Enhanced ethylene selectivity and stability of Mo/ZSM5 upon modification with phosphorus in ethane dehydrogenation
Graphical abstract
Introduction
As an important building block in the chemical industry, ethylene is principally produced by steaming cracking and fluid catalytic cracking of naphtha or gas oil [1]. However, with rising demand for ethylene, the capacity of such petrochemical cracking processes fails to meet the needs of manufacture of basic and intermediate chemical materials such as polymer and ethylene oxide [2], [3]. Furthermore, owing to relatively high feedstock cost and low selectivity to ethylene, the naphtha-based process seems less competitive than that based on ethane, in which the conversion of hydrocarbon reaches 70% with yield up to 50% [2], [4]. Discovery of a large reserve of shale gas has reignited wide interest in thermal cracking using natural-gas-derived ethane as feedstock. However, those pyrolysis processes suffer from high operating temperature and high equipment demand. Catalytic dehydrogenation of light alkanes that proceeds under milder conditions is a promising alternative to alleviate the issue of high energy consumption.
State-of-the-art routes developed for catalytic dehydrogenation of light alkanes include direct dehydrogenation to olefins and dehydroaromatization to aromatics [4], [5], [6], [7], [8], [9], [10]. As to direct dehydrogenation of ethane to ethylene, current research mainly involves Pt-based catalysts. Despite efforts to modify the electronic and geometric structure of Pt-based catalysts with promoters such as Sn, Ga, In, and Ir, the stability is still limited, primarily due to coke deposition [11], [12], [13], [14], [15], [16], [17]. Furthermore, the high temperature required for reaction and catalyst regeneration also provokes aggregation of Pt particles, leading to reduced activity [1], [18]. A novel preparation protocol such as the atomic layer deposition method (ALD) has been proposed to annihilate active sites for hydrogenolysis or coking and sterically isolate and stabilize desired active sites, which could be beneficial in suppressing coking and sintering to prolong the catalyst life [19], [20]. The formidable issue associated with the nonoxidative catalytic conversion of ethane to ethylene lies in its limited equilibrium conversion, which makes it less economically attractive [2], [4], [19], [20].
In comparison with direct dehydrogenation to olefins, dehydroaromatization exhibits a higher equilibrium conversion in the same temperature range [5]. The exemplary Cyclar process developed by BP-UOP is based on bifunctional catalysts, where an initial dehydrogenation product, such as ethylene, is produced on dehydrogenation sites and subsequent oligomerization and aromatization takes place on Brønsted acid sites [9], [21]. However, the paradox occurs that the higher acid density and strength indispensable for facilitating the conversion of ethane to aromatics also contribute to cracking and hydride transfer, leading to easier formation of methane and coke [7], [9]. The reduction of acid density and strength would enhance the ethylene selectivity but lower the ethane conversion [5], [9]. In all, obtaining a high ethylene yield via catalytic dehydrogenation of ethane on zeolite-based catalysts is highly challenging.
Mo/ZSM5 has been proven an effective catalyst for dehydroaromatization of methane [22], [23], [24], [25], [26]. Molybdenum species have been verified to exist in an isolated state after calcination in air; these are efficient in selective activation of CH bonds but not stable and apt to agglomerate with time on stream [25], [27]. Recently, it was shown that activity loss due to coke and aggregation of molybdenum species can be restored by air calcination [25], [28]. However, Kosinov et al. pointed out that the dealumination due to interaction of Mo and framework aluminum under oxidative conditions at high temperature led to irreversible deactivation [29]. To date, there are few reports on conversion of ethane to ethylene or BTX (benzene, toluene, and xylene) catalyzed by Mo/ZSM5 and even fewer on the catalyst stability [8]. Here, we carried out a systematic study of Mo/ZSM5 for ethane dehydrogenation. High ethylene yield and catalyst stability with low carbon loss due to coke and methane formation were obtained on phosphorus-modified Mo/ZSM5. Particular attention was paid to understanding the influence of phosphorus on the catalyst structure and its correlation with the catalytic performance.
Section snippets
Catalyst preparation
Mo/ZSM5 was prepared via a wet impregnation method adapted from our previous work [26]. Briefly, an aqueous solution of (NH4)6Mo7O24 was added to ZSM5 (purchased from Nankai University, SiO2/Al2O3 = 50) with a nominal loading of 4.7 wt.% Mo. Then it was left at room temperature for 12 h and further dried at 110 °C for 12 h. Finally, the catalyst was calcined at 500 °C for 6 h. The obtained catalyst was denoted as Mo/ZSM5. Phosphorus was introduced by impregnating NH4H2PO4 onto Mo/ZSM5 following
Catalytic performance of P-Mo/ZSM5
Fig. 1 shows that modification with P had an obvious effect on the catalytic performance. Compared with Mo/ZSM-5, the initial ethane conversion over 1P-Mo/ZSM-5 increased significantly from 24.0% to 44.1%. However, it still dropped drastically from 44.1% to 12.5% within 260 min (Fig. 1a). In contrast, 2.5P-Mo/ZSM5 exhibited totally different behavior, as its activity rose quickly from 4.4% to 19.5% within 35 min. Then it almost leveled off within 200 min and subsequently slowly decreased from
Conclusions
Non-noble-metal catalysts based on phosphorus-modified Mo/ZSM5 have been developed for nonoxidative conversion of ethane with less carbon loss. The activity and ethylene selectivity were greatly increased by adding 1 wt.% P to Mo/ZSM5, while the most stable catalyst was obtained by adding 2.5 wt.% P to Mo/ZSM5, with ethylene yield reaching 15%. The results indicate that P likely interacts with the framework Al to form thermally stable SAPO-like interfaces, which leads to reduced acid density
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos. 21621063 and 21425312). We thank Professor Xiuwen Han for fruitful discussions.
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