MW-assisted Method Research Articles

Catalytic pyrolysis of high-density polyethylene (HDPE) over hierarchical ZSM-5 zeolites produced by microwave-assisted chelation-alkaline treatment

A new microwave (MW) assisted chelation-alkaline strategy was developed to produce hierarchical ZSM-5 zeolites (with external surface area of 203 m2/g) in minutes, leading to the formation of heterogeneous structure with a high-silica shell and aluminium-rich core (silicon-to-aluminium ratios of 49.2 and 37.6, respectively). The zeolites were assessed comparatively by catalytic pyrolysis of high-density polyethylene (HDPE). Compared to the mesoporous ZSM-5 zeolite prepared by the conventional alkaline treatment (via desilication by the NaOH aqueous solution), the hierarchical zeolite by the MW-assisted method showed comparable kinetics in catalytic pyrolysis of HDPE, but with the improved stability in the cyclic pyrolysis-regeneration tests. The improved stability could be due to the better-preserved structural integrity thanks to the relatively milder surface etching by the MW-assisted chelation dealumination method. In addition, compared to the parent ZSM-5, the product distribution of HDPE catalytic pyrolysis over the hierarchical ZSM-5 zeolite showed a higher portion of oil products (i.e., 12 wt% vs. 6.9 wt%) and an improved alkane-to-olefin ratio in the oil products (2.22 vs. 1.86, respectively) due to the reduced acidity and improved hydrogen transfer ability. The MW-assisted chelation-alkaline treatment could be a new strategy to regulate the trade-off between stability and activity of zeolites for catalytic applications.

Microwave-assisted production of metal-organic frameworks for water purification: A mini-review

Metal-organic frameworks (MOFs) have been demonstrated to be the “star” functional materials for pollutants elimination from wastewater. The microwave (MW)-assisted method can accomplish higher MOFs yield within shorter time than conventional approaches, which can produce desired MOFs with the larger BET surface area and higher catalytic performance. Herin, the various MW-assisted methods, and the corresponding influences of various factors (including synthesis time, solvent, temperature) on the yields and physicochemical properties of the as-prepared MOFs were summarized and discussed in detail. Moreover, the adsorption, catalytic oxidation and catalytic reduction performances of as-prepared MOFs were highlighted. Finally, the future challenges and opportunities of MOFs prepared via MW-assisted method for water purification are proposed, which might provide useful information for designing and producing the versatile MOFs via MW-assisted methods.

On understanding the sequential post-synthetic microwave-assisted dealumination and alkaline treatment of Y zeolite

Systematic investigation was performed to understand the change of physiochemical properties in Y zeolite after the microwave (MW)-assisted dealumination (using mineral acid, HCl, and chelating agent, EDTA4−) and the subsequent alkaline treatment (of the dealuminated zeolites). The findings show that the combination of EDTA4− and hydrogen ions was effective to achieve dealumination of zeolite Y under MW irradiation, which formed complexed framework Al, instead of extra-framework Al (EFAl), to be extracted readily by the sequential alkaline treatment for mesopore formation. Conversely, under the same MW condition, the use of HCl encouraged the formation of EFAl species in the defective Y framework, which did not benefit the mesopore formation. The disclose of the distinct dealumination mechanisms of the MW-assisted method using different agents can benefit the further development of effective MW methods for dealumination of zeolites and/or making mesoporous zeolites.

Synthesis of 3,4-Dihydropyrimidin-2(1H)-one-phosphonates by the Microwave-Assisted Biginelli Reaction

The synthesis of novel 3,4-dihydropyrimidin-2(1H)-one-phosphonates was elaborated by the microwave (MW)-assisted three-component Biginelli reaction of β-ketophosphonates, aromatic or aliphatic aldehydes and urea derivatives. The condensation was optimized on a selected model reaction in respect of the reaction parameters, such as the heating method, the type of the catalyst and solvent, the temperature, the reaction time and the molar ratio of the starting materials. The fast and solvent-free MW-assisted procedure was then extended for the preparation of further new 3,4-dihydropyrimidin-2(1H)-one-phosphonate derivatives starting from different aromatic aldehydes, β-ketophosphonates and urea derivatives to prove the wide scope of the process. As a novel by-product of the Biginelli-type synthesis of 3,4-dihydropyrimidin-2(1H)-one-phosphonates, the 5-diethoxyphosphoryl-4-phenyl-6-styryl-3,4-dihydropyrimidin-2(1H)-one was also isolated and characterized. Our MW-assisted method made also possible the condensation of aliphatic aldehydes, diethyl (2-oxopropyl)phosphonate and urea, which reaction was previously reported to be impossible in the literature.

SYNTHESIS OF MESOPOROUS NANOPARTICLES VIA MICROWAVE-ASSISTED METHOD FOR PHOTOCATALYTIC DEGRADATION OF PHENOL DERIVATIVES

Mesoporous transition metal oxides have gained attention widely since they possess both optical and electronic properties of transition metal oxides especially for photocatalytic degradation application. In this research work, mesoporous titania nanoparticles (MTN) and mesoporous zinc oxide nanoparticles (MZN) were successfully synthesized using microwave (MW)-assisted method to degrade phenol derivatives under visible light irradiation. The microwave sintering effect on the surface of these modified structures was studied to relate with their photocatalytic performance. The characterization results indicated that MW-assisted method was mainly contributed in generating Ti3+ site defects (TSD) and oxygen vacancies (OV) in MTN while for MZN contained only OV as one of the strategies in light-absorption modification for TiO2 and ZnO to enhance their photoactivity. MTN also showed the degradation of 2-chlorophenol was up to 97% while degradation of phenol by MZN was up to 87%.

Facile microwave synthesis of carbon dots powder with enhanced solid-state fluorescence and its applications in rapid fingerprints detection and white-light-emitting diodes

In this report, we successfully developed a simple and fast MW-assisted method for preparing CDs with strong solid-state fluorescence (SSF) by using phthalic acid and piperazine as precursors. The prepared p-CDs can be obtained in high yield (48.7%) and emit bright yellow-green SSF under 365 nm UV light. The absolute PL quantum yield (PLQY) of p-CDs in solid state was measured to be 20.5%, which is much higher than that in aqueous solution. This interesting phenomenon shows that p-CDs not only successfully conquer the aggregation-caused fluorescence quenching (ACQ) effect, but also achieve enhanced fluorescence emission, which was rarely reported in previous literatures as CDs in solid state always reduce their fluorescence emission due to the excessive resonance energy transfer (RET) or direct π-π interactions. In addition, the relationship between the feed ratio of precursors and optical properties of the CDs were also investigated detailedly. Based on their strong SSF, the p-CDs were successfully used in rapid latent fingerprints detection and white light-emitting diodes (WLEDs) preparation with high quality. In summary, this research not only developed a new type of CDs with strong enhanced SSF, but also offered a valuable reference for design SSF-emitting CDs with high yield.

Microwave-Assisted Rapid Synthesis of Graphene-Supported Single Atomic Metals.

Graphene-supported single atomic metals (G-SAMs) have recently attracted considerable research interest for their intriguing catalytic, electronic, and magnetic properties. The development of effective synthetic methodologies toward G-SAMs with monodispersed metal atoms is vital for exploring their fundamental properties and potential applications. A convenient, rapid, and general strategy to synthesize a series of monodispersed atomic transition metals (for example, Co, Ni, Cu) embedded in nitrogen-doped graphene by two-second microwave (MW) heating the mixture of amine-functionalized graphene oxide and metal salts is reported here. The MW heating is able to simultaneously induce the reduction of graphene oxide, the doping of nitrogen, and the incorporation of metal atoms into the graphene lattices in one simple step. The rapid MW process minimizes metal diffusion and aggregation to ensure exclusive single metal atom dispersion in graphene lattices. Electrochemical studies demonstrate that graphene-supported Co atoms can function as highly active electrocatalysts toward the hydrogen evolution reaction. This MW-assisted method provides a rapid and efficient avenue to supported metal atoms for wide ranges of applications.

A self-quenching-resistant carbon dots powder with tunable solid-state fluorescence and their applications in light-emitting diodes and fingerprints detection

In this report, a novel type of carbon dots (CDs) were prepared via a simple MW-assisted method by using citric acid and piperazine as precursors. The CDs can be obtained in large-scale and emit bright solid-state fluorescence (SSF) with different colors. Then the synthetic CDs were fully characterized with PL, UV, XPS, TEM, and FTIR and the relationship between the mass ratio of precursors and physicochemical property of the CDs was also investigated detailedly. The results showed that these CDs prepared at different mass ratios have different element content and functional group content. More importantly, the mass ratio of the precursors not only has a great influence on their particle sizes, but also can affect their PL properties. With the decreased of the mass ratio, their particle size decreased gradually and their emission wavelength gradually red-shift. This interesting finding may be helpful for fabricating CDs with controllable particle size and tunable light-emitting colors. Based on their excellent SSF property, these CDs were successfully used to fabricate light-emitting diodes (LEDs) and detect latent fingerprints (LFPs). In conclusion, the research may offer a valuable reference for obtaining CDs with SSF and developing LEDs phosphors and LFPs detection reagent.

Microwave Assisted Preparation of Thio-Functionalized Polyacrylonitrile Fiber for the Selective and Enhanced Adsorption of Mercury and Cadmium from Water

In order to overcome the drawbacks of conventional heating for performing surface modification process, such as long synthesis time, low ef ciency, and adsorption capacity, we reported a novel approach by introducing a microwave (MW) assisted method to prepare a thio-functionalized fibrous adsorbent (PANMW-Thio) for the enhanced and selective removal of Hg2+ and Cd2+ from water in this study. The properties and morphologies of the adsorbent prepared by the MW-assisted method and conventional heating were compared through FTIR, XRD, SEM characterization, and mechanical tests, and the results revealed that the MW assisted method is a satisfactory technology to enable a higher sulfur content and reinforced mechanical property for the adsorbent. The selective adsorption experiment indicated that PANMW-Thio fibers possess higher affinity toward both Hg2+ and Cd2+ in the mixed solution with Pb2+, Cu2+, Ni2+, and Zn2+. The optimum pH value for the adsorption of Hg2+ and Cd2+ was found to be 7 with maximum adsorp...

Microwave-assisted radiosynthesis of the hypoxia marker 1-α-D-(5- deoxy-5-[18F]fluoroarabinofuranosyl)-2-nitroimidazole ([18F]FAZA).

The routine manufacture of most short-lived positron-emitting radiopharmaceuticals (PERs) involves conventional heating to accelerate the radiolabeling process. Nucleophilic radiofluorination reactions are generally slow at lower temperatures, and are accompanied by thermal decomposition of both precursor and product at higher temperatures. This necessitates HPLC purification and results in lower recovered radiochemical yields (rRCYs). [(18)F]FAZA, a PER for clinical imaging of focal tissue hypoxia, is routinely manufactured in-house in 3-12% rRCY using a Health Canada approved conventional heating procedure. The microwave-assisted (MW) radiosynthesis of [(18)F]FAZA is now reported. Manual (MRDS) and automated (ASU) reagent delivery systems coupled to a commercial MW unit were built in-house. The MW unit controlled power, irradiation time and monitored reaction temperature (Tmax control), while the acetylAZA tosylate precursor and QMA Accel(TM) cartridge eluent reagents (K2CO3, K2.2.2) were dispensed by the MRDS or ASU. The radiofluorination yields (RFYs) and the chemical and radiochemical TLC profiles of the post-labeling reaction mixtures were compared to those obtained using the conventional heating production method and to those reported for optimized literature methods. MW RFYs for [(18)F]FAZA reached >76% (n=3) in 3 min. Post-labeling analysis of the MW-assisted reaction mixtures demonstrated cleaner UV and radiochemical TLC profiles than those obtained from conventional heating in routine production runs; the relatively clean MW reactions allowed rapid HPLC isolation of [(18)F]FAZA in overall rRCYs of 55±4%. In practical terms, the MW process provided only small gains in reaction time and RFY, but produced only a few secondary impurities, thereby improving the rRCY in comparison to conventional heating methods. These findings provide a rationale for adaptation of the MW-assisted method for the routine production of clinical [(18)F]FAZA.

Microwave-Assisted Chemistry: Synthetic Applications for Rapid Assembly of Nanomaterials and Organics

The magic of microwave (MW) heating technique, termed the Bunsen burner of the 21st century, has emerged as a valuable alternative in the synthesis of organic compounds, polymers, inorganic materials, and nanomaterials. Important innovations in MW-assisted chemistry now enable chemists to prepare catalytic materials or nanomaterials and desired organic molecules, selectively, in almost quantitative yields and with greater precision than using conventional heating. By controlling the specific MW parameters (temperature, pressure, and ramping of temperature) and choice of solvents, researchers can now move into the next generation of advanced nanomaterial design and development. Microwave-assisted chemical reactions are now well-established practices in the laboratory setting although some controversy lingers as to how MW irradiation is able to enhance or influence the outcome of chemical reactions. Much of the discussion has focused on whether the observed effects can, in all instances, be rationalized by purely thermal Arrhenius-based phenomena (thermal microwave effects), that is, the importance of the rapid heating and high bulk reaction temperatures that are achievable using MW dielectric heating in sealed reaction vessels, or whether these observations can be explained by so-called "nonthermal" or "specific microwave" effects. In recent years, innovative and significant advances have occurred in MW hardware development to help delineate MW effects, especially the use of silicon carbide (SiC) reaction vessels and the accurate measurement of temperature using fiber optic (FO) temperature probes. SiC reactors appear to be good alternatives to MW transparent borosilicate glass, because of their high microwave absorptivity, and as such they serve as valuable tools to demystify the claimed magical MW effects. This enables one to evaluate the influence of the electromagnetic field on the specific chemical reactions, under truly identical conventional heating conditions, wherein temperature is measured accurately by fiber optic (FO) probe. This Account describes the current status of MW-assisted synthesis highlighting the introduction of various prototypes of equipment, classes of organic reactions pursued using nanomaterials, and the synthesis of unique and multifunctional nanomaterials; the ensuing nanomaterials possess zero-dimensional to three-dimensional shapes, such as spherical, hexagonal, nanoprisms, star shapes, and nanorods. The synthesis of well-defined nanomaterials and nanocatalysts is an integral part of nanotechnology and catalysis science, because it is imperative to control their size, shape, and compositional engineering for unique deployment in the field of nanocatalysis and organic synthesis. MW-assisted methods have been employed for the convenient and reproducible synthesis of well-defined noble and transition core-shell metallic nanoparticles with tunable shell thicknesses. Some of the distinctive attributes of MW-selective heating in the synthesis and applications of magnetic nanocatalysts in organic synthesis under benign reaction conditions are highlighted. Sustainable nanomaterials and their applications in benign media are an ideal blend for the development of greener methodologies in organic synthesis; MW heating provides superb value to the overall sustainable process development via process intensification including the flow systems.

Selective MW-assisted surface chemical tailoring of hydrotalcites for fluorescent and biocompatible nanocomposites

ZnAl based hydrotalcite nanoparticles (ZnAl-HTlc NPs) were covalently modified by an organic oligothiophene fluorescent compound (T4Si) by using direct microwave (MW)-assisted silylation. Morphological and optical characterization proved that the MW-assisted method enables efficient grafting of the target fluorescent dye on the nanoparticles (NPs) surface in a few minutes with a predefined loading ratio only depending by the MW irradiation time. Moreover, the presented approach preserved the HTlc interlayer region, allowing further functionalization. Filmability, fluorescent properties, and biocompatibility of the silylated compound was also demonstrated highlighting the potential of the so-obtained lamellar NPs in applications broadening from diagnostic biomedical tools to photonics and sensing.

Coupling microwave-assisted and classical heating methods for scaling-up MFI zeolite membrane synthesis

Silicalite-1 (S-1) nano-seeds obtained by microwave-assisted (MW) heating have been used to coat industrial supports and to up-scale the synthesis of MFI zeolite membranes by secondary growth applying a classical heating (CH) method. The MW-assisted method was adapted/optimized for fast synthesis and high yield of uniform and non-aggregated S-1 nano-seeds suspensions which were directly used for seeding macroporous industrial α-Al2O3- based supports by dip-coating. The CH secondary growth yielded uniform MFI membranes, by applying optimal reaction conditions while minimizing the reaction time and consumption of chemicals. The protocol coupling MW-assisted and classical heating in zeolite membrane synthesis revealed high reproducibility and has been validated on various industrial ceramic supports: single tubes (Pall-Exekia, Atech, Inocermic, CTI) and also single capillaries and capillary bundles, from Hyflux, providing a higher surface/volume ratio (typically S/V > 100 m2/m3). Membrane homogeneity has been validated by both SEM and single gas permeation measurements with N2 and SF6. The N2/SF6 ideal selectivity was used to predict the ethanol/water separation factor for the prepared MFI membranes.