Elsevier

Catalysis Today

Volume 348, 15 May 2020, Pages 9-14
Catalysis Today

Steam reforming of dimethoxymethane to hydrogen-rich gas over bifunctional CuO-ZnO/ƞ-Al2O3 catalyst-coated FeCrAl wire mesh

https://doi.org/10.1016/j.cattod.2019.08.050Get rights and content

Highlights

  • DMM is a promising green source of hydrogen for fuel cells.

  • Structured CuO-ZnO/ƞ-Al2O3 catalyst-coated FeCrAl wire mesh was studied in DMM SR.

  • The catalyst provides 100% DMM conversion with H2 productivity of 2.5 L/g·h at 300 °C.

  • The catalyst is efficient in DMM SR to H2-rich gas with low CO (<1 vol.%) content.

Abstract

Catalytic steam reforming of dimethoxymethane (DMM) to hydrogen-rich gas was successfully performed using structured CuO-ZnO/ƞ-Al2O3/FeCrAl catalysts. The catalysts containing on their surface both acidic and copper-based sites were active and selective for DMM steam reforming to hydrogen-rich gas with low (< 1 vol.%) CO content. In particular, the CuO-ZnO/ƞ-Al2O3/FeCrAl catalysts provided a 100% DMM conversion with hydrogen production rate of ∼2500 ml H2/(gcat·h) at 300 °C, WHSV =3500 ml/(gcat·h) and molar ratio H2O/DMM = 4.5.

Introduction

In recent years, high temperature proton exchange membrane fuel cells (HT PEMFCs) have attracted a growing interest as an alternative ecologically benign source of electric power due to their high tolerance to hydrogen impurities as compared to the conventional PEMFC [[1], [2], [3]]. Indeed, the HT PEMFCs can be fueled by hydrogen-rich gas containing up to 3 vol.% of CO [1]. Analysis of current literature shows that the gas mixture of that composition can be produced by catalytic steam reforming (SR) of oxygenated compound of C1 chemistry, such as methanol [[4], [5], [6]], dimethyl ether (DME) [[6], [7], [8]] and DMM [[9], [10], [11], [12], [13], [14], [15]]. The hydrogen-rich gas with low CO content produced by these reactions can be used for direct feeding of HT PEMFCs without any further CO removal that makes the HT PEMFC based power units more compact and simple.

Among oxygenated compound of C1 chemistry, DMM is proved to be one of the most promising and environmentally benign materials. Similarly to methanol and DME, DMM is an easy to synthesize compound. It can be produced by condensation of methanol with formaldehyde or direct catalytic oxidation of methanol [16]. Since DMM is relatively inert and non-corrosive liquid material, it can be easily stored and transported. Besides, DMM is considered as a clean substitute for diesel fuel combusting with lower emissions of NOx and CO [17]. These facts, together with the recent data on DMM SR [[9], [10], [11], [12], [13], [14], [15]], predict increased DMM demand that will become a promising raw material for the production of hydrogen-rich gas for HT PEMFCs feeding.

The DMM SR is still studied insufficiently, only a few papers are available so far. The reaction is expressed by equation:CH3OCH2OCH3 + 4H2O = 8H2 + 3CO2

It is generally assumed [[9], [10], [11], [12], [13], [14], [15]] that DMM SR proceeds viaa consecutive two-step reaction mechanism including DMM hydrolysis to methanol and formaldehyde (2) and steam reforming of the formed methanol (3) and formaldehyde (4) to hydrogen-rich gas:CH3OCH2OCH3 + H2O = 2CH3OH + CH2OCH3OH + H2O = 3H2 + CO2CH2O + H2O = 2H2 + CO2

It is well known that solid acids and copper-based catalysts are effective for DMM hydrolysis and methanol/formaldehyde steam reforming reactions, respectively.

In previously published papers, DMM SR was performed over granulated catalysts: mechanical mixed systems comprised of a solid acid and a Cu-based catalyst [[9], [10], [11]] and bifunctional catalysts CuO-CeO2/Al2O3 and CuO-ZnO/Al2O3 [[12], [13], [14], [15]] containing both the acid sites and copper-based species on alumina surface. The mechanically mixed systems provided complete DMM conversion and H2 productivity up to 7400 ml H2/(gcat·h) at 250–300 °C. The bifunctional CuO-CeO2/Al2O3 and CuO-ZnO/Al2O3 catalysts demonstrated H2 productivity ∼15,000 ml H2/(gcat·h) at 300 °C with low CO content. These catalysts contain both the Al2O3 surface Lewis acid sites for DMM hydrolysis and Cu-based (CuO-CeO2 or CuO-ZnO) species for methanol/formaldehyde SR.

Since DMM SR is highly endothermic reaction, the use of conventional granulated catalysts can obviously cause the formation of cold spot temperature gradients along the reactor axis, leading to lower catalyst efficiency. Aimed to resolve this problem, we deposited the active catalytic components on metallic heat-conducting support. In our DMM SR experiments, we used FeCrAl wire mesh support which is known for its high heat conductivity, flexibility and good promises for catalytic technologies [18,19]. Alumina deposited on FeCrAl wire mesh as a coating improves adhesion properties, surface area and porosity [19]. Moreover, in the case of DMM SR, the alumina acid sites play an important role in DMM hydrolysis at the first step of the reaction.

In the present paper, we report the first example of the DMM SR to hydrogen-rich gas over structured CuO-ZnO/ƞ-Al2O3 catalyst-coated FeCrAl wire mesh. Comparative analysis of the performance of CuO-ZnO, CuO and ZnO supported on structured ƞ-Al2O3/FeCrAl catalysts in DMM SR was performed.

Section snippets

Catalyst preparation

The FeCrAl alloy wire mesh provided by “NPO Souznichrom” Inc (Russia) was used as a metallic support. The FeCrAl alloy wire mesh was coated by 6 wt.% of ƞ-Al2O3 (Fig. 1a) according to [19]. The ƞ-Al2O3/FeCrAl sample was twisted into an Archimedean spiral (Fig. 1b) and then calcined at 500 °C for 2 h before being impregnated. The supported ƞ-Al2O3 was mechanically stable during the procedure of twisting and didn't lose its weight. The CuO-ZnO/ƞ-Al2O3/FeCrAl structured catalyst was prepared by

Catalytic performance of ƞ-Al2O3/FeCrAl and ZnO/ƞ-Al2O3/FeCrAl

At first, DMM hydrolysis (2) was performed using ƞ-Al2O3/FeCrAl as a catalyst. Fig. 2 shows the temperature dependencies of DMM conversion, and the CH2O, CH3OH and MF selectivities for DMM hydrolysis over ƞ-Al2O3/FeCrAl. We used selectivity as a catalyst characteristic in order to demonstrate the transformation of the reaction scheme. As the temperature increased from 150 to 300 °C, the DMM conversion increased and reached ∼100%. Methanol and formaldehyde were the main reaction products at

Conclusions

The feasibility of hydrogen production by steam reforming of DMM using structured bifunctional CuO-ZnO/ƞ-Al2O3/FeCrAl catalysts was demonstrated for the first time. The catalysts provided complete conversion of DMM to hydrogen-rich gas with a low CO content at ∼300 °C and molar ratio H2O/DMM = 4.5. Note that the simultaneous presence of copper, zinc oxide, and alumina on the structured FeCrAl wire mesh surface is of key importance for getting the active and stable catalysts for DMM SR.

Acknowledgements

The authors thank Dr. V. N. Rogozhnikov for provided ƞ-Al2O3/FeCrAl support. This work was supported by the Russian Science Foundation (Project N 17-79-300717-79-30071).

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