Biomass transformations with Sn in dealuminated ß zeolites

The concept of biorefinery, transforming biological feedstock into valuable chemicals, has gained in interest the last decades due to depleting cheap oil reserves, but also due to an increasing environmental awareness. An important challenge before switching to a biomass-driven society, is to transform biomolecules like triglycerides and lignocellulosics into added-value chemicals efficiently, for which new developments in the chemical processing of such biomolecules are crucial. The design of new catalytic materials, able to efficiently and selectively convert the intended reactions, is an important topic in this progress. Many new or modified catalytic materials with various catalytic entities and capabilities are being researched that can defy the new challenges associated with biomass conversion. Reagent molecules are now more polar and reactive than those we used to convert with acid/base and redox catalysis, and the new reactions are likely to be processed in aqueous conditions at elevated temperatures. Lewis acidity has always been an important type of catalysis in the fine chemistry, but also in the petrochemistry. In the group of water stable Lewis acid catalysts, Sn-containing materials of which the acid Sn-beta (Snβ) is the most studies member receive a lot of attention currently. Snβ is capable of catalyzing several interesting biomass related reactions, including the isomerization and epimerization of monosaccharides, but is also allows the direct conversion of sugars into lactate esters and condensation C-C coupling chemistry. Furthermore, the materials also display excellent catalytic properties in more organically oriented reactions such as Baeyer-Villiger oxidations and Meerwein-Ponndorf-Verley reductions. Though Snβ is a proven catalysts, its synthesis is arduous. Long synthesis times and hazardous chemicals are needed to incorporate Sn into the zeolite framework, impeding industrial scale use of the catalytic material. The focus of this doctoral research is to find an alternative and more sustainable synthesis procedure for the Sn-containing catalyst. Previous efforts succeeded in shortening the synthesis times by modifying the traditional hydrothermal procedure, but they still need toxic chemicals. Another approach, practiced here in this work, introduces Sn into commercial beta zeolite structures, a so-called post-synthesis procedure. This method prevents the use of hazardous chemicals and only short synthesis times are needed, but the catalytic activity of materials which have been synthesized following this approach until today, is significantly lower than its hydrothermal counterpart. The research presented in this work therefore attempts a novel synthesis method, combining both synthesis times and practical operations from the post-synthetic procedures without compromising the catalytic properties, characterized by a high activity and excellent catalyst stability. The novel synthesis procedure grafts Sn precursor salts onto a dealuminated commercial Beta zeolite during reflux in dry isopropanol. Up to 2 wt% of monoatomic active Sn4+ atoms can be introduced into the zeolite framework without formation of extraframework non-catalytically active Sn-species. Essential steps in the synthesis method and requirements of chemicals and conditions were studied and discussed in this work. The catalytic properties of the material were tested in several biomass and fine chemicals related transformations. Most surprisingly, it appears that different reaction types required somewhat different Snβ catalysts for their optimal performance. Therefore, different Sn active sites, monomeric to oligomeric SnOx in nature, are possible in the same beta framework and each reaction type requires a different optimization synthesis procedure. Biomass conversion usually requires multi-step or cascade type of reactions to form the desired product. One-pot conversions combine many different steps together, which is especially of interest when the intermediate chemicals are very reactive or hazardous. Design of multiple site catalysts is therefore paramount in biomass conversion. The great challenge is to incorporate the different active sites in balance with the requirements of the reaction kinetics. The presented novel synthesis procedure is highly versatile and therefore ideal to design such multi-active site catalysis. Partial, instead of complete, removal of Al in the original structure of Beta allows the presence of Brønsted acidity, next to the Lewis acidity of Sn. This bifunctional catalyst model catalyst showed great potential in the conversion of carbohydrate to lactate esters. In essence, this reaction requires multiple steps, while the reaction rate is determined by a dehydration step. Accelerating this step with the acidity greatly improved the reaction rate. Interestingly, the cooperative effect is only valid when the two different catalytic sites are close to each other, ie. in the same zeolite crystal. Other important reactions like glucose to HMF and levulinic acid might also benefit from the presence of the two sites, though a different balance is anticipated. Though Snβ has already been discovered in 1997, the characteristics of the genuine active Sn site is under debate. Next to the site characteristics, there is also discussion on the importance of the active site pocket, either containing nearby silanols or not. The latter might assist in coordinating the molecules in the active site pocket or it effects the polarity, and thus the adsorption (and intrazeolitic concentration), of molecules. This work noticed that post-synthetic synthesis leads to a different active Sn site. The Sn-atom is less coordinated to the framework, but therefore shows a stronger Lewis acidity. The site appears like a coordinative binding, with separating acid – base entities due to the constraints in the active site, dictated by the rigid zeolite frame. That Sn sites in the new and old material are different might not come to a surprise, as Sn is incorporated in Al-removal sites in the post-synthesis procedure, while the incorporation of Sn in the hydrothermal synthesis runs differently. Catalytic consequences of these differences were demonstrated by detailed kinetic analysis of two reaction types for which a better activity per Sn was observed in the grafted material. In Baeyer-Villiger oxidation, both entropic (transition state stabilization) and enthalpic (increased Lewis acid strength) effects were noticed, whereas only entropic benefits (site accessibility) were found in case of a bimolecular Meerwein-Ponndorf-Verley reaction. The new Sn catalyst presented in this work thus shows great catalytic opportunities in biomass conversion. It is not only very active, its production for large scale application is practical. Moreover, the synthesis procedure allows tailoring the catalytic properties, and can be used for other active site types like Zr, Ce and Hf, which can be of great interest in the development of various biorefinery processes.status: publishe