Cellulose-Silica Hybrid Materials Obtained by Heteropolyacid

Chapter 8

Cellulose-Silica Hybrid Materials Obtained by Heteropolyacid Catalyzed Sol-Gel Synthesis

Downloaded by COLUMBIA UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch008

S. Sequeira, D. V . Evtuguin, and I. Portugal CICECO and Chemistry Department, University of Aveiro, 3810-193 Aveiro, Portugal

Organic-inorganic hybrids based on cellulose fibers from Eucalyptus globulus kraft pulp were prepared at room temperature by a sol-gel method using TEOS as the silica precursor and H3PW12O40 as the catalyst. These cellulose/silica hybrids (CSHs) were chemically and structurally characterized. The absence of unconverted TEOS and the formation of a silica network composed essentially by Q and Q structures were confirmed by FTIR, solid-state C and 2 9 S i N M R . Image analysis (SEM and A F M ) revealed that the silica had deposited on the fibers essentially in the form of a thin film or mesoparticles, which bridged the fibers. The silica network improved hydrophobicity, dimensional and thermal stability and bending strength of the starting fibrous material. The potential applications of these CSHs are discussed. 3

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Hybrids, or composites, are composed by at least two different materials belonging to the same (e.g. two metals) or different classes (e.g., a metal and a ceramic). In particular, organic-inorganic hybrids (OIHs) comprise typically an organic polymer host matrix and embedded inorganic formulations. The organic polymer may be used as a pre-formed material or synthesized in situ during the hybrid preparation. OIHs are a relatively new type of materials with interesting mechanical, optical, electrical, and thermal properties. The inorganic phase © 2007 American Chemical Society

In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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122 usually confers thermal and mechanical resistance to the hybrid, whereas the organic phase governs the hybrid's low density, toughness, and flexibility (1). The properties of hybrid materials depend not only on the properties of the starting materials and the preparation methods used, but also on the composite's phase morphology and interfacial properties. A significant proportion of recently reported OIHs are synthesized using a sol-gel process, which is a rather flexible and versatile technique (1). This allows the incorporation of the inorganic component in thermally unstable organic materials, since the synthesis occurs under mild conditions (room temperature and atmospheric pressure). In a sol-gel process, the in situ generated inorganic particles are evenly dispersed at a nanometer scale in the polymeric host matrix and are bound to it through hydrogen or covalent bonds thus forming a network. A brief outline of the fundamentals of the sol-gel syntheses of OIHs is given below.

Sol-Gel Synthesis of OIHs In a typical sol-gel synthesis of OIHs, a metal alkoxide M ( O R ) (where M is Si, T i , A l , etc. and R is C H , C H , C H , etc.) is mixed with water, an alcohol (or another co-solvent) and a small amount of acid or base as catalyst, in the presence of an organic polymer. Hydrolysis and polycondensation reactions occur simultaneously and a metal oxide three-dimensional network with - O - M O - M - linkages is formed on the polymer surface (2). Silicon alkoxides are most frequently used for these systems and are discussed here as an example. The basic reactions involved in the formation of a silica network, using tetraethyl orthosilicate (TEOS) as chemical precursor, are (where y=n+l): n

3

2

5

3

7

OEt E t — Ο — S i — OEt

OEt +

H 0 2

^

H—0—Si—OEt

OEt

y

H-

•OEt > —i\—ι

+

EtOH

OEt

OEt

I Et— 0— Si-

OEt

(

i

)

OEt 0—Si—

I

-OH

η EtOH

OEt OEt

(2)

The primary hydrolysis of TEOS by water replaces ethoxy moieties with hydroxyl groups (Eq. 1), thereby rendering the silane active for low temperature polymerization (Eq. 2). The hydrolysis reaction is partially reversible and, in the presence of an alcohol as the co-solvent (for example, ethanol), re-etherification


123 takes place (2). The condensation reaction eliminates either water (if two =Si~ O H moieties react) or alcohol (if ^ S i - O H and ^Si-OEt react) to produce -Si-OSi- linkages (Eq. 2). Partial alcoholysis (or hydrolysis) of the -Si-O-Si- bonds constitutes the reverse reactions. TEOS hydrolysis and condensation reactions are promoted by either acids or bases. However, the hydrolysis is faster under acidic conditions than in neutral or weak alkaline media, whereas the condensation is faster under neutral or alkaline conditions. The specific conditions of the hydrolysis and condensation steps profoundly affect the structure of the sol-gel silicates (2). Indeed, these reactions are extremely dependent on the H 0 / T E O S ratio, the TEOS concentration, the proportion and type of organic solvents, the pH, the catalyst and its amount, the temperature, etc. (3). This makes sol-gel syntheses attractive to researchers, since it is possible to manipulate readily the final structure of the materials. In addition to the primary hydrolysis and polycondensation, leading to the formation of a series of siloxane linear and cyclic pre-polymers, polymer clusters or primary particles (sols), a sol-gel synthesis involves other important steps such as gelation (accompanied by a sharp increase in viscosity), aging, drying, stabilization (thermal or chemical) and densification (occurring between 1000 and 1700°C) (2). Gelation may be defined as a collision process of growing sol particles leading to the formation of an infinite silica network - a gel. The further densification of this silica network occurs during aging, which involves the polymerization, coarsening and phase transformation of the gels. This process may take months at room temperature. During aging, the proportions of tertiary (Si(0) OH or Q ) and quaternary (Si(0) or Q ) structures increase dramatically. The finalization of condensation reactions and silica gel shrinkage occur when all solvents are removed during the drying process. The latter can be carried out at higher temperatures (50-150°C) to produce structurally compact microporous silica materials (xerogels), or under supercritical conditions to produce a silica material possessing a structure similar to that of the original sol (aerogels). The surface dehydroxylation o f silica is accomplished at around 700°C (thermal stabilization) and its transformation into solid ceramics of different polymorphs occurs at >1000°C. The S i 0 tetrahedral unit is the fundamental building block of the majority of silicates (Fig.la). S i 0 tetrahedra can be presented in a simplified mode as rigid units (the S i - 0 bond length is 1.62 Â with the ideal tetrahedral angle of 109.5°), which are linked through their corners to form pairs, rings, chains, sheets, or frameworks. When the tetrahedra are linked by their corners, the Si-OSi bond angle 0(Fig. lb) can vary between around 91° (two-membered rings) and 180° (linear chain). Amorphous silica (as in the case of xerogels and aerogels) contains usually four-, five-, six-, seven- and eight- membered rings (2). Due to the low thermal resistance of organic polymers, in most of the synthesized OIHs (bulk materials and films) the bound silica is in the form of xerogel or aerogel.


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In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007. 4

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Figure 1. Polyhedral-filling representation of silica building block: Si0 tetrahedral unit (a), two Si0 linked by their corners (b) and six-membered cyclic structure (c).


tetrahedra

125 However, an organic polymer matrix, and in particular a lignocellulosic material, can also be used as a template to produce solid ceramics with specific microstructure (4). In addition to silicon alkoxides, titanium and zirconium alkoxides are used in the sol-gel synthesis of OIH materials (I).


Cellulose-Based Organic/Inorganic Hybrids Cellulose, the most abundant renewable biopolymer, possesses several unique properties required in different technical areas and biomedicine (5). However, the serious drawback of cellulosic fibers is their hydrophilic nature, which results in a poor compatibility with hydrophobic polymer matrices in composite fabrications. The other disadvantages are the relatively low degradation temperature of cellulose-based materials and the dramatic loss of mechanical properties after moistening (6). One of the promising ways to overcome these deficiencies is a surface modification with an inorganic component. Silica derivatives of cellulose are particularly attractive, since silica substantially improves the thermal stability of the parent polymer, its lipophilic behavior and its affinity towards specific substrates (5, 7). The incorporation of silica into cellulose or cellulose derivatives has been accomplished with a wide range of silylation reagents, the most important being trimethylsilyl chloride (8). However, these reactions usually require an organic solvent, such as pyridine, xylene or D M F , and high temperatures (80-160°C). Alternatively, silica incorporation by milder sol-gel processes has been reported for cellulose derivatives such as hydroxypropyl cellulose (9, 10) and cellulose acetate (11, 12), with the main objective of increasing the mechanical and thermal resistance of the ensuing materials. Tanaka and Kozuka (13) described a sol-gel method for producing hybrid materials from cellulose acetate (CA), which has bioresorption properties, and silica, a bioactive and biocompatible inorganic material, with the aim of achieving mechanical properties similar to those of cortical bones. This work is one of several examples of applications of cellulose derivatives in biomedicine. Similarly, titanium was incorporated into ethyl cellulose, yielding a transparent material with interesting dielectric properties (14). The cellulose acetate/Al 0 hybrid was obtained by a sol-gel procedure using aluminium tetraisopropoxide (15). After the A1 0 modification with γaminopropyltrimethoxysilane, the mixed A l 0 / S i 0 hybrid containing amine groups showed promising results on transition metals adsorption from ethanol solutions. Zeolite (Y and L) /cellulose hybrids were prepared from bleached kraft pulp and zeolite powders (16). These OIHs can be used in the preparation of soft membranes with molecular sieving properties. A n interesting approach to produce molecular imprinted cellulose/silica composites for the selective binding of organic substrates from solutions was recently reported (17). A sol-gel method has also been applied for the deposition of hydrophobic polysiloxane coatings on wood (18) and for the modification of sulphite pulp with water-soluble silicon2

3

2

2

3

3

2



126 containing compounds (19), to increase the hydrophobicity of the final hybrid materials. The modification of cellulose fibers with pre-hydrolyzed trialkoxysilanes bearing variable functions (propylamine, allyl, alkyl, etc.) significantly increases their compatibility to hydrophobic polymer matrices in thermoset composites (20). Modified fibers may be considered as OIHs as well, since they contain bound inorganic components in the form of a discontinuous xerogel film. Based on this research, no information is available on OIHs for composite materials derived from unmodified cellulosic fibers (pulp) and silica. The pulp industry produces annually more than 200 million tons of bleached/unbleached pulp worldwide. Although the majority of pulp fibers are used for papermaking, cellulose/silica hybrid materials with attractive mechanical, thermal and sorption properties might be considered as potentially interesting alternative applications.

Sol-Gel Synthesis of Cellulose/Silica Hybrids from Pulp Synthesis Procedure and Raw Material Requirements Prior to their use in the sol-gel synthesis, the pulp was disintegrated (swollen) in water ( l g of oven dried pulp/100 ml of water) and then washed with ethanol to remove the water excess. The ensuing filtered pulp contained only a small amount of residual solvent. This pre-activation step is necessary to improve the accessibility of cellulose. A pre-determined proportion of TEOS, distilled water, ethanol, and catalyst were added to the pre-activated pulp. The reaction proceeded over 24 hours at room temperature (~20°C) with constant stirring. The final hybrid material was filtered, washed with ethanol to remove the excess of TEOS and dried first at 40°C (24 h) and then at 105°C (24 h). The percentage of silica incorporation was determined by weighing the final hybrid material and/or by weighing the residue after calcination at 525°C (3 h). The success of the cellulose/silica hybrids (CSHs) synthesis depends critically on the state of the fibers' surface (21). Among the studied Eucalyptus globulus kraft pulps (bleached, unbleached and primary sludge fibers) the best results were obtained with the bleached pulp (BP) containing a minimum of residual lignin, extractives, and ash. In the case of the primary sludge fibers, the yield of C S H was minimal (< 5% weight increment) because of the strong surface contamination with precipitated lignin, extractives and minerals (Fig. 2). Pulp beating did not influence significantly the percentage of incorporated silica in the CSHs (21).


127


Figure 2. SEM images of bleached krafi pulp (left) andfibers from the primary sludge (right).

Catalyst Selection A n acid-catalyzed sol-gel synthesis normally involves strong mineral acids, which may damage the acid-labile host polymer matrix during the aging and drying steps. Additionally, mineral acids (especially volatile and thermally unstable compounds) can induce equipment corrosion during production and limit the final material applications due to environmental concerns. In this context the use of strong non-volatile and thermally stable solid acids, such as heteropoly acids (HPAs), deserves attention. The efficiency of sol-gel catalysis with H PA s had not been assessed before this study. In addition to different conventional acids (HC1, H N 0 , H P 0 and H S 0 ) , several heteropoly acids bearing the general formula H [ X M i O ] " (X=Si, P; M= M o , W) were employed in the preparation o f CSHs based on B P . H P A s possess a very low negative charge on the surface bridging oxygens (low basicity) and are therefore strong Bransted acids, even stronger than mineral acids (22). H P A s are thermally stable up to 350-400°C, and unlike most strong mineral acids, have more affinity to silica than to the cellulosic material. The molar concentration of the H P A s was about 10 times lower than that used with conventional acids and corresponded to 30-50 times lower catalyst weight load. Under the same reaction conditions, nitric acid was the most effective catalyst among the examined conventional acids, whereas among the HPAs, the highest catalytic activity was displayed by H P W i O (or simply P W i ) , which was even higher than that of nitric acid (Fig. 3). The efficiency of the TEOS hydrolysis/condensation reactions and of the interaction of siloxane pre-polymers with cellulose in the presence of H P A s increased in the following order: H P W O ( P W ) > H4S1W12O40 (SiW ) > }l&'\Mo O (SiMo ) > Η p Μ θ ι Ο (PMoi ). This is in fair agreement with the reported acidic strength in acetone (pΚ^ values in parentheses), viz. P W i (1.6) > S i W i (2.0) ~ P M o (2.0)>SiMo (2.1)02/ 3

3

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2

4

x

x

2

40

3

3

2

4 0

2

3

3

2

1 2

40

4 0

12

12

X2

A0

12

2

2

2

12


i 2


128

Figure 3. Pulp weight increment (%) after BP reaction with TEOS in the presence of mineral (ca. 0.3 molfl) and heteropoly acids (ca. 3.0 10 mol/l). Reaction condition: 24 h, 20°C, H 0/ TEOS/EtOH molar proportion 3.2/1.0/8.7. 4

2

Effect of H 0 / T E O S Ratio and Catalyst Concentration 2

The water content influences the rate of TEOS hydrolysis, which in turn affects the series of hydrolysis/condensation reactions characteristic of a sol-gel process (3). Theoretically, to convert completely silicon alkoxides to S i 0 , two moles of water are needed per mole of precursor (R = Alk): 2

«Si(OR) + 2 « H 0 -> « S i 0 + 4wROH 4

2

(3)

2

In practice, this molar H 0 / T E O S ratio (r) is not sufficient, probably because of the formation of non-cyclic intermediate species, and should preferably be higher than 2.5. Thus, an excess of water is required to prepare cellulose/silica hybrids. According to the practice of C S H syntheses with P W as catalyst, at r < 3, silica incorporation into the pulp fibers is too slow and at r >4.4 it is difficult to control the spontaneous silica gel formation in bulk. Thus, the acceptable r values varied between 3.0 and 4.4. It was observed that increasing r from 3.2 to 4.4, increased the silica incorporation into B P from 43 to 52% (w/w). These results were attributed to the increase in the molecular weight of the intermediate siloxane polymers, arising from the enhancement of the condensation reactions, promoted by the increase in the H 0 to alkoxysilane molar ration (3). The amount of catalyst in the reaction medium influences significantly both the hydrolysis (Eq.l) and the condensation reactions (Eq.2). Increasing the catalyst to alkoxysilane ratio normally favors the hydrolysis reactions, rather 2

Î 2

2


129 than the condensations. As a consequence, the rate of polymer cluster formation increases, but their intermediate size decreases. This explains the decrease in silica bound to the pulp when the P W i charge was increased above 3.0 * 10" mol/1. It can be concluded that the size of the silica domains, related to each active growing centre on the pulp fiber surface, diminishes as the catalyst load is increased. A maximum incorporation of silica (> 50% wt.) was obtained with B P with a 4.4 H 0 / T E O S molar ratio, 8.3 EtOH/TEOS molar ratio and a P W concentration of 3.0 * 10" mol/1 (optimized conditions). 4

2

2

1 2

4


Characterization of the Cellulose/Silica Hybrids Image Analysis Image analysis showed (Fig. 4) that silica was deposited on the fiber surface as isolated or aggregated mesoparticles (1-4 μηι in diameter). Simultaneously, larger particles of 5-10 μηι were detected, which were mostly localized at the intersection of neighboring fibers (Fig. 4), thus bounding them and reinforcing the composite structure. Large-scale particles were more abundant in hybrids obtained with higher H 0 / T E O S molar ratios. Large areas of the fibers' surface, visually not covered by silica, nevertheless showed the presence of silicon, as revealed by EDS (Fig. 4). The A F M analysis revealed that the silica film comprised conjugated round-shape domains of 0.05-0.3 μηι (Fig. 4). This film was discontinuous and interrupted by uncovered regions of the fiber's surface. 2

F T I R and N M R Analysis 13

The solid-state C - N M R spectrum of B P / S i 0 hybrids obtained under optimized conditions (Fig. 5a) did not show any signals at 16.9 and 58.5 ppm, assigned to methyl and methylene carbons, respectively, in ethoxy moieties, i.e. during the CSHs preparation, the hydrolysis of TEOS was complete. The search for possible cellulose linkages with siloxy moieties, using differential C C P M A S N M R spectra of the initial pulp and the hybrids, failed. However, this fact does not prove the absence of these linkages. Indeed, the extent of cellulosesilica covalent bonds was probably very low (< 3-5 mol-%) and, therefore, not detectable by solid-state C - N M R . In the FTIR spectrum (not shown) of the B P / S i 0 hybrids, the band at 1000-1150 cm" , corresponding to Si-O-C vibration, was not clearly detected, because it overlapped with a broad band at 1000 -1150 cm" assigned to the δ(Ο-Η) mode of primary and secondary 2

1 3

13

1

2

1



Figure 4. SEM images of pulp fibers covered by silica (a, b). EDS spectrum (c) shows the presence of silica film on pulp surface that was assessed by AFM (d) applied in a tapping mode.


131 1

alcohol groups of cellulose, and with a band at 1080 cm' assigned to the asymmetric stretching of the inter-tetrahedral oxygen atoms of silica (23). Additional structural information on the silica portion of the B P / S i 0 hybrids was obtained by solid-state N M R . S i M A S N M R spectra provided information about the proportions of Q species (Q designates a Si atom bonded to η other Si atoms via O-bridges) in silica materials allowing quantification of their crosslink density. A typical spectrum of a B P / S i 0 hybrid (Fig. 5b) showed two major signals at -102 and -111 ppm (Q and Q structures, respectively), a small signal at -91 ppm (Q structures) and the absence of a signal at -81 ppm (Q structures). This indicated the presence of a silica network composed essentially of cyclic units (Q structures) connected by oxygen bridges ( Q structures) (2). The crosslink density (η) may be determined from the N M R spectra. It is defined as the ratio of effective ( £ f ) and potential (/got) functionalities of Si substituted by OSi moieties and is calculated as shown below (x is the mole fraction of Q structures calculated from corresponding peak areas): 2

2 9

w

n

2

3

4

2

1


3

4

n

n

4

fef p o t

where

n = 1

4

"

= 1

For the BP/silica hybrids obtained under our optimized conditions, η was found to be close to 0.83.

Thermal Analysis Thermogravimetric analyses of B P and BP/SÎ02 hybrids (Fig. 6) revealed a first weight loss at 60°C, corresponding to water release. The thermal degradation of the organic material was observed at 305 °C for B P and at 345°C for the hybrid material. This increase in degradation temperature suggests strong organic-inorganic phase interactions (strong hydrogen bonding and, probably, covalent bonding) that greatly influence thermal resistance. The results of the DSC analyses of the hybrid materials corroborated T G A data.

Cellulose-Silica Interactions in Hybrids According to the results obtained by spectroscopic and thermal analyses, it may be proposed that both the xerogel film and the silica particles are attached to the cellulose surface in pulp hybrids essentially by hydrogen bonding, although a small proportion of covalent bonding between cellulose and silica cannot be completely ruled out. X-ray scattering analysis ( W A X R D ) showed a significant


In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007. 13

29

2

Figure 5. Solid-state C NMR (a) and Si NMR (b) spectra of an optimized BP/Si0 hybrid.


Κ»

133

Ο

200

400


TCC)

Figure 6. Thermogravimetric plots of Si0 , BP and BP/Si0 2

2

hybrid.

decrease and broadening in the cellulose crystalline reflection peaks of the hybrid materials. A simplified scheme of cellulose-silica interaction in these hybrid materials is shown in Fig. 7.

Properties of Cellulose/Silica Hybrids A series of composite materials based on CSHs from B P were prepared by molding (Fig. 8) and subsequent pressing at room temperature or at 140°C for 8 minutes, followed by a post-thermal treatment (24). The density (d) of the final materials varied from 0.15 to 0.60 g/cm . Their thermal conductivity, mechanical properties, and dimensional stability were evaluated using 5-mm thick plates. The preparation of C S H formulations was done under optimized conditions as discussed above (material designated as B P / S i 0 ) . In a series of CSHs syntheses, 5% (vol.) of TEOS was substituted by triethoxyoctylsilane (TEOcS) in order to improve the hydrophobicity of the ensuing hybrid materials (designated as BP/Si0 /C ). Some properties of these C S H materials and B P are compared in Table 1. The incorporation of silica into bleached kraft pulp changed radically its hydrophilicity (expressed as Water Retention Value or W R V ) and its dimensional stability (expressed as relative weight (AW%) and volume (AV%) increment after soaking in water). This is especially notable in the case of the B P / S i 0 / C hybrid, which was a highly hydrophobic material. The CSHs also displayed an enhanced bending strength (σ), which was similar to that of medium-density fiberboards, in the case of the B P / S i 0 hybrids, and similar to that of high-density fiberboards in the case of the B P / S i 0 / C hybrids. 3

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8

2

8

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8



134

Figure 7. Schematic representation of cellulose-silica interactions in a bleached pulp/silica hybrid material. The fragment of cellulose chain sitting on the pulp surface is hydrogen bound to a silica fragment composed of two (SiO) cyclic structures. 6

Figure 8. Pre-formed CSH formulation (right) prepared by molding from suspension (left).

Table 1. Some properties of bleached kraft pulp and CSH materials (d=0.45) Si0 %

Hybrid BP BP/Si0 BP/Si0 /C 2

2

8

0 55 54

WRV 2

(%)

σ (MPa)

λ (W/m.K

118 12

Cellulose-Silica Hybrid Materials Obtained by Heteropolyacid

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