Process for the selective catalytic decomposition of alkyl ethers and alcohols in a feedstock originating from the synthesis of methyl tert-amyl ether

Abstract: A process for the selective catalytic decomposition of alkyl ethers and alcohols in a feedstock originating from the synthesis of methyl tert-amyl ether (TAME) is described, consisting of reacting said feedstock in the presence of a catalyst consisting of silica modified by the addition of alumina in a quantity of between 0.1 and 1.5% by weight of the silica, operating at a temperature of between 200 and 250°C and at a space velocity (LHSV) of between 4 and 15 h⁻¹.

Description: This invention relates to a process for the selective catalytic decomposition of alkyl ethers and alcohols in a feedstock originating from the synthesis of methyl tert- amyl ether (TAME). The invention relates in particular to a process for purifying a C₅ olefin cut originating from TAME synthesis to make it suitable for subsequent alkylation with isobutane. The process consists of catalytic treatment for removing oxygenated compounds which poison the alkylation catalyst. It is well known that when methanol is present, branched unsaturated olefinic C₅ hydrocarbons (isoamylenes) give rise to the formation of TAME, the reaction being catalyzed by Lewis acids, mineral acids and organic acids (as described for example in U.K. Patent No. 1 506 596). A suitable refinery stream for TAME production is a C₅ fraction from catalytic cracking containing only small quantities of C₄ and C₅+ hydrocarbons. TAME is used as an octane booster and can either be left in the reaction products or separated by distillation as a bottom product. In this latter case the remaining hydrocarbon products can be effectively used in the alkylation reaction provided they are of high purity (in particular, the oxygenated products must be contained at very low level). In addition to TAME, the oxygenated products which may remain in said fraction are the tertiary amyl alcohol which forms as a by-product in TAME synthesis, and in particular MTBE which forms because of the isobutene present in small quantities in the C₅ cut and which because of its boiling point is difficult to separate from a C₅ fraction. The total removal of C₄ hydrocarbons from the C₅ cut could be a solution, but this would be very costly. A catalyst system has now been found which allows selective decomposition of the oxygenated components present in products originating from TAME synthesis, so making the C₅ feedstock suitable for feeding to alkylation. The process of the invention has high selectivity both with regard to the content of oxygenated components and with regard to the formation of heavy products, which would be detrimental to selectivity and would have no suitable use. The present invention presents two simultaneous innovative aspects: The use of chemical methods (destructive catalytic elimination) to separate the undesirable oxygenated components instead of the use of physical methods such as extraction or distillation. Indeed, given the low content of oxygenated components to be eliminated, it is by no means certain that the result could be attained using the aforesaid conventional physical methods. The use of a highly selective catalyst, which eliminates the oxygenated components by reducing then to a level compatible with the alkylation reaction, while maintaining at a very low level the parallel oligomerization reaction or formation of C₆+ compounds, which as such would only reduce the reaction selectivity, being of low value. The process for the decomposition of alkyl ethers and alcohols in a feedstock originating from TAME synthesis, according to the present invention, consists of reacting said feedstock in the presence of a catalyst consisting of silica modified by the addition of alumina in a quantity of between 0.1 and 1.5% by weight of the silica, operating at a temperature of between 200 and 250°C and at a space velocity (LHSV) of between 4 and 15 h⁻¹. The decomposition is conducted preferably at an operating pressure of between 1 and 2 bars. The catalyst can be easily prepared, starting from a preformed silica of the required purity (this being a commercially available product), thus limiting the preparation to impregnation, drying and calcining. The impregnation is effected with a solution of an aluminium salt (such as the nitrate or isopropylate) to provide the final desired alumina content. It is however advisable to use a high purity silica, ie a silica with an Na₂O content not exceeding 0.12% by weight, an SO₄ content not exceeding 0.15% by weight and an Al₂O₃ content not exceeding 0.30% by weight. The catalysts used in said process are neither toxic nor corrosive, and can operate in the presence of water without giving rise to the formation of acid products. In addition they are stable with time for many thousands of hours and can be easily regenerated. The method of operation and the advantages of the process according to the present invention will be more apparent from the following examples, which are provided to illustrate the invention but without being limitative thereof. EXAMPLES The research was conducted on a synthetic mixture reproducing the composition of the effluent from TAME synthesis. Its composition is as follows: The catalytic tests were conducted in a flow microreactor of plug-flow type containing 1 cm³ of catalyst of suitable particle size, 30-40 mesh (0.4-0.5 mm). EXAMPLE 1 The alumina-modified silica catalyst was prepared in the following manner: 10 g of high purity silica of the following composition: are treated with an aqueous solution containing 0.368 g of aluminium nitrate (added Al₂O₃ = 0.5% by weight of the silica). The material obtained is dried slowly at 120°C for 3 hours, then calcined at 500°C for 4 hours. 1 cm³ of catalyst prepared in this manner (consisting of 0.5% Al₂O₃ on high-purity silica) is placed in the microreactor and heated to a temperature of 240°C. The pressure is 1.6 bars. The reaction mixture is then fed at LHSV = 4, the results obtained being shown in Table 1. EXAMPLE 2 Without any intermediate regeneration, the catalyst of Example 1 is adjusted to a temperature of 230°C and the LHSV raised to 5.6. The pressure is maintained at 1.6 bars. Product analysis is conducted after stabilization. The results are shown in Table 1. EXAMPLE 3 With the other operating conditions of the preceding examples unaltered, the LHSV is raised to 10. Product analysis is conducted after stabilization. The results are shown in Table 1. EXAMPLE 4 With the other operating conditions of the preceding example unaltered, the LHSV is raised to 11. The test results are shown in Table 1. EXAMPLE 5 The test is continued by raising the temperature to 235°C and setting the LHSV at 10. Analysis of the reaction products is conducted after stabilization. The results are shown in Table 1. EXAMPLE 6 The test is continued by further raising the temperature to 240°C and the LHSV to 11.6. Product analysis is conducted after stabilization. The results are shown in Table 1. EXAMPLE 7 - Comparative The catalyst of Example 1 is placed in the microreactor and raised to a temperature of 240°C. The operating pressure is 1.6 bars. The reaction mixture is fed at an LHSV of 0.5. The results obtained are shown in Table 1. EXAMPLE 8 - Comparative The catalyst of Example 1 is placed in the microreactor and raised to a temperature of 240°C. The operating pressure is 1.6 bars. The reaction mixture is fed at an LHSV of 24.0. The results obtained are shown in Table 1. EXAMPLE 9 - Comparative The catalyst of Example 1 is placed in the microreactor and raised to a temperature of 350°C. The operating pressure is 1.6 bars. The reaction mixture is fed at an LHSV of 11.5. The results obtained are shown in Table 1. From the data of Table 1 it can he seen that very high space velocities (LHSV greater than 15) (Example 8) do not result in complete methyl-tertiary-butyl-ether (MTBE) decomposition, making the product unsuitable for alkylation. Too low a space velocity (LHSV less than 4) (Example 7) or too high a temperature (exceeding 250°C) (Example 9) result in a considerable reduction in olefin selectivity due to the greater formation of by-products from side reactions.

Claims: A process for the selective catalytic decomposition of alkyl ethers and alcohols in a feedstock originating from the synthesis of methyl tert-amyl ether (TAME), characterised by reacting said feedstock in the presence of a catalyst consisting of silica modified by the addition of alumina in a quantity of between 0.1 and 1.5% by weight of the silica, operating at a temperature of between 200 and 250°C and at a space velocity (LHSV) of between 4 and 15 h⁻¹. A process as claimed in claim 1, wherein the feedstock is reacted at a pressure of between 1 and 2 bars. A process as claimed in claim 1, wherein the silica forming the catalyst is of high purity. A process as claimed in claim 3, wherein the high purity silica has an Na₂O content not exceeding 0.12% by weight, an SO₄ content not exceeding 0.15% by weight and an Al₂O₃ content not exceeding 0.30% by weight.

Inventors: ANCILLOTTI FRANCESCO, ANCILLOTTI, FRANCESCO
Applicants: SNAM PROGETTI, SNAMPROGETTI S.P.A.
Application year: 1992
Main classification: B01J21
IPC codes: C07C1, C07C7, B01J21