NO790010L - PROCEDURE FOR MAKING FURAN COMPOUNDS - Google Patents

Description

The present invention relates to a method for the production of furan compounds by direct oxidation of conjugated diolefins with air or oxygen in the liquid phase in the presence of a transition metal catalyst system.

The current methods for the direct oxidation of diolefins to furan compounds are mainly vapor phase methods which are generally characterized by low conversions and poor selectivities. These disadvantages are caused by the instability of the furan compounds at higher temperatures in the presence of

oxygen which leads to the formation of resinous products, charring and uncontrolled polymerisation. The liquid phase process according to the present invention eliminates these disadvantages by working at moderate temperatures.

Although several liquid phase processes are known for the preparation of furan compounds, they involve the use of oxygenated compounds as starting materials. US patents 3,932,468 and 3,996,248 relate to the rearrangement of butadiene monoxide, and US patent 3,933,861 relates to the reaction of an alkene and an alkene oxide to obtain substituted furans. Both of these methods require oxygenated starting materials, while according to the present invention furan compounds are obtained by direct oxidation of the conjugated diolefin.

Although Japanese patent no. 77 77,049 relates to a method for the oxidation of butadiene to furan in an aqueous acidic medium, the present method differs from this method in that the present method is carried out in an organic solvent medium in which the catalyst and the furan products are more stable.

According to the present method, acyclic conjugated diolefins containing from 4 to 10 carbon atoms are converted to furan and alkyl-substituted furan compounds by direct oxidation of the diolefin with molecular oxygen in a liquid-phase reaction. The reaction is carried out in a non-aqueous reaction medium in the presence of a transition metal-organometallic catalyst complex•

The liquid phase oxidation reaction according to the present invention is a free radical reaction, and these reactions appear to be initiated by the formation of an initial free radical. This initial free radical can form the desired product (furan) directly, or can lead to the formation of other radical intermediates that can give either furan, other oxygenated products such as a diolefin monoxide, 2,5-dihydrofuran, crotonaldehyde, or oligomers and/or polymers.

The role of the catalyst in the present invention is to react with the initial key radical intermediates, and transfer them directly to furan products before harmful by-products can be formed. It is this selective catalytic behavior, coupled with the specific reaction conditions indicated here, that leads to the increased selectivity of the oxidation of the diolefins to the desired furan compounds.

Suitable feeds in this invention for conversion to furan compounds include acyclic alkadienes with from 4 to 10 carbon atoms. Examples include butadiene-1,3, pentadiene-1,3, isoprene, hexadiene-1,3, decadiene-1,3 and the like, and mixtures thereof. The acyclic alkadienes with 4 or 5 carbon atoms are preferred in the present process. The furanfor compounds produced by the present method have the formula:

where each R is individually selected from the group consisting of hydrogen and an alkyl group with 1-6 carbon atoms, the total number of carbon atoms in the R groups being in the range 0-6. Representative compounds include furan, 2,methylfuran, 3-methylfuran, 2,5-diethylfuran, 2-n-hexylfuran, 2-isopropyl-3-methylfuran, 3,4-n-dipropylfuran, 3-methyl-4-n-butylfuran and such.

The catalysts used according to the invention are organometallic complexes or salts of the metals of groups IVB, VB, VIB, VIIB or VIII of the periodic table.

These complexes have the general formula:

[R M (L) ]

x y z

where R is an organic ligand selected from the group consisting of alkyl, aryl, alkene, diene, triene or alkyne groups of 1-8 carbon atoms;

L is a ligand selected from the group consisting of carbon monoxide and a halogen;

M is a transition metal or a mixture thereof selected from groups IVB, VB, VIB, VIIB and VIII of the periodic table

system;

and where

x is 0 - 2,

»y is 0 - 6,

and x + y is 1 - 6,

and where

z is 1 - 6.

Specific examples of suitable catalysts include OsCl3, Os3(CO)12, [CpMo(CO)3]2 (Cp = cyclopentadienyl radical), CpV(CO)4, CpTiCl2, CpMn(CO)3, (Cp)2Fe, Mo (CO)g, [CpFe (CO)2]2, (C4H5)Fe(CO) 3, Co2(CO)g, Ru3(CO)12, Rhg (CO) lg and W(CO) g .

Although these complexes and salts are effective catalysts in themselves, it may be advantageous to also use certain promoters for these catalysts as indicated by the general formula:

A R™ x

m. n

where A can be mercury, thallium, indium, or a group IV A element such as silicon, germanium, tin or lead;

R can be a hydride, alkyl, aryl or amine group

X can be an anion of a mineral acid or a carboxylic acid,

and where

m is O - 4,

n is 0 - 4 r

and m + n is 1 - 4.

Specific examples of these types of promoters include such compounds as Hg(C2H302)2, SnCl2, (<C>2<H>5)2SnCl2, SnCl4, (CH3)3SnN(CH3)2, Ge<l>2, (n -C4Hg)3GeI, (cr-C5H5)Ge(CH3)3, (C2H5)3PbCl, (CH3)3SiH or SiH3I.

When promoters are used for the catalysts of the present invention, they may be added to the reaction mixture as separate species or they may be reacted with the catalyst to form a separate chemical compound which may be isolated and purified prior to its use as a catalytic agent. Representative examples of compounds formed by reactions occurring between the catalyst and the promoter include: ClHgFe(Cp)2, Hg[Co(CO)4]2, C<l>2<S>n[Fe(CO)2Cp]2, I2Ge [Co(CO)4]2,<[>(<C2H>5)3Pb]2Fe(CO)4, H3SiCo(CO)4, Cl(CH3)2Sn[Mn(CO)5]and [Cp(CO) 3Mo-Sn(CH3)2~Mn(CO)5].

The promoter compounds of the catalyst system are advantageously used in molar ratios of from 0.25 to 2.0 mol of promoter per moles of transition metal catalyst. However, preferred molar ratios of promoter compound to transition metal catalyst are approx. 0.5:1 to 2:1. The catalysts used according to the invention (with or without promoters) can be dissolved in the reaction medium as homogeneous catalysts, suspended in the reaction medium as insoluble, support-free heterogeneous catalysts, or in some cases where it is advantageous they can be placed on supports such as silicon oxide, aluminum oxide or polymers materials and suspended in the reaction medium. However, it is preferred that the catalyst system is a homogeneous system where the catalyst is dissolved in the reaction solvent. Concentration of cataly-— 6

sator in the solvent medium can vary from 10 to 10.0 .mol/liter. Preferably, a catalyst concentration of from approx. 10~5 to 1.0 mol/liter.

The reaction medium suitable for the present method

is an essentially inert, non-coordinating or weakly coordinating organic solvent with a boiling point considerably higher than the boiling points of the feed or those obtained

products. Solvents with boiling points from 130° to 225°C are particularly preferred. Preferably, these solvents have an absence of removable hydrogens which could lead to oxidation of the solvent or binding of the active regions of the metal or metals in the catalyst, thereby deactivating the catalyst. Examples of suitable solvents include paraffinic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons and nitrile-aromatic compounds such as heptanes, decanes and the like; toluene and the xylenes; chlorobenzene, chloroform, carbon tetrachloride, etc.; and benzonitrile, chlorobenzene being the most preferred. Substituted furans such as alkylfurans or 2,3-benzofuran can also serve as suitable solvents in some cases.

The oxidation reaction in the present invention is

very sensitive to reaction conditions, and it is an essential feature of the invention that the reaction is carried out under conditions that maximize selectivity. The reaction can be carried out at temperatures in the range from approx. 20° to 200°C, and preferably at temperatures in the range from approx. 50° to 130°C. Temperatures above this range cause the formation of further oxidation products. such as crotonaldehyde and increases the formation of unwanted polymer.

The reaction pressure can vary from 1 to 20 atmospheres, and preferably from 1 to 10 atmospheres. The partial oxygen pressure is of particular importance for the selectivity of the reaction,

and oxygen pressures of from 0.5 to 5 atmospheres, and in particular oxygen pressures of from 1 to 3 atmospheres, are used with advantage.

Another critical reaction variable that affects the selectivity of the reaction is the ratio of diolefin to oxygen.

■Although the molar ratio of diolefin to oxygen may vary from 0.001 to 100.0, a ratio of 0.33 to 5.0 is preferred.

In cases where the reaction is carried out in a closed reaction vessel, the reaction times can vary from 0.5 to 10 hours, and a reaction time of from 1.0 to 4 hours is preferred. Continuous operation at which reaction does the ion mixture? kept at a constant temperature and pressure is also covered by the invention. Under such conditions, the diolefin and air or oxygen are continuously fed to the reactor, while volatile products and unreacted feed are continuously removed. The volatile products can be collected, and unconverted feed is recycled to the reactor.

The reaction vessel can be constructed of stainless steel, or in certain cases the reaction vessel can be lined with glass, quartz or a stable resin material to reduce side reactions between the reaction intermediates and the walls of the reaction vessel.

Examples 1 - 12

The oxidation of butadiene to furan in the presence of a variety of transition metal catalyst complexes with or without promoter was carried out in a series of experiments according to the following method: An amount of catalyst necessary to obtain a concentration of 1 x 10 mol of catalyst in the reaction solvent was weighed into a stainless steel - reaction tube (180 mm long x 9.5 mm diameter) fitted with a stainless steel ball valve and septum cap. The tube was evacuated and charged with a mixture of butadiene and oxygen in a 1:1 molar ratio at an initial oxygen pressure of 2.2 atmospheres. 4 ml of chlorobenzene solvent was introduced into the tube with a metering pump. The tube and its contents were heated to a temperature of 110°C in a heating block for 2 hours. At the end of this time, the tube was rapidly cooled to room temperature, and the reaction mixture was analyzed by gas chromatography,

The conversion percentage of the butadiene and the selectivity percentage to furan calculated on the percentage of converted butadiene obtained in Examples 1-12 are indicated in the table below.

(Cp = cyclopentadiene)

x The reaction carried out in a resin-coated stainless steel reactor

Claims (11)

1. Process for the transfer of acyclic conjugated diolefinic hydrocarbons containing from 4 to 10 carbon atoms to furan and alkyl-substituted furans, characterized in that the conjugated diolefins are reacted with molecular oxygen in the liquid phase in an inert organic solvent in the presence of a catalyst with the composition: where R is an organic ligand selected from the group consisting of of alkyl, aryl, alkene, diene, triene or alkyne radicals containing from 1 to 8 carbon atoms; L is a ligand selected from the group consisting of carbon- monoxide and a halogen; M is a transition metal or mixture thereof, selected from groups IVB, VB, VIB, VIIB and VIII in the periodic table; and where x is 0 - 2, y is 0 - 6, x + y is 1 - 6, and where z is 1 - 6. 2. Method according to claim 1, characterized in that a promoter is used together with the catalyst which is a compound with the formula: where A is an element selected from the group consisting of quick- silver, thallium, indium, silicon, germanium, tin and lead; R is a hydride, alkyl, aryl or amine radical; and X is an anion of a mineral acid. or a carboxylic acid; and where m and n are each numbers from 0 to 4, and m + n is 1 - 4. 3. Method according to claim 2, characterized in that the promoter is used in a molecular ratio of from 0.25 to 2.0 mol per moles of transition metal catalyst. 4. Method according to claims 1-3, characterized in that the reaction is carried out in the temperature range from 20° to 200°C. 5. Method according to claims 1-4, characterized in that a molar ratio of diolefin to oxygen in the range from 0.001 to 100.0 is used. 6. Method according to claims 1-5, characterized in that the reaction is carried out in an inert organic solvent at a boiling point in the range from 130° to 225°C. 7. Method according to claim 6, characterized in that a solvent selected from the group consisting of paraffinic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons, nitrile-aromatic compounds and alkyl- or aryl-substituted furans is used. 8. Method according to claim 7, characterized in that chlorobenzene is used as solvent. 9. Method according to claims 1-6, characterized in that a catalyst is used which is soluble in the reaction solvent. 10. Method according to claims 1-6, characterized in that a catalyst is used which is suspended in the reaction mixture of fungal solution. 11. Method according to claim 5, characterized in that butadiene is used as diolefin.