SiO2 Catalyst: Effect of P

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Kinetics, Catalysis, and Reaction Engineering

Dehydrogenation of Isobutane over Ni-P/SiO2 Catalyst: Effect of P Addition Qingqing Zhu, Shan Zhang, Huanling Zhang, Guowei Wang, Xiaolin Zhu, and Chunyi Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00032 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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Industrial & Engineering Chemistry Research

Dehydrogenation of Isobutane over Ni-P/SiO2 Catalyst: Effect of P Addition By Qingqing Zhu, Shan Zhang, Huanling Zhang, Guowei Wang*, Xiaolin Zhu, and Chunyi Li* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, PR China

*Corresponding authors:

Guowei Wang (E-mail: [email protected]) & Chunyi Li (E-mail: [email protected])

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The addition of P to Ni/SiO2 catalyst greatly changes isobutane dehydrogenation performance. Ni:P ratio is an important influencing factor for Ni-P/SiO2 catalyst due to the formation of different Ni-P compounds. When Ni:P ratio equals to 1:1, Ni-P/SiO2 catalyst with Ni2P formed on the surface exhibits the optimum dehydrogenation performance (isobutane conversion of 22% and isobutene selectivity of 81.3%). The in-situ FTIR characterization was performed to determine the adsorption mode of isobutane. Isobutane is adsorbed onto the Ni-P surface with one H atom in a methyl group, and the intermediate tends to form isobutene rather than C-C bonds scission. Moreover, the decreased adsorption energy of isobutene on Ni2P surface inhibits its further reactions. The deactivation of Ni-P/SiO2 catalyst is mainly caused by the phase transformation from Ni2P to Ni12P5 and coke deposition. Finally, some general rules are summarized to get a deep understanding of the second component added to Ni/SiO2 catalyst. KEYWORDS: isobutane dehydrogenation, Ni2P, P addition, isobutene, adsorption mode

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1. INTRODUCTION Supported Pt and CrOx catalysts have been widely used in dehydrogenation of alkanes for industrial production.1-7 Due to their high cost and environmental issues, the studies focused on non-noble metal catalysts, such as supported Ni, Mo, Fe, and Sn catalysts, have received widespread attentions in recent years.8-11 Single-component Pt catalysts exhibit relatively low selectivity to olefins and deactivate rapidly for alkane dehydrogenation;12,13 therefore, in order to modify dehydrogenation performances, the addition of the second component is inevitable,14 and the most studied is Sn.15-18 The introduction of Sn can selectively facilitate the formation of olefins by reducing Pt crystallites size and donating an electron from Sn to Pt atom; moreover, Sn is conductive to the migration of coke from active Pt to support to enhance catalyst stability.19,20 Regarding supported Ni catalysts, the rupture of C-C bonds can be effectively inhibited after the addition of Sn.21,22 Sulfur plays a similar role. The addition of sulfur to Ni-based catalysts can significantly improve the selectivity to isobutene in our previous works.8,23 Besides S and Sn, P addition to Ni-based catalysts also exhibit a positive effect in alkane dehydrogenation.24 Supported Ni-P catalysts have been widely used in catalytic hydrogenation in the reported literature.25-29 Since dehydrogenation and hydrogenation are reverse reactions, research on Ni2P for alkane dehydrogenation have been performed in recent years.30-32 Wang and coworkers proved that the selectivity to isobutene for isobutane dehydrogenation over Ni2P supported on activated carbon (AC) was much higher than that over a Ni/AC catalyst under the same reaction conditions.31 Moreover, for ethane dehydrogenation to ethene, P addition to Mo/ZSM-5 catalyst also promoted ethene selectivity and decreased the yield to coke.33

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Although the addition of Sn, P, and S to Ni catalysts all effectively facilitate the formation of olefins in alkane dehydrogenation,8,21-24,30,34 the reaction performance, especially the stability of the modified Ni-based catalyst, is significantly different. For the Ni-Sn/SiO2 catalyst, isobutane conversion along with selectivity to isobutene remains stable for 200 h.21,22 On the contrary, Ni-S and Ni-P catalysts deactivate much more rapidly.23,30,32 The different catalyst stability mainly results from different properties of these three elements, such as atomic radius, electronegativity and interaction strength with Ni. In this paper, Ni-P/SiO2 catalysts with different molar ratio of Ni to P were prepared and evaluated for isobutane dehydrogenation. Characterized by XRD, XPS, and Py-IR, et al, Ni-P/SiO2 catalyst exhibits different Ni-P compounds and acid properties when the molar ratio of Ni to P changes, which exerts a great influence on dehydrogenation performance. To explain the effect of P addition, the adsorption modes of isobutane and isobutene on the catalyst with the optimal Ni:P ratio were determined by in-situ FTIR. In addition, the stability of Ni-P/SiO2 catalyst was investigated, and rapid deactivation is probably caused by the transformation of Ni-P phase and coke deposition. Finally, combined with our previous works, a better understanding of the effect of the secondary component on Ni/SiO2 catalyst is obtained. 2. EXPERIMENTAL 2.1. Catalyst Preparation Ni-P/SiO2 catalysts used in this work were prepared by excessive wetness impregnation. 1.95 g Ni(NO3)2·6H2O and a certain amount of NH4H2PO4 were dissolved in 12.4 g deionized water. Then the obtained solution was added to 9.5 g SiO2 particles, purchased from Qingdao Ocean Chemical Co. LTD (particle size: 60-80 mesh, purity: ≥ 99.9%). After standing for 3 h at room temperature, the mixture was dried at 100 °C overnight and calcined at 650 °C in air for 2 h. The content of NiO in NiO/SiO2 was 5.0 wt%. The catalyst precursors were reduced under H2 atmosphere at 650 °C for 1 h and cooled to room 4


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temperature at N2 flow. These as-prepared catalysts are named as 5Ni-P/SiO2-x, where x represents the molar ratio of Ni to P. The P/SiO2 catalyst was prepared by the same method mentioned above, and the amount of P was the same as that in 5Ni-P/SiO2-1.0 catalyst. 2.2. Catalyst Characterization XRD (X-ray diffraction) of the catalyst was carried out on a HAOYUAN DX-2700BH diffraction system using Cu Kα radiation at 40 mA and 40 kV, running from 35° to 80° with a speed of 0.5° min-1. XPS (X-ray photoelectron spectra) were recorded by using an ESCALab-250Xi apparatus with an AlKα X-ray excitation source. The data processing involved background subtraction by using the Shirley method. N2 adsorption-desorption measurements were carried out with a Quadrasorb SI instrument at 77 K to determine the BET (Brunauer-Emmett-Teller) specific surface area and pore properties of catalysts. In order to remove the adsorbed moisture, all samples were evacuated at 300 °C for 5 h at the pressure of 0.1 mmHg prior to the measurements. The TEM images were obtained using a JEM 2010 high-resolution transmission electron microscope operated at 120.0 kV. The acid sites of catalysts were determined by pyridine-IR. The infrared spectra of adsorbed pyridine on 5Ni-P/SiO2-x catalysts were collected using a Bruker Tensor 27 infrared spectrometer in the range of 4000-600 cm-1. The background spectrum and that of chemisorbed pyridine were recorded at 200 °C. Due to the oxidation of Ni-P compounds in air at high temperature, the amount of coke in deactivated catalyst cannot be detected only by TG-DTA (the thermogravimetry/differential thermal analysis) (Figure S1). The amount of coke deposited on the deactivated catalyst was determined by a 1HW(T) HF infrared absorption C/S instrument. Fully dried CaCO3 powders (50.0 mg) were loaded in a crucible, and the temperature was rapidly raised to 950 °C in order to ensure the complete decomposition of CaCO3. The signal of formed CO2 was detected by an infrared detector at the same time. About 250 mg catalyst was put 5



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in another crucible, and it was heated to 950 °C under O2 flow. The amount of coke can be calculated based on the IR signals of CO2 formed from CaCO3 and catalyst. A detailed description of this method is presented in Supporting Information. The redox performance of 5Ni-P/SiO2-x catalysts were determined by H2-TPR (temperature programmed reduction of H2) at PCA-1200 chemical adsorption analyzer. During the test, 0.1 g catalyst precursor particles (20-60 mesh) were pretreated at a flow of Ar at 600 °C for 30 min with a heating rate of 10 °C min-1, and then temperature was reduced to 100 °C. A mixture of 10 vol.% H2/N2 (30 mL min-1) was brought into contact with catalyst, heated to the final temperature at a rate of 10 °C min-1 subsequently. The TCD signal was recorded in real time. 2.3. Catalytic Activity Test Dehydrogenation of isobutane was conducted in a fixed-bed microreactor with an inner diameter of 12 mm. 2.0 g catalyst particles with the size of 60-80 mesh were located in the reactor. Before reaction, catalysts were pretreated under H2 at 650 °C for 30 min; afterwards, temperature was cooled to required temperature under N2 flow (30 mL min-1). During the test, isobutane without H2 or inert gas was fed to the reactor. Each gas product was collected for 10 min, which means that the conversion and selectivity at each time were the average data in 10 min. Thus, the initial conversion and selectivity were obtained at the first 10 min. The composition of gas products was analyzed by a Bruker 450 Gas Chromatograph. The mass of coke deposited on the catalyst after reaction was determined by carbometer. The conversion of isobutane and product selectivity were calculated as follows (eqs 1 and 2): isobutane conversion (mol%) =

𝑚𝑜𝑙 𝑖𝑠𝑜𝑏𝑢𝑡𝑎𝑛𝑒𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑚𝑜𝑙 𝑖𝑠𝑜𝑏𝑢𝑡𝑎𝑛𝑒𝑓𝑒𝑑

𝑚𝑜𝑙 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑖𝑓𝑜𝑟𝑚𝑒𝑑

selectivity (mol%) = 𝑚𝑜𝑙 𝑖𝑠𝑜𝑏𝑢𝑡𝑎𝑛𝑒𝑟𝑒𝑎𝑐𝑡𝑒𝑑 ×

𝑁𝑖

where Ni represents the number of carbon atoms in product i. 6


4

× 100

× 100%

(1) (2)

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2.4. In-situ FTIR Measurement Infrared spectra of adsorbed isobutane and isobutene on Ni-P/SiO2 catalyst were collected by a Bruker TENSOR 27 FTIR spectrophotometer (with an instrumental resolution of 2 cm-1). Before measurement, 20 mg catalyst powder was pressed into a thin wafer with a diameter of 13 mm, and then located in an in-situ transmission cell provided by DICP. The catalyst wafer was pretreated under H2 flow (30 mL min-1) at 650 °C for 1 h. Afterwards, temperature was cooled down to 25 °C in N2 with the flow of 70 mL min-1, and the background signal was collected. Subsequently, N2 was switched to isobutane (or isobutene) flow (30 mL min-1). After 1 h, gaseous isobutane (isobutene) was gradually removed by vacuum, and IR spectra under different pressures were collected at the same time. The measurement was repeated to exclude the background noise and obtain reproducible infrared spectra. 3. RESULTS AND DISCUSSIONS 3.1. Catalysts Characterization XRD patterns of 5Ni/SiO2 catalysts before and after P addition are shown in Figure 1. Obviously, the characteristic diffraction peaks due to metallic Ni disappear after the introduction of P. Instead, different Ni-P compounds, such as Ni3P, Ni12P5, and Ni2P, can be detected in 5Ni-P/SiO2-x catalysts. As for 5Ni-P/SiO2-3.0 catalyst, diffraction peaks attributed to Ni3P can be observed. With the decrease of the molar ratio of Ni to P to 2.0, Ni12P5 can be detected. Further decreasing the ratio leads to the formation of Ni2P. That is, increased amount of P in catalyst precursor facilitates the formation of Ni2P phase. Even if excess amount of P species is added, no peaks due to phosphorus or phosphate are visible, from which we conclude that these crystallites are too small to be detected. It can be seen from Figure 1 that the addition of P to Ni/SiO2 catalyst changes the existing state of Ni crystallites.

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Figure 1. XRD patterns of 5Ni-P/SiO2-x catalysts. To further study the effect of P, XPS measurements of 5Ni-P/SiO2-x catalysts were carried out. According to the reported literature,22,35 the Ni 2p peak of metallic Ni is located at 852.3 eV, and that of Niδ+ in Ni-P compound can be detected in the range of 853.5-852.2 eV. As shown in Figure 2a, the obvious Ni 2p peaks observed at 853.0 and 852.4 eV indicate the formation of Ni3P and Ni2P,36 which is consistent with the results of XRD patterns shown in Figure 1. That is, due to the decreased electron density of Ni atoms by donating electrons to P atoms,37 Ni atoms exhibit positive partial charges. With regard to P species, the P 2p binding energies of phosphate species and Pδ- in Ni2P are centered at 134.3 and 129.3 eV, respectively.38 In addition, as the H2-TPR profiles of 5Ni-P/SiO2-x catalyst (Figure S2) prove the presence of phosphate. During the preparation process of the catalyst, nickel phosphate can be formed on the catalyst after calcination. According to reported literatures,39-41 in the experiment of H2-TPR, Ni2+ can be reduced to metallic Ni, and then H2 molecules dissociatively adsorb on Ni surface to generate H atoms, which spill over to the adjacent P atoms and reduce them to phosphorus or phosphine. At last, these phosphorus species react with metallic Ni to form Ni-P compounds. As the bond energy of P-O in phosphate is stronger than that of Ni-O bond, a higher reduction temperature is needed. As reported in 8


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literatures,42,43 the peaks in H2-TPR profiles below 550 ºC are attributed to the reduction of NiO, while the reduction temperature of phosphate is generally in the range of 550-800 ºC. According to the H2-TPR profiles in Figure S2, phosphate in the precursors of 5Ni-P/SiO2-1.0 and 5Ni-P/SiO2-1.0 catalysts cannot be completely reduced under the reduction temperature of 650 ºC prior to reaction. As for 5Ni-P/SiO2-3.0 catalyst, the peaks attributed to P species are difficult to be checked. Only one small peak located at 134.3 eV can be detected for 5Ni-P/SiO2-1.0 catalyst. 5Ni-P/SiO2-0.3 catalyst shows a recognizable peak at 129.3 eV attributed to Pδ- of Ni2P, which is consistent with the XRD patterns.

Figure 2. XPS spectra (Ni 2p (a) and P 2p (b)) of 5Ni-P/SiO2-x catalysts: (1) 5Ni-P/SiO2-3.0 catalyst; (2) 5Ni-P/SiO2-1.0 catalyst; (3) 5Ni-P/SiO2-0.3 catalyst. The introduced P species in 5Ni/SiO2 catalyst not only change the existing state of Ni but also generate new acid sites. As shown in Figure 3, the adsorption of pyridine on 5Ni-P/SiO2-x catalysts gives rise to three infrared bands in the range of 1600-1400 cm-1. Specifically, these band centered at 1547, 1492 and 1448 cm-1 are contributed to vibrational modes of the pyridine ring adsorbed onto Brønsted acid sites, Brønsted and Lewis acid sites, and Lewis acid sites, respectively.44 The area of the band at 1547 cm-1 9



increases significantly with the increase of P; while the band areas peaked at 1448 cm-1 increase slowly. Combined with the variation trend of peak area for P5+ species at 134.3 eV (Figure 2b), the Brønsted acid sites in 5Ni-P/SiO2-x are likely to be related with P-OH sites derived from incompletely reduced phosphate (Figure S2). With regard to NH3-TPD profiles of 5Ni-P/SiO2-a catalysts (a = 1.0, 0.5 and 0.3) shown in Figure S3, the peak temperatures are 188, 197 and 198 ºC, respectively, which are belonged to NH3 molecules adsorbed on weak acid sites, and the number of the acid sites is increased with the decrease of Ni:P ratio (Table S1).

Figure 3. Infrared spectra of adsorbed pyridine on 5Ni-P/SiO2-x catalysis (x = 2.0, 1.0, 0.5 and 0.3). 3.2. Catalytic Performance of Ni-P/SiO2 Catalyst for Isobutane Dehydrogenation 3.2.1. Isobutane Dehydrogenation over Ni-P/SiO2 Catalyst with Different Ni:P Ratio As listed in Table S2, SiO2 supported exhibits extremely low activity, and thus, the reactions of isobutane mainly occur on Ni and Ni-P surface. Methane is the only hydrocarbon product for isobutane reacting over 5Ni/SiO2 catalyst at 600 °C under GHSV (gas hourly space velocity) of 150 h-1 (Table 1); even GHSV is increased to 1200 h-1, only trace amount of isobutene is detected (Table S3), which is caused by C-C bonds scission catalyzed by the aggregated Ni particles on SiO2. In our previous works, S or Sn was introduced to Ni-based catalyst to modify its dehydrogenation performance,8,21,22 here P was added instead. 10


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For isobutane dehydrogenation over 5Ni-P/SiO2-1.0 catalyst, the selectivity to isobutene increases sharply to 81.3% (Table 1). In Ni2P, every Ni atom is tetrahedrally surrounded by four P atoms,45,46 indicating that the aggregated Ni atoms active for C-C bonds scission are dispersed on a molecular scale. Therefore, different catalytic performances before and after P addition are probably attributed to the formation of Ni2P. Table 1 Gaseous product distribution of isobutane dehydrogenation over 5Ni-P/SiO2-x catalysts. Ni:P (molar ratio) 1:0

3:1

2:1

1:1

1:2

1:3

100

84.0

10.4

21.6

19.4

17.4

methane

100

93.3

5.9

3.1

4.2

4.3

ethane

0.0

2.0

0.2

0.2

0.4

0.3

ethene

0.0

0.3

0.8

0.7

3.1

2.4

propane

0.0

0.0

3.1

1.8

2.8

3.7

propene

0.0

0.0

11.8

9.5

13.2

14.2

n-butane

0.0

0.0

0.1

0.1

0.2

0.4

n-butenes

0.0

0.0

1.3

2.4

18.7

20.8

isobutene

0.0

4.5

76.1

81.3

54.6

50.0

1,3-butadiene

0.0

0.0

0.7

0.8

2.8

3.9

Conversion(%)

Selectivity (%)

Reaction conditions: temperature = 600 °C, mass of catalyst = 2.0 g, GHSV = 150 h-1. Actually, different Ni-P compounds exhibit different dehydrogenation performance. With regard to isobutane dehydrogenation on Ni3P surface in 5Ni-P/SiO2-3.0 catalyst, the selectivity to isobutene is only 4.5% with isobutane conversion of 84.0%, demonstrating the scission of C-C bonds is inevitable. As Ni3P 11



(Figure S4(a)) surfaces are constituted by Ni4P4 and Ni8-terminated surfaces,47 the existence of assembled multiple Ni atoms active for C-C bonds rupture is quite possible. As for 5Ni-P/SiO2-2.0 catalyst, isobutane conversion reduces to 10.4%, while isobutene selectivity rises to 76.1% due to the formation of Ni12P5. As listed in Table 1, when the molar ratio of Ni to P is reduced to 1:1, the highest selectivity to isobutene (81.3%) can be obtained; and at the same time, the conversion of isobutane is 21.6%. These facts manifest that the catalytic activity of Ni2P for isobutane dehydrogenation is much higher than that of two other Ni-P compounds. Actually, the distribution of atoms and electronic properties are different among these three Ni-P compounds. Specifically, the number of P atoms surrounding Ni atoms increased in the following order: Ni3P, Ni12P5 and Ni2P (Figure S4).37,48 That is, the increased amount of P in Ni-P compounds improves the selectivity to isobutene to some extent. Given the low dehydrogenation activity of P/SiO2 under the same reaction conditions (Table S4), the active sites for isobutane dehydrogenation are probably the Ni atoms in Ni2P crystallites. However, the introduction of excessive P species cannot facilitate the formation of isobutene. The selectivity to isobutene is decreased to 54.6% over 5Ni-P/SiO2-0.5 catalyst, and that to C1-C3 hydrocarbons and linear butenes rise apparently. In addition, C-C bond scission and the formation of n-butenes are further exacerbated over 5Ni-P/SiO2-0.3 catalyst. n-Butenes can be formed by the isomerization of isobutene or dehydrogenation of n-butane. Because of almost no strong acid sites existing on 5Ni-P/SiO2 surface (Figure S3) as well as the low amount of n-butane in the produces (Table 1), the formation of n-butenes is probably caused by the former route, skeleton isomerization of isobutene. According to XPS spectra (Figure 2) and IR spectra of adsorbed pyridine (Figure 3), the superficial OH groups connected to phosphate species which are not reduced promote the formation of byproducts. 3.2.2. Isobutane Dehydrogenation over Ni-P/SiO2 Catalyst under Different Reaction Conditions 12


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Since the 5Ni-P/SiO2-1.0 catalyst exhibits relatively good dehydrogenation performance, the reaction conditions have been optimized. Catalytic dehydrogenation is an endothermic reaction, so the increasing temperature facilitates the activation and rupture of the C-H bond. Consequently, the conversion of isobutane increases from 11.6% to 28.4% when temperature changes in a range of 560-620 °C (Figure 4a). At the same time, it should be noted that higher temperature also promotes C-C bond scission as the secondary reactions of formed isobutene (Table S5), which causes the lower selectivity to isobutene (85.0% → 73.9%). As shown in Figure 4b, the shorter restrained time due to the increased GHSV prevents further reaction of generated isobutene (Table S6). Therefore the selectivity to isobutene is increased and the formation of coke is inhibited.

Figure 4. Catalytic performance of 5Ni-P/SiO2-1.0 catalyst for isobutane dehydrogenation under different reaction conditions: (a) the effect of temperature (mass of catalyst = 2.0 g, GHSV = 150 h-1); (b) the effect of GHSV (mass of catalyst = 2.0 g, temperature = 600 °C). 3.3. Adsorption Modes of Isobutane and Isobutene on Ni-P/SiO2 Catalyst Although the addition of moderate amounts of P to the 5Ni/SiO2 catalyst significantly prevents C-C bonds scission, the selectivity to isobutene is lower than that over the Ni-Sn/SiO2 catalyst in our previous work.21 It is speculated that the intermediates formed on Ni, Ni-P and Ni-Sn surfaces are probably different, which can be determined by the adsorption mode of isobutane. Generally, H atoms instead of C atoms in 13



alkane molecules tend to interact with the metal surface upon adsorption,49,50 and the adsorbed C-H bond leads to a significant shift of the corresponding IR bands. Moreover, for double-site adsorption, the shift of ν(C-C) is obviously larger than that for single-site adsorption.

Figure 5. Infrared spectra of isobutane adsorbed on 5Ni-P/SiO2-1.0 catalyst. The infrared spectra of isobutane adsorbed onto the 5Ni-P/SiO2-1.0 catalyst are shown in Figure 5. For comparison, the assignment of the IR bands of gaseous isobutane is listed in Table S7. In the region of 3000-2800 cm-1, the bands at 2978 and 2952 cm-1 belong to stretching vibration of C-H bonds in methyl are shifted to 2980 and 2958 cm-1, respectively. The band at 1380 cm-1, attributed to δs(CH3), is shifted to 1386 cm-1 after adsorption. Shifts of infrared bands due to the C-H bond in the methylidyne, 2892 cm-1→2890 cm-1, and 1334 cm-1→1336 cm-1, are obviously smaller. Moreover, the bands centered at 2966, 2880, 2870, 1490, 1477, 1466 and 1394 cm-1 exhibit no obvious shifts. That is, the electrons of C-H bonds in methyl undergo the largest change after adsorption. In addition, a ν(C-C) shift of 3 cm-1 is much less than that of

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the double-site adsorption isobutane.21 Thus, the adsorption of isobutane on Ni-P surface may be of the single-site type with one H atom in a methyl group, as shown in Scheme 1a. For isobutane adsorbed on 5Ni/SiO2 catalyst in our previous work,21 two H atoms in different methyl groups are interacting with the Ni surface. The formed intermediate tends to the rupture of the C-C bond, leading to the generation of abundant methane instead of isobutene.51,52 On the contrary, after the addition of P, only one H atom in a methyl is adsorbed on active site. In this case, the rupture of two C-H bonds in both a methyl and the methylidyne to generate isobutene is quite possible, which is consistent with reported literatures.53,54 That is, C-C bonds breaking can be effectively inhibited on the Ni2P surface.

Scheme 1. Adsorption modes of isobutane (a) and isobutene (b) on the 5Ni-P/SiO2-1.0 catalyst. The adsorption mode of isobutene reflects its desorption behaviors to some extent, which influences the product distribution for isobutane dehydrogenation. As for the infrared spectra of isobutene adsorbed onto the 5Ni-P/SiO2-1.0 catalyst shown in Figure 6, the C=C stretching band centered at 1660 cm-1 is shifted to 1648 cm-1, which is the largest shift in the infrared spectra. With regard to the bands due to C-H bonds in vinylidene in the regions of 3100-3000 cm-1 (ν(=CH2)) and 1300-1250 cm-1 (=CH2 in-plane bend)55 listed in Table S8, these shifts are observed as follows: 3097 cm-1→3099 cm-1, 3075 cm-1→3073 cm-1, 1292 cm-1→1290 cm-1, and 1268 cm-1→1265 cm-1. The existence of these bands indicates that the C=C bond is not destroyed during the adsorption. As for the bands assigned to the stretching vibration and bending vibration of C-H bonds in methyl, only three shifts can be observed: 2926 cm-1→2924 cm-1, 1456 15



cm-1→1458 cm-1, and 1392 cm-1→1394 cm-1. In addition, there is a shift of 3 cm-1 contributed to C-C bond centered at 1052 cm-1. The above detected small shifts are generally caused by physical adsorption or background noise. Therefore, on the surface of 5Ni-P/SiO2-1.0 catalyst, isobutene is probably adsorbed in a single-site mode with C=C bond.

Figure 6. Infrared spectra of isobutene adsorbed on 5Ni-P/SiO2-1.0 catalyst. For the π-bonded olefin adsorption species on metal surface, the adsorption energy is generally proportional to the shift of the infrared band attributed to C=C bond.56,57 The shift of ν(C=C) for isobutene adsorbed on Ni2P surface is 12 cm-1, which is obviously smaller than that on Ni surface (18 cm-1).21 The significantly decreased adsorption strength of isobutene can effectively prevent its further reactions. 3.4. The Deactivation Origin of the Ni-P/SiO2 Catalyst The stability of the 5Ni-P/SiO2-1.0 catalyst was investigated under the optimized reaction conditions. As shown in Figure 7, the conversion of isobutane along with the selectivity to isobutene is decreased with 16


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the extension of reaction time. The initial isobutane conversion of 21.6% decreases to 11.0% after 120 min; at the same time, the selectivity to isobutene drops from 81.3% to 76.3%. Compared with the excellent stability of the Ni-Sn/SiO2 catalyst for isobutane dehydrogenation in our previous works,21,22 the deactivation of 5Ni-P/SiO2-1.0 catalyst is much more rapid. In order to determine the origin of catalyst deactivation, the phase analyses after different reaction time were carried out. It is obvious from Figure 8 that the characteristic diffraction peaks attributed to Ni12P5 become increasingly intensified with longer time on stream, while the peaks due to Ni2P diminish simultaneously. P atoms located in the Ni lattice weaken the interaction between Ni atoms, resulting in an unstable crystal structure.45 According to the on-line MS spectra of isobutane dehydrogenation (Figure S5), the intensity of isobutane signal gradually increases after 18 min of continuous reaction, indicative of the deactivation of the catalyst. Furthermore, the MS (mass spectra) signal of PH3 can be detected during the reaction, demonstrating P loss in the form of PH3. That is, during the reaction, the active Ni2P is converted to Ni12P5, with low dehydrogenation activity, which is one reason for catalyst deactivation. Similar to our previous works for isobutane dehydrogenation over Ni-S catalysts, S loss by H2S is also inevitable, leading to the decrease of dehydrogenation activity.8,34 In addition, the average diameter of Ni-P compound is 7.5 nm in the fresh 5Ni-P/SiO2-1.0 catalyst, which increases to 10.9 nm after 120 min reaction (Figure S6); that is, the sintering of Ni-P compound during reaction is inevitable.

17



Figure 7. Catalytic performance of the 5Ni-P/SiO2-1.0 catalyst for isobutane dehydrogenation with time on stream (temperature = 600 °C, GHSV = 150 h-1).

Figure 8. XRD patterns of 5Ni-P/SiO2-1.0 catalysts at different reaction time. As for coke formation, the amount of coke on the catalyst is 0.58 wt% after 120 min reaction. In addition, as the pore properties of 5Ni-P/SiO2-1.0 catalyst before and after reaction exhibit little variation (Table S9), the generated coke is mainly deposited on the catalyst surface rather than plugging pores. The amount of coke formed during the first 30 min is 0.35 wt%, which accounts for 60% of the total amount of coke generated during the reaction shown in Figure 7, and thus the reduction of isobutane conversion in the 18


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first 30 min is much more rapid. Unlike Sn in Pt-Sn-based catalysts,19 P species in Ni2P and Ni12P5 cannot transfer coke from active sites to support; therefore, the decreased Ni-P surface by the coverage of coke lowers the conversion of isobutane. In short, the deactivation of 5Ni-P/SiO2-1.0 catalyst is probably caused by the phase change of Ni-P compounds and coke deposition on active sites. 3.5. The Role of the Second Component Added to Ni/SiO2 for Isobutane Dehydrogenation For the supported Ni catalyst, the addition of Sn, S and P changes the reaction of isobutane from cracking to dehydrogenation.8,22,34,58 According to the reaction and characterization results for isobutane dehydrogenation over Ni/SiO2 catalyst with different components introduced some universal rules can be summarized here. First of all, the amount of A (the added component) atoms embedded in Ni crystallites should be optimized. Ni crystallites cannot be well dispersed with a low amount of A, leading to a high activity for C-C bonds scission. As listed in Table 1, almost no isobutene can be produced on Ni3P surface if the molar ratio of Ni to P equals to 3.0; the similar reaction results have been obtained over Ni3Sn alloy for Ni-Sn-based catalyst.22 The excess amount of A may result in the coverage of active surface and the change of acid properties, which is unfavorable for isobutene formation. Secondly, when an olefin molecule is adsorbed onto the Ni surface, there is the d-π* back-donating bonding between the 3d orbital of the Ni atom and the anti-bonding orbital of C=C bond, and the increased electron density of Ni atom can reduce the strength of this bond.21,53 Although well dispersed Ni crystallites can be formed after the addition of Sn, S and P, the charge properties of Ni species are different. The sequence of the binding energies of Ni 2p increases in the following order: Ni-Sn/SiO2 (851.9 eV), Ni-S/SiO2 (852.4 eV), and Ni-P/SiO2 (852.8 eV),22,23 consistent with the decrease sequence of isobutene selectivity (Table S10). That is, Niδ- is more beneficial to isobutene generation. 19



Last but not least, after the introduction of atom A, the structure stability of the newly formed NixAy compound on the catalyst surface is an extremely important factor regarding the stability of the catalyst. If a stable alloy phase can be formed after the addition of A to supported Ni catalyst, such as Ni-Sn alloy, the stability of the catalyst is much higher.21,22 However, the introduction of large atoms, such as P and S, can significantly weaken metal bonds through occupying the inner space of the metal lattice and forming metal interstitial compounds, which results in an unstable crystal structure and reduce catalytic performance under high temperature. Specially, the loss of P and S23 can be detected during isobutane dehydrogenation over Ni-P/SiO2 and Ni-S/SiO2 catalysts. Therefore, the stability of Ni-A/SiO2 catalyst largely depends on the structure of the NixAy compound produced. 4. CONCLUSIONS The introduction of P to the 5Ni/SiO2 catalyst significantly improves the catalytic performance for isobutane dehydrogenation. 5Ni-P/SiO2 catalysts with different molar ratio of Ni to P were prepared and characterized by XRD, XPS and Py-IR, et al, indicative of the different existing states of Ni and P on the catalyst surface. As for isobutane reacting over 5Ni-P/SiO2-3.0 catalyst, the selectivity to isobutene is only 4.5% with a high isobutane conversion (84.0%); that is, C-C bonds scission is inevitable on the Ni3P surface. With the ratio decreased to 2:1, a Ni12P5 phase is formed, and the conversion reduces to 10.4% while isobutene selectivity rises to 76.1%. The highest selectivity to isobutene of 81.3% is obtained on Ni2P surface for the 5Ni-P/SiO2-1.0 catalyst. As the ratio is further decreased, Brønsted acid sites can be produced inevitably, leading to the aggravation of the secondary reactions of generated isobutene and the rupture of C-C bonds. In order to get a deep understanding of the reaction result, the adsorption modes of isobutane and isobutene on 5Ni-P/SiO2-1.0 catalyst were determined by in-situ FTIR. Isobutane is adsorbed on Ni2P surface with one H atom in a methyl, and the generated intermediate is prone to form isobutene 20


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instead of methane. In addition, the further reactions of isobutene are effectively inhibited as the adsorption energy of isobutene on Ni2P surface is much lower than that on Ni surface. The deactivation of the 5Ni-P/SiO2-1.0 catalyst is mainly caused by the transformation of active Ni2P to low activity Ni12P5 by P loss and the formation of coke on active sites. In combination with our previous works on Ni-Sn/SiO2 and Ni-S/SiO2 catalysts, the above results suggest that the second components added to Ni/SiO2 catalyst obey the following rules: Firstly, the amount of additives should be optimized. Secondly, Niδ- is more beneficial to isobutene production than Niδ+. Last but not least, the structural stability of the compound formed by Ni and the added component is strongly dependent on the stoichiometric ratio. ASSOCIATED CONTENT Supporting Information The principle and method of carbometer, TG-DTA curves of deactivated catalyst, H2-TPR profiles, NH3-TPD curve, real time MS spectra of isobutane dehydrogenation over 5Ni-P/SiO2-1.0 catalyst, reaction results of isobutane on SiO2, Ni/SiO2, and P/SiO2 catalysts, reaction results of isobutene on Ni-P/SiO2 catalyst, assignment of gaseous isobutane and isobutene, pore property of fresh and deactivated Ni-P/SiO2 catalyst, the conversion of isobutane and selectivity to isobutene over 5Ni-P/SiO2, 5Ni-Sn/SiO2 and Ni-S/SiO2 catalysts. AUTHOR INFORMATION Corresponding authors E-mails: [email protected] (G. Wang); [email protected] (C. Li). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS 21



This work was financially supported by the National Natural Science Foundation of China (21606257, 21706288), the Natural Science Foundation of Shandong Province (ZR2017BB020), and the Fundamental Research Funds for the Central Universities (18CX02016A, 17CX02015A). REFERENCES (1) Wang, H.-Z.; Sun, L.-L.; Sui, Z.-J.; Zhu, Y.-A.; Ye, G.-H.; Chen, D.; Zhou, X.-G.; Yuan, W.-K., Coke Formation on Pt–Sn/Al2O3 Catalyst for Propane Dehydrogenation. Ind. Eng. Chemistry Res. 2018, 57(26), 8647-8654. (2) Liu, J.; Zhou, W.; Jiang, D.; Wu, W.; Miao, C.; Wang, Y.; Ma, X., Isobutane Dehydrogenation over InPtSn/ZnAl2O4 Catalysts: Effect of Indium Promoter. Ind. Eng. Chem. Res. 2018, 57(33), 11265-11270. (3) Sattler, J.J.H.B.; Ruiz-Martinez, J.; Santillan-Jimenez, E.; Weckhuysen, B.M., Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides. Chem. Rev. 2014, 114: 10613-10653. (4) Zha, S.; Sun, G.; Wu, T.; Zhao, J.; Zhao, Z.-J.; Gong, J., Identification of Pt-Based Catalysts for Propane Dehydrogenation via a Probability Analysis. Chem. Sci. 2018, 9(16), 3925-3931. (5) Ricca, A.; Palma, V.; Iaquaniello, G.; Palo, E.; Salladini, A., Highly Selective Propylene Production in a Membrane Assisted Catalytic Propane Dehydrogenation. Chem. Eng. J. 2017, 330: 1119-1127. (6) Fridman, V.Z.; Xing, R., Investigating the CrOx/Al2O3 Dehydrogenation Catalyst Model: II. Relative Activity of the Chromium Species on the Catalyst Surface. Appl. Catal. A-Gen. 2017, 530: 154-165. (7) Wan, L.; Zhou, Y.; Zhang, Y.; Duan, Y.; Liu, X.; Xue, M., Influence of Lanthanum Addition on Catalytic Properties of PtSnK/Al2O3 Catalyst for Isobutane Dehydrogenation. Ind. Eng. Chem. Res. 2011, 50(8), 4280-4285. (8) Wang, G.; Meng, Z.; Liu, J.; Li, C.; Shan, H., Promoting Effect of Sulfur Addition on the Catalytic Performance of Ni/MgAl2O4 Catalysts for Isobutane Dehydrogenation. ACS Catal. 2013, 3: 2992-3001. 22


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Abstract Graphic

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SiO2 Catalyst: Effect of P

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