In recent years, the development of chemical reaction processes utilizing plant resources instead of petroleum resources has been actively carried out, with attention also being given to the development and application of novel production processes for sugar alcohols derived from cellulose have been paid attention. One example of an application is a process to obtain a starting material for a useful chemical substance by dehydrating a sugar alcohol. For example, sorbitol or mannitol, each being a sugar alcohol, is obtained by the hydrolysis and hydrogenation of cellulose. Likewise, mannitol can also be obtained by hydrolysis and hydrogenation of hemicellulose that occupies about ¼ of a biomass component (FIG. 1). Alternatively, mannitol can be obtained in a high yield not only from hemicellulose but also from cellulose by adjusting cellulose hydrolysis conditions. When mannitol obtained from biomass in a large amount as above is dehydrated, isomannide (useful as a starting material for functional plastics and pharmaceuticals) and 2,5-sorbitan having wide use applications as an emulsifying agent or a surfactant for food, pharmaceuticals and cosmetics are obtained.
As shown in FIG. 1, although the reaction to obtain 2,5-sorbitan from mannitol proceeds as a one-stage dehydration reaction, the process to obtain isomannide consists of a two-stage dehydration reaction. Since mannitol has 6 hydroxyl groups in a molecule, a plurality of isomers are produced depending on the position of the dehydrated hydroxyl group. 2,5-sorbitan is produced by selecting a reaction condition, and isomannide can be produced through an intermediate 1,4-mannitan by adjusting the reaction condition to a different condition. There are many different kinds of side reactions as described above, and therefore, in order to increase the yield of a specific target substance, development of a catalyst having high selectivity and development of a chemical reaction control technique using said catalyst are required.
For example, Patent Literature 1 discloses a method for obtaining isomannide by dehydrating mannitol in the presence of an acid catalyst that is a homogeneous catalyst, such as sulfuric acid or hydrogen chloride, as a conventional method. If such a homogeneous catalyst is used, a step of removing the acid from a reaction product becomes complicated, the cost of handling equipment for using a liquid or gaseous strong acid increases, and the burden of safety also becomes a big problem.
Patent Literature 2, Example 45 discloses a method for obtaining isomannide from mannitol using, as a catalyst, water-resistant Lewis acid such as bismuth(III) triflate and, according to this, isomannide is obtained in a yield of 61% at a temperature of about 160° C. and under a pressure of 20 torr. However, there are environmental and safety problems because a heavy metal such as a bismuth compound is used. Moreover, even if the catalyst is solid in an unused state, it is dissolved in mannitol during the reaction, and therefore, it is difficult to separate and remove the catalyst from the final product. On that account, a solid catalyst that is easy to recover and that can be reused is strongly desired.
Non-Patent literature 1 describes that mannitol can be converted into 2,5-sorbitan and isomannide by hydrothermal reaction in water at 250° C. for 30 hours under noncatalytic conditions. Although this method is preferable from the viewpoint that a catalyst such as toxic gas is not used, it requires a high temperature and long-term reaction, and not only is the production efficiency low but the energy efficiency is also low, so that it is hard to say that this method is practical.
If dehydration reaction of mannitol is to be carried out in water, the equilibrium shifts to the reverse reaction because of the water, and therefore, the yield is generally lowered. Even if use of an organic solvent other than water is attempted to improve it, a high temperature and a high pressure are required, the cost of the reactor increases, and in addition, such a method is undesirable also from the viewpoints of environmental preservation and safety.
The method for producing the corresponding anhydrosugar from mannitol, described in the Patent or Non Patent Literature mentioned above, is assumed to synthesize a single target substance (e.g. isomannide) from a starting material. However, these production facilities are expensive, and mass production requires a large amount of facility investment. If different useful substances can be synthesized from the same starting material under similar reaction conditions by simply changing a catalyst, the initial investment for the production facilities to produce them can be reduced, so that such synthesis is preferable.
As described above, sulfuric acid, toxic gas catalysts such as hydrogen chloride, and heavy metal complex catalysts such as bismuth are conventionally known as catalysts for the dehydration reaction of mannitol. In actuality, any catalyst having overcome the defects of the conventional catalysts, that is, any catalyst which can be easily separated and removed from a product, is excellent in handling safety and recycling properties, can carry out a reaction at a mild temperature and atmospheric or close to atmospheric pressure, and provides a high reaction yield for a short period of time, has not been realized yet, and achieving such a catalyst has been desired.