Polymeric carbon material from waste sulfuric acid of alkylation and its application in biodiesel production

Highlights

• A novel process of the waste sulfuric acid of alkylation at below 473.15 K is proposed.

• The removal rate of organic matter in waste acid was higher than 99.5 wt % at 453.15K.

• Acid soluble oil was polymerized to form a carbon material with many acidic sites.

• The carbon material is an efficient solid acid catalyst to produce biodiesel.

• The conversion rate of oleic acid remained higher than 91 wt % after reuse for 5 times.

Abstract

The huge waste sulfuric acid from isobutane/butene alkylation, which contains acid soluble oil and can lead environment pollution and waste of resources, is difficult to deal with. For the treatment of alkylation waste sulfuric acid, the process convert waste sulfuric acid into dilute sulfuric acid and polymerized carbon materials through direct polymerization of acid soluble oil at below 473.15 K is proposed. Compared with the traditional pyrolysis process at 1373.15 K, the technology developed in this study can save more energy efficient with reduced carbon emissions. It was found that the removal rate of organic matter in waste acid was higher than 99.5 wt % at 453.15K. The dilute sulfuric acid obtained from the reaction can be used in the synthesis of sulphate. The polymeric carbon material has strong acidity. Further study indicates that the polymeric carbon material can be used as an efficient solid acid catalyst for the esterification of oleic acid and methanol to produce biodiesel. The results showed that conversion of oleic acid could reach 95.03 wt % under the optimized conditions, which remained higher than 91 wt % after reuse for 5 times. This work provides an alternative method to treat waste sulfuric acid and also a feasible and low cost route to obtain polymeric carbon material from the organic resources in the sulfuric acid as solid acid catalyst, which can reduce the manufacturing cost of biodiesel.

Introduction

Alkylation of isobutane with butene using concentrated sulfuric acid is one of the important processes for the production of gasoline components with high octane number (Wang, H. et al., 2017). At the end of the reaction, the waste sulfuric acid is a kind of viscous liquid with black red color, generally referred as “waste sulfuric acid of alkylation” (WSAA). WSAA is a mixture containing about 90 wt % sulfuric acid and 6–7 wt % of acid soluble oil (ASO) and some water. The ASO consists of sulfuric acid esters and cyclic polyolefinic hydrocarbons having 2–4 double bonds, which show a wide range of boiling points (Berenblyum et al., 2002; Simon miron, 1962). In 2016, the capacity of China's sulfuric acid alkylation plants was more than 15 × 10⁶ t, and the annual amount of WSAA was over 10⁶ t, of which more than 60,000 t was ASO. It is costly for industry plants to regenerate the WSAA by high temperature incineration and the contained ASO is usually oxidized into CO₂ and H₂O. Regeneration of WSAA using electrochemical method has been reported (Albright, 2009), and another method is to treat the used acid propylene to form isopropyl sulfates, which was then extracted by isobutene (Chen, 1996). However, these methods can't be industrialized because of low efficiency and high cost. It's necessary to develop alternative technologies to treat WSAA with relatively low cost and obtain some value-added products.

On the other hand, the increasingly serious environmental problems caused by the burning of fossil fuels motivate the imminent development of biofuels (Ragauskas et al., 2006; Savage, 2011). Biodiesel is considered as a typical “green energy” and an alternative source because of its sustainable, biodegradable and non-toxic characteristics (Lai Fatt Chuah, 2017; Lin et al., 2011). In the green chemical industry, solid acid catalysts (SAC) are widely used because of their low corrosion, easy recovery and reusability. Synthesis of biodiesel using SAC as a catalyst can use both high acid value used oils and low grade non-refined oil as the feedstock (Wang, Y.-T. et al., 2017). At present, a series of SACs have been developed for the green synthesis of biodiesel (Bala et al., 2017; Lau et al., 2016) which include acidic oxide, heteropolyacid catalysts, sulfated zirconia based catalysts, organically-functionalized acid or sulfonated carbon materials (Martínez et al., 2018; Su and Guo, 2014). The sulfonated carbon materials (Liu et al., 2013) are a new kind of SAC, which shows good performance and attracts a lot of attention in recent years since they can be produced from renewable materials like sugar (Toda et al., 2005), rice husk (Zeng et al., 2016), cyclodextrin (Fu et al., 2015), biomass (Zhang et al., 2015) or other wastes (Bennett et al., 2016; Nurfitri et al., 2013). However, the preparation of the sulfonated carbon materials uses sulfuric acid as the sulfonate agent, where a large amount of waste sulfuric acid is produced. Therefore, a green, simple and cost efficient process for the production of sulfonated carbon materials is attractive.

In this study, a simple and cost effective process for the treatment of WSAA was proposed. This process was implemented by simply heating the WSAA at appropriate temperatures so that most of the ASO could be converted into polymeric carbon material (PCM), a kind of sulfonated carbon material which could be easily separated from sulfuric acid. The residual sulfuric acid can then be used to produce sulphates, such as ammonium sulfate, and the PCM can be used as SAC with relatively high acidity that could be applied to catalyze biodiesel production. This study provides a novel idea for the effective utilization of the waste materials in the energy production industry so that the amounts of waste chemicals could be significantly reduced.