Yanzhao Zhaiab,
Yongjun Ji*a,
Guangna Wanga,
Yongxia Zhua,
Hezhi Liua,
Ziyi Zhongc and
Fabing Su*a
aState Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China 100190. E-mail: yjji@ipe.ac.cn; fbsu@ipe.ac.cn; Fax: +86-10-82544851; Tel: +86-10-82544850
bSchool of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing, 100083, China
cSchool of Chemical & Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459
First published on 21st August 2015
This work aims to provide a facile, low-cost and scalable method for the preparation of multicomponent Cu–Cu2O–CuO catalysts, which are of high interest to the organosilane industry. A series of submicrometer-sized and Cu-based catalysts containing CuO, Cu2O and Cu, or some combination of them, were synthesized by a simple low-temperature wet chemical method using CuSO4·5H2O as the precursor and N2H4·H2O as a reducing agent. The samples were characterized by X-ray diffraction, thermogravimetric analysis, temperature-programmed reduction, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy techniques. It was observed that the composition of the samples could be tailored by varying the amount of reducing agent at a given reaction temperature and time. These catalysts were then tested in the Rochow reaction, using silicon powder and methyl chloride (MeCl) as reactants to produce dimethyldichlorosilane (M2), which is the most important organosilane monomer in the industry. Compared with bare CuO and Cu particles, the ternary CuO–Cu2O–Cu catalyst displayed much improved M2 selectivity and Si conversion, which can be attributed to the smaller copper particle size and the synergistic effect among the different components in the CuO–Cu2O–Cu catalyst. This catalyst preparation method is expected to yield efficient and low-cost copper catalysts for the organosilane industry.
![]() | (1) |
Among the products from the above Rochow reaction, dimethyldichlorosilane ((CH3)2SiCl2, M2) is the most important monomer used for silicon rubber production in the organosilane industry, and thus, a high M2 yield is highly desired. Besides the accurate control of reaction conditions and the selection of proper reactors, the development of highly efficient catalytic systems is crucial for high M2 selectivity and Si conversion. As reported, the primary catalysts used in the Rochow reaction are Cu-based catalysts including metallic Cu,1 CuCl,4 Cu2O,5 CuO,6 Cu–Si alloy7 and Cu–Cu2O–CuO,8 together with some catalyst promoters.2,6,9–11 In recent years, our group has developed a number of cupreous and copper oxide structures, such as flower-like ZnO grown on urchin-like CuO microspheres,12 porous cubic Cu microparticles,13 mesoporous Cu2O microspheres,5 shape-controlled Cu2O microparticles,14 dandelion-like CuO microspheres,15 flower-like CuO microspheres,16 and CuCl microcrystals with different morphologies17 etc., for M2 synthesis, and found that the particle morphology and size, as well as the catalyst composition, impact on the catalytic properties significantly.
Among the various Cu-based catalysts, the multicomponent Cu-based catalysts containing Cu, Cu2O, and CuO are of great interest due to their superior catalytic performances in the Rochow reaction. For instance, Khitouni et al. prepared such catalysts through a high-energy mechanical milling process, using Cu, Cu2O, and CuO as the precursors.18 Although their method is simple, it still lacks effective control of the solid reaction between Cu, Cu2O, and CuO during the mechanical milling process. Therefore, to meet industrial application, more effective methods for preparing multicomponent Cu-based catalysts should be developed. More recently, we found that the multicomponent Cu–Cu2O–CuO catalyst can also be synthesized by the partial reduction of CuO nanoparticles in H2/N2 gas19 or by the controlled oxidation of copper flakes in O2/N2 gas.20 These methods, however, still suffer from low yields and the use of high-cost equipment together with intensive energy consumption (high temperature), which are troublesome and thus limit their further application. On the other hand, we have noticed that there are scarce reports concerning the preparation of multicomponent Cu-based catalysts via wet chemical approaches.
Herein, we report a facile and low cost preparation of the ternary CuO–Cu2O–Cu catalyst by a wet chemical method with obvious advantages, such as mild reaction conditions (70 °C), large-scale production capability and easy operation. Most importantly, the composition of the samples could be tailored by simply varying the amount of the reducing agent N2H4·H2O. The prepared ternary CuO–Cu2O–Cu catalyst exhibited highly improved M2 selectivity and Si conversion in the Rochow reaction in comparison to those of sole CuO and Cu microparticles, demonstrating the importance of the synergistic catalytic effect among the components. The resulting ternary CuO–Cu2O–Cu catalyst using the present method is a promising catalyst for the industrial Rochow reaction.
![]() | (2) |
![]() | ||
Fig. 2 XRD patterns (a), TG curves (b), and H2-TPR curves (c) of all the samples, and XPS spectra of the Cu 2p peak for the CuO–Cu2O–Cu sample (d). |
Samples | Cua (nm) | Cu2Oa (nm) | CuOa (nm) | Weight increaseb (wt%) | TMc (°C) | STPRc | SBETd (m2 g−1) |
---|---|---|---|---|---|---|---|
a Calculated from the XRD patterns, standard errors: crystal size, 0.5 nm.b Given by TG analysis.c Obtained from H2-TPR curves using the integrated areas under the curves presented.d Calculated with the BET method. | |||||||
CuO | — | — | 18 | — | 264.2 | 53![]() |
15.3 |
CuO–Cu2O | — | 45 | 18 | 4.5 | 258.5 | 34![]() |
10.2 |
CuO–Cu2O–Cu | 35 | 51 | 17 | 11.2 | 241.2 | 23![]() |
3.6 |
Cu2O–Cu | 46 | 41 | — | 17.1 | 214.5 | 9435.0 | 3.4 |
Cu | 51 | — | — | 25.2 | — | — | 2.5 |
Fig. 2b shows the TG curves of all the samples measured in air. It is seen that there is no visible weight change for the CuO sample, suggesting the formation of a nearly pure CuO phase. The CuO–Cu2O sample shows a weight increase of 4.5 wt% over a wide temperature range (200–420 °C) which is derived from the oxidation of Cu2O to CuO. Thus, the estimated content of Cu2O in this sample is about 40.2 wt%. On the other hand, the CuO–Cu2O–Cu and Cu2O–Cu samples exhibit a 11.2 and 17.1 wt% weight increase, respectively, corresponding to the oxidation of Cu2O and Cu to CuO. In the case of Cu, the weight increase reaches 25.2 wt%, which is close to the theoretical value of the oxidation of Cu into CuO (25.0 wt%), confirming that it is metallic Cu.
Fig. 2c shows the H2-TPR curves of all the samples. The hydrogen consumption peak areas (STPR) and peak temperatures (TM) are summarized in Table 1. It is observed that with the decrease of CuO content in the samples, following the order of CuO, CuO–Cu2O, CuO–Cu2O–Cu, Cu2O–Cu and Cu, the reduction temperature and hydrogen consumption are also slightly decreased. For the reduced samples, the reduction peaks move slightly to a lower temperature region with an increase in the amount of reducing agent. Using the ratio of hydrogen consumption peak area to the mole number of O in CuO as a reference, and based on the above TG analysis, it can be calculated that the content of CuO in the CuO–Cu2O and CuO–Cu2O–Cu samples is about 59.4 and 15.7 wt%, respectively. For Cu2O–Cu, the content of Cu2O is 40.6 wt%. Therefore, the above results indicate that the contents of Cu, Cu2O, and CuO in the final products can be well tuned by adjusting the amount of N2H4·H2O in the preparation.
The high-resolution XPS spectra in Fig. 2d show the binding energies of Cu 2p3/2. The peaks at 934.2 eV are characteristic of Cu(II), whereas the ones at 932.9 eV are attributed to Cu (0) or Cu(I).21,22 Unfortunately, from the XPS data alone, it is difficult to distinguish clearly the Cu(0) and Cu(I) species due to the effects of crystal size and surface coverage on the binding energy.23 However, by combining them with the XRD results shown in Fig. 2a, we can judge whether the sample contains Cu (0) or Cu(I), or both of them. In other words, we can conclude that the sample actually contains CuO, Cu2O and Cu.
Fig. 3 shows the SEM images of all the samples. In agreement with the XRD results, the samples all appear to be well crystallized. Fig. 3a presents the SEM image of CuO, which reveals that the sample is made up of a large quantity of different sheet-like submicrometer structures. However, after the addition of a small amount of N2H4·H2O, the SEM image shows that the obtained CuO–Cu2O sample is composed of premature polyhedron structures, the surfaces of which are covered with a sheet-like substance (Fig. 3b), implying that the CuO crystallites are partially reduced to Cu2O and attached to the surface simultaneously during the growth of the polyhedron structures, which coincides with previous studies.24–26 A further increase in the amount of N2H4·H2O leads to obvious changes in the morphology of CuO–Cu2O–Cu (Fig. 3c), in which the sample consists of spheres, cubes, octahedra and other polyhedra in sizes ranging from approximately 0.05 to 2 μm. Fig. 3d displays the SEM image of Cu2O–Cu, in which the majority of the crystals have cubic and octahedral shapes, and their sizes are in the range of 100–500 nm. The Cu sample, as shown in Fig. 3e, exhibits a block-like shape with a size range of 0.1–3 μm.
Fig. 4a presents the TEM image of the CuO–Cu2O–Cu sample, which consists of cubic, octahedral, spherical and other polyhedral crystals, in full agreement with the above SEM result (Fig. 3c). The high-resolution transmission electron microscope (HRTEM) image of CuO–Cu2O in Fig. 4b shows the presence of two different interplanar distances of 0.25 and 0.24 nm, corresponding to the (111) planes of CuO and Cu2O, respectively. Moreover, two sets of diffraction spots in the fast Fourier transform (FFT) patterns further verify the coexistence of the two structures. Similarly, the CuO–Cu2O–Cu sample (Fig. 4c) possesses three apparent lattice spacings, in which the one at 0.21 nm corresponds to the (111) plane of Cu, and the other two, indexed to the (111) planes of CuO and Cu2O, remain the same (0.25 and 0.24 nm) as CuO–Cu2O. In the case of Cu2O–Cu (Fig. 4d), two lattice plane distances of 0.24 and 0.21 nm are observed, which are characteristic of the (111) facets of Cu2O and CuO, respectively. These results prove that the prepared Cu-based samples are composed of polycrystals, while within each multi-component particle there are different Cu component crystals, supporting the successful synthesis of Cu-based catalysts with controllable composition by varying the amount of reducing agent using a wet chemical method.
CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4 | (3) |
Cu(OH)2 → CuO + H2O | (4) |
CuO + N2H4 → 2Cu2O + N2 + 2H2O | (5) |
Cu2O + N2H4 → 2Cu + N2 + H2O | (6) |
The chemical reaction between CuSO4 and NaOH yields cupric hydroxide (Cu(OH)2) (formula (3)), which is a metastable phase. Upon heating in a constant temperature (70 °C) water bath under magnetic stirring, it decomposes into more stable cupric oxide (CuO) (formula (4)),27 which has been also confirmed previously by Cudennec and Lecerf et al.28 Upon the addition of N2H4·H2O, which is employed as a reducing agent, CuO is reduced into Cu2O and Cu, accompanied with the production of N2 and H2O (formulas (5) and (6)).24,25 In addition, we also found that a trace of NH3 as a by-product will also be generated. The formation of various products with distinct morphologies is illustrated in Scheme 1. As mentioned above, Cu(OH)2 is not stable. When heated at 70 °C for 10 min, it is transformed solely into black CuO, which has a sheet-like morphology and a submicrometer size. Upon the addition of a small amount of N2H4·H2O, an obvious change in the product morphology and size is observed (Fig. 3b, SEM image). Clearly, there should be a dissolution–recrystallization process starting from the solid precursor.26 During the reduction process, the added N2H4·H2O reacts with the free Cu2+ ions via coordination and reduction steps. With the progressive dissolution of CuO to yield Cu2+ and the subsequent reduction of Cu2+ to Cu+, the CuO particles are gradually dissolved and recrystallization occurs.25,26 When the amount of N2H4·H2O reaches 1.5 mL, part of the CuO is transformed into Cu2O, forming nanocrystals of both CuO and Cu2O with premature polyhedron structures. Continuously increasing the amount of N2H4·H2O to 2.5 mL, CuO is completely transformed into Cu2O, and even part of the yielded Cu2O is further converted into Cu. Thus, the coexistence of CuO, Cu2O, and Cu grains with structures of spheres, cubes, octahedra and other polyhedra is obtained. It should be noted that the presence of small amounts of ions such as Na+, OH−, etc. has a great influence on the morphology of the Cu2O product.26 Finally, with a further increase of the N2H4·H2O amount to 3.5 mL or 5.0 mL, because there is a surplus of the reducing agent, the generated Cu2O is converted into Cu2O–Cu or pure Cu completely. Therefore, by tuning the supply of N2H4·H2O, it is possible to control the content of CuO, Cu2O, and Cu in the final Cu-based products, as proved by the above comprehensive characterizations with XRD, TG, TPR, XPS and TEM techniques.
Samples | Product composition (%) | ||||||||
---|---|---|---|---|---|---|---|---|---|
M1 | M2 | M3 | M1H | M2H | LBR | HBR | M2 yield | C–Si (%) | |
a Reaction conditions: cat., 1.0 g; catalyst: Si (mass ratio) = 1![]() ![]() |
|||||||||
CuO | 12.5 | 66.1 | 1.4 | 14.9 | 0.8 | 0.3 | 4.0 | 17.5 | 26.4 |
CuO–Cu2O | 11.3 | 68.7 | 1.0 | 9.2 | 0.4 | 0.4 | 9.0 | 18.7 | 27.2 |
CuO–Cu2O–Cu | 10.1 | 80.0 | 1.5 | 7.2 | 0.9 | 0.1 | 0.2 | 41.1 | 51.4 |
Cu2O–Cu | 11.4 | 71.8 | 1.0 | 11.2 | 1.1 | 0.1 | 3.4 | 23.0 | 32.0 |
Cu | 21.9 | 47.3 | 1.0 | 23.0 | 0.9 | 0.3 | 5.6 | 8.8 | 18.7 |
Fig. 5 displays the XRD patterns of the waste contact masses after the reaction, which contain the unreacted Si, the Cu-based catalyst, and a trace amount of the promoter Zn. As shown in Fig. 5a, all the waste contact masses are composed of Si and Cu, but lack the Cu2O and CuO species. The formation of Cu may originate from the reaction of MeCl with the lattice oxygen of the Cu-based catalyst.9 An enlarged view of the XRD patterns in the range of 40–50° (Fig. 5b) shows the presence of η-Cu3Si and Cu6.69Si species, suggesting the formation of alloyed CuxSi active components by the reaction of Cu and Si via diffusion during the reaction. The Cu3Si alloy, generally produced between the Cu catalyst and the Si interface at elevated temperatures,29 is normally regarded as the key catalytic active species in the Rochow reaction,30,31 by which methylchlorosilanes (MCSs), especially M2, are formed. The amount of Cu3Si can substantially affect the Si conversion and M2 selectivity.6 The intensities of the Cu3Si peaks observed for CuO–Cu2O–Cu are much higher than that of the other samples, suggesting that CuO–Cu2O–Cu is more active in generating Cu3Si than the other samples. This may be because the ternary composition of CuO–Cu2O–Cu has a stronger synergistic effect and closer contact between the catalysts and the solid Si, thus promoting the formation of more active Cu3Si phases, which is the key step to enhance the catalytic activity towards M2 production.
![]() | ||
Fig. 5 XRD patterns of the waste contact masses (a) and enlarged view in the 2θ range of 40–50° (b). |
Fig. 6a shows an SEM image of the Si particles after the reaction using CuO–Cu2O–Cu as the catalyst. The smooth region on the surface of the Si particles stems from the unreacted Si, while the coarse or cavity-like region comes from reacted Si, suggesting the occurrence of etching process during the reaction, which is consistent with the so-called anisotropic etching reaction mechanism.14 The element mapping images show the distribution of the elements Si (Fig. 6b), Cu (Fig. 6c) and C (Fig. 6d) on the surface of the waste contact mass. The element Si mapping image clearly shows the reacted (dark yellow) and unreacted (bright yellow) zones of the Si particles, while both Cu (dark cyan) and C (red) are distributed uniformly on the reacted or etched Si surface, indicating the occurrence of a catalytic reaction between the Si particles and Cu-based catalysts. As shown in Scheme 2, a mechanism for the catalytic cycle is proposed. The ternary Cu–Cu2O–CuO catalyst is first transformed to Cu* (active species) via the reaction of MeCl with the lattice oxygen, which then reacts with Si to form the alloyed CuxSi active intermediate. Then, MeCl is adsorbed on the surface and transformed into M2, and meanwhile Cu* is released again.
![]() | ||
Fig. 6 SEM image of Si particles after reaction (a), elemental mapping images of Si (b), Cu (c) and C (d). |
The above results allow us to describe graphically the possible Rochow reaction process in the presence of a Cu-based catalyst (Scheme 3). First, Si is mixed with the reduced Cu particles in submicrometer size before the reaction. With diffusion of Cu into the Si, the CuxSi alloy is gradually formed at the Cu catalyst and the Si interface at elevated temperatures; the alloy then further reacts with the gas MeCl to form MSCs. As the reaction proceeds further, more elemental copper is produced while the Cu in the CuxSi alloy is reduced until deactivation of the catalyst occurs. At the same time, MeCl may be transformed into carbon depositions (C), initiated by the metallic Cu, leading to the deactivation of the contact mass. The presence of the multicomponent particle structure should enhance the gas diffusion into the Cu–Si contact area, resulting in the formation of much more of the active species. Nevertheless, the real synergistic effect is not yet fully understood at present and should be further explored.
This method is simple without use of any complicated and delicate equipment, and the reaction conditions are mild, as the reaction temperature is only 70 °C. More importantly, this method is readily scalable. Our experiment results showed that about 10 g of products could be obtained at one time by using a 500 mL beaker (not shown), indicating its great potentiality for industrial application.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10999j |
This journal is © The Royal Society of Chemistry 2015 |