Tannin Gels and Their Carbon Derivatives: A Review

International audienceTannins are one of the most natural, non-toxic, and highly reactive aromatic biomolecules classified as polyphenols. The reactive phenolic compounds present in their chemical structure can be an alternative precursor for the preparation of several polymeric materials for applications in distinct industries: adhesives and coatings, leather tanning, wood protection, wine manufacture, animal feed industries, and recently also in the production of new porous materials (i.e., foams and gels). Among these new polymeric materials synthesized with tannins, organic and carbon gels have shown remarkable textural and physicochemical properties. Thus, this review presents and discusses the available studies on organic and carbon gels produced from tannin feedstock and how their properties are related to the different operating conditions, hence causing their cross-linking reaction mechanisms. Moreover, the steps during tannin gels preparation, such as the gelation and curing processes (under normal or hydrothermal conditions), solvent extraction, and gel drying approaches (i.e., supercritical, subcritical, and freeze-drying) as well as the methods available for their carbonization (i.e., pyrolysis and activation) are presented and discussed. Findings from organic and carbon tannin gels features demonstrate that their physicochemical and textural properties can vary greatly depending on the synthesis parameters, drying conditions, and carbonization methods. Research is still ongoing on the improvement of tannin gels synthesis and properties, but the review evaluates the application of these highly porous materials in multidisciplinary areas of science and engineering, including thermal insulation, contaminant sorption in drinking water and wastewater, and electrochemistry. Finally, the substitution of phenolic materials (i.e., phenol and resorcinol) by tannin in the production of gels could be beneficial to both the bioeconomy and the environment due to its low-cost, bio-based, non-toxic, and non-carcinogenic characteristics

Study and characterization of carbon xerogels from tannin-formaldehyde system as catalyst support applications

This contribution reports the use of an organic gel, xerogel, as catalyst support for Cu. The xerogel was synthesized from the system tannin-formaldehyde at pH 3, using F-127 Pluronic as surfactant. The surface area values were higher than 900 m2/g even after the Cu (10%, wt) impregnation. The morphological analysis by SEM-FEG revealed the presence of spheres arranged in a tridimensional structure. XRD diffractograms showed the presence of CuO and Cu2O crystalline phases combined with the amorphous structure of the porous carbon. From XRD analysis it is possible to infer the Cu species are heterogeneously dispersed on the support with the co-existence of small and larger clusters, which is in agreement with Raman spectroscopy. Raman study also indicated a highly defect/disorder structure of the xerogel derivatives, ensuing the short-range structural order of the carbonic structure and oxygen groups decorating the carbon surface. XPS results corroborate with XRD and Raman results, detecting the presence of CuO and Cu2O. In addition to the mentioned Cu species, XPS also detected Cu0 which may be originated from the chemical interaction between the electron oxygen groups with the Cu precursor. The high surface area and the thermal stability (~ 300 oC) of Cu/XCTF envisages its feasibly for relevant catalytic applications.Keywords: Carbon xerogels, tannin, renewable process, catalyst support

The conversion of wood residues, using pilot-scale technologies, into porous activated biochars for supercapacitors

In this study, activated biochar was produced using pilot-scale technologies of fast pyrolysis and activation to create desirable morphology, surface chemistry, and adsorptive properties for application in supercapacitors. First, residues from white birch were converted into biochar by fast pyrolysis (~ 450 °C). Then, physical (using CO2) or chemical (using KOH) activation was carried out in a homemade pilot-scale furnace at 900 °C. These synthesized materials presented distinct porosity structures: micro-/mesoporous (CO2 material) and highly microporous (KOH material), reaching surface areas of up to 1700 m2 g−1. Electrochemical results showed that KOH-activated biochar had higher specific electrical capacitance in both acidic and neutral electrolytes with a maximum specific capacitance value of 350 and 118 F g−1 at 1 A g−1, respectively; while, for CO2-activated biochar, the maximum obtained values were 204 and 14 F g−1. The greater proportion of oxygenated and nitrogenated functional groups on the surface of the KOH activated biochar, along with its high surface area (with wider porosity), improved its performance as a supercapacitor electrode. Specifically, the low proportion of ultramicropores was determinant for its better electrochemical behavior, especially in the neutral electrolyte. Indeed, these results are similar to those found in the literature on the electrical capacitance of carbonaceous materials synthesized in a small-scale furnace. Thus, the chemical-activated biochar made from wood residues in pilot-scale furnaces is a promising material for use as electrodes for supercapacitors

Investigation of sustainable porous carbon as radar absorbing material

Radar Absorbing Materials (RAMs) are composite materials able to attenuate an incident electromagnetic wave. Usually, RAMs are made of a polymeric matrix and an electromagnetic absorbent filler, such as silicon carbide or carbon black. Several materials have been investigated as potential fillers, aiming to improve the Reflection Loss (RL) and absorption bandwidth broadening. In this paper, a composite made with silicone rubber and biochar was investigated as a sustainable porous carbon made with biomass waste. Five different composites were characterized, composed of 1 - 5 wt.% of biochar in the silicone rubber. Although the RL of pure biochar composites is not significant, it was demonstrated here how a biochar composite can improve the RL of a RAM material when it is applied as a double-layer structure. While the RL of a ferrite-based RAM with 2.0 mm thickness reaches -28 dB, a combination of this RAM with biochar composite reaches ~ -60 dB with the same thickness. The double-layer structure with 2.3 mm thickness can have an absorption bandwidth of 2.95 GHz over the X-band frequency range, and a structure with 2.6 mm thickness can reach a RL of ~-76 dB. This demonstrates a sustainable, cheaper, and lighter material application (i.e., biochar), which is successfully used in the development of high-efficient electromagnetic shield or sensors.Keywords: Biochar, permittivity, RAM, sustainability, reflectivity.

Electromagnetic properties of Carbon-Graphene Xerogel, Graphite and Ni-Zn Ferrite composites in polystyrene matrix in the X-Band (8.2 – 12.4 GHz)

The electromagnetic properties of Carbon-Graphene Xerogel (CGX), Flaky graphite (GR) and Nickel-Zinc ferrite (FeNiZn) composites in polystyrene (PS) matrix were studied in the X-Band range (8.2 – 12.4 GHz). In this work the Expanded Polystyrene (EPS) waste material was processed into polystyrene through the recycling of EPS. The polystyrene obtained was utilized as dielectric matrix, mainly because PS is a wellknown organic polymer that presents low dielectric loss and light weight, which contribute to applications in composites for the aerospace field. In order to produce the final composite specimens, the CGX additive was previously synthesized through a sustainable method that employed the use of waste from the paper and pulp industry (black liquor). Afterwards, the morphological and structural analysis were made through Scanning Electron Microscope (SEM) and Raman Spectrometer, respectively. On the other hand, the magnetic ferrite material, FeNiZn, was obtained for the composite production through calcination, whereas the GR utilized was commercially obtained. It was observed that the increase of CGX and GR influenced on the increase of the Complex Permittivity, and that 10 wt% CGX + 50wt% FeNiZn composite sample demonstrated an absorption peak at 10.5 GHz. The results are relevant concerning the recycling of EPS waste through its use as dielectric matrix, thus developing greener and low-weight composite materials to be used in microwave applications.Keywords: Carbon-Graphene. Composite. Recycling. Polystyrene. Microwave.

Kinetic Study of the oxidation of lignins from sugarcane bagasse and straw and application of lignin from sugarcane bagasse in natural-materials-based resins

Considerando a grande quantidade de resíduos sólidos, como a palha e o bagaço de cana, gerados devido às atividades agrícolas, procurou-se desenvolver um projeto que utilize esses resíduos para obtenção de produtos com maior valor agregado, visando atender às expectativas em termos econômicos e ambientais. O bagaço da cana é um resíduo gerado em grandes proporções no Brasil. O Estado de São Paulo instituiu uma lei que proíbe a prática da queimada, para fins de colheita, pelo fato de causar sérios problemas ambientais e danos à saúde da população das cidades produtoras de cana, tornando a palha mais um resíduo em abundância. O bagaço e agora a palha da cana são queimados em caldeiras para a geração de energia nas próprias usinas de açúcar e álcool. Os excedentes deste processo podem ser utilizados para a obtenção de produtos de maior valor agregado, desde polpas celulósicas bem como a produção de ligninas que podem ser utilizadas em resinas para fabricação de aglomerados. Para o uso integral da biomassa lignocelulósica é necessário fazer a separação de seus constituintes majoritários: celulose, hemicelulose e lignina. Neste trabalho, os processos de separação utilizados foram a polpação etanol-água e a técnica de explosão a vapor tendo a palha e o bagaço da cana como materiais de partida. As ligninas obtidas pelo processo etanol-água foram oxidadas em meio ácido sob condições diferentes para determinar as cinéticas da oxidação e as energias de ativação destas ligninas. Ligninas oxidadas apresentam fortes propriedades quelantes e podem ser aplicadas no tratamento de efluentes para remoção de metais pesados. A oxidação foi realizada em meio ácido acético utilizando o sitema catalítico Co/Mn/Br à 50, 80 e 115°C por 5 h. A energia de ativação calculada para as ligninas de bagaço e de palha apresentaram um valor de 23,4 kJ/mol e 34,2 kJ/mol, respectivamente, indicando que a lignina de palha é mais reticulada. O estudo cinético da oxidação foi avaliado por UV-Visível. Espectros de infravermelho de várias amostras de ligninas oxidadas foram submetidas à Analise por Componentes Principais (PCA). Os resultados mostraram suaves modificações na estrutura da lignina após a reação de oxidação. A lignina obtida pela técnica de explosão a vapor foi testada em resinas para fabricação de aglomerados, juntamente com resinas à base de farinha de soja e de tanino. Foram ainda fabricadas resinas utilizando o glioxal em substituição do formaldeído, que é um material tóxico. Lignina glioxilada foi adicionada à resina de farinha de soja glioxilada em substituição das resinas PF ou de isocianato (pMDI), visando uma maior utilização de material natural na resina. As formulações que continham 70 ou 80% de material natural apresentaram resultados dentro dos padrões exigidos. As resinas com 70% de material natural podem ser utilizadas em menor porporção na madeira e também podem ser utilizados tempos de prensagem menores, que são industrialmente significativos. A melhor formulação encontrada foi utilizando farinha de soja pré-cozida glioxilada (SG) com a adição de tanino e pMDI, onde as proporções de material foram SG/T/pMDI 54/16/30 (m/m) .Considering the large amount of agricultural residues, such as straw and sugarcane bagasse, generated due to agricultural activities, we sought to develop a project that proposes the use of these residues to obtain products with higher value, to acchieve expectations in terms of cost and environment. Bagasse from sugarcane is a by-product generated in large proportions in Brazil. São Paulo State introduced a law which prohibits the practice of burning for harvesting because of causing serious environmental problems and damage to the health of the population of cities close to cane producers, making straw an other abundant residue. Sugarcane bagasse and straw are burned in boilers for generation of energy in sugar and alcohol industries. However, excess of those by-products could be used to obtain products with higher value, as cellulosic pulps with applications in cardboard packing and the application in resins for the manufacture of particleboard. For the integral use of the vegetable biomass it is necessary separate the major components: cellulose, hemicellulose and lignin, and for this purpose ethanol-water pulping and steam explosion process were used, with the sugarcane bagasse and straw. Lignins obtained by ethanol-water pulping were oxidized in acidic medium under different conditions to study kinetics of the oxidation and calculate the activation energies of these lignins. Oxidized lignins presents very strong chelating properties and could be applied in effluents treatments for heavy metals removal. The oxidation of lignins were performed using acetic acid and Co/Mn/Br catalytical system at 50, 80 and 115°C for 5 h. Activation energy (Ea) was calculated for lignins from sugarcane bagasse and straw and presented Ea of 34.4 kJ.mol-1 and 23.3 kJ.mol-1, respectively, indicating higher crosslinked formation for straw. A kinetic study of the oxidation was evaluated by UV/Visible. FTIR spectra of various samples of oxidized lignins were submitted to Principal Component Analysis (PCA). The results showed slight structure modifications in lignins after oxidation reaction. Lignin obtained by the steam explosion process was tested in resins for the manufacture of particleboards, together with resin-based soy flour and tannin. Resins were manufactured to substitute formaldehyde (a toxic material) with glyoxal. Glyoxalated lignins were added to glyoxalated soy flour in place of PF resins or the isocyanate (pMDI), aiming greater use of natural materials in resins. Adhesive resin formulations in which the total content of natural material is either 70 or 80% of the total resin solids content gave good results. The resins comprising 70% by weight of natural material can be used in a much lower proportion on wood chips and can afford pressing times fast enough to be significant under industrial panel pressing conditions. The best formulation of all the ones tried was the one based on glyoxalated precooked soy flour (SG), to which a condensed tannin was added in water solution and pMDI, where the proportions of the components SG/T/pMDI was 54/16/30 by weight

Kinetic Study of the oxidation of lignins from sugarcane bagasse and straw and application of lignin from sugarcane bagasse in natural-materials-based resins

Considerando a grande quantidade de resíduos sólidos, como a palha e o bagaço de cana, gerados devido às atividades agrícolas, procurou-se desenvolver um projeto que utilize esses resíduos para obtenção de produtos com maior valor agregado, visando atender às expectativas em termos econômicos e ambientais. O bagaço da cana é um resíduo gerado em grandes proporções no Brasil. O Estado de São Paulo instituiu uma lei que proíbe a prática da queimada, para fins de colheita, pelo fato de causar sérios problemas ambientais e danos à saúde da população das cidades produtoras de cana, tornando a palha mais um resíduo em abundância. O bagaço e agora a palha da cana são queimados em caldeiras para a geração de energia nas próprias usinas de açúcar e álcool. Os excedentes deste processo podem ser utilizados para a obtenção de produtos de maior valor agregado, desde polpas celulósicas bem como a produção de ligninas que podem ser utilizadas em resinas para fabricação de aglomerados. Para o uso integral da biomassa lignocelulósica é necessário fazer a separação de seus constituintes majoritários: celulose, hemicelulose e lignina. Neste trabalho, os processos de separação utilizados foram a polpação etanol-água e a técnica de explosão a vapor tendo a palha e o bagaço da cana como materiais de partida. As ligninas obtidas pelo processo etanol-água foram oxidadas em meio ácido sob condições diferentes para determinar as cinéticas da oxidação e as energias de ativação destas ligninas. Ligninas oxidadas apresentam fortes propriedades quelantes e podem ser aplicadas no tratamento de efluentes para remoção de metais pesados. A oxidação foi realizada em meio ácido acético utilizando o sitema catalítico Co/Mn/Br à 50, 80 e 115°C por 5 h. A energia de ativação calculada para as ligninas de bagaço e de palha apresentaram um valor de 23,4 kJ/mol e 34,2 kJ/mol, respectivamente, indicando que a lignina de palha é mais reticulada. O estudo cinético da oxidação foi avaliado por UV-Visível. Espectros de infravermelho de várias amostras de ligninas oxidadas foram submetidas à Analise por Componentes Principais (PCA). Os resultados mostraram suaves modificações na estrutura da lignina após a reação de oxidação. A lignina obtida pela técnica de explosão a vapor foi testada em resinas para fabricação de aglomerados, juntamente com resinas à base de farinha de soja e de tanino. Foram ainda fabricadas resinas utilizando o glioxal em substituição do formaldeído, que é um material tóxico. Lignina glioxilada foi adicionada à resina de farinha de soja glioxilada em substituição das resinas PF ou de isocianato (pMDI), visando uma maior utilização de material natural na resina. As formulações que continham 70 ou 80% de material natural apresentaram resultados dentro dos padrões exigidos. As resinas com 70% de material natural podem ser utilizadas em menor porporção na madeira e também podem ser utilizados tempos de prensagem menores, que são industrialmente significativos. A melhor formulação encontrada foi utilizando farinha de soja pré-cozida glioxilada (SG) com a adição de tanino e pMDI, onde as proporções de material foram SG/T/pMDI 54/16/30 (m/m) .Considering the large amount of agricultural residues, such as straw and sugarcane bagasse, generated due to agricultural activities, we sought to develop a project that proposes the use of these residues to obtain products with higher value, to acchieve expectations in terms of cost and environment. Bagasse from sugarcane is a by-product generated in large proportions in Brazil. São Paulo State introduced a law which prohibits the practice of burning for harvesting because of causing serious environmental problems and damage to the health of the population of cities close to cane producers, making straw an other abundant residue. Sugarcane bagasse and straw are burned in boilers for generation of energy in sugar and alcohol industries. However, excess of those by-products could be used to obtain products with higher value, as cellulosic pulps with applications in cardboard packing and the application in resins for the manufacture of particleboard. For the integral use of the vegetable biomass it is necessary separate the major components: cellulose, hemicellulose and lignin, and for this purpose ethanol-water pulping and steam explosion process were used, with the sugarcane bagasse and straw. Lignins obtained by ethanol-water pulping were oxidized in acidic medium under different conditions to study kinetics of the oxidation and calculate the activation energies of these lignins. Oxidized lignins presents very strong chelating properties and could be applied in effluents treatments for heavy metals removal. The oxidation of lignins were performed using acetic acid and Co/Mn/Br catalytical system at 50, 80 and 115°C for 5 h. Activation energy (Ea) was calculated for lignins from sugarcane bagasse and straw and presented Ea of 34.4 kJ.mol-1 and 23.3 kJ.mol-1, respectively, indicating higher crosslinked formation for straw. A kinetic study of the oxidation was evaluated by UV/Visible. FTIR spectra of various samples of oxidized lignins were submitted to Principal Component Analysis (PCA). The results showed slight structure modifications in lignins after oxidation reaction. Lignin obtained by the steam explosion process was tested in resins for the manufacture of particleboards, together with resin-based soy flour and tannin. Resins were manufactured to substitute formaldehyde (a toxic material) with glyoxal. Glyoxalated lignins were added to glyoxalated soy flour in place of PF resins or the isocyanate (pMDI), aiming greater use of natural materials in resins. Adhesive resin formulations in which the total content of natural material is either 70 or 80% of the total resin solids content gave good results. The resins comprising 70% by weight of natural material can be used in a much lower proportion on wood chips and can afford pressing times fast enough to be significant under industrial panel pressing conditions. The best formulation of all the ones tried was the one based on glyoxalated precooked soy flour (SG), to which a condensed tannin was added in water solution and pMDI, where the proportions of the components SG/T/pMDI was 54/16/30 by weight

New families of carbon gels based on natural resources

4th France-Russia Conference on New Achievements Materials and Environmental Sciences (NAMES), Nancy-Univ, Inst Natl Polytechnique Lorraine (INPL), Nancy, FRANCE, OCT 26-29, 2010International audienceCarbon gels are versatile materials which can be used for many applications. They are extremely expensive, because generally prepared from resorcinol - formaldehyde (RF) resins first gelled and next dried with supercritical carbon dioxide. In the present work, resorcinol has been substituted partly or completely by tannins, a family of molecules extracted from mimosa tree barks. Tannins are natural, non-toxic products, typically thirty times cheaper than resorcinol. Their chemical resemblance with the latter makes them be often called natural resorcinol. Using tannins not only substantially decreases the cost but also allows preparing materials in a much wider range of pHs than that usually employed for RF gels. Consequently the main pore size and the fraction of given families of pores, controlling the carbon gels' properties, are tuned in an easier way, and a much wider range of pore structures is obtained. Finally, two alternative ways of drying are suggested for further decreasing the cost: freeze-drying and supercritical drying in acetone. Both are shown to lead, in some conditions described below, to materials having similar characteristics to those of expensive RF carbon aerogels previously dried in supercritical CO2

Systematic studies of tannin–formaldehyde aerogels: preparation and properties

Gelation of tannin–formaldehyde (TF) solutions was systematically investigated by changing pH and concentration of TF resin in water. In this way we constructed the TF phase diagram, from which chemical hydrogels could be described, and also synthesized thermoreversible tannin-based hydrogels. Conditions of non-gelation were also determined. Hydrogels were dried in supercritical CO2, leading to a broad range of TF aerogels. The latter were investigated for volume shrinkage, total porosity, micro-, meso- and macropore volumes, Brunauer–Emmett–Teller (BET) surface area, microscopic texture, mechanical and thermal properties. All these properties are discussed in relation to each other, leading to an accurate and self-consistent description of these bioresource-based highly porous materials. The conditions for obtaining the highest BET surface area or mesopore volume were determined and explained in relation to the preparation conditions. The highest BET surface area, 880 m2 g−1, is remarkably high for organic aerogels derived from a natural resource

Bimodal activated carbons derived from resorcinol-formaldehyde cryogels

Resorcinol-formaldehyde cryogels prepared at different dilution ratios have been activated with phosphoric acid at 450 °C and compared with their carbonaceous counterparts obtained by pyrolysis at 900 °C. Whereas the latter were, as expected, highly mesoporous carbons, the former cryogels had very different pore textures. Highly diluted cryogels allowed preparation of microporous materials with high surface areas, but activation of initially dense cryogels led to almost non-porous carbons, with much lower surface areas than those obtained by pyrolysis. The optimal acid concentration for activation, corresponding to stoichiometry between molecules of acid and hydroxyl groups, was 2 M l−1, and the acid–cryogel contact time also had an optimal value. Such optimization allowed us to achieve surface areas and micropore volumes among the highest ever obtained by activation with H3PO4, close to 2200 m2 g−1 and 0.7 cm3 g−1, respectively. Activation of diluted cryogels with a lower acid concentration of 1.2 M l−1 led to authentic bimodal activated carbons, having a surface area as high as 1780 m2 g−1 and 0.6 cm3 g−1 of microporous volume easily accessible through a widely developed macroporosity