Pervaporation and Gas Separation Properties of High-Molecular Ladder-like Polyphenylsilsesquioxanes

This paper presents the results of studies on the pervaporation properties (for benzene/hexane mixtures) and gas permeability (for He, H2, N2, O2, CO2, CH4, C2H6, and C4H10) of ladder-like polyphenylsesquioxanes (L-PPSQ) with improved physical and chemical properties. These polymers were obtained by condensation of cis-tetraphenylcyclotetrasiloxanetetraol in ammonia medium. The structure of L-PPSQ was fully confirmed by a combination of physicochemical analysis methods: 1H, 29Si NMR, IR spectroscopy, HPLC, powder XRD, and viscometry in solution. For the first time, a high molecular weight of the polymer (Mn = 238 kDa, Mw = 540 kDa) was achieved, which determines its improved mechanical properties and high potential for use in membrane separation. Using TGA and mechanical analysis methods, it was found that this polymer has high thermal (Td5% = 537 °C) and thermal-oxidative stability (Td5% = 587 °C) and good mechanical properties (Young’s module (E) = 1700 MPa, ultimate tensile stress (σ) = 44 MPa, elongation at break (ε) = 6%), which is important for making membranes workable under various conditions. The polymer showed a high separation factor for a mixture of 10% wt. benzene in n-hexane (126) at a benzene flow of 33 g/(m2h).


Introduction
Membrane separation processes are promising for CO 2 extraction [1] and for the separation of aromatic/aliphatic hydrocarbons [2,3]. Membrane technologies allow separation costs to be reduced significantly owing to their compactness, modularity, higher separation efficiency, and lower energy consumption compared to conventional processes. High permeability, selectivity, and stability at elevated temperatures are the key properties of membrane materials in the development of membranes based on them [4].
Polymer membranes based on glassy polymers currently dominate the market because of their relatively low cost, ease of production, and scalability of the molding process [5]. Though membrane polymers can work at elevated temperatures, only polytetrafluoroethylene (PTFE) and polysulfones are mechanically and hydrolytically resistant to high-temperature wet gases. The creation of membranes based on PTFE for the separation of gases and organic solvents does not appear possible due to its insolubility in organic solvents, acids, and alkalis [6]. The operating temperatures of polysulfones do not exceed 170-180 • C [7][8][9]. high yields of phenyl-containing siloxanes with various structures [41]. Later, a simple and highly efficient method for the synthesis of ladder-like polyphenylsilsesquioxanes with controlled molecular weights in ammonia medium was developed and optimized [42]. The main difference between the L-PPSQ obtained in ammonia medium and the L-PPSQ obtained by other methods is that the former are less brittle, which, combined with their good mechanical and thermal characteristics, makes them promising objects for use in materials science.
The purpose of this work was to obtain and study the gas separation and pervaporation properties of L-PPSQ with improved physical and chemical characteristics.

Materials
Solvents were purified according to the reported procedures [43]. Toluene was distilled from calcium hydride under argon. Sodium hydroxide, pyridine, chlorotrimethylsilane, and HCl were purchased from Aldrich. All-cis-tetraphenylcyclotetrasiloxanol was obtained by our method, published previously [44].

Instrumentation
GPC analysis was performed on a Shimadzu chromatograph using a RID-20A refractometer as the detector, PSS SDV analytical 10 5 Å columns (size 300 × 8 mm), and THF as the eluent.
NMR spectra were recorded on a Bruker Avance™ 600 spectrometer (Bruker Elemental GmbH, Karlsruhe, Germany) operating at 99.36 MHz for 29 Si. The chemical shifts for 29 Si were measured using TMS as the external standard.
Powder X-ray diffraction phase analysis was performed on a D8 Advance Bruker AXS (Bruker Elemental GmbH, Karlsruhe, Germany) diffractometer in Bragg-Brentano focusing geometry using CuKα radiation; the angular step was 0.02 • , and the scan rate was 1-2 deg min −1 . The samples were placed on flat holders. Powder patterns were processed with DIFFRACplus software Bruker AXS.
Thermogravimetric studies were performed using a Derivatograph-C instrument (MOM SZERVIZ KFT, Budapest, Hungary) under air or argon at a heating rate of 10 K min -1 .

Synthesis of Ladder-like Polyphenylsilsesquioxane in Ammonia
The condensation of cis-tetraphenylcyclotetrasiloxanetraol (cis-tetrol) was performed in steel autoclaves (see Figure 1).
Cis-tetrol (1 g) was loaded into an autoclave (1), which was then cooled to −50 • C, and ammonia was pumped in (2) using a gas flow regulator. The reaction was carried out at 150 • C for 4 h (3), after which ammonia was decompressed (4), thus isolating the target product (5). To determine the structure and properties of the resulting L-PPSQ, the residual silanol groups were blocked with chlorotrimethylsilane in the presence of pyridine, and then the resulting product was resuspended in the THF/EtOH system (Scheme 1). Cis-tetrol (1 g) was loaded into an autoclave (1), which was then cooled to −50 °C, and ammonia was pumped in (2) using a gas flow regulator. The reaction was carried out at 150 °C for 4 h (3), after which ammonia was decompressed (4), thus isolating the target product (5). To determine the structure and properties of the resulting L-PPSQ, the residual silanol groups were blocked with chlorotrimethylsilane in the presence of pyridine, and then the resulting product was resuspended in the THF/EtOH system (Scheme 1). Scheme 1. Blocking of residual silanol groups in L-PPSQ followed by reprecipitation of the resulting polymer.
It is important to note that L-PPSQ synthesized in ammonia has a high molecular weight, which cannot be achieved using other modern methods of synthesis of these polymers. The LPPSQ obtained in this work was investigated by uniaxial stretching and TGA methods. It can be seen from the data obtained that the synthesized polymer has high mechanical and thermal characteristics (see Table 1 and Supplementary Materials), which exceed the results published at the moment in the literature [45]. The main difference between L-PPSQ obtained in ammonia and the polymers obtained by other methods is their lower brittleness, which combined with good mechanical and thermal properties makes them promising objects for today's materials science.   Cis-tetrol (1 g) was loaded into an autoclave (1), which was then cooled to −50 °C, and ammonia was pumped in (2) using a gas flow regulator. The reaction was carried out at 150 °C for 4 h (3), after which ammonia was decompressed (4), thus isolating the target product (5). To determine the structure and properties of the resulting L-PPSQ, the residual silanol groups were blocked with chlorotrimethylsilane in the presence of pyridine, and then the resulting product was resuspended in the THF/EtOH system (Scheme 1).

Scheme 1.
Blocking of residual silanol groups in L-PPSQ followed by reprecipitation of the resulting polymer.
It is important to note that L-PPSQ synthesized in ammonia has a high molecular weight, which cannot be achieved using other modern methods of synthesis of these polymers. The LPPSQ obtained in this work was investigated by uniaxial stretching and TGA methods. It can be seen from the data obtained that the synthesized polymer has high mechanical and thermal characteristics (see Table 1 and Supplementary Materials), which exceed the results published at the moment in the literature [45]. The main difference between L-PPSQ obtained in ammonia and the polymers obtained by other methods is their lower brittleness, which combined with good mechanical and thermal properties makes them promising objects for today's materials science. It is important to note that L-PPSQ synthesized in ammonia has a high molecular weight, which cannot be achieved using other modern methods of synthesis of these polymers. The LPPSQ obtained in this work was investigated by uniaxial stretching and TGA methods. It can be seen from the data obtained that the synthesized polymer has high mechanical and thermal characteristics (see Table 1 and Supplementary Materials), which exceed the results published at the moment in the literature [45]. The main difference between L-PPSQ obtained in ammonia and the polymers obtained by other methods is their lower brittleness, which combined with good mechanical and thermal properties makes them promising objects for today's materials science.

Study of L-PPSQ Solubility
The solubility of L-PPSQ was studied in a number of organic solvents: Chloroform, benzene, n-methylpyrrolidone, dimethylacetamide, hexane, acetone, ethanol, and a mixture of benzene and hexane in 10/90 and 30/70 ratios. The Hildebrand and Hansen solubility parameters were analyzed for each solvent and polymer.

Preparation of L-PPSQ Films
An L-PPSQ solution in chloroform (1 wt%) was used to make the films. Dense L-PPSQ films were fabricated by casting L-PPSQ solution in chloroform onto cellophane, followed by drying for 200 h under ambient conditions at room temperature. The initial film diameter was 7.5 cm. The thickness varied in the range of 30-35 µm. The film thickness was measured to within ±1 µm using a Mitutoyo ® 293 Digimatic QuickMike electron micrometer (Takatsu-ku, Kawasaki, Kanagawa, Japan).

Gas Transport Tests
Single gas permeation measurements were carried out at a temperature of 30.0 ± 0.1 • C and at a feed pressure of 0.8 bar using a constant volume/variable pressure experimental setup (GKSS Time-Lag Machine) described elsewhere [46]. The permeability coefficient P expressed in Barrers (1 Barrer = 10 −10 cm 3 (STP) × cm × cm −2 × s −1 × cmHg −1 ) was estimated by linear extrapolation of experimental data to zero trans-membrane pressure. The diffusion coefficient D was estimated using the time-lag method as D = l 2 /6θ, where l is the membrane thickness and θ is the experimental time lag before the attainment of the steady-state permeation regime. The solubility coefficient S was indirectly estimated in terms of the solution-diffusion permeation model: S = P/D. The ideal selectivity of a pair of gases was calculated as the ratio of the permeabilities of the individual gases.

Study of Pervaporative Properties
The transport and separation properties of L-PPSQ membranes were studied by vacuum pervaporation for the separation of a benzene/hexane mixture containing from 5 to 15 wt. % benzene at 20 • C. The membrane thickness was 100 ± 5 µm. Figure 2 shows a schematic diagram of the vacuum evaporation unit described elsewhere [47]. Using an MV-Z gear pump (Ismatec, Zurich, Switzerland) (2), the starting mixture to be separated (flow I) is fed from a thermostatically controlled 1 L container with a stirrer (1) through a heat exchanger (3) to a membrane module (4), and then back to the tank (1) by flow II. The effective membrane area in the membrane module is 13.5 cm 2 ; the volume flow rate of the mixture being separated is 350 mL/min. The permeate passing through the membrane (flow III) in the vapor phase is condensed in glass traps (5) placed in Dewar flasks with liquid nitrogen (−196 • C). To achieve continuity of pervaporative separation, the traps are arranged in parallel and operate alternately throughout the experiment. A LOIP LT-100 liquid thermostat (St. Petersburg, Russia) (6) ensures a constant temperature of the mixture being separated with an accuracy of ±0.1 • C. The creation and maintenance of the difference in partial pressures of the mixture vapors were provided by evacuation of the submembrane space with an Ebara PDV-250 vacuum pump (Ota-ku, Tokyo, Japan) (7). The pressure in the diaphragm space was 0.2 mbar. The safety trap (8) prevents the permeate vapors from entering the vacuum pump. The concentrations of the initial solution, the retentate, and the permeate were determined by gas chromatography on a Crystallux-4000M chromatograph (Yoshkar-ola, Russia) equipped with a flame ionization detector. Analysis was performed using the following parameters: Evaporation temperature of 200 °C, column temperature of 120 °C, and The concentrations of the initial solution, the retentate, and the permeate were determined by gas chromatography on a Crystallux-4000M chromatograph (Yoshkar-ola, Russia) equipped with a flame ionization detector. Analysis was performed using the following parameters: Evaporation temperature of 200 • C, column temperature of 120 • C, and detector temperature of 150 • C. We used a Phenomenex Zebron ZB-FFAP capillary column (length 50 m, diameter 0.32 mm, phase thickness 0.50 µm) and the phase with the following composition: polymeric ester of 2-nitroterephthalic acid + polyethylene glycol.
The total permeate flow J, kg/(m 2 ·h), was determined by the gravimetric method using the Equation (1): where m is the total mass of the permeate that passed through a membrane with area S in time t.
The separation factor β was determined by Equation (2): where x o and x w are the mass fractions of the organic component and water in the mixture being separated, and y o and y w are the mass fractions of the organic component and water in the permeate. The mass flows of the components in the permeate were determined as:

Results and Discussion
We used high-molecular-weight L-PPSQ obtained by condensation of cis-tetraphenylcyclotetrasiloxantetraol (cis-tetrol) in ammonia as the membrane material [42]. The main difference between L-PPSQ obtained in ammonia and the polymers obtained by other methods is that they have lower brittleness, which, combined with good mechanical (E = 1700 MPa, σ p = 44 MPa, ε p = 6%) and thermal characteristics (T d 5% above 500 • C in air and in argon) makes them promising objects for today's materials science (see Supplementary Materials).
An advantage of this method is that the ammonia used in cis-tetrol condensation acts both as a solvent and a catalyst in this process, which eliminates the need for additional reagents. Yet another important advantage of this approach is that ammonia is instantly removed from the reaction zone upon decompression. This makes it possible to obtain target products that require no additional purification.
It is important to note that decompressed ammonia can be regenerated by means of a drying column (Figure 3), which allows it to be used repeatedly in further syntheses. An advantage of this method is that the ammonia used in cis-tetrol condensation acts both as a solvent and a catalyst in this process, which eliminates the need for additional reagents. Yet another important advantage of this approach is that ammonia is instantly removed from the reaction zone upon decompression. This makes it possible to obtain target products that require no additional purification.
It is important to note that decompressed ammonia can be regenerated by means of a drying column (Figure 3), which allows it to be used repeatedly in further syntheses.

Ammonia Regeneration
It has been shown [42] that the presence of water in the reaction of cis-tetrol with ammonia results in products with low molecular weights. Water is also released in this process in the course of homocondensation. Accordingly, additional equipment must be

Ammonia Regeneration
It has been shown [42] that the presence of water in the reaction of cis-tetrol with ammonia results in products with low molecular weights. Water is also released in this process in the course of homocondensation. Accordingly, additional equipment must be used to dry the regenerated ammonia before it is reused.
After the reaction is performed in reactor 2, ammonia is pumped through the drying column 3 to the NH 3 tank 1. After that, ammonia can be reused quantitatively in the next reaction. As can be seen from Figure 4, L-PPSQ obtained using regenerated ammonia has almost the same molecular weight characteristics as the polymer obtained in the reaction with the original ammonia. If ammonia is pumped to the next reaction without a drying column, then products with low molecular weights are obtained (see Figure 4). This important result of our study allows us to state that our suggested approach fully meets modern "green chemistry" requirements.

Ammonia Regeneration
It has been shown [42] that the presence of water in the reaction of cis-tetrol with ammonia results in products with low molecular weights. Water is also released in this process in the course of homocondensation. Accordingly, additional equipment must be used to dry the regenerated ammonia before it is reused.
After the reaction is performed in reactor 2, ammonia is pumped through the drying column 3 to the NH3 tank 1. After that, ammonia can be reused quantitatively in the next reaction. As can be seen from Figure 4, L-PPSQ obtained using regenerated ammonia has almost the same molecular weight characteristics as the polymer obtained in the reaction with the original ammonia. If ammonia is pumped to the next reaction without a drying column, then products with low molecular weights are obtained (see Figure 4). This important result of our study allows us to state that our suggested approach fully meets modern "green chemistry" requirements.

Study of L-PPSQ Solubility
To determine the sorption interaction of the polymer synthesized with various solvents, its solubility was studied in seven organic solvents of different nature (chloroform, benzene, n-methylpyrrolidone (NMP), dimethylacetamide (DMAA), hexane, acetone, and ethanol) and two organic mixtures (benzene/ethanol with different compositions) ( Table 2). Table 2. Estimation of the solubility and long-range interaction of the polymer and solvents of various nature.

Compound Solubility
Strong swelling "+"-dissolves, "−"-insoluble. Table 2 shows that L-PPSQ is soluble in chloroform, benzene, and polar aprotic amide solvents: NMP and DMAA. L-PPSQ is insoluble in hexane, acetone, and ethanol. L-PPSQ is insoluble in a benzene/hexane mixture at a ratio of 10/90, but it swells strongly in the 30/70 mixture. This behavior is characteristic of PPSQ. Various groups of researchers noted the solubility of materials of this class in benzene, chloroform, and aprotic solvents (THF, DMF) and their insolubility in ketones, alcohols, simple ether, and saturated cyclic and aliphatic hydrocarbons [48]. The insolubility of L-PPSQ in a wide range of solvents allows us to expect that the properties of a membrane will be stable when the latter is operated in these media. Apparently, an undoubted advantage of L-PPSQ is that it is soluble in solvents with various viscosities, which would allow the development of membranes with good performance characteristics on its basis.
Based on the values of the long-range interaction parameter obtained, we can say that benzene and chloroform are the best solvents for L-PPSQ in the series studied. The interaction of L-PPSQ with NMP and DMAA solvents appears to occur at the boundary of the region of "good" solvents in terms of the long-range parameter value (MF: 10.6 MPa 1/2 ; DMAA: 10.5 MPa 1/2 ). In the 30 wt.% benzene/70 wt.% n-hexane mixture, L-PPSQ swells strongly but does not dissolve. Thus, it can be assumed that a long-distance parameter value above 10.5-10.6 delineates the region of "bad" solvents.

Gas Transport Properties
The permeability (P) and diffusion (D) coefficients were measured for individual gases: He, H 2 , N 2 , O 2 , CO 2 , CH 4 , C 2 H 6 , and C 4 H 10 . The solubility coefficients (S) were calculated as the ratio of permeability coefficients to diffusion coefficients ( Table 3). The ideal selectivity of permeability (α P ), diffusion (α D ), and solubility (α S ) was defined as the ratio of the permeability, diffusion, and solubility coefficients for the corresponding gas pair (Table 4) according to Equation (4). The largest diffusion coefficients are characteristic of gases with the smallest diffusion diameters, i.e., He and H 2 (d ef (He) = 2.10 Å and d ef (H 2 ) = 1.80 Å). The distribution of diffusion coefficients is characteristic of the silsesquioxane family. Due to the high diffusion of gases with small kinetic diameters, the L-PPSQ synthesized in this work has selective permeability for He and H 2 . Thus, this material can have an ideal permeability selectivity for He/CH 4 = 2.7 and for H 2 /CH 4 = 4.5. In the case of ethane/methane and n-butane/methane, the ideal permeation selectivity is close to 1, despite the high dissolution selectivity (7.7 and 40.8, respectively).
The most interesting property is that the gas permeability coefficient for carbon dioxide is relatively high. It is worth noting that the high-molecular-weight L-PPSQ obtained in this work demonstrates a more than twofold higher permeability coefficient than its crosslinked analogs [49]. This value of the CO 2 permeability coefficient provides a higher ideal selectivity of the material for carbon dioxide: CO 2 /CH 4 = 10.5 and CO 2 /N 2 = 20.9. The separating properties of the material thus obtained outline the range of potential applications for CO 2 extraction from natural and flue gases. Similar results were obtained in the early 1990s. It was shown [50] that the ideal CO 2 /CH 4 gas vapor selectivity of L-PPSQ was about 9. This is noticeably higher than that of other polysiloxanes, such as polydimethylsiloxane, for example. However, the authors mention the low mechanical strength and high brittleness of the L-PPSQ films studied, which made it difficult and even impossible to use them as membrane-forming materials. For comparison, the correlation of CO 2 permeability and selectivity of CO 2 /N 2 and CO 2 /CH 4 of the polymers synthesized in this work and the closest analogs presented in the literature on the Robson diagram [51] is shown ( Figure 5). Based on the correlation shown in Figure 5, it is clear that increasing the molecular weight (Mn = 238 kDa, Mw = 578 kDa) of the L-PPSQ leads to a significant (2-5 times) increase in the permeability of the ladder polymer while maintaining increased CO2 selectivity.

Vacuum Pervaporation
Catalytic reforming is one of the methods used to produce high-octane gasoline fractions. In this process, dehydrogenation of naphthenic hydrocarbons and dehydrocyclization of paraffines result in reformate, a product rich in aromatic hydrocarbons. Along with the advantages due to the presence of aromatic hydrocarbons in gasoline, drawbacks also exist, namely, an increase in the liability of automotive gasoline to produce carbon monoxide. In addition, benzene, whose content in gasoline should not be more than 1%, is isolated separately from aromatic compounds [53]. As an alternative to the traditional methods, i.e., extraction and hydrogenation, a membrane process-vacuum pervaporation-can be used for the separation of aromatic compounds from gasoline. In view of this, in this work, we studied L-PPSQ membranes for vacuum pervaporation in the separation of benzene-hexane mixtures. The concentration of benzene in n-hexane varied from 5 to 15 wt.%. The solution treatment temperature was 20 °C, and the pressure in the submembrane space was 0.2 mbar. Figure 6 shows the permeate flow and the separation factor as functions of the benzene concentration in the solution being processed. As one can see from Figure 6, the permeate flow increases as the benzene content in the solution increases. The maximum permeate flow over the entire range of benzene concentrations in n-hexane studied was 39 g/(m 2 ·h). The partial flow of n-hexane (4.6-5.9 g/(m 2 ·h)) was almost an order of magnitude smaller than that of benzene (30-33 g/(m 2 ·h)) over the entire concentration range studied. The maximum value of the separation factor was 126. It should be noted that the separation factor decreases significantly (4-fold) with an increase Based on the correlation shown in Figure 5, it is clear that increasing the molecular weight (Mn = 238 kDa, Mw = 578 kDa) of the L-PPSQ leads to a significant (2-5 times) increase in the permeability of the ladder polymer while maintaining increased CO 2 selectivity.

Vacuum Pervaporation
Catalytic reforming is one of the methods used to produce high-octane gasoline fractions. In this process, dehydrogenation of naphthenic hydrocarbons and dehydrocyclization of paraffines result in reformate, a product rich in aromatic hydrocarbons. Along with the advantages due to the presence of aromatic hydrocarbons in gasoline, drawbacks also exist, namely, an increase in the liability of automotive gasoline to produce carbon monoxide. In addition, benzene, whose content in gasoline should not be more than 1%, is isolated separately from aromatic compounds [53]. As an alternative to the traditional methods, i.e., extraction and hydrogenation, a membrane process-vacuum pervaporation-can be used for the separation of aromatic compounds from gasoline. In view of this, in this work, we studied L-PPSQ membranes for vacuum pervaporation in the separation of benzene-hexane mixtures. The concentration of benzene in n-hexane varied from 5 to 15 wt.%. The solution treatment temperature was 20 • C, and the pressure in the submembrane space was 0.2 mbar. Figure 6 shows the permeate flow and the separation factor as functions of the benzene concentration in the solution being processed. As one can see from Figure 6, the permeate flow increases as the benzene content in the solution increases. The maximum permeate flow over the entire range of benzene concentrations in n-hexane studied was 39 g/(m 2 ·h). The partial flow of n-hexane (4.6-5.9 g/(m 2 ·h)) was almost an order of magnitude smaller than that of benzene (30-33 g/(m 2 ·h)) over the entire concentration range studied. The maximum value of the separation factor was 126. It should be noted that the separation factor decreases significantly (4-fold) with an increase in the concentration of benzene in hexane. This is probably due to the membrane swelling at higher benzene concentrations in the mixture, which causes a higher penetration of hexane through the membrane. A comparison of our results with the literature data showed that the L-PPSQ membranes studied in this work had the maximum benzene/hexane separation factors (Table 5).     polyurethane Benzene/hexane, 10/90%, T = 25 • C 1000 25 [56] Poly(ethylene oxide imide) Benzene/hexane, 40/60%, T = 60 • C 28 7.5 [54] Poly(vinylalcohol) Benzene/hexane, 20/80%, T = 50 • C 168 3 [57] Polydimethylsiloxane with 6% wt. poly(oligosilsesquioxanes) * Benzene/heptane, 0.6/99.4%, T = 70 • C 12 825 3.5 [58] * Composite membrane.

Conclusions
In this work, a high-molecular-weight LPS was obtained, which has improved thermal and mechanical characteristics compared to literary analogs. These characteristics make it possible to obtain a polymer membrane based on it with good operational properties.
The films of high-molecular-weight L-PPSQ obtained in this work showed an improved selectivity for carbon dioxide (CO 2 /CH 4 = 10.5; CO 2 /N 2 = 20), which makes it a promising material for CO 2 separation tasks. Moreover, films of high molecular weight L-PPSQ showed a record-high separation factor of benzene/n-hexane mixtures (up to 126) at permeate fluxes through the membrane comparable to well-known polymeric membranes. Thus, this membrane material is also promising for the separation of aromatic and aliphatic hydrocarbon mixtures.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/polym15153277/s1, Figure S1: GPC curve of L-PPSQ; Figure S2: TGA curves of L-PPSQ obtained at a heating rate of 10 degrees/min in air and in argon; Figure S3: Stretching curve of L-PPSQ; Table S1: Molecular weight characteristics of L-PPSQ.