Remarkable crystal phase effect of Cu/TiO₂ catalysts on the selective hydrogenation of dimethyl oxalate

Abstract

A series of Cu-based TiO₂-supported catalysts were synthesized via facile ammonia evaporation for the selective hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). A 100% conversion of DMO, 99% selectivity for EG, and super stability were achieved over the 20% Cu/P25 TiO₂ catalyst. The effect of the crystal phase of the titania support and the interaction between Cu and P25 were investigated. The active copper species located at the interface between the anatase and rutile phases play important roles in the selective hydrogenation. The excellent catalytic performance was mainly attributed to better copper dispersion and an appropriate ratio of Cu⁰/Cu⁺, which stems from the intimate interaction between Cu and the P25 support.

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1. Introduction

With the soaring demand for petroleum-related derivatives and the increasing shortage of oil resources, an alternative for these products is urgently required. Recently, a catalytic process for coal-to-ethylene glycol (CTE) conversion has attracted attention for many applications of ethylene glycol (EG), such as antifreeze, coolant, polyester, and solvents,¹ and also for its environmental friendliness and lower cost, because the CTE process is superior to the traditional ethylene oxide hydration route.

The CTE approach comprises two steps: coupling of CO with nitrite esters to obtain dimethyl oxalate (DMO), which was produced on an industrial scale of 10 000 tons per year in 2010;² and then the sequential hydrogenation of DMO to the desired products, which is now the focus of academic and industrial research.

Many studies have investigated various catalysts from the viewpoint of texture, structure, catalytic activity and stability. Previous research revealed that copper-based catalysts showed excellent catalytic activity and better stability,^3–7 especially for silica-supported catalysts.^8–10 Chen et al.⁸ explored the effect of ammonia-evaporation (AE) temperature and the cooperative effect between Cu⁰ and Cu⁺ in the catalysts. Yin et al. used hexagonal mesoporous silica (HMS) as a support, significantly increasing the catalytic performance.⁹ Thereafter, various catalysts have been synthesized with HMS support.^10–14 Unfortunately, the catalyst tends to deactivate because of the easy erosion of the SiO₂ support, along with the aggregation of copper species at elevated reaction temperatures.^2,6 Therefore, it is important to devise a catalyst that is supported on a non-silicon matrix.

Titania, which is a versatile material in photocatalytic, gas-sensing and heterogeneous reactions,^15–17 has found increasingly wide applications in catalytic systems. Numerous investigations have concentrated on exploiting the interaction between the active species and the TiO₂ matrix, particularly P25 TiO₂, a universal titania that comprises around 80% anatase and 20% rutile phases by weight. Xia et al.¹⁸ achieved ultrahigh stability in Pt/CNT@TiO₂ and attributed it to the strong metal–support interaction between Pt and the TiO₂-based support. Analogous metal–support interactions were observed by Jiang and co-workers and Kou and co-workers.^19,20 Jovic et al.²¹ measured the electron transfer between the anatase and rutile interface of Au/P25 TiO₂ in H₂ production from ethanol. Intimate contact was discovered among anatase, rutile and Au particles, which may be responsible for the high catalytic performance. Murdoch and co-workers observed a marked difference in H₂ production rate on Au/TiO₂ anatase and Au/TiO₂ rutile,²² which was attributed to variations in electron–hole recombination rates. Similar results were also found by several other groups.^23–25 In addition, much work has revealed that the moderate surface and textural properties may induce a strong metal support interaction (SMSI).^26,27

Based on these advantages, it is expected that the titania support should play a part in an even wider range of chemistry, namely catalysis. However, to the best of our knowledge, despite the intimate interaction between the metal and TiO₂ support, the performance of Cu/TiO₂ in the vapor-phase hydrogenation of DMO to EG has not been reported. Herein, we report a Cu/TiO₂ catalytic system with perfect catalytic performance, and investigate the support morphology with various characterization methods. We expect that our results will aid future exploration of titania-supported heterogeneous systems.

2. Experimental

2.1 Catalyst preparation

All the reagents were purchased from Sinopharm Chemical Reagent Co., Ltd without further purification, unless otherwise specified.

The Cu/P25 TiO₂ (Degussa Co., Ltd) catalyst precursor with different support morphologies is prepared by the AE method.⁸ Firstly, Cu(NO₃)₂·3H₂O (6.1 g) was dissolved in deionized water (200 mL), and then aqueous ammonia (23 mL, 25 wt%) was added and stirred for 0.5 h at 333 K. The initial pH of the suspension was 11–12. P25 TiO₂ (6.4 g) was added to the solution and the mixture was stirred for another 4 h. The suspension was then preheated at 363 K to evaporate the ammonia, decrease the pH, and deposit the copper species on titania. When the pH value of the suspension was decreased to 6–7, the evaporation process was terminated. The residue was washed with deionized water three times and ethanol once, and was then dried at 393 K overnight. The catalyst precursors were calcined at a 823 K for 4 h, pelletized, crushed, sieved through 40–60 meshes, and denoted as 20CuP25-823 (simplified as Cu/P25), where 20 and 823 denote the copper mass concentration and calcination temperature, respectively.

For comparison, the copper-based catalysts supported on pure anatase TiO₂ (Aladdin Co., Ltd), pure rutile TiO₂ (Aladdin Co., Ltd), and TiO₂ that was mechanically mixed with A and R (A/R = 80 : 20) were denoted as 20Cu/A-823, 20Cu/R-823 and 20Cu/M-823 (simplified as Cu/A, Cu/R and Cu/M), respectively, were also prepared by the AE method.

2.2 Catalyst characterization

The BET surface area (S_BET) was measured by using N₂ physisorption at 77 K on a Micromeritics Tristar 3000 apparatus. The pore size distributions were obtained from the desorption isotherm branch of the nitrogen isotherms by using the Barrett–Joyner–Halenda (BJH) method.

The wide-angle XRD patterns were collected on a Bruker D8 Advance X-ray diffractometer by using nickel-filtered Cu Kα radiation (λ = 0.15406 nm) with a scanning angle (2θ) of 20–80°, a scanning speed of 4° min⁻¹, a voltage of 40 kV and a current of 40 mA.

TEM characterization was performed on a JEOL JEM 2010 microscope. Samples for electron microscopy observation were prepared by grinding and dispersing the powder in ethanol, and then applying a drop of the very dilute suspension to a carbon-coated grid. For the reduced samples, all samples were reduced in H₂ at 573 K for 4 h.

TPR profiles were obtained on a Tianjin XQ TP 5080 auto adsorption apparatus. A 15 mg sample of the calcinated catalyst was outgassed at 473 K under an Ar flow for 2 h. After the sample was cooled to room temperature under an Ar flow, the in-line gas was switched to 5% H₂/Ar, and the sample was heated to 773 K at a ramping rate of 10 K min⁻¹. The H₂ consumption was monitored by a thermal conductivity detector (TCD). The copper dispersion and the specific surface area of metallic copper (SA_Cu) of the catalysts were measured by dissociative N₂O adsorption.²⁸ The specific surface area of metallic copper was calculated from the amount of H₂ consumption 1.46 × 10¹⁹ with copper atoms per square meter.²⁹

XPS experiments were carried out with a PerkinElmer PHI 5000C ESCA system equipped with a hemispherical electron energy analyzer. The Mg Kα (hν = 1253.6 eV) anode was operated at 14 kV and 20 mA. All binding energies were calibrated by using the carbonaceous C 1s line at 284.6 eV as reference. The experimental errors were within ±0.2 eV.

2.3 Activity test

The catalytic activity test was performed by using a fixed-bed microreactor. Typically, a catalyst (0.6 g, 40–60 meshes) sample was loaded into a stainless steel tubular reactor with the thermocouple inserted into the catalyst bed for better control of the pretreatment and reaction temperature. Catalyst activation was performed at 573 K for 4 h with a ramping rate of 2 K min⁻¹ from room temperature under a 5% H₂/Ar (v/v) atmosphere. After cooling to the reaction temperature, 5 wt% DMO (>99%) in methanol and H₂ were fed into the reactor at a H₂/DMO molar ratio of 100 and a system pressure of 2.5 MPa. The reaction temperature was first set at 493 K and the LHSV of DMO ranged from 0.08 to 1.00 h⁻¹. For the TOF value calculation, the LHSV of DMO was set at 2.80 h⁻¹ to keep the initial conversion lower than 20%. The products were condensed, and analyzed on a gas chromatograph (Finnigan Trace GC ultra, Thermo Scientific) fitted with an HP-5 capillary column and a flame ionization detector (FID).

All the catalysts are reduced at 573 K for 4 h under a 5% H₂/Ar (v/v) atmosphere prior to the catalytic test.

3. Results

3.1 Structural and textural properties

The physicochemical parameters of the catalysts are listed in Table 1. Pure P25 had a BET specific surface area of 44.0 m² g⁻¹ and no substantial deviation from that value was observed in the Cu/P25 catalyst. In addition to the variation in BET value, a similar trend in pore volume and pore diameter was observed among the four catalysts, consistent with the results for the Au/P25 catalysts observed by Jovic and co-workers.²¹ They reported a similar trend in the physicochemical parameters, which they attributed to the trends in the physicochemical properties of different titania supports. The N₂ adsorption–desorption isotherms of the catalysts (Fig. S1, ESI†) show that they exhibit Langmuir type IV isotherms, with an H1-type hysteresis loop, corresponding to a typical large-pore mesoporous material.¹⁴ Additionally, each sample displayed a significant N₂ adsorption–desorption hysteresis at a high relative pressure of P/P₀ > 0.9, which indicates the high degree of textural porosity.¹⁴ Notably, catalysts that have similar S_BET data show sharply different D_Cu and SA_Cu values, which should be assigned to the difference in interaction between copper species and supports. An intimate metal–support interaction leads to a high metal dispersion, whereas a poor interaction induces a poor dispersion, despite the high specific area of the catalyst.

Table 1 Textural properties and chemical composition of the catalysts

Sample	SBET (m^2 g^−1)	Vpore (cm^3 g^−1)	Dpore (nm)	DCu^a (%)	SACu^a (m^2 g^−1)	BECu^b (eV)	BECu^c (eV)	BETi^b (eV)	BETi^c (eV)	TOF^d (h^−1)
a Cu dispersion (DCu) and Cu metal surface area (SACu) determined by N2O titration method.b Binding energy (Cu 2p3/2 and Ti 2p3/2) of catalysts calcinated at 823 K.c Binding energy (Cu 2p3/2 and Ti 2p3/2) of catalysts reduced at 573 K.d Reaction conditions: P = 2.5 MPa, H2/DMO = 100 mol/mol, LHSV of DMO = 2.80 h^−1, and T = 493 K.
P25	44.0	0.36	27.4	—	—	—	—	—	—	—
Cu/P25	49.0	0.35	28.7	28.3	38.3	933.8	933.0	460.8	459.8	9.1
Cu/M	45.6	0.19	12.6	17.8	24.1	933.9	932.8	460.5	459.1	0.59
Cu/A	42.7	0.24	12.5	19.5	26.3	933.8	932.9	461.0	459.0	0.46
Cu/R	24.6	0.11	3.76	11.3	15.3	934.0	933.0	459.5	459.0	8.0

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The XRD characterizations of these catalysts calcinated at 823 K are shown in Fig. 1A. All samples showed strong diffraction lines assigned to anatase and rutile phases, indicating that no obvious phase transformation occurred in the structural and textural properties of P25, pure anatase and rutile supports and the mixed support. Weak diffraction peaks around 35.5°, 38.7° and 48.7°, characteristic of monoclinic CuO (JCPDS 48-1548), were observed for all of such catalysts, which indicates the slight aggregation of copper oxide species. XRD patterns of the reduced Cu/TiO₂ catalysts are shown in Fig. 1B. Upon reduction at 573 K, the copper oxide peaks (2θ = 35.5°, 38.7°, 48.7°) for the calcinated samples disappeared, indicating the good reduction of the copper oxide species. For the reduced catalysts, peaks (2θ = 43.5°, 50.4°, 74.1°) ascribed to Cu (111), Cu (200) and Cu (220) diffraction peaks (JCPDS 01-1241) appeared, and the metallic copper diffraction line in the Cu/P25 was much weaker than those in the other three catalysts, indicating the better dispersion of the copper species. This is consistent with the copper dispersion and SA_Cu data from N₂O titration results listed in Table 1.

Fig. 1 XRD patterns of different catalysts (A) calcinated at 823 K (inset: magnified XRD image, 35–40°) and (B) reduced at 573 K.

As the TEM images of the reduced catalysts in Fig. 2 showed, P25 comprises small spherical anatase crystallites and larger angular rutile crystallites.²¹ Previous research on the Au/TiO₂ system concluded that it is the active metal species preferentially located at the interface between the anatase and rutile phase that acts as a reactive center.²¹ In Fig. 2E, the HRTEM image of the reduced Cu/P25 catalyst contains lattice fringes of 0.232 and 0.350 nm, which fits well with the rutile (220) and anatase (101) planes respectively, confirming the coexistence of the anatase and rutile phases in the P25-supported system. The lattice fringe of 0.208 nm that corresponds to the metallic Cu (111) planes at the interface of the anatase and rutile phases reveals the successful synthesis of the P25-supported copper catalysts. Changes in the interactions among the cupreous species and support significantly affects the stability and redox properties of the catalyst.³⁰ Similar observations were made by Tsukamoto et al.,³¹ Jovic et al.²¹ and Akita et al.³² for the Au/TiO₂ photocatalysts, and their work confirmed a preferential location of Au nanoparticles at the interface between anatase and rutile titania. However, for the Cu/M, we observe no similar location at these interfaces after we counted more than thirty images under different magnifications, which only showed amalgamations of copper species compared with the well dispersion of Cu in Cu/P25. Analogously, for the Cu/A and Cu/R catalysts, we merely observed sintering and aggregation of the copper species with no metals at the interface. TEM images under different magnification revealed similar results (Fig. S2, ESI†). These results show that Cu/P25 displays a better dispersion, consistent with the XRD and N₂O titration results. In addition, the better dispersion and location at the interface may result from the intimate interaction between copper and the support.

Fig. 2 TEM images for the reduced catalysts (A) Cu/P25; (B) Cu/M; (C) Cu/A; (D) Cu/R; and (E) HRTEM of Cu/P25. All samples were pre-treated in H₂ at 573 K for 4 h.

3.2 Redox behaviour

TPR characterization is carried out to investigate the redox properties of the copper-based catalysts. The results are shown in Fig. 3. A dominant peak at around 522 K with a shoulder peak at about 433 K are visible for Cu/P25, indicating the coexistence of distinct well dispersed and bulk copper species. For the Cu/R and Cu/M catalysts, similar results were obtained. However, for Cu/R and Cu/M we observed a higher reduction temperature compared with that of Cu/P25, which may imply higher reduction ability arising from the intimate interaction between the copper and supports in Cu/P25. This is in accordance with studies by Wen et al.³³ and Vaudagna et al.,³⁴ who also observed an increased reduction of copper species arising from the intimate interaction between the metal and supports. The result was also consistent with the TEM images, indicating that an intimate interaction would occur in the Cu/P25 system. Notably, for the Cu/A catalyst, broad peaks originating from three distinct copper species occurred. We assigned the three peaks as follows: the shoulder peak at about 468 K is from well-dispersed copper species, whereas the peaks at 487 and 560 K are from copper oxide species that interact better with the support and bulk CuO particles. We conclude that compared with the other three catalysts, there is an intimate interaction between Cu and the support in the Cu/P25 catalyst, which may also account for its remarkable performance in activity tests.

Fig. 3 TPR profiles of Cu/P25, Cu/M, Cu/R and Cu/A.

3.3 Chemical state and surface composition

The Cu 2p XPS results of the calcinated catalysts are shown in Table 1 and Fig. 4A, and the binding energy of the Cu 2p_3/2 peak at around 933.8 eV along with the presence of the characteristic shakeup satellite peaks suggests that the copper oxidation state is +2 in all four catalysts.¹⁴ We can clearly see that in the Cu/TiO₂ catalysts there is a slightly higher binding energy for Cu 2p_3/2 compared with those of Cu/SiO₂ in the literature,³⁵ indicating that compared with the silica-supported system, a stronger interaction between the copper oxide and the titania supports occurred, which is consistent with observations reported by Wen et al.³⁵ and Chen et al.³⁶ Fig. 4B shows the Cu 2p XPS spectra of the reduced catalysts, all of which were pre-treated in H₂ at 573 K for 4 h. The peak at approximately 932.8 eV was assigned to metallic Cu⁰ and/or Cu⁺, because the binding energies of Cu⁰ and Cu⁺ cannot be distinguished based on the Cu 2p_3/2 peak.^37,38 In addition, the shake-up satellite peaks evident in the calcinated samples disappeared, which also confirms that most of the surface copper species in the catalysts were in the reduced state.³⁵ There is a wide weak peak between 942 and 944 eV for some reduced catalysts, indicating the presence of some Cu²⁺ species, which may stem from minor oxidation during the sample preparation or baseline oscillations in the XPS apparatus. However, based upon previous studies, these peaks are too small to affect the peak fitting, and thus can be ignored.

Fig. 4 Cu 2p XPS spectra of catalysts (A) calcinated at 823 K and (B) reduced at 573 K.

From the Cu 2p XPS results, clear shifts of the binding energy can be observed in the reduced samples. The Cu/P25 sample displays a higher BE compared with other samples, which have similar BE values of Cu 2p at 932.8 eV (Fig. 4B). In the Ti 2p XPS profiles of the reduced samples in Fig. 5, a clear chemical shift of the BE deriving from the reduction of Ti⁴⁺ to Ti³⁺ in the TiO₂ or the electron transfer from Cu to TiO₂ is visible. A clear BE shift of Ti 2p of about 1.8 eV occurs after reduction in both Cu/A and Cu/M, whereas this BE shift is negligible in Cu/R. As Panpranot et al. proved, it is more difficult to reduce Ti⁴⁺ to Ti³⁺ in rutile TiO₂, because the rutile phase is more thermodynamically and structurally stable than the anatase phase,³⁹ confirming that there was almost no decrease in the BEs of Ti 2p in Cu/R after reduction. Therefore, the greater BE shift in Cu/A and Cu/M are mainly attributed to the reduction of Ti⁴⁺ to Ti³⁺ in TiO₂. However, in Cu/P25, the Cu 2p peak shows a higher BE value compared with the other samples, and the shift of the Ti 2p peak to a lower BE is observed, showing that the electrons in Cu are transferred to the P25 support because of the preferential location at the interface. Wen et al. also observed this in Cu/TiO₂–SiO₂ catalysts.³⁵ The preferential location of copper at the interface between the anatase and rutile phases is beneficial for electron transfer from Cu to P25 TiO₂, enhancing the metal–support interaction, and thus contributing to the higher activity of Cu/P25.

Fig. 5 Ti 2p XPS spectra of catalysts (A) calcinated at 823 K and (B) reduced at 573 K.

To investigate the synergetic effect between Cu⁰ and Cu⁺ further, the Cu LMM X-ray induced Auger spectra (XAES) was obtained (Fig. S3, ESI†). The kinetic energies of 916.6 and 918.5 eV strongly suggest the co-existence of the Cu⁺ and Cu⁰ species in the reduced catalysts.¹⁴ Deconvolution of the original Cu LMM peaks was performed and the peak positions and their contributions extracted from the deconvolution are listed in Table 2. The Auger parameter of copper remained at 1851–1852 eV, suggesting the presence of Cu⁺ species along with Cu⁰.⁴⁰ In addition, the asymmetric Cu LMM peaks and the modified Auger parameter, α′, at about 1851.0 eV attributed to Cu⁰, and about 1849.0 eV attributed to Cu⁺, verified the co-existence of the Cu⁰ and Cu⁺ species on the surface of the catalysts.¹⁴ The crystal phases strongly affect the distributions of surface Cu⁺ and Cu⁰, as the molar ratio of surface Cu⁺/Cu⁰ varied in the samples. A higher amount of surface Cu⁺ species was observed in Cu/P25.

Table 2 Surface Cu components of catalysts reduced at 573 K based on Cu LMM deconvolution

Sample	KE^a (eV)		AP^b (eV)		BECu 2p3/2^c (eV)	Cu^0/Cu^+^d
	Cu^+	Cu^0	Cu^+	Cu^0
a Kinetic energy.b Auger parameter.c Binding energy of the reduced samples.d Cu^0/(Cu^+) × 100%.
Cu/P25	916.0	918.6	1849.0	1851.6	933.0	0.857
Cu/M	916.2	919.1	1848.9	1851.8	932.7	1.021
Cu/A	916.1	918.8	1848.9	1851.6	932.8	0.932
Cu/R	916.0	918.9	1849.0	1851.9	933.0	0.904

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3.4 Catalytic activities and stability

To understand the origin of the catalytic reactivity over Cu/TiO₂ catalysts, we investigated the relationship between the catalytic performance and intrinsic structure of catalysts with a probe reaction of the gas-phase hydrogenation of DMO by tuning the LHSV value.

Fig. 6 shows the conversion of DMO and the selectivity for EG over various catalysts. Cu/P25 showed optimal catalytic activity, even at a high LHSV of 1.0 h⁻¹, whereas the other three catalysts were markedly deactivated. For Cu/P25, a yield of 98.9% was available under optimized reaction conditions. These results clearly demonstrate the high efficiency of the Cu/P25 catalyst during the selective hydrogenation of DMO to EG. The optimal catalytic performance of Cu/P25 may arise from the strong interaction between copper which located at the interface of anatase and rutile phases, and P25 titania support. This finding is also visible in the HRTEM images and TPR profiles. The physicochemical properties show that the Cu/P25 catalyst has the largest BET area, SA_Cu, and copper dispersion. Combining these observations with the TEM and TPR results, we conclude that the interaction between Cu and P25 induces a better dispersion as well as an optimized Cu⁰/Cu⁺ distribution, which produces a better catalytic performance. The P25-supported catalyst is expected to be capable of increasing the amount of copper species with partial positive charges resulting from a better metal support interaction as well as the electron transfer between the copper and P25 titania. Similar electron transfer was observed by Jovic et al.²¹ and Tsukamoto et al.³¹ in the Au/TiO₂ system.

Fig. 6 Catalytic performance of the catalysts. Reaction conditions: P = 2.5 MPa, T = 493 K, H₂/DMO = 100 (mol/mol).

To gain further insight into the effect of the crystalline TiO₂ supports, the TOF values were investigated to compare the catalytic performance of the catalyst based on the moles of DMO converted per mole of surface sites per hour (Table 1). Cu/P25 showed the highest TOF value of 9.1 h⁻¹, which is 13.8% higher than that of Cu/R, and more than 10 times larger than those of Cu/A and Cu/M.

To investigate the catalysts more thoroughly, we also conducted a test of long-term catalytic performance (Fig. S4, ESI†) at 493 K with a LHSV of 0.3 h⁻¹. During the reaction process, we discovered no obvious deactivation of Cu/P25, even after a period of 96 h, and the yield of EG remained around 95%, indicating the long-term catalytic life span of the P25-supported copper-based catalyst, which was also attributed to the intimate interaction between the Cu species and P25 support.

To exclude the effects of titania supports on the catalytic performance of DMO hydrogenation to EG, we examined pure P25 as a catalyst for the hydrogenation of dimethyl oxalate, only to find too slight DMO conversion less than 1.0% under reaction conditions identical to that of the stability test (Fig. S1, ESI†). Thus, the effect of the titania support could be neglected.

4. Discussion

Although copper-based catalysts for hydrogenation or hydrogenolysis have been extensively studied and various supports, such as Al₂O₃, ZrO₂, SiO₂, and hydroxyapatite, have been used to modify their catalytic performance,^2,4,41,42 studies of titania-supported copper catalysts in ester hydrogenation remain scarce. In our present study, outstanding hydrogenation activity was achieved with the Cu/P25 catalyst. The structural and textural evaluation showed the highest BET area and SA_Cu, whereas the redox behaviour indicates a better metal–support interaction of Cu/P25 compared with the other three catalysts. Notably, the highest dispersion and the largest amount of surface Cu⁺ species were also observed in Cu/P25, as determined by N₂O adsorption and XAES.

The subtle information about catalytic activities for the catalysts is indicated by the TOF values, of which Cu/P25 displayed the highest value of 9.1 h⁻¹ (Table 1), suggesting the strong effect of the titania crystal phase on the catalytic system for DMO hydrogenation. Although there appears to be no obvious variation in physicochemical factors, such as S_BET, D_pore and V_pore, the TOF values differ sharply and this confirmed the superior properties of Cu/P25.

Numerous studies have been conducted to investigate the catalytic role of Cu⁺ and Cu⁰ in ester hydrogenation.^8,14,41,42 Poels et al. suggest that the Cu⁰ sites dissociatively adsorb hydrogen molecules, and that the Cu⁺ sites strongly bind and activate the ester and acyl groups.⁴³ Dai and colleagues reported that Cu⁺ may function as electrophilic or Lewis acid sites to polarize the C=O bond via the electron lone pair on oxygen, thus improving the reactivity of the ester group in DMO.¹⁴ Gong et al. revealed a cooperative effect between surface Cu⁰ and Cu⁺ as well as an optimum Cu⁰/Cu⁺ ratio in the Cu-based catalysts,⁴¹ which was verified by He and co-workers.⁴² Accordingly, the efficiency of DMO hydrogenation to EG may depend heavily on the synergy of Cu⁰ and Cu⁺ sites on the catalyst surface and optimized Cu⁰/Cu⁺ distributions may result in a high activity. In our present work, the highest amount of surface Cu⁺ species was observed in Cu/P25, which produces the highest yield of EG. However, this does not apply to all cases. The P25-supported Cu/P25 catalyst is expected to increase the amount of copper species with partial positive charges, resulting from the better metal support interactions and the electron transfer, whereas no similar behavior was observed for the other catalysts, which may account for the highest amount of Cu⁺ in the Cu/P25 system. A similar result was observed by He et al. in the CuB/SiO₂ system, and they ascribed this to the greater acidity and electron affinity of boric oxide.⁴²

We also note that copper dispersion and the specific surface area of metallic copper play a significant role in the catalytic performance, which has been confirmed in various studies.^8,30,35,42 The highest SA_Cu was achieved in the Cu/P25 catalyst, which results from the better dispersion of copper species on the support. Additionally, the TEM images show the better dispersion of copper species in Cu/P25, and the HRTEM images clearly show the location of active copper at the interface between anatase and rutile titania, indicating an intimate interaction between Cu and the support in the Cu/P25 catalyst. This may be responsible for the optimized catalytic activity. In short, a strong interaction between Cu and the P25 support greatly increases the dispersion of metallic copper, which also contributes to the higher catalytic activity and stability.

5. Conclusions

In summary, a series of copper-based titania-supported catalysts were synthesized to achieve the optimum catalytic performance. The crystalline phases of TiO₂ play an important role in the activity for of the vapor-phase hydrogenation of DMO to EG, because it affects the interaction between copper and the support, and thus affects the metallic dispersion of copper and the ratio of active Cu⁰/Cu⁺. The P25-supported copper catalyst displays excellent catalytic performance because of the intimate interaction between copper and the P25 support. This interaction was validated by the location of Cu at the interface between the anatase and rutile phases, increasing the copper dispersion and inducing an optimized Cu⁰/Cu⁺ distribution.

Acknowledgements

We thank financial support by the Major State Basic Resource Development Program (Grant no. 2012CB224804), Natural Science Foundation of China (Project 21373054, 21173052), and the Natural Science Foundation of Shanghai Science and Technology Committee (08DZ2270500).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00053j

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