Porous Polymers as Adsorbents

POROUS POLYMERS AS ADSORBENTS 6.13.1 Porous and Coordination Polymers... 

Barns, R. D., Law, L. M., MacLeod, A. J. Comparison of Some Porous Polymers as Adsorbents for Collection of Odor Samples and the Application of the Technique to an Environmental Malodor, Analyst 106, 412 (1981)... 

The use of porous polymers as adsorbents offers a number of advantages, among them the high... 

R. D. Barnes, L. M. Law, and A. J. MacLeod, Comparison of some porous polymers as adsorbents for collection of odor samples and the application of the technique to an environmental malodor. Analyst 106 412 (1981). 

Various sample enrichment techniques are used to isolate volatile organic compounds from mammalian secretions and excretions. The dynamic headspace stripping of volatiles from collected material with purified inert gas and trapping of the volatile compounds on a porous polymer as described by Novotny [3], have been adapted by other workers to concentrate volatiles from various mammalian secretions [4-6]. It is risky to use activated charcoal as an adsorbent in the traps that are used in these methods because of the selective adsorption of compounds with different polarities and molecular sizes on different types of activated charcoal. Due to the high catalytic activity of activated charcoal, thermal conversion can occur if thermal desorption is used to recover the trapped material from such a trap. 

C/W emulsions may be used as templates in the formation of porous polymers, as shown in Figure 5 (52). Polymerization takes plaee in the aqueous phase eontinuous channels between the CO2 droplets. The CO2 is vented and the water is removed to form a porous polymer. The median pore diameter on the order of 1 pm reflects the size of the original CO2 droplets. Polymer foams may be used as adsorbents, as substrates for catalysts, and as scaffolds in biomedical engineering. 

Carbon monoxide is a gas which is very difficult to analyze since it is a strong dipole and is very volatile. On porous polymer type adsorbents, carbon monoxide elutes very quickly and is difficult to quantify in the presence of inert gases. The molecular sieve type of adsorbent generates sufficient retention to separate carbon monoxide, as can be seen in Fig. 7-8. 

The principal adsorbents used in GSC are silica, alumina, graphltlzed carbon blacks, porous polymer beads, zeolites and cyclodextrlns [8,430,431,445]. The bonded phase sorbents discussed in section 2.2.3 could also be considered as modified adsorbents in many respects. 

The retention mechanism of organic solutes by porous polymer beads remains ambiguous [478]. At low temperatures adso tion dominates but at higher temperatures the polymer beads could behave as a highly extended liquid with solvation interactions. The evidence for a partition mechanism is not very strong and its importance, at present, remains speculative. Like other adsorbents it has proven possible to control retention and enhance efficiency by diluting porous polymers with an inert support material (479). 

In gas-solid chromatography (GSC) the stationary phase is a solid adsorbent, such as silica or alumina. The associated virtues associated therewith, namely, cheapness and longevity, are insufficiently appreciated. The disadvantages, surface heterogeneity and irreproducibility, may be overcome by surface modification or coating with small amounts of liquid to reduce heterogeneity and improve reproducibility 4,15). Porous polymers, for example polystyrene and divinyl benzene, are also available. Molecular sieves, discussed in Chapter 17, are used mainly to separate permanent gases. 

As stated earlier the procedure for this analysis is based largely on the methods developed by Hangartner.(4) Figure 1 outlines the scheme utilised for sample processing and analysis. In addition to the detection system already discussed the only other significant difference in this work is the choice of adsorbent which is Carbotrap D-l a graphitised carbon black (GCB). The use of GCB s in environmental analysis is well documented in the literature both as column materials and adsorbants. (7, 8) Initial work within Severn Trent confirmed the claimed superiority of GCB s compared with adsorbents based on porous polymers such as Tenax GC. No evaluation of the relative merits of GCB s and activated carbons have been made at this laboratory but tests with the latter are likely in the future. 

Other solid sorbents have been found more suitable than charcoal for a number of compounds. Silica gel and alumina have been used as a complement to charcoal when sampling polar compounds, but water vapor is strongly adsorbed on these sorbents which leads to deactivation of the sorbent and breakthrough of the compounds by frontal elution. Difficulties also arise with compounds that hydrolyze easily. Alternative sorbents for the collection of polar organic compounds which are sensitive to hydrolysis are porous polymers such as the Chromosorb porous polymer series, Porapak porous polymer series, Tenax-GC and Amberlite XAD sorbent series. 

Porous polymers, such as Porapak and the Chromosorb Century Series, are the most widely used adsorbents. Porapak Q and Chromosorb 102 are both styrenedivinylbenzene polymers, have similar separation characteristics and are the most widely used of the porous polymers. The other members of the respective series differ in composition and consequently in selectivity. 

Adsorbent choice. The choice of adsorbent material depends on the volatile compounds in the food. Of the synthetic porous polymers, the most widely used and best overall adsorbent is Tenax TA (poly-2,6-diphenyl-p-phenylene oxide) 60 to 80 mesh. While Tenax does not show an adsorption capacity for all volatiles, especially very small polar compounds such as acetaldehyde, it has good thermal stability and desorption capabilities. It also traps little water and generates very few artifacts. Table G1.2.2 shows a few limitations and advantages of various adsorbents, all of which can be purchased from chromatography suppliers. If very small volatiles are the goal, various Carbosieves could be used, or traps containing several adsorbents in series. Traps with mixed adsorbents should be desorbed immediately, before transfer between phases occurs. 

More concretely, we are interested in highly cross-linked, permanently porous polymers. These materials, which have a permanent porous structure produced during their synthesis and preserved in the dry state [203], are employed in a broad variety of applications [204] as adsorbents and ion exchangers [191-194],... 

Air samples are usually collected to solid adsorbents such as Tenax, XAD resins, graphitized carbons (e.g. Carbopak), active charcoal, or porous polymers (e.g. Chromosorb). The chemicals are eluted from the adsorbent to a liquid or gas phase by liquid-solid elution or extraction or by thermal desorption. Extraction is the most common method. Thermal desorption can be applied when analysis is by GC (gas chromatography) method, and, recently, the use of automated thermal desorption has been proposed to provide increased sensitivity in GC/MS analysis of a wide range of CWC-related chemicals 8. ... 

The distinguishing features of gas chromatography are a gaseous mobile phase and a solid or immobilized liquid stationary phase. Liquid stationary phases are available in packed or capillary columns. In the packed columns, the liquid phase is deposited on a finely divided, inert solid support, such as diatomaceous earth or porous polymer, which is packed into a column that typically has a 2- to 4-mm id and is 1 to 3 m long. In capillary columns, which contain no particles, the liquid phase is deposited on the inner surface of the fused silica column and may be chemically bonded to it. In gas-solid chromatography, the solid phase is an active adsorbent, such as alumina, silica, or carbon, packed into a column. Polyaromatic porous resins, which are sometimes used in packed columns, are not coated with a liquid phase. 

Adsorbents are used in medicine mainly for the treatment of acute poisoning, whereas other extracorporeal techniques based on physico-chemical principles, such as dialysis and ultrafiltration, currently have much wider clinical applications [1]. Nevertheless, there are medical conditions, such as acute inflammation, hepatic and multi-organ failure and sepsis, for which mortality rates have not improved in the last forty years. These conditions are usually associated with the presence of endotoxin - lipopolysaccharide (LPS) or inflammatory cytokines - molecules of peptide/protein nature [2]. Advantages of adsorption over other extracorporeal techniques include ability to adsorb high molecular mass (HMM) metabolites and toxins. Conventional adsorbents, however, have poor biocompatibility. They are used coated with a semipermeable membrane of a more biocompatible material to allow for a direct contact with blood. Respectively, ability of coated adsorbents to remove HMM solutes is dramatically reduced. In this paper, preliminary results on adsorption of LPS and one of the most common inflammatory cytokines, TNF-a, on uncoated porous polymers and activated carbons, are presented. The aim of this work is to estimate the potential of extracorporeal adsorption technique to remove these substances and to relate it to the porous structure of adsorbents. 

In GSC, separation occurs based on differences in the adsorption of the various components in the sample onto the solid adsorbent. While GSC may not offer as much flexibility in stationary phase functionality as GLC, it has its own advantages. For separation applications, advantages include higher available operating temperatures, higher column efficiencies, and no stationary phase leakage. Typical solid phases for GSC include zeolites, silica gel, activated alumina, carbon, carbon molecular sieves, diatomites, and porous polymers. 

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