Andrea
Laybourn
*ab,
Juliano
Katrib
b,
Rebecca S.
Ferrari-John
b,
Christopher G.
Morris
ac,
Sihai
Yang
ac,
Ofonime
Udoudo
b,
Timothy L.
Easun
ad,
Chris
Dodds
b,
Neil R.
Champness
*a,
Samuel W.
Kingman
*b and
Martin
Schröder
*ac
aSchool of Chemistry, University of Nottingham, Nottingham NG7 2RD, UK. E-mail: Andrea.Laybourn@nottingham.ac.uk; Neil.Champness@nottingham.ac.uk
bFaculty of Engineering, University of Nottingham, Nottingham NG7 2RD, UK. E-mail: Sam.Kingman@nottingham.ac.uk
cSchool of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK. E-mail: M.Schroder@manchester.ac.uk
dSchool of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK
First published on 5th April 2017
Synthesis of metal–organic framework (MOF) materials via microwave heating often involves shorter reaction times and offers enhanced control of particle size compared to conventional heating. However, there is little understanding of the interactions between electromagnetic waves and MOFs, their reactants, and intermediates, all of which are required for successful scale-up to enable production of commercially viable quantities of material. By examining the effect of average absorbed power with a constant total absorbed energy to prepare MIL-53(Al) we have defined a selective heating mechanism that affords control over MOF particle size range and morphology by altering the microwave power. This is the first time a selective mechanism has been established for the preparation of MOFs via microwave heating. This approach has been applied to the very rapid preparation of MIL-53(Al)ta (62 mg in 4.3 seconds) which represents the fastest reported synthesis of a MOF on this scale to date.
Alternative synthetic routes to MOFs include mechanochemical,9 electrochemical,10 sonochemical,11 and microwave12 methods. Microwave heating is an extremely promising technology for MOF synthesis as it potentially offers shorter reaction times,13 control over crystallite size,13,14 and more energy efficiency compared to conventional heating.15 Many of the approaches for microwave synthesis of MOFs provide little understanding of the effect of microwave energy upon the bulk reaction mixture and simply use the average temperature of the reaction mixture as a key indicator.14,16 There is also insufficient knowledge of parameters that are essential for scale-up. These parameters include (i) dielectric properties, defining the efficiency of power coupling and distribution of the electric field within the heating cavity; (ii) penetration depth and (iii) power density in the heated phase. Such information is crucially important when scale-up is considered as all of these variables underpin the design of large scale process systems. Failure to determine this fundamental information ultimately leads to failure in scale-up from the laboratory to industrial scale.
We report the effect of microwave heating to produce MIL-53(Al) (MIL = Materials Institute Lavoisier).17 MIL-53(Al) was chosen since reaction kinetics in solution18 and its properties19 are well understood. We have determined20 that aqueous M(III) salts are strong microwave absorbers (dielectric losses above 35), whereas the ligand, H2BDC (terephthalic acid), exhibits little interaction with the electric field at microwave frequencies (dielectric losses below 0.03) indicative of a selective heating process. We have now constructed a single mode standing wave microwave applicator capable of housing a high-pressure vessel (Fig. 1). This allows superimposition of the electric field maximum on the reaction mixture within the batch reactor, thus ensuring the maximum amount of microwave energy is being absorbed by the reaction mixture during treatment. The position of electric field is particularly important as the power density (process by which heat is generated from electromagnetic energy) is proportional to the electric field squared:
Pd = 2πfε0ε′′E2 |
Fig. 1 Schematic of purpose-built single mode microwave system. Labels are assigned in Table S2 in ESI.† C-1: PC controller; E-1: Teflon cup for microwavable acid digestion vessel (Parr Instruments); E-2: outer section of microwavable acid digestion vessel (Parr Instruments); MW-1: Microwave generator; MW-2: Automatic 3-stub tuner; MW-3: Microwave applicator/cavity section; MW-4: sliding short; S-1: Lexan shield (blast-safe); T-1: Optris CT IR sensor. |
The electric field was pre-matched to the reaction mixture using an automatic 3-stub tuner (S-TEAM).23 The 3-stub tuner was also used to measure the forward and reflected power from which the average absorbed power (average of the forward minus the reflected power) and total absorbed energy were calculated. A microwave frequency of 2.45 GHz was used, consistent with penetration depth measurements. After microwave irradiation, the reaction mixture was allowed to cool and the resulting white powder recovered by filtration, washed with deionised water (ca. 10 mL × 3) and dried in air. A minimum of three separate reactions were carried out at each power and energy. The yields of MIL-53(Al)ta were determined in triplicate using thermogravimetric analysis. Data are given in Table S3† and Fig. 2.
Regression analysis was used to develop a predictive model for the yield of MIL-53(Al). Analysis of variance (ANOVA) showed that the model is statistically significant, with p < 0.00001. Multiple linear regression (MLR) was used to determine the coefficients and significance of absorbed energy and treatment time for all experiments using 41 variables (degrees of freedom) in the final calculation. Absorbed energy was shown to be statistically significant with p < 0.05, as was time with p < 0.00001. The obtained MLR coefficients were used to model the yield at 18.7 kJ (the average energy input across all regressed experiments, i.e. 50.2 kJ mol−1 based on water), at treatment times between 75 and 4.5 s, equivalent to average absorbed powers between 250 W and 4.0 kW, respectively. Results of the modelled yield are plotted as a function of average absorbed power in Fig. 2.
Fig. 2 shows the effect of the average absorbed power upon the average yield of MIL-53(Al)ta at a total absorbed energy of 51 kJ mol−1. It should be noted that although the target energy was 56 kJ mol−1, owing to rapid changes in the dielectric properties of the reaction mixture during microwave irradiation, the match parameters varied significantly and so the average energy absorbed by the reaction mixture experimentally was approximately 51 kJ mol−1 based on water. A rapid increase in yield of MIL-53(Al)ta from 15.9 ± 7.0 to 36.8 ± 4.9% between average absorbed powers of 269 ± 2 and 682 ± 4 W is observed. This trend of increasing yield with increasing power at constant energy shows that a selective heating mechanism of the aqueous metal ions in solution is taking place during the microwave treatment.20 In particular, treatments involving higher powers and shorter reaction times result in highly localised heating whereby the rate of heating is greater than the loss of heat through dissipation,24 thus increasing the overall rate of reaction. At powers above 682 ± 4 W the yield of MIL-53(Al)ta does not increase above ca. 30%. We ascribe the levelling off in yield at 30% to the stoichiometry of the reagents and hence diminishing availability of Al(III) during the reaction. Recently Taddei et al. reported no conversion of reactants using the same ratio as that used in our study, i.e. 1:1 of Al2(SO4)3:H2BDC.25
Using the particle size distribution analysis method of Tsai and Langner,26 our present work shows that both particle size range and morphology of MIL-53(Al)ta can be controlled by altering the microwave power (Fig. 3). As the average absorbed power increases the range of particle sizes observed by scanning electron microscopy (SEM) analysis becomes much narrower. For example, a median particle size of 2 μm2 with a range between 0.9 and 36 μm2 is exhibited by MIL-53(Al)ta particles synthesised at 269 ± 2 W. However, particle sizes recovered from reaction at a power of 1739 ± 44 W have a median size of 5 μm2 with a range between 1 and 12 μm2, as identified by SEM analysis. Interestingly, a change in morphology of particles with power is also observed in the SEM images. At an average absorbed power of 269 ± 2 W the recovered MIL-53(Al)ta is composed of a 50:50 mixture of irregular-shapes and well-defined cubes (Fig. S8 and S9†). As the microwave power increases the ratio of morphologies shifts in favour of formation of well-defined cubes. For example, at an average absorbed power of 1739 ± 44 W the ratio of irregular-shapes to well-defined cubes is 10:90 (Fig. S8–S11†). Particle formation may be a consequence of two different growth mechanisms which are affected by several factors including, but not limited to, reaction kinetics, dissolution of H2BDC and temperature.27
One of the future goals of this work is to develop a procedure for producing industrially meaningful quantities of MOF, and therefore we have compared materials produced using our current method with the current industrial standard for MIL-53(Al) (Basolite A100). SEM image analysis of commercially available MIL-53(Al) material (Basolite A100) shows that it is most similar to MIL-53(Al)ta prepared in water at 269 ± 2 W (Fig. 3, S8 and S12†), and is composed of large aggregates of irregular shaped particles with a median particle size of 1.5 μm2. The range of particle sizes is very broad, between 0.3 and 52 μm2. The irregular shape and lower median particle size exhibited by Basolite A100 leads to broadening of the peaks in the PXRD pattern (Fig. S3 and S4†). Using the method of Vivani et al. the reciprocals of the full width half maximum (1/FWHM) for a selected peak in the PXRD pattern were compared15 (Table 1). FWHM were determined using a split pseudo-Voigt peak fitting function (Fig. S4†). The relative crystallinity 1/FWHM remained consistently between 4 and 6 for materials synthesised using microwave heating, whereas Basolite A100 exhibited low crystallinity (1/FWHM = 2). These data show that microwave heating offers a clear advantage over conventional heating processes for preparation of materials.
Average absorbed power (W) | Relative crystallinity (1/FWHM) |
---|---|
Basolite A100 | 1.79 |
269 ± 2 | 5.20 |
682 ± 4 | 4.31 |
945 ± 11 | 4.76 |
1739 ± 44 | 5.17 |
3819 ± 92 | 4.46 |
Average absorbed power (W) | Langmuir surface areaa,b (m2 g−1) | BET surface areaa,c (m2 g−1) | Micropore volumef,d (V0.1) | Total pore volumef,e (VTot) | V 0.1/VTot |
---|---|---|---|---|---|
a Values given to zero decimal places. b P/P0 range 0.001 to 0.01. c In order to achieve positive values for C for the BET surface area, the Rouquerol plot was used in the P/P0 range 0.001 to 0.01. d Pore volume at P/P0 = 0.1 derived from the Dubinin–Astakhov equation. e Total pore volume at P/P0 = 0.99. f Values are given to two decimal places. g Basolite A100 obtained from Sigma Aldrich. Values in italics and bold are within the reported range of MIL-53(Al)op materials prepared via conventional heating in continuous flow.22 | |||||
Commercial material | 864 | 834 | 0.35 | 0.88 | 0.40 |
269 ± 2 | 685 | 658 | 0.24 | 0.27 | 0.89 |
682 ± 4 | 1020 | 970 | 0.35 | 0.44 | 0.79 |
945 ± 11 | 458 | 436 | 0.16 | 0.19 | 0.84 |
1739 ± 44 | 513 | 504 | 0.18 | 0.22 | 0.81 |
3819 ± 92 | 285 | 280 | 0.10 | 0.12 | 0.83 |
Data in Table 2 show no obvious functional form to the correlation between microwave power and surface area. Materials prepared at powers of 269, 682 and 1739 W all exhibit Brunauer–Emmett–Teller surface areas (SABET) within the range of MIL-53(Al)op materials prepared via conventional heating in continuous flow (459–919 m2 g−1).22 The highest SABET of 970 m2 g−1 was exhibited by MIL-53(Al)op synthesised at 682 ± 4 W. This SABET is higher than both the commercial Basolite A100 material (834 m2 g−1) and MIL-53(Al)op synthesised by continuous flow (919 m2 g−1).22 MIL-53(Al)op produced at 3819 ± 92 W gave the lowest SABET for our materials at 280 m2 g−1. We ascribe this reduction in SABET at high power to material degradation as a result of elevated bulk reaction temperatures as the amount of Al was higher than expected in the thermogravimetric data (Table S3†) and no metal oxide was observed in the PXRD patterns. In addition, the reduction in SABET is consistent with a previous report in which microwave synthesis of MOF-5 at powers above 1000 W led to deterioration of crystal quality and reduction in porosity.16
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta01493g |
This journal is © The Royal Society of Chemistry 2017 |