Anal. Chem. 2007, 79, 1290-1293
Metal-Organic Frameworks as Adsorbents for Trapping and Preconcentration of Organic Phosphonates Zheng Ni,† John P. Jerrell,‡ Keith R. Cadwallader,‡ and Richard I. Masel*,†
Department of Chemical and Biomolecular Engineering, University of Illinois at UrbanasChampaign, 600 South Mathews Avenue, Urbana, Illinois 61801, and Department of Food Science and Human Nutrition, University of Illinois at UrbanasChampaign, 1302 West Pennsylvania Avenue, Urbana, Illinois 61801
Metal-organic frameworks (MOFs) have high surface areas and tailorable molecular properties so they have the potential of being selective adsorbents for preconcentrators. In this paper, IRMOF1 is tested as an adsorbent for preconcentration for the first time using dimethyl methylphosphonate (DMMP) as a test case. We find that DMMP is selectively adsorbed on IRMOF1 and is released upon heating to 250 °C. Concentration gains of more than 5000 are observed for DMMP with a 4-s sampling time. Sorption capacities are 0.95 g of DMMP/g of IRMOF1. By comparison, dodecane shows a preconcentration gain of ∼5 under similar conditions. These results demonstrate that MOFs can be quite useful in selective preconcentrators. The objective of this paper is to evaluate the potential of metalorganic frameworks (MOFs) as novel adsorbents for preconcentration of organic phosphonates. MOFs are a new class of molecular networks, where metal centers and bridging organic ligands are arranged in a three-dimensional skeleton as indicated in Figure 1. They have surface areas ranging from 1000 to 5400 m2/g, tailorable polarity and pore size, and high thermal stability.1-3 Applications discussed in the literature include catalysts for alkynes conversion,4 gases separation,4 and the storage of hydrogen,5 methane,6 and ammonia. We have been interested in these frameworks because they can be tailored to selectively adsorb molecules of interest. Thus, they could be quite useful in the preconcentrators used for the detection of trace impurities; however, at this point, there are no published reports of the use of MOFs in preconcentrators. * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemical and Biomolecular Engineering. ‡ Department of Food Science and Human Nutrition. (1) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705-714. (2) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319-330. (3) Rowsell, J. L. C.; Yaghi, O. M. Microporous Mesoporous Mater. 2004, 73, 3-14. (4) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre, J. J. Mater. Chem. 2006, 16, 626-636. (5) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127-1129. (6) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469-472.
1290 Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
Figure 1. Structure of a typical MOF. The spheres are metal clusters; the rods are organic linkages. In IRMOF1, the spheres are OZn4 clusters, while the linkages are BDC ion (-OOC-Ph-COO-).
In this paper, we explored the MOF’s performance as an adsorbent for dimethylmethyl phosphonate (DMMP) preconcentration. DMMP is a molecule that is commonly used as a simulant of nerve agents. It has polarity and volatility similar to that of sarin, but is much safer to use. We are using a MOF developed by Yaghi et al., IRMOF1.7 IRMOF1 has a high porosity at 2900 m2/g and linking groups that have a polarity similar to those of the phosphonates. Thus, it is a reasonable candidate as an adsorbent for phosphonates. We find that IRMOF1 is an excellent selective adsorbent for phosphonates. We observe preconcentration gains over 5000 for DMMP with sampling times of 4 s. By comparison, if we instead use Tenax TA in the trap, the gain is only 2. Furthermore, IRMOF1 is selective: dodecane shows a gain of only ∼5. Thus, it appears that MOFs are excellent candidates as adsorbents for preconcentrators. EXPERIMENTAL SECTION Materials. IRMOF1 crystals were synthesized according to the reported procedures.6 A 0.200-g aliquot of Zn(NO3)2‚6H2O (0.672 mmol) and 0.084 g of 1,4-benzenedicarboxylate acid ligand (0.50 mmol) were dissolved in 20 mL of N,N′-diethylformamide. The solution was stirred for 10 min and then sealed in a closed vessel. The vessel was heated at 90 °C for 20 h. Small light yellow crystals were collected after the solvothermal treatment (yield in (7) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276279. 10.1021/ac0613075 CCC: $37.00
© 2007 American Chemical Society Published on Web 01/13/2007
Figure 2. Comparison of the GC chromatograms and MS spectra of (A) DMMP directly injected into a GC/MS (dotted line) and (B) DMMP trapped and thermally desorbed (solid line). Notice that there are no satellite peaks.
40%). XRPD patterns were collected on Bruker General Area Detector Diffraction System and showed that we produced singlephase material. Solvated crystals used for XRPD measurements were transferred along with their mother liquor into a 0.7-mm capillary tube. The collected XRPD pattern compared well with a simulated curve based on the previously reported IRMOF1 structure.6 The XRPD spectra are given in our previous paper.9 We generally stored the crystals in air, rather than in dry conditions, to simulate their behavior in a real preconcentrator. This reduces their surface area. After a few days of storage, our crystals had a surface area of 630 m2/g. This compares to a previous report of 2900 m2/g of a fresh sample.6 RESULTS AND DISCUSSION We performed two sets of measurements, one to see if DMMP is effectively adsorbed and desorbed from IRMOF1, and a second to compare the gain of DMMP and dodecane in a model preconcentrator. We also did comparison tests with 60/80 mesh Tenax TA and 20/40 mesh Carbotrap. Figure 2 shows the result of an experiment that was done to see whether DMMP adsorbed on IRMOF1 and whether it desorbed without decomposition. IRMOF1 crystals were first treated under vacuum at 150 °C for 1 day. Then 5 mg of the IRMOF1 was packed into a 3-mm glass GC liner with glass wool used to block both ends. The capillary was then installed in a programmable temperature vaporizer injector (CIS4, Gerstel, Germany) in an Agilent 5973N GC/MS system. A 10-mL sample of DMMP vapor and air mixture was injected through the sample inlet at 50 °C. The sample is kept at 50 °C for 10 min and then heated to 250 °C at 10 °C/s to release the vapor into the GC analytical column (Rtx5ms, 30 m × 0.25 mm i.d. × 0.5 µm film, Restek). A parallel measurement was also carried out with an empty glass GC liner capped by glass wools on both ends. Desorption peaks are compared in Figure 2. A sharp DMMP peak comes out at ∼17.50 min for both capillaries. No “breakdown” peaks were observed during the entire measurement. This result shows that DMMP vapor can be effectively captured by IRMOF1 at 50 °C, and there is no reaction during the adsorption/desorption process. Next we measured the sorption capacity of IRMOF1 by saturating IRMOF1 with DMMP and toluene vapors, respectively. (8) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; John Wiley & Sons, Inc.: New York, 1996; p 513.
Figure 3. TGA spectrum taken by saturating a sample of IRMOF1 with DMMP or toluene and then heating at 10 °C/min to desorb the DMMP or toluene. Notice that a tremendous amount of DMMP desorbssalmost 1 g of DMMP/g of adsorbent.
We then loaded the samples into a thermogravametric analyzer and measured the weight loss during thermal desorption. In detail, IRMOF1 crystals were ground to 325 mesh. The solvates were completely removed by heating the solid at 150 °C under vacuum. A 5-mg sample of IRMOF1 was then packed into a 3-mm glass GC liner as described earlier. Organic solvent vapor was carried by 15 sccm He gas through a saturator and then passed through the sample capillary at 50 °C. After 10 µL of solvent loss was observed from saturator, the samples were removed from system. Thermal gravimetric analyses were then carried out on a PerkinElmer TGA7 instrument heating from 25 to 300 °C at 10 °C/min. Figure 3 shows the results. Notice that 1 g of IRMOF1 can adsorb 0.95 g of DMMP while it only adsorbs ∼0.1 g of toluene. The desorption of toluene does not show any clear features, suggesting that the toluene is only weakly bound. In contrast, the TGA spectrum of DMMP shows an inflection point at ∼140 C. Differentiation of the data and using an analysis as outlined in Masel8 indicates that there is a single binding site for DMMP in IRMOF1 with a binding energy of ∼19 kcal/mol. We have also made identical measurements using 60/80 mesh Tenax TA and 20/40 mesh Carbotrap as adsorbants. We find that much less DMMP adsorbs in Tenax and Carbotrap than in IRMOF1. At saturation, only 0.013 g of DMMP adsorbs on Tenax TA per gram of adsorbent while only 0.02 g of DMMP adsorbs per gram of Carbotrap. This compares to 0.95 g for IRMOF1. Clearly, IRMOF1 has much higher adsorption capacity for DMMP than Tenax or Carbotrap. (9) Ni, Z.; Masel, R. I. J. Am. Chem. Soc. 2006, 128, 12394-12395.
Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
1291
Figure 6. Comparison of the GC chromatograms produced by exposing the IRMOF1 in a groove in a Valco sample valve to a gas stream containing 642 ppb DMMP followed by thermal desorption to that from an empty groove containing 651 ppm DMMP. Figure 4. Schematic of the experimental procedure used to measure the gain of the preconcentrator. (a) First, a helium gas containing ppb levels of DMMP is fed into a groove in a Valco gas sampling valve containing MOF crystals. (b) Second, gas is fed into an empty tube. (c) Then both grooves are rotated against blank openings and the valve is heated to desorb the MOF. (d) Finally, the DMMP adsorbed on the MOF is injected into the column.
Figure 7. Comparison of the GC chromatograms produced by exposing the IRMOF1 in a groove in a Valco sample valve to a gas stream containing 2030 ppb dodecane followed by thermal desorption to that from an empty groove containing 2030 ppb dodecane.
Figure 5. Comparison of the GC chromatograms produced by exposing the IRMOF1 in a groove in a Valco sample valve to a gas stream containing 105 ppb DMMP followed by thermal desorption to that from an empty groove containing 105 ppb DMMP.
Next, a real-time preconcentration measurement was carried out using a novel purge-and-trap system. A few cubic-shaped crystals (size ∼100 µm) or fine MOF powders were mounted in one of the four 0.06-µL grooves on a standard Valcon E rotor from Valco Instruments. The rotor was then installed inside the injection valve and driven by a multiposition electronic actuator. Figure 4 shows the schematic configuration used for the performance test. The organic vapor was introduced through a bubbler in an Isotemp water bath and diluted to the appropriate concentration. This sample gas was then connected to the sampling inlet port of the valve. A flame ionization detector (FID) was connected to the injection outlet port for data collection. Helium gas was applied as both the carrier gas and injection gas at pressures of 20 and 10 psi, respectively. In a typical measurement, the MOF adsorbent trap was treated by heating at 250 °C while purging with pure helium gas (Figure 4d). The adsorbents were confirmed to be cleaned when the signal 1292 Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
read from detector dropped to the baseline. The trap was then actuated to a sampling position as shown in Figure 4a. A gas sample containing DMMP vapors at a few hundred ppb level flowed through the trap at 30 °C for 4-60 s. Next, the trap was actuated to a sealed position; meanwhile, an empty groove underwent the same sampling procedure as that of the MOF trap (Figure 4b). Then both trap and reference were sealed in the valve system and heated to 250 °C for thermal desorption (Figure 4c), followed by trap injection (Figure 4d) and reference injection (Figure 4a), respectively. Figure 5 shows the results of an experiment where 30 sccm of 107 ppb DMMP was allowed to flow into the trap for 1 min, the trap was then sealed and heated to 250 °C, and then DMMP was injected into an Aligent 5973N GC/MS. A second pulse created by following the same procedure with an empty slot is also shown. Notice, there is obviously a large preconcentration gain although quantification is not possible, since we are outside of the linear response region of the detector. In order to quantify the results, we compared the GC spectrum of 642 ppb DMMP with a trap and DMMP vapor at 651 ppm with no trap. A sampling time of 4 s was used for both experiments, and the sample gas flow rate was 150 sccm in each case. The MOF was ground to 325 mesh so the desorbed peak shows much
less tailing effect than that in Figure 5. Figure 6 compares the peaks seen in both samples. Notice that the GC peak measured by adsorbing 642 ppb DMMP in IRMOF1 for 4 s and then desorbing is ∼5 times larger than from the empty groove measured with 651 ppm DMMP. Therefore, we conclude that preconcentration gains of over 5000 are possible using only 5 µg of IRMOF1 and a 4-s sample time. Figure 7 shows the results of a similar experiment with dodecane vapor. In this case, we flowed 100 sccm of a gas stream containing 2028 ppb dodecane through the trap for 1 min and repeated the experiments above. Notice that the dodecane peak is only enhanced by a factor of 5 when the preconcentrator is used compared to a gain of over 5000 for DMMP. Thus, it is clear that that while IRMOF1 adsorbs DMMP very rapidly, it only slowly adsorbs dodecane, presumably because DMMP can have strong dipole-dipole interactions with the framework while the dodecane only interacts via van der Waals forces. We have also made identical measurements using Tenax TA rather than IRMOF1 in the trap. In this case, we only observe a concentration gain for DMMP of a factor of 2 compared to 5000 with IRMOF1. Clearly, the IRMOF1 is much more effective in a DMMP preconcentrator than Tenax TA. CONCLUSION In summary, a metal organic framework can be applied as good adsorbents for preconcentrators in micro GC systems. These
adsorbents have a tremendous capacity: almost 1 g of DMMP/g of adsorbent, equivalent to ∼0.7 g of DMMP/mL of trap. Furthermore, the adsorption process is selective. We observe 5000 preconcentrator gain for DMMP and only 5 gain for toluene. Low gains are seen with TenaxTA. These results indicate that MOFs are effective adsorbents for use in preconcentors. ACKNOWLEDGMENT We gratefully acknowledge financial support from the Defense Advanced Research Projects Agency (DARPA) under U.S. Air Force grant FA8650-04-1-7121. Any opinions, findings, and conclusions or recommendations expressed in the manuscript are those of the authors and do not necessarily reflect the views of DARPA or the U.S. Air Force. NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on January 13, 2007, with IRMOF5 in two sentences instead of IRMOF1. The correct version was posted to the Web on January 18, 2007.
Received for review July 19, 2006. Accepted December 6, 2006. AC0613075
Analytical Chemistry, Vol. 79, No. 4, February 15, 2007
1293
