Elsevier

Catalysis Today

Volume 369, 1 June 2021, Pages 88-94
Catalysis Today

More efficient ethanol synthesis from dimethyl ether and syngas over the combined nano-sized ZSM-35 zeolite with CuZnAl catalyst

https://doi.org/10.1016/j.cattod.2020.03.035Get rights and content

Highlights

  • A nano-sized ZSM-35 zeolite with abundant active sites for dimethyl ether carbonylation was prepared.

  • An auto-reduced CuZnAl catalyst with excellent hydrogenation ability was prepared by a simple solid-state method.

  • The proximity effect for both nano-sized ZSM-35 and CuZnAl catalysts on ethanol synthesis was clarified.

  • Combined nano-sized ZSM-35 and CuZnAl catalysts realized more efficient ethanol synthesis.

Abstract

Converting syngas into ethanol (EtOH) is highly attractive but remains challenge. Dimethyl ether (DME) carbonylation with CO to methyl acetate (MA) on zeolite and its further hydrogenation to EtOH on Cu-based catalyst open a new EtOH synthesis route from syngas. In this work, a nano-sized ZSM-35 (NZ35) zeolite, possessing abundant active sites and porosity and short diffusion path, is found to realize much better activity of DME to MA than that of the conventional ZSM-35 zeolite (CZ35). In addition, a simple formic-acid-assisted solid-state method is employed for preparation an auto-reduced CuZnAl (CZAargon) catalyst under argon atmosphere. The prepared CZAargon catalyst exhibits an excellent catalytic activity for conversion of produced MA to EtOH. By investigating the effects of different integration manners of NZ35 zeolite and CZAargon catalyst, we find that EtOH can be synthesized only when the NZ35 zeolite and CZAargon catalyst pack in a dual-catalyst bed reactor. After optimizing the reaction conditions for EtOH synthesis with the combination of NZ35 zeolite and CZAargon catalyst, it is found that the DME conversion and MA selectivity are stabilized at 47.0 % and 45.6 % respectively, at 220 °C and 2.5 MPa.

Introduction

Ethanol (EtOH), as a renewable energy source, has attracted increasing attentions because of its environmental and long-term economic advantages [[1], [2], [3]]. As an alternative route, syngas (CO+H2) convertion to EtOH is regarded as one of the most promising EtOH synthesis methods. This production process of EtOH is highly competitive compared with fermentation of sugars or corns, and hydration of ethylene because syngas can be efficiently produced from natural/shale gas, coal, biomass, and even CO2 [[4], [5], [6]]. At present, direct synthesis of EtOH from syngas is limited because of the usage of the expensive Rh-based catalyst and the low yield of EtOH [2,7]. Moreover, some indirect methods for EtOH synthesis from syngas have been developed. The typical alternative processes are using methanol (MeOH) or dimethyl oxalate (DMO) as raw materials [8,9]. However, their usage of noble-metal or corrosive catalysts and the complicated processes usually limit their application.

Dimethyl ether (DME) can be easily synthesized by one-step from syngas through H-ZSM-5/CuZnAl capsule catalyst, which makes it an attractive candidate as clean and high efficiency energy [10]. Recently, it is reported that the zeolite catalyst such as MOR, FER, and EU-12 are discovered their carbonylation ability for converting DME to methyl acetate (MA), which provides a halide-free and noble metal-free method to produce MA [11,12]. Considering that the DME carbonylation to MA and ester hydrogenation to alcohols occur at a similar temperature, a new route for direct synthesis of EtOH from DME and syngas was proposed in our previous research [13]. In this method, a dual-catalyst bed reactor with combined zeolite and Cu-based catalyst was designed. MA is firstly produced by DME carbonylation over the upper zeolite catalyst, and then the formed MA is further hydrogenation over the low Cu-based catalyst bed to generate EtOH and MeOH [13,14]. It should be noted that the formed main byproduct of MeOH can be recycled easily through dehydration reaction to produce the feedstock of DME over solid acid catalysts, significantly improving the utilization efficiency of raw materials. For DME carbonylation on zeolite, it is still difficult to realize excellent catalytic stability and activity simultaneously though many post-synthesis modification approaches, such as ion-exchange with metals, selective dealumination, selective poisoning with pyridine, are adopted for improving their carbonylation ability [[15], [16], [17]]. Therefore, preparation of zeolite catalyst with both high activity and stability in DME carbonylation is very attractive but challenging. Nano-sized zeolites attract considerable attention because their larger accessible surface area and fewer diffusion limitations than conventional ones [[18], [19], [20]]. In contrast to the enormous investigations on DME carbonylation over nano-sized H-MOR zeolite [20,21], the study of DME carbonylation over nano-sized ZSM-35 zeolite has not been widely studied. Therefore, it is highly desirable to develop a nano-sized ZSM-35 zeolite for achieving more efficient DME carbonylation.

Moreover, the MA hydrogenation to MeOH and EtOH is also an important step in this new EtOH synthesis route. The employed CuZnAl (CZA) catalyst prepared by conventional co-precipitation method is considered as highly active for converting MA to EtOH [22]. However, this method creates large amounts of concentrated nitrate wastewater and consumes extra cost of reduction energy, which makes the whole reaction process inefficient. In our previous research, Cu-based catalyst, prepared by a novel formic acid-assisted solid-state method, can be directly used for MeOH synthesis from syngas [23]. As compared with the co-precipitation method, this novel method is easily operated. Moreover, the prepared Cu-based catalyst can be automatically reduced in situ by the released H2 and CO during the metal-formic acid precursor calcination in argon atmosphere. Therefore, this novel and simple means will be suitable for preparing CZA catalyst, which is employed for MA hydrogenation to EtOH and MeOH.

In the present study, a nano-sized ZSM-35 zeolite (NZ35) is directly synthesized by hydrothermal method. The influences of temperature on DME carbonylation performance over NZ35 zeolite are investigated. The NZ35 zeolite shows remarkably high activity and stability with respect to the conversional ZSM-35 (CZ35) zeolite for DME carbonylation reaction. The CZA catalyst is designed and prepared by a novel formic-acid-assisted solid-state method. In this method, the auto-reduced CZA catalyst is calcined in argon atmosphere (CZAargon).This method is simple, waste water free and without further reduction. The prepared CZAargon catalyst exhibits excellent hydrogenation ability for converting MA to EtOH. The combined catalysts in a dual-catalyst bed reactor with tandem of NZ35 zeolite and CZAargon catalyst realize more efficient EtOH synthesis using DME and syngas as starting feedstocks.

Section snippets

Catalysts preparation

The NZ35 zeolite was prepared via a direct hydrothermal synthesis route and the CZAargon catalyst was prepared by a novel solid-state method with metal nitrates and formic acid. The detailed preparation processes and catalyst characterization were described in Supplementary data.

Catalytic performance evaluation

All the catalytic performance evaluation reactions were carried out in a fixed-bed steel reactor with inner diameter of 9.5 mm. For the single DME carbonylation reaction, only 0.5 g zeolite (40–60 mesh) was packed in

Structure and texture properties of the NZ35 and CZ35 zeolite

Fig. 1(a) presents the XRD patterns of NZ35 and CZ35 zeolite. All the characteristic peaks in the 2θ region from 5° to 50° can be indexed to the typical FER zeolite structure [25]. From the images displayed in Fig. 1(b, c), it is evident that the NZ35 zeolite consists of nanoparticles about 80 nm while the CZ35 zeolite has a cuboid-shaped morphology in size of nearly 250 nm. The textural properties are summarized in Table 1. The bulk SiO2/Al2O3 ratio, measured by XRF, is 15.3 for NZ35 and 16.2

Conclusions

A more efficient and economical EtOH synthesis route, different with the traditional method of direct conversion syngas to EtOH using Rh-based catalyst, is realized from syngas and DME in a dual-catalyst bed reactor containing NZ35 zeolite and CZAargon catalyst. The NZ35 zeolite, which possesses abundant active sites and porosity and short diffusion path, exhibits much better DME carbonylation performance than that of the CZ35 zeolite. The CZAargon catalyst is directly prepared by a simple

CRediT authorship contribution statement

Xiaobo Feng: Conceptualization, Writing - original draft. Jie Yao: Formal analysis. Yan Zeng: Software. Yu Cui: Data curation. Shun Kazumi: Investigation. Reubroycharoen Prasert: Visualization. Guangbo Liu: Resources. Jinhu Wu: Validation. Guohui Yang: Methodology, Supervision. Noritatsu Tsubaki: Writing - review & editing.

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

X. F. thanks the China Scholarship Council (CSC) for financial support. This study was financially supported by the grant from JST-ACT-C of Japan Sci. & Tech. Agency (JST).

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