Zeolite Crystals Research Articles

Generating zeolite crystals with mesoscopic aperiodic structure to promote catalytic hydroisomerization

Generating hierarchical zeolites provides an effective way to enhance their transport and catalytic properties. Several seminal methods have been reported to fabricate them with mesoscopically periodic order using surfactant-like structure...

Promoted propane dehydrogenation over Co confined within core–shell silicalite-1 zeolite crystals

Silicalite-1 coating boosted the catalytic activity of Co-loaded silicalite-1 zeolite for propane dehydrogenation reaction.

Morphology Regulation of Zeolite MWW via Classical/Nonclassical Crystallization Pathways.

The morphology and porosity of zeolites have an important effect on adsorption and catalytic performance. In the work, simple inorganic salts, i.e., Na salts were used to synthesize MWW zeolite using the organic compound 1-Butyl-2,3-dimethyl-1H-imidazol-3-ium hydroxide as a structure-directing agent and the morphology was regulated by the alkali metals. The sample synthesized without Na salts shows a dense hexagon morphology, while different morphologies like ellipsoid, wool ball, and uniform hexagon appear when using NaOH, Na2CO3, and NaHCO3, respectively. Moreover, the impact of Na salts on the induction, nucleation, and the evolution of crystal growth was studied. Different kinds of Na salts have a different impact on the crystalline induction time in the order of NaHCO3 (36 h) < Na2CO3 (72 h) = NaOH (72 h). Meanwhile, the crystalline mechanism with the cooperation of inorganic salts and the organic SDAs is proposed. NaOH- and Na2CO3-MWW zeolite crystallized with a network of hydrogel via the nonclassical pathway in the system; however, the product is synthesized via a classical route in the NaHCO3 environment. This work provides information about MWW zeolite crystallization and modulating diverse morphologies by adjusting the process.

Micron-Sized Hierarchical Beta Zeolites Templated by Mesoscale Cationic Polymers as Robust Catalysts for Acylation of Anisole with Acetic Anhydride

Hierarchical Beta zeolites with interconnected intracrystalline mesopores and high structural stability are highly attractive for catalytic applications involving bulky reactants. Here, by introducing a suitable amount of polydiallyldimethylammonium chloride into the initial synthesis system, micron-sized Beta zeolite crystals with abundant hierarchical porosity (Beta-H) were hydrothermally synthesized. The sample named Beta-H_1 exhibited very high catalytic activity and durability for the Friedel–Crafts acylation of anisole with acetic anhydride. A 92% conversion rate of acetic anhydride could be achieved after 1 h of reaction in a fixed bed reactor, and 71% conversion still remained after 10 h, much better than the rate for conventional Beta zeolite (which decreased rapidly from 85% to 37% within 10 h). The enhanced catalytic performance of Beta-H zeolites could be mainly attributed to the relatively lower strong acid density and the faster transport rate of the hierarchical zeolites. In addition, Beta-H showed high structural stability and could be easily regenerated via high-temperature calcination without obvious loss in catalytic activity, demonstrating its great potential for catalytic applications in the industrially important Friedel–Crafts acylation process.

Influence of ZSM-5 Crystal Size on Methanol-to-Olefin (MTO) vs. Ethanol-to-Aromatics (ETA) Conversion.

Crystal size is a key parameter of zeolites applied as catalysts. Herein, ZSM-5 crystals with similar physicochemical and acid properties, few defects, and aluminum exclusively in tetrahedral coordination are synthesized and the influence of the crystal size on the MTO and ETA conversion is investigated. Short olefins are the main products of the MTO conversion, whereas larger olefins and aromatics dominate the products after ETA conversion. In the case of both feeds, an increased crystal size decreases the catalyst's lifetime. The MTO conversion over larger ZSM-5 altered the product distribution, which was not the case for the ETA conversion. The reason is that the instantly available aromatics during ETA conversion lead to fast coking and zeolite crystals only active in the outer layers. Thus, the different reactivity of different-sized ZSM-5 is direct proof of a different conversion mechanism for both alcohols.

Synthesis of SAPO-34 Zeolite Membrane: Influence of Sources of Silica

The research described the production and characterization of various materials, particularly alpha-alumina ceramic supports, zeolite SAPO-34, and zeolite membranes. Ceramic supports were manufactured through dry uniaxial compaction. Sintering of the supports was carried out at 1300°C for 2 h. SAPO-34 zeolites and zeolite membranes were synthesized through a hydrothermal process involving two steps: a first step at 38°C for 24 h and a second step at 200°C for 24 h. The research aimed to determine how different silica sources, namely Aerosil 380, colloidal silica, and TEOS, influenced the outcome of the synthesis. The study identified that Aerosil 380 silica was the most suitable source for synthesizing SAPO-34 zeolites and membranes. Zeolite membranes (SAPO-34/alpha-alumina) displayed a uniform and homogeneous distribution of SAPO-34 phase zeolitic crystals. The absence of defects or cracks in these membranes confirmed the successful formation of the SAPO-34 zeolite membrane structure. This research has significant implications, particularly in materials science and applications utilizing zeolites and membranes. The choice of silica source plays a crucial role in determining the quality and properties of the synthesized materials, and the detailed characterization provides valuable insights into their performance in practical applications. Overall, the research contributes to the understanding and optimization of zeolite synthesis processes.

Insight on MWW siliceous zeolites: From 2D precursors toward 3D structure

ITQ-1 layered zeolites, with MWW framework, were prepared using different synthesis and calcination procedures and fully characterized by means of X-ray diffraction, scanning electron microscopy, N2 sorption and vibrational and nuclear magnetic resonance spectroscopies. Exploring different compositions of the precursor gel, the role of the Organic Structure Directing Agent (OSDA) was definitely disclosed. We proved that the concentration of OSDA in the synthesis gel affects the short-range crystalline order of zeolite crystals. In particular, diluted precursor gels led to the formation of low-density materials in the form of hollow spheres, with partially disordered layers, thinner crystals and high defectivity. In general, all ITQ-1 samples had the same crystalline structure but different morphology, either rose-like or hollow spheres. Moreover, the calcination procedure had an impact on the structural defects of the ITQ-1 zeolite: the higher the removal rate of the OSDA, the higher the defectivity degree of the zeolite.

Artificial Sodium Channels for Enhanced Osmotic Energy Harvesting.

Inspired by the ion channels of electric eels, we can use biomimetic nanofluidic materials to harvest the osmotic power released by mixing seawater and river water. While biological ion channels have both cation/anion and inter-cation selectivity, previous nanofluidic materials neglected the latter. As a result, NaCl solutions were generally used to simulate river water, ignoring the fact that the dominating cation in river water is typically Ca2+. In this work, we show that the different ionic compositions of seawater and river water can be exploited to improve osmotic power density by employing biomimetic sodium selective materials. Inspired by a range of properties of biological sodium channels, we constructed artificial sodium channels with zeolitic imidazolate framework-65 crystals, which selectively transport Na+ but almost completely block Ca2+. Resultantly, the effective concentration gradient of seawater/river water is dramatically increased by preventing the major cations in the river water from participating in the ion diffusion. As a result, the osmotic power density can be increased by more than 1 order of magnitude. These results should open new avenues to develop high-performance osmotic generators and may advance other applications based on biomimetic ion channels such as neuromorphic information processing.

Zeolitic germanosilicate analogue to pharmacosiderite crystallized in an acidic medium

Zeolites are typically synthesized in alkaline or fluoride-containing near-neutral media. Sophisticated organic structure-directing agents have been investigated for such systems with the aim of discovering materials with unprecedented structures and properties for novel technical applications. In contrast, zeolite crystallization in strongly acidic media has yet to be explored. This study demonstrates that a zeolitic silicate phase crystallizes from acidic gels using trimethylamine as an organic additive with the composition 1 SiO2:0.3 TMA:0.3 HCl: 0.15 HF:55 H2O:(0.1–0.4) GeO2. This phase has an interrupted four-connected framework analog to the octahedron/tetrahedron-mixed framework of the mineral family pharmacosiderite. In comparison to the pharmacosiderite-type HK3(Ge7O16)(H2O)4, the four GeO6-octahedra forming the central [HGe4O4O12]-cluster are replaced by four SiO4-tetrahedra in a [Si4O6(OH)2.89]-unit in the new phase. However, the structure is distorted and may contain connectivity and point defects; thus, healing by the occasional incorporation of GeO6-units is necessary. The refined unit cell has a cubic symmetry, space group P-43m (#215), with a = 7.7005(1) Å. Acidic-medium synthesis is a useful way to find new zeolites that move in a fundamentally different direction from sophisticated organic structure-directing agents.

Self-limiting growth of thin dense LTA membranes boosts H2 gas separation performance

The preparation of dense LTA-type zeolite membranes with a thickness in the sub-micron range remains extremely difficult due to their high electronegativity. Herein, thin dense LTA membranes were prepared through the secondary growth of nano LTA seeds in a highly reactive gel. The uniformity of the nano seed layer contributed to the growth of continuous membranes. The self-limiting growth of the zeolite membrane layer was controlled by the synergistic effect of using a highly reactive gel and nano seeds, and resulted in the preparation of thin membranes. Optimized aging times of the gel further enhanced the packing density of the membranes via an efficient intergrowth of the zeolite crystals. This is the first time to obtain single-phase LTA membranes with a thickness of less than 1 μm. The ∼ 470 nm-thick membrane exhibited a H2 permeance of 1.5 × 10-6 mol m−2 s−1 Pa−1, which is tenfold higher than results for literature-reported membranes, a H2/CO2 selectivity of 20.9, and a H2/CH4 selectivity of 17.3. The approach presented offers an alternative pathway for the preparation of thin zeolite membranes with superior permeability and selectivity in gas separation applications.

Hydrogenation of Carbon Dioxide to Dimethyl Ether on CuO–ZnO/ZSM-5 Catalysts: Comparison of Powder and Electrospun Structures

The promising direct dimethyl ether (DME) production through CO2 hydrogenation was systematically analyzed in this research by synthesizing, characterizing, and testing several catalytic structures. In doing so, various combinations of precipitation and impregnation of copper- and zinc-oxides (CuO–ZnO) over a ZSM-5 zeolite structure were applied to synthesize the hybrid catalysts capable of hydrogenating carbon dioxide to methanol and dehydrating it to DME. The resulting catalytic structures, including the co-precipitated, sequentially precipitated, and sequentially impregnated CuO–ZnO/ZSM-5 catalysts, were prepared in the form of particle and electrospun fibers with distinguished chemical and structural features. They were then characterized using XRD, BET, XPS, ICP, TGA, SEM, and FIB-SEM/EDS analyses. Their catalytic performances were also tested and analyzed in light of their observed characteristics. It was observed that it is crucial to establish relatively small-size and well-distributed zeolite crystals across a hybrid catalytic structure to secure a distinguished DME selectivity and yield. This approach, along with other observed behaviors and the involved phenomena like catalyst particles and fibers, clusters of catalyst particles, or the whole catalytic bed, were analyzed and explained. In particular, the desired characteristics of a CuO–ZnO/ZSM-5 hybrid catalyst, synthesized in a single-pot processing of the precursors of all involved catalytically active elements, were found to be promising in guiding the future efforts in tailoring an efficient catalyst for this system.

Enhanced bio-oil upgrading by sub-microscale dispersed silanol-free ZSM-5 nanosheets and evidence for revealing an unconventional mechanism

Silanol-free ZSM-5 nanosheet with merely 35 nm thickness in the b-axis was synthesized by seeded growth from defect-free 10 nm silicalite-1 in a fluoride medium (denoted as F35). Submicron-scaled stöber sol silica beads were introduced into the gap between nanosheets to serve as a spacer for the prevention of neighboring crystals' close contact with each other, which can improve the heat and mass transportation during the catalytic reactions when compared to the aggregated zeolite crystals. Methanol-to-hydrocarbons (MTH) and model bio-oil upgrading were conducted by using dispersed F35, respectively, and the results were compared to the aggregated ZSM-5. The effluent from bio-oil upgrading through dispersed F35 contains no heavier compound, which was considered as the coke precursor, and significantly reduced amount of unreacted feeding molecule. GC–MS revealed that the coke solution from dispersed F35 contains a considerably reduced (61 %) amount of heavier carbon species. Among them, some species have molecular configurations highly consistent with the zeolite channel structure. The dispersed F35 shows a 33 % longer life in catalytic reactions and a 60 % decreased amount of external thermal cock. An unconventional ‘temperature-determined flexible channel’ mechanism that explains the ‘host–guest’ interactive behavior under catalytic reaction was proposed.

A fast synthetic strategy for quick preparation and optimization of platelike MFI crystals

The demand for high aspect ratio zeolite crystals has been increasing dramatically for many applications, ranging from catalysis to membrane separations. However, manufacturing such platelike crystals usually requires energy-intensive and time-consuming synthesis procedures. Here, by employing a thin-wall tubular reactor, we report for the first time a fast synthetic strategy that yields preferentially-b-oriented platelike MFI crystals within minutes of hydrothermal treatment. Various synthesis conditions such as hydrolysis, precursor aging, chemical compositions, etc. can be quickly optimized within days. Such optimization resulted in highly crystalline platelike crystals with lateral dimensions up to ∼5 μm length in c-axis or ultra-thin crystals with ∼25 nm thickness. Meanwhile, non-platelike MFI crystals can even reach 135 μm lateral size in 24 h. This fast synthetic strategy can be further generalized for the preparation and optimization of diverse important materials including MOFs, COFs, and other zeolites, benefiting both fundamental research and practical applications.

Analysis and Augmentation of Guest–Host Interaction Energy Models as CHA and AEI Zeolite Crystallization Phase Predictors

Analysis and Augmentation of Guest–Host Interaction Energy Models as CHA and AEI Zeolite Crystallization Phase Predictors

Evolution of nucleation/growth kinetics for NaP zeolite crystals and the adsorption removal of phenol compound

AbstractThe NaP zeolite with excellent crystallinity and adsorption performance was obtained by adopting the Na2SiO3 and NaAlO2 from coal fly ash as the raw materials. As the synthesis condition of aging for 0.5 h at 30°C and crystalizing for 8 h at 120°C with n(H2O)/n(SiO2) = 116, n(Na2O)/n(SiO2) = 1.4, and n(SiO2)/n(Al2O3) = 2.8, NaP zeolite owned higher crystallinity (98.98%) and spherical morphology (∼2.78 μm). During the crystallization, the nucleation/growth kinetics was calculated to survey the crystallization mechanism via the theoretical calculation and characterization methods. The diffusion of reactants in the solid–liquid‐phase interface was dominant, wherein the heterogeneous nucleation and solid‐phase transformation were also proposed based on the fitting results of Avarami–Erofeev and simple kinetic equations. The activation energy results of nucleation (En, 27.67 kJ/mol) and growth (Eg, 13.31 kJ/mol) demonstrated that the induced nucleation period was the critical step during the crystallization. Hence, the method of rapid nucleation was introduced and the induced nucleation time was shortened 50% (from ∼2.0 to ∼1.0 h) as the crystal seed was added. Moreover, the grain size of NaP‐Seed zeolite reduced to ∼2.02 μm. Meanwhile, the synthesized NaP zeolite adsorbent was employed for the removal of phenol compound from the aqueous solution. Under the synergistic effect of electrostatic interaction and pore filling effect, NaP zeolite exhibited better adsorption with the removal rate of 82.63% (pH = 5.0 and 40 min) and stability in the five consecutive cycles.

Catalytic pyrolysis of plastic waste to gasoline, jet fuel and diesel with nano MOF derived-loaded Y zeolite: Evaluation of temperature, zeolite crystallization and catalyst loading effects

Catalytic pyrolysis of municipal plastic waste can serve as a means of refining nature from the non-degradable plastic pollutants while producing a significant amount of liquid fuel required for the transportation industry. The low quality of the produced liquid fuel is often a fundamental challenge in pyrolysis, which can be overcome by utilizing catalysts. In this study, an upgraded zeolite Y catalyst has been employed. However, one of the main issues with zeolites is their small pore size, which restricts the entry of heavy polymer compounds and only allows small molecules to enter. To address this limitation, an innovative approach has been applied in this research where MIL-53 (Cu) is loaded onto the surface of the zeolite. After pyrolysis, it was determined that a composite of copper and its oxides, enclosed in a carbon nanocarbon shell C60 and C70, has been loaded onto the surface of the zeolite. This inexpensive composite exhibit behavior similar to noble and expensive metals due to its high electron transfer density. Additionally, analyses XRD, FTIR, BET, XRF, EDX, GC–MS, NH3-TPD and CHNSO were utilized for the characterization of the catalyst and the liquid fuel. Furthermore, three key parameters were selected for experimental design: temperature (375–525 °C), catalyst loading in the reactor (2.5–17.5 %), and support crystallinity percentage (0–100 %). Using a response surface approach, the impact of these parameters on the liquid fuel production efficiency was investigated. Ultimately, the produced liquid fuel was separated into three fractions: gasoline, jet fuel, and diesel. The effects of these primary parameters on the efficiency of each fuel type were determined using the response surface methodology.

Improved hydrothermal stability of LTA zeolite membrane by forming an acrylamide protective layer for pervaporation dehydration of water-rich acrylamide and ethanol solutions

It is a great challenge to maintain the hydrothermal stability of LTA zeolite membrane in pervaporation (PV) dehydration of water-rich solutions at high temperatures. In this work, high selective LTA zeolite membranes fabricated on coarse α-alumina supports were successfully applied for dehydration of water-rich acrylamide (AM) solutions. For dehydration of a 70 wt% AM/H2O solution at 353 K, the LTA zeolite membrane exhibited incredibly superior hydrothermal stability for running 240 h. The membrane exhibited a superb permeation flux of 7.96 kg m−2 h−1 and a high separation factor of more than 3800. The mechanism of stable existence of membrane crystals in the water-rich solutions was further investigated in detail. The results demonstrated that AM dynamically formed a new protective layer on LTA zeolite membrane surface, which effectively inhibited the degradation of zeolite crystals by high-temperature water. Importantly, the application of LTA zeolite membrane in the separation of other water-rich solutions could be further expanded by adding a small amount of AM. This study provides a new and simple method to improve the hydrothermal stability and application of LTA zeolite membranes in water-rich solutions at high temperature, which could expand the scope of industrial application and extend membrane lifetime.

Anchoring bimetallic PtAu nanoparticles on hierarchical zeolite as efficient electrocatalyst for the enhancement of ethanol electrooxidation

Zeolite materials as nanoreactor are promising candidates as catalysts for the oxidation of alcohol fuels. In the present work, the hierarchical zeolite was prepared via a hydrothermal technique. Then, the bimetallic Pt-Au-zeolite composite as electrocatalysts were synthesized by co-precipitation route. The surface morphology, composition and structure of the resulting composite were investigated by FE-SEM, EDS, XRD, XPS, BET&BJH and FT-IR. The catalytic performance of the composite has been examined in ethanol electrooxidation reactions under alkaline media. Pt-Au-zeolite exhibits superior selectivity for the electrooxidation of the ethanol, outperforming both conventional Au and Pt nanoparticles supported on zeolite crystals catalyst. The peak electrooxidation current of ethanol with Pt-Au-zeolite catalyst is 3.63 times that of Pt-zeolite with a scan rate of 50 mVs−1, which is due to the synergistic effect of Au and Pt in zeolite framework. It could be attributed to the incorporation of Au and Pt into zeolite framework, which was beneficial to the charge transferring the surface of Pt-Au-zeolite electrocatalyst and facilitated the ethanol oxidation. Besides, the significant electrochemical activity can be due to the production of OH–, which leads to the species present on the surface of the electrocatalysts during the activation process that controlled by the zeolite framework.

Constructing Intrapenetrated Hierarchical Zeolites with Highly Complete Framework via Protozeolite Seeding.

Creation of intrapenetrated mesopores with open highway from external surface into the interior of zeolite crystals are highly desirable that can significantly improve the molecular transport and active sites accessibility of microporous zeolites to afford enhanced catalytic properties. Here, different from traditional zeolite-seeded methods that generally produced isolated mesopores in zeolites, nanosized amorphous protozeolites with embryo structure of zeolites were used as seeds for the construction of single-crystalline hierarchical ZSM-5 zeolites with intrapenetrated mesopores (mesopore volume of 0.51 cm3 g-1 ) and highly complete framework. In this strategy, in contrast to the conventional synthesis, only a small amount of organic structure directing agents and a low crystallization temperature were adopted to promise the protozeolites as the dominant growth directing sites to induce crystallization. The protozeolite nanoseeds provided abundant nucleation sites for surrounding precursors to be crystallized, followed by oriented coalescence of crystallites resulting in the formation of intrapenetrated mesopores. The as-prepared hierarchical ZSM-5 zeolites exhibited ultra-long lifetime of 443.9 hours and a high propylene selectivity of 47.92 % at a WHSV of 2 h-1 in the methanol-to-propylene reaction. This work provides a facile protozeolite-seeded strategy for the synthesis of intrapenetrated hierarchical zeolites that are highly effective for catalytic applications.

Constructing Intrapenetrated Hierarchical Zeolites with Highly Complete Framework via Protozeolite Seeding

AbstractCreation of intrapenetrated mesopores with open highway from external surface into the interior of zeolite crystals are highly desirable that can significantly improve the molecular transport and active sites accessibility of microporous zeolites to afford enhanced catalytic properties. Here, different from traditional zeolite‐seeded methods that generally produced isolated mesopores in zeolites, nanosized amorphous protozeolites with embryo structure of zeolites were used as seeds for the construction of single‐crystalline hierarchical ZSM‐5 zeolites with intrapenetrated mesopores (mesopore volume of 0.51 cm3 g−1) and highly complete framework. In this strategy, in contrast to the conventional synthesis, only a small amount of organic structure directing agents and a low crystallization temperature were adopted to promise the protozeolites as the dominant growth directing sites to induce crystallization. The protozeolite nanoseeds provided abundant nucleation sites for surrounding precursors to be crystallized, followed by oriented coalescence of crystallites resulting in the formation of intrapenetrated mesopores. The as‐prepared hierarchical ZSM‐5 zeolites exhibited ultra‐long lifetime of 443.9 hours and a high propylene selectivity of 47.92 % at a WHSV of 2 h−1 in the methanol‐to‐propylene reaction. This work provides a facile protozeolite‐seeded strategy for the synthesis of intrapenetrated hierarchical zeolites that are highly effective for catalytic applications.