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 Al
3+ sites will be preserved, which have a negative effect on the isomerization of glucose to fructose (see previous sections). Nevertheless, the combination of such Al
3+ sites and Lewis acidic Sn
4+ 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 Sn
4+ into framework sites, which results in lower catalytic activity. On the other hand, a longer crystallization time will incorporate more Sn
4+ 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].