Process for making isoidide

Abstract

A process is provided for making isoidide, wherein a dextrose feed is first converted by a combination of epimerizaton and hydrogenation to an iditol-enriched mixture of hexitols in the presence of hydrogen and in the further presence of a single, dual purpose catalyst, and the iditol is then conventionally fully dehydrated to provide isoidide. In one general embodiment, the dehydration is performed on the iditol-enriched mixture of hexitols to provide an isohexides product mixture including isoidide, and the isoidide is recovered therefrom. In another general embodiment, iditol is separated from the iditol-enriched mixture of hexitols, and at least a portion of this iditol is then dehydrated to provide an isoidide product.

Description

TECHNICAL FIELD[0001]The present invention broadly relates to the synthesis of sugar alcohols, and in particular, to the synthesis of hexitols such as sorbitol, mannitol and iditol from the corresponding hexose sugars such as dextrose, and to the subsequent production of the corresponding isohexides from such hexitols.BACKGROUND ART[0002]The Value of Isoidide and Previous Methods for Making Via Iditol:[0003]There are fundamentally three isohexides: isomannide, isoidide, and isosorbide. Isosorbide is a commercially-produced, bicyclic diol that is made by first hydrogenating dextrose to sorbitol, then double dehydrating the sorbitol to yield isosorbide. Its double hydroxyl function would make isosorbide of interest as a building block for polymerization. However, the making of polymers of suitable properties from isosorbide is hampered by the molecule's stereochemistry, with one endo hydroxyl group and one exo hydroxyl group, resulting in asymmetrical reactivity and amorphous polymers...

AI Technical Summary

Problems solved by technology

However, the making of polymers of suitable properties from isosorbide is hampered by the molecule's stereochemistry, with one endo hydroxyl group and one exo hydroxyl group, resulting in asymmetrical reactivity and amorphous polymers (due to the lack of symmetry).

Its epimer isomannide, which has two endo hydroxyl groups, has proven to be unfavorable for polymerization due to low reactivity and low linearity.

Isoidide, however, is not currently manufactured on a commercial scale, in part (but not exclusively) because of the high cost of the synthetic precursor iditol from which isoidide might be made by an analogous double dehydration pathway as employed for making isosorbide.

The hydrogenation of L-idose is mentioned briefly as a possible method, but as Fuertes notes L-idose is not a naturally-occurring sugar that can be economically recovered from plants in the manner of other sugars such as dextrose.

Both of these pathways are said to prove iditol in “noticeable amounts”, however, they are also dismissed as commercially impractical because of poor purity and poor yield considerations due to the appearance or the persistence in the reaction medium of other polyols which are indicated as separated from the desired iditol product only with considerable difficulty.

2, lines 39-41; one method in this regard was said to provide 50 grams / liter of iditol from 150 g / l of L-sorbose in five days (with no remaining L-sorbose), but direct crystallization of the iditol from the fermentation medium was termed “impossible”, while the second method reviewed was described as “extremely complex and delicate” and as providing no teaching regarding recovery of the iditol from the fermentation medium.

One perceived challenge to the practical commercial fulfillment of this alternative pathway has been the difficulty, in light of the very high purities demanded by polymer manufacturers for monomer feedstocks, of producing a highly pure isosorbide feed for being epimerized to in turn produce the needed very high purity isoidide.

In particular, sorbitans are the primary byproducts in the isosorbide synthesis from sorbitol, and have been recognized as causing significant problems with color development and isosorbide degradation over time.

A further perceived challenge to the realization of a commercial process for producing isoidide via epimerization from isosorbide has come from the fact that even when highly pure isosorbide is obtained and used in an epimerization process as originally described by Le Notre et al. in their journal article, a set of byproducts are nevertheless formed that with the isomannide and isosorbide epimers pose still further difficulties for obtaining the ultimately desired monomer grade purity isoidide product.

Yet another challenge to an epimerization-based method for making isoidide from isosorbide relates to the separation of the isomeric products isosorbide, isoidide and isomannide from the crude epimerization product mixture.

The separation of these isomers has generally been by fractional distillation; while distillation is effective to a certain extent, the isohexides have relatively close boiling points at elevated temperatures and reduced pressures, resulting in added cost and complexity for carrying out the separation by distillation alone.

Images