Effects of Cl− on Cu2O nanocubes for direct epoxidation of propylene by molecular oxygen

doi:10.1016/j.catcom.2019.105897

Catalysis Communications

Volume 135, February 2020, 105897

https://doi.org/10.1016/j.catcom.2019.105897 Get rights and content

Highlights

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Using Cu₂O nanocubes enclosed by (100) facets as model catalytic systems.
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NH₄Cl doping played a positive role for direct epoxidation of propylene.
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The activity and selectivity critically depended on the loading amount of NH₄Cl.

Abstract

Promoters have been widely used in industrial catalysis but understanding the mechanism behind their function is still a challenging subject. As ideal model catalysts, nanocrystal catalysts with well−defined exposing facets and relatively large specific surface have attracted intense attention. We explored the effect of Cl⁻ loading on Cu₂O nanocubes enclosed by (100) facets for the direct epoxidation of propylene by molecular oxygen. Interestingly, a volcano−type curve was obtained when the selectivity and conversion are plotted as a function of the loading amount of Cl⁻, indicating that optimal surface O/Cl⁻ ratio is critical for desirable catalytic performance.

Graphical abstract

Introduction

Propylene oxide (PO) is one of the bulk chemicals with about ten million tons annual production [1,2]. As compared to the industrialized chlorohydrin and Halcon method, direct epoxidation of propylene (DEP) with molecular oxygen appears an ideal alternative [3,4]. The DEP reaction has been studied mainly on Cu−, Ag− and Au−based catalysts [[3], [4], [5], [6], [7], [8], [9], [10], [11]]. Among them, Cu−based catalysts are claimed to be promising because the oxygen atoms thereon were suggested to be more electrophilic, thus more selective for DEP [4,10,12,13]. However, despite the much effort made, including crystal facet regulation [14] and particle size control [8,15], the unmodified Cu−based catalysts are still unsatisfactory for the reaction. The high PO selectivity (40% ~ 50%) could only be achieved at very low propylene conversion levels (usually <1%); while when the conversion is increased, the PO selectivity dramatically decreases.

The mechanism(s) of DEP reaction appear(s) very complicated. One widely accepted mechanism involves the oxametallacycle (OMP) as the key intermediate to generate PO, which usually takes place on electrophilic oxygen species [5,13,[16], [17], [18]]. On the other hand, allylic C single bond H bond activation occurring on the nucleophilic oxygen species would be a competitive route, which leads to the formation of acrolein (ACR). Experimentally, the most common strategy to enhance the DEP reaction is to introduce chemical promoters to tune the chemical behavior of surface oxygen, and Cl⁻ and alkali metal ions have been frequently employed [[5], [6], [7],[19], [20], [21]]. For example, Wang et al. [6] suggested that alkali metal addition could reduce Lewis acidity of CuO_x/SBA − 15 and thus increase the PO selectivity. In addition, Zhang et al. [7] proposed that the introduction of Cl⁻ effectively regulated the electrophilicity of Ag/BaCO₃ catalysts and suppressed deep oxidation of propylene. However, it is very difficult to decouple the roles of chlorine anions and metal cations when metal chlorides are used as promoters.

In this contribution, we systematically investigated the effect of Cl⁻ for DEP over Cu₂O model catalysts. In particular, Cu₂O nanocubes (Cu₂O−NCs) enclosed by (100) facets were synthesized via a surfactant−free wet chemical method. To rule out the influence of metal cations, the as synthesized Cu₂O−NCs were impregnated with the NH₄Cl solution with different concentrations. Through advanced characterizations, we found that the optimal ratio of O/Cl⁻ on the surface is the key to achieve the desirable catalytic performance. And the Cu₂O−NCs loaded with 0.33 wt% NH₄Cl at 150 °C displayed the best performance with 57% PO selectivity and a TOF of 3.4 × 10⁻⁴ s⁻¹. Our work provides a signpost for exploring promoter effects by using well−defined nanocrystals.

Section snippets

Chemicals and catalysts preparation

Chemicals: Copper nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99.0%), sodium hydroxide (NaOH, 96.0%) and D − (+) − glucose (C₆H₁₂O₆, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ammonium chloride (NH₄Cl, 99.5%) was purchased from Guangdong Guanghua Sci−Tech Co., Ltd., and all chemicals were used as received without further purification.

Cu₂O−NCs: Cu₂O−NCs were prepared via a surfactant−free wet chemical method with a revised protocol based on previous report [14]. In a typical synthesis,

Characterization of Cu₂O−NCs with and without Cl⁻ and their catalytic performance

The as prepared Cu₂O−NCs were highly uniformed with edge length in the range of 200–400 nm as shown in Fig. S1(a−c). The TEM and SAED images confirmed that the Cu₂O−NCs are enclosed by (100) facets. Fig. S1(d) and (e) illustrate that some small NH₄Cl particles appear on the surface of Cu₂O−NCs after NH₄Cl loading, and no significant corrosion of surface is observed. Powder XRD measurements illustrated that both unmodified Cu₂O−NCs and 1 wt% NH₄Cl−Cu₂O−NCs have nearly produced very similar

Conclusions

To summarize, the promotion effect of Cl⁻ in the DEP reaction was systematically demonstrated by using Cu₂O−NCs with (100) exposing facets as model catalyst. We disclosed that optimal surface O/Cl⁻ ratio was critical for obtaining high selectivity values of PO. Cu₂O−NCs with 0.33 wt% NH₄Cl loading displayed the best catalytic performance with 57% PO selectivity and a TOF of 3.4 × 10⁻⁴ s⁻¹. Our work provides a very powerful method to investigate the structure – catalytic property relationship of

Declarations of Competing Interest

None.

Acknowledgements

The authors acknowledge support from the National Key Research and Development Program of China (2017YFA0206801, 2017YFA0207303), the National Natural Science Foundation of China (21721001, 21931009, 91845102 and 21773190). The authors thank Dr. Ye Wang for helping in constructing the catalytic equipments, Wei Zhou for the measurement of TPD−MS.

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The direct epoxidation of propylene by O₂ is a significant and challenging topic. The key factor for this homogeneous aerobic epoxidation is the activation of molecular oxygen under mild conditions. In this work, the aerobic epoxidation of propylene catalyzed by manganese porphyrins was achieved in the presence of isoprene. Isoprene contains an allyl methyl group, and the α-H can be easily removed to achieve the activation of molecular oxygen. The conversion of propylene was 38% and the selectivity toward propylene oxide (PO) was up to 87%. The role of isoprene was demonstrated, and a plausible mechanism was proposed. The protocol reported herein is expected to provide a strategy for the simultaneous preparation of propylene oxide and isoprene monoxide.
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However, this approach still presents significant challenges and is regarded as the holy grail of industrial production [16]. In recent years, research on the gas-phase epoxidation of propylene mainly focuses on coinage metals, such as Cu, Ag, Au, and their alloys, and the influence of metal particle size [9,17–20], morphological characteristics [21], catalyst preparation method [22–25], additives, [10,15,26,27] and supporting effect [28,29] on the reaction. Ag-based catalysts have been successfully applied to the theoretical investigation and industrial application of ethylene epoxidation [30–34].
The direct epoxidation of propylene on Ag(1 1 1) under different oxygen coverage conditions was systemically studied on the basis of density functional theory calculation and the general trends of oxygen coverage on the two parallel competitive reaction paths in propylene oxide (PO) formation: α-H stripping versus epoxidation and the generation of PO versus acetone (AC) were explored. Then, the PO formation selectivity was investigated. Results showed that increase in oxygen coverage enhanced the adsorption strength of adsorbed species, except the O* species. In the primary reaction of propylene epoxidation, high oxygen coverage facilitates the first α-H stripping process, and opposite trend was observed in epoxidation process. As for the secondary chemistry in propylene epoxidation, increase in oxygen coverage benefits PO and AC formation. The coverage of O* has considerable influences on the primary reaction but small influence on the secondary reaction. Although a high oxygen coverage benefits the first α-H stripping, the second α-H stripping has higher barrier than that in secondary chemistry, and thus the PO formation would be better than that of acrolein formation at higher oxygen coverage.
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Overall, the results showed that the reaction selectivity could be controlled by using specific crystal facets. Wang et al. [73] explored the effects of adding Cl− as a promoter to cubic Cu2O crystals with selectively exposed (100) facets. They observed a maximum of PO selectivity at intermediate Cl− loading, indicating there existed an optimal O/Cl− ratio for propylene epoxidation.
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Copper based materials have been studied as potential catalysts for the direct propylene epoxidation using molecular oxygen. The less reactive copper-bound oxygen (compared to silver-bound) allows for the formation of the oxametallacycle (OMC) intermediate, which is generally considered as crucial in the formation of PO [12-16]. The role of different promotors has also been investigated, focusing on the effects of alkali and alkaline earth salts on Cu, Fe, Ru and Sn containing catalysts [17-21].
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Short communicationEffects of Cl− on Cu2O nanocubes for direct epoxidation of propylene by molecular oxygen

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemicals and catalysts preparation

Characterization of Cu2O−NCs with and without Cl− and their catalytic performance

Conclusions

Declarations of Competing Interest

Acknowledgements

J. Catal.

Catal. Today

Chin. J. Catal.

Catal. Commun.

Chin. Chem. Lett.

J. Catal.

Appl. Surf. Sci.

Angew. Chem. Int. Ed.

J. Catal.

J. Catal.

J. Catal.

Gas−phase propene epoxidation over coinage metal catalysts

Res. Chem. Intermed.

Short communication
Effects of Cl⁻ on Cu₂O nanocubes for direct epoxidation of propylene by molecular oxygen

Characterization of Cu₂O−NCs with and without Cl⁻ and their catalytic performance