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

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,

O R I G I N A L P A P E R

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: 2 April 2015 / Accepted: 2 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

ana-lyzed 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

chitosan-containing 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

silica-based 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,

20/80, and 0/100, respectively Their compositions and structures as well as adsorption properties to Congo red were analyzed and compared in detail

Keywords Aerogel Composites  Chitosan  Silica  Adsorption  Congo red

1 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

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

1 Beijing Engineering Research Center of Cellulose and Its

Derivatives, School of Materials Science and Engineering,

Beijing Institute of Technology, Beijing 100081, China

2 Aerospace Research Institute of Materials and Processing

Technology, China Aerospace Science and Industry

Corporation, Beijing 100074, China

DOI 10.1007/s10971-015-3800-7

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

pre-cursors 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

polyconden-sation 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

aero-gels, 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

tech-nique 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

pro-moted 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

aero-gels, 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 5 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

2 Materials and method

2.1 Materials

Chitosan (deacetylation degree [95 %) and

tetram-ethoxysilane (TMOS) were purchased from Shanghai

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

5 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 SiO2was kept 5 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 formu-lations of different samples are listed in Table1 After 48-h gelation of all samples, they were soaked in 2 wt% tri-ethanolamine in ethanol for a week and successively in ethanol for another week with daily exchange in both cases Afterward, they were dried from supercritical CO2at 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 measur-ing the weights and volumes of the samples Referrmeasur-ing to literature [28], porosity of the aerogel could be calculated according to Eq (1)

Porosity¼ 1 q

wSqSþ wCqC

where q, qS, and qC are the individual densities of the aerogel, silica and CS; wSand wCare 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

monitored by determining the dye concentration of

super-natant at predetermined time The adsorption capacities of

Congo red onto aerogels were calculated by Eq (2)

Qt¼ C0 Ct

m

where Qt refers to adsorption capacity (mg/g) of every

aerogel sample at time t; C0and 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

concentra-tions 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

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

8 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

scan-ning electron microscopy (SEM, JSM-6700F), and the

sam-ples 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)

3 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

Table1 The use of triethanolamine in post-treatment aims

to neutralize the introduced acetic acid as well as to

pro-mote 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, tri-ethylamine, 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 solu-tion 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 flowa-bility 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.1 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 SiO2aerogel (C0S5), characteristic absorptions at 470 and 800 cm-1 are attrib-uted 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 SiO2in the other three samples proves successful combination of both components In addition, the sharp peak between 3200 and 3600 cm-1 is related with hydro-gen 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.2 According

to TGA curves in Fig.2a, the CS contents in various composite samples were calculated, and the results are listed in Table 2 It can be observed that the relative CS contents are below or close to theoretical ones for three

composite aerogels, i.e., C4S1, C1S1, and C1S4, in which

theoretical CS contents are 80, 50, and 20 wt%,

respec-tively 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

temper-atures of initial decomposition and maximum

decomposi-tion 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

chi-tosan thermostabilization, in accordance with that of

cel-lulose–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

character-ized, and the results are listed in Table2, 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

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

Table 1 Detailed synthetic

formulations of various aerogels Sample 5 wt% CS/g H2O/g EtOH/g TMOS/g Gelation time/h

a 1 wt% glyoxal aqueous solution

Fig 1 FTIR spectra of aerogel samples

Fig 2 TGA and DTG curves for aerogel samples

reveal increasing values as silica content rises, as displayed

in Table2

In order to observe internal network structures of

aero-gels more directly, SEM measurement was taken, and their

photographs are shown in Fig.4 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 cross-linked by a small amount of glyoxal, forming a primary non-flowable gel Subsequently, immersion in tri-ethanolamine/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 incor-porated, the hydrolysis and polycondensation of this pre-cursor produced a great deal of silanol groups, which would form hydrogen bonds, ionic bonds, and even cova-lent 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 agglomera-tion of CS chains is restrained efficiently, forming regular cellular network structure In C1S1 network with 57 wt% SiO2 inside, so much amount of SiO2is 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 sub-stances 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 car-rying 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

Table 2 Some physical properties of various aerogels

Sample CS contenta/wt% Bulk density/(g/cm3) Porosity/% Pore volume/(cm3/g) BET surface area/(m2/g)

a Calculation based on TGA measurements

Fig 3 XRD of aerogel samples

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

maxi-mum 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

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

dQt

Qt¼ Q1e 1 ek1 t

ð6Þ t

Qt

k2Q2e2

þ 1

Qe

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 Table3 together with the measured equilibrium value, were derived through fitting the data of Fig.5using 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

Fig 4 SEM photographs of aerogel samples

Fig 5 Congo red adsorption of different aerogels as a function of

time

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 k2values 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

driv-ing 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 Table2, so the changing trend of k2depends 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 k2values than C5S0 and C4S1,

as shown in Table3 For equilibrium adsorption capacity

(Qe), whether theoretical or experimental, the distinction

of various CS-containing aerogels is very little (Table3),

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

4 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 porosi-ties, pore volumes, and surface areas increase conse-quently; 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 chemisorption suggested by pseudo-second-order kinetics model In addition, the composition and pore structure only affect adsorption rate constants but have no influ-ence on equilibrium adsorption capacities for the CS-containing aerogels studied in this research, which pro-vides a reference for their applications as novel adsor-bent materials

Acknowledgments We would like to thank the anonymous reviewers and the editor for their constructive comments This work was financially supported by National Natural Science Foundation of China (21004005), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (20120932003).

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