Zeolitic materials for the separation of carbon dioxide from methane

Eduardo Pérez-Botella, Miguel Palomino, Susana Valencia, Fernando Rey

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Zeolites are adsorbents with very interesting properties which have allowed them to be used in industrial separation applications since their commercialization in the 1950s. The discovery of new zeolite structures and compositional analogues opens the range of separations that can be realised by these fascinating group of materials. The upgrading of biogas and natural gas is necessary for their use as fuels and it involves a separation step for CO2 removal, which can be carried out using zeolites. Here, I will present my work on pure-silica zeolites, aluminophosphates (AlPOs) and silicoaluminophosphates (SAPOs) as promising CO2 adsorbents for its removal from methane-rich mixtures. Small pore zeolites with narrow channels (channel-like topology) present an intrinsically high structural selectivity towards CO2, which makes their performance comparable to that of aluminosilicate zeolites. AlPOs and SAPOs can be superior to analogous zeolites, in which they present similar CO2/CH4 selectivities but lower heats of adsorption, thus lowering the energy required for the regeneration step. Introduction Zeolites are microporous crystalline aluminosilicates, which have found extended use as adsorbents and catalysts. Their structure consists of TO4 corner-sharing tetrahedra, where T is usually Si or Al [1]. The way in which the tetrahedra are connected gives rise to different structures with well-defined pore sizes which allow for size discrimination of molecules with a precision of tenths of Å, which is the main reason why zeolites have been known for many years as molecular sieves. Besides the enormous amount of conceivable possible zeolite structures [2], out of which around 250 are known to exist [3], their composition is another key aspect of their adaptability and variety. Charge distribution in zeolites can be modified through their composition to tune the surface polarity. Zeolitic structural analogues, such as pure silica zeolites, aluminophosphates (AlPOs) or silicoaluminophosphates (SAPOs), present notably unique properties which can be exploited to carry out less energy intensive separations [4]. Methane is a fuel that can be of renewable (biogas) or fossil (natural gas, coalbed methane, associated gas) origin. It is one of the most used fuels, and will still be due to its higher H/C relation, which lowers the amount of CO2 produced upon combustion [5]. In methane rich mixtures, one of the major components besides methane is carbon dioxide, which needs to be removed. Traditionally this is done by aqueous amine absorption, which is an energy-intensive process and also involves high operational costs. An alternative method for carrying out this separation is physisorption on zeolites [6]. In this talk I will show how pure silica zeolites with the right pore size and topology can be used to separate CO2 from CH4 with performances comparable to those of aluminosilicate zeolites. Additionally, I will show that some AlPOs and SAPOs present very particular CO2 adsorption behaviour which makes them interesting adsorbents for achieving efficient regeneration through pressure swing. Experimental Pure silica zeolites with ITW, IHW, CHA, LTA, MTF, AFI and RWR structures and ALPOs and SAPOs with CHA, LTA and AFI structure were synthesized following previously described procedures and fully characterized in terms of crystalline structure, framework chemical environment and textural properties. Adsorption isotherms of CO2, CH4 and H2O were measured. Breakthrough experiments were carried out at 25 °C, with a 2-step regeneration procedure which involved a purge gas at ambient temperature followed by direct heating of the adsorbent column. Results Out of the small pore pure silica materials tested, materials with channel-like topology (RWR and ITW) present the largest ideal CO2/CH4 selectivities. Nonetheless, the best performances in competitive adsorption experiments are not obtained with this material. Si-ITW is superior to the other studied materials, in which it presents certain degree of kinetic exclusion of CH4, together with a high affinity towards CO2 due to the close-fitting of this adsorbate inside the channels. Despite its moderate adsorption capacity, its CO2/CH4 selectivity makes it competitive with intermediate Si/Al ratio zeolites, such as UZM-9 (relabeled here as LTA-6), and clearly superior to other pure silica zeolites, such as Si-RWR and Si-LTA. On the other hand, AlPOs and SAPOs with AFI, CHA and LTA structures present an unusually low CO2 isosteric heat of adsorption, when compared to analogous zeolites. Some LTA and CHA ALPOs and SAPOs present similar or even higher selectivity towards carbon dioxide and higher working capacities, which makes them also very promising adsorbents, as the energy required for regeneration of a hypothetical process could be notably reduced. Conclusions Low polarity small pore zeolitic materials can be effectively used to selectively adsorb CO2 from its mixtures with methane. High selectivities and moderate working capacities can be obtained with materials, such as Si-ITW, the pore size and topology of which favour the adsorption of CO2. Their hydrophobicity and relatively low heat of adsorption compared to aluminosilicate zeolites makes them a promising alternative, as no drying step would be needed prior to the adsorption of CO2 and lower energy requirements are needed for regeneration. Also, despite not being hydrophobic, SAPOs and AlPOs present notably lower CO2 isosteric adsorption heats than zeolites with the same framework charge. This also makes them an interesting alternative for cases where a very low energy input for regeneration is desired. Acknowledgments I want to thank the Spanish Ministry of Education and Professional Training for the grant FPU15/01602 which allowed me to carry out my PhD thesis at the ITQ. References [1] E.M. Flanigen, R.W. Broach, S.T. Wilson, Introduction, in: S. Kulprathipanja (Ed.), Zeolites Ind. Sep. Catal., Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2010: pp. 1–26. [2] M.D. Foster, M.M.J. Treacy, A Database of Hypothetical Zeolite Structures. http://www.hypotheticalzeolites.net/. [3] Ch. Baerlocher, L.B. McCusker, Database of Zeolite Structures. http://www.iza-structure.org/databases/. [4] E. Pérez-Botella, M. Palomino, S. Valencia, F. Rey, Zeolites and other adsorbents, in: K. Kaneko, F. Rodríguez-Reinoso (Eds.), Nanoporous Mater. Gas Storage, Springer Singapore, Singapore, 2019: pp. 173–208. [5] World Energy Council, Full report: The Role of Natural Gas, London, UK, 2017. [6] M. Tagliabue, D. Farrusseng, S. Valencia, S. Aguado, U. Ravon, C. Rizzo, A. Corma, C. Mirodatos, Natural gas treating by selective adsorption : Material science and chemical engineering interplay, Chem. Eng. J. 155 (2009) 553–566.
Originele taal-2English
Titel42 RIA - Book of Abstracts
Plaats van productieValencia
UitgeverijSpanish Chemistry and Physics Royal Societies (RSEQ and RSEF)
Pagina's18-18
Aantal pagina's1
StatusPublished - sep 2022
Evenementthe XLII Iberian Adsorption Meeting (42RIA) - Universitat Politècnica de València (UPV), Valencia, Spain
Duur: 13 sep 202216 sep 2022

Conference

Conferencethe XLII Iberian Adsorption Meeting (42RIA)
Land/RegioSpain
StadValencia
Periode13/09/2216/09/22

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