Morphology (Powder) Paper

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Morphology and production mechanism of the functionalized carboxylate alumoxanemicro and nanostructures

Ali Ashraf Derakhshan, Laleh Rajabi ⁎, Hanieh KarimnezhadPolymer Research Center, Department of Chemical Engineering, College of Engineering, Razi University, Kermanshah, Iran

a b s t r a c ta r t i c l e i n f o

Article history:Received 2 June 2011Received in revised form 24 March 2012Accepted 1 April 2012Available online 6 April 2012

Keywords:Carboxylate-alumoxaneNanostructuresSEMTEMTG-DTA analysis

Four types of carboxylate alumoxanes, namely, para hydroxybenzoate alumoxane (PHBA), salicylate alumox-ane, para aminobenzoate alumoxane (PABA) and fumarate alumoxane were successfully synthesized fromboehmite nanoparticles and the corresponding carboxylic acids and characterized, two of which (salicylateand fumarate-alumoxanes) have not been reported in the literature so far. Boehmite nanoparticles wereused as the starting material. The structures were characterized, using FTIR, SEM, TEM and TG-DTA. Mecha-nisms were proposed for the formation of the synthesized structures. TEM micrographs of salicylate alumox-ane confirmed the morphologies such as nano-sheets, nanorods and nanoribbons. Nanothreads wereobserved in the TEM micrographs of fumarate alumoxane. Cubic agglomerates of fumarate alumoxanewere detected. PHBA with structures similar to hexagonal prism were detected. Mechanisms were proposedfor the formation of these products. Microstructure analysis explained well the experimental parametersobtained from thermal analysis.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Carboxylate-alumoxanes are prepared from the reaction of boehmite[Al(O)(OH)]n, with carboxylic acid (HO2CR). Although, they are giventhe general formula, [Al(O)x(OH)y(O2CR)z]n where 2x+y+z=3 andR=C1–C14 [1], carboxylate-alumoxanes are in fact alumina nanoparti-cles between 5 and 200 nm in diameter. The surface of the nanoparticleis covered with covalently bound carboxylate groups. The physical andchemical properties of the alumoxane can be controlled by the choiceof organicmolecule, and range fromwater soluble powders to hard solidsthat are resistant to acids, bases and organic solvents [2,3]. The carboxylicacid selected in the synthesis of alumoxane, may be simple or the onewith active functional groups. The selection of carboxylic acid should bepurposefully done. Some of the simple carboxylic acids which havebeen used are: acetic acid, methoxyacetic acid, methoxy(ethoxy)aceticacid, methoxy(ethoxy ethoxy)acetic acid, hexanoic acid etc. Some ofthe carboxylic acids containing other functionalized groups are:4-hydroxybenzoic acid, 4-aminobenzoic acid,methacrylic acid, hydroxyl-acetic acid, aminoacetic acid, 6-aminohexanoic acid, lactic acid, L-lysineetc. Carboxylate-alumoxanes can be used in various applications suchas: nanocomposites, membranes, coating, biocompatible materialsand can be used as precursor in preparation of alumina, catalysts etc.Reaction of 4-hydroxybenzoate-alumoxane (4HB-Alumoxane) nano-particles, formed from the reaction of the acid with boehmite, with

methylalumoxane (MAO) resulted in a solid nanoparticle-based MAO(n-MAO) which reacted readily with zirconocenes, including: Cp2ZrCl2,Cp2ZrMe2 and (n-BuCp)2ZrCl2, tomake an active solid catalyst for olefinpolymerization. The catalytic activity of the n-MAO based catalystas reported, was greater than the homogeneous analog under similarAl(MAO):Zr ratio and was comparable to that of a traditional silica sup-ported catalyst but offered the potential of being easily chemically mod-ified [4]. Itwas reported that, Lysine-alumoxane and4-hydroxybenzoate-alumoxane could potentially react with epoxy resins to providecovalent bonding of the nanoparticles to the polymer lattice [5]. Severalalumoxanes have been successfully prepared and incorporated into ure-thane and epoxy resin systems to give coatingswith improved properties.These alumoxanes included4-hydroxybenzoate, lysine, 4-aminobenzoateand dimethylol propionate-alumoxane and improved properties ofcoatings such as: toughness, abrasion resistance, hardness, flamma-bility, chemical resistance, corrosion resistance and thermal stability[6]. Christopher D. Jones et al. [7] investigated the use of a series ofcarboxylate-alumoxanes with varying chain length of the organic sub-stituent, for the synthesis of alumina. They reported that, themethodol-ogy of the carboxylate-alumoxane system allowed for chemical controlwhere the choice of the carboxylate periphery determined the proper-ties of the alumina formed during thermolysis. A.S. Mistry et al. evaluat-ed the degradation and biocompatibility of a novel composite materialcomposed of a biodegradable polymer and ceramic nanoparticles(acryloyl undecanoic amino acid-alumoxane) for bone tissue engineeringscaffolds. The incorporation of functionalized alumoxane nanopar-ticles into the PPF-based polymer (poly(propylene fumarate)) wasshown to significantly increase the material's flexural mechanical

Powder Technology 225 (2012) 156–166

⁎ Corresponding author. Tel.: +98 831 4274535; fax: +98 831 4274542.E-mail address: [email protected] (L. Rajabi).

0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2012.04.003

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properties [8]. Carboxylate-alumoxanes have been previously reportedto be formed by a number of routes [9]. Perhaps the synthetic route sub-jected to the most detailed analysis is the process reported by Kimuraand coworkers [10–12]. In their studies the alumoxanes were preparedvia a multi-step synthesis involving the reaction of a carboxylic acidwith an alkoxyalumoxane. Naureen Shahid et al. produced solventfree carboxylate-alumoxane nanoparticles using a rheomixer. Thismethod has been demonstrated for lysine, stearate and 4-hydroxybenzoate-alumoxanes. They controlled the reaction rate andparticle size by a combination of temperature and shear rate [13].Apblett et al. described a method for the synthesis of carboxylate-

alumoxanes through the reaction of pseudo boehmite and carboxylicacids in refluxing xylenes [14].

The aim of this work was to analyze and compare the morphology offour carboxylate alumoxanes in detail, since knowledgeof themicrostruc-ture of thesematerials is of great importance from the structure–propertypoint of view. The synthesis of carboxylate alumoxanes, although simple,may lead to products with different sizes and morphologies. Thus, pre-dicting the mechanisms through which various morphologies result, canbe very useful for structure–property relations including these materials.This work considered four alumoxanes, two of which (salicylate andfumarate-alumoxanes) have not been synthesized and studied so far.The boehmite nanoparticles used in this work were synthesized in thelaboratory, based on the method published previously [15].

2. Experimental

2.1. Materials

Salicylic acid (2-hydroxybenzoic acid), fumaric acid, 4-hydroxybenzoicacid and 4-aminobenzoic acid were purchased fromMerck.

Fig. 1. Structures of the carboxylate alumoxanes.

Fig. 2. The three possible modes of carboxylate linkages (a, b and c). The changes in the absorbing frequencies of the carbonyl groups in the carboxylic acids and alumoxanes (d).

Table 1The appropriate conditions for synthesis of carboxylate alumoxanes.

Alumoxanesample

Boehmite(g)

Carboxylic acid Weight ofcarboxylicacid (g)

Distilledwater(ml)

Time ofrefluxion(h)

PHB-A 6 4-hydroxybenzoic acid 14 200 14PAB-A 5 4-aminobenzoic acid 6 500 12Sal-A 6 2-hydroxybenzoic acid 12 200 16Fum-A 7.8 Fumaric acid 5 200 18.5

157A.A. Derakhshan et al. / Powder Technology 225 (2012) 156–166


Fig. 3. Comparison of the FTIR spectra of the carboxylate alumoxanes.

Fig. 4. SEM micrographs of the PHBA at different magnifications.

158 A.A. Derakhshan et al. / Powder Technology 225 (2012) 156–166


2.2. Synthesis of carboxylate alumoxanes

The appropriate conditions for synthesis of carboxylate alumox-anes are given in Table 1. Boehmite and specific carboxylic acidwere mixed thoroughly with distilled water in a 500 ml round-bottomed flask. The resulting suspension was sonicated for 20 minin an ultrasonic bath (The SONREX, DT52 H Bandelin Company) andthen refluxed at its boiling point after which it was allowed to coolto room temperature and filtered. The precipitate was mixed wellwith 200 ml of ethanol to dissolve the unreacted carboxylic acids.The obtained mixture was filtered and dried in the air oven at 80 °C.

2.3. Characterization

The IR spectra of the samples were obtained using ABB BomemFourier Transform Infrared (MB-104) spectrophotometer. The mor-phology of the powders synthesized was examined by scanning elec-tron microscopy (SEM, TESCAN VEGA TS 5136MM) and transmissionelectron microscopy (TEM, Philips CM10, high tension 100 kV). Ther-mal analysis of the powders was performed by TG-DTA analyzer(Pyris Diamond).

3. Results and discussion

Fig. 1 presents the structures of the carboxylate alumoxanes.

3.1. FTIR analysis of carboxylate alumoxanes

FTIR spectroscopy is a very useful method of characterization.Comparison of the FTIR spectra of the starting materials (boehmiteand carboxylic acid) with those of the products (carboxylate alumox-ane) can tell us how successfully the synthesis process has beenachieved. Carboxylate linkages on the surface of boehmite are the re-sult of the reactions between the hydroxyl groups of boehmite andcarboxylic groups of the corresponding carboxylic acids. Formationof the carboxylate groups on the surface of boehmite can be per-formed in three possible ways, as shown in Fig. 2 (a, b, c). Carboxylatelinkages form either through unidentate (Fig. 2a) or bidentate(Fig. 2b, c) modes, among which the linkage to two Al atoms(Fig. 2b) seems to be the most stable mode. Thus, carboxylates getbonded to the surface of boehmite mostly via this mode. The reasonfor more stability of this mode of linkage is the formation of six-membered rings which are stabilized through resonance.

Fig. 5. The proposed mechanism of the formation of PHBA hexagonal prisms.



Carboxylate bonds show specific absorbing frequencies in the FTIRspectra, the most informative ones in the characterization of alumoxanesbeing the symmetric and asymmetric stretching vibration frequencies.Fig. 2d shows the changes in the absorbing frequencies of the carbonylgroups in the carboxylic acids and alumoxanes. Thus, it is expected to ob-serve two strong peaks in the regions of 1400 cm−1 and 1600 cm−1 inthe FTIR spectra of carboxylate-alumoxanes. Absorbing frequencies atabout 1460 cm−1 and 1580 cm−1 have been reported previously byLandry et al. [1] for various alumoxanes.

Fig. 3 is a comparison of the FTIR spectra of the carboxylate alu-moxanes and the corresponding carboxylic acids used as precursors.The absorbing frequencies of the carbonyl groups of the carboxylicacids appeared below 1700 cm−1, due to resonance with the benzenerings through conjugated double bonds. The absorbing peaks belong-ing to the carbonyls of the carboxylic acids are of high intensities (thepink colored peaks of Fig. 3e, f, g and h). Whereas, in the FTIR spectraof carboxylate alumoxanes, the carbonyl peaks appeared with lowerintensities at higher frequencies related to the stretching vibrationsof the carbonyls in the unidentated carboxylates (Fig. 3a). Lower in-tensity of the peaks indicated the fewer existence of unidentatedmode in the synthesized alumoxanes. As mentioned before, in thesynthesized carboxylate alumoxanes, carboxylates are bonded to thesurface of boehmite mostly through bidentated modes (Fig. 3a andb). The double peaks related to these carboxylate groups (linked viabidentated modes) are marked as green in Fig. 3 (a, b, c and d). Thehigher intensities of these peaks indicated the bidentated modes ofthe linkage of the carboxylates to the surface of boehmite.

3.2. SEM and TEM analyses of carboxylate alumoxanes

The morphology, size, dimension and also the mechanism of theformation of the synthesized alumoxanes are discussed with thehelp of the corresponding SEM and TEM micrographs.

3.2.1. Para hydroxybenzoate alumoxaneFig. 4 presents the SEMmicrographs of the PHBA at different mag-

nifications. Structures similar to the hexagonal prisms were obtainedcovered with small particles. The average cross section and length ofthese prisms were about 400 nm and 4 μm respectively. Fig. 5 pre-sents the proposed mechanism of the formation of PHBA hexagonalprisms. The molecules of para hydroxybenzoic acid contain two dif-ferent polar ends and the interactions between their polar ends deter-mine the arrangement of these molecules with respect to one another(Fig. 5a). However, the different polar ends and steric effects causedthe molecules to arrange themselves as rings (Fig. 5a). This type ofregular cyclic arrangements was also observed for PHBA structures.Therefore, the prismatic structure of PHBA was related to the polarityof their corresponding carboxylates.

The SEMmicrograph (Fig. 5b) shows the cross section of one of theseprisms of about 600 nm, clearly indicating the hexagonal surface. The av-erage size of the precursor boehmite nanoparticle was about 30 nmwithcrystal structures almost similar to one another. Boehmite nanoparticlesbear OH groups on their surfaces; thus, have the tendency to form stronghydrogen bondswith other surrounding polar structures. As shown sche-matically in Fig. 5b, on refluxing the boehmite nanoparticles with para

Fig. 6. TEM pictures of PHBA at different magnifications (a) and (b), strong physical interactions between the PHBA prism structures and the much smaller needle like nanoobjects (c).



hydroxybenzoic acid, strong interactions occurred between the two typesofmolecules, as a result ofwhich, boehmite nanoparticles arranged them-selves in a regular manner which on heating got linked together throughoxide linkages. The molecules of para hydroxybenzoic acid surroundedthe boehmite nanoparticles from their carboxylic ends and thus, the reg-ular crystal structures of boehmite nanoparticles gathered due to the im-posed pressure of the surrounding layer of the hydroxy benzoatemolecules as shown in Fig. 5b. Thus, the more regular the structures ofboehmite nanoparticles, themore regularwould be the shape anddimen-sions of the resulting prisms. The concentration of the starting materials,duration of heating and the reflux temperature strongly affected themorphology.

The small nanoparticles on the surfaces of the PHBA prismstructures were characterized with the help of TEM micrographs.Fig. 6(a, b) presents the TEM pictures of PHBA at different magnifica-tions. Two prism structures with the cross sections of 400 nm and800 nm, and lengths of 4.5 μm and 6 μm respectively are shown inFig. 6a. The needle like nano objects on the surfaces of the PHBA

structures and the surroundings are shown in Fig. 6b with highermagnification. These needles like nano objects are of the averagecross section and length of 10 nm and 60 nm respectively.

The strong physical interactions between the PHBA prism struc-tures and the much smaller needle like nano objects are more clearlyseen in the TEM micrograph presented in Fig. 6c.

Fig. 7a shows the SEM micrographs of salicylate-alumoxane. Thepictures showed morphologies such as sheets, rods, ribbons andmostly layers of nanosheets on top of one another. Fig. 7b alsoshows SEM pictures of salicylate-alumoxane at various magnifica-tions, where layers of nanosheets of the corresponding alumoxanecan be seen more clearly. The average thickness and length of thesenanosheets were about 70–100 nm and 4 μm respectively. However,smaller nanostructures were also seen in the pictures.

Fig. 8 presents the TEM micrographs of salicylate-alumoxane.The pictures confirmed the morphologies such as nanosheets, rodsand ribbons as were detected in the SEM micrographs (Fig. 7a, b).Nanorods with the average cross section and length of 40–50 nm

Fig. 7. SEM micrograph of salicylate alumoxane (a) and different magnifications of the same, showing layers on top of one another (b).



and 0.5–1.5 μm, and nanoribbons with the average width and lengthof 100–300 nm and 0.5–4 μm respectively were detected. However,mostly nanosheets of the average thickness, length and width of70–100 nm, and 3–4 μm and 1–3 μm of salicylate-alumoxane wereobserved in the TEM pictures (Fig. 8).

The proposedmechanism for the formation of the nanosheets is givenin Fig. 9a. Salicylic acid has a structure like surfactants, that is, one polarand one nonpolar end. Thus, the molecules of salicylic acid tended to di-rect their polar ends towards polar groups and nonpolar heads towardsnonpolar groups in solutions. Such type of tendency to interact caused

Fig. 8. TEM micrographs of salicylate alumoxane.

Fig. 9. The proposed mechanism of formation of salicylate alumoxane nanosheet: effect of polarity (a), effect of hydrogen bonding between nanoribbons (b) and interaction be-tween layers (c).



Fig. 10. SEM micrographs of the fumarate alumoxane at different magnifications.

Fig. 11. TEM micrograph and the proposed mechanism of the fumarate alumoxane.



regular arrangement of molecules in solution. Therefore, the plates likestructure of salicylate alumoxanes were related to the polarity of theircorresponding carboxylates. Another interesting point in the mechanismof formation of salicylate alumoxane nanosheets is shown schematicallyin Fig. 9 (b and c). Due to regular arrangement of themolecules of salicylicacid in solution (explained above), the boehmite nanoparticles were firstformed as rod and ribbon structures which were surrounded by salicylicacid molecules. Then, due to hydrogen bonds between the hydroxylgroups of these acid molecules, the rod and ribbon structures were heldstrongly next to one another. Thus, the resulting structures after heatingwere in the form of sheets. Next, the physical interactions between therings on the surfaces of the nanosheets also forced the layers of thenanosheets to stand tightly on top of one another as shown in Fig. 9c.

3.2.2. Fumarate alumoxaneFig. 10 presents the SEM micrographs of the fumarate-alumoxane

at different magnifications. Cubic structures with the dimensions ofabout 600 nm and perforated surfaces were detected. These cubicstructures seemed to be the result of agglomeration of the smallerfumarate-alumoxane nanostructures. There also existed some threadlike nanostructures as layers on the surfaces of the cubic structures.

More details of the structure of fumarate-alumoxane and theirformation mechanism are shown in Fig. 11. Some short nanothreadswith an average cross section of 10–30 nm were observed in theTEM pictures. These short nanofibers on interacting with one anotherin plane, formed layers which further interactions (hydrogen bonds)between them (the layers) resulted in the formation of cubicagglomerates. The figure also presents a more detailed analysis andmechanism of the formation of the above mentioned structures. The

molecules of fumaric acid may interact with the boehmite structuresfrom their both ends, bearing the carboxylic group. However, most ofthe acid molecules must have interacted with the boehmite struc-tures from one end only, due to high concentration of the acid onreflux.

Thus, the fibrous structures of fumarate-alumoxane got linked inplane to each other through their fumarate linkages. The fibrousplates could further formmultilayer structures due to the interactions(hydrogen bonds) between the carboxylic groups on the surfaces ofthe plates and finally formed cubic structures shown in Figs. 10 and11. This mechanism also explains the formation of some cavities onthe surfaces of some of the cubes.

3.2.3. Para aminobenzoate alumoxaneFig. 12 shows the SEM and TEM micrographs of PABA at different

magnifications. Large size agglomerates were formed as a result of ad-hesion of smaller structures. Needle like nanorods with the averagelength of 50 nm and diameter of 15 nm were observed in the TEMpictures. Among the needle like nanorods some free boehmite nano-particles could also be seen, the existence of which was confirmed bythe FTIR studies.

3.3. TG-DTA analysis

Fig. 13 presents the TG-DTA thermograms of the boehmite andalumoxane samples. A two-step weight loss was detected for theboehmite sample, due to dehydroxylation process of boehmite.Three steps of weight loss were detected for PHBA, being thermallystable up to 333.9 °C. PHBA started to phase transfer at 400 °C. This

Fig. 12. SEM and TEM micrographs of PABA.



indicates that, alumoxanes are transferred to alumina at lower tem-peratures as compared to boehmite. Two step weight loss wasdetected for Sal-A which was thermally stable up to 219 °C. Consider-ing the fact that, the structures of para hydroxyl benzoate-alumoxane

and salicylate-alumoxane are almost similar to one another; thequestion is that, why salicylate-alumoxane was thermally less stablethan PHBA?! The answer lies in the position of the phenolic groupwith respect to the carboxylate group on the benzene ring.

Fig. 14 well describes the cause of thermal degradation of salicy-late alumoxane at lower temperature. As shown, due to steric effectexisting between phenolic and carboxylate groups, the latter cannotposition itself in the plane of benzene ring and thus, the resonance ef-fect between the carboxylate and ring is reduced to some extent.Whereas, the functional groups in PHBA, all exist in the plane of res-onance. The weight loss for PABA and Fum-A occurred in a single step(Fig. 13).

Fig. 15(a) compares the ceramic yields of the boehmite and thefour types of alumoxanes. Alumoxanes loose more weight on thermo-lysis than boehmite, due to their higher molecular weights as a resultof bearing carboxylate groups. Comparison of the amount of the func-tional groups (in millimole) on the surface of one gram of the boehm-ite and the corresponding alumoxanes is shown in Fig. 15(b). Thisinformation is useful in using these materials as precursors for furtherreactions. All these materials contain functional groups on their sur-face. Boehmite has OH groups on its surface. On converting boehmite toalumoxane, the hydroxyls of boehmite react with the corresponding

Fig. 13. TG-DTA thermograms of the boehmite and alumoxane samples.

Fig. 14. Position of the phenolic group with respect to the carboxylate group on thebenzene ring in salicylate alumoxane and PHB alumoxane.



carboxylates. Twomoles of OHare consumed to formonemole of carbox-ylate on the surface of boehmite, especially when bidentated bonds areformed. Thus, it is expected that, the number of carboxylate groups onthe surface of the alumoxane be half that of the hydroxyl on the surfaceof boehmite, as almost observed from Fig. 15(b). However, other factorssuch as steric effect in some alumoxanes can decrease the number ofcarboxylate groups on the surface of the corresponding alumoxane.

Fig. 15(c) is a comparison of thermal stability of all the samples. On thewhole, presence of carboxylate groups has increased the thermal stabilityof boehmite with an exception of salicylate-alumoxane.

4. Conclusions

A detailed analysis of the morphology of four carboxylate alumox-anes was presented. The synthesis of carboxylate alumoxanes, leadsto products with different sizes and morphologies. Characterizationof the synthesized materials proved the formation of nano-sheets,nanorods and nanoribbons of salicylate-alumoxane and nanothreadsof fumarate-alumoxane. Morphologies such as hexagonal prismsand needle like structures were characterized for PHBA and PABA re-spectively. Mechanismswere proposed explaining the formation of theproducts. Except salicylate alumoxane, the other three alumoxanesshowed improved thermal stability compared to boehmite. Study ofthe microstructure matched well with the obtained results from thethermal analysis.

References

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