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Al ratio
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Review
Enhanced catalytic activity in propene oxidation over NaZSM-5 zeolitesupported Pt nanoparticles by increasing the zeolite Si/Al ratio ⁎
⁎
Yiwen Jiang, Ling Zhang, Yiquan Xie, Shichao Han, Qiuyan Zhu, Xiangju Meng , Feng-Shou Xiao Key Laboratory of Applied Chemistry of Zhejiang Province, Zhejiang University, Hangzhou 310028, China
A R T I C LE I N FO
A B S T R A C T
Keywords: VOCs abatement Propene oxidation NaZSM-5 zeolite Si/Al ratio Pt nanoparticles
A series of zeolite-supported Pt nanoparticles (Pt/NaZSM-5) have been prepared by adjusting Si/Al ratio of NaZSM-5 zeolite from 60 to pure silica. Catalytic tests in propene oxidation as an important model reaction for removal of hydrocarbons emitted from automobiles during cold-start, show that the catalytic activities significantly increase with Si/Al ratios of NaZSM-5 zeolite in these Pt/NaZSM-5 catalysts. As a typical example, Silicalite-1 supported Pt nanoparticles exhibits the highest activity, giving the 50% conversion temperature (T50) and 90% conversion temperature (T90) at 112 °C and 118 °C, respectively. Characterizations of these catalysts with multiple techniques show that the enhanced catalytic activities are strongly related to the zeolite Si/Al ratios of the catalysts, where an increase of zeolite Si/Al ratio is favorable for the adsorption of propene, promoting the propene oxidation. This feature might offer a good opportunity to design and prepare highly efficient heterogeneous catalysts for removal of light olefins in the future.
1. Introduction With an increase of environmental awareness, more stringent policies have been introduced to protect human environment. As one of the major pollutants in the automobile exhaust [1], hydrocarbon is strictly limited in emission regulations due to their toxic, carcinogenic, mutagenic, and teratogenic nature [2]. Propene, a typical hydrocarbon, mainly comes from the automobile exhaust during cold-start, and it has been recognized as a polluting molecule due to its high photochemical ozone creativity potential (POCP), making its abatement from such source imperative [3]. Among proposed routes for eliminating hydrocarbons, catalytic oxidation stands out. It has obvious advantages such as full conversion of hydrocarbons, exceptional persistence, and economic efficiency, in comparison to other techniques such as physical adsorption and plasma elimination [4–7], It is worth noting that currently available oxidation catalysts fail to achieve the complete elimination of propene under the engine coldstart conditions. Therefore, it is strongly desirable to develop highly efficient heterogeneous catalysts for the propene oxidation [8]. Generally, oxidation catalysts used in volatile organic compounds (VOCs) abatement mainly include Perovskite [9–11], supported noble metal catalysts, and transition metal oxides [12–20]. Among them, supported noble metal catalysts have been intensively investigated for
⁎
the propene oxidation due to its excellent activity at relatively low temperature and extraordinary stability [21]. In these cases, conventional supports are inorganic oxides such as alumina, silica, titania, and ceria [11,22–24]. Compared with these oxides, zeolites as catalyst supports are getting more and more attention owing to their large surface area, strong adsorption capacity, high thermal and hydrothermal stabilities, uniform and intricate channels, and controllable acidic property [25–30]. For example, Hao et al. reported that NaZSM-5 supported Ag catalyst is highly active for C2H4 combustion [31]. In this work, we have prepared a series of Pt-based catalysts supported by NaZSM-5 zeolites with different Si/Al ratios from 60 to pure silica. Catalytic tests in propene oxidation show that the increase of the Si/Al ratios in the catalysts leads to the enhancement of catalyst activity. When the Silicalite-1 is employed as the support, the catalyst is most active, rendering the T90 as low as 118 °C. 2. Experimental 2.1. Materials NaAlO2, TPAOH (40 wt.%), and TEOS were purchased from Sinopharm Chemical Reagent. H2PtCl6∙6H2O, CH3COONH4, and 100–140 mesh amorphous silica was purchased from Aladdin Chem.
Corresponding authors. E-mail addresses: [email protected] (X. Meng), [email protected] (F.-S. Xiao).
https://doi.org/10.1016/j.cattod.2019.06.075 Received 25 January 2019; Received in revised form 14 May 2019; Accepted 18 June 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Yiwen Jiang, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.06.075
Catalysis Today xxx (xxxx) xxx–xxx
Y. Jiang, et al.
Table 1 Textural parameters of the as-synthesized catalysts. Catalyst
Si/Al
BET surface area, m2/g
Micropore volume, cm3/g
Pt/NaZSM-5-60 Pt/NaZSM-5-100 Pt/NaZSM-5-200 Pt/Silicalite-1
52 87 205 > 2000
332 331 407 398
0.16 0.16 0.18 0.17
hydrothermal synthesis [32]. Typically, a certain amount of NaAlO2 was dissolved in 32.24 mL of water, followed by the addition of 5.54 mL of TPAOH. After stirring for 15 min at room temperature, 10.13 mL of TEOS were introduced into the mixture. After stirring for 5.5 h at room temperature, the resultant gel was transferred into an autoclave for crystallization at 180 °C for 4 days. The product was collected by centrifuging, washing, drying at 80 °C in air for 12 h, and calcining at 550 °C for 4 h. The HZSM-5-60 zeolites were prepared by ion-exchange from NH4+-exchange of NaZSM-5-60, followed by calcination at 500 °C for 4 h. The NaZSM-5 zeolites and amorphous silica supported Pt catalysts were prepared through the incipient wetness impregnation method, with H2PtCl6 as the Pt precursor. After calcination at 450 °C for 4 h in air and reduction under H2 flow (80 mL/min) at 300 °C for 2.5 h, the supported Pt catalysts (denoted as Pt/support) were finally obtained, where the loading of Pt was about 1 wt%. Fig. 1. XRD patterns of the (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/ NaZSM-5-200, and (d) Pt/Silicalite-1 catalysts.
2.3. Catalyst characterization Propene was supplied by Hangzhou New Century Hybrid Gas Company. All the chemicals were of analytical-reagent grade and were used without further purification.
X-ray diffraction (XRD) pattern was carried out with a RIGAKU Ultimate IV diffractometer with Cu Kα radiation. Nitrogen adsorptiondesorption isotherms were acquired by Belsorp Max at −196 °C, and the specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. The inductively coupled plasma atomic emission spectrometry (ICP-AES) was obtained using a Perkin-Elmer plasma 8000 optical emission spectrometer to determine the Si/Al ratios of the zeolites and the Pt content in the catalysts. High-resolution TEM images
2.2. Catalyst preparation The NaZSM-5-x zeolites (x stands for Si/Al ratio at 60, 100, and 200, as well as pure silica in the starting gels) were prepared from
Fig. 2. SEM images of the (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/NaZSM-5-200, and (d) Pt/Silicalite-1 catalysts. 2
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Fig. 3. (A) TEM image of the Pt/Silicalite-1 and (B) Pt size distributions of (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/NaZSM-5-200, and (d) Pt/Silicalite-1 catalysts.
catalyst (40–60 mesh) was used with a total flow-rate of feed gases at 50 mL/min (1000 ppm propene in standard air), a space velocity (SV) of 30,000 mL/(g·h), and 50% relative humidity (RH). The concentration of propene in the tail gases was analyzed by a gas chromatograph (Fuli, GC9790) equipped with a flame ionization detector and a 19091N-113 INNOWAX capillary column (Agilent, 30 m × 0.32 mm × 0.25 mm i.d.). The conversion of propene was calculated by the difference between inlet and outlet propene concentration. The catalytic activities were determined by the values of T50 and T90, defined as the temperatures at 50% and 90% of propene conversion. The relative humidity was determined by a handheld humidity detector. The relative
were obtained on a JEOL 2100 F electron microscope at 200 kV. Scanning electron microscopy (SEM) images of the samples were recorded on a Hitachi SU 1510 apparatus. Contact angles of catalysts were measured on a Shanghai Zhongchen JC2000X Optical Contact Angle Meter. 2.4. Propene oxidation Propene oxidation at atmospheric pressure was performed in a continuous-flow fixed bed micro-reactor of a quartz tube (6 mm i.d.) with the catalyst placed at the center (Fig. S1). In a typical run, 100 mg 3
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catalysts are detailed in Table 1. ICP analyses show that these synthesized zeolites have distinguishable Si/Al ratios while for each case the difference between the ratio in the starting gels and the corresponding zeolite product is expected and acceptable (Table 1). Fig. 3A shows TEM image of Pt/Silicalite-1, evidencing the microporosity of zeolite and nanoparticles of Pt. Clearly, the Pt nanoparticles are uniformly distributed on the Silicalite-1 zeolite. Furthermore, the Pt size distribution on various zeolites have been compared, as shown in Fig. 3B. Notably, they have very similar mean size distribution of Pt nanoparticles around 2–3 nm. To sum up the characterizations results, these catalysts have similar zeolite crystallinity, crystal size, textural parameters, Pt size distribution, but distinguishable Si/Al ratios. 3.2. Catalytic evaluation Fig. 4 shows catalytic data in propene combustion over various studied catalysts. The T50 and T90 values of these catalysts are presented in Table 2, for the purpose of clarity and comparison. Notably, these zeolite-supported Pt catalysts exhibit much higher activities than other supported noble metal catalysts reported previously [33–35]. Among these zeolite-supported Pt catalysts, it is particularly observed that the catalytic activity was strongly influence by the Si/Al ratio in the catalysts. Specifically, when Si/Al ratio of the starting gel for the synthesis of NaZSM-5 zeolite is 60, the T50 and T90 values of Pt/NaZSM-5-60 are 145,°C and 152,°C; when Silicalite-1 is used, the T50 and T90 values of Pt/Silicalite-1 are 112,°C and 118,°C. The remarkable performance of the Pt/Silicalite-1 should be potentially important for complete removal of light olefins at relatively low temperature in the future. To understand the effect of reactants on catalytic activity, we have measured the reactant orders (Table 3), which could be estimated from the slope of a graph of lnR as a function of ln[A], where R is the reaction rate and [A] stands for the reactant concentration, as given in Fig. S3. Notably, compared with Pt/NaZSM-5-60, the propene order over Pt/ Silicalite-1 is remarkably reduced, while the oxygen order over Pt/Silicalite-1 is obviously enhanced. According to the reported literature [36], the reduced reaction order means the enhanced activity of the reactant and relatively high Si/Al ratios are favorable for the adsorption of propene, which should be attributed to the increase of sample hydrophobicity, in good agreement with those reported previously [7]. Considering the relationship between the Si/Al ratios of the catalysts and the catalytic activities, it is suggested that higher activity over Pt/ Silicalite-1 than that over Pt/NaZSM-5-60 is mainly attributed to the change in propene order rather than oxygen order. To understand the difference in propene order over various catalysts, we have managed to obtain their adsorption breakthrough curves of propene, as shown in Fig. 5. Interestingly, propene permeation in Pt/ Silicalite-1 is much slower than that in Pt/NaZSM-5-60, indicating that Pt/Silicalite-1 has larger adsorption capacity than Pt/NaZSM-5-60. This feature might be reasonably related to their distinguishable catalyst wettability, where relatively hydrophobic zeolites might be helpful for adsorption of propene. To verify the difference of catalyst wettability, we have measured contacting angle of water on various catalyst surface, as given in Fig.
Fig. 4. Dependences of propene conversion on reaction temperature in propene oxidation over the (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/NaZSM-5200, and (d) Pt/Silicalite-1 catalysts under propene concentration of 1000 ppm in standard air, flow rate of 50 mL/min, and space velocity of 30,000 mL/(g·h).
humidity of inlet gas without addition of water vapor was 50%. When additional water vapor was introduced from a bubbling method, the relative humidity reached 70%. Reaction rates of propene oxidation over propene and oxygen were both measured in a continuous-flow fixed bed micro-reactor of a quartz tube (6 mm i.d.) with the catalyst placed at the center. In a typical run, 500 mg catalyst (40–60 mesh) was used with feed gases composed by 2000 ppm propene and 5% oxygen (He as a balance gas). To test the oxygen rate, the flow rate of propene was kept at 25 mL/min, and the flow rate of oxygen was adjusted from 4.0 mL/min to 3.5, 3.0, 2.5, and 2.0 mL/min, respectively. To measure the propene rate, the flow rate of oxygen was kept at 25 mL/min, and the flow rate of propene was adjusted from 22.5 mL/min to 20, 17.5, 15 mL/min. The kinetic measurements for propene oxidation were implemented from the propene conversion below 15%. Adsorption breakthrough curves of propene for different catalysts were performed with the same equipment at room temperature by flowing 40 ppm propene. 3. Results and discussion 3.1. Catalyst characterization Fig. 1 presents XRD patterns of the zeolite-supported Pt catalysts including Pt/NaZSM-5-60, Pt/NaZSM-5-100, Pt/NaZSM-5-200, and Pt/ Silicalite-1, giving similar characteristic peaks associated with MFI zeolite structure. Fig. 2 shows SEM images of these catalysts, exhibiting uniform crystals of 300–400 nm. Fig. S2 shows N2 sorption isotherms of these catalysts, having typical Langmuir-type adsorption. A steep N2 uptake occurs at relative pressure (P/P0) less than 0.01, assigned to the micropore filling of nitrogen. The textural parameters of studied Table 2 Catalytic data for oxidation of propene over various catalysts. Catalyst
Noble metal, wt%
T50, °C
T90, °C
GHSV, mL/(g·h)
Concentration, ppm
Ref.
Pt/NaZSM-5-60 Pt/NaZSM-5-100 Pt/NaZSM-5-200 Pt/Silicalite-1 Pt/Al2O3 Pd/TiO2 Pd/Al-PILC Pt/Al-PILC
1 1 1 1 5 0.8 2 2
145 124 122 112 210 150 210 310
152 130 128 118 225 160 260 325
30,000 30,000 30,000 30,000 60000 35000 20000 20000
1000 1000 1000 1000 800 1000 5000 5000
This work This work This work This work [33] [34] [35] [35]
4
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effect of water on catalyst activity is pretty obvious in previous reports [37]. As observed from the aforementioned results, the Si/Al ratio in the MFI zeolites strongly influences the catalytic activity, where pure silica zeolite has the highest activity. To understand the attribution of MFI channels for the catalytical activity, amorphous silica (AM) is employed as a control support for preparation of supported Pt catalysts. As shown in Fig. S6, the amorphous silica supported Pt catalyst (Pt/AM) exhibits higher activity than Pt/NaZSM-5-60, but much less than Pt/Silicalite-1. Compared with the amorphous silica, the channels of the Silicalite-1 zeolite are very favorable for mass transfer in the catalytic reactions. As a result, there is an obvious by-product in the Pt/AM-catalyzed reaction, while it is undetectable on the Pt/Silicalite-1 catalyst.
Table 3 Reactant order in propene combustion. Catalyst
Propene
Oxygen
Pt/NaZSM-5-60 Pt/NaZSM-5-100 Pt/NaZSM-5-200 Pt/Silicalite-1
2.5 3.0 2.5 1.5
1.9 2.1 2.2 2.9
4. Conclusion A set of NaZSM-5 zeolite-supported Pt catalysts have been prepared by adjusting Si/Al ratio in the starting gels from 60 to pure silica. Catalytic tests in propene oxidation show that the Silicalite-1 supported Pt catalyst (Pt/ Silicalite-1) is extraordinarily active, giving the values of T50 and T90 at 112 °C and 118 °C. Moreover, this catalyst is highly durable even in the presence of high humidity. The combination of high activity and long-time stability makes this catalyst potentially important for propene oxidation at relatively low temperature, as a key model reaction for removal of hydrocarbons emitted from cold-start engines. Fig. 5. Adsorption breakthrough curves of propene for the (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/NaZSM-5-200, and (d) Pt/Silicalite-1 catalysts under propene concentration of 40 ppm.
Acknowledgment This work is supported by National Key Research and Development Program of China (2017YFC0211101) and National Natural Science Foundation of China (21673205, 21720102001 and 91634201). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.06.075. References [1] R.M. Heck, R.J. Farrauto, Appl Catal A-Gen. 221 (2001) 443–457. [2] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165–2180. [3] R.G. Derwent, M.E. Jenkin, S.M. Saunders, M.J. Pilling, Atmos. Environ. 32 (1998) 2429–2441. [4] S. Pasquiers, Eur. Phys. J.-Appl. Phys 28 (2004) 319–324. [5] C. Ayrault, J. Barrault, N. Blin-Simiand, F. Jorand, S. Pasquiers, A. Rousseau, Catal. Today 89 (2004) 75–81. [6] I. Yet-Pole, Environ. Sci. Technol. 38 (2004) 3785–3791. [7] C.Y. Chen, X. Wang, J. Zhang, S.X. Pan, C.Q. Bian, L. Wang, F. Chen, X.J. Meng, X.M. Zheng, X.H. Gao, F.S. Xiao, Catal. Lett. 144 (2014) 1851–1859. [8] D.H. Kim, M.C. Kung, A. Kozlova, S.D. Yuan, H.H. Kung, Catal. Lett. 98 (2004) 11–15. [9] R.J.H. Voorhoeve, D.W. Johnson, J.P. Remeika, P.K. Gallagher, Science 195 (1997) 827–833. [10] W.B. Li, J.X. Wang, H. Gong, Catal. Today 148 (2009) 81–87. [11] M. Alifanti, M. Florea, V.I. Parvulescu, Appl. Catal. B-Environ. 70 (2007) 400–405. [12] L.F. Liotta, Appl. Catal. B-Environ. 100 (2010) 403–412. [13] C. Lahousse, A. Bernier, P. Grange, B. Delmon, P. Papaefthimiou, T. Ioannides, X. Verykios, J. Catal. 178 (1998) 214–225. [14] S. Scire, L.F. Liotta, Appl. Catal. B-Environ. 125 (2012) 222–246. [15] V.P. Santos, S.A.C. Carabineiro, P.B. Tavares, M.F.R. Pereira, J.J.M. Orfao, J.L. Figueiredo, Appl. Catal. B-Environ. 99 (2010) 198–205. [16] F. Bertinchamps, C. Gregoire, E.M. Gaigneaux, Appl. Catal. B-Environ. 66 (2006) 1–9. [17] D. Delimaris, T. Ioannides, Appl. Catal. B-Environ. 84 (2008) 303–312. [18] Y.X. Liu, H.X. Dai, Y.C. Du, J.G. Deng, L. Zhang, Z.X. Zhao, C.T. Au, J. Catal. 287 (2012) 149–160. [19] B. de Rivas, R. Lopez-Fonseca, C. Jimenez-Gonzalez, J.I. Gutierrez-Ortiz, J. Catal. 281 (2011) 88–97. [20] M. Hosseini, T. Barakat, R. Cousin, A. Aboukais, B.L. Su, G. De Weireld, S. Siffert, Appl. Catal. B-Environ. 111 (2012) 218–224. [21] J. Wan, R. Ran, M. Li, X.D. Wu, D. Weng, J. Mol. Catal. A-Chem. 383 (2014) 194–202.
Fig. 6. Dependences of propene conversion on reaction time over Pt/Silicalite-1 catalyst in the relatively humidity (RH) of (a) 50% and (b) 70%.
S4. Clearly, Pt/Silicalite-1 exhibits higher contacting angle than Pt/ NaZSM-5-60, confirming relatively higher hydrophobicity of Pt/ Silicalite-1 than that of Pt/NaZSM-5-60. In addition, the acidity of the catalysts is also important for the catalysis. Therefore, we have compared the catalytic activity of Pt nanoparticles supported on MFI zeolite with Na+ and H+ cations. Notably, the T90 and T50 (147 °C and 129 °C) of the Pt/HZSM-5-60 are slightly lower than those (152 °C and 145 °C) of the Pt/NaZSM-5-60 (Fig. S5, Table S1), but much higher than those (118 °C and 112 °C) of the Pt/Silicalite-1. These results indicate that the zeolite hydrophobicity is more important than zeolite cations for the contribution of catalytic activities. Fig. 6 shows dependences of the catalytic activity on reaction time in propene oxidation at 140 °C over the Pt/Silicalite-1 catalyst under the condition of 50% and 70% RH. It is of great importance that there is almost no loss in catalytic activity within 100 h tests, demonstrating that the Pt/Silicalite-1 catalyst is durable. In contrast, the inhibition 5
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[30] L. Wang, S.D. Xu, S.X. He, F.S. Xiao, Nano Today 20 (2018) 74–83. [31] H.L. Yang, C.Y. Ma, Y. Li, J.H. Wang, X. Zhang, G. Wang, N.L. Qiao, Y.G. Sun, J. Cheng, Z.P. Hao, Chem. Eng. J. 347 (2018) 808–818. [32] C.Y. Chen, X. Wang, J. Zhang, S.X. Pan, C.Q. Bian, L. Wang, F. Chen, X.J. Meng, X.M. Zheng, X.H. Gao, F.S. Xiao, Catal. Lett. 144 (2014) 1851–1859. [33] J. Wan, R. Ran, M. Li, X.D. Wu, D. Weng, J. Mol. Catal. A-Chem. 383 (2014) 194–202. [34] S. Gil, J.M. Garcia-Vargas, L.F. Liotta, G. Pantaleo, M. Ousmane, L. Retailleau, A. Giroir-Fendler, Catalysts 5 (2015) 671–689. [35] A. Aznarez, A. Gil, S.A. Korili, RSC Adv. 5 (2015) 82296–82309. [36] L. Zhang, L. Chen, Y.B. Li, Y.X. Peng, F. Chen, L. Wang, Cb. Zhang, X.G. Meng, H. He, F.S. Xiao, Appl. Catal. B: Environmental 219 (2017) 200–208. [37] A.Z. Abdullah, M.Z. Abu Bakar, S. Bhatia, J. Hazard. Mater. 129 (2006) 39–49.
[22] E.M. Cordi, J.L. Falconer, J. Catal. 162 (1996) 104–117. [23] B. Solsona, T.E. Davies, T. Garcia, I. Vazquez, A. Dejoz, S.H. Taylor, Appl. Catal. BEnviron. 84 (2008) 176–184. [24] B.M. Reddy, A. Khan, Catal. Rev.-Sci. Eng. 47 (2005) 257–296. [25] L. Zhang, L. Chen, Y.B. Li, Y.X. Peng, F. Chen, L. Wang, C.B. Zhang, X.J. Meng, H. He, F.S. Xiao, Appl. Catal. B-Environ. 219 (2017) 200–208. [26] L. Zhang, Y.X. Peng, J. Zhang, L. Chen, X.J. Meng, F.S. Xiao, Chin. J. Catal. 37 (2016) 800–809. [27] C.Y. Chen, F. Chen, L. Zhang, S.X. Pan, C.Q. Bian, X.M. Zheng, X.J. Meng, F.S. Xiao, Chem. Commun. 51 (2015) 5936–5938. [28] C.Y. Chen, Q.M. Wu, F. Chen, L. Zhang, S.X. Pan, C.Q. Bian, X.M. Zheng, X.J. Meng, F.S. Xiao, J. Mater. Chem. A 3 (2015) 5556–5562. [29] C.Y. Chen, J. Zhu, F. Chen, X.J. Meng, X.M. Zheng, X.H. Gao, F.S. Xiao, Appl. Catal. B-Environ. 140 (2013) 199–205.
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Review
Enhanced catalytic activity in propene oxidation over NaZSM-5 zeolitesupported Pt nanoparticles by increasing the zeolite Si/Al ratio ⁎
⁎
Yiwen Jiang, Ling Zhang, Yiquan Xie, Shichao Han, Qiuyan Zhu, Xiangju Meng , Feng-Shou Xiao Key Laboratory of Applied Chemistry of Zhejiang Province, Zhejiang University, Hangzhou 310028, China
A R T I C LE I N FO
A B S T R A C T
Keywords: VOCs abatement Propene oxidation NaZSM-5 zeolite Si/Al ratio Pt nanoparticles
A series of zeolite-supported Pt nanoparticles (Pt/NaZSM-5) have been prepared by adjusting Si/Al ratio of NaZSM-5 zeolite from 60 to pure silica. Catalytic tests in propene oxidation as an important model reaction for removal of hydrocarbons emitted from automobiles during cold-start, show that the catalytic activities significantly increase with Si/Al ratios of NaZSM-5 zeolite in these Pt/NaZSM-5 catalysts. As a typical example, Silicalite-1 supported Pt nanoparticles exhibits the highest activity, giving the 50% conversion temperature (T50) and 90% conversion temperature (T90) at 112 °C and 118 °C, respectively. Characterizations of these catalysts with multiple techniques show that the enhanced catalytic activities are strongly related to the zeolite Si/Al ratios of the catalysts, where an increase of zeolite Si/Al ratio is favorable for the adsorption of propene, promoting the propene oxidation. This feature might offer a good opportunity to design and prepare highly efficient heterogeneous catalysts for removal of light olefins in the future.
1. Introduction With an increase of environmental awareness, more stringent policies have been introduced to protect human environment. As one of the major pollutants in the automobile exhaust [1], hydrocarbon is strictly limited in emission regulations due to their toxic, carcinogenic, mutagenic, and teratogenic nature [2]. Propene, a typical hydrocarbon, mainly comes from the automobile exhaust during cold-start, and it has been recognized as a polluting molecule due to its high photochemical ozone creativity potential (POCP), making its abatement from such source imperative [3]. Among proposed routes for eliminating hydrocarbons, catalytic oxidation stands out. It has obvious advantages such as full conversion of hydrocarbons, exceptional persistence, and economic efficiency, in comparison to other techniques such as physical adsorption and plasma elimination [4–7], It is worth noting that currently available oxidation catalysts fail to achieve the complete elimination of propene under the engine coldstart conditions. Therefore, it is strongly desirable to develop highly efficient heterogeneous catalysts for the propene oxidation [8]. Generally, oxidation catalysts used in volatile organic compounds (VOCs) abatement mainly include Perovskite [9–11], supported noble metal catalysts, and transition metal oxides [12–20]. Among them, supported noble metal catalysts have been intensively investigated for
⁎
the propene oxidation due to its excellent activity at relatively low temperature and extraordinary stability [21]. In these cases, conventional supports are inorganic oxides such as alumina, silica, titania, and ceria [11,22–24]. Compared with these oxides, zeolites as catalyst supports are getting more and more attention owing to their large surface area, strong adsorption capacity, high thermal and hydrothermal stabilities, uniform and intricate channels, and controllable acidic property [25–30]. For example, Hao et al. reported that NaZSM-5 supported Ag catalyst is highly active for C2H4 combustion [31]. In this work, we have prepared a series of Pt-based catalysts supported by NaZSM-5 zeolites with different Si/Al ratios from 60 to pure silica. Catalytic tests in propene oxidation show that the increase of the Si/Al ratios in the catalysts leads to the enhancement of catalyst activity. When the Silicalite-1 is employed as the support, the catalyst is most active, rendering the T90 as low as 118 °C. 2. Experimental 2.1. Materials NaAlO2, TPAOH (40 wt.%), and TEOS were purchased from Sinopharm Chemical Reagent. H2PtCl6∙6H2O, CH3COONH4, and 100–140 mesh amorphous silica was purchased from Aladdin Chem.
Corresponding authors. E-mail addresses: [email protected] (X. Meng), [email protected] (F.-S. Xiao).
https://doi.org/10.1016/j.cattod.2019.06.075 Received 25 January 2019; Received in revised form 14 May 2019; Accepted 18 June 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Yiwen Jiang, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.06.075
Catalysis Today xxx (xxxx) xxx–xxx
Y. Jiang, et al.
Table 1 Textural parameters of the as-synthesized catalysts. Catalyst
Si/Al
BET surface area, m2/g
Micropore volume, cm3/g
Pt/NaZSM-5-60 Pt/NaZSM-5-100 Pt/NaZSM-5-200 Pt/Silicalite-1
52 87 205 > 2000
332 331 407 398
0.16 0.16 0.18 0.17
hydrothermal synthesis [32]. Typically, a certain amount of NaAlO2 was dissolved in 32.24 mL of water, followed by the addition of 5.54 mL of TPAOH. After stirring for 15 min at room temperature, 10.13 mL of TEOS were introduced into the mixture. After stirring for 5.5 h at room temperature, the resultant gel was transferred into an autoclave for crystallization at 180 °C for 4 days. The product was collected by centrifuging, washing, drying at 80 °C in air for 12 h, and calcining at 550 °C for 4 h. The HZSM-5-60 zeolites were prepared by ion-exchange from NH4+-exchange of NaZSM-5-60, followed by calcination at 500 °C for 4 h. The NaZSM-5 zeolites and amorphous silica supported Pt catalysts were prepared through the incipient wetness impregnation method, with H2PtCl6 as the Pt precursor. After calcination at 450 °C for 4 h in air and reduction under H2 flow (80 mL/min) at 300 °C for 2.5 h, the supported Pt catalysts (denoted as Pt/support) were finally obtained, where the loading of Pt was about 1 wt%. Fig. 1. XRD patterns of the (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/ NaZSM-5-200, and (d) Pt/Silicalite-1 catalysts.
2.3. Catalyst characterization Propene was supplied by Hangzhou New Century Hybrid Gas Company. All the chemicals were of analytical-reagent grade and were used without further purification.
X-ray diffraction (XRD) pattern was carried out with a RIGAKU Ultimate IV diffractometer with Cu Kα radiation. Nitrogen adsorptiondesorption isotherms were acquired by Belsorp Max at −196 °C, and the specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. The inductively coupled plasma atomic emission spectrometry (ICP-AES) was obtained using a Perkin-Elmer plasma 8000 optical emission spectrometer to determine the Si/Al ratios of the zeolites and the Pt content in the catalysts. High-resolution TEM images
2.2. Catalyst preparation The NaZSM-5-x zeolites (x stands for Si/Al ratio at 60, 100, and 200, as well as pure silica in the starting gels) were prepared from
Fig. 2. SEM images of the (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/NaZSM-5-200, and (d) Pt/Silicalite-1 catalysts. 2
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Fig. 3. (A) TEM image of the Pt/Silicalite-1 and (B) Pt size distributions of (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/NaZSM-5-200, and (d) Pt/Silicalite-1 catalysts.
catalyst (40–60 mesh) was used with a total flow-rate of feed gases at 50 mL/min (1000 ppm propene in standard air), a space velocity (SV) of 30,000 mL/(g·h), and 50% relative humidity (RH). The concentration of propene in the tail gases was analyzed by a gas chromatograph (Fuli, GC9790) equipped with a flame ionization detector and a 19091N-113 INNOWAX capillary column (Agilent, 30 m × 0.32 mm × 0.25 mm i.d.). The conversion of propene was calculated by the difference between inlet and outlet propene concentration. The catalytic activities were determined by the values of T50 and T90, defined as the temperatures at 50% and 90% of propene conversion. The relative humidity was determined by a handheld humidity detector. The relative
were obtained on a JEOL 2100 F electron microscope at 200 kV. Scanning electron microscopy (SEM) images of the samples were recorded on a Hitachi SU 1510 apparatus. Contact angles of catalysts were measured on a Shanghai Zhongchen JC2000X Optical Contact Angle Meter. 2.4. Propene oxidation Propene oxidation at atmospheric pressure was performed in a continuous-flow fixed bed micro-reactor of a quartz tube (6 mm i.d.) with the catalyst placed at the center (Fig. S1). In a typical run, 100 mg 3
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catalysts are detailed in Table 1. ICP analyses show that these synthesized zeolites have distinguishable Si/Al ratios while for each case the difference between the ratio in the starting gels and the corresponding zeolite product is expected and acceptable (Table 1). Fig. 3A shows TEM image of Pt/Silicalite-1, evidencing the microporosity of zeolite and nanoparticles of Pt. Clearly, the Pt nanoparticles are uniformly distributed on the Silicalite-1 zeolite. Furthermore, the Pt size distribution on various zeolites have been compared, as shown in Fig. 3B. Notably, they have very similar mean size distribution of Pt nanoparticles around 2–3 nm. To sum up the characterizations results, these catalysts have similar zeolite crystallinity, crystal size, textural parameters, Pt size distribution, but distinguishable Si/Al ratios. 3.2. Catalytic evaluation Fig. 4 shows catalytic data in propene combustion over various studied catalysts. The T50 and T90 values of these catalysts are presented in Table 2, for the purpose of clarity and comparison. Notably, these zeolite-supported Pt catalysts exhibit much higher activities than other supported noble metal catalysts reported previously [33–35]. Among these zeolite-supported Pt catalysts, it is particularly observed that the catalytic activity was strongly influence by the Si/Al ratio in the catalysts. Specifically, when Si/Al ratio of the starting gel for the synthesis of NaZSM-5 zeolite is 60, the T50 and T90 values of Pt/NaZSM-5-60 are 145,°C and 152,°C; when Silicalite-1 is used, the T50 and T90 values of Pt/Silicalite-1 are 112,°C and 118,°C. The remarkable performance of the Pt/Silicalite-1 should be potentially important for complete removal of light olefins at relatively low temperature in the future. To understand the effect of reactants on catalytic activity, we have measured the reactant orders (Table 3), which could be estimated from the slope of a graph of lnR as a function of ln[A], where R is the reaction rate and [A] stands for the reactant concentration, as given in Fig. S3. Notably, compared with Pt/NaZSM-5-60, the propene order over Pt/ Silicalite-1 is remarkably reduced, while the oxygen order over Pt/Silicalite-1 is obviously enhanced. According to the reported literature [36], the reduced reaction order means the enhanced activity of the reactant and relatively high Si/Al ratios are favorable for the adsorption of propene, which should be attributed to the increase of sample hydrophobicity, in good agreement with those reported previously [7]. Considering the relationship between the Si/Al ratios of the catalysts and the catalytic activities, it is suggested that higher activity over Pt/ Silicalite-1 than that over Pt/NaZSM-5-60 is mainly attributed to the change in propene order rather than oxygen order. To understand the difference in propene order over various catalysts, we have managed to obtain their adsorption breakthrough curves of propene, as shown in Fig. 5. Interestingly, propene permeation in Pt/ Silicalite-1 is much slower than that in Pt/NaZSM-5-60, indicating that Pt/Silicalite-1 has larger adsorption capacity than Pt/NaZSM-5-60. This feature might be reasonably related to their distinguishable catalyst wettability, where relatively hydrophobic zeolites might be helpful for adsorption of propene. To verify the difference of catalyst wettability, we have measured contacting angle of water on various catalyst surface, as given in Fig.
Fig. 4. Dependences of propene conversion on reaction temperature in propene oxidation over the (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/NaZSM-5200, and (d) Pt/Silicalite-1 catalysts under propene concentration of 1000 ppm in standard air, flow rate of 50 mL/min, and space velocity of 30,000 mL/(g·h).
humidity of inlet gas without addition of water vapor was 50%. When additional water vapor was introduced from a bubbling method, the relative humidity reached 70%. Reaction rates of propene oxidation over propene and oxygen were both measured in a continuous-flow fixed bed micro-reactor of a quartz tube (6 mm i.d.) with the catalyst placed at the center. In a typical run, 500 mg catalyst (40–60 mesh) was used with feed gases composed by 2000 ppm propene and 5% oxygen (He as a balance gas). To test the oxygen rate, the flow rate of propene was kept at 25 mL/min, and the flow rate of oxygen was adjusted from 4.0 mL/min to 3.5, 3.0, 2.5, and 2.0 mL/min, respectively. To measure the propene rate, the flow rate of oxygen was kept at 25 mL/min, and the flow rate of propene was adjusted from 22.5 mL/min to 20, 17.5, 15 mL/min. The kinetic measurements for propene oxidation were implemented from the propene conversion below 15%. Adsorption breakthrough curves of propene for different catalysts were performed with the same equipment at room temperature by flowing 40 ppm propene. 3. Results and discussion 3.1. Catalyst characterization Fig. 1 presents XRD patterns of the zeolite-supported Pt catalysts including Pt/NaZSM-5-60, Pt/NaZSM-5-100, Pt/NaZSM-5-200, and Pt/ Silicalite-1, giving similar characteristic peaks associated with MFI zeolite structure. Fig. 2 shows SEM images of these catalysts, exhibiting uniform crystals of 300–400 nm. Fig. S2 shows N2 sorption isotherms of these catalysts, having typical Langmuir-type adsorption. A steep N2 uptake occurs at relative pressure (P/P0) less than 0.01, assigned to the micropore filling of nitrogen. The textural parameters of studied Table 2 Catalytic data for oxidation of propene over various catalysts. Catalyst
Noble metal, wt%
T50, °C
T90, °C
GHSV, mL/(g·h)
Concentration, ppm
Ref.
Pt/NaZSM-5-60 Pt/NaZSM-5-100 Pt/NaZSM-5-200 Pt/Silicalite-1 Pt/Al2O3 Pd/TiO2 Pd/Al-PILC Pt/Al-PILC
1 1 1 1 5 0.8 2 2
145 124 122 112 210 150 210 310
152 130 128 118 225 160 260 325
30,000 30,000 30,000 30,000 60000 35000 20000 20000
1000 1000 1000 1000 800 1000 5000 5000
This work This work This work This work [33] [34] [35] [35]
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effect of water on catalyst activity is pretty obvious in previous reports [37]. As observed from the aforementioned results, the Si/Al ratio in the MFI zeolites strongly influences the catalytic activity, where pure silica zeolite has the highest activity. To understand the attribution of MFI channels for the catalytical activity, amorphous silica (AM) is employed as a control support for preparation of supported Pt catalysts. As shown in Fig. S6, the amorphous silica supported Pt catalyst (Pt/AM) exhibits higher activity than Pt/NaZSM-5-60, but much less than Pt/Silicalite-1. Compared with the amorphous silica, the channels of the Silicalite-1 zeolite are very favorable for mass transfer in the catalytic reactions. As a result, there is an obvious by-product in the Pt/AM-catalyzed reaction, while it is undetectable on the Pt/Silicalite-1 catalyst.
Table 3 Reactant order in propene combustion. Catalyst
Propene
Oxygen
Pt/NaZSM-5-60 Pt/NaZSM-5-100 Pt/NaZSM-5-200 Pt/Silicalite-1
2.5 3.0 2.5 1.5
1.9 2.1 2.2 2.9
4. Conclusion A set of NaZSM-5 zeolite-supported Pt catalysts have been prepared by adjusting Si/Al ratio in the starting gels from 60 to pure silica. Catalytic tests in propene oxidation show that the Silicalite-1 supported Pt catalyst (Pt/ Silicalite-1) is extraordinarily active, giving the values of T50 and T90 at 112 °C and 118 °C. Moreover, this catalyst is highly durable even in the presence of high humidity. The combination of high activity and long-time stability makes this catalyst potentially important for propene oxidation at relatively low temperature, as a key model reaction for removal of hydrocarbons emitted from cold-start engines. Fig. 5. Adsorption breakthrough curves of propene for the (a) Pt/NaZSM-5-60, (b) Pt/NaZSM-5-100, (c) Pt/NaZSM-5-200, and (d) Pt/Silicalite-1 catalysts under propene concentration of 40 ppm.
Acknowledgment This work is supported by National Key Research and Development Program of China (2017YFC0211101) and National Natural Science Foundation of China (21673205, 21720102001 and 91634201). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.06.075. References [1] R.M. Heck, R.J. Farrauto, Appl Catal A-Gen. 221 (2001) 443–457. [2] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165–2180. [3] R.G. Derwent, M.E. Jenkin, S.M. Saunders, M.J. Pilling, Atmos. Environ. 32 (1998) 2429–2441. [4] S. Pasquiers, Eur. Phys. J.-Appl. Phys 28 (2004) 319–324. [5] C. Ayrault, J. Barrault, N. Blin-Simiand, F. Jorand, S. Pasquiers, A. Rousseau, Catal. Today 89 (2004) 75–81. [6] I. Yet-Pole, Environ. Sci. Technol. 38 (2004) 3785–3791. [7] C.Y. Chen, X. Wang, J. Zhang, S.X. Pan, C.Q. Bian, L. Wang, F. Chen, X.J. Meng, X.M. Zheng, X.H. Gao, F.S. Xiao, Catal. Lett. 144 (2014) 1851–1859. [8] D.H. Kim, M.C. Kung, A. Kozlova, S.D. Yuan, H.H. Kung, Catal. Lett. 98 (2004) 11–15. [9] R.J.H. Voorhoeve, D.W. Johnson, J.P. Remeika, P.K. Gallagher, Science 195 (1997) 827–833. [10] W.B. Li, J.X. Wang, H. Gong, Catal. Today 148 (2009) 81–87. [11] M. Alifanti, M. Florea, V.I. Parvulescu, Appl. Catal. B-Environ. 70 (2007) 400–405. [12] L.F. Liotta, Appl. Catal. B-Environ. 100 (2010) 403–412. [13] C. Lahousse, A. Bernier, P. Grange, B. Delmon, P. Papaefthimiou, T. Ioannides, X. Verykios, J. Catal. 178 (1998) 214–225. [14] S. Scire, L.F. Liotta, Appl. Catal. B-Environ. 125 (2012) 222–246. [15] V.P. Santos, S.A.C. Carabineiro, P.B. Tavares, M.F.R. Pereira, J.J.M. Orfao, J.L. Figueiredo, Appl. Catal. B-Environ. 99 (2010) 198–205. [16] F. Bertinchamps, C. Gregoire, E.M. Gaigneaux, Appl. Catal. B-Environ. 66 (2006) 1–9. [17] D. Delimaris, T. Ioannides, Appl. Catal. B-Environ. 84 (2008) 303–312. [18] Y.X. Liu, H.X. Dai, Y.C. Du, J.G. Deng, L. Zhang, Z.X. Zhao, C.T. Au, J. Catal. 287 (2012) 149–160. [19] B. de Rivas, R. Lopez-Fonseca, C. Jimenez-Gonzalez, J.I. Gutierrez-Ortiz, J. Catal. 281 (2011) 88–97. [20] M. Hosseini, T. Barakat, R. Cousin, A. Aboukais, B.L. Su, G. De Weireld, S. Siffert, Appl. Catal. B-Environ. 111 (2012) 218–224. [21] J. Wan, R. Ran, M. Li, X.D. Wu, D. Weng, J. Mol. Catal. A-Chem. 383 (2014) 194–202.
Fig. 6. Dependences of propene conversion on reaction time over Pt/Silicalite-1 catalyst in the relatively humidity (RH) of (a) 50% and (b) 70%.
S4. Clearly, Pt/Silicalite-1 exhibits higher contacting angle than Pt/ NaZSM-5-60, confirming relatively higher hydrophobicity of Pt/ Silicalite-1 than that of Pt/NaZSM-5-60. In addition, the acidity of the catalysts is also important for the catalysis. Therefore, we have compared the catalytic activity of Pt nanoparticles supported on MFI zeolite with Na+ and H+ cations. Notably, the T90 and T50 (147 °C and 129 °C) of the Pt/HZSM-5-60 are slightly lower than those (152 °C and 145 °C) of the Pt/NaZSM-5-60 (Fig. S5, Table S1), but much higher than those (118 °C and 112 °C) of the Pt/Silicalite-1. These results indicate that the zeolite hydrophobicity is more important than zeolite cations for the contribution of catalytic activities. Fig. 6 shows dependences of the catalytic activity on reaction time in propene oxidation at 140 °C over the Pt/Silicalite-1 catalyst under the condition of 50% and 70% RH. It is of great importance that there is almost no loss in catalytic activity within 100 h tests, demonstrating that the Pt/Silicalite-1 catalyst is durable. In contrast, the inhibition 5
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[30] L. Wang, S.D. Xu, S.X. He, F.S. Xiao, Nano Today 20 (2018) 74–83. [31] H.L. Yang, C.Y. Ma, Y. Li, J.H. Wang, X. Zhang, G. Wang, N.L. Qiao, Y.G. Sun, J. Cheng, Z.P. Hao, Chem. Eng. J. 347 (2018) 808–818. [32] C.Y. Chen, X. Wang, J. Zhang, S.X. Pan, C.Q. Bian, L. Wang, F. Chen, X.J. Meng, X.M. Zheng, X.H. Gao, F.S. Xiao, Catal. Lett. 144 (2014) 1851–1859. [33] J. Wan, R. Ran, M. Li, X.D. Wu, D. Weng, J. Mol. Catal. A-Chem. 383 (2014) 194–202. [34] S. Gil, J.M. Garcia-Vargas, L.F. Liotta, G. Pantaleo, M. Ousmane, L. Retailleau, A. Giroir-Fendler, Catalysts 5 (2015) 671–689. [35] A. Aznarez, A. Gil, S.A. Korili, RSC Adv. 5 (2015) 82296–82309. [36] L. Zhang, L. Chen, Y.B. Li, Y.X. Peng, F. Chen, L. Wang, Cb. Zhang, X.G. Meng, H. He, F.S. Xiao, Appl. Catal. B: Environmental 219 (2017) 200–208. [37] A.Z. Abdullah, M.Z. Abu Bakar, S. Bhatia, J. Hazard. Mater. 129 (2006) 39–49.
[22] E.M. Cordi, J.L. Falconer, J. Catal. 162 (1996) 104–117. [23] B. Solsona, T.E. Davies, T. Garcia, I. Vazquez, A. Dejoz, S.H. Taylor, Appl. Catal. BEnviron. 84 (2008) 176–184. [24] B.M. Reddy, A. Khan, Catal. Rev.-Sci. Eng. 47 (2005) 257–296. [25] L. Zhang, L. Chen, Y.B. Li, Y.X. Peng, F. Chen, L. Wang, C.B. Zhang, X.J. Meng, H. He, F.S. Xiao, Appl. Catal. B-Environ. 219 (2017) 200–208. [26] L. Zhang, Y.X. Peng, J. Zhang, L. Chen, X.J. Meng, F.S. Xiao, Chin. J. Catal. 37 (2016) 800–809. [27] C.Y. Chen, F. Chen, L. Zhang, S.X. Pan, C.Q. Bian, X.M. Zheng, X.J. Meng, F.S. Xiao, Chem. Commun. 51 (2015) 5936–5938. [28] C.Y. Chen, Q.M. Wu, F. Chen, L. Zhang, S.X. Pan, C.Q. Bian, X.M. Zheng, X.J. Meng, F.S. Xiao, J. Mater. Chem. A 3 (2015) 5556–5562. [29] C.Y. Chen, J. Zhu, F. Chen, X.J. Meng, X.M. Zheng, X.H. Gao, F.S. Xiao, Appl. Catal. B-Environ. 140 (2013) 199–205.
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