Dan Fărcaşiu

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.

The effects of reaction conditions and catalyst deactivation on the mechanism of n-butane isomerization on sulfated zirconia

Abstract The mechanism of n-butane isomerization on a widely studied unpromoted sulfated zirconia catalyst was studied at low conversions in a once-through reactor using 13C-labeled n-butane as a reactant. The comparison of results from GC/MS analysis of isobutane produced from 12C containing n-butane and 13C-labelled n-butane suggests that, at reaction temperatures ≤250°C, the reaction proceeds mainly via the bimolecular mechanism. The decrease in n-butane isomerization activity with time-on-stream did not appear to cause a change in the reaction mechanism.

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.

Acid strength of tetrafluoroboric acid The hydronium ion as a superacid and the inapplicability of water as an indicator of acid strength

The Hammett acidity function, H 0 , of 50.7% to 79.2% aqueous fluoroboric acid has been determined by 13 C NMR spectroscopy (the Δδ 0 method) with mesityl oxide as indicator. At all concentrations, the H 0 values are more negative than those of sulfuric or trifluoromethanesulfonic acid solutions, increasingly so at higher concentrations. The acid strengths measured represent the acidities of various clusters of hydronium ions and water, ion-paired with fluoroborate anions; in such systems the acid strength is also dependent upon the anion. At 79.2% concentration (0.28 mol excess water per mol hydronium ions) the solution is as strong as, or slightly stronger than, pure sulfuric acid, meaning that mixtures with less excess of water are superacidic. The hydronium ion (as fluoroborate) being a stronger acid than 100% sulfuric acid means that water dissolved in hydronium fluoroborate is a weaker base than the hydrogen sulfate ion in sulfuric acid. At the same concentration (wt.%), ‘fluoroboric acid’ is stronger than perchloric acid, but the two are equally strong when concentration is measured in molar ratios water:acid, meaning that perchloric acid is also fully converted in concentrated solutions to hydronium perchlorate. In acetic acid solution, hydronium perchlorate and fluoroborate are again equally strong, as expected, but they are significantly weaker than phosphotungstic acid, even though the hydron-donating species should be H 3 O + ·xH 2 O, in all cases. The acid strength of H 3 O + is thus strongly dependent upon the nature of the anion, which means that the basicity of water is strongly dependent upon the acid. Water is, therefore, not useful as a probe base for estimating acid strengths.

Reaction mechanisms on liquid and solid acid catalysts. Correlation with acidity

The probe bases used for measurement of strength of acid catalysts need to be structurally similar to the catalytic substrates. Traditional acidity measurements are inapplicable when the hydron transfer forms a tight ion pair, as on solid acids. Also, solid acids are much weaker than liquid acids of similar structure. Alkane activation by C–H and C–C cleavage, evidenced in strong superacids, does not occur in less strongly acidic media. The reactivity patterns and structure–reactivity relationships are the same for the cationoidic species (weakly coordinated carbocations) occurring in the less strong acids as for the full-fledged carbocations intervening in superacids. Even in trifluoromethanesulfonic acid, a weak superacid, the first step in alkane activation is an oxidation, forming unsaturated carbocations. On sulfated zirconia, the first step of alkane reactions is a one-electron oxidation. Mechanistic features incompatible with standard (or “traditional”) carbocationic processes, are observed in the reaction of 3-methylpentane on the zeolite HZSM-5.

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.

Mechanistic study of the acid catalyzed formation and hydrolysis of MTBE in nonpolar media

The acid catalyzed hydrolysis of MTBE using p-toluenesulfonic acid was studied in nonpolar medium to ascertain the effect of nucleophile upon reaction. NMR measurements showed protonation of both MTBE and methanol by p-toluenesulfonic acid in nonpolar medium but no nucleophile-electrophile interaction of MTBE with p-toluenesulfonic acid methyl ester as potential nucleophile. Addition or in situ generation of a nucleophile did not accelerate the reaction. At low conversions and low catalyst concentration the reaction exhibited pseudo zero order with respect to reactant concentration and second order with respect to catalyst concentration. The results suggest protonation of MTBE in a fast preequilibrium step in which the acid catalyst exhibits cooperative effect. No nucleophilic assistance in the transition state of MTBE hydrolysis in nonpolar medium occurs.

Absence of correlation of hydrogen bond donor ability with acid strength and catalytic activity of acid catalysts

Two liquids, acetic acid and hexafluoroisopropanol (HFIP), and two solids, silica gel and polymethacrylic acid (PMA), were compared for hydrogen bond donor ability, acid strength, and catalytic activity in typical acid‐catalyzed reactions, inversion of sugar and cleavage of acetone dimethyl ketal. In each pair, the weaker acid (HFIP and silica gel, respectively) was much the stronger hydrogen bond donor, but was totally devoid of catalytic activity, which the poor hydrogen bond donor but stronger acids (acetic and methacrylic acid, respectively) exhibited. A strong hydrogen bond donor (e.g., HFIP) enhances, however, the catalytic activity of the acid catalyst (AcOH). Thus, hydrogen bond donor ability is not a measure of acid strength. A correlation of the two properties is possible only when each group (acids and bases) involved in the comparison consists of very close structural relatives. Such a correlation cannot be extrapolated to any other case.

Solvent effects on 15N NMR chemical shifts of 2,6-di-tert-butylpyridine. Absence of hydrogen bonding with the nitrogen atom

Comparison of the 15N chemical shifts of 2,6-di-tert-butylpyridine (DTBP) and pyridine (Py) in a series of solvents demonstrates the inability of the nitrogen atom in DTBP to participate in hydrogen bonds. Both bases are fully hydronated in trifluoromethanesulfonic acid (TFMSA) and trifluoroacetic acid (TFA) solutions, but the chemical shift values for Py differ in the two media, indicating hydrogen bonding between the N–H group of the pyridinium ion and the trifluoroacetate anion in the ion pair, which is absent in hydronated DTBP.

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.