Water Can Increase Zeolite Catalyst Reactivity

Historically, water is considered to reduce activity in zeolite-based hydrocarbon catalysis. Currently, there exists widespread interest in understanding synergistic impacts of water in catalytic transformation of non-traditional hydrocarbon feedstocks by zeolites, e.g. biomass feedstocks, since significant amounts of water are liberated in initial reaction stages. Questions regarding water's active or passive role in solid acid catalysis have prompted our work on studying fundamentals of water-zeolite interactions by in-situ magnetic resonance methods. We have developed multiple methods of introducing water into zeolite from trace amounts to access amounts. The small loading results, below 0.5 water molecules per acid site, have shown interesting onsite interaction information at molecular level, for example, suggesting the acid site proton can be deprotonated by a single water molecule. In addition, the introduction of water to hydrophobic organosilane modified zeolites shows liquid water can be blocked outside the crystallites, implying potential application of the hydrophobically modified zeolite. For water's positive impact on hydrocarbon reactions, we have experimentally shown that trace amounts of water enhance isobutane reactivity in HZSM-5 by up to an order of magnitude (ACS Catalysis 2014, 4, 3039). Subsequently, active sites were characterized in the presence of water for hydrophilic and hydrophobically-modified zeolites (ACS Catalysis 2015, 5, 7480). From that work, we determined that only vapor-phase water could access acid sites in organosilane modified catalysts, while liquid-phase water was excluded from the catalyst interior volume, leading to increased catalyst lifetimes in water-rich environments (JACS 2015, 137, 11810). Moving forward, we recognize that aromatic reaction centers are common to many important hydrocarbon conversions in zeolites. Specifically, alkylation-dealkylation steps have been shown as key steps in methonal-to-hydrocarbon (MTH) conversions. Aromatic alkylation-dealkylation reactions are investigated as test reactions to probe whether water can play an active role in lowering activation energies for the critical side-chain alkylation and dealkylation steps.Chemistr

Assessment, Control, and Impact of Brønsted Acid Site Heterogeneity in Zeolite HZSM‑5

Acidic zeolites are solid aluminosilicate catalysts whose utility arises from Brønsted sites that predominately reside in their crystalline framework structure. Data described herein indicate that extra-framework aluminum (EFAl) moieties, often proposed as important species in overall catalyst activity via Brønsted–Lewis synergies, can themselves contribute protons that are also reactive Brønsted acid centers. While the MFI family of zeolites is a relatively simple channel-structure type, the quantitative spectroscopic detection of all protons shows that the distribution of reactive Brønsted acid site protons arising from framework and extra-framework moieties can be complex. Experiments show that postsynthetic treatments can be used to modify this distribution, in theory enabling routes to HZSM-5 catalysts with only one type of reactive Brønsted site. Quantitative spin-counting NMR experiments combined with chemical washing using ammonium hexafluorosilicate (AHFS) show that the number of framework bridging acid sites (BAS) in typical commercial MFI catalysts (Si/Al equal 15 and 40) is between 50 and 60% of that expected based on the total Al content. Acidic protons from EFAl constitute the major fraction of remaining Brønsted sites. Probe-molecule reactions demonstrate that catalysts with only framework BAS are significantly less reactive than those with both extra-framework and framework Brønsted acid sites. Various postsynthetic methods are compared to optimize the desired Brønsted acid site distribution in MFI catalysts, including both removal and re-introduction of acidic protons from EFAl sites

Ultrahigh-field 67Zn NMR reveals short-range disorder in zeolitic imidazolate framework glasses

The structure of melt-quenched zeolitic imidazole framework (ZIF) glasses can provide insights into their glass-formation mechanism. We directly detected short-range disorder in ZIF glasses using ultrahigh-field zinc-67 solid-state nuclear magnetic resonance spectroscopy. Two distinct Zn sites characteristic of the parent crystals transformed upon melting into a single tetrahedral site with a broad distribution of structural parameters. Moreover, the ligand chemistry in ZIFs appeared to have no controlling effect on the short-range disorder, although the former affected their phase-transition behavior. These findings reveal structure-property relations and could help design metal-organic framework glasses

Ultrahigh-Field ⁶⁷Zn NMR reveals short-range disorder in zeolitic imidazolate framework glasses

The structure of melt-quenched zeolitic imidazole framework (ZIF) glasses can provide insights into their glass-formation mechanism. We directly detected short-range disorder in ZIF glasses using ultrahigh-field zinc-67 solid-state nuclear magnetic resonance spectroscopy. Two distinct Zn sites characteristic of the parent crystals transformed upon melting into a single tetrahedral site with a broad distribution of structural parameters. Moreover, the ligand chemistry in ZIFs appeared to have no controlling effect on the short-range disorder, although the former affected their phase-transition behavior. These findings reveal structure-property relations and could help design metal-organic framework glasses

Water Interactions in Zeolite Catalysts and Their Hydrophobically Modified Analogues

Renewed interest in zeolite catalyst performance in the presence of variable amounts of water has prompted solid-state NMR experiments designed to identify the nature of water interaction with and within conventional and chemically modified H-ZSM-5 zeolites. Recent work has demonstrated that water can positively influence reaction rates in zeolite-catalyzed chemistries, and new interest in catalytic processing of molecules derived from biomass requires understanding the fate of water in and on zeolite catalysts, as a function of water loading. The contribution of acid site density to water adsorption within zeolites is assessed by comparing bulk uptake and molecular experiments at varying Si:Al ratios, and interpreting those results in the context of solid-state NMR results that reveal strongly adsorbed water molecules and water clusters. <i>In situ</i> magic-angle spinning (MAS) NMR experiments for water loadings ranging from ca. 4 to 500 water molecules per zeolite unit cell indicate the following: (1) the dominant interaction is from water adsorbed from the vapor phase at an interior acid site, and unique signals for both the water and acid site are resolved at low loadings; (2) the exchanged-averaged water/acid site chemical shift at higher loadings can be used to measure acid site titration by water; and (3) silane-treated hydrophobically modified H-ZSM-5 does not allow liquid-phase water to access interior acid sites. The <i>in situ</i> ¹H MAS NMR method indicates that as-synthesized acidic zeolites can be rendered hydrophobic in the presence of liquid-phase water, with only a minimal reduction in the total number of acid sites

Untangling Framework Confinements: A Dynamical Study on Bulky Aromatic Molecules in MFI Zeolites

In MFI zeolites, differentiating the molecular dynamics as a function of pore structures, that is, straight-channel versus zigzag-channel versus channel-intersection, has always been challenging, mainly due to small differences in pore size but is of great interest because these subtle structural differences can remarkably influence shape selectivity. Herein, 1,2,4-trimethyl benzene (1,2,4-TMB), a characteristic molecule larger than the 10-MR channel diameter, while smaller than the channel intersection, is chosen to probe the pore-confined dynamical behaviors in MFI via 2H NMR spectroscopy and density functional theory calculations. Our results show that in the absence of acid sites, that is, in the siliceous MFI silicalite-1, 1,2,4-TMBs can only diffuse along the straight channels; while at equilibrium, they incline to occupy the channel intersections with structure-defined orientations. Furthermore, a series of dynamic motions of 1,2,4-TMBs under different types of pore confinements are revealed and evaluated at a molecular level over a wide range of timescales, concluded, in short, as methyl C3-rotation > 112°-flip > 90°-flip > translational diffusion. With the presence of acid sites, that is, in H-ZSM-5; however, 1,2,4-TMBs are strongly adsorbed on Brønsted acid sites and the confined motions are further impeded. The findings in this work may provide insights to the catalytic roles of polymethyl-benzene intermediates, including 1,2,4-TMB, which usually serve as active centers or deactivation precursors in zeolite-based hydrocarbon conversion processes