Effects of Void Environment and Acid Strength on Alkene Oligomerization Selectivity

Abstract

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...

Stability of bound species during alkene reactions on solid acids

“…TON (Si/Al = 39) (26,27) -exchanged zeolitic samples, Al-NMR, deconvolution of FTIR spectra, and in situ titrations, which have been reported previously for these samples (6,28 , and isobutene (99.9%; Praxair) were metered into He flow using electronic mass controllers, and products were brought into a gas chromatograph (Agilent 6890) through lines held at 373 K. Concentrations were measured by flame ionization after chromatographic separation in a methyl silicone capillary (Agilent HP-1 column; 50 m × 0.32 mm × 1.05-μm film). The system pressure was maintained using a dome-loaded regulator (Tempresco).…”

“…Alkenes interact with protons via H bonding or π-interactions, as well as by proton transfer from strong acids to form alkoxides; the intermediate between these states is a positively charged carbenium ion. The thermodynamics of alkoxide formation remain uncertain because of their very fast oligomerization and β-scission reactions (5,6), thus requiring indirect inferences from kinetic, spectroscopic, or theoretical methods. This uncertainty is particularly prevalent in isobutene-derived species, for which steric effects may disfavor the formation of covalently bound butoxides (7)(8)(9).…”

Mesostructured Catalysts Based on the BEA Topology for Olefin Oligomerisation

“…The gas phase (non-condensed compounds) was sampled in regular intervals ( % 1h)f or time-on-stream (TOS) of % 8h,u sing loops (heated at 200 8C) that were connected on-line to aD ANI Master Fast GC gas-chromatography instrument equipped with ac apillary column ValcoBond VB-1 (60 m 0.25 mm 1.50 mm), FID detector, and asplit/sliptless injector.Q uantifications were based on external calibration curves using pure 1C 4 ;t he experimental range of error was less than 5%.F or the different catalysts prepared, the distributions of butene isomers (C 4 )w ere always similar,c orresponding to thermodynamic equilibrium compositions, which is in agreement with that reported by Sarazen et al [82] in that isomer distributions with chains of ag iven size are independent of the type of solid acid catalyst. The catalytic results were expressed as conversion of butenes (X C4 ), which did not react to give higher molar mass products, [27,32,83] using Equation (1).…”

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

“…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. 5,6 Oligomerization is particularly important at low temperatures: the small-pore zeolite ZSM-5 can convert propene even at room temperature, where it is reported to produce polypropylene chains of varying lengths, 5 but above 473 K the dominant product is reported as mainly C 6 isomers due to the increasing importance of b-scission reactions and isomerization to the equilibrium.…”

“…5,6 Oligomerization is particularly important at low temperatures: the small-pore zeolite ZSM-5 can convert propene even at room temperature, where it is reported to produce polypropylene chains of varying lengths, 5 but above 473 K the dominant product is reported as mainly C 6 isomers due to the increasing importance of b-scission reactions and isomerization to the equilibrium. 4 This communication concentrates on the interaction of propene with ZSM-5 over the temperature range 140-373 K. In 1994 Spoto and co-workers used FTIR spectroscopy to investigate the oligomerization of ethene and propene on ZSM-5 at room temperature, noting that the conned structure of the zeolite inuenced the degree of branching, distribution and length of the products. 5 In the specic case of propene, branching was observed with approximately equal concentrations of CH 3 and CH 2 groups.…”