13C CPMAS NMR Research Articles

Naked-Eye Heterogeneous Sensing of Fluoride Ions by Co-Polymeric Nanosponge Systems Comprising Aromatic-Imide-Functionalized Nanocellulose and Branched Polyethyleneimine.

Heterogeneous colorimetric sensors for fluoride ions were obtained by cross-linking TEMPO-oxidized cellulose nanofibers (TOCNF) with chemically modified branched polyethyleneimine 25 kDa (bPEI). Functionalization of bPEI primary amino groups with aromatic anhydrides led to the formation of the corresponding mono- and bis-imides on the grafted polymers (f-bPEI). A microwave-assisted procedure allowed the optimization of the synthetic protocol by reducing reaction time from 17 h to 30 minutes. Hydrogels obtained by mixing different ratios of TOCNF, bPEI and f-bPEI were lyophilized and thermally treated at about 100 °C to promote the formation of amide bonds between the amino groups of poly-cationic polymers and the carboxylic groups of cellulose nanofibers. This approach generated a series of cellulose nanosponges S1-S3 which were characterized by FT-IR and by solid state 13 C CPMAS NMR. These sponge materials can act as colorimetric sensors for the selective naked-eye recognition of fluoride ions over chloride, phosphate and acetate ions at concentrations of up to 0.05 M in DMSO. Moreover, when the sponges were functionalized with perylene tetracarboxylic diimide, successful naked-eye detection was achieved with only 0.02 % w/w of chromophore units per gram of material.

3-Arm PEG Based Amphiphilic Polymer Sorbents for Polar and Non-Polar Liquids

Crosslinked polymer gels are gaining technological and industrial attention due to their sophisticated application features. Herein, a highly effective amphiphilic polymeric network capable of absorbing polar and nonpolar solvents was produced using hydrophilic glycerol ethoxylate, a member of star type polyethylene glycol (s-PEG), and different hydrophobic organosilane monomers by bulk polymerization as utilized under an inert atmosphere. In this context, the preparation of amphiphilic polymeric networks was achieved by the reaction between hydroxyl terminated s-PEG and ethoxysilane terminated organosilane without using any initiator, activator or a catalyst. SEM, FTIR, 13C and 29Si CPMAS NMR and TGA are used for morphological, structural and thermal characterization of the prepared materials. Likewise, the absorption behavior of amphiphilic polymeric networks was determined by measuring their swelling ratios in polar and nonpolar solvents. Because of having a hydrophobic and hydrophilic structure in their 3D polymeric structure, the obtained polymeric networks behave as an organogel in oil medium, while they can prepared as a hydrogel in aqueous medium. The effect of crosslinker’s difference on the swelling capacity of sorbent were researched by comparing different molecular weight of monomers, and it was found that the molecular structure of organosilane monomers had significant influence on the swelling ability of obtained amphiphilic sorbents. By the use of easy preparations, not needing extra agents for the synthesis, having high and quick absorption abilities in organic solvents showed that the resulting amphiphilic sorbent have potential as absorbents for different organic solvents. Also, the degradable features of amphiphilic polymer networks in water open up optimum application opportunities for additional uses, such as with drug delivery agents.

Characterization of Handmade Papers (13th–15th century) from Camerino and Fabriano (Marche, Italy)

The dispute over the paternity of western paper is a longstanding problem. In Italy, a country that has a long tradition of papermaking, many cities compete for the leadership in the production of this material; on the other hand, several studies attribute to Fabriano (13th century), a little center in Marche region, the birth of modern paper and its diffusion throughout Europe. During the 14thcentury, the nearby town of Camerino imposed itself on the paper market, triggering a historic rivalry with the manufacture of Fabriano. As scientific studies on these different productions are missing, a comparative, multidisciplinary investigation has been carried out to characterize a set of thirteenth-fifteenth century handmade papers from Camerino-Fabriano area. Several analytical techniques–some non-invasive such as Fourier-transform infrared spectroscopy (FTIR) and portable magnetic resonance (unilateral NMR), joined to micro-destructive ones such as scanning electron microscopy (SEM), 13C CPMAS NMR spectroscopy, thermo-gravimetric analysis (TGA), sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and reverse-phase liquid chromatography (RP-HPLC)–have been employed in order to study the morphology of the paper documents, to identify the substances used in production process and to define their preservation state. Moreover principal component analysis (PCA) has been applied to the spectroscopic data, allowing the clustering of Fabriano documents with respect to Camerino ones and dividing the samples in two sets. This work improves and confirms knowledges on paper production in Marche region, and the experimental data collected highlight similarities and differences between Camerino and Fabriano manufactures.

Recyclable Mesoporous Organosilica Nanoparticles Derived from Proline-Valinol Amides for Asymmetric Organocatalysis

Going a step further from bulk organosilicas to nanosized materials, we describe herein the preparation of mesoporous organosilica nanoparticles derived from mono- and bis-silylated proline-valinol amides, both by grafting on preformed mesoporous silica nanoparticles (MSN) and by a co-condensation method in neutral medium using Brij-56/CTAB as templates. This is the first report on the obtention of functionalized MSN by a co-condensation procedure with a structurally complex chiral precursor. The functionalized MSN have been characterized by elemental analysis, 29Si and 13C CP MAS NMR, transmission electron microscopy, scanning electron microscopy, N2-sorption measurements, dynamic light scattering, ζ-potential, and powder X-ray diffraction. We have evaluated the activity of these materials as recyclable catalysts in the asymmetric aldol reaction. The best organocatalysts are those derived from the monosilylated precursor, observing good diastereo- and enantiomeric ratios with a simple and environmentally friendly optimized protocol (water, 0 °C, absence of cocatalyst). The nanocatalyst was easily recovered by centrifugation and recycled up to five runs without loss of activity and selectivity. The use of organosilica nanoparticles reduces the problems of diffusion and low reaction rates encountered with bulk organosilicas.

Pd-chelated 1,3,5-triazine organosilica as an active catalyst for Suzuki and Heck reactions

Design of novel functional periodic mesoporous organosilica with molecular level control finds applications in several fields of energy and environmental research including adsorption, catalysis, nanotechnology, and energy harvesting. Herein we present a melamine (1,3,5-triazine) functionalized periodic mesoporous silica (MPMO) by self-assembly of N2,N4,N6-tris(3-(triethoxysilyl)propyl)-1,3,5-triazine-2,4,6-triamine (TTET) with tetraethylorthosilicate (TEOS) via cocondensation strategy. The TTET silsesquioxane precursor was synthesized by the condensation reaction between cyanuric chloride and 3-aminopropyl triethoxysilane. The resultant MPMO material serves as an effective solid chelating agent through amine and triazine functionalities for Pd(II) to provide Pd-MPMO. The Pd-MPMO material was thoroughly characterized by a small-angle XRD, HRTEM, N2 sorption, 13C CP-MAS NMR, 29Si CP-MAS NMR, and ICP analyses. The Pd-MPMO serves as an active catalyst for CC bond formation reactions by Suzuki- and Heck cross-coupling methodologies under ligand- and cocatalyst-free conditions. Notably, the present catalytic protocol demonstrates a wide spectrum of substrate scope towards Suzuki coupling between aryl halides (I−, Br−, Cl−) and aryl boronic acids with high turn-over-number (TON) in aqueous phase under air ambience. Whereas for Heck-coupling reaction, the phenyl iodides furnished high TON than the other aryl halides. Investigation of Pd-leaching by a hot filtration test as well as reusability experiments confirms the true heterogeneous nature of present Pd-MPMO and its robustness in terms of substrate scope, catalyst stability, and durability.

Evaluation of molecular properties of humic acids from vermicompost by 13 C-CPMAS-NMR spectroscopy and thermochemolysis–GC–MS

The molecular characteristics of different vermicomposts, produced with variable rates of cattle manure and cotton residues, and of related humic acids were determined by 13C-CPMAS NMR and Off-line pyrolysis THM-GC–MS. The analyses highlighted a composition of mature vermicomposts based on differential amounts of lignocellulosic fraction and alkyl compounds inherited from original biomasses, thus revealing a decrease of overall hydrophobic character at increasing amounts of cotton input. The evaluation of molecular parameters derived from both solid state NMR spectra and from the specific biomarkers released by thermochemolysis suggested that the humification process was mainly characterized by the selective preservation of specific lignin derivatives. The potential bioactive properties of humic acids were hence evaluated in a bioassays test on maize seedlings. The humic extracts showed a differential response in root growth-promoting effect, depending on the specific molecular features. The higher bioactivity found in humic components isolated from vermicompost with no or low addition of cotton residues, supported the role of the overall hydrophobic character and of the large content of bioavailable lignin units and nitrogen moieties. These results confirm that the combination of solid state NMR and thermochemolysis are valuable and efficient tools to assess the molecular quality of natural organic matter. The detailed molecular characterization of compost materials may hence represent a useful requirement to select the most suitable application of bulk biomasses and derived fractions for soil organic matter managements.

Predicting nitrogen mineralization from organic amendments: beyond C/N ratio by 13C-CPMAS NMR approach

The processes of nitrogen (N) mineralization from organic amendments (OA) is important to provide mineral forms (NH4+ and NO3−) of N for plants. Assessment of N mineralization is usually done by using the C/N ratio of OAs but limits to the predicting capability of such indicator have been reported. Here, we propose a new definition of organic carbon (C) quality based on 13C-CPMAS NMR that provide a more accurate prediction of N mineralization. A mesocosm incubation experiment was carried out to assess the effects of ten OA types on soil N mineralization in three different soils. OAs were chemically characterized by C/N ratio and in solid state spectroscopy by 13C-CPMAS NMR. Nitrogen mineralization in terms of NH4+ and NO3− was monitored after 3, 10, 30, 100, and 300 days of incubation. N mineralization was fast for OAs with high C quality coupled with high N content (e.g., meat powder, fish meal and alfalfa litter), while the same process was slow for amendment with low C quality even with high N content (e.g. humus, alfalfa biochar). On the contrary, a rapid but short-term N immobilization was found for OAs with high C quality but low N content (e.g., glucose). OAs with low C quality and low N content (e.g., sawdust, cellulose, wood biochar and grass litter) showed a slow, but long-lasting N immobilization. We found that C/N ratio was unable to predict the N mineralization for OAs with low C quality. Considering 13C-CPMAS NMR spectral regions, the carbonyl C, and N-alkyl and methoxyl C regions had the most significant positive correlation with N mineralization, while the di-O-alkyl C and O-alkyl C were strongly associated with N immobilization. This study demonstrates that the biochemical quality of organic C defined by 13C-CPMAS NMR is capable of predicting N dynamic pattern better than C/N ratio.

Composition and turnover time of organic matter in soil fractions with different magnetic susceptibility

The composition and turnover time (TT) of organic matter in soil fractions with different magnetic susceptibility were studied in a tropical primary forest in Ghana. The starting hypothesis was that soil organic matter (SOM) composition and properties depend on the magnetic susceptibility of the minerals SOM is associated with. Soil samples from 0 to 5, 5–15, 15–30, and 30–50 cm depth intervals were sieved to remove rock fragments (>2.0 mm) and then separated into two size fractions (0.5–2.0 mm and <0.5 mm) that were processed by a High Gradient Magnetic Separator (HGMS) to finally obtain four fractions with different size and/or magnetic susceptibility. All fractions were analysed for their mineral composition, 14C concentration, and spectroscopic properties of SOM (13C CPMAS NMR).From a mineralogical point of view, the magnetic (MA) fractions essentially differed from the non-magnetic (NM) ones for a higher presence of oxides and kaolinite, which per se is non-magnetic. In terms of chemical composition of SOM, the MA fractions showed higher contribution of labile compounds than the NM ones. At all depths, the 14C concentration revealed shortest TT of SOM in the <0.5 mm MA fraction and longest TT in the <0.5 mm NM fraction, while the 0.5–2.0 mm fractions showed intermediate and similar TT's. At depths <5 cm, the fine NM fraction was not significantly influenced by the carbon fixed after the 1950s (“bomb carbon”), having TT of almost 1000 years and suggesting that in this tropical soil some SOM can be stabilized also in relatively superficial horizons.Our findings support the hypothesis that minerals are driving factors of the fate of SOM. As a consequence, soil fractionation based on magnetic susceptibility might be a meaningful procedure for having insight into SOM dynamics.

Amine functionalized mesoporous hybrid materials: influence of KCl and xylene on the textural characteristics and CO2 sorption

Amine-functionalized mesoporous silica materials were prepared by sol–gel method in the system tetraethylortosilicate: bis[(3- trimethoxysilyl)propyl]amine (TEOS: BTPA). The hybrid amine functionalized silica materials were synthesized by co-condensation reaction between TEOS and BTPA precursors in acidic media. Soft template approach for pore formation was applied and as a structural directing agent was used surfactant Pluronic P123. For improving of pore ordering and textural characteristics xylene and KCl were used and their influence on the structural and morphological characteristics was investigated by FTIR, 13C CP MAS NMR, 29Si MAS NMR, BET, PSD and XRD techniques. CO2 adsorption properties of the synthesized amine functionalized hybrid materials were investigated. The simultaneous presence of xylene and KCl leads to improvement of the pore ordering, increasing of the textural characteristics values and formation of cylindrical pores. The determined heats of CO2 adsorption evidenced chemisorption process between CO2 and the amine groups of the hybrid materials.

Solid-state 1H and 13C NMR of corn starch plasticized with glycerol and urea

1H and 13C NMR spectroscopy was used for the study of corn starch plasticized with urea and/or glycerol. The information on the interactions between plasticizers and starch macromolecules as well as the arrangement of starch macromolecules depending on the storage time (from 1 day to 52 weeks), were obtained from the broad-line and magic-angle spinning 1H NMR spectra measured at 22 °C and within the range from 30 to 140 °C and the cross-polarization magic-angle spinning (CP MAS) 13C NMR spectra measured at 30 °C. The 1H chemical shifts for the groups of the plasticizer molecules enabled characterization of the interactions between urea and glycerol molecules in starch plasticized with a mixture of urea and glycerol. With increasing temperature, the signal related to water molecules gradually vanished due to water evaporation, and resolved 1H peaks associated with starch macromolecules were detected above 100 °C. The free water amount in the thermoplastic starch samples was also estimated using MAS 1H NMR spectra. Formation of double helices due to sample ageing was observed through the CP MAS 13C NMR spectra.

Facile preparation of functionalized poly(amidoamine)s with biocidal activity on wood substrates

Linear poly(amidoamine)s (PAAs), functionalized in the main chain with hydroxyl and/or siloxane groups, were obtained by single step addition of primary amines (ethanolamine, 3-aminopropyltriethoxysilane) to N,N’-methylenebisacrylamide, in water or low molecular weight alcohols, without catalysts. The polymers were characterized by FTIR, Raman, 1H and 13C NMR, CP-MAS NMR and ESI-MS. The coordination ability of PAAs towards Cu2+ in water was confirmed by the Job’s plot method.The PAAs alone and added with Cu2+ ions or Ag-nanoparticles were applied on wood as preservatives for fungi and insects. The siloxane hybrid formulations applied on wood are effective against C. puteana and T. versicolor fungi, even after a leaching procedure, in particular with copper addition.The PAA functionalized with OH groups are effective as wood preservatives before the leaching procedure, in particular with Ag addition. All the formulations give good results against wood-boring beetles T. holosericeus and H. bajulus (Cerambicidae species).

Synthesis and Physical Properties of Proton Conducting Polymer Electrolytes Comprising PAM Cross-Linked Flexible Spacers

The design of novel proton exchange membranes with high conductivity and better dimensional stability has become increasingly important due to the need for applications in different devices. The present work shows the acid-doped and crosslinked polyacrylamide (PAM) networks including flexible spaces. To this end PAM was modified with 1,4-butanediol diglycidyl ether (BG) to form PAM35BG and PAM50BG networks, which would afford more space for protonated solvents. The reaction of PAM with BG was monitored by FTIR and 13C CP-MAS NMR. The polymer electrolytes were produced by acid doping at several stoichiometric ratios with respect to the monomer repeat unit of a host polymer. The resulting materials exhibited better thermal, chemical, and electrochemical stabilities and had distinct Tg values. Additionally, the pores of the PAM-BG materials were filled with H3PO4 to get PAM35BG0.5H3PO4 and PAM50BG1.0H3PO4. The doping enhanced the proton conductivities of the membranes as high as 0.003 S/cm at 120 °C under an anhydrous atmosphere. The proton diffusion mechanism and the dielectric relaxation were further examined using the complex modulus formalism, M*.

African tree bark exudate extracts as biohardeners of fully biosourced thermoset tannin adhesives for wood panels

A totally “bio” hardener derived from the exudate extract of the African trees Vachellia nilotica and Senegalia senegal was investigated. It hardened a maritime pine tannin resin without any obvious presence of an aldehyde thus obtaining a completely “bio” adhesive for wood panels. MALDI-ToF and CP MAS 13C NMR analysis indicated that the hardeners cross-linking species was 2,5-dihydroxymethyl furan linked onto carbohydrate chains. The hydroxymethyl furan moieties reacted both after detachment from the carbohydrate chains but also with carbohydrate chains still attached. The carbohydrates too were then involved as part of the binder. The gel times were slower than what was obtainable with paraformaldehyde but sufficiently fast to possibly be of industrial significance.

Efficient Removal of Arsenic Using Zinc Oxide Nanocrystal-Decorated Regenerated Microfibrillated Cellulose Scaffolds

Regenerated microfibrillated cellulose (R-MFC) fibers were prepared successfully by a combined dissolution and regeneration approach using phosphoric acid/ethanol treatment on jute cellulose. The prepared R-MFC fibers possessed high surface area (10.74 m2/g), good aspect ratio (L/D = 30), and excellent thermal stability (Tmax = 352 °C). In addition, the fibers exhibited 3.84 wt % of phosphate groups (PO42–) with a ζ-potential of −8.4 mV and low crystallinity index (CI) of 47.5%. These R-MFC fibers were in the cellulose II polymorph, confirmed by 13C CPMAS NMR and WAXD measurements, and they were effective to anchor the growth of ZnO nanocrystals. WAXD and TEM examinations on the imbedded ZnO nanocrystals indicated that they possessed the hexagonal wurtzite crystal structure and could assemble into a flower-like morphology in the R-MFC scaffold. A R-MFC composite containing 41 wt % of ZnO nanocrystals was found to be very efficient to remove arsenic (As(V)) ions from water with the maximum capacity of 4,42...

5-Hydroxymethyl furfural modified melamine glyoxal resin

ABSTRACT Glyoxal as a nonvolatile and nontoxic aldehyde, which can be used as a substitution of formaldehyde in wood industry to prepare novel melamine-glyoxal (MG) resins. However, for the reason of the lower reactivity of the glyoxal compared with formaldehyde, the performances of the MG resin is not as good as expected. 5-Hydroxymethyl furfural (HMF) was used as a modifier to improve the properties of melamine-glyoxal (MG) resins to prepare a hydroxymethyl furfural modified melamine-glyoxal (HMFMG) resin. This was tried as a plywood adhesive. Matrix-assisted laser desorption ionization time of flight (MALDI-TOF), Cross Polarisation-Magic Angle Spinning Nuclear Magnetic Resonance (CP-MAS 13C NMR), Differential Scanning Calorimetry (DSC), and thermomechanical analysis (TMA) were used to analyze the structure of the oligomers formed and the thermomechanical properties of the resins. The curing activation energy of HMFMG resin was found to be lower than MG resin, and it also significantly with a better plywood bonding performance and water resistance.

Tautomeric Equilibria in Solutions of 2-Phenacylbenzimidazoles

Detailed NMR spectral analysis of DMSO-d6 solutions of the series of substituted 2-phenacylbenzimidazoles (ketimine form, K) reveals two from three tautomeric forms. Integrals of the 1H NMR signals are used in establishing the molar ratio of tautomers. The experimental analyses are supported by quantum-chemical calculations, which satisfactorily reproduced the experimental trends. Although the reported semiempirical quantum-chemical calculations show that enaminone E, i.e., 2-(1,3-dihydro-2H-benzo[d]imidazol-2-ylidene)-1-phenylethan-1-one, was thermodynamically most stable, the results of MP2 ab initio calculations reveal the following order of stability: ketimine &gt; enolimine &gt; enaminone (substituents do not affect this sequence). 13C CPMAS NMR spectral data reveal that in the crystalline state the enolimine tautomer O is predominant in the p-CH3 and p-NO2 substituted congeners.

Effects of Experimental Warming and CO2 Concentration Doubling on 13C CPMAS NMR Spectra of Humin in Coniferous Forest Ecosystems of the Eastern Tibetan Plateau in China

Effects of Experimental Warming and CO2 Concentration Doubling on 13C CPMAS NMR Spectra of Humin in Coniferous Forest Ecosystems of the Eastern Tibetan Plateau in China

Decomposition of maize stover varies with maize type and stover management strategies: A microcosm study on a Black soil (Mollisol) in northeast China

Crop residue decomposition has an important impact on soil organic carbon (SOC) sequestration and CO2 emission. Residue quality and management strategies are two important factors regulating decomposition process and SOC mineralization and greenhouse gas emission. In this study, a microcosm experiment in field condition was conducted on a silty loam (a Black soil) in Northeast China to investigate stover decomposition and soil CO2 emission characteristics as influenced by different crop cultivars and stover field incorporation methods. Stover from two popular maize cultivars Xianyu335 (XY) and Liangyu99 (LY) were applied in two modes (soil surface application vs soil incorporation) at a rate of 11 t ha−1, and CO2 efflux was monitored during the decomposition duration of 144 days. The structural transformation of carbon functional groups in maize stover were evaluated using solid state 13C-CPMAS NMR and elemental analysis techniques. Results showed that up to 71.7%∼86.9% (weight basis) of C and N in soil-incorporated stover was decomposed during the study period, which was significantly greater than the losses (32.8%∼55.3%) of C or N from the surface-applied stover for both maize cultivars; decomposition rates of main C functional groups were significantly higher in soil incorporation (71.1%∼88.8%) than in surface application (20.9%∼60.2%) systems. The concentrations of SOC, total N, available N, and microbial biomass C and N in soil were also higher with stover incorporation than surface application. Stover incorporation resulted in a notably lower CO2 emission rate and accumulative CO2 efflux (53.9–55.4 mol m−2) during the stover decomposition compared with surface application (57.4–67.0 mol m−2). Between the two maize cultivars, the LY stover showed a higher decomposition rate and greater capacity for SOC sequestration when incorporated into soil. The LY stover induced higher (16.8%) CO2 emission than XY when applied on soil surface, but no significant difference was found between the two cultivars when incorporated into soil. The results suggested that cultivar selection and stover management strategies have great potential in reducing soil CO2 emission while improving soil biochemical properties. Incorporating the LY stover into soil rather than surface mulching could enhance SOC sequestration and reduce CO2 emission.

A hybrid perchlorate with a low band gap: Crystal structure, physicochemical characterization and Hirshfeld surfaces study

The anhydrous hybrid material (C6H10N2) (ClO4)2 has been synthesized by the slow evaporation method. X-ray diffraction on a single crystal shows that this compound crystallizes in the monoclinic system with the centrosymmetric space group P21/c. The 2D supramolecular network is stabilized by NH⋯O interactions whereas the van der Waals contacts play a key role in the consolidation of the 3D packing as verified by Hirshfeld surface analysis in combination with 2D fingerprint plots. The 13C CP-MAS NMR spectrum is in agreement with the X-ray structure. DFT calculations allowed the attribution of the NMR peaks. FT-IR spectrum confirms the presence of various functional groups in the synthesized material. The thermal stability, phase transition and degradation, of this material has been studied using simultaneous TG-DTA. The gap between the frontier molecular orbitals is about 1.53 eV suggesting that the studied compound is a semiconducting material.

CHARACTERIZATION OF SEVERAL GENERATIONS OF NiMo HYDROPROCESSING CATALYSTS EMPLOYED IN THE SAME HYDROTREATER

This work details the chemical characterization of four NiMo/Al2O3 hydroprocessing generation catalysts (1979-1986, 1993-1998, 2003-2008 and 2008-2011) employed to perform hydrodesulfurization (HDS) of diesel fractions in the same hydrotreater of a Brazilian refinery. The basic differences are the quality of the feedstocks and the HDS experimental conditions. Samples were characterized by X-ray fluorescence, X-ray diffraction, 13C CP-MAS NMR, elemental analysis, loss of volatiles, specific surface area and average pore volume. The amount and variety of foreign elements deposited on the catalyst increased in the most recent generations due to the higher amounts of impurities in the processed feedstocks. Coke became more aromatic and loss of textural properties was more prominent in these samples as expected from the more drastic HDS conditions and the quality of the crude diesel. On the other hand, loss of volatiles was lower due to the low carbon content, partial oxidation of sulfur to sulfate species and oxidation of some elements deposited on the catalyst. Coke tended to ignite as it became more aromatic. For this reason, leaching of decoked samples from the most recent generations by sulfuric acid presented very low yields due to the formation of refractory oxides that are insoluble in the leachant.