Synthesis, Characterization and Evaluation of Tin-containing Silicates for Biomass Conversion

Søren Tolborg

Research output: Book/ReportPh.D. thesisResearch

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Abstract

The transition to renewable carbon sources such as biomass will require entirely new catalytic processes and result in completely new products. An entire industry is built up around the chemicals that are available from fossil resources but will be unfeasible to prepare from other carbon sources. This dissertation describes the preparation and use of several important stannosilicate materials, known to transform carbohydrates into different valuable products. Several aspects of the tin-containing catalysts are investigated and discussed and new insight into the conversion of sugars is provided. The catalyst Sn-Beta is an important and highly active catalyst in a number of reactions. By carefully investigating the fluoride-route synthesis, the active component tin was discovered to influence the crystal morphology by changing the growth of the crystals. Even a small increase in tin content lead to much longer crystallization times (up to 60 days). Tin was furthermore not evenly distributed within the crystals, but instead found as an enriched outer shell. Small amounts of alkali were found to limit the growth retardation, enabling the preparations of Sn-Beta materials with high tin content.
For the catalytic conversion of sugars, the addition of alkali to the reaction media was also found to have a large effect on the product distribution. Having small concentrations of alkali present modified Sn-Beta to favor retro-aldol/aldol condensation reactions resulting in up to 75% methyl lactate from sucrose at optimized conditions. The effect of alkali was found to transcend to a variety of sugars, solvents and other stannosilicates such as Sn-MCM-41 or Sn-Beta prepared by a post-synthesis methodology. The latter makes it possible to use industrially relevant tin-containing catalysts to achieved high yields of methyl lactate simply by optimizing the amount of cosolute. In the absence of alkali, instead of retro-aldol reaction, hexoses and pentoses were found to undergo β-dehydration leading to several new and highly functional products. These include the trans-2,5,6-trihydroxy-3-hexenoic acid methyl ester (from hexoses) or trans-2,5-dihydroxy-3-pentenoic acid methyl ester (from pentoses) in acceptable yields (18-33%). Several additional products and intermediates were identified and quantified, providing a better understanding of the transformation of sugars catalyzed by tin. By using a zeolite with narrow channels (Sn-MFI), shape selectivity could be exploited for the valorization of small sugars. Glycolaldehyde (GA) could selectively undergo aldol condensation to give high yields of the rare tetroses (74%). With larger pore systems and channels either substantial yields of hexoses were formed from subsequent condensation reactions (Sn-MCM-41 and Sn-Beta): In the particular case of Sn-MFI and Sn-Beta, additional conversion of the tetroses lead to vinyl glycolic acid and α-hydroxy-γ-butyrolactone (HBL). Changing the conditions, it was possible to form up to 44% of VGA directly from GA using Sn-MFI. An important part of making the transition to more renewable resources is to have attractive alternatives to switch to. This means new, interesting chemicals easily obtainable from the most abundant biomass-derived sugars need to be found. The findings and processes investigated and discussed here should be a move in this direction.
Original languageEnglish
PublisherDTU Chemistry
Number of pages199
Publication statusPublished - 2016

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