PROCESS FOR THE OXIDATION OF PRIMARY ALCOHOLS TO CARBOXYLIC ACIDS

Abstract

The present invention relates to a process for preparing carboxylic acids by oxidizing primary alcohols in the liquid phase in the presence of ruthenium dioxide as a catalyst.

Claims

1. A process for the preparation of carboxylic acids of formula (I)
R—COOH  (I), wherein R denotes alkyl, which is optionally further substituted once, twice or more than twice by aryl, hydroxy, halogen, cyano, alkoxy or aryloxy the process comprising at least the step of reacting alcohols of formula (II) in the liquid phase
R—CH.sub.2—OH  (II), wherein R has the meaning defined above with a gas comprising molecular oxygen, preferably dioxygen (O.sub.2) in the presence of ruthenium dioxide (RuO.sub.2).

2. The process according to claim 1, wherein in formulae (I) and (II) R is C.sub.1-C.sub.8-alkyl which is either not or substituted once by alkoxy.

3. The process according to claim 1, wherein in formulae (I) and (II) R is methyl, n-propyl or isopropyl, preferably methyl.

4. The process according to claim 3, wherein the compounds of formula (II) employed are prepared from renewable resources, preferably by fermentation of monosaccharides by yeasts and bacteria.

5. The process according to claim 1, wherein the compounds of formula (II) are water miscible and are employed as an aqueous solution at a level of from 0.5 to 95 vol.-%, preferably of from 1 to 90 vol.-%, more preferably of from 2 to 50 vol.-% and even more preferably of from 5 to 20 vol.-%.

6. The process according to claim 1, wherein the gas comprising molecular oxygen, preferably dioxygen (O.sub.2) is selected from pure dioxygen, mixtures of dioxygen and at least one inert gas, or air each of them whether dried or not.

7. The process according to claim 1, wherein the partial pressure of molecular oxygen, preferably dioxygen (O.sub.2) is typically from 10 hPa to 10 MPa, preferably from 200 hPa to 1 MPa, more preferably from 0.1 MPa to 5 MPa and yet even more preferably from 0.5 MPa to 5 MPa.

8. The process according to claim 1, wherein the ruthenium dioxide has specific surface area of at least 1 m.sup.2/g, preferably 1 to 300 m.sup.2/g and more preferably 2 to 200 m.sup.2/g, and even more preferably 10 to 200 m.sup.2/g as measured by gas adsorption according to BET method (ISO 9277:2010).

9. The process according to claim 1, wherein the ruthenium dioxide has crystal size as measured by powder X-ray diffraction of 0.5 nm to 200 nm, preferably 1 nm to 50 nm and even more preferably 1 nm to 30 nm.

10. The process according to claim 1, wherein the ruthenium dioxide is diluted with or supported on inert solid materials.

11. The process according to claim 1 being performed batchwise or continuously, preferably continuously.

12. The process according to claim 1 being performed continuously with residence times of 1 minute to three hours.

13. The process according to claim 1 being performed such that at least 20% of the mass of compound of formula (II) employed is converted, preferably 20 to 100%, more preferably 30 to 80%, even more preferably 30 to 60% and yet even more preferably 35 to 50%.

14. The process according to claim 1, wherein a three-phase fixed bed reactor, a trickle flow reactor, a fluidized bed reactor or a suspension reactor a stirred tank with gas inlet is employed to host the reaction.

15. The process according to claim 1, wherein the reaction temperature is 75 to 250° C., preferably 100 to 220° C., more preferably 130 to 220° C. and even more preferably 140 to 200° C.

Description

EXPERIMENTAL PART—EXAMPLES

[0059] General

[0060] A) RuO.sub.2 Catalysts

[0061] The RuO.sub.2 catalysts employed herein were commercially obtained from [0062] 1) Acros Organics, with a purity of 99.5+%, a crystal size t of 27.0 nm as measured by X-ray diffraction and a specific surface area S.sub.BET of 17 m.sup.2/g (hereinafter referred to as RuO.sub.2—Type 1) [0063] 2) Alfa Aesar with a purity of 99.95%, a crystal size t of 1.5 nm as estimated by X-ray diffraction and a specific surface area S.sub.BET of 107 m.sup.2/g (hereinafter referred to as RuO.sub.2—Type 2)
or derived from the aforementioned by [0064] 3) Calcinating RuO.sub.2—Type 2 at a temperature of 823 K for 5 hours, thereby producing a ruthenium dioxide having a crystal size t of 31.3 nm as measured by X-ray diffraction and a specific surface area S.sub.BET of 2.8 m.sup.2/g (hereinafter referred to as RuO.sub.2—Type 3)

[0065] Notes:

[0066] The specific surface areas were determined by gas adsorption—BET method (ISO 9277:2010).

[0067] The crystal size t was evaluated by the reflection at about 54.5° 2θ of the X-ray diffraction after fitting the Bragg reflections to Gaussian and Lorentzian functions using the software TOPAS 6 [A. A. Coelho, J. Appl. Crystallogr. 2018, 51, 210-218]. The full width at half maximum was deconvoluted from these fittings and the crystal size was calculated by using Scherrer equation according to the method disclosed in P. Scherrer, Kolloidchem. Ein Lehrbuch (Ed.: R. Zsigmondy), Springer Berlin Heidelberg, Berlin, Heidelberg, 1912, pp. 387-409. The X-ray diffraction patterns were acquired on a PANalytical X'Pert PRO-MPD diffractometer, operating with Cu(Kα) radiation. Data were recorded in the 10-70° 2θ range with an angular step size of 0.050° and a counting time of 2 s per step.

[0068] B) The Reactor

[0069] The experiments were performed in a custom-made trickle-flow reactor.

[0070] FIG. 1 shows a simplified depiction thereof. The numerals indicate the following: [0071] 1 reactor [0072] 2 metal plate heater [0073] 3 catalyst bed [0074] 4 oxygen bottle [0075] 5 ethanol supply [0076] 6a high-pressure liquid chromatography pump [0077] 6b mass flow controller [0078] 7 back-pressure regulator [0079] 8 product collection

[0080] The reactor was operated as follows:

[0081] Oxygen (PanGas, 99.999%) was supplied from an oxygen bottle 4 via a mass flow controller 6b by Bronkhorst, calibrated for oxygen flow at 20 bar (4). The liquid flow (5.sub.±0.3 wt.-% ethanol solution (Fluka, >99.8%)) was introduced from the ethanol reservoir 5 with a KNAUER AZURA® P 4.1S high-pressure liquid chromatography pump, equipped with a titanium 10 ml pump head 6a. The two phases were met and introduced to the reactor 1 via Swagelok ⅛″ stainless steel tubing. Before entering the reactor 1, the reactant stream was preheated to about 120° C. The catalyst bed 3 (150.sub.±0.1 mg catalyst diluted 1:1 with SiC) was fixed with quartz wool inside a 4 mm inner diameter stainless steel tube (reactor); The bed was stabilized in the middle of the heating zone by a hollow stainless-steel rod of ca. 1.5 outer diameter, which ensured a constant height and minimum back pressure (maximum 0.2 bar). A custom-made metal plate heater 2 heated the reactor; it was controlled by a temperature controller (TC). An Equilibar® U3L Series precision back-pressure regulator 7, equipped with a PTFE glass laminated diaphragm, maintained the pressure of the system. The back-pressure regulator 7 was controlled by a Bronkhorst process pressure controller EL-PRESS P-802CV (PC). The flows, temperature of the heater, and the pressure of the back-pressure regulator were all recorded and controlled via a custom-made LabVIEW™ program. The temperature of the catalyst bed was recorded with a K-type thermocouple. The system pressure was also recorded before the reactor with a Keller Digital Manometer dV-2 PS. The product stream was collected in the product collection 8 and cooled down below 280 K by a dry ice-water bath and prepared for sampling.

[0082] If not indicated otherwise the process conditions were as follows:

TABLE-US-00001 Reaction Pressure   17 ± 1.0 Temperature (° C.) 150 ± 3 Ruthenium dioxide mass (mg) 150 ± 2 Ruthenium dioxide dilution ratio with SiC 1:1 Ethanol concentration (wt.-%)    5 ± 0.3 Oxygen purity (vol. %) 100 Liquid flow (mL/min) 0.3 Gas flow (mL/min) 5 Residence time (min) 5.5 Product cooling (K) 288 ± 2

[0083] C) Product Analyses

[0084] The liquid products were analyzed with an Agilent 7890A gas chromatographer equipped with a flame ionization detector (FID). 0.5 μL of the sample were injected at 343 K and carried in a 2 mL/min helium flow through the column DB-WAX. The temperature of the column was constant at 313 K for 2 min and was then heated at 8 K/min up to 409 K. The FID was fed by 30 mL/min hydrogen mixed in 400 mL/min air at 573 K. The signal of each compound was calibrated and the calibration line used for quantification was determined by linear regression.

[0085] The quantification of the compounds was used to determine the ethanol conversion (X) and the product selectivity (S.sub.i), wherein EtOH is ethanol and AcOH is acetic acid.

X (%)=EtOHin(EtOHmol/l).sub.in-EtOH(mol/lout) (mol/l)×100%

SAcOH (%)=EtOHin (AcOHmol/lout)-EtOH(mol/lout) (mol/l)×100%

Examples 1 to 8

[0086] Examples 1 to 8 were run using the above mentioned setup at different temperatures using RuO.sub.2—Type 1 diluted with SiC (1:1) as a catalyst.

[0087] The results are given in table 1

TABLE-US-00002 TABLE 1 Temperature Conversion Example [C.] (X) Y_AcOH S_AcOH 1 100 6.8 3.9 56.6 2 100 9.3 5.6 60.0 3 125 18.3 13.3 72.6 4 150 21.5 18.3 85.0 5 150 22.9 17.7 77.2 6 150 25.1 20.1 80.2 7 150 27.5 25.4 92.3 8 170 37.9 37.7 99.3

[0088] It's apparent from the results that the selectivity to acetic acid is almost quantitative at conversions about and above 35% of ethanol which is achieved in this experimental setup at temperatures of 170° C.

Examples 9 to 11

[0089] Examples 9 to 11 were run using the above mentioned setup with various types of ruthenium dioxide diluted with SiC (1:1) at 150° C.

[0090] The results are given in table 2.

TABLE-US-00003 TABLE 2 Conversion g AcOH Example Catalyst [X) Y_AcOH S_AcOH g RuO.sub.2 × h 9 RuO.sub.2 - Type 1 25.4 17.3 68.1 1.2 10 RuO.sub.2 - Type 2 48.4 39.5 81.6 2.4 11 RuO.sub.2 - Type 3 17.2 11.0 64.1 0.6

Example 12

[0091] Example 12 was performed using the above mentioned setup with ruthenium dioxide—type 1 diluted with SiC (1:1) at 150° C. for more than 24 hours.

[0092] The results are given in table 3:

TABLE-US-00004 TABLE 3 Time Conversion [h] [X) Y_(AcOH) S (AcOH) 7 25.6 18.4 72.0 11 21.5 18.3 85.0 19 22.9 17.7 77.2 27 20.9 18.7 89.6

[0093] It is apparent from table 3 that the catalyst is stable over time and does virtually not deactivate.