Analysis of recycled poly (styrene-co-butadiene) sulfonation: a new approach in solid catalysts for biodiesel production
© Aguilar-Garnica et al.; licensee Springer. 2013
Received: 21 June 2013
Accepted: 14 September 2013
Published: 21 September 2013
The disposal of solid waste is a serious problem worldwide that is made worse in developing countries due to inadequate planning and unsustainable solid waste management. In Mexico, only 2% of total urban solid waste is recycled. One non-recyclable material is poly (styrene-co-butadiene), which is commonly used in consumer products (like components of appliances and toys), in the automotive industry (in instrument panels) and in food services (e.g. hot and cold drinking cups and glasses). In this paper, a lab-scale strategy is proposed for recycling poly (styrene-co-butadiene) waste by sulfonation with fuming sulfuric acid. Tests of the sulfonation strategy were carried out at various reaction conditions. The results show that 75°C and 2.5 h are the operating conditions that maximize the sulfonation level expressed as number of acid sites. The modified resin is tested as a heterogeneous catalyst in the first step (known as esterification) of biodiesel production from a mixture containing tallow fat and canola oil with 59% of free fatty acids. The preliminary results show that esterification can reach 91% conversion in the presence of the sulfonated polymeric catalyst compared with 67% conversion when the reaction is performed without catalyst.
KeywordsPoly (styrene-co-butadiene) Sulfonation Recycling Catalyst Biodiesel Esterification
The Guadalajara Metropolitan Zone (GMZ) is the second largest urban area in México. More than 4 million inhabitants in the GMZ generate approximately 0.508 kgperson-1day-1 of household solid waste. The major components of the household solid waste are putrescible elements (53%), different types of paper (10%) and plastics (9%). From all of this waste, only 2.2% is separated for reuse/recycling, whereas the rest is deposited in municipal landfills. Rigid plastics, including poly (styrene-co-butadiene), represent approximately 1% of the non-recyclable materials (Bernache-Pérez et al. 2001).
On the other hand, it is well known that the widespread use of fossil fuel reserves has increased the air pollution levels worldwide, affecting global climate. These reserves (including petroleum-based diesel or petrodiesel) are being rapidly depleted. Biodiesel has been proposed as a renewable, biodegradable, non-toxic and non-inflammable alternative to petrodiesel. Chemically, biodiesel is a mixture of alkyl esters that is traditionally produced in a process known as transesterification in which refined plant oils or animal fats (i.e., triglycerides) are mixed with alcohol and heated in the presence of an alkaline catalyst. The relatively high cost of oils and fats contribute 60-80% of the total biodiesel cost, making it non-competitive with petrodiesel (Wen et al. 2010). To address the cost issue, it has been proposed that biodiesel be produced from cheaper feedstock, such as waste cooking oils (Liang 2013), grease from grease traps or animal fats (Canoira et al. 2008) that are characterized by their high (>1%) amount of free fatty acids (FFAs). However, the application of the alkaline transesterification technology to transform the aforementioned raw materials into biodiesel is not recommended because the reaction between FFAs and the alkaline catalyst makes soap, thereby reducing biodiesel conversion and creating difficulties in separating and purifying the product (Marchetti et al. 2007). To avoid this problem, it has been proposed that a pretreatment step be introduced before the conventional transesterification process. This pretreatment stage is known as esterification and is usually catalyzed by sulfuric acid, with reaction yields over 95% (Canacki and Van Gerpen 2001). Solid-acid catalysts, such as Dowex monosphere 550A, Dowex upcore Mono A-625, Amberlyst-15, Amberlyst-16, Amberlyst-35, Dowex HCR-W2, mesoporous aluminosilicates, Amberlyst 131, Relite CFS, ZrO2-supported metal oxide and mesoporous organosilicas, have also been considered (Özbay et al. 2008, Carmo et al. 2009, Tesser et al. 2010, Morales et al. 2010, Kim et al. 2011). More recently layered bismuth carboxylates, has also been used in esterification of fatty acids (Rosa da Silva et al. 2013). Compared with sulfuric acid, solid-acid catalysts have lower reaction rates, but they are often preferred over sulfuric acid because they are easily separated from the product, prevent corrosion (Silva and Rodrigues 2006) and can be reused (Vieira Grossi et al. 2010).
In this paper, we present a lab-scale strategy for recycling poly (styrene-co-butadiene) waste. Although the strategy is conceived to mitigate the solid-waste disposal problem in the GMZ, it could be extended to any city that has a similar situation. In this strategy, the poly (styrene-co-butadiene) waste is sulfonated with fuming sulfuric acid. The sulfonation method proposed here has been previously studied (Inagaki et al. 1999; Inagaki & Kiuchi 2001) but, to the best of our knowledge, the conditions (temperature and time) that maximize the number of acid sites in the sulfonation are not reported yet. Therefore, the behavior of the poly (styrene-co-butadiene) waste sulfonation under time and temperature variations is analyzed in the present work. This analysis is conducted with an experimental design from which is possible to deduce a mathematical model that adequately describes the sulfonation process. The theoretical optimal conditions for the sulfonation experimental runs are obtained from this model and the resulting product sulfonated in these conditions is then used as a solid-acid catalyst to produce biodiesel in the esterification of feedstock with a high content of FFAs.
Materials and methods
Poly (styrene-co-butadiene) waste was collected in the form of disposable cups. Chloroform, sulfuric acid and potassium hydroxide were provided by Analytyka (México). Methanol and phenolphtalein were obtained from Karal (México). Finally, fuming sulfuric acid and potassium bromide were provided by JT Baker (USA).
Qualitative characterization of poly (styrene-co-butadiene) waste
Poly (styrene-co-butadiene) waste cups contain residual accumulations of carbonated drinks or natural/artificial juice. These cups are crushed to a particle size of approximately 0.60-0.80 cm2, washed in a detergent solution and then rinsed. The clean plastic pieces are dried to a constant weight and are qualitatively analyzed as follows to confirm the presence of butadiene. First, a 0.20 g aliquot of the polymer is dissolved in 2.5 mL of chloroform. The polymer is extracted from this solution with 10 mL of methanol to ensure that additives are not interfering with the characterization (Lacoste et al. 1996). The extract is dried and dissolved again in 2.5 mL of chloroform to generate two samples. A film obtained from the first sample is further analyzed with a Perkin-Elmer FT-IR spectrophotometer, whereas the second sample is added to bromine water.
The sulfonation of poly (styrene-co-butadiene) waste which is already clean and dry, is carried out with fuming sulfuric acid (10 mL/g plastic) as the sulfonation agent. Different combinations of temperature (30°C, 70°C, 110°C) and time (1.0 h, 3.0 h, 5.0 h) are considered. Once the reactions are finished, the products are washed with distilled water and then dried to a constant weight.
Sulfonation level determination
The sulfonation level of the sulfonated products is commonly known as number of acid sites and is expressed in terms of the number of milliequivalents of ~SO3H groups (m eq SO 3 H) per gram of sulfonated product. In this work, the sulfonation level was determined in a titration procedure with 0.1 N alkaline solution of potassium hydroxide using phenolphthalein as an indicator.
Optimization of the sulfonation process
It is well known that the chemical reaction yield is strongly affected by temperature and time. Nevertheless the influence of these factors on the sulfonation process of poly (styrene-co-butadiene) is not reported yet. To cover this lack of information it is proposed in this work to conduct an experimental design considering three levels for each factor and the number of acid sites as response variable. The main objective of this 3×3 experimental design is to verify if a combination of temperature and time could maximize the number of acid sites. Previous experiments on the sulfonation process showed that a very low number of acid sites is obtained when the reaction is carried out at 30°C and 1.0 h. As a consequence, these operating conditions are set as the lowest point in the proposed design. Additionally, the highest point (i.e., 110°C and 5.0 h) is selected because it has been previously reported (Inagaki & Watanabe2003; Inagaki and Noguchi 2003).
Characterization of the product obtained under optimal sulfonation conditions
Poly (styrene-co-butadiene) waste is sulfonated at the optimal conditions that maximize the number of acid sites to both quantitatively and qualitatively characterize the sulfonated product and to verify whether it can act as catalyst in esterification reactions. The results of the quantitative characterization are expressed not only in terms of the number of acid sites but also in terms of methanol and water absorption, which are calculated following the “tea bag” method (Hosseinzadeh 2011). The qualitative characterization is carried out by using a sample to form a potassium bromide pellet whose infrared spectrum is recorded using a Perkin-Elmer FT-IR spectrophotometer (Martins et al. 2003).
where A 0 is the initial content of FFAs for the feedstock and A f is the final content of FFAs of the fluid extracted from the bottom of the separatory funnel (biodiesel phase). The values for A 0 and A f are obtained as described in the American Oil Chemists’ Society Official Method Cd 3d-63.
Results and discussion
Results of the sulfonation experiments expressed in terms of the number of milliequivalents of ~SO 3 H groups per gram of sulfonated polymer
0.42 ± 0.01
4.30 ± 0.17
2.66 ± 0.15
1.23 ± 0.06
5.13 ± 0.21
2.40 ± 0.20
0.82 ± 0.03
3.83 ± 0.12
ANOVA for testing the significance of the individual regression coefficients
ANOVA for testing the significance of the proposed mathematical model
Source of variation
Sum of squares
Degrees of freedom
Conclusions and perspectives
Poly (styrene-co-butadiene) waste was sulfonated with fuming sulfuric acid under varying times and temperatures. The objective of these experimental runs was to apply a 3×3 experimental design to deduce a mathematical model that adequately represents the sulfonation data at 95% confidence. From this model is possible to derive the operating conditions (75°C and 2.5 h) maximizing the number of acid sites in the sulfonated polymer. Then, it was verified that the sulfonated polymer waste under these conditions was able to act as catalyst in the esterification of a synthetic mixture of tallow fat and canola oil with a high FFAs content. Thus, it was demonstrated that this type of rigid plastic waste can be treated and further applied in a process that produces biodiesel. However, there is another issue related to the proposed catalyst that remains to be examined: its stability (i.e., the catalytic activity when the catalyst is reused). We are currently studying this issue, along with the economic feasibility of the proposed catalyst which is crucial to scale up the strategy proposed here from lab-scale to pilot-plant scale. Another future research topic is a comparison between the activity of the proposed catalyst and the activity of other catalysts that are either reported in the literature or commercially available.
GMZ Guadalajara Metropolitan Zone.
FFAs Free Fatty Acids.
A O, A f Initial and final content of FFAs in the esterification experiments, respectively.
N.A.S Number of Acid Sites.
T, t Temperature and time, respectively.
S ST , S SM , S E Total sum of squares, sum of squares due to the model and sum of squares due to error, respectively.
y i , ŷ i Experimental data and estimated value, respectively.
F 0 Stastistic and significance level, respectively.
The authors gratefully acknowledge financial support from the Consejo Estatal de Ciencia y Tecnología de Jalisco (COECyTJAL) through the Project FOMIXJAL2009-05-122665. The authors also express their thanks to Quimikao for providing tallow fat samples.
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