1. Introduction
Geraniol (GA) (C
10H
18O, (2
E)-3,7-dimethylocta-2,6-dien-1-ol) is a chemical compound from the group of monoterpene alcohols. It is a commonly used fragrance ingredient in perfumes, creams, powders, shampoos, and toilet soaps [
1,
2,
3]. On an industrial scale GA is isolated from such plants as
Pelargonium graveolens or
Cymbopogon winterianus [
4,
5,
6]. GA shows a variety of biological effects (
Figure 1) [
7,
8,
9,
10]. Literature reports confirmed its effective action in the treatment of intestinal, skin, liver, kidney and prostate cancer [
11,
12,
13,
14]. GA also has anti-inflammatory, antibacterial and antioxidant properties [
15,
16].
Terpenes, including GA, are natural products that can be easily transformed into valuable compounds, which are then used in industrial methods of producing perfumes, various types of aromatic or therapeutic agents [
2]. Geraniol undergoes numerous changes in the presence of porous catalysts, hence the process, which was originally to be carried out as the isomerization process of this compound, becomes a very complicated process to describe. For geraniol the following possible transformations can be described: dehydration (products in the form of β-pinene (the equivalent name is beta-pinene) (BP) and ocimene (OC)), oxidation (products in the form of (cis-, trans-) citral (CI)), isomerization (products in the form of linalool (LO) and nerol (NO)), dimerization (geranylgeraniol (GG)), cyclization and fragmentation carbon chain (6,11-dimethyldodeca-2,6,10-trien-1-ol (DC) and thumbergol (TH)) (
Figure 2).
The aim of further research on this process should be to find a very efficient heterogeneous catalyst that would permit 1, maximum 3 products obtained with high selectivity in this process.
One of the products of dehydration of geraniol is β-pinene (main product obtained in our work during studies in the presence of diatomite). β-Pinene is an organic compound from the group of bicyclic monoterpenes. Due to its properties, it is used in the cosmetics industry, as a raw material for the production of other fragrances such as: bergaptol, limonene and terpineol. Additionally, it is used as an anti-inflammatory, anti-cancer, expectorant and bronchodilator [
17]. It was described that beta-pinene is used in the production of drugs applied in treatment of the liver and kidneys [
18] and also it was shown that this compound may be applied as an effective antiallergic agent [
19]. The anxiolytic properties of pinene were also confirmed [
20]. The potential for pain relief of neuropathic and inflammatory types was investigated using the essential oil of
Ugni myricoides leaves. Researchers attribute the obtained effects to the presence of pinene [
21]. Beta-pinene can be also used as an antibacterial agent [
22,
23] and this compound may be helpful in the treatment of Alzheimer’s and other neurodegenerative diseases [
24].
Other products obtained as a result of the transformation of GA are cymenes-organic chemical compounds belonging to the group of monoterpenes. They are mainly used in the perfume industry [
25].
Linalool (isomerization product) belongs to aliphatic unsaturated alcohols. As a fragrance or as linaloolyl acetate, it has found application in the perfume industry. This compound is also used in treatment of leukemia or cervical cancer [
26].
Nerol oil is known for its properties supporting the renewal of skin cells, which translates into improvement of its elasticity, maintenance of the appropriate serum level, reduction of wrinkles and scars. It is used in treatment of bacterial skin infections caused by fungi, bacteria or yeasts [
27].
Another valuable product of GA transformation is citral (oxidation products). Citrals are used in the perfume and food industry and also in medicine-citrals have antibacterial properties and, moreover, are used in cancer treatment [
28].
Thumbergol (one of the main products obtained in our work for studied in the presence of alum) is diterpene monocyclic alcohol used in cancer treatment This compound also shows neuroprotective [
29] and antibacterial properties [
30,
31].
Literature data on the transformation process of GA are not sufficiently described. Yu et al. [
32] presented the transformation of GA with the presence of the FeCl
2 × 6H
2O as the catalyst. The main products obtained in this process were linalool and α-terpineol. If the process was carried out in acetonitrile with water (5%), linalool yield was 26%, while if the process was carried out in anhydrous acetonitrile, linalool yield was 4%, and α-terpineol yield was 53%. Studies in the presence of mineral showed that the only product was α-terpineol which was formed with less than 10% yield.
Haese et al. [
33] presented studies on transformations of pure GA, and in mixture with nerol. The isomerization was carried out at 160 °C and at vacuum. As the catalyst oxodiperoxotungstic acid was used. The main product for pure Ga and Ga in mixture with nerol was linalool.
Srivastava et al. [
34] presented the transformation of GA under the influence of gamma radiation, the source of which was
60Co. As a result of irradiation of the GA-methanol solution, GA was converted to nerol and linalool, and the conversion of GA reached value of 30%.
Ramishvili et al. [
35] and Tsitsishvili et al. [
36] transformed GA in the presence of micro- and mesoporous zeolites of the BEA type. The obtained results show that GA was converted mainly to linalool and nerol, as well as to: (2
E,6
E)-6,11-dimethyldodeca-2,6,10-triene-1-ol and (
trans, trans-farnesol-(2
E,6
E))-3,7,11-trimethyldodeca-2,6,10-trien-1-ol). During studies the conversion of GA reached 99%.
Fajdek-Bieda et al. [
37] carried out the process of transformations of GA in the presence of sepiolite. The main products in the process were: β-pinene, ocimenes, linalool, nerol, citrals and thumbergol. The highest value of selectivity was obtained for linalool (19 mol%) at the GA conversion amounted to 100 mol%.
In the article by Fajdek-Bieda et al. [
38] was described the process of GA transformation, which was performed in the presence of clinoptilolite. The main products were DC and TH. Optimal conditions for obtaining of DC and TH were: temperature 140 °C, catalyst content 12.5 wt % and the reaction time of 180 min. At these conditions GA conversion was 98 mol%, and the selectivities of DC and TH were 14 and 47 mol%, respectively.
Natural minerals are very interesting heterogeneous catalysts used in catalytic reactions, mainly because their availability in the form of numerous deposits and relatively low price [
39]. Diatomite (SiO
2·nH
2O) is a mineral from the group of siliceous sedimentary rocks, consisting mainly of opal and cristobalite. Diatomite can be used as the porous catalyst in many organic syntheses [
40,
41,
42,
43,
44,
45]. In Poland, its deposits occur in the south of Poland, near Krosno [
46,
47,
48]. Diatomite is also used as the filtering agent and as the absorbent for liquid fertilizers, disinfectants and insecticides [
49,
50,
51,
52]. Alum (potassium aluminum sulphate dodecahydrate) is a natural mineral with the chemical formula KAl (SO
4)
2 · 12 H
2O) [
53]. It occurs in the form of a crystal that is brittle and easily soluble in water [
54,
55]. This mineral crystallizes in the form of regular octagonal crystals. Due to its properties, it is used for the treatment of fireproof fabrics as well as for clarifying cloudy water [
56,
57,
58,
59]. Alum is used in the synthesis of synthetic ethyl alcohol, but scientific literature lacks a large amount of information on the catalytic use of alum. One of the few literature reports shows the use of potash alum as a sustainable heterogeneous catalyst in a one-pot synthesis of highly functionalized pyrrol-2-ones and furan-2-ones [
60]. Examples of alum occurrence are: Uzbekistan, Italy—Vesuvius, while in Poland Sandomierz, Międzyzdroje and Lower Silesia [
61,
62].
Natural minerals are very interesting heterogeneous catalysts used in catalytic reactions, mainly because their availability in the form of numerous deposits and relatively low price [
39]. Diatomite (SiO
2·nH
2O) is a mineral from the group of siliceous sedimentary rocks, consisting mainly of opal and cristobalite. Diatomite can be used as the porous catalyst in many organic syntheses [
40,
41,
42,
43,
44,
45]. In Poland, its deposits occur in the south of Poland, near Krosno [
46,
47,
48]. Diatomite is also used as the filtering agent and as the absorbent for liquid fertilizers, disinfectants and insecticides [
49,
50,
51,
52]. Alum (potassium aluminum sulphate dodecahydrate) is a natural mineral with the chemical formula KAl (SO
4)
2 · 12 H
2O) [
53]. It occurs in the form of a crystal that is brittle and easily soluble in water [
54,
55]. This mineral crystallizes in the form of regular octagonal crystals. Due to its properties, it is used for the treatment of fireproof fabrics as well as for clarifying cloudy water [
56,
57,
58,
59]. Alum is used in the synthesis of synthetic ethyl alcohol, but scientific literature lacks a large amount of information on the catalytic use of alum. One of the few literature reports shows the use of potash alum as a sustainable heterogeneous catalyst in a one-pot synthesis of highly functionalized pyrrol-2-ones and furan-2-ones [
60]. Examples of alum occurrence are: Uzbekistan, Italy—Vesuvius, while in Poland Sandomierz, Międzyzdroje and Lower Silesia [
61,
62].
In this work, we investigated the transformations of GA in the presence of natural catalysts in the form of diatomite and alum. The studies tested the influence of temperature, catalyst content, and reaction time on the course of the GA transformation process. The syntheses were carried out under the atmospheric pressure, in the air atmosphere and without the use of any solvent. The absence of solvent is advantageous as this eliminates the possibility of the reaction used solvent with the GA transformation products as well as the need to recover and recycle the solvent to the process. The aim of the study was to compare the catalytic activity of diatomite and alum and to find the most favorable conditions for the transformation of GA on these catalysts. Before catalytic studies with GA the catalysts were described with the following instrumental methods: XRD, SEM/EDX, FTIR and XRF methods. Thanks to the use of these methods a full characterization of their physicochemical properties was prepared.
2. Materials and Methods
2.1. Raw Materials
The syntheses were performed in the presence of diatomite (100% pure, Nanga, Hobbs, New Mexico) and aluminum potassium alum (100% pure, Nanga, Ufa, Russia) as the catalysts. The organic raw material used in our tests was GA (99% pure, from Acros Organics, Milwaukee, USA). For quantitative analysis which were performed by the gas chromatography method (GC), standards in the form of: citronellol (95% pure, from Sigma Aldrich, Steinheim, Germany), citral (95% pure, Sigma Aldrich, Steinheim, Germany), ocimen (90% pure, from Sigma Aldrich, Milwaukee, WI, USA), beta-pinene (95% pure, from Fluka, Milwaukee, WI, USA), linalool (97% pure, from Acros, Steinheim, Germany), farnesol (96% pure, from Acros, Steinheim, Germany), nerol (97% pure, from Acros, Steinheim, Germany), myrcene (pure technical from Sigma Aldrich, Steinheim, Germany) and geranylgeraniol (85% pure, from Sigma Aldrich, Milwaukee, WI, USA) were used.
2.2. Characteristics of Diatomite and Alum
For the characteristc of diatomite and ałum the following method were used:
- —
X-ray diffractometry (XRD)—Empyrean X-ray diffractometer with Cu Kα radiation source (Malvern Panalytical, Grovewood, UK); analysis of samples in the temperature range of 5–30° in 0.02° steps;
- —
Specific area (SSA), total pore volume (TPV) and micropore volume (MV)—nitrogen adsorption method at 350 °C using the QUADRASORB evoTM Gas Sorption Surface and Pore Size Analyzer (Quantachrome Instruments, Boynton Beach, FL, USA), prior to analysis samples were degassed at 250 °C for 20 h in atm. N2;
- —
Mapping of elements—scanning electron microscopy (SEM) and EDX surface spectra-SEM apparatus (JEOL company, JSM-6010LA, Tokyo, Japan) with a secondary electron detector;
- —
Elemental analysis performed with Epsilon3 energy dispersed X-ray fluorescence spectrometer (EDXRF) (Malvern Panalytical, Grovewood, UK);
- —
FT-IR infrared spectorscopy (Thermo Nicolet 380 apparatus, Malente, Germany)—wavenumber range from 400 to 4000 cm−1.
2.3. Method of Transformations of Geraniol and Analyses of the Post-Reaction Mixtures
The syntheses were carried out in a glass reactor with a capacity of 25 cm3, which was equipped with a reflux condenser and a magnetic stirrer with heating function. The ranges of the studied parameters were as follows: temperature 80–150 °C, catalyst content 5–15 wt %, reaction time from 15 min to 24 h. In order to perform a qualitative and quantitative analyses, the sample of the post-reaction mixture was first centrifuged and then it was dissolved in acetone in the ratio 1:3.
Qualitative analyses were performed using the GC-MS method on a ThermoQuest apparatus with a Voyager detector and a DB-5 column (filled with phenylmethylsiloxanes, 30 m × 0.25 mm × 0.5 mm). Analysis parameters: helium flow 1 mL/min, sample chamber temperature 200 °C, detector temperature 250 °C, oven temperature—isothermally for 2.5 min at 50 °C, then heating at the rate of 10 °C/min to 300 °C. Quantitative analyses were performed with help of Thermo Electron FOCUS chromatograph with FID detector and TR-FAME column (cyanopropylphenyl packed, 30 m × 0.25 mm × 0.25 mm). The analysis parameters were as follows: helium flow 0.7 mL/min, sample chamber temperature 200 °C, detector temperature 250 °C, oven temperature—isothermally for 7 min at 60 °C, then heating at the rate of 15 °C/min to 240 °C. The FID temperature was kept at the level of 250 °C. Examples of GC-MS analyzes in the presence of alum and diatomite are included in the
Supplementary Material.
The quantitative analyses of the products were performed using the external and internal standard method. In case of the first method, 8-point calibration curves were performed for each compound in the concentration range of 0–33 wt %. After the chromatographic analyses the mass balances for each synthesis were prepared. The mass balances allowed us to calculate the main functions describing the process:
4. Discussion
The conducted studies have shown that both diatomite and potassium aluminum alum are the active catalysts in the process of GA transformations and that all tested parameters (temperature, amount of the catalyst and reaction time) have influence on the course of GA transformations and values of GA conversion and selectivities of the main products. The conducted research shows that the most favorable conditions of the GA transformation process were obtained: at the temperature of 80 °C (for both tested catalysts), with a catalyst content of 1 wt % (for both tested catalysts) and for 1 h (for diatomite) and for 3 h (for alum). The obtained conditions permit the obtainment of reaction products, mainly BP, DC and TH with the highest possible selectivity (99% mol, 44% mol and 53% mol), with a high level of GA conversion (in the range of 96–98% mol). The use of both higher temperature and higher catalyst content and longer reaction times can give rise to undesirable reaction products. Due to the more developed surface, diatomite catalyses the formation of beta-pinene (a product with a smaller molecule) in its pores. On the other hand, the reactions occurring on alum are catalyzed by active centers located on its surface, which allows to obtain, as a result of the transformation of geraniol and small-molecule products formed during its transformation, products with larger particles. The mechanism of changes in geraniol on diatomite and alum, however, requires further detailed research. It is interesting, however, that replacing the catalyst with diatomite allows to obtain a low molecular weight product, which is beta-pinene, using the shape-selective action of the pores present in the diatomite.
Comparing the results presented in our previous publications, which described the process of transformation of GA with the use of sepiolite (a mineral from the silicate group, classified as clay minerals) [
29] and clinoptilolite (a mineral from the group of silicates, included in the group of zeolites) [
30], with these described in this paper, it was observed that the previously tested catalysts required longer reaction time, a much higher catalyst content and higher temperature to obtain high values of selectivity of the transformation to the appropriate products and the conversion of GA. In general, the presented studies showed that increasing in the temperature or extending the reaction time causes a decrease in the value of the selectivity of the formation of compounds that most likely decompose because their structure is not stable.
Undoubtedly, the advantage of the proposed method of GA transformation is the lack of solvent in the reaction medium. Its presence could cause additional reactions with GA and its transformation products, which would increase the number of products produced.