Full-Component Acetylation of Corncob Residue into Acetone-Dissolvable Composite Resin by Titanium Oxysulfate Reagent

Abstract

Herein, all components of corncob residues were acetylated to synthesize an acetone-soluble resin material. Moreover, titanium oxysulfate (TiOSO₄), a low-cost intermediate for the industrial production of TiO₂, was first used as an acetylation reagent. Through optimizing reagent dosages and reaction times, above 90% of hydroxyl groups in the corncob residue can be substituted by acetyl groups. During the acetylation reaction, TiOSO₄ was transformed into TiO₂ and uniformly distributed within the acetylated corncob residue. The resulting product, owing to its solubility in acetone, can be employed to fabricate a composite film with excellent mechanical properties, achieving an increase of 85% in tensile strength and 90% in strain rate compared to commercial cellulose acetate film. By this preparation technique, the industrial-grade corncob residue as raw material can be converted to acetylated composite films. Further analysis indicates that the coexistence of acetylated lignin and TiO₂ plays a pivotal role in enhancing the mechanical properties of acetylated corncob residue composite film. Additionally, this material exhibits substantial degradation within 28 days under natural environmental conditions, whereas commercial cellulose acetate shows no significant changes even after 60 days. The present achievements are a significant breakthrough in the high-value technologies for the conversion of corncob residues.

1. Introduction

Corncob is a significant agricultural solid waste generated from corn production, and it is primarily composed of cellulose (32–36%), hemicellulose (35–40%), and lignin (7–20%) [1,2]. Common corncob management practices include its utilization as heating fuel or in low-value applications, such as substrates for edible mushrooms, agricultural fertilizers, and papermaking materials [3,4,5]. Furthermore, a portion of corncobs have been processed to produce industrial furfural through the hydrolysis of their hemicelluloses, illustrating a valuable pathway for their utilization [6,7,8]. However, approximately 23 million tons of corncob residue is classified as solid waste each year, with the majority being disposed of via centralized incineration or landfilling [9,10,11]. This practice leads to resource wastage and environmental pollution, apart from hindering the advancement of industrial furfural derived from corncobs.

To address these challenges, researchers have conducted extensive technological investigations aimed at the high-value utilization of corncob residue [12]. For instance, corncob residue has been employed as a filler in combination with polypropylene to produce an eco-friendly composite material [13]. Furthermore, various carbonization technologies have transformed corncob residue into biochar with diverse structural characteristics [14,15]. These methods were primarily focused on the comprehensive utilization of all components of corncob residue. In contrast, other approaches emphasize the selective extraction and utilization of specific components from corncob residues. For example, enzymes have been used to hydrolyze cellulose from corncob residue for bioethanol production as a fuel source [16]. Additionally, cellulose derived from corncob residues can serve as a precursor for various bio-based materials [17]. A range of physicochemical and biological pretreatment technologies are utilized to remove lignin and isolate cellulose suitable for applications in the pulp manufacturing industry and beyond [18,19]. It is noteworthy that chemical modifications of cellulose obtained from biomass waste can yield high-value-added resin materials. As a widely utilized biomass modification technique [20,21], acetylation is one of the important methods for maximizing the value derived from biomass waste, which has attracted attention from researchers and practitioners alike [22].

In industry, the acetylation of biomass usually uses liquid sulfuric acid as a catalyst to promote the reaction of biomass with the acetylation agent [23]. However, the corrosive nature of liquid sulfuric acid poses significant challenges for production equipment and complicates subsequent treatment processes. To address these issues, research has been conducted on the application of various catalysts, such as 1-methylimidazole and 4-(dimethylamino) pyridine (DMAP), in biomass acetylation [24,25]. Notably, alternatives to liquid sulfuric acid have been investigated, including mesoporous carbon-based solid sulfonic acids, sulfate-promoted solid superacid, and so on, for synthesizing cellulose acetate [26]. These sulfur-containing solid acids have been demonstrated to exhibit excellent catalytic performance, despite exhibiting varying acid properties that arise from the distinct interaction mechanisms between sulfur species and different supports [27,28,29]. However, the prices of these solid acids are relatively high. TiOSO₄ is an intermediate in the industrial production of TiO₂ through the sulfuric acid method, which possesses advantages of strong acidity, high yield, and low cost. To date, there have been no reported studies on its application as an acetylation reagent.

To enhance the high-value utilization of corncob residue and improve its overall utilization rate, this study introduced low-cost TiOSO₄ as a reagent, for the first time, for the acetylation of all components of corncob residue into a dissolvable resin material. The effect of reaction conditions on the acetylation of corncob residue was studied, and the structures of resulting acetylated products were systemically analyzed by various characterizations. The mechanical properties of the corresponding film were evaluated with an in-depth analysis of the underlying structure–performance relationships. Finally, the degradation performance of the acetylated corncob residue was further assessed under natural conditions.

2. Results and Discussion

2.1. Structural Analysis of Model Compound

To avoid the influence of impurities in industrial compound, a model compound derived from corncob was utilized as the raw material for acetylation. The process from corncob to acetylated corncob residue is illustrated in Figure 1a. The chemical composition of the model corncob residue (LC) and industrial corncob residue (ILC) was analyzed by sulfuric acid hydrolysis. The cellulose, hemicellulose, and lignin contents of the corncob were determined by adopting the standard National Renewable Energy Laboratory (NREL) procedures [30]. After the treatment, the hemicellulose content of the corncob was effectively reduced from 44.6% to 3.3%. The commercial results are shown in Figure 1b. This significant decrease indicates that the LC can serve as an appropriate model for ILC, owing to its low hemicellulose content. Notably, the lignin component remained largely intact before and after treatment. The increased proportion of cellulose within LC can be attributed to the removal of hemicellulose from the corncob.

The structural parameters of LC in comparison to corncob were analyzed by XRD. As shown in Figure 1c, both samples exhibited three peaks at 15°, 22°, and 34.7° in the XRD patterns, which are characteristic of crystalline cellulose I [31]. In comparison, the peaks of the LC demonstrate higher intensity than those of corncobs, indicating higher degree of crystallinity of cellulose. Moreover, as shown in Figure 1d, the FTIR spectrum of LC exhibited peaks at 1376 cm⁻¹, 1062 cm⁻¹, and 890 cm⁻¹. These peaks are attributed to the bending vibration of C-H bonds in cellulose, the stretching vibration of C-O bonds, and the deformation vibration of C-O-C bonds on the glycosidic linkage, respectively [32,33]. It is noteworthy that a significant decline in peak intensity is observed at 1730 cm⁻¹, which corresponds to a characteristic peak of acetyl groups in hemicellulose [34,35]. This indicates a reduced content of hemicellulose in LC compared to that of corncob. The low content of hemicellulose allows LC to model the important structural characteristics of industrial corncob residue.

2.2. Evaluation of Acetylation Degree

Upon addition of TiOSO₄, the acetylation reaction of LC commences. TiOSO₄ could be dissociated into TiO²⁺ with strong Lewis acidity and SO₄²⁻. The SO₄²⁻ anion, through its interaction with water or hydroxyl groups, exhibits Brønsted acidity [36]. With these acidic characteristics, the hydroxyl groups of LC were substituted with acetyl groups, thereby resulting in a reduction in the hydroxyl value.

Table 1. Hydroxyl value and acetylation degree of various acetylated products under different dosages of TiOSO₄ and reaction times.

Sample	Hydroxyl Value (mg KOH/g)		Acetylation Degree (%)
	60 min	120 min	60 min	120 min
ALC-TiO2-30	101	57	80.7	89.1
ALC-TiO2-40	93	37	82.3	92.9
ALC-TiO2-50	47	50	91.0	90.5
ALC-TiO2-60	41	53	92.2	89.9
ALC-H	93	36	82.3	93.1

presents that, initially, the hydroxyl value of LC is 524 mg KOH/g. It is evident that the incorporation of 30% dosage of TiOSO₄ results in a significant reduction in hydroxyl value of ALC-TiO₂-30 composite after 60 min of reaction, decreasing to 101 mg KOH/g. In terms of acetylation degree, this indicates that 80.7% of hydroxyl groups in LC undergo acetylation by TiOSO₄. With the extension of reaction time and the increase in TiOSO₄ dosage, more hydroxyl groups in LC are substituted by acetyl groups. The acetylation degree of the ALC-TiO₂ composite initially increases and subsequently decreases, suggesting the presence of optimal reaction conditions. Notably, when the dosage of TiOSO₄ reaches 40% and the reaction time extends to 120 min, the resulting ACL-TiO₂-40 composite achieves a hydroxyl substitution degree of 92.9%. This result is comparable to that obtained under similar conditions of acid quantity using liquid sulfuric acid (93.1%), indicating that TiOSO₄ exhibits excellent performance for biomass acetylation.

2.3. Structural and Compositional Analyses of ALC-TiO₂ Composites

The acetylated products were subjected to structural analyses. XRD patterns of ALC-TiO₂ composites are depicted in Figure 2a. The intensities of the three major peaks at 15°, 22°, and 34.7° of LC decrease or even vanish compared with those of ALC-TiO₂ composites. Additionally, the XRD results of ALC-TiO₂ composites exhibit characteristic peaks within the range of 2θ = 17.4°~22.3°, aligning with those observed for commercial cellulose diacetate (CDA) [37,38]. This finding confirms the successful acetylation of cellulose in LC by TiOSO₄. Notably, peaks at 8.4°, 18.3°, and 26.6° corresponding to the diffraction characteristics of anatase TiO₂ are detected in the XRD patterns of ALC-TiO₂ (PDF#21-1272) [39], providing evidence that TiOSO₄ can undergo in situ hydrolysis to form TiO₂ during the acetylation reaction.

Figure 2b shows the FTIR spectra of various ALC-TiO₂ composites. The prominent absorption peak at 3440 cm⁻¹ is attributed to the stretching vibration of the -OH group, while the absorption peak at 2900 cm⁻¹ is attributed to the stretching vibration of the C-H bond in CH₂ [40]. Moreover, the peak at 1750 cm⁻¹ is associated with the stretching of the C-O bond within ester carbonyl groups present in various ALC-TiO₂ composites [41,42]. The peak at 1360 cm⁻¹ corresponds to bending vibrations of the C-H bond in methyl groups found in acetyl structures, while the peak at 1220 cm⁻¹ can be attributed to the stretching vibration of C-O bonds at the junctions of acetyl groups [42]. These observations collectively indicate that LC has undergone acetylation catalyzed by TiOSO₄. Note that as TiOSO₄ dosage increases, the relative peak intensities of C-O to -CH₃ groups also increase distinctly, suggesting a higher content of acetyl groups in the corresponding ALC-TiO₂ composite [43,44], a result which is consistent with the analyses mentioned above.

The DSC curves presented in Figure 2c indicate the presence of an endothermic peak at approximately 200 °C. This temperature marks the transition of the ALC-TiO₂ composites from a solid crystalline state to a liquid amorphous state [45], which aligns with the results obtained from commercial cellulose diacetate. In contrast, no corresponding step peak is evident in the DSC curve of LC. This finding suggests that cellulose and lignin within ALC-TiO₂ composites have undergone acetylation.

The thermal stability of the composite is a critical factor for its practical applications. Figure 2d shows the TG curves of various ALC-TiO₂ composites apart from the control samples. A slight mass reduction is observed for all TG curves at temperatures below 100 °C, which can be attributed to the evaporation of adsorbed water from the sample surfaces. Subsequently, a significant decline in mass occurs when the temperature exceeds 200 °C, indicating the onset of thermal degradation in all samples [12,46]. The maximum rate of mass loss for each sample is noted between 250 °C and 350 °C. Beyond this range, specifically at a temperature above 350 °C, the mass loss stabilizes gradually for each sample, signifying that their thermal decomposition processes have been completed. Importantly, the thermal decomposition behaviors exhibited by ALC-TiO₂ composites closely resemble those of commercial cellulose diacetate, while markedly differing from those associated with LC. This variation can be ascribed to structural changes occurring in cellulose and lignin during acetylation [47]. Furthermore, apart from ALC-TiO₂-30, all other ALC-TiO₂ composites have higher thermal decomposition temperatures compared to those recorded for commercial cellulose diacetate. This observation suggests the superior thermal stability of ALC-TiO₂ composites.

The micro-morphological transformation from corncob into ALC-TiO₂ composite is illustrated in Figure 3. As previously demonstrated, during the acetylation reaction process, hydroxyl groups present in cellulose or lignin are gradually substituted by acetyl groups. This substitution weakens their hydrogen bonds within both intra- and inter-molecular networks. Consequently, the acetylated products can dissolve in the reaction medium [23,48]. As displayed in Figure 3c, the morphology of ALC-TiO₂ markedly differs from that of both corncob (Figure 3a) and LC (Figure 3b). This indicates that after acetylation, the acetylated cellulose and lignin undergo molecular structural reconstruction, resulting in discernible differences in their morphologies. Moreover, many micron-scale columnar particles are dispersed throughout the matrix of the ALC-TiO₂ composite (Figure 3d). To further elucidate the composition of these particles, EDS mapping analyses were conducted on a randomly selected particle (Figure 3d). The results presented in Figure 3e,f indicate the pronounced signals for titanium and oxygen elements at this location. This finding confirms that the particles within the ALC-TiO₂ matrix consist primarily of TiO₂, suggesting that TiOSO₄ undergoes transformation into TiO₂ after acetylation reaction.

2.4. Analyses of the Mechanical Properties of Acetylated Corncob Residue and TiO₂ Composites

The mechanical properties, which are an important parameter of films, are depicted in Figure 4a. Comparative analysis reveals that with the increases in TiOSO₄ content, the tensile stress value of the ALC-TiO₂ composite film increases initially before subsequently declining. This trend is reflected in the corresponding strain values. Notably, the tensile strength of the ALC-TiO₂-40 composite film can reach 37.6 MPa, representing an improvement of 20% to 85% compared to those of other composites (22.6~31.4 MPa) and commercial cellulose diacetate (20.3 MPa).

The Young’s modulus, an important physical parameter for thin films, is listed in Figure 4b. It can be observed that the Young’s modulus of ALC-TiO₂-40 composite film is comparable to that of ALC-TiO₂-30 composite film, and dramatically superior to those of other samples. Therefore, the ALC-TiO₂ composite can possess the optimal rigidity when the dosage of TiOSO₄ is 40% in the acetylation reaction. Furthermore, this specific reagent dosage of TiOSO₄ is utilized for catalytically acetylating industrial corncob residue. The resulting IALC-TiO₂ composite film exhibits mechanical properties comparable to those of the ALC-TiO₂ composite using the LC as raw material. Apparently, the present approach to acetylize corncob residue demonstrates significant potential for further industrial application.

Nanoindentation was further used to analyze the mechanical properties of the composite film [49]. Figure 5 presents representative load–displacement curves for ALC-TiO₂ composites and commercial cellulose diacetate. Notably, the unloading curves do not coincide with the loading curves, indicating that ALC-TiO₂ composites exhibit not only elastic behavior but also plastic characteristics. It is worth noting that the values of indentation depth (h_r) and peak indentation depth (h_p) of ALC-TiO₂ composites initially decreased and subsequently increased with the rising amount of titanium oxide sulfate. Both the h_r and h_p values of all ALC-TiO₂ composites are significantly smaller than those of the commercial cellulose diacetate. Smaller h_r and h_p values indicate higher mechanical strength of the material [50].

Based on the abovementioned mechanical properties, the underlying reasons behind the superior mechanical properties of the ALC-TiO₂-40 composite film can be explained. The physical appearance of the ALC-TiO₂-40 composite film was further investigated, and the results are depicted in Figure 6a, where it is evident that the surface of the sample is exceptionally smooth and flat. Magnified SEM images show that the microscopic surface of the ALC-TiO₂-40 composite film is equally uniform and devoid of any defects. Further analysis of the internal morphology of the film reveals that, as illustrated in the cross-sectional view (Figure 6c), the ALC-TiO₂-40 composite film also exhibits a continuous homogeneous morphology without any defects. This morphological integrity provides evidence of excellent mechanical properties of the ALC-TiO₂-40 composite film.

From the perspective of composition, the exceptional mechanical properties of ALC-TiO₂ composite film are attributable to the concurrent presence of acetylated lignin, acetylated cellulose, and TiO₂, along with their synergistic interactions. ALC-H composite is synthesized by catalyzing the acetylation of LC using liquid sulfuric acid. In comparison to CDA, ALC-H contains additional acetylated lignin. As illustrated in Figure 4a, the mechanical results show that the tensile strength of ALC-H composite film is slightly higher than that of CDA film, and its strain at break also has obvious advantages. This suggests that the presence of acetylated lignin contributes positively to enhancing the toughness of acetylated cellulose. Further comparison between the mechanical properties of ALC-TiO₂-40 and ALC-H composite films clearly demonstrates that the superior tensile stress of ALC-TiO₂-40 composite film is related to the presence of TiO₂. This aligns with similar mechanisms observed in organic–inorganic composite materials where TiO₂ plays an essential role as an inorganic filler [51]. EDX mapping (Figure 6b,d) confirms that TiO₂ is uniformly distributed both throughout and on the surface of the ALC-TiO₂-40 composite, film without any indications of agglomeration. Upon further magnification of the cross-sections, as shown in Figure 6e,f, it becomes clear that acetylated components form a dense matrix with TiO₂.

Thus, as illustrated in mechanism schematic Figure 6g, it can be concluded that acetylated lignin, acetylated cellulose, and TiO₂ are uniformly distributed in the ALC-TiO₂-40 composite film, and they can be tightly bonded to each other without a phase-separation phenomenon and structural defects. These structural, morphological, and compositional characteristics contribute significantly to achieving favorable mechanical properties in the acetylated corncob residue–TiO₂ composite film.

2.5. Environmental Impacts

As an alternative to traditional petroleum-based polymers, the degradation of materials derived from biomass under natural conditions has persistently been a focal point of research. To this end, we assessed the degradability of the as-prepared composite film. Generally, cellulose acetate exhibits biodegradability but requires a complete degradation period ranging from 4 to 9 months [52]. As illustrated in Figure 7, the appearance of CDA remains largely unchanged after 60 days of natural degradation, which is consistent with previously reported studies. In contrast, the ALC-TiO₂-40 composite film begins to show cracks and signs of degradation after 60 days. These fissures become increasingly pronounced beyond the 60-day mark. It is evident that cracking initiates within the film and progressively extends toward its central region, resulting in a noticeable degree of bending on the surface. Further observations of the micro-morphology at the crack of ALC-TiO₂ reveal the presence of additional finer cracks (Figure 7e). Concurrently, the fractography (Figure 7f) indicates a significant occurrence of voids within ALC-TiO₂, which markedly contrasts with the dense structure observed in freshly prepared ALC-TiO₂, as mentioned above (Figure 6c). The FTIR analysis (Figure 7g) shows that, in comparison to freshly prepared ALC-TiO₂, the signal peak intensities at 1116 cm⁻¹ and 1460 cm⁻¹ in the ALC-TiO₂ after 60 days reduces dramatically. These two signal peaks are ascribed to the aromatic C-H in-plane deformation (S-units) and the C-H deformation of acetylated lignin, respectively [53,54]. Apparently, the degradation of acetylated lignin occurred during the 60-day experiment, resulting in the formation of cracks within the ALC-TiO₂. These findings indicate that the ALC-TiO₂ composites possess superior degradation performance compared to commercial cellulose diacetate.

3. Materials and Methods

3.1. Materials

Corncobs (Suyu 27) were sourced from Lianyungang City, Jiangsu Province, China. Industrial corncob residue was produced by Rizhao Force Biological Technology Co., Ltd. (Rizhao, China). Acetic acid (99.5%), potassium hydroxide (1 mol/L) and sodium hydroxide (0.5 mol/L) standard solutions, ethanol (99.8%), and acetic anhydride (98.5%) were supplied by Shanghai Testing Laboratory Equipment Co., Ltd. (Shanghai, China). Acetone (99.5%), toluene (99.8%), and sulfuric acid (98%) were procured from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Titanium oxysulfate was obtained from Aladdin Reagent (Shanghai, China).

3.2. Preparation of Corncob Residue Model Compound

Suyu 27 corncob was finely crushed into a powder and subsequently sieved through a 100-mesh screen. The corncob powder was then placed in a Soxhlet extractor equipped with a filter cartridge. Initially, the corncob was extracted using 150 mL of an ethanol–toluene solution with a volume ratio of ethanol to toluene of 1:0.427. The heating powder was set to ensure that siphoning was performed 4~6 times per hour. After 24 h, the treated sample was dried at 65 °C for 4 h and then crushed to obtain a solid powder. Subsequently, the powder was immersed in a 2% NaOH aqueous solution at a mass ratio of 1:30 (powder/2% NaOH), with continuous stirring at 90 °C for 1 h to facilitate the removal of hemicellulose. The resultants were collected by filtration and washed with deionized water until neutral pH value of elution solution was achieved. Followed by being dried at 65 °C for 12 h and grinding, the final powder was achieved to model the corncob residue, which was designated as LC.

3.3. Acetylation of Corncob Residue

The acetylation of corncob residue was carried out in a 250 mL three-necked flask. In all, 2 g of corncob residue and 16 mL of acetic acid were added to the flask, and the mixture was stirred at 45 °C for 2 h. Subsequently, 8 mL of acetic anhydride and varying amounts of TiOSO₄ (30, 40, 50, and 60 wt.% in LC) were introduced into the flask to start the acetylation reaction. After 2 h, a solution containing excessive magnesium acetate (5 wt.% in LC) and acetic acid (20 wt.% in LC) was promptly added to the flask to terminate the acetylation reaction. The resulting mixture was then poured into water at 90 °C to precipitate the acetylated species, which were washed with distilled water until the pH value of the rinse solution reached neutral. After being vacuum-dried at 105 °C, the acetylated corncob residue–TiO₂ composite material was obtained and named ALC-TiO₂-30, 40, 50, and 60.

For comparison, H₂SO₄ was utilized as the catalyst for the synthesis of an acetylated corncob residue, designated as ALC-H. All the experimental procedures were consistent with those described above, and the dosage of H₂SO₄ employed was 17 μL. Furthermore, industrial corncob residue (ILC) served as the raw material for acetylation. Prior to the reaction, ILC was washed with deionized water to eliminate weakly adsorbed impurities and subsequently dried at 100 °C for 12 h. The remaining experimental operations followed those outlined for ALC-TiO₂, as previously mentioned. The resulting acetylated sample was labeled IALC-TiO₂.

3.4. Characterization

The functional groups of the samples were analyzed by Fourier-transform infrared spectroscopy (FTIR) on a Nicolet 6700 spectrometer at a wavenumber range of 2000~400 cm⁻¹ with a resolution of 4 cm⁻¹. The crystal structures of the samples were characterized utilizing a Rigaku Ultima IV X-ray diffractometer (XRD, Rigaku, Akishima, Japan) at a scanning rate of 10°/min and a step size of 0.02°. The surface and cross-sectional morphological of the samples were analyzed by field emission scanning electron microscopy (FESEM) (Regulus 8100, Hitachi, Tokyo, Japan) at an accelerating voltage of 30 kV. The thermal properties and composition of the samples were investigated by thermogravimetric analysis (TGA) (STA2500, NETZSCH, Selb, Germany) under a nitrogen atmosphere, at a heating rate of 10 °C/min. The thermal characteristics of the samples were assessed utilizing a differential scanning calorimeter (DSC) (Model Photo-DSC 204 F1 Phoenix^® NETZSCH). Samples underwent heating from 30 °C to 150 °C at a rate of 10 °C/min, followed by cooling to 20 °C at a rate of 20 °C/min. Finally, the temperature was increased to 300 °C again at the same rate.

3.5. Preparation of Composite Film

To examine the mechanical properties, the abovementioned ALC-TiO₂ composites were prepared into films using a conventional casting method. First, 15.2 g of ALC-TiO₂ composite was thoroughly dispersed in a mixture solution of acetone and water (95:5, w/w) under vigorous magnetic stirring, followed by ultrasonic treatment for 30 min to eliminate bubbles. The resulting mixture was then cast onto a glass plate, utilizing a film scraper to ensure uniform surface thickness, and subsequently was placed in a vacuum oven at 40 °C for solvent evaporation. Finally, the ALC-TiO₂ composite film was obtained. Moreover, IALC-TiO₂ and ALC-H composite films were fabricated via the same operations.

3.6. Determination of Acetylation Degree

The acetylating ability of the reagent TiOSO₄ was evaluated by the residual hydroxyl value of the acetylated products. First, 9.75 g of LC was placed in a 250 mL three-necked flask with a condensing tube, and 14 mL of acetic anhydride and 2 mL of pyridine were precisely added. The mixture was heated to 80 °C and maintained for 1 h. After cooling to room temperature, both the condenser tube and the three-necked flask were rinsed with ethanol, and phenolphthalein was introduced into the system as an indicator. The resulting solution was titrated with a standard potassium hydroxide–ethanol solution. A blank test was conducted under identical conditions. The hydroxyl value (X) of various samples was calculated as follows:

$$\mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{l} \mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}\left(\mathrm{X}\right)={\displaystyle \frac{\left(V_1-V_2\right)\times N\times 56.1}{G}}+A$$

(1)

where V₁ (mL) represents the volume of titrated potassium hydroxide–ethanol solution, V₂ (mL) denotes the volume of potassium hydroxide–ethanol solution used in the blank test, N indicates the normal concentration of potassium hydroxide in ethanol solution, A signifies the acid value of the sample, and G (g) refers to the weight of the sample.

Acetylation degree of various products was calculated as follows:

$$\mathrm{Acetylation} \mathrm{degree}={\displaystyle \frac{{\mathrm{X}}_0-{\mathrm{X}}_{\mathrm{i}}}{{\mathrm{X}}_0}}\times 100\%$$

(2)

where X₀ is the hydroxyl value of the LC, and X_i is the hydroxyl value of the acetylated product.

3.7. Mechanical Property

Prior to the test, the ALC-TiO₂ composite film was cut into a rectangular sample measuring approximately 4 cm in length and 1 cm in width. The film thickness was accurately measured using a digital vernier caliper before testing. The tensile strength and strain of the films were tested using a universal testing machine (HK-308, Huakai Co., Ltd., Guangzhou, China) with a 500 N load cell at room temperature. The strain rate was set to 5 mm/min, and the measurements were repeated three times for each sample.

Furthermore, the mechanical properties of the films were analyzed by a nanoindentation instrument, using an iMicro nanoindenter (KLA, Milpitas, CA, USA). The experimental procedure was conducted under an indentation load of approximately 20 mN, with no fewer than six measurement points on each film.

3.8. Experiments of Environmental Impact

The films of the ALC-TiO₂-40 composite and commercial cellulose acetate (CDA) were cut into circular discs with a diameter of 5 cm. Then, the films were placed in a square experimental chamber (25 cm × 25 cm × 15 cm) filled with 2 kg of soil collected from Nanjing City, China (32°03′ N, 118°46′ E). Under simulated natural environmental conditions, the morphological changes in the films were recorded to evaluate their degradation performances.

4. Conclusions

In summary, the corncob residue was completely transformed into an acetylated composite through TiOSO₄ catalysis. The reaction conditions, including reagent (TiOSO₄) dosage and reaction time, were optimized based on the acetylation degree. Additionally, structural changes in the corncob residue before and after the reaction were systemically analyzed. It was demonstrated that both cellulose and lignin within corncob residue underwent successful acetylation, and TiO₂ generated from TiOSO₄ hydrolysis was uniformly dispersed throughout the acetylated corncob residue. The resulting composite film exhibited superior mechanical properties to those from commercial cellulose acetate and acetylated corncob residue by liquid sulfuric acid catalysis. Moreover, this composite has been shown to possess excellent degradation performance under natural conditions. These research findings provide valuable guidance for advancing novel utilization in agricultural solid wastes.