Preparation of High Pore Volume Pseudoboehmite Doped with

Nov 12, 2012 - School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou ... ionic surfactant Triton X-100 through microemulsion metho...
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Preparation of High Pore Volume Pseudoboehmite Doped with Transition Metal Ions through Direct Precipitation Method Fan Yang,† Qiang Wang,† Jinglong Yan,† Jian Fang,† Jihua Zhao,*,† and Weiguo Shen‡ †

School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China School of Chemistry and Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China

‡

S Supporting Information *

ABSTRACT: Mesoporous pseudoboehmite with novel pore properties was prepared via the direct precipitation method using aluminum nitrate nonahydrate as the inorganic alumina precursor and different surfactants containing bis (2-ethylhexyl) sulfosuccinate sodium salt (AOT), cetyltrimethylammonium bromide (CTAB), and tert-octylphenoxypolyethoxyethanol (Triton X-100) as the structure-directing agents. The as-synthesized mesoporous products were characterized by wide-angle X-ray diffraction (XRD) and transmission electron microscope (TEM) imaging. Pure pseudoboehmite could be obtained when the final pH was between 8 and 10.5, and the presence of different surfactant micelles played an important role in the morphology and growth of pseudoboehmite. In addition, the pore properties could be enhanced significantly by the presence of transition metal ions. Particularly, when nickel nitrate was added to the aluminum nitrate solution at the molar ratio of 0.0040, the specific surface area, the pore volume, and the average pore diameter of pseudoboehmite reached significantly large values of 381 m2/g, 1.18 cm3/g, and 9 nm, respectively.

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makes the first method less attractive for industrial applications. Furthermore, the toxicity of some byproducts also renders such an approach not widely supported. On the other hand, the neutralization method requires a perfect control of several experimental parameters, such as the rate of gas flow, which has a great effect on the nucleation time, or the final pH, to avoid the formation of aluminum hydroxides such as gibbsite or bayerite.14 Moreover, in the process of rehydration, rigorous control of temperature, time, and pH are demanded to prepare pure products. In contrast to the former three methods, the direct precipitation method is more accessible and reproducible to synthesize pseudoboehmite, alumina, and other oxides.24 From the environmental and economical viewpoints, inorganic aluminum salts are inexpensive and easily available; it has hence received much attention these days. In addition, to obtain well-defined mesoporous aluminum oxides with a fine control of the morphology and particle size, surfactants are frequently employed as templates in the synthesis of pseudoboehmite and alumina crystals. The surfactants are usually inexpensive, accessible, and can afford relatively uniform and large mesoporous materials. For instance, Kuang et al.25 reported the synthesis of boehmite nanotubes by employing cationic surfactant CTAB. Zhao et al.26 successfully prepared reticulate-like γ-alumina with nonionic surfactant Triton X-100 through microemulsion method. Hyun Chul Lee et al.27 investigated the influence of cationic, anionic, neutral, and nonionic surfactants on boehmite and alumina nanoparticles synthesized under hydrothermal conditions. Zhang et al.28 reported that γ-Al2O3 with disordered

INTRODUCTION High surface area transition aluminas, known as activated aluminas, are disordered crystalline phases formed by thermal transformation of aluminum hydroxides and oxyhydroxides into thermodynamically stable corundum or α-alumina.1−3 These aluminum oxide reagents play commercial roles as adsorbents, catalysts, or catalyst supports in many chemical processes. So far, at least seven different alumina phases have been described, including γ-, η-, δ-, θ-, κ-, and χ-phases, as well as the stable αalumina phase.4−7 Among these oxides, γ-alumina is perhaps the most extensively used in heterogeneous catalysis and adsorptive processes because of its comparatively large surface area, its unique surface characteristics, and an exceptional structural stability. γ-Alumina is usually synthesized through the dehydration of pseudoboehmite under high temperature conditions. Therefore, its properties are greatly determined by the quality and the characteristics of this precursor.8−11 Generally speaking, the pseudoboehmite powders used as the precursor for γ-alumina are produced by four main methods: (1) hydrolysis of an aluminum alkoxide;12,13 (2) neutralization of a NaAlO2 solution with CO2 gas,14,15 by controlling the reaction conditions, i.e. the final pH, the flow rate of the gas phase, and the concentration of the NaAlO2 solution; (3) rehydration of the products of gibbsite impulse decomposition, by controlling the decomposition temperature, the residence time, the heating rate, and the pH of the experiment environment;16,17 (4) precipitation of acidic aluminum aqueous solutions (Al2(SO4)3, Al(NO3)3, AlCl3·6H2O),18−23 by controlling the amount of basic reactants (NaAlO2, NaOH, NH3·H2O, Na2CO3), the temperature, the addition rate, the aging time, and the final pH. These methods lead to precursors with different shapes, morphologies, and surface properties, but unfortunately, each of them has its own intrinsic drawbacks. For example, the comparatively high price of aluminum alkoxides © 2012 American Chemical Society

Received: Revised: Accepted: Published: 15386

July 4, 2012 November 5, 2012 November 12, 2012 November 12, 2012 dx.doi.org/10.1021/ie3017626 | Ind. Eng. Chem. Res. 2012, 51, 15386−15392

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scanning electron microscopy (FESEM, JEOL JSM-6701F, Japan). Transmission electron microscopy (TEM) images were obtained on a Hitachi H-600 with an accelerating voltage of 80 kV to distinguish the morphology and the dimension of the pseudoboehmite nanostructures. The samples for TEM analysis were prepared via sonication in ethanol for about 3 min; a drop of the as-prepared solution was then deposited on a copper grid. Nitrogen adsorption−desorption isotherms for the surface area, pore volume, and pore size distributions were measured using an ASAP 2010 analyzer at −196 °C with ultrahigh-purity nitrogen gas. Prior to adsorption measurements all the samples were outgassed under vacuum at 100 °C for 5 h. The Brunauer−Emmett−Teller (BET) specific surface area was calculated from N2 adsorption isotherms in the relative pressure range of 0.05−0.20.34 The single-point pore volume was collected at a partial pressure of 0.98. Pore size distributions were calculated from the adsorption isotherms by the Barret− Joyner−Hallender (BJH) model. The metal content of samples was determined by inductively coupled plasma optical emission spectroscopy using an IRIS Advantage ER/S (Thermo Jarrell Ash, USA). Synthesis Procedure. Aluminum nitrate nonahydrate dissolved in ethanol at a concentration of 0.2 mol/L was used as the alumina precursor. Ammonia (28 wt %) dissolved in deionized water at a concentration of 0.62 mol/L was used as alkali. The procedure to synthesize the pseudoboehmite nanoparticles was as follows: A solution containing the desired surfactant (0.01 mol/L, 6 mL) was added dropwise into the homogeneous aluminum nitrate solution (0.2 mol/L, 12 mL) at 27 °C under vigorous stirring. After stirring for half an hour, an aqueous solution of ammonia was added slowly to the clear sol. The pH was adjusted to a certain value (between 8 and 11 depending on the experiment) and then the sol was gently stirred at 27 °C for 40 min. The final white gel was aged for 4 h at 65 °C, then centrifuged and washed several times by absolute ethanol and deionized water. Thereafter, the product was dispersed in ethanol and dried at 60 °C for 24 h. Under this condition, ethanol gradually evaporated and the gel was converted to xerogel, which was further dried at 120 °C under ambient conditions for 48 h. In the present paper, the labeling of the compounds starts with PB (pseudoboehmite), continues with the type of surfactants (A, C, T referring to AOT, CTAB, and Triton X-100, respectively), and ends with the type of metal ions (Cu, Ni, Co referring to copper ion, nickel ion, and cobalt ion, respectively).

mesoporosity was prepared by using a nonionic Tergitol 15-S7-mediated scaffolding of peptized forms of commercially available pseudoboehmite nanoparticles. Cai et al.29 first synthesized mesostructural alumina with nonionic structuredirecting agents and explored its adsorption properties. The results demonstrated that the morphologies and surface properties of the products mostly depended on the nature of the involved surfactants. Besides morphologies, the pore size distribution, the pore volume, and the specific surface area also play a definitive role in the function of pseudoboehmite as catalysts, catalyst supports, and adsorbents. Conventional pseudoboehmite usually exhibits a pore volume lower than 0.45 cm3/g and a small pore diameter ranging from 2 to 6 nm.30 Therefore, numerous research groups have made efforts to prepare pseudoboehmite with large pore volume and high surface area under a series of different synthesis conditions to enlarge their applications in both fundamental and practical aspects.14,15,31,32 Tsukada et al.33 confirmed that higher specific surface area and pore volume pseudoboehmite could be produced by adding other metallic salts when mixing an acidic aluminum aqueous solution with an alkaline aluminum aqueous solution, or maturing the aluminum hydroxide sludge. In this study we incorporated transition metal components including copper, nickel, and cobalt to influence the characteristics of pseudoboehmite. We produced homogeneous pseudoboehmite via direct precipitation method with Al(NO3)3·9H2O as the raw acidic aluminum solution, NH3·H2O as the basic reactant, and different surfactants such as an anionic surfactant (AOT), a nonionic surfactant (Triton X-100), and a cationic surfactant (CTAB) as structure-directing agents. The specific surface area and the pore volume increased distinctly once the transition metal ion was added, especially with nickel ion, which suggested that the interaction between pseudoboehmite and transition metal ions had significant influence on the pseudoboehmite dispersion, as well as on the physicochemical properties such as surface area and pore volume.

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EXPERIMENTAL SECTION Chemicals. The nonionic surfactant Triton X-100 [p-tertC8 H17 C6 H4 (OC 2H4 ) nOH, n = 9−10] and the anionic surfactant AOT (C20H37NaO7S, 96% mass fraction), which was sliced up and dried by P2O5 in a desiccator for 2 weeks before use, were obtained from Sigma-Aldrich. The cationic surfactant CTAB (C19H42BrN) and cobalt nitrate hexahydrate (Co(NO3)2·6H2O, A.R.) were bought from Sinopharm Chemical Reagent Co., Ltd. Aluminum nitrate nonahydrate (Al(NO3)3·9H2O, A.R.) was purchased from Xi′an Chemical Reagent Factory. Concentrated ammonium hydroxide (28 wt % NH3, A.R.) was provided by Baiyin Liangyou Chemical Reagent Limited Company. Cupric nitrate (Cu(NO3)2·3H2O, 99.5%, A.R.) was received from the Third Branch of Tianjin Chemical Reagent Liu Chang. Nickel nitrate (Ni(NO3)2·6H2O, A.R.) was purchased from Tianjin Guangfu Technology Development Co., Ltd. Materials Characterization. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku D/MAX2400 X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 0.15406 nm) from 10° to 80°. Measurements were conducted using a voltage of 40 kV and a current setting of 40 mA. The surface morphologies of the as-prepared pseudoboehmite nanoparticles were observed by field emission

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RESULTS AND DISCUSSION

Effect of the Final pH on Crystal Form and Morphology. Figure 1a displays the XRD patterns for the products prepared with AOT under different final pH, which played a crucial role in the formation of pseudoboehmite (PB) nanostructures. When the pH was equal to 11.0, the crystal form of the product was a mixture of bayerite and PB, and as the pH was equal to 10.5 or lower, pure PB could be obtained (a1−a5 Figure 1). All the diffraction peaks can be indexed to the orthorhombic unit cell of AlOOH (space group Cmcm). Rousseaux et al.35 reported the synthesis of aluminum hydrate precipitated at room temperature from Al(NO3)3 and NH4OH, and represented the transformation sequence as follows: 15387

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nanofibers with 2−5 nm diameters could be observed. With further increase of the pH to 10.5, the nanoparticles of pseudoboehmite changed to an aggregated particle-like morphology, attributed to the cancellation of the interactions between the surfactant of the micelles and the nanoparticles at high pH value. Table S1 in the Supporting Information demonstrated that the final pH had an effect on the pore properties. From 8 to 10.5, with the increase of the pH, the specific surface area increased, but the pore size first decreased and then increased. Taking into account the experimental operations and the practical applications, we chose 9.7 as the suitable pH for the following experiments. Effect of Surfactant. Samples synthesized with Triton X100 or CTAB as surfactant at the final pH of 9.7 had an aggregated particle-like morphology, as shown in Figure 3a and b. In contrast, the TEM micrograph displayed in Figure 2b showed that pseudoboehmite prepared with anionic surfactant at pH equal to 9.7 was structured as densely distributed nanofibers with 2−5 nm diameters. Different surfactants could lead to distinct structures of pseudoboehmite nanoparticles. Generally, in a templating synthesis, the surfactant properties greatly influence the pore characteristics of the products. In the surfactant−pseudoboehmite system, the surfactant was expected to be in an intermediate state with more close packing than in liquid phase due to the interaction of the surfactant with the pseudoboehmite crystallite surfaces.36 The solid-state 1H NMR results confirmed that two 6-coordinate Al−OH groups (an octahedral AlOH site and an octahedral Al2OH site) exist on the pseudoboehmite surface.37 In this case, the surfactant dispersed in polar solutions can interact with the hydroxyl groups on pseudoboehmite nanoparticles surface through hydrogen bonding and is thus closely bonded to the surface. In other words, the interaction between pseudoboehmite surface and surfactant constrains the crystallites growth to the formation of the surfactant micelles. Consequently, the particlelike pseudoboehmite synthesized with Triton X-100 or CTAB reflected the granular morphology of the micelles, as proved by the SEM images (Figure 3c and d). Fibrous samples were obtained in the AOT system because the small particles aggregated and grew along one direction as the AOT micelle is not very stable. Figure 1b shows the XRD patterns for the pseudoboehmite mesophases prepared from different surfactants. Samples synthesized with different surfactants exhibited the same typical diffraction peaks corresponding to pure pseudoboehmite. The interactions between the surfactants and the surface of pseudoboehmite only influenced the growth of nanoparticles and thus the morphology, while the nature of the crystallizing material remained the same. The textural properties of PB synthesized with different surfactants are summarized in Table 1. The values of pore size, surface area, and pore volume illustrated that surfactants had a tiny influence on the pore properties of products. Effect of Transition Metals on Pore Properties. To obtain the pseudoboehmite nanoparticles with large specific surface area and large pore volume, we attempted to add transition metal ions to the aluminum nitrate solution, such as Ni2+, Cu2+, and Co2+. The TEM images of PB-A-Cu, PB-A-Ni, and PB-A-Co (see Figure S1 in the Supporting Information) depicted that all samples exhibited fiber-like mesoporous framework. As shown by the pore properties reported in Table S2, compared with the molar ratio of Cu(NO3)2/ Al(NO3)3 equaled 0, the values from 0.002 to 0.01 implied a

Figure 1. Wide-angle range powder X-ray diffraction (XRD) patterns for PB: (a) XRD spectra of pseudoboehmite with AOT under different final pH; (b) XRD spectra of pseudoboehmite prepared with different surfactants at pH = 9.7; (c) XRD spectra of pseudoboehmite containing transition metals with AOT at pH = 9.7. pH = 8

Al3 + + 3OH‐ ⎯⎯⎯⎯⎯→ amorphous gel pH = 9

⎯⎯⎯⎯⎯→ pseudoboehmite pH = 10

⎯⎯⎯⎯⎯⎯→ bayerite or nordstrandite

Our investigation implies some modifications of this scheme. Pure PB could be obtained within a wider pH range from 8 to 10.5, preferably 9 to 10.5, which made the experiment process easier and more accessible. Figure 2 shows the TEM and SEM images of the pseudoboehmite samples synthesized with AOT. It can be seen that the morphology of aluminum hydrate was basically determined by the final pH of the mixture. As displayed in Figure 2a, b, and c, when the final pH was lower than 10.5, the particles of pseudoboehmite were fiber-like with massive pores. Particularly, when the pH was equal to 9.7, densely distributed 15388

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Figure 2. TEM images of the pseudoboehmite prepared at different final pH with AOT: (a) pH = 9, (b) pH = 9.7, (c) pH = 10, (d) pH = 10.5. SEM images of pseudoboehmite prepared at different final pH with AOT: (e) pH = 9.7, (f) pH = 10.5.

certain degree increase in pore sizes, pore volumes, and surface areas. When the molar ratio was 0.01, PB sample exhibited the highest textural properties, whereas, apparent impurity peaks of CuO were observed in the XRD pattern (Figure S2 and inset of Figure S2). It demonstrated that the appropriate molar ratios to obtain pure PB were 0.002−0.008, and we chose 0.004 as the experimental molar ratio. The XRD patterns for pseudoboehmite containing transition metal ions are displayed in Figure 1c. No characteristic diffraction peaks indicating transition metals compounds were observed in these powders. It was speculated that the content of compounds was far below the detection limitation of X-ray diffraction. For example, the content of Ni2+ in pseudoboehmite, measured by inductively coupled plasma optical emission spectroscopy, was 0.24%. Figure 4 provides nitrogen adsorption−desorption isotherms for a series of pseudoboehmite derived in the presence of transition metal ions, and Table 2 summarizes the values of pore sizes, BET surface areas, and pore volumes of

pseudoboehmite. Figure 4a shows that all the samples exhibited type IV isotherm with well-developed H1-shaped hysteresis loops for PB-A, PB-A-Cu, PB-A-Ni, and PB-A-Co.34 The curves of PB-A and PB-A-Co had a similar intense mesoporous filling step at P/Po ≈ 0.5, and accordingly, the fundamental particle size of PB was also similar for these two samples which had almost identical BET surfaces areas of 302 and 296 m2/g, respectively. However, products prepared with Cu2+ and Ni2+ exhibited a distinct condensation step at P/Po ≈ 0.75 and P/Po ≈ 0.8 (Figure 4a, curves (b) and (c)). As a result, these two samples had larger surface areas and higher mesopore size compared with pseudoboehmite prepared without metal ions. It was noteworthy that pseudoboehmite synthesized with Ni2+ had a much larger BET surface areas (381 m2/g), a much higher pore volume (1.18 cm3/g), and a much larger average BJH mesopore size (9.0 nm) (Figure 4b). The novel pore properties of products prepared with transition metal ions may be attributed to three main causes. 15389

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Figure 3. TEM images of pseudoboehmite prepared with different surfactants as pH was equal to 9.7: (a) nonionic surfactant (TritonX-100), (b) cationic surfactant (CTAB). SEM images of pseudoboehmite prepared with different surfactants: (c) nonionic surfactant (TritonX-100), (d) cationic surfactant (CTAB).

complexes could be decomposed once the temperature increased. Generally, the stability constants sequence of the amine complexes is Ni(NH3)62+ < Cu(NH3)42+ < Co(NH3)63+. The hexamine complex of Ni began to decompose and release NH3 at lower temperature than Cu(NH3)42+.45 Nevertheless, the complex of Co(NH3)63+ was too stable to decompose (the decomposition temperature of the cobalt ammine complexes was about 280 ◦C). Therefore, when nickel nitrate or copper nitrate was added, the special surface areas, the pore volumes, and mesopore sizes increased as NH3 was released. The last but not least cause can be the modification of the surface tension between water and the surface hydroxyl groups of PB after addition of Cu2+, Co2+, or Ni2+. In conclusion, the abovementioned three reasons can be the cause of the novel pore characteristics with incorporation of transition metal ions.

Table 1. Textural Properties of PB Synthesized with Different Surfactants with Final pH at 9.7 sample

SBET (m2/g)

pore volume (cm3/g)

pore size (nm)

crystallite size (nm)

PB-C PB-T PB-A

297.3 296.8 296.3

0.41 0.40 0.36

5.2 5.4 4.3

4.27 4.46 4.10

The first cause can be the formation of hydrotalcite-like compounds dispersed on the surface of pseudoboehmite, with the general chemical composition [M2+1−xM3+x(OH)2]− (An−)x/n·mH2O, where M2+ and M3+ are divalent and trivalent cations.38−41 When transition metal ions reached a certain amount, we speculated that the products would ultimately exist in the form of hydrotalcite-like compounds. The second cause can be the decomposition of amine complexes. In the initial reaction process, Al3+ and transition metal ions coprecipitated. Coprecipitation can entail that the transition metal not only adsorbs onto the freshly formed hydrous oxide colloids but also incorporates into the hydrous oxide lattice.42 As the ammonia concentration in the solution increased, the Cu2+, Co2+, and Ni2+ transformed into the more stable forms Cu(NH3)42+, Co(NH3)62+, and Ni(NH3)62+, respectively. Co(NH3)62+ was very unstable in air and facilely oxidized to Co(NH3)63+, and we hypothesized that Co(NH3)62+ was transformed into Co(NH3)63+.43,44 Partial amine complexes can still exist in the layer structure of pseudoboehmite. During drying, amine

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CONCLUSION

A facile and applicable procedure has been developed to prepare mesoporous pseudoboehmite with aluminum nitrate via direct precipitation method. Pure pseudoboehmite could be obtained within the pH range of 8−10.5. The morphologies of the crystallites are distinct from those obtained with different surfactants as structure-directing agents and templates. In addition, in this study, we investigated the influence of transition metal ions on the pore properties of pseudoboehmite, particularly Ni2+, Cu2+, and Co2+. Once nickel ion was added, the specific surface area, the pore volume, and the 15390

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Article

ASSOCIATED CONTENT

S Supporting Information *

Table S1: Effect of final pH on the pore properties of pseudoboehmite prepared with AOT as surfactant. Table S2: The textural properties for PB synthesized with different molar ratio of Cu(NO3)2/Al(NO3)3 with AOT and the final pH at 9.7. Figure S1: TEM images of pseudoboehmite prepared with different surfactants as pH was 9.7: (a) PB-A-Cu, (b) PB-A-Ni, (c) PB-A-Co. Figure S2: XRD patterns for PB made with AOT at final pH = 9.7 and the molar ratio of Cu(NO3)2/Al(NO3)3 was 0.01. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-931-8912541. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the National Natural Science Foundation of China (Projects 20603014, 20673059, 20973061, and 20903051), the Chinese Ministry of Education (Key project 105074), the Committee of Science and Technology of Shanghai (Projects 0652 nm010 and 08JC1408100), and the Fundamental Research Funds for the Central Universities (Project lzujbky-2011-116 and lzujbky2010-36).

Figure 4. (a) Nitrogen adsorption−desorption isotherms and (b) BJH adsorption pore size distribution curves of pseudoboehmite prepared with different transition metals, with AOT as surfactant and the final pH at 9.7.

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Table 2. Adsorption Parameters Evaluated from Nitrogen Adsorption Isotherms for PB Prepared with Addition of Different Transition Metals, with AOT as Surfactant and the Final pH at 9.7 SBET (m2/g)

pore volume (cm3/g)

pore size (nm)

crystallite size (nm)

0.0042

296.3 334.7

0.36 0.62

4.3 6.1

4.10 4.13

0.0036

302.4

0.36

4.1

4.10

0.0040

381.4

1.18

9.0

4.45

Me(NO3)2/Al(NO3)3 sample molar ratio PB-A PB-ACu PB-ACo PB-ANi

REFERENCES

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average pore diameter of pseudoboehmite reached remarkably large values of 381 m2/g, 1.18 cm3/g, and 9 nm, respectively. It is speculated that the transition metal ions have a significant influence on the pore characteristics through the interaction between pseudoboehmite nanoparticles and metal ions. The essentially improved mesoporosity, exceptionally high pore volume, and high surface areas of pseudoboehmite could promote the dispersion of catalytic components on the samples and the diffusion of reactants or products. In consequence, the mesoporous pseudoboehmite described in the present work can potentially be used not only in chemical catalysis but also in adsorptive applications. 15391

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dx.doi.org/10.1021/ie3017626 | Ind. Eng. Chem. Res. 2012, 51, 15386−15392