Elsevier

Chemical Engineering Journal

Volume 361, 1 April 2019, Pages 450-460
Chemical Engineering Journal

Catalytic conversion of furfuryl alcohol to n-hexyl levulinate using modified dendritic fibrous nanosilica

https://doi.org/10.1016/j.cej.2018.12.102Get rights and content

Highlights

  • Bi-functional catalysts (Al/DFNS/Pr-SO3H) were prepared with different Si/Al molar ratio.

  • These catalysts were characterized by various methods and used for the synthesis of nHL.

  • The highest yield of nHL was achieved in the presence of 40Al/DFNS/Pr-SO3H catalyst.

  • 40Al/DFNS/Pr-SO3H catalyst was reused four times without notable decrease in catalytic activity.

Abstract

Dendritic fibrous nanosilica (DFNS) was used as an efficient support to prepare a series of Al/DFNS/Pr-SO3H catalysts with different Si/Al molar ratios. The textural properties, morphology, structural feathers, acidity, and heterogeneous nature of the catalysts were characterized. The Al/DFNS/Pr-SO3H catalysts were utilized in the synthesis of n-hexyl levulinate (nHL) from alcoholysis of furfuryl alcohol with n-hexanol. Among these catalysts, 40Al/DFNS/Pr-SO3H, a Lewis and Bronsted bi-functional solid acid catalyst, showed the highest catalytic performance and the reaction conditions were optimized in the presence of this catalyst. Under optimum reaction conditions, n-hexyl levulinate yield and furfuryl alcohol conversion were 93.5% and 99.9%, respectively. Also, reusability experiments affirmed that 40Al/DFNS/Pr-SO3H catalyst can be recycled and reused for four times with an insignificant decrease in yield of product.

Introduction

As noted in many articles written about fossil fuel resources, we are facing serious challenges over the next decades concerning the reduction of these non-renewable resources, increasing the global need for energy and environmental issues associated with global warming. Therefore, in recent years, a significant area of research has devoted to seek the new sustainable sources of energy and chemicals [1], [2], [3], [4], [5]. Based on the review of current research, biomass seems to be a good substitute for fossil fuel resources because this renewable source is not only cheap, abundant and accessible, but also dissolves the difficulty of agricultural waste use. Alternatively, biomass is unparalleled in providing the only concentrated source of fixed carbon which can produce liquid transport fuel and valuable chemicals with a minimum amount of environmental impacts [6], [7], [8], [9]. Among different high-value organic chemicals derived from biomass, special recent attention has been dedicated to levulinate esters owing to their many applications in industry, including production of diesel fuels, resins, cancer therapeutics, herbicides, fragrances, and so on [10], [11], [12], [13], [14], [15], [16], [17].

One of the potential routes for the production of levulinate esters from biomass resources is alcoholysis of cellulosic biomass-derived furfuryl alcohol (Fig. 1). This reaction was carried out in the presence of homogeneous or heterogeneous acid catalysts [18], [19]. However, most of the researchers prefer to use heterogeneous acid catalysts to avoid corrosion of the instruments, pollution problems and the troubles related to separation and recovery of the products [20], [21], [22]. Hence, in the last decade, a variety of heterogeneous acid catalysts, such as functionalized organosilica nanotubes (ArSO3H-Si(Et)Si-Ph-NTs) [23], heteropoly acids (i.e., H5AlW12O40) and POM-based ionic liquids (i.e., [MIMBS]5[AlW12O40]) [24], zeolites (i.e., Fe/USY, hierarchical H-ZSM-5) [25], [26], sulfated zirconia (SZ) [27], metal salts [28], SO4−2/TiO2 [29] Titanium exchanged heteropoly tungstophosphoric acid (TPA) and porous aluminosilicate (Al-TUD-1) have been reported for production of levulinate esters from furfuryl alcohol [30], [31]. Although, the efficiency of these catalysts has affirmed in furfuryl alcohol alcoholysis, most of them are plagued with arduous preparation procedures, high cost and difficult recovery. To improve these problems, some silica mesoporous materials have been investigated as effective supports to synthesize heterogeneous acid catalysts. Thus, many researchers have studied the catalytic activity of modified mesoporous silica for production of alkyl levulinates from furfuryl alcohol. For example, Enumula et al. used Al2O3-SBA-15 as a catalyst for this reaction. They produced n-butyl levulinate with 94% selectivity at 110 °C after 6 [32]. Sankar et al. synthesized n-butyl levulinate over SBA-16 supported tungstophosphoric acid catalyst with 97% selectivity at 110 °C after 3 h [33]. Appaturi et al. prepared mesoporous Ti-KIT-6 and used in the reaction of furfuryl alcohol and butanol. The selectivity of n-butyl levulinate was 100% at 110 °C after 5 h [34].

Among mesoporous silica, dendritic fibrous nanosilica possesses more unique properties relative to other popular silica supports. For instance, in comparison to SBA-15 and MCM-41, the active sites of DFNS are available from all directions due to its special fibrous morphology. This feature and its adjustable pore size and pore volume increase the loading of modifier molecules (e.g., metals, organic molecules, metal oxides, inorganic complexes and so on) onto the surface of silica support. Moreover, this environmentally friendly non-toxic silica has good mechanical, chemical, and thermal stability compered to known mesoporous materials [35], [36]. Regarding these good features, it seems that DFNS can be a good candidate to synthesize an efficient heterogeneous catalyst for production of alkyl levulinates.

Up to now, our group has applied various kinds of functionalized mesoporous silica in some biomass conversions, including production of n-butyl levulinate from levulinic acid, conversion of fructose to ethyl levulinate, dehydration of glucose to 5-hydroxymethylfurfural and so on [21], [37], [38], [39], [40]. According to the results of our previous works, we concluded that these reactions were effectively carried out in the presence of functionalized mesoporous silica. In this context, we prepared a novel bi-functional heterogeneous acid catalyst using DFNS as an efficient support and investigated the effect of the simultaneous presence of two acidic functions on the catalytic performance of this catalyst in the reaction of furfuryl alcohol and n-hexanol for the first time. For this purpose, surface modification was carried out in two steps: direct synthesis of Al/DFNS with different Si/Al molar ratios and synthesis of Al/DFNS/Pr-SO3H by a post-grafting method. Then this catalyst was applied in the reaction of furfuryl alcohol and n-hexanol and effect of different parameters on the yield of n-hexyl levulinate was assayed. Catalyst reusability was also investigated.

Section snippets

Materials

Furfuryl alcohol (98%), n-hexanol, toluene, 1-pentanol, cyclohexane, aluminiumisopropoxide and other chemicals (analytical grade) were acquired from Merck. Tetraethyl orthosilicate (TEOS, >98%) cetyltrimethylammonium bromide (CTAB, greater than99%) and urea were supplied by Dae-Jung, S. Korea. (3-Mercaptopropyl) trimethoxysilane was gained from Aldrich. n-Hexyl levulinate (nHL) as the standard for calibration curves was synthesized by esterification of levulinic acid with n-hexanol (synthesis

Catalyst characterization

In order to evaluate the properties of synthesized catalysts and the effects of modification on the nature of DFNS, the samples were investigated by following methods:

Conclusion

In this work, Al/DFNS/Pr-SO3H catalysts with different Si/Al molar ratios were prepared using dendritic fibrous nanosilica (DFNS) as an efficient support. Characterization of these catalysts demonstrated that the original framework of DFNS has not changed after modification. The results of Py-FTIR, TGA and titration methods revealed the high acidity of synthesized catalysts and showed that the modification approach was successfully performed due to special morphology of DFNS. The catalytic

Acknowledgment

We would like to thank Isfahan University of Technology (Research Council Grant) for the financial support of this work.

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