Abdelhamid Sayari

Sulfated Zirconia-Based Strong Solid-Acid Catalysts: Recent Progress

Abstract This review article deals with recent progress in the preparation of sulfated zirconia (SZ)-bassed, strong solid-acid catalysts, the characterization of their physicochemical properties and the evaluation of their catalytic performance in various promising applications. Strong emphasis was put on discussion of controversial issues such as the strength of acid sites, the nature of active sites, the reaction mechanism, and the role and state of supported platinum. An important part of this work was devoted to recent catalytic applications.

Composition–activity effects of Mn–Ce–O composites on phenol catalytic wet oxidation

Abstract Mn–Ce–O composite catalysts have been widely used in sub- and supercritical catalytic wet oxidation of toxic organics contained in aqueous streams. In order to investigate their composition–activity relationship, 11 samples with Ce/(Mn+Ce) atomic bulk ratios ranging from 0 to 100% were prepared by co-precipitation. Phenol was selected as a model pollutant and the catalytic oxidation was carried out in a batch slurry reactor using oxygen as the oxidizing agent under mild reaction conditions. The results showed that the catalytic activity was greatly influenced by the catalyst composition. The catalyst with Mn/Ce ratio=6/4 was found to be the most active in reducing both phenol concentration and total organic carbon (TOC). All catalysts were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature programmed reduction (TPR) and nitrogen adsorption techniques. Systematic shifts in binding energy, diffraction angle, and reduction temperature were observed in the XPS, XRD and TPR spectra, respectively. XPS and XRD data revealed the occurrence of significant interactions between Mn and Ce oxides, resulting in the evolution of textural, structural and oxidation state with composition. TPR analysis showed that the interaction between Mn and Ce greatly improved the oxygen storage capacity of manganese and cerium oxides as well as oxygen mobility on the surface of catalyst. Catalytic active sites have been ascribed to manganese oxide species exhibiting higher oxidation state. Furthermore, XPS revealed that the most active catalyst, i.e. Mn/Ce 6/4, exhibits an electron-rich surface which may be very important in the activation of adsorbed oxygen.

Non-silica periodic mesostructured materials: recent progress

Summary The discovery of M41S periodic mesoporous silicates in 1991–1992 based on the use of surfactant supramolecular templates had an immediate strong impact on the area of mesostructured inorganic materials. A variety of related synthesis strategies have been developed while a great diversity of materials in terms of both composition and structure has been achieved. This review deals with non-silica mesostructured materials. Supramolecular templating techniques based on electrostatic interactions (ionic bonding), van der Waals interactions (hydrogen bonding) and direct covalent bonding between organic and inorganic species are presented, discussed and evaluated. Important issues, particularly regarding thermal stability of the materials, have been identified for future research.

Periodic mesoporous materials: synthesis, characterization and potential applications

Publisher Summary This chapter discusses the particular class of molecular sieves, having periodic mesoporous structure, with pore sizes in the range of 2 to 10 nm. They are comprised of the M41S mesoporous molecular sieves and solids, with related structures. The discovery of M41S has created new opportunities in several areas. By its simplicity and diversity, the synthesis strategy aroused the interest of zeolite synthesis scientists in the rich chemistry of surfactant-inorganic systems. This effort has already led to several important findings, in particular: (1) the design of new synthesis routes, using cheap polymer surfactants, (2) the discovery of new morphologies without lyotropic surfactant counterparts, and (3) the synthesis of some stable non-silica based mesoporous materials. Silicate-based materials and non-silicate materials are dealt with separately. The chapter discusses the preparation methods and characterization techniques of M41S and related materials. Potential catalytic applications of M41S are also discussed in this chapter.

Catalytic wet oxidation of phenol over PtxAg1-xMnO2/CeO2 catalysts

Abstract Catalytic wet oxidation reactions of aqueous phenol over unpromoted, base- and noble-metal promoted MnO2/CeO2 catalysts were carried out under mild conditions (80–130°C, 0.5xa0MPa O2) in a batch slurry reactor. Even though the catalyst-mediated oxidation was very effective in destroying phenol, only a moderate selectivity toward complete mineralization into CO2 and H2O was attained due to parallel formation of deactivating carbonaceous deposits. Promotion of the mixed-oxide catalysts with platinum and/or silver enhanced the mineralization selectivity and reduced appreciably the amount of deposits.

Vanadium containing crystalline mesoporous molecular sieves Leaching of vanadium in liquid phase reactions

Abstract Vanadium (V) containing mesoporous molecular sieves, V-HMS have been synthesized at room temperature in the presence of dodecylamine as template and vanadyl(IV) sulfate hydrate or vanadium(V) triisopropoxide as vanadium source. The presence of V in the silicate matrix was investigated by UV-visible spectroscopy, 51V MAS NMR and catalytic measurements. Samples were also characterized using chemical analysis, XRD, SEM, FTIR and adsorption properties. It was found that the vanadium centers in V-HMS molecular sieves consist of three-legged (SiO)3Vue5fbO species, similar to those in amorphous V SiO 2 . V-HMS materials catalyze the oxidation of organic substrates such as phenol, 2,6-di-tert-butyl phenol, naphthalene and cyclododecanol in the presence of either dilute hydrogen peroxide or tert-butyl hydroperoxide as the oxidant. Leaching of active vanadium species from the silicate matrix during reaction was investigated. The stability of vanadium during liquid phase reactions was found to depend on the nature of the substrate, the solvent and the oxidant.

Influence of hydrothermal restructuring conditions on structural properties of mesoporous molecular sieves

Abstract MCM-41 mesoporous molecular sieves were synthesized at low temperature (343xa0K) in the presence of cetyltrimethylammonium bromide (CTMABr) and subjected to postsynthesis hydrothermal treatment at elevated temperatures (413xa0K and 423xa0K). The hydrothermal stability of good quality large pore MCM-41 obtained by restructuring at 423xa0K was found to be comparable with that for the most stable MCM-41 materials reported in the literature. However, restructuring in the mother liquor at 413xa0K did not lead to good quality large pore MCM-41 materials. Adsorption measurements revealed limited pore size enlargement, accompanied by the development of microporosity and appearance of constrictions in the mesoporous structure. The similarity of adsorption isotherms for some of these materials and those of the ordered mesoporous molecular sieve SBA-2 with a three-dimensional cage-like pore structure suggests the presence of disordered arrays of mesoporous cages with no well-defined shape, size and connectivity. It was also shown that the hydrothermal treatment in water at 423xa0K led to initial improvement of the ordering, but the pore size increase was relatively small (ca. 10%). Extended treatments afforded materials with significantly enlarged unit cells, but with lower degree of pore structure ordering. Attempts were made to compare different approaches to the synthesis of good quality large unit cell MCM-41 materials templated by CTMABr.

Nanoporous zirconium oxide prepared using the supramolecular templating approach

Hexagonal (Hx-ZrO2) and lamellar (L-ZrO2) phases of zirconium oxide have been prepared using the supramolecular templating approach. Long chain primary alkyl amines led to the formation of lamellar phases, while quaternary ammonium surfactants gave hexagonal phases. The materials have been characterized by XRD, TG/DTG, IR, XPS, SEM and EDX techniques. The influence of various synthesis parameters such as (i) the ZrO2 surfactant ratio, (ii) the surfactant/water ratio, (iii) the nature of surfactant, (iv) the crystallization temperature and (v) the crystallization time have been investigated. The final solid products were found to be thermally unstable regardless of their structure. Removal of the surfactant from the mesopores by solvent extraction without damaging the structure was not possible.

New insights into pore-size expansion of mesoporous silicates using long-chain amines

Abstract The possibility of preparation of MCM-41 silicas with extra-large mesopore volume using either direct synthesis in the presence of long-chain amines or post-synthesis treatment was explored. It was shown that the use of large amounts of dimethyldecylamine (DMDA) to restructure MCM-41 with 3.5xa0nm pores prepared under low-temperature conditions (343xa0K) affords mesoporous silicas with up to 13.5xa0nm pores, relatively narrow pore-size distributions and extremely high total pore volumes up to 3.3xa0cm3/g. The application of large amounts of dimethylhexadecylamine (DMHA) was somewhat less effective, yet suitable for the preparation of materials with up to 8.5xa0nm pores and total pore volumes as large as 2.3xa0cm3/g. Large-pore silicas with narrow pore-size distributions were also obtained using direct synthesis in the presence of large amounts of DMHA along with cetyltrimethylammonium bromide in the gel. The current and earlier studies clearly demonstrate the feasibility of the preparation of disordered silicas with narrow pore-size distributions centered in the range from about 6 to 13.5xa0nm by a judicious choice of direct synthesis or restructuring procedures involving long-chain amines, such as DMDA or DMHA. Thermogravimetric analysis of as-prepared amine-expanded materials was shown to provide useful information about the template/silica mass ratio, which in turn was found to be related to the pore volume after calcination. Moreover, the weight-change derivatives provided some information regarding the organization of micelles inside the amine-swollen materials.

Catalytic wet oxidation of high-strength alcohol-distillery liquors

Abstract This work demonstrated that the catalytic wet oxidation (CWO) of high-strength alcohol-distillery waste liquors is feasible. Two distillery liquors (TOC=10xa0500; 22xa0500xa0mg/l; COD=26xa0000; 50xa0000xa0mg/l) produced by alcohol fermentation of enzymatic hydrolyzates from steam- and ammonia-exploded timothy grass were treated in a batch stirred autoclave over three different catalysts, 1% Pt/Al 2 O 3 , Mn/Ce oxides, and Cu(II)-exchanged NaY zeolite. The temperature and oxygen partial pressure ranges were (453–523xa0K) and (0.5–2.5xa0MPa), respectively. Effects of temperature, oxygen partial pressure, TOC content, and type of catalyst on the removal of total organic carbon (TOC) were investigated. The CWO initial rates were found to be first-order with respect to TOC, and the activation energies ranged from 34 to 51xa0kJ/mol. The highest TOC removal was achieved with Mn/Ce oxides and Cu(II)/NaY catalysts. The catalysts were found to be very effective for short contact times, while for prolonged exposures coke laydown was shown to be the prime factor responsible for the loss of catalyst activity. The carbonaceous deposits were characterized by electron spectroscopy for chemical analysis (ESCA) and temperature-programmed oxidation (TPO).