Intercrystalline Mesopores Research Articles

Synthesis of hierarchical zeolite Beta with low organic template content via the steam-assisted conversion method

We report the synthesis of hierarchical zeolite Beta with low organic template (tetraethylammonium hydroxide, TEAOH) content by the steam-assisted conversion (SAC) method. Neither zeolite seeds nor the secondary mesoporogen were required. Only a bit amount of organic template (denoted as TEAOH/SiO2 ratio=0.06) was used to obtain highly crystalline zeolite Beta, which was remarkably lower than the reported lowest value (0.10). Physicochemical properties of the resulting samples were investigated by comprehensive characterizations including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, solid state 27Al MAS NMR spectra, nitrogen adsorption–desorption, X-ray fluorescence, temperature-programmed desorption of ammonia and infrared spectra of pyridine adsorption. These characterization results showed that the SAC method led to hierarchical crystalline zeolite Beta aggregates assembled by 20–40nm nanosized crystals with controllable SiO2/Al2O3 ratios among 29–47 by varying the compositions of synthetic gels with the Na2O/SiO2 ratio of 0.12 and the H2O/dry gelratios at 0.20–0.30. These zeolite Beta aggregates possessed large external surface area (158–180m2/g) and mesoporous volume (0.27–0.30cm3/g) resulted from the formation of intercrystalline mesopores and of the small zeolite crystal sizes. The obtained hierarchical zeolite Beta showed 41.2% conversion in the esterification reaction of levulinic acid with ethanol and four catalytic runs without obvious decrease in conversion, which were better than those on the zeolite Beta samples from the conventional hydrothermal synthesis. This was due to the promotion of the accessibility of reactants to the acid sites distributed on the external surface of zeolite nanocrystals and of the prominent diffusion in intercrystalline mesopores.

Selective p-xylene production from biomass-derived dimethylfuran and ethylene over zeolite beta nanosponge catalysts

p-Xylene can be synthesized via a renewable route from biomass-derived 2,5-dimethylfuran (DMF) and ethylene with zeolite catalysts. Here, we propose mesoporous beta zeolite with a nanosponge-like morphology (NSP-BEA) as a highly efficient catalyst for the cycloaddition of DMF with ethylene. The NSP-BEA zeolite was synthesized through a hydrothermal synthesis route using multi-ammonium surfactants as a meso–micro dual structure-directing agent. From structural analyses and measurements of the surface acidity, the NSP-BEA was found to be composed of ultrathin zeolitic nanocrystals with intercrystalline mesopores (∼4.5nm). It also possessed a large number of Brønsted acid sites on its external surfaces and in its internal micropores. This NSP-BEA catalyst exhibited high catalytic performance upon a cycloaddition of DMF with ethylene as compared to the commercial beta zeolite, alumina, and silica–alumina catalysts reported previously.

Effect of vanadium contamination on the framework and micropore structure of ultra stable Y-zeolite

Y-zeolites are the main component of fluid catalytic cracking (FCC) catalyst for conversion of crude petroleum to products of high demand including transportation fuel. We investigated effects of vanadium which is present as one of the impurities in FCC feedstock on the framework and micropore structure of ultra-stable (US) Y-zeolite. The zeolite samples were prepared and characterized using standard techniques including: (1) X-ray diffraction, (2) N2 adsorption employing non local density functional theory method, NLDFT, (3) Transmittance and Pyridine FTIR, (4) Transmittance electron microscopy (TEM), and (5) 27Al and 29Si MAS-NMR. Results revealed that in the presence of steam, vanadium caused excessive evolution of non inter-crystalline mesopores and structural damage. The evolved mesopore size averaged about 25.0nm at 0.5wt.% vanadium loading, far larger than mesopore size in zeolitic materials with improved hydrothermal stability and performance for FCC catalyst. A mechanism of mesopore formation based on accelerated dealumination has been proposed and discussed. Vanadium immobilization experiments conducted to mitigate vanadium migration into the framework clearly showed vanadium is mobile at reaction conditions. From the results, interaction of vanadium with the passivator limits and decreases mobility and activity of vanadium into inner cavities of the zeolite capable of causing huge structure breakdown and acid sites destruction. This study therefore deepens insight into the causes of alteration in activity and selectivity of vanadium contaminated catalyst and hints on a possible mechanism of passivation in vanadium passivated FCC catalyst.

Synthesis of inter-crystalline mesoporous ZSM-5 generated by self-interlocked MFI nanosheet stacks

ZSM-5 products with inter-crystalline mesopores originating from the SI-ONSs can be produced in the ternary composition domain established.

Facile control of inter-crystalline porosity in the synthesis of size-controlled mesoporous MFI zeolites via in situ conversion of silica gel into zeolite nanocrystals for catalytic cracking

We proposed a general strategy for the green synthesis of MFI zeolite nanocrystals with controllable inter-crystalline mesopores.

Ultratrace detection of toxic heavy metal ions found in water bodies using hydroxyapatite supported nanocrystalline ZSM-5 modified electrodes

Hydroxyapatite nanoparticles, intercrystalline mesopores of Ag-Nano-ZSM-5, and lower electron transfer resistance of the material are responsible for high electro-catalytic activity.

Direct Synthesis of Hierarchical ZSM-5 Zeolite and Its Performance in Catalyzing Methanol to Gasoline Conversion

A simple method was developed and tested to synthesize hierarchical ZSM-5 zeolites using tetrapropylammonium hydroxide as a sole structure directing agent to generate intercrystalline mesopores. XRD analysis confirmed the formation of ZSM-5 zeolites. SEM and TEM imaging analyses revealed the presence of hierarchical structure, formed by self-assembly of highly crystalline, nanosized primary particles. Solid-state MAS NMR analysis showed the vast majority of Al atoms were tetrahedrally incorporated with Si atoms in the zeolite framework. N2 physisorption studies indicated that the mesopores of an average pore size of 30 nm were created in the hierarchical ZSM-5 zeolites with an abundance of micropores. The performance of these zeolites in catalyzing methanol to gasoline conversion was evaluated using a fixed-bed reactor operating at 350 °C with ca. 16.5% methanol in nitrogen at a total pressure of 1.1 MPa and WHSV of 1.2 h–1. The hierarchical ZSM-5 zeolites exhibited almost 100% methanol conversion and 59% selectivity toward the gasoline range hydrocarbons. The coke formation on the used catalyst was studied using a N2 physisorption technique and a thermogravimetric analyzer. The hierarchical ZSM-5 zeolite showed less than 2% coke formation and deposition over 24 h on stream, much less compared to the conventional ZSM-5 zeolite with nearly 8% carbon deposition under the same reaction conditions.

Crystallite-pore network model of transport and reaction of multicomponent gas mixtures in polycrystalline microporous media

Abstract A three-dimensional pore network model has been developed to simulate anisotropic multicomponent diffusion and reaction in polycrystalline microporous media with coexisting intracrystalline micropores and intercrystalline mesopores (i.e., defects). Transport in these pores is modeled with the generalized Maxwell–Stefan surface diffusion model proposed by Krishna [11] and the Knudsen diffusion model, respectively. A new feature highlight of this model is the representation of polycrystalline media with a crystallite-pore network model. In contrast to previous pore network models, the crystallite-pore network model has the novel aspect of modeling the anisotropic transport inside the crystallites forming a polycrystalline layer by assigning to every crystallite two parameters to describe its orientation. The model was applied to simulate xylene isomerization in a polycrystalline ZSM-5 zeolite membrane, which had been experimentally investigated in a Wicke-Kallenbach cell by Haag et al. [13] . First, their experimental data were used to estimate adsorption and diffusion parameters of the xylene isomers in the ZSM-5 membrane via fitting single-gas permeance data of the xylene isomers. Second, adopting these parameters, the experimental data for xylene isomerization were used to determine kinetic parameters for xylene isomerization in the ZSM-5 membrane. Finally, effects of selected structural parameters – concentration of defects, connectivity of defects, crystallite orientation, and crystallite size – were investigated using the obtained adsorption, diffusion, and reaction parameters. The simulation results show that high selectivity towards p-xylene requires a low concentration of defects in the polycrystalline layer and a low loading of xylene isomers in the membrane. The novel crystallite-pore network model is also applicable to many other reaction systems.

Micron ZSM-11 microspheres seed-assisted synthesis of hierarchical submicron ZSM-11 with intergrowth morphology

Hierarchical submicron ZSM-11 particles with intergrowth morphology and intercrystalline mesopores were prepared by adding micron ZSM-11 microspheres (6.5–7.5μm) to the TBA-gel precursors. The particle size of products could be feasibly adjusted from 700 to 200nm with the increase of seeds content from 1wt% to 10wt%. Compared to seed particles, the obtained submicron product particles exhibited both enhanced external surface area by 10–30m2/g and increased mesopore volume by 0.09–0.20cm3/g. According to the follow-up investigation, the existence of seed particles was found to induce the nucleation, which was responsible for the fast crystallization rate as well as the small particle size of products. In addition, the amount of template ((TBA)2O/SiO2) used in the seeded system was able to be reduced as low as 0.005 (molar ratio).

Synthesis of Micro-Sized MEL-Type Zeolite Aggregates

Hierarchical micro-sized zeolite aggregates with the MEL structure were prepared hydrothermally from a tetrabutylammonium hydroxide(TBAOH)-tetraethyl orthosilicate(TEOS)-H2O system. The obtained microsized silicalite-2 microspheres had sizes larger than 10 μm, high BET surface area(460 m2·g-1), and large pore volume(0.74 cm3·g-1). The formation of micro-sized spheres alleviates preparation and application difficulties.The presence of inter-crystalline mesopores originating from the spontaneous assembly of nano-sized primary particles during hydrothermal synthesis gives the advantages of nanoparticles, reducing diffusion limitations.The introduction of titanium does not strongly affect the morphology and textural properties of the MEL-type zeolite, which are quite similar to those of silicalite-2 aggregates. The micro-sized titanium silicalite-2(TS-2)microspheres showed comparable catalytic activity in phenol hydroxylation to that of titanium silicalite-1(TS-1) of size 100-200 nm, and were easily recovered by traditional filtration, simplifying the separation and recovery compared with nano-sized TS-1.

Formation mechanism and catalytic application of hierarchical structured FeAlPO-5 molecular sieve by microwave-assisted ionothermal synthesis

At atmospheric pressure, FeAlPO-5 molecular sieve with hierarchical micro- and meso-porous structure has been ionothermally synthesized by microwave heating with eutectic mixture as the solvent. Among the synthesis parameters investigated, the ratio of P2O5/Al2O3, aluminum source, and heating methods significantly affected the mesoporosity of FeAlPO-5 molecular sieve. N2 Physisorption, SEM and TEM characterizations showed that the resultant material possessed inter- and intra-crystalline mesopores simultaneously. The larger intercrystalline mesopores in size resulted from the intercrystalline void space between bar crystals and nanoparticles. The calcination removed the organic species embedded in the crystals of FeAlPO-5 molecular sieve, leading to the formation of smaller intracrystalline mesopores. The catalytic properties of hierarchical FeAlPO-5 molecular sieve were investigated in the hydroxylation of phenol with microporous FeAlPO-5 as a reference. The results showed that the catalytic performance of the former catalyst was superior to the latter.

Synthesis of Transition‐Metal Exchanged Nanocrystalline ZSM‐5 and Their Application in Electrochemical Oxidation of Glucose and Methanol

AbstractNanocrystalline ZSM‐5 was prepared using propyltriethoxysilane. Materials were characterized by a complementary combination of X‐ray diffraction, nitrogen sorption, and scanning electron microscopy. Transition‐metal ion exchanged nanocrystalline ZSM‐5‐modified electrodes were constructed for the electrocatalytic oxidation of glucose and methanol. A non‐enzymatic electrochemical sensor based on a Ni2+‐exchanged nanocrystalline ZSM‐5‐modified electrode exhibits the highest sensing ability, whereas the corresponding Cu2+‐exchanged electrode exhibits the highest current sensitivity for glucose oxidation. Among the variety of electrodes modified with transition‐metal ion exchanged nanocrystalline ZSM‐5, the Ni2+‐exchanged electrode exhibits high current sensitivity and sensing ability in methanol oxidation. Electrocatalytic activity of conventional ZSM‐5‐modified electrodes was significantly low compared to nanocrystalline ZSM‐5‐modified electrodes. Enhancement in the electrocatalytic activities of nanocrystalline ZSM‐5‐modified electrodes can be correlated with the enhanced accessibility of glucose/methanol to M2+ active centers in the nanocrystalline ZSM‐5 owing to its large specific surface area and intercrystalline mesopores. The sensor was applied directly to determine glucose concentration in adult human blood serum, and the precision of the method was found to be satisfactory. The non‐enzymatic sensor exhibited excellent reproducibility, repeatability, stability, and antifouling ability for direct determination of glucose in human blood serum.

Etherification of glycerol for improved production of oxygenates

Various resin and zeolite based catalysts are prepared, characterized and evaluated for their catalytic performance towards the etherification of glycerol with tertiary butyl alcohol. Resin based catalysts exhibited higher glycerol conversions and ether selectivities at relatively lower reaction temperature (75°C) than the zeolite based catalysts (90° to 110°C) but the zeolite based catalysts performed better in terms of stability in activity to produce high amounts of di-tertiary butyl (DTBG) and tri-tertiary butyl ether (TTBG) of glycerol (oxygenate stock). Though acidity is important for the etherification reaction, the zeolites exhibiting better porosity rather than acidity could yield higher oxygenate. The presence of inter-crystalline meso-pores in N-BEA further improved the oxygenate production and time-on-stream activity of the catalyst. At optimized catalyst parameters and reaction conditions, Nano-beta (N-BEA) zeolite catalyst exhibited above 95% conversion of glycerol with more than 45% and 54% selectivities to DTBG and TTBG, which are potential blending stocks for diesel.

Zeolite with trimodal porosity by desilication of zeolite nanocrystals aggregate

Zeolite with trimodal porosity can be synthesized by desilication of zeolite nanocrystal aggregate. In the desilication process, the originally existed intercrystalline mesopores of zeolite nanocrystal aggregate were enlarged into large mesopore, and the new small intracrystalline mesopore channel was created, thus the Zeolite with trimodal porosity was formed. The structure of resulted zeolite, both on aggregate and mesopore level can be fine tuned by the desilication degree.

Fabrication of a Novel Catalyst Support with Hierarchical Pore Structure

A novel catalyst support with trimodal pore structure was hydrothermally synthesized by building up nano-particles of titanium silicalite-1 inside the macroporous system of silica gel using the tetrapropylammonium hydroxide (TPAOH) as template, titanium (IV) tetrabutoxide monomer (TBOT) as titanium source and the silica gel with macropores as in situ silicon source as well as matrices. The prepared support exhibited intrinsic pore of TS-1, intercrystalline mesopores and residual macropores of silica gel, accomplishing the interconnection of micro-, meso-, and macrpores.

Trimodal pore catalyst preparation method by growth of zeolite inside macroporous matrices of silica

In this paper, a novel catalyst support with trimodal pore structure was hydrothermally synthesized by building up nano-particles of titanium silicalite-1 inside the macroporous system of silica gel using the tetrapropylammonium hydroxide (TPAOH) as template, titanium (IV) tetrabutoxide monomer (TBOT) as titanium source and the silica gel with macropores as in situ silicon source as well as matrices. The prepared support exhibited intrinsic pore of TS-1, intercrystalline mesopores and residual macropores of silica gel. When loaded with Co, the trimodal catalyst was employed to catalyze the methanation of carbon monoxide, showing higher CO conversion and CH 4 selectivity than the bimodal and unimodal catalysts due to the favored metallic Co surface area and dispersion.

Alkaline treatment of template containing zeolites: Introducing mesoporosity while preserving acidity

Alkaline treatment (desilication) is an effective treatment to increase mesoporosity. However, the concomitant decrease in Si/Al ratio affects the strengths of the acidic sites and hence catalytic activity and selectivity. Therefore instead we subjected template containing zeolites to 1 M NaOH to induce additional porosity. All zeolites tested (ZSM-5, ZSM-12 and Beta) consisted of small crystallites (30–200 nm) that were agglomerated into larger particles between 1 and 5 μm. Mesopore formation occurred by slowly removing outer layers of individual crystallites, thereby preserving the Si/Al ratio as well as the crystallinity. Inter-crystalline mesopores were formed for all zeolites, but the treatment was most effective for zeolites with small crystallites. The external surface area of ZSM-5 was increased from 90 to 149 m 2 g −1 and for zeolite Beta from 70 to 158 m 2 g −1. The catalytic performance was tested at 413 K for 3 h for etherification of 1,2-propylene glycol with 1-octene to form octyl-ether. For ZSM-5, conversion increased from 1.2% to 5.6% upon alkaline treatment in the presence of a template, while only to 5% for the template-free treated sample with a significantly higher external surface area (205 m 2 g −1). The parent zeolite Beta was significantly more active (30% conversion, 88% selectivity), which can be ascribed to its larger pore size. Nevertheless also in this case alkaline treatment in the presence of the template significantly increased the activity to 40% conversion with similar selectivity. As only the mesoporosity was changed upon alkaline treatment it suggests that the etherification of 1,2-propylene glycol with 1-octene is affected by intra-crystalline diffusion. Our work illustrates the possibilities to use alkaline treatment of templated zeolites to decouple accessibility changes from acidity, and to gain further insight in zeolite catalysis.

Oxidation of methane to methanol and formaldehyde over Co–ZSM-5 molecular sieves: Tuning the reactivity and selectivity by alkaline and acid treatments of the zeolite ZSM-5 agglomerates

Alkaline treatment of templated Na–ZSM-5 zeolites with NaOH solutions resulted in the creation of intercrystalline mesopores within the zeolite agglomerates, while preserving the micropore volume and crystallinity. It was found that the zeolite external surface area increased with increasing NaOH concentration and pretreatment time. These alkaline-treated samples were loaded with cobalt and a linear relationship between the number of cobalt oxidic species and the external surface area of the zeolite could be established. This in turn leads to a linear relationship between the ZSM-5 surface area and the amount of methanol produced over Co–ZSM-5 from methane and oxygen at 423 K. Attempts to remove extra framework alumina species by an acid treatment were successful. However, this acid treatment increased, after Co-deposition, the amount of highly dispersed Co 2+ inside the ZSM-5 channels, which resulted in a higher selectivity towards formaldehyde. The nature of the charge-compensating cation also determined the cobalt speciation. Co–H–ZSM-5 samples contained more cobalt inside the channels and were more selective towards formaldehyde formation than Co–Na–ZSM-5 samples.

Non-templated intercrystalline mesoporosity in heteroatom-doped AlPO 4-5 using N-methyldicyclohexylamine as structure-directing agent

Non-doped and 11 (Si, Co, Mg, Zn, Mn, Sn(II), Sn(IV), Cr, Fe, V and Zr) heteroatom-doped AlPO 4-5 samples have been crystallized in a high purity using N-methyldicyclohexylamine as structure-directing agent by simply changing the heteroatom source. Needle- or rod-like crystals of nano size, arranged in cylindrical or spherical agglomerates containing intercrystalline mesopores, compose all samples, excepting VAPO-5, whose nanocrystals and agglomerates are shapeless, and ZnAPO-5 composed by larger crystals not defining any mesorpore in their arrangement. Crystal sizes and dimensions were evaluated by direct application of Scherrer equation to the most intense XRD peaks and by Williamson–Hall plots of h 0 0 and 0 0 l reflections, respectively. Scherrer equation systematically underestimated crystal sizes. Williamson–Hall method gives different crystal dimensions, which were much closer to those observed by SEM and TEM studies. Moreover, Williamson–Hall plots consider separately crystal dimensions and strain/stress contributions to XRD peaks broadening, which have been used as key information to rationally understand the role of different heteroatom ions in the sample morphology and their mesoporosity. Intercrystalline mesoporosity could be studied by N 2 adsorption/desorption isotherms in as-prepared samples, as well as in calcined ones, confirming the stability of mesopores under calcinations conditions. Both maxima of pore size distribution and external (extra-micropore) area well correlate with crystal size of MeAPO-5 with a given agglomerates morphology, suggesting different compact packing for each morphology.

One-Step Synthesis of Mesoporous Beta Zeolite Using Phosphate

The addition of sodium phosphate monobasic (NaH2PO4) to a synthetic mixture allows a one-step synthesis of mesoporous Beta zeolite. Aggregation of the nanosized Beta zeolites allows the filtration difficulties to be overcome while the intercrystalline mesopores reduce the diffusion limitation and thus enhance the catalytic activity. The obtained Beta zeolite samples were characterized by X-ray diffraction, scanning electron microscopy, N2 adsorption-desorption, and temperature-programmed desorption of ammonia. The amount of phosphate strongly influenced the mesopore volume of the Beta zeolites. When the molar ratio of NaH2PO4/SiO2 was 0.1, the sample had good textural and acidic properties and exhibited the best catalytic activity for the alkylation of phenol with tert-butanol.