Carbenium Ion Reaction Research Articles

On the generation of light hydrocarbons from the closed pores of Jurassic strata, Sichuan Basin

Light hydrocarbons (C1-C9) offer a specific opportunity to study petroleum generation mechanisms. However, a significant amount of light hydrocarbons evaporate during sample collection, preservation, and preparation. This study aims to identify the origin of light hydrocarbons preserved in the closed pores of Jurassic shale strata from the Sichuan Basin where evaporative losses have been minimized. Twenty-one samples of various maturity levels and lithofacies were investigated using a new approach based on online decrepitation-gas chromatography. Light hydrocarbons were released from the closed pores of finely stratified mudstone, limestone, and siltstone. Among these, gaseous hydrocarbons in the C1-C5 range were utilized to discriminate between petroleum generated by two opposing reactions, i.e., free radical and carbenium ion cracking. Accordingly, in situ petroleum can be divided into three types. (1) Free radical cracking type: a predominance of methane and straight-chain alkanes in the gaseous hydrocarbon range; (2) Carbenium ion cracking type: a deficiency of methane and straight-chain alkanes in gaseous hydrocarbons; (3) Mixed cracking type: their gaseous hydrocarbons exhibit intermediate compositions between those of free radical and carbenium ion cracking types. Differences in the chemical composition of the gaseous hydrocarbons, which can be used to discriminate between the two generation mechanisms, were further supported by variations in the mineral composition, e.g., calcite and mixed-layer clay minerals are characteristic of free radical reaction and carbenium ion reaction, respectively. The present classification diagrams for gaseous hydrocarbons are based on the Jurassic shale strata of the Sichuan Basin. This novel approach for determining the whole range of hydrocarbons from closed pores shows promising prospects for deciphering the origin of light hydrocarbons and thus may be extended to other regions of interest.

Exploration of reaction mechanisms on the plastic waste polyethylene terephthalate (PET) dissolved in phenol steam reforming reaction to produce hydrogen and valuable liquid fuels

The recycling of plastic waste is a good idea for countries which concern air pollution, CO2 emission, and public health. In this research article, PET, as one of the significant plastic waste, was selected to dissolve in phenol for value‐added liquid fuels and hydrogen gas production. This research aims to provide insight into the mechanism of PET waste and phenol steam reforming reaction. The catalytic steam reforming of PET dissolved in phenol employing Ni-Pt/Al-Ti catalyst was conducted in a fixed bed reactor. The reaction test illustrated that Ni-Pt/Al-Ti catalyst possesses high activity and stability in a long time on stream experiment. It was found that valuable liquid fuels such as styrene, acetic acid, benzoic, benzene, and many other components successfully produced from the PET-phenol steam reforming reaction. It was found that during the catalytic reforming of PET, several types of reactions such as dehydrogenation reaction, thermal cracking, steam reforming, water gas shift reaction, carbenium ion reaction, free radical reaction, dihydroxylation reaction, and coke formation occurred.

Decoupling and Functionalization of Coupled Polyisobutylene via Alkoxybenzene Quenching

During living polymerization of isobutylene, marginal conditions can lead to reaction of carbenium ions with exo-olefin and production of coupled polyisobutylene (PIB). When living PIB containing a coupled fraction is subjected to end-quenching with an alkoxybenzene compound in the presence of TiCl4, the coupling reaction is quantitatively reversed to form the two original chain ends, and the regenerated chains are observed to possess the desired alkoxyphenyl functionality. To demonstrate this phenomenon, various PIB samples were prepared to purposefully contain significant coupled fractions, from 10 to 55 mol %, by quenching living PIB with hindered nucleophiles under marginal conditions. The coupled fractions were quantified by 1H NMR and GPC. Coupled PIB samples were reacted with (3-bromopropoxy)benzene, methoxybenzene, or 4-phenoxy-1-butyl acrylate in the presence of TiCl4 and a source of protic catalyst. Various protic sources were found to be effective including externally added water, adventitious ...

Study on the hydrotreatment of C9 aromatics over supported multi-metal catalysts on γ-Al2O3

With the dramatic increasing of economy, the conflict between the lack of petroleum resource and sharply increasing demand for gasoline and diesel has restricted the continuous development of economy and energy of our world. The catalytic conversion of C9 aromatics into clean fuel was studied in our laboratory using hydrotreating catalysts which were prepared by new synthesis technologies that combine vacuum-impregnation and temperature-programmed calcinations. Characterization results indicate that these catalysts have a high surface area, and the activity sites dispersed well on the supporter. Hydrogenation performance of reaction conditions was carried out in two-stage fixed beds. Products of gasoline (<180 °C) and diesel (180–360 °C) fractions were separated from intermediate products via distillation, and the analysis results demonstrate that S/N content, density, and viscosity decreased. However, the H/C molar ratio increased. The main reactions of isomerization, disproportionation, and dealkylation occurred during the conversion of C9 aromatics and the activity order accorded the mechanism of carbenium ion reaction. This study indicates that C9 aromatics could be considerably upgraded through catalytic hydroprocessing to high-quality fuel in the presence of high-performance catalysts and appropriate reaction conditions. Thus, they are promising catalytic technologies and materials. Furthermore, the products could substitute gasoline and diesel partly.

Selective production of ethylene and propylene via monomolecular cracking of pentene over proton-exchanged zeolites: Pentene cracking mechanism determined by spatial volume of zeolite cavity

The influence of the pore structures of zeolites on their selectivities for the formation of ethylene and propylene was examined in the conversion of 2-methyl-2-butene and 2-pentene over a wide range of pentene conversion. The selectivities for ethylene and propylene were highly dependent on both the spatial volume of the zeolite cavity and the dimensionality of the pore structure. Only when the specific spatial volume of zeolite, with one- or two-dimensional pore structure, was almost the same as the volume of pentyl cations did the selective production of both ethylene and propylene proceed via the monomolecular cracking of pentene. When the spatial volume of the zeolite cavity was enough larger than the volume of pentyl carbenium ions, the reaction of pentyl carbenium ions with pentene could proceed to produce decyl carbenium ions, and by following their β-scission, butenes and hexenes were preferentially formed, together with ethylene and propylene.

ChemInform Abstract: Heterolysis and Homolysis Energies for Some Carbon-Oxygen Bonds

Methods described previously for obtaining heterolysis (ΔH het ) and homolysis (ΔH homo ) enthalpies for bonds that can be cleaved to produce resonance-stabilized carbenium ions, anions, and radicals are extended to the study of carbon-oxygen bonds through the reactions of resonance-stabilized carbenium ions with substituted phenoxide ions. Titration calorimetry was used to obtain the heat of heterolysis, and the second-harmonic ac voltammetry (SHACV) method was used to obtain reversible oxidation potentials for the anions

ChemInform Abstract: Synthesis, Characterization, and Catalytic Properties of Clean and Oxygen-Modified Tungsten Carbides

Abstract High surface area WC and β-W 2 C powders (30–100 m 2 g −1 ) were prepared by direct isothermal carburization of WO 3 and W 2 N in CH 4 -H 2 mixtures. After surface cleaning with H 2 , their surfaces are equilibrated with bulk stoichiometric carbides and free of polymeric carbon; they chemisorb 0.2–0.4 monolayers of CO and H. These carbides catalyze neopentane hydrogenolysis with high selectivity. Chemisorbed oxygen also inhibits hydrogenolysis rates and introduces surface sites for neopentane isomerization, a reaction that occurs only on Pt, Ir, and Au metals. Chemisorbed oxygen also inhibits hydrogenolysis of n-hexane and n-heptane on tungsten carbides and introduces surface sites that lead to high isomerization selectivity (70–99%). Kinetic and isotopic tracer studies of n-heptane, 3,3 dimethylpentane, methylcyclohexane, propylene, and methanol reactions show that dehydrogenation reactions and methyl-shifts of unsaturated intermediates occur on oxygen-modified WC powders. Carbidic sites (WC x ) catalyze C-H activation reactions; chemisorbed oxygen titrates such WC x sites and introduces Bronsted acid surface sites (WO x ). Thus, these materials catalyze both dehydrogenation and carbenium-ion reactions, reflecting the bifunctional nature of oxygen-modified transition metal carbide surfaces.

End-Quenching of Quasiliving Carbocationic Isobutylene Polymerization with Hindered Bases: Quantitative Formation of exo-Olefin-Terminated Polyisobutylene

Polyisobutylenes possessing exclusively exo-olefin end groups were created by end-quenching TiCl4-catalyzed quasiliving isobutylene (IB) polymerizations with a hindered base at −60 to −40 °C. Polymerizations were initiated from either 2-chloro-2,4,4-trimethylpentane (TMPCl) or 1,3-bis(2-chloro-2-propyl)-5-tert-butylbenzene, in 60/40 hexane/methyl chloride in the presence of 2,6-dimethylpyridine, allowed to reach 98+% isobutylene conversion, and then reacted with either 2,5-dimethylpyrrole, 1,2,2,6,6-pentamethylpiperidine, or 2-tert-butylpyridine for various times ranging from 10 to 170 min. Typical reaction molar concentrations were [IB] = 0.5, [TMPCl] = 0.014, [26Lut] = 0.010, [TiCl4] = 0.083, and [hindered base] = 0.02−0.04 M. In some cases, minor amounts of coupled PIB were produced through reaction of carbenium ions with exo-olefin. Coupling was suppressed by higher temperature and lower chain end concentration. 2,5-Dimethylpyrrole was the most effective quencher under the conditions studied, yielding...

First Lewis Acid Catalyzed Generation and Reaction of α‐Organylsulfanyl and α‐Organylselanyl Carbenium Ions Using Ethyl α‐Fluoroacetate Derivatives.

Lewis acid catalyzed generation and reactions of α-organylsulfanyl and α-organylselanyl carbenium ions with nucleophiles proceeded in high yields using α-fluoro-α-organylsulfanyl- and α-fluoro-α-organylselanylacetate and 5 mol-% scandium triflate. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Hydrotreating of Cracked Naphtha over Ni/HZSM-5 Catalyst

A series of Ni/HZSM-5 bifunctional catalysts were prepared and were characterized by infrared spectroscopy of adsorbed pyridine as well as by X-ray powder diffraction analysis. The activity of the catalysts was investigated in hydrogenation and aromatization for the hydrotreating of cracked naphtha. It was revealed that the aromatization activity of the Ni/HZSM-5 catalyst decreases with the increase of catalyst acidity. The production of aromatics in hydrotreated naphtha with relatively low reaction temperature is related to a complicated scheme of carbenium ion reactions taking place on the catalyst surface and which are associated with the cooperation of the L acid site and the B acid site in the catalyst. The experimental results of hydrotreating of FCC naphtha show that the Ni/HZSM-5 catalyst has high aromatization and moderate hydrogenation activities; with the increase of reaction temperature the RON of the hydrotreated naphtha increases, mainly because of the production of aromatics, but at the sam...

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.

Hydrogen spillover effect on cumene cracking and n-pentane hydroisomerization over Pt/SiO2 + H-Beta

The effects of catalytic component, hydrogen partial pressure, metal amount and dose amount on cumene cracking and n-pentane hydroisomerization over Pt/SiO2+H-Beta physically mixed catalyst were studied with a pulse reactor system to examine the function of spilt-over hydrogen species. In cumene cracking, high catalytic activity was observed on H-Beta containing catalysts. On the contrary, platinum, hydrogen and H-Beta were indispensable for high catalytic activity in n-pentane hydroisomerization. Hydrogen spillover phenomenon inhibited the deactivation in both reactions. Cumene cracking activity decreased with higher hydrogen pressure on the catalyst with larger amount of Pt/SiO2. It was found that the negative reaction order with respect to hydrogen was always negative in cumene cracking. In contrast, in n-pentane hydroisomerization, the positive reaction order with respect to hydrogen was observed under rather low hydrogen pressure, though the order was negative under high hydrogen pressure. This positive tendency became more significant over the catalyst with larger amount of Pt/SiO2.The negative reaction order with respect to hydrogen is suggested to be due to the supply of spilt-over hydride to carbenium ion reaction intermediate before the cracking and isomerization reaction. At the same time, there seems to be another effect of spilt-over hydride. This inhibits the desorption of the carbenium ion as alkene, which is related to the catalyst deactivation. On the other hand, the positive reaction order with respect to hydrogen in n-pentane hydroisomerization is thought to be due to the enhancement of the acid strength by spilt-over hydrogen.On Pt/SO42−-ZrO2 catalyst, the promoting effect of hydrogen on the activity was significant in cumene cracking, however, not in n-pentane hydroisomerization. The effect of hydrogen spillover on the reaction was different. This can be due to the higher concentration of spilt-over hydrogen over on Pt/SiO2+SO42−-ZrO2 than that over Pt/SiO2+H-Beta.

In situ analysis of volatiles obtained from the catalytic cracking of polyethylene

The effects of the solid-acid-catalyst pore size and acidity on polyethylene catalytic cracking were examined with a comparison of the temperature-dependent volatile-product-slate changes when the polymer was cracked with HZSM-5 and HY zeolites and the protonated form of MCM-41. Volatile-product distributions depended on the catalyst acidity and pore size. With HZSM-5, paraffins were detected initially, and olefins were produced at somewhat higher temperatures. Aromatics were formed at temperatures 30–40°C higher than those required for olefin production. Small olefins (C3–C5) were the most abundant products when HZSM-5 and MCM-41 catalysts were employed for cracking polyethylene. In contrast, cracking with HY produced primarily paraffin volatile products (C4–C8). HY pores were large enough and the acid sites were strong enough to promote disproportionation reactions, which led to the formation of volatile paraffins. Compared with the other catalysts, HZSM-5 with its smaller pores inhibited residue formation and facilitated the production of small alkyl aromatics. Volatile-product variations could be rationalized by a consideration of the combined effects of catalyst acidity and pore size on carbenium ion reaction pathways. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3118–3125, 2001

In situ analysis of volatiles obtained from the catalytic cracking of polyethylene

AbstractThe effects of the solid‐acid‐catalyst pore size and acidity on polyethylene catalytic cracking were examined with a comparison of the temperature‐dependent volatile‐product‐slate changes when the polymer was cracked with HZSM‐5 and HY zeolites and the protonated form of MCM‐41. Volatile‐product distributions depended on the catalyst acidity and pore size. With HZSM‐5, paraffins were detected initially, and olefins were produced at somewhat higher temperatures. Aromatics were formed at temperatures 30–40°C higher than those required for olefin production. Small olefins (C3–C5) were the most abundant products when HZSM‐5 and MCM‐41 catalysts were employed for cracking polyethylene. In contrast, cracking with HY produced primarily paraffin volatile products (C4–C8). HY pores were large enough and the acid sites were strong enough to promote disproportionation reactions, which led to the formation of volatile paraffins. Compared with the other catalysts, HZSM‐5 with its smaller pores inhibited residue formation and facilitated the production of small alkyl aromatics. Volatile‐product variations could be rationalized by a consideration of the combined effects of catalyst acidity and pore size on carbenium ion reaction pathways. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3118–3125, 2001

Conversion of n-pentane and of n-butane catalyzed by platinum-containing WOx/TiO2

Tungstated titania, with and without platinum, was used to catalyze the conversion of n-butane and of n-pentane at atmospheric pressure and temperatures in the range of 423–573 K, both in the presence and in the absence of H2 in the feed stream to a flow reactor. The catalysts were active for isomerization and cracking, and the products included alkenes, in contrast to those observed in catalysis by tungstated zirconia. The catalytic activity of tungstated titania is much less than that of sulfated zirconia and similar to that of tungstated zirconia. The results characterizing tungstated titania indicate a bifunctional reaction network involving metal and acidic sites responsible for hydrogenation/dehydrogenation and carbenium ion reactions, respectively. Platinum in the catalyst and H2 in the feed strongly suppress the otherwise rapid catalyst deactivation associated with coke deposition. The catalyst was found to be stable after a period of initial deactivation, and kinetics data are reported for partially deactivated catalysts. High platinum contents lead to high selectivities for cracking in the presence of H2 and high selectivities for unsaturated products in the absence of H2. Increasing tungsten loadings raise catalyst acidity and favor isomerization. The performance of tungstated titania containing platinum resembles that of the zirconia-based catalysts less than it resembles the performance of platinum supported on (chlorided) alumina, a well-known naphtha reforming catalyst.

NMR study of the role of isopropylsulfates in the two-step “conjunct oligomerization” of propylene and isopentane–propylene alkylation catalyzed by 95% sulfuric acid

“Conjunct oligomerization” of propylene or the isopentane‐propylene alkylation catalyzed by an excess of 95% sulfuric acid was performed in two consecutive steps. First di-isopropylsulfate was prepared by interaction of sulfuric acid with propylene. The ester was then either decomposed at room temperature in the presence of the 5‐10 molar excess of 95% acid or was used in the acid-catalyzed alkylation of isopentane. In situ 1 Ha nd 13 C NMR study of the reaction mixture of “conjunct oligomerization” indicated that the diester participates in two equilibria with sulfuric acid. The first one transforms the diester into a monoester. The second equilibrium corresponds to protonation of the monoester with an excess of sulfuric acid. This converts a minor fraction of the mono-alkylsulfate into isopropyl carbenium ions that are only weakly solvated with sulfuric acid: C3H7HSO4 + H2SO4 C3H + H2SO4 + HSO 4 . The subsequent reactions of alkyl carbenium ions with the non-protonated alkylsulfate result in final products of “conjunct oligomerization” while in the presence in the reaction mixture of isopentane, alkylation with the predominant formation of C 8 branched paraffins takes place. A very low yield of propane indicates a minor role of hydride transfer in alkylation. Another unexpected result is the absence in both reaction mixtures of propylene. These findings are in contradiction with the classical mechanism of isoparaffin‐olefin alkylation by Schmerling. Therefore, an alternative mechanism of this reaction is suggested via a direct alkylation of isopentane with the mono-alkylsulfate.

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.

Effects of catalyst acidity and HZSM‐5 channel volume on polypropylene cracking

Effects of catalyst acidity and the restricted reaction volume afforded by HZSM-5 on the catalytic cracking of polypropylene are described. Polypropylene cracking by silica—alumina and HZSM-5 catalysts yields olefins as primary volatile products. In addition, HZSM-5 channels restrict carbenium ion rearrangements and facilitate formation of significant amounts of propene and alkyl aromatic volatile products. The higher acidity of sulfated zirconia compared to the other catalysts results in an increase in the frequency of hydride abstractions, resulting in the formation of significant yields of saturated hydrocarbons and organic residue for this catalyst. Primary polypropylene cracking products can be derived from carbenium ion reaction mechanisms. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 341–348, 1998

Effects of catalyst acidity and HZSM-5 channel volume on polypropylene cracking

Effects of catalyst acidity and the restricted reaction volume afforded by HZSM-5 on the catalytic cracking of polypropylene are described. Polypropylene cracking by silica—alumina and HZSM-5 catalysts yields olefins as primary volatile products. In addition, HZSM-5 channels restrict carbenium ion rearrangements and facilitate formation of significant amounts of propene and alkyl aromatic volatile products. The higher acidity of sulfated zirconia compared to the other catalysts results in an increase in the frequency of hydride abstractions, resulting in the formation of significant yields of saturated hydrocarbons and organic residue for this catalyst. Primary polypropylene cracking products can be derived from carbenium ion reaction mechanisms. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 341–348, 1998