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GV/l Eur°Pean Patent Office < * S Office europeen des brevets (11) E P 0 5 5 0 4 1 5 B 1

(12) EUROPEAN PATENT S P E C I F I C A T I O N

(45) Date of publication and mention (51) int. CI.6: C01 B 13/14, C0 1 F 7/00, of the grant of the patent: B01J 29/04, C 0 1 G 9 / 0 0 09.07.1997 Bulletin 1997/28

(21) Application number: 93104694.0

(22) Date of filing: 06.05.1991

(54) Polyoxometalate intercalated layered double hydroxides

Schichtenformige Doppelhydroxide mit zwischengeschalteten Polyoxometalaten Hydroxides doubles en couches avec polyoxometalate intercale

(84) Designated Contracting States: AT BE CH DE DK ES FR GB GR IT LI LU NL SE

(30) Priority: 25.05.1990 US 528763

(43) Date of publication of application: 07.07.1993 Bulletin 1993/27

(62) Document number(s) of the earlier application(s) in accordance with Art. 76 EPC: 91910073.5/0 484 495

(73) Proprietor: MICHIGAN STATE UNIVERSITY East Lansing, Michigan 48824 (US)

(72) Inventors: • Pinnavaia, Thomas J.

East Lansing, Ml 48823 (US) • Kwon, Taehyun

Dae Dog Dan 305-343 (KR) • Dimotakis, Emmanuel, Dr.

Urbana, lllionois 61801 (US) • Amarasekera, Jayantha

East Lansing, Ml 48823 (US)

CO 10

10 10 o Q_ LU

(74) Representative: Grunecker, Kinkeldey, Stockmair & Schwanhausser Anwaltssozietat Maximilianstrasse 58 80538 Munchen (DE)

(56) References cited: US-A- 4 454 244 US-A-4 774 212

• CHEMISTRY OF MATERIALS vol. 1, no. 4, 1989, pages 381 - 383 T. KWON ET AL. 'Pillaring of a Layered Double Hydroxide by Polyoxometalates with Keggin-ion structures'

• M.T. POPE 'Heteropoly and Isopoly Oxometalates.' 1983 , SPRINGER- VERLAG , BERLIN

• JOURNAL OF THE AMERICAN CHEMICAL SOCIETY vol. 110, 1988, pages 3653 - 3654 T. KWON ET AL. 'Pillaring of layered double hydroxides (LDH's) by polyoxometalate anions.'

• SYNTHETIC METALS vol. 34, no. 1-3, 1989, pages 609 - 615 M. DOEUFF ET AL. 'Layered double hydroxides pillared by polyoxometalate anions: EXAFS studies and chemical synthesis.'

• INORG.CHEM.,vol 27 (1988),pp 4628-4632; by Drezdon

Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).

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Description

BACKGROUND OF THE INVENTION

5 This invention relates to the preparation of inorganic-oxometalate anion pillared clay compositions having the hydrotalcite type layered double hydroxide crystal structure and more particularly, anionic zinc-aluminum hydrotalcite clays containing large inorganic polyoxometalate anions (POMs) with Keggin-type structures intercalated between pos- itively charged layers of metal hydroxides.

In recent years inorganic materials such as pillared smectite clays have been extensively used as catalytic materi- 10 als in varying applications. These materials comprise negatively charged metal silicate sheets intercalated or pillared

with hydrated cations. See Pinnavaia T. J., Science, 220, 365 (1 983) for a review on these clays. The techniques for the intercalation of these clays are well-established and a wide variety of cations may be incorporated into clays such as montmorillonite. Through the changes in the size of the pillar used to separate the sheets in the clay structure, the pore size of the pillared clay may be tailored to a particular application. Porous clay materials with a high surface area have

15 been prepared using organic or organometallic cations, metal chelates, polyoxometalate cations and transition metal halide clusters. The synthesis of several such systems has been disclosed in several patent literatures including the ones by Pinnavaia et. al., in U.S. Patent Nos. 4,665,045; 4,665,044 and 4,621,070.

Polyoxometalate anions are another class of pillars that are suitable for lamellar solids. POMs containing early tran- sition metals form water soluble anions with the general formula [MmOy]p" (isopolyanions) and [XxMmOy]q" (x<m) (het-

20 eropolyanions). M is usually molybdenum or tungsten, less frequently vanadium, niobium or tantalum, or mixtures of these elements, in their highest oxidation slates. For general review on polyoxometalate anions see, Pope, M. P., Het- eropoly and Isopoly Oxometalates, Springer- Verlag, New York, (1 983). POMs forms a structurally distinct class of com- plexes based predominantly, although not exclusive, upon quasi-octahedral-coordinated metal atoms (M06).

The simplest POMs have the hexametalate structure M60i9, where the oxygen atoms are in closed-packed 25 arrangement with six M06 octahedra (Figure 1A). Some of the isopoly anions include [Nb60i9]8", [Ta60i9]8", [Mo60-|9]2"

etc. The decavanadate anion V10O286" has a related structure (Figure 1B). Similarly, seven edge-shared octahedra form the Anderson-type structures (Figure 1C) such as in [Mo2024]6".

The most extensively studied POM compounds are those with Keggin-type structures (Figure 2). At least two iso- mers of the Keggin structure are known and Fig 2A represents the a-form. The structure has overall Td symmetry and

30 is based on a central X04 tetrahedron surrounded by twelve M06 octahedra arranged in four groups of three edge shared octahedra, M30i3. These groups ("M3 triplets") are linked by several corners to each other and to the central X04 tetrahedron. Most of these types of Keggin ions are either molybdates or tungstates with the general formula [XM12O40]n" where M is Mo or W. For M=W, anions with X=H, B, Al, Ga(lll), Si, Ge(IV), P(V), As(V), V(V), Cr(lll), Fe(lll), Co(lll), Co(ll), Cu(ll), Cu(l), or Zn have been reported. Similarly for M=Mo anions with X= Si, Ge(IV), P(V), As(V), V(V),

35 Ti(V), Zr(IV), In (III) are known The second isomer has the p-Keggin structure, where one of the edge-shared M3O13 triplets of the a-structure rotated by 60° around the C3 axis, thereby reduction of overall symmetry of the anion from Td to C3v (Figure 2B). This structure is known for several tungstates (X=B, Si, Ge, H2) and molybdates (X= Si, Ge, P, As).

Another class of POM is known in which a single M06 octahedron is deficient (Figure 2C). These are known as lac- unary (defect) Keggin POM anions and have the general structures such as [XWii039]n" (represents as XM ) where

40 X=P, As, Si, Ge, B, Al, Ga, Fe(lll), Co(lll), Co(ll), Zn, H2, Sb(lll), Bi(lll); or ^MonO^]"" where X= P, As, Si, Ge. These anions are stable in aqueous solutions and can be isolated in pure forms. Removal of a trigonal group of three adjacent M06 octahedra from the Keggin structure derive an another class of lacunary structures, which lead to the XMg struc- ture (Figure 2D). The anion [PW9034]9" represents one such example. The POM [P2W18Oi8062]6" consists of two PW9 lacunary units fused into a cluster of virtual D3h symmetry (Figure 2E). This unit is now known as the Dawson structure,

45 and has two types of W atoms , six "polar" and twelve "equatorial". Removal of one M06 octahedra from this Dawson structure results in [X2W1706i]10" type lacunary anions.

There are several other POMs which are closely related to Keggin type structure. For example, the anion PV140429" has a "bicapped Keggin" structure. In this POM twelve vanadium atoms form the usual Keggin structure and the remain- ing two V atoms occupy the pits on the Keggin molecule where a C4 axis is passing, forming trigonal bipyramidal caps

so (Figure 2F). The POM anion NaP5W3O11014" has an approximate D5h symmetry and consists of a cyclic assembly of five PW6022 units, each derived from the Keggin anion, [Pw12O40]3" , by removal of two sets of three corner-shared W06 octahedra which leads to the PW6 moiety the P2W18 (Dawson) anion. The sodium ion is located within the poly- anion on the five fold axis and is 1 .25 A above the pseudomirror plane that contains the five phosphorous atoms.

The structures of heteropoly and isopoly oxometalates are not confined to the structure-types described above. 55 There are several other variations. For example, the paratungstate anion [H2W12042]10" has a different arrangement of

its twelve M06 octahedra than in a typical Keggin-type anion. Here, M06 octahedra are arranged in four groups of three edge-shared octahedra to form a central cavity. It has been suggested that the two protons are attached to the oxygen atoms inside the cavity and help stabilize the somewhat open structure by hydrogen-bonding (Figure 1D).

Anionic POM compounds have been extensively used as heterogeneous catalysts for a broad variety of reactions.

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Examples include the oxidation of propylene and isobutylene to acrylic and methacrylic acids, ammoxidation of acrylo- nitrile; oxidation of aromatic hydrocarbons; olefin polymerization and epoxidation, and hydrodesulfurization, Thus neg- atively charged polyoxometalates would present a wider range of thermally stable, catalytically active pillars, provided a suitable host clay material is utilized.

5 Layered double hydroxides (LDHs), which are also referred to as anionic clays, represent a potentially important class of lamellar ionic solids for forming pillared derivatives with anionic POMs. These clays have positively charged lay- ers of metal hydroxides between which are located anions and some water molecules, Most common LDHs are based on double hydroxides of such main group metals as Mg, and Al and transition metals such as Ni, Co, Cr, Zn and Fe etc. These clays have structures similar to brucite (Mg(OH)2) in which the magnesium ions are octahedrally surrounded by

10 hydroxyl groups with the resulting octahedra snaring edges to form infinite sheets. In the LDHs , some of the magne- sium is isomorphously replaced by a trivalent ion say Al3+. The Mg2+, Al3+, OH" layers are then positively charged necessitating charge balancing by insertion of anions between the layers. One such clay is hydrotalcite in which the the carbonate ion is the interstitial anion, and has the idealized unit cell formula [Mg6AI2(OH)16]C03.4H20. However, the ratio of Mg/AI in hydrotalcite can vary between 1 .7 and 4 and various other divalent and trivalent ions may be substituted

15 for Mg and Al. The preparation of LDHs is described in many prior art publications, particular reference being made to following

review journal articles by S. L Suib et. al., in Solid State Ionics, 26 (1988), 77 and W. T. Reichel in CHEMITECH, 58 (1986). An important aspect of the synthesis of these materials, which is particularly relevant to this disclosure, is the variation of the nature of the interstitial anion. The preparation of hydrotalcite-like materials with anions other than car-

20 bonate in pure form requires special procedures, because LDH incorporates carbonate in preference to other anions. Most of the time the smaller anions are introduced to the LDH structure via the precipitation method by using the desired anion solutions instead of carbonate. In this manner the carbonate anion in the hydrotalcite can be varied in synthesis by a large number of smaller anions such as N03", CI", OH", S042" etc. However, in these methods the syn- thesis has to be carried out in an anaerobic condition to prevent carbonate contamination from the atmospheric carbon

25 dioxide. Miyata et. al. in U. S. Pat. Nos. 3,796,792, 3,879,523, and 3,879,525 describes hydrotalcite-like derivatives with anionic substitution including the smaller transition metal anions like Cr042", Mo042", and Mo2072".

The incorporation of larger anions into LDH galleries, such as transition metal polyoxoanions, is not easy. This requires ion-exchange techniques subsequent to the LDH synthesis. The work by Miata et al., in Clays and Clay Minerals, 31, 305 (1983) indicated that the order of ion exchange capability of the gallery anions in hydrotalcite-like

30 derivatives is OH"<F"<CI"<Br"<N03"<l" and for divalent anions, C032"<S042". Monovalent anions can be easily replaced by di- or poly-valent anions. Using this strategy, Pinnavaia and Kwon in J. Am. Chem. Soc, 110, 3653 (1988) have dem- onstrated the pillaring of several polyoxometales including V10O286" into the hydrotalcite structure containing Zn and Al metal ions in the layers. This pillared hydrotalcite-like material catalyzes the photooxidation of isopropanol to acetone.

Chemistry of Materials, Vol. 1 , no. 4, July/August 1 989, pages 381 -383 relates to LDH's of the Zn2AI-type with gal- 35 lery anions selected from a-[H2W12O40]6", a-[SiV3W9O40]7", [PCuW11039(H20)]5" (only partial intercalation), p-

[SiW10O39]8" and [PW9034]9" having pillar heights of greater than about 9A. Basal spacings of greater than about 14A are also obtained.

According to said document these LDH's can be prepared by exchange of the N03" in [Zn2AI(OH)6]N03 • 2H20 by the gallery anion(s) at room temperature but with a boiling suspension of Zn2AI[N03]".

40 Synthetic Metals, Vol. 34, nos. 1-3, 1989, pages 609-615, discloses LDH's of the Zn2AI-type wherein the gallery anions are selected from [V10O28]6", [PCuW11039(H20)]5" (only partial exchange with N03"), a-[H2W12O40]6" and <x- [SiV3W9O40]7", aN having pillar heights of greater than about 9 A and some having basal spacings of greater than 14 A.

These LDH's are prepared as in J. Am. Chem. Soc, Vol. 110, 1988, pages 3653-3654, i.e. by ion exchange at pH 4.5 or nearer to neutral conditions. It is disclosed that the LDH's of the Zn2AI-type have a "potential utility for shape-

45 selective adsorption and catalysis". U. S. Pat. No. 4,454,244 by Woltermann discloses the preparation of several polyoxometalate-LDH reaction prod-

ucts. However, no XRD or analytical data were given to support his assumption that the POMs were intercalated into the galleries of a crystalline LDH host. During the course of this work, we reproduced some of the synthetic procedures disclosed by Wolterman and found the products to be largely amorphous and impure (see the description of the inven-

50 tion below). Recently, in U. S. Pat. No. 4,774,212 by Drezdon, the preparation of several Mg/AI hydrotalcite-like materials with

gallery height about 12 A containing transition metal polyoxoanions such as V10O266", Mo70246" and W70246" with Anderson type structures (Figure 1), is disclosed.

We disclose here a process for preparing Zn/AI LDHs intercalated with transition metal containing large POM ani- 55 ons. The products isolated were pure and gave well-defined XRD peaks corresponding to uniformly crystalline layered

double hydroxide products. The basal spacings of these materials agreed with POM-intercalated structures. The replacement of smaller anions, in hydrotalcite-like LDHs with much larger POMs, particularly with POMs with Keggin type structures, produce structures with increased gallery spacings of 14 A or more. Such materials should exhibit intracrystalline microporosity accessible for the adsorption or diffusion of molecules into the structure from the outside,

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The accessibility of the intracrystalline structure to guest molecules should provide new opportunities for molecular seiving of gas mixtures and for heterogeneous catalysis.

This invention relates to an intercalated uniform crystalline layered double hydroxide clay compositions conforming to the formula [Zn1.xAlx(OH)2]Ax/n .yH20 wherein A is a polyoxometalate anion of charge n", x is between 0.12 and 0.8,

5 and which provides a pillar height greater than 9 A and an X-ray diffraction basal spacing value greater than 14 A, A being selected from H2W120421°-, PV1409-, NaP5W30O11014", SiFeCIIIKSCyWuOsg7-, BVCVJWuO^6", PV3W9O406", PMo2W90397", BCoCIIJWuOsg7", BCuCIIJWuOsg7", EW^C^9" and BCoOIIJWuOsg6" and the use thereof as catalyst in the oxidation of methane, sulfur oxides and nitrogen oxides.

10 IN THE DRAWINGS

Figures 1 A to 1 D are perspective views showing the structures of some common POMs and in particular: 1A the structure of [M6Oig]n"; 1 B structure of V10O286"; 1 C structure of an Anderson type anion; 1 D structure of [H2W12042]10" . The positions of the protons are marked by open circles.

15 Figures 2A to 2F are perspective views showing Keggin and related structures and in particular isomers of the Keg- gin structure: 2A alpha-isomer; 2B beta-isomer, where one M3O13 group (shown unshaded) has been rotated by 60°; 2C a lacunary structure derived from alpha-Keggin structure 2A by removal of one M06 octahedron to result XM^ structure; 2D a lacunary structure derived from alpha-Keggin structure 2A by removal of three adjacent M06 octahedra to result XMg structure; 2E alpha-[P2W18062]6", the Dawson structure; 2F structure of [PV14042]9" based on alpha-Keg-

20 gin structure with two additional vanadium atoms occupying trans-related V05 trigonal bipyramids. Figures 3A to 3C are graphs showing x-ray diffraction patterns for oriented film samples of

3A Zn2AI-N03LDH and Zn2AI-LDH intercalated with 3B Keggin ion alpha-SiV3W9O407"

25 3CV10O286-.

Figures 4A and 4B are graphs showing x-ray diffraction patterns for oriented film samples of Zn2AI-LDH interca- lated by 2A. Lacunary Keggin ion BCu(ll)W110397" 2B POM anion NaP5W30O11014".

Figures 5A to 5D show x-ray diffraction patterns for film samples of 5A Zn2AI-N03 LDH and the POM-LDH reaction 30 products formed by the reaction of the LDH with 5B V10O286"; 5C PW12O403"; and 5D SiV3W9O407" intercalates pre-

pared according to the methods described in U.S. Patent No. 4,454,244.

DESCRIPTION OF THE INVENTION

35 In certain embodiments described in this invention we describe the preparation of POM-intercalated crystalline Zn/AI-LDH compounds of the formula [Zn2AI(OH)6]A1/nn".yH20, where A is a POM anion of negative charge n, and y is a positive number. The incorporation of the guest POM anions into the Zn/AI LDH was carried out using LDHs of the type Zn2AI(0H)6X, [ abbreviated as Zn2AI-X] where X" = CI" or N03"; and N03" is the preferred anion. The polyoxomet- alate anions selected contained at least one transition metal and possessed preferably a Keggin or lacunary Keggin-

40 type structure or other POM anions with structures related to Keggin ions. The anion X in Zn2AI-X is readily exchange- able with POM to give the desired POM intercalated Zn2AI-LDH.

The precursor LDHs Zn2AI(0H)6CI or Zn2AI(0H)6NO3 were prepared using the induced precipitation method. Accordingly, a 0. 1 M Al3+ solution containing the desired anion (CI" or N03") was added to 1 M NaOH until a pH of 7 was achieved. This white slurry [AI(OH)3] was then treated with a 0.3M Zn2+ solution while maintaining the pH of the slurry

45 between 6-7, and more preferably between 6.0-6.2. It is desirable to maintain the pH between 6-6.5 in this step to elim- inate the formation of undesired products. The resultant slurry was then digested, preferably between 60-1 00°C, for a period of 1 8 h to a day to obtain a good crystalline material. This crystallization process for the LDH - chloride or nitrate is important for the subsequent synthesis of well-crystalized pillared POM derivatives from these materials. Shorter durations of digestion resulted in amorphous materials, but extended digestions, for example, through a period of week,

so increased the crystallinity of the final products. All of the manipulations were carried out under an atmosphere of nitro- gen gas and the solvents were degassed prior to the use to avoid possible contamination of C032" from atmospheric C02. An oriented film sample of the Zn2AI-N03 LDH reaction product showed a well-defined X-ray powder patterns cor- responding to the LDH structure (Figure 3A). Chemical analysis revealed that the Zn/AI ratio of this material to be 2. These CI" and N03" precursor Zn2AI-LDHs were stored as aqueous slurries for the subsequent anion exchange reac-

55 tions with POM anions as discussed below. Anion exchange reactions were carried out by adding an aqueous hot suspension of [Zn2AI(OH)6]X.zH20 (X = CI"

and N03") into an aqueous solution containing POM anions under anaerobic conditions. Preferably, a stoichiometric excess of the anion pillaring material over the hydrotalcite-type clay is used, for example, about 1 to about 2 molar excess, although a stoichiometric amount can also be used. The exchange reaction depends on the temperature and

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the pH of the solution. When the exchange reaction was carried out at ambient temperatures, the products obtained showed incomplete exchange. Zn2AI-LDHs are amphoteric and stable only within the pH range of 5.8 to 10. Moreover, most POMs are unstable or undergo hydrolysis at high basic conditions, Therefore, exchange reactions were carried out under slight acidic conditions at a pH in the range of about 6 to 4.

5 Aqueous hot suspensions of [Zn2AI(OH)6]N03.2H20 were found to undergo complete intercalative ion exchange reactions with aqueous solutions of POM anions such as SiFe(lll)(S03)W110397-, with Keggin structure. These inter- calated products were crystallographically well-ordered phases. Similarly, POM anions such as PMo2W90397-, Bco(ll)W110397-, BCu(ll)W110397-, BW-|i0399-, with a lacunary (defect) Keggin structure can be intercalated in Zn2AI- LDH structure to give pure products with basal spacings around 14 A. Furthermore, robust POMs with fused Keggin-

10 type structures such as H2W12O4210- (Figure 1D), PV140429- (Figure 2E). NaP5W30Oii014- also readily undergo intercalation to give pure crystalline products.

The anions with lower charge, for example a charge less than 7-, such as [PW12O40]3" and [SiW12O40]4" show no ion exchange whereas, intermediate anions show partial intercalation (eg. [PCuW11039(H20)]5"). Partial intercalation also was observed with several POMs with 6- charge such as BV^W^O^6", BCollllJWuOsg6" and PV3W9O406". X-

15 ray diffraction patterns of these materials showed the presence of small amounts of non-intercalated starting material Zn2AI-N03, along with intercalated product. These observations can be explained considering the guest anion size and host layer charge density. If one assumes a triangular arrangement of Keggin ions with a 9.8 A diameter in the LDH gal- leries, then the area needed to accommodate each ion is 83 A2. Since the area per unit layer charge for [Zn2AI(OH)6]N03.2H20 is 16.6 A2, Keggin ions with a charge less than 5- such as [PW12O40]3" and [SiW12O40]4", are

20 spatially incapable of balancing the host layer charge making their intercalation in the LDH structure impossible. It is also apparent that the stacking symmetry of these Keggin ions plays a role in the intercalation reactions. The

oxygen frame work of the POM anions with the a-Keggin structure define a polyhedral form with Td symmetry. Thus, there are two plausible orientations for a-isomers of Keggin structure in the LDH galleries. One orientation suggests the C3 axis is orthogonal to the layers, and the other orientation the C2 axis is orthogonal. The C3 orthogonal orientation of

25 the a-isomer allows the hydroxyl groups in the LDH to undergo H-bonding to six oxygens of the upper M3 triad and to three terminal oxygens on the opposing side of the POM. Conversely, the C2 orthogonal orientation permits 12 H-bond- ing interactions to 8 terminal and 4 bridging POM oxygen atoms. Thus, a larger number of H-bonds should favor the C2 orthogonal orientation. Accordingly, the intercalation of p-isomers of Keggin type POM was slow and incomplete. For example, the p-[SiV3W9O40]7" POM anion which has a C3v oxygen framework symmetry, undergoes partial intercala-

30 tion in Zn2AI-LDHs. As discussed earlier, in the p-Keggin structure, one of the edge-shared M30-|3 triplets of the <x- structure is rotated by 60° around the C3 axis (Figure 2B). Thus once the p-isomer is intercalated inside the LDH gal- leries with an orientation similar to that of the a-isomer, its preferred orientation is one with the C3 axis inclined. In this orientation the p-Keggin ion is capable of mimicking only half the H-bonding interaction encountered for a C2 orthogonal orientation of the a-isomer. One face of the POM experiences guest-host interactions equivalent to those found for the

35 C3 orthogonal orientation of a, but the H-bonding pattern on the opposite face of the POM is completely disrupted. As a result POM anions such as p-SiWi-|0398", p-SiV3W9O407", with p-Keggin type structures undergo only partial interca- lation.

The anions that posses structures other than Keggin- or lacunary Keggin-type also can be intercalated using this procedure. For example intercalation of smaller POM anions with Anderson type structures such as V10O286" give crys-

40 talline products with basal spacing of about 12 A (Figure 3C). This process of incorporating large POM anions into hydrotalcite structure has several novel features. Most impor-

tantly, the anion exchange reactions do not require controlled pH conditions, and the methods are convenient and fast. Furthermore, all the products prepared according to this invention afford pure and completely intercalated crystalline products with basal spacings of 14 A or higher with high surface area values (Table 1). Although not discussed here, all

45 the materials were further characterized by NMR, IR and other techniques. The types of POMs that can be intercalated are not limited to the ones described in this invention. The processes described could be adapted for the intercalation of any suitable POM anion.

The preparation of materials prepared by the reaction of polyoxometalate anion with Zn/AI -LDH has been dis- closed in U.S. Pat. No. 4,774,212 by Drezdon. In this latter work pillaring by relatively small POMs with Anderson type

so structures such as V10O286", Mo70246" and W70246" were disclosed. In the preparation of these materials, controlled acidic conditions and LDH precursors intercalated by dicarboxylate anions were used. Hydrotalcite-like materials were first interlayered by dicarboxylate anions to obtain organic anion derivatives with basal spacings of about 14 A. These dicarboxylate anion pillared LDHs were then treated with vanadates, molybdates or tungstates at controlled acidic pH conditions to intercalate V10O266", Mo70246" and W70246" anions (Figure 1), with basal spacings of about 12 A. By fol-

55 lowing the teachings of our disclosure we can also intercalate these smaller ions into LDH structures (Figure 3C). The emphasis of our invention was the synthesis of POM-intercalated LDHs that result in higher basal spacings of 14 A or more. The polyoxometalates selected have Keggin-type structures or related ones.

A second patent, U.S. Patent No. 4,454,244 by Wolterman, also has claimed the preparation of several POM-LDH reaction products. The POMs used by Wolterman were mostly non-Keggin type anions such as V10O286", Mo2076",

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Ta6018(OH)7" etc. His starting materials were either the nitrates or chlorides of ZN/AI or Mg/AI-LDHs. These solid LDHs were slurried in solutions containing POM anions to obtain POM-containing products. However, no XRD or analytical data were given in the patent to support the presence of a pillared crystalline phase. The pillaring of a layered solid by a gallery ion requires that the product be crystalline with the host structure still intact following insertion of the guest.

5 Depending on the pH, however, the reaction of a LDH and a POM can lead to hydrolysis products that are amorphous non-pillared phases and to diffusely crystalline impurities. Thus, we have made several attempts as described below to characterize some of Wolterman's materials by following the teachings of his patent.

Following the conditions given in U.S. Patent No. 4,454,244, we have found the reaction products to be largely X- ray amorphous and impure. For example, the yellow product said to be V10O286" intercalated into a Zn2AI-LDH was

10 found to be X-ray amorphous as judged by the absence of distinct Bragg reflections. The X-ray diffraction pattern for this product is shown in Figure 6B. The diffuse reflection near 10-11 A are coincident with the reflections for poorly formed Mg or Al salts of the POM; they are not characteristic of a pillared LDH-V10O286". In contrast, the V10O286" inter- calated Zn2AI-LDH prepared according to our teachings as disclosed in this patent was an analytically pure, crystalline layered material. The XRD of the [Zn2AI(OH)6](V10O28)1/6.xH2O product synthesized according to our teachings

15 showed four orders of 00I harmonics corresponding to a basal spacing of 1 1 .9 A (Figure 3C). This basal spacings cor- responded to gallery heights of 7.1 A (three oxygen planes) and to a V10O286" orientation in which the C2 axis is parallel to the host layers.

U.S. Patent No, 4,454,244 further claimed the intercalation of four Keggin-type POM anions. These included, PMoi2O403", PW12O403", PMo6V6O405" and PMo6W6O405". We believe that one of these anions, namely PMo6V6O405"

20 is not known to the public. The known P, Mo, V containing Keggin-type POMs include PMo-| 1 VO404", PMo10V2O405", and PMo9V3O406", but not PMo6V6O405". Moreover, a POM that contained six V, six Mo and one P, if it existed, should have an overall anion charge of minus 9, i.e., PMo6V6O409".

Our attempts to intercalate a Keggin-type polyoxometalate according to Wolterman's method also resulted in an X- ray amorphous materials. For example the reaction of, [PW12O40]3" with Zn2AI-N03 according to the procedures given

25 in example 5 of U.S. Patent 4,454,244 resulted in a material that was largely X-ray amorphous (Figure 5C). Further- more, the reaction of Zn2AI-N03 and the Keggin ion <x-SiV3W9O407", resulted in a product which showed diffuse scat- tering features (Figure 5D) that were different from those observed for an authentic sample of pillared [Zn2AI(OH)6](SiV3W9O40)i/7.xH2O which we discussed earlier (cf., Figure 3B). The diffuse XRD peaks in Figure 5C and 5D were consistent with those observed for the Mg2+ or Al3+ salts of the POM.

30 We have also found that the products prepared according to Wolterman's teachings were not only amorphous and impure, but also exhibited physical properties different from an authentic, crystalline pillared LDH-POM. For example, the N2 BET surface area of 155 m2/g was observed for the crystalline <x-SiV3W9O407" intercalated LDH prepared according to our teachings, when out gassing was done at 150°C under vacuum. However a much lower surface area of 47 m2/g was observed under analogous conditions for the amorphous material prepared according to Wolterman's

35 method. The above observations clearly demonstrate that the POM intercalated LDH materials we disclose here have com-

positions and properties different from the ones resulting from the work of Drezdon (U.S. Patent No. 4,774,212) and the work of Wolterman (U.S. Patent No. 4,454,244). We have presented in this disclosure, evidence to prove that our mate- rials contain Keggin ions intercalated between LDH layers with retention of crystallinity. We believe that the Keggin-

40 anion products claimed in U.S. Patent No. 4,454,244 were amorphous and impure MN and MMI-POM sans outside the composition of matter represented by our materials.

On the basis of the known catalytic properties of POMs and hydrotalcites, the POM-intercalated hydrotalcites of this invention may be useful as catalysts for many industrially important processes including the oxidation of methane, sulfur oxides, nitrogen oxides etc., most likely at temperatures in the range of from 100-1000°C.

45 The following examples will serve to illustrate certain embodiments of the herein disclosed invention.

EXAMPLE 1

The preparation of [Zn2AI(OH)6]X.zH20 (X = N03, CI) is described in this example. so All the manipulations were carried out under a N2 atmosphere, and the water used as a solvent was pre-boiled for

about 2 hours under N2 before using. To a 200-ml solution of 0.1 M AI(N03)3.9H20 was added a 1 .0 M solution of NaOH until the pH of the solution was

7. The white slurry was stirred for one hour, and a 200-ml solution of 0.3M Zn(N03)2 was added drop-wise. The pH of the mixture was maintained at about 6.0, by adding NaOH. The resulting slurry was boiled for 24h under a nitrogen

55 atmosphere. (Boiling this suspension for one week produced products with high crystallinity.) The product, [Zn2AI(OH)6]N03 zH20, was washed several times with water by centrifugation, and stored as an aqueous slurry. A por- tion of the slurry was dried in air. The X-ray diffraction powder pattern of the dried solid corresponds in to a LDH struc- ture with a basal spacing value of 8.9 A. By employing a similar method, the CI" derivative, [Zn2AI(OH)6]Cl.zH20, can be prepared using AICI3 and MgCI2 as starting materials.

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EXAMPLE 2

The general preparation of polyoxometalate-intercalated Zn/AI LDH materials is described in this example. A boiling solution containing about a 5 mequiv. portion of a Zn2AI-X (X = N03, or CI) LDH slurry, prepared by the

5 method in Example 1 , was added drop-wise to a stirred aqueous solution containing about 7.5 mequiv. of the desired polyoxometalate anion. After the additions were complete, the pH of the resultant slurries were adjusted to about 6 by adding dilute HN03 acid. The slurries were stirred for about 1 h and the solid products were isolated and washed thor- oughly with water by centrifugation, The X-ray powder diffraction patterns of the dried solids correspond to hydrotalcite- like layered structures, with polyoxometalate anions in the gallery (Figures 3B, 3C, 4A and 4B). The basal spacings are

10 given in Table 1 . Chemical analyses conformed to the structure Zn2AI(OH)6[POM ]i/n.YH20, where POM represent the polyoxometalate with a negative charge of n. The N2 BET surface area for selected products outgassed at 150°C were also determined (Table 1).

TABLE 1

Basal Spacings and Surface Areas of Polyoxometalate Pillared Layered Double Hydroxides of the Type [Ml, Mx ( O H ) ^ . YH20

Layer Metals Gallery Anion An" Basal Spacing Surface Area BET N2(m2/g)

M" Mim

Zn Al SiFe(lll)(S03)W110397- 14.7

PV140429- 14.5

H2W120421°- 14.1 15

NaP5W30O11014- 16.5 8

PMo2W90397" 14.5

BCo(ll)W110397- 14.3

BCu(ll)W110397- 14.4

B\N^0399- 14.5 96

Claims

1. An intercalated uniform crystalline layered double hydroxide clay composition conforming to the formula [Zn^ 40 xAlx(0H)2lAx/n * yH20 wherein A is a polyoxometalate anion of charge n", x is between 0.12 and 0.8, and which pro-

vides a pillar height greater than 9 A and an X-ray diffraction basal spacing value greater than 14 A, A being selected from H2W120421°-, PV140429-, NaP5W30O11014", SiFe(lll)(S03)W110397-, BVfVJWuO^6", PV3W9O406", PMo2W90397-, BCo(ll)W110397-, BCu(ll)W110397-, EW^O^9" and BCo(lll)W110396-.

45 2. The use of an intercalated uniform crytalline layered double-hydroxide clay composition according to claim 1 as cat- alyst in the oxidation of methane, sulfur oxides and nitrogen oxides.

Patentanspruche

so 1. Eingelagerte gleichformige kristalline Zweischichthydroxidtonzusammensetzung entsprechend der Formel [Zn^ xAyOH jAx/n • yH20, wobei A ein Polyoxometalatanion mit der Ladung n" ist, x zwischen 0,12 und 0,8 liegt, und welche eine Saulenhohe von mehr als 9A und einen Basisabstandes von mehr als 14A bei der Rontgenbeugung aufweist, und A unter H2W120421°-, PV140429-, NaP5W30O11014", SiFe(lll)(S03)W110397-, BV^WnO^6", PV3W9O406", PMo2W90397-, BCo(ll)W110397-,BCu(ll)W110397-, EW^O^7", BWu0399- und BCo(lll)W110396-

55 ausgewahlt ist.

2. Verwendung der eingelagerten gleichformigen kristallinen Zweischichthydroxidtonzusammensetzung nach Anspruch 1, als Katalysator bei der Oxidation von Methan, Schwefeloxiden und Stickstoffoxiden.

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Revendications

1. Composition d'argile cristalline uniforme intercalee a hydroxyde double en couches correspondant a la formule [Zn1.xAlx(OH)2]Ax/n.yH20 ou A est un anion polyoxometallate de charge n", x est entre 0,12 et 0,8, et qui fournit une hauteur de pilier superieure a 9 A et une valeur d'espacement basal par diffraction de rayons X superieure a 14 A, A etant choisi parmi H2W120421°-, PV140429-, NaP5W30O11014", SiFeCIIIXSCyW^COsg7-, BVCVJWuO^6", PV3W9O406", PMo2W90397-, BCo{\\)^lu0397-, BCuCIIJWuOsg7-, BWnOsg9" et BCoCIIIJWuOsg6-.

2. Utilisation d'une composition d'argile cristalline uniforme intercalee a hydroxyde double en couches selon la reven- dication 1 en tant que catalyseur dans I'oxydation du methane, des oxydes de soufre et des oxydes d'azote.

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