Unified Catalytic OxidationâAdsorption Desulfurization Process Using

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A unified catalytic oxidation-adsorption desulfurization process using cumene hydroperoxide as oxidant and vanadate based poly ionic liquid as catalyst and sorbent Yong Chen, Hong-Yan Song, Ying-zhou Lu, Hong Meng, Chunxi Li, Zhi-gang Lei, and Biao-hua Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02464 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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Industrial & Engineering Chemistry Research

A unified catalytic oxidation-adsorption desulfurization process using cumene hydroperoxide as oxidant and vanadate based poly ionic liquid as catalyst and sorbent Yong Chen1,2, Hong-yan Song 3, Ying-zhou Lu2, Hong Meng2, Chun-xi Li*,1,2, Zhi-gang Lei1,2, Biao-hua Chen1,2 1

State Key Laboratory of Chemical Resource Engineering; 2College of Chemical Engineering and 3Faculty of Science, Beijing University of Chemical Technology, Beijing 100029, P. R. China

E-mail: [email protected]

KEYWORDS: poly ionic liquid, adsorbent, catalyst, vanadate, oxidative desulfurization, cumene hydroperoxide

ABSTRACT: For the deep desulfurization of fuel oil, adsorptive desulfurization (ADS) and oxidative desulfurization (ODS) may be the most viable alternatives to the conventional HDS process. In this work, a highly cross-linked vanadate based poly ionic liquid (V-PIL) was 1

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synthesized by reacting poly vinylimidazole (PVIM) with peroxy-vanadic acid. The as-prepared V-PIL is mesoporous with specific area of 163 m2∙g-1 and good adsorptivity for the oxidized sulfur compounds. Further, it can efficiently catalyze the oxidation of dibenzothiophene and benzothiophene with cumene hydroperoxide (CHP), achieving a unified desulfurization processes of ODS and ADS. Its catalytic activity and reusability is superior to the supported vanadate catalysts, e.g. V2O5@active carbon and V2O5@zeolite. The desulfurization capacity of V-PIL is 99% in the first run under specified conditions, i.e. stirring 4 h at 323K for 500 ppmS DBT oil, with CHP/S being 4 in mole ratio, and 3 wt% V-PIL with respect to oil. And over 80% of its desulfurization capacity can be retained after five successive uses without regeneration under the above conditions. The V-PIL may represent a new type of bulk catalyst with potential application considering its rich porosity, well dispersed and accessible catalytic sites, and no soluble loss of the bonded vanadates in the recycling uses.

1. INTRODUCTION Diesel oil contains various fused ring thiophenic sulfurs, such as benzothiophene (BT), dibenzothiophene (DBT) and their alkyl substituted derivatives,1,2 which can hardly be removed efficiently by conventional hydrodesulfurization (HDS).3 However, faced with the increasing strict environmental regulations, it is imperative to develop alternative or supplementary desulfurization technologies for the production of ultralow S-content oils. Given the inherent 2


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difficulties of HDS, development of various non-HDS technologies is in the ascendant, such as extractive desulfurization (EDS),4-6 adsorptive desulfurization (ADS),7,8 and oxidative desulfurization (ODS).9,10 These desulfurization processes can be run in milder conditions, and thus are feasible for practical uses. Besides, their appropriate combination may have better performance due to the synergetic effect, e.g. extractive catalytic oxidative desulfurization (ECODS),11,12 where the EDS equilibrium is broken dynamically by the oxidation of sulfur compounds. At present, study of ADS13-15 is prosperous toward to finding novel sorbents with high adsorptivity and selectivity for the fused ring thiophenic sulfurs, and some ADS processes enhanced with catalytic oxidation have been reported in recent years.16,17 ODS with H2O2 as oxidant is a promising process,18,19 whereby deep desulfurization can be achieved facilely in laboratory provided H2O2 can be well dispersed. Nevertheless, its practical use is confronted with great challenges due to the low efficient dispersion of small amount of H2O2 in oil by conventional mixing equipment. To tackle this problem, some researchers use surfactant to improve the dispersion of H2O2,20,21 which however arises a new problem for treating the emulsified oil. Other researchers use a polar solvent or ionic liquid (IL) as extractant to intensify the ODS performance, forming a so called ECODS process.12,19 For the ODS process with H2O2 as oxidant, metal heteropolyacid salts are widely used catalyst. In addition, V2O5 and vanadate also show good catalysis,22,23 and the supports, e.g. alumina, titania, and silica,24-26 show definite influence on the catalytic performance. For instance, almost all of thiophenic 3



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S-compounds in model oil can be removed within 1 h by ECODS at 303 K with V2O5 as catalyst and [BMIM]BF4 as extractant.27 Compared to H2O2, hydrophobic oxidant is more suitable for the homogeneous oxidation of sulfur compounds, and the desulfurization process can be simplified further if the oxidized sulfur compounds can be adsorbed efficiently on the catalyst, forming an integrated process of ODS and ADS. Till now, such a unified desulfurization process with cumene hydroperoxide (CHP) as oxidant and vanadate based poly ionic liquid (V-PIL) as catalyst, has not been reported. Besides, as a commodity chemical and intermediate of phenol production, CHP is cheap and commercially available. Moreover, its reductive product, 2-phenyl-2-propanol, can be used directly as a fuel component with high value of octane number, or at least will not deteriorate the oil quality. Further, the imidazolium based V-PIL might be a good sorbent for the fused ring thiophenes as inferred from the good EDS performance of alkyl imidazoles and the imidazolium based ILs. In this regard, a highly cross-linked V-PIL catalyst with porous structure was synthesized by reacting PVIM and peroxy-vanadic acid, and characterized by XRD, FT-IR, SEM, EDS and N2-adsorption, respectively. Its catalysis for the oxidation of DBT in model oil with CHP as oxidant was studied at varying conditions, including catalyst usage, temperature, CHP/S (mole ratio), and reaction time. Its reusability and regeneration was also investigated, and compared with the supported catalysts of V2O5 on active carbon and 4A zeolite. At last, its applicability to the desulfurization of real diesel oil is validated. 4


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2. EXPERIMENTAL SECTION 2.1. Chemical Materials 1-vinylimidazole

(VIM,

>99%)

was

purchased

from

Aladdin

Ind.

Co.

2,2-azobisisobutyronitrile (AIBN, AR), vanadium pentoxide (V2O5, AR), and aluminum chloride anhydrous (AlCl3, >99%) were bought from Tianjin Guangfu Fine Chem. Res. Inst. 3-methylthiophene

(3-MT

ACROS,

>99%),

benzothiophene

(BT

ACROS,

>99%),

dibenzothiophene (DBT ACROS, >99%), and cumene hydroperoxide (CHP ACROS, 80%) were purchased from J & K Sci. Ltd. Ethyl alcohol (AR), ethyl acetate (AR), and n-octane (AR) were the products of Tianjin Damao Chem. Hydrogen peroxide (H2O2, 30%) was from Shantou Xilong Chem. Ltd., and 1-hexene (98%) was made by Alfa Aesar. Toluene (AR) and chloroform (AR)

were

from

Beijing

Chem.

Works.

Active

carbon(AC)

was

a

commercial

coconut-shell-derived carbon from Tangshan Hua Neng Tech. Carbon Co. Ltd. 4A zeolite (ZMS) and cyclohexene (98%) were from Sinopharm Chem. Reagent Co. Ltd. Diesel was bought from gas station of Sinopec Group. All chemicals were used as received. 2.2. Synthesis of Catalysts 2.2.1. Synthesis of the V-PIL The V-PIL catalyst was synthesized by reacting PVIM and peroxy-vanadic acid (1:1 mole ratio) at room temperature. As shown in Scheme 1, PVIM was first prepared through radical polymerization of VIM. Specifically, 30 g VIM, 150 g toluene, and 0.15 g AIBN were added into 5



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a 250 ml flask at nitrogen atmosphere, and then reacted at 343 K for 12 h.28 The PVIM was obtained by vacuum filtration, washed thrice with 50 g ethyl acetate each time, and dried in a vacuum oven for 12 h at 353 K. Then peroxy-vanadic acid was prepared through oxidation of 0.49 g V2O5 (0.00269 mol) with 20 g of 15 wt% H2O2 at 293 K under magnetic stirring for 1 h, forming a transparent purple solution.29 The peroxy-vanadic acid solution was added dropwise into the PVIM dilute solution containing 0.5 g PVIM (0.00531 mol) and 200 g ethanol at room temperature, and then kept stirring for 4 h. Finally, a highly cross-linked V-PIL was obtained after filtration, washing thrice with 20 g ethanol each time and drying in vacuum oven at 353 K for 12 h. Scheme 1. Schematic for the Synthesis of V-PIL Catalyst

2.2.2. Preparation of V2O5@AC and V2O5@ZMS Aqueous peroxy-vanadic acid was prepared by adding 0.2 g V2O5 into 10 g 15 wt% H2O2 solution and stirring 1 h at 293 K, to which 1 g AC powder was added and impregnated for 12 h. The above mixture was dried by rotary evaporator at 373 K and the solid was calcined at 473 K for 4 h, forming V2O5@AC catalyst with V-loading of 0.002 mol V∙(g catalyst)-1. The 6


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V2O5@ZMS catalyst with the same V-loading was prepared following the same procedure as V2O5@AC just by replacing AC with ZMS. 2.3. Characterization of the V-PIL Catalyst The X-Ray Diffraction patterns (XRD) were recorded on a Bruker AXS D8 FOCUS diffractometer using Cu Kα (λ=1.5406 A) radiation run at 40 kV and 40 mA. The Fourier Transform Infrared Spectroscopy (FTIR) was recorded by mixing with KBr in a 1:100 (w/w) ratio using a Thermo Electron Nicoiet 8700 spectrometer. The Brunauer Emmett Teller (BET) specific area and pore volume were determined using Micromeritics ASAP 2020 physical adsorption apparatus at 77 K. The Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) images were acquired by placing the sample under a ZEISS supra 55 electron microscope with Oxford X-ray energy dispersive spectrometer. 2.4. Catalytic Oxidative Desulfurization Experiments The model oil used here is a binary mixture of n-octane and an aromatic S-compound, namely, 3-MT, BT, or DBT, with its initial S-content all being 500 μg/g (ppmS). For a typical experiment, 15 g oil, and a certain amount of CHP and V-PIL catalyst were added directly into a 50 ml conical flask. The flask was closed with glass cork and vibrated ultrasonically for 1 min, and then put into a thermostatic bath at specific temperature, and stirred magnetically at about 200 rpm. Samples (0.2 mL for each) were taken out after a certain time by a syringe with microfiltration head. The S-content with respect to 3-MT, BT, and DBT in the oils was measured 7



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by HPLC using an external standard method (Shimadzu 10A-VP, equipped with UV-vis detector and a C-18 column; wavelength 242 nm for 3-MT, 251 nm for BT, and 310 nm for DBT; methanol/water = 8/2 for 3-MT and BT, methanol/water = 9/1 for DBT; flow rate=1 ml/min). For each sample, HPLC analysis was repeated twice to obtain the average S-content. The detection limit of S-content is within 0.2 ppm for all sulfur compounds, and the maximum relative errors are 2% (100–1000 ppmS), 4% (10–100 ppmS), 10% (1–10 ppmS), and 30% ( BT >> 3-MT, which is consistent with other ODS processes.22 For the first use, the V-PIL catalyst is slightly better than V2O5@AC and V2O5@ZMS, while for the continuous uses, its catalytic performance is superior to the supported ones and follows the order of V-PIL > V2O5@AC > V2O5@ZMS, as shown in Figure 13. Besides, all the catalysts show a decreased desulfurization capacity for the successive use without regeneration, but the catalysis of V-PIL could be kept to the utmost extent. The rapid decline of the catalysis of V2O5@AC and V2O5@ZMS may be due to the dissolution loss of the

22


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loaded vanadates, as manifested by the light yellow color of the treated oil, in contrast, the oil treated with V-PIL catalyst is colorless and transparent.

Figure 12. Desulfurization ratio of DBT, BT and 3-MT by different catalysts. Experimental conditions: 500 ppm oil 15 g, catalyst 0.3g, CHP/S = 4 (mole ratio), 323 K, magnetic stirring 4 h.

Figure 13. Desulfurization performance of three catalysts for DBT oil for five successive uses. Experimental conditions: 500 ppmS oil 15 g, catalyst 0.3 g, CHP/S = 4 (mole ratio), 323 K, magnetic stirring 4 h. 23



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The DBT adsorbance of the catalysts was measured under the catalytic oxidative conditions, i.e. 500 ppmS oil 15 g, catalyst 0.3 g, 323 K, magnetic stirring 4 h. The adsorptivity of V2O5@AC, V-PIL and V2O5@ZMS were found to be 12.43, 2.21 and 1.49 mgS.g-1, respectively, and the highest adsorptivity of V2O5@AC may be due to the high adsorptivity of AC. This suggests that there is little relevance between DBT adsorptivity and ODS performance in the present desulfurization process, and the key is the dispersity and availability of vanadate catalytic centers. In short, the V-PIL catalyst is superior to V2O5@AC and V2O5@ZMS from reusability and stability, which arises mainly from its unique structure. Specifically, the vanadates with multiple crosslinking sites are well dispersed among the chains of protonated PVIM, forming a highly cross-linked ionic network with fully accessible vanadates for the catalytic reaction. In contrast, the vanadates are physically dispersed on the surface of V2O5@AC and V2O5@ZMS, and only the exposed vanadates are applicable, leading to a lower usability and efficiency. Besides, the vanadates on the supported catalysts can be lost easily in the recycling uses, due to the lack of basic sites on AC and ZMS for the acid-base strong interactions, resulting in a declining catalytic activity. 3.5. Desulfurization of the Present Process for Real Diesel Considering the promising desulfurization performance of the present process for the fused ring thiophenic sulfurs in model oil, we also studied its applicability for real oil. Before 24


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experiment, 100 g commercial diesel oil was treated three times by anhydrous AlCl3 at 303 K. Each time 3 g AlCl3 was added to the oil and stirred magnetically for 2 h followed by filtration to get the treated oil, whereby the O- and N-containing components can be removed efficiently by the adsorption of AlCl3 due to the strong Lewis acid-base interaction among them14. Then, additional DBT was added to the as-treated oil to an overall S-content of 720 ppm. The desulfurization experiment was carried out under the optimal condition for the model oil, i.e. 15 g oil, 0.3 g V-PIL catalyst, CHP/S = 4 (mole ratio) and 323 K, and 81.8% desulfurization ratio can be obtained. To reveal the declining performance of the process for real oil, we prepared three DBT model oils containing 6.67 wt% of toluene, 1-hexene, and cyclohexene respectively. As shown in Figure 14, existence of the unsaturated components has a negative effect on the desulfurization capacity, especially for cyclohexene which reduces the desulfurization ratio to 83.8%. This may be ascribed to the much easier oxidation of cyclohexene than octane, and even the oxidation of octane is suppressed in the presence of cyclohexene, as shown by comparing the HPLC chromatogram for the oxidized oils with and without cyclohexene. For detail, please see Figure S1 in Supporting Information. Therefore, the declined desulfurization performance for the real diesel oil can be relative to the competitive oxidation between unsaturated components and S-compounds.

25



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Figure 14. Influence of different model oil on the desulfurization ratio. Experimental conditions: 15 g oil, V-PIL 0.3 g, CHP/S = 4 (mole ratio), 323 K, magnetic stirring 4 h. 3.6. Supposed Desulfurization Mechanism of CHP and V-PIL for DBT oil To reveal the independent contribution of CHP oxidant, catalysis of V2O5, adsorption of V-PIL, and the dual effect of V-PIL for catalysis and adsorption, six control experiments were done, and the results are shown in Table 2. As shown from entries 1 and 2, CHP is unable to oxidize DBT without catalyst, and V2O5 shows negligible adsorptivity for DBT. Besides, V-PIL shows weak adsorptivity for DBT with desulfurization ratio being 4.7% for the regenerated V-PIL and 9.1% for the fresh V-PIL. The higher desulfurization ratio of the fresh V-PIL may be ascribed to the additional oxidizing ability of the peroxy groups whereon, in contrast the peroxy groups in the regenerated V-PIL is disappeared. This can be justified by the disappeared or weakened characteristic peaks of V(O2) at 808.7 cm-1 and 594.9 cm-1 and the enhanced peaks of V2O at 753.3 cm-1 and 453.4 cm-1 for the regenerated V-PIL catalyst,36,40,41 as shown in Figure 15. 26


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Further, the V-PIL’s catalytic activity is much higher than neat V2O5 owing to its well dispersed catalytic sites and high adsorptivity for DBTO2. Table 2. Desulfurization results of different reaction systemsa Entry

a

Reaction System

Desulfurization Ratio (%)

1

Oil + CHP

0

2

Oil +

V2O5

3

Oil +

Fresh V-PIL

9.1

4

Oil +

Regenerated V-PILb

4.7

5

Oil + CHP + V2O5

30.3

6

Oil + CHP + V-PIL

99.0

0

Experimental conditions: 15 g 500 ppmS DBT model oil, 0.3 g V-PIL or 0.11 g V2O5 with

equivalent vanadate, CHP/S = 4 (mole ratio), 323 K, magnetic stirring 2 h. b

Regenerated V-PIL after experiment of entry 3.

27



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Figure 15. FTIR spectra for fresh and regenerated V-PIL. The catalytic mechanism of the present system is proposed based on the above analysis and the catalytic oxidation mechanism of vanadates or V2O5 reported in similar systems.27,30 As presented in Scheme 3, the overall catalytic oxidation process is assumed to be composed of three parts. (1) The adsorption of DBT and CHP on the V-PIL catalyst. This process enhances the concentration of the reactants, i.e. CHP and DBT, on the V-PIL catalyst, and promotes reaction rate accordingly. (2) Oxidation of DBT by peroxy-vanadates (e.g. V4O164-) and the dynamic peroxidation of vanadate by CHP. In this process, DBT is oxidized firstly to DBTO and then to DBTO2 by peroxy vanadate, and remains in the V-PIL, meanwhile the vanadates are peroxidized by excessive CHP, forming an oxidation loop. (3) Mutual promotion of the ADS and ODS in the present process. As the catalytic oxidation proceeds, the adsorption equilibrium of V-PIL is broken dynamically, whereby the DBT and CHP in oil are transferred to the V-PIL and 28


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converted to DBTO2 and 2-phenyl-2-propanol, respectively, leading to a deep desulfurization. Therefore, the declining catalytic activity of the V-PIL catalyst in the successive uses originates mainly from the blanketing of the catalytic sites by the accumulated DBTO2, and the desulfurization capacity of the used V-PIL can be recovered easily via chloroform washing, as shown in Figure 10. Scheme 3. Supposed desulfurization mechanism of DBT by CHP and V-PIL

4. CONCLUSION A novel and promising desulfurization process is developed based on the catalytic oxidation and adsorption mechanism, which can efficiently remove the fused ring thiophenes under mild conditions without cross contamination to the oil. This process uses oil-soluble CHP as oxidant and the vanadate based PIL as both catalyst and sorbent, and its desulfurization ability follows the order DBT > BT >> 3-MT. CHP is unable to oxidize DBT without V2O5 or vanadate catalyst. V-PIL shows lower adsorptivity for DBT but high adsorptivity for DBTO2. The catalytic stability 29



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and reusability of the V-PIL are superior to the physically supported V2O5 catalysts. The DBT removal ratio can be still as high as 80% after five successive uses of the V-PIL catalyst without regeneration, and the used catalyst can be regenerated effectively by chloroform washing. ASSOCIATED CONTENT Supporting Information Detailed analysis data of HPLC chromatogram of Figure 8, comparison of HPLC chromatogram for the oxidized oils with and without cyclohexene. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel./Fax: +86-10-64410308. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful for the support from the National Natural Science Foundation of China (Nos. 21376011).

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Abstract graphic

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Unified Catalytic OxidationâAdsorption Desulfurization Process Using

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