Ionic Liquid-Based Route for the Preparation of Catalytically Active

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Article

Ionic liquid - based route for the preparation of catalytically active cellulose – TiO porous films and spheres 2

Alexandra Smarandita Maria Wittmar, and Mathias Ulbricht Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04720 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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Industrial & Engineering Chemistry Research

Ionic liquid - based route for the preparation of catalytically active cellulose – TiO2 porous films and spheres Alexandra S. M. Wittmara,b,* and Mathias Ulbrichta,b* a

b

Lehrstuhl für Technische Chemie II, Universität Duisburg-Essen, 45141 Essen, Germany

CENIDE – Center for Nanointegration Duisburg-Essen, NETZ – NanoEnergieTechnikZentrum, 47057 Duisburg, Germany *Corresponding authors: Fax: +49 – 201 – 183 3147, e-mail: [email protected]; [email protected]

Keywords: TiO2 nanoparticles, cellulose nanocomposites, ionic liquids, non-solvent induced phase separation, photocatalysis

Abstract

The present work evaluates the possibilities to process cellulose with ionic liquids and functional nanoparticles like TiO2 towards a new generation of porous nanocomposites, shaped as films or spheres, which may find direct applications in water purification, catalysis and selfcleaning materials. The focus was set on the factors controlling the formation of the porous film

ACS Paragon Plus Environment

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structure during the non-solvent induced phase separation (NIPS) process from polymer solutions in ionic liquids via immersion in water and during the porous film drying step. Temperature and co-solvent addition facilitate cellulose solubilization and help controlling the phase separation by improving the mass transfer. The complex relation between the catalytic activity of the porous TiO2-cellulose nanocomposite materials obtained under different processing conditions and their structure has been studied during the photo-degradation of model organic dyes like rhodamine B and methylene blue. After drying, the catalytic activity of the nanocomposites decreases as a consequence of the reformation of the intra- and inter-molecular hydrogen bonds in cellulose which diminish the flexibility and the mobility of the fine cellulose fibrils network.

1. INTRODUCTION The term “ionic liquids” (ILs) usually defines a class of organic salts consisting of bulky asymmetric organic cations and a wide range of inorganic and organic anions, which are liquid at ambient temperature1,2. The asymmetry of the cation is considered the main cause of the low melting point of the ILs while the nature of the anion is responsible for some of the physical properties like their hydrophilicity and their miscibility with conventional solvents3. Due to their advantageous properties like very low vapor pressure, ability to dissolve a wide range of materials (organic, inorganic or

polymers) and high thermal stability, the ILs have been

considered as very interesting “green alternatives” to conventional volatile organic solvents (VOCs) for applications in synthesis, electrochemical and separation processes4. One of the main drawbacks of the ILs is their high viscosity which is several orders of magnitude higher than that of the conventional solvents5. On the other hand, Swatloski et al. draw the attention on the fact


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that even if the use of ILs reduces the VOCs emissions, not all of them can be considered as “green solvents”; the emission of HF observed by using ILs with PF6- anion is a strong example which supports this opinion6. Yun and collaborators also discussed the “pseudo-green” character of ILs in relation with their biodegradation: on the one hand, the biodegradability of ILs is increasing with the length of the alkyl chain of the cation, on the other hand, this change leads also to increasing of their toxicity7. Cellulose is one of the most widespread natural polymers and due to its biocompatibility and high availability is one of the preferred raw materials for many applications. The good chemical and mechanical stability of cellulose together with its poor solubility in water and in common conventional solvents is strongly connected with high number of intra- and intermolecular hydrogen bonds8. The stability of pure cellulose against most common organic solvents is responsible for the processing difficulties. The classical methods for cellulose processing from solution involve its solubilization in NaOH – thiourea aqueous solutions, LiCl – dimethylacetamide mixtures or aqueous xanthate solutions9. More recently it had been reported that the addition of a triethyloctylammonium chloride salt to acetone also allows the dissolution of cellulose into this solvent by increasing polarity10. A wide range of ionic liquids have been investigated for the dissolution of cellulose. Among them the best results have been obtained when [Mmim][Cl], [Bmim][Cl], [Allylmim][Cl], [Mmim][OAc] and [Bmim][OAc] (M = methyl; B = butyl; mim = methylimidazolium; Cl = chloride; OAc = acetate) were employed11. The highest fraction of cellulose dissolved (~ 25 wt %) was reported for [Bmim][Cl] and obtained by microwave heating12.

The dissolution

mechanism of cellulose in ionic liquids involves the oxygen and hydrogen of the cellulose hydroxyl groups for the formation of the electron donor – electron acceptor pairs which interact


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with the IL. The cations of the IL solvents are acting as electron acceptor centers while the anions are electron donor centers. During the interaction of the IL with the hydroxyl groups of the cellulose an opening of the hydrogen bonds between the macromolecular chains takes place and therefore the cellulose can be dissolved13. Recent studies have indicated that the addition of a co-solvent significantly influences the solubility of cellulose in an IL. Polar aprotic solvents such as dimethylsulfoxide (DMSO) and dimethylformamide (DMF) clearly have a beneficial influence because they can dissociate [Bmim][OAc] and therefore the free anion can more readily interact with cellulose14,15. A recent study has shown that during the re-precipitation of cellulose from ILs solution, the dissolution temperature and dissolution time influence the polymerization degree (DP) of the regenerated polymer: the higher is the dissolution temperature and the longer is the dissolution time the more pronounced is the decrease of DP which is observed16. The use of cellulose solutions in ionic liquids for the membrane preparation has been studied for a while now17 and the successful preparation of high-flux antifouling membranes has been reported18. More recently supported multilayer and self-standing membranes have been prepared from cellulose solutions in [Mmim][OAc] by Nunes and collaborators19. Further modification of the porous films and membranes prepared by phase separation from polymer solutions in ionic liquids by addition of metal oxide nanoparticles in the casting solution is starting to raise the interest of the scientific community. Among other studied metal oxides, TiO2 and ZnO may introduce many beneficial properties to the membranes due their hydrophilicity, photocatalytic activity or bactericide effect. To the best of our knowledge, our group reported in 2015 for the first time the successful preparation of porous cellulose films doped with commercially available nanoparticles by phase separation from [Bmim][OAc] and [Bmim][Cl] ionic liquids. The


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influence of the ionic liquid on the porous film formation was discussed and the catalytic activity of the nanocomposites during photo-degradation of organic dyes such as rhodamine B and methylene blue was demonstrated20. Later in the same year, Pihlajamäki and collaborators have obtained TiO2 doped cellulose ultrafiltration membranes with good mechanical stability and excellent fouling resistance by a similar method21. In the present work we expand the investigations started in our previous study towards the factors controlling the formation of the porous film structure during the non-solvent induced phase separation (NIPS) process from polymer solutions in ionic liquids via immersion in water and during the porous film drying step. Additionally, the influence of the use of a co-solvent on the formation of TiO2 doped cellulose porous films in the presence of the ionic liquids is studied. The highly porous TiO2 doped cellulose porous films have shown excellent photocatalytic activity against organic dyes combined with recyclability (Fig. 1). Spherical porous nanocomposite beads have been also prepared from the same polymer solutions and the formation of their porous structure during NIPS process was investigated.

Figure 1. Photo-degradation of organic pollutants on TiO2 doped porous films

2. MATERIALS AND METHODS 2.1. Materials


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Microcrystalline cellulose with degree of polymerization, DP 210 – 230 (according to supplier specification)

was

purchased

from

Merck

Millipore.

The

ionic

liquid

1-butyl-3-

methylimidazolium acetate ([Bmim][OAc]) in BASF quality (≥ 95 %) was purchased from Sigma-Aldrich and used without further purification. Both ionic liquid and cellulose are hygroscopic. Because water presence as traces does not hinder the cellulose dissolution in ionic liquids, it had been decided to use all precursors “as received” in order to avoid difficult and costly purification processes which would hinder a facile up-scaling of the studied processes. In some experiments dimethylsulfoxide (DMSO) (analytical reagent, ≥ 99.5 %) from VWR International was used as co-solvent for the cellulose dissolution. As catalytically active additive the TiO2 Aeroxide P90 (≥ 99.5 %) from Evonik Industries was used. The dyes used for the catalytic activity tests were methylene blue (hydrate; ≥ 95 %) from Fluka and rhodamine B (96 %) from Alfa Aesar. Both dyes were used as aqueous solutions in the concentration range from 2 x 10-5 M to 5 x 10-5 M. The water used for the preparation of the dye solutions was purified by a Milli Q Reference water purification system (Merck-Millipore) in order to have a resistivity of ~ 18.2 MΩ⋅cm @ 25 °C and a TOC value below 5 ppb.

2.2. Preparation of cellulose based catalytic porous films and spheres The nanoparticles and the cellulose at the desired ratios (C:TiO2 = 10:1; 10:1.5; 10:2) were mixed by grinding them together in a mortar for ca. 10 min in absence of the solvent. Hereafter, the corresponding amount of ionic liquid (8 wt % polymer in ionic liquid) was added into the mortar and the mixture was further grinded until a homogenous paste without lumps and large agglomerates was obtained. For selected samples the paste was diluted with the corresponding amount of DMSO (IL:DMSO = 2:1, 3:1 or 4:1). The final paste was transferred to a snap-cap


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vial and magnetically stirred for ca. 5 h at 70 °C until the complete solubilization of the cellulose. The polymer films were casted on glass substrates with the help of a motorized film applicator (model AB3400 from TQC), using a casting knife with gap width of 300 µm and a speed of 20 mm/min. Before casting, the glass plate, the casting knife and the polymer solution were all preheated to 70 °C in order to ensure the fluidity of the polymer solution. The glass supported polymer films were immediately immersed in the coagulation bath consisting of distilled water at room temperature (RT) and were left there for ca. 24 h in order to allow completion of phase separation. Subsequently, the porous films were washed with fresh distilled water and dried in air at room temperature or freeze-dried in an Alpha 1-2 freeze dryer from Christ. From the pre-heated C:TiO2 = 10:2 polymer solutions in [Bmim][OAc]:DMSO = 4:1 polymer spheres were also prepared using drop formation cum phase separation technique in coagulation bath consisting of distilled water at RT. The polymer droplets were formed using a 10 ml syringe with a needle with 0.70 (width) x 30 (length) mm2. The spheres were left in water overnight for the completion of the phase separation process and afterwards washed with fresh distilled water and dried in air at RT or freeze-dried in an Alpha 1-2 freeze dryer from Christ.

2.3. Characterization methods The rheology of the polymer solutions in ionic liquid or ionic liquid/co-solvent mixture was studied in rotation mode using an Anton Paar Physica MCR301 rheometer with cone and plate geometry (1°) or with a plate and plate geometry and Peltier temperature control system. To eliminate any previous shear histories and to allow the samples to establish the equilibrium structures, a steady pre-shear was applied at a shear rate of 1 s-1 for 60 s followed by a 120 s rest period before each dynamic rheological measurement. In viscosity versus shear rate scans, the


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shear rates were varied between 0.1 and 1000 s-1. For polymer solutions in ionic liquids, viscosity versus temperature studies were performed over a temperature interval from 20 °C to 100 °C, at a constant rotation speed of 9.4 min-1 and a heating rate of 10 °C/min. Scanning electron micrographs of the porous films at different magnifications were taken with a FEI ESEM Quanta 400 FEG instrument. For the cross section measurements the samples were broken in liquid nitrogen. The samples were sputtered with Au/Pd (80/20) at 0.1 mbar and 30 mA for 30 sec until a layer of 2 – 3 nm was obtained. The catalytic activity of the TiO2 doped cellulose-based porous films was evaluated in the photo-degradation of organic dyes like methylene blue or rhodamine B using the slightly modified two methods described in our previous papers20,22. Porous film pieces of 1 cm2 were impregnated with aqueous solutions of organic dyes (2 x 10-5 M): each piece was immersed in 30 ml of dye solution and was maintained there in dark conditions for 30 min. The porous film pieces after removal from dye solution were placed in an open crystallizing dish and exposed to direct sunlight for 15 min (around midday in May). Digital pictures were taken before and after sunlight exposure (Test 1). In order to assess the photo-degradation efficiency in a heterogeneously catalyzed water treatment process, pieces of porous film (0.5 g of wet porous film or 0.1 g dry porous film), were inserted in 30 ml of aqueous dye solution (5 x 10-5 M). The mixtures were first stirred in dark conditions until the adsorption–desorption equilibrium had been reached, then the samples were exposed for a certain amount of time to UV light (365 nm) in a TLC CN-15 viewing cabinet from Vilber Lourmat GmbH equipped with 2 x 15 W lamps and yielding an UV intensity at location of the sample of 1050 µW/cm2. Small samples of dye solution were taken periodically by a syringe and the catalyst was filtered off with a 0.2 µm cellulose acetate microfiltration


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membrane (Sartorius). The variation of dye concentration with irradiation time was followed by UV spectroscopy using an UviLine 9400 Spectrometer from Schott Instruments. The measurements were performed in the 400 – 800 nm spectral range with a resolution of 1 nm. For comparison purposes, the photo-degradation of 30 ml aqueous dye solution (5 x 10-5 M) with 0.009 g pure TiO2 P90 powder was analyzed in analogous manner (Test 2). The Test 2 was performed also for the TiO2 doped cellulose spheres: 0.1 g dry spheres (containing 0.02 g TiO2) were added to 30 ml of aqueous dye solution (5 x 10-5 M). The mixtures were first stirred in dark conditions until the adsorption–desorption equilibrium had been reached, then the samples were exposed for a certain amount of time to UV light (365 nm) in a TLC CN-15 viewing cabinet from Vilber Lourmat GmbH equipped with 2 x 15 W lamps and yielding an UV intensity at location of the sample of 1050 µW/cm2. Small samples of dye solution were taken periodically by a syringe. In this case the filtration was not necessary because the spheres were large and stable. The variation of dye concentration with irradiation time was followed by UV spectroscopy as described above.

3. RESULTS AND DISCUSSION 3.1. Characterization of cellulose solutions and respective nanoparticle dispersions The TiO2 P90 powder used for the preparation of nanocomposite porous films and spheres is constituted from mixture of anatase (ca. 97 %, with primary particle size 10 – 14 nm) and rutile (ca. 3 %, primary particle size 12 – 17 nm). These particle size distributions calculated from the XRD patterns are in good agreement with the observations in transmission electron micrographs20.


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The water content in the ionic liquid, determined by Karl Fischer titration, was ~ 11000 ppm. Because up-scalable processes were envisaged, the ionic liquid was used as received without further purification. The TiO2 P90 could be very well dispersed in the pure ionic liquids showing and average agglomerate size about 22 nm (see Supporting information Fig. SI-1.). The obtained results are in good agreement with our previous studies concerning the dispersibility of TiO2 nanoparticles in hydrophilic ionic liquids23. One precondition for obtaining well-defined porous structures, either polymeric or polymer nanocomposite, is the state of the polymer and that of the nanomaterial in the solvent system. Therefore, cellulose solutions and cellulose - TiO2 solvent systems were investigated by rheology. In most of the polymer solutions at low shear rates the molecular associations blocked the shear flow and therefore the viscosity was very high. With the increase of shear rate the macromolecular chains started to align in the flow direction which was marked by a drop in viscosity (shear thinning) due to the advanced disassociation of molecules at very high shear rate (Fig. 2). The viscosity of the polymer solutions depends not only of the shear rate but also of the temperature; the fluidity of the solutions was improved by increasing the temperature. The viscosimetric characterization of the polymer solutions gave valuable information about the quality of the used solvent and about the chain conformation of the molecules in the polymer solutions.


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Cellulose solutions in [Bmim][OAc] @ 9.4 min-1

Cellulose solutions in [Bmim][OAc] @ RT 100

100

0 wt% 2 wt% 4 wt% 6 wt% 8 wt% 10 wt% 12 wt%

10

0 wt% 2 wt% 4 wt% 6 wt% 8 wt% 10 wt% 12 wt%

10

Viscosity, Pa.s

Viscosity, Pa.s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

1

0,1

0,01

0,1

0,001 0,1

1

10

Shear rate, s

100

1000

50

-1

60

70

80

90

100

Temperature, °C

Figure 2. Rheological properties of the cellulose solutions in [Bmim][OAc] The viscosity of the ionic liquid based dope solution can be reduced by the addition of appropriate amounts of a good co-solvent (polar organic solvent) which does not diminish the solvation ability of the ionic liquid24. The study of Zhang and collaborators has shown that the viscosities if the IL/DMSO solutions decreases with the increase of the DMSO fraction23. Furthermore, for cellulose/IL/DMSO solutions a decreased monomer friction coefficient by adding DMSO and no significant change in the entanglement state of cellulose in IL were recorded; therefore the thermodynamic properties of ILs-DMSO mixtures with respect to solubilizing cellulose were similar with those of ILs. Our own rheological experiments have shown a consistent decrease of the polymer solution viscosity with the increase of the DMSO content in the solvent (see Supporting information Fig. SI-2.). As observed in Fig. 3, the viscosity profiles of the polymer solutions in IL – co-solvent mixtures revealed the important decrease in the viscosity (as expected) as well as the diminishing of the shear thinning effect with the increase of the co-solvent ratio. The presence of the cosolvent in high amount induces the formation of an additional solvation layer between the polymer chains improving the processability of the polymer solutions at room temperature. Because the DMSO is not a solvent for the pure cellulose, its addition to the IL will change the


11


ionic liquid solvent quality. The addition of small amounts of DMSO will enhance the solvent quality of the ILs (high degree of shear thinning) while the addition of larger amounts of DMSO will certainly decrease the quality of the solvent (less pronounced swelling of the polymer coils and therefore low degree of shear thinning). The addition of TiO2 to the polymer solutions was marked by a slight increase in the viscosities but no modification of the curve profile (cf. Fig. 3, left); therefore no major disruption of the cellulose inter-molecular hydrogen bonding due to the nanoparticles presence can be envisaged. A measure of the entanglement of the polymer chains in a solution can be given by the activation energy of the viscous flow which can be calculated from the Arrhenius plot (Fig. 3, right); the data are presented in Table 1. The DMSO addition clearly reduced the chain’s entanglement as indicated by the reduction of the activation energy. Also, the nanoparticle addition slightly seemed to diminish the polymer chain entanglement.

Arrhenius plot for 20 - 100°C interval @ 9.4 min-1

8 wt % polymer in solution @ RT 7 6

10

5 4

C in [Bmim][OAc] C in [Bmim][OAc]/DMSO 4/1 C in [Bmim][OAc]/DMSO 2/1 C:TiO2=10:1 in [Bmim][OAc]/DMSO 4/1 C:TiO2=10:1 in [Bmim][OAc]/DMSO 2/1

3

ln η

Viscosity, Pa.s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

2 1

C in [Bmim][OAc] C in [Bmim][OAc]/DMSO = 4/1 C in [Bmim][OAc]/ DMSO = 2/1 C:TiO2 = 10:1 in [Bmim][OAc]/DMSO = 4/1 C:TiO2 = 10:1 in [Bmim][OAc]/DMSO = 2/1 0,1 0,1

1

10

100

1000

0 -1 -2 -3 0,0026

0,0028

-1

0,0030

0,0032

0,0034

1/T [K-1]

Shear rate, s

Figure 3. Rheological properties of cellulose solutions in [Bmim][OAc] in the presence and absence of DMSO co-solvent as well as the respective systems with added TiO2 nanoparticles


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Table 1. Activation energy of viscous flow for cellulose solutions in [Bmim][OAc] in the presence and absence of DMSO co-solvent based on Arrhenius plot for 20 – 100 °C interval @ 9.4 min-1 Sample

Ea [kJ/mol]

[Bmim][OAc]

~ 42

[Bmim][OAc]:DMSO 4/1

~ 31

[Bmim][OAc]/DMSO 2/1

~ 27

C in [Bmim][OAc]

~ 52

C in [Bmim][OAc]/DMSO 4/1

~ 41

C in [Bmim][OAc]/DMSO 2/1

~ 35

C:TiO2 = 10:1 in [Bmim][OAc]/DMSO 4/1

~ 38

C:TiO2 = 10:1 in [Bmim][OAc]/DMSO 2/1

~ 33

3.2. Porous cellulose nanocomposite films By using systems characterized in detail in section 3.1., nanocomposite porous films were prepared via the NIPS process with water as coagulation bath. The preservation of the wet film porous structure in the dried materials is strongly influenced by the choice of the drying technique. It was demonstrated that air-drying shrinks the surface pore size and collapses the bulk porous structure of the film. Freeze drying shrinks the pores on the surface but the collapse of the bulk porous structure of the film is much reduced compared to the air-drying25. In our previous works20 we have observed that the addition of the TiO2 nanoparticles has a minimal influence on the structure formation.


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Figure 4. Cellulose – TiO2 nanocomposite porous films from ILs solutions (8 wt% polymer solution); phase separation in water @ RT; effect of different drying methods: a) air dried, and b) freeze dried In the present work we have observed that for a cellulose porous film prepared by phase separation from a polymer solution in the [Bmim][OAc] ionic liquid, by regular air-drying a relatively compact structure has been obtained while by freeze-drying a fine spongy structure with thin not collapsed fibers and some macrovoids has been obtained (Fig. 4). Similar findings26 have been reported previously for bacterial cellulose membranes where it was observed that freeze-drying reduced the swellability of the membrane by a factor 5 while the evaporationdrying by a factor of 50. This strong decrease of swellability of the air-dried porous film is correlated with the collapse of network meshes by complete aggregation of the polymer, as demonstrated by an increase of the intramolecular hydrogen bonding of cellulose filaments. After the freeze-drying process only a slight increase of the size of the network meshes due to a partial aggregation of free polymer strands was observed.


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Figure 5. Cellulose porous films from ILs solutions (8 wt% polymer solution); phase separation in water @ RT, freeze-dried; effect of dilution with co-solvent As discussed in section 3.1., by the addition of DMSO at fractions lower than 30 % to the cellulose solutions in [Bmim][OAc] only an important improvement of the processing efficiency with a certain influence to the porous film formation process would be expected. Fig. 5 confirms these expectations, and the same fine spongy structure with thin not collapsed fibers in combination with more pronounced macrovoids has been obtained also in the presence of DMSO as co-solvent. The more pronounced macrovoids also point to the effect of lower viscosity causing faster solvent exchange during the NIPS process. Even higher concentrations of cosolvent will reduce the solubility of cellulose in the solvent mixture and will therefore have an even stronger impact on the pore structure formation during phase separation. Previous studies related to the fabrication of cellulose acetate porous films from [Bmim][OAc] polymer solutions have shown that nanoparticle (TiO2) addition leads to a slight increase in the film porosity for the air-dried porous films, probably due to the addition of the hydrophilic nanoparticles (see Supporting Information Fig. SI-3). In the case of the freeze-dried cellulose porous films prepared from a [Bmim][OAc]/DMSO (4/1) polymer solution no significant difference in the film porosity was observed due to the changed dopant content (Fig. 6). At


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higher concentrations of TiO2 a slightly larger density of catalyst agglomerates sizes within the polymer and especially at film surface has been observed (Fig. 6b).

Figure 6. Cellulose – TiO2 nanocomposite porous films (top surface view) from [Bmim][OAc]:DMSO = 4:1 solutions (8 wt% polymer solution); phase separation in water @ RT, freeze-dried; effect of different TiO2 content: a) C:TiO2 = 10:1.5 and b) C:TiO2 = 10:2

3.3. Feasibility of porous cellulose nanocomposite sphere preparation For comparison purposes, cellulose spheres have been prepared by a shaping procedure based on drop formation and subsequent NIPS with water as coagulation bath. Drops of cellulose solution in IL/DMSO solvent mixture were formed with the help of a syringe and the droplets were released into the coagulation bath. The dispensed drops had weights around 15.4 ± 2.1 mg and yielded dry spheres with weights around 1.21 ± 0.15 mg. This weight of a dry sphere represents ~ 8 % of the weight of the corresponding drop, being in very good agreement with the solid content of the casting solution. Due to the high viscosity of the cellulose solution in the


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IL/DMSO mixture, the droplets release speed was very low and only relatively large spheres could be obtained. The high viscosity of the dope is also responsible for the formation of a small tail at surface of the spheres. The reduction of tail formation could be achieved by increasing the gap between the syringe needle and the surface of the non-solvent (for our system the evaporation of the solvent is expected to be minimal). The physical properties of the cellulose macrospheres like porous structure, apparent density and specific surface area are strongly influenced by the dope concentration and by the coagulation process. By increasing the dope concentration the porosity of the resulted beads may be reduced and by tuning the temperature and the composition of the coagulation bath the morphology and the pore size distribution can be adjusted. The addition of metal oxide nanoparticles like TiO2 to the dope solution influences the properties of the resulted nanocomposite sphere: decrease of the porosity and specific surface, increase of the density27. The dried polymer spheres generated in this work have a size around 1 mm and their structure consists of numerous inter-connected and dead-ended open macropores (Fig. 7). The TiO2 agglomerates are homogenously dispersed within the body of the microsphere and no tendency to agglomerate at the sphere surface has been observed, unlike in the case of the corresponding porous film. The structure of the inter-connected open pores within the porous film is identical with the one of the inter-connected open pores in the sphere (see Supporting information Fig. SI4) suggesting a similar phase separation process for both geometries. However differences in the surface porosities should be not ignored (the sphere surface is more porous than the film surface). These differences may be induced by the different immersions speeds: the polymer droplets are immersed with high speed (dropping) while the polymer films on glass substrate are gently immersed in non-solvent.


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Figure 7. Cellulose – TiO2 (C:TiO2 = 10:2) nanocomposite spheres from ILs solutions (8 wt% polymer solution); phase separation in water @ RT, freeze-dried. Surface –top pictures– and cross-section –bottom pictures– at different magnifications

3.4. Catalytic activity and correlations with nanocomposite structure Catalytic activity of the nanocomposite materials was evaluated with respect to photodegradation of organic dyes in two different formats, as model systems for processed such as advanced oxidation in water purification or “self-cleaning” (under sun light) materials. As observed in Fig. 8, the dried porous film had very poor catalytic activity while the wet one exhibited a moderate catalytic activity. The poor catalytic activity of the dried films we consider to be connected with the cellulose crystallization upon drying. The addition of the DMSO cosolvent to the polymer solution had a noticeable effect on the catalytic activity of the wet porous


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films, expressed by a slight increase of the rate constant from 0.00202 min-1 to 0.00274 min-1 (Table 2). Rhodamin B (5 x 10-5) degradation by excitation @ 365 nm

Rhodamin B 5x10-5 degradation by UV excitation at 365 nm 1,0

1,0

a)

0,9

b)

0,9

0,8

without catalyst C:TiO2 =10:1 from [Bmim][OAc] - air dried

0,8

C:TiO2 =10:2 from [Bmim][OAc]/DMSO - wet

0,7

C:TiO2 =10:1 from [Bmim][OAc] - wet

0,7

C:TiO2 =10:1.5 from [Bmim][OAc]/DMSO - wet

0,6

C:TiO2 =10:1 from [Bmim][OAc]/DMSO - wet

0,6

C:TiO2 =10:2 from [Bmim][OAc]/DMSO - freeze dried

ln(C0/C)

ln(C0/C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,5 0,4

0,4

0,3

0,3

0,2

0,2

0,1

0,1

0,0

0,0

-0,1

-0,1 0

20

40

60

Time, min

80

100

120

C:TiO2 =10:1.5 from [Bmim][OAc]/DMSO - freeze dried

0,5

0

20

40

60

80

100

120

Time, min

Figure 8. Rhodamin B 5 x 10-5 M photodegradation with an 8 wt% cellulose porous film depending on different processing parameters; equivalent of 0.013 g TiO2 (C:TiO2=10:1.5) and 0.02 g TiO2 (C:TiO2=10:2) The differences in catalytic activity can be explained by the fact that for the less porous material only a small amount of dye is able to come in direct contact with the active oxide species and the efficiency of removal of degradation products from the surface is also poor. Many other additional factors like the agglomeration of the catalyst nanoparticles or the diffusion of the nanoparticles to the film surface may also be responsible for the differences in the catalytic activity. A complete understanding of the complex interdependence of these factors requires further in-depth studies. The increase of the TiO2 fraction from 13 wt% to 20 wt% led to an increase of the catalytic activity (Fig. 8b and Table 2): the rate constant k increased from 0.00272 min-1 to 0.00413 min1

. However, as we had demonstrated in our previous work21, the improvement of the catalytic

activity by increasing the TiO2 content has its limits: a too high concentration of the dopant may


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lead to extended agglomeration and therefore to a loss of the available catalytically active surface.

Table 2. Rate constant k for the degradation of Rhodamin B (5 x 10 -5 M in water) with different TiO2 based photocatalytic porous films; equivalent of 0.009 (C:TiO2 = 10:1), 0.013 (C:TiO2 = 10:1.5) and 0.02 (C:TiO2 = 10:2) g TiO2 [Bmim][OAc]:DMSO ratio

k [min-1]

TiO2 P90

-

0.01579 ± 0.00075

C:TiO2 (10:1) without DMSO (AD)

-

near detection limit

C:TiO2 (10:1) without DMSO (W)

-

0.00202 ± 0.00038

C:TiO2 (10:1) with DMSO (W)

2:1

0.00274 ± 0.00015

C:TiO2 (10:1.5) with DMSO (W)

4:1

0.00272 ± 0.00020

C:TiO2 (10:2) with DMSO (W)

4:1

0.00413 ± 0.00059

C:TiO2 (10:1.5) with DMSO (FD)

4:1

near detection limit

C:TiO2 (10:2) with DMSO (FD)

4:1

0.00038 ± 0.00015

C:TiO2 (10:2) with DMSO spheres

4:1

0.00052 ± 0.00025

Sample

(FD) AD = air dried; W = wet; FD = freeze dried.

The TiO2 agglomerates are homogenously dispersed within the body of the microsphere and no tendency to concentrate at the sphere surface has been observed, unlike in the case of the corresponding porous film. However, it was observed that in the dried state (after freeze drying) the porous films and spheres have similarly low catalytic activity (Fig. 9 and Table 2). Furthermore, the particles have a diameter of about 1 mm, and hence the pathway into the


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structure is significantly longer than for the films with thicknesses between 0.1 – 0.15 mm. Therefore, a less efficient excitation of TiO2 nanoparticles deeper in the material is expected to lower overall photo-catalytic activity. The apparently better catalytic activity of the spheres may be only due to a better absorptivity of the material in the spherical geometry. Rhodamin B (5 x 10-5 M) degradation by excitation @ 365 nm 0,4

0,3

C:TiO2 (10:2) porous film ln(C0/C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C:TiO2 (10:2) sphere

0,2

0,1

0,0

-0,1 0

20

40

60

80

100

120

Time, min

Figure 9. Rhodamin B 5 x 10-5 M photodegradation in the presence of photocatalytic C:TiO2 (10:2) freeze dried porous film and sphere from [Bmim][OAc]:DMSO = 4:1; equivalent of 0.02 g TiO2 The photocatalytic activity of the cellulose – TiO2 nanocomposite porous films prepared from polymer solutions in ILs has been one more time demonstrated also by the results obtained in “Test 1”. By simple irradiation with sunlight, the dye adsorbed on the porous films surface was decomposed in short time – ca. 15 min (Fig. 10). For this test the methylene blue dye was chosen because it offers a better contrast in the digital pictures. The test was repeated for several immersions followed by sun light irradiation of a porous film piece, and the dye was successfully decomposed each time, by this demonstrating the recyclability of the catalytic nanocomposite porous film in more than three successive cycles (Fig. 10). This type of experiment was


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performed only to confirm the existence of the catalytic activity and to prove the recyclability of the catalyst and had no quantitative aspect.

Figure 10. Recyclability of the C:TiO2 (10:1) wet porous films from [Bmim][OAc] solution used for the photodegradation of methylene blue

4. CONCLUSIONS In the present work we discussed in detail a new potentially environmentally friendly route for the preparation of TiO2 – cellulose porous nanocomposites (porous films and spheres) with potential applications in the fields of catalysis or as self-cleaning materials. The fabrication is based on the NIPS process as a facile way for tailoring material’s pore structures. The main advantages of the described method are connected with the use of a class of solvents with lower volatility – ionic liquids – and therefore minimizing the use of the VOC compounds, combined with the use of the large scale produced, commercially available catalyst nanoparticles, which


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allows the facile upscaling of the process. Special attention has been given to the use of a cosolvent to the structure formation of the porous nanocomposites and the influence of the drying method, both in relation with the obtained photocatalytic activity. The porous films obtained after the addition of a polar organic solvent to the dope solution also showed a finger like – filigree structure, slightly more relaxed than in case of the films obtained without co-solvent. A strong positive effect on the material shaping by a pronounced reduction of the viscosity due to the co-solvent could be noticed. The impact of the co-solvent on the porous film catalytic activity was minimal. The drying method plays an essential role for the structure evolution of the final nanocomposite material. It was observed that drying under ambient conditions leads to a dramatic collapse of the pores within the nanocomposite, dense materials with poor catalytic activity are obtained in this case. By freeze-drying of the nanocomposite, the collapse of the pores is prevented and highly porous structures are obtained. However the presence of the pores does not lead to an increase of the catalytic activity of the dried porous films most probably due to the cellulose crystallization by drying. After drying, as a consequence of the reformation of the intra-molecular and inter-molecular hydrogen bonds (leading also to a partial crystallization of the material) the flexibility and the mobility of the fine cellulose fibrils network is irreversibly lost and the access of the rhodamine to the catalytically active centers more difficult. The wet porous films obtained at the end of the phase separation process have good catalytic activity most probably due to the more relaxed structure of the cellulose matrix. The increase of TiO2 concentration in the porous film was observed to yield a significant improvement of the photocatalytic activity. In further studies possible leaching of the TiO2 nanoparticles from the wet porous films will be analyzed. In dried state, shaping of the porous material as a flat sheet does not confer it a significantly better catalytic activity compared to spheres obtained from the


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same solutions/dispersions and analogous phase separation conditions.. However, the porous nanocomposite spheres can be made much smaller and may thus have great potential per se, but also as intermediates for the preparation of carbon based porous composites by appropriate stabilization and carbonization processes.

Supporting Information. Particle size distribution (DLS) of TiO2 P90 nanoparticles in [Bmim][OAc] ionic liquid; viscosity of a cellulose solution in [Bmim][OAc]/DMSO mixture as a function of the DMSO fraction; SEM cross-section and surface images of cellulose acetate – TiO2 nanocomposites; SEM cross-section images of TiO2 doped cellulose based spheres and porous films (PDF).

AUTHOR INFORMATION Corresponding Authors * Tel: +49 203 379 8226. Email: [email protected] (Dr. Alexandra Wittmar) * Tel: +49 201 183-3151. Email: [email protected] (Prof. Mathias Ulbricht) Notes The authors declare no competing financial interest.

Funding Sources The work is supported by the Deutsche Forschungsgemeinschaft (DFG), grant number WI 4325/2-1.


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ACKNOWLEDGMENTS The financial support through the Deutsche Forschungsgemeinschaft (DFG), grant number WI 4325/2-1, is kindly acknowledged. We gratefully acknowledge the collaboration with Mr. Smail Boukercha (SEM characterization) at the University of Duisburg-Essen. Thanks are also due to Steffen Köcher who did several experiments during his advanced studies project at University Duisburg-Essen.

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TABLE OF CONTENTS ABSTRACT GRAPHICS


29

Ionic Liquid-Based Route for the Preparation of Catalytically Active

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