From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis

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<ul><li><p>From Microporous to Mesoporous Molecular Sieve Materials and TheirUse in Catalysis</p><p>Avelino CormaInstituto de Tecnologa Qumica, UPV-CSIC, Universidad Politecnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain</p><p>Received April 7, 1997 (Revised Manuscript Received June 16, 1997)</p><p>ContentsI. Introduction 2373II. Zeolites Containing Mesopores 2374III. Pillared Layered Solids 2376</p><p>A. Pillar-Layered Silicates as Acid Catalysts 23771. Nature of the Acid Sites 23772. Influence of Activation Conditions on</p><p>Acidity2378</p><p>3. Catalytic Activity of Acid PLS 2379B. Pillar Layered Silicates as Redox Catalysis 2383C. New Perspectives for Pillared Materials 2383</p><p>III. Silica-Aluminas with Narrower Pore SizeDistribution</p><p>2385</p><p>IV. Ordered Mesoporous Materials 2386A. Synthesis of Silica M41S Molecular Sieves</p><p>Materials2386</p><p>1. Direct Synthesis 23862. Indirect Synthesis 2390</p><p>B. Synthesis of Mesoporous Molecular SievesContaining Elements Other Than Silica</p><p>2391</p><p>C. Characterization of Mesoporous MolecularSieves</p><p>2395</p><p>D. Catalytic Properties of Mesoporous Materialswith Long-Range Crystallinity</p><p>2400</p><p>1. Acid Catalysis 24002. Base Catalysis 24063. Redox Catalysis 2407</p><p>E. Ordered Mesoporous as Support 24101. Supporting Acids and Bases 24102. Supporting Metals and Oxides 24113. Supporting Active Species for Selective</p><p>Oxidation2412</p><p>V. Conclusions and Perspectives 2413VI. Acknowledgments 2414VII. References 2414</p><p>I. IntroductionIt is possible to say that zeolites are the most</p><p>widely used catalysts in industry. They are crystal-line microporous materials which have become ex-tremely successful as catalysts for oil refining, pet-rochemistry, and organic synthesis in the productionof fine and speciality chemicals, particularly whendealing with molecules having kinetic diametersbelow 10 . The reason for their success in catalysisis related to the following specific features of thesematerials:1 (1) They have very high surface area andadsorption capacity. (2) The adsorption propertiesof the zeolites can be controlled, and they can bevaried from hydrophobic to hydrophilic type materi-</p><p>als. (3) Active sites, such as acid sites for instance,can be generated in the framework and their strengthand concentration can be tailored for a particularapplication. (4) The sizes of their channels andcavities are in the range typical for many moleculesof interest (5-12 ), and the strong electric fields2existing in those micropores together with an elec-tronic confinement of the guest molecules3 are re-sponsible for a preactivation of the reactants. (5)Their intricate channel structure allows the zeolitesto present different types of shape selectivity, i.e.,product, reactant, and transition state, which can beused to direct a given catalytic reaction toward thedesired product avoiding undesired side reactions. (6)All of these properties of zeolites, which are ofparamount importance in catalysis and make themattractive choices for the types of processes listedabove, are ultimately dependent on the thermal andhydrothermal stability of these materials. In the caseof zeolites, they can be activated to produce verystable materials not just resistant to heat and steambut also to chemical attacks.</p><p>Avelino Corma Canos was born in Moncofar, Spain, in 1951. He studiedchemistry at the Universidad de Valencia (19671973) and received hisPh.D. at the Universidad Complutense de Madrid in 1976. He becamedirector of the Instituto de Tecnologa Qumica (UPV-CSIC) at theUniversidad Politecnica de Valencia in 1990. His current research fieldis zeolites as catalysts, covering aspects of synthesis, characterizationand reactivity in acidbase and redox catalysis. A. Corma has writtenabout 250 articles on these subjects in international journals, three books,and a number of reviews and book chapters. He is a member of theEditorial Board of Zeolites, Catalysis Review Science and Engineering,Catalysis Letters, Applied Catalysis, Journal of Molecular Catalysis,Research Trends, CaTTech, and Journal of the Chemical Society,Chemical Communications. A. Corma is coauthor of 20 patents, five ofthem being for commercial applications. He has been awarded with theDupont Award on new materials (1995), and the Spanish National AwardLeonardo Torres Quevedo on Technology Research (1996).</p><p>2373Chem. Rev. 1997, 97, 23732419</p><p>S0009-2665(96)00406-2 CCC: $28.00 1997 American Chemical Society</p></li><li><p>Despite these catalytically desirable properties ofzeolites they become inadequate when reactants withsizes above the dimensions of the pores have to beprocessed. In this case the rational approach toovercome such a limitation would be to maintain theporous structure, which is responsible for the benefitsdescribed above, but to increase their diameter tobring them into the mesoporous region. The strategyused by the scientist to do this was based on the factthat most of the organic templates used to synthesizezeolites affect the gel chemistry and act as void fillersin the growing porous solids. Consequently, attemptswere made that employed larger organic templateswhich would not, as was hoped, hence result in largervoids in the synthesized material. This approach didnot give positive results in the case of zeolites, butin contrast was quite successful when using Al andP or Ga and P as framework elements.4-14 Only veryrecently a 14-member ring (MR) unidirectional zeolite(UTD-1) could be synthesized using a Co organome-tallic complex as the template15,16 (Table 1). Thetemplate can be removed, and the thermal stabilityof the framework of the organometallic-free materialis high, resisting calcination temperatures up to 1000C. The presence of framework tetrahedral Al gener-ates Brnsted acidity which is strong enough to carryout the cracking of paraffins.In a general way, we have summarized in Table 2</p><p>the different strategies directed toward the synthe-size of ultralarge pore zeolites.However, when the zeolite and zeotypes with the</p><p>largest known diameters are considered in the con-text of their possible uses as catalysts the followingcan be said: cacoxenite,17 which is a naturallyoccurring mineral with a 15 pore system, is</p><p>thermally unstable and thus cannot be used as acatalyst. Cloverite, although this structure haspotentially large pores, the diffusion of large mol-ecules is restricted, owing to the unusual shape ofthe pore openings which are altered due to protrudinghydroxyl groups. Likewise in VPI-5, stacking disor-der or deformation of some of the 18-member ringsduring dehydration results in a decrease in the poresize from 12 to about 8 . In the case of the newzeolite UTD-1, the fact that it has to be synthesizedwith an organometallic Co complex, which has thento be destroyed, and the Co left has to be acid leachedraises strong questions concerning its practical ap-plication, which remains in doubt unless a moresuitable template and activation procedure can befound.In conclusion, it can be said that despite the</p><p>outstanding progress made in producing large poremolecular sieves, the materials so far synthesized arestill not suitable to be used in the context of currentcatalytic processes. Largely for this reason aloneanother approach has been undertaken in order toincrease the activity of the existing microporousmaterials for processing large molecules such asthose existing, for instance, in vacuum gas oil andwhich need to be cracked and hydrocracked. Thisapproach involves the generation of mesopores in thecrystallites of the microporous zeolites.</p><p>II. Zeolites Containing MesoporesFollowing the definition accepted by the Interna-</p><p>tional Union of Pure and Applied Chemistry, porousmaterials can be grouped into three classes based ontheir pore diameter (d): microporous, d &lt; 2.0 nm;</p><p>Table 1. The Typical Larger Pore Zeolites/Zeotypes</p><p>material ring sizeyear</p><p>discovered synthesis media</p><p>inorganicframeworkcomposition channels/pores</p><p>cacoxenite 20-TO4 ring naturally occurring Al, Fe, P 14.2 pore diameterzeolites X/Y (FAU) 12-TO4 ring 1950s Al; Si 7 diameter pore</p><p>12 diameter cavity3D channel system</p><p>AlPO4-8 (AET)* 14-TO4 ring 1982 n-dipropylamine template Al, P 1D channel systemVPI-5 (VFI)* 18-TO4 ring 1988 tetrabutylammonium/</p><p>n-dipropylamine templatesAl, P 13 channel diameter</p><p>hexagonal arrangement of 1Dchannel system</p><p>cloverite (CLO)* 20-TO4 ring 1991 (a) quiniclidinium template largest aperture of window is 13 (b) F- rather than OH asmineralizer</p><p>Ga, P 30 cavities</p><p>3-D channel systemJDF-20 20-TO4 ring 1992 (a) triethylamine template Al, P hydroxyl groups protruding</p><p>into channel system(b) glycol solvent</p><p>UTD-1 14-TO4 ring 1996 [(Cp*)2Co]OH Si, Al 1-D channel system7.5 10 </p><p>Table 2. The Major Routes Employed by Zeolite Synthetic Chemists To Increase the Pore Size of MicroporousZeolites and Zeotypes</p><p>method employed toincrease pore size examples structures</p><p>use specific spacing units tobuild the inorganic framework</p><p>addition of further four ring buildingunits to six ring units in porousaluminophosphates</p><p>AlPO4-5 framework further extended toVPI-5 (refs 182, 183, and 188)</p><p>use different oxide systems use two sorts of tetrahedral atomsto yield diffrent T-O bond lengths</p><p>VPI-5 (aluminum and phosphorous)cloverite (gallium and phosphorous)</p><p>use specially designedtemplates</p><p>exploit the specific structure directingeffect of an organic template</p><p>use of quinuclidine to form cloverite</p><p>2374 Chemical Reviews, 1997, Vol. 97, No. 6 Corma</p></li><li><p>mesoporous, 2.0 e d e 50 nm; macroporous, d &gt; 50nm. In the case of zeolites, for example zeolite Y andCSZ-1, it was shown18,19 that during the dealumina-tion of the zeolite by steam mesopores of mixed sizesin the range 10-20 nm were formed which could becharacterized by different techniques including gasadsorption, high-resolution electron microscopy, andanalytical electron microscopy.19 When a large num-ber of defects occur in a small area it can lead tocoalescence of mesopores, with the formation ofchannels and cracks in the crystallite of the zeolite(Figure 1).The presence of the mesopores in the crystallites</p><p>of a given zeolite should basically increase theaccessibility of large molecules to the external open-ing of the pores. In other words, and from the standpoint of large reactant molecules, the presence ofmesopores in the crystallites of the zeolite would beequivalent to increasing the external surface of thezeolite making a larger number of pore openingsaccessible to the reactant. The beneficial effect of thecombination of micro and mesoporous region in thezeolite crystallites was shown by comparing thecracking activity of two series of Y zeolite dealumi-nated by SiCl4 and steam.20 The dealumination bySiCl4 generated little mesoporosity and preservedmost of the microporosity of the zeolite. On the otherhand, the dealumination by steam produced manymore mesoporous areas within the material whilesome of the microporosity was destroyed (Figure 2).When the catalytic activity of the two series ofsamples was compared for cracking a small reactantmolecule (n-heptane) which can easily penetratethrough the pores of the zeolite Y, it was found thatthe samples dealuminated by SiCl4 treatment, andthese, which have a greater microporosity, were more</p><p>active than those dealuminated by steam. However,when the two series of dealuminated Y zeolites wereused to crack a vacuum gas oil, containing moleculestoo large to penetrate deep into the microporoussystem, the steam-dealuminated samples, whichcontain a greater proportion of mesoporosity, gavea higher conversion (Figure 3a,b).Increasing accessibility by producing mesopores</p><p>during the activation of zeolites can have quite aprofound impact in the case of fine and specialitychemical production, in which the performed catalystwill be working at low reaction temperatures andonly moderate regeneration temperatures will benecessary. This is for instance the case for theesterification of fatty acids with alcohols.21 Thus,because of the formation of mesopores by steamtreating zeolite Y during the activation process, themodified catalyst gives good activity and selectivity,with a definitive optimum existing between the totalnumber of acid sites and those accessible to the largemolecules through the generated mesopores.Unfortunately, in processes where the catalyst</p><p>regeneration occurs at high temperatures the meso-porosity of the catalyst changes during the regenera-tion and in cases such as the FCC, this occurs in anuncontrollable way. It appears then that a procedureinvolving the formation of a secondary mesoporoussystem by steaming the microporous solid can onlybe adequate for some special cases and therefore,other more general solutions should be explored.</p><p>Figure 1. Schematic representation of mesopores formedin steamed zeolites.</p><p>Figure 2. Pore volume in the mesoporous region for poresbetween 40 and 120 , of zeolites USY dealuminated bysteam (0) and by SiCl4 (9).</p><p>b</p><p>a</p><p>Figure 3. (a) Cracking of n-heptane on USY dealuminatedby SiCl4 (b), and by steam (O) and (b) cracking of gas oilon USY dealuminated by steam (O), and by SiCl4 (b).</p><p>Molecular Sieve Materials and Their Use in Catalysis Chemical Reviews, 1997, Vol. 97, No. 6 2375</p></li><li><p>III. Pillared Layered SolidsOwing to the difficulties to synthesize zeolites</p><p>having the required extra large channel and cavitysizes, a group of ultralarge pore materials consistingof layered structures with pillars in the interlamellarregion, the so-called pillared-layered structures(PLS),22-28 have been synthesized. The layered com-pounds typically used involve smectites, metal (Zr,Ti, etc.) phosphates, double hydroxides, silicas, andmetal oxides. If the PLS is to be used for molecularsieve applications, the pillared material must havethe following characteristics: uniform spacing be-tween the pillars, suitable gallery heights, and layerrigidity. Among the different layered phases, smec-tites are probably the ones which best fulfill theserequirements. The family of minerals known assmectites includes beidellite, hectorite, fluorhectorite,saponite, sauconite, montmorillonite, and nontronite.Smectites can be described simply on the basis oflayers containing two sheets of silica sandwiching alayer of octahedral Al or Mg (2:1 layered clays).Substitution of some of the Al3+ for Mg2+ or Li+, orthe isomorphous replacement of tetrahedral Si4+ forAl3+ results in an amount of total negative chargeon the layer, compensated in turn by the presence ofhydrated cations in the interlayer region.It was Barrer and McCleod29 who first prepared</p><p>pillared materials by exchanging alkali and alkaline-earth cations in a montmorillonite clay for quater-nary ammonium compounds. However, the resultingmaterial was thermally unstable and therefore, of nopractical use for catalysis. An important qualitativeadvance was provided by the use of oxyhydroxyalu-minum cations as the pillaring agent.30,31 The gen-eral procedure for preparing these clays consists32 ofexchanging the cations in the interlamellar position,i.e., Na+, K+, and Ca2+, with larger inorganic hydroxycations. These hydroxy species are polymeri...</p></li></ul>


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