Fe-Mo-oxide of spinel structure for methanol oxidation

Abstract

The aim of this master thesis was to investigate a Fe-Mo-oxide catalyst of spinel structure for methanol oxidation to produce formaldehyde and determine if the spinel could be an alternative to the commercial Formox catalyst used today. This included firstly, synthesising the catalyst to get more knowledge of the formation of the structure and secondly, determine the activity, selectivity to formaldehyde and stability of the catalyst. A literature review was performed and several synthesis methods to form a Fe/Mo spinel were found. The method that was most commonly used in previous work was precipitation in combination with H2-reduction and thus it was selected to use for synthesis in this project. Spray drying in combination with H2-reduction was also chosen to move forward with. The experimental part of the project focused on: synthesis of spinel catalyst, catalyst characterisation and activity and stability measurements. The catalyst synthesis included formation of precursors (precipitation or spray drying), which was prepared and performed before this project, and reduction by hydrogen. For the reduction experiments, some parameters were chosen to vary to learn more about at what conditions the spinel phase is favoured. All synthesised catalysts were characterised; to obtain the present phases in the catalyst an X-ray diffraction analysis was performed and to measure the specific surfaces a Brunauer-Emmett-Teller-analysis was executed. The results showed that it was more difficult to synthesise pure spinel phase when the Mo-content was high.

After reduction and characterisation some catalysts were selected for activity and stability measurements. These measurements aimed to find the catalytic performance of the spinels and compare it to the commercial catalyst used in the process. The activity and ageing tests (stability measurements) were thus constructed to imitate the Formox process in terms of temperatures and reactor feed. Results from the activity tests showed that the selectivity to formaldehyde is lower than for the present Formox catalyst. The most promising spinel from the activity measurements was then used for stability measurements and this test showed better results; the formaldehyde selectivity increased over time. An inductively coupled plasma analysis was performed and it was found that all molybdenum was maintained from the ageing test which is a promising catalyst quality.

Popular Abstract

Fe-Mo-oxide of spinel structure – a catalyst for the future

The use of products such as plastics, table napkins and paint is an everyday event worldwide. What you might not think of is how these products are manufactured or what they are made from. Several chemicals have been added together and many process steps have led to the production of all these products. One thing that all above mentioned products have in common is that the bulk chemical formaldehyde has been used somewhere in the manufacturing process. A bulk chemical is defined as a chemical that is produced in a very large scale, and today 45 million tons of formaldehyde is produced annually!

One crucial ingredient in producing formaldehyde and the heart of the process is the catalyst. A catalyst is something that facilitates or speeds up a chemical reaction without being consumed. The commercial catalyst used for selective methanol oxidation to form formaldehyde is a mixture of metal oxides, where the metals in this case are iron, Fe, and molybdenum, Mo. The formaldehyde selectivity, which is the ratio of formaldehyde formed from methanol, is high for this catalyst, 92-95%.

One of the leaders in process and catalyst knowledge for formaldehyde production by selective methanol oxidation is Johnson Matthey Formox. This company sells formaldehyde plants, but also catalyst to costumers all over the world. To provide a catalyst that has a high standard and remains competitive, research on composition and structure of the catalyst to improve performance is ongoing. Potential improvements of the commercial catalyst that have been targeted are phase stability and reducing Mo-loss from the catalyst in order to increase the lifetime of the catalyst. Therefore, these two areas are the background to and motivation for the project: ‘Fe-Mo-oxide of spinel structure for methanol oxidation’.

A spinel structure is a structure where metal oxides are coordinated in a specific way, enabling cation vacancies to form when the structure is oxidised. The cation vacancies are voids where the metals normally would be and these voids allow the structure to be very flexible. The atoms of the structure can easily diffuse through the structure without causing the catalyst structure to fall apart. This is the quality that provides a Fe-Mo-oxide catalyst of spinel structure high phase stability and low molybdenum losses. Results from experiments in this project indicated that Mo diffused through the structure to the surface which increased the formaldehyde selectivity over time. Also, results showed that Mo was maintained in the structure over time. However, the formaldehyde selectivity of the spinels was still lower than for the commercial catalyst and therefore improvements of the spinel catalyst are still required. It was also shown in this project that the synthesis of a spinel of high Mo-content was complicated. Future work should therefore focus on improving the synthesis method so that a Fe-Mo-oxide of spinel structure with a higher Mo content can be synthesised and investigated.

• author: Thulin, Maria
• supervisor: Christian Hulteberg
• organization: Chemical Engineering (M.Sc.Eng.)
• course: KET920 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: iron molybdate catalyst, methanol oxidation, Fe-Mo-oxide, spinel structure, formaldehyde catalyst, chemical engineering, kemiteknik
• language: English
• id: 8883499
• date added to LUP: 2016-06-28 11:32:52
• date last changed: 2016-06-28 11:32:52