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Ethanol transformation into higher hydrocarbons over HZSM-5 zeolite: Direct detection of radical species by in situ EPR spectroscopy
Catalysis Communications 27 (2012) 119–123
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Ethanol transformation into higher hydrocarbons over HZSM-5 zeolite: Direct detection of radical species by in situ EPR spectroscopy Karima Ben Tayeb a, b,⁎, Ludovic Pinard a, Nadia Touati b, Hervé Vezin b, Sylvie Maury c, Olivier Delpoux c a b c
IC2MP, UMR 7285, Université de Poitiers, 4 rue Michel Brunet, 86022 Poitiers, France LASIR, UMR 8516, Université des Sciences et Technologies de Lille, Cité Scientifique, Bâtiment C4, 59655 Villeneuve d'Ascq, France IFP Energies Nouvelles, Rond-point de l'échangeur de Solaize, BP3, 69390 Vernaison, France
a r t i c l e
i n f o
Article history: Received 10 April 2012 Received in revised form 15 June 2012 Accepted 3 July 2012 Available online 13 July 2012 Keywords: Brønsted acidity Coke composition In situ EPR Radicals Zeolite Ethylene oligomerization
a b s t r a c t HZSM-5 (Si/Al ratio = 40) zeolite is an efficient catalyst for ethanol transformation due to its capacity to maintain a high activity in C3+ hydrocarbons formation with time-on-stream (TOS) and this in spite of a great loss of acidity and microporosity and a high coke content deposited inside the pores of the zeolite. A study by in situ Electron Paramagnetic Resonance (EPR) spectroscopy in ethylene conversion to hydrocarbons showed that a fraction of radicals was active. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The strong need in fuel oils and the growing availability of ethanol at a competitive price encourage researchers to find technical solutions to transform this alcohol to incorporate it in the fuel oil pool [1]. Because of its properties, ethanol can naturally be added to the gasoline pool [2] but its incorporation in large proportion requires the modification of the motorization system. Also, to continue using the existent motorizations, all new fuels have to obey certain criteria, namely physical and combustion properties, and so, it would be more interesting to transform ethanol into incorporable compounds. Thus, it is claimed that the ethanolto-gasoline (ETG) process transforms ethanol coming from natural carbohydrates into gasoline over heterogeneous acid catalysts such as HZSM-5 zeolites [3–9]. Many similarities between methanol conversion into olefins (extensively studied) and ethanol transformation were proposed [10], but the fundamental difference for the ethanol case is the existence of a carbon–carbon liaison at the start of the reaction. This allows an olefin formation which can undergo oligomerization reactions through classical acid catalysis ways. Nevertheless, the acid mechanism is not the only mechanism that occurs during the ethylene oligomerization on HZSM-5
⁎ Corresponding author at: LASIR, UMR 8516, Université des Sciences et Technologies de Lille, Cité Scientifique, Bâtiment C4, 59655 Villeneuve d'Ascq, France. Tel.: + 33 3 20 43 68 92; fax: + 33 3 20 43 67 55. E-mail addresses: [email protected] (K. Ben Tayeb), [email protected] (L. Pinard), [email protected] (H. Vezin), [email protected] (S. Maury). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.07.005
zeolite, since the activity is maintained in spite of the nearly total loss of Brönsted acidity [11–14]. The existence of radicals has been detected on the coked catalysts [12–14]. These radicals could play an important role in the understanding of ethylene transformation into hydrocarbons on a coked zeolite. Nevertheless, a number of key questions remains unanswered, for instance: Is the catalytic behavior only due to the presence of radicals? What is the real nature and concentration of radicals at reaction temperature? What is the percentage of active radicals? For this, Continuous Wave-EPR (CW-EPR) experiments of ethylene transformation at 623 K on fresh and coked HZSM-5 (40) zeolite were performed in order to get the necessary data to understand: the spontaneous formation of radicals in zeolite pores, and the reactivity of radicals. 2. Experimental 2.1. Catalytic experiments The HZSM-5(40) zeolite is a commercial material from Zeolyst International and the parent form is protonic. Consequently, the zeolite is used without any particular treatment. The solid exhibits a threedimensional structure with an Si/Al ratio of 40. The solid was powdered, pressed, crushed and sieved to obtain 0.2–0.4 mm homogeneous particles. Prior to catalytic testing, it was activated in situ under nitrogen flow (3.3 L h−1) at 773 K and a total pressure of 30 bar. The catalytic test was carried out in a continuous down-flow fixed bed reactor under a total pressure of 30 bar and at 623 K. The catalyst (0.3 g) was placed in the middle of the reactor between layers of glass beads
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using 1.19 mm diameter balls (stainless steel, 40 cm long, 1.3 cm internal diameter and 1.7 cm external diameter). Ethanol (96%, V/V) was fed into the reactor at 2 ml h−1 corresponding to an N2/ethanol molar ratio of 4 and a weight hourly space velocity (WHSV) of 15 h−1. Reaction products were analyzed by on-line gas chromatography using a VARIAN 3800 gas chromatograph equipped with two detectors: a FID detector (J&W PONA capillary column) and a TCD detector (5A sieve+Porabond Q). 2.2. Characterization techniques The acidity measurements were determined by pyridine adsorption at 423 K followed by Infrared (IR) spectroscopy using a Nicolet Magna FTIR 550 spectrometer. The samples of fresh and used catalysts were first pressed into thin wafers and activated in situ in the IR cell under secondary vacuum (10 − 6 mbar) at 623 K (fresh sample) and 423 K (coked samples). The interaction between the pyridine molecules and the acid sites of the zeolite is responsible for the appearance of some characteristic bands of pyridine adsorbed on Brønsted and Lewis acid sites in the 1300–1700 cm − 1 region, corresponding to pyridinium ions (1490, 1545, 1640 cm − 1) and pyridine coordinated to Lewis acid sites (1455, 1490, 1600–1630 cm − 1) [15,16]. The extinction coefficients used for the estimation of the Brønsted and Lewis acid sites were the ones previously determined by Guisnet et al. [17]: 1.13 cm μmol − 1 and 1.28 cm μmol − 1 for respectively Brønsted and Lewis acidities. Nitrogen adsorption measurements were carried out at 77 K on an automatic Micrometrics ASAP 2010 apparatus. The carbon content retained within the zeolite was analyzed by a total combustion at 1293 K with a mixture of helium and oxygen, in a Thermoquest NA2100. The extraction of the soluble coke molecules from the zeolite micropores and the subsequent analysis by GC–MS of the extract recovered, allowed the identification of the different compounds present in the coke. The method originally developed at University of Poitiers was used [18]. The CW-EPR experiments were performed with a Bruker ELEXSYS 580-FT spectrometer operating in X-band. The catalyst (0.11 g) was deposited on a sintered glass of a homemade quartz EPR cell. This cell consists of a U-tube (Fig. 1) and was placed into the cavity of the EPR spectrometer. The ethylene conversion was carried out at 623 K and under atmospheric pressure. The ethylene was fed at a rate of 5 mL min − 1 which corresponds to a WHSV of 3.2 h − 1. The spectra were recorded at a 100 KHz modulation field frequency and a microwave frequency of 9.59 GHz, with an amplitude modulation of 0.4 mT and a microwave power of 1.26 mW corresponding to
Fig. 1. Cell dedicated to the EPR experiments.
non-saturation conditions. The Weak Pitch from Bruker was used as standard reference and contained a known concentration of spin/ mass (1.29∙10 13 spins/g). It was analyzed under the exact same EPR operating conditions as the catalysts. The spin concentration is given by the double integration of the first derivative of the EPR signal. The signal of weak pitch drops rapidly at 623 K and can enhance the source of error due to very weak signal detected. Moreover at such temperature weak pitch is partially destroyed, effectively a loss of 20% of signal is measured after go back to room temperature (RT). So consequently this is why the determination of spin concentration is performed at room temperature using Curie Weiss law [19]. Furthermore, for this study we have verified and confirmed beforehand that all radicals remain stable during the in situ EPR analysis. 3. Results and discussion Under our operating conditions, ethanol was totally converted into ethylene, C3+ hydrocarbons, water (stoichiometric quantity from ethanol dehydration) and traces of diethylether. Ethanol conversion remained total (100%) with TOS and no deactivation was observed on HZSM-5 (40) zeolite even after 24 h of running. Among the hydrocarbons, there are C1–C12 paraffins (P), C2–C12 olefins (O), C6–C10 naphthenes (N), and C6–C18 aromatics (A). During the first hour, the ethylene yield decreased progressively coinciding with the increase of C3+ hydrocarbons yield (Fig. 2a). After 1 h of reaction, ethylene was totally transformed into higher hydrocarbons and no deactivation was observed. As mentioned by Takahara et al. [20], diethylether was transformed into ethylene which explained the low yield (~0.1%). Fig. 2b shows the evolution of the fractions selectivity with TOS: C3–C4 (methane and ethane were produced in negligible amount and ethylene was not taken into account being a primary product of ethanol dehydration and was considered as a reactant), C5–C11 and C12+. During the first hour, the C3–C4 fraction decreased progressively while the C5–C11 and C12+ fractions increased. Independent of time-on-stream, the C5–C11 fraction is predominant [11]. After 1 h of TOS, a quasi-plateau is observed indicating that no evolution occurred for the long times. The distribution in P-O-N-A is represented in Fig. 2c. Initially, selectivities in paraffins and olefins dropped whereas the selectivity in aromatics strongly increased. For naphtene compounds, a very small amount was observed. No evolution occurred after 1 h of TOS. Extensive characterization of coked samples at different reaction times was carried out in order to better understand the catalytic performances. The catalysts after 0.5 h and 24 h of reaction present respectively a carbon content of 2.2% and 6.1% (Table 1). We reported that the coking phenomenon is done very rapidly from the beginning of the reaction (increase from 0 to 2.3% between 0 to 1 h) and then it is mitigated with TOS (increase from 2.3 to 6.1% between 1 and 24 h). As indicated in the literature [21,22], the formation rate of coke depends on the crystallite size, the porous structure and the acidity of zeolite. The loss of acidity and microporosity is also remarkably fast and important (Table 1), which is in agreement with the rapid deposit of carbonaceous compounds. No dealumination of zeolitic framework has been observed and therefore does not explain the loss of acidity and porosity. This loss is due to an acid site poisoning and a pore blockage by coke molecules [23]. The fresh and coked samples were analyzed by the CW-EPR technique. No paramagnetic signal was detected for the fresh sample (no EPR signal was also detected for the fresh sample after in situ activation), and therefore, all EPR signals detected on the coked samples are due to the existence of paramagnetic species present in the carbon deposit. The radical species concentration increased with TOS from 0.94 to 4.6 μmol g−1 (Table 1). This increase was correlated to the carbon content [13,24]. Infrared analyses of the spent catalysts were analyzed and compared to the fresh zeolite (Fig. 3). The spectra could be divided into two parts: bands at 1500–1650 cm−1 are generally attributed to the aromatic rings and those at 1340–1470 cm−1 are associated with the alkyl branches of the aromatics (CH stretching δs (CH2), CH
K. Ben Tayeb et al. / Catalysis Communications 27 (2012) 119–123
a
Table 1 Physical–chemical characterizations of the fresh and coked HZSM-5(40) zeolite.
14 100
12
Zeolite
C3+ Hydrocarbons
10
%C (wt)
8 90
6 4
85 2
Ethylene 0 5
10
15
Brønsted Lewis
Fresh / Coked 2.2 (0.5 h) Coked (1 h) 2.3
0.177 0.154 (13%)
Coked (24 h)
0.081 (54%)
297 247 (17%) 241 (20%) 72 (76%)
0.120 (32%)
6.1
Amount of rad. species (μmol g−1 catalyst)
47 34 (28%) 32 (32%) 27 (43%)
/ 0.94 0.99 4.6
(xx %) = loss. a Number of acid sites able to retain pyridine at 423 K.
80 0
Aciditya (μmol g−1)
Micropore volume (cm3 g−1)
95
Yield (%)
Yield (%)
121
25
20
TOS (h)
b 100
1606
Selectivity (%)
1386 1372
C5-C11
80
1510
1460 1448
TOS=24 h
60 TOS=0.5 h
1390
40 Fresh
C12+
20
1700
C3-C4
0 0
5
10
15
20
1600
1500
1400
1300
-1
Wavenumber (cm )
25
TOS (h)
Fig. 3. Infrared spectra of the 1300−1700 cm−1 region for the HZSM-5(40) fresh and coked catalysts after 0.5 h and 24 h of TOS.
c 100
of the coke molecules size and the channels size of zeolite, the possibility of polyaromatic compounds desorption did not take into account. Furthermore, the nature of coke molecules changed with TOS: the molecules became more condensed. Despite the presence of voluminous coke molecules, the C3+ hydrocarbons yield remained total. These results seemed to support the hypothesis previously mentioned [13] of a
Aromatics 80
Selectivity (%)
1572
60
40
20
Table 2 Coke composition determined for coked HZSM-5(40) zeolite after 0.5 h and 24 h of TOS by GC–MS coupling (HF solubilization and CH2Cl2 extraction).
Paraffins Naphtenes Olefins
0
TOS (h) 0.5
0
5
10
15
20
25
Coke structure H3C
H3C
H3C
TOS (h)
CH3
Fig. 2. Ethanol conversion over HZSM-5(40) into ethylene and C3+ hydrocarbons yield (a); C3+ hydrocarbons distribution by number of carbon atoms (b) and chemical family (PONA) (c).
stretching δs (CH3), and/or δs ([CH3]2 Cb). Vibration band at about 1600 cm−1 was often attributed to the more condensed aromatics (polyaromatics) and that at about 1500 cm−1 to the single ring aromatics. The used catalysts present bands (more intense for the coked catalyst after 24 h) in the region attributed to C\C aromatic bonds (around 1600 cm−1), which indicate the presence of aromatic compounds [13]. The results are corroborated by analyses GC–MS coupling (Table 2). The compounds obtained after 0.5 h corresponded mostly to highly alkyl-substituted monoaromatics. After 24 h, more condensed aromatics and highly alkyl-substituted were observed (monoaromatic compounds are still detected). These compounds are very bulky and responsible for the pores blockage and the acid sites poisoning. In view
CH3 CH3
24
CH3
H3C
CH3
CH3
CH3
H3C
H3C
CH3
CH3 CH3
CH3
CH3
CH3
CH3
R R
R
R=R1-R6
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K. Ben Tayeb et al. / Catalysis Communications 27 (2012) 119–123
a
a
Fresh-Reference 330 min (623 K) 690 min (623 K) 720 min (RT)
338
339
340
341
342
343
344
345
0 min (Reference-RT) 0 min (Reference-623 K) 240 min (623 K) 270 min (RT)
338
339
340
Magnetic field (mT)
b Relative amount of radical species (µmol.g-1 catalyst)
Amount of radical species (µmol.g-1 catalyst)
b 0,24 0,22 0,20 0,18 0,16 0,14 0,12 0,10 0,08 0,06 0,04 0,02 0,00 0
100
200
300
400
500
341
342
343
344
345
Magnetic field (mT)
600
700
800
TOS (min)
1,0
0,8
0,6
0,4
0,2
0,0 0
50
100
150
200
250
300
TOS (min)
Fig. 4. CW-EPR spectra at various TOS (a) and amount of radical species (b) for the HZSM-5(40) fresh catalyst (from 0 to 720 min).
Fig. 5. CW-EPR spectra at various TOS (a) and relative amount of radical species (b) for the HZSM-5(40) coked catalyst (from 0 to 270 min).
possible relation between the radical species and the catalytic results (namely, the C3+ hydrocarbons formation). The mechanism of ethanol transformation probably involved two types of active sites. The first sites are Brønsted acid sites (weak Brønsted acid sites: ethanol dehydration while strong acid sites: ethylene oligomerization), that are active when the carbon content is low. During the reaction, the molecules are trapped in the pore volume of the zeolite (called coke molecules) which became the organic radicals formed by spontaneous ionization as observed by Moissette et al. [25]. These species became active sites even in presence of high carbon content. For a better understanding of the impact of radicals in ethylene oligomerization, a study by in situ EPR spectroscopy was carried out. The experiments were first performed on the fresh catalyst. The products formed during ethylene transformation were not analyzed but the presence of coked molecules (8% after 720 min of TOS) confirmed that the transformation took place. The EPR spectra are recorded at various TOS: the spectrum recorded at RT after 720 min of TOS was used to normalize the EPR spectra recorded at 623 K (Fig. 4a). The most intense signal (in green) displayed small shoulders which can be due to non-resolved interactions with protons. The amount detected increased with TOS up to a plateau after 350 min at 0.14 μmol g−1 (gray zone in Fig. 4b). The radical formation rate was of 0.4·10−3 μmol g−1 min−1. After stopping the reaction (the feed gas was shutting off and we come back at RT), several spectra were recorded at different times. The spin concentration was determined at room temperature using Curie Weiss law. For all spectra, the radical concentration is equivalent and became higher (0.18 μmol g−1) as observed in green zone in Fig. 4b. This shows that one part of the radicals formed is consumed during the ethylene transformation (0.045 μmol g−1 that is to say 25%).
We repeated the experiments on a coked catalyst (1 h), which contained 2.3% of carbon and an amount of radical species of 0.99 μmol g−1 of catalyst (Table 1). For both catalysts (fresh and coked), the line width varied between 1 mT and 0.7 mT (Figs. 4a and 5a) which suggested a similar nature of the radical species with a g factor of 2.003. The EPR signal recorded at RT after 270 min of TOS (in green) was more intense compared to what was measured at the reaction beginning (in black), which indicated that the radical species were generated during the ethylene transformation over the coked catalyst. Fig. 5b presents the evolution of relative amount of radical species with TOS. The detected amount increased (14% in absolute value) after 250 min of TOS at 0.16 μmol g−1 (absolute amount of 1.15 μmol g−1) which corresponds to the radical formation rate of 0.54·10−3 μmol g−1 min−1 (gray zone). It is worth noting that the radical formation rate is almost similar for both catalysts (0.4·10−3 against 0.54·10−3 μmol g−1 min−1). As previously observed, when the feed gas was stopped (green zone), the amount of radical species became more important. Therefore, a part of the radicals was involved in the ethylene oligomerization because they were consumed during the reaction (0.71 μmol g−1 whether 38% in absolute amount). This fraction of radicals was lower over the fresh catalyst (25% versus 38%) because probably Brönsted acid sites were still involved together with radicals. As expected, we can notice that the amount of radical species is more important in the case of coked catalyst compared to the fresh catalyst. Moreover, we observed an increase of carbon content from 2.3 to 9.9% between the beginning and the end of reaction [11–14]. The existence of reactive radical species explains the high catalytic performances of the catalysts, considering the losses in acidity and microporosity [13,26–30].
K. Ben Tayeb et al. / Catalysis Communications 27 (2012) 119–123
4. Conclusions In summary, after 24 h of reaction there was still complete ethanol transformation into C3+ hydrocarbons over HZSM-5(40) zeolite, even though a 54% loss of microporosity and 76% loss of Brønsted acidity. All results seem to indicate that two types of active sites exist for ethylene oligomerization into C3+ hydrocarbons: the Brønsted acid sites and the radicals. The Brønsted acid sites are active when the carbon content is low while the radicals operate at high carbon content when most of Brønsted acid sites are deactivated. References [1] J. Duplan, Institut Français du Pétrole report, 2005. [2] R. Le Van Mao, P. Levesque, G. McLaughlin, L.H. Dao, Applied Catalysis 34 (1987) 163–179. [3] D.R. Whitcraft, X.E. Verykios, R. Mutharasan, Industrial and Engineering Chemistry Process Design and Development 22 (1983) 452–457. [4] G.A. Aldridge, X.E. Verykios, R. Mutharasan, Industrial and Engineering Chemistry Process Design and Development 23 (1984) 733–737. [5] A.T. Aguayo, A.G. Gayubo, A. Atutxa, B. Valle, J. Bilbao, Catalysis Today 107–108 (2005) 410–416. [6] J. Schulz, F. Bandermann, Chemical Engineering and Technology 16 (1993) 332–337. [7] J.P. Marques, I. Gener, P. Ayrault, J.C. Bordado, J.M. Lopes, F.R. Ribeiro, M. Guisnet, Microporous and Mesoporous Materials 60 (2003) 251–262. [8] A.G. Gayubo, A. Alonso, B. Vale, A.T. Aguayo, J. Bilbao, Applied Catalysis B: Environmental 97 (2010) 299–306. [9] A.T. Aguayo, A.G. Gayubo, A.M. Tarrio, J. Bilbao, Journal of Chemical Technology and Biotechnology 77 (2002) 211–216. [10] E.G. Derouane, J.B. Nagy, P. Dejaifve, J.H.C. Van Hooff, B.P. Spekman, J.C. Vedrine, C. Naccache, Journal of Catalysis 53 (1978) 40–55.
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[11] F. Ferreira Madeira, N.S. Gnep, P. Magnoux, S. Maury, N. Cadran, Applied Catalysis A: General 367 (2009) 39–46. [12] F. Ferreira Madeira, N.S. Gnep, P. Magnoux, H. Vezin, S. Maury, N. Cadran, Chemical Engineering Journal 161 (2010) 403–408. [13] F. Ferreira Madeira, H. Vezin, N.S. Gnep, P. Magnoux, S. Maury, N. Cadran, ACS Catalysis 1 (2011) 417–424. [14] F. Ferreira Madeira, Ph.D. thesis, Universite de Poitiers, Poitiers, France, 2009. [15] F. Bauer, H.G. Karge, In: in: H.G. Karge, J. Weitkamp (Eds.), Molecular Sieves, Characterization II, vol. 5, Springer, Berlin, 1999, p. 249. [16] F. Thybault-Starzyk, Les Matériaux Micro et Mésoporeux – Caractérisation, EDP Sciences, Les Ulis, France, 2004. [17] M. Guisnet, P. Ayrault, J. Datka, Polish Journal of Chemistry 71 (1997) 1455–1461. [18] P. Magnoux, P. Roger, C. Canaff, V. Fouché, N.S. Gnep, M. Guisnet, Studies in Surface Science and Catalysis 34 (1987) 317–330. [19] M. Brandhorst, S. Cristol, M. Capron, C. Dujardin, H. Vezin, G. Le bourdon, E. Payen, Catalysis Today 113 (2006) 34–39. [20] I. Takahara, M. Saito, M. Inaba, K. Murata, Catalysis Letters 105 (2005) 249–252. [21] D. Chen, K. Moljord, T. Fuglerud, A. Holmen, Microporous and Mesoporous Materials 29 (1999) 191–203. [22] M. Guisnet, P. Magnoux, D. Martin, Studies in Surface Science and Catalysis 111 (1997) 1–19. [23] G.F. Froment, Studies in Surface Science and Catalysis 6 (1980) 1–19. [24] H.G. Karge, Studies in Surface Science and Catalysis 58 (1991) 531–570. [25] A. Moissette, F. Luchez, C. Brémard, H. Vezin, M. Hureau, Physical Chemistry Chemical Physics 11 (2009) 4286–4297. [26] J.K.A. Clarke, R. Darcy, B.F. Hegarty, E. O'Donoghue, V. Amir-Ebrahimi, J.J. Rooney, Journal of the Chemical Society, Chemical Communications 5 (1986) 425–426. [27] C.D. Chang, S.D. Hellring, J.A. Pearson, Journal of Catalysis 115 (1989) 282–285. [28] I.M. Dahl, S. Kolboe, Catalysis Letters 20 (1993) 329–336. [29] M. Stocker, Microporous and Mesoporous Materials 29 (1999) 3–48. [30] S.J. Kim, H.G. Jang, J.K. Lee, H.K. Min, S.B. Hong, G. Seo, Chemical Communications 47 (2011) 9498–9500.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Ethanol transformation into higher hydrocarbons over HZSM-5 zeolite: Direct detection of radical species by in situ EPR spectroscopy Karima Ben Tayeb a, b,⁎, Ludovic Pinard a, Nadia Touati b, Hervé Vezin b, Sylvie Maury c, Olivier Delpoux c a b c
IC2MP, UMR 7285, Université de Poitiers, 4 rue Michel Brunet, 86022 Poitiers, France LASIR, UMR 8516, Université des Sciences et Technologies de Lille, Cité Scientifique, Bâtiment C4, 59655 Villeneuve d'Ascq, France IFP Energies Nouvelles, Rond-point de l'échangeur de Solaize, BP3, 69390 Vernaison, France
a r t i c l e
i n f o
Article history: Received 10 April 2012 Received in revised form 15 June 2012 Accepted 3 July 2012 Available online 13 July 2012 Keywords: Brønsted acidity Coke composition In situ EPR Radicals Zeolite Ethylene oligomerization
a b s t r a c t HZSM-5 (Si/Al ratio = 40) zeolite is an efficient catalyst for ethanol transformation due to its capacity to maintain a high activity in C3+ hydrocarbons formation with time-on-stream (TOS) and this in spite of a great loss of acidity and microporosity and a high coke content deposited inside the pores of the zeolite. A study by in situ Electron Paramagnetic Resonance (EPR) spectroscopy in ethylene conversion to hydrocarbons showed that a fraction of radicals was active. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The strong need in fuel oils and the growing availability of ethanol at a competitive price encourage researchers to find technical solutions to transform this alcohol to incorporate it in the fuel oil pool [1]. Because of its properties, ethanol can naturally be added to the gasoline pool [2] but its incorporation in large proportion requires the modification of the motorization system. Also, to continue using the existent motorizations, all new fuels have to obey certain criteria, namely physical and combustion properties, and so, it would be more interesting to transform ethanol into incorporable compounds. Thus, it is claimed that the ethanolto-gasoline (ETG) process transforms ethanol coming from natural carbohydrates into gasoline over heterogeneous acid catalysts such as HZSM-5 zeolites [3–9]. Many similarities between methanol conversion into olefins (extensively studied) and ethanol transformation were proposed [10], but the fundamental difference for the ethanol case is the existence of a carbon–carbon liaison at the start of the reaction. This allows an olefin formation which can undergo oligomerization reactions through classical acid catalysis ways. Nevertheless, the acid mechanism is not the only mechanism that occurs during the ethylene oligomerization on HZSM-5
⁎ Corresponding author at: LASIR, UMR 8516, Université des Sciences et Technologies de Lille, Cité Scientifique, Bâtiment C4, 59655 Villeneuve d'Ascq, France. Tel.: + 33 3 20 43 68 92; fax: + 33 3 20 43 67 55. E-mail addresses: [email protected] (K. Ben Tayeb), [email protected] (L. Pinard), [email protected] (H. Vezin), [email protected] (S. Maury). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.07.005
zeolite, since the activity is maintained in spite of the nearly total loss of Brönsted acidity [11–14]. The existence of radicals has been detected on the coked catalysts [12–14]. These radicals could play an important role in the understanding of ethylene transformation into hydrocarbons on a coked zeolite. Nevertheless, a number of key questions remains unanswered, for instance: Is the catalytic behavior only due to the presence of radicals? What is the real nature and concentration of radicals at reaction temperature? What is the percentage of active radicals? For this, Continuous Wave-EPR (CW-EPR) experiments of ethylene transformation at 623 K on fresh and coked HZSM-5 (40) zeolite were performed in order to get the necessary data to understand: the spontaneous formation of radicals in zeolite pores, and the reactivity of radicals. 2. Experimental 2.1. Catalytic experiments The HZSM-5(40) zeolite is a commercial material from Zeolyst International and the parent form is protonic. Consequently, the zeolite is used without any particular treatment. The solid exhibits a threedimensional structure with an Si/Al ratio of 40. The solid was powdered, pressed, crushed and sieved to obtain 0.2–0.4 mm homogeneous particles. Prior to catalytic testing, it was activated in situ under nitrogen flow (3.3 L h−1) at 773 K and a total pressure of 30 bar. The catalytic test was carried out in a continuous down-flow fixed bed reactor under a total pressure of 30 bar and at 623 K. The catalyst (0.3 g) was placed in the middle of the reactor between layers of glass beads
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K. Ben Tayeb et al. / Catalysis Communications 27 (2012) 119–123
using 1.19 mm diameter balls (stainless steel, 40 cm long, 1.3 cm internal diameter and 1.7 cm external diameter). Ethanol (96%, V/V) was fed into the reactor at 2 ml h−1 corresponding to an N2/ethanol molar ratio of 4 and a weight hourly space velocity (WHSV) of 15 h−1. Reaction products were analyzed by on-line gas chromatography using a VARIAN 3800 gas chromatograph equipped with two detectors: a FID detector (J&W PONA capillary column) and a TCD detector (5A sieve+Porabond Q). 2.2. Characterization techniques The acidity measurements were determined by pyridine adsorption at 423 K followed by Infrared (IR) spectroscopy using a Nicolet Magna FTIR 550 spectrometer. The samples of fresh and used catalysts were first pressed into thin wafers and activated in situ in the IR cell under secondary vacuum (10 − 6 mbar) at 623 K (fresh sample) and 423 K (coked samples). The interaction between the pyridine molecules and the acid sites of the zeolite is responsible for the appearance of some characteristic bands of pyridine adsorbed on Brønsted and Lewis acid sites in the 1300–1700 cm − 1 region, corresponding to pyridinium ions (1490, 1545, 1640 cm − 1) and pyridine coordinated to Lewis acid sites (1455, 1490, 1600–1630 cm − 1) [15,16]. The extinction coefficients used for the estimation of the Brønsted and Lewis acid sites were the ones previously determined by Guisnet et al. [17]: 1.13 cm μmol − 1 and 1.28 cm μmol − 1 for respectively Brønsted and Lewis acidities. Nitrogen adsorption measurements were carried out at 77 K on an automatic Micrometrics ASAP 2010 apparatus. The carbon content retained within the zeolite was analyzed by a total combustion at 1293 K with a mixture of helium and oxygen, in a Thermoquest NA2100. The extraction of the soluble coke molecules from the zeolite micropores and the subsequent analysis by GC–MS of the extract recovered, allowed the identification of the different compounds present in the coke. The method originally developed at University of Poitiers was used [18]. The CW-EPR experiments were performed with a Bruker ELEXSYS 580-FT spectrometer operating in X-band. The catalyst (0.11 g) was deposited on a sintered glass of a homemade quartz EPR cell. This cell consists of a U-tube (Fig. 1) and was placed into the cavity of the EPR spectrometer. The ethylene conversion was carried out at 623 K and under atmospheric pressure. The ethylene was fed at a rate of 5 mL min − 1 which corresponds to a WHSV of 3.2 h − 1. The spectra were recorded at a 100 KHz modulation field frequency and a microwave frequency of 9.59 GHz, with an amplitude modulation of 0.4 mT and a microwave power of 1.26 mW corresponding to
Fig. 1. Cell dedicated to the EPR experiments.
non-saturation conditions. The Weak Pitch from Bruker was used as standard reference and contained a known concentration of spin/ mass (1.29∙10 13 spins/g). It was analyzed under the exact same EPR operating conditions as the catalysts. The spin concentration is given by the double integration of the first derivative of the EPR signal. The signal of weak pitch drops rapidly at 623 K and can enhance the source of error due to very weak signal detected. Moreover at such temperature weak pitch is partially destroyed, effectively a loss of 20% of signal is measured after go back to room temperature (RT). So consequently this is why the determination of spin concentration is performed at room temperature using Curie Weiss law [19]. Furthermore, for this study we have verified and confirmed beforehand that all radicals remain stable during the in situ EPR analysis. 3. Results and discussion Under our operating conditions, ethanol was totally converted into ethylene, C3+ hydrocarbons, water (stoichiometric quantity from ethanol dehydration) and traces of diethylether. Ethanol conversion remained total (100%) with TOS and no deactivation was observed on HZSM-5 (40) zeolite even after 24 h of running. Among the hydrocarbons, there are C1–C12 paraffins (P), C2–C12 olefins (O), C6–C10 naphthenes (N), and C6–C18 aromatics (A). During the first hour, the ethylene yield decreased progressively coinciding with the increase of C3+ hydrocarbons yield (Fig. 2a). After 1 h of reaction, ethylene was totally transformed into higher hydrocarbons and no deactivation was observed. As mentioned by Takahara et al. [20], diethylether was transformed into ethylene which explained the low yield (~0.1%). Fig. 2b shows the evolution of the fractions selectivity with TOS: C3–C4 (methane and ethane were produced in negligible amount and ethylene was not taken into account being a primary product of ethanol dehydration and was considered as a reactant), C5–C11 and C12+. During the first hour, the C3–C4 fraction decreased progressively while the C5–C11 and C12+ fractions increased. Independent of time-on-stream, the C5–C11 fraction is predominant [11]. After 1 h of TOS, a quasi-plateau is observed indicating that no evolution occurred for the long times. The distribution in P-O-N-A is represented in Fig. 2c. Initially, selectivities in paraffins and olefins dropped whereas the selectivity in aromatics strongly increased. For naphtene compounds, a very small amount was observed. No evolution occurred after 1 h of TOS. Extensive characterization of coked samples at different reaction times was carried out in order to better understand the catalytic performances. The catalysts after 0.5 h and 24 h of reaction present respectively a carbon content of 2.2% and 6.1% (Table 1). We reported that the coking phenomenon is done very rapidly from the beginning of the reaction (increase from 0 to 2.3% between 0 to 1 h) and then it is mitigated with TOS (increase from 2.3 to 6.1% between 1 and 24 h). As indicated in the literature [21,22], the formation rate of coke depends on the crystallite size, the porous structure and the acidity of zeolite. The loss of acidity and microporosity is also remarkably fast and important (Table 1), which is in agreement with the rapid deposit of carbonaceous compounds. No dealumination of zeolitic framework has been observed and therefore does not explain the loss of acidity and porosity. This loss is due to an acid site poisoning and a pore blockage by coke molecules [23]. The fresh and coked samples were analyzed by the CW-EPR technique. No paramagnetic signal was detected for the fresh sample (no EPR signal was also detected for the fresh sample after in situ activation), and therefore, all EPR signals detected on the coked samples are due to the existence of paramagnetic species present in the carbon deposit. The radical species concentration increased with TOS from 0.94 to 4.6 μmol g−1 (Table 1). This increase was correlated to the carbon content [13,24]. Infrared analyses of the spent catalysts were analyzed and compared to the fresh zeolite (Fig. 3). The spectra could be divided into two parts: bands at 1500–1650 cm−1 are generally attributed to the aromatic rings and those at 1340–1470 cm−1 are associated with the alkyl branches of the aromatics (CH stretching δs (CH2), CH
K. Ben Tayeb et al. / Catalysis Communications 27 (2012) 119–123
a
Table 1 Physical–chemical characterizations of the fresh and coked HZSM-5(40) zeolite.
14 100
12
Zeolite
C3+ Hydrocarbons
10
%C (wt)
8 90
6 4
85 2
Ethylene 0 5
10
15
Brønsted Lewis
Fresh / Coked 2.2 (0.5 h) Coked (1 h) 2.3
0.177 0.154 (13%)
Coked (24 h)
0.081 (54%)
297 247 (17%) 241 (20%) 72 (76%)
0.120 (32%)
6.1
Amount of rad. species (μmol g−1 catalyst)
47 34 (28%) 32 (32%) 27 (43%)
/ 0.94 0.99 4.6
(xx %) = loss. a Number of acid sites able to retain pyridine at 423 K.
80 0
Aciditya (μmol g−1)
Micropore volume (cm3 g−1)
95
Yield (%)
Yield (%)
121
25
20
TOS (h)
b 100
1606
Selectivity (%)
1386 1372
C5-C11
80
1510
1460 1448
TOS=24 h
60 TOS=0.5 h
1390
40 Fresh
C12+
20
1700
C3-C4
0 0
5
10
15
20
1600
1500
1400
1300
-1
Wavenumber (cm )
25
TOS (h)
Fig. 3. Infrared spectra of the 1300−1700 cm−1 region for the HZSM-5(40) fresh and coked catalysts after 0.5 h and 24 h of TOS.
c 100
of the coke molecules size and the channels size of zeolite, the possibility of polyaromatic compounds desorption did not take into account. Furthermore, the nature of coke molecules changed with TOS: the molecules became more condensed. Despite the presence of voluminous coke molecules, the C3+ hydrocarbons yield remained total. These results seemed to support the hypothesis previously mentioned [13] of a
Aromatics 80
Selectivity (%)
1572
60
40
20
Table 2 Coke composition determined for coked HZSM-5(40) zeolite after 0.5 h and 24 h of TOS by GC–MS coupling (HF solubilization and CH2Cl2 extraction).
Paraffins Naphtenes Olefins
0
TOS (h) 0.5
0
5
10
15
20
25
Coke structure H3C
H3C
H3C
TOS (h)
CH3
Fig. 2. Ethanol conversion over HZSM-5(40) into ethylene and C3+ hydrocarbons yield (a); C3+ hydrocarbons distribution by number of carbon atoms (b) and chemical family (PONA) (c).
stretching δs (CH3), and/or δs ([CH3]2 Cb). Vibration band at about 1600 cm−1 was often attributed to the more condensed aromatics (polyaromatics) and that at about 1500 cm−1 to the single ring aromatics. The used catalysts present bands (more intense for the coked catalyst after 24 h) in the region attributed to C\C aromatic bonds (around 1600 cm−1), which indicate the presence of aromatic compounds [13]. The results are corroborated by analyses GC–MS coupling (Table 2). The compounds obtained after 0.5 h corresponded mostly to highly alkyl-substituted monoaromatics. After 24 h, more condensed aromatics and highly alkyl-substituted were observed (monoaromatic compounds are still detected). These compounds are very bulky and responsible for the pores blockage and the acid sites poisoning. In view
CH3 CH3
24
CH3
H3C
CH3
CH3
CH3
H3C
H3C
CH3
CH3 CH3
CH3
CH3
CH3
CH3
R R
R
R=R1-R6
122
K. Ben Tayeb et al. / Catalysis Communications 27 (2012) 119–123
a
a
Fresh-Reference 330 min (623 K) 690 min (623 K) 720 min (RT)
338
339
340
341
342
343
344
345
0 min (Reference-RT) 0 min (Reference-623 K) 240 min (623 K) 270 min (RT)
338
339
340
Magnetic field (mT)
b Relative amount of radical species (µmol.g-1 catalyst)
Amount of radical species (µmol.g-1 catalyst)
b 0,24 0,22 0,20 0,18 0,16 0,14 0,12 0,10 0,08 0,06 0,04 0,02 0,00 0
100
200
300
400
500
341
342
343
344
345
Magnetic field (mT)
600
700
800
TOS (min)
1,0
0,8
0,6
0,4
0,2
0,0 0
50
100
150
200
250
300
TOS (min)
Fig. 4. CW-EPR spectra at various TOS (a) and amount of radical species (b) for the HZSM-5(40) fresh catalyst (from 0 to 720 min).
Fig. 5. CW-EPR spectra at various TOS (a) and relative amount of radical species (b) for the HZSM-5(40) coked catalyst (from 0 to 270 min).
possible relation between the radical species and the catalytic results (namely, the C3+ hydrocarbons formation). The mechanism of ethanol transformation probably involved two types of active sites. The first sites are Brønsted acid sites (weak Brønsted acid sites: ethanol dehydration while strong acid sites: ethylene oligomerization), that are active when the carbon content is low. During the reaction, the molecules are trapped in the pore volume of the zeolite (called coke molecules) which became the organic radicals formed by spontaneous ionization as observed by Moissette et al. [25]. These species became active sites even in presence of high carbon content. For a better understanding of the impact of radicals in ethylene oligomerization, a study by in situ EPR spectroscopy was carried out. The experiments were first performed on the fresh catalyst. The products formed during ethylene transformation were not analyzed but the presence of coked molecules (8% after 720 min of TOS) confirmed that the transformation took place. The EPR spectra are recorded at various TOS: the spectrum recorded at RT after 720 min of TOS was used to normalize the EPR spectra recorded at 623 K (Fig. 4a). The most intense signal (in green) displayed small shoulders which can be due to non-resolved interactions with protons. The amount detected increased with TOS up to a plateau after 350 min at 0.14 μmol g−1 (gray zone in Fig. 4b). The radical formation rate was of 0.4·10−3 μmol g−1 min−1. After stopping the reaction (the feed gas was shutting off and we come back at RT), several spectra were recorded at different times. The spin concentration was determined at room temperature using Curie Weiss law. For all spectra, the radical concentration is equivalent and became higher (0.18 μmol g−1) as observed in green zone in Fig. 4b. This shows that one part of the radicals formed is consumed during the ethylene transformation (0.045 μmol g−1 that is to say 25%).
We repeated the experiments on a coked catalyst (1 h), which contained 2.3% of carbon and an amount of radical species of 0.99 μmol g−1 of catalyst (Table 1). For both catalysts (fresh and coked), the line width varied between 1 mT and 0.7 mT (Figs. 4a and 5a) which suggested a similar nature of the radical species with a g factor of 2.003. The EPR signal recorded at RT after 270 min of TOS (in green) was more intense compared to what was measured at the reaction beginning (in black), which indicated that the radical species were generated during the ethylene transformation over the coked catalyst. Fig. 5b presents the evolution of relative amount of radical species with TOS. The detected amount increased (14% in absolute value) after 250 min of TOS at 0.16 μmol g−1 (absolute amount of 1.15 μmol g−1) which corresponds to the radical formation rate of 0.54·10−3 μmol g−1 min−1 (gray zone). It is worth noting that the radical formation rate is almost similar for both catalysts (0.4·10−3 against 0.54·10−3 μmol g−1 min−1). As previously observed, when the feed gas was stopped (green zone), the amount of radical species became more important. Therefore, a part of the radicals was involved in the ethylene oligomerization because they were consumed during the reaction (0.71 μmol g−1 whether 38% in absolute amount). This fraction of radicals was lower over the fresh catalyst (25% versus 38%) because probably Brönsted acid sites were still involved together with radicals. As expected, we can notice that the amount of radical species is more important in the case of coked catalyst compared to the fresh catalyst. Moreover, we observed an increase of carbon content from 2.3 to 9.9% between the beginning and the end of reaction [11–14]. The existence of reactive radical species explains the high catalytic performances of the catalysts, considering the losses in acidity and microporosity [13,26–30].
K. Ben Tayeb et al. / Catalysis Communications 27 (2012) 119–123
4. Conclusions In summary, after 24 h of reaction there was still complete ethanol transformation into C3+ hydrocarbons over HZSM-5(40) zeolite, even though a 54% loss of microporosity and 76% loss of Brønsted acidity. All results seem to indicate that two types of active sites exist for ethylene oligomerization into C3+ hydrocarbons: the Brønsted acid sites and the radicals. The Brønsted acid sites are active when the carbon content is low while the radicals operate at high carbon content when most of Brønsted acid sites are deactivated. References [1] J. Duplan, Institut Français du Pétrole report, 2005. [2] R. Le Van Mao, P. Levesque, G. McLaughlin, L.H. Dao, Applied Catalysis 34 (1987) 163–179. [3] D.R. Whitcraft, X.E. Verykios, R. Mutharasan, Industrial and Engineering Chemistry Process Design and Development 22 (1983) 452–457. [4] G.A. Aldridge, X.E. Verykios, R. Mutharasan, Industrial and Engineering Chemistry Process Design and Development 23 (1984) 733–737. [5] A.T. Aguayo, A.G. Gayubo, A. Atutxa, B. Valle, J. Bilbao, Catalysis Today 107–108 (2005) 410–416. [6] J. Schulz, F. Bandermann, Chemical Engineering and Technology 16 (1993) 332–337. [7] J.P. Marques, I. Gener, P. Ayrault, J.C. Bordado, J.M. Lopes, F.R. Ribeiro, M. Guisnet, Microporous and Mesoporous Materials 60 (2003) 251–262. [8] A.G. Gayubo, A. Alonso, B. Vale, A.T. Aguayo, J. Bilbao, Applied Catalysis B: Environmental 97 (2010) 299–306. [9] A.T. Aguayo, A.G. Gayubo, A.M. Tarrio, J. Bilbao, Journal of Chemical Technology and Biotechnology 77 (2002) 211–216. [10] E.G. Derouane, J.B. Nagy, P. Dejaifve, J.H.C. Van Hooff, B.P. Spekman, J.C. Vedrine, C. Naccache, Journal of Catalysis 53 (1978) 40–55.
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