Acidity in Zeolite ZSM-5

Skeels, G.W.; Flank, William H.

doi:10.1021/bk-1983-0218.ch022

Cited by 5 publications

(3 citation statements)

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“…Reaction I explains why suspensions of exchanged zeolite crystals in distilled water tend to be basic, whereas at the same time the proton concentration and, therefore, the acidity inside the cavity increases. For example, calcination of zeolite ZSM‐5 at 400 °C leads to the highest acidity due to the formation of Brønsted sites, whereas calcination at 700 °C leads to fully hydrated predominance of Lewis acid sites 3…”

Section: Introductionmentioning

confidence: 99%

Proton Activity Inside the Channels of Zeolite L

Albuquerque

Calzaferri

2007

Chemistry A European J

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The proton activity inside the channels of zeolite L has been studied by investigating dye-loaded zeolite L crystals under different conditions, such as water content, nature of the counterions, and nature of the solvent. The discussion is made within the frame of three types of dye-loaded zeolite L systems, classified according to their ability to exchange matter (dyes, cations, solvent, and other small molecules) with the environment. The classification refers to dye-loaded zeolites. The term "closed" and "semi-open" characterize different possibilities of the channels to exchange small molecules, cations, and solvent molecules with the environment, but not dyes. The "open" systems also allow for dye exchange. UV-visible and fluorescence spectroscopy have been used to observe the proton activity inside the zeolite L channels. The influence of the proton activity on the luminescence of encapsulated dyes is discussed, special attention being given to luminescence quenching by excited-state protonation. Partially proton-exchanged zeolite L can be a superacid, whereas for the M-exchanged form (M: K(+), Li(+), Cs(+), Mg(2+), Ca(2+)) the pH ranges from about 2.5 to 3.5. For these last forms, the differences in pH are due to the acid-base reactions of the respective metal cations with water inside the zeolite. Finally, we describe an easy experimental procedure that can be used to tune the proton activity inside the zeolite L to a considerable extent.

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Section: Introductionmentioning

confidence: 99%

Proton Activity Inside the Channels of Zeolite L

Albuquerque

Calzaferri

2007

Chemistry A European J

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“…Silica gel was transformed into microcrystalline balls of silicalite (34) and into various ordered shapes (35). ZSM-5 crystals were de-aluminated by acid leaching/steaming/acid leaching and other methods (36)(37)(38)(39)(40)(41). From the complex literature on adsorption of organic molecules into the ZSM-5-silicalite and catalytic activity, the following representative papers are selected: linear decrease of hydrophobicity with Al substitution (42); methanol adsorption on acidic ZSM-5 and conversion to gasoline, xylene, etc.…”

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confidence: 99%

Biochemical evolution. I. Polymerization on internal, organophilic silica surfaces of dealuminated zeolites and feldspars

Smith

1998

Proc. Natl. Acad. Sci. U.S.A.

117

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Catalysis at mineral surfaces might generate replicating biopolymers from simple chemicals supplied by meteorites, volcanic gases, and photochemical gas reactions. Many ideas are implausible in detail because the proposed mineral surfaces strongly prefer water and other ionic species to organic ones. The molecular sieve silicalite (Union Carbide; ‫؍‬ Al-free Mobil ZSM-5 zeolite) has a three-dimensional, 10-ring channel system whose electrically neutral Si-O surface strongly adsorbs organic species over water. Three -O-Si tetrahedral bonds lie in the surface, and the fourth Si-O points inwards. In contrast, the outward Si-OH of simple quartz and feldspar crystals generates their ionic organophobicity. The ZSM-5-type zeolite mutinaite occurs in Antarctica with boggsite and tschernichite (Al-analog of Mobil Beta). Archean mutinaite might have become de-aluminated toward silicalite during hot/cold/wet/dry cycles. Catalytic activity of silicalite increases linearly with Al-OH substitution for Si, and Al atoms tend to avoid each other. Adjacent organophilic and catalytic Al-OH regions in nanometer channels might have scavenged organic species for catalytic assembly into specific polymers protected from prompt photochemical destruction. Polymer migration along weathered silicic surfaces of micrometer-wide channels of feldspars might have led to assembly of replicating catalytic biomolecules and perhaps primitive cellular organisms. Silica-rich volcanic glasses should have been abundant on the early Earth, ready for crystallization into zeolites and feldspars, as in present continental basins. Abundant chert from weakly metamorphosed Archaean rocks might retain microscopic clues to the proposed mineral adsorbent/catalysts. Other framework silicas are possible, including ones with laevo/dextro one-dimensional channels. Organic molecules, transition-metal ions, and P occur inside modern feldspars.

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“…By contrast, DTA or DTG can be extremely useful in studies of the acidity of ZSM-5 zeolite. Acidic centres have been characterized in terms of their spatial, strength and catalytic activity distribution patterns by running ammonia adsorption and temperature-programmed desorption (TPD) (Anderson et aL, 1979;Tops~e et al, 1981;Von Ballmoos, 1981;Penchev et at., 1981;Inui et al, 1983;Skeels & Flank, 1983;Post & Van Hooff, 1984).…”

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The use of combined thermal analysis to study crystallization, pore structure, catalytic activity and deactivation of synthetic zeolites

et al. 1984

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Emphasis is placed on the advantages of combining simultaneous thermal analysis methods (TG-DTA-DTG) with other complementary physico-chemical techniques for investigating synthesis and various properties of zeolites belonging to the pentasil family. TG and DTA give quantitative information on the dehydration and decomposition of organic guest molecules that interact with intermediate phases obtained during hydrothermal transformation of amorphous aluminosilicate gels into crystalline zeolites. In particular, weight losses (TG) and heat effects (DTA) due to the oxidative decomposition of tetrapropylammonium ions occluded in an intermediate phase have been related to the amount of zeolite ZSM-5 present. As a result, very small particles of this zeolite, amorphous to X-rays, could be detected in the early stages of the crystallization process. Isothermal sorption of small hydrocarbon molecules (n-hexane, 3-methyl-pentane) is used to probe the intracrystalline pore volume of zeolite ZSM-5. The total hydrocarbon uptake (TG) and the shape of the corresponding DTA peak are sensitive to steric modifications of the ZSM-5 channel system by various chemicals, while the sorption rates are better correlated to the extent of zeolite surface poisoning by boron or carbon. TG-DTA data provide an easy means of describing the filling and packing of n-hexane in ZSM-5 pore structure. Various steps characterizing isothermal competitive sorptions (nitrogen, ethylene, water) or catalytic conversions at high temperature (ethylene, methanol) over H-ZSM-5 are described. Finally, the progressive formation (removal) of carbonaceous residues resulting from these transformations are related to the actual rate of de-activation (re-activation) of a zeolitic material. In this respect, three different synthetic zeolites, namely mordenite, offretite and H ZSM 5, which differ by their shape-selective properties and structure of their channel network, are investigated by TG.Combined thermal analysis methods have been used extensively to characterize natural minerals, including various zeolites and clays (Mackenzie, 1970;Smykatz-Kloss, 1974; Todor, 1978; Mackenzie & Caillere, 1979). In particular, simultaneous TG-DTA measurements have supplied useful qualitative or semi-quantitative information about dehydration processes (mechanisms) or high-temperature stabilities of various zeolitic

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Acidity in Zeolite ZSM-5

Cited by 5 publications

References 0 publications

Proton Activity Inside the Channels of Zeolite L

Proton Activity Inside the Channels of Zeolite L

Biochemical evolution. I. Polymerization on internal, organophilic silica surfaces of dealuminated zeolites and feldspars

The use of combined thermal analysis to study crystallization, pore structure, catalytic activity and deactivation of synthetic zeolites

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