Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

The Brønsted acidity of proton-exchanged zeolites has historically led to the most impactful applications of these materials in heterogeneous catalysis, mainly in the fields of transformations of hydrocarbons and oxygenates. Unravelling the mechanisms at the atomic scale of these transformations has been the object of tremendous efforts in the last decades. Such investigations have extended our fundamental knowledge about the respective roles of acidity and confinement in the catalytic properties of proton exchanged zeolites. The emerging concepts are of general relevance at the crossroad of heterogeneous catalysis and molecular chemistry. In the present review, emphasis is given to molecular views on the mechanism of generic transformations catalyzed by Brønsted acid sites of zeolites, combining the information gained from advanced kinetic analysis, in situ, and operando spectroscopies, and quantum chemistry calculations. After reviewing the current knowledge on the nature of the Brønsted acid sites themselves, and the key parameters in catalysis by zeolites, a focus is made on reactions undergone by alkenes, alkanes, aromatic molecules, alcohols, and polyhydroxy molecules. Elementary events of C-C, C-H, and C-O bond breaking and formation are at the core of these reactions. Outlooks are given to take up the future challenges in the field, aiming at getting ever more accurate views on these mechanisms, and as the ultimate goal, to provide rational tools for the design of improved zeolite-based Brønsted acid catalysts.

Facile synthesis of nanosized mordenite and beta zeolites with improved catalytic performance: non-surfactant diquaternary ammonium compounds as structure-directing agents

Nanosized MOR and BEA zeolites were directly synthesized using simple diquaternary ammonium compounds. The nanosized zeolites show improved catalytic performance in Friedel–Crafts and n-alkane hydroconversion reactions.

Hierarchical Zeolites as Catalysts for Biodiesel Production from Waste Frying Oils to Overcome Mass Transfer Limitations

Hierarchical crystals with short diffusion path, conventional microcrystals and nanocrystals of ZSM-5 zeolites were used for biodiesel production from waste frying oils and were assessed for their catalytic activity in regard to their pore structure and acidic properties. Produced zeolites were characterized using XRD, nitrogen adsorption–desorption, SEM, TEM, X-ray fluorescence, and FTIR. Pore size effect on molecular diffusion limitation was assessed by Thiele modulus calculations and turnover frequencies (TOF) were used to discuss the correlation between acidic character and catalytic performance of the zeolites. Owing to the enhanced accessibility and mass transfer of triglycerides and free fatty acids to the elemental active zeolitic structure, the catalytic performance of nanosponge and nanosheet hierarchical zeolites was the highest. A maximum yield of 48.29% was reached for the transesterification of waste frying oils (WFOs) using HZSM-5 nanosheets at 12:1 methanol to WFOs molar ratio, 180 °C, 10 wt % catalyst loading, and 4 h reaction time. Although HZSM-5 nanosponges achieved high conversions, these more hydrophilic zeolites did not function according to their entire acidic strength in comparison to HZSM-5 nanosheets. NSh-HZSM5 catalytic performance was still high after 4 consecutive cycles as a result of the zeolite regeneration.

Synthesis of Hierarchical MOR-Type Zeolites with Improved Catalytic Properties

Hierarchical MOR-type zeolites were synthesized in the presence of hexadecyltrimethylammonium bromide (CTAB) as a porogen agent. XRD proved that the concentration of CTAB in the synthesis medium plays an essential role in forming pure hierarchical MOR-type material. Above a CTAB concentration of 0.04 mol·L-1, amorphous materials are observed. These hierarchical mordenite possess a higher porous volume compared to its counterpart conventional micrometer crystals. Nitrogen sorption showed the presence of mesoporosity for all mordenite samples synthesized in the presence of CTAB. The creation of mesopores due to the presence of CTAB in the synthesis medium does not occur at the expense of zeolite micropores. In addition, mesoporous volume and BET surface seem to increase upon the increase of CTAB concentration in the synthesis medium. The Si/Al ratio of the zeolite framework can be increased from 5.5 to 9.1 by halving the aluminum content present in the synthesis gel. These synthesized hierarchical MOR-type zeolites possess an improved catalytic activity for n-hexane cracking compared to large zeolite crystals obtained in the absence of CTAB.

Effect of zeolite morphology on charge separated states: ZSM-5-type nanocrystals, nanosheets and nanosponges

In the present work, we investigate the electron transfer occurring in the porous void of three MFI-type zeolite (ZSM-5) nanomaterials (nanocrystals, nanosheets and nanosponges) after adsorption and photoexcitation of t-stilbene (t-St). ZSM-5 nanosheets are constituted of lamellar stacking of several nanosheets (20-40 nm) where each nanosheet has a thickness of 2 nm. Nanosponges are composed of ZSM-5 nanocrystals (2-3 nm) separated by mesoporous holes of 5.8 nm facilitating the synthesis of hierachical materials. While the nanosheets show microporosity similar to that observed for the ZSM-5 nanocrystals, the absorption isotherms of the nanosponges show the existence of secondary micropores. After photoirradiation of t-St, UV-vis absorption spectroscopy shows the formation of charge separated states (radical cation and charge transfer complex) in the nanocrystals and in the nanosheets whereas no ionized species is detected in the nanosponges. The radical cation (RC) is stabilized in the nanosheets while it evolves very rapidly towards a Charge Transfer Complex (CTC) in the nanocrocrystals. The particular morphology of the nanosheets and nanosponges is put forward to explain this result since all host materials are of the MFI-type. To investigate ultra-short phenomena in the three nanomaterials, the UV-vis transient spectra were recorded between 2 and 450 μs after photoexcitation by nanosecond laser pulses. In the nanocrystals and nanosheets only the RC is detected whereas CTC formation is not observed. Photoexcitation of t-St in the nanosponges also leads to the formation of a RC but it recombines completely within 70 μs. This suggests the preferential location of t-St in the secondary micropores with pores larger than the micropores of the MFI-type framework and possibly in the mesopores of the nanosponges.

Study on the catalytic performance of different crystal morphologies of HZSM-5 zeolites for the production of biodiesel: a strategy to increase catalyst effectiveness

Strategies to improve molecular transport and accessibility of ZSM-5 zeolites were investigated for the model reaction of esterification of linoleic acid with methanol for biodiesel production.

Bifunctional catalysts for the hydroisomerization of n-alkanes: the effects of metal–acid balance and textural structure

The summary of recent advances reveals excellent potentials for the preparation of novel bifunctional catalysts with excellent catalytic performances for n-alkane hydroisomerization.

Hydroisomerization of n-hexadecane over a Pd–Ni2P/SAPO-31 bifunctional catalyst: synergistic effects of bimetallic active sites

The Pd-Ni2P/SAPO-31 catalyst shows excellent performance in n-hexadecane hydroisomerization due to the synergistic effects of Pd–Ni2P and improved metal/acid balance.

Enhanced n-dodecane hydroisomerization performance by tailoring acid sites on bifunctional Pt/ZSM-22 via alkaline treatment

Acidity controlled Pt/ZSM-22 catalysts for n-dodecane hydroisomerization were explored through a simple and efficient post-treatment with different NaOH concentrations, and then the resulting samples were characterized.

Synthesis of hierarchical ZSM-48 nano-zeolites

A specific gemini-type surfactant that acts as dual functions template to generate *MRE-type zeolitic micropores and mesopores simultaneously.

Formation domain of SDA-free Y faujasite small crystals

The ternary phase diagram shows that the domain for obtaining Y faujasite small crystals is very narrow.