Zeolite color changes

A very convenient method to quantitatively determined the number of Bronsted add sites in the often used photochemical nano-vessels, zeolites X and Y, is available.28 This method take advantage of indicator/probe molecules which undergo an intense color change upon protonation within the zeolite pore network. The amount of a base necessary to quench the color change gives a direct measure of the concentration of acidic sites. The base used to titrate the Bronsted sites must be more basic than the probe molecule and sufficiently basic to be completely protonated. 

These oxidations suffer from the fact that the high selectivities are only observed at low conversions (<7%). At higher conversions, the carboxylic acid products leach the transition metals out of the zeolite framework into solution where the selectivity index is much lower [63]. As these reactions proceed, the 3 -I- oxidation states of the metal ions return to their 2 -I- states, accompanied by their characteristic color change. In the case of MnAlPO-18, the spent catalyst (Mn ) was washed with methanol and reactivated in dry air at 550°C and successfully recycled (Mn Mn ) twice without appreciable loss of activity [64]. 

Acidity and basicity are paired concepts that are very often invoked to explain the catalytic properties of divided metal oxides and zeolites. The concept of acids and bases has been important since ancient times. It has been used to correlate large amounts of data and to predict trends. During the early development of acid-base theory, experimental observations included the sour taste of acids and the bitter taste of bases, color changes in indicators caused by acids and bases, and the reaction of acids with bases to form salts. 

Ultramarines are zeolites, though lattice paths are restricted by 0.4 nm diameter channels. The sodium ions can be exchanged for other metal ions (e.g., silver, potassium, lithium, copper). Although this produces marked color change, none of the products have commercial value. 

The natural clay minerals are hydrous aluminum silicates with iron or magnesium replacing aluminum wholly or in part, and with alkali or alkaline earth metals present as essential constituents in some others. Their acidic properties and natural abundance have favored their use as catalysts for cracking of heavy petroleum fractions. With the exception of zeolites and some specially treated mixed oxides for which superacid properties have been claimed, the acidity as measured by the color changes of absorbed Hammett bases is generally far below the superacidity range. They are inactive for alkane isomerization and cracking below 100 °C and need co-acids to reach superacidity. 

In 1962, Ralek et al. reported that after dehydration-rehydration treatment, the silver-exchanged zeolite A changed from white (hydrated) to yellow to orange and finally to brick-red in color.1241 In 1977, Kim and Seff observed a similar phenomenon for Ag-A single crystal when it was treated carefully at 400 °C, and they revealed that after dehydration there exist Ag6° metal clusters in the Ag-A zeolite on the basis of XRD analysis (Figure 9.8).[25] Later, Jacobs et al. revealed that Ag32+ clusters may be present in the zeolite.[261... 

To qualitatively test for Bronsted acidity using the colorimetric method, 50 mg of the zeolite sample was pretreated at 300°C under vacuum for approximately 12 hrs. 1 mL of a dilute solution of retinol (Aldrich) or retinyl acetate (Aldrich) in dry hexane was injected onto the activated zeolite (10,11). A color change to blue indicated the presence of Bremsted acid sites. 

H2O + Si-O-Al M(OH) + Si-0(H )-Al where Si-O-Al represents a part of the zeolite firework (10). It has also been shown that Bronsted acid sites can be present in low concentrations in alkali-metal zeolites. For example, Ramamurthy and coworkers have established the presence of low levels of Bronsted acidity in NaY and NaX zeolites using the color change of a base indicator (10, 11, 19). The acidic forms of retinyl acetate, retinol and retinyl Schiff bases are colored blue and are easily identified by both visual inspection and spectrophotometrically. Using these bases, they were able to determine whether or not Bronsted acids were present in low concentrations in various zeolite samples. Importantly, these studies showed that the presence of small quantities of acid sites... 

The monomer-loaded zeolite samples display dramatic color changes from white to different hues of blue and green when (a) aniline in different acidic zeolite forms is treated with the oxidant, or when (b) pyrrole or thiophene monomers are admitted into Cu(II)/Fe(III)-containing zeolites Y or MOR from the vapor phase or from hexane (or other hydrocarbon) solutions (Table 1). These color changes correspond to those observed in bulk synthesis reactions.H l No reaction is observed with the zeolite sodium forms, indicating that the polymerizations proceed only in the presence of intfazeolite protons and/or appropriate oxidants. No polymer formation is detected in zeolite Cu(II)A (pore size 0.4 nm, smaller than pyrrole or thiophene). This is... 

FIGURE 11. (a) A chromogenic hybrid material containing a pyrylium dye anchored on the zeolite Beta walls, (b) The color changes observed in this hybrid material upon addition of certain amines. 

Herron et al. dried Cd-Y zeolite prior to sulfidation in vacuiun and then, after cooling to 100°C, exposed the material to flowing H2S for 30 min. Finally, the still-white zeolite was evacuated, sealed and transferred to a dry nitrogen glove box for storage [301]. During the final evacuation the white zeolite became pale-yellow. This color change was found to be reversible, i.e., the addition of H2S turned the color of the zeolite back to white [321]. The reaction... 

Because in zeolites the silver clusters change their color when in contact with water, it has been suggested that these compounds may be used as water-vapor-sensing materials. Ozin et al. observed the vapor-pressure chromic, cathode-ray chromic, water chromic, photochromic, and thermochromic properties for silver-sodalite, and they proposed that these compounds hold promise for use as sensors.[30] In addition, the silver clusters in zeolites are also sensitive to other molecules, and therefore, they have great potential as chemical-sensing materials. 

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