Jun Yua,
Dongsen Mao*a,
Dan Dinga and
Guanzhong Lu*ab
aResearch Institute of Applied Catalysis, School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, P. R. China. E-mail: dsmao@sit.edu.cn; gzhlu@ecust.edu.cn; Fax: +86-21-60873625; Fax: +86-21-64253824
bKey Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, P. R. China
First published on 9th August 2016
The effects of surface properties of the monodispersed SiO2 prepared using different ammonia concentrations on the catalytic performance of Rh–Mn–Li/SiO2 for CO hydrogenation to C2 oxygenates were investigated. The investigation based on the catalytic performance and characterizations of the catalysts suggested that the catalytic performance was relatively stable on the Rh–Mn–Li catalyst supported on the SiO2 prepared using high ammonia concentration, which is attributed to the steady state of the metals supported on it. However, the rough surface of SiO2 prepared using a low ammonia concentration resulted in easy agglomeration of the metals. Correspondingly, the size of Rh particles increased and the isolated Rh+ sites decreased in the reaction process, leading to the decrease of C2 oxygenate selectivity of the catalyst as a function of time.
In our previous studies, it has been found that the catalytic performance of the Rh–Mn–Li/SiO2 catalyst for the synthesis of C2 oxygenates from CO hydrogenation was enhanced greatly when a commercial SiO2 was replaced by a monodispersed SiO2 prepared by the Stöber method.18,19 The excellent performance of this catalyst can be mainly attributed to the special surface properties of the monodispersed SiO2. It is widely accepted that the properties of supports have a considerable influence on the Rh-based catalysts, which finally leads to disparities in the catalytic performance. Solymosi et al.11 proposed that surface hydroxyl groups on silica play an important role in the change in the oxidation state of the metal, and the active sites of Rh+ can be formed via an oxidation of the Rh0 clusters by the surface OH groups of SiO2. Chen et al.20 observed that the Rh particles were more homogeneously dispersed when 14–20 mesh silica was used instead of 20–40 mesh as a support for Rh–Mn–Li catalyst. As a result, the space time yield and selectivity of C2+ oxygenates were significantly increased from 338.6 g (kg−1 h−1) and 49.2% to 618.4 g (kg−1 h−1) and 54.6%, respectively (T = 300 °C, P = 3 MPa, SV = 12500 h−1). Bao and co-workers found that the different electronic properties of inside and outer carbon nanotubes surface can change the Rh–Mn interaction, which resulted in the different CO adsorption behavior and catalytic performance.21
In fact, the morphology and surface properties of SiO2 prepared by the Stöber method can be modified by the ammonia concentration, which may influence on the supported metal components. Thus, the effects of surface properties of the monodispersed SiO2 prepared by different ammonia concentration on the catalytic performance of Rh–Mn–Li/SiO2 catalysts were investigated in this work. Then, CO hydrogenation performance of the catalysts was correlated with the interaction extents among Rh, Mn and SiO2.
RhCl3 hydrate (Rh ∼36 wt%, Fluka), Mn(NO3)2·6H2O (99.99%, SCRC), Li2CO3 (99.5%, SCRC), and SiO2 mentioned above were used in catalyst preparations. Catalysts were prepared by co-impregnation to incipient wetness of the prepared SiO2 (1.0 g) with an aqueous solution of RhCl3 hydrate and precursors of the promoters (Mn and Li), followed by drying at 90 °C for 4 h, and then at 120 °C overnight before being calcined in air at 350 °C for 4 h. For both of the catalysts, Rh loading was 1.5 wt% and the weight ratio of Rh:
Mn
:
Li = 1.5
:
1.5
:
0.07. The obtained catalysts were denoted as RML/SiO2(H) and RML/SiO2(L). In addition, the used catalysts were denoted as RML/SiO2(H)-UD and RML/SiO2(L)-UD.
H2 temperature-programmed reduction (H2-TPR) was carried out in a quartz micro-reactor. 0.1 g of the sample was first pretreated at 350 °C in O2/N2 (20% v/v) for 1 h prior to a TPR measurement. During the TPR experiment, H2/N2 (10% v/v) was used at 50 ml min−1 and the temperature was ramped from room temperature (RT) to 650 °C at 10 °C min−1 while the effluent gas was analyzed with a TCD detector.
CO adsorption was studied using a Nicolet 6700 FTIR spectrometer equipped with a DRIFT (diffuse reflectance infrared Fourier transform) cell with CaF2 windows. The sample in the cell was pretreated in H2/N2 (10% v/v) at 400 °C for 2 h, and then the temperature was dropped to RT. After the cell was outgassed in vacuum to <10−3 Pa, the background was scanned. After introducing CO into the IR cell (pCO = 8.0 × 103 Pa), the IR spectrum of CO adsorbed on the catalyst was recorded. The concentration of CO was higher than 99.97%, and it was pretreated by dehydration and deoxygenization before being used. The spectral resolution was 4 cm−1 and the scan times were 64.
The temperature-programmed surface reaction (TPSR) experiment was carried out as follows: after the catalyst was reduced at 400 °C in H2/N2 (10% v/v) for 2 h, it was cooled down to RT and CO was introduced for adsorption for 0.5 h; afterward, the H2/N2 was swept again, and the temperature was increased at the rate of 10 °C min−1 with the QMS as the detector to monitor the signal of CH4 (m/z = 15).
Catalyst | CO conv. (C%) | Selectivity of productsb (C%) | STY (C2 Oxy) (g (kg−1 h)−1) | STY (HC) (g (kg−1 h−1)) | ||||||
---|---|---|---|---|---|---|---|---|---|---|
CO2 | MeOH | AcH | EtOH | C2 Oxyc | HCd | Other Oxye | ||||
a Reaction conditions: T = 300 °C, P = 3 MPa, catalyst: 0.3 g, and flow rate = 50 ml min−1 (H2/CO = 2). Experimental error: ±5%.b Based on carbon efficiency, carbon selectivity = niCi/∑niCi.c C2 Oxy denotes oxygenates containing two carbon atoms.d HC denotes all hydrocarbons.e Other Oxy denotes oxygenates containing more than two carbon atoms. | ||||||||||
RML/SiO2(L) | 15.7 | 6.7 | 1.7 | 35.2 | 17.5 | 57.7 | 31.2 | 2.7 | 303.1 | 87.4 |
RML/SiO2(H) | 16.6 | 3.8 | 1.1 | 35.7 | 18.1 | 59.8 | 32.1 | 3.2 | 327.8 | 101.1 |
The reaction results showed that the catalytic performance was relatively stable on SiO2(H) supported catalyst, however, the reactivity of the RML/SiO2(L) catalyst changed obviously as the function of time. It is suggested that the changing of ammonia concentration played an important role in the modification of the properties of SiO2. Correspondingly, the structure and state of the supported metals could be different on the two supports, resulting in the different catalytic performance.
N2 adsorption–desorption was carried out to characterize the textural properties of the samples. It can be seen from Table 2 that the supports of SiO2(H) and SiO2(L) kept a similar specific surface area, average pore diameter, and pore volume. Upon being loaded with metal components, there was a slight decrease in the surface area and pore diameter for both of the catalysts. It is supposed that the loading of metal components did not have major influence on the textural properties of the supports.
Sample | SBET (m2 g−1) | Vp (cm3 g−1) | Dp (nm) |
---|---|---|---|
SiO2(H) | 11.6 | 0.021 | 7.9 |
RML/SiO2(H) | 10.4 | 0.021 | 6.7 |
SiO2(L) | 20.8 | 0.036 | 7.1 |
RML/SiO2(L) | 19.3 | 0.031 | 6.3 |
The IR spectra of SiO2 and the corresponding catalysts in N2 atmosphere at 300 °C are presented in Fig. 3. According to the previous studies,23,24 the region in the range 3500–2750 cm−1 with a maximum at 3400 cm−1, originates from the absorption of H2O and –OH interacted with hydrogen bond; and the band at 950 cm−1, arises from the absorption of Si–OH. As shown in Fig. 3, it is obvious that there were more hydroxyl groups on the surface of SiO2(L) than that of SiO2(H), suggesting that the surface Si–OH groups on SiO2(H) condensed more significantly. Moreover, it can be found that the IR profiles of catalysts did not change obviously compared with the corresponding supports, but the intensity of the hydroxyl groups on SiO2 decreased after supporting metal components, suggesting that the metal components were fixed by the hydroxyl–metal interaction.
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Fig. 4 TEM images and Rh particle size distributions of different catalysts: (a) RML/SiO2(H), (b) RML/SiO2(L), (c) RML/SiO2(H)-UD, (d) RML/SiO2(L)-UD. |
It can be seen that the Rh nanoparticles were highly dispersed on the fresh catalysts, and the particle size fell sharp in the range of 1–4 nm. The mean sizes of Rh particles over RML/SiO2(H) and RML/SiO2(L) were very close, which were 2.6 and 3.1 nm, respectively. Compared with the fresh catalysts, the mean size of Rh particles in the RML/SiO2(H)-UD catalyst became slightly larger (3.5 nm), whereas the mean size of Rh particles in the RML/SiO2(L)-UD increased strikingly to 8.1 nm. It is inferred that, compared with the smooth convex surface of SiO2(H), the rough surface of SiO2(L) looked like a huge amounts of hollows in the surface, which should be more conducive to the agglomeration of metal particles in these hollows.
Combined with the catalytic results, it is obvious that the size of Rh particles has a remarkable influence on the activity and selectivity of Rh–Mn–Li/SiO2 for CO hydrogenation. It is conceivable that, because of the similar Rh particle size of the fresh catalysts, the initial catalytic performance over the two catalysts were almost the same. Furthermore, the catalytic activities of the RML/SiO2(H) catalyst were found to be quite stable with the time, which should be attributed to the stable Rh particle size on the SiO2(H) support. In contrast, the mean size of Rh particles supported on SiO2(L) increased remarkably during the reaction process, which, in turn, was reflected by the gradual decrease of C2 oxygenates selectivity along with the increase in hydrocarbons selectivity. As reported in the literature,25–27 the optimal Rh particle size for the formation of C2 oxygenates is 2–6 nm. Meanwhile, Ojeda et al.28 found that as Rh particle size increased, the higher hydrocarbons formation was favored at expenses of the oxygenated compounds. These conclusions are consistent with our results. Moreover, it is noteworthy that although the selectivities of RML/SiO2(L) changed dramatically as the function of time, but the conversion rate was relatively stable, indicating that CO conversion is little influenced by the Rh particle size, consistent with the suggestion reported by Underwood et al.29 that for Rh/SiO2, the dispersion of Rh had little effect on the total catalyst activity, but did strongly influence product selectivity.
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Fig. 5 H2-TPR profiles of the catalysts: (A) RML/SiO2(H) and RML/SiO2(H)-UD, (B) RML/SiO2(L) and RML/SiO2(L)-UD. |
The spectra of the adsorbed CO on the used catalysts in Fig. 6b showed that the intensity of adsorbed CO decreased severely compared with the fresh catalysts, suggesting that the active sites of Rh lost part of activity or were covered partly. Meanwhile, the peak area ratio of CO(gdc)/CO(l) also decreased after reaction. In more detail, the peak area ratio of CO(gdc)/CO(l) on RML/SiO2(L)-UD become much smaller than that of RML/SiO2(H)-UD. Considering that the CO(gdc) species can only be formed on the highly dispersed Rh+ sites, it is concluded that the Rh supported on SiO2(L) is easier to aggregation compared with those supported on SiO2(H), which is also consistent with the result of TEM.
Combined with the reaction data, it is inferred that the relatively stable ratio of Rh+/Rh0 is correlated to the steady catalytic performance of RML/SiO2(H) in the reaction process. Ichikawa and co-workers35 proposed that Rh0 is active for CO dissociation and Rh+ is active for CO insertion, and, correspondingly, the changing of ratio of Rh+/Rh0 will change the catalytic properties. Chuang and Pien also suggested that the presence of isolated Rh+ sites may contribute to the increased selectivity for higher oxygenates.36 According to the above propositions, we inferred that, because the ratio of Rh+/Rh0 on RML/SiO2(L) would decrease markedly during the reaction, the hydrocarbons formation was favored at expenses of oxygenated compounds as the function of time.
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Fig. 7 TPSR profiles of the catalysts for CH4 formation: (a) RML/SiO2(H) and RML/SiO2(H)-UD, (b) RML/SiO2(L) and RML/SiO2(L)-UD. |
The TPSR experiments over the used catalysts are also showed in Fig. 7 in order to compare with the fresh samples. Compared with the catalyst of RML/SiO2(H), one peak of CH4 formation at about 240 °C also appeared on RML/SiO2(H)-UD, however, the peak area decreased strikingly. It is inferred that the state of Rh may not change evidently, but the number of Rh sites on RML/SiO2(H) decreased after reaction caused by the coverage of the high boiling products, which is in line with the result of IR. On the profile of RML/SiO2(L)-UD, the CH4 peak was centered at around 275 °C, which shifted to the higher temperature compared to that of RML/SiO2(L). The shifting of the CH4 peak should be attributed to the aggregation of Rh particles supported on SiO2(L), which is good agreement with the results of TEM and TPR. By combining the catalytic properties of the catalysts and above discussions, it can be suggested that the stable catalytic properties of RML/SiO2(H) is attributed to the steady state of Rh; on the contrary, the aggregation of Rh particles supported on SiO2(L) is viewed as an important factor affecting selectivity.
In addition, a wide peak of CH4 at the range of 300–700 °C appeared on both of the used catalysts. Since the peak area was too large, it should not be formed by the hydrogenation of adsorbed CO. In order to confirm where it came from, the temperature procedure experiment over the used catalysts in the flow of H2 without CO pro-absorption was used to compare with the TPSR profiles, as shown in Fig. 8. It can be seen that the profiles obtained without CO pro-absorption just appeared one peak at the range of 300–700 °C, which coincided with the second peak of TPSR profiles of the used catalysts. This observation clearly indicated that the peak at the range of 300–700 °C should be formed by the hydrogenation of carbon disposition on the used catalysts.
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Fig. 8 The TPSR profiles (the straight lines) and temperature procedure experiment profiles in the flow of H2 without CO pro-absorption (the dash lines): (A) RML/SiO2(H)-UD; (B) RML/SiO2(L)-UD. |
The results proved that the changing of ammonia concentration played an important role in the modification of the properties of SiO2. Correspondingly, the structure and state of the supported metals could be different on the two supports. The stable catalytic properties of RML/SiO2(H) is attributed to the relatively steady state of Rh in the reaction process. On the contrary, the rough surface of SiO2(L) resulted in easy agglomeration of the metals and decrease of the isolated Rh+ sites, thus the higher hydrocarbons formation was favored at expenses of the oxygenated compounds over the RML/SiO2(H) catalyst as the function of time.
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