Vincent A. Durante

Active Iron Oxo Centers for the Selective Catalytic Oxidation of Alkanes

Much work has been done in an effort to understand the nature of iron oxo complexes and their roles in the selective catalytic oxidation of alkanes. Iron oxo (ferryl) species (Fe=O) have been proposed to be the active intermediates responsible for both the enzymatic and biomimetic oxidations of alkanes to alcohols, while it is generally accepted that iron(III) μ-oxo species [Fe(III)-O-Fe(III)] are not catalytically active. We have synthesized a number of iron complexes having μ-oxo bridges in several molecular environments including porphyrinato, polyoxometallate, and silicometallate structures and examined the catalytic activity of these compounds for alkane oxidation in both liquid and vapor phase. The activity and selectivity of these catalysts depend upon the molecular environment of the μ-oxo species used as the catalyst precursor. In some instances in situ conversion of μ-oxo to ferryl oxo species may be the key to catalysts capable of direct hydroxylation of alkanes with air or oxygen.

Isobutane dehydrogenation over sulfided nickel catalysts

Abstract We have studied the dehydrogenation of isobutane to isobutylene over heavily sulfided Ni catalysts at 873 K. In the absence of sulfur but in the presence of hydrogen, supported metallic nickel catalyzes the rapid hydrogenolysis of alkanes, resulting in exceedingly poor selectivity toward dehydrogenation products. However, the hydrogenolysis activity can be essentially eliminated by proper sulfidation, resulting in high selectivity toward isobutylene production. Due to thermodynamic limitations, elevated temperatures are necessary to attain reasonable conversions to isobutylene. Under these conditions, the formation of coke is rapid and is the main cause of catalyst deactivation. Treatment of nickel catalysts with relatively large amounts of sulfur-containing reagents results not only in dramatically improved selectivity but also in a decreased rate of coke formation. The catalysts exhibited activation periods of several hours until a maximum was reached when they were placed in the reaction environment at 873 K. This activation period did not depend upon the previous reduction treatment but did depend upon the time on stream. A correlation was found between the increase in activity and the growth of a carbidic phase on the catalyst. This species, characterized by TPO and XPS, reached a saturation value at a C/Ni ratio of unity, which was coincidental with the amount of carbon on the catalyst when it achieved its maximum activity. Two alternative explanations for the activation process are discussed. The first considers the participation of a nickel-carbidic-carbon complex in the reaction scheme. The second considers the creation of sulfur vacancies during the initial coke deposition period which would increase the exposure of a catalytically active NiS moiety.

Platinum-Nickel/L-zeolite Bimetallic Catalysts: Effect of Sulfur Exposure on Metal Particle Size and n-Hexane Aromatization Activity and Selectivity

Abstract Experimental evidence that Ni can be used to stablizie Pt particles supported in L-zeolite against deactivation by sulfur is presented. A series of Pt-Ni/L-zeolite bimetallic catalysts prepared by co-impregnation were tested for n-hexane aromatization and characterized by Extended X-ray Absorption Fine Structure (EXAFS), hydrogen chemisorption and Transmission Electron Microscopy (TEM) before and after sulfiding. It was found that bimetallic particles are significantly less subject of sulfur-induced particle growth than their monometallic (pt) counterpart. This mechanism of catalyst fouling is known to cause deterioration of Pt/L aromatization catalysts.