Olefin Polymerization over Synthetic Molecular Sieves

Norton, Charles J.

doi:10.1021/i260011a009

Cited by 31 publications

(9 citation statements)

References 8 publications

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“…Solid Brønsted acids such as zeolites are able to catalyse the oligomerization of light olens such as propene to longer chain alkyl hydrocarbons. 1,2 This reaction is of interest both as a means of C-C bond formation for the valorisation of light hydrocarbons and oxygenates and because it is the inverse reaction to the b-scission mechanism by which alkenes are cracked in commercial uidised catalytic cracking (FCC) reactors and therefore has an effect on the nal product composition of this important commercial process. 3,4 The reaction proceeds via protonation of the olen to a carbenium ion which reacts with other olen molecules to give alkyl chains, with the exact composition being determined by shape selective effects from the zeolite pore.…”

Section: Introductionmentioning

confidence: 99%

Low-temperature studies of propene oligomerization in ZSM-5 by inelastic neutron scattering spectroscopy

et al. 2019

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Observation of the oligomerization of propene in ZSM-5 at 293 K by neutron vibrational spectroscopy shows that the product species are linear alkyl chains. No evidence is found for the formation of branched products. The selective formation of linear alkyl chains is attributed to a confinement effect within the zeolite pore structure. A role for zeolite crystallite size, a controllable parameter within the catalyst preparative stage, in being able to influence the product composition in technically relevant olefin oligomerization reactions is considered.

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Section: Introductionmentioning

confidence: 99%

Low-temperature studies of propene oligomerization in ZSM-5 by inelastic neutron scattering spectroscopy

et al. 2019

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“…Light alkene oligomerization is an attractive route for the synthesis of chemical feedstocks and transportation fuels from undervalued precursors [1][2][3][4]. Turnover rates (per proton) for reactions of hydrocarbons and oxygenates on solid Brønsted acids depend sensitively on the size and shape of the molecular-sized voids that contain the active protons in microporous aluminosilicates [5][6][7].…”

Section: Introductionmentioning

confidence: 99%

“…Extensive studies of alkene oligomerization catalysis have broadly explored the reactive and selective properties of acid catalysts [1][2][3]13,[15][16][17][18][19], but without the broad range of materials or the systematic and purposeful variations in void environment and acid strength, the detailed measurements and mechanistic interpretation of turnover rates, or the theoretical treatments that we bring together in the present study.…”

Section: Introductionmentioning

confidence: 99%

Catalysis on solid acids: Mechanism and catalyst descriptors in oligomerization reactions of light alkenes

Sarazen

Doskocil²,

Iglesia

2016

Journal of Catalysis

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This study addresses fundamental descriptions of confinement and acid strength effects on stability for transition states and intermediates involved in alkene oligomerization on solid acids. Kinetic and infrared data and theoretical treatments that account for dispersive interactions show that turnover rates (per H +) on aluminosilicates and heterosilicates with microporous voids (TON, MFI, BEA, FAU) and on mesoporous acids (amorphous silica-alumina, dispersed polyoxometalates) reflect the free energy of CC bond formation transition states referenced to gaseous alkenes and bound alkene-derived precursors present at saturation coverages. These free energy barriers decrease as the size of confining voids decrease in aluminosilicates containing acid sites of similar acid strength and approach bimolecular transition state (TS) sizes derived from density functional theory (DFT) for propene and isobutene reactants. Such TS structures are preferentially stabilized over smaller bound precursors via contacts with the confining framework. These effects of size, typically based on heuristic geometric analogies, are described here instead by the dispersive component of DFT-derived energies for TS and intermediates, which bring together the effects of size and the shape, for different framework voids and TS and precursor structures derived from alkenes of different size; these organic moieties differ in "fit" within voids but also in their proton affinity, as a result of the ion-pair character of TS structures. The larger charge in TS structures relative to their alkene-derived precursors cause free energy barriers to decrease as conjugate anions become more stable in stronger acids. Consequently, oligomerization rate constants decrease exponentially with increasing deprotonation energy on unconfined acid sites in polyoxometalates and silica-alumina and on confined sites within MFI frameworks with Al, Ga, Fe, or B heteroatoms. Reactivity descriptions based on geometry or acid strength are replaced by their more relevant energetic descriptors-van der Waals confinement energies, proton affinities of organic molecules, and deprotonation energies-to account for reactivity, here for different reactants on diverse solid acids, but in general for acid catalysis.

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“…The oligomerization of alkenes on solid Brønsted acids provides an effective strategy to form new C–C bonds from small hydrocarbons. − These processes become attractive as such small molecules are excluded from fuels, because of vapor pressure restrictions, and as small alkenes become available from biomass-derived oxygenates. Solid acids, such as the acid forms of zeolites, catalyze these reactions .…”

Section: Introductionmentioning

confidence: 99%

Effects of Void Environment and Acid Strength on Alkene Oligomerization Selectivity

Doskocil²,

2016

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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) Brønsted 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 oligomerization catalysis through shape-selective effects conferred by void environments may have used inaccurate tabulated thermodynamics, as we show in this study. The void environment, however, influences the size distribution of the chains formed in these acid-catalyzed alkene reactions. One-dimensional microporous aluminosilicates predominantly form true oligomers, those expected from dimerization and subsequent oligomerization events for a given reactant alkene; such chains are preserved because they cannot grow to sizes that would inhibit their diffusion through essentially cylindrical channels in these frameworks. Amorphous SiAl and colloidal silica-supported POM clusters contain acid sites of very different strength; both exhibit size variations across the void space, but at length scales much larger than molecular diameters, thus preserving true oligomers by allowing them to egress the void before β-scission events. Mesoporous acids of very different strength (POM, SiAl) give similar true isomer selectivities, as also observed on MFI structures with different heteroatoms (X-MFI, where X = Al, Ga, Fe, B), which also differ in acid strength; this insensitivity reflects oligomerization and β-scission reactions that involve similar ion-pair transition states and therefore depend similarly on the stability of the conjugate anion. Three-dimensional microporous frameworks contain voids larger than their interconnecting paths, an inherent consequence of intersecting channels and cage–window structures. As a result, oligomers can reach sizes that restrict their diffusion through the interconnections, until β-scission events form smaller and faster diffusing chains. These undulations are of molecular dimensions and their magnitude, which is defined here as the ratio of the largest scale to the smallest scale along intracrystal diffusion paths, determines the extent to which oligomerization–scission cycles contribute to the size distribution of products. These contributions are evident in the extent to which chain siz...

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Olefin Polymerization over Synthetic Molecular Sieves

Cited by 31 publications

References 8 publications

Low-temperature studies of propene oligomerization in ZSM-5 by inelastic neutron scattering spectroscopy

Low-temperature studies of propene oligomerization in ZSM-5 by inelastic neutron scattering spectroscopy

Catalysis on solid acids: Mechanism and catalyst descriptors in oligomerization reactions of light alkenes

Effects of Void Environment and Acid Strength on Alkene Oligomerization Selectivity

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