Amorphous, turbostratic and crystalline carbon membranes with hydrogen selectivity

doi:10.1016/j.carbon.2016.04.062

Carbon

Volume 106, September 2016, Pages 93-105

https://doi.org/10.1016/j.carbon.2016.04.062 Get rights and content

Abstract

Hydrogen production by catalytic steam reforming of renewable hydrocarbons like bio-methane or bio-ethanol has become an attractive goal of sustainable chemistry. Side reactions as in ethanol steam reforming decrease the hydrogen selectivity. A low-temperature catalytic membrane reactor with a hydrogen-selective membrane is expected to solve this problem. Three different carbon membranes are investigated with respect to their performance to extract hydrogen selectively from the binary and ternary reaction mixtures (H₂/CO₂), (H₂/CO₂/H₂O), and (H₂/ethanol) as model systems for bio-ethanol steam reforming. The three carbon membranes under study are (i) an amorphous carbon layer prepared by physical vapour deposition (PVD) of carbon on an porous alumina support using a carbon fibre yarn, (ii) a turbostratic carbon layer obtained by pyrolysis of a supported organic polymer blend as precursor, and (iii) a crystalline carbon prepared by pressing of graphite flakes into a self-supporting disc. For the equimolar binary feed mixture (H₂/CO₂) all carbon membranes were found to be hydrogen selective. For the ternary feed mixture (41vol.-% H₂/41vol.-% CO₂/18vol.-% H₂O), in the case of the amorphous and crystalline carbon membrane, the hydrogen selectivity remains also in the presence of steam. The turbostratic carbon membrane separates preferentially steam (H₂O) from the ternary feed mixture (H₂/CO₂/H₂O).

Introduction

In reforming reactions, steam is reacted with hydrocarbons such as methane (natural gas or bio-gas) or (bio-) ethanol to give hydrogen and carbon dioxide according to CH₄ + 2H₂O ⇆ 4H₂ + CO₂ or C₂H₅OH + 3H₂O ⇆ 6H₂ + 2 CO₂. Due to the endothermic character of reforming, the reactions takes place at high temperatures. Moreover, carbon monoxide is also formed, which reacts with steam in the exothermic water gas shift reaction at lower temperatures to hydrogen and carbon dioxide [1], [2], [3], [4], [5], [6]. If the reforming reaction of methane or ethanol is conducted at lower temperatures, the formation of by-products is suppressed, but the conversion is low [7], [8], [9], [10], [11]. However, when the produced hydrogen is extracted from the reactor through a hydrogen-selective membrane, the reaction mixture can be pushed towards complete conversion.

For the hydrogen extraction from a catalytic membrane reactor, a membrane has to be found that transports selectively hydrogen in the presence of steam, carbon dioxide and hydrocarbons (methane, ethanol). Current state of the art technique is reported by the application of Pd alloys as hydrogen-selective membrane (100% H₂ selectivity) for methane and also for ethanol reforming [12], [13], [14], [15], [16], [17]. However, Pd is expensive and not long-time stable in the presence of hydrocarbons at temperatures above 400 °C [18], [19], [20]. Molecular sieve membranes (zeolite, metal-organic framework) separate according to the kinetic diameter of a molecule. This means, such membranes separate preferentially the smaller water molecule (2.6 Å) rather than the slightly larger hydrogen molecule (2.9 Å) and also carbon dioxide (3.3 Å) [21]. The other reaction components are less critical because of their much larger kinetic diameters (CO: 3.8 Å, CH₄: 3.8 Å, C₂H₅OH: 4.5 Å).

In this context, the selective separation of hydrogen faces also different challenges by the strong adsorption of water or steam in highly polar membranes like aluminosilicate zeolites. In the case of metal-organic frameworks (MOFs) this effect of blocking water is probably reduced, but steam containing atmospheres in combination with high temperatures (up to 400 °C) are unfavorable for the stability of these materials.

Carbon-based materials offer some suitable properties like chemical and thermal stability, hydrogen selectivity and hydrophobic character in general. Thus, we decided to evaluate different carbon-based membranes in terms of hydrogen-selectivity for the mixture H₂/CO₂ in the presence of steam. Three different carbon membranes were prepared and tested. Among them, a new type of carbon membrane was prepared by physical vapour deposition (PVD) using the sublimation of carbon through resistance heating and subsequent deposition of amorphous carbon layers on a porous alumina support as demonstrated in Ref. [22]. A further carbon membrane was obtained by controlled pyrolysis of an organic polymer blend layer on a porous alumina support as it has been proposed in literature especially for air separation [23], [24], [25]. This procedure results in a turbostratic arrangement of carbon layers. A third carbon membrane was obtained by pressing of commercial graphite flakes into self-supporting disc, we name it crystalline carbon in this study. In a previous paper on such pressed graphite membranes, we found a hydrogen selectivity for binary (H₂/CO₂) and ternary (H₂/CO₂/H₂O) mixtures independent of temperature [26].

The aim of this work is to compare the separation behaviour of these three differently structured carbon membranes. As model systems for the ethanol steam reforming process, the binary equimolar mixtures H₂/CO₂ and H₂/EtOH as well as the ternary gas mixture (41 vol.-% H₂/41 vol.-% CO₂/18 vol.-% H₂O) were selected.

Section snippets

Supported amorphous carbon membrane by sublimation and deposition of carbon using resistance heating

Porous α-Al₂O₃ discs (Fraunhofer Institute for Ceramic Technologies and Systems IKTS, prior HITK/Inocermic, Hermsdorf, Germany) with a diameter of 18 mm, a thickness of 1.0 mm, and a pore size in the top layer of around 70 nm, were used as supports for carbon deposition. These supports were coated with amorphous carbon in a high vacuum sputter coater (Leica EM SCD500) using a carbon fibre yarn (Leica: 16LZ02308VN) with a static arrangement of the support. Before sublimation the chamber was

Macroscopic description (SEM)

The three carbon membranes differ in thickness and shape as a result of their different procedures in synthesis and preparation described in detail in Section 2.1. The amorphous carbon membrane was generated by physical vapour deposition which results in a crack-free carbon layer of about 300 nm on a porous α-Al₂O₃ disc (see Fig. 1a). The synthesis of the turbostratic carbon membrane also gave a crack-free carbon layer of about 8 μm on a porous α-Al₂O₃ disc (see Fig. 1b) and the crystalline

Conclusions

Three different carbon membranes were investigated for the in-situ separation of hydrogen from the reaction mixture of bio-ethanol steam reforming for an intended catalytic membrane reactor operating at low temperatures (≤ 400 °C). Amorphous carbon made by physical vapour deposition (PVD) of sublimated carbon from a glowing carbon fibre yarn on a porous alumina support, turbostratic carbon generated by pyrolysis of a supported polymer precursor film on a porous alumina support, and crystalline

Acknowledgements

The authors thank the Deutsche Forschungsgemeinschaft (DFG, Ca 147/19-1 and FE 928/7-1) and the National Natural Science Foundation of China (NSFC, 21322603) for financing the project “Hydrogen production from bio-ethane and bio-ethanol in catalytic membrane reactors”. The project partner X. Zhu (Dalian) is thanked for stimulating discussions.

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View full text

Amorphous, turbostratic and crystalline carbon membranes with hydrogen selectivity

Abstract

Introduction

Section snippets

Supported amorphous carbon membrane by sublimation and deposition of carbon using resistance heating

Macroscopic description (SEM)

Conclusions

Acknowledgements

A review on reforming bio-ethanol for hydrogen production

Int. J. Hydrogen Energy

Ethanol steam reforming in a microchannel reactor

IChemE

Hydrogen production by catalytic ethanol steam reforming

ScienceAsia

Ethanol steam reforming on Rh/Al2O3 catalysts

Energy & Fuels

Hydrogen production by ethanol steam reforming on Ni/oxide catalysts

AIP Conf. Proc.

Synthesis gas production by steam reforming of ethanol

Appl. Catal. A General

Catalyst deactivation and regeneration in low temperature ethanol steam reforming with Rh/CeO2–ZrO2 catalysts

Catal. Lett.

Low temperature and H2 selective catalysts for ethanol steam reforming

Catal. Lett.

Low temperature catalytic steam reforming of ethanol. 1. The effect of the support on the activity and stability of Pt catalysts

Appl. Catal. B Environ.

Low temperature catalytic steam reforming of ethanol. 2. Preliminary kinetic investigation of Pt/CeO2 catalysts

Appl. Catal. B Environ.

Catalytic activity of CeO2 supported Pt-Ni and Pt-Co catalysts in the low temperature bio-ethanol steam reforming

Chem. Engin. Trans.

Methanol and ethanol steam reforming in membrane reactors: an experimental study

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Low temperature ethanol steam reforming in a Pd-Ag membrane reactor Part 1: Ru-based catalyst

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Hydrogen production from ethanol steam reforming: energy efficiency analysis of traditional and membrane processes

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Effect of oxygen addition on the hydrogen production from ethanol steam reforming in a Pd-Ag membrane reactor

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Design and process study of Pd membrane reactors

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Overview of Pd-based membranes for producing pure hydrogen and state of art at ENEA laboratories

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Effects of co-exisiting hydrocarbons on hydrogen permeation through a palladium membrane

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Brief review of steam reforming using a metal membrane reactor

T. Catal.

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Ethanol steam reforming on Rh/Al₂O₃ catalysts

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Low temperature and H₂ selective catalysts for ethanol steam reforming

Low temperature catalytic steam reforming of ethanol. 2. Preliminary kinetic investigation of Pt/CeO₂ catalysts

Catalytic activity of CeO₂ supported Pt-Ni and Pt-Co catalysts in the low temperature bio-ethanol steam reforming