Sol-gel Polycondensation Of Resorcinol Research Articles

Resorcinol-Formaldehyde-Derived Carbon Xerogels: Preparation, Functionalization, and Application Aspects.

Carbon xerogels (CXs) are materials obtained via the pyrolysis of resins prepared via the sol-gel polycondensation of resorcinol and formaldehyde. These materials attract great attention as adsorbents, catalyst supports, and energy storage materials. One of the most interesting features of CXs is the possibility of fine-tuning their structures and textures by changing the synthesis conditions in the sol-gel stage. Thus, the first part of this review is devoted to the processes taking place in the polycondensation stage of organic precursors. The formation of hydroxymethyl derivatives of resorcinol and their polycondensation take place at this stage. Both of these processes are catalyzed by acids or bases. It is revealed that the sol-gel synthesis conditions, such as pH, the formaldehyde/resorcinol ratio, concentration, and the type of basic modifier, all affect the texture of the materials being prepared. The variation in these parameters allows one to obtain CXs with pore sizes ranging from 2-3 nm to 100-200 nm. The possibility of using other precursors for the preparation of organic aerogels is examined as well. For instance, if phenol is used instead of resorcinol, the capabilities of the sol-gel method become rather limited. At the same time, other phenolic compounds can be applied with great efficiency. The methods of gel drying and the pyrolysis conditions are also reviewed. Another important aspect analyzed within this review is the surface modification of CXs by introducing various functional groups and heteroatoms. It is shown that compounds containing nitrogen, sulfur, boron, or phosphorus can be introduced at the polycondensation stage to incorporate these elements into the gel structure. Thus, the highest surface amount of nitrogen (6-11 at%) was achieved in the case of the polycondensation of formaldehyde with melamine and hydroxyaniline. Finally, the methods of preparing metal-doped CXs are overviewed. Special attention is paid to the introduction of a metal precursor in the gelation step. The elements of the iron subgroup (Fe, Ni, Co) were found to catalyze carbon graphitization. Therefore, their introduction can be useful for enhancing the electrochemical properties of CXs. However, since the metal surface is often covered by carbon, such materials are poorly applicable to conventional catalytic processes. In summary, the applications of CXs and metal-doped CXs are briefly mentioned. Among the promising application areas, Li-ion batteries, supercapacitors, fuel cells, and adsorbents are of special interest.

Engineering single-atom Fe–N active sites on hollow carbon spheres for oxygen reduction reaction

Seeking alternatives to noble metals-based electrocatalysts for oxygen reduction reaction (ORR), hollow carbon spheres (CSs) were finely tuned with stable single-atom Fe–N species through a synthesis methodology requiring only earth-abundant metal precursors. CSs with different sizes were synthesized by sol-gel polycondensation of resorcinol with formaldehyde over silica nanoparticles, followed by thermal annealing and silica etching. A catalyst screening revealed the positive impact of both the hollow core and structural defects of the CSs for ORR. Single-atom Fe–N active sites were introduced on the best performing CSs through simultaneous incorporation of iron and nitrogen precursors, and glucose. A significant enhancement in ORR activity was observed despite the small iron load introduced (0.12 wt%). ORR performance indicators, advanced characterization, and molecular simulation studies revealed nitrogen's crucial role in anchoring individual iron atoms and modulating the charge density nearby the active sites (increase of 80 mV in the half-wave potential). Adding glucose as a chelating agent enhances the metal-heteroatom coordination and subsequent dispersion of iron, accounting for an increase of 20 mV in the half-wave potential, an average of electrons transferred as high as 3.9 (at 0.4 V vs. RHE), and higher stability (99%) than that of a platinum-based (20 wt%) electrocatalyst (92%).

Synthesis of bio-based xerogels from lignin precipitated from the black liquor of the paper industry for supercapacitors electrodes

In this paper, lignin-activated carbon xerogels from the black liquor of the paper industry were synthesized by the sol-gel polycondensation of resorcinol and lignin with formaldehyde, NaOH, and water. The sol-gel was prepared at three Lignin/(Lignin+Resorcinol) mass ratios of 0, 13, and 27%. A portion of sol-gel was functionalized with a no-common crosslinking agent, cyanuric chloride (CC) saturated in 1,4-dioxane, to introduce some nitrogen functional groups on the surface. The sol-gels non- and functionalized were dried and activated with H3PO4 at a mass ratio of 1:1 at 450 °C. After the functionalization with the CC and activation process, the superficial nitrogen content percentage increased to 7 wt%, where the most prevalent functionality was N-pyrrole. The sample with a high percentage of nitrogen groups did not show the best electrochemical behavior (112 F g−1 at 0.125 A g−1). However, increasing lignin content up to 27% without functionalization improved the gravimetric capacitance value of 149.5 F −1. These results are mainly attributed to electrostatic effects and the high presence of phosphorus functionalities introduced after the activation process that enhance the surface wettability of the electrodes. Furthermore, the analysis of the variance confirmed that the CC impregnation process did not favor gravimetric capacitance. Lignin can be considered as an excellent substitute for resorcinol partially to produce bio-based activated carbon xerogels and use them as electrodes for supercapacitors and reduce production costs.

Pyrolysis of Porous Organic Polymers under a Chlorine Atmosphere to Produce Heteroatom-Doped Microporous Carbons.

Three types of cross-linked porous organic polymers (either oxygen-, nitrogen-, or sulfur-doped) were carbonized under a chlorine atmosphere to obtain chars in the form of microporous heteroatom-doped carbons. The studied organic polymers constitute thermosetting resins obtained via sol-gel polycondensation of resorcinol and five-membered heterocyclic aldehydes (either furan, pyrrole, or thiophene). Carbonization under highly oxidative chlorine (concentrated and diluted Cl2 atmosphere) was compared with pyrolysis under an inert helium atmosphere. All pyrolyzed samples were additionally annealed under NH3. The influence of pyrolysis and additional annealing conditions on the carbon materials’ porosity and chemical composition was elucidated.

Synthesis of monolithic carbon aerogels with high mechanical strength via ambient pressure drying without solvent exchange

A simple, fast and cost-effective method for monolithic carbon aerogels (CAs) preparation was proposed through sol-gel polycondensation of resorcinol with formaldehyde in a basic aqueous solution followed by ambient pressure drying without solvent exchange, and carbonization. The microstructure and network strength of CAs were tailored by adjusting the catalyst concentration ([resorcinol]/[sodium carbonate] in the range of 300–2000), water content ([deionized water]/[resorcinol] equals to 17 and 24, respectively), and gelation temperature (Tgel in the range of 30−90 °C). Resultantly, the CAs with a wide range of density (0.30–1.13 g/cm3), high specific surface area (465−616 m2/g), high compressive strength (6.5–147.4 MPa) and low thermal conductivity (0.065-0.120 Wm−1 K−1) were obtained in this work. Moreover, the large-sized CAs (100 × 100 × 20 mm3) can also be prepared by this method since the formed robust skeleton network can resist shrinkage/collapse of pore structure and prevent cracking during drying. The improved mechanical strength and monolithic forming abilities could be mainly attributed to the uniform arrangement of carbon particles and pores, fine particle size, abundant network structure and enhanced particle neck.

Origin of microporosity in chalcogen-doped carbon materials: The case of selenium-doped carbogels

The outstanding properties of carbon materials doped with p-block elements are usually attributed directly to the presence of certain foreign atoms. By using the example of selenium-doped carbon gels, we hereby postulate that in many cases the properties of doped carbon materials are not determined by the presence of the heteroatom itself, but rather by how the element affects the carbon structure during high-temperature synthesis. Se-doped microporous carbogels were obtained via sol-gel polycondensation of resorcinol and 2-formylselenophene followed by pyrolysis at 600–1200 °C. Pyrolysis temperatures above 800 °C result in a decrease in the Se-content accompanied by a distinct increase in ultramicroporosity. A comparative analysis of oxygen-, sulfur- and selenium-doped carbogels shows that chalcogens increase the porosity of carbonaceous materials via the high-temperature oxidation of carbon to corresponding dichalcogenides (CO2, CS2 or CSe2), but their oxidative power diminishes with increasing atomic mass. Consequently, S and Se are more selective carbon oxidants.

Influence of catalyst amount on properties of resorcinol-formaldehyde xerogels

The effect of catalyst amount on properties of resorcinol-formaldehyde xerogels was investigated. The samples were prepared by sol-gel polycondensation of resorcinol with formaldehyde at three different levels of catalyst (Na2CO3, molar ratios of resorcinol/catalyst R/C = 1000, 2000, 3000). The influence on mechanical, thermal and thermomechanical properties of organic xerogels and on properties of carbon xerogels (surface, adsorption and oxyreactivity) was evaluated. The higher R/C ratio leads to increased particle size of the materials, which affects mechanical strength, residual humidity and shrinkage during heating (all parameters decrease). The R/C ratio does not affect significantly pyrolysis process and microporosity of the materials, the effect on oxyreactivity was negligible. Contrary, higher catalyst content (R/C = 1000) lead to relative high volume of ink-bottle shaped mesopores, but this pores are practically unavailable for adsorption of large molecules of methylene blue or hydrated chromate anions, which negatively influenced adsorption of both adsorbates from aqueous solution on the carbon xerogels.

Synthesis of mesoporous carbon aerogels based on metal-containing ionic liquid and its application for electrochemical capacitors

Mesoporous carbon aerogels were prepared by sol-gel polycondensation of resorcinol and formaldehyde through ionic liquid/metal ions synergistic effect. The as-prepared carbon aerogels had a maximum specific surface area of 825 m2 g−1, total pore volume of 0.951 cm3 g−1, and average pore size of 4.6 nm. In addition, a high specific capacitance of 188 F g−1 at a current density of 1 A g−1, high specific energy of 9.08 Wh kg−1 at a specific power of 6250 W kg−1, and excellent cyclic stability could be found.

Carbon Aerogels Synthesized via Ambient Drying as Supercapacitor Electrode Materials with High Specific Capacitance

Carbon Aerogels (CAs) are potential candidates for electrochemical double layer electrode materials due to their unique properties such as high Specific Surface Area (SSA) up to ~1000 m2/g, controllable interconnected 3D-mesoporosity, good electrical conductivity and longer cycle stability. CAs are commonly prepared through a sol-gel polycondensation of resorcinol (R) with formaldehyde (F) and water by using Na2CO3 catalyst. The synthesis involves four steps: sol-gel formation, solvent exchange, supercritical drying, and pyrolysis. Supercritical drying is vital in creating mesoporosity while relative composition of R-F to water dictates pore-size distribution [1]. Recent studies [2] suggest that micropores of size comparable to that of electrolyte ions are crucial in obtaining high specific capacitance due to partial/complete de-solvation of ions into micropores. Therefore, our study is focused on synthesis of microporous CAs by R-F polymerization using (NH4)2CO3catalyst that allows drying aqua gel precursors at ambient conditions. Ambient drying has an additional advantage of producing high microporosity while eliminating two synthesis steps, e.g. solvent exchange and supercritical drying, which would significantly reduce the cost of the materials.Organic gels (OGs) were formed by polymerization of resorcinol and formaldehyde with varying amounts of distilled water (0.883g in Gel1, 2.894g in Gel2 and 4.904g in Gel3 respectively) using (NH4)2CO3 catalyst. The organic gels formed after polymerization were dried at ambient conditions for two days followed by pyrolysis at 900oC in inert (N2) atmosphere. Nitrogen adsorption-desorption measurements indicate that all three CA samples dried at ambient conditions have high microporosity and exhibit Langmuir type of monolayer adsorption isotherms. However, SSA of the materials decreases with amount of water content. BET analysis, SEM and TEM data will be presented for understanding the differences implied by the relative amount of R-F to water content. Supercapacitor electrodes were made from slurry prepared by mixing CA materials in 1wt.% PVDF in NMP and deposited onto etched copper foil (from MTI®) using doctor-blade followed by drying in vacuum and hot pressing at 2500 PSI. The electrochemical properties of the cells assembled in symmetric CR 2325 coin cell supercapacitor geometry using Celgard® polypropylene (PP) membrane separator will be discussed in terms of cyclic voltammetry, galvanostatic charging-discharging, electrochemical impedance and durability tests. The structure dependent electrochemical properties such as specific capacitance, energy density, power density, and cycle stability in aqueous (6M KOH) and organic electrolyte (NEt4BF4/1M Acetonitrile) will also be discussed in comparison to graphene platelets from STEM Chemicals as a baseline.

The Effect of Different Ratio in Carbon Aerogel on Pore Structure in Ambient Dry

Organic aerogels were prepared by sol-gel polycondensation of resorcinol and formaldehyde using sodium carbonate as a catalyst and dried under ambient pressure. Carbon aerogels were obtained by the dried organic aerogels carbonized under nitrogen atmosphere at high temperature. The pore structures and surface characteristic of the carbon aerogels obtained were investigated by N2 adsorption isotherms, scanning electron microscope (SEM). Through controlling the concentrations of the main ingredients (resorcinol and formaldehyde), catalyst concentration, we can find out the best concentration which the specific surface area can reach the highest. In this experiment the highest specific surface area was 1645m2/g. In addition, the results show that catalyst for the cross link effect between colloidal microspheres is greater than the growth of the colloid itself, and the effect of water solvent plays an important role in the density of carbon aerogel.

Optimal electrochemical performances of CO2 activated carbon aerogels for supercapacitors

Activated carbon aerogels(ACAs) derived from sol-gel polycondensation of resorcinol (R) and formaldehyde (F) were pyrolyzed under Ar flow and activated in CO2 atmosphere. The morphology of ACAs was characterized by scanning electron microscopy (SEM) and the structural properties were determined by N2 adsorption at 77 K. The results show that ACAs have a typical three-dimensional nanonetwork structure composing of cross-linking of carbon nanoparticles. The specific surface area and the total pore volume remarkably increase with increasing activation time while the previous porous structure still remains. The specific capacitance of the 950-10-ACA electrode can reach up to 212.3 F/g in 6 mol/L KOH electrolyte. The results of constant-current charge-discharge testing indicate that the ACAs electrodes present fast charge-discharge rate and long cycle life (about 98% capacitance retained after 3000 charge-discharge cycles at 1.25 mA/cm2). Lower internal resistances can be achieved for 950-10-ACA electrode in KOH electrolyte. Our investigations are very important to improve the wettability and electrochemical performance of electrode for supercapacitors.

Synthesis and characterization of resorcinol formaldehyde carbon cryogel as efficient sorbent for imidacloprid removal

AbstractCarbon cryogel (CC) has been prepared through sol gel polycondensation of resorcinol with formaldehyde in basic aqueous solution followed by freeze-drying and carbonization. Porous properties of CC analyzed by gravimetric McBain method confirmed the expected surface area and degree of mesoporosity. Obtained powdered material was used for the removal of pesticide imidacloprid from aqueous solutions under regular batch adsorption procedure. A set of several isotherm models including Langmuir, Freundlich, Temkin, Dubinin–Radushkevich, Brouers–Sotolongo, Redlich–Peterson, Sips, Toth, Jovanovic, Hurkins–Jura, Halsey and Radke–Prausnitz model was used for nonlinear fitting of equilibrium data. Several models fitted with quite high correlation coefficients. Bangham and Elovich model fitted suitably for kinetic study indicating that pesticide diffusion into carbons pores is important step which dictates the process rate. Suitability of kinetic models decreased with the increase in adsorbent dosage. Imidac...

超高比表面积氮掺杂碳气凝胶的制备及其电化学性能

Activated N-doped carbon aerogel (N-ACA) is synthesized by the sol-gel polycondensation of resorcinol with formaldehyde using the melamine as nitrogen source, and then the carbon derivative being activated in CO2 flow. The surface morphology and porous structure are analyzed using scanning electron microscopy (SEM) and N2 adsorption at 77 K. The contained nitrogen of surface and monolith is tested by X-ray photoelectron spectroscopy (XPS) and elemental analysis. The electrochemical performances of N-dope carbon aerogels as electrodes are confirmed using constant-current charge-discharge test (GV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It reveals that the rich porous structure and the chemical state of nitrogen functionalities have crucial effect on the electrochemical performance of the electric double-layer capacitors. The appropriate CO2 activation can increase the number of micropores and mesopores as well as the specific surface area (4082 m2g-1).Furthermore, the presence of amount of pyrrolic nitrogen is reinforced in the electron donor capacity. The specific capacitance of N-ACA electrode is up to 211.9 Fg-1 and retains about 98% after 500 charge-discharge cycles.

Fabrication of Mesoporous Titanium Dioxide Assisted by Resorcinol/Formaldehyde Gel

Mesoporous structures of materials have attracted considerable attention recently because of their potential in a variety of applications. In this work, resorcinol/formaldehyde (RF) gel was used as a template for the synthesis of mesoporous titania, which is in great interest in catalysis, photocatalysis, gas sensor and photovoltaic applications. RF-gel was prepared by the sol-gel polycondensation of resorcinol (R) and formaldehyde (F). Titanium tetraisopropoxide (TTIP) was used as precursor for titania. However, direct incorporation of TTIP into RF-gel was limited by a spontaneous reaction between TTIP and RF gel, which consequently resulted in rapid solidification of the gel. Titania sol was firstly prepared from TTIP via sol-gel process. After certain period of aging time, the titania sol was added into RF mixture, which had been aged for predetermined period of time as well. After that, the mixture was further aged for another 36 hours before being dried at 80°C. Finally, the dried gel was calcined at 500°C for 4 hours to remove the RF template. The obtained titania powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), surface area measurement via nitrogen adsorption, and Fourier transform infrared spectroscopy (FTIR). The specific surface area of the samples was in the range of 30-33 m2/g. and the average diameter of about 8.8-37.7 nm. It was found that the rate of addition for titania sol into the RF-gel had influence on the phase of the final product. The increased rate favored the formation of titania in rutile phase.

Synthesis of C-Doped TiO2 Nanoparticles by Novel Sol-Gel Polycondensation of Resorcinol with Formaldehyde for Visible-Light Photocatalysis

Nanosized C-doped TiO 2 was prepared by a new method based on resorcinol-formaldehyde (RF) gel formation in situ during hydrolysis of Ti precursor (TTIP, titanium tetraisopropoxide). XRD, DRS, EDX, SEM and HRTEM results confirmed that the carbon in the sample existed as free carbon (C–C), carbonate carbon (–CO 3 ) and doped into lattice of TiO 2 (Ti–C), resulting in red shift of optical adsorption edge and band gap narrowing. The photo activity of the C-doped TiO 2 , tested on 2-chlorophenol, revealed that it was more active under visible light (k vis = 3.50 × 10 −3 min −1 ).

Preparation and Electrochemical Properties of Carbon Cryogel for Supercapacitor

Electrochemical properties of carbon cryogel electrode for the application of composite electrode materials mixed with metal oxide in supercapacitor have been studied. Carbon cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde, followed by a freeze drying, and then pyrolysis in an inert atmosphere. Physical properties of carbon cryogel were characterized by BET, X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that carbon cryogel is amorphous material. The electrochemical properties of carbon cryogel were measured by cyclic voltammetry as a function of concentration of liquid electrolyte, galvanostatic charge-discharge with different scan rates and electrochemical impedance measurements. The result of cyclic voltammetry indicated that the specific capacitance value of a carbon cryogel electrode was approximately 150.2 F/g (at 5 mV/s in 6 M KOH electrolyte).

Preparation and structure characterization of carbons prepared from resorcinol-formaldehyde resin by CO2 activation

In this work, carbon xerogels with a high pore volume and surface area (up to 2.58 cm3/g and 3200 m2/g respectively) have been synthesized using the sol-gel polycondensation of resorcinol (R) with formaldehyde (F) in a basic medium of monoethanolamine (MEA), followed by drying and pyrolysis. This medium (MEA) has not been used in previous investigations. The effect of activation with CO2 on the pore size distribution and the chemical functional groups has been investigated using N2 (77 K) adsorption, FTIR and elemental analysis techniques. A series of experiments has been conducted to investigate the effect of activation time and activation temperature. Activation of the samples was carried out at 850, 900 and 980 °C for times ranging from one to three hours. Within the range of activation conditions, an increase in activation time at 850 °C results in a continuous steady rise of the BET surface area and total pore volume. However, at the two higher temperatures, the surface area shows a maximum when plotted against activation time. FT-IR results show that the use of MEA as a catalyst leads to the formation of nitrogen functional groups in the surface of the resin.

Characterization of Carbon Cryogel Synthesized by Sol-Gel Polycondensation and Freeze-Drying

Resorcinol-formaldehyde (RF) cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze-drying carried out with t-butanol. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Characterization by nitrogen adsorption showed that carbon cryogels were micro- and mesoporous materials with high surface areas (500 m2/g< SBET <700 m2/g). Cyclic voltammetry experiments at various scan rates (2 to 200 mV s-1) were carried out to study the electrical double-layer charging of carbon cryogel electrodes in 0.5 mol dm-3 HClO4 solution. It has been demonstrated that the total specific capacitance can further be divided into mesoporous and microporous specific capacitance by analyzing the linear dependence of the specific capacitance (C) on the reciprocal of the square root of the potential scan rate ( ν -1/2), as well as linear dependence of the reciprocal specific charge (1/C) on the square root of the potential scan rate ( ν 1/2).

Characterization of carbon cryogels synthesized by sol-gel polycondensation

Resorcinol-formaldehyde (RF) cryogels were synthesized by the sol-gel polycondensation of resorcinol (R) with formaldehyde (F) and freeze-drying was carried out with t-butanol. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Characterization by nitrogen adsorption showed that the carbon cryogels were micro and mesoporous materials with high specific surface areas (SBET ? 550 m2/g). Cyclic voltammeter experiments at various scan rates (2 to 200 mV s-1) were performed to study the electrical double-layer charging of carbon cryogel electrodes in 0.5 mol dm-3 HClO4 solution. It has been demonstrated that it is possible to sub-divide the total specific capacitance into the mesoporous and the microporous specific capacitance by analyzing the linear dependence of the charge (q) on the reciprocal of the square root of the potential scan rate (?-1/2), and the linear dependence of the reciprocal charge (1/q) on the square root of the potential scan rate (?-1/2). The specific capacitance was found to be constant over a wide range of sample weight (12.5 to 50.0 _g) and a very promising specific capacitance value of 150 F/g, was found for this material operating in an acidic 0.5 mol dm-3 HClO4 solution at room temperature.

FREEZE DRYING FOR PREPARATION OF AEROGEL-LIKE CARBON

RF hydrogels were synthesized by the sol-gel polycondensation of resorcinol with formaldehyde and RF cryogels were prepared by freeze drying of the hydrogels with t-butanol. The cryogels were characterized by nitrogen adsorption, density measurements, and scanning electron microscope. Their porous properties were compared with those of the aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >5.8× 10−4m3/kg. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Aerogel-like carbons (carbon cryogels) were obtained by pyrolyzing RF cryogels in an inert atmosphere. The carbon cryogels were mesoporous materials with high surface areas >8.0× 105m2/kg and large mesopore volumes >5.5× 10−4m3/kg. When pyrolyzed, micropores were formed inside the cryogels more easily than inside the aerogels.