Søren Tolborg

Tin‐containing Silicates: Alkali Salts Improve Methyl Lactate Yield from Sugars

This study focuses on increasing the selectivity to methyl lactate from sugars using stannosilicates as heterogeneous catalyst. All group I ions are found to have a promoting effect on the resulting methyl lactate yield. Besides, the alkali ions can be added both during the preparation of the catalyst or directly to the solvent mixture to achieve the highest reported yield of methyl lactate (ca. 75 %) from sucrose at 170 °C in methanol. The beneficial effect of adding alkali to the reaction media applies not only to highly defect-free Sn-Beta prepared through the fluoride route, but also to materials prepared by post-treatment of dealuminated commercial Beta zeolites, as well as ordered mesoporous stannosilicates, in this case Sn-MCM-41 and Sn-SBA-15. These findings open the door to the possibility of using other preparation methods or different Sn-containing silicates with equally high methyl lactate yields as Sn-Beta.

Incorporation of tin affects crystallization, morphology, and crystal composition of Sn-Beta

The crystallization of Sn-Beta in fluoride medium is greatly influenced by the amount and type of tin source present in the synthesis gel. By varying the amount of tin in the form of tin(IV) chloride pentahydrate, the time required for crystallization was studied. It was found that tin not only drastically affects the time required for crystallization, but also that the presence of tin changes the morphology of the formed Sn-Beta crystals. For low amounts of tin (Si/Sn = 400) crystallization occurs within four days and the Sn-Beta crystals are capped bipyramidal in shape, whereas for high amounts of tin (Si/Sn = 100) it takes about sixty days to reach full crystallinity and the resulting crystals are highly truncated, almost plate-like in shape. Using SEM-WDS to investigate the tin distribution along transverse sections of the Sn-Beta crystals, a gradient distribution of tin was found in all cases. It was observed that the tin density in the outer parts of the Sn-Beta crystals is roughly twice as high as in the tin depleted core of the crystals. Sn-Beta was found to obtain its maximum catalytic activity for the conversion of dihydroxyacetone to methyl lactate close to the minimum time required for obtaining full crystallinity. At excessive crystallization times, the catalytic activity decreased, presumably due to Ostwald ripening.

Tin-containing silicates: identification of a glycolytic pathway via 3-deoxyglucosone

Inorganic glycolytic systems, capable of transforming glucose through a cascade of catalytic steps, can lead to efficient chemical processes utilising carbohydrates as feedstock. Tin-containing silicates, such as Sn-Beta, are showing potential for the production of lactates from sugars through a cascade of four to five sequential steps. Currently, there is a limited understanding of the competing glycolytic pathways within these systems. Here we identify dehydration of glucose to 3-deoxyglucosone as an important pathway that occurs in addition to retro-aldol reaction of hexoses when using tin-containing silicates. It is possible to influence the relative carbon flux through these pathways by controlling the amount of alkali metal salts present in the reaction mixture. In the absence of added potassium carbonate, at least 15–30% carbon flux via 3-deoxyglucosone is observed. Addition of just a few ppm of potassium carbonate makes retro-aldol pathways dominant and responsible for about 60–70% of the overall carbon flux. The 3-deoxyglucosone pathway results in new types of chemical products accessible directly from glucose. Furthermore, it is argued that 3-deoxyglucosone is a contributing source of some of the methyl lactate formed from hexoses using tin-containing silicates in the presence of alkali metal salts. Further catalyst design and system tuning will permit even better control between these two different glycolytic pathways and will enable highly selective catalytic transformations of glucose to a variety of chemical products using tin-containing silicates.

Application of 119Sn CPMG MAS NMR for Fast Characterization of Sn Sites in Zeolites with Natural 119Sn Isotope Abundance

(119)Sn CPMG MAS NMR is demonstrated to be a fast and efficient method for characterization of Sn-sites in Sn-containing zeolites. Tuning of the CPMG echo-train sequence decreases the experimental time by a factor of 5-40 in the case of as-synthesized and hydrated Sn-BEA samples and by 3 orders of magnitude in the case of dehydrated Sn-BEA samples as compared to conventional methods. In the latter case, the reconstruction of the quantitative spectrum without the loss of sensitivity is shown to be possible. The method proposed allows obtaining (119)Sn MAS NMR spectra with improved resolution for Sn-BEA zeolites with natural (119)Sn isotope abundance using conventional MAS NMR equipment.

Synthesis of a novel polyester building block from pentoses by tin-containing silicates

We report here the direct formation of the new chemical product trans-2,5-dihydroxy-3-pentenoic acid methyl ester from pentoses using tin-containing silicates as catalysts. The product is formed under alkali-free conditions in methanol at temperatures in the range 140–180 °C. The highest yields are found using Sn-Beta as the catalyst. Under optimised conditions, a yield of 33% is achieved. Purified trans-2,5-dihydroxy-3-pentenoic acid methyl ester was used for co-polymerisation studies with ethyl 6-hydroxyhexanoate using Candida antarctica lipase B as the catalyst. The co-polymerisation yields a product containing functional groups originating from trans-2,5-dihydroxy-3-pentenoic acid methyl ester in the polyester backbone. The reactivity of the incorporated olefin and hydroxyl moieties was investigated using trifluoroacetic anhydride and thiol–ene chemistry, thus illustrating the potential for functionalising the new co-polymers.

Accelerated synthesis of Sn-BEA in fluoride media: effect of H2O content in the gel

Tin-containing zeotypes, particularly Sn-BEA, are promising heterogeneous catalysts for a number of important industrially relevant reactions. However, the direct hydrothermal synthesis of these materials requires unfavourably long times, which is an obstacle for their industrial application. In the present study we show that up to 4-fold reduction of the crystallization time can be achieved by a decrease of the H2O/SiO2 ratio in the synthesis gel from 7.5 to 5.6. The crystallization kinetics has been studied for five series of gels containing 1.0SiO2 : 0.27TEA2O : xSnO2 : 0.54HF : yH2O, for which y was fixed to 5.6, 6.8 and 7.5 at x = 0.005 and to 5.6 and 6.8 at x = 0.010. The crystallization time was varied within 0.5–60 days. The intermediate and final products obtained were investigated using XRD, FTIR, XRF, SEM, UV-Vis, MAS NMR spectroscopy and nitrogen adsorption–desorption techniques. The products obtained with lower water content are shown to have the same structure, textural properties and morphology as materials synthesized with higher water content. Although the size of the crystals is found to decrease with water content in the gel, it does not affect the Sn coordination and environment as confirmed by 119Sn MAS NMR.

Shape-selective Valorization of Biomass-derived Glycolaldehyde using Tin-containing Zeolites

A highly selective self-condensation of glycolaldehyde to different C4 molecules has been achieved using Lewis acidic stannosilicate catalysts in water at moderate temperatures (40-100 °C). The medium-sized zeolite pores (10-membered ring framework) in Sn-MFI facilitate the formation of tetrose sugars while hindering consecutive aldol reactions leading to hexose sugars. High yields of tetrose sugars (74 %) with minor amounts of vinyl glycolic acid (VGA), an α-hydroxyacid, are obtained using Sn-MFI with selectivities towards C4 products reaching 97 %. Tin catalysts having large pores or no pore structure (Sn-Beta, Sn-MCM-41, Sn-SBA-15, tin chloride) led to lower selectivities for C4 sugars due to formation of hexose sugars. In the case of Sn-Beta, VGA is the main product (30 %), illustrating differences in selectivity of the Sn sites in the different frameworks. Under optimized conditions, GA can undergo further conversion, leading to yields of up to 44 % of VGA using Sn-MFI in water. The use of Sn-MFI offers multiple possibilities for valorization of biomass-derived GA in water under mild conditions selectively producing C4 molecules.

Quantitative NMR Approach to Optimize the Formation of Chemical Building Blocks from Abundant Carbohydrates

The future role of biomass-derived chemicals relies on the formation of diverse functional monomers in high yields from carbohydrates. Recently, it has become clear that a series of α-hydroxy acids, esters, and lactones can be formed from carbohydrates in alcohol and water solvents using tin-containing catalysts such as Sn-Beta. These compounds are potential building blocks for polyesters bearing additional olefin and alcohol functionalities. An NMR approach was used to identify, quantify, and optimize the formation of these building blocks in the Sn-Beta-catalyzed transformation of abundant carbohydrates. Record yields of the target molecules can be achieved by obstructing competing reactions through solvent selection.

Overcoming catalyst deactivation during the continuous conversion of sugars to chemicals: maximising the performance of Sn-Beta with a little drop of water

Producing chemicals from renewable resources represents one of the key challenges in chemical science. Whilst catalytic methods for converting renewables to chemicals offer several advantages over biological approaches, the solid catalysts developed to date are typically plagued by rapid rates of deactivation, prohibiting their greater exploitation. Here, we demonstrate, for the first time, that a Sn-containing zeolite, Sn-Beta, is capable of continuously converting saccharide solutions to value added chemicals with high levels of activity, selectivity and stability. For both the isomerisation of glucose to fructose, and the conversion of fructose to alkyl lactates, we observe that the addition of up to 10% of water to the methanol/sugar reaction feed increases reactivity by a factor of 2.5, and catalyst stability by one order of magnitude. Continuous operation for up to 1366 h (57 days) is demonstrated, with only limited loss of activity being observed over this period of time. Post-reaction characterisation indicates that the addition of water influences several elements of the catalytic system, which cooperatively result in improved performance.

Direct Observation of Tin in Different T-Sites of Sn-BEA by One- and Two-Dimensional 119Sn MAS NMR Spectroscopy

The direct and quantitative identification of active sites is crucial for the development of zeolite catalysts and their implementation in industry. Herein we report on the application of one-dimensional 119Sn direct polarization (DP) and rotational echo double-resonance (REDOR) and two-dimensional 119Sn magic-angle tuning (MAT) NMR spectroscopy for the identification of different Sn sites in fully dehydrated Sn-BEA zeolite. It is demonstrated that 119Sn magic-angle spinning (MAS) NMR techniques, modified by Carr-Purcell-Meiboom-Gill (CPMG) echo-train acquisition allow to resolve three groups of NMR signals, which can be attributed to three groups of nonequivalent T-sites based on the existing theoretical predictions: (I) T9, T4, and T3; (II) T2, T1, and T8; and (III) T7, T5, and T6. Results suggest that the sites attributed to group III are the most populated in Sn-BEA samples obtained via the fluoride route. The attribution of NMR lines to different T-sites in the structure of BEA allows for the establishment of structure-reactivity relationship and therefore for further improvement of Sn-BEA catalysts.