Catalyst – solid sorbent – or support therefor: product or process – Solid sorbent – Inorganic gel containing
Reexamination Certificate
2000-10-10
2003-11-11
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Solid sorbent
Inorganic gel containing
C502S407000, C502S408000, C502S172000
Reexamination Certificate
active
06645908
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to sorbents used for the analysis of organic contaminants, particularly to sol-gel derived sorbents and more particularly to copolymerized sol-gel derived sorbents with controlled pore sizes and surface areas used as air-sampling sorbents for the analysis of organic contaminants including organic explosives.
BACKGROUND OF THE INVENTION
Until recently, the needs for improved solid sorbent sampling media have been largely unrecognized. Dramatic improvements in sensor and analytical instrument sensitivity have relegated interest in improved sampling materials to secondary status. However, existing regulatory drivers and/or remediation/containment monitoring requirements are forcing the environmental analytical chemist to detect analytes at lower levels where instrumental sensitivity improvements are more difficult to achieve. Sample concentration, prior to sensing or analytical detection, will be required to achieve the more sensitive detection limits.
Solid sorbents have been used for a number of years for sampling of environmental contaminants. The use of small, robust multisorbent traps already has found application in mainstream analytical methodologies and exhibited the potential for substantial cost savings. For example, described in the
Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air
are U.S. Environmental Protection Agency methods for monitoring volatile and semivolatile organic compounds in air samples, several of which employ sorbent sampling techniques. In fact, each of the first two methods calls for a different sorbent sampling technique, followed by thermal desorption to a capillary gas chromatograph column for analysis. In Method TO-1, compounds are trapped on TENAX™ (poly(2,6-diphenyl)-phenylene oxide, Enka Research Institute Arnhem), a porous polymer adsorbent, transferred to a cold trap, and desorbed to the column. In Method TO-2, compounds are trapped on a carbon molecular sieve adsorbent, transferred to a cold trap, and desorbed to the column.
Various materials have been developed over the years for use as sorbents for capturing and releasing organic analytes. Activated charcoal was one of the first widely used solid sorbent materials. However, its properties are such that, for many analytes, it does not release the sorbate under a solvent wash, and can often promote chemical transformation of the sorbed analytes. Today, activated charcoal is typically used for industrial hygiene applications, where the stability of the analyte in question has been confirmed, and quantities of the target analyte are not small. Porous polymers, such as the CHROMOSORB 106™ (styrene/divinylbenzene copolymer, Manville Corporation) and TENAX-GC™, increased in popularity because they could be thermally desorbed and reused. However, TENAX-GC™, the most widely used of these sorbents, suffers of poor retention of polar molecules and more volatile species. More importantly, TENAX™ can decompose, slightly, when it is heated, resulting in a number of artifacts being observed in supposed blank samples.
In the late 1980's,nonspecific carbon-based sorbents, based on sintered carbon blacks and carbon molecular sieves, became commercially available. These materials have excellent thermal stability, and because of their nonspecific adsorption characteristics, are useful for collection of several types of organic volatiles or semivolatiles. Carbon sorbents are also known to have “good” thermal stability and are good sorbents for a wide range of organics such as hydrocarbons, chlorinated solvents, nitriles, ketones, etc. However, the carbon-based solid sorbents utilized in currently used systems suffer from some important limitations. The volatility range of analytes which any one sorbent can collect is relatively narrow, necessitating the use of traps filled with the multiple sorbents described above in a “parfait” configuration. That is, the sorberts are packed sequentially in a bed, with increasingly retentive sorbents downstream. The traps can be thermally desorbed, during which time the desorption flow is the reverse of that used for sampling. While these so-called “multi-sorbent traps” are extremely useful in a variety of active air sampling applications, the multilayer configuration of the traps precludes their use for passive sampling. Also, if the thermal pulse during the desorption phase of the analysis is not closely synchronized with the desorption flow, sorbates can be pushed off one sorbent bed and be irreversibly sorbed on a more retentive sorbent bed.
The silicate sol-gel technique is a process whereby a tetraalkylorthosilicate such as tetramethylorthosilicate (TMOS), is hydrolyzed under acid or base catalysis to first produce a sol which subsequently gels and proceeds to form a xerogel. The sol is a colloidal suspension of small (1 nm to 1000 nm diameter) particles of polymerized silicate. The suspension is thermodynamically stable and upon further reaction proceeds to produce a gel. The gel is a solid structure formed from the reacting silicate and containing within it a continuous liquid phase. Further curing of the gel by removal of all trapped water leads to the formation of a xerogel. The low processing temperatures used to form these glasses allow for construction of inorganic materials with entrapped organic guests. The sol-gel process is typically used to produce highly hydroxylated materials.
Materials for use as organic analyte sorbents must contain covalently bonded nonpolar moieties to promote favorable analyte-substrate interactions. Several methods have been developed that allow for the production of composite inorganic-organic materials. Alkyltrialkoxysilanes, such as methyltrimethoxysilane (MTMS), can be polymerized to form xerogels wherein an alkyl substituent is attached directly to silicon (Si) atoms in the backbone of the polymer network. These materials have organic moieties throughout and are substantially different from surface derivatized materials. Another approach to preparing composite inorganic-organic materials involves derivatization of the surface of inorganic oxides with a variety of organic reagents. This approach has been widely employed to modify the surface of chromatographic silica gels, for example. A variety of technologies for preparation of siloxane, [SiO
2
]—O—Si—R, bonded materials exists. These methods often produce materials having varied thermal and hydrolytic instabilities. One reason for their instability is that methods based on derivatizing surface hydroxyl groups of silica-based gels are not able to cap all hydroxyl groups as a result of the bulky size of the derivatizing agents. Underivatized and exposed surface hydroxyl groups contribute to the thermal and hydrolytic instability of many supports.
An approach that has been used to circumvent this problem involves the use of trifunctional silanes such as RSiCl
3
and RSi(OR′)
3
as surface modifying reagents. It has been reported in
Analytical Chemistry
, v. 65 (1993), pp. 822-826, that trichlorosilanes can be induced to undergo attachment with simultaneous lateral polymerization to form a siloxane coating, when the reaction is conducted on a silica gel that contains a monolayer of water on its surface. Although not studied in detail, the polymeric bonded phases were reported to have substantially improved hydrolytic stabilities even at pH 1.8 and 10.0. This research was aimed at making high carbon density surfaces with mixed C
3
and C
18
coatings for chromatography purposes. However, the procedure is general for other “R” groups.
Another intriguing approach stems from a recent report in
Journal of American Chemical Society
, vol. 117, pp. 2112-2113 on the reactions of ethoxysilanes with silica gels. Highly dehydroxylated silica gels were prepared by heating at 600° C. Reaction of this material with a monoethoxysilane reagent gave a highly derivatized surface, as determined by solid-state NMR, that quantitatively retained the ethoxy groups. This finding was inter
Dindal Amy B.
Sigman Michael E.
Johnson Edward M.
Silverman Stanley S.
Stafford Shelley L.
UT-Battelle LLC
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