Direct low temperature oxidation of methane to methanol via copper zeolite materials

This work investigated the development of a material and system suitable for direct low temperature (200 °C) oxidation of methane to methanol under ambient conditions with molecular oxygen. The approach was inspired by an enzymatic system, the methane mono oxygenase, which utilises iron or copper complexes for this reaction. It was found that zeolite structures ion-exchanged with copper or iron can facilitate similar structures to those found in those enzymatic systems. To create such complexes, different precursor salts and zeolites were tested alongside different preparation techniques and processing parameters. The effects of each parameter were investigated and optimised. The thesis is split up into four research chapters:

“Effects of Material Preparation Techniques”: Three different hydrated copper(II) precursor salts were tested (sulphate, nitrate and acetate form). Three different aqueous ion-exchange procedures were studied: single ion exchange, single ion exchange with pH modification and multi-ion exchange. Solids prepared using copper acetate achieved highest copper loading and best methane activation performance. X-ray photoelectron spectroscopy indicated that the multi-ion exchanged material formed more active sites for methane activation compared to pH modified materials despite similar copper loading.

“Effects of Zeolite Supports”: Three different zeolites multi-ion exchanged with copper acetate were tested: mordenite, ZSM-5 and zeolite Beta. The materials prepared with mordenite led to the highest product formation and showed on XPS studies the highest formation of active Cu(II) species.

“Influence of Material Pre-treatment on Methane at Low Activation Temperatures”: calcination and activation atmosphere and time as well as reaction and desorption temperatures were optimised. Cyclic testing was shown to be successful and reaction mechanisms were proposed. An optimal parameter processing window was identified.

“Bimetallic and Iron Materials”: The copper containing enzymatic system contains zinc, therefore bi-metallic materials containing both were prepared. Zinc only materials were in-active. However, when zinc added in small doses to copper exchanged zeolites, an increase in product formation occurred. A proposed mechanism suggests that adding zinc may break up inactive copper clusters to smaller active copper sites. The prepared iron materials were not successful.