Resorcinol-formaldehyde Carbon Aerogels Research Articles

Iodide Removal by Resorcinol-Formaldehyde Carbon Aerogels.

The adsorption technique is widely used in water purification, and its efficiency can be significantly improved by target-specific adsorbent design. Research on iodine and its ion removal from water has attracted a great deal of interest due to increased concentrations in the environment and acute toxic effects, e.g., in human thyroid cells. In this work, the iodide removal performance of two high-surface-area resorcinol–formaldehyde-based carbon aerogels was studied under acidic conditions. The BET surface area was 790 m2/g (RF_ac) and 375 m2/g (RMF-GO), with a corresponding micropore ratio of 36 and 26%, respectively. Both aerogels showed outstanding adsorption capacity, exceeding the reported performance of other carbons and Ag-doped materials. Owing to its basic nature, the RMF-GO carbon aerogel showed higher I− capacity, up to 97 mg/g, than the acidic RF_ac, which reached a capacity of 82 mg/g. The surface chemistry of the aerogels also played a distinct role in the removal. In terms of kinetics, RF_ac removed 60% of the iodide ions and RMF-GO 30% within 8 h. The removal kinetics was of the first order, with a half-life of 1.94 and 1.70 h, respectively.

Interaction of resorcinol-formaldehyde carbon aerogels with water: A comprehensive NMR study

Carbon aerogels prepared from resorcinol-formaldehyde aerogels are promising platforms for electrodes, catalysts, adsorbents in environmental chemistry and as electric conductors. For these applications the knowledge of their structure and behavior in aqueous medium is essential. In this work two resorcinol-formaldehyde (RF) carbon aerogels prepared in different ways were characterized with various NMR methods while their pore structure was stepwise saturated with water. The wetting properties were studied by vapor adsorption and low-field NMR relaxometry, while the morphology was followed by NMR cryoporometry during the hydration process. At several water saturation levels the self-diffusion of water was measured. The comprehensive evaluation of the results led to a detailed description of the wetting process of these carbon aerogels beyond the pore size distributions. At low hydration level water clusters formed on and around the hydrophilic functional groups of the surface being able to adsorb water, but no continuous water layer developed on the surface. With increasing water content, spherical water drops formed inside the pore system, and vapor phase diffusion was observed in the partially filled pores. Subsequently the interconnected pore structure was saturated. The determined wet structure was compared to low temperature nitrogen gas adsorption results and scanning electron microscopy images.

Water-Ionic Liquid Binary Mixture Tailored Resorcinol-Formaldehyde Carbon Aerogels without Added Catalyst.

The potential applications of mesoporous carbon aerogels are wide-ranging. These gels are often obtained from resorcinol-formaldehyde (RF) hydrogel precursors. The sol-gel method in this synthesis provides an efficient and versatile means of product control through systematic variation of process conditions, such as pH, stoichiometry, concentration, catalyst, further additives, etc., in addition to the drying and pyrolytic conditions. Here, a novel means of tuning the texture of carbon aerogels is proposed. Water-1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO4] mixtures constitutes a polycondensation medium that requires no added catalyst, thus yielding an intrinsically metal-free carbon aerogel after pyrolysis. We also show that the carbon morphology is tailored by the supramolecular structure of the aqueous ionic liquid. The results of scanning electron micrographs, low-temperature nitrogen adsorption/desorption isotherms, and small-angle X-ray scattering (SAXS) confirm that changing the initial water concentration from 9 to 55 wt % gives rise to systematic alteration of the mesopore size and volume, as well as of the bead size. The pore structure becomes consolidated only when the water content exceeds 25 wt %. When the water content reaches 55 wt %, the bead size increases by two orders of magnitude. The electrocatalytic performance, however, is compromised, most probably by structural defects.

Enhanced hydrogen storage properties of 2LiBH4-LiAlH4 nanoconfined in resorcinol formaldehyde carbon aerogel

Resorcinol formaldehyde carbon aerogel (RFC)was synthesized in this paper to be used as a scaffold for nanoconfining 2LiBH4-LiAlH4composite. LiBH4 and LiAlH4 were nanoconfined into the pores of the prepared resorcinol formaldehyde carbon aerogel by a two-step melt-infiltration process. The microstructure evolution was investigated by XRD, FTIR analysis, and BET measurements. The hydrogen storage properties were studied by TPD, TG/DSC/MS measurements. The experimental results show that LiBH4 and LiAlH4 are well dispersed in the scaffold after the melt infiltration process. The nano-confined 2LiBH4-LiAlH4 composite (denoted as 2LiBH4-LiAlH4/RFC) shows a two-step dehydrogenation process. The onset dehydrogenation temperatures for the two steps are 100 °C and 280 °C, respectively, which are 70 °C and 170 °C lower than that of the milled 2LiBH4-LiAlH4 mixture. The formation of AlB2 during the second dehydrogenation step alters the reaction pathway of LiBH4. The kinetic properties of the composite are greatly enhanced compared to the physical mixture of LiBH4 and LiAlH4. The nanoconfinement and the formation of AlB2 have a combined effect on the improvement of hydrogen storage properties for nanoconfined 2LiBH4-LiAlH4/RFC. It could be rehydrogenated at 350 °C and 5 MPa H2. A reversible hydrogen storage capacity of 5.7 wt.% is achieved.

Hydrogen storage properties of nanoconfined LiBH4–NaBH4

In this study a eutectic melting composite of 0.62LiBH4–0.38NaBH4 has been infiltrated in two nanoporous resorcinol formaldehyde carbon aerogel scaffolds with similar pore sizes (37 and 38 nm) but different BET surface areas (690 and 2358 m2/g) and pore volumes (1.03 and 2.64 mL/g). This investigation clearly shows decreased temperature of hydrogen desorption, and improved cycling stability during hydrogen release and uptake of bulk 0.62LiBH4–0.38NaBH4 when nanoconfined into carbon nanopores. The hydrogen desorption temperature of bulk 0.62LiBH4–0.38NaBH4 is reduced by ∼107 °C with the presence of carbon, although a minor kinetic variation is observed between the two carbon scaffolds. This corresponds to apparent activation energies, EA, of 139 kJ mol−1 (bulk) and 116–118 kJ mol−1 (with carbon aerogel). Bulk 0.62LiBH4–0.38NaBH4 has poor reversibility during continuous hydrogen release and uptake cycling, maintaining 22% H2 capacity after four hydrogen desorptions (1.6 wt.% H2). In contrast, nanoconfinement into the high surface area carbon aerogel scaffold significantly stabilizes the hydrogen storage capacity, maintaining ∼70% of the initial capacity after four cycles (4.3 wt.% H2).

Nanoconfined NaAlH4: prolific effects from increased surface area and pore volume

Nanoconfinement is a promising technique to improve the properties of nanomaterials such as the kinetics for hydrogen release and uptake and the stability during cycling. Here we present a systematic study of nanoconfined NaAlH4 in nanoporous scaffolds with increasing surface area and pore volume and almost constant pore sizes in the range of 8 to 11 nm. A resorcinol formaldehyde carbon aerogel was CO2-activated under different conditions and provided aerogels with BET surface areas of 704, 1267 and 2246 m(2) g(-1) and total pore volumes of 0.91, 1.30 and 2.21 mL g(-1), respectively. Nanoconfinement of NaAlH4 was achieved by melt infiltration and (27)Al MAS NMR reveals that the respective scaffolds incorporate 68, 82 and 91 wt% NaAlH4, for the above-mentioned samples, while the remaining fraction decomposes to metallic Al indicating that increasing CO2-activation tends to facilitate the infiltration process. The frequencies for the (23)Na and (27)Al MAS NMR centerband resonances from NaAlH4 vary systematically for the infiltrated samples and are shifted towards higher frequency and become more narrow with increasing degree of CO2 activation of the scaffolds. This new effect is attributed to increasing interactions with conduction electrons from increasingly graphite-/graphene-like scaffolds. The bulk versus nanoconfined ratio of NaAlH4 was investigated using Rietveld refinement, revealing that the majority of added NaAlH4 is confined inside the nanopores. The hydrogen desorption kinetics decreased with increasing surface area and the hydrogen storage capacity is more stable and decreases less during continuous hydrogen release and uptake cycles. In fact, the available amount of hydrogen (2.7 wt% H2) was more than doubled compared to the nanoconfinement in the non-activated carbon aerogel (1.3 wt% H2). Furthermore, it was demonstrated that Ti-functionalization of the CO2-activated aerogels combines the high storage capacity with fast hydrogen release kinetics from NaAlH4 which fully decomposes into Na3AlH6 at T ≤ 100 °C.

New families of carbon gels based on natural resources

Carbon 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.

Nanoconfined 2LiBH4–MgH2–TiCl3 in carbon aerogel scaffold for reversible hydrogen storage

Nanoconfinement of 2LiBH4–MgH2–TiCl3 in resorcinol–formaldehyde carbon aerogel scaffold (RF–CAS) for reversible hydrogen storage applications is proposed. RF–CAS is encapsulated with approximately 1.6 wt. % TiCl3 by solution impregnation technique, and it is further nanoconfined with bulk 2LiBH4–MgH2 via melt infiltration. Faster dehydrogenation kinetics is obtained after TiCl3 impregnation, for example, nanoconfined 2LiBH4–MgH2–TiCl3 requires ∼1 and 4.5 h, respectively, to release 95% of the total hydrogen content during the 1st and 2nd cycles, while nanoconfined 2LiBH4–MgH2 (∼2.5 and 7 h, respectively) and bulk material (∼23 and 22 h, respectively) take considerably longer. Moreover, 95–98.6% of the theoretical H2 storage capacity (3.6–3.75 wt. % H2) is reproduced after four hydrogen release and uptake cycles of the nanoconfined 2LiBH4–MgH2–TiCl3. The reversibility of this hydrogen storage material is confirmed by the formation of LiBH4 and MgH2 after rehydrogenation using FTIR and SR-PXD techniques, respectively.

Nanoconfined NaAlH4: Determination of Distinct Prolific Effects from Pore Size, Crystallite Size, and Surface Interactions

Nanoconfinement is a new method to improve the hydrogen storage properties for metal hydrides. A systematic study of melt-infiltrated NaAlH4 in resorcinol formaldehyde carbon aerogels with pore sizes of Dmax = 4, 7, 10, 13, 19, 22, 26, 39 and >100 nm is presented. A linear correlation between pore size and crystalline domain size is observed using PXD. The distinct effects from pore size, crystallite size, and interfacial contact between NaAlH4 and carbon aerogel on hydrogen release and uptake properties are investigated. In situ synchrotron powder X-ray diffraction shows that formation of crystalline NaAlH4 nanoparticles only occurs in the nanoporous scaffolds (4 100 nm) as compared to bulk NaAlH4, i.e., reduction of the temperature for maximum hydrogen release rate of ΔTmax = −90 °C. Additional improvement is induced by reducing the pore sizes in the range 7 ≤ Dmax ≤ 39 nm and...

Nanoconfined 2LiBH4–MgH2 Prepared by Direct Melt Infiltration into Nanoporous Materials

Nanoconfined 2LiBH(4)-MgH2 is prepared by direct melt infiltration of bulk 2LiBH(4)-MgH2 into an inert nanoporous resorcinol-formaldehyde carbon aerogel scaffold material. Scanning electron microscopy (SEM) micrographs and energy dispersive X-ray spectroscopy (EDS) mapping reveal homogeneous dispersion of Mg (from MgH2) and B (from LiBH4) inside the carbon aerogel scaffold. Moreover, nanoconfinement of LiBH4 in the carbon aerogel scaffold is confirmed by differential scanning calorimetry (DSC). The hydrogen desorption kinetics of the nanoconfined 2LiBH(4)-MgH2 is significantly improved as compared to bulk 2LiBH(4)-MgH2. For instance, the nanoconfined 2LiBH(4)-MgH2 releases 90% of the total hydrogen storage capacity within 90 mm, whereas the bulk material releases only 34% (at T = 425 degrees C and p(H-2) = 3.4 bar). A reversible gravimetric hydrogen storage capacity of 10.8 wt % H-2, calculated with respect to the metal hydride content, is preserved over four hydrogen release and uptake cycles. (Less)

Improved Hydrogen Storage Kinetics of Nanoconfined NaAlH4 Catalyzed with TiCl3 Nanoparticles

Nanoparticles of NaAlH(4) have been infiltrated in nanoporous carbon aerogel with TiCl(3) nanoparticles in order to explore possible synergetic effects between nanoconfinement and a functionalized catalytic scaffold. Resorcinol formaldehyde carbon aerogels with an average pore size of 17 nm and total pore volume of 1.26 mL/g were infiltrated with TiCl(3) to obtain an aerogel doped with 3.0 wt % TiCl(3) nanoparticles. NaAlH(4) was melt-infiltrated into the functionalized carbon aerogel at 189 °C and p(H(2)) ∼ 186-199 bar. Energy-dispersive spectrometry (EDS) combined with focused ion beam (FIB) techniques revealed the presence of Na, Al, Ti, and Cl inside the aerogel scaffold material. The infiltrated NaAlH(4) was X-ray amorphous, whereas (27)Al magic-angle spinning (MAS) NMR spectroscopy confirmed the presence of nanoconfined NaAlH(4). Temperature-programmed desorption mass spectroscopy (TPD-MS) and Sieverts' measurements demonstrated significantly improved hydrogen desorption kinetics for this new nanoconfined NaAlH(4)-TiCl(3) material as compared to nanoconfined NaAlH(4) without the catalysts TiCl(3) and to bulk ball-milled samples of NaAlH(4)-TiCl(3). We find that the onset temperature for hydrogen release was close to room temperature (T(onset) = 33 °C), and the hydrogen release rate reached a maximum value at 125 °C, which demonstrates favorable synergetic effects between nanoconfinement and catalyst addition.

Confinement of MgH2Nanoclusters within Nanoporous Aerogel Scaffold Materials

Nanoparticles of magnesium hydride were embedded in nanoporous carbon aerogel scaffold materials in order to explore the kinetic properties of hydrogen uptake and release. A new modified procedure for the synthesis of magnesium hydride nanoparticles is presented. The procedure makes use of monoliths (approximately 0.4 cm(3)) of two distinct types of nanoporous resorcinol-formaldehyde carbon aerogels loaded with dibutylmagnesium, MgBu(2). Excess MgBu(2) was removed mechanically, and the increase in mass was used as a measure of the amount of embedded MgH(2). Energy-dispersive spectrometry revealed that MgH(2) was uniformly distributed within the aerogel material. In situ synchrotron radiation powder X-ray diffraction showed that MgBu(2) transformed directly to MgH(2) at T approximately 137 degrees C and p(H(2)) = 50 bar. Two distinct aerogel samples, denoted X1 and X2, with pore volumes of 1.27 and 0.65 mL/g and average pore sizes of 22 and 7 nm, respectively, were selected. In these samples, the uptake of magnesium hydride was found to be proportional to the pore volume, and aerogels X1 and X2 incorporated 18.2 and 10.0 wt % of MgH(2), respectively. For the two samples, the volumetric MgH(2) uptake was similar, approximately 12 vol %. The hydrogen storage properties of nanoconfined MgH(2) were studied by Sieverts' measurements and thermal desorption spectroscopy, which clearly demonstrated that the dehydrogenation kinetics of the confined hydride depends on the pore size distribution of the scaffold material; that is, smaller pores mediated faster desorption rates possibly due to a size reduction of the confined magnesium hydride.

The control of porosity at nano scale in resorcinol formaldehyde carbon aerogels

Organic aerogels were synthesized by sol–gel polymerization of resorcinol (R) with formaldehyde (F) catalyzed by sodium carbonate (C) followed by vacuum drying. The influence of the resorcinol/sodium carbonate ratio (R/C) on the porous structure of the resultant aerogels was investigated. The nitrogen adsorption–desorption measurements show that the aerogels possess a well developed porous structure and mesoporosity was found to increase with increasing the R/C ratio. Carbon aerogels were obtained by carbonization of RF aerogels. The carbonization temperature impacts the microstructure of the aerogels by pore transformations during carbonization probably due to the formation of micropores and shrinkage of the gel structure. The results showed that a temperature of 1073 K is more effective in the development of the pore structure of the gel. Activated carbon aerogels were obtained from the CO2 activation of carbon aerogels. Activation results in an increase in the number of both micropores and mesopores, indicative of pore creation in the structure of the carbon. Activation at higher temperatures results in a higher degree of burn off and increases the pore volume and the surface area remarkably without change of the basic porous structure, pore size, and pore size distribution.

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.