PROBE REACTION

The final probe of molecular clusters is that of selected chemical reactions. The use of probe reactions to study supported cluster catalysts is well established, and we are attempting the development of similar probes of unsupported clusters. The first steps in this direction are the design of a pulsed chemical reactor to go with the pulsed cluster source and the development of criteria for reactions. It is important to recall that at present... 

The probe reaction utilized a 1/1 molar mixture of methanol and isobutanol over H-mordenite, a strongly acidic zeolite comprised of linear one-dimensional channels made up of 12-ring 6.5 by 7.0 A windows [8]. There is a side-pocket system in H-... 

Clearly, mechanistic investigations can provide circumstantial evidence for the participation of particular intermediates in a reaction but, here, we are concerned with the definitive observation of these species. If the intermediates are relatively stable then direct spectroscopic observation of the species during a room-temperature reaction may be possible As a rather extreme example of this, the zero-valent manganese radicals, Mn(CO>3L2 (L phosphine) can be photochemically generated from Mh2(CO)gL2, and, in the absence of O2 or other radical scavengers, are stable in hydrocarbon solution for several weeks (2, 3) However, we are usually more anxious to probe reactions in which unstable intermediates are postulated. There are, broadly speaking, three approaches - continuous generation, instantaneous methods and matrix isolation. 

Kalakkad, Datye, and coworkers—TEM indicates strong metal-support interactions in Pt/ceria and Pt/Ce/Al catalysts. Kalakkad et al 362,369 published TEM and probe reaction studies on 0.6%Pt/CeO2 catalysts relevant to ACC catalysis. The... 

LFP-Probe Method. In cases where the radicals of interest do not contain a useful chromophore, the LFP technique can be modified by incorporation of a probe radical reaction that gives a product with a chromophore. The probe reaction can be unimolecular or bimolecular, a constant concentration of probe reagent is employed in the latter case. Formation of the detectable species occurs with an observed first-order or pseudo-first-order rate constant equal to k0. In the presence of another reagent X that reacts with the original radical, the rate constant for formation of detectable species is kohs = k0 + kx [X], and the bimolecular rate constant is determined (as before) by conducting the reaction at varying concentrations of X. Note that the LFP-probe technique is a direct method even though the reactant or product of interest is not monitored. 

Rate constants for reactions of Bu3SnH with some a-substituted carbon-centered radicals have been determined. These values were obtained by initially calibrating a substituted radical clock on an absolute kinetic scale and then using the clock in competition kinetic studies with Bu3SnH. Radical clocks 24 and 25 were calibrated by kinetic ESR spectroscopy,88 whereas rate constants for clocks 26-31 were measured directly by LFP.19,89 90 For one case, reaction of Bu3SnH with radical 29, a rate constant was measured directly by LFP using the cyclization of 29 as the probe reaction.19... 

Progress of the reaction can be followed by gas chromatography, TLC (ethyl acetate/hexane 1 5) or 1H NMR. (Probe reactions can be carried out in CDCI3 in an NMR tube). 

In most cases, introduction of fluorine adjacent to sulfur can be monitored by proton NMR for small-scale probe reactions run in CDCI3. The CHF peaks in the... 

L. Lahav, M. Probing reaction pathways via asymmetric transformations in third and centro-symmetric crystals in Photochemistry in Organized and Constrained Media Ramamurthy, V., Ed. VCH Publishers New York, New York, 1991, pp 248-302. 

A typical probe reaction for estimating catalytic properties in selective hydrogenations is the hydrogenation of cinnamaldehyde. This molecule contains both a C=C and a C=0 double bond, thus the formation of hydrocinnamaldehyde and/or cinnamyl alcohol by reduction of the one or the other, or the formation of phenyl propanol in the case of complete reduction may indicate the potential of the catalyst for other fine chemical transformations. Indeed, this reaction was one of the first to be tested by CNT-supported catalysts [120]. Noble metals show a high activity in this reaction and... 

Study of fhe mechanism of MeOH oxidation over Pt and PtRu surfaces has recenfly been given new insights using a combination of experimental and theoretical approaches. The use of electrochemically linked mass spectroscopy techniques (e.g., differential electrochemical mass spectroscopy— DBMS) has allowed the quantification of the MeOH oxidation reaction in terms of comparing CO2 yields with electrons passed. In addition, detection and quantification of reaction intermediates has also been demonstrated. In addition, use of theorefical fechniques such as DFT has allowed calculation of adsorbafe energies, probing reaction pathways, and activation of H2O to provide active OH species. 

Vaughan, D.E.W., Strohmaier, K.C., Kliewer, W.R., and Leta, D.P. (2005) Methylcyclohexane ring-contraction a sensitive solid acidity and shape selectivity probe reaction. J. Phys. Chem. B, 109 (6), 2222-2226. 

Usually, the PFR serves as a probe reaction for the study of the morphology of the matrix. This has been demonstrated by Weiss and co-workers in a series of recent articles on the photoreactivity of esters included in several polyethylene films. Low-density polyethylene (LDPE) films hosting 2-naphthyl esters bring about different selectivity in the PFR as compared with the reaction in solution. In addition, the selectivity is different if the polymer is stretched [286,287], Table 16 indicates the different product distributions upon irradiation of 2-naphthyl esters, depending on the nature of the solvent or matrix. The most striking fact is that irradiation of 2-naphthyl myristate leads to the coupling at the position 1 in ferr-butanol with 86% yield, whereas this product is absent in the irradiations in polyethylene films. Moreover, the product of coupling at position 3 is absent... 

Most of the pseudounimolecular hydrolytic probe reactions involve water-catalyzed pH-independent hydrolysis of activated esters, amides, or acid chlorides " " (Scheme 9). The hydrolysis of these probes occurs via a dipolar... 

As the reasons for rate retardations have been discussed for pseudounimolecular probe reactions already, we focus on the reported increased bimolecular rate constants. Two main reasons for increases in bimolecular rate constants come to the fore (1) dehydration of the reactive counterions and (2) charge delocalization during the activation process leading to the transition state. An intriguing third reason (although, admittedly, not strictly equating to an increased bimolecular rate constant) is (3) the increase in local counterion concentration as a result of comoving counterions. We will discuss these three effects in order. 

Methanol conversion on ZSM5 and other catalysts is found to be a very elucidating probing reaction for catalyst activity, internal and external active sites and spacious constraints. Deactivation mechanisms in different temperature ranges are specified. Basic reaction steps and reaction pathways are understood from the detailed time resolved selectivity data. 

To ensure the system is probing reactions in a kinetically controlled regime, the reaction conditions must be calculated to determine the value of the Wiesz-Prater criterion. This criterion uses measured values of the rate of reaction to determine if internal dififusion has an influence. Internal mass transfer effects can be neglected for values of the dimensionless number lower than 0.1. For example, taking a measured CPOX rate of 5.9 x 10 molcH4 s g results... 

Vaida, M., Popovitz-Biro, R., Leiserowitz, L., and Lahav, M. (1991). Probing reaction pathways via asymmetric transformations in chiral and centrosymmetric crystals. In Photochemistry in Organized and Condensed Media, ed. V. Ramamurthy. VCH, pp. 248-302. 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

Xylenes. Because of the practical significance of xylenes, isomerization of xylenes over zeolites is frequently studied.348 The aim is to modify zeolite properties to enhance shape selectivity, that is, to increase the selectivity of the formation of the para isomer, which is the starting material to produce terephthalic acid. In addition, m-xylene isomerization is used as a probe reaction to characterize acidic zeolites.349,350... 

Probing reaction dynamics with Rydberg states The ring opening reaction of 1,3-cyclohexadiene... 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

The idea of using spectroscopy of the environment to probe reaction leads to the main topic of this chapter, which is using spectroscopy of reaction products to probe the environment in which reaction occurs. 

The transient nature of the cavitation event precludes conventional measurement of the conditions generated during bubble collapse. Chemical reactions themselves, however, can be used to probe reaction conditions. The effective temperature realized by the collapse of clouds of cavitating bubbles can be determined by the use of competing unimolecular reactions whose rate dependencies on temperature have already been measured. The sonochemical ligand substitutions of volatile metal carbonyls were used as... 

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