Applied Catalysis A, General (v.336, #1-2)
Editorial Board (CO2).
Preface for Jean Sommer's special issue by Pierre Mothe Esteves; Stephane Walspurger; Benoit Louis (1).
Are carbenium and carbonium ions reaction intermediates in zeolite-catalyzed reactions? by M. Boronat; A. Corma (2-10).
The mechanism of acid-catalyzed hydrocarbon reactions such as skeletal isomerization, hydride transfer, alkylation, dehydrogenation and cracking in superacid media and over solid zeolites is analyzed and compared. In particular, the stability, rearrangements and nature (reaction intermediates or transition states) of trivalent carbenium ions and non-classical pentacoordinated carbonium ions in homogeneous and heterogeneous phase are discussed.▪The mechanism of acid-catalyzed hydrocarbon reactions such as skeletal isomerization, hydride transfer, alkylation, dehydrogenation and cracking in superacid media and over solid zeolites is analyzed and compared. In particular, the stability, rearrangements and nature (reaction intermediates or transition states) of trivalent carbenium ions and non-classical pentacoordinated carbonium ions in homogeneous and heterogeneous phase are discussed. It is concluded that both carbenium and carbonium ions may exist as true reaction intermediates in zeolite-catalyzed processes only when the positive charge is not easily accessible to framework oxygen atoms.
Keywords: Zeolites; Carbocations; Acid catalysis; DFT;
Friedel-Crafts alkylations over hierarchical zeolite catalysts by Yinyong Sun; Roel Prins (11-16).
Hierarchical zeolite M-ZSM-5 exhibited a much higher catalytic activity than conventional ZSM-5 in the benzylation of benzene with benzyl alcohol. The apparent rate constant for M-ZSM-5 is 23 times larger than that for conventional ZSM-5 and six times larger than that for ASA, which is attributed to the combination of mesoporosity and strong acidity.▪Hierarchical mesoporous ZSM-5 (M-ZSM-5) was prepared using amphiphilic organosilane as a mesoporous structure-directing agent, and characterized by XRD, N2 sorption, 27Al and 29Si MAS NMR spectroscopy, and 2-methyl-2-pentene isomerization. The XRD and N2 sorption data indicate that M-ZSM-5 is a mesoporous material with ZSM-5 framework. 29Si MAS NMR spectroscopy revealed that M-ZSM-5 was highly crystalline, like conventional ZSM-5. 27Al MAS NMR spectroscopy showed that most of the aluminum atoms were in tetrahedral coordination and 2-methyl-2-pentene isomerization exhibited that M-ZSM-5 possessed strong acidity similar to conventional ZSM-5.The benzylation reaction of benzene by benzyl alcohol was used to evaluate the catalytic reactivity of the hierarchical zeolite catalyst in Friedel-Craft alkylations. The results showed that M-ZSM-5 had a much higher catalytic activity than conventional ZSM-5. The apparent rate constant for M-ZSM-5 is 23 times larger than that for conventional ZSM-5. Moreover, the catalyst could be reused and maintain the initial activity even after three cycles. These results indicate that hierarchical zeolites have potential application in Friedel-Crafts alkylations, especially of large molecules.
Keywords: Hierarchical; Zeolites; Friedel-Crafts; Benzyl alcohol; Benzylation;
A density functional study on the nature of the adsorption complex between isobutane and H-ZSM5 and its implication for the mechanism of activation of alkane molecules over H-ZSM5 zeolite by Ivan Milas; Alexander Martins Silva; Marco Antonio Chaer Nascimento (17-22).
Calculations on the nature of the isobutane–H-ZSM5 adsorption complex and on the transitions states for dehydrogenation reactions of isobutane and methane in H-ZSM5 have been performed. The results strongly support the experimental observations that a specific complex is formed between the zeolite and the isobutane molecule and that the zeolite has enough acid strength to promote direct protonation of alkanes. ▪Theoretical calculations on the nature of the adsorption complex between isobutane and H-ZSM5 and on the dehydrogenation reactions of isobutane and methane in this zeolite have been performed, using the 20T cluster to model the acid site of the zeolite. We considered the adsorption process and the chemical exchange reactions to take place through the hydrogen atom bonded to either a primary or the tertiary C atom of isobutane. The calculations were performed using density functional theory (DFT) with the hybrid functional X3LYP and the 6-31G** basis set. NMR chemical shifts were calculated at the X3LYP/6-31G** optimized geometries using an extended basis set (6-311G**++), in order to investigate the nature of the adsorption complex between the reagent molecules and the H-ZSM5 acid site. The results of the adsorption studies showed that within the range of the experimental and theoretical uncertainties, it is not possible to distinguish if the complex formed involves the primary or the tertiary atom of isobutane. In fact, the similarity of the adsorption energies suggests that both complexes could be formed. The same conclusion can be drawn from the analysis of the chemical shifts results. The activation energies for the exchange reactions at the primary and tertiary centers are also very similar and both transition states can be stabilized by a weak hydrogen bond. Therefore, the preference for the H/D exchange at the primary center observed at room temperature cannot be attributed to that effect. The activation energy for the methane exchange reaction was found to be larger than that for the isobutane molecule, as expected. However, while the predicted difference is in the range of 5.3–7.8 kcal/mol, the most favorable difference that one obtains from the experimental results is of 14.2 kcal/mol, a result which is difficult to understand if one considers the nature of the transition states and the differences in bond energies and in the steric hindrance experienced by the molecules. In conclusion, the theoretical calculations strongly support the experimental observations that a specific complex is formed between the zeolite and the isobutane molecule and also that the zeolite has enough acid strength to promote direct protonation of alkanes. On the other hand, the fact that, at room temperature, no H/D exchange was observed at the tertiary carbon atom, does not exclude the possibility that other reactions which occur at higher temperatures, such as dehydrogenation and cracking, will involve the tertiary center.
Keywords: Zeolites; Exchange reaction; Isobutane; DFT calculations; Adsorption complex;
Hydroisomerization of long-chain n-alkanes on bifunctional Pt/zeolite catalysts: Effect of the zeolite structure on the product selectivity and on the reaction mechanism by A. Soualah; J.L. Lemberton; L. Pinard; M. Chater; P. Magnoux; K. Moljord (23-28).
The transformation of n-decane, n-tetradecane and n-hexadecane was carried out in a fixed-bed reactor at 220 °C under a 30 bar total pressure on bifunctional Pt-exchanged HBEA, HMCM-22 and HZSM-5 zeolite catalysts. The activities of the catalysts and especially the reaction scheme depended strongly on the zeolite structure. Whatever the reactant, monobranched isomers were the only primary reaction products on PtHBEA, while cracking was the main reaction observed on HMCM-22 and HZSM-5 with n-tetradecane and n-hexadecane. This was explained in terms of diffusion of the reaction intermediates inside the zeolites porosities: rapid diffusion inside the large channels of the HBEA zeolite, improved by the small size of the crystallites, blocking of the reaction intermediates inside the porosity of HMCM-22 and HZSM-5 yielding extensive cracking.The transformation of n-decane, n-tetradecane and n-hexadecane was carried out in a fixed-bed reactor at 220 °C under a 30 bar total pressure on bifunctional Pt-exchanged HBEA, HMCM-22 and HZSM-5 zeolite catalysts. Whatever the reactant, isomerization was the main reaction observed on PtHBEA, while it was cracking on HMCM-22 and HZSM-5. This was explained in terms of diffusion of the reaction intermediates inside the zeolites porosities.▪
Keywords: Hydroisomerization; n-Alkanes; Platinum; Zeolites; Bifunctional catalysts;
Efficient cleavage of cumene hydroperoxide over HUSY zeolites: The role of Brønsted acidity by Konstantin Yu. Koltunov; Vladimir I. Sobolev (29-34).
An efficient cleavage of cumene hydroperoxide to form phenol and acetone was achieved by using HUSY zeolites of high Si/Al ratio. The efficiency of this catalyst was comparable to that of sulfuric acid.▪The cleavage of cumene hydroperoxide in the presence of H-form USY zeolites to form phenol and acetone has been examined. For comparison, the same reaction, catalyzed by Y-, ZSM-5, β-zeolites, as well as sulfuric and heteropolyacids is performed. As appeared, HUSY zeolites of high Si/Al ratio (15 and 40) are sufficiently active catalysts, and their efficiency is comparable to that of sulfuric acid. Such outstanding catalytic performance of HUSY should be attributed to combination of favourable porous topology, sorption properties and relatively strong acidity of the internal surface. The acid strength of HY and HUSY zeolites is roughly estimated in terms of H/D exchange between deuterated acetone and water over the solid acid, while their sorption properties are estimated based on H2O-TPD measurements.
Keywords: Cumene hydroperoxide; Cleavage; Phenol; HUSY zeolites; Acidity; Proton deuterium exchange;
Catalytic activity of MnIII(Salen) and FeIII(Salen) complexes encapsulated in zeolite Y by R.J. Corrêa; G.C. Salomão; M.H.N. Olsen; L. Cardozo Filho; V. Drago; C. Fernandes; O.A.C. Antunes (35-39).
MnIII(Salen) and FeIII(Salen) complexes were synthesized, encapsulated in zeolite Y and employed as catalysts in the cyclohexane oxidation reactions, using hydrogen peroxide (H2O2, 30% in water) or tert-butylhydroperoxide (TBHP, 70% in water). Best yield (7.9 %) was obtained with MnIII(Salen) using TBHP.DFT calculations show that the iron and manganese complexes occupy two cavities of the Y zeolite. ▪Ship-in-a-bottle complexes of iron (III) and manganese (III) containing the ligand N,N’-(salicylaldehyde)ethylenediamine (H2SALEN) were synthesized in zeolite Y (ship-in-a-bottle), and employed as catalysts in the cyclohexane oxidation reactions, using hydrogen peroxide (H2O2, 30% in water) or tert-butylhydroperoxide (TBHP, 70% in water). The complexes, [FeIII(SALEN)Y] and [MnIII(SALEN)Y], where H2SALEN = N,N’-bis(salicylidene)-1,2-phenylenediimine, were characterized by IR and Mössbauer spectroscopy, by N2 BET analysis, X-ray powder diffraction, X-ray fluorescence and elemental analysis. DFT (density functional theory) calculations show that the iron and manganese complexes occupy two cavities of the Y zeolite.
Keywords: Zeolite; Biomimetic catalyst; DFT; Ship-in-a-bottle;
Mechanistic insights of CO2-coke reaction during the regeneration step of the fluid cracking catalyst by Luciana Tavares dos Santos; Fabiana M. Santos; Raphael S. Silva; Thiana S. Gomes; Pierre M. Esteves; Ricardo D.M. Pimenta; Sônia M.C. Menezes; Oscar R. Chamberlain; Yiu L. Lam; Marcelo Maciel Pereira (40-47).
Coked FCC catalyst modified with pre-calcination treatment were burned under O2/He and CO2/He atmosphere. CO2 preferentially reacts in the beginning of coke burning and a remarkable decreasing of CO2 activity is observed with pre-calcination coked catalyst. Regeneration under 13CO2/He revealed a first kinetics order for coke-CO2 reaction. The CO2 molecule react dissociatively introducing oxygenated functionality in the coke. ▪The regeneration of a coked fluid catalytic cracking (FCC) catalyst under O2/He and CO2/He atmospheres from room temperature up to 1000 °C was studied using online mass spectroscopy. The catalyst was also thermally treated at temperatures between 300 °C and 500 °C in order to promote coke modification. It was verified that the amount of aliphatic carbon decreased for higher temperatures. These treatments have a more significant impact on the coke-CO2 reaction than on the coke-O2 reaction. The former reaction was the focus of the present investigation and the results were compared to the latter reaction. CO2 preferentially reacts in the beginning of coke burning and a remarkable decrease of CO2 activity is observed with the calcinated coked catalyst. Regeneration under 13CO2/He at constant temperatures between 600 °C and 940 °C revealed a first-order kinetics for the coke-CO2 reaction. The CO2 molecule reacts dissociatively introducing oxygenated functionality into the coke. These species are decomposed at high temperature forming CO and no water is observed during the CO2-coke reaction.
Keywords: CO2; FCC; Coke;
Zinc catalyzed conversion of methanol–methyl iodide to hydrocarbons with increased formation of triptane by Stéphane Walspurger; G.K. Surya Prakash; George A. Olah (48-53).
At 200 °C under autogeneous pressure, mixtures of methanol and methyl halides are converted with zinc to a mixture of hydrocarbons. The reaction of methanol and methyl iodide mixtures over zinc or zinc oxide gives 2,2,3-trimethylbutane (triptane), a desirable high-octane compound in significant selectivity. The mechanism of these reactions under amphoteric conditions is discussed. ▪At 200 °C under autogeneous pressure, mixtures of methanol and methyl halides are converted with zinc to a mixture of hydrocarbons. The reaction of methanol and methyl iodide mixtures over zinc or zinc oxide gives 2,2,3-trimethylbutane (triptane), a desirable high-octane compound in significant selectivity. As alternative to previously known ZnI2 or ZnBr2/methanol conversion, the present protocol does not require the use of metal halide catalysts. The initial step of the mechanism of conversion methanol/methyl iodide mixtures to hydrocarbons does not involve strongly acidic species. On the basis of the obtained experimental data with both zinc and zinc oxide, which are amphoteric in nature, the intermediacy of a zinc methoxy species is considered to be the key step for the formation of hydrocarbons. The proposed formation of hydrocarbons is considered as a parallel reaction to ethylene oligomerization to aromatic hydrocarbons.
Keywords: Methanol; Zinc; Zinc oxide; Methanol to hydrocarbon; Methanol to olefin;
A DFT–ONIOM study on the effect of extra-framework aluminum on USY zeolite acidity by Nilton Rosenbach; Claudio J.A. Mota (54-60).
The effect of Al(OH)2 + EFAL species in the acid strength of USY zeolite was studied by the ONIOM scheme, at PBE1PBE/6-31G(d,p) level. The calculations indicated that role of the EFAL is not to related to the Brϕnsted/Lewis acid synergism, but to stabilize the conjugated base by hydrogen bonding and nucleophilic interaction with the framework oxygen atom.▪The effect of Al(OH)2 + EFAL (extra-framework aluminum) species in the acid strength of USY zeolite was studied by the ONIOM scheme, at PBE1PBE/6-31G(d,p) level. The model consists of a hexagonal prism and the sodalite cage (T30), representing a real part of the zeolite Y system that allows calculation of different acid sites (O1 and O3). The acid strength was calculated by computing the deprotonating energy and comparing the results with the data obtained for an isolated acid site. The calculations indicated that there occurs an increase in acid strength, depending on the relative position between the EFAL and the acid site. However, the role of Al(OH)2 + EFAL is not to increase the energy of the acid form, through Brϕnsted/Lewis synergism, but to stabilize the conjugated base, formed upon deprotonation, by hydrogen bonding and nucleophilic interaction with the framework oxygen atom.
Keywords: Zeolite; Acidity; Extra-framework aluminum; DFT;
Continuous supercritical i C 4 / C 4 = alkylation over H-Beta and H-USY by A.L. Mota Salinas; G. Sapaly; Y. Ben Taarit; J.C. Vedrine; N. Essayem (61-71).
This study presents a comparison of the catalytic performances and stabilities of H-Beta and H-USY zeolites for i C 4 / C 4 = alkylation in supercritical isobutane phase. The experimental results clearly show that the zeolite structure strongly influences the performance and deactivation behavior of the zeolite in i C 4 / C 4 = alkylation performed in SC conditions. H-Beta outperformed H-USY zeolite as regards to its lower deactivation and the stable quality of the alkylate, composed mainly of isoparaffins while olefin di-/oligomerization was shown to be responsible of the H-USY deactivation with time-on-stream. Characterization of the spent catalysts gave clues to the differentiated performance of the two zeolites. The stable activity of H-Beta was attributed to the effective extractive effect of the supercritical iC4 media to “clean” the acid sites located on the external surface of the small crystallites of H-Beta or located at the pore mouth. By contrast, the deactivation observed for H-USY was caused by olefin oligomerization inside the zeolite supercages. The supercritical isobutane media was shown to be unable to prevent the oligomers formation inside the zeolite supercages and/or to in situ extract these oligomers located in the supercages of the large crystals of the H-USY zeolite.The catalytic activities of H-Beta and H-USY zeolites were compared for i C 4 / C 4 = alkylation in SC iC4 phase. The zeolite structure was shown to strongly influence the performance and deactivation behavior of the zeolite. H-Beta outperformed H-USY zeolite as regards to its lower deactivation and the stable quality of the alkylate. The sustainable activity of H-Beta in SC iC4 was attributed to its large external surface area where the effect of the SC Fluid is indisputably efficient to prevent the site poisoning by carbonaceous accumulation. By contrast, SC iC4 was shown to be unable to prevent or extract carbon deposits located in the supercages of H-USY.▪
Keywords: Isobutane-butene alkylation; Supercritical fluids; Beta zeolite; USY zeolite; Deactivation; Coke extraction;
Quantum calculations on the acid catalyzed rearrangements of norborn-5-en-2-one, 7-oxanorborn-5-en-2-one and 7-azanorborn-5-en-2-one by José Ángel Sordo; Adrián Varela-Álvarez; Samuele Giani; Pierre Vogel (72-78).
Protonation of norborn-5-en-2-one gives 6-oxo-2-norbornyl cation (2) as most stable C7H9O+ cation. Quantum calculations at B3LYP/6-31G(d) level (gas phase, 1 atm, 25 °C) predict a C S-structure for 2 that can be represented as an intramolecular π-complex (Dewar π-complex model) of an acetylium ion with an alkene (cyclopent-3-enyl moiety). Isomeric 5-oxo-2-norbornyl (1) cation is calculated to be 16.7 kcal/mol less stable than 2. Part of the relatively high stability of 2 arises from the electron-releasing carbonyl group through n(C＝O) ↔ σ ↔ 2p(C+) hyperconjugation. This effect is also present in more stable 6-oxo-3-oxa (14) and 6-oxo-3-aza-2-norbornyl cation (19) as these ions are calculated to be 7.4 and 3.3 kcal/mol, respectively, more stable than their 5-oxo isomers.
Keywords: Carbocations; Frangomeric effect; Hyperconjugation; Wagner-Meerwein rearrangement norbornylcation; π-complexes;
Methane dehydro-aromatization on Mo/ZSM-5: About the hidden role of Brønsted acid sites by Jean-Philippe Tessonnier; Benoît Louis; Séverinne Rigolet; Marc Jacques Ledoux; Cuong Pham-Huu (79-88).
Methane dehydro-aromatization reaction remains an interesting solution to convert methane obtained as by-product of oil extraction and thus fight against flaring and global warming. Until now, it was generally accepted that the molybdenum is anchored on the Brønsted acid sites of the zeolite as (Mo2O5)2+ species. The molybdenum performs the dehydrogenation and coupling of CH4 to ethylene which is consecutively oligomerized to benzene over the Brønsted acid sites. We bring evidence that this picture is actually more complicated than it seems. The Si/Al ratio of the zeolite, and consequently the density of available Brønsted acid sites, plays a dramatic role on the anchoring mode of the molybdenum and on its catalytic activity. ▪Whereas the hegemony of the Fischer–Tropsch synthesis is well established for the valorization of methane in the case of important gas fields, the methane dehydro-aromatization reaction remains an interesting solution to convert methane obtained as by-product of oil extraction and thus fight against flaring and global warming. Until now, it was generally accepted that the molybdenum is anchored on the Brønsted acid sites of the zeolite as (Mo2O5)2+ species. The molybdenum performs the dehydrogenation and coupling of CH4 to ethylene which is consecutively oligomerized to benzene over the Brønsted acid sites. In the present work, we bring evidence that this picture is actually more complicated than it seems. The Si/Al ratio of the zeolite, and consequently the density of available Brønsted acid sites, plays a dramatic role on the anchoring mode of the molybdenum and on its catalytic activity.
Keywords: Methane valorization; Aromatization; Molybdenum; ZSM-5; Acid sites;
Comparison of zeolites LaX and LaY as catalysts for isobutane/2-butene alkylation by Carsten Sievers; Jürgen S. Liebert; Manuel M. Stratmann; Roberta Olindo; Johannes A. Lercher (89-100).
Lanthanum exchanged X and Y type zeolites were prepared by ion exchange and investigated as catalysts for isobutane/2-butene alkylation. With the reactions performed in a continuously operated stirred tank reactor under industrially relevant conditions (T = 348 K, p = 20 bar, paraffin/olefin molar ratio = 10, olefin weight hourly space velocity = 0.2 h−1) the catalyst lifetime of LaX was nearly twice as long as that of LaY. Moreover, a much higher yield of octane isomers was observed with LaX. The product distributions showed that LaX had a high activity for hydride transfer and “self-alkylation” as well as a higher concentration of strong Brønsted acid sites. These differences are related to a higher residual concentration of sodium cations in LaY leading not only to less, but also weaker strong Brønsted acid sites in LaY than in LaX. The replacement of the residual sodium cations by lanthanum cations is less favorable in LaY due to the lower concentration of appropriate sites to accommodate multivalent cations.Lanthanum exchanged X and Y type zeolites were investigated as catalysts for isobutane/2-butene alkylation. The catalyst lifetime of LaX was nearly twice as long as that of LaY due to the higher strength of its Brønsted acid sites. The Brønsted acidity of LaY is markedly influenced by the presence of residual sodium cations.▪
Keywords: Acidity; Hydride transfer; MAS NMR; Lanthanum; Brønsted acid site; Alkylation; Ion exchange;
Behavior of arylvinylketones in zeolites: A systematic study by Abdelkarim Sani-Souna-Sido; Stefan Chassaing; Patrick Pale; Jean Sommer (101-108).
Arylvinylketones were efficiently converted to indanones through zeolite-promoted cyclization. The zeolite nature and the substitution pattern in the starting materials proved critical, the 2-substituted arylvinylketones reacting more readily in H-ZSM5, while the 3-substituted required H-USY. The characterization of rearranged product and solvent addition formed as side-products in a few cases offer insight into the mechanism, suggesting dicationic intermediates.▪The zeolite-promoted cyclization of arylvinylketones to indanones has been investigated, regarding zeolite nature and substitution pattern in the starting materials. 2-Substituted arylvinylketones reacted more readily in H-ZSM5, giving high yield (>90%) of the corresponding indanones, while the 3-substituted are more efficiently converted to the corresponding indanones by H-USY. Rearranged product and solvent addition products were also isolated in a few cases, giving insight into cyclization mechanisms.
Keywords: Zeolites; Arylvinylketones; Indanones; Cyclization; Superelectrophile; Dication;
Insights into the structure of active sites in metal-doped solid acid catalysts by Stéphane Walspurger; Benoît Louis (109-115).
Based on our previous H/D exchange studies devoted to the quantification of the number of Brönsted acid sites in solid acids, we report here an innovative approach to determine both the amount and the localisation of different metal atoms inside zeolite materials, or deposited on solid acids. As a case study, the influence of Mo introduction in the MFI framework with different molybdenum loading and Si/Al ratio was investigated in details. Depending on the Si/Al ratio of the zeolite, i.e. the proximity of two Brönsted acid sites, the Mo-atoms substitute a different quantity of OH groups. Consequently, a chemical structure was proposed to ascertain the geometry of the Mo-complex in the channels of the ZSM-5 zeolite. An overview of the possibilities of this H/D exchange method was also given to show its versatility and flexibility to elucidate the structure and bonding of metal cations and oxides on different kinds of solid acids.
Keywords: Zeolite; Solid acids; Catalyst; Design; H/D exchange; Anchoring; Acidity; Titration;
Intrinsic gas-phase acidity and electrophilicity of model heterocations and carbocations relative to pyridine: Adduct formation versus α- or β-(proton transfer) elimination by Mario Benassi; Patricia V. Abdelnur; Marcos N. Eberlin; Takao Okazaki; Kenneth K. Laali (116-127).
Ion/molecule reactions of gaseous silylium ions (Me3Si+, Cl3Si+, Br3Si+, and bridgehead 3,5,7-trimethyl-1,3,5,7-tetrasilaadamant-1-yl cation), carbocations (tBu+, iPr+, Et+, (MeO)2CH+, CCl3 +, CF3 +, Ph+, PhCH2 +, C7H7 +, and 3,5-dimethyl-1-adamantyl cation), and the dicoordinated boron cation B(OMe)2 + with pyridine were investigated. Three competitive channels: (a) electrophilic addition that forms stable adducts, (b) direct β-elimination that leads to proton transfer, or (c) α-elimination that leads to the transfer of a substituent directly attached to the charge center were identified.▪Ion/molecule reactions with pyridine (Py) of several gaseous silylium ions (Me3Si+, Cl3Si+, Br3Si+, and bridgehead 3,5,7-trimethyl-1,3,5,7-tetrasilaadamant-1-yl cation), carbocations (tBu+, iPr+, Et+, (MeO)2CH+, CCl3 +, CF3 +, Ph+, PhCH2 +, and C7H7 +, and 3,5-dimethyl-1-adamantyl cation), and the dicoordinated boron cation B(OMe)2 + were investigated. The silylium cations, including the unprecedented bridgehead 3,5,7-trimethyl-1,3,5,7-tetrasilaadamantyl gaseous ion, reacted to form mainly the adduct [R3Si–Py]+ via electrophilic attack. Alkyl cations (tBu+, iPr+, and Et+) reacted to give mainly [PyH]+ via β-elimination. CCl3 + forms both [Py–CCl3]+ and [PyH]+, whereas [PyH]+ was the dominant product for the reactions of Ph+, p-Cl–C6H4 +, p-H2N–C6H4 +, PhCH2 +, and the tropylium ion (Tr+). The 3,5-dimethyl-1-adamantyl cation was found to be nearly unreactive towards pyridine. The (MeO)2B+ ion gave a set of three major products: the adduct [(MeO)2B–Py]+, [PyH]+, and [PyMe]+. Reactions involving isotopically labeled pyridine-d 5 and CD3CD2 + were used to investigate the source of protons leading to [PyH]+ in reactions of protic and aprotic cations. Great variations of intrinsic reactivity were observed for the cations investigated, even within the same class. These changes were also found difficult to predict, as they were not always in accord with those predicted based on calculated (DFT) reaction thermochemistries.
Keywords: Pyridine basicity; Pyridine nucleophilicity; Ion/molecule reactions; Tandem mass spectrometry;
Reactions of acenaphthenequinone and aceanthrenequinone with arenes in superacid by Douglas A. Klumpp; Yiliang Zhang; Dat Do; Rendy Kartika (128-132).
The hydroxyalkylation reactions of aceanthrenequinone (6) and acenapthenequinone (7) with a series of arenes have been studied. In reactions with the Brønsted superacid CF3SO3H, the condensation products are formed in good yields (58–99%, 10 examples) with high regioselectivity. The results from the condensation reactions and also theoretical calculations are consistent with the formation of superelectrophilic species. ▪The hydroxyalkylation reactions of aceanthrenequinone (6) and acenapthenequinone (7) with a series of arenes have been studied. In reactions with the Brønsted superacid CF3SO3H (triflic acid), the condensation products are formed in good yields (58–99%, 10 examples) with high regioselectivity. Computational studies were also done to examine the structures and energies of mono- and diprotonated species from 6 and 7. The results from the condensation reactions are consistent with the formation of superelectrophilic species involving protosolvation of carboxonium ion intermediates.
Keywords: Superacid; Superelectrophile; Condensation; Quinone;
Catalytic activities and properties of sulfated zirconia supported on mesostructured γ-Al2O3 by Jie Zhao; Yinghong Yue; Weiming Hua; Heyong He; Zi Gao (133-139).
A novel mesoporous solid acid catalyst system was designed by loading sulfated zirconia into mesostructured γ-Al2O3. The results show that novel mesoporous solid acids exhibit higher catalytic activities than sulfated zirconia and MCM-41 supported sulfated zirconia. The enhanced activities can be attributed to more acid sites and/or stronger acid strength.▪A novel mesoporous solid acid catalyst system was designed by loading sulfated zirconia into mesostructured γ-Al2O3. Their catalytic activities for conversion of n-pentane, Friedel–Crafts benzoylation of anisole with benzoyl chloride and dealkylation of 1,3,5-tri-tert-butyl-benzene were investigated. For comparison, sulfated zirconia and MCM-41 supported sulfated zirconia were also examined. Textural and structural characterizations of the catalysts were performed by means of N2 adsorption, X-ray diffraction and diffuse reflectance UV–vis spectroscopy. Temperature programmed desorption of ammonia and FTIR spectra of adsorbed pyridine were used to determine the acid properties. The results show that novel mesoporous solid acids exhibit superior catalytic performances to sulfated zirconia and MCM-41 supported sulfated zirconia. The enhanced performances of the novel mesoporous acid catalysts can be attributed to more acid sites and/or stronger acid strength.
Keywords: Mesoporous solid acid; Sulfated zirconia; Mesostructured γ-Al2O3; Acid catalyzed reactions;
Different reactivities of acetylene carbonyl compounds under the catalysis by Bronsted superacids and Lewis acids by Andrey O. Shchukin; Aleksander V. Vasilyev (140-147).
Acetylene carbonyl compounds react with arenes in Bronsted and conjugate Bronsted–Lewis superacids leading to aromatic alkenylation products. Under the catalysis by Lewis acids the reactions of the acetylene compounds with arenes result in the formation of substituted indenes.▪Acetylene carbonyl compounds ArCCCOR (R = H, Me, Ph) form vinyl cations ArC+ ＝CHCOR or ArC+ ＝CHC(OH)+R at the protonation in Bronsted and conjugate Bronsted–Lewis superacids (protic superacids: HSO3F, CF3SO3H, HSO3F–SbF5, CF3SO3H–SbF5, HAlBr4) with a wide range of acidity (H o ∼ −11 to −22). The vinyl cations react with arenes Ar’H with the formation of alkenylation products ArAr’C＝CHCOR. On the contrary to protic superacids Lewis acids (AlBr3, AlCl3) activate additionally electrophilic center at carbonyl carbon of the compounds ArCCCOR that leads to the formation of substituted indenes in their reactions with arenes.
Keywords: Acetylene compounds; Bronsted superacids; Lewis acids; Arene alkenylation; Indene synthesis;
Orthoamides, LXVI by Willi Kantlehner; Hans-Jörg Lehmann; Kai Edelmann; Jochen Mezger; Ivo C. Ivanov (148-154).
The course of condensation reactions of alkyne carboxylic acid orthoamides 17a,b can be catalyzed by low acidic polar solvents and also by boric acid esters.▪The course of condensation reactions of alkyne carboxylic acid orthoamides 17a,b is strongly influenced by the nature of the solvent used. Non-acidic or at least low-acidic polar solvents possessing high E T(30) values are best suited for these reactions. The condensation reactions can also be catalyzed by boric acid esters. Bicyclic boric acid esters such as 25 and 27, derived from trioles, are the most effective catalysts. The use of these catalysts allows to prepare conveniently 5,5-bis(dimethylamino)butadienes 18a,b from the acetophenones 16a,b and the orthoamides 17a,b.
Keywords: Alkyne carboxylic acid orthoamides; CH2-acidic compounds; Boric acid esters; Ketene aminals;