Polymer–Metal Complexes in Polyelectrolyte Multilayer Films as

Article pubs.acs.org/Langmuir

Polymer−Metal Complexes in Polyelectrolyte Multilayer Films as Catalysts for Oxidation of Toluene Almagul Mentbayeva,† Alyiya Ospanova,† Zheneta Tashmuhambetova,† Valeria Sokolova,† and Svetlana Sukhishvili*,‡ †

Department of Chemistry and Chemical Technology, Al-Farabi Kazakh National University, Almaty, 050038, Kazakhstan Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States

‡

S Supporting Information *

ABSTRACT: We report on the binding of metal ions (Me2+; Co2+ and Cu2+) with weak polyelectrolyte multilayers (PEMs), as well as on catalytic activity of PEM−Me2+ films for oxidation of toluene. Using several types of PEM films constructed using branched polyethyleneimine (BPEI) or quaterinized poly-4vinylpyridines (QPVPs) as polycations and poly(acrylic acid) (PAA) or poly(styrene sulfonate) (PSS) as polyanions, we found that binding of Co2+ and Cu2+ ions with a PEM matrix can occur both through coordination to polycationic amino groups and/or ionic binding to polyacid groups. The amount of metal ions loaded within the film increased linearly with film thickness and was strongly dependent on polyelectrolyte type, film assembly pH, and fraction of permanent charge in polymer chains. Among various PEM−Me2+ systems, BPEI/PAA−Co2+ films assembled at pH 8.5 show the best catalytic performance, probably because of the preservation of high mobility of Co2+ ions coordinated to amino groups of BPEI in these films. With BPEI/PAA−Co2+ films, we demonstrated that films were highly permeable to reagents and reaction products within hundreds of nanometers of the film bulk; i.e., film catalytic activity increased linearly with layer number up to 30 bilayers and slowed for thicker films.

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INTRODUCTION The alternating layer-by-layer (LbL) deposition of polycations and polyanions at solid substrates is an attractive technique for preparation of nanostructured surface coatings of controlled thickness. As in the majority of cases, LbL films are composed of ionized or ionizable polymers, and such films are able to controllably interact with small ions of various valency. This important property has been used to regulate polymer deposition within films, as well as to construct novel types of polyelectrolyte multilayers (PEMs) acting as ion-selective membranes1−3 useful in water softening and seawater desalination. Multivalent ions were also used in LbL deposition as polymer “cross-linkers”, enabling construction of fluorescent coatings with metal ion sensing capabilities4 or development of inorganic ion/polymer assemblies for tunable release of biological molecules.5 Metal ions infiltrated within PEMs can also serve as precursors for synthesis of film-embedded metal or metal oxide nanoparticles useful in optical and electronic applications.6−9 Recently, this latter approach has also been used to engineer catalytically active PEMs.10 Catalyst-carrying PEM films share several common features with more traditional heterogeneous catalysts immobilized at insoluble supports,11 including ease of catalyst separation, recovery, and reuse. In the case when a catalyst is a multivalent metal ion, immobilization of these ions at/within an insoluble © 2012 American Chemical Society

matrix through strong metal ion−polymer binding also prevents contamination of the product with trace amounts of metal ions.12,13 In addition, earlier approaches to immobilization of metal complexes via covalent attachment to functionalized polymers demonstrated another advantage of polymerbound metal catalysts, rooted in the unique microenvironments created for the reactants within the polymer support. In particular, improved catalyst stability14 and increased selectivity for intramolecular reactions15 have been reported for polymermatrix-embedded catalytic systems. As compared with noble metal catalysts used for oxidative conversion of toluene to benzaldehyde, benzyl alcohol, and benzoic acid,16,17 polymer-supported transition metal complexes (such as those based on Co2+, Cu2+, and Fe3+) present a cheap and efficient alternative enabling oxidation under mild conditions.18 Usually, complexes of these metals are supported on zeolitic imidazolate frameworks,19 polystyrene resin,12 or immobilized by covalent grafting on polymer substrates.20 Specific examples of polymer/transition metal catalytic complexes used in homogeneous oxidation of organic compounds include complexes of a temperature-responsive Received: June 22, 2012 Revised: July 18, 2012 Published: July 20, 2012 11948

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the precipitate was filtered, dissolved in water, and dialyzed using a 25 kDa cutoff dialysis membrane. Quaternization degrees of QX polymers were determined using FTIR as described elsewhere.25 QX polycations with X < 15 were not soluble in aqueous solutions at basic pH because of insufficiently small charge density. Therefore, polycations soluble at basic pH values having percentages of quaternized units higher than 15 (Q17, Q25, Q45, and Q65, synthesized with molar ratios of P4VP to C3H7Br of 1:0.23, 1:0.5, 1:1, and 1:3.33, respectively) were used in this study. Multilayer Deposition. PEMs were deposited onto silicon wafers and glass slides. Silicon wafers and glass slides were precleaned under a quartz UV lamp for at least 2 h, soaked in concentrated sulfuric acid for 1 h, and then carefully rinsed with Milli-Q water. Then, the silicon wafers and glass slides were immersed in 0.25 M NaOH solution for 10 min, rinsed with water, and dried under a flow of nitrogen. Preparation of multilayers at the surface of substrates was performed using 0.3 mg/mL of polyelectrolyte solutions at pH 8.5 in 0.01 M Trizma buffer. Polycations and polyanions were allowed to sequentially adsorb for 10 min. Each deposition step was followed by rinsing with buffer solution at the same pH value. All films were then stabilized by thermal cross-linking in an oven at 125 °C for 1 h. Incorporation of Metal Cations within the Films. To load a metal cation, multilayer films were exposed to 0.05 M solutions of CoCl2 and CuCl2 (in aqueous buffer at pH 5.0 or in ethanol) for 10 h to achieve complete absorption within the films, followed by rinsing with a solvent. Leachability of Metal from the Films. To study the stability of catalysts within the film, glass slides with deposited metal-loaded films were immersed in 10 mL of acetonitrile at 75 °C and shaken for 2 h. Substrates with deposited films were then removed from the solvent, immersed in fresh acetonitrile, and the shaking cycle was repeated. Acetonitrile solutions were evaporated, and the residue was dissolved in 5 mL of 5% nitric acid solution. To determine the total amount of Co2+ or Cu2+ ions within the PEM films, samples were treated with UV radiation for 10 h and dissolved in 50 mL of 5% nitric acid. The solutions were then analyzed for Co2+ and Cu2+ by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Varian Vista MPX instrument. To determine cation concentrations, we used calibration curves obtained for Co2+ and Cu2+ ions at wavelengths 231.2 and 324.7 nm, respectively. Scanning Electron Microscopy (SEM). SEM was performed using a Zeiss Auriga Dual-Beam FIB-SEM. Silicon wafers were glued to the SEM stage by a conductive tape. Prior to imaging, Au−Pt alloy was sputtered onto sample surfaces using an RF-plasma chamber for 10 s. The applied voltage was varied from 1 to 3 kV. Atomic Force Microscopy (AFM). AFM measurements were performed in air at room temperature using a NSCRIPTOR dip pen nanolithography system (Nanoink) operating in ac (tapping) mode. Phase-Modulated Ellipsometry Measurements of Dry Films. Ellipsometry measurements were performed using a home-built, single-wavelength, phase-modulated ellipsometer at 65° incidence. Optical properties of substrates and oxide layer thickness were determined prior to polymer deposition. In measurements of dry film thickness, the refractive index was fixed at a value of 1.5. Catalytic Activity Measurements. Oxygen was used as an environmentally benign, readily available oxidant, which acquires selectivity when used with transition metal complexes.26 Oxidation of toluene was carried out during 1 h at 348 K under an atmospheric pressure of O2 in a fixed-bed U-shaped reactor (with the total volume of 350 mL) containing 10 mL of a mixture of toluene (0.95 mol/L) and acetonitrile (17.2 mol/L) used as a solvent. The mixture was shaken at 400 times per minute. Prior to the addition of a catalyst, the mixture was purged with oxygen in order to remove air from the system. Both the oxygen buret and the reactor were temperature controlled using a circulating water bath. Concentration of consumed oxygen (QO2) (mol/L) was determined from consumption of oxygen using a gas buret (the data points were taken at a 1-min intervals during 1 h) and calculated as the ratio of the amount of consumed oxygen (in mol) to the volume of the reaction mixture (0.01 L).

polymer [poly(N-isopropylacrylamide)] with Co2+ ions21 or coordination of Cu2+ ions within a cross-linked poly(4vinylpyridine) matrix.22 PEMs have been previously explored as matrices for in situ synthesis of zero-charge noble metal nanoparticles (NPs), such as Pt, Pd, or Au, with an intention to generate large surface area heterogeneous catalyst. One interesting approach has been proposed by Wang and Lee, who synthesized nanosized Pd within PEMs deposited on magnetic nanoparticles and tested these nanocomposite catalysts in the hydrogenation reaction of olefinic alchohols.10 In this example, however, deposition of multiple layers did not result in enhanced catalytic activity of the nanocomposite, as only Pd NPs within the outermost film layer retained their activity, with PEM-embedded NPs remaining catalytically inactive.10 In another example, the LbL approach has been used in surface modification of hollow fiber microfiltration membranes through depositing a couple of polyelectrolyte layers, followed by in situ growth of polymersupported Pd NPs,23 but thicker, i.e., multilayer, films have not been studied. Here, we explore the construction of PEM-based catalytic coatings that involves the use of transition metal ions such as Co2+ and Cu2+ rather than zero-charge noble metal nanoparticles as catalytic centers. Similar to Au and Pd nanoparticles, salts of transition metals are resistant to oxidation, but when compared with noble metal NPs, they present cheaper alternative catalysts for oxidation reactions of organic compounds. We study interactions of transition metal salts with several types of PEMs constructed using weak polyelectrolytes and show that both film assembly conditions and polyelectrolyte type strongly affect a film’s capacity to absorb Co2+ and Cu2+, as well as the catalytic activity of immobilized ions. We demonstrate that PEM−Me2+ (where Me2+ represents metal ion) films are highly permeable to reactants and reaction products and are catalytically active within hundreds of nanometers film thickness. The latter is in contrast with earlier reported PEMs with embedded NPs of noble metals, whose catalytic activity was restricted to the outer film surface.

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EXPERIMENTAL SECTION

Materials. Branched poly(ethyleneimine) (BPEI; Mw 65 kDa), poly(4-vinylpyridine) (P4VP; Mw 160 kDa), 1-propyl bromide, acetonitrile, toluene, Trizma hydrochloride, hydrochloric acid, sodium hydroxide, nitric acid, cobalt(II) chloride, copper(II) chloride, and toluene were purchased from Sigma Aldrich. Acetic acid was received from Fluka. Poly(acrylic acid) (PAA; Mw 450 kDa) and polystyrene sulfonate sodium salt (PSS; Mw 500 kDa) were purchased from Scientific Polymer Products, Inc. All chemicals were used without any further purification. Millipore (Milli-Q system) filtered water with a resistivity of 18.2 MΩ was used in all experiments. Silicon (110) wafers (prime grade, p-type with boron dopant, 525 ± 25 μm thick, with native oxide layer of ∼2 nm thick) were bought from Cemat Silicon S.A. On the basis of the parent P4VP polymer, a series of quaternized products (QX polymers, where X denotes a molar percentage of quaternized units) were synthesized by reacting poly(4-vinylpyridine) with 1-propyl bromide in DMSO solution using a procedure described earlier.24 A typical synthesis procedure included placing a solution of 1 g of P4VP in 16 mL of DMSO in a double-neck flask, freezing the solution in liquid nitrogen, and adding different amounts of 1-propyl bromide to achieve various quaternization degrees. After reaction completion, solutions were purged with nitrogen for 15 min until melted. The flask was sealed and placed in an oil bath at 40 °C for 24 h. Polymer solutions were then precipitated in cold diethyl ether, and 11949

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Figure 2 shows dry thicknesses of 50-bilayer polycation/PAA films as a function of deposition pH (Figure 2A, BPEI/PAA system), as well as a function of polycation permanent charge density (Figure 2B, QX/PAA systems). Film thicknesses were quantified using AFM as the heights of steps cut in the film with a razor blade. For deposition of BPEI/PAA films, polymer solutions and 0.01 M Trizma buffer rinsing solutions were adjusted to pH 5.0 or 9.5. In agreement with earlier findings by Rubner and co-workers for a similar type of weak polyelectrolyte multilayers,32 at neutral pH values, both BPEI and PAA were fully ionized and formed ionically stitched layers with the smallest thickness. At pH 7, thickness of (BPEI/PAA)50 film increased as a result of neutralization of PAA (pKa ∼ 5.0) and BPEI (pKb ∼ 8.8), respectively. In these thicker multilayers, a significant fraction of polymer segments is included within low-charge-density “loops”. In the case of QX/PAA systems (Figure 2B), films were deposited at pH 8.5 to avoid protonation of unquaternized pyridine groups (pKa ∼ 3.5) and ensure their availability for binding with metal cations. Consistent with prior studies,31 the thickest films were formed in the case of Q17, which had the lowest charge density and the largest fraction of segments in the loops. Specifically, the thickness of Q17/PAA films was ∼1.8fold larger than that of Q65/PAA multilayers. It was also interesting to explore the role of the polyelectrolyte type on LbL deposition. Therefore, LbL films were assembled with either PAA (a weak polyelectrolyte) or PSS (a strong polyelectrolyte). To maximize the number of deprotonated amino groups within PEM films that are available for binding with metal cations, as well as to ensure that PAA and PSS have the same charge density, LbL deposition was performed at pH 8.5. Figure 3 shows that all PEM types BPEI/PAA, BPEI/PSS, Q17/PAA, and Q17/PSSdemonstrated a linear increase in dry thickness with bilayer number. These data are consistent with previous reports of predominantly linear growth of BPEI/PAA films.33 Importantly, in spite of equal charge density in PSS and PAA chains at pH 8.5, films with PAA were significantly thicker than those with PSS (inset in Figure 3). This observation agrees with our previous report that the LbL structure and properties are dependent not only on the polyelectrolyte charge density but also on the chemical nature of assembled chains.34 Specifically, stronger binding of polycations with SO3−- and SO4−-containing polyanions as compared to binding with polycarboxylic acids results in flatter chain conformations and smaller total polymer amounts deposited within LbL films. Binding of Me2+ within PEM Films. The amount of Cu2+ and Co2+ loaded within PEM systems can be controlled by PEM assembly conditions and types of assembled polyelectrolytes. To prevent decomposition of films during catalytic reaction (PAA−containing films were partially destroyed after the first cycle of oxidation), all films were thermally pretreated as described in the Experimental Section. Metal cations were allowed to absorb within BPEI/PAA, BPEI/PSS, Q17/PAA, and Q17/PSS LbL films from aqueous solutions of CoCl2 and CuCl2 at pH 5.0 for 10 h. Figure 4 schematically shows examples of multilayer systems used for metal ion loading. Cu2+ and Co2+ ions can be electrostatically bound by carboxylic (or sulfate) groups and/or involved into chelate complexes with amino groups of BPEI or QX. In all cases, the amount of metal ions loaded within films increased linearly with layer number, as confirmed for BPEI/ PAA films constructed at pH 8.5 using UV−vis spectrometry

Values of QO2 were then used to calculate the reaction rate (WO2) (mol/L·s) using the following equation:

WO2 =

ΔQ O

2

Δt

The turnover frequency (TOF) was calculated as number of toluene molecules converted to an oxidation product per metal ion catalytic site per second as TOF = [N0(toluene) − Nt(toluene)]/tNMe2+, where N0 is initial number of toluene molecules in the mixture, Nt is number of toluene molecules remaining after reaction time t, and NMe2+ is number of metal ions within the film. Values N0 and Nt were determined by GC chromatography from the area of the toluene elution peaks, and NMe2+ was quantified using ICP-OES.

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RESULTS AND DISCUSSION Construction of Metal Ion-Absorbing PEMs. In order to create multilayer films able to accommodate catalytically active Cu2+ and Co2+ ions, we explored the use of several polyelectrolytes such as branched poly(ethyleneimine) (BPEI), quaternized poly(4-vinylpyridine) (QX, where X is denotes the quaternization degree), poly(acrylic acid) (PAA), and polystyrene sulfonate sodium salt (PSS). Many of these polymers, such as BPEI, P4VP, and PAA, have amino and carboxylic functional groups that can form chelation complexes with transition metal ions, including Co2+ and Cu2+.27−29 Figure 1 illustrates deposition of PEMs at a substrate (using the BPEI/PAA system as an example) and loading of metal ions within the film.

Figure 1. Schematic representation of constructing catalytic PEM films containing Me2+ ions.

We then aimed to construct LbL films with various amounts of “free” polymer segments included in polymer loops that are not involved in intermolecular binding and are therefore available for chelating metal cations. Our strategy for film construction was first based on earlier studies on assembly of weak polyelectrolytes, which showed that film thickness, molecular organization, and amount of polymer adsorbed can all be controlled by simple adjustments of the pH of the dipping solution.30−32 Second, we also relied on prior studies on PEM assemblies of polyelectrolytes with different permanent charge densities, which reported thicker films for polymers with lower degrees of permanent charge.31 11950

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Figure 2. Dependence of film thickness of (BPEI/PAA)50 films on pH (A) and of (QX/PAA)50 films deposited at pH 8.5 on the polycation quaternization degree (B).

range, BPEI/PAA films absorbed 2.5−4-fold larger amounts of Cu2+ than Co2+. This shows the higher chelating strength of both BPEI and PAA to Cu2+. Second, while (BPEI/PAA)50 films deposited at pH 9.5 and 5.0 absorb almost the same amount of Co2+, absorption of Cu2+ is almost twice as high at basic pH values (Figure 5A), indicating stronger chelating of metal ions to amino than to carboxylic groups. Figure 5B shows that, as expected, the amount of metal cations in QX/PAA films decreases with an increase in quaternization degree of QPVP, as the films became thinner and contained fewer amino groups available for coordination with metal ions. It was also interesting to observe that the relative binding capacity of QPVP/PAA system to Co2+ and Cu2+ ions was very different from that observed with BPEI/ PAA films; i.e., (QPVP/PAA)50 multilayers absorbed ∼30% more Co2+ than Cu2+. Note that in the case of QX/PAA and QX/PSS films, Co2+ and Cu2+ were loaded from ethanol solution of salts, since very small amounts of metal ions could be absorbed by films in aqueous solutions. Note that inclusion of Cu2+ and Co2+ ions within BPEI/PAA films assembled at pH 8.5 (and therefore enriched with BPEI) resulted in blue and orange film colors [λmax(BPEI/PAA−Co2+) = 523 nm; λmax(BPEI/PAA−Cu2+) = 684 nm, Supporting Information, Figure S1], also seen with BPEI−Cu2+ and BPEI− Co2+ complexes in solution. As film assembly pH decreased, the colors faded as the content of BPEI within the film decreased, and metal ions were now mostly bound with PAA units. In the case of QPVP/PAA−Me2+ systems, Cu2+- and Co2+-containing films were light greenish and blue, respectively, which is typical for chelate compounds of these metal ions with organic ligands. Finally, we have explored the effect of polyanion type on film loading capacity to metal ions. In these experiments, metalabsorbing capacity for 40-bilayer films of different compositions was compared, while the amount of metal cations was normalized to dry film thickness. Figure 6 shows that the amounts of Co2+ and Cu2+ ions bound within PEM films are smaller when PSS rather than PAA is used in film assembly. Moreover, this difference is especially large (∼10-fold) for the Q17/polyanion system (Figure 6B). Obviously, the film capacity to bind metal cations is strongly dependent on the nature of polyanions used in film assembly. The likely reason for this behavior is probably rooted in stronger binding of PSS with polycations.31,34 The latter can affect the fraction of polybase units included within the film during assembly and available for chelation of metal cations. It is also likely that not only basic groups abundant within the film assembled at basic

Figure 3. Thicknesses of dry LbL films as a function of bilayer number: BPEI/PAA (squares), BPEI/PSS (circles), Q17/PAA (triangles), and Q17/PSS (diamonds). Thicknesses were measured by ellipsometry (open symbols) and AFM (filled symbols). All films were deposited from 0.3 mg/mL polymer solutions in 0.01 M Trizma buffer solutions at pH 8.5.

and ICP-OES (Supporting Information, Figure S1). The data obtained with these techniques were in good agreement. However, BPEI/PAA films constructed at acidic and neutral pH values turned cloudy after binding of Me2+, impeding the use of UV−vis spectrometry for quantifying the amount of bound ions. Therefore, for consistency, metal ion content in films of all types was quantified using ICP-OES. Figure 5 shows the dependences of the amounts of metal ion inclusion within the mutlilayers on polycation type and film assembly pH. Overall, the amount of ions included followed the dependences of PEMs film thicknesses on pH and the polycation quaternization degree (see Figure 2). BPEI/PAA films assembled at neutral pH from 6 to 7 have the smallest absorption capacity for Me2+ ions (Figure 2A), as polymer layers are thin and films are highly ionically cross-linked. In BPEI/PAA films assembled at basic pH values, PAA is fully ionized and is not available for binding with Me2+. At the same time, BPEI is weakly charged and brings in large amounts of free amino groups that are available for coordination with metal cations. The data suggests that Cu2+ and Co2+ can bind with excess carboxylic groups of PAA (contained within BPEI/PAA films assembled at pH 5.0), as well as with the excess BPEI units (contained within PEMs constructed at basic pH). A first important observation in Figure 5A is that within the entire pH 11951

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Figure 4. Schematic representation of internal structure of BPEI/PAA and QX/PAA films deposited under different conditions, as well as binding of metal ions within PEM films.

Figure 5. Amounts of Co2+ and Cu2+ ions absorbed within (BPEI/PAA)50 films as a function of film deposition pH (A) and absorbed within (QX/ PAA)50 as a function of a polycation quaternization degree X (B). Images of metal-loaded LbL films deposited on glass slides (C and D). QX/PAA films were assembled at pH 8.5, and the amounts of Co2+ and Cu2+ were determined by ICP-OES. Metal cations were loaded within BPEI/PAA films from aqueous salt solutions at pH 5.0 and within QX/PAA films from metal salt solutions in ethanol.

pH participate in coordination of metal ions but also polyanion groups which become available for binding with Co2+ and Cu2+ after dissociation of polymer−polymer ionic pairs in metal salt solutions. Binding of metal cations with carboxylic groups of PAA is suggested by the data in Figure 5. Since ionic pairs of PSS with polycations are more difficult to dissociate compared to PAA/polycation ionic pairs,31,34 polycation/PSS systems

have a smaller total amount of groups (basic or anionic) available for binding with Co2+ and Cu2+ ions. Retention of Catalysts within PEM Film. Film Catalytic Activity and Reusability. Retention of immobilized catalysts within a substrate is one of the most important requirements in efficient heterogeneous catalytic systems. Thus, we have studied leaching of metal cations from (BPEI/PAA)30, (Q17/PAA)30, 11952

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Figure 6. The effect of polyanion type on the amount of metal cations absorbed within 40-bilayer BPEI/polyanion and Q17/polyanion films (A and B), as well as images of metal-loaded PEM films deposited at glass slides. In all systems PEMs were deposited from polymer solutions at pH 8.5. Metal ion concentrations were calculated using ICP-OES.

and (BPEI/PSS)30 PEMs films during four 2-h cycles of extraction in acetonitrile at 75 °C (Supporting Information, Figure S2). Acetonitrile solutions of leached metal cations were tested by ICP-OES. The degree of leaching was relatively low, with the highest percentage of ions eluted during the first cycle. The system Q17/PAA−Me2+ showed the lowest retention of metal ions after a first cycle: 9% and 7% of Cu2+ and Co2+ eluted, respectively. The first cycle leachability is related to different ion binding strengths with PEMs in different solvents (in the case of Q17/PAA−Me2+ systems, between ethanol used for ion loading and acetonitrile used in catalytic reaction). After two initial extraction cycles, however, metal ions remained strongly bound with the films. BPEI/PAA−Me2+ films eluted less than 7% of metal cations after multiple extraction cycles. We then studied activity of Co2+ and Cu2+ ions loaded within various types of multilayers assembled at pH 8.5 or 5.0 in the catalytic reaction of toluene oxidation. The reaction was performed at atmospheric pressure and a moderate temperature of 75 °C, as described in the Experimental Section. A combination of FTIR and GC showed that the main product of this reaction was benzaldehyde. This suggestion was made from the appearance of the absorption band at 1690 cm−1, which is assigned to carbonyl groups in benzaldehyde. The identity of the product was then confirmed by GC, as the product of catalytic reaction had the same retention time as a control benzaldehyde sample (data not shown). Figure 7 presents the data expressed as catalytic TOF, i.e., number of toluene molecules converted to an oxidation product per metal ion catalytic site per second. Me2+ ions bound within (BPEI/ PAA)30 multilayers deposited at pH 8.5 were more catalytically active compared to those embedded in films assembled at pH 5.0. For films assembled at pH 8.5 or 5.0, Me2+ ions are coordinated with amino or carboxylic groups, respectively. Within (BPEI/PAA)30−Me2+ and (Q17/PAA)30−Me2+ films deposited at pH 8.5, Co2+ and Cu2+ ions are coordinated within amino groups of polycations through donor−acceptor bonds. This type of binding allows sufficient mobility of metal ions within the coordination sphere of the metal complex, required for high catalytic performance. Binding of Me2+ with negatively

Figure 7. The maximum reaction rate (WO2) (filled bars) and TOF (unfilled bars) of toluene oxidation reaction during 1 h of catalytic reaction using various PEM systems deposited on glass slides.

charged carboxylic acid groups of PAA is more of an ionic nature and results in slightly weaker ion mobility.35,36 However, in PSS-containing matrices, metal ion mobility is drastically reduced and catalytic activity is suppressed, probably as a result of strong ionic pairing of polycations and metal ions with PSS. Another important observation is that Co2+-containing PEMs films were almost twice as active as those containing Cu2+ ions, again due to higher mobility of Co2+ ion within the coordination sphere of the metal complex. Among different multilayer types, (BPEI/PAA)30−Co2+ systems deposited at pH 8.5 had the highest catalytic activity. This observation is consistent with an earlier report on oxygenbinding properties of aqueous BPEI−Co2+ complexes and their ability to enhance the electric current for oxygen reduction at the electrode surface.37 With the best-performing (BPEI/ PAA)30−Co2+ system, we have explored the dependence of the catalytic activity on the number of layers deposited within the film. Figure 8 shows the dependence of the maximum reaction rate (Wo2) on number of BPEI/PAA bilayers. The catalytic 11953

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catalytic cycles. Facile synthesis, relatively mild conditions used in the catalytic reaction, and reusability of PEM-supported Me2+ systems make them attractive candidates for use in oxidation of organic compounds.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis and ICP-OES data on the amounts of metal ions bound within films as a function of layer number, ICP-OES data on leachability of Co2+ and Cu2+ from catalytic PEMs, and catalytic activity of PEM−Me2+ films in several catalytic cycles. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 8. Catalytic activity of (BPEI/PAA)n−Co2+ films for the toluene oxidation reaction as a function of bilayer number.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. activity of these films increases with thickness for films up to 30 bilayers, and then slightly slows for thicker films, in spite of a continued linear increase in the amount of Co2+ absorbed within PEMs shown in Figure S1 (Supporting Information). A slower increase in the rate of the catalytic reaction for thicker films is probably due to restricted diffusion of toluene and/or benzaldehyde within (BPEI/PAA)n−Co2+ films of thicknesses higher than ∼200 nm. Importantly, however, films thinner than ∼200 nm remained permeable to reactant and product molecules, as a result of the small size of Me2+. Note that previous studies involving PEM-embedded NPs reported catalytic activity supplied by the film’s top layer,10 while the bulk of the film remained inactive. Advantageously, in our case, with films not exceeding ∼200 nm thickness, the catalytic activity was supplied by the entire film and can be easily controlled by adjusting the number of polymer layers deposited within a polymer matrix. Finally, we tested the reusability of our catalytically active PEM films in repeated catalytic reactions. After completion of a catalytic reaction, glass slides covered with (BPEI/PAA)30− Co2+ films were separated from the benzaldehyde/acetonitrile solution by filtration, washed with acetonitrile, dried, and then reused in another catalytic cycle using fresh toluene/acetonitrile mixture. As shown in Table S1 (Supporting Information), after four catalytic cycles performed with the same (BPEI/PAA)30− Co2+ films deposited on glass substrates, both values of WO2 or TOF decreased by ∼20%. This moderate loss in catalytic activity is probably due to changes in microenvironments of Co2+ catalytic centers and/or transport properties of the film to reactants and products of the catalytic reaction, caused by exposure of the multilayer to increased temperature and mechanical agitation.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Aliaksandr Zhuk for his help in sample characterization and preparation of the manuscript and to Li Xu for help with AFM measurements. We also thank the Ministry of Education and Science of the Republic of Kazakhstan for providing a scientific internship for A.M. to visit Stevens Institute of Technology.

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REFERENCES

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CONCLUSION We have demonstrated that immobilization of transition metal ions within LbL matrices provides a facile and efficient way for constructing recyclable catalysts useful for oxidation of organic compounds. We found that the amount of captured metal ions, their stability within PEMs, as well as the catalytic activity of metal ion-saturated films are strongly dependent not only on the nature of the metal ion but also on the polyelectrolyte type and conditions of film assembly. We show that with the BPEI/ PAA−Co2+ system, which demonstrated the best performance, films were catalytically active within tens to hundreds of nanometers of film thickness and could be reused in multiple 11954

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