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Catalytic Synthesis of Methanethiol from H2S-Rich Syngas Over Sulfided Sio2-Supported Mo-Based Catalysts

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Catalytic Synthesis of Methanethiol from H2S-Rich Syngas Over Sulfided Sio2-Supported Mo-Based Catalysts Catal Lett (2008) 121:260–265 DOI 10.1007/s10562-007-9326-z Catalytic Synthesis of Methanethiol from H2S-rich Syngas Over Sulfided SiO2-supported Mo-based Catalysts Ai Ping Chen Æ Qi Wang Æ Ying Juan Hao Æ Wei Ping Fang Æ Yi Quan Yang Received: 11 September 2007 / Accepted: 22 October 2007 / Published online: 6 November 2007 Ó Springer Science+Business Media, LLC 2007 Abstract The synthesis of methanethiol from H2S-rich well-known intermediate used in the synthesis of organo- syngas was investigated over sulfided Mo-based catalysts sulfur compounds, such as methionine, dimethyl sulfoxide, supported on SiO2. At CO/H2/H2S = 1/1/2, 0.2 MPa, and dimethyl sulfone [9]. Methanethiol is industrially -1 3,000 h , and 300 °C, mainly CH3SH, COS, and CO2 produced by the reaction of methanol and hydrogen sulfide were formed, along with small amounts of hydrocarbons over supported metal oxide catalysts, such as K2WO4/ and thioethers over potassium-promoted Mo-based cata- Al2O3 and Cs2WO4/Al2O3 [10]. Obviously, compared to lysts. Studies of the reaction pathway show that COS is a the CH3OH-H2S route for the production of methanethiol, primary product, which is hydrogenated to CH3SH and it would be attractive and promising if a simple feedstock, H2O. Most of CO2 originates from water-gas shift reaction. such as H2S-rich syngas (i.e., high H2S-containing syngas The hydrocarbons and thioethers originate from the or CO/H2/H2S mixtures), could be used. hydrogenation of CH3SH. Based on different product distribution and catalysts, different pathways for the synthesis of methanethiol have Keywords Methanethiol synthesis Á Hydrogen sulfide Á been proposed. Based on results of the co-adsorption of CO Syngas Á Mo-based catalysts and H2S over anatase and rutile, Beck et al. [11] proposed a reaction pathway for the formation of CH3SH from the hydrogenation of carbonyl sulfide (COS), which proceeded 1 Introduction via a thioformic acid (HSCHOads) and a surface methyl- thiolate (CH3Sads) intermediate. Moreover, Barrault et al. Supported Mo-based catalysts have been extensively [12] investigated the synthesis of methanethiol from carbon studied in Fischer–Tropsch (F–T) reaction for the synthesis oxides (CO and CO2), H2S and H2 over tungsten-based of mixed alcohols from syngas (CO + H2)[1–6]. In con- catalysts. They proposed that carbon oxides react with H2S trast, the studies of the reactions of the syngas in the to produce COS and that COS was hydrogenated to existence of H2S are very limited. Previously, we have CH3SH. Recently, Mul et al. [13] reported a similar path- studied the effects of H2S concentration in syngas on the way for the CH3SH synthesis from CO and H2S over performance of Mo-based catalysts. In this case, we acci- vanadium-based catalysts. However, based on an approxi- dentally found that mixed alcohols disappeared and mate Anderson–Schulz–Flory product distribution, Zhang methanethiol became the dominant product when the et al. [14, 15] proposed a modified mechanism of the F–T concentration of H2S in the syngas was over 1.6% [7, 8]. process incorporating a reaction of a C1 intermediate with a Methanethiol, also referred to as methyl mercaptan, is a surface –SH group for the synthesis of CH3SH over unmodified a-Al2O3 catalysts. In this paper, the SiO2-supported Mo-based catalysts A. P. Chen Á Q. Wang Á Y. J. Hao Á W. P. Fang Á were chosed for the direct synthesis of methanethiol from & Y. Q. Yang ( ) H S-rich syngas. The catalysts modified with potassium College of Chemistry and Chemical Engineering, 2 Xiamen University, Xiamen 361005, P.R. China and cobalt exhibit a higher catalytic activity than those e-mail: [email protected] reported in recent literature and patents. The products were 123 Catalytic Synthesis of Methanethiol 261 identified, for the first time, on the K–Mo–Co/SiO2 cata- standard. N2, CO, and CO2 were analyzed by an on-line gas lyst. The source of desired products and by-products and chromatographs (GC) fitted with thermal conductivity the pathway of the CH3SH synthesis from H2S-rich syngas detector (TCD) (carrier: H2; a carbon molecular sieve are discussed. column: 1.5 m 9 8 mm; 120 °C (5 min) to 180 °C (9 min) at 10 °C/min). The hydrocarbons were analyzed by an on-line GC equipped with a flame ionization detector 2 Experimental (FID) (carrier: N2; a Porapak Q column: 1.5 m 9 8 mm; 120 °C (5 min) to 180 °C (9 min) at 10 °C/min). The 2.1 Catalyst Preparation sulfur-containing compounds were analyzed by an on-line GC fitted with a flame photometric detector (FPD) (carrier: Catalysts were prepared by the incipient wetness impreg- N2; a HP-Plot/Q capillary column: 30 m 9 0.539 mm 9 nation method and SiO2 (commercial sample, SBET = 40.00 lm; 80 °C (5 min) to 180 °C (5 min) at 10 °C/min). 261 m2/g, 20–45 mesh) was used as the support. In a typical The three detectors were installed in series. All lines from preparation of the K2O–MoO3–CoO/SiO2 catalyst, the the reactor to the gas sampling valves were kept at required quantities of K2CO3, (NH4)6Mo7O24Á4H2O, and *150 °C by using a thermostated heating belt to prevent Co(NO3)2Á6H2O were dissolved in distilled water, to which product condensation. CO and CO2 were quantitatively ammonia was added until the precipitate was dissolved analyzed by an internal standard method. CH4,C2H4, under stirring to yield an aqueous solution. Subsequently, C2H6,CH3SH, H2S, COS, and CS2 were measured by an SiO2 was impregnated for 12 h, followed by drying and external standard method. The mass balances were closed calcining at 400 °C for 4 h. The amount of precursors was within 3%. All the activity and selectivity data were taken expressed as that of corresponding oxides. For the sake of when the steady state was achieved. brevity, the samples thus prepared K2O/SiO2 (15/100, wt/wt), The reaction products were collected with an ice-salt bath CoO/SiO2 (5/100, wt/wt), K2O–CoO/SiO2 (15–5/100, wt/wt), and analyzed off-line with a Varian GC3900/SATUM 2100w MoO3/SiO2 (25/100, wt/wt), MoO3–CoO/SiO2 (25–5/100, GC-MS spectrometer (carrier: He; a HP-2MS capillary wt/wt), K2O–MoO3/SiO2 (15–25/100, wt/wt), and K2O– column: 30 m 9 0.25 mm 9 0.25 lm; 40 °C (10 min) to MoO3–CoO/SiO2 (15–25–5/100, wt/wt) are denoted as 150 °Cat20°C/min; mass scan range: 40–300 AMU; K/SiO2, Co/SiO2, KCo/SiO2, Mo/SiO2, MoCo/SiO2, acetone was as a standard). KMo/SiO2, and KMoCo/SiO2, respectively. 3 Results 2.2 Reaction Conditions 3.1 Product Distribution The reaction was carried out in a stainless-steel tubular fixed-bed flow microreactor with 0.5 ml of catalyst. Before Table 1 shows the distribution of carbon-containing and evaluation, the catalysts were reduced with hydrogen at sulfur-containing products in the reaction of CO/H2/H2Sat 420 °C for 4 h followed by sulfidation with the feedstock 300 °C and 0.2 MPa over different catalysts. The reaction (H2S/H2/CO = 2/1/1, v/v) at 300 °C for 4 h. The feedstock of CO and H2S even took place in the empty reactor (blank of H2S, CO, and H2 was mixed into a cylinder beforehand, test) to form exclusively COS, but the conversion of CO is to which 2–3% (volume %) of N2 was added as internal very low (1.2%). With unmodified SiO2, K/SiO2, Co/SiO2, Table 1 Product distribution Catalysts Selectivity (%) (H O excluded) Conversion of and catalyst activity for the 2 b CO (%) synthesis of methanethiol from CH4 C2H4 C2H6 COS CO2 CH3SH CS2 Thioethers a H2S-rich syngas Blank Trace – – 100 – – – – 1.2 SiO2 3.8 – – 94.7 Trace 1.5 – – 1.7 K/SiO2 1.8 – – 96.0 Trace 2.2 – – 3.7 Co/SiO2 2.2 – – 95.0 Trace 2.8 – – 3.2 KCo/SiO2 1.6 – – 93.2 Trace 5.2 – – 3.1 a Reaction conditions: Mo/SiO2 1.5 – – 96.2 Trace 2.3 – – 4.6 CO/H2/H2S = 1/1/2 (v/v), MoCo/SiO2 2.7 Trace – 94.6 Trace 2.7 – – 8.3 0.2 MPa, 300 °C, 3,000 h-1 KMo/SiO2 0.3 0.1 Trace 19.1 31.7 48.5 0.2 0.1 42.7 b CH3SCH3 +CH3SSCH3 + KMoCo/SiO2 0.6 0.1 Trace 16.6 36.6 45.7 0.3 0.1 62.4 CH3SSSCH3 123 262 A. P. Chen et al. KCo/SiO2, Mo/SiO2 and MoCo/SiO2, a slight increase of CO conversion and the following order of CO conver- sion was found: MoCo/SiO2[ Mo/SiO2[ KCo/SiO2[ K/SiO2[ Co/SiO2[ SiO2. Moreover, COS was still the dominant product (selectivity [ 90%), along with small amounts of CH3SH and CH4. However, the CO conversion increased markedly over the potassium-promoted Mo-based catalysts (KMo/SiO2 and KMoCo/SiO2). Meanwhile, the product distribution changed significantly. It was found that COS, CO2, and CH3SH become the main products, with a decrease of the selectivity of CH4. Surprisingly, C2H4,C2H6, CS2,H2O, and some unidentified products (could not be separated in the GC chromatogram) were also detected in the reaction tail gas. To identify the nature of the species in Fig. 1 Effect of H2 concentration on the catalyst activity and the the overlapping peaks, the products were collected in an ice– product selectivity.
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