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...