Catalytic crosslinking of a regenerated hydrophobic benzylated cellulose and nano TiO₂ composite for enhanced oil absorbency

1 Introduction

Cellulose is a biopolymeric raw material that is heavily used in the textile, construction, paper and pharmaceutical industries (1, 2) for the production of valuable cellulose-based products in the form of threads or fibers, films and stable microspheres for industrial and domestic use (3, 4). Benzylated celluloses are an interesting set of hydrophobic materials with unique applications as packaging materials, membranes (5, 6) and rheology modifiers with improved surface properties. The benzylation of cellulose increases its plasticity by substituting its hydroxyl group with non-polar benzyl groups which reduces the level of hydrogen bonding in the polymeric state of cellulose.

Titanium oxide is a unique material with a direct band gap (3.07 eV) and large excitation binding energy of 60 meV with long-term chemical stability when exposed to acidic and basic compounds (7). It is widely used in near-UV emission, gas sensors, transparent conductor and piezoelectric applications (8, 9). Titanium oxide nanoparticles have potential applications in various areas including optical, piezoelectric and gas sensing and they also exhibit high catalytic efficiency and strong adsorption ability (10, 11).

Areas which experience oil spillage are characterized with heavy pollution index which affect drinking water, plants, animals and humans that inhabit the oil environment (12). Water pollution by petroleum, spent lubricants and greases is a major challenge to oil producing countries of the world, therefore there is a need to synthesize smart materials with enhanced oil absorptive abilities that can be easily disposed and reused (13). Oil absorbing polymers are crosslinked three-dimensional hydrophobic networks with good swelling properties. The different oil adsorbents which are already in use for clean up purposes at regions with pronounced oil pollution include polypropylene, polyesters, polyurethanes sheets and foams are difficult to store after use while the long chain alkyl acrylate polymers have a tendency to exist in their crystalline state, a property that reduces their oil absorbing capacity (14). The aim of this research is to synthesize new cellulose-based biopolymer composites with good absorbency that can be reused and disposed of properly.

2 Materials and methods

3 Results and discussion

The presence of hydroxyl and carbonyl groups is seen at 3423 and 1729 cm⁻¹, respectively. The stretching vibration of C-O-C of the anhydroglucose unit is seen at 1043 cm⁻¹ while the peaks at 2916 and 2840 cm⁻¹ represent the C-H stretch (19). Various bending vibrations were observed at 977, 821 and 724 cm⁻¹ for the various CH₂ groups in the cellulose biopolymer (8). The appearance of transmittance peaks at 1643 and 882 cm⁻¹ confirms the presence of C=C stretch and bend of a phenyl group and the disappearance of peaks at 2800–2900 cm⁻¹ further confirms the synthesis of a regenerated benzyl cellulose (9, 20). The hydroxyl and alkyl functional groups at 3462, 2913 and 2842 cm⁻¹, respectively were also seen after benzylation of cellulose. After the synthesis of the benzyl cellulose and nano-TiO₂ composite, the transmittance peaks at 2336 and 2354 cm⁻¹ for cellulose and the benzylated cellulose reduces greatly while the appearance of new peaks at 816, 769 and 726 cm⁻¹ for Ti-O stretching vibrations confirms the successful synthesis of the composite (21) (Figure 1).

Figure 1:

Infrared spectra of cellulose, benzyl cellulose and benzyl cellulose-TiO₂.

The SEM images in Figure 2 shows different morphological features which differentiates cellulose from benzyl cellulose. The cellulose isolated, which looks like an amorphous matter with a loose foam structure, was transformed into a benzylated form which resembles foam covered with solid bubbles and irregularly sized cavities. From Figures 3–8, after compositing with TiO₂, the composite solidifies into a solid hydrophobic structure with a more open cavity. From the SEM photograph, the major thing which fascinates the observer is the disappearance of the ruggedness of the benzyl cellulose surface to form a composite with a much smoother surface and well arranged cavities (22, 23). The composite appears to be more tightly held at the surface as a result of the crosslinking effect. This information from the SEM results show that the composite is fit for absorbing both small and large molecules in their liquid phases (11, 24). The energy dispersive X-ray (EDX) which gives information about the elemental constituents of composite materials also confirms the successful binding of TiO₂ to the benzyl surface. The peak at 7 keV reveals the presence of titanium in the composite.

Figure 2:

SEM image of (A) cellulose (B) and (C) benzyl cellulose at different magnifications (D) benzyl cellulose-TiO₂ nanoparticles composite.

Figure 3:

EDX of benzyl cellulose-TiO₂ nanoparticles.

Figure 4:

The XRD pattern of cellulose, benzyl cellulose and benzyl cellulose-TiO₂.

Figure 5:

Absorption profile of benzyl cellulose-TiO₂ crosslinked with (A) adipic acid-water (B) adipic acid-glycerol (C) adipic acid-glycerol-aluminum sulfate catalyst (0.1 g) (D) adipic acid-glycerol-aluminum sulfate catalyst (0.2 g).

Figure 6:

First order absorption kinetics of benzyl cellulose-TiO₂ crosslinked with (A) adipic acid-water (B) adipic acid-glycerol (C) adipic acid-glycerol-aluminum sulfate catalyst (0.1 g) (D) adipic acid-glycerol-aluminum sulfate catalyst (0.2 g).

Figure 7:

Second order absorption kinetics of benzyl cellulose-TiO₂ crosslinked with (A) adipic acid-water (B) adipic acid-glycerol (C) adipic acid-glycerol-aluminum sulfate catalyst (0.1 g) (D) adipic acid-glycerol-aluminum sulfate catalyst (0.2 g).

Figure 8:

The oil desorption profile.

The peaks at 2θ=13°, 23°, 26°, 33.5° and 39° typical for cellulose I corresponds to (101), (002), (030) and (040), respectively, while the peaks at 2θ=23°, 33°, 40° with corresponding crystallographic planes at (200), (220) and (222) confirms the synthesis of benzyl cellulose (25, 26). The appearance of peaks at 18°, 21°, 39° and 46.5° confirms the presence of TiO₂ in the final composite formed.

The figures below show the oil absorption profile of regenerated benzylated cellulose crosslinked with adipic acid-water, adipic acid-glycerol, and adipic acid-glycerol-aluminum sulfate. The quantity of oil absorbed by the hydrophobic matrices increases as the retention time increases. In general, light oils tend to be easily absorbed and desorbed from hydrophobic composites while the heavy oils take time to be absorbed into the pores of oil absorbing polymers; rather the heavy oils stays adsorbed at the surfaces due to their bulky nature (17). Maximum absorption was obtained with kerosene while minimum absorption was obtained with toluene across all the composites prepared. The oil absorbency increased rapidly at the beginning due to the solvation of the network chains. The main driving force of this process was the change in the free energies of mixing and elastic deformation. However, the swelling was limited, that is, with the time increasing the absorption rate became slower and finally reached saturation (18).

For chloroform, the maximum quantity of oil absorbed increased from 20.83 to 23.81 when crosslinked with Al₂(SO)₄, 22.64–24.195 for toluene, 23.09–26.08 for xylene, 20.85–22.44 for petrol and 21.76–23.17 for kerosene. The improved oil absorption ability might be as a result of the pore regulation of the benzyl cellulose and titanium oxide composite by the positive charge effect of Al³⁺ on the surface of the composites. The pores tighten up by the Ti⁴⁺ and Al³⁺ interactions and enables the composite to hold more of the oils within its pores. The amount of oil absorbed across all the composite crosslinked at different conditions which ranged from 11.01 to 21.24 shows that the composite is oleophilic (oil loving). Enhanced oil absorption was seen as the amount of the aluminum sulfate catalyst was doubled. A high Q_max of 24.16, 25.81, 27.22, 24.03 and 24.43 g/g was obtained when the amount of catalyst used was doubled; hence, aluminum sulfate can be used to improve the oil absorption capacity of hydrophobic cellulosic composites. From previous work, octadecyl acrylate copolymer which is also an oil absorbent had an oil absorption capacity in the range of 15–21 g/g while isooctyl acrylate copolymer had oil absorption capacity in the range of 18–26 g/g (27).

Absorption of oil onto the absorbent was studied in terms of the kinetics of the absorption mechanism by using two models. Assuming that the absorption kinetic process obeyed the first-order model, the kinetics equation was:

ln(qm−qt)=−k1t+A

where q_t (g/g) was the amount of oil absorbed at time t (h), k₁ was the rate constant of the first order equation and q_m was the maximum amount of absorption (g/g). The absorption rate constant k₁ could be determined experimentally by plotting of ln (q_e −q_t ) versus t.

Similarly, if the absorption obeyed the second-order kinetics model, kinetics equation became following form:

1qe−qt=k2t+B

where k₂ (g/g·h) was the rate constant of the second order equation. The rate constant k₂ could be obtained by plotting 1/(q_e−q_t) versus t. k₁ and k₂ for the oils used were calculated from the slopes. The correlation coefficients for the first-order model were better than the second-order model, indicating that the first order model was more suitable to describe the absorption process (18, 27).

Chloroform absorption for all the crosslinked benzylated cellulose-TiO₂ followed first order kinetics except the adipic acid-glycerol crosslinked while petrol absorption followed the first order for all the composites without any exception. The absorption kinetics of kerosene and toluene as shown in the Table 1 followed by second order kinetics except for the absorption by the adipic acid-water crosslinked composite while that of xylene showed first order absorption with the catalytically crosslinked composite and second order absorption for the uncatalytically crosslinked composite.

4 Conclusion

The composite obtained from benzyl cellulose and TiO₂ is a new material with good oil absorbency and can be used to mop up oil spillages in regions experiencing oil pollution. The aluminum sulfate used is a more economical catalyst that has caused an improvement in the absorbency properties of the composite on oils after being crosslinked.