Remarkable enhancement of Cu catalyst activity in hydrogenation of dimethyl oxalate to ethylene glycol using gold

Abstract

The performance of an SBA-15-supported Cu catalyst for hydrogenation of dimethyl oxalate to ethylene glycol is markedly promoted with Au. A key genesis of the high activity of the catalyst is ascribed to the formation of Cu–Au alloy nanoparticles which stabilize the active species and retard their agglomeration during the hydrogenation process.

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In contrast to the process of petroleum-derived ethylene glycol (EG), the indirect synthesis of EG from coal is a promising alternative process which involves the gasification of coal to syngas, followed by the coupling of CO with nitrite esters to oxalates and then the hydrogenation of oxalates to EG.¹ However, there are still numerous technical problems to be solved or strategies that need improvement. One of the most difficult problems is the catalyst without satisfactory performance for the vapor phase hydrogenation of oxalates to EG. At present, Cu–Cr catalysts are still the preferred industrial catalyst for the hydrogenation because they have a relatively high catalytic stability and durability.² Recently, to get rid of using toxic Cr, Cr-free Cu-based catalysts have been extensively studied as one of the most active materials for the vapor phase hydrogenation of dimethyl oxalate (DMO) to EG,^3,4 but they have inherent disadvantages of poor stability and short lifetime.⁴ Moreover, several works reveal that an appropriate Cu⁰ and Cu⁺ distribution is key to gaining excellent catalytic activity.^3a–c,4a Nonetheless, more distinct evidence needs to be accumulated.

On the other hand, Au containing bimetallic nanocrystals have generated immense interest in catalysis research because they offer a way to fine-tune the catalytic properties by controlling the composition and active species.⁵ Bimetallic Cu–Au systems are widely known to exhibit improved activity and selectivity in a number of selective catalytic reactions including CO oxidation,⁶ propene epoxidation,⁷ selective alcohol oxidation,⁸ and selective hydrogenation.⁹ However, the assignment of active species of Cu–Au catalysts for some reactions is still controversial or ambiguous.^6d,e,9

Herein, we report that an SBA-15-supported Cu–Au bimetallic catalyst with Cu loading of 6 wt% and with a proper amount of Au prepared by a two-step adsorption–NaBH₄ reduction process (see ESI† for details) exhibits remarkable activity and thermal stability for the hydrogenation of DMO to EG. The SBA-15 supported Cu–Au catalysts were labelled as xCuyAu/SBA-15, where x and y denote the weight loadings of Cu and Au, respectively. The physicochemical properties of the catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and nitrogen sorption (Fig. 1S to 3S and Table 1S, ESI†). The results indicate the successful synthesis of CuAu/SBA-15 catalysts.

The vapor phase DMO hydrogenation is known to comprise several continuous reactions, including DMO hydrogenation to methyl glycol (MG), MG hydrogenation to EG, and excessive hydrogenation to ethanol (EtOH).^3,4,9 Furthermore, byproducts such as 1,2-propanediol (1,2-PDO) and 1,2-butanediol (1,2-BDO) are also produced.^3,4 As shown in Table 1, the 6Cu/SBA-15 catalyst yielded a DMO conversion of 75.4% and 39.2% selectivity to EG, whereas the 6Au/SBA-15 catalyst exhibited a much lower DMO conversion of 5.4% and about 80% selectivity to MG under the conditions of 453 K, 3.0 MPa, H₂/DMO molar ratio of 80, and 0.6 h⁻¹ weight liquid hourly space velocity (WLHSV_DMO). When Au was introduced to 6Cu/SBA-15, the generated CuAu/SBA-15 catalysts were much more active than 6Cu/SBA-15 and 6Au/SBA-15 under identical reaction conditions. Their hydrogenation performance passed through a volcano-type improvement as a function of the Au content, as indicated by the turnover frequency (TOF) in Table 1 based on the number of surface metal atoms estimated by metal dispersion according to the equation in literature.¹⁰ The 6Cu1.9Au/SBA-15 catalyst with a nominal atomic ratio of Au/Cu = 0.1 (actual Au/Cu ratio was 0.08 by inductively coupled plasma optical emission spectrometer (ICP-OES), Table 1S, ESI†) gave the highest performance of 100% conversion and 99.1% selectivity to EG. The enhancement became less apparent when the Au content was further increased. In addition, when the 4.6Cu1.4Au/SBA-15 catalyst with a lower metal loading and an Au/Cu ratio at 0.1 was employed, it presented DMO conversion and EG selectivity close to those with 6Cu1.9Au/SBA-15. The results indicate that the Cu/Au atomic ratio has a significant effect on the catalytic behaviour of the CuAu/SBA-15 catalysts.

Table 1 Catalytic performance of reduced catalysts for the hydrogenation of DMO^a

Catalyst	Conversion/%	Selectivity/%				TOF^b/h^−1
		EG	MG	EtOH	Others^c
a Reaction conditions: T = 453 K, WLHSVDMO = 0.6 h^−1, P (H2) = 3.0 MPa, H2/DMO = 80. b Turnover frequency (TOF) was obtained by keeping DMO conversion below 30% (see ESI for details†). c Others included the by-products 1,2-PDO and 1,2-BDO.
6Cu/SBA-15	75.4	39.2	60.8	0	0	23
6Cu0.9Au/SBA-15	84.7	41.8	58.2	0	0	37
6Cu1.4Au/SBA-15	96.6	93.1	3.5	0.4	3.0	73
6Cu1.9Au/SBA-15	100	99.1	0	0.8	0.1	121
6Cu3.7Au/SBA-15	100	98.7	1.2	0	0.1	86
6Au/SBA-15	5.4	19.7	80.3	0	0	12
4.6Cu1.4Au/SBA-15	100	97.6	2.2	0	0.2	83

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The 6Cu1.9Au/SBA-15 catalyst showed a highly stable and excellent catalytic performance at 453 K for over 240 h, whereas the 6 wt% Cu/SBA-15 catalyst gave a stable activity only at about 75% conversion (Fig. 4S, ESI†). When the Cu loading was increased to 10 wt%, the catalyst generated (10Cu/SBA-15) gave a DMO conversion of over 90% and more than 85% selectivity to EG, but it deactivated within 60 h. The essential reason for the decline of the activity might be the obvious aggregation and coagulation of Cu nanoparticles (NPs) during the hydrogenation process (Fig. 5S, ESI†). The stability of 6Cu1.9Au/SBA-15 catalyst was further proven by its catalytic performance after heat treatment at 623 K for 24 h. The change of EG yield with the 6Cu1.9Au/SBA-15 catalyst before and after heat treatment was insignificant compared with 6Cu/SBA-15 and 10Cu/SBA-15 (Table S2†). In the case of 0.6 h⁻¹ WLHSV_DMO, the selectivity to EG with the 6Cu1.9Au/SBA-15 catalyst was higher than 98% at temperatures ranging from 453 K to 493 K (Fig. 6S, ESI†). When the temperature was higher than 493 K, a main by-product of EtOH was produced due to the excessive hydrogenation of EG.^4a Furthermore, when the reaction temperature was set at 473 K, the 6Cu1.9Au/SBA-15 catalyst yielded both DMO conversion and EG selectivity to as high as 98% under a wide range of WLHSV_DMO from 0.3 h⁻¹ to 3.0 h⁻¹, wherein the space time yield (STY) of EG was exceptionally larger than 1500 mg g⁻¹-cat h⁻¹ (Fig 7S, ESI†).

To investigate the reason behind the improved catalytic performance, in situ XRD, high-resolution TEM (HRTEM), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (XAES) measurements were carried out to get detailed information about the structure of the catalysts. The XRD patterns of the as-calcined 6Cu1.9Au/SBA-15 sample during reduction indicated the remarkable transformations of the diffraction lines of Cu and Au species when the reduction temperature was higher than 423 K (Fig. 8S, ESI†). Further increasing the reduction temperature up to 673 K did not result in obvious changes in the XRD patterns. The CuAu/SBA-15 catalysts showed a broad diffraction line centered at around 39–42.8° (Fig. 1). With the increase of Au content, the broad XRD diffraction line slightly shifted to a higher angle and reached a maximum in the case of the 6Cu1.9Au/SBA-15 catalyst. This is an indication of the formation of Au–Cu alloy after reduction and the fluctuation of the Au–Cu alloy composition.^6c,11,12 For the UV-Vis DRS (Fig. 9S, ESI†), the CuAu/SBA-15 samples had an absorption band positioned between 6Au/SBA-15 and 6Cu/SBA-15, which suggested the formation of an Au–Cu alloy.^6d,11 The higher content of alloying resulted in a larger shift of the absorption peak. Results of the UV-Vis DRS had the same trend as that of the XRD patterns, proving the occurrence of alloying in the CuAu/SBA-15 catalysts after reduction.

Fig. 1 XRD patterns of reduced catalysts of (a) 6Cu/SBA-15; (b) 6Cu0.9Au/SBA-15; (c) 6Cu1.4Au/SBA-15; (d) 6Cu1.9Au/SBA-15; (e) 6Cu3.7Au/SBA-15; (f) 6Au/SBA-15.

TEM images revealed that the metal particles were uniformly dispersed on the SBA-15 support (Fig. 2, and 3S, ESI†). With the increase of Au content, the average particle size slightly increased from 2.8 to 3.7 nm, which was smaller than the Au particle size of 4.9 nm in 6Au/SBA-15. Unlike 6Au/SBA-15 or 6Cu/SBA-15 (Fig. 10S, images a and b, ESI†), the HRTEM image of the 6Cu1.9Au/SBA-15 catalyst in Fig. 2 indicates the two intervals of 0.227 and 0.194 nm between the corresponding lattice fringes were smaller than those of the monometallic Au(111) (0.236 nm) and Au(220) (0.204 nm), but larger than those of the monometallic Cu(111) (0.209 nm) and Cu(200) (0.181 nm). The included angle around 52° of the two facets was very close to the theoretical value of 54.7° between the (111) and (200) facets of the facial center cubic structure, implying that the exposed crystal planes of the 6Cu1.9Au/SBA-15 catalyst were dominated by (111) and (200) facets of the Au–Cu alloy. When the Au content was increased to 3.7 wt%, however, separated Au NPs could be detected besides the Cu–Au alloy entities (Fig. 10S, image e, ESI†).

Fig. 2 Images of (a) TEM and (b) HRTEM of the reduced 6Cu1.9Au/SBA-15 catalyst.

Fig. 3 displays the FT-IR spectra of CO absorption on the as-reduced CuAu/SBA-15 catalysts. The assignment of CO absorptions onto Cu⁰, Cu⁺, and Cu²⁺ surfaces has been well-documented in previous work.^4a,13,14 According to these assignments, the CO absorption peaks at 2122 cm⁻¹ observed on the reduced catalysts should be assigned to Cu⁺–CO species. As shown in Fig. 3, the intensity of Cu⁺–CO on the CuAu/SBA-15 catalysts is obviously enhanced compared with that on the 6Cu/SBA-15 catalyst, which may be related to the formation of the Cu–Au alloy. However, with the increase of Au content, the Cu⁺–CO intensity decreased gradually, which could be ascribed to the increase of Au concentration on the catalyst surfaces. Nevertheless, the CO adsorption on either Cu²⁺ and metallic Cu⁰ was predominantly weak and reversible at room temperature.^4a,13,14 Therefore, the presence and the distribution of Cu⁰ and Cu⁺ was further proven by the Cu XPS and LMM XAES of the catalysts treated in situ in 90 kPa H₂ (5%)–Ar (95%) at 623 K for 4 h (Fig. 11S and 12S, ESI†). From the deconvolution results (Table 3S, ESI†), the distributions of Cu⁺ and Cu⁰ species were affected by the Au content. More Cu⁺ species were present when the Au loading was lower than 3.7 wt%. Herein, the smaller value of the Auger parameter (AP) for Cu⁺ than the bulk value was attributed to the strong interactions among Cu⁺, the SBA-15 support, and Au. When Cu species in +2, +1, and 0 valences are in highly dispersed states and in intimate contact with the supports, the AP value can be 2 eV to 3 eV lower than that of the bulk state.¹⁵ Hence, taking the catalytic results into account, we conclude that, in addition to the contribution of Cu–Au alloy NPs, the coexistence of Cu⁺ and Cu⁰ species in a proper distribution on the catalyst surfaces is indispensable for the hydrogenation of DMO to EG.

Fig. 3 In situ FT-IR spectra of CO absorption on 623 K reduced catalysts after evacuation for 30 min at 298 K.

In summary, the findings in this study show that with the introduction of a small amount of Au to 6Cu/SBA-15, the derived 6Cu1.9Au/SBA-15 catalyst exhibits remarkable enhancement in its catalytic performance for the hydrogenation of DMO to EG. This catalyst is very active and robust at temperatures ranging from 453 to 493 K and can provide exceptional EG STY of more than 1500 mg g⁻¹-cat h⁻¹ when the DMO conversion and EG selectivity are higher than 98%. The Cu–Au alloy NPs are formed on the catalyst surface, and are believed to be beneficial for stabilizing the Cu⁰ and Cu⁺ proportion and for retarding the surface transmigration of Cu species during the hydrogenation process.

This work is supported by the MOST of China (2011CBA00508), the NSFC (21173175), the Research Fund for the Doctoral Program of Higher Education (20110121130002), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1036).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterization data. See DOI: 10.1039/c2cy20154b

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