Efficient Tuning of Surface Nickel Species of Ni-Phyllosilicates

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Efficient Tuning of Surface Nickel Species of Ni-Phyllosilicates Catalyst for the Hydrogenation of Maleic Anhydride Jing Jing Tan, Xiaoli Xia, Jinglei Cui, Wenjun Yan, Zheng Jiang, and Yongxiang Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11972 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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The Journal of Physical Chemistry

Efficient Tuning of Surface Nickel Species of Ni‐Phyllosilicates Cata‐ lyst for the Hydrogenation of Maleic Anhydride Jingjing Tan*,†,┴, Xiaoli Xia†,┴, Jinglei Cui‡, Wenjun Yan§, Zheng Jiang*,†,∮ Yongxiang Zhao† † Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi University, Taiyuan 030006, PR China. ‡ State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, In‐ stitute of Resources and Environmental Engineering, Shanxi University, Taiyuan 030006, PR China. § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China.

∮Faculty of Engineering and the Environment, University of Southampton, Highfield, Southampton, SO17 1BJ, UK. ABSTRACT: It is vital for the rational design of the catalyst in the hydrogenation of Maleic anhydride (MA) to chemicals, as its special construction with one C=C bond and two C=O bonds. Herein, the catalyst with nickel nanoparticles (NPs) inlaid nickel phyllosilicate (Ni‐PS) featured with properly Ni0 and assembled Lewis acid sites, are proposed for the efficient hydrogenation of MA to succinic anhydride (SA) and γ‐butyrolactone (GBL). The remarkable efficiency is attributed to the synergistic effect between the surface Ni0 and the Lewis acid sites induced from partially reduced Ni and coordinatively unsaturated Ni (II) in Ni‐PS. The acid sites adsorb and activate the C=C and C=O groups, while the Ni0 facilitates the H2 decomposition. In‐situ XPS demonstrated that the ratio of surface Ni0 and Lewis acid sites can be modulated effectively by changing the Ni/Si molar ratio in the catalysts, and they exhibited significantly different catalytic activity in the hydrogena‐ tion of MA. Under the lower temperature, the hydrogenation activity of MA to SA linearly increases with increasing the Lewis acid sites surface area when the accessible Ni0 surface area is enough, whereas it is primarily affected by the Ni0 sites under high temperature for the hydrogenation MA to GBL.

1. INTRODUCTION Maleic anhydride (MA) has been regarded as a good feedstock since it can be produced at a low cost and on a large scale by partial oxidation of n‐butane instead of ben‐ zene1, 2. Moreover, it is recently reported that MA can be obtained by oxidation of biomass‐derived chemical plat‐ forms such as biobutanol, 5‐hydromethylfurfural (HMF), and furfural3‐8. At present, the annual yield of MA has reached 920000 t, while the demand of MA is limited and the production capacity is overplus. Compared with MA, its downstream products such as succinic anhydride (SA), γ‐butyrolactone (GBL), 1,4‐butanediol (1,4‐BDO) and tet‐ rahydrofuran (THF) are all important chemical raw mate‐ rials (Scheme 1) 2,9‐11, which are not adequate to the indus‐ trial demand. Especially, SA is considered as intermediate for the chemical, pharmaceutical and food industries. GBL is currently one of the most valuable alternatives to the environmentally harmful chlorinated solvents, which is widely used in the polymer and paint industries. It is thus great significance and valuable to catalyze the hydrogena‐ tion of MA to SA and GBL. Several metal catalysts have been reported to be active in MA hydrogenation to SA or GBL, including noble metal catalysts and non‐noble metal catalysts. Noble metal cat‐ alysts, including Pt, Pd, Rh, Ru and Au are highly active for MA hydrogenation but relatively expensive, and it is difficult to achieve application on a large scale12‐15. To lower the cost of the production, the cheaper Cu‐based

Scheme 1. Schematic pathways of MA hydrogenation and Ni‐based catalysts have attracted more attentions. The Cu‐Cr catalyst can efficiently catalyze the hydrogena‐ tion of MA, but the Cr element has toxicity and is poten‐ tially harmful to the environment16. For developing envi‐ ronmentally friendly catalyst, various supported Cr‐free Cu‐based catalysts, such as Cu‐Zn‐M (M=Al, Zr, Ti)17, Cu/SiO29, Cu/ZrO218 and Cu‐Ce‐Al2O319 were proposed for the production of GBL at high temperatures (210‐280oC). However, the poor activity was obtained at low tempera‐ ture for the hydrogenation of MA. Compared with the Cu‐ based catalysts, the Ni‐based catalysts exhibited relatively high catalytic efficiency under the same reaction condi‐ tions and have received more attentions. For example, various of supported Ni‐based catalysts were developed for the hydrogenation of MA to SA or GBL, such as Ni/Al2O320, Ni/CeO221,22, Ni/HY‐Al2O323, Ni/TiO224 and Ni/SiO2‐Al2O325. However, the supported Ni‐based cata‐ lysts suffer from sintering of metal particles at high tem‐ peratures, leading to the catalyst deactivation. Therefore,

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the improvement of the catalyst stability is required. Ad‐ ditionally, the selectivity of GBL is low at high tempera‐ tures since the various hydrogenation functional groups (C=O, C=C and furan ring) in MA. Moreover, the low‐tem‐ perature activities of these catalysts were too low as the low dispersion of Ni nanoparticles (NPs) on the surface of the catalyst. Hence, it would be highly desirable but more challenging to develop a novel Ni‐based catalytic system with highly activity at low temperature and high selectiv‐ ity and stability at high temperature for the hydrogena‐ tion of MA. In general, the high dispersion of Ni NPs is crucial for highly efficient of the catalyst, and the strong metal‐sup‐ port interaction can significantly enhance the dispersion and stability of the active metal. Moreover, efficient hy‐ drogenation of MA to SA involves C=C hydrogenation, while synthesis of GBL with high selectivity involves the hydrogenation of unsaturated bonds (C=C and C=O bonds) and subsequent C‐O hydrogenolysis to C‐H effi‐ ciently. Hence, the desired active sites for the synthesis of SA and GBL are different in considering the different pur‐ pose. The former usually requires balanced metal‐acid sites and the latter needs the cooperation of more highly dispersed metal sites with proper acid sites. Therefore, the development of metal‐acid bifunctional catalyst with strong metal‐support interaction and tunable surface metal and acidic sites properties is a prerequisite for the efficient hydrogenation of MA to SA and GBL. Nickel phyllosilicate (PS) has been paid more attentions in catalysis as its unique structural characteristics27‐30. It forms 1:1 or 2:1 lamellar structure. For 1:1 type, a sheet con‐ sisted of one tetrahedral SiO4 interacted with one sheet contained Ni(II) in octahedral coordination. For 2:1 type, one octahedral sheet of Ni(II) sandwiched between two tetrahedral sheets of SiO4, as shown in Figure 1. The lamel‐ lar structure of PS offers strong metal‐support interac‐ tions and a confinement effect to anchor the active metal, enhancing its dispersion and stability. Moreover, it pos‐ sesses tunable surface metal and acidic sites properties. So far, nickel phyllosilicate derived catalysts have not been investigated for the hydrogenation of MA. Herein, a series of nickel phyllosilicate catalysts with different Ni/Si molar ratio prepared by ammonia evapora‐ tion method were proposed for the hydrogenation of MA. These catalysts were featured with highly dispersed Ni NPs and plenty of proximal Lewis acid sites. These Lewis acid sites originated from the partial reduced Ni and the coordinatively unsaturated Ni (II) sites located at the nickel phyllosilicate structure. The unique lamellar struc‐ ture and the synergistic effect between the surface Ni0 and the Lewis acid sites endow the catalyst with high catalytic activity and stability during the hydrogenation processes. Moreover, the distribution of Ni species (metal and acidic sites) on the surface of the catalysts can be effectively modulated by changing the Ni/Si molar ratio, leading to

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the different catalytic performance. At 80oC, the hydro‐ genation of MA to SA shows the trend of a volcanic curve with increasing the Ni/Si molar ratio. The catalyst with a 0.48 Ni/Si molar ratio displays the highest activity, the conversion of MA was up to 95% within 3 h. Nevertheless, the hydrogenation of MA to GBL increased linearly with increasing the Ni/Si molar ratio at 200oC, and the yield of GBL was achieved to 70% within 3 h over the catalyst with 1.30 of Ni/Si molar ratio.

Figure 1. The structure of Ni‐Containing PS

2. RESULTS AND DISCUSSION 2.1 Textural Properties of the Catalysts The main physicochemical properties of pure SiO2 and the as‐prepared Ni‐PS‐x catalysts were listed in Table 1. It was found that the actual Ni loading determined by ICP‐ OES was close to the nominal value. The SBET (BET surface area) of the catalysts increased with increasing the Ni/Si molar ratio, and then decreased with further increasing its ratio. The sample with a 0.48 value of Ni/Si molar ratio possesses the maximum value for SBET. Such phenomenon is related to the special porous structure of the nickel phyllosilicate. This result indicated that the formation of more integrated nickel phyllosilicate structure by increas‐ ing the Ni/Si molar ratio at the range of 0~0.48. However, the NiO species were found on the surface of the catalyst with further increasing the Ni/Si molar ratio. These NiO species can cover the pores of the nickel silicate structure and then reduce the specific surface area of the catalyst, well in line with the previous studies28. Interestingly, it was observed that the pure SiO2 has the smallest SBET com‐ pared with all the Ni‐PS catalysts. This result suggested that the impaction of Ni NPs can enormously improve the SBET of Ni‐PS catalysts as the formation of the lamellar structure. Additionally, all the catalysts displayed a type IV Langmuir isotherm with an H4‐type hysteresis loop (Figure S1 in ESI). The pore size distribution curves of all the samples are also provided (Figure S2 in ESI). The pore size decreased firstly and then increased with increasing the Ni/Si molar ratio. The catalyst with the Ni/Si molar ratio of 0.48 owns the smallest average pore diameter, which is about 3.5 nm.


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Table 1. Physicochemical properties of catalysts

Catalyst

Ni loading a

SBET 2

Vp b

3

Dp b

dNi b

SNi c

Sacid

(mmol/g)

H2 uptake (mmol/g)e

(mmol NH3/g)f

C (µmol/g)g

d

(wt%)

(m /g)

(cm /g)

SiO2

‐

174

0.3

4.6

‐

‐

‐

‐

‐

PS‐0.09

7.1(8)

227

0.5

7.6

2.7

0.28

0.59

1.62

239.8

PS‐0.19

13.7(16)

270

0.6

7.1

2.9

0.32

0.79

1.68

456.3

PS‐0.32

22.0(24)

312.

0.8

7.8

3.4

0.47

1.03

1.79

746.4

PS‐0.48

31.5(32)

347

0.6

7.7

3.9

0.80

1.13

2.89

1381.5

PS‐0.68

38.8(40)

336

0.8

7.8

4.3

0.88

2.18

2.90

941.9

PS‐0.94

46.0(48)

326

0.7

7.2

4.6

0.87

2.84

2.99

875.8

PS‐1.30

54.5(56)

243

0.5

7.0

4.9

1.13

3.64

2.90

(nm) (nm)

a

423.4 b

Elements content was measured by ICP‐OES, the value in bracket was the theoretical value of Ni loading. The BET surface area, pore volume, and pore size were determined by N2 physical‐adsorption. c Diameter of Ni NPs was calculated according to the statistical results of HRTEM. d SNi was calculated based on H2‐TPD results. e Determined from the amount of H2 consumed in TPR. f Sacid was calculated based on NH3‐TPD results of reduced catalysts. g C was calculated based on Py‐IR results of reduced catalysts.

2.3 FTIR analysis

2.2 XRD analysis The XRD patterns of Ni‐PS catalyst with varying molar ratio of Ni/Si, which dried at 80oC, calcined at 600oC and reduced at 550oC, are presented in Figure 2, respectively. It was observed that the obvious diffraction peaks centered at 26.7°, 33.7°, 39.7°, 53.2° and 60.9° were appeared for all the catalysts, which were assigned to the presence of 2:1 Ni‐ PS structures (JADE‐PDF43‐0664)27. These results sug‐ gested that the successful preparation of nickel phyllosili‐ cate in their precursors. Additionally, it was found that the amorphous SiO2 were obvious when the Ni/Si molar ratio is less than 0.48 due to the Ni loading is too low to form nickel phyllosilicate with SiO2. According to Figure 2b, the diffraction peaks of the calcined samples (Ni‐PS‐0.09 to 0.68) were almost identical to those of the dried samples, suggesting that the structure of these Ni‐PS samples were not be damaged at high temperatures. However, the dif‐ fraction peak centered at 43.3o, which was assigned to NiO, were detected when the Ni/Si molar ratio increased to 0.94‐1.30. This result reflected that the excess Ni species on the surface of these catalysts gathered into large NiO par‐ ticles since the weaker interactions between excess Ni spe‐ cies and SiO2. After reduction, the diffraction peak of nickel phyllosilicate is gradually weakened for all the cata‐ lysts. Noted that no diffraction peaks of Ni NPs were de‐ tected when the Ni/Si molar ratio was less than 0.48. This implied that the Ni NPs were highly dispersed on these cat‐ alysts as the formation of nickel phyllosilicate. Neverthe‐ less, a wide diffraction peaks of Ni were appeared and sharpened with further increasing the Ni/Si molar ratio (>0.48), reflecting that the size of the Ni NPs increased with increasing the Ni content as the excess Ni aggregated and grew on the surface of the catalysts.

The FTIR spectra of dried (Figure 3a) and calcined (Fig‐ ure 3b) Ni‐PS catalysts were shown in Figure 3. The vibra‐ tion bands at 800 cm‐1 and 1109 cm‐1 are attributed to the νSiO vibration in amorphous SiO2 (Si‐O‐Si)31. The vibration bands at 670 cm‐1, 1024 cm‐1 and 3627 cm‐1 are assigned to the δOH vibration, νSiO vibration and the νOH vibration in the phyllosilicate structures, respectively32. The band at 1385 cm‐1 is the vibration of the NO3‐ 32,33. The appearance of characteristic bands at 670 cm−1, 1024 cm−1, and 3627 cm−1 indicated the formation of nickel phyllosilicate in precur‐ sors for all the samples with varying Ni/Si molar ratio (Fig‐ ure 3a), which are consistent with the XRD results. Mean‐ while, it was found that the shoulder peaks centered at 800 cm‐1 and 1109 cm‐1 belonged to amorphous SiO2 were grad‐ ually disappeared, while the peaks of nickel phyllosilicate were gradually sharpened with increasing the Ni/Si molar ratio. These results reflected that the species of Si are mainly in the form of nickel phyllosilicate with increasing the Ni/Si molar ratio. The pure nickel phyllosilicate phase was formed as the ratio of Ni/Si increased to 0.48~1.30. Moreover, such nickel phyllosilicate was preserved after calcination at 600oC (Figure 3b), further suggesting the highly stable of this structure at high temperatures, well in accordance with the XRD results. 2.4 H2‐TPR analysis The reducibility of the calcined catalysts with different Ni/Si molar ratio were studied by H2‐TPR. The profiles and the amount of H2 uptake are shown in Figure 4 and Table 1, respectively. Each sample displays two hydrogen con‐ sumption peaks, and the shapes of the peaks differ between each one. These findings demonstrated that two different



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Figure 3. FTIR spectra of different catalysts. (a) FTIR spec‐ tra of different precursors; (b) FTIR spectra of calcined cat‐ alysts.

Figure 2. XRD patterns of Ni‐PS‐x. (a) XRD patterns of pre‐ cursors, (b) XRD patterns of calcined and (c) XRD patterns of reduced catalysts. Ni species are present in each sample. The former peaks from 200°C to 500°C can be ascribed to the reduction of NiO on the surface of the catalyst or the Ni2+ with weak in‐ teraction with SiO2. Meanwhile, the latter peaks at 500°C to 800°C can be related to the reduction of Ni2+ located at the nickel phyllosilicate, which is coincident with the pre‐ vious study33. The high reduction temperature of these cat‐ alysts reflected the presence of the strong interaction be‐ tween Ni and SiO2 species. That is, Ni species are embed‐ ded into the Ni‐PS structures, and these Ni species are

difficult to be reduced. With the increase of Ni/Si molar ratio in the range of 0.09‐0.48, the latter reduction peak of Ni species shifted to higher temperature. This indicated that the Ni‐O‐Si structure increases gradually with increas‐ ing the Ni content, and the nickel phyllosilicate structure tends to be the integrated 2:1 structure. When the Ni/Si molar ratio exceeds 0.48, the latter reduction peak of Ni species slightly shifted to lower temperature. The reason is that the excessive Ni existed in the form of NiO on the sur‐ face of the catalysts, and they are easy to be reduced and lead to hydrogen spillover during the reduction process. The overflowing active hydrogen has super reduction abil‐ ity, which can lower the reduction temperature of nickel in the Ni‐PS structure. Although the center of the high tem‐ perature reduction peak shifts with the change of Ni/Si molar ratio, it is still in the range of 600‐800 °C. It is thus induced that the Ni species in the nickel phyllosilicate were not completely reduced at 550°C, and lots of Niδ+ ions are exposed at the edge and surface of the catalyst28.


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Figure 4. H2‐TPR results of catalysts 2.5 TEM analysis To further understand the dispersion of the Ni NPs and study the structure and morphology of the catalysts, the TEM analysis of the reduced samples was performed. The images were depicted in Figure 5 and Figure S3 (in ESI). Figure 5(a1)~(g1) display the presence of characteristic fi‐ brous‐like structures (labeled by red ellipse in Figure S3, in ESI) as the formation of Ni‐containing phyllosilicate35,36. These results implied that the Ni species in Ni‐PS structure were not fully reduced at 550oC for 2 h, which are well in line with the TPR results and previous study27. It is re‐ ported that the Ni inlaid nickel phyllosilicate cannot be completely reduced when the reduction temperatures were lower than 700oC27,29,37,38. The unreduced nickel phyl‐ losilicate could act as an auxiliary guest for Ni0 NPs, which would exhibit different properties during the catalytic re‐ action processes. Moreover, it was observed directly the dispersion of Ni NPs on the surface of catalyst from the TEM images. Small Ni particles are uniformly distributed on the support for the samples with Ni/Si molar ratio in the range of 0.09 to 0.68 (Figure 5a‐1~d‐1). The formation of these small Ni NPs without aggregation were related to the strong interaction between Ni2+ ions and SiO2 in nickel phyllosilicate structure, which slows the reduction and nu‐ cleation process during reduction. However, the aggre‐ gates of relatively bulk Ni NPs were obtained on the surface of catalyst when the Ni/Si molar ratio increased up to 0.94~1.30. The reason is that the excess Ni species in these catalysts exist in the form of NiO on the surface. Such NiO are easily to be reduced to metal Ni, and they are prone to aggregate to form large particles under high temperatures. Additionally, the HRTEM were performed to determine the size distribution of Ni NPs (Figure5a‐2~g‐2). The corre‐ sponding size distribution histogram showed that the size of Ni NPs was slightly increased from 2.7 nm to 4.9 nm with increasing the molar ratio of Ni/Si from 0.09 to 1.30, which was well in line with the results of XRD. It is thus con‐ firmed that highly dispersed active sites Ni can be obtained from the nickel phyllosilicate precursor. The number of ac‐ tive metal of Ni was calculated by H2‐TPD, and the results were listed in Table 1. It was found that the exposed surface metallic Ni concentration increased from 0.28 mmol/g to 1.13 mmol/g with increasing the Ni/Si molar ratio from 0.09‐1.30.

Figure 5. The morphology of catalysts and particle size dis‐ tribution histogram of reduced catalysts: (a)PS‐0.09, (b)PS‐0.19, (c)PS‐0.32, (d)PS‐0.48, (e)PS‐0.68, (f)PS‐0.94, (g)PS‐1.30.



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Figure 6. NH3‐TPD patterns of the reduced catalysts 2.6 NH3‐TPD analysis The surface acidic properties of the reduced catalysts were explored by NH3‐TPD (Figure 6 and Table 1). As shown in Figure 6, two main peaks around 190°C~450°C and 450°C~650°C were observed for all samples with vary‐ ing Ni/Si molar ratio, corresponding to medium strong acid sites and strong acid sites, respectively. Note that the intensity of the total desorption peak is increased with in‐ creasing the Ni/Si molar ratio (at the rage of 0.09‐0.48), while the trend is not obvious when the ratio is above 0.48. Moreover, it was observed that the strong acid sites were gradually decreased, while the medium strong acid sites significantly increased with increasing the Ni/Si molar ra‐ tio. These results are in line with the H2‐TPR results, which demonstrated that a large fraction of Ni2+ in nickel phyllo‐ silicate were not reduced at 550oC and the lamellar struc‐ ture was preserved, especially for the samples with 0.48 of Ni/Si molar ratio. Typically, the acidity of Ni‐PS catalysts is derived from two different positions: one resulted from the ‐OH groups on the surface of the catalyst (Brønsted acid), the other is the coordinatively unsaturated Ni2+ sites on the edges/surfaces of PS (Lewis acid), including the unreduced or partially reduced Ni2+ ions in Ni‐PS 33. 2.7 Py‐IR analysis To further determine the nature of acid sites, the Py‐IR spectra of nickel phyllosilicate catalysts with different Ni/Si molar ratio were conducted, the spectra and the acid content are shown in Figure 7 and Table 1, respectively. The bands centered at 1450 cm−1, 1610 cm−1 and 1490 cm−1 are consistent with the absorption of pyridine on Lewis acid sites. The band at 1540 cm−1 assigned to Brønsted acid caused by surface ‐OH groups were not appeared. It is thus verified that the surface acid sites are mainly in the form of Lewis acids (L acid). Such acids were originated from the partially reduced Ni2+ ions in nickel PS and the coordina‐ tively unsaturated Ni2+ sites located at the remanent nickel phyllosilicate structure33,39‐40. Moreover, it was observed that the intensity of these three peaks (1450 cm−1, 1610 cm−1 and 1490 cm−1) were increased firstly and then decreased with increasing the molar ratio of Ni/Si. The sample with a 0.48 Ni/Si molar ratio owns the strongest intensities of the peaks, namely, it possesses the maximum acidic sites (as seen in Table 1). Such phenomenon is related to the change

Figure 7. Py‐IR spectra of reduced catalysts of the special layered phyllosilicate structure. The more nickel phyllosilicate phase, the more coordinatively un‐ saturated Ni2+ and the stronger acidity for the catalyst. Ob‐ viously, nickel and silicon could be completely used to form the integrated nickel phyllosilicate phase as the Ni/Si molar ratio is about 0.48. The strongly interaction between Ni2+ ion and SiO2 in the nickel phyllosilicate phase causes the difficulty of Ni species to be reduced, leading to more unreduced Ni to act as Lewis acid sites. This interaction likely become weak when the Ni species were excessive in these catalysts. Such exceed Ni species will be present in the form of NiO on the catalyst surface, which are prone to be reduced, leading to fewer unreduced Ni and lowering the density of Lewis acid sites. These results were con‐ sistent with the above H2‐TPR and NH3‐TPD results. 2.8 XPS analysis In‐situ X‐ray photoelectron spectroscopy (XPS) was per‐ formed to further verify the reducibility and chemical state of the Ni species in the catalysts with varying Ni/Si molar ratio. As described in Figure 8, the Ni 2p spectra of all the samples were deconvoluted into four peaks, which were centered at 852.7 eV, 855.4 eV, 857.5 eV and 862 eV, respec‐ tively. The first binding energy (BE) positioned at 852.7 eV is attributed to the presence of Ni0 species41,42. The peak

Figure 8. In‐situ XPS spectra of reduced catalysts


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centered at 855.4 eV is assigned to the partial reduced Ni species in nickel phyllosilicate, namely, Niδ+ species43. The peak at 857.5 eV belongs to the Ni2+ species which interact strongly with SiO2 in Ni‐PS catalysts43,44. The peak posi‐ tioned at 862 eV is ascribed to the satellite peak of Ni2p as the electron shake‐up45. Moreover, it was observed that the peak intensities of Ni0 and Niδ+ species were increased lin‐ early, while that of Ni2+ located at nickel phyllosilicate were increased firstly and then decreased with increasing the Ni/Si molar ratio, which were well consistent with the characterization of H2‐TPR results. 2.9 Catalytic performance To explore the catalytic performance, the hydrogenation of MA was performed over these reduced Ni‐PS catalysts (Table 2 and Table S1‐S2 in ESI). It was observed that the conversion of MA increased firstly and then decreased with increasing the Ni/Si molar ratio (entries 1~7). The catalyst with a Ni/Si molar ratio of 0.48 exhibited the highest activ‐ ity for the hydrogenation of MA to SA at 80oC. A 95% con‐ version of MA together with a SA selectivity of 100% was obtained within 3 h, which were greatly higher than that of previous reports20‐22. However, it was found that the hydro‐ genation activity of MA to GBL at 200oC was positively cor‐ related with the Ni/Si molar ratio. Namely, the GBL yield increased linearly with increasing the Ni/Si molar ratio (entries 9~15). The highest yield of GBL reached 70% within 3 h over the catalyst with a 1.30 of Ni/Si molar ratio. These results suggested that the distribution of Ni species (metal Ni0 sites and the Lewis acid sites) have a significant effect on their catalytic activity. To find the proper distri‐ bution of Ni species that enhances the catalytic perfor‐ mance of these catalysts with different Ni/Si molar ratio, the relationship of the structure and catalytic performance Table 2. Influence of Ni/Si molar ratio on the hydrogenation of MAa Entry

MA SA GBL Con.% Sel.% Sel.% 1 PS‐0.09 81 100 0 2 PS‐0.19 86 100 0 3 PS‐0.32 91 100 0 4 PS‐0.48 95 100 0 5 PS‐0.68 92 100 0 6 PS‐0.94 87 100 0 7 PS‐1.30 81 100 0 8 Ni/SiO2Ib 74 100 0 9 PS‐0.09c 87 88 12 10 PS‐0.19 c 93 82 18 11 PS‐0.32 c 96 74 26 12 PS‐0.48 c 98 69 31 13 PS‐0.68 c 100 37 63 14 PS‐0.94 c 100 33 67 15 PS‐1.30 c 100 30 70 16 Ni/SiO2Ic 84 98 2 a: Reaction conditions: MA: 4.9 g; catalyst: 0.3 g; stirrer: 400 r/min; H2: 5 MPa; THF 40 ml as solvent; Reaction temper‐ ature: 80oC; t=3 h. GBL: γ‐butyrolactone. b: the catalyst pre‐ pared by impregnation method, 80oC, c: 200oC; the carbon balance in all reactions ≥97%

Catalyst

Figure 9a. Effects of Ni/Si molar ratio on the conversion of MA at 80oC for 3 h and the Normalized acid sites (NiP2+ in PS and Niδ+) of the catalysts

Figure 9b. Effects of Ni /Si molar ratio on the yield of GBL at 200oC for 3 h and the Ni0 of the catalysts for the hydrogenation MA to SA and GBL were constructed, respectively. Firstly, the MA conversion and the distribu‐ tion of Ni species (Lewis acid sites) as a function of Ni/Si molar ratio were displayed in Figure 9a. Interestingly, it was found that the same trend was appeared for the con‐ version of MA and the surface Lewis acid concentration with varying the Ni/Si molar ratio. That is to say, the two curves increased firstly, and then decreased with increas‐ ing the molar ratio of Ni/Si. The highest conversion of MA and the highest number of Lewis acid sites were occurred simultaneously over the catalyst with the Ni/Si molar ratio of 0.48. The improved catalytic activities of the catalysts can be ascribed to the strong synergistic effect between Ni0 and the Ni species with Lewis acid properties. Typically, Ni0 is used to activate dissociated hydrogen, and Ni species (electron‐deficient) are used to adsorb and activate the C=C group (electron donor) during the hydrogenation pro‐ cesses46. This was further verified by comparison with the activity of Ni/SiO2I catalyst prepared by impregnation method, which has the same active sites of Ni as Ni‐PS‐0.48 but without Lewis acid site (Table 1 entry 8). Its catalytic activity for the hydrogenation of MA to SA at 80oC is much lower than that of Ni‐PS catalysts. However, the XPS re‐ sults indicated that the metal sites of Ni0 were increased monodirectional with increasing the Ni/Si molar ratio at



the range of 0.32 to 1.30. It is thus induced that the catalytic activities efficiently changed according to the number of Lewis acid sites when the accessible Ni0 is enough during the hydrogenation of MA to SA at low temperatures. In order to explore the role of Ni0 sites, the distribution of Ni species (Ni0) and the yield of GBL as a function of Ni/Si molar ratio were also correlated, as shown in Figure 9b. It was found that the yield of GBL produced at high temperature (200oC) were increased with increasing the Ni/Si molar ratio. Meanwhile, the number of metal Ni sites were also increased with increasing the Ni/Si molar ratio at the range of 0.32 to 1.30. Similarly, the synergistic effect be‐ tween Ni0 and the adjacent Lewis acid is vital to realize the hydrogenation of MA to GBL at high temperatures. Ni0 fa‐ cilitates the H2 decomposition, and Ni species with Lewis properties are applied to adsorb and activate the C=O group (electron donor) during the hydrogenation pro‐ cesses. This was also verified by the low catalytic activity of Ni/SiO2I catalyst with the same active sites of Ni as Ni‐PS‐ 0.48 but without Lewis acid site (Table 2 entry 16). Never‐ theless, the results of Py‐IR, XPS and Figure 9a exhibited that the Lewis acid sites of the catalysts increased firstly and then decreased with increasing the Ni/Si molar ratio. All of the results reflected that the catalytic activity of the catalysts no longer remained consistent with the Lewis acid sites, while it primarily increased with the Ni0 sites. That is to say, the catalytic activity depended on the Ni0 sites when the accessible Lewis acid sites are enough dur‐ ing the hydrogenation of MA to GBL at high temperature. The reason is that more activated hydrogen was required for the hydrogenation of both C=C and C=O at high tem‐ perature31,46. The stability of catalyst is a primary challenge for the liq‐ uid hydrogenation reaction over the Ni‐based catalyst. As shown in Figure 10, the recyclability of the Ni‐PS catalyst with the Ni/Si molar ratio of 0.48 was investigated under high temperature (200oC). After each reaction, the catalyst was separated from the reaction mixture by filtration and washed with THF for three times. Then, the recycled cata‐ lyst was reused for the next run. It was observed that the activity of the catalyst maintained at a high level after eight

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successive recycling runs. Furthermore, the XRD and TEM analysis of the catalyst used 8 times were performed, sug‐ gesting that the average particle sizes were slightly in‐ creased from 3.9 nm to 4.4 nm (Figure S4 and Figure S5 in ESI). Moreover, there was no obvious change in the struc‐ ture and morphology of the catalyst before and after the reaction, as verified by the TEM and FTIR results (Figure S5 and Figure S6 in ESI). These results reflect the high sta‐ bility and reusability of the catalyst. The high stability of Ni‐PS‐0.48 can be ascribed to the strong metal‐support in‐ teraction and the confinement effect of phyllosilicate structure in the catalyst, preventing the loss and sintering of active metal Ni species during the reaction. 3. CONCLUSIONS A series of nickel phyllosilicate (Ni‐PS) catalysts with varying Ni/Si molar ratio are developed for the efficient hy‐ drogenation of MA. The catalysts were featured with highly dispersed Ni NPs and plenty of Lewis acid sites. The results demonstrated that the Ni/Si molar ratio has a significant influence on the catalyst structure and catalytic activities. The catalyst with Ni/Si molar ratio of 0.48 is prone to form the integrated nickel phyllosilicate. It exhibited the highest activity in the hydrogenation of MA to SA at low tempera‐ ture, the yield of SA was up to 95% within 3 h at 80oC. The catalyst with Ni/Si molar ratio of 1.30 displayed the highest activity in the hydrogenation of MA to GBL under high temperature, the yield of GBL reached 70% within 3 h at 200oC. The remarkable efficiency of these catalysts is as‐ cribed to the synergistic effect between the surface Ni0 and the Lewis acid sites induced from partially reduced Ni and the remained phyllosilicate. In‐situ XPS characterization clarified that the number of Ni0 and Lewis acid sites can be modulated effectively by changing the molar ratio of Ni/Si in the Ni‐PS catalysts. The surface area of Ni0 increased mo‐ notonously, while the Lewis acid sites increased firstly and then decreased with increasing the Ni/Si molar ratio. It was concluded that the catalytic activity of hydrogenation for MA to SA at lower temperature linearly increases with in‐ creasing the Lewis acid sites when the accessible Ni0 is suf‐ ficient. However, it is primarily affected by the Ni0 sites un‐ der higher temperature for the hydrogenation MA to GBL when the Lewis acid sites are enough. The reason is that more activated hydrogen was needed for achieving both hydrogenation of C=C and C=O. Besides its effectiveness, the Ni‐PS catalyst displayed an excellent stability under high temperature. This is due to the strong metal‐support interaction and the confinement effect of phyllosilicate structure for the catalyst. The attractive properties of Ni‐ PS might be promising for hydrogenation of other α, β‐un‐ saturated carbonyl compound, providing guiding princi‐ ples for the design of metal‐acid multifunctional catalyst in future.

ASSOCIATED CONTENT Figure 10 The recycling test of Ni‐PS‐0.48 catalyst Reaction conditions: MA: 4.9g; catalyst: 0.3 g; stirrer: 400 r∙min‐1; H2: 5 MPa; THF 40 ml as solvent; Reaction temper‐ ature: 200oC; t= 3 h. MA: maleic anhydride, GBL: ‐butyro‐ lactone.

Supporting Information. The Experimental and Figure S1 to S6, Table S1‐S2 can be found in Electronic Supporting Infor‐ mation (ESI). This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * E‐mail: [email protected], [email protected]

Author Contributions ┴These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge Shanxi Provincial Natu‐ ral Science Foundation of China (No. 201701D221030), the fi‐ nancial support of the Natural Science Foundation of China (No. 21703275 and No. U1710221).

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The Journal of Physical Chemistry TOC Graphic


Efficient Tuning of Surface Nickel Species of Ni-Phyllosilicates

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