Catalytic properties of the platinum catalyst supported on alumina modified by oxalic acid in n -heptane reforming
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Catalytic properties of the platinum catalyst supported on alumina modified by oxalic acid in n-heptane reforming* V. Yu. Tregubenko, N. V. Vinichenko, V. P. Talzi, and A. S. Belyi Center of New Chemical Technologies BIC**, Boreskov Institute of Catalysis, 54 ul. Neftezavodskaya, 644040 Omsk, Russian Federation. Fax: +7 (381 2) 67 3334. E-mail: [email protected] Structural modification of alumina was carried out by the decomposition of the products of the reaction of aluminum hydroxide with the organic acid used simultaneously as a peptizer and a "burnable" additive. The modification changes the acidity of the support surface and induces the formation of pentacoordinate Al3+ cations interacting with platinum, which increases the fraction of platinum atoms with electron deficiency. The alumina—platinum catalyst based on the support treated with oxalic acid shows an enhanced selectivity to aromatization in n-heptane reforming. Key words: defects, γ-alumina, platinum, acidity, 27Al MAS NMR spectroscopy, reforming.
The catalysts based on γ-Al2O3 find wide use in catalytic processes, such as oil processing and reduction of emissions from automobile engines. The formation and thermal stability of γ-Al2O3 were studied in many works.1—3 The thermal dehydroxylation of the alumina surface results in the formation of surface defects including aluminum atoms with various degrees of coordinative unsaturation and electron-acceptor ability.4 The modification of the support can favor the formation of additional Lewis acid sites (LAS).5 It has previously been shown6 by the pair distribution function (PDF) method that alumina with a decreased helium pycnometry density is characterized by the local structure. Many cationic vacancies are localized in the octahedral positions of this structure. For the majority of porous solids, the helium pycnometry density is accepted to be the true density and can qualitatively reflect the degree of defectness of the support. When the platinum complexes are loaded on the defect supports, surface compounds were found in which platinum localized in the octahedral vacancies of the support structure is in the electron-deficient state. These catalysts have high activity and selectivity in n-alkane aromatization.6 The examination of the IR spectra of pyridine adsorbed on the γ-alumina surface7 by the diffuse reflectance method found LAS of three types. They were attributed to tri-, * Based on the materials of the XXI Mendeleev Congress on General and Applied Chemistry (September 9—13, 2019, St. Petersburg, Russia). ** Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences.
tetra-, and pentacoordinate Al3+ ions. The calculations by the ab initio3 and molecular dynamic modeling methods8 confirm the existence of stable pentacoordinate Al3+ ions on the alumina surface. Additional proofs for the presence of special sites on the supports where platinum atoms (so-called "defects") are immobilized were obtained by high-angle annular dark-field scanning transmission electron microscopy (HAAD
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