Skeletal Isomerization Research Articles

Catalytic diversity conferred by confinement of protons within porous aluminosilicates in Prins condensation reactions

The effects of confinement of transition states and bound intermediates are described here based on kinetic and theoretical studies for Prins condensation and oligomerization reactions of isobutanal-isobutene reactant mixtures catalyzed by Brønsted acid sites of similar strength but confined within diverse microporous (TON, BEA, MFI, FAU) and mesoporous (Al-MCM-41) aluminosilicate structures. Kinetic data and density functional theory (DFT) show that smaller voids lead to more stable bound monomers derived from isobutene and isobutanal and to protons nearly saturated with H-bonded isobutanal. Prins condensation forms 2,5-dimethyl-hexadiene regioisomers (2,5-DMH) with rate constants that reflect the different stability of the kinetically-relevant CC bond formation transition states and their H-bonded isobutanal precursors. Their differences in size lead to reactivities that depend sensitively on the size and shape of the confining voids in each aluminosilicate and even on the location of the Al-atoms (and the protons) within a given framework. Specifically, FAU leads to the highest turnover rates, because larger voids in Al-MCM-41 form less effective van der Waals contacts with the transition state, while the smaller voids within TON and BEA lead to enthalpic and entropic penalties brought forth by host-guest distortions. MFI cage-like intersections provide more effective stabilization than BEA cylindrical channels of similar size and lead to higher turnover rates and smaller DFT-derived activation free energies; these subtle effects of void shape on reactivity are neglected in descriptions of the confining frameworks based on spherical constructs. Reactivity descriptors require instead theoretical treatments that rigorously account for van der Waals interactions and for the entropy losses and host-guest distortions required for confinement, as reported here from confinement free energies ensemble-averaged over all crystallographic locations in each aluminosilicate structure. The ratios of rate constants for these parallel isobutene dimerization and Prins condensation reactions depend only weakly on the nature of the aluminosilicate framework, because of the similar size and shape of their respective transition states and bound precursors. Transition states for secondary skeletal isomerization and cyclization of 2,5-DMH condensation products, however, are stabilized more effectively (relative to condensation transition states) within voids of intermediate size (BEA, MFI), leading to higher selectivity to secondary products. MFI cage-like structures specifically catalyze the conversion of 2,5-DMH to cyclic 1,4-dimethyl-cyclohexene isomers, direct precursors to p-xylene, with unique specificity.

Water effects on the acidic property of typical solid acid catalysts by 3,3-dimethylbut-1-ene isomerization and 2-propanol dehydration reactions

Two model reactions, namely the Brønsted-acidity-specific skeletal isomerization of 3,3-dimethylbut-1-ene for evaluating the Brønsted acidity and dehydration of 2-propanol for the overall acidity at the catalyst surface, were employed in this work to measure the effect of water on the surface acidic property of typical solid catalysts (HZSM-5, γ-Al2O3 and Nb2O5). For the isomerization reaction, water addition in the reaction feed produced little effect on the catalysis of HZSM-5 but significantly positive effect on the catalysis of Nb2O5; the typical Lewis acidic γ-Al2O3 showed no activity for this reaction in either the presence or absence of water. For the dehydration reaction, the effect of water was generally negative on the catalysis of every catalyst, demonstrating that water presence resulted in poisoning more or less of the Lewis acidity on the catalyst surface. Water poisoning of the Lewis acidity on Nb2O5 also resulted in generation of Brønsted acidity and the higher the H2O partial pressure (or H2O-to-substrate ratio) the higher the newly generated Brønsted acidity. However, generation of Brønsted acidity did not happen on γ-Al2O3 and HZSM-5, regardless of the H2O partial pressure. Three Nb2O5 samples with varying calcination temperature were involved, and their behavior clearly indicated that the water effects on Nb2O5 also depended sensitively on the sample calcination temperature. Moreover, water presence increased the activation energy and selectivity for di-isopropyl ether of the dehydration reaction over γ-Al2O3 and Nb2O5, and thus affected the acidity distribution at the catalyst surface during the reaction.

Influence of the kind of VOx structures in VOx/MCM-41 on activity, selectivity and stability in dehydrogenation of propane and isobutane

Vanadyl acetylacetonate was carefully grafted on mesoporous silica (MCM-41) to create a series of catalysts with VOx surface site density varying from 0.05 to 1.67nm−2. The catalysts were characterized by UV–vis spectroscopy, temperature-programmed reduction (TPR) with H2, and temperature-programmed desorption (TPD) of NH3 as well as tested for their activity, selectivity, and onstream stability in non-oxidative dehydrogenation (DH) of propane and isobutane. Lewis-acidic V3+ and V4+ were identified as DH active sites. For both dehydrogenation reactions, a direct correlation between apparent turnover frequency and the mean number of VOV bonds around a central V cation was established. It suggests that isolated VOx species possess higher intrinsic activity in comparison with their oligomerized counterparts. However, selectivity over the former species in isobutane dehydrogenation is lower due to their higher Lewis acidity and, consequently, increased activity for skeletal isomerization of isobutene and for cracking reactions. Since isomerization reactions are impossible for propene its selectivity is only slightly influenced by degree of aggregation of VOx species. Besides activity and selectivity, the rate of catalyst deactivation is also influenced by mean number of VOV bonds; deactivation is low for single VOx sites but increases with rising degree of aggregation of VOx species.

Theoretical Investigation of Electrophilic Transannular Addition Reactions of Bromine to Face-to-Face (Juxtaposed) Double Bonds in Strained Polycyclic Hydrocarbons.

Transannular electrophilic addition reaction of halogens to face-to-face(juxtaposed) double bonded strained alkenes were theoretically investigated. General rules that allow us to stipulate the factors that direct the main steps of the energy hypersurface of reactions as well as the products were established. Direction of the reaction flow is determined by direction of intramolecular skeletal isomerisation of cyclic-bridged halogenium cation and isomerisation takes place to create a more stable skeletal structure. Stability of resultant skeletal structure is determined by the number of σ bonds between isolated double bonds of the alkene and bonding-type of double bonds (N- and U-type). When the number of σ bonds between double bonds of the alkene is three (m = 3), the reaction takes place to predominantly give an N-type product, and when four (m = 4), N- and U-type products are formed. Structure and stability of cation intermediates (bridged, N- and U-type cations) of electrophilic addition reaction of homohipostrofen molecule, whose double bonds were linked by three σ bonds, with bromine were investigated by DFT methods in detail. Also the addition reaction of endo,endo-tetracyclo[6.2.2.23,6.02,7]tetradeca-4,9-dien molecule, whose double bonds were linked by four σ bonds, with bromine were investigated by quantum chemistry.

Isoskeletomerism as an intermediate concept for mediating between stereoisomerism and isomerism. A remedy for conceptual defects of organic chemistry

After isoskeletomerism is introduced as an intermediate concept for mediating between stereoisomerism and isomerism, so-called ‘constitutional isomerism’ is critically discussed as one of the roots of confusion in organic chemistry. The convention that isomerism is subdivided into stereoisomerism and ‘constitutional isomerism’ is concluded to be misleading, because these concepts are conceptually distinct from a viewpoint of the concepts of equivalence relationships and equivalence classes. The indifference toward these concepts is one of the conceptual defects of organic chemistry. A new flowchart for judging enantiomerism, RS-stereoisomerism, stereoisomerism, isoskeletomerism, and isomerism is discussed to generate an isomer-classification diagram, where RS-stereoisomerism and isoskeletomerism are contained as new matters for remedying the conceptual defects. Skeletons are derived from basic skeletons by three operations (the bond operation, the replacement operation, and the substitutive operation), and they are used to examine the action of isoskeletomeric relationships. Equivalent molecular entities under an isoskeletomeric relationship are collected to give a set of isoskeletomers, which is concluded to be an equivalence class. The confusion caused by so-called ‘constitutional isomerism’ and its subcategories (‘skeletal isomerism’, ‘positional isomerism’, and so on) is critically discussed from the viewpoint of isoskeletomerism as a missing link. The double-entendre of the term constitution is examined when applied to both a 3D entity (a set of stereoisomers) and a 2D entity (a graph). Taxonomy of organic compounds and rational formulas, nomenclature of organic compounds, and combinatorial enumeration of chemical compounds are discussed from the viewpoint of isoskeletomerism.

Catalytic Performance of Ferrierite with Different Crystal Grain Sizes during Skeletal Isomerization of 1‐Butene

AbstractFerrierite (FER) zeolite with small crystal grain size was successfully prepared by a kind of hydrothermal synthesis with aging procedure using ethylenediamine (EDA) as template. The zeolite was characterized by a series of analyses. Aging procedure decreased the crystal grain sizes and no significant change was found in the ratio of Si/Al and acid sites. Eventually, FER zeolite with small crystal grain size ranging from 150nm to 800nm showed high isobutene selectivity and good stability during skeletal isomerization of 1‐butene. The reaction was divided into two stages because of the change in the conversion of 1‐butene. Coke deposited at reaction stage II primarily caused the deactivation of FER zeolite. FER Zeolite with small crystal grain led to not only a lower rate of coke deposition but also more coke content than FER zeolite with normal crystal grain size.

Skeletal isomerization of n-pentane: A comparative study on catalytic properties of Pt/WOx–ZrO2 and Pt/ZSM-22

Pt/WOx–ZrO2 and Pt/ZSM-22 were prepared and used as catalysts for the n-pentane hydroisomerization reactions. The physico-chemical properties of the supports were characterized by XRD, N2 adsorption, NH3-TPD and 1-pentene isomerization reaction. The reducibility and adsorbability of the catalysts were evaluated via H2-TPR, H2-TPD and CO adsorption. Under the reaction condition of WHSV=1h−1, pn-pentane,0=0.24bar and ambient pressure, the yields of isopentane over Pt/WOx−ZrO2 and Pt/ZSM-22 can reach 52% (at 300°C) and 61% (at 340°C) respectively. The selectivities to isopentane over these two catalysts are well above 95%. The hydroisomerization mechanisms over these catalysts are discussed.

Synthesis of ordered porous zirconia containing sulfate ions and evaluation of its surface acidic properties

The synthesis and characterization of sulfated ZrO2 with ordered pores as a solid acid catalyst were investigated. Ordered porous ZrO2 containing sulfate ions was synthesized by hydrolysis of zirconium n-propoxide in the presence of HCl, surfactant and (NH4)2SO4 dissolved in distilled water, when the SO4/Zr molar ratio was changed in the range of 0.8–1.4. BET surface area of sulfated ZrO2 thus synthesized was found to be 150–160 m2 g−1. The presence of sulfate ions in ZrO2 was confirmed by an appearance of IR band at ca. 1398 cm−1 in the IR spectra of sulfated ZrO2. Since two IR bands due to surface hydroxyl groups were still observed for sulfated ZrO2, the possibility that sulfate ions are highly dispersed on the surface of ordered porous ZrO2 was suggested. From the evaluation of surface acidity by NH3-TPD measurements, not only the amount of NH3 desorption but also the strength of acid sites related to NH3 desorption temperature were found to be increased with increasing the SO4/Zr ratio up to 1.2. On the other hand, FTIR spectroscopy following pyridine adsorption revealed that the maximum surface density of Bronsted acid sites was attained for sulfated ZrO2 with SO4/Zr ratio of 1.0. Sulfate species interacting strongly with nano-sized tetragonal ZrO2 was considered to be responsible for the creation of Bronsted acid sites. In accordance with the surface density of Bronsted acid sites, sulfated ZrO2 with SO4/Zr ratio of 1.0 showed the highest activity for the skeletal isomerization of 3,3-dimethylbut-1-ene, which predominantly occurs on Bronsted acid sites.

Superior activity of non-interacting close acidic protons in Al-rich Pt/H-*BEA zeolite in isomerization of n-hexane

Skeletal isomerization of linear alkanes, an essential reaction for the production of gasoline, relies on environmentally questionable chlorinated catalysts, whose activity exceeds that of alternative zeolite catalysts. This work describes an attempt to understand relations between the local arrangement of active sites and skeletal isomerization of n-hexane in order to adapt the structure of zeolite catalysts to increase the reaction rates. For this purpose, we used a combination of synthesis of zeolites of *BEA structural topology with unique density and distribution of strongly acid sites, analysis of the nature of the acid sites by 1H MAS NMR spectroscopy and FTIR spectroscopy of the OH groups and adsorbed d3-acetonitrile, UV–vis-NIR spectroscopy of carbocations formed by protonization, and kinetic analysis. We demonstrate that the high density of non-interacting but close and strongly acidic structural hydroxyl groups significantly lower the activation barrier in the isomerization reaction compared to far-distant acid sites. The organotemplate-free synthesized Al-rich Pt/H-*BEA zeolite (Si/Al 4.2) with an unparalleled high concentration of the non-interacting close H+ ions balancing the charge of the Al-Si-Al sequences forming a wall between the two channels yields 6 times higher reaction rates compared to state-of-the-art Si-rich Pt/H-zeolite catalysts.

Designing ferrierite-based catalysts with improved properties for skeletal isomerization of n-butene to isobutene

The crystal morphology and size of ferrierite catalysts were controlled by a suite of crystallization methods and structure-directing agents.

Density functional theory studies on the skeletal isomerization of 1-butene catalyzed by HZSM-23 and HZSM-48 zeolites

This work reveals the topological effect of zeolites on both enthalpy and entropy of skeletal isomerization of 1-butene.

Catalytic routes to fuels from C1 and oxygenate molecules.

This account illustrates concepts in chemical kinetics underpinned by the formalism of transition state theory using catalytic processes that enable the synthesis of molecules suitable as fuels from C1 and oxygenate reactants. Such feedstocks provide an essential bridge towards a carbon-free energy future, but their volatility and low energy density require the formation of new C-C bonds and the removal of oxygen. These transformations are described here through recent advances in our understanding of the mechanisms and site requirements in catalysis by surfaces, with emphasis on enabling concepts that tackle ubiquitous reactivity and selectivity challenges. The hurdles in forming the first C-C bond from C1 molecules are illustrated by the oxidative coupling of methane, in which surface O-atoms form OH radicals from O2 and H2O molecules. These gaseous OH species act as strong H-abstractors and activate C-H bonds with earlier transition states than oxide surfaces, thus rendering activation rates less sensitive to the weaker C-H bonds in larger alkane products than in CH4 reactants. Anhydrous carbonylation of dimethyl ether forms a single C-C bond on protons residing within inorganic voids that preferentially stabilize the kinetically-relevant transition state through van der Waals interactions that compensate for the weak CO nucleophile. Similar solvation effects, but by intrapore liquids instead of inorganic hosts, also become evident as alkenes condense within MCM-41 channels containing isolated Ni2+ active sites during dimerization reactions. Intrapore liquids preferentially stabilize transition states for C-C bond formation and product desorption, leading to unprecedented reactivity and site stability at sub-ambient temperatures and to 1-alkene dimer selectivities previously achieved only on organometallic systems with co-catalysts or activators. C1 homologation selectively forms C4 and C7 chains with a specific backbone (isobutane, triptane) on solid acids, because of methylative growth and hydride transfer rates that reflect the stability of their carbenium ion transition states and are unperturbed by side reactions at low temperatures. Aldol condensation of carbonyl compounds and ketonization of carboxylic acids form new C-C bonds concurrently with O-removal. These reactions involve analogous elementary steps and occur on acid-base site pairs on TiO2 and ZrO2 catalysts. Condensations are limited by α-H abstraction to form enolates via concerted interactions with predominantly unoccupied acid-base pairs. Ketonization is mediated instead by C-C bond formation between hydroxy-enolates and monodentate carboxylates on site pairs nearly saturated by carboxylates. Both reactions are rendered practical through bifunctional strategies, in which H2 and a Cu catalyst function scavenge unreactive intermediates, prevent sequential reactions and concomitant deactivation, and remove thermodynamic bottlenecks. Alkanal-alkene Prins condensations on solid acids occur concurrently with alkene dimerization and form molecules with new C-C bonds as skeletal isomers unattainable by other routes. Their respective transition states are of similar size, leading to selectivities that cannot sense the presence of a confining host. Prins condensation reactions benefit from weaker acid sites because their transition states are less charged than those for oligomerization and consequently less sensitive to conjugate anions that become less stable as acids weaken.

Mechanisms for the Reverse Skeletal Isomerization of n‐Butenes to Isobutene over Zeolite Catalysts

AbstractThe use of 13C‐enriched isobutene in the reverse skeletal isomerization of 1‐butene over zeolite catalysts allowed us to corroborate that the mechanisms for the reverse reaction were essentially the same as those for the forward reaction, regardless of the structure type of the catalyst employed.

Hydrophilic interaction and reversed phase mixed-mode liquid chromatography coupled to high resolution tandem mass spectrometry for polar lipids analysis

A hydrophilic interaction liquid chromatography (HILIC) fused-core column (150×2.1mm ID, 2.7μm particle size) and a short reversed-phase liquid chromatography (RPLC) column (20mm×2.1mm ID, 1.9μm) were serially coupled to perform mixed-mode chromatography (MMC) on complex mixtures of phospholipids (PL). Mobile phase composition and gradient elution program were, preliminarily, optimized using a mixture of phosphatidylcholines (PC), phosphatidylethanolamines (PE), their corresponding lyso-forms (LPC and LPE), and sphingomyelins (SM). Thus a mixture of PC extracted from soybean was characterized by MMC coupled to electrospray ionization (ESI) high-resolution Fourier-transform mass spectrometry (FTMS) using an orbital trap analyzer. Several previously undiscovered PC, including positional isomers (i.e. 16:0/19:1 and 19:1/16:0) of PC 35:1 and skeletal isomers (i.e. 18:1/18:2 and 18:0/18:3) of PC 36:3 were identified. Therefore, high-resolution MS/MS spectra unveiled the occurrence of isomers for several overall side chain compositions. The proposed MMC-ESI-FTMS/MS approach revealed an unprecedented capability in disclosing complexity of an actual lipid extract, thus representing a very promising approach to lipidomics.

Size-controlled synthesis of hierarchical ferrierite zeolite and its catalytic application in 1-butene skeletal isomerization

Nano-ferrierite zeolite aggregates with hierarchical porosity were successfully prepared by using pyrrolidine as sole organic structure-directing agent. SEM and TEM images revealed that nano-sized crystals (40–60 nm) stacked together loosely to form irregular clusters. Influence of crystallization temperature and initial gel compositions (pyrrolidine content, alkalinity, water content) on the synthesis of ferrierite zeolite were investigated in detail. Low crystallization temperature and high alkalinity were beneficial to form nano-ferrierite zeolite aggregates with high hierarchical porosity (total pore volume: 0.461 cm3/g). Compared with commercialized ferrierite zeolite, hierarchical nano-ferrierite aggregates showed better catalytic stability and product selectivity in the 1-butene skeletal isomerization reaction.

Mechanism of Isobutanal–Isobutene Prins Condensation Reactions on Solid Brønsted Acids

The selectivity to 2,5-dimethyl-hexadiene isomers (2,5-DMH) via acid-catalyzed isobutanal–isobutene Prins condensation is limited by isobutene oligomerization reactions (to 2,4,4-trimethyl-pentene isomers) and by skeletal isomerization and cyclization of the primary 2,5-DMH products of Prins condensation. Experiment and theory are used here to assess and interpret acid strength effects on the reactivity and selectivity for isobutanal–isobutene Prins condensation routes to 2,5-DMH, useful as precursors to p-xylene. Non-coordinating 2,6-di-tert-butylpyridine titrants fully suppress reactivity on Keggin heteropolyacids, niobic acid, and mesoporous and microporous aluminosilicates, indicating that Prins condensation, parallel isobutene oligomerization, and secondary skeletal isomerization and cyclization of primary 2,5-DMH products occur exclusively on Bronsted acid sites. The number of titrants required to suppress rates allows site counts for active protons, a requirement for comparing reactivity among soli...

Effects of Void Environment and Acid Strength on Alkene Oligomerization Selectivity

The effects of channel connectivity, void environment, and acid strength on the relative rates of oligomerization, β-scission, and isomerization reactions during light alkene conversion (ethene, propene, isobutene; 2–400 kPa alkene; 473–533 K) were examined on microporous (TON, MFI, MOR, BEA, FAU) and mesoporous (amorphous silica–alumina (SiAl), MCM-41, Keggin POM) Bronsted acids with a broad range of confining voids and acid strength. Skeletal and regioisomers equilibrate under all conditions of pressure and conversion and on all catalysts, irrespective of their acid strength, void size, or framework connectivity, consistent with rapid hydride and methyl shifts of alkoxides intermediates and with their fast adsorption–desorption steps. Such equilibration is evident from detailed chemical speciation of the products and also from intramolecular isotopic scrambling in all oligomers formed from 2-13C-propene on TON, MFI, SiAl, and POM clusters. Previous claims of kinetic control of skeletal isomers in oligom...

Effect of the degree of zeolite recrystallization into micro–mesoporous materials on their catalytic properties in petroleum refining and petroleum chemistry processes

It has been shown that by varying the degree of recrystallization of zeolites, it is possible to obtain mesostructured zeolites (RZEO-1), micro–mesoporous nanocomposites (RZEO-2), and mesoporous materials with zeolite fragments (RZEO-3). The main features of the effect of recrystallization degree of MOR, BEA, and FER zeolites on their catalytic properties in the processes of cracking of 1,3,5-triisopropylbenzene, skeletal isomerization of butene-1, hydroisomerization of n-alkanes, alkylation of benzene with dodecene-1, alkylation of naphthalene with cyclohexene, and disproportionation of cumene have been revealed. It has been found that each type of catalytic reaction requires a micro–mesoporous catalyst with an optimal degree of recrystallization. Zeolites RZEO-1 are the most efficient in the reactions that require strong acidity; zeolites RZEO-2 are the most promising in consecutive reactions and reactions that proceed in pore mouths; and RZEO-3 are optimal for transformations of bulky molecules.

Generation of Strong Acid Sites on Yttrium-Doped Tetragonal ZrO2-Supported Tungsten Oxides: Effects of Dopant Amounts on Acidity, Crystalline Phase, Kinds of Tungsten Species, and Their Dispersion

A tungsten oxide catalysts supported on yttrium- or ytterbium-doped crystalline zirconium oxide (WOx/Y(Yb)-ZrO2) with different dopant concentrations but the same tungsten loadings and calcination temperatures were prepared to investigate the mechanism responsible for the strong solid acidity of so-called tungstated zirconia. The surface acid–base properties of WOx/Y(Yb)-ZrO2 and the support were characterized by catalyses of n-butane skeletal isomerization, alkylation of anisole with benzyl alcohol, and 2-butanol decomposition. The structural characterization and quantitative analysis were conducted by XRD, XAFS, and UV–Vis techniques. Crystalline zirconium oxides were effective as the starting material for tungsten–zirconium-based strong solid acid catalysts instead of amorphous Zr(OH)x if the crystalline form was tetragonal. Tetragonal-phase Y(Yb)-ZrO2 substitutional solid solutions with dopant concentrations of 3–5 mol % was a suitable support for stabilization of active small WO3 clusters. Tungsten species aggregated to form inactive large WO3 particles on the monoclinic ZrO2 polymorph with Y-dopant amount less than 2 mol % even though the surface density of tungsten species and calcination temperature were the same as those for the active catalysts. Inactive yttrium–tungsten binary oxides formed when the yttrium doping level exceeded 10 mol %. Intrinsic role of crystalline phase of tetragonal zirconia on generating the strong solid acidity is confirmed to stabilize WO3 small clusters without aggregation.

Preparation of NiAlO x and NiSiO x Complex Oxides with High Surface Areas for the Isomerization Reactions of 1-Hexene

The NiAlO x and NiSiO x complex oxides with high surface areas were prepared by the co-precipitation method through the control of acidity of solutions. Nickel species were found to be highly dispersed in these complex oxides and a remarkable feature was that the incorporation of large amount of nickel cations into SiO2 generated the great amount of surface acid sites. Upon the sulfidation by CS2, the sulfided catalysts were found to be highly active for the conversion of 1-hexene to different products formed from the three main competitive reactions (skeletal isomerization, double bond isomerization and hydrogenation of all hexenes). The high yield of skeletal isomerization of 1-hexene on a sulfided NiSiO x (55 %) could be attributed to the high density of acid sites for the adsorption of 1-hexene as well as the low hydrogenation activity of nickel species on this catalyst. In contrast, the high yields of direct hydrogenation and double bond isomerization of 1-hexene on the sulfided NiAlO x might be due to the high hydrogenation activity of nickel species as well as the surface acidity and basicity on the catalyst. The sulfided NiAlO x and NiSiO x complex oxides were highly active for the conversion of 1-hexene to different products. The high yield of skeletal isomerization of 1-hexene on the sulfided NS10 (55 %) could be attributed to the high density of acid sites for the adsorption of 1-hexene.