Carboxymethylcellulose template synthesis of porous aluminium
oxide from hybrid spheres: influence of the degree of substitution
and polymerization
Monickarla Teixeira Pegado da Silva1•Julyana Cardoso Carvalho1• Sibele Berenice Castella˜ Pergher1•Tiago Pinheiro Braga1
Ó Springer Science+Business Media New York 2016
Abstract Catalytic supports based on aluminum oxide were synthesized by the method of hybrid spheres using carboxymethylcellulose as organic precursor (template) and aluminum nitrate as inorganic precursor. The charac-terizations were performed by analysis of TG, XRD, FTIR, SEM and N2 physisorption. The study of synthesis indi-cated that the characteristics of the biopolymer (degree of substitution and polymerization) directly influence on the limit ratio between organic and inorganic precursor in order to observe the formation of the hybrid spheres. The physicochemical properties of the final material (structure by XRD, texture by N2physisorption and morphology by SEM) showed a direct dependence with the biopolymer properties, indicating the versatility of this synthetic route. FTIR spectra confirm the formation of a hybrid material, comparing the pure CMC spectrum with the solids after drying. N2adsorption/desorption isotherm and SEM ima-ges confirm the formation of highly porous materials with a specific surface area between 50 and 162 m2/g.
Keywords Catalytic supports Hybrid spheres Aluminum oxide Carboxymethylcellulose Pores
1 Introduction
The materials used to sustain an active phase of a supported catalyst are known as catalytic supports [1,2]. These solids must contain specific physicochemical properties, such as thermal stability, the resistance to chemical attacks, as well as structural, textural, morphological and acid–base char-acteristics appropriate to the type of applied reaction [3–9]. Silica, alumina, amorphous silica-alumina, zeolites, molecular sieves, activated carbon and clays are the catalytic supports most commonly used in the chemical industry and frequently studied by the scientific community [1,2,10–15]. It is well known that aluminium oxide can act as a appro-priate material for the distribution of active sites as a catalyst support due to its favorable textual properties, high thermal stability and moderate Lewis acidity [10,11].
The use of supports in the catalysts composition as aluminium oxide normally provides numerous advantages such as high specific surface area of the active phase, porosity appropriate to the type of chemical reaction, controlled acidity/basicity, minimize catalytic sintering of the active site due to metal-support interaction, and in some cases, reduce the excessive coke deposition, which nor-mally lead to excellent results of activity, selectivity and stability for the solids [16–22].
The literature shows different methods for the synthesis of alternative catalytic oxide supports, such as the copre-cipitation method [22, 23], conventional sol–gel [23, 24], polymeric precursors (Pechini) [22, 25], polyalcohol [26, 27], homogeneous hydrolysis [28], among others.
Among these routes used to prepare catalysts, there is an intense concern to make these methods more efficient, economic, fast and that at least one of the precursors of the synthesis is derived from a renewable source in order to obtain a more environmentally friendly process [29–33]. & Tiago Pinheiro Braga
tiagoquimicaufrn@gmail.com
1 LABPEMOL-Laborato´rio de Peneiras Moleculares, Instituto
de Quı´mica, Universidade Federal do Rio Grande do Norte, Natal, RN CEP 59078-970, Brazil
The interest in new methods of synthesis using organic precursors (templates), normally organic polymers, com-bined with inorganic precursors aims to obtain materials with a high specific area and an excellent pore size distri-bution as well as allows the control of the structural (crystallite size) and morphological properties, which is possible due to the controlled decomposition/oxidation of the organic matter during calcinations [29]. Thus, the type of polymer and its chemical characteristic, such as its structure (functional groups present) and molecular weight (chain length) may directly influence the physicochemical properties of the final material [33].
On the other hand, most of these routes presented in the literature, which use this type of synthesis, utilize organic precursors from non-renewable sources. Therefore, the development of new synthetic routes using alternative organic biopolymers remains a challenge to be explored.
Carboxymethylcellulose (CMC) is a biopolymer derived from natural cellulose and due to the insertion of car-boxylic groups along its chain, Fig.1, become more sol-uble in water and acquires the potential to act as an excellent complexing agent of metallic cations such as Al3?, which is very interesting during the synthesis of metal oxide catalysts [34,35].
This biomaterial, CMC, has unique properties such as high viscosity, transparency, non-toxic, hydrophilicity, biocompatibility, biodegradability and excellent film forming ability. Furthermore, CMC has been often used as a food additive in products like ice cream, white wine, sparkling, among others [36,37]. However, the feasibility of using the CMC as an organic biotemplate for the preparation of porous catalytic supports (oxides) with high metal oxide dispersion still remains an unexplored research topic. It is important to emphasize that the flexibility to achieve different degrees of polymerization (molecular weight) and different degrees of substitution (the number of carboxymethyl groups) for the CMC makes this biopolymer a potential organic template for the preparation
of catalytic supports with interesting physicochemical properties. Previous studies demonstrate that molecular weight, degree of substitution and the molar ratio between organic (CMC) and inorganic (metallic cations) may influence the interaction of carboximetilcelusose with other compounds [33,38–41].
Thus, the purpose of this work is to prepare hybrid spheres from carboxymethylcellulose and Al3?in order to obtain aluminum oxide using an alternative synthetic route. Furthermore, it was studied the influences of the degree of polymerization and substitution (biopolymer properties) on the ideal relationship between organic and inorganic material to favor the formation of the hybrid spheres and in parallel the effects of these characteristics on the physic-ochemical properties of the final material.
2 Experimental
2.1 Support preparation
2.1.1 Hybrid spheres method
The synthesis of aluminum oxide were performed from a hybrid sphere, using different molar stoichiometric ratios between the inorganic/organic components in order to study the effect of the inorganic/organic relationship on the structural, textural and morphological proprieties. Three different samples of biopolymers have been used, varying the degree of polymerization and substitution of CMC. For the CMC1 and CMC2 were varied the degree of poly-merization (DP) and the degree of substitution (DS) was kept fixed. However, for the CMC1 and CMC3 were varied the degree of substitution and the degree of polymerization was kept fixed (CMC1: DP = 954 and DS = 0.7; CMC2:
DP = 343 and DS = 0.7; CMC3: DP = 954 and
DS = 1.2). The DS and DP values were obtained from the reagent bottle according to sigma-Aldrich.
Fig. 1 Monomer unit of the carboxymethylcellulose
For each CMC was varied the inorganic/organic rela-tionship in order to show that the degree of substitution and the degree of polymerization affect on the characteristics of the supports and on formation of the hybrid spheres during the dripping. For each biopolymer were prepared five samples. The different solids were designated by AlX:Y-CMCZ, which X:Y is the molar relationship between inorganic:organic precursor and Z is the type of car-boxymethylcellulose. The following relationships were prepared for CMC1: Al1:1-CMC1, Al1:1.5-CMC1, Al1:2-CMC1, Al1:2.5-CMC1 and Al1:3.5-CMC1; for the CMC2: Al1:1.5-CMC2, Al1:2-CMC2, Al1:2.5-CMC2, Al1:3.5-CMC2 and Al1:4-Al1:3.5-CMC2; for the CMC3: Al1:0.5-CMC3, Al1:1-CMC3, Al1:1.5-CMC3, Al1:2-CMC3 and Al1:2,5-CMC3. It was kept constant the amount of the inorganic precursor and it was varied the amount of the organic precursor.
The formation of spheres follows the scheme presented in the flowchart of Fig.2, which initially dissolves an adequate amount of nitrate aluminium salt into the beaker containing distilled water and concomitantly in another beaker occurs the biopolymer dispersion in distilled water (50 mL of water for each gram of CMC) under constant temperature of 60°C and stirring to observe its total dis-persion. The same process was repeated for all the other synthesis.
Subsequently, with the aid of a burette, a solution taining CMC was dropped steadily into the beaker con-taining the aluminum nitrate precursor under light and constant stirring, immediately leading to the formation of the hybrid spheres due to the interaction of the functional groups of CMC with Al3?ions. After, the spheres remain in nitrate solution to guarantee complete complexation of Al3? ions with functional groups of CMC. Finally, the hybrid spheres were removed from the nitrate solution using a sieve, dried at room temperature for 24 h and calcined at 600°C for 2 h with a heating rate of 4 °C/min in a muffle furnace under an air atmosphere.
2.2 Support characterization
Thermogravimetric analysis (TG) were performed in a equipment NETZSCH TG 209F3 model, using a platinum crucible, air flow, and a heating ramp of 25–1000°C with a heating rate of 10°C/min in order to observe weight loss with temperature variation and the minimum temperature for complete decomposition/oxidation of the biopolymer.
The X-ray diffraction analyses were carried out on a Bruker D2 Phaser diffractometer using CuKa radiation (k = 1,54A˚ ) with an Ni filter, with step 0,028, current of 10 mA, voltage of 30 kV, using a Lynxeye detector to determine the crystal structure of the synthesized solid. The analysis was performed with an angle 2h range from 10 to 90 degree. The crystallite size calculated by using Scherrer equation.
FTIR analyses were performed to determine stretches present in the samples before and after calcinations as well as for the pure CMC. The range of wavenumber was 698 to 4000 cm-1 for transmittance using a spectrometer Perkin Elmer model in order to confirm the elimination of the CMC after calcination and the formation of a hybrid material before calcination. The analyses were performed in KBr pellets containing 1.0 wt% of the sample, before and after calcinations process for comparative analysis.
The scanning electron microscopy was used to visualize the morphology and the visual porosity of the materials. SEM was performed with a JEOL JSM 6060 microscope. The accelerating voltage was 20 kV and it was applied different magnifications. The ultra high resolution Schottky Field Emission Scanning Electron Microscope analyses were made using a field electron gun (FEG-SEM) from Tescan-MIRA3.
The N2 physisorption analysis were carried out at a temperature of 77 K (-196°C) in a gas adsorption ana-lyzer Autosorb-1C model, Quantachrome Instruments to obtain the textural properties of the support. Prior to analysis, the samples were degassed under vacuum at 200 °C for 2 h. This treatment aims to remove moisture and CO2 from the solid surface. From the obtained iso-therms were extracted the specific surface area from BET method, pore volume and pore diameter values from BJH method.
3 Results and discussion
3.1 Synthesis study
The initial results of synthesis study for the CMC1, CMC2 and CMC3, it was performed in order to confirm the effect of the degree of substitution and polymerization for the carboxymethylcellulose on the ideal molar ratio between Fig. 2 Scheme of the synthesis for the aluminium oxide spheres
inorganic and organic precursors to observe the formation of the hybrid spheres, Table1. The results show that for the CMC1, the ratio limits for the formation of the uniform hybrid spheres was 1.0 mol of carboxymethylcellulose (monomer) for 2.5 mol of Al3?. For larger amounts of CMC, a homogeneous solution was not obtained, and the flocculation of nitrate solution was observed, since the solution became very turbid after addition of a certain amount of CMC. For CMC2, the results indicated that the relationship limits, in this case, was 3.5 mol of CMC for 1.0 mol of Al3? (higher compared to CMC1). Finally, for the CMC3 the relationship limits was 2.0 mol of CMC for 1.0 mol of Al3?(lower related to CMC1).
Comparing the CMC1 and CMC2, which it was varied the degree of polymerization (DP) and the degree of sub-stitution (DS) was kept fixed, it was observed that the degree of polymerization for CMC influences directly the maximum amount of CMC (organic molar fraction), for formation of the homogeneous hybrid sphere, since the CMC1 has the ability to retain more Al3?ions compared to CMC2 due to higher amount of complexing radicals (car-boxymethyl groups), presenting lower ratio limit and consuming the total amount of Al3?previously. CMC1 has a larger amount of monomeric units compared to CMC2 and consequently more complexing groups.
Similarly, comparing the CMC1 and CMC3, which it was varied the degree of substitution and the degree of polymerization was kept fixed, it was perceived that the
degree of substitution also influences directly the maxi-mum amount of CMC (organic molar fraction), for for-mation of the homogeneous hybrid sphere, since the CMC3 retain more ions aluminium related to CMC1 due to the fact that the carboxymethylcellulose with a higher degree of substitution has a greater amount of carboxymethyl groups and consequently consumes the total amount of Al3? earlier, requiring less amount of organic (CMC) to complex all the ions Al present.
Additionally, similar to Table 1, the results in Fig.3 also illustrate the effect of the degree of substitution and polymerization for the carboxymethylcellulose on the ideal molar ratio between organic and inorganic precursors. The data of the Table1were plotted in graphical form in order to observe the regions where there is the formation of homogeneous hybrid spheres, depending on characteristics of the biopolymer. It is observed that the area to the left, Fig.3a, are values of aluminum molar fraction and degree of polymerization for the CMC1 and CMC2 samples, which there is the formation of uniform hybrid spheres (according to images shown in Fig.4a). However, the area to the right does not occur the complete formation of uniform hybrid sphere after the addition of the total amount of Al3? (as can be seen in Fig.4b), observing the floccu-lation of the solution in the beaker. The intermediary area in view, between the formation and non formation of hybrid spheres, it was not explored in this initial study, since the chosen values were enough to clearly show the tendency.
Correspondingly, Fig. 3b shows the regions where there is the full formation of hybrid spheres (no flocculation of aluminum nitrate solution) and the region where does not occur the complete formation of uniform hybrid spheres, however, in this case, the values are a function of alu-minum molar fraction and the degree of substitution. Hence, it is extremely important to check the DP and DS values for the CMC before synthesis in order to choose the appropriate relationship to obtain uniform hybrid spheres and consequently homogeneous aluminum oxide after calcination.
3.2 Characterizations
3.2.1 Thermogravimetric analysis (TG)
Analyzing the curves shown by TG, Fig.5, it was observed that the degradation occurs in four major steps, presenting distinct mass loss rates of hybrid spheres. The first region between 50 and 135 °C is due to the release of water physically adsorbed with 2.7 % of mass loss, characteriz-ing as the region with the lowest mass loss. The second stage of weight loss occurred in the temperature range of 135–205 °C, with a maximum removal rate at 170 °C, Table 1 Relationship between the aluminum ions:monomer of CMC
with the formation of the uniform hybrid spheres for the biopolymers 1, 2 and 3
Type of CMC Molar relation* Formation of the spheres**
1 1:1,0 ? 1 1:1,5 ? 1 1:2,0 ? 1 1:2,5 ? 1 1:3,0 -2 1:1,5 ? 2 1:2,0 ? 2 1:2,5 ? 2 1:3,5 ? 2 1:4,0 -3 1:0,5 ? 3 1:1,0 ? 3 1:1,5 ? 3 1:2,0 ? 3 1:2,5
-* The relation was established between the aluminium ions and the monomer of CMC
** ? = There was the formation of the uniform hybrid spher-es, - = there was not the formation of the uniform hybrid spheres
correlates with the liberation of water molecules physically linked in the sample and the beginning of the nitrate decomposition. This region was observed approximately 3.6 % of mass loss.
The third mass loss range is due to the beginning of organic material decomposition, which occurs between 205 and 290°C, with a maximum rate of elimination at 250°C. Additionally, this second stage may also be
associated with the decomposition of nitrates (NOx) from the inorganic precursor, Al(NO3)39H2O [40–42]. This temperature range was presented approximately 6.1 % of mass loss.
The fourth step (last stage in two events), between 290 and 650°C, most of the organic material is released due to the oxidation/decomposition of organic matter from car-boxymethylcellulose leading to the formation of pores and concomitantly the aluminum oxide. The last range was observed approximately 8.8 % of mass loss, indicating the region with the higher mass loss [42–44].
The complete liberation of the organic and inorganic precursors takes place at approximately 650°C. It is important to emphasize that the complete release of organic/inorganic compounds during calcinations is essential for the formation of a highly porous solid from the cavities generated during decomposition/oxidation of CMC and Al(NO3)39H2O [29,31,33].
3.2.2 X-ray diffraction (XRD)
XRD results are presented in Fig.6, which it was observed clear differences in the XRD patterns of the various cata-lyst supports, depending on the molar ratio between the Fig. 3 aDegree of polymerization according to the mole fraction of
CMC for the CMC 1 and 2. b Degree of substitution according to the mole fraction of CMC for the CMC 1 and 3. (open square) Region
that there is the formation of hybrid spheres, (filled square) region that there is not formation of uniform hybrid spheres
Fig. 4 Image of the hybrid spheres immediately after the synthesis process. a uniform hybrid spheres, b heterogeneous hybrid spheres
0 100 200 300 400 500 600 700 800 900 1000 70 75 80 85 90 95 100 105 110 TG DTG DTG Temperature (°C) Mass (%) Al-1:1-CMC1 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
organic and inorganic material as well as it depends on characteristics of the biopolymer. In the XRD pattern of Fig.6a is shown the XRD patterns of pure CMC and the solid Al1:1-CMC1, Al1:1.5-CMC1 and Al1:2.5-CMC1, respectively. It is clearly noted a profile related to an amorphous solid even after calcination and liberation of the CMC for the Al1:2.5-CMC1.
This may be due to the small particle size, which cannot be identified due to limitation of the technique. It is important to highlight that, for this type of synthesis, the biopolymer acts hindering the organization and packaging for the formation of the crystalline structure (aluminium oxide), leading to the formation of extremely small crys-tallites with very high metal oxide dispersion and/or with a large number of defects (microstrain).
Despite the difficulty to identify crystalline phases in Fig.6a, the XRD pattern of Al1:2.5-CMC1 sample has been expanded 39 and it was perceived that they differ from the other XRD pattern, since have a profile of semi-amorphous solid with the presence of two broader
and less intense peaks (2h angles of 15.4 and 31.8°), which were identified as aluminum oxide and/or hydrated aluminum oxide compared to the standards of the HighScore software package (Al2O3nH2O, JCPDS-01-070-0384 and/or Al2O3, JCPDS-047-1292), accord-ing to Fig.6a. However, the solids Al1:1-CMC1 and Al1:1.5-CMC1 showed an XRD pattern of crystalline material related to the formation of hydrated aluminum oxide (Al2O3.3.H2O, JCPDS-00-047-1292). Thus, it was confirmed that increasing the amount of organic (CMC) in relation to the inorganic (Al3?) hinders the crystallization of aluminum oxide during calcinations and consequently prevents the observation of diffrac-tograms with crystalline diffraction profiles (defined peaks). The nanoparticles formation was confirmed by calculating the crystallite size using the Scherrer equation and the values are presented in Table 3. It observed a similar effect for the crystallites size related to the crystallinity depending on characteristics of the biopolymer (CMC). CMC1 a {101} {111} {100} {001} Al1:1-CMC1 {112} Al1:2,5-CMC1 x3 Intensity (a.u) Al1:1,5-CMC1 10 20 30 40 50 60 70 80 90 Al2O3.3H2O Na2O2 2θ (°) Al1:3.5-CMC2 b Intensity (a.u) Al1:2.5-CMC-2 {220} {101} {001} {100} {111} Al1:1.5-CMC2 CMC 2 10 20 30 40 50 60 70 80 90 Al2O3 Al2O3.3H2O Al2O3 Al2O3.nH2O 2θ (°) {112} {220} {111} {100} Al1:1-CMC 3 {001} c Intensity (a.u) Al1:2-CMC 3 Al1:1.5-CMC 3 10 15 20 25 30 35 40 45 50 55 60 65 70 Al2O3.3H2O Al2O3 2θ (°)
On the other hand, the diffractograms of Fig.6b, for the samples synthesized from CMC2, clearly shows the effect of the molar ratio between inorganic and organic precursors on the structural properties of the final oxide. It is noticed that increasing the amount of organic related to inorganic, it was observed an increase in the amount of aluminium oxide compared to hydrated aluminum oxide, since the solids Al1:1.5-CMC2 and Al1:2.5-CMC2 presented practically the formation of hydrated aluminum oxide (Al2O33H2O, JCPDS-00-047-1292), however, the support Al1:3.5-CMC2 indicated the mixture of aluminum oxide (Al2O3, JCPDS-047-1292) and hydrated aluminum oxide (bayerite). Com-paring the XRD pattern of the sample Al1:3.5-CMC2 with the materials Al1:1.5-CMC2 and Al1:2.5-CMC2, it was visualized a decrease of the peaks related to hydrated alu-minum oxide (18.3° and 20.1° 2h) and an increase of the reflections concerning the aluminum oxide (15.4° and 31.8° 2h). Fixing the type of CMC and increasing the amount of organic, the complexation of the inorganic fraction may increase, which may explain the results observed in Fig.6b. For samples prepared from CMC3, Fig.6c, the influence of the ratio between inorganic and organic on the crystallinity was not noticeable, as seen in the Fig.6b, since for all three diffractograms the most representative peaks are relating to the hydrated aluminum oxide, bayerite (Al2O33H2O, JCPDS-00-047-1292), presenting almost no difference among them.
However, fixing the relationship between inorganic and organic of 1:1.5, considering that all CMC used this rela-tionship, it is possible to confirm that the physicochemical properties of the biopolymer (degree of substitution and polymerization) besides the molar ratio also affect the structural characteristics of the final oxide, since the solids Al1:1.5-CMC2 with the lowest degree of substitution and polymerization showed a better crystallinity and smaller crystallite size (most intense peaks) compared to the sup-port Al1:1.5-CMC3 with highest degree of substitution and polymerization probably due to the fact that with a lower molar mass (lower repetition units of monomer) and an inferior amount of carboxymethyl groups, the metallic dispersion of aluminum ions in the polymer matrix is more concentrated (more aggregated), facilitating its crystal-lization during the calcination step.
The formation of hydrated aluminium oxide at 600°C, which it is not consistent with the normal sequence of the transition aluminas, may be related to the synthesis method employed. The use of organic agent during the synthesis hinders the sintering/coalescence of particles and conse-quently disfavoring the crystallization of the inorganic fraction, justifying the permanence of Al2O33H2O phase even after calcination at 600°C. From the thermogravi-metric analysis, Fig.3, it is clearly observed that at 600°C still remains a small fraction of the organic portion, which is completely removed between 600 and 700°C.
3.2.3 Fourier transform infrared spectroscopy (FTIR)
In carboxymethylcellulose spectrum, Fig. 7, it was clearly observed bands near 1588, 1411 and 1023 cm-1 assigned the following vibrations massimCO (–COO–), msimCO (–COO–) and tco corresponding to the C–O–C groups of ether radicals of the polysaccharide, respectively.
In the spectrum of spheres after drying step, it can observe the 3377 cm-1interaction related to the mOH(H2O and –OH). The carboxymethylcellulose interactions with Al3? ions were confirmed by infrared analysis due to the absorption bands characteristics of the functional groups of the CMC were also observed in samples after drying, however, become less intense and shifted toward shorter wavelengths, which characterizes the interaction of CMC with Al3?, Fig.7. This information confirms the formation of a hybrid material [45–48].
The infrared spectra after calcinations, Fig.7, confirm the complete decomposition of CMC, since it cannot be seen stretches concerning the functional groups of car-boxymethylcellulose, which are in accordance with the TG result (Fig.5), indicating that there was the formation of aluminum oxide, without or with an insignificant amount of CMC.
3.2.4 Scanning electron microscope
From the analysis by scanning electron microscopy, shown in Fig.8, it was observed that the average diameter of spheres is 1.5 mm for the sample after drying. On the other hand, for the same sample after calcinations, it can be
4000 3500 3000 2500 2000 1500 1000 Transmitance (a.u) wavenumber (cm-1) CMC COO --CH2 -C-O-C-H2O and -OH -CH
Al1:1-CMC1 (after drying) Al1:1-CMC1 (after calcination)
Fig. 7 Fourier transform infrared spectroscopy results. Before calci-nation and after calcicalci-nation
observed cracks and fissures in the exterior surface of the sphere demonstrating its low mechanical resistance.
Figure8 shows the morphology of the supports after calcinations at 600°C and maceration of the spheres. The spheres were broken to observe its porosity.
As can be seen in Fig.8, the solids have a sponge-like morphology, which is justified by the use of the organic template (CMC) during the preparation of catalytic sup-ports. The pores are formed by removal of volatile mate-rials during the calcination step, producing a spongy morphology of aluminum oxide due to the cavities gener-ated [29, 31, 33]. It is expected that a large amount of organic matter complexed with aluminum ions per mono-meric unit result in a lower final porosity of the material after heat treatment, since Al3? ions are closer to each
other (lower metallic dispersion in the CMC matrix) and when the samples undergo thermal treatment lead to the formation of less porous solids of aluminium oxide. Hence, the control of the physicochemical properties of the biopolymer such as the degree of substitution and poly-merization also affect the porosity of the final material.
3.2.5 N2physisorption analysis
The samples CMC1, CMC2, Al1:1.5-CMC3 and Al1:0.5-Al1:1.5-CMC3 were selected to be analyzed by nitrogen adsorption–desorption isotherms in order to obtain information about the effect of the physicochemical prop-erties of the biopolymer on textural propprop-erties of the sup-ports, Fig. 9.
Fig. 8 Scanning Electron Microscopy images of the spheres with different magnifications
The result presented by the nitrogen adsorption–des-orption isotherms indicate a curve with type IV profile according to the classification of IUPAC, characteristics of mesoporous materials and hysteresis loop in this relative pressure region (between 0.3 and 1.0, depending on the type of biopolymer). Furthermore, for small values of rel-ative pressure (P/Po), it is clearly perceives the presence of micropores, presenting adsorption at very low relative pressure (P/P0\ 0.1). The samples presented H3 hysteresis loop, indicating the existence of non-rigid aggregates of plate-like particles or assemblages of slit-shaped pores. Therefore, this support has a non-homogeneous distribu-tion of pores containing micro-mesoporous and the majority of mesopores are located in the range between 20 and 310 A˚ (0.002 and 0.031 lm). The irregularity in pore size is also visually observed in the SEM images (Fig.8), since it was seen very different sizes of the cavities. However, it is important to emphasize that, in spite of the comparison between the two techniques, the magnitude of the pores visualized by SEM are quite different from those observed by nitrogen physisorption, since the measures have different operating principles and by SEM can be particularly observed the presence of macropores.
The textural properties (specific surface area, pore vol-ume and diameter) are listed in Table2. It is noticed that the obtained specific area value is very interesting com-pared to other aluminium oxide supports described in the literature, especially for the solid Al1:1.5-CMC1 [49–56]. Table3 shows values of surface area and porosity for different solids described in the literature, confirming the
viability of this synthesis route for the support preparation. It is important to emphasize that our results are related to the hydrated aluminum oxide which have a lower surface area compared to dehydrated aluminum oxide like c-Al2O3 which have a high surface area.
Comparing the Al1:1.5-CMC1, Al1:1.5-CMC2 and Al1:1.5-CMC3, which it was varied the type of CMC and the relationship between organic and inorganic material was kept fixed, it was observed that the degree of poly-merization and the degree of substitution for the CMC also influences on the textural properties of the aluminium oxide.
For the samples Al1:1.5-CMC1 and Al1:1.5-CMC2, where it was varied the degree of polymerization and the degree of substitution was kept fixed, it was noted that the biopolymer with higher molecular weight (larger amount of monomeric units), Al1:1.5-CMC1, leads to a better specific surface area compared to the solid Al1:1.5-CMC2 with an inferior molecular weight, indicating that the concentration of Al3?in the CMC matrix is greater with a small amount of monomeric units and consequently a greater aggregation of aluminium ions in the organic matrix, which will result in a smaller surface area after the liberation of organic material during calcination.
Similarly, comparing the solids Al1:1.5-CMC1 and Al1:1.5-CMC3, which it was varied the degree of substi-tution and the degree of polymerization was kept fixed, it was perceived that the sample Al1:1.5-CMC3 with a greater amount of carboxymethyl groups (high ability to complexation) presented smaller specific surface area and
0 200 400 600 800 1000 0.0 0.2 0.4 0.6 0.8 1.0 Al1:1.5-CMC1 a b Al1:0.5-CMC3 Volume (cm 3/g) Al1:1.5-CMC2 Al1:1.5-CMC2 Al1:0.5-CMC3 ∆V/∆logd (cm 3/g) Al1:1.5-CMC3 Pore diameter (Å) p/p0 Al1:1.5-CMC3 AL1:1.5-CMC1
Fig. 9 aNitrogen adsorption– desorption isotherms of the solids, bpore size distribution curves
pore volume compared to the Al1:1.5-CMC1 with a minor amount of carboxymethyl groups, confirming that greater complexation ability results in lower metallic dispersion in the polymer matrix and consequently causes a smaller surface area after heat treatment.
Furthermore, correlating the Al1:1.5-CMC3 and Al1:0.5-CMC3, which it was varied the relationship between organic and inorganic material and the type of
CMC was kept fixed, it was noticed that the relationship between organic and inorganic practically did not affect on the textural properties of the final oxide, since the surface area and pore volume values were very close, indicating that organic and inorganic ratio it is not a major factor compared to the biopolymer characteristics which influ-ence on the textural properties.
The scheme of Fig. 10illustrates the mechanism for the formation of pores with a high dispersion of metal oxide according to the obtained results from characterizations (TG, XRD, FTIR, SEM and N2 physisorption), justifying the important role of CMC in the oxide properties (struc-ture, texture and morphology).
The carboxylic acid groups present in carboxymethyl-cellulose (CMC) structure are responsible for complexa-tion of Al3? ions and subsequent the formation of the hybrid spheres (FTIR results), allowing the uniform dis-tribution of Al3?in the biopolymer matrix, considering that the metallic dispersion (aggregation) in organic matter is a function of the biopolymer characteristics as described previously (XRD and N2 physisorption results). The
Table 2 Textural properties from N2 physisorption analysis and
crystallite size from XRD
Samples SBET(m2/g) Vp(cm3/g) Dp(A˚ ) Dc(nm)
Al1:1.5-CMC1 162 0.28 98 62
Al1:1.5-CMC2 60 0.43 87 99
Al1:1.5-CMC3 50 0.18 143 40
Al1:0.5-CMC3 64 0.14 90 *
SBET specific surface area, Vp pore volume, Dppore diameter, Dc
crystallite size
* It was not possible to calculate the diameter for the sample Al1:0.5-CMC3 due to the absence of peaks in the diffractograms
Table 3 Comparison of the surface area and pore volume with previously reported values Samples Biotemplate Calcination temperature
(°C)/crystalline phase
SBET(m2/g) Vp(cm3/g) References
Al1:1.5-CMC1 Carboxymethylcellulose 600/bayerite 162 0.28 This study
Al700 Chitosan 700/c-Al2O3 231 0.34 [52]
Co/MAl2O3/P123 Natural rubber latex 400/c-Al2O3 180 0.35 [51]
c-Al2O3ceramic foam Polyurethane sponge 600/c-Al2O3 179 0.48 [50]
ATH-2/500 Glucose 500/c-Al2O3 251 0.39 [49]
Sample B * 200/Al(OH)3 103 ** [55]
V2 * 70/Al(OH)3 44 ** [56]
Bayerite * 80/bayerite 28 0.15 [57]
* It was not used a biotemplate route ** It was not reported the total pore volume
organic fraction difficult the sintering and crystallization process of the inorganic fraction during calcination, maintaining the high metal oxide dispersion even after calcination at 600°C and simultaneously conducting the formation of very small nanocrystallites (XRD results). The formation of the pores is explained by the structure, texture and morphology of the starting template (CMC), since it is generated vacancies (pores) due to the elimina-tion of the organic fracelimina-tion during calcinaelimina-tion (TG, FTIR and SEM images) and simultaneously causing the forma-tion of aluminum oxide (XRD results). The formaforma-tion mechanism of highly porous solid following the biopoly-mer template synthesis was previously described [29, 31, 33,57].
4 Conclusions
Aluminum oxide was synthesized by the hybrid spheres route, which it was used the biopolymer carboxymethylcel-lulose from celcarboxymethylcel-lulose (renewable source) as organic template, becoming an environmentally friendly synthesis method.
The results of this study showed that the degree of polymerization and the degree of substitution for the CMC significantly influences on the formation of uniform hybrid spheres (without flocculation of the solution). A superior molar mass of biopolymer requires a smaller molar fraction of organic material in order to form uniform spheres. Correspondingly, a higher degree of substitution of CMC causes a decrease of the organic molar fraction to obtain uniform hybrid spheres.
Thermogravimetric analysis showed the distinct regions of weight loss related to the decomposition/oxidation of the organic and/or inorganic compounds, which are completely released around 650°C. FTIR spectra confirm the com-plexation of the inorganic compound (Al3?) with the organic biopolymer (CMC) and the formation of a hybrid material, however, all organic material is decomposed in the calcination temperature imposed according to FTIR of the spheres after calcinations.
In the XRD analysis were presented diffraction patterns profile of amorphous solids, for the solids synthesized from the CMC1, and this fact may be explained by the small size and high disorganization (microstrain) of the crystallites. This type of diffractogram characterizes the formation of phases with high metal oxide dispersion. Moreover, for some samples, it was possible to observe the presence of peaks related to the hydrated alumina phase and aluminium oxide, depending on the type of CMC used and the rela-tionship between organic and inorganic material.
SEM images showed that the solid has a sponge-like morphology relative to a support with high porosity, pre-senting cavities of different sizes. The N2 physisorption
isotherms presented a micro-mesoporous profile with inter-esting specific surface area values for the aluminum oxide. The textural properties have shown a strong dependence with the physicochemical properties of the biopolymer.
Thus, the present study confirmed that this alternative method of synthesis is simple and versatile, moreover, allows to obtain a material with interesting structural, textural and morphological properties, depending on polymer characteristics.
Acknowledgments All the members of LABPEMOL, where the experiments and analyzes were performed, and the analytical center of UFRN for the TGA and FTIR analysis. Regina C. dos Santos for the N2physisorption analysis.
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