Crystalline Zeolite Research Articles

Preactivated zeolite nanosheet plate-tiled membrane on porous PVDF film: Synthesis and study of proton-selective ion conduction

Preactivated MFI zeolite nanosheet plates (ZNPs) with large areas (∼2.0 × 2.0 μm2) and nanometer thicknesses (∼60 nm) were prepared directly in liquid-dispersed state. The individual ZNP was a stack of 4-nm-thick single crystalline zeolite nanosheets (ZNs) interlinked by Si–O–Si bonds between neighboring ZN surfaces. The dispersed open-pore ZNPs allowed formulating suspensions for tiling ZNP membranes on polymer substrates without requiring post-coating activation. A pinhole-free ZNP-tiled (ZNPT) membrane has been achieved on macroporous PVDF film by a self-repairing vacuum-assisted filtration coating method. The resultant ZNPT layer was ∼500 nm-thick consisting of about 7 ZNP layers with inter-ZNP width and inter-ZNP entrance porosity around 9 nm and 2%, respectively. The ZNPT-PVDF membrane exhibited high selectivity to proton transport over vanadyl ion and low resistances to proton conduction in aqueous solutions. The membrane also demonstrated to function as an efficient ion separator for the vanadium redox flow battery. The reported synthesis of preactivated ZNP suspension may overcome the major hurdle to realizing polymer-supported ZN membranes, which is the inefficiency of existing methods in obtaining readily dispersible open-pore ZNs. The ZNPT-PVDF membrane can be a more affordable and sustainable alternative to the Nafion-based ion separation membranes.

Na2WO4/Mn supported on all-silica delaminated zeolite for the optimal oxidative coupling of methane via the effective stabilization of tetrahedral WO4: Elucidating effects of support precursors with different crystal structures, Al-addition, and morphologies

The oxidative coupling of methane (OCM) converts this abundant natural feedstock into value-added products and is typically performed in the presence of catalysts such as SiO2-supported Na2WO4/Mn to suppress the undesired deep oxidation affording CO and CO2. Currently, the phase transformation of SiO2 supports to α-cristobalite is known to be important for securing high OCM activity, although the underlying reasons of this influence remain debatable. Herein, Na2WO4/Mn catalysts supported on several SiO2-based materials including conventional amorphous SiO2 and crystalline zeolites were prepared to close the above knowledge gap and thus promote the design of more efficient OCM catalysts. The best support was identified as calcined D-ITQ-1, which is a well-developed delaminated zeolite with a thin basal all-silica nanosheet structure facilitating the transition to the α-cristobalite phase. The corresponding catalyst retained high activity over 100 h and exhibited one of the highest yields (25.8%) of C2+ hydrocarbons (paraffins and olefins) reported for dopant-free Na2WO4/Mn/SiO2 catalysts to date. The results of XRD, Raman, and 29Si NMR indicated that this high performance can be ascribed to the stabilization of tetrahedral WO4 units due to the transition from uncalcined silica to α-cristobalite. Thus, the present work advances our understanding of structural phase transitions and the concomitant formation of the desired active species in Na2WO4/Mn/SiO2 catalysts and paves the way to the design of better catalysts through the modification of zeolite supports.

Characterizing crystalline phase transitions in zeolitization of coal fly ash using focused ion beam-scanning electron microscopy

Coal fly ash (CFA) accounts for a significant portion of coal ash, which is a by-product of thermal power plants. Herein, CFA collected in Korea was used to synthesize a zeolitic material via fusion and hydrothermal synthesis. The crystallographic characteristics of CFA, alkali-fused CFA, and synthesized zeolitic material from CFA (SZC) were investigated via cross-section analysis using focused ion beam-scanning electron microscopy (FIB-SEM) with energy-dispersive X-ray spectroscopy (EDS), which has not been attempted in previous studies. Cross-section milling and EDS line scan were performed using FIB-SEM to develop the existing analysis methods, focusing on the crystalline phase and chemical composition of the surface of synthetic materials. Following the alkali treatment, the amorphous CFA exhibited a biphase comprising both crystalline and amorphous phases. After the synthesis process, both the surface and the interior exhibited a uniform Na-A zeolite crystalline structure with sharp-edged cubes. Furthermore, the SZC comprised crystals of Na-A zeolite with a uniform structure and a Si/Al molar ratio of approximately 1.4.

Effect of the SiO2/Al2O3 Molar Ratio on the Microstructure and Properties of Clay-based Geopolymers: A Comparative Study of Kaolinite-based and Halloysite-based Geopolymers

AbstractAs 1:1 dioctahedral clay minerals, kaolinite and halloysite have similar chemical compositions. However, halloysite often possesses a nanotubular structure and special surface reactivity compared to platy kaolinite. The objective of this current work was to determine the effect of the SiO2/Al2O3 ratio on the microstructure and properties of geopolymers derived from two kinds of kaolin: platy kaolinite and nanotubular halloysite. The chemical structures and compositions of the geopolymers obtained were characterized through X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR), whereas the microstructural analysis was performed by scanning electron microscopy (SEM), the Brunauer–Emmett–Teller (BET) method, and N2 physisorption analysis. The results indicated that calcined halloysite showed greater geopolymerization reactivity than calcined kaolinite. In addition, the mechanical properties of the clay-based geopolymers depended not only on the SiO2/Al2O3 ratio but also on the morphology of the clay. Crystalline zeolite A and geopolymer were produced after alkali-activation of kaolin with a SiO2/Al2O3 ratio of 2.5; these products possessed porous and heterogeneous microstructures having poor compressive strength. As SiO2/Al2O3 ratios increased to >2.5, geopolymers with compact microstructure and high compressive strength were produced after alkali-activation of kaolin. Notably, at a given condition, halloysite-based geopolymers exhibited greater early compressive strength, more compactness, and more homogeneous microstructure than kaolinite-based geopolymers. This can be attributed to the nanotubular microstructure of halloysite, which can release more Si and Al during alkali activation than platy kaolinite. These results indicated that the various morphologies and microstructures among clays have significant impact on the microstructure and compressive strength of geopolymers.

Boosting the propylene selectivity over embryonic borosilicate zeolite catalyst for oxidative dehydrogenation of propane

Boron-containing zeolites have been proven potential in catalyzing the oxidative dehydrogenation (ODH) process and the boron sites in open coordination are considered as the active center. By considering the diversity and adjustability in the crystal structure of zeolite, there is a great opportunity to further tune the activity and selectivity. Herein, the embryonic borosilicate zeolite with abundant defective sites and developed large porosity is prepared for catalyzing ODH of propane reaction. An ∼ 20 % improvement in propylene selectivity is achieved at propane conversion of 21.5 % at 540 °C, with propylene productivity nearly an order of magnitude higher compared with crystalline MFI-type borosilicate zeolite. Solid-state NMR reveals the formation of boron sites in open coordination derived from the defect-rich structure of embryonic borosilicate zeolite, guaranteeing ample exposure of the active center. Combined with kinetic measurements, the reaction in the gas phase initiated by the active boron sites is mainly responsible for the high catalytic performance in ODH of propane reaction. The designed experiment indicates the strengthened gas-phase reaction facilitates the activation of propane, consequently resulting in the increase of reaction order of propane from 1.06 to 2.30. This work reveals the significant role of gas-phase reactions during the ODH of propane reaction and provides a new insight for boosting the propylene selectivity of zeolite-based catalysts.

Tailored alkyl-imidazolium templating synthesis of extra-large-pore germanosilicate zeolite ITT as Baeyer-Villiger oxidation catalyst

Extra-large-pore germanosilicate zeolite ITT was prepared with several structure-tailored alkyl-substituted imidazolium templates. The effect of alkyl chain length and substitution number of template on zeolite topology structure was studied with the variation of synthesis Si/Ge ratio. 1-hexyl-3-methyl-imidazaolium (1H3M) exhibited superior templating effect on the formation of ITT structure within the widest range of synthesis Si/Ge molar ratio of about 2–6. 1H→13C CP/MAS NMR, TGA and molecular simulation results revealed that two 1H3M was occluded in the 18-ring channels per unit cell, presenting the lowest interaction energy with framework. With the combination of XRD, SEM, EDS, ICP, 29Si, 19F MAS NMR, FTIR, N2 physisorption results, it was found that the increase of synthesis Si/Ge ratio improved framework Si/Ge ratio within d4r unit to a certain degree, and in the meantime resulted in the decrease of zeolite crystallinity with the formation of some amorphous phase. ITT showed better Lewis-acid catalytic activity and lactone selectivity for B–V oxidation of 2-adamantanone than BEC, the best germanosilicate zeolite ever reported. The excellent catalytic performance was ascribed to the presence of 18-ring channels in ITT, which facilitated the diffusion of bulky ketone substrate and lactone product. The structure stability of ITT during the reaction was evaluated with the use of either aqueous H2O2 or non-aqueous tert-butyl hydroperoxide (TBHP) as oxidant.

Process optimisation of low silica zeolite synthesis from spodumene leachate residue

Lithium extraction from hard rock sources, such as spodumene, generates an aluminosilicate waste that can be converted into zeolite materials. Increased lithium demand in recent years highlights a growing need to develop a residue mitigation strategy. Zeolite synthesis from aluminosilicate waste is more complex than conventional synthetic zeolite production routes and typically involves an activation step, such as alkali fusion. In this context, a key challenge to increase product quality involved understanding the influence of factors such as aluminium supplement addition and alkali fusion processing options. Zeolite X crystalline content and particle morphology was compared in relation to various reaction gel compositions. Zeolite X was produced from alkali-fused SLR which was found to contain 60 wt% crystalline zeolite X after adjusting key synthesis parameters to 2.0 Si:Al ratio, 1.4 Na2O:SiO2 ratio, 30 H2O:Na2O. The product was highly agglomerated (D[4,3] = 51.1 µm) with small crystal sizes of< 2 µm. Holding this gel composition constant and introducing the aluminium supplementation during gel aging, the average particle agglomeration was reduced (D[4,3] = 14.1 µm). Further processing of the fused SLR qualitatively reduced agglomeration and increased zeolite X crystal size, with little impact on the crystalline zeolite X content; the latter strategy produced a zeolite X material with similar morphological characteristics to a commercially available analogue. This work demonstrated that controlling the processing and additions of activated aluminosilicate was a key consideration in terms of controlling zeolite crystal morphology.

Synthesis of NaA zeolite via the mesoscale reorganization of submolten salt depolymerized kaolin: A mechanistic study

Deep understanding of the formation mechanism is crucial to the rational design and controllable synthesis of zeolites with desired pore architecture and acidity. Herein, we take the synthesis of NaA zeolite from a submolten salt depolymerized kaolin (SMS-K) as an example to elucidate the formation mechanism of NaA zeolite. By using SMS-K as sole Si and Al sources, a highly pure and crystalline NaA zeolite with superior ion exchange ability was synthesized. By using various ex-situ and in-situ characterizations and studying the crystallization kinetics, a plausible formation mechanism of NaA zeolite via the mesoscale reorganization of SMS-K was proposed. The results indicate that the reorganization of SMS-K into NaA zeolite follows the solid-phase transformation route. Upon being mixed with H2O, SMS-K that is mainly composed of silicate monomers and chain/ring microstructures is in-situ transformed rapidly into mesoscale intermediates containing double four-membered rings and β cages, sufficient stirring during aging generates more mesoscale intermediates. During the subsequent crystallization process, the mesoscale intermediates are further self-assembled into NaA nanoparticles when theirs amount reaches a maximum, and finally the nanoparticles grow into well-shaped cubic NaA zeolite particles with smooth surfaces through layer spreading. Our work provides the theoretical foundation for the design and green synthesis of zeolites directly from the natural minerals.

Formation of USY at lower steam temperature by embedding of Ga3+ into framework of NaY and its catalytic cracking performance

• Zeolite [Ga,Al]-Y with high content of 24.4 wt% Ga 2 O 3 can be synthesized by hydrothermal method. • [Ga,Al]-Y can be operated at lower steam temperature to form [Ga,Al]-USY due to distorted structure. • [Ga,Al]-USY prepared at lower steam temperature has good catalytic performance for cracking of 1,3,5-TIPB and n -octane. Decreasing steam treating temperature of zeolite Y to prepare USY has great attraction for industrial production of FCC catalysts, which is often operated at high temperature thus leading to huge energy consumption. Heteroatoms in the framework of zeolite Y are easier to be removed under steam condition, but the introduction of heteroatoms often leads to the decrease of relative crystallinity of zeolite. When Ga 3+ is introduced into the framework of zeolite Y, high crystallinity is well maintained in this work. Interestingly [Al, Ga]-NaY can be treated at low steam temperature to form [Al, Ga]-USY with very good mesoporosity and high framework SiO 2 /Al 2 O 3 ratio. The sample [Al, Ga]-NaY-8 has a distinctive micropore size distribution at 0.92 nm which is much bigger than conventional microporous NaY. DFT study disclosed the reason of low temperature steam treatment. The catalytic cracking of 1,3,5-TIPB and n -octane showed that the catalyst [Al, Ga]-USY obtained at lower steam treating temperature presented good performance.

Crystalline Zeolite Layers on the Surface of Titanium Alloys in Biomedical Applications: Current Knowledge and Possible Directions of Development

In this review, the main focus was on the use of zeolites as layers to modify the surface of titanium implants. The article discusses the basic methods for modifying the functional properties of zeolites (e.g., ion exchange) as well as biomedical applications of zeolites (e.g., drug delivery systems and biosensors). The article reviews the surface modifications of titanium alloys prepared so far with the use of various types of zeolites and selected examples are presented. This review shows the significant impact of titanium surface modification with zeolites, as well as their post-synthetic modification on implant properties, for instance, better biocompatibility, faster osseointegration, better cell adhesion, and corrosion resistance properties. The results of the research presented so far in this review show that the modification of titanium with zeolite layers is a very prospective subject, but underdeveloped, as evidenced by a small number of studies on this subject. We have shown that the prepared layers can be continuously improved and used, e.g., as local delivery systems for various active pharmaceutical ingredients (APIs). We hope that the prepared review will help research groups around the world in the preparation of implants modified with zeolites with even better properties and utility applications.

Acidic-treated TS-1 zeolites with high titanium for cyclohexanone efficient oximation

Successful preparation of acidic-treated TS-1 zeolites with high titanium by hydrothermal synthesis in the presence of cetyltrimethylammonium bromide and tetrapropylammonium hydroxide as templates and followed by treatment with 1 M hydrochloric acid were reported in this paper. Many characterization techniques, including XRD, N2 adsorption-desorption, ICP, UV-vis, FT-IR, SEM, Raman, XPS and TEM, were applied to characterize their physical and chemical properties, and the cyclohexanone oximation was also conducted to investigate the catalytic performance. The results showed that although the crystalline TS-1 zeolite with high titanium can be obtained with double templates, the extra-framework TiOx would be formed based on the SEM images and UV-vis spectra. The MFI topological structure with the acidic treatment can be kept and all the titanium species can be almost incorporated into the framework of TS-1 zeolite, whose relative content for S-0.2-CTS-0.04-160 sample was about 96%. The S-0.2-CTS-0.04-160 sample with the skeleton titanium species as high as 3.38 mol.% resulting in more defects showed higher catalytic performance than other catalysts, whose cyclohexanone conversion of 97.6 % and caprolactam yield of 94.6 % can be obtained. Compared to classical TS-1 zeolite, the S-0.2-CTS-0.04-160 catalyst showed good stability with 89.8 % of cyclohexanone conversion and 84.9 % of caprolactam yield even though after repeated use for 5 times.

Ion-exchange modification of zeolite for the controllable catalytic transformation of diols and selective conversion of 1,2-butanediol to butanal

For the controllable transformation of 1,2-propanediol (PDO) and 1,2-butanediol (BDO) in ethylene glycol (EG), the acidity of H-Beta zeolite was modified via ion exchange with NaH2PO4 and Na2HPO4. After the modification, the separation factors for PDO and BDO reached 7.7 and 11.6 (vs 3 on the parent H-Beta). The selectivity to butanal-derived acetal declined to 4% (vs 42% on the parent H-Beta). Therefore, EG consumption was greatly reduced. Lower temperatures were favorable to the formation of acetone, butanone, and 1,3-butadiene, while higher temperatures promoted the acetalization and oligomerization reactions. The ion-exchange treatment increased the stability of zeolite crystalline structure. Microcalorimetry and NH3-TPD results indicated that the strong and moderate acid sites with NH3 adsorption heat ≥140–160 kJ/mol and of 100–110 kJ/mol were responsible for the dehydration and butanal-related acetalization reactions, respectively. In contrast, the propanal-related acetalization reactions were less sensitive to the strength and type of acid sites. On the other hand, butanal can be selectively synthesized from BDO with this catalyst. The acidity of zeolite catalysts can be modulated with this simple approach for the improvement of selectivity to desirable products.

Modification and Functionalization of Zeolites for Curcumin Uptake.

This work shows that hierarchical zeolites are promising systems for the delivery of biologically relevant hydrophobic substances, such as curcumin. The validity of using piperine as a promoter of curcumin adsorption was also evaluated. The use of pure curcumin is not medically applicable due to its low bioavailability and poor water solubility. To improve the undesirable properties of curcumin, special carriers are used to overcome these shortcomings. Hierarchical zeolites possessing secondary mesoporosity are used as pharmaceutical carrier systems for encapsulating active substances with low water solubility. This porosity facilitates access of larger reagent molecules to the active sites of the material, preserving desirable adsorption properties, acidity, and crystallinity of zeolites. In this work, methods are proposed to synthesize hierarchical zeolites based on a commercial FAU-type zeolite. Studies on the application and adsorption kinetics of curcumin using commercial FAU-type zeolite and hierarchical zeolites based on commercial FAU-type zeolite are also included.

Enhancing the CO2 adsorption kinetics on Na-RHO and Cs-MER zeolites by NH4F/H2O2 etching induced mesoporosity

The Na+-form (Na-RHO) of zeolite rho has emerged as a potential CO2 adsorbent with excellent selectivities toward CH4 and N2, but the very slow CO2 adsorption kinetics hinders its practical use. Here we report that mesoporous Na-RHO, prepared by NH4F/H2O2 etching, shows remarkably enhanced CO2 adsorption kinetics (ca. 7 min vs > 2 h) compared to the Na-RHO, which was also observed for the NH4F/H2O2-etched, Cs+-form (Cs-MER) of merlinoite. A combination of SEM, TEM, and N2 adsorption measurements demonstrates that such kinetics enhancement originated from the mesopore creation by fluoride medium etching, while minimizing the loss of zeolite crystallinity. The observed differences in the zeolite crystal morphology of Na-RHO and Cs-MER and in the characteristics of their intrinsic defect sites appear to be responsible for those in the shape and average size of the secondary mesopores created. The overall results of this study suggest that NH4F/H2O2 etching can be also applied to other small-pore zeolites to improve CO2 adsorption kinetics.

Binder‐free Zeolite Beta Beads with Hierarchical Porosity: Synthesis and Application as Heterogeneous Catalysts for Anisole Acylation

AbstractThree zeolites (H‐Beta, H‐ZSM‐5 and H‐Y) were synthesized in the form of binder‐free macroscopic beads (d=215–840 μm) using a hydrothermal method employing anion‐exchange resin beads as hard template. The beads obtained after removal of the hard template by calcination consisted of crystalline zeolite domains connected with each other to form a hierarchical porous network in which the zeolitic micropores are accessible through meso‐ and macropores, as proven by characterization with XRD, N2 physisorption, SEM, and TEM. The composition, the nature and amount of acid sites and the degree of hydrophobicity of these beads were investigated by means of XRF, solid‐state NMR, pyridine‐FTIR and TGA. The zeolite beads were tested as heterogeneous catalysts in the Friedel‐Crafts acylation of anisole with acetic anhydride to produce para‐methoxyacetophenone. H‐Beta‐Beads displayed the best catalytic performance with 95 % conversion of acetic anhydride and 76 % yield of para‐methoxyacetophenone in a batch reactor test (90 °C, 6 h). Next, the catalytic performance of H‐Beta‐Beads was compared in both batch and continuous‐flow mode to extrudates prepared by mixing zeolite Beta powder with either kaolin or bentonite binders. H‐Beta‐Beads outperformed the extrudates in batch‐mode reactions and could be reused in multiple runs without discernible loss of activity. In the continuous‐flow test, H‐Beta‐Beads demonstrated higher average activity but deactivated more rapidly than the extrudates.

Comparative catalytic study on butene/isobutane alkylation over LaX and CeX zeolites: The influence of calcination atmosphere

Lanthanum-containing (LaX) and cerium-containing X zeolites (CeX) were prepared by a double-exchange, double-calcination method. By changing the calcination atmospheres between nitrogen and air, the CeIV contents in CeX zeolites were adjusted and their impacts on physicochemical properties and catalytic performance in isobutane alkylation were established. The crystallinity of CeX zeolite was found to be negatively correlated with the CeIV content. This i s believed to be due to the water formed during the oxidation of CeIII, which facilitates the framework dealumination. As a consequence, calcining in air resulted in a great elimination of strong Brønsted acid sites while under nitrogen protection, this phenomenon was mostly hindered and the sample's acidity was preserved. When tested in a continuously flowed slurry reactor, the catalyst lifetime for isobutane alkylation was found to be linearly related to the strong Brønsted acid concentration. In addition, Ce3+ was found more benefit for the hydride transfer compared with La3+, which is ascribed to the stronger polarization effect on the CH bond of isobutane. Moreover, the decline of hydride transfer activity can be slowed down by the catalytic cracking of the bulky molecules. Based on the product distribution, a new catalytic cycle of dimethylhexanes (DMHs) involving a direct formation of isobutene rather than tert-butyl carbocation was proposed in isobutane alkylation.

In Situ Growth of Crystalline and Polymer-Incorporated Amorphous ZIFs in Polybenzimidazole Achieving Hierarchical Nanostructures for Carbon Capture.

Mixed matrix materials (MMMs) hold great potential for membrane gas separations by merging nanofillers with unique nanostructures and polymers with excellent processability. In situ growth of the nanofillers is adapted to mitigate interfacial incompatibility to avoid the selectivity loss. Surprisingly, functional polymers have not been exploited to co-grow the nanofillers for membrane applications. Herein, in situ synergistic growth of crystalline zeolite imidazole framework-8 (ZIF-8) in polybenzimidazole (PBI), creating highly porous structures with high gas permeability, is demonstrated. More importantly, PBI contains benzimidazole groups (similar to the precursor for ZIF-8, i.e., 2-methylimidazole) and induces the formation of amorphous ZIFs, enhancing interfacial compatibility and creating highly size-discriminating bottlenecks. For instance, the formation of 15 mass% ZIF-8 in PBI improves H2 permeability and H2 /CO2 selectivity by ≈100% at 35°C, breaking the permeability/selectivity tradeoff. This work unveils a new platform of MMMs comprising functional polymer-incorporated amorphous ZIFs with hierarchical nanostructures for various applications.

Mo-modified hierarchical FAU zeolite: A catalyst-adsorbent for oxidative desulfurization of fuel oil

In this study, highly crystalline hierarchical FAU zeolite was prepared and activated by MoO3 (Meso-Mo-FAU) for oxidative desulfurization (ODS) of fuel oils. Various characterization techniques such as XRD, SEM, TEM, NH3-TPD, TGA/DTA, EDS mapping, FT-IR and N2 sorption were employed to identify the structural properties of the catalyst and supposed mechanism of the ODS process. The effects of different parameters on the efficiency of the ODS system were examined. The results revealed that Meso-Mo-FAU with 19 ​wt.% molybdenum exhibited high catalytic activity at 70 ​°C. Moreover, the structural stability and reusability of the catalyst were conveniently investigated. Hot filtration test confirmed the spent catalyst after six ODS cycles had a robust heterogeneous nature due to no leaching of Mo species into the reaction medium. Furthermore, 99.83% ODS efficiency for real gasoline made an impressive view of this desulfurization system, including catalyst-adsorbent Meso-Mo-FAU, for industrial application.

Kinetic and Thermodynamic Factors Influencing Palladium Nanoparticle Redispersion into Mononuclear Pd(II) Cations in Zeolite Supports

Palladium cations and nanoparticles supported on oxides and zeolites are used as catalysts and adsorbents for a wide range of chemical reactions of practical importance, with various and distinct active site requirements. Consequently, sintering and redispersion processes that interconvert site-isolated Pd cations and agglomerated nanoparticles underpin catalyst activation and deactivation phenomena, yet the influence of Pd nanoparticle size distribution and gas conditions on the thermodynamic and kinetic factors influencing such interconversion is imprecisely understood. Here, we prepare Pd nanoparticles of different particle size and distribution (normal, log–normal) supported on high-symmetry crystalline zeolites to access well-defined materials whose Pd site distributions can be quantitatively characterized by experiments and modeled accurately by theory. Ab initio thermodynamic modeling and isothermal (593–973 K) wet and dry air treatments of Pd-zeolites reveal that the widely observed deactivation in low-temperature hydrous environments reflects site interconversion thermodynamics that favor the agglomeration of isolated Pd cations into nanoparticles. Under high-temperature anhydrous conditions, experimental kinetic measurements and kinetic Monte Carlo simulations evince the preeminence of kinetic factors on Pd nanoparticle redispersion into cations, which proceeds at rates that are strongly influenced by the initial Pd particle size distribution and via a substrate diffusion-mediated Ostwald ripening process whereby Pd monomers are captured in an atom trapping process at anionic exchange sites (framework Al) in the zeolite support. These findings resolve longstanding questions regarding the roles of H2O and support interactions in Pd redispersion processes and identify strategies to enhance or suppress Pd site interconversion by modifying oxide supports and gas conditions.

Conversion of Green Silica from Corn Leaf into Zeolites Na A-X

Combustion of corn leaf as a model of biomass from agricultural waste is a simple way to obtain the energy. It produces a low-value by-product of ash and is rich in silica that can become a precursor for zeolite production. In this study, acid-treated corn leaves combustion was performed to produce high purity silica (SiO2). The diffraction pattern suggested that the extracted silica was amorphous without the impurities phase. Additionally, the nitrogen isotherm indicated that the material was highly mesoporous silica with a total surface area of 200 m2/g. The hydrothermal method was then applied with a composition molar ratio of 1.25SiO2:1Al2O3:5Na2O:250H2O to synthesize zeolites from the silica. The temperature and time effect on the hydrothermal zeolite’s synthesis was investigated. The diffraction pattern shows that high crystalline zeolite Na A-X was produced at temperatures of 100°C and 8 h hydrothermal time. According to nitrogen physisorption analysis, the zeolite Na A-X consisted of micropores with a total surface area of 270 m2/g. The morphology of zeolite Na A-X was cube for the Na-A and octahedral for the Na-X. The hydrothermal temperature and time highly affected the zeolite formed. This research suggested that the ash waste could be valorized through conversion into a high economic value zeolites.