Enantioselective adsorption of limonenes and α-pinenes on germanium oxide metal-organic framework

doi:10.21203/rs.3.rs-7470067/v1

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Enantioselective adsorption of limonenes and α-pinenes on germanium oxide metal-organic framework

https://doi.org/10.21203/rs.3.rs-7470067/v1

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Chiral metal-organic frameworks (CMOFs) are promising materials for catalysis, chromatography, and sensor applications. Typically, chirality in such frameworks is achieved by using chiral ligands. However, there are rare examples of CMOFs that form without any chiral source, usually via spontaneous symmetry breaking, resulting in supramolecular chirality. Germanium oxide framework SU-MB exhibits a unique mechanism of chirality emergence: during synthesis, only micropores of one handedness are selectively filled. This paper investigates the adsorption of limonene and α-pinene enantiomers on SU-MB. The framework demonstrated consistent enantioselectivity for both pairs of enantiomers, with a higher selectivity coefficient observed for α-pinenes than for limonenes. This difference arises from distinct trends in the isosteric heats of adsorption (Q_st): for limonenes and (−)-α-pinene, Q_st approaches the heat of liquefaction with increasing adsorption, whereas for (+)-α-pinene, Q_st remains unchanged. These findings show that even a non-chiral crystal without chiral centers can exhibit enantioselectivity in adsorption processes, because of helical pores of one handedness.

chiral metal-organic frameworks

helical pores

adsorption isotherms

enantioselectivity

Enantiomer adsorption on germanium oxide framework SU-MB was studied

SU-MB framework lacks asymmetric carbon, but has helical micropores

Only one type of chiral micropores is available for adsorption

A selective adsorption of limonene and α-pinene enantiomers was revealed

For α-pinenes selectivity was higher because of the difference in heats of adsorption

Since the beginning of the 20th century, the industrial application of adsorbents has driven the need for porous materials with tailored properties. One of the key requirements for adsorbents is uniform pore structure [1]. The first materials with homogeneous pores were zeolites [2, 3]. However, their low pore variability and high polarity limited their widespread use. In 1992, mesostructured silicates of the MCM-41 type were introduced [4, 5]. These adsorbents exhibited high uniformity and larger pore sizes compared to zeolites, with specific surface areas exceeding 1000 m²/g [6, 7]. Nevertheless, their high cost, limited tunability in pore diameter and polarity, and excessive adsorption activity hindered broad adoption.

The challenge of pore heterogeneity can be addressed using metal-organic frameworks (MOFs). The first MOF was reported by Yaghi in 1995 [8]. MOFs represent a promising class of porous materials that enable precise control over pore size and architecture [9]. By selecting specific metal ions and organic ligands, one can tailor pore polarity and structure [10, 11]. The unique properties of MOFs make them suitable for gas capture and storage, chromatography, catalysis, chemical sensing, drug delivery, extraction and even hydrogen isotope separation [9, 12–15].

All MOFs are crystalline, meaning they exhibit characteristic features such as space groups and crystal chirality [16]. Chirality in crystals can be considered at two hierarchical levels: molecular chirality (e.g., asymmetric carbon centers), and supramolecular chirality, arising from the asymmetric arrangement of atoms, molecules, or ions, leading to chiral crystal units [17–20] or nanostructures [21–23]. Supramolecular chirality often manifests as helical structures [24, 25]. Chiral MOFs (CMOFs) are promising for enantioselective adsorption, chromatographic separation, and asymmetric catalysis [26–29]. Conventionally, a MOF is termed "chiral" if it contains asymmetric carbon atoms, typically resulting in both molecular and supramolecular chirality. However, some MOFs lack chiral centers yet exhibit crystal-level chirality due to supramolecular asymmetry, requiring them to belong to one of the 65 Sohncke space groups [30].

This class of CMOFs remains understudied, with only a few reported examples of CMOFs without asymmetric carbons [31–35]. This is due to formation of chiral racemate in crystal in most cases; lacking the external source of chirality during solvothermal reaction; unknown mechanism of spontaneous symmetry breaking [36–38]. Even when chiral conglomerates form, without external influence, they typically yield a 1:1 mixture of dextrorotatory and levorotatory crystals. To obtain CMOFs from achiral precursors, several strategies have been explored. For instance, Huang et al. synthesized a nontrivial porous CMOF from achiral CuCl₂·(H₂O)₂ and 3-amino-5-carboxy-1,2,4-triazole under hydrothermal conditions [33]. This CMOF forms a racemic conglomerate (space group I4₁22) with two distinct pore types: right-handed with diameter of 14 Å and left-handed of 4.9 Å. By selecting guest molecules sized between 4.9 Å and 14 Å, this MOF demonstrates enantioselectivity, as confirmed by limonene adsorption experiments [39]. Another example is a 3D chiral porous framework based on In(III) and 1,2,4,5-benzenetetracarboxylic acid [31]. While this CMOF crystallizes as a conglomerate, the enantiomeric ratio deviated from the expected 50:50 distribution. The observed Cotton effect confirmed spontaneous symmetry breaking during synthesis, though the mechanism remains unclear. Similar findings have been reported by other groups [34, 37, 38, 40].

In this paper a CMOF based from achiral germanium dioxide and 2-methylpentamethylenediamine was studied. This CMOF (SU-MB) was firstly suggested by Zou [32]. SU-MB features two gyroidal micropore types with opposite chirality and pore diameter 12 Å. In the presence of hydrofluoric acid, one micropore type becomes selectively filled with additional clusters. This feature can be used to achieve selective adsorption of enantiomers.

The Soai group previously demonstrated that chiral crystals can determine molecular chirality when acting as catalysts in the Soai reaction [41–44]. It proved the capability of surface with supramolecular chirality to recognize enantiomers. Our previous studies revealed differential adsorption of limonene and α-pinene enantiomers on some supramolecular CMOF [39], zeotype adsorbent [45], and non-porous crystals [46, 47]. All of them hadn’t asymmetric carbon as SU-MB. But in the case of SU-MB the way of chirality emergence is unique. So, it was interesting to ascertain the chiral recognition mechanism during adsorption on this CMOF.

2.1. Materials

GeO₂ (99,99%, Acros Organics, China, CAS 1310-53-8), MPMD (2-methylpentamethylenediamine, 98,0%, TCI, Japan, CAS 15520-10-2), hydrofluoric acid 99%, Reahim CAS 7664-39-3). Highly purified water obtained on a DV-10UV water deionizer (TsvetChrom, Dzerzhinsk, Russia) was used.

In the inverse gas chromatography experiment, adsorption isotherms were measured for D-limonene (97%, Sigma-Aldrich, Milwaukee, USA, CAS 5969-27-5) and L-limonene (96%, SigmaAldrich, Milwaukee, USA, CAS 5989-54-8), as well as for α-(-)-pinene (99%, Sigma-Aldrich, Milwaukee, USA, CAS 7785-26-4) and α-(+)-pinene (99%, Sigma-Aldrich, Milwaukee, USA, CAS 7785-70-8). The enantiomers chosen have dimensions allowing to adsorb in 12 Å micropores of SU-MB. Helium (99.995%, Techgas, Orenburg, Russia, CAS 14762-55-1) was used as the carrier gas.

2.2. Methods

According to [32] GeO₂ (1 g), MPMD (12,9 mL), water (6.88 mL), hydrofluoric acid (0.24 mL) were loaded in 100 mL autoclaves under autogenous pressure and heated at 165 °С during 11 days. Crystals were washed with water and dried at air.

X-ray powder diffraction data for MOF studied were recorded on a Shimadzu XRD 7000 X-ray powder diffractometer for CuKα. Recording conditions: scanning angles: 5–40 deg, scanning step: 1 deg⸱min^− 1.

The size and morphological features of zeotype material were detailed by field emission scanning electron microscopy (FE-SEM) with a Hitachi Regulus SU8220 scanning electron microscope (Tokyo, Japan). The images were taken in the mode of registration of secondary electrons at 2 kV accelerating voltage.

Thermal stability of MOFs was evaluated by thermogravimetric analysis (TGA) with a TGA Q500 instrument (TA Instruments, US). The experiments were carried out in open platinum crucibles under argon flow in a temperature range of 25–670°C at a heating rate of 10°C⸱min^− 1.

DSC experiments were conducted by DSC on a DSC Q-100 instrument (TA Instruments) at a constant heating rate 10 K⸱min^− 1 in the temperature range from 10 to 300°C. The processing of the experimental data was performed using the TA Universal Analysis 2000 v.4.5A software.

In order to ascertain the enantioselectivity, the adsorption isotherm analysis was used. Isotherms were obtained by inverse gas chromatography (IGC) [48, 49]. IGC allows to obtain adsorption isotherms of vapors, much quickly than by other methods. Sample studied is packed in chromatographic column, and well-known substances are injected.

The fraction of crystals powder of MOF studied was filled into a 300⸱3 mm stainless steel column. The column was conditioned at a temperature of 150 ˚С for 10 h; helium was used as a carrier gas. IGC studies were carried out on a CrystalLyuks 4000M gas chromatograph (Meta-Chrom, Yoshkor-Ola, Russia) equipped with a thermal conductivity detector. The column temperature ranged from 90 to 160 ˚С. The temperature of the injector and detector was 200 ˚С. The helium carrier gas flow rate varied from 2 to 20 mL min⁻1. Enantiomers samples were injected as liquids with volume ranged from 1 to 10 µL.

2.3. Calculations

The number of adsorbed enantiomers molecules and the vapors partial pressure were calculated described in [50]. The experimental adsorption isotherms were approximated by well-known Langmuir and Freundlich equations.

From experimental chromatograms specific retention volumes, mL⸱g^− 1, were calculated:

$$\:{V}_{g}^{0}=\frac{j\left(t-{t}_{m}\right)\omega\:}{{m}_{a}}$$

t is the retention time, s, t_m is the dead retention time, s, ω is the carrier gas flow rate, mL⸱s^− 1, m_a is the mass of the adsorbent in the column, j is the James-Martin coefficient.

The enantioselectivity coefficient was calculated as follows:

$$\:\alpha\:=\frac{{V}_{g2}^{0}}{{V}_{g1}^{0}}$$

where $\:{V}_{g2}^{0}$ >$\:{V}_{g1}^{0}$. In order to prove the significance of enantioselectivity, t-test was used.

The isosteric heats of adsorption q_st, kJ⸱mol^− 1, were calculated from the slope of lnp vs. 1/T dependence[51]:

$$\:{\left(\frac{dlnp}{dt}\right)}_{a}=\frac{{q}_{st}}{R{T}^{2}}$$

To determine whether a significant difference in adsorption values existed, a t-test was employed. This method was used to evaluate the null hypothesis that the adsorption values for the two enantiomers were statistically identical, by p-value calculating. The p-value was compared against a predetermined significance level (α = 0.05)[52]. When the p-value was found to be lower than α, the null hypothesis was rejected, indicating that a statistically significant difference in adsorption values between the enantiomers was present. The t-test was applied to all individual data points across the different adsorption isotherms.

In order to prove the accurate CMOF synthesis powder XRD and SEM images data were obtained. The comparison with [32] has shown the triangular form of the crystals in both cases (Figs. 1). Additionally, five peaks at 6.9, 8.0, 9.8, 12.2 and 14.4 deg were found (Fig. 2). It corresponds to XRD data from[32]. So, SEM and XRD data have supported the CMOF accurate synthesis.

In order to estimate the temperature limit for measurements, TGA analysis was performed. The data on weight losses are presented in Fig. 3 (differential curve) and Fig. 1S (integral curve). Up to 150°C, the mass loss was insignificant (less than 3%). This allows the determination of a temperature range for IGC measurements up to 150°C. Two maxima are observed on the curve (Fig. 3). The first one appears at 250°C, and the second one at 400°C. The first maximum may be attributed to the loss of crystal water, while the second could result from partial decomposition of MPMD. The weight loss at temperatures above 600°C is likely due to the complete destruction of the framework.

A difference in the adsorption of limonene enantiomers was observed on SU-MB (Fig. 4, Figs. 3S–5S). At 130 and 150°C, the apparent difference in limonene adsorption was maximal. As with other samples exhibiting supramolecular chirality [47, 53, 54], enantioselectivity increased with adsorption values. To assess the reliability of the observed chiral recognition, a t-test was employed. The p-values are listed in Table 1 and Tables 1S–4S. At 110°C, no chiral recognition was observed up to 400 Pa (Table 1). However, at higher pressures, a statistically significant difference in adsorption values was detected. At other temperatures, p < α at all pressures, indicating enantioselectivity even in cases where no visible difference was apparent in the figures. Enantioselectivity in a wide range of pressures is observed only for porous chiral crystals [39, 45]. In pores, a smaller amount of substance is required to form an enantiomer layer, enabling chiral recognition even at low enantiomer concentrations.

For limonenes, no dependence of the enantioselectivity coefficient (α) on temperature was observed. The α values ranged from 1.06 at 110°C to 1.07 at 150°C. This is unusual, as enantioselectivity generally decreases with temperature^[55]. The achieved enantioselectivity was similar to that reported for the [{Cu₁₂^I(trz)₈}4Cl 8H₂O]_n MOF sample [39]. Thus, for limonenes, the studied sample exhibited a typical enantioselectivity level.

All isotherms in Figs. 4 and 3S–5S can be classified as BET type III. Such isotherms are well approximated by the Freundlich equation. The results of the approximation are presented in Table 2. All isotherms were fitted with high correlation coefficients (R ≥ 0.99). The Freundlich constants differed beyond the margin of error at 130, 140, and 150°C, corresponding to the isotherms with the highest enantioselectivity. Generally, adsorption isotherm constants derived from experimental data are less sensitive to enantioselectivity, and at low enantioselectivity levels, they rarely differ significantly.

For α-pinene, the difference in enantiomer vapor adsorption was higher than for limonenes (Figs. 5–6, Figs. 6S–10S). At 90 and 110°C, the first adsorption isotherm measurements for α-pinenes exhibited BET type I behavior (Fig. 5, Fig. 6S). Differences in enantiomer adsorption were observed at both low and high pressures (Table 3, Table 4S). However, at certain pressures (up to 292.3 Pa, 113–226.1 Pa, and 146.1 Pa at 90°C; 153.8 and 217.5 Pa at 110°C), no significant difference was detected.

Subsequent measurements at 120°C still showed BET type I isotherms (Fig. 9S). At this temperature, the adsorption of α-pinene enantiomers was statistically different at all partial pressures. However, after heating at 120°C, the isotherm shape at 90–110°C changed to BET type III (Fig. 6, Figs. 7S–8S). This shift from type I to type III indicates weakened adsorbate-adsorbent interactions, likely due to the collapse of the smallest pores at 120°C. Statistical analysis of enantiomer adsorption differences in subsequent measurements revealed a broader enantioselectivity range. For instance, at 110°C, adsorption values differed significantly at all pressures (Table 4), as was the case at other temperatures (Tables 5S–6S).

In all measurements at 120 and 130°C, the isotherms had a BET type I shape. The change in isotherm shape upon reaching 120°C suggests an uneven weakening of adsorbate-adsorbate and adsorbate-adsorbent interactions with increasing temperature. Since adsorbate-adsorbate interactions weaken faster than adsorbate-adsorbent interactions, the latter eventually dominate, altering the isotherm shape.

Table 1
P-values for pairs of points of limonenes vapors adsorption at 110°C (α = 0.05, n = 5)
Р, Pa	р	Р, Pa	р	Р, Pa	р
227	0.0906	245	0.1699	263	0.2861
335	0.9337	400	0.004	417	0.0045
435	0.0049	453	0.005	471	0.005
489	0.0046	508	0.0043	526	0.0035

Table 2
Approximation parameters of limonenes adsorption isotherms by Freundlich equation
	S-(-)-limonene			R-(+)-limonene
	K_f· 10⁴	n	R	K_f· 10⁴	n	R
110	80 ± 10	1.34 ± 0.02	0.998	100 ± 10	1.28 ± 0.02	0.9993
130	43 ± 6	1.30 ± 0.02	0.9981	65 ± 7	1.23 ± 0.02	0.9982
140	51 ± 6	1.23 ± 0.02	0.9987	69 ± 6	1.18 ± 0.01	0.9993
150	59 ± 5	1.16 ± 0.01	0.9992	79 ± 4	1.11 ± 0.01	0.9998

Table 3
P-values for pairs of points of α-pinene vapors adsorption at 110°C (α = 0.05, n = 5, first measurement)
Р, Pa	р	Р, Pa	р	Р, Pa	Р
64	0.0451	154	0.23	218	0.057
281	0.0227	371	0.0296	435	0.0122
499	0.0058	589	0.0045	653	0.0028

This phenomenon has been previously reported by our group for both non-porous and porous chiral crystals [45, 46]. At 120°C, the adsorption isotherms of α-pinene enantiomers differed at all partial pressures, whereas at 130°C, the adsorption values coincided. The decrease of chiral recognition at higher temperatures can be attributed to the proximity to α-pinene’s boiling point. A surface with supramolecular chirality cannot recognize enantiomers in the gaseous state

Table 4
P-values for pairs of points of α-pinene vapors adsorption at 110°C (α = 0.05, n = 5, second and any further measurements)
Р, Pa	р	Р,kPa	р	Р, Pa	р
108	0.00001	328	0.00001	392	0.00004
431	0.0001	504	0.0001	537	0.0001
574	0.0003	593	0.0003	610	0.0005

Table 5
Approximation parameters of α-pinenes adsorption isotherms by the Freundlich equation
T	(-)-α-pinene			(+)-α- pinene
T	K_f·10⁴	n	R	K_f·10⁴	n	R
90	272 ± 2	1.10 ± 0.01	0.9999	220 ± 10	1.15 ± 0.01	0.9996
100	243 ± 5	1.07 ± 0.01	0.9999	180 ± 8	1.14 ± 0.01	0.9997
110	189 ± 4	1.06 ± 0.01	0.9999	136 ± 6	1.13 ± 0.01	0.9998

Table 6
Approximation parameters of α-pinenes adsorption isotherms by the Langmuir equation
T	(-)-α-pinene			(+)-α- pinene
T	K_L·10²	a_m	R	K_L·10²	a_m	R
90	64 ± 2	34 ± 3	0.9996	57 ± 2	39 ± 3	0.9995
110	11 ± 1	35 ± 3	0.9998	11 ± 1	38 ± 3	0.9996
120	2.3 ± 0.5	42 ± 6	0.9822	0.19 ± 0.02	49 ± 5	0.9901
130	0.17 ± 0.02	108 ± 9	0.9955	0.17 ± 0.02	112 ± 9	0.9961

because the adsorbed layer behaves as a two-dimensional van der Waals gas, lacking short- or long-range order [56].Comparison of limonene and α-pinene isotherms reveals that for limonenes, chiral recognition persisted even at 150°C (27°C below their boiling point), particularly at high partial pressures. In contrast, for α-pinenes at 130°C (25°C below boiling), no chiral recognition was observed. This suggests that the adsorption layer of the bulkier α-pinene molecule is less stable than that of limonene.

Adsorption isotherms at 90–110°C from subsequent measurements were approximated using the Freundlich equation (Table 5). The approximation quality was high (R ≥ 0.9996), and adsorption constants differed at all temperatures, decreasing with temperature for both enantiomers. For BET type I isotherms (90–130°C), R values were also high. The monolayer capacity (a_m) remained constant at 90 and 110°C but increased at higher temperatures (Table 6). At all temperatures, a_m values for both enantiomers

Table 7
Enantioselectivity coefficients for limonenes and α-pinenes
T	α
T	limonenes	α-pinenes
90	-	1.12
100	-	1.09
110	1.06	1.15
120	-	1.06
130	1.07	1.00
140	1.05	-
150	1.07	-
160	1.05	-

coincided, as expected. However, Langmuir constants at 90 and 120°C were not equal, supporting the conclusion of enantioselectivity.

When comparing enantioselectivity of limonenes and α-pinenes, α-pinenes exhibited a maximum α of 1.15 at 110°C, whereas limonenes reached only 1.07 (Table 7). Data at 130°C could not be compared due to the loss of α-pinene enantioselectivity near its boiling point. The higher enantioselectivity for α-pinenes may result from a greater difference in the isosteric heats of adsorption (Q_st) between enantiomers (Fig. 7). For limonenes, Q_st increased with adsorption, but the difference was notable only at low coverages. In contrast, for α-pinenes, Q_st exhibited a more pronounced divergence, suggesting a different adsorption mechanism.

For limonene enantiomers, Q_st values at high coverages approached the heat of liquefaction (L), consistent with typical adsorption behavior. At coverages > 1, the second and further adsorption layers form, for which Q_st≈L. A similar trend was observed for (−)-α-pinene, but not for (+)-α-pinene, indicating that the latter did not form a complete monolayer. This difference in monolayer formation likely contributed to the additional enantioselectivity observed for α-pinenes.

On the SU-MB sample studied, differences in the adsorption of both limonene and α-pinene enantiomers were observed. The maximal enantioselectivity coefficient (α) was found to be 1.15. The reliability of enantioselectivity was confirmed by t-test results. This CMOF sample exhibited consistent enantioselectivity, which is unusual given the absence of asymmetric carbon atoms. The observed chirality was of supramolecular origin. In this sample, half of the cavities were occupied, and all of them exhibited the same handedness [32], leading to the emergence of supramolecular chirality. This CMOF is of particular interest because chirality was manifested solely through helical cavities, while the crystal as a whole remained achiral.

These findings demonstrate that the presence of an asymmetric structural element, such as pore helicity, is sufficient for enantiomer recognition during adsorption. This expands the range of adsorbents that can be utilized for enantioselective adsorption processes.

Funding

This work was supported by the Russian Science Foundation (project No. 25-13-20076).

The authors have no competing interests to declare that are relevant to the content of this article.

Acknowledgements

This work was performed with the aid of equipment of the Shared Facility Center CKP FMI of the Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences.

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Enantioselective adsorption of limonenes and α-pinenes on germanium oxide metal-organic framework

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Abstract

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1. Introduction

2. Materials and Methods

2.1. Materials

2.2. Methods

2.3. Calculations

3. Results and discussion

4. Conclusion

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References

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