Catalytic performance in DMO hydrogenation
The conversion and product distribution of DMO hydrogenation reaction are shown in Fig. 4 (A and B). The process of DMO hydrogenation is a typical temperature-controlled reaction as the hydrogenation ability of Cu catalyst is very sensitive to temperature (14). For the AE-Cu/SiO2 catalyst, the DMO hydrogenation as a tandem reaction first generate the partial hydrogenation product MG and immediately convert to EG at a low temperature range of 170° to 230°C. After those deep hydrogenation products, alcohols are generated at a high temperature above 230°C. With the temperature decreasing from 230° to 170°C, EG selectivity is gradually promoted with a maximum value of 96% at 240°C. But if the temperature further decreases to 175°C, then both EG selectivity and DMO conversion will quickly drop to 77 and 71%, respectively, in parallel with an enhanced MG selectivity to 21%. MG will rapidly become prominent in the final product when keeping a very low temperature below 175°C for several hours (fig. S3A). This reflects a typical inactivation for DMO hydrogenation into EG.
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 MG continuously stays above 50% even when the temperature reaches 280°C. The product distribution in Fig. 4 (C and D) shows that deep hydrogenation happens on the AE catalyst with EO and low-carbon alcohols (C3-4 alcohol) as primary products at 240°C. It seems that the EG and EO as the deep hydrogenation products still cannot climb to the maximum selectivity even at 280°C. In addition, the selectivity of MG is compared at a similar low conversion using AE-Cu/SiO2 to that of the SP-made catalyst. In a low conversion of ca. 20% over the AE catalyst at below 175°C, the MG selectivity can reach 86%, which is very close to the optimal value of 87% over the SP catalyst at 250°C. However, the similar performance is only achieved during the inactivation process at a low temperature for DMO hydrogenation with a rapidly decreasing activity. In general, the superiority of the SP catalysts is apparent in terms of a very wide temperature range (230° to 290°C) for producing MG.
From the previous analysis, Cu shows different states in DMO hydrogenation in two catalysts. For the SP-Cu/SiO2, the metallic Cu exhibits an extreme oxidation resistance property even exposing to DMO at high temperature, while most Cu0 are easily oxidized to Cu+ species over the conventional AE-Cu/SiO2. Another interesting result is shown when using fresh as-prepared SP-Cu/SiO2 as an efficient catalyst without any reduction (Fig. 4C). It also exhibits a high MG selectivity of 79% with a DMO conversion of near 15%, which is close to that of the reduced catalyst. The similar product distribution results from a large amount of frozen metallic Cu in the as-prepared SP catalyst. Therefore, it is of great importance to maintain the stability of initial metallic Cu0 for preliminary hydrogenation.
We investigate stability in DMO hydrogenation for these catalysts. In a time on stream of near 30 hours (Fig. 4E), two catalysts show relative stable catalytic performance. With the same reaction conditions, the DMO conversion and MG selectivity on the AE catalyst remain at ca. 100% and below 2%, while those on the fresh SP one remain at 16% and near 80%, respectively. The comparison of BET surface area, crystallite size, and loading amount before and after the reactions are listed in table S3. All the BET surface areas for the used catalysts are slightly decreased if compared with the fresh one. The physically sputtered Cu is anchored on surface of the silica support with a weaker interaction than the AE catalyst, resulting in an inevitable growth for crystallite size after exposure to a higher temperature of 290°C. However, the results of time on stream reaction show that the catalytic performance is stable after near 30 hours, suggesting that particle size is not the main factor that affects the catalytic stability. In the future work, the introduction of a structural promoter may be a useful pathway to avoid the excessive growth of copper nanoparticles.
Furthermore, the TPO–mass spectrometry (MS) experiments (fig. S3B) for the used SP and AE catalysts after stability tests are supplemented to investigate the possible coke effect. The TPO-MS indicates a notable comparison between two used catalysts. The AE catalyst exhibits a CO2 release at 200° to 300°C and 400° to 500°C accompanied by a water release in a wider temperature range, suggesting possible adsorption of carbon species on the surface. However, no obvious CO2 and water MS signals are detected over the SP catalyst. Running in a high selectivity toward MG usually causes a quick and irreversible inactivation on Cu/SiO2 catalysts. But for the sputtered catalyst, no coke deposition and well stability are observed, which make it distinct 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 C–O species on the surface, which is one possible reason for no coke formation.
To illustrate the importance of initial copper state, we use two physically mixed catalysts (PM-Cu/SiO2, CuO and SiO2 with reduction treatment; PM-Cu+/SiO2, Cu2O and SiO2 without reduction) in DMO hydrogenation and characterize their chemical state at different stages using XRD and TPR (fig. S4). Both catalysts exhibit wider reduction temperature than that for the AE catalyst, ranging from 220° to 450°C (fig. S4C). In addition, the reduction peaks of PM-Cu/SiO2 and PM-Cu+/SiO2 shift toward higher temperatures of more than 300°C as compared with the AE and SP catalysts, because of the poor dispersion and large copper particles (more than 45 nm) on physically mixed catalysts.
For the case of PM-Cu/SiO2, a pure Cu0 phase without Cu+ is shown in the initial reaction stage (fig. S4A) owing to the absence of a Cu+ stabilizer (such as copper phyllosilicate in the precursor of AE-Cu/SiO2) and the strong interaction between copper and silica, which make it distinct from the AE catalyst. Because it suffers from the poor dispersion of copper, the maximum MG selectivity over the PM catalyst is only ca. 60%, which is lower than that over the SP catalyst. However, with increasing temperature, part of unstable Cu0 species in the PM catalyst is further oxidized by DMO, resulting in the coexistence of a large amount of Cu0 and few Cu+ observed from the used XRD pattern, EG and EO selectivities are gradually enhanced as reaction temperature increases, and MG selectivity drops to ca. 30% at 290°C (fig. S4D). For the case of PM-Cu+/SiO2, we choose Cu2O as an initial state of copper without reduction. In the starting stage of reactions at 220° to 230°C, Cu+ species are active sites, transforming MG into the deep hydrogenation product EG, with a selectivity of 67.8%. The absence of Cu0 results in a low MG selectivity (ca. 20%), far less than that of more than 85% over the SP catalyst. With the increasing temperature and time on stream, unstable Cu+ species are further reduced to Cu0 by H2 in feedstock (fig. S4B) since there is no strong interaction between copper and silica, similar to the PM-Cu/SiO2 catalyst. Although the maximum of MG selectivity reaches 84.8%, it is not stable if temperature further increases. The MG yield markedly falls down to a value as low as below 10% at 280°C (fig. S4E), which is much faster than the SP-Cu/SiO2 and PM-Cu/SiO2 catalysts.
In comparison, the selectivity of MG over the SP catalyst continuously still stays above 50% even if the temperature reaches 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 (35, 36). 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 (37, 38). 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 (13, 14). 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.