Boosting the propylene selectivity over embryonic borosilicate zeolite catalyst for oxidative dehydrogenation of propane
Graphical abstract
Introduction
Boron-based catalysts have received significant attention for their inhibition of deep oxidation of olefins in the oxidative dehydrogenation of propane (ODHP) process for the production of propylene [1], [2], [3], [4], [5], [6], [7], [8], [9]. Recently, boron-containing zeolite catalysts with diversity and adjustability crystal structures are capable of catalyzing ODHP reaction and provide a unique perspective to understand the reaction process and further develop highly active and selective catalysts [10], [11], [12], [13], [14], [15]. The aggregated boron species embedded in MWW- and MFI-typed zeolite displayed high efficiency in activating propane [11], [13], [14]. However, the hydrolysis-prone nature of BOB unit in the aggregated boron species tend to raise stability problem of the catalysts in humid atmosphere. The silanol-rich structure was shown to disperse boron species and limit their detachment from the catalytic system through the redispersion of over-hydrolyzed boron species. Lately, the isolated boron site with B[OH…O(H)Si]2 structure in borosilicate zeolite showed durability in long period test and enabled the synergistic conversion of oxygen and propane through three hydrogen bonds on the boron site [12]. In addition, the selectivity of propylene in zeolite-based catalysts was much lower under identical propane conversion compared with other boron-based catalysts. It was noteworthy that propane reaction orders of 1 and 1.5 were observed in zeolite-based catalysts, suggesting the surface activation of propane was more favourable [12], [13]. These results enlightened that the activation of propane on the surface might dominate the ODHP process over zeolite-based catalysts, resulting in the distinctive catalytic properties and distinguishing them from other boron-based catalysts.
For hexagonal boron nitride-based (h-BN) and B2O3-based catalysts, it is gradually recognized that, in addition to surface-catalyzed pathway, gas-phase radical reactions are also the important issue during the ODHP process, which could influence the overall catalytic activity [8], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Wu lately reported the improvement of ODHP activity via the gas-phase radical chemistry over VOx-modified BN catalyst, evidenced by the formation of NO according to the observation of synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS) [25]. Moreover, a 1.4-order dependent on the propane partial pressure was observed, which was caused by the enhanced surface contribution catalyzed by VOx site. Therefore, it can be reasonably deduced that the lower reaction order of propane and catalytic activity over zeolite-based catalysts could result from the limited contribution of gas-phase reactions. In consideration of the reaction pathways, the construction of highly exposed active boron centers and enhanced gas-phase reactions might be a feasible way to construct highly active and selective ODHP catalysts.
Compared with crystalline zeolite with microporous structure and saturated coordination framework atoms, embryonic zeolitic materials have the feature of extra-large microporosity with several unit cell dimensions of size. The greatly exposed small unit cells allow guest molecules easy to diffuse and the extra-large micropores improve the efficiency of mass transfer and accessibility of the active sites. More importantly, after embedding boron atoms, the defect-rich zeolitic structures offer abundant SiOH and BOH groups, which contributed to the stabilization of boron species and construction of active boron centers with proton transfer with adjacent SiOH through hydrogen bonds. In addition, the large microporous spaces provide free environments for gas-phase reactions, allowing X-ray amorphous zeolitic material as a model system for the fundamental investigation of ODHP reaction.
Herein, the embryonic borosilicate zeolite (EBZ) was prepared and used for investigating the ODHP reaction. Structure characterizations and kinetic measurements revealed that the H-bonded BOH groups in defect tri-coordination boron species were responsible for activating propane on the catalyst’s surface and gas phase, and the reactions in the gas phase were mainly responsible for the high performance in ODHP reaction. The strengthened contribution of gas-phase reaction facilitates the activation of propane, consequently resulting in higher reaction order of propane. It enables the enhanced catalytic performance of embryonic borosilicate zeolite catalyst in terms of propane conversion and propylene selectivity.
Section snippets
Catalyst preparation
The amorphous embryonic borosilicate zeolite catalysts were synthesized using tetrapropylammonium hydroxide (TPAOH) as organic structure-directing agents (OSDA) from clear homogeneous sols with a molar composition of SiO2: B2O3: TPAOH: H2O: EtOH = 1: x: 0.14: 21: 4 (x = 0.084, 0.168, 0.252 and 0.336). Typically, boric acid (Sinopharm) was dissolved in 4.8 g aqueous solution of TPAOH (25 wt%, Guangfu) and 24 mL deionized water with stirring at room temperature. Then, 8.68 g tetraethyl
Structure and property of EBZ catalysts
The structure and property of EBZ catalysts were characterized and the results were shown in Fig. 1. All the harvested EBZ catalysts showed X-ray amorphous nature and no typical diffraction peaks of MFI were observed (Fig. 1a), due to the size of several zeolite units below the detection limit by XRD. FTIR was used to characterize the short-range orders of the EBZ catalysts. As seen in Fig. 1b, the absorption bands around 1080 and 800 cm−1 correspond to the asymmetric and symmetric SiO
Conclusions
In this work, the embryonic borosilicate zeolite catalyst with small zeolite units, developed porosity, and active boron species was prepared by using TPA+ as the organic structure directing agent for catalyzing ODHP reaction. Such catalyst was convenient to prepare and showed ∼ 20 % improvement in propylene selectivity at propane conversion of 21.5 % at 540 °C and displayed nearly an order of magnitude higher of propylene productivity compared with crystalline MFI-type borosilicate zeolite.
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 study was supported by state key program of National Natural Science Foundation of China (21733002), National Key Research and Development Program of China (2018YFA0209404), the Program for Liaoning Innovative Research Team in University (LT2016001).
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