Preparation of 4A-zeolite-based Ag nanoparticle composite catalyst and research of the catalytic properties

Abstract

A 4A-zeolite-based silver nanoparticle (Ag NPs) composite catalyst was prepared and its catalytic properties were studied. A hydrothermal method was used to synthesis 4A-zeolite from coal gangue raw material. Ag NPs were prepared by an in situ reduction method. The 4A-zeolite-based Ag NP composite catalyst was prepared by impregnation method. The catalyst was analyzed by a series of characterization techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible spectroscopy (UV-vis), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), etc. The characterization results indicated that the Ag NPs were uniformly distributed on the 4A-zeolite. The catalyst was then used to catalyze the epoxidation of styrene. The oxidant was tert-butyl hydroperoxide (TBHP), and isopropanol was used as the solvent. This demonstrated the excellent catalytic properties of the composite catalyst. The results showed very high conversion of styrene (72.1%) and excellent styrene oxide (SO) selectivity (59.5%). In addition, the joint selectivity of SO and benzaldehyde (BZ) reached 93.3%.

---

1. Introduction

4A-zeolite has good adsorption performance and ion exchange properties,^1–3 and has been widely used in the petrochemical industry, environmental protection and other fields in recent years.⁴

Silver catalysts show good catalytic activity for oxidation reactions, but their large-scale application is limited because the price of silver is high. The development of nanotechnology has, however, made this application possible. The physical and chemical properties of nano-silver are relatively stable, and the electronic and catalytic performance of nano-silver is excellent. It has broad prospective applications in battery electrode materials, catalyst materials, medical materials, etc.^5–7

Preparation of nano-silver can be accomplished through both physical and chemical methods. Physical methods mainly include laser ablation of metal^8–11 and metal vapor deposition.^12,13 Chemical methods include gas phase reduction, liquid phase reduction and solid phase reduction.^14–17 The liquid phase reduction method has been widely applied because of its low cost, simple operation, and low requirements for equipment. The size and shape of the nanoparticles, as well as the morphology of particle aggregations, were effectively controlled by altering the proportion of reducing agent and protective agent.

The specific surface area of nanoparticles and their catalytic performance are improved when nano-silver is loaded on a 4A-zeolite, and this can be used to catalyze oxidation reaction as a high efficient catalyst.^18–20 In addition, the combination of the two substances allows the catalyst to be recycled, and the preparation cost of the catalyst will also decrease.

Epoxides are important intermediates for the manufacture of a range of modern commercial products, such as epoxy resins, textiles, surface coatings, and biological chemicals.²¹ SO and BZ are important intermediates for medicine and spice. In addition, BZ is an important chemical raw material. In recent years, more and more researchers have studied styrene epoxidation.^22–27

First of all, 4A-zeolite was synthesized from coal gangue and Ag NPs were prepared using an in situ reduction method. The two substances were then mixed, and nano-silver was loaded onto the 4A-zeolite. The 4A-zeolite-based Ag NP composite catalyst was used to catalyze the epoxidation reaction of styrene (Fig. 1) and BZ and SO were obtained, among other products.^28–32

Fig. 1 Scheme showing preparation of the Ag NPs/4A-zeolite composite catalyst and the chemical equation of styrene epoxidation.

2. Experimental

2.1. Materials

Sodium hydroxide (NaOH, AR), polyvinylpyrrolidone (PVP, M_w = 10 000, AR) and absolute ethyl alcohol (C₂H₅OH, AR) were purchased from China Medicine Group. Deionized water was prepared by our university. Silver nitrate (AgNO₃, AR, 99.8%) was provided by Tianjin Yingda Sparseness & Noble Reagent Chemical Factory. Styrene (C₈H₈, AR, 99.0%) was purchased from Sinopharm Chemical Reagent Co. Ltd. Isopropanol ((CH₃)₂CHOH, AR, 99.5%) was purchased from Tianjin Beichen Founder Reagent Plant. tert-Butyl hydroperoxide ((CH₃)₃COOH, AR, 70%) was purchased from Tianjin Alfa Aesar Chemical Co. Ltd. All the chemicals were used as received without further purification.

2.2. Preparation of 4A-zeolite

Coal gangue powder was obtained by crushing coal gangue and sieving the resulting powder through a 200 mesh sieve. Activated coal gangue powder was then obtained by calcining the coal gangue powder for 1 h under 720 °C. The activated coal gangue powder, sodium hydroxide and deionized water were mixed in certain proportions, and the mixture was stirred for 1 h under 25 °C. The molar ratio of Si : Al : Na : H₂O was 1 : 1.25 : 3.25 : 65. The mixture was then transferred to a hydrothermal reaction kettle. The hydrothermal reaction kettle was placed in the vacuum drying oven, and the kettle was maintained at 100 °C for 6.5 h. Finally, the 4A-zeolite was obtained by suction filtration and drying. The pH had to be kept at approximately 8 during the suction filtration process.

2.3. Preparation of silver nanoparticles

First of all, a certain amount of PVP (1.3040 g) and anhydrous ethanol (25 ml) were mixed for 12 h at 25 °C, until PVP was fully dissolved in the anhydrous ethanol. A certain amount of silver nitrate was then added to the mixed solution, such that the proportion of silver nitrate to PVP was 1 : 10. This mixed solution was heated for 20 minutes using a circulating water bath at 80 °C. It was important to maintain a seal during this experiment.

2.4. Preparation of 4A-zeolite-based Ag NP composite catalyst and catalytic performance

The 4A-zeolite (1 g) which had been prepared was added to the solution containing the Ag NPs. The solution was stirred for 24 h at room temperature, and the 4A-zeolite-based Ag NP composite catalyst could be obtained after the suction filtration and drying process. The 4A-zeolite was activated for 2 h at 400 °C before it was placed in the solution containing the Ag NPs.

The styrene epoxidation was carried out at atmospheric pressure. The reaction mixture, which contained 1 ml styrene, 5 ml TBHP, 5 ml isopropanol and a certain amount of catalyst (0.1–0.5 g), was placed in a 25 ml round bottom flask. The reaction then took place under reflux (at 82 °C) with stirring for 12–48 h. The composite catalyst of Ag NPs/4A-zeolite was used in the styrene epoxidation. The catalyst was then separated from the reaction mixture by filtration. The reaction products and unconverted reactants were analyzed by an Agilent (7890A) gas chromatograph (GC) and Agilent (5975C) gas chromatography mass spectrometer (GC-MS). The GC was equipped with a flame ionization detector (FID) which contained capillary column (SE-30) and N₂ as carrier gas. The conversion of styrene and the content of each component in the products were calculated by an area normalization method. The styrene conversion and product selectivity were calculated as follows:

Styrene conversion (%) = [(areas of reactant converted) × 100]/[areas of reactant used]

Product selectivity (%) = [(areas of product formed) × 100]/[areas of reactant converted]

2.5. Characterization of catalyst

The samples were investigated by SEM (Hitachi, S-3400N, Japan), which was equipped with an energy dispersive spectrometer (EDS), and TEM (Jeol, JEM-2010, Japan and Fei, F20S-TWIN, Tecnai). The samples for TEM were dispersed in ethanol by ultrasonic treatment and a drop of this dispersion was deposited on a TEM carbon-coated Cu grid and dried at room temperature. The UV absorbance of the obtained Ag/4A-zeolite composite catalyst was measured by SHIMADZU, UV-3600 UV-vis recording spectrophotometer (Japan) with a variable wavelength between 200 and 600 nm, which was applied to detect UV absorbance of a solid sample. The UV absorbance of the liquid sample was measured by SHIMADZU, UV3150 UV-visible spectrophotometer with a variable wavelength between 200 and 600 nm using 10 mm quartz cells. The phase and crystalline structures of 4A-zeolite and Ag NPs were characterized by XRD (PIGAKV, D/MAX-2500/PC, Japan) over a range of 2θ angles from 5°to 80°. The XPS measurements were performed using a Thermo fisher Scientific ESCALAB 250 spectrometer.

3. Results and discussion

3.1. Characterization of catalyst

3.1.1. SEM study. Fig. 2 shows the SEM images of the 4A-zeolite (Fig. 2A) and the Ag NPs/4A-zeolite composite catalyst (Fig. 2B). From Fig. 2A, it can be seen that 4A-zeolite has a cube structure and that the size of the 4A-zeolite particles ranged from 1 μm to 1.5 μm. The images indicate that and uniform diameter 4A-zeolite with a smooth surface was prepared successfully. In addition, aggregation of the zeolite cannot be seen in Fig. 2A and B and the zeolite had a well-defined shape. It can also be seen that the 4A-zeolite was fairly well dispersed. After comparing Fig. 2A and B, it was noted that the morphology of the zeolite had not changed, which indicated that the addition of Ag NPs had not affected the structure of the 4A-zeolite. This could be attributed to the relatively stable zeolite framework. Analysis of the Ag NPs/4A-zeolite composite catalyst was performed on an EDS in the SEM. The EDS analysis revealed that the obtained Ag NPs/4A-zeolite composite catalyst contained Si, Al, O, Na and Ag elements. It also showed that silver had been loaded onto the zeolite, as can be seen in Fig. 2C

Fig. 2 SEM images of the 4A-zeolite (A) and Ag NPs/4A-zeolite composite catalyst (B); EDS spectra of Ag NPs/4A-zeolite composite catalyst (C).

3.1.2. Powder X-ray diffraction study. The low angle powder XRD patterns of 4A-zeolite (Fig. 3A) and the Ag NPs/4A-zeolite composite catalyst (Fig. 3B) are shown in Fig. 3. All the absorption peaks were detected between 5° and 80°. The absorption peaks of 4A-zeolite appear at 2θ = 7.2°, 10.2°, 12.5°, 16.1°, 21.7°, 24.0°, 27.2°, 29.9° and 34.2° (JCPDS 39-0223). From Fig. 3A, it can be seen that the pattern shows all the characteristic absorption peaks of 4A-zeolite. It also shows that the 4A-zeolite is highly crystalline. After comparing Fig. 3A and B, the peaks around 2θ = 38.1°, 44.3° and 64.5° were assigned to the Ag (111), (200) and (220) crystal planes, respectively. These peaks correspond to the cubic crystal structure of silver (JCPDS 04-0783). The measured crystal lattice spacing was 0.21 nm, 0.24 nm and 0.145 nm, corresponding to the Ag (111), (200) and (220) crystal planes, respectively. No other peaks characteristic of impurities were observed, which indicated that the metallic Ag was the only other species in the catalyst. The diffraction peaks of Ag species were relatively small, which indicated that the Ag species were highly dispersed on the 4A-zeolite. The results indicated that the impregnation method could effectively load Ag onto the 4A-zeolite and that the silver crystalline phase was formed in the zeolite framework.

Fig. 3 XRD patterns of the 4A-zeolite (A) and the Ag NPs/4A-zeolite composite catalyst (B).

3.1.3. UV-vis spectral study. The UV-vis spectra of the Ag NPs/4A-zeolite composite catalyst (Fig. 4A) and the filtrate (Fig. 4B; the liquid which was acquired in the process of preparing the 4A-zeolite-based Ag NP composite catalyst) is shown in Fig. 4. From Fig. 4A, it can be seen that a broad and characteristic absorption band associated with the formation of Ag NPs appeared at 383 nm, which can be attributed to the existence of Ag NPs. In addition, we concluded from the highly symmetrical peak that the formed Ag NPs were rather well dispersed on the 4A-zeolite surface was found. In addition, the diameter of the Ag NPs appeared to be relatively small. As shown in Fig. 4B, an obvious absorption band at 413 nm could be attributed to the Ag NPs. After comparing Fig. 4B with A, it was shown that the peak position moved to a longer wavelength, for which a reasonable explanation was the aggregation of silver particles in the filtrate. The existence of Ag NPs on the zeolite could be inferred from the UV-vis spectra.

Fig. 4 Solid-UV spectra pattern of the Ag NPs/4A-zeolite composite catalyst (A) and liquid-UV spectra pattern of the filtrate (B).

3.1.4. TEM study. The morphology and size of the nano-silver was detected and analyzed by TEM. The TEM of the Ag NPs/4A-zeolite composite catalyst is shown in Fig. 5. It was obvious that the structure of the 4A-zeolite had not changed. It was clearly observed that there were a large number of small dark spots on the surface of zeolite, which were the Ag NPs on the surface of the 4A-zeolite. The surface of the 4A-zeolite was covered with Ag NPs and the Ag particles were distributed very uniformly. In addition, aggregation of the particles was not seen in Fig. 5A–C, due to the addition of a protective agent (PVP) when the Ag NPs were prepared. Fig. 5D shows a HRTEM image of the Ag NPs/4A-zeolite composite catalyst. The crystal lattice can be clearly on the surface of the Ag NPs/4A-zeolite composite catalyst, and the measured spacing is 0.21 nm, corresponding to the interplanar (200) spacing of Ag NPs, as observed from the XRD patterns described previously. It could thus be confirmed that Ag NPs with an average size of about 5–10 nm were uniformly distributed on the surface of 4A-zeolite.

Fig. 5 TEM images of Ag NPs/4A-zeolite composite catalyst (A–C) and HRTEM image of Ag NPs/4A-zeolite composite catalyst (D).

3.1.5. XPS study. XPS characterization was performed in order to confirm the electronic states of Ag on the catalyst. Fig. 6 shows the XPS patterns for the Ag NPs/4A-zeolite composite catalyst. Fig. 6A shows the fully scanned spectra of the Ag/4A-zeolite in the range of 0–1300 eV. The elements C, Si, Al, O, Na and Ag were demonstrated to exist in the Ag NPs/4A-zeolite composite catalyst. The chemical elements Si, Al, O and Na were attributed to 4A-zeolite. In Fig. 6B, two peaks with high intensity were observed at 368.5 eV and 374.5 eV, which were assigned to the photoelectron spectra of Ag 3d. These peaks correspond well to Ag 3d_5/2 and Ag 3d_3/2 binding energies, respectively. It was shown that Ag⁰ was the only state of Ag that existed in the composite catalyst. The conclusion was that Ag⁰ had been loaded onto the 4A-zeolite. It indicated that Ag⁰ was the only active component that promoted styrene epoxidation on the surface of 4A-zeolite.

Fig. 6 XPS pattern of the Ag NPs/4A-zeolite composite catalyst.

3.2. Catalytic activity

Many solid catalysts reported in the literatures show high product selectivity with TBHP as the oxidant.^22,23,25,33 In this work, the catalytic activities of Ag NPs/4A-zeolite composite catalyst in the epoxidation of styrene were thus tested with TBHP as the oxidant. In order to find the optimal reaction conditions, different parameters, such as catalyst amount and reaction time, were investigated. From analysis of GC and GC-MS detection, the main products of the reaction were found to be styrene oxide (SO) and benzaldehyde (BZ).

The influence of catalyst amount on the catalytic performance was investigated by varying the catalyst amount from 0.1 g to 0.5 g and the results are shown in Table 1. The reaction time was 24 h and the temperature was kept at 82 °C. From the table, it can be seen that the highest conversion of styrene (55.1%) was achieved when 0.1 g catalyst was used. The conversion of styrene did not improve as the amount of catalyst increased, but the selectivity of SO and BZ increased slightly. It is worth noting that the selectivity of SO and BZ decreased noticeably with 0.5 g catalyst. The optimal catalyst amount was considered to be 0.1 g, so we used this value to continue our examination of reaction time.

Table 1 Epoxidation of styrene under various catalyst dosages

Entry	Catalyst (g)	Time (h)	Conversion (%)	Selectivity (%)
				SO	BZ	Others
1	0.1	24	55.1	50.4	32.8	16.8
2	0.2	24	53.2	56.1	35.7	8.2
3	0.3	24	53.9	59.0	33.2	7.8
4	0.4	24	53.0	58.1	35.6	6.3
5	0.5	24	53.3	53.0	27.7	19.3

---

The effect of reaction time on catalytic performance when 0.1 g catalyst was added to the reaction was investigated, and the results are shown in Table 2. The reaction temperature was kept at 82 °C. From the table, it can be seen that reaction time had an obvious effect on catalytic performance. The conversion of styrene and SO selectivity noticeably improved with an increase in reaction time. The highest selectivity of SO was found at 48 h. However, the conversion of styrene increased slowly while the selectivity of SO and BZ decreased noticeably from 48 h to 60 h. This indicates that SO and BZ started to turn into other by-products. Further prolonging the reaction time for styrene epoxidation after 48 h therefore made no sense. There is no doubt that the reaction time plays a key role in styrene epoxidation reaction. The results showed that the optimal reaction time was 48 h. The conversion of styrene was 72.1% and the selectivity of SO was 59.5%. The yield of SO was 42.9%. In addition, the joint selectivity of SO and BZ reached 93.3%.

Table 2 Epoxidation of styrene over various reaction times

Entry	Catalyst (g)	Time (h)	Conversion (%)	Selectivity (%)
				SO	BZ	Others
1	0.1	12	35.9	45.5	40.4	14.1
2	0.1	24	55.1	50.4	32.8	16.8
3	0.1	36	66.4	56.4	30.3	13.3
4	0.1	48	72.1	59.5	33.8	6.7
5	0.1	60	77.8	45.9	28.7	25.3

---

In addition, the catalytic performance of 4A-zeolite and a blank control group were investigated under the 24 h. The catalytic results of 4A-zeolite, Ag/4A-zeolite and blank control are shown in Table 3. Compared with the blank control, 4A-zeolite slightly improved the conversion of styrene and selectivity of SO. It can be seen that the conversion of styrene increased slightly while the selectivity of SO increased obviously from 18.2% to 50.4% when Ag NPs were added to the 4A-zeolite. In addition, the selectivity of BZ and other by-products decreased obviously. The yield of SO increased from 9.7% to 27.8%. The Ag NPs/4A-zeolite composite catalyst therefore showed a significant improvment in styrene epoxidation.

Table 3 Comparison of Ag/4A-zeolite catalyst with 4A-zeolite and a blank control for the styrene epoxidation reaction

Entry	Catalyst	Time (h)	Conversion (%)	Selectivity (%)
				SO	BZ	Others
1	—	24	51.9	16.3	46.2	37.5
2	4A-zeolite (0.1 g)	24	53.1	18.2	46.4	35.4
3	Ag/4A-zeolite (0.1 g)	24	55.1	50.4	32.8	16.8

---

4. Conclusions

A kind of composite catalyst consisting of Ag NPs supported on a 4A-zeolite was prepared. The catalyst was characterized by SEM, XRD, UV, TEM and XPS. After analysis of all the characterization, the results showed that the 4A-zeolite morphology and crystallinity was improved and that Ag NPs had been loaded onto the 4A-zeolite and were evenly distributed. It was shown that the diameter of the Ag NPs was from 5 to 15 nm, according to the particle size distribution. The composite catalyst was then used for the epoxidation of styrene with TBHP as oxidant. Different parameters such as catalyst amount and reaction time were investigated. The optimal catalyst amount was 0.1 g and the optimal reaction time was 48 h. The Ag NPs/4A-zeolite composite catalyst showed good catalytic activity for the epoxidation of styrene. The conversion of styrene was 72.1% and the selectivity of SO was 59.5%. In addition, the joint selectivity of SO and BZ was 93.3%. We concluded that the Ag⁰, which was added to the 4A-zeolite, promoted the epoxidation of styrene. Loading the Ag NPs onto the 4A-zeolite support improved the specific surface area of the nanoparticles and increased the contact area between the Ag NPs. In addition, the catalytic performance was further improved as the Ag NPs were distributed very uniformly on the surface of 4A-zeolite. The highly dispersed Ag NPs significantly improved the yield of SO.

Acknowledgements

The authors gratefully acknowledge the support from the National Natural Science Foundation of China (21366016).

References

1. A. Mayoral, T. Carey and P. A. Anderson, Microporous Mesoporous Mater., 2012, 166, 117–122.
2. D. H. Kim and J. Kim, J. Phys. Chem. Solids, 2007, 68, 734.
3. C. W. Kim, J. S. Park and K. S. Lee, Power Sources, 2006, 163, 144.
4. D. M. Bibby and M. P. Dale, Nature, 1985, 317, 157–158.
5. G. G. Lenzi and C. V. B. Favero, Desalination, 2011, 270(1–3), 241–247.
6. H. Shiomi and H. Furukawa, J. Mater. Sci.: Mater. Electron., 2000, 11(1), 7–31.
7. M. D. Morris, Proc. SPIE-Int. Soc. Opt. Eng., 1999, 224–229.
8. S. Yang, W. Cai and G. Liu, J. Phys. Chem. C, 2009, 113(16), 6480–6484.
9. H. Muto, K. Yamada, K. Miyajima and F. Mafune, J. Phys. Chem. C, 2007, 111(46), 17221–17226.
10. P. Liu, W. Cai and H. Zeng, J. Phys. Chem. C, 2008, 112(9), 3261–3266.
11. S. Hashimoto, T. Uwada, H. Masuhara and T. Asahi, J. Phys. Chem. C, 2008, 112(39), 15089–15093.
12. K. Wegner, B. Walker, S. Tsantilis and S. E. Pratsinis, Chem. Eng. Sci., 2002, 57(10), 1753–1762.
13. C. C. Chen and C. C. Yeh, Adv. Mater., 2000, 12(10), 738–741.
14. Z. S. Pillai and P. V. Kamat, J. Phys. Chem. B, 2004, 108(3), 945–951.
15. Y. Sun, B. Mayers and T. Herricks, Nano Lett., 2003, 3(7), 955–960.
16. D. H. Chen and Y. W. Huang, J. Colloid Interface Sci., 2002, 255(2), 299–302.
17. H. J. Jiang, K. S. Moon and Z. Q. Zhang, J. Nanopart. Res., 2006, 8, 117–124.
18. C. Zakri and P. Poulin, Nano Lett., 2005, 5(11), 2212–2215.
19. K. A. Bethke and H. H. Kung, J. Catal., 1997, 172(1), 93–102.
20. Z. Li and S. M. Flytzani, J. Catal., 1999, 182(2), 313–327.
21. H. Nakajima, S. Takase, H. Terano and H. Tanaka, J. Antibiot., 1997, 50, 96–99.
22. H. Liu, J. Bai, C. P. Li, W. Xu, W. Y. Sun, T. Xu, Y. R. Huang and H. Q. Li, RSC Adv., 2014, 4, 3195–3200.
23. B. Singh and A. K. Sinha, J. Mater. Chem. A, 2014, 2, 1930–1939.
24. Z. Q. Shi, L. X. Jiao, J. Sun, Z. B. Chen, Y. Z. Chen, X. H. Zhu, J. H. Zhou, X. C. Zhou, X. Z. Li and R. Li, RSC Adv., 2014, 4, 47–53.
25. H. Liu, J. Bai, S. Wang, C. P. Li, L. P. Guo, H. O. Liang, T. Xu, W. Y. Sun and H. Q. Li, Colloids Surf., A, 2014, 448, 154–159.
26. H. Lin, Y. Liu and Z. L. Wu, Chem. Commun., 2011, 47, 2610–2612.
27. L. Zhou, C. F. Gorin and R. J. Madix, J. Am. Chem. Soc., 2010, 132, 434–435.
28. M. Silva, C. Freire, B. De Castro and J. L. Figueiredo, J. Mol. Catal. A: Chem., 2006, 258, 327–333.
29. X. T. Wang, Z. Q. Liang, F. Z. Zhang, L. Yang and S. L. Xu, J. Mater. Sci., 2013, 48, 5899–5903.
30. J. Sun, Q. B. Kan, Z. F. Li, G. L. Yu, H. Liu, X. Y. Yang, Q. S. Huo and J. Q. Guan, RSC Adv., 2013, 4, 2310–2317.
31. M. M. Hussain and P. J. Walsh, Acc. Chem. Res., 2008, 41, 883–893.
32. D. S. Pinnaduwage, L. Zhou, W. W. Gao and C. M. Friend, J. Am. Chem. Soc., 2007, 129, 1872–1873.
33. J. H. Liu, F. Wang, S. S. Qi, Z. G. Gu and G. D. Wu, New J. Chem., 2013, 37, 769–774.

---