Amorphous meso-macroporous

Amorphous silica-alumina (Si02-AI2O3) has been also tested for the catalytic cracking of polyolefins [36, 37, 43]. This acid solid is featured by having a broad distribution of pore sizes, which is determined by the synthesis procedure. Moreover, the occurrence of a bimodal pore size distribntion (e.g. meso-macroporous) is usually present. The aluminium... 

Meso-Macroporous Yttrium Oxides The self-formation phenomenon was also used for the preparation of hierarchically porous yttriiun oxides by a controlled polymerization of yttrium butoxide in aqueous media [12,141,144], The synthesized yttrium oxides are 0.5-2 pm in size and are covered by a smooth surface. The fissure particles with funnel-hke and parallel macrochannels below the smooth surfeice were observed by higher resolution SEM observations (Figure 32.13b). The yield of the synthesis as well as the amount of macropores per particle continuously decreases with increasing initial synthesis pH values. The macropore diameters are 1-5, 2-8, and 5-10 pm for syntheses carried out in acidic, neutral, and basic media, respectively. The macropore walls are formed by a regation of mes-ostructured nanoparticles giving a supplementary interparticle porosity centered at 30 nm. A third level of porosity is demonstrated by the inhomogeneous pores centered at 3-7 nm for syntheses in acidic and neutral media and 5-15 nm in an alkaline medium. As for all the previously described compositions, the meso-macroporous yttria structures are amorphous at the atomic scale. 

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

A hierarchically macro-/meso-/microporous structured catalyst, Hp-ZSM, has been recently reported by. Li et al. [173]. The catalyst was synthesized using cetyltrimethylammonium bromide (CTAB) and TBAOH as the meso- and micropore templates. Ethanol was used to generate macropores, probably via an ethanol-in-water microemulsion mechanism. The hierarchical porous zeolite shows higher hydrothermal stability and sustained higher catalytic activity than either the amorphous aluminosilicate ZSM-5 or meso-ZSM-5 catalysts used in the reactions involving large molecules. 

Precipitated silicas are also synthetic amorphous sihcas but have a wide range of pore sizes (meso to macroporous Figure 3.6) compared to siHca gels, which generally have a narrow range of pore sizes (microporous to mesoporous)... 

---