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(Yujie Ban, Meng Zhao, Weishen Yang, pp. 188‒215)
CO2 capture is a hot topic in research and industry. It typically refers to the splitting of CO2/N2, H2/CO2 and CO2/CH4, and is one of the most desirable separation technologies in environment and energy sectors. Membrane-based separations are energy-efficient separation methods cutting the energy consumption of traditional distillation by nearly 90%, whic
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The creation of intergrown layer-like zeolite crystals is one route to form hierarchical zeolites. Faujasite-type (FAU-type) zeolites are among the industrially most important zeolites and the implementation of hierarchical porosity is a promising way to optimise their catalytic and adsorptive performance. After a short general survey into routes for the preparation of hierarchical pore systems in FAU, we will review the currently existing strategies for the synthesis of FAU with layer-like morphology. Those strategies are mainly based on the presence of morphology modifying agents in the synthesis mixture. However, a very recent approach is the synthesis of layer-like FAU-type zeolite crystals assembled in an intergrown manner in the absence of such additives, just by finely adjusting the crystallization temperature. This additive-free preparation route for layer-like FAU, which appears very attractive from an ecological as well as economic point of view, is highlighted in this review. Concluding, a comparison, including powder X-ray diffraction, scanning and transmission electron microscopy, nitrogen physisorption and elemental analysis, between conventional FAU and three layer-like FAU obtained by different synthesis routes was carried out to show the structural, morphological and textural differences and similarities of these materials.
As an important zeolite material, MFI zeolites, as well as their controllable synthesis, are of great interest in both basic and applied science. Among the developed synthetic approaches, the seed-induced method has gradually evolved into a facile, low-cost, and even green alternative to give zeolites the desirable physicochemical properties. In this review, we briefly summarize the development of seed-induced syntheses of diverse functional MFI zeolites, where the “living” seed crystals not only direct the formation of zeolitic framework but also function as special “templates” or “units” to fine-tune the zeolite materials with diverse sizes, shapes, compositions, morphologies and pore structures. Moreover, on the basis of their structural features and crystallization behaviors in seed-induced synthesis, we reveal the roles of seeds and discuss the related crystallization mechanisms including both classical and non-classical pathways. We also want to guide readers to investigate the structure-performance relationships between these functional MFI zeolite catalysts and suitable catalytic reactions.
Metal-containing zeolite catalysts have found a wide range of applications in heterogeneous catalysis. To understand the nature of metal active sites and the reaction mechanism over such catalysts is of great importance for the establishment of structure-activity relationship. The advanced solid-state NMR (SSNMR) spectroscopy is robust in the study of zeolites and zeolite-catalyzed reactions. In this review, we summarize recent developments and applications of SSNMR for exploring the structure and property of active sites in metal-containing zeolites. Moreover, detailed information on host-guest interactions in the relevant zeolite catalysis obtained by SSNMR is also discussed. Finally, we highlight the mechanistic understanding of catalytic reactions on metal-containing zeolites based on the observation of key surface species and active intermediates.
A low-carbon economy calls for CO2 capture technologies. Membrane separations represent an energy-efficient and environment-friendly process compared with distillations and solvent absorptions. Metal-organic frameworks (MOFs), as a novel type of porous materials, are being generated at a rapid and growing pace, which provide more opportunities for high-efficiency CO2 capture. In this review, we illustrate a conceptional framework from material design and membrane separation application for CO2 capture, and emphasize two importance themes, namely (i) design and modification of CO2-philic MOF materials that targets secondary building units, pore structure, topology and hybridization and (ii) construction of crack-free membranes through chemical epitaxy growth of active building blocks, interfacial assembly, ultrathin two-dimensional nanosheet assembly and mixed-matrix integration strategies, which would give rise to the most promising membrane performances for CO2 capture, and be expected to overcome the bottleneck of permeability-selectivity limitations.
Metal-organic frameworks (MOFs) have emerged as a class of promising membrane materials. UiO-66 is a prototypical and stable MOF material with a number of analogues. In this article, we review five approaches for fabricating UiO-66 polycrystalline membranes including in situ synthesis, secondary synthesis, biphase synthesis, gas-phase deposition and electrochemical deposition, as well as their applications in gas separation, pervaporation, nanofiltration and ion separation. On this basis, we propose possible methods for scalable synthesis of UiO-66 membranes and their potential separation applications in the future.
Cu3(BTC)2, a common type of metal organic framework (MOF), was synthesized through electrochemical route for CO2 capture and its separation from N2. Taguchi method was employed for optimization of key parameters affecting the synthesis of Cu3(BTC)2. The results indicated that the optimum synthesis conditions with the highest CO2 selectivity can be obtained using 1 g of ligand, applied voltage of 25 V, synthesis time of 2 h, and electrode length of 3 cm. The single gas sorption capacity of the synthetized microstructure Cu3(BTC)2 for CO2 (at 298 K and 1 bar) was a considerable value of 4.40 mmol·g−1. The isosteric heat of adsorption of both gases was calculated by inserting temperature-dependent form of Langmuir isotherm model in the Clausius-Clapeyron equation. The adsorption of CO2/N2 binary mixture with a concentration ratio of 15/85 vol-% was also studied experimentally and the result was in a good agreement with the predicted value of IAST method. Moreover, Cu3(BTC)2 showed no considerable loss in CO2 adsorption after six sequential cycles. In addition, artificial neural networks (ANNs) were also applied to predict the separation behavior of CO2/N2 mixture by MOFs and the results revealed that ANNs could serve as an appropriate tool to predict the adsorptive selectivity of the binary gas mixture in the absence of experimental data.
Hierarchical ZSM-5 zeolite with radial mesopores is controllably synthesized using piperidine in a NaOH solution. The piperidine molecules enter the zeolite micropores and protect the zeolite framework from extensive desilication. The areas containing fewer aluminum atoms contain fewer piperidine protectant molecules and so they dissolve first. Small amounts of mesopores are then gradually generated in areas with more aluminum atoms and more piperidine protectant. In this manner, radial mesopores are formed in the ZSM-5 zeolite with a maximal preservation of the micropores and active sites. The optimal hierarchical ZSM-5 zeolite, prepared with a molar ratio of piperidine to zeolite of 0.03, had a mesopore surface area of 136 m2·g−1 and a solid yield of 80%. The incorporation of the radial mesopores results in micropores that are interconnected which shortened the average diffusion path length. Compared to the parent zeolite, the hierarchical ZSM-5 zeolite possesses more accessible acid sites and has a higher catalytic activity and a longer lifetime for the alkylation of benzene.
Hierarchical core/shell Zeolite Socony Mobil-five (ZSM-5) zeolite was hydrothermally postsythesized in the solution of NaOH and diammonium surfactant via a dissolution-reassembly strategy. The silica and alumina species were firstly dissolved partially from the bulky ZSM-5 crystals and then were in situ reassembled into the MFI-type nanosheets with the structure-directing effect of diammonium surfactant, attaching to the out-surface of ZSM-5 core crystals. The mesopores thus were generated in both the core and shell part, giving rise to a micropore/mesopore composite material. The micropore volume and the acidity of the resultant hybrid were well-preserved during this in situ recrystallization process. Possessing the multiple mesopores and enlarged external surface area, the core/shell ZSM-5 zeolite exhibited higher activity in the ketalation and acetalization reactions involving bulky molecules in comparison to the pristine ZSM-5.
SSZ-39 zeolite with AEI framework structure is a good catalyst candidate for the methanol-to-olefins (MTO) reaction. However, the diffusion limitation and coke formation often results in fast deactivation of the SSZ-39 zeolite catalyst. One solution for this challenge is to introduce mesoporosity in the SSZ-39 zeolite. Herein, we report the synthesis of mesoporous SSZ-39 zeolite using an organosilane surfactant, N,N-dimethyl-N-(3-(trimethoxysilyl)propyl)octan-1-aminium chloride, as a mesopore template and N,N-dimethyl-cis-2,6-dimethylpiperidinium as a micropore template. The obtained zeolites were characterized by X-ray diffraction, N2 sorption, scanning electron microscopy, temperature programmed desorption of ammonia, and magic angle spinning nuclear magnetic resonance of 27Al. The results show that the mesoporous SSZ-39 zeolite has high crystallinity, meso/microporosity, high surface area, cuboid morphology, and abundant acidic sites. More importantly, this mesoporous SSZ-39 zeolite exhibits enhanced catalyst lifetime in the MTO reaction due to the presence of mesoporosity for fast mass transfer, compared with a conventional SSZ-39 zeolite without mesoporosity.
Mesoporous Y zeolites were prepared by the sequential chemical dealumination (using chelating agents such as ethylenediaminetetraacetic acid, H4EDTA, and citric acid aqueous solutions) and alkaline desilication (using sodium hydroxide, NaOH, aqueous solutions) treatments. Specifically, the ultrasound-assisted alkaline treatment (i.e., ultrasonic treatment) was proposed as the alternative to conventional alkaline treatments which are performed under hydrothermal conditions. In comparison with the hydrothermal alkaline treatment, the ultrasonic treatment showed the comparatively enhanced efficiency (with the reduced treatment time, i.e., 5 min vs. 30 min, all with 0.2 mol·L−1 NaOH at 65°C) in treating the dealuminated Y zeolites for creating mesoporosity. For example, after the treatment of a dealuminated zeolite Y (using 0.1 mol·L−1 H4EDTA at 100°C for 6 h), the ultrasonic treatment produced the mesoporous zeolite Y with the specific external surface area (Sexternal) of 160 m2·g−1 and mesopore volume (Vmeso) of 0.22 cm3·g−1, being slightly higher than that by the conventional method (i.e., Sexternal = 128 m2·g−1 and Vmeso = 0.19 cm3·g−1). The acidic property and catalytic activity (in catalytic cracking of n-octane) of mesoporous Y zeolites obtained by the two methods were comparable. The ultrasonic desilication treatment was found to be generic, also being effective to treat the dealuminated Y zeolites by citric acid. Additionally, the first step of chemical dealumination treatment was crucial to enable the effective creation of mesopores in the parent Y zeolite (with a silicon-to-aluminium ratio, Si/Al= 2.6) regardless of the subsequent alkaline desilication treatment (i.e., ultrasonic or hydrothermal). Therefore, appropriate selection of the condition of the chemical dealumination treatment based on the property of parent zeolites, such as Si/Al ratio and crystallinity, is important for making mesoporous zeolites effectively.
A synthetic mixture of real waste packaging plastics representative of the residue from a material recovery facility (plasmix) was submitted to thermal and catalytic pyrolysis. Preliminary thermogravimetry experiments coupled with Fourier transform infrared spectroscopy were performed to evaluate the effects of the catalysts on the polymers’ degradation temperatures and to determine the main compounds produced during pyrolysis. The thermal and catalytic experiments were conducted at 370°C, 450°C and 650°C using a bench scale reactor. The oil, gas, and char yields were analyzed and the compositions of the reaction products were compared. The primary aim of this study was to understand the effects of zeolitic hydrogen ultra stable zeolite Y (HUSY) and hydrogen zeolite socony mobil-5 (HZSM5) catalysts with high silica content on the pyrolysis process and the products’ quality. Thermogravimetry showed that HUSY significantly reduces the degradation temperature of all the polymers—particularly the polyolefines. HZSM5 had a significant effect on the degradation of polyethylene due to its smaller pore size. Mass balance showed that oil is always the main product of pyrolysis, regardless of the process conditions. However, all pyrolysis runs performed at 370°C were incomplete. The use of either zeolites resulted in a decrease in the heavy oil fraction and the prevention of wax formation. HUSY has the best performance in terms of the total monoaromatic yield (29 wt-% at 450°C), while HZSM5 promoted the production of gases (41 wt-% at 650°C). Plasmix is a potential input material for pyrolysis that is positively affected by the presence of the two tested zeolites. A more effective separation of polyethylene terephthalate during the selection process could lead to higher quality pyrolysis products.
