Hybrid MoâCT Nanowires as Highly Efficient Catalysts for Direct

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Hybrid Mo-CT nanowires as highly efficient catalysts for direct dehydrogenation of isobutane Jiali Mu, Junjun Shi, Liam John France, Yongshan Wu, Qiang Zeng, Baoan Liu, Lilong Jiang, Jinxing Long, and Xuehui Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05273 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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ACS Applied Materials & Interfaces

Hybrid Mo-CT nanowires as highly efficient catalysts for direct dehydrogenation of isobutane Jiali Mu,† Junjun Shi,† Liam John France,† Yongshan Wu,† Qiang Zeng,† Baoan Liu,† Lilong Jiang,‡ Jinxing Long,† Xuehui Li*† †

School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp &

Paper Engineering, South China University of Technology, Guangzhou 510640, P. R. China. ‡

National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou

University, Fuzhou 350002, P. R. China.

Abstract

Direct dehydrogenation of isobutane to isobutene has drawn extensive attention for synthesizing various chemicals. The Mo-based catalysts hold promise as an alternative to the toxic CrOx- and scarce Pt-based catalysts. However, the low activity and rapid deactivation of the Mo-based catalysts greatly hinder their practical applications. Herein, we demonstrate a feasible approach towards the development of efficient and non-noble metal dehydrogenation catalysts basing on Mo-CT hybrid nanowires calcined at different temperatures. In particular, the optimal Mo-C700 catalyst exhibits isobutane consumption rate of 3.9 mmol g-1 h-1, and isobutene selectivity of 73% with production rate of 2.8 mmol g-1 h-1. The catalyst maintained 90% of its initial activity after 50 h of reaction. Extensive characterizations reveal that such prominent performance is well-correlated with the adsorption abilities of isobutane and isobutene, and the formation of η-MoC species. By contrast, the generation of β-Mo2C crystalline phase during long-term reaction causes minor 1

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decline in activity. Compared to MoO2 and β-Mo2C, η-MoC plays a role more likely in suppressing the cracking reaction. This work demonstrates a feasible approach towards the development of efficient and non-noble metal dehydrogenation catalysts.

Keywords: molybdenum, carbide, isobutane, isobutene, dehydrogenation Introduction Isobutene is an important petrochemical feedstock for the production of methacrylates, butyl rubber and polyisobutene. The direct dehydrogenation (DDH) of isobutane for the production of isobutene commercially has garnered considerable attention as a suitable alternative approach to petroleum refinery derived byproducts.1,

2

In this

application, Pt- and CrOx-based catalysts have been commercialized as their remarkably high catalytic activity. Unfortunately, the expense of the former and hazardous nature of the latter economically offset their perceived advantages. As such, the design of low-cost and environmentally friendly catalysts is of paramount importance from the perspective of process economics.3-6 Many efforts have been made to reduce or replace the use of Pt- and CrOx- based catalysts in the DDH process. For example, transition-metal oxides (VOx, ZrO and ZnO),7-12 sulfides (FeS, CoS and NiS),13,

14

bimetallic oxides12,

15, 16

and carbon

materials17, 18 have been widely studied and they exhibited good catalytic performance, though most suffer from poor thermal stability. Mo is earth abundant and has a similar electronic structure to Cr. Oxides, carbides and nitrides of molybdenum have thus been widely utilized in catalytic applications, which but are not limited to alkene isomerization, hydrogenation, hydrogenolysis and hydrodesulphurization.19-21 By contrast, as a candidate for catalytic dehydrogenation. Mo-based catalyst have been mainly studied in oxidative dehydrogenation on ethane,22-24 propane25-27 and n-butane in previous studies.28, 29 For example, the catalytic performance of molybdenum oxides including MoO3 crystallites, MoOx monomers or polymers are typically affected by the 2


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supporting species, the molybdenum oxidation state and the dispersion of the metals. Wang and co-workers showed that the Mo/MgAl2O4 catalyst can achieve ca. 78% selectivity to isobutene at isobutane conversion of 45% in a circulation fluidized-bed unit.30 Compared with the unordered mesoporous molybdena-alumina catalyst, moderately or strongly acidic sites coupled with strong metal-support interaction are beneficial

to

stability

and

coking

resistance

for

ordered

mesoporous

molybdena-alumina catalysts.31 When Mo-based catalysts are sulfurized by H2S, they yield up to 56.3 wt.% isobutene with improved activation ability of the C-H bond over the C-C bond. Nevertheless, measures for sulfur replenishment are required for these catalysts as the sulfur loss leads to significant decrease in catalytic activity.13, 32 Considering the noble-like properties of molybdenum carbide and nitrides, the investigations mainly focus on oxidative dehydrogenation of propane and n-butane, the direct dehydrogenation of isobutane has been studied scarcely. In addition, MoOx, MoxC or Mo2N particles are easy to agglomerate via the traditional incipient wetness impregnation method, resulting that CH4, H2 or NH3 have been adopted to generate metal carbide and nitride catalysts. Hence, in this study, an economical and safe Mo-based catalyst for DDH of isobutane was developed with tunable active components by changing annealing temperature. The catalyst exhibit promising catalytic performance in term of selectivity and stability. The correlation between catalytic performances and physicochemical properties of the catalysts, and the possible dehydrogenation active sites were all intensively investigated. The results elucidated that the η-MoC species plays a key role in catalytic activity. Material and methods Catalyst preparation The precursor of Mo3O10(C6H8N)2·2H2O hybrid nanowire was synthesized according to the reference.33 In a typical synthesis process, 2.48 g (2.0 mmol) of ammonium heptamolybdate [(NH4)6Mo7O24·4H2O] and 3.34 g (36 mmol) aniline (C6H7N) were 3



Page 4 of 26

dissolved in 40 mL of distilled water. Then a 1.0 M HCl aqueous solution was dropwise added with magnetic stirring at room temperature until white precipitate started to form (pH 4~5). After stirring at 50 oC in an oil bath for 3 h, the solid product was filtrated, washed with ethanol, and then dried at 60 oC for 12 h. The obtained precursor was calcined at a specific temperature for 5 h with a ramp rate of 2 oC min-1 under Ar flow (100 mL min-1). Then resulting Mo-CT was removed under ambient conditions and stores in an airtight container for further use, where T denoted the calcination temperature of precursor. Characterization X-ray diffraction (XRD) measurements were undertaken on a D 8 Advance o

diffractometer (Bruker, Germany) using Cu Kα (λ = 1.5418 A) radiation at 40 kV and 40 mA, from 10

o

to 90

o

with a scanning speed of 10

o

min-1. Raman spectra of

catalysts were obtained with a LabRAM Aramis spectrometer (Horiba Jobin Yvon, France) using 633 nm excitation at 25 mW laser power and a 2.5 cm-1 resolution. Brunauer-Emmett-Teller (BET) specific surface area and pore-size distributions were measured by a Tristar II 3020 (Micromeritics, USA). Prior to the measurements, all samples were evacuated at 150 oC for 6 h at a pressure of 1.0 × 10−3 kPa to ensure complete removal of adsorbed moisture. Thermogravimetric analysis was undertaken on a STA 449C (Netzsch, Germany) from 40 to 800 oC in an air flow (20 mL min-1) at a ramp rate of 10 oC min-1. The composition of nanowires precursor and as-prepared Mo-CT samples were determined, using ICP-AES (Leeman, USA) for Mo and CHN elemental analysis (Vario EL III element analyzer, Germany) for C, H and N. Scanning electron microscopy (SEM) images were obtained with a Merlin field emission scanning electron microscope (Zeiss, Germany). Samples were dispersed in ethanol at an appropriate concentration were cast onto an aluminum foil, followed by solvent evaporation under vacuum at room temperature. Transmission electron 4


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microscopy (TEM) was undertaken on a JME-2100F (JEOL, Japan) instrument operated at 200 kV. Samples were dispersed in ethanol at an appropriate concentration and cast onto a carbon-coated copper foil, followed by evaporation of the solvent under vacuum at room temperature. X-ray photoelectron spectroscopy (XPS) was carried out on a Kratos Axis Ultra DLD system (Kratos, UK) equipped with a hemispherical electron analyzer and a Al Kα 450-W X-ray source. Binding energies were referenced to the C 1s signal of adventitious carbon (284.8 eV) to a precision of 0.3 eV. Peak fitting deconvolution was performed using a combination of Gaussian-Lorentzian curves with shirley background subtraction. Hydrogen temperature-programmed reduction (H2-TPR) was performed on a Micromeritics Autochem II instrument (Micromeritics, USA), equipped with a thermal conductivity detector (TCD). Approximately 50 mg of catalyst sample was loaded in a U-shape quartz reactor, pre-treated in a flow of He (flow rate: 30 mL min-1) at 200 oC for 1 h, then cooled to 80 oC. Subsequently, the He flow was switched to a mixture of 10 vol% H2/Ar (flow rate: 30 mL min-1) and held at 80 oC for 2 h. The sample was then heated under 10 vol% H2/Ar (flow rate: 30 mL min-1) from 80 to 800 o

C at a ramp rate of 10 oC min-1 and held at the final temperature for 10 min. The

temperature-programmed desorption of ammonia (NH3-TPD) was performed on an Autochem 2910. The sample (~0.1 g) was pretreated at 200 oC for 1 h under He flow (30 mL min-1), then cooled to 80 oC. NH3 gas was injected to saturate the sample, then purged with He flow for 2 h to remove weakly bound NH3 at 100 oC. The desorption profile of NH3 was measured using TCD from 80 to 800 oC at 10 oC min-1. Temperature-programmed desorption (TPD) of isobutane and isobutene was carried out on a home-built four-channel reactor equipped with a HPRO 20 mass spectrometer (HIDEN, UK). 200 mg of sample was treated at 200 oC in a He flow of 20 mL min-1 for 0.5 h to remove adsorbed water and other gaseous species. Then pure isobutane or isobutene gas was introduced and absorbed at room temperature for 0.5 h and 5



Page 6 of 26

subsequently purged with He (20 mL min-1) for 2 h to remove physical adsorption of species until the baseline becomes stable, followed by a temperature ramp from room temperature to 700 oC at a rate of 10 oC min-1. TPD traces were recorded by MS (m/z 42 for isobutane and m/z 41 for isobutene). Catalytic test The direct dehydrogenation of isobutane was conducted according to our reported method with modification.34 Typically, 200 mg of catalyst was fixed on quartz-wool plugs in a conventional fixed-bed quartz tubular reactor of 8 mm inner diameter. The reaction temperature was set at 600 oC. The reaction mixture consisting of isobutane (20 vol%) and N2 balance was fed at 3 L h-1 g-1. The composition of the outlet gas was analyzed on-line by a 7890A gas chromatography (Agilent, USA), equipped with a flame ionization detector and a HP-AL/S, 50 m, 0.32 mm × 8 µm plot column. All data were collected after 10 min at the chosen temperature, the conversion of isobutane (eq. 1), selectivity of isobutene (eq. 2), yield of isobutene (eq. 3), rate of isobutane conversion (eq. 4) and isobutene production (eq. 5) were calculated as follows:

C(%)=

S(%)=

[ Fi−C4 H10 ]in − [ Fi−C4 H10 ]out [ Fi−C4 H10 ]in

× 100

(Fx )out × 100 [ Fi−C4 H10 ]in − [ Fi−C4 H10 ]out

Y(%)=Ci−C4H10 × Si−C4H8

ri−C4 H10 =

ri −C4 H 8 =

[ Fi−C4 H10 ]in × Ci−C4 H10 mcatalyst [ Fi −C4 H10 ]in × Yi −C4 H 8 mcatalyst 6


(1)

(2)

(3)

(4)

(5)

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Where C, S and Y represent the isobutane conversion, product selectivity and isobutene yield, respectively, r is the isobutane conversion/isobutene production rate, respectively. x represents hydrocarbon products in the effluent gas stream, Fx is the corresponding molar flow rate, and m is the mass of the catalyst (g). Results and discussion The composition of Mo3O10(C6H8N)2·2H2O nanowire determined by ICP and CHN elemental analysis was listed in Table S1. The morphologies of nanowires calcined at 600, 650, 700, 750 and 800 oC are shown in TEM images (Figure 1, S1 and S2). For example, Mo-C700 sample (calcined at 700 oC), a nanowire structure and element composition are observed in TEM and EDS images (Figure 1a and 1b). In the HRTEM images (Figure 1c and 1d), the lattice spacing of 0.34 and 0.24 nm is assigned to the (011) and (002) facets of MoO2, respectively, and the 0.21 nm consistent with the (200) crystal plane of Mo2N. Accordingly, HAADF-STEM shows that Mo, O, C and N elements are distributed uniformly, with MoO2 and Mo2N occur in the center of the nanowire, and some larger MoO2 nanoparticles are present at the edge and along the nanowire (Figure 1e and 1f). When calcination temperature changes from 600 to 800 oC, the the as-synthesized nanowire precursors are mainly converted to crystalline β-Mo2C, which is significantly aggregated at 800 oC (Figure S2a-d). XRD (Figure 1g) and HRTEM images (Figure S2e-h) demonstrated that the composition of Mo-C600 is MoO2, Mo2N and MoO2 coexisted in Mo-C650; nevertheless, η-MoC and β-Mo2C can be observed in Mo-C750, and Mo-C800 is consist of β-Mo2C.

7



♣

(b)

(c)

♦

(g)

♥

: MoO2

∆

: Mo2N

: η-MoC

♣

: β-Mo2C

♣♣

Intensity (a.u.)

(a)

♣ ♣ ♣♥ ♥

(d) (d)

♦

♦∆

♦

♦∆

♦

♦

20

30

♣

∆

♦

∆

♦

♣

♣ ♣♣ Mo-C 800

♣♥

♣ ♣ Mo-C750

∆

∆

∆

♦ 40

50

Mo-C700 Mo-C650 Mo-C600

60

70

80

90

o

2θ( ) 2+

(h)

Mo

Mo3d

3+

Mo

4+

(e)

Mo

Intensity (a.u.)

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

Page 8 of 26

(f)

240

6+

Mo

238

236 234 232 230 Binding Energy (eV)

228

226

Figure 1. Catalyst characterization: (a) TEM image, (b) EDS spectrum, (c and d) HRTEM images, (e) HAADF-STEM image and (f) EDS elemental mapping of Mo (red), O (blue), C (green) and N (yellow) of Mo-C700; (g) XRD patterns of Mo-CT samples; (h) XPS Mo 3d spectra of Mo-C700.

In agreement with SEM and TEM (Figure S1), XRD patterns of the as-prepared precursor exhibits the monoclinic Mo3O10 (C6H8N)2·2H2O structure (Figure S3). Heat treatment of the precursor in Ar could thermally decomposes aniline, in which aniline acts as both the reducing agent and carbon source. However, different annealing temperatures are known to influence the distribution of Mo species in the resulting materials.35 As shown in Figure 1g, Mo-C600 exhibits a significant MoO2 fraction, while Mo-C650 and Mo-C700 possess an additional phase corresponding to Mo2N. Further increase in annealing temperature results in the formation of a dominant β-Mo2C phase, as shown in Mo-C750 and Mo-C800. However, η-MoC phase is exclusively found in Mo-C750. Analysis of the textural properties of Mo-CT samples (Figure S4) revealed the highest surface area for Mo-C700 sample (51 m2 g-1), and TGA spectra of Mo-CT further demonstrate the previous structure and composition results (Figure S5). 8


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Although XRD and TEM analysis confirm the composition of Mo-CT catalysts, the surface chemical state cannot be determined. Hence, XPS was used to investigate the chemical composition and oxidation state. Typically, the Mo 3d spectrum (Figure 1h) shows the characteristic peaks of Mo2+ (228.8 and 231.9 eV), Mo3+ (229.6 and 232.7 eV), Mo4+ (231.0 and 234.1 eV) and Mo6+ (232.9 and 236.0 eV), with a spin energy separation between 3d5/2 and 3d3/2 of 3.1 eV. The fact that Mo6+ species in the form of MoO3, usually generated from the oxidation of the metastable MoO2 or MoxC in air,36 does not show a visible diffraction peak, suggesting it is well dispersed throughout the catalyst. Based on the above characterization results, it is concluded that Mo-CT with different morphology and composition have been synthesized successfully via adjustment of the annealing temperature. The isobutane dehydrogenation performance of the Mo-CT catalysts is presented in Figure 2. The catalytic activity of the catalyst prepared at 600 oC has positive correlation with calcination temperature up to 700 oC, where a maximized isobutane rate of 3.9 mmol g-1 h-1 (Figure 2a), isobutene selectivity of 73% (Figure 2b) and formation rate of 2.8 mmol g-1 h-1 (Figure 2c) are obtained. However, catalytic activities decrease as materials prepared at higher temperatures. For example, Mo-C800 has an isobutene formation rate of 2.0 mmol g-1 h-1, which is significantly aggregated with reduced surface area (Table S2). In particular, two most active catalysts of Mo-C650 and Mo-C700 comprise of MoO2 and Mo2N, while Mo-C750 and Mo-C800 are predominantly β-Mo2C. It thus suggests that the initial crystalline phases may have an important impact upon catalytic performance.

9



(b)

5

Selectivity of isobutene (%)

ri-C4H10 (10 mmol g-1 h-1)

(a)

4 3 Mo-C600 Mo-C650

2

Mo-C700 1

Mo-C750 Mo-C800

80 70 60

Mo-C600 Mo-C650

50

Mo-C700

40

Mo-C750 Mo-C800

30 20

0 0

20

40

60

80

100

0

120

20

4

60

80

100

120

(d) 100

3

80

2

(%)

(c)

40

Time on stream (min)


ri-C4H8 (mmol g-1 h-1)

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

Page 10 of 26

Mo-C600 Mo-C650

Carbon balance Selectivity Conversion Yield

60 20

Mo-C700

1

10

Mo-C750 Mo-C800

0 0

20

40

60

80

100

0 0

120

10

20

30

40

50

Time on stream (h)


Figure 2. The dehydrogenation performance of Mo-CT catalysts. (a) Rate of isobutane conversion. (b) Isobutene selectivity. (c) Rate of isobutene production. (d) Carbon balance (△), isobutene selectivity ( ◇), isobutane conversion (●) and isobutene yield (☆) for Mo-C700 as a function of time.

In this study, Mo-C700 was chosen for long-term stability test due to its highest activity. Figure 2d shows that this catalyst possesses excellent stability over 50 h, with a carbon balance of 99%±2% and an isobutane conversion ca. 15%. The selectivity of isobutene started at approximately 73%, and slowly decreased and maintained at 68% with a 10% isobutene yield. Interestingly, the surface area of this catalyst does not change significantly after 50 h (Table S2), indicating that it exhibits excellent stability, due to minimization of carbon deposition. As reported previously, MoOx and Mo2C species have good catalytic activity for propane and n-butane dehydrogenation reactions (Table S3), but these catalysts often need frequent regeneration due to their low stability. Notably, the Mo-C700 catalyst prepared by a simple synthetic process, demonstrates suitable catalytic properties for direct isobutane dehydrogenation, whilst exhibiting excellent time on stream stability in comparison to those of Mo2C catalysts 10


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prepared by the carburation of calcined MoO3 or molybdate under CH4/H2 atmosphere. (b)

Isobutane

10

Mo-C600

8

Mo-C650

o

205 C

Mo-C750 Mo-C800

4 2

o

320 C

90 235 C

o

500

o

30

0 100

Mo-C650 Mo-C700

6

Mo-C750 Mo-C800

4

o

187 C

10

Mo-C600

200 300 400 o Temperature ( C)

12

(d)

Isobutane

113 C

Mo-C800

230 C

600

185 C o

8

o

305 C

2

600

Mo-C600 o

255 C

8

500

Isobutene

10

10

10

300 400 o Temperature ( C)

Mo-C650 Mo-C750

60

MS Signal (×10 ) /mass 41

(c)

200

Mo-C600 Mo-C700

o

0 100

Isobutene

o

246 C

10

Mo-C700

6

120 MS Signal (× 10 ) /mass 41

10

MS Signal (× 10 ) /mass 42

(a)

MS Signal (× 10 ) /mass 42

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


Mo-C650 Mo-C700 Mo-C750

6 o

Mo-C800

203 C

4 2 0

0 100


500

100

600


500

Figure 3. Isobutane (m/z 42) and isobutene (m/z 41) TPD profiles over Mo-CT catalysts. (a and b) Before reaction. (c and d) After reaction for 2 h.

Significantly, the preceding catalytic performance is dependent upon the types of Mo-CT catalysts. Hence, the desorption behaviors of these catalysts for isobutane and isobutene were investigated by TPD experiments. As shown in Figure 3a, Mo-C600, Mo-C650, Mo-C700 and Mo-C750 catalysts have desorption peaks for isobutane ca. 205 o

C, of which Mo-C700 exhibits the stronger desorption feature than either Mo-C750 and

Mo-C800. From isobutene TPD results (Figure 3b), it is interesting to see that catalysts calcinated at lower temperature (Mo-C600, Mo-C650 and Mo-C700) showed relatively weak interaction with isobutene, exhibiting weaker features at approximately 230 oC. These results coincide well with the maximum activity and selectivity found for Mo-C700, where stronger adsorption of isobutane and weaker adsorption for isobutene 11


600


are anticipated to be favorable properties for this reaction. Intensive investigation of the TPD performance for catalysts obtained after 2 h reaction can further verify the characteristics described above. Accordingly, Mo-C700 showed a relatively small change in adsorption behavior compared to the fresh one, while other catalysts showed either significantly weakened interaction or reduced isobutane adsorption capacity (Figure 3c). Furthermore, all catalysts exhibit decreased adsorption capacity and weaker interaction with isobutene (Figure 3d), which are undoubtedly beneficial for the dehydrogenation reaction. XRD patterns of the catalysts after reaction for 2 h (Figure 4a) demonstrate that negligible changes occur in bulk crystal structures of the materials prepared at 600, 750 and 800 oC. However, Mo-C650 and Mo-C700 possess a new crystalline phase, coinciding with η-MoC in addition to a substantial decline in diffraction peaks associated with Mo2N. It is worth noting that the surface area for Mo-C600, Mo-C650 and Mo-C700 catalysts increased after reaction (Table S2), which can be ascribed to the transformation of MoO2 to MoxC, as demonstrated by aforementioned XRD results and the further decomposition of the catalyst precursor during reaction. On the other hand, the catalysts calcined at higher temperature showed a minor decrease in surface area (Mo-C750 and Mo-C800), which can be assigned to particle aggregation. It is well known that coke formation usually occurs during this type of dehydrogenation process. Coke formation will cause a decline in surface area and pore volume, while increasing the pore size. In this study, the unchanged surface area and decreased pore size of Mo-C700 catalyst after 50 h implied that coke formation is negligible during this DDH process. Considering the superior catalytic properties of Mo-C700, a more thorough investigation of the crystalline phase transition during reaction is conducted. In Figure 4b and 4c, the bulk structure remains intact up to 20 min, however, at 30 min the mixed oxide/nitride phase partially transforms to a Mo0.42C0.58 phase (JCPDS: 12


Page 12 of 26

Page 13 of 26

36-0863). After 60 min, η-MoC starts to form, while after 2 h a substantial decline of the Mo2N phase is found (Figure 4c). It can be seen that the isobutene selectivity (Figure 2d) stabilizes with the evolution of η-MoC, which may play an important role in isobutane dehydrogenation. (b)

♣

(a)

MoN, JCPDS: 25-1367 η -MoC, JCPDS: 08-0384

♦: MoO2 ♥ : η -MoC ♣: β -Mo2C

♣

1h

Mo-C800

♣

♣ ♣♣

♣

♣♣

♣

♥ ♣ ♣♥

Mo-C750

♦

♦ ♦

♥

♦

♥

♦ ♦♥ ♦

♦

♦

♦♥ ♦

♦

♦

20

♣

Mo0.42C0.58, JCPDS: 36-0863

Intensity (a.u.)

Intensity (a.u.)

♣♣

Mo-C700 ♥

30 min MoO2, JCPDS: 78-1069 Mo2N, JCPDS: 25-1366

20 min

10 min

Mo-C650 ♥

0 min

♦

30

40

50

Mo-C600 ♦ ♦ o

60

70

80

20

90

30

40

50 h

(d)

β -Mo2C, JCPDS: 35-0787

25 h

60

5h

30

Mo-C750

MoO2, JCPDS: 78-1069

40

50

60

90

η -MoC

Mo-C700

η -MoC, JCPDS: 08-0384

20

80

β -Mo2C Mo-C800

Mo3C2, JCPDS: 42-0890

2h

70

o

Intensity (a.u.)

(c)

50

2θ( )

2θ( )

Intensity (a.u.)

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


70

80

90

20

o

30

40

50

60

70

80

o

2θ( )

2θ( )

Figure 4. XRD patterns of (a) Mo-CT catalysts after reaction for 2 h. (b-c) Ex situ XRD patterns of Mo-C700 after different reaction times. (d) Mo-C700 after reaction for 50 h, Mo-C750 and Mo-C800 after reaction for 2 h.

In spite of the changes in Mo phases, the Mo-C700 catalyst remains relatively stable (Figure 2d). Therefore, it is reasonable to be believed that substantial structural changes may occur at reaction time longer than 2 h (Figure 4c). After 5 h reaction, the 13



Page 14 of 26

catalyst contains neither crystalline MoO2 nor residual nitride. The primary phase coincides with η-MoC, while an additional metastable Mo3C2 phase is also present. For even longer reaction time, β-Mo2C becomes evident and it suggest that the metastable phase may be an intermediate in the transition from η-MoC to β-Mo2C. This implies that the evolution of the carbide phase may lead to a material with less selectivity and activity, by considering the general decline in isobutene yield from 11.6 to 10.0% after 50 h. As shown in Figure 4d, β-Mo2C is the predominant phase upon Mo-C750 and Mo-C800, while Mo-C750 comprise an amount of η-MoC, which is responsible for a higher activity and isobutene yield. Interestingly, η-MoC is found to be dominant phase for Mo-C700 after 50 h reaction, though some β-Mo2C still exists (Figure 4d and Figure S6). On this basis, it is believed that η-MoC is more active and selective phase than β-Mo2C carbide phase. Table 1. Mo 3d XPS analysis of as-prepared and after 2 h reaction Mo-CT catalysts. Total Mo Sample

Species (Fresh/Used, %)

atom % Mo

2+

3+

Mo

Mo

4+

n2+ /3+

T2+ +3+ (%)

6+

Mo

Mo-C600

10.60/7.85

N. D./N. D.

2.28/2.67

1.06/0.87

7.25/4.31

N. D./N. D.

2.28/2.67

Mo-C650

10.31/9.31

N. D./1.66

1.82/2.75

0.78/0.98

7.72/3.92

N. D./0.60

1.82/4.41

Mo-C700

18.94/18.82

6.25/12.10

4.03/3.74

1.30/1.40

7.37/1.61

1.55/3.24

10.28/15.84

Mo-C750

15.50/17.22

5.31/10.20

2.91/3.37

N. D./N. D.

7.27/3.61

1.83/3.03

8.22/13.57

Mo-C800

15.58/18.73

5.41/9.92

2.40/4.81

N. D./N. D.

7.77/4.00

2.25/2.06

7.81/14.73

N. D.: not detected

To further probe above hypothesis, the elemental surface compositions and Mo valance states of Mo-CT catalysts are summarized (Figure 5 and S7-S10, Table 1 and 14


Page 15 of 26 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


S4). Fresh Mo-C600 and Mo-C650, contain 1.06% and 0.78% of Mo4+ (MoO2), respectively, while no Mo2+ was detected (Figure 5a and Table 1). When the annealing temperature increased to 700 oC, Mo2+ starts to appear and the amount of Mo4+ decreases accordingly. As discussed above, the gradual formation of η-MoC during the reaction (Figure 4) reveals the conversion of Mo oxide and nitride to carbide. This further means that η-MoC is responsible for the catalytic activity. Thus, more attention has been paid to Mo2+ and Mo3+. It is obvious that the ratio of Mo2+/Mo3+ ( n2+ /3+ ) tends to increase with an increase of calcination temperature (Table 1) for the fresh sample. After 2 h of reaction, catalytic activity exhibits a stabilized state (Figure 2d), where the content of Mo6+ is significantly declined, and Mo2+ and Mo3+ species gradually increase for all catalysts, except Mo-C600. Specifically, the n2+ /3+ values visibly increase except for Mo-C800, while the total content of Mo2+ and Mo3+ ( T2+ +3+ ) species increased obviously (Figure 5b and Table 1). A direct comparison of the catalytic performance for Mo-CT with different n2+ /3+ and T2+ +3+ values are illustrated in Figure S11. It was found that, the dehydrogenation activities of Mo-CT are dependent on the variation of n2+ /3+ and T2+ +3+ values on the surface. The activity is optimized at intermediate values of n2+ /3+ = 3.24 and T2+ +3+ = 15.84, which coincides with Mo-C700.

15



2+

Mo

(a)

4+

Mo

3+

2+

Mo

4+

Mo

3+

Mo

(b)

Mo Mo

Mo/C800

Mo/C800

Mo/C750

Mo/C750

2+

Mo

4+

Mo

Mo

(c)

6+

Mo

6+

Page 16 of 26

Mo

3+

6+

50 h

25 h Mo/C700

Mo/C700 Mo/C650

Mo/C650

0h

Mo/C600

Mo/C600 226 228 230 232 234 236 238 240 Binding Energy (eV)

2h

226 228 230 232 234 236 238 240

226 228 230 232 234 236 238 240 Binding Energy (eV)

Binding Energy (eV)

Figure 5. Mo 3d XPS spectra. (a) Before and (b) after 2 h reaction for Mo-CT catalysts. (c) Mo-C700 after different reaction times.

Table 2. Mo 3d XPS analysis of as-prepared and after 2, 25 and 50 h reaction for Mo-C700 catalyst.

Species (%) Reaction time (h) Mo

2+

Mo

3+

Mo

4+

Mo

n2+ /3+

T2+ +3+ (%)

6+

0

6.25

4.03

1.30

7.37

1.55

10.28

2

12.10

3.74

1.40

1.61

3.23

15.84

25

12.80

3.94

N. D.

N. D.

3.25

16.74

50

17.60

4.70

N. D.

N. D.

3.74

22.30

N. D.: not detected

The reaction time effect of Mo-C700 catalyst was also examined by XPS (Figure 5c) and the results are summarized in Tables 2 and S5 by using the substantial different in the Mo 3d peak between the fresh and used catalysts. After 2 h reaction, an obvious increase in Mo2+, Mo3+, Mo4+ and a reduction in Mo6+ (1.61%) are observed. At longer reaction time (25 h) Mo2+ and Mo3+ are the only found Mo valance states, suggesting that the primary crystalline phases are associated with carbides, such as, η-MoC and 16


Page 17 of 26

β-Mo2C. This observation shows a good agreement with XRD results in Figure 4. The high stability of the Mo-C700 catalyst is also in accordance with the increased values of

n2+ /3+ and T2+ +3+ for 50 h reaction. Raman spectra of fresh and spent Mo-CT catalysts (Figure S12) were obtained to examine the degree of graphitization of the samples. During the stability test for Mo-C700, bands at 141, 277 and 985 cm-1 correspondingly shift to 137, 273 and 983 cm-1(Figure 6a). In line with the XRD results (Figure 1g), the initial one corresponds to the generation of new MoxC phases after 50 h reaction due to the rearrangement of MoO2 from oxygen vacancies and the latter two variations are related to the substitution of oxygen with carbon.37 In this study, characteristic D and G carbon bands are observed via Raman spectroscopy at approximately 1330 and 1600 cm-1, respectively (Figure 6b and S12). It is interest to note that the relative intensity ratio of D to G band (ID/IG) exhibits a typical volcano trend, where it reaches to the maximum value for the material prepared at 700 oC (Figure S12b). It indicates that the treatment of calcination at different temperatures has a significant effect on the nature of the resulting carbon as well. For spent catalysts, there are significant decreases in ID/IG values (Figure S12d), reflecting the increase of graphitization degree. However, the Mo-C700 catalyst possesses the highest value of ID/IG=3.77 after 2 h, which gradually declined to 2.86 in conjunction with a degradation of catalytic activity after 50 h (Figure 2d). These observations suggest that more structural defects are beneficial for catalytic performance.

137

(b)

273

25 h 2h

100

277

300

700

-1

900

50 h

2.86

25 h

2.95 3.77

2h

4.41

0h

985

0h

500

G ID/IG

50 h

141

D

983

Intensity (a.u.)

(a) Intensity (a.u.)

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


1000

1100

1200

1400

1600 -1

Raman shift (cm )

Raman shift (cm )

17


1800


0.05

o

354 C o 260 C

o

o

717 C

(d)

560 C

Mo-C800

0.01

o

250 C

Mo-C800

o

667 C

o

425 C

(e)

0.01

o

697 C

o

o

o 362 C 267 C

632 C

o

o

50 h 195 C

245 C

Mo-C750

Intensity (a.u.)

Mo-C750 o

383 C Mo-C700 o

366 C Mo-C650

Mo-C700

o

237 C o

240 C

o

Mo-C650

686 C

o

253 C

o

395 C

o

695 C

Intensity (a.u.)

(c)

Intensity (a.u.)

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

Page 18 of 26

o

685 C o

25 h 195 C o

632 C

o

654 C

o

Mo-C600

237 C

Mo-C600

o

425 C

2h

100 200 300 400 500 600 700 800 o

Temperature ( C)

100 200 300 400 500 600 700 800 o

Temperature ( C)

100 200 300 400 500 600 700 800 o

Temperature ( C)

Figure 6. Raman spectra of Mo-C700 after different reaction times: (a) 100-1100 cm-1, (b) 1100-1800 cm-1. H2-TPR profiles of Mo-CT catalysts: (c) Before and (d) after 2 h reaction, (e) Mo-C700 after different reaction times.

Additionally, catalyst reducibility was examined by TPR. As shown in Figure 6c, the adsorptive carbon gradually generated when the calcination temperature increased from 600 to 750 oC, with the reduction peaks at 267 and 260 oC for Mo-C750 and Mo-C800 catalysts, respectively.38 After 2 h of reaction, more oxygen consumed and all Mo-CT catalysts showed the same peaks at 240-260 oC, due to the reduction of adsorptive carbon (Figure 6d). On the basis, with the calcination temperature increasing, the intensity of the first peak decreased for the spent catalysts. This observation indicated that higher calcination temperature leads to a decrease of the adsorptive carbon and stable MoC1-x gradually generated as observed in above XRD results (Figure 4). For fresh catalysts (Figure 6c), the distinct peaks at 350-400 oC correspond to the reduction of Mo6+ to Mo4+, and the second peak at 500-720 oC is consistent with the reduction of pyrolytic carbon from thermal cracking of hydrocarbons.38-41 Moreover, higher temperatures are needed to further reduction of Mo4+ to Mo0.42 As proved by XPS and XRD, the fact that less existed Mo6+ species and more gradually generated MoC1-x for all spent catalysts, can be well reflected by the decreased peak intensity for the reduction of Mo6+ to Mo4+ (Figure 6d). Likewise, the weakened reduction peaks of 18


Page 19 of 26 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


pyrolytic carbon at 600-700oC reveal that less pyrolytic carbons are generated as some of them can be transformed to graphitic carbon and decomposed at high temperature (800-900 oC).40 During the long stability reaction for Mo-C700 catalyst (Figure 6e), the reduction peak of Mo6+ to Mo4+ is hardly observed and it is verified by XPS that no Mo6+ species existed after 25 h. It is also interesting to see that there are no significant changes observed by comparing samples obtained after 25 and 50 h reaction, revealing that the catalyst can reach a steady state after 25 h. Furthermore, there are no obvious changes on the reduction peaks of pyrolytic carbon after 50 h reaction, suggesting that no increased coke occurs on the surface. This observation can be used as the evidence to explain the stable catalytic performance of the Mo-C700 catalyst. The acidic properties of the Mo-CT catalysts were investigated by NH3-TPD and the results are shown in Figure S13. As can be seen from the profiles, Mo-C600, Mo-C650 and Mo-C700 exhibit desorption peaks at around 250-350 oC and 350-450 oC, which can be ascribed to moderate and strong acid sites, respectively.31, 43 With the increase of calcination temperature, the intensity of moderate and strong acid desorption peaks decreases and shifts towards higher temperature, except for the Mo-C750. The acidic strength distribution, determined via the Gaussian curve fitting method was conducted10, 44 and the data are summarized in Table S6. It shows that the temperature of maximal peak intensity (TM) increases and the total number of acid sites decrease with increasing treatment temperature other than Mo-C700. According to the peak area fraction of different acid sites, the Mo-C700 catalyst exhibited a medium acid site density and a larger fraction of moderate acid sites (Table S6). It has been reported previously that stronger acid sites facilitate the cracking reaction,43, Mo-C700 should promote the dehydrogenation reaction over cracking.

19


45

therefore,


i-C4H8

CH4

C2H4

C2H6

C3H6

C3H8

Conversion

100

30 25

80 20 60 15 40

10

20

5

0

Conversion of isobutane (%)

(b)

(a)

Selectivity (%)

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

Page 20 of 26

0 Mo-C600

Mo-C650

Mo-C700

Mo-C750

Mo-C800

Samples

Figure 7 (a) Performance of different catalyst compositions obtained by variations of preparation temperature and reaction times. (b) The catalytic performance of Mo-CT catalysts after reaction for 2 h.

In an effort to correlate phases with differences in activity and selectivity, three catalysts obtained after 2 h time on stream (Mo-C600, Mo-C700 and Mo-C800) were utilized as materials representing MoO2, MoO2+η-MoC and β-Mo2C phases, respectively, while η-MoC+β-Mo2C combination phases have been generated under the same experimental conditions after 50 h for Mo-C700. According to the presented data in Figure 7a, the evaluation of the influence of active phase upon catalytic parameters is allowed. It showed that β-Mo2C (Mo-C800) has the lowest catalytic activity amongst all samples (Figure 2 and Figure 7a) and a reasonable enhancement in propylene selectivity is exhibited when the catalyst composition contains β-Mo2C, while the presence of MoO2 appears to suppress the formation propylene. When considering the selectivity of methane, an opposite trend is found. Under the same reaction condition, the selectivity to isobutene, propylene and methane is over 97%, while the selectivity to isomerization products can be neglected (Figure 7b). Taking a closer look at the overall selectivity of [CH4+C3H6], it is noticed that MoO2, MoO2+η-MoC, η-MoC+β-Mo2C and β-Mo2C give total values of 29.8%, 24.8%, 29.8% and 32.3%, respectively. Notably, the presence of η-MoC for MoO2+η-MoC and η-MoC+β-Mo2C combinations display lower [C3H6+CH4] selectivity than either pure MoO2 and β-Mo2C, respectively.

7, 46, 47

In general, sensible changes in selectivity towards 20


Page 21 of 26 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


isobutene occur across the explored range, but match the trend found for the combined selectivity of propylene and methane (Figure 7a). Moreover, the best selectivity is observed when both MoO2 and η-MoC are presented in the catalysts composition, this indicates the substantial potential of the new carbide phase (η-MoC) towards realizing a more selective isobutane DDH catalyst though the role of pure η-MoC phase has not been exclusively demonstrated at present, but work is currently ongoing in the hope of achieving this in the near future. In summary, we have demonstrated a method for the synthesis of Mo-CT catalysts for isobutane DDH. Different fractions of MoO2, Mo2N, η-MoC and β-Mo2C were obtained by altering the calcination temperatures. Of the series of tested catalysts, Mo-C700 showed superior catalytic performance, with approximately 15% conversion and 73% isobutene selectivity. The main by-products are methane and propylene, indicating cracking reaction is more active than isomerization reaction. The evolution of Mo-C700 catalyst was probed over 50 h, XRD, XPS and H2-TPR measurements indicates that Mo6+ species are not stable under conditions, reducing to lower oxidation states (Mo2+ and Mo3+). After 50 h, the crystalline phase Figure 2 shows the reaction rate changed to a mixture of η-MoC and β-Mo2C coinciding with a minor loss in isobutene selectivity. TPD results suggested that Mo-C700 catalyst has a stronger adsorption for isobutane and weaker adsorption for isobutene. Consequently, the cracking reaction is partially suppressed in the presence of η-MoC. Therefore, an optimal Mo-CT with a suitable calcination temperature (700 oC) is the key to generate an abundance Mo2+and Mo3+ species with a suitable n2+ /3+ ratio. Efforts towards synthesizing a pure η-MoC have not been successful to date, however, work is still ongoing. Regardless, careful evolution of the change in crystal structure and product selectivity convincingly demonstrates the significant potential of this compared to the commonly studied β-Mo2C. Supporting information 21



Details on sample characterization such as BET, TGA-DSC, XPS and Raman spectra are included in the Supporting Information section. Author Information *Corresponding author Tel: 0086 20 8711 4704. Fax: 0086 20 8711 4707 E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Grant No. 21576096 and 21706077), Natural Science Foundation of Guangdong Province (Grant No. 2017A0303135053). References (1) Baek, J.; Yun, H. J.; Yun, D.; Choi, Y. and Yi, J., Preparation of highly dispersed chromium oxide catalysts supported on mesoporous silica for the oxidative dehydrogenation of propane using CO2: Insight into the nature of catalytically active chromium sites. ACS Catal., 2012, 2, 1893-1903. (2) Im, J. and Choi, M., Physicochemical Stabilization of Pt against Sintering for a Dehydrogenation Catalyst with High Activity, Selectivity, and Durability. ACS Catal., 2016, 6, 2819-2826. (3) Zhang, J.; Su, D.; Zhang, A.; Wang, D.; Schlögl, R. and Hébert, C., Nanocarbon as robust catalyst: Mechanistic insight into carbon-mediated catalysis. Angew. Chem. Int. Edit., 2007, 46, 7319-7323. (4) Frank, B.; Morassutto, M.; Schomäcker, R.; Schlögl, R. and Su, D. S., Oxidative Dehydrogenation of Ethane over Multiwalled Carbon Nanotubes. ChemCatChem, 2010, 2, 644-648. (5) Grant, J. T.; Carrero, C. A.; Goeltl, F.; Venegas, J.; Mueller, P.; Burt, S. P.; Specht, S. E.; McDermott, W. P.; Chieregato, A. and Hermans, I., Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts. Science, 2016, 354, 1570-1573. (6) Venegas, J. M.; Grant, J. T.; McDermott, W. P.; Burt, S. P.; Micka, J.; Carrero, C. A. and Hermans, I., Selective Oxidation of n‐Butane and Isobutane Catalyzed by Boron Nitride. ChemCatChem, 2017, 9, 2118-2127. (7) Rodemerck, U.; Sokolov, S.; Stoyanova, M.; Bentrup, U.; Linke, D. and Kondratenko, E. V., 22


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