Molecular sieve catalysts for the regioselective and shape- selective oxyfunctionalization of alkanes in air

Heterogeneous catalysis: enigmas, illusions, challenges, realities, and emergent strategies of design

Predominantly this article deals with the question of how to design new solid catalysts for a variety of industrial and laboratory-orientated purposes. A generally applicable strategy, illustrated by numerous examples, is made possible based on the use of nanoporous materials on to the (high-area) inner surfaces of which well-defined (experimentally and computationally) active centers are placed in a spatially separated fashion. Such single-site catalysts, which have much in common with metal-centered homogenous catalysts and enzymes, enable a wide range of new catalysts to be designed for a variety of selective oxidations, hydrogenations, hydrations and hydrodewaxing, and other reactions that the "greening" of industrial processes demand. Examples are given of new shape-selective, regio-selective, and enantioselective catalysts, many of which operate under mild, environmentally benign conditions. Also considered are some of the reasons why detailed studies of adsorption and stoichiometric reactions at single-crystal surfaces have, disappointingly, not hitherto paved the way to the design and production of many new heterogeneous catalysts. Recent work of a theoretical and high-throughout nature, allied to some experimental studies of well-chosen model systems, holds promise for the identification of new catalysts for simple, but industrially important reactions.

Nanofibrous and nanotubular supports for the immobilization of metalloporphyrins as oxidation catalysts

Nanofibrous and nanotubular materials, natural and synthetic, are important alternative matrices for the immobilization of metallocomplexes, especially metalloporphyrins, as oxidation catalysts. The process permits a regular and controllable distribution of the active phase at the outer and/or inner surfaces of the tubes, promoting a special environment for the approximation of a substrate to the catalytic active species. The immobilization also prevents the molecular aggregation and bimolecular self-destruction reactions, facilitates the recovery and reuse of the catalyst, reduce de cost of material preparation and environmental concerns. A variety of nanofibrous and nanotubular structures are presented and specific examples of immobilization of iron porphyrins in different supports and their oxidation catalytic activities are presented and discussed.

Oxidation of N-heterocyclics: A green approach

chemical structure image Environmentally benign oxidation methods satisfy the postulates of green chemistry. Heterocyclic Noxides have applications in synthetic organic chemistry, chemotherapy and agrochemicals. Synthesis of Noxides using green oxidants will be attractive over the conventional methods. The presence of the N-oxide group in the azine ring makes it more subject to electrophilic and nucleophilic attack and substantially expands the synthetic approaches for the modification of nitrogen-containing heterocyclics. That is the reason for the increasing interest in the chemistry of heterocyclic N-oxides. Some reactions adopted for oxidation of N-heterocyclics have been discussed. Stereochemical and spectroscopic aspects have been mentioned. It will be advantageous if anchored catalysts are employed for industrial exploitation. Several physiochemical aspects of various methods have been discussed.

Solvent-Free Aerobic Oxidation of n-Alkane by Iron(III)-Substituted Polyoxotungstates Immobilized on SBA-15

The tetrairon(III)-substituted polytungstates [Fe(4)(H(2)O)(10)(beta-XW(9)O(33))(2)](n-) (n = 6, X = As(III), Sb(III); n = 4, X = Se(IV), Te(IV)) were immobilized on (3-aminopropyl)triethoxysilane-modified SBA-15 and showed an excellent catalytic performance for solvent-free aerobic oxidation of long-chain n-alkanes using air as the oxidant under ambient conditions through a classical free-radical chain autoxidation mechanism.

Unique organic-inorganic interactions leading to a structure-directed microporous aluminophosphate crystallization as observed with in situ Raman spectroscopy

We report on the direct observation of key organic template-framework interactions leading to the formation of specific aluminophosphate structures. In particular, we show how MeAPO-34 formation was governed by an interaction between the divalent framework substituted metal ion and the template conformation, while for AlPO-5 the structure formation was determined by the template conformation alone. Understanding such interactions therefore appears to be important for the rationalization of microporous material formation.

Syntheses and characterizations of three low-dimensional chloride-rich zincophosphates assembled about [d-Co(en)(3)](3+) and [dl-Co(en)(3)](3+) complex cations

Three new chloride-rich zincophosphates including two zero-dimensional (0D) clusters [dl-Co(en)3]2[Zn4(H2PO4)2(HPO4)2Cl8] (denoted ZnPO-CJ33) and [d-Co(en)3]2[Zn4(H2PO4)2(HPO4)2Cl8] (denoted ZnPO-CJ34), and one-dimensional (1D) zincophosphate chain [dl-Co(en)3][Zn2(H2PO4)(HPO4)2Cl2] (denoted ZnPO-CJ35) have been solvothermally prepared. ZnPO-CJ33 and ZnPO-CJ34 possess the same cluster structure as the macroanionic [Zn4H6P4O16Cl8]6- unit formed by alternation of ZnOCl3/ZnO3Cl and HPO4/H2PO4 tetrahedra but differ in the countercations. The racemic [dl-Co(en)3]3+ cations are located among the clusters of ZnPO-CJ33, whereas chiral [d-Co(en)3]3+ cations are located among the clusters of ZnPO-CJ34. ZnPO-CJ34 templated by the optically pure chiral [d-Co(en)3]3+ cations is the first chiral monomeric zincophosphate. ZnPO-CJ35 templated by the racemic [dl-Co(en)3]3+ cations possesses a 1D infinite chain structure formed by the alternation of ZnO3Cl and HPO4/H2PO4 tetrahedra. The 1D chain structure of ZnPO-CJ35 can also be viewed as generated from the condensation of 0D clusters of ZnPO-CJ33, with the terminal Cl ions replaced by HPO4 groups. Experimentally, the structural transformation from ZnPO-CJ33 to ZnPO-CJ35 has been investigated.

Structure and nuclearity of active sites in Fe-zeolites: comparison with iron sites in enzymes and homogeneous catalysts

Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe(2+) homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe(2+) grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe(2+) presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N(2)O decomposition on isolated sites, whose properties are more similar to those of the (FeO)(2+) in cationic complexes (included the [(H(2)O)(5)FeO](2+)"brown ring" complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.

Synthesis and characterization of a new layered fluoroaluminophosphate (C4H11NOH)3.5[Al4(PO4)5F] x 0.5H3O with extra-large 16-rings

A new two-dimensional-layered fluoroaluminophosphate (C4H11NOH)3.5[Al4(PO4)5F] x 0.5H3O (denoted as AlPO-CJ20) with an Al/P ratio of 4:5 has been synthesized solvothermally by using 2-amino-2-methyl-1-propanol as the structure-directing agent. Its structure was determined by single-crystal X-ray diffraction analysis and further characterized by solid-state NMR techniques, including 27Al, 19F --> 27Al cross-polarization, and 31P magic angle spinning NMR. The alternation of Al-centered tetrahedra (AlO4 and AlO3F) and PO3(=O) tetrahedra gives rise to a new type of 4.6.16-net sheet. The inorganic sheets are stacked in an ABAB sequence along the [010] direction and further held together through strong H bonds between protonated template molecules and P=O groups in the inorganic layers. Except for Mu-4, AlPO-CJ20 is the second layered aluminophosphate with an Al/P ratio of 4:5, and it contains the largest pore opening of 16-rings in the known layered aluminophosphates. Furthermore, the coordination of Al and P of fluoroaluminophosphates is summarized. Crystal data: (C4H11NOH)3.5[Al4(PO4)5F] x 0.5H3O, monoclinic, C2/c (No. 15), a = 32.678(7) A, b = 12.956(3) A, c = 21.045(4) A, beta = 115.17(3) degrees, V = 8064(3) A3, Z = 8, R1 = 0.0837 [I > 2sigma(I)], and wR2 = 0.2428 (all data).

A Comparative Investigation of Co2+ and Mn2+ Incorporation into Aluminophosphates by in Situ XAS and DFT Computation

The incorporation processes of Mn2+ and Co2+ into the framework of aluminophosphate molecular sieve AlPO4-5, at the onset of crystallization, were investigated by in situ synchrotron X-ray absorption spectroscopy (XAS) and density functional theory (DFT) computation. The results indicated that the syntheses of MnAPO-5 and CoAPO-5 were different in the incorporation mechanism of metal ions. For the synthesis of CoAPO-5, Co2+ transferred from an octahedral into tetrahedral structure with crystal formation, while, for MnAPO-5, the Mn2+ transition to the tetrahedral structure was much more difficult and it occurred after the appearance of long-range ordered microporous structure. The DFT computations of model intermediates involved in the synthesis process suggested that much higher transformation energy of [Mn(OP(OH)3)4]2+ than that of [Co(OP(OH)3)4]2+ was responsible for the diversity of the incorporation behaviors.

Selective Oxidation of Alkanes with Molecular Oxygen and Acetaldehyde in Compressed (Supercritical) Carbon Dioxide as Reaction Medium

The oxidation of cycloalkanes or alkylarenes with molecular oxygen and acetaldehyde as sacrificial co-reductant occurs efficiently in compressed (supercritical) carbon dioxide (scCO2) under mild multiphase conditions. No catalyst is required and high-pressure ATR-FTIR online measurements show that a radical reaction pathway is heterogeneously initiated by the stainless steel of the reactor walls. For secondary carbon atoms, high ketone to alcohol ratios are observed (3.5-7.9), most probably due to fast consecutive oxidation of alcoholic intermediates. Since C--C scission reactions are detected only to a very small extent, tertiary carbon atoms are transformed into the corresponding alcohols with high selectivity. Detailed analysis of the product distributions and other mechanistic evidence suggest that acetaldehyde acts not only as the sacrificial oxygen acceptor, but also as an efficient H-atom donor for peroxo and oxo radicals and as a crucial reductant for hydroperoxo intermediates. In comparison to other inert gases such as compressed N2 or Ar, the use of carbon dioxide was shown to increase the yields of alkane oxygenates under identical reaction conditions.

Syntheses, Structures, Ionic Conductivities, and Magnetic Properties of Three New Transition-Metal Borophosphates Na5(H3O){MII3[B3O3(OH)]3(PO4)6}·2H2O (MII = Mn, Co, Ni)

Three new open-framework transition-metal borophosphates Na5(H3O){M(II)3[B3O3(OH)]3(PO4)6}.2H2O (M(II) = Mn, Co, Ni) (denoted as MBPO-CJ25) have been synthesized under mild hydrothermal conditions. Single-crystal X-ray diffraction analyses reveal that the three compounds possess isostructural three-dimensional (3D) open frameworks with one-dimensional 12-ring channels along the [001] direction. Notably, the structure can also be viewed as composed of metal phosphate layers [M(II)(PO4)2]4- with Kagomé topology, which are further connected by [B3O7(OH)] triborates, giving rise to a 3D open framework. The guest water molecules locate in the 12-ring channels. Partial Na+ ions reside in the 10-ring side pockets within the wall of the 12-ring channels, and the other Na+ ions and protonated water molecules locate in the 6-ring windows delimited by MO6 and PO4 polyhedra to compensate for the negative charges of the anionic framework. These compounds show a high thermal stability and are stable upon calcinations at ca. 500 degrees C. Ionic conductivities, due to the motion of Na+ ions, are measured for these three compounds. They have similar activation energies of 1.13-1.25 eV and conductivities of 2.7 x 10(-7)-9.9 x 10(-7) S cm(-1) at 300 degrees C. Magnetic measurements reveal that there are very weak antiferromagnetic interactions among the metal centers of the three compounds. Crystal data: MnBPO-CJ25, hexagonal, P6(3)/m (No. 176), a = 11.9683(5) A, c = 12.1303(6) A, and Z = 2; CoBPO-CJ25, hexagonal, P6(3)/m (No. 176), a = 11.7691(15) A, c = 12.112(2) A, and Z = 2; NiBPO-CJ25, hexagonal, P6(3)/m (No. 176), a = 11.7171(5) A, c = 12.0759(7) A, and Z = 2.

Solid-State NMR Spectroscopy of Anionic Framework Aluminophosphates: A New Method to Determine the Al/P Ratio

A series of anionic framework aluminophosphates, with different Al/P ratios, have been investigated by various solid-state NMR techniques, including 27Al, 31P magic angle spinning (MAS), 27Al-->31P cross polarization (CP), 27Al{31P} rotational echo double resonance (REDOR), and 31P{27Al} transfer of population double resonance (TRAPDOR). Different Al coordinations (AlO4b, AlO5b, and AlO6b) and P coordinations (PO4b, PO3bOt, PO2bO2t, and PObO3t), where b represents bridging oxygens and t represents terminal oxygens, can be unambiguously determined based on the solid-state NMR spectroscopy. Furthermore, a new method to determine the Al/P ratio of open-framework aluminophosphates has been established, which is useful for the understanding of unknown aluminophosphate structures.

Highly efficient one-step conversion of cyclohexane to adipic acid using single-site heterogeneous catalysts

A solid source of 'active' oxygen (acetylperoxyborate, APB), when dissolved in aqueous solution in the presence of a single-site microporous catalyst containing redox centres (Fe(III)AlPO-31, Mn(III)AlPO-5, Fe(III)AlPO-5), converts cyclohexane with high efficiency (ca. 88%) and exceptionally high selectivity (ca. 81%) to adipic acid at 383 K; this procedure is also effective in converting styrene to styrene oxide and -pinene and (+)-limonene to their corresponding epoxides.

Progress towards the easier use of P450 enzymes

The cytochrome P450 enzymes (P450s or CYPs) form a large family of heme proteins involved in drug metabolism and in the biosynthesis of steroids, lipids, vitamins and natural products. Their remarkable ability to catalyze the insertion of oxygen into non-activated C-H bonds has attracted the interest of chemists for several decades. Very few chemical methods exist that directly hydroxylate aliphatic or aromatic C-H bonds, and most of them are not selective or of limited scope. Biocatalysts such as P450s represent a promising alternative: however, their applications have been limited by substrate specificity, low activity, poor stability and the need for cofactors. This review covers the attempts to overcome these limitations using approaches such as mutagenesis, chemical modifications, conditions engineering and immobilization.

Insight into the construction of open-framework aluminophosphates

Over the past twenty five years, a class of open-framework aluminophosphates, denoted AlPOs, has been prepared with neutral zeolitic frameworks and anionic frameworks showing wonderfully complex structural and compositional diversity. An insight into the construction of open-framework AlPOs revealing their general structural features and topological chemistry is provided in this tutorial review, and the role of templating and the designed construction and synthesis of AlPOs are discussed.

Synthesis, Characterization, and Catalytic Performance of Cr-Incorporated Aluminoborate Octahedral Molecular Sieves

A series of Cr-incorporated PKU-1 molecular sieves (Cr-PKU-1) were synthesized by using boric acid as a flux, and the physicochemical properties were characterized by XRD, ICP, SEM, XPS, UV-vis, and NH3-TPD methods. The morphology of Cr-PKU-1 is a needlelike hexagonal prism with uniform size of about 2 microm in diameter and 20-50 microm in length. XRD and UV-vis provide direct evidence that Cr ions have been successfully incorporated into the framework of PKU-1. NH3-TPD shows a dramatic increase of acidic sites in the Cr-PKU-1 in comparison with PKU-1, indicating that the Cr incorporation can significantly modify the acidity of the compound. In addition, the incorporated Cr ions may act as redox centers, thus catalytic performance of Cr-PKU-1 molecular sieve was investigated by the selective oxidation of styrene under mild reaction conditions.

Design of a "green" one-step catalytic production of epsilon-caprolactam (precursor of nylon-6)

The ever-increasing industrial demand for nylon-6 (polycaprolactam) necessitates the development of environmentally benign methods of producing its precursor, epsilon-caprolactam, from cyclohexanone. It is currently manufactured in two popular double-step processes, each of which uses highly aggressive reagents, and each generates substantial quantities of largely unwanted ammonium sulfate as by-product. Here we describe a viable laboratory-scale, single-step, solvent-free process of producing epsilon-caprolactam using a family of designed bifunctional, heterogeneous, nanoporous catalysts containing isolated acidic and redox sites, which smoothly convert cyclohexanone to epsilon-caprolactam with selectivities in the range 65-78% in air and ammonia at 80 degrees C. The catalysts are microporous (pore diameter 7.3 A) aluminophosphates in which small fractions of the Al(III)O4(5-) and P(V)O4(3-) tetrahedra constituting the 4-connected open framework are replaced by Co(III)PO4(5-) and Si(IV)O4(4-) tetrahedra, which become the loci of the redox and acidic centers, respectively. The catalysts may be further optimized, and already may be so designed as to generate selectivities of approximately 80% for the intermediate oxime, formed from NH2OH, which is produced in situ within the pore system. The advantages of such designed heterogeneous catalysts, and their application to a range of other chemical conversions, are also adumbrated.