The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

It is possible to say that zeolites are the most widely used catalysts in industry They are crystalline microporous materials which have become extremely successful as catalysts for oil refining, petrochemistry, and organic synthesis in the production of fine and speciality chemicals, particularly when dealing with molecules having kinetic diameters below 10 A The reason for their success in catalysis is related to the following specific features of these materials:1 (1) They have very high surface area and adsorption capacity (2) The adsorption properties of the zeolites can be controlled, and they can be varied from hydrophobic to hydrophilic type materials (3) Active sites, such as acid sites for instance, can be generated in the framework and their strength and concentration can be tailored for a particular application (4) The sizes of their channels and cavities are in the range typical for many molecules of interest (5-12 A), and the strong electric fields2 existing in those micropores together with an electronic confinement of the guest molecules3 are responsible for a preactivation of the reactants (5) Their intricate channel structure allows the zeolites to present different types of shape selectivity, ie, product, reactant, and transition state, which can be used to direct a given catalytic reaction toward the desired product avoiding undesired side reactions (6) All of these properties of zeolites, which are of paramount importance in catalysis and make them attractive choices for the types of processes listed above, are ultimately dependent on the thermal and hydrothermal stability of these materials In the case of zeolites, they can be activated to produce very stable materials not just resistant to heat and steam but also to chemical attacks Avelino Corma Canos was born in Moncofar, Spain, in 1951 He studied chemistry at the Universidad de Valencia (1967−1973) and received his PhD at the Universidad Complutense de Madrid in 1976 He became director of the Instituto de Tecnologia Quimica (UPV-CSIC) at the Universidad Politecnica de Valencia in 1990 His current research field is zeolites as catalysts, covering aspects of synthesis, characterization and reactivity in acid−base and redox catalysis A Corma has written about 250 articles on these subjects in international journals, three books, and a number of reviews and book chapters He is a member of the Editorial 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 of them being for commercial applications He has been awarded with the Dupont Award on new materials (1995), and the Spanish National Award “Leonardo Torres Quevedo” on Technology Research (1996) 2373 Chem Rev 1997, 97, 2373−2419

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

Coagulation by hydrolysing metal salts

While the nanocatalysis field has undergone an explosive growth during the past decade, there have been very few studies in the area of shape-dependent catalysis and the effect of the catalytic process on the shape and size of transition metal nanoparticles as well as their recycling potential. Metal nanoparticles of different shapes have different crystallographic facets and have different fraction of surface atoms on their corners and edges, which makes it interesting to study the effect of metal nanoparticle shape on the catalytic activity of various organic and inorganic reactions. Transition metal nanoparticles are attractive to use as catalysts due to their high surface-to-volume ratio compared to bulk catalytic materials, but their surface atoms could be so active that changes in the size and shape of the nanoparticles could occur during the course of their catalytic function, which could also affect their recycling potential. In this Feature Article, we review our work on the effect of the shape of the colloidal nanocatalyst on the catalytic activity as well as the effect of the catalytic process on the shape and size of the colloidal transition metal nanocatalysts and their recycling potential. These studies provide important clues on the mechanism of the reactions we studied and also can be very useful in the process of designing better catalysts in the future.

Catalysis with transition metal nanoparticles in colloidal solution: nanoparticle shape dependence and stability.

Layered double hydroxides (LDHs) have been investigated for many years as host materials for a range of anion exchange intercalation reactions. In this role they have been used extensively as ion-exchange materials, catalysts, sorbents and halogen absorbers. More recently, there have been a tremendous number of new developments using these materials to store and deliver biologically active materials in vivo. Significant advances have been made recently on the characterisation of these materials, including structural studies and on the mechanism of intercalation using in situ techniques.

Intercalation chemistry of layered double hydroxides: recent developments and applications

A Fourier-transform infrared spectral study of propene reactions on acidic zeolites

Infrared, powder X-ray diffraction, and 27Al and 71Ga MAS NMR studies have been undertaken of gel phases formed upon the base hydrolysis of Al3+ and GaO4Al12(OH)24(H2O)]7+12 aqueous solutions. These studies have proven that the fresh gels (those not allowed to age) are formed through the consolidation of tridecamers, and therefore have "pseudo-spinel" structures. Aging of the pseudo-spinel gels forms a pseudo-boehmite phase which subsequently rearranges to form a hydroxide phase. It is proposed that these phase transformations involve cleavage of Al-O-Al and Al- H O -Al bonds in layers parallel to the pseudo-spinel 111 planes, with the resultant formation of the oxyhydroxide, or pseudo-boehmite, phase first, followed by continuing cleavage ultimately forming the hydroxide, or bayerite, phase.

The Structure of Al Gels Formed through the Base Hydrolysis of Al3+ Aqueous Solutions

Block copolymers self-assemble in bulk, forming a number of intricate block segregation patterns Such assembled structures can serve as precursors to nano- or microstructures with numerous potential applications For many applications, such as catalysis, a high surface area to volume ratio is desirable, which requires “breaking up” copolymer solid into dispersed particles Reported in this letter is the preparation of water-dispersible block copolymer microspheres containing Pd nanoparticles The Pd nanoparticles are a more efficient catalyst for alkene hydrogenation than is the commercial Pd black catalyst

Palladium Nanoparticle Catalyst Prepared in Poly(Acrylic Acid)-lined Channels of Diblock Copolymer Microspheres

It has been recognized for some time that the incorporation of fluorine in oxide catalysts (for example, alumina, silica-alumina, or zeolites) enhances their activity for acid-catalyzed reactions such as cracking, isomerization, alkylation, polymerization, and disproportionation. These reactions are thought to proceed via carbocation intermediates which are formed and stabilized on surface protonic sites. The incorporation of fluorine increases the activity by enhancing the acidic properties of the catalyst. Fluorine incorporated in an oxide catalyst replaces surface O or OH, and because fluorine is very electronegative, it polarizes the lattice more than the groups it replaces, and this increases the acidity of both protonic (Bronsted) and nonprotonic (Lewis) sites on the surface. As will be seen, pure alumina is inactive or only slightly active for acid-catalyzed reactions. In contrast, it has been shown repeatedly that fluorinated alumina is a very active, selective, and stable catalyst for su...

Fluorine-Promoted Catalysts

Three different preparation methods for Ni-Mo/Al 2 O 3 catalysts containing between 0 and 7.0 wt.-% phosphorus are compared in thiophene hydrodesulphurization (HDS) and diisopropylbenzene (DIPB) hydrocracking reactions. A promotional effect of phosphorus is seen in thiophene HDS regardless of preparation method, with maximum activity at around 1 wt.-% phosphorus. P/Al 2 O 3 catalysts (P = 0–7 wt.-% ) have little activity in DIPB hydrocracking indicating that phosphorus has very little effect on the Bronsted acidity of alumina. An increase in DIPB hydrocracking is observed for P-Ni-Mo catalysts with low phosphorus loadings, however, and so phosphorus does have an indirect effect on the Bronsted acidity of these catalysts. Impregnation of phosphorus prior to the metal species leads to the most active catalysts for both thiophene HDS and DIPB hydrocracking and this is explained in terms of a dual promotional effect of phosphorus; phosphorus increases both the dispersion and reducibility/sulphidibility of molybdate and increases the amount of nickel available to form the Ni-Mo-S active HDS phase by impeding nickel aluminate formation. One series of catalysts was investigated in the hydroprocessing of a Syncrude combined gas oil-fuel oil, and the same gas oil spiked with 5 vol.-% quinoline. The promotional effect of phosphorus in HDS was in good agreement with the model compound studies: 1 wt.-% phosphorus was the most active HDS catalyst in all cases. Furthermore, phosphorus loadings of between 1 and 3 wt.-% gave a product with lowest nitrogen content, and aromaticity. These loadings also gave the greatest sulphur removal and maintained good hydrogenation in the presence of quinoline, and gave the best quinoline hydrodenitrogenation, suggesting optimum surface acidity and hydrogenation properties. Optimum loadings of phosphorus (around 3 wt.-%) also improve the longevity of the catalyst by maintaining high surface area during the hydroprocessing reaction.

Ronald A. Kydd

Papers

A Fourier-transform infrared spectral study of propene reactions on acidic zeolites

The Structure of Al Gels Formed through the Base Hydrolysis of Al3+ Aqueous Solutions

Palladium Nanoparticle Catalyst Prepared in Poly(Acrylic Acid)-lined Channels of Diblock Copolymer Microspheres

Fluorine-Promoted Catalysts

Phosphorus promotion in nickel-molybdenum/alumina catalysts: model compound reactions and gas oil hydroprocessing