Introducción

The isomerisation reaction is the most feasible and useful α-pinene transformation, which leads to two different classes of valuable compounds, called monocyclic and polycyclic monoterpenes, whereby limonene and camphene are the major compounds respectively. Figure 1.9 reports the major products obtained during the isomerisation, discerning between polycyclic compounds: camphene, β-pinene, triclycene and

fenchene, and monocyclic compounds: limonene, terpinolene, α- and γ-terpinene and p- cymene.

Figure 1.9 – α-Pinene isomerisation reaction mechanism.

The liquid phase α-pinene isomerisation reaction, employing a heterogeneous catalyst, was first investigated by Gurvich et al. in 1915 [124]. From that time, a paramount number of catalysts and different reaction conditions have been investigated to govern the reaction mechanism and terpene rearrangements.

In 1972, Stanislaus and Yeddanapalli [125] studied the vapour phase catalytic isomerisation of α-pinene using alumina-based materials as solid acid catalysts. They were the first to prove that acid strength has a strong influence on product distribution, concluding that isomerisation of the bi- and tricyclic compounds (camphene family) occurred only over strong acid sites. Ohnishi et al. [126] observed comparable results and studied the possible correlation between acidity and catalytic activity/selectivity of NiSO4, SiO2-Al2O3, and ZnS (calcined at different temperatures) as heterogeneous

catalysts, with both Brønsted and Lewis acidic properties. They concluded that the isomerisation reaction does not occur in the presence of Lewis acids in an aprotic solvent, but starts with the irreversible protonation of both α- and β-pinenes, investigated separately, over Brønsted acid sites, which is the beginning of the isomerisation pathway. The industrial conversion of α-pinene occurs under mild conditions, employing acidified TiO2 treated with an acid solution of sulfuric acid, at 150 °C, which reaches 30%

conversion and revealed a zero order of reaction [127]. Severino et al [128, 129] tried to improve the catalytic performances of acidic titania by varying the amount of sulfuric acid and studying the effect of catalyst activation temperature on conversion and selectivity. Results revealed that catalyst activity followed a volcano-like trend as a function of the amount of acid, observing a maximum at 5%, whilst higher catalyst activation temperatures led to higher camphene selectivity at the expense of conversions. The

Camphene

Limonene Terpinolene -Terpinene p-Cymene

Polycyclics Monocyclics -Pinene  + -Pinene  + + +   -Terpinene Triclycene + Fenchene +

increased selectivity was attributed to increase of accessibility to catalyst pore structure. Findik et al. [130] studied the activity of 100 g of wood turpentine (containing 85% of α- pinene) with different heterogeneous solid acid catalysts (2.5 g) such as halloysite treated with acetic acid, HCl activated clay, activated TiO2 on SiO2 with NaOH and HCI,

activated carbon and clinoptilolite (natural zeolites). Their results showed that turpentine conversion to camphene is increased with reaction temperature, reaching the maximum at 155 °C, with clinoptilolite being the most active catalyst; whereas activated carbon and titanium-based catalysts treated with NaOH were inactive. To shed further insight on the greater performances of clinoptilolite catalysts, Allahverdiev et al. [131, 132] carried out the solventless isomerisation of α-pinene in autoclave (in N2 atmosphere)

with pressures ranging, 1-20 bar, employing 2 g of catalysts, activated at 520 °C prior analysis. It was found that reaction perform increases with pressure over the range 1-10 bar, above this value rates were independent on pressure; reaction kinetics was described as first order against α-pinene consumption, with activation energy being 80.9 kJ mol-1_{. It was proposed that upon α-pinene transformation, camphene slowly}

isomerises to tricyclene, and eventually reaches equilibrium concentration; in contrast limonene rapidly isomerises to other monocyclic products, via double bond migration.

The catalytic activity of zeolites towards α-pinene isomerisation was investigated by Lopez et al. [133] employing dealuminated mordenites (MOR), faujasite (FAU) and an amorphous 13% alumina aluminosilicate, at 120 °C in a batch reactor. Mordenite proved to be the most promising catalyst, giving a maximum of 54% selectivity to camphene calculated based on the sum of camphene and limonene yields (68%). Undefined by- products are also observed over microporous zeolites, with the degree of their formation proportional to pore diameters. Furthermore, α-pinene isomerisation rate was normalised to the number of Al3+ _{per unit cell (Al}3+ _{density) in order to compare activities}

obtained for different zeolites structures. A volcano-like curve was observed displaying a maximum of activity for about 1 and 6 Al3+ _{per unit cell, as calculated in the MOR and}

FAU structures, respectively. Mesoporous silicas for the isomerisation of α-pinene were firstly investigated by Yamamoto et al. [134] employing 50 mg of FMS-16 modified with Al. Pretreatment temperature dependence on catalytic activity was investigated showing a maximum at 400 °C, for which 77.8% conversion and 41% of selectivity towards camphene were obtained. Additionally, although the number of Lewis acid sites increased with the calcination temperature, camphene selectivity was reported being unchanged, suggesting that Lewis acid sites did not participate in the reaction. The role of Brønsted and Lewis acid sites is extremely controversial and still under debate. In 2002, Besun et al. [135] investigated in detail the effects of surface area, pore size

distribution and ratio Brønsted /Lewis on α-pinene isomerisation employing acid- activated montmorillonite (clay). As a result, they concluded that pure strong Brønsted acid catalysts with high pore volumes and mesopore diameters (> 2 nm), with low dealumination degrees, are good catalysts for the production of camphene. Interestingly, for materials that showed both types of acidity, via pyridine chemisorption and IR analysis, the ones with the highest ratio L/B exhibit higher selectivity towards camphene, and that increasing the Brønsted acidity of a catalyst results in a great degree of the secondary reaction of limonene. Ecormier et al. [136, 137] studied the isomerisation reaction at 60 °C employing 100 mg of sulfated zirconia as Brønsted and Lewis acid catalysts. They observed that increasing the amount of sulfur the selectivity ratio camphene: limonene decreases. Furthermore, the group also hypothesized that at low S wt%, weak Brønsted acid sites are formed, which favour camphene, while high S loadings promote limonene production due to the presence of stronger Brønsted acid sites. Hammett indicators were used as a method to discern strong/weak acid sites and no further study was made on the role of Lewis acid sites on selectivity nor upon the ratio Brønsted: Lewis sites. Further studies on sulfated zirconia did not consider the Brønsted: Lewis ratio but obtained similar results to previous literature [138]. In 2003, Masini et al. [139] first employed SiO2 supported phosphotungstic acid (HPW) as heterogeneous acid

catalyst for the isomerisation of α-pinene, investigating the effect of calcination temperature (300-500 °C) on catalytic performances. In the study, they observed that decreasing the calcination temperature, conversion is higher and reached its highest value of 40%. Further studies [140, 141] of HPW on different supports, TiO2, ZrO2, and

SiO2 proved that SiO2 is a better support for HPW because showed greater

performances for α-pinene activity. Newman et al. [86, 87] studied the activity of HPW commercial silica with pores sized 10 nm at different wt%, investigating the effect of HPW dispersion on catalytic activity. They suggested that an optimum catalyst activity is observed for the sub-monolayer coverage when HPW form tetramers that trap crystalline water. Conversion reached 45% for the ~30 wt% of HPW on silica, with almost a 1 to 1 selectivity towards camphene and limonene. Cs-doped HPW [82], different HPAs supported on natural zeolites and MCMs materials [142-144] have been employed to improve catalyst activity, lowering the ratio catalyst: substrate and investigating the effects of the ratio Brønsted: Lewis acid sites on the isomerisation pathways. Pd-Zn/Al SBA-15 catalysts were employed by Golets et al. [145] for the selective production of p- cymene from α-pinene, by isomerisation and dehydrogenation of the substrate.