Discovery and Exploration of the Efficient Acyclic Dehydration of Hexoses in Dimethyl Sulfoxide/Water.

Current gaps in the development of sustainable processes include a lack of strategies to systematically identify and optimize the formation of new products. The dehydration of hexoses to 5-hydroxymethylfurfural (HMF) is a particularly widely studied process. In an attempt to identify a new high-selectivity conversion of glucose, quantitative NMR spectroscopy is used to screen conditions that have been reported to yield high conversions of glucose but low formation of HMF. In this manner, an olefinic six-carbon byproduct is identified. By adding water, selectivity for the compound was nearly tripled relative to previous reports. The detection of high-yielding side reactions in the formation of HMF is remarkable, considering how extensively HMF formation has been studied. High selectivity for the acyclic pathway allows hitherto unobserved intermediates in this pathway to be identified by using in situ NMR spectroscopy. An additional, presumably cyclic, pathway contributes to HMF formation.

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.

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.

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.

Dehydration of xylose to furfural over MCM-41-supported niobium-oxide catalysts.

A series of silica-based MCM-41-supported niobium-oxide catalysts are prepared, characterized by using XRD, N2 adsorption-desorption, X-ray photoelectron spectroscopy, Raman spectroscopy, and pyridine adsorption coupled to FTIR spectroscopy, and tested for the dehydration of D-xylose to furfural. Under the operating conditions used all materials are active in the dehydration of xylose to furfural (excluding the MCM-41 silica support). The xylose conversion increases with increasing Nb2 O5 content. At a loading of 16 wt % Nb2 O5 , 74.5 % conversion and a furfural yield of 36.5 % is achieved at 170 °C, after 180 min reaction time. Moreover, xylose conversion and furfural yield increase with the reaction time and temperature, attaining 82.8 and 46.2 %, respectively, at 190 °C and after 100 min reaction time. Notably, the presence of NaCl in the reaction medium further increases the furfural yield (59.9 % at 170 °C after 180 min reaction time). Moreover, catalyst reutilization is demonstrated by performing at least three runs with no loss of catalytic activity and without the requirement for an intermediate regeneration step. No significant niobium leaching is observed, and a relationship between the structure of the catalyst and the activity is proposed.

Preparation and characterization of Mg-Zr mixed oxide aerogels and their application as aldol condensation catalysts.

A series of Mg-Zr mixed oxides with different nominal Mg/(Mg+Zr) atomic ratios, namely 0, 0.1, 0.2, 0.4, 0.85, and 1, is prepared by alcogel methodology and fundamental insights into the phases obtained and resulting active sites are studied. Characterization is performed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, N(2) adsorption-desorption isotherms, and thermal and chemical analysis. Cubic Mg(x)Zr(1-x)O(2-x) solid solution, which results from the dissolution of Mg(2+) cations within the cubic ZrO(2) structure, is the main phase detected for the solids with theoretical Mg/(Mg+Zr) atomic ratio ≤0.4. In contrast, the cubic periclase (c-MgO) phase derived from hydroxynitrates or hydroxy precursors predominates in the solid with Mg/(Mg+Zr)=0.85. c-MgO is also incipiently detected in samples with Mg/(Mg+Zr)=0.2 and 0.4, but in these solids the c-MgO phase mostly arises from the segregation of Mg atoms out of the alcogel-derived c-Mg(x)Zr(1-x)O(2-x) phase during the calcination process, and therefore the species c-MgO and c-Mg(x)Zr(1-x)O(2-x) are in close contact. Regarding the intrinsic activity in furfural-acetone aldol condensation in the aqueous phase, these Mg-O-Zr sites located at the interface between c-Mg(x)Zr(1-x)O(2-x) and segregated c-MgO display a much larger intrinsic activity than the other noninterface sites that are present in these catalysts: Mg-O-Mg sites on c-MgO and Mg-O-Zr sites on c-Mg(x)Zr(1-x)O(2-x). The very active Mg-O-Zr sites rapidly deactivate in the furfural-acetone condensation due to the leaching of active phases, deposition of heavy hydrocarbonaceous compounds, and hydration of the c-MgO phase. Nonetheless, these Mg-Zr materials with very high specific surface areas would be suitable solid catalysts for other relevant reactions catalyzed by strong basic sites in nonaqueous environments.

Catalytic dehydration of xylose to furfural: vanadyl pyrophosphate as source of active soluble species.

The acid-catalysed, aqueous phase dehydration of xylose (a monosaccharide obtainable from hemicelluloses, e.g., xylan) to furfural was investigated using vanadium phosphates (VPO) as catalysts: the precursors, VOPO(4)·2H(2)O, VOHPO(4)·0.5H(2)O and VO(H(2)PO(4))(2), and the materials prepared by calcination of these precursors, that is, γ-VOPO(4), (VO)(2)P(2)O(7) and VO(PO(3))(2), respectively. The VPO precursors were completely soluble in the reaction medium. In contrast, the orthorhombic vanadyl pyrophosphate (VO)(2)P(2)O(7), prepared by calcination of VOHPO(4)·0.5H(2)O at 550°C/2 h, could be recycled by simply separating the solid acid from the reaction mixture by centrifugation, and no drop in catalytic activity and furfural yields was observed in consecutive 4 h-batch runs (ca. 53% furfural yield, at 170°C). However, detailed catalytic/characterisation studies revealed that the vanadyl pyrophosphate acts as a source of active water-soluble species in this reaction. For a concentration of (VO)(2)P(2)O(7) as low as 5 mM, the catalytic reaction of xylose (ca. 0.67 M xylose in water, and toluene as solvent for the in situ extraction of furfural) gave ca. 56% furfural yield, at 170°C/6 h reaction.