Elsevier

Renewable Energy

Volume 151, May 2020, Pages 1092-1101
Renewable Energy

Propyl-SO3H functionalized graphene oxide as multipurpose solid acid catalyst for biodiesel synthesis and acid-catalyzed esterification and acetalization reactions

https://doi.org/10.1016/j.renene.2019.11.108Get rights and content

Highlights

  • GO-PrSO3H acid catalyst was prepared using sulfonic acid groups as active acid sites.

  • Esterification and acetalization reactions were performed in presence of GO-PrSO3H.

  • A clean fuel i.e. biodiesel was produced via oleic acid esterification with methanol.

Abstract

A graphene based acid catalyst, GO-PrSO3H, was prepared through a simple two-step process. Surface modification with (3-mercaptopropyl) trimethoxysilane followed by oxidation of sulfide groups led to the production of sulfonic acid sites on graphene oxide nanosheets. The results of various physicochemical techniques approved the synthesis of desired catalyst. The amount of acid sites was measured via the acid-base treatment with triethylamine which exhibited 1.07 mmol/g H+ in the catalyst structure. Two kinds of acid-catalyzed reactions i.e. esterification and acetalization were adopted to evaluate the catalytic performance of prepared catalyst. More than 90% conversion was achieved for butyl acetate production in acetic acid esterification with n-butanol. Moreover, methyl oleate as one of the main components of biodiesel was produced with good yield over the prepared catalyst via oleic acid esterification with methanol. The 1H NMR technique was also conducted to characterize and determine the amount of produced methyl oleate. Finally, the benzaldehyde acetalization with ethylene glycol was performed which high conversion (92%) was obtained at 3 h. The catalyst reusability for both esterification and acetalization reactions demonstrated the catalyst stability after five reaction cycles.

Introduction

Esterification of carboxylic acids with alcohols has attracted much attention for industrial manufacturing valuable chemicals such as solvents, fragrances, polymers, biodiesel, etc. in recent years [1]. Because of the increase in energy consumption and considering the depletion of non-renewable fossil fuels in future, biodiesel has been found to be a good alternative in industry. Biodiesel is sustainable, renewable, clean, and sulfur-free fuel composed of fatty acid monoalkyl esters, mainly produced via esterification of fatty acids or trans-esterification of triglycerides available in edible or non-edible oils with short-chain primary alcohols using acid or base catalysts [[2], [3], [4], [5]]. However, it is important to esterificate free fatty acids such as oleic acid, linoleic acid, palmitic acid etc. in incompatible feedstocks prior to use base catalysts for transesterification reaction due to the soap formation. Hence, acid-catalysis is more appropriate for biodiesel production [6,7].

On the other hand, acid-catalyzed acetal formation is one of the most important steps in protecting carbonyl groups of aldehydes or ketones with various alcohols or diols in multistep organic syntheses for manufacturing fine chemicals [[8], [9], [10]]. Typically, mineral acids such as H2SO4, H3PO4, and p-toluene sulfonic acid are exploited as homogeneous catalysts to promote the productivity of esterification and acetalization reactions [1,11]. Despite the high efficiency of homogeneous catalytic processes, they suffer from some drawbacks such as difficult separation and recycling of catalyst, severe corrosion, and large production of waste arising from neutralization processes. In order to eliminate these problems for moving toward clean processes, insoluble solid acid catalysts are considered as suitable alternatives in organic transformations due to their facile separation, reusability, non-toxic property, and corrosion elimination as well as waste decrement [12,13].

Scientific researches on designing catalysts with specific morphological and textural properties has grown dramatically over the last few decades. The structural properties mainly affect the catalytic activity which in turn is relevant to the number of active sites and their availability for reactant molecules [[14], [15], [16], [17]]. Solid carbon materials e.g. activated carbon, carbon nanotube, fullerene, and graphene have been used as efficient supports to immobilize catalytically active components which consequently leads to promote the selective formation of desired products [[18], [19], [20], [21], [22]]. Graphene oxide (GO) containing a lot of oxygen functional groups has been utilized to prepare graphene based acid catalysts. To date, the most utilized reagents for sulfonation of the graphene nanosheets are sulfuric acid [[23], [24], [25], [26], [27], [28], [29], [30]], fuming sulfuric acid [31,32], chlorosulfonic acid [27,[33], [34], [35], [36], [37], [38]], and diazonium salt of sulfanilic acid [34,[39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52]]. There are some reports on using graphene based solid acid catalysts in acid-catalyzed reactions. For example, Zhang et al. [23] prepared GO supported dual sulfonic/carboxylic acids to catalyze oleic acid esterification with methanol for biodiesel synthesis. In another work conducted by Liu et al. [31], sulfated graphene was hydrothermally synthesized by using fuming sulfuric acid and exploited as acid catalyst for acetic acid esterification with cyclohexanol and 1-butanol, Pechmann reaction of resorcinol with ethyl acetoacetate and hydration of propylene oxide. Moreover, the use of chlorosulfonic acid as sulfonation reagent was reported by Wei et al. [36] to synthesis sulfonated graphene oxide as a solid acid catalyst in hydrolysis of cellobiose, hydrolysis of isoflavone glycoside, and acetic acid esterification with ethanol. Oger et al. [47] and Zhang et al. [48] prepared graphene supported aryl sulfonic acid with using diazonium salt of sulfanilic acid for acid-catalyzed acetalization of carbonyl compounds with glycerol and additive esterification of carboxylic acids with olefins, respectively.

Despite the several use of graphene based acid catalysts in various organic reactions, the use of immobilized propyl sulfonic acid on the surface of graphene oxide (GO-PrSO3H) is only limited to catalyze the synthesis of star-shape phenolic compounds [53], and bisphenolic antioxidants [27]. To best of our knowledge, there is no any report on utilizing GO-PrSO3H acid catalyst in the esterification and acetalization reactions. In this research, we aim to prepare and fully characterize propyl sulfonic acid functionalized graphene oxide and examine its catalytic activity in acetic acid esterification with n-butanol, biodiesel synthesis through the oleic acid esterification with methanol, and benzaldehyde acetalization with ethylene glycol. This solid acid catalyst exhibited good catalytic performance compared with some other solid acid catalysts reported earlier.

Section snippets

Synthesis of propyl sulfonic acid functionalized graphene oxide (GO-PrSO3H)

Graphene oxide (GO) was synthesized with modified hummers method [54]. Surface silylation of GO was done by reacting excess amount of (3-mercaptopropyl) trimethoxysilane (5 mmol, 1 g) with 1 g of ultrasonically dispersed GO in dry toluene (30 ml). The mixture was refluxed for 24 h under N2 atmosphere. The obtained GO-PrSH was isolated by centrifugation, soxhlet washed with chloroform, and dried in vacuum oven at 70 °C overnight. In order to convert the –SH groups to –SO3H acidic sites, the

Preparation and characterization of GO-PrSO3H

Taking into account the fact that the surface of GO is rich in hydroxyl groups, it is known as a suitable support for incorporating various active species via surface post modification. The procedure used for preparing the GO based acid catalyst was shown in Fig. 1. At first, (3-mercaptopropyl) trimethoxysilane was selected as silylating reagent containing thiol group. The reaction was performed in dry toluene to prevent the possible side reaction of coupling the silyl groups together by

Conclusion

In summary, introducing propyl sulfonic acid groups as active acidic sites on the surface of graphene oxide (GO) produced a graphene based acid catalyst (GO-PrSO3H) with high surface acidity. Graphene oxide was selected as a proper solid support due to its high surface area (249 m2/g) and bearing several hydroxyl functional groups appropriate for surface post modification. The existence of covalently bound propyl-SO3H groups on the surface of GO was corroborated by FT-IR spectroscopy and EDX

Author contribution statement

Majid Masteri-Farahani: Supervision and corresponding author.

Mahdiyeh-Sadat Hosseini: Investigation and writing.

Newsha Forouzeshfar: Investigation and data collection.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge financial support from the Iran National Science Foundation (INSF) [Grant No. 96006456].

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