GB1564223A

GB1564223A - Catalyst comprising phosphoric acid on silica gel its preparation and use

Info

Publication number: GB1564223A
Application number: GB1772376A
Authority: GB
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1976-04-30
Filing date: 1976-04-30
Publication date: 1980-04-02
Also published as: CA1100477A; FR2349359A1; JPS52133095A; ES458266A1; BR7702712A; AU2466877A; NL7704657A; DE2719055A1; AU509571B2; IT1075516B; BE853785A

Description

(54) CATALYST COMPRISING PHOSPHORIC ACID ON SILICA GEL, ITS PREPARATION AND USE (71) We, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V., a company organised under the laws of The Netherlands, of 30 Carel van Bylandtlaan, The Hague, The Netherlands, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a novel catalyst comprising phosphoric acid impregnated on a porous silica gel support, and to a process for the preparation thereof. It relates, moreover, to the use of said catalysts, in particular to the production of alkanols, such as ethyl or isopropyl alcohol, by hydration of the corresponding olefins in the presence of such a catalyst.

From the U. K. Patent Specifications 1,371,905 and 1,306,141 it is known to use a silica gel, such as "Davison Grade 57", impregnated with phosphoric acid as a catalyst for the preparation of alcohols by olefin hydration.

It has now been found that important properties of the catalysts in question, particularly the water resistance and the bulk crushing strength of the catalyst particles, can be much improved if the porous silica gel used as the carrier material for the phosphoric acid is prepared by a special procedure.

The present invention provides a catalyst comprising phosphoric acid supported on porous silica gel particles, which may optionally contain small amounts of a filler, the silica gel particles having both a high water resistance (as defined herein) and high bulk crushing strength, (as defined herein), and a pore volume of at least 0.6 ml/g, said particles having been obtained by the following successive steps: (a) preparing a silica hydrosol by mixing an aqueous solution of an alkali metal silicate with an aqueous solution of an acid, (b) converting the hydrosol into droplet form, and (c) allowing the droplets to gel in a liquid which is immiscible with water, whereupon the hydrogel particles emerging from said liquid have been separated off and freed from water and alkali metal compounds.

According to a particularly preferred procedure. the removal of the water and the alkali metal compounds from said hydrogel particles has been carried out by a special sequence of consecutive steps, as described per se in the co-pending application No. 47991/74, as yet unpublished, which comprises: (Serial No. 1525386) (I) removing by evaporation at least 25% of the amount of water present in the hydrogel particles, (II) decreasing the alkali metal content of the hydrogel particles in an aqueous medium to less than 1%w, calculated on dry material, and (III) drying and calcining the silica particles, which may be globular silica particles.

The novel catalyst, in accordance with the invention, has proved to possess superior properties as compared with those of the catalysts described in the prior art. This applies in particular to the bulk crushing strength, both before and after use in the hydration of olefins, such as ethylene, to the corresponding alcohol.

Globular silica gel particles preferably used as the support for the phosphoric acid may be obtained as follows: First of all a silica hydrosol is prepared by mixing an aqueous solution of an alkali metal silicate with an aqueous solution of an acid. This may very suitably be performed by leading the starting solutions separately into a mixing chamber where mixing of the solutions takes place by stirring. As the alkali metal silicate and the acid, sodium silicate and sulphuric acid, respectively, are very suitable. After the silica hydrosol has been formed, it is converted into droplet form and allowed to gel in a liquid which is immiscible, or substantially immiscible, with water. This may very suitably be performed by introducing the hydrosol via a small aperture in the bottom of the mixing chamber into the upper end of a vertically disposed tube filled with oil. Gelation occurs while the hydrosol droplets move downwards through the oil. At the bottom of the tube the globular hydrogel particles may be caught in an aqueous phase, such as water or, preferably, an aqueous solution of a salt such as Na2SO4, particularly a salt solution having substantially the same salt concentration as that present in the hydrogel particles. The hydrogel particles are then separated from the aqueous phase, e.g. by filtration, whereupon they are subjected to the water removal step.

It is also possible to carry out the water removal step in the oil after gelation has taken place.

In the water removal step, according to a preferred embodiment of the invention, at least 25%, and preferably at least 50%, of the water present in the hydrogel particles is removed therefrom by evaporation, removal of from 60 to 90% of the water being most preferred.

This water removal step may be carried out in various ways. Water may advantageously be removed from the hydrogel particles by contacting them with a gas stream, e.g. a stream of air, either or not at elevated temperature. Water may also be removed from the hydrogel particles by heating them at atmospheric pressure or at reduced or elevated pressure. Other ways of removing water from the hydrogel particles are by contacting them with an inert liquid at a temperature above 100"C. or by contacting them at an elevated temperature with steam or a steam-containing gas stream. Examples of treatments which may very suitably be applied for removing at least 25% of the water present in the hydrogel particles are the following: a) heating the hydrogel particles at a temperature of about 100"C at reduced pressure, b) heating the hydrogel particles at a temperature above 100"C in a stream of air, c) heating the hydrogel particles at a temperature of about 100"C at reduced pressure followed by heating the particles at a temperature of about 500"C in a stream of air, d) contacting the hydrogel particles with a hydrocarbon oil at a temperature above 100"C, e) heating the hydrogel particles at a temperature above 1000C in an autoclave at autogenous pressure. and f) heating the hydrogel particles in a stream of air and steam.

It is particularly recommended to remove the water by contacting the hydrogel particles with a gas stream in such a way, that the particles are heated to a temperature in the range 55 to 1800C, and more preferably in the range 100 to 1700C. Excellent results are obtained by using a stream of air having a temperature of from 100 to 300"C, especially from 180 to 220"C.

The temperature to which the hydrogel particles are heated has a significant influence on the texture. i.e. pore volume and pore diameter, of the catalyst support to be prepared. For example, larger or smaller pore volumes (corresponding with smaller or larger pore diameters) may be obtained. according as said temperature is on the lower or higher side, respectively, of the ranges indicated above. Thus, when it is desired to prepare a catalyst having a relatively large pore volume. heating e.g. to about 135"C has proved to be very suitable.

After the treating step. in which at least 25% of the water present in the hydrogel particles is removed therefrom by evaporation, the alkali metal content of the hydrogel particles is decreased in an aqueous medium to less than l6Xcw calculated on dry material.

This alkali metal removal may very suitably be performed by treating the hydrogel particles with an aqueous solution of an ammonium salt, e.g. the nitrate or the sulphate, ammonium hydroxide. or an inorganic or organic acid. for example, hydrochloric, sulphuric. nitric or acetic acid. Subsequently. a water wash may be applied, if necessary. Treatment with an aqueous solution of NH4NO3 has given excellent results. Suitably. the treatment is carried out by percolation of the aqueous solution in question through the hydrogel particles.

Although the operations for removing at least part of the water and for reducing the alkali metal content are most preferably carried out in the consecutive order described above. a reverse sequence of operations is not excluded.

Finally the hydrogel particles are dried and calcined. Drying and calcining of the hydrogel particles may e.g. be carried out by heating the particles for a certain period of time at a temperature of 100-200 C and 450-550"C. respectively. If desired, the calcination may be followed bv a hvdrothermal treatment. e.g. a treatment with steam.

If desired, a small amount of a filler may be incorporated into the silica particles according to the invention. Incorporation of a filler may be attractive for various reasons. In the first place the porosity of the ultimate silica particles may be influenced by this measure, with the result, for example, of a larger pore volume. Further, for certain applications of the silica particles, the presence of e.g. an alumina filler therein may be attractive. It is also possible to decrease the cost of preparation of the silica particles by incorporating therein a cheap filler. The incorporation of the filler into the silica particles may very suitably be performed by adding the filler to the aqueous solution of the alkali metal silicate and/or to the aqueous solution of the acid from which the hydrosol is prepared by mixing. Examples of suitable fillers are kaolin, montmorillonite, bentonite, precipitated silica fillers, aluminas, zeolites and amorphous precipitated silica-aluminas.

With respect to the amount of filler which may be incorporated into the silica particles according to the invention, it has been found that the presence of a filler in the silica particles reduces the bulk crushing strength of the particles, which effect is more pronounced according as the filler content of the particles is higher. Since, however, the sol-gel method as a rule provides globular silica particles with a very high bulk crushing strength, a small decrease is of no importance, and filler-containing globular silica particles which possess the high bulk crushing strength aimed at (see below) can easily be prepared, provided that the quantity of filler incorporated therein amounts to not more than 25%w of the quantity of silica present in the hydrosol from which the silica particles are prepared.

Incorporation of larger amounts of filler in the silica particles entails the risk that silica particles with an inferior bulk crushing strength are obtained.

The silica gel particles prepared according to the procedure described above are usually obtained in a globular form or in the form of pellets of more or less spheroidal shape, and they have a very water resistance and bulk crushing strength. This applies also to the catalyst of the invention, which comprises phosphoric acid supported on said silica particles.

The pore volume of the silica which should be at least 0.6 ml/g, does not exceed, as a rule, 2.2 ml/g and usually is in the range 0.8 to 1.5 ml/g, a pore volume ranging from 1.0 to 1.4 being preferred.

The expression "particles with a high water resistance" used in this patent application refers to particles having a water resistance of at least 80%, preferably of at least 90%. The water resistance of the silica particles, for example, globular silica particles, is determined in a standard test in which 100 of the globular silica particles are contacted for 5 minutes at room temperature with a volume of water which amounts to 5 times the volume of the 100 globular silica particles. Thereafter the particles are inspected to determine the amount of particles which show cracks or have disintegrated. The water resistance of the globular silica particles is expressed as the percentage of particles which have not been damaged by the contact with water.

The expression "particles with a high bulk crushing strength" used in this patent application refers to particles having a bulk crushing stength (BCS) of at least 10 kg/cm2, preferably of at least 11 kg/cm2. The BCS which is a measure of the mechanical strength of the particles is defined as the pressure exerted by a plunger on the sample contained in a cylinder, at which the quantity of fines passing through a 425 Fm sieve amounts to 0.5% (w/w) of the sample.

It is to be understood in this connection that a relation exists between the bulk crushing strength and the pore volume, in the sense that, as a rule, an increase in pore volume results in a decrease of the BCS and vice-versa. Since, in accordance with the present invention, the silica gel particles are to be impregnated with phosphoric acid, it is desirable that they have a relatively high pore volume, say larger than 1.0 ml/g, in order to produce highly active catalysts which are suitable for the hydration of alkenes to alkanols. Silica gel particles having the most favourable pore volumes can be prepared, however, only at the expense of the BCS which is lower than the attainable maximum. The figures for the BCS and the pore volume mentioned above, therefore, represent a compromise in respect of the desired properties.

The novel catalyst of the invention may be prepared by means of any of the conventional techniques. According to a preferred method, silica gel particles, prepared as hereinbefore described are impregnated with phosphoric acid. Suitably, an aqueous phosphoric acid is used having a concentration of, for example, from 20 to 85%, preferably from 55 to 75%.

Good results are obtained by immersion of the globular silica gel support in the aqueous phosphoric acid, e.g. for 0.5 to 5 hours, followed by draining off the excess acid and drying the impregnated support in the usual way, for example by heating at about 1500C.

The catalyst of the present invention may be applied in various chemical reactions, e.g. in the polymerization or oligomerization of lower olefins, but especially in the hydration of olefins to the corresponding alcohols.

Although it is recommended to apply the catalyst of the invention in the globular or spheroidal form in which it is usually obtained by the procedure decribed above, it may also - at least in part - be present in another form, e.g in the form of smaller particles which may be produced by fragmentation, either before or after the impregnation with phosphoric acid, of the globular particles initially obtained. Alterations in the shape may, moreover, be brought about in the course of the production of the hydrogel particles, if desired.

According to a particular important aspect of the invention, there is provided a process for the preparation of an alkanol, in which process an olefin and water are contacted at elevated temperature and pressure, in the presence of a catalyst as hereinbefore described.

Suitable olefins are those containing from 2 to 10 carbon atoms, especially those having from 2 to 5 carbon atoms. Most preferred are ethylene and propylene.

The reactants are generally applied in the gaseous state. The feed ratio and the reaction conditions may vary widely, depending inter alia on the starting material used. Thus, ethanol may suitably be prepared using a molar ratio water/ethylene in the range 0.2:1 to 1.0:1, preferably 0.3:1 to 0.6:1, a temperature in the range 200 to 3()()0C, preferably 220 to 27() C, and a pressure of from 50 to 90 atg. Isopropyl alcohol may be prepared for examle, using a mole ratio water/propylene ranging from 0.1:1 to 0.5:1, a temperature of from 140 to 2500C, and a pressure of from 15 to 50 atg. The gas space velocity of the feed mixture may range from e.g. 5 to 100 mien', preferably from 8 to 35 mien~ ', and in particular from 15 to 35 min-l.

A process for the preparation of ethanol by hydration of ethylene, using as the catalyst a diatomaceous earth, has been described, for example, by C. R. Nelson and M. L. Courter in Chem. Engng.Progr. 50 (1954) pp. 526 to 531.

Example I (a) Preparatioii of silica gel support An aqueous sodium waterglass solution comprising 12%w SiO2 and having a Na2O/SiO2 molar ratio of 0.3 was mixed contiuously in a mixing chamber with an aqueous 1.2N sulphuric acid solution in a volume ratio acid solution/waterglass solution of 0.75. After a residence time of a few seconds in the mixing chamber, the hydrosol was continuously ejected from the mixing chamber through an aperture in the bottom thereof into a vertically disposed tube with a length of 1.8 m filled with a paraffinic oil at 25"C ("ONDINA 33", marketed by SHELL). The hydrosol jet was thus converted into droplet form, and the hydrosol droplets - which had an average diameter of 6 mm - were allowed to pass through the oil by gravity. During the fall through the tube gelation occurred. The globular hydrogel particles were caught at the bottom of the tube in an aqueous 0.25 M Na,SO4 solution at 250C and separated by filtration. The water content of these globular hydrogel particles was determined in a standard test in which a sample was heated in three hours from room temperature to 600"C and thereafter kept at 6000C for one hour. The water content of the hydrogel particles appeared to be 90%w.

The silica hydrogel particles were dried for 0.5 hours in a stream of air having a velocity of 2240 N1/h and a temperature of 200"C, the temperature of the hydrogel particles being allowed to rise to 135"C. After this treatment the water content of the hydrogel particles amounted to 14Zw. Subsequently, an aqueous 0.1 M solution of ammonium nitrate was percolated through the hydrogel particles, which were then washed with water, dried for 2 hours at 100"C and calcined for 3 hours at 500"C. A reduction of the sodium content to about 0.02%w was thus achieved. The globular silica particles obtained showed a water resistance of 98%. They had an average particle size of 3.5 mm, a pore volume of 1.19 ml/g, and a bulk crushing strength of 11.2 kg/cm-.

The "pore volume" is defined herein as the specific pore volume, determined by means of a water titration method in which the amount of water taken up by a dried sample under specified conditions has been measured.

(b) Imprgnatiosl with phosphoric acid The resulting support. as well as a commercially available silica gel support of comparable size and texture, was impregnated with 60% aqueous phosphoric acid by immersion therein, followed by draining off the excess acid and drying at 1500C. The commercial support. used for comparison only, was "Davison Grade 57" ex W. R. Grace Ltd.

In Table I below, the bulk crushing strengh (BCS) - both before and after impregnation with phosphoric acid - is shown. as well as the average particle size, pore volume and average pore diameter of the silica support.

The results obtained show, inter alia, the superior BCS of the impregnated support of the invention (sample A) as compared with that of the commercial support (sample B).

TABLE I Support average particle pore volume1) average pore B C S size (ml/g) diameter (mm) (nm) before im- after impregnation pregnation with H3PO4 (kg/cm) (kg/cm) A (Example I a) 3.5 1.19 13.0 11.2 15.9 B ("Davison 57") 2.5 1.17 13.9 5.4 4.3 1) measured with H2O Example II Preparation of Ethanol Silica gel particles impregnated with phosphoric acid, which had been prepared as described in Example I, were applied as catalysts in the preparation of ethanol by hydration of ethylene as follows: The catalyst (25 ml or 20 g) was charged to a vertically disposed reactor tube of stainless steel (AISI 316) having a diameter of 25 mm and a length of 300 mm, the catalyst bed being enclosed by stainless steel balls of 2 mm diameter which occupied a volume of 75 ml both above and below the catalyst bed. Temperature measurements were made by means of a pyrometer tube having an outer diameter of 6 mm and which was fitted in the centre of the catalyst bed.

A mixture of gaseous ethylene and water in a molar ratio water/ethylene of 0.56: 1 was passed continuously through the reactor tube in an upward direction at an average temperature of 270"C. The space velocity was 40 min-1 which corresponds with 18.5 ml of liquid water per hour and to 40 N1 of ethylene per hour at a total pressure of 60 bar.

As appears from a GLC analysis after a reaction period of 31 hours, ethanol is formed with very high selectivity. No by-products have been detected and notably, no ether formation has taken place.

Further results of the experiment are given in table 2 below.

TABLE 2 Catalyst acid loading ethanol production ethanol content B C S support (%w) rate of product (g/1.h) (%w) before use after use (kg/cm) (kg/cm) A (Example I(a)) 50 180 19.5 15.9 5.7 B ("Davison 57") 51 185 22.0 4.3 3.1 From table 2 it is seen that the BSC after use of the catalyst for the production of ethanol is much higher when applying the catalyst of the invention, as compared with a catalyst prepared from a commercial carrier.

Claims

WHAT WE CLAIM IS:

1. A catalyst comprising phosphoric acid supported on porous silica gel particles the silica gel particles having both a high water resistance (as defined herein) and high bulk crushing strength (as defined herein, and a pore volume of at least 0.6 ml/g, said particles having been obtained by the following successive steps: (a) preparing a silica hydrosol by mixing an aqueous solution of an alkali metal silicate with an aqueous solution of an acid, b) converting the hydrosol into droplet form, and c) allowing the droplets to gel in a liquid which is immiscible with water, whereupon the hydrogel particles emerging from said liquid have been separated off and substantially freed from water and alkali metal compounds.

2. A catalyst as claimed in Claim 1, characterized in that the removal of the water and the alkali metal compounds from said hydrogel particles has been carried out by the following sequence of consecutive steps: (I) removing by evaporation at least 25% of the amount of water present in the hydrogel particles, (II) decreasing the alkali metal content of the hydrogel particles in an aqueous medium to less than 1%w, calculated on dry material, and (III) drying the calcining the silica particles.

3. A catalyst as claimed in Claim 2, characterized in that at least 50% of the amount of water present in the hydrogel particles has been removed by evaporation.

4. A catalyst as claimed in Claim 3, characterized in that 60 to 90% of the amount of water present in the hydrogel has been removed by evaporation.

5. A catalyst as claimed in any one of claims 2-4, characterized in that the evaporation of water from the hydrogel particles has been effected by contacting them with a gas stream.

6. A catalyst as claimed in any one of claims 2-5, characterized in that during the water removal by evaporation the temperature of the hydrogel particles was allowed to rise to 55 to 1800C.

7. A catalyst as claimed in claim 6, characterized in that the temperature of the hydrogel particles was allowed to rise to 100 to 1700C.

8. A catalyst as claimed in anv one of claims 5-7, characterized in that the gas stream was a stream of air of from 10 to 3000C.

9. A catalyst as claimed in claim 8, characterized in that the stream of air had a temperature of from 1800 to 220"C.

10. A catalyst as claimed in any one of claims 1-9, characterized in that the alkali metal compounds have been removed by treatment of the hydrogel particles with an aqueous solution of ammonium nitrate.

11. A catalyst as claimed in any one of claims 1 to 10, wherein a filler is incorporated into the silica gel particles in an amount of not more than 25%w of the quantity of silica present in the silica hydrosol.

12. A process for the preparation of a catalyst as claimed in any one of claims 1-11, characterized in that silica gel particles obtained according to the procedure defined therein, are impregnated with the phosphoric acid.

13. A process as claimed in claim 12, characterized in that the impregnation is carried out by immersion of said particles in aqueous phosphoric acid of 20 to 85%w concentration, followed by draining off the excess acid and drying the impregnated support.

14. A process as claimed in claim 13, characterized in that the concentration of the aqueous phosphoric acid is in the range 55 to 75%w.

15. A process for the preparation of a catalyst according to any one of claims 12 to 14, substantially as hereinbefore described with special reference to Example I.

16. A catalyst whenever prepared by a process according to any one of claims 12 to 15.

17. A process for the production of an alkanol, characterized in that an olefin and water are contacted at elevated temperature and pressure in the presence of a catalyst as claimed in any one of claims 1-11 and 16.

18. A process as claimed in claim 17, characterized in that an olefin having from 2 to 5 carbon atoms is used.

19. A process as claimed in claim 18, characterized in that the olefin is ethylene or propylene.

20. A process as claimed in claim 19, characterized in that the reaction is carried out at a temperature in the range 200 to 300"C, at a pressure of from 50 to 90 atg, and using a water/ethylene molar ratio in the range 0.2:1 to 1.0:1.

21. A process as claimed in claim 20, characterized in that the temperature is in the range 220 to 270 C, and the water/ethylene molar ratio is in the range 0.3:1 to 0.6:1.

22. A process as claimed in any one of claims 17-21 characterized in that the gas space velocity of the feed mixture ranges from 8 to 35 min , preferably from 15 to 35 min

23. A process for the production of an alkanol according to any one of claims 17 to 22, substantially as hereinbefore described with special reference to Example II.

24. Alkanols whenever produced by a process according to any one of claims 17-23.