Sieve , molecular

Molecular sieve carbons contain ultraflne micropores that permit the penetration of smaller molecules but impede the entry of larger molecules. The diffusion of certain gases and vapors into these materials is an activated process and is strongly temperature dependent. Thus, Marsh and Wyne-Jones observed that the uptake of nitrogen at 77 K for a polymer carbon was much smaller than its uptake of CO2 at 195 K. Kipling et al. observed that the surface area of a polymer charcoal prepared by carbonization at 700°C was very low when measured by adsorption of nitrogen at 77 K than when measured by the adsorption of CO2 at 195 K. 

Molecular sieves are synthetically produced zeolites characterized by pores and crystalline cavities of extremely uniform dimensions. Molecular sieves are available in four different grades. These grades are different from one another because of their chemical composition and pore size. Molecular sieves are ceramic-appearing pellets or balls in diameters of 0.159 mm (1/16 in.) or 3.175 mm (1/8 in.). Molecular sieves have the lowest dusting factor of any commercially available desiccant moreover, the pellets or balls do not change size or shape upon reaching saturation. 

Type 4A (four angstroms). Molecular sieve is the sodium form of the zeolite. Type 4A adsorbs molecules having a critical diameter of less than four angstroms (e.g., NH3). A type 4A molecular sieve is typically used in regenerable drying systems to remove water vapor or contaminants which have a smaller critical diameter than four angstroms. 

Type 13X (ten angstroms). Type 13X is a modified form of the sodium zeolite with a pore diameter of ten angstroms. Molecules of chloroform, carbon tetrachloride and benzene can be adsorbed on type 13X molecular sieves. Type 13X is used commercially for general gas drying, air plant feed purification (i.e., simultaneous removal of H O and CO ) and liquid hydrocarbon and natural gas sweetening (i.e., H S and mercaptan removal). All molecules which can be adsorbed on molecular sieves 3A, 4A and 5A can be adsorbed on type 13X. In addition, type 13X can adsorb molecules of larger critical diameters, such as aromatics and branched-chain hydrocarbons. 

Activated alumina AI.O3 neutral, acid 0.003 (2 to 5 X 10 ) 1 X10 260 (0.2) 100 physisorption More efficient than molecular sieves but with lower capacity. Regeneration by heating at 175°C under vacuum during 24 h. Incompatible with oxidable polar substances. Suitable for hydrocarbons. 

Carbon molecular sieves have a carbon skeletal structure formed by the pyrolysis of proprietary polymeric materials. Further processing provides a pelletised form with a surface area of 1000-1500m g. Carbon molecular sieves such as Carboxen and Carbosieve (Supelco Inc.) are designed for the separation of C1-C3 hydrocarbons and permanent gases. Higher molecular weight material is frequently adsorbed irreversibly. 

The relatively recent synthesis of a family of silica based molecular sieve materials (designated M41S) has attracted considerable interest because of the potential of these materials for use as larger pore catalysts. There have been several reports describing the synthesis of these materials and it is now recognised that there are a variety of routes by which they may be prepared.11 

The acidic properties of mesoporous molecular sieves rely on the presence of active sites in their framework. In the case of MCM-41 active sites are generated by the introduction of heteroatoms into the structure. In particular, Bronsted acid sites are introduced by isomorphous substitution of A1 for Si which is achieved by hydrothermal synthesis in which charged quaternary ammonium micelles are used as the template for charged alumino-silicate inorganic precursors. 

Tanev et al. have reported the synthesis of mesoporous materials via a route which involves self-assembly between neutral primary amines and neutral inorganic framework precursors.12 The regularity of the pore structure in these materials has been illustrated by lattice images which show a honeycomb like structure. The system of channels of these molecular sieves produces solids with very high internal surface area and pore volume. This fact combined with the possibility of generating active sites within the channels produces a very unique type of acid catalyst. In the case of transition metal substituted M41S, the principal interest lies in their potential as oxidation catalysts, especially Ti and V substituted MCM and HMS type materials, and more recently synthesised large pore materials.13 

Zeolitic materials are most notably used in catalysis for shape-selective reactions. These reactions are mainly acid catalysed however, base-catalysed and oxidation reactions have also been reported and will be discussed here. 

Many of the reported examples of acid-catalysed reactions using zeolites are relevant to large-scale chemical manufacturing rather than fine or speciality chemical synthesis. These reactions will be briefly surveyed here since much of the chemistry may be more widely applicable, especially when applied to the larger pore zeotypes. 

The term molecular sieve refers to the fact that their empty spaces are so small that they allow the passage of small molecules only, such as CH4, CO, N2 or CO2. Typical molecular sieves are dehydrated zeolites, aluminosilicates whose structure (with around 130 variations) is formed by a three-dimensional framework of [SiOJ and [AIO4], coordination tetrahedra, interconnected by oxygen bridges. In this spatial structure, there are interconnected cages and channels with diameters from around 0.3 to 1 nm (Fig. 4.6), either empty or containing weakly bound cations. 

Activated carbon adsorbents generally show very little selectivity in the adsorption of molecules of different size. However, by special activation procedures it is possible to prepare carbon adsorbents with a very narrow distribution of micropore size and which therefore behave as molecular sieves. The earliest examples of carbon molecular sieves appear to have been prepared by decomposition of polyvinylidene dichloride (Saran) but more recently a wide variety of starting materials have been used. Most commercial carbon sieves are prepared from anthracite or hard coal by controlled oxidation and subsequent thermal treatment. The pore structure may be modified to some extent by subsequent treatment including controlled cracking of hydrocarbons within the micropore system and partial gasification under carefully regulated conditions.  

The mole sieve dehydration is a semi-batch process using a solid adsorbent to remove water from a fluid stream. The water adsorbs onto the solid. 

The simplest solid desiccant fixed bed system consists of a minimum of two dryer vessels or towers. One vessel is drying the process 

The flow of the process gas stream through the dryer bed is normally top to bottom. The wet gas stream entering the dryer could either be saturated or slightly unsaturated. As the process stream travels down the dryer bed, moisture is picked up by the desiccant and the gas stream becomes increasingly drier. The process gas stream exits the dryer bottom eventually moisture free. 

All fixed bed diyer designs are based on a time cycle. The time cycle is distributed between the on-line and the on-regeneration. A 48 hour cycle will allocate 24 hours to on-line, when the dryer is drying the process stream, and 24 hours on regeneration, when the dryer bed is being reactivated. 

A simplified process flow diagram for a two-bed absorber is shown in figure 7.4. 

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Gel permeation chromatography, exclusion chromatography. gel filtration chromatography. A technique for separating the components of a mixture according to molecular volume differences. A porous solid phase (a polymer, molecular sieve) is used which can physically entrap small molecules in the pores whilst large molecules pass down the column more rapidly. A solvent pressure up to 1000 psi may be used. 

For physical processes, two examples are the elimination of normal paraffins from a mixture by their adsorption on 5 A molecular sieves or by their selective formation of solids with urea (clathrates)... 

This type of analysis requires several chromatographic columns and detectors. Hydrocarbons are measured with the aid of a flame ionization detector FID, while the other gases are analyzed using a katharometer. A large number of combinations of columns is possible considering the commutations between columns and, potentially, backflushing of the carrier gas. As an example, the hydrocarbons can be separated by a column packed with silicone or alumina while O2, N2 and CO will require a molecular sieve column. H2S is a special case because this gas is fixed irreversibly on a number of chromatographic supports. Its separation can be achieved on certain kinds of supports such as Porapak which are styrene-divinylbenzene copolymers. This type of phase is also used to analyze CO2 and water. 

Feedstocks are natural gas, refinery fuel gas, LPG and paraffinic naphthas. After elimination of CO2, the last traces of contaminants are converted to methane (methanation) or eliminated by adsorption on molecular sieves (PSA process). 

Gases which are high in FIjS are subject to a de-sulphurisation process in which H2S is converted into elemental sulphur or a metal sulphide. There are a number of processes based on absorption in contactors, adsorption (to a surface) in molecular sieves or chemical reaction (e.g. with zinc oxide). 

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

D. W. Breck, Zeolite Molecular Sieves, Wiley-Interscience, New York, 1974. 

W. H. Flank and T. Whyte, Perspective in Molecular Sieve Science, ACS Symposium Series, No. 135, American Chemical Society, Washington, DC, 1980. 

Zeolites (section C2.13) are unique because they have regular pores as part of their crystalline stmctures. The pores are so small (about 1 nm in diameter) that zeolites are molecular sieves, allowing small molecules to enter the pores, whereas larger ones are sieved out. The stmctures are built up of linked SiO and AlO tetrahedra that share O ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The stmcture of faujasite is represented in figure C2.7.11 and that of ZSM-5 in figure C2.7.12. The points of intersection of the lines represent Si or A1 ions oxygen is present at the centre of each line. This depiction emphasizes the zeolite framework stmcture and shows the presence of the intracrystalline pore stmcture. In the centre of the faujasite stmcture is an open space (supercage) with a diameter of about 1.2 nm. The pore stmcture is three dimensional. 

Sumelius I G 1992 Attempted translation of the original old-Swedish paper by Cronstedt Molecular Sieves ed M L Ocelli and H E Robson (New York Van Nostrand Reinhold) pp 1-5... 

Casci J L 1994 The preparation and potential applications of ultra-large pore molecular sieves a review Stud. Surf. Sc/. Catai. 85 329-56... 

Kresge C T, Leonowicz M E, Roth W J, Vartuli J C and Beck J S 1992 Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism Nature 359 710-12... 

Flanigen E M, Bennet J M, Grose R W, Cohen J P, Patton R L, Kirchner R M and Smith J V 1978 Silicalite a new hydrophobic crystalline silica molecular sieve Nature 271 512-16... 

Perego G, Millini R and Bellussi G 1998 Synthesis and characterization of molecular sieves containing transition metals in the framework Moiecuiar Sieves Science and Technoiogy vol 1, ed FI G Karge and J Weitkamp (Berlin ... 

Schunk S A and Schuth F 1998 Synthesis of zeolite-like inorganic compounds Molecular Sieves Science and Technology vo 1, ed H G Karge and J Weitkamp (Berlin Springer) pp 229-63... 

Martens J A and Jacobs P A 1999 Phosphate-based zeolites and molecular sieves Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 53-80... 

McDaniel C V and Maher P K 1968 New ultra-stable form of fau]asite Molecular Sieves (London Society of Chemical Industry) pp 186-95... 

Nadimi S, Oliver S, Kuperman A, Lough A, Ozin G A, Garces J M, Olken M M and Rudolf P 1994 Nonaqueous synthesis of large zeolite and molecular sieve crystals Stud. Surf. Sol. Catal. 84 93-100... 

This book concentrates on synthesis and identification methods for molecular sieves including nonaluminosilicate molecular sieves and gives a good overview of structures and patented materials. 

Catalytic system addition of molecular sieves to "soak" up any water with 3A sieves, 5-10 mol % catalyst is used,... 

The reaction of a halide with 2-butene-1,4-diol (104) affords the aldehyde 105, which is converted into the 4-substituted 2-hydroxytetrahydrofuran 106, and oxidized to the 3-aryl-7-butyrolactone 107[94], Asymmetric arylation of the cyclic acetal 108 with phenyl triflate[95] using Pd-BINAP afforded 109, which was converted into the 3-phenyllactone 110 in 72% ee[96]. Addition of a molecular sieve (MS3A) shows a favorable effect on this arylation. The reaction of the 3-siloxycyclopentene 111 with an alkenyl iodide affords the. silyl... 

The carbonylation of some alkyl halides such as iodocyclohexane (911) can be carried out under neutral conditions in the presence of N,N,N.N-tetre,-methylurea (TMU), which is a neutral compound, but catches generated hydrogen halide. Molecular sieves (MS-4A) are used for the same pur-pose[768]. Very reactive ethyl 3-iodobutyrate (912) is carbonylated to give ethyl methylsuccinate (913) in the presence of TMU. The expected elimination of HI to form crotonate, followed by carbonylation, does not occur. 

Molecular sieves 4X Molecules with effective diameter >4A 0.001 0.18 250... 

R. M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves , p. 174, Academic Press, London and New York (1978). 

Fig. 4.20 DR plots for carbon dioxide adsorbed at 293 K on Linde molecular sieves. O, powder SA , powder 4A. (Reduced from the original diagram of Lamond and Marsh. )...

The lower pressure sub-region is characterized by a considerable enhancement of the interaction potential (Chapter 1) and therefore of the enthalpy of adsorption consequently the pore becomes completely full at very low relative pressure (sometimes 0 01 or less), so that the isotherm rises steeply from the origin. This behaviour is observed with molecular sieve zeolites, the enhancement of the adsorption energy and the steepness of the isotherm being dependent on the nature of the adsorbent-adsorbate interaction and the polarizability of the adsorbate. -... 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

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