Christel Peuker

Decomposition of template in SAPO-5 and AlPO4-5 molecular sieves studied by IR and Raman spectroscopy

Abstract The state of the triethylamine (TEA) template in AlPO 4 -5 and SAPO-5 and the characteristics of the framework structure of the molecular sieves were studied by means of IR and Raman spectroscopy. Decomposition of TEA and the formation of OH groups were investigated by DRIFT measurements carried out within the temperature range from 300 to 633 K. It has been found that TEA exists as triethylammonium ions in the as-synthesized samples, and its decomposition occurs via abstraction of ethylene and the stepwise formation of diethyl- and ethylammonium cations. The decomposition mode under a nitrogen atmosphere is different from that under air and requires another temperature. A thermal treatment of both molecular sieves at above 970 K results in a transformation of the structure.

Catalytic and infrared spectroscopic investigations of the molecular sieve types SAPO-34 and SAPO-11

The molecular sieves SAPO-34 and SAPO-11 have been synthesized by means of hydrothermal procedures. The acidic properties of the samples are investigated by infrared spectroscopy and thermodesorption of ammonia. SAPO-34 contains significantly more Bronsted acid sites than SAPO-11 and is able to transform methanol to alkenes with high selectivity. In situ infrared investigations on SAPO-34 show the existence of two forms of adsorbed methanol. At 493 K, dimethyl ether (DME) is formed predominantly, probably by a reaction between physisorbed methanol and methoxy groups. Up to 563 K, the more strongly adsorbed species react to form alkenic intermediates and the formation of coke begins, which proceeds very rapidly at 623 K. The spectrum of the coke is similar to that of aromatic compounds with long aliphatic side chains.

Disruption of the MFI framework by the incorporation of gallium

The influence of isomorphous substitution of silicon by gallium in the MFI framework on lattice crystallinity and defect sites is investigated. The Ga-ZSM-5 zeolites are characterized by XRD, n-hexane adsorption, IR, TPD of ammonia, TG–DTA, MAS NMR, and chemical analysis. The incorporation of the large gallium atom is found to be disruptive. It leads to greater lattice distortion. However, the number of lattice defects decreases with growing gallium content. The sum of lattice defects and incorporated gallium atoms is nearly unchanged. It is suggested that gallium atoms and defect sites are located at similar lattice positions.