The Studies Of The Adsorbed Layers And Heterogeneous Properties Of The Mcm-41 Molecular Sieves Using Thermogravimetry And Nitrogen Adsorption Methods
P. Staszczuk, T. Danielkiewicz
Department of Physicochemistry of Solid Surface, Faculty of Chemistry,
Maria Curie-Sklodowska University,
Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
In the past decade great interest in properties of a new generation of the molecular sieve surfaces was observed. It results from cognitive aspects and their application as adsorbents and catalysts. In the early ninties a new method of production of micro- and macroporous catalytic materials (e.g. MCM-41 molecular sieves) with a definite pore shape were introduced. It is so-called the Zebedde method (zeolities evolutionary de novo design) using template, i.e. organic molecule compounds carrying out the functions of patterns. A porous structure of synthetic zeolite from inorganic gel forms around the organic molecule. Template uses largely alkalies and other organic compounds, e.g. 2-metylcyclohexamine and/or alkyltrimethyl-ammonium bromides. When the synthesis is over, the above compounds are removed by burning from the sample. It is a result of the regular net of empty places in the zeolite grains. So far, the techniques of porous structure zeolites production have been of empirical character using the method of tests and errors. The Zebedde method makes it possible to project such porous materials which are indispensable in a given catalytic reaction. The MCM-41 molecular sieves are easy to synthesize and can be modified by different heteroatoms such as galium. The Zebedde method has already been used to obtain catalysts of practical application in the process of ethylene and propylene production from methanol. It is very essential in this process, that the size of the catalyst pores cannot be very large, because such compounds as benzene and naphthalene could be formed. The MCM-41 molecular sieves possess a compact chemical construction and exact pore size and structure. Moreover, they have unique
adsorption, catalytic and ion exchange properties which decide about various possibilities of their application in many spheres of life.
Application of adsorbents and/or catalysts require knowledge of many surface physicochemical parameters, mainly their adsorption properties and porosity as well as selectivity, catalytic activity and properties of the surface active sites. For estimation of their nature, quality, localization and energy of active centers (Levis and/or Brönsted acid type) adsorption and microcalorimetric methods are most frequently used. Lately, a special thermal analysis technique has been succesfully adopted to study the liquid/solid systems, especially for characterization of the heterogeneous properties of materials. The effects taking place during thermodesorption of liquids from solid samples were registered and used in the practical calculation of the physicochemical parameters. During the thermodesorption process of adsorbed liquid films from solid surfaces the physical bonds (first of all hydrogen bonds, which are 10 times weaker than the chemical ones) are disrupted. The experimental results obtained so far showed that the simple thermogravimetry method of programmed thermodesorption of liquids from material surfaces in quasi-isothermal condition could be applied to study the adsorbed liquid films on the MCM-41 surfaces and its heterogeneous properties.
The studies were made using Na+[Si/Al] MCM-41-10 and Na+[Si/Al] MCM-41-20 molecular sieves. In the studies of liquids thermodesorption process the samples were prepared as follows. To obtain a desirable thickness of the liquid adsorption films on the surfaces, the samples were wetted with liquids to complete saturation and/or vacuum dessicator where the relative liquid vapor pressure p/po was 1. Measurements of programmed thermodesorption of the polar and apolar liquids from the molecular sieve samples under the quasi-isothermal conditions were made using the simultaneous derivatograph Q-1500D (MOM, Budapest, Hungary). The Derivatograph Q-1500D was connected with computer, via the interface, equipped with a suitable Derivat program to register the sample mass loss Q-TG curves as a function of temperature and time and to determine Q-DTG curves with respect to temperature and time as well as to integrate corresponding areas on the differential thermal analysis DTA curves during the classical measurements. The sample mass loss Q-TG and the differential Q-DTG curves in relation to temperature and time were registered and printed using a special platinum crucible and the computer system.
The porosity of molecular sieves MCM-41 were estimated by means of nitrogen adsorption at 77.35 K, with BET and BJH equations employed using sorptomat apparatus type ASAP 2010 V.2.00A (Micrometrics, Inc., USA).
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