Quantum-chemical study of the nonclassical carbonium ion-like transition states in isobutane cracking on zeolites

The ''ab initio'' quantum chemical calculations at MP2/6- 31++G**//HF/6-31G* level were performed for some elementary steps of isobutane cracking on zeolites. The results demonstrated that adsorbed carbonium ions represent not the observable reaction intermediates, but the high-energy transition states of the corresponding elementary reactions. Calculated activation energies are in reasonable agreement with experimental dat

In situ 13c NMR study of "conjunct polymerization" of i-pentene in 95% sulfuric acid

13C NMR study of \"conjunct polymn.\" of 1-pentene in 95% sulfuric acid demonstrated that this is a carbenium ion reaction, where the esters are the precursors of carbenium ions. In the first step of the reaction a complicated mixt. of primary and secondary mono- and dipentyl esters is formed. In an excess of the acid the esters are decompd. yielding aliph. carbenium ions, which are then involved in secondary reactions resulting in the final products. Similar to heterogeneous catalysis on zeolites, the steady state concn. of the aliph. carbenium ions during the reaction is, however, too low for their direct detection by NM

In situ 13c NMR study of "conjunct polymerization" of i-pentene in 95% sulfuric acid

13C NMR study of \"conjunct polymn.\" of 1-pentene in 95% sulfuric acid demonstrated that this is a carbenium ion reaction, where the esters are the precursors of carbenium ions. In the first step of the reaction a complicated mixt. of primary and secondary mono- and dipentyl esters is formed. In an excess of the acid the esters are decompd. yielding aliph. carbenium ions, which are then involved in secondary reactions resulting in the final products. Similar to heterogeneous catalysis on zeolites, the steady state concn. of the aliph. carbenium ions during the reaction is, however, too low for their direct detection by NM

A quantum-chemical study of hydride transfer in catalitic transformations of paraffins on zeolites. Pathways through adsorbed non-classical carbonium ions

A quantum-chemical investigation of the hydride transfer reaction in catalytic transformations of hydrocarbons on zeolites has been performed. Ab initio calculations at the MP2/6-31++G**//HF/6-31G* level demonstrated that the activated complexes of hydride transfer reaction in catalytic transformations of paraffins on zeolites very much resemble adsorbed nonclassical carbonium ions. However, these transient species are strongly held at the surface active sites by the Coulomb interaction. The calculated activation energies for reactions involving propane and isobutane are in a reasonable agreement with the experimental dat

A quantum-chemical study of adsorbed nonclassical carbonium ions as active intermediates in catalytic transformations of paraffins. II. Protolytic dehydrogenation and hydrogen-deuterium hetero-isotope exchange of paraffins on high-silica zeolites

HF/3-21G quantum-chem. anal. of the protolytic attack of acid protons in zeolites at the C-H bonds in methane and ethane indicated that the resulting transition states depend on the sign of the bond polarization. If a hydride ion is split off from the paraffin, then the transition state resembles the adsorbed carbonium ion and the reaction results in mol. hydrogen and in formation of the surface alkoxy group. The case, when a proton tends to split off from the paraffin, corresponds to the hetero-isotope exchange of paraffins with surface OH groups. This is a concerted acid-base reaction with a transition state different from adsorbed carbonium ion. [on SciFinder (R)

Quantum-chemical study of the isobutane cracking on zeolites

An ab initio quantum chem. investigation of the elementary steps of catalytic isobutane cracking is presented. A reasonable agreement between exptl. and theor. activation energies is found. The results demonstrate that the adsorbed carbenium and carbonium ions represent not the existing reaction intermediates, but the high-energy transition states of the corresponding elementary reactions. This results in much higher activation energies than for the similar reactions in homogeneous superacid soln

A quantum-chemical study of hydride transfer in catalitic transformations of paraffins on zeolites. Pathways through adsorbed non-classical carbonium ions

A quantum-chemical investigation of the hydride transfer reaction in catalytic transformations of hydrocarbons on zeolites has been performed. Ab initio calculations at the MP2/6-31++G**//HF/6-31G* level demonstrated that the activated complexes of hydride transfer reaction in catalytic transformations of paraffins on zeolites very much resemble adsorbed nonclassical carbonium ions. However, these transient species are strongly held at the surface active sites by the Coulomb interaction. The calculated activation energies for reactions involving propane and isobutane are in a reasonable agreement with the experimental dat

The Sites of Molecular and Dissociative Hydrogen Adsorption in High-Silica Zeolites Modified with Zinc Ions. III DRIFT Study of H2 Adsorption by the Zeolites with Different Zinc Content and Si/Al Ratios in the Framework

DRIFT study of dihydrogen adsorbed at 77 K by the zinc-modified hydrogen forms of mordenite and ZSM-5 zeolites with the different Si/Al ratios in the framework indicated the existence of several adsorption sites connected with the modifying zinc ions. The fraction of the sites of the strongest perturbation of adsorbed mol. hydrogen with the H-H stretching frequency of ca. 3930-3950 cm-1 is the highest at the highest Si/Al ratios of 41 or 80. These sites dissociatively adsorb hydrogen at moderately elevated temps. Adsorption of hydrogen with the higher H-H stretching frequency of about 3970-4000 cm-1 predominates at the lower Si/Al ratios. It does not result in the dissocn. of adsorbed mols. It was concluded that the most active sites are most probably connected with the zinc ions, which compensate two distantly sepd. neg. charged [AlO4]- tetrahedra localized in the adjacent five- or six-membered rings on the walls of the large channels of the pentasil's framework. The sites of the weaker perturbation of adsorbed hydrogen are most probably connected with Zn+2 ions at the conventional exchangeable cationic positions with two aluminum atoms in the same five- or six-membered ring

The Sites of Molecular and Dissociative Hydrogen Adsorption in High-Silica Zeolites Modified with Zinc Ions. III DRIFT Study of H2 Adsorption by the Zeolites with Different Zinc Content and Si/Al Ratios in the Framework

DRIFT study of dihydrogen adsorbed at 77 K by the zinc-modified hydrogen forms of mordenite and ZSM-5 zeolites with the different Si/Al ratios in the framework indicated the existence of several adsorption sites connected with the modifying zinc ions. The fraction of the sites of the strongest perturbation of adsorbed mol. hydrogen with the H-H stretching frequency of ca. 3930-3950 cm-1 is the highest at the highest Si/Al ratios of 41 or 80. These sites dissociatively adsorb hydrogen at moderately elevated temps. Adsorption of hydrogen with the higher H-H stretching frequency of about 3970-4000 cm-1 predominates at the lower Si/Al ratios. It does not result in the dissocn. of adsorbed mols. It was concluded that the most active sites are most probably connected with the zinc ions, which compensate two distantly sepd. neg. charged [AlO4]- tetrahedra localized in the adjacent five- or six-membered rings on the walls of the large channels of the pentasil's framework. The sites of the weaker perturbation of adsorbed hydrogen are most probably connected with Zn+2 ions at the conventional exchangeable cationic positions with two aluminum atoms in the same five- or six-membered ring