Adsorbents zeolite-based

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

One of the parameters in the broad class of liquid adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. The acidity of zeolitic adsorbent is normally affected by the zeolite Si02/Al203 molar ratio, the ionic radii and the valence of the cations exchanged into the zeolite. In this contribution, Sanderson s model of intermediate electronegativity of zeolitic adsorbent acidity (SjJ can be calculated as a representation of the strength of the adsorbent acidity based on the following equation ... 

Citric acid separation from fermentation broth employs the full allotment of Sorbex beds in addition to the four basic Sorbex zones. The process utilizes a resin instead of a zeolite based adsorbent. The resin is a nonionic cross-linked polystyrene polyvinyl benzene formulation. Operating temperatures for this process are sufficient to overcome diffusion limitations with a corresponding operating pressure to maintain liquid-phase operation. The desorbent consists of water blended with acetone. Subsequent processing steps remove the desorbent from the desired extract product citric acid. 

Zeolites have an enormous impact on our daily lives, both directly and indirectly. For example, upstream hydrocarbons such as aromatics and olefins are produced using zeolite catalysts. The aromatics or olefins are then separated from the reaction mixtures using zeolite adsorbents. The purified components produced by these zeolite-based methods are then used in downstream processes to produce products that we use daily, such as clothes, furniture, foods, construchon materials and materials to build roads, automobile parts, fuels, gasoline, etc. In addihon to the indirect impacts mentioned above, zeolites also have a direct impact on our daily lives. For example, zeolites are used as builders in detergent formulations. 

Chapter 3 outlines zeolite synthesis, modification and the manufacturing of zeolite-based catalysts and adsorbents. Extensive patent references are given to provide the reader with a historical perspective. Some of the pitfalls associated with the operation of synthesis and manufacturing units are also described. 

UV spectroscopy of adsorbed Hammett bases has also been used to estimate the acidity of solids such as zeolites.38... 

Zeolite-based materials are also promising for the removal of organic compounds from industrial waste water (13). This is particularly true for chlorinated pollutants and the preferred process is based on adsorption/separation using hydrophobic molecular sieves (HMS). Compared to carbon adsorbents, HMS presents a good compromise between sorption capacities, selectivity to organics compared to water, and regenerability (vide infra, section 16.3.1.). 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

Cobalt, copper and nickel metal ions were deposited by two different methods, ionic exchange and impregnation, on an amorphous silica-alumina and a ZSM-5 zeolite. The adsorption properties towards NH3 and NO were determined at 353 and 313 K, respectively, by coupled calorimetric-volumetric measurements. The average acid strength of the catalysts supported on silica-alumina was stronger than that of the parent support, while the zeolite-based catalysts had (with the exception of the nickel sample) weaker acid sites than the parent ZSM-5. The oxide materials used as supports adsorbed NO in very small amounts only, and the presence of metal cations improved the NO adsorption [70]. 

A. V. Kiselev Zeolites are porous crystals. This means that we can find the molecular field distribution in their channels. The advantage of describing the adsorption on zeolites using the molecular theory consists in obtaining the constants which have a definite physical meaning (for example, the Henry constant and second virial coefficient). Further development of the theory needs a further improvement of the model based on the investigation of the adsorbate-zeolite systems by the use of modern physical methods. 

Velu, S., Ma, X., and Song, C. Selective adsorption for removing sulfur from jet fuel over zeolite-based adsorbents. Industrial Engineering Chemistry Research, 2003, 42, 5293. 

Recently, great attention has been paid to the ambient temperature adsorption for removing sulfur compounds from natural gas, as the system is simple and is quick and easy to be started up. The major adsorbents that were used in the ambient temperature adsorption are AC-based and zeolite-based materials. The Osaka Gas mixed metal and metal oxide catalyst provides a low-temperature method of desulfurization. The catalysts (or adsorbents) is claimed to remove organic sulfur and H2S at room temperature.115... 

Configurationally biased Monte Carlo techniques [63-65] have made it possible to compute adsorption isotherms for linear and branched hydrocarbons in the micropores of a siliceous zeolite framework. Apart from Monte Carlo techniques, docking techniques [69] have also been implemented in some available computer codes. Docking techniques are convenient techniques that determine, by simulated annealing and subsequent freezing techniques, local energy minima of adsorbed molecules based on Lennard-Jones-or Buckingham-type interaction potentials. 

The Lewis acid-base properties of zeolites received a growing interest recently. A series of works " suggested that the infrared and XPS spectra of adsorbed pyrrole can characterize both the Lewis basicity and the Lewis acidity of alkali-exchanged zeolites. The zeolite base strength decreases with an increase in Si/AI ratio of the framework, and increases when the electropositivity of the counter alkali cation is raised. The Lewis acid strength of counter cations increases with Si/AI ratio and decreases with electropositivity. In other words, the alkali-exchanged zeolites have both Lewis acid and Lewis base centres and provide an easy way to tune up the relative strengths of these two sites. 

Zeolite-based materials are extremely versatile uses include detergent manufacture, ion-exchange resins (ie, water softeners), catalytic applications in the petroleum industry, separation processes (ie, molecular sieves), and as an adsorbent for water, carbon dioxide, mercaptans, and hydrogen sulfide. 

Adsorption of pyridine bases is generally applied to test the surface acid properties of zeolites. Adsorbed pyridine base molecules interact with the surface acid site and the strength of the interaction can be monitored by spectroscopic methods. Komiyama et al. [47] obtained in situ molecular AFM images of well ordered arrays of pyridine and P-picoline on the (010)... 

The nonuniform distribution of the proton-active centers in zeolites can be measured by temperature-controlled desorption of adsorbed organic bases. The bases that are adsorbed on the centers of highest activity require the highest temperature for desorption. The IR spectra of adsorbed bases such as ammonia and pyridine give information about the nature of the adsorption centers. For example, the pyridi-nimn ion is indicative of proton-donor sites. NMR and ESR spectroscopy are also useful for elucidating the nature of acid centers. 

---