Phenolic Carbons Research Articles

Improving the Oxidation Resistance of Phenolic Resin Pyrolytic Carbons by In Situ Catalytic Formation of Carbon Nanofibers via Copper Nitrate.

Phenolic resin pyrolytic carbons were obtained by catalytic pyrolysis of phenolic resin at 500 °C, 600 °C, 700 °C, and 800 °C for 3 h in an argon atmosphere using copper nitrate as a catalyst precursor. The effects of copper salts on the pyrolysis process of phenolic resin as well as the structural evolution and oxidation resistance of phenolic resin pyrolytic carbons were studied. The results showed that copper oxide (CuO) generated from the thermal decomposition of copper nitrate was reduced to copper (Cu) by the gas generated from the thermal decomposition of the phenolic resin. Carbon nanofibers with tapered structures were synthesized by Cu catalysis of pyrolysis gas at 500-800 °C. The catalytic pyrolysis of phenolic resin with Cu increased the graphitization degree and reduced the pore volume of the phenolic resin pyrolytic carbons. The combined action improved the oxidation resistance of phenolic resin pyrolytic carbons.

Novel Nanomaterials Made of Humic Substances from Green Composts and Chitosan Exerting Antibacterial Activity

Green compost represents a sustainable source of humic substances (HS), which are supramolecular associations of heterogeneous natural compounds bearing a variety of functional groups. Novel humic-chitosan nanoconjugates (NC) were synthesized by using HS from three green composts [coffee husks (HS-COF), artichoke residues (HS-CYN), and fennel residues (HS-FEN)]. Solid-state NMR spectra showed that HS-CYN and HS-FEN contained the largest relative amount of polysaccharidic and phenolic carbons, whereas HS-COF had the greatest relative content of alkyl and carboxyl groups. While size and z-potential of NC depended upon the HS origin, NC-HS-FEN revealed the greatest thermal stability, followed by NC-HS-CYN and NC-HS-COF. Scanning electron microscopy and transmission electron microscopy analyses suggested the formation of spherical nanoparticles only for NC-HS-FEN and NC-HS-CYN. This was explained by more stable conformations of HS-FEN and HS-CYN that enabled regular spherical nanoparticles, as assessed by high-pressure size-exclusion chromatography measurements. NC bioactivity against bacterial human pathogens was strain-specific, and the inhibition of bacterial development was attributed to the positive z-potentials that facilitated NC adhesion on bacterial cell walls, smaller nanosizes favoring cell penetration, and subsequent release in the cell of NC toxic components that altered microbial biochemical functions. This study indicates that not only novel NCs can be obtained by humic matter derived from recycled biomass, such as green compost, but they can also be employed as effective and sustainable antimicrobial agents.

Precisely tuning porosity and outstanding supercapacitor performance of phenolic resin-based carbons via citrate activation

Developing a facile, inexpensive, and efficient approach for preparing porous carbon materials with high electrochemical properties is critical to the commercial viability of supercapacitors (SCs). Herein, a novel hierarchical porous carbon with extraordinary SCs performance is developed via a two-step carbonization process by employing low-temperature self-made phenolic resin as raw materials. Therein, the introduction of citrate is the key to generate distinct configurations and obtain high electrochemical properties. Particularly, the potassium citrate and sodium citrate (mass ratio of 3:1) activated sample (RFN-KNa) exhibits large specific surface area (1089.78 m2 g−1), high nitrogen content (2.91 at.%) and outstanding specific capacitance (280 F g−1 at 1 A g−1). Additionally, the RFN-KNa-based symmetric SCs also possesses ultrahigh energy densities, which are found to be 24.79 Wh kg−1 (at 900.1 W kg−1) and 17.42 Wh kg−1 (at 703.4 W kg−1) in 1 M Na2SO4 and 6 M KOH electrolyte, respectively. Meanwhile, the SCs also displays eminent rate capability and cycle stability. This work offers a new insight for preparing hierarchical porous carbon materials and opens up a new avenue for applying resin materials in the field of energy storage.

Structural Complexity of Graphene Oxide: The Kirigami Model.

Investigation of highly oxidized graphene oxide (GO) by solid-state nuclear magnetic resonance (NMR) spectroscopy has revealed an exceptional level of hitherto undiscovered structural complexity. A number of chemical moieties were observed for the first time, such as terminal esters, furanic carbons, phenolic carbons, and three distinct aromatic and two distinct alkoxy carbon moieties. Quantitative one-dimensional (1D) and two-dimensional (2D) 13C{1H} NMR spectroscopy established the relative populations and connectivity of these different moieties to provide a consistent "local" chemical structure model. An inferred 2 nm GO sheet size from a very large (∼20%) edge carbon fraction by NMR analysis is at odds with the >20 nm sheet size determined from microscopy and dynamic light scattering. A proposed kirigami model where extensive internal cuts/tears in the basal plane provide the necessary edge sites is presented as a resolution to these divergent results. We expect this work to expand the fundamental understanding of this complex material and enable greater control of the GO structure.

Transformation of catechol coupled to redox alteration of humic acids and the effects of Cu and Fe cations

Humic substances are reactive during redox alteration. However, the role of this reactivity in the transformation of organic compounds and in carbon cycling in the environment is still unclear. Here, we used 14C-radioactive tracer to study abiotic transformation and humification of catechol, a representative of naturally occurring monomeric phenols and phenolic pollutants, in suspensions of humic acids (HAs) at original and H2/Pd-reduced redox states with flux of air (HAorg-Air and HAred-Air, respectively) or N2 (HAorg-N2 and HAred-N2, respectively) for 20 min in absence and presence of Cu(II) and Fe(II). Both HAorg and HAred can transform catechol in the absence of O2 to 19% and 25% of the initially applied amount, respectively. The transformation of catechol strongly increased when air was introduced, amounting to 75% in HAred-Air treatment and 43% in HAorg-Air treatment, owing to the generation of reactive oxygen species. Considerable amounts of catecholic carbons were incorporated into HAs (26% for HAred-Air and 19% for HAorg-Air), constituting humification of catechol. The presence of Cu(II) strongly inhibited the overall transformation and humification of catechol, although it significantly increased humification at the start of incubation. The presence of Fe(II) overall enhanced both the transformation and humification. The results provide first insights into the impacts of redox alteration of humic substances together with the presence of metal ions with variable valences on the fate of phenolic compounds in the environment. This study points out that redox alteration-induced abiotic transformation may be one important process for dissipation of phenolic pollutants and humification of phenolic carbons in environments rich in HAs and subject to redox fluctuation.

Soil washing with solutions of humic substances from manure compost removes heavy metal contaminants as a function of humic molecular composition

Humic Substances (HS) from Leonardite and two different composts were used as biosurfactants to wash heavy metals (Cu, Pb, Zn, Cd, Cr) from a soil added with two metals concentrations and aged for 4 and 12 months. Composts were obtained by mixing manure with either 40 (CM-I) and 20 (CM-II) % of straw as structuring material. For both aging periods and both metal concentrations, HS from CM-I removed more metals than from Leonardite, whereas the washing capacity of HS from CM-II was negligible. 13C-CPMAS-NMR spectra of HS indicated that while aromatic moieties for CM-I and Leonardite were more abundant than CM-II, HS from CM-I was largest in carboxyl and phenolic carbons. Hence, HS from CM-I had a greater complexing capacity than from both Leonardite and CM-II and effectively displaced heavy metals from soil during the washing treatment. Moreover, the amount of metals removed by solutions of ammonium acetate (AA) and diethylenetriaminepentaacetic acid (DTPA), was found invariably smaller than by HS from CM-I, thereby indicating that HS removed more than one metal specie. The combined washing with HS from CM-I before and after soil treatment by either AA and DTPA revealed significant larger metal removals than by single solutions alone. This shows that humic soil washing also renders residual metals potentially more available to subsequent soil remediation approaches, such as phytoextraction. These results suggest a novel, efficient, and molecularly-based technology to remediate soils from heavy metals can be based on a low-cost and sustainable humic matter produced from recycled biomasses.

Model Studies on the Effect of Aldehyde Structure on Their Selective Trapping by Phenolic Compounds.

The reaction among flavor-relevant saturated aldehydes (propanal, 2-methylpropanal, butanal, 2-methylbutanal, 3-methylbutanal, pentanal, hexanal, and glyoxal) and phenolic compounds (resorcinol, 2-methylresorcinol, 2,5-dimethylresorcinol, and orcinol) was studied both to identify and to characterize the formed carbonyl-phenol adducts and to understand the differences in the carbonyl-trapping abilities of phenolic compounds. The obtained results showed that carbonyl-trapping by phenolics is selective and that the formation of carbonyl-phenol adducts depends on the structures of both the phenol and aldehyde involved. In relation to the phenolic derivative, the presence of groups that increase the nucleophilicity of phenolic carbons will increase the carbonyl-trapping ability of these compounds. On the other hand, the presence of groups that increase the steric hindrance of these positions without affecting nucleophilicity will inhibit the reaction. Analogously, the presence of branching at position 2 of the aldehyde will also inhibit the reaction by steric hindrance. All of these results suggest that the addition of phenolics to foods may change food flavor not only because of their sensory properties but also because they can modify the ratio among food odorants by selective reaction of phenolics with determined carbonyl compounds.

Microstructures and mechanical properties of pyrocarbons produced from phenolic resin with added Ni(NO3)2

A refractory containing graphite is commonly used in the metallurgical industry in locations subject to severe thermal shock because of the high thermal conductivity and good thermal shock resistance of graphite. However, a refractory that uses phenolic resin as the carbon precursor is brittle, and to improve its strength and toughness, Ni(NO3)2 is added to the resin to catalyze the in-situ formation of carbon nanofibers/nanotubes. The microstructure and mechanical properties of the Ni(NO3)2-modified phenolic resin carbons were characterized by XRD, SEM, TEM and mechanical tests. Results indicate that carbon nanofibers/nanotubes (2% by volume) were formed within the pyrocarbons as a result of the nickel catalyst and these are interconnected to form a network structure. The nanocarbon fibers/tubes significantly improve the bend strength, elastic modulus, tensile strength and fracture toughness of the pyrocarbons and their fracture energies are increased accordingly.

Phenolic carbon tailored for the removal of polar organic contaminants from water: A solution to the metaldehyde problem?

Current water treatment technologies are inefficient at treating water contaminated with metaldehyde, an 8-member cyclic tetramer of acetaldehyde widely used as a molluscicide in large-scale agriculture and in gardens, and which has been frequently observed to breach European regulatory limits in the UK due to its high solubility and frequent use. Here, we examine the controls on metaldehyde adsorption onto activated phenolic carbon, namely the influence of activation degree, pore size distribution, particle size, point of zero charge and surface functionalisation, by synthesising “tailored” carbons from phenolic resin. Metaldehyde adsorption has been found to be independent of specific surface area (SBET), which is highly unusual for an adsorption process, and is favoured in carbons with (a) high microporosity with narrow pore size distribution, (b) presence of mesopores which allow efficient diffusive transport, and (c) an absence of negatively charged functional groups. The maximum adsorption capacity of the phenolic resin-derived carbons, tested at an elevated (i.e. exceeding environmental levels) water concentration of 64 mg metaldehyde/L, was 76 mg metaldehyde/g carbon compared with 13 mg metaldehyde/g carbon in industrial granular activated carbon (GAC). The phenolic resin-derived carbons and GAC showed similar adsorption kinetics with maximum metaldehyde uptake occurring within 30 min under batch adsorption conditions, although adsorption isotherms indicate much stronger adsorption of metaldehyde on the phenolic resin-derived carbons. Adsorption efficiency for metaldehyde was maintained even in the presence of high background concentrations of organic matter and inorganic salts, indicating the potential utility of these “designer” carbons in waste and/or drinking water treatment.

Microporosity development in phenolic resin-based mesoporous carbons for enhancing CO2 adsorption at ambient conditions

Soft-templating method was used to prepare mesoporous carbons. The synthesis in the presence of hydrochloric and citric acids involved resorcinol and formaldehyde as carbon precursors and triblock copolymer Pluronic F127 as a template. The as-synthesized samples underwent carbonization in flowing nitrogen at various temperatures; namely 600°C, 700°C and 800°C. Two routes were used to develop microporosity in the mesoporous carbons studied. The first one involved introduction of tetraethyl orthosilicate to the reaction system. After silica dissolution with NaOH, an increase in microporosity was observed. The second method, chemical activation with KOH at 700°C, was explored as an alternative approach to create microporosity. It is noteworthy that the TEOS addition not only led to the development of microporosity but also to some improvement of mesoporosity. The post-synthesis KOH activation resulted in more significant increase in the microporosity as compared to the samples obtained by TEOS-assisted synthesis. The mesopore volume was somewhat lower for activated carbons as compared to that in mesoporous carbons. Both methods resulted in micro-mesoporous carbons with good adsorption properties; for instance, in the case of carbons prepared in the presence of TEOS, the best sample exhibited BET surface area of 1463m2/g and the total pore volume of 1.31cm3/g. For the KOH activated carbons the best adsorption parameters were as follows: the specific surface area=1906m2/g, and the total pore volume=0.98cm3/g. Both procedures used for microporosity development afforded carbons with good adsorption properties that can be useful for applications such as CO2 adsorption, air and water purification.

Enhancement of CO2 adsorption on phenolic resin-based mesoporous carbons by KOH activation

Carbons with high surface area and large volume of ultramicropores were synthesized for CO2 adsorption. First, mesoporous carbons were produced by soft-templating method using triblock copolymer Pluronic F127 as a structure directing agent and formaldehyde and either phloroglucinol or resorcinol as carbon precursors. The resulting carbons were mainly mesoporous with well-developed surface area, large total pore volume, and only moderate CO2 uptake. To improve CO2 adsorption, these carbons were subjected to KOH activation to enhance their microporosity. Activated carbons showed 2–3-fold increase in the specific surface area, resulting from substantial development of microporosity (3–5-fold increase in the micropore volume). KOH activation resulted in enhanced CO2 adsorption at 760mmHg pressure: 4.4mmolg−1 at 25°C, and 7mmolg−1 at 0°C. This substantial increase in the CO2 uptake was achieved due to the development of ultramicroporosity, which was shown to be beneficial for CO2 physisorption at low pressures. The resulting materials were investigated using low-temperature nitrogen physisorption, CO2 sorption, and small-angle powder X-ray diffraction. High CO2 uptake and good cyclability (without noticeable loss in CO2 uptake after five runs) render ultramicroporous carbons as efficient CO2 adsorbents at ambient conditions.

Effect of the addition of carbon black and carbon nanotubes on the structure and oxidation resistance of pyrolysed phenolic carbons

Carbon black (CB) and carbon nanotubes (CNTs) were used as additives in phenolic resin. The effects of the additives and heat treatment temperature on the oxidation resistance and structure of carbon from the pyrolysed phenolic resin were investigated by differential scanning calorimetric analysis, X-ray diffraction, scanning electron microscopy, and mercury intrusion porosimetry. It was observed that both CNTs and CB improved the graphitization degree and oxidation resistance of the pyrolytic carbons. The graphitization degree of carbons containing CNTs was higher than that of those with CB, but the oxidation resistance of the former was lower due to the higher porosity in the microstructure. An increase in the heat treatment temperature also resulted in an improvement in the graphitization degree and oxidation resistance of the carbons.

Phenolic resin-based carbons with ultra-large mesopores prepared in the presence of poly(ethylene oxide)–poly(butylene oxide)–poly(ethylene oxide) triblock copolymer and trimethyl benzene

The evaporation-induced self assembly (EISA) strategy has been successfully employed to obtain ultra-large mesoporous carbons (UMC) by using Vorasurf 504 poly(ethylene oxide)–poly(butylene oxide)–poly(ethylene oxide), (PEO–PBO–PEO) triblock copolymer as a template. Resorcinol and formaldehyde were used as carbon precursors, whereas trimethyl benzene (TMB) was used as a pore expander. Nitrogen adsorption isotherms revealed that the resulting UMC materials possess ultra-large mesopores (about 27nm) and relatively narrow pore size distribution. Mesoporous carbons synthesized without TMB show also fairly large mesopores (about 20nm). These carbons were examined for adsorption of Lysozyme (Lz) from buffered solution at pH 10.8. All Lz adsorption isotherms were reasonably well fitted by Langmuir equation, giving high maximum adsorption capacity (31μmol/g).

Effect of the addition of carbon black and carbon nanotubes on the structure and oxidation resistance of pyrolysed phenolic carbons

Carbon black (CB) and carbon nanotubes (CNTs) were used as additives in phenolic resin. The effects of the additives and heat treatment temperature on the oxidation resistance and structure of carbon from the pyrolysed phenolic resin were investigated by differential scanning calorimetric analysis, X-ray diffraction, scanning electron microscopy, and mercury intrusion porosimetry. It was observed that both CNTs and CB improved the graphitization degree and oxidation resistance of the pyrolytic carbons. The graphitization degree of carbons containing CNTs was higher than that of those with CB, but the oxidation resistance of the former was lower due to the higher porosity in the microstructure. An increase in the heat treatment temperature also resulted in an improvement in the graphitization degree and oxidation resistance of the carbons.

Proposed Benchmark Methods for Analyzing Acai (Euterpe oleraceae Mart.)

Methods are proposed for analyzing acai. They start with gravimetric methods for total lipids and insoluble solids, 1H-NMR to determine the relative distribution of the types of lipids, CPMAS 13C-NMR to analyze insoluble solids, and ICP-AES to measure the amounts of total Ca, Mg, K and P. The ratio of K to Ca can be used to distinguish between acai products and grape juice. These methods were used to analyze samples. The total lipids were 46.7±2.5%, as triglycerides. The triglycerides contained 24.8±2.8% saturated fatty acyls, 66.1±2.8% mono-, 7.58±0.52% di- and 1.46±0.05% omega-3 unsaturated fatty acyls. A CPMAS 13C-NMR spectrum of the insoluble solids contained peaks due to C=O and/or phenolic carbons, aromatic carbons, glycosidic carbons and aliphatic carbons, with chemical shifts and peak sizes that were different from those of other types of insoluble fibers. HPLC and LC–MS/MS found cyanidin-3-Oglucoside and cyanidin-3-O-rutinoside. Anion chromatography found lactate, acetate, formate, galacturonate, chloride, sulfate, malate, oxalate, phosphate, inositol hexaphosphate (phytic acid), citrate and isocitrate. No Salmonella, coliform bacteria, pesticides, herbicides, fungicides or industrial chemicals were found in genuine Brazilian acai. Keywords: Acai, triglycerides, NMR, ion chromatography, phytic acid, potassium, calcium, pulp, aroma, magnsium

Investigating the structure of non-graphitising carbons using electron energy loss spectroscopy in the transmission electron microscope

We have investigated the structure and bonding in phenolic resin-based carbons as a function of heat treatment using a combination of high resolution transmission electron microscopy, electron energy loss spectroscopy, X-ray diffraction, gas adsorption and density measurements. On heat treatment, the initially amorphous material transforms to a micro-porous material containing small graphitic packets enclosing elongated pores, with a relatively small change in density but an appreciable change in planar carbon sp 2-content as well as the development of significant curvature in the graphitic layers. The results from the different characterisation techniques are discussed and wherever possible, cross-correlated. They are also compared with proposed models for the microstructural development in non-graphitising carbons and also with the corresponding data obtained from graphitisable carbons. Overall the data provides strong evidence for a fullerene-based model for the microstructural development in non-graphitising carbons and more generally, the interpretation of the current EELS data has important implications for the spatially localised analysis of poorly crystalline carbon materials.

Hierarchically porous phenolic resin-based carbons obtained by block copolymer-colloidal silica templating and post-synthesis activation with carbon dioxide and water vapor

A series of hierarchically porous carbons was synthesized by self-assembly of polymeric carbon precursors and block copolymer template in the presence of tetraethyl orthosilicate (TEOS) and colloidal silica under acidic conditions. Resorcinol and formaldehyde were used as carbon precursors, poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer was employed as a soft template, and TEOS-generated silica and colloidal silica were used as hard templates. The carbon precursors were polymerized in hydrophilic domains of block copolymer, followed by carbonization and silica dissolution. This resulted in carbons possessing cylindrical (∼12 nm) and spherical (20 or 50 nm) mesopores created by thermal decomposition of the soft template and by the dissolution of colloidal silica, respectively; fine pores were also formed by the dissolution of the TEOS-generated silica (∼2 nm). A further increase in fine porosity was achieved by post-synthesis activation of the carbons with carbon dioxide and/or water vapor, which resulted in hierarchical carbons with a surface area and pore volume approaching 2800 m 2/g and 6.0 cm 3/g, respectively.

Adsorption properties of phenolic resin-based mesoporous carbons obtained by using mixed templates of Pluronic F127 and Brij 58 or Brij 78 polymers

Phenolic resin-based mesoporous carbons were synthesized by using mixed templates of Pluronic F127 and Brij 58 or Brij 78. For the purpose of comparison three samples of nanoporous carbons were prepared by using single templates of Pluronic F127, Brij 58 and Brij 78 polymers, respectively. Adsorption properties of the aforementioned carbons were studied by nitrogen adsorption at −196 °C. The resulting carbons featured high specific surface areas ranging from 641 m2/g for the sample obtained in the presence of Brij 58 polymer to 825 m2/g for the carbon prepared by using a mixed template containing 23% of Pluronic F127 and 77% of Brij 58 and from 588 m2/g for the sample obtained in the presence of Brij 78 polymer to 813 m2/g for the carbon prepared by using Pluronic F127 only. It was shown that the width of mesopores increases with increasing amount of Brij 58 or Brij 78 in the mixture of one of these polymers with Pluronic F127, suggesting that Brij polymers act also as micelle expanders.

Polymer-templated mesoporous carbons with nickel nanoparticles

Soft-templating synthesis of mesoporous carbons in the presence of tetraethyl orthosilicate (TEOS) and nickel nitrate was carried out to introduce nickel nanoparticles and create additional microporosity in these materials. This strategy was employed to synthesize phenolic resin-based mesoporous carbons with two different loadings of nickel nanoparticles and to obtain nickel-containing mesoporous carbon–silica hybrids. Removal of silica with NaOH solution from the hybrid composites gave mesoporous carbons with Ni particles and inversely, burn off carbon from the aforementioned composites gave NiO-containing mesoporous silicas. Nitrogen adsorption, small and wide angle X-ray diffraction, transmission electron microscopy and thermogravimetric analysis showed good adsorption and structural properties of the aforementioned materials.

Solid-state NMR and ESR studies of activated carbons produced from pecan shells

A large number of solid-state NMR and ESR experiments were explored as potential tools to study chemical structure, mobility, and pore volume of activated carbon. We used a model system where pecan shells were activated with phosphoric acid, and carbonized at 450°C for 4h with varying amounts of air flow. Through the use of different NMR experiments (e.g., CP-MAS, SPE-MAS, and DD-MAS) several structural parameters were calculated such as mole fraction of bridgehead aromatic carbons, number of carbons per aromatic ring system, and number of phenolic carbons per aromatic ring system. The relaxation time measurements (T1, TCH, and T1ρH) were indicative of the relative mobility of different structural units. ESR spectra showed the presence of π-type aromatic free radicals in the carbonized samples with a slight shift in g value with increasing oxidation. The combined NMR and ESR data give a consistent picture of the carbon structure and the carbonization process, which is not easily available otherwise. In addition, the 1H NMR data on adsorbed water are shown to be consistent with the trends in the amount of pore volumes for different samples of activated carbons.