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Electrochemical hydrogenation of coal on Ni-based catalysts
Fuel 122 (2014) 54–59
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Fuel journal homepage: www.elsevier.com/locate/fuel
Electrochemical hydrogenation of coal on Ni-based catalysts Yanlan Liu a, Wei Zhou a, Chinbay Q. Fan b, Renhe Yin a,⇑ a b
Department of Chemistry, Shanghai University, Shanghai 200444, China Gas Technology Institute, 1700 S. Mt. Prospect Rd, Des Plaines, IL 60018, USA
h i g h l i g h t s NiB, PVP-NiB and PVP-NiB/SiO2 catalysts were prepared with chemical reduction. PVP-NiB/SiO2 catalysts with amorphous alloy structure, high B content and larger SBET were obtained. PVP-NiB/SiO2 could improve the cleavage of bridge bonds and the hydrogenation of aromatic rings and unsaturated bond. The electrolytic yield of coal with PVP-NiB/SiO2 catalyst could reach up to 57%.
a r t i c l e
i n f o
Article history: Received 12 September 2013 Received in revised form 3 January 2014 Accepted 4 January 2014 Available online 18 January 2014 Keywords: Catalyst Coal liquefaction Electrochemical Hydrogenation NiB
a b s t r a c t The electrochemical hydrogenation of coal on NiB, polyvinylpyrrolidone (PVP) bond NiB and PVP-NiB/ SiO2 amorphous alloy catalysts was systematically investigated in the alkaline electrolyte. The catalysts were prepared by Watanabe method using borohydride reduction. Structural properties of the catalysts were characterized using XRD, IR, ICP, BET, TEM and XPS techniques. The activity of the catalysts has been evaluated by measuring the yield of coal hydrogenation. The results show that Ni-based catalysts could catalyze the breaking of bridge bonds in coal and add hydrogen to the derivatives of the cracked coal during the coal liquefaction. PVP-NiB/SiO2 exhibits the highest activity among NiB, PVP-NiB and PVP-NiB/ SiO2 catalysts. The high activity could be attributed to larger BET surface areas, better dispersion of the Ni active nano-particles and back-donating electronic effect. The effects of the amount of catalyst and reaction temperature on the reaction were also probed. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Direct coal liquefaction (DCL) techniques have been developed since 1920s. The techniques were used to convert solid coal into liquid fuel as well as wide variety of chemical products [1]. DCL refers to hydrogenation of coal performed usually at 400–470 °C and under a high H2 pressure [2]. The process results in a decrease of molecule size and an increase of solubility of the products in organic solvent because of the hydro-cracking, ring-opening and hydrogenation reactions of coal [3]. Though the conventional coal gasification/liquefaction methods can obtain acceptable conversion rate and efficiency, the technologies are capital intensive and need high temperature and high pressure [4]. In recent years, electrochemical reduction method under clean and mild reaction conditions has been employed to facilitate the process of coal hydrogenation and attracted much attention. In 2000, Galan et al. investigated the electrochemical hydrogenation on coal macromolecules [5]. They found that the electrochemical reduction of coal increases the ratio of hydrogen ⇑ Corresponding author. Tel./fax: +86 021 66133517. E-mail address: [email protected] (R. Yin). 0016-2361/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2014.01.004
in aliphatic and aromatic species. However the conventional coal gasification uses eutectic NaCl–NaCO3 salt with high pH due to its high surface adhesion [6]. In this paper, the anodic and cathodic reaction involving coal in NaOH system are as following [5,7–9].
Anode : 4OH ! O2 þ 2H2 O þ 4e Cathode : Coal þ 4H2 O þ 4e ! H Coal H þ H2 þ 4OH Total : Coal þ 2H2 O ! H Coal H þ H2 þ O2 For DCL, catalyst plays an important role. In 2002, Kaneko et al. studied the catalytic activity of limonite catalysts for DCL and found that Ni containing limonite showed a high liquefaction activity for Indonesia coal [10]. In 2011, Kan et al. used W-Ni/c-Al2O3 catalyst for the hydrogenation of coal tar and found that the raw coal tar could be considerably upgraded through catalytic hydroprocessing and high-quality fuels were obtained [11]. In 2011, Lei et al. reported that NiW/SBA-15 catalyst had high catalytic activities in hydrotreatment of heavy oil from a DCL process [12]. NiB has been widely investigated as the catalyst for hydrogenation of unsaturated compounds, such as furfural [13], citral [14], cyclopentadiene [15], and cyclohexene [16]. So far, there has been no
Y. Liu et al. / Fuel 122 (2014) 54–59
report about the catalytic activity of NiB for coal liquefaction. In this work, we focus on studying the catalytic effect of NiB catalyst for the coal liquefaction by an electrochemical reduction method.
2. Experimental 2.1. Coal samples Coal samples used were Shenhua coal provided by Chemical Research Institute of East China University of Science and Technology. The pretreatment of the samples has been reported elsewhere [7]. The coal had been crushed, ball milled for 24 h, washed with 4 M HCl in 343 K, filtered, and dried in a vacuum for 24 h at 393 K. The resulted particle size was less than 16 lm.
2.2. Catalyst preparation The amorphous catalyst was prepared by a chemical reaction referring to the literature [13]. An appropriate quantity of polyvinylpyrrolidone (PVP, M.W. 58,000) served as a protective reagent was dissolved into 0.1 M nickel acetate aqueous solution. An adequate amount of SiO2 was then added to the mixture. The molar ratio of PVP (monomer):Ni:SiO2 is 40:2:5. After being stirred for several hours, the mixture was gradually added into 1 M sodium borohydride under vigorous stirring. The reaction was allowed at 298 K until no bubbles were generated. The resulting black solid was separated with a high-speed centrifuge and thoroughly washed with deionized water and ethanol, and finally stored in ethanol for further use. The as-prepared NiB amorphous alloy with PVP is assigned to PVP-NiB/SiO2. PVP-NiB represents the catalyst without SiO2 and NiB is the catalyst without PVP and SiO2. 2.3. Catalyst characterization The crystal structure of the catalysts was obtained on a DX2700 X-ray diffractometer with Cu Ka radiation, operated at 40 kV and 40 mA. IR spectrometer used was Avatar 370 (Nicolet Inc.). The chemical composition of the catalysts was analyzed by inductively coupled plasma (ICP, Thermo Fisher, ICAP6300). The surface area (SBET) of the catalysts was identified by nitrogen adsorption at 196 °C using the Brunauer–Emmett–Teller (BET, Beishide Instrument, 3H-2000III) method. Transmission electron microscopy (TEM) photographs were observed with a NEC JEM200CX apparatus. X-ray photoelectron spectroscopy (XPS) was recorded with a Perkin–Elmer PHI 5000C photoelectron spectrometer using Mg Ka radiation.
2.5. Extraction of products After electrolysis the catholyte mixture was separated into liquid and solid products by filtering. Since tetrahydrofuran (THF) is a generally used solvent to evaluate solubility in coal liquefaction industry [17], the products were extracted directly by THF (363 K, 500 r. min1, and 24 h) [18] to generate THF soluble fractions (TSi) and THF insoluble fractions (TISi). After extraction step, TSi were obtained by rotary evaporation to remove the solvent [4], TISi collected in a cylindrical filter paper were dried under vacuum at 393 K for 24 h and weighed to calculate the extract yield. The yield of the electrochemical reaction is determined as:
Yield ¼ ðM C M TISi Þ=M C 100% where MC: coal quality, MTISi: THF insoluble quality, i: 1, 2, 3, 4 TIS1, TIS2, TIS3 and TIS4 represented THF insoluble of the original coal, electrolytic coal, electrolytic coal with 20 wt.% PVP-NiB catalyst and electrolytic coal with 20 wt.% PVP-NiB/SiO2 catalyst, respectively. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD Fig. 1 shows the XRD patterns of the NiB, PVP-NiB and PVP-NiB/ SiO2 catalysts. For NiB, besides the one broad peak around 2h = 45° corresponding to a characteristic of typical amorphous alloy structure [19], there were two small shoulder peaks at 33° and 60°, which indexed as hexagonal phase of Ni(OH)2 (JCDPS card file No. 14-0117) [20]. It can be clearly seen that there was only one broad peak at around 2h = 45° in PVP-NiB spectrum, confirming the formation of NiB amorphous alloy. There was a broad peak at around 2h = 22° appearing on the pattern of the SiO2. After subtracting the background spectrum of SiO2, no obvious diffraction peaks were observed for the PVP-NiB/SiO2, which was assumed that the particles of PVP-NiB/SiO2 alloy might become smaller and are well dispersed on SiO2 support [16]. 3.1.2. IR spectra In order to confirm the NiB particles supported on SiO2, we investigate the IR spectra of the catalysts (Fig. 2) respectively. In the spectrum of SiO2, a broad peak at ca. 1103 cm1 and a sharp peak at ca. 468 cm1 can be assigned to SiAOASi asymmetric
2.4. Electrochemical setup The catalyst activity was evaluated for coal electrochemical hydrogenation/liquefaction reactions conducted in an H-type electrolytic cell, whose compartments were separated by a porous frit. The solution used for the anode was 1.0 M, 100 mL NaOH and the cathode solution consisting of 3 g coal sample suspended in 1.0 M, 100 mL NaOH, which was stirred at a constant rate. The catalyst NiB, PVP-NiB and PVP-NiB/SiO2 were added to the cathode solution to evaluate the effect on coal reduction, respectively. An iridium oxide/iridium foil (3 cm ⁄ 3 cm), a Pt sheet and a saturated calomel electrode were used as working, counter, and reference electrode, respectively. An electrochemical workstation (CHI660B, Shanghai Chenhua Instrument Company) was used for the electrolytic experiments and records the linear sweep voltammetry (LSV) [8]. The LSV scan rate was 0.1 V s1.
55
Fig. 1. XRD patterns of the NiB, PVP-NiB and PVP-NiB/SiO2 catalysts.
56
Y. Liu et al. / Fuel 122 (2014) 54–59
188.4 eV and 192.6 eV, respectively. In comparison with the standard BE of pure B (187.1 eV) and Ni 2p3/2 (853.1 eV) [24], the BE of elementary B in the PVP-NiB/SiO2 catalyst positively shifted by 1.3 eV and that of metallic Ni 2p3/2 negatively shifted by 0.6 eV. According to the law of chemical shift, partial electron might transfer from B to Ni, leading to B electron-deficient and Ni electronenriched, which agreed with earlier investigation [25]. Such electronic modifications may facilitate the coal electrochemical hydrogenation/liquefaction [22]. 3.2. Effect of catalyst on coal liquefaction
Fig. 2. IR of amorphous alloy catalysts.
stretching vibration and SiAOASi bending vibration, respectively [21]. And the two characteristic absorption peak (1103 cm1, 468 cm1) also can be obviously seen in PVP-NiB/SiO2, suggesting that NiB alloy are in good dispersion on SiO2 support, which is also confirmed by the XRD analysis in Fig. 1. 3.1.3. Results of BET and ICP measurements Table 1 lists the composition and the BET surface areas (SBET) of the NiB, PVP-NiB and PVP-NiB/SiO2 catalysts. The ratios of Ni/B of the three catalysts are nearly the same, i.e. about 1.0. The high content of B (50%), which is significantly higher than that of previously reported [22], is usually helpful to protect the active Ni centers from oxidation and to increase the hydrogenation activity [22]. As shown in Table 1, the SBET of PVP-NiB/SiO2 drastically increase to 244.9 m2 g1, which was more than four times the SBET of NiB (63.5 m2 g1) and twice that of PVP-NiB (113.5 m2 g1). The results can be explained that the support SiO2 can prevent the agglomerate of the NiB particles and promote the well dispersion of the catalysts. In general, a larger SBET is beneficial for enhancing the catalytic activity [23]. 3.1.4. TEM A further investigation of morphology was made by TEM. As shown in Fig. 3, the particle size of PVP-NiB amorphous alloy was about 20–25 nm, and 10–15 nm for PVP-NiB/SiO2 sample. Combined with the previous analysis, the addition of SiO2 inhibits the aggregation of Ni-B particles, leading to a better dispersion and smaller size of the PVP-NiB/SiO2 [16]. And such morphology is helpful to enhance the active surface area of catalysts. 3.1.5. XPS In order to evaluate the electronic state of the PVP-NiB/SiO2 amorphous alloy, the binding energies (BE) of the relative substances were determined by XPS under present conditions. As shown in Fig. 4, there is a peak at 852.5 eV for the Ni 2p3/2, which is assigned to metallic Ni. In the result of B 1s level, two peaks corresponding to the elementary B and oxidized B were observed at Table 1 Bulk compositions and surface areas of the NiB, PVP-NiB and PVP-NiB/SiO2 catalysts. Catalyst
Composition (at.%) Ni
B
SiO2 NiB PVP-NiB PVP-NiB/SiO2
– 50.83 51.75 51.94
– 49.17 48.25 48.06
Ni/B (at. ratio)
SBET (m2 g1)
– 1.03 1.07 1.08
427.5 63.5 113.5 244.9
Fig. 5 describes the effect of catalyst on the coal electrochemical hydrogenation/liquefaction reactions. A 1 M NaOH solution was used for the blank test. LSV curves a, b, c and d are assigned to the electrolysis test of blank, NiB, PVP-NiB and PVP-NiB/SiO2 in 1 M NaOH, respectively. The cathode solution of curves e, f, g and h consists of coal, NiB with coal, PVP-NiB with coal and PVP-NiB/ SiO2 with coal in 1 M NaOH, respectively. As shown in Fig. 5, the current density in the curves of a, b, c and d increased from 0.8 V indicated hydrogen evolution reaction occurred. But they are almost the same, implying that the catalysts did not take reactions. Compared to a curve, the current density in e curve increased, which combined with the yield results and IR analysis illustrates a coal reaction. It can be clearly found that the catalysts have good catalytic activity for the hydrogenation/liquefaction of coal because the current density of f, g and h are higher than that of e. Compared to NiB, PVP-NiB and PVP-NiB/SiO2 catalysts exhibit higher activity. Thus, we take PVP-NiB and PVP-NiB/SiO2 as the major research object in the following study. 3.3. Effect of the amount of catalyst Fig. 6 shows the effect of the amount catalyst on current density of coal liquefaction. As shown in Fig. 6A, the current density of c (10%), d (20%) and e (30%) increased, which can be attributed to the adding of catalyst. An increasing in the amount of PVP-NiB seemed to improve the current density, however, the current density decreased when the amount of PVP-NiB was increased to 30%. The PVP-NiB/SiO2 (Fig. 6B) followed the same rule as that of PVPNiB, resulting in an increase in the current density and the distinction was more evident than that of PVP-NiB. It is possible that excessive catalyst was easy to agglomerate, consequently leading to the active sites was covered and the activity was decreased. 3.4. Effect of reaction temperature Fig. 7 displays the change of electro-reduction current density along with temperature ranging from 303 K to 363 K. We can see that the current density increases at first but then remains stable even decreases as the electrolysis temperature increases to 363 K. And maximum current density occurs at 343 K. The results imply that reaction temperature is an important parameter strongly affected the rates of chemical reactions. And the higher temperature not always leads to faster reactions. The reason may be that the viscosity of Shenhua coal increases and the catalysts tend to crystallize and agglomerate as the temperature increases [26]. 3.5. Yield The electrolysis condition as followed: electric potential was 1.6 V; temperature was 343 K; the amount of catalyst was 20 wt.%. The PVP-NiB, PVP-NiB/SiO2 were used as the catalysts for coal liquefaction. The results are shown in Fig. 8. THF soluble of the original coal is 4%. In non-catalytic electrolysis reaction,
Y. Liu et al. / Fuel 122 (2014) 54–59
57
Fig. 3. TEM images of amorphous alloy catalysts. (A) PVP-NiB; (B) PVP-NiB/SiO2.
Fig. 4. XPS spectra of PVP-NiB/SiO2 amorphous alloy.
Fig. 5. LSV results: (a) Blank; (b) NiB; (c) PVP-NiB; (d) PVP-NiB/SiO2; (e) Coal; (f) NiB with coal; (g) PVP-NiB with coal; (h) PVP-NiB/SiO2 with coal. m (cat) : m (coal) = 20%. Scan rate: 0.1 V s1, 0.45 V to 1.60 V vs. HgO/Hg, T = 343 K.
the liquefaction yield is 32%; With the addition of 20 wt.% PVP-NiB, or PVP-NiB/SiO2, the liquefaction yield are 43% and 57%, respectively, which is distinct higher than that of the previous works reported [27]. The results show that there is a very notable change in the yield with the catalysts (4% vs. 43% and 57%), which implies that the PVP-NiB and PVP-NiB/SiO2 catalysts could improve the coal liquefaction yield and efficiency. 3.6. Analysis of products TS1, TS2, TS3 and TS4 represented THF soluble of the original coal, electrolytic coal, electrolytic coal with 20 wt.% PVP-NiB
catalyst and electrolytic coal with 20 wt.% PVP-NiB/SiO2 catalyst, respectively. The products TS1, TS2, TS3 and TS4 were analyzed by IR shown in Fig. 9. The IR peaks were not influenced in any way by simply dissolving PVP during the reactions. Detailed information for the IR is demonstrated in Table 2. For the TS2, TS3 and TS4, three similar IR spectra with different intensities of peaks were obtained. Comparing to the TS1, some changes occurred: (1) The peak at 2958 and 2920 cm1 for the alkanes are increased, accompanied by the significantly smaller peaks at 803 cm1, indicating the possibility of reduction and destruction of aromatic groups and the bridge bond connecting the CH2; (2) The peak at 1700 cm1 and 1630 cm1 become much weaker, indicating the possibility that the catalysts could promote H atoms insert into C@O and C@C, leading to the reduction of unsaturated bonds. (3) The peaks at 1261, 1096 and 1024 cm1 are not very visible, denoting that most of CAOAC is reduced and destructed. Taking all above analyses into consideration, the results conclude that electro-reduction can promote the cracking of bridge band and the transformation of unsaturated bond into saturated bonds, of course, the addition of catalyst could make the changes more obvious. Elemental analyses are used to study the change of H/C ratio. It can be found that the H/C ratio of THF soluble increases after the electrolysis (Table 3). H/C ratio of TS2 increases to 1.51. Moreover, H/C ratio of TS3 increases from 0.98 (TS1) to 1.71, and that of TS4 up to 1.95, much higher than that of TS1 (0.98), indicating 97 H [i.e. (1.95–0.98) 100] are added per 100 C after the electrolysis, which is significant higher than that of the previous works reported [32]. The results demonstrate that THF soluble compounds were electrochemically reduced and catalyst, especially PVP-NiB/SiO2 could improve the cleavage of bridge bonds and the hydrogenation of aromatic rings and unsaturated bond.
58
Y. Liu et al. / Fuel 122 (2014) 54–59
Fig. 6. Effect of the amount catalyst on current density of coal liquefaction: (A) PVP-NiB; (B) PVP-NiB/SiO2; a. Blank; b, c, d, e are described as m (cat): m (coal) = 0%, 10%, 20%, 30%, respectively. Scan rate: 0.1 V s1, 0.45 V to 1.60 V vs. HgO/Hg, T = 343 K.
Fig. 7. Effect of electrolysis temperature on coal liquefaction. (A) PVP-NiB; (B) PVP-NiB/SiO2; a: 303 K, b: 323 K, c: 343 K, d: 363 K. m (cat): m (coal) = 20%. Scan rate: 0.1 V s1, 0.45 V to 1.60 V vs. HgO/Hg.
Fig. 8. Results of yield. m (cat): m (coal) = 20%. Electric potential: 1.60 V vs. HgO/ Hg. T = 343 K.
4. Mechanism We come up with a possible catalytic mechanism of Ni-based catalysts for coal hydrogenation/liquefaction according to the above analysis. In electrolysis process, the reaction of ring-opening, bridge cracking, dealkylation and so on makes coal cleave radical fragments (Fig. 9). And hydrogen evolution reaction occurred
Fig. 9. Infrared spectra of TS1, TS2 and TS3. TS1: THF soluble of the original coal; TS2: THF soluble of electrolytic coal; TS3: THF soluble of electrolytic coal with 20 wt.% PVP-NiB catalyst; TS4: THF soluble of electrolytic coal with 20 wt.% PVPNiB/SiO2 catalyst.
(Fig. 5). Ni active sites in the NiB amorphous alloys have unpaired d electron (Fig. 4), which could form chemisorption bond with hydrogen. In the role of chemisorption bond, hydrogen break down to active hydrogen atoms [33]. Then the coal radical fragments can be easily stabilized by active hydrogen atoms to form the coal liquids (Fig. 8). The reaction between NaOH, coal and catalysts are as follows:
Y. Liu et al. / Fuel 122 (2014) 54–59 Table 2 Vibrational assignments. Vibration (cm1)
Assignment
Ref.
2958 2920 1700 1630 1462 1377 1261, 1096, 1024 803
CH3 (asym) CH2 (asym) C@O C@C (skeletal vibration) CH2 (bending vibration) CH3 (sym) CAOAC Ar–X
[28] [28] [29] [30] [11] [11] [31] [11]
THF soluble
C (wt.%)
H (wt.%)
N (wt.%)
H/C (at.)
TS1 TS2 TS3 TS4
61.79 79.52 82.98 61.85
5.06 10.03 11.82 10.03
1.05 0.41 0.36 0.52
0.98 1.51 1.71 1.95
Electrolysis
4H2 O þ 4e ! 4OH þ 2H2 Catalysis
H2 ! H þ H Electrolysis X
R þ H
the adsorbed hydrogen and promote the hydrogenation of coal. Though the efficiency of electrochemical reduction of coal may be too low comparing to that of conventional DCL performed at high temperature and high pressure, it is very possible to obtain a higher yield with improved conditions, such as catalyst, electrode and electrolytic cell, of course, the preliminary treatment of coal is useful, which need our further research. Acknowledgements The authors gratefully acknowledge the National Science Foundation of China (Nos. 21173144 and 20873083), the Instrumental Analysis and Research Center of Shanghai University, and the State Key Laboratory of Chemical Engineering (No. SKL-ChE-08A01).
Table 3 Element analyses of THF soluble of coal.
Coal !
59
R
Hydrogenation
! RH
5. Conclusions In summary, the activity NiB, PVP-NiB and PVP-NiB/SiO2 amorphous alloy catalysts can be prepared by chemical reduction, characterized by XRD, IR, BET, ICP and XPS. The catalytic activity of NiB, PVP-NiB and PVP-NiB/SiO2 for the electro-reduction of coal was investigated. The results show that PVP-NiB/SiO2 exhibits higher electrochemical catalytic activity between the three catalysts studied, which was confirmed by LSV test, yield, IR and elemental analyses. With PVP-NiB/SiO2, the yield of coal liquefaction can be drastically improved to 57%, H/C ratio of the products can be up to 1.95. It attributes to both structural effect and electronic effect. For structural effect, the PVP-NiB amorphous alloy can be well dispersed on SiO2 and stabilized in solution. For electronic effect, electron-enriched Ni and weaker bond strength of NiAH activate
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Electrochemical hydrogenation of coal on Ni-based catalysts Yanlan Liu a, Wei Zhou a, Chinbay Q. Fan b, Renhe Yin a,⇑ a b
Department of Chemistry, Shanghai University, Shanghai 200444, China Gas Technology Institute, 1700 S. Mt. Prospect Rd, Des Plaines, IL 60018, USA
h i g h l i g h t s NiB, PVP-NiB and PVP-NiB/SiO2 catalysts were prepared with chemical reduction. PVP-NiB/SiO2 catalysts with amorphous alloy structure, high B content and larger SBET were obtained. PVP-NiB/SiO2 could improve the cleavage of bridge bonds and the hydrogenation of aromatic rings and unsaturated bond. The electrolytic yield of coal with PVP-NiB/SiO2 catalyst could reach up to 57%.
a r t i c l e
i n f o
Article history: Received 12 September 2013 Received in revised form 3 January 2014 Accepted 4 January 2014 Available online 18 January 2014 Keywords: Catalyst Coal liquefaction Electrochemical Hydrogenation NiB
a b s t r a c t The electrochemical hydrogenation of coal on NiB, polyvinylpyrrolidone (PVP) bond NiB and PVP-NiB/ SiO2 amorphous alloy catalysts was systematically investigated in the alkaline electrolyte. The catalysts were prepared by Watanabe method using borohydride reduction. Structural properties of the catalysts were characterized using XRD, IR, ICP, BET, TEM and XPS techniques. The activity of the catalysts has been evaluated by measuring the yield of coal hydrogenation. The results show that Ni-based catalysts could catalyze the breaking of bridge bonds in coal and add hydrogen to the derivatives of the cracked coal during the coal liquefaction. PVP-NiB/SiO2 exhibits the highest activity among NiB, PVP-NiB and PVP-NiB/ SiO2 catalysts. The high activity could be attributed to larger BET surface areas, better dispersion of the Ni active nano-particles and back-donating electronic effect. The effects of the amount of catalyst and reaction temperature on the reaction were also probed. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Direct coal liquefaction (DCL) techniques have been developed since 1920s. The techniques were used to convert solid coal into liquid fuel as well as wide variety of chemical products [1]. DCL refers to hydrogenation of coal performed usually at 400–470 °C and under a high H2 pressure [2]. The process results in a decrease of molecule size and an increase of solubility of the products in organic solvent because of the hydro-cracking, ring-opening and hydrogenation reactions of coal [3]. Though the conventional coal gasification/liquefaction methods can obtain acceptable conversion rate and efficiency, the technologies are capital intensive and need high temperature and high pressure [4]. In recent years, electrochemical reduction method under clean and mild reaction conditions has been employed to facilitate the process of coal hydrogenation and attracted much attention. In 2000, Galan et al. investigated the electrochemical hydrogenation on coal macromolecules [5]. They found that the electrochemical reduction of coal increases the ratio of hydrogen ⇑ Corresponding author. Tel./fax: +86 021 66133517. E-mail address: [email protected] (R. Yin). 0016-2361/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2014.01.004
in aliphatic and aromatic species. However the conventional coal gasification uses eutectic NaCl–NaCO3 salt with high pH due to its high surface adhesion [6]. In this paper, the anodic and cathodic reaction involving coal in NaOH system are as following [5,7–9].
Anode : 4OH ! O2 þ 2H2 O þ 4e Cathode : Coal þ 4H2 O þ 4e ! H Coal H þ H2 þ 4OH Total : Coal þ 2H2 O ! H Coal H þ H2 þ O2 For DCL, catalyst plays an important role. In 2002, Kaneko et al. studied the catalytic activity of limonite catalysts for DCL and found that Ni containing limonite showed a high liquefaction activity for Indonesia coal [10]. In 2011, Kan et al. used W-Ni/c-Al2O3 catalyst for the hydrogenation of coal tar and found that the raw coal tar could be considerably upgraded through catalytic hydroprocessing and high-quality fuels were obtained [11]. In 2011, Lei et al. reported that NiW/SBA-15 catalyst had high catalytic activities in hydrotreatment of heavy oil from a DCL process [12]. NiB has been widely investigated as the catalyst for hydrogenation of unsaturated compounds, such as furfural [13], citral [14], cyclopentadiene [15], and cyclohexene [16]. So far, there has been no
Y. Liu et al. / Fuel 122 (2014) 54–59
report about the catalytic activity of NiB for coal liquefaction. In this work, we focus on studying the catalytic effect of NiB catalyst for the coal liquefaction by an electrochemical reduction method.
2. Experimental 2.1. Coal samples Coal samples used were Shenhua coal provided by Chemical Research Institute of East China University of Science and Technology. The pretreatment of the samples has been reported elsewhere [7]. The coal had been crushed, ball milled for 24 h, washed with 4 M HCl in 343 K, filtered, and dried in a vacuum for 24 h at 393 K. The resulted particle size was less than 16 lm.
2.2. Catalyst preparation The amorphous catalyst was prepared by a chemical reaction referring to the literature [13]. An appropriate quantity of polyvinylpyrrolidone (PVP, M.W. 58,000) served as a protective reagent was dissolved into 0.1 M nickel acetate aqueous solution. An adequate amount of SiO2 was then added to the mixture. The molar ratio of PVP (monomer):Ni:SiO2 is 40:2:5. After being stirred for several hours, the mixture was gradually added into 1 M sodium borohydride under vigorous stirring. The reaction was allowed at 298 K until no bubbles were generated. The resulting black solid was separated with a high-speed centrifuge and thoroughly washed with deionized water and ethanol, and finally stored in ethanol for further use. The as-prepared NiB amorphous alloy with PVP is assigned to PVP-NiB/SiO2. PVP-NiB represents the catalyst without SiO2 and NiB is the catalyst without PVP and SiO2. 2.3. Catalyst characterization The crystal structure of the catalysts was obtained on a DX2700 X-ray diffractometer with Cu Ka radiation, operated at 40 kV and 40 mA. IR spectrometer used was Avatar 370 (Nicolet Inc.). The chemical composition of the catalysts was analyzed by inductively coupled plasma (ICP, Thermo Fisher, ICAP6300). The surface area (SBET) of the catalysts was identified by nitrogen adsorption at 196 °C using the Brunauer–Emmett–Teller (BET, Beishide Instrument, 3H-2000III) method. Transmission electron microscopy (TEM) photographs were observed with a NEC JEM200CX apparatus. X-ray photoelectron spectroscopy (XPS) was recorded with a Perkin–Elmer PHI 5000C photoelectron spectrometer using Mg Ka radiation.
2.5. Extraction of products After electrolysis the catholyte mixture was separated into liquid and solid products by filtering. Since tetrahydrofuran (THF) is a generally used solvent to evaluate solubility in coal liquefaction industry [17], the products were extracted directly by THF (363 K, 500 r. min1, and 24 h) [18] to generate THF soluble fractions (TSi) and THF insoluble fractions (TISi). After extraction step, TSi were obtained by rotary evaporation to remove the solvent [4], TISi collected in a cylindrical filter paper were dried under vacuum at 393 K for 24 h and weighed to calculate the extract yield. The yield of the electrochemical reaction is determined as:
Yield ¼ ðM C M TISi Þ=M C 100% where MC: coal quality, MTISi: THF insoluble quality, i: 1, 2, 3, 4 TIS1, TIS2, TIS3 and TIS4 represented THF insoluble of the original coal, electrolytic coal, electrolytic coal with 20 wt.% PVP-NiB catalyst and electrolytic coal with 20 wt.% PVP-NiB/SiO2 catalyst, respectively. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD Fig. 1 shows the XRD patterns of the NiB, PVP-NiB and PVP-NiB/ SiO2 catalysts. For NiB, besides the one broad peak around 2h = 45° corresponding to a characteristic of typical amorphous alloy structure [19], there were two small shoulder peaks at 33° and 60°, which indexed as hexagonal phase of Ni(OH)2 (JCDPS card file No. 14-0117) [20]. It can be clearly seen that there was only one broad peak at around 2h = 45° in PVP-NiB spectrum, confirming the formation of NiB amorphous alloy. There was a broad peak at around 2h = 22° appearing on the pattern of the SiO2. After subtracting the background spectrum of SiO2, no obvious diffraction peaks were observed for the PVP-NiB/SiO2, which was assumed that the particles of PVP-NiB/SiO2 alloy might become smaller and are well dispersed on SiO2 support [16]. 3.1.2. IR spectra In order to confirm the NiB particles supported on SiO2, we investigate the IR spectra of the catalysts (Fig. 2) respectively. In the spectrum of SiO2, a broad peak at ca. 1103 cm1 and a sharp peak at ca. 468 cm1 can be assigned to SiAOASi asymmetric
2.4. Electrochemical setup The catalyst activity was evaluated for coal electrochemical hydrogenation/liquefaction reactions conducted in an H-type electrolytic cell, whose compartments were separated by a porous frit. The solution used for the anode was 1.0 M, 100 mL NaOH and the cathode solution consisting of 3 g coal sample suspended in 1.0 M, 100 mL NaOH, which was stirred at a constant rate. The catalyst NiB, PVP-NiB and PVP-NiB/SiO2 were added to the cathode solution to evaluate the effect on coal reduction, respectively. An iridium oxide/iridium foil (3 cm ⁄ 3 cm), a Pt sheet and a saturated calomel electrode were used as working, counter, and reference electrode, respectively. An electrochemical workstation (CHI660B, Shanghai Chenhua Instrument Company) was used for the electrolytic experiments and records the linear sweep voltammetry (LSV) [8]. The LSV scan rate was 0.1 V s1.
55
Fig. 1. XRD patterns of the NiB, PVP-NiB and PVP-NiB/SiO2 catalysts.
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188.4 eV and 192.6 eV, respectively. In comparison with the standard BE of pure B (187.1 eV) and Ni 2p3/2 (853.1 eV) [24], the BE of elementary B in the PVP-NiB/SiO2 catalyst positively shifted by 1.3 eV and that of metallic Ni 2p3/2 negatively shifted by 0.6 eV. According to the law of chemical shift, partial electron might transfer from B to Ni, leading to B electron-deficient and Ni electronenriched, which agreed with earlier investigation [25]. Such electronic modifications may facilitate the coal electrochemical hydrogenation/liquefaction [22]. 3.2. Effect of catalyst on coal liquefaction
Fig. 2. IR of amorphous alloy catalysts.
stretching vibration and SiAOASi bending vibration, respectively [21]. And the two characteristic absorption peak (1103 cm1, 468 cm1) also can be obviously seen in PVP-NiB/SiO2, suggesting that NiB alloy are in good dispersion on SiO2 support, which is also confirmed by the XRD analysis in Fig. 1. 3.1.3. Results of BET and ICP measurements Table 1 lists the composition and the BET surface areas (SBET) of the NiB, PVP-NiB and PVP-NiB/SiO2 catalysts. The ratios of Ni/B of the three catalysts are nearly the same, i.e. about 1.0. The high content of B (50%), which is significantly higher than that of previously reported [22], is usually helpful to protect the active Ni centers from oxidation and to increase the hydrogenation activity [22]. As shown in Table 1, the SBET of PVP-NiB/SiO2 drastically increase to 244.9 m2 g1, which was more than four times the SBET of NiB (63.5 m2 g1) and twice that of PVP-NiB (113.5 m2 g1). The results can be explained that the support SiO2 can prevent the agglomerate of the NiB particles and promote the well dispersion of the catalysts. In general, a larger SBET is beneficial for enhancing the catalytic activity [23]. 3.1.4. TEM A further investigation of morphology was made by TEM. As shown in Fig. 3, the particle size of PVP-NiB amorphous alloy was about 20–25 nm, and 10–15 nm for PVP-NiB/SiO2 sample. Combined with the previous analysis, the addition of SiO2 inhibits the aggregation of Ni-B particles, leading to a better dispersion and smaller size of the PVP-NiB/SiO2 [16]. And such morphology is helpful to enhance the active surface area of catalysts. 3.1.5. XPS In order to evaluate the electronic state of the PVP-NiB/SiO2 amorphous alloy, the binding energies (BE) of the relative substances were determined by XPS under present conditions. As shown in Fig. 4, there is a peak at 852.5 eV for the Ni 2p3/2, which is assigned to metallic Ni. In the result of B 1s level, two peaks corresponding to the elementary B and oxidized B were observed at Table 1 Bulk compositions and surface areas of the NiB, PVP-NiB and PVP-NiB/SiO2 catalysts. Catalyst
Composition (at.%) Ni
B
SiO2 NiB PVP-NiB PVP-NiB/SiO2
– 50.83 51.75 51.94
– 49.17 48.25 48.06
Ni/B (at. ratio)
SBET (m2 g1)
– 1.03 1.07 1.08
427.5 63.5 113.5 244.9
Fig. 5 describes the effect of catalyst on the coal electrochemical hydrogenation/liquefaction reactions. A 1 M NaOH solution was used for the blank test. LSV curves a, b, c and d are assigned to the electrolysis test of blank, NiB, PVP-NiB and PVP-NiB/SiO2 in 1 M NaOH, respectively. The cathode solution of curves e, f, g and h consists of coal, NiB with coal, PVP-NiB with coal and PVP-NiB/ SiO2 with coal in 1 M NaOH, respectively. As shown in Fig. 5, the current density in the curves of a, b, c and d increased from 0.8 V indicated hydrogen evolution reaction occurred. But they are almost the same, implying that the catalysts did not take reactions. Compared to a curve, the current density in e curve increased, which combined with the yield results and IR analysis illustrates a coal reaction. It can be clearly found that the catalysts have good catalytic activity for the hydrogenation/liquefaction of coal because the current density of f, g and h are higher than that of e. Compared to NiB, PVP-NiB and PVP-NiB/SiO2 catalysts exhibit higher activity. Thus, we take PVP-NiB and PVP-NiB/SiO2 as the major research object in the following study. 3.3. Effect of the amount of catalyst Fig. 6 shows the effect of the amount catalyst on current density of coal liquefaction. As shown in Fig. 6A, the current density of c (10%), d (20%) and e (30%) increased, which can be attributed to the adding of catalyst. An increasing in the amount of PVP-NiB seemed to improve the current density, however, the current density decreased when the amount of PVP-NiB was increased to 30%. The PVP-NiB/SiO2 (Fig. 6B) followed the same rule as that of PVPNiB, resulting in an increase in the current density and the distinction was more evident than that of PVP-NiB. It is possible that excessive catalyst was easy to agglomerate, consequently leading to the active sites was covered and the activity was decreased. 3.4. Effect of reaction temperature Fig. 7 displays the change of electro-reduction current density along with temperature ranging from 303 K to 363 K. We can see that the current density increases at first but then remains stable even decreases as the electrolysis temperature increases to 363 K. And maximum current density occurs at 343 K. The results imply that reaction temperature is an important parameter strongly affected the rates of chemical reactions. And the higher temperature not always leads to faster reactions. The reason may be that the viscosity of Shenhua coal increases and the catalysts tend to crystallize and agglomerate as the temperature increases [26]. 3.5. Yield The electrolysis condition as followed: electric potential was 1.6 V; temperature was 343 K; the amount of catalyst was 20 wt.%. The PVP-NiB, PVP-NiB/SiO2 were used as the catalysts for coal liquefaction. The results are shown in Fig. 8. THF soluble of the original coal is 4%. In non-catalytic electrolysis reaction,
Y. Liu et al. / Fuel 122 (2014) 54–59
57
Fig. 3. TEM images of amorphous alloy catalysts. (A) PVP-NiB; (B) PVP-NiB/SiO2.
Fig. 4. XPS spectra of PVP-NiB/SiO2 amorphous alloy.
Fig. 5. LSV results: (a) Blank; (b) NiB; (c) PVP-NiB; (d) PVP-NiB/SiO2; (e) Coal; (f) NiB with coal; (g) PVP-NiB with coal; (h) PVP-NiB/SiO2 with coal. m (cat) : m (coal) = 20%. Scan rate: 0.1 V s1, 0.45 V to 1.60 V vs. HgO/Hg, T = 343 K.
the liquefaction yield is 32%; With the addition of 20 wt.% PVP-NiB, or PVP-NiB/SiO2, the liquefaction yield are 43% and 57%, respectively, which is distinct higher than that of the previous works reported [27]. The results show that there is a very notable change in the yield with the catalysts (4% vs. 43% and 57%), which implies that the PVP-NiB and PVP-NiB/SiO2 catalysts could improve the coal liquefaction yield and efficiency. 3.6. Analysis of products TS1, TS2, TS3 and TS4 represented THF soluble of the original coal, electrolytic coal, electrolytic coal with 20 wt.% PVP-NiB
catalyst and electrolytic coal with 20 wt.% PVP-NiB/SiO2 catalyst, respectively. The products TS1, TS2, TS3 and TS4 were analyzed by IR shown in Fig. 9. The IR peaks were not influenced in any way by simply dissolving PVP during the reactions. Detailed information for the IR is demonstrated in Table 2. For the TS2, TS3 and TS4, three similar IR spectra with different intensities of peaks were obtained. Comparing to the TS1, some changes occurred: (1) The peak at 2958 and 2920 cm1 for the alkanes are increased, accompanied by the significantly smaller peaks at 803 cm1, indicating the possibility of reduction and destruction of aromatic groups and the bridge bond connecting the CH2; (2) The peak at 1700 cm1 and 1630 cm1 become much weaker, indicating the possibility that the catalysts could promote H atoms insert into C@O and C@C, leading to the reduction of unsaturated bonds. (3) The peaks at 1261, 1096 and 1024 cm1 are not very visible, denoting that most of CAOAC is reduced and destructed. Taking all above analyses into consideration, the results conclude that electro-reduction can promote the cracking of bridge band and the transformation of unsaturated bond into saturated bonds, of course, the addition of catalyst could make the changes more obvious. Elemental analyses are used to study the change of H/C ratio. It can be found that the H/C ratio of THF soluble increases after the electrolysis (Table 3). H/C ratio of TS2 increases to 1.51. Moreover, H/C ratio of TS3 increases from 0.98 (TS1) to 1.71, and that of TS4 up to 1.95, much higher than that of TS1 (0.98), indicating 97 H [i.e. (1.95–0.98) 100] are added per 100 C after the electrolysis, which is significant higher than that of the previous works reported [32]. The results demonstrate that THF soluble compounds were electrochemically reduced and catalyst, especially PVP-NiB/SiO2 could improve the cleavage of bridge bonds and the hydrogenation of aromatic rings and unsaturated bond.
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Fig. 6. Effect of the amount catalyst on current density of coal liquefaction: (A) PVP-NiB; (B) PVP-NiB/SiO2; a. Blank; b, c, d, e are described as m (cat): m (coal) = 0%, 10%, 20%, 30%, respectively. Scan rate: 0.1 V s1, 0.45 V to 1.60 V vs. HgO/Hg, T = 343 K.
Fig. 7. Effect of electrolysis temperature on coal liquefaction. (A) PVP-NiB; (B) PVP-NiB/SiO2; a: 303 K, b: 323 K, c: 343 K, d: 363 K. m (cat): m (coal) = 20%. Scan rate: 0.1 V s1, 0.45 V to 1.60 V vs. HgO/Hg.
Fig. 8. Results of yield. m (cat): m (coal) = 20%. Electric potential: 1.60 V vs. HgO/ Hg. T = 343 K.
4. Mechanism We come up with a possible catalytic mechanism of Ni-based catalysts for coal hydrogenation/liquefaction according to the above analysis. In electrolysis process, the reaction of ring-opening, bridge cracking, dealkylation and so on makes coal cleave radical fragments (Fig. 9). And hydrogen evolution reaction occurred
Fig. 9. Infrared spectra of TS1, TS2 and TS3. TS1: THF soluble of the original coal; TS2: THF soluble of electrolytic coal; TS3: THF soluble of electrolytic coal with 20 wt.% PVP-NiB catalyst; TS4: THF soluble of electrolytic coal with 20 wt.% PVPNiB/SiO2 catalyst.
(Fig. 5). Ni active sites in the NiB amorphous alloys have unpaired d electron (Fig. 4), which could form chemisorption bond with hydrogen. In the role of chemisorption bond, hydrogen break down to active hydrogen atoms [33]. Then the coal radical fragments can be easily stabilized by active hydrogen atoms to form the coal liquids (Fig. 8). The reaction between NaOH, coal and catalysts are as follows:
Y. Liu et al. / Fuel 122 (2014) 54–59 Table 2 Vibrational assignments. Vibration (cm1)
Assignment
Ref.
2958 2920 1700 1630 1462 1377 1261, 1096, 1024 803
CH3 (asym) CH2 (asym) C@O C@C (skeletal vibration) CH2 (bending vibration) CH3 (sym) CAOAC Ar–X
[28] [28] [29] [30] [11] [11] [31] [11]
THF soluble
C (wt.%)
H (wt.%)
N (wt.%)
H/C (at.)
TS1 TS2 TS3 TS4
61.79 79.52 82.98 61.85
5.06 10.03 11.82 10.03
1.05 0.41 0.36 0.52
0.98 1.51 1.71 1.95
Electrolysis
4H2 O þ 4e ! 4OH þ 2H2 Catalysis
H2 ! H þ H Electrolysis X
R þ H
the adsorbed hydrogen and promote the hydrogenation of coal. Though the efficiency of electrochemical reduction of coal may be too low comparing to that of conventional DCL performed at high temperature and high pressure, it is very possible to obtain a higher yield with improved conditions, such as catalyst, electrode and electrolytic cell, of course, the preliminary treatment of coal is useful, which need our further research. Acknowledgements The authors gratefully acknowledge the National Science Foundation of China (Nos. 21173144 and 20873083), the Instrumental Analysis and Research Center of Shanghai University, and the State Key Laboratory of Chemical Engineering (No. SKL-ChE-08A01).
Table 3 Element analyses of THF soluble of coal.
Coal !
59
R
Hydrogenation
! RH
5. Conclusions In summary, the activity NiB, PVP-NiB and PVP-NiB/SiO2 amorphous alloy catalysts can be prepared by chemical reduction, characterized by XRD, IR, BET, ICP and XPS. The catalytic activity of NiB, PVP-NiB and PVP-NiB/SiO2 for the electro-reduction of coal was investigated. The results show that PVP-NiB/SiO2 exhibits higher electrochemical catalytic activity between the three catalysts studied, which was confirmed by LSV test, yield, IR and elemental analyses. With PVP-NiB/SiO2, the yield of coal liquefaction can be drastically improved to 57%, H/C ratio of the products can be up to 1.95. It attributes to both structural effect and electronic effect. For structural effect, the PVP-NiB amorphous alloy can be well dispersed on SiO2 and stabilized in solution. For electronic effect, electron-enriched Ni and weaker bond strength of NiAH activate
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