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Hierarchical porous zeolite structures for pressure swing adsorption applications Benjamin Besser, Henrique Akira Tajiri, Gerd Mikolajczyk, Jens Möllmer, Thomas Schumacher, Stefan Odenbach, Roger Glaeser, Stephen Kroll, and Kurosch Rezwan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11120 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 19, 2016
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Hierarchical Porous Zeolite Structures for Pressure Swing Adsorption Applications Benjamin Besser,† Henrique Akira Tajiri,† Gerd Mikolajczyk,‡ Jens M¨ollmer,¶ Thomas C. Schumacher,† Stefan Odenbach,‡ Roger Gl¨aser,¶ Stephen Kroll,∗,†,§ and Kurosch Rezwan†,§ University of Bremen, Dresden University of Technology, and Universit¨ at Leipzig E-mail:
[email protected]* Phone: +49 421 218 64933. Fax: +49 421 218 64932
Abstract Porous adsorbents with hierarchical structured macropores ranging from 1 to 100 µm are prepared using a combination of freeze casting and additional sacrificial templating of polyurethane foams, with a zeolite 13X powder serving as adsorbent. The pore system of the prepared monoliths features micropores assigned to the zeolite 13X particle framework, interparticular pores of ∼1-2 µm, lamellar pores derived from freeze casting of ∼10 µm and an interconnected pore network obtained from the sacrificial templates ranging from around 100 to 200 µm with a total porosity of 71 %. Gas permeation measurements show an increase in intrinsic permeability by a factor of 14 for monoliths ∗
To whom correspondence should be addressed Advanced Ceramics, University of Bremen, Am Biologischen Garten 2, 28359 Bremen, Germany ‡ Institute of Fluid Mechanics, Dresden University of Technology, George-B¨ahr-Straße 3, 01062 Dresden, Germany ¶ Institut f¨ ur Nichtklassische Chemie e.V and der Universit¨at Leipzig, Permoserstraße 15, 04318 Leipzig, Germany § Centre for Materials and Processes (MAPEX), University of Bremen, Bibliothekstraße 1, 28359 Bremen, Germany †
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prepared with an additional sacrificial templated foam compared to monoliths solely providing freeze casting pores. Cyclic CO2 adsorption and desorption tests where pressure swings between 8 and 140 kPa reveal constant working capacities over multiple cycles. Furthermore, the monoliths feature a high volumetric working capacity of ∼1.34 mmol/cm3 which is competitive to packed beds made of commercially available zeolite 13X beads (∼1.28 mmol/cm3 ). Combined with the faster CO2 -uptake showing an adsorption of 50 % within 5-8 s (beads ∼10 s) the monoliths show great potential for pressure swing adsorption applications, where high volumetric working capacities, fast uptakes and low pressure drops are needed for a high system performance.
Keywords Hierarchial porous structure, zeolite 13X, freeze casting, sacrificial template, gas permeation, CO2 -capacity, CO2 -uptake, volumetric working capacity
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Introduction
With the contribution of about 60 % to the effect of greenhouse gases on climate change, carbon dioxide (CO2 ) is supposed to be one of the mayor actors in global warming. 1 This triggered global efforts to reduce the concentration of atmospheric CO2 and one option to reduce CO2 -emissions aiming to mitigate global climate change is CO2 capture and utilization. 2 Among many others, pressure swing adsorption (PSA) is a possibility to recover CO2 from flue gas using the mechanism of preferentially adsorption on a solid adsorbent. 3,4 Up to now, different materials are investigated for this purpose, for example zeolites, 5–7 activated carbons, 8,9 metal-organic frameworks 10,11 or mesoporous silicas. 12,13 In particular, zeolite 13X (13X) provides promising properties while combining low cost, high CO2 -uptake and CO2 /N2 selectivity, as well as sufficient thermal and mechanical stability. 2,14–16 PSA processes have already been proven advantageous for different applications and have
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been commercialized to produce O2 -enriched gas or for H2 -purification. 17,18 To achieve this, major innovations for handling the gas streams have been required which are reported in a series of patents by Keefer et al. 19–22 and Golden et al. 23–25 However, until now, PSA is not commonly used for the recovery of CO2 from flue gas due to certain material and process technology limitations. 26 High working loads and rapid adsorption/desorption cycles are required to compete with other techniques such as cryogenic distillation or membrane separation 1–3 and further improvements in this technology will require the continued development of new structured, multilayered adsorbents as pointed out by Ritter et al. 18 In industry, solid adsorbents are usually employed as beads, pellets or granules assembled to packed beds. 27–29 However, swing adsorption processes on conventional packed beds suffer from mass transfer limitations related to gas diffusion in and out of the bulk material, thus limiting the overall system performance. 29,30 Reducing the size of the beads only solves the problem partly by resulting in a higher pressure drop, in turn limiting the process efficiency as well. 29 Therefore, Coppens and coworkers extensively investigated the optimization of spacial pore size and porosity distributions in pellets to overcome mass transfer limitations in the field of catalysis. 31–35 A general conclusion of their work is that hierarchical pore networks can provide structural properties which lead to enhanced catalyst performance. In most cases, structuring beads or pellets is accompanied with an increase in porosity which results in a smaller volumetric working capacity. An ideal structure for adsorbents combines low pressure drops and fast mass transfer kinetics to ensure rapid adsorption/desorption cycles, while possessing a high volumetric working capacity, i.e. high amount of adsorbent per volume unit, enabling small system sizes. Porous structured monoliths are one possibility to achieve fast mass transfer kinetics, because diffusion paths on small scales can be reduced to short lengths. Furthermore, monoliths feature superior properties in terms of heat transfer for non adiabatic processes. 36,37 Various types of structured monoliths with tailored pore size and porosities such as laminates or foam structures are investigated. 27 Especially Rezaei and coworkers studied the influence of different
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structural configurations of adsorbents for PSA applications. 28–30,38,39 These studies indicate that, for example, laminate structures are very promising for adsorbents when made with small spacings and sheet width (
