Synthesis and Characterization of CoMo and NiMo

TOPICS IN CHEMISTRY AND MATERIAL SCIENCE, Vol. 6 (2011) pp. 197–204 Advanced Micro- and Mesoporous Materials – 11 eds. K. Hadjiivanov, V. Valtchev, S. Mintova, G. Vayssilov

Synthesis and Characterization of CoMo and NiMo/Violet-Type Ultramarine and Its use as Catalysts of the Thiophene Hydrodesulfurization Reaction R. Zacarías, R. Álvarez, F. Ocanto, C.F. Linares Laboratorio de Catálisis y Metales de Transición, Facultad Experimental de Ciencias y Tecnología, Departamento de Química, Universidad de Carabobo, Valencia, Postal Code: 2123, Spain Abstract Violet ultramarine-type sodalites were synthesized from commercial blue ultramarines by mixing NH4 Cl (6w/w%) at 200◦ C. These solids were impregnated with Mo as active phase, and Co or Ni as promoters. These catalytic precursors were characterized by: FT-IR, X-ray diffraction and vis-reflectance. These techniques allowed characterizing the structure’s type, color and catalytic behaviour of these solids. These catalytic precursors were tested in the hydrodesulfurization (HDS) reaction of thiophene. Results showed that the NiMo/violet ultramarine catalyst was more active that the CoMo/violet ultramarine catalyst. Likewise, it was determined a less catalytic deactivation by using the CoMo/violet ultramarine in comparison to the NiMo/violet ultramarine.

1 Introduction The hydrotreating (HDT) reaction is a crucial process in the oil crude refining which is able to remove some heteroatoms such as: S, N, metals, aromatics and others from some oil crude cuts, by using high H2 pressures and temperatures, and a specific catalyst. As environmental regulations are stricter, catalytic exigencies for hydrotreating catalysts will be increased. This could explain the high interest for the synthesis of new catalysts which should be more actives, selective and stables for the hydrotreating reactions [1,2]. Actually, Co-Mo or Ni-Mo pairs supported on alumina are traditionally used catalysts [2] for hydrotreating reactions. In that sense, alternative supports such as ultramarine-type materials could be employed. Ultramarines are sodalites containing encapsulated sulphur species which produce intense colors [3]. The typical formula of this mineral is Na8−x [SiAlO4 ]6 ·[S2 ,S3 ,SO4 ,Cl]2−x . Ultramarine can be defined as a silicoaluminate structure formed by sodalite-type cages β which contains cations and sulphur anions. Si and Al atoms are conISSN 1314–0795©2011 Heron Press Ltd.

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R. Zacarías, R. Álvarez, F. Ocanto, C.F. Linares nected by oxygen bridges forming cubic (Al2 Si2 O8 4-members ) and hexagonal (Al3 Si3 O12 6-members) rings. Sodium cations balance the negative charge of sulphur anions. Chromophore species, such as radical sulphur anions, are found inside the sodalite cage forming sodium sulphur salts. These structures are chemical and thermally stable [3]. Conducted studies have showed that pre− − dominant sulphur species are S− 3 and S2 . The S3 radical is responsible of the high absorption in the visible region; therefore this specie produces a intense blue color [4,5]. Clark et al. [5] showed the nature of sulphur species responsible for the color in the varieties of blue, green, violet and pink ultramarines. S− 3 and − − S− chromophores are present for diverse ultramarines. S and S radicals are in 2 3 2 equal proportion for green ultramarines. For blue ultramarines, S− 3 are in major proportion that the S− 2 radicals. For violet and pink ultramarines, other different radicals are present which offer the violet or pink tone. These tones can be archived by chemical treatment. For the pink and violet ultramarines, these chromophores are not completely defined [5]. Ultramarines are often employed as pigment due to the fact of their exciting colors. However, other applications have been few documented. Wells [6] carried out the hydrodesulfurization reaction of thiophene using different ultramarine-types such as: blue, pale blue, pink and white ultramarine in presence of a H2 atmosphere and different temperatures. Violet ultramarines were not investigated in this work. Results showed that white and pink ultramarines were more active than blue ultramarines at 400◦ C. On the other hand, Mo and Co or Ni oxides are often impregnated on diverse supports and tested in the HDT reactions [7,8]. These metals could improve catalytic properties of these ultramarines in HDT reactions. The goal of this work was to synthesize and characterize CoMo/violet ultramarines and NiMo/violet ultramarines and to test them in the hydrodesulfurization (HDS) reaction of thiophene. 2 Experimental 2.1 Synthesis of violet ultramarine and the catalytic precursors Violet ultramarine was synthesized from commercial blue ultramarine. The blue ultramarine was mixed with NH4 Cl in a 6 w/w% proportion. This mixture was placed in a glass reactor at 200◦ C for 2 h in a glycerin bath. During the heating process, the blue color of the ultramarine was changed to violet. After that, the violet ultramarine was impregnated with a molybdenum solution ((NH4 )6 Mo7 O24 ·4H2 O, 8% as MoO3 ). This solid was dried for 24 h at room temperature. Then, impregnated solids with Mo were divided in two portions. One portion, was impregnated with a Co solution (Co(NO3 )2 ·6H2 O), and the other one, was impregnated with a Ni solution (Ni(NO3 )2 ·6H2 O). A atomic 1:3 Co (Ni):Mo proportion was used. Solids were also dried at room temperature for 24h and, then, they were calcined at 450◦ C for 4 h.

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Synthesis and Characterization of CoMo and NiMo/Violet-Type Ultramarine ... 2.2 Characterization of solids The as-synthesized violet ultramarine and their catalytic precursors, before and after HDS reaction, were characterized by different techniques such as: Xray diffraction (XRD, Phillips PW3710 BASED, CuK(α) : 1,542), FT-IR spectroscopy (Shimadzu 8400s IRFT) and visible spectroscopy(Mira Skan, Hunter Lab, diffuse reflectance). 2.3 HDS reaction Co(Ni)Mo/violet-ultramarines and as-synthesized violet ultramarine were tested in the HDS reaction of thiophene. Thiophene HDS was carried out on a continuous flow reactor working at atmospheric pressure. All catalysts were presulfided prior to catalytic tests using a CS2 (10 v/v%, 10 mL/h)/heptane solution under H2 stream (100 mL/min). The temperature was increased up to 350◦ C, at a rate of 0.0833◦C/s, and kept at these conditions for 3 h. Then, the reaction was performed on 200 mg of catalyst using a liquid feed (2.7 × 10−4 cm3 /s) composed of 10 v/v% of thiophene in n-heptane and H2 (0.25 cm3 /s) at 325◦ C. The system was covered with a heating mantle (150◦C) in order to avoid any condensation of the reaction products. Reaction products were injected to a Varian 3800 (AutoSystem XL) gas chromatographer equipped with a flame ionization detector. 3 Results and Discussion 3.1 Characterization of solids Catalytic precursors showed a nominal chemical composition as 8% MoO3 , 0,83% as NiO or 0,81% as CoO. Figure 1 shows the FT-IR spectrum of an as-synthesized violet ultramarine. As a rule, all solids: violet ultramarine and Co(Ni) Mo/violet ultramarines showed same bands (the violet ultramarine is only shown for clarity). A centered band at 3464 cm−1 corresponds to OH− groups from water molecules occluded

Figure 1. FT-IR spectrum of violet ultramarine.

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Figure 2. Vis-reflectance spectra of synthesized samples and catalytic precursors.

in the solid. Bands between 1140 and 422 cm−1 were attributed to Si-O-Al bonds of the sodalite structure. Bands between 723 and 656 cm−1 correspond to the sodalite “fingerprint” [9]. A very small band around 585 cm−1 is usually attributed to encapsulated sulphur species inside the sodalite cage; however, this band was not observed. A possible explanation could be associated to its low intensity [3]. Figure 2 shows Vis-reflectance spectra of synthesized samples and catalytic precursors. A maximum around 600 nm was attributed to S3− radicals responsible for a blue color. Violet ultramarine, CoMo and NiMo/violet ultramarine showed a less absorbance in comparison to the blue ultramarine (at 600 nm). For the violet ultramarine, in addition to S3− radicals which are found in high − proportion, other sulphur radicals as S− 2 and S4 can be also determined in a low − − proportion. The presence of S2 and S4 radicals could promote a low absorbance in the band placed to 600 nm [9].

Figure 3. XRD pattern of CoMo/violet ultramarines. Unmarked peaks correspond to violet ultramarine.

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Synthesis and Characterization of CoMo and NiMo/Violet-Type Ultramarine ...

Figure 4. XRD pattern of NiMo/violet ultramarines. Unmarked peaks correspond to violet ultramarine.

The presence of Co, Mo and Ni oxides could decrease the intensity of the band maximum at 600 nm by dilution. Likewise, Figures 3 and 4 show the XRD patterns of synthesized CoMo/violet ultramarine and NiMo/violet ultramarine. In both diffractograms were observed the sodalite structure corresponding to violet ultramarine which was preserved during the calcination process. When violet ultramarines were impregnated with Mo and Co or Ni, it was identified, together to the sodalite structure, several metallic oxides (Figures 3 and 4). These oxides were analyzed by their PDF files. For CoMo/violet ultramarine and NiMo/violet ultramarine were identified: CoO (PDF-75-04519, 75-0912) and NiO2 (PDF-85-1977). Mox Oy species were diverse, and by using PDF files were possible to identify: MoO3 (PDF-75-0912, PDF 85-2405 and PDF-80-0347), Mo9 O26 (PDF-86-1269) and Mo4 O11 (PDF-73-1538). Also, Co or Ni molibdate were identified: NiMoO4 (PDF-45-0142) and CoMoO4 (PDF73-1331). 3.2 Catalytic test At the beginning, the violet ultramarine support was tested in the HDS of thiophene. Reaction was carried out sulfided and no sulfided previously the violet ultramarine. Results showed very low conversions (∼ 0%) in the thiophene HDS reaction in both cases. These results are opposite to those reported by Wells [6]. According to [6], green, blue, white, pink ultramarines were actives to the thiophene HDS reaction using reaction temperatures close to 400◦ C. These results were attributed to sulphur radicals from the sodalite structure which were responsible of the catalytic activity. Then, CoMo/violet ultramarines and NiMo/violet ultramarines were also tested in the HDS reactions. Solids were divided in two portions. One portion was previously sulfided and the other one was not sulfide. When solids were 201

R. Zacarías, R. Álvarez, F. Ocanto, C.F. Linares

Figure 5. HDS of thiophene (6 v/v%) using NiMo/violet ultramarine and NiMo/violet ultramarine catalysts.

not sulfided, the catalytic reaction was negligible. This result shows that sulphur radicals from violet ultramarine do not migrate to the surface, and they are not able to sulfide the impregnated metals. Likewise, the sulfided CoMo and NiMo/violet ultramarines were tested by using a 6 and 1% thiophene/hexane flow. When the reaction was carried out by using a 6% thiophene flow, the conversion was very low. Better results were obtained with the commercial catalyst (CoMo/alumina) as was expected (Figure 5). The low results obtained by using the modified ultramarines could be associated to the low surface areas showed by these sodalites [3]. To improve the catalytic activity of these modified ultramarines, the thiophene feed was diluted to 1% (Figure 6). Initially, NiMo/violet ultramarine was more active than CoMo/violet ultra-

Figure 6. HDS of thiophene (1 v/v%) using NiMo/violet ultramarine and NiMo/violet ultramarine catalysts.

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Synthesis and Characterization of CoMo and NiMo/Violet-Type Ultramarine ...

Figure 7. FT-IR spectrum of NiMo/violet ultramarine after HDS of thiophene (6% v/v).

marine, but NiMo/violet ultramarine is deactivated faster than CoMo/violet ultramarines (Figure 6). The presence of the coke deposition could cause this deactivation. Solids were analyzed before and after reaction by FT-IR. Results showed that the sodalite structure remained unchanged. After reaction, the color of ultramarine was changed from violet to black. The black color is due to the coke deposition on the ultramarine surface. However, FT-IR spectra did not show appreciable changes. All bands of the sodalite structure, identified by FT-IR, were found before and after reaction. Therefore, acidic conditions (H2 S) and high temperatures of reaction did not affect the sodalite structure (Figure 7). 4 Conclusions It was possible to synthesize and characterize violet ultramarine from the blue ultramarine. The violet ultramarine did not show catalytic activity for the HDS reaction of thiophene; however the presence the Mo and Ni(Co) oxides on the violet ultramarine improve the catalytic activity of these solids when these catalytic precursor were sulphided. CoMo/violet ultramarine were more stable to thiophene HDS reaction than NiMo/violet ultramarine. Acknowledgements The authors are grateful to CDCH-UC for funding the research carried out in this work. References [1] G. Alves, R. García and R. Cid, Bol. Soc. Chi. Quim. 44 (1999) 337-344. [2] D.D. Whitehurst, H. Farang, T. Nagamatsu, K. Sakanishi and I. Mochida, Catal. Today 45 (1998) 299-305

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R. Zacarías, R. Álvarez, F. Ocanto, C.F. Linares [3] L. Goncalves, “Síntesis de Pigmentos Inorgánicos del Tipo Azul Ultramarino Utilizando Materia Prima Nacional” Licenciature Dissertation, Chemistry Dept., Carabobo Univ., Venezuela (2007). [4] N. Gobeltz, A. Demortier, J.P. Lelieur, and C.J. Duhayon, Soc. Faraday Trans. 94 (1998) 2257-2260. [5] R. Clark, T. Dines, and M. Kurmoo, Inorg. Chemi. 22 (1983) 2766-2772. [6] P.B. Wells, J. Catal. 19 (1970) 372-377. [7] M.J. Ledoux, O. Michaux, G. Agostini,and P.J. Panissod, J. Catal. 102 (1986) 275288. [8] S. Harris and R.R. Chianelli, J. Catal. 98 (1986) 17-31. [9] S. Kowalak, A. Jankowska, and S. Zeidler, Microporous Mesoporous Mater. 93 (2006) 111-118.

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