Application of Sulfated Tin (IV) Oxide Solid Superacid Catalyst to Partial Coupling Reaction of α-Pinene to Produce Less Viscous High-Density Fuel

Abstract

Brønsted acid-catalyzed reactions of α-pinene have been studied because of their ability to produce various types of fragrance molecules. Beyond this application, dimeric hydrocarbon products produced from coupling reactions of α-pinene have been suggested as renewable high-density fuel molecules. In this context, this paper presents the application of a sulfated tin(IV) oxide catalyst for the partial coupling reaction of α-pinene from turpentine. Brønsted acid sites inherent in this solid superacid catalyst calcined at 550 °C successfully catalyzed the reaction, giving the largest yield of dimeric products (49.6%) at 120 °C over a reaction time of 4 h. Given that the low-temperature viscosity of the mentioned dimeric products is too high for their use as a fuel in transportation engines, lowering the viscosity is an important avenue of study. Therefore, our partial coupling reaction of α-pinene provides a possible solution as a considerable amount of the isomers of α-pinene still remained after the reaction, which reduces the low-temperature viscosity. On the basis of a comparison of the reaction products, a plausible mechanism for the reaction involving coinstantaneous isomerization and coupling reaction of α-pinene was elucidated.

1. Introduction

Turpentine, one of the most widely produced plant-derived secondary metabolites, is a mixture of monoterpenes. It is made up mainly of α-pinene and its isomers, such as β-pinene and camphene. Depending on the production method, it is categorized as gum turpentine (produced from oleoresins of conifers), wood turpentine (produced from aged pine stumps), sulfate turpentine (produced by the kraft pulping process), and sulfite turpentine (produced by the sulfite pulping process) [1]. Concerns about fossil fuel depletion and environmental destruction urge us to develop alternative energy resources. In this regard, turpentine, in which C10 hydrocarbons form major components, has been reported as a potential candidate for providing biofuel blends for fueling both spark ignition engines and compression ignition engines [2,3,4,5].

A coupling reaction by which C20 hydrocarbons can be synthesized from renewable α-pinene has been devised for ramjet propulsion [6,7,8], but not for conventional jet fuels. This is because the relatively high carbon number of these compared to that of petroleum-derived fuels tremendously increases their low-temperature viscosity, which limits the suitability of using dimeric products alone for transport fuel [9]. Thus, the partial coupling of monoterpene hydrocarbons was suggested as one possible solution [10]. This agrees with the results reported by Harvey et al., who suggested blending dimeric products with monoterpene hydrocarbons such as α-pinene, thereby resolving the viscosity problem of dimeric products [6].

Coupling of monoterpene hydrocarbons has been studied using various types of heterogeneous acid catalysts, such as Nafion, Nafion SAC-13, montmorillonite K10, Al-incorporated MCM-41, Pd-Al-incorporated MCM-41, phosphotungstic acid supported on MCM-41, phosphotungstic acid supported on SiO₂, Zeolite Hβ, and silica-alumina aerosol [6,7,8,11,12,13,14]. Given that the isomerization of monoterpene hydrocarbons is usually attributed to Brønsted acid activity and that Brønsted acids can also assist in their coupling reactions undertaken in harsh conditions, it has been thought that the catalytic activities of the above catalysts are caused by Brønsted acid sites and the role of Lewis acid sites is less significant.

Herein, we used sulfated tin(IV) oxide (SO₄²⁻/SnO₂) as a solid superacid catalyst to carry out partial coupling of α-pinene for the production of less viscous high-density fuel molecules. SO₄²⁻/SnO₂ catalysts have been used in various types of organic reactions, such as dehydration of sorbitol and xylose to isosorbide and furfural, respectively, esterification of free fatty acids, the Penchmann condensation reaction, and deprotection of silyl ether groups [15,16,17,18,19]. Successive chemical precipitation and immersion in diluted sulfuric acid yielded this catalyst from tin chloride pentahydrate in a facile procedure. In this study, the catalyst successfully furnished dimeric products from α-pinene with its isomers in solventless conditions. On the basis of the results, a plausible mechanism for the isomerization and coupling reaction, in which Brønsted acid catalysis plays a central role, was also suggested. To our knowledge, no attempt has been made so far to propose a mechanism considering both reactions together.

2. Materials and Methods

2.1. Catalyst Preparation

To prepare the catalysts, tin oxide powder obtained from tin (IV) chloride pentahydrate (SnCl₄·5H₂O) by chemical precipitation, followed by drying, was used as a precursor. Briefly, a transparent 0.1 M tin chloride solution was prepared by dissolving SnCl₄·5H₂O in distilled water. To hydrolyze the tin chloride complex, a 28 wt% aqueous ammonia solution was added dropwise under vigorous stirring. The addition was stopped when the pH of the solution reached approx. 8. After a precipitate was separated from clear supernatant liquid, thorough washing was carried out with a 4 wt% ammonium acetate solution by centrifugation. The white product was then dried in an oven at 105 °C for 24 h and ground into a fine powder. The prepared tin oxide (SnO₂) powder (10 g) was placed in a round flask containing 3 g of sulfuric acid diluted with 20 mL of distilled water. After sufficient stirring at 80 °C, water was removed in vacuo and sulfuric acid-treated SnO₂ was dried and stored in an oven at 65 °C. This precatalyst was dried further at 120 °C for 12 h, followed by calcination at 450, 550, 600, and 650 °C for 4 h.

2.2. Catalyst Characterization

To understand the properties of the prepared catalysts and elucidate their catalytic activity, we conducted X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA), and temperature-programmed desorption of ammonia (NH₃-TPD). The XRD patterns of the catalysts were collected to compare the crystal structures. The surface morphology of the catalysts was investigated using FE-SEM. Energy dispersive X-ray spectroscopy (EDS) and elemental mapping were also performed as part of the SEM investigation. To study the thermal properties of the catalysts, TGA was carried out. NH₃-TPD was performed to characterize the improvement in the catalyst acidity. The detailed analysis conditions are described in the Supplementary Materials.

2.3. Catalytic Tests

In a typical experiment, turpentine (5 g), tridecane (2 g, GC internal standard), and catalysts (0.1 g) were added without any solvent to a 50 mL glass flask equipped with a magnetic Teflon-coated stirrer and a reflux condenser. The reactor was then loaded on a preheated aluminum heating block and stirred vigorously. Upon completion of the reaction, the reactor was removed from the heating block and immediately cooled to room temperature using a cold-water bath. After cooling, the crude reaction mixture was diluted with n-hexane (100 mL) and filtered over a Celite pad. To estimate the product composition by the internal standard method, a diluted reaction mixture was analyzed using a gas chromatograph (7890B) equipped with a DB-5ms column (30 m × 250 μm, 0.25 μm thickness), a mass spectrometer detector (5977A), and a flame ionization detector (Agilent Technologies, Santa Clara, CA, USA). The yield of the products and the conversion rate of α-pinene were calculated using the following equations:

$$\mathrm{Yield}\mathrm{of}\mathrm{product}i(\%)=\frac{\mathrm{Weight}\mathrm{of}\mathrm{product}i}{\mathrm{Weight}\mathrm{of}\mathrm{crude}\mathrm{reaction}\mathrm{mixture}}\text{×}100,$$

(1)

$$\mathrm{Conversion}\mathrm{rate}\mathrm{of}\mathsf{\alpha }-\mathrm{pinene}(\%)=\frac{\mathrm{Consumed}\mathrm{weight}\mathrm{of}\mathsf{\alpha }-\mathrm{pinene}}{\mathrm{Initial}\mathrm{weight}\mathrm{of}\mathsf{\alpha }-\mathrm{pinene}}\times 100.$$

(2)

3. Results and Discussion

3.1. Catalyst Characteristics

3.1.1. Catalyst Surface Morphology

To investigate the surface morphological properties of intact tin oxide (SnO₂) and sulfated tin oxide (SO₄²⁻/SnO₂), field emission-scanning electron microscopy (FE-SEM) observation was carried out. The SEM images in Figure 1a,b show that intact SnO₂ has a rough surface, whereas SO₄²⁻/SnO₂ exhibits a smooth one because, although both surfaces consist of globular nanoparticles, the size of the nanoparticles in SO₄²⁻/SnO₂ was much smaller than that of the nanoparticles in intact SnO₂. Energy dispersive spectroscopy (EDS) analysis confirmed the presence of sulfur-containing groups on the surface of SO₄²⁻/SnO₂ (Figure S1a–d). In addition, elemental mapping images clearly show a uniform distribution of sulfur atoms on the surface (Figure 1c and Figure S1e).

3.1.2. Catalyst Crystal Structure

The crystal structures of tin oxide (SnO₂) and sulfated tin oxide (SO₄²⁻/SnO₂) calcined at 550 °C was characterized using powder X-ray diffraction, as shown in Figure 2. There were obvious differences in the diffraction peaks for the species. The diffractogram of intact SnO₂ shows a good match to the ICDD powder diffraction file of cassiterite SnO₂ (PDF 00-041-1445), which means that it has a tetragonal crystal structure similar to that of cassiterite SnO₂ (P42/mnm space group). Major diffraction peaks observed at 2θ = 27°, 34°, and 52° could be indexed to the (110), (101), and (211) planes of cassiterite SnO₂, respectively. Immersion in diluted sulfuric acid before calcination significantly influenced the crystal structure. Even though the characteristic diffraction peaks of cassiterite SnO₂ were detected in the diffractogram of SO₄²⁻/SnO₂, the intensities of the peaks decreased significantly and their breadth broadened considerably. Because this weakening and broadening denotes diminished crystallinity and crystallite size [16,20], Figure 2 suggests an amorphous structure and small-sized crystallites of SO₄²⁻/SnO₂. In accordance with the results presented in previous studies related to sulfated metal oxides such as SnO₂ and ZrO₂, sulfate groups on the surface of sulfuric acid immersed SnO₂ seem to hamper both aggregation and crystallization themselves during calcination [17,21,22]. This result also coincides with the FE-SEM images shown in Figure 1, which shows the difference in the size of globular nanoparticles making up the surfaces.

3.1.3. Catalyst Thermostability

The catalyst was designed to be applied to the coupling of α-pinene under somewhat harsh reaction conditions (≥100 °C). Our query was whether the catalytic activity over sulfated tin oxide (SO₄²⁻/SnO₂) could be sustained during the reaction. Because sulfate groups introduced on the surface of the SO₄²⁻/SnO₂ support by immersion in diluted sulfuric acid are responsible for the catalytic activity [23], the thermal decomposition behavior of sulfate groups was investigated by thermogravimetric analysis (TGA). Figure S2 shows the TGA graph of intact SnO₂ and SO₄²⁻/SnO₂ calcined at 550 °C. SO₄²⁻/SnO₂ displayed two distinguishable weight loss sections, whereas intact SnO₂ presented gradual weight reduction throughout the temperature range. The weight reduction of intact SnO₂ and the first weight loss of SO₄²⁻/SnO₂ from 120 to 550 °C can be explained by the desorption of chemisorbed water molecules and dehydroxylation on the surface of the SnO₂ support [20,22]. The difference between the extents of the weight reduction experienced by them may be attributed to the differences in the surface functional groups, especially sulfate groups, on which water can interact [23]. In addition, significant weight loss was observed at temperatures higher than 600 °C in the case of SO₄²⁻/SnO₂, which is attributed to the decomposition of sulfate groups on the surface of the SnO₂ support [20,22].

3.1.4. Catalyst Acidity

To qualitatively evaluate the acidities of the intact tin oxide (SnO₂) and sulfated tin oxide (SO₄²⁻/SnO₂) catalysts, we carried out temperature programmed desorption of ammonia (NH₃-TPD). The TPD profiles of desorbed ammonia clearly show an improvement in the acid strength of the catalysts when SnO₂ was immersed in diluted sulfuric acid before calcination (Figure 3a). Peak deconvolution can help us interpret the meaning of the overlapped peaks. As can be seen in Figure 3b, the profile of intact SnO₂ calcined at 550 °C consisted mainly of two peaks centered at approx. 372 °C (dark yellow) and 513 °C (dark cyan), which are attributed to weak and strong chemisorption of ammonia. On the other hand, the SO₄²⁻/SnO₂ calcined at 550 °C had four kinds of acid sites which present four peaks centered at approx. 240 °C (magenta), 353 °C (blue), 531 °C (purple), and 736 °C (orange) (Figure 3c). In particular, the strongest acid sites were attributed to the desorption of ammonia from sulfate groups on the surface of SO₄²⁻/SnO₂ [24]. This interpretation coincided with the TGA results for SO₄²⁻/SnO₂ shown in Figure S2, which displays a significant weight loss at temperatures higher than 600 °C due to the decomposition of sulfate groups on the surface. In addition to the obvious differences in the peak number and overall peak intensities in the TPD profiles, the amount of NH₃-uptake by the catalysts also suggests that immersion in diluted sulfuric acid generates much more acid sites than those developed in intact SnO₂ (

Table 1. NH₃ uptake of intact SnO₂ and SO₄²⁻/SnO₂ calcined at different temperatures.

Catalyst	Intact SnO2 1	SO42−/SnO2
		450 °C 2	550 °C	600 °C	650 °C
NH3 uptake (μmol g−1)	171.1 3	1161.8	760.2	400.1	425.0

¹ Intact SnO₂ was calcined at 550 °C. ² Calcination temperature. ³ Calculated based on the NH₃–TPD results.

).

The effect of the calcination temperature on the acidity of the sulfated catalyst was evaluated based on the amount of NH₃ uptake (Table 1). Generally, increasing the calcination temperature reduces the total acid sites of sulfated or phosphated solid acid catalysts [25,26]. Our results coincide with those reported. However, one difference was found above the 600 °C calcination temperature, with a new peak centered at approx. 696 °C appearing and the disappearance of the peak centered at approx. 736 °C in the TPD profiles (Figure 3d).

3.2. Catalytic Tests and Reaction Mechanism

3.2.1. Effect of Catalyst Calcination Temperature on Partial Coupling Reaction of α-Pinene

When it comes to preparing a sulfated metal oxide catalyst using sulfuric acid treatment, a sintering process is important because the promotion of sulfate groups on the surface of metal oxide occurs during calcination and the acid sites generated by these sulfate groups offer the essential catalytic activity [23]. Figure 4 shows stark differences in the effect of the calcination temperature on the catalytic activity of sulfated tin oxide (SO₄²⁻/SnO₂). The yield of dimeric products from α-pinene (1) was the largest for the catalyst calcined at 550 °C (48.9 ± 1.2%), while almost no conversion to dimeric products and significantly lowered production were observed below and above this temperature, respectively. When we tried to correlate the yield of dimeric products with the amount of the total acid sites of catalysts, unlike previous papers [25,26], there was a large discrepancy in the calcination temperature range from 450 to 550 °C (Table 1 and Figure 4). Although the catalyst calcined at 450 °C showed the highest NH₃ uptake, it was not able to furnish dimeric products at all. Since a large amount of sulfuric acid can hinder the growth of SnO₂ crystals (Figure 2), thereby leading to poor promotion of sulfate groups on the surface of SO₄²⁻/SnO₂, the negligible catalytic activity of catalyst calcined at 450 °C (α-pinene conversion rate <20%) was attributed to sulfuric acid remaining in large quantities during the calcination process. Zhang et al. reported that increasing the calcination temperature from 150 to 550 °C made sulfate groups of SO₄²⁻/CeO₂ transition from surface sulfate states to bulk sulfate states; the catalyst mainly possessing sulfate groups as surface sulfate states worked well in catalytic reduction of NO by NH₃. According to the Raman spectra in which only the catalyst calcined at 550 °C presented the peaks denoting bulk sulfate states, this transition seems to occur abruptly when the calcination temperature increased from 450 °C to 550 °C [27]. In this respect, the discrepancy between the yield of dimeric products and the amount of the total acid sites of catalysts can be understood by considering that the catalyst possessing sulfate groups as bulk sulfate states is effective in the coupling reaction of α-pinene. On the other hand, calcination temperatures higher than 550 °C caused significant decomposition of sulfuric acid and even sulfate groups on the surface of SO₄²⁻/SnO₂ (Figure S2). Therefore, when the same amount of sulfuric acid was treated, a much higher calcination temperature made the catalyst lose sulfate groups. In this regard, the less effective catalytic activity of catalysts calcined at 600 and 650 °C can be understood. Finally, to compare the effect of the promotion of sulfate groups on the catalytic activity, we conducted the reaction with intact SnO₂ calcined at 550 °C, which showed negligible α-pinene conversion (data not shown).

In contrast to dimeric products, catalysts prepared at higher calcination temperatures (600 and 650 °C) more readily furnished camphene (6) from α-pinene (1) and the catalyst calcined at 550 °C also yielded a considerable amount of compound 6 (Figure 4). In other words, the conversion to compound 6 competed with the production of dimeric products from compound 1, and once formed, compound 6 was thought to be indifferent to the homocoupling reaction with these catalysts. This is supported by the fact that several catalysts lack the ability to catalyze homocoupling of compound 6 to form dimeric hydrocarbon products [11].

3.2.2. Effect of Reaction Time and Temperature on Partial Coupling Reaction of α-Pinene

To investigate the effects of reaction time and temperature on the yield of dimeric products, we selected the sulfated tin oxide (SO₄²⁻/SnO₂) catalyst calcined at 550 °C based on the results shown in Figure 4. As one can see in Figure 5, the conversion of α-pinene (1) was almost 100 ± 0% only after 30 min except for the reaction at 100 °C. This consumption furnished dimeric products almost entirely at the beginning of the reaction (Figure 5a). The additional increase in the yield continued up to 3 h at all reaction temperatures. A quite interesting feature was that after 3 h, there was no significant difference among the yields of dimeric products at 100, 110, and 130 °C, although the increase in the yield of dimeric products during the reaction was the highest at 100 °C (from 13.6 ± 3.2% to 45.0 ± 0.7%). A significant difference after 3 h was only observed at 120 °C, which produced the highest yield of all (48.9 ± 1.2%). However, when we carried out the reaction further in these conditions, there was a poor improvement (from 48.9 ± 1.2% to 49.6 ± 0.7%). In other words, only around half of compound 1 yielded dimeric products and therefore our catalyst did not seem suitable for the coupling reaction. However, considering the low-temperature viscosity of dimeric products [9], this partial coupling will be even more appropriate for fuel applications [10].

Figure 5b shows that the yields of camphene (6) decreased depending on the reaction temperature and time. Given compound 6 seems to be less reactive to homocoupling with our catalyst, its decrease during the reaction time implies the possibility of heterocoupling of compound 6 with other species, even including dimeric products. Because such heterocoupling can consume dimeric products and furnish oligomeric products, reaction temperatures higher than 120 °C produced a lower yield of dimeric products.

In order to investigate why the yields of both dimeric products and camphene did not show major changes after 3 h, we conducted a reusable test of the catalyst to prove whether or not the deactivation of the catalyst occurred during the reaction. Unfortunately, the used catalyst did not furnish dimeric products (data not shown). We thought that this was due to the sludge generated by the condensation of various substrates covering the acid sites of the catalyst. An interesting result was that thorough washing with acetone also cannot restore the activity of the used catalyst, but rather eliminated the activity. This is because the acetone washing removed not only the sludge but also the sulfate groups of bulk sulfate states [27]. As mentioned before, sulfate groups as bulk sulfate states of catalysts seem to have key role in the coupling of α-pinene (1).

3.2.3. Mechanism of Isomerization of α-Pinene over Sulfated Tin Oxide

Finally, the mechanism of isomerization and coupling reaction of α-pinene (1) over the sulfated tin oxide (SO₄²⁻/SnO₂) catalyst was considered. The generally accepted mechanism of acid-catalyzed isomerization of compound 1 includes two distinct pathways [28]: ring enlargement rearrangement (path A), wherein tri- or bicyclic compounds are produced; and ring opening rearrangement (path B), in which monocyclic compounds are formed (Scheme 1). Both pathways are initiated by protonation of olefin in compound 1 generating pinanyl cation (2). If the highly strained 4-membered ring in carbocation 2 suffers from ring enlargement by Wagner–Meerwein rearrangement, bornanyl cation (3) is formed. This carbocation 3 can produce tricyclene (4) via direct deprotonation or camphene (6) via deprotonation of isocamphanyl cation (5) resulting from 1,2-sigmatropic rearrangement of carbocation 3. Alternatively, carbocation 2 can be stabilized by the opening of the four-membered ring, which results in p-mentha-1-en-8-yl cation (7). In light of the similarity in structure and stability between carbocation 7 and p-mentha-1-en-4-yl cation (8), the 1,2-hydride shift between them is considered reversible. These carbocations (7 and 8) can be deprotonated to furnish monocyclic compounds (9–12).

In the above mechanism, the selectivities of camphene (6) from path A and limonene (9) from path B are of interest, and it is undeniable that the selectivities depend on what catalyst is used. Because compound 9 can further isomerize to other monocyclic compounds, catalysts showing the higher (but not by much) selectivity for compound 6 compared to that for compound 9 have been reported more commonly. Kitano et al. reported isomerization of α-pinene (1) over an Al₂O₃-supported MoO₃ catalyst, which presented a slightly higher selectivity toward compound 6 than for compound 9, although the conversion of compound 1 was not good enough [29]. SiO₂- or MCM-41-supported H₃PW₁₂O₄₀ and MSU-S BEA or Y catalysts showed over 90% conversion of compound 1 and a slightly higher selectivity for compound 6 [12,30,31]. Furthermore, very powerful catalysts such as Fe³⁺-exchanged clinoptilolite, sulfated zirconia, SBA-15 supported TiO₂, and sulfuric acid-treated montmorillonite clay have been suggested for the production of compound 6 with prominent selectivity [28,32,33,34,35]. However, catalysts that showed the higher selectivity for compound 9 compared to compound 6 have been reported much less frequently. Yamamoto et al. developed a SiO₂-supported Pr₂O₃ catalyst, which showed very high selectivity for compound 9 although the conversion of compound 1 was notably low [36]. In addition to this catalyst, a SiO₂-supported AlCl₃ catalyst showed higher (but not by much) selectivity for compound 9 with the varying conversion of compound 1. In this study, SO₄²⁻/SnO₂ showed much higher selectivity for compound 6 than for compound 9 with 100% conversion of compound 1 (Figure 5b and Figure S3). This tendency can be justified by the difference between compounds 6 and 9 in reactivity for further isomerization as mentioned previously. It has also been reported that Al-MCM-41 lacks the ability to catalyze homocoupling of compound 6 formed by the isomerization of compound 1 [11]. Given the significant amount of compound 6 still remaining after the coupling reaction was over, not only further isomerization but also homocoupling of compound 6 seems to be difficult with our catalyst, as mentioned in the previous section.

In some papers, isoterpinolene (15) has been also suggested as a co-product. One plausible mechanism for the formation of compound 15 starts from protonation of terpinolene (10), resulting in p-mentha-4(8)-en-2-yl cation (13) [28]. Not only can direct deprotonation of carbocation 13 generate compound 15, but also deprotonation of allylic carbocation I (14) resulting from carbocation 13 via 1,2-hydride shift can do the same. However, this suggestion has been controversial considering various isoterpinolene/terpinolene concentration ratios, either higher or lower than 1 [37]. A second possible mechanism is initiated by protonation of α-terpinene (11) or γ-terpinene (12), which gives allylic carbocation III (18) and p-mentha-3-en-1-yl cation (19). This proposal was supported by Salacinski’s results which showed the chemical equilibria of p-menthadiene species under sulfuric acid at 67 °C [38]. When compound 11 or 12 reacted under this condition as a sole starting material, the chemical equilibrium consisted of only compounds 11, 12, 15, and a small amount of p-mentha-3,8-diene (20), where stabilization by the formation of a conjugated diene was considered the driving force. In addition, the author described reaction coordinate diagrams with allylic carbocations (14 and 18) as reaction intermediates. The presence of compound 20 in our results, even though the quantity of it was relatively small, also seems to indicate this mechanism. Additionally, the successive transformation of p-mentha-1-en-8-yl cation (7) into allylic carbocation II (16) and p-mentha-2-en-8-yl cation (14) via 1,3- and 1,5-hydride shifts, respectively, was suggested as the other possible route for the formation of compound 15 [39]. Behr and Wintzer also reported that compound 15 was formed as a major side product when the hydroaminomethylation of compound 9 was carried out with a [Rh(cod)Cl]₂/TPPTS catalyst [40], which means that compound 9 can be a linchpin when it comes to the production of compound 15. As seen in Figure S3, the yield of compound 15 seems to follow the same trend as that of 9 along the reaction time. This also suggests that the third suggested pathway chiefly occurs during the isomerization of α-pinene (1) in the case of our catalyst.

In addition to the above-isomerized products, p-cymene (21) and p-menthene isomers (22a–c) were detected in the reaction mixture. The simultaneous formation of compounds 21 and 22a–c can be explained by disproportionation between α-terpinene (11) and γ-terpinene (12) [6,11]. Moreover, dehydrogenation of compounds 11 and 12 was suggested to justify the production of compound 21 with the generation of hydrogen gas [41,42]. Given the para position of methyl and isopropyl groups therein, compound 21 was generally reported as the target product not only when α-pinene (1) was used in neat form [41,43] or as a major constituent of crude sulfate turpentine [44], but also when limonene (9) was used as a sole starting material [42,45]. The concentration of compound 21 in the reaction mixture gradually increased with reaction time (Figure S3). This is because it did not participate in further reactions, including both the isomerization and coupling reaction in our catalytic system [7,12].

3.2.4. Mechanism of Coupling of α-Pinene over Sulfated Tin Oxide

The lack of knowledge about the molecular structure of dimeric products obtained from monoterpene is attributed to the simultaneous homo- and heterocoupling that occurs for the starting materials and the isomers therein [6,11,12]. Furthermore, it being hard to isolate only one dimeric product from a product mixture, the study of their molecular structure has proven difficult. These phenomena were also observed in our results; although the reaction started with α-pinene (1) as a sole substance, besides isomerization, a variety of dimeric products were concurrently produced (Figure S4). Nevertheless, some reports have suggested several possible molecular structures without an understanding of the complicated reaction system [7,11,46].

Acid-catalyzed coupling reactions of monoterpene hydrocarbons generally involve three steps: first, the protonation of olefin in monoterpene giving carbocations; next, the attack of olefin (nucleophile) in another monoterpene on the previous carbocation (electrophile) furnishing a dimeric carbocation with the formation of a new C–C bond between the nucleophile and the electrophile; and, finally, the deprotonation of this carbocation giving dimeric products. Of course, dimeric carbocations formed by nucleophilic attack on monomeric carbocations, or even by the re-protonation of dimeric products, can suffer from isomerization and further coupling reactions, which is one reason for the complexity of the reaction mechanism. In addition to coupling reactions that involve protonation/deprotonation, the Diels–Alder reaction between monoterpenes, especially α-terpinene (11), has also been suggested as a possible mechanism for the coupling reaction of monoterpenes [47].

In light of the results obtained for a reaction at a relatively low temperature (100 °C, Figure S3a), although the concentration of α-pinene (1) in the reaction mixture precipitously decreased with reaction time, considering almost all of the dimeric products were yielded in just 30 min, compound dimeric structure (PD1). We can also imagine that further isomerization of this possible dimeric product gives, for example, PD1a and PD1b via ring enlargement and ring opening, respectively.

The isomers of α-pinene (1), such as camphene (6), limonene (9), terpinolene (10), α-terpinene (11), γ-terpinene (12), and isoterpinolene (15), and p-mentha-3,8-diene (20), all of which have olefin in their structure, can partake in coupling reactions as compound 1 does. As electrophiles, stable allylic carbocations (14, 16, and 18) derived from these monocyclic isomers were thought to play a pivotal role in the coupling reaction [38,39]. This type of coupling can occur from the beginning to the end of a reaction, particularly in a predominant coupling reaction that occurs after compound 1 is completely consumed. In Scheme 2, we suggest some possible dimeric structures (PD2–4), showing that the isomers react as nucleophiles and electrophiles (carbocations).

Although we are not sure whether camphene (6) reacts as a nucleophile or electrophile (as isocamphanyl cation (5)) during the coupling reaction, it is clear that compound 6 predominantly participates in a heterocoupling (Scheme 2, PD4) rather than a homocoupling reaction considering that a significant amount of compound 6 still remained after the reaction. This is in agreement with the results obtained using Al-incorporated MCM-41 [11]. Meylemans et al. asserted that this phenomenon is attributed to the low basicity of compound 6, which causes poor interactions between the external olefin and the acid sites of the catalyst, thereby making the protonation of compound 6 difficult [7].

4. Conclusions

In summary, a sulfated tin(IV) oxide catalyst prepared using a facile procedure was applied to the partial coupling reaction of α-pinene to furnish a renewable and less viscous high-density fuel. To evaluate the catalytic activity of the catalyst, we considered the effect of the calcination temperature, reaction time, and reaction temperature, and attempted to rationalize the results using the catalyst characteristics. The catalyst calcination temperature had an enormous influence on the production of dimeric hydrocarbon products, while reaction times and temperatures exceeding 1 h and 100 °C affected the reaction to a lesser extent. The highest yield of dimeric products (49.6%) was obtained when the catalyst was calcined at 550 °C and the reaction was carried out at 120 °C for 4 h. Although the yield was less than half, we think this value is enough to consider utilizing the reaction products as renewable fuels, because it is not known that the dimeric products alone have a low-temperature viscosity too high for use as a fuel in transportation engines. In other words, the mixture with the isomers of α-pinene can drag down the low-temperature viscosity to the range of transportation fuels. Finally, we described the possible mechanism of the coinstantaneous isomerization and coupling reaction of α-pinene owing to our catalyst acting as a Brønsted acid.