Identification of the strong Brønsted acid site in a metal–organic framework solid acid catalyst

Abstract

It remains difficult to understand the surface of solid acid catalysts at the molecular level, despite their importance for industrial catalytic applications. A sulfated zirconium-based metal-organic framework, MOF-808-SO4, has previously been shown to be a strong solid Brønsted acid material. In this report, we probe the origin of its acidity through an array of spectroscopic, crystallographic, and computational characterization techniques. The strongest Brønsted acid site is shown to consist of a specific arrangement of adsorbed water and sulfate moieties on the zirconium clusters. When a water molecule adsorbs to one zirconium atom, it participates in a hydrogen bond with a sulfate moiety that is chelated to a neighboring zirconium atom; this motif in turn results in the presence of a strongly acidic proton. On dehydration, the material loses its acidity. The hydrated sulfated MOF exhibits good catalytic performance for the dimerization of isobutene (2-methyl-1-propene), achieving 100% selectivity for C8 products with good conversion efficiency. The chemistry at the surface of solid acid catalysts is of vital importance for industrial catalytic applications, yet a precise molecular picture of these surfaces remains elusive. Attempts to obtain a clear view of the Brønsted acid sites in solid acids such as sulfated zirconia have resulted in multiple proposed models, in part due to the difficulty in characterizing the structure of this

Reversible Structural Transformations of Metal–Organic Frameworks as Artificial Switchable Catalysts for Dynamic Control of Selectively Cyanation Reaction

“…[13][14][15][16][17][18] MOFs, with site isolation and adjacent active catalytic sites, which have been obtained throughd irect use or postsynthetic functionalization, as single-site and cooperative catalysts for chemical transformations have been investigated. [19][20][21][22][23][24][25][26][27] Ta king advantage of their frequently occurring reversible structural transformations, through single-crystal to single-crystal processes,t he uniquef lipping motion between structurala nd topologyc onnectionsa re similart oe ach otherm akes MOFs a highly appropriate choice for building molecular devices,s uch as switching or sensing materials. [28][29][30][31][32] Meanwhile,s olvent-induced dissolution-exchange-crystallization behavior is another important,but difficult, approach to regulating reversible structural transformations because the connection modesb etween metal ions and ligandsa re very difficult to re-edit.…”

Developing two-dimensional solid superacids with enhanced mass transport, extremely high acid strength and superior catalytic performance

“…The acidity of the synthesized 2D solid acids was investigated by using 1 H and 31 P MAS NMR, a reliable and state-of-the-art technique for determination of the acidity of solid acid catalysts. 6,9,24,38,39 In particular, 31 P trialkylphosphine (TMP) NMR has been used to distinguish Brønsted and/or Lewis acid types. When TMP molecules are bound to Lewis acid sites, they give rise to 31 P resonance ranging from ca.…”

Efficient Production of Ethyl Levulinate from Furfuryl Alcohol Catalyzed by Modified Zirconium Phosphate

“…The schematic representation was shown in Figure . The Brønsted acid site not only came from P‐OH group, but also arose from the hydrogen‐bonding interaction between water and sulfate ,, , . Trickett et al.…”

“…concluded that the Brønsted acid site was correlated with the number of binding water in the catalyst. The Brønsted acid site in the sulfated catalyst originated from a weakly bound proton between two bases (a sulfate and a hydroxyl from water molecule) . In this work, the O−H bond from water molecule formed hydrogen bond with adjacent sulfate, resulting in the increase of Brønsted acid sites.…”