Structure porosity

Long-term grasslands provide plenty of organic matter for humus formation. If the soil is not acidic or waterlogged, the soil fauna and flora create humus and the subsequent useful development of a porous crumb and granular structure. Porosity encourages root growth and the uptake of soil nutrients. 

Pores are found in many solids and the term porosity is often used quite arbitrarily to describe many different properties of such materials. Occasionally, it is used to indicate the mere presence of pores in a material, sometimes as a measure for the size of the pores, and often as a measure for the amount of pores present in a material. The latter is closest to its physical definition. The porosity of a material is defined as the ratio between the pore volume of a particle and its total volume (pore volume + volume of solid) [1]. A certain porosity is a common feature of most heterogeneous catalysts. The pores are either formed by voids between small aggregated particles (textural porosity) or they are intrinsic structural features of the materials (structural porosity). According to the IUPAC notation, porous materials are classified with respect to their sizes into three groups microporous, mesoporous, and macroporous materials [2], Microporous materials have pores with diameters < 2 nm, mesoporous materials have pore diameters between 2 and 50 nm, and macroporous materials have pore diameters > 50 nm. Nowadays, some authors use the term nanoporosity which, however, has no clear definition but is typically used in combination with nanotechnology and nanochemistry for materials with pore sizes in the nanometer range, i.e., 0.1 to 100 nm. Nanoporous could thus mean everything from microporous to macroporous. 

Textural mesoporosity is a feature that is quite frequently found in materials consisting of particles with sizes on the nanometer scale. For such materials, the voids in between the particles form a quasi-pore system. The dimensions of the voids are in the nanometer range. However, the particles themselves are typically dense bodies without an intrinsic porosity. This type of material is quite frequently found in catalysis, e.g., oxidic catalyst supports, but will not be dealt with in the present chapter. Here, we will learn that some materials possess a structural porosity with pore sizes in the mesopore range (2 to 50 nm). The pore sizes of these materials are tunable and the pore size distribution of a given material is typically uniform and very narrow. The dimensions of the pores and the easy control of their pore sizes make these materials very promising candidates for catalytic applications. The present chapter will describe these rather novel classes of mesoporous silica and carbon materials, and discuss their structural and catalytic properties. 

For the special case of the sediment-water interface, DA is determined by the aqueous diffusivity, the sediment structure (porosity, tortuosity, pore size), and the sorption property of the chemical. Let us demonstrate this by applying the theory of transport of sorbing chemicals in fluid-filled porous media, which we have derived in Chapter 18.4 and Box 18.5, to the special case of diffusion in the sediment column. Since for this particular situation the fluid in the pore space is water, the subscript f (fluid) is replaced by w (water) while the superscripts sc and op mean sediment column and open water. 

Higher intrapellet residence times increase the contribution of chain initiation by a-olefins to chain growth pathways. This intrapellet delay, caused by the slow diffusion of large hydrocarbons, leads to non-Flory carbon number distributions and to increasingly paraffinic long hydrocarbon chains during FT synthesis. But intrapellet residence time also depends on the effective diameter and on the physical structure (porosity and tortuosity) of the support pellets. The severity of transport restrictions and the probability that a-olefins initiate a surface chain as they diffuse out of a pellet also de-... 

Void spaces created by the arrangement of the atoms in a crystal (structural porosity) this is then called intragranular porosity... 

MFI zeolite membranes (silicalite-1, ZSM-5), on either flat or tubular porous supports, have been the most investigated for gas separation, catalytic reactors, and pervaporation applications. The structural porosity of MFI zeolite consists of channels of about 5.5 A, in diameter, the sihca-rich compositions induce... 

In other respects, we can consider zeohte membranes as pertaining to the ceramic material category. Indeed zeolites are classified for the most part as microporous, crystalline silico-aluminate stmctures with different alumininum/silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeohtes are crystalline materials and they have a structural porosity very different from microporous amorphous silica [124]. Zeohte membranes are well adapted to the separation of gases, in particular H2 from hydrocarbons, but these membranes are not very selective for the separation of mixtures of noncondensable gases. 

In the first, the pores may be an inherent feature of crystalline structures (e.g. zeolites, clay minerals). Such intracrystalline pores are generally of molecular dimensions and result in very regular networks often described as "structural" porosity. 

The effects of polymer and nonsolvent concentration upon cell structure, porosity and permeability are discussed next. 

Recent sol-gel methods have been recognized as promising procedures to prepare catalysts [12-14]. The sol-gel methods allows a unique way of catalyst design, because they represent an ab initio synthesis of the final solid from well defined molecular compounds [13]. By suitable choice of reagents, reaction and drying conditions, such technique allows to predefine pore structure, porosity, composition, surface polarity and crystallinity or amorphicity of metal oxides [12]. In principle, any metal that forms stable oxides can be forced to copolymerise with other metals in sol-gel procedures to provide mixed metal oxides [13]. 

The relationship between other flammability characteristics and the polymer tendency for carbonization has also been studied 112-114). Flame retardant effectiveness of coke is, primarily, related to the reduced release of fuels into the gas-phase, and to the ability of the coke to undergo heterogeneous oxidation. Therefore, not only the composition is important but also the morphological structure, porosity, specific surface area and thermophysical properties of the carbonaceous residue115, U6). 

In fact Lp contains information abont the porons structure (porosity, pore size, and tortuosity) of the manbrane as well as the viscosity of the filtrated liquid varying nonlinearly with the tanperature. For the sake of comparison between membranes exhibiting different thicknesses, the membrane thickness Ajc can be introduced in the permeability coefficient whose dimension becomes (m m/(m /Pa/s)). 

Ritter N, Senkovskal, Kaskel S, Weber J. Intrinsically microporous poly(imide)s structure-porosity relationship studied by gas sorption and X-ray scattering. Macromolecules 2011 44(7) 2025-33. 

Mass transfer rates in gas-flowing solids-fixed bed contactors are expected to be high, according to fluid dynamics and heat transfer behavior. Somewhat lower values of mass transfer coefficients than those expected were reported in the literature [6,35-37]. The reasons for that are the effects of segregation as well as strong influence of axial backmixing. Apart from this, mass transfer rates depend on size and structure (porosity) of flowing solids [36]. 

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