chemengineering Article Investigation of Thermal Behavior of Layered Double Hydroxides Intercalated with Carboxymethylcellulose Aiming Bio-Carbon Based Nanocomposites Vagner R. Magri 1 , Alfredo Duarte 1, Gustavo F. Perotti 1,2 and Vera R.L. Constantino 1,* 1 Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo (USP), Av. Prof. Lineu Prestes 748, São Paulo CEP 05508-000, SP, Brazil; [email protected] (V.R.M.); [email protected] (A.D.); [email protected] (G.F.P.) 2 Instituto de Ciências Exatas e Tecnologia, Universidade Federal do Amazonas (UFAM), Rua Nossa Senhora do Rosário 3863, Itacoatiara CEP 69103-128, AM, Brazil * Correspondence: [email protected]; Tel.: +55-11-3091-9152 Received: 31 March 2019; Accepted: 27 May 2019; Published: 1 June 2019 Abstract: Carboxymethylcellulose (CMC), a polymer derived from biomass, was intercalated into 2+ 3+ layered double hydroxides (LDH) composed by M /Al (M2Al-CMC, M = Mg or Zn) and evaluated as precursors for the preparation of biocarbon-based nanocomposites by pyrolysis. M2Al-CMC hybrids were obtained by coprecipitation and characterized by X ray diffraction (XRD), vibrational spectroscopies, chemical analysis, and thermal analysis coupled to mass spectrometry. Following, pyrolyzed materials obtained between 500–1000 ◦C were characterized by XRD, Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Above 600 ◦C, Raman spectra of all samples showed the presence of graphitic carbon, which plays a role in the degree of crystallinity of produced inorganic phases (for comparison purposes, M2Al-CO3 materials were investigated after calcination in the same experimental conditions). XRD patterns of Mg2Al-CMC pyrolyzed between 600–1000 ◦C showed poorly crystallized MgO and absence of spinel reflections, whereas for Zn2Al-CMC, it was observed well crystallized nanometric ZnO at 800 ◦C, and ZnAl2O4 and γ-Al2O3 phases at 1000 ◦C. Above 800 ◦C, the carbothermic reaction was noticed, transforming ZnO to zinc vapour. This study opens perspectives for nanocomposites preparation based on carbon and inorganic (mixed) oxides through precursors having organic-inorganic interactions at the nanoscale domain. Keywords: intercalation compounds; layered materials; layered double hydroxides; hydrotalcite; thermal analysis; carboxymethylcellulose; carbon-based nanocomposites 1. Introduction More than forty minerals have been identified as layered double hydroxide (LDH) phases which structures are analogous to brucite mineral, Mg(OH)2 [1]. In nature, partial replacement of magnesium ions by Al3+, Fe3+, Cr3+, Mn3+ or Co3+ produces an excess of positive charges in the structure that are neutralized by simple counter ions such as carbonate, hydroxide, sulphate or chloride. In some minerals, divalent cations like Zn2+, Ni2+, Fe2+ or Mn2+ are present instead of magnesium. These materials are also called as hydrotalcite-type compounds given that the first identified LDH phase was nominated hydrotalcite (due to its textural resemblance with talc) [2]. The chemical composition of LDH materials is usually represented by the general formula 2+ 3+ x+ m x 2+ 3+ [(M (1 x)M x)(OH)2] [A −x/m nH2O] − and denoted as M RM -A, where M is the metal ion, − 2+ 3+ · m 2+ 3+ R is the M /M molar ratio and A − is a hydrated anion [2,3]. The M /M molar ratio in the minerals is usually 3 or 2 as for example in hydrotalcite ([Mg Al (OH) ]CO 4H O) and quintinite 6 2 16 3· 2 ChemEngineering 2019, 3, 55; doi:10.3390/chemengineering3020055 www.mdpi.com/journal/chemengineering ChemEngineering 2019, 3, 55 22 of of 17 usually 3 or 2 as for example in hydrotalcite ([Mg6Al2(OH)16]CO3·4H2O) and quintinite ([Mg Al (OH) ]CO 3H O), respectively [1]. In LDH structure, hydroxide ions are in a hexagonal ([Mg4Al22(OH)1212]CO3·3H3· 2O),2 respectively [1]. In LDH structure, hydroxide ions are in a hexagonal close-packed arrangement arrangement in in two two adjacent adjacent planes planes in in such such a way a way that that octahedral octahedral sites sites are are occupied occupied by metalby metal cations cations (Figure (Figure 1a).1a). The The octahedral octahedral are are edge-sharing edge-sharing along along the the crystallographic crystallographic axes axes a anda and b, formingb, forming two-dimensional two-dimensional (2D) (2D) structures structures as shown as shown in Figure in Figure 1b [3].1b Positive [ 3]. Positive layers layers are stacked are stacked along thealong c-axis the andc-axis the and anions the anions are intercalated are intercalated between between them [3]. them Different [3]. Di fflayerserent layersstacking stacking of the ofsame the materialsame material can produce can produce LDH LDH structures structures with with different different dimensions dimensions along along c cdirection,direction, i.e., i.e., distinct polytypes: 3R (rhombohedral symmetry) or 2H (hexagonal symmetry) [[3].3]. Figure 1. Schematic representation of (a) hydroxide ions coordinated to metal cation in an octahedral Figurearrangement, 1. Schematic (b) layered representation double of hydroxide (a) hydroxide (LDH) ions structure coordinated comprising to metal stacked cation in positive an octahedral layers arrangementintercalated by, negative(b) layered ions, double (c) monomeric hydroxide unit (LDH) of carboxymethylcellulose structure comprising (CMC).stacked positive layers intercalated by negative ions, (c) monomeric unit of carboxymethylcellulose (CMC). From the 1970s, synthetic materials similar to LDH minerals have been prepared in a laboratory andFrom characterized the 1970s, by synthetic increasingly materi advancedals similar physicochemical to LDH minerals techniques have been prepared [4]. Such in materials a laboratory are andvery characterized versatile in terms by increasingly of the chemical advanced composition physicoc ofhemical layers, techniques as well as M[4].2+ /SuchM3+ materialsmolar ratio, are and very a versatilegreat number in terms of combinations of the chemical with composition different anions of layers, can beas well employed as M2+ in/M LDH3+ molar synthesis. ratio, and Instead a great of numbersimple anions, of combinations LDH hybrid with materials different can anions be synthesized can be employed by the intercalationin LDH synthesis. of polyoxometalates Instead of simple [5], anions,anionic metalLDH hybrid complexes materials [6] and can organic be synthesized species like by drugs, the dyes,intercalation amino acids,of polyoxometalates synthetic polymers [5], anionicand biopolymers metal complexes [4,7,8]. [6] Owing and toorganic their compositionalspecies like drugs, versatility, dyes, amino a variety acids, of LDHsynthetic with polymers different andphysical-chemical biopolymers [4,7,8]. properties Owing can to be their obtained compositio for diversenal versatility, applications, a variety ranging of fromLDH heterogeneouswith different physical-chemicalcatalysis [9], environmental properties remediation can be obtained [10], for energy diverse production applications, [11], drugranging carrier from and heterogeneous diagnosis in catalysismedicine [9], [12 ]environmental up to nanoreactors remediation for prebiotic [10], lifeenergy [13]. production [11], drug carrier and diagnosis in medicineLDH [12] organic-inorganic up to nanoreactors hybrid for prebiotic materials life [13]. have significant potential as precursors for nanocompositesLDH organic-inorganic based on mixed hybrid metal materials oxides (MMO)have significant and carbonaceous potential species as precursors (C). Through for 2+ 2 nanocompositescalcination, LDH based can be on converted mixed intometal oxides oxides (MO (MMO)x) and spinelsand carbonaceous (M1M2O4) which species display (C). MThrough–O − 3+ 2 2+ 2− calcination,and M –O LDH− acid-basic can be sites, converted high specific into oxides surface (MO area,x) and well spinels dispersed (M metals1M2O4)and which fine display tuning interfaceM –O andof components M3+–O2− acid-basic species suitablesites, high for adsorptionspecific surface and/ orarea, heterogeneous well dispersed catalysis metals in and a wide fine range tuning of interfacereactions [of14 –components16]. Some studies species have suitable reported for theadso preparationrption and/or of MMO heterogeneous/C composites catalysis by the bottom-upin a wide rangemethod of wherereactions an [14–16]. organic Some source studies is carbonized have repo withinrted the the preparation confined space of MMO/C between composites LDH layers by [ the17]. bottom-upThis approach method reduces where the numberan organic of synthetic source is steps carbonized compared within to the the physical confined mixture space of between pre-prepared LDH layersLDH and [17]. nanocarbon This approach form reduces (e.g., carbon the number nanotubes of synthetic or graphene steps compared oxide), allowing to the physical isolation mixture of porous of pre-preparedcarbon and yielding LDH moreand nanocarbon homogeneous form materials (e.g., atca submicroscale,rbon nanotubes fostering or graphene synergistic oxide), effects. allowing Metal isolationoxides (or of hydroxides) porous carbon and carbonaceousand yielding materialsmore homogeneous show exciting materials adsorption at submicroscale, and catalytic properties fostering synergisticsuitable for applicationseffects. Metal in environmentaloxides (or hydroxides) remediation and or developmentcarbonaceous of sensorsmaterials [17 ].show exciting adsorptionIn this study,and catalytic LDH materials properties composed suitable by for