Nadhita
Chanchaona
a,
Liang
Ding
a,
Shiliang
Lin
a,
Sulaiman
Sarwar
a,
Simone
Dimartino
a,
Ashleigh J.
Fletcher
b,
Daniel M.
Dawson
c,
Kristina
Konstas
d,
Matthew R.
Hill
d and
Cher Hon
Lau
*a
aSchool of Engineering, University of Edinburgh, Robert Stevenson Road, EH9 3FK, UK. E-mail: cherhon.lau@ed.ac.uk
bDepartment of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, G1 1XJ, UK
cSchool of Chemistry, EaStCHEM, Centre of Magnetic Resonance, University of St. Andrews, KY16 9ST, UK
dCSIRO Manufacturing Flagship, Private Bag 10, Clayton South, Melbourne, Victoria 3169, Australia
First published on 27th January 2023
Hypercrosslinked polymers (HCPs) are typically synthesised over 24 hour batch reactions, limiting productivity rates during scale-up production. Continuous flow synthesis can potentially overcome this limitation. However, the formation of insoluble HCP products, compounded by HCP expansion due to solvent adsorption during synthesis can clog flow reactors. Here, we overcome clogging issues through reactor design and optimisation of synthesis parameters. Using this reactor, we synthesised HCPs via internal, post-, and external crosslinking strategies underpinned by Friedel–Crafts alkylation over various synthesis parameters – residence time, substrate concentration, reagent ratio, and temperature. The space-time-yield (STY) values, a key parameter for productivity rates, of flow synthesis were 32–117 fold higher than those in batch reactions. HCPs produced via internal crosslinking in flow synthesis contained additional microporosity that enhanced CO2/N2 selectivity at 298 K by 850% when compared to HCPs produced in batch reactions. Outcomes from this work could enable high productivity scale-up production of HCPs for post-carbon capture.
Most HCPs are produced via batch reactions over 18–24 hours9 to ensure extensive crosslinking10 and this may be inadequate to produce enough HCPs to meet the growing global demand. In the first hour of batch reactions, more than 80% of the reagents are used up in cross-linking. The pores of the initial polymer will adsorb solvent molecules, creating a gel-like structure. More solvent is then added to disperse the initially-formed polymers, breaking up the gel-like material to ensure complete crosslinking.
Most studies in this field are focused on developing new HCP materials with unprecedented adsorption capacity,11,12 identifying alternative reagents (solvents, monomers) for the sustainable synthesis of HCPs,13 and improving batch productivity rates.14 For example, low HCP productivity rates production can be improved by reducing synthesis time from 24 hours to 5–35 minutes via solvent-free mechanochemical ball milling and liquid-assisted grinding.15 This significant reduction in synthesis duration does not impact HCP quality, with specific surface areas reaching 625–1720 m2 g−1. Another technique suitable for improving the productivity of HCP production is flow synthesis.
The key benefit of replacing batch reactions with flow synthesis is improving productivity rates. To date, flow chemistry had been used for synthesising microporous materials, such as polymers of intrinsic microporosity (PIM),16 metal–organic frameworks (MOFs),17 covalent organic frameworks (COFs),18 and HCPs.10 Flow syntheses of MOFs and COFs enhanced the space-time yield (STY) – a key indicator of productivity rates, comprising product yield, operating time, and reactor volume,17 by 30 fold,19 and 94 fold,18 respectively. These significant STY improvements were attributed to a reduced production time. Meanwhile, Fritsch deployed continuous flow reactions to reduce the PIM synthesis time by 90%.16 The first reported example of flow synthesis of HCPs was reported in 2000 when Vincent and co-workers reported using a tubular reactor to control the particle size of HCPs formed via post-crosslinking during suspension polymerisation.20 However, this work mainly focused on narrowing the particle size distribution. Our group previously deployed flow synthesis to produce HCPs via the external crosslinking method.10 A key limitation of this proof-of-concept was reactor clogging by the newly formed insoluble HCP products that expanded upon solvent adsorption within the reactor.
In this work, we overcome reactor clogging by optimising reactor design and synthesis parameters – residence time, temperature, reagent concentration, and ratio for flow synthesis of HCPs via internal crosslinking of α,α′-dichloro-p-xylene (DCX), post- and external crosslinking of waste Styrofoam in a bespoke coil reactor. To demonstrate the effects of flow synthesis on HCP quality and productivity rates, we compared the specific surface areas, micropore volumes, and STY value of HCPs produced from the flow synthesis with batch reactions. Results from this work support the theory that flow synthesis could be used to scale the production of good-quality HCPs. This is different from existing approaches that rely on HCP functionalisation,21–23 utilisation of bespoke24 or novel25,26 monomers in HCP synthesis, and modified synthesis protocols.15 Our approach establishes a new method to tailor and enhance HCP CO2/N2 selectivity via reactor design and flow synthesis.
Polytetrafluoroethylene (PTFE) tubes with dimensions of 1/8 in × 0.063 in × 50 ft were purchased from Supelco Inc., while PTFE tube fittings (Y-junction connectors, female union connectors, ferrules, and nuts) were purchased from Nanjing Runze Fluid Control Equipment Co., Ltd. Polypropylene syringes were purchased from MB Fibreglass, while 21G needles were supplied from Becton, Dickinson, and Company. A tube holder was fabricated from polylactic acid (PLA) filaments using a Prusa i3 MK3S 3D printer. The design was based on two different shapes with 7 individual parts, a central ring, and holder wings, which can be hinged on to each other. A peristaltic dosing pump (SEKO Kronos 50) was used to pump the monomer solution as well as cleaning solvents after the operation. A syringe pump (Cole-Parmer® SP210iwz) was deployed to inject the catalyst solution into the reactor.
(i) Internal crosslinking – 2.884 g of DCX and 2.672 g of FeCl3 were dissolved in 100 mL DCE. The mixture was heated to 373 K (ref. 24) and continuously stirred with a magnetic stirrer for 24 hours.
(ii) Post-crosslinking – 0.385 g of PS, 0.385 g of FeCl3, and 0.385 g of DCX were first dissolved in 100 mL of DCE. The mixture was heated to 353 K (ref. 27) via an oil bath and continuously stirred with a magnetic stirrer for 24 hours.
(iii) External crosslinking – 0.762 g of PS, 3.810 g of FeCl3, and 3.810 g of FDA were dissolved in 100 mL of DCE. The mixture was heated to 353 K (ref. 28) via an oil bath and continuously stirred with a magnetic stirrer for 24 hours.
(i) Internal crosslinking – the substrate solution was prepared by dissolving 1.154 g of DCX in 20 mL of DCE. A peristaltic pump was used to feed the substrate solution into the system. The catalyst solution was prepared by mixing 1.069 g of FeCl3 in 20 mL of DCE and then loaded into a syringe and pumped into the system by a syringe pump. All reactants were mixed in a Y-junction connector prior to the entry into the reactor at the total volumetric flow rate of 5.40 mL min−1. The reactor was submerged in a heated oil bath setting at 343 K. The slurry product was collected at the end of the reactor.
(ii) Post-crosslinking – the substrate solution was prepared by dissolving 0.154 g of PS in 20 mL of DCE. The catalyst solution was prepared by mixing 1.524 g of FeCl3 and 1.524 g of DCX in 20 mL of DCE and then loaded into a syringe. The substrate solution was fed into the system via a peristaltic pump, while the catalyst solution was fed via a syringe pump. Both solutions were fed into the reactor at the total volumetric flow rate of 2.94 mL min−1 and mixed at a Y-junction. The reactor was submerged in a pre-heated oil bath at 343 K. The slurry product was collected at the end of the reactor.
(iii) External crosslinking – the substrate solution was prepared by dissolving 0.305 g of PS in 20 mL of DCE. The catalyst solution was prepared by mixing 1.524 g of FeCl3 and 1.524 g of FDA in 20 mL of DCE and loaded into a syringe. A peristaltic pump was used for feeding substrate solution into the reactor. A syringe pump was used for feeding the catalyst solution into the system. The total volumetric flow rate was 1.94 mL min−1. Both solutions were mixed at the Y-junction before entering the reactor, which was pre-heated inside the oil bath at 343 K. The slurry product was collected at the end of the reactor.
The reactor volume was approximately 30 mL (1.58 mm inner diameter, 15.21 m length). Our reactor comprised a 50 mm pitch helical coil with multiple curvature radii (34, 39, 44, 49, and 54 mm) designed to minimise the spatial footprint.
Since the reactor volume (V) was fixed, a change in the residence time (τ) depended on the total volumetric feed flow rate (F), as shown in the following equation:
The operating time of flow synthesis (top) was calculated using the following equation where Vfeed was denoted as the volume of the feed solution:
Upon completion of each synthesis process, either in batch or flow, products were washed with chloroform and methanol until the washings became clear. Then, the samples were the soaked overnight in methanol and rewashed in the same way with methanol and acetone. Finally, samples were dried in an oven at 333 K for 8 hours and collected for further characterisation.
STY is the term used in flow chemistry to determine how much product, i.e., yield is produced in a reactor volume within a time period. As batch reactions here took place over 24 hours, the operating period for the calculation of the flow system was then fixed to per day unit. The STY calculations for batch and flow systems17,31 were shown in the following equations:
Fourier transform infrared spectroscopy (FTIR) was used to investigate the chemical structure of the reagents and HCPs synthesised in this work. This was achieved with an ATR-FTIR (Nicolet iS10 with Smart iTX Diamond accessory, Thermo Fisher Scientific, Waltham, MA) with 64 scans per sample and a resolution of 0.48 cm−1. The chemical structures of the synthesised HCPs were validated with solid-state 13C cross polarisation (CP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy using a Bruker Avance III spectrometer with a 9.4 T superconducting magnet.
N2 and CO2 adsorption studies at 298 K were performed and the adsorption isotherms were used to calculate CO2/N2 selectivity (SCO2/N2) based on the ideal adsorption solution theory (IAST). The standard conditions34 of 298 K and 15:85 molar ratio of the inlet gas mixture (yCO2 = 0.15, yN2 = 0.85) were chosen for this calculation using following equations:
Ptyi = P0ixi |
xCO2 + xN2 = 1 |
yCO2 + yN2 = 1 |
The parameters from the SSLF model equation, q, k, and n, represented the maximum-adsorbed amount of pure gas at the adsorption site, the affinity parameter, and the solid heterogeneity respectively. These parameters were obtained from the regression analysis of the experimental adsorption isotherm data.
Transmission electron spectroscopy (TEM) was deployed to characterise HCP morphology. The samples were dispersed in DCE and deposited onto carbon-coated copper grids (Agar Scientific). Excess solvent was blotted away and the grid was air-dried. Samples were imaged in an FEI F20 microscope operating at 200 kV. The images were taken under low-dose conditions using a Gatan Rio CCD camera. The micrographs (Fig. S8) are shown in ESI.†
Crosslinking approach | Substrate–crosslinker type | Substrate concentration [% w/v] | Substrate:FeCl3:crosslinker | Temperature [K] | Volumetric flow rate [mL min−1] | Reaction timea [min] | HCP yield [%] | Sampleb |
---|---|---|---|---|---|---|---|---|
a Residence time is an equivalent term for reaction time in the flow system. b The samples were named X-Y-HCP in which X denotes hypercrosslinking approach (I for internal crosslinking, P for post crosslinking, E for external crosslinking) and Y denotes synthesis system (B for batch reactions, F for flow synthesis). | ||||||||
Internal crosslinking | DCX–N.A. | 2.884 | 1:1:0 (mol) | 373 | N.A. | 1440 | 58.50 | I-B-HCP |
2.884 | 1:1:0 (mol) | 343 | 5.40 | 5 | 7.24 | I-F-HCP | ||
Post-crosslinking | PS–DCX | 0.385 | 1:1:1 (wt) | 353 | N.A. | 1440 | 155.38 | P-B-HCP |
0.385 | 1:1:1 (wt) | 343 | 2.94 | 10 | 126.72 | P-F-HCP | ||
External crosslinking | PS–FDA | 0.762 | 1:5:5 (wt) | 353 | N.A. | 1440 | 156.63 | E-B-HCP |
0.762 | 1:5:5 (wt) | 343 | 1.94 | 15 | 114.30 | E-F-HCP |
In our work, heat transfer was mainly impacted by this significant difference in reactor-specific surface area. The larger specific surface area of the flow reactor enhanced heat transfer, i.e., the energy flow from the oil bath to the reactor walls that were in close contact with the reactants inside the reactor. At the same time, the small volume of the flow reactor offered short diffusion pathways (better mass transfer) to enable faster mixing.37 The combination of these factors was favourable for polymerisation/crosslinking kinetics. The effects of the thermal conductivity of the reactor material and heat source temperature were negligible here. The difference in thermal conductivities between the glass batch reactor (1 W m−1 K−1)38 and polytetrafluoroethylene (PTFE) flow reactor (0.25 W m−1 K−1)39 was 4 times, while the difference in heat source temperatures used in both batch and flow syntheses was only 10 K.
In terms of mass transfer, a higher degree of mixing is favourable for the reaction as reagent particles have more access to the reactor surface where the highest heat energy is provided.17 The mixing efficiency could be correlated to the dimensionless Reynolds number (Re) which describes fluid flow patterns – laminar, transient, and turbulence. Turbulent flow is typically associated with good mixing as an external driving force is applied to induce particle diffusion, rather than relying solely on molecular diffusion.17 In a stirred tank reactor, the turbulent region is where Re > 10000,40 while turbulent fluid flow in a helical coil reactor is where Re > 40.41
Using an equation for a propeller turbine mixer tank,42 we estimated that the Re number in our batch reactor reached a value of 6524 (see ESI†). This indicated transient fluid flow (the transition region between laminar and turbulent flow pattern, 100 < Re < 10000) in our batch reactor, i.e., moderate mixing efficiency. Meanwhile, the Re number in our coil flow reactor (Table S5†) during the synthesis of E-F-HCP (15 minutes residence time) was estimated to be 65. This indicated turbulent mixing in our flow reactor. Clearly, the coiled reactor used in this work provided better mixing rates, enhancing polymerisation/crosslinking kinetics.20
The advantage of short reaction times in flow synthesis resulted in an increase in STY (Fig. 3). With flow synthesis, the STY value improved by 32 fold for internal crosslinking, 117 fold for post-crosslinking, and 68 fold for external crosslinking. This addressed the issue of low HCP productivity rates due to time constraints required in batch reactions that we have now overcome with flow synthesis.
Comparing the FTIR spectra of HCPs synthesised by internal crosslinking in batch (I-B-HCP) and continuous flow (I-F-HCP) reactions, we observed that in both samples, the infrared peak intensities correlated to C–Cl vibrations27,43,44 at 668 and 1260 cm−1 and C–H bending44 at 854 cm−1 were reduced when compared to the unreacted DCX precursor (Fig. 4a). This was ascribed to chloromethyl substitution at the para position, suggesting that the chloro group was eliminated during Friedel–Crafts alkylation. The intensities of peaks correlated to the skeletal benzene rings, such as aromatic C–H stretching43 at 2971 and 2866 cm−1 and aromatic CC stretching43,45 at 1421, 1445, and 1512 cm−1 were also reduced. This suggested the success of DCX crosslinking.45 At the same time, the peak corresponding to C–H stretching45 at 2923 cm−1 from HCP broadened when compared to that of DCX. This was due to the overlapping original C–H stretching peak with the newly formed methylene bridge (–CH2CH2–) crosslinks.45 This type of crosslinked bridge was formed from DCE molecules that functioned as both a solvent and a crosslinker reagent.46
Fig. 4 FTIR spectra of the HCPs and their precursors: (a) internal crosslinking (p-DCX), (b) post-crosslinking (PS + DCX crosslinker), (c) external crosslinking (PS + FDA crosslinker). |
The FTIR spectra of HCPs synthesised via post-crosslinking, P-B-HCP, and P-F-HCP (Fig. 4b), resembled that of PS. We observed peaks that corresponded to aromatic C–H stretching at 3087, 3061, 3027 cm−1, C–H bending at 756, 696 cm−1, aromatic CC stretching at 1603, 1493, 1453 cm−1, C–H bending at 854 cm−1 and backbone C–H stretching at 2921 and 2849 cm−1.28 However, these peaks were less intense than those of PS. These indicated that the aromatic benzene rings of PS were modified during post-crosslinking. These trends were also presented in the FTIR spectra of HCPs synthesised via external crosslinking, E-B-HCP, and E-F-HCP (Fig. 4c). The broadened peak corresponding to C–H stretching45 centred at 2921 cm−1 similar to the spectra of internally crosslinked HCPs was observed in these spectra. This indicated that –CH2CH2– crosslinks were also formed in the externally crosslinked HCP. The presence of newly formed crosslinking methylene bridges in these HCPs was also validated with 13C-NMR.
The 13C-NMR spectra of I-B-HCP and I-F-HCP (Fig. 5a) contained major peaks centred at 138, 35, and 16 ppm. These peaks were correlated to the substituted aromatic carbon,47 the crosslinked bridges from via FDA28,47 and DCE,27 respectively. There were also unreacted DCX24,48 in these samples as we also observed peaks centred at 43 ppm (carbon in –CH2Cl) and 128 ppm (non-substituted aromatic ring). The spectra of P-B-HCP and P-F-HCP (Fig. 5b) also contained a peak centred at 146 ppm correlated to the quaternary non-substituted aromatic carbon48,49 from the parent PS molecule. These spectra also contained peaks at 136 and 128 ppm, which corresponded to substituted aromatic carbon and non-substituted aromatic rings from both PS and DCX, respectively. The broad peak centred at 40 ppm in these spectra was a mixture of signals attributed to aliphatic methylene carbon, methine backbone carbon, and methylene crosslink bridges.48 We did not observe any peaks corresponding to chloromethyl carbon in these samples. The 13C-NMR spectra of E-B-HCP and E-F-HCP (Fig. 5c) were like those of HCPs produced from post-crosslinking.
Fig. 5 13C-NMR spectra of the HCPs: (a) internal crosslinking (p-DCX), (b) post-crosslinking (PS + DCX crosslinker), (c) external crosslinking (PS + FDA crosslinker). |
Samples | BET surface area [m2 g−1] | DFT total pore volume [cm3 g−1] | DFT micropore volume [cm3 g−1] |
---|---|---|---|
a DFT micropore volume data were obtained from the cumulative pore volume of the pore size up to 2 nm. | |||
I-B-HCP | 964 | 1.263 | 0.042 |
I-F-HCP | 950 | 1.014 | 0.281 |
(−1.5%) | (−19.7%) | (+569%) | |
P-B-HCP | 903 | 0.788 | 0.203 |
P-F-HCP | 816 | 0.650 | 0.195 |
(−9.6%) | (−17.3%) | (−3.9%) | |
E-B-HCP | 1162 | 1.219 | 0.184 |
E-F-HCP | 1092 | 0.917 | 0.293 |
(−6.0%) | (−24.8%) | (+59.2%) |
The existence of micropores was confirmed through the steep slope at lower relative pressure, where P/P0 < 0.05 (Fig. S5†). Based on IUPAC classification,51 the isotherms of internally crosslinked HCPs were a mixture of type I and IV. Moreover, the presence of macropores (>50 nm) in the internally crosslinked HCPs could be observed in the isotherm at P/P0 > 0.9. It should be noted that this was the only HCP type in this study that formed macropores. The isotherms of HCPs synthesised by post-crosslinking and external crosslinking were classified as type IV, where ink bottle-like pore shapes were more prevalent. The hysteresis loops of these HCPs indicated that there were significant amounts of small mesopores (2–10 nm) within these HCPs, which correlated to the broad peaks in the pore size distributions (Fig. 6).
The microporosity of HCPs studied here could be tailored by varying synthesis parameters: residence time, feed solution composition and concentration, and temperature. Here we observed that at a fixed substrate concentration, reagent ratios, and temperatures, an increase in residence time from 10 to 15 minutes typically enhanced the micropore volume of externally crosslinked HCPs by 595%. However, a further increase of residence time to 20 minutes reduced micropore volume by 65.5% (Table S3†). Similar trends were observed in batch reactions over 12 and 24 hour periods.1 Prolonged reaction times could lead to excessive crosslinking, forming pores that are smaller than the kinetic diameter of N2, the gas used to probe porosity content during gas adsorption experiments. Another reason for an optimised micropore volume in HCPs produced via 15 minutes of residence time in flow reactors could be the mixing intensity. Our calculation (Table S5†) showed that with a residence time of 15 minutes, the Reynolds number of our reactor reached a value of 65. This was closest to the optimal Reynolds number of 40 in helical reactors.41 Here it is important to highlight that the optimisation of HCP porosity by tailoring residence time did not drastically affect STY, i.e., productivity rate. The highest STY value of 965 mg mL−1 day−1 was achieved with a residence time of 10 minutes. The increase of residence time to 15 minutes reduced this STY value by 28.3%, reaching 692 mg mL−1 day−1.
The enhancements in both micropore volume and STY were observed (Table S3†) when the weight ratio of PS by FeCl3 by FDA increased from 1:1:1 to 1:5:5 while fixing the other three synthesis parameters. No solid was formed at the weight ratio of 1:1:1. This indicates that crosslinking did not occur. Although a similar test in the batch system showed that the crosslinking using a 1:1:1 ratio of PS to FeCl3 to FDA is achievable,28 shorter residence time and lower temperature in flow reactors might be insufficient to induce crosslinking when there is a low ratio of catalyst and crosslinker to the substrate. HCP products were obtained when the weight ratio was increased to 1:3:3, with a STY of 981 mg mL−1 day−1. However, the micropore volume of 0.006543 cm3 g−1 was the lowest among all the externally crosslinked HCPs. The increase in micropore volume and the STY when increasing the ratio from 1:3:3 to 1:5:5 was 1493% and 47.5%, respectively. Similar trends showing the improvement of the specific surface area, as a result of pore generation, and product yield were also observed in batch HCP synthesis.24,28 The higher amount of the catalyst and crosslinker in the system indicated a higher possibility of the substrate getting involved in the reaction. Hence, the suitable weight ratio of substrate to the catalyst to crosslinker for flow synthesis in our system was 1:5:5.
Increasing the reaction temperature while fixing the residence time, substrate concentration, and chemical ratio did not enhance STY (average of 830 mg mL−1 day−1), however, micropore volumes were enhanced. The increase in the temperature from 333 to 343 K increased the micropore volume by 84.8% from 0.059 to 0.109 cm3 g−1. This indicated that elevating temperature by a 10 K difference in our system affected the crosslinking degree in HCPs.
The impact of PS concentration on micropore volume was subtle compared with the other three synthetic parameters. The amount of micropore volume was increased by just 37.5% from 0.4% to 0.6% w/v of PS. Increasing the substrate concentration from 0.6 to 0.8% w/v and then to 1.0% w/v slightly increased and decreased micropore volume by 5.7% and 9.5%, respectively. This trend was also observed in batch synthesis,24,46 which suggested that the fabrication of microporous polymers, i.e., HCP is not sensitive to the monomer concentration as much as macroporous polymers.24 On the other hand, the substrate concentration caused the largest influence on STY among all four synthetic parameters, as the correlation between PS concentration and STY was directly proportional. Increasing the PS concentration from 0.4% to 1.0% w/v enhanced STY by 3 fold from 457 to 1447 mg mL−1 day−1.
With these optimal synthesis parameters, we were able to enhance the STY of HCP synthesis using flow reactions, as shown earlier in Fig. 3. This inferred that we were able to not only enhance the microporosity of the internally crosslinked HCPs but also improve the STY of such HCPs.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta09253k |
This journal is © The Royal Society of Chemistry 2023 |