Isomerisation of cresols over zeolites HZSM5, HBeta and HMordenite
In the natural (phenolic material obtained from coal) phenolics industry two problems are experienced with regards to cresol isomers. Firstly, insufficient quantities are recovered from natural sources such as coal tar and refinery caustics and secondly the isomer distribution is inflexible and does not always meet market demand. Thus, having technology which provides alternative routes to certain cresol isomers is of utmost importance to natural cresol producers. The isomerisation of one cresol isomer into one or more of the others would enable a producer to swing (to a limited extent) the distribution of isomers to suite the demand of customers. For this study, the isomerisation and disproportionation of the three cresol isomers were investigated over three types of zeolites under liquid as well as vapour phase (only o-cresol isomer) reaction conditions. The best activity for o-cresol isomerisation, in the liquid phase, was initially HMordenite but because the catalyst deactivates so quickly, HZSM5 was thus regarded as the best catalyst. HZSM5 gave the best activity for both m-and p-cresol isomerisation in the liquid phase. The activity over HBeta was between that of HZSM5 and HMordenite for all the cresol isomer conversions. Expected from the shape selectivity of the zeolite, the best isomerisation selectivity (>86.0%) was obtained with HZSM5. HBeta gave both isomerisation and disproportionation but the favored pathway is disproportionation, while HMordenite, due to the deactivation, proved to be suitable for neither isomerisation nor disproportionation. It can thus be concluded that HZSM5 gave the best conversion for 0-, m-and p-cresol in terms of the desired isomerised products and that it is the preferred catalyst for isomerisation in the liquid phase. The average activity for o-cresol conversion in the vapour phase followed the order HZSM5 > HBeta = HMordenite. HZSM5 proved to be the best isomerisation catalyst for o-cresol and high selectivity (>95%) and stability was obtained over five days on stream, while HBeta showed a gradual decline in isomerisation selectivity and stability. A rapid decrease was noticeable over HMordenite in terms of selectivity and stability. The activity, stability and isomerisation selectivity data obtained, is therefore conclusive with regards to HZSM5 being the preferred zeolite for isomerisation of o-cresol. Further work could entail optimisation of the p-selectivity, from o-cresol and m-cresol transformation, and lifetime studies of the preferred zeolite HZSM5. It is thus recommended to investigate CVD (chemical vapour deposition) to deactivate the external surface of the catalyst as well as noble metal impregnation and/or the use of hydrogen as a carrier gas.
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