Microwave-assisted synthesis of a thermally stable Zn-containing aluminophosphate with ERI-zeotype structure templated by diquaternary alkylammonium

Abstract

Microwave-assisted hydrothermal synthesis of a Zn-containing aluminophosphate with ERI-zeotype structure, was successfully prepared by using N,N,N,N′,N′,N′-hexamethyl-1,6-hexanediammonium (Me₆-diquat-6) as the template. The as-synthesized zeolite possesses high thermal stability, large surface area and weak and medium acid sites, which will be potentially interesting for applications in adsorption and catalysis.

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Aluminophosphate molecular sieves (AlPO₄-n) constructed by alternating connected AlO₄ and PO₄ tetrahedra are an important class of microporous materials.^1–4 They possess neutral frameworks without Bronsted acidity, and thus exhibit low catalytic activity compared to their aluminosilicate zeolite counterparts. Metal substitution of aluminium can modify the properties of AlPO₄-n, which enables them with remarkable catalytic performance. For example, MnAPO-5 showed good catalytic activity in the alkylation of benzene,⁵ the dehydrogenation of ethane to ethane,⁶ the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone (KA-oil),⁷ and the oxidation of p-cresol to p-hydroxylbenzaldehyde.⁸

The AlPO₄-17 with ERI zeolite framework type has 8-ring channels of 3.6 × 5.1 Å in window size, and is featured by the quasi-closed cavities in the framework.^9–12 Some metal substituted AlPO₄-17 have been tested in various catalytic reactions. For instance, the nickel-modified SAPO-17 (ERI) exhibited catalytic performance in the methanol-to-olefins reaction, which was influenced by the amount of incorporated Ni atoms, and it showed that an optimum Ni concentration can lead to a maximum ethylene selectivity.¹³

AlPO₄-17 is generally synthesized under conventional electric heating mode in the presence of piperidine,⁹ quinuclidine,¹⁰ cyclohexylamine,¹¹ and N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA).¹² It is widely reported that microwave synthesis of zeolites offers many distinct advantages over conventional synthesis such as rapid crystallization, increased phase purity and phase selectivity, narrow particle size distribution and facile optimization of synthesis conditions.^14–19 Microwave technique is not only an efficient synthesis method, but also energy efficient and economical. However, microwave-assisted synthesis of AlPO₄-17 and metal-substituted AlPO₄-17 have not yet been performed to our knowledge. Therefore, it is of interest to investigate the microwave-assisted hydrothermal synthesis of metal-substituted AlPO₄-17.

In this work, we report the successful synthesis of Zn-substituted AlPO₄-17 by using diquaternary ammonium as the template under microwave irradiation. The result product is denoted as ZnAPO-ERI-MW, which possesses high thermal stability and accessible porosity in contrast to that synthesized under similar conditions by conventional heating (denoted as ZnAPO-ERI-CV).

The SEM images of ZnAPO-ERI-CV and ZnAPO-ERI-MW are shown in Fig. 1a and b, respectively. It revealed that the ZnAPO-ERI-CV particle size obtained by conventional hydrothermal synthesis is of 1 μm, while the ZnAPO-ERI-MW particle size obtained under microwave heating process is of 200–400 nm. In comparison, the ZnAPO-ERI-MW crystals have more regular shape, morphology and uniform particle size. Their difference in crystal size and morphology might be caused by the different crystallization kinetics that take place in conventional and microwave heating processes.

Fig. 1 SEM images of (a) ZnAPO-ERI-CV (b) ZnAPO-ERI-MW.

Structure and phase purity were studied by powder X-ray diffraction (XRD) analysis. Fig. 2a shows the simulated XRD pattern of AlPO₄-17 (ERI) generated based on the reported crystal structure data.¹⁸ The experimental XRD patterns of the ZnAPO-ERI-CV products conducted in conventional oven (Fig. 2b) and ZnAPO-ERI-MW products conducted in microwave oven (Fig. 2c) are consistent with the simulated one, indicating that the as-synthesized products are pure phase. Inductively negatively coupled plasma (ICP) analyses results indicate that the ratio of Zn/Al in both ZnAPO-ERI-MW and ZnAPO-ERI-CV are 0.10. Fig. 2d showed that the structure of ZnAPO-ERI-MW remains intact at 540 °C with the decomposition of SDAs, while the samples of ZnAPO-ERI-CV began to transform into amorphous gradually at about 480 °C (Fig. 2e), which demonstrate the samples of ZnAPO-ERI-MW prepared under microwave heating are more stable than ZnAPO-ERI-CV conducted by convention heating process. Therefore, we mainly focused on the investigations concerning analyses and properties of ZnAPO-ERI-MW samples.

Fig. 2 XRD patterns of (a) simulated XRD of AlPO₄-17 (b) ZnAPO-ERI-CV (c) ZnAPO-ERI-MW (d) calcined ZnAPO-ERI-MW under 540 °C (e) calcined ZnAPO-ERI-CV under 480 °C.

Elemental analysis gives the ratio of C/H/N for ZnAPO-ERI-MW is 6.16/17.28/1, which is in agreement with the C₁₂H₃₂N₂O₂ formula of Me₆-diquat-6 dihydroxide. In order to prove the SDA molecules remained intact after the hydrothermal process, the liquid-state ¹H NMR and ¹³C NMR were carried out for pure SDA molecules and the organic species extracted after dissolving the framework (Fig. S1 and S2†). The as-synthesized ZnAPO-ERI-MW compound was first dissolved in 5 M HF solution to liberate the organic species, and then the solution was neutralized by addition of 2 M sodium hydroxide solution and further extracted with CH₂Cl₂. After evaporating the solvent, the liquid-state ¹H NMR and ¹³C NMR measurements were performed on the residual solid dissolved in D₂O (0.5 mL). The resultant ¹H NMR and ¹³C NMR spectrum is identical to that of Me₆-diquat-6 in D₂O solution, suggesting that the Me₆-diquat-6 cations remain intact in the structure of ZnAPO-ERI-MW.

Thermogravimetric (TG) analyses of ZnAPO-ERI-MW and ZnAPO-ERI-CV were performed in the air with a heating rate of 10 K min⁻¹ (Fig. S3†). For ZnAPO-ERI-MW, a weight loss of 3.81% between 25 °C and 100 °C corresponds to the removal of physically adsorbed water molecules, then two steps were observed in the TG trace for the decomposition of SDA molecules, a rapid weight loss step of 9.36% between 100 °C and 400 °C and a gradual weight loss step of 8.46% between 400 °C and 700 °C. However, for ZnAPO-ERI-CV, the first weight loss of 3.66% between 25 °C and 150 °C corresponds to the desorption of physically adsorbed water, then a one-step rapid weight loss of 15.90% between 150 °C and 500 °C was observed in the TG curves for the decomposition of SDA molecules, which might result in its diminished thermal stability. It has been reported that the hydrothermal stability of zeolites will increase with particle size decreases,²⁰ which is in accordance with the experimental result that the sample of ZnAPO-ERI-CV with larger particle size is less thermally stable.

Nitrogen adsorption–desorption isotherm of ZnAPO-ERI-MW was measured on ASAP 2020 V3.02 micromeritics surface and porosity analyzer as shown in Fig. 3. The BET surface area and the t-Plot micropore volume are 648.2 m² g⁻¹ and 0.249 cm³ g⁻¹, respectively. The pore diameter inserted in Fig. 3 ranged from 3.9 Å to 5.1 Å, which lies between the range of 3.6 × 5.1 Å reported by the Structure Commission of the International Zeolite Association (IZA-SC).²¹

Fig. 3 N₂ adsorption and desorption isotherms and pore diameter (insert) of ZnAPO-ERI-MW.

To investigate whether Zn is substituting into the aluminophate lattice, FT-IR spectra of AlPO₄-ERI, ZnAPO-ERI-CV and ZnAPO-ERI-MW were performed, respectively (Fig. S4†). The sample of AlPO₄-ERI was synthesized according to the literature.¹⁸ The changes in Fig. S4† to note are that there is a significant broadening and small red shift in the T–O–T asymmetric stretching region (1030–1220 cm⁻¹) of the ZnAPO-ERI-CV and ZnAPO-ERI-MW spectra when compared to the spectra of AlPO₄-ERI, which indicates that Zn is successfully involved in the aluminophate lattice.²²

Ammonia temperature-programmed desorption (NH₃-TPD) curves of AlPO₄-ERI and ZnAPO-ERI-MW are shown in Fig. 4. The NH₃ desorbed at low temperatures can be assigned to the physisorption of NH₃, while NH₃ desorbed at high temperatures correspond to the chemisorption of NH₃ and the area represents the relative acid-site density or the number of each acid-site.^23–28 For AlPO₄-ERI, only one desorption peak at 150–250 °C is observed, corresponding to the physisorption of NH₃. However, for ZnAPO-ERI-MW, there are two regions, region-I (150–250 °C) and region-II (250–350 °C), are observed in its acid profiles. These temperature ranges represent weak and medium acid strength, respectively, which could be attributed to the successful substitution of Zn atoms into the aluminophate lattice.^29,30

Fig. 4 NH₃-TPD profiles of AlPO₄-ERI and ZnAPO-ERI-MW.

In conclusion, thermally-stable zeolite with ERI framework topology is an important candidate for improving separation and catalytic processes. Microwave-assisted hydrothermal synthesis of Zn-containing aluminophosphate ZnAPO-ERI-MW with ERI-zeotype structure was successfully synthesized by using N,N,N,N′,N′,N′-hexamethyl-1,6-hexanediammonium as structure directing agents. Compared to AlPO₄-ERI and ZnAPO-ERI-CV synthesized under conventional heating process, ZnAPO-ERI-MW prepared by microwave radiation exhibits shorter synthesis time, higher thermal stability upon 540 °C with the removal of templates, and large surface area with the BET surface area of 648.2 m² g⁻¹. NH₃-TPD analysis of ZnAPO-ERI-MW shows that some weak and medium acid sites are present in the structure, which might contribute to its applications in adsorption or catalytic processes.

Experimental

General procedure for the preparation of organic SDA

The organic structure directing agent (SDA) of N,N,N,N′,N′,N′-hexamethyl-1,6-hexanediammonium (Me₆-diquat-6) cation was prepared by reacting 1,6-dibromohexane (97%, Aldrich) with an excess of trimethylamine (99%) in ethanol as a solvent with rapid stirring at room temperature overnight. Then the solvent was removed by evaporation under vacuum, and the resulting white solid was washed with diethyl ether until unreacted amine was completely removed from the product. The compound was verified by ¹H and ¹³C NMR. The obtained dibromide salt was converted into the dihydroxide by anion exchange in distilled water solution using an OH⁻ resin. The concentrated by rotoevaporation under vacuum with mild heating. The final concentration was determined by titration using a certified HCl solution and phenolphthalein. The concentration of Me₆-diquat-6 dihydroxide is 0.6 mol L⁻¹.

Preparation of ZnAPO-ERI-MW and ZnAPO-ERI-CV zeolites

Typically, 0.784 g of boehmite (Catapal® B, 72.6 wt% Al₂O₃, Sasol) was dissolved in a 4.65 g 0.6 mol L⁻¹ aqueous solution of Me₆-diquat-6 dihydroxide, and then 1.30 g H₃PO₄ (85%) and 0.832 g Zn(NO₃)₂·6H₂O were added. The mixture was stirred until a homogeneous solution was obtained. Then the gel was evaporated under infrared lamp with stirring until the final desired water quantity was achieved. The molar ratio of the final gel composition was 0.25Zn(NO₃)₂ : 0.50Al₂O₃ : 1.0H₃PO₄ : 0.25Me₆-diquat-6 : 45H₂O. If the ratio of H₂O/H₃PO₄ is higher than 45, a dense phase will be formed. The precursor gel was loaded in a 100 mL Teflon autoclave, which was then sealed and placed in a microwave oven (Milestone ETHOS-D). The mixture was heated to the reaction temperature of 160 °C in 2 min (microwave power was 400 W) and maintained at that temperature for 2 h (microwave power was 200 W).³¹ For comparison, the same reaction mixture was sealed in a 15 mL Teflon-lined stainless steel autoclave and heated in a conventional oven at 160 °C for 6 days. After cooling to room temperature, the product was obtained by centrifugation, then washed by distilled water and ethanol and dried at room temperature. The ZnAPO-ERI samples synthesized under microwave irradiation method and conventional hydrothermal method are named as ZnAPO-ERI-MW and ZnAPO-ERI-CV, respectively.

Acknowledgements

We thank the State Basic Research Project of China (Grant no. 2011CB808703), National Natural Science Foundation of China (no. 21001050), and the Fundamental Research Funds for the Central Universities (no. N130305003).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10541a

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