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Coking and deactivation behavior of ZSM-5 during the isomerization of styrene oxide to phenylacetaldehyde
Catalysis Communications 98 (2017) 116–120
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Coking and deactivation behavior of ZSM-5 during the isomerization of styrene oxide to phenylacetaldehyde
MARK
Ming-Lei Gou⁎, Junqing Cai, Wensheng Song, Zhen Liu, Yun-Lai Ren, Bingli Pan, Qingshan Niu School of Chemical Engineering and Pharmaceutics, Henan University of Science and Technology, Luoyang, Henan 471023, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Styrene oxide Phenylacetaldehyde ZSM-5 Coking Deactivation behavior
Coking and deactivation behavior of ZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde were investigated under gas-phase free of solvents. More soft coke was firstly formed on HZSM-5 and then partly converted to hard coke with the reaction carried through. The soft coke, which can be removed via desorption at 200–400 °C, had little effect on the catalyst activity and selectivity. While the hard coke, which had a certain degree of crystallization and must be completely removed via combustion with oxygen, would cause major catalyst deactivation. By confirmed, pore blocking was the predominant mode leading to deactivation of catalyst.
1. Introduction Isomerization of styrene oxide and derivatives to aldehydes yields valuable compounds or intermediates used for production of fragrances, pharmaceuticals, insecticides, fungicides and herbicides, particularly the halogenated derivatives of aldehydes are more needed [1]. Various solid acid catalysts, including mixed-metal oxides [2], silicaalumina gels [3,4], natural silicates [5], Nafion-H [6], heteropoly acids [7] and zeolites [8–10], etc., have been tried to catalyze this process. Among them, zeolites of the mordenite, erionite, chabazite and pentasil types are all superior to others in view of suppressing side reactions by shape selectivity, particularly the pentasil zeolites have the best performance with phenylacetaldehyde yield of over 90% at 200 °C and WHSV = 3 h− 1, however the catalysts begin to show deactivation indicating by decrease of the styrene oxide conversion after running for 6 h time on stream [11]. Gou et al. [12] have investigated the effect of acidity of ZSM-5 (SiO2/Al2O3 = 25–360) in the isomerization of styrene oxide to phenylacetaldehyde and find out that the catalyst lifetimes are affected by both acid strength and concentration. Increasing the acid concentration while decreasing their acid strength can prolong the catalyst lifetimes. A series of zeolites modified by different methods, such as dealumination [13], fluorine modification [14], phosphorus modification [15] and alkali treatment [16], have been applied for the reaction. However, the catalyst lifetimes have shown only limited improvement. Although effects of acidity and structure of ZSM-5 on the isomerization of styrene oxide to phenylacetaldehyde have been widely studied, only a small quantity of reports focus on the coke species ⁎
Corresponding author. E-mail address: [email protected] (M.-L. Gou).
http://dx.doi.org/10.1016/j.catcom.2017.04.035 Received 3 January 2017; Received in revised form 21 March 2017; Accepted 17 April 2017 Available online 18 April 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
and deactivation behaviors of catalysts. The deactivation mechanisms of ZSM-5 are not yet clear. So in the present study, coke species and deactivation behavior of ZSM-5 were identified using N2 adsorption, SEM, XRD, XPS, TG-DTA and NH3-TPD/titration. 2. Experiment Styrene oxide (> 98 wt%) was purchased from TCI (shanghai) development Co., Ltd. and used without further purification. HZSM-5 (SiO2/Al2O3 = 135) were obtained from Catalyst Plant of Nankai University (Tianjin, People's Republic of China) and used as catalyst. Isomerization of styrene oxide was carried out in a continuous flow fixed-bed reactor (stainless steel tube, i.d. = 9 mm) operated at 200 °C, WHSV = 3.0 h− 1 and flow rate of N2 = 120 ml/min. The catalyst (0.5 g, 20–30 mesh) was loaded in the constant-temperature zone and pretreated at 500 °C for 2 h in nitrogen flow. Styrene oxide, free of any solvents, was introduced into the reactor with flowrate of 0.025 ml/min using a HPLC pump (model Series II, LabAlliance, USA). At end of each experiment, the used catalyst was unloaded after purging with nitrogen at 200 °C for 2 h to remove any possible residual reactants. Scanning electron microscope (SEM) experiments were made on a Hitachi SU-8010 scanning electron microscope (FE-SEM, 5 kV). The powder X-ray diffraction (XRD) patterns were collected on a PANalytical X'Pert Pro diffractometer in 2θ range of 5–50° with Co Kα radiation source (λ = 0.1789 nm). N2 adsorption was measured at 77 K on a Quantachrome Autosorb-1 instrument. The surface area was calculated according to the BET equation, and the pore volume and size were determined using the HK method.
Catalysis Communications 98 (2017) 116–120
M.-L. Gou et al.
X-ray photoelectron spectra (XPS) were performed on a PHI-1600 ESCA spectrometer with Mg Kα radiation source (1253.6 eV). The binding energies were determined using C 1s peak of contaminant carbon (BE = 284.6 eV) as standard. Thermogravimetry-differential thermal analysis (TG-DTA) measurements were conducted on a Perkin-Elmer Pyris 6 thermogravimetric analyzer at a heating rate of 10 °C/min from room temperature to 800 °C in air or nitrogen flow. In order to investigate the acidity changes of the fresh and used catalysts, temperature programmed desorption of ammonia (NH3-TPD) was performed on a conventional apparatus equipped with a thermal conductivity detector. The sample (100 mg, 20–30 mesh) was pretreated at 200 °C in nitrogen and then cooled down to ambient temperature. Sufficient NH3 was supplied into the system followed by flushing with nitrogen at 150 °C for 1 h. The TPD profile was obtained by heating the sample from 150 to 500 °C at a rate of 15 °C/min. Simultaneously, the desorbed NH3 was trapped in boric acid, followed by titration with a standard H2SO4 solution [17]. 3. Results and discussion As shown in Fig. 1, catalytic testing of HZSM-5 was carried out at 200 °C, WHSV = 3.0 h− 1 and flow rate of N2 = 120 ml/min. In a blank test without catalyst, the conversion of styrene oxide was below 1% all the time. While the initial conversion of styrene oxide (TOS = 1.0 h) over HZSM-5 was over 99%, suggesting that this reaction was a catalysis process. The styrene oxide conversion remained unchanged for the first 6 h, but thereafter the catalyst began to show deactivation as indicated by decreasing of the styrene oxide conversion. In order to compare the changes of the catalyst properties, the used catalysts before (used after 4 h time on stream) and after (used after 8 h time on stream) deactivation were unloaded from the reactor in two separate experiments. The phenylacetaldehyde selectivity kept > 96% from beginning to end of the reaction. The main by-product was the dimer (2,4diphenyl-2-butenal) formed via aldol condensation of phenylacetaldehyde, maintained its selectivity of 1–3%. Some other by-products, such as phenylethanol, phenylethanediol, styrene and so on, were also detected by GC–MS with their total selectivity of < 1%. But the trimer (2,4,6-tribenzyl-s-trioxane) formed via trimerization of phenylacetaldehyde at external acid sites [12] could not be detected since the catalyst contained only a trace amount of external acid sites. For verification the deactivation behavior of HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde, several charac-
Fig. 2. SEM micrographs of the fresh and used HZSM-5.
terization techniques were implemented over the fresh and used catalysts. The SEM micrographs of the fresh and used catalysts are presented in Fig. 2. The borderline among the fresh catalyst granules was clear. While after reaction 4 h time on stream, the borderline among the catalyst particles became ambiguous. Increasing of the reaction time to 8 h, more and more catalyst particles were buried in the coke. Simultaneously, the activity of the catalyst gradually decreased with accumulation of the coke deposited on the catalyst surface. The filamentous carbon and other crystalline carbon were not observed on the used catalysts. But the structure of the coke can be
Fig. 1. Performances of HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde. Reaction conditions: T = 200 °C, P = 1 atm, catalyst loading = 0.5 g, flow-rate of N2 = 120 ml/min, WHSV = 3.0 h− 1.
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Fig. 3. XRD patterns of the fresh and used HZSM-5.
further examined via XRD. The XRD patterns of the fresh and used catalysts are shown in Fig. 3. The distinct peaks at 2θ = 9–11° and 26–29° were all agree with the standard MFI structure (JCPDS-PDF: 00-049-0657). But the intensity of peaks at 2θ = 9–11° over used HZSM-5 decreased slightly causing by some amorphous coke formed in larger pores and decreased the longrange ordering of HZSM-5. A new peak at 2θ = 31.1° (labeled by the dashed line) over the used catalysts was similar with the characteristic peak of graphite at 2θ = 30.7° (JCPDS-PDF: 00-001-0640), which can be called pregraphite-like carbon [18]. So the coke formation on HZSM5 in the isomerization of styrene oxide has a certain degree of aromatization. The different configurations of carbons have different binding energies and can be distinguished by C 1s XPS analysis. The binding energy and percent area of each peak for the used catalysts are listed in Table 1. There were mainly four peaks in the C 1s spectra: dehydrogenated carbon species (~ 282.0 eV) [18], pregraphite-like carbon (~ 284.1 eV) [19], oxidized carbon in OeCeO (~ 286.5) and oxidized carbon in C]O (~ 287.7 eV) [20]. The surface coke adjoining 282.0 and 284.1 eV was the main coke species. The relative percentage of peak area at 282.0 eV (56.1%) was higher than that of peak area at 284.1 eV (36.2%) on the HZSM-5 after 4 h time on stream. Along with the reaction time extending to 8 h, the relative percentages of peak areas at 282.0 eV (46.1%) and 284.1 eV (44.0%) were almost the same. The amount of coke was higher for the sample obtained after 8 h time on stream (as indicated by the TG-DTA curves below). So it is assumed that some dehydrogenated carbon species can convert to pregraphitelike carbon with the reaction going on. During this period, the styrene oxide conversion decreased gradually. Apparently, there is a close relationship between the content of pregraphite-like carbon and catalyst deactivation. The TG-DTA curves of the used catalysts in air or nitrogen are shown in Fig. 4. The weight loss below 200 °C was attributed to the loss of adsorbed water or some volatile species. The coke deposition over
Fig. 4. TG-DTA curves of the used HZSM-5 in air or nitrogen atmosphere.
the used HZSM-5 were completely removed at around 200–700 °C in air. There were two main stages in the TG curves: 200–400 °C (so-called soft coke) and 400–700 °C (so-called hard coke). The DTA curves showed three main exothermic peaks at around 220, 390 and 550 °C with some small shoulder peaks at 250, 290, 320 and 490 °C, meaning that removes of the soft and hard coke were muti-step reaction processes. The content of soft coke (5.8%) was higher than that of hard coke (5.0%) on the used HZSM-5 after 4 h. While when the
Table 1 Quantitative evaluative of C 1s XPS spectra of used HZSM-5 catalysts. Catalysts
Used HZSM-5 after 4 h Used HZSM-5 after 8 h
BE(eV) ~ 282.0
~ 284.1
~ 286.5
~ 287.7
56.1% 46.1%
36.2% 44.0%
4.6% 5.9%
3.1% 4.0%
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reaction time increased to 8 h, the content of soft coke slightly decreased to 5.5%, and the content of hard coke obviously increased to 6.4%. Simultaneously, the styrene oxide conversion decreased from 99% to 93%. From above, it can be concluded that more soft coke was firstly formed on HZSM-5, and with the reaction carried through, some soft coke gradually converted to the hard coke which would cause major catalyst deactivation. As shown in Fig. 4, the weight loss differences of used HZSM-5 after 8 h time on stream were observed in air and nitrogen atmosphere. The weight loss in the stage of 200–400 °C was similar whether in air (5.5%) or nitrogen (5.6%), indicating that the soft coke can be completely removed via desorption from the catalysts at an elevated temperature (200–400 °C). However, the weight loss in the stage of 400–700 °C in nitrogen (3.1%) was lower than that in air (6.4%), indicating that the hard coke removal was somewhat faster under air. The TG profile above 700 °C was not a horizontal line in nitrogen, because of part hard coke (1.7%) slowly decomposing to some volatile species. Based on the results of XRD and XPS, the coke formation on HZSM-5 has a certain degree of crystallization which must be removed via burning in air. By comparing the weight loss differences in air and nitrogen, approximately 1.6% amount of coke still existed on the catalyst after treatment in nitrogen, which must be removed via burning in air, too. Therefore, it is estimated that approximately 1.6% of the pregraphite-like carbon was formed over HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde at 200 °C, WHSV = 3.0 h− 1 and flow-rate of N2 = 120 ml/min. Table 2 shows the textural changes of the fresh and used HZSM-5. Styrene oxide is a highly reactive substance and can be completely converted to phenylacetaldehyde and other by-products on relatively weak acid sites at 70 °C and WHSV = 0.5–15 h− 1 [21,22]. Although the BET surface area, pore volume and pore size of catalyst remarkably decreased to 68.6 m2/g, 0.011 cm3/g and 1.7 Å after running for 4 h, the rest active sites can almost completely catalyzed this reaction. While when the reaction was continuously operated for 8 h, the BET surface area (10.3 m2/g) and pore volume (0.002 cm3/g) almost reduced to zero and the catalyst showed deactivation as indicated by decreasing of the styrene oxide conversion. Deactivation of zeolites caused by coke usually has two mechanisms: active site suppression and pore blocking [23]. The loss of sorption capacity of the spent catalysts can find out which deactivation mode is predominant. When coke deposition takes place by simple active site suppression, the ratio of coke volume (Vc) to the volume not accessible to the adsorbate (Vna) should be around 1. However, when coke deposition leads to pore blocking and there are not coke deposits in intersections, a ratio of Vc/ Vna lower than 1 is expected. Especially, if most of coke deposits on the external surface of zeolites, blocking the entrances to the internal structure, a much lower value of Vc/Vna will be obtained. The weight loss between 200 °C and 700 °C was attributed to coke deposition by TG. The weight percentage of coke was converted to coke volume (Vc) assuming soft coke density of 1.22 g/cm3 [24,25] and hard coke density of 1.60 g/cm3 [26]. As shown in Table 2, the ratio Vc/Vna for used HZSM-5 after 4 h and 8 h was found to be around 0.60, indicating that the carbonaceous compounds blocked the diffusion of reactant molecules into the channels and intersections. Therefore, pore blocking by coke is the predominant mode leading to deactivation of HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde.
Fig. 5. NH3-TPD profiles of the fresh and used HZSM-5.
As shown in Fig. 5, the changes of acidic properties for the fresh and used HZSM-5 were characterized by NH3-TPD/titration, and the acid concentrations with differing strengths are given in Table 2. Compared with HZSM-5, the used HZSM-5 showed an obvious decrease in the intensity of peaks at 240 °C and 398 °C which represented weak and strong acid sites, respectively. The concentrations of acid sites were probed via NH3 trapping in boric acid and then titration with a standard H2SO4 solution [17]. As shown in Table 2, the used HZSM-5 after 4 h and 8 h contained trace amount of weak acid sites and a few strong acid sites. According to the previous study [12], the isomerization of styrene oxide can be completely catalyzed by the strong acid sites of HZSM-5 (0.048–0.606 mmol/g). The used HZSM-5 after 8 h time on stream also contained some amount of strong acid sites (0.118 mmol/g), but gradually lost its activity. So it is also proved that the coke deposited on the surface of catalyst blocks the diffusion of reactant molecules into the micropores and results in the deactivation of catalysts. 4. Conclusion Coke deposition is the main cause of the deactivation of HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde. TG results show that there are two types of coke on the used HZSM-5: soft coke and hard coke. More soft coke was firstly formed on HZSM-5 and then partly converted to hard coke with the reaction carried through. The soft coke, which can be removed via desorption at an elevated temperature (200–400 °C), has little effect on the catalyst activity and selectivity. While the hard coke, which has a certain degree of crystallization (i.e., pregraphite-like carbon by XRD and XPS) and must be removed via combustion with oxygen, would cause major catalyst deactivation. By confirmed, pore blocking by coke is the predominant mode leading to deactivation of HZSM-5. Acknowledgment The authors acknowledge the Doctoral Scientific Research
Table 2 Textural and acidic properties of the fresh and used HZSM-5. Sample
BET surface area (m2/ g)
Pore volume (cm3/ g)
Pore size (Å)
Vc (cm3/g)
Vna (cm3/g)
Vc/Vna
Weak acid sites (mmol/ g)
Strong acid sites (mmol/ g)
HZSM-5 Used HZSM-5 after 4 h Used HZSM-5 after 8 h
290.4 68.6 10.3
0.144 0.011 0.002
4.6 1.7 1.5
– 0.079 0.085
– 0.133 0.142
– 0.59 0.60
0.114 (240 °C) Trace Trace
0.256 (398 °C) 0.164 (397 °C) 0.118 (397 °C)
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[11] W. Hoelderich, N. Goetz, L. Hupfer, R. Kropp, H. Theobald, B. Wolf, US5225602, 1993. [12] M.L. Gou, R. Wang, Q. Qiao, X. Yang, Catal. Commun. 56 (2014) 143–147. [13] M.D. González, Y. Cesteros, P. Salagre, F. Medina, J.E. Sueiras, Microporous Mesoporous Mater. 118 (2009) 341–347. [14] I. Salla, O. Bergada, P. Salagre, Y. Cesteros, F. Medina, J.E. Sueiras, T. Montanari, J. Catal. 232 (2005) 239–245. [15] M.L. Gou, R. Wang, Q. Qiao, X. Yang, Appl. Catal. A 482 (2014) 1–7. [16] M.L. Gou, R. Wang, Q. Qiao, X. Yang, Microporous Mesoporous Mater. 206 (2015) 170–176. [17] N.R. Meshram, S.G. Hegde, S.B. Kulkarni, P. Ratnasamy, Appl. Catal. A 8 (1983) 359–367. [18] B.K. Vu, M.B. Song, I.Y. Ahn, Y.W. Suh, D.J. Suh, J.S. Kim, E.W. Shin, J. Ind. Eng. Chem. 17 (2011) 71–76. [19] B.M. Weckhuysen, M.P. Rosynek, J.H. Lunsford, Catal. Lett. 52 (1998) 31–36. [20] S. Hamoudi, F.I. Larachi, A. Adnot, A. Sayari, J. Catal. 185 (1999) 333–344. [21] C. Neri, F. Buonomo, US4495371, 1985. [22] H. Smuda, W. Hoelderich, N. Goetz, H.G. Recker, US4929765, 1990. [23] A. de-Lucas, P. Canizares, A. Duran, A. Carrero, Appl. Catal. A 156 (1997) 299–317. [24] J.L. Sotelo, M.A. Uguina, J.L. Valverde, D.P. Serrano, Appl. Catal. A 114 (1994) 273–285. [25] D. Bibby, N. Milestone, J. Patterson, L. Aldridge, J. Catal. 97 (1986) 493–502. [26] D. Dong, S. Jeong, F.E. Massoth, Catal. Today 37 (1997) 267–275.
Foundation (No. 4008/13480049) and Youth Foundation (No. 2015QN009) of Henan University of Science and Technology, and the National Natural Science Foundation of China (No. 51105130). References [1] W.F. Hölderich, U. Barsnick, R.A. Sheldon, H. van Bekkum (Eds.), Fine Chemicals Through Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2001, pp. 217–231. [2] H. Kochkar, J.M. Clacens, F. Figueras, Catal. Lett. 78 (2002) 91–94. [3] B.G. Pope, US4650908, 1987. [4] F. Zaccheria, R. Psaro, N. Ravasio, L. Sordelli, F. Santoro, Catal. Lett. 141 (2011) 587–591. [5] E. Ruiz-Hitzky, B. Casal, J. Catal. 92 (1985) 291–295. [6] G.K.S. Prakash, T. Mathew, S. Krishnaraj, E.R. Marinez, G.A. Olah, Appl. Catal. A 181 (1999) 283–288. [7] V.V. Costa, K.A. da Silva Rocha, I.V. Kozhevnikov, E.V. Gusevskaya, Appl. Catal. A 383 (2010) 217–220. [8] D. Brunel, M. Chamoumi, B. Chiche, A. Finiels, C. Gauthier, P. Geneste, P. Graffin, F. Marichez, P. Moreau, Stud. Surf. Sci. Catal. 52 (1989) 139–149. [9] M. Chamoumi, D. Brunel, P. Geneste, P. Moreau, J. Solofo, Stud. Surf. Sci. Catal. 59 (1991) 573–579. [10] K. Smith, G.A. El-Hiti, M. Al-Shamali, Catal. Lett. 109 (2006) 77–82.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Coking and deactivation behavior of ZSM-5 during the isomerization of styrene oxide to phenylacetaldehyde
MARK
Ming-Lei Gou⁎, Junqing Cai, Wensheng Song, Zhen Liu, Yun-Lai Ren, Bingli Pan, Qingshan Niu School of Chemical Engineering and Pharmaceutics, Henan University of Science and Technology, Luoyang, Henan 471023, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Styrene oxide Phenylacetaldehyde ZSM-5 Coking Deactivation behavior
Coking and deactivation behavior of ZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde were investigated under gas-phase free of solvents. More soft coke was firstly formed on HZSM-5 and then partly converted to hard coke with the reaction carried through. The soft coke, which can be removed via desorption at 200–400 °C, had little effect on the catalyst activity and selectivity. While the hard coke, which had a certain degree of crystallization and must be completely removed via combustion with oxygen, would cause major catalyst deactivation. By confirmed, pore blocking was the predominant mode leading to deactivation of catalyst.
1. Introduction Isomerization of styrene oxide and derivatives to aldehydes yields valuable compounds or intermediates used for production of fragrances, pharmaceuticals, insecticides, fungicides and herbicides, particularly the halogenated derivatives of aldehydes are more needed [1]. Various solid acid catalysts, including mixed-metal oxides [2], silicaalumina gels [3,4], natural silicates [5], Nafion-H [6], heteropoly acids [7] and zeolites [8–10], etc., have been tried to catalyze this process. Among them, zeolites of the mordenite, erionite, chabazite and pentasil types are all superior to others in view of suppressing side reactions by shape selectivity, particularly the pentasil zeolites have the best performance with phenylacetaldehyde yield of over 90% at 200 °C and WHSV = 3 h− 1, however the catalysts begin to show deactivation indicating by decrease of the styrene oxide conversion after running for 6 h time on stream [11]. Gou et al. [12] have investigated the effect of acidity of ZSM-5 (SiO2/Al2O3 = 25–360) in the isomerization of styrene oxide to phenylacetaldehyde and find out that the catalyst lifetimes are affected by both acid strength and concentration. Increasing the acid concentration while decreasing their acid strength can prolong the catalyst lifetimes. A series of zeolites modified by different methods, such as dealumination [13], fluorine modification [14], phosphorus modification [15] and alkali treatment [16], have been applied for the reaction. However, the catalyst lifetimes have shown only limited improvement. Although effects of acidity and structure of ZSM-5 on the isomerization of styrene oxide to phenylacetaldehyde have been widely studied, only a small quantity of reports focus on the coke species ⁎
Corresponding author. E-mail address: [email protected] (M.-L. Gou).
http://dx.doi.org/10.1016/j.catcom.2017.04.035 Received 3 January 2017; Received in revised form 21 March 2017; Accepted 17 April 2017 Available online 18 April 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
and deactivation behaviors of catalysts. The deactivation mechanisms of ZSM-5 are not yet clear. So in the present study, coke species and deactivation behavior of ZSM-5 were identified using N2 adsorption, SEM, XRD, XPS, TG-DTA and NH3-TPD/titration. 2. Experiment Styrene oxide (> 98 wt%) was purchased from TCI (shanghai) development Co., Ltd. and used without further purification. HZSM-5 (SiO2/Al2O3 = 135) were obtained from Catalyst Plant of Nankai University (Tianjin, People's Republic of China) and used as catalyst. Isomerization of styrene oxide was carried out in a continuous flow fixed-bed reactor (stainless steel tube, i.d. = 9 mm) operated at 200 °C, WHSV = 3.0 h− 1 and flow rate of N2 = 120 ml/min. The catalyst (0.5 g, 20–30 mesh) was loaded in the constant-temperature zone and pretreated at 500 °C for 2 h in nitrogen flow. Styrene oxide, free of any solvents, was introduced into the reactor with flowrate of 0.025 ml/min using a HPLC pump (model Series II, LabAlliance, USA). At end of each experiment, the used catalyst was unloaded after purging with nitrogen at 200 °C for 2 h to remove any possible residual reactants. Scanning electron microscope (SEM) experiments were made on a Hitachi SU-8010 scanning electron microscope (FE-SEM, 5 kV). The powder X-ray diffraction (XRD) patterns were collected on a PANalytical X'Pert Pro diffractometer in 2θ range of 5–50° with Co Kα radiation source (λ = 0.1789 nm). N2 adsorption was measured at 77 K on a Quantachrome Autosorb-1 instrument. The surface area was calculated according to the BET equation, and the pore volume and size were determined using the HK method.
Catalysis Communications 98 (2017) 116–120
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X-ray photoelectron spectra (XPS) were performed on a PHI-1600 ESCA spectrometer with Mg Kα radiation source (1253.6 eV). The binding energies were determined using C 1s peak of contaminant carbon (BE = 284.6 eV) as standard. Thermogravimetry-differential thermal analysis (TG-DTA) measurements were conducted on a Perkin-Elmer Pyris 6 thermogravimetric analyzer at a heating rate of 10 °C/min from room temperature to 800 °C in air or nitrogen flow. In order to investigate the acidity changes of the fresh and used catalysts, temperature programmed desorption of ammonia (NH3-TPD) was performed on a conventional apparatus equipped with a thermal conductivity detector. The sample (100 mg, 20–30 mesh) was pretreated at 200 °C in nitrogen and then cooled down to ambient temperature. Sufficient NH3 was supplied into the system followed by flushing with nitrogen at 150 °C for 1 h. The TPD profile was obtained by heating the sample from 150 to 500 °C at a rate of 15 °C/min. Simultaneously, the desorbed NH3 was trapped in boric acid, followed by titration with a standard H2SO4 solution [17]. 3. Results and discussion As shown in Fig. 1, catalytic testing of HZSM-5 was carried out at 200 °C, WHSV = 3.0 h− 1 and flow rate of N2 = 120 ml/min. In a blank test without catalyst, the conversion of styrene oxide was below 1% all the time. While the initial conversion of styrene oxide (TOS = 1.0 h) over HZSM-5 was over 99%, suggesting that this reaction was a catalysis process. The styrene oxide conversion remained unchanged for the first 6 h, but thereafter the catalyst began to show deactivation as indicated by decreasing of the styrene oxide conversion. In order to compare the changes of the catalyst properties, the used catalysts before (used after 4 h time on stream) and after (used after 8 h time on stream) deactivation were unloaded from the reactor in two separate experiments. The phenylacetaldehyde selectivity kept > 96% from beginning to end of the reaction. The main by-product was the dimer (2,4diphenyl-2-butenal) formed via aldol condensation of phenylacetaldehyde, maintained its selectivity of 1–3%. Some other by-products, such as phenylethanol, phenylethanediol, styrene and so on, were also detected by GC–MS with their total selectivity of < 1%. But the trimer (2,4,6-tribenzyl-s-trioxane) formed via trimerization of phenylacetaldehyde at external acid sites [12] could not be detected since the catalyst contained only a trace amount of external acid sites. For verification the deactivation behavior of HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde, several charac-
Fig. 2. SEM micrographs of the fresh and used HZSM-5.
terization techniques were implemented over the fresh and used catalysts. The SEM micrographs of the fresh and used catalysts are presented in Fig. 2. The borderline among the fresh catalyst granules was clear. While after reaction 4 h time on stream, the borderline among the catalyst particles became ambiguous. Increasing of the reaction time to 8 h, more and more catalyst particles were buried in the coke. Simultaneously, the activity of the catalyst gradually decreased with accumulation of the coke deposited on the catalyst surface. The filamentous carbon and other crystalline carbon were not observed on the used catalysts. But the structure of the coke can be
Fig. 1. Performances of HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde. Reaction conditions: T = 200 °C, P = 1 atm, catalyst loading = 0.5 g, flow-rate of N2 = 120 ml/min, WHSV = 3.0 h− 1.
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Fig. 3. XRD patterns of the fresh and used HZSM-5.
further examined via XRD. The XRD patterns of the fresh and used catalysts are shown in Fig. 3. The distinct peaks at 2θ = 9–11° and 26–29° were all agree with the standard MFI structure (JCPDS-PDF: 00-049-0657). But the intensity of peaks at 2θ = 9–11° over used HZSM-5 decreased slightly causing by some amorphous coke formed in larger pores and decreased the longrange ordering of HZSM-5. A new peak at 2θ = 31.1° (labeled by the dashed line) over the used catalysts was similar with the characteristic peak of graphite at 2θ = 30.7° (JCPDS-PDF: 00-001-0640), which can be called pregraphite-like carbon [18]. So the coke formation on HZSM5 in the isomerization of styrene oxide has a certain degree of aromatization. The different configurations of carbons have different binding energies and can be distinguished by C 1s XPS analysis. The binding energy and percent area of each peak for the used catalysts are listed in Table 1. There were mainly four peaks in the C 1s spectra: dehydrogenated carbon species (~ 282.0 eV) [18], pregraphite-like carbon (~ 284.1 eV) [19], oxidized carbon in OeCeO (~ 286.5) and oxidized carbon in C]O (~ 287.7 eV) [20]. The surface coke adjoining 282.0 and 284.1 eV was the main coke species. The relative percentage of peak area at 282.0 eV (56.1%) was higher than that of peak area at 284.1 eV (36.2%) on the HZSM-5 after 4 h time on stream. Along with the reaction time extending to 8 h, the relative percentages of peak areas at 282.0 eV (46.1%) and 284.1 eV (44.0%) were almost the same. The amount of coke was higher for the sample obtained after 8 h time on stream (as indicated by the TG-DTA curves below). So it is assumed that some dehydrogenated carbon species can convert to pregraphitelike carbon with the reaction going on. During this period, the styrene oxide conversion decreased gradually. Apparently, there is a close relationship between the content of pregraphite-like carbon and catalyst deactivation. The TG-DTA curves of the used catalysts in air or nitrogen are shown in Fig. 4. The weight loss below 200 °C was attributed to the loss of adsorbed water or some volatile species. The coke deposition over
Fig. 4. TG-DTA curves of the used HZSM-5 in air or nitrogen atmosphere.
the used HZSM-5 were completely removed at around 200–700 °C in air. There were two main stages in the TG curves: 200–400 °C (so-called soft coke) and 400–700 °C (so-called hard coke). The DTA curves showed three main exothermic peaks at around 220, 390 and 550 °C with some small shoulder peaks at 250, 290, 320 and 490 °C, meaning that removes of the soft and hard coke were muti-step reaction processes. The content of soft coke (5.8%) was higher than that of hard coke (5.0%) on the used HZSM-5 after 4 h. While when the
Table 1 Quantitative evaluative of C 1s XPS spectra of used HZSM-5 catalysts. Catalysts
Used HZSM-5 after 4 h Used HZSM-5 after 8 h
BE(eV) ~ 282.0
~ 284.1
~ 286.5
~ 287.7
56.1% 46.1%
36.2% 44.0%
4.6% 5.9%
3.1% 4.0%
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reaction time increased to 8 h, the content of soft coke slightly decreased to 5.5%, and the content of hard coke obviously increased to 6.4%. Simultaneously, the styrene oxide conversion decreased from 99% to 93%. From above, it can be concluded that more soft coke was firstly formed on HZSM-5, and with the reaction carried through, some soft coke gradually converted to the hard coke which would cause major catalyst deactivation. As shown in Fig. 4, the weight loss differences of used HZSM-5 after 8 h time on stream were observed in air and nitrogen atmosphere. The weight loss in the stage of 200–400 °C was similar whether in air (5.5%) or nitrogen (5.6%), indicating that the soft coke can be completely removed via desorption from the catalysts at an elevated temperature (200–400 °C). However, the weight loss in the stage of 400–700 °C in nitrogen (3.1%) was lower than that in air (6.4%), indicating that the hard coke removal was somewhat faster under air. The TG profile above 700 °C was not a horizontal line in nitrogen, because of part hard coke (1.7%) slowly decomposing to some volatile species. Based on the results of XRD and XPS, the coke formation on HZSM-5 has a certain degree of crystallization which must be removed via burning in air. By comparing the weight loss differences in air and nitrogen, approximately 1.6% amount of coke still existed on the catalyst after treatment in nitrogen, which must be removed via burning in air, too. Therefore, it is estimated that approximately 1.6% of the pregraphite-like carbon was formed over HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde at 200 °C, WHSV = 3.0 h− 1 and flow-rate of N2 = 120 ml/min. Table 2 shows the textural changes of the fresh and used HZSM-5. Styrene oxide is a highly reactive substance and can be completely converted to phenylacetaldehyde and other by-products on relatively weak acid sites at 70 °C and WHSV = 0.5–15 h− 1 [21,22]. Although the BET surface area, pore volume and pore size of catalyst remarkably decreased to 68.6 m2/g, 0.011 cm3/g and 1.7 Å after running for 4 h, the rest active sites can almost completely catalyzed this reaction. While when the reaction was continuously operated for 8 h, the BET surface area (10.3 m2/g) and pore volume (0.002 cm3/g) almost reduced to zero and the catalyst showed deactivation as indicated by decreasing of the styrene oxide conversion. Deactivation of zeolites caused by coke usually has two mechanisms: active site suppression and pore blocking [23]. The loss of sorption capacity of the spent catalysts can find out which deactivation mode is predominant. When coke deposition takes place by simple active site suppression, the ratio of coke volume (Vc) to the volume not accessible to the adsorbate (Vna) should be around 1. However, when coke deposition leads to pore blocking and there are not coke deposits in intersections, a ratio of Vc/ Vna lower than 1 is expected. Especially, if most of coke deposits on the external surface of zeolites, blocking the entrances to the internal structure, a much lower value of Vc/Vna will be obtained. The weight loss between 200 °C and 700 °C was attributed to coke deposition by TG. The weight percentage of coke was converted to coke volume (Vc) assuming soft coke density of 1.22 g/cm3 [24,25] and hard coke density of 1.60 g/cm3 [26]. As shown in Table 2, the ratio Vc/Vna for used HZSM-5 after 4 h and 8 h was found to be around 0.60, indicating that the carbonaceous compounds blocked the diffusion of reactant molecules into the channels and intersections. Therefore, pore blocking by coke is the predominant mode leading to deactivation of HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde.
Fig. 5. NH3-TPD profiles of the fresh and used HZSM-5.
As shown in Fig. 5, the changes of acidic properties for the fresh and used HZSM-5 were characterized by NH3-TPD/titration, and the acid concentrations with differing strengths are given in Table 2. Compared with HZSM-5, the used HZSM-5 showed an obvious decrease in the intensity of peaks at 240 °C and 398 °C which represented weak and strong acid sites, respectively. The concentrations of acid sites were probed via NH3 trapping in boric acid and then titration with a standard H2SO4 solution [17]. As shown in Table 2, the used HZSM-5 after 4 h and 8 h contained trace amount of weak acid sites and a few strong acid sites. According to the previous study [12], the isomerization of styrene oxide can be completely catalyzed by the strong acid sites of HZSM-5 (0.048–0.606 mmol/g). The used HZSM-5 after 8 h time on stream also contained some amount of strong acid sites (0.118 mmol/g), but gradually lost its activity. So it is also proved that the coke deposited on the surface of catalyst blocks the diffusion of reactant molecules into the micropores and results in the deactivation of catalysts. 4. Conclusion Coke deposition is the main cause of the deactivation of HZSM-5 in the isomerization of styrene oxide to phenylacetaldehyde. TG results show that there are two types of coke on the used HZSM-5: soft coke and hard coke. More soft coke was firstly formed on HZSM-5 and then partly converted to hard coke with the reaction carried through. The soft coke, which can be removed via desorption at an elevated temperature (200–400 °C), has little effect on the catalyst activity and selectivity. While the hard coke, which has a certain degree of crystallization (i.e., pregraphite-like carbon by XRD and XPS) and must be removed via combustion with oxygen, would cause major catalyst deactivation. By confirmed, pore blocking by coke is the predominant mode leading to deactivation of HZSM-5. Acknowledgment The authors acknowledge the Doctoral Scientific Research
Table 2 Textural and acidic properties of the fresh and used HZSM-5. Sample
BET surface area (m2/ g)
Pore volume (cm3/ g)
Pore size (Å)
Vc (cm3/g)
Vna (cm3/g)
Vc/Vna
Weak acid sites (mmol/ g)
Strong acid sites (mmol/ g)
HZSM-5 Used HZSM-5 after 4 h Used HZSM-5 after 8 h
290.4 68.6 10.3
0.144 0.011 0.002
4.6 1.7 1.5
– 0.079 0.085
– 0.133 0.142
– 0.59 0.60
0.114 (240 °C) Trace Trace
0.256 (398 °C) 0.164 (397 °C) 0.118 (397 °C)
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[11] W. Hoelderich, N. Goetz, L. Hupfer, R. Kropp, H. Theobald, B. Wolf, US5225602, 1993. [12] M.L. Gou, R. Wang, Q. Qiao, X. Yang, Catal. Commun. 56 (2014) 143–147. [13] M.D. González, Y. Cesteros, P. Salagre, F. Medina, J.E. Sueiras, Microporous Mesoporous Mater. 118 (2009) 341–347. [14] I. Salla, O. Bergada, P. Salagre, Y. Cesteros, F. Medina, J.E. Sueiras, T. Montanari, J. Catal. 232 (2005) 239–245. [15] M.L. Gou, R. Wang, Q. Qiao, X. Yang, Appl. Catal. A 482 (2014) 1–7. [16] M.L. Gou, R. Wang, Q. Qiao, X. Yang, Microporous Mesoporous Mater. 206 (2015) 170–176. [17] N.R. Meshram, S.G. Hegde, S.B. Kulkarni, P. Ratnasamy, Appl. Catal. A 8 (1983) 359–367. [18] B.K. Vu, M.B. Song, I.Y. Ahn, Y.W. Suh, D.J. Suh, J.S. Kim, E.W. Shin, J. Ind. Eng. Chem. 17 (2011) 71–76. [19] B.M. Weckhuysen, M.P. Rosynek, J.H. Lunsford, Catal. Lett. 52 (1998) 31–36. [20] S. Hamoudi, F.I. Larachi, A. Adnot, A. Sayari, J. Catal. 185 (1999) 333–344. [21] C. Neri, F. Buonomo, US4495371, 1985. [22] H. Smuda, W. Hoelderich, N. Goetz, H.G. Recker, US4929765, 1990. [23] A. de-Lucas, P. Canizares, A. Duran, A. Carrero, Appl. Catal. A 156 (1997) 299–317. [24] J.L. Sotelo, M.A. Uguina, J.L. Valverde, D.P. Serrano, Appl. Catal. A 114 (1994) 273–285. [25] D. Bibby, N. Milestone, J. Patterson, L. Aldridge, J. Catal. 97 (1986) 493–502. [26] D. Dong, S. Jeong, F.E. Massoth, Catal. Today 37 (1997) 267–275.
Foundation (No. 4008/13480049) and Youth Foundation (No. 2015QN009) of Henan University of Science and Technology, and the National Natural Science Foundation of China (No. 51105130). References [1] W.F. Hölderich, U. Barsnick, R.A. Sheldon, H. van Bekkum (Eds.), Fine Chemicals Through Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2001, pp. 217–231. [2] H. Kochkar, J.M. Clacens, F. Figueras, Catal. Lett. 78 (2002) 91–94. [3] B.G. Pope, US4650908, 1987. [4] F. Zaccheria, R. Psaro, N. Ravasio, L. Sordelli, F. Santoro, Catal. Lett. 141 (2011) 587–591. [5] E. Ruiz-Hitzky, B. Casal, J. Catal. 92 (1985) 291–295. [6] G.K.S. Prakash, T. Mathew, S. Krishnaraj, E.R. Marinez, G.A. Olah, Appl. Catal. A 181 (1999) 283–288. [7] V.V. Costa, K.A. da Silva Rocha, I.V. Kozhevnikov, E.V. Gusevskaya, Appl. Catal. A 383 (2010) 217–220. [8] D. Brunel, M. Chamoumi, B. Chiche, A. Finiels, C. Gauthier, P. Geneste, P. Graffin, F. Marichez, P. Moreau, Stud. Surf. Sci. Catal. 52 (1989) 139–149. [9] M. Chamoumi, D. Brunel, P. Geneste, P. Moreau, J. Solofo, Stud. Surf. Sci. Catal. 59 (1991) 573–579. [10] K. Smith, G.A. El-Hiti, M. Al-Shamali, Catal. Lett. 109 (2006) 77–82.
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