Ethylene, catalytic oxidation kinetics

Although the main routes to propylene oxide formation are not based on direct catalytic oxidation of propylene, the direct epox-idation of propylene on silver would be financially preferable if high yield and selectivity to propylene oxide could be achieved. Similarly to ethylene oxidation on silver part of the undesirable byproduct CO2 comes from the secondary oxidation of propylene oxide (2,3). The kinetics of the secondary silver catalyzed oxidation of propylene oxide to CO2 and H2O have been studied by very few investigators (2). 

Oxidation of ethylene according to Eq. (9.2) occurs stoichiometrically. A catalytic oxidation is possible if palladium(O) is reoxidized immediately. This happens in the presence of oxidizing agents such as cupric and ferric chlorides ferric sulfate chromates heteropoly acids of phosphoric, molybdic, and vanadic acids peroxides and others. Benzoquinone is used by Moiseev et al. for their kinetic investigations [11]. Gaseous oxygen does not oxidize palladium black in a sufficiently short time. 

Gold catalyzed oxidations of alcohols and diols possess great potential towards selectivity [10]. As example, chemical kinetics was measured during the catalytic oxidation of ethylene glycol based on the O2 consumptiom Deposited Au on the porous glass shows no catalytic activity while the Au loaded ceria replica shows an initial activity of 53 mmol-gAu min within the first 10 minutes of the reaction. In the latter case, after 60 minutes a conversion of 59% was obtained. As main prodnct glycolic acid was produced at a selectivity of 94%. Oxalic acid and glyoxylic acid were detected as side products. 

Methane can be oxidatively coupled to ethylene with very high yield using the novel gas recycle electrocatalytic or catalytic reactor separator. The ethylene yield is up to 85% for batch operation and up to 50% for continuous flow operation. These promising results, which stem from the novel reactor design and from the adsorptive properties of the molecular sieve material, can be rationalized in terms of a simple macroscopic kinetic model. Such simplified models may be useful for scale up purposes. For practical applications it would be desirable to reduce the recycle ratio p to lower values (e.g. 5-8). This requires a single-pass C2 yield of the order of 15-20%. The Sr-doped La203... 

Freeder, B. G. et al., J. Loss Prev. Process Ind., 1988, 1, 164-168 Accidental contamination of a 90 kg cylinder of ethylene oxide with a little sodium hydroxide solution led to explosive failure of the cylinder over 8 hours later [1], Based on later studies of the kinetics and heat release of the poly condensation reaction, it was estimated that after 8 hours and 1 min, some 12.7% of the oxide had condensed with an increase in temperature from 20 to 100°C. At this point the heat release rate was calculated to be 2.1 MJ/min, and 100 s later the temperature and heat release rate would be 160° and 1.67 MJ/s respectively, with 28% condensation. Complete reaction would have been attained some 16 s later at a temperature of 700°C [2], Precautions designed to prevent explosive polymerisation of ethylene oxide are discussed, including rigid exclusion of acids covalent halides, such as aluminium chloride, iron(III) chloride, tin(IV) chloride basic materials like alkali hydroxides, ammonia, amines, metallic potassium and catalytically active solids such as aluminium oxide, iron oxide, or rust [1] A comparative study of the runaway exothermic polymerisation of ethylene oxide and of propylene oxide by 10 wt% of solutions of sodium hydroxide of various concentrations has been done using ARC. Results below show onset temperatures/corrected adiabatic exotherm/maximum pressure attained and heat of polymerisation for the least (0.125 M) and most (1 M) concentrated alkali solutions used as catalysts. 

Qince the discovery (6) of supported chromium oxide catalysts for polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16, 19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 

For reactions that are the same on metal and other catalytic sites (e.g., hydrogenation or total oxidation), the reaction may seem to proceed in a similar fashion on the metallic source of spillover and on the diluent support. Some careful studies may be able to discriminate between activity on the metal and the spillover-induced sites. As an example, hydrogenation of ethylene occurs on Pt (or Ni) and on silica or alumina activated by spillover. The product (i.e., only ethane) is the same, as the kinetics often are (rate = /c[C2H4]°[H2]1), but the specific mechanism is different. Deuteration is able to discriminate between the relative rate of alkyl reversal. Deuteration of ethylene on an activated silica produces d2-ethane as the initial product (137), contrary to the results for metal-catalyzed ethylene hydrogenation (2). 

Subsequent experiment on the kinetics of dehydration of ethanol on silica-alumina and on the initial oxides confirms this opinion. As a matter of fact, silica gel starts exhibiting appreciable catalytic properties in this reaction only at temperatures in excess of 400 C. We believe that the behavior of the surface compound with respect to desorption into vacuum upon heating is closely related to the catalytic properties of the solid. Silica gel at 400°C converts methanol into ether and water alumina gel and silica-alumina, in accordance with our data, under the same conditions carry the reaction all the way to ethylene and water. In this connection, the first stage in both reactions is the formation of a surface compound of the type of alcoholate ... 

Comparison of the kinetics for the most feasible stereochemical pathways of the alternative la—>4 and lb 2 routes clearly shows that the octadienediyl-Ni" complex is preferably generated via the ethylene-assisted coupling of two -butadienes along lb—>2. Thus, the thermodynamically favorable [Ni (ix -butadiene)2(ethylene)] form of lb also represents the catalytically active species for oxidative coupling. The [Ni (ix -s/ ,ri (C )>A-cis,-octadienediyl)-(ethylene)] species 2 is almost exclusively formed in a thermoneutral process (AG= -0.1 kcal mol 1) that requires a moderate activation free-energy of 12.8 kcal mol This indicates the oxidative coupling as a facile, reversible step. 

Catalytic properties of complexes of multi-valenced metals with poly(ethylene glycol) (PEG) and polyurethane (PU) have been studied during liquid-phase oxidation processes such as the liquid-phase oxidation of hydrocarbons (phenanthrene, tetralin, cyclohexene), decomposition of hydroperoxides, hydrocarbons and decomposition of hydrogen peroxide [101 -106]. The kinetics of these reactions have been studied. The rate and selectivity of a particular reaction process depend not only on the properties... 

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