Identifying the Active Site in a Metal-Organic Framework Heterogeneous Acid Catalyst
Sulfated MOF-808 displays a distinct crystalline backbone, decorated with a disordered array of many molecular groups. Which group is the source of its strong Brønsted acidity?
Heterogeneous acid catalyst materials are behind many of the world’s most important industrial-scale chemical transformations. One example is sulfated zirconia, known for its strong Brønsted acidity that makes it capable of efficient isomerization and dimerization of light olefins. The surface environment of a material such as sulfated zirconia is complex, with many different ways to arrange the many sulfates, water, and hydroxides that terminate the surface. The material clearly demonstrates strong acidity, but it isn’t exactly obvious where that acidity is coming from at the molecular level. Many molecular models for the strong acid site have been proposed over the years, with lots of compelling evidence, but despite this, it is still not definitively known what molecular structure is responsible for the material’s acidity. In a catalyst like an enzyme, there is a distinct active site where the critical step of catalysis takes place, and through the use of techniques such as X-ray crystallography and NMR, many enzyme active sites are known down to the atom-level details. For sulfated zirconia and many other heterogeneous catalysts, we just don’t have that same level of information.
In our recent work, we have done for a heterogeneous acid catalytic metal-organic framework (MOF) what has been so difficult to do for other catalysts like sulfated zirconia: we have definitively identified the strong acid site. The material we studied, MOF-808-SO4, may be thought of as a crystalline “MOF-analogue” of sulfated zirconia. Its parent material, MOF-808, has zirconium clusters that are connected to other clusters via organic linking units, but critically, there are 6 sites on these clusters which are surface sites. In MOF-808, these sites are occupied by formate molecules, but can be exchanged away and replaced by 2-3 sulfate groups to make MOF-808-SO4.
MOF-808-SO4 is a crystalline material, and single crystal X-ray diffraction reveals very clearly the MOF-808 backbone structure. However, the decoration of these clusters at their surface sites with sulfates and other moieties varies from cluster to cluster. This is a similar scenario to the surface of sulfated zirconia, making it a challenge to directly observe the strong acid site. But because the number and type of moieties on these clusters is limited, the possible sources of acidity are constrained, making it our task to sequentially narrow down the possibilities until we are left with what must be the acid site.
We discovered that adsorbed water was central to the material’s strong acidity. With this knowledge, we set about to discover the feasibility of several configurations of sulfates, hydroxides, and water molecules on the clusters using density functional theory, paying close attention to the water molecules. One configuration stuck out: when water was adsorbed adjacent to a sulfate, it would participate in a hydrogen bond that significantly lengthened the O-H bond length. This breaks the symmetry of the proton sites in the water molecule, and accordingly we were able to observe these two distinct protons on a single water molecule using 1H double-quantum solid state NMR. We tested MOF-808-SO4 in catalyzing the dimerization of isobutene to isooctene, where it exhibits good conversion efficiency and excellent selectivity. Significantly, the material shows severely reduced activity after heating to dehydrate the material, confirming the important role of water in the active site.
Our research has now been published in Nature Chemistry https://www.nature.com/articles/s41557-018-0171-z.