Ti-incorporated SBA-15 mesoporous silica as an efficient and robust Lewis solid acid catalyst for the production of high-quality biodiesel fuels

Highlights

• High-quality BDFs derived from various vegetable oils are efficiently synthesized over innovative Ti-SBA-15 catalysts.

• Catalytically active sites are mostly related to the tetrahedral Ti⁴⁺ species with weak Levis acid character.

• The hydrophilic silica framework and weak acidity of Ti-SBA-15 catalysts give excellent water and FFA tolerance levels.

Abstract

One-pot synthesized Ti-SBA-15 mesoporous materials with various Ti loadings of 0.808–6.78 mol% were applied as heterogeneous solid acid catalysts for simultaneous esterification and transesterification of vegetable oils with methanol into high-quality biodiesel fuel (BDF) at 200 °C under autogeneous pressure. According to the diffuse-reflectance (DR) UV–vis spectra, diffuse-reflectance infrared Fourier transform (DRIFT) spectra and pulsed ammonia (NH₃) chemisorption studies combined with other conventional characterizations, the catalytically active site for high-quality BDF synthesis was mostly related to the tetrahedral Ti⁴⁺ species with weak Lewis acid character, which differential heat of NH₃ adsorption was lower than 90 kJ mol⁻¹. Due to that the tetrahedral Ti⁴⁺ species were accessible on largely mesoporous framework, the Ti-SBA-15 catalyst gave much higher activity in transesterification of crude Jatropha oil (CJO) with methanol than microporous titanosilicate of TS-1 and commercial TiO₂ nanocrystallites. Among them, the 3Ti-SBA-15 catalyst with a Ti loading of 2.46 mol% showed a highest fatty acid methyl ester (FAME) content of 90 mass% at 200 °C for 3 h using a methanol-to-oil molar ratio of 27. When the reaction period and methanol-to-oil molar ratio were increased to 3–6 h and 108, respectively, a great variety of edible and non-edible vegetable oils with various acid values (0.06–190 mg KOH g^−l), including refined soybean oil (RSO), refined rapeseed oil (RRO), waste cooking oil (WCO), crude palm oil (CPO), CJO and palm fatty acid distillates (PFAD), was directly transformed into high-quality BDFs, which met with a European standard (EN 14214:2009), over 3Ti-SBA-15 catalyst at 200 °C. The used 3Ti-SBA-15 catalyst was easily regenerated by calcination and its high activity was maintained. Most importantly, the 3Ti-SBA-15 catalysts could resist 5 wt% of water or 30 wt% of free fatty acid (FFA), which tolerance levels were several ten times better than those of homogeneous and heterogeneous catalysts in the current BDF production technology.

Introduction

BDF which mainly consists of FAME is considered as a sustainable bio-fuel for transportation sector due to lower emitting of green house gases, such as carbon dioxide and methane, and hazardous exhaust gases, such as sulfur oxides and nitrogen oxides, in comparison to burning petro-diesel [1]. Conventional BDF is synthesized by transesterification of high-grade vegetable oils with excess amounts of anhydrous methanol catalyzed by homogeneous alkali bases, such as NaOH, KOH or NaOCH₃, at mild reaction condition of around 60–80 °C and 1 bar [2], [3], [4], [5]. To synthesize high-quality BDF with diminishing waste water and catalysts, the French Institute of Petroleum (IFP) have commercialized several Esterfip-H plants in European union using a heterogeneous base of zinc aluminate mixed oxide under severe reaction condition of 170–230 °C and 10–60 bar [6], [7]. Due to that FFA and water easily cause saponification and catalyst deactivation, only high-grade vegetable oils, such as refined soybean, rapeseed, palm and coconut oils, can be fed in base-catalyzed transesterification, where the upper limits of FFA and water contents are 0.5 wt% and 0.3 wt%, respectively. Besides high production cost, recent reports argued that BDF made by edible vegetable oils might not be sustainable enough to be used in European Union [8], [9]. Furthermore, the increased demand for planting and irrigation may destroy tropical rainforest, namely the main carbon reducer in earth, and make a significant increase in water and nitrogen fertilizer use, yielding plenty of edible vegetable oils [10].

On purpose of reducing production cost, pressure on environment, and competition for food supply, recent studies have focused on efficient and economical way to transform non-edible vegetable oils, WCO and animal fats with high FFA and water contents into high-quality BDFs [2], [3], [4], [5], [6], [7], [11], [12], [13], [14], [15], [16], [17]. For example, Jatropha curcas can be grown on dry areas without careful irrigation and its seeds contain around 25–30 wt% of non-edible Jatropha oil with high FFA content up to 15 wt% [5]. It is generally agreed that Jatropha BDF has a little impact on food supply and environment. However, the removal of a large amount of FFA from CJO or other low-grade feedstock is essential for minimizing the formation of unwanted soap, which makes separation and purification a difficult task. The current way is to pre-esterify FFA of low-grade feedstock catalyzed by liquid acids, such as sulfuric acid or para-toluenesulfonic acid, before based-catalyzed transesterification [16]. Unfortunately, liquid acid and base catalysts are corrosive and difficult to be recycled. The production cost is greatly increased by the complicated processing steps and costly anticorrosive facilities. Large amounts of waste catalysts and water are formed, which have seriously polluted our environment. The life times of vehicle's engine and exhaust system are likely shortened by burning alkali- and acid-contaminated BDF products. Numerous reports have studied solid acid catalysts for simultaneous esterification and transesterification [18], [19], [20], [21]. However, the solid acid-catalyzed BDF production process is mostly carried out at severe condition using excess amounts of catalyst and methanol because of slow reaction rate. Macario and Giordano lately indicated that Amberlyst-15 acidic resin was active in esterification of FFA feed into BDF but it was quite poor in transesterification of triglycerides (TG) [18]. Albe-Rubio et al. further reported that the sulfonic acid-functionalized mesoporous silica was deactivated due to leaching of active phases and deposition of organic species during BDF synthesis [19]. To overcome these drawbacks, the development of innovative solid acid catalysts for simultaneous esterification and transterification of a great diversity of low-grade feedstock with high acid value at moderate condition has been an emergent and challenge field of research.

Ti-incorporated porous silica materials, particularly MFI- and MWW-type titanosilicates, have been industrially applied as green solid catalysts in clean synthesis of fine chemicals through selective oxidation, hydroxylation and ammoxidation [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. In contrast, only limited numbers of studies have dealt with BDF production technology because the Ti-based catalysts have been regarded to be poor in acid-catalyzed reactions [33], [34], [35], [36], [37]. Oku et al. showed that TS-1 and Ti-impregnated amorphous silica gave ca. 74–79 mol% of FAME yield in transesterification of triolein with subcritrical methanol at 200 °C and 60 bar [33], [34]. However, Siano et al. reported that only 59–64% of refined soybean oil were transesterified with methanol into FAME catalyzed by Ti-grafted amorphous silica at 180 °C under autogenous pressure [35], [36], [37]. In addition to serious leaching of Ti species, the FAME yields dropped ca. 27% and 20% by adding 5 wt% of FFA and 0.5 wt% of water to the reaction mixtures, respectively. There is still room for improving the activity and reusability of Ti-containing catalysts with high water and FFA tolerance levels in high-quality BDF synthesis. In a short communication, we lately demonstrated that the synthesis of high-quality BDF can be carried out over Ti-containing catalysts at 200 °C under autogeneous pressure [38]. Herein a detailed study on the application of mesoporous Ti-SBA-15 materials as weak Lewis solid acid catalysts in synthesis of high-quality BDFs derived from a great variety of non-edible and edible vegetable oils property was presented, in comparison to mesoporous silica of SBA-15, micrporous aluminasilicate of H-ZSM-5, microporous titanosilicate of TS-1 and commercial TiO₂ nanocrystallites as reference catalysts. The influences of structural property and acidic nature on the catalytic performance and the water and FFA tolerance levels were particularly reported. The reaction mechanism of high-quality BDF synthesis over Ti-SBA-15 catalysts was proposed.