Carbenium Ion Mechanism Research Articles

Design synthesis of ZSM-5 zeolite with yolk-shell structure by Oswald ripening for aromatization of coal-oil co-refining oils

Aiming to improve the catalytic performance of HZSM-5 in the catalytic conversion of coal-oil co-refining oil, a range of zeolite catalysts (HZ5-xD) were prepared by adjusting the recrystallization times of dissolution–recrystallization of HZSM-5 basing on the Oswald ripening mechanism. The results show that HZ5-xD processes more easily accessible active sites and mixed microporous-mesoporous structures. Larger pore size and moderate acidity were beneficial for the generation of benzene, toluene, ethylbenzene and xylene (BTEX). The selectivity and yield of reached as high as 29.76 % and 236.83 mg·g−1 over HZ5-2D at 700 °C, respectively. Both HZSM-5 and HZ5-xD demonstrate high selectivity to C3 gaseous hydrocarbons, likely due to the obedience of the carbenium ion mechanism. HZ5-2D still demonstrated more excellent catalytic performance than HZSM-5 after five regenerations. Collectively, these finding illustrates the broad application potential of Oswald ripening in special morphologies of zeolite to improve the conversion efficiency of coal tar to BTEX.

Roles of Molecular Structure in the Catalytic Cracking of n‐Heptane, Methylcyclohexane and Cyclopentene over HZSM‐5 Zeolites

AbstractCatalytic cracking technology is an important candidate for the crude‐to‐chemicals, which is promising to convert oil into chemicals by a clean and efficient way. n‐Heptane, methylcyclohexane and cyclopentene catalytic cracking over HZSM‐5 zeolites were studied at 320–550 °C under atmosphere, and a particular attention was paid to the effects of reaction temperature and time on stream. The cracked gas was analyzed by an online gas chromatograph, and the spent HZSM‐5 zeolites were characterized by TG, N2 physisorption and NH3‐TPD. It provided detailed information about the conversion, product distribution, coke formation and catalyst deactivation. Furthermore, the roles of molecular structure in the reaction network were discussed based on the carbenium ion mechanism. It was found that the protolytic cracking and hydride transfer was favored by n‐heptane due to the linear structure; the demethylation, protolytic ring‐opening and protolytic dehydrogenation was favored by methylcyclohexane due to the methyl group and cyclic structure; the protonation and oligomerization was favored by cyclopentene due to the cyclic and C=C structure. This made a significant difference on the catalytic performance: The activity was in a descending order of n‐heptane≈cyclopentene>methylcyclohexane. Alkane formation was in a descending order of n‐heptane>methylcyclohexane>cyclopentene. Aromatic and coke formation were in a descending order of cyclopentene>methylcyclohexane>n‐heptane.

Integrating continuous-stirred microwave pyrolysis with ex-situ catalytic upgrading for linear low-density polyethylene conversion: Effects of parameter conditions

A continuous-stirred microwave pyrolysis (CSMP) reactor coupled with an ex-situ catalytic bed was developed for the conversion of linear low-density polyethylene (LLDPE) to high-grade fuels. Effects of catalyst-to-reactant ratio and feeding rate on product yield and chemical selectivity were studied. The CSMP of LLDPE produced up to 84.1 wt.% condensate product composed of hydrocarbon fuels. With the assistance of the ex-situ catalytic process with HZSM-5, more refined chemicals dominated by gasoline-range hydrocarbons (C5–C12 hydrocarbons) were observed. When the catalyst-to-reactant ratio was 15 % and the feeding rate was 6 g/min, the condensate oil with a high higher heating value (45.23 MJ/kg) showed the highest selectivity of gasoline-range hydrocarbons (98.0 %). The pyrolysis processes yielded ethylene as the main gas product. With the addition of HZSM-5, propylene and propane were the primary gas components. The plausible pathway for the conversion of LLDPE mainly proceeded through radical mechanism for pyrolysis and carbenium ion mechanism for catalysis was elaborated. The potential net energy gain increased from 34.16 MJ/kg to 48.91 MJ/kg with growing feeding rate from 6 g/min to 30 g/min.

Effects of the acidity and shape selectivity of dealuminated zeolite beta on butene transformations

The acidity and shape selectivity of zeolites are two important catalytic properties, and they have been extensively applied to the chemical and petrochemical industries. Here, we investigated the various effect mechanisms of Brønsted acid sites (BASs) from bridged hydroxyl groups in zeolite Beta and Lewis acid sites (LASs) from silanol nests in Si-Beta on butene transformation and their reaction pathways by diffuse-reflectance infrared Fourier-transform spectroscopy. BAS catalysis of butenes double bond activation and isomerization follows the classical carbenium ion mechanism, while catalysis by silanol nests as weak LASs due to hydrogen bond interaction is achieved by formation of allyl species and alkoxyl groups. Moreover, the oligomerization reactions of butene isomers are affected by the dynamic diameters and activation energies of the butene isomers and the synergy between acidity and shape selectivity of the zeolite. Oligomerization of n-butene and cis-2-butene is more likely to be affected by acids on the sample, while oligomerization of trans-2-butene and isobutene is influenced by the shape selectivity of the channel, in addition to the acidity on partly dealuminated zeolite Beta.

Octane compositions in sulfuric acid catalyzed isobutane/butene alkylation products: experimental and quantum chemistry studies

Octanes in alkylation products obtained from industrial alkylation were studied by batch experiments. More than eight octane isomers were identified and quantified by gas chromatography-mass spectrometry. Based on a classic carbenium ion mechanism, the carbocation transition states in concentrated sulfuric acid catalyzed alkylation were investigated using quantum-chemical simulations and predicted the concentration and octane isomerization products including trimethylpentane and dimethylhexane as well as the formation of heavier compounds that resulted from the oligomerization of octane and butene. The agreement between model calculations and experimental data was quite satisfactory. Calculation results indicated that composition and content of trimethylpentanes in the alkylation products were 2,2,4-trimethylpentane > 2,3,3-trimethylpentane > 2,3,4-trimethylpentane > 2,2,3-trimethylpentane whether the 2-butene or i-butene acts as olefin. Heavier compounds in the alkylate were primarily formed by the oligomerization of dimethylhexane with 1-butene. Hopefully, the carbocation transition state models developed in this work will be useful for understanding the product distributions of octane in alkylation products.

Roles of the free radical and carbenium ion mechanisms in pentane cracking to produce light olefins

In order to explore the roles of the free radical and carbenium ion mechanisms in alkane cracking to produce light olefins, pentane cracking tests have been carried out over a zeolite catalyst at 600–800 °C, and a particular attention has been paid to the measurement of the product distribution. The cracking modes for pentane included “uncatalytic pyrolysis” following the free radical mechanism, “catalytic cracking” following the carbenium ion mechanism, and “catalytic pyrolysis” following the dual mechanisms (involving the concurrence of the free radical and carbenium ion mechanisms): (1) The free radical mechanism did not occur until 700 °C under the studied conditions, and it led to an ethylene-rich product distribution. (2) Zeolite catalyst initiated the carbenium ion mechanism that led to a propene-rich product distribution. (3) The dual mechanisms exhibited a wide distribution of ethylene and propene, which approximated to that of the free radical mechanism from that of the carbenium ion mechanism with increasing reaction temperature. The kc/kt and kcp/(kt+kc) parameters have been proposed to determine the relationship between the free radical and carbenium ion mechanisms. It was found that the carbenium ion mechanism greatly contributed to pentane cracking at the low temperatures, and the free radical mechanism took over the dominant position at the high temperatures. A proper proportion between the carbenium ion and free radical mechanisms promoted pentane cracking. In addition, yield of propene plus ethylene (P + E) and ratio of propene to ethylene (P/E) have been employed to evaluate the light olefins production. In the research scope, pentane cracking at 740 °C over 200 mg K-ZSM-22 zeolites seemed the optimal process, which has taken the consumption of energy and catalyst as well as the production and distribution of light olefins into consideration.

Catalytic cracking of 1, 3, 5-triisopropylbenzene over silicoaluminophosphate with hierarchical pore structure

In order to prepare a highly active catalyst for the catalytic cracking of larger molecules, a novel micro-mesoporous silicoaluminophosphate composite (define as mesoporous SAPO-5) with hierarchical tri-modal pore size distributions has been firstly synthesized via post-synthetic method in acidic condition and subsequently characterized. Morphology control of the composite is attempted by adjusting pH value of the synthetic system. Three different morphologies of composite, including sphere-, rod- and net-like, are obtained in the different conditions. Possible mechanism for the formation of mesoporous SAPO-5 has been proposed. The mesoporous SAPO-5 exhibits higher cracking activity than conventional microporous SAPO-5 for cracking of 1, 3, 5-triisopropylbenzene (1, 3, 5-TIPB) under the same reaction conditions. The result indicates that the mesoporous SAPO-5 with hierarchical pore structure is favorable for catalytic cracking of large molecule. When the cumene as the reaction molecule, the microporous SAPO-5 catalyst exhibits higher conversion in catalytic cracking of cumene compared to the mesoporous SAPO-5, and the result may be attributed to that microporous SAPO-5 has much stronger acidity and specific selectivity than mesoporous SAPO-5 catalyst in catalytic cracking of cumene. Meanwhile, corresponding carbenium ion mechanism can account for the products formed during the whole reaction process.

Catalysis of CexZr1–xO2 for di-iso-propyl-ether hydration

The catalytic potential of CexZr1–xO2 in isopropyl ether (DIPE) hydration was explored. While the acidic H-zeolite catalyst was favorable for propylene formation through carbenium ion mechanism, Ce-ZSM-5 and CexZr1–xO2 catalysts improved product selectivity of isopropyl alcohol (IPA) through redox mechanism. The catalytic property of CexZr1–xO2 depended on the preparation method and variable, type of cerium precursor and Ce/Zr ratio. By means of characterizations with X-ray diffraction, N2 adsorption isotherms and NH3 temperature programmed desorption, tetragonal phase of CexZr1–xO2 was proposed as the active phase in which CeO2 and ZrO2 catalyzed synergistically the DIPE hydration with IPA product selectivity. The CexZr1–xO2 prepared from cerium sulfate precursor with co-precipitation hydrothermal method exhibited maximum catalytic activity and IPA product selectivity. The precursor effect was attributed to the stabilization of SO42– species on the tetragonal phase of CexZr1–xO2 and super solid acidity.

ChemInform Abstract: The Conversion of Butanes in HZSM-5.

The distribution of products from the conversion of iso- and normal butane on HZSM-5 has been determined from 300 to 550°C over a wide range of conversions. The results clearly indicate that the nature of the products changes significantly in altering both the temperature and conversion conditions. High temperatures and low conversions favor low molecular weight products expected from a protolytic or radical attack on the butane. Reverse conditions favor C3 and higher hydrocarbons. These products are indicative of a classical carbenium ion mechanism. As conditions were changed the transition in mechanism was indicated by the amount of methane produced. For the first time H2 was detected as a primary product in the conversion of normal butane indicating that a tertiary C-H bond is not required for its production.

Catalytic Cracking of 1-Hexene to Propylene Using SAPO-34 Catalysts with Different Bulk Topologies

Abstract Three SAPO-34 catalysts, 100% SAPO-34, 30% SAPO-34, and meso-SAPO-34, with different bulk topologies were prepared. The catalysts were characterized by N 2 adsorption, scanning electron microscopy, X-ray diffraction, and infrared spectroscopy techniques. The pore size, total acidity, and internal cage structure of the catalysts were almost identical, but they had different bulk appearances. The role of the bulk topology/structure of the catalysts was studied using 1-hexene cracking. On 30% SAPO-34, the surface acidity and diffusion rate decreased due to blocking by binder, which adversely affected catalytic activity. 100% SAPO-34 gave better cracking ability and higher propylene selectivity because of suitable acid sites and effective shape selectivity, respectively. In order to study the effect of diffusion, meso-SAPO-34 was used. The different bulk structure gave different feed conversion and selectivity profiles. A superior control of the stereochemistry was observed in the cracking by the meso-SAPO-34 and 100% SAPO-34 catalysts, in which enhanced diffusion mass transport played an appreciable role. Most of the propylene was produced by the direct cracking pathway by the β-scission carbenium ion mechanism. Hydrogen transfer reactions became significant at higher conversions. Decreasing the residence time to a certain extend is an appropriate way to obtain high propylene yield and selectivity. Activity and selectivity patterns for 1-hexene cracking to propylene were compared to justify superior SAPO-34 topology for 1-hexene cracking to propylene.

In-situ NMR studies of isobutane activation and exchange in zeolite beta

1H solid-state NMR techniques have been used to simultaneously detect the reactivity of both catalyst and alkane reactant protons in an in-situ experimental design. Specifically, the activation of isobutane C–H bonds by the solid acid zeolite H-Beta is directly observed while the reaction is in progress, and the rate of proton transfer between the solid catalyst surface and gaseous isobutane is quantitatively measured using isotopic 1H/ 2H exchange methods. Arrhenius analysis of isothermal kinetic runs revealed an apparent activation barrier of 70 kJ/mole for the exchange process between isobutane and the 12-membered ring H-Beta, which exceeds our previously determined value of 57 kJ/mole for isobutane in the 10-membered ring H-ZSM-5 (JACS 2006, v. 128, p. 1848). Estimation of true activation energies using heat of adsorption data from the literature combined with the experimentally measured apparent E a suggests that the true activation barrier differs by only 6–7 kJ/mole in the two catalysts. We discuss the possibility that subtle shape selectivity, or inverse shape selectivity, and lattice solvation differences between the two catalysts account for the enhanced solvation of the isobutane transition state in HZSM-5 compared to the larger channel H-Beta. In all experiments, the isobutane reagent was treated to eliminate any unsaturated impurities that might serve as initiators for carbenium-ion mechanisms, and the active catalyst was free of any organic contaminants that might serve as a source of unsaturated initiators.

Alkane C−H Bond Activation in Zeolites: Evidence for Direct Protium Exchange

The mechanism of alkane C-H bond activation in heterogeneous acid catalysis is unknown. (1)H solid-state NMR techniques have been used to simultaneously detect the reactivity of both catalyst and alkane reactant protons in a true in-situ experimental design. Specifically, the activation of isobutane C-H bonds by the solid acid zeolite HZSM-5 is directly observed, and the rate of proton transfer between the solid catalyst surface and gaseous isobutane is quantitatively measured using isotopic (1)H/(2)H exchange methods. An observable adsorption complex forms between the isobutane and the primary Bronsted acid site of ZSM-5, which leads to proton exchange between the zeolite surface and the isobutane methyl groups at temperatures (273 K) much lower than previously reported. The secondary acid site in ZSM-5 is less accessible to or less reactive with the isobutane molecule. Simultaneous detection of protium loss from the Bronsted acid site and protium gain by perdeuterated isobutane reveals a common rate constant equal to 4.1-4.6 x 10(-4) s(-1) at 298 K, but at lower temperatures, the transition between this and a much slower rate process is resolved. The measured activation energy for isobutane H/D exchange is 57 kJ/mol. In all experiments, the isobutane reagent was purified to eliminate any unsaturated impurities that might serve as initiators for carbenium-ion mechanisms, and the active catalyst was free of any organic contaminants that might serve as a source of unsaturated initiators. In total, our results are consistent with direct proton exchange between the zeolite surface and the methyl groups of isobutane.

The reduction of isomerisation activity on a WO 3/SiO 2 metathesis catalyst

By increasing the WO 3 loading on WO 3/SiO 2 metathesis catalysts it was observed that metathesis activity is closely related to the tungsten surface compound and not to the crystalline WO 3. It was observed that metathesis activity reached a maximum while WO 3 crystalline material continued to increase with increasing WO 3 loading. At high WO 3 loadings the formation of crystalline material also protected the active tungsten surface compound from deactivation by being the preferred species for over-reduction. The occurrence of branched metathesis products was attributed to an increase in Brønsted acidity indicating an alkoxide (carbenium ion) mechanism responsible for skeletal isomerisation. Reduction of branched metathesis products were achieved with the addition of small amounts of an alkali-metal ion. Large amounts of the alkali-metal ion resulted in a sharp decline in conversion indicating destruction of the Lewis acidity necessary for metathesis activity. Preparation of WO 3/SiO 2 catalysts at a pH above the iso-electric point of silica also had a significant effect on catalyst selectivity.

Solvation as a Main Factor That Determines the Strength of Liquid Superacids and the Selectivity of the Acid‐Catalyzed Reactions of Olefins

AbstractFor Abstract see ChemInform Abstract in Full Text.

Solvation as a main factor that determines the strength of liquid superacids and the selectivity of the acid-catalyzed reactions of olefins

Analysis of literature and ab initio quantum chemical calculations indicates that the solvation of protons in anhydrous H 2SO 4 or HF is considerably weaker than in aqueous solutions. This results in very low constants of autoprotolysis of anhydrous superacids. In contrast, hydration of protons in moderately concentrated acids is much stronger, resulting in higher dissociation constants. Therefore, the strength of superacids is connected to the high chemical activity of the weakly solvated protons, rather than to the amount of protons. In a similar way, solvation of protonated active intermediates of the acid-catalyzed transformations of olefins in anhydrous superacids is also weaker than in aqueous solutions. This results in the real carbenium ion mechanisms of the isoparaffin–olefin alkylation or “conjunct oligomerization” of olefins. In contrast, solvation with water in moderately concentrated or dilute sulfuric acid transforms alkyl carbenium ions into oxonium ions. Therefore, cationic polymerization of olefins in moderately concentrated sulfuric acid is not a real carbenium ion reaction. Similar to the acid-catalyzed transformations of olefins on zeolites, it involves oxonium ions as the stable active intermediates and alkyl carbenium ions as transition states.

Kinetic Modeling of the Methanol to Olefins Process. 1. Model Formulation

Detailed kinetic models at the elementary step level have been formulated for the methanol to olefins (MTO) process over HZSM-5 catalyst with a Si/A1 ratio of 200. Starting from plausible mechanisms, the formation of primary products has been modeled rigorously by means of the Hougen−Watson formalism. The formation of higher olefins has been expressed in terms of carbenium ion mechanisms. A computer algorithm has been used to generate the reaction network. The rate coefficient of each elementary step has been formulated according to the single-event approach. The number of single events for each elementary step was calculated from the structure of the activated complex determined by quantum chemical calculations. Activation energies for each elementary step were obtained through the Evans−Polanyi relation that accounts for the various energy leve1s of carbenium ions and olefin isomers. The single-event kinetics combined with the Evans−Polanyi relation provides a tremendous reduction of the number of parameters to be estimated. Thermodynamic constraints further restrict the number of independent parameters to 33.

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.

Is the cationic polymerization of iso-olefins in a moderately concentrated sulfuric acid a real carbenium-ion reaction?

The different reaction products resulting from 2-methyl-2-butene transformations in 95 and 65% sulfuric acid at 0°C clearly indicate the different mechanisms of the acid catalyzed ‘conjunct olygomerization’ and cationic polymerization of olefins in the concentrated and more dilute acid. The formation of paraffinic oligomers with even and odd numbers of carbon atoms in the 95% acid indicates the predominant role of hydride transfer and cracking. This should be considered as an evidence of the real carbenium-ion mechanism of ‘conjunct olygomerization’, although the steady state concentration of alkyl carbenium ions is below the limit of 13 C NMR detection. The absence of these reactions in a moderately dilute acid indicates a different mechanism of cationic polymerization. The latter represents an oxonium ion reaction where similar to transformations of olefins on zeolites the carbenium ions are formed only as transition states.

Hydrogen production in the conversion of 2-methylbutane over a series of acid catalysts

The production of hydrogen from the conversion of 2-methylbutane was studied over a series of acid catalysts in a recirculation reactor system. Conversion of 2-methylbutane over an amorphous silica–alumina catalyst and ZSM-5 zeolite resulted in significant amounts of hydrogen. This supports a carbonium ion mechanism with a penta-coordinated carbonium ion intermediate. The conversion of 2-methylbutane over the USY zeolite and sulfated zirconia did not result in hydrogen being produced thus supporting the bimolecular carbenium ion mechanism.

Mechanistic Considerations in Acid-Catalyzed Cracking of Olefins

The relative rates of cracking and resultant product distributions for cracking C5–C8olefins over ZSM-5 at 510°C were quantified and rationalized in terms of carbenium ion mechanisms. Conditions were chosen to minimize bimolecular reactions. Cracking rates increase more dramatically with carbon number for olefins than for monomolecular cracking of paraffins, as more energetically favorable modes become available for β-scission of the carbenium ion formed by proton donation to the olefin. Product distributions were used to determine the relative rates of various modes of β-scission, as classified by the types of carbenium ions involved. For hexene and heptene feeds, the most-favorable β-scission mode available (C-type, involving just secondary carbenium ions, for hexene feed; B-type, involving secondary plus tertiary carbenium ions for heptene) accounted for 70–80% of the cracking. Product distribution was independent of which hexene or heptene isomer was fed, since double-bond and skeletal isomerization precedes significant cracking. For 1-octene feed, however, the olefin was nearly all cracked via secondary-tertiary and tertiary-secondary β-scission (after isomerizing to a dimethylhexene) before it isomerized further to the 2,4,4-trimethylpentene isomer, which would be required to undergo the most energetically favored (tertiary-tertiary) form of cracking. A semiquantitative prediction of rates and product distribution for 1-octene cracking could be made, using rates for the various types of β-scission calculated from results with C6–C7feeds.