carbon catalysts using rice grains as an exotemplate and carbon precursor

Applied Catalysis A: General 460–461 (2013) 26–35

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Preparation of shaped magnesium oxide/carbon catalysts using rice grains as an exotemplate and carbon precursor Li Ma, Xiaoyue Zhang, Dan Lin, Yuan Chun ∗ , Qinhua Xu Key Laboratory of Mesoscopic Chemistry of the Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China

a r t i c l e

i n f o

Article history: Received 12 January 2013 Received in revised form 8 April 2013 Accepted 11 April 2013 Available online 25 April 2013 Keywords: Magnesium oxide Rice grains In situ transformation Exotemplate Black carbon

a b s t r a c t Biomorphic magnesium oxide and magnesium oxide/carbon catalysts have been synthesized via an in situ transformation technique using rice grains as a template and carbon precursor. During preparation, MgO precursors are dispersed on the surface of rice grains with the aid of the rice-cooking process, and further transformed into solid bases upon calcination. Biomorphic MgO/carbon materials are formed when calcination is performed in a nitrogen atmosphere. These particles are spindle shaped catalysts with a circumference of several millimeters. Pyrolysis at higher temperature aids in the formation of porous structures, and the resulting MgO/carbon materials display high specific surface area (>260 m2 /g) and more strongly basic sites. The presence of black carbon affects the catalytic behavior of MgO in the methylation of cyclopentadiene. These shaped MgO/carbon materials exhibit much higher catalytic performance than MgO at lower reaction temperatures. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Solid strong bases are extremely desirable for developing environmentally benign processes to catalyze various reactions under mild conditions while minimizing the production of pollutants [1]. A conventional solid strong base, MgO is a widely used catalyst [2–11], catalyst support [12–15] and adsorbent [16–18]. Commercial MgO is usually produced by thermal decomposition of magnesium hydroxide or carbonate. However, MgO prepared by this method has a small specific surface area, and inhomogeneous morphology and grain size, which are disadvantageous for these applications [14]. Thus, it is very desirable to develop alternative methods to produce abundantly porous MgO with high specific surface area in a controllable manner [13]. The sol–gel technique has been developed to prepare nanoscale MgO powders, and the specific surface area of the resulting MgO can reach 650 m2 /g using a modified autoclave hypercritical drying procedure [15–18]. The chemical vapor deposition approach can generate high-specificsurface-area MgO with considerably reduced surface heterogeneity [19,20]. Nanoporous MgO film prepared by reactive ballistic deposition displays specific surface areas as high as 1000 m2 /g [21,22]. Nevertheless, these approaches are too expensive for conventional catalytic applications. Exotemplating pathways are thus attracting considerable attention due to their simplicity [13]. Several porous

∗ Corresponding author. Tel.: +86 25 83686501; fax: +86 25 83317761. E-mail address: [email protected] (Y. Chun). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.04.018

materials, such as porous silica [23], carbon [24–26], filter paper [27], and diatoms [28] have been used as exotemplates to synthesize nanoporous MgO with higher specific surface area (decades to 201 m2 /g) and wide pore size distributions (PSDs). The resulting materials serve as negative replicas of exotemplates. The exotemplate pathway can even be used to prepare ordered mesoporous MgO materials with surface areas around 300 m2 /g. To achieve this aim, mesoporous SBA-15 silica and CMK-3 carbon are successively used as hard structure matrixes via a double replication procedure [29,30]. In our previous work, natural plant materials such as cotton fiber and pine-wood were used as exotemplates to prepare biomorphic MgO particles [31,32]. These materials possessed a nanocrystalline assembled mesoporous structure and exhibited high specific surface area and high basic catalytic performance in the decomposition of isopropanol. Compared with man-made template materials, the natural plant materials are cheap and diverse, with structural and compositional hierarchical order [33]. Thus, they have been widely used for producing biomorphic carbides, oxide ceramics [33–36] and high-specific-surface-area oxides [37–40]. Due to the desire to disperse the precursor, the exotemplates are usually chosen from the porous materials containing voids, e.g. silica and wood materials. However, porous plant materials are somewhat limited, and it is thus worth developing a new strategy to expand exotemplating to nonporous plant materials, e.g. plant seeds. Commercial MgO is a powdery solid. The fine powder must be shaped into granules, spheres or extrudates prior to use as catalysts in practical processes [41]. The exotemplate pathway has the

L. Ma et al. / Applied Catalysis A: General 460–461 (2013) 26–35

advantages of duplicating the templates, which makes it possible to directly synthesize shaped catalyst with a suitable exotemplate. Rice grains, which are spindly particles with a circumference of several millimeters, might be a good choice. However, rice grains are nonporous and cannot be used as conventional exotemplates. Fortunately, rice can be cooked in boiling water, and a great deal of water is absorbed during this cooking process, providing a route to dispersing precursor into the rice grains. In this paper, we described a novel in situ transformation to prepare biomorphic MgO and MgO/carbon materials based on this rice-cooking process. We found that the resulting shaped solid bases exhibited good basic catalytic performance.

2. Experimental 2.1. Synthesis Commercial MgO (98%) with a specific surface area of 7 m2 /g was purchased from ACROS Corp. Biomorphic MgO and MgO/Carbon materials were prepared using magnesium acetate tetrahydrate (Mg(Ac)2 ·4H2 O, SCRC, 99%) as MgO precursor and rice grains purchased from a supermarket in Nanjing, China, as exotemplate and carbon precursor. Typically, a certain amount of Mg(Ac)2 ·4H2 O was dissolved into 10 mL of distilled water in a 100-mL beaker, and 20 g of rice grains, pre-washed with distilled water six times, were added to the solution and covered with a watch glass. The mixture was boiled until the solution was almost completely absorbed by the rice, usually ca. 20 min. The resulting cooked rice-like mixture was dried at ambient temperature for 12 h and then calcined at 773 K in air or N2 (80 mL/min) for 3 h. The resulting materials were denoted MgO(O)-x and MgO(C)-x for air and N2 , respectively, where x indicates the mass ratio of Mg(Ac)2 ·4H2 O and rice grains. A portion of the MgO(C)-x samples were calcined at 973 K in N2 (80 mL/min), and denoted MgO(C)-x-973.

27

For comparison, the same cooking process was used to prepare reference carbon (C) from rice grains. A portion of the reference carbon samples was used to impregnate magnesium acetate tetrahydrate. After calcination at 973 K in N2 (80 mL/min), the resulting sample was denoted MgO/C(I)-x-973, where I and x indicate the impregnation method and the mass ratio of Mg(Ac)2 ·4H2 O and rice grains, respectively.

2.2. Characterization X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku, D/max-RA diffractometer with Cu K␣ radiation in the 2Â range from 5◦ to 60◦ . N2 adsorption and desorption isotherms were measured using a Micrometrics ASAP 2020 system at 77 K. Samples were evacuated at 573 K for 4 h prior to testing. Specific surface area was determined using the Brunauer–Emmett–Teller (BET) equation with adsorption branches in the relative pressure range from 0.04 to 0.2. Pore size distribution (PSD) was calculated using the Barrett–Joyner–Halenda (BJH) algorithm with desorption branches. The % carbon and hydrogen content of samples was measured on a CHN-O-RARID elemental analyzer. Thermal analysis (thermogravimetry (TG) and differential scanning calorimetry (DSC)) were performed using a NETZSCH STA 449 C thermal analyzer under Ar at a heating ratio of 10 K/min. Scanning electron microscopy (SEM) investigations were carried out on a Hitachi S-4800 field-emission instrument at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were performed using JEOL JEM-2100 transmission electron microscope operating at 200 kV. The samples were dispersed in ethanol and dripped onto holey carbon copper grids. The basicity of samples was determined using temperature-programmed desorption (TPD) of CO2 [42]. For comparison, 100 mg of MgO(C)-0.5 was used for each experiment, and samples were pretreated under three different conditions before the TPD measurement: (1) activating at 773 K in N2 for 1 h; (2) activating at 973 K in N2 for 1 h, (MgO(C)-0.5-973 formed); (3) calcining

Fig. 1. Photographs of (a) raw rice grains, (b) Mg(CH3 COO)2 /cooked rice, (c) MgO(O)-0.5 and (d) MgO(C)-0.5 samples.

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L. Ma et al. / Applied Catalysis A: General 460–461 (2013) 26–35

at 773 K in air for 3 h, (MgO(O)-0.5 formed). The resulting samples were further activated in He (50 mL/min) at 873 K for 1 h. After cooling to room temperature, the blank TPD profiles were measured by increasing temperature to 873 K at 8 K/min. The system was cooled to room temperature again before injecting 40 mL of CO2 . Weakly adsorbed CO2 was removed by flowing He at 30 mL/min for 1 h, and temperature was then increased to 873 K at 8 K/min. The liberated CO2 was detected by an on-line gas chromatograph with a thermal conductivity detector (TCD) and quantitatively measured using the external standard method. The TPD profiles of samples shown in the figure were obtained by subtracting their blank profiles. 2.3. Catalytic testing The vapor-phase methylation of cyclopentadiene (CPD) with methanol was carried out in a conventional flow reactor at atmospheric pressure [42,43]. The catalyst (50 mg) was thermally activated at 873 K for 1 h in N2 and cooled to the reaction temperature. CPD was obtained by thermal depolymerization of dicyclopentadiene (Aldrich) at 873 K, and the reactants CPD and methanol (Nanjing Chemical Reagent, China, ≥99.7%) were introduced using syringe pumps with a molar ratio of 1.8 (space velocity of 2.5 h−1 ) in the flow of N2 (20 mL/min). The reaction was carried out at various temperatures with a reaction time of 10 min. The reaction mixture was analyzed using on-line GC (Varian 3700). The conversion and selectivity in this reaction were defined as Ci = moles of substrate reacted per mole of CPD in the feed, and Si = moles of product i formed per mole of CPD reacted, respectively.

Table 1 Preparation conditions and texture parameters of biomorphic MgO(O) and MgO(C) samples. Sample

Mass loss (%)

MgO content (%)

Crystalline size (nm)

ABET (m2 /g)a

MgO(O)-0.05 MgO(O)-0.125 MgO(O)-0.25 MgO(O)-0.375 MgO(O)-0.5 MgO(C)-0.05 MgO(C)-0.125 MgO(C)-0.25 MgO(C)-0.375 MgO(C)-0.5 MgO(C)-0.25-973 MgO(C)-0.5-973 MgO(C)-1.0-973

98.9 97.5 96.0 94.8 93.7 81.1 81.9 79.9 78.6 78.4 81.3 79.9 81.4

– – – – – 4.7 12 19 24 29 20 31 51

7.7 9.9 8.9 9.0 8.7 – – 3.8 5.6 5.3 4.4 5.5 7.9

81 104 106 142 166 5 5 14 28 56 288 263 297

a

Specific surface area calculated using BET theory.

distinguished from the rice-like particles in sample MgO(C)-1.0. The data in Table 1 also indicate that the pyrolysis temperature has an effect on mass loss. Compared with MgO(C)-0.5, MgO(C)-0.5973 has higher MgO content, indicating that carbonaceous species formed at 773 K can be further carbonized at 973 K. Nevertheless, this mass change is not pronounced.

3. Results and discussion

MgO(O)-0.50

3.1. In situ preparation of biomorphic MgO(O) and MgO(C)

MgO(O)-0.375

Rice grains have a spindly appearance, with a ca. 2-mm diameter and ca. 5-mm length that varies with growing location and variety. Raw rice grains absorb water poorly, and the magnesium acetate is not well dispersed on the surface. However, raw rice grains absorb water during the hydrothermal treatment, and magnesium acetate is well dispersed on this swollen cooked rice. White MgO(O) and black MgO(C) particles were obtained by calcining these magnesium acetate/cooked rice grains in air and nitrogen, respectively and are shown in Fig. 1. Calcination in air or N2 can generate rice-like spindly particles. However, the choice of atmosphere affects mechanical strength. The MgO(O) particles prepared in air atmosphere are somewhat fragile, while the MgO(C) particles prepared in N2 exhibit considerable mechanical strength due to the existence of carbon matrix, and can be directly used as catalysts. It should be pointed out that it is difficult to disperse magnesium acetate into cooked rice grains. If the raw rice grains are cooked before adding magnesium acetate, the resulting sample is a mixture of spindly carbon and powdery MgO particles. Therefore, in situ formation of magnesium acetate/cooked rice particles during hydrothermal treatment, i.e. the cooking process, is a key step to generating these biomorphic materials. Table 1 summarizes the preparation conditions. The mass losses for MgO(O) and MgO(C) are significantly different; the former is higher than 93%, while the latter is no more than 82%. MgO(C) samples are dark, indicating the existence of carbonaceous species, which are derived from the pyrolysis of rice grains in N2 . Calculation based on mass loss during preparation indicates that MgO content in MgO(C) samples varies from 4.7% to 51%, according to the dosage ratio of tetrahydrate magnesium acetate and rice. However, it is difficult to disperse all the magnesium salt when the dosage ratio exceeds 0.5, and thus some inhomogeneous particles can be

A

* Periclase MgO

*

*

*

MgO(O)-0.250 MgO(O)-0.125 MgO(O)-0.050 20

30

40

50

60

70

2θ ( ) o

* Periclase MgO

*

B

* MgO(C)-1.0-973

*

MgO(C)-0.5-973 MgO(C)-0.25-973 MgO(C)-0.5 MgO(C)-0.375 MgO(C)-0.25 MgO(C)-0.125 MgO(C)-0.05 20

30

40

50

60

70

2θ ( o ) Fig. 2. XRD patterns of biomorphic (A) MgO(O) and (B) MgO(C) samples.

L. Ma et al. / Applied Catalysis A: General 460–461 (2013) 26–35 160

200

150

100

50

0 0.0

0.2

MgO(C)-0.25-973 MgO(C)-0.5-973 MgO(C)-1-973 MgO(C)-0.5

A

MgO(O)-0.05 MgO(O)-0.125 MgO(O)-0.25 MgO(O)-0.375

Amount adsorbed (cm3/g STP)

Amount adsorbed (cm 3/g STP)

29

0.4

0.6

0.8

120

80

40

0 0.0

1.0

0.2

0.4

0.6

0.8

1.0

p/ps

Relative Pressure 0.3

0.5

MgO(O)-0.05 MgO(O)-0.125 MgO(O)-0.25 MgO(O)-0.375

MgO(C)-0.25-973 MgO(C)-0.5-973 MgO(C)-1-973 MgO(C)-0.5

B dV/dlog(W) (cm3/g)

0.4

dV/dlog(W) (cm 3/g)

C

0.3

0.2

D

0.2

0.1

0.1

0.0

0.0 1

10

100

1

10

100

Pore diameter (nm)

Pore diameter (nm)

Fig. 3. (A) Nitrogen adsorption–desorption isotherms at 77 K and (B) pore-size distributions on biomorphic MgO(O) samples; (C) nitrogen adsorption isotherms at 77 K and (D) pore-size distributions on biomorphic MgO(C) samples.

100 0.0 80 -0.1 60 -0.2 40 -0.3

DSC (mW/mg)

Fig. 2 shows wide-angle XRD patterns for the prepared MgO(O) and MgO(C) samples. As shown in Fig. 2(A), all MgO(O) peaks can be readily assigned to a pure phase of periclase MgO (JCPDS card number 89-4248), indicating that magnesium acetate was converted into homogeneous MgO with a rock salt structure [44]. Calculations using the Scherrer equation indicate these MgO(O) particles have an average size between 7.7 and 9.9 nm (as shown in Table 1), much smaller than MgO particles (18 nm) prepared by direct calcination of magnesium acetate[32]. These MgO(O) particles are also smaller than the mesoporous MgO derived from cotton fiber [32] or mesoporous carbon templates [13]. Therefore, as an exotemplate, rice grains have the advantage of controlling MgO particle size. As shown in Fig. 2(B), MgO particles can be in situ-dispersed on amorphous carbonaceous materials derived from rice grains, with a threshold value of ca. 12% (sample MgO(C)-0.125), similar to the dispersion of MgO on porous carbon by impregnation [11]. The phase of periclase MgO is formed at a dosage ratio of magnesium acetate/rice grain over 0.125. Nevertheless, the particle size of this MgO is apparently smaller than on MgO(O) prepared at the same dosage ratio of magnesium acetate/rice grain, due to dispersion on the carbonaceous materials. Fig. 3 shows the nitrogen adsorption-desorption isotherms and pore size distributions of these biomorphic MgO(O) and MgO(C) samples. The adsorption isotherms of prepared MgO(O) samples exhibit type II characteristics (Fig. 3(A)). However, apparent hysteresis loops can be distinguished on the adsorption–desorption isotherms of these MgO(O) samples, indicating the existence of

mesoporous structures [45]. As shown in Fig. 3(B), these samples have a bimodal mesoporous structure, with one narrow smaller mesopore centered at 3.5 nm and another broad larger mesopore centered in the 5–60 nm region, which is most likely caused by a network effect [45]. This kind of PSD is different from that of MgO materials prepared with a mesoporous carbon aerogel as a template, which presents only one broad PSD [13]. The pore size for the broad mesopore is enhanced as rice grain dosage increases, indicating a template effect of the rice grain. As shown in Fig. 3(C), MgO(C) samples show adsorption isotherms more characteristic

Mass fraction (%)

3.2. Characterizations of biomorphic MgO(C) and MgO(O)

20 -0.4 0 300

400

500

600

700

800

900

1000

Temperature (K) Fig. 4. TG-DSC curves of oven-dried Mg(CH3 COO)2 ·4H2 O/cooked rice grain, weight ratio 0.5.

30

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Fig. 5. SEM images of biomorphic MgO(O), MgO(C) and black carbon.

of type I, indicating that microporous structures predominate in these samples. This kind of adsorption isotherm mainly represents the adsorption characteristics of the carbonaceous species. On the other hand, a mesopore centered at 4 nm can be distinguished from

the PSD curve of these MgO(C) samples (Fig. 3(D)), and should be ascribed to MgO. The BET specific surface areas of MgO(O) and MgO(C) samples are listed in Table 1, as calculated from their adsorption

L. Ma et al. / Applied Catalysis A: General 460–461 (2013) 26–35

31

Fig. 6. TEM images of biomorphic C-973 (a), MgO(C)-0.05-973 (b) and MgO(C)-0.5-973 (c and d); HR-TEM images of biomorphic C-973 (e) and MgO(C)-0.5-973 (f).

isotherms. Generally, the BET specific surface area increases almost monotonously with the dosage of MgO precursor. Calcination temperature has a dramatic effect on the textural parameters of MgO(C). MgO(C) samples prepared at 773 K show a specific surface area lower than 60 m2 /g, while MgO(C) samples prepared at 973 K exhibit a specific surface area higher than 260 m2 /g. Elemental analysis gives a molar H/C ratio of 0.75 for MgO(C)-0.5 (data not shown), much higher than for active carbon [46]. This indicates that MgO(C) samples contain a certain amount of organic residue derived from incomplete carbonization of the cooked rice. These organic residues may block pores, decreasing the specific surface area. The H/C ratio decreases to 0.46 with increasing calcination temperature from 773 to 973 K, indicating the further carbonization of the chars [46]. Accordingly, the specific surface area improves dramatically. Table 1 also indicates that MgO(O) samples exhibit a specific surface area higher than MgO(C) prepared at 773 K. This can be ascribed to the removal of carbonaceous species by O2 . These MgO samples show higher specific surface areas than MgO prepared by direct decomposition of magnesium acetate, which has a specific surface area of 73 m2 /g [32]. This demonstrates that rice grains have a template effect on the formation of porous MgO, similar to other plant templates such as dry adsorbent cotton [32]. Fig. 4 depicts the TG-DSC curve for oven-dried magnesium acetate/cooked rice grains. Ca 20% of mass loss is observed when temperature increases from 300 to 473 K under Ar atmosphere, and an endothermic peak appears on DSC curve simultaneously. Thus, the mass loss mainly derives from the removal of water. When

temperatures exceed 523 K, the mass fraction decreases dramatically, and only 24% of the solid remains when temperatures reach 773 K. Two endothermic peaks can be distinguished on the DSC curve at 547 K and 652 K, indicating pyrolysis of rice grain and Mg(CH3 COO)·4H2 O in this temperature range [31,47]. The mass fraction decreases smoothly and slowly to 20% as temperature increases to 973 K, indicating further carbonization, as the organic species on the magnesium acetate/cooked rice grains are converted to black carbon under these calcination conditions. Fig. 5 shows SEM images of biomorphic MgO(C) and MgO(O) samples. Due to the small size of individual carbon particles, it is difficult to identify black carbon particle features, which were prepared by pyrolysis of rice grains at 773 K. Black carbon features can still be distinguished from the SEM images of MgO(C). However, many gibbous inserted particles cover the surfaces, especially those with higher MgO content. Sea-urchin-like particles cover the surfaces of MgO(C)-1 sample. Similar particles have been observed on MgO(C)-0.5(not shown in Figures), but not on samples containing less MgO; these sea-urchin-like particles should be ascribed to surface aggregation of excess MgO. A comparison of the SEM images of MgO(C)-0.5 and MgO(C)-0.5-973 indicates that calcination at 973 K generates larger pores in the MgO(C) sample, derived from the further carbonization of chars. However, due to the limitation of SEM measurement, we can barely distinguish the change in micropores in this sample that is responsible for the increase in specific surface area, as indicated in Table 1. MgO(O) samples exhibit morphologies similar to MgO(C), due to the template effect of the rice grains.

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20

Conversion of CPD (%)

a b

c

15

A

MgO(O)-0.05 MgO(O)-0.125 MgO(O)-0.25 MgO(O)-0.375 MgO(O)-0.5 Commercial MgO

10

5

d 0 650 300

400

500

600

700

800

750

800

Temperature (K)

Temperature (K)

60

Selectivity for MCPD (%)

Fig. 7. TPD profiles of CO2 on (a) MgO(C)-0.5-973, (b)MgO(C)-0.5, (c) MgO(O)-0.5 and (d) C.

Nevertheless macropores can be observed between the irregular particles in the MgO(O) samples, especially for those with a lower dosage ratio of magnesium acetate/rice grain. These macropores derived from the removal of the rice template. TEM images of biomorphic MgO(C) and C are presented in Fig. 6. The black carbon derived from rice grains exhibits a relatively smooth surface (Fig. 6(a)), in agreement with its SEM image. The morphology of MgO(C)-0.05-973 is similar to that of C-973. It is difficult to distinguish the MgO particles from the carbon matrix, indicating good dispersion of MgO at this dosage ratio of MgO precursor and rice grain and in agreement with the XRD results. Contrarily, MgO particles inserted into the carbon matrix (Fig. 6 (c)) or covering the sample surfaces (Fig. 6(d)), can be observed throughout the MgO(C)-0.5-973 sample. The HRTEM image (Fig. 6(f)) reveals lattice fringes with a periodic spacing of 0.21 nm corresponding to the (200) plane of these MgO particles. These lattice fringes cannot be distinguished from the HRTEM image of C-973 (Fig. 6(e)) and MgO(C)-0.05-973 (not given in Figures). In addition, the HRTEM image of MgO(C)-0.5-973 indicates that the particle size of MgO is around 5–6 nm, in good agreement with the 5.5 nm value provided by XRD (Table 1). The TPD profiles of CO2 pre-adsorbed at room temperature on selected samples are presented in Fig. 7. The TPD profile of black carbon presents only one weak peak around 360 K, indicating that the basicity of the carbon derived from the pyrolysis of cooked rice is very weak, coincident with their relatively inert surfaces. On the other hand, three desorption peaks with peak temperature around 370, 460 and 550 K were deconvoluted in the TPD profile for MgO(C)-0.5 and are attributed to bicarbonates formed on Brønsted OH groups, bidentate carbonates desorbed from metal–oxygen pairs and unidentate carbonates released from low coordination oxygen anions, respectively [1,48]. We found that the desorption peak around 370 K was higher than others when activated at 873 K, demonstrating that the basicity of MgO(C)-0.5 is mostly due to bicarbonates formed on Brønsted OH groups. The relative contribution of the peak at 370 K decreased when activated at 973 K, and the peaks at higher temperatures were strengthened. Therefore, higher activation temperature aids the formation of strongly basic sites. The weaker basicity of Mg(C)-0.5 activated at 873 K can be ascribed to organic residues that cover the surface of MgO and reduce its basicity. As discussed above, these organic residues can be further carbonized at 973 K, exposing more “clean” surfaces to give stronger basicity. It is important to note that CO2 adsorption decreases dramatically when the carbonaceous species on

700

40

MgO(O)-0.05 MgO(O)-0.125 MgO(O)-0.25 MgO(O)-0.375 MgO(O)-0.5 Commercial MgO

B

20

0 650

700

750

800

Temperature (K) Fig. 8. (A) Conversion of CPD and (B) selectivity for MCPD with methanol/CPD ratio of 1.8 (space velocity = 2.5 h−1 ) as functions of temperature on biomorphic MgO(O) samples (p = 101 kPa).

MgO(C)-0.5-973 are removed by calcination at 873 K in air, indicating that the MgO in the MgO(C)-0.5-973 sample is more strongly basic than pure MgO, due to the template and dispersion effects of black carbon. 3.3. Catalytic behaviors of biomorphic MgO(C) and MgO(O) The catalytic behavior of the prepared MgO(O) and MgO(C) samples in methylation of CPD with methanol is shown in Figs. 8–10. The target products of this reaction are methylcyclopentadienes (MCPD) formed via co-operative action of acid/base pairs or strongly basic sites [42]. The by-products consist mainly of dimethylcyclopentadienes (DMCPD) and cyclopentene, which derive from the deep methylation of MCPD and hydrogenation of CPD, respectively. Some light compounds, e.g. CO, CH4 , CO2 and H2 , can also be produced through decomposition of methanol on basic sites. Fig. 8 depicts the catalytic behavior of commercial MgO and biomorphic MgO(O) samples in the methylation of CPD with methanol. Commercial MgO shows poor conversion of CPD and selectivity for MCPD from 673 to 773 K, which should be ascribed to its small specific surface area (7 m2 /g). The biomorphic MgO(O) samples exhibit catalytic performance that is clearly superior to commercial MgO, especially at higher temperatures. The dosage of the rice template has an apparent effect on the conversion of CPD. Catalytic performance on MgO(O) samples increases in an nearly proportional fashion with the mass ratio of Mg(Ac)2 ·4H2 O and rice

L. Ma et al. / Applied Catalysis A: General 460–461 (2013) 26–35

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20

12

8

4

0 650

700

750

30

20

10

0 650

800

B 80

60

40

20

0 650

750

800

100

MgO(C)-0.5 MgO(C)-0.5-973 MgO(O)-0.5 Commercial MgO C

Selectivity for MCPD (%)

Selectivity for MCPD (%)

80

700

Temperature (K)

Temperature (K) 100

C

MgO(C)-0.5-973 MgO(C)-0.5-973(0.05g MgO) MgO(C)-0.5-973(O2) MgO/C(I)-0.5-973

40

Conversion of CPD (%)

Conversion of CPD (%)

16

A

MgO(C)-0.5 MgO(C)-0.5-973 MgO(O)-0.5 Commercial MgO C

700

750

800

Temperature (K)

MgO(C)-0.5-973 MgO(C)-0.5-973(0.05gMgO) MgO(C)-0.5-973(O2) MgO/C(I)-0.5-973

D

60

40

20

0 650

700

750

800

Temperature (K)

Fig. 9. (A and C) Conversion of CPD and (B and D) selectivity for MCPD with methanol/CPD ratio of 1.8 (space velocity = 2.5 h−1 ) as functions of temperature on various catalysts (p = 101 kPa). MgO(C)-0.5-973 (0.05 g MgO) was measured based on catalyst with 0.05 g MgO.

grains, which is coincident with the increase in specific surface area (Table 1). Thus, the change in specific surface area should be responsible for the differing activity of these MgO(O) samples. As shown in Fig. 9, black carbon prepared from the pyrolysis of cooked rice is inert to MCPD formation, coincident with its weak basicity. This is evident in the CO2 -TPD result. When activated at 873 K, the biomorphic MgO(C)-0.5 shows conversion of CPD (e.g. 4.8%, at a reaction temperature of 723 K) and higher selectivity for MCPD (e.g. 59% at 723 K). The catalytic activity of MgO(C)-0.5 increases greatly when the activation temperature reaches 973 K, and the resulting MgO(C)-0.5-973 Exhibits 10% conversion of CPD and 62% selectivity for MCPD at 623 K. This dramatic change can be ascribed to the huge increase in specific surface area (from 56 to 263 cm2 /g) and the enhancement of basicity upon this treatment at 973 K. The catalytic result on MgO(C)-0.5-973 is much better than on MgO(O)-0.5 and commercial MgO, indicating that the presence of black carbon can improve catalytic performance of MgO. It should be pointed out that MgO content on MgO(C)-0.5 is only 31%. The other 69% is black carbon, which is inactive in the reaction. If comparing the catalytic performance based on the same MgO mass, conversion of CPD at 723 K is 30% and 3.3% on MgO(C)-0.5-973 and MgO(O)-0.5, respectively. Viewed this way, catalytic enhancement of the black carbon is more pronounced. It is well known that the carbon deposits deactivate solid base catalysts. However, in the current study, the presence of black carbon on MgO is advantageous for CPD methylation. As shown in Fig. 9(C), catalytic performance decreases dramatically if

MgO(C)-0.5-973 is activated at 973 K in O2 to remove carbon, which is apparently different from the effect of calcination in O2 on spent catalyst to remove coke deposits [42]. This indicates that the black carbon derived from rice grains plays an important role in the catalytic behavior of MgO(C)-0.5-973. The CO2 -TPD result indicates that MgO(C)-0.5-973 has more strongly basic sites than MgO(O)0.5, and thus more active sites on the surface of MgO(C)-0.5-973. Since the black carbon is inactive in this reaction, the promotion of activity should not be attributed to active sites on black carbon. More possibly, it derives from a dispersion effect from the black carbon. As indicated in Table 1, the particle size of MgO(C) samples is smaller than that of MgO(O) samples; more basic sites are exposed on the surfaces of these particles. Moreover, smaller particles lead to increased polarization due to larger curvature, which also contributes to stronger basicity [32]. Fig. 8(C) and (D) depicts the catalytic performance of MgO/C(I)-0.5-973, which has the same components as MgO(C)-0.5-973 but is prepared by impregnation of magnesium acetate on black carbon derived from the cooked rice grain. The catalytic activity of MgO/C(I)-0.5-973 was much inferior to MgO(C)-0.5-973, indicating that in situ transformation is key to generating solid bases with higher catalytic activity. As discussed above, magnesium acetate can be completely absorbed by swelling rice grains during the rice-cooking process. MgO particles are thus formed on the black carbon surfaces after calcination at 973 K. However, it is difficult to disperse MgO precursors via impregnation if the black carbon has already formed; the resulting MgO/C(I)-0.5973 is more like a mixture of spindly carbon and powdery MgO

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L. Ma et al. / Applied Catalysis A: General 460–461 (2013) 26–35

MgO(C)-1-973 is distinct from the MgO that covers the black carbon surface. Similar to pure MgO, this MgO should have catalytic performance much inferior to MgO(C)-0.5-973. Therefore, the catalytic activity on MgO(C)-0.5-973 is higher than MgO(C)-1-973.

Conversion of CPD (%)

20

15

A

MgO(C)-0.05-973 MgO(C)-0.25-973 MgO(C)-0.375-973 MgO(C)-0.5-973 MgO(C)-1-973

4. Conclusions

10

5

0 650

700

750

800

Temperature (K) 100

Selectivity for MCPD (%)

80

MgO(C)-0.05-973 MgO(C)-0.25-973 MgO(C)-0.375-973 MgO(C)-0.5-973 MgO(C)-1.0-973

B

Biomorphic MgO and MgO/carbon materials with a uniformly spindly appearance can be synthesized via “in situ transformation” using rice grains as the template and carbon precursor. Calcination conditions have an effect on the formed solid base materials. After pyrolysis at 973 K in nitrogen, the resulting MgO/carbon materials have high specific surface area and exhibit catalytic performance for CPD methylation superior to pure MgO. The presence of black carbon plays an important role in the promotion of MgO catalytic activity. MgO particle size decreases due to dispersion on the black carbon, and the catalyst basicity is enhanced. The present approach extends the usage range of plant material exotemplates. The seeds of plants with varying shapes and sizes can provide a diversity of exotemplates. This strategy is not limited to the synthesis of MgO, and should be applicable to other biomorphic porous materials, e.g. Al2 O3 , ZnO, etc. Acknowledgements

60

This work was supported by the National Natural Science Foundation of China (21273108, 20873061), the Fundamental Research Funds for the Central Universities (1116020503), and the Testing Fund of Nanjing University.

40

20

References 0 650

700

750

800

Temperature (K) Fig. 10. (A) Conversion of CPD and (B) selectivity for MCPD with methanol/CPD ratio of 1.8 (space velocity = 2.5 h−1 ) as functions of temperature on biomorphic MgO(C) samples (p = 101 kPa).

[1] [2] [3] [4] [5] [6] [7]

particles. Therefore, MgO/C(I)-0.5-973 shows catalytic activity similar to a mixture of MgO and black carbon. More work is still needed to further interpret the effect of black carbon. Fig. 10 illustrates the catalytic behavior of other biomorphic MgO(C)-x-973 samples in the methylation of CPD. The dosage of rice grains affects the catalytic performance of these biomorphic MgO/carbon materials. Very little MCPD and DMCPD forms on MgO(C)-0.05-973, demonstrating that this sample is inert to CPD methylation. When the mass ratio of Mg(Ac)2 ·4H2 O and rice reaches 0.25, the resulting MgO(C)-0.25-973 shows increased catalytic activity. Further increasing the mass ratio to 0.5, the resulting MgO(C)-0.5-973 exhibits its best catalytic performance, with 14% conversion and 59% selectivity for MCPD at 773 K. The change in catalytic activity with rice grain dosage cannot be ascribed to a change in specific surface area, since the MgO(C)-x-973 samples have similarly high specific surface area (Table 1). The XRD result indicates that MgO is highly dispersed on black carbon on MgO(C)-0.05973, while nanocrystalline MgO is formed on other MgO(C)-x-973 with higher MgO content. This suggests that the catalytic activity of MgO(C)-x-973 samples derives from the crystalline phase of MgO. It should be pointed out that MgO(C)-1-973 shows the catalytic performance inferior to MgO(C)-0.5-973, although its MgO content is much higher. As shown in Table 1, MgO particle size on MgO(C)-1-973 is 7.9 nm, much larger than the 5.5 nm for MgO(C)0.5-973, indicating that MgO with weaker basicity was formed on MgO(C)-1-973. Moreover, as mentioned above, the extra MgO in

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

[29]

V.K. Diez, C.R. Apesteguia, J.I. Di Cosimo, J. Catal. 240 (2006) 235–244. H. Hattori, Chem. Rev. 95 (1995) 537–550. M. Tu, R.J. Davis, J. Catal. 199 (2001) 85–91. Y. Ono, J. Catal. 216 (2003) 406–415. Y. Wang, J.H. Zhu, J.M. Cao, Y. Chun, Q.H. Xu, Microporous Mesoporous Mater. 26 (1998) 175–184. N. Sutradhar, A. Sinhamahapatra, S.K. Pahari, P. Pal, H.C. Bajaj, I. Mukhopadhyay, A.B. Panda, J. Phys. Chem. C 115 (2011) 12308–12316. B.M. Choudary, R.S. Mulukutla, K.J. Klabunde, J. Am. Chem. Soc. 125 (2003) 2020–2021. M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, J.R. Ruiz, F.J. Urbano, Appl. Catal. A 244 (2003) 207–215. Gy. Szollosi, M. Bartok, Appl. Catal. A 169 (1998) 263–269. Z. Liu, W. Li, C. Pan, P. Chen, H. Lou, X. Zheng, Catal. Commun. 15 (2011) 82–87. G. Zhao, J. Shi, G. Liu, Y. Liu, Z. Wang, W. Zhang, M. Jia, J. Mol. Catal. A 327 (2010) 32–37. B.Q. Xu, J.M. Wei, H.Y. Wang, K.Q. Sun, Q.M. Zhu, Catal. Today 68 (2001) 217–225. W.C. Li, A.-H. Lu, C. Weidenthaler, F. Schuth, Chem. Mater. 16 (2004) 5676–5681. K.T. Ranjit, K.J. Klabunde, Chem. Mater. 17 (2005) 65–73. S. Utamapanya, K.J. Klabunde, J.R. Schlup, Chem. Mater. 3 (1991) 175–181. R.M. Richards, A.M. Volodin, A.F. Bedilo, K.J. Klabunde, Phys. Chem. Chem. Phys. 5 (2003) 4299–4305. M.E. Martin, R.M. Narske, K.J. Klabunde, Microporous Mesoporous Mater. 83 (2005) 47–50. R. Richards, W. Li, S. Decker, C. Davidson, O. Koper, V. Zaikovski, A. Volodin, T. Rieker, K.J. Klabunde, J. Am. Chem. Soc. 122 (2000) 4921–4925. E. Knozinger, K.H. Jacob, P. Hofmann, J. Chem Soc, Faraday Trans. I89 (1993) 1101–1107. E. Knozinger, O. Diwald, M. Sterrer, J. Mol. Catal. A 162 (2000) 83–95. Z. Dohnalek, G.A. Kimmel, D.E. McCready, J.S. Young, A. Dohnalkova, R.S. Smith, B.D. Kay, J. Phys. Chem. B 106 (2002) 3526–3529. J. Kim, Z. Dohnalek, J.M. White, B.D. Kay, J. Phys. Chem. B 108 (2004) 11666–11671. J. Lee, S. Han, T. Hyeon, J. Mater. Chem. 14 (2004) 478–486. F. Schuth, Angew. Chem. Int. Ed. 42 (2003) 3604–3622. R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677–681. M. Schwickardi, T. Johann, W. Schmidt, F. Schuth, Chem. Mater. 14 (2002) 3913–3919. A.N. Shigapov, G.W. Graham, R.W. McCabe, H.K. Plummer Jr., Appl. Catal. A 210 (2001) 287–300. K.H. Sandhage, M.B. Dickerson, P.M. Huseman, M.A. Caranna, J.D. Clifton, T.A. Bull, T.J. Heibel, W.R. Overton, M.E.A. Schoenwaelder, Adv. Mater. 14 (2002) 429–433. J. Roggenbuck, M. Tiemann, J. Am. Chem. Soc. 127 (2005) 1096–1097.

L. Ma et al. / Applied Catalysis A: General 460–461 (2013) 26–35 [30] J. Roggenbuck, G. Koch, M. Tiemann, Chem. Mater. 18 (2006) 4151–4156. [31] R.Q. Sun, X. Zhou, L.B. Sun, H. Wu, Y. Chun, Q.H. Xu, Chem. J. Chin. Univ. 28 (2007) 2333–2337. [32] R.Q. Sun, L.B. Sun, Y. Chun, Q.H. Xu, H. Wu, Microporous Mesoporous Mater. 111 (2008) 314–322. [33] Z. Liu, T. Fan, D. Zhang, J. Am. Ceram. Soc. 89 (2006) 662–665. [34] H. Sieber, C. Hoffmann, A. Kaindl, P. Greil, Adv. Eng. Mater. 2 (2000) 105–109. [35] C.R. Rambo, H. Sieber, Adv. Mater. 17 (2005) 1088–1091. [36] T. Ota, M. Takahashi, T. Hibi, M. Ozawa, S. Suzuki, Y. Hikichi, H. Suzuki, J. Am. Ceram. Soc. 78 (1995) 3409–3411. [37] H. Sieber, Mater. Sci. Eng. A 412 (2005) 43–47. [38] B.-H. Sun, T.-X. Fan, J.-Q. Xu, D. Zhang, Mater. Lett. 59 (2005) 2325–2328. [39] G.-J. Zhang, J.-F. Yang, T. Ohji, J. Am. Ceram. Soc. 84 (2001) 1395–1397.

35

[40] M. Kh Aminian, N. Taghavinia, A. Iraji-zad, S.M. Mahdavi, M. Chavoshi, S. Ahmadian, Nanotechnology 17 (2006) 520–525. [41] G. Chandrasekar, M. Hartmann, V. Murugesan, J. Porous Mater. 16 (2009) 175–183. [42] D.X. Lan, L. Ma, Y. Chun, C. Wu, L.B. Sun, J.H. Zhu, J. Catal. 275 (2010) 257–269. [43] L.B. Sun, Z.Y. Wu, J.H. Kou, Y. Chun, Y. Wang, J.H. Zhu, Z.G. Zou, Chin. J. Catal. 27 (2006) 725–731. [44] J.C. Yu, A. Xu, L. Zhang, R. Song, L. Wu, J. Phys. Chem. B 108 (2004) 64–70. [45] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169–3183. [46] Y. Chun, G. Sheng, C.T. Chiou, B. Xing, Environ. Sci. Technol. 38 (2004) 4649–4655. [47] M. Singh, B.M. Yee, J. Eur. Ceram. Soc. 24 (2004) 209–217. [48] V.K. Diez, C.R. Apesteguia, J.I. Di Cosimo, J. Catal. 215 (2003) 220–233.