New, more powerful zeolite catalysts developed
The world would come to a standstill without zeolites. They play a crucial role in the production of petrol, are used in washing powder to remove calcium from water, dry dishes in dishwashers and are used to produce pure oxygen. Researchers at FAU have recently managed to produce a new zeolite with a layered structure which could be used to make chemical processes faster, less expensive and more environmentally friendly.
Zeolites are silicates with a solid but porous framework structure. These minerals occur naturally in various forms almost all over the earth's crust. Since the beginning of the 1950s, they have also been widely produced artificially. Chemists use the special properties of zeolites in different ways. The fine pores of zeolite crystals have a diameter of less than a nanometre and are used in many chemical processes as a filter - a kind of sieve for molecules which allows small molecules to pass through and keeps big ones out. They are also great catalysts, as molecules can accumulate on their surfaces which then triggers highly specific reactions.
No petrol and no washing powder without zeolites
'Zeolites are the workhorses of modern chemistry,' says Prof. Dr. Wilhelm Schwieger from FAU's Chair of Chemical Reaction Engineering. 'Each drop of petrol that we use has flowed through a zeolite crystal. And around 30 percent of standard washing powder is made up of zeolites.' There are currently 218 different known zeolite structures. This great variety and the wide range of possible applications make these minerals a particularly interesting subject for research. 'We are trying to produce materials which have highly specific properties in order to trigger, optimise and better control chemical reactions,' explains Schwieger.
Layered structures direct molecules more effectively
Together with Prof. Dr. Martin Hartmann from Erlangen Catalysis Resource Center (ECRC), Schwieger has been working on a new type of catalyst as part of the research cluster 'Engineering of Advanced Material' (EAM). The special feature of this catalyst is that it is made of layers of zeolite crystals which lie on top of one another. This creates a system with pores which vary greatly in size. 'You can think of the large pores that are several nanometres in diameter as being the motorways for molecules. They allow the molecules to get to a certain point quickly. The small pores that are less than a nanometre in diameter are like countless footpaths which only certain molecules can go along,' says Schwieger.
The advantage of this hierarchical structure is that exactly the right molecules can get to the active centres - the built in substances which act as a catalyst - much quicker. 'From a molecule's perspective, a zeolite crystal is huge,' explains Martin Hartmann. 'In comparison, the surface area of the walls of the pores in five grams of zeolite is equivalent to the size of a football pitch. It takes a long time for the molecules to get through it and during this time there may be undesired reactions. As we can now set the pore sizes exactly and make them progressively smaller using a suitable combination of various zeolites, the speed at which the molecules travel and the path which they take can be controlled exactly by the crystal structure.'
Fewer by-products, less resources used
This hierarchical filter function means that only the desired molecules are transported to the catalytic centres. 'This selection enables us to produce exactly the products that we want - without the by-products which usually result from chemical reactions,' says Schwieger. Other considerable advantages of these specially designed catalysts are that they have a longer service life, age slower and form less deposits. 'If one day these new catalysts were used in industrial processes, for example, they could help us to use less natural resources more. And because they work more efficiently, costs would probably also decrease,' says Hartmann.
Source: Universität Erlangen-Nürnberg
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