Reactive Adsorption of Thiophene on ZnNi/Diatomite- Pseudo-Boehmite Adsorbents

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1 Scientific Research 2012,Vol. 14, No. 3, pp September 30, 2012 Reactive Adsorption of Thiophene on ZnNi/Diatomite- Pseudo-Boehmite Adsorbents Meng Xuan; Weng Huixin; Shi Li (The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai ) Abstract: The thiophene removal ability of the synthesized ZnNi/diatomite-pseudo-boehmite adsorbent was tested in a labscale fixed-bed reaction system. X-ray diffractograms (XRD) were used to characterize the adsorbent samples. The effects of Zn/Ni molar ratio, various model fuels and regeneration patterns on the RADS tests were studied. The adsorption mechanism was investigated by XRD and MS analyses. The results indicted that thiophene in the model fuel was first decomposed on the surface Ni of the adsorbent to form Ni 3 S 2 while the hydrocarbon portion of the molecule was released back into the process stream, followed by reduction of Ni 3 S 2 to form H 2 S in the presence of H 2, and then H 2 S is stored in the adsorbent accompanied by the conversion of ZnO into ZnS. Key words: reactive adsorption; desulfurization; thiophene; olefin; regeneration 1 Introduction Sulfur compounds existing in the fuels can lead to SO x -related air pollution generated by vehicle engines. In order to minimize the negative health and environmental effects of automotive exhaust emissions, the sulfur level in motor fuels should be minimized. In fact, the zero-emission and, as a consequence, zero levels of S compounds are called for worldwide in the forthcoming 5 10 years. Such ultra low-sulfur fuels requirements will have great impact on the oil refining processes. Efficiency of the desulfurization technologies becomes a key issue. The conventional hydrodesulfurization (HDS) process is highly efficient in removing thiols, sulfides, and disulfides, but is less effective for treating aromatic thiophenes and thiophene derivatives (especially benzothiophene, dibenzothiophene, and their alkylated derivatives). The efficiency of HDS decreases substantially when it is used to produce ultralowsulfur (ULS) transportation fuels [1]. Several non-hds-based desulfurization technologies such as adsorptive desulfurization, charge-transfer complex formation, extraction using ionic liquids, and biocatalytic treatment, have been proposed recently for the desulfurization of liquid fuels [2-4]. Among them, the reactive adsorption desulfurization (RADS) is considered to be a promising approach for deep desulfurization of liquid fuels because it combines the advantages of both catalytic HDS technology and adsorptive desulfurization process [5]. The S Zorb process based on reactive adsorption is the first real commercial process of importance. The S Zorb process is carried out in the presence of hydrogen and zinc oxide with a metal or metal oxide serving as a promoter on a carrier. Sulfur from the sulfur compounds is transformed to hydrogen sulfide, which is by chemisorption bound with zinc oxide to form zinc sulfide. Although there have been many patents and patent applications in the field of desulfurization by the S Zorb process, but only a few reports with articles are available about the effect of adsorbent components and reaction conditions on the reactive adsorption of the sulfur compounds. In the present work, a systematic study to understand the effects of Zn/Ni molar ratio, various model fuels and regeneration times on the performance of the ZnNi/ diatomite-pseudo-boehmite adsorbent was conducted, and the results of this study were discussed in this paper. In addition, we also tried to explore the reactive adsorption mechanism. Corrresponding Author: Dr. Shi Li, Telephone: ; yyshi@ecust.edu.cn. 33

2 2012,14(3): Experimental 2.1 Model fuels Model fuels (MF-I and MF-II) containing thiophene as a model sulfur compound with a sulfur concentration of μg/g were prepared by using n-octane as the solvent. The major difference between MF-I and MF-II is that MF-II contained 10 m% of 1-octene as the olefin compound. All the sulfur compounds and hydrocarbon compounds used for preparing the model fuels were purchased from Aldrich and were used without further purification. 2.2 Preparation of adsorbents The zinc and nickel-based adsorbents used in this study were prepared by the kneading method. Zinc oxide, nickel sesquioxide, diatomite and pseudo-boehmite were thoroughly mixed with an inorganic binder, sesbania powder, for 0.5 h. Furthermore, a liquid binder-dilute nitric acidwas added to the mixture in order to make a slurry (all chemicals were purchased from the Sinopharm Chemical Reagent Co., Ltd.). An extruder was used to form pellets with an outer diameter of 1 mm from the slurry, followed by drying at 120 for 5 h. Finally, the resulted materials were calcined at 600 for 1 h. The calcined materials were then ground, and the particles with a grain size of mesh were collected through sieving. The molar ratio of Zn to Ni varied at 0, 0.1, 0.4, 0.7 and 1.0 with the amount of diatomite and pseudo-boehmite equating to 25 m% each, respectively. 2.3 Characterization of adsorbents X-ray powder diffraction (XRD) patterns were obtained with a Bruker D8 diffractometer, using Cu-Kα radiation ( nm) operating at 40 kv and 40 ma and a secondary graphite monochromator operating with a scanning rate of 5( )/min ranging from10 to Adsorption experiments About 1.0 g of the adsorbent was put in a stainless steel column having a bed dimension of 6 mm in i.d. and 250 mm in length. The packed column was placed in a multichannel convection oven designed in our laboratory for performing the adsorption experiments. In order to ensure that the Ni species in the Ni-based adsorbent were in the reduced form, the adsorbent bed was pretreated with H 2 gas at a flow rate of 30 ml/min under a pressure of 0.5 MPa at 370, with this temperature maintained for about 1 h. After the pretreatment, the oven temperature and pressure were increased to the desired adsorption temperature and pressure in the H 2 stream. During the adsorption experiments, the model fuel was pumped to the adsorbent column by a micro-injection pump, and routed through the adsorbent bed at a weight hourly space velocity (WHSV) of 4 h -1 and a H 2 /oil volume ratio of 400. The liquid products were collected in a cryogenic trap cooled with an ice water bath and subjected to analyses periodically. The treated fuel samples were analyzed quantitatively using a Varian 3800 gas chromatograph and qualitatively using a Finnigan SSQ710 GC-MS. The distribution of sulfur compounds in the products was analyzed by an Agilent 6890A gas chromatograph equipped with a 355 SCD detector. After the RADS experiments, the used adsorbent was regenerated online using air to generate gaseous SO x -containing compounds such as sulfur dioxide, while burning off any remaining hydrocarbon deposits that might be present. The regeneration experiments were performed at 550 at ambient pressure with air stream at a flow rate of 300 ml/min for 2 h and repeated reduction of adsorbent before RADS reaction should be conducted as necessary. 3 Results and Discussion 3.1 Characteristics of the sample The XRD patterns of the adsorbents with different Zn/ Ni molar ratios are shown in Figure 1. The diffraction lines of the adsorbent after calcination were attributed to the crystalline phases of NiO (JCPDF file: # ), ZnO (JCPDF file: # ) and SiO 2 (JCPDF file: # ). As the Zn/Ni molar ratio increased, the intensity of peaks corresponding to ZnO also increased. When the Zn/Ni molar ratio in adsorbent was below 0.4, the XRD peaks for ZnO were too broad to be noticed and the ZnO crystallite size was relatively small. When the Zn/Ni molar ratio increased from 0.4 to 0.7, the XRD peak for ZnO became much sharper, which denoted that higher concentration of ZnO might induce the formation 34

3 Meng Xuan, et al. Reactive Adsorption of Thiophene on ZnNi/Diatomite-Pseudo-Boehmite Adsorbents of ZnO crystallites with large crystal size, suggesting that agglomeration of ZnO particles could occur outside the mesoporous structure of diatomite-pseudo-boehmite support. The formation of large particles at higher Zn/ Ni molar ratio, such as 0.7, could reduce the surface adsorption sites of ZnO particles, which was supposed to be a disadvantage for the adsorption of sulfur compounds. Zn/Ni molar ratio should be the main cause leading to a drastic decrease of the desulfurization rate. Figure 1 X-ray diffraction patterns of adsorbents with different Zn/Ni molar ratios NiO; ZnO; SiO Effect of Zn/Ni molar ratio Figure 2 shows the curve for the adsorptive removal of thiophene from the MF-1 fuel over ZnNi/diatomite-pseudo-boehmite adsorbents with variable Zn/Ni molar ratios synthesized in our laboratory. It can be seen from Figure 2 that the initial desulfurization rate of the adsorbent with zero Zn/Ni molar ratio was quite high, but the adsorbent was deactivated quickly. The performance of the ZnNi/ diatomite-pseudo-boehmite adsorbent was more effective as compared with the adsorbent that was not modified by ZnO. Tawara K., et al. [5] reported an effective adsorptive catalyst with alumina supported high concentration of Ni species for UD-HDS of kerosene although its service life was quite short. Based on further experimental studies, they found out that the ZnO support was capable of regenerating Ni catalyst automatically during the HDS reaction and the ZnO content would be a key for determining the catalyst life. For the adsorbents incorporated with ZnO in the range of Zn/Ni molar ratio 0.4, the desulfurization rate on adsorbents increased with an increasing Zn/Ni molar ratio, and in the range of Zn/Ni molar ratio 0.4 the desulfurization rate on adsorbents decreased with an increasing Zn/Ni molar ratio. As it can be seen from the XRD results, the formation of ZnO crystallites at high Figure 2 RADS of thiophene in MF-1 as a function of time on stream on ZnNi/diatomite- pseudo-boehmite adsorbents with variable Zn/Ni molar ratios (at T=370, P=1 MPa, WHSV=4 h -1 and H 2 /oil=400) Zn/Ni=0; Zn/Ni=0.1; Zn/Ni=0.4; Zn/Ni=0.7; Zn/Ni=1 3.3 Effect of olefin on RADS of model fuel Effect of olefin on the adsorptive desulfurization over ZnNi/diatomite-pseudo-boehmite adsorbent with a Zn/Ni molar ratio of 0.4 was examined by using the model fuel MF-II, which contained 10 m% of 1-octene. The RADS of MF-II over ZnNi/diatomite-pseudo-boehmite adsorbent was performed at a temperature of 400, a pressure of 1 MPa, a WHSV of 4 h -1, and a H 2 /oil ratio of 400. The adsorptive desulfurization curves are shown in Figure 3 in comparison with the adsorptive desulfurization of MF-I under the same conditions. The desulfurization rate Figure 3 Effect of olefin on conversion of thiophene in MF-Ⅰ and MF-Ⅱ over ZnNi/ diatomite-pseudo-boehmite adsorbent MF-Ⅰ; MF-Ⅱ 35

4 2012,14(3):33-38 of MF-II on the adsorbent decreased significantly and the corresponding thiophene conversion decreased from 97% to 50%. This adsorptive desulfurization rate was about 1.5 times less than the case of MF-1 desulfurization in the absence of 1-octene, indicating that the olefin existing in the commercial gasoline could decrease the desulfurization capacity of ZnNi/diatomite-pseudo-boehmite adsorbent substantially because of the competitive adsorption for the activated hydrogen on the surface by 1-octene [6]. 3.4 Desulfurization of MF-Ⅱ on repeatedly regenerated adsorbent Regeneration of the ZnNi/diatomite-pseudo-boehmite adsorbent with a Zn/Ni molar ratio of 0.4 was achieved in a two-step mode, i. e.: (a) oxidation in dry air at 550 for 2 h; and (b) reduction of nickel oxide species to Ni 0 in hydrogen stream at 370 for 1 h. Results of three cycles of desulfurization are shown in Figure 4 and are expressed in terms of corresponding thiophene conversion rates. The adsorbent demonstrated a good and reproducible performance, which was a clear indication of complete regeneration of the adsorbent. In fact, there was a slight increase in thiophene conversion in the second cycle of desulfurization reaction on the adsorbent as compared with the results for the first cycle of desulfurization reaction, which might indicate that some changes of the adsorbent after multiple cycles of regeneration could enhance the diffusion and adsorption of thiophene. However, there was a slight Figure 4 Adsorptive desulfurization of MF-Ⅱ amid successive RADS cycles on ZnNi/diatomite-pseudo-boehmite adsorbent (at T=400, P=1 MPa WHSV=4 h -1, and H 2 /oil=400) First Cycle; Second Cycle; Third Cycle decrease in thiophene conversion in the third cycle of desulfurization as compared to the results of the second cycle of desulfurization. It can be concluded that regeneration of the adsorbent through two cycles could bring back the adsorbent capacity to its original level, indicating that the adsorbed thiophene molecules were eliminated through combustion and reduction reactions. 3.5 Adsorption mechanism The XRD patterns of the calcined ZnNi/diatomite-- pseudo-boehmite sample at a Zn/Ni molar ratio of 0.4 and the used sample after RADS of MF-Ⅱ at different reaction times on stream (at 6 h and 24 h, respectively) are shown in Figure 5. The diffraction lines of ZnNi/diatomitepseudo-boehmite samples after calcination were mainly attributed to the crystalline phases of tetragonal SiO 2, cubic NiO (2θ=37.3, 43.3, 62.9, 75.4, and 79.4, JCPDF file: # ) and hexagonal ZnO (2θ=31.8, 34.5, 36.3, 47.7, and 56.7, JCPDF file: # ). Other crystalline phases such as those of Al 2 O 3 were not present, indicating that Al 2 O 3 existed in an amorphous state; in addition, the cubic ZnAl 2 O 4 (JCPDF file: # ) was formed in the calcined sample. After reaction with thiophene a complex XRD pattern was observed (Figure 5b and 5c), which demonstrated that the characteristic diffraction peaks of ZnO tended to disappear after reaction, while those of NiO could not be observed in the used adsorbent samples. In addition, new crystallite phases of Ni, ZnS and Ni 3 S 2 in the used adsorbent samples had been identified. Peaks at 44.49, and that corresponded to Ni metal were observed in the adsorbent after RADS, indicating that NiO was converted into Ni metal completely through the H 2 activation treatment. The most intensive signals for ZnS and Ni 3 S 2 were located at the 2θ of 28.6 and 32.1, respectively. With the time on stream increasing from 6 h to 24 h, the intensity of ZnS peak increased significantly, indicating to a great increase of ZnS content in the adsorbent, coupled with gradual decrease of ZnO peak, indicating to a monotonous decrease of ZnO content with an increase in the time on stream. After RADS in 6 h, Ni 3 S 2 appeared and the intensity of Ni 3 S 2 peak increased significantly, indicating to a marked increase of Ni 3 S 2 content in the adsorbent, while that of 36

5 Meng Xuan, et al. Reactive Adsorption of Thiophene on ZnNi/Diatomite-Pseudo-Boehmite Adsorbents Ni peak decreased gradually, indicating to a monotonous decrease of Ni content with an increase in the time on stream. For the ZnNi/diatomite-pseudo-boehmite system, the formation of ZnS and Ni 3 S 2 indicated that the synergistic catalytic effect on thiophene desulfurization might occur over Ni metal and ZnO existing in the adsorbent. Tawara K, et al. [5] reported that Ni/ZnO is an autoregenerative adsorptive UD-HDS catalyst, the ZnO support is supposed to be capable of regenerating the sulfurpoisoned surface Ni into active surface Ni by accepting H 2 S. In order to verify the sulfur transfer mechanism in the Ni/ZnO-based adsorbent, we had conducted additional tests. The used adsorbent sample after RADS of MF- Ⅱ for 24 h was tested once again in the same reactor and then the adsorbent bed was treated with H 2 gas at a flow rate of 40 ml/min under a pressure of 1 MPa and at a temperature of 400 without introducing the model fuel. The XRD patterns of this adsorbent sample are shown in Figure 5d. Upon comparing the XRD patterns of the used adsorbent sample before and after H 2 treatment (Figure 5c and d) it can be seen that the intensity of Ni 3 S 2 and ZnO peak decreased significantly, while that of ZnS peak increased slightly, indicating that sulfur was transferred from Ni 3 S 2 to ZnO with formation of ZnS in the presence of hydrogen. Ryzhikov A, et al. [7] studied the reactive adsorption of thiophene on Ni/ZnO, they found out that the major hydrocarbon products were butenes (trans-2-butene, cis-2-butene and 1-butene), with butane and isobutene formed in minor quantities. It should be noted that butane, etc., could be detected in the liquid products (the results analyzed by mass spectrometry were not shown in this paper). These new phases (according to XRD analyses) and gases (according to MS analyses) relating to our adsorbent samples revealed that with regard to the RADS reaction over our adsorbent, thiophene in the model fuel was first decomposed on surface Ni of the adsorbent to form Ni 3 S 2, while the hydrocarbon portion of the molecule was released back into the process stream. Secondly, Ni 3 S 2 was regenerated to form H 2 S in the presence of H 2. Finally, H 2 S was stored quickly in the adsorbent accompanied by the conversion of ZnO into ZnS. 2C 4 H 4 S+3Ni+6H 2 Ni 3 S 2 +2C 4 H 10 Ni 3 S 2 +2ZnO+2H 2 3Ni+2ZnS+2H 2 O Figure 5 XRD patterns of the calcined ZnNi/diatomitepseudo-boehmite sample (a); the used sample after RADS of MF-Ⅱ for 6 h (b); the used sample after RADS of MF-Ⅱ for 24 h (c), and the used sample through post-treatment with H 2 (d) SiO 2 ; NiO; * ZnO; Ni; + ZnS; # Ni 3 S 2 4 Conclusions In this work, the removal of thiophene was conducted in a lab-scale fixed-bed reactor system over ZnNi/diatomite- -pseudo-boehmite adsorbent samples with variable Zn/Ni molar ratios that were synthesized in our laboratory. The optimum Zn/Ni molar ratio for achieving the maximum desulfurization rate was determined to be approximately 0.4. Olefins in gasoline demonstrated a strong effect on the desulfurization performance of the ZnNi-based adsorbent, which might probably cause a competitive vying for the activated hydrogen on the surface. Two-cycle schemes for regeneration of adsorbent are deemed to be appropriate to restore the adsorbent activity to its original level, so that it can be reused in the absorber. XRD and MS results indicted that for the RADS reaction over adsorbent samples, thiophene in the model fuel was first decomposed on surface Ni of the adsorbent to form Ni 3 S 2, while the hydrocarbon portion of the molecule was released back into the process stream, followed by the reduction of Ni 3 S 2 to form H 2 S in the presence of H 2, and then H 2 S was stored in the adsorbent accompanied by the conversion of ZnO into ZnS. References [1] Song C S. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel[j]. Catal Today, 2003, 86: [2] Babich J A, Moulijn J A. Science and technology of novel processes for deep desulfurization of oil refinery streams: A 37

6 2012,14(3):33-38 review[j]. Fuel, 2003, 82: [3] Hernandez-Maldonado A J, Yang R T. Desulfurization of diesel fuels via π-complexation with nickel (II)-exchanged X- and Y-zeolites[J]. Ind Eng Chem Res, 2004, 43: [4] Bösman A, Datsevich L, Jess A, et al. Deep desulfurization of diesel fuel by extraction with ionic liquids[j]. Chem Commun, 2001, 46: [5] Tawara K, Nishimura T, Iwanami H. Ultra-deep hydrodesulfurization of kerosene for fuel cell system (Part 2): Regeneration of sulfur-poisoned nickel catalyst in hydrogen and finding of auto-regenerative nickel catalyst[j]. Sekiyu Gakkaishi, 2000, 43(2): [6] Ma X L, Michael S, Song C S. Deep desulfurization of gasoline by selective adsorption over nickel-based adsorbent for fuel cell applications [J]. Ind Eng Chem Res, 2005, 44: [7] Ryzhikov A, Bezverkhyy I, Bellat J P. Reactive adsorption of thiophene on Ni/ZnO: Role of hydrogen pretreatment and nature of the rate determining step[j]. Applied Catalysis B: Environmental, 2008, 84: New Generation Catalyst for Methanol Pyrolysis Developed by CAS Passed Appraisal On June 4, 2012 the Science and Technology Department of Shanxi province organized an meeting for assessing the achievements of the project Research and development of a new-generation high-selectivity catalyst for methanol pyrolysis undertaken by the CAS Shanxi Coal Chemical Institute. Experts attending the appraisal meeting have unanimously recognized that compared with the existing commercial catalysts, the new-generation catalyst for methanol pyrolysis features high selectivity and convenience in operation with its catalytic performance indicators assuming a domestically leading position, and meanwhile they all have proposed an accelerated efforts to apply and disseminate this technology. This project after three years of R&D work on the basis of optimum conditions of small scale experiments had prepared in an enlarged scale more than 100 kg of catalyst sample by means of impregnation to carry out the simulated commercial single-tube tests to acquire satisfactory results. The catalyst developed thereby is characteristic of a simple preparation process and its content of active component copper is significantly reduced to be ready for direct use in the reactor without pre-reduction by hydrogen gas stream. This catalyst for pyrolysis of methanol has a high selectivity (reaching about more than 98%) and can retain its catalytic performance after regeneration which is suitable for repeated use in the pyrolysis process. Energy-Saving Process for Ethylene Glycol Manufacture Coupled with Dimethyl Carbonate Production Developed by CAS In an attempt to eradicate high energy consumption in the course of ethylene glycol production, the team engaging in the ionic liquid cleanup process and energy-saving innovations at the Process Engineering Institute of CAS has developed a high-efficiency and energy-saving integrated ethylene glycol (EG)/dimethyl carbonate (DMC) manufacture technology. It is learned that compared to the existing direct hydration of ethylene oxide, this technology features high yield (close to 100%), good product quality (complying with the national standard for EG: GB/T , and provisional standard for DMC: YS/T ), over 30% reduction in energy consumption, effective utilization of CO 2 -containing waste gas discharged in the course of manufacturing ethylene oxide through ethylene oxidation, and flexibility in adjusting production plan in response to the market need. At present a commercial sideline test of 25 t/a EG has been completed, a 200 kt/a EG basic design and a feasibility study report have been prepared. The Process Engineering Institute of CAS has filed 16 invention patents including an international patent application (among which 4 patents have been authorized). 38