We have undertaken a laser ablation Fourier transform mass spectrometric study of intra MFI-zeolites containing occluded tetra-propyl-ammonium (TPA) and ethylene polymers. The TPA molecules act as structure directing agents in the synthesis of MFI-zeolites. In such a synthesis, most of the template cations remain inside zeolite channel intersections and counterbalance nearby lattice negative charges. Since the aperture size of the MFI-zeolite channel is smaller than the kinetic diameter of the TPA, these template ions become encapsulated inside the channel intersections like so-called "ship-in-a-bottle" molecules. The analysis of such molecules is usually carried out by extraction treatment after dissolving the zeolite wall structure in strong acid such as hydrofluoric acid. This aqueous treatment does not guarantee the integrity of the analyte molecular species. Furthermore, such thermally unstable and involatile species cannot be analysed by GC/MS.
A goal of our research has been to use laser ablation FT-mass spectrometry to study such zeolite occluded molecules. In our experiments the zeolite lattice is photoablated by an IR laser pulse. If desorption of the occluded species occurs rapidly and before sufficient laser energy accumulates in molecular bonds leading to photodecomposition, the intra-zeolite molecules can be desorbed as adduct ions. In this context, the intra-MFI-zeolite TPA ions represent a model compound for laser based desorption studies of other zeolite occluded molecules.
In the present study, intact occluded TPA ions have been observed only after laser photo-decomposition of the MFI-zeolite structure. Furthermore, we have obtained exciting preliminary results which reveal that we are able to desorb intact polyethylene polymer adduct ions [CnH2n+1]+ which have been synthesised as occluded species in an MFI-zeolite. These experiments represent the first direct analysis of species synthesised in constrained environments and have significance for zeolite catalysis in the petroleum industry.