FTIR sorption kinetics

The sorption kinetics of n-hexane in MFI-type zeolites of different sizes have been measured by means of micro-FTIR spectroscopy. To check for an influence of the Si/Al ratio, nsj/Ai, on the sorption characteristics, a sample of silicalite was also investigated. The measured transport diffiisivities show ndther a dependence on the crystal size nor on the Si/Al ratio. The temperature dependence is shown to follow an Arrhenius-type law. The results of this study compare well with literature data obtained by different techniques. 

The pore diameters of MFI-type zeolites are comparable to the size of many commercially important molecules, such as aromatics or linear or branched hydrocarbons [1]. Thus, the study of the difiusion of reactive molecules in the channel system of zeolite catalysts is of considerable interest for the understanding of the catalyst performance. A variety of methods has been developed and applied to the measurement of diffii-sion coefficients, amongst others gravimetric techniques [2], neutron scattering [3], NMR [4] and Frequency Response [5]. The FTIR technique offers the possibility to study sorption and sorption kinetics under conditions close to those of catalytic experiments. By the use of an IR microscope, single crystals have become accessible to the FTIR technique. 

Fig. 4 a Schematic setup for experimental studies of sorption and sorption kinetics by micro -FTIR spectroscopy (so-called FTIR microscopy), b Flow cell for micro-FTIR investigations of sorption and sorption kinetics in zeolites [30,31]... 

A method is presented based on multiple internal refiection-FTIR to measure the diffusion coefficients of liquid in polymer, e.g. PMMA or PS, films applied to a substrate. The experimental method here described was designed primarily to measure in-situ hquid water mass uptake and diffusion based on sorption kinetics, but it can also be suitable for vapours, or for other liquids. 24 refs. 

Suarez et al. (36) use a combination of FTIR spectroscopy, electrophoretic mobility and pH titration data to deduce the specific nature of anionic surface species sorbed to aluminum and silicon oxide minerals. Phosphate, carbonate, borate, selenate, selenite and molybdate data are reviewed and new data on arsenate and arsenite sorption are presented. In all cases the surface species formed are inner-sphere complexes, both monodentate and bidentate. Two step kinetics is typical with monodentate species forming during the initial, rapid sorption step. Subsequent slow sorption is presumed due to the formation of a bidentate surface complex, or in some cases to diffusion controlled sorption to internal sites on poorly crystalline solids. 

The Fourier Transform Infra-Red spectroscopy (FTIR) technique is a valuable method for investigating the kinetics of sorption of organic molecules in polymers, hi further works (Safa and Abbes, 2002 Safa et al., 2007 Zaki et al. 2009), we have used such method to study the sorption rate of different esters by polypropylene. The ester function is characterized by absorption band in regions where the polypropylene does not absorb. The sorption can be evaluated by the ratio of the carbonyl ester (CO) band area at 1747 cm to the area of a characteristic peak of the PP at 841 cm used as a reference band. The polymer samples are cut-out in thin films of 50 pm thickness by using a microtome. These films are then put on a support adapted for the spectroscopic analysis. Figure 5 shows FTIR spectra of a polypropylene sample after contact with 5000 ppm of amyl acetate solution at 23°C after 5 hours, 4 days and 15 days of immersion. The absorption characteristic band of esters at 1747 cm and its growth is clearly observed. 

An elegant alternative method to measure sorption into polymers is the Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) method. It allows in situ acquisition of the kinetic data and at the same time records the changes that occur in the polymer matrix due to the influence of the diffusant. Effects such as swelling, changes in morphology and polymer solvent interactions can all be simultaneously monitored. To calculate the diffusion coefficients from ATR-FTIR data, the mass uptake equation used in gravimetric diffusion experiments has to be modified to take into account the convolution of the evanescent field with the diffusion profile. 

---