Mechanistic Studies of Continuous Glucose Upgrading over Lewis Acidic Silicates by Operando UV–Vis and HSQC NMR

Abstract

Lewis acidic silicates are effective catalysts for the conversion of glucose to bio-molecules of industrial interest, such as methyl lactate and methyl vinyl glycolate. Although well studied in recent years, open questions of these processes remain, particularly in regards to elements of the reaction mechanisms, and how the catalysts deactivate during continuous operation. Such questions endure due to the lack of spectroscopic techniques capable of providing insight into the catalytic reaction at operational conditions. In this study, we follow the catalytic conversion of glucose over the Lewis acidic silicates Sn-Beta and Hf-Beta in a continuous flow reactor equipped with operando UV-Vis spectroscopy. In doing so, we identify several transient absorption features related to the activation and conversion of glucose at various operational conditions (<170 °C, <25 bar). Additional spectroscopic studies (high-field 1 H-13 C HSQC NMR) and kinetic cross-experiments allow each Snglucose interaction to be assigned to a particular class of product, and hence to various selective or non-selective reaction pathways. Based on these assignments, operando UV-Vis is used to probe how partial deactivation of the stannosilicate catalyst affects the overall performance of the glucose upgrading reaction during continuous operation. We demonstrate how deactivation during glucose conversion is non-uniform, with different reaction pathways losing activity at different rates. These findings are shown to have consequences with respect to improving the selectivity of catalysts for glucose upgrading during continuous operation.

High-Productivity Continuous Conversion of Glucose to α-Hydroxy Esters over Postsynthetic and Hydrothermal Sn-Beta Catalysts

“…We note that the low level of selectivity both reflects the cascade nature of the reaction process ( Scheme 1 ) and further emphasizes the high reactivity and multifunctionality of Sn-Beta in being able to catalyze different reaction pathways. 12 , 30 Consequently, despite the improved level of stability achieved upon addition of water, the selectivity of the process to retro-aldol products in methanol/water (99:1) still has margins for improvements.…”

“…Generally, the hydrothermal synthesis of Sn-Beta is known to generate an active and selective catalyst for retro-aldol fragmentation, due to the particular active site speciation and the defect-free nature of its hydrophobic framework. 28 − 30 However, despite its typically excellent levels of performance, the synthesis of this material has several drawbacks that complicate its industrial use. 31 − 33 Therefore, many studies have focused on the synthesis of Sn-Beta materials made by alternative, postsynthetic procedures.…”

Highly Dispersed Sn-beta Zeolites as Active Catalysts for Baeyer–Villiger Oxidation: The Role of Mobile, In Situ Sn(II)O Species in Solid-State Stannation

“…Lewis acidic Sn in β zeolites (BEA framework), in particular, is highly active and selective in a variety of biomass conversion reactions, including (i) sugar isomerization and epimerization of mono- and disaccharides, − (ii) conversion of sugar­(-derived) molecules into lactic acid, − (iii) aldol reactions, , (iv) Meerwein–Ponndorf–Verley (MPV) reduction of aldehydes and ketones, − and (v) H 2 O 2 -mediated Baeyer–Villiger oxidation (BVO) of ketones. − …”

Tandem Reduction–Reoxidation Augments the Catalytic Activity of Sn-Beta Zeolites by Redispersion and Respeciation of SnO2 Clusters

“…Lewis acidic Sn incorporated in a beta (β) zeolite is highly active in industrially relevant reactions such as the reduction of ketones/aldehydes to alcohols by Meerwein-Ponndorf-Verley reaction (MPV), [1][2][3] the oxidation of ketones to esters with H2O2 (Baeyer-Villiger oxidation, BVO), [4][5][6][7][8] the isomerization and epimerization of sugars, [9][10][11][12][13][14][15][16][17][18] the conversion of carbohydrates into lactic acid, [19][20][21] and aldol condensations. 22,23 There is general consensus that Snβ catalysts contain Lewis acidity (LA), but the exact structural properties of the active Sn site is still debated.…”

“…Mössbauer data, with only 5-8% of Sn(II) remaining (Table1, entry[15][16][17][18][19]. Note that at a reoxidation temperature…”