Ortho-selectivity in aluminophosphate molecular sieves: A molecular simulation study
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Ortho-selectivity in aluminophosphate molecular sieves: A molecular simulation study S. Mardˆonio P. Lucena · C´elio L. Cavalcante Jr · Jo˜ao A. F. R. Pereira
C Springer Science + Business Media, LLC 2006
Abstract Monte Carlo adsorption simulations of xylenes have been performed in aluminophosphate molecular sieve structures. A new force field fitted for o-xylene in AlPO4 -5 was used. It is shown that force fields have good transferability among the aluminophosphate sieves series and the new force field adequately describes the experimentally observed adsorption isotherms for xylene/AlPO4 -5. A previous investigation of adsorption isotherms and structural analysis has been extended to AlPO4 -8 and VPI-5 sieves. In AlPO4 -8, like in AlPO4 -5, the variations in the channels diameters and the corresponding interaction energy of the molecule-crystal lattice drive all molecular positioning. In VPI-5, the modulation between wide and narrow regions becomes negligible due to the larger pore diameter, so no ortho-selectivity was observed. The simulations confirm the ortho-selectivity mechanism proposed to aluminophosphates. Keywords Aluminophosphates . Adsorption . Xylenes . Molecular simulation S. M. P. Lucena . C. L. Cavalcante Jr () Universidade Federal do Cear´a, Dept. Eng. Qu´ımica, Grupo de Pesquisas em Separa¸co˜ es por Adsor¸ca˜ o, Campus do Pici, Bl. 709, 60455-760, Fortaleza, CE, Brazil e-mail: [email protected] J. A. F. R. Pereira Universidade Estadual de Campinas, Faculdade de Engenharia Qu´ımica, Campinas, SP Brazil
1 Introduction The present study is related to the xylenes adsorption simulation in aluminophosphates. The aluminophosphates represent a series of molecular sieves first synthesized by Wilson et al. (1982) in the decade of 80. Most of the representatives of this series present unidimensional channels and a neutral framework. Some macroscopic measurements like Infra-Red (Barthomeuf and Mallmann, 1990), thermodynamic properties (Eder and Lercher, 1996; J¨anchen et al., 1996) and diffusion coefficient measurements (Chiang et al., 1991; Cavalcante et al., 2000) have been carried out on aromatic molecules adsorbed in aluminophosphate, in order to understand their sorption and diffusion properties. Some of these studies (Barthomeuf and Mallmann, 1990; Chiang et al., 1991) were performed to investigate the ortho-selectivity behavior in aluminophosphates. Unlike silicalite that has p-xylene selectivity and NaY that has m-xylene selectivity, AlPO4 -5 has shown a surprising o-xylene selectivity. There have been reports on molecular simulation of the adsorption of xylenes in silicalite (Snurr et al., 1993a,b) and in NaY (Lachet et al., 1998, 1999). Recently, we have reported preliminary studies on the adsorption of xylenes in aluminophosphates (Lucena et al., 2006). Using a simplified force field, it was observed that the channels modulation of the aluminophosphates and the corresponding interaction of the xylenes with the lattice drive all molecular
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positioning which results in the ortho-
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