Structure Of Molecular Sieves Research Articles

Construction of silicon-rich hollow nanotubes embedded in hierarchical SSZ-13 and enhanced SCR high-temperature activity

The meso-channel structure of the SSZ-13 molecular sieve can both slightly speed up the diffusion of reactants or products and lower the chance of pore blockage. Micro-mesoporous SSZ-13 was constructed using hydroxylated carbon nanotubes (CNTs) as the hard template. CNTs not only introduced silica-rich and aluminum-poor hollow nanotubes to SSZ-13, but also shortened the crystallization period of SSZ-13 by no less than 24 h. SSZ-13–0.04 underwent a “crystallization-dissolution-re-crystallization” process during the 24th and 96th hours of crystallization. Compared with the conventional SSZ-13, the SSZ-13 constructed with adamantane-carbon nanotubes as the double template showed better adsorption capacity for NO, stronger acidity, and higher SCR high-temperature activity.

Semantic mask-based two-step approach: a general framework for X-ray diffraction peak search in high-throughput molecular sieve synthetic system

X-ray diffraction (XRD) is used for characterizing the crystal structure of molecular sieves after synthetic experiments. However, for a high-throughput molecular sieve synthetic system, the huge amount of data derived from large throughput capacity makes it difficult to analyze timely. While the kernel step of XRD analysis is to search peaks, an automatic way for peak search is needed. Thus, we proposed a novel semantic mask-based two-step framework for peak search in XRD patterns: (1) mask generation, we proposed a multi-resolution net (MRN) to classify the data points of XRD patterns into binary masks (peak/background). (2) Peak search, based on the generated masks, the background points are used to fit an n-order polynomial background curve and estimate the random noises in XRD patterns. Then we proposed three rules named mask, shape, and intensity to screening peaks from initial peak candidates generated by maximum search. Besides, a voting strategy is proposed in peak screening to obtain a precise peak search result. Experiments show that the proposed MRN achieves the state-of-the-art performance compared with other semantic segmentation methods and the proposed peak search method performs better than Jade when using f1 score as the evaluation index.

Self-synthesis and performance analysis of a Cu-Fe composite ZSM-5 zeolitic catalyst for NOx reduction and particulate matter removal using NH3 SCR

The NH3-SCR has gained widespread usage due to its exceptional denitrification efficiency. It achieves high nitrogen oxides (NOx) reduction rates within the temperature range of 300⁰C to 400℃. Nonetheless, most of the catalyst have a limited temperature range for effective NOx removal applications. For sustainable utilization of common combustion units, it has become essential to develop a catalyst which can provide a broader spectrum of temperature range for NOx reduction. This study is thus aimed to enhance the structure of ZSM-5 zeolite molecular sieves using tetrapropylammonium bromide. Copper and iron ions (total 3% weight) were incorporated to boost the reaction at temperatures of 180⁰C, 250⁰C and 320⁰C. The overall efficiency of NOx removal was above 81%, highest being 97.6%. The carrier significantly impacts the NOx conversion rate with increased copper ion content in the samples at medium and low temperatures. The Particulate Matter removal efficiency ranged from 90.1% to 93.1%.

A Layered Aluminophosphate [C17H21N2]3[Al3(PO4)4]·5H2O with Fluorescent Property Derived from Carbazolyl-Modified Template.

Designing and innovating organic structure-directing agents is the key to synthesizing novel molecular sieve structures. Herein, we design a novel carbazolyl-modified template and further synthesize a two-dimensional layered aluminophosphate with [C17H21N2]3[Al3(PO4)4]·5H2O (denoted as ZHKU-2). ZHKU-2 is composed of AA-stacked [Al3P4O16]3- layers constructed from alternating AlO4 and PO3(=O) tetrahedrons to form a 4.6.8 network featured by capped six-ring secondary building units. Carbazolyl-templated ZHKU-2 exhibits strong purple fluorescence with a high quantum yield of 25.98%. This work expands aluminophosphate materials of the [Al3P4O16]3- family and provides a view for synthesizing new molecular sieves by exploring the organic luminescence structure-directing agents.

Moisture Adsorption Mechanism of Molecular Sieves in Oil–Paper Insulation: Simulation on Static Adsorption Characteristics

Oil-paper insulation structures are widely utilized inside power transformers. Moisture speeds up the deterioration of the paper insulation and significantly degrades the performance of the transformers’ oil-paper insulation. The molecular sieve is a widely used material for moisture removal from gas and liquid, but its mechanism in moisture removal in transformer insulation is still not clear, especially from a microscopic perspective. In this paper, the density functional theory and molecular dynamics simulation were adopted to calculate the forces of water molecules in the oil-paper insulation system. Also, the models of different A-type molecular sieves were constructed, and the steady-state adsorption characteristics of water molecules in the sieves were obtained by the Monte Carlo method. The results show that the method of calculating the adsorption energy to characterize the adsorption capacity of moisture by different objects is reasonable. The basic reason why molecular sieves can effectively adsorb moisture in the oil-paper structure is that the adsorption energy between the water molecule and the molecular sieve structure is higher than that between the water molecule and other molecules in the oil-paper insulation. Compared with 3A molecular sieves, 4A molecular sieves have a larger free volume, more adsorption sites, and higher adsorption energy, which indicates that 4A molecular sieves have better moisture adsorption characteristics. This research gives a microcosmic explanation of the moisture adsorption characteristics of the oil-paper insulation structure and the moisture adsorption mechanism of molecular sieve, which may provide a theoretical basis for the application and selection of molecular sieves in the online moisture removal of oil-paper insulation structure.

Preparation of ZSM-5 molecular sieve modified by kaolin and its CO2 adsorption performance investigation

The ZSM-5 molecular sieve was prepared by the crystal species-assisted hydrothermal synthesis method using kaolin as raw material, and the molecular sieve were prepared by monometallic Na, Ni and bimetallic Ni–Na modification of the molecular sieve when using the isovolumetric impregnation method. It was found that the modified molecular sieve maintained the MFI structure of ZSM-5 molecular sieve, some mesopores appeared, and Na and Ni were uniformly dispersed on the inner and outer surfaces of the carrier by using XRD, ICP, FI-IR,UV–vis and BET. Moreover the presence of Na could increase the loading of metallic Ni. The effect of the modified molecular sieve on the CO2 adsorption performance was investigated using CO2 programmed temperature rise desorption (CO2-TPD). Na and Ni modification can increase the number of adsorption centers and improve the activity of adsorption centers on the surface of the adsorbent. Then Ni as an additive can enhance the interaction between metal and carrier, increase the density of active sites on the molecular sieve surface, and improve the adsorption performance on CO2. Among them, 1.5 Ni-0.6 Na/ZSM-5 adsorption has the better performance for CO2 adsorption with the bettert adsorption capacity of 1.97 mmol/g, which is 74% higher than ZSM-5, 84% higher than 2 Ni/ZSM-5, and 25% higher than 0.6 Na/ZSM-5.

Effects of Aluminum Source with Different Properties on the Properties of Pd/SAPO-41 and its Catalytic Hydroisomerization Reaction

Aluminium source is one of the main factors affecting the physicochemical properties of SAPO-41.Silicoaluminophosphate molecular sieves-SAPO-41 were prepared via hydrothermal synthesis using di-n-butylamine (DBA) as templating agent, pseudo-boehmites, phosphoric acid and silica sol as aluminum, phosphorus and silicon sources respectively. Using XRD, N2 physical adsorption, SEM and Py-IR analysis media, the effects of aluminum sources on the crystal and pore structure, morphology, B-acid center strength of the prepared SAPO-41 molecular sieves. The catalytic hydroisomerization performance over Pd/SAPO-41 bifunctional catalysts were investigated. Pseudo-boehmite type has little effect on the crystal structure and chemical composition of SAPO-41 molecular sieve, but has different effects on its morphology, pore structure, B-acid strength of SAPO-41, and Pd dispersion on Pd/SAPO-41. The hydroisomerization conversion of n-decane over 0.5Pd/S41-A1 can reach 88.56 %, and the selectivity of isoparaffin can reach 90.88%.This work provides a reference for selecting an ideal aluminum source for the synthesis of SAPO-41.

Direct evidence of the ultramicroporous structure of carbon molecular sieves

We utilize gas sorption experiments, neutron pair distribution function, and small-angle X-ray scattering studies on carbon molecular sieves (CMSs) to gain new insight into their ultramicroporous structure. CMS materials derived from the pyrolysis of PIM-1 (PIM = polymer of intrinsic microporosity) under an inert atmosphere (PIM-1-CMS) and H2 atmosphere (4% H2-PIM-1-CMS) were studied. Neutron total scattering studies of these materials reveal these CMS materials to be mainly graphene-like ribbons with short-range atomic ordering. Small-angle X-ray scattering and low angle diffraction peaks corroborate the presence of well-defined ultramicroporosity observed in carbon dioxide and cryogenic neon adsorption studies suggesting, expectedly, that these materials are non-graphitizing carbon structures. Our findings provide microscopic evidence of the non-graphitizing and ribbon-like structure of these CMS materials and suggest a possible hypothetical microporous structure.

Identifying the promotional mechanism for the activity and SO2 resistance of Ce-doped K-OMS-2 for the low-temperature SCR of NO with NH3

Mn-based catalysts for the low-temperature selective catalytic reduction (SCR) of NOx by NH3 can be strongly deactivated due to the generation of MnSO4 derived from upstream SO2 and H2O. In this study, a catalyst architecture based on a Ce-doped manganese oxide octahedral molecular sieve (Ce-OMS-2) was successfully synthesized to promote NH3-SCR activity and enhance SO2 tolerance at low temperatures. We investigated the role of Ce and its specific octahedral molecular sieve structure on the anti-SO2 poisoning effect of Ce-OMS-2. Maintaining the manganese oxide octahedral molecular sieve structure with polyvalent manganese species and active oxygen species and the charge transfer between Mn and Ce via the Ce-O-Mn bond in the Ce-OMS-2 catalyst are found to be the key factors for the realization of excellent NH3-SCR activity and SO2 tolerance. In-depth in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) show that the number of Brønsted acid sites in Ce-OMS-2 can also favor SO2 resistance. This study provides new insight into the mechanism for de-NOx activity and SO2 tolerance on Ce-OMS-2 catalyst and is beneficial for the design of anti-SO2 NH3-SCR catalysts.

Functional mesoporous materials in clean energy: an interview with Dongyuan Zhao

ABSTRACT National Science Review invited Prof. Dongyuan Zhao of Fudan University for an interview focusing on his team's renowned research on functional mesoporous materials and energy-related applications. Prof. Zhao is a professor of chemistry and materials science, and a member of the Chinese Academy of Sciences. He received his PhD in chemistry from Jilin University in 1990. He has since focused his research on the synthesis and structure of porous materials and molecular sieves. His team received a first-tier national science award in 2021 for their contribution to the research and development of mesoscopic materials. They discovered a method of synthesizing mesoporous organic polymers and carbonaceous materials using organic-organic self-assembly. This work was published in 2005 and since then it has turned into a vibrant new field of more than 40 000 publications so far. His team has named more than 20 of their inventions after Fudan University: the FDU mesoporous series.

Study on the effect of physical structure and chemical modification of molecular sieve on the adsorption properties of C4F7N and its decomposition products

The environmentally friendly insulation medium C4F7N mixed gas decomposes after long-term overheating and partial discharge (PD). Some of the decomposition products not only affect the stable operation of gas insulated switchgear but also cause personal safety hazards. Therefore, the adsorption materials need to be equipped for adsorbing decomposition products in the equipment. Molecular sieves are considered to be the most promising adsorbent materials with wide variety and ionic modification space. In order to better apply the molecular sieve to adsorb the decomposition products of C4F7N, the effects of its physical structure and chemical modification on the adsorption of C3F8, C3F6, C2F6, and CF4 are investigated based on Grand Canonical Monte Carlo (GCMC) and Density functional theory (DFT) in this paper. Firstly, the adsorption amounts and sites of various pure silica-type molecular sieves (ATS, BEA, BOG, CAN, CHA, LAU, LTA, MTW, STT, TON, and ZSM) are investigated by GCMC. Then the adsorption energy, charge transfer, and density of states of the molecular sieve modified by cations (H+, Na+, Mn2+, and Fe2+) for C4F7N and its decomposition products are obtained based on DFT. The results reveal that the molecular sieve topology, size of gas molecules and molecular sieve windows are important factors affecting the adsorption process. After modification with Mn2+ and Fe2+, the adsorption energy and electron transfer of C4F7N decomposition products by ZSM molecular sieve increased significantly and the adsorption distance became closer. The main action site of C4F7N is CN bond, the main action sites of C2F6 are CC double bond and F atom, and C3F8, C2F6 and CF4 is F atom.

The Role of Intermediate Phases in the Crystallization of Aluminophosphate Sieves on Examples of AlPO-11 and AlPO-41

The formation of intermediate phases during aging of the reaction gel composition 1.0Al2O3•1.0P2O5•1.5DPA•40H2O and its subsequent crystallization into molecular sieves AlPO-11 and AlPO-41 was studied in this work. The initial gels and crystallization products were characterized by XRD; MAS NMR 27Al and 31P; scanning electron microscopy (SEM); transmission electron microscopy (TEM); and N2-physical adsorption. It has been found that the nature of the aluminum source used to prepare the gel has a significant effect on the properties of the resulting intermediate phases. It is shown that by changing the chemical and phase composition of the intermediate aluminophosphate, it is possible to control the morphology and characteristics of the secondary porous structure of the AlPO-11 molecular sieve. The formation of the intermediate phases with a layered structure opens the possibility to synthesize high-phase purity AlPO-41 at the di-n-propylamine/Al2O3 ratio = 1.5. The formation mechanisms of AlPO-11 and AlPO-41 are proposed depending on the phase composition of the intermediate phases.

La doping of manganese oxide leads to a dramatically promoted catalytic performance for UV–visible-infrared photothermocatalytic ethyl acetate abatement

La-doped manganese oxide catalysts with nanorod morphology, cryptomelane octahedral molecular sieve structure (OMS-2) and La/Mn atomic ratios of 0.0045, 0.0097, and 0.0459 (denoted as La-OMS-2-A, La-OMS-2-B, and La-OMS-2-C) were synthesized with the reaction among KMnO4, La(NO3)3, and Mn(NO3)2 at 90 °C. They were used for photothermocatalytic oxidation of ethyl acetate (typical organic pollutant) merely with UV–visible-infrared (UV–vis-IR) illumination. An appropriate amount of La doping was found to dramatically enhance photothermocatalytic activity and diminish the selectivity of more toxic by-product acetaldehyde. The optimal La-OMS-2-B demonstrated exceptionally large photothermocatalytic activity, low acetaldehyde selectivity, and good catalytic stability. Within initial five minutes, its CO2 formation rate (rCO2) was exceptionally high (1362.0 μmol g−1 min−1), a 67-fold increase as compared to pure OMS-2, while its acetaldehyde selectivity was very low (1.9%), a 37-fold decrease as compared to pure OMS-2. Even with λ > 830 nm IR illumination, it still possessed good photothermocatalytic performance. The photothermocatalytic oxidation abided by a light-driven thermocatalysis mechanism. The dramatic catalytic enhancement by the La-doping arose from a considerable promoted thermocatalytic activity due to the oxygen activity of OMS-2 being substantially improved with La-doping. A photoactivation on La-OMS-2-B, quite unlike photocatalysis on semiconductor photocatalysts, was discovered that further substantially improved catalytic activity and decreased acetaldehyde selectivity due to the oxygen activity of OMS-2 being considerably promoted with UV–vis-IR illumination.

Role of the Aluminophosphate Gel Aging Stage in Controlling the Morphology and Secondary Pore Structure of AlPO4-11 Molecular Sieves

Role of the Aluminophosphate Gel Aging Stage in Controlling the Morphology and Secondary Pore Structure of AlPO4-11 Molecular Sieves

MOF-derived nanocomposites functionalized carbon molecular sieve membrane for enhanced ethylene/ethane separation

Fine tailoring the pore structure and sorption capability of carbon molecular sieve (CMS) membrane is highly desirable for efficient separation of C2H4/C2H6 gas pair. In this work, a facile approach was proposed to enrich the ultramicroporosity of CMS membrane and enhance the affinity adsorption capacity for C2H4, in which ZIF-8 nanoparticles were incorporated into a novel precursor polymer of phenolphthalein-based poly (arylene ether ketone) (PEK-C) followed by carbonization to prepare mixed matrix CMS membranes to achieve simultaneously high C2H4 permeability and high C2H4/C2H6 selectivity. The ZIF-8-derived nanocomposites partially maintain the pore structure of ZIF-8 precursor, and have good compatibility with the carbon matrix. The BET surface areas, micropore volume and the number of ultramicropore with molecular sieving ability of CMS membrane are greatly improved due to the introduction of ZIF-8-derived microporous carbons, resulting in a significant increase in the C2H4 permeability and C2H4/C2H6 diffusion selectivity. The C2H4/C2H6 sorption selectivity of CMS membrane is also enhanced owing to the selective adsorption affinity of the ZIF-8-derived Zn2+ metal oxides for C2H4. The C2H4 permeability of mixed matrix CMS membrane with 10 wt% ZIF-8 loading is increased by 42% compared to that of neat CMS membrane, and the C2H4/C2H6 selectivity is increased by 102%.

Conversion of methanol to propylene over SAPO-14: Reaction mechanism and deactivation

Methanol to olefins (MTO) reaction as an important non-oil route to produce light olefins has been industrialized, and received over 80% ethylene plus propylene selectivity. However, to achieve high single ethylene or propylene selectivity towards the fluctuated market demand is still full of challenge. Small-pore SAPO-14 molecular sieve is a rare MTO catalyst exhibiting extra-high propylene selectivity. It provides us a valuable clue for further understanding of the relationship between molecular sieve structure and MTO catalytic performance. In this work, a seconds-level sampling fixed-bed reactor was used to capture real-time product distributions, which help to achieve more selectivity data in response to very short catalytic life of SAPO-14. Changes in product distribution, especially during the low activity stage, reflect valuable information on the reaction pathway. Combined with in situ diffuse reflectance infrared Fourier-transform spectroscopy, in situ ultraviolet Raman measurements and 12C/13C isotopic switch experiments, a reaction pathway evolution from dual cycle to olefins-based cycle dominant was revealed. In addition, the deactivation behaviors of SAPO-14 were also investigated, which revealed that polymethylbenzenes have been the deactivated species in such a situation. This work provides helpful hints on the development of characteristic methanol to propylene (MTP) catalysts.

Adsorption Properties of ZSM-5 Molecular Sieve for Perfluoroisobutyronitrile Mixtures and Its Fluorocarbon Decomposition Products

Perfluoroisobutyronitrile (C4F7N), an environment-friendly insulating gas, has excellent insulating properties and has the potential to be used in gas-insulated equipment when mixed with CO2. Selecting suitable adsorption materials to adsorb the decomposition products of the C4F7N mixture can ensure the safe and stable operation of the gas-insulated equipment and the personal safety of the operators in the electric power industry. The adsorption characteristics of the ZSM-5 molecular sieve on C4F7N and its five fluorocarbon decomposition products were investigated by adsorption experiments. The results show that the ZSM-5 molecular sieve has a certain adsorption effect on six fluorocarbon gases; the adsorption performance of C3F6 and C3F8 are the best, with an adsorption efficiency over 85%, while the concentration of CO2 and C4F7N is affected by the ZSM-5 molecular sieve. At the same time, the paper based on the Metropolis Monte Carlo simulation of Materials Studio software found that the ZSM-5 molecular sieve has the strongest adsorption effect on C4F7N molecules and the weakest adsorption effect on CO2 molecules. The stronger the polarity of the gas molecule, the more obvious the adsorption effect of molecular sieve structure on it. As a result, the ZSM-5 molecular sieve could be used in tail gas purification of insulated equipment, as well as to provide solutions for the development and production of protective equipment.

Stable EMT type zeolite/CsPbBr3 perovskite quantum dot nanocomposites for highly sensitive humidity sensors

Perovskite quantum dots (PQDs) have been widely studied due to the outstanding light emission properties including high quantum efficiency, narrow linewidths and electron transport properties. However, poor stability limits their implication in optical devices, especially working at ambient conditions in the presence of moisture that rapidly attenuate their performance. In this work, PQDs were loaded in nanosized EMT zeolite crystals synthesized from template-free precursor systems resulting in a composite EMT-CsPbBr3. We found and studied for the first time that, in the pores of the zeolites, a small amount of water molecules can promote the crystallization of perovskite nanocrystals. The energy and bond length of perovskite CsPbBr3 confined in the cages of EMT zeolite were calculated in the presence of water molecules, corresponding to the effect of humidity. Crucially, the pore structure of EMT molecular sieve provides an important research model. The great stability and reliability of the EMT-CsPbBr3 as humidity sensor is presented. The luminous intensity efficiency of the EMT-CsPbBr3 composite was maintained at nearly 90% after continuous usage for 6 months. Both the theoretical and experimental results show that a trace amount of water enhances the luminescence of perovskite stabilized in the hydrophilic EMT zeolite.

Relation between Morphology and Porous Structure of SAPO-11 Molecular Sieves and Chemical and Phase Composition of Silicoaluminophosphate Gels.

The formation of silicoaluminophosphate gels using boehmite, Al isopropoxide, and di-n-propylamine as a template of silicoaluminophosphate gels as well as their subsequent crystallization into SAPO-11 molecular sieves was studied in detail using X-ray fluorescence spectroscopy (XRF), powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption–desorption methods. The effect of the chemical and phase composition of silicoaluminophosphate gels on the physicochemical properties of SAPO-11 molecular sieves was shown. The secondary structural units that the AEL lattice is composed of were found to be formed at the initial stage of preparation involving aluminum isopropoxide. Several approaches to control their morphology and secondary porous structure are also proposed.

Synthesis and hydrogen isomerization performance of ordered mesoporous nanosheet SAPO-11 molecular sieves

In this study, ordered mesoporous nanosheet SAPO-11 molecular sieves were synthesized using uncalcined SBA-15 molecular sieves as a Si source. The effects of different Si sources on the structure and properties of SAPO-11 molecular sieves were examined, and their growth mechanism was elucidated. An electrostatically polarized region was formed between a P123 template (triblock copolymer) contained in uncalcined SBA-15 and two-dimensional channels of SBA-15 molecular sieves. When AlO2− and PO2+ species diffused into this region, its electrostatic polarization promoted the preferential growth of SAPO-11 molecular sieves along the c-axis, which contained a large number of exposed catalytically active sites with acidic properties. During the isomerization of long-chain alkanes, the synthesized sieves exhibited high isomerization activity and selectivity. When the conversion rate of n-dodecane reached 87.96%, the yield of i-dodecane was 68.08%, and the yield of multi-branched i-dodecane was 31.65%.