Characterization of essential oil
The yield of essential was obtained from C. winterianus (CIM-Jeeva: 1.2% and BIO-13: 1.3%) and C. citriodora (2.8%), respectively. Citronellal is a major compound in these essential oils and their compositions before and after conversion are presented in Table 1. Citronella oil was contained 38.9-41.4% of citronellal with (+)-citronellal (91%) as the predominant enantiomer followed by (-)-citronellal (9%). Whereas, eucalyptus oil was possessed 69.5% of citronellal with (-)-citronellal (48%) and (+)-citronellal (52%) in enantiomeric contour. In toxicity assay, citronellal was found to be a harmful compound (LC50: 0.04) as compared to menthol (LC50: 0.39) [13]. Therefore, menthol is the best-suited compound for pharmaceutical and nutraceutical applications.
Characterization of composites
The surface morphology of the composites (Sn-B-NaYZE, Al-B-NaYZE, Sn-B-YZE, and Al-B-YZE) were examined by SEM and TEM. The SEM micrograph revealed that all particles persist in a well shaped structure, size ranging from 0.54 to 0.78 µm (Fig. S2). Also, the SEM images of 1%Pd/AC and 1%Ru/AC have been shown the proper incorporation of Pd and Ru on AC surface (Fig. S2c,f). TEM images of composites is present in Supplementary Fig. S3, and it reveals that the composites of Al-B-NaYZE and Al-B-YZE were homogeneous and smooth as compared to Sn-B-NaYZE and Sn-B-YZE. HRTEM images of Sn-B-NaYZE, which indicated the proper incorporation of metals (Sn, B) on NaYZE, the variant lattice pattern is indicated as polycrystalline nature, which is further confirmed by the SAED ring pattern (Fig. 1c). A high-resolution image suggested that the uniform distribution of sodium borate as black lines on the surface of Sn impregnated ZE. On the other hand, the HRTEM image of Al-B-NaYZE shows the high dispersion of particles. It was predicted that AlO and BO on NaYZE made the composite smooth to form indistinguishable morphology. In Sn-B-NaYZE, the HRTEM micrographs reveal the lattice constants of 0.25 nm and 0.35 nm were attributed to the plane of 101 and 110 of SnO2 assigned to the d spacing. The lattice fringe of the HRTEM of SnO2 was complimented well to the XRD data. Fig. 1f-I shows the individual maping of elements present in Sn-B-NaYZE composite. Moreover, the elemental distribution of composites is determined through SEM-EDAX analysis (Fig. S4 and S5). The obtained metal weight % was in correlation with XPS results.
XRD data of prepared catalysts Al-B-NaYZE, Sn-B-NaYZE, Al-B-YZE, and Sn-B-YZE were presented in Fig. S6. For Al-B-NaYZE and Sn-B-NaYZE, the 2θ values recorded at 6.1°, 11.8°, 15.5°, 20.2°, 23.4°, resemble the peaks observed in a typical NaYZE. These values of 2θ were matched to the hkl values of 111, 311, 331, 440, and 533, respectively [14,15]. The lack of any hump and soft horizontal line in the background was indicated the complete crystallinity in composites without any amorphous species (SiO2) [16,17]. In Sn-B-NaYZE has been shown 2θ correspond to 26.4 and 33.9, these are matched with lattice planes of 110 and 101 of SnO2, respectively [18]. The resemblance of XRD patterns of Sn-B-NaYZE and Sn-B-YZE have indicated the successful incorporation of Sn into the NaYZE and YZE framework, and it is supported by decreasing the molar ratio of SiO2/Al2O3 in Sn-B-NaYZE and Sn-B-YZE (Table 2). Al/Sn impregnated ZE had reduced crystallinity in comparison to the normal one, which was reflected by decreasing the intensity of diffraction peaks. Sn-B-NaYZE was less crystalline as compared to Al-B-NaYZE and Na-YZE. The incorporation of Sn is led to enhance the Na content (Table 2). The conventional YZE framework was composed of SiO4 and AlO4 species, which were arranged in tetrahedral symmetry. Due to the different oxidation states of Al3+ and Si4+, which were impacted the overall negative charge to the framework, thereby need monovalent ion (Na+) for maintaining the charge balance. For Sn2+ composite, the framework has shifted the burden to increase the negative charge, hence additional monovalent Na+ was needed for maintaining the charge neutrality to facilitate better incorporation of Sn2+ [16]. The intensity of characteristic peak was indicated the extent, as well as the concentration of crystallinity in composites could be expressed according to the equation described by Pal et al [19]. The absence of a any extra peak at 28o was indicated that no more free BA in the composites [20]. Similarly, 1%Pd/AC has shown the characteristics 2θ peaks at 40.5o, 47.0o, and 68.5o.
The data for NH3-TPD of Al-B-Na-YZE, Sn-B-Na-YZE, Al-B-YZE and Sn-B-YZE depicted two peaks: one narrow peak at 150 oC and another broad peak at 500 oC. The low temperature peak was derived from the weakly adsorbed NH3 molecules via H-bonding. Whereas, the high temperature peak was ascribed to strong Bronsted acid sites. As shown in Fig. 2a, the total peak area corresponding to Al-B-YZE, Sn-B-YZE, Al-B-Na-YZE and Sn-B-Na-YZE are 4326.2, 9600.6, 30369.0 and 41108.1 sq. unit respectively, which indicated that the Na composites have enhanced acidic character. Besides acidic character, Al-B-NaYZE and Sn-B-NaYZE have appeared analogous to the cubic structure of aluminosilicate, which might be facilitated higher Bronsted acidic characteristics [21]. The relative abundance of Lewis acid concentrations at 150 oC were 7,334.1 and 1,241.6 sq. units for Al-B-NaYZE and Al-B-YZE, respectively. Similarly, the amounts of Bronsted acid concentrations at 500 oC were 22,290.9 and 3,082.4 sq. units for Al-B-NaYZE and Al-B-YZE, respectively. It is clear that Al-B-NaYZE is a strong acidic composite as compared to Al-B-YZE. Amounts of weak acid concentrations at 150 oC are 9,199.2 and 3,206.6 sq. units for Sn-B-NaYZE and Sn-B-YZE, respectively. Similarly, the amounts of strong acid concentrations at 500 oC are 30,572.1 and 6,365.2 sq. units for Sn-B-NaYZE and Sn-B-YZE, respectively. From the above results, Sn-B-NaYZE is a strong acidic composite as compared to Sn-B-YZE. The lower percentage of B loading was slightly impacted the Si/B ratio, which helped to shift the orientation towards the tetrahedral coordination. On the other hand, higher B loading was decreased the Si/B ratio, hence facilitating the trigonal forms of coordination [22]. Therefore a definite percentage of B incorporation has been shown to enhance the Prins reaction, due to its proper balance in between the trigonal and tetrahedral coordination forms. Trigonal B has represented the Lewis acidic character, whereas B is exhibited the Bronsted acidic behavior in dynamic equilibrium to favor the extra-ordinary activity of these composites [23].
The incorporation of Sn into the ZE (zeolite) framework is verified by examining the bands formed by charge transfer from ligand to the metal as analyzed from the UV-Vis spectra (Fig. 2b). Sn-B-NaYZE and Sn-B-YZE exhibited a strong UV-Vis characteristic absorption band centered at 219 nm, which was featured in the presence of tetracoordinate Sn4+ in the ZE framework. The intensity of this band was proportional to the loading of tetrahedrally coordinated Sn4+ ion.23 The band at 250 nm was attributed to the polymeric SnO2 species. Therefore, the stronger band intensity of Sn-B-NaYZE in comparison to that of the Sn-B-YZE suggested the greater formation of Sn4+ species during its incorporation procedure [24]. The absorption spectra of pure SnO2 demonstrated an extensive band at ~240-400 nm, which was allocated to the polymeric Sn-O-Sn category species. The absorption spectra of Sn-B-NaYZE and Sn-B-YZE exhibited a blue shift relative to the pure SnO2. This blue shift could be attributed to small SnO2 species, and is largely dependent on the size-quantization of the SnO2 particles in the nanometer regime. SnO2 species were smaller in size and impacted a stronger blue shift [18]. The blue shift is more significant for Sn-B-NaYZE, which indicated the presence of smaller SnO2 particles (Fig. 2b). The absorption spectrum of Sn-B-NaYZE has contained a strong band at ~205 nm with very weak shoulder bands in between ~290-420 nm. The findings indicated that most of the Sn was inserted into the tetrahedral framework, and very few of them were present in the extra framework. However, SnO2 phases were not detected in Sn-B-YZE. Furthermore, the spectra of Al-B-NaYZE and Al-B-YZE were quite similar to YZE spectra, which means no such changes were observed after Al impregnation on the ZE framework.
XPS spectra of composites are presented in Supplementary Fig. S7. The XPS spectra illustrated two principal signals that related to Sn 3d5/2 and Sn 3d3/2 photoelectrons with binding energies of 487.0 and 495.5 eV, respectively (Fig. 2c), which indicated the existence of Sn4+ particles on the surface [23]. The count Sn-B-NaYZE was higher as compared to the Sn-YZE. The consequences implied that the Sn-B-NaYZE is acquired more framework of Sn particles on the surface, which is complied with the results of UV-Vis and SEM-EDX (Table 2). The peaks at the binding energy of 74.0, 74.8, 75.2, and 75.8 eV have confirmed the presence of Al-O and Al-OH (Al 2p3/2) species. The peaks of Al 2p (76.2) and Si 2p (103.3) are confirmed that these elements are properly integrated with YZE in Sn-B-NaYZE and Al-B-NaYZE (Supplementary Fig. S7b). The counts of Al-B-NaYZE and Al-B-YZE are higher than that of Sn-B-NaYZE and Sn-B-YZE, which suggested the proper impregnation of Al on the ZE framework, and hence the presence of Al extra framework. The signal at 192.8 eV in all samples has confirmed the presence of B species in the form of B2O3 (Fig. S7c).
FT-IR spectra of the composites are given in Fig. S8a,b† The band at 1150 and 1017 cm-1 were assigned to asymmetric stretching vibration Al-O-Si, while 791 and 450 cm-1 were allocated to symmetric stretching and bending vibration of Al-O-Si. Sn impregnated composites have been shown the characteristic peaks at 592, 915, and 1627 cm-1. Al impregnated composites have been given a characteristic peak at 1110 cm-1. The peak observed at 460 cm-1 was indicated the Si-O bending vibration, and at 580 cm-1 was originated due to the double six-membered ring structure of the ZE [25]. All the composites have been displayed a distortion in between the 910-950 cm-1, which is allocated to asymmetrical stretching of Si-O-M(M: Al or Sn). Sn-B-YZE and Al-B-YZE composites were shown bands at 3434, 3227, and 2260 cm-1, which were arisen due to stretching vibration of SiO-H, BO-H, and B-OH, respectively. The B-OH bending vibration was assigned at 1187 cm-1[20]. Whereas, these bands are completely absent in Al-B-NaYZE and Sn-B-NaYZE composites. The stretching and bending vibration of Si-O-B (borosilicate linkage) are observed at 927 and 649 cm-1, respectively (Fig. S8a,b). In addition, the composites were incorporated B in the crystal lattices in the form of trigonal as well as tetrahedral valency states [22]. Thus, tetrahedral B was assigned the peak at 920-930 cm-1, whereas trigonal B was at 1395 cm-1.
Nitrogen adsorption-desorption was carried out to determine the SA and pore volume. The isotherms are investigated at a relative pressure of P/Po ranging from 0 to 0.9 (Supplementary Fig. S9a,b). The SA is calculated using Eq. (4), the SA, pore-volume, and pore radius are presented in Table 3.
Where SA: Surface area m2/g, Vm: Volume of monolayer m3, Am: Area occupied by one molecular of nitrogen in monolayer is 0.162 nm2, N: Avogadro's number of a mole (6.02×1023 molecules).
The N2 adsorption reveals that Al/Sn-B-Na-YZE belongs to type-1 isotherm and Al/Sn-B-YZE composite belongs to type-IV isotherm (Fig. S9a,b). Isotherms of NaYZE and Al/Sn-B-NaYZE exhibited microporous adsorption behavior as missing hysteresis loops within the framework, and the pattern of pore size distribution was indicated the lack of mesoporous behavior, which thereby confirmed the microporous nature. While, the isotherms of YZE and Al/Sn-B-YZE indicated the presence of some mesopores, due to the presence of hysteresis loop in the framework. The isotherm was raised at a low-pressure range with an increase in relative pressure (P/Po), and on further increasing the relative pressure the N2 adsorption was increased, which confirmed the presence of mesopores in the composites [21]. The small change in SA and micropore volume after adsorption of Sn and Al indicated the successful incorporation of Sn and Al on ZE (NaYZE and YZE) (Table 3). Whereas, the significant decrease in SA and micropore volume was recorded when B was incorporated into the Sn-NaYZE framework which might be formed complex oxides of metal-Na-B. It is interestingly observed that the Sn-B-NaYZE and Al-B-NaYZE have very low BET SA (18.4, 4.3 m2g-1), Langmuir SA (28.4, 6.9 m2g-1), total pore volume (0.022, 0.006 cm3g-1) with improved pore diameter (503.4, 583.8Ao), respectively. On the other hand, 1%Pd or 1%Ru reduced catalysts are prepared using inexpensive AC as a major percentage, which gave improved SA and porosity for enhancing the activity (Table 3).
Composites have been shown thermo-stability behavior up to 1000 oC (Fig. S10). The total mass losses of composites such as Sn-B-NaYZE, Al-B-NaYZE, Sn-B-YZE, and Al-B-YZE were 3.5%, 5.2%, 4.1%, and 6.7%, respectively in the entire temperature range, which indicated prepared catalysts were thermally more stable as compared to the respective base materials. The difference in the stability of base material catalyst and the metal incorporated base material catalyst is due to hinderance in diffusion of components by the incorporated metals, also hindering the phase transition kinetics. It was validated that Na was firmly associated with the composites to give extra stability and helped to generate the acidic sites. The phase transition kinetics is shown by the TGA evaluation of composites before and after Py adsorption (Fig. S11). The initial mass loss after Py adsorption was attributed to desorption from weakly acid sites, and the mass loss at a higher temperature (>400 oC) was attributed to desorption at strongly acidic sites up to 700 oC, which was well correlated the findings of NH3-TPD result.
Metal loadings of the composites are determined by ICP-OES and SEM-EDX (Table 2, Table S5). The composites are contained the rational percentage of Al, Sn, Na, and B, which formed the complex architecture. Therefore, the reduction of SA and pore volume may be due to the partial pore blockage of ZE with these complex oxides [24]. It is justified that the drastic reduction (>50 times) of micropore volume and also the SA in Sn-B-NaYZE and Al-B-NaYZE (Table 3). But, the pore diameter (503.4-583.8 Ao) of the above composites was enhanced more than two times.
Carbonyl-ene rearrangement of citronellal
The carbonyl-ene rearrangement of citronellal is studied in Al-B-NaYZE, Sn-B-NaYZE, Al-B-YZE, and Sn-B-YZE composites (Scheme 2). Among these, Al-B-NaYZE and Sn-B-NaYZE are shown better activity (Table S2). Order followed by the catalytic activity of prepared composites are in this form Sn-B-NaYZE > Al-B-NaYZE > Sn-B-YZE > Al-B-YZE. Further, the elemental ratios of both these composites have been optimized for getting higher citronellal cyclization. The ratios of Sn-B-NaYZE (0.02:0.05:0.5) and Al-B-NaYZE (0.01:0.05:0.45) are optimum for performing a better activity (Fig. 3). This transformation has been studied in CHCl3, CH2Cl2, and cyclohexane. It has been observed that cyclohexane gave 96% conversion of citronellal to about 98% of isopulegol isomers in 2 h at 70 oC (Fig. S12). The kinetics of transformation from citronellal to isopulegol has been optimized to obtain maximum conversion in 2 h (Fig. S13).
For a greener approach, the above chemical kinetics was studied over liquid-CO2 medium for 60 min. There is 99% conversion of citronellal to about 99% of selectivity towards isopulegol isomers is attained in 45 min (Fig. 4a,b). Sn-B-NaYZE have been shown promising results with atmost enantioselectivity to (-)-isopulegol (Table 4). The enantiospecific analysis was revealed (-)-isopulegol (94%), (+)-neoisopulegol (3%), (+)-neoisoisopulegol (2%), and (+)-isoisopulegol (1%) from (+)-citronellal. Thus, the present composites have lower SA, but improved pore diameter and strong acidic character, which facilitated the exceptional conversion rate and selectivity to isopulegol. From the HRTEM image, it was observed that the Sn preferred spherical layer type of arrangements in Sn-B-NaYZE with lattice planes distance in between 0.25-0.35 nm to facilitate the carbonyl-ene rearrangement. The reaction probably took place on Lewis and Bronsted acidic sites of the composites leading to a protonated citronellal, which is further transformed to more stable carbocation via intermolecular rearrangement followed by deprotonation to isopulegol (Scheme S2a). Further, the experimental figures obtained against logarithmic of reactant concentration vs time revealed that the reaction is followed by first-order kinetics (Fig. 4e). Therefore, the reaction was dependent on the initial concentration of citronellal, and the decrease of concentration was measured along with the progress of the reaction. As displayed, both the composites are associated with trigonal and tetragonal geometry, and there is a high possibility for the effective and selective transformation of carbonyl-ene rearrangement to isopulegol (Fig. 5). The present catalytic synthesis was equally effective for the conversion of either (+)-citronellal or its racemic mixture. Similarly, the developed composites were not only selective and efficient for the conversion of pure citronellal, but they were also equally effective in citronellal-rich essential oils.
Table S8, shows the comparison of catalyst activity in cyclization of citronellal to isopulegol with reported literature. All the works were reported the conversion in organic solvent medium with poor (%) enantioselectivity to (-)-isopulegol. While in our process, we have utilized the greener approach (liquid CO2 medium) and a novel catalyst which is giving the prominent transformation of citronellal (99%) to enantiospecific (-)-isopulegol (95%) at a moderate reaction condition. Further these isoulegol isomers were utilized to produce menthol isomers (98%).
Reduction of isopulegol isomers
For reduction of isopulegol isomers, a different metal doped activated carbon catalysts were prepared and utilized at fixed reaction condition inorder to screan out the effective one (Fig. S1). Among, 1%Pd/AC and 1%Ru/AC catalysts were shown the better activity. Furhter, the effect of temperature and pressure are studied, and the results are listed in Table S4. The kinetics of reaction and distribution of menthol enantiomers with time using 1%Pd/AC and 1%Ru/AC catalysts are presented in Fig. 6. This reduction was attained by 99% conversion of isopulegols to menthols in 1 h at 60 oC and 40 psi H2 pressure using 1%Pd/AC with 98% selectivity towards (±)-menthol. While, in case of 1%Ru/AC catalyst the conversion percentage of isopulegol was quite comparable but the selectivity to (±)-menthol isomers was reduced. From the comparative study, it was noticed that the high metal loading and high temperature lead to enhance the yield of un-desirable menthol isomers. Therefore optimized temperature, pressure and catalyst loading not only helped in conversion efficiency but also lead the pathway for semi-synthesis of enantiospecific (-)-menthol.
Composites activity
The rearrangement (Prins reaction) of citronellal to isopulegol is the rate-limiting step (Fig. 4e). These composites were associated with both Lewis and Bronsted acid sites, which facilitated the cyclization of citronellal. The composites comprised of Al, Si, and Sn were provided Lewis acid sites and properly coped with B for the generation of Bronsted acid sites. Composites prepared using different ratios of Al/Sn, YZE/Na-YZE, B have been studied for the conversion of citronellal to isopulegol (Fig. 5, Table S1). The results showed that 0.5-1.0%B and 4-6% of Al/Sn in YZE/NaYZE are effective combinations for the conversion of citronellal to isopulegol (Table S1). The composites Sn-B-NaYZE and Al-B-NaYZE have been shown the best catalytic activity. Most likely, these two composites are oriented towards trigonal and tetragonal geometry as presented in Fig. 5a, which may facilitate the enhancement of the Lewis and Bronsted acidity. Therefore, these composites followed a peculiar mechanism in which both the trigonal and tetragonal configured species are actively participated in a complex form to show exceptional selectivity towards the cyclization of citronellal (Fig. 5b). Finally, the composites associated with high Lewis and Bronsted acidity, trigonal and tetragonal Metal-Na-B architecture, and improved pore diameter were facilitated the composite activities towards the Prins rearrangement.
Metal impreginated ZE were effective catalysts for monoterpene valorization [26,27]. Azkaar et al [26]. utilized Ru/H-beta-300 extrudates catalyst for citronellal conversion in a continuous flow reactor and they had obtained 67-73% of (-)-menthol. Maki-Arvela et al [9]. reported the isopulegol dimers (di-isopulegyl ethers) in the process of cyclization, but the present composites are very selective towards isopulegol without any unwanted dimers. Whereas in current process, the total 98% conversion to menthol isomers is attanined with 95% of enantioselectivty to (-)-menthol.
Enantiomeric selectivity and enrichment of (-)-menthol
The reaction was carried out with two substrates as (+)-citronellal and (±)-citronellal. The possible isopulegol enantiomers identified after completion of reaction are presented in Scheme S2b. (+)-Citronellal is found to transform 94% (-)-isopulegol as a major enantiomer. The other unfavorable diastereomers viz., (+)-neoisopulegol (3%), (+)-isoisopulegol (2%), and (+)-neoisoisopulegol (1%) are detected in trace (Fig. S14). These isopulegol enantiomers are transformed to the corresponding menthol (step 2) by reduction reaction (Fig. 7). Therefore (+)-citronellal was identified as an ideal substrate for the production of (-)-menthol in a two-step process (Scheme S3a). Menthol enantiomers obtained from the pure citronellal as well as from eucalyptus and citronella oils are estimated using chiral-GC-FID analysis. Menthol possesses eight diastereomers (four pairs of enantiomers) like (±)-menthol, (±)-isomenthol, (±)-neoisomenthol, and (±)-isoisomenthol (Fig. 7). But, (-)-menthol is the only pharmaceutically active enantiomer, which is accumulated by the Mentha plant through a biosynthetic enzymatic pathway [6]. But, this green processed (-)-menthol is further purified through derivatization. In the present process, menthyl acetate derivatives of all isomers of menthol have been prepared. But, only (-)-menthyl acetate is crystallized at 35 oC (Scheme S3b). This selective crystallization is achieved due to the structural orientation of (-)-menthyl acetate as presented in Scheme S3c. The (-)-menthyl acetate was followed a suitable chair conformer, where all bulky groups preferred to be equatorial in position; hence, the crystallization was easily attained. Whereas, (+)-menthyl acetate revealed the bulky groups in the axial position, hence the crystallization is difficult. Finally, (-)-menthyl acetate is hydrolyzed to obtain (-)-menthol as presented in Scheme S3b. This methodology was followed for the isolation of crude menthol (13 g) from Eucalyptus oil. After enrichment, the yield of (-)-menthol was 6 g. Similarly, the isolated yields of crude menthol and (-)-menthol were 5 g and 4 g from citronella oils, respectively. Therefore, citronella oil is a better substrate for the semi-synthesis to (-)-menthol as compared to the eucalyptus essential oil (Fig. 7).
NMR study of natural and synthetic menthol
(-)-Menthol, sourced from semi-synthesized essential oils (present process) and also from the petrochemical origin (synthetic available in the market) have been displayed in the same 13C-NMR spectra. 13C-NMR (δ) 31.6 (C-1), 45.1 (C-2), 71.6 (C-3), 50.2 (C-4), 23.2 (C-5), 34.6 (C-6), 22.2 (C-7), 25.9 (C-8), 16.1 (C-9), 21.0 (C-10) (Supplementary Fig. S15a,b). From the DEPT study, it is found that C-2, C-5, and C-6 are secondary carbon and others are either primary or tertiary carbons (Fig. S15c†). It is clear that menthol from natural (essential oil) and synthetic (petrochemical origin) (-)-menthol cannot be differentiated by chiral-GC-FID and NMR analyses.
Biobased index
The enantiomers of menthol are differentiated by Chiral-GC-FID and 13C-NMR, however, the 14C-radiocarbon analysis of synthetic (petrochemicals) and natural (-)-menthols are expected to give dissimilar biobased index (14C content). The bio-based index (100%) of the isolated (-)-menthol is shown a higher pMC value (>99%), and it is very close to the menthol obtained from the M. arvensis essential oil. On the other hand, the synthetic menthol prepared by the chemical industry from petrochemicals has been shown a biobased value of 0% (Table 5). This is an interesting report on origin authentication of (-)-menthol using 14C-radiocarbon-dating to validate as nature identical.
Chemical compositions and volatility of modified essential oil
The geraniol in citronella oil is partially transformed to dihydro-citronellol (Scheme 4a). In the process of catalytic modification, the partial reduction of geranyl acetate and citronellyl acetate to 3,7-dimethyloctyl acetate, 2,7-dimethylheptanol, etc is noticed (Scheme 4b). The catalytically modified oil was frozen (-40 oC) to crystallize the menthol. In addition, the spent oil contained only 13-15% menthol in dissolution state, and it is hardly crystallized due to its lower concentration. After isolation of menthol, the spent oil was enriched with other terpene alcohols (d-terpineol, terpinen-4-ol, a-terpineol, dihydro-citronellol, citronellol, geraniol), terpinyl acetate (3,7-dimethyloctyl acetate, citronellyl acetate, geranyl acetate), and sesquiterpenoids (9.8-1.5%). Interestingly, spend citronella oil was contained enrich percentage of (-)-citronellol (13.5-14.4%) and (-)-menthol (14.0-14.2%) along with linalool (1.8-1.9%), dihydro-citronellol (7.0-7.5%), geraniol (14.5-15.1%), 3,7-dimethyloctyl acetate (3.4-3.9%), citronellyl acetate (5.0-5.9%) and geranyl acetate (4.4-4.5%) are an ideal combination for improving olfactometry profile. This oil has smelled refreshing, rosy, and peppermint type with a cooling sensation. In general practice, isolation of (-)-menthol from M. arvensis essential oil, the remaining spent oil known as dementholized oil (DMO) contained 14-18% of menthol are very much in use where a low percentage of menthol is desired [5,6]. In terms of mentha oil trade, menthol crystal and DMO are equally important. Citronella essential oil contained only 38.9-41.4% of citronellal along with geraniol (20.8-21.8%), citronellol (9.9-10.0%), citronellyl acetate (4.0-4.2%), geranyl acetate (4.2-4.6%), elemol (4.2-5.1%), etc (Table 1). In the process of catalytic transformation, the above terpenoids were partially rearranged or reduced to comparatively higher volatile compounds (Scheme S4). As a result, the volatility of modified oils has been increased. Therefore, the chemical compositions are fully complying with the TGA and DTG results (Fig. S16).
From Fig. S16, the eucalyptus essential oil has displayed high volatility as compared to its modified oil. The high volatile isopulegol (10.9%) and citronellal (69.5%) are transformed to crystalline menthol (Table 1). The partial isolation of menthol as crystal was led to enhancement of the concentration of other constituents in the spent oil marked by 1,8-cineole (1.3%), d-terpineol (8.9%), a-terpineol (1.8%), terpinen-4-ol (5.9%), menthol (13.1%), citronellol (15.5%), geraniol (1.2%), citronellyl acetate (9.8%), b-caryophyllene (9.7%) and b-caryophyllene oxide (1.2%) might be responsible for enhancing the olfactometry value. Similarly, the modified oil was appended structurally related compounds such as 2,6-dimethyl heptanol (1.8-1.9%), dihydro-citronellol (7.0-7.5%), and 3,7-dimethyl octyl acetate (3.4-3.9%) due to the process of catalytic transformation (Table 1, Scheme S4). Therefore, it has been displayed a better fragrance profile due to menthol along with the above bioactive terpene alcohols.
Anti-microbial and anti-oxidant activities
In current investigations, moderate activity was exhibited by the eucalyptus oil against Gram-(+)-bacteria (Staphylococcus aureus) and also fungus viz., Candida albicans. The activity displayed by the eucalyptus essential oil has been accredited to the synergistic impact of the two major compounds viz., citronellal (69.5%) and citronellol (5.3%) [28].Further, the antibacterial activity of essential oil is found to be enhanced when the isopulegol and citronellal are converted to menthol (79.5%) (Table S7a). This activity was significantly enhanced in the spent oil against the S. aureuas after the separation of menthol crystal. The partial isolation of menthol as crystal was led to an increase in the concentration of other compounds in the spent oil such as 1,8-cineole (1.3%), d-terpineol (8.9%), a-terpineol (1.8%), terpinen-4-ol (5.9%), menthol (14.1%), citronellol (15.5%), geraniol (1.2%), citronellyl acetate (9.8%), b-caryophyllene (9.7%), and b-caryophyllene oxide (1.2%) might be responsible for enhancing the antimicrobial activities.
Citronella oils (CIM-Jeeva and BIO-13) were exhibited significant antimicrobial activities against C. albicans, S. aureus, and S. typhimurium. The observed activities in citronella oil were due to the presence of a majority of oxygenated terpenoid such as linalool (1.0-1.1%), citronellol (9.9-10%), geraniol (20.8-21.8%), citronellyl acetate (4.0-4.2%), geranyl acetate (4.2-4.6%), elemol (4.2-5.1%), a-cadinol (1.1-1.7%), etc [29]. The modified oil was appended structurally related compounds such as 2,6-dimethyl heptanol (1.8-1.9%), dihydro-citronellol (7.0-7.5%), and 3,7-dimethyl octyl acetate (3.4-3.9%) due to the process of catalytic transformation (Table 1, Scheme S4). Therefore, it has been displayed significant antifungal activity may be due to menthol along with the above bioactive terpene alcohols (Table S7b†). These spent oils are closed to the M. piperita essential oil, and are better suitable as a food-grade preservative without much altering the sensory value of the food products [1,3].
The antioxidant activity of the fresh essential oil, catalytically modified oil and spent oil of C. citriodora, CIM-Jeeva, and BIO-13 are shown in Fig. S17. For each of the three different essential oils, the modified and spent oil exhibit low IC: 50 values, and hence marked by high antioxidant activity in comparison to the fresh essential oils. The modified and spent oils have also shown high antioxidant activity at a concentration of 5 mg mL-1. Menthol has been enhanced the activity in the modified and spent oil. A similar pattern in activities was observed at lower concentrations (2.5, 1.25, and 0.6 mg mL-1) of all three essential oils.
Re-usability of catalyst
The reusailty of the heterogenous catalyst is the most promising feature of this conversion process. The isopulegol/menthol conversion rate for both the steps using Sn/Al-B-Na-YZE or 1%Pd/AC were stable upto four consecutive runs with only maxium 5% loss in yield indicating the high reusability potential of above catalysts, thereby making the overall process economical (Fig. 8). The reuse catalysts SAA, pore volume upto three consecutive runs are presented (Table 6). Only some slight variation in SAA of reused catalyst was observed as compared to the fresh catalyst, which signified of their true hereogeneous nature of catalysts.