Method for recovering catalyst

Abstract

A catalyst is recovered from an aqueous reaction mixture comprising heterocyclic nitroxyl catalyst and oxidized cellulose, by: separating the oxidized cellulose from the reaction mixture, contacting the reaction mixture with solid hydrophobic adsorbent particles with particle sizes below 350, preferably below 200 m, more preferably below 100 m, said particles being silica particles provided with functionalized hydrophobicity, adsorbing the catalyst to the hydrophobic adsorbent particles, and eluting the catalyst from the adsorbent particles with an organic solvent.

Claims

1. A method for recovering catalyst from an aqueous reaction mixture comprising heterocyclic nitroxyl catalyst and oxidized cellulose, the method comprising: separating the oxidized cellulose from the reaction mixture, after the separation of the oxidized cellulose from the reaction mixture, contacting the reaction mixture with solid hydrophobic adsorbent particles with particle sizes below 350 m, said particles having surfaces made hydrophobic by functional groups, adsorbing the catalyst to the hydrophobic adsorbent particles, and eluting the catalyst from the adsorbent particles with an organic solvent.

2. The method according to claim 1, wherein the functionalized hydrophobicity is carbon-chain functionalized hydrophobicity.

3. The method according to claim 2, wherein the carbon-chain functionalized hydrophobicity is C6-C18 hydrophobicity.

4. The method according to claim 1, wherein the hydrophobic particles are silica particles with functionalized hydrophobicity.

5. The method according to claim 1, wherein the organic solvent is water soluble.

6. The method according to claim 5, wherein the water-soluble organic solvent is ethanol, methanol or acetone.

7. The method according to claim 1, wherein the reaction mixture is introduced through a column packed with the solid adsorbent particles, whereafter the organic solvent is introduced through the same column.

8. The method according to claim 1, wherein after the separation of the oxidized cellulose, colloidal matter is removed from the reaction mixture before the reaction mixture is contacted with solid hydrophobic adsorbent particles.

9. The method according to claim 1, wherein the recovered catalyst is recycled to catalytic oxidation of cellulose.

10. The method according to claim 1, wherein the oxidized cellulose exists in fibers in the reaction mixture, and the oxidized cellulose is separated from the reaction mixture by separating the fibers from the reaction mixture.

11. The method according to claim 1, wherein the particle sizes of the solid hydrophobic adsorbent particles are below 200 m.

12. The method according to claim 1, wherein the particle sizes of the solid hydrophobic adsorbent particles are below 100 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the invention will be described with reference to the appended drawings, where

(2) FIG. 1 illustrates the recovery of the catalyst in example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) In the following disclosure, all percent values are by weight, if not indicated otherwise. Further, all numerical ranges given include the upper and lower values of the ranges, if not indicated otherwise.

(4) In the present application all results shown and calculations made, whenever they are related to the amount of pulp, are made on the basis of dried pulp.

(5) General Description of Catalytic Oxidation and Catalyst

(6) In the invention, the catalyst to be recovered is the result of a catalytic oxidation process where primary hydroxyl groups of cellulose are oxidized catalytically by a heterocyclic nitroxyl compound, for example 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical, TEMPO. Other heterocyclic nitroxyl compounds known to have selectivity in the oxidation of the hydroxyl groups of C-6 carbon of the glucose units of the cellulose can also be used, and these compounds are widely cited in the literature. Hereinafter, the oxidation of cellulose refers to the oxidation of these hydroxyl groups to aldehydes and/or carboxyl groups.

(7) Whenever the catalyst TEMPO is mentioned in this disclosure, it is evident that all measures and operations where TEMPO is involved apply equally and analogously to any derivative of TEMPO or any heterocyclic nitroxyl radical capable of catalyzing selectively the oxidation of the hydroxyl groups of C-6 carbon in cellulose after it has been activated. Other known members of this group are the TEMPO derivatives 4-methoxy-TEMPO and 4-acetamido-TEMPO.

(8) In this disclosure, catalytic oxidation refers to nitroxyl-mediated (such as TEMPO-mediated) oxidation of hydroxyl groups. The catalytic oxidation of fibers or fibrous material in turn refers to material which contains cellulose that is oxidized by nitroxyl-mediated (such as TEMPO-mediated) oxidation of hydroxyl groups of the cellulose.

(9) The heterocyclic nitroxyl compound used as catalyst in the oxidation process (such as TEMPO) is stable in its neutral, radical form, and it can be stored in that form. After the catalyst is activated to the oxidized form, it can participate at once in the reaction, and the oxidation process of the cellulose starts quickly.

(10) The structural formula of TEMPO in its radical form is given below

(11) ##STR00001##

(12) The catalyst can be activated in various ways. After the activation, the oxidation reaction can be started and performed to completion to a desired conversion degree in a reaction medium in the presence of the catalyst, cellulose and main oxidant. The reaction medium can be water-based medium where the materials are dissolved and suspended. In the case of pulp raw material, the cellulose exists in fibre form as suspension in water in a suitable consistency, whereas the catalyst and the main oxidant are dissolved in the water. The pH of the reaction medium is controlled during the reaction to keep it in the optimum range. Temperature of the reaction medium may also be controlled.

(13) Recovery of the Catalyst

(14) After the catalytic oxidation of cellulose, the oxidized cellulose in fibrous from is separated from the reaction mixture. Colloidal suspended matter, which could not be separated together with the fibrous matter, mainly cellulose and hemicellulose, is removed by more accurate separation methods, such as fine filtration of centrifugation.

(15) A clear aqueous reaction mixture containing remaining dissolved matter, including the catalyst, is introduced through a packed column of a solid adsorbent, which consists of particles having surfaces that are hydrophobic through functionalization made on the particle surfaces. The column can be a chromatography column, through which the reaction mixture is pumped so that it flows from the top downwards. In course of the flow through the column, the catalyst, which is in the reduced neutral form, separates as it is retained by the hydrophobic adsorbent to which it has affinity while the dissolved salts travel through the column along with the flow of the aqueous mixture. Visually the separated catalyst can be detected as a coloured zone in the column.

(16) The catalyst is eluted from the adsorbent particles by introducing a small volume of organic solvent, preferably methanol or ethanol, through the packed column. The organic solvent is preferably water-soluble. The catalyst in solid form can be recovered by evaporating the solvent.

(17) The size of the adsorbent particles is below 350 m, preferably below 200 m, and more preferably below 100 m, which makes the surface area of adsorbent in the packed column large with regard to the volume of the packed column. The small particle size is easy to achieve with material that is not inherently hydrophobic. The particles can be made hydrophobic chemically by functionalization, as explained above.

(18) The functionalization is carried out through carbon chains chemically bonded to the surfaces of the particles. Alkyl chains between C6-C18 can be used.

(19) Suitable adsorbent particles include silica-based particles with carbon-chain functionalized hydrophobicity on their surfaces, especially C16-C18 functionalized.

(20) For example with C-18 functionalized silica particles, a separation efficiency above 99.5% (amount of catalyst adsorbed/initial amount of catalyst in the reaction mixture) is achieved. The enrichment factor, which can be calculated as ratio reaction mixture volume/solvent volume, is normally over 30, which means that the liquid volume where the catalyst is dissolved is reduced to less than 1/30 parts of the original, meaning that the concentration of the catalyst is increased over 30-fold, which makes the catalyst reusable in a subsequent catalytic oxidation process of cellulose. The catalyst can be concentrated further by evaporating the solvent, which is easy if the solvent is an organic volatile solvent, such as ethanol, methanol, THF or acetone. Acetone seems to be a good solvent in view of the easiness to remove it by evaporation.

(21) NFC

(22) The heterocyclic nitroxyl catalyst is recovered from and it can be reused in a process for catalytic oxidation of cellulose for the purpose of making nanofibrillar cellulose (NFC). The oxidation process takes place in an aqueous reaction medium containing the cellulose as fibrous raw material, the catalyst, which may have been activated in advance or is activated in the reaction medium, and the main oxidant, preferably hypochlorite.

(23) The fibrous raw material is suspended in the reaction medium can be any fibres consisting mainly of cellulose, especially fibres of plant origin. The fibres, when suspended in the aqueous reaction medium, form a pulp of given consistency. The fibers can be especially from wood. Chemical pulp, such as softwood or hardwood pulp, for example bleached birch pulp, can be used.

(24) The oxidation reaction is allowed to proceed till a required conversion degree (oxidation level) has been achieved. As expressed in carboxylate groups generated as the result of oxidation, this is normally 0.5-1.4 mmol COOH/g pulp.

(25) For the purpose of making NFC, it has been found that the oxidation level (conversion degree) of 0.5-1.1 mmol COOH/g pulp, preferably 0.6-0.95 and most preferably 0.7-0.9 is already sufficient that the cellulose fibers can be easily disintegrated to fibrils by mechanical energy.

(26) After the desired conversion degree has been attained, the reaction medium is taken out from the reactor. The fibres containing the oxidized cellulose are separated from the reaction medium, and the reaction medium is subjected to further purification to remove the colloidal matter before the recovery of the catalyst as explained above.

(27) The fibres are washed to remove the remnants of the chemicals and processed further to NFC.

(28) The term nanofibrillar cellulose refers to a collection of isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. Microfibrils have typically high aspect ratio: the length might exceed one micrometer while the number-average diameter is typically below 200 nm. The diameter of microfibril bundles can also be larger but generally less than 1 m. The smallest microfibrils are similar to so called elementary fibrils, which are typically 2-12 nm in diameter. The dimensions of the fibrils or fibril bundles are dependent on raw material and disintegration method. The nanofibrillar cellulose may also contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Fibers containing oxidized cellulose are more easily disintegrated to nanofibrillar cellulose because the carboxylic groups formed in the cellulose weaken the internal bonds of the fiber.

EXAMPLES

(29) The recovery method is described by way of following examples, which shall not be interpreted as limiting the method.

(30) The glass columns were filled with octadecyl silane sorbent (C18 sorbent) with the particle size of 40 m and nominal porosity of 60 . During the packing, the sorbent was wetted with water to ensure dense packing. The C18 sorbent consists of non-polar octadecylsilane-bonded, irregular silica gel (silica) particles.

Example 1 (TEMPO Extraction)

(31) 90 liter of process water used in TEMPO oxidation has been first separated by decanter centrifuge to collect the colloidal substances of the oxidation process. The colloidal substances are mainly cellulose and xylans. After the separation the solution has been pumped through C18 column, where the separation of TEMPO molecules from the solution takes place. The separated TEMPO can be seen as symmetrical red zone in the used column.

(32) The extraction is performed with ethanol and the extraction takes place downward. 1.8 liter ethanol is enough to extract the TEMPO from the column and in the extractant was a precipitate. The extractant was distilled with vacuum (33-40 mbar and 16-19 C.). The separation efficiency has been followed by measuring the TEMPO concentration in different solutions. The starting TEMPO concentration was 160 mg/liter in the process solution and after the column treatments the TEMPO concentration in process water was decreased to 0.05 mg/liter. This means that the trapping efficiency is 99.97% of TEMPO to C18 adsorbent material. The TEMPO concentration in ethanol was 7200 mg/liter. The extraction efficiency was 93% and the enrichment factor 90:1.8=50.

Example 2 (TEMPO Extraction)

(33) 90 liter of process water used in TEMPO oxidation has been first separated by decanter centrifuge to collect the colloidal substances of the oxidation process. The colloidal substances are mainly cellulose and xylans. After the separation the solution has been pumped through C18 column, where the separation of TEMPO molecules from the solution takes place. The separated TEMPO can be seen as asymmetrical red zone in the used column due to the unwanted flow phenomenon in the column. The extraction is performed with ethanol and the extraction takes place downward. 1.8 liter ethanol was used to extract the TEMPO from the column and the extractant has a precipitate. The TEMPO concentration in ethanol was 5100 mg/liter. In the water/ethanol solution was 4200 mg/liter TEMPO. The extraction efficiency was 93% and the enrichment factor 90:(1.8+1)=34.

Example 3 (TEMPO Extraction)

(34) 70 liter of process water used in TEMPO oxidation has been first separated by decanter centrifuge to collect the colloidal substances of the oxidation process. After the separation the solution has been pumped through narrow C18 column (capacity of 886 cm.sup.3, 47 cm long, diameter 4.9 cm), where the separation of TEMPO molecules from the solution takes place. The separated TEMPO can be seen as symmetrical red zone in the used column.

(35) The extraction is performed with acetone and the extraction takes place downward in the speed of 90 ml in minute. 1 liter acetone is enough to extract the TEMPO from the column and the extractant contained 11.2 g of TEMPO. The extractant was distilled with vacuum (10-35 mbar and 15-19 C.). The easy distillate fraction was bright up to 54% of the initial volume and from 54 to 90% it got more yellow colour. The separation efficiency has been followed by measuring the TEMPO concentration in different solutions. After distillation of the readily distillable fraction of the acetone, the residue acetone solution containing TEMPO was was evaporated in fume hood under atmospheric temperature and pressure with airflow. The almost dry TEMPO was collected. The recovery of solid TEMPO was 9.1 g, which means 81% recovery field of solid TEMPO. The extraction efficiency was 93% and the enrichment factor 70:1=70.

Example 4 (Oxidation of Pulp)

(36) Activation of radical TEMPO was first carried out. 0.375 g radical TEMPO was weighted and transferred to closed glass bottle. 50 ml of water was added to bottle. 4 ml of NaClO (12.9%) solution was added to TEMPO solution. pH of TEMPO solution was adjusted to 7.5 by 1 M H.sub.2SO.sub.4 using pH meter. Solution was mixed strongly until all radical TEMPO was dissolved.

(37) 243 g (48 g as dry) never-dried birch pulp was weighted in closed vessel. Activated TEMPO solution was mixed with pulp. Pulp was shifted to Buchi reactor and 819 ml water was mixed with pulp. Temperature of pulp was set to 18 C. 63 ml (12.9%) NaClO was added to reactor by pump while pulp was mixed strongly. The pH was kept under 9 during NaClO addition by controlling pumping speed. Temperature of pulp was lifted to 25 C. after NaClO addition and pH was controlled by titrator (pH 9, 1 M NaOH) until all NaClO was consumed. Active chlorine titration was used to monitor NaClO consumption during oxidation process. Strong mixing was continued until all NaClO was consumed. Pulp was washed with ion changed water after oxidation. Carboxylate content of pulp (conductometric titration) was determined after pulp consistency determination.

(38) Conversion of residual aldehydes to carboxylates by acidic phase oxidation was carried out in the second stage oxidation. 10 g (calculated as dry) of TEMPO oxidized pulp was weighted and shifted to Buchi reactor. Pulp was diluted by 1000 ml of water. 0.6 g NaClO.sub.2 and 2 ml DMSO was mixed with pulp solution. pH of solution was adjusted to 3 by 1 M H.sub.2SO.sub.4 using pH meter. Temperature of pulp solution was adjusted to 50 C. and solution was mixed 2 hours until oxidation was completed. Pulp was washed with ion-changed water after oxidation. CED-viscosity and carboxylate content of pulp (conductometric titration) was determined after pulp consistency determination.

(39) The following table compares the performance of fresh catalyst (reference) with the performance of recovered and recycled catalyst.

(40) TABLE-US-00001 TABLE 1 Oxidation parameters in oxidation with pure fresh TEMPO (reference) and recycled TEMPO (examples 1 and 3) recycled recycled REF TEMPO TEMPO Experiment oxidation (E1) (E3) Reaction time (min) 154 173 171 HOCl addition (mmol NaClO/g 2.4 2.8 2.4 pulp) TEMPO addition (mmol/g pulp) 0.05 0.05 0.045 mmol COOH/g pulp (1st stage 0.87 0.82 0.91 oxidation) mmol COOH/g pulp (2nd stage 0.98 0.95 1.04 oxidation) Temperature ( C.) 25 25 25 pH 9 9 9

Example 5. Fluidisation

(41) Pulp consistency of the oxidized pulp sample was adjusted to approximately 1.5% by water. Sample was mixed by Turrax 10 min. pH was adjusted to 9 by NaOH and pH meter. Pulp solution was forced by 2000 bar pressure through 200 m chamber and 100 m chamber (=1 pass) of fluidizator (Microfluidics M110P). Pulp dispersion formed a gel in fluidization.