Cobalt Supported on Alkaline-Activated Montmorillonite as an Efficient

Sep 13, 2013 - (4-9) However, the contact between the FT catalyst and zeolite in the ... MMT an excellent support or component of catalysts for FT syn...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/EF

Cobalt Supported on Alkaline-Activated Montmorillonite as an Efficient Catalyst for Fischer−Tropsch Synthesis Yong-Hua Zhao,†,‡ Qing-Qing Hao,† Yong-Hong Song,† Wei-Bin Fan,§ Zhao-Tie Liu,† and Zhong-Wen Liu*,† †

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education (MOE), School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, Shaanx, People’s Republic of China ‡ School of Chemistry and Environmental Engineering, Liaoning University of Technology, Jinzhou 121001, Liaoning, People’s Republic of China § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, People’s Republic of China S Supporting Information *

ABSTRACT: Cobalt (10 wt %) was supported on KOH-activated Ca-type montmorillonite (Ca-MMT) as a catalyst for Fischer−Tropsch (FT) synthesis. The material was systematically characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray fluorescence (XRF), scanning electron microscopy (SEM), N2 adsorption−desorption at low temperature, temperature-programmed reduction of hydrogen (H2-TPR), and temperature-programmed desorption of ammonia (NH3-TPD) techniques. It was found that the acidity and textural properties of the activated MMT were strongly dependent upon the conditions of alkaline treatment (concentration of KOH solution, time, and temperature), which determines the reduction behavior of the cobalt oxide and, thus, the conversion of CO and the selectivity of FT products. As a result, efficient Co/MMT catalysts for FT synthesis were obtained by optimizing the alkaline treatment of MMT.

1. INTRODUCTION Fischer−Tropsch (FT) synthesis, which converts synthesis gas (syngas) to a mixture of hydrocarbons, is an important process for the production of clean transportation fuels free of sulfur, nitrogen, and aromatics.1,2 However, as a polymerization process, the FT reaction follows the Anderson−Schulz−Flory (ASF) kinetics, resulting in products with a poor selectivity to any specific group of hydrocarbons, such as gasoline and diesel.1,3 Therefore, a downstream refinery is usually required for the production of liquid fuels with FT synthesis. To improve the efficiency of the FT process, it is desirable to develop the one-step synthesis of liquid fuels by minimizing the production of gaseous (C1−C3) and solid (C21+) hydrocarbons. Thus far, various strategies have been applied to regulate the product distribution of FT synthesis.2 Being a promising approach, the combination of zeolites and FT catalysts (such as Fe, Co, and Ru) is extensively investigated for FT synthesis. In the case of physical mixtures of zeolites and FT catalysts, the selectivity of gasoline is greatly enhanced because of the secondary cracking reactions of the primary FT longchain hydrocarbons catalyzed by acidic sites over zeolites.4−9 However, the contact between the FT catalyst and zeolite in the physical mixture is poor, which affects the conversion of reactants and the selectivity of products.9 Alternatively, FT metals are directly supported on zeolites.10 In this case, FT catalysts (such as cobalt) are difficult to be reduced when they are supported on traditional microporous zeolites, such as ZSM-5,10 leading to a low FT activity and a high CH4 selectivity. To solve these issues, mesopore zeolites have been extensively investigated in recent years.11,12 Nevertheless, the © XXXX American Chemical Society

relatively low thermal/hydrothermal stability and weak acidity of the zeolites are two main obstacles to achieve a high selectivity of the targeted product. The desilication (extraction of part of framework Si) of zeolites by alkaline treatment has widely been investigated as an effective method to tailor their acidic and textural properties.13 The alkaline treatment for the desilication can create mesopores and modify the acidity of zeolites without a significant decrease in the thermal stability of zeolites.13,14 For example, under mild conditions (0.1−1.0 mol/L NaOH and 70 °C), the creation of mesopores and the decrease in acidity for H-ZSM-5 (Si/Al = 26) have been observed after the desilication.12 Consequently, the Ru-supported H-ZSM-5 catalyst exhibited a very high selectivity to C5−C11 hydrocarbons (∼80% at a maximum) under typical FT conditions. Montmorillonite (MMT) is a layered aluminosilicate clay composed of two tetrahedral sheets with Si in cationic sites sandwiching an octahedral Al sheet.15 The layered structure and the exchangeable interlayer cations make MMT an excellent support or component of catalysts for FT synthesis. In our previous works,16−20 MMT was tailored by acid activation,16 ion exchange,17,20 pillaring,18 and template-directed pillaring19 to develop Co-based catalysts for FT synthesis. In this work, the alkaline activation of MMT was explored for the creation of acidic sites and mesopores, which are desirable for FT Special Issue: Ultraclean Fuels Production and Utilization Received: June 22, 2013 Revised: September 12, 2013

A

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

at 1.0 mL and 1.6 cm, respectively. The catalyst was reduced at atmospheric pressure in a flow of pure H2 (50 cm3/min) at 400 °C for 4 h. After reduction, syngas (H2/CO = 2, with 4% Ar as an internal standard) was switched and the FT reaction was carried out under the conditions of W/F = 5.05 g h mol−1, T = 235 °C, and P = 1.0 MPa. To prevent condensation of the products, the line between the outlet of the reactor and the inlet of the gas chromatography (GC) column was heated at 180 °C. The effluent products were analyzed by two online GC (GC-9560, Shanghai Huaai Chromatographic Analysis Co., Ltd.). Hydrocarbons were analyzed with an HP-PONA capillary column (0.20 mm × 50 m, 0.5 μm) and a flame ionization detector (FID). CO, CH4, Ar, and CO2 in the effluent after cooling in an ice−water trap were analyzed with a packed activated-carbon column and a TCD. The selectivity for hydrocarbons was calculated on the basis of the carbon number.

synthesis. Furthermore, cobalt was supported on alkalineactivated MMT as a catalyst, which exhibits promising catalytic performance for FT synthesis.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The purified Ca-type MMT (CaMMT) was provided by Zhejiang Sanding Group Co., Ltd. The content of MMT in the purified mineral was determined to be 98.8 wt %. To activate Ca-MMT with alkaline, 5 g of Ca-MMT was magnetically stirred in 500 mL of KOH solution (5−15 mol/L) at 60−100 °C for 1−12 h. After activation, the slurry was centrifuged, sufficiently washed with deionized water until neutral pH, and then ion-exchanged in 1 mol/L NH4NO3 solution at 80 °C for 12 h. After this, the suspension was centrifuged, washed, dried, and calcined at 500 °C for 4 h. The obtained material was abbreviated as MMT-m-t/ T, where m, t, and T were the concentration of KOH solution, time, and temperature during the alkaline treatment, respectively. For comparison, the raw Ca-MMT was exchanged in 1 mol/L NH4NO3 solution at 80 °C for 12 h and abbreviated as MMT-0. The cobalt catalysts were prepared via the incipient impregnation method using cobalt nitrate as a precursor of Co. After impregnation, the samples were dried in an oven at 120 °C for 12 h and then calcined in air at 200 °C for 2 h by increasing the temperature at a controlled heating rate of 2 °C/min. The metallic cobalt loading was kept at 10 wt % for all of the catalysts. 2.2. Characterization Techniques. N2 adsorption−desorption isotherms were measured with BelSorp-Max (Bel Japan, Inc.) at −196 °C. Prior to analysis, each sample (ca. 150 mg) was degassed at 350 °C for 5 h. The surface area was calculated by the Brunauer−Emmett− Teller (BET) method, and the pore size distribution was determined on the basis of the Barrett−Joyner−Halenda (BJH) method using the data of adsorption branches. X-ray diffraction (XRD) patterns were obtained at room temperature on a Bruker D8 Advance X-ray diffractometer using monochromatized Cu Kα radiation (40 kV and 40 mA). The samples were scanned with a step size of 0.02° and a counting time of 0.2 s per step. The crystal size of Co3O4 over the calcined catalysts was calculated according to the Scherrer formula and the 440 diffraction (2θ at about 65.2°). The crystal size of the metallic cobalt in the reduced catalysts was estimated according to the equation of d(Co0) = 0.75d(Co3O4). Fourier transform infrared (FTIR) spectroscopic analysis was conducted on a Nicolet Avatar 360 spectrometer. FTIR spectra in the transmittance mode were recorded in the range of 400−4000 cm−1 at a resolution of 4 cm−1 using the KBr-pressed disk technique. The temperature-programmed reduction of hydrogen (H2-TPR) was carried out on a Micromeritics Autochem 2920 apparatus. The samples (ca. 50 mg) were initially flushed with Ar at room temperature for 30 min. Then, the gas was switched to 10 vol % H2 in Ar, and the temperature increased up to 900 °C at a ramping rate of 10 °C/min. The water generated during the reduction was retained by a downstream 2-propanol/liquid N2 trap. The H2 consumption rate was monitored with a thermal conductivity detector (TCD) previously calibrated using the reduction of CuO as a reference. The temperature-programmed desorption of ammonia (NH3-TPD) was measured with a Micromeritics Autochem 2920 instrument. Typically, 50.0 ± 0.5 mg of sample was first preheated at 550 °C under flowing He for 60 min and then cooled to 120 °C. Subsequently, the sample was exposed to 5% NH3 in He for 30 min. After this, the system was purged for 2 h in a flow of He at the same temperature. Finally, NH3-TPD was carried out from 120 to 550 °C at a heating rate of 10 °C/min under a helium flow of 30 mL/min. Scanning electron microscopy (SEM) images were obtained on a Philips-FEI model Quanta 200. X-ray fluorescence (XRF) spectra were recorded on a Philips MagiX X-ray fluorescence spectrometer. 2.3. FT Reaction. The catalytic tests were performed in a stainlesssteel tubular reactor (inner diameter = 10 mm) by loading 0.5 g of catalyst (40−60 mesh) diluted with quartz sands. For all of the tests, the total volume and height of the catalyst bed in the reactor were kept

3. RESULTS 3.1. Textural and Structural Properties. 3.1.1. Effect of Alkaline Activation on the Structure of MMTs. The XRD patterns of MMTs calcinated at 500 °C are given in Figure 1.

Figure 1. XRD patterns for (a) Ca-MMT, (b) MMT-0, (c) MMT-53/80, (d) MMT-10-3/80, (e) MMT-15-3/60, (f) MMT-15-1/80, (g) MMT-15-12/80, (h) MMT-15-3/80, and (i) MMT-15-3/100.

The changes of the MMT-layered structure by alkaline treatment under different conditions are clearly observed from the characteristic (001) diffraction. The d001 spacing for MMT-0 and the alkaline-activated MMTs under mild conditions (MMT-5-3/80, MMT-10-3/80, and MMT-15-3/ 60) is almost identical (0.96 nm), which is similar to that of the raw Ca-MMT. This result can be reasonably ascribed to the loss of the hydrated water molecules in MMTs after calcination at 500 °C. Moreover, the intensity of the (001) diffractions of MMT-5-3/80, MMT-10-3/80, and MMT-15-3/60 is not clearly decreased in comparison to that of Ca-MMT. This indicates that the layered structure of MMT is not destroyed after alkaline activation under mild conditions, although the desilication and dealumination clearly occur according to the decreased content of SiO2 and Al2O3 in MMT (Table 1). In the cases of the MMTs activated under severe conditions, i.e., MMT-15-1/80, MMT-15-3/80, MMT-15-12/80, and MMT-15-3/100, almost the same (about 1.00 nm) d001 spacing is observed and slightly higher than that of the MMTs activated under mild conditions (Figure 1). Moreover, with increasing the severity of activation conditions, the intensity of the (001) diffractions is clearly decreased in the order of MMT-15-1/80 > MMT-15-3/80 > MMT-15-12/80 > MMT-15-3/100. These observations indicate that the layered MMT structure may be B

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

Table 1. Summary of the Textural Properties of Different MMTs

samples

BET surface area (m2 g−1)

pore volume (cm3 g−1)

average pore size (nm)

Ca-MMT MMT-0 MMT-15-3/60 MMT-15-3/80 MMT-15-3/100 MMT-15-1/80 MMT-15-12/80 MMT-5-3/80 MMT-10-3/80

82.3 120.9 128.4 161.4 117.5 135.9 116.4 129.2 137.3

0.20 0.18 0.21 0.26 0.47 0.21 0.47 0.20 0.23

9.5 6.1 6.7 6.4 16.1 6.0 16.3 6.1 6.8

SiO2 (wt %)a

Al2O3 (wt %)a

65.0

21.9

54.3 38.4 46.9 40.9 29.5 44.4 41.2

13.2 5.7 10.3 6.0 4.0 6.6 6.5

a

The content of SiO2 and Al2O3 over the MMTs was determined by XRF. Figure 2. FTIR spectra of (a) Ca-MMT, (b) MMT-5-3/80, (c) MMT10-3/80, (d) MMT-15-3/60, (e) MMT-15-1/80, (f) MMT-15-3/80, (g) MMT-15-12/80, and (h) MMT-15-3/100.

partially destroyed as a result of the obvious desilication and dealumination (Table 1). Moreover, on the basis of the MMT structural information, the silica and alumina species from the desilication and dealumination can be intercalated into the interlayer region of MMT and oxide pillars are formed after calcination. Thus, the slightly expanded interlayer space of the MMTs activated under severe conditions may be originated from the oxide pillars in the interlayer region. The peaks at 2θ of about 20.8° and 26.5° are reasonably assigned to the diffractions of quartz [Joint Committee on Powder Diffraction Standards (JCPDS) 46-1045], which is as an impurity present in the MMTs. Moreover, the impure quartz is not removed even under the severest activation conditions. On the contrary, its intensity is clearly increased with further increasing the severity of the alkaline treatment, indicating that the desilication and dealumination of MMT significantly occur. This is clearly reflected from the XRF results. As shown in Table 1, the content of SiO2 and Al2O3 for alkaline-activated MMT is gradually decreased with increasing the severity of the alkaline activation. However, for MMT-15-3/100, the content of SiO2 and Al2O3 is actually increased, which may be due to the recrystallization of the leached silica and alumina under the conditions of strong alkaline and high temperature.21 To further reveal the impact of alkaline treatment on the chemical and structural properties of MMT, FTIR studies were carried out and the results are given in Figure 2. For the raw Ca-MMT, the characteristic absorption bands of Al−OH−Al (915 cm−1), Al−OH−Mg (845 cm−1), Si−O in tetrahedral sheets (1030 cm−1), Si−O−Si (465 cm−1), Si−O−Al (Al in octahedral) (523 cm−1), the bridging hydroxyl groups (3623 cm−1), and the hydroxyl groups in molecular water (3450 and 1640 cm−1) are clearly observed.16,22−26 Following these characteristic bands, the effect of alkaline activation on the structural and compositional changes of the raw Ca-MMT can be determined from the variation of the infrared (IR) spectra. With increasing the severity of the alkaline activation, the intensity of the IR bands for bridging hydroxyl groups (3623, 915, and 845 cm−1) and Si−O in tetrahedral sheets (1030 cm−1) are gradually decreased. Moreover, the vibrational intensity of the hydroxyl groups in molecular water (3450 and 1640 cm−1) is decreased, indicating that the amount of water coordinated to the octahedral cations is decreased. Moreover, bending vibrations of Si−O−Si, Si−O−Al, and Al− OH-Al (465, 523, and 915 cm−1) are weakened. These changes suggest that the progressively increased leaching of Si4+, Al3+,

and Mg2+ clearly occur, which is confirmed by the XRF results (Table 1). However, the intensity of the band at about 1110 cm−1 assigned to the Si−O vibrations of amorphous silica with a three-dimensional framework and the characteristic band for amorphous silica at 800 cm−1 are gradually increased. Furthermore, with increasing the severity of the activation conditions, a clear band at about 3690 cm−1 ascribed to vibrations of external silanol groups appear over MMT-15-12/ 80 and MMT-15-3/100. For the MMTs alkaline activated at the same temperature and time, the intensity of the 3690 cm−1 peak is increased with increasing the concentration of KOH solution (curves b, c, and f in Figure 2). Additionally, the appearance of the 3690 cm−1 band is accompanied with a clear decrease in the intensity of bridging hydroxyl groups (3623, 915, and 845 cm−1). Thus, these observations strongly suggest that the 3690 cm−1 band is very possibly originated from the broken Si−Al−OH bonds during the activation of MMT in a high concentration of KOH solution. In summary, a part of silica and alumina in the MMT layer is dissolved in alkaline solutions, the extent of which is clearly dependent upon the conditions of alkaline activation. Moreover, the layered structure of MMT is still preserved even under the severest conditions of alkaline activation, although a partial destruction of the MMT layer clearly occurred under the conditions of relatively higher temperature and higher concentration of KOH solution. These observations are obviously different from those of the activation of MMT with acid, i.e., the easily complete destruction of the layered MMT structure.16 3.1.2. Textural Properties of the Alkaline-Activated MMTs. The N2 adsorption−desorption isotherms of the MMTs are displayed in Figure 3. As indicated from the isotherm, the raw Ca-MMT shows only a small volume of N2 adsorbed in the lower relative pressure range (p/p0 < 0.1), indicating the presence of a very limited amount of micropores. For MMT-0, the volume of the adsorbed N2 is clearly increased with increasing p/p0 until 0.1. This indicates that the amount of micropores derived from the narrow slit-like pores is increased. However, for the alkaline-activated MMTs, the amount of N2 adsorbed in relatively lower pressure range (p/p0 < 0.1) is not increased clearly in comparison to that of MMT-0. Thus, C

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

Figure 3. N2 adsorption−desorption isotherms of (a) Ca-MMT, (b) MMT-0, (c) MMT-5-3/80, (d) MMT-10-3/80, (e) MMT-15-3/60, (f) MMT-15-1/80, (g) MMT-15-3/80, (h) MMT-15-12/80, and (i) MMT-15-3/100.

Figure 4. Mesopore size distribution calculated by the BJH method using adsorption branches of (a) Ca-MMT, (b) MMT-0, (c) MMT-53/80, (d) MMT-10-3/80, (e) MMT-15-3/60, (f) MMT-15-1/80, (g) MMT-15-3/80, (h) MMT-15-12/80, and (i) MMT-15-3/100.

micropores cannot be significantly created by the alkaline activation of Ca-MMT. On the contrary, with increasing the severity of alkaline activation, e.g., MMT-15-12/80 and MMT15-3/100, a clear decrease of the amount of N2 adsorbed in the range of p/p0 < 0.1 is observed, leading to the decreased amount of micropores. On the basis of the International Union of Pure and Applied Chemistry (IUPAC) classification,27 all of the MMTs show a type IV isotherm, together with a hysteresis loop in the relative pressure range of 0.4−1.0. However, the size and shape of the hysteresis loops are strongly dependent upon the activation conditions. In the cases of MMT-0, MMT-5-3/80, MMT-10-3/ 80, MMT-15-1/80, and MMT-15-3/60, a typical H4 hysteresis loop can be suggested because the branches of the adsorption and desorption are nearly horizontal and parallel in a wide range of p/p0, which is indicative of the narrow slit-like pores.27 However, with increasing the severity of the alkaline activation, a gradual change of the isotherm from H4 to H3 hysteresis loop is observed at a higher relative pressure. This is clearly reflected from the isotherms of MMT-15-3/80, MMT-15-12/80, and MMT-15-3/100. Generally, the slit-shaped pores originated from loosely coherent plate-like particles are interpreted for an H3 hysteresis loop.27 Thus, the delamination of the MMT laminar structure under severe conditions of alkaline activation, i.e., decreasing the number of layers per stack, is obvious for MMT-15-12/80 and MMT-15-3/100. This explanation is wellconfirmed from the BJH pore size distribution (Figure 4). As shown in Figure 4, the alkaline-activated MMTs under mild conditions (MMT-5-3/80, MMT-10-3/80, MMT-15-1/80, and MMT-15-3/60) show very similar pore size distributions and the peak pore size is centered at about 2.1 nm. On the contrary, MMT-15-12/80 and MMT-15-3/100, which are activated under severe conditions, show very broad pore size

distributions and a clear shift toward larger pore size is observable. Thus, with increasing the severity of the conditions of alkaline activation, more mesopores are created over the activated MMT and the pore size distribution becomes wider because of the delamination of the MMT laminar structure, which is consistent with the XRD and FTIR results. The calculated textural data of MMTs are summarized in Table 1. In the case of the raw Ca-MMT, its surface area is about 82 m2/g. On the contrary, by simply ion exchanging the Ca-MMT with NH4+, i.e., MMT-0, its BET surface area is increased to about 121 m2/g. For the MMT-15-3/T, the surface area is increased with increasing the activation temperature from 60 to 80 °C. However, a further increase of the activation temperature to 100 °C induces a significant decrease of the surface area of MMT-15-3/100. Similarly, for the MMT-15-t/ 80, the surface area is increased first and then decreased with the increasing of the activation time from 1 to 12 h. Moreover, the pore volumes of the alkaline-activated MMTs are monotonically increased with increasing the severity of the activation conditions. The clearly larger average pore size of MMT-15-3/100 and MMT-15-12/80 (∼16 nm) may be due to the partial destruction of the layered MMT structure, which is supported by the results of XRD and XRF. To reveal the effect of the alkaline activation on the morphologic changes, the alkaline-activated MMTs were subjected to SEM observations, and the results are given in Figure 5. Generally, the SEM images of the MMT before and after alkaline activation are greatly different. The typical SEM image of the raw Ca-MMT represents irregular flakes with small patches of platelets. However, for MMT-15-3/60 and MMT10-3/80, clumps of agglomerated particles attached on MMT flakes are clearly observable, reflecting the partial collapse of the MMT laminar structure. Upon further increasing the severity of the treatment conditions, more agglomerates and some cracks D

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

Figure 5. SEM images of (a) Ca-MMT, (b) MMT-15-3/60, (c) MMT-10-3/80, (d) MMT-15-3/80, (e) MMT-15-12/80, and (f) MMT-15-3/100.

the crystal size of Co3O4 calculated by the Scherrer equation is very similar (∼12−20 nm) for all of the catalysts. 3.2. Acidic Property. The acidic properties of the MMTs evaluated by NH3-TPD are shown in Figure 7, from which two distinct NH3 desorption peaks are seen for all of the MMTs. The peaks centered at about 150−210 °C and at about 380 °C correspond to the weak and strong acid sites, respectively. The acidity of Ca-MMT is low, which can be reasonably ascribed to the basic interlayer cations of calcium. For MMT-0, weak acid sites are enhanced because of the NH4+ ions entered into the MMT interlayer, which are decomposed into protons after calcining at 500 °C. In the case of the alkaline-activated MMTs, significant amounts of weak and strong acidic sites are created because of the desiliconization and dealumination of the MMT. For the MMTs activated under relatively mild conditions (MMT-5-3/80, MMT-10-3/80, MMT-15-3/60, MMT-15-1/ 80, and MMT-15-3/80), substantial amounts of strong acidic sites are created. However, the MMTs activated under severer conditions (MMT-15-12/80 and MMT-15-3/100) have few strong acidic sites. Moreover, the peaks of weak acid sites for alkaline-activated MMT are moved toward a lower temperature, indicating a decreased acidity. To obtain the distribution of the acidic site with different strengths quantitatively, the amounts of the total, weak, and strong acidic sites were calculated by integrating the NH3-TPD peaks, and the results are given in Table 3. As shown in Table 3, MMT-15-3/60 shows the highest amount of total acidic sites. However, the maximum amount of strong and weak acidic sites is observed for MMT-15-3/60 and MMT-15-3/100, respectively. 3.3. Reduction Behavior. The H2-TPR profiles of different Co/MMT catalysts are presented in Figure 8. Two main reduction peaks appear at 215−247 °C (peak I) and 309−456 °C (peak II), which can be attributed to the two-step reduction of Co3O4, i.e., Co3O4 to CoO and CoO to Co, respectively. It is noticeable that the peak profiles of Co3O4 to CoO for all catalysts are very similar, although the temperatures at peak maximum are slightly varied (Table 2). This indicates the very limited effect of the alkaline activation on the reduction of Co3O4 to CoO over Co/MMT catalysts. However, the shape

are found for MMT-15-3/80. Furthermore, MMT-15-12/80 looks to be melted to such an extent that the original shape of the raw Ca-MMT can hardly be seen. Under the severest treatment conditions, MMT-15-3/100 shows big clumps of amorphousness-like agglomerates. Thus, damage of the alkaline activation on the MMT laminar structure can be distinctly determined from the SEM observations, which is agreeable with the results of XRD and FTIR. 3.1.3. Structure of the Catalysts. The XRD patterns of the cobalt-supported MMTs are given in Figure 6. The (001)

Figure 6. XRD patterns for 10 wt % Co loaded on (a) Ca-MMT, (b) MMT-0, (c) MMT-5-3/80, (d) MMT-10-3/80, (e) MMT-15-1/80, (f) MMT-15-12/80, (g) MMT-15-3/60, (h) MMT-15-3/80, and (i) MMT-15-3/100.

diffractions of MMTs are still seen for all of the catalysts, although their intensities are clearly decreased in comparison to those shown in Figure 1. This suggests that the regularity of the layered structure of MMTs is decreased after the introduction of cobalt. Moreover, the peaks at 2θ of 19.1°, 31.4°, 36.9°, 44.9°, 59.6°, and 65.5° are well-agreeable with the standard pattern of cubic Co3O4 (JCPDS 65-3103), indicating that pure Co3O4 is formed over all of the catalysts. As shown in Table 2, E

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

Table 2. Crystal Properties of Cobalt Species and H2-TPR Characteristics of Different Co/MMT Catalysts temperature at peak maximum (°C) catalyst

d(Co3O4) (nm)a

d(Co0) (nm)b

dispersion (%)c

Co3O4 to CoO

CoO to Co

Co/Ca-MMT Co/MMT-0 Co/MMT-15-3/60 Co/MMT-15-3/80 Co/MMT-15-3/100 Co/MMT-15-1/80 Co/MMT-15-12/80 Co/MMT-5-3/80 Co/MMT-10-3/80

15.74 16.87 16.40 16.37 14.48 19.21 12.85 20.45 18.65

11.81 12.65 12.30 12.28 10.86 14.41 9.64 15.34 13.40

8.13 7.59 7.80 7.82 8.84 6.66 9.96 6.26 7.16

231 241 240 247 238 244 224 215 230

333 321 309 446 456 324 456 334 343

a

The Co3O4 particle size for the corresponding catalyst was estimated from Scherrer’s equation. bThe crystal size of the metallic Co was calculated from d(Co0) = 0.75d(Co3O4). cThe dispersion was calculated from 96/d(Co0).

Figure 7. NH3-TPD profiles of (a) Ca-MMT, (b) MMT-0, (c) MMT15-12/80, (d) MMT-15-3/100, (e) MMT-10-3/80, (f) MMT-5-3/80, (g) MMT-15-3/80, (h) MMT-15-1/80, and (i) MMT-15-3/60.

Figure 8. H2-TPR profiles for 10 wt % Co loaded on (a) Ca-MMT, (b) MMT-0, (c) MMT-5-3/80, (d) MMT-10-3/80, (e) MMT-15-1/ 80, (f) MMT-15-12/80, (g) MMT-15-3/60, (h) MMT-15-3/80, and (i) MMT-15-3/100.

Table 3. Acidic Properties of MMTs sample

total (mmol/g)

weak acid (mmol/g)

strong acid (mmol/g)

Ca-MMT MMT-0 MMT-15-3/60 MMT-15-3/80 MMT-15-3/100 MMT-15-1/80 MMT-15-12/80 MMT-5-3/80 MMT-10-3/80

0.45 0.46 0.68 0.65 0.58 0.60 0.56 0.57 0.65

0.19 0.21 0.24 0.27 0.32 0.24 0.32 0.21 0.24

0.26 0.25 0.44 0.38 0.26 0.36 0.24 0.36 0.41

80, the maximum temperature at peak II for Co/MMT-10-3/80 is increased from 334 to 343 °C. This phenomenon can be explained on the basis of the porous properties and chemical compositions of the activated MMTs. From the results of XRD and the pore size distributions, the mesopores created because of the desilication and dealumination of MMT are increased with increasing the severity of the treatment conditions. Thus, the cobalt species can be easily dispersed into the mesopores on the framework of the MMT layers, leading to the retarded reduction of the cobalt species because of the strong Co− support interactions. Furthermore, it is noteworthy that a small but clear reduction peak at about 825 °C is observed over all of the catalysts. This can be reasonably ascribed to the reduction of the cobalt oxide located in the MMT interlayer, indicating that ion exchange of Co2+ occurred during the impregnation of Co(NO3)2. 3.4. FT Performance. 3.4.1. Activity. From the time-onstream (TOS) catalytic activity results shown in Figure 9, CO conversion reaches steady state after a TOS of about 6 h. The steady CO conversion over Co/Ca-MMT is very low (about 7%). However, the CO conversion over Co/MMT-0 is significantly increased. This observation is very similar to our previous results using the NH4+-exchanged Na-MMT as a support of cobalt for FT synthesis, which has been explained as the negative influence of alkali and alkaline earth cations.20 In the cases of the alkaline-activated MMT-based catalysts, the CO conversion is between those over Co/Ca-MMT and Co/

and temperatures at peak maximum for the reduction of CoO to Co are greatly dependent upon the activation conditions of MMTs (Figure 8 and Table 2). In the case of Co/Ca-MMT, the maximum temperatures for peak I and II are at about 231 and 333 °C, respectively. For Co/MMT-0, the maximum temperature at peak II is decreased to 321 °C. However, for the catalysts based on the alkaline-activated MMTs, the general observations are that the maximum temperatures for peak II are increased and the shape of peak II becomes broader with increasing the severity of the alkaline activation conditions (Figure 8 and Table 2). For Co/MMT15-3/80, Co/MMT-15-3/100, and Co/MMT-15-12/80, peak II is very broad, spanning from 300 to 590 °C with a peak maximum of about 456 °C, while it is decreased to about 309 °C for Co/MMT-15-3/60. In comparison to Co/MMT-5-3/ F

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

the FT test. Thus, the very broad H2-TPR peak with a peak maximum at about 455 °C for Co/MMT-15-12/80 and Co/ MMT-15-3/100 suggests an incomplete reduction of CoO to Co under the reduction conditions applied. This may account for the slightly lower CO conversion over Co/MMT-15-12/80 and Co/MMT-15-3/100. On the contrary, the maximum temperature of peak II for Co/MMT-5-3/80, Co/MMT-15-1/ 80, and Co/MMT-15-3/60 is significantly decreased to about 330 °C (Figure 8 and Table 2), which is clearly lower than that of the reduction temperature of the catalyst. Consequently, the easier reduction of CoO to Co is reasonably expected at the reduction temperature of 400 °C, leading to the slightly higher CO conversion over these catalysts. Thus, the reduction behavior of the catalyst may be the main factor in determining the FT activity over Co/MMT catalysts. 3.4.2. Product Distribution. The selectivity of the grouped hydrocarbons over different catalysts was calculated, and the results are summarized in Table 4. As shown in Table 4, all of the catalysts show a low CO2 selectivity because of the low catalytic activity of cobalt for the water-gas shift reaction. In the case of methane, Co/Ca-MMT gives a lower CH4 selectivity (13%). However, the CH4 selectivity over Co/MMT-0 and Co/alkaline-activated MMT catalysts is apparently increased in comparison to that over Co/Ca-MMT. Generally, the higher CH4 selectivity can be induced by the confinement effect of micropores and the incompletely reduced cobalt species.29,30 Following this general rule, the slightly varied CH4 selectivity over different catalysts can be well-explained when the N2 adsorption isotherms (Figure 3), pore properties (Table 1), and H2-TPR results (Figure 8 and Table 2) are taken into account. For Co/MMT-0, Co/MMT-15-3/60, Co/MMT-15-1/80, Co/ MMT-5-3/80, and Co/MMT-10-3/80, the small pore size of MMTs (about 6.5 nm; Table 1) should be the main reason for their relatively higher CH4 selectivity. Apparently, the similarly high CH4 selectivity over Co/MMT-15-12/80 and Co/MMT15-3/100 is contradictory to the bigger pore size of MMT-1512/80 and MMT-15-3/100 (about 16 nm; Table 1). However, it is reasonable when the incomplete reduction of CoO to Co is taken into account, which has been discussed in section 3.4.1. Moreover, the highest CH4 selectivity of 21.14% over Co/ MMT-15-3/80 can be reasonably ascribed to the combination of the smaller pore size of MMT-15-3/80 (6.4 nm; Table 1) and the incomplete reduction of CoO at 400 °C (Figure 8). In comparison to the results over Co/SiO2,16 all Co/MMT catalysts (Table 4) exhibit a higher selectivity of C5−C12 (36− 42%) and a lower selectivity of C21+ hydrocarbons (15−22%).

Figure 9. TOS CO conversion over 10 wt % Co loaded on MMTderived materials.

MMT-0. Moreover, the CO conversion over Co/MMT-5-3/80, Co/MMT-15-3/80, Co/MMT-15-1/80, and Co/MMT-15-3/ 60 is almost identical and clearly higher than that over Co/ MMT-15-3/100 or Co/MMT-15-12/80. To explain these activity results, the pore size, BET surface area, and pore volume of the catalysts are correlated with the CO conversions and TOFs. As indicated from Figures S1−S3 of the Supporting Information, it is clear that the porous properties of the catalysts are not key factors in determining the FT activity. It is well-established that metallic Co is the active site for the FT reaction. As shown in Table 2, the crystal size of Co over all of the catalysts is very similar, indicating that the crystal size of Co plays a less important role in determining the FT activity over Co/MMT catalysts. This is clearly reflected by correlating the crystal size of Co with the CO conversion over different catalysts (see Figure S4 of the Supporting Information). Moreover, this explanation can be further confirmed by the calculated turnover frequency (TOF) based on XRD results (Table 4). Considering the approximate errors, the TOF is kept nearly constant when Co particles are varied from 9 to 15 nm. This is well-agreeable with the results that the TOF for CO conversion over Co supported on carbon nanofibers is kept almost constant when the Co particles are varied from 6 to 27 nm.28 As discussed in section 3.3 (Figure 8 and Table 2), for the alkaline-activated MMT-based catalysts, the shape and temperature at peak maximum for the reduction of CoO to Co are greatly dependent upon the activation conditions of MMTs. However, all Co/MMT catalysts were reduced at 400 °C before

Table 4. Main Results of the FT Synthesis over Different Catalystsa hydrocarbon distribution (%) −3

−1 b

catalyst

CO conversion (%)

TOF (10 , s )

CO2 selectivity (%)

C1

C2−C4

C5−C12

C13−C20

C21+

Co/Ca-MMTc Co/MMT-0 Co/MMT-15-3/60 Co/MMT-15-3/80 Co/MMT-15-3/100 Co/MMT-15-1/80 Co/MMT-15-12/80 Co/MMT-5-3/80 Co/MMT-10-3/80

7.5 37.6 32.6 32.0 26.8 34.0 24.4 33.0 31.4

10.5 56.6 47.7 46.7 34.6 58.3 28.0 60.2 50.0

0.98 0.77 0.59 0.75 1.14 0.63 0.99 0.23 0.24

13.10 20.04 17.88 21.14 19.67 17.80 16.84 16.24 16.22

23.30 13.94 12.99 14.92 13.82 13.05 13.70 12.89 13.59

39.52 39.30 37.33 37.08 37.71 36.55 35.99 40.95 41.49

4.84 8.48 16.82 10.78 10.17 14.81 14.63 7.57 8.39

19.25 18.25 14.98 16.08 18.63 17.80 18.84 22.35 20.31

Operating conditions: W/F = 5.05 g h mol−1; P, 1 MPa; T = 235 °C; and TOS = 10 h. bTOF was based on the dispersion given in Table 2. cTOS = 7.5 h.

a

G

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

Figure 10. Carbon number distribution of FT hydrocarbons at TOS of 10 h over 10 wt % Co loaded on (a) Ca-MMT, (b) MMT-0, (c) MMT-5-3/ 80, (d) MMT-10-3/80, (e) MMT-15-1/80, (f) MMT-15-12/80, (g) MMT-15-3/60, (h) MMT-15-3/80, and (i) MMT-15-3/100.

Supporting Information) indicates that they should not be the main factor in determining the product selectivity of FT synthesis over the Co/MMT catalysts. On the contrary, as discussed in our previous works,4,18,20 depending upon the acidity of the catalysts, cracking/isomerization reactions of

Moreover, all of the catalysts based on the alkaline-activated MMT show a very similar selectivity of C2−C4 hydrocarbons (12−15%), which is obviously higher than that over Co/SiO2. The correlation between the observed product selectivity and the porous properties of MMTs (see Figures S1−S3 of the H

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

explained as the relatively low reaction temperature and the weak acidity of the MMTs, which has been discussed in our previous work.18

long-chain FT hydrocarbons has occurred at different extents. As shown in Table 3, the amount of acidic sites over MMT-15t/T is decreased in the order of MMT-15-3/60 > MMT-15-3/ 80 > MMT-15-1/80 > MMT-15-3/100 > MMT-15-12/80, which is consistent with the increased order of C21+ selectivity over the corresponding catalyst (Table 4). This changing pattern can be directly observed when the amount of total acidic sites is plotted against the C21+ selectivity over the corresponding catalyst (see Figure S5 of the Supporting Information). Thus, the cracking of the long-chain FT hydrocarbons really occur over the acidic sites, the extent of which is dependent upon the amount of acidic sites. This observation is similar to our previous results, in which the acidactivated Na-MMTs are applied for FT synthesis over cobalt.16 However, the selectivity of C21+ hydrocarbons over the Co/ alkaline-activated MMT catalysts is clearly lower than that over the Co/acid-activated MMT catalysts.16 In this case, the alkaline activation is more effective for tailoring the acidic properties of MMT, such that the FT synthesis over a Coloaded catalyst is promising for selectively synthesizing liquid hydrocarbons. To obtain more information on the product composition, detailed carbon number distributions of C2−C12 hydrocarbons at steady state were calculated, and the results are given in Figure 10. In comparison to Co/SiO2,16 a significantly increased selectivity of C2−C9 hydrocarbons is observed over all of the Co/alkaline-activated MMT catalysts, indicating that cracking reactions obviously occur. However, changes in the selectivity of olefins and normal paraffins are clearly dependent upon the alkaline activation conditions of the MMTs. In the cases of Co/MMT-5-3/80, Co/MMT-15-1/80, and Co/MMT15-3/60, the selectivity of C4−C8 olefins and normal paraffins presents a symmetric increase, indicating that the main reaction is the catalytic cracking of long-chain FT hydrocarbons over acidic sites. On the contrary, over Co/MMT-10-3/80, Co/ MMT-15-3/80, Co/MMT-15-12/80, and Co/MMT-15-3/100, the selectivity of C5−C12 normal paraffins is sharply increased, while the selectivity of the corresponding olefins is only slightly varied. This indicates that the hydrocracking of the long-chain FT hydrocarbons overwhelmingly occurred, although the catalytic cracking cannot be totally ruled out. As reported in many works, the extents of cracking/isomerization reactions of long-chain FT hydrocarbons are closely dependent upon the acidity of the catalysts.7,31 Moreover, following the spillover mechanism of hydrocracking reactions,32 the distance between acidic sites and Co over the catalyst plays a key role in determining the extent of hydrocracking/hydroisomerization reactions. In accordance with this explanation, the specific product distribution of the FT reaction over different catalysts is understandable by taking into account the CO conversion and the acidic and structural properties of the catalysts. As discussed in section 3.1, the relatively rich mesopores and the partial destruction of the MMT layer for Co/MMT-15-3/80, Co/MMT-15-12/80, and Co/MMT-15-3/100 may facilitate the hydrogen spillover from Co to acidic sites, leading to the dominant hydrocracking reactions. In the cases of Co/MMT-0, Co/MMT-5-3/80, Co/MMT-15-3/60, and Co/MMT-15-1/ 80, hydrogen spillover may be impeded by the well-preserved MMT interlayer. Thus, cracking reactions of the long-chain FT hydrocarbons are dominant, which is agreeable with our previous results.16 When the selectivity of isoparaffins is concerned, it is very low and almost independent of the specific catalyst, as shown in Figure 10. This can be well-

4. CONCLUSION The structural, textural, and acidic properties of Ca-MMT activated with a KOH solution under different conditions were quantitatively investigated, and the materials were evaluated for FT synthesis over cobalt for the first time. On the basis of the results and discussion, the main conclusions are summarized as follows: (1) Under the alkaline activation conditions investigated, i.e., T = 60−100 °C, t = 1−12 h, and KOH concentration = 5−15 mol/L, the layered structure of CaMMT is not fully destroyed, although desilication and dealumination obviously occur. However, the partial destruction of the layered MMT structure is evident under severer alkaline activation conditions. Moreover, a part of the leached silica and alumina during the alkaline activation is redeposited over the MMT under the conditions of strong alkaline and high temperature. With increasing the severity of the alkaline activation conditions, the pore volume of the alkaline-activated MMTs is monotonically increased and the pore size distribution becomes broader. (2) The acidity of Ca-MMT is low. However, the amount of both weak and strong acidic sites is significantly increased over the alkaline-activated MMT, the extent of which is strongly dependent upon the alkaline activation conditions. Over the MMTs activated under relatively mild conditions (MMT-5-3/80, MMT-10-3/80, MMT-15-3/60, MMT-15-1/80, and MMT-15-3/80), substantial amounts of strong acidic sites are formed. However, the MMTs activated under severer conditions (MMT-15-12/80 and MMT-15-3/100) have few strong acidic sites. The maximum amounts of strong and weak acidic sites are observed over MMT-15-3/60 and MMT-15-3/100, respectively. (3) Although Co/Ca-MMT is an inefficient catalyst for FT synthesis, a significantly increased FT activity is achieved over the catalysts based on the alkaline-activated MMTs. The CO conversion is decreased in the order of Co/MMT-0 > Co/ MMT-15-1/80 ≈ Co/MMT-5-3/80 ≈ Co/MMT-15-3/60 ≈ Co/MMT-15-3/80 ≈ Co/MMT-10-3/80 > Co/MMT-15-3/ 100 > Co/MMT-15-12/80 > Co/Ca-MMT. This is mainly caused from the impact of the composition and pore properties of the alkaline-activated MMT on the reduction of cobalt species over the catalysts. (4) As a result of the cracking of the long-chain FT hydrocarbons, the alkaline-activated MMT catalysts show 36−42% selectivity of C5−C12 hydrocarbons, 8−17% selectivity of C13−C20 hydrocarbons, and 15−22% selectivity of C21+ hydrocarbons. Moreover, hydrocracking reactions are dominant over Co/MMT-15-3/80, Co/MMT-1512/80, and Co/MMT-15-3/100, while the catalytic cracking mainly occur over Co/MMT-0, Co/MMT-5-3/80, Co/MMT15-3/60, and Co/MMT-15-1/80. Because of the relatively low reaction temperature and weak acidity of the MMTs, the selectivity of isoparaffins is very low over all of the catalysts.

■

ASSOCIATED CONTENT

S Supporting Information *

Correlation between the FT performance and the physical/ chemical properties of the Co/alkaline-activated MMT catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. I

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

■

Article

(24) Madejova, J.; Bujdak, J.; Janek, M.; Komadel, P. Spectrochim. Acta, Part A 1998, 54, 1397−1406. (25) Sevim, A.; Tanil, A. J. Mol. Struct. 2004, 705, 147−151. (26) Flessner, U.; Jones, D. J.; Rozière, J.; Zajac, J.; Storaro, L.; Lenarda, M.; Pavanc, M.; Jiménez, L. A.; Rodrıguez, C. E.; Trombetta, M.; Busca, G. J. Mol. Catal. A: Chem. 2001, 68, 247−256. (27) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603−619. (28) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; Dillen, A. J.; Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956−3964. (29) Xiong, H.; Zhang, Y.; Liew, K.; Li, J. J. Mol. Catal. A: Chem. 2005, 231, 145−151. (30) Liu, Y.; Fang, K.; Chen, J.; Sun, Y. Green Chem. 2007, 9, 611− 615. (31) Bao, J.; He, J.; Zhang, Y.; Yoneyama, Y.; Tsubaki, N. Angew. Chem., Int. Ed. 2008, 47, 353−356. (32) Kusakari, K.; Tomishige, K.; Fujimoto, K. Appl. Catal., A 2002, 224, 219−228.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-29-8153-0801. Fax: +86-29-8153-0727. Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support by the National Natural Science Foundation of China (20876095), the State Key Laboratory of Coal Conversion (J12-13-606), the Fundamental Research Funds for the Central Universities (GK201002043 and GK201305012), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1070) is highly appreciated. Yong-Hua Zhao thanks the Foundation for the Excellent Doctoral Dissertation of Shaanxi Normal University (X2011YB06).

■

REFERENCES

(1) Dry, M. E. Catal. Today 2002, 71, 227−241. (2) Zhang, Q.; Kang, J.; Wang, Y. ChemCatChem 2010, 2, 1030− 1058. (3) Dry, M. E. Appl. Catal., A 2004, 276, 1−3. (4) Liu, Z.-W.; Li, X.; Asami, K.; Fujimoto, K. Energy Fuels 2005, 19, 1790−1794. (5) Liu, Z.-W.; Li, X.; Asami, K.; Fujimoto, K. Appl. Catal., A 2006, 300, 162−169. (6) Li, X.; Luo, M.; Asami, K. Catal. Today 2004, 89, 439−446. (7) Liu, Z.-W.; Li, X.; Asami, K.; Fujimoto, K. Catal. Today 2005, 104, 41−47. (8) Tsubaki, N.; Yoneyama, Y.; Michiki, K.; Fujimoto, K. Catal. Commun. 2003, 4, 108−111. (9) Liu, Z.-W.; Li, X.; Asami, K.; Fujimoto, K. Ind. Eng. Chem. Res. 2005, 44, 7329−7336. (10) Koh, D. J.; Chung, J. S.; Kim, Y. G. Ind. Eng. Chem. Res. 1995, 34, 1969−1975. (11) Cheng, K.; Kang, J.; Huang, S.; You, Z.; Zhang, Q.; Ding, J.; Hua, W.; Lou, Y.; Deng, W.; Wang, Y. ACS Catal. 2012, 2, 441−449. (12) Kang, J.; Cheng, K.; Zhang, L.; Zhang, Q.; Ding, J.; Hua, W.; Lou, Y.; Zhai, Q.; Wang, Y. Angew. Chem., Int. Ed. 2011, 50, 5200− 5203. (13) Wei, X. T.; Smirniotis, P. G. Microporous Mesoporous Mater. 2006, 97, 97−106. (14) Ogura, M.; Shinomiya, S.; Tateno, J.; Nara, Y.; Nomura, M.; Kikuchi, E.; Matsukata, M. Appl. Catal., A 2001, 219, 33−43. (15) Su, H.; Zeng, S.; Dong, H.; Du, Y.; Zhang, Y.; Hu, R. Appl. Clay Sci. 2009, 46, 325−329. (16) Hao, Q.-Q.; Wang, G.-W.; Liu, Z.-T.; Liu, Z.-W. Nanocatalysis for Fuels and Chemicals; Dalai, A. K., Ed.; American Chemical Society (ACS): Washington, D.C., 2012; ACS Symposium Series, Vol. 1092, Chapter 11, pp 167−193. (17) Hao, Q.-Q.; Wang, G.-W.; Zhao, Y.-H.; Liu, Z.-T.; Liu, Z.-W. Fuel 2013, 109, 33−42. (18) Hao, Q.-Q.; Wang, G.-W.; Liu, Z.-T.; Lu, J.; Liu, Z.-W. Ind. Eng. Chem. Res. 2010, 49, 9004−9011. (19) Hao, Q.-Q.; Liu, Z.-W.; Zhang, B.; Wang, G.-W.; Ma, C.; Frandsen, W.; Li, J.; Liu, Z.-T.; Hao, Z.; Su, D. S. Chem. Mater. 2012, 24, 972−974. (20) Wang, G.-W.; Hao, Q.-Q.; Liu, Z.-T.; Liu, Z.-W. Appl. Catal., A 2011, 405, 45−54. (21) Wang, Y.; Lin, M.; Tuel, A. Microporous Mesoporous Mater. 2007, 102, 80−85. (22) Breen, C.; Madejova, J.; Komadel, P. Appl. Clay Sci. 1995, 10, 219−230. (23) Tyagi, B.; Chudasama, C. D.; Jasra, R. V. Spectrochim. Acta, Part A 2006, 64, 273−278. J

dx.doi.org/10.1021/ef401291q | Energy Fuels XXXX, XXX, XXX−XXX