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Graphical abstract Highlights • A part of extraframework Al (EFAl) is close to strong Brønsted acid site (SBAS). • H-MFI with different concentrations of EFAl-SBAS sites are prepared by steaming. • Pentane cracking rate on EFAl-SBAS is 50-fold higher than on isolated SBAS. • Pentane dehydrogenation rate on EFAl-SBAS is 80-fold higher than on isolated SBAS. • The higher rate on EFAl-SBAS is caused by increased activation entropy. Abstract The activity of strong Brønsted acid sites in H-MFI zeolites for pentane cracking and dehydrogenation is enhanced by the presence of extra-framework aluminum in spatial proximity. The steric proximity of these two sites is deduced from the perturbation of OH groups on extra-framework aluminum oxide by pyridine or pentane adsorbed on Brønsted acid sites manifested in the IR spectra. The turnover frequencies of overall cracking and dehydrogenation on such sites are about 50 and 80 times higher than on isolated strong Brønsted acid sites. While pentane does not adsorb stronger, the site pair stabilizes cracking and dehydrogenation transition states mainly via increasing the activation entropy. This is interpreted as a later transition state for the cracking and dehydrogenation. The results suggest that controlled steaming can be used to enhance catalytic activity of zeolite Brønsted acid sites. [ABSTRACT FROM AUTHOR]

Cracking of light alkenes on solid acids and, in particular on zeolites, is ubiquitous in refineries. Different zeolite framework topologies in the catalysts used provide spatial constraints for reactants and products, leading to discrete reaction routes and varying selectivities. The conversion of alkenes in cracking proceeds either via monomolecular or bimolecular reaction pathways. In both routes, the reacting alkene adsorbs first on Brønsted acid sites forming the initial state (e.g., π-complex, carbenium ion or alkoxide), followed by β -cleavage in the carbenium transition state, generating a smaller alkene and a carbenium ion (monomolecular route) or forming a dimer by alkene addition that cracks subsequently (bimolecular route). The current state of understanding alkene sorption and cracking is critically discussed and four questions are formulated that may be the central questions from our perspective in further developing new generations of catalysts and processes for alkene conversion. [Display omitted] • State to art of the study of catalytic cracking of alkenes in zeolites. • Discussion of the role of zeolite pore size and acid strength of the catalysts. • Discussion of the adsorption state of alkenes on BAS. • Discussion of the elementary steps of alkene cracking on BAS for an in-depth understanding of the reaction mechanism. [ABSTRACT FROM AUTHOR]

Nano-sized zeolite Beta aggregates with interparticle mesopores were synthesized by the aerosol-assisted method. The amorphous powders in the Na 2 O–Al 2 O 3 −SiO 2 system, which was obtained from an aerosol drying process, were used as the precursor. The zeolite was crystallized in a high concentration system with high alkalinity. This can not only improve the autoclave utilization and the crystallization rate, but also reduce the amount of both template and waste water. The obtained zeolite Beta is spherical nano-aggregates with interparticle mesopores. It shows high BET surface area and good Al species distribution. The study on crystallization process indicated that the precursor gradually dissolved under the strong alkaline condition and the nano-sized zeolite crystals were formed in the liquid phase as time prolonged. Therefore, the transformation mechanism mainly followed the liquid-phase mechanism. More importantly, the zeolite beta synthesized by aerosol-assisted method exhibited good catalytic activity and stability in the cracking reaction of 1,3,5-TIPB. [ABSTRACT FROM AUTHOR]