METHOD FOR THE SEPARATION OF LOW MOLECULAR-WEIGHT AROMATIC COMPOUNDS

Abstract: Method for the separation of low molecularweight aromatic compounds, e.g. vanillin and dehydrodivanillin, from lignin, alkali and from each other, in alkaline aqueous solutions by the aid of ion-exchange resin, for solving the problems cause by: neutralizing the alkaline solution with concurrent precipitation of the lignin; extracting low molecular aromatic compounds from large amounts of dilute solutions containing large amounts of precipitated lignin; and regenerating the ion-exchange resin after each batch. According to the method, the separation is effected by means of a cationic ion-exchange resin in its sodium salt form, e.g. of a strong sulphonic acid type or weak carbonic acid type, from which the substances adsorbed are removed by elution. The resin may be crosslinked to the extent of 2 - 16% divinylbenzene. The substances adsorbed onto the ion-exchange resin are appropriately eluted by either water alone, or by application of an aqueous sodium-salt solution, e.g. Na<u2>uCO<u3>u followed by water.

Description: METHOD FOR THE SEPARATION OF LOW MOLECULAR- WEIGHT AROMATIC COMPOUNDS This invention relates to a method for the sepa- ration of low molecular-weight aromatic compounds, e.g. vanillin and dehydrodivanillin, from lignin, alkali and from each other, in alkaline aqueous solutions by the aid of a cationic ion-exchange resin. Background of the Invention It is known from prior art that vanillin can be manufactured from solutions with a content of lignosul- phonic acids and their salts, for example from spent sul- phite liquor, by air oxidation of the substrate mixed with either sodium hydroxide or calcium hydroxide plus sodium carbonate. After the oxidation, the vanillin thus formed from lignin is present in the alkaline solution as water- soluble sodium vanillate. The vanillin can be isolated- frαn the oxidised solution by acidifying for example with carbon dioxide and sulphuric acid, separating the lignin precipitated, and extracting the vanillin by the aid of a suitable solvent, e.g. benzene or toluene. (Hibbert, H. , To linson, G..Jr., US pat. 2 069 185, Jan. 26, 1937). Methods are also known, according to which the vanillin is extracted from the oxidised solution as its sodium salt by using a higher alcohol (e.g. n-butanol or isopropanol) (Sandborn, L.R., Salvesen, J.R., Howard, G.C., US pat. 2 057 117, Oct. 13, 1936; Bryan, CC, Can.pat. 528 837, Aug. 7, 1956) . It is also known that carbonyl compounds can be isolated by the aid of cation exchangers, on which the resin first is allowed to react with certain nitrogen- containing compounds, for example with hydroxylamine and hydrazine, which alter the chemical character of the resin in a way such that it is capable of reacting quantitativ¬ ely with the carbonyl compounds (Tδppel, O. , US pat. 2 897 238, July 28, 1959) . A method of isolation is also known by which the sodium salt of vanillin is transformed into vanillin by th aid of weakly protonised cation exchange resins, which at the same time are transformed into their sodium salt mod¬ ification. The inherent disadvantage of the methods de¬ scribed above is the need of neutralisation in order to isolate the vanillin, with concurrent precipitation of the lignin, extraction of vanillin from large volumes of dil¬ ute vanillin solutions containing large amounts of pre¬ cipitated lignin, chemical transformation of the ion ex¬ change resin into a modification that makes it possible to effect adsorption, or regeneration of the ion-ex- change resin into its original ionic modification after each batch. Summary of the Invention In the present invention it has been noted that sodium vanillate can be adsorbed onto a cation ex¬ cchhaannggeerr iijn Na -form direct from the oxidised alkaline solution. The invention is characterised by the ion-ex- change resin being in its same cation form all through the process, so that no separate regeneration step is required. On the basis of the experiments made it has been established that large quantities of oxidised sol- ution can be fed into the column whereby the ligno- sulphonates, sodium hydroxide and sodium carbonate are eluted before the sodium vanillate which is adsorbed onto the surface of the resin. The sodium vanillate ad¬ sorbed can later be eluted with either water of dilute alkali or alkali salt solution or by combination of these making it practicable to enrich the sodium vanillate for further treatment. Moreover, we have noted _ OM y wip that by this procedure more than half of the alkali in the oxidised solution is recoverable along with the lignin. This ' liquor, freed from sodium vanillate, can be led direct to evaporation and regeneration of chemicals, whereby the alkali can be recycled to the sulphate pulp mill without raising the total sulphur content. We have also estab¬ lished that the amount of acid required for the neutra¬ lisation of the vanillate fraction has usually diminished by more than 60 % from the original consumption, and that precipitation of lignin does not occur in connection with the neutralisation of the vanillate fraction. This is of considerable advantage, as in processes of vanillin manu¬ facture according to current technology, the neutralis¬ ation and isolation of the vanillin from the lignin pre- σipitated are considerable cost factors and present technical problems. If the substances adsorbed in the column are eluted with a sodium salt solution, the sodium vanillate can be separated also from other oxidation products. Description of Preferred Embodiment The ion-exchange resins utilised in the ex¬ periments made have been either of the stro sulphonic acid type or cation-exchangers of weak carbonic acid type. Dowex-50W, X-8, has proved to be a particularly appro¬ priate strong cation-exchanger owing -to its low proport¬ ion of crosslinks, resulting in considerable contraction in NaOH-solution and swelling in aqueous solution. On the other hand, the adsorption of sodium vanillate onto the resin diminishes as the proportion of divinyl benzene crosslinks increases in the resin. The invention is directable to any cation-ex¬ change resin that can be transformed into Na -modification and that unchanged withstands conditions under which the pH of the alkaline solution is higher than 10. In the method covered by the present invention, the vanillin-containing lignin solution obtained in the oxidation-reactor is fed into a column filled with the cation-exchanger. The sodium vanillate and related subs¬ tances become adsorbed onto the surface of the resin while sodium lignosulphonates, sodium hydroxide and sodium car¬ bonate are eluted before the sodium vanillate. Solutions with a content of vanillin can be introduced into the column as long as adsorption takes place. Description of the Drawings Figs. 1-8 illustrate the elution of samples in¬ vestigated through a column filled with a cation exchange resin in its Na -modification. The oxidation products ad¬ sorbed onto the surface of the resin are eluted with water or with a suitable water solution that retains the resin i its Na -form. Thus, no separate regeneration steps is req¬ uired. No precipitation occurs during the course of the adsorption and elution stages. Consequently, the oxidation products can be concentrated into a small quantity of elua while, at the same time, the column is purified for the next adsorption stage. The present invention will now be more particul¬ arly described with reference to the accompanying drawings wherein the following examples are used in detailed de- scription of the invention, but are not to be considered a limiting the scope of this invention to the amounts of substances, column measures, or running conditions cited in the examples. Example 1: A glass column, with a diameter of 2 cm and length of 40 cm, was mounted in an upright position so tha a hose pump could be connected to the bottom of the column making it possible to adjust the rate of flow of the sol- ution. The column was filled with Dowex-50W, X-2 ion-ex¬ change resin, 200 - 400 mesh, the height of the resin column being 25 cm. Prior to filling, the ion-exchange res was transformed into its Na -form by treating it with so¬ dium hydroxide solution (3 moles/litre) . 2 ml of oxidised lignosulphonate solution, with a vanillin content amounting to 9.9 g vanillin/ml, was fed into the column from the top at a rate of 21 ml/h (6.69 2 ml/h.cm ) . Since the absorptivity of vanillin in alkaline solution at 348 nm wavelength is 14.6 times higher than the absorptivity at 280 nm wavelength, the separation of, sodium vanillate can be followed spectrophotometrically. The sol- ution tha had passed the column was collected as 4 ml frac¬ tions which were analysed for their content of sodium and for the separation of lignin from vanillate by measurement of the absorbances at 280 and 348 nm wavelengths, respect¬ ively. When- the 2 ml sample had flown into- the column, the run was continued by feeding distilled water into the column at tha same rate. It was then noted that the lignin part moved in the column considerably faster than the vanillate part (Fig. 1) . As the inner volume of the column was 26 ml, the main proportion of the lignin was collected within the elution volume range of 26 - 45 ml, while the sodium vanillate was collected within the elution volume range 55 - 65 ml. The. sodium peak was located immediately before the sodium vanillate at 58 ml elution volume in a way such that 68 % of the sodium contained in the sample had been eluted from the column prior to the vanillate fraction. The area of the vanillate region was determined by planimetry, and was 28 units. The lignosulphonate fraction had been fully se- parated from the sodium vanillate and no precipitation was observed during the process. Example 2: The procedure in Example 1 was followed except that the volume of the sample was 16 ml. In this experiment/ the lignosulphonate fraction broadened to extend over the elution volume range 26 - 75 ml. The vanillate fraction be¬ came eluted after this at the volume 75 - 90 ml (Fig. 2) . nillate region. The area of the vanillate region was 268 units. The vanillate fraction did not precipitate upon neutralisation with sulphuric acid. Example 3: The procedure in Example 1 was followed, except that the volume of the sample was 32 ml. The lignin fract- ion broadened strongly to extend over the entire elution range. However, the main proportion of the lignin came out from the column prior to the sodium vanillate (Fig. 3) . The lignin contained in the vanillate fraction did not pre¬ cipitate upon neutralisation with sulphuric acid. The area of the vanillate region was planimetered to.528 units. Example 4: The procedure in Example 1 was followed, except that the volume of the oxidised lignosulphonate solution was 64 ml. Again, the lignin fraction broadened strongly; nevertheless, the vanillate fraction did not appear until after the lignin, at the elution volume 120 - 160 ml (Fig. 4) . Since the column was incapable of adsorbing all of the sodium vanillate contained in the sample, two different peaks are observable in the vanillate region in Fig. 4, of which the larger peak depicts the sodium vanillate boun to the column. The lignin contained in the vanillate did not precipitate upon neutralisation with sulphuric acid. The area of the vanillate region was 734 units. When the planimetered vanillate regions in Ex¬ amples 1 - 4 are considered as a function of the sample volume, it can be noted that the area increases linearly as long as there occurs adsorption onto the resin. When th resin no longer is capable of binding more sodium vanillat the area no more grows despite increases in sample volume. On the basis of Examples 1 - 4, a curve can be drawn il¬ lustrating the relationship between the area of the va- O nillate region and the sample volume (Fig. 5) . From this curve, it ' can be deduced that the adsorption capacity of the ion exchange resin used in the previous examples had been fully utilised at a sample volume of 42.7 ml. Since the sample solution had a content of 9.9 mg vanillin/ml, it can be calculated that the adsorption capacity of the column in respect of sodium vanillate amounted to 423 g, or 5.4 kg sodium vanillate per cubic metre of resin. Example 5: In the 5th example a column was employed whose diameter was 2 cm and- length 95 cm. The column was filled with 300 cm of Dowex-50W, X-2, 200 - 400 mesh cation-ex- change resin in the sodium form. 6 ml oxidised lignosulpho¬ nate solution was fed into the column at the rate of 20 2 ml/h (6.37 ml/h.cm ), and, after the sample had flowed into the column, distilled water was introduced at the same rate until the column had again become purified. In this ex- periment it was observed that the lignosulphonate fraction became somewhat extended (elution colume 100 - 150 ml) ; however, the vanillate part was distinctly separate from the lignin (the elution volume of the sodium vanillate was 195 - 205 ml) , and the sodium vanillate appeared immediately after the maximum of the alkali concentration (Fig. 6) . This test implies that an optimum can be found by changing the volume and diameter of the column, so that the separation of sodium vanillate from lignin is as complete as possible under the experimental conditions applied. Example 6 A column was used with a diameter of 1.09 cm and length 35 cm. The column was filled with 25 cm of Dowex-50W, X-8, 200 - 400 mesh cation-exchange resin in its sodium form. Oxidised lignosulphonate solution with a content of 10.2 mg vanillin/ml was fed into the column at 2 the rate of 25.8 ml/h (27.7 ml/h.cm ), until the adsorpt¬ ion capacity of the resin had been fully consumed. The amound of sodium vanillate bound by the column at the sat¬ uration point was calculated. It was established that the cation exchange resin used was capable of adsorbing 5.5 kg sodium vanillate/m resin. Example 7: A glass column, diameter 4 cm and length 200 cm, was filled with 2500 cm of Dowex-50W, X-8, ' 200 - 400 mesh strong sulphonic acid-type cation-exchange resin in the Na -form. 800 ml of oxidised lignosulphonate solution with content of 22 g vanillin per litre was fed into the column 2 at the rate of 60 ml/h (4.8 ml/h.cm ). After the sample ha been introduced, the run was continued by pumping distille water into the column at the same rate until the column ha become purified. The solution that passed the column was collected as 40 ml fractions, which were subjected to spectrophoto etric determination of the absorbancies at 280 and 348 nm wavelengths and analysed for their contents of oxidation products and consumption of sulphuric acid on neutralisatio . In this test it was noted that the lignosulpho¬ nate part was very broad, and that the vanillate part appeared after the lignin area, althought these were partl superimposed (Fig. 7) . The pH of the lignin area was 12 or higher, while that of the vanillin area ranged from 7 to 12. No precipitation took effect upon neutralisation with sulphuric acid. The amount of sulphuric acid consumed in the neutralisation of the vanillate area had diminished by 62.5 % as compared with the amount of sulphuric acid con- sumed in the neutralisation of the entire sample. At the peak of the vanillate region, the concentrations of the oxidation products were more than twice as high as those in the original lignosulphonate solution fed into the co¬ lumn. In the elution of the adsorbed oxidation products from the column, the quantity of distilled water required equalled the quantity of oxidised solution introduced in the experiment. Example 8: The procedure of Example 6 was followed, except that the column was filled with Dowex-50W, X-16, 200 - 400 mesh cation-exhanger in the Na -form. It was established that the column was capable of adsorbing 1.3 kg sodium va- 3 nillate per m resin. Example 9: The procedure of Example 6 was followed, except that the column was filled with Amberlite-200 macroporous, strong sulphonic acid type cation-exchanger in the Na - form. It was established that the column was capable of ad- 3 εorbing 2.4 kg sodium vanillate per m resin. Example 10: The procedure of Example 6 was followed, except that the cojumn was filled with AG-MP-50, 200 - 400 mesh macroporous, strong sulphonic acid-type cation-exchanger + in the Na -form (Bio-Rad) . It was " establisσhed in this test that the resin was capable of adsorbing 2.0 kg sodium va- nillate per m 3. Example 11: The procedure of Example 6 was followed, except that the column was filled with Amberlite-CG-50, 200 mesh macroporous metacrylic acid type weak cation-exchanger in the Na -form. It was noted that the column was capable of adsorbing 2.0 kg sodium vanillate per m Example 12 : A glass tube, diameter 2 cm and length 40 cm, wa filled with 78.5 cm 3 of Dowex-50W, X-2, 200 - 400 mesh + strong cation-exchange resin in the Na -form. 4 ml of oxidised lignosulphonate solution, with a vanillin content of 11 mg/1, was introduced into the co- 2 lumn at the rate of 20.2 ml/h (6.44 ml/h.cm ) . After the sample, 75 ml of an aqueous sodium carbonate solution (1 mole/1) was fed into the column at the same rate. This was. followed by introduction of distilled water until all of the substances adsorbed onto the resin had been eluted from the column. The solution that had passed through the column was collected as 4 ml fractions, which were sub- jected to spectrophotometric determination of the absorb- ance at wavelengths 280 and 348 nm. On the basis of this test it was established tha the lignin is eluted by the sodium carbonate solution ex¬ actly as in the previous examples by water, whereas the oxidation products are eluted more slowly than by water an distinctly after the lignosulphonate part (Fig. 8) . More¬ over, it was established that as the elution is continued with distilled water, sodium vanillate is the first oxi¬ dation product removed from the column, and is followed by the sodium salt of dehydrodivanillin. Upon neutralisation of the vanillate region with sulphuric acid, carbon di¬ oxide was liberated but no precipitation was detectable. On the basis of the test it was established that the strong cation-exchange resin in the sodium form was capable of removing the oxidation products from the oxi¬ dised sodiumlignosulphonate solution, the oxidation prod¬ ucts being adsorbed onto the surface of the resin. The object of introduction of sodium carbonate solution into the column, after the sample, was that of purifying the column from the excess of lignin solution. The object of the introduction of distillied water was that of eluting the sodium vanillate and related compounds from the column ' O On the basis of the above examples it can be stated that the invention is applicable in the separation of oxidation products, by which is understood sodium va¬ nillate, sodium vanillate type aromatic compounds and di- mers or trimers formed during the oxidation of lignin, from an oxidised lignin solution that can moreover be collected for further treatment. By changing the eluant, the oxidation products are furthermore isolable as aromatic monomers, such as so- dium vanillate, and other oxidation products formed in the oxidation of lignin, such as the sodium salt of dehydrodi- vanillin.

Claims: (received by the Intemational Bureau on 30 June 1981 (30.06.81)) Claims 1 to 9 (cancelled) 10. A method for the separation of sodium vanillate from lignin and alkali in alkaline aqueous solutions, characterized by feeding an alkaline aqueous solution contai sodium vanillate and a material selected from the groups of lignin and alkali into a bed of a cation exchange resin in i sodium salt form, said sodium vanillate and said material be adsorbed on the resin at different strength levels, and elut the adsorbed sodium vanillate and said material from the res by aqueous solution, said sodium vanillate being collected primarily in one elution volume and said material being collected primarily in a second elution volume. 11- The method according to claim 1 , characterized by eluting the sodium vanillate and said material from the resin with an aqueous sodium salt solution followed by water 12. The method according to claim 1 or 2 , character by eluting the sodium vanillate and said material from the resin with an aqueous solution of sodium carbonate. 13. The method according to anyone of claims 1-3, characterized by utilizing a strong sulphonic acid-type cati exchange resin to adsorb said sodium vanillate and said mate 14. The method according to anyone of claims 1-3, characterized by utilizing a macroporous metacrylic acid-typ weak cationic exchange resin to adsorb said sodium vanillate and said material. 15. The method according to anyone of claims 1-5, characterized by maintaining the pH of said aqueous solution at a value above 12.0.

Inventors: FORSS K, FREMER K, TALKA E
Applicants: FORSS K, FREMER K, TALKA E
Application year: 1981
Main classification: B01D15
IPC codes: B01J47, C07C45, C07C47