Take the straight path: Furfural was converted into γ‐valerolactone (GVL) through sequential transfer‐hydrogenation and hydrolysis reactions catalyzed by zeolites with Lewis and Brønsted acid sites (see picture). Together, Zr‐Beta and Al‐MFI nanosheets generated GVL in 78 % yield without the use of precious metals or molecular H2. This system offers an attractive streamlined strategy for the production of GVL from biomass‐derived molecules.

Hf-, Zr- and Sn-Beta zeolites effectively catalyze the coupled transfer hydrogenation and etherification of 5-(hydroxymethyl)furfural with primary and secondary alcohols into 2,5-bis(alkoxymethyl)furans, thus making it possible to generate renewable fuel additives without the use of external hydrogen sources or precious metals. Continuous flow experiments reveal nonuniform changes in the relative deactivation rates of the transfer hydrogenation and etherification reactions, which impact the observed product distribution over time. We found that the catalysts undergo a drastic deactivation for the etherification step while maintaining catalytic activity for the transfer hydrogenation step. (119) Sn and (29) Si magic angle spinning (MAS) NMR studies show that this deactivation can be attributed to changes in the local environment of the metal sites. Additional insights were gained by studying effects of various alcohols and water concentration on the catalytic reactivity.

Carbohydrate epimerization is an essential technology for the widespread production of rare sugars. In contrast to other enzymes, most epimerases are only active on sugars substituted with phosphate or nucleotide groups, thus drastically restricting their use. Here we show that Sn-Beta zeolite in the presence of sodium tetraborate catalyses the selective epimerization of aldoses in aqueous media. Specifically, a 5 wt% aldose (for example, glucose, xylose or arabinose) solution with a 4:1 aldose:sodium tetraborate molar ratio reacted with catalytic amounts of Sn-Beta yields near-equilibrium epimerization product distributions. The reaction proceeds by way of a 1,2 carbon shift wherein the bond between C-2 and C-3 is cleaved and a new bond between C-1 and C-3 is formed, with C-1 moving to the C-2 position with an inverted configuration. This work provides a general method of performing carbohydrate epimerizations that surmounts the main disadvantages of current enzymatic and inorganic processes.

The catalytic activity of tin-containing zeolites, such as Sn-Beta, is critically dependent on the successful incorporation of the tin metal center into the zeolite framework. However, synchrotron-based techniques or solid-state nuclear magnetic resonance (ssNMR) of samples enriched with 119Sn isotopes are the only reliable methods to verify framework incorporation. This work demonstrates, for the first time, the use of dynamic nuclear polarization (DNP) NMR for characterizing zeolites containing ∼2 wt % of natural abundance Sn without the need for 119Sn isotopic enrichment. The biradicals TOTAPOL, bTbK, bCTbK, and SPIROPOL functioned effectively as polarizing sources, and the solvent enabled proper transfer of spin polarization from the radical’s unpaired electrons to the target nuclei. Using bCTbK led to an enhancement (ε) of 75, allowing the characterization of natural-abundance 119Sn-Beta with excellent signal-to-noise ratios in <24 h. Without DNP, no 119Sn resonances were detected after 10 days of continuous analysis.

M-Beta Lewis acid catalyst Zr Hf Sn Ti

The Lewis acidity of isolated framework metal sites in Beta zeolites was characterized with 15N isotopically labeled pyridine adsorption coupled with magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The 15N chemical shift of adsorbed pyridine was found to scale with the acid character of both Lewis (Ti, Hf, Zr, Nb, Ta, and Sn) and Brønsted (B, Ga, and Al) acidic heteroatoms. The 15N chemical shift showed a linear correlation with Mulliken electronegativity of the metal center in the order Ti < Hf < Zr < Nb < Ta < Sn < H+. Theoretical calculations using density functional theory (DFT) showed a strong correlation between experimental 15N chemical shift and the calculated metal–nitrogen bond dissociation energy, and revealed the importance of active site reorganization when determining adsorption strength. The relationships found between 15N pyridine chemical shift and intrinsic chemical descriptors of metal framework sites complement adsorption equilibrium data and provide a robust method to characterize, and ultimately optimize, metal-reactant binding and activation for Lewis acid zeolites. Direct 15N MAS NMR detection protocols applied to the Lewis acid–base adducts allowed the differentiation and quantification of framework metal sites in the presence of extraframework oxides, including highly quadrupolar nuclei that are not amenable for quantification with conventional NMR methods.

The selective low temperature oxidation of methane is an attractive yet challenging pathway to convert abundant natural gas into value added chemicals. Copper-exchanged ZSM-5 and mordenite (MOR) zeolites have received attention due to their ability to oxidize methane into methanol using molecular oxygen. In this work, the conversion of methane into acetic acid is demonstrated using Cu-MOR by coupling oxidation with carbonylation reactions. The carbonylation reaction, known to occur predominantly in the 8-membered ring (8MR) pockets of MOR, is used as a site-specific probe to gain insight into important mechanistic differences existing between Cu-MOR and Cu-ZSM-5 during methane oxidation. For the tandem reaction sequence, Cu-MOR generated drastically higher amounts of acetic acid when compared to Cu-ZSM-5 (22 vs 4 μmol/g). Preferential titration with sodium showed a direct correlation between the number of acid sites in the 8MR pockets in MOR and acetic acid yield, indicating that methoxy species present in the MOR side pockets undergo carbonylation. Coupled spectroscopic and reactivity measurements were used to identify the genesis of the oxidation sites and to validate the migration of methoxy species from the oxidation site to the carbonylation site. Our results indicate that the CuII–O–CuII sites previously associated with methane oxidation in both Cu-MOR and Cu-ZSM-5 are oxidation active but carbonylation inactive. In turn, combined UV–vis and EPR spectroscopic studies showed that a novel Cu2+ site is formed at Cu/Al <0.2 in MOR. These sites oxidize methane and promote the migration of the product to a Brønsted acid site in the 8MR to undergo carbonylation.

Stannosilicate zeolites with nanosheet morphology and MFI topology (Sn-MFI-ns) are successfully synthesized with an amphiphilic organic structure-directing agent. Sn source, Si/Sn ratio, crystallization time and temperature impact framework Sn incorporation and nanosheet morphology. Optimal synthesis conditions generate Sn-MFI-ns with high crystallinity and isolated framework Sn sites. Nanosheets are ∼2 nm thick in the (010) direction, resulting in large intersheet mesopore volumes and external surface areas exceeding 430 m2/g. Unlike bulk Sn-MFI, Sn-MFI-ns is highly active in the Baeyer–Villiger oxidation of bulky cyclic ketones using hydrogen peroxide (H2O2). Although Sn-MFI-ns and Sn-MCM-41 have comparable activity and oxidant efficiency, the nanosheets exhibit drastically higher thermal and hydrothermal stability.