Charles Gardner Coe

Preparation of carbon molecular sieves, I. Two-step hydrocarbon deposition with a single hydrocarbon

Abstract A process is described for preparing a carbon molecular sieve that is suitable for the kinetic separation of gases, such as oxygen from nitrogen. The process involves modifying a carbon support, having a majority of micropores with an effective pore size of about 4.5 to 20 A, using a two-step process in which the sieve is contacted with two different concentrations of a volatile carbon-containing organic compound. The concentration of the carbon-containing compound used in the first step is larger than that in the second step, so that the pore openings of the micropores of the support are narrowed successively in two distinct steps without excessively filling the micropores themselves.

Granular carbon molecular sieves

Abstract Any future carbon molecular sieve (CMS) for use in the commercial production of N2 should have a high volumetric O2 equilibrium capacity. We developed a procedure for producing a high capacity coconut shell char that can be converted into a high capacity CMS for air separation. Granules of coconut shell char are heated in flowing inert gas at about 2 to 12°C per minute to a peak temperature of 775° to 900°C. After holding from 1 to 8 hours, the char is cooled in an inert gas atmosphere. The granular char thus produced has an oxygen capacity in excess of 8.0 cc/cc. Contacting the char with an oxidizing atmosphere containing CO2, H2O, or O2 at 650° to 900°C increases the O2 capacity to greater than 9.0 cc/cc. The coconut shell char can be converted to a CMS by treatment with a volatile carbon-containing organic compound that, when pyrolyzed, deposits carbon within the interior of the carbon granules. The granular CMS thus produced can be used in a nitrogen pressure swing adsorption column without the need for pelletization.

High micropore volume low silica EMT-containing metallosilicates

The present invention is a composition, a synthesis of the composition and a method of using the composition for selectively adsorptively separating nitrogen from oxygen wherein the composition is a crystalline EMT with a Si/Al ratio less than 2.0 and a micropore volume determined in the sodium and/or potassium form of at least 0.20 cm3 /g and a lithium cation exchange of at least 80%, preferably including an intergrowth with a crystalline FAU structure, wherein the pure or intergrowth compositions have the chemical formula: M.sub.2/n O:X.sub.2 O.sub.3 :(2.0 to <4.0)SiO.sub.2 wherein M=one or more metal cations having a valence of n, and X is selected from the group consisting of aluminum, gallium and boron, preferably aluminum.

Measurement of intracrystalline diffusion of nitrogen in zeolites NaX and NaCaA using pulsed field gradient n.m.r.

The measurement of intracrystalline diffusion (D intra ) of nitrogen in zeolites by 15 N pulsed field gradient (p.f.g.) n.m.r. is complicated by the small magnetogyric ratio. This causes a large reduction in signal sensitivity, necessitating a significant enhancement of the field gradient pulse intensity in comparison with 1 H p.f.g. n.m.r. Owing to recent progress in measuring techniques, these technical problems were overcome and direct measurement of the intracrystalline self-diffusion coefficients of nitrogen in 12- to 15-μm crystals of zeolite NaX and NaCaA in a temperature range between 135 and 300 K was completed. The intracrystalline diffusion for both these zeolites was found to be a hundred times slower than the long-range diffusion within the crystal bed. At present, these measurements are not possible for typical A- and X-type zeolites, which have diameters less than 10 μm.

DRIFTS and Raman Investigation of N2 and O2 Adsorption on Zeolites at Ambient Temperature

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and Raman spectroscopy were used to examine N2 and O2 adsorption on cation-exchanged (K, Na, Sr, Ca, and Li) low silica X (LSX) zeolites. IR and Raman absorption bands were observed for the molecular vibration of adsorbed N2 and O2 at room temperature and atmospheric pressure. The intensity (in Kubelka-Munk units) of the IR band increased with N2 pressure and mirrored the adsorption isotherm for N2. Both O2 and N2 displayed a similar dependence of the molecular vibrational frequency on cation charge density, which suggests that both gases are interacting directly with the cations. The vibrational frequencies for adsorbed N2 and O2 were more sensitive to the cation charge density than to framework Al content. Infrared studies of N2 and O2 on mixed cation forms of LSX show that N2 interaction was localized near individual cations within the sorption cavity of the zeolite. Thus, adsorbed N2 can be used to probe accessibility of specific cations within the zeolite framework. The spectroscopic data are consistent with the theory that the stronger interaction of N2 over O2 is caused by the stronger influence of the electric field with the larger quadrupole of N2.

Defining effective microporosity in carbon molecular sieves

Abstract A pseudo equilibrium method is described for establishing the micropore distribution in carbon molecular sieves (CMS) having micropore sizes below 6 A. The method uses a series of carefully selected molecular probes varying in their minimum Van der Waals diameter between 3.7 and 6.0 A. Various small-pore zeolites were used to validate the method. Over the critical micropore size range these probes can establish the “effective” microporosity which can be modified by post-treatment to produce an O 2 -selective CMS useful for air separation. This method coupled with kinetic measurements of O 2 and N 2 uptake indicates that pores larger than 4 A are not highly O 2 -selective. Results from these molecular probe studies in combination with porosimetry and pycnometry measurements show that the O 2 -selective carbons have an unusual bimodal pore distribution with no significant mesoporosity. We found obtaining pore distributions from equilibrium analysis of N 2 or O 2 isotherms at cryogenic conditions was inappropriate on these carbons. The study clearly demonstrates the importance of the method which monitors the size and relative amounts of micropores which are O 2 -selective.

Structural Effects on the Adsorptive Properties of Molecular Sieves for Air Separation

The structural variability of microporous adsorbents provides a means for controlling the equilibrium and kinetic adsorption parameters over a wide range. Specific structural features can be combined to provide materials having adsorption properties best suited for a particular gas separation application. Results from the development of adsorbents for producing oxygen and nitrogen from air will be given to illustrate the importance of molecular structure on selectivity, gas capacity, and uptake rates. Zeolitic adsorbents are used to produce oxygen and operate under equilibrium conditions. Both N2 capacity and selectivity are strongly influenced by the type, size, location, and number of accessible cations present in zeolitic adsorbents as well as the structure type. Carbon molecular sieves (CMS) are a subgroup of activated carbon which are only mildly selective for O2 over N2 at equilibrium; however, the pore structure is carefully controlled to allow O2 to adsorb 30 times faster than N2 providing a means to produce N2 via a kinetic-based process. The effective micropore size of the CMS can be controlled by the deposition of hydrocarbons. The ability to control the size of the micropore openings without significantly altering the rate of gas uptake is a major synthetic challenge. Molecular probe studies illustrate how changes of 0.3A in the effective micropore size dramatically alter the separating ability of the CMS. Tailoring adsorbent structure and composition leads to advanced materials with improved properties for producing oxygen and nitrogen from air.