Communication pubs.acs.org/IC
Amino- and Sulfo-Bifunctionalized Metal−Organic Frameworks: OnePot Tandem Catalysis and the Catalytic Sites Hui Liu, Fu-Gui Xi, Wei Sun, Ning-Ning Yang, and En-Qing Gao* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, College of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, P. R. China S Supporting Information *
the possibility of proton transfer. Here we report on new amino− sulfo-bifunctionalized MIL-101 MOFs fabricated by a convenient PSM approach. The MOFs show tunable acid−base catalytic properties. Remarkably, we demonstrated that the MOFs take the zwitterionic form, with the catalytic acid site being the ammonium group rather than the sulfo one. The bifunctional MOFs were prepared by tandem PSM (Figure 1). MIL-101-NO2 was synthesized and reduced to MIL-
ABSTRACT: New MIL-101 metal−organic frameworks (MOFs) dually functionalized with amino and sulfo groups were fabricated by postsynthetic modification and used to catalyze one-pot deacetalization−Knoevenagel condensation. We proved that the MOFs take the zwitterionic form, with the catalytic acid site being the ammonium group rather than the sulfo one. The acid and base concentrations in the materials are correlated, and the ratio can be readily tuned to achieve optimal catalytic performance.
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ne-pot tandem reactions have attracted intensive attention as an approach of chemical process intensification.1 Multifunctional heterogeneous catalysts are highly appealing for tandem reactions that require different catalytic sites.1c However, the fabrication of such catalysts is not as straightforward as it looks, especially when the different sites tend to quench each other, such as Brønsted acid and base. Nevertheless, different solid materials including mesoporous silicas2 mesoporous organosilicas, 3 porous organic frameworks,4 and montmorillonite5 have been functionalized to contain coexistent acidic sulfo and basic amino sites accessible for tandem catalysis. Metal−organic frameworks (MOFs), which feature ordered hybrid frameworks, high surface area, tunable pore size, and tailorable functionalities, are currently actively studied for catalysis.6 Catalysis with MOFs can be achieved through inherent metal centers or prefunctionalized organic ligands. Furthermore, catalytic sites can be introduced by postsynthetic modification (PSM) at the metal or organic component or within the pore.7 Thus, MOFs are promising platforms for the design of sophisticated nanoreactors furnished with multiple catalytic sites for tandem reactions.8,9 For potential Brønsted acid−base catalysts, a few MOFs dually functionalized with amino and sulfo groups have been reported, 10 including MOF-5-NH(CH2)3SO3H,10a UiO-66-NH(CH2)3SO3H,10b and two MIL101-NH2SO3H species synthesized by different methods.10c,d Only the MIL-101 derivatives have been tested as acid−base bifunctional catalysts for tandem reactions. Despite increasing interest in the design of acid−base catalysts by incorporating amino and sulfo groups onto various solids, the nature of the catalytic sites has not been examined seriously. Sulfonic acids are generally stronger than organic ammonium in acidity, as indicated by pKa (Table S1), so it is expected that amino groups can easily take protons from sulfo groups. However, the acid and base activities of the catalysts have been attributed to SO3H and NH2, respectively, without mentioning © XXXX American Chemical Society
Figure 1. Synthesis of the bifunctional MOFs and 1H NMR spectra of various MOFs after digestion with NaOD/D2O.
101-NH2 according to literature procedures.11 The latter MOF was then modified through the ring-opening reaction of 1,3propanesultone with an amino group. The new MOFs show new IR bands attributable to the sulfo group (1043 and 1185 cm−1; Figure S1a). 1H NMR spectra show two sets of signals assignable to unmodified and sulfo-modified linkers (Figure 1). The materials are denoted as MIL-101-AB-x, where A = acid, B = base, and x is the molar fraction of the sulfo-modified linker calculated from NMR integrals. Several materials with x = 0.26− 0.52 were prepared by controlling the reaction time and the dosage of 1,3-propanesultone (Table S2). According to nitrogen adsorption studies (Figure S2), MIL-101-AB-0.32 and MIL-101NH2 exhibit Brunauer−Emmett−Teller specific surface areas of 1732 and 2277 m2 g−1 and total pore volumes of 1.02 and 1.33 cm3 g−1, respectively. The reduction in these parameters after PSM is due to the introduction of a sulfopropyl group. Powder XReceived: April 28, 2016
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DOI: 10.1021/acs.inorgchem.6b01057 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
envisioned. (i) The graft of sulfopropyl groups reduces the pore size and makes the amino site less accessible. At high modification ratios, the steric effects may become significant and disfavor the bimolecular Knoevenagel reaction. (ii) Because the pKa value of RSO3H (EtSO3H, −1.61) is much smaller than those of PhNH3+ (4.63) and PhNH2R+ (PhNH2Et+, 5.12; Table S1), the amino group (especially the secondary one) can easily take a proton from the sulfo one, affording the zwitterionic MOF with negative sulfonate and positive ammonium groups (Figure 3). The zwitterionic form means that the content of basic sites
ray diffraction (PXRD) measurements confirmed that the MIL101 framework is maintained after modification (Figure S1b). The modified MOF shows thermogravimetric behavior similar to that of the mother material (Figure S3). The tandem deacetalization−Knoevenagel reaction (Figure 2) was studied to check the acid−base catalytic performance of
Figure 3. Formation of the zwitterionic MOF. Figure 2. Tandem deacetalization−Knoevenagel reaction (top). Plots of the conversion of 1 (a) and the yield of 3 (b) at different times versus the modification ratio (x).
(free amino) in the catalysts is reduced by, rather than independent of, modification. A high modification ratio can reduce the base concentration to the extent that the Knoevenagel reaction is not efficiently catalyzed. Meanwhile, the zwitterionic form means that the predominant acid sites should be an ammonium group rather than a sulfo group. Solid-supported ammonium sites catalyzing deacetalization have been demonstrated elsewhere.12 Obviously, the contents of the acid and base sites in the catalysts are correlated to each other. For a given tandem reaction, there is an optimal acid-to-base ratio, at which the two steps compromise to achieve the best performance. Control tests were designed to clarify the possibilities. Knoevenagel condensation of 2 with malononitrile was performed over MIL-101-AB-0.52 (Figure S6). Under the same conditions as those used for tandem reactions, the yield of 3 was only 18% after 30 min, so the highly modified MOF is not a good base catalyst. In another test, MIL-101-AB-0.52 was treated with excessive NaOH, thoroughly washed to remove residual NaOH, and then used for the Knoevenagel reaction. The yield of 3 reached 98% within 30 min, so the material became an excellent base catalyst after alkali treatment. PXRD confirmed that the framework was retained after treatment. Because treatment neither destroys the sulfopropyl group nor significantly changes the pore size, the significant difference in the catalytic activity before and after treatment precludes the possibility of steric effects. Instead, the results support the zwitterionic proposal: alkali treatment converts acidic ammonium to basic amino and thus enhances the basic activity. X-ray photoelectron spectroscopy (XPS) studies provided unequivocal evidence for the zwitterionic proposal (Figure 4). MIL-101-AB-0.32 shows two N 1s bands at 399.2 and 401.4 eV, attributable to amino and ammonium groups, respectively.13 After alkali treatment, the ammonium band disappears. In
MIL-101-AB-x. First, catalysts with different modification ratios (x) were compared under identical conditions. According to Figure 2, conversion of 1 at given times increases with x. For instance, the conversion after 5 min increases from 45% for x = 0.26 to 87% for x = 0.52. This is as expected and simply reflects the fact that the acid concentration increases with x, in favor of deacetalization. At first thought, the total base concentration, including primary (unmodified) and secondary (modified) amino sites, should remain constant, independent of the modification. One can expect that the yield of 3 after a given time should also increase with x because 2 is generated faster when x is higher. This trend was indeed observed at low modification ratios. For instance, the yield of 3 after 2 h increases from 91 to 99% with increasing x from 0.26 to 0.32. However, further increasing x above 0.32 led to a significant decrease in the yield, with a larger amount of 2 not reacted (Figure 2 and Table S3). Therefore, a high modification ratio is conducive to deacetalization but has an adverse effect on Knoevenagel condensation. The best performance at the moderate modification ratio of x = 0.32 is the result of a compromise between the two steps. To confirm the bifunctional nature of the catalyst, the reaction was performed over MIL-101-AB-0.32 in the presence of ptoluenesulfonic acid or ethylenediamine, which serves to quench the base or acid site in the catalyst. No evident conversion of 1 was detected in the presence of the base. When the acid was added, the first step completed very quickly (in 30 min), but the second step became much slower (the yield of 3 was only 41% after 2 h). The results confirm the roles of the different sites: the acid sites catalyze deacetalization, and the base sites catalyze Knoevenagel condensation. To confirm the heterogeneity, the solid was filtered off after a reaction over MIL-101-AB-0.32 proceeded for 5 min. The reaction in the filtrate became much slower, similar to the reaction with no catalyst (Figure S4), so the active species is in the solid. Besides, the catalyst has been reused for three cycles without significant reduction in the catalytic activity and structural integrity (Figure S5). The unexpected diverse effect of the modification above x = 0.32 is worth further investigation. Two possible origins can be
Figure 4. XPS core-level spectra of MIL-101-AB-0.32. B
DOI: 10.1021/acs.inorgchem.6b01057 Inorg. Chem. XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21173083 and 21471057).
contrast, the S 2p and O 1s bands remain essentially unchanged. The analysis clearly supports that the ammonium (rather than the sulfo) group is present as an acid site in the untreated material. On the basis of the proposal, the catalytic performance of the highly modified MOFs for the deacetalization−Knoevenagel reaction can be enhanced by controlled alkali treatment, which converts not all but a portion of the ammonium groups to amino. Indeed, treating MIL-101-AB-0.52 with an inadequate amount (relative to the acid in the MOF) of NaOH lead to a much improved performance (Figure S7). As mentioned above, previous studies with amino−sulfo solid catalysts have attributed the acid activity to SO3H, without mentioning the possibility of proton transfer. Considering the pKa values (Table S1), proton transfer is very likely, and the materials should be in the self-neutralized “zwitterionic” form, or in equilibrium in favor of the zwitterionic form, with ammonium as the predominant acid site. Moreover, the preparation of these catalysts often involved the use of highly polar solvents (water and/or alcohols), which are good proton mediators. Therefore, proton transfer should readily occur even in cases where the two groups are “site-isolated” (separately grafted within the pore). As we have revealed for MIL-101-AB-x, the sulfo-to-amino ratio in the zwitterionic materials should be a key factor determining the tandem catalytic properties. Unfortunately, the effect of the ratio was not reported for previous materials. We noticed that the materials usually contain excessive amino groups relative to the sulfo groups. It can be envisioned that the excess is important in ensuring the presence of sufficient free amino sites for base catalysis. One exception is the mesoporous silica nanoparticles in which SO3H and NH2 were separately grafted on the internal and external surfaces. The material contains the two groups in the 1:1 ratio but still shows base activity.2b In this case, the special site isolation may impede proton transfer from SO3H to NH2: transfer would cause charges of the same sign to accumulate in the pore, so repulsion between like charges sets a barrier to transfer. In conclusion, new MIL-101 MOFs dually functionalized with amino and sulfo groups have been fabricated by tandem PSM. The acid-to-base ratio can be tuned to achieve optimal performance for tandem catalysis. We revealed that the MOFs take the zwitterionic form, with the ammonium (rather than sulfo) group being the catalytic acid site. This is likely applicable to other amino−sulfo-bifunctionalized solids. The work demonstrates the great versatility of MOFs as platforms for the design of new catalysts and offers a better understanding of acid− base bifunctional catalysts.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01057. Experimental details and additional graphics (PDF)
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DOI: 10.1021/acs.inorgchem.6b01057 Inorg. Chem. XXXX, XXX, XXX−XXX
