Amorphous mesoporous materials

This paper describes the unique properties of TUD-1, a new, amorphous mesoporous material. We will highlight several organic synthesis applications. (We discnss petrochemical and refining applications of TUD-1 in more detail elsewhere (7).)... 

If the origin of micropores is in crystalline material e.g. zeolite in amorphous matrix, their presence can be controlled by XRD. As it is seen from Fig.4, the highest peak 111 of NaY zeolite shows observable intensity for a content of zeolite of about 5%. But if micropores are part of amorphous mesoporous material, XRD is ineffective. 

Thommes, M. 2004. Physical adsorption characterization of ordered and amorphous mesoporous materials. In Nanoporous Materials Science and Engineering, edited by Lu, G. Q. Zhao, X. S. Imperial College Press, London, pp. 317-364. 

Bellussi and coworkers prepared a strongly acidic amorphous mesoporous material [53]. The pore radius of this amorphous zeolite precursor is about 20 A. It has very high propylene oligomerization activity at 120°C, a temperature much lower than what H-ZSM-5 needs for comparable activity. In benzene propagation, at conditions where H-ZSM-5 produces only 13 % cumene, this material gives 95 % yield. It is possible that very small, XRD invisible ZSM-5 domains cause this high acid activity. 

The ordered mesoporous materials (or crystalline mesoporous materials) such as MCM-41 (MCM stands for Mobil composite of matter), MCM-48 and SBA-15 (SBA stands for University of California, Santa Barbara) are a new generation of materials that are different from nonordered (amorphous) mesoporous materials. They are amorphous and not ordered at the atomic level from a classical crystallographic view point, but their regular channels or pores are ordered at the nanometer level. Because of this, these materials have certain characteristics of crystalline solids. Their structural information can be obtained by diffraction methods and other structural analysis techniques. The discovery of periodic mesoporous structures is a major advance in composite organic-inorganic materials synthesis. 

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. 

A joint research project between Lummus Technology and the Delft University of Technology led to the discovery of a new mesoporous material, named TUD-1 (8). TUD-1 is a three-dimensional amorphous structure of random, intercoimecting pores. The original emphasis was on the silica version, which has since been extended to about 20 chemical variants (e.g., Al, Al-Si, Ti-Si, etc.). 

Figure 41.4 shows a typical XRD (X-Ray Diffraction) pattern of TUD-1, along with a TEM image (12). Similar to other mesoporous materials, TUD-1 has a broad peak at low 20. However, it has a broad background peak, commonly called an amorphous halo, and lacks any secondary peaks that are evident for example in the hexagonal MCM-41 and cubic MCM-48 structures. The TEM shows that the pores have no apparent periodicity. In this example the pore diameter is about 5 nm. 

Most examples discussed so far made use of amorphous inorganic supports or sol-gel processed hybrid polymers. Highly disperse materials have recently become accessible via standard processes and, as a result, materials with various controlled particle size, pore diameter are now available. Micelle-templated synthesis of inorganic materials leads to mesoporous materials such as MCM-41, MCM-48, MSU, and these have been extensively used as solid supports for catalysis [52]. Modifications of the polarity of the material can increase the reactivity of the embedded centre, or can decrease its susceptibility to deactivation. In rare cases, enhanced stereo- or even... 

Although the mesoporous materials, such as Ti-MCM-41, have lower intrinsic epoxidation selectivity than TS-1 and Ti-beta, they must nevertheless be used as catalysts for reactions of large molecules typical in the fine chemicals industry. It is, therefore, interesting to elucidate how these ordered mesoporous materials compare with the earlier generation of amorphous titania-silica catalysts. Guidotti et al (189) recently compared Ti-MCM-41 with a series of amorphous titania-silica catalysts for the epoxidation of six terpene molecules of interest in the perfumery industry (Scheme 6). Anhydrous TBHP was used as the oxidant because the catalytic materials are unstable in water. The physical characteristics of these catalysts are compared in Table XII. 

Insertion of nano-materials into mesoporous materials-. Ultrasonic radiation has been used for the insertion of amorphous nano-sized catalysts into the mesopores. 

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts... 

The acidity of thermally stable mesoporous aluminophosphates (AlPO) and sili-coaluminophosphates (SAPO) has also been stndied by microcalorimetry [245]. By contrast with microporous crystalline alnminophosphate molecnlar sieves, mesoporous compounds are amorphous and characterized by Al/P ratios greater than 1. These particularities are responsible for a strong Lewis acidity, making these mesoporous materials more acidic than the microporons analognes, with an amonnt of strong acid sites that increases with the silicon content. 

Remarkably, in 2002, Inagaki and co-workers reported that, starting from 1,2-bis (triethoxysilyl)benzene as a siliceous precursor, mesoporous benzene-silica with crystal-like pore walls (Ph-PMO) can be prepared (Fig. 2) [35]. Owing to their crystallinity, these new hybrid organic-inorganic materials were much more stable in water than the amorphous mesoporous silica-supported sulfonic sites described above [36-39]. 

The XANES and EXAFS data indicate that Ti,v in these mesoporous materials is in tetrahedral coordination and highly dispersed (Thomas et al., 1994 Maschmeyer et al., 1995 Blasco et al., 1995 Liu et al., 1996). Also the UV visible spectra, with absorption edges at 48,000 cm 1 and 43,500 cm-1, indicate the presence of isolated Tilv in tetrahedral and octahedral coordination. A moderate increase in Q4-Si content along with a broadening of the Q4-Si NMR peak indicates that a part of the Tiiv can be embedded in the amorphous silica walls (Zhang et al., 1996). 

In comparison to the mesoporous materials, less research has been published on the functionalization of microporous materials by direct grafting (excluding various types of ion exchange). It has been stated that some of the newly modified mesoporous materials suffer from the adsorption of products and by-products onto the amorphous walls of the support structure.11031 Microporous zeolitic... 

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