Zeolite nanofiber assemblies as acid catalysts with high activity for the acetalization of carbonyl compounds with alcohols

Abstract

Zeolite nanofiber assemblies (HNB-MOR) as efficient heterogeneous catalysts for the formation of a range of acetals in good yields. The mesoporosity of HNB-MOR benefits mass transfer, and the strong acidic sites on HNB-MOR facilitate acetalization activity. The catalyst can be reused 10 times without loss of activity.

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Introduction

Acetals are an important class of fine chemicals.¹ In multi-stage synthesis, acetalization is also an important protection strategy for carbonyl groups.² Generally, acetalization can be catalyzed efficiently by conventional mineral acids with high activity. However, these acid catalysts are toxic, corrosive and difficult to remove from the reaction medium, which strongly limits their practical applications in industry. One solution to these problems is to replace mineral acids with highly active solid acid catalysts, which are advantageous as isolation and purification of the intermediates is not required.

Different kinds of solid catalysts such as heteropoly acids,³ imidazolium salts,⁴ sulfated zirconia,⁵ montmorillonite,⁶ envirocat EPZG,⁷ alumina/KSF⁸ and natural kaolinitic clay⁹ have been employed for the acetalization of carbonyl compounds. However, these solid acid catalysts have lower activity due to their relatively small surface area. Other solid acids such as cationic exchange resins¹⁰ and metal organic frameworks¹¹ show better activity toward acetalization reactions, but the activity of such catalysts cannot be regenerated by calcination after catalyst deactivation due to their relatively lower thermostability.

Great attention has been dedicated to the uses of acidic aluminosilicate zeolites to achieve the acetalization reaction.¹² However, one disadvantage of conventional micropore zeolites is that the pore sizes are too small to allow bulky molecules access. Ordered mesopore molecular sieves such as Al-MCM-41¹³ and Al-SBA-15¹⁴ could overcome this pore size limitation, but higher activity was not achieved by these catalysts for acetal formation. This is attributed to the lower acidity of ordered mesoporous materials due to the amorphous nature of mesoporous walls.^15,16 Therefore, the development of an economic, green and sustainable catalytic system for this fundamental transformation is highly desirable.

Recently, we reported a facile method for the synthesis of nanofiber assemblies of mordenite (NB-MOR).¹⁷ This material has parallel-mesopore channels in the nanofiber assemblies of the microporous mordenite. After ion-exchange of NH₄⁺ and calcination, the H-form of NB-MOR (HNB-MOR) was obtained. The HNB-MOR catalyst is strongly acidic and shows high activity in the acetalization of various carbonyl compounds with a series of alcohols involving bulky organic substrates under mild reaction temperatures (50 °C), compared with HAlMCM-41 and mesopore-free HMOR catalysts.

Experimental

Material synthesis

Mordenite nanofiber assemblies (NB-MOR) as well as mesopore-free mordenite (MOR) were synthesized in a similar way to previous work.¹⁶ Mesoporous aluminosilicate (Al-MCM-41) was synthesized according to the literature.¹⁵ The H-form of the samples was obtained by ion-exchanging twice with NH₄NO₃ solution (1 M) at 80 °C for 4 h, followed by calcination at 550 °C for 4 h.

Catalyst characterization

Nitrogen adsorption–desorption isotherms were obtained using a Micromeritics ASAP 2020M apparatus at liquid nitrogen temperature (−196 °C). Specific surface areas were calculated from the adsorption data using the Brunauer–Emmett–Teller (BET) equation. The mesopore size distributions were calculated using the Barrett–Joyner–Halenda (BJH) model. The acidity of the materials was determined by stepwise temperature-programmed desorption of ammonia (NH₃-STPD) on a Micromeritics ASAP 2920 instrument. The detailed operating procedure and conditions are described in a previous work.¹⁸

Reaction and analysis

In a typical experimental procedure for an acetalization reaction in the presence of a catalyst (30 mg), the carbonyl compound (0.2 mmol) and alcohol (2.0 mL) were placed in a 10 mL glass vessel. The reaction was then allowed to proceed at 50 °C for 12 h. After the reaction finished, the catalyst was separated by centrifugation and the liquid phase was passed through filter paper. The liquid was analyzed with an Agilent 7890A GC equipped with a FID detector and mass spectrometer. The products were obtained by flash chromatography (hexane : EtOAc). ¹H NMR (500 MHz) and ¹³C NMR (125 MHz) were performed by spectrometers at 20 °C using CDCl₃ as the solvent. Chemical shifts are given in parts per million relative to TMS as the internal standard at room temperature.

Results and discussion

Fig. 1 shows the nitrogen adsorption isotherm, pore size distribution and NH₃-STPD profiles of the samples. The nitrogen sorption isotherm of HNB-MOR exhibits a hysteresis loop at a relative pressure of 0.85–0.95, which is typically assigned to the presence of a mesoporous structure (Fig. 1a). The mesopore-size distributions of HNB-MOR and HAlMCM-41 are mainly centered at 33 and 2.5 nm (Fig. 1b and c), respectively. Sample textural parameters are presented in Table 1. The NH₃-STPD profiles of the samples are shown in Fig. 1d. For the characterization of acidity by NH₃-STPD, the acidic strength can be differentiated as weak, middle and strong according to the desorption temperature.¹⁹ Clearly, the concentrations of relatively strong (250–350 °C) and strong (>350 °C) acidic sites of HNB-MOR and HMOR catalysts are much higher than those of HAlMCM-41.

Fig. 1 (a) N₂ adsorption isotherm, (b) pore size distribution of HNB-MOR, (c) pore size distribution of HAlMCM-41, (d) NH₃-STPD curves of the samples.

Table 1 Textural parameters of the catalyst samples

Samples	SBET^a (m^2 g^−1)	Sexter.^b (m^2 g^−1)	Vmeso^c (cm^3 g^−1)	Vmicro^d (cm^3 g^−1)
a BET surface area.b External surface area including the mesoporous surface area.c Mesoporous volume.d Microporous volume.
HMOR	346	12	0.002	0.13
HNB-MOR	474	158	0.50	0.13
HAlMCM-41	876	824	0.67	0.01

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The activity and scope of the acetalization of various aldehydes with a series of alcohols over HNB-MOR, HMOR and HAlMCM-41 catalysts are shown in Tables 2–5. The blank experiment showed that no product was obtained when the reaction was carried out in the absence of a catalyst. Table 2 shows the reaction activity of the acetal formation of various aldehydes with methanol over HNB-MOR, HMOR and HAlMCM-41 catalysts. Clearly, the HNB-MOR, HMOR and HAlMCM-41 catalysts gave high yields for aldehydes not only with electron-withdrawing groups as the halogen (entries 1–3) but also with electron-donating groups as –CH₃ and –CH₂CH₃ (entries 4–6). However, when the aldehyde with an electron-withdrawing nitro substituent was used as the substrate, the product yields over HNB-MOR reached 97–100%, and the yields over HMOR and HAlMCM-41 catalysts were only 48 and 14% (entries 7 and 8). Meanwhile, the activity of zeolite HNB-MOR and HMOR catalysts was much higher than HAlMCM-41 in the reaction entries 5–8.

Table 2 Acetal formation of aldehydes with methanol over HNB-MOR, HMOR and HAlMCM-41 catalysts^a

Entry	Product	Yields [%]
		a/b/c^b
a Reaction conditions: aldehyde (0.2 mmol), methanol (2 mL), catalyst (30 mg), 50 °C, 12 h. The yields were obtained by GC analysis (isolated yields in parentheses).b a) HNB-MOR as the catalyst; b) HMOR as the catalyst; c) HAlMCM-41 as the catalyst.
1		100(94)/100/100
2		100(91)/100/99
3		100(93)/100/100
4		100(92)/100/100
5		100(89)/95/90
6		100(87)/97/94
7		100(95)/93/58
8		97(91)/48/14

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Table 3 shows the reaction activity of the acetal formation of p-nitrobenzaldehyde with various alcohols over HNB-MOR, HMOR and HAlMCM-41 catalysts. The reactions with ethanol, ethanediol, n-propanol, n-butanol and benzyl alcohol over the HNB-MOR catalyst proceeded readily and gave the corresponding acetals in very high yields. In contrast, the product yields over HMOR and HAlMCM-41 catalysts are very low except for the reaction using ethanol as a substrate (entry 1). In addition, the product yields over the HMOR catalyst were reduced with increasing molecule dimensions of the alcohols. Particularly when aromatic alcohol was used as a substrate, the yields were very low over the HMOR and HAlMCM-41 catalysts. The results from Tables 2 and 3 indicate that the HNB-MOR catalyst shows excellent activity in the acetalization with aldehydes and alcohols including the bulky organic substrates.

Table 3 Acetal formation of p-nitrobenzaldehyde with various alcohols over HNB-MOR, HMOR and HAlMCM-41 catalysts^a

Entry	Product	Time (h)	Yields^b^,^c^,^d [%]
			a/b/c^b
a Reaction conditions: p-nitrobenzaldehyde (0.2 mmol), HNB-MOR (30 mg), alcohol (2 mL), 50 °C, 12 h. The yields were obtained by GC analysis (isolated yields in parentheses).b a) HNB-MOR as the catalyst; b) HMOR as the catalyst; c) HAlMCM-41 as the catalyst.c Benzyl alcohol (1.2 mmol), CH3CN (1.5 mL).d Ethanediol (0.6 mmol), CH3CN (1.5 mL).
1		12	100(93)/98/84
2		12	100(91)/83/72
3		18	100(89)/77/65
4^c		12	87(81)/37/11
5^d		12	99(90)/70/53

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It is worth mentioning that not only aldehydes, but also ketones are good substrates for the HNB-MOR-catalyzed acetalization reaction relative to HMOR and HAlMCM-41 catalysts. Table 4 clearly shows that the HNB-MOR catalyst gives the highest conversions in the acetalization of ketone with a series of alcohols. These results indicate that the HNB-MOR catalyst shows good activity in the acetalization of ketone with alcohols. The reusability of HNB-MOR catalysts was also surveyed. After the reaction, the catalyst was simply separated by filtration and washed with ethanol, dried at 50 °C, and reused 10 times without loss of activity (Table 5). These results indicate that the HNB-MOR catalyst has a good catalyst life, which is one of the key features of catalysts for industrial applications.

Table 4 HNB-MOR-catalyzed acetal formation of ketone with series alcohols^a

Entry	Product	Yield [%]
		a/b/c^b
a Reaction conditions: cyclohexanone (0.2 mmol), HNB-MOR (30 mg), alcohol (2 mL), 50 °C, 12 h. The yields were obtained by GC analysis and the products were detected by GC-MS.b a) HNB-MOR as the catalyst; b) HMOR as the catalyst; c) HAlMCM-41 as the catalyst.c Ethanediol (0.6 mmol), CH3CN (1.5 mL).d Benzyl alcohol (1.2 mmol), CH3CN (1.5 mL).
1^c		91/53/38
2		98/97/94
3		82/75/64
4		69/65/25
5^d		42/27/5

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Table 5 Recyclability experiments of the HNB-MOR system^a

Entry	Recycle	Yield [%]
a Reaction conditions: p-nitrobenzaldehyde (0.2 mmol), HNB-MOR (30 mg), ethanol (2 mL), 50 °C, 12 h. The yields were obtained by GC analysis.
1	Run 1	100
2	Run 2	100
3	Run 3	100
4	Run 4	>99
5	Run 5	100
6	Run 6	100
7	Run 7	100
8	Run 8	100
9	Run 9	100
10	Run 10	>99

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Generally, the catalytic performance of zeolite in the acetalization reaction can be co-influenced by acidity and pore structure. For the acetalization of aldehydes with small dimensions of alcohols, the reactant as well as the product can diffuse into the micropores in HMOR so that HNB-MOR and HMOR have a comparable catalytic activity (entries 1–6, Table 2). However, when using a substrate alcohol with large molecular dimensions, the conversion of aldehyde over HMOR is much lower than the conversion over HNB-MOR (entries 2–5, Table 3). This is due to the difference in mesoporosity of both catalysts. HNB-MOR has a mesopore surface area of 158 m² g⁻¹ (Table 1) which could favour mass-transfer, while HMOR has an external surface area of only 12 m² g⁻¹. It is reasonable that the HNB-MOR catalyst gives higher conversion compared with mesopore-free HMOR. Compared with HNB-MOR, although HAlMCM-41 has a relatively high mesopore volume (0.67 m³ g⁻¹) and mesopore surface area (876 m² g⁻¹), it still showed a lower acetalization capability with the bulky substrate, which may be related to the difference in acidity between the HNB-MOR and HAl-MCM-41 catalysts. It has been reported that acidic sites on catalysts facilitate acetalization reactions.²⁰ In this work, the HNB-MOR zeolite with strong and abundant acidic sites gave a high product yield, compared with HAlMCM-41 which has a relatively weak acidity (Fig. 1d). Particularly for the acetalization reaction using aldehyde with a nitro substituent as the substrate, the activity of the HAlMCM-41 catalyst was even lower than HMOR (entries 7 and 8, Table 2 and entries 1–5, Table 3). These phenomena indicate that a strong acidic catalyst is desirable for acetalization reactions, especially for aldehydes with electron-withdrawing nitro substituents. These results further demonstrate that strong acidic sites on the catalyst play an important role in acetal formation reactions.

Conclusions

In summary, we have developed an example of a zeolite material (HNB-MOR) as an outstanding catalyst for the synthesis of acetals with excellent yields under mild conditions. The mesoporosity of HNB-MOR benefits mass transfer and the strong acidic sites on HNB-MOR facilitate its acetalization activity.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (U1162115, 21076163 and 21371137) and the Science and Technology Program of Zhejiang Province (2010C31096).

Notes and references

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