Introduction

The rapid development of modern industry is often accompanied by the pollution of water resources. This problem is especially problematic for water resources used extensively in dye processing industries [1,2,3,4]. Filtration [5], centrifugation [5], adsorption [6, 7], chemical coagulation [8, 9], photodegradation [10,11,12] and biodegradation [13] have been applied to restore the purity of wastewater. Among them, adsorption of porous materials is regarded as one of the most effective and simplest methods to remove pollutants [13].

In recent years, research on porous materials has developed nearly explosively in gas storage and separation [14,15,16,17,18], drug delivery and release [19, 20], heterogeneous catalysis [21,22,23,24], optoelectronics [25, 26], chemosensors [27, 28] and proton exchange [29]. These high-value applications drive more attention on the development of advanced porous materials with designed pore architectures and functionalities.

Cubic silsesquioxanes are considered as multifunctional building units for the construction of porous materials due to their unique structural features [30, 31]. They possess well-defined 3D hybrid core–shell structures: inorganic Si–O-Si core and peripheral organic substituents that can be further functionalized. The combination of a rigid support structure and multiple reactive antennae on one molecule offers the possibility of the construction of porous materials by different synthetic methods. Octavinylsilsesquioxane (OVS), as a typical example, has been successfully applied to prepare a series of porous materials via hydrosilylation [32], radical polymerization [33], Heck reaction [34], Friedel–Crafts reaction [35], click reaction [36] and cationic polymerization [37].

The reticular approach offers a new route to the design and synthesis of porous materials with controlled pore structures and functional groups by regulating topological construction nodes. The strategy has also been used in silsesquioxanes-based hybrid porous materials. For example, some porous silsesquioxane-imine networks were prepared through the condensation of octa(3-aminopropyl)silsesquioxane and selected multitopic aldehydes and their porous properties can be controlled by the strut length and rigidity of linkers [38]. Some luminescent silsesquioxanes-based porous materials were also prepared by the topology combination of cubic OVS and halogenated aromatic compounds via Heck coupling reaction [39, 40]. However, the catalytic cross-coupling reaction requires expensive catalysts; moreover, the halogenated aromatic compounds are also expensive or not easily available. Therefore, it is necessary to seek some economic methods to prepare and tune silsesquioxanes-based porous materials through the reticular approach.

In this work, we report the synthesis of two series of silsesquioxanes-based hybrid porous materials by employing Friedel–Crafts reaction between OVS and tetrabiphenylsilane (TBPS) or octabiphenylsilsesquioxane (OBPS). Considering a large number of reactive sites in TBPS or OBPS, we have performed the reactions under different molar ratios of the reactants to allow modulation in the local cross-link densities and consequently porosity of the resulting materials. These synthesized hybrid polymers possess micropores at about 1.4 nm and mesopores at about 4.0 nm, respectively, high surface areas of 1185–1500 m2 g−1 and a large pore volume range of 0.79–1.30 cm3 g−1. The utility of these porous materials for CO2, dye and ion adsorption was further investigated. The results showed that these hybrid porous materials possessed excellent adsorption ability for dye and heavy metal ion, indicating that silsesquioxanes-based porous materials are promising candidates for water purification.

Results and discussion

Characterization

The hybrid porous polymers TBPS-HPPs and OBPS-HPPs were synthesized via Friedel–Crafts reaction of OVS with TBPS and OBPS, respectively (Scheme 1). All products were yellow powders and did not dissolve in organic solvents. To confirm their structures, they were fully characterized by FTIR, solid-state 13C CP/MAS NMR and 29Si MAS NMR spectroscopy. Considering the spectral similarity of the porous materials in the series of TBPS-HPPs or OBPS-HPPs, TBPS-HPP-4 or OBPS-HPP-3 were selected as representative examples to analyze. Figure S1 shows the FTIR spectra of OBPS, TBPS, OVS, TBPS-HPP-4 and OBPS-HPP-3. Compared with OVS, the intensities of the vinyl group peaks of TBPS-HPP-4 and OBPS-HPP-3 at 1280, 1412, 1603 and 3063 cm−1 decrease, and a new methylene (Si–CH2–) band at 2963 cm−1 appears, confirming the success of Friedel–Crafts reaction. A broad peak around 1108 cm−1 is attributed to characteristic νSi-O-Si, suggesting the formation of cross-linked network polymers [34, 41, 42].

Scheme 1
scheme 1

Synthetic routes and possible structures of hybrid porous polymers

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The results obtained from 13C CP/MAS NMR and 29Si MAS NMR spectroscopy provide useful information on the structure and composition of the porous polymers. The 13C CP/MAS spectra of TBPS-HPP-4 and OBPS-HPP-3 are shown in Fig. 1. The signals for the carbon atoms of the bridging biphenyl units from TBPS or OBPS and of the unreacted double bonds from OVS are observed in the range of 120 to 160 ppm. The peaks from 0–50 ppm are mainly ascribed to the methylene carbons (Si–CH2–CH2–Ar–), confirming the formation of single C–C bonds after Friedel–Crafts reaction [41, 42]. Chemical shifts in the range of 0–50 ppm are relatively complex because of the random reaction positions on the phenyl groups in TBPS or OBPS units.

Figure 1
figure 1

Solid-state 13C CP/MAS NMR spectra of TBPS-HPP-4, OBPS-HPP-3 and OVS (*spinning sideband)

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The solid-state 29Si NMR spectra of TBPS-HPP-4 and OBPS-HPP-3 are shown in Fig. 2. For both of them, signals at ca. − 80 ppm are assigned to the silicon atoms of T3 units [Tn: CSi(OSi)n(OH)3-n] from unreacted Si–CH=CH2 units [35]; signals at ca. − 67.5 ppm are T3 silicon connected to ethylene units formed after reaction [43,44,45]. Different from TBPS-HPP-4, a moderate intensity peak at − 47 ppm attributed to the silicon atoms of T1 units is observed for OBPS-HPP-3 and the presence of T1 units indicated that partial destruction of cage during the network formation of OBPS-HPP-3 [46,47,48]. Compared to TBPS-HPP-4, the two following factors help to produce more structure stress in OBPS-HPP-3, which results in cage collapse. One is there are more reactive sites in OBPS, which results in higher local cross-linked density [34]; the other is that cubic OBPS is more rigid than tetrahedral TBPS [47, 49]. Moreover, the relative intensities of signals for the Si–CH=CH2 units (~ − 80 ppm) of OBPS-HPP-3 are stronger than that of TBPS-HPP-4, which likely results from the overlap with silicon atoms associated with biphenyl groups.

Figure 2
figure 2

Solid-state 29Si MAS NMR spectra of TBPS-HPP-4, OBPS-HPP-3 and OVS (* satellite peaks)

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Thermal stability and morphology

TGA was used to evaluate the thermal stabilities of the hybrid porous polymers. In Figure S2, TBPS-HPPs exhibit high thermal stabilities and negligible mass loss before 400 °C. The decomposition temperatures (Td) at 5 wt% for TBPS-HPP-1, TBPS-HPP-2, TBPS-HPP-3 and TBPS-HPP-4 are all about 500 °C, suggesting highly cross-linked networks [41, 42, 50]. However, the decomposition of OBPS-HPP-1, OBPS-HPP-2, OBPS-HPP-3 and OBPS-HPP-4 begins at ~ 200 °C. This is expected due to the presence of incompletely condensed silanols as a result of partial collapse of silsesquioxane in OBPS-HPP-3, as evident from solid-state 29Si MAS NMR spectra.

Although OVS, TBPS and OBPS are crystalline, the hybrid porous polymers TBPS-HPP-4 and OBPS-HPP-3, as expected, exhibit nearly amorphous features, as evidenced by powder X-ray diffraction (PXRD) in Figure S3 and small-angle X-ray scattering (SAXS) in Figure S4. From PXRD patterns, the broad diffraction peaks at around 2θ ≈ 22° corresponding to d-spacing of 4 Å, are attributed to Si–O–Si linkage. The results are consistent with those reported earlier for related silsesquioxanes-based porous materials [46, 50]. In addition, no maxima are observed in SAXS patterns in keeping with the amorphous character of these materials. However, different from the typical amorphous porous polymers, one peak is observed at 2θ < 1.5° (Figure S5) in lower angle PXRD, which is likely attributed to the partially periodic arrangement of silica cages [39, 51, 52].

Figure S6 provides FE-SEM images of particle size and morphology. TBPS-HPP-4 and OBPS-HPP-3 aggregate into interlinking irregular shapes with nanostructure appearance and a wide range of diameters from 500 nm to several micrometers [41, 50, 51, 53]. The worm-like texture shown in translucent HR-TEM images (Fig. 3 and Fig. 4) indicates a porous structure with relatively uniform pore diameters. When the HR-TEM images are extended to a certain extent, it is interesting to observe that the arrangement direction of porous channels exhibited a certain orientation, in accordance with the results of lower angle PXRD [54]. Therefore, both TBPS-HPP-4 and OBPS-HPP-3 exhibit long-range disordered features and short-range ordered features.

Figure 3
figure 3

HR-TEM images of TBPS-HPP-4 in different scales

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Figure 4
figure 4

HR-TEM images of OBPS-HPP-3 in different scales

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Porosities

The BET surface areas (SBET) and pore structures of TBPS-HPPs and OBPS-HPPs were evaluated by N2 adsorption and desorption isotherms experiments at 77 K. All samples exhibit type IV shape nitrogen adsorption isotherms in Figs. 5 and S7, suggesting micro- and mesoporosity [50]. The detailed porosity data are given in Table 1. SBET of TBPS-HPP-1 ~ TBPS-HPP-4 and OBPS-HPP-1 ~ OBPS-HPP-4 is calculated as 1185, 1186, 1254, 1321, 1442, 1473, 1500 and 1433 m2 g−1, respectively. The respective micropore surface areas (Smicro) are calculated to be 294, 323, 227, 290, 343, 139, 111 and 145 m2 g−1 by t-plot method.

Figure 5
figure 5

Nitrogen adsorption and desorption isotherms of a TBPS-HPP-4 and b OBPS-HPP-3 (insets are their NL-DFT pore size distribution)

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Table 1 Porosity data of TBPS-HPPs and OBPS-HPPs
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Pore size distributions (PSD) were confirmed from the nonlocal density functional theory (NL-DFT). As shown in Figs. 5 and S8, the porous polymers exhibit similar PSD curves with bimodal porosities, which indicate uniformity of micro- and mesopores in the networks, in accordance with the N2 adsorption and desorption isotherms [50]. TBPS-HPPs show bimodal pore structures with uniform average microporosities centered at 1.4 nm and uniform mesoporosities centered at about 3.9 nm. The pores are formed by cage stacking through hypercrosslinking of OVS with TBPS or OBTP, and nanosized cage and relative flexible linker of Si–CH2–CH2–Ar between cage and nodes should be responsible for this typical porosity.

OBPS-HPPs exhibit similar pore structure with TBPS-HPPs, whose microporosities were centered at 1.4 nm and uniform mesoporosities were centered at about 3.6 nm. The total pore volumes (Vtotal) are 0.79, 0.87, 0.96 and 1.02 cm3 g−1 for TBPS-HPP-1, TBPS-HPP-2, TBPS-HPP-3 and TBPS-HPP-4, respectively; the Vtotal of OBPS-HPP-1, OBPS-HPP-2, OBPS-HPP-3 and OBPS-HPP-4 are 1.17, 1.25, 1.30 and 1.23 cm3 g−1, respectively.

The ratios of the micropore volume (Vmicro) to Vtotal for TBPS-HPP-1 to TBPS-HPP-4 are 0.16, 0.16, 0.10 and 0.12 (Table 1), respectively. The ratios of Vmicro to Vtotal for OBPS-HPP-1 to OBPS-HPP-4 are 0.13, 0.04, 0.02 and 0.04 (Table 1), respectively. According to the IUPAC classification, TBPS-HPPs and OBPS-HPPs can be regarded as mesoporous materials because of the low Vmicro/Vtotal ratios.

For TBPS-HPPs and OBPS-HPPs, SBET and Vtotal increase monotonically with increasing molar ratio of OVS to TBPS (or OBPS). For TBPS-HPPs, from TBPS-HPP-1 to TBPS-HPP-4, a maximum SBET of 1320 m2 g−1 and a maximum Vtotal of 1.02 cm3 g−1 are achieved when the molar ratio of OVS to TBPS is 1.0:0.33, i.e., TBPS-HPP-4, compared with the hybrid porous materials constructed by OVS and tetraphenylsilane (TPS) (the range of SBET, Vtotal and Vmicro/Vtotal is 518–989 m2 g−1, 0.35–0.76 cm3 g−1 and 0.15–0.27, respectively) [41].

It is interesting that TBPS-HPPs exhibit higher SBET, larger total pore volumes and lower Vmicro/Vtotal. TBPS possesses the same rigid tetrahedral geometry with TPS, but it has a longer strut and contains eight biphenyl groups, which provide more reactive sites during Friedel–Crafts reactions. Usually, increasing the length of nodes’ building blocks and local cross-linked density within a certain extent can be helpful in improving surface areas and pore volumes. In addition, moderate flexibility in the rigid strut could help form mesopores [41]. One phenyl could rotate to some extent relative to the linked other phenyl in biphenyl group; therefore, TBPS shows more flexibilities than TPS. In summary, these factors help to improve surface areas and porous volumes.

For OBPS-HPPs, OBPS-HPP-3 exhibited a maximum SBET of 1500 m2 g−1 and a maximum Vtotal of 1.30 cm3 g−1 and its corresponding molar ratio of OVS to OBPS was 1.0:0.14. Increasing molar ratio of OVS to OBPS increases the number of reactive sites on the biphenyl groups in OBPS. Generally, increased local cross-linked densities result in higher surface areas. However, further increasing the molar ratio of OVS to OBPS greater than 1:0.14 would increase steric hindrance and result in framework interpenetration, leading to the partial distortion and collapse of Si–O-Si cages. This leads to an increase in free volume and a decrease in surface area for OBPS-HPP-4. Compared to TBPS, OBPS possesses a rigid cubic geometry, larger molecular volume and twice the number of biphenyl groups.

Both OVS and OBPS are nanometer-sized, bulky rigid cages, which can only pack to a limited density. Significant pore space originates from packing persistent in the dry state [32]. Replacing TBPS with OBPS as nodes to construct porous materials with OVS further increases local cross-linked density, favoring formation of mesopores [41, 42]. Therefore, higher surface areas and more mesopores are achieved.

In summary, the synthesized TBPS-HPPs and OBPS-HPPs possess high surface areas and unique bimodal structures; however, OBPS-HPPs exhibit higher surface areas, larger pore volume and more mesopores compared with TBPS-HPPs (Table 1). Therefore, the porosities of these hybrid porous polymers could be systematically tuned by selecting the building block of node with different geometries and changing their molar ratio.

Fluorescence properties

The fluorescent emission spectra of these polymers and node molecules are recorded in the suspension of ethanol and water (Vethanol/Vwater = 9:1) (Fig. 6). The fluorescent emission spectra of TBPS-HPPs are red-shifted approximately 50 nm compared with TBPS monomers. This red shift is mainly due to the introduction of rigid and bulky cage units (OVS) into TBPS-HPPs, which inhibits the aggregation of TBPS units [55].

Figure 6
figure 6

Emission spectra for a TBPS-HPPs and b OBPS-HPPs with excitation held at 360 and 274 nm in the suspension of ethanol and water (Vethanol/Vwater = 9:1, 0.5 mg mL−1)

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However, the red-shifted phenomenon of fluorescent emission spectra for OBPS-HPPs is not obvious. This may be due to the node building blocks containing the rigid and bulky cage; the red shift derived from the OVS cage is weakened. Therefore, OBPS-HPPs emitted broad fluorescence within 350–370 nm in the suspension of ethanol and water (Fig. 6). Surprisingly, although TBPS-HPPs and OBPS-HPPs possess the same fluorophores, the maximal emission wavelengths of TBPS-HPPs are larger nearly 100 nm than those of OBPS-HPPs; the red shift is observed in TBPS-HPPs compared with OBPS-HPPs, which is probably ascribed to electron-withdrawing properties of cage [56, 57]. More interestingly, for TBPS-HPPs, the fluorescent emission nearly did not shift with the increasing local cross-linking density; for OBPS-HPPs, the fluorescent emission are red-shifted with the increasing local cross-linking density.

CO 2 storage

One of the most important potential applications for porous materials is CO2 storage, because CO2 is a key anthropogenic greenhouse gas and considered a cause of global warming [14]. TBPS-HPP-4 and OBPS-HPP-3 are selected for CO2 capture test as they possess the highest SBET in TBPS-HPPs and OBPS-HPPs, respectively. The CO2 capture capacities for TBPS-HPP-4 and OBPS-HPP-3 are 5.51 wt% (1.25 mmol g−1) and 5.43 wt% (1.24 mmol g−1) at 273.0 K/101 kPa (Figure S9). Though the surface area of OBPS-HPP-3 is higher than that of TBPS-HPP-4, TBPS-HPP-4 exhibits a little higher CO2 capture capacity due to its higher micropore area and volume [56, 58,59,60,61].

Dye adsorption

Water contamination caused by toxic organic dyes remains a serious environmental and public problem, because clean water resources are very vital for human health. Adsorption is the simplest and widely adopted technology for rapid and convenient removal of organic dyes from wastewater [62, 63]. The highest porosity and large accessible pores of TBPS-HPP-4 and OBPS-HPP-3 inspired us to investigate their potential utility for adsorption efficiency of different dyes, including rhodamine B (RB), methylene blue (MEB), Congo red (CR), crystal violet (CV) and methyl orange (MO), whose molecular sizes are in the order of CR > RB > CV > MEB > MO (Table S2) [64,65,66].

From Table 2, it can be seen that the saturation equilibrium adsorption capacities (Qe) are 1325, 1069, 830, 114 and 2230 mg g−1 for RB, MEB, CV, MO and CR onto TBPS-HPP-4, respectively, and 1485, 1026, 817, 78 and 1055 mg g−1 for RB, MEB, CV, MO and CR onto OBPS-HPP-3, respectively (Table 3). The adsorption isotherms are shown in Figure S11 and Figure S12. Obviously, both polymers exhibit an excellent adsorption efficiency toward RB, MEB, CV and CR rather than MO. This means they prefer to adsorb dyes with big sizes, which should be related to their pore structures. They possess higher mesopore contents, which usually favor adsorbing bulky dyes. Bigger sized dyes can block micropore orifices or bind on the mesopores and further increase adsorption amount [67]. In contrast, smaller molecular sized dyes, i.e., MEB and MO, show low adsorption capacities because they are easily in and out of pores due to small spatial hindrance.

Table 2 Parameters of Langmuir and Freundlich isotherms for dyes adsorption of TBPS-HPP-4
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Table 3 Parameters of Langmuir and Freundlich isotherms for dyes adsorption of OBPS-HPP-3
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Besides dye size, the adsorption capacity is also related to the nature of porous material and the ionic characteristics of dye. Compared with TBPS-HPP-4, OBPS-HPP-3 is more electronegative since its higher oxygen content, which does not favor adsorbing anionic dyes due to electrostatic repulsion. This explains the adsorption capacity of CR (anionic dye) onto TBPS-HPP-4 is twice that of OBPS-HPP-3 although SBET ofOBPS-HPP-3 is larger than TBPS-HPP-4 [35, 66, 68]. It also explains the adsorption capacity of MEB (cationic) is much larger than MO (anionic) although they have similar sizes, regardless of TBPS-HPP-4 or OBPS-HPP-3.

Langmuir isotherm model and Freundlich isotherm model are used to investigate their adsorption behaviors (Figure S14, S15, S16 and S17, Tables 2 and 3) [69,70,71]. It is found that the correlation coefficients of Langmuir model are generally a little higher than those of the Freundlich, but most of them are over 0.90, which indicates that the adsorption process is mainly monolayer adsorption and with partial multilayer adsorption phenomenon.

We also performed a detailed kinetic study to understand the efficiency of TBPS-HPP-4 and OBPS-HPP-3 for RB (Figure S10). The results show that this adsorption procedure reaches equilibrium in just 20 min for TBPS-HPP-4 and OBPS-HPP-3. The variation of RB, MEB, CV, MO and CR removal efficiencies with different initial concentrations is shown in Figure S13, Tables 2 and 3. For each set of adsorption experiment, the amount of hybrid porous material is fixed. Therefore, with the initial concentration of the dyes increasing, the removal efficiency of TBPS-HPP-4 and OBPS-HPP-3 gradually decreases. TBPS-HPP-4 and OBPS-HPP-3 also exhibit an excellent removal efficiency toward RB, MEB, CV and CR rather than MO.

Pb 2+ adsorption

Lead, as one of the most important and versatile metal ions, is a carcinogenic and non-biodegradable substance. Every year, a large quantity of lead has been discharged into effluent in the form of Pb (II), resulting in serious risks to the ecological environment and human health. Therefore, it is crucial and necessary to remove the lead ions from wastewater [72, 73]. Figure S18 (a) shows the equilibrium adsorption isotherm of TBPS-HPP-4 and OBPS-HPP-3 for Pb2+. It could be seen that the experimental maximum adsorption capacity for Pb2+ was 302 mg g−1 for TBPS-HPP-4 and 277 mg g−1 for OBPS-HPP-3, which were higher than some other adsorbents (Table S3) [73,74,75,76,77]. Results showed that their adsorption behaviors fitted better to Langmuir isotherm model than Freundlich isotherm model (Figure S18 (b), Figure S18 (c) and Table 4). The high surface areas, extended π-conjugation networks as well as electron-rich feature should account for their high absorption of Pb2+ [78].

Table 4 Parameters of Langmuir and Freundlich isotherms for Pb2+ adsorption onto TBPS-HPP-4 and OBPS-3
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Regeneration

For the full use of the adsorbents, the regeneration study was conducted by adsorption–desorption experiments for several times. Cyclic adsorption–regeneration tests were carried out for the simulated wastewater of CR, RB and Pb2+. The desorption experiments were performed by washing contaminants-loaded sample with hot ethanol and 2 M hydrochloric acid. As shown in Figure S19, the removal efficiency of OBPS-HPP-3 and TBPS-HPP-4 for dyes remained 100% after five cycles, while the removal efficiency for Pb2+ decreased. These results showed that the adsorption ability of OBPS-HPP-3 and TBPS-HPP-4 did not decrease drastically even after five consecutive adsorption/desorption cycles. It can be concluded that OBPS-HPP-3 and TBPS-HPP-4 could be regenerated and reused in dyes and heavy metal ions removal.

Conclusions

Two kinds of hybrid porous polymers (TBPS-HPPs and OBPS-HPPs) were easily constructed via topology combination of cubic octavinylsilsesquioxane with tetrahedral tetrabiphenylylsilane and cubic octabiphenylsilsesquioxane as node building blocks, respectively. The synthesized hybrid porous polymers exhibited high surface areas and mesoporous structure. Importantly, the porosities of porous polymers could be systematically tuned by adjusting the building block nodes with different geometries and changing their molar ratio to OVS. They could be applied for CO2 adsorption and water treatment. Result showed they could efficiently remove bulky dyes, such as Congo red (CR), rhodamine B (RB) and crystal violet (CV). They also exhibited excellent adsorption capacities for Pb2+, and the maximum adsorption capacity achieved was 300 mg g−1. They are powerful candidates for water purification in the near future.

Experimental section

Experimental section is provided in Supporting Information.