Towards a methanol economy: Zeolite catalyzed production of synthetic fuels

The main focus of this thesis is zeolite catalyzed conversion of oxygenates to hydrocarbon fuels and chemicals. Furthermore, conversion of ethane to higher hydrocarbons has also been studied. After a brief introduction to the concept of “the methanol economy” in the first chapter, the second chapter is a literature study of Mobil’s “methanol to hydrocarbons” (MTH) process, giving an overview of the history of the process, the nature of the employed catalysts, and the reaction mechanism. In the third chapter, a series of experiments concerning co conversion of ethane and methanol over a commercial H-ZSM-5 zeolite impregnated with gallium and/or molybdenum is described. The object was to investigate if the presence of methanol in the feed could enhance the conversion of ethane, but in all cases the opposite is observed; the presence of methanol actually suppresses the conversion of ethane. This suppression of ethane conversion is most likely due to simple composition for the active sites. Isotopic labeling studies performed with ¹³C labeled methanol and unlabeled ethane showed that in the very first minutes of the reaction, large amounts of carbon atoms from ethane are incorporated into the products. This observation is attributed to a stoichiometric reaction between ethane and the metal oxide species in the catalyst, leading to the oxidation of ethane to ethene, which is readily converted and incorporated into the products.
Conversion of methanol, ethanol, 2-propanol, and 1 butanol to hydrocarbons over various zeolite catalysts is studied in Chapter 4. When 2-propanol or 1-butanol is converted over H-ZSM-5, the total conversion capacities of the catalyst are more than 25 times higher than for conversion of methanol and ethanol. Furthermore, for conversion of C₃₊ alcohols, the selectivity shifts during the experiment, and after around one third of the total run time, the product mixture consists almost exclusively of alkenes, which is in stark contrast to conversion of methanol and ethanol where the catalyst produces large amounts of aromatics until full deactivation. When zeolite H Beta is employed, the conversion capacities for all four alcohols are markedly lower than for H-ZSM-5, and H Beta has higher conversion capacity for methanol than the other alcohols. Furthermore, conventional and mesoporous H Ga MFI was employed in the conversion of methanol and 2 propanol. These catalysts showed a lower selectivity towards aromatics than H-ZSM-5 and the mesoporous H-Ga-MFI deactivated extremely slowly during the conversion of 2-propanol and only very small amounts of coke were deposited on the gallium based zeolites compared to H-ZSM-5. In the fifth chapter the direct zeolite catalyzed production of hydrocarbons from other oxygenates such as glycerol, methyl lactate, and acetic acid is studied. In general, very fast deactivation due to coke deposition is observed, but dilution of the reactants in methanol has a distinct positive effect on this, and reasonable conversion capacities are achieved in this way. The incorporation of the carbon atoms from the oxygenates into the hydrocarbon products was confirmed by isotopic labeling experiments. More than 25 different oxygenates are converted over H-ZSM-5 as 10 % solutions in methanol, and the effect of the H/C atomic ratio of the dehydrated reaction mixture and the specific functional groups in the reactant on the catalyst lifetime, product selectivity, and formation of coke is addressed.