Influence of active-site proximity in zeolites on Brønsted acid-catalyzed reactions at the microscopic and mesoscopic levels

Selective Light Hydrocarbon Production from CO2 Hydrogenation over Na/ZnFe2O4 and CHA-Zeolite Hybrid Catalysts

“…Additionally, the unmodified internal structure and CHA-morphology of SSZ-13 (Figures S8 and S9) imply that the enhanced ethylene-to-propane reaction seems to be a result of modified acidity due to the dealumination of the framework. Hence, the acid site modifications exhibited in Figures S12–S14 are presumed to be responsible for the enhanced ethylene-to-propane conversion rates in St-SSZ-13. − This underscores again that the strong acid sites in SSZ-13, a feature absent in SAPO-34, are pivotal for ethylene-to-propane conversion reaction.…”

The Overlooked Potential of Sulfated Zirconia: Reexamining Solid Superacidity Toward the Controlled Depolymerization of Polyolefins

“…As only weaker acids remain, this also causes a shift in selectivity without extensive engineering controls. ,, Higher temperatures and pressures will shift selectivity due to both reaction thermodynamics and the rate of coke build up. Long-term activity can be retained by pressurization with H 2 and doping with metal nanoparticles that remove coke from Lewis acid sites, but these nanoparticles also block access to the surface’s existing active sites. , Alternatively, selectivity on solid acids can be controlled by synthetically altering the expression of Brønsted or Lewis acidity, acid strength, and acid density. ,, For example, in isobutene alkylation with SZrO, cracking products are observed during reaction onset as a result of SZrO’s strong Lewis acid sites. However, the yield of cracking products rapidly decreases with time on stream, as coking that blocks acid site accessibility shifts selectivity to alkylation catalyzed by weaker acids.…”

“…Diffusion of hydrocarbons in zeolites is influenced by the pore dimensions as well as the density of sorption sites lining the pores. More polar surfaces facilitate rapid diffusion of reactants, reduce dwell time on active sites, and impede coking, driving remarkable catalytic activity through mass transfer even with relatively weak acidity. , At the melt temperature, the weak intermolecular forces that hold POs together are broken, and diffusion into polar pores becomes favorable. ,, While large-molecular-weight species like straight-chain POs diffuse rapidly due to abundant interactions with pore walls that promote continued feeding of the untangled polymer into the pore, low molecular weight species diffuse slowly. ,, Slower diffusion of cracked products facilitates secondary cracking events, coking, and a slower turnover rate as products impede the rapid diffusion of larger molecular weight reactants. To enable cracking of PE to small olefins by acidic zeolites, short pores and temperatures exceeding 300 °C are needed, as well as high H 2 pressures to break up coke. , In terms of pore diameter, narrow pores not only prohibit large intermediate and product formation but also interactions with branched or bulky reactants.…”

Paired Site Characterization in Aluminosilicate Zeolites through Limited Alkylammonium Stability

“…Two framework Al atoms proximal as next-nearest neighbors are considered to be paired, while a framework Al atom with no nearest neighbors other than Si is considered isolated (Scheme ). The influence of proximate Al pairs has been shown for methanol to aromatics conversion, , benzene alkylation, , alkane/alkene cracking, , olefin aromatization, and alcohol dehydration . For applications involving the inclusion of metal cations at proximate sites, reduced Ga species over Al pairs have been shown as active centers for alkane dehydrocyclization and dehydrogenation. , The relevance of Cu balanced by paired sites in zeolites has been investigated for low-temperature ammonia selective catalytic reduction, − as well as the impact of water vapor for the ion-mediated chemistries over proximate Al pairs .…”

“…Two framework Al atoms proximal as next-nearest neighbors are considered to be paired, while a framework Al atom with no nearest neighbors other than Si is considered isolated (Scheme 1). The influence of proximate Al pairs has been shown for methanol to aromatics conversion, 47,48 benzene alkylation, 49,50 ing, 51,52 olefin aromatization, 53 and alcohol dehydration. 54 For applications involving the inclusion of metal cations at proximate sites, reduced Ga species over Al pairs have been shown as active centers for alkane dehydrocyclization and dehydrogenation.…”