Povilas Lukinskas

Theoretical study of hydrogen dissociation on dialuminum oxide clusters and aluminum–silicon–oxygen clusters, as models of extraframework aluminum species and zeolite lattice

Abstract The reactivity of coordinatively unsaturated centers of aluminum oxide clusters, found for example in extraframework aluminum species of steamed zeolites, was examined theoretically on the example of the reaction of a hydrogen molecule with dialuminum hydroxide clusters (HO) 2 (H 2 O) n AlOAl(OH) 2 (H 2 O) ( 1 ). In the cluster, one aluminum atom is tricoordinated and the other is tetracoordinated ( 2 , n =0), or both aluminum atoms are tetracoordinated ( 3 , n =1). The system studied is also a model for transitional aluminas, in which two or three tetracoordinated aluminum centers can occur next to each other. Density functional theory calculations with electron correlation at the B3LYP/6–31G ∗∗ level have identified a complex with physisorbed hydrogen and a complex with chemisorbed hydrogen in each case. Each of them was more stable than the corresponding complex formed by the corresponding one-aluminum cluster. The transition structures for chemisorption were identified. The reaction coordinate for chemisorption revealed that the reaction is a case of metal ion catalysis, rather than an acid–base reaction with heterolytic dissociation of hydrogen. The potential energy barrier (PEB) for hydrogen chemisorption was lower for the two-Al clusters than for the one-Al clusters. The chemisorption on a silicon–aluminum cluster, (HO) 2 (H 2 O)AlOSi(OH) 3 was also found to occur, but it had a higher PEB than for the corresponding two-aluminum cluster. Thus, zeolites can also exchange hydrogen, albeit less effectively than alumina, whereas the extraframework aluminum species in steamed zeolites should exchange hydrogen easier than the intact zeolite.

Toward a quantitative evaluation of Lewis acid strength. A 13CNMR study of the interaction of boron trifluoride with diethyl ether

Abstract The rate of exchange between boron trifluoride diethyl etherate and free diethyl ether can be conveniently measured by 13 C dynamic NMR spectroscopy (DNMR) between 25°C and −50°C in dichloromethane solution. The same rates and activation parameters are found by the line shape analysis of either the methyl or the methylene group. The rates do not vary with the ratio of free ether to complex between 0.61 and 2.4, indicating a rate-determining unimolecular decomposition of the latter. The literature claim of a bimolecular mechanism for the ether exchange is thus found to be incorrect. An enthalpy of activation (ΔH‡) of 9.7 kcal/mol was calculated for this decomposition. This value is a good measure of the strength of interaction of BF3 with diethyl ether, because the recombination to form the complex (2) should have a very low energy barrier, and it is close to the ΔH° value of 10.9 kcal/mol determined by Brown and Adams for the decomposition of 2 to gaseous diethyl ether and BF3. The 13 C DNMR method can be applied in principle to the interaction of ether with other Lewis acids, thus providing a method for comparison of Lewis acid strengths.

The Hydronium Tetrafluoroborate Dimer in Nonpolar Media and its Proton NMR Spectrum

Abstract Hydronium tetrafluoroborate ion pairs, H3O+·BF4- have been shown computationally to be unstable toward decomposition, in the absence of solvation or electrostatic interactions existing in crystals. As the proton NMR spectrum of a hydronium salt with the octanesulfonate-antimony pentachloride complex anion was reported in freon solution, we investigated the hypothesis that larger ionic clusters were present in the nonpolar solvent. It was found that the dimer (H3O+·BF4-)2 was stable at the MP2/6-31G* level. GIAO-B3LYP chemical shift calculations with the same basis set and also with the 6-31G**, 6-31++G**, 6-311++G**, dzvp, tzp, tz2p, and qz2p basis sets conducted on the hydronium fluoroborate dimer reproduce the main features of the experimental spectrum: the existence of two signals with a two-to-one intensity ratio and the more intense resonance at higher frequency (more deshielded). The alternative structures, of hydronium tetrafluoroborate ion pairs with one and with two hydrogen bonds between anion and cation, give calculated chemical shifts which are farther from the experimental values.

The stability of the hydronium tetrafluoroborate dimer determined by theoretical calculations. Implications for water protonation in nonpolar solvents and on solid acids

Upon geometry optimization with no constraints (MP2/6-31G*), hydronium fluoroborate decomposed, but the dimer (H3O+·BF4−)2 was stable. In the dimer, the anions face each other and create a cavity containing the two cations. Density functional theory calculations (B3LYP/6-31G* to /6-31++G**) gave somewhat different arrangements of clusters and ion geometries. The MP2/6-31G* geometry was symmetrical, with a boron to boron distance of 4.650 A and the closest interionic fluorine to fluorine distance of 2.783 A. Hydrogen bonds connect ions of opposite sign. Two groups of equivalent hydrogens generate an AX2-type 1H NMR spectrum, as found for a hydronium salt in Freon solution; dimeric clusters should also be present in nonpolar solvents. In concentrated aqueous solution, however, the 17O NMR spectrum indicates the presence of ion pairs. The double cluster is stable at B···B distances of 5.00, 5.50 and 6.00 A (F···F distances of 3.10, 3.91 and 4.55 A), but decomposes to a solvated ion pair at B···B of 6.50 A. Cooperation of acid sites inside porous solids should also facilitate water protonation, to form double anion–hydronium ion clusters. Whether sites of equal strength protonate water at stoichiometric ratio depends upon the channel size and disposition of sites in it. The expectation of narrowly defined spectral properties for protonated water in solid acids is not warranted. The protonating ability is determined by intrinsic strength, distance between, and relative orientation of sites.

Steric requirements of the alkene–carbocation alkylation in relation to the hydride transfer and proton cleavage of carbon–carbon bonds. Significance for the reactions of alkanes in zeolites

The reaction of the 2-propyl cation with propene has been investigated by MP2/6-31G** and B3LYP/6-31G** calculations. An ion–molecule complex stabilized by 10–11 kcal mol−1 over the isolated reactants was identified. It continued over a barrier and gave 1,1,3-trimethyl-1-protonated cyclopropane as the reaction product. The process read in the opposite direction represents the cracking of 2-methylpentane, occurring nominally from the 2-methyl-4-pentyl cation (open structure of 1,1,3-trimethyl-1-protonated cyclopropane). The geometries predicted by the MP2 and B3LYP calculations were characteristically different, both for the reaction product and for the ion–molecule complex, with the MP2 calculations showing a stronger stabilization of hydrogen- and carbon-bridged species. The calculated volumes (packed-cell dimensions) of the transition state for the isopropyl cation–propene alkylation and for the isopropyl cation–propane hydride exchange were compared. The former had higher steric requirements in the transversal cross-section. Therefore, in the cracking of alkanes on medium pore zeolites such as HZSM-5, the β cracking step should be more sensitive to the existence of methyl side-chains than the hydride transfer step. The cracking mechanism of alkanes and alkenes on medium-pore zeolites is discussed based on these findings.

Metal ion catalysis in chemisorption anddehydrogenation of alkanes on aluminium hydroxide clusters, revealed bytheoretical calculations

Electron-correlated DFT calculations with a large basis set show that propane adds to coordinatively unsaturated aluminium, as in the clusters (HO)3Al(OH2)x (x = 0, 1), by aluminium insertion into a C–H bond, followed by hydrogen migration to an oxygen atom and predict correctly experimental observations; the alternative pathway involving alkyl–oxygen interaction has a much higher energy barrier and does not predict correctly the experimental results.

The effect of cycloalkanes on the reaction of hexane with trifluoromethanesulfonic acid

The chain reaction of hexane catalyzed by trifluoromethanesulfonic acid (TFMSA) under mild conditions (disproportionation and cracking) was suppressed by small amounts (1–5%) of cycloalkanes, namely methylcyclopentane (MCP), cyclohexane (CH) and cyclopentane (CP). The reaction then proceeded in the isomerization mode. The addition of ferric ions to the acid overcomes the stabilizing effect of cycloalkanes. The formation of substituted allyl cations during the induction period for cracking was also inhibited by very small amounts of MCP. Addition of MCP or 3-methylhexane had the same effect on reduction of the induction period and (minor) on the increase in rate of the reaction stabilized against cracking by CP addition. Incorporation of deuterium into the products from hexane and MCP-1-d shows that MCP acts as a hydride relay in the reaction. Hydride transfer catalysis by the reaction product is disproved for any catalyst. Both the reaction of hexane with MCP-1-d and of uniformly labeled hexane-U-d4.3 with MCP showed H/D scrambling which is extensive among all products (2- and 3-methylpentane, cracking products and CH) and the acid catalyst, smaller in MCP, and is virtually zero in hexane. Thus, the main features of the reaction are the initiation by oxidation and the formation and hydronation of alkenes.