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•Hierarchical zeolite for biofuel production from biomass is discussed.•Several major reaction routes from different biomass sources are elaborated.•The presence of hierarchical ...porosity significantly enhances the catalytic performance.•It is vital to incorporate hierarchical zeolite in the upscale production of biofuel.•There is plenty of room for improvement to increase the efficiency.
The development of biofuel production from biomass has become a promising breakthrough and could tremendously enhance the potential of industrial technology. In this case, zeolites have emerged as suitable materials for catalyzing biomass conversion due to their outstanding catalytic properties, including the presence of the intrinsic acid sites, shape-selectivity properties, and high thermal stability. However, the sole micropores in zeolite cause diffusional limitation issues, especially for the bulky molecules involved in biomass feedstocks. For instance, many oxygenate molecules are too large to enter the micropores, so they cannot be converted into the product. In this case, hierarchical porosity could facilitate these molecules to access the acid site within the zeolite crystals. Research on synthetic strategies, modifications, and evaluations of their catalytic properties has consistently grown every year. This article reviews the recent development of hierarchical zeolite catalysts for biomass conversion to biofuel. Numerous strategies of hierarchical zeolite fabrication (bottom-up, top-down methods, and green synthesis approaches) and its modification (metals and functionalization with organic materials) and their characteristics are comprehensively reviewed. The key point in the fabrication of hierarchical zeolites is the development of notable mesopores while preserving the intrinsic micropores. Moreover, the role of hierarchical zeolites in various biofuel and bio-based chemicals for biofuel and biofuel additives production reactions, i.e., pyrolysis, hydrolysis, esterification and transesterification, isomerization, condensation, upgrading of bio-oil, and catalytic cracking reactions, are discussed in detail. Finally, the remaining challenges and insight that can be considered for further improvement are provided.
Hierarchical core–shell zeolite–zeolite composite was fabricated by Y overgrown with a thin layer of nano-β: The extracted aluminum species by depolymerizing NaY zeolite crystals react with the ...silicon species near the interface of the NaY zeolite core and the precursor yielding the β zeolite layers, which induces and promotes the growth of β zeolite crystals over the core zeolite. Steric hindrance provoked by the concurrently growing crystals in the shell gives the β zeolite phase in the composite a relatively smaller size; A hierarchical pore system resulted from alkaline etching of the core Y and intercrystalline void of nano-β in the shells is introduced into the as-synthesized zeolite–zeolite composites.
•Zeolite composite is fabricated by which Y is overgrown with a thin layer of nano-β.•Incompatibility of core and shell is circumvented using core as nutrients for growth of shell.•Hierarchical pore system contributes to enhancing the accessibilities of Brönsted-acid site.•Depolymerization of core contributes to transmission of Al-species from cores to shell layers.•A high-efficiency hydrocracking catalyst of heavy oil producing good liquid yield.
Zeolite–zeolite composites composed of Y zeolite cores and polycrystalline β zeolite shells were prepared, in which Y zeolite was overgrown with a thin layer of nano-β zeolite. The incompatibility of the cores and the shells with different zeolite structure types, chemical compositions and crystallization conditions was circumvented by using Y zeolite as the nutrients for the growth of β zeolite crystals since the dissolution of the core zeolites was inevitable during the formation of the shell layers. The structural, crystalline, and textural properties of the as-synthesized samples were characterized by XRD, SEM, TEM, EDS, SAED, N2 adsorption–desorption, FT-IR, in situ IR spectra of pyridine and di-tert-butyl pyridine. The performances of the as-synthesized zeolite–zeolite composite supported by Ni–Mo active components were investigated during the hydrocracking of Iran VGO oil. The results displayed that the core–shell structured composites with a high-silicon β zeolite shell displayed a better performance in terms of conversion of VGO oil and yield of jet fuel, middle distillate yields and middle distillate selectivity than the reference catalyst and the composite catalyst with a low-silicon β zeolite shell.
The objective of this review is to highlight the need for further investigation of microbial toxicity caused by desorption of surfactant from Surfactant Modified Zeolite (SMZ). SMZ is a low cost, ...versatile permeable reactive media which has the potential to treat multiple classes of contaminants. With this combination of characteristics, SMZ has significant potential to enhance water and wastewater treatment processes. Surfactant desorption has been identified as a potential issue for the ongoing usability of SMZ. Few studies have investigated the toxicity of surfactants used in zeolite modification towards microorganisms and fewer have drawn linkages between surfactant desorption and surfactant toxicity. This review provides an overview of natural zeolite chemistry, characteristics and practical applications. The chemistry of commonly used surfactants is outlined, along with the kinetics that drive their adsorption to the zeolite surface. Methodologies to characterise this surfactant loading are also described. Applications of SMZ in water remediation are highlighted, giving focus to applications which deal with biological pollutants and where microorganisms play a role in the remediation process. Studies that have identified surfactant desorption from SMZ are outlined. Finally, the toxicity of a commonly used cationic surfactant towards microorganisms is discussed. This review highlights the potential for surfactant to desorb from the zeolite surface and the need for further research into the toxicity of this desorbed surfactant towards microorganisms, including pathogens and environmental microbes.
•Water remediation applications for Surfactant Modified Zeolite (SMZ) are reviewed.•Desorption of surfactant from SMZ highlighted as a potential environmental issue.•Quantifying desorbed surfactant toxicity crucial to understanding SMZ's useability.•Toxicity of desorbed surfactants towards microorganisms requires further research.
Membrane pervaporation is an emerging technique for the separation of water from alcohols, and zeolites with ordered ultramicropores (pore size <0.7 nm) have shown particular promise for use as ...membrane materials. This is the first paper to propose the use of NaP1 zeolite in membrane pervaporation. NaP1 zeolite has a GIS type topology with an 8-member ring that forms a pore limiting diameter of 3.0 Å, which is ideal for the separation of water from alcohols. NaP1 zeolite membranes with various Si/Al ratios (1.9, 3.3, and 3.9) were made via the seeded growth. Pervaporation tests were performed using an aqueous feed solution of 90 wt% ethanol or IPA at 348 K. The sample with a Si/Al ratio of 3.3 achieved separation factors surpassing most existing zeolite membranes: water/ethanol (200,000) and water/isopropanol (36,000). Nano-beam X-ray diffraction was used to map grain sizes in membrane samples, due to its role in surface hydrophilicity. State-of-the-art molecular simulations provided valuable insights into the diffusion and adsorption of water/alcohol molecules in NaP1 zeolite. From simulations, NaP1 zeolite presented a high water diffusivity and a high adsorption selectivity of water over ethanol or isopropanol. Experiment and computation results demonstrate the potential of NaP1 zeolite as a membrane material for alcohol dehydration.
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•This is the first ever report on the fabrication of zeolite NaP1 (GIS-type) membranes for pervaporation.•The optimized membrane achieves a separation factor of exceeding 200,000 for water/ethanol.•Molecular simulations unveil the transport mechanism for this system.
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► Pure zeolite Na–X and Na–A using silica precursor from fine particles of bagasse fly ash waste. ► Prepared zeolites have higher specific surface area than the most previous reports. ...► The degree of anions effect on zeolite crystallization increase with the increase of their molecular weight.
Bagasse fly ash (BFA), a solid waste from sugar cane industries, is still underutilized leaving disposal problems. In fact, high silica content inside this material opens a possibility to become a potential zeolites precursor. In this study, fine particles of BFA has been reused to prepare zeolite Na–X and Na–A with high purity. After segregated by mechanical sieving, the small particle fraction of BFA has been subjected to alkali fusion treatment followed by dissolution and then residue separation to produce clear silicate supernatant as the silica source for preparing the low-silica zeolites. High purity of Na–X and Na–A zeolites were prepared using hydrothermal treatment by optimizing the process condition, i.e., curing time, hydrothermal temperature and Si/Al molar ratio in the reaction mixture. Furthermore, the effect of several anions on the rate of zeolite X crystallization was also investigated.
Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for ...the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O
Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al
O
sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.
MFI zeolite nanosheets with a single-unit-cell crystal thickness were syn-thesized under hydrothermal synthesis conditions using diquaternary ammonium surfactants as the zeolite structure-generating ...agent. The zeolite structure generation process was analyzed from X-ray powder diffraction, N2 adsorption isotherms, scanning electron micrographs, and transmission electron micrographs of the porous materials generated at various reaction times. This analysis indicated that the nanosheets were initially generated as a disordered assembly that transformed into an ordered multilamellar mesostructure through a dissolution−recrystallization process upon prolonged hydrothermal aging. Synthesis factors affecting the rates of the initial generation of the nanosheets and their restructuring process were investigated while varying the temperature, structure of the surfactant tail, synthesis compositions, and basicity. On the basis of the result, it was possible to synthesize MFI zeolite nanosheets as a fully disordered assembly possessing a large mesopore volume even under synthesis conditions using the bromide form of the surfactant and a sodium-containing silica source such as water glass.
► We have synthesized zeolite NaA from metakaolin. ► Magnetic zeolite NaA was synthesized by adding Fe3O4 in the precursor of the zeolite. ► Optimum synthesis conditions were determined. ► Magnetic ...zeolite can be used to remove heavy metals Cu2+, Pb2+ from water.
The optimum parameters for synthesis of zeolite NaA based on metakaolin were investigated according to results of cation exchange capacity and static water adsorption of all synthesis products and selected X-ray diffraction (XRD). Magnetic zeolite NaA was synthesized by adding Fe3O4 in the precursor of zeolite. Zeolite NaA and magnetic zeolite NaA were characterized with scanning electron microscopy (SEM) and XRD. Magnetic zeolite NaA with different Fe3O4 loadings was prepared and used for removal of heavy metals (Cu2+, Pb2+). The results show the optimum parameters for synthesis zeolite NaA are SiO2/Al2O3=2.3, Na2O/SiO2=1.4, H2O/Na2O=50, crystallization time 8h, crystallization temperature 95°C. The addition of Fe3O4 makes the NaA zeolite with good magnetic susceptibility and good magnetic stability regardless of the Fe3O4 loading, confirming the considerable separation efficiency. Additionally, Fe3O4 loading had a little effect on removal of heavy metal by magnetic zeolite, however, the adsorption capacity still reaches 2.3mmolg−1 for Cu2+, Pb2+ with a removal efficiency of over 95% in spite of 4.7% Fe3O4 loading. This indicates magnetic zeolite can be used to remove metal heavy at least Cu2+, Pb2+ from water with metallic contaminants and can be separated easily after a magnetic process.
Fluoride in drinking water above permissible levels is responsible for human dental and skeletal fluorosis. In this study, therefore, the large internal surface area of zeolite was utilized to create ...active sites for fluoride sorption by exchanging Na
+-bound zeolite with Al
3+ or La
3+ ions. Fluoride removal from water using Al
3+- and La
3+-exchanged zeolite F-9 particles was subsequently investigated to evaluate the fluoride sorption characteristics of the sorbents. Equilibrium isotherms such as the two-site Langmuir (L), Freundlich (F), Langmuir–Freundlich (LF), Redlich–Peterson (RP), Tóth (T), and Dubinin–Radushkevitch (DR) were successfully used to model the experimental data. Modeling results showed that the isotherm parameters weakly depended on the solution temperature. From the DR isotherm parameters, it was considered that the uptake of fluoride by Al
3+-exchanged zeolite proceeded by an ion-exchange mechanism (
E
=
11.32
−
12.13
kJ
/
mol
), while fluoride–La
3+-exchanged zeolite interaction proceeded by physical adsorption (
E
=
7.41
−
7.72
kJ
/
mol
). Factors from the solution chemistry that affected fluoride removal from water were the solution pH and bicarbonate content. The latter factor buffered the system pH at higher values and thus diminished the affinity of the active sites for fluoride. Natural groundwater samples from two Kenyan tube wells were tested and results are discussed in relation to solution chemistry. In overall, Al
3+-exchanged zeolite was found to be superior to La
3+-exchanged zeolite in fluoride uptake within the tested concentration range.
ZSM-5 zeolite is widely used in catalytic cracking of hydrocarbon, but the conventional ZSM-5 zeolite deactivates quickly due to its simple microporous and long diffusion pathway. Many studies have ...been done to overcome these disadvantages recently. In this review, four main approaches for enhancing the catalytic performance, namely synthesis of ZSM-5 zeolite with special morphology, hierarchical ZSM-5 zeolite, nano-sized ZSM-5 zeolite and optimization of acid properties, are discussed. ZSM-5 with special morphology such as hollow, composite and nanosheet structure can effectively increase the diffusion efficiency and accessibility of acid sites, giving high catalytic activity. The accessibility of acid sites and diffusion efficiency can also be enhanced by introducing additional mesopores or macropores. By decreasing the crystal size to nanoscale, the diffusion length can be shortened. The catalytic activity increases and the amount of carbon deposition decreases with the decrease of crystal size. By regulating the acid properties of ZSM-5 with element or compound modification, the overreaction of reactants and formation of carbon deposition could be suppressed, thus enhancing the catalytic activity and light alkene selectivity. Besides, some future needs and perspectives of ZSM-5 with excellent cracking activity are addressed for researchers’ consideration.
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