Application of photoacoustic spectroscopy to heterogeneous catalysts

Abstract

Photoacoustic signal generation depends on thermal as well as optical properties of the sample. This enables the photoacoustic spectroscopy techniques to apply well in case of powdered samples which are optically opaque and have an irregular surface such as coked catalysts. Even in case of photoacoustic signal saturation can be avoiding by diluting the sample with inert diluent. Photoacoustic signal of Cr-impregnated HZSM-5 zeolite diluted with magnesium oxide showed a linear relationship between the signal intensity and the Cr content. Photoacoustic determination of coke content on HZSM-5 zeolite catalyst was carried out using activated carbon and heavily coked catalyst as standards. The results were in agreement with that of gravimetric analyses. Photoacoustic spectroscopy was used to investigate the deposition and distribution of coke materials on zeolites during methanol conversion reaction using a photoacoustic cell allowing gas flow. When the reaction was carried out at 300℃ there were two distinct absorption bands having maxima at 327 and 402 nm which were caused by polyalkylaromatics and condensed aromatic species, representatives of high temperature coke. Below 200℃, only one absorption peak was observed around 360 nm arising from olefinic species, a characteristic species for low temperature coke. Transformation of low temperature coke to high temperature coke on ZSM-5 zeolite was observed by raising the temperature stepwise, after loading methanol at room temperature. It seemed that low temperature coke formed in internal pores of HZSM-5 zeolite migrated to the external surface transforming into more condensed coke materials. The fact that HY zeolite coked at 300℃ showed larger phase lag around 400 nm in phase spectrum suggests that the coke materials exist in deeper sites. These were considered in terms of difference in distribution of coke materials resulting from the different pore structures of HZSM-5 and HY zeolites. In case of ZSM-5 zeolite,...