Particular properties of the coke formed on nano-sponge *BEA zeolite during ethanol-to-hydrocarbons transformation

A reusable, impurity-tolerant and noble metal-free catalyst for hydrocracking of waste polyolefins

One-step conversion of low-purity polyolefins to value-added products without pretreatments represents a great opportunity for chemical recycling of waste plastics. However, additives, contaminants, and heteroatom-linking polymers tend to be incompatible with catalysts that break down polyolefins. Here, we disclose a reusable, noble metal-free and impurity-tolerant bifunctional catalyst, MoS_x-Hbeta, for hydroconversion of polyolefins into branched liquid alkanes under mild conditions. The catalyst works for a wide scope of polyolefins, including different kinds of high-molecular weight polyolefins, polyolefins mixed with various heteroatom-linking polymers, contaminated polyolefins, and postconsumer polyolefins with/without cleaning under 250°C and 20 to 30 bar H₂ in 6 to 12 hours. A 96% yield of small alkanes was successfully achieved even at a temperature as low as 180°C. These results demonstrate the great potentials of hydroconversion in practical use of waste plastics as a largely untapped carbon feedstock.

Roles of ethanol in coke formation and HZSM-5 deactivation during n-heptane catalytic cracking

Compared with n-heptane, ethanol easily interacted with the acid site of HZSM-5, promoting alkene and external coke formation.

Bioethanol Upgrading to Renewable Monomers Using Hierarchical Zeolites: Catalyst Preparation, Characterization, and Catalytic Studies

Bioethanol is one of the most promising renewable resources for the production of important monomers. To date, there have been various processes proposed for bioethanol conversion to renewable monomers. In this review, the catalytic bioethanol upgrading to various types of monomers using hierarchical zeolites as catalysts is illustrated, including the recent design and preparation of hierarchical zeolites for these catalytic processes. The characterizations of catalysts including textural properties, pore architectures, acidic properties, and active species are also exemplified. Moreover, the catalytic studies with various processes of monomer production from bioethanol including bioethanol dehydration, bioethanol to hydrocarbons, and bioethanol to butadiene are revealed in terms of catalytic activities and mechanistic studies. In addition, the future perspectives of these catalytic circumstances are proposed in both economic and sustainable development contexts.

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.

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.

The effect of nanostructures on high pressure intrusion–extrusion of water and electrolyte solutions in hierarchical nanoboxes of silicalite-1

A drastic effect of the presence of cavities on high pressure intrusion–extrusion of LiCl aqueous solution in silicalite-1 nanoboxes is observed.

Understanding the Role of Internal Diffusion Barriers in Pt/Beta Zeolite Catalyzed Isomerization of n-Heptane

Applications of zeolites in catalysis are plagued by strong diffusion resistance, which results from limitations to molecular transport in micropores, across external crystal surfaces, but also across internal interfaces. The first type of diffusion resistance is well understood, the second is receiving increasing attention, while the diffusion barriers at internal interfaces remain largely unclear. We take Pt/Beta catalyzed isomerization of n-heptane as the model system to explore the role of internal diffusion barriers in zeolite catalysis. The two as-synthesized Pt/Beta catalysts have an identical Pt loading, similar Beta particle size and acidity, but different internal structures. A Pt/Beta crystal with no observable internal interfaces can be 180 % higher in activity and 22 % higher in selectivity than its counterpart with numerous internal interfaces. This can only be attributed to the strong transport barriers across internal interfaces, as supported by directly comparing the apparent diffusivities of the two Beta samples.

Impact of the Framework Type on the Regeneration of Coked Zeolites by Non-Thermal Plasma in a Fixed Bed Dielectric Barrier Reactor

The formation of coke as a result of propene transformation at 623 K on zeolites results from a product shape selectivity mechanism of which the products are polyaromatic molecules, such as pyrene on MFI, anthracene on MOR, pyrene and coronene on FAU. Zeolite regeneration can be achieved by using non-thermal plasma (NTP), with decreased energy consumption, employing a fixed bed dielectric barrier reactor. The efficiency of this alternative regeneration process depends on the coke toxicity. On MFI and FAU (featuring three-dimensional 10 and 12 ring channel systems, respectively) coking occurs by poisoning the Brønsted acid sites; on MOR, (presenting a one-dimensional 12 ring channel system) pore blocking takes place, leading to higher coke toxicity. A complete coke removal is achieved on MFI and FAU zeolites using NTP within 3 h, while for MOR coke, removal proceeds slower and is incomplete after 3 h on stream. Hence, the efficiency of regeneration is impacted by the accessibility of active oxygenated species generated under plasma (e.g., O*, O2+) to coke molecules.

Performance of surfactant-modified *BEA-type zeolite nanosponges for the removal of nitrate in contaminated water: Effect of the external surface

Hierarchical *BEA-type nanosponges zeolite with a high external surface area (116 m².g^-1) and small crystal size, synthesized in the presence of a dual-porogenic organic compound, were modified with a cationic surfactant (HDTMA⁺Br^-: hexadecyltrimethyl ammonium bromide) in order to create a new anion exchanger system: the surfactant-modified zeolite nanosponges (SMZ_NS). For comparison, two other surfactant-modified *BEA-type zeolite materials, SMZ_MC and SMZ_NC, were obtained by modifying the synthesized conventional micron-size microcrytals and nanocrystals *BEA-type zeolite with HDTMA⁺Br^-, respectively. Textural and structural properties were determined for the three prepared materials using N₂ adsorption/desorption analysis, XRD, SEM, and TEM. Nitrate adsorption isotherms were drawn in a large concentration range [0.8-24.2 mmol.L^-1] and fitted with Langmuir isotherm model. The maximum nitrate removal capacity (1338 mmol.Kg^-1/83 mg.g^-1) was obtained for SMZ_NS material. This value is the highest ever observed for nitrate removal using surfactant-modified zeolite. The nitrate removal kinetics were fitted with the pseudo second-order model for both materials SMZ_NS and SMZ_NC.

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.

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.

Influence of LiCl aqueous solution concentration on the energetic performances of pure silica chabazite

Stored energy is increased by a factor of 5.5 for the “Si-CHA–20 M aqueous LiCl” system compared to the “Si-CHA–water” system.

Oxygen-containing coke species in zeolite-catalyzed conversion of methanol to hydrocarbons

Oxygen-containing coke species are identified during methanol-to-hydrocarbons reactions, and their influence on the deactivation of catalyst is investigated.

Catalytic cracking of n-pentane over CLD modified HZSM-5 zeolites

The possible role of CLD modification on H-ZSM5 zeolite in n-pentane cracking was proposed.

Impact of extreme downsizing of *BEA-type zeolite crystals on n-hexadecane hydroisomerization

The extreme diminution of the crystallite size increases accessibility of Brønsted acid sites but decreases their strength.