Carbenium Ion Intermediates Research Articles

Cyclohexane conversion over H-zeolite supported platinum

The catalytic conversion of cyclohexane over H-ZSM-5 in hydrogen or nitrogen and in hydrogen atmosphere over a series of H-zeolites loaded with platinum was studied using a flow-through microreactor/GC on-line system at pressures of 13 bar. The distribution of the products formed via hydrodecyclization was found to depend definitely on whether or not dimerization of primarily formed hexyl-carbenium ions is sterically hindered in the zeolitic pore system. Reaction routes are suggested which involve, as an essential step, the hydrogenation of intermediate carbenium ions, formed either primarily by reactant adsorption or cracking, by spill-over hydrogen activated on the metallic catalyst component. The basically different product distribution obtained by decyclization of cyclohexane in nitrogen atmosphere or over monofunctional H-ZSM-5 is attributed to disproportionation of primarily formed hexyl-carbenium ions and subsequent hydrogen transfer reactions which results in a broad spectrum of paraffins and aromatics in the effluent and in carbon-rich deposits on the catalyst surface causing fast catalyst deactivation. This process does not manifest itself in bifunctional catalysis due to the much faster hydrogenation and, hence, elimination of carbenium ions by hydrogenation.

The Role of Platinum in Hexane Isomerization over Pt/FeOy/WOx/ZrO2

Hexane isomerization was investigated in Pt-free and Pt-containing highly acidic iron oxide/tungsten oxide zirconia (FeOy/WOx/ZrO2) catalysts. It has been found that hexane isomerization on WOx/ZrO2_based catalysts takes place mainly on monofunctional acid sites via intermolecular hydride transfer reactions between neutral molecules and carbenium ions. The highly acidic character of WOx/ZrO2 is such that hexane isomerization does not proceed via the classical dual functional mechanism. These results are consistent with those obtained in strong acid systems, and contrast those of zeolite-based catalysts, where the presence of metal is required to achieve high conversions. The relative rate of hydrogen transfer and the thermodynamic limitation imposed on the hexane isomers dictate the product distribution. The presence of Pt is found to specifically facilitate the approach to equilibrium of the 2,2-dimethylbutane isomer. This isomer is shown to form from the most sterically-hindered carbenium ion intermediate. The results suggest that Pt-catalyzed hydrogen activation generates substantial amounts of surface atomic hydrogen which readily undergoes hydrogen transfer with this hindered intermediate, which would otherwise undergo very slow hydride transfer with neutral molecules. Our results also suggest that Pt-catalyzed hydrogen activation may play a more classical role in keeping the surface concentration of hexyl cations low, and consequently secondary reactions, oligomerization/cracking, that give rise to cracked products, are inhibited. The results of deuterium exchange measurements in D2 at low conversion, are best explained by invoking both acid- and metal-facilitated deuteration pathways. The presence of Pt results in a moderately higher level of deuterium exchange in the isohexane products, but a large increase in the deuteration of the normal hexane starting material. At ∼20% conversion, nearly all of the unreacted normal hexane molecules contain at least one deuterium atom over Pt/FeOy/WOx/ZrO2, while only 4–5% of the hexane is deuterated over Pt-free FeOy/WOx/ZrO2. These results can be attributed to branched isomer exchange with deuterated Brønsted sites, supplemented by non-isomer-selective C–H bond activation by Pt.

Structural characterization of the enzyme-substrate, enzyme-intermediate, and enzyme-product complexes of thiamin phosphate synthase.

Thiamin phosphate synthase catalyzes the formation of thiamin phosphate from 4-amino-5-(hydroxymethyl)-2-methylpyrimidine pyrophosphate and 5-(hydroxyethyl)-4-methylthiazole phosphate. Several lines of evidence suggest that the reaction proceeds via a dissociative mechanism. The previously determined crystal structure of thiamin phosphate synthase in complex with the reaction products, thiamin phosphate and magnesium pyrophosphate, provided a view of the active site and suggested a number of additional experiments. We report here seven new crystal structures primarily involving crystals of S130A thiamin phosphate synthase soaked in solutions containing substrates or products. We prepared S130A thiamin phosphate synthase with the intent of characterizing the enzyme-substrate complex. Surprisingly, in three thiamin phosphate synthase structures, the active site density cannot be modeled as either substrates or products. For these structures, the best fit to the electron density is provided by a model that consists of independent pyrimidine, pyrophosphate, and thiazole phosphate fragments, consistent with a carbenium ion intermediate. The resulting carbenium ion is likely to be further stabilized by proton transfer from the pyrimidine amino group to the pyrophosphate to give the pyrimidine iminemethide, which we believe is the species that is observed in the crystal structures.

CHEMICAL MECHANISMS OF CATALYTIC CRACKING OVER SOLID ACIDIC CATALYSTS: ALKANES AND ALKENES

The review discusses chemical mechanisms of most important reactions that take place in the course of catalytic cracking of alkenes and alkanes over solid acidic catalysts. The main subject of the review is the mechanism of the principal cracking step, fission of C─C bonds in aliphatic hydrocarbons. The cracking mechanism of alkenes via carbenium ion intermediates (with complications caused by oligomerization reactions) is well established. In contrast, the C─C bond fission mechanism in alkanes is still a controversial subject. The review compares merits and difficulties of different mechanisms proposed in the literature: several carbenium-ion mechanisms, the carbonium-ion mechanism, the chain-reactions mechanism with participation of both carbonium and carbenium ions, and the oxonium-ion mechanism. Reactions accompanying C─C-bond fission in cracking reactions, hydrogen redistribution, formation of cycloalkanes, light aromatic compounds, and coke are also briefly discussed.

Sterol C-methyl transferase from Prototheca wickerhamii mechanism, sterol specificity and inhibition

The membrane-bound sterol methyl transferase (SMT) enzyme from Prototheca wickerhamii, a non-photosynthetic, yeast-like alga, was found to C-methylate appropriate Δ24(25)-sterol acceptor molecules to Δ25(27)-24β-methyl products stereoselectively. Incubation with pairs of substrates—[2H3-methyl]AdoMet and cycloartenol, and AdoMet and [27-13C]lanosterol—followed by 1H and 13C NMR analysis of the isotopically labeled products demonstrated the si-face (β-face attack) mechanism of C-methylation and the regiospecificity of Δ25(27)-double bond formation from the pro-Z methyl group (C27) on lanosterol. The enzyme has a substrate preference for a sterol with a 3β-hydroxyl group, a planar nucleus and a side chain oriented into a ‘right-handed’ structure (20R-chirality)—characteristic of the native substrate, cycloartenol. The apparent native molecular weight of the SMT was determined to be approximately 154,000, as measured by Superose 6 FPLC. A series of sterol analogues which contain heteroatoms substituted for C24 and C25 or related structural modifications, including steroidal alkaloids, havs been used to probe further the active site and mechanism of action of the SMT enzyme. Sterol side chains containing isoelectronic modifications of a positively charged moiety in the form of an ammonium group substituted for carbon at C25, C24, C23 or C22 are particularly potent non-competitive inhibitors (Ki for the most potent inhibitor tested, 25-azacycloartanol, was ca. 2nM, four orders of magnitude less than the Km for cycloartenol of 28μM), supporting the intermediacy of the 24-methyl C24(25)-carbenium ion intermediate. Ergosterol, but neither cholesterol nor sitosterol, was found to inhibit SMT activity (Ki=80μM). The combination of results suggests that the interrelationships of substrate functional groups within the active center of a Δ24(25) to Δ25(27) 24β-methyl-SMT could be approximated thereby allowing the rational design of C-methylation inhibitors to be formulated and tested.

Body nitrosation potential measured by a novel 15N breath test

Oxygenated nitrogen species, for example, the protonated form of nitrous acid (H 2ONO +), dinitrogentrioxide (N 2O 3), dinitrogentetroxide (N 2O 4 ), or peroxynitrite (ONOO −), can react with amines to form molecular nitrogen. These reactions can occur spontaneously with primary aliphatic amines or via cytochrome P450 catalysed reactions with secondary amines. In principle measurements of the excretion of the molecular nitrogen generated by these reactions could be used as an index of the levels of oxygenated nitrogen compounds acting as nitrosating agents. To test this idea, [ 15N 2]urea (3 mmol) was administered orally to five patients infected with Helicobacter pylori (as diagnosed by the [ 13C]urea breath test) and to four healthy volunteers. All participants ingested 3-mmol sodium nitrate as a precursor for NA 5 min before the ingestion of the nitrogen tracer. During the test the participants breathed 100% oxygen to increase the sensitivity of detection of endogenous molecular nitrogen. After the administration of [ 15N 2]urea, the patients with H. pylori showed significantly increased 15N enrichments of exhaled N 2, expressed as δ value (‰), compared with healthy volunteers (patients: 3.5 ± 0.9 vs. volunteers: 1.3 ± 0.4; p < .05). We speculate that the endogenous production of molecular nitrogen is a protective process controlling the body NO and nitrite levels. The 15N breath technique allows the noninvasive estimation of the body nitrosation and could indicate the health risk, possibly the oxidative stress status, caused by highly reactive oxygenated nitrogen species and carbenium ion intermediates.

In situ MAS NMR investigations of molecular sieves and zeolite-catalyzed reactions

Solid-state MAS NMR is a powerful technique to study heterogeneous catalysts and the way by which they operate. In situ MAS NMR has been demonstrated to be a powerful method to understand reaction mechanisms, to study the nature, dynamics and reactivity of surface intermediates and active sites, and to characterize structural modifications in the catalyst itself, in particular when using 13C strategically labelled substrates. In this paper, three examples selected from our own work are used to illustrate the potential of in situ MAS NMR. They are the formation of cumene and its isomerization to n-propylbenzene on zeolite H-ZSM-11, the activation of propane at low temperature and the alkylation of benzene with propane on zeolite H-ZSM-5, and the characterization of the aluminophosphate molecular sieve VPI-5 structure with temperature. Studies of the alkylation of benzene with propene confirmed that cumene was the primary reaction product. The undesired n-propylbenzene by-product results from the intermolecular reaction between cumene and benzene, enhanced by molecular shape-selective effects in medium pore size zeolites (e.g., H-ZSM-11). It explains why large pore zeolites, e.g., zeolite Beta, are used commercially today for this process. Propane can be activated at low temperature (ca. 573 K) on bifunctional medium pore size zeolites possessing intimately related acidic Bronsted sites and a dehydrogenation function provided by Ga or Zn species. In Ga/H-ZSM-5 catalysts, at 573 K, the activation of propane was shown to occur via a protonated pseudocyclopropane (PPCP) intermediate (or transition state). The latter evolves in a manner that can be formally described by the formation of CH3+, C2H2+, and C3H7+ carbenium ion intermediates. These species can react with olefins, alkanes, or other electron-rich molecules such as benzene. The primary reaction products of the reaction of propane with benzene are n-propylbenzene (in small amount), ethylbenzene and toluene. Their subsequent reactions lead eventually to toluene and xylenes as the final products. In the structural characterization of VPI-5, 27Al, 31P, and 27Al nutation MAS NMR spectra show that, at 294 K, fully hydrated VPI-5 contains three equally populated Al and P crystallographic sites and that one-third of Al is 6-coordinate. The VPI-5 structure then belongs to the P63 space group. Above 353 K, VPI-5, fully or partially hydrated, undergoes a structural transformation to a higher framework symmetry, i.e., the P63cm space group. The transformation occurs at nearly the same temperature in both cases, indicating that the breakdown of the hydrogen-bonded helical water structure inside the VPI-5 pores is not a factor in the process.

Comparative study of the transformation of n-butane, n-hexane and n-heptane over H-MOR zeolites with various Si/Al ratios

The transformations of n-butane, n-hexane and n-heptane were carried out in a flow reactor at 523 K, p alkane=0.1 bar, p nitrogen=0.9 bar over a series of H-mordenite samples with framework Si/Al ratios from 6.6 to 80. Because of the rapid deactivation of the samples (mostly during n-hexane and n-heptane transformations), a series of product analyses was performed at a very short time-on-stream in order to obtain, with good accuracy, the activity and selectivity of the fresh samples. With all the samples, n-heptane is slightly more reactive than n-hexane and much more reactive than n-butane (15–100 times). The effect of the acid-site density on the mordenite activity is different for n-butane, n-hexane and n-heptane transformations, which suggests that these reactions occur through different mechanisms: bimolecular with n-butane; monomolecular with n-hexane and n-heptane. The bimolecular mechanism of n-butane transformation is confirmed by simultaneous formation of isobutane, propane and pentanes as primary products. With all the H-mordenite samples, isomers and C 3–C 5 alkanes appear as primary products of n-hexane transformation. From n-heptane, C 3–C 5 alkenes are observed as primary products as well as isomers and C 3–C 6 alkanes. The isomer/light products ratio is approximately equal to 2 from n-hexane and 0.2 from n-heptane, as is expected from the relative difficulty in the modes of cracking: difficult C mode (involving two secondary carbenium–ion intermediates) from n-hexane and relatively easy B mode (one tertiary and one secondary carbenium–ion intermediates) from n-heptane. However, most of the light products do not result from direct cracking of C 6 and C 7 compounds. Whatever the reactant, the product distribution is practically identical for all the dealuminated samples. Very different distributions of the C 3–C 6 products are observed with the non-dealuminated sample: faster formation of C 3 at the expense of C 4–C 6, in particular C 4. This large change in selectivity should be due to the presence of mesopores in the dealuminated samples rather than the larger density of acid sites in the non-dealuminated one.

Pinacol rearrangement over metal-substituted aluminophosphate molecular sieves

Pinacol rearrangement was found to proceed at relatively mild temperature over metal-substituted aluminophosphate molecular sieves. Pinacolone and 2,3-dimethyl-1,4-butadiene were the two main products obtained. The conversion of pinacol and the selectivities of these two products were found to be influenced by the metal species, the metal content, the crystalline structure of aluminophosphate and the solvent used in the liquid-phase reaction. Among the first row transition metal elements substituted in the AFI crystal structure, late transition metals seem to give better catalytic activities than the early ones. Fe 3+, Cu 2+ and Ni 2+ in order are the three which give the highest pinacol conversions and pinacolone selectivities. The catalytic activity was found to have no direct correlation with the acid strength or amount of the acid sites on the catalysts. A mechanism involving the redox ability of Fe(III) and its stabilization of the carbenium ion intermediates was proposed. Besides, in comparison of the catalytic activities of Fe-substituted molecular sieves of different crystalline structures, the activity decreased in the order of APO-5> APO-11 ≥ APO-8>VPI-5>silicalite-1. Since the catalytic activity was independent of pore diameter, the liquid-phase reaction was considered to proceed mainly on the outer surfaces of the catalysts. The hydrophilicity of the aluminophosphate surface was in favor of catalyzing the pinacol reaction, which involves polar reactants and less polar products. Moreover, high pinacol conversion was achieved by using solvents of low polarity.

Selective ethane conversion into aromatic hydrocarbons over Zn-ZSM-11 zeolite

The conversion of ethane into aromatic compounds at 550°C and atmospheric pressure over Zn-ZSM-11 zeolite (Si/Al=17; 2.5 wt.% of Zn as counter ion) has been studied in a flow reactor at different partial pressures of ethane. The observed products at different ethane conversion levels were formed through a variety of processes including ethane dehydrogenation, producing ethylene as the only primary unstable product. Ethylene underwent very rapid reactions through carbenium ion intermediates, producing aromatic hydrocarbons and C1-C4 hydrocarbons as secondary products.

N-pentane isomerization and disproportionation catalyzed by promoted and unpromoted sulfated zirconia

Isomerization and disproportionation of n-pentane were catalyzed by sulfated zirconia, Fe- and Mn-promoted sulfated zirconia, and Pt-, Fe-, and Mn-promoted sulfated zirconia in a flow reactor at temperatures of −25 to 50°C and n-pentane partial pressures of 0.005–0.01 atm. Incorporation of the Fe and Mn promoters increased the activity of the sulfated zirconia by two orders of magnitude at 50°C; addition of Pt to the latter catalyst increased the activity only slightly. The primary reactions, disproportionation (to give butanes and hexanes) and isomerization, occurred in parallel; secondary disproportionation reactions gave heptanes, propane, butanes, and pentanes. The data are consistent with acid-base catalysis and carbenium ion intermediates, and the isomerization is inferred to proceed both by unimolecular and bimolecular mechanisms. H2 in the feed stream and Pt in the catalyst both led to reductions in the rate of catalyst deactivation.

Formation of 3-Methylene-1,4-cyclohexadienes from α,α,α′,α′-Tetraaryl-1,4-benzenedimethanols by the Intramolecular Migration of an Aryl Group

Abstract By reactions of diethyl p-phthalate with 2,6-dimethoxyphenyllithium, ΦLi, a tri-substituted compound, p-HOCΦ2C6H4C(O)Φ (1), or tetra-substituted 1,4-benzenedimethanol, p-HOCΦ2C6H4CΦ2OH (2a), were obtained. Compound 1 further reacted with aryllithium, ArLi, to give the 1,4-benzenedimethanols, p-HOCΦ2C6H4CArΦOH (Ar = 4-MeC6H42b, 4-MeOC6H42c). When 2a was treated with acid in dimethyl sulfoxide, 3-(2,6-dimethoxybenzoyl)-3-(2,6-dimethoxyphenyl)-6-[bis(2,6-dimethoxyphenyl)methylene]-1,4-cyclohexadiene, (4a) was obtained in quantitative yield under a variety of conditions. The reaction is understood to involve an initial formation of carbenium ion, [p-Φ2C+–C6H4CΦ2OH] (3a), of which the positive charge must be partly shared with the other end of the p-phenylene ring, where one of the neighboring Φ-groups migrates. Reactions in acetone resulted to give 4a or dark-violet crystals of (2′,6′-dimethoxy-4-biphenylyl)bis(2,6-dimethoxyphenyl)carbenium perchlorate, [(4-ΦC6H4)CΦ2]ClO4, depending on the reaction conditions. Reactions in methanol, ethanol, or in hot 2-propanol resulted to give the reduced compound, (4-ΦC6H4)Φ2CH. The 1,4-benzenedimethanols, 2b and 2c, also reacted with perchloric acid in dimethyl sulfoxide to form the 3-methylene-1,4-cyclohexadiene derivatives, (Ar = 4-MeC6H44b, 4-MeOC6H44c), respectively. Evidence is given by the 1H NMR spectrum, which shows the formation of intermediate carbenium ions 3a—c.

Characterization of the active site of Schwanniomyces occidentalis glucoamylase by in vitro mutagenesis.

Site-directed mutagenesis was performed to define the active site of the Schwanniomyces occidentalis glucoamylase. The mutated GAM1 genes were expressed in Saccharomyces cerevisiae, and enzymatic and growth properties of the transformants were determined. Mutants were transcribed and translated similar to the wild-type glucoamylase. Therefore, all effects on enzymatic activity could be referred to single amino acid substitutions. Asp470 was shown to be essential for the enzyme activity. Replacement of Asp470 by glycine led to a complete loss of activity. We suppose that Asp470 serves as a general acid-base and stabilizes the formation of the intermediate carbenium ion. Substitution of Trp468 by alanine affected predominantly the alpha-1,6 activity and not the alpha-1,4 activity of the enzyme. The exchange impaired substrate binding as well as enzymatic catalysis. An influence of amino acid 474 on the substrate specificity could not be demonstrated. Exchanges at position 474 exhibited K(m) and Vmax values similar to wild-type glucoamylase.

3,4-Dihydroquinolinium salts: preparation by reaction of N-Arylnitrilium salts with alkenes

N-Arylnitrilium salts 1 react with nucleophilic alkenes 2 to afford 3,4-dihydroquinolinium salts 3, which can be transformed into the free bases with aqueous sodium hydroxide. Dehydrogenation of the 3,4-dihydroquinolinium salts 3 with 2,3-dichloro-5,6-dicyano-p-benzoquinone furnishes quinolinium salts 7. If the intermediate carbenium ion A formed by electrophilic attack of 1 on the alkene 2 is conjugatively or hyperconjugatively stabilized, instead of 3,4-dihydroquinolinium salts 3 iminium salts 4 resulting from a Houben-Hoesch reaction, or iminium salts 5 arising from a formal ene reaction are formed. For the 3,4-dihydorquinolinium salt 3ac X-ray structural analysis has been carried out.

An unstrained homoallylic secondary adamantyl toluene-p-sulfonate solvolyses with σ-bond migration to give a protoadamantyl substituted allylic carbenium ion

The aqueous ethanolysis of 4-adamantylidene-2ax-adamantyl toluene-p-sulfonate (tosylate) (1a–OTs) shows a sensitivity (m value) of 0.86 ± 0.05 to the ionizing power of the solvent. 1a–OTs reacts approximately 4.7-fold faster than 2-adamantyl tosylate in 80∶20 v/v ethanol–water at 25 °C. The solvolysis reactions of 1a–OTs result in complete (>95%) rearrangement of the substituted adamantyl ring into a protoadamantyl ring via a solvent-equilibrated allylic carbenium ion intermediate. The extrapolated α-secondary deuterium kinetic isotope effect for solvolysis of 1a–OTs in 60∶40 v/v ethanol–water at 25 °C is 1.16 ± 0.02, while comparable values for solvolysis of the epimeric 2eq tosylate 1e–OTs measured in several ethanol–water mixtures at 25 °C fall in the range 1.14–1.17. These results are consistent with a 1a–OTs solvolysis reaction transition state that involves concerted σ-bond participation and departure of the tosylate leaving group.

Hydroisomerization and Hydrocracking of Alkanes: 7. Influence of the Balance between Acid and Hydrogenating Functions on the Transformation ofn-Decane on PtHY Catalysts

The transformation ofn-decane was studied at 473 K, 101 kPa andpH2/pn-decane=9 on a series of PtHY catalysts containing from 0.02 to 1.5 wt% platinum and with Si/Al atomic ratios of 3, 9, or 35. The ratio between the number of accessible Pt atoms and the number of acid sites on which the heat of ammonia adsorption is greater than 100 kJ mol−1(nPt/nA) was chosen for characterizing the balance between the hydrogenating and the acid functions. The activities, stabilities, and selectivities of the catalysts are definitely governed by this balance. For low values ofnPt/nA(<0.03), the activity per acid site is low, the deactivation is rapid andn-decane leads directly to all the isomerization and cracking products. For high values (≥0.17) the activity per acid site is maximal, the deactivation is very slow andn-decane transforms successively into monobranched isomers, dibranched isomers, and tribranched isomers plus cracking products. In this latter case the catalyst can be considered as anidealbifunctional catalyst, namely a catalyst on which only one transformation of the alkene intermediates on the acid sites can occur during their diffusion from the platinum sites on which they are generated to those on which they are hydrogenated. Therefore the reaction scheme ofn-decane transformation matches the reaction scheme of olefinic intermediates. From the product distribution established on ideal catalysts the mechanism of the transformation of olefinic intermediates can be deduced. Branching isomerization occurs through protonated cyclopropane and cyclobutane intermediates. The cracking products result from the β-scission steps of tribranched decenes which involve two tertiary carbenium ion intermediates (mode A) and from the β-scission steps of dibranched decenes which involve one tertiary and one secondary carbenium ion intermediate (mode B). The participation of the β-scission of mono, di, and tribranched decenes through mode C (via two secondary carbenium ion intermediates) is negligible. The rate constants of the various steps involved inn-decane transformation are estimated. They can be classified in the following order: A cracking>Methyl shift>Branching>B cracking>>C cracking.

Elementary reactions and thermodynamic effects in the conversion of propene over an acidic A1MFI

The conversion of propene over H-A1MFI zeolites was investigated at various temperatures and subatmospheric pressure. The overall conversion scheme was cracked down into a set of well known reactions: namely a primary dimerization step, two secondary or tertiary disproportionation reactions involving intermediate carbenium ions within the C 6–C 9 range and, in addition, a number of hydrogen transfer reactions. Quantitative analysis of the product distributions provided an evaluation of the relative weight of each step depending on the temperature. The variation of the overall conversion with temperature was interpreted in terms of a thermodynamically limited process above 573 K.

Reactions involved in the alkylation of isobutane with 2-butene and with propene on a USHY zeolite

The transformation of an isobutane-2-butene mixture with a 41 molar ratio was carried out in liquid phase at 323 K on a USHY zeolite with a framework Si/Al ratio of 4.5. C 5–C 14 isoalkanes with about 70 wt.-% of C 8 and a small amount of C 8 alkenes were obtained. Moreover, about 7% of butene was transformed into undesorbed C 10–C 30 products containing three unsaturations or cycles. The C 5–C 14 products resulted from the following reactions: alkylation of isobutane with 2-butene, dimerization of butene, oligomerization-cracking and self-alkylation of isobutane. In order to determine the significance of this latter reaction, the transformation of an isobutane-propene mixture was investigated under similar operating conditions. Trimethylpentanes, and particularly the 2,2,4 isomer, were formed, which confirmed that isobutane self-alkylation was taking place. This reaction was about twice slower than the alkylation of isobutane with propene and twice faster than the dimerization of propene. From the hydrogen balance it was concluded that self-alkylation of isobutane supplied the main part of the hydrogen necessary for the saturation of the products of alkene dimerization and oligomerization-cracking. Hydride transfer would be the limiting step of the formation of isoalkanes. However, isomerization through hydride and methyl shifts of carbenium ion intermediates played also a significant role in the product distribution. This reaction occurred at a rate similar to that of hydride transfer to tertiary carbenium ions but much faster than hydride transfer to secondary carbenium ions.

Peculiarities Observed in H-D Exchange Between Perdeuterioisobutane and H-Zeolites

H-D exchange between perdeuterioisobutane and NaH-Y-zeolites and H-mordenite (H-M) was studied using the pulse technique. Nine of the ten D atoms in the isobutane molecule were exchanged for H on the most acidic Y-zeolites at the lowest temperature (200°C) where H-D exchange was observed, while other transformations of isobutane were not. The reaction is thought to be initiated by protonation of the tertiary C-D bond on the Brønsted acid sites of the catalyst and the exchange to proceed via the t-butyl carbenium ion intermediate. Release of the ion from the surface occurs via hydride transfer conserving the identity of the tertiary position. At higher temperatures and over the more acidic H-mordenite at even lower temperature (125°C) an extensive intermolecular H-D exchange occurs between perdeuterioisobutane and its exchanged products. A more detailed understanding of this reaction chemistry is developed herein.

NMR Observation of Indanyl Carbenium Ion Intermediates in the Reactions of Hydrocarbons on Acidic Zeolites

In situ MAS-NMR was used to study the reactions of styrene and methylstyrene on the acidic zeolites HZSM-5, HY, and H-mordenite. The results are introduced and discussed in the context of our recently proposed reclassification of typical zeolites as strong acids rather than superacids. We also show that the observation of indanyl cations but not simply phenylcarbenium ions as persistent species in high concentration is an expected consequence of the relative acid strengths of the zeolites and carbenium ions. 55 refs., 8 figs., 2 tabs.