(631e) Synthesis of Methyl Levulinate from Glucose Using Solid Acid Catalyst

1. Introduction Levulinic acid or alkyl levulinate is one of the important chemicals, which can be an intermediate of many types of chemical products. The production method of levulinic acid is, for example, treating saccharide with mineral acids. However, the recovery of levulinic acid after using homogeneous catalyst is generally difficult, and there are few reports about recovery or further purification of produced levulinic acid. In this study, the production method of methyl levulinate using solid acid catalyst was examined. Furthermore, the purification of methyl levulinate and the reusability of recovered catalyst and methanol was examined as well.

2. Experimental method Glucose was used as a saccharide material. 4 types of zeolites consist of different SiO₂/Al₂O₃ ratio (Name: Cation type: SiO₂/Al₂O₃ = HS-320: NH₄⁺: 6, HS-341: H⁺: 7, HS-500: K⁺: 5.5, HS-690: H⁺: 200) were used as solid acid catalysts after the calcination under air at 550°C for 6 h. A certain amount of glucose, catalyst, and methanol were sealed in a batch reactor, and then plunged into preheated oil bath. After the reaction, a reactor was cooled in a water bath, and then products were recovered by vacuum filtration. A recovered catalyst was again calcined under the same condition for reuse. A filtrate was distilled at 80°C and then at 120°C for the recovery of methanol and the purification of the product.

3. Results and discussion

3.1 Conversion of glucose into methyl levulinate using zeolite 0.2 g of glucose, 0.1-1.0 g of zeolite, and 10 mL of methanol were reacted at 80-160°C for 1 h. The conversion pathways of glucose into methyl levulinate consist of the isomerization of glucose to fructose, dehydration of fructose to 5-hydroxymethylfurfural, hydration of 5-hydroxymethylfurfural to levulinic acid, and at any time intermediates form dehydration products of methyl-intermediates. When 0.1 g of HS-320 was used at 120°C, fructose was obtained at the yield of 42mol% and the selectivity of 78%. This yield was close to glucose-fructose equilibrium which indicated that HS-320 was an effective catalyst for glucose isomerization. Lewis acidity of zeolite is expressed by the cleavage of Si-Al bonds, therefore, Lewis acidity of zeolite is considered to be stronger as its SiO₂/Al₂O₃ ratio is closer to 1. Isomerization of glucose to fructose is generally promoted by Lewis acid, therefore, HS-320 whose SiO₂/Al₂O₃ ratio was the closest to 1 among examined catalysts resulted in the best yield of fructose. The best yield of methyl levulinate among the examined condition was 54mol% which was presented by 0.5 g of HS-341 via 4 h of the reaction at 160°C.

3.2 Conversion of glucose into methyl levulinate using several types of zeolites To increase the yield of methyl levulinate, the use of several types of zeolites was examined. Firstly, sequential reaction of isomerization using HS-320 and conversion using HS-341 was examined (â??2-steps synthesisâ?). As the 1st step, 0.2 g of glucose was treated with 0.1 g of HS-320 and 10 mL of methanol at 120°C for 1 h, and then as the 2nd step, HS-320 was exchanged with 0.5 g of HS-341 and treated at 160°C for 0.5-4 h. Secondly, simultaneous reaction of isomerization using HS-320 and conversion using HS-341 was examined (â??one-pot synthesisâ?). 0.2 g of glucose was treated with 0.5 g of blended catalyst and 10 mL of methanol at 160°C for 0.5-4 h. The blended catalyst consisted of 0.1 g of HS-320 and 0.4 g of HS-341. Through â??2-step synthesisâ?, the yield of methyl levulinate was lower than that obtained using only HS-341 at any reaction time. This is probably because of the material loss at the 1st step as byproducts. On the other hand, the yield was slightly increased by â??one-pot synthesisâ?, which resulted in the yield of 58mol% after 4 h of the reaction.

3.3 Recovery and reuse of catalyst

 During the reaction at 160°C for 4 h, the catalyst HS-341 changed its color from white into brown. This was because undesirable byproduct of humin was adsorbed on the surface of the catalyst. The brown catalyst turned back into white after re-calcination, however, the yield of methyl levulinate changed from 54mol% to 45, 41, 34mol% after 1st, 2nd, 3rd time of reuse, respectively. For the reuse of the catalyst, therefore, it was implied that the further examination of the calcination condition which could only remove humins was indispensable.

3.4 Purification of methyl levulinate and reusability of recovered methanol Filtrate obtained after the reaction at 160°C for 4 h with HS-341 contained 1wt% of methyl levulinate. Through the simple distillations at 80°C and 120°C, colorless transparent liquid which was considered as methanol was obtained, and the concentration of methyl levulinate increased to 33wt%. The obtained clear liquid through the distillation at 80°C was then used as a reaction medium. 0.2 g of glucose was treated with 0.5 g of HS-341 and 10 mL of obtained liquid at 160°C for 4 h. As the result, 51mol% of glucose was converted into methyl levulinate. The yield decreased by 3mol% comparing with the yield using pure methanol, however, the decrease was not considerable. Therefore, it could be concluded that some amounts of methanol was reusable in this process.

4. Conclusions Glucose was converted into methyl levulinate using solid acid catalyst at the yield of 58mol%. The possibility of reuse of the catalyst was indicated. The purification of the product was indicated to be possible through the simple distillation, and the reusability of methanol was confirmed.