CA1094778A - Synthetic imogolite - Google Patents

Abstract

ABSTRACT

An inorganic material related to imogolite, a rare constituent of soil, which forms gels with under 1% solids concentration, is synthesised by digesting (at pH 3.1 to 5.0) soluble hydroxyaluminium complexes freshly formed at pH 3.2 to 5.5 in aqueous solution with restricted aluminium concentration.

Description

10~477B

This invention relates to an inorganic material, which is a fibrous product having tubular structure related 'o, or resembling, the natural product imogolite.

Imogolite is a regular polymeric hydrated aluminium silicate which is found principally in weathered volcanic deposits, often in the form of thin gelatinous films, and consists of long tubes (for example 1 to 10 micrometres) with an outside diameter of around 22A

and inside diameter about lOAo The tubes are partially aligned in bundles giving a highly porous material (pore space around 60%) with pores of effectively about 9A diameter, affording a surface area of about 1000 m g s containing in the natural state from 10 to 45g water per 1009 dry mineral at relative humidity from 0.03 to 1. The water can be pumped off under vacuum or by heating in air to 150 C, the dried material remaining stable up to about 300 C.

The tubes can be dispersed by subjecting the material to ultrasonic treatment in an acidic solution of a pH of 3 to 5, and can be re-coagulated on adjusting the pH to 7~5 or more, this being the reverse of usual behaviour in clays.

On the basis of electron diffraction pattern, composition and the proven presence of orthosilicat.e anions, it has been proposed 4~k ~0~ 78 that the walls of the tubes have a structure like that of a single sheet of gibbsite (Al(OH)3) with orthosilicate groups replacing the inner hydroxyl surface of the gibbsite tube. This gives an empirical formula (HO)3Al203SiOH, which is also the sequence of atoms encountered on passing from the outer to the inner surface of the tubular model. Natural samples have compositions in the range Al23~Si2)1 o - 1.2 ( 2 2~3 - 3Ø
Therefore this invention consists of a method of synthesising an inorganic material which is a fibrous product having tubular structure related to~ or resembling9 the natural product imogolite, (for example being 10 - 15% larger in tube diameter), the method comprising digesting a solution of pH from 3.1 to 5.0, containing soluble hydroxyaluminium silicate (preferably orthosilicate) complexes formed at pH 3.2to 505 in aqueous solution, (the digestion being preferably in the presence of sufficient dissolved silica to inhibit boehmite formation), the concentration of Al (other than in already-formed imogolite or stable complexes) being not more than 50 ~preferably not more than 25, more preferably not more than 15) millimoles per litre, and the product displaying discernible electron diffraction peaks at 1 4A, 2.~A and 4~2A.

109 ~7~78 Thus, in accordance with the present teachings, a method is provided of synthesising an organic material which is a fibrous product and displays discernible electron diffraction peaks at o o o 1.4A, 2.lA and 4.2A and which resembles imogolite. The method comprises forming at pH 3.2 to 5.5 soluble hydroxyaluminium silicate complexes in aqueous solution and digesting the solution at pH from 3.1 to 5.0, the concentration of available aluminium being up to 50 millimolar.
By a further embodiment of the present teachings, an organic material is provided which is a fibrous product and has a tubular structure resembling the natural product imogolite but has 10 to 15% larger tube diameter and displays discernible electron diffrac-o o otion peaks at 1.4A, 2.lA and 4.2A with the walls of the tube having a structure like that of a single sheet of gibbsite with ortho-silicate groups replacing the hydroxyl groups on the inner surfacesof the tubes.

-2a-10~ 78 Further aqueous hydroxyaluminium silicate solution may be added in steps during the digestion to keep the concentration continuously high but within the above limits. Hydroxyaluminium silicate solutions are best freshly formed and kept cool.
Preferably the adjusted-pH solution is held at 40 C to 170 C, preferably 90C to 130C, conveniently 95C to 100C, at least until the yield of product reaches a maximum (typically taking 1 to 60 days). Exemplary durations are 20 days at 60C and 1-3 days at 100C. The temperature range is not mandatory, but, at excessive temperatures in the wet, imogolite decomposes to bohemite or kaolinite and much silica remains in solution, while at lower temperatures the reaction times become prolonged. The preferred pH of formation of the soluble hydroxyaluminium silicate complexes is around 4.5, approached gradually from lower pH. At lower pH, especially less than 4.0, formation of complexes is likely to be incomplete or slow unless the solution has experienced an excursion to higher pH values, whilst at higher pH, especially over 5.0, precipitates are likely to form, all the more so in more concentrated solutions.
Any precipitate should be redissolved or redispersed quickly to a pH more than 3.5. Redispersal may be in perchloric, hydrochloric, nitric, formic or benzoic acids, preferably in a mixture of acetic with nitric or perchloric acids. Formation of synthetic imogolite at pH 4.7 to 5.0 is erratic and depends on the exact history of the solutions.
Thus, in that pH range, imogolite forms with difficulty if the solutions were too alkaline and had been re-acidified or if the solutions were allowed to age. Therefore the preferred pH for the digestion is not more than 4.6, and this digestion pH may be reached by acidification with non-complexing acids.
Further, since the solution tends to become more acidic during digestion, the preferred starting pH is not less than 3.5.
~ eating excessively concentrated hydroxyaluminium sllicate solutions produces weakly fibrous or non-fibrous products which do not form gels, and the situation cannot be retrieved by subsequent dilution. The preferred atomic ratio of Si to Al in the method is from 0.42 to 0.58, and should not exceed 0.8.
Chloride ion, if present, preferably does not exceed 25 millimolar. Acetic acid or similar weak organic acid is advantageously present preferably in a molar concentration preferably about half and preferably not more than quadruple that of the aluminium, provided sufficient mineral acid, e.g.
perchloric acid, is present to inhibit organic anion formation.
Where acetic acid or other weak organic acid exceeds double the aluminium, the reaction is slower. The organic acid should not exceed 25mM or twice the aluminium concentration (whichever is the greater) as higher concentrations can decrease the rate of reaction.

109~778 - The product may be isolated from its colloidal solution, preferably by drying, for example spray-drying, or by freeze-drying after precipitating a gel with alkali or added salt e. g.
chloride or phosphate and centrifuging (which may be repeated after washing and mechanical agitation of the gel), or alternatively by foam-flotation using an anionic detergent. In this isolated form, it may find application as a molecular sieve, catalyst support, coagulant (i.e. gel-former) or sorbent.
Synthetic imogolite may incorporate other ions replacing Al or Si by isomorphous substitution (e.g. Cr(III) or Fe(III), or Ge or Ti respectively) and may be activated for catalysis by heating or exposing to hydrogen.

Coherent films can be formed by evaporating imogolite colloidal so~ution~ on to a flat surface; such films may find application as membranes.

Instead of this isolation, the solution held for a while at 60 C

to 140 C may be made alkaline, for example with ammonia. A gel results, which may find application in its own right.

The product need not be isolated from its colloidal solution.

Instead, for example, it may be used as a flocculant, a hydrophilizer ~0~477B

or a thic~ener The invention extends to the product of this method, optionally isolated as set forth, to synthetic imogolite howsoever made, and to its dispersions.

The invention also extends to a gel comprising synthetic imogolite, including a gel with a solids concentration of under 1% by weight e.g. under 0.5% e.g. 0.1%.

The invention will now be described by way of example.

An aqueous solution was prepared containing 1.4 millimolar silica SiO2 in the form of Si(OH)4 monomer (silicic acid) and 2.4 millimolar aluminium trichloride A~C13. As chloride ions in excess of about Z5 millimolar appear to inhibit formation of imogolite, aluminium trichloride solutions should be not more concentrated than 8mM. On partial neutralisation, the aluminium trichloride gives rise in solution to hydroxyaluminium cations, which combine with the silicic acid to form a soluble hydroxyaluminium ortho~ilicate complex. A

slight excess of silica over the theoretical requirement for imogolite wa~ used to inhibit formation of boehmite ( ~AlOOH). The solution was adjusted to pH 5 with 1 molar sodium hydroxide. Then 1000 ml of 109~7'78 the solution wa~ acidified by adding 1 millimole of hydrochloric acid and 1 millimole of acetic acid: the resultant pH was 4.35.

The acidified solution was heated up to 95C in an inert vessel and maintained at that temperature, either under reflux conditions or in a qealed pre~sure vessel, each giving the same results.

After 5 days the bulk of gel precipitated by ammonia reached a maximum and quantities of a synthetic mineral related to, or resembling, the natural product imogolite hsd formed. The fibrous morphology, electron diffraction pattern ( with sharp peaks at 1.4A ,

2.1A and 4.2A) and infrared spectrum all supported this conclusion.

The vessel containing the synthetic mineral had its contents rendered slightly alkaline by addition of ammonia; then the contents were centrifuged. A bulky gel resulted, believed to incorporate an open network of oross-lined synthetic-imogolite tubes, and having a solids content by weight of 0.1%.

EXAMPLES 2 to 9 ~xample 1 wa~ repeated, except that the amount of the hydrochloric scid was varied.

109`~778 Example No. mmol HCl Initial pH Time for pH Volume of added per maximum There- gel obtained 1000 ml gel bulk after (arbitrary units) 2 0 4.55 5 days4.0 14

3 0.5 4.~5 5 days3.6 24

4 o.68 4.1 3 days3.5 20 1.02 4.0 3 days3.4 20 6 1.35 3.9 3 day93-3 20 7 2.04 3.8 3 days3.2 19 8 2.72 3.7 3 days3.2 18 9 3.40 3.6 3 days3.2 15 EXAMPLES 10 to 15 Exa~ple 1 was repeated, except that the amount of acetic acid was varied substantially, under two different amounts of hydrochloric acid. Acetic acid can be seen to be inessential although advanta~eous. Also, the solution containing Si(OH)4 and AlCl3 was adjusted to pH 5 before adding the hydrochloric and acetic acids, and wa~ then heated as in Example 1, to 95 C.

~ qr~t ~r ~
Example Si(OH)4 AlCl3 HCl HAc Initial Days for Final Volume No. pHmax. gel pH of gel .
1.~3 2.~ 1 0 3.9 3 3-3 16 11 1.33 2.4 1 2.2 4.1 5 3.5 18 12 1-33 2.4 1 22.0 3.6 3 3.2 20 13 1.33 2.4 2.5 0 3-6 5 3.2 18 14 1.33 2.4 2.52.2 3.8 3 3.2 18 1-33 2.4 2.522.0 3.5 3 3.2 20 (~ arbitrary unit~).

109~778 EXAMPLES 16 to 19 From these Examples, it will be seen that NaCl (preferably chloride ion generally) inhibits imogolite formation and so limits the concentration of AlCl3 which can be used. Procedure was as in Example 10, except that NaCl wa~ added before the adjustment to pH 5. Note thnt 7 millimoles NaCl were unavoidably present derived from the AlCl3 in each of Examples 16 - 19 before any was added, so that the total amount~ present were 7, 17, 37 and 107 millimoles respectively.

~ ~s ~9~ p~ li~
., .
Example Si(OH)4 AlCl NaCl HCI HAc Initial Days for Final Volume No 3 pH max. gel ~H of gel . ..
16 1.33 2.4 0 1 2.2 4.5 2 3.3 15 17 1-33 2.4 10 1 2.2 4.5 5 3.3 14 18 1.33 2.4 30 1 2.2 4.5 5 3-2 7 19 1.33 2.4 100 1 Z.2 4.4 5 3-2 4 ~ ~ . . . . . ..

EXAMPLES 20 to 25 These Examples show that synthetic imogolite forms in the presence of 0.1M NaCl04, and that Al(C104)3 of at least 9.6mM can be used in the synthesi3. Procedure was as in Examples 16 to 19, but replacing AlCl3 by Al(Cl04)3 and NaCl by NaClO4. Solutions were diluted where necessary to equalise Al and Si concentrations 109~778 before measuring yield of gel.
~a~s ~9~
NaC104 Example Si(OH)4 Al(C104)3 added totalHC104 Initial Days for Final Volume No. pH max gel pH of gel 1.33 2.4 7.2 1 4.4 1 3.2 12 21 2.66 4.8 0 14.4 2 4.3 5 3.2 20 22 4- 7.2 21.6 3 4.3 5 3.3 20 23 5.33 9.6 0 28.8 4 4.4 5 3.7 14 24 2.66 4.830 44 2 4.3 7 3'4 17 2.66 4.8100 114 2 4.3 7 3.8 15 EXAMPLES 26 to 31 In these Examples, Example 1 was repeated except that the aluminium salt was nc~ aluminium iso-propoxide. Also, instead of hydrochloric and acetic acids, a variety of acids was used on~at a time. The aluminium iso-propoxide was added as a O.lM solution in iso-propanol to an aQ~eous Si(OH)4 solution, then the acid was added and the solution haated to 95 C.
~illi~S p~ l~Y
. _ .
Example Si(OH) Al(iso-Pr) Acid Initial Days for Final Volume No, 4 3 (all 2 mmol) pHmax gel pH f gel 26 2.66 4.8 none 5.5 - 5~4 27 2.66 4.8 HCl 4.4 2 3.4 14 28 2.66 4.8 HC104 4~5 2 3.5 14 29 2.66 4.8 Acetic 4.3 2 4.1 14 2.66 4.8 Trichlor- 4.5 1 4.9 7 acetic 31 2.66 408 Oxalic 5.5 - 4.9 0 10~4778 Acids which failed (as did oxalic) include phthalic, citric and lactic, which all form strong aluminium complexes. Successful acids (such as HCl) also include formic and benzoic, which do not form significant aluminium complexes. Salicylic acid, which forms a weak aluminium complex, was slightly successful. Presumably any complexant must be displaceable by silica if imogolite is to form.

EXAMPLES 32 to 34 These Examples show the use of alumina-silica precipitate to avoid excess salt concentrations which inhibit imogolite formation.
:.
10 In each, 500ml of a sodium silicate solution, made from a fusion of 0.5g quartz powder in 2.5g Na2C03, was slowly added to 100ml of a solution containing 15 mmol Al(C104)3 and 10 mmol HCl04, giving a solution of pH 4.4 containing a soluble hydroxyaluminium silicate complex. The solution was then adjusted to pH 5.5, and 15 the resultant precipitate was separated from the supernatant by centrifuging. The supernatant was discarded and the precipitate immediately redispersed in 300 ml of 20 n~ HCl04. This dispersion and two dilutions from it were then heated at 95 C, when the precipitate rapidly redissolved. Synthetic imogolite formed in 20 the diluted solutions as follows.
Solution composition, mmoles/~-t~
Ex~leSiOz l(A120 ) HCl04 Initial Heating Final Volume No. 3 pH time Of gel (days) 32 107 4 2 4.6 5 3.2 19 33 5.1 12 6 4.3 5 3.1 25 34 17 40 20 4.1 5 3.5 11 All volumes increased between 2 and 5 days. Gel volumes were measured on solutions diluted to a standard 2.0 mmol Al/lOOOml.

` ` 10~778 In further experiments (not described in detail) based on Examples 32 - 34, aluminà and silica were more completely precipitated from solutions adjusted to pH 6.5 or pH 8, rather than pH 5.5, and the precipitates obtained were as satisfactory for imogolite synthesis j provided they were immediately redissolved in acid as described for Examples 32 to 34 above.

Other methods of reducing salt concentrations (seen from Examples 24 and 24 to inhibit imogolite formation) should also be satisfactory in promoting formation of imogolite: e.g. partial neutralisation of the aluminium-salt and silica solution with an anion-exchange re~in in the OH form, rather than with NaOH, Na2C03, or the like; or ensuring that the salt formed is sparingly soluble (e.g. potassium perchlorate), so that it can be removed by filtration, or using an aluminium alkoXite, e.g. with tetraalkyl silicate, instead of an inorganic aluminium salt. In further trials, Example 33 was repeated but, in place of the perchloric acid for redispersing the precipitate, there were succeisfully used hydrochloric, nitric, formic and benzoic acids in turn. No gel was obtained when sulphuric acid was usedl and a reduced yield was obtained with acetic acid alone.

The best yield were obtained with a mixture of perchloric acid and ~094778 acetic acid, and these conditions were then standardised in later experiments.

EXAMPLES 35 to 39 Each of these Examples 35 to 39 has three versions, a 96 C, a 110C and a 120 C version.

A preparation of a reactive alumina-silica sol is as follows:-

5 litres of solution containing 1000 ppm SiO2 (prepared as described below) were added over 30 minutes with vigorous stirring, to 150 m moles (56.27 9) AltN03)3.9H20 dissolved in 126 ml of lM HC104 and diluted to 1 litre. This gave 6 litres of a slightly opalescent sol which cleared in about 20 minutes. After standing 1 hour, lM NaOH was added dropwise to pH 4.5, when the 501 wa~ again allowed to clear by standing for 1 hour. Drop-wise addition of lM NaOH was then continued to pH 6.8. The resulting precipitate was spun down in 6 x 1 litre polypropylene bottles at 2000 rpm for 30 minutes. The clear supernatants were discarded, and the precipitates combined by dispersing them in a solution containing 30 ml lM HClO~ and 43 ml of 1.74M acetic acid, finally adjusting the total volume to one litre.

This dispersion rapidly cleared.

The concentrated stock solution, containing 150 n~ Al, about 80 ~0~778 mM Si, 30 mM HCL04 and 75 mM CH3COOH, was stored in a cold room and used in subsequent experiments. Its pH was around 3.9 - 4.2.

The lOQO ppm SiO2 solution was prepared by fusing 59 quartz with 259 anhydrous Na2C03 and then dissolving the melt in 5~ distilled water. An alternative procedure for preparing the lQOO ppm SiO2 solution would have been to dissolve 22g Na2SiO3.9H20 (containing 59 SiO2 by analysis) in 5 litres distilled water containing 160 ml lM

Na2C03. The presence of the carbonate in this procedure helps in obtaining a clear reactive final dispersion.

In making the above stock solution, the silicate solution is added to the acidified A~(N03)3 solution, so that the reaction forming the alumina-silica complex proceeds always in acid solution.

Reversing the order of ~ddition~ i.e. adding the Al(N03)3 solution (without added acid~ to the sodium silicate, gives a precipitate formed at pH over 7. This precipitate can also be dispersed in perchloric And acetic acid, as described above to give opalescent concentrated ~tock solution, which also gives synthetic imoaolite wh~n d11uted to lQ mM A~ and heated at 96 C, but at a lower yield.

Th~ conc~ntr~ted stock solution was diluted to give the 20 c~ncentrations below, and then heated in inert plasti~s bombs in an 10~778 oven at 96C, or in an autoclave for 110 C and 120 C. The results indicated that there was little or no advantage in working at temperature above 100 C, and that the yield3 of gel decreased at concentrations higher than 10 mM Al. An opalescence, probably due to boehmite, appeared at 120 C in the more dilute solution~.

Volume Or gel, measurFd on solutions diluted to 1 mmol Al/1000 ml.

.
m mol Al day 1day 2 day 3day 1day 2 day 3day 1 day 2 day 3 Example per 1000 No. ml 39 30 ,7 8 8 4 6 7 4 5 6 Gel volumas decrea~ed rapidly at temperatures below 90 C. At 60 C, a gel volume of only 6 unit~ (mea~ured at 2.5 m mol Al/1000 ~) was obtained after incubating solutions containing 10 mM and 5 mM per 1000 ml for as long a~ 1 month. This volume did not increa~e a M er long incubation.
A ~ood yiold Or gel w~ however obtained at 60 C by incubating a ~olutlQn pr~ar~d ~ follow~s A ~olution containing 2.5 mM Al(Cl04~3 10~4778 and 1.3mM Si(OH)4 (prepared by hydrolysis of tetraethoxysilane) was adjusted to pH 5, then re-acidified (to pH 4.3) to give concentrations of 0.5 mM Perchloric acid and 1.25 mM acetic acid. After 21 day~, the volume of gel, measured on the undiluted solution, was 23 units.

Since gel formation is inhibited by high concentrations of reagent, a procedure was developed in which the concentration of imogolite wa~ built up in increments, each addition of the concentrated stock solution of Example 35 being converted to synthetic imogolite under reflux before more reagent was added. In the present Example, the volumes of stock solution added were adjusted to introduce 10 m mol/1000 ml of re~ctive A~, and other components of the stock solution in proportion, at each addition. Initially (day O), 50 ml stock solution was diluted to give 750 ml containing 10 m mol/litre of A~, and the solution heated to boil gently under reflux. Subsequent additions of stock solutions were added at 2-day intervals, and the gel volumes were monitored as below:-10~778 Before stock solution added Day m mol AlGel volume Stock solution ~er litre(1 mM Al~ increment ml 4 19.4 23 57

6 28 22 62 8 34.6 28 66 ~2.3 25 71 12 49.5 21 76 14 56.3 20 81 16 62.5 18 87 18 68.3 18 93 28 ~3 ~ 20 Thus ~ubstantial concentrations of synthetic imogolite can be prepared. The pH of the starting solution was 4.5, but later remained in the range 3.3 - 3.6 during the additions.

Other patterns of addition of stock solution attempting to increase the concentration more rapidly gave lower gel volumes e.g.

by increasing the frequency of additions to one day intervals instead of two, or by increasing the volumes of the stock solution added at each 2-day increment by 50%.

EXAMPLES 41 to 48 Examples 20 to 23 were repeated, but replacing Al(C104)3 and HC104 with either Al(N03)3 and HN03 (Examples 41 to 44), or AlC13 with HCl (Examples 45 to 48). These established that perchlorate systems are better than nitrate systems and nitrate systems are better than chloride systems.

m mol rea~ent per 1000 ml Days f or Volume of ExampleSi(OH)4Al(N03)3 HN03Initial pH max. gel Final pH gel ~.5m~Al) 41 1.33 2.4 1 ~.6 1 3.2 14 42 2.66 4.8 2 4.5 5 3~3 20 43 4.00 7.2 3 4.4 7 3.8 14 44 5.33 9.6 4 4.4 7 3.8 8 Si(OH)4 AlC13 HCl 1.33 2.4 1 4.7 2 3.2 15 46 2~66 4.8 2 4.6 2 3.2 15 47 4.00 7.2 3 4.6 5 3.6 8 48 5.33 9.6 4 4.5 5 3.7 7 ~XAMPLES 49 to 54 The~e ~x~mple~ were performed to confirm that the atomic ratie o~ Sl to ~l in the reactin~ solution~ should be near 0.5, preferably within th~ li~it~ 0.42 - 0.58 for optimum yields. Excess ~illc~ is pre~erable to too little ~ilic~. The ~eneral procadure followed that ~0~7'78 of Example 23. Examples 49 to 54 are according to the invention but 49, 50 and 54 are non-preferred.

m mol reagent per 1000 ~
Days for Final Volume of Example Al(C104)3Si(OH)4HCl04 Initial pHmax. gel pH Si:Al gel (2.5 mM

49 10 7.5 5 4.3 2 3.80.75 7 6.7 5 4.3 2 3.7o.67 11 51 10 5-8 5 4.3 2 3.50.58 18 52 10 5.0 5 4.3 2 3.30.50 20 53 10 4.~ 5 4.3 2 3.10.42 17 54 10 3-3 5 4.3 1 3.10.33 6 EXAMPLES 55 and 56 These Examples examine the nature of the products formed at high concentrations of the stock solution described in Examples.35 - 39.

This ~tock solution was heated undiluted and at various dilutions at 96 C for 7 days:-Examplem moles reagent/litre pH Gel Volume No.Al Si HCl04 HAcinitial final (1 mM Al) 5510 5.3 2 5 4.3 3.5 23 563 15.9 6 15 4.2 4.0 7 Comparative 90 47.7 18 ~5 4.1 4.1 0 Comparative 150 79.5 30 75 4.0 4.4 0 After heating, the undiluted stock solution was no longer reactive, and no gel formed when it was diluted to the 10 mM Al level and heated at 96 C.

10~4778 The products formed in the above four experiment~ were compared by electron micro~copy, electron diffraction, and infrared ~pectro4copy.

At the 10 mM Al level well-ordered ~ynthetic imogolite had formed with the characteri~tic diffraction pattern and morphology. At higher 5 concentrations the fibrou~ morphology wa~ feebly developed or ab~ent.

The characteri~tic diffraction features of ~ynthetic imogolite at ~ 2.1A, and 4.2A (a~ociated with repeat di~tance~ along the fibre axi~) became increa~ingly broad at 30 mM and not all were di~cernible at 90 ~M Al or 150 mM A~. Certain broad diffraction band~ of ~ynthetic imogolite, mo~t obviou~ly tho~e near 2.3A ~nd 3.45R, per~i~ted at all concentration~ and were given al~o by the unheated ~tock ~olution. Stock ~olution diluted to the 10 ~M A~

l~vel and heated to 60 C for 1 month gave a product with diffraction ~imilar to the 90 mM A~ level at 96C.

~O

Claims (22)

I CLAIM:-

1. A method of synthesising an inorganic material which is a fibrous product displaying discernible electron diffraction peaks at 1.4.ANG., 2.1.ANG. and 4.2.ANG., and which resembles imogolite, the method comprising forming at pH 3.2 to 5.5 soluble hydroxyaluminium silicate complexes in aqueous solution, and digesting said solution at pH
from 3.1 to 5.0, the concentration of available aluminium being up to 50 millimolar.

2. A method according to claim 1, wherein the digestion is performed at 40° to 170°C.

3. A method according to claim 2, wherein the digestion is performed at 90°C to 130°C.

4. A method according to claim 3, wherein the digestion is performed at 95°C to 100°C.

5. A method according to claim 1, wherein the digestion is in the presence of sufficient dissolved silica to inhibit boehmite formation.

6. A method according to claim 1, wherein the hydroxyaluminium silicate complexes are hydroxyaluminium orthosilicate complexes.

7. A method according to claim 1, wherein the said concentration of Al is up to 25 millimoles per litre.

8. A method according to claim 7, wherein the said concentration of A1 is up to 15 millimoles per litre.

9. A method according to claim 1, wherein any precipitation accompanying formation of the hydroxyaluminium silicate complexes is quickly redissolved or redispersed in acid.

10. A method according to claim 9, wherein any precipitation accompanying or following formation of the hydroxyaluminium silicate complexes is quickly redissolved or redispered in a mixture of acetic with one of nitric and perchloric acids.

11. A method according to claim 1, further comprising adding an aqueous solution containing a hydroxyaluminium silicate complex in steps to the reaction mixture, but keeping within the said concentration of A1.

12. A method according to claim 1, wherein the pH of digestion is up to 4.6.

13. A method according to claim 1, wherein chloride ion, if present, does not exceed a concentration of 25 millimolar.

14. A method according to claim 1, wherein the digestion is performed in the presence of acetic acid.

15. A method according to claim 14, wherein the molar concentration of the acetic acid is not more than quadruple that of the available aluminium, and wherein there is also present sufficient acid to inhibit formation of acetate anion.

16. A method according to claim 1, wherein the digestion is allowed to continue for from 1 to 60 days.

17. A method according to claim 1, further comprising removing the product by precipitation with alkali or by foam-flotation using an anionic detergent.

18. A method according to claim 1, further comprising forming an aqueous gel having an imogolite concentration of under 1% by weight.

19. A method according to claim 1, further comprising casting the product on a surface, drying it, and removing it as a coherent film.

20. An inorganic material, which is fibrous product having a tubular structure resembling the natural product imogolite but having 10-15% larger tube diameter and displaying discern-ible electron diffraction peaks at 1.4.ANG., 2.1.ANG. and 4.2.ANG., the walls of said tubes having a structure like that of a single sheet of gibbsite with orthosilicate groups replacing the hydroxyl groups on the inner surfaces of said tubes.

21. A membrane composed of the synthetic imogolite material of claim 20 cast into a coherent film.

22. A colloidal solution of the synthetic imogolite-like material of claim 20.