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DOI: 10.1039/C5RA09942K (Paper) RSC Adv., 2015, 5, 63209-63214

Germanosilicate zeolite ITQ-44 with extra-large 18-rings synthesized using a commercial quaternary ammonium as a structure-directing agent†

Kun Qian*ab, Yilin Wangb, Zhiqiang Liangb and Jiyang Li*b
aLiaoning Medical University, Jinzhou 121001, P. R. China
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China. E-mail: lijiyang@jlu.edu.cn

Received 27th May 2015 , Accepted 9th July 2015

First published on 9th July 2015

Abstract

Germanosilicate zeolite ITQ-44 with extra-large 18-rings has been hydrothermally synthesized by using a commercial benzyltriethylammonium bromide (denoted as SDA-1) as structure-directing agent (SDA). The crystallization field and the influence of various synthetic factors on the synthesis of ITQ-44, such as the crystallization temperature, the crystallization time, the Si/Ge ratio, and the amount of the F− and water have been studied. The as-synthesized ITQ-44 has been characterized by XRD, ICP, CHN, TG, SEM, NMR, N2 adsorption and NH3-TPD. The results indicate that use of SDA-1 cations could lead to a pure phase of ITQ-44 in a wide synthetic range and remain intact in the final product. The framework structure of ITQ-44 is maintained at 500 °C. B, Al and Ga atoms can be introduced into the framework of germanosilicate ITQ-44, which produces Lewis acid sites in the framework. This work offers the possibility to synthesize extra-large pore germanosilicate zeolites with simple and commercial SDAs, which is important for the practical applications of germanosilicate zeolites.

1. Introduction

Extra-large pore (>12-rings) zeolites are highly desirable in both the petrochemical and fine chemical industries due to their potential applications of catalysis, adsorption and separation in dealing with bulkier molecules.1–4 However, among 229 distinct zeolite framework types approved by the Commission of the International Zeolite Association (IZA-SC),5 extra-large pore zeolites, particularly those with low framework density and more than 18-ring opening windows are limited. The only typical examples known are 18-ring silicate ECR-34 (ETR),6 20-ring gallophosphate Cloverite (-CLO),7–9 and 18-ring aluminophosphate VPI-5 (VFI).10–12 In recent years, several synthetic strategies have been developed towards the design and synthesis of zeolites.13–19 Particularly, a series of extra-large pore germanosilicates have been synthesized using pre-designed organic cations as structure-directing agents (SDAs). Significant examples are ITQ-37 (-ITV)20–22 with 30-rings, ITQ-43 (ref. 21 and 23) with 28-rings, ITQ-33 (ref. 24 and 25) and NUD-1 (ref. 26) with 18-rings, ITQ-54 (ref. 27) with 20-rings, and ITQ-44 (ref. 21 and 28) with 18-rings, etc. Note that the synthesis of these extra-large pore germanosilicates depends mainly on the use of complex organic SDAs, and most of such SDAs are prepared by complex reaction schemes and are not commercially available. Hence a great interest has been aroused to explore simple and cheap SDAs to direct the synthesis of these extra-large pore zeolites, which is important for their practical applications.

Germanosilicate ITQ-44 (IRR) is the first zeolite synthesized possessing a three-dimensional (3D) pore system made of intersecting 18- and 12-ring channels. Its inorganic framework is constructed by double 4-rings (d4rs) and double 3-rings (d3rs), and has a low framework density of 12.3 T per 1000 Å3. To date, synthesis of B or Al incorporated silicogermanate ITQ-44 has been reported by using complex and bulky (2′-(R),6′-(S))-2′,6′-dimethylspiro[isoindole-2,1′-piperidin-1′-ium] as the SDA. Such an SDA is not commercially available; it is prepared at laboratory scale by the reaction of 1,2-bis(bromomethyl)benzene and cis-2,6-dimethylpiperidine with chloroform and other necessary organic solutions. Moreover, the crystalline zone for the pure phase of ITQ-44 is very narrow and the B or Al heteroatoms are necessary in the synthesis.

In this work we report for the first time the synthesis of pure germanosilicate ITQ-44 by utilization of cheap and commercial benzyltriethylammonium bromide (SDA-1) as the SDA. In the synthesis, Ga, B, and Al can also be introduced into the framework of germanosilicate ITQ-44 as heteroatoms, leading to a potential solid-state acid catalyst. The influence of various synthetic factors on the formation of ITQ-44, such as the crystallization temperature and crystallization time, Si/Ge ratio, and amount of F− and water, has been carefully studied to achieve the optimal crystallization field of ITQ-44. Several characterization techniques including XRD, ICP, CHN, TG, SEM, NMR, N2 adsorption and NH3-TPD have been employed to characterize the as-synthesized products.

2. Experimental section

2.1. Chemicals

Colloidal silica (Aldrich, Ludox AS-40), GeO2 (Yunnan Lincang Xinyuan Germanium Industrial Co., Ltd, 99.99%), Ga2O3 (Shanghai Chemical Reagent Co., Ltd, 99.99%), benzyltriethylammonium bromide (Tianjin Guangfu Fine Chemical Research Institute, 99%). NH4Cl (99.5%), NH4F (96%), H3BO3 (99.5%), Al2O3 (AR), Al(OH)3 (AR), Al(NO3)3 (99.0%) and AlCl3 (97.0%) were purchased from Beijing Chemical Works. These chemicals were used as received without further purification. Fresh Al(OH)3 was prepared by adding ammonia dropwise to an AlCl3 solution. The gel was filtered, washed with deionized water and alcohol, and then dried at room temperature.

2.2. Synthesis of ITQ-44

The gel molar composition for the synthesis of germanosilicate ITQ-44 was as follows: 0.67SiO2[thin space (1/6-em)]:[thin space (1/6-em)]0.33GeO2[thin space (1/6-em)]:[thin space (1/6-em)]0.25SDAOH[thin space (1/6-em)]:[thin space (1/6-em)]yNH4F[thin space (1/6-em)]:[thin space (1/6-em)](0.25 − y)NH4Cl[thin space (1/6-em)]:[thin space (1/6-em)]xH2O (x = 1–5 and y = 0–0.25). The commercial benzyltriethylammonium bromide (SDA-1) was exchanged to the hydroxide form with an anionic exchange resin first. Then, the colloidal silica and GeO2 were dissolved in a solution of the SDA-1. Finally, the NH4F and NH4Cl were added and the reaction mixture was stirred to obtain an even gel. Some of the water in the gel was evaporated under an infrared lamp with stirring until the final desired water quantity was achieved. The dry gel was transferred into an 18 mL Teflon-lined stainless steel autoclave, which was put into an oven for crystallization at 140–200 °C for 1–10 days. The final white colored crystalline product, was washed with water and ethanol, and dried at an ambient temperature.

B-ITQ-44, Al-ITQ-44 and Ga-ITQ-44 can be synthesized from the gel with a molar composition of 0.67SiO2[thin space (1/6-em)]:[thin space (1/6-em)]0.33GeO2[thin space (1/6-em)]:[thin space (1/6-em)]zM2O3[thin space (1/6-em)]:[thin space (1/6-em)]0.25SDAOH[thin space (1/6-em)]:[thin space (1/6-em)]yNH4F[thin space (1/6-em)]:[thin space (1/6-em)](0.25 − y)NH4Cl[thin space (1/6-em)]:[thin space (1/6-em)]xH2O (x = 1–5; y = 0–0.25; z = 0.005–0.050; M = Ga, Al, B) at 170 °C for 1 day.

2.3. Characterization

Powder X-ray diffraction (XRD) and in situ variable temperature X-ray diffraction data were collected on a Rigaku D-Max 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å) to identify the phase and study the thermal stability, respectively. The scanning electron microscopy (SEM) image was taken on a S4800 scanning electron microscope. The solid 13C and 27Al MAS NMR spectra were recorded on a Bruker AVANCE III 400 WB spectrometer. Nitrogen adsorption–desorption isotherms were measured on an ASAP 2020 V3.02 micromeritics surface and porosity analyzer. The inductively coupled plasma (ICP) analysis was carried out on a Perkin-Elmer Optima 3300 DV. The NH3 temperature programmed desorption (NH3-TPD) measurement was performed on a micromeritics AutoChem II 2920 Chemisorption Analyzer. The CHN element analysis was performed on a Vario Micro Cube elemental analyzer. Thermogravimetric analysis (TG) was carried out on a NETZSCH STA449C TG/DTA analyzer in air with a heating rate of 10 °C min−1.

2.4. Simulation method

Molecular dynamics (MD) calculations were performed to simulate the location of SDA-1 in zeolite ITQ-44. The energy calculations were done using the UFF force field within the Forcite program.29 Periodic boundary conditions were applied in one unit cell. The inorganic framework was fixed in order to optimize the position of SDA-1. The inorganic framework was considered as an all Si composition and F− ions were added together with SDA-1 to achieve the charge balance. Charge equilibration was used in simulation to redistribute the overall charge on the guest molecules and atoms of the framework.30 Times steps of 1 fs and a million steps were used to find the stationary point within the NVE ensemble during the dynamical simulation. The calculated electrostatic interaction was evaluated through the Ewald sum method and van der Waals interactions with a 4.5 Å cutoff. The total simulation time was 500 ps for each structure. For comparison, the interaction of SDA-1 and ITQ-17 zeolite had also been calculated. All the computational studies were carried out by using Material Studio software.31

3. Results and discussion

3.1. Synthesis of ITQ-44

In this work, the commercial benzyltriethylammonium bromide (Fig. 1b) instead of complex 2′,6′-dimethylspiro[isoindoline-2,1′-piperidin]-1′-ium (Fig. 1a) has been used as the SDA to synthesize ITQ-44. The two different SDAs have a similar ratio of C/N, hydrophobic properties and spatial configurations.
image file: c5ra09942k-f1.tif
Fig. 1 The structures of (a) 2′,6′-dimethylspiro[isoindoline-2,1′-piperidin]-1′-ium and (b) SDA-1.

The influence of various synthetic factors on the synthesis of ITQ-44, such as the crystallization temperature, crystallization time, Si/Ge ratio, and amount of F− and water has been carefully studied. In these studies, the H2O/T (T = Si + Ge) ratios are selected as 1, 2, 3 and 5; the Ge/Si ratios are selected as 0, 0.1, 0.5 and 1.0; the F−/T ratios change from 0 to 0.25, and the crystallization is performed at 140–200 °C for 1–10 days. The resultant phases are identified by powder X-ray diffraction analysis.

First, the influence of the amount of water on the synthesis of ITQ-44 was investigated. As shown in Table 1, when the H2O/T ratio is 1, a pure phase of ITQ-44 can be obtained. On increasing the water amount, germanosilicate ITQ-17 (ref. 32 and 33) with a BEC topology and 3D 12-ring channels, begins to appear in the product mixed with ITQ-44. By further enhancing the H2O/T ratio to 5, pure ITQ-17 is produced with no ITQ-44. This indicates that the water amount is a vital factor for the phase selectivity of the product, and that the pore opening of the final product decreases with the increase of the water amount added in the synthesis. It has been noted that at a suitable water amount, the existence of heteroatoms does not affect the formation of the final product. The molecular simulation study indicates that SDA-1 has a low interaction energy with the frameworks of ITQ-44 (−55.2 kcal mol−1, per unit cell) and ITQ-17 (−32.61 kcal mol−1, per unit cell), indicating SDA-1 is suitable for their synthesis.

Table 1 Products obtained with a composition of 0.67SiO2[thin space (1/6-em)]:[thin space (1/6-em)]0.33GeO2[thin space (1/6-em)]:[thin space (1/6-em)]zM2O3[thin space (1/6-em)]:[thin space (1/6-em)]0.25SDAOH[thin space (1/6-em)]:[thin space (1/6-em)]0.25NH4F[thin space (1/6-em)]:[thin space (1/6-em)]xH2O (x = 1–5; z = 0–0.025; M = Ga, Al, B) at 170 °C for 1 day
M content H2O/T ratio
1 3 5
None   ITQ-44 ITQ-44 + ITQ-17 ITQ-17
B/T 0.01 ITQ-44 ITQ-44 + ITQ-17 ITQ-17
0.05 ITQ-44 ITQ-44 + ITQ-17 ITQ-17
Al/T 0.01 ITQ-44 ITQ-44 + ITQ-17 ITQ-17
0.05 ITQ-44 ITQ-44 + ITQ-17 ITQ-17
Ga/T 0.01 ITQ-44 ITQ-44 + ITQ-17 ITQ-17
0.05 ITQ-44 ITQ-44 + ITQ-17 ITQ-17

In contrast to the synthesis reported in ref. 21 and 28, the pure germanosilicate ITQ-44 can be prepared in the presence of SDA-1. Previous studies have shown that the Ge/Si ratio is important to the synthesis of the pure phase of germanosilicate ITQ-44. As shown in Table 2, there is no crystalline product when the Ge content is very small (Ge/Si ratio < 0.1). The pure ITQ-44 can be obtained when the Ge/Si content is in between 0.5 and 1.0, and the H2O/T ratio is 1–2. The importance of the role of Ge atoms in the formation of ITQ-44 is due to the large number of d3r and d4r cages in the structure of ITQ-44; the Ge atoms can stabilize these small rings.21 It should be noted that the use of F− ions is necessary in such syntheses. Pure ITQ-44 can be produced in a wide range of F−/T ratios, from 0.05 to 0.25 (see Table S1†). In the absence of F− ions, the main product is GeO2. Considering that the framework of ITQ-44 contains a large number of d4rs, the F− ions may locate in the center of these d4rs and stabilize the final structure, which is consistent with the previous reports.21,28

Table 2 The products obtained in the reaction gel with molar composition of mSiO2[thin space (1/6-em)]:[thin space (1/6-em)]nGeO2[thin space (1/6-em)]:[thin space (1/6-em)]0.25SDAOH[thin space (1/6-em)]:[thin space (1/6-em)]0.25NH4F[thin space (1/6-em)]:[thin space (1/6-em)]xH2O (x = 1–3; m + n = 1, n/m = 0–1.0) at 170 °C for 1 day
H2O/T Ge/Si
0 0.1 0.5 1.0
1 Amorphous Amorphous ITQ-44 ITQ-44
2 Amorphous Amorphous ITQ-44 ITQ-44
3 Amorphous Amorphous ITQ-44 + ITQ-17 ITQ-44 + ITQ-17

As shown in Tables S2 and S3,† a minor effect of the crystallization temperature and crystallization time has been found on the synthesis of ITQ-44. The pure phase of ITQ-44 can be obtained at 150–200 °C for 1–10 days. However, a low Ge/Si ratio and low crystallization temperature (140 °C) does not favour the formation of a pure phase of ITQ-44, instead an amorphous phase of ITQ-44 will be observed.

Heteroatoms can be inserted in the framework of ITQ-44 from a gel with molar composition of 0.67SiO2[thin space (1/6-em)]:[thin space (1/6-em)]0.33GeO2[thin space (1/6-em)]:[thin space (1/6-em)]zM2O3[thin space (1/6-em)]:[thin space (1/6-em)]0.25SDAOH[thin space (1/6-em)]:[thin space (1/6-em)]0.25NH4F[thin space (1/6-em)]:[thin space (1/6-em)]xH2O (x = 1–2, z = 0.005–0.050, M = Ga, Al, B) at 170 °C for 1 day (Table 1). In the synthesis of Al-ITQ-44, the effects of Al source, Al/T ratio and H2O/T ratio have been studied. As shown in Table 3, under a suitable H2O/T ratio of 1.5–2, using the Al(OH)3 and fresh Al(OH)3 as an Al source can lead to pure Al-ITQ-44, while an amorphous phase is obtained when using Al(NO3) or Al2(SO4)3 as an Al source under similar conditions. Higher Al amounts, such as Al/T ratio of 0.1, will produce the mixture of Al-ITQ-44/Al2O3 or Al-ITQ-44/GeO2. This suggests that only a limited amount of the Al atoms can be incorporated into ITQ-44. This result is further confirmed by the ICP analysis (Table S4†). It shows that the content of Al atoms in the product increases with the increase of the molar ratio of M/(Si + Ge) in the reaction gel, but that the total amount of Al atoms in the product is less than that initially added in the reaction gel. Besides Al atoms, B and Ga atoms can also be introduced into the framework of germanosilicate ITQ-44. The amounts of B and Ga atoms in the product are less than that of Al atoms in Al-ITQ-44.

Table 3 Products obtained in the reaction gel with composition of 0.67SiO2[thin space (1/6-em)]:[thin space (1/6-em)]0.33GeO2[thin space (1/6-em)]:[thin space (1/6-em)]zAl2O3[thin space (1/6-em)]:[thin space (1/6-em)]0.25SDAOH[thin space (1/6-em)]:[thin space (1/6-em)]0.25NH4F[thin space (1/6-em)]:[thin space (1/6-em)]xH2O (x = 1.5, 2; z = 0.025, 0.05) at 170 °C for 1 day
Al source Al/T = 0.05 Al/T = 0.10
H2O/T = 1.5 H2O/T = 2.0 H2O/T = 1.5 H2O/T = 2.0
Al(OH)3 ITQ-44 ITQ-44 ITQ-44 + Al2O3 ITQ-44 + Al2O3
Fresh Al(OH)3 ITQ-44 ITQ-44 ITQ-44 + GeO2 ITQ-44 + GeO2
Al(NO3)3 Amorphous Amorphous Amorphous Amorphous
Al2(SO4)3 Amorphous Amorphous Amorphous Amorphous

Overall, the pure and M-doped germanosilicates ITQ-44 (M = Al, B, Ga) can be synthesized in a wide synthetic range by using commercial SDA-1. Among various synthetic factors, the water amount and the Si/Ge ratio are important for the phase selectivity. Al, B, and Ga atoms have been successfully introduced into ITQ-44. The presence of the trivalent metals promotes the formation of acidic sites in the framework, making the product useful in catalysis.

3.2. Characterization of ITQ-44

The experimental XRD pattern of the as-synthesized products is consistent with the simulated one generated on the basis of the reported crystal structure data of ITQ-44,28 proving that the as-synthesized ITQ-44 is in a pure phase (Fig. 2). SEM image of ITQ-44 shows that the particle size is of micrometer scale, and the morphology is hexagonal prism, which corresponds to its crystallographic structure crystallized in P6/mmm space group (Fig. 3).
image file: c5ra09942k-f2.tif
Fig. 2 The XRD patterns of as-synthesized ITQ-44 and the simulated ITQ-44 based on the reported crystal structure.

image file: c5ra09942k-f3.tif
Fig. 3 The SEM image of as-synthesized ITQ-44.

The solid state 13C NMR spectrum of ITQ-44, as shown in Fig. 4, indicates that the organic SDA-1 cations are kept intact in the final product. CHN analysis of the as-synthesized ITQ-44 reveals that the content of C, N, and H is 11.09, 0.94 and 1.71, respectively, giving rise to the molar ratio of C/N of 13.7, which is close to the C/N ratio in SDA-1.


image file: c5ra09942k-f4.tif
Fig. 4 The solid state 13C NMR spectrum of ITQ-44.

The TG curve in Fig. S1† shows a total weight loss of ca. 13% for the as-synthesized ITQ-44, which mainly corresponds to the decomposition of the occluded organic species in the framework. On base of the empirical formula of ITQ-44,28 as well as the TG and compositional analyses (Si/Ge = 2), the formula of the as-synthesized ITQ-44 can be given as [C13H22N]17[Ge100Si200O600F17]. The positions of the SDA-1 cations occluded in ITQ-44 are theoretically simulated by using Materials Studio Software (Fig. S2†).

The in situ variable temperature XRD analysis reveals that the as-synthesized ITQ-44 has good thermal stability (Fig. 5). Its framework structure can remain in temperatures of up to 500 °C in air. At 600 °C, the crystallinity of ITQ-44 decreases greatly. The SDA-1 cations occluded in ITQ-44 can be removed by calcination at 500 °C for 5 hours in O2. The N2 adsorption of calcined ITQ-44 (Fig. S3†) gives the BET surface area of 350 m2 g−1, and the t-plot micropore volume of 0.13 cm3 g−1. Such a BET surface area is lower than that of the reported ITQ-44 (BET, 470 m2 g−1), which could have been underestimated owing to some loss of crystallinity during the calcination process.


image file: c5ra09942k-f5.tif
Fig. 5 The in situ variable temperature XRD patterns of ITQ-44.

Fig. S4† gives the XRD patterns of the as-synthesized M-ITQ-44 (M = Al, B, Ga). Ga-incorporated ITQ-44 has not been reported previously.34 The 27Al MAS NMR spectrum of Al-ITQ-44 provides direct evidence for the existence of Al species in the as-synthesized sample (Fig. 6). There are two kinds of coordinated Al atoms in the product. The small signal with a chemical shift at 50 ppm corresponds to the four-coordinated Al atoms in the framework, while the peak around 15 ppm corresponds to the non-framework five-coordinated Al atoms. The 27Al MAS NMR results reveal that only a limited amount of the tetrahedrally coordinated Al atoms are introduced into the ITQ-44 framework.


image file: c5ra09942k-f6.tif
Fig. 6 The 27Al MAS NMR spectrum of the as-synthesized Al-ITQ-44.

NH3-TPD measurements were carried out to investigate the acidic property of the calcined germanosilicate ITQ-44 and M-ITQ-44 (M = Al, B, Ga). Obvious peaks between 170 and 220 °C resulted from Lewis acid sites can be observed for M-ITQ-44 when compared with ITQ-44. The acidity sites of Al-ITQ-44 are much greater than those of B/Ga-ITQ-44 (Fig. 7). This indicates that the Al-ITQ-44 will be more efficient than the B/Ga-ITQ-44 in a weak acid reaction. However, the strong Brönsted acid sites can not be observed clearly in these compounds, which may be due to too few four-coordinated heteroatoms in the framework. In addition, it should be noted that some amorphous phase (SiO2 or M2O3) could be observed in M-ITQ-44, which may also contribute to the formation of Lewis sites.35,36 A detailed investigation is underway.


image file: c5ra09942k-f7.tif
Fig. 7 The NH3-TPD curve of calcined ITQ-44 and M-ITQ-44 (M = Al, B, Ga).

4. Conclusions

This work reports for the first time the use of a commercial benzyltriethylammonium bromide as the SDA to synthesize pure germanosilicate ITQ-44, and Al, B, Ga-doped ITQ-44 zeolites. In the synthesis, the amount of water and the ratio of Ge/Si are of great importance for the phase selectivity of ITQ-44. The as-synthesized ITQ-44 has good thermal stability, since its structure can be retained upon calcinations in the air at 500 °C. A few tetrahedrally coordinated heteroatoms can be introduced into the framework of ITQ-44, which mainly leads to Lewis acid sites. This work offers a feasible route for the synthesis of extra-large pore ITQ-44, and may be useful to explore its applications in the future.

Acknowledgements

This work is supported by National Natural Science Foundation of China (No. 21271081) and Ahon Pharma Funding of Liaoning Medical University (No. XZJJ 20140104).

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Footnote

† Electronic supplementary information (ESI) available: The influence of F− ions, crystallization temperature and time on the synthesis of ITQ-44. ICP analysis, TG and N2 adsorption of ITQ-44, XRD of M-ITQ-44, and location of SDA-1 in ITQ-44. See DOI: 10.1039/c5ra09942k
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