Using 1-octene as a representative petrochemical feedstock, a group of researchers has been able to investigate what happens to hydrocarbon molecules when they are adsorbed onto the industrial zeolite catalyst ZSM-5, which is used to combine them to form longer molecules (oligomers). They found that the shape of the pore inside the zeolite catalyst meant that only linear oligomers were formed, not branched ones. They were able to see the chain grow until the size of the molecule made it unable to move along the pore channel, although they did see that the molecules still had room to rotate inside the pore.
To understand the chemistry happening on the surface of the ZSM-5, they kept the sample at a relatively low temperature, with the sample experiencing no prior steam treatment, as it would do in industry. Their aim for future studies is to examine steamed ZSM-5 samples that more closely replicate what happens in industry.
Zeolites as catalysts
Zeolites are minerals whose structure contains pores that provide a space for molecules to be adsorbed. This had led to huge scientific interest, and a wide range of industrial applications including gas storage and nuclear waste disposal. Because their pores give the material a large surface area, another major use is for catalysis. The different pore sizes of different zeolites can also mean that they can be designed with size selectivity towards certain molecules.
The zeolite ZSM-5 is widely used in the petrochemical industry for increasing the octane number of fuel during commercialised fluidized catalytic cracking (FCC). However, it is increasingly being used in FCC for its preference for producing a higher ratio of shorter chain hydrocarbons. These shorter chain molecules are useful chemical starting blocks for many industries, and increasing the efficiency of their production would be an extremely valuable development.
The selectivity of ZSM-5 towards producing short chain hydrocarbons is thought to be due to the confined volume within the zeolite pore structure. Because of the significant influence of these structural factors, understanding how the hydrocarbon molecules interact with, and diffuse within, the zeolite structure are important factors in understanding the zeolite's catalytic activity.
Using neutrons for investigation
Neutrons interact very differently to hydrogen (1H) and deuterium (2H). 1H has a large incoherent cross section, whereas the scattering from 2H is mainly coherent scattering. When studying the interactions of hydrocarbon reactants, products and intermediates with an inorganic metal/metal oxide catalyst, the scientists can assume that any incoherent neutron scattering is because of the hydrogen, representing just the organic species and their protons. This allows selective spectroscopic measurements with neutrons in cases where other techniques, such as NMR or optical spectroscopy are incompatible with the inorganic catalyst.
ZSM-5 and 1-octene
Inelastic neutron scattering (INS) occurs when the neutron exchanges energy with the atom. INS using MAPS and TOSCA provided the group with information on the vibrational motions in the zeolite catalyst. For the first time, for this experiment, the group used INS to provide insight into the interactions of this type of hydrocarbon with a cracking catalyst.
Quasielastic neutron scattering (QENS) measures the change in energy and momentum of neutrons scattered from moving atoms, providing a measure of both the size and direction of the motion in the sample. This study used included QENS measurements on OSIRIS to provide an insight into what would occur in a perfect bulk zeolite, enabling the scientists to neglect effects due to particle size and bulk materials transfer, and providing a good test of computational theory.
The INS data, together with the complete absence of translational mobility visible on QENS time scales, showed the scientists that the formation of oligomers goes to completion on addition to the activated zeolite, even at 293 K. Because of the shape of the pore structure, the reaction goes via an end-to-end mechanism, producing linear products.
New molecules are added to the chain until it becomes large enough that the oligomer does not have any long-range movement. Even at this length, the group were still able to see the rotations of the molecule inside the pores.
This study provides an awareness of how a reagent such as 1-octene chemisorbs and interacts with an activated ZSM-5 catalyst. By using INS in this way, this work pioneers the technique as a method for providing information on the hydrocarbon–zeolite interaction in this kind of catalyst, giving valuable insight both in explaining the results of QENS-derived dynamical measurements and in investigating the role played by structural changes in the zeolite in affecting the cracking reaction.
The full paper can be found at DOI: 10.1021/acs.jpcc.8b08420
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