chemengineering

Article Investigation of Thermal Behavior of Layered Double Hydroxides Intercalated with Carboxymethylcellulose Aiming Bio- 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) 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 mineral, Mg(OH)2 [1]. In nature, partial replacement of 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 ) [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 , − 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 ), 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 zinc, applications magnesium in and environmental aluminum cations remediation intercalated or withdevelopment carboxymethylcellulose of sensors [17]. (CMC) polymer were chosen to investigate the thermal preparation of MMOIn/C this composites. study, LDH CMC materials is a water-soluble composed anionicby zinc, polymer magnesium obtained and aluminum through chemical cations modificationintercalated withof cellulose carboxymethylcellulose polysaccharide, the (CMC) most abundantpolymer were natural chosen polymer to investigate [18]. Such the polymer thermal is constituted preparation by of a MMO/Cchain of glucosecomposites. rings connectedCMC is bya βwater-soluble-1,4 glycosidic anionic linkages betweenpolymer C(1)obtained and C(4) through of adjacent chemical units, withmodification a carboxymethyl-substituted of cellulose polysaccharide, group mainly the most at C(6)abundant position natural (Figure polymer1c). As [18]. a biomass Such derivative,polymer is constituted by a chain of glucose rings connected by β-1,4 glycosidic linkages between C(1) and C(4) ChemEngineering 2019, 3, 55 3 of 17

ChemEngineering 2019, 3, 55 3 of 17 of adjacent units, with a carboxymethyl-substituted group mainly at C(6) position (Figure 1c). As a biomass derivative, CMC is a low-cost, abundant and sustainable source for biocarbon production CMCand, as is an a low-cost, anionic polyelectrolyte, abundant and sustainablecan be intercalated source for into biocarbon LDH structure. production Small and, organic as an anionicspecies polyelectrolyte,intercalated into can LDH be intercalatedcan be volatilized into LDH even structure. under pyrolytic Small organic conditions species precluding intercalated the into carbon LDH canformation. be volatilized The usage even of underpolymeric pyrolytic organic conditions species increases precluding the thepossibility carbon of formation. carbon production The usage in of a polymericnon-oxidizing organic atmosphere. species increases the possibility of carbon production in a non-oxidizing atmosphere. Few studies were reported about systems comprisingcomprising LDH and CMC but none of them focused on the preparation ofof MMOMMO/C/C nanocomposites.nanocomposites. MgMg1.151.15Al-CMCAl-CMC material material was prepared by exfoliationexfoliation of nitrate-LDH in formamide and posterior restacking in a CMC solution also in the organic solvent [[19].19]. Later M2Al-CMC (M = Mg,Mg, Zn, Zn, Cu, Ni) materials were isolated by coprecipitationcoprecipitation method [[20,21].20,21]. In thisthis work,work, compositecomposite materials based on MMOMMO/C/C were prepared atat fourfour temperaturetemperature valuesvalues 2+2+ 22++ (500,(500, 600,600, 800800 andand 10001000◦ °C)C) throughthrough pyrolysis pyrolysis of of M M2Al-LDH2Al-LDH (M (M= =Mg Mg oror ZnZn ) intercalated withwith CMC, asas representedrepresented in in Figure Figure2. 2. LDH LDH materials materials constituted constituted by by Mg Mg2+/2+Al/Al3+3+and and Zn Zn2+2+//AlAl33++ werewere chosen owing toto dissimilar dissimilar characteristics characteristics as thermalas thermal stability stability [22] and[22] topological and topological transformation transformation from LDH from to MMOLDH to [23 MMO,24]. Otherwise,[23,24]. Otherwise, for comparison for comparison purposes, purposes, LDH materials LDH containingmaterials containing intercalated intercalated carbonate insteadcarbonate of instead CMC were of CMC also preparedwere also toprepared investigate to investigate the probable the influence probable ofinfluence CMC on of the CMC inorganic on the phasesinorganic achieved phases inachieved the pyrolysis in the pyrolysis process. process. Nanocomposites Nanocomposites comprising comprisi of graphiticng of graphitic carbon, carbon, metal oxidesmetal oxides and spinel and phasesspinel phases were produced were produced within thewithin 600–1000 the 600–1000◦C range. °C For range. the LDHFor the hybrid LDH material hybrid 2+ 2+ containingmaterial containing Zn ions, Zn ZnO ions, nanocrystals ZnO nanocrystals were observed were observed when pyrolysis when pyrolysis was performed was performed at 800 ◦ C;at zinc800 °C; oxide zinc underwent oxide underwent a carbothermic a carbothermic reaction reaction at higher at higher temperatures. temperatures. Whereas, Whereas, XRD XRD pattern pattern of Mgof Mg2Al-CMC2Al-CMC exhibited exhibited only only MgO MgO phase phase within within the 600–1000the 600–1000◦C range, °C range, indicating indicating that pyrolyzed that pyrolyzed CMC hinderedCMC hindered the spinel the spinel phase phase crystallization. crystallization.

2+ (Mg2+ or Zn2+) 3+ =M =Al Coprecipitation Pyrolysis Nanocomposite: at pH constant N2 metal oxide + spinel + nanocarbon

c b =Na+ = -COO- a Carboxymethylcellulose (CMC) Layered Double Hydroxide Precursor hybrid material: M Al-CMC 2 Figure 2. Schematic representation of the two-step synthetic approach used to produce MMO/carbon Figure 2. Schematic representation of the two-step synthetic approach used to produce MMO/carbon nanocomposites from layered double hydroxides. nanocomposites from layered double hydroxides. 2. Materials and Methods 2. Materials and Methods 2.1. Chemicals 2.1. ChemicalsMagnesium nitrate hexahydrate (Mg(NO ) 6H O, Merck), aluminum nitrate nonahydrate 3 2· 2 (Al(NOMagnesium3)3 9H2O, nitrate Merck), hexahydrate zinc nitrate (Mg(NO hexahydrate3)26H2O, (Zn(NOMerck), 3)aluminum2 6H2O, Merck),nitrate nonahydrate magnesium · · chloride(Al(NO3) hexahydrate39H2O, Merck), (MgCl zinc2 6H nitr2O,ate Sigma-Aldrich), hexahydrate (Zn(NO aluminum3)26H chloride2O, Merck), hexahydrate magnesium (AlCl chloride3 6H2O, · · Sigma-Aldrich),hexahydrate (MgCl zinc2 chloride6H2O, (ZnClSigma-Aldrich),2, Sigma-Aldrich), aluminum sodium chloride hydroxide hexahydrate (NaOH, Sigma-Aldrich), (AlCl36H2O, sodiumSigma-Aldrich), carbonate zinc (Na chloride2CO3, Merck),(ZnCl2, Sigma-Aldrich), sodium carboxymethylcellulose—CMC sodium hydroxide (NaOH, (90 kDa,Sigma-Aldrich), Degree of substitutionsodium carbonate 0.7, Sigma-Aldrich) (Na2CO3, Merck), were used sodium without carboxymethylcel further purification.lulose—CMC Deionized (90 water kDa, was Degree used inof allsubstitution experimental 0.7, procedures.Sigma-Aldrich) were used without further purification. Deionized water was used in all experimental procedures. 2.2. Preparation of LDH-CMC Hybrid Materials 2.2. PreparationThe intercalation of LDH-CMC of CMC Hybrid polymer Materials into LDH was carried out by co-precipitation method and the best experimentalThe intercalation conditions of CMC are polymer described into ahead. LDH was For thecarried preparation out by co-precipitation of the Mg2Al-CMC method sample, and 3.070the best g of experimental CMC was solubilized conditions into are 250 desc mL ofribed deionized ahead. waterFor the and preparation the pH value of of the the solutionMg2Al-CMC was adjusted to 9.5 by the addition of NaOH 0.2 mol L 1 solution. Afterwards, 250 mL of an aqueous sample, 3.070 g of CMC was solubilized into 250· mL− of deionized water and the pH value of the ChemEngineering 2019, 3, 55 4 of 17 solution containing 16.67 mmol of MgCl 6H O and 8.83 mmol of AlCl 6H O was added dropwise 2· 2 3· 2 to CMC solution under a nitrogen atmosphere and vigorous stirring. The pH value was maintained constant by the addition of NaOH 0.2 mol L 1 solution. The co-precipitation reaction was carried at · − 60 ◦C and, after the complete addition of the metal cations solution, the suspension was maintained for 6 h at 60 ◦C. The white precipitate was centrifuged, washed four times with deionized water and dried at room temperature under reduced pressure. For the preparation of the Zn2Al-CMC sample, 20 g of CMC was solubilized into 850 mL of deionized water and the pH value of the solution was adjusted to 7.5 by the addition of NaOH 0.5 mol L 1 solution. Afterwards, 520 mL of an aqueous solution containing 41.20 mmol of ZnCl · − 2 and 21.41 mmol of AlCl 6H O was added dropwise to CMC solution under a nitrogen atmosphere, 3· 2 vigorous stirring and at room temperature. The pH value was maintained constant by the addition of NaOH 0.5 mol L 1 solution. After addition of the metal cations solution, the suspension was dialyzed · − for 3 days changing the water after each 12 h. The white precipitate was then freeze dried. For comparison purpose, Mg2+/Al3+ and Zn2+/Al3+ LDH materials intercalated with carbonate anion (Mg2Al-CO3 and Zn2Al-CO3) were synthesized as described by Miyata [25] with modifications. Typically, the pH value of 250 mL of water was adjusted to 9.5 (for Mg2Al-CO3) or 9.0 (for Zn2Al-CO3) by the addition of Na CO 0.2 mol L 1 solution. To this solution, 250 mL of a solution containing 2 3 · − 16.67 mmol of Mg(NO ) 6H O (or Zn(NO ) 6H O) and 8.33 mmol of AlCl 6H O was added dropwise 3 2· 2 3 2· 2 3· 2 under stirring. The co-precipitation reaction was carried out at an ambient atmosphere and room temperature. The pH value was maintained constant by the addition of Na CO 0.2 mol L 1 solution. 2 3 · − After addition of the metal cations solutions, the suspension was maintained under stirring at ambient atmosphere and room temperature for 2 h. The white precipitate was centrifuged, washed five times with deionized water, one time with ethanol and dried at room temperature under reduced pressure.

2.3. Preparation of MMO/C Composites

Mg2Al-CMC and Zn2Al-CMC precursors were pyrolyzed on a Shimadzu TGA-50 furnace under 1 1 a molecular nitrogen flow of 50 mL min− , using alumina crucibles and a heating rate of 10 ◦C min− , at temperatures of 500, 600, 800 and 1000 ◦C. For comparison purpose, Mg2Al-CO3 and Zn2Al-CO3 were calcined under nitrogen flow using the same parameters. Besides, Mg2Al-CMC and Zn2Al-CMC precursors were also calcined under synthetic air at 1000 ◦C, using the same conditions described to pyrolysis at N2 environment. Afterwards, heated samples were stored in a desiccator containing silica gel under reduced pressure.

2.4. Physical Measurements Elemental chemical analysis (carbon and hydrogen) were performed on a Perking Elmer model 2400 analyzer. Magnesium, zinc and aluminum contents were determined in triplicate by inductively coupled plasma optical emission spectroscopy (ICP OES) on a Spectro Arcos spectrometer. Both analyses were performed at the Central Analítica of Instituto de Química da Universidade de São Paulo (CA-IQUSP). Mass spectrometry coupled to simultaneous thermogravimetric analysis and differential scanning calorimetry (TG-DSC-MS) were performed on a Netzsch thermoanalyzer model TGA/DSC 409 PC 1 Luxx coupled to an Aëolos 403 C mass spectrometer under synthetic air or N2 flow of 50 mL min− employing alumina crucibles and a heating rate of 10 C min 1. ◦ · − X ray diffraction (XRD) patterns of powdered samples were recorded on a Bruker diffractometer model D8 DISCOVER equipped with Nickel-filtered CuKα radiation (0.15418 nm), operating at 40 kV, 30 mA, sample rotating at 20 rpm, from 1.5 to 70◦ (2θ) at 0.05◦ intervals, 3 s per interval. 1 Fourier transform infrared (FTIR) spectra of powdered samples were recorded in the 4000–400 cm− range on a Bruker Alpha spectrometer using a single bounce attenuated total diffuse reflectance (ATR) mode with a diamond crystal. Fourier transform Raman (FT-Raman) spectra were recorded on an FT-Raman Bruker FRS-100/S instrument using 1064 nm excitation radiation (Nd3+/YAG laser) and liquid ChemEngineering 2019, 3, 55 5 of 17

1 nitrogen cooled Ge detector. Raman spectra of pyrolyzed samples were recorded in the 3500–100 cm− range on a Renishaw inVia instrument coupled to a confocal Leica microscope and a CCD detector, using 532 nm excitation radiation (Nd3+/YAG laser). Scanning Electron (SEM) micrographs were collected using a JEOL JSM7401F instrument, equipped with a field emission gun (FEG). Accelerating voltages (kV) are reported on each micrograph. Energy Dispersive X ray Spectra (EDS) were collected simultaneously on a Thermo Fischer Scientific detector, Pioneer model, 30 mm2 Si Crystal, coupled to a Noran System Six signal processor (NSS). Powder samples were pulverized onto carbon conductive tapes prior to SEM imaging.

3. Results and Discussion

3.1. Characterization of LDH-CMC Hybrid Materials

XRD patterns of LDH-CO3 and LDH-CMC synthesized samples showed profiles characteristic of layered double hydroxides (Figure3). LDH-CO 3 samples are more crystalline than LDH-CMC, as evidenced by the high intensity and narrow diffraction peaks which can be indexed assuming a hexagonal cell with rhombohedral symmetry (3R polytype) and R-3m space group. The reflections observed at 2θ values below 30◦ are assigned to the (00l) planes that are related to interlayer distance (dbasal) and the cell parameter c (Figure1b). The reflection observed in 2 θ values in the region of 60◦ is attributed to the (110) plane related to the cell parameter a that gives the average distance between metal cations in the layer. The presence of broad XRD peaks associated with both (003) and (110) planes of LDH-CMC samples indicates that the crystal dimensions are both reduced in the crystallographic directions a and c in comparison to LDH-CO3 materials [26]. Table S1 (Supplementary Materials) shows the 2θ values, the respective interplane distances (dhkl) and the calculated unit cell parameters for LDH materials. For Zn2Al-CO3 and Mg2Al-CO3 samples, the d003 spacing values were respectively 0.754 nm and 0.765 nm. Samples Zn2Al-CMC and Mg2Al-CMC showed a shift of 003 diffraction peaks to 2.492 and 1.715 nm, respectively, indicating an expansion of the interlayer space in order to accommodate the macromolecules. XRD pattern of Mg2Al-CMC sample prepared by Yadollahi and Namazi [20] presented d003 spacing value equal to 1.73 nm while Zn2Al-CMC sample reported by Yadollahi et al. [21] showed the d003 value of 1.58 nm. Considering that CMC is a polymer that may present different arrangements, the proposal of a conformation for the chains between the LDH layers is not straightforward. The basal spacing difference between Mg2Al-CMC and Zn2Al-CMC materials prepared in this work may be related to the higher water and CMC contents in the zinc matrix. The calculated d110 values for Zn2Al-CO3 and Zn2Al-CMC were 0.153 nm, while for Mg2Al-CO3 and Mg2Al-CMC were 0.152 nm and 0.151 nm, respectively, which means that the molar ratio between the metal cations is similar in the samples. These points will be discussed later when chemical elemental analysis will be presented. XRD pattern of NaCMC exhibits mainly one broad peak around 2θ value of 20◦ (Figure S1, Supplementary Materials) which is associated with the low degree of organization of the macromolecules [27]. Considering the XRD patterns of LDH-CMC samples, it is observed a broad hump at (2θ) 20◦ that could suggest that part of the polymer chains might not be confined within the 2+ interlayer space but rather adsorbed over LDH particles. XRD patterns of M2Al-CMC (M = Mg, Zn, Ni, Cu) samples reported in the literature [21] also showed a broad peak in the same region. The infrared and FT-Raman spectra of Zn2Al-CO3 and Mg2Al-CO3 are shown in Figure4. The discussion about vibrational data is presented in Supplementary Materials, as well as a tentative attribution of the main vibrational bands (Table S2). ChemEngineering 2019, 3, 55 6 of 17 ChemEngineering 2019, 3, 55 6 of 17

d = 0.754 nm d = 0.765 nm (003) Zn2Al-CO3 (003) Mg2Al-CO3 (003) (003) 5000 cps 1000 cps ) (006) (006) cps (012) ( ) (012) y it

) (1013 ens t (015) (015) ) n (1013 I Intensity (cps)

2x (018)

3x (113) (018) (009) (0114 ) ) (110) (113) ) ) ) (110) (101) (1010 (104) (1010 (116) (0111 (116) (0012 (0111 (107)

5 10152025303540455055606570 5 10152025303540455055606570 θ θ 2 (degree) 2 (degree) (a) (b) Zn Al-CMC Mg Al-CMC 2 d = 1.750 nm 2 d(003) = 2.492 nm (003) (003) (003) 1000 cps 1000 cps ) cps ( ty i

ntens Intensity (cps) I (012) (012) (110) (110)

5 10152025303540455055606570 5 10152025303540455055606570 θ θ 2 (degree) 2 (degree) (c) (d)

Figure 3. XRD patterns of Zn2Al-CO3 (a), Mg2Al-CO3 (b), Zn2Al-CMC (c) and Mg2Al-CMC (d) Figure 3. XRD patterns of Zn2Al-CO3 (a), Mg2Al-CO3 (b), Zn2Al-CMC (c) and Mg2Al-CMC (d) samples.samples. TheThe di diffrationffration peaks peaks in in (a, b(a)) were and indexed(b) were according indexed toaccording ICSD 190041 to ICSD and 86655,190041 respectively. and 86655, respectively. The spectroscopic characterization of sodium CMC and LDH-CMC samples was evaluated by FTIR-ATRXRD andpattern FT-Raman of NaCMC spectroscopies exhibits mainly (Figure one4). Spectrabroad peak of the around polymer 2θ and value hybrid of 20° materials (Figure are S1, verySupplementary similar, and Materials) the results clearlywhich indicateis associated the presence with ofthe CMC low indegree the LDH of structure.organization A tentative of the attributionmacromolecules regarding [27]. cellulose Considering and carboxymethylcellulosethe XRD patterns of LDH-CMC spectroscopic samples, data isit presentedis observed in a Table broad1. Thehump NaCMC at (2θ) bands20° that attributed could suggest to antisymmetric that part of the and poly symmetricmer chains stretching might not of carboxylate be confined groups within arethe 1 1 2+ noticedinterlayer at 1585space cm but− rather(νasCOO adsorbed−) and 1412 over cm LDH− ( νparticles.sCOO−), XRD respectively. patterns Inof theM2Al-CMC LDH-CMC (M spectra, = Mg, theZn, 1 νNi,asCOO Cu) −samplesband is reported shifted toin 1577the literature cm− . The [21]∆ν alsoCOO showed− values a broad calculated peak byin the same formula region.∆νCOO − = νas Theνs are infrared very close and toFT-Raman that one spectra of NaCMC, of Zn indicating2Al-CO3 and an Mg electrostatic2Al-CO3 are interaction shown in of Figure carboxylate 4. The − 1 groupsdiscussion with about LDH layers.vibrational Bands data in theis presented 1100–980 cmin −Supplementaryregion are assigned Materials, to stretching as well as and a bendingtentative vibrationattribution of of C–O the bondmain fromvibrational the glycosidic bands (Table ring and S2). hydroxyl groups. FT-Raman spectra of NaCMC 1 and theThe hybrid spectroscopic materials characterization show very similar of bands.sodium The CMC band and at LDH-CMC 2907 cm− is samples related towas C–H evaluated stretching, by 1 atFTIR-ATR 1118 cm− andis attributedFT-Raman tospectroscopies symmetric stretching (Figure 4). of Spectra C(1)–O–C(4) of the grouppolymer and and ring hybrid breathing, materials and are at 1 918very cm similar,− is assigned and the to results bending clearly C(5)C(6)–H indicate and the HC(6)O.presence of CMC in the LDH structure. A tentative attribution regarding cellulose and carboxymethylcellulose spectroscopic data is presented in Table 1. The NaCMC bands attributed to antisymmetric and symmetric stretching of carboxylate groups are noticed at 1585 cm−1 (νasCOO−) and 1412 cm−1 (νsCOO−), respectively. In the LDH-CMC spectra, the νasCOO− band is shifted to 1577 cm−1. The ΔνCOO− values calculated by the formula ΔνCOO− = νas − νs are very close to that one of NaCMC, indicating an electrostatic interaction of carboxylate groups with LDH layers. Bands in the 1100–980 cm−1 region are assigned to stretching and bending vibration of C–O bond from the glycosidic ring and hydroxyl groups. FT-Raman spectra of NaCMC and the ChemEngineering 2019, 3, 55 7 of 17 hybrid materials show very similar bands. The band at 2907 cm−1 is related to C–H stretching, at 1118 ChemEngineeringcm−1 is attributed2019 ,to3, 55symmetric stretching of C(1)–O–C(4) group and ring breathing, and at 9187 cm of 17−1 is assigned to bending C(5)C(6)–H and HC(6)O. Zn

2 674 Al-CO 946 3410 1358 2092 525 423 1058 863 630 688 1632 NaCMC 3 1400

1377 556 Mg 760 1118 1415 1327 918

2 Al-CO Al-CMC 1150 918 3026 1262 1458 2 1026 1595 Mg 3 1577

Zn 995 2 Al-CMC 1046 Al-CMC 2 Zn

3 Mg 556 2 1060 Al-CMC 155 Al-CO 2 495 2869 2922 1448 1393 698 Mg

1265

Normalized RamanNormalized Intensity

1374 1150 3 NaCMC 895 Normalized Transmitance (%) 3218 3143 1320 1412 440 1095 Al-CO 703 2 1585

1024 Zn

4000 3500 3000 1750 1500 1250 1000 750 500 250 4000 3500 3000 1750 1500 1250 1000 750 500 250 -1 wavenumber (cm-1) wavenumber (cm ) (a) (b)

Figure 4. FTIR-ATR (a) and FT-Raman (b; λexc = 1064 nm) spectra of Zn2Al-CO3 (black), Mg2Al-CO3 Figure 4. FTIR-ATR (a) and FT-Raman ((b); λexc = 1064 nm) spectra of Zn2Al-CO3 (black), Mg2Al-CO3 (grey), Zn2Al-CMC (red), Mg2Al-CMC (blue) and sodium carboxymethylcellulose (NaCMC, green). (grey), Zn2Al-CMC (red), Mg2Al-CMC (blue) and sodium carboxymethylcellulose (NaCMC, green).

Table 1. Vibrational spectroscopic data of sodium carboxymethylcellulose. Table 1. Vibrational spectroscopic data of sodium carboxymethylcellulose. Raman IR Tentative of Attribution Reference Raman3400–3200 3600–3300 IR (bb) νOH Tentative free or hydrogen of Attribution bonded [28–30] Reference 3400–32002902 3600–3300-- (bb) νOH free or hydrogenνCH bonded[28,30] [28– 30] 2902-- –2922 νasνC(6)HCH2 [28] [28 ,30] – 2922 νasC(6)H2 [28] -- 2869 νsC(6)H2 [28] – 2869 νsC(6)H2 [28] 15951595 15851585 νasasCOO- [31] [ 31] δ 2 1458(sh)1458(sh) – -- δscC(6)H2 [30,32][30,32 ] δ δ –-- 1448(sh)1448(sh) δscscCHCH2 andand δipCOH [29,32] [29,32 ] δ δ δ δ 14151415 – -- δCHCH2 2andand δCCH,CCH, δHCO,HCO, δCOHCOH [30,32] [30,32 ] ν –-- 14121412 νssCOO- [31] [ 31] 1377 1374(sh) δδCH2 andδδCCH, δHCO, δδCOH [30,32] 13271377 1374(sh) 1320 δwC(6)HCH2 andand CCH,δCCH, HCO,δHCO, δCOHCOH [30,32] [30,32 ] δ 2 δ δ δ 12621327 12641320 wC(6)HδtwC(6)H2 2andand δCCH,CCH, δHCO, δ COHCOH [30,32] [30,32 ] 1150(sh)1262 1150(sh)1264 δtwC(6)HνasC-C and2 andνas δC-OCCH, (ring δHCO, breathing) δCOH [30,32] [29,32 ] 11181150(sh) 1150(sh) – νasνC-CsC(1)-O-C(4) and νasC-O and s (ringring breathingbreathing) [29,32] [32 ] ν –1118 1095(sh)-- νsC(1)-O-C(4) andC(2)-O s ring breathing [32] [29 ,33] δ δ – 1046(sh) C(6)-Oν and C(5)-O [33] 1026(sh)-- 1095(sh) 1019 δC(2)-OC(3)-O [29,33] [33 ] –-- 9951046(sh) (sh) δC(6)-OνC(1)-O and δC(5)-O [33] [ 33] 9181026(sh) –1019 δC(5)C(6)-HδC(3)-O and δ HC(6)O[33] [ 30] 896-- 995 895 (sh) νδC(1)-OC(1)-H [33] [ 32] 703 δrkCH and δoopCOH (C6 and C3) [29,34] 918 -- δC(5)C(6)-H2 and δHC(6)O [30] bb = broad896 band; sh = shoulder;895ν = stretching; as = antisymmetric;δC(1)-Hs = symmetric; δ = bending;[32] w = wagging; tw = twisting; sc = scissoring; ip = in-plane; oop = out-of-plane; rk = rocking. 703 δrkCH2 and δoopCOH (C6 and C3) [29,34] bb = broad band; sh = shoulder; ν = stretching; as = antisymmetric; s = symmetric; δ = bending; w = Thermal analysis technique indicated significant changes in the CMC thermal profile before and wagging; tw = twisting; sc = scissoring; ip = in-plane; oop = out-of-plane; rk = rocking. after the intercalation process. In the pristine form and under N2 atmosphere, NaCMC exhibits four eventsThermal of mass analysis loss until technique 1000 ◦C (Figureindicated S2, significan Supplementaryt changes Materials). in the CMC The first thermal event, profile endothermic, before occurringand after the from intercalation room temperature process. up In tothe about pristine 170 form◦C is and attributed under toN2 the atmosphere, release of aNaCMC total of 7.3exhibits % of thefour initial events mass of asmass water loss molecules until 1000 (release °C of(Figure fragment S2, mSupplementary/z = 18) presented Materials). initially inThe the hydrophilicfirst event, polymer.endothermic, According occurring to TGAfrom androom DSC temperature data, the up polymer to about decomposition 170 °C is attributed occurs into twothe release exothermic of a steps [35,36]. The first step, steeper, in the 240–320 ◦C range, promotes release of water (m/z = 18), ChemEngineering 2019, 3, 55 8 of 17

decarboxylation with CO2 (m/z = 44) release and a low amount of C3H5 (m/z = 41); the second step, in the 320–530 ◦C range, yields a charred residue. The last event of mass loss, also endothermic, recorded above 800 ◦C can be related to the decomposition of carbonaceous matter to Na2CO3 [35]. CMC used in this study has 0.7 of the degree of substitution (DS) of carboxymethyl group and, as a result, the repeating unit (RU) can be represented by C6H10O5(C2H2O2Na)0.7. Considering this composition, the expected percentage of Na2CO3 residue (13.7%) is very close to the experimental value (13.5%) obtained by TGA analysis (Figure S2). The knowledge of the thermal behavior of carbonate LDH phases and the products produced in distinct temperatures is important to contrast with thermal properties of LDH-CMC precursors. According to TGA-MS results, for both carbonate-based layered structures, three main events of mass loss can be described (Figure5). The first event, ranging from room temperature up to 220 ◦C is related to the release of interlayer and adsorbed water molecules [37], as indicated for MS curve (release of m/z = 18 fragment). The initial structure is proven to be modified even at temperatures above 100 ◦C owing to the detection of growing quantities of m/z = 44 (CO2) fragment in MS analysis. In the second event of mass loss at temperatures above 200 ◦C, composition integrity of the layered structure is severely affected owing to the release of water molecules (m/z = 18) through a mechanism known as dehydroxylation [37]. For Zn2Al-CO3, the onset temperature for dehydroxylation was observed at a lower temperature (215 ◦C) in comparison to Mg2Al-CO3 (282 ◦C), and thus it can be inferred that the Mg2Al-CO3 structure is thermally more stable than the analogous phase of zinc. ChemEngineering 2019, 3, 55 9 of 17

100 100 Zn2Al-CO Mg2Al-CO 3 Exo 1 3 Exo 0 ) ) -1 -1 -1 0 80 66.8% of res. mass -2 80

-1

Ton-set 282°C -3 Mass (%) 55.5% of res. mass Mass (%) Ton-set 215°C -2 60 -4 DSC (mW mg (mW DSC DSC (mW mg (mW DSC -3 -5 16.3% 11.4% 2.6% 2.4% 17.4% 23.2% 3.9% 60 0 0 2.5 1.6 ) ) -1 -1 -1 1.4 -1 A) 590°C 752°C A) -2 258°C 2.0 114°C -9 -9 329°C 1.2 393°C -3 1.5 -2 1.0 160°C 190°C -4 m/z 18 H O m/z 18 H2O 0.8

2 -5 1.0 0.6 m/z 44 CO (x10) -3 m/z 44 CO2 (x10) -6 2 0.5 0.4

-7 min DTG (mg

-4 0.2 Current (x10 DTG (mg min (mg DTG -8 0.0 (x10 Current 0.0 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000

Temperature (°C) Temperature (°C) (a) (b) 100 Zn Al-CMC 100 Mg Al-CMC 0.9 2 Exo 1.5 2 ) 80 ) -1 1.0 -1 80 Exo 0.6 0.5 60 28.5% of res. mass 0.0 60 Mass (%) Mass (%) Mass 40% of res. mass 0.3 40 -0.5 DSC (mW mg (mW DSC 40 mg DSC (mW -1.0 20 7.6% 7.4% 14.6% 20.5% 21.6% 8.8% 22.9% 22.5% 5.8% 0.0 0 5 0 8

) )

-1 400°C -1 A) -1 4 A) -1

218°C -9 94°C -9 99°C 6

864°C -2 3 -2 480°C

306°C 434°C m/z 18 H2O 4

304°C m/z 18 H O -3 2 -3 2 m/z 44 CO2 (x2) m/z 44 CO2 (x2) m/z 41 C H (x100) 2 -4 3 5 1 -4 m/z 41 C H (x100)

DTG (mg min DTG (mg 3 5 Current (x10 Current Current (x10 DTG (mg min (mg DTG -5 0 -5 0

100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 Temperature (°C) Temperature (°C) (c) (d)

Figure 5. TG-DSC and DTG-MS curves of Zn2Al-CO3 (a), Mg2Al-CO3 (b), Zn2Al-CMC (c) and Figure 5. TG-DSC and DTG-MS curves of Zn2Al-CO3 (a), Mg2Al-CO3 (b), Zn2Al-CMC (c) and Mg2Al-CMC (d) samples under N2 atmosphere. Mg2Al-CMC (d) samples under N2 atmosphere.

For Zn2Al-CMC, the dehydration step occurs within the 30–250 °C temperature range, closely followed by Mg2Al-CMC in the 30–225 °C range according to the increase in the amount of the m/z = 18 (H2O) fragment detected (Figure 5). The second event of mass loss (from 250 to 380 °C) is related to the thermal decomposition of polymer chains through release of water molecules (m/z = 18) from hydroxyl groups in the polymer composition and decarboxylation of side groups (m/z = 44) for both layered systems, according to DTG data. Although the beginning of this decomposition event is not significantly shifted towards higher temperatures, when pristine NaCMC is compared to the intercalated polymer into LDH galleries, it is noticed that the polymer decomposition event exhibits larger thermal range after immobilization. Hence, the interaction between both inorganic and organic phases delays the start of polymer fragmentation. Above 380 °C, MS curves of M2Al-CMC phases are altered substantially depending on the composition of the inorganic layers (Figure 5). At higher temperature values, glucose chains undergo fragmentation in both LDH samples yielding not only water and molecules but also C3H5 fragments (m/z = 41). Even though the beginning of this event is around the same value for both Mg and Zn-based LDH, it is observed that this second decomposition step is extended to a wider temperature range for Mg2Al-CMC (380–605 °C) than for Zn2Al-CMC (380–520 °C). Mass changes are negligible above 605 °C for Mg-based hybrid material (Figure 5). On the other hand, the TGA curve of Zn2Al-CMC reveals one event of mass loss above 760 °C which is not observed in the same experiment conducted at air atmosphere (data not shown). This observation is surprising ChemEngineering 2019, 3, 55 9 of 17

Around the same temperature value and at higher temperatures, the release of m/z = 44 fragment (CO2) is intensified for Mg2Al-CO3 as the third event of mass loss (decarbonation process) occurs. At around 480 ◦C, dehydroxylation is ended for Mg2Al-CO3 sample but carbonate decomposition (release of m/z = 44 fragment) is still observed at temperatures up to 700 ◦C (even though the majority of carbonate decomposes at around 440 ◦C). On the other hand, Zn2Al-CO3 material exhibits a distinct thermal behavior. Even though it dehydroxylates and is partially decarbonated (since the m/z = 44 fragment analysis reveals the increase of CO2 detection above 200 ◦C), only a small fraction of carbonate ions is released. Just at temperature values higher than 520 ◦C, m/z = 44 fragment is detected in large quantities in an apparent two distinct mechanisms of mass loss up to 840 ◦C. Such differences in the decarbonation process for Zn2Al-CO3 compared to Mg2Al-CO3 can be a direct consequence of the occurrence of dehydroxylation of the Zn-based LDH at lower temperatures than the thermal 2 decomposition of carbonate, which allows the coordination of CO3 − ion to metals present in the formed mixed oxide structure, increasing the anion thermal stability [22,23]. As the temperature of the analysis is increased, after dehydroxylation of the LDH is completed, it is formed an amorphous M2+(Al)O oxide phase, followed by the formation of crystalline oxide phases. Above temperature values of 500 and 800 ◦C, spinel phases are obtained respectively for Zn2Al-CO3 and Mg2Al-CO3 [23,38,39]. For Zn2Al-CMC, the dehydration step occurs within the 30–250 ◦C temperature range, closely followed by Mg2Al-CMC in the 30–225 ◦C range according to the increase in the amount of the m/z = 18 (H2O) fragment detected (Figure5). The second event of mass loss (from 250 to 380 ◦C) is related to the thermal decomposition of polymer chains through release of water molecules (m/z = 18) from hydroxyl groups in the polymer composition and decarboxylation of side groups (m/z = 44) for both layered systems, according to DTG data. Although the beginning of this decomposition event is not significantly shifted towards higher temperatures, when pristine NaCMC is compared to the intercalated polymer into LDH galleries, it is noticed that the polymer decomposition event exhibits larger thermal range after immobilization. Hence, the interaction between both inorganic and organic phases delays the start of polymer fragmentation. Above 380 ◦C, MS curves of M2Al-CMC phases are altered substantially depending on the composition of the inorganic layers (Figure5). At higher temperature values, glucose chains undergo fragmentation in both LDH samples yielding not only water and carbon dioxide molecules but also C3H5 fragments (m/z = 41). Even though the beginning of this event is around the same value for both Mg and Zn-based LDH, it is observed that this second decomposition step is extended to a wider temperature range for Mg2Al-CMC (380–605 ◦C) than for Zn2Al-CMC (380–520 ◦C). Mass changes are negligible above 605 ◦C for Mg-based hybrid material (Figure5). On the other hand, the TGA curve of Zn2Al-CMC reveals one event of mass loss above 760 ◦C which is not observed in the same experiment conducted at air atmosphere (data not shown). This observation is surprising considering that it is expected the release of less volatile compounds at N2 atmosphere than at molecular oxygen environment. The ascription of this event will be clarified ahead in view of XRD and SEM data of pyrolyzed materials. Based on the dehydration process observed by TGA and data from the chemical elementary analysis, a formula is proposed for LDH-CO3 and LDH-CMC materials synthesized in this work (Table S3). Experimental M2+/Al3+ molar ratio is very close to the nominal value for carbonate or CMC intercalated LDH materials. Precursor hybrid materials present about 20–22% in mass of carbon element.

3.2. Characterization of MMO/C Composites

LDH-CMC samples pyrolyzed at 500, 600, 800 and 1000 ◦C (MMO/C nanocomposites) were evaluated by Raman spectroscopy to assess the type of carbonaceous material obtained (Figure6). Raman spectra of both LDH-CMC pyrolyzed at 500 ◦C still show fluorescence that suppressed vibrational bands, as observed for NaCMC and LDH-CMC precursors (Figure S3). ChemEngineering 2019, 3, 55 10 of 17

considering that it is expected the release of less volatile compounds at N2 atmosphere than at molecular oxygen environment. The ascription of this event will be clarified ahead in view of XRD and SEM data of pyrolyzed materials. Based on the dehydration process observed by TGA and data from the chemical elementary analysis, a formula is proposed for LDH-CO3 and LDH-CMC materials synthesized in this work (Table S3). Experimental M2+/Al3+ molar ratio is very close to the nominal value for carbonate or CMC intercalated LDH materials. Precursor hybrid materials present about 20–22% in mass of carbon element.

3.2. Characterization of MMO/C Composites LDH-CMC samples pyrolyzed at 500, 600, 800 and 1000 °C (MMO/C nanocomposites) were evaluated by Raman spectroscopy to assess the type of carbonaceous material obtained (Figure 6). ChemEngineeringRaman spectra2019 ,of3, 55both LDH-CMC pyrolyzed at 500 °C still show fluorescence that suppressed10 of 17 vibrational bands, as observed for NaCMC and LDH-CMC precursors (Figure S3).

5000 5000 Zn2Al-CMC Mg Al-CMC G 2 D G D 5000

2D

500°C 1000 500°C 2D ID/IG = 0.60

ID/IG = 0.63 600°C 500 600°C

500 ID/IG = 0.78 ID/IG = 0.86

Raman Intensity Raman Intensity 800°C 800°C Raman Intensity 500 500 ID/IG = 0.94

I /I = 0.86 D G 1000°C 1000°C

800 1200 1600 2000 2400 2800 3200 800 1200 1600 2000 2400 2800 3200 -1 wavenumber (cm ) wavenumber (cm-1) (a) (b)

Figure 6. Raman spectra of Zn2Al-CMC (a) and Mg2Al-CMC (b) samples pyrolyzed at 500, 600, 800 Figure 6. Raman spectra of Zn2Al-CMC (a) and Mg2Al-CMC (b) samples pyrolyzed at 500, 600, 800 and 1000 °C under N2 atmosphere (λexc = 532 nm). and 1000 ◦C under N2 atmosphere (λexc = 532 nm).

TheThe characteristic characteristic spectral spectral profileprofile ofof graphitic materials materials is is observed observed for for samples samples obtained obtained in the in the 600–1000 °C range (Figure 6): bands D (1340–1350 cm−1), G (1590–1600 cm−1) 1and 2D (2500–2800 cm−1). 1 600–1000 ◦C range (Figure6): bands D (1340–1350 cm − ), G (1590–1600 cm− ) and 2D (2500–2800 cm− ). 2 BandBand G isG assignedis assigned to theto the in planein plane stretching stretching of theof the C= CC=C bonds bonds (Csp (Csp2) and) and is characteristic is characteristic for for all formsall forms of sp2 carbon. Band D is attributed to ring breathing and requires defects for its activation [40– of sp2 carbon. Band D is attributed to ring breathing and requires defects for its activation [40–42]. 42]. However, besides the graphitic bands, Raman spectra of pyrolyzed samples at 600 °C also However, besides the graphitic bands, Raman spectra of pyrolyzed samples at 600 C also exhibit exhibit shoulders attributed to amorphous hydrogenated carbonaceous material at 1180◦ cm−1 (C–H shoulders attributed to amorphous hydrogenated carbonaceous material at 1180 cm 1 (C–H in aromatic in aromatic ring structure and correlated to a rich sp3 structure), 1430 cm−1 (aromatic− ring breathing ring structure and correlated to a rich sp3 structure), 1430 cm 1 (aromatic ring breathing for rings for rings containing more than two fused aromatic rings), and− 1250 cm−1 (rich sp3 structure as an 1 3 containingaryl-alkyl moreC–C (like than polyacetylene) two fused aromatic bonded rings), to arom andatic 1250species) cm −[42,43].(rich These sp structure bands disappear as an aryl-alkyl with C–C (like polyacetylene) bonded to aromatic species) [42,43]. These bands disappear with temperature temperature increasing up to 1000 °C. The intensity ratio between bands D and G (ID/IG) is also increasingincreased up with to 1000temperature◦C. The progression, intensity ratio pointing between out bandsdehydrogenation D and G (I Dof/I Gcarbonaceous) is also increased material with temperatureand intensification progression, of aromatic pointing rings out [43]. dehydrogenation Hence, amorphous of carbonaceous carbon is produced material between and intensification 500 and of600 aromatic °C, and rings yield [ 43a more]. Hence, organized amorphous structure carbon as the istemperature produced betweenis further 500increased. and 600 ◦C, and yield a more organizedFTIR-ATR structure spectra of as theLDH-CMC temperature pyrolyzed is further mate increased.rials exhibit weak bands assigned to the organicFTIR-ATR polymer spectra (Figure of LDH-CMC7). A significant pyrolyzed change materials is observed exhibit at 1150–990 weak bands cm−1 assignedregion, attributed to the organic to 1 polymerC–O–C (Figureand C–O7). Avibration significant modes change of glycoside is observed ring ats 1150–990and to hydroxyl cm − region, groups, attributed indicating to C–O–Ctheir andconversion C–O vibration to other modes compounds of glycoside as the temperature rings and tois hydroxylincreased. groups,In general, indicating bands observed their conversion bellow −1 1 to900 other cm compounds are assigned as to the vibrational temperature modes is increased. of M–O bonds In general, as already bands discussed observed in Supplementary bellow 900 cm − are assigned to vibrational modes of M–O bonds as already discussed in Supplementary Materials (Table S2). For comparison purpose, the FTIR-ATR spectra of LDH-CO3 calcined samples (Figure S4) and the discussion about the results are presented in Supplementary Materials. To investigate the differences between the thermal transformations of Mg2Al-LDH and Zn2Al-LDH materials, XRD patterns of samples calcined between 500–1000 ◦C under N2 flow were recorded (Figure8). Zn 2Al-CMC pyrolyzed and Zn2Al-CO3 calcined at 500 and 600 ◦C are converted into a poor crystalized ZnO phase. However, at higher temperature values, the products from pyrolysis are very distinctive indicating an influence of the organic polymer in the structure and composition of the inorganic portion of the produced composite. ZnO phase produced from the hybrid LDH at 800 ◦C is significantly more crystalline than that one obtained from Zn2Al-CO3. On the other hand, Zn2Al-CO3 is transformed into a well crystalized ZnO and ZnAl2O4 (spinel) phases at 1000 ◦C, while XRD pattern of Zn2Al-CMC presents peaks attributed to ZnAl2O4 and poor crystalized γ-Al2O3 phase. From 800 to 1000 ◦C, Zn2Al-CMC material seems to lose the ZnO phase and this event should be correlated to that one noticed by thermal analysis in the same temperature range (Figure5). ChemEngineering 2019, 3, 55 11 of 17

ChemEngineering 2019, 3, 55 11 of 17 Materials (Table S2). For comparison purpose, the FTIR-ATR spectra of LDH-CO3 calcined samples (Figure S4) and the discussion about the results are presented in Supplementary Materials.

Zn Al-CMC 2 Mg2Al-CMC

ν(OH) ν(OH) ν( ν( CH2) CH2) - - Normalized Transmitance(%) ν (COO ) ν (COO ) Normalized Transmitance (%) as as ν(C-O-C)

500 °C 500 °C ν (COO-) ν(C-O) s ν( 600 °C C-O-C) 600 °C ν (COO-) δ( ν( s C-O) 800 °C C-O) 800 °C δ(C-O) 1000 °C 1000 °C

3600 3000 1600 1400 1200 1000 800 600 400 3600 3000 1600 1400 1200 1000 800 600 400 -1 wavenumber (cm ) wavenumber (cm-1) (a) (b)

Figure 7. FTIR-ATR spectra of Zn2Al-CMC (a) and Mg2Al-CMC (b) samples pyrolyzed at 500, 600, Figure 7. FTIR-ATR spectra of Zn2Al-CMC (a) and Mg2Al-CMC (b) samples pyrolyzed at 500, 600, 800 800 and 1000 °C under N2 atmosphere. ChemEngineeringand 1000 ◦C under2019, 3, 55 N 2 atmosphere. 12 of 17

To investigate the differences ZnbetweenAl-CO the thermal transformations of Mg2Al-LDH and 2 3 Mg2Al-CO3 Zn2Al-LDH materials, XRD patterns of samples calcined between 500–1000 °C under N2 flow were 500 °C 500 °C 500 cps 300 cps recorded (Figure 8). Zn2Al-CMC pyrolyzed and Zn2Al-CO3 calcined at 500 and 600 °C are converted into a poor crystalized ZnO phase. However, at higher temperature values, the products from

500 cps 600 °C pyrolysis600 are°C very distinctive indicating an influence of the organic polymer in300 the cps structure and composition of the inorganic portion of the produced composite. ZnO phase produced from the 1000 cps 800 °C 500 cps hybrid 800LDH °C at 800 °C is significantly more crystalline than that one obtained from Zn2Al-CO3. On Δ # Δ Δ Intensity (cps) the otherIntensity (cps) OZn hand, Zn2Al-CO3 is transformed into a well crystalized ZnO and ZnAl2O4 (spinel) phases at ∗ # MgO @ MgAl O ∗ 2000 cps 2 4 # 1000 °C,ZnAl while2O4 XRD∗ Δ pattern of Zn2Al-CMC presents peaks attributed# to@ ZnAl2O4 and poor Δ Δ @ Δ ∗ ∗ Δ @ @ 500 cps @ crystalized1000 °Cγ-Al2O3 phase. From∗ ∗ 800 ∗to 1000 Δ°C,Δ Zn2Al-CMC1000 material °C seems to lose the ZnO phase

and this15 event 20 25 should 30 35 be 40correlated 45 50 to 55 that 60 one 65 no 70 ticed15 by 20thermal 25 30 analysis 35 40 in 45 the 50 same 55 temperature 60 65 70

θ range (Figure 5). 2 (degree) 2θ (degree) XRD patterns of (Mga) 2Al-CMC pyrolyzed between 500–1000 °C indicate(b) the formation MgO

phase which crystallinity is increasedZn Al-CMC with the temperature500 °C evolution (Figure Mg8). Al-CMCSimilarly, 500 °C 2 2 Mg2Al-CO3 sample calcined between 500–800 °C shows reflections of MgO. However, when calcined 200 cps 200 cps at 1000 °C, MgO and MgAl2O4 (spinel) phases are observed. The organic polymer intercalated into LDH modified the degree of organization of the inorganic portion600 °C of pyrolyzed products. If calcined Δ 300 cps under an air600 atmosphere°C Δ at 1000 °C, both LDH-CMC materials produce metal200 oxide cps and spinel Δ phasesΔ (Figure S5a) likewise LDH-CO3 samples2000 cps under nitrogen atmosphere (Figure 8). In addition, ZnO Δ Δ Δ Δ 800 °C whereas the800 Mg °C2Al-CMC sample calcined underΔ synthetic air presented less residual mass than that Δ # 300 cps Intensity (cps) calcinedIntensity (cps) under N2 at 1000∗ °C, as it is expected due to carbonized matter, curiously the opposite was ∗ ∗ # MgO 1000 °C γ 200 cps # observed for Zn2Al-CMCγ sampleγ (Figure S5b). γ ∗γ ∗ 1000 °C # ∗ γ ∗ 300 cps ZnAl2O4 -Al2O3 15 20 25 30 35 40 45 50 55 60 65 70 15 20 25 30 35 40 45 50 55 60 65 70 2θ (degree) 2θ (degree) (c) (d)

Figure 8. XRD patterns of Zn2Al-CO3 (a), Mg2Al-CO3 (b), Zn2Al-CMC (c) and Mg2Al-CMC (d) Figure 8. XRD patterns of Zn2Al-CO3 (a), Mg2Al-CO3 (b), Zn2Al-CMC (c) and Mg2Al-CMC (d) samples samples pyrolyzed at 500, 600, 800 and 1000 °C under N2 atmosphere. The phases were indicated pyrolyzed at 500, 600, 800 and 1000 ◦C under N2 atmosphere. The phases were indicated according according to ZnO (wurtzita; ICSD 26170), ZnAl2O4 (ICSD 75091), MgO (periclase; ICSD 9863), to ZnO (wurtzita; ICSD 26170), ZnAl2O4 (ICSD 75091), MgO (periclase; ICSD 9863), MgAl2O4 (ICSD MgAl2O4 (ICSD 31373) and γ-Al2O3 (ICSD 249140). 31373) and γ-Al2O3 (ICSD 249140). It can be proposed that graphitic carbonaceous material formed during the pyrolysis of hybrid LDH, and evidenced by Raman spectroscopy (Figure 6), hinders the formation of crystalline spinel phase in the obtained MMO/C nanocomposites, and allows the production of well crystallized ZnO particles at lower temperatures compared to Zn2Al-CO3. Besides that, comparing the results of thermal analysis (Figure 5), Raman spectroscopy (Figure 6) and XRD (Figure 8), it is proposed that a carbothermic reaction is occurring between 800–1000 °C, i.e., ZnO is reduced to metal form by carbon (from CMC pyrolysis) and released as vapour (zinc boiling point is 920 °C), as represented by the following chemical equation:

ZnO(s) + C(gr) → Zn(v) + CO(g). (1) Metal zinc is produced by five industrial processes involving mainly the pyrometallurgical method. Carbon is mixed with ZnO and heated at about 1000–1300 °C to produce the metal in the vapour phase [44]. The presence of aluminum oxide at Zn2Al-CMC pyrolyzed at 1000 °C can indicate the partial reduction of spinel phase as observed for the mineral gahnite (a zinc aluminate of ZnAl2O4 composition) in the presence of carbon [45]. Hence, a spinel phase decomposition could be represented by the following chemical equation:

ZnAl2O4(s) + C(gr) → Al2O3(s) + Zn(v) + CO(g). (2)

A discussion about thermodynamic aspects of reaction (1) can be found in Supplementary Materials (Table S4 and Table S5). The carbothermic reaction can explain the disappearance of ZnO ChemEngineering 2019, 3, 55 12 of 17

XRD patterns of Mg2Al-CMC pyrolyzed between 500–1000 ◦C indicate the formation MgO phase which crystallinity is increased with the temperature evolution (Figure8). Similarly, Mg 2Al-CO3 sample calcined between 500–800 ◦C shows reflections of MgO. However, when calcined at 1000 ◦C, MgO and MgAl2O4 (spinel) phases are observed. The organic polymer intercalated into LDH modified the degree of organization of the inorganic portion of pyrolyzed products. If calcined under an air atmosphere at 1000 ◦C, both LDH-CMC materials produce metal oxide and spinel phases (Figure S5a) likewise LDH-CO3 samples under nitrogen atmosphere (Figure8). In addition, whereas the Mg 2Al-CMC sample calcined under synthetic air presented less residual mass than that calcined under N2 at 1000 ◦C, as it is expected due to carbonized matter, curiously the opposite was observed for Zn2Al-CMC sample (Figure S5b). It can be proposed that graphitic carbonaceous material formed during the pyrolysis of hybrid LDH, and evidenced by Raman spectroscopy (Figure6), hinders the formation of crystalline spinel phase in the obtained MMO/C nanocomposites, and allows the production of well crystallized ZnO particles at lower temperatures compared to Zn2Al-CO3. Besides that, comparing the results of thermal analysis (Figure5), Raman spectroscopy (Figure6) and XRD (Figure8), it is proposed that a carbothermic reaction is occurring between 800–1000 ◦C, i.e., ZnO is reduced to metal form by carbon (from CMC pyrolysis) and released as vapour (zinc boiling point is 920 ◦C), as represented by the following chemical equation: ZnO + C Zn + CO . (1) (s) (gr) → (v) (g) Metal zinc is produced by five industrial processes involving mainly the pyrometallurgical method. Carbon is mixed with ZnO and heated at about 1000–1300 ◦C to produce the metal in the vapour phase [44]. The presence of aluminum oxide at Zn2Al-CMC pyrolyzed at 1000 ◦C can indicate the partial reduction of spinel phase as observed for the mineral gahnite (a zinc aluminate of ZnAl2O4 composition) in the presence of carbon [45]. Hence, a spinel phase decomposition could be represented by the following chemical equation:

ZnAl O + C Al O + Zn + CO . (2) 2 4(s) (gr) → 2 3(s) (v) (g) A discussion about thermodynamic aspects of reaction (1) can be found in Supplementary Materials (Tables S4 and S5). The carbothermic reaction can explain the disappearance of ZnO diffraction peaks (Figure8) and the formation of γ-Al2O3 phase. As it is expected, under synthetic air this reaction is not observed because CMC is fully decomposed at 600 ◦C (data not shown). To the best of our knowledge, this is the first time that carbothermic reaction is reported for zinc-LDH intercalated with organic species. SEM micrographs shown different morphological transition from LDH to oxides phases according to M2+ cation composition (M2+ = Mg2+ or Zn2+). The micrographs of LDH-CMC precursor samples (Figure9a,b) exhibit a lack of surface distinction among particles, showing up as a continuous mass. This observation can be interpreted as a consequence of small particles formation as already suggested by XRD patterns (Figure3); morphology of LDH-CMC samples are very distinct of M 2Al-CO3 phases, which interparticle frontiers are usually easily distinguished (Figure S6). The presence of the polymer chains also on the external surfaces of LDH particles can contribute to the continuous mass aspect seen in the images. Through an agglomeration process, which occurs along with the sedimentation of particles from reactional media, high surface energies acting over a large number of very small particles can generate such aspect, and behave as a cohesive mass. ChemEngineering 2019, 3, 55 13 of 17 ChemEngineering 2019, 3, 55 14 of 17

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 9. SEM micrographs of MMO/C samples obtained from Zn Al-CMC (left) and Mg Al-CMC Figure 9. SEM micrographs of MMO/C samples obtained from Zn2Al-CMC (left) and Mg2Al-CMC (right) as synthesized (a,b) and pyrolyzed at 800 (c–f) and 1000 C(g,h). (right) as synthesized (a, b) and pyrolyzed at 800 (c, d, e, f) and ◦1000 °C (g, h).

ChemEngineering 2019, 3, 55 14 of 17

Morphological alterations are evident when the pyrolysis process is carried out at 800 ◦C, producing distinguishable particles at a nanoscale (Figure9c–f). Thermal decomposition of Zn 2Al-CMC agglomerates gives rise to denser and zinc/oxygen richer particles as shown respectively by brighter secondary electron contrast (Figure9c) and EDS spectra (Figure S7). Zn 2Al-CMC pyrolyzed at 800 ◦C is composed mainly by well-defined crystallites reaching up to hundreds of nanometers in size and connected though their grain boundaries (Figure9e). These observations agree with its respective XRD pattern (Figure6), which evidences the arising of a high crystalline ZnO phase. In comparison, the absence of prominent morphologies is noticed for Mg2Al-CMC pyrolyzed at 800 ◦C (Figure9d), the sole new feature is the peeling appearance (Figure9f). Possibly CMC thermal decomposition promotes the weakening and rupture of interfacial interactions among particles, previously acting as an interparticle connector; the subsequent gas evolution from polymer decomposition and matrix dehydroxylation (Figure5) can also corroborate to the breakdown of the cohesive mass. A new set of superficial transformations takes place when Zn2Al-CMC material is pyrolyzed at 1000 ◦C (Figure9g). The zinc rich phase previously observed at 800 ◦C completely vanishes and a new one emerges, as can be visually depicted mostly as a collection of anisotropic acicular nanocrystals, assembled as branched particles onto the MMO/C composite surface. Besides, EDS spectra collected from this sample reveal considerable decrease in Zn signal (Figure S8). This observation is indicative of ZnO depletion owing to the carbothermic reduction, as suggested by thermal analysis (Figure5), and XRD pattern recorded for Zn2Al-CMC pyrolyzed at 1000 ◦C (Figure8). When comparing EDS spectra of acicular particles (Figure S8) to those observed at 800 ◦C, it is observed an inversion in the intensity between Zn and Al peaks. Branched particles produce EDS spectra with higher aluminum and oxygen intensities compared to those collected from their vicinity, indicating they are composed by Al2O3 (higher Al and O content) as observed by XRD for Zn2Al-CMC pyrolyzed at 1000 ◦C (Figure8). The superficial features of Mg2Al-CMC pyrolyzed at 800 ◦C are preserved when heated up to 1000 ◦C (Figure9h). Also, no significant changes are detected in Mg /Al/O composition by EDS spectroscopy (data not shown) for samples obtained at both high temperature values. Nanocomposites prepared in this study from a carbon precursor derived from biomass intercalated between LDH based on magnesium and aluminum in the 600–1000 ◦C range can be explored as adsorbent or catalyst. For example, composite constituted by MgO and biochar obtained by the electrochemical method, which was used for phosphate removal from water [46]. Nanocomposites obtained by Zn2Al-CMC pyrolysis in the 600–800 ◦C range produce incrusted ZnO particles in a matrix of graphitic carbon and spinel phase. is a semiconductor with potential to be employed in devices for energy production such as dye-sensitized solar cells [47].

4. Conclusions LDH materials intercalated with CMC polymer, a carbon source derived from biomass, were evaluated as a precursor for the preparation of MMO/C nanocomposites. Best experimental conditions were achieved to get carbon chains between the LDH layers to assure an organic-inorganic interaction at a nanoscale domain. Pyrolysis step conducted above 600 ◦C produced materials comprising graphitic carbon, which structural organization is increased at higher temperature values. In samples obtained from Mg2Al-CMC, the biocarbon presence delays the crystallization of MgO when compared to Mg2Al-CO3 calcined at the same temperature, and also precludes the crystallization of MgAl2O4. On the other hand, biocarbon formed from Zn2Al-CMC pyrolyzed at 800 ◦C promotes the generation of high crystalline ZnO particles if compared to Zn2Al-CO3 calcined at the same temperature, and also prevents crystallization of ZnAl2O4 phase. Above about 880 ◦C (endothermic peak in DSC curve), ZnO is reduced by graphitic carbon producing zinc vapour (carbothermic reaction); poor crystallized ZnAl2O4 spinel and γ-Al2O3 phases are noticed after pyrolysis at 1000 ◦C. Considering the results obtained in this study, to prepare nanocomposites constituted by graphitic carbonaceous material and metal oxides from CMC hybrid materials, the temperature range of pyrolysis process should be 600–800 ◦C for Zn2Al-CMC and 600–1000 ◦C for Mg2Al-CMC. Future studies should investigate ChemEngineering 2019, 3, 55 15 of 17 the surface properties of these nanocomposites to explore application in adsorption processes, basic catalysis or photodevices.

Supplementary Materials: The following are available online at http://www.mdpi.com/2305-7084/3/2/55/s1, Figure S1: XRD pattern of sodium carboxymethylcellulose (NaCMC), Figure S2: TG-DSC and DTG-MS curves of sodium carboxymethylcellulose (NaCMC) under N2 atmosphere, Figure S3: Raman spectra (λexc = 532 nm) of NaCMC and LDH-CMC samples, Figure S4: FTIR-ATR spectra of Zn2Al-CO3 and Mg2Al-CO3 samples calcined at 500, 600, 800 and 1000 ◦C under N2 atmosphere, Figure S5: XRD patterns of M2Al-CMC samples calcined at 1000 ◦C under synthetic air. Pictures of M2Al-CMC samples calcined at 1000 ◦C under synthetic air or nitrogen atmosphere, indicating the percentage of residue after each heating process, Figure S6: SEM image of Mg2Al-CO3 sample, Figure S7: EDS spectra of Zn2Al-CMC pyrolyzed at 800 ◦C, Figure S8: EDS spectra of Zn2Al-CMC pyrolyzed at 1000 ◦C; Table S1: Interplanar distances dhkl and 2θ (CuKα) values from X-ray diffraction data of LDH-CO3 and LDH-CMC, Table S2: Vibrational data of Zn2Al-CO3 and Mg2Al-CO3, Table S3: Chemical analysis data and proposed formula for LDH-CO3 and LDH-CMC, Table S4: Standard molar thermodynamic parameters of substances at 25 ◦C, Table S5: Standard molar thermodynamic values for the carbothermic reaction between graphitic carbon and zinc or magnesium oxides at 25 ◦C. Values are calculated using the data presented in Table S4. Author Contributions: Conceptualization, V.R.L.C., G.F.P. and V.R.M.; methodology, V.R.L.C., G.F.P. and V.R.M; formal analysis, G.F.P., V.R.M. and A.D.; investigation, G.F.P. and V.R.M.; resources, V.R.L.C.; data curation, V.R.L.C.; writing—original draft preparation, V.R.L.C., G.F.P., V.R.M. and A.D.; writing—review and editing, V.R.L.C., G.F.P., V.R.M. and A.D.; visualization, V.R.L.C., G.F.P., V.R.M. and A.D.; supervision, V.R.L.C.; project administration, V.R.L.C. Acknowledgments: This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 33002010191P0—Química (scholarship to V.R.M.). V.R.L.C. is thankful to the Conselho Nacional de Desenvolvimento Científico e Tecnológico—Brasil for the research grant (CNPq 305446/2017-7). The authors are also grateful to the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP 2011/50318-1) for financial support and to the Laboratório de Espectroscopia Molecular (LEM-USP) for the Raman and FTIR spectra recording. Conflicts of Interest: The authors declare no conflict of interest.

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