Role of Surface Area in Dehydrocyclization Catalysis

Abstract

THE deposition of active agents onto adsorbentshas been employed widely to provide catalysts of increased activity, improved stability, greater selectivity, more convenient physical form, and lower cost (11). The development of a simple method for measuring surface areas of porous materials (1) provided a basis for correlating information on the action of adsorbents in this application. This work was undertaken to examine the relation of adsorbent area to catalyst activity for molybdena impregnated onto activated alumina as a catalyst for the dehydrocyclization of n-heptane to toluene. This reaction was reported with a variety of catalysts prepared in diverse manners on a number of supports (4, 5, 7, 9, 14, 15). investigation of the reaction mechanism indicates that an initial dehydrogenation is followed by cyclization and further dehydrogenation (S, 17, 18). Carbon skeleton rearrangement can take olace on a chromia-alumina catalyst (10). MATERIALS, APPARATUS, AND ANALYSISThe activated aluminas were products of the Aluminum Ore Company. The activated alumina F series analyzed: 2% loss on ignition, 0.1% Na20, 0.1% Si02, 0.05% FeA, and 0.7% Cl.The activated alumina H series analyzed: 4% loss on ignition, 0.1% Na»0, 7% Si02, 0.2% Fe203, '.3% CaO, 0.2% MgO, and 0.3% S03. Activated aluminas of the H series have one and a half to three times the surface area and pore volume, and more uniform pore widths as determined by sorption, than do those of the F series. Molybdena was obtained as Baker's Analyzed ammonium molybdate: 83% MoOs, 0.01% SO,, all other reported impurities less than 0.01%. Ammonia was Baker's Analyzed (meeting A.C.S. standards). The n-heptane, supplied by Westvaco Chlorine Products Company, had boiling point 98,42 °C., freezing point -90.65°C ., d^°0 .68371 gram per ml., and 21.38766. Tank hydrogen passed over hot copper and dried with activated alumina was used for catalyst reduction.Catalysts were prepared as follows: Ammonium paramolybdate was dissolved in distilled water (0.5 ml. per gram of alumina in the completed catalyst), and the quantity of concentrated ammonia necessary to convert the paramolybdate to the normal molybdate was added. The solution was boiled and the alumina added immediately wdth vigorous stirring. The excess water was removed by drying at 70-90°C. under overhead heat, with occasional stirring. "When the alumina appeared dry, in 4-6 hours, the sample was calcined 16 hours at 500°C.

Effect of oxomolybdate species dispersion on direct methanol oxidation to dimethoxymethane over MoOx/TiO2 catalysts

“…The geometrical monolayer coverage was calculated assuming the formation of MoO 3 species according to an occupancy of 15.4 Å 2 and, obviously, to the SSA of the support. The results showed that the impregnation on the high surface titania enabled the synthesis catalysts with coverage varying from 0.09 monolayers (1 wt.% Mo) up to 1.26 monolayers (14 wt.% Mo) (Table , ).…”

Dispersed Vanadium in Three-Dimensional Dendritic Mesoporous Silica Nanospheres: Active and Stable Catalysts for the Oxidative Dehydrogenation of Propane in the Presence of CO2

“…were catalytically active for the ODHP reaction. − Especially, the vanadium oxide catalysts have been widely studied for the ODHP reaction. , For the VO x /SiO 2 supported catalysts, it is well known that the surface structure of the supported metal oxide is crucial to the catalytic performance. Russell et al first proposed the concept of monolayer coverage in 1946, which represented the maximum amount of two-dimensional (2D) vanadium oxide species on a SiO 2 support just before the formation of 3D V 2 O 5 crystallites on the surface. The isolated monomeric OV­(O−) 3 species together with dimeric and oligomeric species were present on SiO 2 support surfaces below the monolayer coverage with low vanadium loading .…”

Monolayer- and crystal-type MoO3 catalysts: Their catalytic properties in relation to their surface structures

“…In addition to the experimental results of Russell and Stokes (1), many other facts recently reported point to the existence of a monomolecular layer of (reduced) molybdenum oxide on alumina (2)(3)(4)(5)(6)(7)(8). Sonnemans and Mars (5) showed that complete monolayers of MoOa on alumina may be prepared by adsorption either of MoO2(OH)2 from the gas phase or of polymolybdate ions from aqueous solutions.…”