Preparation and Characterization of a Selective Nitric Oxide Adsorbent

Sep 20, 2008 - Maine, Orono, Maine 04469, and Philip Morris U.S.A. Research Center, P.O. Box 26583, Richmond, Virginia. 23261. We report the synthesis...
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Ind. Eng. Chem. Res. 2008, 47, 7857–7861

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Preparation and Characterization of a Selective Nitric Oxide Adsorbent Based on Cobalt(II) Phthalocyanine Tetrasulfonic Acid W. J. DeSisto,*,† R. Cashon,‡ D. Cassidy,† N. Hill,† D. M. Ruthven,† J. B. Paine III,§ and J. A. Fournier§ Department of Chemical and Biological Engineering and the Laboratory for Surface Science and Technology, Jenness Hall, Department of Biochemistry, Microbiology and Molecular Biology, Hitchner Hall, UniVersity of Maine, Orono, Maine 04469, and Philip Morris U.S.A. Research Center, P.O. Box 26583, Richmond, Virginia 23261

We report the synthesis and characterization of a selective nitric oxide (NO) adsorbent based on the covalent binding of cobalt(II) phthalocyanine tetrasulfonic acid (CoPcS) onto imidazole-functionalized mesoporous silica gel beads. The equilibrium data indicated that the adsorption of NO was highly favorable, approaching irreversible adsorption. The breakthrough curves for NO were modeled assuming irreversible adsorption and resulted in pore diffusivities through the packed bed of 0.1-0.3 cm2 · s-1. Adsorption experiments of NO in the presence of nitrogen, water vapor, and ammonia indicated that the adsorption was selective for NO. Introduction Nitric oxide, NO, has received considerable attention as a molecule due to its importance in physiological function as well as its potential harm to humans.1 This has spurred significant research into the controlled capture and release of NO, which has been well-reviewed.2 One focus has been on aqueous media because of the importance of NO in biological systems. In addition to understanding NO capture and release in aqueous media, it is also important to understand NO capture and release on solid-state media (thin films, powders, etc.) to enable NO separation, capture, and sensing.3 Such applications include detection and analysis of NO from breath (as this has been correlated with a number of health issues),4-6 NO removal from flue gas arising from burning of fossil fuels,7-9 and the removal of NO from cigarette smoke. Metalloporphyrins (for example, hemin) and metallophthalocyanines are classes of compounds known to bind NO.2,10-13 For these classes of compounds, the oxidation state of the metal center strongly influences NO binding. For example, Co(II), Fe(II), and Fe(III) complexes are known to bind NO.12 Nitric oxide binding can result in metal-NO adducts ranging from metalsNO+ to metalsNO- species. The majority, if not all, of these studies were conducted in solution, where the metal oxidation state can be manipulated and equilibrium can be established. An interesting compound that has potential for gas-phase adsorption of NO is the water-soluble cobalt(II) phthalocyanine tetrasulfonic acid, CoPcS, complex shown in Figure 1. This compound is known to react with NO when in solution.14,15 An additional study was performed on the removal of NO from a gas stream using CoPcS bound to an imidazole-functionalized silica support in an aqueous suspension.16 These studies indicated that the Co(II)PcS form is stable in solution and effectively binds NO and imidazole ligands. * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemical and Biological Engineering and the Laboratory for Surface Science and Technology, University of Maine. ‡ Department of Biochemistry, Microbiology and Molecular Biology, University of Maine. § Philip Morris U.S.A. Research Center.

In contrast to the work done in aqueous media, we have undertaken the study of metal porphyrin compounds that have the potential to bind NO from a complex gas stream in the solid state. For direct adsorption from a gas stream there are important physical and chemical properties of the adsorbent that must be considered. The physical properties include porosity, particle size and shape, surface area, etc. For metal porphyrin compounds the important chemical properties of the adsorbent include metal oxidation state, surface chemistry, and molecular interactions between the porphyrin molecule and the support (if used) as well as interactions between neighboring metal porphyrin molecules. For example, in hemin it is well-known that interactions between hemin molecules can lead to the formation of µ-oxo dimers that reduce ligand binding capability. In this paper, we describe the preparation of Co(II)PcS compounds supported on different silica supports and their characterization as potential NO adsorbents. The cobalt(II) phthalocyanine complexes were either impregnated onto silica gel or covalently grafted onto imidazole-functionalized silica gel. The materials were characterized by porosimetry, UV-vis spectroscopy, and NO breakthrough curves. The breakthrough curves were modeled to determine values for the diffusivity and saturation capacity of the adsorbent. The cobalt phthalocyanine tetrasulfonic acid grafted onto imidazole-functionalized mesoporous silica gel showed significant NO adsorption. This

Figure 1. Cobalt phthalocyanine tetrasulfonic acid (M ) Co).

10.1021/ie8000293 CCC: $40.75  2008 American Chemical Society Published on Web 09/20/2008

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compound has potential as a selective gas-phase adsorbent of NO in a complex gas stream. Experimental Section Materials. Silica gel was used as a support for the metal phthalocyanine compounds. Two sources of silica gel were used in our experiments: silica gel beads from Qingdao Haiyang Chemical Company Ltd. (silica gel no. 775, 30-50 mesh, 300-700 µm particle size) and silica gel granules from GraceDavison (silica gel no. 646, 35-60 mesh, 150 Å pore size). The Qingdao silica gel was functionalized with imidazole (Silicycle, Inc.) to have 2.3 × 10-3 mol of nitrogen/g of silica. Both silica gels were also functionalized with imidazole using 3-N-imidazolylpropyl triethoxysilane (IPTES). IPTES was added to a toluene solution at 15 mM. The silica gel beads were refluxed in this solution for 24 h at 90 °C. The beads were then baked dry for 24 h at 110 °C. The Grace-Davison granules and Qingdao beads were functionalized with 1.33 × 10-3 and 1.49 × 10-3 mol of nitrogen/g of silica. CoPcS (Porphyrin Products) was used as received. Deionized water (18 MΩ) was used in all preparations. In a typical preparation of CoPcS loaded onto imidazolefunctionalized silica (Im-silica), 0.0635 g of CoPcS was dissolved in 30 mL of H2O. To this solution 0.5 g of Im-silica was added. The solution was stirred for 3 days at room temperature or 6 h at 70 °C. Both preparations resulted in almost complete loading of the CoPcS onto the Im-silica. For the impregnation of CoPcS onto silica (not functionalized) a typical preparation involved adding 0.0635 g of CoPcS to 15 mL of H2O containing 0.5 g of silica gel beads. This solution was evaporated to dryness under stirring. Estimations of compound loading on various Co(II)PcS-imidazole-silica samples were made by measuring the concentration of remaining Co(II)PcS in the filtrate. This was done by UV-vis spectroscopy. Standard solutions were prepared to provide an extinction coefficient. The extinction coefficient, ε, measured at 670 nm, was 66 324.6 M-1. Materials Characterization. Spectra of the CoPcS compound in solution were characterized using a transmission UV-vis spectrophotometer. Spectra were recorded from 350 to 700 nm. Spectra of CoPcS compound loaded onto silica and grafted onto Im-silica powders were characterized using a fiberoptic-based reflection UV-vis spectrophotometer (Ocean Optics). All IR spectra were recorded on ABB FTLA 2000 spectrometers. The membranes were analyzed with DRIFT using a Praying Mantis diffuse reflectance apparatus from Harrick Scientific. KBr powder was used to record the reference for the DRIFT spectra. All spectra were recorded at 4 cm-1 resolution using 100 scans requiring approximately 2 min of collection time. Nitrogen porosimetry measurements were made with a Micromeretics ASAP 2020 instrument. Nitric oxide breakthrough curves were obtained by flowing a dilute NO stream over the sample and analyzing the effluent using a calibrated NO analyzer (Seivers). A tank of 5 ppm NO/ N2 was connected to a vertical adsorption column. The powder was weighed (approximately 50 mg samples) and transferred to the column supported by a porous nickel frit. The dilute NO stream was flowed (50-100 sccm) down through the powder to avoid creating a fluidized bed. In some experiments a small amount of air (20%) was added to the gas flow. The concentration of NO was recorded every 1 s for periods up to 8 h. Nitric oxide adsorption experiments were performed at room temperature under flowing N2.

Table 1. BET Surface Area of NO Adsorbents and Supports sample

surface area (m2/g)

Qingdao silica gel beads Grace-Davison silica gel granules imidazole-functionalized Qingdao beads imidazole-functionalized Grace-Davison granules CoPcS-imidazole-silica gel (Qingdao) CoPcS-imidazole-silica gel (Grace-Davison)

452 297 261 179 165 131

In order to assess NO adsorption in the presence of potential competitive interferences, NO adsorption was undertaken in the presence of water vapor and ammonia. A glass bubbler was filled with a 0.1 M NH4OH aqueous solution. Through this solution a gas mixture of N2 with 5 ppm of NO was bubbled, saturating it with water vapor and a small amount of NH3. We ran a control experiment without the adsorbent, and the bubbling of the NO/N2 mixture through the 0.01 M NH4OH solution did not change the detected NO concentration as measured when bypassing the bubbler. The concentrations of NH3 and H2O were estimated based on thermodynamic data on ammonia/water solutions.17 An additional assumption is that the carrier gas was saturated with vapor from the bubbler (thermodynamic equilibrium). At our flow rate of 50 sccm, this is a reasonable assumption. The concentrations of the gases and vapors in the mixed stream in mol/L were the following: NH3, 6.6 × 10-4; H2O, 1.0 × 10-3; NO, 2.2 × 10-7; N2, 4.5 × 10-2. Results The BET surface area of the silica gel supports, imidazolefunctionalized supports, and CoPcS-loaded supports are given in Table 1. Both silica gels used had high surface areas. Pore size distribution data for the supports determined using the BJH data reduction of the isotherms is shown in Figure 2. The Qingdao beads contained a wide pore size distribution with the majority of pores ranging from 100 to 150 Å. The GraceDavison granules contained larger pores, the bulk of which were greater than 150 Å. Functionalizing these gels with n-propylimidazole resulted in a surface modification. The resulting decrease in surface area is shown in Table 1. FTIR analysis of the functionalized beads indicated C-H stretching peaks in the region of ∼2900 cm-1 indicative of surface modification. Nitrogen analysis of the functionalized powders indicated 1.33 × 10-3 and 1.49 × 10-3 mol of nitrogen/g of silica in the GraceDavison and Qingdao silica gel, respectively. The addition of CoPcS resulted in additional decrease in the surface area of the beads as shown in Table 1. The diffuse reflectance UV-vis spectra for CoPcS grafted onto Im-silica is shown in Figure 3. The adsorption bands at 600-680 are consistent with the spectra for the CoPcS

Figure 2. Pore size distribution of silica gel supports.

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Figure 3. UV-vis spectra of CoPcS-imidazole-silica gel. Figure 5. Equilibrium data for CoPcS-imidazole-silica gel samples: Qingdao beads (filled squares) and Grace-Davison granules (filled diamonds).

Figure 4. NO breakthrough curves for CoPcS-imidazole-silica gel samples indicating significant NO adsorption. Two different silica gel supports were used: Qingdao beads (open squares) and Grace-Davison granules (filled diamonds).

molecule.12,13 These adsorption bands correspond to the Q bands that arise from π-π* transitions in the phthalocyanine ring. For CoPcS in aqueous solution,11 the QR band appears at 660 nm. The Qβ band appears as an inflection of the QR band between 630 and 650 nm. When imidazole is added to the CoPcS solution the QR band shifts from 660 to 665 nm. The subtle shift in the QR band from 660 to 665 nm observed for the imidazole-CoPcS complex in solution was not observed with the reflectance spectra (Figure 3); however, the absorption band for the solid compound does overlap this region. In addition, the electronic absorption of tetrasulfonated phthalocyanines in aqueous solution is explained in terms of monomer/ dimer equilibrium where the monomeric species adsorbs at higher energy than the dimeric species. In the spectra of Figure 3, both species are present. The spectra taken in Figure 2 were of compounds with different CoPcS loading. In all these samples there was no variation in the spectra. The CoPcS molecule remains intact while grafted onto the silica gel. CoPcS was impregnated onto Grace-Davison silica gel, and its NO adsorption properties were measured. The volume of NO adsorbed during this experiment was 1.9 × 10-6 mol · cm-3 corresponding to a NO/Co molar ratio of ∼0.044. Nitric oxide breakthrough curves for CoPcS grafted onto imidazole-functionalized silica gel are shown in Figure 4. For beads the CoPcS loading was ∼10%. Breakthrough curves for samples prepared on Grace-Davison and Qingdao functionalized silica gel are shown together for comparison. Samples prepared in this manner consistently adsorbed significantly greater amounts of NO when compared to the impregnated or complexed samples described above. The amounts of NO adsorbed for the CoPcS loaded onto functionalized Grace-Davison and Qingdao silica gel were 5.5 × 10-6 and 3.3 × 10-6 mol · cm-3, respectively. The molar ratios of NO/Co were 0.17 and 0.083, respectively. Equilibrium Data. Equilibrium data were extracted from the NO response curves obtained at varying initial NO concentrations taken at 298 K. For the CoPcS-imidazole-silica gel beads the equilibrium data is shown in Figure 5. The data taken over

the concentration region shown indicate that the adsorbed concentration is in the favorable range for adsorption. Data taken for higher concentrations show a weak dependence of loading on concentration. Desorption experiments at 298 K under pure nitrogen indicated that little or no NO was desorbed from the sample. From these data, it appears that the NO adsorption is almost irreversible at 298 K. Analysis of Response Curves for NO. The breakthrough curves were modeled to determine the diffusivity of NO within the adsorbent particles. On the basis of the assumption of irreversible equilibrium, analytical expressions for the breakthrough curve are available.18 Two models were used: (1) the column was treated as a well-mixed cell where the rate of accumulation of NO within the volume element is negligible, and (2) diffusion was treated as occurring in the fluid phase with a distributed adsorption along the pore walls according to a pore diffusion model developed by Cooper and Liberman.19 For short columns and high flow rates model 1 was appropriate. For slightly longer columns where the breakthrough was delayed model 2 was used. For shorter columns both models provided similar values for the diffusivity. In model 1 the column was treated as a well-mixed cell where the rate of accumulation of NO within the volume element is negligible. The resulting mass balance within the column is given as Fc0 ) VS

dqj + Fc dt

(1)

where F is the carrier flow rate, V is the bed volume, c0 is the NO feed concentration, and qj is the sorbate (NO) concentration averaged over the column. VS is the volume of solid sorbant related to the total volume by ε, the bed voidage. Equation 1 can be rearranged as

()

VSqS d qj c )1c0 Fc0 dt qS

(2)

where qS is the saturation capacity of the adsorbant. In analyzing adsorption behavior of strongly adsorbed species, it is convenient to assume a rectangular isotherm.18 A model for the analysis of experimental response curves based on this assumption has been reported20 along with the explicit solution to the model expressed in terms of the amount absorbed as a function of time21 as

{

]}

1 π 1 qj ) 1 - + cos + arccos(1 - 12τ) qS 2 3 3

[

3

(3)

and τ/t ) ((εpDp)/Rp2)(c0/qS) where Rp is the particle radius, εp is the porosity, and Dp is the pore diffusivity. Differentiating

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Figure 6. NO breakthrough curve for CoPcS-imidazole-silica gel (Qingdao beads) fitted with model 1: experimental data (dashed line); model (filled squares with solid line).

Figure 8. NO breakthrough curves for CoPcS-imidazole-silica gel (GraceDavison granules) in the presence of interferences: NO data (solid triangles); H2O and NO (open circles); H2O, NH3, and NO (open squares).

103 and 3.0 × 103, respectively. The presence of water vapor reduced the binding capacity of CoPcS-imidazole-silica to nearly 50% of the original value. Additional addition of NH3 did not significantly affect the NO binding capacity further. Because the experiment was rapid, with a time scale on the order of ∼10 s, we did not detect decomposition of the NO molecule prior to reaching the detector; however, the presence of water vapor did lower the NO value observed at the detector slightly, by approximately 500 ppb. Discussion Figure 7. NO breakthrough curve for CoPcS-imidazole-silica gel (GraceDavison granules) fitted with model 2: experimental data (dashed line); model (filled squares with solid line).

eq 3 and combining with eq 1 yields an expression for the response curve in terms of adsorbate concentration:

( )

εpDpVS c ) c0 Rp2F π 1 π 1 sin + arccos(1 - 12τ) sin + arccos(1 - 12τ) 3 3 3 3

1-

[

] [

√1 - (1 - 12τ)2

] (4)

This expression was fitted to the experimental response curve with the parameters (εpDpVS)/(Rp2F) and t/τ. Because of the assumption of a rectangular isotherm, the model does not work at small time values. In our analyses, we fitted curves between c ) 0.1 and c ) 0.9 of full scale values. An example of a fit using model 1 is shown in Figure 6 resulting in a pore diffusivity of 0.11 cm2 · s-1. For the analyses of breakthrough curves using model 2 we followed the method of Cooper and Liberman, the details of which are provided in ref 15. In order to fit the data to model 2, k and kc0/qS were used as fitting parameters with k ) 15Dp/ Rp2. An example of this fit for a delayed breakthrough is shown in Figure 7 resulting in a pore diffusivity of 0.33 cm2 · s-1. Adsorption of NO in a Complex Gas/Vapor Stream. In addition to assessing the NO adsorption properties of the CoPcS-imidazole-silica molecule, we examined NO adsorption in a complex gas/vapor stream. To assess the ability of CoPcS-imidazole-silica to bind NO in the presence of potential interferences, we undertook qualitatiVe experiments using water vapor with ammonia. Figure 8 shows the results NO adsorption in the presence of water vapor and ammonia. The NO adsorption is shown in the presence of H2O, H2O/NH3, and N2. The molar ratios of H2O/NO and NH3/NO were 4.5 ×

Grafting the CoPcS molecule to an imidazole-silica support increased the capacity to bind NO relative to an impregnated sample. The poorer performance of the impregnated sample could be a result of increased Co-Co interactions and dimerization. This dimerization has been shown to occur for CoPcS in solution at high concentrations.11 In contrast, the functionalization of silica gel with imidazole provided a site to bind the metal center of CoPcS to the support, disposed in such a manner as to make the binding of a second imidazole ligand unlikely. Such a binding would leave one final binding site on the sixcoordinated cobalt cation available for ligand binding to ligands such as NO. If the silica is overloaded with CoPcS this may encourage dimerization and reduction in NO binding capability. This may be approached at higher loadings of CoPcS onto the silica gel. This was confirmed on experiments with higher loading on functionalized silica gel powder. Higher loadings, up to 20%, were achieved on powdered samples prepared by crushing the beads. With these samples we observed a general trend toward increasing NO binding efficiency, measured by the NO/Co ratio, when decreasing the Co loading on the functionalized silica gel support. The presence of imidazole functionality was confirmed with FTIR spectroscopy. Cobalt was loaded onto the functionalized mesoporous silica gel (in bead form) with up to ∼10% loading. Attempts to increase the loading were unsuccessful due to either diffusion limitation into the mesopores or a limited amount of imidazole sites that actually participated in binding the CoPcS molecules. For an ideal structure there should be a 1:1 ratio of NO to cobalt. In our samples, the NO/Co was between 0.083 and 0.17 for CoPcS loaded onto functionalized silica gel beads. This inefficiency may be due to several factors including the presence of dimerization of the CoPcS molecule and the lack of appreciable diffusion of NO into the mesoporous silica gel. The samples prepared on the Grace-Davison silica gel were repeatedly superior to those prepared on the Qingdao silica gel beads. On the basis of porosimetry data the Grace-Davison silica gel contained larger pores (greater than 150 Å) which could

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allow for the CoPcS binding to occur deeper within the pore. In contrast the Qingdao silica gel support contained smaller pores where crowding of the CoPcS at the pore mouth could have occurred, resulting in a less efficient adsorbent. Analyses of the breakthrough curves resulted in pore diffusivities ranging from 0.1 to 0.4 cm2 · s-1. The molecular diffusivity of NO in N2 was calculated as 0.22 cm2 · s-1, whereas the Knudsen diffusivity was estimated as 0.046 cm2 · s-1 assuming a pore radius of 15 nm, based on data from the granular Grace-Davison silica gel. As expected for a packed bed the bulk of the diffusion occurs through the voids in the bed as this path is least resistive; thus, the diffusivity through these voids was determined to be essentially molecular. The assumption of complete mixing was valid for shorter column lengths where data from models 1 and 2 were in agreement. The assumption of irreversible equilibrium led to the model slightly overestimating the saturation capacity of the sorbant. Experimental evidence is provided for the favorable binding of NO in the presence of NH3 and H2O. This is consistent with reports of Co(II) porphyrins and phthalocyanines binding of NO in aqueous solutions.16 On the basis of a report of NH3 binding by [Co(III)Pc]Br and the stability of Co(II)Pc compounds,22 the favorable binding of NO is also consistent with the literature. It is unclear at this time the role of water vapor in the reduction of NO binding capacity; however, the retention of significant NO binding with H2O/NO molar ratios on the order of 1000 is encouraging. Conclusion A selective nitric oxide (NO) adsorbent based on the covalent binding of CoPcS onto imidazole-functionalized mesoporous silica gel beads was prepared and characterized. This material was shown to adsorb stoichiometrically significant amounts of NO, corresponding to NO/Co ratios up to 0.17. Additionally, potential NO adsorbents based on the same CoPcS compound were prepared by either impregnation of CoPcS or imidazolecomplexed CoPcS onto silica gel. When impregnated CoPcS/ silica gel adsorbents were compared to CoPcS-imidazolefunctionalized silica gel adsorbents, the NO adsorption was decreased 5-fold. The equilibrium data for the CoPcS-imidazolefunctionalized mesoporous silica gel indicated that the adsorption of NO was highly favorable, approaching irreversible adsorption. Data were collected for two types of silica gel beads. The breakthrough curves for NO were modeled assuming irreversible adsorption and resulted in pore diffusivities through the packed bed of 0.1-0.4 cm2 · s-1. Adsorption experiments of NO in the presence of significant excess of nitrogen, water vapor, and ammonia indicated that NO adsorption in this atmosphere was selective for NO. This material has the potential application for selective adsorption in cigarette smoke or other gas streams and in sensors. Acknowledgment Research described in this article was supported by Philip Morris U.S.A. Inc. The authors acknowledge Professor Carl Tripp for assistance with the FTIR measurements.

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ReceiVed for reView January 07, 2008 ReVised manuscript receiVed July 16, 2008 Accepted August 07, 2008 IE8000293