Mesoporous xEr₂O₃·CoTiO₃ composite oxide catalysts for low temperature dehydrogenation of ethylbenzene to styrene using CO₂ as a soft oxidant

Abstract

A series of mesoporous xEr₂O₃·CoTiO₃ composite oxide catalysts have been prepared using a template method and tested as a new type of catalyst for the oxidative dehydrogenation of ethylbenzene to styrene by using CO₂ as a soft oxidant. Among the catalysts tested, the 0.25Er₂O₃·CoTiO₃ sample with a ratio of 1 : 4 : 4 content and calcined at 600 °C exhibited the highest ethylbenzene conversion (58%) and remarkable styrene selectivity (95%) at low temperature (450 °C).

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Introduction

Styrene is an important monomer in the petrochemicals industry, and is mainly used for polymeric materials such as polystyrene resin, acrylonitrile–butadiene–styrene resin, and styrene–butadiene rubber used in toys, television shells, and insulation foam.¹ The styrene production volume is about 23 million tons annually; and more than 90% of the industrial production of styrene follows from the catalytic dehydrogenation of ethylbenzene (EB) in the presence of steam over iron oxide based catalysts.² Since 1957, potassium promoted iron oxide (Fe–K) has been applied for dehydrogenation of ethylbenzene, either adiabatically or isothermally over a fixed bed reactor in which the reactants are passed over the catalyst bed employing radial or axial flow. The reaction cycle was proposed that ethylbenzene adsorbed onto a terrace Fe³⁺ site; then C–H in the ethyl group was deprotonated by lattice oxygen at a step site. H₂O containing a lattice oxygen was postulated to desorb on the catalyst instead of H₂, meaning that oxidative dehydrogenation proceeded on the Fe–K catalyst.³ Several metal oxide catalysts, such as MgO, Cr₂O₃, CeO₂ and MoO₃, also have been studied, both with and without promoters, such as small amounts of noble metals of Pd and Pt.⁴ In order to lower capital and lower energy cost, there is a strong need to develop catalysts, such as composite metal oxides, that have high activity and selectivity for styrene conversion at low temperatures.⁵

EB dehydrogenation process involves a highly endothermic reaction (ΔH = 129.4 kJ mol⁻¹) carried out in the vapor phase over a solid catalyst.⁶ Styrene plants run their reactors under isothermal or adiabatic conditions at 600 to 650 °C with a short contact time between the feedstock and the catalyst in order to prevent polymerization of styrene, with a typical conversion and yield being 90% and 65%, respectively, on a per pass basis. During the dehydrogenation reaction, a sufficient amount of steam is required to provide energy and improve the stability of metal catalysts. Therefore, there is a great effort to develop an applicable alternative route with less energy consumption, i.e., oxidative dehydrogenation (ODH) of ethylbenzene, by using an oxidant.⁷ The oxidative dehydrogenation (ODH) process of ethylbenzene to styrene in the presence of oxidant have attracted considerable attention since it can be operated at lower temperatures and ethylbenzene conversion would not be equilibrium limited.^8,9 A variety of oxidizing agents, such as O₂, N₂O, SO₂ and CO₂ and so on, have been used for the oxidative dehydrogenation of ethylbenzene at a lower reaction temperature.^10–14 However, molecular oxygen always causes deep oxidation into CO_x, which greatly lowers the atomic efficiency of feedstock. In another study, nanocrystalline ceria catalysts with defect sites, promoted the dehydrogenation of EB at 598 K, which is far lower than the temperatures normally encountered in the existing conventional processes for the dehydrogenation of EB (873 K), however the reaction needed extra N₂O as an oxidant.¹⁵ Until now, it has been challenging to achieve high selectivity as well as a favorable yield of styrene. Exploitation of CO₂ as a soft oxidant for the commercially catalytic ODH of ethylbenzene to styrene has received enormous interest recently because of several benefits to using CO₂, compared to other oxidants; CO₂ can act as an oxidant as well as a diluent. In addition, CO₂ stays in the gas form throughout the reaction and CO₂ can also decrease the partial pressure of reactants more effectively than steam.^13,16 Further, in the presence of CO₂, coupling with the reverse water gas shift reaction CO₂ + H₂ ↔ CO + H₂O, the ethylbenzene dehydrogenation reaction becomes more favorable.^17,18

Metal oxides are very important for potential applications in adsorption, separation, electric double-layer capacitors, lithium-ion batteries, and catalysis.^19–22 Among them, rare earth metal oxides have been widely applied as catalysts for dehydrogenation of ethylbenzene. Furthermore, in most of the oxidation reactions involving metal oxides as catalysts, basic sites and acidic sites on the catalyst surface strongly influence ethylbenzene conversion.²³ In this direction, herein we report the synthesis of mesoporous Er : Ti : Co composite metal oxides using high molecular weight surfactant Pluronic F123 as template agent.²⁴ For the first time, the catalytic performance of these rare earth and transition metal mixed oxides have been studied for the dehydrogenation of ethylbenzene to styrene by using CO₂ as a soft oxidant. In particular, in the absence of diluent steam and using CO₂ to mitigate coke formation, the conversion of ethylbenzene reached up to 50–60% with a 95–99% selectivity levels of styrene, at a temperature of 450 °C.

The mesoporous Er₂O₃·CoTiO₃ composite oxides were synthesized using Pluronic F123 as a templating agent, and titanium(IV) iso-propoxide, erbium(III) nitrate pentahydrate and cobalt nitrate hexahydrate as the precursors. The suspension of metal salts and template agent in ethanol was aged for 24 hours under mild condition and dried in 80 °C oven for another 24 hours before calcination, under desired temperatures (800 °C, 700 °C, and 600 °C) (Scheme 1a). This series of mesoporous Er : Ti : Co composite metal oxides were prepared from different molar ratio of metal salts (Scheme 1b) and labelled as xEr₂O₃·CoTiO₃-T, in which x stand for the atomic ratio of Er to Ti and Co, and T stands for the calcination temperature in celsius.

Scheme 1 Schematic illustration of the formation of the mesoporous metal oxides using Pluronic F123 as the template agent (a) and the ternary diagram depicting the molar ratio variables of the starting metal salts (b).

The mesoporous xEr₂O₃·CoTiO₃ series were found to have an X-ray diffraction (XRD) spectrum corresponding to rhombohedral CoTiO₃ (ICSD 01-077-1373), with impurities of TiO₂ and Co₂O₃ at the lowest Er concentration (Fig. S1 in ESI†). The peaks of Er₂O₃ were not detected for any of the materials, indicating that the amorphous erbium oxide was uniformly dispersed in the main phase of CoTiO₃. The metal oxides from sole metal salt precursor, i.e., Er₂O₃, TiO₂ and Co₂O₃ (Fig. S2, ESI†), and 0.5Er₂O₃·CoTiO₃-700 and 0.5Er₂O₃·CoTiO₃-800 were also characterized by XRD. The mixture of Er₂Ti₂O₇ (ICSD 00-055-0813), CoTiO₃ and amorphous cobalt oxides rather than CoTiO₃ will be obtained under high temperature calcinations, indicating that dispersed amorphous erbium oxide related materials will transform into crystalline Er₂Ti₂O₇ if it is annealed at 800 °C (Fig. S3, ESI†).

The mesostructure of the xEr₂O₃·CoTiO₃ composite oxides is seen by scanning electron microscopic (SEM) and transmission electron microscopy (TEM) (Fig. 1). As observed in the SEM and TEM images, the mesopores were formed from discrete interconnected particles. Interestingly, the widths of disordered mesopores, as determined from the pore size distributions (PSD) given in Table 1, gradually increased with the decreasing the erbium content, i.e., with the mesopores widths transformed from 5–10 nm of Er₂O₃·CoTiO₃-600 and 0.5Er₂O₃·CoTiO₃-600 to 15–45 nm of 0.025Er₂O₃·CoTiO₃-600. For the typical 0.5Er₂O₃·CoTiO₃-600, elemental mappings at the microstructural level by energy-dispersive X-ray spectroscopy (EDX) revealed that the spatial distribution of the metal elements is quite uniform in the product (Fig. S4†).

Fig. 1 SEM images ((a), Er₂O₃·CoTiO₃-600; (c), 0.5Er₂O₃·CoTiO₃-600; (e), 0.25Er₂O₃·CoTiO₃-600; (g), 0.125Er₂O₃·CoTiO₃-600; (i), 0.025Er₂O₃·CoTiO₃-600) and TEM images ((b), Er₂O₃·CoTiO₃-600; (d), 0.5Er₂O₃·CoTiO₃-600; (f), 0.25Er₂O₃·CoTiO₃-600; (h), 0.125Er₂O₃·CoTiO₃-600; (j), 0.025Er₂O₃·CoTiO₃-600) of the mesoporous xEr₂O₃·CoTiO₃ composite oxides.

Table 1 The porosity details and the styrene production rate of ethylbenzene dehydrogenation at 450 °C over different composite metal oxides

Sample	SBET^a (m^2 g^−1)	VSP^b (cm^3 g^−1)	w^c (nm)	Styrene production rate^d (μmol g^−1 h^−1)
a Specific surface area calculated in the p/p0 range of 0.05–0.20.b Single point pore volume taken at p/p0 ∼ 0.98.c Mesopore width from the maximum of the calculated PSDs.d Styrene production rate at 450 °C.e Metal oxide prepared under 600 °C calcination.f Data not obtained due to very low nitrogen uptake.
Er2O3·CoTiO3-600	27.7	0.047	5.0	138
0.5Er2O3·CoTiO3-600	32.7	0.052	5.0	289
0.25Er2O3·CoTiO3-600	69.9	0.168	9.6	326
0.125Er2O3·CoTiO3-600	37.7	0.111	12.4	253
0.025Er2O3·CoTiO3-600	28.5	0.104	26.5	167
0.5Er2O3·CoTiO3-700	8.0	0.065	29.5	149
0.5Er2O3·CoTiO3-800	8.8	0.086	109.1	54
Er2O3-600^e	11.6	0.056	30.8	9.6
TiO2-600^e	5.9	0.019	23.5	32
Co3O4-600^e	0.2	—^f	—^f	3

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Fig. 2 shows the nitrogen adsorption isotherms of these metal oxide composites. All the samples show a steep increase in the curve under different medium relative pressures with a hysteresis, indicating the presence of mesopores. Surface area and pore volume of xEr₂O₃·CoTiO₃-T series decrease with increasing calcination temperatures as shown in Fig. S5, ESI.† 0.25Er₂O₃·CoTiO₃-600 exhibit a highest Brunauer–Emmett–Teller (BET) surface area and pore volume, being 69.9 m² g⁻¹ and 0.168 m³ g⁻¹, respectively (Fig. 2 and Table 1). Moreover, the PSD curves indicate the widths of disordered mesopores gradually decreased with the increasing erbium content (Fig. 2b). These observations, consistent with the pore sizes transformation on the TEM images, indicate that the surface area and pore size distribution of the final products strongly depends on the molar ratio of the metal salt precursors. In contrast, the metal oxides from sole metal salt precursor show very low BET surface areas and pore volumes, indicating that these final products (Er₂O₃, TiO₂ and Co₂O₃) are composed of aggregated particles (Table 1, Fig. S6, ESI†).

Fig. 2 N₂ 77 K adsorption isotherms (a) and corresponding pore-size distributions (b) for the xEr₂O₃·CoTiO₃-600 composite oxides.

The catalytic performance of the synthesized Er₂O₃·CoTiO₃ sample was measured under steady state conditions between 400 and 500 °C using a CO₂/ethylbenzene molar ratio of ∼100. In all cases, the major product in the reaction is styrene, accompanied with some amount of benzene, and toluene as by-products.

Ethylbenzene conversion (X_EB), styrene selectivity (S_ST) and styrene yield (Y_ST), are shown in the following equations:²⁵

X_EB = (1 − ethylbenzene out/ethylbenzene in) × 100

S_ST = (product out/(ethylbenzene in − ethylbenzene out)) × 100

Y_ST = (X_EB × S_ST)/100

As shown in Fig. 3a, the pure Er₂O₃ and TiO₂ give a very low ethylbenzene conversion compared with the pure Co₃O₄ (Fig. 3a) in the whole temperature range. However the mixed xEr₂O₃·CoTiO₃-600 metal oxides show a much better catalytic performance for ethylbenzene dehydrogenation, especially the 0.25Er₂O₃·CoTiO₃-600 catalyst. The catalytic activity increased with increasing temperature, the highest styrene yield was obtained at 475 °C with ethylbenzene conversion of ∼80% and styrene selectivity of ∼90% over the 0.25Er₂O₃·CoTiO₃-600 catalyst. At 500 °C, the styrene yield decreased due to decreased styrene selectivity. Table 1 summarizes the styrene production rates at 450 °C as calculated from the styrene selectivity and conversion for our reaction conditions (see ESI†). It is apparent from Table 1 that the styrene production rate approximately correlates with the BET surface area although there is deviation (see Fig. S7, ESI†). This indicates that the main reason of the higher ethylbenzene conversion and yield of styrene over the 0.25Er₂O₃·CoTiO₃-600 catalyst is mainly due to the larger surface area of the sample.

Fig. 3 Catalytic activity (a), styrene selectivity (b) and (c) yield of synthesized xEr₂O₃·CoTiO₃-T samples. Reaction conditions: EB, 2.204 μmol min⁻¹; CO₂, 5 mL min⁻¹; Ar, 15 mL min⁻¹ as the carrier, with a total flow rate of 20 mL min⁻¹. Catalyst: ∼100 mg mixed with 200 mg quartz sand (60–80 mesh). CO₂ : EB molar ratio of ∼100.

Comparing the test results of ODH reaction over the 0.5Er₂O₃·CoTiO₃-600 catalysts calcined under different temperatures in Fig. 3, we found that performance of this series of samples strongly depends on the calcination temperature. It has been discovered that the sample calcined at 600 °C, shows the better activity than the other two samples calcined at 700 and 800 °C individually. As shown in Table 1, the higher calcination temperatures caused loss of BET surface area, decreasing the catalytic activity accordingly. Through the comparison of the conversion of xEr₂O₃·CoTiO₃ catalysts with that for equal weights of pure Er₂O₃, Co₃O₄ or TiO₂, each made from the same template method, these mixed components enhanced the yield of the styrene over the catalysts. The 0.25Er₂O₃·CoTiO₃-600 has a highest conversion of the ethylbenzene. Further, increasing the ratio of Er : Ti : Co does not increase the ethylbenzene conversion any more, while the selectivity to styrene decreased (Fig. 3b). The selectivity to styrene over the pure Co₃O₄ catalyst is very low, while it is much higher over the pure TiO₂, but the highest selectivity to styrene is obtained over Er₂O₃ although the conversion of ethylbenzene is low. Overall, 0.25Er₂O₃·CoTiO₃-600 shows the best yield of styrene in the total temperature range. The 0.25Er₂O₃·CoTiO₃-600 has a higher conversion and comparable selectivity of the dehydrogenation of ethylbenzene with other reported porous rare earth based catalysts.^26,27 For instance, V₂O₅–CeO₂/TiO₂–ZrO₂ composites have a conversion less than 60% at the reaction temperature being 600 °C;²⁶ the conversion of SBA-15 supported CeO₂–ZrO₂ is 63.7% and selectivity is 93.0% with time on stream in 10 h in the presence of CO₂.²⁷

In summary, we prepared a series of xEr₂O₃·CoTiO₃ mixed oxides based on a simple template method, with mesoporous or macroporous texture obtained by varying the composition. The catalytic performance of rare earth and transition metal mixed oxides were studied on the dehydrogenation of ethylbenzene to styrene. In particular, in the presence of CO₂, the specific activity up to 370 μmol g⁻¹ h⁻¹ can be obtained with at 95–99% selectivity to styrene, at a low temperature of 450 °C.

Acknowledgements

This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. TEM (J. C.) and SEM (D. K. H.) experiments were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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Footnote

† Electronic supplementary information (ESI) available: Experimental and measurements details, XRD patterns of metal oxides prepared from with different molar ratio, XRD patterns of metal oxides prepared from sole metal salt precursor, XRD patterns of 0.5Er₂O₃·CoTiO₃-T, elemental analysis of the typical 0.25Er₂O₃·CoTiO₃-600, porosity measurements of 0.5Er₂O₃·CoTiO₃ prepared under different calcination temperature, porosity measurements of metal oxides, porosity measurements of metal oxides, and plot of the styrene production rate vs. surface area. See DOI: 10.1039/c6ra04228g

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