Micro-mesoporous catalysts

Hydroisomerization of n-octane over Pt-containing micro/mesoporous catalysts obtained by recrystallization of zeolites BEA and MOR was investigated in the temperature range of 200-250 °C under 1-20 bar. Composite materials showed remarkably high activity and selectivity with respect to both pure microporous and pure mesoporous materials. The effect is due to high zeolitic acidity combined with improved accessibility of active sites and transport of bulky molecules provided by mesopores. 

Nickel containing MCM-36 zeolite was used as new catalyst in the ethylene oligomerization reaction performed in slurry semi-batch mode. This catalyst, with micro-mesoporous structure, mild acidity and well balanced Ni2+/acid sites ratio, showed good activity (46 g of oligomers/gcataLh) and selectivity (100% olefins with even number of carbon atoms). The NiMCM-36 behaviour was compared to those obtained with NiMCM-22, NiY, NiMCM-41 and NiMCM-48 catalysts. 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

Partial recrystallization of zeolites into composite micro/mesoporous materials leads to 1,3-2 fold increase of n-octane conversion and 2-3 fold increase of the yield of target products - branched octanes, indicating improved accessibility of active sites and transport of bulky molecules provided by mesopores. In the case of BEA series recrystallization in mild conditions leads to remarkable increase in selectivity to i-octane from 40 to 67%. On the contrary, complete recrystallization results in low catalytic activity, comparable with MCM-41 catalyst. 

In view of the numerous advantages of POMs the development of strategies for converting them to solid catalysts is of primaiy interest. First, catalytically active POMs can be heterogenized in the form of insoluble salts using Cs, Ag, K, NH/ and some organic cations [37,49, 58-64]. Such salts possess micro/mesoporous structure and their smface area is typically in the range of 10-150 mVg. 

Reactions for the synthesis of fine chemicals differ in many aspects from the hydrocarbon reactions that constitute today the major application of zeolites and other micro- or mesoporous catalysts, as they often involve the transformation of molecules with several functional groups. Chemoselectivity is therefore of prime importance. These reactions are generally operated in rather mild conditions and condensed media (rather than vapour phase) to avoid undesired secondary reactions. The use of solvents can have major impacts on the activity and selectivity of these catalysts as they may affect the adsorption and desorption of reactants and products on these catalysts. 

Zeolites and related materials prepared by isomorphous substitution of lattice atoms can also be considered as sohd solutions and yield specifically designed micro- and mesoporous catalysts. 

Dias, A. S., Pillinger, M., and Valente, A. A., Dehydration of xylose into furfural over micro-mesoporous sulfonic acid catalysts. J Catalysis 2005, 229 (2), 414-423. 

Fig. 15. T-O-S values of the toluene conversion in alkylation of toluene with propylene over different Beta related catalysts (toluene/propylene molar ratio of 2.0, WHSV 20 h 1, reaction temperature 250 °C) over nanosized Beta B1 - 0, industrial Bela iB2 - o, micro/mesoporous composite BM3 - V, and mesoporous Al-MCM-41 - x.

The alumina aerogel (490 m2.g-l, 1.3 cm. g-l micro-mesopore volume) was prepared from aluminum sec - butoxide dissolved in sec - butanol by hydrolysis with a stoichiometric amount of water. The alcogel was then dried in an autoclave under supercritical conditions with respect to sec - butanol [4]. Two conventional alumina xerogels ( Degussa 110 m2.g-l and R.P. 100 m2.g-i ) were also used as supports of Pd and Pt-Rli catalysts. 

Selective Oxidation over Micro- and Mesoporous Catalysts (Thursday)... 

Hierarchical porous materials are predominately based on zeolitic systems where mainly mesopores have been introduced in the microporous framework of a zeolite crystal in a similar way as motorways would intersect a narrow road system of a downtown area [110-113]. The hierarchy in such materials will result in an optimized performance in transport-limited applications. Thus, hierarchical zeolite-containing materials combine characteristics of pore size regimes of at least two different length scales [113, 114]. It has already been proved that such micro/mesoporous bimodal systems reduce the diffusion limitations for molecules within zeolite catalysts [115-118]. Several methods for the implementation of additional transport pores have been developed during the past few years, however. 

Mesoporous materials as the catalyst have good application prospects in the important multiphase reaction. The transport of media in the mesoporous materials is different from that in macro the complex structure of interface affects the reaction and transport, which makes the transport of the same importance as the reaction. The activity of catalyst could be improved by mesoporous Ti02- But various crystal facets make the recognizing of surface property difficult. The experimental phenomena are due to both complex structures and interactions in varied scales. It is necessary to care about not only catalysis mechanism but also transport for the design of catalyst with nano-/micro-/mesopore structures. 

Ivanova, I. I., Kuznetsov, A. S., Yuschenko, V. V., and Knyazeva, E. E. (2004). Design of composite micro/mesoporous molecular sieve catalysts. PureAppl. Chem. 76(9), 1647-1658. 

Figure 3.15 is a two-dimensional representation of the structure of a catalyst, which is made up of primary particles containing micro/mesopores which are aggregated together, separated by macropores, to form a pellet. This structure can be mapped as a lattice having the same pore size distribution and the same co-ordination number as the catalyst. Each pore becomes a bond in one of the lattices and each pore junction becomes a node. The size of the pores is assigned to the bonds so that the real structure and the lattice structure have the same pore size distribution, and the mean coordination numbers are the same. 

The integration of the favorable acidic and diffusion properties of the MCM-22 zeolite and the MCM-41 material, respectively, promised a composite, which is better catalyst than the components alone. This study deals with the synthesis of micro/ mesoporous MCM-22/MCM-41 aluminosilicate composite material, and with the catalytic behavior of the Ni,H-form composite and zeolite in the hydroconversion of n-heptane (11-C7). 

The lower rate of cracking in the micro/mesoporous composite relative to the microporous zeolite can be explained by the shorter residence time of the C7 caibenium ions and C products in the mesoporous particles having lower diffusion resistance. If the residence time was equalized by adjusting the space time to get similar conversions the selectivity difference of the catalysts disappeared. 

Hierarchical micro/mesopore structure can be obtained by forming composite from delaminated layered-stmcture zeohte and a mesoporous MTS material. At the same space time the bifunctional zeolite catalyst, having hierarchical micro/mesoporous stractme, show lower n-C hydroconversion activity and higher hydroisomerization selectivity than the corresponding microporous zeolite catalyst. 

Grabowska et al. (2008) prepared a composite oxide ZnAl O by microwave-assisted hydrothermal treatment of a precursor mixture of lydroxides obtained by precipitation of aluminum and zinc nitrates. Various studies show that ZnAl O is nanosized and is a micro/mesoporous material with large a suifaee area (140 mVg). The gas phase catalytic methylation of 4-hydroxypyridine in the presence of the ZnAljO catalyst was performed in a continuous process at atmospherie pressure in the temperature range of 240-360 °C. A mixture of O- and N-alkylated products, namely 4-methoxypyridine and N-methyl-4-pyridone were obtained. The alkylation of 4-hydroxypyridine with methanol at 345 °C offered 87.6% selectivity towards N-methyl-4-pyiidone with about 89% 4-methoxypyridine conversioa... 

More recently, bis(oxazoline)-metal complexes supported in micro- and mesoporous solids have been used as catalysts of hetero-Diels-Alder and ene reactions. 

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

Third Concept in Catalyst Design. Fenton Detemplation. Mild Organic Template Removal in Micro- and Mesoporous Molecular Sieves... 

I 6 Oxidation Tools in the Synthesis of Catalysts and Related Functional Materials Table 6.1 Detemplation approaches of micro- and mesoporous materials. 

Pore volumes are determined by forcing N2 (for micro- and mesoporous materials) or Hg (macroporous materials) under pressure into the pores. The quantity of N2 or Hg entering the catalyst is directly related to the pressure and the radius of the pores. The Kelvin equation describes this ... 

The object of the present study was to use in the above mentioned hydrogenations improved carbon supported catalysts, which could compete with the Pd black catalyst. Carbon materials are common supports, their surface properties can be modified easily and it is possible to prepare carbons with different proportion of micro-, meso- and macropores, which can be key factors influencing their performances. A highly mesoporous carbon was synthesised and used as support of Pd catalysts in the enantioselective hydrogenations. To our knowledge this is the first report on the use of highly mesoporous carbon for the preparation of Pd catalysts for liquid-phase hydrogenation. 

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