The rocky road to a greener future

Chlorinated hydrocarbons are important environmental contaminants. Whilst chlorine itself is vital in safeguarding drinking water, by-products such as halomethanes pose a threat both to human health and the environment. Compounds such as these are formed through the reaction of chlorine with naturally occurring substances such as decomposing plant material. They are also the by-product of various industrial processes. Heightened concerns over the long term impact of these compounds have fuelled research into ways of removing them from water sources. And it appears that a solution may have been found –removing the pollutants from water by passing it over some rocks! Not just any rocks though – a special type of mineral known as a zeolite which is playing an increasing role in tackling environmental problems.

Crystal of mordenite (left) and atomic structure (right)

Zeolites are a class of naturally occurring minerals found in volcanic ash. Although physically zeolites look solid, at the atomic level they are full of holes! Zeolites are built up of tetrahedral units of silica (SiO₄) and alumina (AlO₄) which arrange to form a network of interconnected pores and channels of molecular dimensions. The framework carries an excess negative charge and this is the key to the catalytic and adsorption properties that arise. In order to preserve charge neutrality within the zeolite lattice charge compensating cations are loosely held in the pores. If the cation happens to be a proton then a special type of active site, known as a brønsted acid site is formed. This site is able to interact with molecules and initiate a catalytic breakdown through an initial proton transfer from the acid site to the sorbate. The unique porous structure of a zeolite coupled with the huge surface area gives rise to a vast number of applications.



Zeolites were first discovered by Swedish mineralogist Axel F Cronsedt in 1756. The name ‘Zeolites’ comes from the Greek ‘Zeo’ (to boil) and ‘lithos’ (stone) because the stones appeared to boil when heated.

	A brønsted acid site is formed when the framework charge is compensated for by a proton.

The geometry of the zeolite channels and pores control the way in which molecules react, allowing access to molecules of a specific size and shape and encouraging molecules of a particular shape to form. This size and shape selectivity can effectively be used to tailor a reaction to yield selected products and it is for this reason that zeolites are often referred to as molecular sieves. These clever rocks act not only as catalysts providing sites for molecules to react but also as adsorbents – molecular sponges that can hold onto molecules of a specific size within their pores. It is this dual-function nature, combined with the ability to utilise the shape selective properties to tailor chemical reactions that is drawing interest into possible applications of these minerals in environmental cleanup.

The primary building units of the zeolite lattice are the silica and alumina tetrahedra. These tetrahedral units assemble into secondary building units (SBUs) comprising of simple polyhedra which come together to form either an array of interconnecting channels or a system of cage-like voids

How zeolites are built

Environmental applications of zeolites

Application	Details
Detergents	Zeolites have replaced environmentally harsh phosphates. They act by exchanging sodium atoms in the zeolite void for calcium and magnesium ions that lead to hard water and therefore poor laundry performance.
Removal of nitrogen oxides	Oxides of nitrogen react to form photochemical oxidants such as ozone. They also form acid rain. Zeolites are used as catalysts to reduce the nitrogen oxides.
Removal of volatile organic compounds from car exhausts	Zeolites are used to trap the organic compounds.


“Rarely in our technological society does the discovery of a new class of inorganic material result in such a wide scientific interest and kaleidoscope development of applications as has happened with the zeolite molecular sieves.” D.W Breck Zeolite molecular sieves: Structure, chemistry and use, New York, p1, 1974

Our research is focussed on investigating the adsorption and catalytic properties of three chlorinated hydrocarbons; dichloromethane, 1,2-dichloroethane and trichloroethylene in three acidic zeolite frameworks – Mordenite, Faujasite and ZSM-5. Each framework structure can be made with a high silica content which makes the zeolites hydrophobic. This is a particularly useful property for removing pollutants from water as the zeolites will adsorb organics in preference to water. Since proton transfer from the brønsted acid site to the sorbate is a key initiation step for the reaction it is crucial to understand features of sorbate adsorption such as preferential adsorption sites and how these sites are approached. Zeolites are particularly advantageous in this application since they offer the possibility of tuning selectivity to give the favoured products; HCl, CO₂ and H₂O. Zeolites also offer several advantages over other catalysts, including their thermal stability, total regenerability and controlled reactions thereby minimising the formation of unwanted side products.

Using state of the art computational techniques we are probing the mechanisms of adsorption at the microscopic level. Molecular modelling is becoming an increasingly important tool in obtaining detailed information about each stage of a reaction such as adsorption sites, reaction intermediates and transition states. Since zeolites contain many hundreds of atoms the theoretical calculations would take a very long time if performed on all the atoms. To speed things up a cluster of the zeolite is extracted around the active site. The exact number of silcon and aluminium atoms in a unit cell, known as the Si/Al ratio is an important quantity since specific reactions require specific ratios. To increase the accuracy of our model we have distributed aluminium atoms around the main cluster ring according to experimental Si/Al ratios. The interaction between the sorbate molecule and the most thermodynamic acid site pointing into the ring was then modelled using quantum mechanical techniques.

An obvious limitation in this method is the neglect of the long-range electrostatic effects of the zeolite crystal. Initial results obtained indicate that there is no general trend in the adsorption energies for the systems under study. This indicates that the interaction between the sorbate and the acid site is not the only contributing factor to the adsorption energy and that long-range effects are important. To try and improve the model the same calculations are being repeated using an embedded approach which accounts for the long – range effects of the zeolite crystal. The next stage of calculations will attempt to model the reaction pathway to gain further understanding of how these chlorinated compounds are broken down.

Misbah Sarwar - Davy Faraday Research Laboratory, Royal Institution of Great Britain