Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Creating Hydrothermally Stable Inorganic Membrane Interlayers by Limiting the Anatase-to-Rutile (ATR) Transition Temperature in Doped-Titania Dana L. Martens,†,‡ Julius Motuzas,† Simon Smart,*,†,‡ and João C. Diniz da Costa†,‡ †
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The University of Queensland, FIM2Lab − Functional Interfacial Materials and Membrane Laboratory, School of Chemical Engineering, Brisbane, Queensland 4072, Australia ‡ Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Carlton, Victoria 3053, Australia S Supporting Information *
ABSTRACT: This work investigates the hydrostability of interlayers for inorganic membranes under harsh, yet realistic wet gas separation conditions of 550 °C and 75 mol % H2O. In order to avoid hydrothermal failure of traditional interlayers like γ-Al2O3, this work pursues titania as an interlayer with a strategy of either doping (5 mol %) or forming composites (25−50 mol %) with zirconium and silicon to increase the anatase-to-rutile (ATR) transition temperature and thereby improve stability. A series of powdered samples containing xTi:yZr:zSi were prepared and characterized pre-and post-hydrothermal exposure. The doped samples demonstrated an increased ATR transition temperature and no phase change even under harsh hydrothermal testing. The composite samples by contrast showed dramatically altered structural characteristics that are unsuitable for use as interlayers. Overall, the TiO2 doped with Zr achieved superior structural integrity compared to 95:5 Ti/Si or 90:5:5 Ti/Zr/Si. Substrates coated with interlayers were tested for gas (He and N2) permeation from 100 to 500 °C. The He/N2 selectivity for all membranes and conditions were around the ideal Knudsen selectivity for this gas pair. N2 permeation pre- and posthydrothermal treatment were almost the same for all membranes, while He permeation slightly increased, particularly for the 95:5 Ti/Si membranes. These results confirm that only minor structural changes occurred after the harsh hydrothermal treatment, and that the best hydrothermal stability was provided by 95:5 Ti/Zr materials and membranes.
1. INTRODUCTION Separation technologies form a significant part of almost all industrial processes, accounting for between 40 and 70% of capital and operating costs for chemical plants1 and up to 15% of global energy usage.2 Membrane technologies that can separate molecules without resorting to phase changes offer the opportunity to drastically reduce the energy consumption of chemical processes.3 Inorganic membranes, in particular, can facilitate these separations at high temperatures and high pressures and in corrosive environments, improving industrial efficiencies and offering new process routes.4 Microporous inorganic membranes, i.e., those with pore sizes dp < 2 nm, have been intensively investigated for high temperature wet gas separation. Examples include silica membranes doped with oxides of cobalt,5,6 zirconia,7 niobium,8 and nickel,9 in addition to silica-based membrane reactors.10,11 Despite all of these reports, silica-derived membranes remain underutilized in industrial settings due to issues with cost and stability.12 The performance of microporous inorganic membranes for gas separation is almost entirely dependent on producing ultrathin, defect-free membranes.13 To survive the rigors of industrial applications, these ultrathin membrane layers are coated onto porous supports. The latter gives the mechanical strength while the former confers the separation of gas © XXXX American Chemical Society
molecules generally by a molecular sieving mechanism. An interlayer is sandwiched between the rough porous substrate and the top ultrathin film. This type of inorganic membrane architecture is called asymmetric, as supports have a large pore size (dp ∼ 500 nm); the interlayers greatly reduce the pore sizes to dp < 10 nm and the top selective layer to dp ∼ 0.3−0.5 nm. Therefore, asymmetric inorganic membranes are characterized by a layered system of stacked particles with different grain sizes in order to obtain very narrow pore size distributions.14 The major role of the interlayers in multilayered asymmetric membranes is to provide a sufficiently smooth and flawless surface to coat the selective layer without defects.15 Following these principles of microporous inorganic membrane assembly established in the 1980s, there is an ample number of reports in the literature on the preparation of high quality microporous inorganic membranes. For instance, the most common are silica membranes delivering very high gas selectivities ranging from 100 to 1000. Most silica Received: Revised: Accepted: Published: A
March 21, 2018 June 21, 2018 July 26, 2018 July 26, 2018 DOI: 10.1021/acs.iecr.8b01260 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research membranes are prepared on α-Al2O3 porous supports, which is coated with an alumina boehmite solution and forms smooth γ-Al2O3 interlayers. This type of silica membrane configuration has been so successful that by the early 2010s, Yacou and coworkers16 reported the production of a multitube silica membrane module reaching H2 gas selectivities up to 1000 for over 2000 h of stable operation. This concerted research effort has returned excellent results for dry gas separations; however, it is an entirely different story in the case of wet gas separation. Ulhmann et al.5 exposed tubular silica membranes containing γ-Al2O3 interlayers to high temperature (500 °C) wet mixed gas streams, but gas selectivity decayed with time. This was attributed to the hydrothermal instability of the γ-Al2O3 interlayers, which suffers from densification and pore broadening upon contact with steam at elevated temperatures.17,18 In the presence of steam, γ-Al2O3 undergoes a hydrolytic attack, resulting in the phase change to bayerite (β-Al(OH)3).19 Therefore, changes in interlayer structure/phase propagate to the top selective layer, causing disruptions and defects and rendering the membrane ineffective for gas separation (Figure 1).
anatase crystallographic lattice23 to create binary and ternary mixed metal oxide systems. Therefore, this work focusses solely on finding suitable interlayer materials that (a) are able to be coated onto porous supports, forming a defect free, homogeneous layer, (b) have a pore size and surface roughness suitable for an interlayer, and (c) remain stable under harsh hydrothermal testing conditions. Specifically we propose to investigate the preparation of doped and composite TiO2-derived interlayers against those criteria. Zirconium and silicon dopants were chosen as they have been linked to an increase in the ATR transition temperature.23 The materials were prepared by a sol−gel method using chelating agents to produce doped TiO2 (
