cyclohexene using calcined Co/Fe/Al, hydrotalcite supported CoMo catalysts. Edwin Oviedo1, Sylvette Brunet2, Carlos Linares1. 1 Department of Chemistry, ...
Hydrodesulfurization
of
thiophene
and
hydrogenation
of
cyclohexene using calcined Co/Fe/Al, hydrotalcite supported CoMo catalysts Edwin Oviedo1, Sylvette Brunet2, Carlos Linares1 1 Department 2
of Chemistry, University of Carabobo, Venezuela
Institut de Chimie des Millieux et Materiaux de Poitiers (IC2MP), UMR, 7285 CNRS-
Université de Poitiers, France.
ABSTRACT Co2+, Fe3+, Al3+ tertiary hydrotalcite-type materials were synthesized with a molar ratio Fe3+/Al3+ at: 0.21, 0.42, 0.50. Then, these synthesized hydrotalcites were calcined and impregnated with Mo (15.wt.% MoO3) and Co (3Mo: Co) and calcined again, to obtain the catalytic precursors: CoMo/CoFeAl(0.21), CoMo/CoFeAl(0.42), CoMo/CoFeAl(0.50) . Assynthesized hydrotalcites and catalytic precursors were characterized by different physicalchemical techniques: surface area BET, elemental analysis, temperature programmed desorption of carbon dioxide (CO2-TPD), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). These calcined hydrotalcites supported CoMo were tested in reactions of thiophene hydrodesulfurization (HDS) and cyclohexene hydrogenation (HYD). Results indicated these catalysts presented few HYD pathway active sites. The CoMo/calcined CoFeAl hydrotalcite presented lower HDS activity than CoMo/Al2O3 (reference catalyst) but the CoMo/CoFeAl(0.42) and CoMo/CoFeAl(0.50) were highly selective towards HDS pathway, which their HDS/HYD ratio were 8.85 and 6.32 respectively, superior to the 1.05 of CoMo/Al2O3..
Keywords:
Hydrotalcite,
Hydrogenation, Iron, basicity.
Thiophene,
Cyclohexene,
Hydrodesulfurization,
1.-Introduction Nowadays, several environmental regulations demand of gasoline and their derivate with low sulfur contents. Gasoline is a mixture of products of different processes from the refinery industry. Hence, the major contribution of sulfur content in the gasoline pool is coming of the FCC unit which provide of a 90 % sulfur compounds and C5-C7 olefins. Olefins are useful because increase the gasoline quality due to increasing of octane number increased. Therefore, hydrodesulfurization (HDS) reactions of gasoline should carefully be controlled in order to increase the desulfurization process keeping a low hydrogenation (HYD) of olefins. In that sense, it is mandatory to develop new hydrotreating catalysts for achieving the permitted sulfur limits without a deep hydrogenation of olefins, i.e. highly-selective HDS catalysts for FCC gasoline. Alumina is the preferred support for the synthesis of hydrotreatment catalysts. Many investigations have been focused in the modifying the alumina or to test another supports in order to enhance the catalytic yield. Previous investigations have found that the pore structure of support and the interaction between support and active phase are two essential factors that play a vital role in the performance of hydrotreatment catalysts. The support has a key function in the activity and selectivity of the catalyst. Therefore, the goal of this work is to study the effect of mixed oxides from calcined CoFeAl hydrotalcites with different Fe3+/Al3+ proportions in order to determine the influence of Fe in the thiophene hydrodesulfurization (HDS) and cyclohexene hydrogenation (HYD) reaction. 2.-Materials and Method 2.2. Preparation of catalytic precursors Calcined HTLc CoFeAl(0.21), calcined HTLc CoFeAl(0.42) and calcined HTLc CoFeAl(0.50) were impregnated by the incipient wetness method using aqueous solutions of ammonium heptamolybdate tetrahydrate (Riedel-de Haën, 98%) and cobalt nitrate hexahydrate (BDH Chemical Ltd, 97%) in order to get 15 wt % of MoO3 in the catalyst and a Mo/Co molar ratio of 3, which was kept constant in all formulations. Solids were dried and calcined to get the final catalytic precursors: CoMo/CoFeAl(0.21), CoMo/CoFeAl(0.42) and CoMo/CoFeAl(0.50). El γ-Al2O3 is the commercial-support
2.3. Characterization of solids obtained Tertiary CoFeAl hydrotalcites and catalytic precursor samples were characterized by different physical-chemical techniques such as: X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), CO2 temperature programmed desorption (CO2-TPD), surface measurements (BET) and chemical analysis. 2.3.1.-Chemical analysis Catalytic precursors were analyzed by X-fluorescence (XRF) by using a S4 Explorer equipment for determining the chemical composition of the catalysts. This equipment was operated using an X-ray tube of rhodium anode and scintillation detector operating on a 20-mA current and 50 kV voltage to obtain the samples spectra. 2.3.2.-. Surface measurements (BET) Surface measurements were carried out in a Tristar 3000 equipment using N2 gas as adsorbent test molecule. Previously, samples were heating at 200 °C by 12 h. 2.3.3.- Fourier- transformed infrared spectroscopy (FT-IR) FT-IR spectroscopy was carried out in Fourier Perkin Elmer model A Analyst 200 FT-IR. Samples were previously mixed with KBr in proportion to 1:3 (sample: KBr) forming a fine pellet. FT-IR analysis was carried out between 500 and 4000 cm-1.
2.3.4.- X-ray diffraction (XRD) X-ray diffractions were carried out in a Philipp PW3442 using a Cu wavelength CuK(α) 1.54060. Samples were molted to obtain a fine powder for their analysis. The identification of phases was done by using PDF files.
2.3.5.- Carbon dioxide temperature programmed desorption (CO2-TPD) Carbon dioxide thermal desorption (CO2-TPD) measurements are performed on a micromeritics Autochem II 2920. Samples were placed into a quartz cell and activated at atmospheric pressure by heating up to 350 °C with a rate of 10 °C.min-1 (helium flow rate: 20
mL.min-1). Samples were kept for 30 min at 350 °C, then cooled at -60 °C for 30 min under a helium flow. Then, catalytic precursors were saturated with 10%CO2/He mixture gas, during 30 min at -60 °C (gas flow: 20 mL.min-1). The injected quantities (2.55 mol) at 100 mBar were higher than those observed in thermogravimetric experiments at the same pressure (about 3.50 mmol. g-1). The catalytic precursors have been swept for 2 h by a stream of dry helium in order to remove reversible adsorbed CO2 at the temperature of -60 °C and clean the cell. The experiments were carried out in a pure helium stream (20 mL.min-1) using a heating rate of 10 °C.min-1 up to 600 °C.
2.4. Catalytic activity measurements Catalytic activity measurements were carried out in a continuous flow reactor working at the atmospheric pressure and 325 °C with 200 mg of catalytic precursor. Before the test, the catalytic precursors samples were presulfided in situ at 400 °C using a vaporized stream of 10 mL.h-1 of a CS2 (10 vol.%)/n-heptane solution mixed with a H2 stream (100 mL.min-1) at atmospheric pressure for 2 h. The reaction of thiophene hydrodesulfurization was carried out using a thiophene (6 vol.%)/heptane solution, i.e. 33939 ppm S with a feed rate of 2.7x10-4 cm3.s-1 mixed with a H2 stream (0.25cm3.s-1) at 325 °C. For the hydrogenation reaction, cyclohexene was used as model feed composed by a cyclohexene (6 vol.%)/heptane solution, i.e. 7wt.% cyclohexene a feed rate of 2.7x10-4cm3.s-1 mixed with a H2 stream (0.25 cm3.s-1) at 325 °C. Reactions products and unreacted feed were analyzed with a Varian 3800 (AutoSystem XL) gas chromatograph equipped with a flame ionization detector and a capillary column (25m, 0.25µm i.d., CP-Sil 5 CB).
3.-Results and discussion 3.1.- Catalytic evaluation 3.1.1.-Catalytic activity Table 1.- Catalytic activities and selectivity (HDS/HYD) for CoMo/CoFeAl for thiophene HDS and cyclohexene HYD reactions at T=325°C, P=1 Bar, H2/feed=1800, solvent: heptane
CoMo/CoFeAl(0.21)
Thiophene HDS Activity (mmol/h/m2) 4.78
Thiophene HDS Activity (mmol/h/g) 181.64
Cyclohexene HYD Activity (mmol/h/m2) 4.55
Cyclohexene HYD Activity (mmol/h/g) 172.9
CoMo/CoFeAl(0.42)
29.61
1273.23
3.34
143.62
8.85
CoMo/CoFeAl(0.50)
29.34
909.54
4.64
143.84
6.32
CoMo/Al2O3
7.67
1564.68
5.65
1152.6
1.36
Catalyst
Selectivity HDS/HYD 1.05
The catalyst CoMo/Al2O3 presented the highest HDS catalytic activity, as can be seen in Table 1. However the CoMo/CoFeAl(0.42) and CoMo/CoFeAl(0.50) were more active than CoMo/Al2O3 per surface area, as can be observed in Table 1, this could be due to their largest pore diameter (Table 2),
Activity (mmol/h/m2)
35 30
HDS Thiophene
25
HYD cyclohexene
29.61
29.34
20 15 10
7.67 5.65
5
4.78 4.55
4.64
3.34
0 CoMo/Al2O3
CoMo/CoFeAl(0.21)
CoMo/CoFeAl(0.42)
CoMo/CoFeAl(0.50)
Figure 1.- Catalytic Activity of CoMo / calcined CoFeAl hydrotalcites as thiophene HDS as cyclohexene HYD at T=325°C, P=1 Bar, H2 / feed=1800, 33939ppmS / thiophene, 7.wt.%
HDS/HYD
cyclohexene, solvent: heptane
10 9 8 7 6 5 4 3 2 1 0
8.85
6.32
1.36
CoMo/Al2O3
1.05
CoMo/CoFeAl(0.21)
CoMo/CoFeAl(0.42)
CoMo/CoFeAl(0.50)
Figura 2.-Selectivity HDS/HYD ratio T=325°C, P=1 Bar, H2/feed=1800, 33939ppm S / thiophene, 7.wt.%, cyclohexene, solvent: heptane
3.1.2.-Selectivity The precursors CoMo/CoFeAl(0.42) and CoFeAl(0.50) presented the highest HDS/HYD ratio 8.85 and 6.32 respectively (Table 1). This fact of an increasing of the HDS/HYD relationship is desirable to preserve the octane number of the gasoline and to avoid the hydrogen consumption by hydrogenation. Indeed, these mixed oxides, possessed the optima relationship among metals, promoting HDS reaction and diminishing the olefins hydrogenation, favoring the selectivity towards HDS instead HYD, maximum HDS/HYD.
4.-Conclusions The CoMo/CoFeAl(0.42) was the most active among catalysts from calcined hydrotalcites used as supports. The CoMo/calcined CoFeAl hydrotalcite were few actives to HYD reaction of cyclohexene. The HDS activity of thiophene (converted mmol per surface area) was higher for the CoMo/calcined CoFeAl hydrotalcites than CoMo/Al2O3, except CoMo/CoFeAl (0.21) The CoMo/CoFeAl(0.42) presented the highest HDS/HYD ratio (equal to 8.85), around 8 fold than CoMo/Al2O3, what is desirable to preserve the gasoline octane number, promoting less consumption of hydrogen during hydrotreatment. Also, the CoMo/CoFeAl(0.50) presented a higher HDS/HYD ratio (equal to 6.32) than CoMo/Al2O3 The HDS activity of thiophene increased as well as Fe3+/Al3+ ratio was increased HDS/HYD ratio increased as well as Fe3+/Al3+ ratio was increased The HDS activity of thiophene is favored over CoMo/calcined CoFeAl hydrotalcite compared to CoMo/Al2O3 catalyst
