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Facile preparation of hierarchically porous carbon using diatomite as both template and catalyst and methylene blue adsorption of carbon products
Journal of Colloid and Interface Science 388 (2012) 176–184
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Facile preparation of hierarchically porous carbon using diatomite as both template and catalyst and methylene blue adsorption of carbon products Dong Liu a, Peng Yuan a,⇑, Daoyong Tan a,b, Hongmei Liu a,b, Tong Wang a, Mingde Fan c, Jianxi Zhu a, Hongping He a a b c
CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, China Graduate School of the Chinese Academy of Science, Beijing 100049, China College of Environment and Resources, Inner Mongolia University, Hohhot 010021, China
a r t i c l e
i n f o
Article history: Received 28 April 2012 Accepted 9 August 2012 Available online 29 August 2012 Keywords: Hierarchically porous carbon Diatomite Solid acidity Adsorption Methylene blue
a b s t r a c t Hierarchically porous carbons were prepared using a facile preparation method in which diatomite was utilized as both template and catalyst. The porous structures of the carbon products and their formation mechanisms were investigated. The macroporosity and microporosity of the diatomite-templated carbons were derived from replication of diatom shell and structure-reconfiguration of the carbon film, respectively. The macroporosity of carbons was strongly dependent on the original morphology of the diatomite template. The macroporous structure composed of carbon plates connected by the pillarand tube-like macropores resulted from the replication of the central and edge pores of the diatom shells with disk-shaped morphology, respectively. And another macroporous carbon tubes were also replicated from canoe-shaped diatom shells. The acidity of diatomite dramatically affected the porosity of the carbons, more acid sites of diatomite template resulted in higher surface area and pore volume of the carbon products. The diatomite-templated carbons exhibited higher adsorption capacity for methylene blue than the commercial activated carbon (CAC), although the specific surface area was much smaller than that of CAC, due to the hierarchical porosity of diatomite-templated carbons. And the carbons were readily reclaimed and regenerated. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Porous carbon materials have received a great deal of attention in recent years because of their extensive applications in gas separation, water purification, catalytic reaction, and electrochemical processing [1]. The templating technique used for the preparation of porous carbons has been well-established among the various methods already proposed. Many organic or inorganic templates with porous structures have been utilized in templating route [2–6], and the basic processes in producing the templated carbon involve the following: (i) impregnation of a polymer precursor into the porous space of a template; (ii) polymerization of the precursor by using an acid catalyst; (iii) carbonization under an inert atmosphere; and (iv) liberation of the resultant carbon network via removal of the template [5,6]. For the purpose of actual applications, one of the major challenges in the synthesis of the templated carbon is the selection of templates with desirable porosity and economical viability [6].
⇑ Corresponding author. Fax: +86 20 85290341. E-mail address: [email protected] (P. Yuan). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.08.023
Naturally-occurring porous minerals with high porosity, such as zeolite [7], montmorillonite [8], and diatomite [9], for being used as templates are attracting increasing attention. Among them, diatomite is one of the few templates for the preparation of macroporous carbon materials [9,10], because of its high macroporosity. Diatomite, also known as diatomaceous earth or kieselgur, is a fossil assemblage of diatom shells [11]. It is the most abundant form of silica on earth and is easily obtained at very low cost [9]. Composed of biogenetic amorphous silica and classified as noncrystalline opal-A in mineralogy, diatom shell is characteristic of highly developed porosity and particularly macroporous structure [12–14] and thus has been used as the template for the preparation of macroporous materials [15,16]. The sizes of the macropores of diatomite range from nanometric to micrometric domains. Holmes et al. [9] and Cai et al. [10] first reported the synthesis of macroporous diatomite-templated carbon, and Perez-Cabero et al. [17] prepared porous carbons using cultivated diatom as the template. In these studies, additional strong acid catalyst such as sulfuric acid was used without exception to catalyze the impregnated carbon precursor on the diatom shell surface [9,10]. However, the addition of liquid acid is potentially harmful to the environment and makes the preparation unnecessarily complicated. In fact, our previous
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report has revealed that diatomite itself contains both Brønsted and Lewis acidity: the former origins from the diatomaceous silica and the very tiny clay mineral particles strongly adhered to diatom shells and the latter from the clay minerals in diatomite [13]. Therefore, it is of great practical significance to utilize these inherent acid sites of diatomite rather than any additive catalysts to catalyze the carbon precursor. It is also of meaning in avoiding the potential threats to environment and lowering the production cost because diatomite is readily available in tonne-scales. The correlation between the structure and acidity of diatomite and the property of the diatomite-templated carbon is particularly deserved to be studied because diatoms are diverse in morphology and porosity. In addition, investigation into the adsorption properties of the diatomite-templated carbons is very necessary for the related uses. However, these mentioned studies have not been sufficiently conducted, to the best of our knowledge [18]. In the present work, the hierarchically porous carbon was prepared by a facile method, in which diatomite was utilized as template and catalyst simultaneously. The preparation mechanisms, in particular, the influences of the solid acidity of the diatomite templates on the porosity of the carbon products, were investigated. The adsorption capacity of the diatomite-templated carbon was evaluated by the model contaminant, methylene blue (MB). Two diatomite samples possessing diatoms of different genus were used to make clear the effects of morphology and porosity of diatom on the structure of the carbon products as well as their adsorption performances. The structural and adsorptive properties of the porous carbon were studied based on combined characterization methods, including X-ray fluorescence spectrometer (XRF), CHO elemental analysis, powder X-ray diffraction (XRD), thermogravimetric (TG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption– desorption, and mercury intrusion method.
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reagents used in this process are of analytical grade and used as received. 2.2. Preparation of the diatomite-templated porous carbon Furfuryl alcohol (FA) was used as the carbon precursor because it can be readily catalyzed by both Brønsted and Lewis acid sites [20,21]. The porous carbon product was prepared as follows [18]: diatomite and FA were mixed in ceramic boats with a solid/liquid ratio of 1 g: 5 mL, and then stirred for 1.5 h at room temperature, followed by heating at 95 °C for 24 h. The mixture was further heated at 150 °C under vacuum conditions for 1 h to promote cross-linking. The resultant was transferred into a tubular oven and heated at 700 °C (the optimized temperature selected from a series of preliminary experiments, see Supplementary Data, Fig. S1) for 3 h under N2 atmosphere (99.999%) for complete carbonization. The obtained product was dissolved in 40% HF solution for 12 h (or in 4 mol/g NaOH solution at 80 °C for 80 h; see Supplementary Data, Fig. S2) to remove the diatomite template (1 g solid in 20 mL aqueous HF solution or in 40 mL aqueous NaOH). The obtained carbons were differentiated by prefixing ‘‘C/’’ to the corresponding diatomite templates, for example, C/Dt(JL) denoted the Dt(JL)-templated carbon. The result of the blank experiment in which the diatomite was not added showed that the polymerization of FA did not occur under the above experimental conditions, indicating the indispensability of the diatomite as the catalyst. For comparison purpose, we performed a carbon preparation experiment in which 25 mg oxalic acid was used as the catalyst instead of diatomite, and the preparation steps were the same as the above-described. The obtained carbon was denoted C/FA. All chemicals and reagents used in the preparation are of analytical grade and used as received. 2.3. MB Adsorption and regeneration tests
2. Materials and methods 2.1. Diatomite samples Two raw diatomite samples were obtained from Qingshanyuan diatomite Co., Ltd. (Jilin provinces, China) and Shanwang diatomite Co., Ltd. (Shandong provinces, China), respectively, and purified by sedimentation method [19]; the purified samples were denoted Dt(JL) and Dt(SD), respectively. The dominant diatoms of Dt(JL) and Dt(SD) are the genus Coscinodiscus Ehrenberg (Centrales) and Synedra Ehrenberg (Pennales), respectively. The chemical composition (wt%) of Dt(JL) was determined as follows: SiO2, 86.18; Al2O3, 3.08; Fe2O3, 1.47; K2O, 0.51; CaO, 0.37; MgO, 0.33; Na2O, 0.06; ignition loss, 8.00. And that of Dt(SD) is as follows: SiO2, 87.14; Al2O3, 2.92; Fe2O3, 1.18; K2O, 0.41; CaO, 0.40; MgO, 0.57; Na2O, 0.07; ignition loss, 7.31. Three derivates of Dt(JL) were prepared by thermal and acid treatment [18]. Thermal treatment was performed in a programmed temperature-controlled muffle oven at 650 °C for 3 h, and the obtained sample was denoted Dt(JL)/T. The acid treated samples were prepared by an acid-washing process, in which 15 mL hydrochloric acid solution (2 mol/L) was mixed with 1 g of Dt(JL), and the mixture was kept at 105 °C for 4 h. The obtained mixture was divided into two parts. One part was further stirred for 1 h, and then the solid was separated by centrifugation and dried at 60 °C for 12 h, followed by heating at 550 °C for 5 h in the muffle furnace. The product was ground into powder in mortar and denoted Dt(JL)/A. The other part of the mixture was centrifugated and washed by distilled water repetitiously until free of Cl (tested by AgNO3). After that, the sample was dried at 150 °C for 24 h. The product was denoted Dt(JL)/P. All chemicals and
In a typical run of MB (C16H18ClN3S23H2O; Molecule Weight: 373.9; Tianjin Kermel Chemical Reagent Co., LTD., China) adsorption kinetics test, 0.1 g of carbon products was added into 15 mL MB solution (concentration: 500 mg/L) and then the mixture was strongly shaken at a rate of 200 rpm in an oscillator to ensure a complete mixing at room temperature. The contact time was in the range from 5 to 180 min. At timed intervals, the mixture was centrifuged and the supernatant was used for the determination of the MB content, which was performed on a UV–vis spectroscopy at wavelength of 665 nm. The adsorption amount (qe, mg MB/g carbon) was calculated using the following equation:
qe ¼
ðC 0 C t ÞV M
ð1Þ
where C0 and Ct (mg/L) are the MB concentration in the reaction solution before and after adsorption, respectively; V (L) is the solution volume, and M (g) is the amount of the adsorbent, 0.1 g. The rate expression of the adsorption kinetics simulated by the pseudo-second-order model is given by:
dqt ¼ kðqe-mod qt Þ2 dt
ð2Þ
where k is the second order rate constant (g mg1 min1), qt and qe-mod are the amount of MB adsorbed on per unit mass (mg/g) of carbon products at time t and at equilibrium, respectively. The integrated Eq. (2) for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt is given:
t 1 1 ¼ þ t qt kq2e-mod qe-mod
ð3Þ
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Applying the initial adsorption rate (mg g1 min1) h = k2 q2e-mod , the linear form of Eq. (3) is:
t 1 1 ¼ þ t qt h qe-mod
ð4Þ
Adsorption isotherms were obtained following a very similar procedure except for scheduled concentration (100–1400 mg/L) and the equilibrium time (120 min). The adsorption isotherm of MB on a commercial activated carbon (CAC) for comparison purpose was obtained following the same steps as above-described. After MB adsorption, the carbon adsorbents were reclaimed for further characterization and regeneration tests. And the MB-adsorbed carbons derived from Dt(JL) and Dt(SD) were denoted C/Dt(JL)-MB and C/Dt(SD)-MB, respectively. The corresponding adsorption conditions are as follows: MB concentration, 500 mg/L; and adsorption time, 180 min. The adsorption isotherm is simulated by the Langmuir model, and the linear form of the Langmuir model is given as
Ce 1 1 ¼ þ Ce Q e kL Q m Q m
ð5Þ
where kL (L/g) and Qm (mg/g) are the Langmuir constant and the monolayer adsorption capacity, respectively, and Ce (mg/L) is the equilibrium concentration of the MB solution. The regeneration of the MB-adsorbed carbon product (here take C/Dt(JL)-MB and C/Dt(SD)-MB as the samples) was performed at 700 °C for 3 h under N2 atmosphere (99.999%) in a tubular oven [22]. The products were denoted C/Dt(JL)-R and C/Dt(SD)-R. The regeneration efficiency was evaluated by comparing the surface area and pore volume of the regenerated carbon with those of the original one. 2.4. Characterization methods Elemental analysis of diatomite was performed using a Rigaku 100e X-ray fluorescence spectrometer. TG analysis was conducted on a Netzsch STA 449C instrument. Approximately, 10 mg of finely samples was heated in a corundum crucible from 30 to 1000 °C at a heating rate of 10 °C/min under highly pure N2 atmosphere (for the diatomite samples) or air atmosphere (for the carbon products). The CHO elemental analysis of carbon products was performed on a Vario EL III Universal Elemental Analyzer. SEM micrographs were obtained using a 5 kV FEI-Sirion 200 field emission scanning electron microscope attached to an Oxford INCA energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100 electron microscope operating at an acceleration voltage of 200 kV. The specimens for TEM observation were prepared by the following procedure. The sample was ultrasonically dispersed in ethanol for 5 min, and then, a drop of sample suspension was dropped onto a carbon-coated copper grid, which was left to stand for 10 min and transferred into the microscope. The XRD patterns were taken on a Bruker D8 Advance diffractometer with Ni filter and Cu Ka radiation (k = 0.154 nm) with a generator voltage of 40 kV and a generator current of 40 mA. A scan rate of 1° (2h)/min was applied for the determination. The solid acidity of diatomite was determined by the titration method as previously described by Benesi et al. [23], as detailed in our previous report [18]. The N2 adsorption–desorption isotherms were measured on a Micromeritics ASAP 2020 system (Micromeritics Co. Norcross, USA) at liquid nitrogen temperature. Samples were outgassed at 150 °C for 12 h at the degas port and then transferred to the analysis port to degas further for 6 h below a relative pressure of 0.01 before measurement. The specific surface area, SBET, was calculated
using the multiple-point Brunauer–Emmett–Teller (BET) method, and the total pore volume, Vpore, was evaluated from nitrogen uptake at a relative pressure of approximately 0.99. The micropore volume, Vmicropore, was estimated using the HK methods [24]. Micropore size distributions from 0.35 to 2 nm were determined using non-local density functional theory (DFT) [25]. The volume (Vmacropore) and size distribution of macropore was determined by the mercury intrusion method using a Micromeritics Autopore IV 9500 porosimeter. The pressure cycle used in predefined steps was from 0.5 to 60,000 psia. 3. Results and discussion 3.1. Characterization of diatomite and diatomite-templated carbons 3.1.1. XRD The XRD patterns of both Dt(JL) and Dt(SD) show the main phase of non-crystalline opal-A, characteristic of the broad diffraction at approximately 22° (2h) (Fig. 1a and b). Montmorillonite and quartz impurities are also found in the diatomite samples, and the content (wt%) of these impurities was semi-quantitatively determined as follows: Dt(JL), montmorillonite 4.9% and quartz 2.6%; Dt(SD), montmorillonite 1.1% and quartz 4.2%. The diatomite-based carbon products exhibit two broad XRD diffractions at approximately 24° and 43° (Fig. 1), attributed to the (0 0 2) and (1 0 0) reflections of the graphite, respectively. The d(002) value is traditionally used to estimate the graphitization degree of the carbon [8,26–29]. The d(002) values, 0.382 nm for Dt(JL) and 0.388 nm for C/Dt(SD), are larger than that of the ideal graphite (0.335 nm) [8]. These larger d(002) values indicate that the carbon products were composed of graphite microcrystals, which were disorderly stacked and did not form a complete graphite layer [30,31]. The diffractions of the diatom shells as well as the impurities disappear in the XRD patterns of the carbons, indicating the removal of the minerals during the process of HF washing. 3.1.2. TG and CHO elemental analyzes Two dramatic mass-losses at the similar temperature range are showed in the TG curves of the diatomite samples, and the corresponding DTG peaks centered at approximately 160 °C for Dt(JL) and 180 °C for Dt(SD) (Fig. 2a). They are attributed to the removal of physically-adsorbed water molecules [11]. Moreover, the massloss in the range 250–800 °C is associated with a very broad DTG
Fig. 1. XRD patterns of diatomite and diatomite-templated carbons.
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inorganics in the carbons. The higher decomposition temperature of C/Dt(SD) than C/Dt(JL) indicates the frameworks of the carbon derived from Dt(SD) were more stable than that from Dt(JL).
Fig. 2. TG and DTG curves of diatomite and diatomite-templated carbon products.
peak, implying multiple and partially overlapped mass-loss events, such as the dehydration of capping water and the dehydroxylation of different silanols [13,32]. The total mass-losses of C/Dt(JL) and C/Dt(SD) are 97.2% and 98.4% (Fig. 2b), respectively, confirming the most removal of the inorganic component in the final carbon products. This result is in agreement with the XRD results. The CHO elemental analysis shows that the composition (wt%) of C/Dt(JL) is as follows: C, 84.01%; H, 2.47%; and O, 9.94%; C/Dt(SD): C, 81.93%; H, 2.49%; and O, 9.96%. This result also supports the most removal of the
3.1.3. SEM and TEM The SEM image (Fig. 3a) shows that the diatom shells in Dt(JL) are disk-shaped and relatively uniform in diameter (20–40 lm) and thickness (1.2–1.8 lm). The frustule possesses a highly developed porous structure in which two types of macroporous structure are exhibited. The macropores in the center of shell arrays not so regularly and their pore sizes are concentrated at 300– 800 nm (Fig. 3a1). In contrast, the macropores in the edge show an ordered array with the pore size 100–250 nm (Fig. 3a2). The diatomite-carbon composite (before the removal of templates) is composed of the diatom shell and the carbon film coated on the shell surface (Fig. 3b). This structure is similar to that of the diatomite-charcoal composite prepared by the pyrolysis method [33]. The carbon coating is very thin because the locations of the macropores of the shell are still identifiable on the surface of the composite (Fig. 3b). The thickness of the carbon film is determined to be 10–50 nm based on TEM observation (see Supplementary Data, Fig. S3). In addition, EDS analysis indicates that the surface of the diatomite-carbon composite is chemically homogenous, and the Si/C molar ratio is approximately 0.12. There exists a high morphological dependency between the diatomite template and its derivate porous carbon. As exhibited in the top view of a carbon particle in C/Dt(JL) (Fig. 3c), the carbon particle consists of two carbon platforms connected by some carbon pillars. This structure resulted from the negative reproduction of the original porous structure in the center of the diatom shell, because the diameter (250–750 nm) and height (1.2–1.6 lm) of the carbon pillars correspond very well with the size of the central pores and the thickness of the shell, respectively, as displayed by the single carbon pillar (Fig. 3c, inset). There is another type of macroporous structure in the porous carbon (Fig. 3d). It is derived
Fig. 3. SEM images of (a) diatom shell of Dt(JL), (a1) central macropores of diatom shell, (a2) edge macropores of diatom shell, (b) Dt(JL) and carbon composite, (c) carbon pillars replicated from the central macropores of the diatom shell of Dt(JL) (inset: single carbon pillar), and (d) carbon tubes replicated from edge macropores of the diatom shell of Dt(JL); TEM images of (e) carbon tubes of C/Dt(JL), (f) carbon pillars of C/Dt(JL) (inset: selected area electron diffraction (SAED) pattern), (g) mesopores in the carbon pillar of C/Dt(JL), and (h) micropores of C/Dt(JL).
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from the edge pores and presents an ordered array, composing of regular hollow carbon tubes interconnected by carbon walls. The tube diameter (80–200 nm) is in good agreement with the original size of the edge pores. TEM images (Fig. 3e) clearly confirm the hollow structure of the tube-like macropores corresponding to the edge pores. However, the carbon pillars derived from the central pores are mostly solid but hollow at both ends (Fig. 3f). The partially solid structure was generated by the ‘‘in situ’’ polymerization of furfuryl alcohol at the middle part of the pillars, due to that the sufficiently deep macroporous channels prevented furfuryl alcohol from evaporation; in contrast, the furfuryl alcohol molecules in both openings of the channels were mostly evaporated under high temperature. Compared to central pores, the edge pores were too shallow to hold the furfuryl alcohol from evaporating, thus result-
ing in a completely hollow macroporous structure of carbon products. The wall of carbon tubes and pillars is composed of graphite microcrystals, as revealed by the faint and consecutive diffraction rings in the SAED pattern (Fig. 3f, inset). The disordered mesoporous and wormhole-like microporous [34] structures appear in the carbon wall (Fig. 3g and h), which resulted from the stacking of the carbon species and structure-reconfiguration of the carbon film during the removal of the diatom template, respectively [18,31]. The diatom shells in Dt(SD) are canoe-shaped and their width and length are 3–20 and 40–120 lm, respectively (Fig. 4a). Abundant macropores are distributed on the frustule surface, which array in rows and are line-interlaced on each side of the frustule valve (Fig. 4a, inset). The pore size of these macropores
Fig. 4. SEM images of (a) diatom shell of Dt(SD); (b) Dt(SD) and carbon composite; (c) C/Dt(SD); and (d) carbon tubes replicated from macropores of the diatom shell of Dt(SD).
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is 250–600 nm. In the diatomite-carbon composite, some macropores are still visible, indicating that the pore channels were not completely filled (Fig. 4b). The resultant porous carbon consists of the platy carbon substrate and the end-closed carbon tubes perpendicularly connected with the carbon substrate (Fig. 4c). Some carbon tubes should be hollow, as revealed by the exposed cavities of some end-broken tubes (Fig. 4d). The size of the tubular macropores of the carbon is 200–500 nm (Fig. 4c and d), smaller than that of the original frustule in Dt(SD). TEM images of C/Dt(SD) (not showed) also shows the mesopores generated from the stacking of the graphite microcrystals and the micropores with the wormhole-like structures as those exhibited in C/Dt(JL). This suggests
that the mesoporous and microporous structure in C/Dt(SD) were formed following a similar mechanism to that in C/Dt(JL). 3.1.4. N2 adsorption and mercury intrusion The carbon materials, as well as the diatomite templates, exhibit type II adsorption isotherms with hysteresis loops [8,35], reflecting the microporous and mesoporous structures of the carbons (Fig. 5a). The sharp increase in the N2 adsorbed quantity near the relative pressure of 1 indicates the existence of macropores in all samples. According to IUPAC-classification, the hysteresis loop of the obtained carbons is typical H3 [36], implying that the pores of the carbons are mainly narrow and slit-like [31,35]. These pores may be given by the stacking of platy carbon microparticles. The porous parameters of diatomite and carbon products are summarized in Table 1. The SBET values of C/Dt(JL) and C/Dt(SD) are 270 and 335 m2/g, respectively, which are tens of times greater than those of diatomite templates (Dt(JL), 19 m2/g and Dt(SD), 30 m2/g). These values are much higher than that of the diatom-templated carbon (169 m2/g) prepared by using AlCl3 as the catalyst [17]. The values of Vpore and Vmicropore of the carbons are significantly larger than those of the corresponding diatomite templates (Table 1), indicating that the porosity of carbon was not mainly derived from the reproduction of diatomite. It did not result from the direct carbonization of polyfurfuryl alcohol either, due to sample C/FA shows a very low Vpore value (Table 1). Considering impossibility of creating micropores by water derived from dehydroxylation [37], due to quite insufficient hydroxyl groups of diatomite (Fig. 2a), most micropores in the carbons should be formed due to the structure-reconfiguration of the carbon film, which occurred during the removal of the diatomite template. As showed in the FTIR spectra of C/Dt(JL) (see Supplementary Data, Fig. S4 and Table S1), various functional groups such as CAOH and C@O [38] exist in the obtained carbon film. They resulted from the polymerization of furfuryl alcohol and the following carbonization. The silanols (Si-OHs) of diatomite, which played as the acid sites for the polymerization of furfuryl alcohol, were tightly combined with the functional groups of the carbon film by hydrogen bonds, forming the stable carbon-diatomite composition. As a result, when these hydrogen bonds were broken during the removal of the diatomite template, the silanols were drawn from the carbon film and plenty of micropores were generated. This proposal is supported by the correlation between the amount of the solid acid sites of the diatomite template and the porosity of the carbon product. The acidity values of Dt(JL)-P, Dt(JL), Dt(JL)-T, and Dt(JL)-A are determined as 0.07, 0.09, 0.17, and 0.31 mmol/g, respectively. The result that Dt(JL)-P contains
Table 1 Porous parameters of diatomite and carbon products. Sample
Fig. 5. N2 adsorption isotherms of templates and diatomite-templated carbon products at 196 °C; micropore (b) and mesopore (c) size distributions of diatomite-templated carbon products.
SBET (m2/ Vpore (cm3/ Vmicropore g) g) (cm3/g)
Dt(JL) 19 Dt(JL)-P 29 Dt(JL)-A 16 Dt(JL)-T 17 C/Dt(JL) 270 C/Dt(JL)-P 255 C/Dt(JL)-A 426 C/Dt(JL)-T 406 C/Dt(JL)161 MB Dt(SD) 30 C/Dt(SD) 335 C/Dt(SD)207 MB C/FA 10 CAC 1013
Vmacropore (cm3/g)
Dmacropore (nm)
0.045 0.056 0.048 0.042 0.212 0.196 0.470 0.348 0.140
0.003 0.005 0.003 0.003 0.127 0.127 0.153 0.195 0.081
2.21 2.33 2.73 2.55 1.66 1.70 5.66 1.71 –
686 696 565 575 551 674 422 575 –
0.053 0.264 0.169
0.004 0.162 0.098
2.28 1.88 –
729 568 –
0.009 0.686
0.007 0.597
– –
– –
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fewer acid sites than Dt(JL) is due to the loss of some solid sites sourced from the tiny montmorillonite particles after purification treatment. Moreover, 650 °C heating made more acid sites exposed at the surface of diatom shells and montmorillonite particles, so that the acid amount of Dt(JL)-T is higher than that of Dt(JL) [13]. And acid washing resulted in the introduction of Al species onto the surface or implant into the structure [39] of diatom shell. These Al species were derived from montmorillonite by acid leach [40], and they possessed high acid strength so that increased the acidity of the diatomite template. In agreement with the order of acid amounts between different diatomite samples, the Vpore value of the carbons follows an order as: C/Dt(JL)-P (0.196 cm3/g) < C/Dt(JL) (0.212 cm3/g) < C/Dt(JL)-T (0.348 cm3/g) < C/Dt(JL)-A (0.470 cm3/g) (Table 1). It is evident that there is the positive correlation between the acid amount of the template and the resulting porous parameter of the carbon product, providing an evidence for the above proposal. Fig. 5b shows the micropore size distributions of the carbon products. One primary population appears at approximately 0.74 nm for C/Dt(JL) and two populations at approximately 0.70 and 1.74 nm for C/Dt(SD), respectively. These populations mainly correspond to the microporous generated by the structure-reconfiguration of the carbon film during the removal of diatomite template. The difference of the micropore size distributions of the two carbons might be relevant with the combination of carbon with diatomite, which were affected by the surface acid sites of the diatomite templates as previously described; however, the specific attribution of these populations is still difficult at this stage. The diatomite-templated carbons exhibit a wide distribution of the mesopore sizes (Fig. 5c), supporting the proposal that the mesopores were derived from the disordered stacking of the carbon species. The macropore volumes of the carbon products are 1.66 cm3/g for C/Dt(JL) and 1.88 cm3/g for C/Dt(SD), and the mean macropore diameter is 551 and 568 nm, respectively (Table 1). These macropore sizes are smaller than those of the corresponding templates (686 nm of Dt(JL) and 729 nm of Dt(SD) Table 1), which is consistent with the above-mentioned SEM observation and due to the formation of carbon tubes and pillars in the macropores of diatom shell. These fundamental results indicate that hierarchically porous carbon can be readily prepared by using diatomite as template and catalyst, and the porosity of obtained carbon is strongly dependent on morphology and the solid acidity of diatomite. The main mechanism is proposed that the furfuryl alcohol is catalyzed by inherent or enhanced acid sites of diatomite and the macro, meso, and microporosity of obtained carbon products result from the replication of diatomite templates, staking of carbon species, and the structure-reconfiguration of the carbon film, respectively.
higher than 0.999, and the simulated adsorption capacity (qe-mod) is pretty close to the experimental data (qe-exp) (Table 2). The good fitting of the pseudo-second-order kinetics implies the chemical adsorption process is the rate-limiting step during the MB adsorption [41–43]. The chemical adsorption took place between the MB molecules and the carbon-surface functional groups. It is noteworthy that C/Dt(SD) exhibits higher initial adsorption rate (53.2 mg g1 min1) than C/Dt(JL) (39.6 mg g1 min1) (Table 2). This result is mainly due to the diffused rate of MB in C/Dt(SD) was higher than that in C/Dt(JL), because MB molecules were more readily entered into the larger micropores in C/Dt(SD) than that in C/Dt(JL). Fig. 6c displays the MB adsorption isotherms of C/Dt(JL), C/ Dt(SD), and CAC. Langmuir model shows the best fitting (R2 > 0.99) in quantitatively describing the adsorption data (Fig. 6d and Table 2) as compared with Freundlich and Redlich-Peterson models (not showed). The Qm values for the three adsorbents follow an order of C/Dt(SD) (333 mg/g) > C/Dt(JL) = CAC (250 mg/ g). This result indicates the diatomite-templated carbons possess higher MB adsorption capacity than commercial activated carbon and many other reported microporous carbons [44], although the SBET and VPORE values of the diatomite-templated carbons are much smaller than those of CAC (SBET = 1013 m2/g and Vpore = 0.686 cm3/ g) and the reported carbons. The relatively low adsorption of MB on microporous CAC is due to that its pore size is concentrated on the range of 0.48–0.58 nm (Fig. 5b, inset), which means MB (molecule size is approximately 1.700.760.33 nm [45,46]) cannot be held by the micropores in CAC. In contrast, the abundant macropores and mesopores in the diatomite-templated carbons provide sufficient places for the adsorption of MB. In addition, some larger sized micropores (diameter P 0.76 nm) are also available for the capturing of MB molecules. This is evidenced by the fact that the micropores with pore size of approximately 1.74 nm in C/Dt(SD) disappeared after MB adsorption, and some new micropores with pore size of 0.93 nm appeared (Fig. 5b), which reflects the filling of MB molecules in 1.74 nm micropores. The decrease in Vpore of C/Dt(SD) from 0.264 to 0.169 cm3/g after MB adsorption also supports the filling of MB molecules in micropores. Similar results are observed for C/Dt(JL) where Vpore decreases from 0.212 to 0.140 cm3/g. However, C/Dt(JL) exhibits less developed porosity and particularly the 1.74 nm microspores than C/Dt(SD), leading to lower MB adsorption. The affinity between MB and the diatomite-templated carbons are evaluated based on the separation factor, RL, determined from the Langmuir adsorption isotherm model [47,48]. RL is defined by the following equation:
3.2. MB Adsorption and regeneration of the diatomite-templated porous carbons
where C0 is the initial concentration (mg L1) of MB, and KL is the Langmuir constant related to the energy of adsorption (L mg1). Different affinity between adsorbent and adsorbate is classified according to the value of RL: unfavorable adsorption (RL > 1), linear adsorption (RL = 1), favorable adsorption (0 < RL < 1), or irreversible adsorption (RL 0) [48]. As showed in Table 2, the RL values corresponding to both diatomite-templated carbons are in the range of 0–1, indicating the favorable MB uptake. In addition, the RL values are close to zero implies the irreversibility of the adsorption processes. This result is in good agreement with the proposed chemical adsorption process. As showed in Table 1, the SBET and Vpore values of the regenerated carbons are determined as follows: C/Dt(SD)-R, 354 m2/g and 0.303 cm3/g; C/Dt(JL)-R, 309 m2/g and 0.223 cm3/g. These values are close to those of C/Dt(SD) and C/Dt(JL) and much higher than those of C/Dt(SD)-MB and C/Dt(JL)-MB (Table 1), indicating
C/Dt(JL) and C/Dt(SD) were used in the MB adsorption tests to evaluate the adsorption capacity of the carbons. Fig. 6a shows the adsorption of MB by the carbons at different adsorption time. The initial MB concentration was set as 500 mg/L. The MB adsorption on both samples reached equilibrium state within 20 min, indicating a fast adsorption process. The carbons show good adsorption for MB, and the MB uptakes at the point of 20 min are determined as follows: C/Dt(JL), 134.7 mg/g; and C/Dt(SD), 144.0 mg/g. As compared to another two models, pseudo-first-order and intraparticle diffusion models (not showed), the pseudo-secondorder simulation model shows the best fitting for the kinetics of the MB adsorption on the carbons (Fig. 6b). The value of R2 is
RL ¼
1 ð1 þ kL C 0 Þ
ð6Þ
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183
Fig. 6. (a) Effect of agitation time on the MB removal efficiency of C/Dt(JL) and C/Dt(SD); (b) linear fitting plots based on pseudo-second-order kinetic model for the adsorption of MB; (c) the MB adsorption isotherms of C/Dt(JL), C/Dt(SD) and CAC; and (d) linear fitting plots based on Langmuir isotherm model for MB adsorption.
Table 2 Kinetics constants and Langmuir equation parameters for MB adsorption on various adsorbents. Model
Pseudo-secondorder
Langmuir
Parameters
Adsorbents C/Dt(JL)MB
C/Dt(SD)MB
CAC
R2 h (mg g1 min1) qe-mod (mg g1) qe-exp (mg g1)
0.9998 53.2
0.9995 39.6
– –
142.2 138.1
155.5 148.9
– –
R2 KL Qm (mgg1) RL
0.9906 0.015 333.3 0.045– 0.40
0.9910 0.010 250.0 0.066– 0.50
0.9987 0.042 250.0 0.017– 0.19
that most of the adsorbed MB molecules have been removed. It is noteworthy that the SBET values of the regenerated carbons are even slightly larger than the carbons before MB adsorption (Table 1). One possible reason is that the adsorbed MB molecules underwent carbonization during the regeneration process, creating some new micropores and mesopores and thus enhanced the porosity of the regenerated carbon.
4. Conclusions A facile method for the preparation of the porous carbon was proposed, and in this method, the natural diatomite was utilized as both template and catalyst. The obtained carbon product showed hierarchically porous structure, possessing macropores, mesopores, and micropores, simultaneously. The macroporous structure of the diatomite-templated carbon was derived from the direct replication of the diatomite template. The partly solid carbon pillars and ordered hollow carbon tubes interconnected by thin carbon walls were replicated from the central and edge pores of diatom shell of genus Coscinodiscus Ehrenberg, respectively. The end-closed carbon tubes were derived
from the replication of diatom shell of genus Synedra Ehrenberg. The mesopores and size-uniform wormhole-like micropores resulted from the stacking of the graphite microcrystals and the structure-reconfiguration of the carbon film during the removal of templates, respectively. The surface area and pore volume of the carbon products increased with the increment of the acid amount of templates, showing a positive correlation between the acidity of the diatomite template and the pore parameters of resulting carbons. The hierarchically porous carbon had good adsorption capacity for MB. The MB uptake followed the pseudo-second-order model during which chemical adsorption of MB took place on the carbon surface. Langmuir adsorption isotherm model well fit in the adsorption data. The adsorption capacities of MB on the diatomite-templated carbons were dramatically higher than that of commercial activated carbon because of the hierarchically porous structure of the diatomite-templated carbons. In addition, the regeneration of the carbons was highly efficient because the reused carbons exhibited higher surface area and pore volume than those of initial carbons. In conclusion, the morphology, porosity, and solid acidity of initial diatomite have significantly effects on the textural property of diatomite-templated carbons, which further affect the MB adsorption capacity of the carbons. These fundamental results demonstrate that the diatomite-templated carbon is promising for applications in the fields of adsorption and purification. Acknowledgments The financial supports from the National Natural Scientific Foundation of China (Grant No. 40872042) and Natural Science Foundation of Guangdong Province, China (Grant No. 8151064004000007) are gratefully acknowledged. This is a contribution (No. IS1545) from GIGCAS. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.08.023.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile preparation of hierarchically porous carbon using diatomite as both template and catalyst and methylene blue adsorption of carbon products Dong Liu a, Peng Yuan a,⇑, Daoyong Tan a,b, Hongmei Liu a,b, Tong Wang a, Mingde Fan c, Jianxi Zhu a, Hongping He a a b c
CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, China Graduate School of the Chinese Academy of Science, Beijing 100049, China College of Environment and Resources, Inner Mongolia University, Hohhot 010021, China
a r t i c l e
i n f o
Article history: Received 28 April 2012 Accepted 9 August 2012 Available online 29 August 2012 Keywords: Hierarchically porous carbon Diatomite Solid acidity Adsorption Methylene blue
a b s t r a c t Hierarchically porous carbons were prepared using a facile preparation method in which diatomite was utilized as both template and catalyst. The porous structures of the carbon products and their formation mechanisms were investigated. The macroporosity and microporosity of the diatomite-templated carbons were derived from replication of diatom shell and structure-reconfiguration of the carbon film, respectively. The macroporosity of carbons was strongly dependent on the original morphology of the diatomite template. The macroporous structure composed of carbon plates connected by the pillarand tube-like macropores resulted from the replication of the central and edge pores of the diatom shells with disk-shaped morphology, respectively. And another macroporous carbon tubes were also replicated from canoe-shaped diatom shells. The acidity of diatomite dramatically affected the porosity of the carbons, more acid sites of diatomite template resulted in higher surface area and pore volume of the carbon products. The diatomite-templated carbons exhibited higher adsorption capacity for methylene blue than the commercial activated carbon (CAC), although the specific surface area was much smaller than that of CAC, due to the hierarchical porosity of diatomite-templated carbons. And the carbons were readily reclaimed and regenerated. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Porous carbon materials have received a great deal of attention in recent years because of their extensive applications in gas separation, water purification, catalytic reaction, and electrochemical processing [1]. The templating technique used for the preparation of porous carbons has been well-established among the various methods already proposed. Many organic or inorganic templates with porous structures have been utilized in templating route [2–6], and the basic processes in producing the templated carbon involve the following: (i) impregnation of a polymer precursor into the porous space of a template; (ii) polymerization of the precursor by using an acid catalyst; (iii) carbonization under an inert atmosphere; and (iv) liberation of the resultant carbon network via removal of the template [5,6]. For the purpose of actual applications, one of the major challenges in the synthesis of the templated carbon is the selection of templates with desirable porosity and economical viability [6].
⇑ Corresponding author. Fax: +86 20 85290341. E-mail address: [email protected] (P. Yuan). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.08.023
Naturally-occurring porous minerals with high porosity, such as zeolite [7], montmorillonite [8], and diatomite [9], for being used as templates are attracting increasing attention. Among them, diatomite is one of the few templates for the preparation of macroporous carbon materials [9,10], because of its high macroporosity. Diatomite, also known as diatomaceous earth or kieselgur, is a fossil assemblage of diatom shells [11]. It is the most abundant form of silica on earth and is easily obtained at very low cost [9]. Composed of biogenetic amorphous silica and classified as noncrystalline opal-A in mineralogy, diatom shell is characteristic of highly developed porosity and particularly macroporous structure [12–14] and thus has been used as the template for the preparation of macroporous materials [15,16]. The sizes of the macropores of diatomite range from nanometric to micrometric domains. Holmes et al. [9] and Cai et al. [10] first reported the synthesis of macroporous diatomite-templated carbon, and Perez-Cabero et al. [17] prepared porous carbons using cultivated diatom as the template. In these studies, additional strong acid catalyst such as sulfuric acid was used without exception to catalyze the impregnated carbon precursor on the diatom shell surface [9,10]. However, the addition of liquid acid is potentially harmful to the environment and makes the preparation unnecessarily complicated. In fact, our previous
D. Liu et al. / Journal of Colloid and Interface Science 388 (2012) 176–184
report has revealed that diatomite itself contains both Brønsted and Lewis acidity: the former origins from the diatomaceous silica and the very tiny clay mineral particles strongly adhered to diatom shells and the latter from the clay minerals in diatomite [13]. Therefore, it is of great practical significance to utilize these inherent acid sites of diatomite rather than any additive catalysts to catalyze the carbon precursor. It is also of meaning in avoiding the potential threats to environment and lowering the production cost because diatomite is readily available in tonne-scales. The correlation between the structure and acidity of diatomite and the property of the diatomite-templated carbon is particularly deserved to be studied because diatoms are diverse in morphology and porosity. In addition, investigation into the adsorption properties of the diatomite-templated carbons is very necessary for the related uses. However, these mentioned studies have not been sufficiently conducted, to the best of our knowledge [18]. In the present work, the hierarchically porous carbon was prepared by a facile method, in which diatomite was utilized as template and catalyst simultaneously. The preparation mechanisms, in particular, the influences of the solid acidity of the diatomite templates on the porosity of the carbon products, were investigated. The adsorption capacity of the diatomite-templated carbon was evaluated by the model contaminant, methylene blue (MB). Two diatomite samples possessing diatoms of different genus were used to make clear the effects of morphology and porosity of diatom on the structure of the carbon products as well as their adsorption performances. The structural and adsorptive properties of the porous carbon were studied based on combined characterization methods, including X-ray fluorescence spectrometer (XRF), CHO elemental analysis, powder X-ray diffraction (XRD), thermogravimetric (TG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption– desorption, and mercury intrusion method.
177
reagents used in this process are of analytical grade and used as received. 2.2. Preparation of the diatomite-templated porous carbon Furfuryl alcohol (FA) was used as the carbon precursor because it can be readily catalyzed by both Brønsted and Lewis acid sites [20,21]. The porous carbon product was prepared as follows [18]: diatomite and FA were mixed in ceramic boats with a solid/liquid ratio of 1 g: 5 mL, and then stirred for 1.5 h at room temperature, followed by heating at 95 °C for 24 h. The mixture was further heated at 150 °C under vacuum conditions for 1 h to promote cross-linking. The resultant was transferred into a tubular oven and heated at 700 °C (the optimized temperature selected from a series of preliminary experiments, see Supplementary Data, Fig. S1) for 3 h under N2 atmosphere (99.999%) for complete carbonization. The obtained product was dissolved in 40% HF solution for 12 h (or in 4 mol/g NaOH solution at 80 °C for 80 h; see Supplementary Data, Fig. S2) to remove the diatomite template (1 g solid in 20 mL aqueous HF solution or in 40 mL aqueous NaOH). The obtained carbons were differentiated by prefixing ‘‘C/’’ to the corresponding diatomite templates, for example, C/Dt(JL) denoted the Dt(JL)-templated carbon. The result of the blank experiment in which the diatomite was not added showed that the polymerization of FA did not occur under the above experimental conditions, indicating the indispensability of the diatomite as the catalyst. For comparison purpose, we performed a carbon preparation experiment in which 25 mg oxalic acid was used as the catalyst instead of diatomite, and the preparation steps were the same as the above-described. The obtained carbon was denoted C/FA. All chemicals and reagents used in the preparation are of analytical grade and used as received. 2.3. MB Adsorption and regeneration tests
2. Materials and methods 2.1. Diatomite samples Two raw diatomite samples were obtained from Qingshanyuan diatomite Co., Ltd. (Jilin provinces, China) and Shanwang diatomite Co., Ltd. (Shandong provinces, China), respectively, and purified by sedimentation method [19]; the purified samples were denoted Dt(JL) and Dt(SD), respectively. The dominant diatoms of Dt(JL) and Dt(SD) are the genus Coscinodiscus Ehrenberg (Centrales) and Synedra Ehrenberg (Pennales), respectively. The chemical composition (wt%) of Dt(JL) was determined as follows: SiO2, 86.18; Al2O3, 3.08; Fe2O3, 1.47; K2O, 0.51; CaO, 0.37; MgO, 0.33; Na2O, 0.06; ignition loss, 8.00. And that of Dt(SD) is as follows: SiO2, 87.14; Al2O3, 2.92; Fe2O3, 1.18; K2O, 0.41; CaO, 0.40; MgO, 0.57; Na2O, 0.07; ignition loss, 7.31. Three derivates of Dt(JL) were prepared by thermal and acid treatment [18]. Thermal treatment was performed in a programmed temperature-controlled muffle oven at 650 °C for 3 h, and the obtained sample was denoted Dt(JL)/T. The acid treated samples were prepared by an acid-washing process, in which 15 mL hydrochloric acid solution (2 mol/L) was mixed with 1 g of Dt(JL), and the mixture was kept at 105 °C for 4 h. The obtained mixture was divided into two parts. One part was further stirred for 1 h, and then the solid was separated by centrifugation and dried at 60 °C for 12 h, followed by heating at 550 °C for 5 h in the muffle furnace. The product was ground into powder in mortar and denoted Dt(JL)/A. The other part of the mixture was centrifugated and washed by distilled water repetitiously until free of Cl (tested by AgNO3). After that, the sample was dried at 150 °C for 24 h. The product was denoted Dt(JL)/P. All chemicals and
In a typical run of MB (C16H18ClN3S23H2O; Molecule Weight: 373.9; Tianjin Kermel Chemical Reagent Co., LTD., China) adsorption kinetics test, 0.1 g of carbon products was added into 15 mL MB solution (concentration: 500 mg/L) and then the mixture was strongly shaken at a rate of 200 rpm in an oscillator to ensure a complete mixing at room temperature. The contact time was in the range from 5 to 180 min. At timed intervals, the mixture was centrifuged and the supernatant was used for the determination of the MB content, which was performed on a UV–vis spectroscopy at wavelength of 665 nm. The adsorption amount (qe, mg MB/g carbon) was calculated using the following equation:
qe ¼
ðC 0 C t ÞV M
ð1Þ
where C0 and Ct (mg/L) are the MB concentration in the reaction solution before and after adsorption, respectively; V (L) is the solution volume, and M (g) is the amount of the adsorbent, 0.1 g. The rate expression of the adsorption kinetics simulated by the pseudo-second-order model is given by:
dqt ¼ kðqe-mod qt Þ2 dt
ð2Þ
where k is the second order rate constant (g mg1 min1), qt and qe-mod are the amount of MB adsorbed on per unit mass (mg/g) of carbon products at time t and at equilibrium, respectively. The integrated Eq. (2) for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt is given:
t 1 1 ¼ þ t qt kq2e-mod qe-mod
ð3Þ
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Applying the initial adsorption rate (mg g1 min1) h = k2 q2e-mod , the linear form of Eq. (3) is:
t 1 1 ¼ þ t qt h qe-mod
ð4Þ
Adsorption isotherms were obtained following a very similar procedure except for scheduled concentration (100–1400 mg/L) and the equilibrium time (120 min). The adsorption isotherm of MB on a commercial activated carbon (CAC) for comparison purpose was obtained following the same steps as above-described. After MB adsorption, the carbon adsorbents were reclaimed for further characterization and regeneration tests. And the MB-adsorbed carbons derived from Dt(JL) and Dt(SD) were denoted C/Dt(JL)-MB and C/Dt(SD)-MB, respectively. The corresponding adsorption conditions are as follows: MB concentration, 500 mg/L; and adsorption time, 180 min. The adsorption isotherm is simulated by the Langmuir model, and the linear form of the Langmuir model is given as
Ce 1 1 ¼ þ Ce Q e kL Q m Q m
ð5Þ
where kL (L/g) and Qm (mg/g) are the Langmuir constant and the monolayer adsorption capacity, respectively, and Ce (mg/L) is the equilibrium concentration of the MB solution. The regeneration of the MB-adsorbed carbon product (here take C/Dt(JL)-MB and C/Dt(SD)-MB as the samples) was performed at 700 °C for 3 h under N2 atmosphere (99.999%) in a tubular oven [22]. The products were denoted C/Dt(JL)-R and C/Dt(SD)-R. The regeneration efficiency was evaluated by comparing the surface area and pore volume of the regenerated carbon with those of the original one. 2.4. Characterization methods Elemental analysis of diatomite was performed using a Rigaku 100e X-ray fluorescence spectrometer. TG analysis was conducted on a Netzsch STA 449C instrument. Approximately, 10 mg of finely samples was heated in a corundum crucible from 30 to 1000 °C at a heating rate of 10 °C/min under highly pure N2 atmosphere (for the diatomite samples) or air atmosphere (for the carbon products). The CHO elemental analysis of carbon products was performed on a Vario EL III Universal Elemental Analyzer. SEM micrographs were obtained using a 5 kV FEI-Sirion 200 field emission scanning electron microscope attached to an Oxford INCA energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100 electron microscope operating at an acceleration voltage of 200 kV. The specimens for TEM observation were prepared by the following procedure. The sample was ultrasonically dispersed in ethanol for 5 min, and then, a drop of sample suspension was dropped onto a carbon-coated copper grid, which was left to stand for 10 min and transferred into the microscope. The XRD patterns were taken on a Bruker D8 Advance diffractometer with Ni filter and Cu Ka radiation (k = 0.154 nm) with a generator voltage of 40 kV and a generator current of 40 mA. A scan rate of 1° (2h)/min was applied for the determination. The solid acidity of diatomite was determined by the titration method as previously described by Benesi et al. [23], as detailed in our previous report [18]. The N2 adsorption–desorption isotherms were measured on a Micromeritics ASAP 2020 system (Micromeritics Co. Norcross, USA) at liquid nitrogen temperature. Samples were outgassed at 150 °C for 12 h at the degas port and then transferred to the analysis port to degas further for 6 h below a relative pressure of 0.01 before measurement. The specific surface area, SBET, was calculated
using the multiple-point Brunauer–Emmett–Teller (BET) method, and the total pore volume, Vpore, was evaluated from nitrogen uptake at a relative pressure of approximately 0.99. The micropore volume, Vmicropore, was estimated using the HK methods [24]. Micropore size distributions from 0.35 to 2 nm were determined using non-local density functional theory (DFT) [25]. The volume (Vmacropore) and size distribution of macropore was determined by the mercury intrusion method using a Micromeritics Autopore IV 9500 porosimeter. The pressure cycle used in predefined steps was from 0.5 to 60,000 psia. 3. Results and discussion 3.1. Characterization of diatomite and diatomite-templated carbons 3.1.1. XRD The XRD patterns of both Dt(JL) and Dt(SD) show the main phase of non-crystalline opal-A, characteristic of the broad diffraction at approximately 22° (2h) (Fig. 1a and b). Montmorillonite and quartz impurities are also found in the diatomite samples, and the content (wt%) of these impurities was semi-quantitatively determined as follows: Dt(JL), montmorillonite 4.9% and quartz 2.6%; Dt(SD), montmorillonite 1.1% and quartz 4.2%. The diatomite-based carbon products exhibit two broad XRD diffractions at approximately 24° and 43° (Fig. 1), attributed to the (0 0 2) and (1 0 0) reflections of the graphite, respectively. The d(002) value is traditionally used to estimate the graphitization degree of the carbon [8,26–29]. The d(002) values, 0.382 nm for Dt(JL) and 0.388 nm for C/Dt(SD), are larger than that of the ideal graphite (0.335 nm) [8]. These larger d(002) values indicate that the carbon products were composed of graphite microcrystals, which were disorderly stacked and did not form a complete graphite layer [30,31]. The diffractions of the diatom shells as well as the impurities disappear in the XRD patterns of the carbons, indicating the removal of the minerals during the process of HF washing. 3.1.2. TG and CHO elemental analyzes Two dramatic mass-losses at the similar temperature range are showed in the TG curves of the diatomite samples, and the corresponding DTG peaks centered at approximately 160 °C for Dt(JL) and 180 °C for Dt(SD) (Fig. 2a). They are attributed to the removal of physically-adsorbed water molecules [11]. Moreover, the massloss in the range 250–800 °C is associated with a very broad DTG
Fig. 1. XRD patterns of diatomite and diatomite-templated carbons.
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inorganics in the carbons. The higher decomposition temperature of C/Dt(SD) than C/Dt(JL) indicates the frameworks of the carbon derived from Dt(SD) were more stable than that from Dt(JL).
Fig. 2. TG and DTG curves of diatomite and diatomite-templated carbon products.
peak, implying multiple and partially overlapped mass-loss events, such as the dehydration of capping water and the dehydroxylation of different silanols [13,32]. The total mass-losses of C/Dt(JL) and C/Dt(SD) are 97.2% and 98.4% (Fig. 2b), respectively, confirming the most removal of the inorganic component in the final carbon products. This result is in agreement with the XRD results. The CHO elemental analysis shows that the composition (wt%) of C/Dt(JL) is as follows: C, 84.01%; H, 2.47%; and O, 9.94%; C/Dt(SD): C, 81.93%; H, 2.49%; and O, 9.96%. This result also supports the most removal of the
3.1.3. SEM and TEM The SEM image (Fig. 3a) shows that the diatom shells in Dt(JL) are disk-shaped and relatively uniform in diameter (20–40 lm) and thickness (1.2–1.8 lm). The frustule possesses a highly developed porous structure in which two types of macroporous structure are exhibited. The macropores in the center of shell arrays not so regularly and their pore sizes are concentrated at 300– 800 nm (Fig. 3a1). In contrast, the macropores in the edge show an ordered array with the pore size 100–250 nm (Fig. 3a2). The diatomite-carbon composite (before the removal of templates) is composed of the diatom shell and the carbon film coated on the shell surface (Fig. 3b). This structure is similar to that of the diatomite-charcoal composite prepared by the pyrolysis method [33]. The carbon coating is very thin because the locations of the macropores of the shell are still identifiable on the surface of the composite (Fig. 3b). The thickness of the carbon film is determined to be 10–50 nm based on TEM observation (see Supplementary Data, Fig. S3). In addition, EDS analysis indicates that the surface of the diatomite-carbon composite is chemically homogenous, and the Si/C molar ratio is approximately 0.12. There exists a high morphological dependency between the diatomite template and its derivate porous carbon. As exhibited in the top view of a carbon particle in C/Dt(JL) (Fig. 3c), the carbon particle consists of two carbon platforms connected by some carbon pillars. This structure resulted from the negative reproduction of the original porous structure in the center of the diatom shell, because the diameter (250–750 nm) and height (1.2–1.6 lm) of the carbon pillars correspond very well with the size of the central pores and the thickness of the shell, respectively, as displayed by the single carbon pillar (Fig. 3c, inset). There is another type of macroporous structure in the porous carbon (Fig. 3d). It is derived
Fig. 3. SEM images of (a) diatom shell of Dt(JL), (a1) central macropores of diatom shell, (a2) edge macropores of diatom shell, (b) Dt(JL) and carbon composite, (c) carbon pillars replicated from the central macropores of the diatom shell of Dt(JL) (inset: single carbon pillar), and (d) carbon tubes replicated from edge macropores of the diatom shell of Dt(JL); TEM images of (e) carbon tubes of C/Dt(JL), (f) carbon pillars of C/Dt(JL) (inset: selected area electron diffraction (SAED) pattern), (g) mesopores in the carbon pillar of C/Dt(JL), and (h) micropores of C/Dt(JL).
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from the edge pores and presents an ordered array, composing of regular hollow carbon tubes interconnected by carbon walls. The tube diameter (80–200 nm) is in good agreement with the original size of the edge pores. TEM images (Fig. 3e) clearly confirm the hollow structure of the tube-like macropores corresponding to the edge pores. However, the carbon pillars derived from the central pores are mostly solid but hollow at both ends (Fig. 3f). The partially solid structure was generated by the ‘‘in situ’’ polymerization of furfuryl alcohol at the middle part of the pillars, due to that the sufficiently deep macroporous channels prevented furfuryl alcohol from evaporation; in contrast, the furfuryl alcohol molecules in both openings of the channels were mostly evaporated under high temperature. Compared to central pores, the edge pores were too shallow to hold the furfuryl alcohol from evaporating, thus result-
ing in a completely hollow macroporous structure of carbon products. The wall of carbon tubes and pillars is composed of graphite microcrystals, as revealed by the faint and consecutive diffraction rings in the SAED pattern (Fig. 3f, inset). The disordered mesoporous and wormhole-like microporous [34] structures appear in the carbon wall (Fig. 3g and h), which resulted from the stacking of the carbon species and structure-reconfiguration of the carbon film during the removal of the diatom template, respectively [18,31]. The diatom shells in Dt(SD) are canoe-shaped and their width and length are 3–20 and 40–120 lm, respectively (Fig. 4a). Abundant macropores are distributed on the frustule surface, which array in rows and are line-interlaced on each side of the frustule valve (Fig. 4a, inset). The pore size of these macropores
Fig. 4. SEM images of (a) diatom shell of Dt(SD); (b) Dt(SD) and carbon composite; (c) C/Dt(SD); and (d) carbon tubes replicated from macropores of the diatom shell of Dt(SD).
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is 250–600 nm. In the diatomite-carbon composite, some macropores are still visible, indicating that the pore channels were not completely filled (Fig. 4b). The resultant porous carbon consists of the platy carbon substrate and the end-closed carbon tubes perpendicularly connected with the carbon substrate (Fig. 4c). Some carbon tubes should be hollow, as revealed by the exposed cavities of some end-broken tubes (Fig. 4d). The size of the tubular macropores of the carbon is 200–500 nm (Fig. 4c and d), smaller than that of the original frustule in Dt(SD). TEM images of C/Dt(SD) (not showed) also shows the mesopores generated from the stacking of the graphite microcrystals and the micropores with the wormhole-like structures as those exhibited in C/Dt(JL). This suggests
that the mesoporous and microporous structure in C/Dt(SD) were formed following a similar mechanism to that in C/Dt(JL). 3.1.4. N2 adsorption and mercury intrusion The carbon materials, as well as the diatomite templates, exhibit type II adsorption isotherms with hysteresis loops [8,35], reflecting the microporous and mesoporous structures of the carbons (Fig. 5a). The sharp increase in the N2 adsorbed quantity near the relative pressure of 1 indicates the existence of macropores in all samples. According to IUPAC-classification, the hysteresis loop of the obtained carbons is typical H3 [36], implying that the pores of the carbons are mainly narrow and slit-like [31,35]. These pores may be given by the stacking of platy carbon microparticles. The porous parameters of diatomite and carbon products are summarized in Table 1. The SBET values of C/Dt(JL) and C/Dt(SD) are 270 and 335 m2/g, respectively, which are tens of times greater than those of diatomite templates (Dt(JL), 19 m2/g and Dt(SD), 30 m2/g). These values are much higher than that of the diatom-templated carbon (169 m2/g) prepared by using AlCl3 as the catalyst [17]. The values of Vpore and Vmicropore of the carbons are significantly larger than those of the corresponding diatomite templates (Table 1), indicating that the porosity of carbon was not mainly derived from the reproduction of diatomite. It did not result from the direct carbonization of polyfurfuryl alcohol either, due to sample C/FA shows a very low Vpore value (Table 1). Considering impossibility of creating micropores by water derived from dehydroxylation [37], due to quite insufficient hydroxyl groups of diatomite (Fig. 2a), most micropores in the carbons should be formed due to the structure-reconfiguration of the carbon film, which occurred during the removal of the diatomite template. As showed in the FTIR spectra of C/Dt(JL) (see Supplementary Data, Fig. S4 and Table S1), various functional groups such as CAOH and C@O [38] exist in the obtained carbon film. They resulted from the polymerization of furfuryl alcohol and the following carbonization. The silanols (Si-OHs) of diatomite, which played as the acid sites for the polymerization of furfuryl alcohol, were tightly combined with the functional groups of the carbon film by hydrogen bonds, forming the stable carbon-diatomite composition. As a result, when these hydrogen bonds were broken during the removal of the diatomite template, the silanols were drawn from the carbon film and plenty of micropores were generated. This proposal is supported by the correlation between the amount of the solid acid sites of the diatomite template and the porosity of the carbon product. The acidity values of Dt(JL)-P, Dt(JL), Dt(JL)-T, and Dt(JL)-A are determined as 0.07, 0.09, 0.17, and 0.31 mmol/g, respectively. The result that Dt(JL)-P contains
Table 1 Porous parameters of diatomite and carbon products. Sample
Fig. 5. N2 adsorption isotherms of templates and diatomite-templated carbon products at 196 °C; micropore (b) and mesopore (c) size distributions of diatomite-templated carbon products.
SBET (m2/ Vpore (cm3/ Vmicropore g) g) (cm3/g)
Dt(JL) 19 Dt(JL)-P 29 Dt(JL)-A 16 Dt(JL)-T 17 C/Dt(JL) 270 C/Dt(JL)-P 255 C/Dt(JL)-A 426 C/Dt(JL)-T 406 C/Dt(JL)161 MB Dt(SD) 30 C/Dt(SD) 335 C/Dt(SD)207 MB C/FA 10 CAC 1013
Vmacropore (cm3/g)
Dmacropore (nm)
0.045 0.056 0.048 0.042 0.212 0.196 0.470 0.348 0.140
0.003 0.005 0.003 0.003 0.127 0.127 0.153 0.195 0.081
2.21 2.33 2.73 2.55 1.66 1.70 5.66 1.71 –
686 696 565 575 551 674 422 575 –
0.053 0.264 0.169
0.004 0.162 0.098
2.28 1.88 –
729 568 –
0.009 0.686
0.007 0.597
– –
– –
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fewer acid sites than Dt(JL) is due to the loss of some solid sites sourced from the tiny montmorillonite particles after purification treatment. Moreover, 650 °C heating made more acid sites exposed at the surface of diatom shells and montmorillonite particles, so that the acid amount of Dt(JL)-T is higher than that of Dt(JL) [13]. And acid washing resulted in the introduction of Al species onto the surface or implant into the structure [39] of diatom shell. These Al species were derived from montmorillonite by acid leach [40], and they possessed high acid strength so that increased the acidity of the diatomite template. In agreement with the order of acid amounts between different diatomite samples, the Vpore value of the carbons follows an order as: C/Dt(JL)-P (0.196 cm3/g) < C/Dt(JL) (0.212 cm3/g) < C/Dt(JL)-T (0.348 cm3/g) < C/Dt(JL)-A (0.470 cm3/g) (Table 1). It is evident that there is the positive correlation between the acid amount of the template and the resulting porous parameter of the carbon product, providing an evidence for the above proposal. Fig. 5b shows the micropore size distributions of the carbon products. One primary population appears at approximately 0.74 nm for C/Dt(JL) and two populations at approximately 0.70 and 1.74 nm for C/Dt(SD), respectively. These populations mainly correspond to the microporous generated by the structure-reconfiguration of the carbon film during the removal of diatomite template. The difference of the micropore size distributions of the two carbons might be relevant with the combination of carbon with diatomite, which were affected by the surface acid sites of the diatomite templates as previously described; however, the specific attribution of these populations is still difficult at this stage. The diatomite-templated carbons exhibit a wide distribution of the mesopore sizes (Fig. 5c), supporting the proposal that the mesopores were derived from the disordered stacking of the carbon species. The macropore volumes of the carbon products are 1.66 cm3/g for C/Dt(JL) and 1.88 cm3/g for C/Dt(SD), and the mean macropore diameter is 551 and 568 nm, respectively (Table 1). These macropore sizes are smaller than those of the corresponding templates (686 nm of Dt(JL) and 729 nm of Dt(SD) Table 1), which is consistent with the above-mentioned SEM observation and due to the formation of carbon tubes and pillars in the macropores of diatom shell. These fundamental results indicate that hierarchically porous carbon can be readily prepared by using diatomite as template and catalyst, and the porosity of obtained carbon is strongly dependent on morphology and the solid acidity of diatomite. The main mechanism is proposed that the furfuryl alcohol is catalyzed by inherent or enhanced acid sites of diatomite and the macro, meso, and microporosity of obtained carbon products result from the replication of diatomite templates, staking of carbon species, and the structure-reconfiguration of the carbon film, respectively.
higher than 0.999, and the simulated adsorption capacity (qe-mod) is pretty close to the experimental data (qe-exp) (Table 2). The good fitting of the pseudo-second-order kinetics implies the chemical adsorption process is the rate-limiting step during the MB adsorption [41–43]. The chemical adsorption took place between the MB molecules and the carbon-surface functional groups. It is noteworthy that C/Dt(SD) exhibits higher initial adsorption rate (53.2 mg g1 min1) than C/Dt(JL) (39.6 mg g1 min1) (Table 2). This result is mainly due to the diffused rate of MB in C/Dt(SD) was higher than that in C/Dt(JL), because MB molecules were more readily entered into the larger micropores in C/Dt(SD) than that in C/Dt(JL). Fig. 6c displays the MB adsorption isotherms of C/Dt(JL), C/ Dt(SD), and CAC. Langmuir model shows the best fitting (R2 > 0.99) in quantitatively describing the adsorption data (Fig. 6d and Table 2) as compared with Freundlich and Redlich-Peterson models (not showed). The Qm values for the three adsorbents follow an order of C/Dt(SD) (333 mg/g) > C/Dt(JL) = CAC (250 mg/ g). This result indicates the diatomite-templated carbons possess higher MB adsorption capacity than commercial activated carbon and many other reported microporous carbons [44], although the SBET and VPORE values of the diatomite-templated carbons are much smaller than those of CAC (SBET = 1013 m2/g and Vpore = 0.686 cm3/ g) and the reported carbons. The relatively low adsorption of MB on microporous CAC is due to that its pore size is concentrated on the range of 0.48–0.58 nm (Fig. 5b, inset), which means MB (molecule size is approximately 1.700.760.33 nm [45,46]) cannot be held by the micropores in CAC. In contrast, the abundant macropores and mesopores in the diatomite-templated carbons provide sufficient places for the adsorption of MB. In addition, some larger sized micropores (diameter P 0.76 nm) are also available for the capturing of MB molecules. This is evidenced by the fact that the micropores with pore size of approximately 1.74 nm in C/Dt(SD) disappeared after MB adsorption, and some new micropores with pore size of 0.93 nm appeared (Fig. 5b), which reflects the filling of MB molecules in 1.74 nm micropores. The decrease in Vpore of C/Dt(SD) from 0.264 to 0.169 cm3/g after MB adsorption also supports the filling of MB molecules in micropores. Similar results are observed for C/Dt(JL) where Vpore decreases from 0.212 to 0.140 cm3/g. However, C/Dt(JL) exhibits less developed porosity and particularly the 1.74 nm microspores than C/Dt(SD), leading to lower MB adsorption. The affinity between MB and the diatomite-templated carbons are evaluated based on the separation factor, RL, determined from the Langmuir adsorption isotherm model [47,48]. RL is defined by the following equation:
3.2. MB Adsorption and regeneration of the diatomite-templated porous carbons
where C0 is the initial concentration (mg L1) of MB, and KL is the Langmuir constant related to the energy of adsorption (L mg1). Different affinity between adsorbent and adsorbate is classified according to the value of RL: unfavorable adsorption (RL > 1), linear adsorption (RL = 1), favorable adsorption (0 < RL < 1), or irreversible adsorption (RL 0) [48]. As showed in Table 2, the RL values corresponding to both diatomite-templated carbons are in the range of 0–1, indicating the favorable MB uptake. In addition, the RL values are close to zero implies the irreversibility of the adsorption processes. This result is in good agreement with the proposed chemical adsorption process. As showed in Table 1, the SBET and Vpore values of the regenerated carbons are determined as follows: C/Dt(SD)-R, 354 m2/g and 0.303 cm3/g; C/Dt(JL)-R, 309 m2/g and 0.223 cm3/g. These values are close to those of C/Dt(SD) and C/Dt(JL) and much higher than those of C/Dt(SD)-MB and C/Dt(JL)-MB (Table 1), indicating
C/Dt(JL) and C/Dt(SD) were used in the MB adsorption tests to evaluate the adsorption capacity of the carbons. Fig. 6a shows the adsorption of MB by the carbons at different adsorption time. The initial MB concentration was set as 500 mg/L. The MB adsorption on both samples reached equilibrium state within 20 min, indicating a fast adsorption process. The carbons show good adsorption for MB, and the MB uptakes at the point of 20 min are determined as follows: C/Dt(JL), 134.7 mg/g; and C/Dt(SD), 144.0 mg/g. As compared to another two models, pseudo-first-order and intraparticle diffusion models (not showed), the pseudo-secondorder simulation model shows the best fitting for the kinetics of the MB adsorption on the carbons (Fig. 6b). The value of R2 is
RL ¼
1 ð1 þ kL C 0 Þ
ð6Þ
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Fig. 6. (a) Effect of agitation time on the MB removal efficiency of C/Dt(JL) and C/Dt(SD); (b) linear fitting plots based on pseudo-second-order kinetic model for the adsorption of MB; (c) the MB adsorption isotherms of C/Dt(JL), C/Dt(SD) and CAC; and (d) linear fitting plots based on Langmuir isotherm model for MB adsorption.
Table 2 Kinetics constants and Langmuir equation parameters for MB adsorption on various adsorbents. Model
Pseudo-secondorder
Langmuir
Parameters
Adsorbents C/Dt(JL)MB
C/Dt(SD)MB
CAC
R2 h (mg g1 min1) qe-mod (mg g1) qe-exp (mg g1)
0.9998 53.2
0.9995 39.6
– –
142.2 138.1
155.5 148.9
– –
R2 KL Qm (mgg1) RL
0.9906 0.015 333.3 0.045– 0.40
0.9910 0.010 250.0 0.066– 0.50
0.9987 0.042 250.0 0.017– 0.19
that most of the adsorbed MB molecules have been removed. It is noteworthy that the SBET values of the regenerated carbons are even slightly larger than the carbons before MB adsorption (Table 1). One possible reason is that the adsorbed MB molecules underwent carbonization during the regeneration process, creating some new micropores and mesopores and thus enhanced the porosity of the regenerated carbon.
4. Conclusions A facile method for the preparation of the porous carbon was proposed, and in this method, the natural diatomite was utilized as both template and catalyst. The obtained carbon product showed hierarchically porous structure, possessing macropores, mesopores, and micropores, simultaneously. The macroporous structure of the diatomite-templated carbon was derived from the direct replication of the diatomite template. The partly solid carbon pillars and ordered hollow carbon tubes interconnected by thin carbon walls were replicated from the central and edge pores of diatom shell of genus Coscinodiscus Ehrenberg, respectively. The end-closed carbon tubes were derived
from the replication of diatom shell of genus Synedra Ehrenberg. The mesopores and size-uniform wormhole-like micropores resulted from the stacking of the graphite microcrystals and the structure-reconfiguration of the carbon film during the removal of templates, respectively. The surface area and pore volume of the carbon products increased with the increment of the acid amount of templates, showing a positive correlation between the acidity of the diatomite template and the pore parameters of resulting carbons. The hierarchically porous carbon had good adsorption capacity for MB. The MB uptake followed the pseudo-second-order model during which chemical adsorption of MB took place on the carbon surface. Langmuir adsorption isotherm model well fit in the adsorption data. The adsorption capacities of MB on the diatomite-templated carbons were dramatically higher than that of commercial activated carbon because of the hierarchically porous structure of the diatomite-templated carbons. In addition, the regeneration of the carbons was highly efficient because the reused carbons exhibited higher surface area and pore volume than those of initial carbons. In conclusion, the morphology, porosity, and solid acidity of initial diatomite have significantly effects on the textural property of diatomite-templated carbons, which further affect the MB adsorption capacity of the carbons. These fundamental results demonstrate that the diatomite-templated carbon is promising for applications in the fields of adsorption and purification. Acknowledgments The financial supports from the National Natural Scientific Foundation of China (Grant No. 40872042) and Natural Science Foundation of Guangdong Province, China (Grant No. 8151064004000007) are gratefully acknowledged. This is a contribution (No. IS1545) from GIGCAS. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.08.023.
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