Crystalline pore structure

As previously published [9], the MCMoidal material is classified as bridging the gap between highly ordered (MCM-41, MCM-48) and totally amorphous (xerogel) silica adsorbents in respect to their crystallinity, pore structure and behaviour towards water. 

The scaling down of the calcination process for industrial catalyst manufacturing requires knowledge of both the processing characteristics of the commercial rotary kiln and, for each different catalyst material, the physical and chemical processes Caking place during Che calcination. In this paper the elements of the model will be described in more detail and the problems of its validation discussed. It should be realized that the model is still in the development phase. Therefore, the most important heat and mass transfer phenomena occurring in a rotary kiln must be described properly first. A description of the development of important catalytic properties such as surface chemistry, crystallinity, pore structure and metal dispersion is still beyond the scope of the present model. 

Industrial carbon anodes and artificial graphites are not a single material but are rather members of a broad family of essentially pure carbon. Fortunately, artificial graphites can be tailored to vary widely in their strength, density, conductivity, pore structure, and crystalline development. These attributes contribute to their widespread applicability. Specific characteristics are imparted to the fmished product by conti ollmg the selection of precursor materials and the method of processing [19]... 

Zeolites are crystalline alumina-silicates having a regular pore structure. Their basic building blocks are silica and alumina tetrahedra. Each tetrahedron consists of silicon or aluminum atoms at the center of the tetrahedron with oxygen atoms at the comers. Because silicon and aluminum are in a +4 and +3 oxidation state, respectively, a net charge of -1 must be balanced by a cation to maintain electrical neutrality. 

Ni [182], V [183], and A1 [184]. SU-M [185] is a mesoporous germanium oxide with crystalline pore walls, possessing one of the largest primitive cells and the lowest framework density of any inorganic material. The channels are defined by 30-rings. Structural and thermal information show that there exists a mismatch between framework stability and template decomposition. The latter requires temperatures higher than 450 °C, while the structure is preserved only until 300 °C. 

Fig. 4.2 Periodic mesoporous organosilicates (PMO) (A) various framework structures (B) crystalline pore wall composed of phenyl rings and silica. Adapted from [52], A. Vinu et al.,...

The number of publications concerning utilization of the EISA process for fabrication of different structured materials is counted in the hundreds, which is far beyond the possibilities of this chapter to review in depth. Rather, we intend to provide a brief introduction into EISA and its application to the fabrication of functional thin films for electronic applications (e.g., electro-chromic layers and solar cells), with a special focus on fabrication of crystalline mesoporous films of metal oxides. Attention will also be given to techniques used to evaluate the pore structure of the thin films. For the other aspects of the EISA process, for example its mechanism,4 strategies for preparation of crystalline porous metal oxides,5 mesoporous nanohybrid materials,6 periodic organic silica materials,7,8 or postgrafting functionalization of mesoporous framework,9 we kindly recommend the reader to refer to the referenced comprehensive reviews. 

When type X is utQized, in any of its ion exchange forms, for dehydration or possibly for sweetening (sulfur removal), there is little likelihood that the intracrystalline diffusion will be the dominant resistance to mass transfer. Large aromatic sulfurs would of course be an exception. When type X is used for adsorption of hydrocarbons or aromatics then it is possible that the micro-pore diffusion might dominate. When type A is used there is always a distinct possibility that intra-crystalline diffusion will be slow and may dominate the mass transfer, even for relatively small molecules. This is especially true when the chosen structure is a K A or type 3A. Selection of other small pore structures, for separations or purification applications can also create situations where the dominant resistance is found in the crystaUites. 

XRD patterns of Pt/FSM-16 [25] (and HMM-1 [32]) show no significant change at 26 = 1-10° before and after the incorporation of metal nanowires and nanoparticles (Figure 15.7). This indicates that the pore structures and mesoporous channels of FSM-16 (and HMM-1) remained unchanged in the synthesis of the Pt wires and Pt particles [18-20, 23, 24] by wet photo-irradiation with methanol -i- water vapor and dry H2 reduction, respectively. Furthermore, in the high 26 region, typical peaks assigned to Pt fee crystalline are observed for both samples of Pt nanowire/FSM-16 and Pt nanoparticles/FSM-16 [25]. 

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

Zeolites are aluminosilicate crystallines consisting of pores of molecular dimensions, interconnected by small windows(5-8A diameter). Strict regularity of the pore structure enables higher slectivities to be achieved in both catalysis and sorption processes. The intrazeolite circumstances alike a "solid-solvent" accomodate the selected reactant molecules and promote some inorganic and organic synthetic reactions, similarly in solution. 

An important parameter influencing the mode of action of cellulases is the accessibility of the cellulose to the enzymes. The molecular weights of cellulases range between 30 and 80 kDa. A comparison of the size of cellulase (3-8 nm) and the pore size of cotton swollen in water (1-7 nm) shows very clearly that cellulases can penetrate the cellulose to a limited extent only. In addition, the enzyme reaction takes place preferentially on amorphous cellulose because the more compact, crystalline cellulose structures do not offer any space for such macromolecules. Thus - provided of enzyme and process parameters have been selected correctly -cellulases act mainly on the textile surface. In this way interesting effects on cellu-losic fibers can be achieved. 

The obtained solid phases are characterized by an ordered, not crystalline, pore wall structure, presenting sharp mesopore-size dispersions. These mesoporous materials are ordered, but not crystalline, because of the lack of precise atomic positioning in the pore wall structure as was shown by MAS-NMR and Raman spectroscopy [115]. This gave rise to inorganic solids with enormous differences in morphology and structure [113]. 

Gas, or vapor molecules, after the degasitication process, can go through the pore structure of crystalline and ordered nanoporous materials through a series of channels and/or cavities. Each layer of these channels and cavities is separated by a dense, gas-impermeable division, and within this adsorption space the molecules are subjected to force fields. The interaction with this adsorption field within the adsorption space is the base for the use of these materials in adsorption processes. Sorption operations used for separation processes imply molecular transfer from a gas or a liquid to the adsorbent pore network [2],... 

---