Hydration, of a-pinene

Synthetic pine oil is produced by the acid-catalyzed hydration of a-pinene (Fig. 1). Mineral acids, usually phosphoric acid, are used in concentrations of 20—40 wt % and at temperatures varying from 30—100°C. Depending on the conditions used, alcohols, chiefly a-terpineol (9), are produced along with /)-menthadienes and cineoles, mainly limonene, terpinolene, and 1,4- and 1,8-cineole (46—48). Various grades of pine oil can be produced by fractionation of the cmde products. Formation of terpin hydrate (10) from a-terpineol gives P-terpineol (11) and y-terpineol (12) as a consequence of the reversible... 

A common industrial method of a-terpineol synthesis consists of the hydration of a-pinene or turpentine oil with aqueous mineral acids to give crystalline cis-terpin hydrate mp 117°C), followed by partial dehydration to a-terpineol. Suitable catalysts are weak acids or acid-activated silica gel [83]. 

Terpineol is mainly isolated from plants of the Eucalyptus species and is a mixture of a-terpineol, P-terpineol, 4-terpineol, and 1,4- and 1,8-terpindiols. Both enantiomers of a-terpineol are commercially produced through hydration of a-pinenes.37... 

The hydration of a-pinene (1) with aqueous mineral acids leads to a complex mixture of monoterpenes known as s)mthetic pine oil [2], The main products are monocyclic terpenes, namely a-terpineol (9). The reaction mechanism has been extensively studied [3-7]. It is generally accepted that it proceeds through cation I (Scheme 1). Subsequent carbonium ion rearrangements leads to two parallel pathways. One yields bi- and tricyclic products such as camphene (2), bomeol (3) and isobomeol (4). The other )delds monocyclic products such as li-monene (5), a- (6) and y-terpinenes (7) terpinolene (8), a-terpineol (9)and 1,8-terpine (10) Products from the cyclization of terpineol, like 1,8-cineol, can also be formed. By controlling the many reaction variables the process can be directed to produce a maximum of terpene alcohol s. 

The direct hydration of a-pinene and camphene to bomeol and/or isobomeol is of great interest since presently a two step procedure is used in industrial practice acetolysis of camphene and subsequent hydrolysis of bomyl acetate [8]. 

Zeolites show unique catalytic properties due to their uniform pore size. Namura et al. [10] used various zeolites in the hydration of a-pinene to ct-terpineol. Between the catalysts tested ferrierite made possible to achieve selectivities to terpineol as high as 69 %. 

On the other hand, the selective hydration of a-pinene to bomeol, catalysed by an unspecified high silica zeolite, has been reported by Chen et al. 

Scheme 1 - Acid catalysed isomerization and hydration of a-pinene. 1 a-pinene 2 camphene 3 bomeol 4 isobomeol 5 limonene 6 a-terpinene 7 y-terpinene 8 terpinolene 9 a-terpineol 10 1,8-terpine.

The main product in the hydration of a-pinene for all the catalyst samples is a-terpineol. 

On the other hand, the hydration of camphene leads mainly to isobomeol. The study of catalytic activity towards the hydration of a-pinene or cam-... 

In both cases a pseudo first order kinetics was observed, being a-pinene more reactive than camphene, due to the angle strain of the cyclobutane ring (Figure 2). This kinetic behaviour is in agreement with what was observed for the hydration of a-pinene over zeolite H-beta... 

However, in contrast to what was observed for the isomerization reaction of pure a-pinene over USY [18], the highest activity is now reached at a much lower value of Nai (22 instead of 36 for the isomerization of pure pinene [18]. Also in contrast to the isomerization of pure pinene is the abrupt loss in activity for high Nai values. For Nai around 37 the catalyst samples are even inactive for the hydration of a-pinene. When the reaction is carried out over HY zeolites, no significant conversion is observed even after 170 h. This absence of catalytic activity observed for the parent H-Y or for the low dealumi-nated catalyst samples, can be explained as follows ... 

Although Lewis sites exhibit a very low activity in the hydration reactions of a-pinene and camphene, they are very selective to a-terpineol and isobomeol respectively (Figure 5). The highest selectivities — 70 % to a-terpineol, in the hydration of a-pinene and 90 % to isobor-neol, in the hydration of camphene — are reached at the highest relative concentration of Lewis sites. This concentration, however, corresponds to the lowest catalytic activity. 

The main products of the hydration reactions of a-pinene and of camphene over USY zeolites are a-terpineol and isobomeol, respectively. In the hydration of a-pinene simultaneous isomerization takes place whereas with camphene, hydration is observed nearly exclusively. 

Selectivities to a-terpineol, in the hydration of a-pinene and to isobomeol, in the hydration of camphene, can be as high as 70% and 90%, respectively. Selectivities to both terpene alcohols grow continuously with NEFAL. Apparently they are not affected by the changes in the acid strength. 

Although the Lewis sites give a very small contribution to the catalytic activity, they are very selective to the terpenic alcohols. a-Terpineol is particularly sensitive to changes in the relative concentration of Lewis sites. In the whole range of the relative concentrations of Lewis sites tested, selectivity to a-terpineol in the hydration of a-pinene changes of about 20%. 

P-19 - Hydration of a-pinene over heteropolyacids encaged in USY zeolites... 

P-19 - Hydration of a-pinene over heteropolyacids encaged,in USY zeolites J. Vital, A.M. Ramos, l.F. Silva, J.E. Castanheiro, M.N. Blanco, C. Caceres, P. 

Synthetic pine oil is prepared by the hydration of a-pinene with aqueous mineral acids (Fig. 2.4), and is mainly used in household cleaning and disinfection products [7]. The acidic conditions promote the formation of an into-mediate carbocation which readily undergoes rearrangement to form predominantly the isomaic jp-menthane car-bocation, followed by water addition to generate essentially a-terpineol (Fig. 2.4) and minor amounts of fenchol and bomeol. 

Figure 6.36 shows some of the major products manufactured from a-pinene (65) (Sell, 2003, 2007). Acid-catalyzed hydration of a-pinene gives a-terpineol (74), which is the highest tonnage material of all those described here. Acid-catalyzed rearrangement of a-pinene gives camphene (89)... 

An important advantage of hydroboration-oxidation for hydration of alkenes is that rearrangements of the carbon skeleton do not occur, whereas they do in acid-catalyzed hydration. For example, acid-catalyzed hydration of (+)-a-pinene (65) proceeds with skeletal rearrangements to produce a number of isomeric alcohols. 

Another process of major importance is the acid catalysed hydration of a-pinene to give a-terpineol as is described in section Monocyclic Monoterpenoid Hydrocarbons under a-terpineol. 

Acid-catalyzed hydration of a-pinene produces pine oil and this is one of the largest uses of turpentine [177-179]. Mineral acids are used as catalysts, usually in a concentration of 20-40 wt% and temperatures of 30-100°C. An efficient surfactant, preferably one that is soluble in the acid-phase upon completion of the reaction, is needed to emulsify the a-pinene and acid. The surfactant can then be recycled with the acid. Phosphoric acid is the acid commonly used in the pine oil process. Its mild corrosion characteristics and its moderate strength make it more manageable, especially because the acid concentration is constantly changing in the process by the consumption of water. Phosphoric acid is also mild enough to prevent any significant dehydration of the alcohols formed in the process. Optimization of a process usually involves considerations of acid type and concentration, temperature, surfactant type and amount, and reaction time. The optimum process usually gives a maximum of alcohols with the minimum amount of hydrocarbons and cineoles. 

Contactor-type polymeric membrane reactors have been also applied to liquid-phase reactions other than hydrogenation or oxidation. The hydration of a-pinene has been carried out successfully over polymeric membranes consisting of mixed matrixes of PDMS embedded USY or beta zeolites or sulfonated activated carbon. The membranes were assembled in a flat contactor-type reactor configuration, separating the aqueous and organic phases. Sulfonated PVA membranes were also reported to be effective in the acid catalysed methanolysis of soybean oil carried out in a flat contactor-type membrane reactor configuration. ... 

Wal J, Ramos A M, SUva IF, Valente H and Castanheiro J E (2000), Hydration of a-pinene over zeohtes and activated carbons dispersed in polymeric membranes , Catal Today, 56,167-172. 

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