A novel dual-template method for synthesis of SAPO-44 zeolite

Abstract

The chabazite-like silicoaluminophosphate molecular sieve SAPO-44 was first synthesized using tetraethylenepentamine and N,N,N′,N′-tetramethyl-1,6-hexanediamine as dual-templates. Ammonia temperature-programmed desorption profiles confirmed the presence of abundant strong acid sites due to the higher Si/Al ratio in comparison with using the routine template cyclohexane, suggesting a promising application in solid acid catalysis by SAPO-44.

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The silicoaluminophosphate (SAPO) molecular sieves with well-defined pores and cavities of molecular dimensions have attracted widespread attention in the past decades as significant candidates for application in separation,¹ adsorption² and catalytic reactions.^3–6 The introduction of silicon into aluminophosphate (AlPO) frameworks makes SAPOs with abundant acid sites, which determines their potential in the field of catalysis, for example catalysis of methanol to olefins (MTO)^3,4,7 and selective catalytic reduction of NO_x with NH₃ (NH₃-SCR).^5,8–10 SAPO-44 as one of SAPO members also demonstrates excellent catalytic activity in the field of MTO and NH₃-SCR.^3,5 But reports of its synthesis methods and physicochemical properties have been quite limited.

In general, SAPO-44 is synthesized hydrothermally from a gel containing sources of Al, Si, P and the single-template cyclohexane (CHA). The limitation of available template types inhibits greatly the modulation of physicochemical properties, the molar ratio of Si/Al and the amount of added active ingredients, which further restricts its extensive application in catalyzing reactions such as MTO and NH₃-SCR. Recently, a complex of tetraethylenepentamine (TEPA) and Cu²⁺ (Cu–TEPA) has been employed as a novel template for the synthesis of Cu-SSZ-13 (an aluminosilicate zeolite), which exhibits superior catalytic performance in NH₃-SCR.¹¹ Later, Cu-SAPO-34 was synthesized using the Cu–TEPA complex combined with diethylamine (DEA) as dual-templates, presenting increased hydrothermal stability with very high activity for NH₃-SCR.¹² Most recently, tri-templates, i.e., tetraethyl ammonium hydroxide, triethylamine and morpholine, were also reported to synthesize SAPO-34, which shows high MTO activity resulting from high crystallinity, small crystal size and high surface area.¹³ Unfortunately, to the best of our knowledge, no dual-templates have been reported and employed in the synthesis of SAPO-44.

N,N,N′,N′-Tetramethyl-1,6-hexanediamine (TMHD) is generally used as template for synthesis of SAPO-56 with AFX framework.¹⁴ As discussed above, the combination of two or more kinds of templates possibly produces special effects on physicochemical properties of the final product. Here, we reported a novel method using TEPA and TMHD as dual-templates to synthesize SAPO-44 with high crystallinity and purity. The synergistic effect between TEPA and TMHD greatly influenced the properties, for example, higher Si/Al ratio and acid amount in comparison with using the routine single-template CHA, suggesting a promising application in solid acid catalysis for SAPO-44.

The hydrothermal route as the most commonly used method to prepare molecular sieves was employed in our research. Firstly, a designated amount of TEPA (when used; such as 0.37, 2.37, 3.37 molar ratios) was dissolved in deionized water; then phosphoric acid (85 wt%), pseudo bohemite (78 wt%) and silica solution (29.4 wt%) were mixed into solution in order, stirring vigorously. Afterwards, TMHD or CHA (when used) was added and blended under continuous stirring up to 15 h, where the molar ratio of sources was 0.6 SiO₂ : 0.8 Al₂O₃ : 1 P₂O₅ : 40 H₂O : 2.37 TMHD. The resulting gel was sealed in an autoclave with a Teflon liner, and heated at 200 °C for 96 h. After the reactor had cooled to room temperature naturally, the as-synthesized samples were washed using a centrifuge machine until the pH reached neutral level, and then dried in an oven at 100 °C for 12 h. Finally, the residual organic species were removed by calcination from 30 °C to 550 °C, holding at 550 °C for 6 h in air atmosphere.

The amount of P₂O₅ was set as the criterion to define clearly which kind of template was adapted and the amount of TEPA, TMHD and CHA. For example, TEPA (3.37, alone) means only TEPA was used as template and the molar ratio of TEPA/P₂O₅ is 3.37. TEPA (0.37) + TMHD (2) stands for TEPA and TMHD employed simultaneously, the values of the molar ratio of TEPA/P₂O₅ and TMHD/P₂O₅ being 0.37 and 2, respectively.

X-ray diffraction (XRD) patterns were recorded on a D8 FOCUS X-ray powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) operating at 50 kV and 30 mA in the range 5° < 2θ < 50° with a scanning speed of 0.2°. The Brunauer–Emmett–Teller (BET) surface area and pore size distribution were determined from the linear portion of the BET plots by measuring the N₂ adsorption and desorption isotherms at 77 K using an ASAP 2020 surface area and porosity analyzer. The elemental contents of samples were detected by a PerkinElmer Optima 2100DV based on inductively coupled plasma atomic emission spectrometer (ICP-AES). The morphology was examined using a Hitachi SU-70 field emission scanning electron microscope (SEM). NH₃ temperature-programmed desorption (NH₃-TPD) experiments were performed in a quartz reactor using 50 mg catalyst. NH₃ was monitored using a quadrupole mass spectrometer (OmniStar 200, Balzers). Prior to the experiments, the samples were pretreated at 500 °C for 30 min in 10 vol% O₂/He (50 mL min⁻¹) and then cooled to 100 °C. NH₃ adsorption took place at 4000 ppm NH₃/He (50 mL min⁻¹). Then the samples were purged with highly pure He for 1 h to remove any weakly adsorbed NH₃. Finally, the samples were heated to 800 °C at a ramping rate of 10 °C min⁻¹.

First, the single TEPA template for synthesis of SAPO-44 was investigated. As shown in Fig. 1, the product was a mixture including AlPO₄ (JCPDS 52-1178), SiO₂ (JCPDS 82-1562) and SAPO-44 (JCPDS 47-0630) for TEPA (0.37, alone). On further increasing the amount of TEPA, as for TEPA (2.37, alone), single phase SAPO-44 with poor crystallinity was obtained. However, continually increasing the amount of TEPA would result in difficulty in stirring the precursor gel due to the high viscosity. Furthermore, the crystallinity of TEPA (3.37, alone) showed no obvious improvement compared with TEPA (2.37, alone). In contrast, under the same synthesis conditions using TMHD as single-template, the crystal phase was assigned to SAPO-56 (JCPDS 52-1178), in agreement with a previous study.¹⁴ Interestingly, the high crystallinity of SAPO-44 was present when the dual-templates of TEPA (0.37) + TMHD (2) were substituted for the single-template (TEPA or TMHD). The intensities and positions of XRD peaks for TEPA (0.37) + TMHD (2) were similar to those for CHA. If the amount of TEPA was reduced, namely, by using TEPA (0.15) + TMHD (2.22) (TEPA/P₂O₅ = 0.15) as dual-templates, mixed crystal phases including SAPO-56 and SAPO-44 were observed simultaneously.

Fig. 1 XRD patterns of samples synthesized by single-template and dual-templates of TMHD and TEPA in different amounts.

In the dual-template system, the amount of TMHD was much higher than that of TEPA, but only the diffraction peaks assigned to SAPO-44 rather than SAPO-56 were found in the sample of TEPA (0.37) + TMHD (2). Only when the amount of TEPA was sufficiently low did the crystal phase of SAPO-56 appear in the sample of TEPA (0.15) + TMHD (2.22). This indicates that TEPA plays the role of organic structural directing agent (OSDA), while TMHD serves as an assistant-template to fill space and support the framework during the formation of the chabazite structure. The shape and the size of OSDA could affect the cavities of different small pore zeolites greatly, even though the final products are the same.^7,10 The molecular structure of TEPA is a linear chain, differing from the cyclic annular structure of the CHA molecule. Cyclic structures possibly favor the formation of chabazite cavities for SAPO-44 due to the similarity of structural characteristics. The linear chain of TEPA is smaller than the cavities, but when TMHD is introduced into the system and mixed with TEPA sufficiently, the appropriate size of bimolecular templates could support and fix the formation of the framework tightly, which would contribute to the high crystallinity of SAPO-44. Moreover, TMHD could act as a charge compensator with TEPA for negatively charged lattices of SAPOs in the function of template molecule, as previously reported.¹⁵

Fig. 2a shows N₂ adsorption/desorption properties of the samples. TEPA (0.37) + TMHD (2) shows a typical microporous structure, similar to the CHA (2, alone) sample, even though the BET surface area is lower for the former (367 m² g⁻¹) than the latter (563 m² g⁻¹). Interestingly, the pore size distributions, as shown in Fig. 2b, are quite similar for both TEPA (0.37) + TMHD (2) and CHA (2, alone), showing micropore size mainly focused on 0.68 nm. However, the micropore size is 0.59 nm and the BET surface area is only 234 m² g⁻¹ for TEPA (3.37, alone). In addition, some mesopores and macropores were found in the pore distribution of TEPA (3.37, alone), due to large amounts of uncrystallized particles. However, the same phenomenon did not take place in the samples of TEPA (0.37) + TMHD (2) and CHA (2, alone). It is a common concept that samples with the same topological structures have the similar micropore structure. However, some details may be different. For example, Rostami et al. reported that the SAPO-34 molecular sieves can be synthesized using tetraethyl ammonium hydroxide, morpholine or a mixture of the two as templates. The results show that different pore sizes of SAPO-34 could be obtained from the three templates.⁷ In fact, the different OSDAs with different shape and size play a great role in controlling the structure of products, even though the samples show the same XRD patterns. Importantly, the pore size distribution (Fig. 2b inset) showed the tiny difference in the maxima for TEPA (0.37) + TMHD (2) and CHA (2, alone) in comparison with TEPA (3.37, alone), as indicated above. These results further confirmed that the single-template TEPA is too small to form and support the cavities of SAPO-44, which leads to low crystallinity with the single-template TEPA.

Fig. 2 N₂ adsorption–desorption isotherms (a) and pore diameter distribution (b) of TEPA (3.37, alone), CHA (2, alone) and TEPA (0.37) + TMHD (2).

The amount of acid in zeolites greatly determines the catalytic performance of reactions such as MTO and NH₃-SCR. Fig. 3 shows NH₃-TPD profiles, and the corresponding data including ICP-AES results are collected in Table 1. Two desorption peaks centered at around 200 and 400 °C, could be found in both TEPA (0.37) + TMHD (2) and CHA (2, alone). The first desorption peak is attributed to the hydroxyl groups (–OH) bonded to the defect sites, i.e. P–OH, Si–OH, and Al–OH, while the second peak is assigned to the bridging hydroxyl group, i.e. –SiOHAl–, formed by substitution of phosphorous by silicon, which is responsible for strong acidity of SAPOs.¹⁶ The first desorption peak for TEPA (0.37) + TMHD (2) is quite similar to that of CHA (2, alone), while the second peak is different. Namely, TEPA (0.37) + TMHD (2) shows more strong acid sites than CHA (2, alone) (Table 1). From the ICP results in Table 1, TEPA (0.37) + TMHD (2) shows a much higher Si/Al ratio than CHA (2, alone), which results in a higher substitution of P by Si and so more –SiOHAl–. The higher Si/Al ratios are derived from the higher combination ability of TEPA in TEPA (0.37) + TMHD (2) than of CHA in CHA (2, alone). The relation between acid sites and Si/Al ratio is in accordance with previous results.^6,16–18 It is common that strong acid sites play a major role in determining the catalytic performance of MTO reactions due to the reaction temperature generally being higher than 400 °C.¹⁶ The presence of more acid sites, especially strong acid sites, could be beneficial to the improvement of catalytic performance. The higher number of strong acid sites for TEPA (0.37) + TMHD (2) shows its much greater potential application in the field of solid acid catalysis than CHA (2, alone).

Fig. 3 NH₃-TPD profiles of CHA (2, alone) and TEPA (0.37) + TMHD (2).

Table 1 The quantity of NH₃ desorption from NH₃-TPD profiles and element content from ICP-AES data

Sample	NH3-TPD (μmol g^−1)			Si/Al (molar ratio)
	Total	Weak	Strong
CHA (2, alone)	638	147	491	1.46
TEPA (0.37) + TMHD (2)	967	97	870	1.86

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Fig. 4 shows the morphology of TEPA (0.37) + TMHD (2), which presents a cubic shape typical of SAPO-44 with an average crystal size about 12 μm, consistent with that synthesized by CHA template.¹⁷ The result further illustrated that highly crystalline SAPO-44 with a regular structure also could be synthesized by a dual-template method.

Fig. 4 SEM image of SAPO-44 synthesized by TEPA (0.37) + TMHD (2).

Conclusion

A novel method using dual-templates composed of TEPA and TMHD was employed to prepare microporous SAPO-44 with high crystallinity and purity. TEPA plays the role of OSDA, while TMHD is an assistant-template to fill space and support the framework during the formation of the chabazite structure. Compared with using the routine single-template CHA, a greater abundance of strong acid sites is obtained using dual-templates due to the higher Si/Al ratio, demonstrating the positive modulation of performance by adopting different templates.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 21477046, 21277060 and 21547007) and Open Project from Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials.

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