Dry Gel Research Articles

Dry gel assisting crystallization of bifunctional CuO–ZnO–Al2O3/SiO2–Al2O3 catalysts for CO2 hydrogenation

A series of bifunctional catalysts CuO–ZnO–Al2O3/SiO2–Al2O3 (CZA/SA) with different SA/CZA ratios were prepared by dry gel assisting crystallization method. The catalytic performance of bifunctional catalysts for one-step synthesis of methanol (MeOH) and dimethyl ether (DME) from CO2 hydrogenation was investigated. The results showed that compared with Cu/ZnO/Al2O3, CZA/SA catalyst significantly improved the selectivity of DME and MeOH and reduced the CO selectivity with moderate CO2 conversion. The catalyst structure was characterized by X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (H2-TPR), which further confirmed that the active site of methanol generation was the Cu–ZnO interface with a strong interaction rather than solo Cu or ZnO crystallite, which could enhance the hydrogenation function and inhibit reverse water gas shift reaction. Compared with the physical mixing method, the dry gel assisting crystallization method could flexibly adjusted the acid strength and the amounts of acid sites, which was benefit for enhancing dehydration function to improve the selectivity of DME.

High‐Performance Zeolitic Hollow‐Fiber Membranes by a Viscosity‐Confined Dry Gel Conversion Process for Gas Separation

Zeolite membranes show great potential for gas and hydrocarbon separations, but high manufacturing cost has been one of the main hurdles in their industrial application. Here we demonstrate a method termed viscosity-confined dry gel conversion (VCDGC) for zeolite hollow fiber membrane fabrication. We demonstrate in detail the VCDGC synthesis of small-pore CHA zeolite membranes. Extensive permeation measurements reveal that dry gel-processed CHA zeolite hollow fiber membranes have excellent gas and hydrocarbon separation characteristics well exceeding or comparable to current membranes. Medium-pore MFI membranes are also fabricated, and their favorable hydrocarbon separation characteristics indicate the versatility and reliability of the VCDGC method.

High-Performance Zeolitic Hollow-Fiber Membranes by a Viscosity-Confined Dry Gel Conversion Process for Gas Separation.

Zeolite membranes show great potential for gas and hydrocarbon separations, but high manufacturing cost has been one of the main hurdles in their industrial application. Here we demonstrate a method termed viscosity-confined dry gel conversion (VCDGC) for zeolite hollow fiber membrane fabrication. We demonstrate in detail the VCDGC synthesis of small-pore CHA zeolite membranes. Extensive permeation measurements reveal that dry gel-processed CHA zeolite hollow fiber membranes have excellent gas and hydrocarbon separation characteristics well exceeding or comparable to current membranes. Medium-pore MFI membranes are also fabricated, and their favorable hydrocarbon separation characteristics indicate the versatility and reliability of the VCDGC method.

Springback effect and structural features during the drying of silica aerogels tracked by in-situ synchrotron X-ray scattering

The springback effect during ambient pressure drying of aerogels is an interesting structural phenomenon, consisting of a severe shrinkage followed by almost complete re-expansion. The drying of gels causes shrinkage, whereas re-expansion is believed to be linked to repelling forces on the nanoscale. A multi-scale structural characterization of this significant volume change is key in controlling aerogel processing and properties. In this work, hydrophobic, monolithic silica aerogels with high specific surface areas were synthesized by modification with trimethylchlorosilane and ambient pressure drying. Here, we report a multi-method approach focusing on in-situ X-ray scattering to observe alterations of the nanostructured material during the drying of surface-modified and unmodified silica gels. Both show a porous fractal nanostructure, which partially collapses during drying and only recovers in surface-modified samples during the springback effect. Distinct changes of the X-ray scattering data were reproducibly associated with the shrinkage, re-expansion and drying of the gel network. Our findings may contribute to tailor aerogels with specific functionality, as the springback effect has a direct influence on properties (e.g., porosity, pore size distribution), which is directly affected by the degree of re-expansion.

A Novel Strategy to Enhance Acid Strength of Zeolites by Incorporating Ge into Zeolite Framework

AbstractIn the long history of zeolite research, further enhancement of their acid strength has looked difficult. In this study, we achieved the enhancement by incorporating Ge into the zeolite framework. We synthesised the zeolites by a dry gel conversion method and characterised them in terms of morphology, crystal structure, and composition. We determined the acid strength of these zeolites by calculating the change in the enthalpy of desorption in the temperature‐programmed desorption of ammonia and confirmed that the acid strength of the zeolites with Ge was higher than that of a conventional zeolite. This enhancement of acid strength probably originated from the unprecedented interactions between Al and Ge, unlike the conventional interactions between Al and Si. The enhanced acid strength led to high efficiency of ethanol‐to‐propylene reactions, affording higher yield of propylene than that obtained using conventional zeolites.

Supercritical CO2-assisted processes for development of added-value materials: Optimization of starch aerogels preparation and hemp seed extracts impregnation

The aim of the present study was the utilization of supercritical CO2 as a green medium in various processes: starch gel drying, supercritical extraction from hemp seed flour (SCE process), hemp seed oil (HSO) impregnation (SCI process), as well as integrated process of hemp seed flour (HSF) extraction and starch gels impregnation (SCE-SCI process) for development of added-value materials that can be used as phytopharmaceuticals. Optimization of starch aerogels was performed by variation of temperature (35 and 45 °C) and pressure (8, 10, and 20 MPa) in order to obtain materials with high porosity, which will enable maximal loading capacity for hemp seed extracts. Proposed scCO2-assisted processes of SCE from HSF, SCI and SCE-SCI were performed at pressure of 30 MPa and temperature of 60 °C. It was shown that conditions of starch gel drying significantly influenced material morphology (porosity was in a range of 48–82%, and specific surface area of 71–208 m2/g), which consequently determined aerogel loading capacity. FTIR analysis confirmed that scCO2 did not have effect on polymer composition nor it remained in polymer after drying process. The highest loading of both HSO and HSF extract (24.9% and 29.78%, respectively) was achieved when aerogel obtained at 10 MPa and 45 °C was tested as a carrier. Furthermore, chemical analysis showed that both HSO and HSF extract are rich in unsaturated fatty acids especially linoleic acid (54–59%) and α-linolenic acid (15–18%). These essential fatty acids have well-established health benefits including protection against cardiovascular, neurodegenerative and inflammatory diseases.

Two-Step Dry Gel Method Produces MgAPO-11 with Low Aspect Ratio and Improved Catalytic Performance in the Conversion of Methanol to Hydrocarbons

In this article, the synthesis, characterization and catalytic performance of three MgAPO-11 catalysts with distinct crystal morphologies (sunflower, ball and candy) are presented. Among the three samples, the candy-like MgAPO-11-C, with high crystallinity and uniform particle size (of about 1 µm), was synthesized for the first time by using a unique two-step dry gel method. Despite the similar acid strength of the three samples, the different and distinct morphologies of the catalysts resulted in very different methanol-to-hydrocarbons (MTH) performances. In particular, the candy-like MgAPO-11-C presented the best MTH performance with the highest total conversion capacity (4.4 gMeOH·gcatalyst−1 h−1) and the best selectivity to C5+ aliphatics (64%).

Design of Zr- and Al-Doped *BEA-Type Zeolite to Boost LDPE Cracking.

Nowadays, the increase in plastic waste is causing serious environmental problems. Catalytic cracking has been considered a promising candidate to solve these problems. Catalytic cracking has emerged as an attractive process that can produce valuable products from plastic wastes. Solid acid catalysts such as zeolites decompose the plastic waste at a lower temperature. The lower decomposition temperature may be desirable for practical use. Herein, we synthesized both Zr- and Al-incorporated Beta zeolite using amorphous ZrO2–SiO2. The optimized Zr content in the dry gel allowed the enhancement of Lewis acidity without a significant loss of Brønsted acidity. The enhancement of Lewis acidity was mainly due to Zr species incorporated into the zeolite framework. Thanks to the enhanced Lewis acidity without any significant loss of Brønsted acidity, higher polymer decomposition efficiency was achieved than a conventional Beta zeolite.

Superfast-Expanding Porous Hydrogels: Pushing New Frontiers in Converting Chemical Potential into Useful Mechanical Work.

Superabsorbent polymer gels can absorb large amounts of water (100-1000× their dry weight). For the past 50 years, many scientists such as de Gennes have proposed to extract mechanical work from gel expansion/contraction, which could pave the way for "artificial muscles". However, slow rates of gel expansion have limited these efforts: macroscale (∼cm) gels take over 24 h to expand to their equilibrium size. Gels can be made to expand faster if their characteristic length scale is reduced, e.g., by making a macroscopic gel porous. Still, gels that are both superabsorbent and able to expand rapidly have not yet been realized. Here, we create gels at the macroscale (∼cm or larger) that are porous, highly robust, superabsorbent and expand much faster than any gels thus far. Our approach involves the in situ foaming of a monomer solution (acrylic acid and acrylamide) using a double-barreled syringe that has acid and base in its two barrels. Gas (CO2) is generated at the mixing tip of the syringe by the acid-base reaction, and gas bubbles are stabilized by an amphiphilic polymer in one of the barrels. The monomers are then polymerized by ultraviolet (UV) light to form the gel around the bubbles, and the material is dried under ambient conditions to give a porous solid. When this dry gel is added to water, it absorbs water at a rate of 20 g/g·s until an equilibrium is achieved at ∼300× its weight. In the process, each gel dimension increases by ∼20%/s until its final dimensions are more than 3× larger. Such rapid and appreciable expansion can be easily observed by the eye, and remarkably, the swollen gel is robust enough to be picked up by hand. SEM images reveal a porosity of >90% and an interconnected network of pores. The gels are responsive to pH, and a full cycle of expansion (in regular water) and contraction (at pH 10 or in ethanol) can be completed within about 60 s. We use gel expansion to rapidly lift weights against gravity, resulting in ∼0.4 mJ of work being done over 40 s, which translates to a power density of 260 mW/kg. This ability to harness the chemical potential energy from the gel to do useful mechanical work could enable new designs for mechano-chemical engines─and potentially for artificial muscles.

Removal of Acetic Acid from Bacterial Culture Media by Adsorption onto a Two-Component Composite Polymer Gel.

Organic acids, including acetic acid, are the metabolic products of many microorganisms. Acetic acid is a target product useful in the fermentation process. However, acetic acid has an inhibitory effect on microorganisms and limits fermentation. Thus, it would be beneficial to recover the acid from the culture medium. However, conventional recovery processes are expensive and environmentally unfriendly. Here, we report the use of a two-component hydrogel to adsorb dissociated and undissociated acetic acid from the culture medium. The Langmuir model revealed the maximum adsorption amount to be 44.8 mg acetic acid/g of dry gel at neutral pH value. The adsorption capacity was similar to that of an ion-exchange resin. In addition, the hydrogel maintained its adsorption capability in a culture medium comprising complex components, whereas the ion-exchange did not adsorb in this medium. The adsorbed acetic acid was readily desorbed using a solution containing a high salt concentration. Thus, the recovered acetic acid can be utilized for subsequent processes, and the gel-treated fermentation broth can be reused for the next round of fermentation. Use of this hydrogel may prove to be a more sustainable downstream process to recover biosynthesized acetic acid.

Synthesis of beta nanozeolite aggregates with hierarchical pores via steam-assisted conversion of dry gel and their catalytic properties for Friedel-Crafts acylation

Modulation of the porosity and acidity of zeolites is a very attractive subject for designing highly efficient solid-acid catalysts. In this work, Beta nanozeolite aggregates with hierarchical porous structure were synthesized via steam-assisted conversion (SAC) of dry gel in presence of seeds and a cationic polymer of polydiallyldimethylammonium chloride (PDAD). The influence of various synthesis parameters, such as the alkalinity, the contents of PDAD and zeolite seeds, and the crystallization conditions (temperature, water/dry gel ratio and time) on the formation of Beta zeolites was studied by combining a variety of characterization techniques. The SAC-synthesized Beta zeolites (S-Beta) are assembled from ultra-small nanocrystals around 10 nm to form self-sustaining micron-sized zeolitic aggregates, and possess abundant interconnected mesopores and relatively mild acidity. Compared with the conventional Beta zeolite, S-Beta aggregates show much higher catalytic activity for the Friedel-Crafts acylation of anisole with acetic anhydride. The excellent catalytic performance of S-Beta should be mainly attributed to the enhanced mass transport ability for reagents as well as the mild acidity that can avoid the rapid deactivation of the catalyst under the test conditions.

Preparation of nano-TiN powders by Ni-catalysed carbothermal reduction nitridation

TiN nanoparticles were prepared at 900–1100 °C by Ni-catalysed carbothermal reduction nitridation from sol-gel using tetrabutyl titanate, citric acid monohydrate and nickel chloride hexahydrate as starting materials. The catalytic effects of nickel on the carbothermal reduction nitridation of xerogels were investigated. Ni has a crucial promoting effect on the carbothermal reduction reaction of the xerogels and visibly enhanced carbothermal reduction and nitridation reactions. The dry gel with 5% NiCl2 was added to obtain nano-TiN at 900 °C. The transmission electron microscopy analysis and Materials Studio simulation results showed that the facilitating effect of Ni on carbothermal reduction nitridation reaction was ascribed to the provision of heterogeneous nucleation sites for amorphous titania, promoting crystallisation and adsorption of N2, resulting in its smooth dissociation into highly active N atoms and consequently enhancing the carbothermal reduction nitridation.

Ultrasound-promoted synthesis of high-porosity silica aerogels using embedded recycled PET fibers

Silica aerogel is fabricated through replacement of solvent by the air, so that this stage is of high relevance to achieve appropriate physical properties. Although, evaporation under ambient pressure is considered as simplest drying technique, but leads to irreversible shrinkage and collapse of gel network. Here, we improve the efficiency of ambient pressure drying via increasing the strength of gel network through physically reinforcement of the gel network using recycled polyethylene terephthalate (rPET) fibers. The polymeric phase of rPET was then removed from structure using thermal treatment. Also, the effect of using four different sonication time, from 5 to 35 min, on enhancement of aerogel properties was also investigated. the prepared silica aerogels exhibit a high surface area and porosity and using more sonication time was led to better properties. According to results, surface area and porosity were increased from 593 m 2 /g and 93% up to 691 m 2 /g and 96% by enhancing sonication time from 5 min to 35 min, respectively. The reason is thought to be related to create further dispersion among silanol groups and fibers surface. As well, the fabricated silica aerogels showed mesopores structure along with a narrow and uniform pore size. These results indicate the high efficiency of proposed method for synthesize of silica aerogel. Improvement of the ambient pressure drying of wet gel to produce the silica aerogel via increasing the strength of gel network by recycled PET fibers reinforcement. • Development of a cost-effective and eco-friendly approach to silica aerogel preparation based on ambient pressure drying. • Improve the efficiency of drying through reinforcing the gel network using embedding the PET fibers into gel matrix. • Investigate the effect of ultrasonic and dispersion degree on aerogel properties using two different sonication time. • Obtaining the high specific surface area, low density and high porosity based on the proposed approach.

Phytochemical Composition and Antioxidant Activities of Phosphate Buffered Saline and Aqueous Extracts of Aloe barbadensis Miller Leaf Latex and Gel from Three Counties of Kenya

Aims: Aloe barbadensis Miller (A. barbadensis) is one of the most treasured species from the Aloe genus that has been used in management of various ailments. However, there are few reports on the secondary metabolites, total phenolic content (TPC), total flavonoid content (TFC) and antioxidant activities of its leaf latex and gel. This study aimed at comparing the phytochemicals and antioxidant activity of phosphate buffered saline (PBS) and distilled water extracts of latex and gel of A. barbadensis leaves from Kisumu, Elgeyo Marakwet and Baringo Counties of Kenya, East Africa.
 Study Design: The study employed quantitative and qualitative research designs.
 Place and Duration of Study: This study was conducted at the Department of Chemistry and Biochemistry, School of Sciences and Aerospace Studies, Moi University, Eldoret, Kenya between May 2021 and August 2021.
 Methodology: Leaf samples were extracted by maceration using distilled water and PBS. Phytochemical screening was performed following standard screening procures while TPC, TFC and antioxidant activity were determined using Folin-Ciocalteau method, Aluminum Chloride colorimetric assay and DPPH radical scavenging assay, respectively. One-way analysis of variance (ANOVA) was performed with Tukey post hoc test at P = .05. Correlations among TPC, TFC and antioxidant activity of the extracts were assessed using Pearson’s bivariate correlation. The analyses were performed using GraphPad Prism for windows (v9.0, GraphPad Software, California, USA).
 Results: Phytochemical screening results indicated the presence of alkaloids, phenols, saponins, flavonoids, cardiac glycosides, tannins, steroids, terpenes and quinones as the main secondary metabolites in the extracts. TPC and TFC were found to be highest for the aqueous extracts, with the highest contents (102.393 ± 0.121 mg GAE/ g DW and 47.228 ± 0.248 mg QE / g DW) being for dry latex of leaves from Baringo. The highest antioxidant activity (IC50 = 21.900 ± 0.0594 mg/mL) was for aqueous extract of fresh latex of leaves from Baringo, followed by those from Elgeyo Marakwet and then Kisumu.
 Conclusion: This study established that dry and fresh latex and gel extracts of A. barbadensis leaves possess therapeutic phytochemicals with antioxidant activities, which support their use in traditional phytomedicine in Kenya.

Dry gel synthesis of hierarchical ZSM-5 zeolite using hydroxyl propyl methyl cellulose (HPMC) as a mesoporogen and its catalytic activity in alkylation reactions

In this work, a “green” and facile method for synthesis of hierarchical ZSM-5 zeolite was presented by combination of a bio-mesoporogen and a dry gel conversion (DGC) process. ZSM-5 zeolite with high hierarchy factors and excellent mesoporosity was synthesized by adding hydroxypropylmethylcellulose (HPMC), which originated from cellulose biomass, to a zeolitic synthetic gel. The obtained zeolite samples were analyzed by X-ray diffraction (XRD), nitrogen adsorption–desorption (BET) and scanning electron microscopy (SEM) to determine their properties. The influence of crystallization time on the crystallinity of ZSM-5 zeolite was investigated. Moreover, the results showed that HPMC as a pore directing agent is an important factor for the formation of hierarchical zeolite with high mesoporosity. The as-prepared ZSM-5 sample with high pore volume, large surface area and abundant accessible acid sites, which seriously improves diffusion efficiency and catalytic activity, exhibited high catalytic performance in the benzylation reaction.

Triplet-triplet annihilation upconversion in LAPONITE®/PVP nanocomposites: absolute quantum yields of up to 23.8% in the solid state and application to anti-counterfeiting.

The low quantum efficiency in the solid phase and the highly efficient quenching by oxygen are two major weaknesses limiting the practical applications of triplet-triplet annihilation (TTA) upconversion (UC). Herein, we report an organic-inorganic hybrid nanocomposites fabricated by self-assembly of LAPONITE® clay and poly(N-vinyl-2-pyrrolidone) (PVP), which serves as excellent matrix for solid-state TTA-UC even in air. In the hybrid hydrogel doped by TTA-UC components, the anionic acceptors are arranged in an ordered manner at the nano-disk edge through electrostatic attraction, which avoids haphazard accumulation of the acceptors and allows for highly efficient inter-acceptor triplet energy migration. Moreover, the entangled PVP could not only protect the triplet excitons from oxygen quenching but even proactively eliminate oxygen by photoirradiation. Significantly, the dried gel prepared by completely removing water from the hydrogel gave absolute UC quantum efficiencies of up to 23.8% (out of a 50% maximum), which is the highest TTA-UC efficiency obtained in the solid state. The dried gels are readily made into powder by grinding with maintained UC emissions, making them convenient for application to information encryption and anti-counterfeiting security by virtue of the high UC quantum efficiency and insensitivity to oxygen.

Tpaoh Post-Treatment of Dry Gel to Make Hierarchical Ts-1 Zeolites for Catalysis

Tpaoh Post-Treatment of Dry Gel to Make Hierarchical Ts-1 Zeolites for Catalysis

Defect engineering to boost catalytic activity of Beta zeolite on low-density polyethylene cracking

Catalytic cracking is of considerable interest as a sustainable, renewable, efficient, and eco-friendly chemical conversion method of plastic wastes. In catalytic cracking, plastic wastes can be converted into valuable products such as monomers or petrochemical feedstock. Solid acid catalysts, especially zeolites, significantly lower the thermal decomposition temperature of plastics. However, the catalysts have not shown enough performance to be implemented in society. Therefore, major efforts have recently been devoted to solving this problem by appropriately designing zeolite catalysts. Herein, we report a new defect-rich Beta zeolite synthesized by a dry gel conversion method using methyltriethoxysilane. The obtained catalyst exhibited outstanding activity on low-density polyethylene cracking. We measured Fourier-transform infrared spectra using pyridine as a probe molecule and found that this high activity is contributed by the increase of the amount of Lewis acid sites. It probably arises from the ratios of extra-framework hexacoordinated aluminum and silanol groups higher than the conventional Beta zeolite shown in 27Al and 1H magic angle spinning nuclear magnetic resonance spectra. We can say that the increase of defects of zeolite Beta further accelerated polymer cracking. This work provides a new strategy to improve the catalytic activity of zeolites on polymer cracking.

Silica-Bacterial Cellulose Composite Aerogel Fibers with Excellent Mechanical Properties from Sodium Silicate Precursor.

Forming fibers for fabric insulation is difficult using aerogels, which have excellent thermal insulation performance but poor mechanical properties. A previous study proposed a novel method that could effectively improve the mechanical properties of aerogels and make them into fibers for use in fabric insulation. In this study, composite aerogel fibers (CAFs) with excellent mechanical properties and thermal insulation performance were prepared using a streamlined method. The wet bacterial cellulose (BC) matrix without freeze-drying directly was immersed in an inorganic precursor (silicate) solution, followed by initiating in situ sol-gel reaction under the action of acidic catalyst after secondary shaping. Finally, after surface modification and ambient drying of the wet composite gel, CAFs were obtained. The CAFs prepared by the simplified method still had favorable mechanical properties (tensile strength of 4.5 MPa) and excellent thermal insulation properties under extreme conditions (220 °C and −60 °C). In particular, compared with previous work, the presented CAFs preparation process is simpler and more environmentally friendly. In addition, the experimental costs were reduced. Furthermore, the obtained CAFs had high specific surface area (671.3 m²/g), excellent hydrophobicity, and low density (≤0.154 g/cm3). This streamlined method was proposed to prepare aerogel fibers with excellent performance to meet the requirements of wearable applications.

Self-Etherification of 5-Hydroxymethylfurfural to 5,5′(Oxy-bis(methylene))bis-2-furfural over Hierarchically Micromesoporous ZSM-5: The Role of Brønsted- and Lewis-Acid Sites

The 5,5′(oxy-bis(methylene))bis-2-furfural (OBMF) formation from the 5-hydroxymethylfurfural (HMF) self-etherification over zeolites is an important reaction for the synthesis of value-added chemicals from biomass resources. However, there is still a lack of knowledge about the reaction mechanism and the roles of different acid sites that impedes the exploitation of zeolite catalysts for efficient OBMF production. In the present study, the ZSM-5 (HPZ) catalysts of four hierarchically micromesopores were successfully synthesized in different Si/Al ratios by adjusting the amount of Al in the solvent evaporation with the assistance of a dry gel conversion method. The kinetics of OBMF formation from HMF self-etherification was investigated. The apparent activation energy and the conversion reaction order were calculated to be 104.6 kJ/mol and 0.65, respectively. The intrinsic activity comparison of four catalysts and density functional theory (DFT) calculations showed that HMF adsorption and etherification benefit from Brønsted-acid sites. Besides, the DFT results suggested that the etherification proceeds via a concerted step, instead of the sequential pathways, thus leading to the high selectivity of OBMF on hierarchically micromesoporous ZSM-5 (HPZ) catalysts.