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Microporous and Mesoporous Materials 107 (2008) 3–15

Catalysis by layered materials: A review q

Gabriele Centi, Siglinda Perathoner *

Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Universita di Messina, Salita Sperone 31, 98166 Messina, Italy

Received 15 November 2006; received in revised form 13 February 2007; accepted 8 March 2007Available online 16 March 2007

Abstract

Layered materials, with their structure consisting of stacked sheets, represent an interesting opportunity for developing new materialswith a tailored nano-design, controlled accessibility to the sites and properties, tuneable pore size and volume, and high surface area. Theuse of layered materials (layered perovskite, anionic clays, pillared clays) in catalytic reactions is reviewed with emphasis on the possi-bilities offered from these catalysts to develop new processes for environmental protection, selective oxidation and refinery/biorefinery.� 2007 Elsevier Inc. All rights reserved.

Keywords: Catalysis; Layered materials; Clay; Layered perovskite; PILC; Hydrotalcite

1. Introduction

Starting from the beginning of petrochemical industry,layered materials have been used as catalysts. The firsthydrocracking process developed over 80 years ago wasbased on acid-treated clays [1], but later and still now, zeo-lites and aluminosilicates were used. However, in few casessuch as in treating some heavy fractions, clays are still used.In addition, they are additives in current fluid catalyticcracking (FCC) catalysts; in particular, kaolin or bentoniteis used [2–6]. The discovery around 30 years ago of the pos-sibility of modification of these clays by pillaring the sheetswith metal-oxide or of intercalating between the sheetanions, complexes, and organic chemicals represented abreakdown in the catalytic chemistry of these materials,because introduced new possibilities in mastering the prop-erties and reactivity, although not fully explored [7–11].

Today a large spectrum of structural, textural and com-positional modifications are possible for layered materials,allowing a fine-tuning and control of the catalytic reactivity

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.03.011

q Plenary lecture given at the Workshop ‘‘Innovative Applications of

Layered Materials: from Catalysis to Nanotechnology’’, September 1–2,2006, Alessandria, Italy.

* Corresponding author. Tel.: +39 090 6765609; fax: +39 090 391518.E-mail address: centi@unime.it (S. Perathoner).

[12–33]. This makes these materials very interesting for cat-alytic applications. Worth noting, clays appear to have arelevant role also in catalyzing the starting of the life onthe earth [34]. In fact, is known that the clay montmorillon-ite catalyzes the polymerization of RNA from activatedribonucleotides [35]. Hanczyc et al. [34] observed thatmontmorillonite accelerates the spontaneous conversionof fatty acid micelles into vesicles. Clay particles oftenbecome encapsulated in these vesicles, thus providing apathway for the prebiotic encapsulation of catalyticallyactive surfaces within membrane vesicles. In addition,RNA adsorbed to clay can be encapsulated within vesicles.Once formed, such vesicles can grow by incorporating fattyacid supplied as micelles and can divide without dilution oftheir contents by extrusion through small pores. These pro-cesses mediate vesicle replication through cycles of growthand division. Therefore, clays, probably near to underwa-ter hot springs, probably catalyzed the starting formationof the first cells and thus of the life. Clays and layeredmaterials, in general, have a relevant role in catalyzing bio-logical reactions.

Recently the interest on these materials has beenrenewed for the possibility of using layered materials withintercalated inorganic and/or organic guests to preparehybrid materials, nanocomposite and multifunctionalmaterials [19,36,37] with applications as nonlinear optical

4 G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15

materials, conductors, photoactive materials, nanomag-nets, polymer additives, ion-exchangers, electrodes, etc.,besides to catalysis. An interesting review on the applica-tions of hybrid organic–inorganic nanocomposites and lay-ered nanocomposite has been recently published by C.Sanchez et al. [38].

This review will discuss the use of layered materials incatalytic applications. In fact, various reviews on thesematerials exist in the literature [12–33], but there is the needof a focused overview discussing the recent trends, limitsand opportunities of this class of catalysts. The layeredmaterials may be divided in three main classes: (i) layeredstructures where exists anisotropy of crystal bondingenergy in lattice directions, e.g. where van der Waals forcesare involved along one main crystallographic axis; (ii)multi-layer catalysts, produced by methods such as atomiclayer deposition/epitaxy, ultra-thin films methods, etc., and(iii) delaminated materials, obtained by swelling of layeredmaterials and earlier developments [39]. The latter will bediscussed in another contribution of this issue, and willbe thus not discussed here.

Multi-layer catalysts are at a too early stage of develop-ment, although they quite interesting. Worth note, several‘‘active’’ catalysts show a layered structure and their prop-erties depend on this structure, but mainly because thenature of the surface reflects the order-disorder in the struc-ture. Examples are the (VO)2P2O7 for n-butane oxidationto maleic anhydride and MoVW oxides for acrolein toacrylic acid [40].

Regarding the layered structures, they include a largevariety of materials such as sulfides (molybdenite – MoS2,for example), oxides (brucite – Mg(OH)2, for example),silicates (montmorillonite, Na0.3Al1.7Mg0.3Si4O10(OH)2 ÆH2O, for example) and materials such as graphite andmixed-layer structures. For sake of conciseness, we willlimit discussion here to only three types of layered struc-tures, which are the most relevant for catalysis: (i) hydrotal-cites; (ii) pillared clays and clays, and (iii) layeredperovskite. The first two classes of catalysts are knownand used from several years. The interest on layered per-ovskites is more recent, but their interesting behaviour asnew photocatalysts for water splitting is raising theirinterest.

Layered structures can be classified in those having a (i)neutral layer, e.g. brucite (Mg(OH)2) and other hydroxides,phosphates and chalcogenides, and various metal oxidessuch as V2O5; (ii) negatively charged layers with com-pensating cations in the interlayer space, e.g. widespreadlamellar compounds in nature such as cationic clays (mont-morillonite, hectorite and beidellite, etc.), and (iii) posi-tively charged layers with compensating anions in theinterlayer space, the most common of which are the layereddouble hydroxides called often with the acronym LDH.The second class is commonly found in nature (severalminerals belong to the montmorillonite group – smectites),while LDH (hydrotalcite – HT – anionic clays) are typicallysynthesized in laboratory by coprecipitation followed by

calcination. During this thermal treatment the HT (havingas general formula M2þ

1�xM3þx ðOHÞ2ðA

n�Þx=n � mH2O, where

the most typical M2+ion is Mg2+ and Al3+ for M3+ ion)transforms first to an amorphous oxide and then, at highertemperatures, to a crystalline spinel-like oxide. What isoften indicated in the literature as hydrotalcite catalyst isinstead the amorphous oxide or even the crystalline spi-nel-like oxide. HT derived oxides, however, have thecharacteristics of the possible reconstruction during thecatalytic reaction of a structure resembling the startingHT structure (memory effect), which often is quite relevantin determining the catalytic performances [41–44].

The intercalation by polymeric metal hydroxides of thecationic clays, followed by thermal treatment, allows pillar-ing of the layers, with a modification of the interlayer dis-tance and typically also an increase of the thermalstability. The catalytic behaviour of these pillared clays(PILCs) is usually associated to the modification of theacidity (by substituting tetrahedral and octahedral sites),by active sites introduced during the pillaring process (andlater by substitution) and by intercalation with organo-metallic complexes. The method of pillaring could largelyaffect the final characteristics and performances [31]. Alsoanionic clays could be pillared (pillared layered anionicclays – PILACs – [45,46]). These materials, particularlythose having isopoly- and heteropoly-anions intercalatedbetween the layers, show a quite interesting catalytic behav-iour [47–50]. Also various homogeneous complexes can beintercalated leading to a family of interesting catalysts.

2. Main catalytic uses of layered materials

A literature survey over the period 2000–2006 (limitingto English as language), revealed that nearly 20,000 papershave been published on clays, layered perovskites (LP) pil-lared clays (PILC) and hydrotalcite (HT) materials, ofwhich about 85% were dealing on catalysis. This percent-age increases to about 25% limiting to only LP, PILCand HT materials, evidencing that for the these materialsthe application as catalysts is quite relevant.

Of the publications dealing on catalysis, the percentageof patents were about 2% for LP, 26% for HT and 6%for PILC, while about 40% for clays. This evidences thatthe LP and PILC materials are still mainly at the lab-scaledevelopment stage, while HT and especially clays find abroad range of application.

In terms of topics, limiting the analysis to the more recentyears, clays are mainly used for acid-catalyzed reactions,although for a quite broad range of reactions: ethylation,alkylation, isomerisation, esterification, hydrodealkylation,hydro-dehydrogenation, ring opening, etc. A second mainarea of use is for polymerization and as polymer additives.An emerging area of interest relates to the alcohol (metha-nol, ethanol) to olefins reaction, while there is still largeinterest as additives for fluid catalytic cracking (FCC) cata-lysts. Other applications for which clays were patented

G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15 5

include NOx abatement, Fischer–Tropsch reaction, crack-ing of waste plastics, selective oxidation, and synthesis offine chemicals.

For HT, the main area of patents regarded their use infine chemicals productions, although for a large varietyof reactions. HT is also quite used for polymerizationand as polymer or cracking catalysts additives, as supportfor dehydrogenation and for the reduction of sulphur ingasoline. Other applications include NOx removal,Fischer–Tropsh, partial oxidation of methane, water gasshift, H2 production and a large variety of other uses in cat-alytic reactions. There are thus various similarities in thefield of application of clays and HT as catalysts, but inboth cases there is a broad field of use, and a dominantapplication is absent.

PILC were also patented for a broad range of applica-tions, from NOx abatement to alkylation, photoFenton,ammoxidation, synthesis of fine chemicals, oligomeriza-tion, upgrading of lubrificant oils and hydroconversion.Interesting, in recent years (2000–2006), very few patentsregarded their use in acid-catalyzed reactions (the startingapplications for which they were developed, as mentionedabove), while clays find wider use in acid-catalyzed reac-tions, essentially for their quite low cost and performancesnot so different from PILC.

Clays or clay-modified catalysts (in particular acid-treated clays) are commercially used catalysts [51–53].Acid-treated montmorillonites are offered from variouscompanies as catalysts for hydrocarbon cracking. Kaoli-nites are quite good catalysts for Diels–Alders reactions.They are also excellent supports for Lewis acids (ZnCl2,in particular) or for transition metals (copper and iron, inparticular), to be applied in various organic reactions

Fig. 1. Use of functionalized clays in the synthesis of fine chemicals: reaction scof a 3,4-dihydropyrimidin-2(1H) derivate over 12% ZnCl2/clay catalyst (Envir

(Friedel–Crafts, acylation and alkylation of aromatics,etc.). Depending on the metal, the Bronsted versus Lewisacidity can be tuned, allowing obtaining a family of cata-lysts as for example, Envirocats [54–56].

Reported in Fig. 1 is an example of reaction catalyzedfrom Envirocat EPZ10, a 12% ZnCl2/clay catalyst. Thespecific example is the Biginelli reaction which is used forthe synthesis of a 3,4-dihydropyrimidin-2(1H) derivate, avaluable chemical with antitumoral, antiviral and antihy-pertensive activity. Using this catalyst it is possible to selec-tively synthesize this chemical in a one step reactionbetween an aldehyde, urea and ethyl acetoacetate [56].The example is interesting because illustrates the conceptof how using functionalized clays it is possible to obtainvery interesting performances in the synthesis of finechemicals.

As commented in more detail also later, notwithstand-ing the broad field of use suggested from open and patentliterature, the application of these layered-type catalystsappear to be more promising in the field of the synthesisof fine chemicals, where the potential of making complexreactions in one-pot (cascade reactions) can be bestexploited.

Another interesting example is shown in Fig. 2. Usually,it is very difficult to make cascade reactions, which needboth strong acid and base Bronsted sites, because they tendto self-neutralize. Kaneda et al. [57] used an interestingapproach to solve the problem. They localized strong acidsites inside the layers of montmorillonite clay by insertingTi4+ ions, and used as base Bronsted sites those presenton the external surface of the hydrotalcite particles.Because the Ti(IV)–montmorillonite has active acid sitesin the narrow interlayers, the base sites of large HT

heme and pictorial representation of the Biginelli reaction for the synthesisocat EPZ10). Graph elaborated from the results of Lee and Ko [56].

Ph

O

OPh O

acid catalysis

H2O

Ti4+-montmor.

base catal.

hydrotalcite

NC CN

Ph

CN

CN

93% yield

Fig. 2. Cascade reactions requiring both strong acid and base Bronsted sites: reaction scheme and pictorial representation of tandem deprotection–aldoholreaction with acids and base different steps using a combination of using Ti4+-montomorillonite (Ti4+-mont) and hydrotalcite (HT). Graph elaboratedfrom the results of Kaneda et al. [57].

6 G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15

particles show no interaction with the acid sites. The reac-tion shown in Fig. 2 is an example of tandem deprotection–aldohol reaction with acids and base different steps [57].The overall yield, which was observed using Ti4+-monto-morillonite (Ti4+-mont) and hydrotalcite (HT) was 93%.Using the single catalysts alone or in combination with sol-uble acid or bases, much worse performances wereobtained. Kaneda et al. [57] reported in addition a varietyof acid and base reactions, such as esterification, acetaliza-tion, deacetalization, aldol reaction, Michael reaction, andepoxidation, proceeding using both the Ti4+-mont and theHT in a single reactor.

They also showed the possibility to perform selectivelyvery complex cascade reactions, such as that shown inScheme 1. Using a combination of Ti4+-mont, HT andPd supported on HT (Pd/HT) it was possible in a single

NC

COOH

MeOH

Ti-mont.

NC

COOMeHT

HOOC CN

P

Ph

Pd/HT

CN

Ph

CN

COOMe

CN

88% overall yield

Scheme 1. One-pot synthesis of epoxynitrile in a four sequential acid and bahydrotalcite (HT) and Pd supported on HT (Pd/HT): (i) esterification; (ii) dea

one-pot process (in one reactor) to synthesize epoxynitrilewith 91% overall yield in a four sequential acid and basereactions: (i) esterification; (ii) deacetalization; (iii) aldoholreaction and (iv) epoxidation. 2-Carboximethoxy-2-benzyl-glutaronitrile can be selectively synthetized (88% overallyield) also in a four sequential reaction in which the firsttwo steps were similar to the former, but the unsaturatednitrile is then first reduced with H2 (on supported Pd par-ticles) followed by Michael reaction with acrylonitrile atthe base sites of the Pd/HT.

3. Layered perovskite catalysts

The interest on layered perovskite-type catalysts isgrowing for their performances in the photochemical split-ting of water [58–75], but they show interesting behaviour

Pd/HTH2

h

CN

COOMe

CN

COOMe

HT

H2O2

91% overall yield

Ph

COOMe

CN

O

se reactions using a combination of Ti4+-montomorillonite (Ti4+-mont),cetalization; (iii) aldohol reaction and (iv) epoxidation.

G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15 7

also in the combined removal of particulate and NOx [76],low temperature oxidation of CO [77] and methane oxida-tive coupling [78,79].

One method for preparing these layered perovskites isbased on the conversion by acid treatment of an Aurivilliusphase, Bi2SrTa2O9 for example, which consists of perov-skite-like slabs and bismuth oxide sheets [80,81]. High-res-olution electron microscopy observations showed that thestructure of the perovskite-like slabs is retained after theacid treatment, but part of the bismuth oxide sheets areselectively leached and the protons are introduced intothe interlayer space to form a protonated layered perov-skite, H1.8[Sr0.8Bi0.2Ta2O7] [81]. Interesting, the Aurivilliusphase is not active for photocatalytic H2 evolution, butbecomes active when transformed to the correspondingprotonated form [82].

The interest of these materials for the photochemicalsplitting of water derives from the possibility to use theirinterlayer space for reaction sites, where electron–holerecombination processes could be retarded by physical sep-aration of electron and hole pairs generated by photo-absorption. The first studies on this class of materials werefocused on layered niobate [58–60], which are characterizedby the presence of slabs with a perovskite-type structurethat are separated by planes with additional oxygen. Laterlayered titanate [61] and tantalate [63,64] were alsodeveloped. Layered titanate (Sr3Ti2O7, for example) andtantalite (H2La2/3Ta2O7, for example) belong to theRuddlesden–Popper phases which are layered perovskiteshaving as general formula AO(ABO3)n. The structure ischaracterized by corners sharing BO6 octahedra layers,with A atoms occupying the 9 and 12 coordinate interstitialsites. Usually, NiO is also added as co-catalyst [67].

The characteristics of the layer determine the perfor-mances. Shimizu et al. [69] indicate that the availabilityof interlayer space of layered tantalate as reaction site isan important factor to improve the photocatalytic activityof Ta-based semiconductor materials. The same authorsalso observed [72] the activity in water photosplittingresults from the hydrated layered structure, where thephotogenerated electrons and holes can be effectively trans-ferred to the intercalated substrates (H2O).

Lee et al. [62,67,83,84] found that (110) layered perov-skite materials, a series of homologous structures with ageneric composition of AmBmO3m+2 (m = 4, 5; A = Ca,Sr, La; B = Nb, Ti), exhibited higher water-splitting activ-ities under UV irradiation than (100) layered perovskitematerials. They suggested that the photocatalytic activityin water splitting of these layered perovskites was depen-dent on their electronic band structure, which in turndepend on the structural characteristics. In order to betterunderstand which factors determine the efficiency in watersplitting, Lee et al. [73] have compared the behaviour of aseries of perovskites in the water splitting at room temper-ature under UV irradiation (medium-pressure Hg lamp).They found that in general layered perovskites such asSr2Nb2O7, Sr2Ta2O7 and La2Ti2O7 show much higher

activity in water splitting than analogous bulk-type per-ovskites (SrTiO3, LaTiO3), but layered perovskites suchas Sr3Ti2O7 could also be quite inactive. In fact, an addi-tional requirement is the presence of a hyper-valency inthe electronic structure of perovskite slab. Leet at al. [84]also reported the effect of substitution of Zr for Ti in KLa-TiO4 with a layered perovskite structure on the photocata-lytic decomposition of water under the UV lightirradiation. Both the optical properties and the crystallinityof KLaTiO4 were varied by the substitution of Zr for Ti.As Zr content was increased, the crystallinity of KLaZrx-

Ti1 � xO4 was increased, which had a positive effect onthe photocatalytic activity in the water splitting reaction.However, the direct band gap property was lost graduallywith increase of Zr content, which resulted in lowering ofthe photocatalytic activity. As a result, the highest activitywas obtained for KLaZr0.3Ti0.7O4, but the addition of NiOaround doubles the activity [84].

In a semiconductor–liquid interface, the photogeneratedelectron–hole pairs are separated by the electric field pres-ent in the depletion layer. Since the thickness of this layer isinversely proportional to the electron density, the highlydonor-doped perovskite materials would create a narrowerdepletion layer than perovskites of the normal valency.This depletion layer is a function of both the presence ofcation non-stoichiometry and of a layered structure, whichare thus both required to have active photocatalysts. Theslab thickness of (110) layered perovskites had instead aminor role on water-splitting activity, while other impor-tant factors are the amount of loaded nickel oxide andthe doping with alkaline earth components (Ba, Sr, andCa) [73]. In conclusion, the presence of a layered structureand the interlayer characteristics are important factors forthe photocatalytic activity in water splitting, but still therelationships between photocatalytic behaviour and lay-ered microstructure in these materials need to be furtherinvestigated.

Worth noting is that by creating Ohmic layered nano-composite, e.g. n-type WO3 overlayer on W layer depositedon PbBi2Nb1.9Ti0.1O9 p-type perovskite an unprecedentedhigh activity for the photocatalytic oxidation of water,photocurrent generation, and acetaldehyde decompositionunder visible light irradiation (k P 420 nm) was observed[85]. Kim et al. [86] also reported that nanoislands of p-typeCaFe2O4 interfacing with highly crystalline layered perov-skite based lattice PbBi2Nb1.9W0.1O9 are photocatalyticnanodiodes showing high stability and high activity fordegradation of toxic pollutants, oxidation of H2O, andphotocurrent generation, all under visible light. Thispointed out the potential large interest of layered perovsk-ites photoactive materials.

A recent active direction of research on this class ofmaterials regards the possibility of improving the photo-behaviour under visible light irradiation. Kim et al. [74]showed that nearly complete substitution of lead into lay-ered perovskites is an effective method of visible lightsensitization for these materials. They showed that CaBi4-

8 G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15

Ti4O15 (Aurivillius phase), K0.5La0.5Ca1.5Nb3O10 (Dion–Jacobson phase), and Sr3Ti2O7 (Ruddlesden–Popperphase) are photocatalysts, all working under UV light,but their analogous containing lead (PbBi4Ti4O15,K0.5La0.25Bi0.25Ca0.75 Pb0.75Nb3O10, and PbTiO3) absorbvisible light and exhibit photocatalytic activity of decom-position of water under visible light, a very important goal.A different approach to promote visible light response is bynitrogen doping [70]. Nitrogen doping in Sr2Nb2O7 redshifted the light absorption edge into the visible light rangeand induced visible light photocatalytic activity. There isan optimum amount of nitrogen doping that showed themaximum rate of hydrogen production. Among the poten-tial variables that might cause this activity variation, thecrystal structure appeared to be the most important. Thus,as the extent of N-doping increased, the original ortho-rhombic structure of the layered perovskite was trans-formed into an unlayered cubic oxynitride structure. Themost active catalytic phase was an intermediate phase stillmaintaining the original layered perovskite structure, butwith a part of its oxygen replaced by nitrogen and oxygenvacancy to adjust the charge difference between oxygen anddoped nitrogen. In agreement, hydrous layered perovskiteoxynitride, K2LaTa2O6N Æ xH2O, was recently found toshow high photocatalytic performance in water splitting[87].

In conclusion, layered perovskites are an interestingclass of materials which photocatalytic behaviour andprobably more in general their catalytic performancesdepend on the presence and characteristics of the layeredstructure. However, further studies are needed to improveunderstanding of this relationship. There is in general alarge interest on light-to-chemical energy conversion inlamellar solids [88].

To note that reliable literature data on the comparisonin water splitting behavior between layered perovskitematerials and titania or other semiconductor materialsare few or nearly absent, at least in terms of comparablereaction conditions and absence of sacrificial agents. Inaddition, it is not possible to speak generically of TiO2,because depending on the preparation, nanostructure, dop-ing, etc., completely different results could be obtainedespecially regarding water splitting properties. It is thusnot possible to have conclusive indications that layeredperovskite are better than oxides like Titania, or the con-trary. We believe that both layered perovskites and nano-structured Titania or other semiconductors such as SiCshould be considered interesting directions of research forwater splitting at the present stage of development, but thatan in-depth comparison of their relative performanceswould be necessary.

Layered perovskite, particularly after their modificationby pillaring, may be used also for other type of catalyticapplications. However, still few attempts have been madein this direction. Matsuda et al. [89] have synthesized pil-lared layered perovskite oxides, HLaNb2O7, having SiO2,TiO2 and ZrO2 in the interlayer. The pillared catalysts with

SiO2 and TiO2 exhibited high activity for the dehydrationof methanol and 1-butanol, but the catalyst with ZrO2

did not exhibit high activity.Shimizu et al. [90] have investigated the synthesis of

Ruddlesden–Popper-type layered perovskite tantalates,H2ATa2O7 (A = Sr or La) which have been pillared withn-alkylamines and oxide nanoparticles (Fe2O3 or Fe–Simixed oxide). The pillaring increases the number of acidsites in perovskites, but the acidic property were tested onlyby temperature programmed desorption (TPD) of NH3.No indication was given regarding the catalytic behaviour,although the potential application was suggested.

Various authors have instead investigated the synthesisof pillared layered perovskites for the application asadvanced materials. Chi et al. [91] have investigated thesynthesis and structure of pillared perovskites La5Re3MO16

(M = Mg, Fe, Co, Ni) in relation to their magnetic proper-ties. Chi et al. [91] have observed by neutron diffraction amagnetic structure for the Fe series member of these pil-lared perovskites. This structure consisting of ferrimagneticperovskite layers coupled antiparallel along the stacking c-axis. This suggests a peculiar structure in these pillaredoxides, which could have interesting catalytic reactivity.Hong and Kim [92] have studied a layered perovskite oxide,RbLa2Ti2NbO10, regarding proton exchange and intercala-tion behavior. The protonated form, HLa2Ti2NbO10,exhibits interesting the Bronsted acidity. Polyoxonuclearcation, Al13, may be introduced into the interlayer by reflux-ing octylamine-intercalated compounds with an Al13 pillar-ing solution. The polyoxonuclear cation-pillared materialexhibits a bilayer structure and is thermally stable.

These two examples show that layered perovskites offera large variety of possible modifications and tuning of theproperties, which make them interesting and suitable forcatalytic applications, even if still limited research attentionis given on this topic, apart from the cited activities in thefield of photocatalytic water splitting.

4. Pillared clays (PILCS) as catalysts

Several reviews have discussed the characteristics of pil-lared clays (PILC) and their use as catalysts or catalyticmaterials [12–14,16,18,31,33]. The general conclusionwhich may be derived from these review is that pillaredclays are very interesting and promising catalysts in abroad range of application, for example in acid-catalyzedreaction [14], in the synthesis of bulk and fine chemicals(a broad range of reactions such as dehydrogenation,hydroxylation, disproportionation, esterification, epoxida-tion, alkylation, isomerisation, Fischer–Tropsch, methanereforming, hydrogenation, aromatization, etc. [12,31]),and for the reduction of pollutants (selective catalyticreduction of NOx, and catalytic removal of organic volatilecompounds [16,18]). Generally, it is also claimed that thepillaring procedure significantly improves the perfor-mances with respect to the starting clays. However, thisconclusion is in apparent contrast with the analysis of

G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15 9

patent literature shortly summarized before which evi-dences the much more limited number of patents on pil-lared clays (as catalysts) with respect to the clays itself. Inaddition, PILC-based materials are used as commerciallycatalysts in few cases and it is typically preferred to useclays and not pillared clays as additives for commercialcracking catalysts, as mentioned before. The question isthus whether and when the pillaring procedure introducesnew specific catalytic functionalities and/or allows improv-ing catalyst characteristics (higher thermal stability, forexample) which justify the higher cost of their preparation.

Therefore, notwithstanding the various reviews on thetopic, it is still useful critically analyze some specific exam-ple to discuss this question, which was typically not consid-ered enough in depth previously. The first example regardsthe use of TiO2 pillared clays, often indicated with the acro-nym Ti-PILC. One of the first motivations to develop thesematerials was for the possibility to obtain photocatalystswith improved photo-chemical and -physical properties[93]. Various other research groups have then investigatedthese materials [89–100] which were applied also for otherreactions such as the selective catalytic reduction of NOx(in particular after addition of vanadium or copper ions),oxidation of aniline [100], alkylation of aromatics [99], cat-alytic hydrotreating of heavy vacuum gas oil, acylation ofaromatic compounds, besides to photocatalysis.

Yoneyama et al. [101] reported a greater activity of Ti-PILC in comparison to powder TiO2 in photodecomposi-tion of 2-propanol and n-carboxylic acids. Cheng [13] alsofind that the rate per g TiO2 in photo-mineralization ofphenyl pollutants is higher for Ti-PILC than for TiO2 (bothin anatase and rutile forms), when hydrophobic pollutantsare present (Table 1). However, the reverse behaviour wasobserved for hydrophilic pollutants, as shown from thecomparison of the degradation rates of phenol (hydro-philic) and benzene (hydrophobic) in Table 1. Cheng [13]concluded that the pore structure of PILC facilitates thephoto-degradation by adsorbing favourably the hydropho-bic pollutant molecules onto the surface of silicate, wherethe active TiO2 microcrystallites were adjacent.

From this observation, it is possible to conclude that thenanoarchitecture of Ti-PILC, where Titania pillars ofabout 0.8–1.5 nm height are present between the silicatelayers, is important to develop improved photo-catalysts.However, one may argue that in these photocatalytic pro-cesses the behaviour is determined not from the rate of

Table 1Rate of CO2formation in the photo-degradation of phenyl pollutants inaqueous solutions over TiO2 and Ti-PILC [13]

Pollutant Rate of minerization, mol/min g TiO2

Ti-PILC TiO2 anatase TiO2 rutile

Benzene 4.49 1.90 2.53Chlorobenzene 5.30 3.17 3.661,2-Dichlorobenzene 6.36 3.49 4.80Phenol 2.22 7.08 2.50

adsorption of the organics, but from the rate of recombina-tion of photogenerated electron and holes and on thequenching of the surface photogenerated processesthe adsorbing organics. These processes could depend onthe nano-architecture of the photocatalyst, but in a muchmore complex relationship with respect to what suggested.In addition, the nanosize of titania in the pillars causes ablue shift with respect to anatase TiO2 due to quantumconfinement, e.g. in the opposite direction of going to vis-ible region, the aimed direction to use solar energy. Fromthe application point of view, it should be also remarkedthat it is not relevant the specific activity per g of TiO2,but the overall activity per g of photocatalyst. Further-more, one should be aware that referring generic to TiO2

is not correct, because quite different performances couldbe obtained, depending on the type of pollutant, and typeof Titania (commercial source and/or the preparationmethod). Therefore, from the application point of view bet-ter activities in the photodegradation of these pollutantscould be obtained and may not be supported the claim thatthe nanostructure of these materials allows to improve thephotocatalytic behaviour.

A second example regards the use of PILC in the pro-duction of biofuels. The increasing crude oil prices andenvironmental awareness have increased interest in theupgrading of vegetable oils, including vegetable oil wastes,for use as a fuel or fuel additive. There are several methodsfor the conversion of vegetable oils to biodiesel. The mostcommon is the transesterification process, in which an alco-hol is reacted with the oil to form esters and glycerol in abase catalyzed reaction. However, this process shows sev-eral limits when applied to animal fats or used vegetableoils. Another method for the conversion of vegetable oilsto a useable fuel product is by catalytic cracking reactions.Pillared clays could be potentially interesting in this reac-tion with respect to typical zeolite-based catalysts for thecatalytic cracking of oil, due to their larger pores and usu-ally less severe reaction conditions with respect to fluid cat-alytic cracking (FCC). Kloprogge et al. [102] have recentlycritically discussed the problem of the application of pil-lared clays and related porous materials for cracking ofvegetable oils (Canola, Palm and Sunflowers oils) to pro-duce biofuels, although mainly from the reaction mecha-nism point of view.

With respect to the cracking of oil fractions, the conver-sion of vegetable oils has the additional complexity of theneed of performing also a deoxygenation of the molecules.Therefore, in principle different catalysts from typical FCCcatalysts could be necessary. However, Katikaneni et al.[103] by comparing the performances of various catalysts,including Al-PILC, in the conversion of canola oil to fuelto organic liquid product (OLP) and coke formation,found that HZSM-5 gave the highest yield of OLP(63 wt.%) while the pillared clay gives only 55 wt.% yield.The OLP of the PILC contained more aliphatic hydrocar-bons than the other catalysts and the least amount ofaromatic hydrocarbons. Their results showed that as the

10 G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15

pore size of the catalyst was increased, the conversion ofcanola oil, the coke formation and the selectivity for ali-phatics increased while the yield of hydrocarbons and theselectivity for aromatics decreased. This indicates that med-ium pore catalysts, and not PILC type materials, have theoptimal balance between high reaction rates of crackingand deoxygenation ad a minimum rate of coke formation.However, PILC offer the potential advantages of a multi-functionality that very probably is the key to optimizethe behaviour in these quite complex reactions. Thisrequires first to better understand the reaction mechanismand the type of catalytic functionalities which are needed,and then to design appropriate catalysts. Layered materials(or their combination) have the potential to allow thisadvanced design, but still no serious attempts have beenmade in this direction. PILC materials are thus potentiallyinteresting catalysts in this application, which may be a rel-evant component for future biorefineries, but still theirapplicability has to be explored.

A third example regards the use of PILC as newadvanced catalysts for NOx conversion. Various reviewon pillared clays indicate this as one of the more promisingarea of use of these catalysts [12,14,16–18,31,104] and sev-eral publications and some patents were published recentlyon the topic [105–114]. Examples are the use of (i) Cu-ion-exchanged iron-pillared interlayer clays (Fe-PILCs) for theselective catalytic reduction (SCR) of NO by propene [105];(ii) Rh-based aluminium pillared clays for the SCR of NOxby methane in excess oxygen [107a]; (iii) Pt-doped Cu-loaded ZrPILC for the SCR of propylene and decane[106a]; (iv) pillared Fe-saponite for the reduction of NOxwith propene in the absence of gaseous O2 [108]; (v) Pdsupported on Ti-PILC for the catalytic reduction of NOxwith hydrogen and carbon monoxide in the presence ofexcess oxygen [109]; (vi) Cu-exchanged Ti-PILC for thereduction of NOx with propene [112a], and (vii) Fe3+–and other transition metal ions exchanged Al- or Ti-PILCsfor the SCR of NO by ammonia in the presence of excessoxygen [114].

Patents also were issues recently on the use of PILCs assuperior catalysts for the abatement of nitrogen oxidesfrom power plant emissions [110,111]. Let us thus shortlyanalyze these patented results and particularly the last ones[110]. The claim relates to a vanadium-oxide impregnatedTi-PILC catalyst for removing nitrogen oxides (NOx) byusing ammonia (NH3) as a reducing agent. Specific claimis the superior tolerance against the various deactivationfactors, such as poisoning by SO2 and arsenic, and mechan-ical stability. It is also claimed that the catalyst has anoperational temperature window shifted to the lower reac-tion temperature range, allowing an improved energy econ-omy, lower size of the reactor and more economical andefficient operation of the SCR process.

As discussed more in detail elsewhere [115], currentcommercial V–W-oxide on TiO2 based catalysts for theSCR of NOx with NH3/O2 from power plants have typi-cally life time of over 10 years, and therefore deactivation

do not appear a so strong incentive to change a catalysttechnology which development on an industrial scalerequires long and costly experimentation. The deactivationby SO2 is essentially related to the chemical composition ofthe V2O5–WO3/TiO2 catalyst, and its doping. Some of thecurrent commercial catalyst may operate in the presence ofquite large SO2 concentrations in the flue gas. Dependingon the chemical composition of the V2O5–WO3/TiO2 thetemperature windows can be tuned in the 150–400 �C tem-perature range.

Looking more closely to the results reported in the pat-ent by Long and Yang [110], their results were comparedwith those of a reference commercial V2O5–WO3/TiO2.As mentioned above, do not exists one commercial cata-lyst, but it is a family of catalysts which specific composi-tion may be tuned depending whether the objective is toreduce the rate of SO2 oxidation (real objective, more thanthe resistance to deactivation by SO2 which is not a keyissue), a lower temperature windows, etc. Long and Yang[110] claims that using V2O5/Ti-PILC it is possible toobtain >98% conversion of NO (at a space-velocity of100,000 h�1, 500 ppm of NO and NH3, 0% of water, and5% of oxygen) at 300�C, while about 350–400 �C arerequired for the reference catalyst. However, as pointedout above, >98% conversion of NO in these conditionscould be easy reached also on V2O5–WO3/TiO2 catalysts,but the key industrial issue is the ratio between rate ofNOx and SO2 conversion, not the activity itself. Resistanceto SO2 was evaluated by injection into the reactor at thetemperature of 250�C of 5,000 ppm SO2, an unrealistichigh concentration. In these conditions, the main effect isthe deposition onto the catalyst surface of ammonium sul-phate, which deactivation by fouling. A higher surface areacatalyst such as V2O5/Ti-PILC in comparison with V2O5–WO3/TiO2 catalyst has an apparent lower deactivation rate(in short-term experiments), but this has no relation withthe effective long-term stability of these catalysts.

Several of the remarks made for this specific examplemay be extended to other cases of using pillared clays forthe selective conversion of NOx. Although may be notexcluded that in some specific cases, pillared clays couldbe preferable catalysts for the abatement of NOx, wemay suggest that up to now there is still not clear demon-strations of real advantages of using these materials asimproved catalysts for NOx abatement, in contrast withthat often claimed in the literature.

There are instead cases where pillared clays show clearadvantages with respect to other classes of catalysts. Oneexample is for the wet oxidation of waste in water usingH2O2 as oxidant (wet hydrogen peroxide catalytic oxida-tion, WHPCO [116]). WHPCO is an interesting catalyticmethod for the elimination of organic compounds in water.The main advantages in comparison, for example, to wetair (catalytic) oxidation (WACO) are that it avoids theuse of costly reactors, can be selective towards the conver-sion of specific substrates and is easy manageable. H2O2 isthe precursor for generating hydroxyl radicals which are

Temperature, °C

200 400 600 800 1000

ml H

2m

in-1

2.18

2.20

2.22

2.24

2.26 Cu-PILC (pillaring)

Cu-ZSM-5

Cu-Al2O3

538°C

336°C374°C221°C

451°C

Fig. 3. Temperature programmed reduction curves in H2 flow for 2 wt.%copper samples: (a) Cu2+ supported on alumina; (b) copper ion-exchangedH-ZSM5 and (c) Cu-PILC prepared by introducing copper ions in thepillars [128].

G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15 11

the effective and highly active oxidizing species. This reac-tion involves the so-called Fenton mechanism, where Fe2+

or Cu+ reacts with H2O2 to generate hydroxyl radicals.Due to the presence of a reaction cycle Fe3+ and Cu2+ ionscould be also used (Fenton-like mechanism). However,there are two main side reactions, which compete with thiscatalytic cycle: (i) the oxidation of the reduced transitionmetal ions by oxygen dissolved in water and (ii) the com-plexation of the transition metal ions by some of the inter-mediate oxidation reaction products, such as oxalic acid[117]. The use of solid Fenton-type catalysts instead ofhomogeneous salts in solution could limit these drawbacks,in addition to other advantages such as an easier recover, alower contamination of the effluents with metals, whichnegatively influence the consecutive biological treatmentstep, and a wider range of pH operations [117].

The use of different solid Fenton-type catalysts has beenproposed: (i) transition metal exchanged zeolites [117–119],Cu2+ containing montmorillonites [120,121], and Fe3+ orCu2+ containing pillared clays [122–128] have been pro-posed as active catalysts for the oxidation of differentorganic compounds. Disordered pillared clays, preparedby freeze-drying method, were also earlier employed as cat-alysts in these processes with interesting results, althoughnot enough stability/activity [129].

Me-PILC catalysts are the only one, which showstogether with a good rate of conversion of the pollutants,negligible leaching of the cations, keeping their activityduring successive runs. This is a very relevant aspect,because not only allow operating the process continuously,but also avoid contamination of the water with transitionmetals which abatement is difficult.

The transition metal in Fe-PILC is already present inpart in the origin clay material, both as isolated specieson the clay layer and as oxide clusters, but after the specificpillaring procedure making it active for the WHPCO reac-tion an additional isolated species was detected, probablylocated on the pillars [123]. The latter species catalyzesmore efficiently the degradation of organic waste byH2O2 and results stable to leaching even after long timeexperiments, a result confirmed by various authors.

Also for Cu-PILC, characterization data pointed outthat sites with specific features form when introduced inthe clay pillars [130]. Their reactivity is also differentfrom that of copper ions localized in different positions.This is demonstrated, for example, from the resultsreported in Fig. 3, where the temperature programmedreduction curves for 2% wt. copper samples are compared:(a) Cu2+ supported on alumina (110 m2/g); (b) copperintroduced in the cationic positions of a H-ZSM5 sample(SiO2/Al2O3 ratio of 27) by ion-exchange and (c) Cu-PILCprepared by introducing copper ions in the pillars [128].The selective introduction of copper ions in the pillars con-siderably reduce their reducibility by H2, even though theystill maintain a redox character, as shown from the factthat Cu-PILC show equal or even higher activity in theFenton-type redox mechanism than Cu/Al2O3 and

Cu-ZSM-5. The latter two show, in addition, progressiveleaching of copper in solution during the catalytic WHPCOtests, differently from Cu-PILC [128].

This result shows that the introduction of transitionmetal ions in the pillars of the clay creates specific andstable active sites, which show interesting catalytic proper-ties also in other applications: gas phase synthesis ofmethyl ter-buthyl ether (MTBE) [131], ethylbenzene dehy-drogenation to styrene [132] or nucleophilic ring opening ofoxiranes with aniline [133].

5. Hydrotalcite (HT) based catalysts

Hydrotalcite-like anionic clays (HTs, or layered doublehydroxides, LDHs) are another well-known class oflayered materials with large use in catalysis [15,17,19,24,134–138] although often they are used as precursor forthe catalysts more than as layered material itself. Forexample, in various processes for the use or manufactureof syn gases, several commercial catalysts derive fromhydrotalcite materials: in the partial oxidation or steamreforming of hydrocarbon feed materials, in the methanolor higher alcohol synthesis, in the methanation reactionand in the Fischer–Tropsch synthesis [15,138]. Similarly,several relevant processes use catalysts derived from HT,such as for the reduction of maleic anhydride to c-butirro-lactone, the gas phase carbonilation of methanol or thesynthesis of fine chemicals. However, in these catalysts itis not specifically used the layered structure of the anionicclay, but other features of these materials, such as the pos-sibility of obtaining well homogeneous catalysts, the non-stoichiometry of the mixed oxides and sometimes the mem-ory effect.

They represent also an interesting opportunity for heter-ogeneizing homogeneous catalysts [15,19]. HTs are excel-lent materials to design bifunctional redox-base catalystsor to control acid–base properties around a heterogeneized

12 G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15

metal complex. Applications, although often only poten-tial, range from large-scale base chemicals to the synthesisof small-scale specialty chemicals. Nevertheless, LDHsprecursors have been also successfully used as catalysts inrelevant fine chemical productions (pharmaceutical, fra-grances, foods, etc.).

For conciseness, we will discuss here only selected exam-ples chosen because they represent some of the more prom-ising direction of research for these layered materials. Thefirst concept is the use of multifunctionality of these layeredmaterials to develop complex cascade reactions of interestfor the synthesis of fine chemicals. An interesting examplewas published recently by Kaneda et al. [139] and is picto-rially represented in Fig. 4. Using monomeric RuIV–OHspecies added by grafting on the HT surface (Ru/HT), theywere able to make selective one-pot syntheses as thatshown in Fig. 4 (synthesis of R,R-dialkylated phenylaceto-nitriles). The yield of the final product depends on the nat-ure of the Z substituent. In the case of Z = CN, the totalyield to 2-ethyl-2-phenylglutarodinitrile, a highly usefulintermediate of sedative Glutethimide, was 93% [139], withrespect to conventional synthesis which gives a total yieldof 39%. For this single pot synthesis, the Ru species andbase sites on the Ru/HT surface participate in four sequen-tial reactions: oxidative dehydrogenation, aldol condensa-tion, hydrogenation, and Michael reaction to finallyproduce R,R-dialkylated phenylacetonitriles. This resultfurther shows the concept already presented before of thehuge potential of using this type of functionalized hydrotal-cite materials to perform selectively complex cascade reac-

R2 OH

[Ru]

[Ru]-HR2 O

base sites

R1 CN H2OR1

Ru

Fig. 4. Cascade reactions over functionalized hydrotalcite containing monomsynthesis of R,R-dialkylated phenylacetonitriles. Graph elaborated from the r

tions. Further interesting examples of cascade reactionswere presented by Kaneda et al. [24,57,135,140].

A further interesting example of the possibility of usingmultifunctionality of hydrotalcite-based catalysts wasgiven by de Jong et al. [141] in the synthesis of the basechemical methyl isobuthyl ketone (MIBK). Using carbonnanofibers on which both Pd particles and hydrotalcitesplatelets were deposited [142], it is possible to selectivelyperform the self-condensation of acetone to diacetone alco-hol, which is further dehydrated to mesityl oxide and thenhydrogenated to MIBK in one-step. Supporting the hydro-talcite over the carbon nanofibers allows producing highlyactive base catalysts [142], which are of general importancefor organic synthesis and fine chemicals manufacture. deJong et al. [142] showed that Mg–Al hydrotalcite plateletswith a lateral size of 20 nm can be produced by depositionon carbon nanofibers and the resulting supported catalystexhibited a specific activity in the condensation of acetonewith citral four times that of unsupported hydrotalcites dueto the higher number of active edge sites. They also showed[142b] that activation of Mg–Al hydrotalcites via thermaltreatment followed by rehydration resulted in irregularlystacked platelets showing high activity as base catalystsdue to the distorted edge structure of the platelets. Sup-porting HT on carbon fibers offers thus the possibilitynot only to realize multifunctional catalysts, as shownabove by introducing Pd particles, which are mainly local-ized on the carbon nanofibres, but also to induce distortionin the layered structure of HT leading to an enhancementof its intrinsic activity as base catalyst.

CN

R2 [Ru]-H

[Ru]

base sites

R1 CN

R2

Z

Z

R2

CNR1

eric RuIV–OH species: reaction scheme and pictorial representation of theesults of Kaneda et al. [139].

G. Centi, S. Perathoner / Microporous and Mesoporous Materials 107 (2008) 3–15 13

HT also offer huge potential both as catalyst or todevelop advanced materials by intercalating metal com-plexes, low or medium-nuclearity oxometalates such aschromate or vanadate, respectively, or high-nuclearity oxo-metalates such as iso- and hetero-polyoxometalates such asKeggin units [XM12O40]n�. Rives and Ulibarri [45] haveextensively review the topic. We limit thus here to cite arecent result which demonstrates the high interest for thesematerials. Corma et al. [143] have recently patented a pro-cess for producing cycloalkanol and/or cycloalkanone byoxidizing cycloalkane with molecular oxygen in the pres-ence of CoII-Keggin heteropolyacid intercalated in HT.At 130�C a cyclohexanone selectivity of about 60% withcyclohexanol selectivity of about 29% and a cyclohexaneconversion of about 5% was observed. The reaction is veryinteresting not only from the application point of view, butalso as an example of the possibility of selective oxidationusing molecular oxygen. Intercalation of the CoII-Kegginoxoanion inside the HT layers was essential to obtain thecited performances. These pillared layered anionic clays(PILACs) are a very promising field of research. Otherrecent interesting examples are peroxo-polyoxometalatespillared hydrotalcite, which showed 91.5% selectivity forpropylene oxide at 47.5% propylene conversion for propyl-ene epoxidation by O2 in methanol at 80 �C [144] and thepreparation of aromatic ketones and aldehydes via oxida-tion of alkylarenes by oxygen on polyoxometallate(s) inter-calated in hydrotalcite [145].

The intercalation of polyoxometalate anions into theinterlayer space could be used also to prepare very interest-ing catalytic materials. For example, Tichit et al. [146]showed that metallic nanoparticles could be introducedbetween brucite layers, generating materials with highersurface area and additional functionality. These supportedmetal particles prepared from LDH nanocomposite precur-sors can be employed as electron transfer materials andmetal-base bifunctional catalysts.

6. Conclusions

Layered materials are a great opportunity for catalysiswith a wide range of uses going from refinery to fine chem-icals and environment protection, especially to developmulti-functional catalysts with tailored properties. How-ever, sometimes their practical relevance was overestimated.

In this short review, we have examined some cases todiscuss recent trends, and limits and opportunities offeredfrom this class of materials. Attention was focused on theuse as catalysts of three main types of materials: layeredperovskite, pillared clays and layered double hydroxides(hydrotalcite). The objective was not to make a systematicreview of the topic, also because several reviews have beenalready published, but instead to offer a personal view ofthe more interesting recent direction of research and atthe same time of the area where there is an increasing num-ber of publications, but which do not parallel the practicalrelevance of the topic.

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