Resorcinol-formaldehyde Hydrogels Research Articles

Polyethylene glycol-impregnated carbon quantum dots-phenolic phase change composites for highly efficient thermal energy storage

The development of stable smart thermal energy storage systems is the key to reaching breakthroughs in thermal management technologies based on phase change composites (PCC). In this study, high-performance PCC is developed by impregnating polyethylene glycol (PEG), as a polymeric phase change material (PCM), into the porous structure of modified resorcinol-formaldehyde (RF) hydrogels. Carbon quantum dots (CQDs) and 3-chlorophenol are introduced into the RF structure during the synthesis process to increase energy storage properties and impregnation efficiency. A sol-gel polymerization method is used to synthesize the hydrogels, followed by the impregnation of liquid PEG600 into the hydrogel structure. The low PEG leakage of optimal PCC consisting of 3-chlorophenol and 0.2 wt% CQDs is related to the presence of surface functional groups in the structure of the modified RF hydrogels resulting in the formation of hydrogen bonding and electrostatic forces. The melting enthalpy of the optimal PCC is 67 % higher than the unmodified RF-based PCC, while the encapsulation efficiency is 92.5 %. Our results confirm that thermal energy storage performance can be improved by 40 % through the addition of CQDs and 3-chlorophenol into the polymeric material. These PCC systems show long-term stability in terms of storage/release cycles and have the potential to be used as thermal regulator systems.

Resorcinol formaldehyde hydrogel: Synthesis, polymerization, and application in ceramic gel-casting

In the present study, resorcinol formaldehyde (RF) hydrogel was successfully synthesized and applied in the gel-casting process. The polymerization of the gel system and the effects of catalysts on the mechanical properties of the ceramic preform were investigated. Changing the type of catalyst to control the gel reaction rate enabled the in situ “immobilization” of the B4C particles, thereby enabling the preparation of B4C/C porous preforms with a uniform structure and controllable pore size. The results revealed that using Na2CO3, having a high polarizing power Na+ and a high degree of hydrolysis CO32–, as a catalyst, the B4C/C porous preforms with low porosity and best mechanical properties can be obtained. Further, the flexural strength and compressive strength could reach 21.2 MPa and 52.2 MPa, respectively.

Hybrid Proton-Exchange Membrane Based on Perfluorosulfonated Polymers and Resorcinol-Formaldehyde Hydrogel.

Organic resorcinol–formaldehyde (RF) hydrogels were introduced into a hybrid cation-exchange membrane in order to enhance its following properties: water uptake, thermal stability, and ionic conductivity. This study was aimed to investigate the modifications induced by the RF organic clusters that form a uniform distributed network within the perflourosulfonated acid (PFSA) matrix. RF concentration was controlled by resorcinol and formaldehyde impregnation time using water or ethanol solvents. The specific morphological and structural properties were characterized by atomic force microscopy, UV–Vis, and Fourier transform infrared spectroscopy. Thermo-gravimetric analysis was employed to study the thermal stability and degradation processes of the composite membranes. Proton conductivity, as a function of relative humidity (RH) at 80 °C, was measured using in-plane four-point characterization technique. Compared to the pristine membrane, the PFSA–RF hybrid membranes showed improved thermal stability at up to 46 °C and higher ionic conductivity for low RF content, especially at low relative humidity, when using ethanol-based solvents. Single fuel cell testing on RF-based membrane–electrode assembly revealed impeccable fuel crossover and power performance at 80 °C and 40% relative humidity, delivering a 76% increase in power density compared to a reference assembled with a pristine membrane and the same catalyst loadings.

Polyvinylpyrrolidone-resorcinol-formaldehyde hydrogel system reinforced with bio-synthesized zinc-oxide for water shut-off in heterogeneous reservoir: An experimental investigation

This work aims at evaluating advancement in water shut-off performance using nanocomposite hydrogel (PVP-ZnO:RF) prepared from PolyVinylPyrrolidone (PVP); used as polymer, Resorcinol-Formaldehyde (RF); used as a crosslinker and nano Zinc Oxide (ZnO); used as strength modifier and it was compared with conventional hydrogel (PVP:RF)i.e., hydrogel without ZnO nanofiller. The ZnO, used as a nanofiller in this work, was successfully bio-synthesized (i.e., green route synthesized) from plant extract (Moringa oleifera leaves) and the average size was found to be 10 nm. In this research work, the effect of ZnO nanofiller on gelation time, gel strength, thermal stability, rheological properties and water shut-off performance was systematically evaluated. On the incorporation of ZnO nanofiller, gelation time decreases but gel strength increases. The thermal stability of hydrogel was studied using a Differential Scanning Calorimeter (DSC) that depicts maximum tolerable temperature increases from 86 °C to 92 °C at 0.5 wt.% of ZnO concentration in nanocomposite hydrogel (PVP). The mechanical stability of the nanocomposite hydrogel (PVP-ZnO:RF) demonstrates that infusion of ZnO nanofiller has significantly enhanced the dynamic moduli (i.e., storage modulus (G′) and loss modulus (G″)). Moreover, the optimum results of storage modulus (G′) and loss modulus (G″) are found at 0.5 wt.% of ZnO nanofiller. The water shut-off performance in the high permeable streak, in terms of percentage reduction in permeability, was 97% and 92% for nanocomposite hydrogel (PVP-ZnO:RF) and conventional hydrogel (PVP:RF), respectively. Also, the residual resistance factor is found to be 31.31 and 12.71 for PVP-ZnO:RF and PVP:RF hydrogels. Thus, the developed nanocomposite hydrogel (PVP-ZnO:RF) may be a promising solution to excessive water production in mature oil fields.

Characterization of resorcinol–formaldehyde hydrogel as adsorbent for cesium ion

We evaluated the performance of a resorcinol–formaldehyde (RF) hydrogel as an adsorbent for removing aqueous cesium ions (Cs+), by synthesizing cylindrical pellets of RF gels under different molar ratios (R/C) between resorcinol (R) and sodium carbonate (C) as the catalyst for the sol–gel polycondensation in the range from 2.5 to 400. The RF gel prepared at R/C = 2.5, containing the greatest amount of sodium ion (Na+), had the greatest Cs+ adsorption amount (0.660 mmol g−1), because Cs+ could be adsorbed on the network structure in the gel through exchange with Na+ at an equal molar ratio, but with the smallest effective diffusion coefficient of Cs+ (1.45 × 10–12 m2 s−1). Kinetic analyses based on the Dryden–Kay and intra-particle diffusion models revealed that the diffusivity of Cs+ in the RF gel depended strongly on the density of the network structure, which can be controlled by varying the R/C ratio.

Removal of Rhodamine B by g-C3N4/Co3O4/MWCNT composite stabilized in hydrogel via the synergy of adsorption and photocatalysis under visible light

Hydrogel nanocomposite as a novel porous macromolecular provides potential applications in many different fields. In this paper, Co3O4/g-C3N4/multi wall carbon nanotube (MWCNT) nanocomposites were stabilized in resorcinol formaldehyde hydrogel for a highly efficient removal of Rhodamine B from aqueous solution. Hydrogel, Co3O4 in hydrogel, g-C3N4 in hydrogel, Co3O4/g-C3N4 in hydrogel and Co3O4/g-C3N4/MWCNT in hydrogel were synthesized and characterized by Fourier-transform infrared spectroscopy (FTIR), UV–Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), BET, BJH, Scanning Electron Microscope (SEM) and energy dispersive X ray (EDX). BET results showed that the presence of nanomaterials in synthesis steps of hydrogel, increase specific surface area about 22.5-30 times. DRS results confirmed that the stabilization of g-C3N4 and Co3O4 nanoparticles in hydrogel effectively shifted absorption edge to longer wavelength (red shift). The aim of this study is to eliminate Rhodamine B from solution by semiconducting materials which stabilized in hydrogel. Adsorption and synergistic performance of each sample in removal of 3 ppm solution of Rhodamine B were evaluated by UV–Vis spectrophotometry in dark and under visible light irradiation, respectively. The best removal performance (87%) was obtained by Co3O4/g-C3N4/MWCNT nanocomposites stabilized in hydrogel in 120 min under visible light irradiation. All of the samples follow first order kinetic model in removal of Rhodamine B. This study provides new insights into the stabilization of catalysts in hydrogel as a porous material and facilitates their application in the synergistic removal of dye in the field of water treatment.

Effect of high pressure on growth of colloidal particles during sol–gel phase transition of resorcinol–formaldehyde solution

We examined the effect of high pressure on the sol–gel phase transition of a resorcinol–formaldehyde (RF) aqueous solution to a RF hydrogel, which was hold under a certain pressure varied in the range from the ambient pressure to 400 MPa. By employing the dynamic light scattering method, we analyzed the scattered intensity of a He–Ne laser which was irradiated to the RF solution to determine the hydrodynamic diameter of the structure formed during the phase transition. Before the completion of the phase transition, we measured the transient change in the size of colloidal particles, i.e. the unit of the porous structure formed in the RF hydrogel. It was revealed that the size of the colloidal particles increased with the increase in the pressure added to the RF solution. After the completion of the phase transition, we prepared a RF cryogel by removing solvent from a RF hydrogel by freeze-drying. The specific surface area of the RF cryogel was found to decrease with the increase in the added pressure, as a result of the increase in the size of the unit of the porous structure.

Photocatalytic properties of TiO2@polymer and TiO2@carbon aerogel composites prepared by atomic layer deposition

Monolithic structured TiO2/aerogel composites were prepared from resorcinol-formaldehyde polymer aerogel (RFA) and its carbon aerogel (RFCA) derivative. A resorcinol-formaldehyde hydrogel was synthesized in a sol-gel reaction and transformed into polymer aerogel by supercritical drying. The RFA was converted to carbon aerogel by pyrolysis at 900 °C in dry N2. Amorphous and crystalline TiO2 layers were grown from TiCl4 and H2O precursors by atomic layer deposition (ALD) at 80 °C and 250 °C, respectively, on both RFA and RFCA. The substrates and the composites were studied by N2 adsorption, TG/DTA-MS, Raman, SEM-EDX and TEM techniques. Their photocatalytic activity was compared in the UV catalyzed decomposition reaction of methyl orange dye.

New method for introducing mesopores into carbon microhoneycombs using dextran

Carbon microhoneycombs (CMHs), which can be prepared from a resorcinol–formaldehyde (RF) hydrogel using ice crystals as a template, are attractive materials for industrial applications such as solid catalysts and adsorbents in flow system reactors. For such applications, pores in a material are important for mass transfer, but the mesopore volumes of CMHs are smaller than that of typical RF-derived carbons because ice crystals compress RF gels during freezing, and mesopores partly collapse. The introduction of mesopores into CMHs requires an additional step such as HCl treatment in the preparation process, and this increases the preparation time. In this work, we developed a convenient method for introducing mesopores into CMHs, in which dextran was added to the initial RF solution. This simple step increased the mesopore volume from 0.015 to 0.191 cm3/g. We investigated the porous structure of the sample at each step in the preparation, and found that dextran protected the porous structure of the RF hydrogel from growth of ice crystals, enabling mesopore introduction. A combination of dextran addition and HCl treatment further increased the mesopore volume to 0.753 cm3/g; this cannot be achieved using HCl treatment alone. This may be because the HCl solution can diffuse through the mesopores introduced by dextran addition, and this promotes pore formation.

Chemical bias of electrochemical and photoelectrochemical water splitting using a hydrogel separator

Abstract Resorcinol-formaldehyde hydrogels are shown to be adequate separators in electrochemical and photoelectrochemical water splitting cells. Combined with concentrated buffer electrolytes, they allow the maintenance of relatively stable pH gradients between basic anolytes and acidic catholytes. The water splitting potential at a current density of 3.0 mA/cm 2 applied between two Pt electrodes and a pH bias of 8.1 units retained a value of 2.7 V for several hours. Using iron foam/hematite as photoanode and Pt as cathode, the water splitting photocurrents at an applied potential of 1.23 V reached values of 0.40 and 0.06 mA/cm 2 in the presence and absence, respectively, of this chemical bias.

Marked Increase in Hydrophobicity of Monolithic Carbon Cryogels via HCl Aging of Precursor Resorcinol–Formaldehyde Hydrogels: Application to 1-Butanol Recovery from Dilute Aqueous Solutions

Monolithic carbon cryogels having a honeycomb structure with pore openings a few tens of micrometers in diameter (Carbon MicroHoneycomb, CMH) were synthesized by directional freezing of precursor resorcinol–formaldehyde (RF) hydrogels and subsequent carbonization at temperatures ≥ 674 K. Aging of precursor RF monoliths with 1 N HCl aq. was found to markedly increase the hydrophobicity of the corresponding CMHs as characterized by water vapor adsorption experiments conducted at 298 K. Analysis of the water vapor adsorption data indicates that levels of hydrophobicity of CMHs are similar to those exhibited by other types of highly hydrophobic adsorbents, such as a coal-derived activated carbon and a defect-free pure silica zeolite Beta. HCl aging also drastically changes the porous structure of CMHs from microporous to micro/mesoporous as characterized by nitrogen adsorption/desorption experiments. Because of significantly enhanced hydrophobicity of CMHs as well as hierarchical pore structure (straight macr...

Effect of molybdenum on the structure formation of resorcinol–formaldehyde hydrogel studied by coherent x-ray scattering

A detailed study on the effect of Mo on the gelation process of resorcinol-formaldehyde systems is presented. The evolution of the system was followed by x-ray photon correlation spectroscopy, which allows in situ investigation of the dynamics as well as of the structural evolution in non-equilibrium processes. The Mo was introduced into the system after a pre-polymerization period (PP), the effect of which was also examined. Our results show that the presence of Mo substantially modifies the gelation process by favoring the growth of large compact clusters with weak bonds between them. However, this effect can be reduced by increasing PP.

Influence of Surface Chemistry on Dehydrogenation in Carbon Cryogel Ammonia Borane Nanocomposites

AbstractThis paper reports the synthesis and characterization of boron‐ and nitrogen‐modified carbon cryogel (CC) ammonia borane (AB) nanocomposites (BNCC‐AB) for hydrogen storage. Resorcinol–formaldehyde (RF) derived CCs were modified by homogeneous dispersion of AB in RF hydrogel prior to pyrolysis. Nanocomposites were fabricated by immersing CC in the AB solution. Nitrogen sorption analysis, X‐ray photoelectron spectroscopy, and differential scanning calorimetry at multiple heating rates were used to study the structure and dehydrogenation properties of the nanocomposites. The results demonstrated lower dehydrogenation temperatures and reduced activation energies for AB when confined inside pores of B‐ and N‐modified CCs relative to AB when confined in the unmodified CC with the same pore size.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Water desorption from resorcinol-formaldehyde hydrogels and adsorption in the resulting xerogels

Water desorption isotherms of resorcinol-formaldehyde (RF) hydrogels and subsequent water adsorption isotherms of the resulting xerogels are determined using the static gravimetric method, at various temperatures. Isotherms obtained from samples synthesized at various pH are compared. Two different mechanisms are involved in RF hydrogels water desorption. At large relative humidity the capillary tension resulting from water removal induces a macroscopic shrinkage of the gel, whereas at low humidity water is evaporated with no network deformation. These two mechanisms are analyzed using a plastic deformation model and the Guggenheim-Anderson-de Boer (GAB) model, respectively. Adsorption isotherms of RF xerogels are analyzed using the GAB model.

Towards the production of carbon xerogel monoliths by optimizing convective drying conditions

Resorcinol–formaldehyde hydrogels prepared at various resorcinol/sodium carbonate ratios, R/ C, were convectively air dried. The influence of the drying operating conditions, i.e. air temperature and velocity, on the pore texture, shrinkage and cracking of the dried gels were investigated. Shrinkage was found to be isotropic. The shrinkage behaviour and the textural properties of the gels are independent of the drying operating conditions, but are completely determined by the value of the synthesis variables. The analysis of the drying kinetics shows two main drying periods. During the first phase, shrinkage occurs and the external surface of the material remains completely wet: heat and mass transfers are limited by external resistances located in a boundary layer. When shrinkage stops, the second period begins: the evaporation front recedes inside the solid and internal transfer limitations prevail. The drying time can be reduced by increasing the air temperature and/or velocity, but the temperature increase is limited when monolithicity is required, especially when the pores are small. For example, at a temperature of 160 °C and a velocity of 2 m/s, about 1 h is needed to dry a 2.8 cm in diameter and 1 cm in height cylinder containing macropores (pore width > 50 nm after drying). The same cylinder presenting small mesopores (pore width = 10–15 nm after drying) requires 20 h at 30 °C and 2 m/s to reach complete dryness without the development of cracks.

Evaluation of porous structure of resorcinol-formaldehyde hydrogels by thermoporometry

The porous structures of resorcinol-formaldehyde (RF) hydrogels were evaluated by thermoporometry (TPM) based on the thermodynamic relationship between the size of the pores and the solidification temperature of the freezable pore water in the pores. Adding the thickness of the nonfreezable pore water layer to the ice crystal size distribution of the freezable pore water calculated from the thermogram (a recording of the heat flux during the solidification of the freezable pore water) allowed for the determination of the porous properties of the RF hydrogels. It was confirmed that the RF hydrogels possess a fairly uniform mesoporous structure formed between the interconnected primary particles, and that the size of the mesopores clearly increased with the increase in the molar ratio of resorcinol to catalyst ( R/ C) used in the preparation of the initial RF solution. The dependence of both the mesopore volume and the surface area of a given RF hydrogel on the R/ C ratio were also established.

Influence of drying on the morphology of resorcinol–formaldehyde-based carbon gels

The method of drying resorcinol–formaldehyde (RF) gels influences the structure and gas adsorption capacity of the resulting networks as well as of their carbonized derivatives. Small angle X-ray scattering (SAXS) reveals the presence of submicroscopic clusters both in the RF gels and in the carbon gels, whose size, of the order of tens of nanometres, is consistent with SEM observations. Xerogels, obtained by heating the RF hydrogel in an inert atmosphere, have the most compact structure and display the lowest specific surface area (<900 m 2/g). The highest surface area is found in the gel lyophilized after freeze-drying in t-butanol (>2500 m 2/g), but the resulting carbon cryogel is not structurally stable, the measured surface area decreasing with time. Supercritical extraction with liquid carbon dioxide yields aerogels with an intermediate value for the surface area (ca. 1000 m 2/g). Carbonization causes a reduction in the size of the elementary spheres (from 27 to 8 nm) in the case of the aerogels, while an increase (from 10 to 22 nm) is observed for the xerogels. The adsorption sites in the carbon gels are located principally in irregular spaces between monomer units. For the carbonized cryogels the specific surface area derived from SAXS is greater than that found by low temperature gas adsorption. This phenomenon is interpreted in terms of restricted pore size, which limits the access of gas molecules. By contrast, in the RF gels the SAXS-derived surface areas are substantially smaller than those measured by gas adsorption.

Controllability of pore characteristics of resorcinol–formaldehyde carbon aerogel

Resorcinol–formaldehyde (RF) aerogels were synthesized by sol–gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution, followed by drying with supercritical carbon dioxide. The RF carbon aerogels were prepared by carbonizing the RF aerogels at a high temperature under a nitrogen atmosphere. On changing the catalyst species and the catalyst ratios in synthesizing the RF hydrogels, we investigated their influence on the pore characteristics of the RF carbon aerogels prepared. We found that the order in the magnitude of the peak radii in the size distributions agrees with that in the magnitude of the gyratory radius measured from the small-angle X-ray scattering curves of the RF solutions during their gelation process. The gyratory radii and the form of the primary particles constituting the RF hydrogel were found to determine the pore characteristics of the RF carbon aerogels, relating to the polymerization conditions such as the catalyst species and the catalyst ratios in synthesizing the RF hydrogel.

Size control and characterization of spherical carbon aerogel particles from resorcinol–formaldehyde resin

Spherical resorcinol–formaldehyde (RF) aerogel particles were synthesized by emulsion polymerization of resorcinol with formaldehyde in a slightly basic aqueous solution, followed by supercritical drying with carbon dioxide. RF carbon aerogel particles were prepared by carbonizing of the RF aerogel particles at a high temperature under a nitrogen atmosphere. By changing the viscosity of the RF sol added to the cyclohexane containing a surface-active agent for preparation of the spherical RF hydrogels, we investigated the influence of the apparent viscosity of the RF sol on the size of the generated RF carbon aerogel particles. We could successfully prepare the RF carbon aerogel particles with a truly spherical shape and control their size in the range from about 10 to 500 μm by changing the apparent viscosity of the RF sol. The spherical RF carbon aerogel particles with an average diameter of 20 μm have a BET surface area of about 800 m 2/g and a uniform mesopore radius of 1.78 nm.

Immobilization of heteropoly acids in the network structure of carbon gels

Keggin-type heteropoly acids (HPAs) were immobilized in the unique network structure of resorcinol-formaldehyde (RF)-derived carbon gels. Two methods were employed for immobilization, one by impregnating 12-tungstophosphoric acid (PW12) in a wet RF hydrogel followed by drying and carbonization of the support, and the other by synthesizing 12-molybdophosphoric acid in a slurry mixture of carbon gel particles and deionized water. It was found that significant amounts of the acids could be immobilized to carbon gel supports by both methods, and large amounts of the acids remained in the carbon supports even after long-term hot water extraction. Moreover, the obtained complexes were found to show catalysis in the synthesis of methyl-tert-butyl ether from methyl alcohol and tert-butyl alcohol, indicating that they could be practically used as solid acid catalysts in various reactions which involve water.