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Freezing of copper as a precious metal-like catalyst for preliminary hydrogenation



INTRODUCTION

The pure utilization of carbon resources for synthesizing high value-added chemicals is highly desirable with the increasing energy and environmental problems 1 ). One of the successful industrial uses is the synthesis of alcohols from carbon via syngas (CO and H ), which contains the coupling of CO with nitrite esters to dimethyl oxalate (DMO) and sequential hydrogenation of DMO to alcohols (). 2 ). Typically, the hydrogenation of DMO can typically achieve three major products, including methyl glycolate (MG), ethylene glycol (EG) and ethanol (EO). Now copper-based catalysts have been intensively investigated ( 3 6 ) and very high exchanges of EC (or EO) and rational insights have been achieved in previous work ( 7 ̵

1; 9 ). In Cu-based catalysts, the balance of Cu and Cu + is indispensable for deep hydrogenation to the EG or EO from DMO. MG, as a preliminary DMO hydrogenation product, is an important intermediate of higher commercial price than EG and EO for the synthesis of pharmaceuticals, fine chemicals and perfumes. However, MG is difficult to achieve via copper catalysts, since the thermodynamic constant of the second hydrogenation step is two orders of magnitude greater than that of the first hydrogenation step (). For tandem hydrogenation reactions, the development of an effective catalyst for controlling and regulating target products remains a major challenge for both academia and industry.

Generally, the synthesis of MG via DMO requires a moderate reaction requirement and a catalyst with a relatively weak hydrogenation property, such as Ag-based and Au-Ag based noble metal catalysts ( 10 12 ). and nonsilica-supported Cu catalysts, such as hydroxyapatite and activated carbon (). Silver surfaces, unlike copper, generally lack affinity towards H [dissociation] due to the filled d-band and contribute to MG production (). However, the application of high load noble metal catalysts is undesirable. The highly efficient silver catalyst also suffers from by-products due to its activity in chemoselective hydrogenation of both C = C and C = O bonds ( ). In comparison, copper-based catalysts are still a good alternative in the aspect of product control if their hydrogenation ability can be controlled to a moderate level since the copper sites are highly selective against the hydrogenation of C = O bonds rather than hydrogenolysis of C = C 16 . ).

In this study, freezing of Cu species of a Cu / SiO 2 catalyst in a low reduction metallic state is achieved by argon plasma sputtering (SP) from a copper target of a self-made SP. The sputtered Cu atoms are distributed on SiO 2 evenly with the continuous hexagonal rotation and mechanical vibrations in the polygonal fat-SP process ( 17 – ). This process can be used directly in the DMO hydrogenation reaction without any reduction treatment and is difficult to oxidize even at a high temperature under oxidizing atmosphere. The SP-Cu / SiO 2 catalyst exhibits high selectivity to MG at a wide temperature range, due to its relatively lower reduction than the conventional ammonia evaporation (AE) shaped one. The catalytic behavior of sputtered Cu on product distribution of DMO hydrogenation is very similar to that of precious metals (Ag or Au) derived from the electron structure altered by the bombardment of high energy argon plasma. The present study is expected to show the noble metal-like property of sputtered Cu and dissolve the catalytic mechanism of Cu and Cu + species in preparative and deep hydrogenation processes, exploring the potential applications of sputtered Cu in yield of precious metal catalysts. RESULTS

Chemical state of copper over SP and AE-made Cu / SiO 2 catalysts

We performed X-ray diffraction (XRD) characterizations to understand the nature of copper species and their variations during one DMO hydrogenation reaction (Fig 1A). As previously reported, the weak and broad diffraction at ca. 31.2 ° and 35.8 ° suggest the presence of copper phyllosilicate on fresh AE-Cu / SiO 2 (). These copper compounds must be reduced to metallic Cu or Cu [0] before reaction. Subsequently, Cu [0] 2 species become predominantly during the reaction. For the reduced AE catalyst, it is difficult to separate the copper state from the weak XRD diffraction peaks, since Cu is well dispersed or small. Therefore, we also provided the enhanced X-ray absorption fine structures (EXAFS) results (Fig 1B). It provides further evidence of the coexistence of metallic Cu and Cu [0] species, in which the latter is the primary phase compared to reference Cu and Cu [2]. For SP-Cu / SiO 2 only metallic Cu species are detected in the sample so produced and they remain stable during the reaction. It indicates that metallic Cu can be formed and dispersed on the support in the SP preparation process without any reduction treatments, which is also evidenced by temperature programmed reduction (TPR) in Figure 1C. A large amount of H is consumed at about 240 ° C when Cu precursors are reduced to metallic Cu or Cu [19459] for AE-Cu / SiO 2 2 . However, only a small H [2] 2 consumption peak derived from passivation of active Cu species can be observed on SP-Cu / SiO during reduction from 100 ° to 500 ° C. IB shows the SP used catalyst only Cu-Cu coordination in metallic Cu, suggesting that copper is "frozen" in a metallic state, which is well consistent with XRD analysis.

FIG. Chemical state characterizations of SP and AE-made Cu / SiO 2 catalysts.

( A) XRD pattern of fresh and used Cu / SiO 2 samples. a.u., arbitrary units. ( B) EXAFS spectra of used SP and reduced AE Cu / SiO 2 samples. FT, Fourier transform. ( C ) 2 -TPR results of used SP-Cu / SiO and reduced AE-Cu / SiO 2 samples . ( D) In ​​situ, results of SP and AE-made Cu / SiO 2 samples . (19459033] E ) TEM image of fresh SP-Cu / SiO 2 and ( F ) TEM image of fresh AE-Cu / SiO 2

] In-situ diffuse reflectance infrared (IR) Fourier transform spectra (DRIFTS) results in Fig. 1D showing strong adsorption of CO on Cu + and Cu by AE-Cu / SiO 2 corresponding to the IR absorption peaks at about 2127 and 2102 cm -1 (19459009) -1 ( 13 ). In comparison, no CO adsorption is observed on the SP-Cu / SiO 2 specimen, either on Cu or on Cu + indicating that the interaction between CO and Cu species are very weak. Detailed CO adsorption processes can be seen in Fig. S1. It reveals that the chemical behavior of the Cu [0] species on SP-Cu / SiO and AE-Cu / SiO samples is completely different. The former exhibits non-reactivity upon the supply of electrons, which is anomalous to Cu and more approximately to precious metals. The particle size effects on the adsorption property can be omitted since both Cu particles are centered about 3 to 5 nm with the corresponding copper loading amount and highly dispersed on the silica support reflected from the transmission electron microscope (TEM) images in Figure 1 (E and F). It is noteworthy that the Brunauer-Emmett Teller (BET) surface is slightly reduced by only 5.6% after physical SP of Cu nanoparticles on silica compared to the crude silica support, while markedly increasing from 283 to 401 m 2 g -1 after the AE process. These findings suggest that the copper deposition process on silica is quite different despite the similar dispersion of Cu nanoparticles. Using a powerful Ar plasma stream, metallic copper nanoparticles are shocked and homogeneously dispersed on the silicon surface by physical polygonal rotation and mechanical vibration. This is a physical deposition process without destroying the pore properties of silica. In contrast, the layered copper phyllosilicates [Cu 2 2 2 (OH) 2 ] are formed as a key precursor in the chemical AE , disperses Cu nanoparticles in silica, enlarges the surface and gives stable Cu + in DMO hydrogenation ( 7 ).

Electron structure information of copper [X659014] We performed X-ray photoelectron spectroscopy (XPS) surface analysis to identify the chemical valence and electron structure of copper. Binding energy (BE) peak at approx. 932 to 933 eV in Cu 2p spectra (Fig 2A) are generally referred to the presence of Cu + and / or Cu species on the catalyst surface. The absence of approx. 933.5 eV to Cu 2+ and the characteristic satellite peaks (940-945 eV) show that the copper species in the fresh SP and reduced AE catalysts are of low valence (<2) and that they cannot be oxidized to CuO during the reactions ( 21 ). In detail, Cu 2p [2] 2 [19459] peaks of AE-made catalysts occur at a BE of 933.1 eV, while that of SP-made catalysts alternates with a lower BE of 931.8 eV, suggesting that copper is in a lower valence in the latter.

FIG. 2 Electron structure information of copper.

() Cu 2p XPS spectra. ( B ) Cu L [19459] M [M] 45 [XAESspectraPriortoXPSandXAESexperimentsArionicSPprepressformatwithabeamenergyof3keVandaSPrateof20µm for 7.5 minutes was used to clean the surface. ( C] ) Schematic of improved CK transition and electron penetration effect for SP made Cu / SiO produced with high energy argon plasma bombardment. E Auger Auger electrons; E p incident photon energy. The detailed low valence Cu state can be precisely distinguished by L 3 M M [X459009] X-ray excited Auger electron spectroscopy (XAES) spectra ( Fig. 2B). According to the literature, the asymmetric and broad Auger peak of AE-made Cu / SiO 2 catalysts can be deconvolved in two symmetrical peaks centered at 914.8 and 918.2 eV, corresponding to Cu + . and Cu and 22 respectively. The peak position and their intensity are shown in Table S1. About 56% Cu species in the reduced AE catalyst are Cu + . The increasing content of Cu + from 56.0 to 78.1% during the reaction suggests that about half of Cu [oxidized] to Cu + through DMO in catalytic the hydrogenation reaction. In Figure 2B, the eye peaks of sputtered Cu catalysts can be divided into several Cu terms, primarily at approx. 917 and 919 eV together with small characteristic peaks at 913 to 917 and 921.7 eV. The detailed top information is shown in Table S2. They are very close to the pure Cu bulk of Figure S2 instead of the supported AE-Cu catalysts, showing that Cu is essentially in the metallic state in the case of SP catalysts. It is noteworthy that the kinetic energy of primary Cu [Auger] AE catalyst peak is 0.8 eV lower than that of Cu bulk or sputtered copper, which is probably due to the stronger interaction between Cu and silica. Sputtered copper nanoparticles are anchored on the surface of the silica and interact with silica in weak physical force. It is a universal phenomenon in our earlier SP-made catalysts ( 18 19 ). (2p ] Generally, the Cu Auger junction is produced from a separate L (2p ) corehole decay via the Auger process involving two M (19459009) ( 3d) electrons resulting in a final 3d configuration . In detail, according to the work of Pauly et al . (), the primary five definitive terms in Cu mass spectra as shown in Fig. S3 and Table S2 are observed. The peaks at 919 and 922 eV are defined as 1 G and 3 F, which differs from L-S coupling corresponding to two 3d holes in metallic copper. These two terms are related to "normal" Cu, and their Auger contribution to the measured L 3 M 45 M 45 45 The other three Auger troops in the range of 913 to 917 eV is defined as 2 F *, Sum * and 4 [F]which are characteristic features of L 2 L 3 M Coster-Kronig (CK) transition. From the schematic in Figure 2C, L 2 L 3 M 45 CK transition occurs if the electron energy level of the Auger ionization hole (2p 1/2 ) and fill holes (2p 3/2 ) are on the same core level (2p) and leave an "extra" hole in M ​​ 45 the original state of normal L 3 M M M 26 27 . Thus, the additional Auger vacancy satellite transitions correspond to a configuration after the common process 3 M 45 45 45 shifted to lower kinetic energy due to Coulomb interaction between this spectator guard and Auger electron ( ). The contribution of all additional structures to total Cu contribution (62.9%) in the SP-Cu / SiO catalyst 2 is much higher than that of pure Cu mass (18.1%), suggesting that the CK transition is obviously improved. Further, the composition of various cup operations in the sputtered Cu catalyst remains almost stable even after high temperature DMO hydrogenation (> 280 ° C). The slightly elevated ratio of extra Cu [6245 to 65.5%] may be due to the formation of some Cu + + species with an overlapped Auger peak at ca. 914.8 eV. The elevated CK transition phenomenon is presumably attributed to the increase in the electron density of inner shells in Cu atoms by high energy argon plasma bombardment during the SP process. In general, the simple electron in the 4th orbit has a chance to arrive at the inner shell because of its lower energy level than those in the 3d path, called as "penetration effect". As described in Fig. 2C, the penetration power may also be improved as the electron density of the interior is increased by the continuous plasma bombardment. In this case, the outermost electron in the SP-Cu is more difficult to escape from the Cu atom, making these Cu nanoparticles exhibit a higher oxidation resistance than the AE-Cu and the Cu volume and thus exhibit noble metal-like behaviors. [19659021] Oxidation Resistance and Precious Metal-like Properties [O] 2 The O molecule is well known in the oxidation of Cu to Cu + and is commonly used in the surface area measurement of Cu [species] (). Here, N 2 [temperature-programmedoxidation(N 2 O-TPO) is performed to examine the oxidation behavior of metallic Cu in SP and AE-made Cu / SiO 2 samples via monitoring the released N signal, as shown in Figure 3A. It can be seen that the signal N is apparently detected immediately after the introduction of N 0 into the system. In addition, this oxidation process terminates rapidly within about 10 minutes. It shows that the Cu [A] -cu / SiO species is very active and can be easily oxidized by N [O] even at room temperature. This is consistent with the oxidation of Cu to Cu + with ester or products under a catalytic reaction ( 29 ). In contrast, most of the metallic Cuet cannot be oxidized by N [0] 0 at room temperature. Only a small amount of Cu on the surface of SP-Cu / SiO can be oxidized, which may correspond to the active Cu types reduced in H 2 -TPR measurement (Fig. 1C) . It is observed that the Cu species could react with N at elevation of temperature. However, the oxidation process passes through about 40 minutes until the temperature reaches 200 ° C. It suggests that the metallic Cu produced by the SP method is more difficult to oxidize in an oxidation environment, much like that expressed on stable Au or Ag nanoparticles. It can be responsible for the improved penetration effect, where the isolated electron in a 4s path penetrates into the 2p path, leaving the filled 3d path as the outermost layer. It makes Cu difficult to lose electrons and oxidizes.

FIG. 3 Oxidation resistance and noble metal-like properties for sputtered copper.

( A] [O] TPO results of SP and AE-made Cu / SiO 2 2 2 catalysts, where it the upper left panel shows an N [2] 2 2 2 release at 25 ° C with time on the stream and the right right panel is released with temperature programming from 25 ° to 250 ° C. ( B) UV-Vis adsorption spectra of SP and AE-made Cu / SiO 2 catalysts. The post shows magnification of the selected area. The Cu powder with a purity of 99% is also tested for comparison. The vertical dashed line marks the position of the top surface plasma monosorbent. C ) Photographs of various catalysts.

Generally, the free electrons in precious metals (especially d electrons in silver and gold) are free to move through the material ( 31 ). Light in resonance with the surface plasma monoscillation causes the free electrons in the metal to oscillate. As the wave front of the light passes, the electron density of the particle is polarized to a surface and oscillates in resonance with the frequency of the light, resulting in oscillation ( 31 ). This phenomenon is called surface plasmon resonance (SPR) and exclusively in precious metals, such as Ag and Au, due to their air-stable nature ( 32 ). SPR is reported to be detected even in Cu nanoparticles sputtering at NaCl substrates and determined by an absorption peak at about 3 570 nm in ultraviolet visible (UV vis) spectroscopy ( 33 ). Here, an absorption peak at 563 nm is detected by UV wave spectroscopy in Figure 3B over the used SP-Cu / SiO 2 catalyst, while no apparent adsorption peak signals can be observed on both reduced and used AE-Cu / SiO 2 catalysts. The fresh SP-Cu / SiO 2 catalyst shows an undesirable adsorption peak at the same wavelength as it uses, probably because of its smaller Cu particle size than the latter ( 34 ). We also find that pure Cu powders are difficult to exhibit the SPR phenomenon. These findings reveal that Cu nanoparticles produced by the SP method exhibit different properties from commonly supported Cu nanoparticles or pure Cu powders with respect to the electron structure and oxidation reduction capability. It can also get some clues from the image of these samples in Figure 3C. For the SP-Cu / SiO 2 catalysts, the color changed from dark brown to red, confirming the presence of metallic copper after the reaction. These Cu particles, which have weak interactions with carriers, exhibit properties such as pure Cu. In comparison, the one produced by AE shows black after the reaction that we generally observe on supported Cu nanoparticles. Catalytic Performance in DMO Hydrogenation The conversion and product distribution of the DMO hydrogenation reaction are shown in Figure 4 (A and B). The process of DMO hydrogenation is a typical temperature controlled reaction, since the hydrogenation capacity of the Cu catalyst is very sensitive to temperature (). For the catalyst AE-Cu / SiO the DMO hydrogenation as a tandem reaction first generates the partial hydrogenation product MG and is immediately converted to the EG at a low temperature range of 170 ° to 230 ° C. a high temperature above 230 ° C. When the temperature decreases from 230 to 170 ° C, the EG selectivity is gradually promoted with a maximum value of 96% at 240 ° C. However, if the temperature drops further to 175 ° C, both EC selectivity and DMO conversion rapidly decreased to 77 and 71%, respectively, in parallel with an improved MG selectivity to 21%. MG will quickly appear in the final product when it is at a very low temperature below 175 ° C for several hours (Fig. S3A). This reflects a typical inactivation of DMO hydrogenation in the EC.

FIG. Catalytic performance over various Cu / SiO 2 catalysts.

Conversion (co.) And product distribution of DMO hydrogenation reaction over AE-Cu / SiO 2 ) and SP-Cu-SiO 2 ( B) catalysts. ( C ) and () show product distributions of the tested catalysts at 240 and 280 ° C, respectively. Selectivity, selectivity. (19459033) E [StabilitytestsofAEandSPmadeCu/SiO2catalysts[1965] Very surprisingly, the maximum selectivity of MG over the SP catalyst reaches as high as 87% with a DMO conversion of 29% at 250 ° C. The selectivity of the MG remains continuously above 50% even when the temperature reaches 280 ° C. Figure 4 (C and D) shows that deep hydrogenation occurs on the AE catalyst with EO and low carbon carbohydrates (C3-4 alcohol) as primary products at 240 ° C. It seems that EG and EO as the deep hydrogenation products still do not can climb to maximum selectivity even at 280 ° C. In addition, the selectivity of MG is compared to a similar low conversion using AE-Cu / SiO 2 to the SP-made catalyst. At a low conversion of approx. 20% above the AE catalyst at below 175 ° C, the MG selectivity can reach 86%, which is very close to the optimum value of 87% over the SP catalyst at 250 ° C. However, the same performance is achieved only during the low temperature inactivation process for DMO hydrogenation with a rapidly decreasing activity. In general, the superiority of the SP catalysts is evident in a very wide range of temperatures (230-290 ° C) for the production of MG. From the foregoing analysis, Cu shows various states in DMO hydrogenation in two catalysts. For SP-Cu / SiO the metallic Cuen exhibits an extreme oxidation resistance property also exposed to high temperature DMO while most Cu is easily oxidized to Cu + [species] over the conventional AE-Cu / SiO 2 . Another interesting result is shown when using fresh as prepared SP-Cu / SiO 2 as an effective catalyst without any reduction (Figure 4C). It also exhibits a high MG selectivity of 79% with a DMO conversion of close to 15%, which is close to that of the reduced catalyst. The similar product distribution is derived from a large amount of frozen metallic Cu in the thus-produced SP catalyst. It is therefore of great importance to maintain the stability of the original metallic Cu for preliminary hydrogenation.

We are investigating DMO hydrogenation stability for these catalysts. For a period of nearly 30 hours of power (Fig. 4E), two catalysts show relatively stable catalytic performance. With the same reaction conditions, the DMO conversion and the MG selectivity of the AE catalyst remain at approx. 100% and below 2%, while those on the fresh SP remain at 16% and near 80%, respectively. The comparison of BET surface area, crystallite size and loading amount before and after the reactions is given in Table S3. Any BET surface area of ​​the catalysts used decreases slightly compared to the fresh ones. The physically sputtered Cu is anchored on the surface of the silica support with a weaker interaction than the AE catalyst, resulting in an unavoidable growth of crystallite size after exposure to a higher temperature of 290 ° C. However, the results of time on the current reaction show that catalytic performance is stable after nearly 30 hours, indicating that particle size is not the main factor affecting catalytic stability. In the future work, the introduction of a structural promoter can be a useful way to avoid the excessive growth of copper nanoparticles.

Furthermore, the TPO mass spectrometry experiments (Fig. S3B) for the used SP and AE catalysts after stability testing are supplemented to investigate possible coke effect. TPO-MS indicates a remarkable comparison between two used catalysts. The AE catalyst exhibits a CO [200] to 300 ° C and 400 ° to 500 ° C release followed by a water temperature release in a larger temperature range, suggesting possible adsorption of carbon types on the surface. However, no apparent CO and water MS signals are detected over the SP catalyst. Driving with high selectivity to MG usually causes a rapid and irreversible inactivation on Cu / SiO 2 catalysts. However, for sputtered catalyst, no coke deposition and well stability are observed, making it different from conventional catalysts. The frozen copper in zero valence with the absence of Cu + in catalytic reactions results in no adsorption and activation process of C = O or CO species on the surface, which is a possible cause of no coke formation

To illustrate the weight of the original copper state we use two physically mixed catalysts (PM-Cu / SiO 2 CuO and SiO 2 with reduction treatment; PM-Cu 2 2 2 ] [2] 0 and SiO without reduction in DMO hydrogenation and characterizing their chemical states in various steps using XRD and TPR (Fig. S4). Both catalysts have a wider reduction temperature than that of the AE catalyst ranging from 220 to 450 ° C (Fig. S4C). In addition, the reduction peaks of PM-Cu / SiO 2 and PM-Cu + / SiO 2 are changed to higher than 300 ° C as compared to AE and SP catalysts, due to the poor dispersion and the large copper particles (more than 45 nm) on physically mixed catalysts. For the case of PM-Cu / SiO 2 phase without Cu + in the initial reaction step (Fig. S4A) is shown due to the absence of a Cu + + stabilizer (such as copper phyllosilicate in the precursor of AE-Cu / SiO 2) and the strong interaction between copper and silica, making it distinct from the AE catalyst. Since it suffers from the poor dispersion of copper, the maximum MG selectivity over the PM catalyst is only about 60%, which is lower than that over the SP catalyst. With increasing temperature, some of the unstable Cu species in the PM catalyst are further oxidized by DMO, resulting in the coexistence of a large amount of Cu and obtaining Cu ] + Observed from the XRD pattern used, EG and EO selectivities are gradually selected as reaction temperature increases, and the MG selectivity drops to about 10%. 30% at 290 ° C (Fig. S4D). For the case of PM-Cu + / SiO we select Cu 0 as an initial state of copper without reduction. I startsteget av reaktioner vid 220 till 230 ° C är Cu + arter aktiva ställen, som omvandlar MG till den djupa hydrogeneringsprodukten EG, med en selektivitet av 67,8%. Frånvaron av Cu 0 resulterar i en låg MG-selektivitet (ca 20%), långt mindre än den av mer än 85% över SP-katalysatorn. Med den ökande temperaturen och tiden som är på gång reduceras ostabila Cu + -arterna vidare till Cu 0 av H 2 i råmaterialet (fig S4B) eftersom det finns ingen stark växelverkan mellan koppar och kiseldioxid, liknande PM-Cu / SiO 2 katalysatorn. Även om maximalt MG-selektivitet når 84,8%, är det inte stabilt om temperaturen ökar ytterligare. MG-utbytet faller markant ned till ett värde så lågt som under 10% vid 280 ° C (fig. S4E), vilket är mycket snabbare än SP-Cu / SiO 2 och PM-Cu / SiO 2 katalysatorer.

I jämförelse förblir selektiviteten hos MG över SP-katalysatorn fortfarande kontinuerligt över 50%, även om temperaturen når 280 ° C. The product distribution exhibits more stable than two PM catalysts from a high temperature range from 230° to 290°C, which provides a wider temperature range for producing MG than two PM catalysts. Therefore, it is crucial to stabilize copper into a zero valence for producing MG with a high selectivity in a wide temperature range.

Distinct product separation has been achieved through modifying the metal property, which is very similar to Ag-based or Au–Ag–based catalysts. The major products of Ag/SiO2 are MG and EG at 220° and 280°C, respectively (10). The copper nanoparticles in the SP-Cu/SiO2 catalyst can be successfully frozen in a stable metallic state without oxidation in Cu+ by oxygen in air and DMO in catalytic reactions. Therefore, the sputtered Cu has a potentiality in substitution of noble metals in hydrogenation reactions.

In copper-based catalysts, Cu0 is the active site and primarily responsible for the hydrogenation activity, while Cu+ facilitates the conversion of IMs (3536). Cu+ is the key factor for hydrogenation of DMO to EG or EO by enhancing the activation of the C=O group in DMO (35). In contrast, it is also reported that the addition of Ag in Cu-based catalysts shows a positive effect on the DMO-to-MG reaction due to the enhancement of proportion of Cu+ in Cu (3738). Moreover, nonsilica supports, such as hydroxyapatite and activated carbon, are used to increase the molar ratio of Cu+/Cu0 and constrain hydrogenation activity of copper in MG production (1314). It seems that the catalytic function of Cu+ and Cu0 species in different DMO hydrogenation steps is still unclear. Cu+ species are inevitable in coexistence with Cu0 during the reaction in conventional Cu/SiO2 catalysts due to the relatively high Cu reducibility, which causes problems in deeply understanding the hydrogenation functions of Cu species.

Reaction pathways for frozen Cu0 in DMO hydrogenation

For further understanding the catalytic mechanisms, we performed density functional theory (DFT) calculations to simulate the hydrogenation processes for Cu0-based catalysis. In general, •H was formed at Cu0 active sites in the initial stage (35), which mainly cause the subsequent hydrogenation reaction of organic compounds via the addition of •H to the molecular sites with the highest electron density. Therefore, •H may add to the ketonic and etheric O atoms of DMO and MG to generate MG and EG, respectively. As shown in Fig. 5, we calculated four possible pathways for the •H-initiated atmospheric reactions. The computed Gibbs free energy changes (ΔG), enthalpy changes (ΔH), and activation free energies (ΔG) for the possible reaction pathways with selective optimal structures are shown in Fig. 6 and listed in table S4. The detailed structures and bond lengths of the reactant complexes (RCs), transition states (TSs), IMs, and products for the •H addition reactions of DMO to generate MG are also shown in fig. S5. As indicated by the negative ΔG and ΔH values, all the steps of the four pathways are spontaneous and thermodynamically favorable. The highest ΔG values for pathways A and B are 19.9 and 19.2 kcal/mol, respectively, indicating that MG can be produced via the •H addition at ketonic and etheric O atoms of DMO, while the highest ΔG values for pathways C and D (26.2 and 24.7 kcal/mol) are much higher than that for pathways A and B. Thus, it is difficult for MG to further react with •H to form EG, which is consistent with the experimental results that high selectivity for MG can be obtained over the SP-Cu/SiO2 catalyst.

Fig. 5 DFT calculation and schematic of reaction pathway.

Pathways A and B and pathways C and D for the H-initiated atmospheric reaction for DMO to generate MG and for MG to generate EG, respectively.

Fig. 6 Molecule-level free energy surface in four reaction pathways.

Profiles of free energy surface (FES) along with optimal structures and bond lengths of the RCs, TSs, IMs, and products in (A) pathways A and B and (B) pathways C and D for the H-initiated atmospheric reaction for DMO to generate MG and for MG to generate EG, respectively.

For pathway A, the distance of O6–H15 was reduced from 4.36 Å in RCA1 to 1.56 Å in TSA1 and to 0.98 Å in IMA1falling in the range of an O−H single bond length. The calculated atomic charges (q) and spin densities (ρ) show an obvious charge transfer from •H to DMO, and C1-centered radical (ρ = 0.52 in IMA1) is formed. In the following step, another •H is added to C1forming methyl 2-hydroxy-2-methoxyacetate. This is a biradical reaction without free energy barriers. As the third •H gradually approached O7the distance of O7–H17 was reduced from 3.1 Å in RCA2 to 1.23 Å in TSA2 and to 0.97 Å in IMA2falling in the range of O−H single bond length. Simultaneously, the bond length of C1–O7 was elongated from 1.41 Å in RCA2 to 1.57 Å in TSA2 and to 3.33 Å in IMA2indicating the rupture of the C–O bond. During this reaction step, electron transfer also occurred, and C1 shows radical character in TSA2 and IMA2. Last, MG is formed with the addition of •H. For pathway B, the reaction process is similar except that •H is added to O7 first and then to O6.

MATERIALS AND METHODS

Catalyst preparation

A pure Cu target (purity > 99.9%; 50 mm by 100 mm; Toshima Co. Ltd.) was used to deposit Cu atoms on porous silica pellets CARiACT Q-10. The self-made polygonal rotation SP apparatus is described in fig. S6. After the target was presputtered for 0.5 hours, certain amount of the pretreated SiO2 pellets was loaded into the cavity barrel. Then, the vacuum chamber was evacuated to 9.9 × 10−4 Pa, followed by introducing an Ar (99.995%) flow of 29 ml min−1 into the chamber until the pressure reached 2.0 Pa. The SP experiment started with the input power of 450 W. The hexagonal barrel was rotated at 3.5 rpm and vibrated mechanically to mix the support and deposited metal atoms uniformly. After SP for 3.5 hours, approximately 17 weight % (wt %) of Cu were loaded. Thereafter, a 1.0% O2 (balanced by nitrogen) flow was gradually introduced into the cavity barrel to recover ordinary pressure. Last, the catalyst after SP was pressed and crushed into granules of 20 to 40 mesh before catalytic reaction. The as-prepared catalyst was denoted as SP-Cu/SiO2.

One hundred five milliliters of standard solution (0.3 M) of Cu(NO3)2·3H2O in deionized water was mixed with 10 ml of 25 wt % ammonia aqueous solution and stirred for 30 min to produce uniform copper ammonia complex solution. Subsequently, porous silica pellets CARiACT Q-10 (Fuji Silysia Chemical Ltd.; specific surface area, 283 m2 g−1; mean pore diameter, 10 nm) were slowly added to the copper ammonia complex solution and stirred for 2 hours at room temperature. The suspension was heated in a water bath preheated to 40°C and aged for 4 hours. Afterward, the water bath was continuously heated to 90°C, allowing for the evaporation of ammonia, as well as the consequent deposition of copper species on silica. The evaporation process was terminated as the pH value of the suspension decreased to 6 to 7 (ca. 3 hours). The obtained precipitates were filtrated and washed with deionized water and dried at 120°C for 8 hours. The solid powder was then calcined at 450°C in air for 4 hours. The final catalyst was denoted as AE-Cu/SiO2.

The physically mixed catalysts “PM-Cu/SiO2 and PM-Cu+/SiO2” were prepared by mixing porous silica CARiACT Q-10 and CuO or Cu2O powders with a mortar, respectively. The catalyst was pressed and crushed into granules of 20 to 40 mesh before reduction and catalytic reaction.

Catalyst characterization

XRD spectra were obtained on a PANalytical X’pert Pro diffractometer equipped with Cu Kα (40 kV, 20 mA) irradiation. TEM images were obtained using a JEOL JEM-2100 (120 kV) microscope.

The x-ray absorption data of the reduced AE-Cu/SiO2 and used SP-Cu/SiO2 sample at the Cu K-edge were recorded at room temperature in transmission mode at beam line BL14W1 (39) of the Shanghai Synchrotron Radiation Facility (SSRF) in China.

In situ DRIFTS were conducted on a Bruker TENSOR27 Fourier transform IR spectrometer with a diffuse reflectance attachment and an MCT (mercury cadmium telluride) detector. A ZnSe window was used for the in situ IR cell. The absorbance spectra were collected for 32 scans with a resolution of 2 cm−1. Before CO adsorption, a catalyst of about 0.015 g was heated to 250°C with a N2 flow (99.99%) of 30 ml min−1. After sweeping for 1 hour, a pure H2 flow (99.99%) of 30 ml min−1 was introduced instead of N2 to reduce the catalyst for 1 hour. Subsequently, pure He was flowed into the cell for 30 min, followed by a vacuum desorption for 1 hour to remove H2 residual in cell and adsorbed on the catalyst, followed by cooling down of the temperature to room temperature. After keeping for 0.5 hours, the background spectra under this condition were recorded. Then, a pure CO flow (99.99%) of 30 ml min−1 was introduced into the IR cell for 0.5 hours. After adsorption, a N2 flow of 50 ml min−1 was used to sweep for 2 hours, followed by obtaining desorption spectra of CO desorption.

Hydrogen TPR experiments were performed with the self-made TPR system. Forty-milligram samples were pretreated at 150°C for 1 hour in an Ar flow before the following test. The samples were heated in a 5% H2 (Ar balance, 30 ml min−1) flow from 25° to 500°C with a heating rate of 10°C min−1. Thermal conductivity detector signals were recorded in this process.

XPS and XAES were performed under an ultrahigh vacuum (8 × 10−10 Pa) using a Thermo Fisher Scientific (ESCALAB 250Xi) x-ray photoelectron spectrometer with a monochromatic Al Kα source (hν, 1253.6eV). It operated with a pass energy of 40 eV. Before each test, Ar ionic SP was performed with a beam energy of 3 keV and a SP rate of 20 Å min−1 for 7.5 min. The collected BEs were calibrated using the C 1s peak at 284.8 eV as the reference.

N2O-TPO experiments were performed with the self-made TPO system. First, 40 mg of samples were prereduced at 250°C with 99.99% hydrogen (30 ml min−1). Then, they were purged with Ar flow (30 ml min−1) for 1 hour and cooled down to room temperature. Two mass spectrum signals of 28 (N2) and 44 (N2O) were monitored by an on-line mass spectrometer (Omnisorp Corporation). After the base line getting stable in He, the mixture of 5% N2O (He balance, 30 ml min−1) was introduced instead of He. This process was carried out for 1 hour. Last, the samples were heated in the same atmosphere from 25° to 250°C with a heating rate of 5°C min−1.

The UV-vis spectrometer (Lambda 950) was used to analyze the adsorption range of prepared Cu-based nanoparticles. The adsorption signals were recorded from 200- to 800-nm wavelengths. Metallic Cu powder (99%) was also tested for comparison.

The Cu loading was determined by scanning electron microscope–energy dispersive spectrometer. The specific surface area was analyzed by nitrogen adsorption/desorption at 77 K using the BET method (Autosorb iQ2 by Quantachrome).

Evaluation of DMO hydrogenation reactions

The catalytic performance in DMO hydrogenation was conducted with a fixed-bed reactor. Briefly, 0.5 g of the catalyst (20 to 40 meshes) was packed into a stainless-steel tubular reactor with the thermocouple inserted into the catalyst bed. Catalyst reduction was performed at 250°C for 4 hours with a ramping rate of 2°C min−1 from room temperature in a pure H2 flow. After cooling to the reaction temperature, 8 wt % DMO (purity > 99%) solution in methanol was fed into the fixed-bed reactor with a H2/DMO molar ratio of 150. The reaction pressure was controlled to 3.0 MPa. The liquid hourly space velocity of DMO was set as 0.5 hours−1. The feeding was continuous, and the reaction temperature was ranged from 170° to 290°C. For each temperature, the catalyst was stabilized at least for 3 hours. For the stability test, the temperature was fixed at 240°C. The reaction products were collected with an ice trap and analyzed on an offline gas chromatograph with a flame ionization detector.

The used SP and AE catalyst for characterizations was derived after a continuous reaction with a wide temperature range. For the AE-used catalyst, the continuous temperature range was 170° to 280°C. For the SP-used catalyst, the continuous temperature range was 240° to 290°C.

DFT calculation

The DFT calculations were performed with the Gaussian 09 program suite, using the B3LYP hybrid meta exchange-correlation functional in conjunction with the 6-31+G (d,p) basis set. Frequency calculations were performed to determine the character of stationary points. TSs were characterized with one imaginary vibrational frequency. Intrinsic reaction coordinate analysis was executed to verify that each TS uniquely connected the designated reactants with the products. The profiles of the FES were depicted using relative free energies to the reactants. In FES, the free energies for each species were evaluated at the same atomic type and number. The values of Gibbs free energy and enthalpy were corrected by zero-point energy and thermal energy at 298 K. The Gibbs free energy changes (ΔG) and enthalpy changes (ΔH) were calculated for each reaction step by their changes from RC to IM.

Acknowledgments: This paper is dedicated to the 70th anniversary of Dalian Institute of Chemical Physics, Chinese Academy of Sciences. We would like to thank M. Tan, G. Liu, A. Taguchi, and T. Abe for assistance in the physical sputtering experiment, as well as X. Tong for help in stability tests. We appreciate beamline BL14W1 (SSRF) for providing the beam time to derive XAFS results. Funding: J.S. thanks the financial support of Foundation of State Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering (grant no. 2016-04), the Hundred-Talent Program of Dalian Institute of Chemical Physics, and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2018214). Author contributions: J.S. and J.Y. conceived the research, performed characterizations, analyzed data, and wrote the manuscript. F.M. and Y.S. evaluated catalysts. X.W. executed the DFT calculation and discussed the concept. Q.M. and N.T. helped with the catalyst preparation and characterizations. All the authors contributed to analysis and discussion on the data. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.


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