Synthesis and Characterization of a Layered Silicogermanate PKU-22

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Synthesis and characterization of a layered silicogermanate PKU-22 and its topotactic condensation to a 3D STI-type zeolite Yanping Chen, Shiliang Huang, Xiaoge Wang, Lei Zhang, Ningning Wu, Fuhui Liao, and Yingxia Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01000 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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Crystal Growth & Design

Synthesis and characterization of a layered silicogermanate PKU-22 and its topotactic condensation to a 3D STI-type zeolite Yanping Chen†, Shiliang Huang‡, Xiaoge Wang†, Lei Zhang†, Ningning Wu§, Fuhui Liao† and Yingxia Wang*,† †

College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, PR China. Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, 621900, PR China. § Physicochemical Analysis and Measurements Center, Institute of Chemistry, Chinese Academy of Science, Beijing, 100190, PR China. ‡

KEYWORDS: Layered silicogermanate, PKU-22, Layered silicates, Topotactic condensation, Zeolite ABSTRACT: A new layered silicogermanate, PKU-22, was hydrothermally synthesized under fluoride conditions using tetraethylammonium (TEA+) cation as the structure directing agent (SDA). The crystal structure was determined by single crystal X-ray diffraction. Structure analysis reveals that PKU-22 is constructed by sti layers stacking along the [100] direction in an ···AAAA··· manner, with TEA+ cations occurring in the inter-layer spaces and F- anions residing within the layer and connecting to Ge atoms, which also act as the charge compensation species. In-situ temperature-variable powder X-ray diffraction results indicated that PKU-22 could be transferred into a 3D STI-type zeolite PKU-22a on heating. It is interesting that the TEA+ cations can keep intact in the structure of the condensed product. Solid-state NMR, ICP-AES, TG-DSC and CHN elemental analyses were applied to aid the structure analysis of PKU-22 and illustrate its transformation. The scheme of the topotactic condensation of PKU-22 to PKU22a is proposed.

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INTRODUCTION

Microporous crystalline zeolites are essential candidates as ionexchangers, adsorbents and catalysts in chemical and petrochemical industries, due to their unique structures.1 The synthesis of new zeolites with novel frameworks, special channel systems and active sites have attracted considerable attentions. Various new zeolites have been synthesized by using appropriate structure directing agents (SDAs) and the replacement of Si framework element by heteroatoms such as Ge,2 B,3 Al,4 Be,5 however, the accurate design of zeolites with particular topology and pore system on atomic scale is still a challenge.6 During the synthetic effort, layered silicates have been intensively investigated and the topotactic condensation of layered silicates was found to be an effective way for the formation of new framework zeolites with controlled pore system.7 Layered silicates are constructed by the stacking of 2D layers which are built via the connection of the SiO4 tetrahedra, with the ordered arrangement of the equal amount of terminal silanol or siloxy groups at both sides.8 The low charge density cations, for example, alkali metal or organic ammonium cations, are incorporated into the inter-layer space for charge compensation.9 It is pointed out that at proper conditions, the dehydration condensation reaction occurs between the inter-layer silanol groups.10 As a result, the Si-O-Si bonds are formed and the layer sheets are connected into 3D zeolites, accompanying with the decomposition of the SDA molecules.7a Since the topology of layered sheets is remained without the break or rearrangement of bonds in the intralayer, this structure transformation is named as topotactic condensation, which is promising in the generation of new zeolite materials.1c, 7a, 11 Several zeolite frameworks, such as CAS (IZA framework type code),12 MWW,13 FER,14 SOD,15 AST,16 MTF,8 CDO,14b, 17 NSI,18 RRO19 and RWR,20 have been synthesized with layered silicates as precursors. Among them, CDO, NSI, RRO and RWR zeolites, are new frameworks, which could be only obtained in the topotactic conversion route so far. Recently, much attention has been paid to the ADOR method and inverse

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sigma transformation, which focus on synthesizing new zeolites via selective and controllable disassembly and reassembly of layered sheets from known zeolites,21 leading to several unique 3D frameworks, for example, IPC-2 and isostructural COK-14 (OKO),21a, 21b IPC-4 (PCR),21b IPC-6 and IPC-7,22 IPC-9 and IPC-10,23 IPC-12,24 IPC-15 and IPC-16.25 Wherein, IPC-4, IPC-9, IPC-12 and IPC-15 are orderly distinct zeolites formed by topotactic condensation of disassembled layered sheets. Therefore, the topotactic condensation method, ADOR process and inverse sigma transformation, open up useable pathways in designing zeolites from layered sheets. As the topology of the resulted framework after topotactic conversion strongly depend on the original layers, searching for more new layered silicates is beneficial in deeply understanding the formation of zeolites from layered precursors. However, to date, only a limited number of layered silicate structures have been reported.7a One of the reasons for this is the difficulty of the structure determination of layered silicates.26 The limited crystal sizes of layered silicates in the SiO2-SDA-H2O system generally cause the low resolution of the X-ray diffraction data, preventing the structure solution from single crystal X-ray diffraction.27 Moreover, layered silicates usually suffer from stacking disorder between layers, leading to severe peak broadening in the powder Xray diffraction (PXRD) patterns, which further hampers the structure determination.28 Thus the careful control of the synthetic conditions is prospective for the formation of new wellcrystallized layered silicates, which are potential candidates to be transformed into zeolites and served as suitable models for understanding the condensation process. Here we present a new layered silicogermanate PKU-22 (Peking University, denoted as PKU), and its transformation to a zeolite PKU-22a. PKU-22 is well crystallized and its structure was solved by single crystal X-ray diffraction data. Topotactic condensation was observed upon the heating of PKU-22, and the resulted product PKU-22a possessed a STI-type framework. The SDA molecules were found intact in PKU-22a, indicating that the condensation and the removal process of organic species from

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PKU-22 on heating could be separated in to two steps. The scheme of the topotactic condensation of PKU-22 to PKU-22a is proposed. PXRD, Solid-state NMR, ICP-AES, TG-DSC and CHN elemental analyses were applied to aid the structure analysis and 5 to illustrate the transformation of PKU-22. Germanate was chosen for the reason that Ge favors the formation of particular building units due to the smaller Ge-O-Ge angles and longer Ge-O bond distances comparing to the Si-O-Si angles and Si-O bond distances, which might induce the formation of new structures.29 10

Table 1. Crystallographic data and structure refinement results for PKU-22 Identification code PKU-22 |(C8H20N)(H2O)|[Ge6.67Si2.33O17(OH)2F] Chemical formula Formula weight 1022.91 g/mol 180.0(10) K Temperature Wavelength 0.71073 Å Orthorhombic Crystal system Space group Pbcm 10.6001(6) Å a b 18.3760(11) Å 13.8320(8) Å c Volume 2694.3(3) Å3 4 Z Density (calculated) 2.522 g/cm3 7.538 mm-1 Absorption coefficient F(000) 1976.0 0.01 mm × 0.05 mm × 0.10 mm Crystal size Theta range for data collection 3.28° < θ < 26.37° -13≤ h ≤10, -16 ≤ k ≤ 22, -10 ≤ l ≤ 17 Index ranges Reflections collected 6911 2836 Unique reflections Completeness to theta=26.37° 98.5% Multi-scan Absorption correction Min. and max. transmission 0.519 and 0.928 transmissionmethod Refinement Full-matrix least-squares on F2 Data / restraints / parameters 2836/15/189 Goodness-of-fit on F2 1.058 Final R indices [I>2sigma(I)] R1 =0.0588, wR2 = 0.1500 R indices (all data) R1 =0.0903, wR2 = 0.1766 Largest diff. peak and hole 1.862 and -1.390 e. Å3

EXPERIMENTAL SECTION

Synthesis of PKU-22. Tetraethylammonium hydroxide (TEAOH, 20 wt%, Sinopharm Chemical Reagent Co., Ltd), hydrofluoric acid (Beijing Chemical Works, HF, 40%) and GeO2 (China Germanium Co., Ltd, 99.999%) were commercially avail15 able. Fumed SiO2 was prepared by the method published elsewhere.30 Typically, appropriate amounts of fumed SiO2 and GeO2 were added into a TEAOH solution under stirring. Then hydrofluoric acid was added into the above solution. A gel was formed by heating the solution on a magnetic stirrer at 55 oC. The typical 20 molar composition of the gel was 0.2SiO2: 0.8GeO2: 0.5SDA: 0.5HF: 5-7H2O. Then the gel was transferred into a Teflon-lined stainless steel autoclave and kept in an oven at 165 oC under static conditions for 21 days. Finally, white solid product was separated from mother liquor by centrifugation, washed with deionized wao 25 ter and dried in an oven at 65 C overnight.

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Structure determination. A single crystal with the size of about 0.01 mm × 0.05 mm × 0.10 mm was selected and the single crystal X-ray diffraction data were collected on an Agilent Supernova CCD diffractometer (Mo-Kα, λ=0.71073 Å, graphite monochromator) at 180.0 K. Data reduction was performed using the CrysAlisPro program. The structure was solved by direct methods and refined by minimizing the sum of squared deviations against F2 using a full-matrix technique with the SHELXL-97 program.31 The occupancies of Si/Ge in T1-T5 sites were initially set to a ratio of Si/Ge=0.35 according to the ICP result, and refined automatically for each position only with the constraint on the overall Si/Ge ratio of 0.35. The TEA+ cations were located by the difference Fourier maps. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of TEA+ cations, water guest molecules and terminal hydroxyl groups were geometrically added and restricted to their parent atoms by the rigid model. Crystallographic data and details of the structure reﬁnement are listed in Table 1. The atomic coordinates, thermal parameters and occupancies of PKU-22 are summarized in Table S1 in the SI. The TO bond distances are in the range of 1.666(6)-1.767(6) Å, which is in agreement with those in the GeO2-rich zeolites, as shown in Table S2 in the SI. CCDC-1562626 contains the supplementary crystallographic data for PKU-22. The data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. The experimental PXRD pattern of PKU-22 is consistent with the simulated one based on the crystal structure of PKU-22, as shown in Figure S1 in the SI, indicating that the as-synthesized PKU-22 sample is pure. Condensation and Decomposition of PKU-22. The change of PKU-22 with temperature was detected by in-situ PXRD from 30 to 500 °C on a modified Bruker D8 Advance diffractometer equipped with MRI high temperature attachment, a graphite monochromator and LynxEye detector, using a Cu Kα radiation source (λ=1.5418 Å) in Bragg-Brentano geometry. The data in the 2θ range of 5 to 40° were collected with a step width of 0.02° and the remaining time was 1 s/step. The heating rate was 0.1 °C/s and the sample was equilibrated for 5 min before each data collection. According to the previous results, PKU-22 sample was heated to 260 °C at a slow heating rate of 0.05 °C/s and then

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65 kept at this temperature for 5 h in the MRI high temperature

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chamber. Then the PXRD data of PKU-22a was collected in the 2θ range of 5 to 40° with a step of 0.004° and a remaining time of 4 s each step. The PKU-22a sample for characterizations was obtained by carefully heating PKU-22 at 260 °C for 5 h in an oven in air atmosphere. Other characterizations. A plate-like morphology of PKU-22 was observed on a Hitachi S4800 scanning electron microscope (Figure S2 in the SI). TEM images was taken on a JEOL-JEM 2100 transmission electron microscope at an accelerator voltage of 200 kV. The purity of the products was examined by PXRD on a Rigaku D/Max-2400 diffractometer with a Cu Kα radiation source (λ=1.5418 Å) in a step width of 0.02°. Elemental analysis (EA) for carbon, hydrogen and nitrogen (CHN) was conducted on an ElementarVario EL III microanalyzer. The thermal analysis was measured on TGA Q50 V20.6 with a heating rate of 5 °C/min from room temperature (RT) to 800 °C in air. The content of Si and Ge was carried out on an ESCALAB2000 analyzer by the ICP method. Solid-state 1H-13C CP MAS NMR spectrum of PKU-22 was performed on Bruker AVANCE III 400 MHz NMR spectrometer at spinning rates of 5 kHz, with a frequency of 100.64 MHz, a contact time of 2 ms and a 4 mm Bruker probe. For PKU-22a, the 1 H-13C CP MAS NMR spectrum was measured at the spinning rates of 10 kHz with a contact time of 1 ms. The solid-state 29Si MAS NMR was examined at spinning rates of 5 kHz, with a frequency of 79.30 MHz, a 30o pulse length of 2 µs and a number of scans of 3072. The recycle time was 60 s and a 7mm Bruker probe was used. For quantitative signal intensities, the high power decoupling was used in the 29Si experiment. Furthermore, the 1H29 Si CP MAS NMR spectrum was measured at spinning rates of 5 kHz with a 7mm Bruker probe and a contact time of 5 ms. The solid-state 1H MAS NMR of PKU-22 was operated at the frequen-

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Crystal Growth & Design Table 2. Synthesis conditions of PKU-22 and the resulting products Run no.

Initial gel composition (molar ratio)

Rotating speed /rpma

Temp/ oC

Time/days

Productb

1

SiO2 0.3

GeO2 0.7

SDA 0.5

HF 0.5

H2O 5

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21

PKU-22+U+BEC

2

0.2

0.8

0.5

0.4

5

15

175

21

BEC+R-GeO2 PKU-22+U+BEC+ R-GeO2

3

0.2

0.8

0.5

0.5

3

15

175

21

4c

0.2

0.8

0.5

0.5

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175

21

PKU-22+U

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0.2

0.8

0.5

0.6

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175

21

PKU-22+U+ R-GeO2

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0.2

0.8

0.5

0.5

7

15

175

21

BEC+ R-GeO2

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0.2

0.8

0.5

0.5

5

15

175

7

PKU-22+U+BEC

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0.2

0.8

0.5

0.5

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15

175

14

PKU-22+U+BEC U+ R-GeO2

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0.2

0.8

0.5

0.5

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15

175

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0.1

0.9

0.5

0.5

2.5

15

175

21

R-GeO2

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0.1

0.9

0.5

0.5

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175

21

PKU-22+ R-GeO2

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1

0.5

0.5

2.5

15

175

21

R-GeO2

13

1

0.5

0.5

5

15

175

21

R-GeO2

0.8

0.5

0.5

5-7

165

21

PKU-22

14

d

0.2

After 10 days, the synthesis at 175 oC was transferred from rotating condition to static state in favor of the crystal growth. b U indicates an un known phase. R-GeO2 implies rutile-type GeO2. c Single Crystal for X-ray diffraction data collection was selected from sample (run no. 4). d The sample from Run no. 14 was used for characterization, such as SEM, NMR, TG-DSC, EA, ICP and in-situ PXRD. a

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cy of 399.16 MHz with spinning rates of 12 kHz, a 30o pulse length of 5 µs and a 4mm Bruker probe. The recycle time was 2 s. For PKU-22a, A dumbod2 pulse program was used for homonuclear decoupling to obtain a high resolution 1H spectrum with a 10390 Hz spin rate and a 3s recycle delay, and the 1H chemical shifts were referenced to solid glycine with the resonances of H in -CH2- group at 3.5 ppm and H in -NH3+ group at 7.5 ppm.32 The solid-state 19F MAS NMR was recorded on a Bruker AVANCE III 500 WB (11.75 T) spectrometer with spinning rates of 30 kHz, operated at 470.96 MHz for 19F. A commercial DVT quadruple resonance H/F/X/Y 2.5 mm CP/MAS probe was used with a 90o single pulse excitation applying pulse width of 2.0 µs (π/2) and a recycle delay of 3 s to obtain quantitative results. There is no fluorine background from the H/F/X/Y probe head. The chemical shift of 13C was calibrated with adamantane (38.48 ppm). The 29Si and 1 H chemical shifts of PKU-22 were calibrated with octakis(trimethylsiloxy)silsesquioxane (11.72 ppm) and adamantane (1.91 ppm), respectively. The 19F chemical shift was calibrated with polytetrafluoroethylene (PTFE) by checking the resonance of F in -CF2 groups at -122 ppm.

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RESULTS AND DISCUSSION

Synthesis conditions of PKU-22 and its stability. The synthetic conditions of PKU-22 and the corresponding products are given in Table 2. PKU-22 was firstly obtained at the molar composition of 0.2SiO2: 0.8GeO2: 0.5SDA: 0.5HF: 5H2O (run no. 4) under 175 oC for 21 days, accompanied by an unknown impurity phase, here denoted as U phase. Then a systematic synthetic test was conducted to optimize conditions for the formation of PKU-22. When the amount of Si (run no. 1) or Ge (run no. 10-13) was increased, BEC-type zeolite or rutile-type GeO2 appeared. Thus, the ratio of Si/Ge was fixed at 0.2: 0.8. The decrease of HF (run no. 2) yielded a mixture of BEC-type zeolite and rutile-type GeO2, while the increase of HF (run no. 5) resulted in the mixture of PKU-22, U phase and rutile-type GeO2. Then the ratio of HF/T and SDA/T (T= Si + Ge) were controlled to 0.5. As the effect of water, a mixture of PKU-22, U phase, BEC-type zeolite and rutile-type GeO2 was formed at the low H2O/T ratio of 3 (run no. 3), and only BEC-type zeolite and rutile-type GeO2 emerged but no PKU-22 when increasing water content (run no. 6). The reaction time on the formation of PKU-22 was varied to 7, 14, 21 and 28

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days. PKU-22, U phase and BEC-type zeolite were obtained by 7 or 14 days (run no. 7 and 8). The prolongation of the time to 28 days (run no. 9) resulted in the unknown phase and rutile-type GeO2. It can be seen that it is difficult to obtain the pure phase of PKU-22 under 175 oC. A fine adjustment of the temperature to 165 oC led to the formation of pure phase and then the synthesis of PKU-22 was kept at the molar composition of 0.2SiO2: 0.8GeO2: 0.5SDA: 0.5HF: 5-7H2O (run no. 14) under 165 oC for 21 days. PKU-22 shows a moderate stability to water. A weight of 40 mg PKU-22 sample was dispersed in 10 mL water for 12 h, 24 h, 48 h and 72 h, and then separated from the suspension. PXRD profiles of the solids looked no change comparing with that of the assynthesized PKU-22, as shown in Figure S3 in the SI. Then PKU22 was treated in water at 100 oC for 5 min, 30 min and 1 h, respectively. In boiling water, PKU-22 was stable in short time, and then began to transfer to quartz-type GeO2. The PXRD pattern of the sample treated by 30 minutes presented the emergence of peaks at 2θ=20.6o and 26o relating to quartz-type GeO2. These two peaks became more obvious after one hour, as shown in Figure S3 in the SI. PKU-22 is unstable in boiling water, which further confirms that the introduction of Ge into crystalline porous material would lead to a less stability. Framework structure. PKU-22 crystallizes in the orthorhombic space group Pbcm (No.57) with unit cell parameters of a=10.6001(6) Å, b=18.3760(11) Å and c=13.8320(8) Å. PKU-22 takes a layered structure containing isolated layers similar to those in STI framework structure, as shown in Figure 1. This layer can be illustrated by the connection of bre and sti cages. The bre cage is a typical composite building unit formed by 10 T atoms (Figure 1a), in which Ge atom prefers to reside at T1 position and Si favors the T4 position. The bre cages connect to one another via sti cages (Figure 1b), forming a chain along the [001] direction, and then the chains are further linked by sharing T4 atoms, giving rise to the basic layer of PKU-22 (Figure 1c), denoted as sti layer, since it is the only layer that builds the STI-type framework. In the STI-type framework, neighbouring layers are related by a mirror plane with a ···ABAB··· stacking manner along the c direction and connected with each other via the bridge oxygen atoms into 3D framework, while in PKU-22 the sti layers are isolated, leaving T5 site as a Q3 manner (Figure

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configuration related by mirror symmetry with 50% occupancy for each (Figure S4d in the SI). The distribution of the + TEA cations in PKU-22 is in a tt.tt conformer, which is beneficial to strengthen the electrostatic interactions between the cations and negative framework.37,38 Topotactic Condensation. In-situ PXRD was applied to detect the change of PKU-22 with the temperature from RT to 500 o C (Figure 4 and Figure S7 in the SI). It can be seen that several new peaks at 2θ = 9.1o, 18.5o and 20.8o, appeared at the temperature above 200 oC, which indicated that a condensation process occurred and a new crystalline phase emerged. The transformation was completed at 240 oC, and the new phase was denoted as PKU22a. PKU-22a was stable up to 300 oC and then gradually collapsed to amorphous. Thus PKU-22 was treated carefully at 260 °C for 5 h, and a set of PXRD data for PKU-22a was collected for phase identification. The SEM and TEM images after condensation show that PKU-22a possess a plate-like morphology (Figure S8a and S8c in the SI), very similar to that of PKU-22 (Figure S8b and S8d in the SI). It also implies the transformation of PKU-22 to PKU-22a by condensation is really a topotactic process.

Figure 5. Simulated (a) and experimental (b) PXRD patterns of PKU-22a.

tion of a layered octosilicate into RWR-type zeolite under NMF refluxing, and pointed out that the separation of interlayer condensation and elimination of organic guests was a useful way for the 40 45 formation of zeolites with the highest quality. In current work, the organic species preserved in PKU-22a is the original SDA molecules being applied in the synthesis of PKU-22. The condensation process in our work is similar to that in Asakura’s study, but the occlusion of the SDA species is different. 50

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Since PKU-22a is the condensation product of PKU-22, it is natural to relate it with the structure of STI-type zeolites.39 Accordingly, a simulation was performed with the orthorhombic 25 space group of Cmca and the unit cell parameters of a=13.832 Å, b=19.060 Å and c=18.376 Å, and the comparison of the calculated result and the experimental PXRD pattern of PKU-22a is shown in Figure 5. Although experimental profile suffers from the peak broadening and high background scattering, the characteristic 30 peaks, such as (020), (002), (222), (133), (444) and etc. still can be well identified. It means that PKU-22 really transferred to a 3D STI-type zeolite. The low crystallinity of PKU-22a probably comes from the deformation and imperfect stacking of the sti layers during the condensation.

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Another interesting thing is that the SDA molecules can be maintained during the process, that is, the topotactic condensation process is independent from the removal of organic SDA species. This phenomenon is rare, since organic molecules were generally driven out or decomposed during the condensation of hydrous 40 layered silicates. Asakura et al. firstly observed the retention of organic N-methylformamide (NMF) species during the condensa-

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The TG-DSC data, 1H-13C CP MAS NMR, 1H MAS NMR and elementary analysis data also support the retention of organic species. In the TG-DSC curve of PKU-22, three steps were observed with weight loss of 2.73%, 3.56% and14.48% at the temperature regions of 40-100 oC, 100-290 oC and 290800 oC, respectively (Figure S9 in the SI). The first one is generally due to loss of the physically adsorbed water molecules on the surface of PKU-22.9, 27 The second one corresponds to the removal of guest water molecules between layers and the dehydration condensation of hydroxyls, which is comparable to the calculated value from the model (3.43%). The third one is related to the decomposition of organic SDA molecues and the elimination of F- species, consisting with the calculated value of 14.20%. This result indicates that the SDA molecules are remained in PKU-22a before 290 oC. As the 1H-13C CP MAS NMR and 1H MAS NMR spectra of PKU-22a, the two main peaks at 5.3 and 50 ppm of 13C, + relating to resonances of –CH3 and –CH2– groups of TEA cations (Figure S10 in the SI), are similar to those in PKU-22. Sure the two peaks in the spectrum of PKU-22a are broadened and split, which might be due to the less mobile and more complicated local environments for TEA+ molecules in the condensed products, as compared to the much mobile and distinct position of SDAs in PKU-22.10b In the spectrum of 1 H MAS NMR (Figure S11 in the SI), two peaks at 1.3 and 3.2 ppm corresponding to signals of H in –CH3 and –CH2– groups of TEA+ cations, respectively, are also observed, which further confirm the retention of organic TEA+ cations in the structure of PKU-22a. In addition, the CHN elemental analysis also supported the presence of SDA in PKU-22a. The experimental data of C, H and N are 9.53, 1.95 and 1.39 wt%, respectively, giving rise to a ratio of C/H/N = 8/20/1, which is the same as the theoretical value of the cation TEA+

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Figure 6. Proposed condensation PKU-22 to PKU-22a with the STItype framework. The T (Si or Ge) atoms, oxygen atoms, carbon atoms, nitrogen atoms and F- anions are displayed in green, red, grey, blue and cyan, respectively. The hydrogen atoms are omitted for clarity.

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(C8H20N+). A chemical formula |C8H20N| [Ge6.67Si2.33O18F] can be deduced for PKU-22a on the basis of TG, NMR and the elemental analysis results. Scheme for the Condensation. Marler and Gies proposed four criteria for a successful condensation of layer hydrous silicates: far distance (>ca. 4 Å) of the intra-layer siloxy/silanol groups, ordered layer stacking, strong inter-layer hydrogen bonds and a suitable (small) organic cation between the layers.7a From the structural data, we can see that PKU-22 satisfies the demands: the intra-layer terminal O7···O7 distance in is 4.836 Å and the inter-layer O7···O7 distance is + 2.573 Å; it consists of regularly stacked sti layers with TEA cations between layers. These structural features enable PKU-22 to be a good layered compound for studying the condensation, though it is a silicogermanate. Comparison of the cell parameters of the two compounds, PKU-22 crystallizes in the orthorhombic space group Pbcm (a=10.6001(6) Å, b=18.3760(11) Å, c=13.8320(8) Å), while PKU-22a takes the orthorhombic space group of Cmca (a=13.832 Å, b=19.060 Å, c=18.376 Å). Since the sti layer is parallel to bcplane in PKU-22 but parallel to ac-plane in PKU-22a, the structure setting of PKU-22a is transformed to be the equivalent space group Bbcm with the unit cell parameters of a’=19.060 Å, b’=18.376 Å and c’=13.832 Å, then its layer also stacks along the a direction similar to that in PKU-22. It shows clearly that the sti layer has no change during the condensation, but the inter-layer distance was shrunk about (10.6 – 19.06/2) =1.07 Å. This shrinkage value is quite shorter than the known condensation of layered compounds into well ordered zeolites, such as RUB-39 to RUB-41 (~2.0 Å),19a HPM-2 to MTF-type framework (~2.0 Å),8, 41 DPA-HLS to AST-type framework (~2.7 Å),16a PREFER to FER (~3.7 Å) 14a and Nu-6(1) to Nu-6(2) (~5.2 Å),18 while comparable to that in the conversion of PLS-1 to CDS-1 (~1.3 Å).17a Considering the extreme short hydrogen bond length of 2.4 Å,42 if the hydroxyl groups in one layer directly oppose that in the adjacent layer, the shrinking value of the inter-layer distance during condensation is about 2 Å. When the shrinking value is much less than 2.0 Å, for example 1.0 Å, it is generally related with the relative shift between adjacent layers that separates the terminal oxygen atoms to the distance about 2.5 Å in the layered precursors. Then the layers perform a shift during the condensation. In layered PKU-22, SDA molecules arrange along the c direction and the hydrogen-linked hydroxyl groups from the

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neighbouring layers are not directly opposite one another. For the formation of STI framework by condensation, layers in PKU-22 should take shifts to lead the terminal –OH groups to proper positions. There are two possibilities: one is that neighbouring layers in PKU-22 take a relative b/2 shift, and the other is a c/2 shift. It was found that the shifts of layers along the b direction require passing through the SDA molecules and high energy is needed, while the shift along the c direction by a half unit cell do not need to cross over SDA molecules, which is a low energetic path. Then the condensation process is proposed, as shown in Figure 6. The heating treatment of PKU-22 caused the escape of guest water molecules and the break of the hydrogen bonds, which induced the slipping of the opposed sti layer to a suitable position for the condensation, leading to the formation of PKU-22a. The scheme of the topotactic condensation of PKU-22 to PKU22a is proposed to describe the process, as illustrated in Scheme 1. The translation of the sti layer also results in the change of the stacking manners from ···AAAA··· to ···ABAB···, and then the length of the a-axis is almost doubled in PKU-22a, in which B-centred lattice is formed. Asakura has obtained a well-crystallized RWR zeolite by the condensation of layered precursor with the retention of NMF species,40 however, the preservation of organic SDAs is not commonly observed in the condensed products. It was found that the remaining of inter-layer cations (organic or inorganic cations) would generate siloxy defects for charge compensation, yielding an interrupted framework which might block accesses.7 For example, CDS-1 zeolite contained quite a lot of Q3 groups in its corresponding CDO-type framework due to the existence of K+ cations in the interlayer space of PLS-1.17a However, if anions are incorporated into framework as charge compensation species, a complete condensation might take place for the formation of zeolites with occlusion of SDA cations. In PKU-22, several factors favor the maintenance of the organic species. Firstly, it is the F- anions bonded to Ge atoms in sti cages, which are toward to keep the TEA+ cations. Secondly, the cages in STI-type framework are suitable ones for the residence of the TEA+ cations. Thirdly, the short distance between the adjacent layers benefits the occurrence of the condensation reaction. Fourthly, the involvement of silicon into the framework enhances the stability of PKU-22 that enables the layer to be kept during the condensation. Thus, the transformation of PKU-22 to PKU-22a provides a useful model for understanding topotactic condensation process and shows a nice way to construct zeolites with less or none defects from the layered precursors.

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It is worthy to point out that during our study on the transformation of PKU-22, a new compound BUCT-2 was reported.43 Although PKU-22 has the similar structure layer with BUCT-2, the structure data of BUCT-2 is not completed and 5 the transformation of the structure was not mentioned in the work.

Rey, F.; Rius, J. Nature 2002, 418, 514-517. (c) Corma, A.; Rey, F.; 60 Valencia, S.; Jordá, J. L.; Rius, J. Nat. Mater. 2003, 2, 493-497. (d)

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CONCLUSIONS

We have synthesized a new layered silicogermanate PKU-22 under hydrothermal conditions using TEA+ cations as SDA in the presence of fluoride. Single crystal X-ray diffraction of PKU-22 indicates that PKU-22 takes a layered structure in which separated sti layers are stacked in an ···AAAA·· manner with TEA+ cations located in the inter-layer space. In-situ temperature-variable PXRD patterns revealed that PKU-22 could transfer to a 3D zeolite PKU-22a via topotactic condensation with the preservation of the SDA cations in it. PKU-22a possesses a STI-type framework, with a relative c/2 shift between neighbouring sti layers in comparison with that of PKU-22. An indicative scheme was proposed to show the topotactic condensation process. The remaining of TEA+ cations in the calcined product is mainly due to the existence of F- anions bonded to the framework. The transformation of PKU-22 to PKU-22a provides a useful model for understanding topotactic condensation process and shows a nice way to construct zeolites with less or none defects from the layered precursors.

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ASSOCIATED CONTENT

Supporting Information The atomic coordinates, selected bond distances and angles of 30 PKU-22; the experimental and simulated PXRD patterns of PKU22; SEM images; guest species in PKU-22; 1H MAS NMR and 1 H-13C CP MAS NMR for PKU-22 and PKU-22a; in-situ PXRD; TEM images; TG-DSC curves. This material is available free of charge via the Internet at http://pubs.acs.org. 35

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AUTHOR INFORMATION Corresponding Author * Tel: 86-10-62755538. Fax: 86-10-62751708. E-mail: [email protected].

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Notes 40 The authors declare no competing financial interest.

ACKNOWLEDGMENT

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This work was financially supported by the State Science and Technology Commission of China (Grant No. 2012CB224802), the National Natural Science Foundation of China (Grant No. 45 21171009), and the Presidential Foundation of CAEP (Grant No. 110 201501018). We thank National Center for Magnetic Resonance in Wuhan for 19F MAS NMR measurement. Physicochemical Analysis and Measurements Center in Institute of Chemistry Chinese Academy of Science is acknowledged for the performance of 115 1 13 1 29 1 29 50 solid-state H– C CP, H, Si, and H- Si CP MAS NMR spectra.

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For Table of Contents Use Only Synthesis and characterization of a layered silicogermanate PKU-22 and its topotactic condensation to a 3D STI-type zeolite Yanping Chen†, Shiliang Huang‡, Xiaoge Wang†, Lei Zhang†, Ningning Wu§, Fuhui Liao† and Yingxia ,† 5 Wang* Synopsis PKU-22, a new layered silicogermanate constructed by orderly stacking of sti layers, was transferred to a 3D zeolite PKU-22a on heating. PKU-22a takes the STI-type framework and keeps the SDA cations in its channels. A relative c/2 shift between neighbouring sti layers took place and the condensation process was separated from the removal of SDA 10 molecules.

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Synthesis and Characterization of a Layered Silicogermanate PKU-22

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