Hydrothermal synthesis of boehmite in cellular alumina monoliths for catalytic and separation applications

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Hydrothermal synthesis of boehmite in cellular alumina monoliths for catalytic and separation applications Nuno M.D. Vitorino a,∗ , Andrei V. Kovalevsky a , João C.C. Abrantes a,b , J.R. Frade a a

Aveiro Institute of Materials CICECO-DEMaC, University of Aveiro, 3810 Aveiro, Portugal UIDM, ESTG, Polytechnic Institute of Viana do Castelo, 4900 Viana do Castelo, Portugal

b

Received 5 March 2015; received in revised form 22 April 2015; accepted 24 April 2015

Abstract For the first time, controlled hydrothermal functionalization was performed in porous cellular ␣-Al2 O3 ceramics, targeting potential separation and catalytic processes for sustainable chemistry applications. Growth of micro-nano hierarchical structures of boehmite (␥-AlOOH) with high aspect ratio was assessed by SEM/EDS/XRD/FTIR. The pH and precursors concentration determine the growth mechanism and final functionalization level. The obtained results anticipate strategies for promoting formation of boehmite at the pores surfaces and tuning its morphology, including pre-treatments of the cellular matrix to increase the concentration of the surface defects and preliminary saturation of the pores with appropriate precursors to achieve uniform functionalization. Thus grown boehmite structures can be easily converted to ␥-Al2 O3 by thermal treatment while retaining the former distribution and aspect ratio, opening new possibilities for preparation of ␥-Al2 O3 supported catalysts. © 2015 Elsevier Ltd. All rights reserved. Keywords: Boehmite; Cellular alumina monoliths; Hydrothermal synthesis; ␥-Alumina

1. Introduction Functional materials with controlled dimensionality and morphology are recognized as an essential part of numerous engineering solutions for energy, food, water and environmental problems. Those possessing large surface areas, tunable pore size, appropriate thermal stability and tailored acid/basic functionalities, allowing modification with catalytically active species, are of particular interest for both technology and fundamental research. Transition aluminas, being already known and quite mature materials, are one of the most frequently used catalyst supports [1,2], especially the ␥-alumina polymorph which contributes significantly to performance of supported catalysts in a variety of industries, with emphasis on petroleum, automotive sectors [1,3]. Therefore, reasonable improvements in relevant alumina properties and/or working concepts for catalytic applications are extremely challenging and expected to contribute significantly to green sustainable chemistry,

∗

Corresponding author. Tel.: +351 258 819 700. E-mail address: [email protected] (N.M.D. Vitorino).

encouraging the design of cheap, environmentally-safe and effective systems and processes. A well-established route to induce the ␥-alumina polymorph is based on dehydration of boehmite (AlOOH) at 400–1000 ◦ C [4–7]. Nanostructured boehmite itself represents an interesting material for designing superhydrophobic and self-cleaning surfaces for adhesion and surface oxidation prevention, self-cleaning of windshields, antibiofouling paints, etc. [8–10]. Boehmite is also known for a fairly rare combination of superhydrophobicity, superoleophilicity and relatively good thermal stability [11], very promising for filtration or separation. Most commonly the nanostructured boehmite is synthesized by hydrothermal processes in various conditions and based on different synthesis strategies, as summarized in Table 1. As shown, these strategies are designed to yield boehmite particles of different shapes, seeking requirements of diverse applications. Boehmite can be then converted into ␥-alumina powder with corresponding particle morphologies. For many catalytic applications aluminabased supports with narrow pore size distribution, uniform pore structure with appropriate pore volume are highly desired [12]. Furthermore, as an ultimate goal one may consider 3D

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Table 1 Literature data on the conditions of the hydrothermal synthesis of nanostructured boehmite. Precursors

Hydrothermal conditions Temperature

(◦ C)

Boehmite features

Ref.

[19] [20]

Time (h)

Al(NO3 )3 •9H2 O; NaHCO3

200

24

Al(NO3 )3 •9H2 O; NH3 Camphorsulfonic acid NaAlO2 ; urea

160

24

2D particles with 8 ␮m diameter (low Al concentration, 0.1 M) 1D wire-like particles with 10 ␮m length (high Al concentration, 0.2 M) Nanotubes with ∼500 nm length and ∼20–40 nm diameter

160 180 200 150 160 220 300 160 200 200

3

Flower-like boehmite nanostructures on quartz surface

[21]

12 0.17 1.33 2 12 72 24

Flower-like boehmite structures Plate-like boehmite particles with average particle size in range 4–25 ␮m

[7] [22]

Leaf-like boehmite nanosheets with aspect ratio of 2 and 60–90 nm of thickness Flowerlike nanoarchitetures Cantaloupe-like and hollow microspherical superstructures

[4] [23] [24]

165

3

Hollow core/shell and hollow microspheres

[25]

180

3

[26]

165

24

The morphology evolves from singe nanoflakes, nanoflakes assemblies, and flower-like structures to hollow microspheres with increasing of KAl(SO4 )2 12H2 O Nanorods-like structure

NH4 Al(SO4 )2 •12H2 O; Urea Al(OH)3

AlCl3 ; NaOH; CTAB AlCl3 ; C2 H6 O (ethanol) Al(NO3 )3 •9H2 O; C6 H5 Na3 O7 •2H2 O Al2(SO4)3•18H2 O Urea sodium tartrate Al(NO3 )3 •9H2 O, urea and KAl(SO4 )2 Al2 (SO4 )3 •18H2O; NH3

hierarchical structures, which are ordered in all appropriate length scales, from molecular to nano and macro [13–15]. In particular, the incorporation of macropores into mesoporous architectures can minimize diffusion barriers and potentially enhance the distribution of active sites during modification with catalyst [13]. On the other hand, such hierarchical structures are expected to tackle the challenges of the filtering applications, especially those demanding high medium flow rates. Synthesis of mesoporous aluminas with narrow pore size distribution represents an interesting and challenging approach for the fabrication of catalyst supports [1], especially in the view of great advancements, realized for mesostructured forms of silica [14,16]. For those applications dealing with high medium flow rates and significant pressure changes (filtering of water/oil emulsions, combustion, and other catalytic reactions with volume change), introduction of larger pores is essential to support a reasonable performance, along with providing sufficient mechanical strength and appropriate interconnectivity of the pores. Recently a new facile method for processing of highly porous cellular alumina ceramics by emulsification of the liquid paraffin in ceramic suspensions was developed in our group [17,18]. The approach allows fine 3D-tuning of cavities shape and interconnectivity between pores, and results in mechanically strong ceramics (compressive strength of 10–40 MPa), with a clear potential for further improvement. Thus, to benefit from the synergy of simple and flexible processing, well-tunable pore structure and mechanical resistance, the present work focuses on the bulk modification of the monolithic cellular ceramics with boehmite/␥-alumina nanostructures via hydrothermal treatment. Thus modified monoliths are attractive for variety of catalytic applications, considering that nanostructured boehmite at the

[27]

surface of the pores can be further functionalized by impregnation with various precursors, containing transition metal cations (Ni2+ , Cu2+ , Fe2+/3+ , Co2+/3+ , etc.) and converted to ␥-aluminasupported catalytic layers. Those functionalities can represent a particular interest for performing catalytic combustion in monoliths, where inherent porous microstructure may be adjusted for efficient gas and thermal transport. To our best knowledge, the described strategy represents a first attempt for in situ functionalization of porous ceramic monoliths in hydrothermal conditions. 2. Experimental Porous alumina monoliths were prepared by emulsification of molten paraffin (Merck 1.07337.2500) in aqueous alumina (Alcoa CT3000) suspension (50% vol. of solids content, stabilized with a viscosity close to 0.5 Pa s using Dolapix PC-67 (10% vol.)). Paraffin was melted at 80 ◦ C, prior to addition of the alumina suspension in volume ratio paraffin:suspension 1.5:1, followed by addition of an anionic surfactant (sodium lauryl sulphate, Sigma-Aldrich L-6026) as emulsification agent (6% related to the emulsion volume), and collagen (OXOID LP0008) as shape stabilizer. Emulsification was promoted by mechanical stirring at 1000 rpm for 10 min, while maintaining temperature at 80 ◦ C, to avoid paraffin solidification. After drying, the resulting ceramic emulsions was sintered at 1550 ◦ C for 3 h with heating rate 0f 5 ◦ C min−1 , and a dwell of 2 h at 200 ◦ C to allow paraffin volatilization. Additional details about cellular alumina monoliths preparation can be founded in our recent papers [17,18,28]. The 0.5 cm thick alumina monoliths (∅ 2 cm), sintered at 1550 ◦ C for 2 h, with ∼60–70% porosity were subjected to hydrothermal synthesis in a 100 ml Teflon-lined stainless steel

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Table 2 Sample denominations, concentration of precursors, pH and phase composition of the modified monoliths, estimated by XRD analysis. Sample denomination

Al05Na05 Al01Na05 Al005Na025 Al001Na005 Al0001Na0005 Al1Na05 Al005Na05 Al001Na05

Precursor concentrations, M

pH

c(Al(NO3 )3

c(NaHCO3 )

Al3+ /HCO3 −

0.5 0.1 0.05 0.01 0.001 1 0.05 0.01

0.5

1 0.2

0.25 0.05 0.005 0.5

2 0.1 0.02

autoclave, with an aqueous solution fraction of 40% at room temperature. Hydrothermal treatments were performed at 190 ◦ C for 19 h, using various concentration of Al(NO3 )3 •9H2 O (Sigma Aldrich BCBH6623V) and NaHCO3 (Merck K25746929 843). The pH of solutions was assessed in ambient conditions by after dissolving the precursors and boiling for 1 h under continuous stirring, in order to assess possible effects of basicity on boehmite formation. Still, one should not exclude significant shifts in pH values as hydrothermal synthesis progresses. The concentration of precursors for various treatments and corresponding values of pH are given in Table 2. After hydrothermal treatment the samples were washed by repeated immersions in deionized water to remove residual precursors, and dried at 100 ◦ C. Possibilities for conversion of boehmite to ␥-Al2 O3 were studied at 400–700 ◦ C. Phase composition of dried modified monoliths, was studied by XRD (Bruker D8 Advance DaVinci, in 10–90◦ 2θ range with a step of 0.02◦ and a residence time of 0.5 s), after crushing the monolith to powder. Diffraction patterns were analyzed using ICDD (International Centre of Diffraction Data, PDF 4) in EVA (Bruker) software. The phase content was evaluated by Rietveld analysis, using Topas software (Bruker). The microstructure and phases distribution in monoliths was assessed by electron microscopy (SEM-Hitachi SU1510), and energy dispersive XRay spectroscopy (EDS – Bruker Quantax 200), using fracture surfaces. The quantification of boehmite was also performed by visual counting the specific number of its clusters, to assess the morphology of the synthesized phase. For this quantification, a set 5–10 SEM micrographs, which corresponded to various volume parts of samples, prepared in the same conditions, was selected; the results, obtained for each micrograph, were then averaged. The areas, used for calculations, were additionally mapped by EDS, to avoid confusion between boehmite clusters and remaining traces of spurious phases such as nitrite, nitratine and nahcolite. 3. Results and discussion Typical SEM micrograph of the sintered ceramic monoliths (Fig. 1A) shows well-defined cellular structure with nearly bimodal cell size distribution and good interconnectivity between pores, as previously observed in [17,18]. The

molar ratio 3.34 8.51 8.09 8.20 8.64 2.63 8.36 8.54

Phase composition of monoliths (% wt.) ␣-Alumina

Boehmite

99.1 99.5 99.8 100 100 95.5 99.2 100

0.9 0.5 0.2 0 0 4.5 0.8 0

cited works already demonstrated high flexibility of the developed processing route for tuning microstructural features, with emphasis on cell size distribution, thickness of the cell walls, size of interconnecting windows, percolation of open porosity, and appropriate mechanical strength, aiming to match requirements of various applications. The monoliths with shown microstructure (Fig. 1A) were thus selected as model monoliths, to demonstrate the concept of in situ hydrothermal functionalization of ␣-alumina cellular ceramics with open cell microstructures. The phase composition of the cellular monoliths after performing hydrothermal synthesis is shown in Fig. 2, and corresponding phase fractions are listed in Table 2. Two phases were unambiguously detected by XRD, including the supporting ␣-Al2 O3 as a main component and hydrothermally induced boehmite. Additional peaks of minor intensity may be attributed to remaining traces of reactant (NaHCO3 ) or sodium-containing byproducts (nitrite Na2 CO3 and nitratine NaNO3 ). Broader peaks shape indicates nanostructuring in synthesized boehmite, which is typical for hydrothermally-grown phases. FTIR results also confirm presence of boehmite in modified cellular monoliths (Fig. 3), ascribed as in Refs. [20,29]. The spectrum shows a large band around 3400 cm−1 , assigned to the (Al)O–H vibrations, whereas the small band at 1065 cm−1 is attributed to the Al–O–H symmetric and asymmetric modes of boehmite. The torsional boehmite modes at 798 and 652 cm−1 are also observed in the spectrum. The weak band at 1636 cm−1 corresponds to the stretching and bending modes of adsorbed water. The band at 1382 cm−1 , growing upon increasing the concentration of precursors, may be attributed to the stretching vibration of C O and C–O bonds of CO3 2− ion [30], indicating that a part of carbonate-based residuals is still trapped inside the highly porous cellular monolith even after repeated washing. The results of combined SEM/EDS analysis (Fig. 1B–D) clearly confirm microstructural changes in hydrothermallymodified cellular ceramics, if compared to non-modified one (Fig. 1A). As a unique difference observed is the formation of rods/fibres and flower-like clusters, one should attribute those structures to boehmite grown during the hydrothermal treatment, in accordance with XRD and FTIR results. EDS inspection also suggests that the fibres are mainly composed of aluminium cations and do not result from precipitation of any sodium-containing species (Fig. 4). As a known morphological

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Fig. 1. SEM micrographs of the fractured non-modified cellular ceramics (A) and hydrothermally-modified Al1Na05 (B), Al1Na05 (C) and Al005Na05 (D) samples.

Al0001Na0005 Al01Na05 Al1Na05

1065 798 652

1382

1636

Al2O3 monolith 3445

Transmitance (a.u.)

feature, boehmite crystals form aggregates with nanorod- and nanowire-type structures having high aspect ratio, which have been reported by hydrothermal synthesis in acidic to moderately basic conditions [31–33]. The thickness of boehmite nanorods, grown in the pores of cellular monoliths, is ∼100–500 nm, i.e. one order of magnitude thicker than in above cited Refs. [31–33], and with high aspect ratio. Note that the used range of pH of the precursor’s solution (Table 2) is favourable for development of elongated architectures, while at pH ≥ 10.5 one observed the formation of nanoplates and flake-type boehmite particles [32,33]. In the present work pH was determined by changes in Al(NO3 )3 and NaHCO3 concentrations, which can be

4000 3500 3000 2500 2000 1500 1000 500 Wave number / cm-1 Fig. 3. FTIR spectra of the powdered hydrothermally-modified and nonmodified cellular ceramics samples.

Fig. 2. XRD patterns of the powdered Al1Na05 and Al0001Na0005 samples.

Fig. 4. EDS map of the fracture of Al005Na05 sample.

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understood considering two main reactions with opposite effects on acidity/bacicity: Al3+ + 3H2 O  Al(OH)3 + 3H+ −

HCO3 + H2 O  H2 CO3 + OH

−

(1) (2)

Low Al3+ /HCO3 − ratio (0.02–0.2) results in slightly basic conditions (Table 2), facilitating the formation of colloidal dispersion of Al(OH)3 particles, which eventually may be converted to boehmite according to the reaction: Al(OH)3 → AlOOH + H2 O

(3)

Thus, one may expect rather massive formation of boehmite when shifting to more basic conditions. However, the estimates of the boehmite fraction from XRD data (Table 2) suggest higher modification of the cellular monoliths in acidic conditions, which correspond to the excess of the Al(NO3 )3 . As the precursors/products residues were mostly washed out from the pores after the hydrothermal treatment, one may assume that lower pH might be more favourable for improved bonding of boehmite particles to the monolith walls and further growth with preferential fibre-like orientation. Firstly, acidic conditions may contribute by protonation of the surface oxygen ions in the monolith, thus creating the conditions for further stacking of AlOOH layers along b-axis through the formation of hydrogen bonds. Secondly, preferential growth of boehmite nanostructures with high aspect ratio was shown to be facilitated by selective adsorption of anionic groups (i.e. NO3 − ) in acidic conditions on the (0 1 0) and (0 0 1) facets of boehmite, terminating further progress in these directions, while allowing the growth along (1 0 0) plane [29,32,33]. This mechanism is expected to be relevant for the whole studied pH range (Table 2), taking into account that isoelectric point of boehmite corresponds to 9.1–9.2 [32,34] and further dissolution of aluminium hydroxides in more basic conditions is rather relevant at pH > 8.6, in accordance with Pourbaix diagrams [35]. Still, one should not exclude presence of specific local pH conditions inside the pores of ceramic monolith during hydrothermal treatment, also provided by diffusion limitations for precursors and resulting products. Indeed, further studies are required for detailed information on these mechanisms of hydrothermal synthesis inside porous ceramics, as compared to traditional approaches. From the other hand, the concentration of aluminium precursor in solution during hydrothermal treatment appears to be quite important factor for the surface modification of cellular ceramics, as the highest fractions of boehmite were observed for samples with notations Al1Na05 and Al05Na05. The morphological features of hydrothermally-induced boehmite in the pores are expected to be affected by presence of surface defects and dangling bonds, which may energetically favour the nucleation and growth processes. The results, presented in Fig. 5, show a clear tendency for the amount of boehmite clusters to saturate for higher modification levels. The number of clusters drastically increases only for low modification level, while further increase in boehmite content results in formation of hierarchical structures, including larger clusters and flower-like architectures (Fig. 5), rather than appearance

Fig. 5. Dependence of the area-specific number of boehmite clusters on the modification level of the cellular ceramics. Top left image shows an example of SEM image for visual quantification.

of new nucleation sites. This implies that further improvement in modification level may be achieved by appropriate pre-treatments of the cellular monoliths, to increase the concentration of surface defects, favourable for nucleation. For the latter, one might consider hydrothermal pre-treatment in strongly alkaline or acidic conditions, or even high-temperature annealing in reducing atmospheres. Particular attention should be given to the obvious specifics of the hydrothermally-induced modification in porous cellular ceramics, including different conditions for the reaction at the surface of the sample and inside the pores. The results, presented in this work, describe the “bulk situation” extending over a layer thickness of at least 1–2 mm apart from the surface, i.e. on a distance of around two orders of magnitude larger than the average pore size. Within these boundaries, the results of SEM/XRD analysis confirmed nearly equal level of modification, while at the surface the concentration of boehmite was found to be around 10 times higher. Although one may consider further improvement of the hydrothermal reactor in order to facilitate the diffusion inside pores (i.e., rotation/shaking and/or application of ultrasound, etc.), more feasible approach may include several pre-impregnation of the sample with corresponding solution of the precursors under vacuum, each followed by a drying step. The latter is expected to be more important for the case of thick monoliths, considering intermediate and large-scale separation and catalytic applications. Finally, depending on the target application, the boehmite may be converted to ␥-Al2 O3 by controlled thermal treatment, as demonstrated by the XRD and SEM results shown in Fig. 6. It should be noticed that the nanostructural features of boehmite are nearly retained in resulting ␥-Al2 O3 , with some reduction in fibres size. The optimal temperature range for conversion corresponds to 670–800 K, typical for this phase transformation [5]; further heating results in reversion to ␣-Al2 O3 . As a feasible strategy for producing cellular ␥-alumina supported catalysts, one may thus consider a twosteps modification approach, including simultaneous growth

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financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. SEM facilities were funded by FEDER Funds through QREN – Aviso SAIECT-IEC/2/2010, Operac¸ão NORTE-070162-FEDER-000050. References

Fig. 6. Examples of the XRD patterns for the powders, obtained from Al1Na05 samples after annealing at 773 and 973 K for 2 h. The inset shows the dependence of the ␥-alumina content on the annealing temperature, estimated from XRD data. The SEM images correspond to the fractured ceramic samples after annealing at 773 and 973 K.

and functionalization of boehmite in the solution, containing appropriate precursors, under hydrothermal conditions, followed by thermal treatment to form target supported-catalyst structure. 4. Conclusions Nanostructured boehmite (␥-AlOOH) having rods/fibreand flower like morphology was successfully synthesized by hydrothermal treatment at 463 K for 19 h at the pore surfaces of the cellular ␣-Al2 O3 monoliths, using aqueous solution of Al(NO3 )3 ·9H2 O and NaHCO3 precursors. The amount of the boehmite formed was mainly determined by the concentration of precursors, with a maximum of ∼4.5% from the total weight of the porous alumina matrix. Within the studied range, the pH showed a minor effect on the morphology and modification level, with apparently more facile surface attachment of the boehmite particles in acidic conditions. The general mechanism of the process includes initial nucleation, followed by preferential growth of the clusters and their transformation into more complicated hierarchical structures. In thus modified porous ceramics, the boehmite can be transformed into ␥-alumina phase by a delicate thermal treatment, while retaining favourable micro- and nanostructural features. Acknowledgements This work was developed in the scope of the project CICECOAveiro Institute of Materials (Ref. FCT UID/CTM/50011/2013, PTDC/CTM-ENE/2073/2012 and PEst-C/CTM/LA0011/2013, and grants SFRH/BPD/99367/2013 and IF/00302/2012),

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[19]

[20]

[21]

[22]

[23]

[24]

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Please cite this article in press as: Vitorino NMD, et al. Hydrothermal synthesis of boehmite in cellular alumina monoliths for catalytic and separation applications. J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.04.040