Molecular sieve beads

W. Drost was my synthesis assistant from 1950 to 1952 and later developed our methods for making hard, attrition-resistant molecular sieve beads. 

In adsorption chromatography (NPLC), control of the water content in solvents is important. In some cases, it is preferred to mix known amounts of dry and water saturated solvents together in order to know or control the percentage of water saturation. On the other hand, addition of activated molecular sieve beads (4 A or 5 A) to the solvent storage bottle clearly improves their purity and reduces the water content, hnpiu ities, in addition to water, can often be removed by adsorption methods, particularly frontal analysis, which is utiHzed often in LC. 

Fig. 35.4b) AFM measurements shows the AuNP(2a/6) bind selectively to the SAM from aqueous solution. In the second case (Fig. 35.4a), the calixarene-capped nanoparticles bind specifically to the molecular sieve beads primed with 7xTsO from aqueous solution. Both experiments indicated that the cavity of 2a exhibits its characteristic cation binding properties also in aqueous media. 

Type 3A sieves. A crystalline potassium aluminosilicate with a pore size of about 3 Angstroms. This type of molecular sieves is suitable for drying liquids such as acetone, acetonitrile, methanol, ethanol and 2-propanol, and drying gases such as acetylene, carbon dioxide, ammonia, propylene and butadiene. The material is supplied as beads or pellets. 

Dimethyldioxirane was prepared according to the literature procedure2 as an 0.08M solution and was dried over 4a beaded molecular sieves for 24 hr prior to use. 

Catalytic bead Resistance RKI Instruments 0-5% Yes Uses special molecular sieve filter to select H2 + exclude VOCs, CO ... 

The acceptor resin, in a syringe fitted with a Teflon filter, was washed with MeOH (5x) and CH2C12 (5x) and was dried under vacuum for 24 h. The donor imidate (5 equiv) and 4-A molecular sieves (ca. 2-mm beads) were added and the mixture dried in vacuo for 6 h. Dry CH2C12 (2.5 mL) was added and the mixture was stirred at rt under N2 for 1 h. Freshly distilled TMSOTf (0.15 equiv with respect to donor) was added and the reaction stirred for 2h (reaction time depended on the donor but, on average, all reactions were complete after 2 h). The resin was filtered off, washed successively with MeOH (5x) and CH2C12 (5x), and dried. 

The solvent is available commercially in pure form. Distillation from magnesium ethoxide [64] affords fairly dry material (H20 content = 50 ppm), but a better job can be done H20 content = 18 ppm can be obtained by allowing the alcohol to stand over powdered 3-A molecular sieve [52]. It is important that the molecular sieve be powdered the more commonly found beads only reduce the water content to 99 ppm. 

Zeolite molecular sieves (sodium and calcium aluminosilicates) of nominal pore size 0.3... 0.5 nm, nonnally used as beads except in cases where the use of powdered molecular sieve is essential (marked wifli an asterisk). 

The introduction of Sephadex in 1959 provided the biochemist with a new powerful tool for the separation of complex mixtures of biopolymers on the basis of their molecular size. The scope of the technique was further extended by the introduction of the bead-form agaroses which permit separation of particles and molecules up to 40000000 daltons. Early work showed that separation might be influenced by solute-matrix interactions rather than purely steric factors [171], Increasing work has been done on the nature and extent of these interactions and, more recently, these effects have been used to improve and even effect separations on gels such as Sephadex. These interactions have been reviewed recently [172, 173] and in this section we will briefly consider some aspects of solute-matrix interactions and their application in the separation procedure. Recent developments of new molecular sieve media and some new column techniques are also discussed. 

The media most commonly used in gel chromatography (otherwise termed gel filtration or molecular sieve chromatography) are the cross-linked dextrans (Sephadex), bead-form polyacrylamides (Bio-Gel P) and the bead form agaroses (Sepharose, Bio-Gel A, and Indubiose). It is on these gels that most data have been accumulated and the majority of interactions of interest occur. 

Molecular sieve chromatography, useful for separating proteins of different sizes, operates as follows. The protein sample solution is placed in the top of a cylinder (or column) packed with molecular sieves, as shown at the left of Figure 6.28. The sieves consist of tiny balls (or beads) that contain small pores (or holes) of unifonn size. The size is small enough to permit the entry (or passage) of a small protein... 

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