WO2021032519A1

WO2021032519A1 - Method for purification of isosorbide

Info

Publication number: WO2021032519A1
Application number: PCT/EP2020/072356
Authority: WO
Inventors: Feelly RUETHER; Rene Backes; Claudia STOER; Svenia Duefert; Holger WERHAN; Steffen Mader; Benjamin Bernhard KUEHNE
Original assignee: Basf Se
Priority date: 2019-08-21
Filing date: 2020-08-10
Publication date: 2021-02-25

Abstract

The invention relates to a process for purifying a crude isosorbide, in which the crude isosorbide is melted and converted, by cooling, into a crude isosorbide melt suspension consisting of isosorbide crystals and residual melt, the amount by weight of impurities in the isosorbide crystals being less than the amount by weight of impurities in the residual melt, optionally a part of the residual melt is separated off mechanically from the crude isosorbide suspension, further the isosorbide crystals in the melt isosorbide suspension are purified from residual melt by washing with a washing isosorbide melt, the amount by weight of impurities in the washing isosorbide melt being less than the amount by weight of impurities in the residual melt.

Description

Method for purification of Isosorbide

Description

The present invention relates to a process for purifying a crude isosorbide, in which the crude isosorbide is melted and converted, by cooling, into a crude iso sorbide melt suspension consisting of isosorbide crystals and residual melt, the amount by weight of impurities in the isosorbide crystals being less than the amount by weight of impurities in the residual melt, optionally a part of the residual melt is separated off mechanically from the crude isosorbide melt suspension, further the iso sorbide crystals in the crude melt isosorbide suspension are purified from residual melt by washing with a washing isosorbide melt, the amount by weight of impurities in the washing isosorbide melt being less than the amount by weight of impurities in the re sidual melt.

Isosorbide is especially important for the preparation of polymers for a very wide range of applications (electronic materials, optical devices, glazing, LED covers, bottle raw materials, interior and exterior automotive parts) as well as for cosmetic and pharmaceutical preparations. Furthermore, Isosorbide may be used for the prepara tion of polycarbonates, polyesters, polyurethanes, polysulfones and epoxides.

Isosorbide is obtainable from glucose by hydrogenation which gives sorbitol and sub sequent acid-catalysed dehydration of sorbitol to isosorbide via 1,4 sorbitan as inter mediate (WO2015/112389). However, the obtained isosorbide possesses not the puri ty required for the above described applications. Other components, apart from iso sorbide, contained in the isosorbide, most prove to be disadvantageous during the use of isosorbide. For example, side-products, degradation products or impurities con tained in isosorbide are formic acid, glycerol, sorbitans as well as decomposition prod ucts resulting from oxidation, dehydration or polymerization reactions. Such side products generally influence the degree of polymerization and may also cause colora tions of the polymers.

However, high purification of isosorbide (polymer grade, purity 99.0 % or higher than 99.0 %, < 50 APHA) requires extensive effort in the purification steps:

For example, WO2015/112389 describes several purification steps: distillation, chro matography, solvent crystallization, melt crystallization, and solvent washing and fil tration. US 9,598,325 B2 describes distillation in combination with active carbon treatment and crystallization.

US7, 122,661 describes a purification process for isosorbide comprising distilling, treat ing with a cationic or anionic resin and decolorizing the resulting composition the dis tilled composition with char coal. However, said process comprises many purification steps and requires much effort.

WO 2009/126849 A1 describes a suspension cooling crystallization of isosorbide from aqueous solution with solid-liquid separation on centrifuge and washing of crystals with cold water as wash liquor. However, cold water is not suitable as wash liquor due to high solubility of isosorbide in water leading to loss of product.

These purification steps are required to remove the impurities and side compounds, which affect the color property and the performance of isosorbide during polymeriza tion. The drawback of these purification steps is besides the number of the necessary steps that in addition solvent to wash the crystals is required, which results in loss of product during washing due to the solubility of isosorbide in the solvent.

It is a primary object of the invention to provide a simple and effective method for isosorbide purification while avoiding or reducing loss of product by washing.

This object is achieved by a process for purifying a crude isosorbide comprising: a) melting crude isosorbide, b) cooling the isosorbide melt to a crude isosorbide melt suspension com prising isosorbide crystals and residual melt, c) optionally removing parts of the residual melt by mechanical separation, d) removing the residual melt by washing with a washing isosorbide melt, the amount by weight of impurities in the washing isosorbide melt being less than the amount by weight of impurities in the residual melt and e) obtaining isosorbide crystals.

Crude isosorbide means isosorbide comprising impurities.

In the suspension melt crystallization isosorbide crystals are formed from crude iso sorbide melt by cooling. The isosorbide crystals are suspended in the residual melt. Due to inclusion of the residual melt in isosorbide crystals during crystallization and due to adherent residual melt on isosorbide crystals, the purity of isosorbide crystals after solid-liquid separation is not high. To achieve high purity the isosorbide crystals are washed according to the present invention with washing isosorbide melt, the amount by weight of impurities in the washing isosorbide melt being less than the amount by weight of impurities in the residual melt to be removed by the washing step.

Surprisingly, said purification method for isosorbide provides high-purity isosorbide crystal based on crystallization and does not require several further purification steps like e.g. further crystallization steps, treatment with active carbon, ion exchange res ins and/or distillation. However, additional purification steps like further crystallization steps, treatment with active carbon, ion exchange resins and/or distillation may be performed.

In particular, the process according to the present invention comprises the steps of: a) melting crude isosorbide comprising at least 85 wt.-% isosorbide based on the total weight of the crude isosorbide, b) cooling the isosorbide melt to a crude isosorbide melt suspension com prising isosorbide crystals and residual melt, c) optionally removing parts of the residual melt by mechanical separation, d) removing the residual melt by washing with a washing isosorbide melt comprising at least 99.0 wt.-% pure isosorbide based on the total weight of the washing isosorbide melt, e) obtaining isosorbide crystals.

Advantageously, the method according to the present invention may provide iso sorbide crystals with a purity of at least 99.0 wt.-% isosorbide based on the total weight of the obtained isosorbide crystals.

Preferably, the isosorbide crystals obtained in step e) may have a purity of at least 99.0 wt.-%, at least 99.1 wt.-%, at least 99.2 wt.-%, at least 99.3 wt.-%, at least 99.4 wt.-%, at least 99.5 wt.-%, at least 99.6 wt.-%, at least 99.7 wt.-%, at least 99. 8 wt.-% or at least 99.9 wt.-% isosorbide based on the total weight of the obtained isosorbide crystals. The washing isosorbide melt in step d) may have a purity of at least 99.1 wt.-%, at least 99.2 wt.-%, at least 99.3 wt.-%, at least 99.4 wt.-%, at least 99.5 wt.-%, at least 99.6 wt.-%, at least 99.7 wt.-%, at least 99. 8 wt.-% or at least 99.9 wt.-% iso sorbide based on the total weight of the obtained isosorbide crystals.

Preferably, the isosorbide washing melt in step d) should have at least the same purity as purity of the isosorbide crystals obtained in step e).

Furthermore, method according to the present invention may provide isosorbide crys tals with the following properties: a) The melting point of the isosorbide crystals may be at least 62 °C, pref erably at least 63.3°C, and/or b) the isosorbide crystals may have an APFIA color value of <50 APFIA, measured as melt, heated at 70 °C, in a 1 cm cuvette, and/or c) the phi-value of the isosorbide may be between 7.0 and 9.0, preferably between 8.0 and 8.5 (The phi-value is measured as a 40 wt.-% aqueous solution at 25°C), and/or d) the isosorbide crystals may have an UV-transmission at 275 nm of more than 70%, measured as a 20 wt.-% aqueous solution in a 5 cm cuvette, and/or e) the isosorbide crystals may have an UV-transmission at 350 nm of more than 90%, measured as a 20 wt.-% aqueous solution in a 5 cm cuvette, and/or f) the isosorbide crystals may have a water content of less than 0.5 wt.-% based on the total weight of the isosorbide crystals, and/or g) the isosorbide crystals may comprise based on its total weight, less than 500 ppm, preferably less than 100 ppm by weight of formic ac id and/or, less than 500 ppm, preferably less than 100 ppm by weight of glycerol and/or, less than 500 ppm, preferably less than 100 ppm by weight of sorbitans and/or, less than 500 ppm, preferably less than 100 ppm by weight Isosorbide- monoacetate and/or, less than 500 ppm, preferably less than 100 ppm by weight Isosorbide- monoformates and/or, less than 500 ppm, preferably less than 100 ppm by weight Isosorbide- bisformates and/or, less than 500 ppm, preferably less than 100 ppm by weight Isosorbide- bisacetates, and any combination thereof.

In view of the improved properties of the isosorbide obtainable according to the meth ods according to the present invention said isosorbide may be used for the preparation of polycarbonates, polyesters, polyurethanes, polysulfones and/or epoxides.

The method according to the present invention provides isosorbide crystals which are stable with or without addition of a stabilizer like e.g. BHT (Butylated hydroxytoluene), 2,6-bis(l,l-dimethyl)-4-methylphenol or other hindered phenols like Ethylene bis[3,3- bis[3-(l,l-dimethylethyl)-4-hydroxyphenyl]butanoate], Pentaerythritol tetrakis(3- (3,5-di-tert-butyl-4-hydroxyphenyl)propionat), Didoceyl 3,3'-thiopropionate, Octade- cyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, Tetrakis methylene (3,5-di-t- butyl-4-hydrxyhydrocinnamate)-methane, Octadecyl 3,5-di-t-butyl-4- hydroxyhydrocinnnamate, l,3,5-tris(3,5— butyl-4-hydroxybenzyl)-3-triazine 2,4,6- (1H, 3H, 5H)trione, di-tert-butyl-4-methoxyphenol, butylated hydroxyanisol, 2,6- dimethoxy-4-methylphenol, 2,6-dimethoxyphenol, 2,6-di-tert-butyl-4-methoxyphenol, 4,4'-butylidenebis(6-tert-butyl-m-cresol), or N-[ 1,1,3, 3-Tetramethylbutyl)- phenyl]naphthalin-l-amine, tert-butylhydroquinone, Irganox 5057, Covi-Ox T-90 EU, Irganox 1076, Irganox 1010, Tinuvin 111, Tinuvin 123, Chimasorb 2020, Irgafos 168, monoethanolamine, diethanolamine, triethanolamine, morpholine, ascorbic acid, to copherol, erythorbic acid, lactic acid, citric acid, gallic acid, salicylic acid, sodium sa licylate, Phenothiazine, 4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, NaBH₄, LiBH₄, NaOH, NaB0₂, Na₂HP0₄, H₃C0₃, NH₃, NH₄BH₄ or mixtures thereof.

The above mentioned stabilizers may be added to the isosorbide obtained by the pro cess according to the present invention.

The crude isosorbide used for the process according to the present invention may comprise up to 6 wt.-% or up to 5 wt.-% or up to 4 wt.-% or up to 3 wt.-% or up to 2 wt.-% or up to 1 wt.-% water based on the total weight of crude isosorbide or the crude isosorbide may comprise traces of water.

In particular, the crude isosorbide may comprise from 0.2 to 6 wt.-% water, based on the total weight of the crude isosorbide. More preferably, the crude isosorbide com prises from 0.4 to 5 wt.-% or from 0.6 to 4 wt.-% or from 0.6 to 3 wt.-% or from 0.6 to 2 wt.-% or from 0.75 to 1.5 wt.-% water or from 1.0 to 1.5 wt.-% water, based on the total weight of the crude isosorbide.

The crude isosorbide can deliberately be prepared so that it contains this amount of water as a result of the preparation. Usually, however, the preparation may result in crude isosorbide which is essentially free of water or comprises just traces of water. In these cases and in cases where the amount of water contained in the crude isosorbide is unsatisfactory, it is of course possible, according to the invention, to bring the water content to the desired level by adding water before the preparation of crude iso sorbide. Water may be added to the crude isosorbide to bring the water amount to 0.2 to 6.0 wt.-% water, based on the total weight of the crude isosorbide. More pref erably, water may be added to the crude isosorbide to bring the water amount to a range of 0.4 to 5 wt.-% or a range of 0.6 to 4 wt.-% or to a range of 0.6 to 3 wt.-% or to a range of 0.6 to 2 wt.-% or to a range of 0.75 to 1.5 wt.-% or from 1.0 to 1.5 wt.-% water, based on the total weight of the crude isosorbide.

Cooling in step b) may be performed to a temperature below 62°C. Preferably, the cooling step b) is performed at a temperature from 20 to 61°C, preferably 50 to 60°C.

In the suspension crystallization process in step b) a crystal suspension comprising crystals having a lower impurity content and a residual melt having a higher impurity content is produced by cooling the isosorbide melt containing the impurities. The solid crystals may grow while present directly in suspension or may be deposited as a layer on a cooled wall, from which they are then scratched off and resuspended in the re sidual melt, i.e. the solids formation can be carried out e.g. in cooled stirred kettles, in scraped-surface heat exchangers or in disk crystallizers, as described, for example, in Chem.-Ing.-Techn. 57 (1985) No. 2, 91-102.

Generally, all suspension crystallizers are suitable for the process for the preparation of the isosorbide according to the present invention. In particular, the following sus pension crystallizers of the companies below can be used as suspension crystallizers: Scraped surface crystallizers from GEA or Scraped crystallizers with growth vessels from Sulzer Chemtech Ltc.

The suspension crystallizers can be operated with refrigerant and crude isosorbide may be fed both concurrently and counter currently. The latter is preferred.

Preferably, isosorbide crystals of the crude isosorbide melt suspension have a size from 100 to 2000 pm.

On the way from the crystallizer to the wash column, it is preferred to homogenize the crystal suspension (for example by stirring and/or by means of suitable pumps).

Separation of suspension crystals and residual melt may be carried out either exclu sively by means of washing with a washing isosorbide melt or by prior partial mechan ical separation of residual melt as described in step c) and subsequent by washing with a washing isosorbide melt.

Mechanical separation of the residual melt from the crystals in step c) may be carried out by pressing off, filtration and/or centrifugation (cf. for example Chem.-Ing.-Techn. 57 (1985) No. 2, 91-102). Suitable means for mechanical separation of the crystal phase in step c) may be presses, sieves, centrifuges and filters. For example, belt fil ters, drum filters and curved sieves can be used. Frequently, the mechanical separa tion of the crystal phase from the crude isosorbide suspension is carried out in such a way that the crystal phase is still dripping wet with residual melt. The isosorbide crys tal phase separated from the crude isosorbide suspension may still contain from 5 to 30, or up to 10 wt.-% by weight residual melt, based on the total weight of the iso sorbide crystal and residual melt separated as crystal phase.

According to the invention, it is not required to subject a hereinabove mechanically isolated crystal phase to resuspension before its further purification in a wash column with forced transport, as recommended, for example in WO 98/25889. Of course, such a resuspension could however may be carried out before the use of the wash column purification step.

The crude isosorbide melt suspension in step b) may comprise after cooling 20 to 40 wt.-% solid content, preferably 30 to 40 wt.-% solid content, based on the total weight of the crude isosorbide melt suspension. In step d) the content of washing isosorbide melt may be 1-10 wt.-% of the total weight of the solid content in the crude isosorbide melt suspension.

Frequently, the wash column purification step is carried out in such a way that the difference between the temperature of the crude isosorbide suspension fed to the wash column and that of the wash melt recycled to the wash column (i.e. the temper ature difference) is from 2 to 15°C, frequently from 5 to 10°C, more frequently from 2 to 6°C.

Different types of wash columns may be used. Preferably wash columns with forced transport are used. The wash column may be a mechanical or hydraulic wash column.

In principle, the wash column types are divided into those with forced transport of the suspension crystal bed and those with gravity transport of the suspension crystals (a detailed description of the different wash column types is to be found, inter alia, in Chem.-Ing.-Techn. 57(1985) No. 2, 91-102, in Chemical Engineering Science 50, 1995, No. 17, 2717 to 2729, Elsevier Science Ltd., in Applied Thermal Engineering 17, (1997) No. 8-10, 879-888, Published by Elsevier Science Ltd., and the citations stated in the abovementioned references). In wash columns with forced transport of the sus pension crystal bed, at least one force other than gravitation in the transport direction is used for transporting the suspension crystal bed.

Inside the wash column, the suspension crystals are transported either from top to bottom or from bottom to top. The wash liquid may be passed counter-currently or concurrently to the suspension crystals in the wash column.

Washing with a washing isosorbide melt may be carried out in a wash column in which the wash liquid is passed preferably counter currently to the suspension crystals.

In the case of wash columns with forced transport of the suspension crystal bed, a distinction is made for example between pressure columns (also referred to as hy draulic columns), in which the crystals and the wash melt are transported, for exam ple, externally by pumps and/or hydrostatic level and the mother liquor is generally forced out of the wash column via a filter (on the other side of the filters, atmospheric pressure, reduced pressure or superatmospheric pressure may prevail), and mechani- cal columns having mechanical force transport means for the crystal bed, such as spe cial rams, stirrers, screws, helices or spirals.

Wash columns which are suitable according to the invention and may be used are both hydraulic wash columns, (for example that of the SoliQz B.V. in Rotterdam, The Netherlands (cf. Applied Thermal Engineering 17, No. 8-10, (1997), 879-888, or Chemical Engineering Science 50, No. 17, (1995) 2717-2729, Elsevier Science Ltd., or 4th BIWIC 94/Bremen International Workshop for Industrial Crystallization, Bremen, Sep. 8-9, 1994 at the University of Bremen, Ed.: J. Ulrich, or Trans. I. Chem. E, 72, Part A, September 1994, pages 695 to 702, and Applied Thermal Engineering 17, Nos. 8-10, (1997), 879-888, Elsevier Science Ltd.)) and mechanical wash columns (for ex ample that from GEA Niro PT B.V. 's-Hertogenbosch, The Netherlands). Both types of wash columns are shown in Fig.2.

Those wash columns with forced transport which are described in patents of TNO In stitute, The Netherlands or Niro Process Technology B.V., The Netherlands or other companies are also particularly advantageous according to the present invention (cf. for example EP97405 Al, WO 00/24491, EP920894 Al, EP398437 Al, EP 373720 Al, EP 193226 Al, EP191194 Al, WO 98/27240, EP305 316 Al and US 4,787,985).

The process according to the invention can of course also be carried out in such a way that the crude isosorbide suspension to be washed according to the invention is the result of a fractional crystallization, for example of a fractional suspension crystalliza tion. However, it is important according to the invention that such a fractionation is not essential for a successful purification.

A part of the washed isosorbide crystals obtained in step e) may be melted and part of this melt, typically from 1-10 wt.- % based on the total weight of isosorbide crystals obtained in step e), may be used in the wash column as a wash medium for washing. The other part of the isosorbide crystals my be removed as pure isosorbide.

The residual melt obtained in step b) that is not crystallized may be purified by distilla tion up to crude isosorbide melt comprising at least 85 wt.-% isosorbide based on the total weight of the isosorbide melt. Preferably, the residual melt obtained in step b) that is not crystallized is purified by distillation at a temperature from 120 and 250°C and a pressure from 0.0001 to 0.2 bara, preferably 0.001 to 0.05 bara. Bara means bar absolute. The distilled and purified isosorbide melt may be reintroduced in step a). The crude isosorbide used in the process according to the present invention may be obtained by acid-catalyzed dehydration of sorbitol-syrup to an isosorbide mixture, re moving isosorbide by distillation and obtaining an isosorbide melt.

In particular, the crude isosorbide for step a) may be obtained by:

Method A i) Concentrating sorbitol syrup at 90 to 150°C and up to 0.5 bara, preferably 0.05 to 0.2 bara, to a sorbitol syrup having at least 85 wt.-% sorbitol based on the to tal weight of the sorbitol syrup, ii) Converting the syrup obtained in step i) using an acid catalyst at 100 to 200°C and a pressure up to 0.5 bara, preferably operating at 0.05 to 0.2 bara, to an isosorbide mixture, iii) Neutralization of the acid catalyst in the isosorbide mixture, preferably by addi tion of at least 1.0 equivalent, more preferably 1.0 to 12.0 equivalents, most preferably 2.0 to 4.0 equivalents of an alkaline solution (e.g. NaOH, KOH, Ca(OH)₂), with regard to the amount of added acid catalyst, iv) Removing isosorbide at 120 to 250°C and 0.0001 to 0.2 bara, preferably 0.001 to 0.05 bara, by distillation and v) Obtaining an isosorbide melt comprising 85 to 99 wt.-% isosorbide based on the total weight of the isosorbide melt. or

Method B i) Converting sorbitol syrup, having between 20 and 85 wt.-% sorbitol based on the total weight of the sorbitol syrup, using an acid catalyst at 130 to 250 °C and a pressure up to 40 bara, to an isosorbide mixture, ii) Neutralization of the acid catalyst in the isosorbide mixture, preferably by addi tion of at least 1.0 equivalent, more preferably 1.0 to 12.0 equivalents, most preferably 2.0 to 4.0 equivalents of an aqueous alkaline solution (e.g. NaOH, KOH, Ca(OH)₂), with regard to the amount of added acid catalyst, iii) Removing water from the mixture at 90 to 150 °C and up to 0.5 bara, preferably 0.05 to 0.2 bara, by distillation and iv) Removing isosorbide at 120 to 250 °C and 0.0001 to 0.2 bara, preferably 0.001 to 0.05 bara, by distillation, v) Obtaining an isosorbide melt comprising 85 to 99 wt.-% isosorbide based on the total weight of the isosorbide melt.

Method A: Preferably a reactor is filled with an aqueous solution of isosorbide under inert gas atmosphere. Preferably an aqueous 65-75 wt.-% sorbitol syrup based on the total weight of the sorbitol syrup is concentrated by reducing the pressure to 0.05 to 0.2 bara and heating to 100°C to 120°C, whilst removing water from the mixture to obtain a sorbitol syrup of 85 to 98 wt.-% based on the total weight of the sorbitol syr up.

Preferably an acid catalyst, e.g. para-toluenesulfonic acid, is used to convert sorbitol syrup to isosorbide. Preferably, 0.1 to 3.0 mol% of acid catalyst, in particular para- toluenesulfonic acid, with regards to the amount of sorbitol, is added to the sorbitol syrup. For the conversion of sorbitol to isosorbide the temperature is increased to 150°C to 180°C while maintaining the pressure between 0.05 to 0.2 bara for a reac tion time of 60 to 120 min. After the reaction, the catalyst in the resulting isosorbide mixture is neutralized by the addition of an aqueous solution up to 80 wt.-% base (based on the total weight of the aqueous base solution), e.g. potassium hydroxide, in particular at least 1.0 equivalent, more preferably 1.0 to 12.0 equivalents, most pref erably 2.0 to 4.0 equivalents with regards to the amount of added acid catalyst, and distilled to obtain the crude isosorbide, containing >85 wt.-% isosorbide based on the total amount of the isosorbide. The distillation for removing isosorbide from the reac tion mixture may be performed batch-wise from the batch reactor by increasing the temperature from 180 °C to 250 °C and decreasing the pressure to 0.001 and 0.05 bara or in a continuous process at 170° to 220°C and a pressure between 0.001 and 0.05 bara.

Method B: Preferably a reactor is filled with an aqueous solution of isosorbide under inert gas atmosphere. The concentration of sorbitol may be varied between 20-85 wt.-% based on the total weight of the isosorbide solution. To obtain a 65-85 wt.-% sorbitol solution, based on the total weight of the sorbitol solution, an aqueous 65-75 wt.-% sorbitol solution may be concentrated by reducing the pressure to 0.05 to 0.2 bara and heating to 100 °C to 120 °C, whilst removing water from the mixture to ob tain a sorbitol syrup of up to 85 wt.-% sorbitol. Preferably, the reactor is heated to the applied reaction temperature, which is in the range of 130-250 °C, more preferred 170 to 250°C and a pressure of 4 to 40 bara. Preferably, an acid catalyst, e.g. para- toluenesulfonic acid, is used to convert sorbitol syrup to isosorbide. Preferably, the catalyst acid concentration e.g. para-toluenesulfonic acid, in the reaction mixture is in the range of 0.1-3.0 mol-%, with regards to the amount of sorbitol. Preferably, the reaction time is 120 to 150 min. Preferably, the catalyst in the resulting isosorbide mixture is neutralized after the reaction by the addition of a base as an aqueous solu tion up to 80 wt.-% base (based on the total weight of the aqueous base solu tion), e.g. potassium hydroxide, preferably at least 1.0 equivalent, more preferably 1.0 to 12.0 equivalents, most preferably 2.0 to 4.0 equivalents with regards to the amount of added acid catalyst. Afterwards, preferably water is removed from the mix ture at 90-150 °C using an adjusted pressure of 0.05 to 0.2 bara. Subsequently, pref erably the resulting mixture is distilled to obtain the crude isosorbide melt, containing >85 wt.-% isosorbide based on the total amount of the obtained crude isosorbide. The distillation may be performed batch-wise or in a continuous process at a temperature of 180 °C to 250 °C and a pressure between 0.001 and 0.2 bara or in a continuous process at 170° to 220°C and a pressure between 0.001 and 0.2 bara.

Preferably, the process according to the present invention for purifying crude iso sorbide comprises the following steps: a) melting isosorbide comprising at least 85 wt.-% isosorbide based on the to tal weight of the crude isosorbide, b) cooling the isosorbide melt to a crude isosorbide melt suspension consisting of isosorbide crystals and residual melt, c) removing parts of the residual melt by mechanical separation, d) removing the residual melt by washing with a washing isosorbide melt comprising at least 99.0 wt.-% pure isosorbide, preferably at least 99.5 wt.-% pure isosorbide, based on the total weight of the washing isosorbide melt, and e) obtaining isosorbide crystals comprising at least 99.0 wt.-% pure iso sorbide, preferably at least 99.5 wt.- % pure isosorbide based on the total weight of the obtained isosorbide crystals. wherein the crude isosorbide melt suspension in step b) comprises after cooling 20 to 40 wt.-% solid content based on the total weight of the crude isosorbide melt suspen sion and wherein the content of washing isosorbide melt in step d) is from 1 to 10 wt.-% of the total weight of the solid content in the crude isosorbide melt suspension. The crude isosorbide suspension from suspension melt crystallization may be fed to the wash column from the top. In the mechanical wash column, the isosorbide crys tals maybe transported to the bottom by means of a mechanical ram. The remaining residual melt may be separated from the isosorbide crystals by means of a filter which is placed in the ram. A compact isosorbide crystal solid bed may be formed. At the bottom of the mechanical wash column, the isosorbide crystals may be scraped from the solid bed by means of a rotating knife. The obtained purified isosorbide may be partly or completely melted and part of the said melt may flow in the direction of the filter in contrary to the transport direction of the crystal bed, resulting in counter cur rent washing of the isosorbide crystals with the washing isosorbide melt. The rest of the melt of purified isosorbide crystals may be discharged from the wash column as product. The obtained isosorbide crystals purified may contain > 99.0 wt.-%, prefera bly > 99.5 wt.-% isosorbide and may have a <50 APHA. The residual melt separated in the wash column can be recycled back e.g. to the distillation process to produce crude isosorbide melt. The content of washing isosorbide melt may be from 1 to 10 wt.-% of the total weight of the solid content in the crude isosorbide melt suspen sion.

Advantageously, the above described one-stage suspension crystallization of the iso sorbide provides with minimum apparatus costs, the preparation of an isosorbide quality which contains preferably at least 99.0 wt.-%, preferably at least 99.1 wt.-%, preferably at least 99.2 wt.-%, preferably at least 99.3 wt.-%, preferably at least 99.4 wt.-%, more preferably at least 99.5 wt.-%, more preferably at least 99.6 wt.-%, more preferably at least 99.7 wt.-%, more preferably at least 99.8 wt.-%, most pref erably at least 99.9 wt.-% of its weight of isosorbide.

Figures:

Figure 1 shows a schematic overview of the purification process according to the pre sent invention.

Figure 2 shows different types of wash columns, mechanical and hydraulic wash col umns are useful according to the present invention as well as a gravity wash column. Figure 3 shows the wash process in a mechanical wash column in a scheme. The up per part shows the crystal solid bed with residual melt, the lower part shows the washed crystal bed. The pressure p₂ causes the transport of the washing melt to wash the crystal bed counter-currently. Figure 4 shows gas chromatographic separation chromatogram for crude isosorbide as prepared in Example la.

Figure 5 shows gas chromatographic separation chromatogram for isosorbide purified as described in Example 2.

Examples

Example la (Isosorbide synthesis)

A batch reactor was filled with an aqueous 68 wt.-% sorbitol syrup based on the total weight of the sorbitol syrup under inert gas atmosphere. Vacuum was applied, reduc ing the pressure to 0.1 bara, and the reaction mixture was heated to 114°C, whilst removing water from the mixture and the pressure of 0.1 bara was maintained, re sulting in a concentration of the aqueous sorbitol syrup to a sorbitol syrup containing 95 wt.-% sorbitol based on the total weight of the sorbitol syrup. An acid catalyst (0.25 mol-% of para-toluenesulfonic acid, with regards to the amount of sorbitol) was added to the sorbitol syrup while maintaining the pressure of 0.1 bara. Subsequently, the temperature was increased to 175 °C at 0.1 bar for a reaction time of 90 min, re sulting in the conversion of sorbitol to isosorbide (and the described side products). Herein, the color changed from transparent to faint yellow to dark brown. After 90 min the resulting isosorbide melt was neutralized by the addition of 50 wt.-% aqueous potassium hydroxide solution (based on the total weight of the aqueous base solution) adding 2.0 equivalents potassium hydroxide with regards to the amount of added acid catalyst, at 0.1 bara at 175 °C and distilled to obtain the crude isosorbide melt, con taining 97.7 wt.-% isosorbide based on the total weight of the crude isosorbide melt. The distillation was performed batch-wise from the batch reactor by decreasing the pressure to 0.015 bara and increasing the temperature stepwise to 240 °C, or in a continuous process over a thin film evaporator at 200 °C and a pressure of 0.01 bara. A clear yellow isosorbide melt was obtained, with e.g. the following purity and color (APHA).

Example lb (Isosorbide synthesis)

A batch reactor was filled with an aqueous syrup of sorbitol under inert gas atmos phere. The concentration of sorbitol was 40 wt.-% based on the total weight of the sorbitol syrup. Subsequently, the reactor was heated to the applied reaction tempera- ture at 190 °C. After reaching the set temperature, an acid catalyst (para- toluenesulfonic acid) was dosed into the reactor within a timeframe of 2 min. After completion of the dosing procedure, the acid concentration of the reaction mixture was 0.50 mol-% with regards to the amount of sorbitol. The pressure inside of the reactor was adjusted by the water vapor pressure at the set temperature leading to a maximum pressure of 13 bara. Within a reaction time of 120 min, the color changed from transparent to faint yellow to dark brown. The resulting isosorbide mixture was neutralized by the addition of 50 wt.-% aqueous potassium hydroxide solution 2.0 equivalents potassium hydroxide with regards to the amount of added acid catalyst. Afterwards, water was removed from the mixture at 90 °C using an adjusted pressure of 0.1 bara. Subsequently, the resulting mixture was distilled to obtain the crude iso sorbide melt, containing 97.2 wt.-% isosorbide based on the total weight of the crude isosorbide melt. The distillation was performed batch-wise from the batch reactor by decreasing the pressure to 0.015 bara and increasing the temperature stepwise to 240 °C, or in a continuous process over a thin film evaporator at 200 °C and a pres sure of 0.01 bara. A clear yellow isosorbide melt was obtained, with e.g. the following purity and color (APHA).

Example 2 (Purification of crude isosorbide)

A crude isosorbide melt obtained as described in Example la was fed to a suspension crystallizer at 70°C and atmospheric pressure. The suspension crystallizer was an agi tated vessel with jacket. The vessel was tempered at ca. 56°C and by this cooling iso sorbide crystals were formed from the crude isosorbide melt. The isosorbide crystals were suspended in the remaining residual melt by means of stirring. The amount of isosorbide crystals was ca. 30% by weight based on the total weight of the crude iso sorbide melt suspension. The crystallization enthalpy was dissipated through the wall cooling of the vessel. The crude isosorbide melt suspension from suspension melt crystallization was fed to the wash column from the top. In the mechanical wash col umn, the isosorbide crystals were transported to the bottom by means of a mechani cal ram. The remaining residual melt was separated from the isosorbide crystals by means of a filter which was placed in the ram. A compact isosorbide crystal solid bed was formed. At the bottom of the mechanical wash column, the isosorbide crystals were scraped from the solid bed by means of rotating knife. The scraped isosorbide crystals were then melted. Part of the melt of Isosorbide crystals flowed in the direction of the filter in contrary to the transport direction of the crystal bed, which resulted in counter current washing of the isosorbide crystals with the washing melt of isosorbide crystals. The rest of the melt of purified isosorbide crystals was discharged from the wash column as product.

The analytics of the purified melt Isosorbide crystals resulted in

Example 3 (APHA color values)

Samples of the isosorbide were molten at 70 °C for 30 min. After heating, the molten samples were measured in a 1 cm cuvette in a commercially available colorimeter (Hach Lange Lico 500) at room temperature. The Pt/Co color values (APHA color val- ues/Hazen color values) were determined according to DIN/ISO 6271 (ASTM D 1209- OS) with standardized illuminant C and observer/angle of view of 2 degrees.

Example 4 (Gaschromatography)

Prior to gas chromatographic separation, isosorbide was dissolved in Acetonitrile and derivatized by addition of N-Methyl-N-(trimethylsilyl)-2,2,2-trifluoroacetamide (MSTFA). The derivatization was carried out with 0.1 g sample, 0.05 g l-Methoxy-2- (2-methoxyethoxy)ethane as internal standard, 2 mL Acetonitrile/MSTFA (1:1) for 60 min at 60 °C.

Compositional analysis of the said sample was performed by the means of gas chro matography using flame ionization detection (GC-FID). Known signals were quantified by using l-Methoxy-2-(2-methoxyethoxy)ethane as internal standard. GC separation was achieved using a 30 m x 320 pm x 0.25 pm (length x inner diameter x film thick ness) fused silica capillary coated internally with a stationary phase of 14% cyanopro- pyl-phenylsiloxane/86% methylpolysiloxane. Gas chromatographic separation took place after injecting an aliquot of the trialkylsilylated product at 260 °C, in the chro matographic system with a carrier gas pressure of 670 hPa and a split ratio of 40:1 at an initial oven temperature of 50°C held for 2 min, followed by a temperature ramp of lOK/min up to 250°C and a final isothermal period of 13 min. Eluting compounds were detected by the means of a flame ionization detector operated at 290 °C. Crude Isosorbide produced according to Example la

Purified Isosorbide produced according to Example 2

Example 5 (Melting point) Melting point of samples were measured by Differential Scanning Calorimetry (DSC) Q2000 from TA Instruments. Nitrogen was the carrier gas and calibration was done with indium. Ca. 10 mg of crystallized sample were enclosed in hermetically sealed aluminum pans and run against air (empty pan) as reference. Crystallized samples were heated to 100°C at 10°C/min. Melting curves were recorded from 20 to 100°C. The resulting DSC data was analyzed by peak program and peak temperature, onset temperature and melting temperature were recorded. Melting temperature was de fined as the onset temperature, which is the inflection point of melting curve and solid line. Example 6 (pH-value)

Samples of the isosorbide were dissolved in water (40 wt.-%). The pH-value of the solution was measured at 25°C temperature with a Knick "Portamess Type 911 pH" and a Mettler Toledo "HA405-DPA-SC-S8/120" electrode. Example 7 (UV-transmission)

Samples of the isosorbide were dissolved in water (20 wt.-%). The UV-transmission values of the solution were determined on a Thermo Scientific "Evolution 201" using "Thermo Scientific Insight 2" software for measurement control and analysis of the obtained spectra. Water was used for baseline correction and the isosorbide solution was measured at ambient temperature in quartz cuvettes with a path length of 5 cm (l/Q/50) from Starna GmbH.

Example 8 (Water content)

A defined amount of isosorbide was dissolved in "Hydranal Medium K" by Honeywell. The water contents of isosorbide samples were then determined with a Metrohm "870 KF Titrino plus" using a Metrohm 6.0338.100 electrode using "Hydranal Compo- site 5 K" by Honeywell for titration.

Claims

Claims:

1. A process for purifying a crude isosorbide comprising: a) melting crude isosorbide, b) cooling the isosorbide melt to a crude isosorbide melt suspension compris ing isosorbide crystals and residual melt, c) optionally removing parts of the residual melt by mechanical separation, d) removing the residual melt from the isosorbide crystals by washing with a washing isosorbide melt, the amount by weight of impurities in the washing isosorbide melt being less than the amount by weight of impurities in the residual melt, and e) obtaining isosorbide crystals.

2. A process for purifying a crude isosorbide according to claim 1 comprising: a) melting crude isosorbide comprising at least 85 wt.-% isosorbide based on the total weight of the crude isosorbide, b) cooling the isosorbide melt to a crude isosorbide melt suspension compris ing isosorbide crystals and residual melt, c) optionally removing parts of the residual melt by mechanical separation, d) removing the residual melt from the isosorbide crystals by washing with a washing isosorbide melt comprising at least 99.0 wt.-% pure isosorbide based on the total weight of the washing isosorbide melt, and e) obtaining isosorbide crystals.

3. The process as claimed in anyone of the preceding claims, wherein in step b) cooling is performed to a temperature below 62 °C.

4. The process as claimed in anyone of the preceding claims, wherein the crude isosorbide melt suspension in step b) comprises after cooling 20 to 40 wt.-% sol id content based on the total weight of the crude isosorbide melt suspension.

5. The process as claimed in any of the preceding claims, wherein the content of washing isosorbide melt is between 1-10 wt.-% of the total weight of the solid content in the crude isosorbide melt suspension.

6. The process as clamed in anyone of the preceding claims, wherein the crude iso- sorbide in step a) comprises 6 wt.-% water and impurities or less based on the weight of the crude isosorbide.

7. The process as claimed in anyone of the preceding claims, wherein the crystals in step d) are washed in a wash column in a counter current stream.

8. The process as claimed in claim 7, wherein a mechanical wash column is used for washing.

9. The process as claimed in preceding claim 7, wherein a hydraulic wash column is used for washing.

10. The process as claimed in any of the preceding claims 7 to 9, wherein the differ ence between the temperature of the crude isosorbide suspension fed to the wash column and that of the wash melt recycled to the wash column (i.e. the temperature difference) is from 2 to 15°C.

11. The process as claimed in any of the preceding claims, wherein the in step e) obtained isosorbide crystals are subsequently melted and used at least partially in step d) as washing isosorbide melt.

12. The process as claimed in any of the preceding claims, wherein the in step b) obtained residual melt is purified by distillation up to crude isosorbide melt com prising at least 85 wt.-% isosorbide based on the total weight of the isosorbide melt and reintroduced in step a).

13. The process as claimed in claim 12, wherein the residual melt obtained in step b) that is not crystallized is purified by distillation at a temperature from 120°C to 250°C and a pressure from 0.0001 to 0.2 bara, preferably from 0.001 to 0.05 bara, and reintroduced in step a).

14. The process as claimed in any of the preceding claims, wherein the crude iso sorbide melt used in step a) is produced by i) Concentrating sorbitol syrup at 90 to 150°C and up to 0.5 bara, preferably 0.05 to 0.2 bara, to a sorbitol syrup having at least 85 wt.-% sorbitol based on the total weight of the sorbitol syrup, ii) Converting the syrup obtained in step i) using an acid catalyst at 100 to 200°C and a pressure up to 0.5 bara, preferably operating at 0.05 to 0.2 bara, to an isosorbide mixture, iii) Neutralization of the acid catalyst in the isosorbide mixture by addition of at least 1.0 equivalent of an alkaline solution with regards to the amount of added acid catalyst, iv) Removing isosorbide at 120 to 250°C and 0.0001 to 0.2 bara, preferably 0.001 to 0.05 bara, by distillation, v) Obtaining an isosorbide melt comprising 85 to 99 wt.-% isosorbide based on the total weight of the isosorbide melt.

15. The process as claimed in any of the preceding claims 1 to 13, wherein the crude isosorbide melt used in step a) is produced by h) Converting sorbitol syrup, having between 20 and 85 wt.-% sorbitol based on the total weight of the sorbitol syrup, using an acid catalyst at 130 to 250°C and a pressure up to 40 bara, to an isosorbide mixture, ii) Neutralization of the acid catalyst in the isosorbide mixture by addition of at least 1.0 equivalent of an alkaline solution with regards to the amount of added acid catalyst, iii) Removing water from the mixture at 90 to 150°C and up to 0.5 bara, pref erably 0.05 to 0.2 bara, by distillation, iv) Removing isosorbide at 120 to 250°C and 0.0001 to 0.2 bara, preferably 0.001 to 0.05 bara, by distillation v) Obtaining an isosorbide melt comprising 85 to 99 wt.-% isosorbide based on the total weight of the isosorbide melt.

16. The process according to claim 14 or 15, wherein the acid catalyst is para- toluenesulfonic acid.

17. The process as claimed in any of the preceding claims, wherein the isosorbide crystals obtained in step e) comprise more than 99.0 wt.-% preferably more than 99.5 wt.-% isosorbide based on the total weight of the isosorbide crystals.

18. The process as claimed in any of the preceding claims, wherein the isosorbide crystals obtained in step e) have an APHA color value of <50 APHA.

19. Isosorbide obtainable by a process according to anyone of claims 1 to 18.

20. Isosorbide obtainable by a process according to anyone of claims 1 to 19 having a melting point of at least 62 °C.