Mesoporosity In Zeolites Research Articles

Interplay Between Ni and Brønsted and Lewis Acid Sites in the Hydrodesulfurization of Dibenzothiophene.

Ni-MoS2/γ-Al2O3 catalysts are commonly used in hydrotreating to enhance fossil fuel quality. The extensive research on these catalysts reveals a gap in understanding the role of Ni, often underestimated as an inactive sulfide phase or just a MoS2 promoter. In this work, we focused on analyzing whether well-dispersed supported nickel nanoparticles can be active in the hydrodesulfurization of dibenzothiophene. We dispersed Ni by Strong Electrostatic Adsorption (SEA) method across four supports with different types of acidity: silica (~ neutral acidity), γ-Al2O3 (Lewis acidity), H+-Y zeolite, and microporous-mesoporous H+-Y zeolite (both with Brønsted-Lewis acidity). Our findings reveal that Ni is indeed active in dibenzothiophene hydrodesulfurization, even with alumina and silica as supports, although their catalytic activity declines abruptly in the first hours. Contrastingly, the acid nature of zeolites imparts sustained stability and performance, attributed to robust metal-support interactions. The efficacy of the SEA method and the added mesoporosity in zeolites further amplify catalytic efficiency. Overall, we demonstrate that Ni nanoparticles may perform as a hydrogenating metal in the same manner as noble metals such as Pt and Pd perform in hydrodesulfurization. We discuss some of the probable reasons for such performance and remark on the role of Ni in hydrotreatment.

Optimizing zeolitic hierarchical pore structure to boost the direct conversion of aromatics from syngas over the iron-based/zeolite bifunctional catalysts

The utilization of iron-based/ZSM-5 bifunctional catalysts for converting syngas to aromatics (STA) has garnered significant interest because of its cost-effectiveness. This study aims to elucidate the regulatory mechanism of zeolitic pore structure on the performance of the bifunctional catalyst in STA reaction. The relationship among the mesoporosity and mesoporous diameter of hierarchical ZSM-5 zeolite component, product distribution, and catalyst stability is investigated through a range of analyses techniques, including XRD, SEM, TG, Raman, N2-adsorption and desorption. The results demonstrate that as the mesoporosity of zeolite increased to 78.9%, the lifetime of the iron-based/ZSM-5 bifunctional catalyst extended beyond 96 h and maintained the aromatics selectivity over 40% at 2 MPa, 320 °C, and 3000 h−1. This outcome can be attributed to the role of high mesoporosity in restraining coke formation in zeolites. Moreover, the reduction in mesopore size from 11.1 to 5.8 nm results in an increased aromatics selectivity from 39.6% to 43.4%, and the fraction of light aromatics rose from 48.8% to 54.5%, indicating that small mesopore size can also expedite the production of aromatics, particularly light aromatics. It is thus concluded that the improvement of aromatics selectivity and stability of iron-based/ZSM-5 bifunctional catalysts in STA reaction can be achieved by increasing the mesoporosity and decreasing the mesopore size of the zeolite component.

Influence of catalyst pelletization on propane dehydrogenation over hierarchical MFI zeolite supported with platinum-yttrium nanoparticles

Platinum-yttrium (PtY) nanoparticles supported on a siliceous form of mesoporous MFI zeolite (MZ) were recently found to exhibit excellent catalytic performance in propane dehydrogenation (PDH). Herein, potential shaping of the PtY/MZ catalyst is investigated from the laboratory-synthesized powder form into industrially preferable pellets. The MZ zeolite powder was synthesized using multi-ammonium surfactants as a structure-directing agent and then pelletized in a hydraulic press by itself or with inorganic binders (i.e., pseudoboehmite, fumed silica, or hydrotalcite). As a part of the sequential pelletization process, pressing was followed by a high-temperature treatment (923 K). Addition of 20 wt% pseudoboehmite resulted in the highest mechanical strength without a significant loss of the initial zeolite mesoporosity. The pseudoboehmite-bound pellet also showed high catalytic activity and longevity in PDH application when simultaneously impregnated with 1 wt% Pt [Pt(NH3)4(NO3)2] and 1 wt% Y [Y(NO3)2·6H2O]. Notwithstanding the observed high propane conversion, the pseudoboehmite-bound catalyst produced an unsatisfactory result of low propylene selectivity (∼60%). Image analyses of HAADF-STEM EDS and solid-state 31P NMR spectroscopic data suggest that such findings could be attributed to Al migration from pseudoboehmite onto zeolite regime during the phase transformation of pseudoboehmite in the shaping process. Alternatively, the uses of non-acidic binders, such as fumed silica and hydrotalcite binders were also demonstrated. The silica-bound catalyst yielded high propane conversion with high propylene selectivity although mechanical strengths of the pellet were compromised to a moderate level.

Development and catalytic mechanism of a highly efficient Pt/Kβ catalyst for n-Heptane aromatization

Development of a high-performance naphtha aromatization catalyst has been paid many attentions as this reaction is generally employed to elevate the octane number of FCC gasoline and Fischer-Tropsch-synthesized oil. Here, a hierarchical porous Pt/Kβ catalyst is developed. It shows much higher activity, selectivity and stability than conventional Pt/KL. The catalytic performance of Pt/Kβ depends on Brϕnsted acidity and mesoporosity of zeolite β and nanoparticles (NPs) size and electronic state of Pt. The catalytic mechanism was investigated by combining TPSR, GC–MS, reaction kinetic studies and DFT calculations. Pt NPs catalyze dehydrogenations of alkanes/cycloalkenes to alkenes and dienes, while Brϕnsted acid sites facilitate alkene cyclization and ring-expansion of cycloalkenes despite that they can also dehydrogenate alkanes/alkenes through hydrogen transfer reactions. The n-heptane is mainly aromatized by dehydrogenating to 1-heptene and 2-heptene, 1,6- and 1,5-cyclizing into methylcyclohexane and ethyl-cyclopentane, and successively ring-expanding and/or dehydrogenating to toluene that may be further dealkylated to benzene.

Mesoporosity and Acidity Manipulation in ZSM‐23 and theirn‐Hexadecane Hydroisomerization Performance

AbstractThe enhanced mesoporosity in the networks is expected to offer a potential route to surmount mass‐transport constraints within one‐dimensional microporous structural zeolites. Highly crystalized ZSM‐23 zeolite was synthesized and subsequently post‐treated with alkali‐etching and acid‐etching, the effects of post‐treatments on the physical and chemical properties of the parent zeolite were investigated via XRD, N2physical adsorption, FE‐SEM, TEM,27Al MAS NMR, Py‐IR and NH3‐TPD. The catalytic activity of various Pt supported bifunctional catalysts was investigated vian‐hexadecane hydroisomerization. The post‐treated process not only increased the pore volume as well as the specific surface area, which efficiently improved the diffusion efficiency, but also changed the acidity and acid distribution of zeolite: alkali treatment could increase the content of total Brönsted acid while acid treatment exhibited an opposite trend. Acid treatment effectively promoted the density of weak Brönsted acid sites and suppressed the med‐strong Brönsted acid sites. With the manipulation of acidity and development of mesoporosity in zeolite, the formation of isomers and the suppression of cracked products in then‐hexadecane hydroisomerization were accomplished. The catalytic performance of catalysts showed that acid treatment could effectually promote the formation of isomeric products and inhibit cracking reactions. Especially, Pt/Z(1.0 HN) catalyst with suitable distribution of acid sites and textural properties, exhibited an outstandingn‐C16hydroisomerization performance with the highesti‐C16yield of 64 wt.% at 340 °C.

Facile Morphology and Porosity Regulation of Zeolite ZSM-5 Mesocrystals with Synergistically Enhanced Catalytic Activity and Shape Selectivity

The morphology and mesoporosity of zeolite are two vital properties to determine its performance in diverse applications involving adsorption and catalysis; while it remains a big challenge for the synthesis and regulation of zeolites with exceptional morphology/porosity only through inorganic-ions-based modification. Herein, by simply optimizing the alkali metal type (K+ or Na+), as well as alkali/water ratio and crystallization temperature, the zeolite ZSM-5 mesocrystals with diverse mesostructures are simply and controllably prepared via fine-tuning the crystallization mechanism in an organotemplate-free, ions-mediated seed-assisted system. Moreover, the impacts of these key parameters on the evolution of seed crystals, the development and assembly behavior of aluminosilicate species and the solution-phase process during zeolite crystallization are investigated by means of directional etching in NH4F or NaOH solutions. Except for the morphology/mesoporosity modulation, their physical and chemical properties, such as particle size, microporosity, Si/Al ratio and acidity, can be well maintained at a similar level. As such, the p/o-xylene adsorption and catalytic performance of o-xylene isomerization are used to exhaustively evaluate the synergistically enhanced catalytic activity and shape selectivity of the obtained products. This work demonstrates the possibility of effectively constructing novel zeolite mesostructures by simply altering parameters on simple ions-controlled crystallization and provides good models to inspect the impacts of mesoporosity or morphology on their catalytic performances.

Oxidative carbonylation of methane to acetic acid over micro-mesoporous rhodium-modified zeolites

It was first demonstrated that under liquid-phase conditions, an increase in the mesoporosity of ZSM-5 zeolite boosts a rhodium-containing catalyst activity in the oxidative carbonylation of methane to acetic acid (the acetic acid yield is 570 μmol g cat −1 ). To increase the mesoporosity of zeolite, two approaches were used: ultrasonic treatment and desilication of zeolite. The ultrasonic pretreatment leads to an increase in the acetic acid yield with a decrease in the total yield of oxygenates (methanol/acetic acid molar ratio is 0.2). On the contrary, preliminary desilication contributes to an increase in the total yield of oxygenates, mainly due to methanol (methanol/acetic acid molar ratio is 1.8). The combination of ultrasonic pretreatment and desilication of zeolite improves the total yield of oxygenates with an increase in the acetic acid yield. Such a change in the catalytic properties is attributed both to the acidity of the parent zeolite and the local atomic environment of rhodium. XAS spectroscopy showed that rhodium can be present on the zeolite surface either as nanoclusters giving rise to methanol or as isolated rhodium atoms, which promote the acetic acid formation. • Oxidative conversion of methane to acetic acid with the assistance of СО was studied. • Rhodium-modified microporous and micro-mesoporous zeolite catalysts were used. • Ultrasonic treatment of the zeolite leads to an increase in the yield of acetic acid. • Desilication of the zeolite leads to an increase in the yield of methanol. • The formation of acetic acid is facilitated by the monoatomic distribution of rhodium.

Towards a comprehensive understanding of mesoporosity in zeolite Y at the single particle level

The usage of complementary characterization techniques enabled closing the understanding gap concerning the location and structure of mesopores in zeolites.

Micelle Formation inside Zeolites: A Critical Step in Zeolite Surfactant-Templating Observed by Raman Microspectroscopy.

Micelle formation inside faujasite (FAU) zeolite, a critical step in the introduction of mesoporosity in zeolites by surfactant templating, has been confirmed by both 13C NMR and Raman spectroscopy. Here we provide unambiguous evidence of the incorporation of surfactant molecules inside zeolites during the first step of the surfactant-templating process followed by their self-assembly into micelles after hydrothermal treatment. The homogeneous presence of these micelles throughout zeolite crystals has been directly observed by Raman microspectroscopy, confirming the uniform incorporation of mesoporosity in zeolites by surfactant templating.

Surfactant‐Templated Zeolites: From Thermodynamics to Direct Observation

AbstractSurfactant‐templating is a versatile method to introduce tailored intracrystalline mesoporosity in zeolites. This method has been proven to be extremely effective to finely control both the amount of mesoporosity and the size of their mesopores, while maintaining the main features of the original zeolite, such as crystallinity, acidity, and hydrothermal stability. By building on the knowledge generated during the last 30 years in the field of amorphous mesoporous materials, the surfactant‐templating method has been successfully applied to develop intracrystalline mesoporosity into a variety of zeolite structures including FAU, BEA, MOR, LTL, and MFI. The improvement in both the textural and diffusional properties of the zeolite results in excellent catalytic performances in different industrially relevant processes, ranging from fluid catalytic cracking to the synthesis of active pharmaceutical ingredients. As a result of the excellent catalytic performances shown by the surfactant‐templated mesoporous Y zeolite, this is currently being used in several refineries, being the first industrial and large‐scale application of hierarchical zeolites. In this progress report, the literature regarding the surfactant‐templating method in zeolites is critically reviewed.

A review on the role of hierarchical zeolites in the production of transportation fuels through catalytic fast pyrolysis of biomass

Design of highly active catalysts plays a critical role in catalytic fast pyrolysis (CFP) of lignocellulosic biomass. Advanced catalysts can improve the deoxygenation rate of pyrolysis vapors and boost the production of fuel precursors. Zeolites are suitable frameworks for catalyzing pyrolysis vapors into species of transportation fuel value. However, their nano-sized structure limits the diffusion of reactants into pores and over active sites. Consequently, the aggregation of large molecules outside micropores leads to a massive coke formation, blocking pore channels, and thus, preventing the accessibility to acid sites. This could cause quick deactivation and instability of the catalyst, necessitating frequent catalyst regeneration and replenishment. Hierarchical micro/mesopore-structured zeolites are promising candidates to cope with the mentioned challenges. In addition to their adjusted pore structure and increased accessibility of acid sites, the formation of mesoporosity in zeolites provides an adequate room for the deposition of additional active phases such as metal nanoparticles, further boosting the catalytic activity. Different strategies used for preparing hierarchical zeolites can tremendously alter their attributes, and consequently affect product selectivity during CFP of biomass. Focusing on the precursors of transportation fuels (i.e., aromatic hydrocarbons and olefins), the present paper critically reviews the impacts of methods used for synthesizing hierarchical zeolite, on CFP of bio-based feedstocks. Moreover, the role of metal addition to hierarchical zeolites in biomass catalytic pyrolysis is also discussed briefly. Among the different synthesis techniques, desilication treatment using alkaline solutions is the most promising owing to its simplicity, high productivity, and scalability. In terms of product selectivity, the addition of Ga species to hierarchical zeolites can increase aromatic hydrocarbons while Ce incorporation can increase the yield of valuable oxygenates such as furan. Despite the advantages of mono-metallic hierarchical zeolites in producing cherished chemicals, future studies should scrutinize the influence of bi-metallic hierarchical zeolites in bio-based CFP processes.

USY zeolite mesoporosity probed by NMR cryoporometry

Abstract We report on the application of the NMR cryoporometry (C NMR) technique to probe the micro- and mesopore size distributions in industrially important USY zeolites (Si/Al ratio of 30, 15, and 6). The technique provides the clues to small mesopore structure range (20–110 A) created by various postsynthesis treatments of zeolites like ion exchange, steaming, acid leaching and mesoporization. C NMR is compared with the conventional Barrett-Joyner-Halenda (BJH) model to construct the pore size distributions. Both methods account for the mesopore volume, and under some approximations the micropore volume. The C NMR approach has been effectively applied to the study of extruded samples. It is concluded that the significant increase of mesoporosity in extrudates is more due to the binder possessing this porosity range. The effect of mesoporization when comparing the extrudates of parent sample and mesoporized zeolites is less pronounce than in the initial porous material as observed by both C NMR and BJH pore size distributions. Therefore, an important transport amelioration in extrudates is observed apparently due to alumina playing a role of binder.

Defect-assisted mesopore formation during Y zeolite dealumination: The types of defect matter

Abstract The present work reports an easy and low-cost way of industrial relevance for post-synthetic upgrading of zeolite Y, achieved by the rational application of chemical induced framework defects to engineering zeolite mesoporosity. Si-defects and Al-defects, formed as a result of the selective removal of framework Si and Al atoms by alkaline treatment and oxalic acid leaching, respectively, are introduced into the framework of an as-synthesized zeolite Y. Such defect-containing zeolites were ion-exchanged, steamed and acid washed, and the final products were subjected to a comprehensive physico-chemical characterization. The results show that Al-defects, due to its inclination to condensation during the heating stage before steaming, hamper the migration and coalescence of steaming induced framework defects, hence suppressing the propagation of mesoporosity during steaming. In contrast, the presence of Si-defects favors the formation of mesoporosity in zeolite Y framework during dealumination. Further, we demonstrate that the introduction of Si-defects can be incorporated into the zeolite synthesis process without compromising the ability to enhance zeolite mesoporosity. The derived catalysts show higher gasoline selectivity than the base catalysts in the catalytic cracking of a wax oil from Daqing.

Core–Shell Metal Zeolite Composite Catalysts for In Situ Processing of Fischer–Tropsch Hydrocarbons to Gasoline Type Fuels

Fischer–Tropsch synthesis has two main challenges related to direct production of gasoline fuels. First, the chain length distribution of the products follows a broad and unselective Anderson–Schulz–Flory distribution. Second, mostly linear hydrocarbons are formed in the Fischer–Tropsch reaction, thus requiring isomerization while manufacturing gasoline fuels. The present paper addresses a synthetic strategy for the preparation of hierarchical metal and zeolite nanocomposite catalysts for direct synthesis of iso-paraffins from syngas. The nanocomposites are synthesized in three steps. In the first step, the parent (core) zeolite is etched with an ammonium fluoride solution. The etching creates small mesopores inside the zeolite crystals. In the second step, the Ru nanoparticles prepared using water-in-oil microemulsion are deposited in the mesopores of the zeolite. In the third step, a zeolite shell of MFI-type zeolites (silicalite-1 or ZSM-5) is grown on the parent zeolite crystals coating both the etched surface and metallic nanoparticles. Thus, the metal nanoparticles become entirely encapsulated inside the zeolite matrix. Most important parameters such as ruthenium content, zeolite mesoporosity, and more particularly, the acidity of the catalyst shell, which affect the catalytic performance of the synthesized nanocomposite materials in low-temperature Fischer–Tropsch synthesis were identified in this work. The higher relative amount of iso-paraffins was observed on the catalysts containing a shell of ZSM-5. The proximity between metal and acid sites in the zeolite shell of the nanocomposite catalysts is a crucial parameter for the design of efficient metal zeolite bifunctional catalysts for selective synthesis of gasoline-type fuels via Fischer–Tropsch synthesis, while the acidity of the catalyst core has only a limited impact on the catalytic performance.

Testing the limits of zeolite structural flexibility: ultrafast introduction of mesoporosity in zeolites

Ultrafast Surfactant-Templating (UST) achieved in just 1 minute indicates the high structural flexibility of zeolites for mesopore formation.

Facile synthesis of mesoporous zeolite Y using seed gel and amphiphilic organosilane

Abstract Zeolite Y is widely used as an industrial catalyst for catalytic cracking of heavy feedstocks to lighter olefins. However, incorporation of uniform mesoporosity in zeolite Y without compromising the microporous characteristics is required to improve the catalyst efficiency and minimize catalyst deactivation. Here, we report a facile synthesis method for mesoporous zeolite Y (MPY sg ) using seed gel and amphiphilic organosilane, that results in mesoporous zeolite showing a narrow mesopore size distribution (3–4 nm) with high external surface area (110 m 2 g -1 ) and high micropore surface area (654 m 2 g -1 ). During the crystallization of MPY sg , the organosilane acted as a mesopore-generating agent, and the seed gel provided secondary nuclei that created a uniform mesoporous structure in the zeolite Y nanocrystals. Analysis of aluminosilicate precursors collected during the crystallization revealed that the crystallization process starts with a long nucleation period, followed by rapid crystal growth. Due to the highly accessible strong acid sites present on its external surface, MPY sg shows catalytic activity in the decalin cracking reaction higher than that of conventional microporous zeolite Y.

Facile Fabrication and Morphology Regulation of Crossed MFI Zeolite with Improved Performance on LDPE Cracking

Morphology and mesoporosity of zeolite are two vital properties to determine its performance in diverse applications involving catalysis, adsorption, and separation; therefore, synthesis of zeolite...

Quantification of rhenium oxide dispersion on zeolite: Effect of zeolite acidity and mesoporosity

The anchoring nature and spatial distribution of rhenium oxide (ReOx) supported on zeolites (ReOx/zeolite) were determined as a function of the support acidity and mesoporosity. The ReOx/zeolite samples were characterized collectively using Argon isotherm, XRD, Raman, XAS, DRIFTS, and organic chemical titrations. The results show that the ReOx species formed the isolated distorted tetrahedral ReO4− structure. On the zeolite support, ReOx species anchored predominantly on Brønsted acid sites (Si-OH+-Al) enclosed in micropores (internal acid sites), by replacing the protons in Si-OH+-Al sites at a H+/Re ratio of ∼1.1 in zeolites with high acidity. The decrease in zeolite acidity led to the anchoring of ReOx species onto both Si-OH+-Al and silanol group (Si-OH). The increase in zeolite mesoporosity migrated the ReOx species onto both internal acid sites and those on the external surface and mesopores (external acid sites), as well as Si-OH groups. The implication of distribution of ReOx in zeolite was explored by direct non-oxidative methane conversion reaction.

Hierarchical Low Si/Al Ratio Ferrierite Zeolite by Sequential Postsynthesis Treatment: Catalytic Assessment in Dehydration Reaction of Methanol

In contrast to high silica zeolites, it is difficult to obtain mesoporosity in zeolites with low Si/Al ratio (e.g., <20) via conventional NaOH-based treatment, making the obtainment of hierarchical zeolites with high acidity a challenging target. In this paper, we report the preparation of hierarchical FER-type zeolite at low Si/Al molar ratio (about 10) by postsynthesis etching involving a sequence of three treatments with NaAlO2, HCl, and NaOH solutions and investigate the effect of both NaAlO2 solution concentration and time of treatment on the textural properties. The obtained materials exhibit a mesoporous volume higher than the parent ferrierite with no significant effect on the sample acidity. The catalytic activity of some samples was investigated in vapour-phase methanol dehydration to dimethyl ether, revealing the superiority of hierarchical zeolites in terms of methanol conversion, although the presence of mesopores causes formation of light hydrocarbons at high temperatures.

Direct Production of Iso‐Paraffins from Syngas over Hierarchical Cobalt‐ZSM‐5 Nanocomposites Synthetized by using Carbon Nanotubes as Sacrificial Templates

AbstractThis paper focuses on the effect of synthesis conditions on the structure of cobalt‐ZSM‐5 zeolites and the localization, dispersion, and reducibility of cobalt species and their catalytic performance in Fischer–Tropsch synthesis. The zeolite catalysts were prepared by using carbon nanotubes as sacrificial templates. Addition of pristine carbon nanotubes resulted only in a slight increase in the zeolite mesoporosity, whereas the presence of carbon nanotubes impregnated with cobalt drastically modified the zeolite morphology and texture. The zeolite mesoporous volume increased by 3–4 times. The catalysts prepared by using cobalt‐impregnated carbon nanotubes as templates showed 5–8 times higher Fischer–Tropsch reaction rates normalized by reducible cobalt compared with the catalysts prepared by conventional impregnation. Higher selectivity to isomerized branched hydrocarbons on these zeolites was attributed to the enhanced diffusion favoring removal of isomerized products and avoiding their cracking.