Al2O3 catalyst using mesoporous alumina prepared with a cationic surfactant

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Synthesis of Pt/Al2 O3 catalyst using mesoporous alumina prepared with a cationic surfactant J.L. Contreras a,∗ , G. Gómez a , B. Zeifert b , J. Salmones b , T. Vázquez b , G.A. Fuentes c , ˜ a J. Navarrete d , L. Nuno a

Universidad Autónoma Metropolitana-Azcapotzalco, Av. Sn. Pablo 180, Col. Reynosa, México City P.C. 02200, Mexico Instituto Politécnico Nacional, ESIQIE, U.P. López Mateos Zacatenco, México City, Mexico c Universidad Autónoma Metropolitana-Iztapalapa, CBI-IPH, Mexico City, Mexico d Instituto Mexicano del Petróleo Eje Central Lázaro Cárdenas 152, México City, Mexico b

a r t i c l e

i n f o

Article history: Received 29 June 2014 Received in revised form 9 September 2014 Accepted 7 October 2014 Available online xxx Keywords: Mesoporous Al2 O3 Pt/Al2 O3 Sulfur, cetyl trimethyl ammonium bromide n-Heptane Hydroconversion

a b s t r a c t Pt on alumina catalysts were studied when the texture of the alumina change by addition of a cationic surfactant cetyl-trimethyl ammonium bromide (CTAB). The mesoporous alumina was prepared using CTAB from 0.01 to 0.1 M. The surface area and pore volume increased as CTAB concentration increased passing through a maximum. After calcination at 300 ◦ C, and 550 ◦ C boehmite and ␥-Al2 O3 were found by XRD. After calcination at 800 ◦ C, the transition between ␥- and ␪-Al2 O3 was observed together with an increase of pore volume and pore diameter respect to the alumina without surfactant. The alumina prepared with the lowest concentration of CTAB showed the highest thermal stability. Brønsted and Lewis acidities were found on alumina samples having CTAB concentrations from 0 to 0.075 M. The sample of alumina with the highest CTAB concentration showed the highest population of pentahedral aluminum (AlV ), Lewis acidity and a dense network of wormlike particles by SEM. After H2 reduction of these catalysts, sulfur that came from the alumina support weakened the function of Pt particles to hydrogenolysis reaction whereas the CTAB introduced Brønsted acid sites in the alumina increasing the isomerization reaction. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The scientific advance in the design and the preparation of heterogeneous catalysts has benefitted with the studies in mesoporous materials coprecipitated with cationic surfactants in either SiO2 , Al2 O3 or other oxides as TiO2 and ZrO2 [1]. Mesoporous aluminas have been synthesized using different routes among which we mention three: cationic [2–5], anionic [1,6,7] and non-ionic [8,9] route. In these preparations, the surfactant acts as template agent in aqueous or organic solvents using Al+3 , (Al13 O4 (OH)24 (H2 O)12 )7+ (Keggin cation) or Al-alkoxides and some examples have been cited in the reviews of Cejka [10] and Márquez-Alvarez et al. [11]. Some authors have used organic molecules such as tetra ethylene glycol [12], glucose [13] or even ionic liquids such as 1-methyl-3octylimidazolium chloride have shown to self-assemble with the aluminum moieties, acting as effective templates, which lead to mesoporous aluminas.

∗ Corresponding author. Tel.: +52 5553189065x; fax: +52 5553947378. E-mail address: [email protected] (J.L. Contreras).

The synthesis of mesoporous aluminas with cationic surfactants is not so simple than the synthesis of alumina by coprecipitation without surfactant. After several years of investigation, it has been possible to mature in this area since achievements have been documented in the control of pore diameter, as well as the formation of different active sites on its surface prepared for different catalytic reactions. Supporting this idea in a similar form to what happens with the mesoporous silicates of MCM-41 and aluminosilicates [14], synthesis of organized mesoporous aluminas using alkyltrimethylammonium surfactants could be possible. For example, some authors have prepared aluminas using several types of surfactants: alkyl trimethyl ammonium bromide, octyl trimethylammonium, tetradecyl trimethyl ammonium and cetyl trimethyl ammonium. In the first case, Acosta et al. [3] published the first report on the synthesis of aluminum hydroxide mesophases with the cationic surfactants: octyl [C8 H17 (CH3 )3 N+ Br− ] and tetradecyl trimethyl ammonium bromides [C14 H29 (CH3 )3 N+ Br− ] in aqueous solution. They used AlCl3 as a precursor, and the precipitation was controlled by the addition of urea and a pH close to 7. The authors made a previous neutralization with NH4 OH before the precipitation at an [NH3 ]/Al ratio of 1. This

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ratio was the upper limit to avoid the precipitation of Al hydroxide. The alumina obtained showed poor crystallinity when the sample was heated to remove the surfactant, in N2 flow at 450 ◦ C followed by calcination in air at the same temperature. Apparently, the octyl trimethyl ammonium was not a good structure-directing agent, probably because the molecule has a short hydrophobic carbon chain. In the case of tetradecyl trimethyl ammonium surfactant, the alumina showed an increase in the surface area and the pore volume [3]. The substitution of the octyl by the tetradecyl type surfactant induces an important modification of the porous texture. The porosity doubles, the average equivalent pore diameter increases to 5.5 nm and a strong broadening of the pore size distribution is observed. The total templating effect on porosity is strongly dependent on the surfactant used. In other publications, Cabrera et al. [2,15] have prepared mesoporous aluminas using aqueous solutions of alkyl trimethyl ammonium surfactants and atrane complexes as precursors. They used aminotrialcoxo complexes obtained by the transesterification of a metal alkoxide with triethanolamine aiming to slow down the hydrolysis of the precursor. Their aluminas prepared with an aqueous solution of cetyl trimethyl ammonium bromide (CTAB) as the surfactant exhibited a wormhole-like porous structure after calcination at 500 ◦ C. The water/triethanolamine ratio was found to affect both surface area and pore size. Their results suggested that the porosity was originated by the agglomeration of small alumina crystallites. Complexation of the aluminum precursor with triethanolamine is probably limiting the growth of those crystallites. The synthesis of mesoporous alumina in the aqueous medium is not yet well understood, compared with the synthesis made in other solvents such as alcohols. To obtain mesoporous alumina Kim et al. [16] studied three different surfactants (CH3 (CH2 )n−1 N(CH3 )3 Br, (n = 12, 14 and 16) using a precursor of tri-sec-butoxide of aluminum in 1-butanol under hydrothermal conditions without additives. They obtained a mesoporous alumina synthesized in a much simple manner. The pore structure showed a wormhole-like, high area, thermal stability and different sites of Al coordination. In another study, Kim et al. [17] prepared mesoporous alumina using alkyl carboxylates [CH3 (CH2 )nCOOH]; (n = 4, 10 and 16) as chemical template in sec-butanol as parent alcohol and isooctane as co-solvent. In another solution, the Al precursor was aluminum sec-butoxide. These two solutions were mixed, and small amount of H2 O was added into the mixture. The authors found that the carbon length of the template, the molar concentration of H2 O or co-solvent and calcination condition controlled the pore properties of alumina. In order to study the effect of the solvent, Ray et al. [18] modified the synthesis procedure used by Kim et al. [17] and showed that the change of solvent 1-butanol for 2-butanol led to an increased area in the samples calcined at 500 ◦ C, from 401 to 475 m2 /g. Furthermore, either cationic or non-ionic surfactants produced thermally stable mesoporous alumina after calcination at 500 ◦ C, when the alumina was synthesized with organic solvents. The influence of the thermal treatment over the textural properties of a sol–gel mesoporous ␥-Al2 O3 synthesized in acid medium using cationic surfactants (e.g. hexadecyl trimethyl ammonium chloride/bromide) has been investigated [19]. Among the different heating procedures tested, an isothermal step at 150 ◦ C under flowing N2 , led towards the most remarkable results (mesoporous ␥-Al2 O3 with BET area ∼690 m2 /g). Different alkyl trimethyl ammonium bromide surfactants (alkyl ∼ C10 –C18 ) were used for the preparation of these aluminas, also leading towards meaning better materials under this thermal treatment (570–700 m2 /g) with the only exception of the alumina prepared with C18 alkyl chain surfactant, due to the appearance of a lamellar phase.

In some synthesis of mesoporous alumina made by Gonzalez˜ et al. [20] have explored the effect of the addition of chemical Pena modifiers of aluminum sec-butoxide (as triethanolamine and ethyl acetoacetate) in the path of preparations with non-ionic surfactants. The chemical modification of the Al sec-butoxide promotes the formation of penta and tetracoordinate aluminum and modifies the alumina pore structure. High intrinsic activity was reached with the catalysts prepared by this alumina compared to those prepared from the unmodified alkoxide. The present investigation is a contribution to improving the knowledge about the synthesis of mesoporous alumina using the cationic surfactant cetyl trimethyl ammonium bromide (CTAB). We emphasized the relationship between the surfactant concentration about textural properties, the Lewis and Brønsted acidity, the population of tetrahedral, pentahedral and octahedral aluminum respectively (AlIV , AlV , AlVI ), the morphology by scanning electron microscopy (SEM), and the catalytic activity using n-heptane hydroconversion when Pt particles are supported on different aluminas prepared.

2. Experimental Mesoporous aluminas were synthesized by the precipitation method using Al2 (SO4 )3 and (NH4 )OH and the cationic surfactant cetyl-trimethyl ammonium bromide (CTAB) or (CH3 (CH2 )15 (CH3 )3 NBr). The concentration of CTAB changed from zero to 0.1 M. The H2 O/Al (mole/atom) ratio was 213 (Table 1). Initially, an aqueous solution of Al2 (SO4 )3 (J.T. Baker) with the cationic surfactant CTAB (Sigma-Aldrich) was mixed, agitated and heated during 24 h at 60 ◦ C. This solution was mixed with a solution of NH4 OH (J.T. Baker) dropwise at room temperature until a pH of 8, slowly the boehmite gel was produced. This boehmite gel was aged during 24 h at 25 ◦ C, and after this step the solid was filtered and washed with enough distilled water to remove the (NH4 )2 SO4 and the residual CTAB until no foam was observed. In order to remove the surfactant and not produce large thermal gradients, the samples were dried at 110 ◦ C for 24 h and heated under N2 flow (2 l/h) with a heating rate of 2 ◦ C/min at 300 ◦ C during 1 h. Then N2 was replaced by air flow (1.8 l/h) and the air calcination began using a heating rate of 2 ◦ C/min until a temperature of 550 ◦ C was attained and maintained for 4 h. This last step was intended to remove the surfactant completely and to form ␥-Al2 O3 . The aluminas calcined at 550 ◦ C were impregnated with an aqueous solution of H2 PtCl6 (Aldrich) to get a constant Pt concentration of 0.5 wt% Pt. These samples were dried at 110 ◦ C for 24 h, calcined at 550 ◦ C for 2 h and reduced in H2 flow (Infra-Air products) (1.8 l/h) at 550 ◦ C for 2 h. In order to know the thermal stability after a treatment of high severity, the samples were calcined at 800 ◦ C during a long period (20 h) in a muffle. Measurements of N2 adsorption-desorption isotherms at −196 ◦ C (77 K) were carried out in ASAP-2000 (Micromeritics) equipment. The samples were previously outgassed at 300 ◦ C during 3 h under vacuum of 1 × 10−3 Torr. The surface area values were obtained by application of the BET equation. The micropore volume of the samples was calculated by means of the t-plot method. The calculation of the pore size was carried out by applying the BJH model to desorption branch of the isotherm, assuming cylindrical pore geometry. The X-ray diffraction (XRD) of the samples calcined at 300, 550 ´ and 800 ◦ C were obtained in a Rigaku diffractometer (Phillips Xpert) fitted with a Cu anode tube (30 kV, 20 mA), using CuK␣ radiation. XRD patterns at higher angles (2 ∼ 10–100◦ ) were taken with a step size of 0.02◦ and a counting time of 2 s. The identification of

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Table 1 Name and preparation conditions of aluminas synthesized with the surfactant CTAB. The H2 O/Al (mole/atom ratio) was 213. Name

CTAB (Molar concentration)

CTAB/Al ratioa

Aging time (h)

Heating in N2 at 300 ◦ C (h)

Calcination in air at 550 ◦ C (h)

C1-0 C1-0.01 C1-0.05 C1-0.075 C1-0.10

0 0.01 0.05 0.075 0.10

0 0.03 0.19 0.28 0.38

24 24 24 24 24

1.1 1.0 1.1 1.0 1.0

4.0 4.1 4.2 4.1 4.1

the different crystalline phases was performed by comparison with the corresponding JCPDS diffraction data cards. The thermal gravimetric analysis (TGA) was made using an SDTQ-600 equipment, and the differential thermal analysis (DTA) was made in a Perkin-Elmer 1700 instrument, using alumina as reference. The experiments were made in air flow of 20 cm3 /min with a heating rate of 10 ◦ C/min. The dried samples were heated at a heating rate of 10 ◦ C/min from 25 to 1000 ◦ C in air flow of 20 cm3 /min, and the loss of weight was measured in function of temperature. The DTA was made as a function of temperature. The infrared spectra of calcined samples were obtained in an infrared spectrometer (Spectra-220) to investigate the presence of surfactant and functional groups on alumina. For the spectroscopic examination, each sample was pulverized with KBr in a mortar and pressed up to 700 kg/cm2 to obtain a transparent disk in order to be placed in the infrared beam path of the instrument. The surface acidity of the samples was measured by infrared spectroscopy of pyridine previously adsorbed, using an infrared spectroscope FTIR (Nicolet Model 170-SX). Each sample in pressed powder was put in a special glass cell in which the specimen temperature and the vacuum could be controlled. Pretreatment of the samples prior to the adsorption of pyridine consisted in outgassing (1 × 10−3 Torr) followed by heating to 500 ◦ C at 20 ◦ C/min and cooling to room temperature. After the pretreatment, the samples were exposed to a mixture of pyridine (4%) in N2 during 15 min. for the chemisorption process at 25 ◦ C. The pyridine physically adsorbed was removed by vacuum and the infrared spectra were obtained “in situ” at temperatures from 25 to 400 ◦ C. The Lewis and Brønsted acid sites were identified and calculated [21]. 27 Al NMR spectra of calcined aluminas were recorded in a Bruker 400 (Avance MSL) spectrometer for solids operating at 100 MHz and 27 ◦ C. Scan conditions were 0.35 ␮s pulse with a delay time of 0.5 s. The sample spinning rate was 11 kHz. The unresolved 27 Al NMR spectra were deconvoluted in three peaks by using a standard Lorentzian function. Temperature programmed reduction (TPR) profiles of the calcined Pt/Al2 O3 samples were obtained under H2 flow (10% H2 in Ar) by using a commercial thermodesorption apparatus multipulse RIG model (from ISRI) equipped with a thermal conductivity detector (TCD). Samples of 30 mg and a gas flow rate of 25 cm3 /min were used in the experiments. The TPR profiles were registered by heating 30 mg of sample from 25 to 1000 ◦ C at a rate of 10 ◦ C/min, and the rate of H2 consumption was monitored by a TCD. The amount of H2 consumed was obtained by the deconvolution and integration of the TPR peaks using the Peak Fit program. The calibration was done by measuring the change in weight due to a reduction in H2 of 2 mg of CuO using an electrobalance Cahn-RG. The TPR signal of CuO was made and correlated with the stoichiometric H2 consumption. Chemisorption measurements of H2 were performed using a conventional volumetric glass apparatus (base pressure 1 × 10−5 Torr). The amount of chemisorbed hydrogen was determined from adsorption isotherms measured at room temperature (RT). In a typical experiment, the catalysts (0.5 g) were re-reduced in H2 at 500 ◦ C for 1 h, next evacuated at the same temperature for 2 h and cooled down under vacuum to RT. After that, the

first adsorption isotherm was measured. The catalyst was then evacuated to 1 × 10−5 Torr for 30 min at RT to remove physisorbed species and back-sorption isotherm. The linear parts of the isotherms were extrapolated to zero pressure. The subtraction of the two isotherms gave the amount of hydrogen strongly chemisorbed on metal particles. These values were then used to calculate the Pt dispersion (H/Pt ratio). In preliminary experiments, it was found that chemisorptions of hydrogen on Al2 O3 support was negligible at RT. The uncertainty of the reported uptakes was ±0.45 ␮mol H2 /g cat. The images of scanning electron microscopy (SEM) with field emission and high resolution were taken on a Jeol microscope (model JFM 6701 F) using secondary electrons. The qualitative and quantitative chemical analyzes were obtained by coupling the energy dispersive spectroscopy of X-ray (EDS) to microscope. For the observation, each sample was spread on a graphite tape and to make it conductive, Pd and Au atoms were deposited. Using the same microscope, the Pt atoms distribution in selected regions was analyzed and the chemical composition of these regions was analyzed, by energy dispersive X-ray spectrometry (EDS). The Pt/Al2 O3 catalysts were evaluated in a conventional fixed bed microreactor, 0.1 g of catalyst with a particle size of 0.147 mm (100-Tyler mesh) was evaluated. All the catalysts were tested in the n-heptane hydroconversion, in an equipment operated continuously at an absolute pressure of 0.8 atm using H2 as a carrier gas (1 cm3 /s), connected on-line with a chromatograph (Varian CP3380) utilizing a capillary column of methyl silicon-gum of 30 m. Then n-heptane (Tecsiquim S.A.) was vaporized in a glass saturator at 25 ◦ C and mixed with H2 (99.9% Infra-Air Products), the H2 /nheptane molar ratio was 15.8. In order to prevent the presence of diffusional effects during the evaluation of the catalysts, the best flow conditions and catalyst particle size were determined. The effluent from the reactor was analyzed through a sampling valve. The catalysts were previously activated in H2 flow (1.8 l/h) at 550 ◦ C during 2 h. The reaction temperature was controlled from 350 to 500 ◦ C, and a space velocity WHSV of 4.8 h−1 was used. Products from the reactions of hydrogenolysis, hydrocracking, isomerization and dehydrocyclization were analyzed by gas chromatography as the reaction temperature increased. Both conversion and selectivity for each catalyst and reaction temperature were determined. 3. Results and discussion 3.1. Textural properties The samples exhibited a type IV isotherm with an H2 hysteresis loop (Fig. 1a) which is characteristic of mesoporous solids [22] having bottleneck pores. The alumina without surfactant C1-0 (Fig. 1a) showed a loop shape attributed to slit-shaped pores with parallel walls, wide pores with narrow, short openings. The hysteresis loop shape of the aluminas with surfactant C1-0.01, C1-0.05 and C1-0.075 (Figs. 1b–d) were attributed to spherical cavities of varying radii but constant neck width, for example, ink-bottle pores, voids between close-packed spherical particles [23]. The sample C1-0.1, prepared with the high concentration of CTAB (Fig. 1e) showed a hysteresis loop similar with the loop of the

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Fig. 1. (a) N2 adsorption isotherm at −196 ◦ C and pore size distribution (upper inset) of C1-0 sample calcined at 550 ◦ C; (b) N2 adsorption isotherm at −196 ◦ C and pore size distribution (upper inset) of C1-0.01sample calcined at 550 ◦ C; (c) N2 adsorption isotherm at −196 ◦ C and pore size distribution (upper inset) of C1-0.05 sample calcined at 550 ◦ C; (d) N2 adsorption isotherm at −196 ◦ C and pore size distribution (upper inset) of C1-0.075 sample calcined at 550 ◦ C; (e) N2 adsorption isotherm at −196 ◦ C and pore size distribution (upper inset) of C1-0.1 sample calcined at 550 ◦ C.

alumina prepared without CTAB (sample C1-0). These isotherms were similar with those found by Aguado et al. [24] and Ray et al. [18] who also used CTAB in the preparation of their aluminas. The calculated BJH mesopore diameters (upper inset in Fig. 1a–e) showed different pore distributions as the concentration of the CTAB increased. The presence of CTAB during the synthesis of these samples produced a single pore size distribution, unlike the two distributions observed in the alumina without CTAB. This CTAB effect has already been observed in other studies [11]. For these aluminas, it was apparent that an increase in the surfactant concentration produced a decrease in the pore diameter (Table 2). This effect is shown in Fig. 1b–e, where the

maximum in the pore diameter (Dp) of the each alumina increases ˚ respectively. The last sample showed from 37, 40, 90 and 600 A, a value of Dp opposite to this trend probably because certain CTAB concentration may affect the calcination process producing a overheating in the boehmite precursor and an increase in pore diameter. The total templating effect on porosity was strongly dependent on the surfactant concentration and also Acosta et al. [3] have mentioned the importance of the cationic surfactant used which could produce lyotropic liquid crystal mesophases in the wet and dried gels. The same authors found poorly crystalline anhydrous transition alumina using CTAB.

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a

5

100 CI - 0

95

Zone II

CI-0.01

90

CI-0.05

Weight loss (%)

85

CI-0.075

Zone III

CI-0.1

80 75

Zone I

Zone IV

70 65 60 55 50 45 0

100

200

300

400

500

600

700

800

900

1000

Temperature (°C)

b

900 800

C1-0.1

700

ΔT

600

C1-0.075

500 400

C1-0.05

300

C1-0.01

200 C1-0 100 0 0

100

200

300

400 500 600 Temperature (°C)

700

800

900

1000

Fig. 2. a TGA of the dried (100 ◦ C) samples prepared with the surfactant CTAB; (b) DTA of the dried (100 ◦ C) samples prepared with the surfactant CTAB.

The surface areas of the aluminas prepared with the CTAB surfactant were higher than the area of the alumina without surfactant (Table 2). The surface area of these aluminas calcined at 550 ◦ C increased as the CTAB concentration increased, and a maximum located at 0.05 M of CTAB was found (Table 2). This maximum did not change when the alumina samples were calcined at 800 ◦ C. All the alumina samples promoted with CTAB and calcinated from 300

to 800 ◦ C showed surface areas higher than the area of the sample without surfactant. This behavior using cationic surfactants has been reported previously by Acosta et al. [3]. On the other hand, it was observed that a low concentration of CTAB of the order of 0.01 M (sample C1-0.01) was enough to stabilize the alumina after calcination at 800 ◦ C because the surface area of this sample decreases only 8 m2 /g after calcination up to 800 ◦ C (Table 2).

Table 2 Textural properties of the aluminas prepared with the surfactant CTAB (H2 O/Al = 213) calcinated at 550 ◦ C (4 h) and at 800 ◦ C (24 h) under air flow. Sample

300 ◦ C

550 ◦ C

800 ◦ C

SBET (m2 /g)

Vp (cm3 /g)

Dp (Å)

SBET (m2 /g)

Vp (cm3 /g)

Dp (Å)

SBET (m2 /g)

Vp (cm3 /g)

Dp (Å)

C1-0 C1-0.01 C1-0.05 C1-0.075 C1-0.10

135 250 348 157 140

0.39 0.33 0.53 0.29 0.20

73 37 42 48 106

94 229 331 168 113

0.29 0.31 0.61 0.26 0.46

80 36 41 67 182

90 221 213 143 98

0.30 0.35 0.65 0.34 0.28

93 44 82 56 150

SBET : specific surface area, Vp: pore volume and Dp: average pore diameter.

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Table 3 Weight losses of the samples prepared at different concentrations of CTAB. T (◦ C)

Weight loss (%)

Sample

C1-0

C1-0.01

C1-0.05

C1-0.075

C1-0.1

23–240 240–380 380–630 630–1000

20 7.5 8 11

12.6 9.5 6 11

12.8 13 5 15

16.5 7.5 10 15

26 7 10 15

The TGA results of the dried samples are shown in Fig. 2a and Table 3. The sample prepared with the highest concentration of CTAB (sample C1-0.1) showed the greatest weight change (Table 3). Instead the sample without CTAB (C1-0) showed the smallest change in weight (Fig. 2a). The weight changes of the samples prepared with intermediate concentrations of CTAB (C1-0.05 and C1-0.075) corresponded to intermediate weight changes between the weight losses of the two samples above. According to the DTA and some investigations in the literature [19] the TGA temperature range has been divided into four regions: Zone I (25–180 ◦ C) is attributed to loss of physisorbed water and apparently there may be a loss of physisorbed surfactant. This assumption has been proposed because the weight loss observed for each sample was proportional to the increase in concentration of CTAB used during their preparation and the sample without CTAB showed the lowest weight loss. The zone II is associated with the decomposition of the surfactant (180–400 ◦ C). This fact was supported by the DTA in Fig. 2b and an exothermic peak was produced by the combustion of CTAB in the samples C1-0.01 to C1-0.1. Also, other studies have found combustion peaks attributed to surfactants in this temperature range [19]. The zone III (400–730 ◦ C) is attributed to the removal of nitrogen compounds caused by strongly adsorbed CTAB [19], also carbon residues and oxides of sulfur (at 730 ◦ C) that remained after alumina preparation with Al2 (SO4 )3 and the release of water resulting from the phase change to ␥-Al2 O3 . Specifically, this change is located between 370 and 630 ◦ C, and it is produced by dehydroxylation and transformation of boehmite to ␥-Al2 O3 [25]. In the zone IV (700–1000 ◦ C) are carried out the typical phase changes of ␥ → ␦, ␪ → ␣-Al2 O3 and also the combustion of the last compounds of CTAB observed in the sample prepared with the highest surfactant concentration, sample C1-0.1 (Fig. 2a).

of obtaining mesoporous aluminas; that did not collapse, after calcinations despite using CTAB as template and without the addition of any hydrolysis delaying agents. These samples also exhibit some extremely wide bottom reflections, likely from layered pseudobohemite-like nanodomains, along with a broad peak with maximum at 2 ∼ 18–22◦ corresponding to an amorphous phase [19,31]. During the analysis of the diffraction pattern of X-rays from these samples, there appears to be a proportional relationship between the intensity of the reflection d4 4 0 with surface area BET and inversely proportional to the pore diameter area. In the case of alumina samples calcined at 800 ◦ C, the XRD analysis showed an increase in the intensity of reflections showing an increase in crystallinity (Fig. 5). Again, the three reflections in 2 = 37, 45, 67◦ belonging to the ␥-Al2 O3 appeared [19] also ␪Al2 O3 characterized by the angles 2 = 32, 45, 67◦ . Similarly, the correlation found for samples calcined at 550 ◦ C was observed for the samples calcined at 800 ◦ C between the intensity of the d4 4 0 reflection with the BET surface area.

Intensity (a.u.)

3.2. TGA-DTA

C1-0.075 C1-0.05 C1-0.01 C1-0

0

10

20

30

40

50

60

70

80

90

2θ (°)

3.3. Structure by XRD

Fig. 3. XRD of the dried (100 ◦ C) samples prepared with the surfactant CTAB.

C1-0.075

Intensity

The alumina samples heated at 300 ◦ C under N2 flow, showed the reflections at 2 = 14, 28, 38, 49, 65, 70 and 85◦ (Fig. 3) corresponding to the formation of boehmite [26]. It has also been reported in the literature, the formation of the boehmite phase AlO(OH) at temperatures below 300 ◦ C [27]. On the other hand, it was observed that increasing the concentration of CTAB during the preparation of the samples, the reflection intensity at 2 = 65, 49, 38 and 28◦ decreased suggesting a loss of crystallinity. These results suggest that the surfactant does not alter the formation of the boehmite structure at low temperatures (<300 ◦ C). When these samples were calcined at 550 ◦ C, the intensity of the reflections decreased and were located at 37, 45, 67◦ (Fig. 4) which corresponded with the formation of ␥-Al2 O3 [28–30]. In accordance with Aguado et al. [24] these three main peaks placed at d-spacings of 0.24, 0.19 and 0.14 nm respectively, which correspond to the d3 1 1, d4 0 0 and d4 4 0 reflections of the ␥-Al2 O3 phase. The width of these reflections suggests (according to the Scherrer equation) the occurrence of nanometer size crystals and a low ordered ␥-Al2 O3 phase. These results showed the possibility

C1-0.05 C1-0.01 C1-0 0

10

20

30

40

50

60

70

80

90

100

2θ Fig. 4. XRD of the samples prepared with the surfactant CTAB and calcined at 550 ◦ C.

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1.2 C1-0.1 1

C1-0.075

Absorbance (a.u.)

C-0.1

0.8

C1-0.075

0.6 C1-0.05 0.4 C1-0.01

C1-0.05

0.2

C1-0

C1-0.01

0

500 C1-0

10

20

30

40

50

60

70

80

90

1000

1500

2000

Wavelenght 100

2θ (°) Fig. 5. XRD of the samples prepared with the surfactant CTAB and calcined at 800 ◦ C.

3.4. IR spectra after calcination Infrared spectra of the samples calcined at 550 ◦ C (Fig. 6) show the following species: band between 2200 and 3800 cm−1

2500

3000

3500

4000

(cm-1)

Fig. 6. Infrared spectra of the samples prepared with CTAB after calcination at 550 ◦ C.

attributable to OH species [32], peak at 2350 cm−1 due to CO2 molecular, band between 1550 and 1750 cm−1 attributed to water of hydration [32], bands between 1000 and 1300 cm−1 due to vibrations of the groups Al O, Al OH, OH, band at 700 cm−1 due to vibrations of Al = O and AlO(OH) [33]. No bands were observed between 1400 and 1550 cm−1 due to C H species [34] that could

Fig. 7. (a) IR spectra of pyridine chemisorbed on the sample C1-0 at temperatures from 25 to 400 ◦ C; (b) IR spectra of pyridine chemisorbed on the sample C1-0.05 at temperatures from 25 to 400 ◦ C; (c) IR spectra of pyridine chemisorbed on the sample C1-0.075 at temperatures from 25 to 400 ◦ C; (d) IR spectra of pyridine chemisorbed on the sample C1-0.1 at temperatures from 25 to 400 ◦ C. gr7d.

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b

700

250 B

B

600

L

500

Pyridine adsorbed ( µmole/g)

Pyridine adsorbed(umol/g)

a

B+L Total

400 300 200 100 0 0

100

200

300

L

200

B+L Total

150 100 50 0

400

0

Temperature (°C)

d 350

350 B

300

Pyridine adsorbed (umol/g)

Pyridine adsorbed (umol/g)

c

L

250

B+L

200

Total

100 200 300 Temperature (°C)

150 100 50

400

B L B+L Total

300 250 200 150 100 50 0

0 0

100 200 300 Temperature (°C)

0

400

200 Temperature (°C)

400

Fig. 8. Quantitative determination of the Brønsted and Lewis acidity of the aluminas calcined at 550 ◦ C and prepared with the surfactant CTAB (a) C1-0, (b) C1-0.05, (c) C1-0.075, (d) C1-0.1.

come from the degradation or combustion of CTAB and, therefore, the calcination in air at 550 ◦ C for 4 h was enough to remove the surfactant CTAB.

3.5. Acidity by adsorption of pyridine All infrared spectra of pyridine showed the band at 1450 cm−1 (Fig. 7) attributed to the presence of Lewis acid sites (L) [21]. Also, all the samples showed the band between 1490 and 1510 cm−1 attributed to the presence of both Lewis and Brønsted acid sites. Only the infrared spectra of the samples C1-0 (Fig. 7a), sample C10.05 (Fig. 7b) and mainly sample C1-0.075 (Fig. 7c) showed the band located at 1540 cm−1 attributed to Brønsted acid sites (B). The quantitative analysis of these samples is shown in Fig. 8. The sample C1-0 prepared without surfactant shows a small amount of Brønsted acid sites (Fig. 7a) and Fig. 8a, and the proportion of Lewis acid sites is 7.8 times greater at 100 ◦ C and is much higher at 25 ◦ C. This distribution is already known in the literature; however the effect of the surfactant CTAB is reflected in the amount of both acid sites and the proportion or ratio of Brønsted to Lewis acid sites. In the case of the sample prepared with a concentration of CTAB of 0.05 M (sample C1-0.05) the amount of Brønsted acid sites increased to 1.5 times the amount observed for the alumina C1-0. When the concentration of CTAB increased at 0.075 M in the preparation of the aluminas, the amount of Brønsted acid sites increased to three times the amount observed for the alumina C1-0 prepared without CTAB (Fig. 8c) and the amount of Lewis acid sites was almost the same. Therefore, the ratio of Brønsted acid sites to Lewis acid sites is the largest.

On the other hand, the infrared spectra of the sample C10.1(Fig. 7d and d) showed that the band due to Lewis acid sites (at 1450 cm−1 ) did not decrease as in the other samples as the temperature increases. It was surprising that the absorbance of the band almost did not decrease when the temperature increased from 100 ◦ C to 400 ◦ C and the same behavior can be observed in the band located at 1580 cm−1 that is also attributed to Lewis acid sites [21]. It was also observed that increasing the temperature of the sample, the absorbance of the band at 1490 cm−1 did not decrease compared with the decrease observed in the other samples (Fig. 8d). This sample (C1-0.1) showed the highest amount of Lewis acid sites when the temperature was higher than 300 ◦ C, and it did not show Brønsted acid sites.

3.6.

27 Al

NMR

The presence of Al in octahedral coordination (AlVI ) near 0 ppm, Al in pentahedral coordination (AlV ) about 30 ppm and Al in tetrahedral coordination (AlIV ) about 60 ppm can be observed for the samples heated at 300 ◦ C in Fig. 9. These signals have been reported in other studies [9,10,19,35] and are present in samples C1-0, C10.075 and C1-0.1. Pentahedral coordination (AlV ) is absent in the samples C1-0.01 and C1-0.05, and the main peak in all the samples corresponded with the octahedral coordination (AlVI ). Previous analysis of 27 Al NMR (not shown) of the dried samples at 100 ◦ C (boehmite AlO(OH)) showed only the octahedral coordination AlVI . The 27 Al NMR spectra of calcined samples at 550 ◦ C are shown in Fig. 10 and the deconvolution values for each spectrum of the

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Al

Al

IV

Al

V

C1-0.1

Intensity (a.u.)

C1-0.075

C1-0.05

C1-0.01

C1-0

200

150

100

50

0

-50

-100

-150

-200

ppm

Chemical shift (ppm) Fig. 9. 27 Al NMR spectra of the samples heated at 300 ◦ C and prepared with the surfactant CTAB.

9

concentration of 0.05 M of CTAB (Fig. 9) and the coordination of AlVI come from the process of dehydration and dehydroxylation of the aluminum hydroxide and the origin of AlV coordination is the result of AlIV migration towards AlVI sites [36]. During the synthesis of alumina, the formation of AlV comes from the unsaturated Al atoms which in turn come from the formation of oxygen vacancies during the dehydroxylation of aluminum precursors. This AlV serves as an electron acceptor and act as Lewis acid site [37]. In the case of AlIV , it has been reported that its presence in the alumina produce Brønsted acidity while the presence of aluminum in coordination AlV provides Lewis acidity [38]. These 27 Al NMR results confirmed that the sample of alumina C1-0.1 having the highest population of AlV (Table 5) showed only Lewis acidity and the Brønsted acidity was absent (Fig. 8d) also the C1-0.05 sample that showed the highest proportion of AlIV (or the highest ratio of AlIV /AlV = 2.8) and this sample did not show the highest Brønsted acidity which corresponded to the sample C10.075, however both samples showed the same relative AlVI /AlIV ratio of 4 (or AlIV /AlVI = 0.25). We have constructed Table 5 with Brønsted (B) and Lewis (L) acidity of the alumina at 100 ◦ C likewise; we have included the relationship of sites of AlIV to AlV by 27 Al NMR. Brønsted sites are related with AlIV [38] and Lewis sites are related with AlV [37]. It is noted that increasing the concentration of surfactant CTAB, the ratio of B/L acid sites increases and likewise the ratio of AlIV /AlV . However, this behavior disappears in the sample C1-0.1 (with Brønsted acidity = 0) which was prepared with the maximum concentration of CTAB. 3.7. SEM

Fig. 10. 27 Al NMR spectra of the samples calcined at 550 ◦ C and prepared with the surfactant CTAB.

Al coordination are shown in Table 4. When the concentration of CTAB increased during the synthesis of aluminas the proportion of octahedral coordination AlVI decreased (Fig. 9 and Table 4) and the concentration of pentahedral (AlV ) and tetrahedral coordination (AlIV ) increased (with the exception of sample C1-0.05). Pentahedral (AlV ) was not found in the sample prepared with a Table 4 Population in (%) of each deconvoluted spectral Al from the 27 Al NMR spectra of the samples calcined at 550 ◦ C with their AlVI /AlIV ratio and H2 consumption from TPR analysis. Sample

Population (%) IV

C1-0 C1-0.01 C1-0.05 C1-0.075 C1-0.10 a

TPR V

Al

Al

Al

Al /Al

H2 /Pta

12 14 17 14 15

15 9 6 19 34

73 77 77 67 51

6 5 4 4 3

1.11 – 0.75 0.80 1.03

Mol of H2 /atom of Pt up to 700 ◦ C.

VI

VI

IV

For the samples without CTAB (samples C1-0) and with CTAB (sample C1-0.01) heated at 300 ◦ C with a boehmite-like structure by XRD, two morphologies were observed (Fig. 11a and b). In the C1-0 sample, plate-shaped particles with a flowerlike threedimensional nanoarchitecture having pores between nanoplates. This structure is in accordance with the slit-shaped pores with parallel walls with narrow, short openings suggested during the study of N2 physisorption made previously in this work (Section 3.1). In the sample C1-0.01 made with CTAB, the structure was similar to the previous one having laminar walls, but in this sample the pores were narrower. This sample showed high roughness laminar walls, unlike the previous sample where the laminar walls were smooth. The laminar walls morphologies of boehmite have been seen in other investigations [39,40]. Also, flowerlike three-dimensional nanoarchitecture of boehmite have been observed by Zhang et al. [41]. Other authors had found nanosheets when their preparation of boehmite samples reached a pH near 7 [42] and also plate-shaped particles when their preparation was made with NaOH at pH of 7 [18]. In the case of calcined aluminas, the morphologies gradually changed when they were prepared with the surfactant CTAB. In the samples prepared with and without surfactant (C1-0, and C1-0.01) many hemispherical clusters bonded together to form a cauliflower-like structure were observed (Fig. 11c and d). The spheroidal particles with diameters between 30 and 54 nm forming a solid structure were observed. In the case of aluminas prepared with a higher concentration of CTAB, (samples C1-0.05, and C1-0.075), hemispherical particle clusters were also observed attached to cylindrical structures where these clusters of cylinders or rods were connected with the hemispheres (Fig. 12a and b). In the sample prepared with the highest concentration of CTAB (sample C1-0.1) a dense network of wormlike particles was observed (Fig. 12c). For the same sample an image of higher

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Fig. 11. (a) SEM micrograph of the sample C1-0 heated at 300 ◦ C; (b) SEM micrograph of the sample C1-0.01 heated at 300 ◦ C; (c) SEM micrograph of the sample C1-0 calcined at 550 ◦ C; (d) SEM micrograph of the sample C1-0.01 calcined at 550 ◦ C.

resolution (Fig. 12d) shows the presence of structures attached to the worm-like formations clustered and separated from one another. These wormlike particles have also been observed in aluminas prepared by the sol–gel method using the cationic surfactant hexadecyl trimethyl ammonium bromide [19]. These structures probably came from the uncontrolled coalescence of the aluminum species forming the pore walls, during calcination [43]. We observed the presence of sulfur in Fig. 13b and c, this sulfur comes from alumina because Al2 (SO4 )3 was used as the precursor. Also, this result confirms the observation that the peak in the TPR located at 750–800 ◦ C (Fig. 15) was due to the reduction of S [44,45]. Also, the presence of sulfur together with Pt atoms was confirmed in this study (Fig. 14). This result is in accordance with the TPR experiments of Pt/Al2 O3 catalysts because the peak at 570 ◦ C could be due to the reduction of PtS [46]. By EDS studies, the presence of Pt dispersed in the sample and its composition is observed (Fig. 14); also a sulfur composition was close to 2 wt% while the Pt was 9 wt%. These two elements were found in the same cluster (Fig. 14a–c), therefore, Pt metal and sulfate (or Pt sulfite or Pt-sulfur) after H2 reduction could be present on the catalyst surface. This fact was very important in order to explain the low selectivity to hydrogenolysis-hydrocracking and high selectivity to isomerization of n-heptane. Using SEM, these results demonstrated that particles having the wormlike morphology (catalyst C1-0.1P) against particles having the hemispherical shaped morphology (catalysts C1-0.05P and C10.075P) can serve as an element of referring to select a good catalyst for paraffin isomerization.

3.8. TPR The H2 consumption for the samples impregnated with 0.5% Pt is shown in Fig. 15. The TPR of the alumina (sample C1-0) showed one peak at 750–800 ◦ C attributed to the reduction of S by H2 producing H2 S at high temperature [45,46]. These authors obtained H2 S from the reduction of elemental sulfur or surface sulfate formed on strong surface sites of the alumina. The TPR analysis of the Pt/Al2 O3 catalysts showed two peaks: one located at 570 ◦ C and the second between 750 and 800 ◦ C. As mentioned, this second peak is due to the reduction of sulfur derived from sulfate groups. The peak at 570 ◦ C could be due to the reduction of PtS obtained during the reduction of sulfated alumina [46]. In the absence of sulfur the Pt precursors on alumina (PtOxCly) after the air calcination at 500 ◦ C are reduced at temperatures of 250 and 450 ◦ C [44,47], we did not find these peaks. The presence of Pt and sulfur were in intimate contact in our catalysts after reduction by SEM-EDS (Fig. 14). The sulfur could be as sulfate, sulfite, H2 S or sulfur [46], depending on the temperature and atmosphere of pretreatment. The small Pt particles can be strongly affected by the presence of sulfur; it is known that sulfur poisons the Pt surface because sulfur modifies the electronic density of Pt in its external d orbital [48]. The poison tolerance of Pt is strongly affected by the kind of precursor salt and the pretreatment employed during the preparation of the Pt catalyst. In the case of the sulfur poisoning tests made by Badano et al. [48], the Pt acted as an electron donor centre. Conversely, with oxygenated poisons (for example tetrahydrofuran) the metal acts as an electron

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Fig. 12. (a) SEM micrograph of the sample C1-0.05 calcined at 550 ◦ C; (b) SEM micrograph of the sample C1-0.075 calcined at 550 ◦ C; (c) SEM micrograph of the sample C1-0.1 calcined at 550 ◦ C. (Magnification ×50,000.); (d) SEM micrograph of the sample C1-0.1 calcined at 550 ◦ C. (Magnification ×100,000.)

acceptor center. Thiophene interacts with the metal d electrons in a planar way through the ␲ electrons of the aromatic nucleus (weak bond). Thiophene and tetrahydrofuran were directly adsorbed on the metal particles via a ␴ bond (strong bond). In studies of sulfur poisoning over a Pt/Al2 O3 catalyst made by Gracia et al. [49] in the oxidation of CO, the sulfur not only affects the bonding of adsorbed CO molecules on the Pt surface, but also the poisoning made a blockage of the active sites where O2 adsorbs dissociatively, increasing the ignition temperature. The initial extent of the poisoning was determined by the storage capacity of the support. In order to find an explanation to the results of H2 consumption in these catalysts, we also observed a relationship between the H2 consumption by TPR and the population of AlV by 27 Al NMR (Table 4). The samples C1-0P and C1-0.1P showed high H2 consumption and high population of AlV , while the other samples (C1-0.05P and C1-0.075P) showed lower values in both analysis techniques. These results suggest that Pt precursors were chemisorbed on Lewis acid sites in the sample C1-0.1 with the highest number of Lewis sites (Figs. 7d and 8d) also showed the largest population of AlV (Fig. 10 and Table 4). The effect of CTAB in the synthesis of catalysts appears to be indirect because although the surface area increases, so does the population of Brønsted sites to a concentration of 0.075 M. In the case of the sample having the highest concentration of CTAB

(sample C1-0.1P), the effect was inversely because only Lewis sites and high H2 reduction of Pt precursors were observed. The alumina of this catalyst showed worm-like shapes by SEM, forming pores derived from the elongated morphology and was very different from the morphology of pores and particles of the catalyst C1-0P that was prepared without CTAB. 3.9. Hydrogen chemisorption The Pt dispersion (H/Pt), the metallic area (S) and the crystallite diameter (d) where calculated (Table 6) using the hydrogen chemisorption, the Pt concentration in the sample and the equations proposed by Paryjczac and Szymura [50]. The Pt dispersion did not show a proportional trend with the concentration of CTAB. From crystallite diameter values (11 A˚ average) we can deduce that the crystal size is very small and that could favor hydrogenolysis reaction, and in this respect the role of sulfur will be relevant. The Pt metal-support interaction using this mesoporous Al2 O3 showed that Pt particles are present in a high dispersion state. We did not find any strong-metal support interaction (SMSI) as in the case of Pt supported on TiO2 [51] because the H2 consumption by TPR was almost proportional with the Pt dispersion. As is known, the SMSI effect exists when the Pt oxychloro-complexes are reduced totally to metal and the Pt dispersion assessed by H2 chemisorption is very low.

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Fig. 13. Sulfur composition in the alumina calcined at 550 ◦ C (Sample C1-0); (a) SEM of the sample C1-0; (b) Mapping of S atoms in the sample C1-0; (c) EDS of the sample C1-0.

Fig. 14. SEM and EDS of the C1-0.05P catalyst; (a) SEM of C1-0.05P catalyst. (b) Pt mapping of the C1-0.05P catalyst; (c) EDS of C1-0.05P catalyst.

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Table 5 Brønsted/Lewis acid ratio at 100 ◦ C and AlIV /AlV ratio for the catalysts prepared with CTAB. Catalyst

CTAB Conc. (M)

Brønsted Sites (␮molPy /gcat )

Lewis sites (␮molPy /gcat )

Brønsted/Lewis acid ratio

AlIV /AV

C1-0P C1-0.01P C1-0.05P C1-0.075P C1-0.1P

0 0.01 0.05 0.075 0.100

19 31 29 31 0

149 132 120 118 197

0.12 0.23 0.24 0.26 0

0.80 1.55 2.83 0.73 0.44

Table 6 H2 chemisorption of the Pt/Al2 O3 catalysts in function of the CTAB concentration. Catalyst

QH2 (mol/gcat )

t (gatmPt /gcat )

−6

−5

6.43 × 10 6.82 × 10−6 6.38 × 10−6 4.56 × 10−6 6.38 × 10−6

C1-0P C1-0.01P C1-0.05P C1-0.075P C1-0.1P

1.25 × 10 1.68 × 10−5 1.30 × 10−5 1.28 × 10−5 1.27 × 10−5

3.10. n-Heptane hydroconversion

70

The catalytic selectivity (Figs. 16–19) and conversion (Fig. 20) for these catalysts showed a strong effect of the sulfur presence and the CTAB concentration during the preparation of the aluminas. The hydrogenolysis-hydrocracking products were: methane and ethane (C1 + C2), propane (C3), butane (n-C4),

60

H/Pt

S (m2 /g)

dPt (Å)

1.030 0.810 0.978 0.712 0.998

254 200 241 175 246

9.1 11.6 9.6 13.2 9.4

Sh

30

10

C1 - 0P

4000

(μmol/g)

40

20

5000

H2 consumption

Sc

50 Selectivity (%)

6000

Si

C1 - 0.1P

0

3000

0

0.02

C1 - 0.075P

0.04 0.06 Concentration of CTAB (M)

0.08

0.1

2000

Fig. 17. Selectivity of catalysts evaluated at 400 ◦ C, H2 /n-heptane molar ratio was 15.8, WHSV of 4.8 h−1 .

C1 - 0.05P 1000

C1 - 0

0 0

200

400

600

800

1000

Temperature ( °C )

80 Fig. 15. TPR traces of the catalysts: C1-0.1P, C1-0.075P and C1-0.05P and the sample C1-0P.

Sh 70 Si

100

60

Sc

Selectivity (%)

90

Selectivity (%)

80 70 60

Sh

50

Si

40

40 30 20

Sc

30

50

20

10

10

0

0 0

0.02

0.04 0.06 Concentration of CTAB (M)

0.08

0.1

Fig. 16. Selectivity of catalysts evaluated at 350 ◦ C, H2 /n-heptane molar ratio of 15.8, WHSV of 4.8 h−1 .

0

0.02

0.04 0.06 Concentration of CTAB (M)

0.08

0.1

Fig. 18. Selectivity of catalysts evaluated at 450 ◦ C, H2 /n-heptane molar ratio was 15.8, WHSV of 4.8 h−1 .

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80 70 60 Selectivity (%)

Sh 50

Si

40

Sc

30 20 10 0 0

0.02

0.04 0.06 Concentration of CTAB (M)

0.08

0.1

Fig. 19. Selectivity of catalysts evaluated at 500 ◦ C, H2 /n-heptane molar ratio was 15.8, WHSV of 4.8 h−1 .

pentane (n-C5) and hexane (n-C6). Isomerization products were: (i-C4), 2-methylpentane (2-MP), 3-methylpentane (3-MP), 2,3-dimethylpentane (2,3-DMP), 3 ethylpentane (3, ETP). Dehydrocyclization products were: cyclohexane (CH) and toluene (T) and very small amounts of benzene were observed. An analysis of the reactions from the results at 350 ◦ C (Fig. 16) showed that at low temperatures, the isomerization reaction is thermodynamically favored [49]. The catalysts made with the samples C1-0.01P, C1-0.05 and C1-0.075P showed high selectivitiy to isomers 2-MP and 3-MP (isomerization selectivity Si). These representative isomers passed through a maximum for C10.01P, C1-0.05P and C1-0.075P catalysts, in which the presence of Brønsted sites were observed (Fig. 8b–d). It was noted that the selectivity to isomers of the three catalysts mentioned above is greater than the selectivity of the catalyst made with C1-0P alumina prepared without CTAB. In the case of catalyst, C1-0.1P the production of C1 + C2 (hydrogenolysis selectivity, Sh) and toluene (deshydrociclization selectivity, Sc) in small quantities were the main products. This catalyst showed a great amount of Lewis acid sites and none Brønsted acid sites (Fig. 8d) and the hydrogenolysis-hydrocracking reaction were more important than the isomerization reaction. For the evaluation at 400 ◦ C the isomers 2-MP and 3-MP (Fig. 17) were still present but the hydrogenolysis (C1 + C2), hydrocracking 60 500°C

Conversion (%)

55

450°C

50

400°C

45

350°C

40 35 30 25 20 15 10 0

0.02

0.04 0.06 Concentration of CTAB (M)

0.08

0.1

Fig. 20. Conversion of the catalysts evaluated from 350 to 500 ◦ C, H2 /n-heptane molar ratio was 15.8, WHSV of 4.8 h−1 .

(C3) and toluene (T) increased because these reactions were thermodynamically favored [49]. Increasing the temperature to 450 ◦ C, again increases hydrogenolysis products but has increased especially dehydrocyclization reaction represented by toluene T (Fig. 18), mainly due to the decomposition of the isomers formed giving side reactions. Finally at 500 ◦ C, all catalysts shown the main reaction products represented by C1 + C2 and C3 (grouped as Sh) which were produced from the hydrogenolysis and hydrocracking reactions (Fig. 19). Also the dehydrocyclization products represented as Sc and a minor amount of isomerization products (2-MP, 3-MP and 2,3 DMP) represented as Si. The presence of hydrogenolysis products such as methane (C1) and to a lesser extent ethane (C2) are mainly due to small Pt particles that are the most active in this reaction [50]. Hydrogenolysis reaction require high temperature and strong bonds of the reactants with the catalyst sites. This reaction is more difficult to carry out than the hydrogenation reactions, which occur more easily. From the TPR results the samples C1-0P and C1-0.1P showed the highest H2 consumption (Table 4) and also the highest Pt dispersions (Table 6), and therefore they showed the smallest Pt particle diameters. In the case of the formation of hydrocracking products, they mainly occur on the acid centers of the alumina support [52]. In the same manner as the isomerization reaction, both reactions are facilitated by paraffin dehydrogenation to olefins catalyzed by Pt and, therefore, the presence of Pt is very important. The formation of C3 to C4 and C2 to C5 came from secondary reactions of hydrocracking of n-heptane and were obtained due to the presence of Lewis acid sites and Pt particles. The selectivity to C3 (and to a lesser extent C2) were the products more important of this reaction. A maximum was observed in toluene selectivity (or Sc) for the catalyst C1-0.01 that has shown Brønsted acid sites and a relative amount of Pt. The dehydrocyclization reaction of nheptane is typically represented by the formation of toluene and is favored at high temperatures (Fig. 19). At 500 ◦ C is observed that increasing the CTAB concentration, the dehydrocyclization selectivity, slightly increases and the presence of Pt sites was very important because toluene may be produced either by a bifunctional mechanism (Pt + acid sites) or monofunctional mechanism (Pt) both being equivalent mechanisms in the n-heptane hydroconversion [52]. In our results the C1-0.01P catalyst showed the highest selectivity to aromatics (Sc = 33%) due to a good Pt dispersion (H/Pt = 0.81) and an excellent B/L acid ratio of 0.23. Moreover we can observe (Figs. 16–19) that a large effect of the concentration of CTAB or the ratio of acid sites of B/L together the presence of Pt-sulfur sites were responsible to the high selectivity to isomers (at 350 ◦ C) and the high selectivity to aromatics (at 500 ◦ C) as was showed in Fig. 19. In the case of catalyst C1-0.1P evaluated at 500 ◦ C the hydrogenolysis-hycrocracking selectivity was the main reaction due to the presence of Lewis acid sites and small Pt crystallites. The dehydrocyclization reaction to aromatics (toluene) depended of Pt particles as well as acidity (Brønsted) if the reaction mechanism through the formation of “methylcyclopentane” is accepted instead of reaction mechanism of free radicals which can be performed only with the presence of metal sites. From these results we demonstrated that the synthesis of mesoporous aluminas using the Al2 (SO4 )3 and CTAB as precursors, was suitable to provide a catalyst with excellent selectivity to isomers (especially at low temperature, T = 350 ◦ C) and may also provide catalysts with high selectivity for the production of aromatics (toluene).

Please cite this article in press as: J.L. Contreras, et al., Synthesis of Pt/Al2 O3 catalyst using mesoporous alumina prepared with a cationic surfactant, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.10.010

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4. Conclusions The surface area and pore volume increased as CTAB concentration increased passing through a maximum. After calcination at 300 ◦ C boehmite was found by XRD and ␥-Al2 O3 was produced at 550 ◦ C. The transition between ␥ → ␪-Al2 O3 was also observed after calcination at 800 ◦ C together with an increase of the pore volume and pore diameter respect to the alumina prepared without CTAB. The alumina prepared with the lowest concentration of CTAB showed the highest thermal stability. The effect of the concentration of CTAB in the synthesis of the aluminas resulted in the presence of acid Brønsted sites at concentrations between 0.01 M and 0.075 M, however, when the concentration of CTAB was 0.1 M, only Lewis acid sites were observed which were stable at temperatures above 100 ◦ C. Another important effect was observed when the concentration of CTAB was 0.1 and 0 M. High reducibility of Pt precursors together with a high population of AlV was obtained. A high hydrocracking selectivity for the catalyst made with the highest concentration of CTAB was observed. Aluminas prepared with intermediate CTAB concentrations between 0.05 and 0.075 M produced the best catalysts to produce isomerization products. At low CTAB concentration, the dehydrocyclization selectivity passed through a maximum and the presence of sulfur together Pt sites was very important to minimize the hydrogenolysis selectivity increasing the isomerization selectivity. References [1] F. Vaudry, S. Khodabandeh, M.E. Davis, Chem. Mater. 8 (1996) 1451. [2] S. Cabrera, J.E. Haskouri, J. Alamo, A. Beltrán, D. Beltrán, S. Mendioroz, M.D. Marcos, P. Amoros, Adv. Mater. 11 (1999) 379. [3] S. Acosta, A. Ayral, C. Guizard, L. Cot, J. Sol–Gel Sci. Technol. 8 (1996) 195. [4] M. Kamal, S. Khalil, Appl. Surf. Sci. 255 (2008) 2874. [5] Q. Liu, A. Wang, X. Wang, T. Zhang, Microporous Mesoporous Mater. (2007) 35. [6] M. Yada, M. Ohya, M. Machida, T. Kijima, Chem. Commun. (1998) 1941. [7] J. Cejka, N. Zilkova, J. Rathousky, A. Zukal, Phys. Chem. Chem. Phys. 3 (2001) 5076. [8] S.A. Bagshaw, T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 35 (1996) 1102. ˜ I. Díaz, C. Márquez-Alvarez, E. Sastre, J. Pérez-Pariente, [9] V. Gonzalez-Pena, Microporous Mesoporous Mater. 44 (2001) 203. [10] J. Cejka, Appl. Catal., A: Gen. 254 (2003) 327. ˇ [11] C. Márquez-Alvarez, N. Zˇ ilková, J. Pérez-Pariente, J. Cejka, Catal. Rev. Sci. Eng. 50 (2) (2008) 222–286. [12] Z. Shan, J.C. Jansen, W. Zhou, T. Maschmeyer, Appl. Catal. 254 (2003) 339. [13] B. Xu, T. Xiao, Z. Yan, X. Sun, J. Sloan, S.L. Gonzalez-Cortés, F. Alshahrani, M.L.H. Green, Microporous Mesoporous Mater. 91 (2006) 293. [14] Q.S. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P.Y. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schüth, G.D. Stucky, Chem. Mater. 6 (1994) 1176.

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Please cite this article in press as: J.L. Contreras, et al., Synthesis of Pt/Al2 O3 catalyst using mesoporous alumina prepared with a cationic surfactant, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.10.010