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Highly active Ni2P catalyst supported on core-shell structured Al2O3@TiO2 and its performance for benzofuran hydrodeoxygenation Bolong Jiang, Jing Gong, Jian Zhang, Feng Li, Jiaojing Zhang, Yanxiu Liu, Yanguang Chen, and Hua Song Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02018 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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Industrial & Engineering Chemistry Research

Highly active Ni2P catalyst supported on core-shell structured Al2O3@TiO2 and its performance for benzofuran hydrodeoxygenation Bolong Jianga, Jing Gongc, Jian Zhanga, Feng Lia,b, Jiaojing Zhang a, Yanxiu Liu a, Yanguang Chena, Hua Songa,b,∗ a

b

College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China

Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, Heilongjing, China c

PetroChina Daqing Refining Petrochemical Company, Daqing 163318, Heilongjing, China

*

To whom correspondence should be addressed. Email: [email protected]. Tel: +86 0459 6503167; Fax: +86 0459 6506498.

Abstract Highly active Ni2P catalyst supported on Al2O3@TiO2 (core@shell) for hydrodeoxygenation (HDO) of benzofuran (BF) was described. The effect of the TiO2 shell thickness on the structure and HDO properties of Ni2P/Al2O3@TiO2 were studied. The results showed that an appropriate TiO2 shell thickness can effectively suppress the formation of AlPO4 and change the transmission mechanism of Ni2P particles, which is beneficial to formation of highly dispersed Ni2P particles on the Al2O3 core. The Ni2P/A@T-25 showed the best HDO activity of 95% with oxygen-free products yield of 87% among the as-prepared Ni2P/Al2O3@TiO2 catalysts. As compared with Ni2P/Al2O3 (BF conversion of 78% with oxygen-free products yield of 47%), the BF conversion and yields of oxygen-free products over Ni2P/A@T-25 catalyst were increased by 22% and 85%, respectively.

Key words：core-shell; N i2P/Al2O3@TiO2; Benzofuran; Hydrodeoxygenation.

1. Introduction The growing demand for fossil fuels, especially oil and its derivatives, has generated the need to seek new

1

ACS Paragon Plus Environment


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sources to offset oil depletion, including biomass. 1 The pyrolysis is known as one method of biomass conversion. However, the oxygen content in liquid products, which produced via pyrolysis, is high (35–50 wt.%), which results in a serious problem because oxygen compounds degrade the product quality, leading to low pH, low heating value, high viscosity, and low chemical and thermal stability.

2

The hydrodeoxygenation (HDO) is

generally used to upgrade pyrolysis oil. The catalytic removal of oxygen used hydrogen, which is operated at a high hydrogen pressure (3–10 MPa) and moderate temperature (573–773 K). 3 Conventional HDO catalysts which based on sulfidized CoMo

4

or NiMo 5, usually supported on Al2O3, exhibited a good performance for the

hydrocarbons production from bio-oil. However, due to oxidation of the active phase sulfide catalysts showed a progressive lost of their activities during the HDO reaction. 6 Alternatively, catalysts based on noble metals have also been efficiently applied to the HDO of bio-oil.

7,8

However, these are expensive, and therefore their

application may affect the economy of the process. The low-cost transition metal phosphides, such as Ni2P, have been regarded as viable alternatives to precious metals.

9,10

It is well known that bulk Ni2P catalysts have a low

surface area and that high reduction temperatures are needed to obtain the bulk material (because of the strong P–O bond and its high reduction temperatures), which may lead to low dispersion and relatively low activity of the obtained catalyst.

11

Therefore, the alteration of the catalytic activity on a high-surface-area support can be

considered to be a result of changes in dispersion and morphology. 12 In recent years, macroporous γ-Al2O3, which is often used as a support for commercial catalysts, has been tested as a support for Ni2P catalysts because of its relatively high surface area, high mechanical strength, good thermal stability, and cheap.

13,14

However, phosphorus can strongly interact with γ-Al2O3 and lead to the

formation of AlPO4 on the surface of the support. It was reported that TiO2 support can serve as both a conventional support and an electronic accelerator in hydrogenation process. 15 However, pure TiO2 supports have some drawbacks, such as low surface area (which is 25 µm. This result indicated that increasing x can effectively suppress the formation of AlPO4.

Figure 4 IR spectra of Ni2P/Al2O3 and Ni2P/A@T(x) catalysts: (1) Ni2P/Al2O3, (2) Ni2P/A@T-1, (3) Ni2P/A@T-4, (4) Ni2P/A@T-25, (5) Ni2P/A@T-40.

3.6. X-ray photoelectron spectroscopy The X-ray photoelectron spectroscopy (XPS) spectrum of fresh and spent samples were carried out, and the results were listed in Table 2 and Table 3. For all the samples, Ni 2p3/2 core-level spectrum possessed three components. The first one which centered at 851.6–853.5 eV can be attributed to Niδ+ in the Ni2P phase. The second which centered at 856.3–857.6 eV can be assigned to the interaction of Ni2+ ions with phosphate ions, as a result of superficial passivation, along with the broad satellite at approximately 6.0 eV higher than that of the Ni2+ species, and this shake-up peak is attributed to divalent species. 27 For P 2p core-level spectrum, the peak at 129.5 eV can be attributed to Pδ- of the metal phosphides and the peak at 134.5eV can be attributed to surface metal phosphate species due to the superficial oxidation of the nickel phosphide . 11 As shown in Figure 5a, the XPS pattern for the fresh Ni2P/Al2O3 catalyst exhibited Ni 2p3/2 peaks at 852.3 and 856.4 eV, which can be assigned to the Niδ+ band of Ni2P and Ni2+species, respectively. Interestingly, for the fresh Ni2P/A@T-x samples, the binding energy of Ni2+ was 856.2 eV, which shifted to a slightly lower value with increasing x, indicating the presence of an electronic effect which was caused by Ti species in the shell. This may 11



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result in electron enrichment of Ni species on the surface. Our previous study showed that the Ti incorporated into Ti-MCM-41-supported Ni2P samples could act as an electronic promoter.

11

Compared with the respective fresh

catalysts, the intensities of the Niδ+ species contributions of the corresponding spent samples were stronger, showing that more Ni2P particles were formed on the surface after HDO. In addition, as compared to the spent Ni2P/Al2O3 catalyst, the signal intensities of the Niδ+ for the spent Ni2P/A@T-x samples were stronger. Furthermore, with increasing x, the signal intensity of the Niδ+ for the spent Ni2P/A@T-x samples became distinctly stronger, showing that Ti species in the shell could significantly promote the formation of more Ni2P particles on the surface of Ni2P/A@T-x during the HDO process. As shown in Figure 5(b), for all of the fresh catalysts, peaks centered at 134.6 eV for PO43− were detected, together with a weak peak at 129.3 eV which can be attributed to the Pδ– in Ni2P. Compared with the fresh Ni2P/A@T-x, the intensity of the Pδ+ signal for the corresponding spent Ni2P/A@T-x was increased with increasing x. However, this effect does not occur for Ni2P/Al2O3, indicating that Ti species present in the shell play a role in the formation of Ni2P during the HDO reaction. Interestingly, this result is consistent with the conclusion concerning Ni species drawn above, which reached a maximum at x = 25, showing that the higher value of the x, the stronger this effect. This is possibly because Ti4+ species present in the shell could be partially reduced to Ti3+ under the HDO reaction conditions. The 3d electron in Ti3+ might then be transferred through the conduction band of A@T-x and injected into the Ni 3d conduction band. The increase in electron density will lead to minor stabilization of the Ni 3d levels and a small Ni→P charge transfer, reduction of PO43- to Ni2P.

12


11,28

which is beneficial for the

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Figure 5 XPS spectra in the Ni(2p) and P(2p) regions for Ni2P/Al2O3 and Ni2P/A@T-x catalysts (a) Ni 2p core level spectra, (b) P 2p core level spectra, (1) Ni2P/Al2O3-fresh, (2) Ni2P/A@T-4-fresh, (3) Ni2P/A@T-25-fresh, (4) Ni2P/Al2O3-spent, (5) Ni2P/A@T-4-spent, (6) Ni2P/A@T-25-spent. As can be seen from Table 3, all samples showed lower Ni/P values than theoretical Ni/P ratio of 1/2, which likely due to the enrichment of phosphorous on the surface of the samples. The Ni/P ratios of all spent samples were higher than those of corresponding fresh samples. This possibly can be explained by a partial loss of phosphorous caused by the formation of volatile P species (such as PH3) during HDO reaction, which would cause a decrease in the P amount on the surface. In addition, both the spent Ni2P/A@T-4 and spent Ni2P/A@T-25 samples showed considerably higher Ni/P atomic ratios than that of the spent Ni2P/Al2O3. This shows that the P amounts on the surface of Ni2P/A@T-x were decreased significantly during the HDO process. In addition, the ratio of the surface Ti/Al increased with the increasing of thickness of the TiO2 shell showing that TiO2 shell is uniform. Both the fresh Ni2P/A@T-25 and Ni2P/A@T-4 samples showed slightly lower Ti/Al ratios after reaction. The fresh Ni2P/A@T-x catalysts have comparatively higher Niδ+/Ni2+, Pδ-/ PO43− and Ni2P content compared to the fresh Ni2P/Al2O3 catalyst, reaching maximum over fresh Ni2P/A@T-25, which indicated that more Ni2P phase was formed on core-shell Ni2P/A@T-x catalysts. In addition, for spent catalysts the Ni2P contents were higher than those of corresponding fresh ones. Therefore one concludes that the high Ni2P contents of all spent samples can be attributed to the formation of Ni2P particles by reduction of Ni2+ and PO43− during the HDO 13



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process. Table 2 Spectral parameters obtained by XPS analysis Binding energy (eV) Superficial atomic ratio Sample

Ni 2p3/2

P 2p

Ni2+

Ni2P

PO43-

Ni2P

Ni/P

Ti/Al

Ni2P/Al2O3-fresh

856.4

852.3

134.3

129.3

1/2.39

-

Ni2P/A@T-4-fresh

856.2

853.4

134.5

129.3

1/2.79

1/19.63

Ni2P/A@T-25-fresh

856.2

852.3

134.5

129.3

1/2.64

1/10.21

Ni2P/Al2O3-spent

856.3

852.3

134.3

129.3

1/2.24

-

Ni2P/A@T-4-spent

856.4

852.2

134.3

129.3

1/2.19

1/21.16

Ni2P/A@T-25-spent

856.4

852.3

134.3

129.3

1/2.07

1/11.75

Table 3 The Niδ+/Ni2+, Pδ-/ PO43− and Ni2P content obtained by XPS analysis Sample

Niδ+/Ni2+

Pδ-/ PO43−

Ni2P (%)

Ni2P/Al2O3-fresh

0.10

0.06

1.01

Ni2P/A@T-4-fresh

0.14

0.07

1.37

Ni2P/A@T-25-fresh

0.23

0.08

2.08

Ni2P/Al2O3-spent

0.34

0.06

2.82

Ni2P/A@T-4-spent

0.48

0.11

3.61

Ni2P/A@T-25-spent

0.71

0.33

4.62

3.7. CO uptake The CO uptakes at room temperature of the catalysts are listed in column 2 of Table 4. The CO molecules 14


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would mainly be adsorbed on Ni sites and the amounts of CO molecules which are adsorbed on P sites are possibly very small.

29

The CO uptake of Ni2P/A@T-25 was 15 µmol·g-1, which was slightly lower than that of

Ni2P/Al2O3 (17 µmol·g-1). A similar result was obtained in our previous study, whereby the CO uptake of Ni2P/MCM-41 was found to be much higher than that of Ni2P/Ti-MCM-41. Oyama et al. 24 found that higher SBET of support was responsible to the greater dispersion of the Ni2P phase and increased CO uptakes, which is consistent with our results. The SBET of Ni2P/A@T-25 is 246 m2·g-1, which is lower than that of Ni2P/Al2O3 (257 m2·g-1). In addition, the Ni/P surface molar ratio for Ni2P/A@T-25 is comparatively lower than that of Ni2P/Al2O3, indicating greater enrichment of P species on the surface of Ni2P/A@T-25. This may be another reason for the lower CO uptake of Ni2P/A@T-25 when compared to the Ni2P/Al2O3. Oyama et al. 24 have reported the similar findings. Surface P species could block some of the exposed nickel atoms and lead to a decrease in CO uptake. Chen et al. 30 attributed this phenomenon to the ensemble effect of P. The presence of P species on the surface of the Ni2P catalysts reduced the number of CO chemisorption sites. In addition, surface nickel sites could also be blocked by surplus P. Table 4 Results of BF HDO over Ni2P/Al2O3 and Ni2P/A@T-x catalysts. Yields of Conversion

Product distributions

CO uptake

Catalysts

Oxygen-free products （%）

-1

(µmol·g )

MCH ECH EB 2,3-DHBF 2-EtPh （%）

Ni2P/Al2O3

78

17

47

8

43

8

16

25

Ni2P/A@T-1

84

-

64

7

58

11

9

15

Ni2P/A@T-4

86

-

71

9

62

12

8

9

Ni2P/A@T-25

95

15

87

9

71

12

3

5

Ni2P/A@T-40

84

-

63

8

60

7

7

18

15



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3.8. Catalytic results Figure 6 presents the conversion of BF, and the yields of oxygen-free products as function of reaction time over the Ni2P/Al2O3 and Ni2P/A@T-x catalysts under the condition of 573 K, 3.0 MPa, weight hourly space velocity (WHSV) of 4.0 h–1 and hydrogen/oil ratio=500 (V/V). As expected, the conversion was greatly increased for all the catalysts during initial stage of HDO reaction. It is well known that the HDS activity over Ni2P catalysts increased initially with time on stream and the explanation of this behavior as result of the formation of an active superﬁcial phospho sulﬁde phase with a stoichiometry represented by NiPxSy

31

. Therefore, it is reasonable to

explain the increase in HDO activity during initial stage of HDO reaction as that an intermediate phase, which is more active than Ni2P, was also formed. This intermediate phase was possibly formed by the surface reconstruction of Ni2P at the reaction conditions, which confirmed by the change of the superficial atomic ratio after reaction. The Ni2P/A@T-x catalysts exhibited higher BF conversion with higher yields of oxygen-free products than the Ni2P/Al2O3 catalyst for tested reaction time. This possibly because the TiO2 shell weakened the strong interaction between Al2O3 and Ni2P to suppress the formation of AlPO4 (Figure 4), and TiO2 species may act as an electronic promoter, favoring the formation of more uniform Ni2P particles. It is suggested that the unique core-shell structure of Ni2P/A@T-x could account for the superior HDO performance.

Figure 6 Variation of the transformation of BF (a), and the yields of oxygen-free products (b). (Reaction temperature=573 K, pressure=3.0 MPa, WHSV=4.0 h–1, and hydrogen/oil ratio=500 (V/V)). 16


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Table 3 and Figure 7 shows the reactivities of Ni2P/Al2O3 and Ni2P/A@T-x catalysts for HDO of BF after 8 h when the steady reaction conditions had been reached. The BF conversion increased with increasing x and then decreased slightly with further increasing x. The HDO activity reached a maximum of 95% at x=25, which is increased by 22% as compared to that over Ni2P/Al2O3 (78%). All of the catalysts supported on an A@T-x core-shell structure showed enhanced HDO performance as compared to those directly supported on Al2O3, especially the ethylcyclohexane (ECH) selectivity increased remarkably, showing great promise for deep HDO catalysis. The selectivity for ECH was 71% over Ni2P/A@T-25 catalyst, which was increased by 65% as compared to that over Ni2P/Al2O3 (43%). However, the selectivities to methylcyclohexane (MCH) and to EB over Ni2P/A@T-x catalysts were both changed little. This indicates that the transformation of ethylbenzene (EB) to ECH was improved significantly by an A@T-x core-shell structure support, whereas that of ECH to MCH was almost remained unchanged at the reaction conditions. With increasing x, the yield of oxygen-free products (EB, ECH and MCH) increased and then decreased (Figure 7). The yields of oxygen-free products reached a maximum of 87% over Ni2P/A@T- Ni2P/A@T-x with a shell thickness of 25 µm, which was increased by 85% as compared to that over Ni2P/Al2O3 (47%). With increasing x, the selectivities for the O-containing intermediates 2,3-DHBF and 2-EtPh decreased initially and then increased, reaching minimum over Ni2P/A@T-25. This observation is consistent with the results of Wang et al.

32

An over-thick shell (> 25 µm) increases the transport resistance and

decreases the surface area of the catalyst. therefore, there is an optimum value. For Ni2P/A@T-x, the optimum thickness is 25 µm in the present case.

17



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Figure 7 Reactivities of Ni2P/Al2O3 and Ni2P/A@T-x catalysts for BF HDO As a model reaction, the BF HDO over Ni2P/MCM-41 was investigated in our previous work, and a reaction network was proposed. 21 The results exhibited that the oxygen-free products detected were mainly: MCH, EB and ECH.

The

oxygen-containing

intermediates

detected

were

mainly

2-ethylphenol

(2-EtPh)

and

2,3-dihydrobenzofuran (2,3-DHBF). The first step in the transformation of BF over Ni2P/MCM-41 is hydrogenation of the C=C bond of BF, which lead to 2,3-DHBF. And 2-EtPh is obtained as a secondary product by cleavage of the C–O bond in 2,3-DHBF. Further, 2-EtPh is converted into EB via the direct deoxygenation (DDO) pathway. The formed EB is transformed to ECH via hydrogenation. Based on our previous work, according to the product distribution and reaction routes previously which reported in the literature, 24 a possible reaction pathway for the BF HDO over Ni2P/A@T-x catalysts is shown in Scheme 1. There are a few mechanisms pertinent to C–O bond-cleavage reactions, including Hofmann-type elimination reaction (E2) and nucleophilic substitution reaction (SN2).

33

Our results have indicated that C–O bond cleavage on Ni2P/A@T-x is enhanced

with core-shell structured catalysts. Edelman

34

reported that after ring-opening of 2, 3-DHBF, the subsequent

HDO of the 2-EtPh is control step for overall BF HDO. The 2-EtPh in the hydrogenation pathway can, then, directly undergo an E2 or SN2 reaction to give the hydrocarbons. As evidenced by XPS measurements, the TiO2 shells possibly act as an electron donor may transform the Ni2P into an electron-rich state (in accordance with the results of XPS analysis). The electron-rich Ni2P species could enhance both E2 and SN2 reactions. 18


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Scheme 1 Proposed reaction pathway for BF HDO over Ni2P/A@T-x. On the basis of the above experimental results and theoretical analysis, a possible pathway for the HDO of BF over the Ni2P/Al2O3@TiO2 core-shell structured catalysts is proposed, which accounts for the increase in performance in four ways. (i) During the HDO process, hydrogen molecules dissociate into hydrogen atoms, which are attached at the active sites of the catalyst and then transfer to the adjacent core surface via spillover and diffusion. Meanwhile, BF molecules are predominantly adsorbed on the surface of the catalyst shell, where both their diffusion and reaction can readily take place. Finally, the dissociated H atoms flow over the active sites and participate in the hydrogenation reaction to form water. According to the TEM results, Ni2P particles can be located on the TiO2 shell or on the Al2O3 core. The HDO activity of Ni2P/Al2O3 is expected to be low, because they are directly exposed to BF and H2O, resulting in a serious poisoning from water. Due to the unique core-shell structure of Ni2P/A@T-x catalysts, Ni2P particles located on the Al2O3 core can be less affected by water (ii) The TiO2 shell can weaken the strong interaction between Al2O3 and P to suppress the formation of AlPO4 and change the transmission mechanism of Ni2P particles, which is beneficial to the formation of more dispersed Ni2P particles on the Al2O3 core. According to the TEM results, compared with Ni2P/Al2O3 catalyst, Ni2P/A@T-4 and Ni2P/A@T-25 cores show smaller and more uniform Ni2P particles. This confirms that TiO2 shell can promote the dispersion of Ni2P, which is possibly because the TiO2 shell can change the transmission mechanism of Ni2P particles. (iii) In addition, some of the Ti4+ may be partially reduced to Ti3+. As Ti3+ is an electron donor and Ni2P is an excellent electrical conductor, therefore the electrons in Ti3+ can be easily transferred through the Ti3+ conduction band and injected into the Ni conduction band (in accordance with the results of XPS analysis). The Al2O3@TiO2 core surface is enriched with the dangling bonds of Ni and P atoms. This could enhance the 19



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adsorption ability of BF on the catalyst, ultimately resulting in an increase in catalytic performance. This phenomenon is similar to our previously reported results observed with Ni2P/Ti-MCM-41 catalysts.

11

Ti can

modify the surface electronic atmosphere of the Ni2P/MCM-41 catalyst and enhance the adsorption ability of DBT thereon, thereby enhancing the catalytic activity of DBT HDS.

4. Conclusions In the present work, a series of Ni2P/Al2O3@TiO2 core-shell structured catalysts for HDO of BF was described. SEM images showed that TiO2 shells of different thicknesses were successfully deposited on the surface of Al2O3 cores. IR analysis showed that an appropriate shell thickness could effectively suppress the formation of AlPO4. The TEM analysis showed that deposition of TiO2 shell on the surface of Al2O3 cores obviously promotes the formation of smaller and uniform Ni2P particles, facilitates their better dispersion. The Ni2P/A@T-25 catalyst ranged Ni2P particle sizes from approximately 2 to 6 nm. With increasing x, the signal intensity of the Niδ+ in the spent Ni2P/A@T-x samples became distinctly stronger, showing that Ti species in the shell could significantly promote the formation of more Ni2P particles on the surface of Ni2P/A@T-x during the HDO process. Compared with the Ni2P/Al2O3 catalyst, the Ni2P/A@T-25 catalyst showed a high BF conversion of 95%, with an oxygen-free product yield of 87% under relatively mild conditions of 573 K, 3.0 MPa, WHSV 4 h-1, and a hydrogen/feed ratio of 500 (V/V). As compared with Ni2P/Al2O3 (BF conversion of 78% with oxygen-free products yield of 47%), the BF conversion and yields of oxygen-free products over Ni2P/A@T-25 catalyst were increased by 22% and 85%, respectively. A possible pathway to explain the improved BF HDO performance of Ni2P/Al2O3@TiO2 core-shell structured catalysts involved three aspects: (i) The role of TiO2 shell is twofold, it can weaken the strong interaction between Al2O3 and P to suppress the formation of AlPO4 and change the transmission mechanism of Ni2P particles, which contributes to the formation of highly dispersed Ni2P particles

20


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on the Al2O3 core. (ii) During the HDO process, Ti4+ present in the shell is reduced to Ti3+ which acts as an electron donor during the reaction. ASSOCIATED CONTENT Supporting Information Details on methods to describe the synthetic procedure of the A@T-x core-shell structured supports and demonstrate

uniform TiO2 shell which deposited on the surface of the Al2O3 core. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments The authors acknowledge the financial supports from the National Natural Science Foundation of China (21276048).

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Shu,

Y.;

Lee,

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K.;

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Structure-sensitivity

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