Solid Acid Catalyst Based on Single-Layer α-Zirconium Phosphate Nanosheets for Biodiesel Production via Esterification

Abstract

In this study, a solid acid was prepared by the sulfonation of surface modified α-zirconium phosphate (ZrP) single-layer nanosheets (SO₃H@ZrP), and the prepared solid acid was investigated for the esterification of oleic acid with methanol to produce biodiesel. For comparison, liquid H₂SO₄ and commercial Amberlyst^® 15 catalyst were also evaluated for the same reaction under the same conditions. The experimental results showed that the SO₃H@ZrP solid acid catalyst has a superior catalytic efficiency for the esterification reaction, as well as excellent recyclability. The SO₃H@ZrP single-layer solid acid catalyst can be uniformly dispersed in the reaction media, but remains heterogeneous and thus can be easily separated and recycled.

1. Introduction

In the past decade, biofuels have emerged as an alternative energy source to fossil fuels to relieve the energy crisis and environmental pollution [1,2]. One of the most promising biofuels is biodiesel, which is usually derived from renewable feedstock, such as plant lipids, animal fats/oils, cooking oils/wastes, and inedible oils, etc. [2,3,4]. The most readily implemented and low-cost route for the synthesis of biodiesel is catalytic esterification and transesterification of free fatty acids (FFAs) [5]. Basic catalysts, including NaOH, KOH, NaOCH₃, and KOCH₃, are primarily employed [3]. However, these basic catalysts could easily react with fatty acids to form soap, which is unfavorable for the product separation. Likewise, acid catalysts, such as H₂SO₄, HCl, BF₃, and H₃PO₄, are adopted to promote the esterification of FFAs. However, corrosion resistant equipment is required because of the high acidity of the above acid catalysts. In addition, extra separation and neutralization steps generate a large quantity of wastewater. Therefore, the development of heterogeneous acid catalysts are highly desirable to both lower the equipment requirement and minimize the negative impact on the environment.

To date, numerous solid acid catalysts have been developed, including zeolite [6], heteropolyacids [7], ion exchange polymer resins [8,9,10], carbon-based solid acids [11], and zinc lanthanum mixed oxides [12]. These solid acid catalysts lead to a more facile process and a lower production cost for esterification reactions. However, the limited active sites of these heterogeneous solid acid catalysts prohibit their widespread application. In addition, their relatively low porosity is not favorable for the mass transfer of reactants by diffusion, which leads to a low reaction rate. As a result, the development of solid acid catalysts with a high concentration of acid sites that are easily accessible is highly desired.

α-Zirconium phosphate [ZrP, Zr(HPO₄)₂·H₂O] is a layered compound with a high ion exchange capacity, which can be exfoliated into single-layer nanosheets [13]. The crystallinity and size of ZrP can be controlled by varying the synthetic conditions [14,15,16]. Therefore, ZrP has been widely applied in constructing versatile functional materials, such as catalysts [17,18,19,20], polymer nanocomposites [21,22,23,24,25,26,27,28], and intercalation compounds [23,29,30,31], etc. The unique structure of ZrP allows it to be an excellent support for the anchoring of functional groups. Moreover, the exfoliated ZrP single-layer nanosheets can be readily and uniformly dispersed in a reaction system, which allows for a high accessibility of the functional groups.

In this work, the solid acid catalyst based on sulfonic acid functionalized ZrP single-layer nanosheets was applied as a catalyst for the esterification of oleic acid with methanol. The solid acid exhibited comparable catalytic activity to liquid H₂SO₄, and even higher activity than the commercial Amberlyst^® 15.

2. Results and Discussion

The procedures for the preparation of SO₃H@ZrP are illustrated in Figure 1. The pristine ZrP (Figure 2a) was exhibited as hexagonal crystal discs with an average lateral dimension of ca. 450 nm, and Figure 2b shows the size distribution of the discs. The micro-crystals were exfoliated by TBA to form individual single-layer nanosheets (Figure 2c) [29,32]. Because of the ultrasonication treatment, the single-layer nanosheets were broken down into smaller pieces, with an average lateral dimension of ca. 66 nm (Figure 2d). After grafting γ-propyl mercaptotrimethoxysilane onto the nanosheet surface and the subsequent oxidation to form SO₃H@ZrP, the ZrP host maintained a sheet structure (Figure 2e). The –SO₃H groups could interact with water molecules to form hydrogen bonds, which facilitated the re-stacking of SO₃H@ZrP nanosheets during drying (Figure 2e). Note that the morphology shown in Figure 2e only represents the state of SO₃H@ZrP nanosheets in the solid phase. When SO₃H@ZrP nanosheets are dispersed in the liquid reaction system (a mixture of oleic acid and methanol), one can clearly observe the Tyndall effect (Figure 2f), suggesting that SO₃H@ZrP nanosheets could be uniformly dispersed to form a homogeneous dispersion. Meanwhile, SO₃H@ZrP nanosheets can be easily separated from the dispersion via centrifugation, exhibiting advantages of both homogeneous and heterogeneous catalysts.

In order to verify the grafting of functional groups onto the ZrP sheet surface, FTIR characterization was performed, and the spectra are shown in Figure 3. After the reaction with γ-propyl mercaptotrimethoxysilane, the two peaks at 3512 and 3595 cm⁻¹ ascribed to the interlayer molecular water and the out-of-plane vibration of the P–OH groups of ZrP at 970 cm⁻¹ vanished [18,29]. Meanwhile, the characteristic absorption peak of –SH located at ca. 2558 cm^−1, and methyl groups located in the range of 2834–3020 cm⁻¹ could be observed. These changes confirmed the successful grafting of –SH groups onto ZrP nanosheets. After oxidation, the peak belonging to –SH disappeared. Instead, a new peak located at ca. 1195 cm⁻¹ assigned to S=O appeared, suggesting that the –SH groups were successfully oxidized into –SO₃H groups.

The esterification reaction results of oleic acid with methanol catalyzed by SO₃H@ZrP, H₂SO₄, and Amberlyst^® 15 are shown in Figure 4. The conversion increased steadily, and gradually slowed down and plateaued for each catalyzed reaction. Liquid H₂SO₄ exhibited the highest catalytic activity of virtually 100% oleic conversion. The catalytic activity of SO₃H@ZrP solid acid is slightly lower, which resulted in a ca. 89% conversion rate after 5 h of reaction. The slightly lower catalytic activity of SO₃H@ZrP in comparison to liquid H₂SO₄ is expected, as liquid H₂SO₄ exhibits the highest possible miscibility and accessibility, as well as a much higher mole amount of functional –SO₃H per unit mass, while in SO₃H@ZrP, the –SO₃H groups are supported on ZrP nanosheets. However, SO₃H@ZrP exhibited higher catalytic activity than the commercial Amberlyst^® 15 catalyst, one of the most popular commercial solid acid catalysts. This can be ascribed to the high density of the sulfonic acid groups on the surface of ZrP nanosheets, and their high dispersion and thus high accessibility in the reaction system.

While liquid H₂SO₄ exhibited the highest catalytic activity, its liquid nature prohibited effective separation after the completion of the reaction. This is a major issue for its commercial applications. In contrast, solid acid catalysts such as SO₃H@ZrP and Amberlyst^® 15 can be easily separated from the reaction system via simple centrifugation and reused for the next reaction.

The recyclability of SO₃H@ZrP solid acid in the esterification of oleic acid with methanol is shown in Figure 5. There is a loss of ca. 9% in the conversion rate after five cycles of reaction, and a loss of ca. 25% in the conversion rate after seven cycles. During catalyst recovery using centrifuge, a trace amount of catalyst was attached to the centrifuge tube, which did not participate in the next catalytic run. So the loss of catalytic activity is believed to be due to the leaching of catalyst during separation at a lab scale, which can be minimized on a large industry scale, rather than from the deactivation of the functional groups. Note that after each cycle of reaction, the SO₃H@ZrP solid acid catalyst was simply washed with CH₂Cl₂ to prepare for the next cycle, but no attempt to regenerate the catalyst was made.

3. Conclusions

In summary, the SO₃H@ZrP solid acid was prepared by the surface modification of ZrP single-layer nanosheets and was evaluated as a solid catalyst for the esterification of oleic acid with methanol. The SO₃H@ZrP solid acid catalyst exhibited higher catalytic activity than the commercial Amberlyst^® 15. An oleic acid conversion rate of 89% was achieved, and the solid acid also exhibited a high recyclability. As such, the SO₃H@ZrP solid acid based on single-layer ZrP nanosheets is a promising solid acid catalyst for acid catalyzed reactions.

4. Experimental

4.1. Materials

γ-Propyl mercaptotrimethoxysilane (MPTMS, 95%, Sigma-Aldrich, Saint Louis, MO, USA), hydrogen peroxide (H₂O₂, 30 wt%, Fisher Scientific, Hampton, VA, USA), zirconyl chloride octahydrate (ZrOCl₂·8H₂O, 98%, Sigma-Aldrich, Saint Louis, MO, USA), phosphoric acid (85%, Sigma-Aldrich, Saint Louis, MO, USA), tetra-n-butylammonium hydroxide (TBA, 0.10 M, Sigma-Aldrich, Saint Louis, MO, USA), dichloromethane (CH₂Cl₂, ≥99.5%, Sigma-Aldrich, Saint Louis, MO, USA), methanol (99%, Alfa Aesar, Haverhill, MA, USA), and oleic acid (OA, ≥99%, Sigma-Aldrich, Saint Louis, MO, USA) were used as received without further purification. Common chemicals including sulfonic acid (98 wt%), hydrochloric acid (37 wt%), ethanol, toluene, and acetone were obtained from commercial vendors and used as received. Amberlyst^® 15 was purchased from Sigma-Aldrich.

4.2. Preparation of ZrP Nanosheets

ZrP microcrystals were first synthesized by a hydrothermal reaction of ZrOCl₂·8H₂O with 6.0 M H₃PO₄ at 200 °C for 24 h in a sealed Teflon-lined pressure vessel according to a reported method [18]. Then, 0.10 M TBA (3.3 mL) was added to 0.10 g of the synthesized ZrP microcrystals dispersed in 6.7 mL H₂O, followed by ultrasonication in an ice bath for 15 min to exfoliate ZrP [29,32]. Subsequently, 0.10 M HCl (3.3 mL) was added into the above mixture to protonate the TBA exfoliated ZrP single-layer nanosheets [27]. The thickness of each ZrP single-layer nanosheet was circa 6.3 Å [33,34].

The protonated ZrP nanosheets were further washed with copious water to eliminate chloride ions, and the obtained gel-like ZrP nanosheets were then rinsed with acetone three times. Subsequently, the gel-like ZrP nanosheets dispersed in acetone were washed by toluene and centrifuged to exchange the solvent from acetone to toluene. The final ZrP nanosheets dispersed in toluene were ready for the immobilization of thiol groups.

4.3. Preparation and Characterization of Solid Acid

A sample of 75 mmol γ-propyl mercaptotrimethoxysilane was mixed with 0.90 g ZrP nanosheets dispersed in 300 mL toluene, and the mixture was refluxed at 90 °C for 24 h under N₂ atmosphere with constant stirring to obtain thiol group grafted ZrP nanosheets (SH@ZrP) [18]. The obtained SH@ZrP sample was washed with toluene, centrifuged, and dried. At the last step, 0.60 g SH@ZrP was dispersed in 24 mL methanol with the assistance of ultrasonication, followed by the addition of 11.0 mL H₂O₂ (30 wt%). The above mixture was aged for 24 h with constant stirring. After that, 10.0 mL HCl (37 wt%) was added to ensure complete oxidation and protonation of the thiol groups. The obtained sample was washed with abundant water and centrifuged until the supernatant was neutral. After being dried, sulfonic acid functionalized ZrP nanosheets (SO₃H@ZrP) were obtained.

The morphology of the samples was characterized by an FEI Helios NanoLab^TM 400 DualBeam^TM scanning electron microscope (SEM) (FEI Company, Hillsboro, OR, USA) and a JEOL 2010F field-emission transmission electron microscopy (TEM) (JEOL Ltd., Tokyo, Japan). The Fourier transform infrared spectroscopy (FTIR) (Analect LLC, Huntington, NY, USA) spectra of the samples were recorded on an Analect RFX-65A FTIR spectrophotometer.

4.4. Esterification of Oleic Acid with Methanol

Esterification of oleic acid was carried out under the catalysis of SO₃H@ZrP at 65 °C in a flask fitted with a water-cooled reflux condenser. For comparison, liquid H₂SO₄ and commercial Amberlyst^® 15 were also evaluated as alternative catalysts. The mole ratio of methanol to oleic acid was maintained at 9:1. All of the catalysts were used at the same mass concentration, 5.0% (w/w), with respect to oleic acid. The samples collected periodically were centrifuged to form two layers: the upper layer being water and methanol, and the bottom layer being methyl esters of oleic acid. The bottom layer was titrated to determine the amount of remaining oleic acid to calculate the rate of conversion. After each reaction, the SO₃H@ZrP catalyst was recycled by centrifugation and washed with CH₂Cl₂ for the next catalytic run.