Hydrothermal stability, mesoporous

This chapter discusses the synthesis, characterization and applications of a very unique mesoporous material, TUD-1. This amorphous material possesses three-dimensional intercoimecting pores with narrow pore size distribution and excellent thermal and hydrothermal stabilities. The basic material is Si-TUD-1 however, many versions of TUD-1 using different metal variants have been prepared, characterized, and evaluated for a wide variety of hydrocarbon processing applications. Also, zeolitic material can be incorporated into the mesoporous TUD-1 to take the advantage of its mesopores to facilitate the reaction of large molecules, and enhance the mass transfer of reactants, intermediates and products. Examples of preparation and application of many different TUD-1 are described in this chapter. 

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. 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

The mesoporous materials reported above are usually prepared from relatively expensive surfactants. Some of them have poor hydrothermal stability. Furthermore, the MCM-41 host structure has a one-dimensional pore system with consequent pore blockage and diffusion limitations. Shan et al. (52) reported the synthesis of a three-dimensional and randomly connected mesoporous titano-silicate (Ti-TUD-1, mesopore wall thickness = 2.5-4 nm, surface area — 700-1000 m2/g, tunable pore size —4.5-5.7 nm) from triethanolamine (TEA). Ti-TUD-1 showed higher activity (about 5.6 times) for cyclohexene epoxidation than the framework-substituted Ti-MCM-41. Its activity was similar to that of the Ti-grafted MCM-41 (52). 

Recently, mesoporous aluminosilicates with strong acidity and high hydrothermal stability have been synthesized via self-assembly of aluminosilicate nanoclusters with templating micelles. The materials were found to contain both micro- and mesopores, and the pore walls consist of primary and secondary building units, which might be responsible for the acidity and stability (181). These materials were tested in isobutane/n-butene alkylation at 298 K, showing a similar time-on-stream behavior to that of zeolite BEA. No details of the product distribution were given. 

The addition of water-soluble polymers such as polyethylene oxide (PEO) or polyvinyl alcohol (PVA) into the synthetic mixture of the C TMAX-HN03-TE0S-H20 system (n = 16 or 18 X = Br or Cl) under shear flow is found to promote uniformity and elongation of rope-like mesoporous silica. The millimeter-scaled mesoporous silica ropes are found to possess a three-level hierarchical structure. The addition of water-soluble polymer does not affect the physical properties of the silica ropes. Moreover, further hydrothermal treatment of the acid-made material under basic ammonia conditions effectively promotes reconstruction of the silica nanochannels while maintaining the rope-like morphology. As a result, a notable enhancement in both thermal and hydrothermal stability is found. 

Figure 3. (A) XRD patterns and (B) N2 adsorption/desorption isotherms of the calcined mesoporous silica ropes obtained from the C,6TMAB-HNO3-TEOS-(PEO-6000)-H2O system before and after the post-synthesis ammonia treatment at 100 °C and a further hydrothermal stability test in water at 100 °C.

Ryoo et al. [6] reported an improvement of hydrothermal stability of MCM-41 via so-called salt effects their XRD results didn t show any structural losses for the materials treated in boiling water for only 12 h. Other researchers [1,7,8] also claimed that the hydrophobicity and hydrothermal stability could be improved by trimethylsilylation. However, Pan et al. [9] found that the BET surface area and pore size of the silylated MCM-48 by (CH3)3Si-Cl decreased markedly after silylation, the BET surface area of MCM-48 dropped to about 45%, and pore diameter decreased from 27 A to <16 A, thus retarding the advantages of MCM mesoporous materials possessing high BET surface area and big pore size. 

Improvement of Hydrothermal Stability of Mesoporous Molecular Sieves of MCM-41 Type... 

Substitution of silicon by other atoms like Ti or Al was reported to improve the thermal and hydrothermal stability to some extent [6]. It was also reported that improved hydrothermal stability could be achieved by adjusting the gel pH several times during crystallization process [7], Post-synthesis silylation technique has also been reported to enhance the hydrothermal stability of mesoporous materials by increasing the hydrophobicity of the samples [8,9]. However, it is most desirable to develop a method for preparing hydrother-mally stable mesoporous material by direct synthesis route. 

Mesopore size distribution analysis showed a very narrow distribution with a peak pore diameter of about 27 A for the calcined samples. It can be seen that the pore wall thickness of the calcined samples was in the 18-19 A and did not change much by adding additional cations. The increased hydrothermal stability observed was, therefore, not due to any increase in pore wall thickness. For the water-treated samples, the adsorption isotherms and pore size distribution curves showed that after hydrothermal treatment the pore size distribution was... 

In situ Synthesis of Micro- and Mesoporous Al-MFI / MCM-41 like Phases with High Hydrothermal Stability. 

The samples obtained by this procedure, containing various ratios of zeolite and mesoporous materials, were tested in 100% steam for 5 hours at temperatures of 650 and 815°C, respectively, in a fixed bed reactor. Prior to the tests the samples were calcined at 550°C in N2 (1 hour) and dry air (4 hours). Synthesis and characterization data for the most interesting samples with regard to improved hydrothermal stability are listed in Tables 1 and 2. 

The unique properties of lanthanide-based materials, e.g., lanthanide-silicates and lanthanide-doped silicas, can be related to the special properties of the 4f" orbitals. Among lanthanide oxides, only Ce, Pr and Tb form dioxides, which crystallize in one simple structure with M4+ ions showing octahedral coordination [17]. For instance, cerium dioxide exhibits an 8 4 catiomanion coordination [18]. Its characteristic feature is the ability to undergo oxidation-reduction cycles in a reversible way [19], It was shown that the presence of Ce and La additives in mesoporous silicas, e.g., MCM-41 [10,11] and MSU-X [12], improves their thermal and hydrothermal stability. 

These studies established that M41S type material is potentially very useful as a support material for energy/environmental catalysts. However, long term hydrothermal stability of the mesoporous support materials should be evaluated. In addition, forming process of the mesoporous powder need to be developed for pressure drop consideration. 

Subsequent synthesis efforts have produced new materials such as SBA-15. This mesoporous silica presents a well-defined hexagonal structure, large surface area, and high hydrothermal stability, which gives them high potential for a variety of applications [120,121],... 

Somewhat surprisingly, however, only a very limited amount of literature is available on hydrothermal stability of even the most commonly applied mesoporous membrane type, namely y-alumina membranes on OC-AI2O3 supports. These mesoporous y-alumina membranes are the common supports for the microporous silica membranes to be used in membrane steam reformers. In the investigations that finally led to the present study, delamination of the y-alumina membrane from the OC-AI2O3 supports in hot steam was found to be a major compli-... 

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