Synthesis porous carbon-based solid acid from rice husk for esterification of fatty acids

https://doi.org/10.1016/j.micromeso.2015.07.028Get rights and content

Highlights

  • A novel solid acid was prepared from rice husk.

  • The solid acid shows ultra high surface area of 1233 m2/g.

  • Its acid strength is stronger than that of HZSM-5 zeolite.

  • The solid acid exhibits excellent activity and stability for esterification.

Abstract

A porous carbon solid acid was synthesized from biomass rice husk by incompletely carbonization, sodium hydroxide leaching and concentrated H2SO4 sulfonation. The solid acid was characterized by XRD, FT-IR, N2 adsorption-desorption and solid-state NMR spectroscopy. The characterization results reveal that the carbon solid acid shows ultra high surface area of 1233 m2/g and stronger acid strength than that of HZSM-5(Si/Al = 38) zeolite. The catalytic performance was tested by the esterification of oleic acid with methanol. The results indicate that this solid acid catalyst is an excellent catalyst compared with other conventional solid acid.

Introduction

Currently industrial esterification processes are carried out by the catalysis of homogeneous Brønsted acids such as sulfuric acid. However, these homogeneous acid catalysts are difficult to be separated, and also cause serious environmental and corrosion problems. Recently, the application of solid catalysts instead of homogeneous liquid catalysts has been paid much attention in view of their convenience of separation and lack of corrosion or toxicity problems [1], [2], [3], [4], [5].

Due to the low densities of effective acid sites, inorganic-oxide solid acids such as zeolite or composite oxide cannot satisfy adequate requirement in esterification reactions [6], [7]. Although strong acidic ion-exchange resins such as Nafion contain abundant sulfonic acid groups (–SO3H), that function as strong acid sites, their catalytic activities are generally much lower for their very low surface area [8]. These limitations have restricted the practical utility of acidic cation-exchangeable resins. Recently, a new type of sulfonated carbons derived from incomplete carbonization of simple natural product such as sugar, starch or cellulose, has been reported to show better catalytic performance for esterification of fatty acids, and higher stability than sulfonated mesoporous silica [5], [9], [10]. However, such materials were nonporous and exhibited low surface area, which may limit the accessibility to the active sites. The carbon solid acid with unique porous properties is synthesized here as an ideal candidate for the development of the high active catalyst.

Every year three million tons of rice husk (RH) are produced in China. So far, such a resource is mainly considered as a waste, and consequently burnt without any profit, except in a few cases of domestic uses for cooking and heating [11]. The main objective of the present work was to prepare carbon based solid acid from rice husk by incomplete carbonization, leaching and sulfonation. The solid acid catalyst was also characterized by X-ray diffraction (XRD), Fourier-transform infrared spectra (FT-IR), scan electron microscope (SEM) and solid state nuclear magnetic resonance (NMR) spectroscopy. The research results will be helpful to obtain the fundamental information of the roles of surface functional groups in the solid acid, which is crucial in the design of a novel carbon-based solid acid for industrial application.

Section snippets

Sample preparation

Rice husk from a grain depot in Wuhan was used as raw material. Rice husk was calcined at 450 °C for 15 h under a N2 flow, and then followed by grinding and leaching with 1 M NaOH at 100 °C K for 5 h at the ratio of solid to liquid of 1 g: 10 ml to remove silica in the rice husk. After carefully washing with deionized water, the dried rice husk carbon was sulfonated with concentrated H2SO4 (98wt%), at 150 °C under a N2 flow for 8 h at the ratio of solid to liquid of 1 g: 10 ml. At last, the

Results and discussion

Fig. 1 illustrates the XRD patterns of rice husk, rice husk carbon and the carbon solid acid. The diffraction peak arising at around 2θ = 25° is corresponded to the diffraction of C (002) [13]. Compared with that of rice husk, the peak of the impurities (2θ = 18°, silica) was disappeared in the spectrum of rice husk carbon and the solid acid. According to Dahn's conclusion [14], it indicates that the solid acid and rice husk carbon consist of a single layer of polyhexagonal carbon atoms after

Conclusions

In summary, a novel carbon solid acid was prepared from biomass rice husk by incompletely carbonization, sodium hydroxide leaching and concentrated H2SO4 sulfonation. The solid acid was characterized by XRD, FT-IR, N2 adsorption-desorption and solid-state NMR. The characterization results show that the carbon solid acid exhibits ultra high surface area of 1233 m2/g and stronger acid strength than that of HZSM-5 zeolite. The catalytic performance was tested by the esterification of oleic acid

Acknowledgments

We acknowledge the financial supports from the National Natural Science Foundation of China (21473126), the Fund of Hubei Provincial Department of Education (B2014094) and the Open Research Fund of Hubei Province Key Laboratory of Coal Conversion and New Carbon Material (WKDM2013010).

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