Zeolite Precursor Research Articles

Tetrapropylammonium Hydroxide Treatment of Aged Dry Gel to Make Hierarchical TS-1 Zeolites for Catalysis.

This work presents the development and systematic study of a method to prepare hierarchical titanium silicalite-1 (TS-1) zeolites with high tetra-coordinated framework Ti species content. The new method involves (i) the synthesis of the aged dry gel by treating the zeolite precursor at 90 °C for 24 h; and (ii) the synthesis of hierarchical TS-1 by treating the aged dry gel using tetrapropylammonium hydroxide (TPAOH) solution under hydrothermal conditions. Systematic studies were conducted to understand the effect of the synthesis conditions (including the TPAOH concentration, liquid-to-solid ratio, and treatment time) on the physiochemical properties of the resulting TS-1 zeolites, and the results showed that the condition of a TPAOH concentration of 0.1 M, liquid-to-solid ratio of 1.0, and treatment time of 9 h was ideal to enable the synthesis of hierarchical TS-1 with a Si/Ti ratio of 44. Importantly, the aged dry gel was beneficial to the quick crystallization of zeolite and assembly of nanosized TS-1 crystals with a hierarchical structure (S ext = 315 m2 g-1 and V meso = 0.70 cm3 g-1, respectively) and high framework Ti Species content, making the accessible active sites ready for promoting oxidation catalysis.

Composition-adjustable Mo6Co6C2/Co@carbon nanocage for enhanced oxygen reduction and evolution reactions

Bifunctional oxygen electrocatalysts that hold outstanding activity and stability are highly crucial for the development of efficient rechargeable Zn-air batteries. Herein, cobalt-molybdenum-based bimetallic carbide and cobalt nanoparticles embedded N-doped carbon nanocages are synthesized via the pyrolysis of functionalized zeolitic imidazolate framework precursor originated from zeolitic imidazolate framework sequentially coated with polydopamine and phosphomolybdic acid. Furthermore, we revealed the composition-performance relationship based on the exploration of bifunctional performance on the pyrolysis products. More importantly, the synergy of multiple active sites with hollow structure gives the prepared catalyst a low overpotential (284 mV) for oxygen evolution reaction and high half-wave potential (0.865 V) for oxygen reduction reaction, besides an excellent bifunctional durability. Furthermore, the prepared catalyst as a cathode electrocatalyst grants the assembled rechargeable Zn-air batteries a high open-circuit voltage, power density, specific capacity, and remarkable charge-discharge cycle stability. This work provides a strategy for the integration and active-adjustment of bifunctional catalyst and its potential applications in water splitting and other catalytic reactions.

Zeolitization of Diatomite Residues by a Simple Method

The possibility of transforming a diatomite-rich waste from the brewing industry into synthetic zeolites has been investigated. After precalcination at 550 °C to eliminate the retained organic matter, the clean diatomite (Dt; with a Si/Al molar ratio of 17.4), was hydrothermally treated for 24 h with continuous stirring in a 3M NaOH solution at 80 °C. The results of mineralogical characterization by X-ray diffraction with Rietveld refinement have shown a crystallization of 55% of zeolite P, which was neoformed from the amorphous phase, opal-CT and quartz of the starting sample. The spectra obtained by Fourier Transform Infrared Spectrometry have corroborated such zeolitization. N2 adsorption–desorption isotherms have shown that the zeolitized material (Dt-Z) is mesoporous, with almost 60% more specific surface area than Dt (62.6 m2/g vs. 39.4 m2/g), greater microporosity and 40% smaller average pore size than Dt (71 Å vs. 118 Å). This study is a first approximation to know the potential of diatomite wastes as zeolite precursors, for which additional research including an aluminum source will be required.

Direct TEM Observation of Vacancy‐Mediated Heteroatom Incorporation into a Zeolite Framework: Towards Microscopic Design of Zeolite Catalysts

AbstractIncorporating hetero‐metal‐atom, e.g., titanium, into zeolite frameworks can enhance the catalytic activity and selectivity in oxidation reactions. However, the rational design of zeolites containing titanium at specific sites is difficult because the precise atomic structure during synthesis process remained unclear. Here, a titanosilicate with predictable titanium distribution was synthesized by mediating vacancies in a defective MSE‐type zeolite precursor, based on a pre‐designed synthetic route including modification of vacancies followed by titanium insertion, where electron microscopy (EM) plays a key role at each step resolving the atomic structure. Point defects including vacancies in the precursor and titanium incorporated into the vacancy‐related positions have been directly observed. The results provide insights into the role of point defects in zeolites towards the rational synthesis of zeolites with desired microscopic arrangement of catalytically active sites.

Direct TEM Observation of Vacancy-Mediated Heteroatom Incorporation into a Zeolite Framework: Towards Microscopic Design of Zeolite Catalysts.

Incorporating hetero-metal-atom, e.g., titanium, into zeolite frameworks can enhance the catalytic activity and selectivity in oxidation reactions. However, the rational design of zeolites containing titanium at specific sites is difficult because the precise atomic structure during synthesis process remained unclear. Here, a titanosilicate with predictable titanium distribution was synthesized by mediating vacancies in a defective MSE-type zeolite precursor, based on a pre-designed synthetic route including modification of vacancies followed by titanium insertion, where electron microscopy (EM) plays a key role at each step resolving the atomic structure. Point defects including vacancies in the precursor and titanium incorporated into the vacancy-related positions have been directly observed. The results provide insights into the role of point defects in zeolites towards the rational synthesis of zeolites with desired microscopic arrangement of catalytically active sites.

Cenosphere-Based Zeolite Precursors of Lutetium Encapsulated Aluminosilicate Microspheres for Application in Brachytherapy.

Coal fly ash hollow aluminosilicate microspheres (cenospheres) of stabilized composition (glass phase—95.4; (SiO2/Al2O3)glass—3.1; (Si/Al)at. = 2.6) were used to fabricate lutetium-176 encapsulated aluminosilicate microspheres as precursors of radiolabeled microspheres applied for selective irradiation of tumors. To incorporate Lu3+ ions into cenosphere’s aluminosilicate material, the following strategy was realized: (i) chemical modification of cenosphere globules by conversion of aluminosilicate glass into zeolites preserving a spherical form of cenospheres; (ii) loading of zeolitized microspheres with Lu3+ by means of ion exchange 3Na+ ↔ Lu3+; (iii) Lu3+ encapsulation in an aluminosilicate matrix by solid-phase transformation of the Lu3+ loaded microspheres under thermal treatment at 1273–1473 K. Two types of zeolitized products, such as NaX (FAU) and NaP1 (GIS) bearing microspheres having the specific surface area of 204 and 33 m2/g, accordingly, were prepared and their Lu3+ sorption abilities were studied. As revealed, the Lu3+ sorption capacities of the zeolitized products are about 130 and 70 mg/g Lu3+ for NaX and NaP1 microspheres, respectively. It was found that the long-time heating of the Lu3+-loaded zeolite precursors at 1273 K in a fixed bed resulted in the crystallization of monoclinic Lu2Si2O7 in both zeolite systems, which is a major component of crystalline constituents of the calcined microspheres. The fast heating–cooling cycle at 1473 K in a moving bed resulted in the amorphization of zeolite components in both precursors and softening glass crystalline matter of the NaX-bearing precursor with preserving its spherical form and partial elimination of surface open pores. The NaX-bearing microspheres, compared to NaP1-based precursor, are characterized by uneven Lu distribution over the zeolite-derived layer. The precursor based on gismondin-type zeolite provides a near-uniform Lu distribution and acceptable Lu content (up to 15 mol.% Lu2O3) in the solid phase.

A nitrite sensor based on bimetallic zeolitic imidazole framework derived Co/porous carbon nanorods

Measurement of nitrite is needed for the control of nitrite pollution and to decrease the health risks and protect environment. It is attractive to design novel electrocatalysts for the efficient electrochemical quantification of nitrite. Herein, a sequence of Co, N-doped-porous carbon rods (CoN-PCRs) were designed and prepared through the calcination of rod-shaped mixed-metal zeolitic imidazolate framework precursors (ZIF-L) with different Co/Zn ratios. It was observed that the highest surface area and best response were obtained with the CoN-PCRs-0.6. Furthermore, its morphology and structure were characterized by several techniques. Additionally, the electrochemical sensing performances of CoN-PCRs-0.6 to nitrite were studied. Benefiting from the structural merits, the constructed sensor showed a low detection limit of 0.14 μM and achieved two wide dynamic ranges 0.2–4000 μM and 4000–100000 μM. The sensor also possessed good specificity, reproducibility, and stability toward nitrite quantification. Ultimately, the rapid and accurate measurement of nitrite in tap water was successfully realized.

Interfacial nanoarchitectonics for accelerated growth of zeolite via enlarging gel-liquid interfacial area using rotating packed bed

Typically, silica-alumina zeolites are hydrothermally synthesized in the sodium-rich dense hydrogel systems with a relatively long crystallization time in industry. The fast and economical synthesis of silica-alumina zeolites is still a big challenge. Herein, we develop a universal method to tackle this issue using the rotating packed bed (RPB). An interesting phenomenon is found that gel-liquid interfacial areas in the zeolite precursor are obviously enlarged when RPB is employed, and thus the following crystallization rates of zeolites are greatly improved, especially for the initial nucleation process. Also, we successfully control the growth rates of silica-alumina zeolite by adjusting the gel-liquid interfacial areas. Furthermore, we find that the transfer rate of OH− and hydrated sodium ions from liquid part to gel phase is enhanced by RPB in the initial nucleation process, which greatly promotes the nucleation rate of zeolites. The density functional theory (DFT) calculations demonstrate that hydrated sodium ions can induce the dehydrogenation of Si–OH to generate reactive oxygen radicals, thus lowering the energy barrier of the polymerization reaction and consequently accelerating the nucleation process. Moreover, NaY, NaZSM-5, and NaA zeolites with larger surface areas and more pore volumes are successfully synthesized by this RPB-enhanced-interface strategy, and their crystallization processes are all greatly accelerated.

Efficient Dry Impregnation of Zirconium into H‐ZSM‐5 Zeolites

AbstractThere is considerable interest in adjusting the acidity and pore architecture of zeolites. Herein, we present solventless incorporation of zirconium (∼2–10 wt.%) in a precursor and desilicated (0.2 and 0.4 mol L−1 NaOH) H‐ZSM‐5 zeolite. Infrared and UV‐vis‐DR spectroscopy confirmed the grafting of highly dispersed Zr in all the zeolites. XRD indicated the presence of ZrO2 in zeolites with higher Zr contents. The micropore volume of the precursor zeolite decreased by ∼10 %, suggesting the presence of Zr mainly on the external surface, while it decreased more than 20 % for the desilicated sample, suggesting Zr had access to the micropores at higher Zr contents. Infrared spectroscopy after pyridine adsorption provided evidence for the Brønsted and Lewis acid sites in all zeolites, but the Brønsted acid‐to‐Lewis acid site ratio decreased by >50 % depending on the Zr and NaOH concentrations. The presence of Zr critically changed the predominant nature of the acid sites of the desilicated zeolites.

Construction of high performance binder-free zeolite monolith

Compared to conventional spheres, strips, and granules, structural adsorbents offer shaping of the adsorbent materials. In this work, we report a straightforward procedure to shape binder-free zeolite monolith (BF-ZM). During the synthesis, hydroxypropyl methylcellulose was applied as a binder, and the zeolite precursor material halloysite nanotubes and Y zeolites were mixed to produce the initial zeolite monolith. The binder was removed by high-temperature calcination to gain preliminary mechanical strength, followed by hydrothermal crystallization to convert the binder into Y zeolites to obtain a 100% pure Y zeolite structure. The as-synthesized BF-ZM exhibits excellent mechanical properties, showing negligible powdering after 1.34 × 105 cycles of accelerated pressurization/depressurization test; in comparison, ordinary packed columns using zeolite beads suffered from 3.51% loss due to powdering. This result indicates that BF-ZM is favorable for filling large pressure swing adsorption columns and can significantly alleviate the issue of adsorbent chalking and slabbing caused by the reciprocal pressure swing of the gas flow. The BF-ZM also achieved a high CO2 adsorption capacity of 6.4 mmol·g−1 at 1.2 bar and 273 K, superior to commercial Y zeolite pellets. The results of the breakthrough experiments showed a CO2 adsorption capacity of 4 mmol·g−1 at 333 K and 1.25 bar before the permeation started. The selectivities (CO2/N2:15/85) of BF-ZM have been calculated using the IAST method, showing 74 excellent selectivity at 333 K and 1 bar. This study provides a straightforward approach to the design and fabrication of high-strength binder-free adsorbents for large-scale industrial gas separations.

Anchoring Effect of Organosilanes on Hierarchical ZSM-5 Zeolite for Catalytic Fast Pyrolysis of Cellulose to Aromatics.

As an essential chemical feedstock, aromatics can be obtained from biomass by catalytic fast pyrolysis (CFP) technology, in which diffusion limitation is still a problem. In this study, several ZSM-5 zeolites with intercrystal stacking macropores were synthesized by adding organosilanes (OSAs) with different alkyl chain groups. Due to the structure-directing effect of the OSA, the prepared ZSM-5 zeolites possess a larger external surface area and pore volume than Blank-Z5. Moreover, the pore size is related to the extent of anchoring of the OSA and silicon–aluminum species in the zeolite precursor. Pyridine Fourier transform infrared (Py-FTIR) and NH3-temperature-programmed desorption (TPD) analyses show that the obtained ZSM-5 zeolites have a higher Brønsted acidity and total number of acid sites. In addition, excessive addition of OSA is not conducive to the growth of ZSM-5 zeolites. The catalytic performance of the synthesized ZSM-5 zeolites was evaluated by Py-GC/MS. The larger external surface area and pore volume improve the accessibility of the acid sites and thus promote the conversion of biomass into aromatics.

Synthesis of industrial solid wastes based geopolymer foams for building energy conservation: Effects of metallic aluminium and reclaimed materials

Thermal insulation materials with less energy consumption and low CO2 emissions have been paid great attention. This study was devoted to assessing the feasibility of utilizing industrial solid wastes for the preparation of sustainable geopolymer foams to make up for the weak link of using industrial solid waste in green low-carbon building energy-saving materials research. Efforts were made to investigate the effects of aluminium powder content and substitution ratio of reclaimed materials on slurry performance, physic-mechanical properties and formation of gel species by a series of characterization methods. Results revealed that the geopolymer foams characterized by compressive strength of 2.83 MPa, bulk density of 507 kg/m3 and thermal conductivity of 0.167 W/(m·K) were successfully produced by external incorporation of 0.17 % dosage of Al powder. 10 % substitution ratio of reclaimed materials presented no obvious negative effect on fluidity, initial setting time, bulk density, compressive strength, thermal conductivity, pore structure and microstructure of the specimens. The pore structure, morphology and polymeric gels of pore partition matrix co-determined the macroscopic properties of geopolymer foams. The existence of semi-crystalline zeolite precursors in reclaimed materials may induce the formation of zeolite-like grains on the inner surface of pores. The present work was in agreement with the strategic objective of resources recycling, solid waste utilization, lower carbon emission and building energy saving, which was conducive to the development of the circular and green economy.

On the chemical condensation of the layers of zeolite precursor MCM-22(P)

Chemical condensation of layers of silicates has been often proposed as an alternative to thermal condensation, finding limited success. The formation of the industrially relevant MCM-22 zeolite (MWW IZA code) from a mild hydrothermal precursor is the most important example of 2D-3D aluminosilicate condensation. Silanols of opposed layers have been condensed by acid-driven dehydration in concentrated nitric acid, as confirmed by powder XRD and 29Si NMR spectroscopy, implying interlayer template extraction and corresponding dealumination. Milder acid treatments favour template extraction and shrinkage of interlayer distance, but do not provide significant silanol condensation. Template extraction is further favoured by degradation of the organics in the presence of a Cu2+ homogeneous catalyst.

Effect of milling time on mechanical properties of anorthite obtained by thermal transformation of Ca-LTA zeolite

Anorthite ceramics was fabricated from calcium exchanged Na-LTA zeolite. The powder compacts of Ca-LTA zeolite were sintered at different temperatures ranging between 1100 and 1400?C. It was found that the temperature of 1100?C was sufficiently high to trigger formation of anorthite which stayed stable even at temperature as high as 1400?C. The highest relative density and the lowest open porosity were measured in samples sintered at 1200?C for 3 h. The effect of milling time of Ca-LTA zeolite precursor on density, microstructure and mechanical properties of samples sintered at 1200?C for 3 h was investigated. The particle size refinement appeared to be beneficial in accelerating densification process and improving mechanical properties. The density, compressive strength and hardness of anorthite ceramics obtained from non-milled precursor were measured to be 70.5 %TD, 64 MPa and 1.45 GPa, respectively. On the other side the anorthite ceramics obtained from 24-hour-long milled zeolite precursor had density of 83.9 %TD, compressive strength of 101 MPa and hardness of 3.44 GPa.

Green solvothermal synthesis and characterisation of surface organosilylated hierarchical nanozeolite ZSM-5

Micellization of the exterior layer of zeolite precursor seeds with surface passivating organosilane groups is an effective technique to reduce aggregation of growing of crystallites. Simultaneously, this technique produces hierarchically porous material with additional mesopores, to minimise typical diffusional limitations encountered with conventional microporous zeolites. The average particle size and the pore architecture of the zeolite can be altered by changing the synthesis environment of the surface passivation process. In this study, low hazard organic solvents with different hydrophobicities (log P) have been used as an effective matrix for the surface organosilanization and crystallization processes of zeolitic species. A clear interaction of organosilane moieties on the exterior region of hierarchical nanozeolite ZSM-5 clusters is evidenced by solid-state nuclear magnetic resonance (NMR) analysis. In addition, hydrolysis of the organosilane molecules has been followed by mass spectroscopy analysis on the synthesis gel solution, leading to a mechanism proposed for the surface silanization processes of the zeolite nanoparticles using the soft-template surface passivating technique.

A shortcut to high-quality gmelinite through steam-assisted interzeolite transformation

Mechanochemical activation and steam-assisted interzeolite transformation offer a simple, fast and cost-efficient synthesis route to gmelinite (GME) zeolite. The use of faujasite (FAU) as the parent zeolite precursor facilitates the reaction and results in high yields of high-quality GME in less than 20 h under optimized conditions. The accessible porosity and amounts of stacking-faults were determined by N2 physisorption and XRD analysis, respectively. The results show that the quality of the prepared materials easily competes with GME zeolites synthesized by conventional hydrothermal synthesis, which encourages further research in this area. By changing the reaction parameters, the approach may also be used for the production of several other types of zeolites, including MOR, MFI and ANA.

Coarse-Grained Model for the Hydrothermal Synthesis of Zeolites

Zeolites are the most used solid catalysts in the chemical industry. The hydrothermal synthesis of these porous aluminosilicate crystals is a multistep process that involves the polymerization of silica from solution to form amorphous aggregates, their crystallization, growth by oriented attachment, and eventually dehydration by heating. The molecular pathways by which zeolites are formed from the precursor solution are not yet fully understood. Molecular simulations can play an important role in elucidating these microscopic processes but are limited by the lack of models able to represent both the polymerization of silica and its crystallization with feasible computational costs. Here, we present a simple, computationally efficient coarse-grained model for the hydrothermal synthesis of zeolites from aqueous solutions. Our TS + W model has three components: silica, a structure-directing agent, and water, each represented by a single particle with short-range interactions. The model quantitatively reproduces the experimental evolution of silica species during the polymerization from solution, yielding amorphous nanoparticles with shape, composition, and silica speciation in agreement with the amorphous precursors of zeolites in experiments. Importantly, the TS + W model spontaneously nucleates and grows zeolites from the amorphous precursor phase and reproduces the temperature required for their dehydration. Our simulations indicate that the preferential formation of zeolites at the amorphous–water interface does not arise from an ability of that surface to heterogeneously nucleate the zeolite. The simplicity and computational efficiency of the TS + W model make it ideal to investigate the interplay between polymerization and crystallization in zeolite synthesis.

2D-to-3D zeolite transformation for the preparation of Pd@MWW catalysts with tuneable acidity

2D-to-3D transformation of layered zeolite precursors (e.g., MCM-22 P, IPC-1 P, pre-FER) into three-dimensional frameworks in combination with metal encapsulation has been recently shown as efficient synthetic strategy to stabilize metal clusters in a zeolite matrix. Even though, tailoring the number of acid sites in a zeolite support and its effect on the properties of thus prepared metal@zeolite materials (e.g., metal dispersion and catalytic activity) remained unrevealed. Herein, we report on the synthesis of Pd@MCM-22 catalysts via modified 2D-to-3D transformation method, which allows to achieve close to nominal loading of noble metal (e.g., 0.1, 0.3, 0.8 wt%) by time-separation of swelling of MCM-22 P layered zeolite precursor and subsequent incorporation with metal. In turn, alternating Si/Al ratio of MCM-22 P (e.g., SiAl = 15, 20, 30) enabled the preparation of Pd@MCM-22 catalysts with variable concentration of acid sites (0.28 – 0.55 mmol/g), while not affecting the Pd dispersion. Decrease in Pd dispersion with increasing metal loading was accompanied with declining catalytic activity in a model reaction of selective hydrogenation of nitro-group in 3-nitrotoluene: 0.1 wt% Pd (dispersion = 52.6%, reaction rate = 21.2 mol·g−1Pd·h−1) > 0.3 wt% Pd (9.6 mol·g−1Pd·h−1) > 0.8 wt% Pd (23.2%, 3.7 mol·g−1Pd·h−1). The results of this study reveal that the reported synthesis strategy can be successfully used to confine small metal nanoparticles in a zeolite matrix while tuning its acidic characteristics for designing bifunctional metal-acidic catalysts.

The silanization process for the hydrothermal synthesis of hierarchical titanium silicalite-1

The silanization is an important approach for hierarchical zeolite preparation. Although the influences of the silanization conditions have been detailed studied, the differences in the reactivity of the zeolite precursor should further be analyzed. In this paper, hierarchical titanium silicalite-1 (HTS) was prepared with trimethoxy[3-(phenylamino)propyl]silane (TMPAPS) under the nTMPAPS:nSi = 0.04–0.20 and nTi:nSi = 0.005–0.04 conditions, the changes of the corresponding physicochemical properties were analyzed, and the influences of the condensation activities between silicon/titanium to the organosialne were deduced and verified by density functional theory (DFT) calculations. Generally, more supermicropores and mesopores can be created in the metastable sponge-like HTS crystals with higher TMPAPS and titanium contents. In comparison, the free energy barrier in the condensation reaction with titanium (1.31 eV) is lower than silicon (1.55 eV), it's preferential to prepare HTS with more intracrystalline secondary porosity by increasing the nTi:nSi. More importantly, the titanium can be isolated by the organosilane, and HTS with more framework titanium can be synthesized by silanization.

Fine property-tuning through Ca content control in a facile synthesis of glasses-based self-setting injectable hydrogel

Self-setting injectable composite hydrogels are promising materials in tissue engineering. In this study a facile synthetic approach was designed based on traditional sol–gel method to produce particulate calcium aluminosilicate-type glasses, which were then formulated with potassium alginate to form a self-setting injectable hydrogel. In order to control the setting-time and mechanical properties of the composite materials, a series of samples were produced with different starting chemical composition. The change in chemical composition and morphology of the glass particles in response to varying initial Ca2+ content were further elucidated by a combination of EM, XRF and NMR spectroscopy, revealing that the binary aluminosilicate glass was preferably formed when low Ca content (<20%) was used in the formulation. At higher Ca content (>30%), calcium aluminosilicates were predominantly formed due to the change in the type of zeolite precursor formed during the low temperature nucleation process in the sol–gel approach. It was also found the hardening times of the alginate gels was positively correlated to the mole ratio of Ca/Al due to high acid resistance of high coordination number Al species. For hydrogels made with glass particle additions, the mechanical strengths were also positively correlated with the Ca/Al ratio for all formulations.