Liquid-phase thiophene adsorption on MCM-22 zeolites. Acidity, adsorption behaviour and nature of the adsorbed products

Microporous and Mesoporous Materials (Impact Factor: 3.37). 09/2009; DOI: 10.1016/j.micromeso.2008.08.008

ABSTRACT a b s t r a c t The liquid-phase adsorption of thiophene from thiophene/iso-octane solutions has been investigated in batch conditions at room temperature and atmospheric pressure on MCM-22 zeolites with Si/Al in the 9–46 range. Thiophene adsorption was found to occur in two steps whatever the Si/Al ratio of the adsor-bent. The presence of ferrierite besides the MCM-22 phase caused a significant loss of the adsorption per-formance. For pure MCM-22 samples, the Si/Al ratio influenced the adsorption performance. Based on the acid properties of the samples, investigated by adsorption microcalorimetry of ammonia, the adsorption features were interpreted by assuming that positively charged species were originated during the first step; these species underwent successive reaction with weakly adsorbed species formed in the second step, leading to heavy molecular weight organosulphur compounds. Direct evidence for the occurrence of reactive adsorption of thiophene involving its transformation into heavy molecular weight organosul-phur compounds was obtained by GC/MS investigation of the nature of the adsorbed material recovered after the adsorption experiments. The peculiar structure of MCM-22 zeolites made possible the formation of long-sized organosulphur compounds. Due to the mechanism by which thiophene is transformed (i.e. progressive addition of other thiophene molecules), the size of the resulting products was found to depend also on the concentration of the weakly adsorbed thiophene molecules able to interact with those already activated through protonation.

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    ABSTRACT: Adsorption from the liquid phase on nanoporous solids such as zeolites is associated with a high filling degree of the materials’ nanopores. Given the limited space in those pores, the organization and packing of the molecules in the pores becomes an important or even dominant factor. Therefore, specific selectivity effects occur in such conditions, often completely different from the selectivity patterns observed in gas phase conditions. In this chapter, selected examples are given for packing-induced selectivity effects in hydrocarbon adsorption in zeolites.
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