Creation of Zeolites with Improved Catalytic Properties

According to the studies of a group of scientists in the Netherlands, Belgium, and France, by using the logics of standard chemical reactions it is possible to modify the pore structure of zeolites, which leads to the production of crystals with pores in three size ranges. The study demonstrates a simple method of setting the porosity in the crystals and could allow for the creation of new types of industrial catalysts.

Zeolites are a class of crystal aluminosilicates which are very commonly used as catalysts in oil processing and petrochemical synthesis. In zeolites the desired chemical transformations take place due to the presence of catalytic sectors localised within the micropores, and researchers have for a long time been conducting experiments on methods of adapting the pores to increase catalyst productivity and expand their use.

The main problem is that the short internal channels within the zeolites have a diameter of no more than 1 nm, which hinders the movement of the free flow of reagents from the catalytic centres into the crystal.

In order to produce greater molecular permeability, scientists have invented ways of preparing zeolites, which also have mesopores and crystalline paths, for molecules that measure many nanometres in diameter. The network consisting of large pores open to the surface of the crystal and connected with the catalytic micropores inside the crystal is similar to strategically situated highways connected to the central streets of a city. This design allows for the avoidance of narrow bottlenecks for molecules and in the same way increases the activity of the catalyst.

This type of pore structure, which is based on template synthesis and subsequent synthetic modification methods, has achieved success only in a limited number of cases. Several types of zeolites with useful combinations of two sizes of pores have been created. There are still no common methods for the creation of zeolites that have pores in three or more ranges, despite technical calculations which predict that such ‘hierarchical porosity’ could lead to advantages in catalysis.

In a study published in Angewandte Chemie, a group of scientists, including Krijn de Jong and Jovana Zečević from the University of Utrecht in the Netherlands, developed a method to give zeolite Y ‘tri-modal porosity’. This zeolite is widely used in petroleum chemistry cracking, the process which converts high molecular hydrocarbons into low molecular hydrocarbons.

With the help of electron microscopy, tomography and other types of analysis, the group that also included Sander van Donk of the Petrochemical Research Centre in Feluy (Belgium), Francois Fajula from the Charles Gerhardt Institute in Montpellier, France, and their colleagues, showed that the combination of acid leaching, which dealuminises zeolites, and the evaporation of micropores of 1 nm in size and larger channels allows for the production of channels 30 nm in diameter.

The subsequent treatment of such examples with alkaline solution, which leaches silica, slightly increases the size of large pores and adds a network of channels of average size in the 5 nm range, which is connected to other types of pores. The group of scientists also notes that in a series of catalytic cracking tests, the ‘tri-modal zeolites’ are less effective in terms of catalytic activity and selectivity of the obtained products than the control samples.

Due to the important connection between the chemical reaction and diffusion in the zeolite crystals, this work undoubtedly deserves attention, according to Frick Kaptein of the Technological University of Delft. The indubitable advantages of this method are the relatively simple ways of changing the structural and catalytic properties of zeolites. However, the method proposed is not a panacea and is not suitable for all types of catalysts.