Preparation and Characterization of a Sulfonated Carbonbased Solid Acid Microspheric Material (SCSAM) and its use for the Esterification of Oleic Acid with Methanol

Research Article

Austin Chem Eng. 2016; 3(1): 1024.

# Preparation and Characterization of a Sulfonated Carbonbased Solid Acid Microspheric Material (SCSAM) and its use for the Esterification of Oleic Acid with Methanol

Honglei Zhang1,2, Xiang Luo1,2, Xinwei Li1,2, George Z Chen2,3, Feng He4* and Tao Wu1*

1Key Laboratory of Clean Energy Conversion Technologies, The University of Nottingham Ningbo China, P.R. China

2Department of Chemical and Environmental Engineering, Energy and Sustainability Research Division, The University of Nottingham, UK

3International Academy of Marine Economy and Technology, The University of Nottingham Ningbo China, P.R. China

4College of Biological and Environmental Engineering, Zhejiang University of Technology, China

*Corresponding author: Tao Wu, Key Laboratory of Clean Energy Conversion Technologies, The University of Nottingham Ningbo China, 199 Taikang East Road Ningbo, 315100, P.R. China

Feng He, College of Biological and Environmental Engineering, Zhejiang University of Technology, 18 ChaoWang Road, Hangzhou, Zhejiang 310032, P. R. China

Received: February 02, 2016; Accepted: February 19, 2016; Published: February 22, 2016

## Abstract

In this study, a sulfonated group (-SO3H) rich carbon-based solid acid microspheric material was prepared by hydrothermal method followed by sulfonation using glucose as the raw material. Such a green, non-corrosive, and renewable carbon material was used as a heterogeneous catalyst for the esterification of oleic acid with methanol for the production of biodiesel. The carbon microspheres were characterized systematically. It was found that the carbon microspheres prepared under the optimal reaction conditions exhibited smooth surfaces, uniform particle sizes and good dispersion. The sulfonated carbon-based solid acid microspheric materials showed high acidity and good catalytic activities for the esterification of oleic acid with methanol. The influence of reaction operating conditions on the performance of esterification was studied. The optimal esterification reaction conditions were found to be: methanol/oleic acid molar ratio 12:1, catalyst loading 0.25 g (0.05 mmol H+), reaction temperature 65 °C, reaction time 8 h and mechanical stirring rate 360 rpm. It was found that the catalyst demonstrated very good reusability although there was noticeable loss in acidity due to the leaching of active sites.

Keywords: Biodiesel; Carbon-based material; Esterification; Solid acid; Reusability

## Abbreviations

-SO3H: Sulfonated groups; CMM: Carbon-based Microspheric Material; SCSAM: Sulfonated Carbon-based Solid Acid Microspheric Material; FESEM: Field-Emission Scanning Electron Microscope; TEM: Transmission Electron Microscope; FTIR: Fourier Transform Infrared Spectroscopy; TGA: Thermo gravimetric Analysis; DSC: Differential Scanning Calorimeter; BET: Brunauer-Emett-Teller; FFAs: Free Fatty Acids; GC: Gas chromatography; WCO: Waste cooking oils; FID: Flame ionization detector; FAME: Free Acid Methyl Ester; rpm: Revolutions per Minute

## Introduction

The increasing demand for energy and depleting fossil fuel resources had led to the search for alternative fuels very imperative. Such alternative fuels must be technically feasible to be produced in large scale. They must also be economically competitive, environmental-friendly, and readily available. Biodiesel is a fuel consisting of alkyl esters derived from renewable lipid feedstock such as vegetable oil or animal fat [1]. Compared with regular diesel, biodiesel is biodegradable, the utilization of biodiesel as a fuel normally leads to lower CO2 and sulfur emissions and almost none particulate pollutants [2,3].

The most frequently used method for industrial scale biodiesel production is via transesterification reaction, which leads to a high yield of biodiesel in a very short time under the presence of base catalysts like NaOH and KOH [3]. But transesterification catalyzed by base catalysts requires raw oil to be of low Free Fatty Acids (FFAs) because the free fatty acids could react with the catalyst to produce soap, which makes the separation of product and unreacted raw oil very difficult.

However, biodiesel is not competitive compared with fossil fuels due to the high cost of raw material and cost of production [4]. One way to reduce the cost is to use cheap raw materials such as waste cooking oils (WCO) [5-7]. However, the use of WCO as the raw material for biodiesel manufacture is problematic due to its high free fatty acids (FFAs) content [8-10]. Therefore, the pretreatment of FFAs in WCO before transesterification is necessary, which is to convert the free fatty acids into methyl ester in the presence of homogeneous strong acid-catalysts, such as sulfuric acid and hydrochloric acid [11]. However, the use of liquid acids causes many problems, such as the difficulty in separating liquid acids from reaction medium, the formation of a large quantity of wastewater and the significant corrosion of equipment [12]. The use of heterogeneous catalysts is therefore preferred because it eliminates the need for the washing process and the catalysts can be easily recycled and reused. Other advantages of such include the simple downstream operations, better process economics and the yield of better quality biodiesel. Consequently, it is desirable to develop a highly active, inexpensive, green and reusable heterogeneous acid catalyst [13].

To date, many solid acid catalysts have been proposed to replace liquid acids for the esterification process, which include ion-exchange resin [7,10], acid zeolites [2,14], meso-structured silica functionalized with sulfonic groups, tungstated zirconia, sulfated zirconia, sulfonated polymers (Amberlyst-15), meso-porous materials [4,15- 17] etc. However, most solid-acid catalysts developed so far are expensive and involve complex synthetic procedures, which impede their commercialization in industrial scale production of biodiesel.

Recently, the synthesis of carbon-based solid acids and their application as heterogeneous catalysts in the production of biodiesel has attracted significant attention because carbon is a cheap and widely available material, and can be easily functionalized with functional groups [13,18-26]. Among them, SO3H-functionalized carbon microspheric material with regular spherical shapes and controllable sizes is attractive because of its metal-free, stable and recyclable nature. This catalyst has been used as a stable and highly active acid catalyst for various acid-catalyzed reactions such as esterification, hydrolysis and dehydration. To date, some methods have been developed for the synthesis of carbon microspheric materials. The hydrothermal method is commonly adopted for the preparation of carbon microspheric materials because of its simple steps and mild reaction conditions [27,28].

In this study, a sulfonated carbon-based solid acid microspheric material was prepared by using glucose as the raw material. This material was used as the catalyst in the esterification of oleic acid with methanol for the production of biodiesel. The structure and properties of the carbon material were characterized systematically. The catalytic performance (catalytic activity and reusability) of this carbon microspheric material was also investigated.

## Materials

Chemicals used in this study, methanol, oleic acid, glucose and concentrated sulfuric acid were purchased from Sigma–Aldrich (USA). All the solvents and reagents used were either of HPLC grade or AR grade. All chemicals used were Analytical Reagent (AR) grade and were used without further purification.

## Preparation of the SCSAM

The procedure for the preparation of sulfonated carbonbased solid acid microspheric material is listed in Figure 1. The hydrothermal carbonization of glucose started from about 9 g of glucose being dissolved in water (50 mL) to form a homogeneous aqueous glucose solution. It was then transferred to a stainless steel autoclave (80 mL capacity), in which the solution was heated up to 200 °C and kept isothermal for 4 hrs allow hydrothermal reactions to occur, which include polymerization reaction and carbonization reaction. Some aromatic compounds and oligosaccharides were formed in the polymerization step. In the carbonization step, carbon spheres covered by hydrophilic groups might arise from crosslinking induced by intermolecular dehydration of linear or branch like oligosaccharides. After the hydrothermal reactions, the reaction solution was cooled down naturally to room temperature. The black precipitate was collected by filtration, sequentially washed with boiling water, pure ethanol, and acetone to prepare Carbon-based Microspheric Material (CMM). The prepared sample was then dried in a vacuum at 60 °C for 24 h. About 1 g of the solid sample was then sulfonated by soaking in 60 mL concentrated sulfuric acid (98 %) at 200 °C under nitrogen atmosphere for 15 h. The sulfonated sample was collected by filtration, washed in succession with boiling water for 5-10 times, and then refluxed with toluene to remove unbonded polycyclic aromatic compounds. The filter cake was vacuumdried over night at 60 °C. The sulfonated carbon-based solid acid microspheric material was therefore prepared (Figure 1).