UV-Raman studies of coke formation during hydrocarbon conversion reactions over zeolite H-MFI

The mechanism of coke formation during hydrocarbon conversions over zeolite is of great interest but has never been fully resolved. UV-Raman spectroscopy has been shown to be a powerful tool in studying this reaction. By incorporating an infrared laser as a fast heating source, time-resolved measurements can be made to more closely look at each stage of the reaction.;In order to understand what is happening in methanol-to-hydrocarbon conversions, ethylene, propylene, and methanol were dosed into zeolite H-MFI and heated using an infrared laser. Each stage of the reaction was monitored using UV-Raman spectroscopy.;From the ethylene reaction, it was determined that at least three major classes of compounds are formed during the reaction. Shifts in the data indicate that each class contains several different molecules, making the spectra very complicated to assign. These classes are identified as substituted fluorenes, polyolefin chains, and one-dimensional bent polyaromatic hydrocarbons. Naphthalene was also identified as an early stage reaction product. All of these molecules must be formed inside the zeolite pores due to the nature of the IR laser heating. Fluorene and naphthalene undergo a transformation at room temperature after long times (overnight) but convert back to their original form upon continued laser heating.;In the propylene reaction, very similar results were obtained to the ethylene reaction. Additionally diene molecules were identified as a reaction product not obtained from ethylene conversion. These diene molecules are very reactive with the small aromatic molecules formed, and the reaction goes to completion at a much lower temperature than when ethylene is used as the starting reagent.;Finally the reaction of methanol over H-MFI was studied. Again, similar molecules were formed as in ethylene and propylene. No diene molecules were observed. Unlike in the ethylene and propylene reaction, there is no change in the reaction spectra overnight, indicating that the coke molecules formed in this case are very stable. This is attributed to the presence of water bonding to hydrogens in the zeolite framework.;This work supports a hydrocarbon pool mechanism. Molecules like substituted fluorenes and naphthalenes form regardless of starting material. Additionally, the coke formed during late reaction stages looks the same for ethylene, propylene, and methanol.