Sorption of vapors by sepiolite

Magnesite pebbles in Miocene lacustrine conglomerates in northwest Turkey have been partially to totally replaced by sepiolite. Only 5% of the magnesite pebbles have been converted to essentially pure sepiolite; the rest represent mixtures of magnesite and sepiolite. This process of sepiolitization is documented by X-ray diffraction (XRD), chemical, scanning electron microscopy (SEM) and BrunauerEmmett-Teller (BET) techniques. These techniques show that the sepiolitization proceeds from the rim towards the core of the pebbles. The conglomerates, with pebbles of magnesite and ultramafic rocks, were deposited in a near-shore environment on the margin of a large Miocene lake with an ophiolitic substratum. The diagenetic transformation of magnesite to sepiolite is believed to have been caused by the interaction of mixed meteoric and lacustrine waters, which were undersaturated with respect to magnesite. The sepiolitization occurred during the highstands of the lake, when the near-shore conglomerates were flooded by the silica-rich lake waters. The pH of the water during the sepiolitization was probably on the order of 10.5-11.5. The sedimentary magnesite beds in the center of the Miocene basin show no sepiolitization, which is explained by the presence of pore water saturated with respect to magnesite.The magnesite-sepiolite replacement process is chemically modeled as a 4-stage process from dimerization to polymerization.

Section: Surface Area (Bet) Studiesmentioning

confidence: 95%

Diagenetic Transformation of Magnesite Pebbles and Cobbles to Sepiolite (Meerschaum) in the Miocene Eskişehir Lacustrine Basin, Turkey

Ece

1998

“…Actually, the location of the "micropores" (Sing et al, 1985) responsible for sepiolite's adsorption capacity is somewhat controversial. Barrer and Mackenzie (1954) and Dandy (1968Dandy ( , 1971 suggested that the availability of these structural channels to the nitrogen molecules is limited. The loss in BET-N2 surface area observed on outgassing at >200~ has been attributed to a sintering of the external fiber surface (Dandy and Nadiye-Tabbiruka, 1975).…”

Section: Introductionmentioning

confidence: 99%

Modification of the Porous Structure and Surface Area of Sepiolite under Vacuum Thermal Treatment

Grillet

1988

Abstract--Modifications of the external surface area and the two types of microporosity of sepiolite (structural microporosity and inter-fiber porosity) were examined as a function of the temperature of a vacuum thermal treatment to 500~ The methods used included: reciprocal thermal analysis, N 2 and Ar low-temperature adsorption microcalorimetry, gas adsorption volumetry (for N2, mr, and Kr at 77 K and CO2 at 273 and 293 K), water-vapor adsorption gravimetry, and immersion microcalorimetry into liquid water at 303 K. If the sample was not heated > 100~ only 20% of the structural microporosity was available to N2, whereas 52% was available to CO: at 293 K. In both experiments, the channels filled at very low relative pressures. At > 350~ the structure transformed to anhydrous sepiolite, which showed no structural microporosity. The inter-fiber microporosity decreased from 0.031 to 0.025 cmVg (as seen with N2), and the external specific surface area decreased from 120 to 48 m2/g. The water adsorption isotherms showed a lower and lower affinity of the external surface of fibers for water as the temperature of thermal treatment increased. The thickness of the bound water on the external surface was estimated to be -<3.5 monolayers, i.e., less than l0/~.

“…A similar explanation was proposed by Pines and Manassen (1966) in their review of the dehydration of alcohols over aluminas. When sepiolite is heated in air at temperatures greater than 200~ micropores are destroyed, and the surface area is decreased (Dandy, 1968). This process occurs at lower temperatures if the heating is carried out in vacuo (Dandy, 1971).…”

Section: Catalysis Experimentsmentioning

confidence: 99%

“…On heating the structure loses water, resulting in a decrease in the surface area and micropore accessibility (Hayashi et al, 1969;Martin Vivaldi and Fenoll Hach-Ali, 1969;Nagata et al, 1974;Dandy and Nadiye-Tabbiruka, 1975;Fernandez-Alvarez, 1978;Serratosa, 1979). Sepiolite has a high adsorptive capacity for many gases and vapors (Barrer et al, 1954;Dandy, 1968Dandy, , 1969Dandy, , 1971Fenoll Hach-Ali and Martin Vivaldi, 1970;Serna and Fernandez-Alvarez, 1975;Serna and Vanscoyoc, 1979). Lopez-Gonzalez et al (1978) studied the adsorption of aminobutane (n-butylamine) on acid-activated sepiolite as an indicator of surface acidity.…”

Section: Introductionmentioning

confidence: 99%

Surface Properties of Sepiolite from Amboseli, Tanzania, and Its Catalytic Activity for Ethanol Decomposition

Dandy

Nadiye-Tabbiruka

1982

Abstract--The catalytic activity of sepiolite from Amboseli, Tanzania, for the dehydration and dehydrogenation of ethanol at 150~176 has been studied using a flow reactor. Both reactions occur, but the catalyst activity decreases with use. The products include water, carbon dioxide, ethene, ethanal (acetaldehyde), diethyl ether, but-1,3-diene, but-2-enal (crotonaldehyde), and an unidentified aromatic compound. The proportions change with temperature, the dehydrogenation reaction being favored at the higher temperatures. The BET surface areas of the sepiolite are 316 m2/g (nitrogen adsorption at -197~ and 212 m2/g (ethanol vapor adsorption at 25~ assuming a molecular cross-sectional area of 24.6 A2), indicating a possible greater penetration of pores and channels by nitrogen compared with ethanol vapor under these conditions. The pore-size distribution reveals that approximately 55% of the surface area measured by nitrogen adsorption is contributed by micropores.