Sn-Beta Catalyzed Transformations of Sugars: Comparison
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Beta zeolite modified with Sn in the framework (Sn-Beta) was synthesized and introduced as a heterogeneous catalyst for Baeyer–Villiger oxidations about twenty years ago. Since then, both syntheses strategies, characterization and understanding as well as applications with the material have developed significantly. Remarkably, Sn-Beta zeolite has been discovered to exhibit unprecedented high catalytic efficiency for the transformation of glucose to fructose (i.e., aldoses to ketoses) and lactic acid derivatives in both aqueous and alcoholic media.

  • Sn-Beta zeolite
  • bottom-up/top-down syntheses
  • sugars transformations
  • isomerization
  • dehydration
  • fragmentation
  • furan and lactate derivatives

1. Introduction

An ambition towards accomplishing carbon recycling and CO2 neutrality drives the utilization of fossil fuel-based energy towards replacement with green sustainable technologies. However, existing infrastructures still require hydrocarbon compounds to produce a wide range of products and materials [1]. Lignocellulosic biomass is an abundant carbon-neutral resource which is frequently considered renewable base for the syntheses of sustainable chemicals, polymers, and fuels [2][3]. Sugars and derived polymers, such as glucose, xylose, sucrose and cellulose, are major constituents of lignocellulose (60–75 wt.%), which can be converted into 5-hydroxymethylfurfural (HMF), furfural (FF), levulinic acid, and other value-added chemicals through benign catalytic processes [4][5][6][7].
Zeolites (aluminum silicates) have unique material properties such as, e.g., open crystal frame structures, pore configuration, and high surface area [8]. These properties facilitate the adsorption and transportation of reactants in catalytic transformations, and especially Lewis acidic derived materials (often referred as zeotype materials) provide excellent performance as heterogeneous catalysts in many sugar transformation reactions [9][10]. In particular, the Davis group was the first to report in 2010 that framework Lewis acidic Sn sites in Sn-Beta exhibit high activity for glucose isomerization to fructose in aqueous media [11]. Concurrently, Taarning and coworkers further discovered that Sn-Beta facilitated be conversion of sugars to lactic acid derivatives with high efficiency [12].

2. Syntheses of Beta Zeolite

The synthesis of unmodified high-silica Beta zeolite (Al-Beta) was firstly reported in 1967 by Wadlinger et al. from Mobil Oil Corp [13] using tetraethylammonium cations as template under hydrothermal treatment for 3 to 60 days. However, the structure of the zeolite remained unclear for a long time, and only in 1988 was the three-dimensional 12-ring framework structure determined by Higgins et al. [14] using a combination of model building, distance-least-squares refinement, and powder pattern simulation. Subsequently, van der Waal et al. [15] demonstrated in 1994 that gel seeds of boron-containing Beta (demetallized by acid treatment) could promote the synthesis of all-silica Beta zeolite.

2.1. Bottom-Up Approaches

2.1.1. Seed-Assisted Synthesis

Mal and Ramaswamy [16] were in 1997 the first to report the synthesis of large-pore Sn-Beta and Al-free Sn-Beta zeolites under hydrothermal conditions. The synthesis procedure followed a three-step route where Al/Sn-Beta zeolite (Si/Sn = 78.8, Si/Al = 28.5) was initially synthesized with the assistance of Al3+ ions in basic media (142 °C, 10 days). Then, the Al/Sn-Beta zeolite was refluxed in acid (110 °C, 3 h) to obtain dealuminated Sn-Beta zeolite (deAl-Sn-Beta; Si/Sn = 85.4, Si/Al > 3000), which was used as seeds to synthesize Al-lean Sn-Beta zeolite (Si/Sn = 54.5, Si/Al > 4000) with a moderate crystallinity of 87%. Although the Sn-Beta zeolite was successfully synthesized under the hydrothermal condition trace Al3+ in the framework structure easily forms Brønsted acidic sites, which can affect catalytic performance and product selectivity.

2.1.2. Dry-Gel Conversion Techniques

To shorten the crystallization time during Sn-Beta synthesis, Kang et al. [17] adopted a simple steam-assisted conversion (SAC) method where dry stannosilicate gel was successfully converted to highly crystalline Sn-Beta (Si/Sn = 93) zeolite within 5 h under 180 °C. Dry-gel conversion techniques such as SAC are much faster and more convenient than conventional hydrothermal procedures, but also in SAC do the content of Sn significantly affects synthesis with higher Sn loading resulting in longer crystallization time and lower crystallinity. When using stannosilicate dry gel in SAC with Si/Sn ≤ 75, it has proved impossible to obtain Sn-Beta even after 200 h of crystallization.

2.1.3. Inter-Zeolite Transformations

Hydrothermal synthesis of Sn-Beta zeolites with high Sn content remains an eminent target in literature. Iida et al. [18] obtained mixed Sn-Si oxide composites by mechanochemical treatment and used them as precursor for hydrothermal synthesis of Sn-Beta zeolite at 170 °C under static conditions for 4 to 8 days. The strategy allowed successful incorporation of framework Sn into Sn-Beta with a relatively high Sn content (Si/Sn = 60, 3.17 wt.%). Following an analogous mechano-chemical route and combining it with inter-zeolite transformation, Zhu et al. [19] also obtained highly crystalline Sn-Beta zeolite with a relatively high Sn content (Si/Sn = 63, 3.03 wt.%) but within 3 days. Here, all-silica MWW-type crystals (ITQ-1 zeolite) were the silica source and deAl-Beta was introduced as the seeds. According to the crystallization mechanism, the framework similarity between the MWW crystals and deAl-Beta seeds was a key factor for the successful synthesis of the Sn-Beta zeolite.

2.2. Top-Down Approaches

Atom-planting is a post-synthetic strategy exploited to introduce Lewis acidic metals into zeolitic matrices. In the general procedure, Sn-Beta zeolites (SiO2/Al2O3 ≈ 1900) have been produced by dealumination of Al-Beta with concentrated nitric acid (65 wt.%, 100 °C, 20 h, 20 mL/g) to create deAl-Beta with framework vacancies where Sn atoms were then inserted. Acid-induced dealumination was preferred instead of steaming to remove the framework Al as the latter method can also create ill-defined extra-framework Lewis acidic Al sites with undesired catalytic performance. The Sn insertion routes included both traditional gas-solid deposition (e.g., chemical vapor deposition), solid-state ion (SSI) exchange as well as liquid-phase routes (e.g., impregnation).

3. Catalytic Transformations of Sugars with Sn-Beta Zeolite

3.1. Sugar Isomerization

Sn-containing zeolites exhibit pronounced performance in the catalytic isomerization of sugars with respect to both conversion and selectivity. Generally, the ratio of Lewis and Brønsted acid sites over zeolites can be adjusted by different methods such as steam/thermal treatment, element doping, and acid/base etching [20], which is directly correlated with the product distribution. Incorporation of Sn into the framework increases the Lewis acidity of zeolites, which has been reported to be efficient for the isomerization of C3-C6 sugars such as glucose, xylose, erythrose, and dihydroxyacetone.

3.2. Sugar Dehydration

In the transformation of lignocellulosic biomass to furans derivatives, the Lewis acidic Sn sites in Sn-Beta zeolite efficiently isomerize aldoses to ketoses (see previous section), and the consecutive dehydration of ketoses to the furans derivatives are catalyzed by Brønsted acidic sites. The introduction of Brønsted acidic sites may be obtained by the direct addition of homogeneous or heterogeneous Brønsted acid into the reaction system or, though more challenging to realize, by incorporating Brønsted acidic sites into the Sn-Beta zeolite framework. Main focus in the literature for both of these approaches have been on the transformation of sugars into mainly HMF and FF. Sn-Beta zeolite exhibits excellent thermal and chemical stability under steam conditions so protic acid such as hydrochloric acid (HCl) can be added directly as homogeneous Brønsted into sugar solutions to facilitate dehydration. Highly efficient transformation of glucose into HMF has been achieved with high selectivity using HCl in one-pot biphasic water/tetrahydrofuran (THF) reaction system [21][22]. The HCl may also be applied for the hydrolysis of polysaccharides, cellobiose or starch into monomeric sugars and subsequently catalyze the dehydration of fructose into HMF. Here, a higher yield will be obtained at low pH and saturated aqueous salt solutions. Analogously, a combination of Sn-Beta zeolite and HCl can catalyze the isomerization and dehydration of xylose (C5 aldose) into FF in one-pot aqueous solution with a FF yield of 14.3% at a much lower temperature (110 °C) than in previous reports (>150 °C) [23]. The introduction of solid Brønsted acid catalysts such as resins can replace the homogeneous acid and improve catalyst recyclability and regenerability. Such system combining Sn-Beta zeolite with Amberlyst® 70 has been shown to efficiently transform glucose to HMF in monophasic reaction systems with good yields using solvents such as GVL (59%), GHL (55%), 1:1 THF:MTHF (60%), and THF (63%), respectively [24]. The solid Brønsted acid PTSA-POM (made from the copolymerization of p-toluenesulfonic acid, PTSA, and paraformaldehyde, POM) has also been introduced for the transformation of glucose to HMF due to its low cost, ease of preparation, and high Brønsted acidity. The combination of Sn-Beta and PTSA-POM achieved a good HMF yield of 60.1% at a glucose conversion of 96.3% in GVL/H2O reaction system [25]. The presence of water significantly affected the reactivity with 10 wt.% water providing the best result. Alternatively, alcoholic solvents have also been used often as reaction media and they directly participate in the reaction by etherification of the furans in the presence of Brønsted acid. Hence, employing an ethanolic system with Amberlyst® 131 as the Brønsted acid in combination with Sn-Beta zeolite successfully converted glucose to 5-(ethoxymethyl)furfural (EMF) in a single reactor at 90 °C [26]. The combined catalytic system proved more stable in dioxane and the introduction of small amounts of water significantly increased the selectivity of HMF instead of EMF [27]. Subsequent studies also indicated that the presence of more Brønsted acid sites in Amberlyst® 15 and an appropriate amount of water in organic solvents were beneficial for the transformation of glucose to HMF and 5-(isopropoxymethyl)furfural (IMF) in 53% yield [26]. In comparison to the combined approaches, the introduction of the Brønsted acidic sites directly into the framework structure of Sn-Beta zeolite may possess advantages for sugar dehydration. When deAl-Beta zeolite is applied as seed for the synthesis of Sn-Beta traces of Brønsted acidic Al3+ sites will be preserved, which have a negative effect on the isomerization of glucose to fructose (see previous sections). Nevertheless, the combination of such Al3+ sites and Lewis acidic Sn4+ sites in Al/Sn-Beta zeolite with a suitable Sn/Al molar ratio have been reported to catalyze the one pot conversion of glucose to HMF with a conversion of 60.0% and HMF selectivity of 62.1% under optimized conditions [28]. Also, the physical pore structure of Al/Sn-Beta zeolite will affect the catalytic reactivity due to limited diffusion of reactants and products. Here, the hierarchical Sn-Beta was synthesized from hydrothermally synthesized hierarchical Al-Beta (SDA: polydiallyldimethylammonium chloride) by exposure to SnCl4 vapor (i.e., gas-solid deposition; see previous section), and the resulting catalysts displayed good activity and afforded both higher glucose conversion (99%) and HMF selectivity (42%) than those of conventional Sn-Beta (96% and 32%, respectively) [29]. Similar catalytic performance has also been obtained at lower reaction temperature at prolonged reaction time with analogously in-situ synthesized Sn-Beta zeolites using diquaternary ammonium as the SDA [30].

3.3. Sugar Fragmentation

Lactic acid/lactate salts and derived esters are chemical platform compounds with high potential and multifunctionality, which are widely used in food, pharmaceuticals, cosmetics and as biodegradable polymers and renewable solvents [31]. The fermentative conversion of biomass-derived sugars to lactic acid is the normal approach on a large scale. However, since Holm et al. [12] described that Sn-Beta zeolite could efficiently and direct catalyze common sugars to methyl lactate a lot of work have focused on these chemocatalytic processes and have been summarized several times [5][9][31][32][33][34][35]. The formation of lactic acid (and its derivatives) from sugars such as glucose comprise a series of reactions concerted by the Sn sites in Sn-Beta zeolites, including: (1) glucose isomerization into fructose; (2) fructose degradation into DHA and GA via retro-aldol condensation (GA can be isomerized into DHA via Sn-Beta); and (3) DHA dehydration to form the lactic acid with pyruvaldehyde as intermediate [36][37]. The synthesis parameters used to prepare Sn-Beta can significantly affect its zeolitic structure and reactivity for such conversion [38]. For example, during hydrothermal synthesis a low water amount can shorten the crystallization time but create extra-framework SnO2 and reduce the incorporation of Sn4+ into framework sites, which results in lower catalytic activity. On the other hand, a longer crystallization time will incorporate more Sn4+ species into the framework sites and generate more Lewis acid sites, though open Sn sites with a proximal silanol are unfavorable for the conversion of glucose to methyl lactate. Similar to sugar dehydration, hierarchical structures of Sn-Beta are also beneficial for obtaining higher yield of lactic acid derivatives from sugars [39][40]. Hence, hierarchical Sn-Beta zeolite synthesized with polydiallyldimethylammonium chloride as SDA gave a higher catalytic activity than that of microporous Sn-Beta and post-synthesized meso-Sn-Beta zeolites for glucose conversion, due to a combined promotional effect of mesoporosity, hydrophobicity and Lewis acidity on the retro-aldol condensation of fructose [41]. In addition, to enhance the efficiency of the catalytic transformation of glucose into lactic acid microwave irradiation was introduced for the controlled-isomerization of glucose, facilitating a very high lactic acid yield of 68.3 wt.% at 180 °C in 30 min over a “monolithic” Sn-Beta catalyst in water [42].

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