A Systematic Study of Metal Triflates in Catalytic Transformations of Glucose in Water and Methanol: Identifying the Interplay of Brønsted and Lewis Acidity

Abstract

The specific type of acidity associated with the given metal trifloromethanesulfonates (Brønsted or Lewis acidity) dramatically influences the course of reactions, and it is possible to select for disaccharides, fructose, methyl glucosides, or methyl levulinate. Glucose is transformed into a range of value‐added molecules in water and methanol under the action of acidic metal triflates as catalysts, including their analogous Brønsted acid‐assisted or Brønsted base‐modified systems. A systematic study is presented of a range of metal triflates in methanol and water, pinning down the preferred conditions to select for each product.

High Yielding Acid‐Catalysed Hydrolysis of Cellulosic Polysaccharides and Native Biomass into Low Molecular Weight Sugars in Mixed Ionic Liquid Systems

“…This is an efficient process for the conversion of a range of non-pretreated cellulosic substrates into glucose, involving environmentally benign solvent-catalyst media. The results compare favourably with outcomes in the literature where mineral acids or zeolites are employed as catalysts for cellulose hydrolysis in [C 4 mim]Cl (Table 4, entries [21][22][23]. [11,15] The conversion of native cellulosic biomass is a significant challenge, and we applied our hydrolysis protocols to biomass derived from terrestrial (chips obtained from softwood or corncob) and marine sources (macroalgae Ulva lactuca and microalgae Porphyridium cruentum), respectively.…”

“…Both mixtures showed the appearance of an asymmetrical stretching band at 1620-1600 cm À 1 of the carboxylate anion [30] confirming the complexation. As we have already discussed for the La(OTf) 3 /H 3 PO 4 system, [23,27,34] La holds a special place relating to Lewis acid-assisted Brønsted acidity, providing a complex acid system with exceptional catalyst activity in several different chemical transformations. pH readings of the mixed acid systems (La(OTf) 3 + oxalic acid) show the increased acidity, albeit that it too appears incremental in dilute solution.…”

“…HMF likely derives from fructose, the isomerisation product of glucose, while humins form by condensation of sugars and HMF. [23,24] Scheme 2 summarises the findings. We sought to better understand the origins of the catalyst activity in the ChCl systems.…”

“…Humins are thought to form by the condensation of intermediate sugars and aldehydes. [23,24] Similarly, low molecular weight carbohydrates in ChCl/oxalic acid DES were almost completely transformed (total conversion to product of 96, 97, and 99 % for cellobiose, glucose, and fructose, respectively, Table S1) into a bulk of undesirable humins and small amounts of HMF (yields up to 11 mol%). The results clearly suggest that neither imidazolium-based solvent nor the acidic DESs are suitable reaction media under our conditions, due to a) the low acidity of [C 4 mim]Cl which fails to promote the catalytic conversion of cellulose and b) the high acidity of DESs inducing the formation of by-products.…”

“…The crude product was subjected to flash column chromatography (hexane/ethyl acetate, 60 : 40 v/v) to isolate HMF, which gave satisfactory analytical data. [23] For the conversions in mixed ionic solvent, cellulose (50 mg), [C 4 mim]Cl (1.000 g) and DES (0.100 g), and in some instances metal triflate catalyst (Al(OTf) 3 , Y(OTf) 3 , AgOTf, In(OTf) 3 , Sn(OTf) 2 , La(OTf) 3 , Yb(OTf) 3 , or Hf(OTf) 4 , 10 mol% based on the number of anhydroglucose units present in cellulose), were introduced to a glass pressure tube equipped with a magnetic follower and the reactor was sealed. The mixture was heated and stirred at the predetermined temperature for a fixed period of time.…”