Adsorptive Desulfurization of Diesel Oil by Alkynyl Carbon Materials

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Adsorptive desulfurization of diesel oil by alkynyl carbon materials derived from calcium carbide and polyhalohydrocarbons Wenfeng Li, Yingjie Li, Yong Chen, Qingnan Liu, Ying-zhou Lu, Hong Meng, and Chunxi Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01295 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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Adsorptive desulfurization of diesel oil by alkynyl carbon materials derived from calcium carbide and polyhalohydrocarbons Wenfeng Li1,2, Yingjie Li1,2, Yong Chen1,2, Qingnan Liu1,2, Yingzhou Lu2, Hong Meng2, Chunxi Li1,2 1

State Key Laboratory of Chemical Resource Engineering; 2College of Chemical

Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China.

E-mail: [email protected]

Abstract: Carbon-based materials are a kind of promising sorbent for the desulfurization of fuel oils especially for polycyclic aromatic sulfur heterocycles (PASHs), and development of new sorbent is of great significance. In this paper, six alkynyl carbon materials (ACM-1 to ACM-6) were prepared for the first time through mechanochemical reaction of calcium carbide with six full halogenated hydrocarbons, i.e. CCl4, C2Cl6, C2Cl4, C6Cl6, C6Br6, and C14H4Br10, respectively. Their carbon content and specific surface areas (SSA) were analyzed, and their adsorption performance for thiophenic sulfurs was studied. The ACMs are meso-porous materials with relatively high SSA and favorable adsorptive performance for PASHs with the order of DBT > BT > 3-MT. Their adsorptivity is virtually consistent with 1 ACS Paragon Plus Environment

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the SSA values, and follows the order of ACM-6 > ACM-5 > ACM-4 > ACM-1 > ACM-3 > ACM-2. ACM-6 has the highest SSA (712 m2 g-1) and saturated adsorbance of 21.1 mg-S·g-1 for DBT in octane, as well as 6 mg-S·g-1 for real diesel. As a whole, ACM-6 represents the best ADS sorbent among all meso-porous carbon materials, and may be applicable for real oils after some further modification.

Keywords:

alkynyl

carbon

materials,

sorbent,

desulfurization,

diesel,

dibenzothiopehene

1. INTRODUCTION Polycyclic aromatic sulfur heterocycles (PASHs) like benzothiophene (BT), dibenzothiophene (DBT) and their alkyl-derivatives in fuel oil are difficult to be removed by conventional hydrodesulfurization (HDS) process.1,

2

For their deep

removal, harsh process conditions of higher temperature and pressure, longer reaction time and larger reactor are indispensible.3 Thus, it is necessary to develop alternative technologies4 for the efficient removal of these refractory sulfurs, e.g. adsorption desulfurization (ADS),5-9 oxidative desulfurization (ODS),10-13 and extraction desulfurization (EDS)14-18 . Thereinto, ADS may be more perspective since it can run at ambient temperature and pressure and thus with lower energy consumption.19, 20 For the ADS method, the key is to develop efficient sorbents with high adsorption capacity and selectivity for PASHs. Till now, many kinds of industrial sorbents have been studied for the ADS process, e.g. metal ion exchanged zeolites,21 carbon-based

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materials,22, 23 metal organic framework materials (MOFs),24-26

molecular sieves27

and active alumina7 etc. Carbon-based materials with rich porosity and high specific area are widely used sorbents for their excellent adsorptivity, 28 lower cost and diverse sources. Further, they are of rich types and designable structure. For example, they can be formed as activated carbon (AC), carbon aerogel, active carbon fiber (ACF), carbon nanotube (CNT), graphene, and fullerene, depending on the starting materials and synthesis method used. Many studies have been done on their ADS performance, adsorption mechanism, and modification. Song et al. studied the adsorption of PASHs on a series of ACs and carbon blacks, and their adsorption capacity falls in the range of 1.7 to 7.0 mg-S g-1.29 Haji and Erkey synthesized a meso-porous carbon aerogel with pore size of 22 nm, and adsorption capacity of 15.1 mg-S g-1 for DBT-model oil.30 The maximum S-adsorptivity so far is 46 and 40.5 mg-S g-1 as DBT in hexane, respectively, for the micro-porous ACF and granular coconut-shell AC.31 Crespo and Yang investigated the adsorption properties of single-walled carbon nanotubes (SWCNTs), but their S-adsorptivity is nearly same as the commercial AC.32 For the adsorption of thiophene from octane, graphene is much better than other three specific ACs,33 which may be attributed to its super-conjugated planar structure composed of condensed aromatic rings. ADS of various AC materials originates from the surficial physical and chemical interactions between sorbent and sorbate, forming the so-called physical adsorption and chemical adsorption, respectively. The physical adsorption here arises from the 3 ACS Paragon Plus Environment

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π-π interactions between PASHs and the condensed aromatics of ACs, dispersion force, as well as the capillary condensation of the micro-pores.23 These interactions are non-selective and highly related to the specific surface area (SSA), and especially the micro-porous characteristics. In contrast, the chemical adsorption depends on specific pseudo-chemical interactions, such as H-bonding, Lewis acid-base interaction and complexation between PASHs and specific groups on carbon surface, e.g. carbonyl, carboxyl, and metallic Lewis acid centers. In short, the carbon-based materials with ordered structure, high SSA and dominant micro-pores are helpful for their physical adsorption capacity. However, the chemical constituent of the sorbents is more crucial for their ADS selectivity, owing to the unique interaction between PASHs and specific sites of the sorbent.25 For instance, by loading transition metal ions such as Zn2+, Ni2+, Ag+, and Cu+1 on carbon materials, the adsorption capacity and selectivity can be improved greatly.34 The existing carbon-based sorbents are mostly composed of varying ratios of sp2 hybridized carbon atoms, but few with sp hybridized carbons until now. For instance, the industrial ACs made from various biomasses are highly porous materials with dominant sp3 carbon and a small amount of sp2 carbon in the form of polycyclic aromatics. Comparatively, the carbon materials obtained from volatile organic compounds by the CVD method are mainly composed of sp2 carbon, e.g. carbon nanotubes, spherical fullerenes, and graphenes. These carbon materials are of highly ordered structure and high SSA, ranging from 1000 to 3000 m2·g-1. Actually, the carbon materials with rich alkynyl groups may have better ADS performance 4 ACS Paragon Plus Environment

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considering the following facts. (1) Alkynyl carbon is of higher electron density and stronger dispersion interaction, as manifested by the increasing freezing points from ethane (90.35K) to ethene (104.01K) and to acetylene (192.4K).35 (2) Tubular π electron cloud distribution around C≡C makes its π-π interaction with PASHs possible in all directions. (3) Compared with the aromatic ring, the alkynyl group is more active and easier to make a chemical modification toward a higher ADS selectivity. Nevertheless, this kind of carbon material has not been synthesized and used for ADS of fuel oils heretofore. In this report, six alkynyl carbon materials (ACMs) were prepared for the first time by using mechanochemical reaction of calcium carbide with different polyhalohydrocarbons. Their adsorptive performance for DBT, BT and 3-methylthiophene (3-MT) in model oils and ADS capability for real diesel were investigated. On this basis, we analyzed the relationship between ADS performance and pore size and SSA of the sorbents. The results showed that the ACMs have relatively high adsorptive capacity and selectivity for DBT, BT and other PASHs, along with easy renewability and stable adsorptive property. 2. EXPERIMENTAL

2.1 Chemical materials Industrial calcium carbide (CaC2, 69 wt%) was purchased from Tianjin Fuchen Chem Regents Factory. Carbon tetrachloride (CCl4, ≥99%), p-xylene (C8H10, ≥ 99%), and nitric acid (HNO3, 68 wt%) were produced by Beijing Chem Works. Hexachlorobenzene (C6Cl6, 99%), hexabromobenzene (C6Br6, 98%) were bought 5 ACS Paragon Plus Environment

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from Beijing HWRK Chem Co Ltd and Sahn Chem Tech (Shanghai) Co Ltd, respectively. 1,2-Bis(2,3,4,5,6-pentabromophenyl)ethane (C14H4Br10, 96%) and benzothiophene (BT, 97%) were purchased from Aladdin Ind Co. Ethanol absolute (C2H5OH, ≥99.7%) was product of Tianjin Daomao Chem. Dibenzothiophene (DBT, 98%), 3-Methyl-thiophene (3-MT, ＞99%) were purchased from J&K Scientific Ltd. n-Octane (C8H18, ≥95%), hexachloroethane (C2Cl6, AR), tetrachloroethylene (C2C4, 99%) were supplied by Tianjin Guangfu Fine Chem Ind Inst. The real diesel used here is a kind of heavy pyrolysis oils from Puyang Petrochemical plant. It was pretreated with anhydrous AlCl3 to remove the heteroatom impurities, e.g. O- and N-containing compounds. 100 g crude diesel was pre-treated four times by AlCl3 in a conical flask at ambient temperature, and 5 g anhydrous AlCl3 was added each time and stirred magnetically for 2 h, whereby the black oil became clear and light yellow. The total S-content of the as-treatment diesel was 996 ppm, and used as real diesel for the adsorption experiment. 2.2 Synthesis of ACMs sorbents Six carbon materials (from ACM-1 to ACM-6) were prepared in a planetary ball mill (QXQM-1, Changsha, China) at ambient temperature via mechanochemical reaction of CaC2 with CCl4, C2Cl6, C2Cl4, C6Cl6, C6Br6 and C14H4Br10, respectively. The possible reaction mechanism may be as follows. The crystal structure CaC2 is smashed and destroyed by the successive and intensive mechanical force, producing tiny ionic clusters of CaC2 molecules with net acetylene anions and calcium cations, respectively. The exposed acetylene anions react with halohydrocarbons through 6 ACS Paragon Plus Environment

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nucleophilic substitution, forming new C-C single bonds, and the cations will combine with halogen anions, forming CaX2. Thus, ACMs are formed as the final product, as is exemplified as follows: C6 Cl6 +3CaC2 → [ C6 (C ≡ C)3 ]n + 3CaCl2

Ball milling experiments were carried out at room temperature for 3-4 h under rotation speed of 530~600 rpm, with 50% excess of theoretical amount of CaC2. For the poly halogenated aromatic (PHAs) reagents with lower reactivity, CaC2 was pre-milled for 6 h, and then added PHAs and reacted for another 3-4 h. As such, black carbon materials were formed, along with the resultant CaX2, residue CaC2, and other inorganic impurities from feed-stocks. The crude product powder was soaked by 100 mL absolute ethanol in a beaker, and water was then added drop-wisely for the hydrolysis of the unreacted CaC2 under magnetic stirring. The pH of the above suspension was adjusted to 4-5 by hydrochloric acid, filtered and washed repeatedly with deionized water several times to remove inorganic ions therein. Finally, the carbon materials was obtained via vacuum drying, sealed and kept in dryer for later use. 2.3 Adsorption experiments The model oil (MO) used is a binary mixture of n-octane with DBT, BT or 3-MT. Before experiment, mother liquors with 1000 ppm-S of DBT, BT and 3-MT were prepared by weighing method, which was then diluted by n-octane to make the oil samples with required S-content. Typically, 0.2 g sorbent and 10 g oil were added into a 50 mL sealed conical flask, and stirred magnetically for 4 h in a thermostatic bath 7 ACS Paragon Plus Environment

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for equilibrium. The oil samples were taken from the flask using a syringe with nano-filtration head. The S-content of the oil samples was determined by S-N analytical instrument (KY-3000SN, Jiangyan Keyan Electronic Instrument Ltd., China). The equilibrium adsorbance qe of the sorbent was calculated by eqn. (1):

qe =

m 0 (C0 -Ce ) ma

(1)

where C0 and Ce are the initial and equilibrium S-content in ppmw, respectively, m0 is the mass of oil in g, and ma is the mass of sorbent in mg. 3. RESULT AND DISCUSSION 3.1 Composition and structure of ACMs

The composition and structure of the ACMs were characterized by scanning electron

microscopy

(SEM),

X-ray

energy-dispersive

spectroscopy

(EDS),

high-resolution transmission electron microscopy (HR-TEM), N2 adsorption desorption isotherm analysis, Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Results showed that all the carbon materials are of ultra-fine particles with uniform particle size, and their mean size is found to be in the range of 15 to 25 nm by analyzing the SEM images.36,37 Their carbon content is in the range of 64 wt%-78 wt% owing to mineral residues from the industrial grade CaC2. The halogen residue in all ACMs is quite low, being within 2 % on atomic basis, as identified by the X-ray energy-dispersive spectra (EDS) analysis.36,37 They are meso-porous materials with average pore size of 3.7-7.7 nm and high SSA values


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ranging from 330 to 710 m2·g-1. For more detailed characterization information of the ACMs, please refer to the published article36,37. 3.2 Adsorption performance of ACMs for DBT

The adsorption performance of the ACMs for DBT in n-octane was tested, and the results are shown in Figure 1(a). Obviously, their S-adsorbance is quite different, for example, the S-adsorption amount of ACM-6 is twice that of ACM-2. Their adsorption capacity follows the order of ACM-6 > ACM-5 > ACM-4 > ACM-1 > ACM-3 > ACM-2. In addition, their adsorption capacity is virtually consistent with their SSA. This indicates that the adsorption process is basically a physical one, where the adsorption capacity is determined mainly by the SSA of sorbent and resultant dispersion interaction. Besides, the adsorption capacity of the sorbents in terms of specific surface area of the ACM sorbents, i.e. in mgS.m-2, was presented in Figure 2(b). It is seen that the unit area adsorption capacity is nearly a constant, being about 0.0289 mgS.m-2, and is virtually irrelevant to the total specific surface area (SSA) of the sorbents as manifested by the very low correlation coefficient (R2=0.12). This indicates that the ACM sorbents have similar composition and structure, and thus their specific area contribution to the sulfur adsorptivity is nearly same.


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Figure 1. (a) DBT adsorbance versus SSA of the ACM sorbents. (b) Unit area DBT adsorbance versus SSA of the ACM sorbents.

Experimental conditions: 0.2 g sorbent, 10 g MO (1000 ppm S as DBT), stirring 4 h at 293K.

From the microscopic point of view, the SSA of sorbent is closely related to its micro-structure unit. For ACM-1 and ACM-2, their basic structural unit may be diamondyne,38 which is formed by connecting the tetrahedral sp3 hybridized carbon with sp hybridized C≡C linker. ACM-3, which is prepared from CaC2 and C2Cl4, 10 ACS Paragon Plus Environment

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may possess a planar polycyclic structure by connecting the double and triple carbon-carbon bonds alternatively. ACM-4 and ACM-5 are all synthesized from full halobenzene, and thus they may have the same graphene-like structure unit that is formed by alternatively connecting benzene ring and C≡C group. And ACM-6 has a similar structure unit with ACM-4. Although it is hard that all carbon materials possess such perfect structure units due to the intensive shear stresses under mechanochemical reaction conditions, such structural units may be retained partially at least.

Among these typical structure units, the planner graphene-like structure is of the highest SSA and dispersion energy parameter, and thus ACM-4, ACM-5 and ACM-6 show higher adsorption capacity and surface area than other carbon materials. Besides, the π-π interaction between DBT and benzene ring of the carbon materials can further increase their adsorption capacity, especially for ACM-6 whose adsorption capacity reached up to 21.1 mg-S g-1.

3.3 Adsorption of ACMs for different S-compounds

In order to study the ADS performance of the ACMs for different PASHs, adsorption experiment was carried out for ACM-2 and ACM-6 at 293K using different MOs. As shown in Figure 2, the adsorption capacity of ACM-2 and ACM-6 follows the same order of DBT>BT>3-MT. Actually, this order is same for nearly all sorbents with physic-sorption, which is closely related to their π-electron number and electron density on the S-atom.26 Compared to

BT and 3-MT, DBT has more 11

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aromatic rings and π-electrons, which increase its dispersion and π-π interactions with the sorbent, leading to a higher adsorption capacity.

Figure 2. Adsorption isotherms of ACM-6 (a) and ACM-2 (b) for different S-compounds.

Experimental conditions: 0.2 g sorbent, 10 g MO with varying

S-content, stirring 4 h at 293K. 3.4 Temperature dependence of the adsorption

Temperature dependence of adsorption of ACM-6 was investigated for DBT-model oil at two temperatures. As shown in Figure 3, the adsorption capacity decreases slightly with the rising temperature from 293K to 368K. At low S-concentrations, the adsorption capacity at 293K is a little higher than that at 368 K, but their difference decreases with the increasing S-content and disappears at equilibrium S-content of ca. 600 ppmw.


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Figure 3. Influence of temperature on adsorption of DBT on ACM-6. Experimental conditions: 0.2 g sorbent, 10 g MO (1000 ppm S as DBT), stirring 4 h at 293K and 368K. 3.5 Adsorption isotherms Adsorption isotherms reflect the distribution of adsorbate between the liquid and sorbent phases at equilibrium. Generally, the adsorption isotherm can be described by Langmuir or Freundlich model. The Langmuir relation, eqn. (2), is suitable for the monolayer chemical sorption on the sorbent with homogeneous active sites, while the Freundlich one, eqn. (3), is more suitable for multilayer physic-sorption on heterogeneous surface with multi-scale porosity.

qe =

qmbCe 1 + bCe

qe = kCe1/ n

(2)

(3)

where qe (mg-S g-1) is the amount adsorbed at equilibrium concentration Ce (ppmw), qm (mg-S g-1) is the Langmuir constant representing monolayer adsorption capacity, 13 ACS Paragon Plus Environment

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and b (ppmS-1) is the Langmuir constant related to the energy of adsorption, k and n are Freundlich constants related respectively to the adsorption capacity and intensity. The adsorption isotherms of DBT on six carbon materials are fitted by Langmuir and Freundlich models respectively, and the regressed model parameters are listed in Table 1. All the adsorption isotherms show a similar trend, and the representative isotherms for ACM-2 and ACM-6 at 293 K are presented in Figure 4. As shown from the correlation coefficients (R2) in Table 1 and the graphical fitness of the variation trends in Figure 4, Freundlich model seems more suitable to describe the adsorption isotherms, indicating that DBT adsorption on the ACMs is mainly multilayer physical

Figure 4. Adsorption isotherms of ACM-2 and ACM-6 for DBT at 293 K. Experimental conditions：0.2 g sorbent, 10 g model oil, stirring 4 h. adsorption.39 Furthermore, by comparing the qm values obtained by Langmuir fitting, ACM-6 has the best ADS capacity, and the adsorption ability of the ACMs follows the order of ACM-6> ACM-5> ACM-4> ACM-1> ACM-3> ACM-2.


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Table1-The model constants and the correlation coefficients of the two adsorption models Langmuir Freundlich Adsorbents -1 -1 2 -1 qm/mg-S g b/ppm R k/mg-S g n/ppmS-1 R2 ACM-1 13.14 0.00759 0.956 0.97307 2.64985 0.998 ACM-2 9.68 0.00757 0.959 0.79604 2.78755 0.995 ACM-3 10.20 0.00785 0.967 0.85058 2.79277 0.991 ACM-4 13.76 0.00867 0.949 1.18835 2.80246 0.993 ACM-5 16.74 0.00755 0.950 1.09655 2.50977 0.992 ACM-6 21.09 0.00863 0.962 1.37513 2.45448 0.997

3.6 The adsorption competition between PASHs and PAHs The diesel oil is generally composed of about 17 vol% aromatics, 5% oliefins and 78% saturated hydrocarbons28, and some aromatics are presented as PAHs. To qualitatively illustrate the competitive adsorption of PASHs to PAHs, we prepared a mixed model oil with same molar concentration of 3.2 μmol L-1 toluene, BT, DBT, 4,6-DMDBT, β-methylnaphthalene, and phenanthrene, for the adsorption experiment of ACM-6, and the results were presented in Table 2. The adsorption selectivity is found to follow the order of toluene < BT < β-methylnaphthalene < DBT < phenanthrene < 4, 6-DMDBT, which is consistent with that of Chunshan Song28. The adsorption selectivity is related to the electron properties of the sorbates including the highest bond order, maximal atomic electron density, dipole moment, charge on sulfur, and π electron numbers28, and thus a quantitative relationship has not been available till now. As a whole, the adsorption selectivity of ACM-6 for PASHs with respect to PAHs is not high, which is common for nearly all pure carbon materials, which may deteriorate its actual adsorptive desulfurization capacity for real oils.


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Table 2. Adsorption uptake and selectivity factor (αi−r) relative to β-methylnaphthalene for PAHs and PASHs. PAHs and PASHs (a) Adsorption uptake Adsorptive selectivity (b) (µmol·g-1) toluene 3.2 0.03 benzothiophene 55.4 0.67 β-methylnaphthalene 70.6 1.00 dibenzothiophene 97.0 1.94 phenanthrene 114.7 3.20 4,6-dimethyldibenzothiophene 128.6 5.13 (a) The concentrations of all PAHs and PASHs were 3.2 µmol·g-1. Experimental Conditions: 0.2 g ACM-6, 10 g model oil, stirring 4 h at 293 K, analyzed with GC-FID, SHIMADZU, GC-2010 equipped with a HP-5 capillary column 30 m × 0.32 mm i.d. × 0.25 μm film thickness (b) The adsorptive selectivity factor was calculated by 𝛼𝑖−𝑟 =

𝑞𝑖 ⁄𝑞𝑟

𝐶𝑒,𝑖 ⁄𝐶𝑒,𝑟

, by referring reference28

3.7 The breakthrough curve of ACM-6 for DBT and BT model oils

For assessing the kinetic and equilibrium adsorption performance of the new sorbents, the breakthrough curves of ACM-6 for DBT and BT model oils was determined, and presented in Figure 5. The model oils used herein were mixtures of BT (or DBT) and β-methylnaphthalene (0.30 wt%), tolune (16.28 wt%) and n-octance (83.42 wt%), with their total S-content all being 435 ppm.

As seen from Figure 5, ACM-6 shows much higher adsorption for DBT than BT. DBT in the oil can be adsorbed steadily until saturation at about 65 min, which corresponds to a saturated adsorption capacity of 9.8 mgS.g-1 by numerical integral of the effluent curve using Eqn(4). q=

𝑄𝑜𝑖𝑙 .𝜌𝑜𝑖𝑙 .10−3 𝑤𝑠

𝑡

∫0 (𝐶0 − 𝐶𝑡 )𝑑𝑡

(4)

where 𝑄𝑜𝑖𝑙 (mL/min) is the oil inlet flow rate, 𝜌𝑜𝑖𝑙 (g/mL) is the density of oil, i.e

0.70 g/mL, 𝑤𝑠 (g) is the weight of sorbent loaded into the fixed column, 𝐶0 is the 16

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initial S-content of oil (435 ppm), 𝐶𝑡 is the S-content of the outlet oil at time t (min).

In contrast, the adsorption of BT reaches equilibrium quickly within 35 min, which corresponds to a saturated adsorption of 3.9 mgS.g-1. The adsorption capacity of BT

and DBT calculated here is a little lower but comparable with that shown in Figure 2(a). The lowering adsorption ability of ACM-6 here may be ascribed to the additional competitive adsorption of β-methylnaphthalene in the present model oil, and its competitive ability is even higher than BT as shown in Table 2.

Figure 5. The breakthrough curves of ACM-6 for DBT and BT model oils. Experimental conditions: adsorption column Din×H=3.9×140 mm, adsorbent 0.60 g, packing volume 1.6 mL, 298 K, flow rate 1 mL/min adjusted by Series II pump (Lab Alliance) 3.8 Influence of oil composition on adsorptive desulfurization

Real diesel is mainly composed of alkanes, alkenes, aromatics and polycyclic aromatics, along with small amounts of aromatic sulfides and other N- and O-bearing impurities. These impurities may be adsorbed competitively with sulfides,40 and impose a negative influence on the S-adsorption. Here, xylene was selected as a 17 ACS Paragon Plus Environment

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representative of aromatic hydrocarbons to investigate their influence on ADS capacity. Meanwhile, the ADS ability of ACM-6 for real diesel was also tested. The results are shown in Figure 6, where MO-1 is a mixture of DBT and n-octane, MO-2 refers the same oil as MO-1 but with 20% xylene, RO is the real diesel pre-treated by anhydrous AlCl3. The total S-contents of these oils are all 1000 ppmw. In order to remove the N-, O-bearing organics and non-cyclic sulfides, leaving only BT and DBT derivatives,41 100 g catalytic pyrolysis oil was pre-treated by AlCl3 four times in a conical flask at ambient temperature, and 5 g anhydrous AlCl3 was added each time and stirred magnetically for 2 h, whereby the black oil became clear and light yellow. The

sulfur

compound

distribution

in

the

RO

was

analyzed

by

gas

chromatography-flame photometric detector (GC-FPD), and the sulfur compounds were identified as various alkylated derivatives of BT and DBT, as shown in Figure 10 of reference.42

Figure 6． Adsorption amount of ACM-6 for different oils. Experimental conditions: all oils with 1000 ppm S, 0.2 g sorbent, 10 g oil, stirring 4 h at 293K. 18 ACS Paragon Plus Environment

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As shown in Figure 6, the adsorption amount of ACM-6 in MO-2 is lower than that in MO-1, indicating that xylene can be adsorbed competitively, but its influence is quite mild considering its much higher content than DBT, being 20% of oil. In contrast, the adsorption amount of ACM-6 decreases greatly from 16.2 mg-S g-1 for MO-1 to 6.2 mg-S g-1 for RO, which may be attributed to the presence of rich variety of PASHs and abundant PAHs in the real oil, because of the lower adsorptivity of PASHs than DBT and competitive adsorption of PAHs on the sorbent. Song et al. studied the adsorption of PAHs on different carbon materials, and found that their adsorption selectivity follows the order of DBT>2-methyl naphthalene> naphthalene> BT.29 This means that PAHs such as naphthalenes have comparable adsorption properties with DBT and BT, and thus their presence will result in a substantial decrease of ADS performance for nearly all kinds of sorbents.

3.9 Regeneration and reusability of the sorbent

To regenerate the used sorbent via solvent-washing method, a solvent mixture of methanol and toluene with equal mass fraction was used. The spent ACMs were washed twice in a conical flask at room temperature, and 10 g solvent was added each time, and stirred for 1 h. The washed sorbent was dried at 363 K for 2 h under reduced pressure, and then used for the next round adsorption. The ADS capacity of the thus-treated ACM-6 sorbent was presented in Figure 7. It is seen that adsorption decreases slightly with the increasing number of reuses and over 90% of the ADS performance can be retained after six cycles of reuse.


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Figure 7. Performance of the regenerated ACM-6 for each cycle. 3.10 Adsorption comparison with other carbon materials

According to their physical appearance, ACs can exist in the forms of powder, granular, columnar, fiber, cloth, aerogel and ordered shapes. And their ADS performance is determined mainly by their SSA, pore size, pore volume and functional groups present. Generally, micro-porous materials with higher SSA and pore volume show higher ADS capacity than the meso-porous ones. Table 2 lists the adsorption performance of different carbon materials reported heretofore for DBT model oil and their relevant structural properties.

As shown in Table 3, the first seven sorbents are all commercial ACs with predominant micro-pores and high SSA ranging from 950 to 2300 m2 g-1, and show higher adsorption capacity for DBT from model oils, being in the range of 17～46 mg-S g-1. The remaining five sorbents are all home-made carbon materials with meso-porous structure and lower SSA ranging from 600 to 1300 m2 g-1, and show 20 ACS Paragon Plus Environment

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Energy & Fuels

lower S-adsorption capacity, being in the range of 11-21 mg-S g-1. This manifests the crucial role of micro-structure on the ADS performance of sorbents. Wang et al. investigated ADS capacity of some carbon materials for thiophene from n-octane, it was found that it is the adsorption heat rather than the SSA that determines the S-adsorption capacity.33 For illustration, the adsorption ability of the sorbents follows the order of graphene> CMK-3 >Maxsorb Carbon> AC, which is consistent with their isosteric heat of adsorption, but irrelevant to their SSA. This suggests that the interaction strength between sorbent and adsorbate is more important than the available adsorptive sites. The strong adsorption of graphene mainly arises from the Table3-Comparison of DBT adsorption capacity of different carbonaceous materials at 298 K from model oils No. Ref

Sorbent

Characteristics

qmax

oil

mg-S/g 1

31

ACF

2

31

GCSAC

3

43

AC

4

44

ACC

5

44

ACC-HNO3

6

45

ACs

7

46

8Current study

AC

ACM-6

SBET=2168, commercial AC fiber with 78% micro-pores SBET=1169, commercial granular AC with 77% micro-pores

n-hexane

46

n-Hexane

40.5

SBET= 2330, commercial AC with pore size 0.6-1 nm and micro-pore volume 1.36 mL g-1

dodecane

35.9

SBET=1118, commercial AC cloth with pore volume of 0.54 mL g-1 and dominant micro-pores of < 1nm

n-heptane

25.8

n-heptane

35.1

n-hexadecane

17.8

n-Octane

16.3

n-Octane+ 20%benzene

3.5

n-octane

21.1

n-Octane+ 20% xylene

12

SBET=955, AC cloth treated with HNO3 with pore volume of 0.46 mL g-1 and increased micro-pore volume SBET=1403 commercial AC with 95% micro-porous area, pore volume 0.58mL g-1 PCB-type commercial AC with SBET=1054 with unknown pore structure

SBET= 712, Meso-porous carbon with pore size of 3.71 nm


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real diesel

5.9

CMK-5

Ordered meso-porous carbon made using AlSBA-15 as template with SBET=1312 pore vole of 1.18 mL g-1, and primary pore size of 3.3 nm

n-hexane

21.7

10

47

CMK-3

Ordered meso-porous carbon made using SBA-15 as template with SBET= 947, pore volume of 1.3 mL g-1, and primary pore size of 4.5 nm

n-hexane

10.9

11

30

CA-22

Meso-porous carbon aerogel with pore size of 22 nm, SBET=670, total pore volume 3.63 mL g-1

n-hexadecane

15.1

n-hexadecane

11.2

9

12

47

30

CA-4

Meso-porous carbon aerogel with pore size of 4.3 nm, SBET=741, total pore volume 0.78 mL g-1 The micro-pore volume and surface area are same as CA-22

abundant carbene-type zigzag edge sites and armchair edges that interact strongly with thiophene molecules. Though micro-porous materials have higher ADS capacity, their desorption is very tough with only 30% desorption rate for each elution, 43 and thus their reusability and applicability is not available in literature. Among the meso-porous sorbents, ACM-6 shows the best ADS performance, meanwhile it can be regenerated easily for recycling uses. Further, compared with the commercial ACs, it shows higher adsorptive selectivity for DBT. For instance, as shown in Table 3, the S-adsorbance of PCB-type commercial AC decreased sharply from 16.3 mg-S g-1 in octane to 3.5 mg-S g-1 when 20% xylene present in n-octane, while that of ACM-6 decreased from 21.1 to 12 mg-S g-1 under similar conditions. Even for real diesel, ACM-6 still showed a S-adsorptivity of about 6 mg-S g-1, which is challenging for other carbon sorbents reported, and thus is of practical significance.

4. CONCLUSION


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A series of ACMs were prepared via mechanochemical reaction of CaC2 with different polyhalo-hydrocarbons. The as-prepared ACMs are meso-porous materials with relatively high SSA and favorable adsorptive performance for PASHs. The adsorption performance is consistent with their SSA order, and ACM-6 has the highest SSA (712 m2 g-1) and adsorption capacity of 21.1 mg-S g-1 for DBT in octane. The adsorptivity of these sorbents for different PASHs follows the order of DBT> BT> 3-MT, and shows a little temperature dependence. The used ACM-6 can be regenerated easily via solvent eluting, and over 90% of its adsorption performance can be retained after six cycles of reuse. Further, the adsorption capacity of ACM-6 is about 6 mg-S g-1 even for real diesel with abundant polycyclic aromatics and various organic sulfur species. As a whole, ACM-6 shows the best ADS performance among all meso-porous carbon sorbents, and may be applicable for real oils after further modification.

AUTHOR INFORMATION Corresponding Author *Tel. /Fax: +86-10-64410308.

E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The authors are grateful for the support from the National Natural Science Foundation of China (Nos. 21376011). REFERENCES (1) Ma, X.; Sakanishi, K.; Mochida, I. Hydrodesulfurization reactivities of various sulfur compounds in diesel fuel. Ind. Eng. Chem. Res. 1994, 33, 218-222. (2) Song, C. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211-263. (3) Anbia, M.; Parvin, Z. Desulfurization of fuels by means of a nanoporous carbon adsorbent. Chem. Eng. Res. Des. 2011, 89, 641-647. (4) Srivastava, V. C. An evaluation of desulfurization technologies for sulfur removal from liquid fuels. RSC Adv. 2012, 2, 759-783. (5) Ma, X.; Velu, S.; Kim, J. H.; Song, C. Deep desulfurization of gasoline by selective adsorption over solid adsorbents and impact of analytical methods on ppm-level sulfur quantification for fuel cell applications. Appl. Catal., B 2005, 56, 137-147. (6) Srivastav, A.; Srivastava, V. C. Adsorptive desulfurization by activated alumina. J Hazard Mater 2009, 170, 1133-40. (7) Duan, F.; Chen, C.; Wang, G.; Yang, Y.; Liu, X.; Qin, Y. Efficient adsorptive removal of dibenzothiophene by graphene oxide-based surface molecularly imprinted polymer. RSC Adv. 2014, 4, 1469-1475. (8) Zhang, J.; Xu, H.; Lu, Y.; Meng, H.; Li, C.; Chen, B.; Lei, Z. Adsorptivity of a hyper cross-linked ionic polymer poly(vinyl imidazole)-1,4-bis(chloromethyl)benzene for thiophenic sulfurs in model oil. Energy Fuels 2016, 30, 5035-5041. (9) Xiong, J.; Zhu, W.; Li, H.; Ding, W.; Chao, Y.; Wu, P.; et al. Few-layered graphene-like boron nitride induced a remarkable adsorption capacity for dibenzothiophene in fuels. Green Chem. 2015, 17, 1647-1656. (10) Li, C.; Jiang, Z.; Gao, J.; Yang, Y.; Wang, S.; Tian, F.; et al. Ultra-deep desulfurization of diesel: oxidation with a recoverable catalyst assembled in emulsion. Chem. Eur. J. 2004, 10, 2277-80. (11) Fraile, J. M.; Gil, C.; Mayoral, J. A.; Muel, B.; Roldán, L.; Vispe, E.; et al. Heterogeneous titanium catalysts for oxidation of dibenzothiophene in hydrocarbon solutions with hydrogen peroxide: On the road to oxidative desulfurization. Appl. Catal., B 2016, 180, 680-686. (12) Qiu, L.; Cheng, Y.; Yang, C.; Zeng, G.; Long, Z.; Wei, S.; et al. Oxidative desulfurization of dibenzothiophene using a catalyst of molybdenum supported on modified medicinal stone. RSC Adv. 2016, 6, 17036-17045. (13) Zhang, H.; Gao, J.; Meng, H.; Lu, Y.; Li, C. Catalytic oxidative desulfurization of fuel by H2O2 in situ produced via oxidation of 2-propanol. Ind. Eng. Chem. Res. 2012, 51, 4868-4874.


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Abstract graphic


Adsorptive Desulfurization of Diesel Oil by Alkynyl Carbon Materials

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