Porosity micro

Wagdare, N.A., Marcelis, A.T.M., Ho, O.B., Boom, R.M., and van Rijn, C.J.M. High throughput vegetable oil-in-water emulsification with a high porosity micro-engineered membrane. Journal of Membrane Science 347(1-2) (2010) 1-7. 

GMPL graded porosity micro-porous layer... 

Activated carbon is a very important industrial adsorbent because it exhibits a well developed porosity (micro, meso and macroporosity) and this is coupled with a great thermal and chemical stability, a relatively large hydrophobicity (thus favouring the adsorption of non-polar substances in the presence of humidity), low production cost, etc. Additionally, the surface of activated carbon can be functionalised with different heteroatoms (but mainly oxygen), thus modifying the chemical nature. A large and accessible surface area is a necessary but not sufficient condition for the preparation of activated carbons to be used in industrial adsorption processes (gas and liquid phase purification, separation, environmental control, etc.), since the last few years has shown that the chemical composition of the carbons surface plays a very important role in the process. 

For the future, the development of synthesis methods aimed at improving reproducibility, reliability and stability of membrane top-layers will have a major impact in inorganic and hybrid membrane production at an industrial scale. In the case of ceramic membranes the control of porosity (micro and meso) and the production of stable nanophase structures seems to be the most challenging objectives. The more recent hybrid membranes can benefit from the recent progress in supramolecular chemistry and are candidate materials for microsystems combining separation, reaction and contactor operations at the micrometer scale. Due to its high versatihty, it is likely that the sol-gel process will remain at the center of these developments. 

Microcracked chromium topcoats Historically, microcracked chromium preceded the micro-porous chromium just described, but it is related to it in that the deposition conditions and thickness of the chromium topcoat are controlled to give porosity through a network of very fine cracks. 

Concentration of gel1, % Soil porosity, % Total Micro/total Mean pore diameter, pm Hydraulic conductivity, mday 1 Avialable moisture1, % Rate of evaporation 2, arb. units Transpiration ratio3, g g-1 Water use efficiency4, g kg-1... 

In general, fungal mycelia are filtered relatively easily, because mycelia filter cake has sufficiently large porosity. Yeast and bacteiia are much more difficult to handle because of thefr small size. Alternative filtration methods, which eliminate the filter cake, are becoming more acceptable for bacterial and yeast separation. Micro-filtration is achieved by developing large cross-flow fluid velocities across the filter surface while the velocity vector normal to the surface is relatively small. Build up of filter cake and problems of high cake resistance are therefore prevented. Micro-filtration is not discussed in this section. 

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

Other key features in the analysis of pore structure are the length scales associated with the various micro- (nano)-scale obstacles and pores, the possible larger-scale variations in structure, and the averaging domain over which information is needed [6,341,436], The hterature refers to analysis of homogeneous and heterogeneous porous media, where homogeneous refers to media with no variation in physical properties (e.g., porosity, diffu-... 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

In order to compare the micro-channel and the fixed-bed reactor, the design and operation parameters should be adjusted in such a way that certain key quantities are the same for both reactors. One of those key quantities is the porosity s, defined as the void fraction in the reactor volume, i.e. the fraction of space which is not occupied by catalyst pellets or channel walls. The second quantity is the specific... 

The figure shows the ratio of the widths of initially delta-like concentration tracers at the reactor exits as a function of the micro-channel Peclet number for different values of the porosity. Taking a value of = 0.4 as standard, it becomes apparent that the dispersion in the micro-channel reactor is smaller than that in the fixed-bed reactor in a Peclet number range from 3 to 100. Minimum dispersion is achieved at a Peclet number of about 14, where the tracer width in the micro-channel reactor is reduced by about 40% compared with its fixed-bed counterpart. Hence the conclusion may be drawn that micro-channel reactors bear the potential of a narrower residence time than fixed-bed reactors, where again it should be stressed that reactors with equivalent chemical conversion were chosen for the comparison. 

With the increasing quest for chemical micro processing research on catalyzed gas-phase reactions, both the catalysts themselves and their carriers have become the focus of scientific investigations - on their preparation, morphology, porosity, composition, etc. 

However, in view of the large specific surface area of catalysts used in conventional fixed-bed reactors for this reaction, attempts have to be made to realize catalyst coatings of similar porosity in micro channels [17]. 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

Filter the cold acetone rapidly into a glass-stoppered test tube by means of suction through a 2- or 3-ml. micro Buchner funnel, fitted with a fritted-glass plate of medium porosity. Wash the tube and funnel with 3 ml. of cold acetone and combine it with the filtrate. 

Mean pore size either the pore size distribution or the specific porosity divided by the specific surface area. Pores are divided into three categories macro (30-50 nm), meso (intermediate size), and micro (less than 2 nm). 

Bhattacharya and Gedanken [11] have reported a template-free sonochemical route to synthesize hexagonal-shaped ZnO nanocrystals (6.3 1.2 nm) with a combined micro and mesoporous structure (Fig. 8.1) under Ar gas atmosphere. The higher porosity with Ar gas has been attributed to the higher average specific heat ratio of the Ar which leads to higher bubble collapse temperatures. With an intense bubble collapse temperature, more disorder is created in the product due to the incompleteness of the surface structure that led to greater porosity. Importance of gas atmosphere has been noted when the same process was carried out in the presence of air which results in the formation of ZnO without any porosity. 

The steam treatment does however affect the Al-surroundings in the zeolite crystal. As seen in Fig. 2b, the intensity of both the tetragonally (at 0 ppm) and octahedrally (at 55 ppm) coordinated aluminum species decreases considerably after steam treatments for more than 4 h. Steam treatment for more than 8 h did not lead to a further decrease in the signal intensities. The decreases confirm that aluminum is extracted from the framework during the steaming process, as was also concluded from the 29Si MAS-NMR spectra (Fig. 1 b). This may lead to the formation of additional (micro) porosity, but the aluminum extraction could negatively affect the catalytic activity. 

Acidic micro- and mesoporous materials, and in particular USY type zeolites, are widely used in petroleum refinery and petrochemical industry. Dealumination treatment of Y type zeolites referred to as ultrastabilisation is carried out to tune acidity, porosity and stability of these materials [1]. Dealumination by high temperature treatment in presence of steam creates a secondary mesoporous network inside individual zeolite crystals. In view of catalytic applications, it is essential to characterize those mesopores and to distinguish mesopores connected to the external surface of the zeolite crystal from mesopores present as cavities accessible via micropores only [2]. Externally accessible mesopores increase catalytic effectiveness by lifting diffusion limitation and facilitating desorption of reaction products [3], The aim of this paper is to characterize those mesopores by means of catalytic test reaction and liquid phase breakthrough experiments. 

The studied solids exhibit great differences in term of porosity (purely microporous, micro-mesoporous and mesoporous). As it is well known, mesoporosity of USY (CBV series) increases as the aluminum content decreases. Large part of mesoporosity is in the range 2-3.6 nm according to the difference between mesoporous volume obtained by N2 or Hg measurements. Pore size estimation gave similar results whatever the technique. 

The microwave technique has also been found to be a potential method for the preparation of the catalysts containing highly dispersed metal compounds on high-porosity materials. The process is based on thermal dispersion of active species, facilitated by microwave energy, into the internal pore surface of a microporous support. Dealuminated Y zeolite-supported CuO and CuCl sorbents were prepared by this method and used for S02 removal and industrial gas separation, respectively [5], The results demonstrated the effective preparation of supported sorbents by micro-wave heating. The method was simple, fast, and energy-efficient, because the synthesis of both sorbents required a much lower temperature and much less time compared with conventional thermal dispersion. 

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