Layer structured bifunctional monolith catalysts for energy-efficient conversion of CO2 to dimethyl ether

https://doi.org/10.1016/j.apcata.2023.119140Get rights and content

Highlights

  • A monolith catalyst has been developed for the direct conversion of CO2 to fuels.

  • Different catalytic components are washcoated onto the monolith as separate layers.

  • The layered configuration enhances the synergistic effects between the components.

  • It also minimizes the detrimental interactions between the catalytic components.

  • The monolith catalyst exhibits outstanding on-stream durability in a 146-h test.

Abstract

A monolith supported bifunctional catalyst for the direct conversion of CO2 to dimethyl ether was developed and evaluated. The catalyst consists of a layer structured configuration, in which a CuO/ZnO/ZrO2 component for methanol synthesis using CO2 as feedstock and a Ferrierite zeolite component for the subsequent dehydration reaction are washcoated onto the channel surfaces of a metallic monolith substrate as two consecutive layers. The metal substrate provides heat conduction to regulate the catalyst bed temperature. The layered configuration significantly improves the synergistic effects of the two components, resulting in a 20% increase in the productivity for dimethyl ether at 240 ℃ as compared with the conventional catalysts with the two components being blended in various levels of proximity. Additionally, the layer structured design minimizes the undesirable interaction between the two components and drastically improves the on-stream durability of the catalyst. No activity decline was observed in a 146-h performance test.

Introduction

Transformation of mobility technologies to reach net zero CO2 emissions is crucial in meeting global goals for carbon neutrality by the middle of the century as the transportation sector currently emits roughly 30% of the greenhouse gas emissions that contribute to climate change [1]. While light-duty passenger vehicles and medium-duty trucks can be electrified, certain parts of the transportation sector, such as aviation, maritime, and off-road heavy machinery, will still rely on liquid fuels in the foreseeable future to meet the demand for high energy density [2]. Converting CO2, either from point sources or through direct air capture, to liquid fuels with renewable energy and green hydrogen can be a workable path to accelerate the decarbonization of the transportation sector [3]. CO2 can be hydrogenated to a wide range of fuels or chemicals depending on the catalytic processes chosen. Among the processes, conversion of CO2 to dimethyl ether (DME) is of great interest [4], [5], [6], [7], [8]. It preserves the highest amount of chemical energy of H2, and the chemical reactions can be carried out under moderate pressure (∼20–50 bar) and temperature (∼200–300 ℃), making it more energy efficient [9], [10]. DME is a non-toxic clean fuel with physical properties similar to that of liquid petroleum gas (LPG), so it can be transported through the established LPG storage and distribution infrastructure [4]. It has a high cetane number (>55) and can be used as a substitute for diesel fuel for heavy-duty diesel engines [4], [5]. It is also a versatile chemical intermediate for production of widely used chemicals and can serve as a H2 carrier [6].

The current commonly-used industrial process for DME production utilizes syngas (a mixture of H2, CO, and a low level of CO2) as the feedstock and involves two distinct catalytic reaction steps [7], [8], [11], [12]. First, syngas is converted into methanol over metal oxide catalysts, typically CuO/ZnO/Al2O3:CO+2H2CH3OHΔH°=906kJ/molCO+3H2CH3OH+HO2ΔH°=494kJ/mol

The produced methanol is subsequently dehydrated in a separate reactor to DME over acid catalysts, such as γ-Al2O3 or zeolites:2CH3OHCH3OCH3+H2OΔH°=234kJ/mol

Combining (R2) and (R3) results in a global reaction for the conversion of CO2 to DME:2CO2+6H2CH3OCH3+3H2OΔH°=1222kJ/mol

With CO2 as feedstock, however, the more practical approach is to add an additional step into the process to facilitate the reverse water gas shift reaction:CO2+H2CO+H2OΔH°=412kJ/mol

Water formed in this step is removed before CO is fed to the methanol synthesis step (R1). This improves methanol yield as compared with direct hydrogenation of CO2 to methanol (R2) [12].

A main reason for carrying out these reactions in separate reactors is that each of the catalytic processes can be optimized to achieve the highest efficiency of material conversion [7], [12]. To maximize the overall energy conversion efficiency, these reactors are typically located at a centralized chemical plant. This arrangement requires great capital investment and intense energy supply but provides economies of scale [11], [12]. This approach is not compatible with most renewable energy sources as those are generally decentralized and have intermittent availability. To effectively convert renewable energy into storable chemical energy and to reduce capital and operational costs, it is highly desirable to combine different chemical processes into one integrated modular system.

CO2 can be directly converted to DME in a single reactor, but this will require bifunctional catalysts that contain both metal oxides, such as CuO/ZnO/ZrO2, to catalyze methanol synthesis (R2) and acid components, such as zeolites, for the dehydration step (R3) to complete the two reactions in tandem. Combining the two catalytic components into one system also creates synergies as the in-situ consumption of methanol in (R3) forces (R2) forwards to achieve thermodynamic equilibrium, thus improving one-pass CO2 conversion and energy utilization efficiency [4], [7], [12], [13], [14], [15], [16].

A considerable amount of research effort has been devoted to the development of suitable bifunctional catalysts for the direct synthesis of DME using CO2 as feedstock, but several main challenges remain to be overcome [16], [17], [18]. As the CO2 hydrogenation reaction (R2) is exothermic and leads to a volume reduction, low reaction temperature (<300 ℃) and high pressure (∼20–50 bar) drive the equilibrium toward CO2 conversion. On the other hand, higher temperature can improve the reaction kinetics. To balance the two aspects for a maximum DME yield, the catalyst operating temperature window is generally very narrow. Although the widely studied Cu-based methanol synthesis catalysts, such as CuO/ZnO/ZrO2, do exhibit excellent activities at around 240–260 ℃, the catalysts can suffer from rapid deactivation at temperatures above 300 ℃ due to Cu sintering [18], [19]. To improve heat transfer and to avoid hot spot formation on the catalysts typically encountered with pelletized catalysts in packed-bed reactors, micro-channel reactors, monolithic reactors, and highly conductive structured catalytic reactors have been explored [20], [21], [22], [23], [24], [25]. These designs demonstrated remarkably improved on-stream durability but have not attracted enough attention mainly because of the relatively low space time yield of these systems compared with a packed-bed reactor design. Therefore, these concepts have not been proven in a large-scale reactor.

To maximize the synergistic effects, it is desirable to place the metal oxides near the acid components [26], [27]. When the two components are in close contact with each other, however, detrimental interactions may occur, which hurts the long-term durability of the catalysts [28], [29], [30], [31]. For example, Bonura et al. reported that a bifunctional catalyst with CuO/ZnO/ZrO2 intimately mixed with a Ferrierite zeolite exhibited superior activity for the direct conversion of CO2 to DME, but also noted that the catalyst progressively lost its CO2 hydrogenation activity as well as its selectivity to DME in a 150-h durability test [32], [33], [34]. Migration of Cu to the zeolite component was identified as one of main deactivation mechanisms [34]. To circumvent this issue, bifunctional catalysts with a core-shell structure were explored [30], [35], [36], [37], [38]. Typically, the acid component is placed on the shell covering the metal oxides core. Thus, methanol formed in the core must diffuse through the shell, where it is dehydrated to DME. Because the interaction of the two components is limited to the interfacial region, core-shell catalysts do show better on-stream durability than the conventional catalysts with the two components simply mixed with each other; however, deactivation is still noticeable. Also, preparation of catalysts with a core-shell structure and good mechanical strength is nontrivial. It typically requires additional binding materials and dense packing of the components, which often reduces the gas diffusivity to the catalytic components. As a result, both experimental results and model simulation showed that catalysts with a core-shell structure are generally less effective [13], [39].

In this study, we report a layer structured monolith catalyst design that overcomes the challenges mentioned above. The catalytic activity and selectivity of the new configuration were compared against the conventional pelletized bifunctional catalysts in a packed-bed reactor. The results clearly demonstrated the performance advantages of the new design. More remarkably, 146-h on-stream durability testing demonstrated excellent stability of the layer structured monolith catalysts.

Section snippets

Catalytic components selection and preparation

CuO/ZnO/ZrO2 (CuZnZr) was selected as the methanol synthesis catalytic component as it exhibits better activity and stability than the commercial CuO/ZnO/Al2O3 catalyst for methanol synthesis with CO2 as feedstock (R2) [40], [41], [42], [43], [44], [45]. The specific composition, CuO:ZnO:ZrO2 = 6:3:1 (in moles), used in the this study was based on the work reported by Bonura et al [32], [33], [34], [40], [46]. The sample was prepared by a gel-oxalate coprecipitation method [46], [47]. Nitrate

Evaluation of pelletized catalysts in a packed-bed configuration

First, we assessed the catalytic performance of the catalytic components and a conventional bifunctional catalyst as pelletized particles (250–500 µm) in a packed-bed configuration for the direct conversion of CO2 to DME. Fig. 1 summarizes the CO2 conversion and the product yields for CO, CH3OH, DME and oxygenated hydrocarbons (CH3OH + DME) at 240 ℃ for CuZnZr only, CuZnZr followed by FER as two separate catalyst beds (CuZnZr > FER), and the bifunctional catalyst with CuZnZr directly deposited

Conclusions

A bifunctional monolith catalyst for the direct conversion of CO2 to dimethyl ether has been developed in this study. Different catalytic components, such as CuO/ZnO/ZrO2 for converting CO2 to methanol and Ferrierite zeolite for the subsequent methanol dehydration to dimethyl ether, are washcoated onto a metallic monolith substrate as separated layers. The layer structured configuration enhances the synergistic effects of the two components, resulting in an additional 20% increase in the

Declaration of Competing Interest

The authors declare no competing interests.

Acknowledgements

This work was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. The authors thank Junyan Zhang for conducting the XRD and Xiaohan Ma for the SEM analysis.

Author contributions

H.-Y.C. conceived the monolith catalyst concept, designed, and conducted the experiment. J.P. contributed ideas to the concept and set up the reactor system. T.J.T. contributed ideas to the concept and facilitated experimental

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