Contribution of the lamellar morphology to the photocatalytic activity of alkaline-hydrothermally treated titania in rhodamine B photodegradation.

TiO2 particles with a specific morphology are essential for their accessibility and photoactivity. The present study shows that NH4OH-based alkaline-hydrothermal treatment affects the transformation of their particle morphology. We investigated the effect of NH4OH by varying the synthesis route. We observed that the TiO2 particles with an open channel pore structure only resulted in the alkaline-hydrothermally treated and calcined samples. Based on Raman and XRD analyses, we figured out the titanate layers as an intermediate phase resulting from the alkaline-hydrothermal treatment of the amorphous particles. The hydrothermal treatment changed the particle surface morphology into a lamellar structure with a high specific surface area. These are the anatase precursors with {200} planes that transform into the anatase phase after calcination. The calcination followed by alkaline-hydrothermal treatment converted the crystallinity without significantly changing their morphology. We found that the morphology of TiO2 particles can be modified via hydrothermal treatment using NH4OH as long as the particles remain uncrystallized. We suggested the modification of particle morphology through the swelling and phase segregation process by alkaline-hydrothermal treatment. All final products have been used for the photodegradation of rhodamine B. S-HT-500 and A-HT-500 show the best photocatalytic activity with their rate constants (k) of 47.9 and 30.9 × 10-2 min-1, and their surface area-normalized rate constants (ksa) of 6.5 and 2.6 × 10-3 L m-2 min-1, respectively, and have a photocatalytic efficiency of 90.93% and 67.78%, respectively, after 10 minutes of UV irradiation. This activity is approximately 3.5 times and 1.5 times higher than that of Degussa P25; 30 times and 20 times higher than that without a photocatalyst.

Thermodynamic Picture of Phase Segregation during the Formation of Bicontinuous Concentric Lamellar (bcl) Silica.

The thermodynamic picture describing the formation mechanism of bicontinuous concentric lamellar (bcl) nanostructured silica particles, bcl silica, was investigated thoroughly. A series of classical kinetics of bcl silica by varying the synthesis time were employed to observe the morphological evolution of bcl silica. The formation mechanism of bcl silica is proposed as the hydrolysis and condensation reactions in the reverse micelle, followed by the phase segregation process. The images of the whole part and the cross-section of bcl silica reveal that bcl silica can be obtained just 30 min after the synthesis starts. The particle morphology evolves from bicontinuous lamellar (bl) morphology, with the absence of the dense part in the center of the particle, to bicontinuous concentric lamellar (bcl) morphology. The theoretical part of this study is focused on the phase segregation process of the mixture. This process is divided thermodynamically into several reversible processes based on the reduced Helmholtz free energy state function. The type of the lamellar orientation (i.e., parallel or perpendicular orientation) changed as the stacked lamellae changed in thickness and was followed by the decrease in the free energy. It was merely shown that the segregation of the thin slab of the lamellar polysiloxane stack favors the perpendicular orientation. In contrast, the thick slab of the lamellar polysiloxane stack yields a complex lamellar structure consisting of perpendicular and parallel orientations. A lamellar polymer confined between two planar substrates can experience a topological transformation into a sphere due to an unfavorable environment, i.e., high surface tension. After the topological transformation, lamellae with a perpendicular orientation form bicontinuous lamellae, whereas the complex lamellar structure transforms into a bicontinuous concentric lamellar morphology.

Recent Advances in Photocatalytic Oxidation of Methane to Methanol.

Methane is one of the promising alternatives to non-renewable petroleum resources since it can be transformed into added-value hydrocarbon feedstocks through suitable reactions. The conversion of methane to methanol with a higher chemical value has recently attracted much attention. The selective oxidation of methane to methanol is often considered a "holy grail" reaction in catalysis. However, methanol production through the thermal catalytic process is thermodynamically and economically unfavorable due to its high energy consumption, low catalyst stability, and complex reactor maintenance. Photocatalytic technology offers great potential to carry out unfavorable reactions under mild conditions. Many in-depth studies have been carried out on the photocatalytic conversion of methane to methanol. This review will comprehensively provide recent progress in the photocatalytic oxidation of methane to methanol based on materials and engineering perspectives. Several aspects are considered, such as the type of semiconductor-based photocatalyst (tungsten, titania, zinc, etc.), structure modification of photocatalyst (doping, heterojunction, surface modification, crystal facet re-arrangement, and electron scavenger), factors affecting the reaction process (physiochemical characteristic of photocatalyst, operational condition, and reactor configuration), and briefly proposed reaction mechanism. Analysis of existing challenges and recommendations for the future development of photocatalytic technology for methane to methanol conversion is also highlighted.

Photocatalytic Degradation of Palm Oil Mill Effluent (POME) Waste Using BiVO4 Based Catalysts.

Disposal of palm oil mill effluent (POME), which is highly polluting from the palm oil industry, needs to be handled properly to minimize the harmful impact on the surrounding environment. Photocatalytic technology is one of the advanced technologies that can be developed due to its low operating costs, as well as being sustainable, renewable, and environmentally friendly. This paper reports on the photocatalytic degradation of palm oil mill effluent (POME) using a BiVO4 photocatalyst under UV-visible light irradiation. BiVO4 photocatalysts were synthesized via sol-gel method and their physical and chemical properties were characterized using several characterization tools including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), surface area analysis using the BET method, Raman spectroscopy, electron paramagnetic resonance (EPR), and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). The effect of calcination temperature on the properties and photocatalytic performance for POME degradation using BiVO4 photocatalyst was also studied. XRD characterization data show a phase transformation of BiVO4 from tetragonal to monoclinic phase at a temperature of 450 °C (BV-450). The defect site comprising of vanadium vacancy (Vv) was generated through calcination under air and maxima at the BV-450 sample and proposed as the origin of the highest reaction rate constant (k) of photocatalytic POME removal among various calcination temperature treatments with a k value of 1.04 × 10-3 min-1. These findings provide design guidelines to develop efficient BiVO4-based photocatalyst through defect engineering for potential scalable photocatalytic organic pollutant degradation.

Organically surface engineered mesoporous silica nanoparticles control the release of quercetin by pH stimuli.

Controlling the premature release of hydrophobic drugs like quercetin over physiological conditions remains a challenge motivating the development of smart and responsive drug carriers in recent years. This present work reported a surface modification of mesoporous silica nanoparticles (MSN) by a functional compound having both amines (as a positively charged group) and carboxylic (negatively charged group), namely 4-((2-aminoethyl)amino)-4-oxobut-2-enoic acid (AmEA) prepared via simple mechanochemistry approach. The impact of MSN surface modification on physical, textural, and morphological features was evaluated by TGA, N2 adsorption-desorption, PSA-zeta, SEM, and TEM. The BET surface area of AmEA-modified MSN (MSN-AmEA) was found to be 858.41 m2 g-1 with a pore size of 2.69 nm which could accommodate a high concentration of quercetin 118% higher than MSN. In addition, the colloidal stability of MSN-AmEA was greatly improved as indicated by high zeta potential especially at pH 4 compared to MSN. In contrast to MSN, MSN-AmEA has better in controlling quercetin release triggered by pH, thanks to the presence of the functional groups that have a pose-sensitive interaction hence it may fully control the quercetin release, as elaborated by the DFT study. Therefore, the controlled release of quercetin over MSN-AmEA verified its capability of acting as a smart drug delivery system.

A Mechanistic Investigation of Sustainable Solvent-Free, Seed-Directed Synthesis of ZSM-5 Zeolites in the Absence of an Organic Structure-Directing Agent.

The solvent-free, seed-directed synthesis using natural precursors has emerged as a sustainable route for the synthesis of zeolite. Albeit the significant progress in the synthesis techniques, the crystallization behaviors of zeolites are somewhat elusive. Herein, we performed a detailed investigation of the crystallization behaviors of ZSM-5 zeolites synthesized through the solvent-free, seed-directed route using rice husk silica as starting materials. The crystallization at 180 °C is completed rapidly in 10 h, with an ultrahigh zeolite yield of at least 95%. Moreover, we evaluated the crystallization kinetics at different temperatures using the nonlinear Avrami equation and found instantaneous nucleation with three-dimensional growth in the studied temperature range, with activation energies for nucleation, transition, and crystal growth of 137, 51, and 51 kJ mol-1, respectively, indicating that nucleation is the rate-determining step. Further investigation of the structural and morphological evolution revealed a preference for secondary nucleation over the seed-growth mechanism. Crystallization proceeds via structural rearrangement within the solid system. We anticipate that our work will provide extensive insights that increase the understanding of zeolite crystallization and expand the highly sustainable production of zeolites.

Direct hydrothermal synthesis of hierarchically porous siliceous zeolite by using alkoxysilylated nonionic surfactant.

A hierarchically porous siliceous MFI zeolite (silicalite-1) with narrow mesoporosity has been hydrothermally synthesized by using trialkoxysilylated alkyl poly(oxyethylene ether) as mesopore-directing agent. A mesostructured silica-surfactant composite was formed at the early stage of the reaction, and zeolite crystallization proceeded during subsequent hydrothermal treatment. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations of the crystallized products showed that micro- and mesopores were hierarchically assembled in unique particle morphology with rugged surfaces. Solid-state (29)Si and (13)C NMR revealed that the covalent bonds between the zeolite framework and mesopore-directing agent were present in the products before calcination. The use of nonsilylated alkyl poly(oxyethylene ether) or a silylated alkytrimethyl-ammonium-type cationic surfactant for the synthesis of silicalite-1 resulted in a mixture of mesoporous silica and zeolite as the final product, which suggests that the covalent interaction and nonelectrostatic charge matching interaction favor the formation of hierarchically mesoporous siliceous zeolite. This alkoxysilylated nonionic surfactant can also be extended to synthesize aluminosilicate MFI zeolite (ZSM-5).

Critical nuclei size, initial particle size and packing effect on the phase stability of sol-peptization-gel-derived nanostructured titania.

The influence of the initial particle size and packing of anatase crystallites on the phase stability of nanostructured titania was investigated. Dried anatase gels with different degrees of particle packing were prepared through the peptization-induced electrostatic stabilization of primary particles in the sol. The initial size of anatase primary particles was varied by precalcination prior to the anatase-rutile phase transformation that occurred during final calcination. In the case of well-packed titania, the initial size of anatase primary particles does not influence the phase-transformation behavior whereas loosely packed titania shows a strong initial anatase primary particle size dependence on the phase-transformation behavior.

Revisiting the seed-assisted synthesis of zeolites without organic structure-directing agents: insights from the CHA case.

Herein, the crystallization behaviour of the CHA zeolite synthesized via the seed-assisted method and in the absence of an organic structure-directing agent has been revisited. To date, the working hypothesis of the seed-assisted synthesis method is that the parent gel and seed share the common composite building unit (cbu) of the targeted zeolite crystal. In the case of the CHA zeolite, we reveal that the parent gel in the absence of CHA seeds leads to the formation of the MER zeolite, which does not follow the cbu working hypothesis. It appears that smaller, but essential common units, i.e., 4-membered ring (4-MR) aluminosilicate, play a key role, instead of a larger cbu. The parent gel contains 4-MRs, which can grow into MER and/or CHA, depending on several factors, i.e. alkalinity, Si/Al ratio, synergistic effects of Na+ and K+, and the seeds. In this study, the CHA zeolite with an Si/Al ratio of up to 15 was selectively crystallized in the presence of CHA seeds at suitable alkalinity ((Na2O + K2O)/SiO2 < 0.4) with a fixed point at the tie line of Na2O/SiO2 = 0.3 with K2O/SiO2 = 0.1. Subsequently, the ternary phase diagram was drawn as high alkalinity with (Na2O + K2O)/H2O > 0.4 showing the formation of an MER zeolite, a thermodynamically more stable phase than the CHA zeolite, whereas low alkalinity with (Na2O + K2O)/SiO2 < 0.4 showed the less crystalline CHA zeolite or amorphous products with MER as a competing phase. The crystallization of the CHA zeolite was found to be strongly dependent on the synergistic effects of sodium and potassium ions. The former appears to organize the 4-MRs into essential double-six rings (d6rs), while the latter arranges the formed d6rs into cha cages. The seeds are partially dissolved and provide the outer surface for the crystal growth of CHA. We anticipate that these results may provide useful insight for understanding the crystallization of zeolites and stimulate versatile design in the synthesis of zeolites, particularly for the industrially demanding seed-assisted technique.

pH-Triggered Drug Release Controlled by Poly(Styrene Sulfonate) Growth Hollow Mesoporous Silica Nanoparticles.

In the current report, hollow mesoporous silica (HMS) nanoparticles were successfully prepared by means of a hard-templating method and further modified with poly(styrene sulfonate) (PSS) via radical polymerization. Structural analysis, surface spectroscopy, and thermogravimetric characterization confirmed a successful surface modification of HMS nanoparticles. A hairy PSS was clearly visualized by high-resolution transmission electron microscopy measurement, and it is grown on the surface of HMS nanoparticles. The Brunauer-Emmett-Teller surface area and average pore size of HMS nanoparticles were reduced after surface modification because of the pore-blocking effect, which indicated that the PSS lies on the surface of nanoparticles. Nevertheless, the PSS acts as a "nano-gate" to control the release of curcumin which is triggered by pH. The drug-release profile of unmodified HMS nanoparticles showed a stormed release in both pH 7.4 and 5.0 of phosphate buffer saline buffer solution. However, a slow release (9.92% of cumulative release) of curcumin was observed at pH 7.4 when the surface of HMS nanoparticles was modified by PSS. The kinetic release study showed that the curcumin release mechanism from PSS@HMS nanoparticles followed the Ritger-Peppas kinetic model, which is the non-Fickian diffusion. Therefore, the PSS-decorated HMS nanoparticles demonstrate potential for pH-triggered drug release transport.

Structural alteration within fly ash-based geopolymers governing the adsorption of Cu2+ from aqueous environment: Effect of alkali activation.

Fly-ash based geopolymers have been considered as a low-cost yet effective adsorbent for the removal of heavy metal cations, including Cu2+, from the aqueous environment. In the synthesis of geopolymers, the fly-ash needs to be alkali activated using several systems rich in either Na+ or K+. Herein, we investigate the effect of alkali activation on the structural alteration and its consequence on the adsorption capacity. Based on the series of detailed characterizations, the geopolymers formed in Na+-based alkali system is found to have more organized structure compared to that formed in K+-based alkali system. Moreover, the incorporation of additional silicate creates ancillary structure which positively contributes to the organization of the overall structure. All the samples, fly-ash and geopolymers, exhibits Cu2+ adsorption based on Langmuir isotherm and pseudo-second order kinetic. The geopolymers with more organized structure display higher Cu2+ adsorption capacity, which reaches 40 mg g-1 higher in comparison to 7 mg g-1 for fly ash sample. The structural alteration induces the formation of open-framework structure with more accessible sites which can accommodate more Cu2+. Our study provides a fundamental understanding for the design and fabrication of geopolymers as an effective adsorbent for the removal of heavy metal cations.

Highly crystalline mesoporous SSZ-13 zeolite obtained via controlled post-synthetic treatment.

The generation of mesoporosity in SSZ-13 zeolite by means of desilication via post alkaline treatment normally results in severe damage to the microporous framework hence giving an undesirable decline in catalytic performance. Herein, we propose a post-synthetic desilication treatment that is controllable with an aim to preserve the high crystallinity of SSZ-13 zeolite during the formation of mesopores. The extent of desilication in alkaline media is controlled by deliberately leaving the organics within SSZ-13 frameworks as they can effectively hinder the attack of hydroxyl ions on siloxane bonds. The resulting SSZ-13 exhibits substantial development of mesoporosity with preserved high crystallinity and microporosity that can then be used to relieve the mass transport issues and lead to an increased activity of LDPE pyrolysis.

Effect of Extra-Framework Cations of LTL Nanozeolites to Inhibit Oil Oxidation.

Lubricant oils take significant part in current health and environmental considerations since they are an integral and indispensable component of modern technology. Antioxidants are probably the most important additives used in oils because oxidative deterioration plays a major role in oil degradation. Zeolite nanoparticles (NPs) have been proven as another option as green antioxidants in oil formulation. The anti-oxidative behavior of zeolite NPs is obvious; however, the phenomenon is still under investigation. Herein, a study of the effect of extra-framework cations stabilized on Linde Type L (LTL) zeolite NPs (ca. 20 nm) on inhibition of oxidation in palm oil-based lubricant oil is reported. Hydrophilic LTL zeolites with a Si/Al ratio of 3.2 containing four different inorganic cations (Li(+), Na(+), K(+), Ca(2+)) were applied. The oxidation of the lubricant oil was followed by visual observation, colorimetry, fourier transform infrared (FTIR) spectroscopy, (1)H NMR spectroscopy, total acid number (TAN), and rheology analyses. The effect of extra-framework cations to slow down the rate of oil oxidation and to control the viscosity of oil is demonstrated. The degradation rate of the lubricant oil samples is decreased considerably as the polarizability of cation is increased with the presence of zeolite NPs. More importantly, the microporous zeolite NPs have a great influence in halting the steps that lead to the polymerization of the oils and thus increasing the lifetime of oils.

Inhibition of palm oil oxidation by zeolite nanocrystals.

The efficiency of zeolite X nanocrystals (FAU-type framework structure) containing different extra-framework cations (Li(+), Na(+), K(+), and Ca(2+)) in slowing the thermal oxidation of palm oil is reported. The oxidation study of palm oil is conducted in the presence of zeolite nanocrystals (0.5 wt %) at 150 °C. Several characterization techniques such as visual analysis, colorimetry, rheometry, total acid number (TAN), FT-IR spectroscopy, (1)H NMR spectroscopy, and Karl Fischer analyses are applied to follow the oxidative evolution of the oil. It was found that zeolite nanocrystals decelerate the oxidation of palm oil through stabilization of hydroperoxides, which are the primary oxidation product, and concurrently via adsorption of the secondary oxidation products (alcohols, aldehydes, ketones, carboxylic acids, and esters). In addition to the experimental results, periodic density functional theory (DFT) calculations are performed to elucidate further the oxidation process of the palm oil in the presence of zeolite nanocrystals. The DFT calculations show that the metal complexes formed with peroxides are more stable than the complexes with alkenes with the same ions. The peroxides captured in the zeolite X nanocrystals consequently decelerate further oxidation toward formation of acids. Unlike the monovalent alkali metal cations in the zeolite X nanocrystals (K(+), Na(+), and Li(+)), Ca(2+) reduced the acidity of the oil by neutralizing the acidic carboxylate compounds to COO(-)(Ca(2+))1/2 species.

Eco-friendly synthesis for MCM-41 nanoporous materials using the non-reacted reagents in mother liquor.

Nanoporous materials such as Mobil composite material number 41 (MCM-41) are attractive for applications such as catalysis, adsorption, supports, and carriers. Green synthesis of MCM-41 is particularly appealing because the chemical reagents are useful and valuable. We report on the eco-friendly synthesis of MCM-41 nanoporous materials via multi-cycle approach by re-using the non-reacted reagents in supernatant as mother liquor after separating the solid product. This approach was achieved via minimal requirement of chemical compensation where additional fresh reactants were added into the mother liquor followed by pH adjustment after each cycle of synthesis. The solid product of each successive batch was collected and characterized while the non-reacted reagents in supernatant can be recovered and re-used to produce subsequent cycle of MCM-41. The multi-cycle synthesis is demonstrated up to three times in this research. This approach suggests a low cost and eco-friendly synthesis of nanoporous material since less waste is discarded after the product has been collected, and in addition, product yield can be maintained at the high level.