Elsevier

Journal of Catalysis

Volume 361, May 2018, Pages 94-104
Journal of Catalysis

Enhanced ethylene selectivity and stability of Mo/ZSM5 upon modification with phosphorus in ethane dehydrogenation

https://doi.org/10.1016/j.jcat.2017.12.023Get rights and content

Highlights

  • Ethylene yield and stability are significantly enhanced on 2.5 wt.% P-modified Mo/ZSM5.

  • P reduces the acid strength and density and improves the dispersion of Mo.

  • P changes the ZSM5 channel system by forming SAPO-like interfaces with framework Al.

  • Less reduced but stable Mo species are produced when 2.5 wt.% P is added.

  • Less dense silanol groups and reduction in both channel diameter and internal volume restrict coke formation.

Abstract

Nonoxidative conversion of ethane to ethylene and/or BTX (benzene, toluene, and xylene) suffers rapid deactivation due to coke deposition. We report here the effects of phosphorus modification on the stability and activity of Mo/ZSM5 for nonoxidative conversion of ethane. The results show that the ethylene and BTX yield and stability are significantly enhanced upon modification with 2.5 wt.% P. NH3 TPD, pyridine FTIR, 1H MAS NMR, 27Al MAS NMR, 31P MAS NMR, 129Xe NMR, XPS, UV–visible diffuse reflectance spectra (UV–vis DRS), and nitrogen physisorption were carried out to understand the effects of P on the structure of Mo/ZSM5 and its correlation with catalytic performance. The presence of P reduces the acid strength and density, changes the channel system of ZSM5 by forming thermally stable SAPO-like interfaces with the framework Al, and improves the dispersion of molybdenum. Rapid deactivation still occurs on Mo/ZSM5 with 1 wt.% P due to the existence of denser silanol groups, more isolated Mo species, and reduced aperture size with little change in effective micropore volume. A higher P loading (2.5 wt.%) leads to less dense silanol groups and less reduced but stable molybdenum species, and simultaneously reduces channel diameter and internal volume. Consequently, the ethylene selectivity is enhanced and the formation of coke precursors is restricted, resulting in improved stability.

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 Csingle bondH 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.

References (93)

  • H. Zhu et al.

    J. Catal.

    (2015)
  • F. Solymosi et al.

    Appl. Catal. A Gen.

    (1998)
  • P.P. Sun et al.

    J. Catal.

    (2010)
  • G. Siddiqi et al.

    J. Catal.

    (2010)
  • V. Galvita et al.

    J. Catal.

    (2010)
  • P.P. Sun et al.

    J. Catal.

    (2011)
  • J. Wu et al.

    J. Catal.

    (2014)
  • J. Wu et al.

    Appl. Catal. A Gen.

    (2015)
  • B.S. Fu et al.

    J. Catal.

    (2013)
  • Y.G. Li et al.

    Catal. Commun.

    (2007)
  • Y. Song et al.

    Appl. Catal. A Gen.

    (2007)
  • H. Liu et al.

    J. Catal.

    (2006)
  • N. Kosinov et al.

    J. Catal.

    (2017)
  • S. Kikuchi et al.

    J. Catal.

    (2006)
  • C.A. Emeis

    J. Catal.

    (1993)
  • D. Ma et al.

    J. Catal.

    (2000)
  • D. Wang et al.

    J. Catal.

    (1997)
  • Z. Shen et al.

    Appl. Catal. A Gen.

    (2009)
  • D.C. Phillips et al.

    J. Catal.

    (2002)
  • J.G. Choi et al.

    Appl. Surf. Sci.

    (1996)
  • Y. Teng et al.

    J. Catal.

    (2009)
  • H. Lin et al.

    Chem. Sci.

    (2016)
  • J.P. Thielemann et al.

    Appl. Catal. A Gen.

    (2011)
  • L. Lizama et al.

    Appl. Catal. B Environ.

    (2008)
  • Z. Liu et al.

    J. Catal.

    (1998)
  • M. Fournier et al.

    J. Catal.

    (1989)
  • L. Mosqueira et al.

    Mater. Chem. Phys.

    (2011)
  • E. Mannei et al.

    Micropor. Mesopor. Mater.

    (2016)
  • R.S. Weber

    J. Catal.

    (1995)
  • G.L. Zhao et al.

    J. Catal.

    (2007)
  • M.L. Gou et al.

    Appl. Catal. A Gen.

    (2014)
  • H.L. Janardhan et al.

    Appl. Catal. A Gen.

    (2014)
  • G. Jiang et al.

    Appl. Catal. A Gen.

    (2008)
  • A. Jentys et al.

    Appl. Catal.

    (1989)
  • A. Rahman et al.

    J. Catal.

    (1988)
  • J. Caro et al.

    J. Catal.

    (1990)
  • G. Lischke et al.

    J. Catal.

    (1991)
  • T. Blasco et al.

    J. Catal.

    (2006)
  • M. Hunger et al.

    Micropor. Mater.

    (1996)
  • W. Zhang et al.

    J. Catal.

    (1999)
  • K. Barbera et al.

    J. Catal.

    (2011)
  • W. Ding et al.

    J. Catal.

    (2002)
  • C.H.L. Tempelman et al.

    Micropor. Mesopor. Mater.

    (2015)
  • F. Jin et al.

    Catal. Today

    (2009)
  • S.-W. Choi et al.

    J. Catal.

    (2017)
  • M. Guisnet et al.

    Appl. Catal. A Gen.

    (1992)
  • Cited by (51)

    • Nanosheets-stacked Al<inf>2</inf>O<inf>3</inf>-flower anchoring Pt catalyst for intensified ethylene production from ethane dehydrogenation

      2022, Fuel
      Citation Excerpt :

      Particularly, our catalyst has the lowest Pt loading of only 0.07 wt%, thus giving the highest TOF value of 6084 h−1 at 590 °C. Moreover, our catalyst gives higher ethane conversion of 27.3 % at 590 °C, compared to other reported catalysts (e.g., at 600 °C: PtZn2/Al2O3 (19 %) [3], Pt3Ir/Mg(Al)O (15 %) [4], and P-Mo/ZSM-5 (20 %) [22]; at 650 °C: Au/TiSi-20 (15.8 %) [23] and Au/CeO2 (13.7 %) [24]). Although the yields of some catalysts are higher than our catalyst such as Co/ZSM-5 (47.8 %) [25], but the C2H4 productivity is just about 30 % (0.18 vs 0.63) of our catalyst.

    View all citing articles on Scopus
    View full text