Molecular sieve materials properties

Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

Methane can be oxidatively coupled to ethylene with very high yield using the novel gas recycle electrocatalytic or catalytic reactor separator. The ethylene yield is up to 85% for batch operation and up to 50% for continuous flow operation. These promising results, which stem from the novel reactor design and from the adsorptive properties of the molecular sieve material, can be rationalized in terms of a simple macroscopic kinetic model. Such simplified models may be useful for scale up purposes. For practical applications it would be desirable to reduce the recycle ratio p to lower values (e.g. 5-8). This requires a single-pass C2 yield of the order of 15-20%. The Sr-doped La203... 

The discrepancy in numbers between natural and synthetic varieties is an expression of the usefulness of zeolitic materials in industry, a reflection of their unique physicochemical properties. The crystal chemistry of these aluminosilicates provides selective absorbtion and exchange of a remarkably wide range of molecules. Some zeolites have been called molecular sieves. This property is exploited in the purification and separation of various chemicals, such as in obtaining gasoline from crude petroleum, pollution control, or radioactive waste disposal (Mumpton, 1978). The synthesis of zeolites with a particular crystal structure, and thus specific absorbtion characteristics, has become very competitive (Fox, 1985). Small, often barely detectable, changes in composition and structure are now covered by patents. A brief review of the crystal chemistry of this mineral group illustrates their potential and introduces those that occur as fibers. 

The preparation of the used Fe-loading molecular sieves materials and the catalytic synthesis of carbon nanotubes have been described in detail in our previous report [22]. The textural properties and compositions of catalysts are shown in Table 1. XPS spectra for samples were recorded on a PHI-5300 ESCA system. The pass energy was 71.550 eV. Before the XPS measurement, all the samples were ground and then dried at 393 K for 2 h. For these samples, the C(ls) level (284.4 eV) was taken as the reference binding energy (B.E.). 

In contrast to zeolites, i. e. the traditional molecular sieve materials which are electrical insulators, it has been shown that cetineites are crystalline nanoporous materials with a photosemiconducting host lattice191. The present paper reports on the optical properties of the phases with A = Na, K and X = S, Se from an experimental and theoretical point of view. 

Mixed matrix membranes with molecular sieves incorporated combine the high separation capacity of molecular sieving materials (see Section 4.3.2) with the desirable mechanical properties and economical processing attributes of polymeric materials. 

Foley H.C. (1995) Carbogcnic molecular sieves Synthesis, properties and application. Microporous materials, 4, 407. 

Over the past decade crystalline microporous materials, in particular zeolites, have continued to find new applications in their traditional areas of use such as catalysis, separation and ion exchange due to their unique physicochemical properties and improved structuring in appropriate morphologies.[1,2,3,4,5] The current impact of molecular sieve materials is significant, with applications ranging from petroleum refining for fuels and petrochemical processes for various chemicals to air separation and nuclear waste management. 

Zeolites and molecular sieve materials exhibit fascinating properties particularly as sorbents or catalysts. From a fundamental point of view, they represent a family of crystallographically well-defined solid materials (this is not always the situation for most of the metallic oxides). When considering catalysis, two main factors play an important role, i.e., the nature of the active sites (particularly acidic sites) and the topology (cavities, channels, pores. ..). 

Isomorphous substitution of T element in a molecular sieve material is very interesting in order to modify its acidic or redox catalytic and shape selective properties. Different ways to perform such a substitution are now well established either during synthesis or post synthesis in( luding solid-solid reaction between the zeolite and another oxide. The substituted eliiment may be strongly or weakly bound to the framework i.e. may remain stable or may give rise to well dispersed metallic oxide particles entrapped in the cavities. This results in different catalytic properties and may even lead to bifunctional catalysis as for Ga-ZSM-5 material. 

The purpose of obtaining isomorphous substitution and new molecular sieve materials was obviously to modify the chemical and physical properties of zeolitic -type materials and therefore their catalytic properties. 

In the last two decades there has been considerable progress in the synthesis of molecular sieve materials. One result of these efforts is cloverite, a large-pore gallophosphate molecular sieve with a 20 T-atom cloverleaf-shaped entrance into a supercage of about 30 A diameter, which has attracted attention not only due to potential novel adsorption and catalytic properties and the uptake of bulky adsorbates, but also as a host for the preparation of semiconductor nanoclusters, as an example for the development of new advanced materials. 

Emphasis is placed in this paper on the present tendancy in the field of zeolitic-type materials. Apart from the synthesis of new molecular sieve materials exhibiting different chemical (acidic or basic) properties and different pore sizes and shapes a special effort is given to the modification of the existing or new materials in order to change their catalytic properties. 

Physical techniques appear to be of prime importance to characterize molecular sieve materials, particularly to try to determine the physical and chemical and subsequently catalytic properties. The use of several techniques is even necessary to get a wide view of the materials but it also important to emphasis the study of difhisivity, chemical features and catalytic features which are of prime importance to characterize more deeply zeolitic material. 

To overcome these limitations, mixed matrix membranes (MMM) started to emerge as an alternative approach in membrane technology. In this approach, the superior gas separation properties of the molecular sieve materials combine with the desirable mechanical properties and economical processability of polymers (Moore et al. 2004). A mixed matrix is a blend of inorganic particles (such as nanoparticles) in a polymer matrix, which are well dispersed. The effect of the inorganic dispersed phase on the MMM properties is related to its chemical structure, surface chemistry, and the type of particles. The inorganic materials used... 

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