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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.

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...

Light olefin oligomerization is an important alternative for the production of clean liquid automotive fuels, and can be performed in the presence of solid acid catalysts. Among them, medium pore zeolites, and especially ZSM-5, have been widely described in the open literature. In this work the relative importance of intracrystalline diffusion path lengths for the product molecules (depending on the zeolite crystal size and presence of mesopores in the crystallites) and Brønsted acid site density are discussed for two different olefins, propene and 1-pentene. Thus, ZSM-5 samples with a) the same crystallite size and different acid site density, b) the same density of acid sites and different crystallite size, c) post-synthesis generation of mesopores by different desilication severity, and d) samples with similar crystal size, mesoporosity and acid site density, but with a different ratio of external to internal acid sites, have been prepared and studied for oligomerization of propene and 1-pentene. The results obtained suggest that the properties required for a best performing catalyst (maximum conversion and lowest deactivation rate) are different for these two alkenes. Whereas Brønsted acid site density is determinant for propene oligomerization when intracrystalline diffusion path lengths are below a certain critical value, the presence of a high number of Brønsted acid sites is not sufficient in the case of 1-pentene, and additional mesoporosity becomes crucial. Thus, mesoporous ZSM-5 samples prepared by post-synthesis desilication treatments present a larger improvement in initial conversion and catalyst life for 1-pentene oligomerization than for conversion of propene.

The basicity of alkali-metal-exchanged (Na, K, Cs) zeolites X and Y was probed by UV−vis diffuse reflectance spectroscopy of adsorbed iodine. The observed blue shift in the visible absorption spectrum of adsorbed iodine, compared to gaseous iodine, correlated well with the negative charge on the framework oxygen atoms calculated from the Sanderson electronegativity equalization principle. The blue shifts associated with iodine adsorbed on classical catalytic supports like silica, alumina, and magnesia suggest that the iodine adsorption technique for probing basicity is applicable to a wide variety of solids. Iodine was also adsorbed on X and Y zeolites containing occluded cesium oxide formed by decomposition of impregnated cesium acetate. However, the iodine appeared to irreversibly react on these strongly basic samples, possibly forming an adsorbed triiodide ion. As a complement to the adsorption studies, the activity of alkali-metal-containing zeolites for the base-catalyzed formation of ethylene carbonate from ethylene oxide and carbon dioxide was investigated. Among the ion-exchanged zeolites, the cesium form of zeolite X exhibited the highest activity for ethylene carbonate formation. The catalytic activity of a zeolite containing occluded cesium was even higher than that of a cesium-exchanged zeolite. The presence of water adsorbed in zeolite pores promoted the rate of ethylene carbonate formation for both cesium-exchanged and cesium-impregnated zeolite X.