Hydrothermal stability of cobalt silica membranes in a water gas shift membrane reactor

Abstract

Cobalt silica membranes were fabricated using sol–gel techniques for separation of H₂ in a membrane reactor set up for the low temperature (up to 300 °C) water gas shift (WGS) reaction. Single dry gas testing prior to reaction showed He/N₂ and H₂/CO₂ selectivities increasing from 75–400 to 45–160 as the temperature increased from 100 to 250 °C, respectively. During reaction the membrane delivered a H₂ permeation purity of 89–95% at high conversions, with the higher water ratio conversion providing superior membrane operational performance. Characterisation of bulk gels indicated that the cobalt silica was hydrophilic and exposure to steam at 200 °C resulted in the densification of the film matrix. The cobalt doping allowed for the membrane structural microporosity to be maintained as H₂ selectivity was not affected by steam exposure, though the flux decreased due to pore collapse of the film matrix. A total of 8 thermal cycle testing were carried out from room temperature to 300 °C, and the membrane displayed good hydrothermal stability, maintaining a high H₂ selectivity for over 200 h of operation.

Introduction

Hydrogen is utilised in a number of industries today, including fertilisers, oil refining and petrochemical. Most importantly hydrogen has gained significant interest as a future clean energy carrier, with major advantages possible when used with new fuel cell technology. Hydrogen is primarily produced from fossil fuels, such as natural gas and coal through steam reforming/gasification and water gas shift reactions, and these hydrogen production reactions have thermodynamic limits of equilibrium. This limit can be enhanced through the removal of the H₂ reaction product_, using H₂ selective membranes to potentially drive the reaction to completion. The application of membrane reactors (MR) to this type of yield enhancement reaction has been well documented [1], [2], [3], [4], [5], noting that the membranes’ characteristics for H₂ permeance and selectivity over time is a vital consideration to the commercial feasibility of this technology [6].

Microporous silica membranes have found great potential for use in industrial applications, particularly when used in MR arrangements [5], [7], [8]. These have been shown to provide good permeation and selectivity for H₂ in a variety of temperatures, however are known to suffer from poor hydrothermal stability [9], [10], [11]. This is a large problem as humid atmospheres are more often encountered in industrial applications, especially as steam is a common component in H₂ producing reactions (i.e. steam reformation and water gas shift (WGS)). The WGS (Eq. (1)) is an equilibrium limited, exothermic reaction with high conversion favoured at low temperatures.

$$\text{CO}+{\text{H}}_2\text{O}\leftarrow \to \text{C}{\text{O}}_2+{\text{H}}_2\Delta \text{H}=-41.2\text{kJ}\text{mo}{\text{l}}^{-1}$$

Typically, conventional WGS reactors use excess steam reactant to optimise the equilibrium conversion. However, in a MR this can also be achieved through H₂ removal, allowing excess steam to be minimised. This establishes an interesting balance to the system; excess steam enhances the catalyst efficiency while increasing conversion, but decreases the membrane's separation and can severely limit the membranes operating lifetime [12], [13]. While a water to CO ratio of 1 represents the minimum amount of water necessary for potential complete conversion, excess water would still be used for industrial purposes. Therefore it is a requirement for membranes to provide greater hydrothermal stability for maintained selectivity and flux over their operational lifetime.

The major problem with silica-derived membranes is associated with silanol (Si–OH) groups as Iler [14] proposed that hydroxyl groups are the most active sites for water interaction. On the other hand, silanol groups are necessary for the precise tailoring of the microporous silica films [15], [16]. Early work addressed this problem by heat treatment strategies, thus reducing surface hydroxyl groups [11], [17]. Later work showed superior improvements by adding inorganic and organic precursors to the silica matrix [9], [18], [19], [20]. For instance, the membrane surface gained a degree of hydrophobicity by using a methyl template covalently bonded to the silica, thus reducing the level of adsorbed water [9], [21]. However, no work was carried out on the effect of prolonged steam exposure, at high temperature and pressure, on the membranes selectivity.

Giessler et al. [12] investigated the stability of silica membranes using non-covalently bonded short carbon chain surfactants (i.e. as templates) for the WGS reaction on a MR set up. This work compared the hydrostability of the templated silica against conventional silica membranes, noting that over time the templated silica membranes performed better, though initially gas separation was lower. This was attributed to the slightly pore widening effect of the surfactants on the silica micro-structure. Duke et al. [10], [22] developed a method to improve hydrostability without decreasing initial selectivity. By carbonising the surfactant template in the silica matrix, it was found that selectivity improved over conventional silica membranes while maintaining high hydrothermal stability. This method differed from the hydrophobic membrane approach as hydrostability was due to structural stabilisation of the silica matrix by the carbonised surfactant, as the membrane proved to be hydrophilic.

Inorganic oxides such as TiO₂, ZrO₂, Al₂O₃, MgO, have also been used to improve the performance of membranes in steam [23], [25], [26]. The use of the metal dopants (i.e. NiO and CoO) in silica membranes has led to very high selectivities [27], [28] even in the presence of steam [29], [30] showing great promise for use in commercial membrane and membrane reactor units [31].

In this work, we investigate the hydrothermal stability of cobalt silica membranes in steam and temperature cycling. Cobalt silica tubular membranes were fabricated using sol–gel techniques and tested under hydrothermal conditions in a MR for the low temperature WGS reaction. Xerogel samples were exposed to steam in autoclaves and the structural changes were analysed using nitrogen and water adsorption. The cobalt silica membranes were tested for a range of water to CO feed ratios, and for temperatures between 150 and 300 °C. The stability and selectivity characteristics were examined by observing the gas permeation characteristics of the cobalt silica membrane before (i.e. dry gas testing) and during reaction (i.e. under steam exposure conditions).