Green aromatics for a bio-based economy - Valorization of biomass derived model compounds over zeolites studied by online analysis

Licentiate thesis, 2021

In the strive towards sustainable chemical production, its carbon-based products can no longer be produced from fossil resources but need to be manufactured from a renewable feedstock.

Simple aromatic building blocks like benzene, toluene and xylenes (BTX) can be obtained by catalytic valorization of biomass derived platform molecules such as furans provided efficient catalysts can be formulated.

To evaluate catalysts for the conversion of furans to BTX and study the involved catalytic reactions, advanced analytical methods are needed. The observation of dynamic responses in catalytic activity and selectivity necessitates time-resolved analysis of rather complex hydrocarbon (HC) streams.

In this work, an online analysis method combining Fourier transform infrared spectroscopy and ion molecule reaction mass spectrometry has been developed for the direct monitoring of the effluent stream during conversion of furans over zeolite catalysts.

The sampling frequency is shown to be at least 4 per minute, which is considerably higher compared to that of separation-based methods operating on time scales of several minutes. A wide range of HCs and other smaller molecules is identified and individual species quantified simultaneously. The carbon balance of around 90% shows that the vast majority of the complex HC stream is indeed analysed.

The developed online analysis has been applied to catalytic step-response experiments, where different zeolites were exposed to concentration steps of 2,5-dimethylfuran at steady temperatures. It is found that the HC stream consists of a range of olefins and aromatics, including BTX. Most interestingly, 2,5-dimethylfuran is isomerized to 2,4-dimethylfuran as well as 2- and 3-methyl-2-cyclopenten-1-one. The formation of BTX is linked to the availability of olefins, which is supported by temperature programmed desorption experiments.

Furthermore, to build understanding of the catalytic mechanisms, surface species were characterised in situ by use of diffuse reflectance infrared Fourier transform spectroscopy. It is clear that upon adsorption, 2,5-dimethylfuran interacts with the zeolite structure and undergoes transforming reactions to olefins and aromatics upon temperature increase, but further studies are necessary to comprehend how the rather low selectivity towards BTX can be increased by catalyst design.