A review of zeolite-like porous materials

Microporous and Mesoporous Materials 37 (2000) 243–252www.elsevier.nl/locate/micromeso

A review of zeolite-like porous materials

Ying Ma a, Wei Tong a, Hua Zhou a, Steven L. Suib a,b, c *a Department of Chemistry, University of Connecticut, Storrs, CT 06269-3060, USA

b Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3060, USAc Environmental Reserch Institute, University of Connecticut, Storrs, CT 06269-3060, USA

Received 8 June 1999; accepted for publication 8 October 1999

Abstract

This review focuses on the most recent reports of zeolite-like microporous and mesoporous materials and inparticular their synthesis. We focus on porous transition metal oxides of various types, also including porous pillaredinterlayer materials that have either positive or negative charges. This review is not all-inclusive: it only covers themost recent papers published from about January 1995 to the present. A discussion of new transition metal oxides,porous pillared materials, sulfide layered materials and porous phosphates is included.

Zeolites, isomorphous substitution of zeolites, aluminophosphates, and silicoaluminophosphates will not be coveredsince excellent reviews can be found elsewhere [1,2]. The section on porous transition metal oxides focuses on bothmicroporous and mesoporous transition metal oxides and, in particular, synthetic strategies used to produce newmaterials. The next section concerns pillared materials, including layers that are anionic as well as cationic. Sulfidesystems discussed in a further section are primarily layered phases. The final section concerns porous phosphatematerials. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Oxides; Phosphates; Porous; Sulfides; Transition metal

1. Porous transition metal oxides for mesoporous material synthesis using surfac-tants. In case 1 [Fig. 1(a)], the condensation pro-cess of alkoxides is so rapid, compared with the1.1. Mesoporous transition metal oxideshydrolysis process, that small aggregates areformed instead of a desirable continuous network.Mesoporous silicon oxides, M41S, were firstThe interaction between the aggregates and surfac-obtained by using surfactants as space-filling rea-tant micelles is usually an electrostatic attraction.gents. Different kinds of mesoporous transitionSince the aggregates cannot wrap around themetal oxides [3–11] have been synthesized, basedmicelles in a closed fashion, washing and calcina-on M41S materials. Usually, metal alkoxides aretion often cause a loss of micelles and frameworkused. Hydrolysis and condensation of metal alkox-collapsing. In case 2 [Fig. 1(b)], a continuousides around surfactant micelles provide large voidsoxide network is formed around the surfactantafter surfactant removal.micelles and locks in the micelles. If the oxideAs shown in Fig. 1, there are two extreme casesnetwork is flexible and thermally stable, as is thecase for M41S, removal of surfactants producesordered and uniform mesopores, while the oxide* Corresponding author. Fax: +1-860-486-2981.

E-mail address: [email protected] (S.L. Suib) framework is usually amorphous.

1387-1811/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S1387-1811 ( 99 ) 00199-7

244 Y. Ma et al. / Microporous and Mesoporous Materials 37 (2000) 243–252

Fig. 1. Two extreme cases of synthesis of mesoporous materials: (a) case 1; (b) case 2.

Synthesis of a few mesoporous transition metal upon further acid addition and hydrolysis. Thehexagonal mesostructured vanadium oxide is gen-oxides [4,7,9,10], which are M41S-like, has been

reported. Mesoporous zirconia has been obtained erated by cooperative formation of micellar entitiesand oligomers. The vanadium coordinationwith anionic surfactants [9,10]. Three kinds of

surfactants, CH3(CH2)10OSO3Na, CH3(CH2)14 number of the final product is determined by 51VNMR to be close to 5. The existence of VNOOSO2Na, and (C12H25O)PO(OH)2, were used.

There are not enough peaks in the XRD pattern moieties probably contributes to the poor thermalstability of the final product.to determine the crystalline order of the mesopores,

while HRTEM shows poorly ordered polygonal Addition of coordination reagents slows downthe hydrolysis and condensation process, whichholes. After removal of the surfactants by calcina-

tion, the large d-spacing peak in the XRD pattern often encourages the formation of continuous net-works. Antonelli and Ying [4] reported the synthe-usually remains with its position shifted by

different amounts. There is qualitative consistency sis of hexagonal mesoporous TiO2 by addingacetylacetone. However, Putnam et al. [3] repeatedbetween the pore diameter (20–40 A) from BET

measurements and from XRD data, so a scaffold- some of the syntheses of hexagonal mesoporousTiO2 reported by Antoneli and Ying [4], anding rather than a templating mechanism has been

suggested. raised several questions. The products of Putnamet al. have a lamellar structure instead of a hexago-A hexagonal mesostructured vanadium oxide

can be synthesized by acid-catalyzed hydrolysis of nal structure, as determined by two-dimensionalpowder X-ray diffraction ( XRD) and TEM.ethanolic cetyltrimethylammonium vanadate

(CTAV ) [7]. The formation mechanism was Hydrolysis of titanium tetraisopropoxide[Ti(OPr)4] in the presence of stearic acidstudied in a rather detailed way. The major species

of CTAV ethanol solutions is probably (EtO)2 (C17H35COOH, STA) [6 ] produces TiO2, whichhas a surface area greater than 100 m2/g, and poreVO−

2, which changes to [H2V2O7]2− upon acid

addition. Large oligomers [VO2]n+n

are formed sizes of 70–170 A in diameter. The ratio of stearic

245Y. Ma et al. / Microporous and Mesoporous Materials 37 (2000) 243–252

acid to Ti(OPr)4 determines the pore size, which and the formation of small condensed aggregates.To approach case 2 of Fig. 1 in mesoporous trans-implies that the mesopores are formed upon the

hydrolysis and condensation of Ti(OPr)4–STA ition metal oxide synthesis, two things can be tried:slow down the condensation process by synthesiz-complexes.

As one can see, the above mesoporous transition ing at a pH away from the zero point of charge ifother conditions allow this, and add coordinationmetal oxides are somewhat less satisfactory than

M41S in terms of thermal stability [7], crystallinity groups on the surfactants to re-enforce the inter-action of oxide framework and surfactant micelles,[9,10] and adsorption properties [3,4,7,9,10].

M41S exhibit a sharp increase in the adsorption if possible.If the hydrolysis and condensation of alkoxidesisotherm at P/P0 between 0.3 and 0.6, which clearly

indicates the existence of a large amount of pri- happen to be case 1 of Fig. 1(a) under theemployed conditions, surfactants are not necessarymary uniform mesopores. In transition metal

oxides, a slow and small increase of adsorption in in order to synthesize mesoporous materials.Secondary mesopores between small aggregatesthe same range probably indicates that there is

only a small amount of primary mesopores, which can be retained if the drying process is controlledwell. TiO2 aerogels [5] prepared by Suh and Parkproduces the large d-spacing peak(s) in their XRD

patterns, in addition to a large amount of second- have surface areas greater than 200 m2/g, andmean pore diameters of 170–230 A (see Table 1).ary mesopores, which are not uniform in size.

The synthesis of mesopores transition metal The pore sizes and pore volumes increase withlonger aging times and higher aging temperaturesoxides is probably a mixture of case 1 and case 2

of Fig. 1. This is not unexpected since the forma- of the sol–gel precursors. Zirconia aerogels withhigher surface areas (143–205 m2/g) and largertion of a continuous network from the hydrolysis

and condensation of transition metal alkoxides is pores (170–650 A) can also be prepared by super-critical drying [11].hard to achieve. The formation of condensed crys-

talline oxide phase(s) may also indicate the exis- Aerogel-like vanadium oxides with 8–90%porosity and surface areas between 150 andtence of case 1. A careful survey with TEM may

be able to determine whether there are two phases, 280 m2/g have been synthesized by ambient dryingmethods [8]. The synthesis involves replacing theand how much is mesoporous. Light scattering

and small-angle neutron diffraction (SAND) can pore fluid (water and acetone) with low-surface-tension alkanes, which reduce the capillary pres-be used to study the as-synthesized solution, for

example, to determine the formation of micelles, sures accumulated during the drying process,thereby retaining most of the porosity of the sol–the shape, and distance between them, hydrolysis

and condensation of alkoxide around the micelles, gel precursor.

Table 1Summary of surface areas, pore sizes and structures of porous transition metal oxides

Material Surface areas (m2/g) Pore sizes (A) Structure Reference

TiO2 >200 170–230 Aerogels [4]>100 70–170 Secondary pores [5]

V2O5 150–280 150–200 Aerogel-like [7]MnO2 10 7.1 Layered [12]

2–18 6.98–7.35 Layered [13]48 6.9×9.6 Microporous tunnels [14]0.3–26 2.3×4.6 Various [15]

ZrO2 100–500 20–40 Mesoporous [9,16 ]143–205 170–650 Secondary pores [10,11]

Nb2O5 150 30–100 Secondary pores [11]120–250 8.5–20 Microporous [17,18]


1.2. Microporous transition metal oxides species; (3) the structures of pillared materials arevery sensitive to synthesis procedures and are hardto control.Abe et al. [17] have prepared porous niobium

oxides by a rather unique soft-chemical process. It is beyond the scope of this section to coverevery aspect of the field of pillared materials.H+/K4Nb6O17 is exfoliated in TBAOH aqueous

solution. Then, MgO fine particles (ca. 100 A) are Instead, we will focus on the most recent trendson improving the synthesis process to obtain zeo-added to produce the composite precipitate. The

composite is evacuated at various temperatures lite-like microporous materials.and then treated in acidic solution to removeTBA+ and MgO to generate the product. 2.1. Negatively charged layered materialsH+/K4Nb6O17 has a surface area of less than5 m2/g, while the product can have surface areas Clays and layered oxygen acid salts belong to

the class of negatively charged layered materials.as high as 150 m2/g and various sized pores, whichmay explain its better photocatalytic activity. Pillaring of this class of materials involves exchang-

ing large inorganic species into the interlayerHydrolysis of Nb(OC2H5)5 in the presence ofshort chain amines, such as hexylamine, has pro- spaces to replace small cations. Several preparation

methods have been classified and summarized in aduced microporous Nb2O5 with pore sizes of 8.5–20 A and surface areas of 120–250 m2/g. The pro- recent review paper [20,21].

The pillaring process basically involves reac-ducts have a hexagonal crystal structure that canbe retained at ~400°C. A supramolecular templat- tions between layered host materials and ionic

guest species. The success of a pillaring processing mechanism has been proposed, where aminesbond to the partially hydrolyzed niobium ethoxide depends on the completeness of ion-exchange reac-

tions, well-defined pure pillaring agents (chargeand self-assemble into a supramolecular template.A summary of surface areas, pore sizes, and and size), and the ability to control condensation

between host/guest and guest/guest once the gueststructures of various transition metal oxides thatare discussed above is given in Table 1. It seems species have been intercalated between host layers.

The size and charge of hosts and guests need tothat there is little systematic understanding ofporous transition metal oxide synthesis. A compar- be matched in order to achieve stable pillared

materials.ison across the board of transition metals, even inone triad, would be of great interest. Complex equilibria exist among oligomeric

species in pillaring solutions. To obtain pureAl7+

13, SO2−

4was added to selectively precipitate

Al7+13

and from Al3+ hydroxo oligomers. With2. Porous pillared interlayered materialspurified Al7+

13, better crystallinity was achieved in

pillared smectites, which was responsible forFollowing zeolites [19], pillared interlayeredclays (PILCs) were first synthesized in the late greater microporosity. A surface area of 206 m2/g

out of a total BET surface area of 312 m2/g was1970s and were materials that showed good specificsurface areas. Numerous pillared materials have contributed by internal surfaces. A micropore

volume of 0.100 ml/g resulted in comparison tobeen synthesized so far [20,21]. Although thesematerials show potential applications as catalysts, the 0.014 ml/g micropore volume of the unpillared

clay. In contrast, the use of unpurified solutionsadsorbents, and gas separators, their commercialuses have yet to be developed at this stage for led to BET surface areas of 249 m2/g (148 m2/g

from internal surface) with a micropore volume ofseveral reasons: (1) the porosity is not homogen-eous and spans from micropores to mesopores, 0.082 ml/g [22].

In order to overcome post-hydrolysis problemswhich makes the establishment of structure-prop-erty relationships difficult; (2) the sizes of zeolite- between intercalated hydroxo complex ions and

phosphate layers, Al and mixed Al/Ga pillaringlike pores (micropores) are not predictable fromthe combination of host materials and guest species were fluorinated before being intercalated.


This treatment minimized the possible condensa- rectorite (R) were compared in the intercalationof titania pillars. A higher CEC (cation exchangetion of MMOH (MNAl, Ga) with PMOH and

retarded the spreading of the intercalated species capacity) led to more titania intercalated andhigher thermal stability in the order of M>S>R.throughout the phosphate interlayer region [23].

The resulting materials showed gallery spaces rang- However, no general trends in the relationshipbetween CEC, BET surface, and relative amountsing from 16.5 to 16.9 A, featuring a double-layer

of Al7+13

. Large porosity was produced in this of micropores are readily apparent. Both pillaredM and R showed 66% or larger BET surface areasreadily intercalated a-SnP, with only 0.104 ml/g of

pore volume contributed by micropores in contrast arising from micropores, while only 29% was dueto micropores in pillared S [27].to 0.374 ml/g from mesopores [23]. In an alumina

pillared titanate prepared by adding exfoliated Besides the improvement in preparation param-eters, the choice of proper host materials and guesttitanate sheets into Al7+

13solutions, double-layered

alumina pillars with a height of 19.4 A were also species can lead to materials that primarily containmicropores. Silica pillared manganese titanatesformed, which could be converted to a mesoporous

material by heating at 500°C to produce material with only micropores were prepared using TEOSas a pillaring agent and C

nH2n+1NH3 (n=6–18)with a surface area of 300 m2/g and a pore volume

of 0.25 ml [24]. as pre-swelling agents. The resultant materials havea uniform microporosity with pore sizes close toConcentrations of pillaring solutions can some-

times play an important role in determining the that of mesitylene, regardless of the pre-swellingagents. Moreover, with C6H13NH3 the resultingrelative amounts of micropores and mesopores.

While keeping the molar ratio of Al/Ce at 5 and pillared materials showed a shape selectivity [28].Intercalation of alumina into saponite producedthe ratio of Al/clay at 20 mmol/g, the total concen-

trations of Al/Ce pillaring solutions were varied slit-like pores with pore sizes on the border betweenmicropores and mesopores [29]. Manganese oxidefrom 0.42 to 2.9 M. With a concentration of 2.5 M

or higher, significant amounts of micropores pillared clays using a trinuclear manganese(III )acetate complex as pillaring agents were produced(0.16~0.18 ml/g) were developed in the pillared

materials, as compared to 0.047~0.058 ml/g in the that represent zeolite-like structures with uniformmicropores after calcination at 500°C [30].unpillared clays. An increase in BET surface area

was also detected. The same effect was also Phosphonates constitute a special class of lay-ered materials, whose layers are covalently linkedobserved in alumina pillared titanates. With more

intercalated alumina, the relative amounts of by organic groups (arylene or alkylene groups)functioning as pillars. Unlike other pillared materi-micropores increased [25].

Pre-swelling agents also significantly influence als that are obtained by ion-exchange methods,pillared phosphonates are produced by direct reac-the porosity in the resulting materials. By using

tetrabutylammonium ions (TBA), bimodal pores tions of arylene or alkylene bisophosphonic acidswith metal-ion-containing solutions [31]. Thecharacteristic of mesopores (>40 A) and micro-

pores (<20 A) co-exist in alumina pillared tita- interlayer spaces of these materials are usuallyfilled with closely packed organic groups.nates. With hexylammonium ions, typical

microporous materials with pore sizes of 10~20 A However, by introducing proper spacers such asH, OH, CH3, phosphites, or phosphates to uni-were synthesized. This difference is ascribed to the

much smaller crystallite sizes of TBA intercalated formly replace a fraction of bisphosphonates,micropores with tunable sizes are obtained [31].titanates consisting of only 10–20 titanate sheets

in contrast to hexylammonium ions intercalated Good crystallinity of the pillared phosphonatesis critical in order to achieve uniform microporos-into titanates, where hundreds of sheets were held

together [26 ]. ity [22,31]. Poorly crystallized or amorphous mate-rials usually contain a majority of mesoporesTo study the influence of the host materials on

the structures and properties of the pillared materi- resulting from stacking disorder or packing ofsmall crystallites. A schematic representation of aals, montmorillonite (M), and saponite (S), and


Fig. 2. Schematic representation of the introduction of micropores in alpha-zirconium phenyldiphosphonate (a) alpha-zirconiumphenyl phosphate (b) phosphite groups introduced as spacers to evenly replace diphosphonate groups. After Clearfield [31].

process that could lead to an introduction of 2.2. Positively charged layersmicropores in zirconium phenyl diphosphonate

LDH ( layered double hydroxides) are anionsystems is shown in Fig. 2. Replacement of phenylexchangers with positively charged layers heldgroups with H spacers can lead to new pores.together by interlayer anions. The general formulaSyntheses conducted in the presence of HF greatly

enhanced the crystallinity of zirconium biphenyl for LDH layers is [M2+1−xM3+

x(OH)−

2]x+ , where

M2+=Mg2+, Ni2+, Zn2+ and M3+=Al3+, Fe3+,phosphonates, and a pure bimodal distribution ofmicropores and mesopores was obtained [31]. Ga3+, and others. The interlayer anions include

CO2−3

, SO2−4

, NO−3

, Cl−, and OH−. This class ofFurther modifying the organic pillars by introduc-ing methyl groups into the ring positions a to laminar materials can be pillared by bulky poly-

oxometalates (POM). The molar ratio ofphosphonate groups led to the formation of amicroporous material with an interlayer spacing M2+:M3+ can be varied in a range to obtain

different charge densities of the LDH layers whicharound 14.5 A and a narrow pore size distributioncentered around 5 A. A large surface area of will subsequently influence the lateral spacing

between the pillars [20,21].405 m2/g also resulted mainly due to these micro-pores [32]. Some common features of this class of pillared

materials are the lack of thermal stability of bothDivalent metal (Zn2+, and Cu2+) phosphonateswith a good crystallinity have recently been synthe- host and guest species and a sensitivity to their

chemical environment [20,21,25]. Therefore,sized. Zinc phosphonates pillared by alkylenegroups with an even number of C atoms possessed research conducted recently has focused on the

improvement of preparation methods to obtainslit-like micropores. The coordination environ-ments of the metal ions, partially determined by highly crystalline materials with zeolite-like

micropores.the number of C atoms in the pillar chains, areinfluential in producing micropores. Preparative Besides direct and indirect ion-exchange pro-

cesses that are also used for negatively chargedparameters and pillared structures need to befurther modified and optimized to obtain micro- layered materials [20,21], a new synthesis route

involving a combination of a solid-state reactionpores in zinc and copper phosphonates [33].


and a stepwise anion-exchange process has been increasing amount of attention in recent years.reported [34]. The immediate precursor before pil- Most of the newly synthesized microporous sulfidelaring is adipate-intercalated LDH, which greatly materials have layered structures.facilitates the exchange of POM ions into both SnS is a good semiconductor and is used inbasic and acidic host materials. The resulting pil- chemical sensing devices. Jiang et al. [38] havelared materials showed good crystallinity. A surface prepared large single crystals of SnS-n materials.area as high as 136 m2/g was obtained with a The SnS-1 series A2Sn3S7 (A+=Et4N+, Me4N+,significant contribution (70 m2/g) from micropores. and mixture of NH+

4and Et4N+) has hexagonal-

However, leaching of M2+ ions during the process shaped 24-atom rings with diameters of aboutconstitutes a problem, which might be responsible 10.5 A in the tin sulfide layer, and interlamellarfor the change of charge density and thus the spacings of 8.5–9.0 A depending on the templates.distribution of micropores and mesopores [34]. (Et4N)2Sn3S7, which has 24-atom rings partially

Another novel method was based on the precipi- occupied by the template, selectively adsorb H2Otation of the pre-swelling anionic surfactant with over CO2. The SnS-3 series [A2Sn4S9 (A+=Prn4a cationic surfactant in an organic phase and the N+ and Bun

4N+)] has elliptical 32-atom rings of

concomitant intercalation of the necessary species 20×10 A in size within the tin sulfide layer, andinto the layers. The driving force of this synthesis an interlamellar spacing of 14 A. The tin sulfideis the immediate formation of a precipitate in the

trigonal bipyramidal and tetrahedral connectingorganic solvent. The generality of this method hasunits make the tin sulfide layer very flexible, andbeen demonstrated by the intercalation of a widethis can undergo elastic deformation to alter therange of organic and inorganic species [35].void spaces within and between the layers toCo-precipitation was used to directly synthesizeaccommodate the size and shape of the templates.POM pillared LDHs when the LDH was acidic.

MoS2 with a large surface area (~60 m2/g) andSince most POM systems are also acidic, thepore size (~110 A in diameter) has been synthe-reaction of host and guest does not cause anysized via decomposition of (NH4)2Mo3S13xH2Ostructural deformation [36 ].under heating and vacuum treatment [39]. High-resolution transmission electron microscopy2.3. Scaled-up synthesis(HRTEM) shows an interlayer spacing of 6–7 A,and unusually bent lamellae. This bent lamellarAlthough there is still a great deal of work tostructure makes MoS2 a more effective lubricant.be done to understand how to improve the synthe-Driving out the volatile species from the precursorses and to precisely control product propertiesunder vacuum assists the MoS2 in forming lamellaebefore these pillared materials can be used com-with curvature and large pores.mercially, scaled-up syntheses of pillared smectite

clays have been conducted by several groups, andpromising results have been obtained. By imitatingindustrial production conditions, such as higher 4. Porous phosphate materialsconcentrations of pillaring solutions and clay sus-pensions, larger particle sizes, and as-received Porous metal oxide-phosphate materials areclays, as much as 1 kg of pillared clays could be zeolite-like materials that can be used as ion-synthesized in one batch. Significant amounts of exchangers, adsorbents, and catalysts. Recently,micropores with volumes of 0.130–0.150 ml/g several new studies in the research area of porousresulted with mesopore volumes of about metal oxide-phosphate materials have been0.070 ml/g [37]. reported [40–50].

Syntheses with surfactants, originally utilizedfor the synthesis of mesoporous silica and alumino-3. Sulfide layered materialssilicates, can also be used to produce porous metal-phosphate materials [40]. This type of synthesisClose analogs of oxide-based microporous

materials are sulfides, which have received an usually involves three steps, as shown schemati-


cally in Fig. 3. These steps are follows: (1) long as well as mesoporosity. The final products hadsurface areas ranging from 295 to 366 m2/g, porechain surfactants are intercalated between layers

of phosphates; (2) intercalated phosphates are ion- size distributions of 15–16 A, and a microporevolume between 0.117 and 0.143 cm3/g, which wasexchanged with metal species, in order to obtain

metal species in the phosphate layers; and (3) the about 20–40% of the total pore volume of thematerial. The materials also showed acidity, mainlypillared phosphates are calcined at a specific tem-

perature to remove the remaining surfactants and of the Lewis type, and were tested for oxidativedehydrogenation of isopropyl alcohol at 220°C,obtain porous metal oxide-phosphate composites.

Jimenez-Lopez et al. have reported a sol–gel which gave a conversion close to 100% and 100%selectivity towards propene.synthetic route to generate highly porous

gallium(III )–zirconium phosphate composites In addition, by using a similar synthetic strat-egy, Jimenez-Lopez and co-workers obtained a[43]. In their synthesis, surfactants and layered

zirconium phosphate were used to obtain interca- porous tin phosphate material pillared with mixedGa/Al oxide [44] which is more thermally stablelated materials in which the surfactants were inter-

calated between the layers of zirconium phosphate. than tin phosphate pillared by aluminum oxide.They claimed that the resulting material was meso-Then, the intercalated system was ion-exchanged

with a gallium oligomeric solution to yield zirco- porous with a surface area ranging from 160 to304 m2/g, a pore volume of about 0.478 cm3/g,nium phosphate inserted with oligomeric gallium

species. Finally, the resulting zirconium phosphate and most of the pores having radii in the range of16–22 A. Furthermore, materials containing Lewiswith oligomeric gallium species was calcined at

400°C for a specific period of time, which resulted acid sites were tested for oxidative dehydrogena-tion of isopropyl alcohol, in which about 20%in Ga/Zr phosphate composites with microporosityconversion and 100% selectivity towards propenewere achieved.

Cassagneau et al. have reported the synthesisof a series of porous zirconium phosphates pillaredinto different silicas [45]. Their results showed thatzirconium phosphates pillared in silica are ther-mally stable mesoporous materials with a surfacearea ranging from 175 to 390 m2/g, a total porevolume of about 0.139–0.289 cm3/g, and most ofthe pores having radii in the range of 22.3–36.4 A.

New porous nickel zirconium phosphate materi-als pillared by nickel oxide have been reportedrecently by Shpeizer and co-workers [46 ]. Thenovelty of this material lies in the configuration ofnickel species intercalated between the layers ofzirconium phosphate. As shown in Fig. 4, those

Fig. 3. Scheme for synthesis of porous metal oxide phosphate Fig. 4. Intercalation of nickel polymeric units between layers ofzirconium phosphate. After Ref. [46 ].composites. After Ref. [40].


nickel species were nickel polymers, which consist sieves for the pharmaceutical and biomedicalindustries.of three layers. Two of these are directly bonded

to the phosphate groups, while the third is presum- Kasuga and co-workers [50] have developedmicroporous materials based on the integration ofably situated between the two bonded layers. The

resulting material is predominately microporous, AgTi2(PO4)3 and Ti(HPO4)22H2O crystals. Theunique properties of these materials include con-with pores of about 10 A, and has a surface area

ranging from 60 to 159 m2/g. The materials are tinuous pores of various diameters ranging from10 nm to 2 mm, surface areas of 30 m2/g andactive for aromatization of n-hexane.

In some cases, for materials synthesized using various functions such as bacteriostatic activityand water-repellent properties.surfactants, thermal instability is a large problem,

which is due mainly to the removal of the surfac-tant during the calcination process [48,49].Recently, a novel method for synthesizing porous

Acknowledgementszirconium oxophosphates with a high thermal sta-bility was reported by Ciesla et al. [48]. The

We thank the US Department of Energy, Officesynthetic route consisted of two steps:of Basic Energy Sciences, Division of Chemical1. A zirconium sulfate surfactant composite wasSciences for support of this research. We acknow-prepared using a surfactant. The resulting pre-ledge Dr. S. McClain, S.J. Perrotti, and L.D.cipitate was then washed and dried at roomConde for help in literature searching.temperature. At this stage, the product was a

crystalline material containing the surfactant, azirconium sulfate surfactant composite.

2. The resulting zirconium sulfate surfactant com-References

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A review of zeolite-like porous materials

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