M.Sc Thesis
M.Sc Student Segal Dovrat Preferential Photodegradation of Contaminants by Molecular Imprinting on a Photocatalytic Substrate Department of Chemical Engineering Professor Yaron Paz

Abstract

The photocatalytic degradation of organic contaminants on Titanium dioxide is not selective. This might be a disadvantage when a mixture containing low concentrations of highly toxic substances together with high concentrations of low-toxic organisc has to be treated.

Molecular imprinting is a means to prepare specific binding sites (molecular recognition sites-MRS) by imprinting the structure of given molecules in appropriate matrices. The MRS  are, in fact, cavities which are stable and can not be destroyed by UV irradiation. We examined the degradation of two contaminants: Diisopropyl methyl phosphonate (DIMP) which is a simulant of the nerve agent sarin and Diethyl hydroxymethyl phosphonate (DEHMP).

In order to study the possible effect of the Molecular Imprinting method, comparative experiments of the photodegradation rates were studied by Fourier Transform Infrared (FTIR) spectroscopy. Characterization of the powder was done by XRD, BET, PNMR, XPS, SEM, and FTIR.

Faster degradation rates were obtained due to the effect of the molecular imprinting. This improvement was by a factor higher than 3. A higher degradation improvement was observed when degrading DIMP over DEHMP-templated TiO2. This result is very important since it allows the use of rather non-toxic material in the production of imprinted sites, yet capable of performing high degradation improvement of the toxic contaminant itself.

Degradation of benzene over DEHMP-templated TiO2 or DIMP-templated TiO2 was not enhanced in comparison to the non-imprinted TiO2. These results indicate that there was no preferentiability towards molecules which were different in structure. In the case of degradation of heptane, some degradation improvement was noticed, which was still lower relative to the phosphonates. The small improvement can be attributed to the relatively small size of the heptane molecules, which facilitates the adsorbance of these molecules on the templates, thus achieving higher degradation rates.

Two methods to remove the templated phosphonates were tested: calcination and mild ammonia treatment. It was shown that the template removal method plays an important role in the degradation mechanism. Although the ammonia treatment seemed as lowering the imprinting effect, photodegradation experiments revealed that no intermediates were desorbed into the gas phase. This observation is of a great importance in particular in systems where the intermediate products are more toxic than the starting molecule itself. In addition, the ammonia treatment produces improved substrates that can deal with higher contaminants concentrations, probably due to a higher concentration of active sites.
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