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Enhanced production of aromatic hydrocarbons by rapeseed oil conversion over Ga and Zn modified ZSM-5 catalysts Rubén Ramos, Alicia Garcia, Juan Angel Botas, and David Pedro Serrano Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03050 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 5, 2016

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Industrial & Engineering Chemistry Research

Enhanced production of aromatic hydrocarbons by rapeseed oil conversion over Ga and Zn modified ZSM-5 catalysts Rubén Ramosa‡, Alicia Garcíaa‡, Juan A. Botasa‡ and David P. Serranoa,b *‡ a

Chemical and Environmental Engineering Group, ESCET, Rey Juan Carlos University, c/

Tulipán s/n, 28933 Móstoles, Madrid, Spain b

IMDEA Energy Institute, Avda. Ramón de la Sagra 3, 28933 Móstoles, Madrid, Spain

*To whom correspondence should be addressed e-mail: [email protected] ‡These authors contributed equally

ACS Paragon Plus Environment

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ABSTRACT

Rapeseed oil conversion has been investigated over catalysts based on Ga- and Zn-modified nanocrystalline HZSM-5 zeolite aimed to the production of hydrocarbons that could be used as both raw chemicals and/or fuels. The reactions have been carried out in a fixed bed reactor operating under nitrogen at 550 °C and atmospheric pressure. The incorporation of the metallic species to the parent zeolite was carried out by wetness-impregnation, resulting in a good metal dispersion, which is facilitated by the relatively high external surface area available in the nanocrystalline zeolitic support. This metal addition causes significant changes in the textural and acidic properties of the ZSM-5 zeolite. In all cases, a high deoxygenation degree of the raw oil was achieved through the formation of CO, CO2 and H2O, whereas the main products obtained were light olefins (mainly ethylene and propylene) and aromatic hydrocarbons (BTX). The incorporation of Ga and Zn in the parent zeolite increased the formation of gasoline range hydrocarbons, promoting the conversion of light olefins into aromatics by a sequence of oligomerization/cyclization/aromatization reactions. This effect was especially noticeable over the Zn/HZSM-5 catalyst, showing a strong enhancement in the production of aromatic hydrocarbons, which was assigned to the generation of new acid sites by the incorporation of the Zn species. Moreover, the addition of Ga on the zeolite increases the coke deposition, which may explain the more pronounced decrease in the aromatization activity observed along the time on stream for the Ga/HZSM-5 catalysts compared to the Zn/HZSM-5 ones.

KEYWORDS: Rapeseed oil; gallium; zinc; HZSM-5 zeolite


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1. Introduction Fossil fuels have been the major source of hydrocarbons in the chemical industries for last decades [1]. Nevertheless, global environmental issues have motivated the development of sustainable technologies to produce fuels and chemicals from renewable bio-resources. In this way, catalytic cracking of oleaginous biomass has attracted attention as an alternative route for the production of hydrocarbons without the supply of external hydrogen [2, 3]. This process usually takes place at 400-600 °C, under an inert atmosphere leading to a relatively broad spectrum of hydrocarbons, from gases to diesel range compounds, reflecting the great diversity of possible reaction pathways [4]. One relevant advantage of this alternative is the fact that the so produced hydrocarbons are very similar to those present in standard fuels obtained from fossil resources, so they can be processed and employed using the current infrastructures. However, it suffers of some disadvantages mainly due to the low product selectivity and the extensive formation of coke over the catalysts. A variety of studies has been published addressing the conversion of triglyceride based oils to hydrocarbons using different catalysts, such as those based on zeolites (ZSM-5, Beta, Y, mordenite, etc.) and ordered mesoporous aluminosilicates (Al-MCM-41 and Al-SBA-15) [5-10]. These materials usually exhibit a suitable combination of acidity and textural properties (ordered structure and uniform pore size) to promote the selective deoxygenation and cracking of the feedstock into hydrocarbon products. One of the most active and widely investigated catalysts has been the HZSM-5 zeolite. This material is a crystalline and microporous alumino-silicate (pore size 5.5-5.6 Å), which exhibits an efficient cracking activity and strong resistance to deactivation. Moreover, HZSM-5 zeolite typically presents a good selectivity for light olefins (C2-C4) and aromatic hydrocarbons (mainly benzene, toluene and xylenes) [11, 12]. Both light olefins and


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aromatics are particularly interesting compounds due to their applications as raw materials for the manufacture of a wide range of products. While the performance of bifunctional catalysts in the conversion of oleaginous biomass by hydrotreatment processes has been widely reported in the literature [13-15], the use of metalmodified zeolites in the catalytic cracking of vegetable oils has been less investigated. These catalysts are characterized by possessing active centers of different nature: (i) zeolite acid sites and (ii) metallic centers. The addition of the metallic phase modifies the physicochemical characteristics of the support, which affects to its catalytic activity and changes the predominant reaction pathways. In particular, the presence of metallic species is expected to promote routes leading to the formation of light olefins and aromatics by the control of hydrogen transfer reactions [16]. In a previous work [17], our group has investigated the viability of converting rapeseed oil into hydrocarbons by using bifunctional Mo- and Ni-loaded HZSM-5 catalysts prepared over a HZSM5 nanocrystalline zeolite, which provides a significant proportion of external surface area. The product mixture was mainly composed of light olefins (mostly ethylene and propylene) and aromatic hydrocarbons (BTX). Ni/HZSM-5 catalysts showed a higher selectivity towards light olefins, and Mo/HZSM-5 towards aromatics. Likewise, we have studied for this reaction the use of hierarchical ZSM-5 zeolites, showing a secondary porosity in the mesopore range, as catalytic supports of the Ni particles [18]. The enhancement of the textural properties in hierarchical ZSM5 improved the accessibility of the reactive molecules, along with a better dispersion of the metallic active phase, leading to the selective production of hydrogen, light olefins and aromatic hydrocarbons. Moreover, the deposition of coke on the catalytic system occurred in form of carbon nanotubes, opening a novel route for obtaining a high added-value co-product from vegetable oils.


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The present work reports the catalytic cracking of rapeseed oil over nanocrystalline HZSM-5 zeolite, modified with Ga and Zn. The zeolitic support here employed consists of nanocrystals, providing it with a high external specific surface area. This fact is expected to enhance both the dispersion of the metallic phase and the accessibility of the bulky triglycerides molecules to the active sites. Indeed, the selected metallic species have been successfully tested in similar processes related to the aromatization of light alkanes [17-22]. Accordingly, the major goal of this work is to establish the feasibility of increasing further the production of aromatic hydrocarbons from vegetable oils by modification of the HZSM-5 support with the incorporation of Ga and Zn species. These aromatic compounds are valuable products for being employed as both raw chemicals and in the formulation of advanced biofuels within the gasoline range.

2. Experimental 2.1. Vegetable oil Rapeseed oil was employed as feedstock for the catalytic experiments. A degummed and refined variety of this oil was obtained from GUSTAV HEESS. The elemental composition, determined by CHNS analyses using an ELEMENTAR Vario EL III analyzer apparatus, consists of: 78 wt% C, 12.1 wt% H, 0.8 wt% N, 0.2 wt% S and 8.9 wt% O (the latter determined by difference). The detailed fatty acid composition of the rapeseed oil is given in Table 1.


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Table 1. Fatty acid composition of rapeseed oil. Fatty acid

(wt%)

Lauric acid

(C12:0) 0.01

Myristic acid

(C14:0) 0.06

Palmitic acid

(C16:0) 4.43

Palmitoleic acid

(C16:1) 0.25

Margaric acid

(C17:0) 0.05

Margaroleic acid

(C17:1) 0.05

Stearic acid

(C18:0) 1.6

Oleic acid

(C18:1) 56.89

Linoleic acid

(C18:2) 22.63

Linolenic acid

(C18:3) 11.95

Arachidic acid

(C20:0) 0.51

Gadoleic acid

(C20:1) 1.12

Behenic acid

(C22:0) 0.29

Erucic acid

(C22:1) 0.03

Lignoceric acid

(C24:0) 0.12

2.2. Catalysts preparation The studied catalysts were prepared using as support a nanocrystalline HZSM-5 zeolite (Si/Al=36) provided by SÜD-CHEMIE. Incipient wetness-impregnation was used to incorporate gallium and zinc into the parent HZSM-5 with different levels of metal loadings, using as precursors aqueous solutions of Ga(NO3)3 and Zn(NO3)2, respectively. Firstly, the catalyst support was subjected to an outgassing treatment under vacuum for 1 h at room temperature. The incorporation of the metallic active phase was performed adding dropwise on the support the


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corresponding precursor solution. The total amount of incorporated metallic precursor was controlled through the concentration of the starting solution, varying the precursor/zeolite mass ratio within the ranges 0.12-0.25 and 0.15-0.32 for Zn/ZSM-5 and Ga/ZSM-5, respectively. Once impregnated, the solvent was removed in a rotary evaporator during 1 h at 60 °C under vacuum. Finally, the metal catalyst precursors were calcined in air under static conditions at 550 °C for 5 h (heating rate 1.8 °C·min-1). This calcination treatment allows the metal nitrate to be decomposed leading to the corresponding Ga and Zn oxides.

2.3. Catalysts characterization The prepared catalysts were characterized to determine their morphological, physical and chemical properties. X-ray diffraction was performed in a Philips X´PERT MPD diffractometer using Cu-Kα radiation (λ= 1.5418 Å). Samples were scanned over the range 5-90 ° using a step size and a counting time of 0.1 ° and 10 s, respectively. The chemical composition of the catalysts was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Varian VISTA-AX CCD spectrophotomer, the samples being subjected to a prior acid digestion with H2SO4 and HF. The acidic properties of the catalysts were probed by ammonia temperature programmed desorption (TPD) in a Micromeritics 2910 equipment, using helium as carrier gas. Thereby, the samples were degassed by increasing the temperature at 15 °C·min-1 to 550 °C in flowing He (50 cm3·min-1) and remaining at 550 °C for 30 min. Then, the saturation of the samples under a stream of ammonia (35 cm3·min-1) was carried out at 180 °C for 30 minutes. Next, the physically adsorbed ammonia was removed by flowing He (50 cm3·min-1) at 180 °C during 90 min. Thereafter, the chemisorbed ammonia was desorbed by increasing the temperature up to 550 °C (heating rate of


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15 °C·min-1) in flowing He (50 cm3·min-1). The variation of ammonia concentration in the helium stream, after passing through the sample bed, was recorded using a thermal conductivity detector (TCD). Argon physisorption isotherms at 87 K were measured with a Quantachrome AUTOSORB sorptometer. Prior to the sorption analyses, the samples were kept at 300 °C and P < 10-5 mmHg for 3 h. The total surface area was estimated according to the BET equation, whereas the micropore volume and the distinction between outer and inner surface was calculated by the t-plot method. The size of both zeolite crystals and metallic species was derived from transmission electron microscopy (TEM) images, obtained with a PHILIPS TECNAI 20 electron microscope, operating at 200 kV and 2.7 Å resolution. Previously to the observation, the catalysts particles were dispersed in acetone, stirred in an ultrasonic bath and droplet placed onto a Cu coated grid. Thermogravimetric tests of the used catalysts were carried out on a SDT 2960 DSC-TGA instrument in order to characterize and determine the amount of coke deposited over them. To that end, ca. 10 mg of sample were loaded on a platinum microcrucible and heated up to 1000 °C at 10 °C·min−1 with flowing air at 100 cm3·min−1. Fourier transform infrared (FTIR) spectra of the used catalysts were obtained to determine the nature of the coke. FTIR spectra were collected at room temperature, using a VARIAN EXCALIBUR SERIES 3100–UMA 600 spectrometer with a resolution of 4 cm-1, in a range from 400-4000 cm-1, and 64 scans. The used samples were previously pelletized using KBr as a diluent (proportion 1:200 wt%) and applying mechanical compression of 9000 kg·cm-2.


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2.4. Catalytic experiments and product analysis The activity of the prepared catalysts was studied in a fixed-bed reactor (stainless steel: ID 10 mm and 300 mm length), at atmospheric pressure and under an inert atmosphere of nitrogen. The catalyst (0.25 g each run) was placed over a thin coat of glass wool, which is supported on a stainless steel grid located in the middle of the reactor length. The rapeseed oil was fed using a HPLC pump, previously adjusted to supply a uniform flow between 0.01 and 5 cm3·min-1. After flushing nitrogen for 10 min, the reactor temperature was increased using a tubular ceramic furnace. The final temperature and the heating rate were controlled by means of an inner thermocouple located in the catalyst bed. More details concerning the experimental setup are given elsewhere [17, 18]. Once the desired temperature was reached (550 °C), the supply of nitrogen was stopped and a steady flow of rapeseed oil was fed into the reactor at 7.6 h-1. A stabilization time of 30 min was set before start collecting the samples of reaction products in order to ensure a stationary state. The tests were extended for an overall reaction time of 180 min and samples of products were collected each 60 min. At the outlet of the reactor, the product mixture was separated in a condenser at 0 °C. Thus, the gas phase flowed up to a glass water trap, being analyzed in-situ using a VARIAN micro-GCCP4900 chromatograph. This apparatus was equipped with 3 channels and independent columns: (i) Molsieve 5A (carrier gas Ar) for detecting H2, O2, N2, CO, CO2 and CH4, (ii) PPQ (carrier gas He) for hydrocarbons from C2 to C4 and (iii) CP-SIL (carrier gas He) for C5 and C6 hydrocarbons. The liquid products were kept in the condenser and recovered in glass vials at the established sampling times. This fraction was mostly composed by a mixture of organic compounds and aqueous phases that were separated by decantation. A VARIAN GC-3800 chromatograph was used to quantify the composition of the organic fraction. This instrument was equipped with a CP


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SIL PIONA column (0.25 mm internal diameter and 100 m length) and a flame ionization detector (FID), being previously calibrated to detect and quantify hydrocarbons in the range C2-C16. At the end of the reaction, the supply of oil was stopped and the reactor was left cooling down to room temperature. Afterwards, the used catalyst was recovered, washed with acetone and dried for characterization analyses.

3. Results and discussion 3.1. Catalysts properties Four different catalyst samples were prepared by incorporation of Ga and Zn to the parent HZSM-5 zeolite. As shown in Table 2, for each metal two samples have been synthesized varying the metal loading. The parent zeolite and the impregnated samples were characterized by different techniques in order to study the effects of metal addition. The HZSM-5 sample used as support possesses nanocrystals with 20-80 nm particle size. The presence of a significant contribution of external surface area is expected to improve the dispersion of the metallic species and promote their interaction with the support. The content of the metallic species in the different catalysts was determined by ICP-AES. The amount of the metals effectively incorporated to the catalysts is slightly lower to the nominal values (Table 2).


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Table 2. Physicochemical properties of the ZSM-5 catalysts.

(a) (b) (c)

Catalyst

Nominal Acid sites(c) Metal metal (b) (b) Smic(b) Vmic(b) Acidity (a) SBET Sext content loading Tmax (m2/g) (m2/g) (m2/g) (cm3/g) (meq (wt%) (wt%) (°C) NH3/g)

HZSM-5

-

-

404

100

304

0.123

0.424

365

Ga/HZSM-5 (1) 4

3.87

375

85

290

0.111

0.421

381

Ga/HZSM-5 (2) 8

6.50

361

103

253

0.104

0.341

389

Zn/HZSM-5 (1) 4

3.35

364

96

268

0.109

0.346

287

Zn/HZSM-5 (2) 8

7.01

335

108

227

0.091

0.435

260

Based on ICP-AES measurements. Textural properties from Ar adsorption isotherms at 87 K. Acid properties from ammonia TPD measurements.

The crystallinity of the samples was verified by X-ray diffraction (Figure 1). In all diffractograms, the position of the diffraction signals match with the characteristic pattern of the MFI zeolite structure. The absence of an amorphous baseline, together with the presence of welldefined zeolitic peaks, denotes a high degree of crystallinity of the samples after the impregnation of the metal components. Regarding the incorporated metal phases, it has not been possible to distinguish the corresponding diffraction peaks in the analyzed 2θ range. This fact suggests that a high dispersion of the metal species has been effectively obtained in the samples impregnated with zinc and gallium.


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Zn/HZSM-5 (2)

Zn/HZSM-5 (1)

I (a.u.)

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Ga/HZSM-5 (2)

Ga/HZSM-5 (1)

HZSM-5

5

10

15

20

25

30

35

40

45

50

2

Figure 1. XRD patterns of the HZSM-5 catalysts. All the materials present argon adsorption isotherms at 87 K (Figure 2) with a large adsorption capacity at low and high relative pressures (P/P0 < 0.1 and P/P0 > 0.9), characteristics of microporous materials in which adsorption takes place not only in the micropores but also in interparticles spaces existing between the zeolite nanocrystals. Likewise, a moderate adsorption is also observed at intermediate values of relative pressure (0.1 < P/P0 < 0.9) associated to the external surface area of the nanocrystals. Table 2 summarizes the textural properties of the HZSM-5 catalysts. All the catalysts present a high external surface area with values of Sext in the range 85 108 m2·g-1 due to the nanocrystalline nature of the support. The incorporation of metallic phases into the HZSM-5 zeolite results in a decrease in the adsorption capacity of the catalysts. As expected this effect is more pronounced as the metal loading increases and it results in a reduction of the BET specific surface area (SBET) and micropore volume (Vmic). This fact may be originated


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from the partial blockage of the zeolite microporous system by the metallic phases. The strongest modification of the textural properties is observed with the Zn-containing samples. For these materials, the incorporation of Zn causes a significant reduction of the SBET, Smic and Vmic parameters, whereas the external surface area is little affected, suggesting that the Zn species are preferentially located within the zeolite micropores having, consequently, a high degree of dispersion. This effect is also observed, although less pronounced, for the two samples incorporating Ga. 400 400400

HZSM-5 HZSM-5 HZSM-5 Ga/HZSM-5 (1) Ga/HZSM-5 (1)(1) Ga/HZSM-5 Ga/HZSM-5 (2) Ga/HZSM-5 (2)(2) Ga/HZSM-5

0.16 0.16 0.16

0.16 0.14

dV/dlogD(cm /g) 3 dV/dlogD(cm /g)

0.12

300 300300

3

0.10 0.10

3

3

0.10

Volads (cm3 /g)STP 3 Vol (cm Volads /g)STP (cm/g) ads STP

0.14 0.14

0.12 0.12 0.12

dV/dlogD(cm /g)3/g) dV/dlogD (cm

350 350350

0.08 0.10 0.08

0.08 0.06

250 250250

0.06 0.06

0.04 0.06 0.04

0.04 0.02

0.02 0.02

Ga/HZSM-5 (2) Ga/HZSM-5 (2) (2) Ga/HZSM-5

0.00 0.00 0.00

200 200200

0.00

446

4

66 8

4

1

10 1214 1416 1618 1820 20 12 12 14 16 18 20 8810 1010 14 12 16 18 20 Pore diameter Pore diameter (Å) (Å) Pore diameter (Å) (Å) Pore diameter

6

8

150 150150 100 100100 50 50 50 0 00 0.0 0.00.0

0.16

0.16

0.14

0.14 0.14

0.6 0.60.6

P/P P/P P/P 0 00

0.8 0.80.8

1.0 1.01.0

HZSM-5 HZSM-5 HZSM-5 Zn/HZSM-5 (1) Zn/HZSM-5 (1)(1) Zn/HZSM-5 Zn/HZSM-5 (2) Zn/HZSM-5 (2)(2) Zn/HZSM-5

0.12

0.12

0.12 0.12

0.10 0.10

3

0.10

3

0.4 0.40.4

0.16 0.16

3/g) dV/dlogD(cm dV/dlogD (cm/g)

350 350350

0.2 0.20.2

dV/dlogD(cm /g) 3 dV/dlogD(cm /g)

400 400400

300 0.08 0.080.08 300300 0.10

3

Volads (cm /g)STP 3 3 Vol (cm Volads /g)STP (cm/g) ads STP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06

0.06 0.06

0.04 0.04 250 0.04 250250 0.06 0.02

0.02 0.02

0.00

0.00

0.00 200 0.00 200200 4

Zn/HZSM-5 (2) Zn/HZSM-5 (2) (2) Zn/HZSM-5 4

46

4

6

68

6

8

810

10 12 10

1

12 14 12

14 16 14

8 Pore 10diameter 12 (Å)(Å)14 Pore diameter Pore diameter (Å)

150 150150

16 18 16

16 Pore diameter (Å)

18 20 18

18

20 20

20

100100 100 50 50 50 0 00 0.00.0 0.0

0.20.2 0.2

0.40.4 0.4

0.60.6 0.6

P/P P/P P/P 0 0 0

0.80.8 0.8

1.01.0 1.0

Figure 2. Ar physisorption isotherms and pore size distributions of the HZSM-5 catalysts.


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The acid properties of the catalysts, determined by means of ammonia TPD (Figure 3), are summarized in Table 2. The overall concentration of acid sites (meq NH3·g-1) is determined by the amount of ammonia desorbed, whereas the desorption temperature of the maximum (Tmax) is an indication of the acid strength. HZSM-5 zeolite possesses mainly Brönsted acid sites of high acid strength (0.424 meq NH3·g-1 and 365 °C), which derive from bridging hydroxyls connected to framework aluminium atoms. Nevertheless, the TPD profile of the parent HZSM-5 also shows a shoulder at about 220 °C that could be attributed to weak acid sites. As illustrated in Figure 3, the impregnation of the metallic species causes some changes in the TPD profiles regarding the parent HZSM-5 zeolite. The incorporation of gallium species decreases the amount of desorbed ammonia, which is accompanied by a small shift of the TPD peak maximum towards higher temperatures (Tmax = 381-389 °C). This fact may be related to a partial blocking of the zeolite microporous system, hindering the access to acid centers there located, or to the exchange of some of the compensation zeolitic protons by metal ions during the impregnation process [23, 24]. On the other hand, the addition of Zn significantly modifies the HZSM-5 acidic features as denoted by the changes observed in the ammonia TPD curves. The incorporation of Zn reduces the concentration of strong acid sites, although a new peak appears at lower temperatures (260 - 287 °C), which increases sharply with the zinc loading. This effect has been also observed in previous works, being assigned to new Lewis acid sites generated over the zeolitic catalysts directly associated to the Zn species [25, 26].


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600 HZSM-5 Zn/HZSM-5 (1) Zn/HZSM-5 (2)

550

450 400 350 300

Temperature (ºC)

TCD signal (a.u.)

500

250 200 150 0

10

20

30

40

50

60

Time (min) 600 HZSM-5 Ga/HZSM-5 (1) Ga/HZSM-5 (2)

550

450 400 350 300

Temperature (ºC)

500

TCD signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250 200 150 0

10

20

30

40

50

60

Time (min)

Figure 3. Ammonia TPD curves of the HZSM-5 catalysts. Figure 4 illustrates TEM micrographs of the samples impregnated with the highest metal loadings, being selected as representative in order to show the average crystal size of the zeolitic support (200 nm scale bar) and the dispersion and size of the metal particles (20 nm scale bar). The addition of the metallic phases is denoted by the presence of nanoparticles with a higher contrast over the zeolite crystallites. Thus, metal particles can be clearly appreciated in the Zn/HZSM-5 samples, exhibiting relatively uniform sizes within the 2-5 nm range. However, the


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metal nanoparticles in the Ga/HZSM-5 material are more difficult to be identified in the TEM micrographs (Figure 4a), due to their lower contrast regarding the zeolitic matrix. Whereas a great part of the metal nanoparticles in the TEM images are observed mainly distributed over the outer part of the zeolite particles, the possible presence of smaller and more dispersed metal species within the HZSM-5 micropores cannot be discarded as they would present sizes too small to be detected in the TEM images.

a)

a)

200 nm

b)

20 nm

b)

200 nm

20 nm

Figure 4. TEM images of the catalysts: (A) Ga/HZSM-5 (2) and (B) Zn/HZSM-5 (2). In summary, it can be concluded from the characterization of the catalyst samples that the impregnation of the nanocrystalline HZSM-5 zeolite with the two metals here studied has occurred


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with a high degree of dispersion of the metallic phases, causing significant changes in the acidic and textural properties of the zeolitic support.

3.2. Catalytic conversion of rapeseed oil The behavior of the synthesized catalysts in the conversion of rapeseed oil was tested in a down flow fixed bed reactor. The operation conditions were a reaction temperature of 550 °C, a weight hour space velocity (WHSV) of 7.6 h-1 and a global reaction time of 180 min. All the tests were carried out under an inert atmosphere of nitrogen without any external supply of hydrogen. As a result of the catalytic cracking reactions, the rapeseed oil was converted into a product mixture that consisted of organic liquid products, water, gaseous components and coke. The mass balance between the amount of oil fed into the reactor and the contributions of the detected products (gas, liquid and solid fractions) resulted in errors lower than 5 % for all the experiments. Under the selected operations conditions, the conversion of the raw material was complete, since among the reaction products, triglycerides or carboxylic acids were not detected in the product mixture. In order to discuss the results here obtained, the reaction scheme proposed in an earlier work for the conversion of vegetables oils over heterogeneous acid catalysts, such as HZSM-5, has been taken into account [18]. According to this work, the first step consists of a series of deoxygenation reactions, through which the oxygen contained in the triglycerides molecules is removed as carbon monoxide, carbon dioxide and water. Next, the hydrocarbon chains of the fatty acids are subjected to cracking reactions reducing their molecular weight and leading to the formation of light olefins and aliphatic hydrocarbons. The produced olefins may take part also as reactants in a number of reactions: (i) conversion into the corresponding light paraffins through hydrogen transfer reactions, (ii) formation of naphthenic hydrocarbons by oligomerization and cyclization, and (iii)


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dehydrogenation/aromatization that leads to the production of aromatic hydrocarbons. Finally, carbonaceous residues (coke) can be formed by condensation/oligomerization reactions of both aromatic and olefinic precursors, covering the surface and filling/blocking the pores of the catalyst. Figure 5 illustrates the product distribution obtained with the studied catalysts for the total operation time (0-180 min), in terms of mass percentage (wt%). Since the starting rapeseed oil is mostly composed of carboxylic acids with a number of carbon atoms under 18 (see Table 1), the resulting hydrocarbons usually do not exceed the diesel boiling point range. Thus, the main fractions here obtained correspond with hydrocarbons in the gaseous (C1-C4) and gasoline (C5C10) ranges, coming mostly from the cracking reactions of larger species over the acid centers of the catalysts. The strong acid character of HZSM-5 zeolite promotes extensive cracking leading to a low proportion of heavy hydrocarbons in the range of kerosene and diesel. According to different authors [27-29], alkanes and alkenes may be activated over Brönsted acid sites to form carbonium ion transition states, which subsequently can undergo C-C bond cleavage, yielding shorter hydrocarbons. In particular, the strong acidity of HZSM-5 zeolite favors the production of light olefins, mostly ethylene and propylene, via carbenium ion transition states [27, 28]. These compounds exhibit a high reactivity which may result in the formation of a variety of larger hydrocarbons by means of oligomerization, cyclization and aromatization reactions. Likewise, hydrogen transfer reactions may play an important role determining the degree of aromatization.


18

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HZSM-5 Ga/HZSM-5 (1) Ga/HZSM-5 (2) Zn/HZSM-5 (1) Zn/HZSM-5 (2)

50

40

30

wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

10

0

H2

CO

CO2

H2O

C1-C4

C5-C10

C11-C13

C14+

Coke

Figure 5. Conversion of rapeseed oil over the HZSM-5 catalysts: overall product distribution. (T = 550 ºC, WHSV = 7.6 h-1, WCAT = 0.25 g, t = 180 min).

The oxygen present in the starting raw materials can be removed by decarboxylation, decarbonylation and dehydration, which lead to the formation of carbon dioxide, carbon monoxide and water, respectively (see Figure 5). It can be concluded that a high degree of deoxygenation has been reached for all the catalysts, since the oxygen mass balances, derived from the obtained product distribution (considering CO, CO2 and H2O), are close to the oxygen content of the starting oil (8.9 wt%). For the parent HZSM-5 zeolite, the weight yield of these compounds follow the order CO > H2O > CO2. The relative extent of the deoxygenation pathways is modified by the metal phases incorporated to the zeolitic support. The two metals here investigated cause a decrease in the production of water, denoting a general trend to inhibit the occurrence of dehydration reactions. Regarding the production of CO and CO2, minor variations are denoted for the Ga/HZSM-5 samples. However, the Zn/HZSM-5 materials have a different behavior, showing a strong enhancement in the formation of CO2. This fact is accompanied by the total absence of water in the product distribution and a significant increase in the H2 production, suggesting that


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the presence of Zn phases in the catalyst promotes the occurrence of the water gas shift reaction between CO and H2O leading to a higher production of CO2 (7.1 wt%) and H2 (2 wt%). This observation is in agreement with the use of Zn-containing catalysts in water gas shift reactions [30-32]. The product distribution in terms of hydrocarbon fractions is significantly modified by the metallic species incorporated to the HZSM-5 zeolite, as illustrated in Figure 5. The presence of metallic sites in the catalysts leads to a decrease in the production of gaseous hydrocarbons, whereas the formation of components in the gasoline range is increased. Indeed, Zn/HZSM-5 materials display the highest yields into the C5-C10 fraction, with values close to 50 wt%. The above commented reduction in the concentration of strong acid centers in the impregnated catalysts might lead to a decrease in the cracking reactions shifting the hydrocarbon distribution towards higher molecular weight compounds. According to Figure 6, the main components of the gaseous hydrocarbons are ethylene and propylene, which are characteristic products of cracking reactions over HZSM-5 zeolites [33]. To a lesser extent, it is also noteworthy the formation of light alkanes, such as methane, ethane and propane, and of C4 olefins (isobutene/1-butene). The addition of Ga to the HZM-5 zeolite provokes some changes in the distribution of light hydrocarbons, which are mainly reflected in a decrease of the ethylene and propylene yields. Interestingly, this effect is quite more pronounced for the Zncontaining catalysts since the gaseous olefins yields are about half of those obtained over the parent zeolite. This result is not accompanied by a higher production of ethane and propane, indicating that the hydrogenation of those olefins is not the reason to explain such a high reduction. Consequently, the decrease in the production of light olefins over the Zn/HZSM-5 samples can be assigned to a lower extension of cracking reactions or to consumption of ethylene and propylene


20

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through different reactions (oligomerization, cyclization and aromatization) due to their high reactivity.


40

Gas fraction (mol%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

20

10

0

dro Hy

gen

e e ne ne xid xid yle tha dio ono Eth Me n m o n rb rbo Ca Ca

e e ne ane ane pen pan ute but Eth Pro Pro 1-B / Iso e en but I so

e nes tan ute Bu 2-B

Figure 6. Conversion of rapeseed oil over the HZSM-5 catalysts: gas composition. (T = 550 ºC, WHSV = 7.6 h-1, WCAT = 0.25 g, t = 180 min).

Figure 7 illustrates the nature of the hydrocarbons present in the product stream. In all cases, the main types are light olefins and aromatic hydrocarbons, which can be employed as raw chemicals in the petrochemical industry. The proportion between these two types of hydrocarbons is altered by the generation of metallic sites in the HZSM-5 support. The incorporation of Ga and Zn to the HZSM-5 zeolite increases the production of aromatic hydrocarbons and reduces the formation of gaseous olefins. This effect become more pronounced for the catalysts impregnated with Zn, showing aromatic yields clearly superior to the value corresponding to the parent HZSM-5 zeolite. Interestingly, the enhanced production of aromatics comes with a strong reduction in the light olefins yield, confirming the close relationship between both types of hydrocarbons. It is also worth to note that these effects, increase of aromatics and decrease of light olefins, are slightly


21


lower in the case of the sample with the highest Zn content. This fact may be related to the more pronounced reduction of textural properties caused by the incorporation of Zn in the Zn/HZSM-5 (2) sample (Table 1).

44


40 36 32

wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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28 24 20 16 12 8 4 0

-C 4 -C 13 -C 13 -C 13 -C 4 -C 13 -C 13 s C5 s C2 s C5 s C5 s C5 s C1 s C6 n n e n i n i n c i i i f f n i f f f t f le af he raf Ole ma ara Par pht ht O -Pa Aro ht P Na Lig Iso Lig

-C 16 C 14

wn kno Un

Figure 7. Conversion of rapeseed oil over the HZSM-5 catalysts: product distribution by hydrocarbons groups. (T = 550 ºC, WHSV =7.6 h-1, WCAT = 0.25 g, t = 180 min). These results indicate that one of the main effect of the addition of Ga, and especially, of Zn species to the zeolite is to promote the transformation of the light olefins into aromatic hydrocarbons through a pathway of oligomerization/cyclization/aromatization reactions, although determining the specific role of the metallic species is not simple [22, 34]. The metallic sites promote dehydrogenation reactions by C-H cleavage with the subsequent removal of hydrogen, while Brönsted acid sites show preference for C-C bond cleavage, reducing progressively the size of the reactant molecules. Thus, the combination of both functionalities leads to a high formation of light olefins required in the aromatization steps [35]. Subsequently, these compounds may be


22

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readsorbed in the active sites (both metallic and zeolitic centres) where they are subjected to oligomerization and cyclization reactions under the steric restrictions of the microporous system. The final step in this sequence is aromatization to yield C6-C8 aromatic hydrocarbons by dehydrogenation of the cyclic components, which is especially promoted by active sites linked to Ga and Zn species. In particular, this last effect is strongly enhanced when incorporating Zn species into the HZSM-5 zeolite, yielding a gasoline fraction as main product having a high share of aromatic hydrocarbons (close to 80 wt%). This is a relevant result as these products could be employed as raw chemicals or in the formulation of advanced biofuels.

3.3. Evolution of the product distribution along the time on stream In order to study the effects of coke deposition on the catalyst performance in rapeseed oil conversion, product samples of liquid and gas phases have been taken at different reaction times (every 60 min). The evolution of the yields of the main products along the reaction time is shown in Figure 8.


23


40

wt% Light olefins (C2-C4)

wt% Light paraffins (C1-C4)

35

12 10 8 6 4

b)

0-60 min 60-120 min 120-180 min

a)

0-60 min 60-120 min 120-180 min

14

30 25 20 15 10 5

2

0

0

-5 SM

HZ

G

5 MZS a/H

(1)

-5 SM /HZ Ga

(2)

-5 SM /HZ Zn

(1)

-5 SM /HZ Zn

(2)

-5 SM

HZ

5 MZS

(1)

/H Ga

5 MZS

(2)

-5 SM

(1)

Z /H Zn

/H Ga

5 MZS

(2)

/H Zn

25

50

c)

0-60 min 60-120 min 120-180 min

45

d)

0-60 min 60-120 min 120-180 min 20

wt% Aliphatics (C5-C13)

40 35

wt% Aromatics

30 25 20 15

15

10

5

10 5

0

0 5 M-

S HZ

Z

/H Ga

-5 SM

(1) /H Ga

-5 SM

(2)

Z

Z /H Zn

-5 SM

(1) Z /H Zn

-5 SM

(2)

-5 SM

HZ

5 MZS

(1)

/H Ga

5 MZS

/H Ga

(2)

-5 SM

Z /H Zn

(1)

5 MZS

(2)

/H Zn

e)

0-60 min 60-120 min 120-180 min

2.5

2.0

wt% Hydrogen

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.5

1.0

0.5

0.0

HZ

-5 SM

/HZ Ga

-5 SM

(1)

-5 SM

/HZ Ga

(2)

1)

( -5 SM

/HZ Zn

-5 SM

(2)

/HZ Zn

Figure 8. Conversion of rapeseed oil over the HZSM-5 catalysts: evolution of the product distribution along the time on stream; (a) light paraffins, (b) light olefins, (c) aromatics hydrocarbons (d) aliphatics hydrocarbons and (e) hydrogen. (T = 550 ºC, WHSV = 7.6 h-1, WCAT = 0.25 g).


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For all the catalysts, the production of light paraffins (Figure 8a) is strongly decreased along the reaction time. In the case of Ga-impregnated catalysts, this effect comes with a reduction in the yield of light olefins (Figure 8b). These results show that the cracking activity of the catalysts is significantly affected by the coke deposition. However, a different behaviour is observed for the Zn/HZSM-5 catalysts since in this case the reduction in the light paraffins yield takes place simultaneously with an enhanced production of light olefins. For these materials coke deposition seems to affect in a higher extension to hydrogen transfer reactions, having a negative effect on the formation of light paraffins, whereas the cracking activity is less altered. Regarding the aromatic hydrocarbons fraction (Figure 8c), although some variability is observed among the samples, its yield tends to decrease along the reaction time, suggesting that the formation of aromatic hydrocarbons is negatively affected by the deposition of coke. This fact may be explained taking into account that aromatics are final products in the overall reaction pathway, being originated as a result of a series of catalytic steps (oligomerization, cyclization and aromatization), all of them being in a higher or lower extension negatively affected by the formation of coke. It is interesting to point out that, in the case of the catalyst containing the highest Zn loading, the aromatics yield is little changed during the reaction time, showing that coke deposition in this case does not modify its remarkable aromatization activity. This can be viewed as other positive consequence of the incorporation of Zn species to the HZSM-5 zeolite with the consequent generation of new acid sites. For all the studied catalysts a general trend is observed regarding aliphatic hydrocarbons within the C5-C13 range (Figure 8d). The yield of this fraction increases continuously with the time on stream, which can be rationalized considering the previous comments on the evolution of the other hydrocarbon types. Since these aliphatic hydrocarbons participate as reactants in cracking


25


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reactions leading to light paraffins and olefins, it is easily understood that the deactivation of the acid sites of the catalyst results in a higher proportion of aliphatic hydrocarbons being detected in the liquid fraction so produced. Finally, it is interesting to point out that great differences are observed among the catalysts regarding the hydrogen production (Figure 8e). Thus, for the parent HZSM-5 zeolite just small amounts of hydrogen are present in the gaseous products. In contrast, Ga/HZSM-5 and Zn/HZSM5 materials show enhanced yields of hydrogen, which can be mainly assigned to the activity of the metal sites in these systems for catalyzing dehydrogenation and aromatization reactions. Nevertheless, using the Zn/HZSM-5 samples hydrogen could be also obtained by the water gas shift reaction between CO and H2O, as above commented. Moreover, for both types of catalysts the hydrogen formation strongly decreases with the time stream, denoting that those sites suffer a relatively fast deactivation by coke deposition. For the purpose of obtaining further insights about the deactivation of the catalysts by fouling with coke, the used samples were subjected to thermogravimetric analysis (TG) in air (Figure 9a). The total amount of deposited coke varies from 18.8 to 23.1 wt%, with respect to the spent catalyst weight, Compared with the parent HZSM-5 zeolite, the highest difference in terms of coke formation is observed for Ga as this metal increases significantly the total amount of coke, whereas Zn has an almost neglected effect on the deposition of carbonaceous material. The derivative of the used catalysts weight loss (DTG) shows that the weight loss takes place at different ranges of temperature for each catalyst, indicating significant changes in terms of location and nature of the coke deposits (Figure 9b). Two main peaks are observed in the DTG curve of the studied catalysts. The peak at high temperature (500-650 °C) is typically associated with polyaromatic coke [0, 0], whereas the peaks at temperatures lower than 400 °C usually corresponds with the burning step of


26

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heavy hydrocarbons deposited over the external surface area of the zeolite nanocrystals [0, 0]. Likewise, some authors [0-40] differentiate between coke formed and deposited over active sites linked to metallic species and coke accumulated on the zeolitic acid sites. The former usually is removed at lower temperature since the metal phases may catalyze the combustion of the carbonaceous materials. In this way, it is remarkable that for the Zn/HZSM-5 catalyst the high temperature peak is shifted towards lower temperatures by about 40 ºC in respect to the parent HZSM-5 zeolite, confirming the role of the Zn species in catalyzing coke combustion reactions, while the overall content of carbonaceous residues is practically the same in both samples. 102 100

a)

HZSM-5 Ga/HZSM-5 (2) Zn/HZSM-5 (2)

98 96

TG (mass %)

94 92 90 88 86 84 82 80 78 76 74

b)

DTG (%/ºC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

100

200

300

400

500

600

700

800

900

1000

Temperature (ºC)

Figure 9. Thermogravimetric analysis of the used HZSM-5 based catalysts: (a) weight loss (TG), (b) derivative weight loss (DTG).


27


On the other hand, the FTIR spectra of the spent catalysts reveal significant differences about the type of deposited coke (Figure 10). According to the literature [41-44], the vibrational modes in the interval 1300-1800 cm-1 can be associated to branched paraffinic hydrocarbons (1380 cm-1), alkylaromatic hydrocarbons (1450 cm-1), polyaromatic hydrocarbons (1600 cm-1) and olefins (1750 cm-1). Just slight variations in this region can be observed for the Ga-containig catalyst with a higher proportion of alkylaromatics and olefins in the catalyst impregnated with Ga. However, these differences are much more pronounced in the range 2800-3100 cm-1, where Ga/HZSM-5 (2) shows two major peaks at 2855 and 2925 cm-1 ascribed to the vibrations of –CH2 and –CH groups, together with a minor absorption band related to –CH3 groups (2955 cm-1) [43]. Hence, it can be concluded that, in addition to polyaromatic species, the presence of Ga promotes the formation of coke with an enhanced proportion of long chain aliphatic or naphthenic hydrocarbons. In contrast, the FTIR spectrum of the Zn/HZSM-5 sample is very similar to that of HZSM-5, showing that the incorporation of Zn does not change the nature of the coke deposited over the catalyst.

Zn/HZSM-5 (2)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

Zn/HZSM-5 (2)

Ga/HZSM-5 (2) Ga/HZSM-5 (2)

HZSM-5 HZSM-5

0 0 0 0 0 0 00 00 00 00 00 00 130 140 150 160 170 180 27 28 29 30 31 32 -1

Wave number (cm )

-1

Wave number (cm )

Figure 10. FTIR spectra of the used HZSM-5 catalysts.


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Conclusions The catalysts here employed for the rapeseed oil conversion show a high degree of metal (Ga and Zn) dispersion, which is greatly due to the use of a nanocrystalline HZSM-5 sample as support having a significant amount of external surface area available for the deposition of the metal. A moderate reduction of the textural properties, as well as significant variations of the acidic features, of the parent zeolite takes place upon metal incorporation. The latter effect is more remarkable for the Zn-containing materials as the incorporation of this metal cause the generation of new acid sites. The catalytic results obtained in this work show that, by the use of Ga- and Zn-impregnated HZSM-5 zeolites, the starting rapeseed oil can be converted efficiently and with a high deoxygenation degree into hydrocarbon mixtures, with a high share of light olefins and aromatics. Both are valuable compounds for being employed as raw chemicals and/or in the formulation of advanced fuels. Compared to the parent HZSM-5 zeolite, the metal impregnated materials show an increase in the production of BTX hydrocarbons while decreasing the light olefins yield. This interesting effect is more pronounced when adding Zn to the zeolitic support. Thus, for the Zn/HZSM-5 samples, the production of aromatics reached 43 wt% of the overall product distribution, which was accompanied by a sharp increase in the hydrogen concentration in the gas phase. These results denote that dehydrogenation and aromatization reactions are strongly promoted by the Zn species. Moreover, this high selectivity towards aromatic hydrocarbons is maintained with little variation along the time on stream over the catalyst possessing the highest Zn content. Regarding the coke deposited on the catalysts, the behaviour of both metals is very different. While Zn incorporation does not modify the amount neither the nature of the coke, the addition of


29


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Ga increases the total coke content of the catalyst, favouring the formation of carbonaceous species with enhanced paraffinic and naphthenic features. This fact may explain the more pronounced decrease in the aromatization activity observed along the time on stream for the Ga/HZSM-5 catalysts compared to the Zn/HZSM-5 ones. Acknowledgements This research has been supported by the Spanish Ministry of Economy and Competitiveness (projects ENE2011-29643-C02-01 and ENE2011-29643-C02-02).

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14. Serrano-Ruiz, J.C.; Ramos-Fernandez, E.V.; Sepúlveda-Escribano, A. From biodiesel and bioethanol to liquid hydrocarbon fuels: new hydrotreating and advanced microbial technologies. Energy Environ. Sci. 2012, 5, 5638. 15. Horáček, J.; Tišler, Z.; Rubáš, V.; Kubička, D. HDO catalysts for triglycerides conversion into pyrolysis and isomerization feedstock. Fuel 2014, 121, 57. 16. Rahimi, N.; Karimzadeh, R. Catalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce light olefins: A review. Appl. Catal., A 2011, 398, 1. 17. Botas, J.A.; Serrano, D.P.; García, A.; de Vicente, J.; Ramos, R. Catalytic conversion of rapeseed oil into raw chemicals and fuels over Ni- and Mo-modified nanocrystalline ZSM-5 zeolite. Catal. Today 2012, 195, 59. 18. Botas, J.A.; Serrano, D.P.; García, A.; Ramos, R. Catalytic conversion of rapeseed oil for the production of raw chemicals, fuels and carbon nanotubes over Ni-modified nanocrystalline and hierarchical ZSM-5. Appl. Catal., B 2014, 145, 205. 19. Berndt, H.; Lietz, G.; Völter, J. Zinc promoted H-ZSM-5 catalysts for conversion of propane to aromatics II. Nature of the active sites and their activation. Appl. Catal., A 1996, 146, 365. 20. Lubango, L.M.; Scurrell, M.S. Light alkanes aromatization to BTX over Zn-ZSM-5 catalysts Enhancements in BTX selectivity by means of a second transition metal ion. Appl. Catal., A 2002, 235, 265. 21. Rane, N.; Overweg, A.R.; Kazansky, V.B.; Van Santen, R.A.; Hensen, E.J.M. Characterization and reactivity of Ga+ and GaO+ cations in zeolite ZSM-5. J. Catal. 2006, 239, 478.


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Graphical abstract

H

C

O

C O

R1

H

C

O

C O

R2

H

C

O

C

R3

H

Rapeseed oil

CH3 CH3

Cyclization

O

H

Cracking

Catalytic cracking (T= 550 °C) Fixed Bed Reactor Deoxygenation

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Ga and Zn modified nanocrystalline HZSM-5

CH3

Aromatic hydrocarbons


36

Enhanced Production of Aromatic Hydrocarbons ... - ACS Publications

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