Coke suppression in MTO over hierarchical SAPO-34 zeolites

Abstract

Hierarchical SAPO-34 crystals were synthesized by a facile TEAOH etching post-treatment for the first time. Owing to the diffusion intensification and unchanged catalytic activity, the single-run lifetime of the resultant zeolites was significantly prolonged in the MTO process from 320 to 640 min without sacrificing the excellent methanol conversion and C⁼_2–4 selectivity.

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Methanol to olefins (MTO), which relies on non-petroleum resources, is considered as a strategic technology for olefin production to balance the shortage of petroleum and natural gas. As a result, the commercial MTO facilities with a C⁼_2–3 capacity of over 10 Mt per year have been established in China in the past decade.¹ In the MTO process, high-value light olefins like C⁼_2–4 rather than longer ones are desired.¹ Concerning the catalyst, shape selectivity and moderate acidity are the key requirements to obtain light olefins.¹ SAPO-34 is the representative catalyst for MTO due to its favourable structure with large cages (0.74 × 0.74 × 1.0 nm) and small windows (0.38 × 0.38 nm) as well as its providential acidity. However, it is usually suffering from rapid coking due to the diffusion limitation through the small windows, which drives the costs up for the frequent catalyst reactivation.^1–4

The residence time of intermediates/products on the acid sites is considered as the critical factor of coke formation.¹ Besides acidity tuning, therefore, many efforts have been made to enhance the diffusion efficiency by adjusting the morphology/structure of SAPO-34 to prolong the single-run lifetime, such as narrowing the crystal size or introducing meso-/macro-pores into the crystals.^2–18 Previous studies have shown that nano-sized SAPO-34 can enhance the MTO performance due to the shortened diffusion distance.^6,9 However, the synthesis of nano-sized SAPO-34 is challenging by the low crystallinity, low product yield and impurities.⁹ As an alternative, constructing SAPO-34 crystal with hierarchical structure, which contains macro-/meso- and micro-pores, is a promising approach to intensify the mass transport. Commonly, hierarchical pore structures can be achieved by either the direct synthesis with multi-templates^{2–5,8,10–12,14,19,20} or the indirect post-treatments.²¹ Comparing with the direct method, which employs the traditional templates e.g. TEA⁵ and the additional surfactants like gelatin⁵ to form micro- and meso-pores, respectively, the post-treatments, like dealumination via acid leaching and desilication by alkali etching, have great advantages of high homogeneity, facile processing and easy to scale-up, which are the main industrial strategies employed to fabricate meso-structured zeolites.⁷ In the case of SAPO-34, however, previous studies indicate that preparation of post-treated hierarchical structure with excellent methanol conversion, high light olefins selectivity and prolonged lifetime in MTO application is still a big challenge that has not been solved so far.²¹

In this work, we developed a facile and simple tetraethylammonium hydroxide (TEAOH) etching post-treatment method to obtain SAPO-34 with hierarchical pore structure. The single-run lifetime of the hierarchical zeolites in MTO was significantly prolonged while the excellent catalytic performance was well preserved.

The parent SAPO-34 crystals were prepared in a reaction mixture with a mass ratio of SAPO-34 crystal seeds to synthesis gel (molar composition of 1Al₂O₃ : 0.44SiO₂ : 1.1P₂O₅ : 2.25triethylamine : 35H₂O) of 1 : 400 at 165 °C for 33 hours. The solid product was washed and dried, followed by calcination at 600 °C for 5 h. In a typical post-treatment, 15 g parent SAPO-34 was mixed in 300 mL TEAOH aqueous solution (0.05–0.2 mol L⁻¹) at 90 °C for 3–9 h. The as-treated SAPO-34 was then separated by filtration and followed by washing and drying before use.

The morphologies of SAPO-34 with and without TEAOH treatments were studied by SEM and TEM. As shown in Fig. 1a, parent SAPO-34 crystals with typical cubic shape exhibit smooth surfaces, and the particle sizes are ranged from 2 to 3 μm. Interestingly, after etching in 0.10 mol L⁻¹ TEAOH at 90 °C for 6 h, the surfaces of treated SAPO-34 present two kinds of pore patterns, i.e. scattered pores on the top and bottom surfaces and an hourglass-like, touching triangles pore pattern on the four side surfaces (Fig. 1b and S1†). The surface pore size is distributed in the range of 80–150 nm. TEM images in Fig. 1c and d further present the details of the inner structure features of the treated SAPO-34. The image took from the touching triangles surface show a clear X-type morphology with gradually changed light and shade contrast (Fig. 1c). It reveals that the electron beam is transmitted through the crystal bulk in the area of touching triangles because the surface pores were extended into the crystal bulk forming a hollow structure. As illustrated in Fig. 1c inset, the TEM image suggests that a macropore structure was constructed in SAPO-34, successfully. Additionally, it also indicates that the opposite surface has the same symmetric morphology, which is consistent with SEM results (Fig. 1b). A different morphology was observed for the non-triangle surface where the electron beam can pass through the areas near to the four surface edges (Fig. 1d). It further confirms that the four side surfaces have the same touching triangle morphology as well as a similar inner hollow structure. The pore size has a strongly positive correlation with the TEAOH concentration, treatment time and TEAOH/SAPO-34 ratio. At higher concentration of TEAOH e.g. 0.2 mol L⁻¹, the touching triangle area became empty or the structure even collapsed because of the main body of crystal was etched off (Fig. S2†). Similar trend were also found when the treatment time or the TEAOH/SAPO-34 ratio was increased to 9 h or 40, respectively (Fig. S3 and S4†).

Fig. 1 Scanning electron microscope (SEM) images of SAPO-34 crystals (a) before and (b) after TEAOH etching; transmission electron microscope (TEM) images of (c) a side face with touching triangles and (d) a non-triangle side face. Insets: schematics of (a) regular and (b) hierarchical SAPO-34 crystal, (c) and (d) TEM observation directions and (d, down) selected area electron diffraction (SAED) patterns.

The similar texture of SAPO-34 with four X-shaped hollow pore faces and two central hollow pore faces has also been reported by others through either multi-template synthesis or grow-etching method.^2,4,5,11 Although the exact formation mechanism of SAPO-34 is not clear, a reasonable conjecture is that the crystal skeleton is firstly formed by eight pyramidal sub-crystals followed by the subsequent growth to cubic crystal through filling the voids around the centre of the crystal (illustrated in Fig. 1a inset).¹¹ Therefore, the subsequent formation parts of SAPO-34 were preferentially etched to form the hierarchical structure due to their shorter crystallization time (Fig. 1b–d). The skeleton of crystal also can be obtained with the harsh etching conditions, which is well consistent with the conjectural formation process of SAPO-34 (Fig. S5†).

The XRD patterns of SAPO-34 before and after TEAOH etching show the typical diffraction peaks of the CHA structure, which indicates a high crystallinity of both samples (Fig. 2a). The XRD pattern of the treated SAPO-34 also suggests that the main structure of parent SAPO-34 was well persevered and no impurity crystal or amorphous state was formed in the etching. The SAED pattern can be assigned to the CHA structure along the [001] direction (Fig. 1d, inset).¹⁰ It further confirms the crystallinity and purity of the treated SAPO-34. Moreover, the pure CHA structure was always present in SAPO-34 samples obtained from different TEAOH-treated conditions (Fig. S6a–c†). However, certain lattice planes were disappeared when TEAOH was replaced by sodium hydroxide (Fig. S6d†), which is consistent with the previous studies.²¹ It was found that NaOH treated SAPO-34 lost most of their crystallinity and the leaching completely removed phosphorus from the bulk, yielding an amorphous aluminosilicate.²¹ Therefore, it demonstrates that the choice of alkaline etching agent is a critical factor for controlling preparation of hierarchical SAPO-34.

Fig. 2 (a) X-ray diffraction (XRD) patterns and (b) N₂ adsorption–desorption isotherms of SAPO-34 crystals before and after TEAOH treatments in 0.10 mol L⁻¹ TEAOH at 90 °C for 6 h (quantity adsorbed value of treated SAPO-34 was added 5 for clarity of display); (c) pore size distribution of parent and TEAOH treated SAPO-34 crystals by (c) mercury intrusion porosimetry (MIP) and (d) Barrett–Joyner–Halenda (BJH) pore size distribution method.

The pore structure of SAPO-34 samples was determined by N₂ sorption and MIP (Fig. 2b–d). Both samples show a steep increase in the N₂ adsorption–desorption isotherms at P/P₀ around 0, which is due to the filling of micropores (Fig. 2b). The same slope in this region also indicates that both samples have the same micro-pore size (Fig. 2b). The parent SAPO-34 shows the type-I isotherms while the treated SAPO-34 exhibits the type-IV isotherms with an obvious hysteresis loop in the region 0.7 < P/P₀ < 0.9 due to the capillary condensation in the mesopores. The micropore surface area and micropore volume of treated SAPO-34 are slightly decreased from 666 m² g⁻¹ and 0.251 cm³ g⁻¹ of the parent material to 656 m² g⁻¹ and 0.246 cm³ g⁻¹, respectively. However, the meso/macro surface area and volume of the treated SAPO-34 are increased to 21 m² g⁻¹ and 0.037 cm³ g⁻¹. The MIP curve of the treated SAPO-34 displays two peaks around 100 nm and 1050 nm, respectively (Fig. 2c). Compared with parent SAPO-34, the additional peak at 100 nm of treated SAPO-34 indicates that macropores were formed in the treated SAPO-34, which is consistent with the SEM results in Fig. 1b. The peaks at 1050 nm are attributed to the void spaces between particles (Fig. 1). The BJH results further reveal that the mesopores with an average pore size of 3.5 nm were formed in the treated SAPO-34. Meanwhile, it also confirms the existence of macropore with 100 nm pore size (Fig. 2d). Therefore, in combination with SEM/TEM results, the pore size distribution analysis proves that the hierarchical pore structure consisted of micro-, meso- and macro-pores were successfully formed in the TEAOH treated SAPO-34. The pore structure depended on the etching conditions. As shown in Fig. S7,† for example, no macropores were formed until the TEAOH concentration was higher than 0.05 mol L⁻¹ and the macropore structure started to collapse at TEAOH concentration of 0.15–0.20 mol L⁻¹ due to the excessive etching.

The influence of TEAOH treatment on the SAPO-34 acidity was measured by NH₃-TPD (Fig. 3a). The parent SAPO-34 shows the weak and strong acidity peaks at 181 and 402 °C, respectively.^2,11,14 The hierarchical SAPO-34 yielded exactly the same weak acid peak at 181 °C. But the strong acid peak was shifted to 416 °C, suggesting a slightly higher acidity. Meanwhile, the weak signal of very strong acid at 498 °C became much smaller in hierarchical SAPO-34, which would be of benefit coke suppression during the MTO reaction. Moreover, both samples have the similar peak areas of weak and strong acid sites, indicating that the acid site density is unchanged after the TEAOH treatment. FT-IR spectra in Fig. 3b indicate similar chemical states for both samples, which suggest that TEAOH treatment has no influence on the chemical bonds of the parent crystals. In details, the bands around 1100 and 720 cm⁻¹ are characteristic of SAPO framework stretching vibration. The peaks below 700 cm⁻¹ are assigned to the vibration of double-6 rings and the bending of T–O. The band at 1220 cm⁻¹ is ascribed to the asymmetric stretch of T–O tetrahedral groups.^15,16,20

Fig. 3 (a) Ammonia-temperature programmed desorption (NH₃-TPD) profiles, (b) Fourier transform infrared spectroscopy (FT-IR) spectra and (c) MAS NMR of SAPO-34 samples before and after TEAOH treatments in 0.10 mol L⁻¹ TEAOH at 90 °C for 6 h.

The solid-state ²⁷Al, ³¹P and ²⁹Si MAS NMR was employed to analyse the chemical environments in SAPO-34 samples (Fig. 3c). Intensity of the ²⁷Al MAS NMR peaks at 35–43, 15 and −15 ppm differed between these two samples. The signal at 35–43 ppm, reflecting to Al atoms in a tetrahedral environment, was weakened in hierarchical SAPO-34. A new small peak at 15 ppm appeared in hierarchical SAPO-34, which indicates the presence of a small amount of penta-coordinated Al atoms. The hierarchical sample has also an intensive peak at −10 ppm, which is assigned to octahedrally coordinated Al atoms formed by two water molecules coordinated to tetrahedrally coordinated framework Al atoms.^11,22,23 The difference in chemical shift indicates that the alkali solution greatly influenced on the coordination manner of Al. In the ³¹P MAS NMR spectra, the signals between −26 to −30 ppm were corresponded to P(OAl)₄ species. The broad shoulder peak at −15 ppm for the hierarchical SAPO-34 was attributed to P atoms coordinated with water (P(OAl)_x(H₂O)_y, x + y = 4).^11,23 The signal at −90 ppm of both samples in the ²⁹Si MAS NMR spectra can be assigned to Si(4Al) species. In the hierarchical SAPO-34 this peak is broadened between −75 to −110 ppm, because it included several additional peaks at around −84, −100 and −105 ppm. The peak at −84 ppm is assigned to Si(OAl)_n(OH)_4−n (n = 1 or 2) species from the breaking of Si–O–Al bonds.^11,24 It indicates that hierarchical SAPO-34 has a slight increase on external surface acidity, which agrees well with NH₃-TPD results (Fig. 3b). The tailing peaks at −100 and −105 ppm are assigned to Si(3Al) and Si(2Al), respectively, indicating increased Si ratio in the local environment. In view of a desilicication process by normal alkali etching, the elemental analysis of TEAOH etching SAPO-34 crystals via inductively coupled plasma (ICP) and X-ray fluorescence (XRF) was further conducted, although the above NMR analysis reveals that Al, Si and P in the framework were all etched (Table S1†). XRF and ICP results indicate that the composition of both Al and P in the treated SAPO-34 is similar to those of the parent precursor. It was found that Si amount was slightly decreased from 11.9% in the parent to 10.7% in hierarchical SAPO-34, revealing the desilicication process. At the same time, both Al and P were detected in the treated TEAOH solution after treatment in a ratio very close to the bulk composition of SAPO-34 (Table S1†). This suggests that Al and P were also homogenously etched off during the desilicication. Therefore, the acidity of parent SAPO-34 was well preserved.

The catalytic performance of SAPO-34 in the MTO reaction was tested in a fixed bed reactor at 400 °C. Complete methanol conversion was achieved over both parent and treated SAPO-34 catalysts (Fig. 4a and Table S2†). However, the single-run life time over hierarchical SAPO-34 with >95% methanol conversion was significantly prolonged to 640 min, which is twice as long as the 320 min life time of the parent catalyst. The coking weight of catalysts was measured by TGA. As showed in Fig. S8,† both of the parent and hierarchical SAPO-34 catalysts after inactivation lost ca. 17% weight. It reveals that coking rate of hierarchical SAPO-34 is half of that of parent ones. Without pore structure reconstruction, the micropores of the parent SAPO-34 are easily blocked by coking species, which will increasingly gradually obstruct the access of reagents into the catalyst. In contrast, the abundant micro-, meso-/macro-porous channels in hierarchical SAPO-34 can greatly enhance the diffusion efficiency of reagents/products into and out of the crystals, which suppresses the coke formation. As showed in Table S3,† the MTO performance of our hierarchical SAPO-34 is much better than that of the acid leaching SAPO-34, in which the single life time is 100–130 min.^25,26 At the same time, our results are comparable to that of the multi-template synthesized hierarchical SAPO-34 (280–600 min).^3,10,11,13 The C⁼₂–C⁼₃ selectivity over hierarchical SAPO-34 is around 80% during the single-run time, which is very close to that of the parent material (Fig. 4b). Similarly, C⁼₂–C⁼₄ selectivity over both catalysts are reached 94%. It suggests that the hierarchical SAPO-34 has the similar activity properties as the parent one, coincident with the acidity analysis in Fig. 2. The stability of our hierarchical SAPO-34 was further tested by a reaction–reactivation cycling because the reactivation stability is the key property of MTO catalyst in fluidized bed reactor. As showed in Fig. 4c, the light olefin selectivity was quite stable during six times cycling, in which the selectivity of C⁼₂, C⁼₃ and C⁼₄ were kept stable around 40%, 42% and 12%, respectively. The single-run lifespans of the cycling were averaged at 620 min with a slightly fluctuation between 580 and 660 min, proving a high stability of treated SAPO-34 during the harsh reactivation (coke burning) treatments.

Fig. 4 The methanol conversion (a) and light olefins selectivity (b) of SAPO-34 before and after TEAOH treatments during the single-run of MTO reaction. (c) Olefin selectivity and lifetime of hierarchical SAPO-34 during reactivation cycling.

Conclusions

In summary, a facile post-treatment method has been developed to synthesize SAPO-34 crystals with hierarchical pore structure via TEAOH etching under mild conditions. The resultant SAPO-34 has symmetric, touching triangle pore patterns on four side surfaces and scattered pores on the two left surfaces. The special hierarchical texture of the treated SAPO-34 was constructed by the original micropores, ca. 3.5 nm mesopore and macropore of ca. 100 nm. Meanwhile, the crystallinity, purity and acid properties of parent SAPO-34 were all well protected. The hierarchical SAPO-34 shows a complete methanol conversion and high light olefins selectivity. Profited from the improvement of mass diffusion in the hierarchical structure, the single-run lifetime of hierarchical SAPO-34 was doubly increased. Moreover, the hierarchical SAPO-34 exhibits high stability during the harsh reactivation cycling. This work demonstrated a simple and efficient synthetic route to prepare hierarchical porous SAPO-34 with prolonged lifetime, which is reliable and processing operability.

Acknowledgements

The authors acknowledge the financial supports from Shanghai Sci & Tech Commission (No. 14DZ1203700 & 14DZ1207602) and NSFC (No. 21506243 & 21507141). G. Zeng thanks the support from Youth Innovation Promotion Association of CAS.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, SEM, pore size distribution and XRD results. See DOI: 10.1039/c6ra02282k

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