Propane activation on Zn-modified zeolite. The effect of the nature of Zn-species on the mechanism of H/D hydrogen exchange of the alkane with Brønsted acid sites
Graphical abstract
Introduction
The conversion of C2C4 alkanes represents a promising way to produce highly valuable aromatic hydrocarbons [1], [2], [3], [4], [5]. Currently, Zn-modified zeolites are among the most effective catalysts to perform propane aromatization [6], [7], [8], [9]. Indeed, the catalysts, which are traditionally prepared by the ion exchange or impregnation of ZSM-5 zeolite (ammonia- or acid-form) with aqueous solution of zinc salts, have demonstrated significantly higher activity and selectivity for aromatics as compared to the catalysts based on pure acid-form ZSM-5 zeolite [5], [10]. On the other hand, the samples obtained by the mechanical mixing of zinc oxide and the acid-form zeolite were also found to exhibit satisfactory activity in the alkane conversion similar to that demonstrated by the ion exchanged or impregnated zeolites [5], [7].
The studies of the composition and the state of zinc introduced into the zeolites by the ion exchange methods have shown the presence of different Zn-species: isolated Zn2+ cations and zinc oxide particles of various dimensions and localization. The latter species were found in the form of small ZnO clusters inside the zeolite pores and large ZnO particles on outer surface of zeolite crystals [6], [9], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].
The method of Zn loading in the zeolites by the solid-state exchange reaction of Brønsted acid sites (BAS) with metallic zinc vapor was also proposed [21], [22], [23]. Such approach leads to isolated Zn2+ cations to be the main state of zinc inside the zeolite pores [24], [25], [26], [27], [28], [29]. ZSM-5 zeolite containing only Zn2+ cations and BAS was shown to activate small alkanes, including methane [30], [31], [32]. Zn-modified H-ZSM-5 zeolite containing only zinc oxide ZnmOn clusters and BAS also showed an enhanced catalytic activity in propane conversion towards the aromatics [33].
The studies of C1C3 alkane activation and conversion on acid-form BEA and ZSM-5 zeolites, loaded with both zinc oxide species and isolated Zn2+ cations, assumed the importance of the former Zn-sites, however, possible influence of latter ones cannot be excluded [19], [20], [34], [35], [36], [37], [38]. Recent examination of ZSM-5 zeolites modified with zinc using different approaches (ion exchange, impregnation, solid-state exchange with zinc vapor, and chemical vapor deposition of dimethylzinc) demonstrated that the samples having heterogeneous distribution of extra-framework zinc species (zinc oxide clusters and oxygenated cationic Zn-complexes) were more active than the samples with isolated Zn2+ sites [39]. Hence, these studies evidence that Zn-species of various structure and composition can provide efficient aromatization of C2C4 alkanes.
Numerous investigations have been focused on establishing the mechanism of propane activation and transformations on Zn- and Ga-modified zeolite catalysts with the aim of finding the ways to enhance aromatization efficiency on these catalysts [1], [2], [3], [4], [5], [10], [12]. Generally accepted reaction network of propane conversion includes the stages of initial propane dehydrogenation to form the olefin, the olefin dehydro-oligomerization and the cyclization to produce aromatic hydrocarbons [4], [6], [12]. Regioselectivity of H/D hydrogen exchange of propane with BAS and remarkable acceleration of hydrogen exchange were observed for Zn/H-ZSM-5 [34] and Zn/H-BEA [35] zeolites as compared to pure acid-form zeolites [40], [41]. That was an evidence for the synergy of Zn-sites and BAS for the activation of CH bonds of the methyl groups of propane molecule on these catalysts. The formation of propylene, coordinating to Zn-species, as the intermediate of propane aromatization was detected with NMR spectroscopy [36], [38]. However, it is still unclear which particular species, either Zn2+ or ZnO, provides the synergy of Zn species and BAS for propane CH bond activation resulting to the regioselectivity of the exchange and initial propane dehydrogenation.
In this paper, two samples of H-BEA zeolite, containing exclusively either isolated Zn2+ cations (Zn2+/H-BEA sample) or small ZnO clusters (ZnO/H-BEA sample), were prepared and examined in the reaction of H/D hydrogen exchange of propane-d8 with BAS. Analysis of the kinetics, monitored with 1H MAS NMR in situ, has allowed us to clarify the role of Zn2+ cations and small ZnO clusters in propane activation on Zn2+/H-BEA and ZnO/H-BEA zeolites.
Section snippets
Zeolite sample preparation
Parent BEA zeolite (Na-BEA) was synthesized according to previously described approach [42]. The acid form of the zeolite (H-BEA) was further obtained by Na-BEA sample treatment with ammonium nitrate aqueous solution followed by calcination at 823 K in air flow for 6 h. For H-BEA zeolite, Si/Al ratio and aluminum content were determined to be 23 and 1.57 wt%, respectively, according to chemical analysis data. Si/Al ratio of 22 was confirmed with 29Si MAS NMR method for this sample (Fig. S1).
Peculiarities of the H/D hydrogen exchange of propane with BAS on ZnO/H-BEA and Zn2+/H-BEA zeolites
H/D hydrogen exchange is an excellent test reaction to study the activation of alkane CH bonds by BAS of zeolites. Propane adsorbed on a zeolite exhibits two signals in 1H MAS NMR spectrum: at 1.0 ppm from the methyl (CH3) groups and at 1.5 ppm from the methylene (CH2) group [49]. Hence, it is possible to monitor the exchange reaction selectively for either the CH3 or CH2 groups by NMR in situ [41]. As the result, an important information on the regioselectivity of the H/D exchange of propane
Conclusion
Two Zn-modified BEA zeolites, loaded with different Zn-species, have been examined with respect to their ability to activate CH bonds in propane molecule. This has been done by monitoring the kinetics of H/D hydrogen exchange between propane-d8 and Brønsted acid sites (BAS) of the zeolites with 1H MAS NMR in situ. One of the Zn-modified zeolites (Zn2+/H-BEA sample) contained zinc exclusively in the form of isolated Zn2+ cations which partially substituted BAS. The other one (ZnO/H-BEA sample)
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Acknowledgment
This work was supported by Russian Science Foundation (grant № 19-43-04101) and by the Deutsche Forschungsgemeinschaft (grant № HA 1893/22-1).
Funding sources
Russian Science Foundation (RSF) (grant № 19-43-04101).
Deutsche Forschungsgemeinschaft (DFG) (grant № HA 1893/22-1).
Declaration of Competing Interest
The authors declare no competing financial interest.
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