Fluoride-free synthesis of a Sn-BEA catalyst by dry gel conversion

Abstract

Sn containing molecular sieves with BEA topology (Sn-BEA) are active Lewis catalysts for a large variety of redox reactions. However, the synthesis of Sn-BEA often requires the use of toxic chemicals (e.g. hydrofluoric acid). In this study, we demonstrated that an active Sn-BEA catalyst can be directly synthesized in a non-fluoride medium via a dry gel conversion method with the aid of seed crystals. It was shown that seeding of zeolite BEA crystals can facilitate crystal growth and lead to BEA topology with framework Sn. Furthermore, it was found that ion-exchange with ammonium ions is indispensable to retain the crystal structure during calcination. The catalytic activity of the Sn-BEA catalyst was evaluated by the isomerization of glucose and reaction of pyruvaldehyde in the aqueous phase, and compared with the conventional hydrophobic Sn-BEA synthesized in a fluoride medium. The catalytic activity of the Sn-BEA synthesized in the absence of fluoride was found to be lower than that of the conventional Sn-BEA for catalyzing the isomerization of glucose in the aqueous phase, which could be attributed to its hydrophilic surface. For the reaction of pyruvaldehyde in the aqueous phase, the Sn-BEA synthesized in this study showed similar activity to the conventional hydrophobic Sn-BEA, indicating that the catalytic activity of Sn-BEA catalysts largely depends on the type of reactions. The current work provides the first example of direct preparation of a Sn-BEA catalyst in a caustic medium. † Electronic supplementary information (ESI) available: The procedures for synthesis of Sn-BEA and SnO 2 /Si-BEA in a fluoride medium, the XRD patterns for the sample prepared in the absence of alkali ions and with NH 4 + salts, the SEM image for Sn-BEA-F, the XRD pattern for SnO 2 /Si-BEA, product distribution for the reactions and the 1 H-NMR spectrum for fructose in the isomerization of deuterium-labeled glucose. See

Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability

“…For these comparisons to be meaningful, it is critical to always establish a benchmark in studies comparing multiple materials. For example, the activity of HT Sn-Beta for glucose isomerization in water (82,88,90) featuring a TOF = 100-120 (mol glucose) (mol Sn) −1 h −1 at 368-383 K when using an initial concentration of (mol glucose) (mol Sn) −1 = 100 has been reproduced by several research groups; hence, it can be used as a reliable benchmark. As more alternative synthesis techniques are developed, the consistent use of benchmark materials should be complemented by including comparisons on a metal content or acid site count basis rather than total catalyst mass, so that more relevant conclusions can be drawn about the nature of the active centers.…”

“…In particular, the type of metal salt used as a precursor in hydrothermal syntheses heavily influences the framework content and catalytic activity of the final product (79,80). Recently, many groups have reported dry-gel or postsynthetic methods for metal incorporation in zeolite Beta (21,81,82). These methods can often incorporate a higher metal content into the zeolite framework compared with the hydrothermal method, leading to higher yields normalized by total catalyst mass for certain reactions, but usually have lower TOFs when normalized by metal content at loadings above 3-5% (83-85).…”

Accelerated synthesis of Sn-BEA in fluoride media: effect of H2O content in the gel

“…Significant efforts have been made for shortening the crystallization time by seeded growth methods [22][23][24] . An interesting method was proposed by Chang et al 23 , who demonstrated that Sn-BEA can be synthesized in non-fluoride media via dry gel conversion using a seeded growth procedure. An interesting method was proposed by Chang et al 23 , who demonstrated that Sn-BEA can be synthesized in non-fluoride media via dry gel conversion using a seeded growth procedure.…”

Mechanochemically assisted hydrothermal synthesis of Sn-substituted MFI-type silicates

“…Notably, the absorption band at approximately 260 nm that is attributed to octahedral Sn 4+ spaces is not observed for the calcined Sn-S MC . On the other hand, the band at 240–250 nm is still in debate, however, penta-coordinated Sn species [30,31], extra-framework Sn species [17], and small tin oxide particles [30,32] have been assigned to the peaks at around 240–250 nm. These results strongly support the idea that Sn 4+ ions were introduced into the Sn-MFI framework as tetrahedrally coordinated states.…”

“…The absorbance peaks of CD 3 CN on the ‘closed Sn’ and the ‘open Sn’ sites are at 2309 cm −1 and 2317 cm −1 , respectively. This result suggests that Sn-S MC and Sn-S HT have the ‘closed’ Sn(OSi) 4 structure [31,32]. Furthermore, all samples show no absorbance at 2317 cm −1 due to the ‘open’ Sn site.…”