather unexpectedly, slightly expanded at 150 °C. The angle b, after a small contraction at 50 °C, remained unchanged until 125 °C and then contracted again at 150 °C. The continuous unit cell volume contraction associated with dehydration was 3.8% at 150 °C. The refinement of the water molecules occupancies showed a constant decrease in water content as the temperature increased. At 150 °C, 37% of the water was found, testifying that ECS-3 is far from being fully dehydrated (Fig. 6). Moreover, at this temperature, no evidence was found for new extra-framework cation positions.

Water molecules/unit cell

Intensity[A.U.]

4

V/V0

25

50

75

100

125

150

25

50

75

100

125

150

32 30 28 26 24 22 20 18 16 14 12 10 Temperature (°C)

Fig. 6. Above: relative variations in unit cell parameters at different temperatures. Below: variation in water content per unit cell at different temperatures.

Taking into account the results of the Rietveld refinement, the temperature was then increased to 200 °C in an attempt to remove all the water. Unfortunately, the quality of the XRD pattern worsened (Fig. 5) and the second scan of 200 °C data collection evidenced a shift in diffraction peaks indicating an incipient breakdown of the ECS-3 structure. Consequently, the analysis was interrupted. The poor quality of the data collected at 200 °C did not allow structural refinement, or unit cell parameters estimation. Moreover, the XRD pattern did not improve after cooling to room temperature revealing that the structural changes at 200 °C were irreversible. According to the most recent definition, ‘collapsed’ zeolites may retain some sorption properties and a recognizable X-ray diffraction pattern [5,6]. This condition must be distinguished from structural breakdown, which causes complete amorphization or

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S. Zanardi et al. / Microporous and Mesoporous Materials 172 (2013) 200–205

recrystallization. Since the ECS-3 scaffolding is constituted by inorganic layers separated by phenylene rings and TG-MS data clearly showed that the organic groups were evolved in the initial degradation of the structure, condensation between layers is expected. A decrease in the interlayer distances would affect the diffraction pattern by shifting the 2 0 0 peak (located at  9.2° 2h) towards higher angles. This was not the case, since the diffraction pattern in Fig. 5 clearly shows that the 2 0 0 peak is slightly shifted to lower angles at 150 °C (which increased the unit cell parameter a) and remained almost unchanged at 200 °C. This means that the inorganic layers maintained their interlayer distances while the long-range order was lost, as clearly seen by the disappearance of reflections in the middle to high 2h region (Fig. 5). Thus, short-range order was preserved during the collapse. NMR spectroscopy experiments were made to probe the molecular details of this structural collapse. Accurate fitting of the 13C MAS NMR spectrum for ECS-3 at 126 MHz (Fig. 7) required four peaks spaced closely around 141 and 134 ppm with the latter composite peak making up ca. 70% of the total area (Table 2 and Supplementary Information Fig. S8). This relative intensity suggests that the less intense signals near 141 ppm are due to C atoms directly connected to silicon. The BTEB precursor dissolved in chloroform gives 13C signals at 133.7 and 133.1 ppm (with a 2:1 intensity ratio, not shown). Treatment at 200 °C in air gave a new peak at 129 ppm (Fig. 7, and S9) which accounted for 18% of the total carbons (or 17 C per unit cell). It could arise from: (i) free benzene molecules, formed by cleavage of both Si–C bonds or (ii) phenyl groups, formed by cleavage of one Si–C bond. The 13C shift at 129 ppm indicates the second phenomenon occurred. A similar 13 C signal (at 128.5 ppm) was reported by Esquivel et al., for a calcined phenylene-bridged PMO [19]. It was assigned to a Si-phenyl group by comparison with the spectrum for phenyltriethoxysilane which showed peaks at 134.5, 131, 129 and 127 ppm [19]. 29 Si MAS NMR analysis confirmed partial Si–C bond rupture at 200 °C in air (Fig. 8) with ca. 18% of Si (or six Si per unit cell) going from T to Q sites (Table 3 and Supplementary Information). Signal assignments were made starting with the 80 ppm signal from a T3 (0Al) site, in line with the literature available for this site in purely siliceous phenylene hybrids, e.g. [19–24]. The other signals were assigned applying the group substitution rules and considering the low Si/Al ratio (1.3) found by elemental analysis. Substitution of an OAl group for an OSi group causes a 5 ppm downfield (higher) 29 Si shift, while substitution of an OH group for an OSi group causes a 10 ppm downfield shift. In as-synthesized ECS-3, nearly all the silicon atoms are covalently bound to one carbon giving T species (Fig. 8, Table 3 and Supplementary Information Fig. S10). Heat trea-

Table 2 13 C MAS NMR data obtained from de-convolution of the spectrum in Fig. 7. Peak assigment

Phenylene C⁄–C–Si Phenylene C⁄–C–Si Phenylene C–Si, phenyl-Si Phenylene C–Si Phenyl-Si

150

100

50

ppm

Fig. 7. 13C MAS NMR spectrum of ECS-3 as-synthesized (bottom) and after thermal treatment at 200 °C in air (top). See Table 2 for peak assignments and relative areas.

Treated at 200 °C in air

C shift (ppm)

Relative area (%)

13

C shift (ppm)

Relative area (%)

142.0 139.8 135.1

16 14 33

141.1

19

134.1

63

133.4

37 128.9

18

Fig. 8. 29Si MAS NMR spectra of ECS-3 as-synthesized (bottom) and after thermal treatment at 200 °C in air (top). See Table 3 for peak assignments and relative areas.

Table 3 29 Si MAS NMR data obtained from de-convolution of the spectrum in Fig. 8. Peak assignment

T2 (2Al) T3 (3Al) T3 (3Al) T3 (2Al) Q2 (2Al) Q3 (3Al)

200

As-synthesized 13

As-synthesized 29

Si shift (ppm) 62 65 68 72 80

Treated at 200 °C in air Relative area (%) 2 57 30 8 3

29

Si shift (ppm)

Relative area (%)

62 66 70

4 46 27

80 85

5 18

ted ECS-3 showed two new resonance peaks at 80 and 85 ppm, that are attributed to Q2 (2Al) and Q3 (3Al) sites, respectively (Fig. 8, Table 3 and Supplementary Information Fig. S11). 13 C NMR estimated 34 C per unit cell became phenyl groups (considering that the peak at 129 ppm, with 17 C/unit cell, represents half the phenyl molecules). 29Si NMR estimated six Si per unit cell became Q sites. These nearly equal amounts of Si (6) and phenylene groups (34/6 = 5.7 per unit cell) involved in the chemical change during structural collapse points to phenylene losing only one bond with silicon. One-sided scission of the organic bridging group was recently reported for calcined phenylene- and ethenylene-bridged PMO’s [19,20]. However, the mechanism proposed in

S. Zanardi et al. / Microporous and Mesoporous Materials 172 (2013) 200–205

the latter case involved a metamorphism, with ‘‘proton transfer from silanol groups to ethenylene bridging groups’’ [20], to give Q4 Si sites and terminal vinyl groups. Our 29Si MAS NMR data revealed very few silanol groups before heating (see Table 3). Therefore, the proton transferred to phenylene and OH transferred to Si sites must have come from water molecules, still trapped in the ECS-3 porosity. This drove to the formation of Q3 and Si-phenyl groups, as well sketched in the scheme 2 of reference [19]. In this paper, it was also emphasized that the highest amounts of Q Si species were formed during thermal treatment in air saturated with water [19], supporting the proposed mechanism. Because ECS-3’s porosity is largely due to the phenylene groups separating the inorganic layers, Si–C bond cleavage could influence its adsorption properties. In fact, the nitrogen adsorption/desorption isotherm changed after 200 °C heating in air from type I to type II (Fig. 3), with a dramatic drop in surface area to 3 m2/g. This drop in the textural properties was not caused by the out-gassing procedure (under vacuum at 120 °C), since the XRD and 13C MAS NMR spectra (not shown) were unchanged. Also, as 13C NMR found no changes after N2 adsorption/desorption measurement, the attribution of the 13C resonance peak at 129 ppm to Si-phenyl groups is supported. ‘‘Free’’ benzene molecules would have been lost during the out-gassing procedure. Moreover, 1H transverse relaxation times for the aromatic hydrogen nuclei (ca. 0.35 ms) were much longer after thermal treatment (10 ms) indicating greater rotation freedom for the organic group. This is consistent with a doublyconnected phenylene becoming a singly-attached phenyl group or one-sided cleavage (Fig. S12). The reason for lost N2 adsorption/desorption capacity is not clear yet. Since the quality of the XRD pattern prohibits the Rietveld refinement, we cannot define the structural changes involved. The nearly constant position of the 200 reflection seems to rule out occlusion of the porosity by compression or condensation between inorganic layers. Our hypothesis is that the newly formed phenyl groups obstruct the aperture of the sinusoidal channel’s pore mouth and impede access to even small molecules like nitrogen. Differently to what was observed for the sample heated at 200 °C in air, the nitrogen adsorption/desorption measurement on the sample out-gassed at 200 °C under vacuum did not show dramatic changes in textural properties. Nevertheless, some modifications in the adsorption/desorption isotherm and in the specific surface area (specific surface area = 152 m2 g 1, specific pore volume 0.15 cm3 g 1, see also Fig. S13) were observed. Studies are in progress to understand the reason for these modifications. At the moment we can exclude irreversible structural modification (e.g. cleavage of Si–C or even Si–O bonds), since the XRD pattern of the sample did not change (Fig. S14). Although this finding raises additional questions on the thermal behaviour of ECS-3, it clearly demonstrates that the presence of water during heating is detrimental for the hybrid structure, and that ECS-3 exhibits higher thermal stability under vacuum condition.

4. Conclusions The thermal behaviour of a crystalline hybrid organic–inorganic aluminosilicate was reported for the first time. ECS-3 maintained a high degree of crystallinity up to 150 °C, as found by in situ X-ray powder diffraction analysis in air. At this temperature, 37 wt.% of the initial water molecules were still trapped in the ECS-3 porosity and the unit cell volume was smaller by only about 3.8%. At 200 °C the long-range structural order was dramatically affected, while short-range order was not. The large loss in XRD pattern quality was consistent with a structural collapse. 13C MAS NMR analysis

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showed that the collapse was caused by cleavage of some Si–C bonds (ca. 20%) and the formation of phenyl groups. Structural collapse of ECS-3 at 200 °C (in air) was associated with a large decrease in the textural properties (and specific surface area). The reason for this is uncertain. It is suggested that this mainly results from a change in the position of the pendant phenyl group, formed via one-sided Si–C cleavage, which blocks the aperture of the sinusoidal channel’s pore mouth. The mechanism proposed for Si–C rupture involves an initial proton transfer from nearby water molecules to the phenylene bridging groups with formation of Q3 Si and Si-phenyl groups. This hypothesis is supported by the N2 adsorption/desorption measurement after degassing the sample at 200 °C in vacuum. In this case, ECS-3 maintains its crystal structure and textural properties. This indicates that removal of water molecules increases the ECS-3 thermal stability. Additional studies are necessary to verify the high temperature limit for ECS-3’s stability (and atomic details of its structural modifications) under vacuum conditions and, more in general, the thermal behaviour of other ECS materials. Acknowledgment The authors thank Massimo Nalli for thermal analysis. Two anonymous referees are gratefully thanked for their valuable suggestions and comments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2013. 01.029. References [1] G. Bellussi, A. Carati, E. Di Paola, R. Millini, W.O. Parker Jr., C. Rizzo, S. Zanardi, Microporous Mesoporous Mater. 113 (2008) 252. [2] F. Hofmann, M. Fröba, Chem. Soc. Rev. 40 (2011) 608. [3] G. Bellussi, E. Montanari, E. Di Paola, R. Millini, A. Carati, C. Rizzo, W.O. Parker Jr., M. Gemmi, E. Mugnaioli, U. Kolb, S. Zanardi, Angew. Chem. Int. Ed. 51 (2012) 666. [4] G. Bellussi, R. Millini, E. Montanari, A. Carati, C. Rizzo, W.O. Parker Jr., G. Cruciani, A. de Angelis, L. Bonoldi, S. Zanardi, Chem. Com. 48 (2012) 7356. [5] G. Cruciani, J. Phys. Chem. Sol. 67 (2006) 1973. [6] D.L. Bish, J.W. Carey, America 45 (2001) 403–452. [7] A. Alberti, A. Martucci, Stud. Surf. Sci. Catal. 155 (2005) 19. [8] A. Alberti, A. Martucci, Microporous Mesoporous Mater. 141 (2011) 192. [9] T. Tani, S. Inagaki, J. Mater. Chem. 19 (2009) 4451. [10] N. Mizoshita, T. Tani, S. Inagaki, Chem. Soc. Rev. 40 (2011) 789. [11] S. Inagaki, O. Ohtani, Y. Goto, K. Okamoto, M. Ikai, K. Yamanaka, T. Tani, T. Okada, Angew. Chem. Int. Ed. 48 (2009) 4042. [12] H. Takeda, Y. Goto, Y. Maegawa, T. Ohsuna, T. Tani, K. Matsumoto, T. Shimada, S. Inagaki, Chem. Comm. (2009) 6032. [13] Y. Maegawa, N. Mizoshita, T. Tani, S. Inagaki, J. Mater. Chem. 20 (2010) 4399. [14] R. Millini, C. Perego, W.O. Parker Jr., C. Flego, G. Girotti, Stud. Surf. Sci. Catal. 154 (2004) 1214. [15] A. Alberti, G. Cruciani, E. Galli, R. Millini, S. Zanardi, J. Phys. Chem. C 111 (2007) 4503. [16] A.C. Larson, R.B. Von Dreele, National Laboratory, Report LAUR-86-748, Los Alamos, 2000. [17] B.H. Toby, J. Appl. Crystallogr. 34 (2001). [18] D.G. Cory, W.M. Ritchey, J. Magn. Reson. 80 (1988) 128. [19] D. Esquivel, C. Jimenez-Sanchidrian, F.J. Romero-Salguero, J. Mater. Chem. 21 (2011) 724. [20] C. Vercaemst, J.T.A. Jones, Y.Z. Khimyak, J.C. Martins, F. Verpoorta, P. Van Der Voort, Phys. Chem. Chem. Phys. 10 (2008) 5349. [21] F. Goethals, B. Meeus, A. Verberckmoes, P. Van Der Voort, I. Van Driessche, J. Mater. Chem. 20 (2010) 1709. [22] Y.C. Pan, H.Y. Wu, C.C. Kao, H.M. Kao, Y.N. Shich, G.T.K. Fey, J.H. Chang, H.H.G. Tsai, J. Phys. Chem. 113 (2009) 18251. [23] A. Comotti, S. Bracco, P. Valsesia, L. Ferretti, P. Sozzani, J. Am. Chem. Soc. 129 (2007) 8566. [24] Y. Goto, S. Inagaki, Microporous Mesoporous Mater. 89 (2005) 103–108.