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Influence of Alkaline Earth Cation Exchanged X-Zeolites Towards Ortho-Selectivity in Alkylation of Aromatics: Hard-Soft-Acid-Base Concept

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The present work describes a novel approach through which the acid hardness/softness of NaX zeolite is varied by ion-exchanging with different alkaline divalent metal ions, resulting in different selectivity for ortho-xylene in toluene methylation. It is well known that alkali cation exchanged X-zeolites are already established as solid base catalysts for side chain alkylation of toluene to produce ethyl benzene/styrene. In this work, alkaline earth cation exchanged X-zeolites are interestingly found to be acidic catalysts for ring alkylation of toluene showing different selectivity pattern for xylenes compared to other zeolite catalysts. The alkaline earth metal ion-exchanged X zeolites (MgX, CaX, SrX and BaX) were synthesized by ion exchange method and characterized by XRD, TPD-NH3, SEM and py-FTIR. The catalytic activity and variation in ortho-xylene selectivity was correlated with chemical composition, acidity, basic strength and acid hardness/softness of zeolite catalyst. The toluene conversion was correlated with Brönsted acidity, whereas selectivity for ortho-xylene was explained on the basis of Hard-Soft-Acid-Base (HSAB) concept. Among the different alkaline earth metal ion-exchanged catalysts, SrX gave good catalytic activity with high ortho-xylene selectivity. Further, influence of Si/Al ratio, different structured Sr exchanged zeolites and amount of Sr content on toluene methylation was studied. The influence of acid hardness of the alkaline earth metal ion-exchanged X zeolite on selective synthesis of ortho product also holds true for ethyl benzene ethylation reaction. The current experimental results further confirmed the theory of HSAB concept to explain the observed selectivity of products in heterogeneous catalysis.
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Keywords: ALKALINE EARTH METAL ION; CATALYST; ORTHO-XYLENE; TOLUENE METHYLATION; X ZEOLITE

Document Type: Research Article

Publication date: 01 December 2014

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  • ADVANCED POROUS MATERIALS (APM) is an international peer-reviewed journal that publishes research activities on the fundamental aspects, synthesis, advanced characterization, structural properties, and multiple applications of all kinds of novel micro, meso-, nano- and macro-porous materials. APM also offers unique opportunity to report the energy and environmental related applications of advanced porous materials addressing significant environmental problems as a result of global climate changes. APM publishes reviews, full-length papers, and short communications, covering materials including zeolites, zeotypes, metal organic frameworks, layered materials, porous carbons, nitrides, metals, polymers, phosphides, chalcogenides, transition metal oxides, hydroxyapatite, gels, fibers, ceramics, glasses, membranes, and thermoelectric materials, mesoporous silica, amorphous and crystalline mesoporous metallosilicates, mesoporous hybrid materials, nanocomposites, porous organic molecules, graphenes, and open framework materials, and their applications in catalysis, sensing, adsorption, separation, drug delivery, magnetism, battery, supercapacitors, solar cells, nanodevices, and fine chemical synthesis.
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