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Synthesis of Hybrid Adsorbents Combining Sol-Gel Processing and Molecular Imprinting Applied to Lead Removal from Aqueous Streams Carlos A. Quirarte-Escalante,† Victor Soto,‡ Wencel de la Cruz,§ Gustavo Rangel Porras,| Ricardo Manrı´quez,⊥ and Sergio Gomez-Salazar*,† Departamento de Ingenieria Quimica, UniVersidad de Guadalajara-Centro UniVersitario de Ciencias Exactas e Ingenierı´as, BlVd. Marcelino Garcı´a Barraga´n #1451, Guadalajara, Jalisco, Mexico 44430, Departamento de Quimica, UniVersidad de Guadalajara-Centro UniVersitario de Ciencias Exactas e Ingenierı´as, BlVd. Marcelino Garcı´a Barraga´n #1451, Guadalajara, Jalisco, Mexico 44430, Centro de Ciencias de la Materia Condensada, UniVersidad Nacional Auto´noma de Mexico, km 107 Carretera Tijuana-Ensenada, Ensenada, B.C. Me´xico 22830, Centro de InVestigaciones en Quı´mica Inorganica-UniVersidad de Guanajuato, Noria Alta s/n, Colonia Noria Alta, Guanajuato, Guanajuato, Mexico 36050, and Institut fu¨r Chemie and Biochemie, Freie UniVersita¨t Berlin, Takustrasse 3, Berlin D-14195, Germany ReceiVed May 31, 2008. ReVised Manuscript ReceiVed NoVember 15, 2008
Two high-capacity thiol functionalized adsorbents are prepared, using sol-gel processing, and applied to the removal of lead(II) from aqueous streams. The first adsorbent (SN) is prepared by co-condensing oligomers of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPS); the second adsorbent (MI) is synthesized by a combined co-condensation/molecular imprinting route of TEOS and MPS. The resulting physicochemical properties of adsorbents are investigated by nitrogen sorption measurements, elemental analysis, Fourier transform infrared spectroscopy (FTIR), solid-state 13C and 29Si crosspolarization magic angle spinning nuclear magnetic resonance (13C and 29Si CPMAS NMR, respectively), and X-ray photoelectron spectroscopy (XPS). The adsorbents exhibit high ligand densities (1.19 mmol/g for SN and 1.03 mmol/g for MI), improved Brunauer-Emmett-Teller (BET) surface areas (SBET ) 129 m2/g for SN and 464 m2/g for MI), and highly developed mesoporosity (Dp ) 15.1 nm for SN and 8.3 nm for MI). 29Si CPMAS NMR measurements indicate that the silicon oxide solid structure of adsorbents is not modified by lead adsorption. XPS results indicate the presence of lead acetate species on the surface of adsorbents. Batch adsorption data are explained by a mechanism in which a hydrated species (Pb(OOCCH3)(H2O)5+) forms a monodentate complex with thiol surface groups. Further characterization of the adsorbents shows rapid adsorption kinetics and equilibrium lead(II) adsorption capacities of 1.13 and 0.715 mmol/g for SN and MI. Lead adsorption dynamics in a packed column indicates high lead uptakes (155 and 80 mg Pb/g-adsorbent for SN and MI, respectively). Combined and simple sol-gel synthesis routes for preparation of adsorbents with high ligand densities and mesoporous structures are demonstrated here.
1. Introduction Lead is one of the transition metals that is the most toxic to humans and all living systems.1 This metal is discarded through wastewater streams from industrial processes such as electronics, batteries, pigments, and photographic materials, thus posing a threat to humans and the environment.2 Exposure to low levels of lead is associated with behavioral abnormalities, hematological disorders, liver damage, inhibi* Author to whom correspondence should be addressed. Tel. and Fax: +52 33 3650 1793. E-mail: sergio.gomez @cucei.udg.mx. † Departamento de Ingenieria Quimica, Universidad de Guadalajara-Centro Universitario de Ciencias Exactas e Ingenierı´as. ‡ Departamento de Quimica, Universidad de Guadalajara-Centro Universitario de Ciencias Exactas e Ingenierı´as. § Centro de Ciencias de la Materia Condensada, Universidad Nacional Auto´noma de Mexico. | Centro de Investigaciones en Quı´mica Inorganica-Universidad de Guanajuato. ⊥ Institut fu¨r Chemie and Biochemie, Freie Universita¨t Berlin.
(1) Meena, A. K.; Mishra, G. K.; Rai, P. K.; Rajagopal, C.; Nagar, P. N. J. Hazard. Mater. 2005, 122 (1-2), 161–170. (2) Patterson, J. W. In Metal Speciation, Separation and RecoVery; Lewis Publishers: Chelsea, MI, 1987.
tion of motor development on infants, learning impairment, decreased hearing, and impaired cognitive functions in humans.3-8 As a result, the presence of lead in wastewater streams has prompted federal and international environmental protection agencies to establish strict regulations to avoid contaminating water sources for human consumption (e.g., limits of 15 and 10 µg/L in drinking water).9,10 (3) Dojildo, J. R.; Best, G. A. Chemistry of Water and Water Pollution; Great Britain, 1993; Ch. 2, pp 59-204. (4) Cory-Slechta, D. A.; Pound, J. G. Handbook of Neurotoxicology (Neurological Disease and Therapy); Chang, L. W., Dyer, R. S., Eds.; Marcel Dekker: New York, 1995; pp 61-89. (5) Agelidis, T.; Fytianos, K.; Vasilikiotis, G. EnViron. Pollut. 1988, 50, 243–251. (6) Needleman, H. Annu. ReV. Med. 2004, 55, 209–222. (7) Payne, J. C.; ter Horst, M. A.; Godwin, H. A. J. Am. Chem. Soc. 1999, 121, 6850–6855. (8) Bouton, C. M. L.; Frelin, L. P.; Forde, C. E.; Godwin, H. A.; Pevsner, J. J. Neurochem. 2001, 76, 1724–1735. (9) USEPA Nutrient Criteria Technical Guidance Manual: Lakes and ReserVoirs; United States Environmental Protection Agency (USEPA): Washington, DC, April 1999; DocumentEPA 822-D-99-001.
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A substantial effort has been invested in the development of more-effective techniques to remove lead and other metal ions from aqueous streams at acceptable concentration levels. These include precipitation,11 ion exchange,12 adsorbent zeolites,13 activated carbon,14 mesoporous materials (e.g., MCM-4115 and their modifications using surfactants of different lengths of aliphatic chains as the structure directing agents16), metallic oxides,17 and natural products (such as chitosan or pine crust).18,19 However, most techniques present limitations, ranging from low lead removal efficiency (e.g., precipitation and ion exchange) and cost feasibility (e.g., ion exchange), to a loss of surface area and a reduction of pore diameter and pore volume (e.g., mesoporous molecular sieves). To overcome these limitations, hybrid functionalized adsorbents have been synthesized through both sol-gel processing and molecular imprinting techniques and used in the removal of heavy-metal ions to sub-parts-per-million (sub-ppm) levels.20,21 In the case of sol-gel processing, the method requires the hydrolysis and co-condensation of organosilanes with cross-linking agents to obtain mesoporous hybrid functionalized solids.22-24 The technique has been proven to be highly efficient in the synthesis of adsorbents that feature high mechanical strength, excellent chemical and thermal stabilities, rigid pore structures, and substantially improved metal uptake capacities, because they allow molecular accessibility to large internal surface areas and volumes.23,25-28 On the other hand, molecular imprinting is a synthetic process that leads to the formation of template-defined binding sites in (10) World Health Organization. Guidelines for Drinking Water Quality, Vol. 1,Recommendations, Second Edition; World Health Organization (WHO): Geneva, Switzerland, 1993. (11) Esalah, J. O.; Weber, M. E.; Vera, J. H. Sep. Purif. Technol. 2000, 18, 25–36. (12) Davis, M. L.; Cornwell, D. A. Introduction to EnVironmental Engineering, Third Edition; McGraw-Hill: New York, 1998; 765 pp. (13) Ahmed, S.; Chughtai, S.; Keane, M. Sep. Purif. Technol. 1998, 13, 57. (14) Macias-Garcı´a, A.; Valenzuela-Calahorro, C.; Go´mez-Serrano, V.; Espinosa- Mancilla, A. Carbon 1993, 31 (8), 1249–1255. (15) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (16) (a) Showkat, A. M.; Zhang, Y.-P.; Kim, M. S.; Gopalan, A. I.; Reddy, K. R.; Lee, K.-P. Bull. Korean Chem. Soc. 2007, 28 (11), 1985–1992. (b) Showkat, A. M.; Lee, K.-P.; Gopalan, A.; Kim, S.-H.; Choi, S.-H. Polymer 2005, 46, 1804. (17) Theis, T. L.; Iyer, R.; Ellis, S. K. J. Am. Water Works Assoc. 1992, 101–105. (18) Bailey, S.; Olin, T.; Bricka, M.; Adrian, D. Water Res. 1999, 33 (11), 2469–2479. (19) Meunier, N.; Blais, J.; Tyagi, R. Hydrometallurgy 2002, 67, 19–30. (20) Nam, K. H.; Tavlarides, L. L. Chem. Mater. 2005, 17, 1597–1604. (21) Yoshida, M.; Uezu, K.; Goto, M.; Furusaki, S. Macromolecules 1999, 32, 1237–1243. (22) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (23) Lee, J. S.; Gomez-Salazar, S.; Tavlarides, L. L. React. Funct. Polym. 2001, 49, 159–172. (24) Hench, L.; West, J. Chem. ReV. 1990, 90, 33–72. (25) Gomez-Salazar, S.; Lee, J. S.; Heydweiller, J. C.; Tavlarides, L. L. Ind. Eng. Chem. Res. 2003, 42 (14), 3403. (26) Nam, K. H.; Gomez-Salazar, S.; Tavlarides, L. L. Ind. Eng. Chem. Res. 2003, 42, 1955. (27) Nam, K. H.; Tavlarides, L. L. SolVent Extr. Ion Exch. 2003, 21 (6), 899. (28) Deorkar, N. V.; Tavlarides, L. L. Ind. Eng. Chem. Res. 1997, 36, 399– 406.
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synthetic polymers.29,30 Typically, this involves free-radical copolymerization of a functional monomer and a crosslinking agent in the presence of a target molecule that acts as a molecular template. Subsequent removal of the template molecule from the solid polymer matrix reveals an imprinted recognition site (complementary to the template, in regard to shape and position of functional groups) that is selective for the original molecular template and its structural analogues.31 There are reports of molecularly imprinted adsorbents for metal ion removal/recognition prepared from both organic polymers and preformed silica.32,33 Furthermore, a study indicates that materials prepared by a hierarchically imprinted route appear to have cavities (0.1-0.3 nm in size) that exhibit ionic recognition upon removal of the metal ion from the complex and the removal of surfactant micelles resulted in the formation of relatively large, cylindrical pores (2.5-4.0 nm in diameter) that gave the gel an overall porosity which included large surface areas and excellent metal-ion transport kinetics.34 However, the adsorbents present low metal uptake capacities, mainly because of the low ligand densities obtained. Given our interest in industrial applications of adsorptive solids that can be prepared under mild reaction conditions, two sol-gel synthesis routes are employed here to synthesize thiol-functionalized adsorbents that are expected to obtain high ligand densities, improved metal-ion uptake kinetics, enhanced surface areas, and mesoporous structures to facilitate the entrance of metal ions to chelating sites. The routes include (1) co-condensation of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPS), and (2) combined co-condensation/molecular imprinting of TEOS and MPS; lead acetate is used as the sacrificial spacer to create a more accurately designed template capable of accommodating lead species, either a Pb ion complex (as proposed in this study) or free lead ions.35 Hence, the aim of the present study is to report on the final structural properties of the thiol-functionalized adsorbents that have been synthesized employing the two sol-gel routes. The materials are characterized by N2 adsorption measurements, elemental analysis, solid-state 13C and 29Si cross-polarization magic angle spinning nuclear magnetic resonance (13C and 29 Si CPMAS NMR), and Fourier transform infrared spectroscopy (FTIR) techniques. X-ray photoelectron spectroscopy (XPS) is conducted to examine the chemical states of the lead species adsorbed and the surface functional moieties. An insight of the lead adsorption process is gained through a proposed adsorption mechanism. Finally, application of the (29) (a) Wulff, G. B.; Hcide, G.; Helfmeier, G. J. Am. Chem. Soc. 1986, 108, 1089–1091. (b) Wulff, G. B.; Hcide, G.; Helfmeier, G. React. Polym. Ion Exch. Sorbents 1987, 6, 299-310. (30) Alexander, C.; Andersson, H. S.; Andersson, L. I.; Ansell, R. J.; Kirsch, N.; Nicholls, I. A.; O’Mahony, J.; Whitcombe, M. J. J. Mol. Recognit. 2006, 19, 106–180. (31) Wulff, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812–1832. (32) Burleigh, M. C.; Dai, S.; Hagaman, E. W.; Lin, J. S. Chem. Mater. 2001, 13, 2537–2546. (33) Fang, G.-Z.; Tan, J.; Yan, X.-P. Anal. Chem. 2005, 77, 1734–1739. (34) Dai, S.; Burleigh, M. C.; Ju, Y. H.; Gao, H. J.; Lin, J. S.; Pennycook, S. J.; Barnes, C. E.; Xue, Z. L. J. Am. Chem. Soc. 2000, 122, 992– 993. (35) Whitcombie, M. J.; Rodriguez, M. E.; Villar, P.; Vulfson, E. V. J. Am. Chem. Soc. 1995, 117, 7105–7111.
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adsorbents for lead uptake from aqueous streams is examined through batch and column experiments. 2. Equilibrium Adsorption Model To obtain a quantitative understanding of the adsorption phenomena of Pb ions on the synthesized adsorbents, an adsorption mechanism is proposed, based on the aqueous phase chemical equilibria. The aqueous phase system studied is Pb(NO3)2-CH3COOH/CH3COONa-H2O. The chemical species considered are: Pb2+, PbOH+, Pb(OH)2, Pb(OH)3-, PbOOCCH3+, Pb(OOCCH3)2, Pb(OOCCH3)3-, Pb(OOCCH3)42-, and PbNO3+. The calculation of species concentrations (using MINEQL36) at 303 K included the action mass laws for each species considered, the corresponding mass balances for [Pb(II)]tot, [-OOCCH3]tot, and [NO3-]tot and the solution electroneutrality condition (see Tables S1 and S2 in the Supporting Information). This temperature was chosen to simulate the conditions at which many lead-containing industrial wastes from lead plating baths and battery plants are treated in packed columns before being discharged to the sewage. Chemical speciation calculations indicate that the fully hydrated species with potential to be adsorbed are Pb(NO3)(H2O)5+ and Pb(OOCCH3)(H2O)5+ and are predominant in the pH range of 3-5.5 (see Figure S1 in the Supporting Information). On the other hand, typical pKa values of thiol groups on the solid phase are ∼6.5, which indicates that these groups are predominantly protonated at pH 3-5, where all experiments are conducted.37 This is the pH range at which many wastewaters from industries are disposed.38 As a result, electrostatic forces for the lead complex-thiol association can be neglected and the Pb complex ions must displace the proton from the thiolate site via an ion-exchange reaction. The reaction scheme proposed considers the formation of a mondentate lead complex-thiol surface species, as follows: RSH + R1Pb+ a (RS)PbR1 + H+
Keq )
q(RS)PbR1CH+ qRSHCR1Pb+ (1)
where R1Pb+ can be either Pb(NO3)(H2O)5+ or Pb(OOCCH3)(H2O)5+ (R1 ) NO3- or CH3COO-) in the aqueous phase, RSH is an unreacted thiol ligand on the solid surface, (RS)PbR1 is the lead-thiol complex formed on the solid surface, and R is a propyl group. The parameters q and C refer to concentrations in the solid and liquid phases, respectively, and Keq is the apparent adsorption equilibrium constant. An adsorption isotherm is derived from the reaction scheme described by expression 1, under the assumptions that all surface sites are available for lead adsorption, no electrostatic effects are present, activity coefficients are unity, (36) Schecher, W. D.; McAvoy, D. C. MINEQL+: A Chemical Equilibrium Program for Personal Computers; The Procter and Gamble Company: Cincinnati, OH, November 1998. (37) Gomez-Salazar, S. Ph.D. Dissertation, Syracuse University, Syracuse, NY, 2002. (38) Scoullos, M. J.; Vonkeman G. H.; Thornton I.; Makuch Z. In Mercury-Cadmium-Lead. Handbook for Sustainable HeaVy Metals Policy and regulation; Kluwer Academic Publishers Dordrecht, The Netherlands, 2001.
and geometric restrictions are neglected. The resulting expression is q)
Keq(qRSH)TOTCR1Pb+ CH+ + KeqCR1Pb+
(2)
where q is the amount of lead (present as (RS)PbR1) on the solid surface (q ) q(RS)PbR1, given in units of mmol/g) and (qRSH)TOT is the saturation capacity of the adsorbent (also given in units of mmol/g), which, here, is taken to be the value determined from elemental sulfur analysis. Equation 2 is a modified Langmuir-type isotherm that allows investigation of the effect of solution pH on lead uptake extent. The equilibrium constant (Keq) is evaluated by nonlinear regression of experimental data using eq 2 and the modified nonlinear least-squares algorithm of Levenberg-Marquardt.39 In the regression, Keq is used as the adjusting parameter. The nature of the lead-thiol complex on the surface of adsorbents is elucidated from FTIR and XPS measurements. 3. Experimental Section 3.1. Materials. All of the reagents used in this study were of analytical reagent (AR) grade and are used as received. TEOS, MPS, lead nitrate, and lead acetate were purchased from Sigma Chemical Co. (St. Louis, MO). 3.2. Synthesis of Adsorbents. TEOS is used as a cross-linking agent, MPS was used as a functional precursor, and triethylamine (TEA) was added to the mixture to induce gelation. Two synthesis routes were followed to prepare the adsorbents: (1) co-condensation between MPS and TEOS (the SN adsorbent), and (2) co-condensation/molecular imprinting between MPS and TEOS (the MI adsorbent). Synthesis of SN Adsorbent. (Not shown in Figure 1; details can be found elsewhere.23) Briefly, TEOS and MPS are independently homocondensed in mixtures of acidic water and ethanol to give the desired degree of oligomerization. The molar ratios are given as follows: MPS:EtOH:H2O:HCl:NaCl ) 1:3:3:0.011:0.01 and TEOS:EtOH:H2O:HCl:NaCl ) 2:8:8:0.012:0.02. The partially homocondensed silanes are mixed and stirred to co-condense for 2-5 min at room temperature. A desired amount of TEA is added at this stage. The gelled materials are aged in a reactor for 1 day at 298 K and dried for 24 h at 353 K. Synthesis of MI Adsorbent. (See Figure 1). A molecular template for the Pb complex is obtained by reacting MPS with lead acetate in an alcoholic solution until complete dissolution of the lead salt (see reaction 2.4 in Figure 1). The mixture is hydrolyzed (see reaction 2.5 in Figure 1; the molar ratio of MPS:PbAc:EtOH:H2O is 1:0.125:3:3) and homocondensed (reaction 2.6 in Figure 1). Cocondensation of the mixture is conducted (see reaction 2.7 in Figure 1) with previously hydrolyzed TEOS and homocondensed (see reactions 2.1-2.3 in Figure 1; the molar ratio of TEOS:EtOH:H2O: HCl:NaCl is 1:4:4:0.006:0.01; the molar ratio of MPS:TEOS is 0.5). TEA is added at this stage. The lead template is removed by washing the adsorbent with 5.0 mol dm-3 HCl until the lead concentration in the washings is undetectable by atomic absorption analysis (see reaction 2.8 in Figure 1). The materials prepared via the two synthetic routes are subjected to a hydrothermal treatment, which consists of washings with acetone refluxed at 333 K for 24 h and dried at 333 K. The adsorbents are ground and sieved to obtain particle sizes in the range of 125-180 µm, using ASTM II specifications for sieve trays. (39) OriginPro 7.5; OriginLab Corporation Northampton, MA, 2003.
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Figure 1. Combined co-condensation/molecular imprinting synthesis route used to prepare the MI adsorbent for lead removal from aqueous streams; the scheme for reaction 2.8 in this figure is an idealized representation of the solid obtained.
3.3. Physicochemical Properties of Adsorbents. Nitrogen adsorption isotherms are obtained at 77 K on N2 adsorption equipment (Model ASAP 2020, Micromeritics, Norcross, GA). Approximately 0.5 g of sample is heated at 423 K overnight under vacuum, to remove all adsorbed species from the surface of adsorbents. The Brunauer-Emmett-Teller (BET) surface area and the characteristic parameter of the adsorbent-adsorbate interactions (CBET) are calculated using adsorption data in the relative pressure range of 0.15-0.26 included in the validity domain of the BET equation. Pore volume and pore size distributions are obtained from adsorption measurements by the Barrett-Joyner-Halenda (BJH) method.40 Theoretical metal uptake capacities of the adsorbents are determined from elemental sulfur analysis, using an EA 1108 Fisons instrument. (40) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373–380.
3.4. Lead Adsorption Experiments. Kinetic experiments are conducted in a batch reactor on SN and MI samples at 303 K using 50 cm3 of a 200 ( 1.5 mg dm-3 lead solution at pH 4, buffered with 0.2 mol dm-3 acetic acid/NaOH. The adsorbents are conditioned in deionized (DI) water overnight, prior to contact with the lead solution, to ensure wetting of pores. An adsorbent amount of 0.25 g (with a particle size range of 125-180 µm) is introduced into the 250 cm3 reactor at time zero and samples of the solution are taken at time intervals and filtered prior to analysis of the lead concentrations. Equilibrium adsorption isotherms are obtained in the batch mode to determine the maximum lead uptake at a pH range of 3.0-5.0 to cover the useful pH range of buffer used. Initial solutions are prepared at several metal concentrations (50-3000 mg dm-3) and fixed pH by dilution from a Pb(NO3)2 stock solution. A 0.2 mol dm-3 acetic acid/NaOH buffer solution is used for pH control.
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Equilibrium adsorption experiments are conducted by contacting ∼0.25 g of adsorbent (with a particle size range of 180-125 µm) with 50 cm3 of initial solution for 16 h (as determined from the kinetic measurements; this is the time after which the amount of metal adsorbed remains unchanged and is taken to guarantee that true adsorption equilibrium is reached) in an Environ-Shaker Model 3597 instrument at 303 K. Suspensions are filtered, the pH values of initial and final solutions are measured with a pH meter (Metrohm, Model 827), and isotherms are reported at pHeq; solutions are analyzed for total metal concentration with an atomic absorption spectrophotometer (Model SpectrAA 220, Varian, Victoria, Australia). The amount of metal adsorbed on the surface of adsorbents is calculated by a mass balance (between initial and final solutions) expressed on an adsorbent mass basis as
q)
V(CPb)0 - (CPb)eq m
(3)
where q is the amount of lead adsorbed (given in units of mmol/ g); (CPb)0 and C(Pb)eq are lead concentrations (given in units of mmol dm-3) in the initial and final solutions, respectively; V is the volume of solution that contains the metal ions (given in units of dm3); and m is the weight of adsorbent (in grams). The experiments are conducted in triplicate and averaged values are reported. Adsorbents with lead adsorbed are identified as SN-Pb and MI-Pb samples. 3.5. FTIR Studies. FTIR measurements are performed on the functionalized adsorbents to investigate the complexation interactions between Pb ions and thiol surface groups. The spectra are recorded with a Perkin-Elmer Model Spectrum One spectrophotometer. Prior to the measurements, all of the samples were dried for 48 h at 353 K (at this temperature, decomposition of incorporated MPS moieties on samples SN and MI did not occur, according to thermogravimetric analysis/differential thermal analysis (TGA/ DTA) measurements; see Figure S2 in the Supporting Information). KBr waffles are used as support. During measurements, a total of 30 scans are performed within a frequency range of 450-4500 cm-1. 3.6. Solid-State NMR. 29Si CPMAS NMR is used to obtain information about the solid structure of silica, and 13C CPMAS NMR is used to assess the presence of the mercaptopropyl groups into the adsorbent (kinetics of hydrolysis and condensation reactions between TEOS and MPS in the liquid phase has been reported22,23,41). All of the spectra are acquired under a magnetic field of 7 T with a Bruker Model Avance II 300 MHz spectrometer operated at room temperature, with a spinning speed set to 5 kHz and using tetramethylsilane (TMS) as a reference. The samples are packed into a 4-mm inner diameter (ID) ZrO2 rotor. 29Si CPMAS spectra are obtained at a frequency of 59.5 MHz. The 90° pulse width was 4 µs, with a contact time of 5 ms and an acquisition delay of 10 s. This contact time is long enough to assess full crosspolarization of all different Q and T silicon species.42,43 In the case of 13C CPMAS NMR measurements, the operation frequency was 75.4 MHz and the 90° pulse width was ∼4 µs. The CP contact time was 1 ms, with an acquisition delay of 5 s. 3.7. XPS Measurements. The spent adsorbents were analyzed by XPS to determine the chemical states of the surface functional groups and to elucidate the nature of the lead-thiol surface complex that formed. Measurements were performed with an XPS RIBER (41) El-Nahhal, I.; Yang, J.; Chiang, I.; Maciel, G. J. Non-Cryst. Solids. 1996, 208, 105–118. (42) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Millar, G. J.; Zhu, H. Y. J. Phys. Chem. B 1997, 101, 6525–6531. (43) Shenderovich, I. G.; Mauder, D.; Akcakayiran, D.; Buntkowsky, G.; Limbach, H.-H.; Findenegg, G. H. J. Phys. Chem. B 2007, 111, 12088– 12096.
Figure 2. Nitrogen adsorption isotherms and pore size distributions on SN and MI adsorbents at 77 K. Table 1. Physical Properties of SN and MI Adsorbents theoretical average pore pore volume, SBET (qRSH)TOT CBET sample diameter, Dp (nm) Vp(cm3/g) (m2/g) (mmol/g)a (kcal/mol) SN MI a
15.1 8.3
0.76 1.14
129 464
1.190 1.028
79 71
As determined by elemental analysis.
spectrometer. The base pressure in the analyzer was
