GB2155916A

GB2155916A - Crystalline metallophosphates

Info

Publication number: GB2155916A
Application number: GB08506588A
Authority: GB
Inventors: David Roger Pyke
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1984-03-15
Filing date: 1985-03-14
Publication date: 1985-10-02
Also published as: GB8406768D0; GB2155916B; GB8506588D0

Abstract

Microporous crystalline aluminium phosphates are modified by isomorphous substitution within the aluminium phosphate of ions of other elements, for example silicon, vanadium, iron, manganese, magnesium, zinc, titanium and zirconium. The modified crystalline aluminometallophosphates are prepared from reaction mixtures comprising sources of aluminium, phosphorus,the modifying metal and an organic templating agent for example an alkylammonium compound, an amine or heterocyclic nitrogen compound. The crystalline aluminometallophosphates may be used as hydrocarbon conversion catalysts, for example in xylenes isomerisation and toluene disproportionation.

Description

SPECIFICATION Crystalline Metallophosphates The present invention relates to novel crystalline metallophosphates which are modifications of microporous aluminium phosphates.

In recent years crystalline aluminosilicate molecular sieves of the crystalline zeolite type have attracted a considerable amount of attention and the number of naturally occurring and synthetic zeolites now approaches 200. In general terms, crystalline zeolites are aluminosilicates having frameworks formed from Al04 and SiO4 tetrahedra joined by shared oxygen atoms. The zeolites have significant ion exchange capacity and different zeolites have differing sizes of pore opening thereby leading to differing adsorptive capacities and catalytic performance.

More recently, anew family of aluminophosphate molecular sieves has been described, the members of which have structural and chemical properties similar to those of the zeolites. So far, about 20 members of this family have been discovered, of which at least 14 are microporous and 6 are two-dimensional layer-type materials. The stoichiometry of these crystalline, microporous aluminophosphates is approximately 1:1 and over the lattice as a whole, they are charge neutral; that is, they do not possess ion exchange character and they are not strongly acidic catalysts. For example, crystalline auminophosphates with pore sizes in the range 4 to 8A have been found to be only very slightly active in xylene isomerisation even at high temperatures. Surface modification by acid washing and reaction with thionyl chloride had little effect.

We have now surprisingly found that it is possible to chemically modify microporous crystalline aluminium phosphates so as to improve and extend the range of their chemical and physical properties. It is believed that this arises as a result of isomorphous substitution within the aluminium phosphate lattice.

Substitution of equivalent ions gives an overall neutrally charged lattice framework while substitution of ions of different valency to Al"' and PV requires charge neutralisation by counterbalancing ions held within the pore structure. This is surprising since isomorphous substitution in the aluminium silicate framework of zeolites is generally thought to be very limited and has not been observed for altervalent ions (see for example the discussion by Barrer. "Hydrothermal Chemistry of Zeolites". Academic Press 1982 pages 301-2.) Thus while Ge and Ga can readily be substituted for Si and Al respectively, the evidence for any significant substitution by Fe"', Cr"', ZrlV etc is very ambiguous.While such substitution can be achieved in high temperature syntheisis of denser non zeolitic materials, relatively low temperature hydrothermal synthesis required for the generation of microporous structures does not give rise in general to such substitution.

Surprisingly we have now shown that such substitution can be useful for generating catalytically active centres in modified microporous aluminophosphates.

Accordingly the present invention comprises a crystalline aluminometallophosphate having a composition, expressed in terms of oxides, as follows: Awl203: x M2XnO: y P205 where M is an element, of valency n, selected from, for example, silicon, vanadium, iron, manganese, magnesium, zinc, titanium and zirconium x is 0.001 to 0.8 andy is 0.6 to 1.4. Thus for example, if pv is replaced by Mlv or Al"' by M", acid character and ion exchange properties are generated by the necessary presence of charge balancing protons or cations. While the overall activity of such materials will be governed by the density of such sites in the framework the acid strength of these sites will depend upon the nature (i.e.

the relative electronegativities) of the individual ions involved.

In preferred embodiments of this invention the element M is silicon. Without wishing to be bound by any theory, we believe from evidence hereinafter disclosed that some of the phosphorous (PV) in the aluminophosphate framework has been substituted by silicon (SiV) and that it is possible that some of the framework aluminium may also have been substituted by silicon.

According to a further aspect of the present invention a process for preparing a crystalline aluminometallophosphate having a composition, expressed in terms of oxides, of Al203:xM20:YP2s n where M is an element of valency n selected from silicon, vanadium, iron, manganese, magnesium, zinc, titanium and zirconium, x is 0.001 to 0.08, and y is 0.6 to 1.4 comprises preparing an aqueous reaction mixture containing a source of element M, a source of aluminium, a source of phosphorus and an organic templating agent, the mixture having a composition in terms of mole ratios of oxides of:: M2O/AI203 = 0.01 to 0.6 H20/A1203 = 1000 to 10 n P205/A1203 = 1.5 to 0.6 R2O/AI203 = 0.5 to 2.0 where R is the organic templating agent, heating the reaction mixture at a temperature in the range of 70"C to 220"C under autogenous pressure until crystals of the aluminometallophosphate are formed.

Preferably the element M is silicon and the silica source can be any of those normally considered suitable for use in synthesising zeolites, for example powdered solid silica, silicic acid, colloidal silica or dissolved silica. Among the powdered silicas usable are precipitated silicas, especially those made by precipitation from an alkali metal silicate solution, such as the type known as "KS 300" made by AKZO, and similar products, aerosil silicas fume silicas and silica gels suitably in grades for use in reinforcing pigments for rubber or silicone rubber. Colloidal silicas of various particle sizes may be used, for example 10 to 15 or 40 to 50 microns, as sold under the Registered Trade Marks "LUDOX", "NALCOAG" and "SYTON".The usable dissolved silicas include commercially available waterglass silicates containing 0.5 to 6.0, especially 2.0 to 4.0 mols of SiO2 per mol of alkali metal oxide, "active" alkali metal silicates as defined in UK Patent 1193254, and silicates made by dissolving silica in alkali metal or quaternary mixture.

The alumina source is most conveniently sodium aluminate, but can be aluminium, an aluminium salt, for examplethe chloride, nitrate or sulphate, an aluminium alkoxide or alumina itself which should preferably be in a hydrated or hydratable form such as colloidal alumina, pseudoboehmite, boehmite, gamma alumina or the alpha, or beta trihydrate.

The preferred source of phosphorus is phosphoric acid.

The organic templating agent is selected from those normally considered for use for this purpose and the actual choice will be dependent on the type of modified aluminophosphate which it is desired to synthesise.

Suitable templating agents include tetrapropylammonium compounds, tetraethylammonium compounds, amines, for example triethylamine, tripropylamine, cyclohexylamine, triethanolamine, 2-methylpyridine, 3-methylpyridine, N-methylpyridine.

The Applicants have found that if sodium silicate is used as the silicon source, sodium is found in the product and can be washed out with acid. The acid-washed silicon-substituted aluminophosphate of suitable pore size is an order of magnitude more active in xylenes isomerisation and toluene disproportionation than eitherthe sodium form or the equivalent pure aluminophosphate of the same pore size.

The aluminosilicophosphates prepared according to this invention, unlike pure aluminophosphates, are ion exchangers. X-ray diffraction patterns of the aluminosilicophosphates of this invention show them to be of the same crystalline form as the various aluminophosphate types but with intensity variations in the patterns and lattice distortions.

Microprobe analysis of the aluminosilicophosphateswith a 50A probe diameter Phillips EM400 Transmission Electron Microscope shows the absence of discrete silica phases and shows silicon to be distributed throughout the crystal fragments and not concentrated at surfaces.

For these reasons therefore, the Applicants believe that the silicon in the aluminosilicophosphates of the invention is part of the crystalline framework of the phosphate. This point is illustrated further in the examples below.

Similar considerations apply for modification by other metals. Essentially the same preparative conditions apply, the modifying ion being introduced in a suitable form such as the hydroxide, phosphate, chioride, nitrate, sulphate etc. In the case of iron, for example, it is believed that Fe"' replaces a proportion of the Al"' sites to give an overall charge neutral lattice. Substitution by Zn" in these sites however requires counterbalancing cations external to the framework and in the acid exchanged form this material is a more strongly acidic catalyst than the equivalent silicon modified material.

The invention is further illustrated by the following examples: Note: In the following examples the following common factors apply: 1. Materials were prepared in a stirred stainless steel, PTFE lined autoclave at autogenous pressure.

2. Powder X-ray diffraction was carried out on a diffractometerwith copper Ka radiation and employing an internal calibration standard where appropriate.

3. Adsorption measurements were carried out after careful outgassing to 450"C on a Kahn vacuum microbalance by equilibrating the sample, held at room temperature, with liquid water, n-hexane or p-xylene maintained at 0 C by a ice/water slush bath.

Example I 0.27 mol of 83% orthophosphoric acid was mixed with 36 cm3 of deionised water and stirred with 0.04 mol of 0.79 sodium silicate solution. 0.23 mol of pseudo-boehmite was then stirred in and the entire mixture combined with a mixture of 0.20 mol tetramethyl ammonium hydroxide solution (25%) and 4 cm3 of 35% hydrochloric acid. The reaction mixture was transferred to an autoclave and reacted at 1 500C for 183 hours.

After filtering washing and drying the material was calcined at 600 C. Chemical analysis gave the proportional atomic Al : P : Si : Na = 1.0 : 0.64: 0.17: 0.06 which after washing with dilute hydrochloric acid reduced the sodium content to give a ratio of 1.0 : 0.7 : 0.16 : 0.02.

Powder XRD showed the material to be an essentially crystalline sodalite type (U.C. type AIPO4-20) but with significant difference in peak intensites and d spacings (see Table 1) from the data given by Union Carbide for the latter material. Indexing gave a lattice parameter value of a = 8.958 compared with that calculated forAIPO4-20 of 8.933A.

TABLE 1 Powder data for calcined sodalite type Si modified ALPO4-20

U/C A1P04-20 2 AlPSi/600/H calcined 600 C, acid washed d(A) I/Io d(A) I/Io 6.19 100 6.34 100 4.37 27 4.48 25 4.15 6 3.92 9 4.01 4 3.59 57 3-658 65 3.14 20 a. 166 15 2.79 19 2;833 15 2.56 13 2.586 11 TABLE 2 XRD data for Si modified AlPO,1-18

Silicon Modified Material A1P04-18 (US 4,310,440) calcined 6000C calcined 6000C d 100xI/Io d 100xI/Io 11.86 3 9.23 100 9.31 100 9.23 100 9.31 100 8.35 14 7.83 4 6.80 26 6.83 9 6.21 1 6.56 8 6.11 4 5.48 15 1 5.50 j 11 5.22 1 18 5.06 1 5.16 17 4.97 5 4.62 4 4.65 5 4.51 7 4.44 4.44 6 4.26 27 4.29 12 4.17 15 1 4.06 5 3.96 4 1 3.95 8 3.81 6 3.88 1 9 3.72 12 3.55 7 3.58 3 3.53 3 3.40 11 3.43 8 3.39 7 3.29 4 3.21 7 3.07 7 2.98 1 2.98 8 2.88 18 2.88 13 2.85 14 2.83 5 2.78 7 Adsorption experiments showed the material to be microporous with small pore characteristics taking up 9.7 wt.% of water under the conditions described above.

Example 2 The same preparative method was employed as in Example 1 but employing lower concentrations of pseudo boehmite and a different templating agent, tetraethylammonium hydroxide. The constituents were: 15.4 cm3 of 85% H3P04, 36 cm3 of water, 8.45 gm of Syton X30 colloidal silica (13.3%Si), 18.3 gms pseudo-boehmite, 4 cm3 HCI (35.4%) and 104cm3 of 25% solution of tetraethylammonium hydroxide. After reaction for 7 days at 1 55 C, filtering, washing, drying and calcination at 600"C, the material was found by powder XRD to be essentially crystalline and a modified form of the type described in the literature as AlPO4- 18. A minor proportion of the type Al PD4- 5 was also found to be present.The d-spacings and intensity variations in the powder pattern with respect to the pure AIPO4- 18 type indicate the incorporation of silicon into the lattice (Table 2) Electron microscopy showed this material to be highly crystalline and TEM microanalysis (50 spot size) showed a regular distribution of silicon with the absence of identifiable silica phases. An average of 10 crystals gave the atomic ratio Al : P : Si of 1.0 : 0.89 : 0.17 in close agreement with the bulk chemical analysis 1.0:0.86:0.19.

The ion exchange properties of this material were demonstrated by shaking 1 gm with a 1 mol dm-3 solution of sodium chloride held at 60"C and analysing the solution at intervals. After 5 minutes the material had exchanged 0.03 gms of sodium ion, after an hour a further 0.004 gms and after 24 hours a further 0.003 gms.

The adsorption properties are illustrated by the adsorption of 26.4wt.% water, 11.4wt.% n-hexane and 5.0 wt.% p-xylene under the conditions described above. Nitrogen isotherm determinations gave a value for the micropore volume of 143 cc at NTP.

The improved catalytic performance in acid catalysed isomerisation reactions is illustrated by comparing the conversion of m-xylene to the ortho and para isomers with that of the pure aluminium phosphate of the same structure and similar porosity (micropore volume 175cc of nitrogen). Results are given in Table 3.

Similarly the increased activity for toluene disproportionation as a result of silicon modification is illustrated by a toluene conversion to benzene and xylenes of 9.7% at 500 C and a WHSV of 0.377 compared with negligible conversion over unmodified material of the same structure.

Example 3 A material of the same structure as that of Example 2 but with lower silicon concentration was prepared by the same method but with the following constituents: 18.4 cm3 orthophosphoric acid, 36 cm3 water, 0.56 gm Syton X-3, 18.3 gm pseudo boehmite, 4 cm3 hydrochloric acid and 66.3 cm3 tetraethyl ammonium hydroxide solution (40%). Although this material had greater porosity (32.4% water, 12.2% n hexane, 10.7% p-xylene) the overall activity for xylenes isomerisation was reduced as a result of the lower concentration of silicon (Table Ill). Chemical analysis gave the atomic ratio P : Al : Si = 0.95:1.0 : 0.05.

Example 4 0.32 mol orthophosphoric acid (85%) was diluted with 46 cm3 water and then combined with 19.2 gm pseudoboehmite and 10.6 gm Syton X-30 colloidal silica. (P : Al : Si = 0.32 : 0.28 : 0.05). 18.5 gm of quinuclidine was dissolved in 34 cm3 of deionised water and added to the reaction mixture with stirring. A further 20 cm3 water was added and the mixture autoclaved at 190"C for 99 hours. After filtering, washing, drying and calcining at 550"C for 2 hours powder X-ray diffraction showed the material to be crystalline and of the type described as AIP04-16 but modified by the inclusion of silicon as shown by differences in d spacings and relative peak intensities (see Table 4).The material was shown to contain only small micropores by the adsorption of 24.2 wt.% of water but only 1.0 wt.% of n-hexane.

Example 5 A silicon modified form of AIPO4-5 type was prepared from a mixture with the proportion Al: P: Si of 0.27 : 0.26: 0.014. 17 cm3 of 85% orthophosphoric acid was diluted with 36 cm3 of water and mixed with 2.82 gm of Syton-X30 solution. The mixture was poured onto 18.3 gm of pseudo-boehmite and after mixing thoroughly a solution of 104 cm3 tetraethylammonium hydroxide (0.18 mol) and 4 cm3 hydrochloric acid was added. After autoclaving at 1 600C for 7 days, filtering, washing, drying and calcining at 600"C a crystalline product was obtained which consisted of modified AIPO4-5 type together with a small proportion of modified AlPO4-1 8 type. (Table 5). The material was microporous, absorbing 25.8 wt.% water, 7.4 wt.% n-hexane and 4.0 wt.% p-xylene and N2 absorption gave a micropore volume of 135 cc N2. The enhanced activity for acid catalysed reactions is illustrated by the isomerisation of m-xylene (Table 6) compared to the conventional AIPO4-5 type prepared without silicon incorporation. Chemical analysis gave the atomic ratiosofAl: P: Sito be 1.0:0.82:0.12.

TABLE 3 Isomerisation of m-xylene over A1P04-18 type modified and umodified materials Conversion per pass (%) at various temperatures

Reaction temperature C 300 450 550 Pure AlPO4-18 0 0.04 0.5 AlPO4-18 modified by 3.7 wt% Si 2.2 6.9 27.6 AlP04-18 modified by 1.05 wtZ Si 0 0.6 6.5 TABLE 4 Silicon Modified AlPO,,-16 type X-ray powder data

d(A) I/Io d(A) I/Io 10.07 2 3.76 3 9.48 3 3.53 3 7.96 34 3.35 24 7.71 100 3.26 4 6.49 14 3.11 14 5.05 6 3.07 14 4.72 29 2.98 34 4.21 10 2.81 2 4.02 75 2.73 11 3.86 16 Example 61ron-Modified typesAlPO4 - 21 and 25 A reaction mixture was prepared containing aluminium, iron and phosphorus in the molar ratio 0.36 : 0.04 : 0.04. 24.9 g of pseudoboehmite was mixed with 27.2 cm3 of 85% orthophosphoric acid diluted with 156 cm3 water..To this was added freshly preipitated iron hydroxide prepared by dissolving 6.5 g.FeCI3 in 60 cm3 water, precipitating with ammonium hydroxide solution, filtering and thoroughly washing with deionised water. 17 cm3 of pyrollidine (0.2 mol) was then added and the reaction mixture autoclaved for 130 hours at 1 50 C. The separated and washed product dried to a pale green powder which after calcination for 3 hours at 6000C became a sandy brown colour. Powder XRD (Table 7) showed the dried material to be a modified type AIPO4-21 material and the calcined material a modified type AIPO4-25 material by virtue of intensity variations in the patterns.

Chemical analysis gave the atomic ratio P : Al : Fe to be 1.0 : 0.8 : 0.1. Electron microscopy showed the material to consist of large cubic crystals with a homogenous distribution of iron [11 crystal average Al : P: Fe = 0.81:1.0: 0.14] and the absence of discrete iron containing phases.

TABLE 5 Powder XRD data for silicon modified type AiPO,,-5

d(A) I/Io d(A) Illo 11.90 100 11.90 100 3.82 8 9.21 69 3.53 12 6.84 29 3.44 21 6.27 2 3.40 12 5.96 6 3.27 3 5.47 11 3.19 4 4.94 10 3.14 5 4.60 4 3.07 10 4.51 30 2.98 13 4.26 34 2.89 20 4.19 42 2.85 11 3.97 53 TABLE 6 Isomerisation of m-xylene over Si modified and unmodified AlPO "-5 type

Reaction temperature C 300 400 450 500 Si-modified Conversion x 0.7 4.7 21.3 43.2 A1P04-5 type Unmodified conversion x 0.04 N.D. 1.0 1.5 A1P04-5 type N.D. = Not Determined Example 7 Vanadium Modified types AIPO4-2 1 andAlPO4-25 1.8 g of V2 5 was mixed with 26 cm3 of 85% orthophosphoric acid and 163 cm3 of deionised water and heated to give a clear yellow solution. 27.5 g of pseudo-boehmite was stirred in followed by 16.5 cm3 of pyrollidine and the reaction mixture was autoclaved at 1 500C for 6 days. The dried product was a pale grey colour which XRD showed to be a modifed type AIPO4-21.After calcination at 6000C for 3 hours the product was canary yellow in colour and XRD showed it to be crystalline modified type AlPO4-25 (Table 8). Electron microcopy showed well formed crystals and microanalysis showed the absence of identifiable vanadium oxide phases and a regular distribution ofvanadium(Al :P:V = 0.8 1.0: 0.3) TABLE 7 Iron modified type AlPO,-25. Peaks excluding a small proportion of tridymite and cristobalite phases

d(A) I/lo d(A) I/Io 9.38 39 3.51 20 7.58 3 3.42 12 5.89 56 3.29 8 4.68 48 3.12 11 4.20 100 3.00 6 3.94 64 2.89 22 3.63 14 TABLE 8 Powder XRD data for vanadium modified type AiPO,,-25 excluding a small proportion of cristobalite and tridymite phases

d(A) - I/Iod(A) 1/ lo 12.01 2 3.53 60 9.517 3.43 9 7.66 10 3.30 9 5.94 55 3.23 7 4.90 4 3.14 7 4.71 10 3.00 10 4.21 100 2.90 12 3.95 50 Example 8 lron-Modified typeAlPO4- ?6 Iron hydroxide was prepared from 5.4 g of ferric chloride by neutralising with 25% ammonium hydroxide solution. After thorough washing this was combined with 20.69 of pseudoboehmite and added to 23 cm3 of 85% orthophosphoric acid in 46 cm3 of deionised water. 18.5 g of quinuclidine was then dissolved in 344 cm3 of water and combined with the above mixture. After autoclaving the mixture at 1 50 C for 90 hours the product obtained was filtered and dried at 1000C. After calcination at 6000C for 3 hours a pale beige coloured product was obtained which had the X-ray diffraction pattern given in Table 9.

Transmission electron microscopy showed the material to consist of both large and small crystal fragments. Microprobe analysis showed a homogeneous distribution of iron over all crystallites with an average atomic ratio (9 particles) of P:Al : Fe = 1:0.6:0.1. Adsorption experiments showed that under the standard conditions, 9 wt% of water was absorbed but only 2.3% of n-hexane.

TABLE 9 Powder XRD for iron modified type AlP04-16 excluding contribution from a minor proportion of cristobalite and tridymite types

d(A) I/Io d(A) I/Io 7.74 100 3.87 25 6.69 6 3.35 25 6.48 5 3.07 16 4.74 17 3.00 41 4.04 71 2.73 7 TABLE 10 Iron modified type AlPO,,-5

d(A) I/Io d(A) I/lo 11.79 33 3.41 12 9.33 1 3.23 6 6.82 9 3.06 9 5.93 3 2.95 9 3.95 100 2.90 6 Example 9 lron-modffied type AlPO4- 5 27 cm3 of 85% orthophosphoric acid (0.40 mol P) was diluted with 54 cm3 of water. 24.7 g of pseudoboehmite (0.36 mol Al) was added to 73 cm3 of this solution while 6.49g of FeCI3 (0.04 mol Fe) was mixed with the remaining portion. The two mixtures were then combined and 98 g of 40% tetramethylammonium hydroxide solution added. The reaction mixture was autoclaved at 150"C for 7 days, washed and dried at 100"C to give a sandy coloured material. After calcination at 600"C for three hours the white powder was chemically analysed to give an atomic ratio P:Al : Fe - 1.0:0.87:0.11. Powder XRD showed the material to consist of the dense tridymite phase together with modified AlPO4-5 type phase (Table 10).Microprobe analysis in the transmission electron microscope showed a relatively homogeneous distribution of iron throughout the sample at a level similar to that obtained by chemical analysis. This material adsorbed 6.2 wt% water. 2.6% n-hexane and 3.0 wt% p-xylene under the conditions described above.

Example 10 Zinc-modified type AlPO4-5 12 cm3 of 85% orthophosphoric acid was diluted with 59 cm3 of deionised water. 61 cm3 of this solution was mixed with 35.49 of aluminium isopropoxide (0.173 mol Al) and the resulting precipitate separated and washed. 4.7 g of zinc orthophosphate was dissolved in the remaining 10 cm3 of acid solution and combined with the aluminium phosphate and a further 50 cm3 of water. 51.9 cm3 of a 20% solution of tetrapropylammonium hydroxide templating agent was combined with the reaction mixture which was then heated in an autoclave for 41 hours at 150 C. The white product was filtered, washed and dried at 100 C. After calcination for 3 hours at 500 C the major phases were found by XRD to be a modified AIPO4 type 5 and tridymite.Adsorption measurements showed adsorption of 6.5 wt% water, 1.0 wt% n-hexane and 1.2wt% p-xylene. The material was found to have activity for xylenes isomerisation at relatively low temperatures, giving initially 8.9% conversion of m-xylene at 300"C and 22.6% conversion at 350"C with an ortho/para ratio of about 0.9. Equilibrating this material with a 1.0 molar solution of sodium chloride removed this catalytic activity.

Example 11 Zinc-modified typeAlPO4- 18 12.2 cm3 of 85% orthophosphoric acid was diluted with 13.5 cm3 of deionised water. 11 .7g of pseudoboehmite was mixed with 4.6 g of zinc orthophosphate and stirred into the acid solution.48.6 of 40% tetramethylammonium hydroxide solution was diluted with 78 cm3 of water, mixed with 3 cm3 of 35% hydrochloric acid and the resulting solution combined with the phosphates.The reaction mixture was heated in an autoclave for 170 hours at 155"C. Powder XRD showed that calcination of the white product at 600"C gave only dense phases but calcination at 400"C for three hour gave a crystalline material with d spacings and intensity variations indicating a modified AlPO4 18 type. Adsorption measurements showed 23.8 wt% of water, 6.3 wt% n-hexane and 3.0 wt% p-xylene were adsorbed under the conditions described above. The material was active in the isomerisation of meta-xylene with an enhanced proportion of para-xylene indicating diffusional constraint upon the reaction. Thus at 422"C a 12.6% conversion of meta xylene was achieved with an ortho/para ratio of 0.21.

TABLE 11 Zinc modified type AlPO6-5

d(A) 1/10 d(A) I/Io 11.85 58 3.61 9 6.85 18 3.43 26 5.94 6 3.07 21 4.49 34 2.97 16 4.37 83 2.66 5 3.96 100 2.59 14 Example 12 Titanium-ModifiedtypeAlPO4-5 15.4cm3 of 85% orthophosphoric and was diluted with 36cm3 of deionised water and 4.5 cm3 of titanium tetrachloride solution (0.04 mol) was very slowly added. 0.18 mol of tetraethylammonium hydroxide solution was mixed with 4 cm3 of hydrochloric acid (35%), the two solutions combined and heated in a PTFE lined autoclave for 6 days at 150 C. After thorough washing, drying and calcining at 500"C the white material had a powderXRD pattern indicating a modified AlPO4-5type (table 12) and analysis gavethe ratioAl:P:Ti = 0.86:1.0:0.026. 17.5 wt% water, 4.5 wt% n-hexane and 5.6 wt% p-xylene were adsorbed under the standard conditions described. The material was found to be active for xylenes isomerisation and showed interesting selectivity to para-xylene (Table 13). Washing the metal with dilute hydrochloric acid (4 hours, 60"C, 0.1 mol dm3 HCI) improved the selectivity to p-xylene further. By determining the isomer distribution from pure meta- and pure ortho-xylene feeds at similar conversion levels a value for R was obtained where rate constant for conversion meta- to ortho xylene =0.48 R= =0.48 rate constant for conversion meta- to para xylene at 400VC.

This may be compared with a typical value for amorphorous silica-alumina of R=0.8 and confirms diffusional inhibition in the escape of the ortho isomer from the micropore structure.

TABLE ~ ~ ~ - - TABLi 12 -- ~ XRD data for titanium modified AlPO4-5 type material

d(A) I/Io 11.80 74 6.83 25 5.91 8 4.91 1 4.47 34 4.20 63 3.95 100 3.58 8 3.42 36 3.06 21 2.96 19 2.65 5 2.58 16 2.42 4 2.38 10 2.16 2 2.13 2 2.10 2 TABLE 13 Isomerisation of m-xylene over titanium modified type AlP04-5

Reaction Calcined Sample Acid Washed Sample Temperature Conversion Ratio ortho Conversion Ratio ortho (Z) para (%) para 3000C 4.1 0.49 4.0 0.32 350 C 16.5 0.60 11.1 0.435 3800C ND ND 23.0 0.497 400 C 37.8 0.83 30 0.577 4500C ND ND 36.7 0.68

Claims

1. A crystalline aluminometallophosphate having a composition expressed in terms of oxides, comprising Awl203: xM2O : YP205 n where M is an element of valency n, xis 0.001 to 0.08 and y is 0.6 to 1.4.

2. A crystalline aluminometallophosphate as claimed in claim 1 wherein the element M is selected from silicon, vanadium, iron manganese, magnesium, zinc, titanium and zirconium.

3. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising a sodalite type silicon-modified AIPO4-20.

4. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising silicon-modified AlPO4-18.

5. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising silicon-modified AlPO4-1 6.

6. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising silicon-modified AlPO4-5.

7. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising iron-modified AIPO4-5.

8. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising ironmodified AIPO4-16.

9. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising iron-modified AlPO4-21.

10. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising iron-modified Al PO4-25.

11. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising vanadium-modified AlPO4-21.

12. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising vanadium-modified AIPO4-25.

13. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising zinc-modified AIPO4-5.

14. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising zinc-modified Al PD4- 18.

15. A crystalline aluminometallophosphate as claimed in claim 1 or 2 comprising titanium-modified AIPO4-5.

16. A process for preparing a crystalline aluminometallophosphate having a composition, in terms of oxides, comprising Al203: xM2O : YP205 n where M is an element of valency n, x is 0.001 to 0.8 andy is 0.6 to 1.4 comprises preparing an aqueous reaction mixture containing a source of element M, a source of aluminium, a source of phosphorus and an organic templating agent, the mixture having a composition in terms of moie ratios of oxides M2D/Al2D3 - 0.01 to 0.6 H20/A1203 - 1000 to 10 n P205/A1203 = 1.5 to 0.6 R20/A1203 = 0.5 to 2.0 where R is the organictemplating agent, heating the reaction mixture at a temperature in the range of 70 C to 220"C under autogeneous pressure until crystals of the aluminometallophosphate are formed.

17. A process as claimed in claim 16 wherein the element M is selected from silicon, vanadium, iron, manganese, magnesium, zinc, titanium and zirconium.

18. A process as claimed in 16 or 17 wherein the organic templating agent is selected from tetra alkylammonium compounds, amines and heterocyclic nitrogen compounds.

19. A process as claimed in claim 18 wherein the organic templating agent is selected from tetramethylammonium compounds, tetraethylammonium compounds, tetrapropyiammonium compounds, triethylamine, tripropylamine, cyclohexylamine, triethanolamine, 2-methylpyridine, 3-methylpryridine, N-methylpryridine, pyrollidine and quinuclidinium compounds.

20. A process for the preparation of crystalline aluminometallophosphates substantially as hereinbefore described with reference to any one of examples 1 to 12.

21. A crystalline aluminometallophosphate whenever prepared by a process as claimed in any one of claims 16 to 20.

22. A process for conversion of a hydrocarbon which comprises contacting a hydrocarbon under hydrocarbon conversion conditions with a crystalline aluminometallophosphate as claimed in any one of claims 1 to 15 and 21.

23. A process as claimed in claim 22 wherein the hydrocarbon conversion process comprises xylene isomerisation.

24. A process as claimed in claim 23 wherein the hydrocarbon conversion process comprises xylene isomerisation and the crystalline aluminometallophosphate is selected from silicon-modified AIPO4-5, silicon-modified AlPO4- 18, zinc-modified Al PD4- 5, zinc-modified AlPO4- 18 and titanium-modified AIPO4-5.

25. A process as claimed in claim 22 wherein the hydrocarbon conversion process comprises toluene disproportionation.

26. A process as claimed in claim 25 wherein the hydrocarbon conversion process comprises toluene disproportionation and the crystalline aluminometallophosphate is silicon-modified AIPO4-18.