Promoting effect of copper oxide on CsX zeolite catalyst for side-chain alkylation of toluene with methanol
Introduction
Styrene is one of the important raw materials in chemical industry, and also widely applied in the fields of polymer, agriculture and medicine [1]. In the industry, styrene is primarily produced by the friedel-craft alkylation of benzene with ethylene to ethylbenzene and the following dehydrogenation [2]. However, the energy consumption of this complicated process is extremely high, due to the high reaction temperature of the ethylbenzene dehydrogenation step. The side-chain alkylation of toluene with methanol has been recognized as an alternative route for producing styrene selectively [3].
It is generally believed that, the side-chain alkylation of toluene with methanol is an acid-base synergistic catalytic process [4]. During the reaction, toluene is adsorbed and stabilized by the acid sites, and methanol is dehydrogenated to formaldehyde over the base sites [5]. Simultaneously, the methyl group of toluene is activated by the base sites, and the formed formaldehyde acts as the true alkylating agent and reacts with the activated benzylic carbon to produce styrene [[6], [7], [8]]. A series of side reactions also occur during the side-chain alkylation process, such as the acid-catalyzed ring alkylation of toluene with methanol to form xylenes and direction conversion of methanol to dimethyl ether [[9], [10], [11]], as well as the base-catalyzed self-decomposition of formaldehyde to CO and H2 [8,12] and transfer hydrogenation of styrene with methanol to yield ethylbenzene [13].
The alkali ion-exchanged zeolites have been extensively studied as the catalyst for the side-chain alkylation reaction, due to their outstanding catalytic performance and easily tuned acid-base properties [14]. Cesium ion-exchanged X zeolite (CsX) is widely recognized as an efficient acid-base catalyst and also frequently used as the substrate for further modification and catalyst preparation [9,15,16]. The alkali cations over CsX serve as the Lewis acid sites, and the negatively charged oxygen atoms on zeolite framework act as base sites [6,9,17]. However, over the CsX catalyst, the toluene conversion is extremely low and the as-generated styrene is inevitably converted to ethylbenzene via transfer hydrogenation with methanol. Thereby, the process of side-chain of toluene to produce styrene has not been commercialized in the industry.
To improve the catalytic activity and selectivity to styrene, the further modification of CsX with alkali metals [18,19], boron compounds [20] and metal oxides [[21], [22], [23]] have been attempted and indicated to be effective. The introduced additives influence the performance of CsX catalyst substrate from two aspects. On one hand, the alkalinity of CsX can be sensitively adjusted to reach a matched acid-base equilibrium of the catalytic centers, thereby facilitating the selective side-chain alkylation reaction. On the other hand, the dehydrogenation of methanol to formaldehyde can be promoted, providing more available alkylating agents and consequently enhancing the reactivity. Although a great effort has been devoted to this area recently, the development of high active and selective catalyst for side-chain alkylation is still challenging and necessary.
In this work, a class of novel additive, copper oxides, has been discovered to be efficient in simultaneously improving the reactivity and selectivity to styrene of CsX for toluene side-chain alkylation with methanol. To probe into the promoting effect of CuO additives, a series of CuO modified CsX samples have been prepared, characterized and evaluated. The results indicated that, the introduced Cu species not only enhanced the methanol dehydrogenation to formaldehyde reaction, but also suppressed the undesired transfer hydrogenation of styrene product. Furthermore, the universality of CuO modification in improving toluene side-chain alkylation performance was further proved by loading CuO species over some well recognized catalyst substrates (Cs2O/CsX, B2O3/CsX and ZnO/CsX), and the effects of reaction condition were also investigated.
Section snippets
Materials preparation
The NaX zeolites (SiO2/Al2O3 = 2.4) were obtained from the Catalyst Plant of Nankai University in China. The CsX samples were prepared by ion exchange of NaX with a certain concentration of cesium hydroxide monohydrate (0.2 M) at 80 °C for 2 h. Then the solids were separated from the slurry by centrifugation, and washed with excess deionized water. The above process was repeated for three times. Finally, the solids were dried overnight at 120 °C and calcined at 550 °C for 3 h in air atmosphere.
Characterization of CuO modified CsX zeolites
The XRD patterns of CuO modified CsX zeolites are illustrated in Fig. 1. All these zeolite samples exhibited typical FAU diffraction peaks, indicating that the crystal structure of X zeolite was preserved after CuO modification, in spite that the intensity of characteristic peaks decreased gradually with increasing CuO loading (Fig. S1). The characteristic diffraction peak of CuO was not detected in the spectrum, suggesting that the introduced Cu species were highly dispersed over the zeolite
Conclusion
To sum up, the promoting effect of CuO additive on the toluene side-chain alkylation reaction performance of CsX zeolites could be understood in two aspects. The introduction of Cu species not only promoted the methanol dehydrogenation to formaldehyde, thus enhancing the conversion of toluene, but also induced new Lewis acid sites and modulated the acid-base property moderately, thereby inhibiting the undesired transfer hydrogenation of styrene to ethylbenzene. For CsX catalyst, the optimal
CRediT authorship contribution statement
Ming Cheng: Conceptualization, Investigation, Writing - original draft. Yue Wang: Investigation, Visualization. Weihuan Wang: Validation, Visualization. Guowei Wang: Project administration. Xiaolin Zhu: Conceptualization, Writing - review & editing, Supervision. Chunyi Li: Funding acquisition, Supervision.
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.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (21706288), the Key Research and Development Plan of Shandong Province (2018GGX107011), the Natural Science Foundation of Shandong Province (ZR2017BB020), and the Fundamental Research Funds for the Central Universities (17C×02015A, 18C×02016A).
References (37)
- et al.
J. Catal.
(1997) - et al.
J. Catal.
(1980) - et al.
J. Catal.
(1987) - et al.
J. Catal.
(1987) - et al.
J. Catal.
(1989) - et al.
J. Catal.
(1972) - et al.
J. Catal.
(1981) - et al.
Appl. Catal, A-Gen.
(1997) - et al.
J. Catal.
(1998) - et al.
Microporous Mesoporous Mater.
(2013)
Catal. Today
J. Catal.
Catal. Today
J. Catal.
Appl. Catal. A-Gen.
J. Mol. Catal. Chem.
Microporous Mesoporous Mater.
J. Catal.
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