Inverse shape selectivity

An interesting related study introduced the concept of inverse shape selectivity in molecular sieves [99]. Relative computed adsorption heats for n-hexane and 2,2-dimethylbutane in a series of zeolites with 1-dimensional channels were compared with corresponding experimental adsorption data and data for the relative selectivity to production of these two C6 isomers in hydrocracking of n-Ci6H34. A peak in the relationship between 2,2-dimethylbutane n-hexane selectivity and channel diameter at intermediate pore sizes indicated a channel size domain in which the branched isomer was... 

Studies of this type have been used by Smit and co-workers to explain the so-called inverse shape selectivity observed in the conversion of long chain w-alkanes over acid zeolites. In such reactions, product distributions are found to depend on the pore structure, particularly for medium-pore zeolites such as ZSM-5. In some cases branched alkanes are favoured over linear alkanes in the products of medium-pore zeolites compared to the reaction selectivities of large-pore zeolites such as zeolite Y. For example, doubly branched isomers are favoured over ZSM-5. This is in contrast with what would be expected from dilfusion rates and is attributed to the enhanced thermodynamic stability of some branched intermediates in the medium-pore zeolites that is predicted by configurational bias GCMC. 

To a number of unexpected shape-selectivity phenomena with long alkane hydroconversion the term inverse shape selectivity was coined [62]. In zeolite pore widths of 0.65-0.74 nm, the opposite was encountered, as expected from mere geometric effects, that is, preferential formation of branched over linear hydrocarbons, viz. the formation of high yields of dibranched from n-alkanes over 10-MR zeolites like API (SAPO-5), the phenomenon being confirmed by adsorption experiments [63]. The molecular basis of such effects has been attributed to a purely entropic effect in monodimensional pores at high pressures expelling larger molecules by smaller ones [64]. 

The unreacted ethanol and the diethylether product retained >98% of from the starting 0-ethanol, indicating that no isotope scrambling occurred. Data in Table 4 demonstrate that was retained in the mixed ether and ethanol attack of the acid-activated 2-pentanol via an axial S 2 rear-attack was the predominant synthesis pathway. Evidently, the shape selectivity induced by the 2 M-5 zeolite channel structure (Figure 2) plays an important role in achieving the remarkably higher configuration inversion... 

Ad(ii) On catalysts with pores and cavities of molecular dimensions, exemplified by mordenite and ZSM-5, shape selectivity provides constraints of the transition state on the S 2 path in either preventing axial attack as that of methyl oxonium by isobutanol in mordenite that has to "turn the comer" when switching the direction of fli t through the main channel to the perpendicular attack of methyl oxonium in the side-pocket, or singling out a selective approach from several possible ones as in the chiral inversion in ethanol/2-pentanol coupling in HZSM-5 (14). Both of these types of spatial constraints result in superior selectivities to similar reactions in solutions. 

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. 

Frequency-selective REDOR (fsREDOR) is a very powerful technique developed for the study of 13C and 15N uniformly labeled peptides or proteins [92]. The basic idea of this technique is to combine REDOR and soft n pulses to recouple a selected 13C-15N dipole-dipole interaction in a multiple-spin system. Usually one could use Gaussian shaped pulses to achieve the required selective n inversions. Other band selective shaped pulses have been developed for a more uniform excitation profile [93]. In its original implementation, fsREDOR was used to extract the intemuclear distances of several model crystalline compounds [92], In the past few years, this technique has proven to be very useful for the study of amyloid fibrils as well. For the Ure2p10 39 fibril samples containing 13C and 15N uniformly... 

Until the recent discovery of UTD-1 and CIT-5, the largest pore zeolites known were composed of pore structures having 12-MRs or less. Many of these materials such as zeolite Y have enjoyed immense commercial success as catalysts (2). There is some evidence from catalytic cracking data that suggests the inverse selectivity found with the 12-MR pore ( 7.5 A) structure such as for SSZ-24 (Chevron) might be used to enhance octane values of fuel (3). However, small increases in pore size as well as variations in pore shape and dimensionality could further improve the catalysts. Pores with greater than a 12-MR structure might allow the conversion of... 

Selected entries from Methods in Enzymology [vol, page(s)] Application in fluorescence, 240, 734, 736, 757 convolution, 240, 490-491 in NMR [discrete transform, 239, 319-322 inverse transform, 239, 208, 259 multinuclear multidimensional NMR, 239, 71-73 shift theorem, 239, 210 time-domain shape functions, 239, 208-209] FT infrared spectroscopy [iron-coordinated CO, in difference spectrum of photolyzed carbonmonoxymyo-globin, 232, 186-187 for fatty acyl ester determination in small cell samples, 233, 311-313 myoglobin conformational substrates, 232, 186-187]. 

For selective irradiations with a flip angle of 180°, one can distinguish two groups of shaped pulses inversion pulses, which change the sign of Zeeman... 

Fig. 3. Numerical simulations of four different selective inversion pulses. (Top) Pulse shapes. From left to right 180° rectangular, 180° Gaussian truncated at 2.5%, Quaternion cascade Q, and l-BURP-2. (Middle) Trajectories of Cartesian operators in the rotating frame during...

The peak rf amplitude required to achieve optimum inversion with a selective inversion pulse is given in comparison to the rf amplitude required to achieve an on-resonance 90° flip-angle with a selective rectangular pulse, the simplest conceivable shape. 

Fig. 4 Selective reflection wavelength (optical pitch) as a function of temperature for three samples host material showing SmC and SmCA without dopant, with 1% and 3% dopant The chemical structures of the rod-shaped host and the bent-shaped dopant molecules are also shown. As shown in inset, inverse pitch increases almost linearly with dopant content [4]...

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