Al ratio on catalytic performances and deactivation rate. Study of the radical species role

Applied Catalysis A: General 443–444 (2012) 171–180

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Ethanol transformation into hydrocarbons on ZSM-5 zeolites: Influence of Si/Al ratio on catalytic performances and deactivation rate. Study of the radical species role Filipa Ferreira Madeira a , Karima Ben Tayeb b,∗ , Ludovic Pinard a , Hervé Vezin b , Sylvie Maury c , Nicolas Cadran c a

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 c IFP Energies Nouvelles, Rond-point de l’échangeur de Solaize, BP3, 69390 Vernaison, France b

a r t i c l e

i n f o

Article history: Received 2 March 2012 Received in revised form 24 July 2012 Accepted 29 July 2012 Available online 7 August 2012 Keywords: Brønsted acidity Coke composition Radical Deactivation Ethanol conversion

a b s t r a c t The catalytic performances of HZSM-5 zeolites with Si/Al ratios ranging from 16 to 500 were investigated for ethanol transformation into hydrocarbons. The fresh and used catalysts were characterized by a combination of nitrogen adsorption, pyridine adsorption followed by infrared spectroscopy (IR), gas chromatography–mass spectrometry (GC–MS) coupling and electron paramagnetic resonance (EPR). HZSM-5(Si/Al = 40) was found to be the most stably and selective catalyst due to an optimum balance between the number of Brønsted acid sites and the amount of radicals, which are active sites for ethanol conversion into higher hydrocarbons. However, a change of radical species nature occurred with timeon-stream (TOS) which could be responsible for the deactivation of all catalysts leading to a decrease of C3+ hydrocarbons yield. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Transformation of bioethanol into hydrocarbons has raised considerable interest in scientific community in the last years, particularly since the price of fossil fuels increased and environmental policies became more strict. Because of its properties, ethanol can naturally be added to the gasoline pool [1] 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 ethanol to gasoline process (ETG) transforms ethanol coming from natural carbohydrates into gasoline over heterogeneous acid catalysts such as HZSM-5 zeolites [2,3]. ZSM-5 zeolite is a well-known catalyst for the transformation of methanol into olefins (MTO) [4] and gasoline (MTG) [5–7]. It is also one of the most studied and claimed as one of the best catalyst for ethanol transformation into ethylene [8,9] and/or into higher hydrocarbons [10–12].

∗ Corresponding author. 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). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.07.037

In a previous work [13], we have shown by carrying out this transformation on HFaujasite, HBeta and HZSM-5 zeolites containing the same quantity of Brønsted acid sites, at total pressure of 30 bar and at 623 K, that HZSM-5 zeolite produced large amount of C3 –C18 hydrocarbons, while HFaujasite and HBeta zeolites mainly yielded into ethylene and diethylether. This study concluded that the difference in selectivity in every product of the reaction for these three zeolites was related to their different capacity to prevent coke formation due to their pore structures. The passivation of HZSM-5(Si/Al = 16) catalyst by tetra ethyl ortho silicate (TEOS) deposition permitted to confirm that the reaction did not occur at the outer surface but probably at the pore entry (pore mouth) of the catalyst channels [14]. Recently, several modifications of HZSM-5 with alkaline earth metals [15], iron and/or phosphorus [16–20], and NaOH [21] have been reported to the catalytic conversion of ethanol to olefins in order to reduce the catalyst deactivation (by coke or dealumination) by decreasing the strength of acid sites. Transformation of ethanol into hydrocarbons over HZSM-5(Si/Al = 16) [22] showed very encouraging results and the existence of radical species among the carbon deposit has been proven. A change in the chemical nature of those species coincided with the start of the catalyst deactivation. This study permitted to propose a mechanism involving radical species for the ethanol transformation into C3+ hydrocarbons. It has been shown that over a series of HZSM-5 zeolites (Si/Al ratio ranging from 16 to 95) with

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varying concentrations of strong Brønsted acid sites, formation of higher hydrocarbons from ethanol (at 573 K and under atmospheric pressure) only occurred to a significant extent with zeolites having more than one proton per unit cell [23]. Talukdar et al. [10] have found that ethanol conversion (under 1 bar and a temperature varying between 673 K and 773 K) on HZSM-5 zeolite with SiO2 /Al2 O3 ratio of 206 produced mainly ethylene, while higher hydrocarbons were obtained with HZSM-5 of SiO2 /Al2 O3 ratio of 40. Recent research is focused on catalyst development for conversion into various hydrocarbon products. Catalytic dehydration of ethanol to ethylene, disproportionation of ethanol to propylene and ethanol to aromatics are some of the studies reported in this direction, where the textural properties of ZSM-5 zeolite, especially its framework Si/Al, which is related to the acidity of the catalyst, played a vital role in determining the nature of the product [24–27]. A recent review on the catalytic conversion of bio-ethanol to hydrocarbon fuels reported by Tretyakov et al. summarizes the research work carried out in this area [28]. The acidity modification of ZSM5 by Ni loading is also observed to optimize acidity and to improve the catalyst stability against dealumination [29]. In order to go forward for understanding the impact of protonic sites on the selectivity, activity and deactivation rate of HZSM5 zeolites, ethanol transformation on different HZSM-5 zeolites with Si/Al ratio ranging from 16 to 500 was undertaken. Extensive characterization of coked samples at different reaction times was carried out in order to better understand the correlation between selectivity toward hydrocarbons, Si/Al ratio (therefore acidity), deactivation rate and coke composition. 2. Experimental 2.1. Catalysts and reactant All HZSM-5 zeolites with Si/Al ratios of 16, 40, 140 and 500 are commercial products provided by Zeolyst International (UK) and by IFP Energies Nouvelles (France). The zeolites are under the NH4 form. The protonic form was obtained through in situ activation under nitrogen (30 bar, 3.3 L h−1 ) for 16 h at 773 K in order to generate the protonic form of the zeolite and to desorb any potential pollutants. Before use in catalytic test, these samples were compacted by compression, crushed, and sieved to obtain 0.2–0.4 mm homogeneous particles. The ethanol is a commercial product from Carlo Erba (96%, 4% water (v/v)). It was used without any further purification.

connected to a double column, composed of a 5A sieve (10 m long, 0.32 mm id, 10 ␮m thickness film) and of a Porabond Q (50 m long, 0.53 mm id, 10 ␮m thickness film). The first column allows the detection of aliphatic, naphthenic, aromatic hydrocarbons and oxygenate compounds such as ethanol, diethylether and others. The second one allows the detection of hydrogen, nitrogen, monoxide, carbon dioxide, oxygen and methane. The oven temperature programming starts from 293 K (maintained for 15 min) thanks to a cryogenic system to 523 K (for 15 min). Between these two temperatures, different heating steps are set: 423 K for 10 min, 493 K for 33.75 min, the heating rate being 8 K min−1 for every step. In addition to the on-line analysis, the liquid phase was also recovered and analyzed by gas chromatography–mass spectrometry coupling (GC–MS). Liquid phases (organic and aqueous) were separated and weighted for mass balance purposes. Water content was determined by weighting the aqueous phase after phase’s separation. 2.3. Characterization techniques Nitrogen adsorption measurements were carried out at 77 K on an automatic Micrometrics ASAP 2010 apparatus, with the purpose to determine the porosity of the zeolites. Before adsorption, the fresh zeolite samples were degassed under vacuum at 363 K for 1 h and then at 623 K for 12 h. The coked zeolites were submitted to a less severe pre-treatment under vacuum at 423 K for 1 h in order to prevent the removal of the coke molecules deposited on the zeolites. Pyridine adsorption followed by infrared (IR) spectroscopy (Nicolet Magna FTIR 550 spectrometer) was used to investigate the acid site properties at 423 K. 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 samples) and 423 K (coked samples). The interaction between the pyridine molecules and the acid sites of zeolites 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 ) [32,33]. The extinction coefficients used for the estimation of the Brønsted and Lewis acid sites were the ones previously determined by Guisnet et al. [34]: 1.13 cm ␮mol−1 and 1.28 cm ␮mol−1 for respectively Brønsted and Lewis acidities. The concentration of different acid sites is calculated from the formula presented below (Beer–Lambert’s law): C=

2.2. Catalytic experiments The catalytic tests were carried out in a continuous downflow fixed bed reactor. The reactor was made of stainless steel, 40 cm long with internal and external diameters of 1.3 and 1.7 cm respectively. The catalyst (0.3 g) was placed in the middle of the reactor (isothermal zone), between layers of glass beads of 1.19 mm of diameter. The operating conditions for ethanol transformation were as follows: 623 K, total pressure of 30 bar, ethanol flow rate: 2 ml h−1 , N2 /ethanol molar ratio = 4, and weight hourly space velocity (WHSV) = 15 h−1 . Previous catalytic tests in literature were almost exclusively carried out under ambiance pressure, and there are only so few papers where transformation of ethanol under pressure is described [30]. This pressure domain is chosen in order to obtain distillate-range hydrocarbons having an almost like petrochemical-type structure as stated by Quann et al. [31]. The whole effluent of the reactor was on-line analyzed using a VARIAN 3800 gas chromatograph equipped with two detectors: a FID connected to a J&W PONA capillary column (100 m long, 0.25 mm inner diameter (id) and 0.5 ␮m thickness film), a TCD

S A × × 1000 ε m

where C is the concentration of acid sites (␮mol g−1 ), A is the area of band (cm−1 ), S is the surface of the wafer (2 cm2 ), ε is the molar extinction coefficient (cm ␮mol−1 ), and m is the mass of sample (mg). The carbon content retained within the used zeolites recovered at different time-on-stream (TOS) was analyzed by a total combustion at 1293 K with a mixture of helium and oxygen, in a Thermoquest NA2100. The analytical method is based on the complete and instantaneous oxidation of the sample by “flash combustion”, which converts all organic substances into combustion products. The resulting combustion gases pass through a reduction furnace and are swept into the chromatographic column by the carrier gas (helium), where they are separated and detected by a thermal conductivity detector (TCD), which gives an output signal proportional to the concentration of the individual components of the mixture. The extraction of soluble coke molecules from zeolite micropores and the subsequent analysis of the extract recovered, allowed the identification of different compounds present in the coke. The

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173

Table 1 Physical–chemical characterizations of fresh HZSM-5 zeolites. Zeolite (Si/Al)

HZSM-5(16) HZSM-5(40) HZSM-5(140) HZSM-5(500) a

Unit cell formula

H5,65 Al5,65 Si90,35 O192 H2,34 Al2,34 Si93,66 O192 H0,68 Al0,68 Si95,32 O192 H0,19 Al0,19 Si95,81 O192

Pore volume (cm3 g−1 )

Aciditya (probe: pyridine) (␮mol g−1 )

Total

Micro.

Meso.

Brønsted

Lewis

Hth +

0.267 0.262 0.219 0.228

0.184 0.177 0.158 0.157

0.083 0.085 0.061 0.071

608 297 91 26

50 47 6 6

981 404 118 33

Number of acid sites able to retain pyridine at 423 K; Hth + : theoretical acidity estimated from H+ of the unit cell formula.

method originally developed at University of Poitiers was used [35]. First, the dissolution of the aluminosilicate matrix of the coked zeolite was performed with a hydrofluoric (HF) acid solution (40 wt.% in water) at room temperature. Then, the acid solution containing the coke molecules was neutralized with sodium bicarbonate (NaHCO3 ) and, finally, dichloromethane was added to the solution. According to the solubility into dichloromethane (CH2 Cl2 ), two immiscible phases appeared: the soluble coke and the insoluble coke. The soluble coke was recovered and analyzed in a Varian chromatograph with a DBS capillary column coupled to a Finnigan Incos 500 quadrupole mass spectrometer. The existence of radicals and their quantification were determined by the continuous wave-electron paramagnetic resonance (CW-EPR). Theses experiments were performed with a Brüker ELEXSYS 580-FT spectrometer operating in X-band. All spectra were recorded at a 100 kHz modulation field frequency and a microwave frequency of 9.80 GHz, with an amplitude modulation of 0.1 mT and a microwave power of 5 mW corresponding to non-saturation conditions. The weak pitch from Brüker was used as standard reference and contained a known concentration of spin/mass (1.29 × 1013 spins/g). It was analyzed under the same operating conditions as the catalysts. The spin concentration is given by the double integration of the first derivative of the EPR signal.

possess a very small amount of Lewis acid sites compared to Brønsted acid sites. This can be due to either structure defects or to the presence of terminal aluminium of zeolite crystals. The proportion of Lewis acid sites represents about 6–8% for HZSM-5(16) and (140), but 13–20% for HZSM-5(40) and (500). 3.2. Ethanol transformation Under the operating conditions used, over all HZSM-5 zeolite samples, ethanol was converted into water, diethylether, ethylene, and C3+ hydrocarbons mainly ranging from 3 to 18 carbon atoms. From the mass balance analysis, the water produced was found to be the stoechiometric amount expected from ethanol dehydration into ethylene and diethylether (primary products). This result is in agreement with the one found by Calsavara et al. [38]. Among the hydrocarbons, there are C1 –C12 paraffins (P), C2 –C12 olefins (O), C6 –C10 naphthenes (N), and C6 –C18 aromatics (A). Methane was produced in negligible amount and only a small amount of ethane was detected. On-line TCD analysis allowed us to observe that no CO, CO2 or H2 were produced at any time. These products have been observed at higher temperatures and are formed by dehydrogenation of ethanol into acetaldehyde which can itself crack into CO, CO2 and CH4 . For all catalysts, ethanol transformation is carried out during 30 h, except for HZSM-5(500) with a maximum TOS of 9 h.

3. Results

HZSM-5 zeolites were denominated as HZSM-5(x), x corresponding to the value of Si/Al ratio. Porosity and acidity of all HZSM-5 zeolite samples are shown in Table 1. Microporous volumes of HZSM-5 having a Si/Al ratio of 16 and 40 are similar, but for HZSM-5(140) and HZSM-5(500) the microporous volumes are slightly lower (0.157–0.158 cm3 g−1 against 0.177–0.184 cm3 g−1 for the others). This could be due to structure defects and/or to a partial pore blockage for the zeolites with a high Si/Al ratio. The presence of a small amount of mesoporous pore volume in all HZSM-5 samples is most probably due to structure defects or to intercrystalline volumes. Table 1 shows, as expected, that Brønsted acidity estimated from pyridine adsorption decreases when Si/Al ratio increases. However, this acidity is about 62–80% of the theoretical value calculated from the unit cell formula (the unit cell formulas are nominal and they are based on the values of chemical composition). This phenomenon is well-known and can be explained by the non-accessibility of pyridine molecules to the acid sites [36] or by the very low acidity of some hydroxyl groups which were too weak to retain adsorbed pyridine at 423 K [37]. The extra-framework Al species (EFAL) has not been observed and therefore does not explain the difference between the theoretical values of protonic acidity and those obtained experimentally. The concentrations of acid sites (H+ per unit cell) of HZSM-5(16), (40), (140), and (500) were equal to 3.50, 1.72, 0.45 and 0.15 respectively. These concentrations are close to those reported by Chaudhuri et al. [23]. All HZSM-5 zeolites

3.2.1. Effect of TOS on ethanol conversion Initially (first analysis after 1 h of TOS), all catalysts led to 100% ethanol conversion (Fig. 1). For HZSM-5 having a low Si/Al ratio of 16 and 40, ethanol conversion remained total with TOS, but on the most dealuminated samples, deactivation was observed after 2 and 16 h for HZSM-5(500) and HZSM-5(140) respectively. This deactivation is more pronounced for HZSM-5(500) and could be

(40)

100

(16 ) (14 0)

Ethanol conversion (%)

3.1. Physical–chemical characteristics of fresh HZSM-5 zeolites

80

60 (500)

40

20

0 0

5

10

15

20

25

30

Time-On-Stream (h) Fig. 1. Ethanol conversion versus TOS for all HZSM-5 zeolites (WHSV = 15 h−1 , T = 623 K, P = 30 bar, molar ratio N2 /EtOH = 4).

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100

80

80

(140 )

60

(500 )

(16)

40

(40) 20

Diethylether yield (%)

Ethylene yield (%)

(b)

100

(a)

60

40

20

(50 0) (140)

0

0 0

5

10

15

20

25

30

Time-On-Strea m (h)

0

5

10

15

20

25

(16)

30

Time-On-Stream (h)

Fig. 2. Ethanol’s dehydration products yield: (a) ethylene and (b) diethylether versus TOS for all HZSM-5 zeolites (WHSV = 15 h−1 , T = 623 K, P = 30 bar, molar ratio N2 /EtOH = 4).

related to the low number of Brønsted acid sites initially present on HZSM-5 zeolite. 3.2.2. Evolution of products with TOS At the beginning of reaction (1 h), no ethylene or diethylether formation were observed on HZSM-5(16) and (40), therefore, the yield in C3+ hydrocarbons was 100% (Figs. 2 and 3). With TOS, ethylene yield increased progressively (Fig. 2a): it began to be detected after 6 h of TOS on HZSM-5(16), whereas on HZSM-5(40) it only appeared after 16 h (in a very small quantity: 2%). A decrease in C3+ yield was observed after 30 h of TOS (56% and 80% for respectively HZSM-5(16) and (40), see Fig. 3). However, HZSM-5(40) catalyst remained stable during the first 16 h. The diethylether yield was null for all the run (Fig. 2b). On HZSM-5(140), about 17% of ethylene was detected and no diethylether was formed at the beginning of the reaction (Fig. 2). Therefore, the C3+ hydrocarbons yield is of 83% (Fig. 3). With TOS, ethylene yield progressively increased to reach about 72% after 30 h (Fig. 2a). Diethylether appeared only after 30 h in a negligible amount (Fig. 2b). A significant decrease in C3+ yield was observed compared to HZSM-5(16) and (40) zeolites (Fig. 3).

C3+ hydrocarbons yield (%)

100 (40)

80 (16)

60

40

HZSM-5(500) zeolite behaved differently: initially, a large amount of ethylene was produced (65%), no diethylether was formed and only 35% of C3+ hydrocarbons were formed (Figs. 2 and 3). With TOS, ethylene and C3+ hydrocarbons yields strongly decreased while diethylether appeared after 2 h of reaction and its selectivity increased with TOS. For all catalysts (except HZSM-5(500)), ethylene yield increased with TOS to the detriment of C3+ hydrocarbons, which corresponded to the deactivation of the active sites for ethylene oligomerization. In fact, ethanol dehydration occurs on the weak Brønsted acid sites while strong acid sites allow ethylene oligomerization [39]. It is known that strong Brønsted acid sites are the first to be deactivated by coke deposition [40,41]. The deactivation was more pronounced for zeolites with a low number of Brønsted acid sites such as HZSM-5(500) and (140) while HZSM-5(40) showed the lower deactivation. The deactivation of all four catalysts was essentially due to coke formation. No dealumination of zeolitic framework has been observed and therefore does not explain the catalyst deactivation. The coke content after the end of the reaction (30 h) varied between 5.5% (for the less acidic HZSM-5(140) sample) and 9.1% (HZSM-5(16) zeolite), as reported in Table 2. From Fig. 2, one can confirm that there is a transformation of diethylether into ethylene in agreement with Takahara et al. [42]. In fact, a diminution of ethylene yield is observed, in parallel an increase in diethylether yield is showed. Ethylene can also be produced both directly from dehydration of ethanol or from diethylether. Furthermore, if we consider the concentration of H+ per unit cell of the different zeolites, our results are somewhat different from those reported by Chaudhuri et al. [23] who found that the formation of C3+ hydrocarbons only occurred on zeolites having more than one proton per unit cell. However, their catalytic experiments were carried out in less severe conditions at 573 K and at atmospheric pressure.

(140)

20 (500)

0 0

5

10

15

20

25

30

Time-On-Stream (h) Fig. 3. C3+ hydrocarbons yield versus TOS for all HZSM-5 zeolites (WHSV = 15 h−1 , T = 623 K, P = 30 bar, molar ratio N2 /EtOH = 4).

3.2.3. Effect of TOS on the distribution of C3+ hydrocarbons For a better understanding, C3+ hydrocarbons were divided into 3 fractions according to their carbon number: 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+ , and also according to their chemical structure: paraffins (P), olefins (O), naphthenes (N) and aromatics (A).

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175

Table 2 Physical–chemical characterizations of coked HZSM-5 zeolites at different TOS = 1 h, 16 h and 30 h. Zeolites (Si/Al)

TOS (h)

Acidity (␮mol g−1 )

Coke (%)

Pore volume (cm3 g−1 )

Brønsted

Lewis

Total

Micro.

Meso.

VR /VA

Ck /CAf

HZSM-5(16)

0 1 16 30

– 6.2 7.4 9.1

608 71 21 3

50 33 9 4

0.267 0.093 0.088 0.063

0.184 0.040 0.030 0.015

0.083 0.053 0.058 0.048

– 0.41 0.38 0.34

– 0.63 0.52 0.54

HZSM-5(40)

0 1 16 30

– 4.3 6.5 7.8

297 217 18 23

47 11 8 18

0.262 0.209 0.151 0.123

0.177 0.137 0.080 0.064

0.085 0.072 0.071 0.059

– 0.94 0.53 0.43

– 0.89 0.93 0.94

HZSM-5(140)

0 1 16 30

– 3.1 4.3 5.5

91 46 13 5

6 6 6 3

0.219 0.164 0.131 0.140

0.158 0.117 0.113 0.102

0.071 0.047 0.018 0.037

– 0.65 0.44 0.54

– 2.10 2.00 2.17

C 3-C 4 (a)

TOS = 1h

100

TOS = 16h

100

C 5-C 11 C 12+

80

Selectivity (%)

Selectivity (%)

80

60

40

20

60

40

20

0

0 (16)

(40)

(140)

(500)

(16)

(40)

(140)

(500)

HZSM-5 (Si/Al ratio) TOS = 1h

(b) 100

80

Selectivity (%)

Selectivity (%)

80

P O N A

TOS = 16h

100

60

40

20

60

40

20

0

0 (16)

(40)

(140)

(500)

(16)

(40)

(140)

(500)

HZSM-5 (Si/Al ratio) Fig. 4. C3+ hydrocarbons distribution into (a) fraction [C3 –C4 , C5 –C11 and C12+ ] and (b) chemical families [paraffins (P), olefins (O), naphthenes (N) and aromatics (A)] for TOS = 1 h and 16 h (except for HZSM-5(500), 2nd reaction time = 9 h), for all HZSM-5 zeolites (WHSV = 15 h−1 , T = 623 K, P = 30 bar, molar ratio N2 /EtOH = 4).

F. Ferreira Madeira et al. / Applied Catalysis A: General 443–444 (2012) 171–180

3.3. Characterizations of the spent catalysts 3.3.1. Measurements of residual acidity, microporosity and carbon content determination As it was shown earlier, despite its low acidity in comparison to HZSM-5(16), HZSM-5(40) had a good stability in C3+ hydrocarbons formation (Fig. 3). This stability could be explained by an optimal acid site density for limiting coke formation. In order to understand the catalytic results, experiments with HZSM-5(16), (40) and (140) zeolites with an equivalent number of Brønsted acid sites were carried out. The weight of each zeolite tested was adapted in order to have the same total number of Brønsted acid

100 Selectivity of C3+ hydrocarbons (%)

Fig. 4a shows the evolution of the fractions selectivity for the different zeolites for 1 h and 16 h of TOS (except for HZSM-5(500) for 9 h). Over most of the catalysts, whatever TOS, C5 –C11 fraction is predominant (55–70%) followed by C3 –C4 fraction and a very small quantity of C12+ fraction (only aromatic hydrocarbons). Nevertheless, over HZSM-5(500), because of its fast deactivation, after 9 h of reaction, the C3 –C4 fraction became more important than C5 –C11 fraction (40% and 60% for 1 h and 90% and 10% after 9 h). The C3 –C4 fraction was principally constituted by C4 (butenes) whereas the main products in C5 –C11 fraction were C5 , C6 (mostly alkanes) and C8 –C10 aromatics. It seems that TOS impacts on selectivity of different fractions. It is observed that for zeolites with a low Si/Al ratio (16 and 40) the amount of heavy (C5 –C11 ) hydrocarbons increases with TOS, while for zeolites with a high Si/Al (140 and 500) the tendency is the opposite. The distribution of C3+ hydrocarbons into paraffins, olefins, naphthenes and aromatics is represented in Fig. 4b for 1 h and 16 h of TOS (9 h for HZSM-5(500)). Initially (1 h), for all catalysts, there is an important formation of paraffins and aromatics. This is most probably due to hydrogen transfer (HT) reactions. The molar ratio paraffins/aromatics was always close to 3, which meant that three olefin molecules participated in this reaction leading to one aromatic molecule and three paraffin molecules. For the samples having low Si/Al ratios, which contained more than 1.7 H+ /unit cell (HZSM-5(16) and (40)), only a low amount of olefins still appeared meaning that on these zeolite samples, olefins continued to be transformed into aromatics (and/or coke molecules) and paraffins. On the contrary, on samples having high Si/Al ratios (HZSM-5(140) and (500)), the amount of olefins was more important and it sharply increased with Si/Al ratio. This confirmed that a high quantity of Brønsted acid sites is needed for hydrogen transfer reactions. As mentioned in the literature [23,43,44], the acid sites strength does not play an important role in HT reactions. After 16 h, apart from HZSM-5(40), there was a diminution of the percentage of paraffins and aromatics, while olefins increased. The behavior of HZSM-5(40) was different compared to the other zeolites. Indeed, the proportion of olefins was still very low probably due to a high density of acid sites which still permitted the HT reactions. On the contrary, the proportion of olefins was close to that of paraffins for HZSM5(16) and on the HZSM-5(140) and (500) samples, the proportion of olefins was higher than that of paraffins. This means that when the catalysts were deactivated or when they possessed less acid sites, olefin molecules were much less “consumed” by HT reactions. Note also that paraffins/aromatics ratios were always close to 3, meaning that most of the aromatic hydrocarbons are monoaromatics. The distribution of these monoaromatics was studied for HZSM5(16) and (40). Whatever TOS and the zeolite, C8 (A8 : ethylbenzene, xylenes) and C9 (A9 : trimethylbenzenes, methylethylbenzenes) aromatics were the main products followed by C7 (A7 : toluene). The proportion of the other aromatic hydrocarbons (A6 (benzene), A10–12+ ) was much lower and almost in the same quantity. These data are in agreement with those obtained by Anunziata et al. [45] and Kecskeméti et al. [46].

80

60

40

(40) (140)

20 (500)

(16)

0 0

5

10

15

20

25

Time-on-stream (h) Fig. 5. C3+ hydrocarbons selectivity versus TOS for all HZSM-5 zeolites containing the same amount of Brønsted acid sites per mass (30 ␮mol) (WHSV = 15 h−1 , T = 623 K, P = 30 bar, molar ratio N2 /EtOH = 4).

sites available for the reaction than HZSM-5(500) which possesses a low number of Brønsted acid sites. The operating conditions being kept similar to those previously mentioned. Experiments were undertaken with 30 ␮mol of Brønsted acid sites (corresponding to 0.05 g of HZSM-5(16), 0.1 g of HZSM-5(40) and 0.3 g of HZSM5(140)). As shown in Fig. 5, which represents the selectivity in C3+ hydrocarbons as a function of TOS, HZSM-5(40) and HZSM-5(140) progressively were deactivated while HZSM-5(16) was initially and quickly deactivated (as observed on HZSM-5(500)). Two different rates of deactivation were also observed: a slow acid sites poisoning and/or pore blockage by coke molecules for HZSM-5(40) and (140) while deactivation is faster for HZSM-5(16). Coking was also studied as a function of TOS on these three zeolites (Fig. 6). Coking kinetics were similar: the carbon content increased rapidly during the first hour of reaction but slowing down afterwards. As expected, carbon content was related to initial acidity of catalyst. Indeed, carbon content was more important for HZSM-5(16). Acidity and porosity of these coked samples are given in Table 2. A great loss of acidity and porosity was observed with TOS. This is due to an acid site poisoning and a pore blockage by coke molecules [47]. At the beginning of reaction (1 h), the loss of Brønsted acidity and microporosity is lower on HZSM-5(40) and more pronounced 10 (16)

8 Carbon content (%)

176

(40) (140)

6

4

2

0 0

5

10

15

20

25

30

Time-On-Stream (h) Fig. 6. Total carbon content versus TOS for HZSM-5(16), (40) and (140) zeolites.

F. Ferreira Madeira et al. / Applied Catalysis A: General 443–444 (2012) 171–180

177

a

a

1510

2964 2934

Absorbance

Absorbance

(140)

(40)

3100

3050

1386 1372

1605

2873

(16)

1465

1575 (16) (40)

(140)

3000

2950

2900

2850

2800

2750

2700

1700

1650

1600

-1

1550

1500

1450

1400

1350

1300

-1

Wavenumbers (cm )

Wavenumbers (cm )

b

b 2964

2873

1463 1456

Absorbance

Absorbance

(16)

1386 1372

1620 1608 1571

2934

(40) (140)

(16) (40) (140)

3100

3050

3000

2950

2900

2850

2800

2750

2700

-1

Wavenumbers (cm ) Fig. 7. IR spectra in the region of 2700–3100 cm−1 for HZSM-5(16), (40) and (140) zeolites coked after (a) 1 h and (b) 30 h.

on HZSM-5(16). Indeed, the loss of Brønsted acidity is respectively of 88.3%, 26.9% and 49.5% for HZSM-5(16), (40) and (140). Concerning the loss of microporosity, it is estimated at 78.3%, 22.6% and 25.6% for HZSM-5(16), (40) and (140). As expected, the loss of Brønsted acidity and microporosity increased with TOS. After 30 h of TOS, the loss of Brønsted acidity is more important with 99.5%, 92.2% and 94.5% against a loss of microporosity of 91.8%, 63.8% and 35.4% for respectively HZSM-5(16), (40) and (140). We can assume that HZSM-5(16) which possessed a great acid sites number led to rapid formation of voluminous coke molecules, causing a more important acid site poisoning and a pore blockage. However, for samples HZSM-5(40) and (140), deactivation probably occurred by poisoning site by site and progressive blockage of microporous volume, conducting to a slower deactivation.

3.3.2. Determination of coke nature by IR and GC–MS analyses An extensive study on coked catalysts has been performed to determine the coke nature: first by IR spectroscopy and then by GC–MS coupling (HF solubilization and CH2 Cl2 extraction). The HZSM-5(16), (40) and (140) samples that were coked after 30 h, were characterized by FTIR spectroscopy. Figs. 7 and 8 show IR vibration bands obtained respectively in the region of 2700–3100 cm−1 and of 1300–1700 cm−1 .

1700

1650

1600

1550

1500

1450

1400

1350

1300

-1

Wavenumbers (cm ) Fig. 8. IR spectra in the region of 1300–1700 cm−1 for HZSM-5(16), (40) and (140) zeolites coked after (a) 1 h and (b) 30 h.

The region of 2700–3100 cm−1 (Fig. 7) corresponded to paraffinic species: 2964 cm−1 for as [CH3 ], 2934 cm−1 for as [CH2 ], and 2873 cm−1 for s [CH3 ]. Coke was constituted of highly alkyl-substituted aromatic molecules. After 1 h of TOS (Fig. 7a), the vibration bands intensity is lower for HZSM-5(40). We can notice that vibration bands intensity increased with TOS (from 1 h to 30 h), as observed for HZSM-5(40) and (140) zeolites (Fig. 7a and b). However, for HZSM-5(16), the spectra obtained from 1 h to 30 h were practically the same. This could be explained by a rapid and initial alkylation reactions of the side chain due to its high acidity. For all coked samples after 30 h (Fig. 7b), in this region, bands intensity was almost similar. In the region of 1300–1700 cm−1 (Fig. 8), bands vibrating around 1500–1650 cm−1 generally corresponded to aromatic rings and those around 1350–1480 cm−1 to alkyl groups of aromatics. 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. A band at 1510 cm−1 is observed after 1 h of TOS (Fig. 8a) but no bands are observed after 30 h of TOS (Fig. 8b) in agreement with the coke structure (Table 3). As expected, the vibration bands intensity was more important for HZSM-5(16) compared to HZSM-5(40) and HZSM-5(140). In addition, for all zeolites, an increase of TOS (from 1 h to 30 h) is accompanied by an increase of the intensity of the bands at

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Table 3 Coke nature determined for coked HZSM-5(40) zeolite after 1 h and 30 h of TOS by GC–MS coupling (HF solubilization and CH2 Cl2 extraction). Time-on-stream (h)

Coke structure

1

30

about 1600 and 1300 cm−1 (Fig. 8a and b), which is in agreement with the increase of the carbon content trapped in the zeolite pores. Moreover, the nature of coke molecules changed with TOS: aromatic rings of coke molecules became more condensed. For the coked sample HZSM-5(16), the spectrum obtained after 1 h of TOS, showed that coking was more rapid compared to the two other zeolites, confirming results previously observed. The coke extracted from coked samples was analyzed by GC–MS coupling. For the higher TOS (16 h and 30 h), a very small amount of occluded material could not be retrieved. This material was attributed to graphitic carbon. Table 3 shows the soluble coke nature obtained on HZSM-5(40) zeolite for TOS = 1 h and TOS = 30 h. The compounds obtained after 1 h corresponded mostly to highly alkyl-substituted monoaromatics. These compounds were still detected after 30 h of TOS and also on-line by GC analysis during our catalytic tests. After 30 h, more condensed aromatics and highly alkyl-substituted were observed. These compounds are very bulky and probably responsible for the pore blockage and the acid sites poisoning. Furthermore, the nature of coke molecules changed with TOS: the molecules became more condensed, as also indicated by IR spectroscopy. The results obtained by coke extraction (GC–MS) are correlated with those obtained by IR spectroscopy. We can now conclude that coke is composed of highly alkyl-substituted aromatics (mono or poly) and the coke nature changed with TOS. 3.3.3. EPR analysis of coked samples Despite a great loss of Brønsted acidity and microporosity and carbon deposition, HZSM-5(16) and (40) catalysts continued to show good yield in C3+ hydrocarbons. In order to explain this maintained yield with such degraded acidity and textural properties, investigations were carried out. In a previous study [22], the presence of radical species among the carbon deposit compounds was evidenced. It is now our goal to investigate the presence or absence of these species from the reaction start and their quantification to establish a possible correlation between their existence and the catalytic results. The reactivity of these species could indeed explain

the catalytic results obtained. The fresh and coked samples (HZSM5(16), (40) and (140)) were analyzed by the CW-EPR technique. The spectra obtained at different TOS in the case of HZSM-5(40) are shown in Fig. 9a. No paramagnetic signal was detected for the fresh sample (which is used as a reference), and therefore, all EPR signals detected on the coked samples are due to the existence of paramagnetic species present in the carbon deposit. The existence of a well-defined signal at TOS = 1 h means that the formation of radical species occurred from the reaction beginning. An increase in the signal intensity was observed between 1 h and 16 h of TOS, which indicated an increase in the spin concentration (hence in radical species). When comparing the signals at 16 h and 30 h, the signal shape changed slightly, becoming thinner. In fact, the line width varied with TOS between 0.89 and 0.56 mT, indicating a change in the radical species [22]. All these signals consisted in a single Lorentzian line shape centered at a g factor of 2.007. The evolution of radical species concentration with TOS is represented in Fig. 9b for the three coked solids. The amount of radical species is almost constant with TOS for zeolite (140), it increases continuously for zeolite (40) (although more noticeable up to 16 h and then more slowly), and for zeolite (16) the amount of radical species passes through a maximum at 16 h (as a result of a very pronounced increase between 1 and 16 h, followed by a decrease period from 16 to 30 h). The new species formed after longer reaction times are probably more bulky and more stable, therefore we can imagine that they are also less reactive and can evolve to become large coke molecules with a low concentration of radicals. These molecules are probably blocked inside the pore contributing to the increase of carbon content with TOS. Despite the presence of voluminous coke molecules, the C3+ hydrocarbons yield remained important: 23% on HZSM-5(140), 56% on HZSM-5(16) and 80% on HZSM-5(40). These results seemed to support the hypothesis previously mentioned [22] of a possible relation between the radical species and the catalytic results (namely, the C3+ hydrocarbons formation). The mechanism of ethanol transformation probably

F. Ferreira Madeira et al. / Applied Catalysis A: General 443–444 (2012) 171–180

fresh - reference TOS = 1 h TOS = 16 h TOS = 30 h

a

342

344

346

348

350

352

354

356

Magnetic field (mT)

4,0 3,5

-1

Amount of radical species (µmol.g catalyst)

b

3,0

(40 )

2,5

(16 ) 2,0 1,5

(140 )

1,0 0,5 0,0 0

5

10

15

20

25

30

Time-On-Stream (h) Fig. 9. CW-EPR analysis: (a) EPR spectra for HZSM-5(40) zeolite versus TOS and (b) amount of radical species (obtained from spin concentration) versus TOS for the HZSM-5(16), (40) and (140) zeolites.

involved two types of active sites. The first sites are Brønsted acid sites, that are active when the carbon content is low and are responsible for the formation of radical species. These species became active sites even in presence of high carbon content. A radical mechanism is supposed and a possible reaction pathway is showed below: Coke◦ (orR ◦ ) + C2 H4 → R C C◦ R C C◦ + C2 H4 → R C C C C◦ R C C C C◦ + C2 H4 → R C6 ◦ R C6 ◦ → R ◦ + C6 (olefins) However, HZSM-5(40) is the most selective in C3+ hydrocarbons probably due to an optimum between the number of Brønsted acid sites and the amount of radical species. 4. Discussion This study showed the different catalytic behavior in ethanol transformation of HZSM-5 zeolites (Si/Al ranging from 16 to

179

500) at 623 K and a total pressure of 30 bar. The catalyst HZSM5(40) was found to be more efficient. Indeed, catalytic results showed clearly that this catalyst is more stable and selective in C3+ hydrocarbons than its counterparts. For a better understanding, an extensive characterization of coked catalysts was undertaken. Firstly, adsorption measurements indicated that ethanol conversion is accompanied by a great loss of Brønsted acidity and microporosity. This loss is more important and faster on HZSM5(16) zeolite compared to HZSM-5(40) and (140). This is caused by the deposition of heavy secondary products generally designated as coke which is responsible for the poisoning of the active sites and the blockage of their access. The carbon content after reaction was also very high from the beginning of reaction and more pronounced for HZSM-5(16) catalyst. As mentioned by Guisnet et al. [40], the coking rate is favored by strength and acid site density of zeolite. In fact, one of the main problems with the use of acid zeolite catalysts is their deactivation by coking [48,49]. However, all catalysts remain active for C3+ hydrocarbons formation despite a large amount of carbonaceous products (coke) is retained in the zeolite. Guisnet [50] demonstrated that coke may participate as active species in various acidic reactions. The catalytic action of coke molecules is relatively general because it is observed for reactions carried out in gas as well as in liquid phase and with small, medium or large pore zeolites. Thus, we can assume that the existence of coke molecules could explain the high catalytic performances maintained despite the dramatic loss in Brønsted acidity and microporosity. The nature of these species was also undertaken by IR spectroscopy and GC–MS. The results obtained by IR are in agreement with GC–MS results with the observation of highly alkyl-substituted aromatics. Moreover, the nature of coke molecules is similar for all catalysts and changed with reaction time: aromatic rings of coke molecules became more condensed. The deactivation increased with TOS and depended strongly of the solid. A slower deactivation only occurred after 16 h of TOS for HZSM-5(40) whereas on HZSM-5(16) it appeared after 6 h. In a previous study [22], we have shown the existence of radical species among the carbonaceous deposits. For all catalysts, we have confirmed by CW-EPR that coke is composed of radical species. By combining the different results, the amount of radical species, as well as their chemical nature, an attempt to correlate these informations with the evolution of the catalytic results was made. As previously mentioned [22], a change in the chemical nature of radical species could explain the start of catalysts deactivation. At first, the radical species are active but the new radical species formed after a longer reaction time became more bulky and stable, and therefore less active, which can explain the deactivation observed for all catalysts. Nevertheless, this deactivation rate depended on the zeolite and its Brønsted acidity. It is probable that the mechanism for ethanol transformation involved two types of active sites. The first sites are Brønsted acid sites that are active when the carbon content is low and are responsible for the formation of radical species. These species became active sites even in presence of high carbon content. However, an optimum between the number of Brønsted acid sites and the quantity of radicals is required to maintain a high activity in C3+ hydrocarbons. The zeolite deactivation by coking has been extensively studied by Guisnet et al. [40,51]. Some information on the location and deactivating effect of coke molecules can be proposed by associating to the knowledge of coke composition, data originating from activity and adsorption measurements at different values of TOS, hence of coke content. On ZSM-5 zeolites, Guisnet et al. [40] indicated that deactivation is caused at a low coke content by a site poisoning progressively evolves to a pore blockage with the increase in coke content. From the amount and coke composition,

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the volume really occupied by the coke molecules (VR ) and the concentration of coke molecules (Ck ) can be determined. VR was compared with the volume apparently occupied (VA ) measured by nitrogen adsorption. Ck /CAf represents the ratio between the concentration of coke molecules (Ck ) and Brønsted acid sites of the fresh zeolite (CAf ). Both ratios VR /VA and Ck /CAf versus TOS are summarized in Table 2. For all catalysts, the ratio VR /VA decreased with TOS and carbon content. The high value (0.94) observed on HZSM5(40) after 1 h of TOS indicates that there is a very low pore blockage by coke molecules compared to HZSM-5(16) and (140). With TOS, VR /VA is lower than 1 which indicates a blockage of the access of adsorbate molecules (nitrogen) to part of the porosity unoccupied by coke molecules. This can be explained by considering that coke molecules which are trapped at the channel intersections of the zeolite completely block their access, without occupying the totality of the volume. The values of Ck /CAf ratio showed that the concentration of coke molecules per Brønsted acid site of the fresh zeolite is more important for a zeolite having a low Brønsted acidity such as HZSM-5(140). Whatever the zeolite, it seems that Brønsted acid sites and radicals are active sites for ethylene oligomerization into C3+ hydrocarbons. The evolution of radicals nature with TOS in less active compounds provoked a deactivation for all catalysts which did not occur at the same TOS. HZSM-5(40) zeolite is the most stable and selective catalyst with 80% in C3+ yield after 30 h of TOS. An optimum between the number of Brønsted acid sites and the quantity of radicals permits to justify the catalytic results. 5. Conclusions A series of four HZSM-5 zeolites with Si/Al ratios ranging from 16 to 500 was studied for ethanol transformation into C3+ hydrocarbons. An extensive characterization of coked zeolites was undertaken by a combination of nitrogen adsorption, pyridine adsorption followed by IR spectroscopy, GC–MS coupling and EPR spectroscopy in order to obtain more information about the amount and the nature of the coke molecules as well as their evolution versus TOS. Under our operating conditions, ethanol conversion is accompanied by a great loss of acidity and microporosity due to coke formation which is responsible for the poisoning of the active sites and the blockage of their access. The coke nature was determined by IR spectroscopy and GC–MS coupling. A change of the species nature with TOS was evidenced: highly alkyl-substituted aromatics becoming more condensed (polyaromatics). All coked samples presented an EPR signal indicating the existence of radical species among the carbonaceous deposits. Deactivation observed for all catalysts conducted in a decrease of the C3+ hydrocarbons yield thus resulting in an increase in the ethylene yield. This deactivation could be due to the change of the species nature: the new radical species formed after a longer reaction time, are probably more stable and therefore less reactive. All results seem to indicate a correlation between the formation of C3+ hydrocarbons, the existence of radical species and the Brønsted acidity. 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. HZSM-5(40) is found to be the most selective and stable catalyst which is due to an optimum balance between the number of Brønsted acid sites and the amount of radicals.

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