Article pubs.acs.org/IECR
Hydrogen Peroxide Stability in Silica Hydrogels Fulya Sudur, Brian Pleskowicz, and Nese Orbey* Chemical Engineering Department, University of Massachusetts Lowell, 1 University Avenue, Lowell, Massachusetts 01854, United States S Supporting Information *
ABSTRACT: Hydrogen peroxide (H2O2) entrapment in silica hydrogels has potential to be used in various industrially important applications to increase H2O2 stability. In this study, optimum conditions for hydrogel formation and H2O2 stability were determined by varying the sodium content and initial H2O2 concentration. Higher retention and better stability of H2O2 were achieved with hydrogels at room temperature at low sodium concentration. Retention values of 89% were obtained with initial H2O2 concentrations up to 10 wt %. H2O2 decomposition in hydrogels followed a first-order reaction. Hydrogels were characterized by measuring their surface area, pore size, and pore size distribution by Brunauer−Emmett−Teller analysis and scanning electron microscopy. Mesoporous (3−24 nm) hydrogels with high surface area (1000−1400 m2/g) were obtained. In addition, the melting point of the entrapped H2O2-water mixture in the hydrogels was studied by low temperature differential scanning calorimetry.
1. INTRODUCTION Hydrogen peroxide (H2O2) is an environmentally friendly oxidant with a broad range of industrial applications, such as pulp and textile bleaching, tooth whitening, skin disinfection, and water treatment.1−4 H2O2 is a very attractive candidate as an antifouling agent in marine coatings,5 and it is an efficient oxidant in organic syntheses in the form of the urea−H2O2 adduct.6 Uranium can be recovered from its solid waste as uranium peroxide by using H2O2.7 The recent discovery of solid H2O2 on the surface of Jupiter’s moon Europa has renewed interest in the properties of solid H2O2.8,9 However, H2O2 has very limited stability due to its propensity to disproportionate exothermally into molecular oxygen and water, according to the following reaction: H 2O2(aq) → H 2O(l) + 1/2 O2(g)
Silica xerogels are formed via sol−gel chemistry by using aqueous silicate precursors, a simple, low-cost, and “green” process.14 Sol−gel chemistry has been used extensively to entrap various active materials (e.g., drugs, biomacromolecules, food ingredients, catalysts, etc.) in inert matrices.15−19 “Sol” refers to the formation of a colloidal suspension, and “gel”, to the formation of a network through the gelation of the colloid in a continuous liquid phase. The starting precursor for silica gel formation can be an alkoxide or alkali silicate. Sodium silicate, or “water glass”, is the most commonly used aqueous silicate in sol−gel chemistry. Silanol groups (Si−OH) are formed by removing the sodium ions from sodium silicate with a cation exchange resin. The concentration of sodium ions in sodium silicate can be controlled by using an ion exchange process,20−22 as shown below, Na + + RzSO−3 H+ → H+ + (RzSO3)Na
(1)
where Rz is the styrene-divinylbenzene copolymer and SO3H is the sulfonic acid group. Ion exchange occurs easily because the ionic bond strength of a sodium ion is stronger than that of a hydrogen ion.22 Silanol groups condense to form Si−O−Si bonds and polymerize in the presence of residual sodium ions. Iler23 explained the polymerization of silanol groups in detail. A condensation reaction leads to an inorganic polymerization process, resulting in the formation of silicon dioxide (SiO2) nanoparticles that aggregate and form branched chains. Further bonding between the aggregates leads to the formation of a threedimensional interconnected network and immobilizes the entire liquid. At this point, a strong increase in viscosity is observed. The sol−gel transition point is reached when the last link forms between two giant particle aggregates, resulting in a “wet gel” or “hydrogel”. Upon drying by evaporation, the gel network
The presence of certain catalytic impurities (e.g., metal ions), elevated temperatures, pH levels, solar radiation, and airborne particles can accelerate the decomposition reaction. Moreover, decomposition occurs even at room temperature; thus, H2O2 should be stored at low temperatures (2−8 °C).10 H2O2 stability can be improved by decreasing the decomposition rate. Although there are some applications in which H2O2 decomposition can be useful, such as when H2O2 is used as a biocide and molecular oxygen forms as a decomposition product,11 for most applications, it is desirable that H2O2 be stable. The use of sol−gel processing for silica xerogels has a high potential of increasing the stability or controlling the decomposition rate of H2O2 by forming strong hydrogen bonds with the silica gel surface.12,13 Better stability in the hydrogel matrix would enhance the use of H2O2 in current applications. For example, H2O2 containing hydrogels in municipal water treatment would provide longer periods of disinfection using lower concentrations of H2O2. It would also enable new applications that are currently not practical due to the instability of H2O2 such as injectable hydrogels for wound healing. © 2015 American Chemical Society
(2)
Received: Revised: Accepted: Published: 1930
December 12, 2014 January 14, 2015 January 20, 2015 January 20, 2015 DOI: 10.1021/ie504850n Ind. Eng. Chem. Res. 2015, 54, 1930−1940
Article
Industrial & Engineering Chemistry Research
sol was removed. The pH was recorded before and after ion exchange. Sol was transferred to a glass Petri dish (D = 100 mm, h = 15 mm) and placed in a BlueM gravity oven at 40 °C until gelation was observed. Gelation occurs faster at higher temperatures, but the decomposition of H2O2 is also increased. To balance the decomposition rate and gel time, 40 °C was selected as the experimental temperature. Samples were stored at room temperature. Different initial concentrations of H2O2 (3.3−19.9 wt %) were also studied at a constant sodium content. 2.3. Characterization. Attenuated total reflection FTIR (Thermo Scientific Nicolet 6700 FT-IR Spectrometer) was performed for the silica hydrogel and xerogel. Spectra were obtained in transmission mode from 600 to 4000 cm−1. The sodium content of the silica hydrogels was determined by measuring the sodium content of the sol before gelation with a flame atomic absorption spectrometer (240 AA, Agilent Technologies). The H2O2 content of the hydrogels was measured by KMnO4 titration immediately after gelation and remeasured daily to assess H2O2 retention in the hydrogel. Three repeated titrations were performed for each gel sample. Hydrogels gradually turned into xerogels during storage, due to the loss of H2O2 and water. The morphology, surface area, pore size, and pore size distribution of the hydrogels were studied by SEM and BET analyses. Drying at ambient pressure causes hydrogels to collapse due to capillary pressure, and the hydrogel mechanical structure can be damaged. Therefore, hydrogels were converted into aerogels using a critical-point dryer (Tousimis SAMDRI-795) to preserve the solid network and retain the pore structure. Before drying, hydrogel samples were stored in 10 mL of ethanol (200 proof, anhydrous, >99.5%) overnight for solvent exchange and were dried with supercritical carbon dioxide (CO2). Nitrogen adsorption−desorption measurements were conducted with a Nitrogen Sorption Porosimeter Quantachrome Autosorb-3B instrument. The specific surface area was obtained by BET analysis. Pore size distribution was calculated by the Barret−Joyner−Halenda method. Pore diameter was evaluated from the desorption isotherm. Before analysis, the aerogels were degassed at room temperature under vacuum. The morphology of the aerogels was visualized with a JEOL JSM 7401F scanning electron microscope. All samples were coated with gold using a vacuum sputter coater and imaged with an accelerating voltage of 10−15.0 kV. The DSC measurements were carried out on a TA Instruments Q200 calorimeter. Hydrogels were placed into an aluminum pan. They were cooled from 0 to −80 °C at 20 °C/min and heated from −80 to 20 °C at 1 °C/min in a nitrogen atmosphere.
contracts and expels the liquid in its pores, forming a xerogel. The pH of the sol regulates the formation and growth of the particles, and, therefore influences the properties of the xerogel.24 The synthesis of H2O2-containing xerogels was first reported in 2007 by Ż egliński and colleagues,12 who tested the antibacterial activity of xerogels containing 3.8 to 68 wt % H2O2. Samples were obtained by drying sol−gel at 70 °C until 90% of the total mass was lost. The stability of 3.8 wt % H2O2 in xerogel was 71% after 63 days of storage at room temperature and as high as 94% after storage at 3 °C. Bednardz et al.13 entrapped H2O2 within silica xerogels, which they used as oxidants in organic synthesis. Xerogels were synthesized by drying sol−gel at room temperature for 48 h until 80−90% of the total mass was lost. When xerogels were stored at 5 °C, only half of the initial H2O2 loading of 8 wt % was retained. No stability of H2O2 at room temperature was reported. These previous studies emphasized obtaining a xerogel with the highest H2O2 loading for their intended applications. Samples were obtained in xerogel form because xerogels are easy to handle and contain high amounts of H2O2. A weight loss of up to 90% was used to minimize H2O2 decomposition and maximize H2O2 stability during xerogel formulation. This is a subjective metrics and researchers used different drying conditions to reach this weight loss value. The effect of sodium on gelation in the presence of H2O2 and its consequent effect on its stability have not been studied to date. Conditions for hydrogel synthesis such as composition and temperature need to be well-defined and controlled for large scale applications. The properties of hydrogels can then be tailored for different end-uses by adjusting these variables. In the present work, the gel formation stage before xerogel synthesis and the factors affecting gelation for H2O2−silica hydrogels are studied. Specifically, the effects of the sodium content and the initial H2O2 concentration on the gel formation and stability of entrapped H2O2 at room temperature were investigated. H2O2 stability in silica hydrogels at room temperature was monitored daily by potassium permanganate (KMnO4) titration. Hydrogels were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Brunauer−Emmett−Teller (BET) analysis. The effects of sodium and H2O2 content on the melting point of the entrapped H2O2- H2O mixture in hydrogels were also studied by low-temperature differential scanning calorimetry (DSC).
2. EXPERIMENTAL PROCEDURE 2.1. Materials. The following materials were used: sodium silicate solution (reagent grade, 10.6% Na2O, 26.5% SiO2); Amberlite IR120 (hydrogen form); hydrogen peroxide (contains inhibitor, 30 wt % in H2O, ACS reagent); orthophosphoric acid (85 wt % in H2O, 99.99% trace metals basis); potassium permanganate (ACS reagent >99%); and sulfuric acid (99.999%). All chemicals were purchased from Sigma-Aldrich Chemical Co. and used without further purification. 2.2. Hydrogel Synthesis. The procedure to obtain hydrogels was adopted from the available literature12 with some modifications. Sodium silicate solution, as received, was diluted with deionized water at a volume ratio of 1:5 and combined with 30 wt % H2O2 solution and 7 μL of phosphoric acid (H3PO4). The sodium content of the sodium silicate solution was controlled by adding different amounts of ion-exchange resin (Amberlite IR 120, hydrogen form) to the sol. The mixture was stirred with an overhead mixer for 4 min, the resin was allowed to settle, and the
3. RESULTS AND DISCUSSIONS The effect of sodium concentration on the stability of H2O2 was studied keeping the initial H2O2 concentration constant at 19.9 wt % and changing the sodium concentration between 0.027 and 0.557 wt %. The effect of initial H2O2 concentration on the stability of H2O2 was also studied at concentrations between 3.3 and 19.9 wt % while keeping the sodium concentration constant at 0.027 wt %. 3.1. FTIR. FTIR spectroscopy was performed to investigate the bonds that formed in silica hydrogels and xerogels under comparable conditions (Figure 1). The broad 3700−3000 cm−1 band corresponds to the O−H stretching frequencies that were generated from H2O2, water, and silanol groups. The absence of stretching vibrations beyond 3700 cm−1 indicates the complete saturation of silica with hydrogen bonds.25 The 2900−2700 cm−1 1931
DOI: 10.1021/ie504850n Ind. Eng. Chem. Res. 2015, 54, 1930−1940
Article
Industrial & Engineering Chemistry Research
Ż egliński et al.25 observed that H2O2 decomposed during FTIR analysis because the hydrogen bond between silica and H2O2 was sensitive to IR radiation. We obtained two consecutive IR spectra of the same sample but did not observe any difference in spectra. We concluded that there was no dissociation of the hydrogen bond during our measurements. 3.2. Analysis of Sodium Content. Previous studies have controlled the pH of the silicic acid sol by adding a strongly acidic, weakly acidic, or alkaline solution.23 In this study, the pH of the sol was varied by monitoring the amount of residual sodium in the silicic acid sol. To determine the residual sodium content, sols were analyzed by flame atomic absorption spectrometry before gelation occurred. Figure 2 illustrates the relationship between the pH and sodium content of the sol. As expected, a decrease in the pH of the sol was observed at lower sodium contents because sodium ions were replaced with hydrogen ions as a result of ion exchange. The pH change was not significant until the sodium content reached approximately 0.1 wt %. However, there was a sharp decline in pH at sodium contents below 0.1 wt %, which we believe is due to the high buffer capacity of the sol above 0.1 wt %.27 In the remainder of the manuscript, low sodium content and low pH are used synonymously. 3.3. Relationship between Gel Time and pH. Next, the effect of pH on the gel time of silicic acid−water−H2O2 was studied. As described in section 3.2, the pH of the sol is changed by changing the sodium content. The rate of gelation depends on the type, concentration of silicate and pH, and temperature of the medium. In accordance with a previously reported protocol, the gel time was determined as the moment when the gel broke away from the wall of the Petri dish, instead of flowing as a liquid, when the dish was tilted.28,29 The results plotted in Figure 3 indicate that the silica gel formation was slow when the pH was low (pH 2−4), due to the low concentration of hydroxyl ions. Gelation was most rapid between pH 4 and 6 because the sol passed through the isoelectric point, at which it did not carry a net electrical charge. At higher pH values (pH 6−8), the sol began to adsorb hydroxyl ions, resulting in longer gelation times. Above a pH of 8, the gel time started decreasing again, presumably because the higher sodium concentration at this pH level disrupted the stability of the sol, causing faster gelation.30,31 Similar trends for dependence of gel time on the pH of the medium were observed in literature for silica gel formation without hydrogen peroxide.23,32,33 Our results shifted slightly toward the acidic side. We believe this is due to the presence of H2O2 in the sol.
Figure 1. Spectroscopic characterization of the silica hydrogel and silica xerogel (Na content 0.027 wt % and initial H2O2 concentration 19.9 wt %).
Figure 2. Relationship between the pH and Na content of the sol (initial H2O2 concentration 19.9 wt %).
Figure 3. Relationship between gel time and pH (initial H2O2 concentration 19.9 wt %).
band corresponds to the vibrations of H2O2.26 The 1400−1300 cm−1 band reflects the bending vibrations of H−O−O. The 1700−1600 cm−1 band corresponds to the well-known molecular bending vibrations of water. The obtained spectrum for silica gel was similar to spectra described in the literature.13,25 The silica xerogel and hydrogel had very similar spectra, indicating similar silanol bonding efficiencies. Therefore, either form can be used to stabilize H2O2, depending on the application.
Figure 4. Room temperature stability of H2O2 as a function of Na and H2O2 content of the sol. 1932
DOI: 10.1021/ie504850n Ind. Eng. Chem. Res. 2015, 54, 1930−1940
Article
Industrial & Engineering Chemistry Research
The effect of H2O2 concentration on the gel time was less complicated. Specifically, the gel time decreased as the H2O2 concentration increased. Understanding how the sodium content affects gel time is essential for large scale synthesis of hydrogels to obtain reproducible results and to choose the optimum gel time for a particular application. 3.4. Stability of H2O2. H2O2 leaves the silica network through decomposition and desorption.12 We studied the effects of the sodium content and initial H2O2 concentration on the stability of entrapped H2O2. Decomposition of H2O2 followed a first-order rate law during the first 10 days of storage: Figure 5. Room temperature stability of H2O2 as a function of storage time at changing Na content (initial H2O2 concentration 19.9 wt %).
−
d[H 2O2 ] = k H2O2[H 2O2 ] dt
⎛ [H O ] ⎞ ln⎜ 2 2 ⎟ = −k H2O2t ⎝ [H 2O2 ]0 ⎠
(3)
(4)
where kH2O2 is the observed first-order rate constant and [H2O2] and [H2O2]0 are the H2O2 concentrations in the hydrogels at any time t and time zero, respectively. Figure 4 illustrates the dependence of the decomposition rate constant on the sodium and H2O2 contents of the hydrogels. At a given H2O2 concentration (e.g., 19.9 wt %), as the sodium content decreased, the H2O2 decomposition rate also decreased. At a given sodium content, the H2O2 load did not have any
Figure 6. Room temperature stability of H2O2 as a function of storage time at changing H2O2 concentration (Na content 0.027 wt %).
Figure 7. Structural properties of silica hydrogels with changing Na and H2O2 content (initial H2O2 concentration 19.9 wt % for a, c, and e; Na content 0.027 wt % for b, d, and f). 1933
DOI: 10.1021/ie504850n Ind. Eng. Chem. Res. 2015, 54, 1930−1940
Article
Industrial & Engineering Chemistry Research
Figure 8. SEM images of hydrogels containing different sodium contents: (a) 0.557, (b) 0.196, (c) 0.070, (d) 0.033, (e) 0.027 wt % (initial H2O2 concentration 19.9 wt %).
appreciable effect on the decomposition rate constant for shortterm storage.
To compare our results with those reported in the literature, we attempted to determine the conditions under which the 1934
DOI: 10.1021/ie504850n Ind. Eng. Chem. Res. 2015, 54, 1930−1940
Article
Industrial & Engineering Chemistry Research
Figure 9. SEM images of hydrogels containing different H2O2 concentrations: (a) 3.3, (b) 6.6, (c) 9.9, (d) 13.2, (e) 16.5, (f) 19.9 wt % (Na content 0.027 wt %).
during drying. Ż egliński et al.12 defined the xerogel formation point as the moment when the highest H2O2 concentration is
hydrogel was converted into a xerogel. Xerogel formation is highly dependent on the time and temperature regime used 1935
DOI: 10.1021/ie504850n Ind. Eng. Chem. Res. 2015, 54, 1930−1940
Article
Industrial & Engineering Chemistry Research
Figure 10. Adsorption/desorption isotherms and pore size distributions of hydrogels containing different sodium contents: (a) 0.557, (b) 0.196, (c) 0.070, (d) 0.033, (e) 0.027 wt % (initial H2O2 concentration 19.9 wt %).
temperature), we obtained better room temperature stability for longer storage time at concentrations up to 10 wt %. Higher stability is obtained in this work because the sodium content and gelation conditions (time and temperature) were optimized. The stability of H2O2 was highest when pH value of the sol was varying between 2 and 4 at an initial H2O2 concentration of 19.9 wt %. However, at this pH range, the gel times were very long due to presence of low hydroxyl ion concentration (as also described in section 3.3). Although the fastest gelation occurs between pH values of 4 and 6, the stability of H2O2 was low due to the presence of sodium. 3.5. Morphology and Structure of Hydrogel. The effects of the sodium content and the H2O2/water ratio on the hydrogel structure were studied by BET and SEM analyses. Before the analyses, the samples were dried using supercritical CO2 forming aerogels. Figure 7 and Tables S1 and S2 report the effects of sodium content and H2O2 concentration on the surface area, pore volume, and average pore diameter.
obtained, which, in their study, corresponded to about 90% total weight loss. In our study, the maximum H2O2 concentration was achieved at 80% total weight loss. Figure 5 and 6 show the long-term stability of H2O2 with changing sodium and H2O2 content. At the lowest sodium content, more than 50% of H2O2 was retained at 70 days of storage; whereas at the highest sodium content, H2O2 took only 5 days to decompose completely. As H2O2 concentration increased up to 10 wt %, retention values up to 89% were obtained when the hydrogels were stored at room temperature for 70 days. When the concentration was higher than 10 wt %, a faster decomposition of H2O2 was observed where more than 70% of the H2O2 was retained after 40 days of storage. The room temperature stabilities of the xerogels were compared to those reported in literature. Ż egliński et al.12 reported 71% H2O2 retention after 63 days of storage at room temperature when an initial H2O2 concentration of 0.5 wt % was used. Although the conditions were not the same for xerogel formation (time and 1936
DOI: 10.1021/ie504850n Ind. Eng. Chem. Res. 2015, 54, 1930−1940
Article
Industrial & Engineering Chemistry Research
Figure 11. Adsorption/desorption isotherms and pore size distributions of hydrogels containing different H2O2 concentrations: (a) 3.3, (b) 6.6, (c) 9.9, (d) 13.2, (e) 16.5, (f) 19.9 wt % (Na content 0.027 wt %).
stability is more prominent than the effect of pore volume. The effect of the H2O2/water ratio was much less pronounced than the effect of sodium content on pore volume (Figure 7d). The average pore diameter increased with increasing sodium content causing lower H2O2 stability (Figure 7e). The sample with the highest sodium content or highest pH had the largest pore diameter, due to the dissolution and reprecipitation of silica. We believe that the presence of sodium favored interparticle condensation, resulting in larger pores. At low sodium concentrations or low pH, small oligomers come together and form smaller sized pores due to the low dissolution rate of silica and enhanced the stability of H2O2. The average pore diameter initially decreased as the H2O2 concentration increased, reaching a constant value at about 10 wt % H2O2 (Figure 7f). The gel time also decreased as H2O2 concentration increased. A higher hydrogen bonding capacity of H2O2 will bring the molecules closer and result in smaller pores.
Surface area was not affected at low sodium concentrations, but decreased significantly at higher sodium contents (Figure 7a) due to the high silica dissolution rate in the presence of sodium (∼0.6 wt % and pH 9).34,35 In this state, silica dissolves and reprecipitates, causing a reduction in the surface area and formation of pores with larger diameter.36 The lowest H2O2 stability was observed for samples that have low surface area. For hydrogels with lower sodium content (
