Adsorption of Branched Alkanes in Silicalite-1:  A Temperature-Programmed-Equilibration Study

Abstract

The adsorption properties of several branched paraffins from C4 to C6 on MFI type zeolite were investigated. A temperature-programmed-equilibration method was used for this study, and the analysis of the desorption rates as a function of temperature showed two desorption peaks. The interpretation of this phenomenon is discussed, and it is revealed that the high-temperature desorption occurs in conditions close to equilibrium while low-temperature desorption is subject to diffusional limitations. Furthermore, an XRD study shows that the phase transition in the crystal structure can be lowered by 70 K when 3-methylpentane is adsorbed. This property shows that the low-temperature desorption peak is not due to a change of symmetry in the structure of the zeolite. Modeling of the high-temperature peak provided adsorption isosteric enthalpy and entropy variations. These values are consistent with literature data. For the first time, adsorption isosteric entropy variations of sorption are calculated for branched alkanes in silicalite-1.

Adsorption and Coadsorption of 2-methylpentane and 2,2-dimethylbutane in a ZSM-5 Zeolite

“…Indeed, the Table 1. The adsorption enthalpies and entropies are of the same order of magnitude as those given by other authors (Cavalcante and Ruthven, 1995;Jolimaître et al, 2001;Millot et al, 1999) and are lower with 22DMB than with 2MP. monoclinic-orthorhombic phase transition might not to be completely undergone at 348 K and a major part of the zeolite is still in the monoclinic structure.…”

“…Experimental works (Cavalcante and Ruthven, 1995;Denayer et al, 1998;Millot et al, 1999;Zhu et al, 2001;Jolimaître et al, 2001) as well as molecular simulations (Shenk et al, 2001;Krishna et al, 2002) have been used to study the adsorption of alkanes on ZSM-5 zeolites at the equilibrium. As new legislative standards on pollutants emission impose to remove toxic aromatic compounds in gasoline, it is necessary to increase the content of highly branched alkanes in the detriment of linear and mono-branched ones in order to keep a high octane number.…”

Self-diffusion of n-alkanes in silicalite using molecular dynamics simulation: A comparison between rigid and flexible frameworks

“…5) lowers the transition temperature and that the orthorhombic phase is stable at room temperature from n-heptane to n-dodecane. More recently, Me ´thivier and coworkers 22 showed that the orthorhombic structure is stable above 303 K whatever the temperature and the loading of 3methylpentane and Millot obtained the same results for n-hexane and 2,2-dimethylbutane. 23 On the other hand Mentzen 24 found that all silicalite/alkane systems ranging from n-hexane and its isomers to n-octodecane exhibit monoclinic P2 1 /n.1.1 symmetry at room temperature.…”

Kinetic Monte Carlo Modelling to Study Diffusion in Zeolite. Understanding the Impact of Dual Site Isotherm on the Loading Dependence of n-Hexane and n-Heptane Diffusivities in MFI Zeolite, as Revealed by QENS Experiments

“…These isotherms were obtained from Grand Canonical Monte Carlo (GCMC) calculations made by the group of Krishna and Van Baten (Jobic et al, 2006) and by Pascual et al (2003) and, for nC6, also come from Tapered Element Oscillating Microbalance (TOEM) experiments (Zhu et al, 2001). These isotherms are then represented by a Dual Site Langmuir (DSL) model as demonstrated by Millot et al (1998Millot et al ( , 1999, whose mathematical expression is given by Equation (14): (14) where indices A and B correspond to the two types of adsorption site of the MFI structure, of different capacity and adsorption force. b A and b B are the model parameters, ratios of the adsorption and desorption rates on the site considered.…”