(281b) Modification of Al Pairing and Distribution in CHA Zeolite
Modifying the aluminum distribution in zeolites could allow one to tune the catalytic, adsorption and/or ion exchange characteristics of the materials. It has been postulated that increased Al pairing in CHA zeolites such that the corresponding Cu-CHA catalysts contain more of the Cu in the Z2Cu (i.e. Al2-Cu2+) configuration rather than the Z-Cu(OH) (or Al-[Cu(OH)]+) configuration would lead to improved hydrothermal stability of the catalyst for selective catalytic reduction (SCR). This is because the Cu2+ in the Z2Cu state is more strongly bound to the zeolite framework. The use of alkali (Na+, K+)[2, 3] and Cu2+-containing organic complexes[4, 5] have been suggested as approaches to help alter the Al distribution in CHA zeolites. Each of these techniques appear to have their own limitations â there seems to be an upper limit for Na+/TMAda+ ratio for the use of Na+ in the synthesis of CHA while K+ seems to be leading to the exclusion of TMAda+ from some cages of the CHA zeolite. The use of Cu2+-containing complexes for the synthesis of CHA zeolites seems to require the use of additional alkali to be applicable across a wide range of synthesis conditions. This additional alkali is known to hurt the hydrothermal stability of the resulting zeolite under SCR conditions. An alternate approach to modify Al distribution in CHA zeolites is presented, which may be applicable to a number of other zeolitic systems. The modification of Al distribution is verified by Cu uptake isotherm measurements as well as IR spectroscopy.
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