Effects of Alkali‐Metal Ions and Counter Ions in Sn‐Beta‐Catalyzed Carbohydrate Conversion

Abstract

Alkali-metal ions have recently been shown to strongly influence the catalytic behavior of stannosilicates in the conversion of carbohydrates. An effect of having alkali-metal ions present is a pronounced increase in selectivity towards methyl lactate. Mechanistic details of this effect have remained obscure and are herein addressed experimentally through kinetic experiments and isotope tracking. The presence of alkali-metal ions has a differential effect in competing reaction pathways and promotes the rate of carbon-carbon bond breakage of carbohydrate substrates, but decreases the rates of competing dehydration pathways. Further addition of alkali-metal ions inhibits the activity of Sn-Beta in all major reaction pathways. The alkali-metal effects on product distribution and on the rate of product formation are similar, thus pointing to a kinetic reaction control and to irreversible reaction steps in the main pathways. Additionally, an effect of the accompanying basic anions is shown, supposedly facilitating the cation exchange and eliciting a different concentration-dependent effect to that of neutral alkali-metal salts.

Stoichiometric active site modification observed by alkali ion titrations of Sn-Beta

“…Each catalyst shows a distinct (and different) optimum potassium carbonate concentration for the formation of methyl lactate, consistent with previously described effects for alkali salts of basic anions. 28,29 Addition of minor amounts of potassium carbonate increases the activity for methyl lactate formation, while over-titration reduces the activity for both methyl lactate and direct dehydration products (Scheme S1 and Figures S4-6). The experiments in Figure 1 show that increasing the tin content of the catalyst results in a shift of the optimum alkali concentration towards higher concentrations.…”

“…28 This "alkali effect" has been correlated to initial rate measurements showing that low concentrations of alkali salt increase carbon-carbon bond cleavage, leading to an increase in the rate of methyl lactate formation. 29 Simultaneously, the presence of these alkali salts reduce both the yield and rate of competing reactions, including Lewis acid catalysed formation of longer alpha-hydroxy esters and Brønsted acid catalysed formation of glycosides or furanic compounds. This alkali effect has been shown to depend on the nature of the counter anion, with basic ions such as carbonate leading to stoichiometric ion exchange due to a concurrent acidbase reaction.…”

“…For such alkali salts, an initial steep activation of the catalyst for methyl lactate formation is followed by a second regime of overall catalyst deactivation at increasing concentrations of alkali salt. 28,29 In the current study, we investigate the quantitative relationship between tin in Sn-Beta and added alkali salts, for the conversion of glucose to methyl lactate. A systematic investigation of functional effects was conducted through double-titration of the catalysts by varying both the amounts of active site tin and of the alkali ion dopants.…”

Response Factors Enable Rapid Quantitative 2D NMR Analysis in Catalytic Biomass Conversion to Renewable Chemicals

“…At the same time, the quantitative use of NMR (qNMR) has gained importance in metabolic studies [15][16][17][18], natural products research [19,20], food analysis [21,22] and pharmaceutical research [18,[23][24][25]. Compound identification and quantification in mixtures has gained much attention in biocatalysis, while applications in chemocatalysis have so far remained sparse [26][27][28][29]. Wider implementation of NMR methodology in chemocatalysis could be attractive, considering that both the substrate (mostly carbohydrates) and the interest in catalyst function and dysfunction bear close resemblance in metabolic and chemocatalytic processes.…”

“…When employing a 0.4 M carbohydrate feedstock, this accuracy translates to a LOQ of ≤0.2 mol% carbon. Such high accuracy can be beneficial in various settings, such as initial rate experiments under low conversion or in the tracking of pathway intermediates [26,27]. Scheme 1.…”