Preparation of hierarchical SAPO-18 via alkaline/acid etching

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Highlights

  • Hierarchical SAPO-18 molecular sieves were successfully prepared via alkaline/acid etching.

  • The post-prepared SAPO-18 exhibits good catalytic performance in the benzylation reaction of benzene and benzyl alcohol.

  • The rationale behind the variation of hierarchical SAPO-18 prepared by different alkali/acid treatments was discussed.

  • SAPO-18 was inferred to follow the oriented nano-particle attachment mechanism in the early crystallization stage.

Abstract

Hierarchical SAPO-18 molecular sieve was successfully post-synthesized in the alkaline tetraethyl ammonium hydroxide (TEAOH) solution and hydrochloride (HCl) acid solution. In the TEAOH solution, preferential dissolution occurs surrounding the defect sites in a controlled manner, leading to hierarchical SAPO-18 with well-preserved chemical compositions and crystallinities. In acidic solution, the dissolution is revealed to be sensitive to both the defects and the chemical compositions (selective dissolution of the Si-O-Al domains), which cooperatively contribute to the generation of meso-/macropores in the crystals. The crystallization of SAPO-18 precursor is thus inferred to proceed via an oriented nanoparticle attachment mechanism in the early stage. The twinning and intergrowth of nanoparticles occur in an ordered way, leading to the oriented arrangement of the crystallite boundaries. The defects located at the intergrowth interface are prone to acid/alkaline attacks, and play crucial role in the development of hierarchical pore systems. The hierarchical SAPO-18 sample exhibits excellent catalytic reaction performance in the liquid phase benzylation reaction of benzene and benzyl alcohol.

Graphical abstract

This work reports the preparation of hierarchical SAPO-18 molecular sieve via alkaline/acid leaching, the relevant mechanism behind and the improved catalytic performance of the post-prepared hierarchical products in the liquid phase benzylation reaction.

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Introduction

Molecular sieves are a kind of crystalline materials of uniform pores/channels and 3-dimensional frameworks, which find wide applications in various fields as adsorbents, ion-exchangers and catalysts, mainly due to their shape-selective properties [[1], [2], [3], [4]]. Molecular sieves are endowed with the unique shape-selectivity due to their confined microporous structures of molecular dimensions. However, the innate micropore channels could alternatively lead to significant transport limitations, seriously restricting their applications.

Great efforts have been devoted to improving the transport properties of molecular sieves, focusing on the tailoring of their morphologies. Various strategies have been developed, among which hierarchical zeolite is one of the most attractive approaches [5]. The essence of hierarchical zeolite is to introduce auxiliary larger porosities into original native micropore system. The synthesis strategies of hierarchical zeolites could be generally classified into two categories: the direct synthesis method (bottom-up strategy) and the post-synthetic method (top-down strategy) [[6], [7], [8], [9]]. The former strategy involves the direct bottom-up synthesis of hierarchical zeolite, frequently with the assistance of hard/soft template. Jacobsen et al. prepared hierarchical ZSM-5 single crystals, using nano-sized carbon black particles as the sacrificial mesopore agent [10]. Previously, our group also succeeded in the preparation of mesoporous MOR, Beta and SAPO-34 molecular sieves by using surfactants, cationic polymers or organic silanes as mesoporogens [[11], [12], [13], [14]]. However, the mesoporogens could only function in very limited and rigorous conditions. And the high cost of the mesoporogens restricts the massive applications of the templated synthesis of hierarchical zeolites.

Post-synthesis treatment, e.g. acid leaching or alkaline treatment, is a more cost-effective modification route to tailor the compositional and textural properties of zeolites. In effect, dealumination by acid leaching or steaming treatment is a practical method employed to enhance the Si/Al ratios of aluminosilicate zeolites [15]. Desilication in alkaline was extensively investigated to introduce mesoporosity into the zeolites [16]. Groen et al. created uniform intracrystalline mesopores inside ZSM-5 molecular sieves using NaOH post-synthetic treatment [17,18]. Under suitable conditions, the same group also succeeded in the preparation of mesoporous zeolites with BEA, MOR and FER topologies [19,20]. Verboekend et al. also treated a series of aluminophosphate molecular sieves, including AlPO-5, SAPO-5, SAPO-11 and SAPO-34, by various acids and bases. They found that the composition of the parent molecular sieve is vital for the treatment results [21]. Based on controlled alkaline/acid leaching strategy, we recently reported the successful preparation of hollow SAPO-34 single crystals [22]. However, due to the low stability of SAPOs in alkaline and acid solutions, research on alkaline treatment and acid treatment of SAPO molecular sieves is still rather limited.

In this contribution, SAPO-18 was selected for the first time to undergo alkaline and acid treatments. TEAOH, sodium hydroxide (NaOH), HCl and oxalic acid (H2C2O4) solutions were employed as treatment mediums. The effects of different acid/alkaline mediums were investigated and compared. Mesopores and/or macropores were successfully introduced into the SAPO-18 crystals, and the rationale behind the variation of morphologies and compositions was discussed.

Section snippets

Preparation of the SAPO-18 precursor

The SAPO-18 precursor was hydrothermally synthesized using N, N-diisopropyldiethylamine (DIEA) as the organic template. The molar ratio of the initial gel was DIEA: Al2O3: P2O5: SiO2: H2O = 1.8: 1.0: 1.0: 0.4: 50. The detailed preparation procedure is as follows: deionized water, pseudoboehmite, H3PO4 (85 wt%), tetraethyl orthosilicate (TEOS), and DIEA were sequentially added into a beaker, and the resultant mixture was stirred at room temperature for 3 min until homogeneity. Afterwards, the

XRD analysis

The XRD patterns of the samples before and after alkaline and acid treatments are presented in Fig. S1. Well-resolved diffraction pattern typical for AEI structure can be observed for SAPO-18 precursor (sample SP18-P). No obvious intensity decrease is observed after treatment in TEAOH solution, irrespective of the TEAOH concentration (0.1–0.4 mol/L) employed, implying the well-kept AEI structure of the samples. In contrast, the diffraction intensity drops sharply even in NaOH solution of merely

Conclusions

SAPO-18 molecular sieve with hierarchical pores were prepared for the first time by alkaline and acid treatments. It is found that the use of TEAOH solution could introduce a large number of mesopores in the crystals without significantly changing the composition and acidity of the sample. Preferential dissolution occurs surrounding the defect sites in a controlled manner. In acidic medium, besides the defect-induced dissolution, selective dissolution of the Si-O-Al domains occurs. Both

CRediT authorship contribution statement

Dong Fan: Writing - original draft, Writing - review & editing, Methodology, Formal analysis. Yuyan Qiao: Methodology, Investigation, Visualization, Formal analysis. Kaipeng Cao: Investigation. Lijing Sun: Software. Shutao Xu: Investigation. Peng Tian: Supervision, Writing - review & editing, Methodology, Conceptualization. Zhongmin Liu: Supervision, Writing - review & editing, Methodology, Conceptualization.

Declaration of competing interest

There are no conflicts to declare.

Acknowledgment

The authors acknowledge the National Natural Science Foundation of China (No. 91545104, 21606221, 21676262 and 21991091) and Key Research Program of Frontier Sciences, Chinese Academy of Sciences, (Grant No. QYZDB-SSW-JSC040). The authors also acknowledge funding support from the Sino-French joint laboratory “Zeolites”.

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    (Dong Fan, Yuyan Qiao) These authors contributed equally.

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