Dimethyl ether (DME), as the simplest ether without C-C bond, is expected to replace diesel due to its high cetane number and low toxicant emission. In addition, DME is also attractive as an excellent substitute for liquefied petroleum gases because of its similar physicochemical properties [1, 2]. Traditionally, the production of DME is realized by a two-step method. To be more specific, firstly, the methanol is synthesized from syngas over the Cu-based catalysts, and then DME is produced by methanol dehydration over the solid acid catalysts [3, 4]. Compared with the two-step synthesis of DME, the direct one-step syngas-to-DME (STD) reaction possesses many advantages, such as breaking the thermodynamic equilibrium limitation of methanol synthesis from syngas, promoting the production of DME and decreasing cost for DME producing, etc. [5, 6]. For these reasons, the one-step STD reaction has drawn increasingly attention of worldwide researchers [7, 8].

The commonly applied bifunctional catalyst in the STD reaction is a mixed catalyst, which is prepared by physically mixing methanol synthesis catalyst and methanol dehydration catalyst [9, 10]. The component of methanol synthesis in the STD reaction is mainly Cu-based catalyst [11, 12], and that of methanol dehydration in the STD reaction is solid acid catalyst, such as acidic zeolites [13, 14] or γ-Al₂O₃ [15, 16]. The preparation of mixed catalysts is simple, but there are still some disadvantages. For instance, long distance and random distribution of two active units result in low activity and selectivity. The supported dual-function catalysts can make up for the deficiency [17, 18]. The components for methanol dehydration, such as zeolites or γ-Al₂O₃, have large specific surface areas and well-developed pores. As a support, it can promote the dispersion of active sites of methanol synthesis and improve the utilization of Cu [19, 20]. However, the opening structure of supported catalysts lowers the selectivity of DME in the STD process. The core-shell capsule-structure catalyst is developed to solve the problem [6, 21-27]. The closing-structured capsule-catalysts are prepared by coating the solid acid catalysts (zeolites or Al₂O₃) shell over the outer surface of millimetre-sized Cu-ZnO-Al₂O₃ cores [21-24]. The selectivity of DME over the capsule-catalyst is markedly higher than that over the open-structure bifunctional catalysts, such as hybrid catalysts and supported catalysts. A high-selectivity CuZn@m-Al₂O₃ catalyst for DME synthesis from syngas was also prepared by implanting the Cu-Zn nanoparticles into the matrix of mesoporous alumina [25]. However, the catalytic activity and the utilization efficiency of copper are very unsatisfactory owing to the use of the millimetre-sized Cu-based particles or poorly-dispersed Cu-based nanoparticles as methanol synthesis unit. It can be envisioned that an excellent bifunctional STD catalyst with high activity, Cu utilizing efficiency and high selectivity can be prepared with the highly-dispersed Cu nanoparticles embedded into porous Al₂O₃, and the high selectivity results from well-organized embedding-type structure. Metal-organic frameworks (MOFs) are widely used in gas storage, ion exchange and catalysis [28-31], due to its regular crystalline structure and developed porosity. Especially, the Cu-1, 3, 5-benzenetricarboxylate framework (Cu-BTC MOF) has also been widely applied to synthesize metal oxides, carbon nanodots, metal oxides@silica oxide and films [32-36], owing to its simple synthesis process, cheap reactants, high metal dispersion, ordered pores and large surface areas.

In this work, we, for the first time, prepared a new bifunctional catalyst (Cu@Al₂O₃-dp) with Cu implanted into porous Al₂O₃, derived from Cu-1, 3, 5-benzenetricarboxylate framework (Cu-BTC MOF) by filling MOF with Al₂O₃ using deposition-precipitation method with sodium carbonate solution as the precipitant (Fig. 1a). According to the SEM images and XRD pattern of Cu-BTC shown in Fig. S1 (Supporting information), the Cu-BTC MOF has been successfully synthesized by solvothermal method [10]. From results of N₂ adsorption shown in Fig. S2 (Supporting information), the Cu-BTC MOF contains abundant micropore. After filling MOF with aluminium, the uncalcined precursor of Cu@Al₂O₃-dp (pre-Cu@Al₂O₃-dp) barely has micropore, it may be ascribed to the fully-filled pore by aluminium, indicating aluminium infiltrate into the pore of MOF. In addition, in order to investigate the implanting ways of alumina and the effect of alumina on the catalyst structure and catalytic performance in the STD reaction, the Cu@Al₂O₃-im catalyst was prepared with immersion method where aluminium nitrate solution was used to fill Cu-BTC MOF. From the scanning electron microscope (SEM) images shown in Figs. 1b and c, the Cu@Al₂O₃-dp catalyst shows an irregular cube with the sizes ranging from 5 µm to 10 µm, and the Cu@Al₂O₃-im catalyst features an octahedral structure similar to Cu-BTC MOF, but the sizes are similar to that of Cu@Al₂O₃-dp and smaller than that of Cu-BTC MOF. From Table 1, the actual CuO loadings of Cu@Al₂O₃-dp and Cu@Al₂O₃-im catalysts are 13.0% and 92.9%, respectively. It is suggested that a little alumina is filled within Cu-BTC MOF by the immersion method. In order to clearly show the advantages of the implanting-type catalyst, the supported Cu catalyst on MOF-derived porous Al₂O₃ (Cu/Al₂O₃) was prepared by incipient wetness impregnation (IWI) method (Figs. 1d). The MOF-derived Al₂O₃ (Al₂O₃-DMOF) was obtained by repeatedly etching away the CuO component of the Cu@Al₂O₃-dp catalyst with ammonia water. Since a small amount of CuO in Al₂O₃-DMOF was completely and tightly wrapped by alumina, therefore, it was not dissolved by ammonia water, with the loading of CuO 4.2% (Table S1 in Supporting information). From Tables S1 and S2 (Supporting information), since there is a little exposed surface Cu site in Al₂O₃-DMOF (29.3 µmol/g) and it exhibits poor catalytic performance, the CuO loading similar to the Cu@Al₂O₃-dp catalyst is loaded onto Al₂O₃-DMOF, and the actual CuO loading of Cu/Al₂O₃ is 18.8%. Moreover, from Fig. 1g, it can be clear that there are many small particles accumulated on the outer surface of Cu/Al₂O₃ catalyst.

The microstructure of Cu@Al₂O₃-dp, Cu@Al₂O₃-im and Cu/Al₂O₃ catalysts are illustrated by high resolution transmission electron microscope (HRTEM) images in Figs. 1e-g, from which, it can be seen that the average size of CuO nanoparticles (NPs) of Cu@Al₂O₃-dp is 2.9 nm, which is far smaller than that of Cu/Al₂O₃ (9.3 nm) and Cu@Al₂O₃-im (20-40 nm). Furthermore, compared with intimate contact between CuO NPs of Cu/Al₂O₃ catalyst, the CuO NPs of Cu@Al₂O₃-dp catalyst are separated by Al₂O₃. Although the CuO NPs of Cu@Al₂O₃-im catalyst are separated by thin alumina, the CuO NPs aggregation are very severe, because of too little alumina to disperse CuO NPs and inhibit thermal sintering of CuO NPs. In addition, as shown in Table 1, the amount of exposed Cu active sites on Cu@Al₂O₃-dp catalyst (416.3 µmol/g) is larger than those of the Cu@Al₂O₃-im (105.7 µmol/g) and Cu/Al₂O₃ catalyst (341.5 µmol/g), although Cu@Al₂O₃-dp catalyst has a lower CuO loading. These results confirm that Cu@Al₂O₃-dp catalyst has a higher Cu dispersion. Additionally, the embedding type structure of Cu@Al₂O₃-dp may compress the CuO sintering during the calcination at high temperature in the preparation process, which may partially lead to its smaller CuO NPs compared to Cu/Al₂O₃. In order to verify the conjecture, the Cu@Al₂O₃-dp-400 and Cu/Al₂O₃-400 catalysts were prepared, and the preparation are similar to that of Cu@Al₂O₃-dp and Cu/Al₂O₃, and the only difference is that the former did not be calcined at 600 ℃ for 1 h. The exposed surface Cu sites of Cu@Al₂O₃-dp-400 and Cu/Al₂O₃-400 catalysts are shown in Table S1. From Table 1 and Table S1, it can be seen that the high temperature treatment is necessary for Cu@Al₂O₃-dp, which makes the Cu sites increase obviously from 97.0 µmol/g to 416.3 µmol/g, while the Cu sites of Cu/Al₂O₃ catalyst sinters severely and decreases from 611.8 µmol/g to 341.5 µmol/g. It is clear that the embedding type structure of Cu@Al₂O₃-dp may inhibit the sintering of CuO.

X-ray diffraction (XRD) was applied to further investigate the crystal structure of catalysts. From Fig. 2a and Fig. S3a (Supporting information), compared with Cu/Al₂O₃ and Cu@Al₂O₃-im catalysts with the obvious and sharp diffraction peaks of CuO (PDF#48-1548) [37], Cu@Al₂O₃-dp catalyst has no diffraction peaks of CuO, which shows its smaller grain size and higher dispersion and it agrees with the results of HRTEM and amount of surface active Cu. It is favourable for improving the CO conversion. The results of N₂ adsorption are shown in Fig. 2b, Fig. S3b (Supporting information) and Table 1, the adsorption-desorption isotherms and pore size distributions of Cu@Al₂O₃-dp and Cu@Al₂O₃ catalysts (Fig. 2b) display that both catalysts contain many micropores and mesopores, and the pores size of Cu@Al₂O₃-dp catalyst is larger than that of Cu/Al₂O₃. The types of hysteresis loop of Cu@Al₂O₃-dp and Cu/Al₂O₃ indicate the pores of the former resemble ink-bottle pores in alumina, while that of the latter is more likely to be intercrystalline pore [38]. From Table 1, the values of S_micro and V_micro of Cu@Al₂O₃-dp are higher than those of Cu/Al₂O₃. Although the Cu@Al₂O₃-dp has a slightly lower mesoporous surface area compared with Cu/Al₂O₃, which is owing to the more intercrystalline pore of Cu/Al₂O₃, the total surface area of Cu@Al₂O₃-dp is larger than that of Cu/Al₂O₃. From Fig. S3b and Table 1, it can be seen that the Cu@Al₂O₃-im nearly has neither micropore nor mesopore, which might be ascribed to the fully-filled structure.

H₂ temperature-programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) experiments were conducted to study the reduction behaviour of CuO and metal-support interaction between Cu and alumina of catalysts. From Fig. 2c and Fig. S3c (Supporting information), compared with Cu/Al₂O₃ and Cu@Al₂O₃-im catalysts obviously with two kinds of reduction peaks of CuO, corresponding to the surface and bulk of reducible CuO species [24], only one main symmetrical reduction peak at 247 ℃ is found on Cu@Al₂O₃-dp catalyst, indicating homogenous distribution of CuO nanoparticles embedded in alumina [25]. From Fig. 2d and Fig. S3d (Supporting information), the Cu 2p_3/2 peaks of Cu@Al₂O₃-dp and Cu@Al₂O₃-im catalysts can fitted and divided into two peaks at 932.9 and 934.7 eV, 932.7 and 933.7 eV, respectively, corresponding to Cu⁺ and Cu²⁺ species. The Cu⁺ species is ascribed to the Cu²⁺ reduced by decomposition of benzoate ligands during calcination [34], which can strengthen the CO adsorption and are beneficial to methanol synthesis [37]. From Table 1, the I_Cu/I_Cu intensity ratios of Cu@Al₂O₃-dp are higher that of the Cu@Al₂O₃-im. CO temperature programmed desorption (CO-TPD) profiles were shown in Fig. S4 (Supporting information), it can be seen that much stronger adsorbed CO on Cu@Al₂O₃-dp than that on Cu/Al₂O₃. Both indicate the capacity of CO adsorption on Cu@Al₂O₃-dp is stronger than that on Cu@Al₂O₃-im and Cu/Al₂O₃.

The binding energies of deconvolution peaks belonged to Cu²⁺ species on Cu@Al₂O₃-dp, Cu/Al₂O₃ and Cu@Al₂O₃-im catalysts are 934.7, 933.9 and 933.7 eV, respectively, which demonstrates the Cu@Al₂O₃-dp catalyst possesses stronger Cu-Al interaction compared to Cu/Al₂O₃ and Cu@Al₂O₃-im catalysts, which can improve intrinsic activity of Cu actives in the Cu@Al₂O₃-dp and conduces to the CO conversion [7].

The acidic nature of the catalysts is measured by NH₃-TPD experiments and shown in Table 1 and Fig. S5 (Supporting information). It can be seen that there are two main NH₃ desorption peaks in Cu@Al₂O₃-dp and Cu/Al₂O₃ catalysts. One was at low temperature (about 150-300 ℃), corresponding to the desorption of the adsorbed NH₃ on weak acid sites, and the other was high temperature (about 300-450 ℃), corresponding to the desorption of the adsorbed NH₃ on medium acid sites [24, 25]. Weak and medium acid sites are identified as the ideal acid active sites for methanol dehydration to dimethyl ether [7]. Moreover, few strong acid sites exist in Cu@Al₂O₃-dp and Cu/Al₂O₃ catalysts. From Fig. S5, it can be observed that Cu@Al₂O₃-dp catalyst has more medium acids and less strong acid than Cu/Al₂O₃, which is beneficial to methanol dehydration and inhibiting the formation of methane and coke [7]. In addition, the Cu@Al₂O₃-im hardly possesses acid sites (2.8 µmol/g), which is attributed to the very low Al content and/or very low surface area. From Table 1, although the Cu@Al₂O₃-dp catalyst shows a slightly lower amount of acidic sites (289.7 µmol/g) than Cu/Al₂O₃ (301.4 µmol/g), it shows much higher DME selectivity (90.4%), resulting from the unique Cu-embedding structure in porous Al₂O₃.

The Cu@Al₂O₃-dp, Cu@Al₂O₃-im and Cu/Al₂O₃ catalysts are applied in the STD reaction, and the results are illustrated in Table 2. Thanks to the unique embedding-type structure and its confinement effect, the Cu@Al₂O₃-dp catalyst shows higher dispersion of Cu, more Cu⁺ species and stronger Cu-Al interaction [7, 37]. Moreover, the methanol formed on Cu species can only come out through the pore of alumina, during which they are converted to DME by the acidic sites of alumina, and increased the selectivity of DME and decreased the selectivity of methanol, breaking the thermo-dynamically limitation and promoting CO conversion. However, due to the unrestricted and opened reaction environment of conventional supported Cu/Al₂O₃ catalyst, the methanol can be directly desorbed from the catalyst without contacting the acidic site of alumina, which leads to high selectivity of methanol and low selectivity of DME, which inhibits the methanol synthesis reaction and decreases CO conversion. Therefore, the 25.7% of CO conversion and 0.3 min⁻¹ of turnover frequency (TOF) are observed over Cu@Al₂O₃-dp, which are much higher than those over the Cu@Al₂O₃-im (2.3%, 0.2 min⁻¹) and Cu/Al₂O₃ (15.4%, 0.2 min⁻¹) catalysts. Meanwhile, Cu@Al₂O₃-dp illustrates much higher distribution of DME (90.4%) compared with Cu@Al₂O₃-im (14.2%) and Cu/Al₂O₃ (63.9%). The selectivity of methanol on the Cu@Al₂O₃-dp is lower than that on the Cu/Al₂O₃, which is ascribed to more medium acidic sites and unique embedded-structure [25]. Moreover, the high selectivity of methane on Cu/Al₂O₃ of 25.5% and Cu@Al₂O₃-im of 36.2% in comparison with 4.2% on the Cu@Al₂O₃-dp can be attributed to the large crystallite sizes of metallic Cu and stronger acidic sites, which are active sites for methanation of CO or decomposition of intermediates such as methanol or possibly DME [7]. For comparison, the supported Cu catalyst on commercial γ-Al₂O₃ with CuO loading of 15.0% was also synthesized (Cu/Al₂O₃-com). The CO conversion and DME selectivity over the Cu/Al₂O₃-com are 7.9% and 60.5%, which are lower than those of the Cu@Al₂O₃-dp and Cu/Al₂O₃. Furthermore, from Table S3 (Supporting information), Cu@Al₂O₃-dp indicates much higher production of DME per hour per gram of CuO than the previously reported Cu-based catalysts. The stability of Cu@Al₂O₃-dp and Cu/Al₂O₃ were measured and are shown in Fig. S6 (Supporting information). It is clear that the CO conversion of Cu@Al₂O₃-dp does not obviously decrease during 28 h of time on stream (TOS). The selectivity of DME over Cu@Al₂O₃-dp catalyst slightly decreases during 28 h, while that over Cu/Al₂O₃ catalyst significantly decreases. A large amount of methane and methanol are produced over the Cu/Al₂O₃ catalyst as time on stream prolongs. From Fig. S6 and Table S4 (Supporting information) the slight decrease of DME selectivity on the Cu@Al₂O₃-dp, compared to Cu/Al₂O₃, results from the loss of a few acidic sites and embedding structure. From Table S4, the loss ratio of acidic sites over Cu@Al₂O₃-dp catalyst is 1.7% during 28 h of time on stream, which is more than Cu/Al₂O₃ catalyst (6.2%).

In this work, a novel Cu-embedded porous Al₂O₃ catalyst has been successfully prepared by filling Al(OH)₃ into Cu-BTC framework through the deposition-precipitation followed by a calcination process. The developed Cu@Al₂O₃-dp catalyst displays 25.7% of outstanding CO conversion and 90.4% of DME selectivity in the STD reaction, which are much larger than those over the supported Cu/Al₂O₃ catalyst. Moreover, the Cu@Al₂O₃-dp catalyst also shows higher catalytic stability than Cu/Al₂O₃ catalyst. The outstanding catalytic performance of Cu@Al₂O₃-dp catalyst can be attributed to its unique embedding-type structure, higher Cu dispersion, more Cu⁺ species, stronger Cu-Al interaction and more medium acidic sites. This work not only develops a highly efficient catalyst for direct synthesis of DME from syngas, but also provides a new way for designing other embedding-type bifunctional catalysts for diverse consecutive reactions.

The authors declare no conflicts of interests.

This work was financially supported by the National Natural Science Foundation of China (No. U1610104), Liaoning Revitalization Talents Program (No. XLYC1907053, China) and CAS Youth Innovation Promotion Association (No. 2018220, China).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.10.031