Base-Free Catalytic Aerobic Oxidation of Mercaptans for Gasoline Sweetening Over Htlcs-Derived Cuznal Catalyst

中国科技论文在线 http://www.paper.edu.cn Base-Free Catalytic Aerobic Oxidation of Mercaptans for Gasoline Sweetening over HTLcs-Derived CuZnAl Catalyst Lida Gao, Qingsong Xue, Ye Liu, and Yong Lu Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Dept. of Chemistry, East China Normal University, Shanghai 200062, China DOI 10.1002/aic.11928 Published online August 13, 2009 in Wiley InterScience (www.interscience.wiley.com). An aerobic oxidative removal of mercaptans from gasoline in the absence of liquid base has been demonstrated for gasoline sweetening over CuZnAl catalyst. This pro- À cess could proceed at large WHSV of gasoline (50–70 h 1) with [95% mercaptan conversion at 150 C (or 300 C) using an O2/S molar ratio of 20–40. At 150 C, dimerization of mercaptans occurred dominantly to form their disulfides. At 300C, deep oxidation of the mercaptans to SO2 was the dominant process in the first tens of hours, but it decreased then with prolonged time on stream and meanwhile the dimerization increased. The spent catalyst could be restored to its fresh activity level only through a calcination treatment in air. This process was also demonstrated to be effective and efficient for sweetening of a real cracking gasoline. VC 2009 American Institute of Chemical Engineers AIChE J, 55: 3214–3220, 2009 Keywords: catalysis, gasoline sweetening, aerobic oxidation, mercaptan, hydrotalcite, copper, zinc Introduction captan.2 The reaction mechanism can be summarized as overall reaction: 2RSH þ 1/2O ! RSSR þ H O. Mercaptans are widely distributed in petroleum products, 2 2 In the Merox process, an aqueous base, such as sodium especially in light oil like gasoline. They cause foul odor, hydroxide or ammonia, is used to fulfill the purpose. But the deterioration of additives in finished products except being aqueous bases always cause an environmental problem of corrosive. Therefore, it is necessary to remove them from spent caustic materials. As a consequence, the disposal of the fuels, which is usually called sweetening in the petro- 1 the spent caustics has become more and more difficult and leum industry. The mercaptan removal technology widely expensive. One possible way to solve this problem is to use used so far is the Merox process developed by UOP. The inorganic1–11 solid bases or polymeric9–13 as carriers to sup- process is based on the ability of a metal chelate, i.e., sulfo- port CoPcS for the fuel sweetening. Extended studies1–7 nated cobalt phthalocyanine (CoPcS), to catalytically oxidize have been devoted to the MgO-based solid catalysts, investi- the mercaptans to their corresponding disulfides in alkaline gating the basicity and textural/structural properties as well medium by molecular oxygen. It has been widely accepted as the synergistic effect between basic sites and CoPcS. The that mercaptan reacts with a base to form mercaptide anion previous efforts make the Merox process much cleaner (i.e., and the mercaptide is then oxidized with O to disulfide in 2 the aqueous bases are only required for catalyst regenera- the presence of catalyst such as CoPcS; note that the mer- tion), but we should bear in mind that a typical refinery han- captide formation is of primary importance since mercaptide dles 100,000–200,000 barrels a day. This means that bulk ion (as a ligand) can bind to CoPcS more strongly than mer- amount of spent caustics is still a problem on a long view. Herein, an aerobic oxidation of mercaptans (AOM)14 for Correspondence concerning this article should be addressed to Y. Lu at ylu@ high efficiency gasoline sweetening, without any spent caus- chem.ecnu.edu.cn. tic emission, has been demonstrated by mixing small amount VC 2009 American Institute of Chemical Engineers of air with the fuel and passing through a fixed bed packed 3214 December 2009 Vol. 55, No. 12 AIChE Journal 转载中国科技论文在线 http://www.paper.edu.cn with CuZnAl catalysts derived from hydrotalcite-like com- alysts was 1.0 g if not specified. The CuZnAl catalysts were pounds (HTLcs). Copper-zinc oxide composites have been directly heated and exposed to a feed stream at atmospheric widely used in methanol synthesis industry and have been pressure. Mercaptan-containing gasoline was fed continu- receiving growing interest due to their potential applications ously using a high-performance liquid pump, in parallel with in the hydrogen production from methanol steam-/autother- air (as oxidant) feeding by a calibrated mass flow controller, mal-reforming15,16, catalytic wet oxidation of phenol and CO into the reactor heated to the desired reaction temperature. oxidation.17 Very recently, it has been demonstrated that the Reaction temperature, weight hourly space velocity (WHSV) HTLcs-derived CuZnAl catalysts can selectively and effi- and O2/S molar ratio were varied in range from 150 to ciently convert thiophene and its derivatives in the gasoline 300C, 20 to 70 hÀ1, and 5 to 40, respectively. The control fuel into SO2 at near 300 C and the atmospheric pressure experiments throughout the temperature and O2/S ratio range 18,19 using a high O2/S molar ratio of 50–200. The promotive of interest showed that the quartz tube reactor induced negli- effects have been observed clearly over the HTLCs-derived gible conversion (\5%). Feed stream was the retail gasoline CuZnAl oxide composites for aforementioned reaction proc- (390 lg/g S) blended with various mercaptans such as 1-pen- esses.15–17 The promoted catalytic performance of HTLcs- tanthiol to total sulfur content of 890 lg/g or a real cracking derived CuZnAl catalysts, especially the catalytic oxidation, gasoline (218 lg/g S from mercaptans; taken from SINO- is likely assignable to the increase of the surface concentra- PEC Shanghai Petrochemical Company.). tion of ionized oxygen vacancies.20 As this is sensible, we The residual mercaptan level in gasoline were monitored asked ourselves whether they can directly catalyze the oxida- by sampling at regular time intervals and analyzed by silver 4,13 tion of mercaptan with O2 to form disulfide with omitting the nitrate titration method combined with a Varian CP 3800 formation of mercaptide ion2 from mercaptan with a base. In gas chromatography-pulsed flame photometric detector (GC- the present work, we investigated the effects of reaction PFPD, with a 60-m CP-sil 8 CD capillary column). The sul- parameters, such as weight hourly space velocity, O2/S molar fur contents in the liquid effluent before and after silver ratio and reaction temperature, on the performance of the nitrate titration were detected using a WK-2D microcoulom- HTLcs-derived CuZnAl catalysts for the aerobic oxidation of etry analyzer (Jiangsu Electroanalysis, China), and therefore mercaptans in gasoline. The regenerability of CuZnAl cata- the difference value is the content of mercaptans in the lysts was also studied, showing that the spent catalyst could liquid effluent. The effluent gas was analyzed with the be restored to its fresh activity level only through a calcina- GC-PFPD combined with a HP 6850 gas chromatography- tion treatment in air. thermal conductivity detector (GC-TCD, with a 30 m AT- plot capillary column) for identification of the gaseous sulfur-containing product and CO/CO , respectively. Experimental 2 Preparation of HTLcs-derived CuZnAl catalyst CuZnAl HTLcs were obtained by coprecipitation method Results and Discussion first. An aqueous solution of sodium carbonate (0.5 mol/L) Effects of weight hourly space velocity and O2/S and sodium hydroxide (2.0 mol/L) is used as precipitant. The molar ratio aqueous solution dissolving Cu/Zn/Al nitrates at a Cu/Zn/Al atom ratio of 37:15:48 to a total metal concentration of The effect of WHSV on the performance of aerobic oxida- 2.0 mol/L was dropped, in parallel with the precipitant, into tion of 1-pentanthiol blended in a retail gasoline over the 100 mL deionized water to form precipitate at 60C under HTLcs-derived CuZnAl catalysts has been investigated with vigorously stirring and controlled pH of about 9. The result- the results shown in Figure 1. Clearly, decrease of 1-pentanthiol conversion could be observed with the increase of ing precipitate was aged under continuous stirring in the mother liquor at the same temperature overnight. After filter- WHSV at reaction temperature of 150 C using an O2/S molar ratio of 20. 1-pentanthiol conversion remained at ing, the precipitate was washed thoroughly with deionized À þ 1 water until Na elimination and dried overnight at 100 Cto 100% with the WHSV at or below 30 h and then decreased slowly with further increasing WHSV up to form HTLcs. CuZnAl oxide-composite catalysts were then À1 obtained by thermal decomposition of the HTLcs in air. The 70 h . Note that high 1-pentanthiol conversion could remain at 94% when operating the reaction with a high decomposition procedure was programmed from room tem- À1 perature to 600C at a heating ramp of 1C/min with a hold WHSV of 70 h . For comparison, the space velocity for the traditional Merox process is generally 1–5 hÀ1, being much time of 3 h at the set point. Decomposition of CuZnAl HTLcs 1–13,21 at 600C has proven to be able to endow the resulted cata- lower than the space velocity of our new process. lysts with highly dispersed CuO (copper dispersion: 23.7%), Obviously, the reaction efficiency is at least 10 times higher large specific surface area (102 m2/g), improved CuO–ZnO than the Merox process. In other words, in equivalent bed 15 volume, to sweeten equivalent gasoline that can be done via interaction and promoted catalyst reducibility/re-oxidability. À1 That is why a temperature of 600C was selected to decom- AOM within 100 h at 70 h , traditional Merox process will [ À1 5 pose the CuZnAl precursor for the catalyst preparation.

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