Vanadium mesoporous

To completely optimize the residue catalyst, other parameters than the different surface areas also must be optimized. For a catalyst cracking North Sea atmospheric residues, the pore size distribution also must be optimized. Pores in the mesopore range that is, pores with diameters between 50 and 500 Angstrom, are most important for precracking of resid molecules [21,23]. The possibility to make nickel and vanadium inactive is also important to optimize. 

Since discovery of MCM-41 materials [1,2] many researchers have been concentrated on the improvement of their quality and properties by incorporating heteroatoms such as titanium [3-5], boron [6,7], vanadium [8], gallium [9], and recently lanthanides (mainly La and Ce) [10-13]. Incorporation of these elements into the MCM-41 structure influences its stability as well as adsorption and catalytic properties [13]. The presence of silanol groups on the surface of these mesoporous materials allows for bonding of organic and inorganic ligands [14-16]. 

Sepiolite has been made exchangable by chemical treatments and Mg2+ at the border of the channels has been substituted by Al3+. In this way sepiolite with mild acidity, controlled mesopore, and improved stability has been obtained. This material is active for gasoil cracking, giving a good bottom conversion, and light cycle oil products without excessive gas and coke formation. Meanwhile, it is active for vanadium passivation. 

Shylesh, S. and Singh, A. P. Synthesis, characterization, and catalytic activity of vanadium-incorporated, -grafted, and -immobilized mesoporous MCM-41 in the oxidation, of aromatics, J. Catal., 2004, 228, 333-346. 

Complexation of metal ions and subsequent incorporation of the resulting metal complex into the oxide matrix during surfactant-templated synthesis prevents aggregation and leads to a homogeneous distribution of metal centers in the mesostructure. Copper- and vanadium-substituted mesoporous silicas were prepared in this way [107,108], Such materials have great potential in the field of catalysis. 

Recently, Carreon and Guliants reported novel hexagonal, cubic and lamellar VPO phases, which displayed improved thermal stability, desirable chemistries (i.e. the P/V ratios and vanadium oxidation states), and pore structures for the partial oxidation of n-butane [143-145]. These novel VPO phases displayed the selectivities to maleic anhydride up to 40 mol. % at 673K at 10 % n-butane conversion [146]. A conventional organic VPO catalyst containing well-crystallized vanadyl(IV) pyrophosphate, the proposed active and selective phase for n-butane oxidation to maleic anhydride, displayed the selectivities to maleic anhydride 50 mol. % under the same reaction conditions. The low yields observed for mesoporous VPO catalysts confirmed the critical role of the vanadyl pyrophosphate phase (VO)2P207 in catalyzing the oxidation of -butane to maleic anhydride. Therefore, the amorphous nature of the mesoporous VPO... 

Few redox studies with cubic mesoporous materials have been reported [52]. The large, complex, three-dimensional pore system offers a unique environment. Ti- and Cr-substituted MCM-48 have been studied for the selective oxidation of methyl methacrylate and styrene to methyl pyruvate and benzaldehyde, respectively, using peroxides as oxidants and were found to outperform TS-1. Ti-MCM-48 has also been found to be better than Ti-MCM-41, TS-1 and Ti02 for the photocatalytic reduction of CO2 and H2O to methane and methanol. Ti-grafted MCM-48 has also been reported as the first functional biomimic of vanadium bromoperoxidase, active at neutral pH and used in the peroxidative halogenation of bulky organic dyes. 

Few studies have also been reported with metal oxide clusters in mesoporous materials. MCM-41 modified by deposition of vanadium and titanium oxides exhibited catalytic activity for reduction of nitrogen oxides by NH3 [186]. 

Periodic mesoporous vanadium modified silicates (V-MCM-41 and V-HMS) were synthesized, characterized and tested in our laboratory [93,141,172-174], V-MCM-41 samples were prepared hydrotheimally at 373 K using Cab-O-Sil fumed silica, vanadyl sulfate and dodecyltrimethylammonium bromide [172]. The preparation of V-HMS was carried out at room temperature in the presence of dodecylamine as template [173,174]. 

Further interesting developments in this field are the discovery of the large pore titanium silicate molecular sieve ETS-10 [77] and the synthesis of mesoporous materids containing titanium and vanadium [78-81], Much systematic work will be required to elucidate how useful these new materials are as catalysts in selective oxidation reactions. 

The oxidation of aniline was carried out in the liquid phase over a series of transition metal - substituted molecular sieves. For low oxidant/aniline ratios, azoxybenzene (AZY) was the major product formed over Ti-containing catalysts, the reaction was limited by diffusion for medium pore zeolites like TS-l and mesoporous silicas were preferred as they permitted the use of both H2O2 and tert-butyl hydroperoxide as oxidants. Higher oxidant/aniline ratios (>2) led to the formation of nitrobenzene (NB), whose selectivity was proportional to the catalyst concentration. In contrast, vanadium containing molecular sieves were only active with TBHP and aniline was converted very selectively into nitrobenzene for all oxidant concentrations. 

A possible softening mechanism ia illustrated schematically in Figure 4A. Temperature excursions at the extrudate surface may accelerate the fluxing of vanadium pentoxide (ref. 10) and increase its interaction with other deposited nretals. Aliunlnuin sulfate, absent in the fines, may have decomposed at the particle surface due to the high temperature conditions. If decomposition did occur, temperatures would have been in excess of about 1400 F, and such conditions covild promote the collapse or sintering of the catalyst mesopores (refs. 11,12). Within the extrudate interior, however, conditions were apparently such to stabilize vanadium pentoxide, aluminum sulfate, and the catalyst mesopotes. 

Vanadium pentoxide Flux Aiuminum sulfate Decomposed Mesopores- Enlarged... 

Through studies over the last few decades, several methods have been developed for improvement of the stabilities of mesoporous silica materials. These strategies were shown to increase either the wall thickness or the degree of polymerization of the silica wall. Incorporation of metal oxides, such as zirconia, alumina, and vanadium oxides, was also demonstrated to be an effective method to improve the hydrothermal stability. 

Similarly, 2,3,5-trimethylphenol (239) was converted into the quinone 240 ca 80%) using H3PMoi204o"" or titanium substituted aluminophosphate (TiAPO-5) molecular sieves" . Efficient oxidation of phenols to the corresponding quinones has also been effected with H2O2-V-HMS (vanadium-containing mesoporous molecular sieves)" . 

D. Wei, W. Chuei, G. L. Haller, Catalytic behavior of vanadium substituted mesoporous molecular sieves, Catal. Today 51 (1999) 501. 

CaiTcon, M.A. and Gillian Is, V.V., Mesostructured vanadium-phosphorus-oxide phases, Micropor Mesopor Mater, 55, 297, 2002. 

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