DE1207399B - Process for the preparation of tetrahydrofuran compounds - Google Patents

Description

Process for the production of tetrahydrofuran compounds A number of processes for the production of tetrahydrofuran compounds by hydrogenation of furan compounds are known, some of which are carried out in the vapor phase and some in the liquid phase. In the vapor phase process - e.g. B. in the US Pat. No. 2,458,001 - one generally uses catalysts with relatively low activity, so that the yield and purity and / or the degree of conversion of the hydrogenation product is less than satisfactory. The liquid phase processes require relatively high pressures and consequently expensive high pressure equipment. Furthermore, in those of the known processes which operate at relatively high temperatures, correspondingly high amounts of heat energy and therefore also greater amounts of energy are required for cooling the vapors of the reaction product leaving.

It is also known from British Patent 780 275 to use furan compounds, e.g. B. furfuryl alcohol and furfurol, in the vapor phase with a hydrogen-containing gas at a pressure not significantly higher than atmospheric pressure and at a temperature below 120'C using a highly active nickel catalyst which reduces at a temperature below 450'C, then partially oxidized and reduced again by passing over a gas containing free hydrogen at a temperature below 250 ° C. to hydrogenate. Carrying out this known method, using furan or 2-methylfuran by, as occurs as a result of the structure of these compounds by hydrogenolysis, d. H. a hydrogenative splitting with the formation of low-boiling or gaseous products. This creates uncontrollable hot zones so that carbon deposits occur which cause catalyst plugging and low yields.

Finally, it is known from French patent specification 829 113 to also add tetrahydrofurfuryl alcohol in the hydrogenation of furfurol. In this case, however, the tetrahydrofurfuryl alcohol is used to dilute the reaction medium.

It has now been found that in the hydrogenation of furan and 2-methylfuran in the vapor phase with a hydrogen-containing gas under a pressure which is not significantly higher than atmospheric pressure, at a temperature below 120'C in the presence of a nickel catalyst, which at a temperature below 450'C reduced, then partially oxidized and containing by passing a free hydrogen gas was reduced again at a temperature below 250'C, which can prevent disadvantages of 780,275 as well as those of the other known method characterized in the method of British Patent specification, that furan or 2-methylfuran is hydrogenated in the presence of 0.1 to 0.2 percent by weight of furfuryl alcohol or tetrahydrofurfuryl alcohol as a catalyst poison, based on the weight of the compound to be hydrogenated.

The process according to the invention is practically quantitative Yields of the almost pure hydroxyl product are obtained, and furthermore the furan is obtained or 2-methylfuran practically quantitatively in tetrahydrofuran or 2-methyltetrahydrofuran converted. Another advantage is that the Catalyst is achieved.

In contrast to the process of the above-mentioned French patent specification 829 113 , the tetrahydrofurfuryl alcohol or furfuryl alcohol does not serve as a diluent in the process according to the invention, but rather as catalyst poisons. It could not be foreseen that in the hydrogenation of furan compounds with the nickel catalysts prepared as described above, which are not used or described in the French patent mentioned, in the presence of 0.1 to 0.2 percent by weight of furfuryl alcohol or tetrahydrofurfuryl alcohol as catalyst poisons above advantages can be achieved. It is also possible to use a mixture of the furfuryl alcohol to be used as catalyst poison and tetrahydrofurfuryl alcohol in the specified amounts by weight.

The nickel catalyst to be used in the process according to the invention has been produced by preferably starting from nickel hydrate, reducing this at a temperature below 450 ° C., then partially reoxidizing the nickel catalyst, preferably by passing a gas containing free oxygen over it for so long, until the proportion of the reduced nickel in the total nickel is about 5501, whereupon the catalyst is reduced again by passing through it a gas which initially contains 5% hydrogen and 95 % nitrogen and the hydrogen content gradually increasing to 100% is, conducts, so that the temperature is kept below 250'C .

The nickel catalyst can be used as such or in conjunction with a Carrier material, such as diatomaceous earth, clay, pumice stone, coal, or known natural ones or artificial clay-like carriers can be used.

When carrying out the process according to the invention using furan, the preferred hydrogenation temperature is in the range from about 70 to 80.degree. C. and when using 2-methylfuran in the range from about 60 to 65.degree.

High yields are obtained at a temperature of preferably below 100'C , but also z. B. at temperatures down to 40'C a complete hydrogenation is achieved.

The hydrogenation is preferably carried out with a stoichiometric excess of gaseous hydrogen, which is optionally mixed with an inert gas, e.g. B. nitrogen, can be diluted. The pressure is not significantly above atmospheric pressure; it results from the movement of the vaporous reactants in the catalyst bed and can be up to 2 or 3 atmospheres.

The inventive method can be carried out so that a Mixture of the compound to be hydrogenated and the corresponding catalyst poison is produced and the gaseous mixture under the specified conditions hydrogenated. Another embodiment is the compound to be hydrogenated in the vapor phase with the hydrogenation gas, in which the corresponding catalyst poison is contained in vapor form, to bring into contact. In another embodiment the gas to be used for the hydrogenation, the compound to be hydrogenated and the catalyst poison in each case separately in the vapor phase in the reaction column introduced.

For the production of the nickel catalyst, in the context of the present Invention not seeking protection.

The following examples explain the process according to the invention. The hydrogenation products obtained were each analyzed by gas chromatography. EXAMPLE 1 A nickel catalyst has been produced as follows: Nickel-kieselguhr tablets containing nickel hydrate precipitated on kieselguhr are placed in a vertical reactor, the system is then flushed with nitrogen, heated to 260.degree. C. , then a stream of hydrogen is passed through and the temperature is increased gradually increased to 427'C. This temperature is maintained by passing the hydrogen through an external heating device, the circulating gas also being passed through an external drying device for drying. As soon as the specified temperature is reached, the system is flushed with nitrogen. The reduced nickel catalyst is now partially re-oxidized by adding a small amount of oxygen to the inert gas stream in such a way that the temperature is kept below 57.degree. As soon as this temperature has reached the bottom of the reactor, the stabilization of the catalyst is complete. After the system has stabilized, it is adapted to atmospheric conditions by introducing air. The catalyst now contains about 60 % nickel, the proportion of reduced nickel in the total nickel being about 55 %.

Then, this stabilized catalyst is placed in a heatable by circulating 01 column and thereby reduced again, that is passed through the column from top to pure below hydrogen, wherein the initial temperature of 140'C is, then within 4 hours gradually to about 200 ' C is increased and held at this value until no more water is released. After the reduction of the catalyst has ended, the temperature is allowed to drop to below 120.degree . A vapor mixture consisting of 2-methylfuran (bp = 63 to 65'C, with a content of 0.1 percent by weight furfuryl alcohol ( b.p. = 168 to 170 ° C, as well as hydrogen and which has been obtained by introducing the 2-methylfuran containing the furfuryl alcohol into a glass wool layer through which a preheated hydrogen stream is passed. The 2-methylfuran containing the furfuryl alcohol is introduced at a rate of 0.08 g per gram of catalyst and per hour, the hydrogen flow rate being about 3501 per mole of furfuryl alcohol-containing 2-methylfuran per hour. The column temperature is maintained at 53'C, and the operating pressure is from about 0.0703 to 0,1406atü which is sufficient to the vapor mixture in the system in the circuit ühren to f. The vapor stream leaving the catalyst column is then passed through a condenser, the reaction products being separated off.

The conversion of 2-methylfuran into 2-methyltetrahydrofuran is almost 100%. According to the analysis, the hydrogenation product contains 95.6 parts by weight of 2-methyltetrahydrofuran with a b.p. = 78 to 81- C, In comparison, if the method described above is used with the difference that no furfuryl alcohol is added, the catalyst bed becomes clogged or a low-boiling compound which has not been investigated in more detail is formed. If the hydrogenation temperature is also changed in the above example, for example at 70.degree . C., an undesirable increase in the temperature to 120.degree. C. is observed at the beginning as a result of the formation of a hot zone, and about 2 hours later the catalyst bed was clogged during at a column temperature of 60'C at the beginning a temperature rise to about 80'C occurs; in the latter case, the hydrogenation product contains, according to the analysis, 6.6 percent by weight of a product that has not been investigated and 87.3 percent by weight of 2-methyltetrahydrofuran. In all of these cases the clogging of the catalyst bed is due to the formation of carbon-like deposits on the upper part of the catalyst bed, which means that the hydrogenation must be terminated after 2 hours.

Example 2 This example is carried out according to Example 1 with the only difference that the temperature in the reaction column during the hydrogenation of the 2-methylfuran is 46.degree. The conversion of 2-methylfuran into 2-methyltetrahydrofuran is almost 100%. According to the analysis, the hydrogenation product consists of 96.8 percent by weight of 2-methyltetrahydrofuran with a b.p. # 78 to 81'C, The hydrogenation is carried out in the manner indicated for 6 hours.

Example 3 2-methylfuran is hydrogenated according to the example, with the difference that the hydrogen flow is 909 liters per mole of feed and per hour and the temperature of the reaction column is kept at 65.degree. The conversion of 2-methylfuran into 2-methyltetrahydrofuran is almost 100%. The hydrogenation product consists of 98.2 percent by weight of 2-methyltetrahydrofuran with a boiling point of 78 to 81 ° C. The hydrogenation is carried out in the manner indicated for 6 hours.

Example 4 The example is carried out with the difference that the charge consists of 0.1 percent by weight of tetrahydrofurfuryl alcohol (bp # 176 to 178'C, containing 2-methylfuran and that the temperature of the reaction column is kept at 55'C. The conversion of 2-methylfuran into 2-methyltetrahydrofuran is almost 100%; According to the analysis, the hydrogenation product consists of 95.7 percent by weight of 2-methyltetrahydrofuran with a b.p. # 78 to 81'C, The hydrogenation is carried out for 6 hours.

Example 5 According to Example, 2-methylfuran containing 0.15 weight percent furfuryl alcohol is hydrogenated as a feed, the feed rate being 0.09 g per gram of catalyst per hour and the unreacted hydrogen through the system after preheating at a rate of 1220 l of the hydrogen-containing gas per mole of feed and per hour is recycled. The conversion of 2-methylfuran into 2-methyltetrahydrofuran is almost 100%. According to the analysis, the hydrogenation product contains 96.3 percent by weight of 2-methyltetrahydrofuran with a boiling point of 78 to 81 ° C, The hydrogenation is carried out in the manner indicated for 6 hours.

Example 6 This example is carried out as described in Example 1 , with the difference that instead of 2-methylfuran as the starting material, furan ( b.p. = 27 to 32 ° C, is used which contains 0.2 percent by weight furfuryl alcohol, and that the column temperature is about 85 ° C. The conversion of the furan to tetrahydrofuran is about 99.5% . The hydrogenation is 99.0 percent by weight of tetrahydrofuran by Kp. = 64 to 66'C.

If, in comparison, this example is carried out with the only difference that no furfuryl alcohol is added, carbon deposits rapidly form at the upper end of the catalyst bed, which lead to blockages and require the reaction to be terminated after only 1 hour.

Claims (1)

Claim: Process for the production of tetrahydrofuran compounds by hydrogenation of furan compounds in the vapor phase with a hydrogen-containing gas under a pressure which is not significantly higher than atmospheric pressure, at a temperature below 120'C in the presence of a nickel catalyst, which is at a temperature below 450'C reduced, then partially oxidized and containing by passing a free hydrogen gas was reduced again at a temperature below 250'C, d a d u rch g ekennzeichnet in that furan and 2-methylfuran in the presence of 0.1 to 0.2 weight percent furfuryl alcohol or tetrahydrofurfuryl alcohol as a catalyst poison, based on the weight of the compound to be hydrogenated, hydrogenated. Documents considered: German Patent No. 817 757, French Patent No. 829 113; British Patent No. 780 275; U.S. Patent No. 2,458,001.