PROCESS FOR THE PREPARATION OF GLYCOL ETHERS

Abstract: A process for the preparation of a glycol ether by reacting an olefin oxide with an alcohol over a catalyst; characterised in that the catalyst is an anionic double hydroxide clay with a substantially intact layered structure, the inter-lamellar anions consisting substantially of the anions of the reactant alcohol.

Description: PROCESS FOR THE PREPARATION OF GLYCOL ETHERS This invention relates to a process for the preparation of glycol ethers. Glycol ethers are useful as jet anti-icing fluids, brake fluid blending components and solvents for paints, inks and the like. They may be produced by reacting an alcohol with an olefin oxide in the presence of either a basic or acidic catalyst. Anionic double hydroxide clays are well-known materials. They are described in, for example, "Anionic Clay Minerals", W.T. Reichle, "Chemtec", January 1986. They consist of positively charged metal oxide/hydroxide sheets with intercalated anions and water molecules. In terms of charge they are mirror-images of the much studied family of cationic clay minerals. The structure of anionic double hydroxide clays is related to that of brucite, Mg(OH>2 * In brucite magnesium is octahedrally surrounded by six oxygens in the form of hydroxide; the octahedral units then, through edge sharing, form infinite sheets. The sheets are stacked on top of each other by hydrogen bonds. If some of the magnesium in the lattice is isomorphously replaced by a higher charged cation, e.g. Al^ + , then the resulting overall single Mg^ + -Al^ + -OH layer gains a positive charge. Sorption of an equivalent amount of hydrated anions renders the structure electrically neutral, resulting in an anionic double hydroxide clay. Anionic double hydroxide clays have, in the dehydrated form, the empirical formula: [M a 2+ N b 3 + (OH) (2a+2b )][X]b in which M 2+ is a divalent metal cation; N + is a trivalent metal cation; X is one equivalent of an anion; and a and b represents the relative proportions of M and N in the structure. Typically M 2+ is Mg 2+ „ Fe 2+ , Co 2+ , Ni 2+ and/or Zn 2+ , and N 3+ is Al 3+ , Cr 3+ and/or Fe + . In an alternative form, the divalent metal may be wholly or partly replaced by lithium, the all-lithium form having the empirical formula: [Li a + N b 3+ (OH) (a+b) ][X] 2 b In the naturally-occurring minerals hydrotalcite and mannaseite, M 2+ is Mg 2+ , N + is Al + , X is carbonate, and a/b is in the range of 1:1 to 5:1. Such minerals occur in a hydrated form. A number of publications disclose that calcined anionic double hydroxide clays have catalytic activity. For example, US Patent No. 4458026 discloses that catalysts prepared by calcination of anionic double hydroxide clays may be used to perform aldol condensations. Japanese Patent Application No. 54-111047 discloses that calcined anionic double hydroxide clays may be used to prepare alkylene glycol ether acetates. EP-A-339426 discloses the use of calcined hydrotalcite for the ethoxylation or propoxylation of compounds containing active hydrogen atoms. Anionic double hydroxide clays in their naturally-occurring or as-synthesised form have a layered structure. Calcination of the material leads to collapse of this layered structure (see for example Sato et al, Reactivity of Solids 1986.2 253-260, and Sato et al, Ind. Eng. Chem. Prod. Res. Dev. 1986, _25_, 89-92), and results in a poorly-crystalline magnesium oxide-type structure. All the above documents require that the anionic double hydroxide clay be used as a catalyst in the calcined form, that is, in a form having a collapsed layer structure. Japanese Patent Application No. Hl-304043 discloses that anionic double hydroxide clays carrying copper ions and in which hydroxy ions are present at anion exchange sites catalyse the vapour phase hydrolysis of aromatic halides. We have now found that, contrary to expectations, anionic double hydroxide clays act as effective catalysts in the preparation of glycol ethers in an un-calcined form. The present invention provides a process for the preparation of a glycol ether by reacting an olefin oxide with an alcohol over a catalyst; characterised in that the catalyst is an anionic double hydroxide clay with a substantially intact layered structure, the inter-lameliar anions consisting substantially of the anions of the reactant alcohol. An anionic double hydroxide clay with a substantially intact layered structure can be obtained in two different ways. It can of course be obtained by using the material in its naturally-occurring or as-synthesised form, without heating to a temperature sufficient to collapse the layered structure. In general, heating to a temperature of greater than 300 β C should be avoided. Secondly, the anionic double hydroxide clay may be calcined to a temperature which causes collapse of the layered structure, and the resulting material subsequently rehydrated. This rehydration causes layers to reform substantially intact. Naturally-occurring anionic double hydroxide clays have an intact layered structure; the inter-lamellar anions are mainly carbonate. Such materials have low activity as catalysts for the preparation of glycol ethers; calcination enhances their activity. However, we have found, surprisingly, that when the inter-lamellar carbonate ions are exchanged with the anion of an alcohol, the resulting material has significantly improved activity for the preparation of glycol ethers. Such ion exchange can be carried out prior to use as a catalyst by using ion exchange techniques. It can also, however, be accomplished by passing an alcohol over the anionic double hydroxide clay. One possible embodiment of the latter method is to conduct a process according to the invention except that the catalyst is an anionic double hydroxide clay with a substantially intact layered structure, the inter-lameliar anions being other than the anions of the reactant alcohol. Conversion of the feedstock is initially low; however, after a period of time, the conversion increases as the inter-lamellar anions exchange with the reactant alcohol resulting in an effective catalyst. When operating in such a manner, the reaction is preferably carried out by passing a continuous flow of fresh alcohol feed over the catalyst. Preferably the anionic double hydroxide clay has a framework structure including magnesium, divalent iron, cobalt, zinc and/or lithium together with aluminium, chromium and/or trivalent iron. Preferably the framework structure comprises magnesium and aluminium. Anionic double hydroxide clays can be prepared by known methods, for example by the method described in US 4458026. In general, solutions of soluble salts of the relevant metals are mixed together with an alkali metal hydroxide and an alkali metal carbonate. The resulting mixture is vigorously stirred until a slurry is formed. The slurry is then heated, typically to a temperature between 50 and 100 β C, preferably 60 to 75 β C, until sufficient crystallisation occurs. The resulting product is an anionic double hydroxide clay in which the interlamellar anions are carbonate. Materials containing other ions, for example bicarbonate or carboxylic anions, may be prepared either by exchange of the carbonate anions or by adapting the synthesis method so that the other anions are incorporated. In order to prepare the catalyst required for the process of the invention, the anionic double hydroxide clay prepared for example as described above may be subjected to calcination. Preferably this involves heating to a temperature of at least 300*C, preferably 300 to 550*C, especially 350 to 500 β C, under non-reducing conditions. The heating may be carried out under vacuum, in an inert gas or, preferably, in an oxidising atmosphere, preferably air. Heating is carried out for a period of time typically between 10 and 30 hours. Such treatment causes the collapse of the double hydroxide layered structure and the decomposition of decomposable anions, and, generally, results in an oxide material having a structure related to that of MgO. Subsequent rehydration causes the layered structure to reform. If rehydration is carried out in the presence of decarbonated deionised water, the resulting material will contain interlamellar hydroxide anions. Subsequent treatment with an alcohol, either before use as a catalyst or in situ during the reaction, will lead to ion exchange with the alcohol, producing an effective catalyst. The alcohol used in the process according to the invention may be an aliphatic, cycloaliphatic or aromatic alcohol, preferably having up to 8 carbon atoms. An aliphatic alcohol preferably has up to 6, more preferably up to 4, carbon atoms. Typical aliphatic alcohols include methanol and ethanol. An example of a suitable cycloaliphatic alcohol is cyclohexanol, and an example of a suitable aromatic alcohol is phenol. More than one alcohol group may be present if desired, but preferably the alcohol is a mono alcohol. Mixtures of alcohols may be used if desired. The alcohol is suitably used in excess if it is desired to produce a mono glycol ether and suppress the formation of oligomeric products. Preferably the molar ratio of alcohol to olefin oxide is at least 2:1, especially at least 5:1, most preferably at least 10:1. The olefin oxide preferably has up to 10, especially up to 8, carbon atoms, and may for example by derived from an alkene, for example ethene or propene, or from a arylalkene such as styrene. In a preferred embodiment of the invention, ethanol is reacted with propylene oxide to produce a mixture of the primary and secondary glycol ethers, 2-ethoxypropan-l-ol and l-ethoxypropan-2-ol. It is a major advantage of the process according to the present invention that the reaction proceeds with a very high selectivity to the secondary alcohol product, which is in general the desired product. The reaction may be carried out in the vapour phase or, especially, the liquid phase. The optimum reaction temperature will of course depend upon the particular reactants used, but will in general be within the range of from 0 to 250"C, especially 70 to 200°C. The reaction may be carried out at atmospheric or elevated pressure, for example up to 100 barg. The following Examples illustrate the invention. Example 1 A. Preparation of Anionic Double Hydroxide Clay (Hydrotalcite Structure) A solution of [Mg(H 2 0) 6 ]Cl 2 (407 g, 2.00 moles) and [Al(H2θ)6]Cl3 (161 g, 0.67 moles) in 1.4 dm 3 of distilled water was added at a rate of 12 cm /minute to a vigorously stirred solution of Na 2 C0 3 (200 g, 1.89 moles) and NaOH (280 g, 7.00 moles) in 2.0 dm 3 of distilled water. The resulting reaction mixture was then heated to 65"C for 18 hours. Vigorous stirring was maintained throughout this period. The slurry was then allowed to cool to room temperature. During this time the precipitate partially settles from its mother liquor. The supernatant was then decanted and the slurry was concentrated by centrifuging (2000 rpm, 1120G, 1 hour) and then decanting. The concentrated slurry was then loaded into dialysis tubing (Medicell Visking size 6-27/32"). The sealed tubes were then continuously washed in distilled water until the effluent water was chloride free (tested by 0.1 mol dm -3 AgNθ3 solution) and the conductivity was below 20 μS cm- . The dialysis tubes were then opened and the recovered slurry was slowly dried in a fan oven at 60 β C. Part of the oven dried material was broken down and sieved with the 0.5-1.0 mm size range being collected. The X-ray diffraction pattern (XRD) showed the material to be hydrotalcite with a d(003) spacing of 7.69 Angstroms. B. Calcination Part of the dried cake from Example 1 Section A was calcined at 400*C for 18 hours in air. It was then broken down and sieved with the 0.5-1.0 mm size range being collected. The XRD indicated a poorly crystalline magnesium oxide-like phase. C. Rehydration A sample of distilled water (1 dm 3 ), in an enclosed glass vessel, was thoroughly purged with a stream of nitrogen in order to remove dissolved carbon dioxide. Calcined hydrotalcite (40 cm 3 , 0.50-1.0 mm pellets) from Example 1 Section B was then slowly added to the distilled water. The pellets were left to soak at room temperature for 18 hours under a nitrogen atmosphere. The water was then decanted. Anhydrous ethanol (75 cm 3 ) was then added to the wet pellets and gently agitated in order to remove traces of water before being decanted. The ethanol washing treatment was then repeated three more times. The pellets were then submersed in a further 200 cm 3 of ethanol and left to stand for 3 hours. The ethanol was again decanted and-the pellets transferred to an evaporating basin and dried under flowing nitrogen at 60 β C for 30 minutes. Comparative Material 1 Calcined Hydrotalcite - Commercial Sample A commercially available calcined Hydrotalcite (KW2015) was obtained from Kyowa Chemical Industry Co. Ltd. The sample of KW2015 was supplied as pellets of 1.0-2.0 mm diameter. The XRD, given in Figure 1, indicated a poorly crystalline magnesium oxide-like phase. Example 2 Rehydrated Calcined Hydrotalcite (Commercial Sample) The comparison material 1 was used in this preparation. Pellets of this material (50 cm 3 ) were added to nitrogen purged distilled water, treated with ethanol and dried according to the method described in Example 1 Section C. The XRD pattern, given in Figure 2, was similar to that of hydrotalcite, with a d(003) spacing of 7.72 Angstroms. Example 3 Rehydrated Calcined Hydrotalcite (Commercial Sample) The procedure described in Example 2 was repeated except that after the aqueous rehydration step, the water was decanted and the pellets were dried under flowing nitrogen at lOO'C. Comparison Material 2 Magnesium Hydroxide The preparation described in Example 1 Section A was repeated except that the chloride solution contained only [Mg(H 2 0)5]Cl2 (543 g, 2.67 moles). The resulting material was shown by XRD to have the Brucite structure. Comparison Material 3 Magnesium Hydroxide A commercially available sample of powdered magnesium hydroxide (AnalaR grade from BDH) having the Brucite structure was made into pellets by pressing to 12 tonnes in a 35 mm diameter die. These pellets were broken down and sieved with the 0.5-1.9 mm size range being collected. Examples 4 - 7 and Comparative Tests 1-4 Catalyst Testing Procedure All of the catalysts were tested in a stainless steel reactor (0.9 cm internal diameter) fitted with a thermowell. The catalyst bed volume used was 10 cm 3 in all cases. The liquid feed composition consisted of 10 moles of absolute ethanol to 1 mole of propylene oxide. The run start-up procedure involved pressurising the reactor to 50 barg with the mixed liquid feed at room temperature. When the working pressure was attained, the pump flow rate was adjusted to 20 cm 3 /minute (LHSV = 2). The reactor temperature was then slowly increased to 100*C. Once at condition (i.e. 0 hours-on-streara), regular sampling test periods commenced. The liquid product was analysed by two separate gas chromatographs. A Varian 3700 fitted with a Porapak Q column (1.0 m, 6 mm outside diameter, 2 mm inside diameter, 80-100 mesh) operating with a temperature programme (llO'C for 7 minutes, ramping 60 β C/minute to 220 β C) was used to determine the propylene oxide conversion. A Pye-Unicam 4500 fitted with a WCOT fused silica capillary column (50 m, 0.25 mm internal diameter, CP-Sil-5) operating with a temperature programme (80 β C for 10 minutes, ramping 6°C/minute to 200 β C) was used to determine the relative amounts of l-ethoxypropan-2-ol (1EP2) and 2-ethyoxypropan-l-ol (2EP1). In addition to these isomeric monoglycol ethers, the amount of higher glycol ethers were also determined with this gas chromatograph. Mass balance were typically 98%+ for any test period. The ratio of the two monoglycol ethers are expressed as a percentage of the desired secondary alcohol isomer 1EP2:- Ethoxypropanol Ratio = 100x(lEP2)/(lEP2+2EPl) Example 4 This Example reports the testing of the material from Example 1 Section C. Comparison Test 1 This comparison reports the testing of the material from Example 1 Section B. Example 5 This Example reports the testing of the material from Example 1 Section A. Comparison Test 2 This comparison reports the testing of Comparison Material 2. Comparison Test 3 This comparison reports the testing of Comparison Material 3. Example 6 This Example reports the testing of the material from Example 2. Example 7 This Example reports the testing of the material from Example 3. Comparison Test 4 This comparison reports the testing of comparison material 1. The results of the catalyst testing are given in Table 1. The results clearly show that in all cases, the use of a catalyst according to the invention leads to an improvement in activity and in selectivity to the desired product compared with the use of other catalysts. In particular, the use of a catalyst with a substantially intact hydrotalcite structure gives improved results over the use of a catalyst in which the hydrotalcite structure has been collapsed by calcination. TABLE 1 : TESTING RESULTS % Propylene Products (% Wt) Oxide Ethoxypropanol Isomer Ratio Experiment Reference No. Conversion % 1EP2/(1EP2+2EP1) Ethoxypropanols Higher Glycol Others Ethers Example 4 57 96 98 Comparison Test 1 44 85 97 Example 5 54 96 94 Comparison Test 2 92 95 Comparison Test 3 84 93 Example 6 82 96 97 Example 7 66 96 97 Comparison Test 4 55 84 96

Claims: Claims 1. A process for the preparation of a glycol ether by reacting an olefin oxide with an alcohol over a catalyst; characterised in that the catalyst is an anionic double hydroxide clay with a substantially intact layered structure, the inter-lamellar anions consisting substantially of the anions of the reactant alcohol. 2. A process according to Claim 1, in which the anionic double hydroxide clay has a framework structure including magnesium, divalent iron, cobalt, zinc and/or lithium together with aluminium, chromium and/or trivalent iron. 3. A process according to Claim 2, in which the anionic double hydroxide clay has a framework structure including magnesium and aluminium. 4. A process according to any one of Claims 1 to 3, in which the catalyst has been prepared by heating an anionic double hydroxide clay to a temperature of from 300 to 550 β C under non-reducing conditions; rehydrating the material; and subsequently treating with the reactant alcohol. 5. A process according to any one of Claims 1 to 4, in which the reactant alcohol is an aliphatic, cycloaliphatic or aromatic alcohol having up to 8 carbon atoms. 6. A process according to Claim 5, in which the reactant alcohol is an aliphatic alcohol having up to 6 carbon atoms. 7. A process according to any one of Claims 1 to 6, in which the olefin oxide has up to 10 carbon atoms. 8. A process according to any one of Claims 1 to 7, in which the reactant alcohol is ethanol and the olefin oxide is propylene oxide. 9. A process according to any one of Claims 1 to 8, in which the molar ratio of alcohol to olefin oxide is at least 2:1. 10. A process according to any one of Claims 1 to 9, in which the reaction is carried out at a temperature in the range of from 0 to 250'C.

Inventors: MCATEER COLIN HUGH, MCATEER, COLIN, HUGH
Applicants: BRITISH PETROLEUM CO PLC, THE BRITISH PETROLEUM COMPANY PLC
Application year: 1991
Main classification: C07C43
IPC codes: B01J21, C07B61, C07C41, C07C43