Intracrystalline Mesoporosity Research Articles

Micro-mesoporous ferrierite obtained using the cationic polymer Luviquat and post-synthesis treatment

This study investigated for the first time the synthesis of ferrierite zeolite through the vapor-phase transport method using the cationic polymer Luviquat as a mesopores-generating agent, followed by desilication treatment. The effect of ferrierite's textural and acid properties was then evaluated in the catalytic cracking of ultra-high molecular weight polyethylene (UHMWPE). The synthesized zeolites showed a crystallinity of 91%–100% and similar Si/Al ratios. After the desilication, there was a decrease in these values along with an increase in acidity, attributable to the extraction of structural silicon. The strong interaction between the polymer and the zeolite precursor during the self-assembly process led to the formation of intracrystalline mesoporosity. However, an internal diffusion limit and/or steric hindrance hinders the complete incorporation of the polymer into the ferrierite structure, especially at high polymer concentrations. Thus, the optimal composition was 40% Luviquat, which resulted in twice the volume of mesopores compared to the sample prepared without the polymer. The desilication of the samples synthesized with the Luviquat led to a substantial increase in mesoporosity, ranging from 67% to 183%, relative to microporous ferrierite. The introduction of mesoporosity and the modification of acid properties in ferrierite contributed to a reduction in the temperature for the cracking of UHMWPE, between 66 and 93 °C compared to the pure polymer. However, in all cases, there was an increase in the average apparent activation energy in relation to desilicated microporous ferrierite zeolite, possibly due to changes in product distribution.

Generation of mesoporosity in crystals of zeolite omega by post-synthesis treatments with surfactants

The large-pore zeolite omega (SiAl = 3.1) has been treated with solutions of the surfactant hexadecyltrimethylammonium (CTA) in aqueous ammonia in order to generate intracrystalline mesoporosity. For this purpose, the as-made sample was calcined, transformed in the acid form by ion exchange with ammonium chloride solution followed by calcination, and the resulting acid form subsequently treated with steam at 620 ºC to partially dealuminate the framework. The samples were characterized by XRD, N2 adsorption, TGA, SEM, 29Si and 27Al MAS NMR, pyridine adsorption for acidity determination, and tested in the isomerization/disproportionation of m-xylene. It has been found that partial dealumination of the framework (Si/Al ∼ 7.3) followed by mild acid leaching (HCl = 0.3 M) to remove only the extraframework aluminium species is required for CTA to generate intracrystalline mesopores with a narrow pore diameter of 4 nm. The CTA treatment is ineffective when applied to the acid form (Si/Al ∼ 5.7) or the steamed sample before acid leaching. The concentration of strong acid sites and the average acid strength of the CTA samples are lower than those of the steamed/acid leached sample, which suggests the presence of weak acid sites located at the mesopores. The mesopores greatly enhanced the catalytic activity in the m-xylene conversion by decreasing diffusional constraints. However, the selectivity to the bulky trimethylbenzenes is not improved by the CTA treatment, which suggests that the reaction mainly takes place at the strong acid sites located at the micropores.

Investigation on the catalysis performance of metal-doped zeolites on ring-opening acidolysis reaction

Metal-incorporated zeolites as heterogeneous Lewis acid were applied in the ring-opening acidolysis of ethylene oxide with acrylic acid to prepare 2-hydroxyethyl acrylate (HEA). The catalysis characteristics, including Lewis acid strength and internal mass transfer resistance, were systematically investigated to facilitate the rational design of zeolite catalysts. Through modifying the Lewis acid strength of zeolites via different metals (Sn, Zr and Ti) incorporation and the internal mass transfer resistance of zeolites from the particle size, pore diameter or pore structure, it was found that the increased Lewis acidity can accelerate both the main and side reactions, resulting in the high activity but low selectivity of zeolite catalysts; the enhanced internal diffusion was the key to limiting subsequent cascade alcoholysis side reactions as well as the decline of catalytic performance. Based on these findings, Zr-containing zeolites featured with moderate Lewis acidity and uniform intracrystalline mesoporosity were designed and prepared, presenting excellent catalysis and recycling performance. These results demonstrated the great potential of the developed zeolite catalysts for the green synthesis of HEA as well as the similar ring-opening acidolysis of epoxides.

Hierarchical basic zeolites allow for the solvent-free synthesis of chromene derivatives

The synthesis of 2-amino-chromene with excellent yields (82%) was achieved by utilizing basic hierarchical zeolites prepared by surfactant-templating as catalysts in a multicomponent reaction under solvent-free conditions. The use of these hierarchical zeolites more than doubles the activity of their microporous counterparts. These results highlight the superior diffusivity and performance of these basic hierarchical zeolites, which can be attributed to their well-developed intracrystalline mesoporosity, highly accessible and strong basic sites.

A comprehensive study of product distributions and coke deposition during catalytic cracking of vacuum gas oil over hierarchical zeolites

In this study, zeolites (Z) were used as catalysts in the cracking of a Colombian vacuum gas oil (VGO), with a focus on product distribution and coke deposition. The catalytic tests were carried out in a MAT-type reactor under typical conditions. The zeolites were subjected to alkaline treatment with NaOH at concentrations ranging from 0.05 to 0.4 mol/L, resulting in the creation of several samples (Z-0.05, Z-0.10, Z-0.20, Z-0.30 and Z-0.40) that were then hydrothermally stabilized (Z-0.05-M, Z-0.10-M, Z-0.20-M, Z-0.30-M and Z-0.40-M) to increase mesoporosity and reduced crystallinity. The increase in mesoporosity was accompanied by an improvement in acidity. Despite Z-0.30-M having higher acidity, Z-0.00-M and Z-0.10-M exhibited the highest activity due to their high crystallinity and microporosity, yielding the highest gas yields. Gasoline was the main product, with maximum yields exceeding 30%. Z-0.20-M produced more aromatic and olefin compounds than the others, resulting in higher quality gasoline. Coke formation followed the trend: Z-0.00-M < Z-0.10-M < Z-0.20-M < Z-0.30-M. The higher intracrystalline mesoporosity in the zeolites favored the formation of a more condensed coke.

Toward rational design of narrowly-distributed mesopore on ZSM-22 zeolite for enhanced Pt dispersion and n-alkane isomerization performance

In present work, the narrowly-distributed intracrystalline mesopore was rationally designed and developed on ZSM-22 zeolites via a controllable desilication route. The intracrystalline mesoporosity with a diameter distribution in the range of 2.9, 4.3 and 9.3 nm were precisely exposed on ZSM-22 zeolite due to the stability effect of surfactant on the NaOH-leached structure. Importantly, the controllable desilication not only exposed Brønsted acid sites and preserved the original framework but also enhanced the dispersion and aggregation resistance of Pt particles in bifunctional catalysts through providing confined mesopore space and defect sites for the anchoring of Pt sites. All desilicated catalysts exhibited higher isomerization yield in n -dodecane isomerization in comparison to the purely microporous Pt/ZSM-22 catalyst. Moreover, compared to the conventionally desilicated catalyst with broadly distributed mesopore, well-balanced bifunctional behavior and much higher selectivity of multi-branched isomers were achieved on the controllably-desilicated catalyst with optimally narrow mesopore. Evidently, the highly dispersed and aggregation resistant Pt particles in the narrowly-distributed mesopores satisfied the high C Pt /C H+ ratios and high degree of intimacy between the hydrogenation and acid functions. Therefore, this work provided a detailed guidance for the rational design and fabrication of efficient catalysts for n -alkane isomerization.

Cooperative Surface Passivation and Hierarchical Structuring of Zeolite Beta Catalysts.

We report a method to prepare core-shell zeolite beta (*BEA) with an aluminous core and an epitaxial Si-rich shell. This method capitalizes on the inherent defects in *BEA crystals to simultaneously passivate acid sites on external surfaces and increase intracrystalline mesoporosity through facile post-hydrothermal synthesis modification in alkaline media. This process creates more hydrophobic materials by reducing silanol defects and enriching the shell in silica via a combination of dealumination and the relocation of silica from the core to the shell during intracrystalline mesopore formation. The catalytic consequences of *BEA core-shells relative to conventional analogues were tested using the biomass conversion of levulinic acid and n-butanol to n-butyl levulinate as a benchmark reaction. Our findings reveal that siliceous shells and intracrystalline mesopores synergistically enhance the performance of *BEA catalysts.

Cooperative Surface Passivation and Hierarchical Structuring of Zeolite Beta Catalysts

AbstractWe report a method to prepare core–shell zeolite beta (*BEA) with an aluminous core and an epitaxial Si‐rich shell. This method capitalizes on the inherent defects in *BEA crystals to simultaneously passivate acid sites on external surfaces and increase intracrystalline mesoporosity through facile post‐hydrothermal synthesis modification in alkaline media. This process creates more hydrophobic materials by reducing silanol defects and enriching the shell in silica via a combination of dealumination and the relocation of silica from the core to the shell during intracrystalline mesopore formation. The catalytic consequences of *BEA core–shells relative to conventional analogues were tested using the biomass conversion of levulinic acid and n‐butanol to n‐butyl levulinate as a benchmark reaction. Our findings reveal that siliceous shells and intracrystalline mesopores synergistically enhance the performance of *BEA catalysts.

Preparation of β zeolite with intracrystalline mesoporosity via surfactant -templating strategy and its application in ethanol-acetaldehyde to butadiene

Abstract The conversion of ethanol to butadiene catalyzed by modified β zeolites has attracted wide attention. The construction of intracrystalline mesoporosity in β zeolite is a challenging way to improve the catalytic activity. In this work, a surfactant-template strategy was used to construct intracrystalline mesoporosity successfully in the alkali treatment of β zeolite using long-chain quaternary ammonium salt with different alkyl chain length (C10~C16) as the template. The intracrystalline pore size can be adjusted by varying the surfactant alkyl chain length (C10~C16) (1.9–3.4 nm). To get more information about this process, CTAB was chosen as the template in further investigations. The mesoporosity can be generated within 1 h. The BET surface area can reach up to 600 m2/g while the pore volume can reach up to 0.568 cm3/g, which are 1.23 and 1.5 times that of the parent zeolite, respectively. The template action of CTA+ is essential for forming mesoporosity, and CTA+ interacts with the zeolite during the alkali treatment process, protects the zeolite framework from being etched, and does not form a separated mesoporous phase. To demonstrate the effect of intracrystalline mesoporosity on the catalytic performance, Zr-β zeolite was prepared and used as the catalyst for ethanol-acetaldehyde conversion. The hierarchical Zr-β zeolite was prepared using a dealumination-metallization strategy, and both the zeolite crystallinity and the mesoporosity were preserved in this process. The total conversion increased by 10%, while the BD selectivity was well maintained. The catalytic performance showed that the introduction of mesoporosity effectively mitigated decreasing conversion while preserving BD selectivity.

Ultrafast surfactant-templating of *BEA zeolite: An efficient catalyst for the cracking of polyethylene pyrolysis vapours

Introducing intracrystalline mesoporosity to zeolites is an effective way to reduce their diffusion limitations and enhancing their catalytic performance in reactions where large molecules are involved. The emergence of the so-called surfactant-templating technique has enriched the toolbox of methods to prepare mesoporous zeolites, in which amphiphilic molecules are used to assist in and direct the generation of mesopores in a pre-existing zeolite structure under alkaline conditions. We herein present ultrafast surfactant-templating of *BEA zeolite, which is achieved in just a few minutes with a tubular reactor that features fast heating. Two *BEA zeolites of the same Si/Al ratio, but differing in crystal size, are employed to demonstrate the concept. Tuning the interplay between the surfactant concentration and the alkalinity leads to the ultrafast generation of mesoporosity in the *BEA zeolite with the larger crystal size (BEA (L)) in 5 min; while it fails to generate uniform mesopores to the *BEA zeolite with the smaller crystal size (BEA (S)) under otherwise identical conditions. The ultrafast surfactant-templating of *BEA (S) is achieved only when it is partially dealuminated through a pre-treatment with acid. Our results provide new insights on the synergetic effect between the surfactant and the base that governs the surfactant-templating process and on how to produce intracrystalline mesoporosity with narrow pore size distribution under ultrafast conditions (2 min). The resultant mesoporous *BEA zeolites have been tested in the catalytic cracking of the vapours produced by pyrolyzing low-density polyethylene, a promising reaction for converting waste plastics into fuels. Improved selectivity towards aromatic hydrocarbons and light olefins is observed as a result of enhanced accessibility and availability of active sites, validating the superiority of the surfactant-templated *BEA zeolites.

A Comparative Study on Mesoporous Y Zeolites Prepared by Hard-Templating and Post-Synthetic Treatment Methods

Mesoporous Y zeolites were prepared comparatively using the bottom-up templating (with carbon nanotubes, CNTs, and nanocrystalline cellulose, NCC, as the templates) and the top-down post-synthetic treatment (with the sequential chemical and alkaline treatment) methods and characterised comprehensively using various techniques. The relevant findings show that the mesoporous Y zeolites with the intercrystal mesoporosity were prepared by the templating methods, and the NCC is the more effective, economic and sustainable hard template than the CNTs for promoting the formation of intercrystal mesoporosity. While for the post-synthetic strategy, the microwave (MW)-assisted chemical treatment (using ethylenediaminetetraacetic acid, EDTA, as the chemical agent for dealumination) was much more effective than the conventional hydrothermal (HT) treatment for introducing intracrystalline mesoporosity under the comparable condition, e.g. the external surface area: 287 m2 g−1 by 1 min MW treatment vs. 205 m2 g−1 by 6 h HT treatment. The prepared mesoporous Y zeolites, along with the parent Y, were assessed using the catalytic cracking of 1,3,5 triisopriopylbenzene (TiPBz). It was found that the well-developed intracrystalline mesoporosity promoted by the effective MW-assisted post-synthesis method is highly beneficial to the catalysis involving the bulky molecules, and accordingly showed the comparatively best catalytic performance (regarding the conversion and selectivity) among all zeolites under study in this work.

Hierarchy in zeolite catalysis: The influence of enhanced mesoporosity on the synthesis of renewable fuels and bio-based platform chemicals

Faujasite (FAU), ZSM-5 (MFI), beta (BEA) and mordenite (MOR) zeolites were admitted to a variety of chemical treatments accompanied by surfactant templating strategy, aiming to introduce the intracrystalline mesoporosity effectively. The resulting materials were tested as solid acid catalysts for esterification of the oleic acid as a common model impurities found in bio-oil feedstoks. It was found that the esterification of oleic acid can be enhanced by the presence of strong acid sites in zeolites and their improved accessibility. Overall, mesostructured FAU zeolite demonstrated an improved catalytic performance as a result of increasing accessibility of the zeolite active sites.

Synthesis of hierarchical zeolite templated carbons

The achievement of hierarchical zeolite templated carbons (ZTCs) is firstly reported through the use of mesoporous zeolites as sacrificial scaffolds. The achieved hierarchical ZTCs feature both tailored microporosity and high degree of mesoporosity and allow hence to bridge the gap between classical ZTCs and CMK-like materials. Mesoporosity from steamed zeolites was meticulously transcribed to the ZTC particles. The use of zeolites featuring surfactant-templated mesoporosity allowed for achieving a ZTC with a very high mesopore volume (0.85 cm3 g−1). The rearrangement of surfactant-templated mesoporosity could be evidenced through nitrogen physisorption and transmission electron microscopy on ultramicrotomed samples. The rearrangement of mesoporosity during ZTC formation has been found to impact coherent crystal size and the lattice strain of the zeolite template. The combination of data from X-ray diffraction and nitrogen physisorption allowed further to reveal that the thickness of the ZTC skeleton is impacted by the degree of intracrystalline mesoporosity in the template zeolite. Through comparing the textural characteristics of the zeolite templates and of the achieved hierarchical ZTCs, important evidence was found that the surfactant-templating of USY zeolite leads to the development of a secondary amorphous phase and that the reduction of the microporous volume during this process is merely apparent.

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.

The effect of mesoporosity in the reaction of vinyl acetate and isoamyl alcohol catalyzed by H-ZSM-5 zeolites based on natural Perlite

Diverse ZSM-5 zeolites having a dual porosity and different acidic properties have been prepared using natural Perlite as source of SiO2 and Al2O3. Two traditional methodologies were selected to generate hierarchical porosity in the parent ZSM-5: (1) soft templating approach to produce an ordered micro-mesoporous material (H-ZSM-5/MCM-41) and (2) different duration of alkaline treatment for the generation of intra-crystalline mesoporosity (H-ZSM-5-MS-x, x = 30, 60 and 90). Surface acidity and accessibility indexes were determined by adsorption of pyridine and 2,6-lutidine. The catalytic activity was evaluated in the reaction of vinyl acetate and isoamyl alcohol which produces isoamyl acetate and acetaldehyde diisoamyl acetal by transesterification and ketalization reactions, respectively. Isoamyl acetate has been selectively produced as a consequence of an adequate combination of Brønsted acidity and accessibility, thus H-ZSM-5/MCM-41 and H-ZSM-5-MS-60 presented enhanced catalytic activity and selectivity compared to the parent H-ZSM-5.

Development of hierarchical and phosphorous-modified HZSM-5 zeolites by sequential alkaline/acid treatments and their catalytic performances for methanol-to-olefins

An efficient and simple method was proposed to synthesize hierarchical and phosphorous-modified HZSM-5 zeolites by sequential alkaline and acid treatment. In the first NaOH alkaline treatment, intracrystalline mesoporosity was introduced into the zeolites by the crystal defect formation and growth of crystal defects due to desilication. The excessive defect growth caused amorphization of the zeolites, dramatically decreasing their surface area and micropore volume. The aluminum-related debris resulting from the desilication deposited on the extra-framework positions and had a dramatic influence on the crystallinity and microporosity of the zeolites. In the subsequent H3PO4 treatment for the alkaline-treated HZSM-5, the deposited extra-framework aluminum species were selectively removed, thus increasing the micropore volume and surface area of the zeolites. Along with the dealumination, the phosphorous species were introduced into the framework to occupy the silicon sites, leading to a crystalline structure repair. The post-treatments could modify the structural and acidic properties of the HZSM-5 zeolite. Catalytic tests showed that the hierarchical and phosphorous-modified HZSM-5 zeolite exhibited higher methanol conversion and propylene selectivity in methanol-to-olefins reactions over a longer period compared with the parent HZSM-5.

Tuning the Brønsted and Lewis acid nature in HZSM-5 zeolites by the generation of intracrystalline mesoporosity—Catalytic behavior for the acylation of anisole

Diffusion and acidity of molecules are key parameters in organic synthesis; therefore, highly stable mesoporous HZSM-5 zeolites with ratios of Brønsted (CB) to Lewis (CL) acid sites tuned by alkaline desilication were employed for Friedel–Crafts acylation of anisole with acetic anhydride. Controlled zeolite desilication (Si/Al = 12 and 23) proceeded by varying the NaOH concentration, temperature, and treatment time. SEM/EDS, XRD, N2 physisorption, NH3-TPD, DRIFTS, and 29Si-MAS-NMR data were used to correlate the generated intracrystalline mesoporosity with the new textural, acidic, and catalytic properties. It was evidenced that desilication allowed the CB/CL ratio to vary from 21 to 2.3. During anisole acylation, the specific activity to 4-methoxyacetophenone (4-MAP), on a desilicated zeolite that preserved most of the original microstructure, markedly increased with increasing mesoporosity, even a decrease of the CB/CL ratio had occurred. The strong changes in the acid nature and textural properties did not change the anisole acylation acid mechanism and improved the internal diffusion of reactants and 4-MAP to or from the active sites, respectively. Thus, appropriate levels of mesoporosity with consequent acidity changes may be designed through controlled zeolite desilication, with the resultant hierarchical zeolite capable of being applied as a catalyst to a particular organic reaction mechanism.

Carbon-Templated Mesopores in HZSM-5 Zeolites: Effect on Cyclohexane Cracking

Intracrystalline mesoporosity in HZSM-5 zeolites was generated using carbon nanoparticles as template. The activity of the resulting zeolites was evaluated for the ring opening and cracking of cyclohexane. The zeolites with mesopores showed smaller particle sizes and a decrease of the amount of medium and strong acid sites, which involved a decrease of Bronsted nature. Despite their lower acidity, the mesoporous zeolites exhibited higher activity. The mesoporous zeolite with the lowest acidity also showed the lowest activity, evidencing that generation of mesoporosity must be carefully tuned in order to preserve the intrinsic zeolitic properties. Increase of the mesoporosity led to increased selectivity towards more valuable light olefins, consequently decreasing the selectivity towards C3 and C4 saturated hydrocarbons. Smaller particle sizes and mesopores improved the products diffusion, decreasing the contact time and the oligomerization and bimolecular hydrogen transfer reactions, which were also disfavored by the decrease of the acid sites concentration.

Intra-crystalline mesoporous SAPO-11 prepared by a grinding synthesis method as FCC promoters to increase iso-paraffin of gasoline

Abstract A hierarchical SAPO-11 was prepared by a grinding synthesis method (GSM) and added as a promoter to increase the iso-paraffin yield of gasoline in fluid catalytic cracking (FCC) reaction. Compared to the classical hydrothermal method, the GSM leads to the creation of intra-crystalline mesopores with the assistance of a surfactant (Cetyltrimethyl Ammonium Bromide, CTAB). CTAB acts as a structure directing agent and as a hard template during the crystallization of SAPO-11. CTAB micelles formed the intra-crystalline mesopores in SAPO-11 crystals. The intra-crystalline mesoporosity of the SAPO-11 is preserved at 800 °C under steaming, which renders the material as an appropriate component of the FCC catalyst. Besides, the presence of mesopores provides isomerization active sites and accelerates the desorption of iso-paraffin. The mesoporous SAPO-11 showed a higher selectivity for iso-paraffin and the yield of iso-paraffin increased by 2%.

Development of intracrystalline mesoporosity in NH4HF2-etched NaY zeolites by surfactant-templating and its effect on toluene adsorption

Volatile organic compounds have seriously affected the natural environment and human health, and adsorption is one of the most effective method for controlling VOCs pollution. In this work, the NH4HF2-etched NaY-s-x zeolites with intracrystalline mesoporosity are obtained via the surfactant-templating process in weak base NH4OH solution. The morphology, microstructure, and toluene adsorption property of as-prepared hierarchical Y zeolites are systematically characterized. It is found that the structural and textural properties of NH4HF2-etched NaY samples are significantly changed by the simple surfactant-templated approach and the large-sized mesopores inside NH4HF2-etched NaY sample disappear to form the surfactant-templated mesoporosity with a size of ~4 nm. It is demonstrated that the intracrystalline mesoporosity obviously improves the toluene molecular adsorption properties of zeolite samples. Moreover, the mechanism of the surfactant-templating process is elucidated.