Pooling mechanism

J.F. (2004) Theoretical smdy of the methylbenzane side-chain hydrocarbon pool mechanism in methanol to olefin catalysis. /. Am. Chem. Soc., 126, 2991-3001. 

Methanol conversion to hydrocarbons has been studied In a flow micro reactor using a mixture of C-methanol and ordinary C-ethene (from ethanol) or propene (from Isopropanol) over SAPO-34, H-ZSM-5 and dealumlnated mordenlte catalysts In a temperature range extending from 300 to 450 °C. Space velocities (WHSV) ranged from 1 to 30 h. The products were analyzed with a GC-MS Instrument allowing the determination of the Isotopic composition of the reaction products. The Isotope distribution pattern appear to be consistent with a previously proposed carbon pool mechanism, but not with consecutive-type mechanisms. 

It is shown that superposition of external magnetic field on the melting zone makes it possible to control the depth and shape of the metal pool. Mechanical properties of a new class of titanium alloys with an intermetallic strengthening, produced by MEM method, are given. 

In hepatocytes the period of Cef oscillations correlates with the time required for observing the first peak in C i after the onset of stimulation (Rooney, Sass Thomas, 1989). Specifically, this time interval, called latency, increases roughly linearly with the period of Cdf oscillations as stimulation decreases. Given that this observation brings further insight into the mechanism of signal-induced Ca mobilization and provides an additional test for any theoretical explanation of the oscillatory phenomenon, it is worth examining the relationship between period and latency of Ca transients in the model based on CICR. The model shows that the existence of an approximately linear correlation between these two quantities is a natural consequence of the two-pool mechanism of Ca oscillations (Dupont etal, 1990). 

P-18 - Studies of the methanol to hydrocarbons reaction using isotopic labelling. Mounting evidence for a hydrocarbon pool mechanism... 

The next step in the methanol-to-hydrocarbons reaction, and in fact the crucial one for the generation of hydrocarbon products is C-C bond formation. Very many proposed mechanisms exist for potential routes at the acid sites of the zeolites, but recent evidence suggests that the reaction instead proceeds via a reactive hydrocarbon pool (See Chapter 8). In fact, an extensive series of high-level theoretical calculations suggests that no single combination of direct reaction steps can link methanol to ethene, and so provides strong indirect evidence that the hydrocarbon pool mechanism is the correct one. 

A hydrocarbon-pool mechanism. It is based on a carbonaceous species, (CH2)re. The species is alkylated by methanol or dimethyl ether until it eliminates an olefin and a new catalytic cycle starts. This mechanism can be represented by the scheme as shown in Figure 27. 

The hydrocarbon pool mechanism has also been studied in MTO and ETO... 

Also of interest are models from closely related fields, such as Matrix Assisted Laser Desorption/Ionization (MALDI) and Electro Spray Ionization Mass Spectrometry (ESI-MS). MALDI in particular attracts attention as this technique records the large molecular ion emissions resulting from the localized deposition of energy occurring on laser irradiation of the solid s surface. Indeed, there appear to be many similarities in the recorded emissions from SIMS and MALDI. Examples of areas of interest include processes described within the Cluster-based mechanisms and the pooling mechanism. These along with their applicability to SIMS are discussed in Section 3.3.4.2. 

The pooling mechanism, however, only describes the first step. What follows are further interactions that can occur in the expanding plume. This second step includes the interaction of electrons, protons, and ions with each other which can result in the neutralization of ions already formed and/or the formation of new ions. As with SIMS, it is the final step that dictates the recorded trends. As the recorded emissions follow a Boltzmann-like distribution, thermodynamic type approaches have been applied. Indeed, the LTE concept appears well suited to this scenario as reactions taking place among electrons, protons, and ions within an expanding plume more closely match that of a dense plasma, as outlined in Section 3.3.3.1. 

To extend this multistep mechanism to describe secondary ion emission of complete molecules along with their fragments in SIMS would, however, requires the assumption that a heat spike ensues during the sputtering of matrices in which suf-ficiendy diffusive excitons can be formed. Heat spikes would have to be assumed as this would provide conditions similar to the plume noted in MALDI, whereas highly diffusive excitons must be assumed to allow energy transfer as described within the context of the pooling mechanism. 

Pooling mechanism A model describing molecular secondary ion emission (from MALDl)... 

While the main purpose of the pool mechanism is to provide monetary compensation, it can have a strong socioeconomic effect as well. The Turkish Catastrophe Insurance Pool (TCIP), established in 2000 after the Kocaeli and Diizce... 

A hydrocarbon pool mechanism via alkylation/dealkylation of hydrocarbon scaffolds. Olefins interconversion via methylation, oligomerization, and cracking. 

In the beginning of the 1990s, Dahl and Kolbe [26,90] formalized a hydrocarbon pool mechanism. The publication initiated an immediate growth of the academic investigations in... 

Initially, the experimental evidence for the hydrocarbon pool mechanism was formulated by showing through labeling experiments that C-methanol and ethylene fed together to an H-SAPO-34 catalyst failed to form propylene by methylation of ethylene [26,90]. 

Original scheme of hydrocarbon pool mechanism. Adapted from Dahl IM, Kolboe S. On the reaction mechanism for propene formation in the MTO reaction over SAPO-34. Catal Lett 1993 20 329-36 Dahl IM, Kolboe S. On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34 1. Isotopic labeling studies of the co reaction ofethene and methanol. J Catal 1994 149 458—64. 

Afterward, the notion of unspecified carbon deposition with an olefin-like composition (CH ) has been gradually transformed to Polymethylbenzenes (PMBs) by many research groups [87,97]. Those PMBs serve as scaffolds/cocatalysts, where methanol is added and olefins are eliminated in a closed catalytic cycle [87,98]. It is therefore indicated that the interplay between the inorganic framework and the organic reaction centers dictates the activity and selectivity. However, according to Ref. [97], the role of PMBs as the major hydrocarbon pool species appears to be independent of the zeotype catalyst chosen. Haw et al. [87] provided both experimental and theoretical evidence in favor of PMBs as the driving force for the hydrocarbon pool mechanism. In 1998, by means of pulse-quench reactions on an H-ZSM-5 catalyst and GC-MS and MAS NMR analysis, it was reported... 

In the hydrocarbon pool mechanism (ie, aromatics carbon pool), two parallel routes, namely the side-chain route or exocyclic methylation route and the "paring route, have been suggested for the formation of olefins [97]. 

Some very recent first-principle calculations together with kinetic Monte Carlo simulations have shown that the MBs with five or six methyl groups are not more active than those with fewer methyl groups [103], Propylene is intrinsically more favorable than ethylene when the reaction is not diffusion limited based on a side-chain hydrocarbon pool mechanism. The theoretical results are consistent with some experimental observations and can be rafionafized based on the shape selectivity of key reaction intermediates and transition states in the pore of catalyst [61,103],... 

Recently, Wang et al. [103] suggested that alkene methylation, firstly proposed by Dessau, should receive more attention in the MTO conversion even on SAPO-34 [109]. The overall energy barriers for the production of ethylene and propylene are much lower than those in side chain and paring hydrocarbon pool mechanisms. That is to say, hydrocarbon pool mechanism, where alkenes themselves are the organic active centers, may be operative in the MTO conversion [61,103]. 

Froment, for example, even in his most recent works, reconciled the hydrocarbon pool mechanism even for SAPO-34 [119], They claim that the so-called intermediates in the carbon pool with which methanol reacts are mainly deduced from the analysis of the catalyst after conversion of the methanol in a batch mode, very diluted with inert gas conditions (not steam), and under the conditions far beyond the MTO/MTG modes. PMBs are the stable dead-ends and not the true intermediates. Coke plays an important role, but... 

The induction period predicted here and observed in the olefins yield in MTO on SAPO-34 depends as much on selectivation by deactivation as on the reaction scheme itself [119]. We should emphasize that the behavior described here, at the time, was based on the reaction scheme proposed by Hutchings and Hunter [22] and used by Alwahabi [101], not upon the carbon pool mechanism, often referred to for its explanation. 

More recently, in a theoretic study, Lesthaeghe et al. claimed that no ethylene can be formed through oxonium ylide mechanism, and their findings correspond to the fact that direct mechanisms fail in explaining the ethylene formation in MTO [122]. They found that ethylene cannot be eliminated from the oxonium intermediates and should be produced via hydrocarbon pool mechanism. 

Some very recent first-principle calculations, applied to methanol-to-olefins conversion in H-SAPO-34, showed that propylene is intrinsically more favorable than ethylene when the reaction is not diffusion limited based on side-chain hydrocarbon pool mechanism [103]. 

In the cofeeding reaction system of ethylene and methanol, it should be noted that both methylation, oligomerization/cracking and so-called MTO conversion via the hydrocarbon pool mechanism are acid-catalyzed parallel reactions and require an appropriate nature of acid sites. The above contradictory conclusions from different researchers may come from the different catalysts used and the different conditions studied. 

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