Investigation and Modeling of the Hydrothermal Stability of Technically Relevant Zeolites
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Investigation and Modeling of the Hydrothermal Stability of Technically Relevant Zeolites∗ W. LUTZ, H. TOUFAR AND R. KURZHALS Tricat Zeolites GmbH, Tricat-Straße, 06749 Bitterfeld, Germany M. SUCKOW Fachhochschule Lausitz, Großenhainer Straße 57, 01968 Senftenberg, Germany
Abstract. The dense-structured zeolites of types ZSM-5 (MFI) and mordenite (MOR) of different SiO2 /Al2 O3 moduli are relatively stable under treatment by liquid water for 72 hours up to 513 K. The open-structured zeolites of types Y in dealuminated modification (FAU) and beta (BEA) undergo strong decomposition in the same range. For these two sample types a mathematical model for the decomposition of the zeolite framework is established that takes into account the influence of modulus as well as temperature. Here the kinetic of the solid phase reaction is a superposition of two different mechanisms described by the Monod equation. Keywords: zeolite, hydrothermal treatment, long-term stability, modeling Introduction The interaction of zeolites and water concerns a basic property of these natural or synthetic materials: hydrated zeolites deliver water under heating without destruction of their framework structure. After cooling a renewed hydration takes place. This reversible process of adsorption and desorption is used for selective drying of gases and liquids in laboratory but especially at technical scale (Breck, 1974). An extensive application of zeolites comprises their use as catalysts in chemical processes where water may be one of the feedstocks or reaction products. Examples are the MTP (Rothaemel and Holtmann, 2002), the MTG (Derouane et al., 1981), and the MTO processes (Kvisle et al., 2003). Extreme hydrothermal stress is imposed on the zeolite structure in oxidation reactions where water acts under relative harsh conditions (regeneration of the FC catalyst (O Connor et al., 1998)) or in the catalytic treatment of water-loaded waste gas stream (automotive applications (Traa et al., 1999) and waste gas treatment (Schwefer et al., 2001). ∗ Dedicated
to the late of Wolfgang Schirmer
Comprehensive knowledge of hydrothermal stability of zeolites under such reaction conditions is a mandatory precondition for a successful process layout. Unlike acid and alkaline solutions which attack preferably Al-rich and Si-rich zeolites, respectively, the water molecules may attack the zeolite framework according to the chemical reactions H+ + OH− + Na+ ≡ Al− −O−Si ≡ ↔ OH− + Na+ + ≡ Al + HO−Si ≡
(1)
and H+ + OH− + ≡ Si−O−Si ≡ ↔ ≡ Si−OH + HO−Si ≡
(2)
of both types although to a significant lower effect. Thereby, the protons of water molecules attack the Si O Al bonds over the whole framework whereas the hydroxide ions attack specifically the terminal OH-groups of the surface. The Figs. 1 and 2 give a schematic impression of both processes where water acts like a catalyst (Iler):
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these experimental conditions the most intensive effect is received because of the highest and a constant water concentration on and in the zeolites. This
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