Intracrystalline Mesoporosity Research Articles

Hierarchical Mordenite Dedicated to the Fluid Catalytic Cracking Process: Catalytic Performance Regarding Textural and Acidic Properties

This work was attempted to show that the sequential dealumination and desilication with the use of tetraalkylammonium cations as pore directing agents (PDA) are an effective procedure for the fabrication of hierarchical mordenite zeolites with preserved crystallinity and uniform intracrystalline mesoporosity. Additionally it was demonstrated that desilication performed in the presence of PDAs offered greater mesoporosity development when compared to pure NaOH treatment. IR studies employing ammonia and pyridine as probes exhibited considerably Bronsted acidity of the resulting materials. The strength of protonic sites was reduced upon the treatment; nevertheless their accessibility to hindered 2,6-di-tert-butylpyridine molecules became noticeably high owing to a more open hierarchical structure. Concentration of the acid sites, their strength, and accessibility were reflected in both catalytic activity and selectivity in the cracking of n-decane, 1,3,5-tri-isopropylbenzene, and vacuum gas oil.

A facile top-down protocol for postsynthesis modification of hierarchical aluminum-rich MFI zeolites.

High aluminum content constitutes a major hurdle for the postsynthesis modification of hierarchical zeolites. A facile protocol comprising fluorination and sequential alkaline treatment is presented for the postsynthesis modification of hierarchical Al-rich MFI zeolites. By virtue of this protocol, uniform intracrystalline mesoporosity is introduced in an Al-rich MFI zeolite (Si/Al = 14.3). The obtained hierarchical zeolites exhibit a significant mesopore size distribution, centered around 6 nm, and show improved conversions in catalytic cracking of bulky aromatic molecules. The fundamental implications of the fluorination-alkaline treatment protocol are related to the formation of F-bearing tetrahedral aluminum species in the antecedent fluorination step, which alleviates the resistance of Al sites to the alkaline medium and causes Al-F complexation for regulated hydrolysis of the Al species during the alkaline treatment process. This top-down protocol and the derived mechanistic understandings are expected to be applied in the synthesis of hierarchical Al-rich zeolites with other framework topologies.

Comparison of mesoporous SSZ-13 and SAPO-34 zeolite catalysts for the methanol-to-olefins reaction

Several approaches to improve the catalytic performance of SSZ-13 and SAPO-34 for application as acid catalysts in the methanol-to-olefins (MTO) reaction were explored. Silylation of mesoporous SSZ-13 with a Si/Al ratio of 20 zeolite resulted in increased lifetime in the MTO reaction. Lowering the acidity of SSZ-13 by increasing the Si/Al ratio to 50 also increased the lifetime. The generation of additional mesoporosity in SSZ-13 with a Si/Al ratio of 50 by use of the organosilane octadecyl-(3-trimethoxysilylpropyl)-ammonium chloride (TPOAC) only resulted in a minor improvement of the lifetime. Attempts to synthesize mesoporous SSZ-13 at high Si/Al ratios by use of (C22H45N+(CH3)2C4H8N+(CH3)2C4H9)Br2 (C22-4-4Br2) were unsuccessful, and instead ZSM-5 zeolite was obtained. Similarly, SAPO-34 could not be made hierarchical by using C22-4-4Br2 as a mesoporogen. In this case, other AlPO-phases were obtained. Mesoporous SAPO-34 was synthesized by using TPOAC in the synthesis gel. The additional intracrystalline mesoporosity did not lower the deactivation rate of SAPO-34 as was earlier observed for SSZ-13. The total methanol conversion capacity per acid site for microporous and mesoporous SAPO-34 were however comparable. The lower acidity of the acid sites in SAPO-34 led to the complete utilization of the micropore space. This is to be contrasted to SSZ-13 zeolite, for which the increased rate of coke formation results in more extensive coking deactivation and underutilization of the micropore space.

Preparation of hierarchical mordenite zeolites by sequential steaming-acid leaching-alkaline treatment

A three-step method consisting of steaming-acid leaching-alkaline treatment is used to introduce mesoporosity in a commercially available Al-rich mordenite zeolite (Si/Al=4.8). It has been found that the three-step sequential treatment results in the intra-crystalline mesoporosity with a pore size distribution centered at around 20nm. Single step or binary combination among steaming, acid leaching (treated in 3.0M HNO3 solution at 80°C for 1h) and alkaline treatment (treated in 0.1M NaOH aqueous solution at 85°C for 2h) hardly induces the mesoporosity with a controlled mesopore size distribution. Among the involved three steps, the impact of steaming temperature on the mesoporosity development is studied. The elevation of steaming temperature in the range of 300–600°C improves the susceptibility to the successive alkaline treatment and thus enhances the mesoporosity formation. Moreover, transitions of 29Si NMR spectra demonstrate an extraction-realumination of framework Al species with Si(2Al) configuration during the synthesis of hierarchical mordenite zeolite.

Catalytic cracking performance of alkaline-treated zeolite Beta in the terms of acid sites properties and their accessibility

The zeolite Beta is considered as a promising additive for FCC catalyst in diesel oil production. In this article, it is shown that hierarchical zeolite Beta obtained by an optimized desilication procedure increases diesel and propylene yields during gas–oil cracking reaction. The alkaline treatment of zeolite Beta (Si/Al=22) by desilication with NaOH and NaOH&TBAOH was investigated. The catalytic performance improvement of desilicated zeolite Beta has been rationalized by deep characterization of the samples including X-ray diffraction, low-temperature adsorption of nitrogen, solid-state 29Si MAS NMR and IR studies of acidity. Finally, the catalytic performance of the zeolites Beta was evaluated in the cracking of n-decane, 1,3,5-tri-iso-propylbenzene, and vacuum gas oil. It was found that desilication with NaOH&TBAOH ensures the more uniform intracrystalline mesoporosity with the formation of narrower mesopores, while preserving full crystallinity resulting in catalysts with the most appropriated acidity and then with better catalytic performance.

Facile Synthesis of Mesoporous Zeolite Y with Improved Catalytic Performance for Heavy Oil Fluid Catalytic Cracking

A facile and direct approach for the synthesis of mesoporous zeolite Y by using [(CH3O)3SiC3H6N(CH3)2-C18H37]Cl as template is presented. Under the basic condition of the Y synthesis system, (CH3O)3Si– bonds hydrolyze to −Si–OH, and yield −Si–O–Al– and −Si–O–Si– linkages that anchor the template in zeolite framework. Afterward, micelles formed by −C18H37 lead to the formation of mesoporosity within zeolite Y crystals. This material, which introduces intracrystalline mesoporosity into zeolite Y, shows superior catalytic performance when used in heavy oil catalytic cracking.

Micron-Sized Single-Crystal-like CoAPO-5/Carbon Composites Leading to Hierarchical CoAPO-5 with Both Inter- and Intracrystalline Mesoporosity

Commercial mesopores-containing carbon (carbon black BP2000) has been used as crystallization host as well as hard template of intracrystalline mesopores in CoAPO-5 materials prepared with N-methyldicyclohexylamine as specific structure-directing agent. As a result, micrometer-sized AFI-structured terminated-tip (pencil-like) hexagonal prims have been obtained. Despite the appearance of single crystals, the crystals are actually composites formed by a large number of elongated-shape CoAPO-5 and carbon nanodomains. Indeed, the removal of carbon by calcination leaves an inorganic Co-doped AlPO4-based material having a complex hierarchical pore system, formed by the expected microporosity for an AFI-structured material and both intra- and intercrystalline mesoporosity. In good agreement with the nanocomposite nature of these micrometer-sized crystals, the size of their fibers generated by burning the carbon matrix can be relatively controlled by the conditions of the calcination processes. Under optimized synthesis and calcination conditions, AFI-structured materials with high mesoporous surface area and pore volume can be afforded.

Decoupling porosity and compositional effects on desilicated ZSM-5 zeolites for optimal alkylation performance

Desilication of conventional zeolites in alkaline medium generates variable intracrystalline mesoporosity, but inevitably changes other properties such as the Si/Al ratio and aluminium distribution. Assessing the individual effects of porosity and composition on the catalytic performance of desilicated zeolites is relevant for their optimal design. Herein, we decouple the respective impacts in the acid-catalysed alkylation of toluene (or cyclohexylbenzene) with benzyl alcohol. These reactions experience strong accessibility constraints to the micropores, providing high sensitivity to the properties of the developed mesopore surface. Through strategic comparison of alkaline-treated ZSM-5 zeolites prepared with and without subsequent acid treatment, we show that while acidity is important, the alkylation activity is dominated by the mesoporous surface area. The selectivity to (methylbenzyl)benzene does not depend on the available external surface. Excessively large mesopore volumes are detrimental to the activity, and variation in micropore volume has a minimal effect. Acid-treated mesoporous zeolites exhibit higher catalytic activities primarily due to textural enhancements by removal of aluminium-rich amorphous debris. The catalytic results are rationalised on the basis of extensive characterisation (AAS, N2 sorption, XRD, TEM, 27Al MAS NMR, FTIR, NH3-TPD, and adsorption of toluene and cyclohexylbenzene).

Towards the control of intercrystalline mesoporosity in inorganic microporous materials: The case of CoAPO-5

The generation of mesoporosity in microporous materials has been postulated as a valid strategy to limit the diffusional problems of organic substrates within the pores of these materials, without affecting their intrinsic catalytic behavior. This article shows the effect of systematic variations of different synthesis parameters on the nature of the intercrystalline mesoporosity in CoAPO-5 materials. The hard impositions of this approach, including a structure-directing agent (SDA) possessing enough structural specificity towards AFI materials under a very wide range of synthesis conditions, and at the same time having the ability of generating intercrystalline mesoporosity, severely reduces the possible choices. Fortunately, N-methyldicyclohexylamine (MCHA) simultaneously obeys both conditions. We have examined some parameters conventionally affecting the crystallization process such as time, temperature and dilution, and some other parameters with particular effect on the results of the crystallization of microporous materials such as heteroatom content, heteroatom source, pH of the starting gel and heating source (conventional or assisted by microwave radiation). Except Co source, the rest of the parameters did have a significant influence on the nature and/or magnitude of intracrystalline mesoporosity. The effect of pH and dilution was particularly relevant, so that ordered intracrystalline mesoporosity is favored when CoAPO-5 is crystallized from relatively diluted and acidic gels. In addition, high Co content produces a higher and better-defined mesoporosity, although it implies a modification of the catalytic activity as the number of active centers is inherently varied. Finally, a relevant improvement in the mesoporosity is obtained when heating is assisted by microwave radiation, since the resultant CoAPO-5 is composed by smaller crystals efficiently and orderly agglomerated in pseudospherical particles with a well-defined mesoporosity of narrower pores.

Influence of crystal size and probe molecule on diffusion in hierarchical ZSM-5 zeolites prepared by desilication

The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1 mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites ( ca. 250 nm). The second sample were laboratory-made large crystals with coffin-like shape ( ca. 17 × 4 × 4 μm 3). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120 m 2 g −1 of mesopore surface area, respectively, in contrast to the 62 and 5 m 2 g −1 of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification.

Aromatization of alkanes over Pt promoted conventional and mesoporous gallosilicates of MEL zeolite

Aromatization of hexane and propane was investigated over Pt promoted mesoporous gallium-containing HZSM-11 with controlled mesoporosity generated by desilication. Prepared catalysts were characterized by nitrogen adsorption, X-ray powder diffraction, scanning electron microscopy, Fourier transform infrared of chemisorbed pyridine, and NH 3 temperature programmed desorption confirming the development of intracrystalline mesoporosity of Ga-containing HZSM-11. The catalytic activities, which were compared in the aromatization of n-hexane and propane, increased upon desilication. The aromatization of n-hexane decreased in the following order, Pt/mesoporous GaZSM-11 ≫ Pt/conventional GaZSM-11 ≫ mesoporous GaZSM-11 > conventional GaZSM-11. Hexane conversion reached 70.1% over mesoporous Pt/GaZSM-11 with Si/Ga of 61, as compared with 29.6 and 24.9% for corresponding mesoporous and conventional GaZSM-11 (Si/Ga = 94), respectively, for experiments at liquid hour space velocity of 3.6 h −1, and 540 °C. Comparison of BTX (benzene–toluene–xylene) selectivity at the conversion level of ∼21.0% revealed that Pt/mesoporous GaZSM-11 is more selective than corresponding mesoporous and conventional GaZSM-11. The BTX selectivity over Pt/mesoporous GaZSM-11 (Si/Ga = 94), which showed strong dependence on the conversion, reached 28.2%, whereas over corresponding mesoporous and conventional GaZSM-11catalysts reached 19.1% and 5.5%, respectively. A higher conversion and better selectivity can be attributed to the improved accessibility to the active extra-framework Ga species owing to the generation of mesopores inside the zeolite particles and shortening the contact time. It is worth mentioning that the prepared catalysts exhibited quite low activity in propane aromatization but exhibiting similar trends as for hexane aromatization.

Full Compositional Flexibility in the Preparation of Mesoporous MFI Zeolites by Desilication

We demonstrate that desilication in alkaline medium is a suitable post-synthetic method to introduce intracrystalline mesoporosity in MFI zeolites independent of the Si/Al ratio in the parent mater...

Desilication Mechanism Revisited: Highly Mesoporous All‐Silica Zeolites Enabled Through Pore‐Directing Agents

The role of pore-directing agents (PDAs) in the introduction of hierarchical porosity in silicalite-1 in alkaline medium was investigated. By incorporation of various PDAs in aqueous NaOH, homogenously distributed mesopores were introduced in 2.5 μm silicalite-1 crystals. It was proven for the first time that framework aluminum is not a prerequisite for the introduction of intracrystalline mesoporosity by desilication. The pore-directing role is not directly exerted by framework trivalent cations metals, but by species on the external surface of the zeolite. The inclusion of metal complexes (Al(OH)(4)(-), Ga(OH)(4)(-)) and tetraalkyl ammonium cations (tetramethyl ammonium (TMA(+)), tetrapropyl ammonium (TPA(+))) in the alkaline solution led to distinct mesopore surface areas (up to 286 m(2) g(-1)) and pore sizes centered in the range of 5-20 nm. In the case alkaline treatment was performed in the presence of Al(OH)(4)(-), all aluminum partially integrated in the zeolite giving rise to both Lewis and Brønsted acidity. Apart from the concentration and location, the affinity of the PDA to the zeolite surface plays a crucial role in the pore formation process. If the PDA is attracted too strongly (e.g., TMA(+)), the dissolution is reduced dramatically. When the pore-directing agent is not attracted to the zeolite's external surface, excessive dissolution occurs (standard alkaline treatment). TPA(+) proved to be the most effective PDA as its presence led to high mesopore surface areas (>200 m(2) g(-1)) over a broad range of PDA concentrations (0.003-0.1 M). Importantly, our results enable to extend the suitability of desilication for controlled mesopore formation to all-silica zeolites.

Mesoporous MFI zeolites by microwave induced assembly between sulfonic acid functionalized MFI zeolite nanoparticles and alkyltrimethylammonium cationic surfactants

Mesoporous MFI zeolites (ZSM-5, TS-1, S-1) having intracrystalline mesoporosity within zeolite crystals were synthesized by microwave induced assembly through the ionic interaction between the sulfonic acid functionalized MFI zeolite nanoparticles and alkyltrimethylammonium cationic surfactants.

Aviation fuel production from lipids by a single-step route using hierarchical mesoporous zeolites

Hierarchical mesoporous molecular sieves with tunable zeolitic crystallinity, acidity and porosity were tailored to develop a single-step process for hydroconversion of triglycerides and free fatty acids obtained from algae and Jatropha seeds. Ni-W catalyst supported on acidic zeolitic ZSM-5 support with hierarchical structure and intra-crystalline mesoporosity with composition similar to that for typical hydrocracking catalyst could yield 40–45% C9–C15 hydrocarbons and high isomerization selectivity (isomer/n-alkane, i/n ~ 2–6) from Jatropha oil. While Ni-Mo catalysts on the same support furnished 40–50% kerosene range hydrocarbons with i/n ~ 3–13. For algal oil feed hydroconversion using sulfided Ni-Mo catalyst supported on high surface area semi-crystalline ZSM-5, unexpectedly high yield of jet-fuel range hydrocarbons (77%) with moderately high isomerization selectivity (i/n = 2.5) could be obtained.

Direct oxidation of benzene to phenol by N2O over meso-Fe-ZSM-5 catalysts obtained via alkaline post-treatment

Fe-ZSM-5 zeolites with uniform crystal size and shape were synthesized by a hydrothermal method, and these parent zeolites were desilicated by an alkaline-treatment method to obtain mesoporous Fe-ZSM-5 (meso-Fe-ZSM-5) zeolites. Both Fe-ZSM-5 and meso-Fe-ZSM-5 zeolites were characterized by XRD, N2 adsorption–desorption, ICP-AES, SEM, TEM, UV-vis, and TGA techniques and as catalysts they were used in the direct oxidation of benzene to phenol (BTOP) by N2O as oxidant. The meso-Fe-ZSM-5 catalysts show a much higher catalytic activity and stability, compared to the parent Fe-ZSM-5 catalysts. A comparison study on the uptakes of benzene in the parent and alkali-treated zeolites indicates that the internal mass-transfer limitations of the meso-Fe-ZSM-5 zeolites are significantly improved due to the introduction of intracrystalline mesoporosity, resulting in a better catalytic activity and stability in the BTOP reaction. Desilication in the alkaline medium of Fe-ZSM-5 crystals enables a more efficient utilization of the zeolite in the BTOP reaction that is strongly affected by diffusional limitations.

Hierarchical ZSM‐5 Zeolites in Shape‐Selective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation

The isomerization of o-xylene, a prototypical example of shape-selective catalysis by zeolites, was investigated on hierarchical porous ZSM-5. Extensive intracrystalline mesoporosity in ZSM-5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o-xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p-xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p-xylene selectivity and reduce the deactivation rate. The HCl-washed hierarchical ZSM-5 displayed an approximately twofold increase in p-xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post-synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).

Alkaline treatment on commercially available aluminum rich mordenite

Several commercially available samples consisting of agglomerated small mordenite crystallites with low Si/Al ratios (5.7–10 at/at) have been treated in aqueous NaOH solution. It was found that the porosity can be enhanced when the sodium hydroxide solution is sufficiently concentrated. Treatment in 1 M NaOH for 15 min resulted in inter-crystalline porosity and the mesopore volume was increased from 0.01 to 0.21 cm 3 g −1 together with an increased external surface area from 36 to 85 m 2 g −1. The micropore volume and crystallinity were preserved after the treatment. Both H-MOR and Na-MOR mordenite agglomerates have been successfully treated: the Na-MOR requires a longer contact time to obtain similar porosity. By carefully choosing the alkaline concentration and contact time, intra-crystalline mesoporosity can be obtained for mordenite with Si/Al ratios as low as 10. Catalytic tests with proton mordenite showed that alkaline treatment leads to more than one order of magnitude of activity gain in the liquid-phase alkylation of benzene with propene to form cumene, while selectivity is preserved. These results demonstrate that alkaline treatment also on high-aluminum content mordenites is an effective tool to enhance accessibility and thereby its catalytic performance.

Mesoporous ZSM-5 zeolites prepared by a two-step route comprising sodium aluminate and acid treatments

A two-step route comprising treatment in sodium aluminate followed by acid washing was developed to introduce secondary mesoporosity in ZSM-5 zeolites with different Si/Al ratios. The parent and treated zeolites were characterized by ICP-OES (solids and filtrates), XRD, TGA, TEM, gas adsorption (N2 at 77K and Ar at 87K), FTIR, and NH3-TPD. The contact of the zeolite with NaAlO2 solutions induced mild leaching of framework silicon, viz. desilication. However, the porosity of the resulting samples was mostly blocked by NaAlO2-derived deposits and Si-containing debris. A subsequent acid washing with HCl or oxalic acid removed the blocking species, largely regaining the native microporosity and further uncovering the extensive intracrystalline mesoporosity generated in the first step. In order to maximize the micro- and mesoporosity in the final zeolites, the conditions of the NaAlO2 and acid treatments were varied with respect to concentration, temperature, and time. The efficiency of the two-step treatment was remarkable for zeolites with Si/Al=25 and 40. The mesopore surface area of the hierarchical ZSM-5 was 3–4 times higher than in the parent zeolites, while other intrinsic properties like crystallinity and acidity were largely preserved. The physico-chemical properties of hierarchical ZSM-5 obtained by the two-step route and by conventional desilication in NaOH were compared. Both desilication variants led to equivalent mesopore surface areas, although the two-step route presented a higher solid yield and produced smaller mesopores due to the much lower amount of silicon leached compared to the treatment in sodium hydroxide. Mechanistic and kinetic aspects of silicon dissolution and porosity development in ZSM-5 were discussed on the basis of alkaline treatments in mixtures of NaOH and NaAlO2.

Quantification of enhanced acid site accessibility in hierarchical zeolites – The accessibility index

An accessibility index (ACI) was derived from infrared spectroscopy of substituted alkylpyridines with different size (pyridine: 0.57 nm, 2,6-lutidine: 0.67 nm, and 2,4,6-collidine: 0.74 nm) over hierarchical porous ZSM-5 crystals. The samples were prepared by selective silicon extraction of a commercial sample in NaOH (desilication), and they contained different degrees of intracrystalline mesoporosity. Our results demonstrate the enhanced accessibility of Brønsted acid sites in the hierarchical zeolites. For example, a relatively bulky molecule such as collidine, which probes practically no acid site of the parent medium-pore MFI structure, can access up to 40% of the acid sites in the mesoporous sample. The ACI is a powerful tool to standardize acid site accessibility in zeolites, and can be used to rank the effectiveness of synthetic strategies to prepare hierarchical zeolites.