Structure of complex zeolite explains its role in naphtha-to-diesel conversion

Scientists at the Polytechnic University of Valencia in Spain and Stockholm University in Sweden have found that a new zeolite they have synthesised has the most complex structure reported to date. Using electron crystallography, they determined that the material has a unique structure containing discrete pores of different dimensions that are connected together.
 
This makes the zeolite, dubbed ITQ-39, a promising catalyst for converting naphtha into diesel fuel, a process for which there is no known commercial catalyst. The material could also increase the efficiency of other processes such as the alkylation of aromatics or xylene isomerization, says Avelino Corma of the Polytechnic University of Valencia. ''There are no examples of three-dimensional channel zeolites of pore dimensions like ITQ-39.''
 
Zeolites are aluminosilicates with a very regular structure of molecule-sized channels and cavities that act as sieves and aid the reactivity of molecules by trapping them in small spaces.

They are generally composed of ring-shaped building units, where the ring vertices are Al or Si, and O forms the links. Zeolites are important industrial catalysts since they can impose a strong selectivity towards desired chemical reaction products, increasing process efficiency.
 
Elucidating the structure of zeolites is crucial to understanding their properties and predicting their applications, says Stockholm University's Xiaodong Zou, but it is challenging because the complex materials require single-crystal diffraction for analysis. Unfortunately, single-crystal zeolites are rarely obtained.
 
''X-ray crystallography has been the main tool for structure determination of crystals that are large enough, typically a few micrometers or larger on a synchrotron source,'' Zou says. ''Many zeolites crystallise in nano-sized crystals so it is difficult to solve the structures of such crystals using X-ray crystallography.''
 
Zou and her colleagues developed several electron microscopy methods to analyze the small crystal domains and solve the structure of ITQ-39, as they outline in a Nature Chemistry paper published online on January 29. This included 3D rotation electron diffraction tomography, crystallographic image processing, and through-focus structure projection reconstruction. They combined this with high-resolution TEM images taken from different crystal orientations. The researchers found that ITQ-39 has 12-membered ring channels that occur in pairs and that intersect in three directions with 10-ring channels that have slightly smaller pore openings. The effective diameters are 6.5-7.9 Å for the 12-ring pores and 4.5–6.0 Å for the 10-ring windows.
 
The researchers reported the material's two-step synthesis last year. They first make a structure-directing agent through the reductive amination of piperidone and amine. Then they add silica and alumina sources, hydrofluoric acid and water, heat the mixture in an autoclave, and filter out the resulting solid zeolite.
 
The researchers have found that ITQ-39 gives higher conversion rates and yields and lasts longer than known catalysts for converting naphtha into diesel. The material's structure explains why. The alkylation of aromatics from heavy naphtha and olefins from light naphtha can give diesel, but it is limited because the formation of coke during the process quickly deactivates known catalysts. In ITQ-39, however, the larger 12-ring channels help to form and diffuse the long-chain hydrocarbons in diesel, while the 10-ring channels diffuse naphtha, preventing over-reactions that lead to coke.