AU5189090A - Synthesis of kenyaite-type layered silicate material and pillaring of said material - Google Patents

Description

SYNTHESIS OF KENYAITE-TYPE

LAYERED SILICATE MJVTERIAL

AND PILLARING OF SAID MATERIAL

This invention relates to the synthesis of a Kenyaite-type layered crystalline silicate and to pillaring of the resultant product.

The fundamental unit of crystalline silicate structures is a tetrahedral complex consisting of the Si cation in a tetrahedral coordination with four oxygens. In some structures, the tetrahedra link to form chains which result in fibrous or needlelike morphologies. Single chains result when SiO^ tetrahedra are joined at two oxygen atoms.

In other silicate structures, the tetrahedra are linked in layers or sheets as in mica minerals. Similar arrangements of the tetrahedra are found in clay minerals wherein two types of sheets may exist, one consisting of aluminum, iron or magnesium ions in a six-fold coordination with oxygens. The layer or sheet structures result from linking between three corners of each tetrahedron to neighboring tetrahedra. Breck, Zeolite Molecular Sieves, John Wiley . Sons, New York, London, Sydney, Toronto, p. 31 (1974) reports that these layer or sheet structures do not have three-dimensional stability and may expand if the layers are forced apart by water, other molecules or ions, and thus, differ from silicates referred to as zeolites which have a framework, three-dimensional structure.

Kenyaite-type layered crystalline silicates have, until now, been found in natural deposits (H.P. Eugster, 'Ηydrous Sodium Silicate From Lake Magadii, Kenya; Precursors in Bedded Chert", Science, 157, 1177-1180 (1967)) or have been synthesized from inorganic systems (K. Beneke and G. Lagaly, "Kenyaite-Synthesis and Properties", Amer. Mineralogist, 6 , 818-826 (1983), and WO88/00091). The present invention provides an improved, economical and reproducible method for preparing synthetic Kenyaite-type layered crystalline silicate exhibiting high crystallinity and purity, catalytic activity and other valuable properties. Accordingly, the invention resides in a method for synthesizing a crystalline Kenyaite-type silicate which comprises (i) preparing a mixture capable of forming said silicate, said mixture comprising sources of alkali metal cations (AM), silica, a non-alkali metal (M) of valence n, water and an organic compound (R) selected from an alkylamilne, a trialkylamine, a tetraalkylammonium compoung and trimethylhexamethylene diamine, said alkyl having 1 to 12 carbon atoms, and having a composition, in terms of mole ratios, within the following ranges:

(ii) maintaining said mixture under sufficient conditions until crystals of said silicate are formed and (iii) recovering said crystalline Kenyaite-type silicate.

The invention further resides in a method for pillaring the resulting layered silicate.

The Kenyaite-type silicate prepared by the method of the invention has a characteristic X-ray diffraction pattern including the lines listed in Table 1 below: Table 1

^-

These X-ray diffraction data were collected with a Rigaku diffraction system, equipped with a graphite diffracted beam monochromator and scintillation counter, using copper K-alpha radiation. The diffraction data were recorded by step-scanning at 0.02 degrees of two-theta, where theta is the Bragg angle, and a counting time of 1 second for each step. The interplanar spacings, d's, were calculated in Angstrom units (A), and the relative intensities of the lines, I/I₀, where I is one-hundredth of the intensity of the strongest line, above background, were derived with the use of a profile fitting routine (or second derivative algorithm). The intensities are uncorrected for Lorentz and polarization effects. The relative intensities are given in terms of the symbols vs = very strong (75-100), s = strong (50-74), m = medium (25-49) and w = weak (0-24). It should be understood that diffraction data listed for this sample as single lines may consist of multiple overlapping lines which under certain conditions, such as differences in crystallite sizes or very high experimental resolution or crystallographic changes, may appear as resolved or partially resolved lines. Typically, crystallographic changes can include minor changes in unit cell parameters and/or a change in crystal symmetry, without a change in topology of the structure. These minor effects, including changes in relative intensities, can also occur as a result of differences in cation content, framework composition, nature and degree of pore filling, and thermal and/or hydrothermal history.

Synthesis of a Kenyaite-type layered silicate according to the invention comprises (i) preparing a mixture capable of forming the layered silicate, said mixture comprising sources of alkali metal cations (AM), silica, non-alkali metal (M), water and an organic compound (R), and having a composition, in terms of mole ratios, within the following ranges:

Broad Preferred

wherein n is the valence of M, (ii) maintaining said mixture under sufficient conditions until crystals of said silicate are formed, and (iii) recovering said silicate having a highly crystalline Kenyaite-type layered structure. The quantity of OH^" is calculated only from inorganic sources of alkali without any organic base contribution.

Metal M may include any one or more of metals from Periodic Table Groups IVB (e.g. titanium), IIIA (e.g. aluminum, gallium, indium and thallium), VIB (e.g. chromium), IVA (e.g. germanium, tin and lead) and VIII (e.g. iron). Organic R may include any organic which acts as a mineralizer to promote crystallization from the above reaction mixture. Examples of such organic agents include alkylamine, trialkylamine, tetraalkylammonium compound wherein alkyl has 1 to about 12 carbon atoms, e.g. tetrapropylammoniura, and trimethylhexamethylene diamine.

Hydrothermal reaction conditions for crystallizing the present product from the above mixture include heating the reaction mixture to a temperature of 60°C to 250°C for a period of time of 6 hours to 60 days. A more preferred temperature range is from 100°C to 200°C with the amount of time at a temperature in such range being from 8 hours to 5 days.

The reaction is carried out until a fully crystalline product is formed. The solid product comprising highly crystalline Kenyaite-type layered silicate is recovered from the reaction medium, such as by cooling the whole to room temperature, filtering and water washing.

The reaction mixture composition for the synthesis of synthetic crystalline silicate hereby can be prepared utilizing materials which can supply the appropriate oxide. Such compositions include metal (M) salts or oxides and, when a separate source of aluminum is desired, aluminates or alumina; and silicates, silica hydrosol, silica gel, silicic acid and hydroxides. It will be understood that each oxide component utilized in the reaction mixture for preparing the present silicate can be supplied by one or more essential reactants and they can be mixed together in any order. For example, any oxide can be supplied by an aqueous solution, sodium hydroxide or by an aqueous solution of a suitable silicate. The reaction mixture can be prepared either batchwise or continuously. Crystal size and crystallization time for the product composition comprising the present silicate will vary with the exact nature of the reaction mixture employed.

Useful sources of silicon for the reaction mixture of the present invention include solid silicas or silica precursors. Such sources are cost effective and allow high solids loading of the reaction mixture. The use of a solid silica, e.g. Ultrasil (a precipitated, spray dried silica) or HiSil (a precipitated hydrated Si0₂ containing about 6 weight percent free H-,0 and about 4.5 weight percent bound H₂0 of hydration and having a particle size of about 0.02 micron) as the oxide of silicon source provides economic synthesis. Such solid silica sources are commercially available.

The silica precursor source of silicon for the present reaction mixture is an amorphous silica precipitate made from a solution of a soluble silica source. Conveniently, the solution is an aqueous solution of a pH ranging from 9 to 12. The source of silica can be any soluble silicate and is preferably sodium silicate. The silica precursor is formed by its continuous precipitation from the solution phase. Accordingly, precipitation comprises initiating precipitation and maintaining said precipitation.

Alteration of the composition of the solution of soluble silica source is undertaken by introducing a precipitating reagent. In one embodiment, the precipitating reagent is a source of acid, conveniently a solution of a mineral acid, such as H₂S0., HC1, and HNO,, typically having a pH ranging from essentially 0 to about 6. Thus, precipitation of the silica precursor can be effected by acid neutralization of a basic solution of a silicate. The silica can be precipitated alone in the absence of sources of other framework elements, e.g. aluminum. In this fashion, both the precipitating reagent and the solution of silica source can be free of intentionally added alumina or alumina source. That is, no aluminum is deliberately added to the silica precipitation reaction mixture, in this embodiment; however, aluminum is ubiquitous and the presence of such a material in minor amounts is due to impurities in the precursors of the reactants or impurities extracted from the reaction vessel. When no source of alumina is added, the amount of alumina in the silica precursor precipitate will be less than about 0.5 weight percent, and generally less than 0.2 weight percent. When a source of alumina is added, the amount of alumina in the silica precursor precipitate will be up to about 5 weight percent. Silicate precipitation can be coprecipitation in the presence of soluble sources of other framework elements including gallium, indium, boron, iron and chromium. The soluble source of these other framework components can be, for example, nitrates. The coprecipitation product would be amorphous, for example an amorphous gallosilicate, borosilicate or ferrosilicate.

Continuous precipitation of the amorphous silica precursor may comprise introducing the solution of silica source and the precipitating reagent to a reaction zone while maintaining a molar ratio of silica source to precipitating reagent substantially constant. For example, the precipitating reagent and the silica source are introduced simultaneously into the reaction zone.

The continuous precipitation of silica precursor effects two results. Firstly, silica gel formation is at least substantially eliminated and secondly, precipitated silica precursor particle size exceeds that at which silica gel formation is possible. The precipitated silica precursor comprises agglomerated solids in the shape of microspheres. Suspensions of these particles exhibit low viscosities at high solids loading in the subsequent synthesis reaction mixture of the present invention, even at solids loading equal to or greater than about 20-40%. The particle size of the precipitated silica precursor ranges between 1-500 microns, but the average size is 50-100 microns.

Other conditions affecting precipitation of silica precursor include time, pH and temperature. The temperature of the precipitation mixture can range from 80°F to 300°F (27°C to 150°C). The time of contact of the solution of silica source and the precipitating reagent can range from 10 minutes to several hours at pH maintained from 6 to 11. Generally, the silica precursor is processed by isolating it, for example by filtration, and removing soluble contaminants therefrom by washing and/or ion exchange. This stage can be considered a solids consolidation step.

The Kenyaite-type silicate prepared by the present method has a chemical composition in terms of mole ratios of oxides and in the anhydrous state, as-synthesized, as follows:

(0 - 5)R₂0:(0 - 5)AM₂0: By>_n: x Si0₂ wherein AM, M, n and R are as defined above, and x is at least 40.

The silicate product of the invention is valuable as a catalyst component, e.g., low activity support, for various chemical conversion processes. Such process mechanisms include (1) formation or rupture of carbon-carbon bonds, such as cracking, isomerization, polymerization, alkylation and olefin dismutation; (2) formation or rupture of carbon-hydrogen bonds, such as hydrogenation, dehydrogenation, hydrogen transfer and hydrogenolysis; (3) conversion of oxygenates to other oxygenates or hydrocarbons as well as other reactions involving heteroatom-containing organic compounds; (4) oxidation of alkanes, alkenes and aromatic hydrocarbons; and (5) the reactions of unsaturated hydrocarbons with carbon oxides. The layered silicate product of the present invention can be modified to increase its interlayer spacing by introducing material between the layers. In particular, chalcogenide, and preferably oxide, pillars can be intercalated between the layers to provide a thermally stable, highly porous structure useful as a catalyst or a catalyst support. The preferred method for producing such a chalcogenide pillared product is described in detail in US Patent No. 4600503 and EP-A-205711. Briefly, the method involves physically separating the layers by introducing an organic cationic species, typically an organoammonium compound and preferably a C*-, or larger alkylammonium cation, at anionic sites between the layers and then introducing between the separated layers of the resultant "propped" species, an electrically neutral compound, such as an alkoxide, capable of conversion, preferably by hydrolysis, to the required chalcogenide pillars. Conveniently the hydrolyzable, electrically neutral compound is tetraraethylorthosilicate, tetrapropylorthosilicate or, more preferably, tetraethylorthosilicate.

The invention will now be more particularly described with reference to the following Examples and the accompanying drawing, which is a reproduction of the X-ray diffraction pattern of the product of Example 3.

Examples 1-6 Six separate efforts were made to synthesize Kenyaite-type layered silicate directly under hydrotherraal conditions from reaction mixtures containing an organic agent. The reaction mixture compositions for each example, in mole ratios, are indicated in Table 2. The metal M salt added to the respective reaction mixtures, if any, and time of reaction in days are also indicated. The organic R added to Example 1 was trimethylhexamethylene diamine. Tetrapropylammonium bromide was the organic R added to the reaction mixtures of Examples 2, 5 and 6. Tripropylamine was the organic R added to the Example 3 reaction mixture, and n-propylamine was added to the mixture of Example 4. The silica source in Example 1 was Q-brand sodium silicate

(about 28.5 wt.% Si0₂, 8.8 wt.% Na₂0 and 62.7 wt.% H₂0), and in Examples 2-6, silica gel.

The reaction mixtures were stirred at 400 rpm for the reaction duration and maintained at 160°C.

Each example yielded pure 100% crystalline Kenyaite-type layered silicate.

Table 2

Figure 1 is a reproduction of an X-ray diffraction scan of as-synthesized product from Example 3. The X-ray diffraction pattern of the Example 3 product is set forth in Table 3.

Table 3

11

Example 7

Samples of the crystalline products from Example 1-6 were found by chemical analysis to be composed as shown in Table 4,

Table 4

Moles per Mole MmQn

Based on these analytical results, the organic species appear to be largely excluded from the structure rather than being trapped within the Kenyaite-type silicate product of this invention.

Example 8

The synthetic kenyaite-type product hereof is pillared by the following procedure. The as-synthesized material is dispersed in water and acidified with 0.1 N HC1 to pH=2. The pH of the solution is kept at 2 for 24 hours with addition of acid as needed. The mixture is then filtered, water-washed and vacuum-dried. The dry sample is then treated with a dimethylsulfoxide(DMSO)/n-octylamine mixture at the weight ratio of 1:2:1 (sample:DMS0:amine) at ambient temperature for 24 hours. The sample is then filtered and dried in air at ambient temperature. The sample is finally suspended in tetraethylorthosilicate (TEOS), 5g TEOS/ lg solid, for 24 hours. After this treatment, the sample is filtered, dried and air calcined at 540°F for 3 hours.

Claims (10)

CLAIMS:

1. A method for synthesizing a crystalline Kenyaite-type silicate which comprises (i) preparing a mixture capable of forming said silicate, said mixture comprising sources of alkali metal cations (AM), silica, a non-alkali metal (M) of valence n, water and an organic compound (R), selected from an alkylamine, a trialkylamine, a tetraalkylammonium compound and trimethylhexamethylene diamine, said alkyl having 1 to 12 carbon atoms, and having a composition, in terms of mole ratios, within the following ranges:

(ϋ) maintaining said mixture under sufficient conditions until crystals of said silicate are formed and (iii) recovering said crystalline Kenyaite-type silicate.

2. The method of claim 1 wherein said mixture has a composition within the following mole ratio ranges:

SiO, M 0 40 to 5000

L m n

H₂0/Si0₂ 10 to 100 0H^"/Si0₂ 0.1 to 2

AM/Si0₂ 0.1 to 1

R/Si0₂ 0.05 to 2

3. The method of claim 1 wherein alkyl is propyl.

4. The method of claim 1 wherein the layered silicate material has an X-ray diffraction pattern exhibiting values as set forth in Table 1.

5. A method for preparing a pillared layered silicate material intercalated with a polymeric chalcogenide which comprises:

(a) providing a crystalline silicate material produced by the method of claim 1; (b) enhancing the interlayer distance of said layered silicate by impregnating the layered silicate with an organic compound capable of forming a cationic species; and

(c) introducing between the layers of said impregnated layered silicate a compound capable of conversion to said chalcogenide and converting said compound to said chalcogenide.

6. The method of claim 5 wherein said cationic species is organoammonium cation.

7. The method of claim 6 wherein said organoammonium cation is C, or larger alkylammonium.

8. The method of claim 5 wherein said compound capable of conversion is electrically neutral.

9. The method of claim 5 wherein said compound capable of conversion is hydrolyzable and said product is converted by hydrolysis to form said pillared material.

10. The method of claim 9 wherein said hydrolyzable compound is selected from tetraethylorthosilicate, tetramethylorthosilicate and tetrapropylorthosilicate.

7020h/0515h