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Microporous and Mesoporous Molecular Sieves
Published in Rolando M.A. Roque-Malherbe, Adsorption and Diffusion in Nanoporous Materials, 2018
MCM-48, the cubic material, exhibits an X-ray diffraction pattern consisting of several peaks that can be assigned to the Ia3d space group [87]. The structure of MCM-48 has been proposed to be bicontinuous with a simplified representation of two infinite three-dimensional, mutually intertwined, unconnected network of rods [88]. Meanwhile, the MCM-50, that is, the stabilized lamellar structure shows an X-ray diffraction pattern consisting of several low-angle peaks that can be indexed to (h00) reflections. Hence, this material can be a pillared layered material with inorganic oxide pillars separating a two-dimensional sheet similar to layered silicates, such as magadiite or kenyaite [42]. Alternatively, the lamellar phase could be represented by a variation in the stacking of surfactant rods such that the pores of the inorganic oxide product could be arranged in a layered form.
Dimensionality Transformation of Layered Materials toward the Design of Functional Nanomaterials
Published in Kazuhiro Shikinaka, Functionalization of Molecular Architectures, 2018
SiOH/SiO– groups can be modified covalently with silane coupling agents, such as chlorosilanes and alkoxysilanes. Silylation with silane coupling agents usually proceeds randomly to all SiOH/SiO– groups on the surface of silicate materials, though ordered structures are also controlled by using unique arrangement of SiOH/SiO– groups of layered silicates. Several layered silicates, such as kanemite and layered octosilicate, have closely located two SiOH/SiO– groups on the surface of layers, and the coupled SiOH/SiO– groups are periodically arranged with relatively large distances from one to one. When silane coupling agents are reacted with these layered silicates closely locating SiOH/SiO– groups are grafted with the molecules to form new ring structures. Because possible resultant structures are limited to only one configuration, these layered silicates are uniformly modified with silylating agents by dipodal modification. Mochizuki, Kuroda, and coworkers demonstrated this reaction for the first time [34] and used this concept for the creation of new 3D zeolitic frameworks from 2D layered octosilicate (Fig. 4.3 a) [35]. The layered octosilicate was uniformly modified with monoalkoxytrichlorosilane, in which two SiOH/SiO– groups are reacted with two SiCl groups of the monoalkoxytrichlorosilane. The residual SiCl group present on the surface of the layer is alkoxylated with dodecylalcohol, followed by the hydrolysis and condensation of the SiOC10H21 groups in organic solvents to complete interlayer condensation. These processes lead to the formation of novel 3D crystalline frameworks with cage- type micropores in the interlayer space (Fig. 4.3 b). The solvent molecules are probably trapped in the cage. Consequently, stepwise silylation and interlayer condensation is promising for molecular-level design of 3D crystalline frameworks from 2D layered materials. These processes are generally applicable for other layered silicates, such as magadiite [36], kenyaite [36], and RUB-51 [37].
Spectral indices derived, non-parametric Decision Tree Classification approach to lithological mapping in the Lake Magadi area, Kenya
Published in International Journal of Digital Earth, 2018
Gayantha R. L. Kodikara, Tsehaie Woldai
The Lake Magadi area is one of a number of closed basins in the East African Rift System (EARS) (Figure 1(a)). It is located within block-faulted Pleistocene trachyte flows of the southernmost part of the Kenyan Rift and occupies the deepest depression of the rift (Ibs-von Seht et al. 2001). The shape, morphology and sedimentation of the area are strongly controlled by tectonism and volcanism (Baker 1986). The area consists of intermittently dry Magadi Lake covering an area of 90 km2 and perennially saline Little Magadi Lake which is located at the northern end of the Lake (Figure 1(b)). The geological setting of the study area comprises of: (1) Precambrian metamorphic rocks, (2) Plio- to Pleistocene volcanic rocks and (3) Holocene to recent lake and fluvial sediments (Atmaoui and Hollnack 2003; Baker 1958; Kodikara 2009; Kuria 2011). The most volcanic activity in the area occurred between 1.4 and 0.7 Ma with the formation of the Magadi plateau trachyte series (Baker 1986). The lacustrine sediments are exposed around the Lake Magadi trough in the central axis of the rift floor, a small narrow basin filled by three successions of quaternary fluvio-lacustrine sediments: (1) the Oloronga beds, named by Baker (1958) and located at the SW, N and NE of Lake Magadi as well as in the area of the NW lagoon, were deposited about 0.8 Ma ago in a weakly alkaline lake. It consists of olive green indurated silts, clays, volcaniclastic sands and irregularly interbedded cherts (Eugster 1969; Kodikara 2009). (2) Below the Oloronga beds are the extensive plateau trachyte flows that cover the rift valley floor and are as old as 1.7 Ma. The latter, known as the high Magadi beds dated at 9100 years (Butzer et al. 1972), are subdivided into an upper and a lower unit separated by a black clay-rich layer named Thilapia bed (Butzer et al. 1972; Eugster 1969). They were deposited during a high stand of the lake around 9000–10,000 years ago. The high Magadi bed mineralogy is characterized by the following group of minerals: (i) detrital silicates, (ii) saline minerals, (iii) calcite, (iv) sodium silicate including magadiite (NaSi7O13(OH)3·3H2O), kenyaite (NaSi11O20.5(OH)3·H2O) and makatite (NaSi2O3(OH)3·H2O), (v) quartz and (vi) authogenic zeolites (Surdam and Eugster 1976). (3) The Holocene evaporites series, which has been continuously accumulated to the present day, resulted from the increasing desiccation of the lake during the Holocene. It consists of alternating trona (NaHCO3·Na2CO3·2H2O) sheets containing black mud with a Na2CO3 and Chloride-rich brine (Baker 1958). The sedimentary environment of the Lake Magadi basin is highly characterized by the dominant influence of nearly continuous tectonic movements and volcanism, which created a multitude of small short-lived basins in the main graben and larger longer lived basins in the broad half graben (Baker 1986).