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Structural Investigations of Silica Polyamine Composites: Surface Coverage, Metal Ion Coordination, and Ligand Modification Mark A. Hughes, Dan Nielsen, and Edward Rosenberg* Department of Chemistry, UniVersity of Montana, Missoula, Montana 59812
Roberto Gobetto and Alessandra Viale Dipartimento di Chimica IFM, UniVersita’ di Torino, 10125 Torino, Italy
Sarah D. Burton and Joseph Ferel William R. Wiley EnVironmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratories, Richland, Washington
Silanization of the silica gel surface in the synthesis of silica gel polyamine composites uses (chloropropyl)trichlorosilane (CPTCS). It is possible to substitute a molar fraction of reagent CPTCS with methyltrichlorosilane (MTCS), creating a mixed silane surface layer. Two types of silica gels were modified with a series of MTCS:CPTCS molar ratios. Solid-state CP/MAS 29Si and 13C NMR spectroscopies were used to evaluate the surface silane composition. Surface silane coverage was markedly improved for the resulting gels. When polyamines were grafted to the resultant MTCS:CPTCS silane layers, it was shown that the decrease in the number of propyl attachments to the polyamine resulted in increased quantities of “free amines”. Optimum MTCS:CPTCS ratios were determined for three polyamines grafted onto one silica gel. A substantial free amine increase was observed for poly(allylamine) (PAA). Metal uptake studies show increases in Cu(II) capacity and/or an improvement in Cu(II) mass-transfer kinetics. The effect of polymer molecular weight upon Cu(II) capacity was investigated for each polyamine. Substantial differences in Cu(II) capacity between 50 000 MW poly(vinylamine) (PVA) and >1000 MW PVA were evident. Similar differences between 25 000 MW poly(ethyleneimine) (PEI) and 1200 MW PEI were found. The mass-transfer kinetics was shown to be improved for composites prepared using a large fraction of MTCS in the reagent silane mixture. This resulted in substantial improvements in the 10% breakthrough Cu(II) capacity for PVA (50 000 MW). PEI composites were further modified to form an amino-acetate ligand. The impact of the MTCS:CPTCS silane ratio on the acetate ligand loading and ultimately on the Cu(II) capacity at pH ) 2 was investigated. A ratio of 12.5:1 was shown to result in an acetate modified PEI composite with a Cu(II) capacity 140% of the Cu(II) capacity of the same composite prepared with “CPTCS only”. 1. Introduction The use of solid-phase adsorbents for the removal and recovery of metal cations and oxoanions from industrial and mining waste streams has been gaining in popularity due to the greater efficiency and environmental friendliness of this method relative to bulk hydrometallurgy or solvent extraction.1 In general, the solid phases of choice have been lightly cross-linked polystyrene or methyl methacrylate polymers.2,3 In cases where dilution of a waste stream is not practical, or in the case of waste streams containing valuable metals, ion exchange offers a viable method for offsetting the price of environmental remediation using relatively simple and safe process designs. Moreover, the increased use of oxidative pressure leaching and bioleaching for metal extraction of sulfide ores in the mining industry is even more compatible with ion exchange than with solvent extraction because it avoids the use of flammable solvents and takes metal concentrations to lower effluent values.4,5 Polymer-based matrixes, however, are not particularly wellsuited to these large-scale applications because they often involve the use of hot solutions and wide swings in pH where * To whom correspondence should be addressed. Tel.: 1-406-2432592. Fax: 1-406-243-4227. E-mail:
[email protected].
the shrink-swell properties of the polymers lead to shorter lifetimes or require the use of dead volumes in plant design. To overcome these disadvantages, we have turned to amorphous silica gel polyamine composites.6-18 These materials do not shrink or swell, can be used at higher temperatures (110 °C for silica polyamine composites compared with 70 °C for polystyrene based resins),19 have improved stability with regard to radiolytic decomposition, and have much longer usable lifetimes than their polystyrene counterparts.12 Furthermore, the polar nature of the silica polyamine surface also makes for better mass-transfer kinetics in the case of aqueous solutions, and the polyamine can be easily modified with metal selective functional groups in ways similar to those used for polystyrene resins,20 without the use of swelling solvents, to form robust carbonnitrogen single bonds.7 The advantages of these composites have been demonstrated through studies that directly compare the properties of the polyamine composites with commercially available polystyrene resins.9,12 Admittedly polystyrene resins are stable at high pH, whereas silica polyamine composites are useful for applications below pH ) 13. The synthetic route (Figure 1) yields a silica gel surface with a functionalized silane coating and makes use of a polyamine as a chelating agent.21,22 The polyamine is covalently bound at multiple points to the silane layer. In this way the chelating moiety becomes an integral part of the coating matrix. These
10.1021/ie0601448 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006
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Figure 1. Synthetic scheme for silica polyamine composites. Figure illustrates the key features including acid washing and humidification to promote “lateral polymerization” (a-c) for maximization of surface coverage. The humidified surface is silanized (d), grafted with a polyamine (e), and a selective ligand may then be attached.
are key factors in creating a material with remarkable durability.9,12 For example, the material designated as WP-1 consists of poly(ethyleneimine) (1200 or 25 000 MW) covalently bound to a (chloropropyl)trichlorosilane (CPTCS) layer grafted onto porous silica gel. To demonstrate the long-term stability of silica polyamine composites, these materials have been subjected to many test cycles. A test cycle consists of treating a packed column of silica polyamine composite with a metal ion solution followed by rinsing with water and recovery with an acid solution. The nature of the metal ion solution and the acid strip solution vary depending upon the composite in question. Silica polyamine composites have been tested continuously through 7000 test cycles with no visible degradation of the composite and a negligible loss in the metal ion adsorption capacity (millimoles of metal ion adsorbed per gram of composite).7 The copper capacity of the polyamine composites in their base form is in the range of 0.7-1.2 mmol/g at pH > 2 and depends on the polyamine being used and the type of silica gel.6 To date, we have incorporated three polyamines: poly(ethyleneimine) (PEI), poly(vinylamine) (PVA), and poly(allylamine) (PAA) (Figure 2), thus producing three distinct silica polyamine composites.7 PEI is a highly branched, water soluble amine polymer containing 1°, 2°, and 3° amino groups in a ratio of 0.35:0.35: 0.30, respectively. PVA and PAA are linear water soluble polyamines containing primary amino groups only. The intent of the work presented here was to investigate several aspects of the impact of diluting the number of chloropropyl groups available for attachment to the polyamine. This is achieved by substituting methyltrichlorosilane (MTCS) for the (chloropro-
Figure 2. Polyamines used to synthesize the silica gel polyamine composites, showing the schematic structure. PVA is a water soluble, linear, polyamine containing all primary amino groups directly attached to the backbone. PAA is a water soluble, linear polyamine containing all primary amino groups with one carbon removed from the backbone. PEI is a branched chain water soluble polymer with primary, secondary, and tertiary amines in the ratio 0.35:0.35:0.30, respectively.
pyl)trichlorosilane in various molar ratios. The effect of this substitution on the extent of silane surface coverage and the possible implications on composite stability are investigated.
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Three polyamines have been incorporated onto silica gels functionalized with both MTCS and CPTCS. The effect of silane substitution on the Cu(II) capacity of the polyamine modified material is reported, and the nature of the polyamine attachment to the functionalized silica gel is discussed. Silane mixing is shown to have a profound effect on the performance of aminoacetate modified polyamine composites. Initial investigations into silane surface coverage were conducted with a Qingdao Haiyang silica gel. Subsequent investigations regarding the incorporation of a polyamine and subsequent modification to yield the acetate modified composite were made using a superior silica gel manufactured by the INEOS Co. We have found that this type of silica gel has the best combination of pore diameter, surface area, and particle size. 2. Experimental Section 2.1. Materials. Raw silica gel was obtained from Qingdao Haiyang (QH), China (100-150 µm particle size, 19.4 nm average pore diameter, 493 m2/g surface area) and also from INEOS, U.K. (150-250 µm particle size, 25.0 nm average pore diameter, 320 m2/g surface area). Poly(vinylamines) (1000 MW, 50 000 MW) were obtained from BASF, Germany. Poly(ethyleneimines) (1200 MW, 25 000 MW) were obtained from Aldrich Chemicals. Poly(allylamine) (15 000 MW) was obtained from Summit Chemicals Inc., NJ. All polymers were in the free base form and were used as received. CPTCS, MTCS, and chlorotrimethylsilane (CTMS) were used as received from Aldrich. Solutions of Cu(II) (50 mmol/L) were prepared by dissolving 25 g of reagent grade CuSO4‚5H2O in 2 L of water. Solution pH was adjusted from intrinsic with 0.2 mol/dm3 H2SO4. Base regeneration was conducted with 4 mol/dm3 NH4OH. Stripping and recovery was achieved with 2 mol/dm3 H2SO4. Cu(II) standards were also obtained from Aldrich. 2.2. Solid-State NMR. The CP/MAS (cross-polarization magic angle spinning) 13C NMR spectra were recorded on JEOL GSX-270, Chemagnetics 500 and Bruker AVANCE-500 NMR spectrometers at 67.8 and 125 MHz, respectively. The CPMAS data at 125 MHz were measured using ramped cross-polarization and SPINAL64 and TPPM decoupling techniques with sample spinning speeds of 15 kHz. The data obtained at 67.8 MHz used a contact time of 3.0 ms and BB (broad band) proton decoupling using sample spinning speeds of 5 kHz. The CP/MAS 29Si NMR were measured at 99.5 MHz using the same cross-polarization and decoupling techniques as those for the13C experiments but with spinning speeds of 5 kHz. Bloch decay 29Si experiments (BD/MAS) were conducted with recycle delays of 120 s. 2.3. Preparation of Hydrated Silica Gels. Silica gels were sieved to ensure particle diameters were in the desired range. The particle size range was 100-150 µm for INEOS silica gel and 200-225 µm for QH silica gel. A 200 g amount of sieved silica gel was added to 800 mL of 1.0 mol/dm3 HCl in a 2 L three-neck round-bottom flask. The flask was equipped with an overhead stirrer and a condenser. During mixing the flask was degassed by an applied vacuum (30 mmHg). The reaction flask was restored to atmospheric pressure. The mixture was then brought to reflux (98 °C) for 6 h. Upon cooling the product was filtered and washed successively with three 800 mL water washes and two 800 mL methanol washes. The resulting gel was dried at 120 °C to a constant mass. Air was forced over a bed of water and through a 2 L column containing 200 g of acid-washed silica gel for 72 h, creating a hydration monolayer upon the silica gel surface. The percent weight gain (W) is used to quantify the increase in mass associated with modifying silica gel. W is calculated by
W% ) (Mf - Mi)/Mi × 100%
(1)
in which Mf is the mass of the modified silica gel and Mi is the mass prior to modification. For the hydration of acid-washed silica gel W ) 6%. 2.4. Preparation of “CPTCS Only” Functionalized Silica Gels. “CPTCS only” functionalized silica gels are silica gels functionalized with CPTCS and without MTCS. For the preparation of CPTCS only functionalized silica gels from both Qingdao Haiyang silica gel and INEOS silica gel, a 30 g quantity of hydrated silica gel was placed in a 250 mL three-neck roundbottom flask equipped with an overhead stirrer. A 120 mL aliquot of hexanes was added, and the resulting mixture was stirred under a constant flow of dry N2. A 90 mmol amount (3 mmol/g of silica gel) of CPTCS was dissolved in 30 mL of dry hexanes. The silane solution was then added dropwise to the reaction mixture. As the reaction proceeded, HCl gas was evolved and was forced out of the reaction flask by dry N2 flow. The reaction flask was degassed by an applied vacuum (30 mmHg) and allowed to react for 24 h. The product was filtered and washed three times with 120 mL of dry hexanes and two times with 120 mL of methanol. The product was filtered and then dried at 110 °C until there was a negligible mass decrease. For the modification of hydrated silica gel with CPTCS only W ) 20% (see Table 1 for elemental analysis). 2.5. Preparation of MTCS:CPTCS Functionalized Silica Gels. MTCS:CPTCS functionalized silica gels are silica gels functionalized with MTCS and CPTCS in a specified molar ratio. In keeping with CPTCS only silica gel, a total of 3 mmol of silanes for each gram of silica gel was employed. The MTCS: CPTCS molar ratios applied to Qingdao Haiyang silica gel included 1:1 and 2:1. The MTCS:CPTCS molar ratios for INEOS silica gel were as follows: 0:1, 1:1, 2.5:1, 5:1, 7.5:1, 10:1, and 12.5:1. The silane solution of requisite molar ratio was prepared in 30 mL of hexanes. From this point the procedure described for CPTCS only gels (section 2.4 above) was adhered to. The percent weight gains for hydrated silica gel functionalized with both MTCS and CPTCS in the molar reagent ratios specified above are in the range of W ) 7-20% (see Table 1 for elemental analysis). 2.6. Preparation of “CTMS Only” Silica Gels. The preparation of “CTMS only” functionalized INEOS silica gel is the same as the preparation of CPTCS only silica gel (section 2.4) with the exceptions that CTMS is substituted for all CPTCS and there is no hydration step in the preparation of the raw INEOS silica gel.23 For silica gel functionalized with CTMS only W ) 10%. Elemental analysis: crbon ) 3.02%. 2.7. Preparation of the Polyamine Composites. In a 100 mL three-neck round-bottom flask equipped with a condenser and overhead stirrer, 5 g of CPTCS only INEOS type silica gel or MTCS:CPTCS INEOS type silica gel was added to a 25 mL of aqueous polyamine solution containing 2 mL of methanol as an antifoaming agent. The resulting mixture was stirred for 15 min and degassed by an applied vacuum (30 mmHg). The mixture was continuously stirred at a temperature of 65 °C for 48 h. The mixture was then cooled, allowed to settle, and then carefully decanted. The resulting composite was washed five times with 20 mL portions of water, one 20 mL portion of a 4 mol/dm3 ammonia solution, three 20 mL portions of water, and two 20 mL of methanol. The resulting composites were then dried to constant mass at 60 °C. A sample of each composite underwent elemental analysis (CHN). Polyamines and concentration used include the following: poly(allylamine) (15 000 MW) in a 10% solution, poly(vinylamine) (
