Anca Ghenciu

The mechanism of conversion of hydrocarbons on sulfated metal oxides. Part II. Reaction of benzene on sulfated zirconia [1]

Abstract The conversion of benzene on sulfated zirconia was studied in batch reactor, under mild conditions. The interaction of benzene with the catalyst is a complex process which is initiated by a one-electron oxidation, followed either by trapping of the generated cation-radicals to form sulfite esters on the surface, or by the reaction of the cation-radicals with the excess of benzene, followed by a cascade of coupling and cleavage reactions. The surface esters liberate phenol upon hydrolysis at the end of the reaction. Thus, benzene is retained on sulfated zirconia not because of protonation to benzenium ion, but because of formation of non-volatile products. Together with our previous results on the reaction of adamantane on sulfated zirconia, the present work elucidates the mechanism of conversion of aliphatic hydrocarbons on sulfated metal oxides. The first interaction is a one-electron oxidation of the alkane, leading to an ion-radical pair, followed by recombination and rearrangement to generate sulfite esters on the surface, which are the active intermediates in the mechanism. The high activity of sulfated metal oxides in alkane conversion is due, therefore, to their one-electron oxidizing ability, leading to ion-radicals and then to surface esters. The latter either ionize generating carbocations, or eliminate forming olefins. Both these species can carry on carbocationic reactions with no requirement of superacidity, which these catalysts do not possess. The oxidative mechanism predicts the existence of an induction period in the alkane conversions and a rapid deactivation of the catalyst, both in agreement with the experimental observations.

Evaluation of acidity of strong acid catalysts I. The strength of boron trifluoride-water systems

Abstract The method of determining a thermodynamic acidity function from the chemical shift changes of 13 C signals of unsaturated ketones at infinite dilution in the investigated acid established by the authors was applied to the system boron triftuoride-water ( III ) ranging from the monohydrate (BF 3 · H 2 O) to the trihydrate (BF 3 · 3H 2 O). The indicators used were mesityl oxide and 4-hexen-3-one. It was found that III is significantly stronger than indicated by earlier measurements conducted by the classical Hammett method based on UV-visible spectroscopy. The mixtures with about 1.25 mol of water per mol of BF 3 or less are stronger than pure sulfuric acid and are therefore superacidic. The stronger acidity of III can be understood because boron triftuoride is a much stronger Lewis acid than sulfur trioxide; therefore the complex with a hydroxyl anion of the former (hydroxytrifluoroborate anion) should have a lower affinity for a hydron than the corresponding complex of sulfuric anhydride (bisulfate anion). Preliminary experiments indicate that the 13 C NMR method can be applied successfully to working catalysts based on III , which are colored and contain dissolved organic materials.

Oxidizing ability as the defining factor of reactivity of sulfated zirconia

Reaction of benzene with sulfated zirconia (SZ) is an oxidation to a surface phenyl ester (sulfite or sulfate) which forms phenol by hydrolysis, and a complex mixture of alkylbenzenes and polycyclic aromatics; a mechanism involving a one-electron transfer from benzene to SZ is proposed; oxidation of a fraction of substrate to alkyl surface esters which then ionize to carbocations is the origin of the high catalytic activity of SZ for alkane isomerization.

Evaluation of hydrogen bonding ability of liquids and solids by C-13 NMR. Silica gel as a strong hydrogen bond donor

The chemical shift difference between signals of C(β) and C(α) of unsaturated ketones, Δδ, which we used before to measure acid strengths, has now been used to evaluate the hydrogen bond donor ability of solvents which are not acidic enough to hydronate the indicator. For such solvents there is no general correlation between H-bond donor ability and acid strength: hexa-fluoroisopropanol is a much weaker acid than acetic acid, but it is a stronger H-bond donor. The method can be applied to evaluate the H-bonding properties of solid surfaces, and it was thus found that silica gel has a much stronger H-bond donor ability than methanol or acetic acid.

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.

Strength of liquid acids in solution and on solid supports. The anion stabilization by solvent and its consequences for catalysis

The effect of solvents upon the effective strength of acids in solution was studied in the strong acid range by the measurement of the Δ δ parameter for mesityl oxide at stoichiometric acid/base ratio (Δ δ1) and in the weak superacid range by the measurement of the hydronation of hexamethylbenzene (HMB). The approach is applicable to acids which cannot be described by an acidity function (non-Hammett acids). For sulfuric acid, the strength given by the Δ δ1 parameter changes with solvent in the order: sulfolane < neat acid < SO2 < hexafluoroisopropanol (HFIP). Thus, sulfolane behaves like a basic solvent toward H2SO4. The small effect of SO2 is a result of its being a polar solvent. The large acidity enhancement observed in HFIP solution results from its ability to form strong hydrogen-bonded complexes with the acid anion (anion stabilization). The extent of hydronation of HMB by trifluoromethanesulfonic acid (TFMSA) changes with solvent in the order: SO2, SO2ClF < trifluoroacetic acid (TFA) < HFIP. As TFA is more basic than SO2, this finding demonstrates that TFA and HFIP are particularly good anion stabilizing solvents. Basicity of a solvent is not well described by the pKBH+ measured for that compound as a solute, but can be assessed from the decrease in the extent of hydronation of a probe base by an acid in the solvent. For hydrogen-bond donor solvents, a correction has to be made for the anion stabilizing effect (acidity-enhancing). An empirical relative solvent basicity parameter (SB) was developed from the examination of hydronation of HMB by TFMSA (3 mol) in TFA-CHCl3 solutions, and its suppression by the addition of an amount of solvent i equal to the acid (SB(i, TFA)). TFA is thus taken as the standard, non-basic, solvent, and also provides the anion stabilization. Values of SB(i, TFA) for some carboxylic acids and one nitroalkane are listed. The effect of anion stabilizing solvents as promoters for strong acid catalysts was shown by the acceleration of the transalkylation of p-di-tert-butylbenzene (p-DTBB) with toluene catalyzed by H2SO4, upon addition of small amounts of TFA or HFIP (most effective) to the acid.