{'fraction_non_alphanumeric': 0.023335621139327384, 'fraction_numerical': 0.008236101578586136, 'mean_word_length': 5.505952380952381, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Zeolite Catalyzed Transformation of Carbohydrates to Alkyl Levulinates

The carbon-based chemicals and fuels that are necessary to meet the energy demand for our society originate presently almost exclusively from inexpensive fossil resources. The forecast of diminishing and more expensive petroleum reserves has, however, engaged the chemical industry to find new feasible routes to convert biomass into potential platform chemicals, that can replace the fossil based chemicals in order to leave the chemical supply chain unaffected (value chain). In biomass, the major source of renewable carbon is confined in lignocellulose, but selective hydrolysis combined with efficient separation of the lignocellulosic fractions; lignin, cellulose, and hemicellulose, remain a technical challenging task. In particular, the direct conversion of lignocellulosic biomass to potential chemicals with high selectivity provides a bottleneck, owing to the recalcitrant feedstock. Conversion of the individual monomeric components in lignocellulose to chemicals is an alternative way to make platform molecules and improve the yield of the targeted product. 7] Consequently, we have focused on how levulinic acid esters can be directly derived from mono-, di-, and polysaccharides using heterogeneous catalysis. Levulinic acid has been listed as one of the twelve most important sugar-based building blocks obtainable from biomass, with a variety of potential applications as solvent, food flavoring agent, plasticizer, resin intermediate, and building blocks for, for example, tetrahydrofuran and succinic acid. To produce levulinic acid, carbohydrates are traditionally being treated with aqueous mineral acid (H2SO4 and HCl). [10] However, a major drawback in this homogeneous process is tedious work-up during the separation stages. Previously, few solid-acid catalysts have been investigated for the conversion of glucose/cellulose to levulinic acid in water. Yand Beta-type zeolites gave 10–40 % yield of levulinic acid from glucose/fructose/cellulose in water at temperatures between 110–240 8C. Despite the many advantages of zeolites, they are not hydrothermally stable in water during the reaction, which leads to a loss of structural integrity and can, therefore, not be recycled. Alternatively, Schraufnagel and Rase have reported that sucrose can be transformed into levulinic acid in water using acidic ion-exchange resins as catalysts at temperatures between 100 and 140 8C. However, owing to thermal instability of the resin at elevated temperatures, moderate yield of activity (25 % levulinic acid), and difficulty in removing humin by-products from the catalyst under mild conditions, they are unattractive for this transformation. A quantitative conversion of glucose was observed over Fe-pillared montmorillonite catalyst, but the selectivity to levulinic acid was only 20 % and a large amount of humins were also observed in this study. Recently, Peng et al. reported that glucose can be directly converted to ethyl levulinate (ELevu) over sulfated zirconia catalysts with a moderate yield up to 30 % at 200 8C. Furthermore, we found that sulfonic acid functionalized ionic liquids and solid Brønsted acids are more promising catalysts for the conversion of fructose to ELevu in ethanol than for the analogous conversion to levulinic acid in water. Here we introduce zeolites as catalysts for the conversion of carbohydrates to alkyl levulinates in alcohol medium to enhance the yield as well as to circumvent the drawbacks of hydrothermal and thermal instability of the catalysts. Aluminum-free Lewis acidic zeotype materials and mesoporous molecular sieves containing Sn, Ti, or Zr are capable of isomerizing hexoses, pentoses and trioses and converting them into useful chemicals. 16] Interestingly, we observed in the present study that glucose can efficiently isomerize to form fructose in alcohol media over zeolites without auxiliary Lewis acid metals (such as Sn, Ti, or Zr) other than aluminum. Under optimized reaction conditions, the transformation of glucose to methyl levulinate (MLevu) in methanol over various commercially available zeolites has been performed and the results are presented in Table 1. All of the experiments were performed with quantitative conversion of glucose and other monoand disaccharides causing the reported yields and se-