Characterization mesoporosity

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

Tin incorporated mesoporous Sn-MFI catalysts with different Si/Sn ratio using microwave were synthesized with carbon as hard template. These tin MFI catalysts were characterized using various physicochemical techniques XRD reviled the formation of more crystalline MFI structures which was further supported by the SEM and TEM imaging which clearly showed well ordered zeolite single crystals with mesoporosity. The N2 sorption isothers reviled the formation of bimodal mesoporous zeolites and the presence of tin in tetrahedral site was confirmed by FTIR (970 cm 1) and XPS (3ds/2 and 3 dj 2 electronic states). The thus synthesized mesoporous Sn-MFI catalysts with different Si/Sn ratios were used in studying the catalytic Baeyer-Villiger Oxidation (BVO) of cyclic ketones... 

Since the first synthesis of mesoporous materials MCM-41 at Mobile Coporation,1 most work carried out in this area has focused on the preparation, characterization and applications of silica-based compounds. Recently, the synthesis of metal oxide-based mesostructured materials has attracted research attention due to their catalytic, electric, magnetic and optical properties.2 5 Although metal sulfides have found widespread applications as semiconductors, electro-optical materials and catalysts, to just name a few, only a few attempts have been reported on the synthesis of metal sulfide-based mesostructured materials. Thus far, mesostructured tin sulfides have proven to be most synthetically accessible in aqueous solution at ambient temperatures.6-7 Physical property studies showed that such materials may have potential to be used as semiconducting liquid crystals in electro-optical displays and chemical sensing applications. In addition, mesostructured thiogermanates8-10 and zinc sulfide with textured mesoporosity after surfactant removal11 have been prepared under hydrothermal conditions. 

X-ray scattering from coal was the subject of several early studies which led to the postulation that coal contains aromatic layers about 20 to 30 A in diameter, aligned parallel to near-neighbors at distances of about 3.5 A (Hirsch, 1954). Small-angle x-ray scattering, which permits characterization of the open and closed porosity of coal, has shown a wide size distribution and the radius of gyration appears to be insufficient to describe the pore size. Application of the Fourier transform technique indicated that some coals have a mesoporosity with a mean radius of 80 to 100 A (Guet, 1990). 

The most obvious approach to obtain controlled mesoporosity is the extension of zeolite-like three-dimensional framework structures to larger pore-sizes. Zeolite-like compounds in the micro- and mesoporous range are the aluminophosphate VPI-5, cloverite and ULP (MCM-41). These new materials are characterized by pore diameters between 1.3 nm and 20 nm. 

We consider first the textural properties of a typical wormhole Ti-HMS in comparison to well ordered hexagonal Ti-MCM-41 and Ti-SBA-3 analogs prepared by S+I and S+XT electrostatic assembly pathways. Table 1 provides the surface areas and pore volumes that characterize the framework mesoporosity (Vfr) and textural porosity (Vtx). The total mesoporosity (Vtotai) is the sum of these two values. Each mesostructure contains 2 mole % Ti and exhibits a HK pore size near 2.8 nm. The values in parenthesis in the table are for the corresponding pure silicas. Note the very high ratio of textural to framework mesoporosity for the HMS molecular sieves (Vtx/Vfr = 1.06) compared to the hexagonal molecular sieves (Vtx/Vfr = 0.03). As will be shown below, the textural porosity of HMS catalysts can improve catalytic activity by facilitating substrate transport to the active sites in the mesostructure framework. 

The progress in the determination of porosity of various types of materials has arisen over the past ten years from advances in application of new spectroscopy techniques. In the present paper the application of small angle X-ray scattering (SAXS), positronium annihilation lifetime spectroscopy (PALS) and low temperature nitrogen adsorption methods to the characterization of mesoporosity is reviewed using different types of silica gels with chemically modified surface. The results from the three methods are compared and discussed. 

Klotz M, Ayral A, Guizard C, and Cot L. S3mthesis and characterization of silica membranes exhibiting an ordered mesoporosity. Control of the porous texture and effect on membrane permeability. Sep. Purif. Technol. 2001 25(l-3) 71-78. 

Films from the same research group were subsequently characterized with regards to their porosity, showing both zeolite microporosity and textural mesoporosity.[102] The above concept can be extended towards films with different binders, including organic polymers. Thus, two-component films comprised of nanoscale silicalite-1 and acrylic latex were deposited on silicon wafers via spin-coating.[103] In this case, a purified suspension of colloidal zeolites with sizes of 30 or 60 nm were first deposited followed by calcination. In a second step, a layer of acrylic latex was deposited, resulting in layers with dielectric constants between 2.0 and 2.5. 

Jagiello J, Olivier J P (2009) A Simple Two-Dimensional NLDFT Model of Gas Adsorption in Finite Carbon Pores. Application to Pore Structure Analysis. Journal of Physical Chemistry C 113 19382-19385 Ravikovitch P I, Neimark A V (2001) Characterization of micro- and mesoporosity in SBA-15 materials from adsorption data by the NLDIT method. Journal of Physical Chemistry B 105 6817-6823 Harkins W D, Jura G (1944) Surfaces of Solids. XII. An Absolute Method for the Determination of the Area of a Finely Divided Crystalline Solid. J. Am. Chem. Soc. 66 1362-1366... 

Mikhail R S, Brunauer S, Bodor E E (1968) Characterization of micro- and mesoporosity in SBA-15 materials... 

On dry gels, standard characterization techniques for porous media are used, several of which have been described in Volume 2 of this series helium pycnometry for pore volume determination (Section 6.3.1.2) as well as nitrogen adsorption at 77 K for surface area (Section 6.3.2.2, BET method), for microporosity (Section 6.3.3.2, Dubinin-Radushkevich method), for pore size distribution (Section 6.3.3.3, BJFl method), and for total pore volume (Section 6.3.3.4). When characterizing gels by nitrogen adsorption, other methods are also used for data interpretation, for example, the t-plot method for microporosity (Lippens and de Boer, 1965) and the Dollimore-Heal method (Dollimore and Heal, 1964) or Broekhoff-de Boer theory for mesoporosity (Lecloux, 1981). 

Throughout this book, reference is made to the characterizations or definitions used to describe this porosity. Thus, porosity in carbons is placed into four categories, not for convenience but out of necessity. The size of the pore determines how adsorption takes place in a pore be it narrow and wide microporosity <2.0 nm, mesoporosity 2.0-50 nm and macroporosity >50 nm. Chapter 1 introduces the reader through the sevaal areas of relevant expertise. 

For most purposes, a working characterization of an activated carbon, involves the determination of isotherms of nitrogen (77 K) (which fills micro- and mesoporosity), of carbon dioxide (273 K) which fills narrow microporosity and enters porosity which may not be... 

The partnership (interdependence) of isotherms ofNi (77 K) and CO2 (273 K), has developed into important new roles for both adsorptives, based on the DR and BET equations, in terms of characterization of volumes of microporosity and mesoporosity as well as availability of sites of high adsorption potential taken up at low relative pressures (concentrations) of adsorbate. 

These transitions in pore filling mechanisms are dependent on the size of the adsorbate molecule and so the above definition (>2 nm and <50 nm) is arbitrary being a function of the adsorbate used. However, as nitrogen dominantly is the characterizing adsorptive, these definitions must suffice. From an adsorption point of view, mesoporosity is not that all-important. Its role is as a transport pore (a means of passage), particularly for adsorptions from solutions, to permit rapid access to the retaining microporosity of the adsorbate of the system. 

Whereas the nitrogen isotherm provides quite reliable data for PSD with a mean size of about 20 nm or less, it is much less reliable for jx>res larger than this. Mercury porosimetry is the alternate technique for characterizing the sizes of the larger mesoporosity and dates back to 1921 and developed later by Ritter and Drake in 1945. TTie physical phenomenon of the mercury porosimeter is that mercury essentially does not wet surfaces of solids, it has a contact angle (0) with solids of about 140° and hence an applied force (pressure (/>)) is needed to push mercury into tubes, or pores with size Tp, as shown below ... 

The sample was characterized by nitrogen sorption at 77 K (ASAP 2000, software 2010, Micromeritics) to determine its total mesoporosity, the average pore size and the average size... 

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