Solvothermal Crystallization Kinetics and Control of Crystal Size Distribution of MOF-808 in a Continuous Flow Reactor

TL;DR: In this article, the authors employed a continuous flow reactor to elucidate the kinetics of crystallization for the Zr-based MOF-808 using time-resolved powder X-ray diffraction measurements.

Abstract: A fundamental understanding of the crystallization pathways for metal− organic frameworks (MOFs) allows for exploring the untapped combinatorial space of the organic and inorganic building units, creating possibilities to synthesize highly crystalline frameworks with desired physicochemical properties. In this work, we employ a continuous flow reactor to elucidate the kinetics of crystallization for the Zr-based MOF-808 using time-resolved powder X-ray diffraction measurements. Specifically, we fit the crystallization curves obtained experimentally using the Gualtieri model to determine the rate constants for nucleation (kN) and growth (kG) for different linker concentrations and temperatures. Higher linker concentrations reduce the competitive coordination of the formate ligand (growth modulator) with the secondary building unit, resulting in higher nucleation and growth rates. The activation energies obtained from Arrhenius plots for nucleation (Ea(N)) and growth (Ea(G)) are 64.7 ± 4 and 59.2 ± 5 kJ mol−1, respectively. At constant residence time, temperature, and composition, higher flow velocities increase the advective transport of precursor species to nucleation sites in the slugs resulting in increased crystal growth rates and thus higher average crystal sizes. Variation in the total flow rate from 0.334 to 1.067 mL/min increased the average crystal sizes from ∼105 ± 22 to ∼180 ± 19 nm, with other parameters held constant. We demonstrate that performing crystallization in the flow reactor provides a unique opportunity to tailor MOF crystal sizes. By strictly controlling the temperature, residence time, and mixing parameters, our results showcase the advantages of flow systems for performing rigorous crystallization and structural evolution studies that can be applied for the synthesis of other MOFs with tailored physicochemical properties. ■ INTRODUCTION Metal−organic frameworks (MOFs) are coordination complexes consisting of organic linkers and inorganic polynuclear clusters that form twoand three-dimensional structures. The numerous ways in which the organic and inorganic units can be combined have led to the discovery of thousands of new frameworks with unique properties that can be targeted for use in industrially attractive applications. Zr-based MOFs are particularly important due to their chemical stability, as well as amenability to postsynthetic modification (PSM). MOF-808, first reported by Furukawa et al., is formed from the assembly of a Zr6(μ3-O)4-(μ3-OH)4(CO2)12 (referred to as Zr6-cluster) inorganic secondary building unit and benzene-1,3,5-tricarboxylic acid (H3BTC) organic linkers. 3 The resulting structure features large cavities (diameter of 18.4 Å) and Brunauer− Emmett−Teller (BET) surface areas of ∼2000 m2·g−1. Monocarboxylic ligands, such as formate, acetate, and propionate, are employed as crystal growth modulators in MOF-808 synthesis to regulate crystal size and increase the crystallinity of the framework. Although MOF-808 has been demonstrated for use in a number of industrially relevant applications ranging from catalysis and water harvesting to heavy metal capture and arsenic removal, details pertaining to the kinetics of crystallization or the control of crystal size distributions (CSDs) have not been reported. A thorough understanding of the self-assembly of MOF building units along with the reaction pathways to achieve precise control over the crystallization process would help in optimizing MOF synthesis to yield the desired crystallinity and provide important parameters for synthesis scale-up. Tailored physicochemical properties of Zr-MOFs can be achieved by coordination modulation, using organic ligands with a similar chemical functionality as the linker to compete for coordination sites at the SBU. In the presence of a modulator, nucleation and crystal growth proceed at a reduced rate. The crystallization process requires an equilibrium between crystal formation and dissolution to allow for sufficient reorganization and defect reparation during the early stages of crystal growth. Accordingly, evaluating the amount of modulator, linker concentration, residence time, and temperature in the extent of crystallization is imperative to achieve the desired CSD, crystallinity, and product yields. Received: August 23, 2021 Revised: September 30, 2021 Article pubs.acs.org/crystal © XXXX American Chemical Society A https://doi.org/10.1021/acs.cgd.1c00968 Cryst. Growth Des. XXXX, XXX, XXX−XXX D ow nl oa de d vi a M A SS A C H U SE T T S IN ST O F T E C H N O L O G Y o n O ct ob er 2 1, 2 02 1 at 0 0: 17 :4 5 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.