Huibin Guo,
Lihong Tang*,
Kai Li,
Ping Ning,
Jinhui Peng,
Feng Lu,
Junjie Gu,
Shuangyou Bao,
Ye Liu,
Tingting Zhu and
Zhengyang Duan
Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China. E-mail: luckyman@163.com; Fax: +86-871-65920507; Tel: +86-871-65920507
First published on 3rd February 2015
MgAlCe hydrotalcite-like compounds (HTLCs) were synthesized by following co-precipitation methods based on different proportions of Al:
Ce. Metal oxides derived from HTLCs were tested in the catalytic hydrolysis of carbonyl sulfide (COS) at the relatively low temperature of 50 °C. The removal effect of COS strongly correlated to the proportion of Al
:
Ce, pH during synthesis and calcination temperature. Hydro-thermal temperatures were applied to improve the crystallinity of the materials and the catalysts were characterized by X-ray diffraction (XRD), thermogravimetry (TG) and Brunauer–Emmett–Teller (BET) methods. These results may contribute to a more full understanding of the effect of preparation conditions on the hydrolysis of COS. The optimum preparation conditions were found to be a 16
:
1 ratio of Al
:
Ce, a pH of 8 during synthesis, a hydro-thermal temperature of 140 °C, and a calcination temperature of 600 °C. The catalyst prepared under these conditions demonstrated an impressive efficiency (100%) sustained for approximately 80 minutes at a relatively low temperature of 50 °C and space velocity of 5000 h−1.
Hydrotalcite-like compounds (HTLCs) were recently discovered when the unique hydrotalcite structure was encountered in nature.15 When screening for catalysts, the mixed metal oxides originating from HTLCs were found to be a family of anionic clays containing a unique catalyst structure.16 The general formula of HTLCs is [M1−x2+Mx3+(OH)2]x+[Ax/nn−·yH2O]x− (an empirical formula), among which M2+ and M3+ are metal cations, An− is an n-valent anion, and the molar ratio of M2+ and M3+ is between 2 and 5.17 The general formula offers various compositions for HTLCs leading to a potentially vast selection of catalysts. There are challenges in seeking functional, efficient catalysts utilizing HTLCs as HTLCs lose crystal water, interlayer anions and hydroxyl groups when calcined at high temperature. The hydrotalcite-like layer structure will, as a result, be partially destroyed, and the inter-surface area will increase. In a related study, Wang et al.18 found that CoNiAl hydrotalcite-like compounds modified by cerium exhibited an improved COS removal effect, but experienced a relatively low service life and had a difficult preparation process. Ramesh Chitrakar et al.19 found that the uptake properties of phosphate on a Zr-modified MgFe–LDH(CO3) (LDH = layered double hydroxide) improved removal efficiency, but also experienced a relatively low service life. Based on previous findings, cost-effective MgAl metals were selected to examine the application of MgAl hydrotalcite-like compounds modified by adding cerium for improving COS hydrolysis activity and increasing service life.
Previous studies revealed that mixed oxides derived from HTLCs produced satisfactory results in the hydrolysis of COS.20 MgAl hydrotalcite modified by adding cerium in varying proportions of Al:
Ce is examined in this study. Optimum preparation conditions were determined, including the proportion of Al
:
Ce, the pH during the synthesis, the hydro-thermal temperatures and calcination temperatures, forming a complete experimental process.
The Brunauer–Emmett–Teller (BET) surface areas and average pore diameter of the catalysts were measured by N2 adsorption using an Autosorb-1-C instrument. The samples were first outgassed at 573 K for more than 12 h before adsorption isotherms were generated by dosing the catalysts with nitrogen at 77 K.
Thermogravimetry (TG) was carried out using a Shimadzu DTG-60H. The sample was loaded into a platinum pan and heated from room temperature to 600 °C at a heating rate of 10 °C min−1 under a N2 atmosphere. The sensitivity and precision are 0.001 mg and ±1% respectively. The range of thermography is ±1 to ±1000 μV.
COSinlet (ppm) and COSoutlet (ppm) are the concentrations of COS measured at the inlet and outlet of the reactor, respectively.
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Fig. 2 COS conversion over Mg2AlxCe1−x with different metal contents. Reaction conditions: 470 ppm COS, 50 °C, 5000 h−1, a relative humidity of 2.67% and N2 to balance. |
The influence of cerium on the catalytic activity is studied through characterizing typical samples of Mg2Al0.94Ce0.06 and Mg2Al by XRD. Powder X-ray diffraction patterns of the HTLC precursors (Fig. 3(A)) and the corresponding mixed metal oxides (Fig. 3(B)) were acquired.
As presented in Fig. 3(A), the characteristic diffraction peaks at 2θ = 11°, 23°, 35°, 39°, 47° and 61° correspond to the (003), (006), (009), (015), (018) and (110) reflections, which are signals typical of hydrotalcite-like compounds,22,23 confirming the formation of the precursor layered double hydroxides composed exclusively of Mg and Al.24 A slightly higher crystallinity was observed for the Ce sample compared to MgAl hydrotalcite. Additionally, alteration of the HTLC structure was minimal in the presence of Ce, indicating that the effect of Ce on the precursors was minimal and that there may be no Ce in the brucite layer; the difference in the ionic radii of octahedral coordinated Ce3+ (1.02 Å), resulting in a large distortion of the layers, may provide an explanation.
Fig. 3(B) illustrates the distinct characteristic peaks of crystalline MgO at 2θ = 43°, while the characteristic peaks of Al2O3 are nearly imperceptible. The Al3+ cation is approximately the same size as Mg2+ (ref. 24) thus, the diffraction peak of the Al phase of the catalyst is absent and will present a similar lattice to periclase-like MgO when merged into solid solutions, indistinguishable by XRD. The characteristic peaks of crystalline MgAl2O4 are identified at 2θ = 63°. Minor quantities of CeO2 were obtained simultaneously; these peaks are sharp, indicating that the Ce-containing samples possessed increased crystallinity. Calculated using MDI Jade 5.0, the crystallinity of MgAl was determined to be approximately 37%, while the crystallinity of MgAlCe was determined to be approximately 42%. The degree of crystallinity would, in fact, rise above 90%. The presence of mordenite (unknown peak),25 however, contributed to low crystallinity. It can be explained that increased crystallinity improves catalytic activity and, with the introduction of Ce, though showing minimal effect as a precursor, calcination of HTLCs with mixed oxides occurred and the catalytic activity was enhanced. Catalytic activity enhancement upon introduction of cerium content arose from the differences in structural properties (but not complete reconstruction, due to the presence of minor quantities of CeO2) and oxidative properties of mixed oxide catalysts.
Furthermore, catalytic hydrolysis of COS is regarded as a base-catalysed reaction,26 and the OH group is the active site for the hydrolysis of COS.27 Therefore, the Boehm titration was used to understand how it is modified by the introduction of different contents of Ce, as well as the relationship between basicity and catalytic activity. From Table 1, it is obvious that Mg2Al0.94Ce0.06 had the highest proportion of OH groups compared to other materials; the OH group mass concentration of Mg2Al0.94Ce0.06 is 2.12 mmol g−1. However, when the Ce content increased, the OH group mass concentration decreased. This indicated that a decrease in the Al:
Ce ratio decreases the OH group mass concentration. At the same time, the OH group mass concentration of Mg2Al is 0.40 mmol g−1, which is also lower than the OH group mass concentration of Mg2Al0.94Ce0.06. This is identical to the results in Fig. 2. Although the lactonic groups are unordered, the OH groups play a vital role in the catalytic hydrolysis of COS.
Samples | Lactonic groups (mmol g−1) | OH groups (mmol g−1) |
---|---|---|
Mg2Al | 0.96 | 0.40 |
Mg2Al0.94Ce0.06 | 1.12 | 2.12 |
Mg2Al0.90Ce0.10 | 2.28 | 0.48 |
Mg2Al0.80Ce0.20 | 2.20 | 0.20 |
Mg2Al0.70Ce0.30 | 1.84 | 0.16 |
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Fig. 4 COS conversion over Mg2Al0.94Ce0.06 (Al![]() ![]() |
To better understand the crystallinity of materials synthesized at different pH values, the XRD spectra of the materials synthesized at different pH values were obtained. Fig. 6 shows that the catalyst has a higher crystallinity when synthesized at pH 8 than at other pH values. The catalyst synthesized at a pH value of 8 had two obvious sharp peaks and few scattering peaks, which indicated the improvement in crystallinity of the material. The crystallinity of catalysts synthesized at other pH values was basically identical; what is more, all of them have more scattering peaks than for the catalyst synthesized at pH 8. Synthesis at pH 8 gave a high crystallinity, contributing to high catalytic activity. This is consistent with the experiment showing that synthesis at pH 8 gave a catalyst with higher activity than those synthesised at pH 7, 9, 10 and 11. But as the crystallinity is low for all of them, this may be due to the influence of the scattering peaks. In order to improve the crystallinity of the materials, the effect of different hydro-thermal temperatures during synthesis on the crystallinity was tested.
XRD patterns of samples synthesized at various hydro-thermal temperatures were also characterized. Fig. 8 indicates that the crystallinity of catalysts synthesized at 140 °C and 175 °C are superior to those synthesized at 35, 70 and 105 °C. The characteristic peaks of crystalline Mg6Al2(OH)18 could be easily identified at 2θ = 28°, 43° and 63°. Crystalline Mg2Al(OH)7 could be easily identified for synthesis temperatures below 105 °C, but is barely identified for when hydro-thermal temperatures were 140 and 175 °C. Mg6Al2(OH)18 may be more stable than Mg2Al(OH)7, allowing the crystalline form of Mg2Al(OH)7 to disappear when the hydro-thermal temperatures during synthesis are 140 and 175 °C. The hydro-thermal temperature range may be divided into two parts: hydro-thermal temperatures under 105 °C produce patterns that exhibit low crystallinity as a result of unknown peaks or scattering peaks, while the unknown peak essentially disappears for hydro-thermal temperatures above 140 °C. A higher crystallinity is obtained during synthesis at a hydro-thermal temperature of 140 °C compared to hydro-thermal temperatures below 140 °C, explaining why a hydro-thermal temperature of 140 °C afforded a catalyst exhibiting higher efficiency than hydro-thermal temperatures below 140 °C. An increase in hydro-thermal temperature during synthesis to 175 °C results in a nearly imperceptible change in crystallinity, indicating 140 °C as the superior hydro-thermal temperature for synthesis of the catalyst, consistent with other experimental results.
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Fig. 9 COS conversion over Mg2Al0.94Ce0.06 calcined at different temperatures. Reaction conditions: 470 ppm COS, 50 °C, 5000 h−1, a relative humidity of 2.67% and N2 to balance. |
Thermal decomposition (Fig. 10) can be divided into three stages. Removal of surface water and physically adsorbed water occurred during the first stage (<200 °C) and the loss of mass accounted for 14.15% of the total, but the LDH structure is not destroyed at this stage. During the second stage (200–400 °C), weight loss occurred mainly due to disruption of the hydrotalcite layer structure, while CO32− in the interlayer was released in the form of CO2. The third stage (400–540 °C) leads to the collapse of the laminate structure as loss of the crystalline water in the layer occurred, forming mixed oxides. Weight loss was not evident above 540 °C (Table 2 summarizes each stage of weight loss). The effective temperature from TG may be 540 °C, but the effective temperature from the experiment was 600 °C. The calcination temperature was generally higher (50–100 °C) than the temperature of hydrotalcite-structure collapse, indicating 600 °C as the superior calcination temperature.32 The TG is consistent with the catalytic activity of Mg2Al0.94Ce0.06. In addition, in connection with Fig. 3, the d-spacing of the (003) reflection of these precursors at 2θ = 11°, which corresponds to the thickness between layers, is 2.34–2.37 nm.33,34 The d-spacing of the (110) reflection at 2θ = 61° is related to the average metal–metal distance in the layer, which for these precursors is 0.3 nm; these results are in good agreement with those reported by Serrano-Lotina et al.35 By the information provided from XRD (Fig. 3), the structure of the hydrotalcite-like precursors and the catalyst after calcination is visually described in Scheme 1.
Sample | Total weight loss rate (%) | The first stage of weight loss | The second stage of weight loss | The third stage of weight loss | |||
---|---|---|---|---|---|---|---|
Weight loss rate (%) | Maximum weight loss temperature (°C) | Weight loss rate (%) | Maximum weight loss temperature (°C) | Weight loss rate (%) | Maximum weight loss temperature (°C) | ||
Mg2Al0.94Ce0.06 | 42.68 | 14.15 | 200 | 21.95 | 400 | 6.58 | 540 |
The catalysts following calcination temperatures of 300, 400, 600 and 700 °C were characterized by BET. Surface area, pore volume and average pore diameter of the catalysts are listed in Table 3. Normally, catalytic activity is related to the BET surface area, however, the surface area increased with increasing calcination temperatures as the surface area after calcination at 700 °C was greater than after calcination at 600 °C. The surface area may not act as a main factor leading to increased activity in COS catalytic hydrolysis. Total pore volume and the average pore diameter are related to the activity of COS catalytic hydrolysis. The total pore volume after calcination at 600 °C was 0.69 cm3 g−1, larger than the total pore volume after calcination at 300, 400 and 700 °C. However, the average pore diameter after calcination at 600 °C is lower than for calcination temperatures of 300, 400 and 700 °C. The efficiency of COS catalytic hydrolysis is inversely related to average pore diameter, as indicated when a higher average pore diameter aligns with lower activity of COS catalytic hydrolysis. The surface area is not linked with catalytic activity; however, the total pore volume is sufficient, as supported by results of the pore size distributions presented in Fig. 11. As shown in Fig. 11, all pores of the samples were smaller than 90 nm when the calcination temperature reached 600 °C; the volume of the pores at a range of 15–30 nm was significantly larger than that of those formed at calcination temperatures of 300, 400 and 700 °C. The pore size behavior suggests that mesoporosity may potentially influence COS hydrolysis. N2 adsorption–desorption isotherms of the catalysts (type IV for all samples), representing typical mesoporous materials are presented in Fig. 12, and align with the pore distributions.
Calcination temperature (°C) | Surface area (m2 g−1) | Total pore volume (cm3 g−1) | Average pore diameter (nm) |
---|---|---|---|
300 | 88.41 | 0.51 | 22.71 |
400 | 110.65 | 0.60 | 21.34 |
600 | 116.26 | 0.69 | 20.50 |
700 | 119.68 | 0.60 | 22.29 |
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