Protective desilication of β zeolite: A mechanism study and its application in ethanol-acetaldehyde to 1,3-butadiene
Graphical abstract
In this work, hierarchical β zeolite were prepared through protective desilication by quaternary ammonium hydroxide (TAAOH), and the detailed mechanism was investigated. Adjusting the length of TAAOH alkyl chain can tune the mesoporous volume of β zeolite. These hierarchical β zeolite was converted Zr-incorporated β zeolite, which showed better catalytic performance and coke capacity than Zr-β zeolite with traditional microporous network.
Introduction
Due to the substitution of shale gas for naphtha associated with 1,3-butadiene (BD) production and the boom of bio-ethanol, the conversion of ethanol into BD (ETB) have recently reattracted widely scientific interest [[1], [2], [3], [4]]. However, the low catalytic stability of the current catalysts restricts its further industrial application [5]. The two predominant catalytic systems of ETB are metal oxide catalysts and zeolite catalysts. In the former, most researches focus on preparing catalysts which possess a subtle balance in the number and strength of acidic, basic sites and redox sites, through changing the metal oxide and adjusting preparation method [3,[5], [6], [7], [8]]. However, tuning of these properties is still challenging and detailed active sites of metal catalysts is still unclear. In the zeolite system to catalyze ETB, β zeolite, which comprises of a three-dimensional, 12-membered ring channel system, is most commonly used to act as support to prepare heteroatom-incorporated or loaded zeolite. It is reported that incorporating or loading metal cation such as Ta [9,10], Mg [11], Zn–Y [12,13], Zr [14,15], etc. into β zeolite to form Lewis acid sites can show good catalytic activity on ETB. Ivanova et al. [14,15] used dealumination-metallization strategy to incorporate Zr into dealuminated β zeolite, proved that the open Zr (IV) Lewis acid sites, represented by isolated Zr atoms in tetrahedral positions of the zeolite crystalline structure connected to three –O–Si linkages and one OH group, are the main active sites to catalyze ETB. And they reported that Ag/Zr-β obtain the highest activity performance in comparison with Ag/ZrO2/SiO2 and Ag/ZrMCM-41. Zr-β seems to be the most promising catalyst to catalyze ETB.
The main advantages of traditional zeolite with microporosity are their extremely high surface area and shape selectivity, but diffusional limitations of reactants and products are frequently observed [16], which limited the application of β zeolite in catalyzing ETB [11,17]. Hierarchical zeolite, those combining the micropores with an additional mesopores, have been employed to improve diffusion, activity, selectivity and stability of zeolite catalysts. At present, synthetic (bottom-up approaches) and post-synthetic (top-down approaches) strategies have been applied to prepare hierarchical zeolite [18,19]. Desilication through base leaching, i.e., framework silicon extraction, as a method of post-synthesis, have become the most commonly used method to introduce mesopores into zeolite due to its simplicity and efficiency [20]. Palkovits et al. [11] reported that the desilication of silicon rich H-β280 turned out being too unstable, and the entire zeolite structure collapsed. After the as-made hierarchical zeolites were used as support materials in the ETB reaction, its catalytic performance was unsatisfied, so an intact crystal structure for zeolitic catalyst was needed. For ETB process, β zeolite will undergo dealumination process and metallization process to prepare heteroatom-incorporated zeolite catalyst, both which may damage the structure of β zeolite. Therefore, the hierarchical β zeolite obtained by desilication should maintain well-preserved crystallinity to strengthen following dealumination-metallization process.
In the alkaline treatment, researchers investigated a classical experimental desilication condition (0.2 M of NaOH, 65 °C, 30 min) and a confined range of molar Si/Al ratios (25–50) for which optimal introduction of intracrystalline mesopores could be achieved [21]. Because the lower stability of lattice Al in β zeolite than that of ZSM-5 and mordenite [22], there lies a different alkaline treatment process in β zeolite, proven by an excessive dissolution in the NaOH-treated β zeolite sample [23]. In order to make alkali-treated β zeolite maintain crystallinity and microporous network, various pore directing agents (PDA) were added into NaOH solution to make this process mild and controlled. Pérez Ramírez et al. [24,25] developed a desilication variant involving NaOH leaching in the presence of quaternary ammonium cations (TPA+ or TBA+) to and protect the ZSM-5 zeolite crystal and tune the mesopores formation in zeolite. They attributed the efficiency of TAA+ in desilication to its affinity to the zeolite surface. Then mixture of NaOH and TBAOH was employed as alkaline sources in β zeolite to produce a hierarchical zeolite without damaging the crystal of the parent zeolite [26]. Sammoury et al. proposed that NaOH incorporated with different zeolite surface affinity PDA would result in different alkali-treated effect in β zeolite [27]. Zhang et al. [28,29] tailored hierarchical structures in β zeolite of differing Al contents through a base leaching process with various PDA to protect β zeolite framework while direct the formation of mesopores. More simply, Holm et al. [30] used TMAOH only to extract silicon from β zeolite and avoid a subsequent ion-exchange step, which achieved a high mesoporous formation while maintain a well crystal structure. However, the detailed mechanism of protective desilication remains unclear.
Considering the possible positive impact of introducing mesoporous system into the microporous zeolite in ETB [11,[31], [32], [33]], we executed the alkaline post-treatment through quaternary ammonium hydroxide (TAAOH) over β zeolite to prepare hierarchical β zeolite, then investigated mechanism of protective desilication of TAAOH in detail. Through adjusting the length of alkyl chain of TAAOH, we can tailor the pore size and mesopores volume of β zeolite. Subsequently, parent β and alkali-treated β zeolite experienced dealumination and Zr incorporation. Acquired Zr-β zeolite were used as catalysts in the conversion of ETB reaction to determine the effect of auxiliary mesopores on catalysis performance.
Section snippets
Hierarchical β zeolite by desilication
Commercial H-β with a nominal SiO2/Al2O3 ratio of 60 was designated as “B60” and used as the starting material for preparing hierarchical β zeolite. Desilication was carried out in the desired concentration of alkaline solutions at the temperature of 65 °C for desired time, and the ratio of liquid to zeolite was kept at 30 ml/g. After that, the suspension was filtered, washed with distillate water until neutral pH, and then dried at 110 °C for 6 h. The alkali-treated β zeolite were denoted as
Protective desilication of TAAOH
Evidence of protective desilication of TAAOH. We tried to introduce mesopores into traditional microporous β zeolite to improve its catalytic performance. NaOH was used to treat B60 first, resulting a high mesoporous volume compared to parent B60, 0.483 cm3/g and 0.070 cm3/g respectively (entry 1,2 of Table 1). However, XRD patterns (Fig. 1) shows that β zeolite treated by NaOH experienced severe structural amorphization and a dramatic loss of crystallinity at 29%. Meanwhile, microporous volume
Conclusion
The alkali-treatment on β zeolite by TAAOH can not only introduce mesopores but also preserve crystal structure and chemical environment of Si atoms. In the desilication process, the protection of TAA+ coexists with OH− attacking and there lies the best treatment concentration. In the initial stage of treatment, the cover rate of TAA+ on the microporous entrance of zeolite is faster than the rate of OH− extracting Si atom, and thus the protection of TAA+ on the framework is dominant. After the
CRediT authorship contribution statement
Minhua Zhang: Innovation, Methodology, Data curation, Writing – original draft. Yunan Qin: Formal analysis, Investigation, Data curation, Writing – original draft. Haoxi Jiang: Validation, Data curation, Writing – review & editing. Lingtao Wang: Supervision, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the National Natural Science Foundation of China (21978211).
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