Hydrothermally prepared chromia-alumina (xCr/Al₂O₃) catalysts with hierarchical structure for propane dehydrogenation

Abstract

Propane dehydrogenation was investigated over a series of chromia-alumina (xCr/Al₂O₃) catalysts containing 2.5–10 wt% chromium (Cr), synthesized via a hydrothermal synthesis method. The synthesized xCr/Al₂O₃ catalysts with hierarchical spindle-like morphology have high specific area and highly dispersed chromia on the surface of γ-Al₂O₃. Besides, the higher Cr⁶⁺/Cr³⁺ ratio is advantageous to obtain higher propane conversion and lower propylene selectivity. The synthesized 7.5Cr/Al₂O₃ catalyst exhibits the highest propane conversion of 62% accompanied by 89% propylene selectivity at 873 K. After regenerated by re-calcinating in air, the original activity of the spent catalyst is well recovered.

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1. Introduction

Propylene is an important basic chemical raw material, which can be used to produce polypropylene, epoxypropane, acrylonitrile, acrolein and other downstream products. Because of growing demand for propylene, much effort has been paid to new propylene production routes. Dehydrogenation of propane (DHP) or oxidative dehydrogenation of propane (ODHP) is a promising industrial alternative to produce propylene since it make low-value propane to high valuable propylene, which is now mainly produced by steam cracking and fluid catalytic cracking (FCC) process.

Supported chromia catalysts were intensively studied for the dehydrogenation of light alkanes. However, the deactivation of Cr-based catalysts due to the catalytic cracking and coking is a major challenge in DHP or ODHP process.^1–4 Therefore, it is important to develop supported chromia catalysts with high reactivity, selectivity and stability. To approach this aim, the various strategies, such as exploring preparation methods,^2,5–9 using novel carriers^10–14 and Cr precursors,¹⁵ introducing additives^16–18 etc. have been widely employed.

The chromia-alumina catalysts revealed in previous works are mainly prepared by the methods of co-precipitation, incipient wetness impregnation, atomic layer self-assembly, atomic layer deposition technique and so on.^1,19–22 However, to our best knowledge, the utilization of hydrothermal method to synthesize CrO_x–Al₂O₃ catalysts for the dehydration of light alkanes has not been revealed. The hydrothermally prepared γ-Al₂O₃ with surfactant-assistance with hierarchical nanoarchitecture and high specific surface had been revealed as effective adsorbents.²³ The hydrothermally prepared Cr₂O₃–ZrO₂ exhibited an enhanced catalytic reactivity compared to the conventional samples.⁷ Pan also found that W/γ-Al₂O₃ catalysts prepared by a surfactant-assisted hydrothermal deposition method had higher dispersion of tungsten species, more open pore channels, and more acid sites, and thus presented a significantly enhanced hydrodenitrogenation activity compared with the catalyst prepared by impregnation.²⁴ In this work, the alumina-supported chromium oxides with hierarchical porous structure are synthesized and used as efficient catalysts in the dehydrogenation of propane. The catalysts exhibit high catalytic activity for the dehydrogenation of propane. The structure–activity relationship of the synthesized catalysts is analyzed upon numerous state-of-the-art analytical techniques.

2. Experimental

2.1 Preparation of catalysts

The catalysts were synthesized by direct hydrothermal method. The required amount of Cr(NO₃)₃·9H₂O and Al(NO₃)₃·9H₂O were dissolved in 80.0 g deionized water and stirred for 3 h at 333 K. Then 20.0 g tetrapropylammonium hydroxide (TPAOH) was added to the mixed solution and stirred overnight. The obtained solution was transferred to a teflon-linked stainless-steel autoclave and sealed. The autoclave was heated at 443 K for 3 days, and then cooled down to room temperature. After centrifuged, the obtained sample was dried at 373 K for 24 h, and then calcined at 823 K in air for 5 h. The catalysts were referred to as xCr/Al₂O₃, where x represents the chromium loading (mass percent) on Al₂O₃.

2.2 Characterizations

XRD measurements were performed on a Bragg-Brentano diffractometer (Rigaku D/Max-2000) with monochromatic CuKa radiation (λ = 1.5418 Å) of graphite curve monochromator in 10–80° with a scan speed of 4° min⁻¹. The ultraviolet-visible (UV-vis) spectra of the samples were performed on a spectrometer (UV-3600, Shimazdu) in the range of 230–1500 nm with BaSO₄ as reference. The morphologies of xCr/Al₂O₃ catalysts were examined by a field emission scanning electron microscopy (FESEM, HITACHI S-4800) and transmission electronic microscopy (TEM, JEM-2010). The N₂ adsorption–desorption isotherms were detected on a Quantachrome NOVA 4000e. All samples were activated by degassing under vacuum at 573 K for 5 h before measuring. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI5000 VersaProbe™.

2.3 Propane dehydrogenation reaction

The catalytic propane dehydrogenation was performed at 873 K in a fix-bed quartz reactor (8 mm in diameter) at atmosphere pressure. The feed flow was the mixed gas of propane and Argon (V(C₃H₈) : V(Ar) = 1 : 4) with a feed flow rate of 20 ml min⁻¹. The catalyst (200 mg) was loaded into the reactor and pre-treated under Ar atmosphere for 30 min at 873 K before propane dehydrogenation reaction. The products containing methane, ethylene, ethane, propylene and propane were analyzed with an on-line gas chromatograph (SP-6890, Shandong Lu'nan) with a micro FID detector.

2.4 TGA

The amount of carbon deposit on the spent sample was also determined by thermogravimetric analysis (TGA). TGA was measured in air flow (50 ml min⁻¹) with a SHIMADZU DTG-60H thermogravimetric analyzer from room temperature to 1073 K at the rate of 10 K min⁻¹.

3. Results and discussion

3.1 X-ray diffraction

The XRD patterns of xCr/Al₂O₃ catalysts are shown in Fig. 1. The peaks at 2θ = 37.6°, 45.8° and 67.3° of all samples (including pure γ-Al₂O₃), which are attributed to the characteristic diffractions of γ-Al₂O₃,^23,24 are found in these samples. Furthermore, no diffraction peaks of chromia are observed due to its high dispersion state.²⁵ The absence of CrO_x peaks could be attributed to small Cr particle size.

Fig. 1 XRD patterns of xCr/Al₂O₃ catalysts.

3.2 Morphologies of xCr/Al₂O₃ catalysts and BET analysis

From Fig. 2(a), the FESEM images show that the xCr/Al₂O₃ catalysts exhibit hierarchical spindle-like morphology in this work. The similar nanohierarchical structure of γ-Al₂O₃ via surfactant-assisted hydrothermal method was also revealed.²³ With the addition of Cr, the particles grow and change to be more dispersive. The TEM images in Fig. 2(b) show that these particles are comprised of a large number of nanoflakes with an average size of ca.1.8 μm in length and 400 nm in width. It was reported that the hydrogen bonds between the boehmite (precursor of γ-Al₂O₃) surface and template molecules can reduce the free energy of crystallites to form low dimensional nanoflakes.^33,34 These nanoflakes tend to aggregate and reduce exposed area in order to decrease the surface energy. Consequently, the hierarchical spindle-like particles are formed through the oriented self-assembly mediated by TPAOH.³³

Fig. 2 FESEM (a) and TEM (b) images of xCr/Al₂O₃ catalysts.

The N₂ adsorption–desorption isotherms of xCr/Al₂O₃ catalysts are given in Fig. 3(a). The samples display type IV adsorption isotherms with H₃-type weak hysteresis loops between 0.5 and 0.99, which is related to big slit-like mesopores. When P/P₀ is above 0.9, the isotherms rise sharply and do not appear S shape, indicating the samples contain macropores. These macropores may be produced by the tiny gaps between the neighboring nanoflakes of catalysts. Fig. 3(b) shows that the samples present a comparatively sharper mesopore size distribution centered at 3.6–4.0 nm. When Cr loading exceeds 5.0 wt%, the catalysts exhibit bimodal pore distribution with mesopores (ca. 3.8–4.0 nm) and macropores (ca.15 nm). The textural parameters of xCr/Al₂O₃ catalysts in Table 1 show that all samples have high specific surface area (S_BET), moderate pore volume (V_P), and small average pore size (D_P). Additionally, the S_BET, V_P and D_P exhibit an increasing trend with the increase of Cr loading in the samples. It should be ascribed to the enhanced particle size and the dispersed nanoflakes, which is consistent with the morphologies shown in Fig. 2.

Fig. 3 N₂ adsorption–desorption isotherms (a) and the pore size distributions (b) of xCr/Al₂O₃ catalysts.

Table 1 Textural properties of xCr/Al₂O₃ catalysts

Samples	SBET (m^2 g^−1)	VP (cm^3 g^−1)	DP (nm)
γ-Al2O3	250.232	0.428	3.742
2.5Cr/Al2O3	258.546	0.490	3.772
5.0Cr/Al2O3	261.391	0.505	3.828
6.5Cr/Al2O3	277.115	0.569	3.836
7.5Cr/Al2O3	282.334	0.621	3.840
10Cr/Al2O3	295.196	0.725	3.921

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3.3 UV-vis spectra and XPS analysis of xCr/Al₂O₃ catalysts

To investigate the oxidation state of chromium, the diffuse reflectance UV-vis spectra of xCr/Al₂O₃ catalysts were carried out and shown in Fig. 4(a). All spectra show two intense absorption bands at 270 and 369 nm, which are attributed to O → Cr⁶⁺ charge transfer of chromate, verifying the presence of Cr⁶⁺ on the catalysts.^2,20,26,35 The weak shoulders at 456 nm assigned to d–d transition (A_2g–T_1g) of Cr³⁺ in octahedral symmetry.^3,15 Another weak band at ∼600 nm should be ascribed to another d–d transition (A_2g–T_2g) of Cr³⁺ in octahedral symmetry.^3,15

Fig. 4 Diffuse reflectance UV-vis spectra (a) and XPS spectra (b) of xCr/Al₂O₃catalysts.

The XPS spectra of xCr/Al₂O₃ catalysts are shown in Fig. 4(b). The binding energy values corresponding to Cr2p_3/2 are shown in Table 2. The oxidation states of Cr⁶⁺ and Cr³⁺ are found for all catalysts at ∼579 eV and ∼576 eV.^27–30 The XPS results also verify the existence of Cr⁶⁺ and Cr³⁺, which agrees with the UV-vis spectra of xCr/Al₂O₃ catalysts. The Cr⁶⁺% and Cr⁶⁺/Cr³⁺ ratios are calculated from the deconvoluted Gaussian fitting from the Cr2p_3/2 peaks shown in Fig. 5. It can be seen that the Cr⁶⁺/Cr³⁺ ratio varies with Cr loading, which influences the catalytic activity of Cr-based catalysts.^15,35 In addition, XPS is an effective method to study the dispersion of transition metal oxides on supports. Many researchers adopt the surface atomic ratios of the active metal elements to the element of the support as a measure of the dispersion of the catalysts.^31,32 The atomic ratio of xCr/Al₂O₃ catalysts detected by XPS ((Cr/Al)_XPS) in Table 2 increases almost linearly as Cr loading increases. This indicates that the chromium species is well dispersed over the investigated range of Cr loading, which is consistent with the XRD results.

Table 2 XPS data of xCr/Al₂O₃ catalysts (*the spent catalysts)

Catalysts	Cr 2p3/2 B.E.(eV)		Cr^6+%	Cr^6+/Cr^3+	(Cr/Al)xps
	Cr^6+	Cr^3+
2.5Cr/Al2O3	579.550	576.841	0.779	0.428	0.0426
6.5Cr/Al2O3	579.173	576.651	3.22	1.080	0.0777
7.5Cr/Al2O3	579.141	576.814	4.52	1.314	0.0971
10Cr/Al2O3	579.127	576.864	4.94	1.034	0.1220
6.5Cr/Al2O3*	579.771	576.455	0.402	0.096	—
7.5Cr/Al2O3*	579.589	576.399	0.757	0.176	—

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Fig. 5 Propane conversion (a) and propylene selectivity (b) over xCr/Al₂O₃ catalysts versus time on stream (reaction conditions: V_(C3H8) : V_(Ar) = 1 : 4, total flow rate = 20 ml min⁻¹, T = 873 K).

3.4 Catalytic performances of xCr/Al₂O₃ catalysts on propane dehydrogenation reaction

The propane conversion and propylene selectivity versus time on steam over the xCr/Al₂O₃ catalysts are shown in Fig. 5. It is observed from Fig. 5(a) that the propane conversion increases with increasing Cr loading from 2.5 wt% to 7.5 wt%, and then decreases by further adding Cr to 10.0 wt%. However, the improved propane conversion with increasing Cr loading may be obtained at the cost of propylene selectivity. From Fig. 5(b), the lowest propylene selectivity is obtained over 7.5Cr/Al₂O₃, which has the highest initial propane conversion of 62.2%. It also can be seen that the propane conversion for all samples decreases with time on steam due to the coke and deactivation of catalysts during the reaction process; while the selectivity varies little. The literatures revealed that the oxidative states of chromium species were key to the catalytic dehydrogenation activity of light alkanes though it is still controversial up to now.^1,35,36 As discussed earlier in this work, the UV-vis and XPS data of xCr/Al₂O₃ catalysts demonstrate the existence of Cr⁶⁺ and Cr³⁺ species. The surface Cr⁶⁺/Cr³⁺ ratios of xCr/Al₂O₃ catalysts are different, which influences the reducibility of catalysts. By comprehensively analyzing the catalytic activity and XPS data, the higher Cr⁶⁺/Cr³⁺ leads to a higher propane conversion and a lower Cr⁶⁺/Cr³⁺ propylene selectivity. The Cr⁶⁺% and Cr⁶⁺/Cr³⁺ of the spent catalysts of 6.5Cr/Al₂O₃ and 7.5Cr/Al₂O₃ are much lower than those of the corresponding fresh catalysts (Table 2). Hence, Cr⁶⁺ species is reduced to Cr³⁺ in DHP process. This suggests that Cr⁶⁺ species are important for DHP reaction. The similar catalytic dehydrogenation of isobutene over other Cr₂O₃–Al₂O₃ catalysts were reported by Xu et al.³⁵

The time-on-stream study of 7.5Cr/Al₂O₃ catalyst is carried out for about 10 hours and is presented in Fig. 6. It can be seen that the propane conversion decreases above 62.0% to 25.7% after about 10 hours, but the propylene selectivity has nearly no change.

Fig. 6 Stability test of 7.5Cr/Al₂O₃ catalyst in propane dehydrogenation (reaction conditions: V_(C3H8) : V_(Ar) = 1 : 4, total flow rate = 20 ml min⁻¹, T = 873 K).

The spent 7.5Cr/Al₂O₃ catalyst is regenerated by re-calcinating in air for 5 h. The recycle of 7.5Cr/Al₂O₃ catalyst for propane dehydrogenation reaction is presented in Fig. 7. It can be seen that the initial propane conversions in three recycles are all above 60.0%, and the propylene selectivity varies little, indicating that the original activity of the catalyst is well recovered. Little difference in the activity is observed before and after regeneration.

Fig. 7 Reuse of 7.5Cr/Al₂O₃catalyst (reaction conditions: V_(C3H8) : V_(Ar) = 1 : 4, total flow rate = 20 ml min⁻¹, T = 873 K).

Thermogravimetric analysis of spent 7.5Cr/Al₂O₃ catalyst is performed in air atmosphere and illustrated in Fig. 8. It can be seen that weight loss can be divided into two stages: room temperature to 473 K and after 473 K. The former stage is mainly attributed to the loss of adsorbed water and organic species. The second stage can be attributed to the burning of coke. According to the result in Fig. 8, the deposited coke is about 3.67 wt% for the spent 7.5Cr/Al₂O₃ catalyst.

Fig. 8 Thermogravimetric analysis (TGA) of spent 7.5Cr/Al₂O₃ catalyst.

4. Conclusion

In the present study, the CrO_x–Al₂O₃ catalysts with hierarchical architecture were hydrothermally synthesized and applied to DHP process. The prepared CrO_x–Al₂O₃ catalysts were well characterized by numerous state-of-the-art analytical techniques. The XPS and diffuse reflectance UV-vis spectra confirm that the existence of Cr⁶⁺ and Cr³⁺, and the Cr⁶⁺/Cr³⁺ ratio varies with the Cr loadings of xCr/Al₂O₃ catalysts. The propane conversion increase firstly with Cr loading and attain a peak point with 7.5% Cr loading, and then decreases with the further Cr loading in the catalysts. This variation trend is identical to the Cr⁶⁺/Cr³⁺ ratio in xCr/Al₂O₃ catalysts, implying that Cr⁶⁺ species are important for DHP reaction. The synthesized 7.5Cr/Al₂O₃ catalyst exhibits the highest propane conversion of 62% accompanying with 89% propylene selectivity at 873 K. After regenerated by re-calcinating in air, the original activity of spent catalyst is well recovered.

Acknowledgements

The research is supported by Shanghai Municipal Natural Science Foundation (13ZR1429900), Innovation Program of Shanghai Municipal Education Commission (12ZZ130), Program for Changjiang Scholars and Innovative Research Team in University (IRT1269) and International Joint Laboratory on Resource Chemistry (IJLRC).

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