Investigation of the Sorption Behavior of a Zeolite Using a TGA/DSC 1 Coupled to a Humidity Generator - METTLER TOLEDO

Investigation of the Sorption Behavior of a Zeolite Using a TGA/DSC 1 Coupled to a Humidity Generator

The sorption behavior of a zeolite was investigated using a TGA / DSC 1 coupled to a humidity generator. The sample was first dried in the TGA / DSC 1 and then exposed isothermally to different relative humidities. The sorption capacity was measured as a change of sample mass. Furthermore, sorption enthalpies were also determined.

Figure 1. The schematic structure of a zeolite with pores in which foreign molecules can be stored [1].

 

Introduction

The Swedish mineralogist Axel Fredrick von Cronstedt noted in 1756 that certain stones seemed to boil on heating and at the same time released considerable amounts of water vapor.

He called these « boiling stones » zeolites (a word whose roots are derived from the Greek words « zeein » for boiling and « lithos » for stone) [1, 2].

The phenomenon of the boiling stones can be understood through the special structure of the zeolites. Zeolites are crystalline materials whose crystal lattices contain extensive pores and channels in which specific foreign molecules (e.g. water, ethanol, and ammonia) can be reversibly stored.

As a result of the large internal surface of the pores and channels (up to 1000 m2 / g), zeolites are able to adsorb up to 30 % of their empty weight of foreign molecules depending on the density of the stored species.

Adsorbed molecules can be removed from a zeolite by heating without affecting its structure. Zeolites can be used for any number of adsorption/desorption cycles. Naturally occurring zeolites consist mainly of SiO2- and AlO4- tetrahedrons that are connected to one another via oxygen atoms, so that pores and channels are formed internally (see Figure 1) [3, 4].

The pores often contain free mobile cations (e.g. Na+), that can be exchanged for other ions (e.g. Ca2+) or for free water. Most zeolites used nowadays are made synthetically.

A wide range of zeolites is therefore available with very different chemical compositions and pore sizes optimized for specific applications.

Zeolites have numerous fields of application. Natrolite is from the technical point of view of great importance and is used for hydraulic cement, as a filler material in the manufacture of paper, as an adsorption agent, for water softening and as an ion exchange material [3].

Synthetic zeolites, such as molecular sieves (Zeolite A, Na12[(AlO2)12(SiO2)12] 27H2O), which is available in different pore sizes, is used as drying agent in laboratories or as a catalyst [2].

Furthermore, zeolites are also employed in washing powder (as water softeners) or in cat litter in order to absorb both liquids and unpleasant odors...



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Conclusions

The sorption behavior of a zeolite can be investigated under isothermal conditions at different relative humidities by means of a TGA coupled to a humidity generator.

From the experimental point of view, it is important to remember that zeolites are strongly hygroscopic. Care must therefore be taken not to expose the sample to the surrounding air after the zeolite has been dried at 0 % relative humidity.

This is easily achieved with the instrument setup presented here. First, the sorption isotherm is constructed using the data from the TGA curve. The isotherm describes the water content of the sample as a function of the applied relatherm describes the water content of the sample as a function of the applied relative humidity.

This data is then used to determine the water content at which the entire internal surface of the zeolite is covered with a monolayer of water (for the zeolite investigated here we found a value of 13.5 %). Furthermore, sorption enthalpies can be determined from the DSC signal, which is also simultaneously measured.



Investigation of the Sorption Behavior of a Zeolite using a TGA/DSC 1 Coupled to a Humidity Generator | Thermal Analysis Application No. UC385 | Application published in METTLER TOLEDO Thermal Analysis UserCom 38