DE3934457A1 - Prepn. of sorbitol - by hydrogenation of alpha-D-glucose in aq. soln. on unsupported metal catalyst of iron gp. - Google Patents

Abstract

Sorbitol (D-glucitol) is prepd. from alpha-D-glucose in aq. soln. by continuous hydrogenation at H2 pressure of 150-500 bars and 60-120 deg.C., in a fixed bed process in a reaction zone, using as catalyst carrier-free bodies of elements of the Fe gp., with pressure strength of more than 50 N (100-400 N) on the surface of the body, and an internal surface of 10-90 square m/g. USE/ADVANTAGE - Conversion of glucose is almost complete, and epimerisation, breakage of the C chain, and formation of higher mol. components by condensation and formation of ethers, are avoided. Sorbitol is obtd. in quantitative yield, with above 99% purity, and contg. less than 0.3% of mannitol and less than 0.2% of unconverted glucose; no purificn. is needed. The catalyst is not transferred, in ionic or colloidal form, to the soln. phase of the substrate, which does not become contaminated with heavy metals. The catalysts can easily be processed after use. Sorbitol is a replacement for sugar for diabetics and in low-calory sweeteners. It is less cariogenic than glucose or other sugars.

Description

The invention relates to a method for producing the Sugar alcohol sorbitol (D-glucitol) from α-D-glucose through continuous catalytic hydrogenation with Hydrogen. The course of the reaction can be illustrate the following reaction scheme:

Sorbitol is only found in the human body orderly absorbed and only in this proportion reduced. Therefore, sorbitol is a sugar substitute for Diabetic and suitable as a low-calorie sweetener. It is also less cariogenic than glucose or others Sugar.

The sweetening power of sorbitol reaches about 50-60% of the sweetness due to sucrose. To increase the sweetness, the aqueous solution with artificial sweeteners, for example as cyclohexyl sulfamate or phenylalanine asparagine acid methyl ester, mixed and by common vacuum crystallization can be obtained in crystalline form. But you can also use the artificial sweeteners in solid Mix the mold with the sorbitol crystals. Sorbitol can also be sweet with others in liquid or solid form tasting carbohydrates, such as palatinite, Lactite, xylitol and others can be mixed.

Even in long-term studies, there were no toxic effects are determined (Ullmanns Encyklopadie der tech African chemistry, Volume 24, Weinheim 1983, p. 774), so that versatile applications in life mid-range, in the manufacture of diabetic pro products as well as sugar-free sweets and food means with low nutritional value.

A discount is often used to produce sorbitol nuclear process (batch process) applied, in which a powdery nickel catalyst in a suspension tion process is used.  

Batch processes have the disadvantage that their capacity, relative to the reaction volume, is very small is and therefore a need for large reaction apparatus doors and storage tanks. The energy consumption is uneconomical and staffing requirements are proportionate high.

Continuous powder catalyst process using several hydrogenation reactors connected in cascade avoid some of the disadvantages mentioned. It remains the cumbersome way, the powdery way Targeted activation, pumping and to filter quantitatively from the reaction product, the Catalyst slurry pumps are subject to a high mecha African stress. The quantitative removal of the powdered catalyst is expensive (rough and Fine filtration apparatus in exchangeable version). Further there is a great risk that the catalyst will operations are relatively quick Loses activity (high catalyst consumption). It is therefore desirable to have the reaction over fixed To let catalysts run off, which is a high should have specific activity that is also about for a longer period of 1 to several years should decrease because frequent catalyst changes too are complex in fixed bed reactions.

Even in the case of fixed catalysts, it is common to connect several reactors in a row, whereby there are several reaction zones connected in series (DE-OS 32 14 432).  

Nickel catalysts on support material (SiO ₂ / Al ₂ O ₃ ) with extremely high active surfaces of 140-180 m ² / g are used, so that the catalysts are so active that they have to be stabilized by additional chemical treatment methods for example, by gassing oxygen to form monomolecular oxygen layers on the catalyst surface (DE-OS 31 10 493). The deactivating stabilization of the catalyst then makes reaction temperatures so high when hydrogenating α-D-glucose (130-180 ° C) that uncontrollable side reactions are possible, such as discoloration through caramelization and hydrogenating cracking (hydrogenolysis) of the mon osaccharide alcohol until the formation of methanol and even methane. In addition, in this reaction, heavy metals such as nickel, iron or cobalt always go into solution in ionic or colloidal form, which requires a subsequent activated carbon treatment of the hydrogenated product and, on the other hand, deionization by ion exchangers.

Since the known hydrogenation processes are based on pH values of 7-13 adjusted α-D-glucose solutions are working add alkalis or alkaline earths to the starting solutions give, which is also cumbersome from the end product must be removed (DE-OS 31 10 493; DE-OS 32 14 432).

Furthermore, it is to be expected that among the hydrating Conditions marked epimerization occurs, so that from α-D-glucose besides sorbitol also mannitol is obtained.  

Furthermore, the effect of carbon chain cleavage of sugars in the catalytic hydrogenation on Raney Known nickel; DE-OS 27 56 270 describes this effect on a sugar mixture, like it from the self-condensation tion of formaldehyde, whereby within the scope of exemplary embodiments there a clear difference Exercise from higher C chain numbers to lower observ is tested. According to DE-OS 28 31 659 such a gap not on 8th subgroup precious metal catalysts observed.

EP 1 52 779 describes a process for the hydrogenation of α-D Glucopyranosido-1,6-fructose on fixed kata known analyzers. However, this is a mixture two reduction products, namely α-D-gluco pyranosido-1,6-mannitol and α-D-glucopyranosido-1,6-sorbitol obtained, the ratio of which is approximately 1: 1.

It was therefore surprising that in the present process Not only is glucose almost completely converted, but avoiding epimerization and C-ket cleavage and formation of higher molecular components by condensation reactions with ether formation only one associated sugar alcohol, namely the sorbitol is, so this is not from relocation products slides and is therefore used directly as a dietetic can be.

A process for the production of sorbitol (D-glucitol) from α-D-glucose by catalytic hydrogenation in aqueous solution with hydrogen under elevated pressure and at elevated temperature has been found, which is characterized in that the hydrogenation is carried out continuously at a hydrogen pressure 150-500 bar and temperatures of 60-120 ° C in a fixed bed process in a reaction zone over unsupported molded bodies serving as hydrogenation catalysts with a compressive strength of more than 50 N, preferably 100-400 N, on the molded body surface and an inner surface of Performs 10 to 90 m ² / g of elements of the iron group of the periodic table.

The sorbitol is in the inventive method in almost quantitative yield obtained. This is from of particular importance since the removal of higher molecular weight or less molecular, created by building or dismantling ner disturbing impurities from the reaction product through additional cleaning processes, such as recrystallization sation from solvents, usually a considerable amount ecological effort with regard to their disposal required. Mannitol diastereomeric to sorbitol is at most traces in the reaction product (<0.3%) detectable.

The fixed bed catalysts to be used according to the invention tend to be in contrast to supported catalysts not to "bleed", i.e. H. not the transition from kata analyzer components in ionic or colloidal form in the solution phase of the substrate so that the substrate is not contaminated by heavy metals that usually also difficult, for example with Help from ion exchangers, with ecological effort can be removed from the substrate. This is from of particular importance since sorbitol has its own specific  shaft, with nickel, iron, cobalt and others Metal ions to form complex chelate compounds that difficult to remove from solutions. The Catalysts can be used after their use can be easily worked up because the heavy metals are not cumbersome to be separated from a carrier material have to.

It is therefore possible in the method according to the invention the sorbitol in a purity of over 99% in the To produce dry matter. The content of not vice versa set glucose reaches values of only 0.2% or underneath. The commercial specifications for Sorbitol can therefore normally be used without further ado Cleaning processes can be fulfilled directly. Such spec Examples of complications are in the DAB (Deutsches Arzneimittel general ledger), as well as in USP (United States Pharmacopeia) and FCC (Food Chemicals Codex).

As a starting compound for the Ver pure crystalline α-D-glucose is used. It is so in oxygen-free deionized water solved that a 15-45 wt .-%, preferably 35-40 wt .-% Solution is formed, the pH value of 3.5-7, preferred 5.5-7. Shows crystalline α-D-glucose dissolved in Water with a pH of 7, a neutral or, be due to trace amounts of sugar acid formation, a weakly acidic reaction, but can in a specialist known manner, for example by adding Citro Nenoic acid, lactic or sugar acid, to the desired pH can be adjusted.  

Pure water is used for the hydrogenation according to the invention used to a pressure of 100-500 bar, preferably 150-300 bar, is pre-compressed. The hydrie Fixed bed process takes place continuously catalysts described below by using the Solution to be hydrogenated either in cocurrent from below ascending together with the previously mixed what Hydrogen over the installed in a hydrogenation reactor Catalyst flows (direct current method) or but by taking the ascending to hydrie solution from the inflowing hydrogen from above countercurrent (countercurrent method).

The hydrogenation reactor can either be a single high pressure pipe made of steel or a steel alloy, the completely or partially filled with the catalyst, with certain pipe cross sections also the application the catalyst on trays (wire baskets or similar) can be useful, or a coated high pressure tube bundle, the individual tubes with catalyst entirely or partially filled. Furthermore, instead of a larger single-tube reactor an arrangement of several small single tube reactors in a row Cascade operated.

The unsupported catalysts are made from metal powders the elements of the iron group of the periodic table, so Nickel, cobalt and iron or mixtures thereof, be preferably from nickel, cobalt or their mixtures or Alloys with each other or with iron, to form bodies processed. Here, powders of the individual metals  or powdered alloys of the metals mentioned Come into play. Moldings are produced according to common methods by pressing the Metal powder, for example on tableting or Pelleting machines, under high pressure, whereby ver improved adhesion of the metal particles too Graphite and / or adhesives in amounts of 0.5-3% by weight, based on the total weight of the catalyst bil components that can be used. The Her The molded body is placed in an oxygen free atmosphere to avoid surface oxidation the. Examples of shaped bodies are tablets, spheres or granules with diameters of 3-7 mm. Tableted Moldings can continue to enlarge the outer Surface with an axial hole (hole core) be provided. Such moldings have macroscopic a smooth surface.

The catalysts to be used according to the invention as shaped bodies must be prepared in such a way that the resulting shaped bodies have compressive strengths above 50 N, preferably compressive strengths of 100-400 N, on the surface of the shaped body. This is important because lower compressive strengths lead to disintegration or erosive abrasion of the moldings, which would cause undesirable contamination of the reaction product with metal powder. The shaped catalyst bodies to be used according to the invention furthermore have specific inner surfaces which are at values of 10-90 m ² / g. The inspection of molded articles on such inner surfaces and thus on their suitability for the process according to the invention are carried out by methods by FM Nelsen and FT Eggertsen, Analyte. Chem. 30 (1958), 1387 or by SJ Gregg and SW Sing, Adsorption, Surface Area and Porosity, London 1967, chap. 2 and 8 have been described.

The hydrogenation is carried out at temperatures of 60-120 ° C. preferably 90-115 ° C. Lower temperatures become long dwell times or the waiver of one largely quantitative turnover of α-D-glucose. Higher temperatures lead to uncontrollable additions reactions, such as condensation reactions, caramelisie or cracking, causing discoloration and lead to the formation of unwanted by-products can.

The hourly catalyst load is 25-100 g α-D-glucose per liter of catalyst. If the mentioned reaction conditions are unexpectedly high Catalytic converter service lives of 16,000 hours and more achieve, with specific catalyst consumption of at most 0.1% or less can be achieved. In order to are the technical advantages of the invention Procedure except in the by the almost quantitative Sales-related high yield and ecological Benefits derived from the purity of the manufactured Product result in the cost-saving continuous working method and the extremely low Kataly sator consumption.  

The aqueous solution of sorbitol leaving the reactor can after the relaxation, in which one over the intercept hydrogen and after the successful Compression and supplementation with additional hydrogen can be used again directly as Sugar substitute in liquid form Use fin the.

The water of this solution can also be used on ver different ways, for example via spray dryers, Remove drying rollers or freeze drying. As the colorless and crystal clear solution of sorbitol in a falling film ver steamer or a similar device on one Concentrate sugar alcohol content of about 80 wt .-% and then after further evaporation a vacuum crystallizer with cooling bring complete crystallization. The Kristalli sat can be done by a downstream grinding process and possibly screening to a uniform grain size bring. The product obtained is free-flowing. In Depending on the crystallization conditions made various crystalline modifications , of which the γ-form with a melting point of 101 ° C is the most stable.

example 1

A vertical, heat-insulated high-pressure pipe made of stainless steel of 45 mm inner diameter and 1 m long was filled with 1.4 l of a hydrogenation catalyst produced by tableting nickel powder, which had a compressive strength of 147 at a cylinder height of 5 mm and a diameter of 5 mm N had on the cylinder surface and an inner surface of 33 m ² / g. Through this tube, together with the triple molar amount of high-purity hydrogen under a pressure of 300 bar, 250 ml per hour of a 40% solution of α-D-glucose in deionized oxygen-free drinking water with a pH of 7.0 pumped continuously and ascending from bottom to top.

Aqueous solution and hydrogen were previously shared passed through a heat exchanger and so high he heats them up in the high pressure pipe at a temperature of 95 ° C occurred. The one leaving the high pressure pipe Mixture of aqueous solution and excess water fabric was placed in a separator via a cooler leads from where the hydrogen after replacing the ver needed again together with fresh α-D- Glucose solution in the preheater and from there again in the High pressure pipe was pumped.

The colorless and clear aqueous solution was let down and in a falling film evaporator on a sugar alcohol  content of about 70% and then concentrated after further evaporation in a vacuum crystallizer brought to crystallization with cooling. It became a white, slightly hygroscopic, odorless solid product obtained, ground to a fine crystalline powder has been. The sorbitol (D-glucitol) formed was otherwise highly pure and showed one in the stable γ-form Melting point of 101 ° C. The content of non-hydrogenated α-D-glucose was 0.1%. The Ni content was included <1 ppm. The catalyst was also after a run time of 9800 hours effective unchanged.

Example 2

Through a high pressure tube as in Example 1 at a temperature of 105 ° C and a hydrogen pressure of 150 bar, the hydrogen in the reverse reaction flow, as described in Example 1, counter to the rising solution of α-D-glucose, with an equal amount every hour 40% aqueous solution of α-D-glucose was hydrogenated as in Example 1, which had a pH of 6.5. The catalyst was made by tabletating nickel powder. With a cylinder height of 5 mm and a diameter of 5 mm, the tablets had a compressive strength of 149 N on the cylinder surface and an inner surface area of 62 m ² / g.

After a running time of 4200 hours with undiminished Efficacy, the conversion of α-D-glucose was 99.9%. The content of non-hydrogenated α-β-D-glucose in the crystallized sorbitol (D-glucitol), the one Purity level of 99.5% was 0.1%. The Ni content was <1 ppm.

Example 3

In a high-pressure tube as in Example 1 at an temperature of 115 ° C and a hydrogen pressure of 300 bar in the same way as in Example 1, an equal amount of a 40% aqueous solution of α-D-glucose was hydrogenated, which has a pH Had a value of 7.5. The catalyst was obtained by tableting a powdered nickel-iron alloy. The alloy contained 15% iron in nickel. With a cylinder height of 5 mm and a diameter of 5 mm, the tablets had a compressive strength of 137 N on the surface of the cylinder and an inner surface of 83 m ² / g. The crystalline sorbitol (D-glucitol) obtained in a vacuum crystallizer had a purity of 99.5%. The content of unconverted α-D-glucose was 0.1%. The Ni and Fe contents were together <1 ppm. The catalyst was still effective after a running time of 8400 hours.

Example 4

In a high pressure tube as in Example 1 at a temperature of 110 ° C and a hydrogen pressure of 300 bar in the same way as in Example 1, 150 ml of a 35% aqueous solution of α-D-glucose was hydrogenated, which has a pH of 6.5 had. The catalyst had been obtained by tableting cobalt powder. With a cylinder height of 5 mm and a diameter of 5 mm, the tablets had a compressive strength of 225 N on the surface of the cylinder and an inner surface of 19 m ² / g. The sorbitol (D-glucitol) obtained in a vacuum rotary tube showed an unreacted α-D-glucose content of 0.2%. The Co content was <1 ppm. The catalyst was still undiminished after a running time of 1000 hours.

Example 5 (comparative example)

An equally large amount of a 40% strength aqueous solution of α-D-glucose was hydrogenated through a high-pressure tube as in Example 1 at a temperature of 110 ° C. and a hydrogen pressure of 300 bar in the same way as in Example 1 at the same time, which had a pH of 6.5. The catalyst had been prepared by applying an aqueous nickel salt solution on an inert spherical Al ₂ O _{3 support} (ball diameter: 5 mm) and then converting the nickel to the metallic state by reducing it in a hydrogen stream. The nickel content of the catalyst was 20%. The inner surface of the catalyst was 140 m ² / g. The sorbitol (D-glucitol) obtained in a vacuum crystallizer had a purity of 94.9%. The content of unreacted α-D-glucose was 1.9%. In addition, other foreign sugar constituents were found in an amount of 3.2% - including mannitol in an amount of 2.1% - so that the sorbitol thus obtained could not be used as a sugar substitute in this production form without ecologically complex cleaning procedures. By increasing the reaction temperature from 110 to 125 ° C, the proportion of unreacted α-D-glucose could be reduced to a value of 0.6%, but at the same time the proportion of organic contaminants rose to a value of 6.2 % - including 3.9% of mannitol. In addition, the reaction product was contaminated with 42 ppm Ni.

Example 6 (comparative example)

An equally large amount of a 40% strength aqueous solution of α-D-glucose was hydrogenated through a high-pressure tube as in Example 1 at a temperature of 115 ° C. and a hydrogen pressure of 300 bar in the same manner as in Example 1, which one had a pH of 6.5. The catalyst had been prepared by applying aqueous nickel and iron salt solutions to an inert, spherical Al ₂ O _{3 support} (ball diameter: 5 mm) and then converting the nickel and iron into the metallic state by reducing them in a stream of hydrogen. The nickel content of the catalyst was 16%, the iron content 4%.

The inner surface of the catalyst was 155 m ² / g.

That obtained by evaporation in a vacuum crystallizer Sorbitol (D-glucitol) had a purity of 93.8%. The content of unreacted α-D-glucose was at 1.6%. In addition, organic contaminants in an amount of 4.6% - including mannitol in one Quantity of 2.9% - and Ni impurities of 38 ppm and Fe impurities of 16 ppm, so that the sugar alcohol obtained is not without ecological agile cleaning measures as a sugar substitute could be used. After a term of The Kataly fell noticeably for 800 hours sator activity determined.

Claims (4)

1. A process for the preparation of sorbitol (D-glucitol) from α-D-glucose by catalytic hydrogenation in aqueous solution with hydrogen under elevated pressure and at elevated temperature, characterized in that the hydrogenation is carried out continuously at a hydrogen pressure of 150 to 500 bar and temperatures of 60 to 120 ° C in a fixed bed process in a reaction zone over serving as hydrogenation catalysts unsupported shaped bodies with a compressive strength of more than 50 N, preferably 100 to 400 N, on the molded body surface and an inner surface of 10 to 90 m ² / g of elements of the iron group of the periodic table. 2. The method according to claim 1, characterized in that that the shaped bodies are such Metal powders made of nickel, cobalt or their Mixtures or alloys with each other or with Iron made with macroscopically smoother Surface. 3. The method according to claim 1, characterized in that the hydrogenation catalysts are cylindrical or spherical shaped bodies with a diameter are from 3 to 7 mm.   4. The method according to claim 1, characterized in that the hydrogenation of α-D-glucose in 15 to 45% aqueous solution with a pH of 3.5 to 7.0.