Catalytic applications of iron zeolites
Date
2022
Authors
LaFollette, Mark R.
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Making fuels from natural gas, biomass, and other sources has long been a goal and several solutions have been used. A promising solution is the methanol to hydrocarbons (MTH) reaction. This reaction converts methanol, a compound easily made from natural gas and biomass, into lager hydrocarbons that can be used as fuels or chemicals. This reaction is catalyzed by zeolites and the selectivity can be determined by reaction conditions and by the type of zeolite used. Zeolites have a 3-dimensional microporous crystalline structure and are a type of silicate. They are traditionally referred to as aluminosilicates due to having some of the silicon atoms replaced with aluminum. These aluminum atoms in the framework give rise to a negative charge that is balanced out by a cation which is where the acid catalytic ability of zeolite comes from. When the cation is a proton a Brønsted acid site is formed. Other heteroatoms, such as iron, besides aluminum can be incorporated inside the zeolite framework by doing an isomorphous substitution during the synthesis. These different heteroatoms can change the Brønsted acid site acidity thus changing the catalytic activity of the zeolite and changing the selectivity and the reactivity. ☐ The first process investigated involved the use of iron zeolites in the MTH reaction. Iron zeolites have weaker Brønsted acid sites than their aluminum counterparts and thus in the MTH reaction are selective to olefins over paraffins and are not selective to aromatic products. The downside to iron zeolites is that they are not as reactive as aluminum zeolites and thus require higher temperatures to achieve similar rates and then will deactivate fast. Additionally, with MTH chemistry it is very easy to make small molecules such as ethylene and propylene and more difficult to make the larger olefins. To overcome these difficulties different olefins (C4 through C8) were co-feed into the MTH reaction. It was found that for Fe beta isobutene gave the highest reaction rate although all co-feed olefins increased the reaction rate above the normal MTH reaction rate. The carbon selectivity for isobutene was able to be shifted towards making larger carbon number molecules than with MTH. For the co-feed olefins over Fe ZSM-5 the reaction rate was similar for all co-feed olefins but still higher than MTH and the carbon selectivity’s were more consistent between different co-feed olefins. The co-feeding of olefins was expanded beyond olefins typically seen in MTH chemistry to include cyclic olefins. ☐ The next project discussed involves the synthesis of iron zeolites. Zeolites are normally synthesized using organic structure directing agents (ODSA) which are normally big bulky organic compounds. These molecules are used to compensate for the negative charge of putting a trivalent metal into the zeolite framework. The use of these molecules are expensive, generate organic waste, and they need to be removed by calcining the zeolite. Ideally zeolites would be able to be synthesized without using these compounds. Aluminum zeolites have been synthesized without using an OSDA using a seed assisted method. Work to extend this to iron zeolites is presented. It resulted in a catalytically active material for MTH consisting of a physical mixture of Fe ZSM-5 and sodium silicate. Upon ion exchange with ammonium nitrate the sodium silicate breaks down leaving a broad peak in the XRD pattern that is a silicate peak. ☐ The final project discusses involves constructing a reactor system capable of measuring rates and kinetics over samples suitable for surface science work that have low surface area and few active sites. This is so to allow the catalytic ability of the 2-dimensional “model zeolite” aluminosilicates to be studied at ambient pressure. This reactor system uses a recirculation loop to allow for product concentrations to build up to detectible levels. The online sampling is accompanied by adding a small bit of fresh feed gas to the system after each injection to replace the volume of gas lost due to sampling. The recirculation loop contains a pump to prevent mass transfer limitations and to make the concentration of gases in the sample loop consistent. The reactor system was tested using the CO oxidation reactor over Pt (111) and Pt (111) with a graphene overlayer. The apparent activation energy of Pt (111) is consistent with the literature and the graphene overlayer reduced the apparent activation energy.
Description
Keywords
Gas to liquids, Methanol, Methanol to hydrocarbons, Methylation, Zeolite