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SBA-15 catalyst
Journal of Alloys and Compounds 806 (2019) 254e262
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Hydrodeoxygenation and hydrodesulfurization over Fe promoted Ni2P/SBA-15 catalyst Bolong Jiang, Tianhan Zhu, Hua Song*, Feng Li Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, 163318, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2019 Received in revised form 17 July 2019 Accepted 20 July 2019 Available online 21 July 2019
The Fe-doped Ni2P/Fe-SBA-15 was prepared by the temperature programmed reduction method at a relative low temperature of 673 K. The effect of Fe on the catalytic performance for benzofuran (BF) hydrodeoxygenation (HDO) and dibenzothiophene (DBT) hydrodesulfurization (HDS) were investigated. The catalysts were characterized by means of X-ray diffraction (XRD), N2 adsorption-desorption, inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia (NH3-TPD), and transmission electron microscope (TEM) and CO uptakes. The results indicate the incorporation of Fe into SBA-15 contributes to the exposure of smaller Ni2P particles (5.6 nm) with an uniform dispersion, along with enhancement of weak and medium acid strengths. Compared with Ni2P/SBA-15, the Fe-doped Ni2P/Fe-SBA-15 exhibited a much higher BF conversion of 91.7% with an improved total deoxygenated product yield of 83.3%. The excellent HDO performance of Ni2P/Fe-SBA-15 can be ascribed to the exposure of highly-dispersed smaller Ni2P particles. Meanwhile, the improved dehydration of 2-EtPh to EB and the high deoxygenated product selectivity can be attributed to the enhanced acidity. As compared to the Ni2P/SBA-15, the Fe-doped Ni2P/ Fe-SBA-15 showed a higher DBT HDS activity of 96.3% with the BP formation at a great proportion of 90.3%, indicating that DBT was mainly transformed through the desulfurization pathway during HDS. © 2019 Elsevier B.V. All rights reserved.
Keywords: Nickel phosphide Fe-doped SBA-15 Hydrodeoxygenation Hydrodesulfurization
1. Introduction With regard to the exhaustion of fossil fuels and the stricter environmental regulations, the demands for clean fuels and renewable energy sources become a pressing issue. In recent years, biomass has attracted much attention as a promising resource [1e4]. There are many methods to convert biomass into liquid fuels (bio-oil), such as thermal, biological and physical processes, in which the fast pyrolysis is considered as to be an effective one [5]. However, the bio-oil obtained from pyrolysis contains high oxygenated molecules (35e50 wt% of oxygen) [6], which would result in low heating values, high viscosity, thermal and chemical instability, and poor miscibility with hydrocarbon fuels [7,8]. Therefore, the oxygen removal is necessary for upgrading the pyrolysis oil before used in combustion engines. Many approaches have been introduced to upgrade the pyrolysis oil, such as hydrodeoxygenation (HDO), catalytic cracking, stream reforming,
* Corresponding author. E-mail address: [email protected] (H. Song). https://doi.org/10.1016/j.jallcom.2019.07.242 0925-8388/© 2019 Elsevier B.V. All rights reserved.
aqueous phase reforming, esterification, and emulsification, etc. Among them, the HDO as an efficient one has been extensively studied [9]. Various types of catalysts have been designed and used for HDO, such as conventional sulfide catalysts, noble metal catalysts and transition metal phosphides [10e16]. The conventional sulfide catalysts such as NiMo and CoMo usually require sulfidation and can be deactivated easily during the HDO process. Considering the noble metals are expensive, the nonsulfide low-cost Ni2P catalyst is regarded as the most promising catalyst for bio-oils upgrading in the industrial scope owing to its high HDO catalytic performance and resistance to poisoning during the HDO process [17,18]. In addition, the Ni2P catalyst has been extensively studied for hydrodesulfurization (HDS), which is known to be the most effective process for removing sulfur-containing molecules in oil. Soled [19] found that Ni2P catalysts with small crystallite size shows better HDS performance than that with large crystallite size. Oyama [20] has reported that highly-dispersed Ni2P catalysts are particularly active for hydrogenation and promising in the field of deep HDS. Supporting the Ni2P active phases in mesostructured supports
B. Jiang et al. / Journal of Alloys and Compounds 806 (2019) 254e262
may contribute to taking full advantage of their catalytic properties by improving the dispersion of Ni2P active particles and increasing the amount of exposed active Ni2P sites [21]. The mesoporous support SBA-15, which has high-ordered twodimensional hexagonal p6mm symmetry structure as well as the improved thermal and hydrothermal stability, was found to have advantages over microporous supports in the mass transfer of reactants during reaction [22]. The dispersion of active phases on mesostructured SBA-15 have been widely investigated due to its high surface area [14,23e27]. Yang [14] has reported that Ni2P particles can be distributed rather uniformly inside the SBA-15 channels, and the resulting catalyst showed high selectivity toward HDO products. Tan and co-workers [28] proposed that SBA-15 has nearly no catalytic activity for the hydroxyalkylation of phenol with formaldehyde to bisphenol F. However, the mesoporous M (Al, Zr, AleZr)-SBA-15 catalysts showed high activity for the hydroxyalkylation reaction. Yan and co-workers [29] successfully synthesized the mesoporous Fe-SBA-15. The catalytic activity for oxalic acid oxidation over Fe-SBA-15 was 2.9 times higher than that of SBA-15, showing that incorporation of Fe into SBA-15 plays an important role in catalytic ozonation. However, as far as we know, the studies of HDO and HDS over Ni2P supported on Fe incorporated SBA-15 support have not been reported yet. Therefore, it is of interest to gain insight into the effect of Fe on HDO and HDS performance over a Ni2P/SBA-15 catalyst. In this work, the nickel phosphide catalysts supported on mesoporous SBA-15 and Fe-SBA-15 were synthesized according to a previously described method from ammonium hypophosphite and nickel chloride at a lower reduction temperature of 673 K. To thoroughly understand the effect of Fe on the structure and nature of the active sites, the as-prepared samples were characterized by several techniques, such as XRD, nitrogen adsorption, CO uptake, XPS and TEM. Finally, the effect of Fe on the catalytic performance for BF HDO and DBT HDS were studied in detail.
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2.3. Characterization methods X-ray diffraction (XRD) analysis was carried out on a D/max2200PC-X-ray diffractometer using Cuka radiation under the setting conditions of 40 kV and 30 mA, with scanning range from 10 to 80ºat a rate of 10 /min. The typical physicochemical properties of supports and catalysts were analyzed by BET method using Micromeritics adsorption equipment of micromeritics TriStar II 3020. All the samples were outgassed at 573 K until the vacuum pressure was 6 mmHg. The adsorption isotherms for nitrogen were measured at 77 K. The CO adsorption capacity of the catalysts was measured by pulsing calibrated volumes of CO into a He carrier. CO uptake was calculated by measuring the decrease in the peak areas caused by adsorption in comparison with the area of a calibrated volume. The Fe and Ni metal contents were measured by inductively coupled plasma mass spectrometry (ICP-MS) using a PerkinElmer Nexion 300 instrument. Prior to analysis, the sample was digested with nitric acid using a microwave heating system. X-ray photoelectron spectroscopy (XPS) spectra were acquired using ESCALAB MKII spectrometer under vacuum. XPS measurements have been performed for Mg radiation (E ¼ 1253.6 eV) and equipped with a hemispherical analyzer operating at fixed pass energy of 40 eV. The recorded photoelectron binding energies were referenced against the C 1s contamination line at 284.8 eV. Transmission electron microscope (TEM) examinations were performed using the JEM-1010 instrument supplied by JEOL. The samples were dispersed in ethanol and placed on a carbon grid before TEM examinations. The temperature-programmed desorption of ammonia (NH3TPD) was adopted to measure the catalyst acidity. 100 mg sample was cleaned with helium and adsorption of ammonia at 313 K until the TCD signal was stable. NH3-TPD was performed between 313 and 1073 K at a heating rate of 10 K min1 using a TCD to detect the desorbed NH3.
2. Experimental
2.4. Catalytic activities
2.1. Materials
In this study, the HDO and HDS activities were measured using the as-prepared catalysts. The HDO of BF over prepared catalysts were carried out in a flowing high-pressure fixed-bed stainless steel catalytic reactor (8 mm in diameter, and 400 mm in length), using a feed consisting of a decalin solution of BF (2 wt%). The conditions of the HDO reaction were 573 K, 3.0 MPa, WHSV ¼ 4 h1, and hydrogen/oil ratio of 500 (V/V). The HDS of DBT over prepared catalysts was performed in a flowing high-pressure fixed bed reactor using a feed consisting of a decalin solution of DBT (1 wt%). The conditions of the HDS reaction were 613 K, 3.0 MPa, WHSV ¼ 1.5 h1, and hydrogen/oil ratio of 500 (V/V). The activities of each catalyst were measured at different time. The feed and reaction product was analyzed by FID gas chromatography with a GC-14C-60 column.
NiCl2$6H2O (98%), NH4H2PO2 (97%), benzofuran (99%), dibenzothiophene (99%) were purchased from Aladdin Reagent Co., Ltd. and decahydronaphthalene (99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd. The mesoporous SBA-15 and Fe-SBA-15 (Fe content of 0.070 wt%) employed as support materials were purchased from Nankai University Catalyst Co., Ltd. . All chemicals were directly used without further purification.
2.2. Preparation of catalysts The supported Ni2P catalyst precursor was prepared by impregnating NH4H2PO2 and NiCl2$6H2O solution with the mesoporous support, following the procedures previously described by our group [30]. After the water was evaporated, the impregnated solid was dried at 363 K overnight. The precursor was then pressed in discs, crushed and sieved to 16e20 mesh. For the reduction, the precursor was placed in a fixed-bed reactor by heating from room temperature to 673 K at a rate of 3 K min1 in a flow of H2 (200 mL min1), held for 2 h and then cooled to 373 K in a continuous H2 flow and held for 1 h under flowing air (20 mL min1). The obtained solid catalysts with a theoretical mass ratio of Ni to blank support of 10 wt% and an initial Ni/P molar ratio of 1/2 were designed as Ni2P/SBA-15 and Ni2P/Fe-SBA-15, respectively.
3. Results and discussion 3.1. XRD Fig. 1 shows the XRD patterns of the synthesized catalysts. For both catalysts, the peaks of Ni2P phase were observed at 40.6 , 44.5 , 47.1, 54.1, and 54.8 (PDF ¼ 03e0395), suggesting the formation of Ni2P active phases on the catalysts. No distinct diffraction peaks corresponding to the FeP species can be observed, indicating the Fe species is well below the detectable limits of XRD and highly dispersed on the surface of catalyst. As compared to the Ni2P/SBA15, the main peaks of Ni2P/Fe-SBA-15 were broadened, suggesting
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And the decrease in the specific textural characteristics indicates some pores of the SBA-15 were possibly blocked by incorporation of Fe into the SBA-15. After loading Ni2P, both catalysts experienced a sharp decrease in surface area compared to the corresponding support, which is due to the blockage of the channel by metal Ni or phosphorous. The pore volumes were also decreased from 0.81 cm3 g1 to 0.56 cm3 g1 for Ni2P/SBA-15 and from 0.37 cm3 g1 to 0.17 cm3 g1 for Ni2P/Fe-SBA-15. The related results of all samples were shown in Table 1. 3.3. CO-uptake and ICP
Fig. 1. XRD patterns for Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts.
the incorporation of Fe contributes to the formation of relatively smaller Ni2P particles. The average size of the Ni2P crystallites (column 5 of Table 1) calculated by the Scherrer equation was 17.9 nm for Ni2P/Fe-SBA-15, smaller than that of Ni2P/SBA-15 (18.4 nm), as confirmed by TEM (This will discussed in section 3.5). Therefore, the incorporation of Fe plays a positive role in the formation of relatively smaller Ni2P particles. 3.2. BET Fig. 2 shows the N2 adsorption-desorption isotherms of samples and Table 1 summarizes the textual properties of the samples. As shown in Fig. 2(a), the SBA-15 exhibited a type IV isotherm at a relative pressure (p/p0) ¼ 0.6e0.8 with a standard H1 type hysteresis loop, confirming the mesoporous ordered materials. However, the Fe-doped SBA-15 displayed a type IV isotherm with an H2 type hysteresis loop and the loop shifted to lower p/p0 (0.4e0.8), implying the pores of Fe-SBA-15 (Vp ¼ 0.37 cm3 g1, DBJH ¼ 5.2 nm) were smaller than those of SBA-15 (Vp ¼ 0.81 cm3 g1, DBJH ¼ 6.8 nm). The above N2 physisorption results are in good agreement with results reported by others for similar materials [31,32], where the introduction of metal into SBA-15 led to the decrease in special surface area and pore volume. Mendoza-Nieto et al. [31] reported the decrease in the surface areas of the Zr and Ti modified SBA-15 supports compared to the original SBA-15 indicates some blocking of the SBA-15 pores by the deposited TiO2 or ZrO2 oxide species could have also taken place. In our case, the IV type hysteresis loop of Fe-doped SBA-15 indicated the reservation of original pore structure after the introduction of Fe into SBA-15.
The CO chemisorption was used to determine the density of exposed Ni sites. The CO capacities of the as-prepared samples are shown in column 6 of Table 1. It is assumed that the CO molecules are mainly absorbed at Ni sites rather than P sites [33]. The CO capacities for Ni2P/SBA-15 catalyst was 38 mmol g1, which was slightly higher than that of Ni2P/Fe-SBA-15 (36 mmol g1), showing the CO uptakes of Ni2P/Fe-SBA-15 slightly decreased by the incorporation of Fe, which is not consistent with the result of XRD analysis. This is possibly due to the lower surface area of the catalyst after the incorporation of Fe (Table 1). Similar results were obtained by Oyama et al. [34], who reported that supports with higher surface area gave rise to a higher dispersion of the Ni2P phase and led to higher CO uptakes. However, this result does not agree with our result from TEM image (Fig. 4) and crystallite sizes calculated by Scherrer equation (Fig. 1), which demonstrated that Ni2P/Fe-SBA-15 possesses more uniform and smaller Ni2P particles than Ni2P/SBA-15. Oyama et al. [35] also found that the CO uptakes of the fresh Ni2P catalysts increase with loading, but not as much as expected from the Ni2P crystallite size and concentration. This means the CO uptakes are not directly correlates with Ni2P crystallite size, which seems to be consistent with our results. Additionally, the ICP analysis was carried out to determine the amount of Ni element and the results were shown in Table 1. The Ni content over Ni2P/Fe-SBA-15 catalyst (7.6 wt%) was slightly lower than that of Ni2P/SBA-15 (8.0 wt%). This might be another reason for the tiny decline of CO uptakes for Ni2P/Fe-SBA-15. 3.4. XPS In order to gain insight into the surface composition of the catalysts, the X-ray photoelectron spectroscopy (XPS) analysis was carried out. As shown in Fig. 3(a), the binding energy centered at 852.3e852.5 eV can be assigned to Nidþ (0
Table 1 Surface properties of supports and catalysts. Sample
SBA-15 Ni2P/SBA-15 Fe-SBA-15 Ni2P/Fe-SBA-15 a b c d e
Metal contenta (wt%) Fe
Ni
e e 0.070 0.056
e 8.0 e 7.6
SBET (m2$g1)
Vp (cm3$g1)
DBJHb (nm)
dXRDc (nm)
CO uptaked (mmol$g1)
NH3 uptakee (mmol$g1)
Conversion (%)
619 326 465 119
0.81 0.56 0.37 0.17
6.8 7.3 5.2 6.6
e 18.4 e 17.9
e 38 e 36
e 313 e 362
e 85 e 91
Fe content: obtained from ICP analysis. DBJH: mesopore diameter calculated from the adsorption branch of nitrogen isotherms using BJH method. dXRD: calculated from the Dc ¼ kl/bcos(q) (Scherrer equation) based on the Ni2P{1 1 1}. CO uptakes: determined by CO chemisorption. NH3 uptakes: determined by NH3-TPD.
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Fig. 2. N2 absorption capacity of supports (a) and catalysts (b).
Fig. 3. The core level XPS spectra of Ni 2p3/2 (a); P 2p (b) and Fe 2p3/2 (c).
134.6 eV can be assigned to P5þ due to the superficial oxidation of the Ni2P particles [36,37]. The Fe 2p core level spectra for Fe-SBA-15 and Ni2P/Fe-SBA-15 were shown in Fig. 3(c). For the Fe-SBA-15 support, two peaks at 711.8 and 721.7 eV can be assigned to Fe3þ and satellite, respectively. For Ni2P/Fe-SBA-15, in addition to the peaks of Fe3þ and satellite, a new peak at 706.6 eV was detected and can be attributed to Fedþ in iron phosphide [38]. The Fe species were observed in the majority of Fe3þ with a weak contribution of Fedþ. Interestingly, FeP was proved to be another acceptable active sites for HDO of phenol [39]. Yuan et al. [40] also found FeP phase
was active and stable in hydrogenation reaction and showed better performance in HDS of DBT than that of Fe2P. Upon loading Ni2P phase on Fe-SBA-15, the peak assigned to Fe3þ shifted to the higher binding energy of 713.2 eV. This was possibly contributed to electron transfer from Fe to Ni or P since after addition of Fe the banding energy of Nidþ and Pdþ were both deceased. The atomic ratios were listed in Table 2. All the samples exhibited lower Ni/P atomic ratio than the theoretical Ni/P ratio (1/2). This might due to the enrichment of phosphorous on the surface of the catalysts. Additionally, the Ni/Si atomic ratio of Ni2P/Fe-SBA-15 (1/33) is slightly lower
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Fig. 4. TEM images and particle size distributions of (a) Ni2P/SBA-15 and (b) Ni2P/Fe-SBA-15.
Table 2 Spectral parameters of catalysts obtained by XPS analysis. Sample
Binding energy (eV)
Ni2P/SBA-15 Ni2P/Fe-SBA-15 Fe-SBA-15
Superficial atomic ratio P 2p
Ni 2p3/2
Fe 2p3/2
Nidþ
Ni2þ
Pd-
PO34
Fedþ
Fe3þ
852.5 852.3 e
856.6 856.2 e
129.2 129.1 e
134.6 134.6 e
e 706.6 e
e 713.2 711.8
Ni/P
Ni/Si
Fe/Si
1/3.6 1/3.9 e
1/30 1/33 e
e 1/59 1/48
than that of Ni2P/SBA-15 (1/30), indicating that the incorporation of Fe slightly decreased the exposed Ni sites on the surface of the catalyst, which is consistent with the result of CO uptakes. 3.5. TEM Fig. 4 showed the morphologies and microstructures of Ni2P/ SBA-15 and Ni2P/Fe-SBA-15 catalysts. For Ni2P/SBA-15 (Fig. 4(a)), the Ni2P particles were aggregated to form large Ni2P particles. Therefore, the distribution of Ni2P particles was very wide in size and the average particle size was about 16.3 nm. For Ni2P/Fe-SBA15 (Fig. 4(b)), the distribution of Ni2P particles was much uniform with the average particle size of about 5.6 nm. Combined with the XRD and CO uptake results, it could be concluded that the incorporation of Fe contributes to the highly uniform dispersion of Ni2P particles and the formation of smaller Ni2P particles on the support. Fig. 5. NH3-TPD profiles of Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts.
3.6. NH3-TPD The acidity of catalysts was determined by NH3-TPD and the results were shown in Fig. 5. The Ni2P/SBA-15 showed two peaks at 410 K and 503 K. It was worth noted that the latter was prominent and showed a shoulder at a higher temperature. Generally, the peak lower than 503 K was assigned to weak acid [41]. In the case of Ni2P/SBA-15, the generation of weak acid was due to the PeOH groups, and the shoulder was related to medium strength acid caused by the electron transfer from Ni to P, similar with the results reported by Yang et al. [42]. As contrast, the peaks of Ni2P/Fe-SBA15 were concentrated around 433 K and 503 K. The former slightly red-shifted and the shoulder of prominent peak showed a tendency to higher temperature as compared to that of the Ni2P/SBA-15, which proved the strengths of weak and medium acid were both improved by the incorporation of Fe. The total NH3 uptakes were
quantified and the value was shown in column 8 of Table 1. NH3 uptakes for Ni2P/Fe-SBA-15 was 362 mmol g1, higher than that of Ni2P/SBA-15 (313 mmol g1). This result indicated that the Ni2P/FeSBA-15 possessed higher acid amount than that of Ni2P/SBA-15. 3.7. Catalytic activity The BF HDO catalytic activity of the catalysts was investigated at a reaction temperature of 573 K, a pressure of 3.0 MPa, a WHSV of 4.0 h1 and hydrogen/oil ratio of 500(V/V). As shown in Fig. 6(a), the conversion of BF gradually increased and then remained approximately stable with time on stream, and the BF conversion over Ni2P/SBA-15 was 83.3% after 7 h. Expectedly, the incorporation of Fe was contributed to high activity of the catalyst with the
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Fig. 6. Conversion (a) of BF and the total deoxygenated product yield (b) over as-prepared catalysts. (Reaction conditions: T ¼ 573 K, p ¼ 3.0 MPa, WHSV ¼ 4.0 h1, and hydrogen/oil ratio ¼ 500(V/V)).
conversion of BF over Ni2P/Fe-SBA-15 up to 91.7% after 7 h. Upon incorporation of Fe, the conversion of BF increased by 8.4%. Similarly (Fig. 6(b)), the yields of O-free compounds gradually increased and then remained approximately stable. After 7 h, the total deoxygenated product yield over Ni2P/SBA-15 was 64.3%. In contrast, the total deoxygenated product yield was 83.3% for the Ni2P/Fe-SBA-15, which was increased by 19.0% compared to Ni2P/ SBA-15. The higher HDO activity over Ni2P/Fe-SBA-15 is attributed to the smaller and uniformly dispersed active Ni2P sites, which were proved by the XRD and TEM results. The Ni2P/SBA-15 showed a larger surface area, but the catalytic performance was restricted by poor dispersion caused by agglomerations of active Ni2P sites. In an attempt to gain insight into the effect of Fe on the catalytic reaction, the product distributions of the last 3 h over the asprepared catalysts were recorded and shown in Table 3. As compared to our previous study [30], the product distributions over the Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts did not change significantly and the deoxygenated product ECH was predominant in product, showing the reaction route for BF HDO over as-prepared catalysts was the same as that of previously reported one (Scheme 1) [30]. The selectivity to 2-EtPh over Ni2P/Fe-SBA-15 was only 0.2%, which decreased markedly as compared to Ni2P/SBA-15 (>1.9%). This showed that almost all the formed 2-EtPh was transformed into EB or Ph. The selectivity to Ph and B decreased after introducing the Fe, this confirms that more 2-EtPh was transformed to EB over Ni2P/Fe-SBA-15 than that over Ni2P/SBA-15. It is demonstrated that the introduction of Fe improved the dehydration of 2-EtPh to EB. As a result, the selectivity to deoxygenated products of ECH increased obviously from 49.5 to 78.2% after 7 h on stream (Table 3). The higher catalytic activity of Ni2P/Fe-SBA-15 could be related to smaller and highly dispersed Ni2P active sites (shown in XRD and TEM results). Meanwhile, the enhanced acidity (shown in NH3-TPD analysis) plays a critical role in improving dehydration of 2-EtPh to EB [43,44], which facilitates the increase
Scheme 1. Reaction pathways for HDO of BF [30].
in a total yield of deoxygenated compounds. Additionally, the carbon balance (CB) was another interesting parameter for testing the carbon loss during the catalytic reaction. The calculation of CB was as follows:
P CB ¼
nci ni 100% 8 nBF
where nBF was the moles of BF in the feed, nci was carbon number of product i and ni was the mole of product i. The CB for BF HDO over Ni2P/SBA-15 and Ni2P/Fe-SBA-15
Table 3 Product distribution and CB of the last 3 h over the catalysts. Catalysts and reaction time/ h
B
MCH
MB
ECH
EB
Ph
2,3-DHBF
2-EtPh
CB
Ni2P/SBA-15
4.3 4.7 6.1 1.7 1.3 1.0
0.6 0.9 3.3 4.1 3.4 3.0
0.9 1.0 2.9 1.3 0.6 0.4
58.5 56.9 49.5 65.9 72.0 78.2
14.0 14.6 15.4 13.0 12.4 8.3
15.0 14.7 13.5 8.6 5.6 4.9
4.8 4.8 6.1 5.3 4.6 4.0
1.9 2.3 3.2 0.1 0.2 0.1
95.0 94.8 94.3 96.8 97.9 98.0
Ni2P/Fe-SBA-15
5 6 7 5 6 7
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activity of Ni2P/Fe-SBA-15 is higher than most of the reported catalysts.
catalysts was listed in column 10 of Table 3. Both Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts showed higher CB values, which indicated the low cleavage of CeC bond. The reservation of carbon atom in product contributed to low loss of calorie for the bio-oil. The comparison of the typical catalytic activity of BF HDO from literatures with our present results were summarized in Table 4. It can be seen that the BF HDO activity of Ni2P/Fe-SBA-15 is higher than most of the reported catalysts. The DBT HDS catalytic activity of catalysts was also investigated at a reaction temperature of 613 K, a pressure of 3.0 MPa, a WHSV of 1.5 h1 and hydrogen/oil ratio of 500(V/V) and the results were shown in Fig. 7. The HDS activities of the catalysts first increased and then reminded stable with time on stream. As compared to Ni2P/SBA-15 (92.1%), the DBT conversion of Ni2P/Fe-SBA-15 (96.3%) had an increase by 4.2%, showing that incorporation of Fe could increase the catalytic activity by contributing to the exposure of smaller and uniformly dispersed active Ni2P sites. For the product distribution in the HDS of DBT (Fig. 7(b)), the BP and CHB were dominant products, which represented the direct desulfurization (DDS) and hydrogenation (HYD) route, respectively. The selectivity to BP over Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts was much higher than that for CHB, indicating that DBT was mainly transformed through the DDS route [50]. For Ni2P/Fe-SBA-15, the selectivity to BP maintained at 90.3% after 8 h, higher than that of Ni2P/SBA-15 (81.6%). This showed that incorporation of Fe could increase the BP selectivity via the DDS route [51,52]. The comparison of the typical catalytic activity of DBT HDS from literature with this study was summarized in Table 5. It can be seen that the HDS
4. Conclusion The Fe-doped Ni2P/Fe-SBA-15 was synthesized by temperature programmed reduction at a relatively low reduction temperature of 673 K by using NH4H2PO2 and NiCl2$6H2O. The effect of incorporation of Fe on the catalytic properties for BF HDO and DBT HDS were investigated. The XRD analysis has shown that incorporation of Fe is beneficial to the formation of the smaller size of Ni2P particles. The amounts of CO uptakes for Ni2P/Fe-SBA-15 slightly decreased, which was due to the lower surface area of the catalyst after the incorporation of Fe. The TEM analysis confirmed that smaller and uniformly dispersed Ni2P particles with the average particle size of about 5.6 nm were formed for Ni2P/Fe-SBA-15. Therefore, it could be concluded that the incorporation of Fe contributes to the highly uniform dispersion of Ni2P particles and the formation of smaller Ni2P particles on the support. In addition, the NH3-TPD analysis proved that the strengths of weak and medium acid were both improved by the incorporation of Fe. Compared with Ni2P/SBA-15 (81.6%), the Fe-doped Ni2P/Fe-SBA-15 exhibited a much higher BF conversion of 91.7%. And the total deoxygenated product yield of Ni2P/Fe-SBA-15 was 83.3%, which is much higher than that of Ni2P/SBA-15 (64.3%). The excellent HDO performance of Ni2P/Fe-SBA-15 could be related to highly dispersed active sites (shown in XRD and TEM results). Meanwhile, the improved dehydration of 2-EtPh to EB and the high deoxygenated product
Table 4 Comparison of the typical results of BF HDO from literature with our present results. Samples
Temperature (K)
Pressure (MPa)
Conversion (%)
Ref. no.
Sulfided NiMo/Al2O3a Sulfided NiMo/Al2O3a NiMoP/Al2O3a Pt/SiO2eAl2O3 Pd/SiO2eAl2O3 W2C(Ar-2-1023 K-1 h) Ni2PeN/MCM-41 Ni2PeO/MCM-41 Ni2P/Al2O3 Ni2P/TiO2 Ni2P/Al2O3@TiO2 Ni2P/SBA-15 Ni2P/Fe-SBA-15
553 553 613 553 553 613 493 493 573 573 573 573 573
2.0 5.0 7.0 3.0 3.0 4.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
74.6 82.5 80.7 80 97 41 31 57 78 85 95 85 91
[45] [45] [46] [47] [47] [48] [30] [30] [49] [49] [49] This work This work
a
With H2S in the feed.
Fig. 7. Conversion (a) and product selectivity (b) of DBT HDS over Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts. (Reaction conditions: T ¼ 613 K, p ¼ 3.0 MPa, WHSV ¼ 1.5 h1, and hydrogen/oil ratio ¼ 500(V/V)).
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Table 5 Comparison of the typical results of DBT HDS from literature with our present results. Samples
Temperature (K)
Pressure (MPa)
Conversion (%)
Ref. no.
Sulfided NiMo/g-Al2O3 Pt/MCM-48 PtMo/MCM-48 CoMoW/SBA-15 NiMoW/SBA-15 FeP/C Fe2P/C Bulk Ni2P Ni2P/MCM-41 Ni2P/Ti-MCM-41 (1.5) Ni2P/SBA-15 Ni2P/Fe-SBA-15
573 623 623 573 573 613 613 613 613 613 613 613
7.3 3.0 3.0 7.3 7.3 3.1 3.1 3.0 3.0 3.0 3.0 3.0
93 21.6 75.4 81 88 77 40 62 81.9 99.4 92.1 96.3
[53] [54] [54] [55] [55] [40] [40] [56] [57] [57] This work This work
selectivity can be attributed to the enhanced acidity (shown in NH3-TPD analysis). As compared to Ni2P/SBA-15 (92.1%), the Ni2P/ Fe-SBA-15 showed higher DBT HDS activity of 96.3%, indicating that incorporation of Fe could increase the HDS activity by contributing to the exposure of smaller and uniformly dispersed active Ni2P sites. It was noted that the selectivity to BP over Ni2P/Fe-SBA-15 was increased after the incorporation of Fe and BP was formed at a great proportion of 90.3%, indicating that DBT was mainly transformed through the desulfurization pathway during HDS. Acknowledgments The authors acknowledge the financial supports from the Graduate Innovation Project of Northeast Petroleum University (JYCX_CX03_2018). References [1] F. Bilgili, The impact of biomass consumption on CO2 emissions: cointegration analyses with regime shifts, Renew. Sustain. Energy Rev. 16 (2012) 5349e5354. https://doi.org/10.1016/j.rser.2012.04.021. [2] S. Cheng, L. Wei, J. Julson, M. Rabnawaz, Upgrading pyrolysis bio-oil through hydrodeoxygenation (HDO) using non-sulfided Fe-Co/SiO2 catalyst, Energy Convers. Manag. 150 (2017) 331e342. https://doi.org/10.1016/j.enconman. 2017.08.024. [3] E. Kordouli, L. Sygellou, C. Kordulis, K. Bourikas, A. Lycourghiotis, Probing the synergistic ratio of the NiMo/g-Al2O3 reduced catalysts for the transformation of natural triglycerides into green diesel, Appl. Catal. B Environ. 209 (2017) 12e22. https://doi.org/10.1016/j.apcatb.2017.02.045. [4] H. Lee, Y.M. Kim, K.B. Jung, S.C. Jung, J.K. Jeon, Y.K. Park, Catalytic hydrodeoxygenation of Geodae-Uksae pyrolysis oil over Ni/desilicated HZSM-5, J. Clean. Prod. 174 (2018) 763e770. https://doi.org/10.1016/j.jclepro.2017.10. 315. [5] A.V. Bridgwater, D. Meier, D. Radlein, An overview of fast pyrolysis of biomass, Org. Geochem. 30 (1999) 1479e1493. https://doi.org/10.1016/s01466380(99)00120-5. [6] L. Ingram, D. Mohan, M. Bricka, P. Steele, D. strobel, D. Crocker, B. Mithell, J. Mohammad, K. Cantrell, C.U. Pittman Jr., Pyrolysis of wood and bark in an auger reactor: physical properties and chemical analysis of the produced biooils, Energy Fuel. 22 (2008) 614e625. https://doi.org/10.1021/ef700335k. €, D. Chiaramonti, Review of fuel oil [7] J. Lehto, A. Oasmaa, Y. Solantausta, M. Kyto quality and combustion of fast pyrolysis bio-oils from lignocellulosic biomass, Appl. Energy 116 (2014) 178e190. https://doi.org/10.1016/j.apenergy.2013. 11.040. [8] N. Koike, S. Hosokai, A. Takagaki, S. Nishimura, R. Kikuchi, K. Ebitani, Y. Suzuki, S.T. Oyama, Upgrading of pyrolysis bio-oil using nickel phosphide catalysts, J. Catal. 333 (2016) 115e126. https://doi.org/10.1016/j.jcat.2015.10.022. [9] A. Iino, A. Cho, A. Takagaki, R. Kikuchi, S.T. Oyama, Kinetic studies of hydrodeoxygenation of 2-methyltetrahydrofuran on a Ni2P/SiO2 catalyst at medium pressure, J. Catal. 311 (2014) 17e27. https://doi.org/10.1016/j.jcat.2013.11. 002. [10] M. Toba, Y. Abe, H. Kuramochi, M. Osako, T. Mochizuki, Y. Yoshimuraa, Hydrodeoxygenation of waste vegetable oil over sulfide catalysts, Catal. Today 164 (2011) 533e537. https://doi.org/10.1016/j.cattod.2010.11.049. [11] D. Kubi cka, J. Hor a cek, Deactivation of HDS catalysts in deoxygenation of vegetable oils, Appl. Catal. A Gen. 394 (2011) 9e17. https://doi.org/10.1016/j. apcata.2010.10.034. [12] C. Zhao, J. He, A.A. Lemonidou, X. Li, J.A. Lercher, Aqueous-phase hydrodeoxygenation of bio-derived phenols to cycloalkanes, J. Catal. 280 (2011)
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Hydrodeoxygenation and hydrodesulfurization over Fe promoted Ni2P/SBA-15 catalyst Bolong Jiang, Tianhan Zhu, Hua Song*, Feng Li Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, 163318, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2019 Received in revised form 17 July 2019 Accepted 20 July 2019 Available online 21 July 2019
The Fe-doped Ni2P/Fe-SBA-15 was prepared by the temperature programmed reduction method at a relative low temperature of 673 K. The effect of Fe on the catalytic performance for benzofuran (BF) hydrodeoxygenation (HDO) and dibenzothiophene (DBT) hydrodesulfurization (HDS) were investigated. The catalysts were characterized by means of X-ray diffraction (XRD), N2 adsorption-desorption, inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia (NH3-TPD), and transmission electron microscope (TEM) and CO uptakes. The results indicate the incorporation of Fe into SBA-15 contributes to the exposure of smaller Ni2P particles (5.6 nm) with an uniform dispersion, along with enhancement of weak and medium acid strengths. Compared with Ni2P/SBA-15, the Fe-doped Ni2P/Fe-SBA-15 exhibited a much higher BF conversion of 91.7% with an improved total deoxygenated product yield of 83.3%. The excellent HDO performance of Ni2P/Fe-SBA-15 can be ascribed to the exposure of highly-dispersed smaller Ni2P particles. Meanwhile, the improved dehydration of 2-EtPh to EB and the high deoxygenated product selectivity can be attributed to the enhanced acidity. As compared to the Ni2P/SBA-15, the Fe-doped Ni2P/ Fe-SBA-15 showed a higher DBT HDS activity of 96.3% with the BP formation at a great proportion of 90.3%, indicating that DBT was mainly transformed through the desulfurization pathway during HDS. © 2019 Elsevier B.V. All rights reserved.
Keywords: Nickel phosphide Fe-doped SBA-15 Hydrodeoxygenation Hydrodesulfurization
1. Introduction With regard to the exhaustion of fossil fuels and the stricter environmental regulations, the demands for clean fuels and renewable energy sources become a pressing issue. In recent years, biomass has attracted much attention as a promising resource [1e4]. There are many methods to convert biomass into liquid fuels (bio-oil), such as thermal, biological and physical processes, in which the fast pyrolysis is considered as to be an effective one [5]. However, the bio-oil obtained from pyrolysis contains high oxygenated molecules (35e50 wt% of oxygen) [6], which would result in low heating values, high viscosity, thermal and chemical instability, and poor miscibility with hydrocarbon fuels [7,8]. Therefore, the oxygen removal is necessary for upgrading the pyrolysis oil before used in combustion engines. Many approaches have been introduced to upgrade the pyrolysis oil, such as hydrodeoxygenation (HDO), catalytic cracking, stream reforming,
* Corresponding author. E-mail address: [email protected] (H. Song). https://doi.org/10.1016/j.jallcom.2019.07.242 0925-8388/© 2019 Elsevier B.V. All rights reserved.
aqueous phase reforming, esterification, and emulsification, etc. Among them, the HDO as an efficient one has been extensively studied [9]. Various types of catalysts have been designed and used for HDO, such as conventional sulfide catalysts, noble metal catalysts and transition metal phosphides [10e16]. The conventional sulfide catalysts such as NiMo and CoMo usually require sulfidation and can be deactivated easily during the HDO process. Considering the noble metals are expensive, the nonsulfide low-cost Ni2P catalyst is regarded as the most promising catalyst for bio-oils upgrading in the industrial scope owing to its high HDO catalytic performance and resistance to poisoning during the HDO process [17,18]. In addition, the Ni2P catalyst has been extensively studied for hydrodesulfurization (HDS), which is known to be the most effective process for removing sulfur-containing molecules in oil. Soled [19] found that Ni2P catalysts with small crystallite size shows better HDS performance than that with large crystallite size. Oyama [20] has reported that highly-dispersed Ni2P catalysts are particularly active for hydrogenation and promising in the field of deep HDS. Supporting the Ni2P active phases in mesostructured supports
B. Jiang et al. / Journal of Alloys and Compounds 806 (2019) 254e262
may contribute to taking full advantage of their catalytic properties by improving the dispersion of Ni2P active particles and increasing the amount of exposed active Ni2P sites [21]. The mesoporous support SBA-15, which has high-ordered twodimensional hexagonal p6mm symmetry structure as well as the improved thermal and hydrothermal stability, was found to have advantages over microporous supports in the mass transfer of reactants during reaction [22]. The dispersion of active phases on mesostructured SBA-15 have been widely investigated due to its high surface area [14,23e27]. Yang [14] has reported that Ni2P particles can be distributed rather uniformly inside the SBA-15 channels, and the resulting catalyst showed high selectivity toward HDO products. Tan and co-workers [28] proposed that SBA-15 has nearly no catalytic activity for the hydroxyalkylation of phenol with formaldehyde to bisphenol F. However, the mesoporous M (Al, Zr, AleZr)-SBA-15 catalysts showed high activity for the hydroxyalkylation reaction. Yan and co-workers [29] successfully synthesized the mesoporous Fe-SBA-15. The catalytic activity for oxalic acid oxidation over Fe-SBA-15 was 2.9 times higher than that of SBA-15, showing that incorporation of Fe into SBA-15 plays an important role in catalytic ozonation. However, as far as we know, the studies of HDO and HDS over Ni2P supported on Fe incorporated SBA-15 support have not been reported yet. Therefore, it is of interest to gain insight into the effect of Fe on HDO and HDS performance over a Ni2P/SBA-15 catalyst. In this work, the nickel phosphide catalysts supported on mesoporous SBA-15 and Fe-SBA-15 were synthesized according to a previously described method from ammonium hypophosphite and nickel chloride at a lower reduction temperature of 673 K. To thoroughly understand the effect of Fe on the structure and nature of the active sites, the as-prepared samples were characterized by several techniques, such as XRD, nitrogen adsorption, CO uptake, XPS and TEM. Finally, the effect of Fe on the catalytic performance for BF HDO and DBT HDS were studied in detail.
255
2.3. Characterization methods X-ray diffraction (XRD) analysis was carried out on a D/max2200PC-X-ray diffractometer using Cuka radiation under the setting conditions of 40 kV and 30 mA, with scanning range from 10 to 80ºat a rate of 10 /min. The typical physicochemical properties of supports and catalysts were analyzed by BET method using Micromeritics adsorption equipment of micromeritics TriStar II 3020. All the samples were outgassed at 573 K until the vacuum pressure was 6 mmHg. The adsorption isotherms for nitrogen were measured at 77 K. The CO adsorption capacity of the catalysts was measured by pulsing calibrated volumes of CO into a He carrier. CO uptake was calculated by measuring the decrease in the peak areas caused by adsorption in comparison with the area of a calibrated volume. The Fe and Ni metal contents were measured by inductively coupled plasma mass spectrometry (ICP-MS) using a PerkinElmer Nexion 300 instrument. Prior to analysis, the sample was digested with nitric acid using a microwave heating system. X-ray photoelectron spectroscopy (XPS) spectra were acquired using ESCALAB MKII spectrometer under vacuum. XPS measurements have been performed for Mg radiation (E ¼ 1253.6 eV) and equipped with a hemispherical analyzer operating at fixed pass energy of 40 eV. The recorded photoelectron binding energies were referenced against the C 1s contamination line at 284.8 eV. Transmission electron microscope (TEM) examinations were performed using the JEM-1010 instrument supplied by JEOL. The samples were dispersed in ethanol and placed on a carbon grid before TEM examinations. The temperature-programmed desorption of ammonia (NH3TPD) was adopted to measure the catalyst acidity. 100 mg sample was cleaned with helium and adsorption of ammonia at 313 K until the TCD signal was stable. NH3-TPD was performed between 313 and 1073 K at a heating rate of 10 K min1 using a TCD to detect the desorbed NH3.
2. Experimental
2.4. Catalytic activities
2.1. Materials
In this study, the HDO and HDS activities were measured using the as-prepared catalysts. The HDO of BF over prepared catalysts were carried out in a flowing high-pressure fixed-bed stainless steel catalytic reactor (8 mm in diameter, and 400 mm in length), using a feed consisting of a decalin solution of BF (2 wt%). The conditions of the HDO reaction were 573 K, 3.0 MPa, WHSV ¼ 4 h1, and hydrogen/oil ratio of 500 (V/V). The HDS of DBT over prepared catalysts was performed in a flowing high-pressure fixed bed reactor using a feed consisting of a decalin solution of DBT (1 wt%). The conditions of the HDS reaction were 613 K, 3.0 MPa, WHSV ¼ 1.5 h1, and hydrogen/oil ratio of 500 (V/V). The activities of each catalyst were measured at different time. The feed and reaction product was analyzed by FID gas chromatography with a GC-14C-60 column.
NiCl2$6H2O (98%), NH4H2PO2 (97%), benzofuran (99%), dibenzothiophene (99%) were purchased from Aladdin Reagent Co., Ltd. and decahydronaphthalene (99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd. The mesoporous SBA-15 and Fe-SBA-15 (Fe content of 0.070 wt%) employed as support materials were purchased from Nankai University Catalyst Co., Ltd. . All chemicals were directly used without further purification.
2.2. Preparation of catalysts The supported Ni2P catalyst precursor was prepared by impregnating NH4H2PO2 and NiCl2$6H2O solution with the mesoporous support, following the procedures previously described by our group [30]. After the water was evaporated, the impregnated solid was dried at 363 K overnight. The precursor was then pressed in discs, crushed and sieved to 16e20 mesh. For the reduction, the precursor was placed in a fixed-bed reactor by heating from room temperature to 673 K at a rate of 3 K min1 in a flow of H2 (200 mL min1), held for 2 h and then cooled to 373 K in a continuous H2 flow and held for 1 h under flowing air (20 mL min1). The obtained solid catalysts with a theoretical mass ratio of Ni to blank support of 10 wt% and an initial Ni/P molar ratio of 1/2 were designed as Ni2P/SBA-15 and Ni2P/Fe-SBA-15, respectively.
3. Results and discussion 3.1. XRD Fig. 1 shows the XRD patterns of the synthesized catalysts. For both catalysts, the peaks of Ni2P phase were observed at 40.6 , 44.5 , 47.1, 54.1, and 54.8 (PDF ¼ 03e0395), suggesting the formation of Ni2P active phases on the catalysts. No distinct diffraction peaks corresponding to the FeP species can be observed, indicating the Fe species is well below the detectable limits of XRD and highly dispersed on the surface of catalyst. As compared to the Ni2P/SBA15, the main peaks of Ni2P/Fe-SBA-15 were broadened, suggesting
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And the decrease in the specific textural characteristics indicates some pores of the SBA-15 were possibly blocked by incorporation of Fe into the SBA-15. After loading Ni2P, both catalysts experienced a sharp decrease in surface area compared to the corresponding support, which is due to the blockage of the channel by metal Ni or phosphorous. The pore volumes were also decreased from 0.81 cm3 g1 to 0.56 cm3 g1 for Ni2P/SBA-15 and from 0.37 cm3 g1 to 0.17 cm3 g1 for Ni2P/Fe-SBA-15. The related results of all samples were shown in Table 1. 3.3. CO-uptake and ICP
Fig. 1. XRD patterns for Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts.
the incorporation of Fe contributes to the formation of relatively smaller Ni2P particles. The average size of the Ni2P crystallites (column 5 of Table 1) calculated by the Scherrer equation was 17.9 nm for Ni2P/Fe-SBA-15, smaller than that of Ni2P/SBA-15 (18.4 nm), as confirmed by TEM (This will discussed in section 3.5). Therefore, the incorporation of Fe plays a positive role in the formation of relatively smaller Ni2P particles. 3.2. BET Fig. 2 shows the N2 adsorption-desorption isotherms of samples and Table 1 summarizes the textual properties of the samples. As shown in Fig. 2(a), the SBA-15 exhibited a type IV isotherm at a relative pressure (p/p0) ¼ 0.6e0.8 with a standard H1 type hysteresis loop, confirming the mesoporous ordered materials. However, the Fe-doped SBA-15 displayed a type IV isotherm with an H2 type hysteresis loop and the loop shifted to lower p/p0 (0.4e0.8), implying the pores of Fe-SBA-15 (Vp ¼ 0.37 cm3 g1, DBJH ¼ 5.2 nm) were smaller than those of SBA-15 (Vp ¼ 0.81 cm3 g1, DBJH ¼ 6.8 nm). The above N2 physisorption results are in good agreement with results reported by others for similar materials [31,32], where the introduction of metal into SBA-15 led to the decrease in special surface area and pore volume. Mendoza-Nieto et al. [31] reported the decrease in the surface areas of the Zr and Ti modified SBA-15 supports compared to the original SBA-15 indicates some blocking of the SBA-15 pores by the deposited TiO2 or ZrO2 oxide species could have also taken place. In our case, the IV type hysteresis loop of Fe-doped SBA-15 indicated the reservation of original pore structure after the introduction of Fe into SBA-15.
The CO chemisorption was used to determine the density of exposed Ni sites. The CO capacities of the as-prepared samples are shown in column 6 of Table 1. It is assumed that the CO molecules are mainly absorbed at Ni sites rather than P sites [33]. The CO capacities for Ni2P/SBA-15 catalyst was 38 mmol g1, which was slightly higher than that of Ni2P/Fe-SBA-15 (36 mmol g1), showing the CO uptakes of Ni2P/Fe-SBA-15 slightly decreased by the incorporation of Fe, which is not consistent with the result of XRD analysis. This is possibly due to the lower surface area of the catalyst after the incorporation of Fe (Table 1). Similar results were obtained by Oyama et al. [34], who reported that supports with higher surface area gave rise to a higher dispersion of the Ni2P phase and led to higher CO uptakes. However, this result does not agree with our result from TEM image (Fig. 4) and crystallite sizes calculated by Scherrer equation (Fig. 1), which demonstrated that Ni2P/Fe-SBA-15 possesses more uniform and smaller Ni2P particles than Ni2P/SBA-15. Oyama et al. [35] also found that the CO uptakes of the fresh Ni2P catalysts increase with loading, but not as much as expected from the Ni2P crystallite size and concentration. This means the CO uptakes are not directly correlates with Ni2P crystallite size, which seems to be consistent with our results. Additionally, the ICP analysis was carried out to determine the amount of Ni element and the results were shown in Table 1. The Ni content over Ni2P/Fe-SBA-15 catalyst (7.6 wt%) was slightly lower than that of Ni2P/SBA-15 (8.0 wt%). This might be another reason for the tiny decline of CO uptakes for Ni2P/Fe-SBA-15. 3.4. XPS In order to gain insight into the surface composition of the catalysts, the X-ray photoelectron spectroscopy (XPS) analysis was carried out. As shown in Fig. 3(a), the binding energy centered at 852.3e852.5 eV can be assigned to Nidþ (0
Table 1 Surface properties of supports and catalysts. Sample
SBA-15 Ni2P/SBA-15 Fe-SBA-15 Ni2P/Fe-SBA-15 a b c d e
Metal contenta (wt%) Fe
Ni
e e 0.070 0.056
e 8.0 e 7.6
SBET (m2$g1)
Vp (cm3$g1)
DBJHb (nm)
dXRDc (nm)
CO uptaked (mmol$g1)
NH3 uptakee (mmol$g1)
Conversion (%)
619 326 465 119
0.81 0.56 0.37 0.17
6.8 7.3 5.2 6.6
e 18.4 e 17.9
e 38 e 36
e 313 e 362
e 85 e 91
Fe content: obtained from ICP analysis. DBJH: mesopore diameter calculated from the adsorption branch of nitrogen isotherms using BJH method. dXRD: calculated from the Dc ¼ kl/bcos(q) (Scherrer equation) based on the Ni2P{1 1 1}. CO uptakes: determined by CO chemisorption. NH3 uptakes: determined by NH3-TPD.
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Fig. 2. N2 absorption capacity of supports (a) and catalysts (b).
Fig. 3. The core level XPS spectra of Ni 2p3/2 (a); P 2p (b) and Fe 2p3/2 (c).
134.6 eV can be assigned to P5þ due to the superficial oxidation of the Ni2P particles [36,37]. The Fe 2p core level spectra for Fe-SBA-15 and Ni2P/Fe-SBA-15 were shown in Fig. 3(c). For the Fe-SBA-15 support, two peaks at 711.8 and 721.7 eV can be assigned to Fe3þ and satellite, respectively. For Ni2P/Fe-SBA-15, in addition to the peaks of Fe3þ and satellite, a new peak at 706.6 eV was detected and can be attributed to Fedþ in iron phosphide [38]. The Fe species were observed in the majority of Fe3þ with a weak contribution of Fedþ. Interestingly, FeP was proved to be another acceptable active sites for HDO of phenol [39]. Yuan et al. [40] also found FeP phase
was active and stable in hydrogenation reaction and showed better performance in HDS of DBT than that of Fe2P. Upon loading Ni2P phase on Fe-SBA-15, the peak assigned to Fe3þ shifted to the higher binding energy of 713.2 eV. This was possibly contributed to electron transfer from Fe to Ni or P since after addition of Fe the banding energy of Nidþ and Pdþ were both deceased. The atomic ratios were listed in Table 2. All the samples exhibited lower Ni/P atomic ratio than the theoretical Ni/P ratio (1/2). This might due to the enrichment of phosphorous on the surface of the catalysts. Additionally, the Ni/Si atomic ratio of Ni2P/Fe-SBA-15 (1/33) is slightly lower
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Fig. 4. TEM images and particle size distributions of (a) Ni2P/SBA-15 and (b) Ni2P/Fe-SBA-15.
Table 2 Spectral parameters of catalysts obtained by XPS analysis. Sample
Binding energy (eV)
Ni2P/SBA-15 Ni2P/Fe-SBA-15 Fe-SBA-15
Superficial atomic ratio P 2p
Ni 2p3/2
Fe 2p3/2
Nidþ
Ni2þ
Pd-
PO34
Fedþ
Fe3þ
852.5 852.3 e
856.6 856.2 e
129.2 129.1 e
134.6 134.6 e
e 706.6 e
e 713.2 711.8
Ni/P
Ni/Si
Fe/Si
1/3.6 1/3.9 e
1/30 1/33 e
e 1/59 1/48
than that of Ni2P/SBA-15 (1/30), indicating that the incorporation of Fe slightly decreased the exposed Ni sites on the surface of the catalyst, which is consistent with the result of CO uptakes. 3.5. TEM Fig. 4 showed the morphologies and microstructures of Ni2P/ SBA-15 and Ni2P/Fe-SBA-15 catalysts. For Ni2P/SBA-15 (Fig. 4(a)), the Ni2P particles were aggregated to form large Ni2P particles. Therefore, the distribution of Ni2P particles was very wide in size and the average particle size was about 16.3 nm. For Ni2P/Fe-SBA15 (Fig. 4(b)), the distribution of Ni2P particles was much uniform with the average particle size of about 5.6 nm. Combined with the XRD and CO uptake results, it could be concluded that the incorporation of Fe contributes to the highly uniform dispersion of Ni2P particles and the formation of smaller Ni2P particles on the support. Fig. 5. NH3-TPD profiles of Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts.
3.6. NH3-TPD The acidity of catalysts was determined by NH3-TPD and the results were shown in Fig. 5. The Ni2P/SBA-15 showed two peaks at 410 K and 503 K. It was worth noted that the latter was prominent and showed a shoulder at a higher temperature. Generally, the peak lower than 503 K was assigned to weak acid [41]. In the case of Ni2P/SBA-15, the generation of weak acid was due to the PeOH groups, and the shoulder was related to medium strength acid caused by the electron transfer from Ni to P, similar with the results reported by Yang et al. [42]. As contrast, the peaks of Ni2P/Fe-SBA15 were concentrated around 433 K and 503 K. The former slightly red-shifted and the shoulder of prominent peak showed a tendency to higher temperature as compared to that of the Ni2P/SBA-15, which proved the strengths of weak and medium acid were both improved by the incorporation of Fe. The total NH3 uptakes were
quantified and the value was shown in column 8 of Table 1. NH3 uptakes for Ni2P/Fe-SBA-15 was 362 mmol g1, higher than that of Ni2P/SBA-15 (313 mmol g1). This result indicated that the Ni2P/FeSBA-15 possessed higher acid amount than that of Ni2P/SBA-15. 3.7. Catalytic activity The BF HDO catalytic activity of the catalysts was investigated at a reaction temperature of 573 K, a pressure of 3.0 MPa, a WHSV of 4.0 h1 and hydrogen/oil ratio of 500(V/V). As shown in Fig. 6(a), the conversion of BF gradually increased and then remained approximately stable with time on stream, and the BF conversion over Ni2P/SBA-15 was 83.3% after 7 h. Expectedly, the incorporation of Fe was contributed to high activity of the catalyst with the
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Fig. 6. Conversion (a) of BF and the total deoxygenated product yield (b) over as-prepared catalysts. (Reaction conditions: T ¼ 573 K, p ¼ 3.0 MPa, WHSV ¼ 4.0 h1, and hydrogen/oil ratio ¼ 500(V/V)).
conversion of BF over Ni2P/Fe-SBA-15 up to 91.7% after 7 h. Upon incorporation of Fe, the conversion of BF increased by 8.4%. Similarly (Fig. 6(b)), the yields of O-free compounds gradually increased and then remained approximately stable. After 7 h, the total deoxygenated product yield over Ni2P/SBA-15 was 64.3%. In contrast, the total deoxygenated product yield was 83.3% for the Ni2P/Fe-SBA-15, which was increased by 19.0% compared to Ni2P/ SBA-15. The higher HDO activity over Ni2P/Fe-SBA-15 is attributed to the smaller and uniformly dispersed active Ni2P sites, which were proved by the XRD and TEM results. The Ni2P/SBA-15 showed a larger surface area, but the catalytic performance was restricted by poor dispersion caused by agglomerations of active Ni2P sites. In an attempt to gain insight into the effect of Fe on the catalytic reaction, the product distributions of the last 3 h over the asprepared catalysts were recorded and shown in Table 3. As compared to our previous study [30], the product distributions over the Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts did not change significantly and the deoxygenated product ECH was predominant in product, showing the reaction route for BF HDO over as-prepared catalysts was the same as that of previously reported one (Scheme 1) [30]. The selectivity to 2-EtPh over Ni2P/Fe-SBA-15 was only 0.2%, which decreased markedly as compared to Ni2P/SBA-15 (>1.9%). This showed that almost all the formed 2-EtPh was transformed into EB or Ph. The selectivity to Ph and B decreased after introducing the Fe, this confirms that more 2-EtPh was transformed to EB over Ni2P/Fe-SBA-15 than that over Ni2P/SBA-15. It is demonstrated that the introduction of Fe improved the dehydration of 2-EtPh to EB. As a result, the selectivity to deoxygenated products of ECH increased obviously from 49.5 to 78.2% after 7 h on stream (Table 3). The higher catalytic activity of Ni2P/Fe-SBA-15 could be related to smaller and highly dispersed Ni2P active sites (shown in XRD and TEM results). Meanwhile, the enhanced acidity (shown in NH3-TPD analysis) plays a critical role in improving dehydration of 2-EtPh to EB [43,44], which facilitates the increase
Scheme 1. Reaction pathways for HDO of BF [30].
in a total yield of deoxygenated compounds. Additionally, the carbon balance (CB) was another interesting parameter for testing the carbon loss during the catalytic reaction. The calculation of CB was as follows:
P CB ¼
nci ni 100% 8 nBF
where nBF was the moles of BF in the feed, nci was carbon number of product i and ni was the mole of product i. The CB for BF HDO over Ni2P/SBA-15 and Ni2P/Fe-SBA-15
Table 3 Product distribution and CB of the last 3 h over the catalysts. Catalysts and reaction time/ h
B
MCH
MB
ECH
EB
Ph
2,3-DHBF
2-EtPh
CB
Ni2P/SBA-15
4.3 4.7 6.1 1.7 1.3 1.0
0.6 0.9 3.3 4.1 3.4 3.0
0.9 1.0 2.9 1.3 0.6 0.4
58.5 56.9 49.5 65.9 72.0 78.2
14.0 14.6 15.4 13.0 12.4 8.3
15.0 14.7 13.5 8.6 5.6 4.9
4.8 4.8 6.1 5.3 4.6 4.0
1.9 2.3 3.2 0.1 0.2 0.1
95.0 94.8 94.3 96.8 97.9 98.0
Ni2P/Fe-SBA-15
5 6 7 5 6 7
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activity of Ni2P/Fe-SBA-15 is higher than most of the reported catalysts.
catalysts was listed in column 10 of Table 3. Both Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts showed higher CB values, which indicated the low cleavage of CeC bond. The reservation of carbon atom in product contributed to low loss of calorie for the bio-oil. The comparison of the typical catalytic activity of BF HDO from literatures with our present results were summarized in Table 4. It can be seen that the BF HDO activity of Ni2P/Fe-SBA-15 is higher than most of the reported catalysts. The DBT HDS catalytic activity of catalysts was also investigated at a reaction temperature of 613 K, a pressure of 3.0 MPa, a WHSV of 1.5 h1 and hydrogen/oil ratio of 500(V/V) and the results were shown in Fig. 7. The HDS activities of the catalysts first increased and then reminded stable with time on stream. As compared to Ni2P/SBA-15 (92.1%), the DBT conversion of Ni2P/Fe-SBA-15 (96.3%) had an increase by 4.2%, showing that incorporation of Fe could increase the catalytic activity by contributing to the exposure of smaller and uniformly dispersed active Ni2P sites. For the product distribution in the HDS of DBT (Fig. 7(b)), the BP and CHB were dominant products, which represented the direct desulfurization (DDS) and hydrogenation (HYD) route, respectively. The selectivity to BP over Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts was much higher than that for CHB, indicating that DBT was mainly transformed through the DDS route [50]. For Ni2P/Fe-SBA-15, the selectivity to BP maintained at 90.3% after 8 h, higher than that of Ni2P/SBA-15 (81.6%). This showed that incorporation of Fe could increase the BP selectivity via the DDS route [51,52]. The comparison of the typical catalytic activity of DBT HDS from literature with this study was summarized in Table 5. It can be seen that the HDS
4. Conclusion The Fe-doped Ni2P/Fe-SBA-15 was synthesized by temperature programmed reduction at a relatively low reduction temperature of 673 K by using NH4H2PO2 and NiCl2$6H2O. The effect of incorporation of Fe on the catalytic properties for BF HDO and DBT HDS were investigated. The XRD analysis has shown that incorporation of Fe is beneficial to the formation of the smaller size of Ni2P particles. The amounts of CO uptakes for Ni2P/Fe-SBA-15 slightly decreased, which was due to the lower surface area of the catalyst after the incorporation of Fe. The TEM analysis confirmed that smaller and uniformly dispersed Ni2P particles with the average particle size of about 5.6 nm were formed for Ni2P/Fe-SBA-15. Therefore, it could be concluded that the incorporation of Fe contributes to the highly uniform dispersion of Ni2P particles and the formation of smaller Ni2P particles on the support. In addition, the NH3-TPD analysis proved that the strengths of weak and medium acid were both improved by the incorporation of Fe. Compared with Ni2P/SBA-15 (81.6%), the Fe-doped Ni2P/Fe-SBA-15 exhibited a much higher BF conversion of 91.7%. And the total deoxygenated product yield of Ni2P/Fe-SBA-15 was 83.3%, which is much higher than that of Ni2P/SBA-15 (64.3%). The excellent HDO performance of Ni2P/Fe-SBA-15 could be related to highly dispersed active sites (shown in XRD and TEM results). Meanwhile, the improved dehydration of 2-EtPh to EB and the high deoxygenated product
Table 4 Comparison of the typical results of BF HDO from literature with our present results. Samples
Temperature (K)
Pressure (MPa)
Conversion (%)
Ref. no.
Sulfided NiMo/Al2O3a Sulfided NiMo/Al2O3a NiMoP/Al2O3a Pt/SiO2eAl2O3 Pd/SiO2eAl2O3 W2C(Ar-2-1023 K-1 h) Ni2PeN/MCM-41 Ni2PeO/MCM-41 Ni2P/Al2O3 Ni2P/TiO2 Ni2P/Al2O3@TiO2 Ni2P/SBA-15 Ni2P/Fe-SBA-15
553 553 613 553 553 613 493 493 573 573 573 573 573
2.0 5.0 7.0 3.0 3.0 4.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
74.6 82.5 80.7 80 97 41 31 57 78 85 95 85 91
[45] [45] [46] [47] [47] [48] [30] [30] [49] [49] [49] This work This work
a
With H2S in the feed.
Fig. 7. Conversion (a) and product selectivity (b) of DBT HDS over Ni2P/SBA-15 and Ni2P/Fe-SBA-15 catalysts. (Reaction conditions: T ¼ 613 K, p ¼ 3.0 MPa, WHSV ¼ 1.5 h1, and hydrogen/oil ratio ¼ 500(V/V)).
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Table 5 Comparison of the typical results of DBT HDS from literature with our present results. Samples
Temperature (K)
Pressure (MPa)
Conversion (%)
Ref. no.
Sulfided NiMo/g-Al2O3 Pt/MCM-48 PtMo/MCM-48 CoMoW/SBA-15 NiMoW/SBA-15 FeP/C Fe2P/C Bulk Ni2P Ni2P/MCM-41 Ni2P/Ti-MCM-41 (1.5) Ni2P/SBA-15 Ni2P/Fe-SBA-15
573 623 623 573 573 613 613 613 613 613 613 613
7.3 3.0 3.0 7.3 7.3 3.1 3.1 3.0 3.0 3.0 3.0 3.0
93 21.6 75.4 81 88 77 40 62 81.9 99.4 92.1 96.3
[53] [54] [54] [55] [55] [40] [40] [56] [57] [57] This work This work
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