Sn-β Zeolite Research Articles

Isomerization of Galactose to Tagatose: Recent Advances in Non-enzymatic Isomerization.

The valorization of galactose derived from acid whey to low-calorie tagatose has gained increasing attention. Enzymatic isomerization is of great interest but faces several challenges, such as poor thermal stability of enzymes and a long processing time. In this work, non-enzymatic (supercritical fluids, triethylamine, arginine, boronate affinity, hydrotalcite, Sn-β zeolite, and calcium hydroxide) pathways for galactose to tagatose isomerization were critically discussed. Unfortunately, most of these chemicals showed poor tagatose yields (<30%), except for calcium hydroxide (>70%). The latter is able to form a tagatose-calcium hydroxide-water complex, which stimulates the equilibrium toward tagatose and prevents sugar degradation. Nevertheless, the excessive use of calcium hydroxide may pose challenges in terms of economic and environmental feasibility. Moreover, the proposed mechanisms for the base (enediol intermediate) and Lewis acid (hydride shift between C-2 and C-1) catalysis of galactose were elucidated. Overall, it is crucial to explore novel and effective catalysts as well as integrated systems for isomerizing of galactose to tagatose.

The important role of weak Brønsted acid site of Sn-β in conversion of sucrose to methyl lactate

Sucrose is an abundant, cheap and sustainable substrate for producing high value-added chemicals. In this work, the role of weak Brønsted (B) acid site of Sn-β zeolite was revealed in sucrose conversion to methyl lactate (MLA). Sn-β zeolites were prepared by post-synthesis and hydrothermal synthesis in F‒ media and modified by Mg2+ to tune the B acidity. The acidity of the zeolites was characterized by FT-IR spectroscopy of pyridine/CD3CN adsorption and hydroxyl region. Their catalytic performance for sucrose conversion to MLA in methanol indicates that weak B acid sites of silanols are indispensible for breaking the glycosidic bond of sucrose, so the highest MLA yield was obtained over the postsynthesized Mg-Sn-β-P with moderate amount of weak B acid sites of silanols, strong L acid sites and no strong B acid sites, which juggled the alcoholysis/hydrolysis of sucrose and the retro-aldol of C6 monosaccharide to MLA, and at the same time inhibited the dehydration side-reaction.

Constructing core-shell structured Au/Snβ@mesosilica composite for one-pot base-free conversion of glycerol to methyl lactate

Herein, core-shell structured Snβ@mesosilica (MS) composite was constructed by oriented assembly of mesoporous silica on microporous Snβ zeolite. The mesoporous channels in the shell were verified vertically aligned to the Snβ core, rendering sufficient mass transfer between the shell and core. The resultant Snβ@MS was thereafter employed as the scaffolds for anchoring Au nanoparticles to develop a bifunctional Au/Snβ@MS catalyst for one-pot base-free conversion of glycerol to methyl lactate (ML), which is regarded as a promising and sustainable route to ML production. Under optimum reaction conditions, a remarkable activity with 98.2% glycerol conversion and 91.9% ML selectivity was achieved. More importantly, the introduction of mesoporous silica shell can efficiently modify the Lewis/Brønsted acid ratio of Snβ@MS, thus inhibit the side reactions. While the confinement effects could protect the highly dispersed Au nanoparticles against sintering. Accordingly, a high stability and recyclability was achieved over the Au/Snβ@MS catalyst.

Toward Industrially Relevant Sn-BETA Zeolites: Synthesis, Activity, Stability, and Regeneration

Lewis acidic Snβ zeolites have emerged as highly interesting catalysts for a variety of biomass-related reactions. During the last decades, a vast body of research has focused on unravelling the identity of the (most) active Sn sites. Recent research has shifted its focus from fine-tuning one single type of active Sn-site toward optimizing the overall catalytic activity and stability of the Snβ zeolite, which is more suitable from an industrial point of view. After delving into a discussion of the Sn active sites, this Perspective highlights the recent developments that are essential for industrial implementation of Snβ zeolites, with an emphasis on the most recent insights and findings for improving the catalyst’s productivity and stability.

Efficient Catalytic Conversion of Glucose into Lactic Acid over Y-β and Yb-β Zeolites.

In this work, a new type of modified β zeolites with rare earth elements (ree) was discovered for producing lactic acid from glucose and achieved a good catalytic effect. At first, the catalytic performances of ree-β zeolites, ree oxides, and single-transition-metal-β zeolites were compared, and the result showed that Y-β and Yb-β zeolites had the best catalytic activity under the same reaction conditions. Under the best reaction conditions, the maximum yields of lactic acid with Y-β and Yb-β catalysts were 45.3 and 43.6%, respectively. The acid characterization showed that Y/Yb-β zeolites had a similar number of Lewis acid sites as Sn-β zeolites, and they were also more than other transition-metal-β zeolites. Thus, Y-β and Yb-β zeolites had a higher lactic acid yield than those catalysts. It is interesting to note that Y-β and Yb-β zeolites owned more Brønsted acids but produced fewer byproducts. Combining the decomposition experiment of 5-hydroxymethyl furfural, fewer byproducts were produced with Y-β and Yb-β zeolites because the low amount of Brønsted acid contained could hinder the decomposition of 5-hydroxymethyl furfural, thereby slowing down the side reaction.

Cost-effective and fast synthesis of Sn-β zeolite with less silanol defects for efficient conversion of glucose to methyl lactate

Sn-β with strong Lewis acid sites is considered as the state-of-the-art solid catalyst for converting glucose to alkyl lactate. However, the extensive use of organic template and mineralizing agent and long preparation period in the synthesis of Sn-β brought serious environmental pollution and high synthesis cost. Herein, a cost-effective and fast synthesis strategy using fewer organic template and mineralizing agent, fumed silica as Si source, Mg2+ cation as crystallization promoter and without additional solvent, is proposed to alleviate the aforementioned shortcomings. The results reveal that highly crystalline Mg-Sn-β can be synthesized in 3 days from the gel of 0.005MgO/0.005SnO2/SiO2/0.15TEAOH/0.15NH4F/3.75H2O. Besides accelerating the crystallization, it is found that Mg2+ cation facilitated Sn4+ incorporation into the framework site and lessened the silanol defects of zeolite notably. For conversion of glucose to methyl lactate (MLA), a higher yield of MLA was obtained over 1Mg-Sn-β-3d (54.4%) compared with Sn-β-H-7d (37.2%) and Sn-β-7d (41.3%).

Insights into the mechanochemical synthesis of Sn-β: Solid-state metal incorporation in beta zeolite

Abstract Sn-β zeolite is an active material for the isomerization of glucose to fructose, which is one of the critical reactions for the valorization of biomass. The material is synthesized either by a top-down or bottom-up approach. In this work, we use a top-down approach for the synthesis of Sn-β to incorporate the tin atoms into the *BEA framework. As compared to the literature, we replace the process of manual grinding with the use of ball milling to make the process reproducible, flexible, and scalable. The primary focus of this work is to investigate the processes occurring during the synthesis by a variety of characterization tools. These techniques include thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), physisorption, X-ray diffraction (XRD), and chemisorption monitored by Fourier-transform infrared spectroscopy (FTIR). The synthesis is followed by characterizing the material at various stages of synthesis. Finally, the materials are tested for the isomerization of glucose to fructose to assess the chemical nature of Sn-β zeolites. The results of this investigation provide several insights into the mechanochemical process for the incorporation of atoms in a zeolite framework. For instance, the importance of the size of precursors, distribution of Sn atoms during synthesis, and chemical changes occurring during milling are highlighted. These insights could produce a blueprint for the synthesis of a variety of solid catalysts.

Gem-Diol-Type Intermediate in the Activation of a Ketone on Sn-β Zeolite as Studied by Solid-State NMR Spectroscopy.

Lewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl-containing molecules is of particular importance due to their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surface gem-diol-type species in the activation of acetone on Sn-β zeolite. 13 C, 119 Sn, and 13 C-119 Sn double-resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)3 Sn-OH) on Sn-β is responsible for the formation of the surface species. 13 C MAS NMR experiments together with density functional theory calculations suggest that the gem-diol-type species exhibits high reactivity and can serve as an active intermediate in the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction of acetone with cyclohexanol. The gem-diol-type species offers an energy-preferable pathway for the direct carbon-to-carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl-containing molecules catalyzed by Lewis acid zeolites.

Direct observation of tin sites and their reversible interconversion in zeolites by solid-state NMR spectroscopy

Metal-substituted zeolites are an important type of solid Lewis acid with a wide range of applications. Despite the importance of this type of catalyst, identifying active sites can be challenging because different types of metal sites experience similar environments in zeolites. Here we show direct observation of different tin sites in Sn-β zeolite. Two types of open tin sites are unambiguously identified via correlating the hydroxyl groups to Sn atoms with one- and two-dimensional proton-detected 1H/119Sn correlation solid-state NMR spectroscopy, which only amounts to ca. 17% of the total tin content. A reversible transformation between the open and closed tin site is observed. The results provide valuable insights into the nature of tin sites in Sn-β zeolite and open an avenue for the use of proton-detected solid-state NMR methods for characterization of metal sites in zeolite catalysts.

Hierarchically porous Sn-β zeolites via an OSDA-free synthesis

Hierarchical Sn-β zeolites were synthesized for the first time without using an organic structure directing agent.

Influence of Hydrophilicity on the Snβ‐Catalyzed Baeyer–Villiger Oxidation of Cyclohexanone with Aqueous Hydrogen Peroxide

AbstractSnβ zeolites are amongst the most effective heterogeneous catalysts for the Baeyer–Villiger (BV) oxidation of ketones with aqueous hydrogen peroxide (H2O2). The high selectivity is rooted in the activation of the carbonyl substrate through interaction with the isolated SnIV sites. However, these sites are also accessible to other molecules in the reaction mixture (in particular the co‐solvent and product). In this contribution, we report the impact of competitive adsorption on the Snβ‐catalyzed BV oxidation of cyclohexanone with aqueous H2O2. We furthermore prepared a series of Snβ zeolites with varying amounts of framework silanols and quantified their hydrophilicities with water adsorption and IR experiments. By correlating the results with catalytic data, we show that Snβ zeolites with an intermediate hydrophilicity achieve the highest activity. Our adaptable post‐synthetic synthesis allows us to tune the material synthesis, resulting in enhanced activity compared with conventional hydrothermal Snβ.

One-pot synthesis of 1,3-diaryl but-1-enes from 1-arylethanols over Snβ zeolite

An environmentally benign catalytic protocol has been successfully developed for the one-pot synthesis of 1,3-diaryl but-1-enes from 1-arylethanols via dehydration of 1-arylethanols followed by head-to-tail dimerization of vinylarenes over heterogeneous catalyst (Snβ zeolite). The scope of the reaction was explored for various 1-arylethanols and afforded the corresponding dimers in good to excellent yields with high regio- and stereoselectivity. Moreover, Snβ zeolite could be easily recovered and reused without significant loss in its catalytic activity.

Correlating Synthetic Methods, Morphology, Atomic-Level Structure, and Catalytic Activity of Sn-β Catalysts

Sn-β zeolites prepared using different recipes feature very different catalytic activities for aqueous phase glucose isomerization, suggesting the presence of different active sites. A systematic study of the morphology and atomic-level structure of the materials using DNP NMR spectroscopy in combination with first-principles calculations allows for the discrimination between potential sites and leads to a proposal of specific structural features that are important for high activity. The results indicate that the materials showing the highest activity possess a highly hydrophobic, defect-free zeolite framework. Those materials show so-called closed and associated partially hydrolyzed Sn(IV) sites in the T6 and T5/T7 lattice positions. On the other hand, postsynthetically synthesized Sn-β samples prepared in two steps via dealumination and subsequent solid-state ion exchange from Al-β show significantly lower activity, which is associated with a hydrophilic framework and/or a lower accessibility and differ...

ChemInform Abstract: Chemo‐ and Regioselective Head‐to‐Tail Heterodimerization of Vinylarenes with 1,1‐Diphenylethene over a Heterogeneous Catalyst (Snβ Zeolite).

AbstractAlkenes bearing strong electron‐withdrawing groups and 1‐octene are not applicable as substrates.

Heterodimerization of Vinylarenes with 1,1-Diphenylethene by Snβ Zeolite

Heterodimerization of Vinylarenes with 1,1-Diphenylethene by Snβ Zeolite

Chemo- and regioselective head-to-tail heterodimerization of vinylarenes with 1,1-diphenylethene over a heterogeneous catalyst (Snβ zeolite)

Highly chemo- and regioselective heterodimerization of vinylarenes with 1,1-diphenylethene to form the corresponding alkenes using heterogeneous catalyst (Snβ zeolite) is reported for the first time.

Silica‐Grafted SnIV Catalysts in Hydrogen‐Transfer Reactions

AbstractSnIV‐substituted zeolites show remarkable activity for important Lewis acid catalyzed reactions. In such complex catalytic systems it is, however, difficult to untangle the influence of all parameters contributing to the overall performance, and hence to rationally improve the catalyst. In this work, we studied silica‐grafted SnIV sites to reduce this complexity by eliminating the potential influence of pore confinement. The surface‐anchored SnIV sites were modified by calcination and silylation to increase Lewis acidity and hydrophobicity. Various techniques were used to characterize the materials, including the adsorption of cyclohexanone by using FTIR spectroscopy. We relate our spectroscopic results with catalytic tests and compare our model catalysts with benchmark Snβ zeolite. This suggested that besides active‐site structure, also hydrophobicity and confinement effects influence the activity. Our work demonstrates the utility of model systems to separate the different contributions to catalytic activity.

Zeolite-catalyzed synthesis of 2,3-unsubstituted benzo[b]furans via the intramolecular cyclization of 2-aryloxyacetaldehyde acetals

An efficient and environmentally benign heterogeneous catalytic process for the synthesis of 2,3-unsubstituted benzo[b]furans has been established via the intramolecular cyclization of 2-aryloxyacetaldehyde acetals. By utilizing tin-exchanged H-β zeolite (Sn-β) as catalyst, a wide range of functionalized 2,3-unsubstituted benzo[b]furans could be prepared in good to excellent yields. The Sn-β zeolite catalyst also exhibited excellent shape selectivity on the cyclization of meta-substituted 2-aryloxyacetaldehyde acetals, and 6-substituted isomers were preferably formed up to 97% regio-selectivity. Moreover, Sn-β zeolite could be easily recovered and reused without any noticeable activity loss.

Cooperative Catalysis for Multistep Biomass Conversion with Sn/Al Beta Zeolite

Lewis acid Snβ-type zeolites with varying amounts of Bronsted acid Al in the framework were synthesized using a simple two-step procedure comprising partial dealumination of β zeolite under action of acid, followed by grafting with SnCl4·5H2O in dry isopropanol. Characterization of the thus-prepared Al-containing Snβ (Sn/pDeAlβ) zeolites with ICP, (pyridine probed) FTIR, and 27Al MAS NMR demonstrates the presence of Bronsted acid framework AlIII. Tetrahedral Lewis acidic SnIV is present, as ascertained by a combination of techniques such as EPMA, 119Sn Moβbauer, XPS, (pyridine probed) FTIR, and UV–vis. A closed SnIV configuration was implied by comparing of 119Sn solid-state MAS NMR and deuterated acetonitrile probed FTIR spectra with literature. The catalytic activity of the Al-containing Snβ was tested for the conversion of 1,3-dihydroxyacetone (DHA) into ethyl lactate (ELA), proceeding via pyruvic aldehyde (PAL). Despite the difference in synthesis between the classic hydrothermal Snβ reference and Sn/...

NMR signatures of the active sites in Sn-β zeolite.

Dynamic nuclear polarization surface enhanced NMR (DNP-SENS), Mössbauer spectroscopy, and computational chemistry were combined to obtain structural information on the active-site speciation in Sn-β zeolite. This approach unambiguously shows the presence of framework Sn(IV)-active sites in an octahedral environment, which probably correspond to so-called open and closed sites, respectively (namely, tin bound to three or four siloxy groups of the zeolite framework).