Enhanced MTO performance over acid treated hierarchical SAPO-34

doi:10.1016/S1872-2067(16)62557-3

Chinese Journal of Catalysis

Volume 38, Issue 1, January 2017, Pages 123-130

https://doi.org/10.1016/S1872-2067(16)62557-3 Get rights and content

Abstract

Hierarchical SAPO-34 crystals were synthesized by a facile acid etching post-treatment. Butterfly-shaped porous patterns on four side faces and hierarchical pores composed of micropores, mesopores and macropores were formed after a nitric acid or oxalic acid treatment. The catalytic performance of the hierarchical SAPO-34 for the methanol to olefins (MTO) process showed that the synergistic effect of the hierarchical pores and acid sites resulted in a longer catalyst lifetime (from 210 to 390 min for the acid treated SAPO-34) and higher selectivity to light olefins of 92%–94%. The ethylene selectivity can be adjusted between 37.4% and 51.5% by the pore size. No hierarchical SAPO-34 was obtained after a treatment with butanedioic acid, and with this sample, fast deactivation was detected after 100 min.

Graphical Abstract

Hierarchical SAPO-34 crystals with butterfly-shaped faces were synthesized by acid etching, which increased the catalyst lifetime and light olefin yield in the MTO reaction.

Introduction

Ethylene and propylene, traditionally produced by the petroleum route, are extremely important chemical raw materials. The methanol to olefins (MTO) process is a strategic technology for olefin production from the abundant nonpetroleum sources to balance the shortage of petroleum in China. SAPO-34 is the catalyst of choice for MTO due to its high hydrothermal stability, suitable micropores and moderate acidity [1, 2]. However, it suffers from the rapid coke formation due to diffusion limitation through the small windows (0.38 nm × 0.38 nm), which would drive the process cost up with frequent catalyst regeneration [3]. Constructing SAPO-34 crystal with a hierarchical structure which contains micro-, meso- and macropores is a promising approach to improve mass transport to decrease the regeneration frequency required [4, 5].

Commonly, hierarchical pore structures are achieved by either direct synthesis with multiple templates or an indirect post-treatment. Wang et al. [4] synthesized SAPO-34 nanosheets with a hierarchical structure using the quaternary ammonium-type organosilane surfactant of [3-(trimethoxysilyl)propyl]octadecyl-dimethylammonium chloride (TPOAC) as the mesopore template and diethylamine (DEA) as the micropore template. Wu et al. [6] synthesized SAPO zeolite by adding TPOAC and C_22-4-4Br₂ as the organosilane surfactant for the mesopores, but their methanol conversion and catalyst lifetime were even worse than those of conventional SAPO-34 in the MTO process. Li et al. [5] synthesized hierarchical SAPO-34/18 zeolite by a hydrothermal method, but unfortunately only a low methanol conversion below 20% was reported. Hollow SAPO-34 cubes with butterfly-shaped spots on the faces prepared from solid precursor gels containing gelatin by a vapor phase transport method has been reported [7]. Wang et al. [4, 8] synthesized hierarchical SAPO-34 catalysts using TPOAC as the mesopore template by direct hydrothermal crystallization, which exhibited a remarkably prolonged catalyst lifetime (350–500 min). A HF-assisted in situ growth etching route was developed to synthesize hierarchical SAPO-34 by the hydrothermal route [9, 10], its single-run lifetime for the MTO reactions was up to 600 min. However, the secondary templates or HF used in the direct synthetic system increase the synthesis cost and environmental pollution. Post-synthetic treatment, which provides an alternative approach to get a hollow-structured micro- or nanosized SAPO-34 has been reported. Qiao et al. [11] synthesized hierarchical SAPO-34 by a hydrochloric acid and tetraethylammonium hydroxide (TEAOH) treatment, respectively, but did not report MTO performance.

Butterfly-shaped porous patterns on four side faces were formed after a nitric acid or oxalic acid treatment in our work. In our previous work, we also developed a facile TEAOH etching post-treatment method to get SAPO-34 with a hierarchical pore structure and single-run MTO lifetime up to 600 min [12]. However, this method was limited by the high price of TEAOH and low recovery rate (25%–42%) of SAPO-34 after the alkali treatment [11]. An acid treatment for the controllable preparation of a hierarchical SAPO-34 is a potential route to optimize the diffusion efficiency, suppress coke deposition and prolong catalyst lifetime as well as improve the recovery rate of the modified zeolite in an economic way. However, there have been only limited works on the acid treatment of SAPO-34 for the MTO application reported so far.

In this work, we synthesized hierarchical SAPO-34 with high crystallinity by controlling the acid post-treatment conditions. The effects of the acid types (nitric acid, oxalic acid and butanedioic acid) on the hierarchical SAPO-34 structure and MTO performance were investigated. It was found that butterfly-shaped pore structures were formed after the SAPO-34 was treated by nitric acid and oxalic acid, and a significantly prolonged single-run lifetime for MTO reaction together with improved light olefin selectivity were achieved.

Section snippets

Synthesis of parent SAPO-34

The parent SAPO-34 crystals were prepared by a hydrothermal route. The synthesis gel molar composition was 1 Al₂O₃:0.44 SiO₂:1.1 P₂O₅:2.25 triethylamine:35 H₂O. SAPO-34 crystal seeds with a mass ratio of 1:500 to the gel were mixed in a closed autoclave. Then the mixture was heated from room temperature to 165 °C in 7 h and kept at 165 °C for 33 h before cooling. The solid product was filtered, washed and dried, and followed by calcination at 600 °C for 5 h to obtain the parent SAPO-34, which

Hierarchical SAPO-34 prepared by acid post-treatment

The XRD patterns of SAPO-34 before and after acid etching are shown in Fig. 1. All the samples showed the typical diffraction peaks of the CHA structure at 9.6°, 20.7°, 26° and 31°. No impurity crystal or amorphous phase was found, which indicated that the pure SAPO-34 crystals were well preserved after the acid etching. After the acid treatment, the intensity of the diffraction peaks was reduced, suggesting poorer relative crystallinity. Compared with S-0 (taken as 100%), the relative

Conclusions

Hierarchical SAPO-34 was prepared by hydrothermal synthesis followed by a nitric acid or oxalic acid post-treatment. Their catalytic performance of hierarchical SAPO-34 was investigated in MTO reaction. The SAPO-34 treated with nitric acid or oxalic acid exhibited butterfly-shaped pores on four side faces and the pores comprised micropores, mesopores (40–50 nm) and macropores (62–500 nm). The number of acid sites was also optimized after the nitric acid and oxalic acid treatment. The

Acknowledgments

Thanks to Lu'an Mining Group (Changzhi, China) for supporting our research and technical assistance.

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This work was supported by the National Natural Science Foundation of China (21403279, 21507141, 21506243), and the Science and Technology Commission of Shanghai Municipality (14DZ1207602, 14DZ1203700).

Published 5 January 2017

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ArticleEnhanced MTO performance over acid treated hierarchical SAPO-34

Abstract

Graphical Abstract

Introduction

Section snippets

Synthesis of parent SAPO-34

Hierarchical SAPO-34 prepared by acid post-treatment

Conclusions

Acknowledgments

Microporous Mesoporous Mater.

Catal. Today

Catal. Today

Appl. Catal. A

Appl. Catal. A

Microporous Mesoporous Mater.

Microporous Mesoporous Mater.

Chin. J. Catal.

Chin. J. Catal.

J. Mater. Chem. A

Cryst. Growth Des.

Chem. Commun.

J. Mater. Chem. A

Chem. Commun.

Chem. Commun.

Article
Enhanced MTO performance over acid treated hierarchical SAPO-34