K. van der Wiele

Elementary Reactions and Kinetic Modeling of the Oxidative Coupling of Methane

The oxidative coupling of methane is typically carried out at temperatures of 650–950°C, using a methane-rich mixture of methane and oxygen or air, and with an oxidic catalyst of low porosity. The process is very complex in the sense that reactions at the surface of the catalyst strongly interfere with reactions in the homogeneous gas phase.

Methane Oxidative Coupling Using Li/MgO Catalysts: The Importance of Consecutive Reactions

Summary The importance of consecutive catalytic reactions in the oxidative coupling of methane over Li/MgO catalysts has been elucidated by means of low pressure experiments (10 - 150 Pa). Ethane is converted a factor 4 faster than methane, primarily into ethylene. Ethylene in turn is oxidized a factor 2.6 faster than methane. The latter factor provides an upper limit to the C2+ yield, achievable with a catalyst like Li/MgO. The product selectivity (at 1 bar) can be modelled quite well using a rather simple consecutive reaction scheme. The kethylene/kmethane ratio can be used as a parameter determining the product selectivity. Depending on the catalyst system and the process conditions, this parameter may vary between 2.6 and 19, causing the C2+ yield to vary between 35 and 6%. It seems possible to reach C2+ yield values of >25%, with C2+ selectivities > 65% via optimization of the interaction of homogeneous and heterogeneous reactions. This opens the way for the production of ethylene via the catalytic oxidative coupling of methane.

The Selective Oxidation Of Methyl-alpha;-D-Glucoside On A Carbon Supported Pt Catalyst

Abstract The selective oxidation of methyl-α-D-glucoside to methyl-α-D-glucosiduronate by oxygen using active carbon supported Pt was studied in a three phase stirred tank reactor. The temperature was varied from 293-313 K, the pH from 6-10, the oxygen partial pressure from 5.0 10 3 to 1.0 10 5 Pa at a constant pressure of 1 10 5 Pa, the methyl-α-d-glucoside concentration from 50 to 1000 mol m −3 and the catalyst concentration from 1-5 kg m −3 . The conversion ranged from 0.02 to 0.10. At this conversion range methyl-α-d-glucosiduronate was obtained with a selectivity of 100%. At the investigated reaction conditions the initial reaction rates were free from mass and heat transfer limitations. Methyl-α-d-glucosedialdehyde was found to be a reactive intermediate. The data could be described by a reaction rate equation of the Langmuir-Hinshelwood type, corresponding to a rate-determining step on the Pt surface involving two active sites. No distinction was made between the sites of chemisorption for oxygen and methyl-α-d-glucoside. The rate equation was based on a reaction sequence with two reaction paths, the first involving the adsorbed methyl-α-d-glucoside and dominating at low pH, the second involving the methyl-α-d-glucoside anion and dominating at high pH. The adsorption of methyl-α-d-glucosedialdehyde and of methyl-α-d-glucosiduronate was found to be negligible. The surface was mainly covered with oxygen at low pH and with both oxygen and the methyl-α-d-glucoside anion at high pH.

Working Principle of Li Doped Mgo Applied for The Oxidative Coupling of Methane

The nature of the active compound in Li doped MgO was investigated by comparing the activity and deactivation of Li/MgO catalysts with that of Li 2 CO 3 supported on an inert carrier (ZrO 2 ). The conclusion is that Li 2 CO 3 itself is a very active catalyst (or a catalyst precursor). Also the role of the catalyst in the oxidative coupling of methane was determined: The selectivity of the active catalyst is mainly due to a very high production rate of methyl radicals.

Chapter 2 Heterogeneous Oxidation Processes

Publisher Summary Many substances can be partially oxidized by oxygen if selective catalysts are used. In such a way, oxygen can be introduced in hydrocarbons, such as olefins and aromatics, to synthesize aldehydes (e.g, acrolein and benzaldehyde) and acids (e.g, acrylic acid and phthalic acid anhydride). A selective oxidation can also result in a dehydrogenation (butene → butadiene) or a dealkylation (toluene → benzene). Other molecules can also be selectively attacked by oxygen. Methanol is oxidized to formaldehyde and ammonia to nitrogen oxides. Olefins and aromatics can be oxidized with oxygen together with ammonia to nitriles (ammoxidation). The Gibbs free energy change for the total combustion of such molecules always has a larger negative value than partial oxidation. Hence, a catalyst to control kinetically the oxidation process is an absolute necessity. By choosing the right conditions and the proper type of catalyst, the oxidation process can be directed toward intermediates that do not react further.