Continuous flow synthesis of PCN

Alessio Zuliani *, M. Carmen Castillejos and Noureddine Khiar * Asymmetric Synthesis and Nanosystem Group (Art&Fun), Institute for Chemical Research (IIQ), CSIC-University of Seville, 41092 Sevilla, Spain. E-mail: [email protected]; [email protected]

First published on 21st November 2023

Zirconium-based MOFs, such as PCN-222 (MOF-545), have gained attention due to their regular porosity, tuneable pore size, versatile structure and exceptional thermal/chemical stability. However, the synthesis of PCN-222 using traditional batch processes suffers from limitations in productivity, scalability, and control over size and morphology. This study presents a paradigm shift from batch to continuous flow (c.f.) synthesis for the efficient production of PCN-222. The optimization of batch conditions and exploration of c.f. parameters enabled the synthesis of high-quality PCN-222 with Space–Time–Yield (STY) exceeding 950 kg m−3 day−1. The c.f. approach offered advantages including improved resource efficiency, safer chemical handling, and scalability. The integration of ultrasound-assisted technique into the c.f. facilitated the shaping of PCN-222 into prolate ellipsoids, making it suitable for applications in drug delivery and catalysis. The successful substitution of hazardous solvents with greener alternatives further enhanced the sustainability of the process. Finally, the loading of a thio-N-acetylgalactosamine-PEG-sulfate ligand on PCN-222 in tandem c.f., expanded the potentialities of the flow synthetic approach. This study not only advances the field of MOF synthesis but also paves the way for efficient and sustainable manufacturing processes in c.f. synthesis of nanoparticles and functionalized nanoparticles for active drug delivery.

Among all, zirconium (Zr) based MOFs are gaining prominence due to their unique combination of properties. For instance, Zr-based MOFs have exceptional thermal and chemical stability, making them ideal for high-temperature applications and in harsh chemical environments. In addition, the Zr metal center has a high coordination number and large ionic radius, which can lead to a diverse range of structures by supporting a large variety of organic ligands. A Zr-based MOF having attracted attention in the last decade is the porous coordination network-222 (PCN-222, also known as MOF-545), firstly synthesized in 2012.4–6 PCN-222 is composed of Zr(IV) clusters connected by tetrakis(4-carboxyphenyl)porphyrin (TCPP) ligands through Zr–carboxyl bonds in a three-dimensional porous network structure. The strong coordination bond between Zr and O, rising from the high charge density and bond polarization of the highly oxidated Zr(IV), results in a highly robust structure. The morphology of PCN-222 is mainly reported as micro-dimensional (>1 μm length) needle/rod-shaped crystals or, less frequently, as prolate ellipsoids (ranging from ca. 50 nm up to more than 500 nm), while the strict control of the size-distribution of PCN-222 is hardly achieved.7,8 The main characteristics of PCN-222 include high surface area of up to more than 2000 m2 g−1 and large pore volume. Thus, PCN-222 has been mainly explored for storing gases such as methane, hydrogen, and carbon dioxide and as a catalyst in organic transformations and in the immobilization of enzymes. Concerning the field of biomedicine, PCN-222 has recently attracted considerable attention due to its remarkable acid stability and biocompatibility, which offer the potential for targeted drug delivery to specific sites such as acidic tumour environments. Moreover, PCN-222 can be used as pH-sensitive drug delivery system, further enhancing its therapeutic applications.9 Additionally, PCN-222 finds potential utility in photodynamic therapy (PDT) when exposed to laser irradiation.10 The captivating biomedical applications of PCN-222 are underscored by the prospect of drug-loaded PCN-222 possessing substantial commercial value, offering opportunities for cost-effective production methods. This contrasts with other applications, such as gas storage or catalysis, where achieving economic viability can be more challenging. For example, Fairen-Jimenez et al. loaded a model drug (doxorubicin) on PCN-222 PEGylated with PEG-phosphate ligands forming a composite having excellent water dispersibility at room temperature, improved colloidal stability and delayed drug-release kinetics of the drug.11 Notwithstanding the potential of PCN-222, its synthesis has only been performed using batch processes, which can suffer from several limitations such as low yields and productivity, long reaction times, and difficulty in scaling up. Typically, the solvothermal synthesis of PCN-222 is performed under laboratory conditions by dissolving a Zr-precursor (normally Zr chloride or previously synthesised Zr6 clusters), TCPP and an acid modulator (usually benzoic acid, trifluoroacetic acid or fluorobenzyl acid) in N,N-dimethylformamide (DMF), followed by heating at 393 K under magnetic stirring from a few up to 48 hours.12–14 The purification of the so-synthesized PCN-222 is performed by centrifugation. The slow crystallization rate of PCN-222 together with heat and mass transfer issues limits the scale up of the process, restricting the production to a few mg per batch. Furthermore, the use of high volume of hazardous DMF entails important limits especially considering health and environmental issues.15 Therefore, the design of more efficient, sustainable, and scalable synthetic methods based on alternative techniques such as mechanochemistry, microwave chemistry, ultrasound-assisted synthesis and continuous flow synthesis is crucial to overpass these inefficiencies and to perform accessible production of PCN-222. The first effort for the scalable synthesis of PCN-222 reported up to date exploited microwave chemistry to produce tens of mg of the MOF with yields of 72%, within only 30 minutes.16 However, the poor control on the morphology and on the size-distribution of the MOF cannot be applied for many applications, such as drug delivery, where nanosystems must have controlled size and morphology to regulate their circulation and clearance kinetics in the bloodstream. Indeed, it has been demonstrated that specific shapes, such as prolate ellipsoidal nanoparticles exhibit lower rates of systemic elimination compared to particles with other shapes, indicating that they may circulate in the bloodstream for longer periods.17 In addition, the smaller particle size shows long in vivo circulation time, although this should be carefully checked also considering the hydrodynamic size in the blood stream. Another example for the scalable synthesis of PCN-222 has been recently reported by Yu et al. through an ultrasound-assisted method. In details, the researchers were able to synthesize PCN-222 in high crystal purity and uniform size within 30 minutes.18 Still, the use of batch synthesis limits the potential scale up of the synthesis, with important sustainable drawbacks, including low energy efficiency and high chemical risk.

In the past few years, continuous flow (c.f.) chemistry has emerged as an attractive alternative to batch synthesis methods for the efficient preparation of different chemicals and materials and has been pointed out as promising path for the industrialization of MOFs.19 C.f. chemistry offers several advantages over batch processes including resource efficiency, reduced waste generation, safer chemical handling, energy efficient, scalability and process intensification.20,21 C.f. has been also reported by the IUPAC as one of the “Top Ten Emerging Technologies” with the potential to turn the planet more sustainable.22 Furthermore, external fields such as microwave or ultrasound (US)23 can be coupled to c.f. to enhance heat transfer, facilitate the disruption of agglomerates of particles, boost the mix of the reagents and precursors in the channel and reduce the undesired deposition of solid materials on the channel wall. At the same time, US have been proved to have multiple effects such as accelerating the nucleation and growth to get monodispersed nanoparticles, influencing the crystal morphology and etching the crystals.18,24

Here, the first use of a c.f. reactor for the synthesis of PCN-222 with defined and sized-controlled prolate ellipsoid morphology (down to less than 100 nm) is reported. After optimizing the batch conditions in terms of ligand-to-metal cluster molar ratio (0.3–2.5), sequential experiments in c.f. examined the influence of ligand concentration (3.2–7.1 mM), flow rate (0.1–5 mL min−1), ligand-to-acid modulator molar ratio (0.007–0.054) and temperature (383–418 K). Considerations of some specific parameters on the sustainability of the synthesis were determined by the green metrics Environmental factor (E-factor) and Reaction Mass Efficiency (RME). The best reaction conditions allowed overpassing the challenge of preserving the quality of MOF during the rapid synthesis in c.f. The c.f. method was thus combined with US technique (20 kHz, 20–90% US amplitude) allowing the etching of the crystals and the shaping the MOF into prolate ellipsoids by variations of the dimensions of the minor axis (i.e., the equatorial axis) of the ellipsoids. Greener alternative solvents to hazardous DMF were also evaluated. The c.f. synthesis yielded high-quality PCN-222 with Space–Time–Yield (STY, one of the most common parameters representing the productivity of the synthesis) up to more than 950 kg m−3 day−1, demonstrating the robustness and scalability of the method. The potential biomedical applications of PCN-222 were finally evaluated by the loading a thio-N-acetylgalactosamine-PEG-sulfate ligand (S-GalNAc-PEG-sulfate) in tandem c.f. The ligand was incorporated onto the PCN-222 particles through Zr–sulfate interactions, contributing to the stabilization of the PCN-222 particles. Additionally, the incorporation of thio-N-acetylgalactosamine (S-GalNAc), a glycosidase-resistant analogue of natural GalNAc, holds great promise as it specifically targets the asialoglycoprotein receptor (ASGPR) overexpressed on hepatic cancer cells. This presents an exciting opportunity to efficiently develop a porphyrin-based drug delivery system using continuous flow synthesis.

Firstly, commercially available N-acetylgalactosamine tetra-acetate 1 was treated with Lawesson's Reagent in toluene at 353 K to provide the cyclized per-O-acetylated thiazoline 2 with 71% yield. The heterocycle in compound 2 was subsequently cleaved through acid hydrolysis using TFA and water in methanol (MeOH) at 273 K.27 This process resulted in the production of thiol 3 with an 85% yield. Secondly, octaetyleneglycol 4 was treated with SOCl2 in the presence of 4-dimethylaminopyridine (DMAP) and diisopropylethylamine (DIPEA) in dichloromethane (DCM) from 273 K to r.t., affording cyclic sulfite 5 with 91% yield. Subsequently, the oxidation of 5 was carried out using NaIO4 with a Ru-catalyst in a mixture of acetonitrile (I), trichloromethane (TCM), and water, resulting in the formation of cyclic sulfate 6 with a yield of 79%.28 The condensation of thiol 3 with cyclic sulfate 6, using triethylamine (Et3N) as a base in dimethylformamide (DMF) at r.t., resulted in the formation of compound 7 with a yield of 61%. Lastly, compound 8, i.e., the S-GalNAc-PEG-sulfate ligand, was obtained in quantitative yield though Zemplen deacetylation of 7, carried out using MeONa in MeOH at r.t. Details of the synthesis as well as 1H-NMR, 13C-NMR and mass spectroscopy of the compounds are reported in the ESI.†

In order to evaluate the productivity of the synthesis in c.f., the Space–Time–Yield (STY)29 was calculated according to eqn (1):

Each experiment was repeated at least three times in order to determine the STY, which was thus reported with its corresponding standard deviation.

The metrics were calculated using the mean mass of the synthesized PCN-222 obtained in the different runs of each experiment. As easily understandable, the lower the E-factor the lower the environmental impact of the synthesis. For example, the general E-factor found in different industrial segments varies form ca. 0.1 of oil refining, to 1–50 of fine chemicals and 50–100 of pharmaceuticals.26 On the contrary, the higher the RME, the lower the environmental impact of the synthesis. In the present study, to accentuate the disparity between batch and c.f. synthesis, green metrics were discussed without accounting for solvents. This omission was supported by the ease with which solvents can be recycled after separation from PCN-222, typically through centrifugation, rendering them potentially non-waste materials.26 Green metrics were calculated using the average mass of PCN-222 produced in each experiment. For instance, in an experiment (entry 4 in Tables S2 and S3 in the ESI†), 11.5 mg of PCN-222 (STY: 5.5 kg m−3 day−1) was produced using 22.5 mg of TCPP, 33.0 mg of Zr6 nodes, and 193.7 mg of TFA (130 μL), totalling 249.2 mg of mass of reagents (excluding the solvent used in the reaction, DMF, and the solvents for washing, DMF and EtOH). The waste mass was determined by subtracting the mass of the product and corresponded to 237.7. Thus, the E-factor was 20.7 (calculated as 237.7 divided by 11.5), and the RME was 4.6% (calculated as 11.5 divided by 249.2 and multiplied by 100). To provide a comprehensive overview, the E-factor calculated considering also the solvents can be checked in the ESI (RME does not count solvents).†

In all the experiments, the concentration of the ligand TCPP was kept constant, while the quantity of Zr6 nodes was varied to tune the L:M from 0.3 to 2.5 (see ESI section-S3†).

Considering that the molecular formula of PCN-222 is Zr6(μ3-OH)4(μ3-O)4(C48H26N4O8)2, formed by TCPP4− ligand (C48H26N4O8) and Zr6(μ3-OH)4(μ3-O)4 nodes34 the results showed that, to increase the STY of PCN-222, it was necessary to use less quantity than the (theoretical) stoichiometric amount of the ligand TCPP (i.e., more Zr6 clusters). This was particularly evident when the L:M was equal or less than 1. At the same time, the size of the PCN-222 was observed to increase when more quantity of the metal clusters was used. This behaviour was in accordance with the reported literature related to other MOFs. In details, it is well established that a decrease of the ratio ligand–metal node influences (normally boosts) the nucleation and growth rates of MOF crystals, which in turn can also impact the size and the morphology of the final crystals.35,36 Thus, considering the STY of the synthesis, the best reaction conditions for the highest STY corresponded to the largest quantity of metal clusters. However, SEM images showed a substantial variation of the size-distribution of the PCN-222 when the L:M was ≤0.5. As a result, a L:M of 1 was selected for the shifting to c.f. synthesis, corresponding to the minimum quantity of Zr6 nodes required to obtain particles having sizes with relative SD of ca. 5% (222 ± 12 nm). A smaller L:M was discarded since it implied more consumption of metal nodes and, more importantly, larger SD (e.g., a L:M = 0.7 required 26% more metal nodes and resulted in a SD of ca. 10%). Importantly, the use of smaller L:M was also discarded considering no relevant variations in the green metrics of the synthesis. In details, when a L:M = 1 was used, E-factor = 20.7 and RME = 4.6%, while when L:M = 0.7, E-factor = 19.9 and RME = 4.8% (ESI Table S3†). The optimized batch conditions were thus selected for the shifting to c.f. synthesis. The importance of shifting the synthesis of MOFs to c.f. rises form the different benefits and advantages brought by this type of synthetic approach including improved control and reproducibility, enhanced reaction kinetics and product yield, safer and more controlled conditions, scalability, and production efficiency, reduced environmental impact, and finally versatility and process intensification. These advantages make the c.f. synthesis a promising approach to produce MOF with consistent quality and improved green characteristics and has been already applied in the synthesis of some types of MOFs such as UiO-66, MIL-100, and HKUST-1 (see ESI section-S4† for a detailed list of the most recent works related to the c.f. synthesis of MOFs). However, up to date, no report discusses the c.f. synthesis of the PCN-222/MOF-545.

The reaction was thus shifted to the c.f. system by mixing in a T valve two feeds, one with the ligand TCPP dissolved in DMF and the other with the Zr6 nodes and TFA dissolved in DMF. A T-valve was preferred to a cross-valve (where TFA would have been separated from the Zr6 clusters in another feed) since the Zr6 clusters did not completely dissolve in DMF without the presence of TFA (probably TFA substitutes the benzoate in capping the Zr6 nodes, making the nodes more polar, thus more soluble in DMF). The influence of the concentration of the reagents, the volumetric flow rate, the temperature of the reaction and the amount of employed acid modulator on the STY and on the size of the PCN-222 were thus singularly determined in c.f. Initially, the reaction was shifted to c.f. starting from the optimized batch conditions, which were 393 K reaction temperature, a L:M of 1, and a concentration of 3.6 mM TCPP, using DMF as solvent. According to the results of the preliminary tests, the amount of employed TFA was initially decreased (from a ligand-to-acid modulator molar ratio (L:Mod) of 0.017 to 0.027), aiming at diminishing the size of the particles11 and, more importantly, considering that the c.f. conditions do not imply the liquid–vapour equilibrium of TFA in the head space of the reactor observed in batch conditions (thus less acid was needed to obtain particles with the same dimensions of those obtained in batch). The amount of TFA used was then optimized after studying the effects of the volume of solvent, of the flow rate, and of the temperature.