Dual acidic titania carbocatalyst for cascade reaction of sugar to etherified fuel additives

Highlights

• HMF etherified fuel additives of different carbon lengths are synthesized with high yields.

• Furan rings retained in the etherified products with minimal ring-opening.

• A dual acidic carbocatalyst acts synergistically to achieve products with high selectivity.

• A cascade of conversion of fructose to etherified products is achieved.

Abstract

An inexpensive carbocatalyst containing Brønsted acidic sulfonic acid group and Lewis acidic Ti⁴⁺ is found to be effective for cascade conversion of C₆ sugar to 5-ethoxymethylfurfural (EMF) via sequential dehydration, and etherification reactions. HMF and fructose conversions at mild conditions achieved 91% and 64% EMF yields, respectively. The results indicate that the two acid sites interplay synergistically for high EMF yield and minimal ring-opened product ethyl levulinate (EL), another promising biofuel additive. Etherification of 2,5-bis(hydroxymethyl)furan (BHMF) with alcohols of varying carbon lengths formed alkoxymethylfurans (AMF) with high yields. The catalyst retained good activity upon recycling. The nature and strength of the acid sites are elucidated.

Introduction

The awareness of climate change and high market volatility of conventional fuels, owing to demand-supply imbalance, necessitates to the development of renewable alternatives and sustainable technologies to meet the growing energy demand for future generations [1]. The catalytic conversion of non-food biomass or cellulosic sugars into liquid fuels, e.g., 2,5-dimethylfuran (DMF) [2], 5-ethoxymethylfurtural (EMF) [3,4] and high carbon alkanes (diesel, gasoline, jet fuel) [[5], [6], [7]] has been reported. However, poor process economics is the major challenge towards commercialization. Thus, production of low oxygenated EMF and analogous alkoxymethylfurans (AMF) via cascade catalysis of cellulosic sugars or etherification of sugar derived 5-hydroxymethylfurfural (HMF) can be economically more competitive.

EMF has a similar energy density (8.7 kWhL⁻¹) as standard gasoline (8.8 kWhL⁻¹) and diesel (9.7 kWhL⁻¹) and has superior energy density than ethanol (energy density = 6.1 kWhL⁻¹) [8,9]. Its high boiling point (274 °C) is another advantage. Avantium has tested 20% EMF blended diesel in passenger vehicle engine. They found EMF can reduce SO_x emission, solid particulate contamination, and soot formation [10]. In addition, the engine ran smoothly for many hours using EMF blended fuel [11]. EMF can also be used as a flavor and aroma ingredient in wine and beers [12]. Homogeneous and heterogeneous catalytic systems [13] have been reported for the production of such fuel additives. Homogeneous catalysts, e.g., mineral acids, inorganic salts [14,15], ionic liquids [16,17], organic acids [18,19], have offered moderate to high catalytic performance. The major challenge in homogeneous catalysis is the separation of the catalysts for recycling.

Therefore, solid acid catalysts are promising from economic and environmental standpoint. Zeolites, sulfonated metal oxides, modified mesoporous silica and ion-exchange resins including Amberlyst-15 [[20], [21], [22], [23]] have been used for EMF synthesis. Antunes et al. have used sulfonated graphene oxide for the etherification of HMF, which produced a mixture of EMF (42%), ethyl levulinate (EL) (43%) (another high octane fuel additive) and EMF- diethyl acetal (9%) at 140 °C for 24 h [9]. The catalytic systems containing Lewis and Brønsted acid sites have been reported to be more selective for EMF synthesis. Thus, mesoporous silicas containing Lewis acidic Al³⁺ achieved 68% EMF yield from HMF at 140 °C for 5 h [24]. A significant amount of non-fuel by-products, e.g., ethyl 4-oxopentanoate and 1,1-diethoxy ethane (DE) were formed, which posed a separation challenge and products purification. A one-pot approach of macro-algae derived agar conversion to a mixture of EMF and EL (5:2) using a −SO₃H functionalized Dowex 50WX8 catalyst in the presence of CrCl₂ or 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) has been reported, [25]_, but poor yield (30%) was the main drawback.

Recently, we have reported the synthesis of a sulfonated carbocatalyst containing Brønsted acidic sulfonic acid group and Lewis acidic Ti⁴⁺ (hereto referred as Glu-TsOH-Ti) via a one-pot hydrothermal method [26]. This dual acidic carbocatalyst exhibited excellent catalytic performance for dehydration of glucose and xylose to HMF and furfural, respectively [26], consistent with another report in which a hydrothermally prepared sulfonated carbocatalyst has been reported to be performed better over commercial zeolites, niobic acid or Amberlyst-15 for cellulosic biofuels production [27]. Usually sulfonated carbonaceous materials are synthesized by two-step processes involving preparation of carbonaceous materials in the first step followed by incorporation of SO₃H group under harsh oxidation conditions using concentrated sulfuric acid [28]. To avoid harsh reaction conditions, we reported a one-pot method by heating a homogeneous mixture of glucose (Glu) and p-toluenesulfonic acid (TsOH) and titanium(IV) isopropoxide in a sealed autoclave at 180 °C for 24 h. The ratio of Brønsted to Lewis acid density in the carbocatalyst was 1.2 [26]. In addition, the acid sites were more accessible for cascade isomerization and dehydration of sugars [26]. Herein, we report that the Glu-TsOH-Ti catalyst is effective for the etherification of HMF and 2,5-bis(hydroxymethyl)furan (BHMF) to their corresponding etherified fuel additives.