Molecular sieves Subject

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). 

Natural gas from MESA s wells flows into a gathering system where pressure is increased to 7 bar (100 psig). Multiple booster stations raise it to 34 bar (500 psig) before gas enters the plant for separation. When gas enters the LNG recovery unit, its pressure must be raised again to 66 bar (950 psig). It is then subjected to a molecular sieve process for moisture removal. A series of heat exchangers lowers the temperature to -34°C (-30°E). 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

Hematoporphyrin dimethyl ester (15, 1.52 g, 2.43 mmol) (diastereomeric mixture) and Af.At-dimethyl-acetamide dimethyl acetal (8 mL) were suspended in o-xylcnc (100 mL), degassed and then heated with exclusion of light in a flask equipped with a reflux condenser and a Soxhlet apparatus containing 3 A molecular sieves. The temperature was raised during 15 min from rt to 115 C and kept at this temperature for 30 min. Then the temperature was raised to 155 C and the mixture kept at this temperature for 3 h. The mixture was evaporated in a bulb tube and the residue subjected to column chromatography [silica gel (ICN), CH2Cl2/MeOAc/MeOH 10 5 0.5] with exclusion of light yield of pure 16A 305 mg (17 %) yield of pure 16B, 375 mg (20%) and 187 mg (10%) of a mixture of 16 A and B. 

The composition of the gas stream before and after contact with the catalyst was monitored by subjecting aliquots to gas chromatographic analysis using a Ohkura Model 701 gas chromatograph and two columns. For N2 and O2, a 1 meter molecular sieve 5A column operating at 65°C was employed for N2O, a 2 meter Porapak Q column operating at 89°C was used. 

The Merck process group subsequently published a more detailed route amenable towards multikilogram scales (Blacklock et al., 1988). This synthesis begins with treatment of alanine with phosgene to produce A-carboxyanhydride (NCA) 16 (Scheme 10.3). Under basic aqueous conditions this anhydride is coupled with proline to produce, upon acidic work-up, the dipeptide alanyl-proline (14). Enalapril is then prepared in one synthetic step by a diastereoselective reductive amination between ethyl-2— oxo-4-phenylbutyrate (13) and 14. This reaction was the subject of extensive optimization, and it was found that the highest diastereoselectivity was obtained by hydrogenation over Raney nickel in the presence of acetic acid (25%), KF (4.0 equiv.), and 3 A molecular sieves (17 1 dr). Enalapril is then isolated in diastereomerically pure form as its maleate salt (Huffman and Reider, 1999 Huffman et al., 2000). 

Imbalance in the stoichiometry of polycondensation reactions of AA-BB-type monomers can be overcome by changing to heterofunctional AB-type monomers. Indeed, IIMU has been subjected to bulk polycondensation using lipases as catalyst in the presence of 4 A molecular sieves. At 70 °C, CALB showed 84% monomer conversion and a low molecular weight polymer (Mn 1.1 kDa, PDI 1.9). No significant polymerization was observed with other lipases (except R cepacia lipase, 47% conversion, oligomers only) and in reference reactions with thermally deactivated CALB or in the absence of enzyme. Further optimization of the reaction conditions (60wt% CALB, II0°C, 3 days, 4 A molecular sieves) gave a polymer with Mn of 14.8 kDa (PDI 2.3) in 86% yield after precipitation [42]. 

The physical-chemical properties of a synthetic gallophosphate molecular sieve, the 30-A supercage cloverite , have been assessed [18]. Instead of attempting to list the burgeoning number of fullerene publications, attention is drawn to the formation and characterization of fullerene-like nanocrystals of tungsten disulfide [19,20]. Preparation, characterization and utilization of carbon nanotubes have been the subject of a number of reports from several laboratories [21-27]. 

Tphe breakthrough curve for a fixed-bed adsorption column may be pre-dieted theoretically from the solution of the appropriate mass-transfer rate equation subject to the boundary conditions imposed by the differential fluid phase mass balance for an element of the column. For molecular sieve adsorbents this problem is complicated by the nonlinearity of the equilibrium isotherm which leads to nonlinearities both in the differential equations and in the boundary conditions. This paper summarizes the principal conclusions reached from a recent numerical solution of this problem (1). The approximations involved in the analysis are realistic for many practical systems, and the validity of the theory is confirmed by comparison with experiment. 

MCM-41 and silicalite-1 can be synthesized in the presence of niobium- and tantalum-containing compounds. The results indicated that Nb(V) and Ta(V) are well dispersed in the framework of silicalite-1 and in the amorphous walls of MCM-41 y-irradiation of activated niobium and tantalum molecular sieves show two radiation induced hole centers (V centers) located on Si-O-Si and M-O-Si (M = Nb, Ta) units. True isomorphous substitution as suggested in the literature for Ti(IV), however, is unlikely to be present Nevertheless, interesting chemical and catalytic properties can be expected from these systems and are subject to further studies... 

The development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3], Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fine chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. 

Powdered, particulate MCM-41 molecular sieves (Si/Al = 37) with varied pore diameters (1.80, 2.18, 2.54 and 3.04 nm) were synthesized following the conventional procedure using sodium silicate, sodium aluminate and C TMAB (n = 12, 14, 16 and 18) as the source materials for Si, A1 and quaternary ammonium surfactants, respectively [13]. Each sample was subjected to calcination in air at 560 °C for 6 h to remove the organic templates. The structure of the synthesized material was confirmed by powder X-ray diffraction (XRD) and by scanning/transmission electron microscopy. Their average pore sizes were deduced from the adsorption curve of the N2 adsorption-desorption isotherm obtained at 77 K by means of the BJH method (Table 1). 

To a stirred mixture of 14 [39] (493 mg, 1.0 mmol), NaCNBH, (383 mg, 6.0 mmol) and 3-A molecular sieves in MeCN (20 mL) at room temperature was added as solution, kept at 0°C, of MejSiCl (652 mg, 6.0 mmol) in MeCN (6 mL). The reaction mixture was stirred for 5 h at room temperature, filtered through Celite, and poured into ice-cold saturated aqueous NaHCOj. The aqueous phase was repeatedly extracted with CHjClj. The combined extracts were washed with saturated aqueous NaHC03, dried (MgSOj), filtered, and concentrated. The residue was subjected to silica gel column chromatography (toluene/ethyl acetate 2 1) to yield 15 (375 mg, 76%), [< ] +19.3° (c 1.0, CHClj). Regioisomer 16 (13%) was also obtained. 

Although a number of books now contain chapters or sections dealing with zeolites, this subject has been the primary focus in very few volumes. The most noteworthy of these is the treatise by D. W. Breck, Zeolite Molecular Sieves, published by Wiley in 1974. The earliest modem ones are the Russian texts on Molecular Sieves and Their Use, published by V. A. Sokolov, N. S. Torocheshnikov, and N. V. Kel tsev in 1964, and S. P. Zhdanov s Chemistry of Zeolites in 1968. Other significant works include the 1976 ACS Monograph on Zeolite Chemistry and Catalysis edited by J. A. Rabo, and the volume on Zeolites and Clay Minerals as Sorbents and Molecular Sieves by R. M. Barrer, published in England in 1978. 

Amines. Triethylamine (TEA) (Pennsalt and Eastman), like all tertiary alkylamines, is subject to light-catalyzed air oxidation. These contaminants are removed readily along with moisture by passing through a column of (or standing over) freshly activated silica gel. (Note molecular sieves must not be used here—these seem to accelerate decomposition and color formation.) The amine should be dry and colorless before use. Other amines used in this work were obtained from Eastman and purified by distillation immediately before use. 

The question of which lattice components to include in the interpretive analysis reduces to the experimental problem of comparing systems of similar geometry but which contain different species in the lattice—e.g., Ge or Th in substitution for Si (1) S or F in substitution for O or OH, and say 2Na+ for lCa+2. The effect of change of cation is well known. In limiting cases it modifies the Molecular Sieve effect in zeolites. For host substituents, the availability of suitable systems is limited. The present work offers a comparison between a normal aluminosilicate and the same species after subjection to a substitution of halogen for oxygen and hydroxyl. 

Free formaldehyde is reacted with acetylacetone in the presence of an excess of an ammonium salt to form the yellow fluorescent compound, 3,5-diacetyl-1,4-dihydrolutidine and subsequently determined spectrophotometrically in methods A-E (14). In these methods, the test sample must be colorless and free from other carbonyl compounds. Some other derivatives have been used to analyze formaldehyde. For example, formaldehyde was reacted with sodium 4,5-dihydroxy-2,7-naphthalene disulfonate in sulfuric acid solution to yield a purple color (580 nm) and then subjected to colorimetric analysis. A purple-colored pararosaniline derivative was used to analyze formaldehyde in air (15). Air sample was passed through an aqueous solution which contained 0.4% of 3-methyl-2-benzothiazolone hydrazone hydrochloride and then a dye produced was determined at 635 or 670 nm (16). Molecular sieve (1.6 mm pettet) was used to trap formaldehyde in air samples. The formaldehyde... 

Experience in air separation plant operations and other cryogenic processing plants has shown that local freeze-out of impurities such as carbon dioxide can occur at concentrations well below the solubility limit. For this reason, the carbon dioxide content of the feed gas subject to the minimum operating temperature is usually kept below 50 ppm. The amine process and the molecular sieve adsorption process are the most widely used methods for carbon dioxide removal. The amine process involves adsorption of the impurity by a lean aqueous organic amine solution. With sufficient amine recirculation rate, the carbon dioxide in the treated gas can be reduced to less than 25 ppm. Oxygen is removed by a catalytic reaction with hydrogen to form water. 

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