Elsevier

Catalysis Communications

Volume 144, September 2020, 106091
Catalysis Communications

Short communication
Direct conversion of ethylene to propylene over Ni- and W-based catalysts: An unprecedented behaviour

https://doi.org/10.1016/j.catcom.2020.106091Get rights and content

Highlights

  • This is an efficient catalytic way for converting ethylene to propylene (ETP).

  • The ETP process is conducted in cascade mode, using two catalysts.

  • In the first reactor, over Ni-AlKIT-6, ethylene is selectively converted into n-C4.

  • In the second one, over WOx/KIT-6, the unconverted ethylene with the formed n-C4 will generate propylene.

  • Both catalysts are highly active and stable in the ETP process.

Abstract

A new ethylene to propylene catalytic process based on cascade reactions, with two catalysts and two reactors, has been explored in this study. In the first reactor loaded with Ni-AlKIT-6 catalyst, at 60–120 °C and 3 MPa, part of ethylene has been selectively transformed into 2-butene. In the second reactor, loaded with WOx/KIT-6 catalyst, 2-butene reacted with the unconverted ethylene at 450 °C and 0.1 MPa. Unprecedented results in terms of catalytic activity, selectivity towards the formation of propylene and stability against deactivation were achieved under these conditions. More precisely, an ethylene conversion of ~85% and a selectivity to propylene of 55% and butenes of 30% remained unchanged during 24 h on reaction stream.

Introduction

Propylene, a major chemical intermediate, is usually produced from fossil-based feedstock, using highly efficient processes, such as fluidized catalytic cracking and steam cracking of saturated hydrocarbons. Owing to the strong demand for polypropylene during the last years (average annual demand growth of 4.5%), the supply of propylene is of great concern because the traditional sources will not be sufficient to cover the demand. Research efforts have been made to develop alternative routes to propylene production based on accessible raw materials, such as propane, ca. propane dehydrogenation [1], methanol, ca. methanol to olefins process [2], and ethylene.

One of the most exciting methods is the conversion of ethylene to propylene (ETP) without the addition of other hydrocarbons [[3], [4], [5], [6]]. The sustainability of this technology is guaranteed by the fact that the ethylene production by dehydration of bioethanol has become economically feasible [[7], [8], [9]]. The ETP process consists of catalytic cascade reactions, including either (a) oligomerization/cracking or (b) dimerization/isomerization/metathesis chemical steps. Typically, oligomerization/cracking is catalyzed by acidic microporous materials like zeolites and SAPOs [[10], [11], [12]] or by multifunctional Ni-mesoporous materials [[13], [14], [15], [16]], at temperatures higher than 350 °C. Generally, mixtures of saturated and unsaturated hydrocarbons of low selectivity to propylene, were produced in such ETP processes.

The processes based on the dimerization/isomerization/metathesis chemical steps are operated at lower temperature (50–150 °C) in the presence of transition metal-based catalysts [[17], [18], [19], [20], [21], [22], [23]]. Three consecutive reactions and three catalytic sites are involved in these processes: (i) dimerization of ethylene to 1-butene, (ii) isomerization of 1-butene to 2-butene, and (iii) metathesis reaction of 2-butene with unreacted ethylene to propylene, as described below.

  • (i)

    Ethylene dimerization2C2H4site11C4H8,H2980=107.31kJmol1

  • (ii)

    1-Butene isomerization1C4H8site2trans2C4H8,H2980=8.54kJmol1

1C4H8site2cis2C4H8,H2980=4.35kJmol1
  • (iii)

    Ethylene-butene metathesisC2H4+trans2C4H8site32C3H6,H2980=2.09kJmol1

C2H4+cis2C4H8site32C3H6,H2980=5.68kJmol1

As shown in several studies [[17], [18], [19], [20]], the catalytic sites involved in these reactions could coexist on a single support. Basset and co-workers [17] used a trifunctional WH3/Al2O3 catalyst at 150 °C and 0.1 MPa for the above processes. Li et al. [18] used a single bimetallic supported catalyst, NiSO4/Re2O7/γ-Al2O3 in an ETP process operated at 50 °C and 0.1 MPa. Catalysts like MeO/Re2O7/B2O3-Al2O3 (Me = Pd or Ni) have been also designated by Buluchevskii et al. [19] for the one-stage ETP process performed at 80 °C and 0.1–1 MPa. More recently, Ghashghaee and Farzaneh [20] developed a nanostructured Ru-Ba-K-MgO-Al2O3 catalyst able to convert ethylene into propylene and butenes at 70 °C and 0.1 MPa. These studies showed that using a single catalyst, which provides different active sites, is a challenging concept. In practice, it is very difficult to balance the catalytic functions involved in the ETP process. Consequently, over these catalysts undesired reactions could occur, causing a decrease in propylene selectivity. Additionally, the catalysts rapidly deactivated.

These weaknesses could be circumvented by distributing the different catalytic sites on two catalysts. Our group developed an original ETP process based on two robust and highly active solid catalysts [21,22]. In a single reactor operated at 80 °C and 3 MPa, ethylene was first converted over a Ni-AlSBA-15 catalyst into 2-butenes, which then reacted with the unconverted ethylene over a MoO3–SiO2–Al2O3 catalyst to produce propylene. More recently, we proposed a new efficient heterogeneous catalytic system associating Ni-AlKIT-6 and ReOx/γ-Al2O3 [23,24]. Under identical mild conditions (60 °C, 3 MPa), ethylene was very selectively converted into high added value molecules, i.e. propylene and 1-butene. Despite their remarkable initial performance (the initial ethylene conversion was higher than 70%), the supported MoOx and ReOx catalysts suffered from significant deactivation after 5 h on reaction stream. Working at low temperature, the loss of catalytic activity was most probably due to the poisoning of active centers by strongly adsorbed products.

Beside the Re- and Mo-based catalysts, W-supported oxides are also well-known metathesis catalysts [25]. It should be noted that a WOx/SiO2 catalyst is used in industrial metathesis processes for converting ethylene and 2-butene to propylene [26]. While ReOx and MoOx catalysts operate at low temperatures, i.e. 20–80 °C or 80–250 °C, WOx catalyst is active only at temperatures higher than 400 °C. The long catalyst lifetime compared to MoOx or ReOx, the easy and efficient regeneration and the resiliency to oxygenate molecules in the feed are the major advantages of supported WOx catalysts [27].

In the present study, we prepared and characterized two mesoporous functionalized materials, i.e. Ni-AlKIT-6 and WOx/KIT-6. They were evaluated as catalysts in the ETP process based on cascade reactions: dimerization/isomerization over Ni-AlKIT-6 and metathesis over WOx/KIT-6. The catalytic sites have been probed by ammonia temperature-programmed desorption (NH3-TPD), hydrogen temperature-programmed reduction (H2-TPR), X-photoelectron spectroscopy (XPS) and 27Al MAS NMR.

KIT-6 is a mesoporous material, possessing a bicontinuous cubic structure with Ia3d symmetry and an interpenetrating cylindrical pore system [28]. It is a suitable support for the present catalysts, thanks to its high surface area and large interconnected mesopores, which are key factors for high mass transport properties. As known, these properties are crucial for processes that involve reactive olefins [3,6].

Section snippets

Catalysts preparation

The dimerization/isomerization Ni-AlKIT-6 catalyst has been prepared in three successive steps, as shown in Fig. S1 (ESI). Initially, a pure siliceous material (sample KIT-6) was prepared in water, using (EO)20(PO)70(EO)20 triblock copolymer (Pluronic P123, Mn = 5800 g mol−1, Aldrich), tetraethyl orthosilicate (TEOS 99%, Aldrich), HCl 37.5% (Aldrich) and n-butanol (99%, Alfa Aesar). The molar ratio of reagents was 1 TEOS/0.017 P123/1.9 HCl/1.3 n-butanol/195 H2O. After stirring for 24 h at

Characterization of catalysts and supports

The powder XRD results showed that all samples prepared in this study were well-organized mesoporous materials. Thus, the small-angle XRD patterns (Fig. S3, ESI) showed a diffraction peak d211 (2θ = 0.97°) and a peak d220 (2θ = 1.12°) characteristic of the cubic Ia3d symmetry of KIT-6 materials [28]. No signal was observed in the powder XRD patterns recorded at 2θ > 5°, confirming the absence of crystalline NiO or WOx species (Fig. S4, ESI). Additionally, the nitrogen adsorption-desorption

Conclusions

The direct conversion of ethylene to propylene has been investigated using two dual catalytic systems and two reactors in series. The cascade reactions involved in the process, i.e. dimerization/isomerization/metathesis were catalyzed by Ni2+, H+ (both delivered by Ni-AlKIT-6 catalyst) and W (from WOx/KIT-6 catalyst), respectively. Even if the ETP process was governed by the metathesis step, a perfect balance between the amount of Ni and W sites is essential to efficiently convert ethylene to

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.

Acknowledgments

ERC thanks for financial support project RTI2018-099668-BC22 of Ministerio de Ciencia, Innovación y Universidades and FEDER.

References (35)

  • P. Tian et al.

    Methanol to olefins (MTO): from fundamentals to commercialization

    ACS Catal.

    (2015)
  • V. Hulea

    Toward platform chemicals from bio-based ethylene: heterogeneous catalysts and processes

    ACS Catal.

    (2018)
  • M. Ghashghaee

    Heterogeneous catalysts for gas-phase conversion of ethylene to higher olefins

    Rev. Chem. Eng.

    (2018)
  • V. Blay et al.

    Converting olefins to propene: Ethene to propene and olefin cracking

    Catal. Rev.

    (2018)
  • V. Hulea

    Direct transformation of butenes or ethylene into propylene by cascade catalytic reactions

    Catal. Sci. Technol.

    (2019)
  • J. Sun et al.

    Recent advances in catalytic conversion of ethanol to chemicals

    (2014)
  • M. Zhang et al.

    Dehydration of ethanol to ethylene, bioethylene production from ethanol: a review and techno-economical evaluation

    Ind. Eng. Chem. Res.

    (2013)
  • Cited by (0)

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