State selection of reactants

No State Selection of Reactants and No State Analysis of Products... 

Control of the selectivity in catalytic reactions is one of the challenging topics. For example, the shape selectivity in catalytic reactions was described for the first time by Mobil research workers [3]. At the present, some categories of shape selectivity have been described in the literatures, i.e. reactant shape selectivity, product shape selectivity and transition state shape selectivity [4,5]. Zeolites such as ZSM-5 show the promising catalytic performance for the shape selectivity of reactants or products in the alkylation of aromatics and in the catalytic cracking of hydrocarbons [6]. The shape selectivity by zeolite catalysts... 

Rigorous tests of the assumptions are not easily performed. The first assumption of RRKM theory can be tested only by single quantum state selection of the reactants. Because state selection is possible in only very small molecules, most efforts have been directed at energy selection which produces a distribution of initial states. While not ideal, this does permit testing one of the consequences of the random lifetime assumptions which is that k E) increases monotonically with increasing E. 

Mass-Spectrometer Ion Source—Photoionization Technique Reference is made here to the discussion in Chapter 3 of the photoionization technique, which is clearly the most powerful ion-source method. Not only is state selection of the reactant ion often possible, but operation of the source at 77°K minimizes the initial thermal energy spread to such an extent that unfolding of the excitation function is possible from the measured phenomenological cross section. Moreover, two types of ion source are described which allow direct measurement of the excitation... 

ABSTRACT. Laser and molecular beam techniques allow detailed study of many dynamical properties of single reactive collisions. The chemical scope of these methods is now very wide and includes internal state preparation of reactants, change of collision energies, state detection of products, and thus determination of state-to-state reaction rates. The great impact of laser spectroscopy on knowledge in the field of structure, molecular energy transfer and the mechanism of elementary chemical reactions is illustrated by two selected examples, i.e. studies in which laser-induced fluorescence (LIF) has been used to determine the specific impact parameter dependence of the Ca + HF -> CaF(X) + H reaction and the product state distributions for the reaction of metastable Ca with SF5. 

Already in Section 1.2.4 we have seen that the reaction rate does depend on the internal excitation of the reactants. See also Figure A3.1. Yet the measurement of k T) for reactions in thermal equilibrium can give no indication of such an effect (for a gas in thermal equihbrium, we are unable to vary the 7 ,s independently). In other words, at thermal equilibrium we are unable to state-select the reactants. It is only by imposing non-equilibrium reactant distributions that we can characterize the role of reactant excitation. Otherwise, when we vary T we vary both the occupations Pi of the different internal states of HCl and the kinetic energy of the collision (which is why the state-selected rates ki(T) are T-dependent). The measurement of k(T) only cannot tell the two apart, without making assumptions. 

Figure C2.12.10. Different manifestations of shape-selectivity in zeolite catalysis. Reactant selectivity (top), product selectivity (middle) and transition state selectivity (bottom).

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

Sections 3.1 and 3.2 considered this problem Given a complex kinetic scheme, write the differential rate equations find the integrated rate equations or the concentration-time dependence of reactants, intermediates, and products and obtain estimates of the rate constants from experimental data. Little was said, however, about how the kinetic scheme is to be selected. This subject might be dismissed by stating that one makes use of experimental observations combined with chemical intuition to postulate a reasonable kinetic scheme but this is not veiy helpful, so some amplification is provided here. 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

It is not the catalytic activity itself that make zeolites particularly interesting, but the location of the active site within the well-defined geometry of a zeolite. Owing to the geometrical constraints of the zeolite, the selectivity of a chemical reaction can be increased by three mechanisms reactant selectivity, product selectivity, and transition state selectivity. In the case of reactant selectivity, bulky components in the feed do not enter the zeolite and will have no chance to react. When several products are formed within the zeolite, and only some are able to leave the zeolite, or some leave the zeolite more rapidly, we speak about product selectivity. When the geometrical constraints of the active site within the zeolite prohibit the formation of products or transition states leading to certain products, transition state selectivity applies. 

Figure 5.31 illustrates ho v a zeolite can influence the selectivity of catalytic reactions. In the first case, one of the reactants is excluded because it cannot enter the zeolite. In the second, A reacts to give two produces, B and C, but C is too large to leave the pore. In the third case, the onward reaction of B to C is prohibited, e.g. because the transition state for this step does not fit. 

Steady state measurements of NO decomposition in the absence of CO under potentiostatic conditions gave the expected result, namely rapid self-poisoning of the system by chemisorbed oxygen addition of CO resulted immediately in a finite reaction rate which varied reversibly and reproducibly with changes in catalyst potential (Vwr) and reactant partial pressures. Figure 1 shows steady state (potentiostatic) rate data for CO2, N2 and N2O production as a function of Vwr at 621 K for a constant inlet pressures (P no, P co) of NO and CO of 0.75 k Pa. Also shown is the Vwr dependence of N2 selectivity where the latter quantity is defined as... 

Conclusive evidence has been presented that surface-catalyzed coupling of alcohols to ethers proceeds predominantly the S 2 pathway, in which product composition, oxygen retention, and chiral inversion is controlled 1 "competitive double parkir of reactant alcohols or by transition state shape selectivity. These two features afforded by the use of solid add catalysts result in selectivities that are superior to solution reactions. High resolution XPS data demonstrate that Brpnsted add centers activate the alcohols for ether synthesis over sulfonic add resins, and the reaction conditions in zeolites indicate that Brpnsted adds are active centers therein, too. Two different shape-selectivity effects on the alcohol coupling pathway were observed herein transition-state constraint in HZSM-5 and reactant approach constraint in H-mordenite. None of these effects is a molecular sieving of the reactant molecules in the main zeolite channels, as both methanol and isobutanol have dimensions smaller than the main channel diameters in ZSM-S and mordenite. 

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