Alain Tuel

Heterogeneous oxidation of aromatic compounds catalyzed by metallophthalocyanine functionalized silicas

Controlled procedures for the covalent anchoring of iron tetrasulfophthalocyanine onto amino-modified silicas have been developed to fix the complex either in a monomer or dimer form. Usually considered as a catalytically inert form, dimeric µ-oxo iron tetrasulfophthalocyanine grafted onto amino-modified silica or MCM-41 is shown to be a selective catalyst for the oxidation of 2-methylnaphthalene to 2-methylnaphthaquinone (vitamin K3) and of 2,3,6-trimethylphenol to trimethylbenzoquinone. Catalytic methods involving environmentally friendly oxidants are needed to perform selective oxidation of alkylaromatics. For example, 2-methylnaphthoquinone (Vitamin K3) is still prepared by stoichiometric oxidation of 2-methylnaphthalene (2MN) by CrO3 in H2SO4, thus leading to severe environmental problems. As an alternative, several homogeneous catalytic methods have been developed Oxidation of aromatic compounds to quinones is a multistep reaction and the yields are rarely high because of coupling and over-oxidation reactions. Vitamin K3 is obtained by 2MN oxidation with CrO3–H2SO4 in 38–60% yields. A 46% yield of vitamin K3 was reported in 2MN oxidation by the CH3ReO3–85% H2O2 system Recently, iron tetrasulfophthalocyanine (FePcS) has been shown to be an active catalyst in the oxidative degradation of chlorinated phenol and in the homogeneous oxidation of condensed aromatics The fixation of active catalysts onto appropriate supports is highly desirable to provide a high catalyst stability as well as facile recovery and recycling. Another reason to immobilize homogeneous complexes is that homogeneous solutions of complexes often contain several species, for example, monomers and dimers, while only some species, usually monomeric ones, are catalytically active. For example, among the several monomer and dimer forms of FePcS in aqueous solutions, monomeric FePcS has been proposed to be the catalytically active complex in the oxidative degradation of trichlorophenol Consequently, by using appropriate methods one can prepare a heterogeneous catalyst supporting only the catalytically active form of the complex. We report here the controlled covalent anchoring of phthalocyanine complexes onto mesoporous MCM-41 silica (Scheme 1) and the catalytic properties of these hybrid materials in the oxidation of 2-methylnaphthalene and 2,3,6-trimethylphenol. Recently discovered mesoporous silicas having large ordered hexagonal channels with diameters from 15 to 100 A(MCM-41) and high surface areas (above 700 g-1) are promising supports for different types of catalystsMCM-41 was prepared as previously describe and modified with 3-aminopropyltriethoxysilane to obtain 0.5 mmol of NH2 groups per gram of material. The modification was proven by the decrease of the silanol signals Q2 (-91 ppm) and Q3 (-100 ppm) and the appearance of a signal characteristic of (SiO)3Si–CH2 groups (-60 ppm) in the 29Si MAS NMR spectrum. 13C CP MAS NMR shows aminopropyl carbon signals at 5.5, 18.9 and 40.3 ppm. Textural characteristics of some solid materials are listed in Table 1. A decrease of specific surface area from 860 to 698 g-1 and average pore diameter from 40 to 36 Aas well as a decrease of the mesoporous volume from 1.30 to 1.04 cm3 g-1 also indicates a successful modification of MCM-41.FePcS was converted to iron tetrasulfochlorophthalocyanine, FePc(SO2Cl)4, by treatment with SOCl2 or PCl5. For covalent anchoring of FePc(SO2Cl)4 onto amino-modified MCM-41 two strategies were used in order to fix the phthalocyanine complex either as a monomer or as a dimer. We used UV-vis spectroscopy to identify these species.

Perspectives on zeolite-encapsulated metal nanoparticles and their applications in catalysis

Microporous cavities and channels of zeolites frameworks have been used for decades for the encapsulation of cations, complexes and metals. In the particular case of metals, nanoparticles are usually formed after post-synthesis modifications such as ion-exchange or wetness impregnation. Tough encapsulation can provide a strict control of the nanoparticle size as well as a limitation of aggregation at high temperature; encapsulated nanoparticles are often hardly accessible due to diffusion limitations of reactants in sub-nanometric micropores. Recent developments in zeolite synthesis have offered a new class of materials with hierarchical structures or unusual morphologies in which the mean diffusion path is considerably reduced. Those zeolites with inter-crystalline and/or intra-crystalline mesopore systems are particularly adapted for supporting nanoparticles both in microporous cavities of the framework and in mesoporous channels. More sophisticated architectures such as yolk/core–shell materials, in which nanoparticles are protected against poisoning or sintering by a thin zeolite shell, are particularly attractive. In contrast to nanometric intrinsic cavities of zeolite frameworks, a large internal void is available for chemical reactions and such catalysts can be considered as nanoreactors in which the reaction is essentially governed by the permeability of the shell. This review focuses on the most recent progress as well as perspectives in the design of zeolite-containing metal nanoparticles, which differ from traditional materials by the originality of the synthesis method or the morphology of the support.

Post-synthesis introduction of transition metals in surfactant-containing MCM-41 materials

Abstract V and TiMCM-41 have been prepared by post-synthesis modification of an as-made pure silica material in the liquid phase. Relatively high metal contents can be introduced without affecting the long-range ordering of the parent material. Solids with increasing metal contents have been characterized by X-ray diffraction, UV–VIS, IR and X-ray photoelectron spectroscopies. The results indicate that the metal species are essentially atomically dispersed at low loading (Me/Si

Synthesis and characterization of vanadium silicates with the ZSM-48 structure

Abstract Vanadium silicates with the ZSM-48 structure have been synthesized hydrothermally. The maximum incorporation achieved by this method was 0.8 V atoms/u.c. Different techniques have been used to characterize the location and coordination of V atoms in the zeolite. All these techniques suggest that V atoms are most likely incorporated in the framework in a highly dispersed state. Moreover, these solids have shown to be catalytically active in the hydroxylation of phenol with hydrogen peroxide.

Quasi All‐Silica Zeolite Obtained by Isomorphous Degermanation of an As‐Made Germanium‐Containing Precursor

Ge-containing ITQ-22 zeolites have been almost completely degermanated under strong acidic conditions without modifications of the framework topology. Simultaneous to Ge extraction, the framework was partially dissolved; mesopores were formed but the structure was maintained through the re-incorporation of some of silicon species at vacant sites. The presence of many defects in the degermanated framework enabled the incorporation of tetrahedral aluminum, opening the way to the preparation of new and stable acid catalysts with original topologies.

Alkene and thioether oxidations with H2O2 over Ti-containing mesoporous mesophase catalysts

Abstract Well-organized Ti-containing mesoporous mesophase materials (Ti-MMM) have been synthesized by hydrothermal synthesis using C 16 H 33 N(CH 3 ) 3 Br and characterized by elemental analysis, IR, DRS-UV, XRD, and N 2 adsorption. Catalytic properties of these materials in alkene and thioether oxidations with aqueous H 2 O 2 in CH 3 CN have been studied and compared with those of the Ti-containing hexagonal mesoporous silica (Ti-HMS). The Ti-MMM catalysts with the Si/Ti atomic ratio varying in the range of 49–124 showed the highest catalytic activities in alkene oxidation. Both the structure of a silicate matrix and a degree of isolation of titanium ions in it are crucial factors determining catalytic activity of Ti-MMM. For thioether oxidation, the structural perfection of Ti-MMM catalysts is less important. No leaching of titanium ions occurs during both reactions. The oxidation process proved to be truly heterogeneous.

Aerobic methylcyclohexane-promoted epoxidation of stilbene over gold nanoparticles supported on Gd-doped titania

Aerobic partial oxidations of alkanes and alkenes are important processes of the petrochemical industry. The radical mechanisms involved can be catalyzed by soluble salts of transition metals (Co, Cu, Mn...). We show here that the model methylcyclohexane/stilbene co-oxidation reaction can be efficiently catalyzed at lower temperature by supported gold nanoparticles. The support has little influence on gold intrinsic activity but more on the apparent reaction rates which are a combination of catalytic activity and diffusion limitations. These are here minimized by using gadolinium-doped titania nanocrystallites as support for gold nanoparticles. This material is obtained by mild hydrolysis of a new Gd(4)TiO(O(i)Pr)(14) bimetallic oxoalkoxide. It leads to enhanced wettability of the < 3 nm gold particles in the tert-butyl hydroperoxide (TBHP)-initiated epoxidation of stilbene in methylcyclohexane; Au/TiO(2):Gd(3+) is in turn as active as the state-of-the-art hydrophobic Au/SiO(2) catalyst. The rate-determining step of this reaction is identified as the gold-catalyzed homolytic decomposition of TBHP generating radicals and initiating the methylcyclohexane-mediated epoxidation of stilbene, yielding a methylcyclohexan-1-ol/trans-stilbene oxide mixture. Methylcyclohexan-1-ol can also be obtained in the absence of the alkene in the gold-catalyzed solvent-free autoxidation of methylcyclohexane, evidencing the catalytic potential of gold nanoparticles for low temperature C-H activation.

Synthesis, characterization, and catalytic properties of titanium silicates prepared using phosphonium ions

Abstract A titanium silicate has been synthesized in the presence of tetrabutylphosphonium ions. X-ray diffraction has revealed that this solid was identical to the already reported TS-2, with the MEL structure. The rate of crystallization of the zeolite as well as the maximal amount of Ti atoms that can be incorporated in the lattice are specific of the template. When the synthesis is carried out under stirring conditions, the size of the crystals is comparable to that of a standard TS-2. For similar Ti contents, the catalytic activities and selectivities in the hydroxylation of phenol with hydrogen peroxide are identical.

An NMR‐Driven Crystallography Strategy to Overcome the Computability Limit of Powder Structure Determination: A Layered Aluminophosphate Case

Along with the growing complexity of many inorganic systems, structure determination from powders is getting more and more difficult, and represents a severe obstacle for the discovery of new materials. The lack of single crystals (or crystals of insufficient size or quality) unfortunately often rules out any structural determination. It is acknowledged that it is more difficult to get good-quality single crystals as soon as the cell volume of a structure increases. The challenge of structure elucidation of powders is very well reflected by the small number of new structures published per year that are determined from powder diffraction compared to the number of structures determined from single-crystal diffraction data. Aluminophosphates are no exceptions, with as little as eight structures in total determined from powder diffraction, out of the more than 250 structures reported in the database. For inorganic or hybrid powders, one of the key, but challenging, steps that prevents structure elucidation is the construction of an initial structural model. Strategies for enhancing the efficiency of powder-based structure determination have shown a continuous development, especially by incorporating direct space structural information. To assist this part, diffraction software products based on simulated annealing or Monte–Carlo global optimization in directspace like FOX, ESPOIR, TOPAS, and so on, have proven reliable strategies for structure determination of a wide range of solids. Alternative routes have been proposed for a drastic acceleration of the structure solution when prior knowledge of the constitutive building blocks is available, allowing to find the solution of extremely large cells. However, considering the frequent absence of such prior knowledge, these methods become unsuccessful due to the exponential explosion of computing time with the number of degrees of freedom in the unit cell. This constitutes a limit of computability causing a bottleneck for structure determination from powders. It is therefore necessary to overcome this limit to yield 100 % success by reducing the computational workload. Beyond the performance of computing facilities itself, a successful and rapid search is largely determined by the number of degrees of freedom involved in the crystal structure versus the amount of input (chemical, topological, geometric, and so on) injected in the search. Nuclear magnetic resonance spectroscopy associating state-of-the-art high magnetic fields, ultra-fast magic angle spinning (MAS), and tailored NMR pulse sequences has become an extremely powerful tool to provide information at the atomic level about the local environment of a given nucleus. In this contribution, we shall show with the example of a novel nano-perforated lamellar aluminophosphate, [Al5(OH) ACHTUNGTRENNUNG(PO4)3ACHTUNGTRENNUNG(PO3OH)4] [NH3ACHTUNGTRENNUNG(CH2)2NH3]2 [2H2O], hereafter referred to as AlPO4-(Al5P7)-DAE (DAE= diaminoethane), 1) how topological information extracted from NMR data, that is, an NMR-driven structure resolution, can be directly introduced in the structure-elucidation process, allowing the determination of an initial model, which was otherwise not possible despite the high quality of the synchrotron powder diffraction (SPD) data, 2) how the computation time necessary to search and converge to this model can be further drastically decreased by increasing the amount of NMR-based information up to the whole complex building blocks. This new methodology differs signifi[a] B. Bouchevreau, Dr. C. Martineau, Dr. F. Taulelle Institut Lavoisier de Versailles, UMR CNRS 8180 Universit de Versailles Saint Quentin en Yvelines 45 Avenue des Etats-Unis 78035 Versailles cedex (France) Fax: (+33) 139254277 E-mail : charlotte.martineau@chimie.uvsq.fr taulelle@chimie.uvsq.fr [b] Dr. C. Mellot-Draznieks Coll ge de France, 11 place Marcellin Berthelot Laboratoire de Chimie des Processus Biologiques FRE 34 88 CNRS, 75005, Paris (France) [c] Dr. A. Tuel IRCELYON, CNRS UMR 5256 Universit Lyon 1, 69626 Villeurbanne (France) [d] Dr. M. R. Suchomel Argonne National Laboratory Advanced Photon Source, Argonne, IL 60439 (USA) [e] Dr. J. Tr bosc, Prof. O. Lafon, Prof. J.-P. Amoureux Univ. de Lille Nord de France, UCCS USTL CNRS UMR 8181, 59652 Villeneuve d Ascq (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201203767.

Evidence for F−/SiO− Anion Exchange in the Framework of As‐Synthesized All‐Silica Zeolites

Zeolites are crystalline microporous aluminosilicates with unique properties in adsorption, separation, and catalysis. For several decades, zeolites were prepared under alkaline conditions (OH route) by hydrothermal crystallization of gels containing organic cations (structure-directing agents or SDAs) and inorganic species. At the end of the 1970s, a breakthrough emerged with the introduction of F ions as mineralizing agents in the synthesis of silicalite-1, the all-silica zeolite with the MFI framework type. Since then, the F route led to the discovery of a number of new materials, particularly silica-rich zeolites with low-density frameworks. Many of these new frameworks contain specific subunits such as double four-membered ring units (D4R) with an F ion located in the center. In addition to the discovery of novel structures, the F route leads to large welldefined zeolite crystals with a low density of lattice defects. For zeolites that can be obtained by following both the F and OH routes, the question remains as to whether or not the two routes have some connection. In particular, although F ions are located in the framework of zeolites prepared in fluoride media, this is not the case for SiO defects when the same zeolites are prepared under alkaline conditions. From experimental observations and geometry optimization, Koller et al. have suggested that defect sites were preferentially located in six-ring units of silica frameworks. These rings are actually the best candidates to accommodate the Q sites that result from the cleavage of two Si–O–Si bridges without creating too much strain in the structure. However, techniques such as NMR and IR spectroscopy were not sufficiently structure-sensitive to precisely define which six-rings of the framework SiO defects were preferentially located. Herein, we show for the first time that F and SiO defect sites can be reversibly exchanged in the framework of assynthesized all-silica zeolites, except when F ions are located at the center of D4R units. Preliminary experiments were performed on all-silica zeolites with MFI and MRE framework types, for which both synthesis routes have been widely documented. Silicalite-1, which was prepared in the presence of HF and denoted [F]-MFI, is pure, and SEM images show that it forms large and regular twinned crystals (Figure 1a). The unique