PROCESS FOR THE PRODUCTION OF ALKYL CARBONATE AND THE USE OF ALKYLENE CARBONATE PRODUCED IN THE PREPARATION OF AN ALCANO DIOL AND A DIALKYL CARBONATE DESCRIPTION OF THE INVENTION The present invention describes a process for producing albylene carbonate and the use of alkylene carbonate produced in the preparation of an alkane diol and a dialkyl carbonate. The processes for the production of alkylene carbonates are known. WO-A 2005/003113 discloses a process in which carbon dioxide is contacted with an alkylene oxide in the presence of a suitable catalyst. The described catalyst is a phosphonium tetralkyl compound. This specification discloses that the catalyst is very stable if it is recycled to the preparation of alkylene carbonate in an alcohol, in particular in propylene glycol (1,2-propane diol). In Publication WO-A 2005/051939 it is described that the decomposition of the phosphonium catalyst is lower if the reaction is carried out in the presence of a lower amount of carbonyl compounds, in particular aldehydes. Both documents show the effectiveness of the processes in batch experiments. Although the presence of 1,2-propane diol as a solvent reduces the decomposition of the catalyst Ref .: 194262
phosphonium, has the disadvantage that the compound tends to react with the alkylene oxide. This is more evident in the case of a continuous process in which the catalyst is recycled to the reactor, in which the alkylene carbonate is actually formed. In addition, in a continuous process the reaction product containing alkylene carbonate, 1,2-propane diol and catalyst is subjected to a processing treatment. The processing treatment generally includes one or more distillation steps to separate the product from the reactants. Because the boiling point of 1,2-propane diol is less than that of propylene carbonate, 1,2-propane diol is removed from the propylene carbonate during processing of the reaction product. Therefore, the stabilizing effect of 1,2-propane diol disappears during processing. It has now been found that the stability of the catalyst does not deteriorate if the recycling of the catalyst in the process is carried out in the presence of the alkylene carbonate. Therefore, the present invention provides a process for the production of the alkylene carbonate by reaction of an alkylene oxide with carbon dioxide in the presence of a phosphonium compound as a catalyst, in the process (a) The alkylene oxide, the carbon dioxide and
the phosphonium catalyst is introduced, continuously into a reaction zone, from which a product stream containing alkylene carbonate and catalyst is removed, (b) alkylene carbonate and a mixture of alkylene carbonate and phosphonium catalyst they are separated from the product stream, and (c) The alkylene carbonate, separated in step (b), is recovered as product, and (d) The mixture of alkylene carbonate and phosphonium catalyst is continuously recycled to the zone of reaction . The present process allows a prolonged use of the catalyst, which is continuously recycled to the reaction zone. It is evident that the process provides a great advantage over the batch processes described in documents of previous inventions. Because the formation of alkylene carbonate is a reversible reaction, it is not obvious to recycle the alkylene carbonate to the reaction zone, because the skilled person can expect the risk to decrease the yield of the desired alkylene carbonate product. Another advantage of the present invention arises from the fact that because the separation between the catalyst and the alkylene carbonate should not be complete, a separation method can be employed.
relatively inexpensive It has been found that the combination of the alkylene carbonate and the alkylene oxide can have a detrimental effect on the catalyst if the catalyst is exposed for a prolonged period to a combination of these compounds. Thus, it is preferable that the mixture of alkylene carbonate and phosphonium catalyst contain not more than 1% p of alkylene oxide, preferably, maximum 0.5% p, based on the total weight of the alkylene carbonate and the phosphonium catalyst. More preferably, the mixture is substantially free of alkylene oxide. The catalyst is a phosphonium compound. Catalysts are known, for example, from US Patent Nos. 5,153,333, US-A 2,994,705, US-A 4,434,105, WO-A 99/57108, European Patent EU-A 776,890 and Publication WO-A 2005/003113. Preferably, the catalyst is a phosphonium halide of formula R4PHal, in which Hal is halide and R can be the same or different group and can be selected from alkyl, alkenyl, cyclic or aromatic aliphatic group. The group R contains, suitably, from 1 to 12 carbon atoms. Good results can be obtained when the group Ci_8 is an alkyl group. More preferred R groups are selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and t-butyl. Preferably, the halide ion is bromide. It seems that the
Bromide compounds are more stable than the corresponding chloride compounds and more stable than the corresponding iodide compounds. The most preferred phosphonium catalyst is tetra (n-butyl) phosphonium bromide. The alkylene oxide which is converted in the present process is suitably a C2-4 alkylene oxide, in particular ethylene oxide or propylene oxide or mixtures thereof. The amount of phosphonium catalyst can be conveniently expressed in a molar catalyst per mole of alkylene oxide. Due to the lower amount of side products, the process herein is preferably carried out in the presence of at least 0.0001 mole of phosphonium catalyst per mole of alkylene oxide. Preferably, the amount of phosphonium catalyst present is such that the range is from 0.0001 to 0.1 mole of phosphonium catalyst, more preferably from 0.001 to 0.05, and more preferably from 0.003 to 0.03 mole of phosphonium catalyst per mole of propylene oxide. The reaction of carbon dioxide with the alkylene oxide is reversible. This means that the alkylene carbonate formed can be converted into carbon dioxide and alkylene oxide. The molar ratio of carbon dioxide and alkylene oxide can be as low as 0.5: 1, more suitably 0.75: 1. In view of the reversibility of the
In the reaction, it is preferable to guarantee an excess of carbon dioxide, such as 1.1: 1 to 10: 1, more preferably 1.5: 1 to 5: 1, more preferably 1.5: 1 to 2: 1. A suitable means for establishing an excess of carbon dioxide pressure is to carry out the reaction at high carbon dioxide pressures and to maintain the constant pressure by dosing the carbon dioxide. The total pressure is in the appropriate range of 5 to 200 bar; the partial pressure of the carbon dioxide preferably is from 5 to 70, more preferably from 7 to 50, and more preferably from 10 to 20 bars. The reaction temperature can be selected over a wide range. Suitably, the temperature is selected from 30 to 300aC. The advantage of using a relatively high temperature is reflected in an increase in the reaction rate. However, if the reaction temperature is excessively high, secondary reactions may occur, namely, the degradation of alkylene carbonate to carbon dioxide and propionaldehyde or acetone and the undesired reaction of alkylene oxide with any alkane diol, if exists. Therefore, the reaction temperature is suitably selected from 100 to 220aC. The expert person may adapt other reaction conditions that he considers appropriate. The residence time of the alkylene oxide and carbon dioxide in the area of
reaction can be selected without this unnecessary burden. Generally, the residence time can be varied, between 5 minutes and 24 hours, preferably between 10 minutes and 10 hours. Suitably, the conversion of the alkylene oxide is at least 95%, more preferably at least 98%. The residence time is adapted according to the temperature and pressure. The amount of catalyst can also vary within wide ranges. Suitable concentrations include from 1 to 25% p, based on the total reaction mixture. Good results can be obtained with the amount of catalyst from 2 to 8% p, based on the total reaction mixture. Although the presence of alkylene carbonate already guarantees that the stability of the catalyst is maintained, it is preferable to add an alcohol in a mixture of alkylene carbonate and phosphonium catalyst. To this the alcohol can be added to the mixture before introducing it into the reaction zone. Alternatively, the alcohol can be added directly to the reaction zone at any suitable place, such that the mixture of alkylene carbonate and phosphonium catalyst also contains the alcohol. The alcohol reinforces the stabilizing effect of the phosphonium catalyst at the reaction temperatures. If there is alcohol, the possibility arises that the
Alcohol reacts with the alkylene oxide to form alkoxyalcohol. This is another reason for keeping the reaction temperature relatively low, for example, in the range of 100 to 220SC. Many alcohols can be selected to increase the stability of the phosphonium catalyst. Alcohol can be monovalent, bivalent or multivalent. The alcohol may include a C1-12 aliphatic chain substituted by one or more hydroxyl groups. The aromatic or aromatic alkyl alcohols can also be used, and suitably contain from 6 to 12 carbon atoms. Poly-alkylene glycols or mono-alkyl ethers thereof may also be used. Mixtures can also be used. Preferably, alcohols selected from the group of Ci-6 monoalkanols, C_6 alkanediols, C3-5 alkane polyols, including, glycerol, phenol, substituted phenols Ci-β, cycloaliphatic alcohols C6-i2, and mixtures thereof are used. The C2-6 alkane polyols, in particular, 1,2-ethane diol, 1,2-propane diol, sorbitol and their mixtures are very suitable. The use of ethane or propane diol has the advantage that the reaction mixture is not contaminated with foreign alcohols. Sorbitol provides excellent stability to the phosphonium catalyst. It may be an advantage to use a combination of 1,2-ethane or propane diol and sorbitol. When alcohol is used in the present process,
the skilled person will generally use a molar excess as compared to the amount of phosphonium catalyst. However, there is a certain limit. Generally, the alcohol must be separated from the reaction mixture, in particular the alkylene carbonate product. For economic reasons, the excess should be optimized to offset the benefits of improved stability with the costs associated with the separation. Suitably, the amount of alcohol is in the range of 1 to 100, preferably 2 to 60, more preferably 3 to 15 moles of alcohol per mole of phosphonium catalyst. With regard to the relative amounts of alkylene carbonate and alcohol, large variations are allowed in the proportion. Very good results have been obtained by employing a weight ratio of alkylene carbonate and alcohol of 0.1-10, in particular from 0.2 to 5, more preferably from 0.5 to 2. In view of the possibilities of the occurrence of the undesired reaction between the alkylene oxide and the alcohol in the reaction zone, the amount is suitably maintained at a relatively low level, such as from 1 to 15% p, based on the weight of the alkylene oxide, carbon dioxide, alkylene carbonate and alcohol in the reaction zone. Preferably, the amount of alcohol is in the range of 5 to 10% p. It is an advantage that the catalyst content
Phosphonium in the mixture to be recycled is relatively high. This means that the yield of the alkylene carbonate product is high while the recycling costs are kept at minimum values. Therefore, the amount of phosphonium catalyst in the mixture of phosphonium catalyst and alkylene carbonate is preferably in the range of 1 to 90% p, based on the total mixture, more preferably 5 to 75% p. Because it has been found that the stability of the catalyst is slightly lower when the weight ratio of the alkylene and the catalyst is below 1, the amount of phosphonium catalyst is more preferably 10 to 40% p. The total mixture includes phosphonium catalyst, alkylene carbonate, and optionally, alcohol. The alkylene carbonate produced in the present process can suitably be used to produce alkane diol and dialkyl carbonate. Accordingly, the present invention also provides a process for preparing an alkane diol and dialkyl carbonate which includes the reaction of alkanol and alkylene carbonate with the transesterification catalyst in which the alkylene carbonate was prepared by the process of the present invention, and recovering the alkane diol and the dialkyl carbonate from the resulting reaction mixture. Suitably, alkanol is a C1- alcohol. Preferably, the alkanol is methanol,
Ethanol, or isopropanol. The most preferred alkanols are methanol and ethanol. The transesterification reaction itself is known. In this context reference is made to US Pat. No. 4,691,041, which describes a process for making an ethylene glycol and dimethyl carbonate by the transesterification reaction on a heterogeneous catalyst system, in particular an ion exchange resin with functional groups of tertiary amines, quaternary ammonium, sulfonic acid and carboxylic acid, toric alkaline or alkaline silicates impregnated on silica and ammonia exchange zeolites. U.S. Patent No. 5,359,118 and US Pat. No. 5,231,212 is a continuous process for preparing dialkyl carbonates with a range of catalysts, including alkali metal compounds, in particular alkali metal hydroxides or alcoholates, thallium compounds, bases of nitrogen such as trialkyl amines, phosphines, stibines, arsenines, sulfur or selenium and tin compounds, titanium or zirconium salts. According to WO-A 2005/003113 the reaction is carried out with heterogeneous catalysts, for example, alumina. This specification allows to separate the phosphonium catalyst from the reaction products. It is suggested to remove the phosphonium catalyst together with the alkane diol. Without
However, in the present invention it is preferred to separate the alcohol, in a previous step. According to the present invention, the alcohol is preferably separated from the product stream containing alkylene carbonate and phosphonium catalyst. In this way the amount of alcohol to be recycled can be kept to a minimum. Moreover, any halide compound that can be formed during the reaction as a byproduct is removed from the alkylene carbonate product and can not complicate any subsequent process step. Moreover, it has been found that if the secondary halide product is recycled to the reaction zone together with the alcohol and the phosphonium catalyst they contribute to the catalytic behavior of the system. The Figure presents a diagrammatic perspective of the process of the present invention. The Figure shows a reaction zone 1, to which the alkylene oxide is poured by line 2. The alkylene oxide is combined with a mixture of phosphonium catalyst, for example, tetrabutyl phosphonium bromide, by line 4 and the reactants are transferred together to the reaction zone 1. The mixture in line 4 also contains an alcohol, for example, 1,2-propane diol, and alkylene carbonate, for example, propylene carbonate. Line 3 also passes carbon dioxide to reaction zone 1. Reaction zone 1 may only include one
reactor. It is also possible to carry out the reaction in two or more reactors. In cases it may be an advantage to provide the optimum amount of excess carbon dioxide in the reactors, by removal or addition of carbon dioxide between the reactors. The reactors are suitably operated under plug flow conditions. It is even more preferable to have a reverse mixing reactor, for example, a continuous stirred tank reactor (CSTR), followed by a plug flow reactor. Said combination is described in US Pat. No. 4,314,945. The alkylene carbonate together with the phosphonium catalyst and the alcohol are discharged from the bottom of a line to line 5 from the reaction zone. The content of line 5 is passed to a first separation zone 6 in which it separates the alcohol by line 7 in the most part, or alternatively in the upper part, and from which the mixture of alkylene carbonate and phosphonium catalyst is removed by line 8 in the bottom or in the lower part. Line 7 can remove low-boiling byproducts and / or carbon dioxide in residual excess (not shown). It is observed that this situation can arise when the alcohol has a lower boiling point than the alkylene carbonate, as is the case when the alcohol used is 1,2-propane diol and the alkylene carbonate used is propylene carbonate. When
uses a high boiling alcohol in combination with a low-boiling alcohol, for example, sorbitol in combination with 1,2-propane diol, the effluent in the line
8 includes a high-boiling alcohol. When only a high-boiling alcohol is used, for example, only sorbitol in the preparation of propylene carbonate or ethylene, the separation zone 6 should only be used to remove light by-products and / or excess carbon dioxide . The effluent in line 8 is passed to another separation zone 9 in which alkylene carbonate is separated, discharged in the upper part by line 10, and recovered as product. The residual product of the separation zone
9 includes alkylene carbonate, phosphonium catalyst and, optionally, a high boiling alcohol. This waste product is discharged via line 11. Possibly, the process alcohol can be added via line 12 to line 11 or to any other suitable places in the process. At least part of the alcohol that was separated in the separation zone 6 and which is removed by line 7 is added to the mixture of alkylene carbonate and phosphonium catalyst. The resulting mixture is poured through line 11. Additional processing catalyst can be combined, if
it exists, with a mixture on line 11 and recycled through lines 4 and 2 to reaction zone 1. EXAMPLES
EXAMPLE 1 To demonstrate that the presence of alkylene carbonate maintains the stability of the catalyst, a mixture of tetra-n-butyl phosphonium bromide (TBPB), propylene carbonate (PC) catalyst and, optionally, 1,2-propane diol is stirred. (1 .2 PD) in air at 120aC for 18 hours. Initially the catalyst contains 0. 06% p of tributyl phosphine oxide (TBPO). The amount of TBPO in the catalyst, indicative of degradation of TBPB, is determined after 18 hours by 31P-NMR. The results are shown in Table 1. Table 1
EXAMPLE 2 In a series of experiments the effectiveness of the liquid on the stability of the phosphonium catalyst was demonstrated. A mixture of 150 g of carbonate of
propylene and 50 g of tetrabutyl phosphonium bromide to mimic the reflux stream from the reaction zone in which the propylene oxide reacts with carbon dioxide. Alcohol (8 g) was added to the mixture and the resulting mixture was kept in a closed vessel at atmospheric pressure and at a specific temperature for a period of time as indicated in the following Table 2. Degradation of the phosphonium catalyst was determined by 31 P-NMR. Likewise, the degradation of propylene carbonate was determined by measuring the pressure increase at the end of the period. The increase in pressure was caused by the decomposition of propylene carbonate to aldehyde and carbon dioxide. The results are presented in the Table. Table 2
The above results show that the presence of an alcohol in addition to propylene carbonate has a stabilizing effect on the catalyst, and furthermore, it reduces the degradation of propylene carbonate. EXAMPLE 3 This example demonstrates the negative effect of propylene oxide on the combination of propylene carbonate and phosphonium catalyst. Therefore, 50 g of tetra n-butyl phosphonium bromide (TBPB) catalyst, 150 g of propylene carbonate (PC), 5 g of propylene oxide (PO) and optionally 3 g of 1,2- are mixed. propane diol (1,2 PD) in an autoclave, and heated for a certain period of time at 180 aC. The catalyst contains 0.06% p of tributyl phosphine oxide. (TBPO) at the beginning of the experiment. At the end of the experiment the amount of TBPO was determined by 31P-NMR. The results are shown in Table 3. Table 3
The results show that the combination of propylene oxide and propylene carbonate decreases the stability of the phosphonium catalyst. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.