GB2269377A - Silica gel and process for making it from polysilicic acid - Google Patents

Abstract

Mesoporous silica gel of high surface area and a low range of pore size distribution suitable as a catalyst support can be made by reacting a water soluble polysilicic acid with a gellation agent formed in situ in the reaction medium to produce hydrogel, heating said hydrogel with an organic solvent to entrain water and separating the gel product, and then preferably heating. The polysilicic acid, which preferably has a molecular weight distribution of less than 10, may be made by reacting an acid and a silica source to form silicic acid, and then extracting it into an organic phase. The silica gel may have a mesopore density of 2-6 ml/g, a surface area of 400-1000 m<2>/g and a pore volume of 2-4 ml/g.

Description

SILICA PRODUCT AND PROCESSES This invention relates to a silica product and process for making it particularly of high pore volume, as well as polysilicic acids and their production.

Silica products, eg for use as catalyst supports are made from silica hydrogel, itself made by reaction of a silica source with a mineral acid and gellation. Such silica product tend to have a wide range of pore volumes and be of comparatively low surface area.

We have found mesoporous silica products that can have a high surface area, high pore volume and usually a low distribution of pore volume sizes, as well as a method of making them.

The present invention provides a mesoporous silica gel having a mesopore density of 2-6 ml/g, a surface area of 400-1000 m2/g and a pore volume of 2-4.0 m3/g.

There is also provided a process for making a silica gel which comprises (i) reacting a water soluble polysilicic acid in a liquid medium comprising water with a gellation agent formed in solution in situ in the medium to produce a hydrogel comprising water, (ii) heating said hydrogel with a substantially water immiscible organic solvent and entraining said water to produce a substantially water free suspension of silica gel in said solvent, and (iii) separating said silica gel and said solvent. Preferably the process comprises after step (iii) heating said silica gel to 400-700"C to leave a mesoporous silica gel.

The silica gel is mesoporous with a high surface area and usually with a low range of pore size distribution. The micro pore density can be 0.01-0.4ml/g eg 0.01-0.1 ml/g eg 0.03-0.07 ml/g, the mesopore density can be 2-6ml/g such as 2-4 ml/g eg 2.3-3.0 ml/g, the mean average pore diameter can be 150-550A (Angstrom)eg 150-350 A eg 200-300 A with average pore diameter 100-350A such as 100-250 A eg 130-200 A, the pore diameter at median area is 100-350A such as 100-250 A eg 150-210 A and median pore diameter of 150-450A e.g.

150-350 A such as 200-270 A, and interquartile range of pore diameter (between quarter and three quarters the pore diameter) of 80-160 A such as 100-140 A. The pore surface area is usually 400-1000 or 400-800 m2/g eg 500-720 m2/g, while the pore volume can be 2-4.0 or 2-3.5 m3/g such as 2.3-3.0 m3/g. These pore and surface area properties can be determined by BET measurements with nitrogen.

In the process of the invention the first step involves a water soluble polysilicic acid. This may be made by hydrolysis of a silicate ester such as a tetralkyl silicate, wherein each alkyl hs 1-6 carbon atoms such as methyl, ethyl or butyl, especially tetraethylorthosilicate. Alternatively silicon tetrachloride can be hydrolysed in particular slowly and carefully with cooling, or a metal silicate can be acidified; further details of a preferred acidification and purification process which forms another aspect of this invention are described hereinafter to produce polysilicic acid of narrow molecular weight distribution. Examples of suitable silicates are Group 1A or 2A metal silicates or aluminosilicates eg minerals or clays, which can be reacted with acid eg aqueous inorganic acid such as hydrochloric, nitric or sulphuric acids.The hydrolysis or acidification reactions can produce aqueous media containing the polysilicic acid, and if desired these may be purified from the reaction products before the gelling. If desired the hydrolysis may be performed in the presence of a water soluble organic solvent eg an alkanol of 1-3 carbons such as ethanol, or acetone in order to aid mixing and reaction, and may alter the physical properties eg pore size of the silica product; relative weight amounts of said water soluble solvent to the aqueous acid may be 0.1-10:1 e.g. 5-10:1.The hydrolysis may be performed in the presence of a substantially water insoluble oxygenated organic solvent, such as an alkanol of 4-8 carbons eg butanol, tert butanol, isoamyl alchol, hexanol, 2 ethyl hexanol and n octanol, or a dialkyl ketone of 5-8 carbons such as diethyl ketone or methyl isobutyl ketone, or a dialkyl ether e.g. with 2-6 carbons in each alkyl such as diethyl, dipropyl, diisopropyl or di n butyl ether.

The hydrolysis of the silicate ester may be catalysed by an acid and this acid, or the acid used in the acidification of the silica source, may be an aqueous acid usually an aqueous inorganic acid such as hydrochloric or sulphuric acids usually of 0.5-20% e.g. 2-10%w/w concentration in the reaction medium. The reaction may be performed at 50-120 C eg 60-90"C for 20-0.1 hr such as 8-0.8hr. The reaction may be performed with one liquid phase, or with two liquid phases when the solvent for the polysilicic acid is substantially non water miscible and in the latter case a phase transfer agent e.g. a water soluble organic solvent as above is preferably present; the reaction may be agitated eg mechanically and/or as a result of solvent reflux.At the end of the reaction a poly silicic acid is obtained in solution in the acidification reaction product. The reaction product is preferably cooled eg to less than 50"C before the gellation. The acid reaction or hydrolysis gives silicic acid or a polysilicic acid which polymerises in situ to a polysilicic acid of the desired molecular weight, shorter times and lower temperatures giving lower molecular weight polyacids. When no polymerization is necessary, the acidification can be at 20-70 C for 100-0.01 hr.

If desired, before the gellation step any organic solvents e.g. water soluble or immiscible ones can be removed from the polysilicic acid solution by extraction with a suitable organic water immiscible solvent which does not extract the polysilicic acid, such as a hydrocarbon or chlorohydrocarbon, e.g. chloroform, methylene dichloride or carbon tetrachloride. This removal of solvent can increase the pore size of the product silica.

The gellation of the polysilicic acid occurs slowly as a result of the formation in situ of the gellation agent in the reaction medium from a precursor. The reaction medium is preferably aqueous and especially aqueous acidic, but may be formed from a substantially water insoluble oxygenated organic solvent such as is described above, especially butanol; organic solvent media may be substantially anhydrous, as in the case of these concentrates in solvent described in our copending patent application (Ref 7955) being filed hereiwth or may also contain water e.g. for any hydrolysis of the precursor to form the gellation agent. The precursor can have a solubility in the reaction medium (especially a solubility in water at 60"C) of at least 10%, e.g. 10-80% by weight.The precursor may be mixed with the medium to form a medium containing substantially all of said precursor in solution. The precursor is decomposed or reacted at the gellation temperature e.g. by being acid hydrolysable or thermally labile to release a base which effects the gellation. The basic gellation agent is formed from a precursor which is usually neutral but hydrolyses to a base and a neutral, basic or weakly acidic compound. Preferably the precursors are condensates derived from a base, such as an organic amine e.g. an aliphatic or aromatic amine or ammonia, with a neutral species, such as an aldehyde or ketone, usually with 1-10 e.g. 2-10 carbons, e.g. an aliphatic one preferably formaldehyde, acetaldehyde or acetone.The aliphatic amine is usually a mono or di alkylamine with e.g. 1-6 carbon atoms in each alkyl group and the aromatic amine may contain 6-14 carbon atoms, and be aniline, optionally substituted e.g. by at least one alkyl or halo substituent.

Preferably the base produced by the hydrolysis and especially the other by product e.g. the aldehyde are water soluble, e.g. to at least 5%wt concentration. Examples of the above precursors are formaldehyde condensates with ammonia (hexamine) or ethylene diamine (imidazoline). The hydrolysis in the gellation step slowly releases the base.

Other suitable compounds are water soluble complexes of bases e.g. ammonia or an aliphatic or aromatic amine (e.g. of 1-10 carbons and 1-3 amine groups such as ethylene diamine) and a metal salt e.g. a transition metal, or zinc; thus ammine complexes may be used, especially with metals, which are needed in a catalyst to be made eventually from the silica. Examples of suitable metals are chromium, manganese, iron, cobalt, nickel, copper, zinc, ruthenium, rhodium, palladium, rhenium osmium and platinum. The approach using metal salt complexes is especially useful for making hydrogels containing cogelled metal and hence cogel catalyst.If desired, mixtures of the metal salt complex, providing base and metal for the gellation and the reaction product from ammonia or amine and the neutral species e.g. formaldehyde can be used, the reaction product providing the rest of the base for the gellation; these mixtures may contain 1.99 to 99:1 w/w of said complex and reaction product. The precursor can also be a nitrile of an alkanoic acid of 2-6 carbons, such as acetic or propionic nitrile, which can hydrolyse under the reaction conditions to form an ammonium salt of analkanoic acid, preferably one which is water soluble.

If desired other bases such as ammonia or ammonium or alkali metal hydroxide may be added to the polysilicic acid before, with or after addition of the precursor but only at a time before any gellation starts; the amount of the other base is insufficient to produce gelling so the use of other base and precursor enables one to do initial adjustments of pH without gelling and then allow the precursor to produce the gelling. Thus the other base alone or with precursor can be added at low temperature or the base added at high temperature, followed by the precursor at high temperature.

The gelling may be performed at 50-120"C eg 60-95"C for 30-0.1 hr such as 18-lhr. It is important that the precursor releases base in situ so that apart from any solvent reflux, once gellation has started, the reaction is in a quiescent state and is not substantially agitated eg not subjected to mechanical shearing forces. In the gellation there is a slow increase in pH away from low acid values without shearing. At the end of the gellation the pH of the aqueous medium is usually 6-7.2 so the amount of precursor alone or with other base (see above) is usually adapted so this figure is reached; weight amounts of 10-200 e.g. 50-100X (based on SiO2 in the polysilicic acid) may be used in the case of hexamine and prorata amounts for other precursors.The gellation produces silica hydrogel which is then usually washed with water to remove soluble ions and any water soluble solvent to leave a purified gel.

This gel is then converted into a nonaqueous gel by replacement of the water by a substantially water immiscible oxygenated organic solvent, such as one designated above or an alkyl ester of a saturated aliphatic carboxylic acid with e.g. 1-6 carbons in the alkyl group and 1-8 especially 2-6 carbons in the carboxylic acid e.g. methyl, ethyl propyl and butyl acetate, propionate or formate, or with a substantially water immiscible organic solvent capable of forming an azeotrope with water such as a liquid aromatic hydrocarbon e.g. 6-10 carbon atoms such as benzene, toluene or xylene. The water can be replaced by entrainment e.g. azeotroping with said solvent and removal of displaced water eg in a Dean and Stark apparatus. The suspension of silica obtained in solvent is then treated to separate the silica, which is then heated to remove the solvent.The azeotropic removal of water avoids the collapse of the pores found with simple drying. The solvent may be removed by heating at up to the solvent boiling point preferably under reduced pressure. From the time that gellation starts up to at least the time of removal of the solvent, the silica is preferably subjected to the minimum amount of handling to minimize mechanical shearing.

The gel obtained may be used as such but is preferably heated up to 400-700"C in at least one stage eg 1-4 stages each preferably 80-120 C apart, at which the temperature is held eg for 5-40 mins substantially constant; between each such holding sts is at least one temperature raising step during which the temper"lure is raised eg at 0.5-10 C/min such as 1-5 C/min at temperature above 200"C and 3-7"C/min at temperature below 200 C. The total heating regime may take 2-lOhr eg 4-7hr.

The silica obtained may be used as a support for a catalyst such as a transition metal catalyst eg one of Group IVA-VIA of the Periodic Table especially chromium, titanium, zirconium or vanadium, for polymerising alpha olefines alone or with a cocatalyst of a Group 1A, 2A or 3A organometallic compound such as an organo aluminium compound. Other catalysts on the support may be Group VIII metals such as iron or cobalt for use in the Fischer Tropsch process.

In another aspect the present invention also provides the polysilicic acids of narrow molecular weight distribution and to a special preparation and purification process therefor.

Silica gel can be made by reaction of a silica source and mineral acid which form silicic acid which polymerises under the acidity and concentration conditions to produce polysilicic acids of ever increasing molecular weight and eventually to form silica gel.

But there is no control over the molecular weight distribution of the polysilicic acid and then no control over the porosity of the silica supports made from the silica gel produced. In J. Mater.

Sci. 1991, 1(6) pp 943-946 is described the production of silicic acid from olivine and its controlled polymerisation with trimethylsilylation to produce three dimensional trimethylated polyorganosiloxanes; in this process acid, ethanol and hexamethyldisiloxane were added to olivine in aqueous slurry and the mixture stirred under reflux at 72"C for 6 hr, followed by extraction of the polyorganosiloxane with chloroform.

The present invention also provides a process for producing a polysilicic acid of controlled molecular weight, which comprises reacting an acid and a silica source to form a silicic acid and/or polysilicic acid, and, if required allowing it to polymerise to a polysilicic acid of the desired molecular weight, extracting said polysilicic acid into a polar organic phase to leave an aqueous phase and separating the organic phase from said aqueous phase.

The invention also provides a concentrate of 40-90% w/w of a polysilicic acid in a polar organic solvent, which is preferably an alkanol, and usually substantially anhydrous.

In the method of the invention the silica source may be silica itself eg sand or quartz, a metal silicate eg an alkali metal silicate such as sodium silicate, a silicate mineral eg an orthosilicate such as olivine through to an aluminosilicate such as mica, or a clay mineral, or a reaction product of silica with a base eg a metal carbonate, such as portland cement from sand and chalk; said reaction product is preferred. The silica source is preferably one with a high percentage of the silica components in similar structure silica units eg single silicate anions as with portland cement or cyclic silicate (as in dioptase) or in a cubic cyclic silicate form as described hereafter, rather than with many different structure silica units as with sodium silicate.The silica source usually contains at least 10% of Si02 by weight such as 25-100% or 30-50% especially 35%, or 10-40% such as 10-25% with the remainder preferably forming water soluble salts with the mineral acid.

The acid is usually an inorganic acid, eg a mineral acid such as sulphuric or nitric acids but preferably a hydrohalic acid especially hydrochloric acid. The relative molar amounts of silicon from the silica source and protons from the acid are 0.1-10:1 eg 0.2-3:1 such as 0.3-1:1.

The reaction is performed in the presence of water usually from the aqueous acid but some may instead or in addition be added separately eg with the silica source. The molar amount of water to silica source in the reaction is usually 10-100:1 such as 20-50:1.

The reaction may be performed in many ways. The silica source may be suspended in a liquid medium and the suspension added to the aqueous acid usually with agitation eg stirring and preferably slowly eg over 5-30 minutes at 20-0 C; the silica source may be added as solid to the aqueous acid or the aqueous acid may be added to the silica source. The process may be performed batch wise or semi continuously eg in at least one or a series of continuous stirred tank reactors, or with continuous mixing of streams of silica source and acid and reaction.

The reaction may be performed at -20 to +llO"C eg -10 to +50"C and especially at -10 to +20"C for periods of time of 1000-0.01 mins eg 500-10 mins and especially 300-30 minutes. In this time the silica source reacts to form the silicic acid and the latter starts to polymerise to form the polysilicic acid of the desired molecular weight in solution in the reaction mixture. The degree of polymerisation may be determined by taking a sample, reacting the silicic acids with a trimethylsilylating reagent such as hexamethyl disiloxane to form trimethylsilyl derivatives, extracting the latter into a suitable solvent eg chloroform and then analysing those derivatives by gel permeation chromatography in the manner described in the above J Mater Sci article.After 90 min at 0-10"C, the polysilicic acid of molecular weight 792 can be predominantly made in the presence of water soluble solvent (as described further below); decreasing the time and/or temperature decreases the degree of polymerisation, while increasing either or both and especially reducing the amount of water soluble solvent can increase the degree of polymerisation. The above process describes the polymerization of small silicic or polysilicic acid units, but if desired the silicate source may already contain the desired polysilicic acid units (in salt form) so in this case the acidification reaction is performed for shorter periods to minimize further polymerization, the lower the valency of the metal in the salt the shorter the acidification time.The polysilicic acid is then extracted from the reaction phase containing water into a polar organic phase. This extraction may be achieved by contact of the reaction phase with a polar organic solvent, which may be substantially water immiscible and usually contains at least 1 eg 1-3 or 1 oxygen atom such as an alkanol of 4-10 carbon atoms, eg n-butanol, tert butanol, amyl alcohol, isoamyl alcohol, n hexanol, n-octanol, 2 ethylhexanol, or n-decanol, or a dialkylketone of 5-10 carbon atoms such as diethyl ketone or methyl isobutyl ketone or carboxyl ester such as an alkyl ester of an alkanoic acid with 1-6 e.g. 1-3 carbons in the alkyl group and alkanoic acid, e.g. butyl and amyl acetates, or a dialkyl ether with 1-5 carbons in each alkyl group such as diethyl, dipropyl diisopropyl and di-n-butylethers, or with a more water miscible solvent which is substantially immiscible with the reaction phase, such as n-or isopropanol or a cyclic ether eg of 4-8 carbon atoms such as tetra hydrofuran or dioxan or mono or di alkyl ether of an alkylene diol, with e.g. 1-4 carbons in the alkyl group and 2-4 carbons in the alkylene group such as ethylene glycol mono or dimethyl ether. Particularly in the case of water miscible solvents, the extraction may be aided by the presence in the aqueous phase; e.g. by addition thereto, of an inert water soluble ionic salt eg of a Group 1A or 2A metal such as sodium, potassium, calcium or magnesium such as a halide eg chloride, sulphate or nitrate; sodium chloride, sodium sulphate and calcium chloride are preferred.The weight ratios of the liquid components in the reaction phase and polar organic solvent are usually 0.5-6:1 eg 1.0-3.5:1, while the weight ratio of the ionic salt to water is usually at least 1:10 such as 1:10 to 2:1 especially 1:5 to 1:1.

The contact between the phases may be batch wise or continuous and in at least 1 step eg 1-3 steps and if continuous may be co-current or countercurrent. If desired before separation of the polar organic phase and the aqueous phase, and with or without the presence of added inert ionic salt, the temperature of the aqueous phase may be reduced in order to reduce the polymerization rate and to increase phase separation.

After the contact of the aqueous reaction phase and the organic solvent, there is produced an organic phase containing the polysilicic acid and an aqueous phase containing any residual acid and unreacted silica source and ionic by-products from the acidification eg sodium chloride from reaction of sodium silicate and hydrochloric acid; at least some, if any, ionic salt added also preferably remains in the aqueous phase. The organic phase may if desired be scrubbed with water or an aqueous solution of the ionic salt in order to reduce the content of entrained salts. The organic phase is then dried eg over anhydrous magnesium sulphate or sodium sulphate, or molecular sieves and then at least some of the solvent is usually evaporated to leave a substantially water free polysilicic acid eg in 40-90% concentration in the solvent. The recovered solvent may be recycled to the extraction or acidification stage.

The acidification reaction of the silica source and the acid is preferably performed in the presence of a neutral water miscible solvent, preferably containing at least one O,N and/or S atom, such as one with at least 1 such as 1-4 hydroxyl groups, and especially 1-5 carbon atoms. The solvent may be a lower alkanol eg of 1-3 carbon atoms such as methanol, ethanol, n propanol or isopropanol, a diol such as ethylene glycol or propylene glycol, a triol such as glycerol or a tetraol such as pentaerythritol. The solvent may also be a cyclic ether eg with 1 or 2 oxygen atoms in ether form and 3-5 carbons such as tetrahydrofuran or dioxan or may be an alkanoic acid amide of 1-10 carbon atoms such as dimethyl formamide or dimethyl acetamide, or a alkanoic acid nitrile of 2-6 carbons such as acetonitrile or a sulphoxide or a sulphone, such as dimethyl sulphoxide.The weight amount of the neutral solvent to the rest of the liquid component of reaction phase eg in the water in the acid and optionally added with the silica source is usually 0.2-3:1 eg 0.3-1.2:1. Conveniently the neutral solvent is added with the silica source eg as a slurry such as one of 2-25% w/v concentration. The neutral solvent in the reaction may complex with the silicic and polysilicic acids and reduce the rate and extent of their polymerisation. Preferably the weight ratio of the neutral solvent to the silica source (expressed as silica) is at least 1:1 eg 1-50:1 such as 5-25:1, and ideally is kept at these weight ratios during the reaction.

When the polysilicic acid is extracted into an organic phase from an aqueous phase containing the neutral solvent, the ionic salt may be added to cause or help separation into 2 phases, eg when the solvent is propanol or isopropanol that is incompletely miscible with a concentrated aqueous solution of sodium chloride. In addition to the ionic salt or instead thereof, the polar organic solvent may be added; this extracts the polysilicic acid and may or preferably may not extract at least some of the neutral solvent as well. For example when the neutral solvent is ethanol, and the polar solvent is n-butanol, only small amounts of the ethanol may be extracted.The lower the amount of the neutral solvent extracted the easier any polar solvent purification from the evaporation prior to recycle, but the higher the amount extracted the less contaminated will be the aqueous phase aiding the latters' disposal if it is not to be recycled to the acidification step. Any neutral solvent extracted can be recovered and recycled.

The process of the invention can produce a polysilicic acid of narrow molecular weight distribution, (Mw/Mn) eg less than 10, as measured by reaction with hexamethyldisiloxane and then gel permeation chromatography. The actual distribution changes according to the weight average molecular weight, which can vary between 200 and 10,000 preferably 350-3000 especially 700-2000. The polysilicic acid contains Si-O groups and Si-OH groups usually in unprotected form (i.e. not as their trimethyl silyl derivatives).

The structure of the polysilicic acid may be that of linear and/or cyclic, including fused cyclic, chains of Si-O- groups such as ones in cube or fused cube arrangements with at least one eg 1-10 such as 2-6 fused cubes in a linear or non linear spatial distribution.

Each cube has a silicon atom at each corner or bridge and an oxygen atom between each silicon atom, and one hydroxyl group on each silicon corner atom. The cubes may be joined together by at least one Si-O-Si bond but are preferably fused together with a common plane of a ring of 4-Si-0- groups. Generally the polysilicic acids may have the formula (SiO)4a(SiO)4b(OH)c, where a is 1, b is 0-6 eg 1-4 and c is an integer so that spare valencies on silicon atoms can be satisfied and is usually such that c/2 is an integer of 4-8 especially 4 or 5. A preferred polysilicic acid is one of molecular weight 792, with 2 fused cubes of SiO groups and 8 corner OH groups and is of formula S12020(0H)8.

The polysilicic acids are stable e.g. for up to 6 months, in the absence of acids or bases and water, and are usually stabilised in the polar organic solvent concentrates by the presence of the solvent which may solvate them.

The polysilicic acids of the invention or made by the process of the invention may be used to produce silica gel, preferred having a narrow range of pore sizes. If desired polysilicic acids of the invention having different molecular weight distributions may be mixed in order to provide polysilicic acid feedstocks to give silica gels of a wide range of choices of pore size distributions, whether narrow, broad, and Uni-, Bi-, N multi-modal. The polyacids are gelled with aqueous base to form the silica hydrogel in a mother liquor comprising also the water immiscible solvent; if desired a water miscible solvent eg methanol, ethanol or acetone may be added to aid the gelling.The hydrogel can be separated eg by decantation from any separate liquid, washed to remove inorganic ions and then the pore sizes are retained by azeotropic distillation with a substantially water immiscible solvent eg a non hydroxylic one such as ethyl acetate and removal of displaced water eg in a Dean and Stark apparatus. The suspension of dehydrated gel in solvent is then treated to separate the gel, which is then heated to remove the solvent. The azeotropic removal of water avoids the collapse of the pores found with simple drying.The solvent may be removed by heating at up to the solvent boiling point preferably under reduced pressure, followed by heating up to 400-700 C in at least one stage eg 1-4 stages each preferably 80-120"C apart at which the temperature is held eg for 5-40 mins substantially constant; between each such holding stage is at least one raising step during which the temperature is raised eg at 0.5-10 C/min such as 1-5 C/min at temperature above 200"C and 3-7"C/min at temperature below 200"C.

The total heating regime may take 2-1Ohr eg 4-7hr.

The silica produced may be used as a support for a catalyst such as a transition metal catalyst eg one of Group IVA-VIII of the Periodic Table especially chromium, titanium, zirconium and/or vanadium, for polymerising alpha oleo in alone or with a cocatalyst of a Group 1A, 2A or 3A organometallic compound such as an organo aluminium compound. Other catalysts on the support may be Group VIII metals such as iron and cobalt, for use in the Fischer Tropsch processes.

The invention is illustrated in the following Examples.

Example 1 Tetra ethyl silicate (34.67g) equivalent to 10.00g SiO2 was mixed with ethanol (123.1g) and 11.3M hydrochloric acid (17.7g) and the mixture stirred and heated for 3 hr at 72"C under reflux of ethanol to give a polysilicic acid solution. The solution was cooled to below 30"C and then to it was added a solution of hexamine (7.2g) in water (15ml) and the mixture obtained heated for 4hr at 80"C under reflux without stirring to leave a hydrogel in an aqueous medium at pH 6.5. The hydrogel was carefully washed with water (3 x 1 litre) without mechanical agitation of the gel to reduce the ethanol and soluble ion concentrations and produce a gel product which was mixed with ethyl acetate (300ml) and submitted to The invention is illustrated in the following Examples.

48 hr azeotropic distillation in a Dean and Stark apparatus with periodic removal of water. The substantially anhydrous suspension of gel in the ethyl acetate obtained was filtered to leave a solid silica, which was heated at 20 up to 5500C over 6hr under the following temperature regime; increase at 5 C/min to 230"C, hold at 230"C for 20 min and increase at 2"C/min to 5500C with steps holding at 320"C, 420"C and 500"C, each for 20 min.

The physical properties of the heat treated silica were found by BET measurements with nitrogen to be as follows; micropore volume 0.05 ml/g, mesopore volume 2.68 ml/g, mean average pore diameter 240A, average pore diameter 174A, pore diameter at median area 181A, median pore diameter 233A, surface area 615 m2/g, pore volume 2.68 m3/g, Interquartile range 122A.

Example 2 A slurry of Portland Cement (from heating sand and chalk) (29g containing 10.2g Si02) was made in isopropanol (100 ml). The slurry was added dropwise to stirred 3M aqueous hydrochloric acid (100 ml) kept at 0-10"C. After 90 min, n-butanol (100 ml) and solid sodium chloride (60g) were added with continued stirring for a further 60 min to give a product which was allowed to stand for 30 min. There was produced an organic phase containing polysilicic acid and an aqueous phase. The two phases were separated and the organic one dried over anhydrous sodium sulphate. The drying agent was filtered off and then about 90% of the solvent distilled off under vacuum to leave a polysilicic acid (in about 80% yield as silica) in concentrated solution in the solvent. The distilled solvent can be recycled for use in the extraction.The polysilicic acid was analysed by treatment with hexamethyldisiloxane to form trimethylsilyl derivatives followed by gel permeation chromatography. It was found to be predominantly of formula Si12O18(OH)8 (Molec Wt 792) with a structure having 2 fused cubes of SiO groups.

Examples 3 and 4 The process of Example 2 is repeated (Ex 2) with ethanol instead of isopropanol in the initial slurry in the cement and (Ex 4) with tetrahydrofuran instead of butanol in the extraction step.

Similar results may be obtained.

Example 5 Sodium silicate solution (containing 10.2g SiO2) was acidified at 0-10"C with concentrated hydrochloric acid and then isopropanol (100ml) was added to form a slurry. The rest of the process of Example 2 was then repeated with this slurry rather than that made from Portland Cement and isopropanol. Similar results to those in Example 2 were obtained.

Claims (11)

We Claim

1. A mesoporous silica gel having a mesopore density of 2-6 ml/g, a surface area of 400-1000 m2/g and a pore volume of 2-4 m3/g.

2. A silica gel according to claim 1 with a micropore density of 0.01-0.4 ml/g, a mesopore density of 2-4 ml/g, a mean average pore diameter of 150-550A units with average pore diameter 100-350A, a pore diameter at median area of 100-350A and median pore diameter of 150-450A and interquartile range of pore diameter of 80-160 A such as 100-140 A.

3. A silica gel according to claim 1 or 2 wherein the pore surface area is 400-800 m2/g and the pore volume is 2-3.5 m3/g.

4. A process for making a silica gel which comprises (i) reacting a water soluble polysilicic acid in a liquid medium comprising water with a gellation agent formed in solution in situ in the medium to produce a hydrogel comprising water, (ii) heating said hydrogel with a substantially water immiscible organic solvent and entraining said water to produce a substantially water free suspension of silica gel in said solvent, and (iii) separating said silica gel and said solvent.

5. A process according to claim 4 which comprises after step (iii) heating said silica gel to 400-700"C to leave a mesoporous silica gel.

6. Polysilicic acid of molecular weight distribution (Mw/Mn) of less than 10.

7. Polysilicic acid according to claim 6 wherein the weight average molecular weight is 200-10,000.

8. Polysilicic acid according to claim 6 or 7 wherein the structure has chains of Si-O- groups in cube or fused cube arrangements.

9. A process for producing a polysilicic acid of controlled molecular weight, which comprises reacting an acid and a silica source to form a silicic acid and/or polysilicic acid, and, if required allowing it to polymerise to a polysilicic acid of the desired molecular weight, extracting said polysilicic acid into a polar organic phase to leave an aqueous phase and separating the organic phase from said aqueous phase.

10. A concentrate of 40-90% w/w of a polysilicic acid in a polar organic solvent, which is substantially anhydrous.

11. A concentrate according to claim 10 wherein the polysilicic acid is one according to any one of claims 6-8 or produced by the process of claim 9.