Niobic Acid Nanosheets Synthesized by a Simple Hydrothermal

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Niobic Acid Nanosheets Synthesized by a Simple Hydrothermal Method as Efficient Brønsted Acid Catalysts Wenqing Fan, Qinghong Zhang,* Weiping Deng, and Ye Wang* State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: This paper reports a novel bottom-up hydrothermal route for the synthesis of niobic acid nanosheets. This route is simpler and greener than the conventional top-down and multistep route for the synthesis of hydrated metal oxide nanosheets via exfoliation of layered compounds, which typically requires the use of bulky organic cations. We have clarified that the pH of the suspension for hydrothermal treatment, the hydrothermal temperature and time, and the presence of NH4+ play roles in determining the morphology of the product. We propose that the nanosheet is formed from amorphous niobic acid nanoparticles through a dissolution− crystallization mechanism. The obtained niobic acid nanosheets are uniform with a thickness of ∼2 nm and uniquely possess mainly Brønsted acid sites. As compared to the conventional amorphous niobic acid and several other typical solid acid catalysts, the niobic acid nanosheet synthesized by our bottom-up method exhibits significantly higher activity and selectivity for the Friedel−Crafts alkylation of anisole with benzyl alcohol. We have further demonstrated that our niobic acid nanosheet is a watertolerant and efficient catalyst for the hydrolysis of inulin, a polysaccharide-based biomass, into fructose. KEYWORDS: nanosheet, niobic acid, hydrothermal synthesis, Brønsted acid, green chemistry

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Nb2O5·nH2O is complicated, and this makes it difficult to further tune the acidity by structure manipulation. Nanosheets represent a new form of materials, and the high anisotropy of nanosheets with ultrathin thickness can provide unique physicochemical properties.9 Recent studies demonstrated that some nanosheets of composite transition metal oxides or hydrated metal oxides with bridged OH groups such as HTiNbO5, HNbWO6, HNbMoO6, HTaWO6, and HNb3O8 functioned as efficient solid acid catalysts for Friedel−Crafts alkylation, esterification, hydrolysis, and dehydration.10−13 The two-dimensional structure of nanosheets can not only enhance the accessibility of bulky reactant molecules to almost all the acid sites, increasing the catalytic performance, but also may afford acid sites with unique properties due to the anisotropic feature. The nanosheets of Nb-based composite metal oxides have been prepared mainly by a top-down route, which is a multistep process via exfoliation of lamellar compounds and typically includes: (1) the synthesis of the parent lamellar compounds by a conventional solid-state reaction at high temperatures, (2) the protonation of the layered compounds in acidic solution, (3) the addition of bulky organic cations (e.g., tetrabutylammonium cations) to expand the interlayer spaces and to cause exfoliation, and (4) the addition of acid solution to the nanosheet solution to form aggregated nanosheets.9−13

INTRODUCTION The development of new solid-acid catalytic materials to replace the liquid acids has attracted much attention in both green chemistry and materials chemistry.1−3 Moreover, solid acid catalysts have recently been expected to play crucial roles in the development of new green chemical processes such as the conversion of renewable biomass into chemicals or fuels.4 For this purpose, solid acid catalysts are usually required to be capable of working in water medium under hydrothermal conditions.4 Niobic acid, also known as hydrated niobium oxide (denoted as Nb2O5·nH2O), is a unique water-tolerant solid acid.1,5 While most of the solid acid metal oxides would lose acidity in the presence of water, Nb2O5·nH2O can function well in water medium at least under temperatures ≤373 K.1,5 In addition to functioning as an efficient acid catalyst for many reactions including hydration, dehydration, and hydrolysis in water medium,5−7 Nb2O5·nH2O could also work in the presence of various liquid media with different polarities and proticities such as decane, cyclohexane, toluene, methanol, and isopropanol.6 However, the nature of the acid sites on Nb2O5·nH2O is not fully understood yet. Both Brønsted and Lewis acid sites are known to coexist on Nb2O5·nH2O, and these sites likely correspond to the OH groups associated with highly polarized NbO6 octahedra and the NbO4 tetrahedra.5,8 Nb2O5·nH2O generally functions as an acid catalyst in amorphous state, and the crystalline Nb2O5 after complete dehydration only possesses very weak Lewis acidity. The structure of amorphous © 2013 American Chemical Society

Received: January 17, 2013 Revised: July 25, 2013 Published: July 28, 2013 3277

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NH3 molecules. Fourier-transform infrared (FT-IR) studies of adsorbed pyridine were performed with a Nicolet 6700 instrument equipped with an MCT detector. The sample was pressed into a self-supported wafer and was placed in an in situ IR cell. After pretreatment under vacuum at 423 K for 1 h, the sample was cooled down to 323 K. Then, pyridine was adsorbed onto the sample for a sufficient time at the same temperature. FT-IR spectra were recorded after the gaseous or weakly adsorbed pyridine molecules were removed by evacuation at 323 K. 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were measured at room temperature using a Bruker AVIII-400 M spectrometer. Catalytic Reaction. The catalyst was evacuated at 353 K for 1 h prior to catalytic reactions. The Friedel−Crafts alkylation was performed in a batch-type reactor. Typically, 0.10 g of catalyst was added into a reaction vessel precharged with benzyl alcohol (10 mmol) and anisole (100 mmol). After the air in the reactor was replaced by N2, the reactor was placed in an oil bath and was heated to the reaction temperature (typically 373 K) in about 10 min. We confirmed that the conversion of benzyl alcohol was 99% at temperatures ≥363 K (see Figure S4 in the Supporting Information). The yield of fructose was 87% at 363 K, and a further increase in reaction temperature rather decreased the yield of fructose due to the formation of humin, carbonaceous species, resulting from the side reactions of dehydration and oligomerization.8 Thus, the optimum reaction temperature for the hydrolysis of inulin to fructose was 363 K. We further performed repeated uses of the Nb2O5·xH2O nanosheet for the hydrolysis of inulin. Figure 14 shows that the

zeolite possessed a larger pore size. The use of mesoporous H-ZSM-5 only slightly increased the TOF for ethyl acetate conversion. This is likely because the smaller reactant molecules can also access the acid sites located in the micropores of zeolites easily. Table 4 reveals that the TOFs over the amorphous Nb2O5·nH2O and the Nb2O5·xH2O nanosheet are higher than those over other solid acid catalysts examined. This suggests that niobic acids are more efficient catalysts in water medium. Moreover, the Nb2O5·xH2O nanosheet demonstrates higher catalytic performance than the amorphous Nb2O5·nH2O. Inulin is an inedible polysaccharide consisting of fructose units linked by β-2,1-glycosidic bonds with the polymer chains terminating in a glucose unit (Scheme 2). Inulin exists as the Scheme 2. Hydrolysis of Inulin to Fructose and Glucose

main component in many plants such as helianthus tuberosus and chicory, which are cold- and heat-resistant and are easy to cultivate even in desert areas. The hydrolysis of inulin can produce mainly fructose, which can be used in food and pharmaceutical industries26 or can be exploited for the production of 5-hydroxymethylfurfural (HMF), a versatile intermediate for the production of valuable chemicals and liquid fuels.27 The hydrolysis of inulin could be catalyzed by enzymes under mild conditions but the process required long reaction times up to more than 10 days.26 Few studies have been focused on the hydrolysis of inulin to fructose by using solid acid catalysts. In this work, we found that the Nb2O5·xH2O nanosheet was a highly efficient and water-tolerant catalyst for the hydrolysis of inulin under relatively mild conditions. As displayed in Table 5, the Nb2O5·xH2O nanosheet showed significantly higher conversion of inulin and higher yield of

Figure 14. Repeated uses of the Nb2O5·xH2O nanosheet for hydrolysis of inulin to fructose. Reaction conditions: inulin, 0.10 g; H2O, 10 mL; Ar, 0.5 MPa; catalyst, 0.075 g; temperature, 363 K; time, 2 h.

inulin conversion and the fructose yield only decrease lightly from 90 to 86% and from 78 to 73%, respectively, after the initial three recycles. The inulin conversion of 86% and fructose yield of 73% could be sustained in the subsequent recycling uses. On the other hand, significant decreases in catalytic performances were observed during the repeated uses of Amberlyst-15 for the hydrolysis of inulin (Figure 15). The inulin conversion and the fructose yield decreased gradually

Table 5. Catalytic Behaviors of Some Solid Acid Catalysts for the Hydrolysis of Inulina selectivityb (%) catalyst

conv. (%)

fructose

glucose

fructose yield (%)

acid densityc (mmol g−1)

TOFd (h−1)

amorphous Nb2O5·nH2O Nb2O5·xH2O nanosheet Nb2O5·xH2O nanosheete H-ZSM-5 (16.5) f mesoporous H-ZSM-5 H-beta (25) f Amberlyst-15 Amberlyst-15g

23 >99 68 54 89 14 >99 48

67 87 81 69 79 63 90 80

14 2.3 5.7 7.4 3.5 8.8 3.3 5.5

16 87 55 37 70 8.6 90 38

0.24 0.51 0.51 1.3 1.1 0.96 4.7 4.7

2.1 5.3 6.7 0.88 2.0 0.28 0.59 2.5

a

Reaction conditions: inulin, 0.10 g; H2O, 10 mL; catalyst, 0.10 g; Ar, 0.5 MPa; reaction temperature, 363 K; reaction time, 2 h. bThe residual byproducts were sucrose and humin. cThe acid density was evaluated by NH3-TPD. dTOF was evaluated by the rate of fructose formation per acid site. eCatalyst, 0.050 g. fThe number in the parentheses denotes the Si/Al ratio. gCatalyst, 0.010 g. 3285

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uses than Amberlyst-15, which also showed excellent activity for the conversion of inulin to fructose.

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ASSOCIATED CONTENT

* Supporting Information S

TG curves for several niobic acid samples, distribution of lateral sizes of the Nb2O5·xH2O nanosheets, time courses for the Friedel−Crafts alkylation of anisole with benzyl alcohol, and dependence of catalytic performances of the Nb2O5·xH2O nanosheets for the hydrolysis of inulin on reaction temperature. This material is available free of charge via the Internet at http://pubs.acs.org

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Figure 15. Repeated uses of Amberlyst-15 for hydrolysis of inulin to fructose. Reaction conditions: inulin, 0.25 g; H2O, 10 mL; Ar, 0.5 MPa; catalyst, 0.010 g; temperature, 363 K; time, 2 h.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Q.Z.); [email protected] (Y.W.). Tel: +86-592-2186156. Fax: +86-592-2183047.

from >99 and 88% to 62 and 53%, respectively, after five reaction cycles. This is possibly because of the leaching of SO3H groups from Amberlyst-15 to the aqueous solution during the reaction. These results demonstrate that the Nb2O5·xH2O nanosheet is stable under our reaction conditions and is an efficient water-tolerant Brønsted-acid catalyst.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2013CB933100 and 2010CB732303), the Natural Science Foundation of China (Nos. 21173172, 21103143, 21161130522, 21033006), and the Program for Changjiang Scholars and Innovative Research Team in Chinese University (IRT1036).

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CONCLUSIONS We succeeded in synthesizing niobic acid nanosheets via a novel bottom-up route by simply treating the precursor obtained from the hydrolysis of NbCl5 dissolved in ethanol with NH3·H2O under hydrothermal conditions. The nanosheet was suggested to be formed from amorphous niobic acid nanoparticles via a dissolution−crystallization mechanism. The pH of the suspension, the hydrothermal temperature and time were key factors for the formation of nanosheets. We further clarified that the NH4+ ions played a crucial role in determining the morphology of the product. Organic ammonium cations could also be used to direct the assembly of niobic acid nanosheets instead of the NH4+ ions. Our characterizations revealed that the Nb2O5·xH2O nanosheet possessed a thickness of ∼2.0 nm with an average lateral size of ∼130 nm. The crystalline structure of the Nb2O5·xH2O nanosheet was similar to that of the nanosheet synthesized by the exfoliation of the layered HNb3O8·H2O. The characterization further suggested that the Nb2O5·xH2O nanosheet was composed of two layers of single sheets. NbO6 octahedra were found to be the predominant building units without NbO4 tetrahedra present in our Nb2O5·xH2O nanosheet. As compared to the conventional amorphous Nb2O5·nH2O particles, the Nb2O5·xH2O nanosheet possessed enhanced acid strength and acid density. Furthermore, the Nb2O5·xH2O nanosheet contained mainly Brønsted acid sites and almost no Lewis acid sites. The Nb2O5·xH2O nanosheet showed unique catalytic behaviors in the Friedel− Crafts alkylation of anisole with benzyl alcohol, the hydrolysis of ethyl acetate, and the hydrolysis of inulin to fructose. As compared to several typical solid acids, the Nb2O5·xH2O nanosheet showed higher yields and TOFs for the target products for these reactions. It is proposed that the stronger acidity and the two-dimensional structure of the nanosheet, which makes the acid sites easier to be accessed by the bulky reactant molecules, contribute to its superior catalytic performances. The Nb2O5·xH2O nanosheet also demonstrated a particularly higher selectivity for the Friedel−Crafts alkylation because of the lack of Lewis acid sites. Furthermore, the Nb2O5· xH2O nanosheet displayed higher stability during the repeated

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