Shuzhen Zhoua,
Lihua Kanga,
Zhu Xua and
Mingyuan Zhu*ab
aSchool of Chemistry and Chemical Engineering of Shihezi University, Shihezi, Xinjiang 832003, P. R. China. E-mail: zhuminyuan@shzu.edu.cn; Fax: +86 9932057210; Tel: +86 9932057270
bKey Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Shihezi, Xinjiang 832003, P. R. China
First published on 9th January 2020
Ni/MCM-41 catalysts were prepared by an impregnation method for acetylene hydrogenation to ethylene based on the calcium carbide acetylene route. X-ray diffraction and transmission electron microscopy indicated that Ni was uniformly dispersed on the support. Temperature-programmed reduction and X-ray photoelectron spectroscopy demonstrated a strong interaction between Ni and MCM-41, and Ni(0) and Ni(II) coexisted in the catalyst. We optimized the catalytic activity by optimizing the Ni loading and reaction conditions including temperature, space velocity, and hydrogen/acetylene ratio. The acetylene conversion reached 100%, the ethylene selectivity reached 47%. Additionally, we tested the catalyst stability; the acetylene conversion was maintained at 100% for 25.73 h and was then rapidly reduced. ICP, TEM, FT-IR, thermogravimetric analysis and BET were used to investigate the reasons for catalyst deactivation; it was found that green oil deposition on the catalyst surface was the main reason for the catalyst deactivation.
Although the reaction system is different, the trace acetylene hydrogenation catalyst system can provide a reference for pure acetylene hydrogenation. Since Pd exhibits excellent hydrogenation activity, current research mainly focuses on a Pd catalyst, and the catalytic activity is improved by support modulation3 and addition of Ag,4 Cu,5 and others. Pei et al.5 prepared Cu alloyed with Pd as a single-atom catalyst for semi-hydrogenation of acetylene with high concentrations of hydrogen and ethylene; the acetylene conversion reached 100% and the ethylene selectivity was 85%. Zhou et al.6 prepared a PdZn intermetallic nanostructure; due to a good distribution of Pd in Pd–Zn–Pd, the chemical adsorption of ethylene was reduced and excessive hydrogenation of ethylene was inhibited, ethylene selectivity remained at 90% at relatively low temperatures (60 °C), and ethane output was low even at high temperatures (200 °C) when acetylene was completely converted. However, for pure acetylene hydrogenation, due to the high acetylene concentration, a large number of catalytically activity sites are required, so a large amount of catalyst or a high loading are needed, resulting in a high cost. Therefore, Pd-based catalysts may be unsuitable for pure acetylene hydrogenation from an economic perspective.
In recent years, non-precious metal Ni has attracted much attention in acetylene hydrogenation.7,8 Chen et al.9 prepared Ni–In bimetallic catalysts to study the effect of In addition on the reaction. Ni/SiO2 conversion could be maintained at 100% before deactivation after 2 h. Adding In improved the ethylene selectivity. However, the green oil selectivity exceeded 30%. Green oil contains mainly C4 and C6 hydrocarbons and polymers, and Ahn et al.10 demonstrated that the precursor of green oil is 1, 3-butadiene. Wang et al.11 found that by adding Ga, Ni–Ga intermetallic compounds could be formed to improve stability and ethylene selectivity; however, even at the optimal Ni/Ga molar ratio, the green oil selectivity exceeded 15%. Both studies consider green oil formation, and it was suggested that the green oil selectivity exceeds that of ethane. This indicates that the Ni-based catalyst used for acetylene hydrogenation will generate large amounts of green oil as a by-product. To control Ni dispersion, Dai et al.12 used the metal organic framework ZIF-8 to prepare a single-atom Ni/N–C catalyst for acetylene removal from an ethylene-rich stream, with acetylene conversion and ethylene selectivity both exceeding 90%. However, due to the lower acetylene concentration, they neglected green oil in the reaction calculation. The above studies were all used for acetylene removal from the ethylene-rich stream, and the acetylene concentration was below 2%. In practice, during acetylene hydrogenation, the acetylene concentration strongly affects the green oil selectivity. Trimm et al.13 prepared a Ni/SiO2 catalyst and studied the distribution of acetylene oligomer reaction products with a feed of 25% C2H2/75% H2; they found that approximately half of the acetylene was converted to a liquid oligomer. Therefore, green oil must be considered in the pure acetylene hydrogenation reaction, and as far as we know, almost no one has studied the use of a Ni single metal catalyst in the hydrogenation of pure acetylene to ethylene and discussed the effect of reaction conditions on acetylene conversion. In addition, according to the literature,14 amino-functionalized MCM-41 can strengthen the interaction between Ni nanoparticles and MCM-41, reduce particle size, and improve dispersion of nickel particles.
Therefore, in this work, we used amino functionalized MCM-41 to immobilize Ni particles on the support prepared a uniformly dispersed Ni/MCM-41 catalyst and mainly studied the effects of Ni loading, and the reaction conditions (temperature, space velocity, hydrogen/acetylene ratio) for hydrogenation of pure acetylene to ethylene. The stability and regenerative capacity of the 1%Ni/MCM-41 catalyst and the main reason for the catalyst deactivation in this reaction were investigated and discussed. This work provides a strategy for further study of Ni-based catalysts for hydrogenation of pure acetylene to ethylene.
The MCM-41-supported metallic Ni catalysts were obtained using incipient impregnation method. Amino functionalized MCM-41 (2 g) was added to 30 mL deionized aqueous solution dissolved with a certain amount of nickel nitrate under agitation, and dispersed by ultrasonication for 10 min, and continuously stirred for 24 h, then dried by rotary evaporated at 70 °C after that heat treatment for 4 h at 400 °C in a muffle furnace. The obtained sample was then reduced in a tube furnace under 5% H2/Ar (100 mL min−1) atmosphere at 500 °C for 4 h to obtain the Ni/MCM-41 catalyst. Catalysts with different Ni contents were obtained by adding different amounts of Ni(NO3)2·6H2O as described above and named X%Ni/MCM-41 (X represents the Ni load).
The acetylene conversion (X) and products selectivity (S) were obtained by the following formulas:
Sgreen oil = 1 − SC2H4 − SC2H6 |
In the above formula, green oil represents products other than C2.
Fig. 1 (A) Infrared spectra of MCM-41 before and after modification, (B) XRD patterns of different catalysts, (C) and (D) correspond to TEM images of the 10%Ni/MCM-41 and 1%Ni/MCM-41, respectively. |
Catalyst | Actual loading | Catalyst | Actual loading |
---|---|---|---|
10%Ni/MCM-41 | 9.7% | 0.5%Ni/MCM-41 | 0.57% |
5%Ni/MCM-41 | 4.4% | 0.3%Ni/MCM-41 | 0.33% |
1%Ni/MCM-41 | 1.1% | 0.1%Ni/MCM-41 | 0.11% |
To measure reduction ability of the catalyst and the strength of the interaction of the Ni species with MCM-41 surface, we conducted a TPR test on the catalyst after calcination at 400 °C, the results are shown in Fig. 2(A). The 10%Ni/MCM-41 shows a broad peak from 350–650 °C, the peak at about 400 °C ascribed to reduction of large Ni particles, which have no or little interaction with the support, and the peak above 500 °C was put down to reduction of small Ni oxide particles that interacted strongly with support.21,22 As the Ni loading decreasing, the reduction peak around 400 °C gradually disappeared, and that around 500 °C gradually moved to higher temperature, consistent with the literature results,23 which indicates that the interaction between MCM-41 and Ni becomes stronger and the metal particles become smaller as the load decreases. Meanwhile, as the load decreases, it is more difficult for NiO particles to be reduced to Ni, consistent with the results of TEM and XRD.
Fig. 2 (A) H2-TPR profiles for catalysts with different Ni loads, (B) Ni 2p3/2 XPS spectra for catalysts with different Ni loads. |
To determine the valence state of Ni in the catalysts, we performed XPS characteristic, as displayed in Fig. 2(B). It is clear that the Ni 2p spectra of these four catalysts all can be divided into three peaks, with binding energies of 852.7 and 856.2 eV corresponding to zero-valent metal Ni and Ni oxide with Ni 2p3/2, respectively.24,25 A satellite peak at 861.5 eV was also observed.26 These results indicate that both Ni(0) and Ni(II) are present in the catalyst. This is consistent with the TPR results; NiO nanoparticles are not completely reduced to Ni nanoparticles at the experimental reduction temperature.
To determine the effect of the reaction conditions on the catalytic performance, we selected 1%Ni/MCM-41 for the following experiments. Fig. 4 displays the effect of different reaction temperatures on catalytic activity at 12000 mL g−1 h−1 and v(H2)/v(C2H2) = 2. The acetylene conversion increases with temperature, the acetylene conversion can reaches 100% at 200 °C. As the temperature was increased further, the conversion rate of acetylene remained constant. Simultaneously, as the temperature increased, the catalyst improved and was better at 250 °C (Fig. 4(A)). The ethylene selectivity increased with temperature and then remained constant at approximately 40% (Fig. 4(B)), so we chose 250 °C as the temperature for space velocity optimization.
Fig. 4 Effect of reaction temperature on catalyst performance (reaction conditions: 12000 mL g−1 h−1 and v(H2)/v(C2H2) = 2); (A) acetylene conversion, (B) ethylene selectivity. |
Fig. 5 shows the effect of different space velocities on catalytic activity at 250 °C and v(H2)/v(C2H2) = 2. As the space velocity increased from 6000 to 18000 mL g−1 h−1, the initial acetylene conversion could reach 100%. However, the catalyst stability was significantly reduced. This may be due to the larger the amount of acetylene treated, the more green oil accumulated on the catalyst surface, resulting in faster deactivation of the catalyst in the same time. Continue increase the space velocity to 22000 mL g−1 h−1, the initial acetylene conversion only 85% and rapidly decrease from 85% to 13% within 8 h (Fig. 5(A)). This is because the catalyst does not provide enough active sites for complete conversion of acetylene at high velocity. The ethylene selectivity increased obviously with the increase of space velocity first and then remained at about 41% (Fig. 5(B)), which is consistent with the literature.31,32 Considering the stability of the catalyst and the ethylene selectivity, we optimized the hydrogen/acetylene ratio with a space velocity of 8000 mL g−1 h−1.
Fig. 5 Effect of acetylene space velocity on catalyst performance (reaction conditions: 250 °C and v(H2)/v(C2H2) = 2); (A) acetylene conversion, (B) ethylene selectivity. |
Fig. 6 reveals the effect of different volume ratios of hydrogen to acetylene on the activity of the catalyst at 250 °C and 8000 mL g−1 h−1. As the ratio of hydrogen to acetylene increased, the conversion rate of acetylene tended to increase. When the ratio of hydrogen to acetylene reached 2, the conversion rate of acetylene reached 100%. When the ratio of hydrogen to acetylene was increased further, there was no change in the acetylene conversion, and no significant inactivation occurred after reacting for 8 h (Fig. 6(A)). However, unlike the acetylene conversion, the ethylene selectivity increased gradually with the volume ratio of hydrogen to acetylene and then decreased (Fig. 6(B)), which is consistent with the literature.33,34 This is because, as the amount of hydrogen increases, a large amount of hydrogen competitively adsorbs on the catalyst surface with acetylene, which reduces the acetylene adsorption on the catalyst surface, thus inhibiting green oil formation and improving the stability. However, when the amount of hydrogen reaches a certain value, the undesorbed ethylene which is more easily further hydrogenated to ethane, reducing ethylene selectivity. The ethylene selectivity reached 40.96% at the optimum volume ratio of hydrogen to acetylene.
Fig. 6 Effect of v(H2)/v(C2H2) on catalyst performance (reaction conditions: 250 °C and 8000 mL g−1 h−1); (A) acetylene conversion, (B) ethylene selectivity. |
Finally, we obtained the optimal reaction conditions: T = 250 °C, 8000 mL g−1 h−1, and v(H2)/v(C2H2) = 3.
Fig. 7 shows that the acetylene conversion over the fresh catalyst was maintained at 100% for 25.73 h and was then rapidly reduced from nearly 100% to 48% in the subsequent 14.3 h (Fig. 7(A)), which may be due to green oil accumulating on the catalyst surface during the reaction, gradually blocking the catalyst pores and covering the Ni active sites, resulting in rapid deactivation of the catalyst. As the reaction progressed, the ethylene selectivity increased from 36% to 47% and then slowly decreased to about 43%. In all subsequent regeneration experiments, the product selectivity remained relatively stable, with an ethylene selectivity of about 45% (Fig. 7(B)), ethane selectivity below 10% (Fig. 7(C)), and green oil selectivity exceeding 40% (Fig. 7(D)). However, the catalyst stability showed a different trend. The initial activity of the catalyst reached 100% after the first regeneration and remained stable for a considerable period of time (15.77 h), which may be because, after H2 treatment, part of the green oil decomposed so that the pores were reopened and the active sites were exposed, resulting in recovery of acetylene conversion, which is consistent with the results observed by previous research.35,36 As the regeneration times increased, the stabilization time of the catalyst decreased slowly and the catalytic activity decreased. After the second regeneration, this stabilized above 99% for 7.97 h. However, after the third regeneration, the initial activity of the catalyst was only 96%, and the catalyst was rapidly deactivated after stabilization above 91% for 8.4 h. After each of subsequent regeneration, the activity and stability of the catalyst gradually worsened and even the initial activity did not reach 70% after the fifth regeneration. This may be due to hydrocarbons that cannot be removed by regeneration; as the number of regenerations increases, they may constantly accumulate, progressively shortening the life of the catalyst after regeneration so that it eventually fails to achieve its initial activity and rapidly deactivated.
Fig. 8 TEM images of the (A) 10%Ni/MCM-41 after reaction 7.5 h, (B) 1%Ni/MCM-41 after reaction 126 h. |
We also performed FT-IR on the catalyst after the reaction, and the results are displayed in Fig. 9. Compared with the catalyst before the reaction, that after the reaction showed many different characteristic peaks. The absorption peaks at 2854 and 2924 cm−1, and 2869 and 2955 cm−1, were ascribed to vibration of the –CH2 and –CH3 groups, respectively.38,39 The peak at 1456 cm−1 was ascribed to the CC group, indicating that CC was present in the catalyst after the reaction.40 These results indicated the presence of aliphatic coke on the catalyst surface. As this was soft coke, it could be removed after regeneration, which is why the catalyst could regain some of its activity.
To further determine the amount of hydrocarbon deposition on the catalyst surface after the reaction, TG-DTG characterization of the 1%Ni/MCM-41 catalyst before and after the reaction was performed, as shown in Fig. 10. Both samples showed significant weight loss around 100 °C attribute to the removal of water. The fresh catalyst showed no weight loss above 100 °C (Fig. 10(A)), indicating that it had good thermal stability. However, after the reaction, we can observe from the DTG curve (Fig. 10(B)) that the catalyst had two clear weight losses at 341.7 °C and 605.1 °C, corresponding to different degrees of combustion of acetylene polymer/green oil in air. According to the literature,41–43 the weight loss at lower temperatures in the range 200–470 °C is attributed to soft hydrocarbon adsorbed in the catalyst pores or on the surface, which can be removed by treatment at high temperature or solvent extraction, and the weight loss above 470 °C is due to a hard graphite-like hydrocarbon, which can only be removed by oxygen treatment at high temperature. According to the TG curve that the weight losses of the catalyst in these two temperature ranges were 21.8 wt% and 54.7 wt%, respectively; therefore, most of the weight loss occurs above 470 °C, indicating that after repeated regeneration, the deposits on the catalyst surface are mostly graphite-like hydrocarbons that are difficult to remove. Additionally, Table 2 showed the structural parameters of the catalyst before and after reaction 126 h. The specific surface area and pore volume of the catalyst after the reaction 126 h are almost zero, indicating that the catalyst is completely covered by green oil after the reaction 126 h, which is consistent with TG results.
Catalyst | Surface area (m2 g−1) | Pore volume (cm3 g−1) | Pore size (nm) |
---|---|---|---|
1%Ni/MCM-41 fresh | 793 | 0.6008 | 3.03 |
1%Ni/MCM-41 used | 0.0738 | 0.000551 | — |
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