In Situ Synthesis of Hypercrosslinked Polymer as Stationary Phase for Capillary Gas Chromatography

Abstract

Hypercrosslinked polymers (HCPs) constructed by the Friedel–Crafts alkylation reaction of aromatic compounds have emerged as a new class of porous materials with unique merit. Herein, a HCP named HCP-TPB was coated onto a capillary column through in situ synthesis. The prepared column exhibited a nonpolar nature, and the column efficiency for n-dodecane was 3003 plates m⁻¹. Moreover, the relative standard deviations of retention time and peak area for six replicate injections of the C3–C6 were lower than 0.1% and 1.5%, respectively. The results of this study showed that it is very promising to utilize HCPs as stationary phases for the separation of C3–C6.

1. Introduction

Porous organic polymers (POPs) are a class of microporous materials formed by light elements, such as C, H, O, and N, connected by valence bonds, which have the merit of high porosity, high mechanical strength, and good physicochemical stability [1,2,3]. Among all kinds of POPs, hypercrosslinked polymers (HCPs) have attracted more and more attention because of their light-weight skeleton structure, high specific surface area, good thermal stability, and easy synthesis [4]. HCPs are a kind of new polymer material with porous structure that takes aromatic compounds as precursors and uses the self-contained groups on the precursors or additional cross-linking agents to connect adjacent benzene rings through covalent bonds to form a spatial network based on the reaction mechanism of Friedel–Crafts alkylation. HCPs possess unique characteristics that make them suitable for gas chromatographic separations. HCPs exhibit robust covalent bonds interlinked by organic building blocks, leading to higher stability compared to metal–organic frameworks (MOFs) [5,6]. This stability is crucial for maintaining the integrity of the stationary phase during chromatographic separations. HCPs also have permanent porosity, allowing for the efficient adsorption and separation of analytes based on their interactions with the stationary phase [7]. More, HCPs can be easily functionalized to tailor their properties for specific separation tasks, providing versatility in chromatographic applications [8]. The predominantly microporous structure of HCPs is advantageous for the separation of small molecules and volatile organic compounds commonly analyzed in gas chromatography [9].

Therefore, HCPs attracted more and more interest in gas chromatography (GC) and high-performance liquid chromatography (HPLC) as chromatographic stationary phase to separate the tested substances [10]. Davankov et al. [11] used HPLC to study the adsorption selectivity of a series of organic compounds on hypercrosslinked polystyrene networks (HPNs) with limiting crosslinking degree, and the results showed that aromatic compounds and their sulfur-containing heterocyclic analogs could be adsorbed and separated on HPNs according to the number of p electrons on them. Tian Hong et al. [12] connected polyethylene diethylbenzene with 3-(isobutenyl chloride) propyl trimethoxysilane bonded on the wall of the tube by Fu-K reaction to obtain a capillary column with a firm structure. In the analysis of liquor standard samples, methanol, formaldehyde, ethyl acetate, and other components achieved baseline separation on the chromatographic column. Moreover, the detection operation is more convenient than the existing polyethylene glycol (PEG-20M) column. Hong et al. [13] dynamic coated of KAPs-1 onto a capillary column resulted in a nonpolar stationary phase with high separation performance for volatile organic compounds, including challenging isomers like ethylbenzene and xylene, which could not be resolved on commercial stationary phases. However, most of the above methods use the dynamic coating method, which will have the concern of stationary phase loss. The chromatographic column itself has a certain service life, which needs to be replaced after a period of use, and column loss is the main factor affecting the service life of the chromatographic column. Therefore, in the preparation of the column, in addition to considering how to improve the efficiency of the column, how to slow down the occurrence of column loss should also be considered.

Analyzing C3–C6 hydrocarbons is crucial in natural gas analysis due to their significant role in determining the composition, quality, and economic value of natural gas. These hydrocarbons, which include propane (C3), butane (C4), pentane (C5), and hexane (C6), are essential for various applications and industries. Their concentrations directly impact the energy content (calorific value) of the gas, which is critical for optimizing fuel efficiency and ensuring compliance with regulatory standards. Additionally, C3–C6 hydrocarbons are valuable raw materials in the petrochemical industry, serving as feedstocks for the production of chemicals, plastics, and synthetic materials. Accurate separation and quantification of these molecules help in monitoring the processing and refining stages, ensuring product purity and safety. Moreover, their presence can indicate certain geological conditions and reservoir characteristics, making them important markers in exploration and resource management. Therefore, reliable analysis of C3–C6 hydrocarbons is fundamental to both the energy sector and the broader chemical industry. Commercial gas chromatography (GC) columns currently used for C3–C6 hydrocarbon measurements include traditional packed columns and capillary columns, often coated with stationary phases such as polyethylene glycol, alumina, or modified siloxanes. These columns are widely used due to their established performance and availability; however, they exhibit certain limitations. Packed columns, while robust, often suffer from lower resolution, longer analysis times, and higher sample consumption compared to capillary columns. On the other hand, capillary columns, though capable of providing better separation and faster analysis, can be prone to phase degradation at high temperatures, reducing their long-term stability and selectivity, particularly for volatile hydrocarbon mixtures. Additionally, commercial columns may have limited customization options for specific analytes, requiring the use of multiple columns to achieve optimal separation. These limitations highlight the need for alternative materials, such as hypercrosslinked porous polymers (HCPs), which offer improved thermal stability, tunable porosity, and enhanced separation performance for complex hydrocarbon mixtures like C3–C6.

In this study, a capillary stationary phase with a rich and porous structure was successfully synthesized using a knitting method with a crosslinking agent. The polymerization process utilized 1,3,5-triphenylbenzene as the monomer and 1,4-dimethoxybenzene as the crosslinking agent, followed by an in situ coating of the resulting polymer onto the inner surface of the capillary. The stationary phase produced through this method exhibits a non-polar nature, making it particularly well-suited for the separation of non-polar compounds, such as C3–C6 hydrocarbons. The porous architecture of the stationary phase enhances its adsorption properties, providing an effective medium for separating low-boiling-point, non-polar substances. This characteristic allows the capillary column to achieve efficient separation of various non-polar analytes beyond C3–C6, extending its application to a wide range of non-polar substances commonly encountered in gas chromatography. Its robust separation performance, particularly for low-boiling compounds, makes this stationary phase highly versatile for analytical applications requiring the separation of volatile, non-polar mixtures.

2. Experimental Section

2.1. Chemicals and Instruments

The ultrapure water of 18.2 MΩ·cm was used throughout the experiment. The reagents used in this work are at least analytical grade reagents. Propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, decanol, dimethanol-formaldehyde, and 1,4-dimethoxybenzene were purchased from Sigma Reagent Company (Shanghai, China). Toluene, ethylbenzene, n-propylbenzene, n-butyl benzene, nitrobenzene, 1, 2-45dichloroethane, methanol, ethanol, and phemethylol were purchased from Chengdu Kelong Chemical Company (Chengdu, China). Aniline, phenol, trimethoxysilane, and 1,3,5-triphenyl benzene were purchased from J&K Scientific (Beijing, China). N-hexane, n-heptane, n-octane, n-nonane, decane, undecane, dodecane, n-tridecane, n-tetrane, benzene, n-butanol, 2-pentanone, 1-nitropropane, and pyridine were purchased from Adamas-Beta Reagent (Shanghai, China). The mixture of hydrocarbons, including 10.2 mmol/mol propane, 10.1 mmol/mol butane, 10.1 mmol/mol iso-butane, 5.04 mmol/mol pentane, 4.95 mmol/mol iso-pentane, and 10.1 mmol/mol hexane with N₂ as carrier was purchased from Zhongce Standard Material Co., Ltd. (Hangzhou, China). Fused-silica capillary was purchased from Yongnian Optic Fiber Factory (Hebei, China).

A Fuli GC-9720plus Gas Chromatograph (Fuli Instruments Co., Ltd., Wenling, China) system with a capillary control unit, a split injection port, and a flame ionization detector was used for all GC separations. Highly pure N₂ gas (99.999%) was used as the carrier gas with a linear velocity of 10–25 cm/s. The injection split ratio was 200:1. The SEM images were obtained from a JEOL JSM-7800F scanning electron microscope (Akishima, Japan) at 30.0 kV.

2.2. Synthesis of HCP-TPB

Take 1 g of anhydrous ferric chloride, 0.32 g of 1,3,5-triphenylbenzene, and 0.56 mL of dimethoxymethane, and dissolve them in 10 mL of 1,2-dichloroethane. React at a constant temperature of 80 °C for 24 h. After centrifuging the resulting brown powder, wash it with 0.1 M hydrochloric acid, followed by three washes with methanol. Vacuum dry the product in an oven at 100 °C for 12 h to obtain HCP-TPB.

2.3. Pretreatment of Capillary

To provide a reaction site for subsequent silanization, the silicon–oxygen bridge structure (Si-O-Si) on the capillary wall needs to be opened firstly to increase the density of the silanol group (Si-OH) on the wall. At the flow rate of 20 μL/min, 1 mol/L NaOH solution was filled into the capillary tube (10 m × 0.25 mm i.d.) by injection pump. The two ends were sealed for 2 h. Then, the liquid was washed with ultrapure water at the same flow rate until the liquid was neutral, and then rinsed with 0.1 mol/L hydrochloric acid at the same flow rate for 1 h, and finally washed with ultrapure water until neutral. Due to the rapid decomposition of the silanization reagent in water, in order to eliminate the residual water in the tube, it was necessary to blow dry the residual liquid through N₂ flow and dry it in the oven at 100 °C for 12 h.

To introduce monomers into the reaction system, silanization reagents containing benzene are used, which are stably bonded to the tube wall by condensation reaction of methoxy group with silanol group [14]. After mixing benzene trimethoxysilane and toluene at a volume ratio of 1:1, the mixed solution is filled into the capillary tube at a flow rate of 10 μL/min; both ends are sealed and placed into an oil bath at 110 °C for 12 h. To ensure that the wall of the capillary tube is covered with a uniform phenyl coating, the reaction steps should be repeated after the reaction residue is flushed out with toluene at a flow rate of 5 μL/min. Finally, the residue is flushed out with 1,2-dichloroethane at a flow rate of 5 μL/min and dried by a stream of N₂.

2.4. In Situ Preparation of HCP-TPB Modified Column

The pretreatment of the capillary and the preparation of the column are shown in Scheme 1. A total of 0.5 g FeCl₃ powder was dissolved in 10 mL 1,2-dichloroethane sonicated for 30 min, then centrifuged at 12,000 r/min for 5 min, and the supernatant was removed until use. Furthermore, 0.32 g 1,3,5-triphenylbenzene and 0.86 g 1,4-dimethoxybenzene were dissolved in the solutions and sonicated for 5 min to fully dissolve. The obtained solution was filled into the capillary at a flow rate of 5 μL/min, and both ends were sealed at 70 °C for 24 h. After the reaction was completed, the residual solution was flushed out with methanol at the same flow rate. Then, the capillary was rinsed with 0.1 mol/L HCl at a flow rate of 5 μL/min for 1 h and washed with methanol three times. Finally, the capillary was dried by N₂ flow.

2.5. Test of Column Performance

Using dodecane as the sample to be tested, the column efficiency of the capillary column was tested at flow rates between 0.1 and 0.5 mL/min. Each flow rate was measured five times in parallel, and the theoretical number and height of chromatographic peaks under each condition were calculated. The results were averaged, and the Van Deemter curve was drawn to determine the optimum flow rate for separation using this column.

The McReynolds constant was used to evaluate the polarity of the stationary phase [15], and the retention time of air was used as the dead time to calculate the adjusted retention time of each component. The reference materials selected were benzene, n-butanol, 1-nitropropane, 2-pentanone, and pyridine. The retention time of n-alkane of C6–C14 and the above five substances were measured, respectively (experimental conditions: injection temperature, 280 °C; column temperature, 100 °C; detector type, FID; detector temperature, 280 °C; carrier gas, high purity nitrogen, 1.2 mL/min; split ratio, 400:1; gas, hydrogen, 30 mL/min; air, 300 mL/min).

3. Results and Discussion

3.1. Characterization of the Synthesized HCP-TPB and Coated Column

According to the time sequence and the different types of precursors used, the preparation methods of HCPs can be roughly divided into the following three: (1) post-crosslinking method; (2) one-step self-condensation method; and (3) “knitting” method [16]. Compared with the above two methods, this method further expands the range of reaction precursors, such as benzene, toluene, benzyl alcohol, and other simple aromatic compounds, that can be directly used as precursors for synthesis. On the other hand, the simplification of precursors makes the synthesis of HCPs no longer dependent on the halogen atoms on the monomer or cross-linking agent, which reduces the cost and environmental pollution. In this paper, the HCP-TPB was synthesized as described in Section 2.3 through the knitting method. The specific surface area and pore size distribution of the prepared HCP-TPB can be obtained through an N₂ adsorption–desorption experiment. As shown in Figure 1, the N₂ adsorption–desorption isotherm of HCP-TPB indicates type I adsorption behavior. According to BET analysis, the specific surface area of HCP-TPB is calculated to be 1028 m²/g. And the pore size analysis presents the pore size distribution of HCP-TPB, revealing a characteristic pore size of 0.8 nm, exhibiting sieving effects on the C3–C6 hydrocarbon mixture [17].

As illustrated in Figure 2, the thermogravimetric analysis (TGA) of the HCP-TPB polymer reveals a slight weight loss below 200 °C, which can be attributed to the evaporation of residual solvents that were not fully removed during the polymer synthesis process. This initial weight loss is relatively minor, indicating that the polymer has good thermal stability at lower temperatures. As the temperature increases up to approximately 550 °C, the TGA curve shows a gradual and continuous decrease in weight, likely due to the desorption of solvents that are more deeply embedded within the polymer matrix. These buried solvents are released slowly as the polymer is heated, contributing to the steady weight reduction observed in this temperature range. Beyond 550 °C, the rate of weight loss for the HCP-TPB polymer accelerates significantly. This rapid decomposition suggests the onset of thermal degradation of the polymer’s structure. Based on this sharp increase in weight loss, it can be concluded that the HCP-TPB polymer undergoes substantial breakdown at temperatures above 400 °C. Therefore, for practical applications, particularly when used as the stationary phase in gas chromatography, the operating temperature of columns utilizing HCP-TPB should not exceed 400 °C. This temperature limit ensures that the polymer retains its structural integrity and separation performance during analytical procedures.

The morphology of the synthesized hyper-crosslinked polystyrene-based polymer (HCP-TPB) and the inner wall of the capillary tube was examined, with the results displayed in Figure 3A–C. Figure 3A reveals the surface structure of the HCP-TPB, which exhibits a loose and porous characteristic. This porous architecture is consistent with the anticipated design of the material, as it enhances the polymer’s ability to adsorb analytes when used as a stationary phase. Such a porous structure is highly advantageous for chromatographic applications, as it promotes efficient separation by providing ample surface area for interactions between the stationary phase and the target compounds. Figure 3B,C focus on the inner surface of the capillary tube, where the target polymer was deposited. These images confirm that the HCP-TPB was successfully synthesized and adhered to the inner surface of the capillary. However, it is also evident that the distribution of the polymer is not entirely uniform. In many areas, the polymer appears to grow in a layered manner, with some regions displaying aggregation where the material has clustered into larger blocks. This uneven distribution could potentially impact the performance of the column by creating localized variations in the stationary phase, which may affect the separation efficiency. While the successful synthesis of the polymer is clear, these observations suggest that further optimization may be required to achieve a more homogenous coating of the polymer on the capillary surface. Improving the uniformity of the polymer distribution could enhance the overall chromatographic performance, ensuring more consistent separation and reducing potential variations in analyte retention.

To evaluate the column efficiency of the prepared capillary column, dodecane was used as the test sample, and experiments were conducted at carrier gas flow rates ranging from 0.1 to 0.5 mL/min. For each flow rate, measurements were repeated five times to ensure consistency and reproducibility. The theoretical plate number and peak height were calculated under each condition, providing a quantitative assessment of the column’s performance. The resulting Van Deemter curve, as depicted in Figure 4, illustrates the relationship between flow rate and column efficiency. This curve is a critical tool in chromatographic optimization, showing how flow rate influences factors such as mass transfer, longitudinal diffusion, and eddy diffusion. From the curve, it is clear that the optimal flow rate for the carrier gas is 0.2 mL/min. At this flow rate, the column efficiency reaches its maximum, as indicated by the highest theoretical plate number and minimized peak height. Given these results, a flow rate of 0.2 mL/min will be used for all subsequent separation experiments, as it ensures optimal performance of the capillary column by balancing separation efficiency with analysis time. This optimized flow rate will contribute to achieving accurate and reliable chromatographic separations in further tests.

The polarity of the stationary phase is a critical factor that significantly influences both the separation efficiency and selectivity in gas chromatography. To quantify the polarity of a stationary phase, McReynolds constants are widely used. These constants are determined based on the retention behavior of five specific analytes: benzene, n-butanol, 2-pentanone, 1-nitropropane, and pyridine. Each of these analytes represents a different interaction characteristic: benzene as an electron donor, n-butanol as a proton donor, 2-pentanone for dipole orientation, 1-nitropropane as an electron acceptor, and pyridine as a proton acceptor. In this study, the retention indices of these five analytes were measured using the synthesized HCP-TPB capillary column. To assess the polarity of the HCP-TPB stationary phase, the retention indices of these analytes were compared to those obtained on a non-polar stationary phase, such as squalane. The differences in retention indices, which reflect the polarity of the stationary phase, are provided in

Table 1. The McReynolds constant of the prepared stationary phase.

Component	I	Is	ΔI	ΔIsum	ΔIave
Benzene	587	653	−66	599	120
n-butanol	876	590	286
2-pentone	688	627	61
1-nitropropane	794	652	142
Pyridine	875	699	176

. The calculated average McReynolds constant for the HCP-TPB stationary phase was found to be 120. This value indicates that the HCP-TPB column possesses moderate polarity. This level of polarity makes the HCP-TPB stationary phase particularly suitable for the separation of C3–C6 hydrocarbons, as it balances the interactions needed for effective separation of hydrocarbons with varying polarity and molecular structure. The moderate polarity enhances the column’s ability to provide selective retention, ensuring efficient separation without excessively long analysis times.

3.2. The Separation Performance of HCP-TPB Column

At a constant flow rate of 0.2 mL/min, a series of temperature-controlled separation tests were conducted on a hydrocarbon mixture comprising C3–C6 components (10.2 mmol/mol of C3, 10.1 mmol/mol of C4, 10.1 mmol/mol of iso-C4, 5.04 mmol/mol of C5, 4.95 mmol/mol of iso-C5, and 10.1 mmol/mol of C6) using nitrogen as the carrier gas. The chromatographic separations, shown in the graphic, illustrate how the retention times and peak resolutions vary as a function of temperature. Each separation experiment was repeated five times under identical conditions to ensure reproducibility, and the results are represented in Figure 5. The corresponding separation factors are listed in

Table 2. Resolution at different temperatures.

T/oC	RC3, iso-C4	RisoC4, C4	RC4, iso-C5	Riso-C5, C5	RC5, C6
35	2.824	2.444	9.052	4.707	25.024
40	2.606	2.228	8.000	4.202	24.294
50	1.939	1.657	5.808	2.974	18.688
60	1.515	1.229	4.219	2.135	14.444
70	1.181	0.914	3.194	1.556	10.632
80	0.912	0.641	2.175	1.139	7.627

, which demonstrates that the prepared gas chromatography column achieved baseline separation for the C3–C6 mixture, confirming the column’s high efficiency. The stationary phase material, hyper-crosslinked polystyrene-based (HCP-TPB), plays a critical role in the selective separation of the hydrocarbons, effectively screening the C3–C6 components. The graphic shows that as the temperature increases from 35 °C to 80 °C, the separation factors decrease, indicated by the narrowing of the peak distances. Although higher temperatures reduce selectivity (as evidenced by reduced peak separation), they also significantly shorten the analysis time. This highlights the trade-off between separation efficiency and analysis speed. The optimal separation temperature was determined to be 40 °C, where a balance between resolution and analysis time is achieved. At this temperature, the peaks are well resolved with clear separation, as seen in the graphic’s red chromatogram, making it the most effective for quantitative chromatographic analysis.

To assess the stability and reproducibility of the synthesized gas chromatography column, 11 consecutive injections of the C3–C6 hydrocarbon mixture were conducted. The resulting chromatograms demonstrate consistent separation performance across all tests. The average retention times and retention factors for C3–C6 across these 11 tests are detailed in

Table 3. The average retention times and k of C3–C6 (n = 11) with RSD.

	Retention Time		k		Peak Area
	Average (min)	RSD (%)	Average	RSD (%)	RSD (%)
C3	1.610	0.070	0.337	0.150	0.867
iso-C4	1.696	0.069	0.408	0.330	0.859
C4	1.774	0.075	0.472	0.219	0.798
iso-C5	2.074	0.081	0.721	0.190	0.930
C5	2.240	0.086	0.859	0.150	1.044
C6	3.571	0.075	1.964	0.128	1.450

. Remarkably, the relative standard deviations (RSD) for retention times and retention factors were found to be less than 0.1% and 0.4%, respectively, indicating minimal variation between runs and excellent stability of the column. Additionally, the peak areas, which are crucial for quantitative analysis, also exhibited outstanding reproducibility. The relative standard deviation for peak areas was less than 1.5%, further confirming the reliability of the column for accurate and consistent analytical measurements. This high level of reproducibility is attributed to the stationary phase, HCP-TPB, which provides stable interactions with the C3–C6 components, ensuring uniform separation across repeated injections. The near-identical chromatographic profiles shown in Figure 5 emphasize the robustness of the prepared capillary gas chromatography column. Even after multiple consecutive injections, there is no significant shift in peak retention times or changes in peak shapes, demonstrating that the column maintains its performance under repeated use, a key factor in practical analytical applications.

4. Conclusions

In this study, we successfully synthesized a HCP-TPB as a stationary phase for gas chromatography using a knitting method with 1,3,5-triphenylbenzene as the monomer and 1,4-dimethoxybenzene as the crosslinking agent. The resulting stationary phase demonstrated several advantageous characteristics, including high specific surface area, a rich porous structure, and excellent thermal stability up to 400 °C. These features make HCP-TPB highly suitable for the separation of small, non-polar molecules, particularly low-boiling hydrocarbons such as C3–C6. Through a series of temperature-controlled tests, we demonstrated that the HCP-TPB column achieved efficient baseline separation of the C3–C6 hydrocarbon mixture, with the optimal separation temperature determined to be 40 °C. The column exhibited excellent performance in terms of separation efficiency, with clear peak resolution and consistent results across multiple tests. Additionally, the McReynolds constant analysis indicated that the HCP-TPB stationary phase possesses moderate polarity, making it versatile for separating a broad range of non-polar compounds. Stability and reproducibility tests further confirmed the robustness of the prepared capillary column. The column maintained its performance across 11 consecutive injections of C3–C6 hydrocarbons, showing minimal variation in retention times, retention factors, and peak areas. The low RSDs for these parameters underscore the reliability and reproducibility of the HCP-TPB stationary phase for analytical applications. Overall, the synthesized HCP-TPB capillary column provides a highly efficient and stable solution for the separation of volatile, non-polar compounds in gas chromatography. Its consistent performance, thermal stability, and ease of synthesis make it a promising alternative to existing stationary phases, with broad potential applications in both natural gas analysis and other fields requiring the separation of low-boiling-point hydrocarbons. Future work could focus on optimizing the uniformity of the polymer coating and further expanding the range of analytes that can be efficiently separated using this column.