wang2015 chitosan–silica composite aerogels preparation characterization and congo red adsorption

J Sol-Gel Sci Technol DOI 10.1007/s10971-015-3800-7 ORIGINAL PAPER Chitosan–silica composite aerogels: preparation, characterization and Congo red adsorption Jianquan Wang1 • Qiushi Zhou1 • Danqiao Song1 • Bin Qi1 • Yanjiang Zhang2 Yizhen Shao1 • Ziqiang Shao1 • Received: April 2015 / Accepted: July 2015 Ó Springer Science+Business Media New York 2015 Abstract A series of aerogels composed of chitosan and/ or silica were fabricated by tuning their feeding ratios They were characterized by FTIR, thermogravimetric analysis, and X-ray diffraction; pore structures were analyzed by Brunauer–Emmett–Teller (BET) nitrogen sorption and scanning electron microscopy (SEM); adsorption capacities to Congo red were explored as well The incorporation of silica enhances the thermostabilization of chitosan in gels And as silica content increases, bulk densities of aerogels decrease gradually, while porosities, pore volumes, and surface areas obtained via BET method increase consequently; as well, porous structure becomes more regular and pore size tends to be smaller that was observed by SEM The adsorption capacities of chitosancontaining aerogels to Congo red reach as high as about 150 mg/g, much higher than that of pure silica (17 mg/g), demonstrating their potential as a class of novel adsorbent materials Graphical Abstract A series of chitosan- and/or silicabased aerogels were fabricated, which were named as C5S0, C4S1, C1S1, C1S4, and C0S5, with different designed CS/SiO2 mass ratios of 100/0, 80/20, 50/50, Electronic supplementary material The online version of this article (doi:10.1007/s10971-015-3800-7) contains supplementary material, which is available to authorized users & Jianquan Wang jqwang@bit.edu.cn Beijing Engineering Research Center of Cellulose and Its Derivatives, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China Aerospace Research Institute of Materials and Processing Technology, China Aerospace Science and Industry Corporation, Beijing 100074, China 20/80, and 0/100, respectively Their compositions and structures as well as adsorption properties to Congo red were analyzed and compared in detail C5S0 C4S1 C1S4 C0S5 C1S1 Keywords Aerogel Á Composites Á Chitosan Á Silica Á Adsorption Á Congo red Introduction Aerogels, a class of ultralow-density solids derived from wet gels through replacing the inside liquid component with gas, attract more and more attentions in this new century, because of their wide applications in insulation, absorbent, catalyst, optics, electronics, etc [1] Usually aerogels are categorized into inorganic, organic, and composite ones thereof Silica, alumina, titania, and zirconia aerogels are several typical inorganic oxide ones produced from their corresponding alkoxides [2] Recently, various metal aerogels of iron, cobalt, copper, silver, gold, titanium, and gold–palladium were also successively developed [3–8] Organic aerogels include those of polycondensation products of aldehydes 123 J Sol-Gel Sci Technol with phenols and/or melamine [9–13], and those of natural polysaccharides such as starch, cellulose, chitin, chitosan, alginate, agar [14–18] And some of the above organic aerogels are used to prepare carbon ones through pyrolysis [11–16] In addition, successful development of carbon nanotubes and graphene also opened novel pathways to fabricate carbon aerogels free of using pyrolysis treatment [19] Organic–inorganic composite aerogels refer to those containing at least both components In a variety of aerogels, silica aerogels may be the most extensively studied one Tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS) are the most frequently used precursors to prepare pure silica aerogels with inherent fragility; methyltriethoxysilane, methyltrimethoxysilane, and other alkoxysilane derivatives carrying functional groups are also introduced to participate in the hydrolysis and polycondensation of TMOS or TEOS, yielding aerogels with some improved or additional properties [2, 20, 21] Moreover, incorporation of polymers is an effective method to reinforce silica aerogel Mechanically enhanced composite silica aerogels, which combine polystyrene, polyurethane, epoxy resin, poly(vinyl pyrrolidone), or poly(dimethylsiloxane), were reported in the literatures [22–26] Natural polysaccharides such as cellulose and chitosan (CS) were also successfully composited with silica to get aerogels, in which cellulose– silica composite ones via sequential interpenetrating technique showed excellent performances [27–30], and CS–silica composite ones revealed capabilities to load iron oxide, transition and lanthanide metals attributed to the presence of amine groups on CS backbones [31]; meanwhile, CS promoted the clustering of primary silica particles and in turn silica also helped to restrain severe shrinkage and deformation of CS networks during drying procedure [32, 33] In both cases of the published CS–silica composite aerogels, however, TEOS was adopted and the CS/silica mass ratio range was narrow and incomplete In order to fully study the effect of feed ratio of CS and silane precursor on structure and some properties of aerogels, more detailed research and modified method are conducted in this research Herein, CS–silica mass ratios from 100/0 to 0/100 were designed in the premise of wt% solid content; TMOS was used in order to get more homogeneous systems; organic base was applied in post-treatment; adsorption properties to a certain anionic dye, Congo red, were discussed in addition to structure and composition analysis of obtained aerogels Materials and method 2.1 Materials Chitosan (deacetylation degree [95 %) and tetramethoxysilane (TMOS) were purchased from Shanghai 123 Jingchun Biotech Co., China Other chemicals (AR grade) were bought from Beijing Chemicals Co., China All of the reagents were used as received without purification 2.2 Preparation of aerogels First, CS powder was dissolved in 1.5 wt% aqueous acetic acid solution under vigorous stirring for 12 h, to produce wt% CS solution Required amounts of CS solution, deionized water, ethanol, and TMOS were mixed in a 20-mL sample bottle under stirring, which was further subjected to debubbling using a Thinky AR-310 mixer Then the sol was left to gelate for 48 h Various samples with different CS/ SiO2 ratios were synthesized through controlling the amounts of CS and TMOS, in the principle that the total solid content of CS and SiO2 was kept wt% in reactant systems For pure CS gel, the sol–gel transition was realized via addition of small amount of glyoxal The samples were nominated as C5S0, C4S1, C1S1, C1S4, and C0S5, with different designed CS/SiO2 mass ratios of 100/0, 80/20, 50/50, 20/80, and 0/100, respectively The detailed formulations of different samples are listed in Table After 48-h gelation of all samples, they were soaked in wt% triethanolamine in ethanol for a week and successively in ethanol for another week with daily exchange in both cases Afterward, they were dried from supercritical CO2 at 50 °C and 15 MPa for 24 h in an autoclave 2.3 Determination of bulk density and porosity of aerogels Bulk densities of the aerogels were calculated by measuring the weights and volumes of the samples Referring to literature [28], porosity of the aerogel could be calculated according to Eq (1)   q Porosity ¼ À Â 100 % ð1Þ wS qS þ wC qC where q, qS, and qC are the individual densities of the aerogel, silica and CS; wS and wC are the mass fraction of silica and CS in the aerogel, respectively The respective densities of silica and CS are 2.63 and 1.34 g cm-3 [28, 34], respectively 2.4 Dye adsorption of aerogels Batch adsorption experiments were performed in 250-mL glass conical flasks on a water bath temperature controlled shaker at 25 °C A certain amount of Congo red was dissolved in deionized water to prepare stock solution with 100 mg/L of dye concentration Every exactly weighed aerogel sample (60 mg) was thrown in 100 mL of the above dye solution in a 250-mL glass conical flask The adsorption progress was J Sol-Gel Sci Technol monitored by determining the dye concentration of supernatant at predetermined time The adsorption capacities of Congo red onto aerogels were calculated by Eq (2)   C0 À Ct Qt ¼ V ð2Þ m where Qt refers to adsorption capacity (mg/g) of every aerogel sample at time t; C0 and Ct are the concentration (mg/L) of Congo red at initial and time t, respectively; m means the mass (g) of aerogel sample; V stands for the volume (L) of dye solution During the adsorption procedure, Congo red concentrations at different times were calculated according to linear regression Eq (3), which was obtained through fitting the relationship of UV absorbance values at different Congo red concentrations, as shown in Fig S1 A ¼ 0:0308C ð3Þ where A is UV absorbance at 500 nm and C means Congo red concentration (mg/L) 2.5 Characterization FTIR spectra were recorded on Nicolet iS5 using KBr method X-ray diffraction (XRD) experiments of sample powders were carried out on Anton Paar XPert Thermogravimetric analysis (TGA) measurements were conducted on NETZSCH 209 F1 under air atmosphere at a heating rate of 10 °C/min Surface areas and pore volumes were analyzed via nitrogen sorption measurements performed on a Micromeritics ASAP2020 system All samples were degassed at 80 °C for h under vacuum prior to analysis Surface areas were obtained by Brunauer–Emmett–Teller (BET) method with six points in the range of relative pressure (P/P0) from 0.05 to 0.3, and total pore volumes were calculated at P/P0 = 0.99 The internal morphology of aerogels was characterized by scanning electron microscopy (SEM, JSM-6700F), and the samples were sputtered with gold before SEM observation As for dye adsorption, dye concentrations were determined through Ultraviolet–visible (UV–vis) absorption spectroscopy on PerkinElmer UV/Vis spectrometer (Lambda 35) Results and discussion 3.1 Preparation of aerogels In this study, all of the samples formed gels successfully in 48 h, and C1S1 and C1S4 gelated in \12 h, as shown in Table The use of triethanolamine in post-treatment aims to neutralize the introduced acetic acid as well as to promote polycondensation of unreacted silanol groups under weak base condition In fact, three kinds of dilute base solution in ethanol (2 wt%) such as aqua ammonia, triethylamine, and triethanolamine were tried The former two caused the gels opaque, probably due to the phase separation induced by generated corresponding acetates in the gel system; therefore, the dilute triethanolamine solution was adopted finally Herein, when aqua ammonium was adopted, the phenomena are quite different from that of other’s work, where CS-/TEOS-derived gels were immersed in 0.6 wt% NH4OH in ethanol solution to obtain transparent monoliths [32, 35] The opaque appearance of gels treated by aqua ammonia in our test may be resulted from higher ammonia concentration (2 wt%) in ethanol and/or the difference of precursor In addition, it also needs to mention that no acid was added during the gelation process for pure SiO2 sample (C0S5), which lost flowability in about 20 h That is to say, TMOS can hydrolyze and condense by itself in the presence of sufficient water free of any acid or alkali catalyst Here the molar ratio of H2O to TMOS was nearly 50:1, much greater than that of 4:1 usually used in mostly the published literatures So far, the reported catalyst-free silica gel preparations are limited to those under high temperature and pressure [36] or ultrasonic radiation [37, 38] Undoubtedly, this research provides a good reference for catalyst-free synthesis of silica gels under conventional conditions 3.2 Characterization of aerogels FTIR spectra of aerogel samples are shown in Fig For pure CS aerogel (C5S0), the peaks at 1652 and 1599 cm-1 correspond to I and II types of residual amides on CS skeleton, respectively While for pure SiO2 aerogel (C0S5), characteristic absorptions at 470 and 800 cm-1 are attributed to antisymmetric and symmetric stretching vibrations of Si–O–Si bonds, respectively; 955 cm-1 corresponds to stretching vibration of free Si–OH groups; 1636 cm-1 arises from bending vibration of H2O absorbed in the sample The coexistence of characteristic peaks from both CS and SiO2 in the other three samples proves successful combination of both components In addition, the sharp peak between 3200 and 3600 cm-1 is related with hydrogen bonds involving –NH2 and/or –OH groups for C5S0 sample, while for other samples the peak at this region becomes broader when SiO2 is incorporated due to the participation of Si–OH groups [39, 40] In order to exactly obtain relative contents of CS in composite aerogels and the effect of introduced silica on thermal property of chitosan, TGA measurements were carried out, and the results are shown in Fig According to TGA curves in Fig 2a, the CS contents in various composite samples were calculated, and the results are listed in Table It can be observed that the relative CS contents are below or close to theoretical ones for three 123 J Sol-Gel Sci Technol Table Detailed synthetic formulations of various aerogels Sample wt% CS/g H2O/g EtOH/g TMOS/g C5S0 0.040a / / C4S1 6.4 1.4 0.2 45 C1S1 2.04 1.46 0.5 \12 \12 C1S4 1.6 4.256 1.344 0.8 C0S5 3.546 0.829 0.625 a Gelation time/h 48 20 wt% glyoxal aqueous solution Fig FTIR spectra of aerogel samples composite aerogels, i.e., C4S1, C1S1, and C1S4, in which theoretical CS contents are 80, 50, and 20 wt%, respectively The 6–7 wt% reduction in CS contents compared with theoretical ones for the former two samples may be attributed to the leakage of uncross-linked CS component from wet gels during immersion In addition, DTG traces of aerogel samples reveal that both characteristic temperatures of initial decomposition and maximum decomposition rate were delayed to some extend due to the incorporation of silica, as displayed in Fig 2b This demonstrates that silica exerts a positive influence on chitosan thermostabilization, in accordance with that of cellulose–silica composite aerogels reported by others [28] XRD patterns of aerogels are shown in Fig 3, from which it can be seen that composite ones only demonstrate the overlap of the diffraction peaks from both components, and no new peaks appear Such non-interference between components indicates their homogeneous combination, like the case of cellulose–silica composite aerogels published elsewhere [28, 29] Some physical properties of aerogels were characterized, and the results are listed in Table 2, from which it can be found that bulk densities of aerogels decreased with silica content increasing Comparing mono-component aerogels, C5S0 and C0S5, the former’s bulk density is 123 Fig TGA and DTG curves for aerogel samples about four times as much as that of the latter Such a notable distinction should arise from the compaction of CS component during the process of supercritical drying, considering that their original sizes of wet gels were not so much different From the viewpoint of molecular structures for both components, CS is more flexible than silica assemblies and is prone to form hydrogen bonds with adjacent chains [33], which may result in more shrinkage with more CS content In contrast to the changing tendency of aerogel bulk densities, other physical properties such as porosities, pore volumes, and surface areas of samples J Sol-Gel Sci Technol Table Some physical properties of various aerogels Sample CS contenta/wt% C5S0 100 0.247 81.6 0.753 150 C4S1 74 0.166 90.1 1.013 247 C1S1 43 0.139 93.3 1.092 342 C1S4 21 0.101 95.7 2.121 567 C0S5 0.066 97.5 3.645 739 a Bulk density/(g/cm3) Porosity/% Pore volume/(cm3/g) BET surface area/(m2/g) Calculation based on TGA measurements Fig XRD of aerogel samples reveal increasing values as silica content rises, as displayed in Table In order to observe internal network structures of aerogels more directly, SEM measurement was taken, and their photographs are shown in Fig For C5S0, both CS aggregates and pores are as large as micrometer scale, and the network structure appears irregular While for C0S5 aerogel with 100 wt% silica, the cross-sectional surface is rather smooth and the inside mesopores are too small to be resolved by conventional SEM, like the case of sample studied by others [27] For composite aerogel samples, generally the porous structure becomes more regular and pore size tends to be smaller with an increase in silica content For example, C4S1 sample, which contains 26 wt% of silica, has a honeycomb-like network structure with micrometer-scale pore size, while both C1S1 and C1S4 samples reveal porous structures surrounded by fibrous assembles, whose sizes are at nano- to submicron level, and pore size of the latter sample is smaller than that of the former As for C5S0, CS macromolecules were weakly crosslinked by a small amount of glyoxal, forming a primary non-flowable gel Subsequently, immersion in triethanolamine/ethanol solution and exchange by ethanol enhanced the aggregation of CS chains through hydrogen bonds Such an interaction and further compaction induced by following supercritical drying cause the gel to generate macropores and CS assembles with micrometer scale in its network, as displayed by SEM When TMOS was incorporated, the hydrolysis and polycondensation of this precursor produced a great deal of silanol groups, which would form hydrogen bonds, ionic bonds, and even covalent bonds with hydroxyl and/or amino groups on CS chains [40, 41] Thus, silica aggregates may grow along CS chains preferentially, which in turn prevents CS molecules from accumulating to different extents depending on the relative ratios of CS and silica For C4S1, the agglomeration of CS chains is restrained efficiently, forming regular cellular network structure In C1S1 network with 57 wt% SiO2 inside, so much amount of SiO2 is enough to cover the surface of CS chains, obtaining fibrillar aggregates with nano- to submicroscale As silica content increases further to near 80 wt%, i.e., C1S4 sample, exceeding part of silica nanoparticles may assemble as their own way besides those growing along CS chains, so the micromorphology of its network approaches toward that of C0S5 3.3 Dye adsorption of aerogels The inherent amino groups on CS molecule backbones can impart CS-based materials with the possibility to adsorb anionic matters; meanwhile hydroxyls, as well as amines, also provide hydrogen bonding points with some substances Actually, CS–silica hybrids applied as adsorbents toward some dyes and anionic materials have been reported in some literatures [42–44] In this research, the ability of prepared aerogels to remove Congo red, a toxic dye carrying sulfate, amino, and azo groups, was explored Some representative UV–vis spectra of dye solution at different time during adsorption measurement are revealed in Fig S2 The dye adsorption values at various times were 123 J Sol-Gel Sci Technol C5S0 C4S1 C1S4 C0S5 C1S1 Fig SEM photographs of aerogel samples In general, pseudo-first-order and pseudo-second-order models are the two frequently applied to analyze the adsorption kinetics, represented as Eqs (4) and (5) that can be, respectively, converted to Eqs (6) and (7) after integrating dQt ¼ k1 ðQ1e À Qt Þ dt dQt ¼ k2 ðQ2e À Qt Þ2 dt À Á Qt ¼ Q1e À eÀk1 t t 1 ¼ þ t Qt k2 Q2e Qe Fig Congo red adsorption of different aerogels as a function of time recorded, as shown in Fig 5, from which it can be found that all of the CS-containing aerogels show increasing adsorption for Congo red as time goes on, and the maximum value can reach as high as around 140 mg/g at 50 h, while pure silica aerogel (C0S5) shows very low adsorption capacity of only 18 mg/g or so, demonstrating that CS is responsible for Congo red adsorption, which must be ascribed to functional moieties such as amino and hydroxyl groups on CS chains 123 ð4Þ ð5Þ ð6Þ ð7Þ where Qe and Qt (mg/g) are the adsorption capacities at equilibrium and time t (min), respectively; k1 (1/min) and k2 [g/(mg min)] correspond to individual rate constants of pseudo-first- and pseudo-second-order kinetics Relevant rate constants and equilibrium adsorption capacities of various aerogels, which are listed in Table together with the measured equilibrium value, were derived through fitting the data of Fig using Eqs (6) and (7) It is found that both pseudo-first- and pseudo-second-order models can fit the obtained data very well by judging from their respective correlation coefficients (R2), while the latter is better than the former one Also equilibrium adsorption capacities derived from pseudo-second-order J Sol-Gel Sci Technol Table Results of fitting kinetic data and measured equilibrium adsorption capacities Sample Pseudo-first-order model -3 Qe(exp)/(mg/g) Pseudo-second-order model -5 K1/(10 /min) Q1e/(mg/g) R21 C5S0 4.16 137.85 0.991 3.54 152.91 0.998 147.74 C4S1 4.29 140.63 0.976 3.94 153.61 0.998 152.84 C1S1 2.82 132.25 0.968 2.14 154.80 0.996 150.22 C1S4 2.84 146.84 0.972 1.97 164.74 0.989 155.37 C0S5 / / / / K2/[10 g/(mg min)] Q2e/(mg/g) R22 models are close to experimental ones This result proves that adsorption kinetics of Congo red by CS-based aerogels prepared in this work follow pseudo-second-order model preferentially, implying that the overall rate of adsorption process is controlled by chemisorption, in accordance with the cases of anionic dyes adsorption onto other CS-based materials published elsewhere [45–48] Comparing the rate constants (k) for various samples, which reflect adsorption speed of an adsorbent for some matter, one can observe that k2 values follow the sequence of C4S1 [ C5S0 [ C1S1 [ C1S4, and k2 of the former two are much higher than those of the others This may be explained by their differences in compositions and pore structures The higher content of CS means more bonding regions for dye molecules and accompanied greater driving force of adsorption, while greater porosity and surface area imply wider paths for dye molecules to enter All of the above parameters are in favor of the increase in k values In this work, however, the porosities and surface areas of aerogels decrease with CS content increasing, as shown in Table 2, so the changing trend of k2 depends on the competition actions of above factors For samples C5S0 and C4S1 with high CS content over 74 wt%, the porosity and surface area take more positive effect on adsorption rates, so the latter one appears slightly higher k2 value than the former While this influence becomes not so outstanding when CS content is lower than 43 wt%, and correspondingly the negative effect from CS content decrement became predominant, so C1S1 and C1S4 behave much lower k2 values than C5S0 and C4S1, as shown in Table For equilibrium adsorption capacity (Qe), whether theoretical or experimental, the distinction of various CS-containing aerogels is very little (Table 3), although their CS contents are so much different (19–100 wt%), demonstrating that neither composition nor pore structure has notable effect on Qe as long as CS content reaches near 20 wt% or higher as studied in this work Such a high adsorption capacity of C1S4 aerogel with only 19 wt% of CS content may be attributed to its high surface area / / 17.34 Conclusion A series of aerogels were fabricated via changing the feed ratio of CS and TMOS, and they were characterized through different methods TGA shows shat silica exerts a positive influence on chitosan thermostabilization With silica contents increasing in aerogels, bulk densities become smaller, while other parameters such as porosities, pore volumes, and surface areas increase consequently; porous structure becomes more regular and pore size tends to be smaller The Congo red adsorption onto chitosan-containing aerogels reaches as high as about 150 mg/g, and their adsorption process is controlled by 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Characterization FTIR spectra