Initiation, chain reaction

Owing to its high endothermicity, the chain initiating reaction is not an important route to formation of the radical R once the reaction system has created other radicals. Obviously, the important generation step is a radical attack on the fuel, and the fastest rate of attack is by the hydroxyl radicals since this reaction step is highly exothermic owing to the creation of water as a product. So the system for obtaining R comes from the reactions... 

In addition to these chain propagation reactions the following chain initiation reactions are assumed. 

The results of studies of this reaction in a hollow reactor show high selectivity up to 640 °C in the range of 4-EP volume rate from 0.065 to 0.78 h 1 and at 4-EP 20% aqueous H202 = 1 3 [94], Under optimal conditions at 620 °C, 4-VP yield equals 20.9% with 92% selectivity. Injection of quartz granules to the reactor raises the yield to 44.3% and selectivity to 96%. This is because the total surface on which, probably, the chain initiation reaction ... 

Hydrogen Transfer. Hydrogen transfer (sometimes called self alky-lation of isobutane) occurs with propylene-isobutane mixtures using HF catalyst. This is a chain initiative reaction in that tertiary butyl carbo-nium ions are formed. End products are (I) propxine and (2) 2,2,4-trimethylpientane. Reactions follow ... 

The essence of the energetic studies on TS and 4-BCMU is contained in Fig. 9. In TS formation of the chain initiating species -- a dimer — requires an energy of 1.0 eV. It can be supplied thermally or optically via monomer excitation. In the former case it is this chain initiation reaction that controls the thermal reactivity and its temperature-dependence. Chain initiation can also be produced optically at a yield of order 10 per absorbed UV-quantum. In this case it is chain propagation that determines the temperature dependence of the polymerization yield. However, the activation energy E" need not be and in general is not identical with the energy... 

Parallel to the photoinitiation processes (with hv) photoaddition processes are observed, as shown for example in the Figs. 4, 5, 11 to 13. After dimer initiation, trimer formation from the dimer is possible etc. The chain propagation within the DR or AC series is performed by photoaddition of monomer molecules M adjacent to the reaction centres, given by the dimer (DR2 or AC2), trimer (DR3 or AC3),... molecules. The molecules M are lowered in energy by the perturbation introduced by the reaction centres. They form a trap for the optical excitation energy. They may be excited directly (M + hv -> M ) or indirectly via nonperturbed monomer molecules (M -f- hv -> M ) and subsequent energy transfer (M + M M + M ). The chain propagation reaction therefore is in competition with the chain initiation reaction. 

Internal olefins are much less reactive than a-olefins in chain initiation and secondary hydrogenation reactions. The reactivity of added a-olefins in chain initiation reactions increased in the order ethylene > propylene s 1 -butene C5+ olefins it becomes almost independent of chain size for C5+ a-olefins. The higher reactivity of ethylene and propylene leads to C2 and C3 selectivity below those of the rest of the distribution in Flory plots (Fig. 5) and to the low termination probabilities measured for C2 and C3 surface chains (Fig. 8), as proposed previously by others (115). 

Our readsorption model shows that carbon number distributions can be accurately described using Flory kinetics as long as olefin readsorption does not occur (/3r = 0), because primary chain termination rate constants are independent of chain size (Fig. 24). The resulting constant value of the chain termination probability equals the sum of the intrinsic rates of chain termination to olefins and paraffins (j8o + Ph)- As a result, FT synthesis products become much lighter than those formed on Co catalysts at our reaction conditions (Fig. 24, jSr = 1.2), where chain termination probabilities are much lower than jS -I- Ph for most hydrocarbon chains. The product distribution for /3r = 12 corresponds to the intermediate olefin readsorption rates experimentally observed on Co/Ti02 catalysts, where intrapellet transport restrictions limit the rate of removal of larger olefins, enhance their secondary chain initiation reactions, and increase the average chain size of FT synthesis products. 

The presence of hydrogenation sites outside FT synthesis pellets (e.g., as a physical mixture of FT and hydrogenation pellets) is much less effective (Fig. 25, curve C). Such extrapellet sites merely capture (low fugacity) olefins that leave FT synthesis catalyst pellets unreacted and prevent their subsequent readsorption along the catalyst bed. They do not prevent the chain initiation reactions that olefins undergo predominantly within... 

Diffiisional restrictions increase the effectiveness of olefin interception sites placed within catalyst pellets. Very high olefin hydrogenation turnover rates or site densities within pellets prevent olefin readsorption and lead to Flory distributions of lighter and more paraffinic hydrocarbons. Identical results can be obtained by introducing a double-bond isomerization function into FT catalyst pellets because internal olefins, like paraffins, are much less reactive than a-olefins in chain initiation reactions. However, light paraffins and internal olefins are not particularly useful end-products in many applications of FT synthesis. Yet, similar concepts can be used to intercept reactive olefins and convert them into more useful products (e.g., alcohols) and to shift the carbon number distribution into a more useful range. In the next section, olefin readsorption model simulations are used to explore these options in the control of FT synthesis selectivity. 

Density functional calculations on the chain initiation reaction shown in Scheme 136 for the ethylene polymerization catalyzed by bis-alkoxo titanium complexes have been studied. Activation barriers of 6.4kcal mol-1 are found for the titanium sulfur-bridged catalysts with higher insertion barriers of 10-15 kcalmol-1 for the GH2-bridged catalysts. 

From the analysis of the rate laws in Table II one can write plausible mechanisms for the reaction, but always more than one. The chain initiation reaction is in no case defined by the rate law, and the chain carriers cannot be uniquely fixed from the rate law. Therefore no attempt will be made to give mechanisms until further information is available to limit the possibilities. 

As most of the rate constants for the reactions invoked are unknown, to fully substantiate the 3-phase combustion model requires a systematic study of the chemistries occurring in each of the three phases by high-level quantum-chemical calculations for the key processes involved. They include in particular the determination of the dissociative sublimation rate, AP(s) NH3 + HCIO4, the chain initiation reaction rate, HCIO4 -> OH + CIO4, as a function of temperature and pressure, and the rates of ensuing chain reactions involving ClOx (x = 0 - 4), NHy and NOy (y = 1 - 3) and HOz (z =... 

Even small admixtnres of halogens, like bromine and chlorine, also have a significant negative effect on prodnction and stability of ozone (Benson Axwortly, 1957). Such admixtures sometimes lead to ozone explosions. The halogens stimulate catalytic thermal decomposition of ozone via the fast chain mechanism, which in the case of chlorine, for example, starts with the formation of atomic chlorine and CIO radicals in the chain initiation reactions (Schumacher, 1957) ... 

Metals can initiate fatty add oxidation by reaction with oxygen. The anion thus produced can either lose an electron to give singlet oxygen or react with a proton to form a peroxyl radical, which serves as a good chain initiator (Reaction 12.11). 

Since the rate of cumene oxidation at 100° is rather high, on this system, on-visible, it is necessary to work with small concentrations of nickel catalyst, which can facilitate a high yield of the hydroperoxide [8, 39]. It is evident from analysis of scheme of catalyzed hydrocarbons oxidation, including participation of catalyst in chain initiation reaction under catalyst interaction with ROOH and also in chain propagation (Ct + RO ), that with decrease in [Ct the rate of reaction should be decreased, and [ROOH] should be increased [8, 15]. It has appeared that together with... 

The initiator radicals initially formed in solution are held together briefly in a cage of solvent molecules. This cage effect causes radical molecules to recombine and slows down their diffusion through the solvent. Therefore, the rate of initiation (R.) depends on the rate of decomposition of the initiator (k ) and on the fraction e of the initiator radicals that escape the solvent cage affecting the efficiency of the chain initiation reaction (20). 

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