ARTICLE pubs.acs.org/est
Recycle of Silicate Waste into Mesoporous Materials Jung Ho Kim, Minwoo Kim, and Jong-Sung Yu* Department of Advanced Materials Chemistry, Korea University, 208 Seochang, Jochiwon, ChungNam 339-700, Republic of Korea
bS Supporting Information ABSTRACT: Template synthesis of porous carbon materials usually requires selective removal of template silica from the carbon/silica composites. It not only involves waste of valuable chemicals, but also poses significant environmental concerns including high waste treatment cost. Recycling of silicates released from such nanocasting methods is successfully performed for the first time to regenerate valuable mesoporous MCM and SBA type silica materials, which will not only help in saving valuable chemicals, but also in decreasing chemical waste, contributing in improvement of our environmental standards. This approach can thus improve cost effectiveness for the mass production of nanostructured carbon and others utilizing silica directed nanocasting method by recycling otherwise silicate waste into highly desirable valuable mesoporous silica.
’ INTRODUCTION The discovery of new M41S mesoporous silica families with the pores larger than 2 nm in diameter in 1992 extended the applications into much bigger dimension1 and has brought a new renaissance in the porous material research.2,3 They possess high surface areas and an easily accessible well-ordered array of uniform mesopores good enough as “Holey Grail” sought after by zeolite chemists of the time.4 Most importantly, the pore sizes exceeded those attainable in zeolites and could be tuned in the nanometer range by choosing an appropriate surfactant templating system, sometimes along with a cosolvent or swelling agent. These mesostructures are formed by controlled packing of cylindrical micelles generated from long chain alkylammonium ion surfactants and subsequent surface coverage by amorphous silica. Recent efforts are focused on the mesoporous MCM,5,6 SBA,7,8 and HMS9 type materials, which are synthesized with the help of cooperative surfactant templating. Tetraalkoxysilane such as tetramethylorthosilicate (TMOS) or tetraethylorthosilicate (TEOS) is usually employed as a silica source. Recently, some attempts have been made to develop green synthesis methods for mesoporous silicas by replacing potentially harmful expensive organosilicates with fumed silica, natural silicate minerals,1012 kanemite,13 water glass,14 or silica gel15 as a silica source. For example, Wang et al. reported synthesis of mesoporous AlMCM-41 using natural mineral, metakaolin.16 Kimura and Kuroda reported the development of ordered mesoporous silica prepared using layered silicates along with surfactant.12 Mesoporous materials with high surface area and periodic arrays of uniform pores can be also used as hosts for the synthesis of polymers and carbon possessing regular ordered pore structures through a nanocasting approach.2,57,1720 Particularly, porous carbon materials have attracted considerable attention r 2011 American Chemical Society
because of their remarkable properties such as high specific surface areas, large pore volumes, chemical inertness, and good mechanical stability, which have great potentials in many areas of modern science and technology, including water and air purification, gas separation, catalysis, chromatography, energy storage, and electrode material for battery, solar cell, and fuel cell.2127 Formation of uniform and 3-dimensionally interconnected pores and specific morphologies in these nanostructured carbon materials is usually achieved by nanocasting of inorganic nanostructured materials as hard templates. The whole procedure of the nanocasting synthesis of mesoporous carbon usually involves presynthesis of a mesoporous silica host, infiltration of a carbon precursor into the mesoporous silica host, carbonization of the carbon precursor infiltrated into the silica host and then selective dissolution of the sacrificial silica host from the carbonsilica composite to generate silica-free mesoporous carbon replica as shown in Figure 1a.1720,28 However, the procedure in such silica-directed synthesis of porous carbons usually requires the selective removal of the template silica from the carbon/silica composite by using harmful hydrofluoric acid or NaOH, not only wasting valuable laboratory/industrial chemicals, but also adding up environmental cleaning cost in form of waste treatment. Although the removal of silica framework by dissolution in aqueous NaOH solution has been already documented, hydrofluoric acid is still largely used for the extraction of silica, in which case, a special reactor is also required with extra care. However, to the best of Received: October 18, 2010 Accepted: March 2, 2011 Revised: February 25, 2011 Published: March 21, 2011 3695
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our knowledge, there is no report yet concerning the recycling of silicate waste into valuable functional silica materials. Herein, we report a recycling method for silicate waste selectively etched during the nanocasting method for the formation of silica-free mesoporous carbon replica. The recycling process represents a regeneration of mesoporous silica materials by a hydrothermal cooperative self-assembly process of silicate oligomers from the silicate waste and added structure directing agent. That is, the silicate waste is not wasted, but rather utilized to produce valuable mesoporous silica materials by this approach. Thus, this method can not only prevent environmental contamination, but also help in saving and recycling chemicals, generating valuable functional materials like mesoporous silica as well as mesoporous carbon replica.
’ EXPERIMENTAL SECTION Preparation of SBA-15 Silica Host, Mesoporous Carbon Replicas and Recycling Silicate Sources. Rod-type 2-D hex-
agonally ordered mesoporous silica SBA-15 hosts were synthesized according to a previous report.29 Mesoporous carbon replica was prepared by a simple gas-impregnation nanocasting method using a calcined SBA-15 host as a hard template.28 The silica template was selectively dissolved by stirring the composite in a 3.03.5 M NaOH solution for 10 min and heating in oven at 353 K for overnight. The solid product was filtered, and washed with ethanol, and dried at 353K and identified as a CMK-3 mesoporous carbon by XRD as expected. The first filtrate containing silicate waste was secured for recycling. The raw silicate waste contained ca. 5 wt % carbon debris and thus was purified by short sonication and filtration through ultrafine filter paper (0.05 um polycarbonate membrane filter, Whatman). The so-obtained silica effluent containing less than 0.3 wt % carbon impurity was used as a precursor for subsequent synthesis of mesoporous silica. Fabrication of Recycled Mesoporous Silica. To synthesize recycled MCM-48 silica,30 30 g of the silicate waste solution was added into a mixture solution containing 2.4 g of cetyltrimethylammonium bromide (CTAB), 27.3 g of deionized water and 50 g of absolute ethanol and mixed for 30 min under vigorous stirring. The final molar composition was CTAB: EtOH: H2O: Na2O: SiO2 = 0.04: 6.82: 15.5: 0.42: 1.00. Subsequently, the mixture of the silica and surfactant solution was placed into a Teflon-lined stainless steel autoclave, heated to 393 K, and held at that temperature for 2 days to yield the MCM-48 silica. The synthesis of recycled MCM-41 silica was similar to that described in MCM-48 and involved the starting molar composition of 30 g silicate waste, 2.4 g CTAB and 135 g DI water.11 The final molar composition was CTAB: H2O: Na2O: SiO2 = 0.04: 53.60: 0.42: 1.00. The mixture solution was stirred for 30 min, charged into a stainless steel autoclave, heated to 393 K for 2 days. The product was filtered, washed with water/ethanol and calcined at 873 K for 6 h. In addition, other different mesoporous materials such as SBA-15 and 16 were also synthesized using triblock copolymer surfactant, P123 and F127, and the same silicate waste. (see details in the Supporting Information)
’ RESULTS AND DISCUSSION Figure 1 schematically illustrates a synthesis route for mesoporous carbon through the nanocasting method using a mesoporous silica, along with a recycling process for preparation of
Figure 1. Schematic procedure for conventional nanocasting method of mesoporous carbon (a) and regeneration of mesoporous silicas by recycling of silicate waste (b).
mesoporous silica materials. In brief, process (a) shows a general nanocasting route of SBA-15 for the formation of CMK-3 as previously reported.28 Process (b) reveals a recycling route of the silicate waste to regenerate mesoporous silica materials. The amorphous silica host in the carbon/silica composite was selectively dissolved in basic NaOH solution. The silicate waste is considered to form a kind of sodium silicate oligomers, which are then adjusted to self-assemble themselves cooperatively with structure-directing agent (for example, CTAB molecules or block copolymers) added in a reaction mixture under the basic conditions. Finally, mesoporous silica materials can be regenerated under proper hydrothermal treatment at high temperature and pressure. Figure 2a and b show high resolution transmission electron micrograph (HR-TEM) images for parent SBA-15 silica and mesoporous carbon replica, CMK-3 viewed along and perpendicular to the direction of the hexagonal pore arrangement. The structure of SBA-15 consists of a hexagonal arrangement of cylindrical mesoporous tubes with 8.5 nm in diameter (HR-SEM, SEM and TEM images in Figure S1 in the Supporting Information), which is similar to the structure of honeycomb-like MCM-41 silica except for random interconnection of the tubes by micropores present in the pore walls. The SBA-15 reveals a rod-type structure with ca. 1.2 um in length and ca. 500 nm in diameter. The SEM and TEM images in Figure S2 in the Supporting Information show that CMK-3 have exactly the same rod-type morphology as that of parent SBA-15 silica in Supporting Information Figure S1. As the HR-SEM and TEM images show, the structure of the CMK-3 carbon is exactly an inverse replica of the parent SBA-15. The fact that the carbon particles are not hollow indicates that the formation of the carbon structure occurred uniformly throughout the entire mesopore volume of the SBA-15 particle. The XRD pattern, chemical analysis, and TEM images indicate not only that the sample was truly homogeneous within a selected batch, but also that this 3696
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Figure 2. Typical HR-TEM images of SBA-15 silica (a) and CMK-3 carbon replica (b), and their relative XRD patterns (c), and nitrogen adsorptiondesorption isotherms obtained at 77 K (d).
Table 1. Structural Properties for SBA-15 silica, CMK-3 Carbon Replica and Various Recycled Mesoporous Silica Synthesized in This Work sample SBA-15 silica
BET surface-area (m2/g) mesopore volume (cm3/g) Pore diameter (nm) a d spacing (nm) unit cell parameter (nm) b wall thickness (nm) 690
0.97
8.5
9.71
11.21
2.7
1210
1.21
3.9
9.12
10.53
6.6
recycled MCM-41
710
1.06
3.0
3.84
4.43
1.4
recycled MCM-48
1260
1.32
2.6
3.25
7.96
1.2
recycled SBA-15-like
750
1.09
6.2
9.67
11.17
4.9
recycled SBA-16-like
620
0.93
4.9
7.43
8.58
3.7
CMK-3 carbon
Maximum value of the BJH pore size distribution peak was deduced from the adsorption branch of the N2 isotherm. b XRD, a0 = 2 d100/31/2 (SBA-15, CMK-3 and MCM-41), a0 = 2 d110/31/2 (SBA-16) and a0 = d211 61/2 (MCM-48). a
synthesis was reproducible. The ordered arrangement of mesopore channels in the SBA-15 silica and CMK-3 carbon nanorods gives rise to the well-resolved XRD peaks as shown in Figure 2c, which can be assigned to (100), (110), and (200) diffractions of the 2-D hexagonal space group (P6mm). In Figure 2d, N2 isotherms obtained for CMK-3 are compared with those for SBA-15. The result indicates that the pore size distribution for SBA-15 is centered at 8.5 nm, while the carbon possesses mesopores with quite narrow pore size distribution centered at 3.9 nm as shown in Figure S1d and S2d in the Supporting Information. The N2 adsorption data provide the BET surface area of 1210 m2/g and 690 m2/g, and the mesopore volume of
1.21 cm3/g and 0.97 cm3/g for CMK-3 and SBA-15, respectively. The structural properties for all the samples synthesized in this work are summarized in Table 1. A photograph of the first filtrate containing silicate waste (Figure 3a) confirms that the slightly turbid solution was retrieved from the silicacarbon composite. Figure 3b shows a composition ternary diagram by weight of Na2OSiO2H2O of three different silicate waste and commercial sodium silicate solutions from J. T. Baker and Aldrich. The main compositions in the silicate waste and commercial sodium silicates were determined by ICP-MS analysis. Based on chemical analysis of the silicate waste solutions, the weight ratio of silica (SiO2) to 3697
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Figure 3. Photograph of a silicate waste (a) and Na2OSiO2H2O ternary diagram of three different silicate wastes and commercial sodium silicates (b).
Table 2. Physicochemical Properties of Commercial Sodium Silicates and Silicate Wastes sample
viscosity (mPa s) a
pHb
62.9
1.24
6.15
11.9
61.3
1.25
6.18
SiO2 (wt%)
H2O (wt%)
10.6
26.5
9.1
29.6
silicate waste (a)
13.6
31.4
54.8
0.19
1.36
34.4
12.1
silicate waste (b)
12.8
30.3
56.7
0.17
1.32
33.9
12.0
silicate waste (c)
14.2
33.2
52.4
0.24
1.39
35.3
12.1
sodium silicate (Aldrich) sodium silicate (J. T. Baker)
a
density (g/cm3)a
Na2O (wt%)
carbon (wt%)
11.6
Measured at 23.5 °C. b Measured at 25 °C.
sodium oxide (Na2O) was found to be in the range of 2.312.37, which is less than the values of commercial sodium silicate solutions (SiO2/Na2O = 2.50 and 3.25 for Aldrich and J. T. Baker, respectively). The physicochemical properties of commercial sodium silicates and silicate waste are shown in Table 2. The raw silicate waste sample usually includes approximately 5.0 wt % of carbon debris. However, the final filtered silicate waste obtained through purification contains less than 0.3 wt % carbon impurity. Interestingly, the pH, density, and viscosity of the silicate waste sample solutions are very similar to each other with about 12.0, 1.36 g/cm3, and 34.5 mPa s, respectively. The pH and density values of the silicate waste samples are slightly higher, but the viscosity values are about 5 times higher than those of the commercial sodium silicates probably due to higher Na2O and SiO2 content in the silicate waste. Soluble silicates are complex mixtures of an almost infinite number of silicate anions.31 The larger anions are known to be two- or three-dimensional condensation products of silicate monomer, SiO44. The 29Si NMR spectra provide quantitative information on various silicate species in silicate solution. Figure 4 represents 29Si NMR spectra of the commercial sodium silicate and silicate waste solutions after baseline correction. Standard Qn notation, in which n indicates the number of neighboring silicon atoms connected, through intermediate oxygen atoms, to the atom in question is commonly used to describe 29Si NMR spectra. The 29Si NMR peaks can be assigned as follows according to previous work:31 Q0 ∼ 72 ppm, Q1 ∼ 79 ppm, Q2(3R) (three ring) ∼ 82 ppm, Q2 and Q3(3R) (three ring) ∼ 87 to 91 ppm, Q3(4R) (four ring) ∼ 96 to 98 ppm, Q4 ∼ 108 ppm (broad). With SiO2:Na2O ratio >2.0 like in the current samples, some of the silicates condense to
Figure 4. 29Si NMR spectra of commercial sodium silicate (J. T. Baker) and silicate waste (c).
polymeric (colloidal) silica. The distribution of these silicate anions varies mainly with silicate concentration and the relative molar concentrations of Na cation and silica. The recycled silicate with lower SiO2/Na2O ratio of 2.34 shows relatively sharp and strong Q1, Q2, and Q2(3R), Q3(4R) signals and thus likely contains more low molecular weight silicate anions, that is, dimeric, cyclotrimeric and low membered ring and chain silicate anions. Interestingly, the silicate waste also seems to contain more large polymeric silicate anions corresponding to Q4 probably due to higher silicate concentration (∼10.5 M) than J. T. Baker commercial sodium silicate with 8.0 M silicate concentration. Other silicate wastes show similar distribution of silicate species. On the other hand, although it is still unclear, the commercial sodium silicate with higher SiO2/Na2O ratio of 3.25 seems to show some better preference to medium polymeric silicate anions containing branching and cross-linking units. Based on this data, the silicate waste is considered to be a kind of new 3698
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Environmental Science & Technology sodium silicate, which is different from well-defined commercial sodium silicate as shown in Table 2, exhibiting different physical and chemical properties. The Q4 silicate species correspond to a variety of fully connected Si centers in which virtually every silicon atom connects to another. It is known that the highly polymeric Q4 silicate species are not suitable as silica source for synthesis of ordered mesoporous silicas.32 However, when the Q4 species were decomposed into to small sized Q0∼Q3 states by dissolving colloidal silica suspension with NaOH, mesoporous silica was successfully obtained with the solution. Our silicate waste possesses Q4 silicates species corresponding to 710% of overall Si species by 29Si NMR, while the rest of the species represents lower molecular weight Q0∼Q3 state good for formation of mesoporous structures. It is also likely that heavily oligomeric Q4 species may not participate in the formation of mesoporous structures in accordance with earlier findings.32 The Q4 silicate species have little influence on the formation of the mesoporous structures as proved by XRD and TEM data. In addition, the current recycling synthesis was performed with Teflon-lined stainless steel autoclave at 393 K. Such high temperature conditions may increase the chance for relatively small low molecular weight Q4-state species to participate in the further silica polymerization for the formation of mesoporous silica. As of right now it would not possible to
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speculate with confirmation and accuracy about the size of Q4 silicate species in our etched silica solutions although relatively small low molecular weight Q4-state species less than 1.0 nm in size may participate in the further silica polymerization for the formation of mesoporous silica with wall thickness around 2 nm. Further work on the size and properties of Q4 silicate species is planed as future work with 29Si NMR, FTIR, vapor phase osmometry, GCMS and other analysis techniques. Spherical primary particles of MCM-41 and MCM-48 materials were synthesized using the silicate waste as a silica precursor and CTAB as a structure-directing agent.11,30 Figure 5, and Supporting Information Figures S3, and S4 show the electron micrographs of resulting recycled MCM-41 and MCM-48 primary particles. The particle sizes estimated from the TEM and SEM images are 250600 nm for the MCM-41 and 150500 nm for the MCM-48. Highly ordered mesoporous structures are clearly seen in the HR-TEM images for the recycled MCM-41 and MCM-48 silica. The XRD patterns exhibit primarily a very intense (211) diffraction peak centered at 2θ = 2.72 along with clear (200) and weaker (420) reflections characteristic of MCM-48 framework with a cubic (Ia3d) structure. The MCM-41 also gives rise to the well-resolved XRD peaks shown in Figure 5c, which can be assigned to (100), (110), and (200) diffractions typical of a 2-D hexagonal pore structure.
Figure 5. Typical HR-TEM images of recycled mesoporous MCM-41 (a) and MCM-48 (b), and their comparative XRD patterns (c) and nitrogen adsorptiondesorption isotherms (d). 3699
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Environmental Science & Technology N2 sorption isotherms at 77 K were measured for the recycled MCM-41 and MCM-48 samples (Figure 5d). All the samples exhibited a type IV isotherm characteristic of mesoporous materials according to the IUPAC nomenclature.33 Nitrogen sorption isotherms for the MCM-41 and MCM-48 silica exhibit a steep step for capillary condensation at the relative pressure of ca. 0.20, which is typical for ordered mesoporous silica with a narrow distribution of mesopores. Their corresponding pore size distribution curves are shown at Supporting Information Figures S3d and S4d with narrow pore size distribution centered at 3.0 and 2.6 nm, respectively. The BET surface area and mesopore volume for the MCM41 silica are 710 m2/g and 1.06 cm3/g while those for the MCM-48 are 1260 m2/g and 1.32 cm3/g, respectively. In addition, regeneration of other mesoporous materials such as SBA-15 and SBA-16 was also successfully performed using general triblock copolymer surfactant such as P123 and F127 in the same recycled silicate conditions. SBA-15 and SBA-16 are prepared only in highly acidic conditions. Thus, TEOS has been usually used as a silica source in such acidic synthesis conditions, while highly basic sodium silicates were rarely used for the synthesis of SBA-15 and SBA-16 mesoporous silica. Only a few reports are available for the synthesis of SBA-15 and 16 using sodium silicate in the literature so far.34,35 The most critical part of the synthesis, which employs basic sodium silicate in the syntheses of SBA-15 and SBA-16 was to ensure that the solution environment of the silicate species should change from initial strong basic conditions to final strong acidic conditions. Recycled SBA-15 mesoporous silica was prepared with P123 surfactant under acid conditions as explained in the Supporting Information. Typically, the SEM and TEM micrographs showed evidence of banana-shaped particles consisting of a hexagonally ordered array of mesostructured channels as shown in Supporting Information Figure S5. The XRD pattern for the mesoporous silica exhibits three intense diffraction peaks at 2θ = 0.91o, 1.57o, and 1.81o corresponding to (100), (110), and (200) diffractions, which are characteristic of the 2-D hexagonal structure typical of SBA-15 (Supporting Information Figure S5c). Nitrogen sorption experiments were performed to evaluate the overall porosity of the mesoporous silica material (Supporting Information Figure S5d). The recycled mesoporous silica showed type IV isotherms with parallel hysteresis loops in the P/Po range of 0.40.9, consistent with the mesoporous nature of the skeletal walls. The BET surface area and mesopore volume are 750 m2/g and 1.09 cm3/g, respectively. Compared to typical SBA-15 silica shown in Figure 2d, the recycled mesoporous silica prepared using P123 reveals high intense (110) and (200) diffractions and tilted parallel hysteresis loops indicative of a bit broader pore size distribution. Despite of that, overall, the recycled mesoporous silica with P123 is very similar to SBA-15 along with the bananashaped particle morphology and TEM image for ordered mesopores as shown in Supporting Information Figure S5. SBA-16-like mesoporous silica was also prepared by similar synthesis procedure using triblock surfactant, F127. Like SBA-15, it is synthesized under acidic conditions using nonionic Pluronic surfactant and therefore can possess intrawall complementary pores. The SEM and TEM images in Supporting Information Figure S6 are similar to those reported earlier for SBA-16.36 The XRD pattern of the calcined sample (Supporting Information Figure S6c) shows a strong (110) reflection at 2θ = 1.19o of the cubic Im3m structure along with a small shoulder (200) reflection at 2θ = 1.84o. Both reflections yield an a0 value of 8.58 nm with cubic Im3m structure. The structural assignment of the
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calcined mesoporous sample with a body-centered cubic (Im3m) structure can be verified by TEM image. The N2 adsorptiondesorption isotherm of the calcined sample and its pore size distribution are presented in Supporting Information Figure S6d. Interestingly, the N2 isotherm reveals triangular hysteresis curve at a relative pressure of 0.400.80, which is typical of SBA16. Both the XRD pattern and N2 isotherm are also similar to those of SBA-16 reported previously.36 The pore size distribution calculated on the adsorption isotherm shows a narrow distribution around a pore diameter of 4.9 nm. The surface area as calculated by BET method is 620 m2/g. The mesopore volume of this material is 0.93 cm3/g. In summary, sacrificial silicate waste released from typical template synthesis of nanostructured carbon was recycled to regenerate highly valuable mesoporous silica with several different crystalline phases for the first time. The silicate waste as a silica source is considered to be a kind of new sodium silicate, which is different from well-defined commercial sodium silicate. So-obtained sodium silicate can have different structures and degrees of polymerization in addition to the difference in composition, viscosity and pH compared to commercial one, exhibiting different physical and chemical properties.37 Thus, the current silicate wastes are totally new sodium silicate materials. Synthesis of mesoporous silica is very sensitive to various experimental conditions such as pH, temperature, composition, structure-directing agent and particularly source of silica. Furthermore, this is a novel recycle of otherwise silica waste into valuable mesoporous silica materials, which has never been examined before and has also tremendous scientific interest for the minimization of chemical waste and the benefit of the environment if nanostructured carbon is to be produced in commercial large scale in industry. Silica recovery yields were found to be in the range of 6575% by determining the weight ratios of parent SBA-15 silica and newly synthesized recycled mesoporous silica. In particular, it was 67% for recycled SBA-15 silica. Optimum conditions continue to be sought for the synthesis of higher quality recycled mesoporous silica using the silicate waste. In addition, further work regarding distribution and contribution of silicate species in silica wastes for the synthesis of ordered mesoporous silica is planed as future work.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental and characterization; TEM, SEM and HR-SEM images; N2 adsorption and XRD data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]; phone: (þ81)-41-860-1494; fax: (þ81)-41-867-5396.
’ ACKNOWLEDGMENT We thank National Research Foundation grant funded by the Korean government (KRF 2010-0029245) and Human Resources Development Program (2009) of KETEP for financial support. Special thanks are given to KBSI at Jeonju, Chuncheon and Daegu for TEM and SEM analysis, XRD and 29Si NMR measurements. 3700
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