Article pubs.acs.org/IECR
Effect of Citric Acid on the Hydrodesulfurization Performance of Unsupported Nickel Phosphide Nan Jiang,† Xiao-Wei Xu,†,§ Hua-Lin Song,‡ Hua Song,*,† and Fu-Yong Zhang† †
Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Chemistry & Chemical Engineering College, Northeast Petroleum University, 199 Fazhan Road, High-Tech Zone, Daqing City 163318, Heilongjing Province, People’s Republic of China ‡ Key Laboratory of Cancer Prevention and Treatment of Heilongjiang Province, Department of Pathology, Basic Medical College, Mudanjiang Medical University, No.3 Tongxiang Street, Aimin District Mudanjiang City 157011, Heilongjing Province, People’s Republic of China § Shandong Yuhuang Chemical Company Ltd., Heze City 274000, Shandong Province, People’s Republic of China ABSTRACT: The Ni−P incorporated citric acid (CA) catalysts with different CA/Ni molar ratios (x) were successfully prepared. The introduction of CA can change the structure of the catalyst precursor by complexing, and therefore affect the catalyst pore structures. The incorporation of CA into bulk Ni−P catalyst leads to a dramatic increase in surface area and pore volume. As compared to the Ni−P sample, the surface area of CA(2.0)−Ni−P increases 8.3 times. CA can also promote the formation of smaller, highly dispersed Ni2P particles, favoring more nickel phosphide phase. The DBT conversion of the CA(2.0)−Ni−P catalyst reached 96%, which increased by 34% as compared to the Ni−P sample. min, further heating to 500 °C at a rate of 1 °C/min, and held for 2 h in a flow of H2 (200 mL/min). The resulting catalysts were naturally cooled to room temperature in a continuous H2 flow, and then passivated in O2/N2 mixture (0.5 vol % O2) with a flow rate of 20 mL/min for 2 h. The Ni−P catalysts obtained were named CA(x)−Ni−P, where x represents the CA/Ni molar ratio. 2.2. Characterization of Catalysts. X-ray diffraction (XRD) analysis of the catalysts was carried out on a D/max2200PC X-ray diffractometer. Thermo gravimetric (TG) analysis curves were obtained by a PerkinElmer 7 simultaneous thermal analyzer at a heating rate of 15 °C/min in air atmosphere. The typical physico-chemical properties of samples were performed by BET method using Micromeritics adsorption equipment of NOVA2000e. CO chemisorption uptake measurements were analyzed in a Micromeritics ASAP 2010 apparatus under static volumetric conditions. X-ray photoelectron spectroscopy (XPS) spectra were recorded with an ESCALAB MKII spectrometer. 2.3. Catalytic Activities. The HDS of DBT was carried out in a flowing high-pressure fixed-bed reactor using a feed consisting of DBT (1 wt %) in a decalin solution. The reaction conditions were 340 °C, 3.0 MPa, hydrogen/oil ratio of 500 (v/ v), and weight hourly space velocities (WHSV) of 1.5 h−1. Prior to reaction, 0.8 g of the catalysts was pretreated at 500 °C for 2 h in flowing H2 (40 mL/min). Sampling of liquid products was started 2 h after the steady reaction conditions had been achieved. The liquid samples were collected every hour and
1. INTRODUCTION The Ni2P catalyst is recognized as an alternative class of catalyst that has potential for HDS of transportation fuels.1−3 However, because of the small specific surface area of bulk Ni2P catalyst, the improvement of bulk Ni2P is necessary to achieve a better structure and higher HDS performance. Yang et al.4 prepared bulk Ni2P with a higher surface area by adding a polymer surfactant (Triton X-114) and ethylene glycol to an aqueous solution of Ni(NO3)2 and (NH4)2HPO4 prior to drying and calcination. Chelating agent is widely used in the modification of molecular sieves and preparation of catalysts.5,6 Cheng et al.7 used citric acid (CA) as a structure template or chelating agent to prepare bulk MoP with a maximum BET surface area of 122 m2/g. Smith et al.8,9 have studied the unsupported Ni2P catalysts prepared using CA, and they found that the high BET surface area (220 m2/g) sample was obtained with the CA/Ni ratio of 2 at the reduction temperature of 650 °C. In this study, the Ni−P incorporated citric acid (CA) catalysts with different CA/Ni molar ratios (x) were successfully prepared, and the effects of CA on the structure and HDS performance of the bulk Ni−P catalyst have been investigated. 2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. The oxidic precursor of Ni−P catalyst was prepared with excess phosphorus (Ni:P = 1:1).10 The different amounts of CA were added into the mixture of Ni(NO3)2 and (NH4)2·HPO4 solution under stirring. The solution then was heated until complete evaporation, resulting in the formation of the solid product. The solid obtained was dried at 120 °C for 12 h and calcined in air at 500 °C for 4 h, resulting in the oxidic catalyst precursor. The oxidic precursors were reduced in a fixed-bed reactor by heating from room temperature to 400 °C at a rate of 2.5 °C/ © 2016 American Chemical Society
Received: Revised: Accepted: Published: 555
September 9, 2015 December 25, 2015 January 2, 2016 January 3, 2016 DOI: 10.1021/acs.iecr.5b03359 Ind. Eng. Chem. Res. 2016, 55, 555−559
Article
Industrial & Engineering Chemistry Research analyzed by FID gas chromatography with a GC-14C-60 column.
3. RESULTS AND DISCUSSION 3.1. XRD. The XRD patterns of Ni−P and CA(x)−Ni−P catalysts were performed, and the results are shown in Figure 1.
Figure 1. XRD patterns of the fresh and spent Ni−P and CA(x)−Ni− P catalysts (the dark and red lines are fresh and spent samples, respectively).
For the Ni−P catalyst, the peaks at 2θ = 40.6°, 44.5°, 47.1°, and 54.1° can be ascribed to Ni2P, and the weak diffraction peaks at 2θ = 28.8°, 30.2°, 31.6°, 36.1°, 43.9°, 45.1°, 47.8°, and 53.0° can be assigned to the Ni5P4 phase. Moreover, the Ni−P catalyst exhibits a broad peak at 2θ = 32° due to the amorphous NixPyOz phases.11 The amorphous NixPyOz phase is the oxidic catalyst precursor, which can transform to nickel phosphide phases during the temperature-programmed reduction. The NixPyOz phase in the Ni−P catalyst was not completely reduced. For the CA(x)−Ni−P, the diffraction peak of the amorphous phase of NixPyOz disappears, and the peaks of the Ni5P4 phase gradually weaken with increasing CA/Ni molar ratio and disappear for CA/Ni > 1.0. This suggests that CA could suppress the formation of Ni5P4 and reduction of amorphous NixPyOz, and therefore promote formation of the Ni2P. This is possibly because the CA(x)−Ni−P catalysts possess a higher Ni/P molar ratio (Table 2) due to the loss of phosphate with a large amount of released gases from decomposition of complex during calcinations (Figure 2). By increasing the CA/Ni molar ratio from 0.5 to 2.0, the diffraction peaks of the Ni2P phase become more broadened and weaken, indicating the decrease in crystallinity and particle size. To some extent, the low crystallinity and small size of Ni2P particles are beneficial to disperse the active phase on the surface. XRD patterns were also obtained for the spent catalysts (Figure 1, red lines in each frame). No other new phase can be observed in the spent samples, implying that the main catalytic framework of Ni5P4 and Ni2P remains unchanged. For all of the samples, the Ni5P4 and Ni2P phases become more crystallized after HDS reaction. This indicates that the Ni5P4 and Ni2P particles are not stable due to the prolonged exposure at the hydrotreating temperature during the HDS reaction. It is worth noting that the CA(2.0)−Ni−P still shows the lowest crystallinity among all of the spent catalysts, which would exhibit high HDS activity. 3.2. TG-DTA. For the TG-DTA curve of the uncalcined Ni− P catalyst precursor (Figure 2), the gradual weight losses before 200 °C and two small endothermic peaks observed at 138 and
Figure 2. TG-DTA curves of pure CA, and uncalcined Ni−P, CA(2.0)−Ni, CA(2.0)−P, and CA(2.0)−Ni−P precursors.
168 °C on the DTA curve can be assigned to the removal of adsorbed and crystal water in the precursor, respectively.12 The TG curve indicates an obvious weight loss occurring from 200 to 320 °C, while the DTA curve shows a distinct endothermic peak at 265 °C, which is attributed to the decomposition of NH4NiPO4. A weak exothermic peak at 400 °C is observed in the DTA curve, which is due to the crystallization of NixPyOz. For pure CA, a distinct endothermic peak at 215 °C in the DTA curve and the greatest weight losses in the TG curve can be observed due to the decomposition of CA accompanied by producing a large amount of gases.13 For uncalcined CA(2.0)− Ni, the main weight losses are observed from 150 to 400 °C, while the DTA curve shows a distinct exothermic peak at 378 °C. Klimova et al.14 have studied the NiMo/SBA-15 catalysts prepared with citric acid, and they found that the addition of CA to the impregnation solutions could stabilize the Ni2+ ions in solution in a wide range of pH values, and avoid the precipitation as Ni(OH)2 compound at basic pH. In basic impregnation solution, the Nicit24− complex was formed.14 Therefore, the distinct exothermic peak of uncalcined CA(2.0)−Ni can be assigned to the decomposition of complex of CA and nickel. For uncalcined CA(2.0)−P, the sustained weight losses after 170 °C and two endothermic peaks are observed at 225 and 275 °C in the DTA curve, which may be associated with the decomposition of CA and (NH4)2·HPO4, respectively; a distinct exothermic peak centered at 574 °C contributed to the decomposition of the complex of CA and phosphate. For uncalcined CA(2.0)−Ni−P, continuous weight losses after 170 °C and the distinct exothermic peak at 200 °C can be observed, which is assigned to the decomposition of the triple complex of CA, nickel, and phosphate; the distinct exothermic peak at 580 °C contributed to the decomposition of the complex of CA and phosphate. This observation indicates (1) the introduction of CA has changed the structure of the catalyst precursor by complexing, thus affecting the catalyst 556
DOI: 10.1021/acs.iecr.5b03359 Ind. Eng. Chem. Res. 2016, 55, 555−559
Article
Industrial & Engineering Chemistry Research Table 1. Properties and HDS Catalytic Performance of the Ni−P and CA(x)−Ni−P Catalysts
selectivity (%) sample
SBET (m2 g−1)
Vp (cm3 g−1)
CO uptake (μmol g−1)
conversion (%)
CHB
BP
Ni−P CA(0.3)−Ni−P CA(0.5)−Ni−P CA(1.0)−Ni−P CA(2.0)−Ni−P
6.1 11.5 17.4 54.6 56.5
0.025 0.055 0.076 0.161 0.192
171 210 234 279 328
62 85 91 93 96
35 36 38 40 37
65 64 62 60 63
pore structures; and (2) the complex of CA and phosphate, which decomposes at a temperature higher than 500 °C, is still present in the CA(2.0)−Ni−P after calcination. In addition, the TG curve of CA(2.0)−Ni−P also exhibits an endothermic peak at 320 °C and an exothermic peak at 517 °C, which are assigned to the decomposition of NH4NiPO4 and crystallization of NixPyOz, respectively. The delay of the crystallization temperature of NixPyOz with CA(2.0)−Ni−P precursor as compared to Ni−P precursor is from the addition of CA, which is similar to the observation reported by Song et al.15 3.3. BET. The surface area and pore volume of Ni−P sample are 6.1 m2 g−1 and 0.025 cm3 g−1 (Table 1), respectively. With increasing CA/Ni molar ratio, the specific surface area and pore volume both increased significantly. This is understandable because the introduction of CA can enrich the catalyst pore structure, which is due to the formation of pores after the release of a large amount of gases from decomposition of CA during calcination.16 The dramatic increase in surface area may lead to a high dispersion of the Ni2P particles, which may also influence the DBT HDS conversion and product distribution. The surface area and pore volume of CA(2.0)−Ni−P reach 56.5 m2 g−1 and 0.192 cm3 g−1, respectively. As compared to the Ni−P sample, the surface area of CA(2.0)−Ni−P increases 8.3 times and the pore volume increases 6.7 times, respectively. 3.4. CO Uptake. As can be seen in Table 1, the CO uptake of Ni−P catalyst is determined as 171 μmol g−1. The CO uptake of CA(2.0)−Ni−P is 328 μmol g−1, which is about double that of Ni−P catalyst. The CO uptake of the samples considerably increases with increasing CA/Ni molar ratio. This is possibly because (1) the higher surface area and smaller active particles contributed to the better dispersion of the active phase with increasing CA/Ni molar ratio; and (2) the addition of CA could stabilize the Ni2+ ions in solution during impregnation, which would avoid the precipitation as Ni(OH)2 compound, leading to more exposed nickel atoms over catalysts. 3.5. XPS. As shown in Figure 3a, all of the Ni 2p spectra were decomposed, taking into account the spin−orbital splitting of Ni 2p3/2 and Ni 2p1/2 lines (17.5 eV) and the presence of satellite peaks at about 5 eV higher than the binding energy of the parent signal. For the Ni 2p3/2 spectrum, the first peak centered at 851.9−852.7 eV is assigned to the Niδ+ species (Ni species in nickel phosphide phases),17 and the second one at 855.0−856.7 eV corresponded to the interaction of Ni2+ species with phosphate, as a consequence of a superficial passivation.18,19 As compared to Ni 2p BEs of Ni− P, all of the Ni 2p BEs of Ni−P to CA(2.0)−Ni−P are shifted to a lower value; this indicates more electron transfer from phosphide species to nickel species for CA(2.0)−Ni−P than Ni−P. It is likely that CA(2.0)−Ni−P sample loses more phosphorus with a large amount of released gases from decomposition of complex during calcinations (Figure 2), which causes the shift of the Ni 2p BEs.
Figure 3. XPS spectra of the Ni−P and CA(2.0)−Ni−P catalysts. (a) Ni 2p core level spectra, and (b) P 2p core level spectra.
As shown in Figure 3b, the bands centered at 128.8−129.2 eV can be attributed to Pδ+ species (P species in nickel phosphide phases),20 and the bands at 133.4−134.8 eV can be assigned to phosphate (P5+) forming the superficial oxidation.19 It is worth noting that the CA(2.0)−Ni−P shows a new type of phosphate species at 136.8 eV, which can be attributed to a complex of CA and phosphate, listed as P(complexing) in Table 2. Table 2 shows the superficial atomic ratios calculated by XPS analysis. As compared to the Ni−P sample, the P/Ni molar ratio of CA(2.0)−Ni−P catalyst is lower. This is posibly because the addition of CA could stabilize the Ni2+ ions in solution during impregnation, leading to more exposed nickel atoms over catalysts. It is worth noting that the RNiδ+ of the Ni−P is only 0.041, whereas that of the CA(2.0)−Ni−P reaches 0.343, which is 8 times higher than that of the Ni−P sample. This result suggests that there is more nickel phosphide formed on the CA(2.0)−Ni−P, which would improve the HDS activity. 3.6. HDS Activity. As shown in Figure 4, the Ni−P sample showed a DBT conversion of 62% after 8 h, which is much 557
DOI: 10.1021/acs.iecr.5b03359 Ind. Eng. Chem. Res. 2016, 55, 555−559
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Industrial & Engineering Chemistry Research Table 2. Spectral Parameters of the Ni−P and CA(2.0)−Ni−P Catalysts Obtained by XPS Analysis binding energy (eV) Ni 2p3/2 sample Ni−P CA(2.0)−Ni−P
δ+
2+
Ni
Ni
852.7 851.9
856.7 855.0
P 2p satellite 861.2 859.8
P
δ−
5+
P
128.8 129.2
Figure 4. HDS activity of the Ni−P and CA(x)−Ni−P catalysts. Temperature, 340 °C; pressure, 3.0 MPa; H2/oil ratio, 500 (v/v); WHSV, 1.5 h−1.
superficial molar ratio P(complexing)
P/Ni
Niδ+/∑Ni
136.8
3.0 2.4
0.041 0.343
134.8 133.4
Figure 5. HDS selectivity of the Ni−P and CA(x)−Ni−P catalysts. Temperature, 340 °C; pressure, 3.0 MPa; H2/oil ratio, 500 (v/v); WHSV, 1.5 h−1.
lower than the values of all of the CA(x)−Ni−P catalysts. This is because (1) CA could promote the reduction of the NixPyOz phase in the oxidic catalyst precursors, therefore leading to the more active nickel phosphide phases formed over catalyst; and (2) CA could suppress the formation of Ni5P4, which shows a lower HDS activity than that of the Ni2P phase.21 The DBT conversions of the CA(x)−Ni−P catalysts increased with increasing CA/Ni molar ratio, which is attributed to the smaller size (Figure 1, discussion of the XRD results) and better dispersion of the Ni2P phase (Table 1, the CO uptake results) and the higher surface area of the catalysts (Table 1, the BET results) with increasing CA/Ni molar ratio. The DBT conversion over the CA(2.0)−Ni−P catalyst reached 96% after 6 h and remained stable with time. As compared to the Ni−P catalyst, the DBT conversion increased by 34% over CA(2.0)−Ni−P catalyst. The products of the DBT HDS are mainly cyclohexylbenzene (CHB) and biphenyl (BP). The HDS selectivity of the Ni−P and CA(x)−Ni−P catalysts was shown in Figure 5 and listed in columns 6 and 7 of Table 1, which illustrates the higher BP selectivity for all of the catalysts. This indicates that the direct desulfurization (DDS) pathway is more favored than the hydrogenation desulfurization (HYD) pathway during HDS over the Ni−P and CA(x)−Ni−P catalysts.10 As compared to the Ni−P sample, the product selectivities for the CA(x)−Ni− P catalysts did not show a significant change. This may suggest that the addition of CA scarcely influences the ratio of Ni(1) sites and Ni(2) sites, which are involved in the DDS route and the HYD pathway, respectively.22
crystallite size and higher dispersion of the Ni2P particles. With increasing CA, the number of exposed nickel atoms increases, leading to a higher CO uptake. The CO uptake of CA(2.0)− Ni−P is analyzed as 328 μmol g−1, which is about double that of Ni−P catalyst. The DBT conversion of the CA(2.0)−Ni−P catalyst reached 96%, which increased by 34% as compared to the Ni−P sample.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 459 6503167. Fax: +86 459 6506498. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (21276048), the Natural Science Foundation of Heilongjiang Province (ZD201201), and the Education Department of Heilongjiang Province (12541060).
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REFERENCES
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4. CONCLUSIONS The Ni−P incorporated citric acid (CA) catalysts with different CA/Ni molar ratios (x) were successfully prepared. XRD analysis has shown that addition of CA can suppress the formation of Ni5P4 and amorphous NixPyOz, and therefore promote formation of the Ni2P. The introduction of CA can greatly increase the surface of the catalyst, leading to a smaller 558
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