Introduction

Macroporous α-alumina is a unique material, combining the refractoriness and chemically inert qualities of pure corundum with the advantages of porous materials having an elevated specific surface area. This includes among others adsorptive properties1,2,3, and facilitated mass transfer in combination with thermal stability, which is an essential feature for certain catalyst supports4,5.

Macroporous α-Al2O3 can be obtained via different routes, e.g., transformation of diaspore6,7, hydrothermal methods8,9, or anodic oxidation of aluminum chips and their successive calcination to porous alumina membranes10,11. Yet one of the most versatile methods is found in the sol-gel synthesis in presence of templates, starting from aluminum alkoxides12 or aluminum salts13,14,15. We recently reported a variation of the well-established PEO-templated synthesis with epoxide-mediated gelation in which we replaced the costly polymer by citric acid – a green, inexpensive, and readily available template16.

This article discloses a further improvement of the citric acid-assisted route, using dicarboxylic acids (DCA) as porogenes. The mechanism is closely related to the one investigated for citric acid, as thermal analyses show. However, due to their simpler chemical structure with only two carboxylic acid functional groups and linear composition, oxalic acid and its homologues enable a much more precise control of the pore diameter in the macropore range, along with enhanced pore volumes.

Experimental Section

Reagents

All reagents were used as received, without further purification or treatment. AlCl3·6H2O (99% purity) was purchased from Alfa Aesar. Propylene oxide was delivered by Acros Organics, citric acid (food quality) by purux, and all employed dicarboxylic acids by Merck. Solvents (ethanol and distilled water) were taken from domestic lines.

Synthesis of macroporous α-Al2O3

For a standard procedure, AlCl3·6H2O and distilled water were placed in the reaction vessel and dissolved in ethanol. The respective dicarboxylic acid was added in a molar ratio φAl of Al3+/DCA = 10 to the mixture and dissolved immediately before the reaction. Additionally, sample C2#2 was prepared using twice the amount of oxalic acid. All compositions are listed in Table 1. Propylene oxide was added with a syringe under vigorous stirring to the cooled reaction mixture. After gel ageing, solvent exchange, and drying, calcination was carried out in air at 1200 °C for 6 h. A detailed description of the complete procedure can be found elsewhere 16.

Table 1 Composition of all samples, synthesized according to the standard procedure.
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Characterization of macroporous α-Al2O3

The obtained granular material was characterized by mercury intrusion, SEM, nitrogen sorption, and XRD. Furthermore, thermal analyses (TG and DTA) were conducted on as-synthesized, dried samples.

Mercury intrusion was performed on a Pascal 440 porosimeter by ThermoScientific/Porotec with pressures ranging from 0.2 mbar to 4000 bar. Mercury surface tension was assumed to be 0.484 N/m, its contact angle was set to 141.3°. Samples were outgassed at 0.2 mbar for 10 minutes at ambient temperature prior to filling the dilatometer with mercury.

Scanning electron microscopy (SEM) images were obtained using a Leo Gemini 1530 by Zeiss. Samples were fixated on a carbon foil and vapor coated with a gold film. Accelerating voltage was 10 kV. Secondary electrons were collected by an Everhart-Thornley detector.

Nitrogen sorption was carried out on an ASAP 2000, Micromeritics. Prior to examination, the samples were dried, outgassed, and activated at 300 °C under vacuum. Determination of the specific surface area (ABET) was conducted using the linearized form of the BET equation in the range of 0.05 ≤ p/p0 ≤ 0.30.

For thermal analyses, 25 mg of non-calcined sample were mixed with 25 mg of pure corundum and placed in a corundum crucible in a Netzsch STA 409 TG/DTA device. The continuous air flow rate was 75 mL/min. The heating rate was set to 10 K/min, starting from room temperature up to 1250 °C. All samples were dried to mass constancy at 120 °C prior to thermal analyses.

Complete transformation to the α-Al2O3 phase and phase purity were confirmed by X-ray diffraction on a D8 Discover by Bruker, using a Vantec500-2D detector.

Results and Discussion

Depending on preparation conditions and thermal history, alumina can be generated in different modifications. The thermodynamically stable, intrinsically almost non-porous one is α-Al2O3, which is obtained from dried gels by calcination at 1200 °C for 6 h. We confirmed complete transformation to α-Al2O3 for all synthesized samples by XRD to ensure that none of the generated porosity is due to remaining fractions of transition alumina, which is often the case even in patented processes17. An exemplary diffractogram of sample C3 is shown in Fig. 1.

Figure 1
figure 1

X-ray diffractogram of sample C3 after calcination at 1200 °C for 6 h confirms complete transformation to α-Al2O3 (). All other samples display the same diffraction pattern.

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By adding dicarboxylic acids to the standard sol-gel assay, porosity can be enhanced significantly. The additive-free sample Ref0 exhibits pores of 116 nm width, accumulating to a pore volume Vp of 0.12 cm3/g. Only marginal deviations thereof are observed when 10 mol-% of adipic acid (sample C6, φAl = 10) are added. However, these values can be augmented up to dp = 838 nm, and Vp = 1.18 cm3/g, by adding the same equivalent of oxalic acid (sample C2#1, cf. Table 2). Porosity figures for malonic acid (C3), succinic acid (C4), and glutaric acid (C5) steadily decrease from these peak values down to the ones obtained for C6 and Ref0, as listed in Table 2. The virtual absence of porosity enhancement for C6 can partly be attributed to the poor solubility of adipic acid in water and ethanol. (Dissolution of the additive in the reaction mixture took several hours. We hence refrained from pursuing assays with dicarboxylic acids C7 and higher homologues.) More importantly, however, the general pore widening mechanism by DCA addition in the sol-gel process needs to be elucidated to explain the observed microstructural characteristics.

Table 2 Textural properties of all synthesized gels after calcination at 1200 °C for 6 h. Samples Ref0, CA68, CA34, and CA27 are adapted from16.
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Clearly the enhancing effect on porosity is strongest for the shortest DCA, oxalic acid. In our recently published article on citric acid-assisted synthesis of highly porous α-Al2O3, we provided evidence for the formation of citrate-Al(III)-oligomers, eventually leading to phase separation and thereby an increased porosity16. For DCAs, the same underlying mechanism can be postulated, which is illustrated by SEM images given in Fig. 2.

Figure 2
figure 2

SEM images of samples Ref0, CA27, CA34, and CA68 are adapted from16. SEM images of samples C2#1, C3, and C6 illustrate the increase of pore diameter and pore volume with decreasing DCA chain length. Sample C2#2 with doubled amount of additive exhibits phase separation. Juxtaposition of samples Ref0 and C6, CA27 and C3, CA34 and C2#1, and CA68 and C2#2, respectively, shows virtually identical porosities via different approaches, while pore volumes are consistently larger with DCA templating. All samples were calcined at 1200 °C for 6 h.

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Comparative thermal analyses of samples C3, CA68, and the additive-free reference Ref0 support this interpretation, as they reveal complete incorporation of malonic acid into the alumina network. No discrete mass loss for the DCA is visible in the TG curve in Fig. 3. However, the total weight loss amounts to 44.1 % for sample Ref0, while C3 presents a weight loss of 48.3 %, hence including the 0.34 g of malonic acid. Correspondingly, the weight loss amounts to 54.0 % for sample CA68, containing 0.68 g of citric acid. (The difference to pure Al(OH)3 as starting material, which would result in a weight loss of only 35 %, can be explained by physisorbed excess solvent for all three samples.) Moreover, just like previously observed on sample CA68, the endothermic phase transition peak at 535 °C is missing in the DTA curve of sample C3. This indicates the strong structure directing effect of malonic acid, preventing the formation of a transitional γ-phase, and provides evidence of the incorporation of malonic acid into the alumina network on a molecular level. (For a more detailed discussion of the mechanism, please refer to 16).

Figure 3
figure 3

Thermal Analyses (TG/DTA) of as-synthesized dried gels Ref0-120 (left), C3-120, containing malonic acid (center), and CA68-120, containing citric acid (right). Graphs for Ref0-120 and CA68-120 are adapted from 16.

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Addition of adipic acid then leads to almost no perceptible change in microstructure and porosity, since adipate-Al(III)-oligomers with their aliphatic C4-units are poorly soluble in the strongly polar medium and thus will not grow extensively to form larger particles. A shorter DCA chain facilitates the solubility of forming dicarboxylate-Al(III)-oligomers in a polar medium like the employed reaction mixture. The oligomers hence grow to form larger particles, resulting in a larger volume fraction of nanoparticle-free solvent, which in turn not only yields larger pore diameters but also an increased pore volume. This effect can be seen by comparing the SEM images of C3 and C2#1. Finally, when increasing the DCA concentration, phase separation occurs just like for citric acid-assisted samples. To demonstrate this effect, an additional sample C2#2 was prepared with twice the amount of oxalic acid as in sample C2#1, leading to φAl = 5. The excess oxalic acid then causes the growth of larger primary alumina particles and consequently an eventual enthalpy-driven phase separation, in analogy to the one observed for citric acid16. Juxtaposition of samples CA68 and C2#2 in Fig. 2 shows the almost identical microstructural habitus. Further increase of the added amount of oxalic acid is not possible due to its water solubility of only 90–100 g/L for the non-hydrated form. It would be feasible for malonic acid since its solubility in water is 15 times higher18. However, due to the additional carbon atom in the chain, the generation of larger pores like in sample C2#2 would require the addition of vast amounts of malonic acid.

Despite the closely related mechanism, there is one major difference between the branched tricarboxylic citric acid and the linear dicarboxylic acids presented in this article. The latter ones offer no branching opportunities via the organic linkers, as illustrated in Fig. 4. Particles hence grow more uniformly, resulting in a more homogeneous alumina network, as shown in Figs. 2 and 5 (close-ups). The mercury intrusion histogram of sample C3 depicts a very narrow monomodal pore width distribution, with ≈ 80 % of the pores exhibiting the modal diameter. Citric acid is a branched molecule and thus can link more alumina nuclei, as schematically shown in Fig. 4. CA27 primary particles hence grow to about the same size as for C3 with only 60 % of the carboxylic acid functional groups.

Figure 4
figure 4

Schematic drawing of alumina sol particles (before gelation) with different additives. Individual alumina nuclei (represented by gray circles) containing one or multiple Al(III)-ions are linked via oxo- or hydroxo-bridging. Linear dicarboxylic acids (left, green) form smaller particles than the same amount of citric acid does (right, red), since the latter one is a branched molecule. (Dots represent terminal carboxylic acid groups coordinated to alumina nuclei).

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Figure 5
figure 5

Pore size distributions of samples C3 (left) and corresponding citric acid-templated sample CA27 (right) determined by mercury intrusion. The insets show close-up SEM images depicting the monomodal macropore structure.

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The effect of this unbranched linking in DCA is reflected in larger Vp generated for relatively smaller dp with DCA. For instance, oxalic acid, added with ratio of φAl = 10 (sample C2#1) yields a Vp of 1.18 cm3/g, with a dp of only 838 nm, while for an identical pore volume, CA addition (sample CA68, also with φAl = 10) yields a dp of 3730 nm (cf. Table 2). As a second example, comparison of samples C3 (φAl = 10) and CA34 (φAl = 20) reveals the impact on the resulting ABET. For a slightly smaller Vp (0.75 vs. 0.79 cm3/g), sample C3 exhibits an ABET of 10 m2/g due to its smaller dp of inly 259 nm, while for sample CA34, larger pores (dp = 916 nm) lead to a reduced ABET of 6 m2/g (cf. Table 2).

Moreover, CA already induces phase separation at φAl = 15. Thus, the generation of a controlled monomodal pore system is considerably more difficult than with dicarboxylic acids, since small deviations in the amount of additive entail a significant alteration of the resulting porosity. The use of DCA thus broadens the range of attainable pore diameters, with both monomodal pore size distributions and enhanced pore volumes, offering new opportunities in terms of their use as catalyst support material, for instance4.

Apart from the limited solubility of DCA with even carbon chain numbers and their tendency to esterify with solvent alcohols, there are no major disadvantages to the DCA route. On the contrary, the two major improvements using DCA as templates in comparison with citric acid consist in increased pore volumes and more uniform monomodal pore structures. Sample C3, for instance, is the one with the highest BET surface area of all DCA-templated α-Al2O3 gels due to its drastically increased pore volume in combination with a still relatively small pore diameter, which is yet sufficiently large to withstand sintering at higher temperatures ( < 1350 °C). This renders DCA-templated α-Al2O3 particularly apt for further high temperature applications in refractory ceramic foams4, which then may be used as catalyst supports in catalytic cracking, for instance17,19.

Conclusion

Dicarboxylic acids were employed for the first time as porogenes in the sol-gel synthesis of macroporous α-Al2O3. The corresponding mechanism likely proceeds via dicarboxylate-Al(III)-oligomers, which yield larger primary particles, and consequently result in larger pore widths. With increasing carbon chain length, this effect decreases and almost vanishes for adipic acid, which forms only small linked structures, since larger ones would be insoluble.

Best results in adjusting pore diameters and increasing pore volumes are obtained for oxalic and malonic acid, leading to very narrow monomodal pore structures with pore sizes in the range of 200 to 900 nm (for 10 mol-% of DCA). When increasing the amount of additive, phase separation can be induced, resulting in similar structures as the ones obtained for citric acid, with pore diameters going up to 2.5 µm. However, larger amounts of DCA are necessary to attain the same effect as with the branched citric acid, which can link more alumina nuclei with its additional carboxylic acid group. In summary, dicarboxylic acids enable a more precise control of monomodal pore size and the generation of higher pore volumes in macroporous α-Al2O3 synthesized via the sol-gel process than the previously reported routes involving citric acid or PEO.