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

Simple NMR Experiments Reveal the Influence of Chain Length and Chain Architecture on the Crystalline/Amorphous Interface in Polyethylenes

The distribution of polyethylene (PE) chain segments between the crystalline, noncrystalline, and interfacial morphological regions is an old question that continues to intrigue the polymer science community. Simple solid state NMR experiments described here reveal that even in the case of linear PE, four distinct chain components may be resolved and reliably quantified. The amounts of rigid crystalline chains in all-trans conformations, amorphous chains with increased equilibrium gauche conformer content undergoing essentially isotropic reorientation, mobile all-trans chains (termed mobile trans), and less mobile noncrystalline chains (termed constrained amorphous) can be quantified by simple ¹³C NMR experiments on solid polymer samples. A version of the EASY background suppression pulse sequence [Jaeger and Hemmann Solid State Nucl. Magn. Reson. 2014, 57–58, 22], modified to eliminate transient Overhauser effects, is used to obtain all of the data in a single experimental acquisition. Using a broad range of well-characterized linear metallocene PE’s, the method reveals that the constrained amorphous and the mobile all-trans fractions, i.e., the total interface content, increases essentially linearly with increasing <i>M</i>_w. Topologically modified PE’s, at similar <i>M</i>_w’s, that contain short-chain branches (SCB), long-chain branches (LCB), or long-chain branches with SCB’s (LCB + SCB) have significantly larger interfacial content per unit molecular weight and most significantly so for the LCB + SCB polymers

Controlling Macroscopic Properties by Tailoring Nanoscopic Interfaces in Tapered Copolymers

Systematic variation of comonomer concentration in individual polymer chains is responsible for unique phase behaviors in different types of copolymers. Controlled self-assembly of gradient copolymers into desired morphologies is theoretically understood but practically challenging, and identifying heterogeneous phase partitioning of individual comonomers in those resulting morphological regions can be difficult. Building on previous work where improved methods were used to elucidate heterogeneous comonomer partitioning in styrene–butadiene gradient copolymers [Clough Macromolecules 2014, 47, 2625], the arrangement of the styrene and butadiene monomers in only a fraction of the total chain length is used here to significantly perturb the overall morphology and physical properties of copolymers. Importantly, the chemical composition of all copolymers was held nearly fixed in this study. The resulting tapered and inverse tapered block copolymers contain nanometer length scale interfaces that differ from one another and differ dramatically from that observed in a control block copolymer composed of chains with a discrete interface. Evidence is presented that butadiene can reside in rigid environments, styrene can reside in mobile environments, and their relative amounts can be varied based on the gradient design. The connection between the molecular design of the gradient, the resulting nanometer-length scale interfacial structures, and mechanical properties is demonstrated using a combination of variable temperature solid-state NMR, modulated DSC (differential scanning calorimetry), AFM (atomic force microscopy), and rheology experiments

Characterization of Kerogen and Source Rock Maturation Using Solid-State NMR Spectroscopy

Solid-state NMR methods common to the analysis of polymers and other rigid solids are utilized for the study of kerogen, bitumen, and the organic content in source rocks. The use of straightforward nondestructive techniques, primarily employing solid-state NMR, is shown to provide useful information about both individual samples and changes between samples that cover a range of thermal maturities of type II kerogen. In addition to aromatic fraction and chemical structure, one of the most striking changes to isolated kerogen with maturity is the distribution of pore sizes, studied with both ¹²⁹Xe NMR and complementary nitrogen physisorption, that may help to understand the process of bitumen generation. Ultimately, direct in situ analysis of source rock samples that allow kerogen and bitumen to be distinguished is desirable, as it would eliminate the time and effort to isolate and prepare kerogen samples. By proper consideration and removal of the background, we find that a clear ¹³C NMR signal can be obtained from source rock with total organic carbon weight as low as 2%. Simple ¹H NMR methods are shown to quickly provide a qualitative measurement of the bitumen in source rocks, while ¹³C cross-polarization is found to be an easy method to distinguish kerogen from bitumen

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

Multiblock Inverse-Tapered Copolymers: Glass Transition Temperatures and Dynamic Heterogeneity as a Function of Chain Architecture

Systematic variation of the size and number of inverse-tapered blocks in styrene–butadiene copolymers results in a wide range of accessible glass-transition temperatures (<i>T</i>_g), including <i>T</i>_g’s approaching that predicted by the Fox equation. Composition-weighted average <i>T</i>_g’s are expected for miscible blends or random copolymers, but such behavior has not previously been reported for block copolymers made from immiscible styrene and butadiene segments. In this work, 50:50 wt % multiblock copolymers with <i>M</i>_n = 120 000 kg/mol were synthesized using an inverse-tapered block design for all blocks except the end blocks. The total composition and molecular weight were held constant, but the type and number of blocks were systematically varied in order to compare contributions from the inverse-tapered chain interfaces to the overall glass transition behavior. Discrete copolymers of similar block number and length were investigated as controls to help separate contributions from the inverse-tapered design and the molecular weight of individual blocks. Some copolymers were intentionally designed such that individual block molecular weights were between the entanglement molecular weight (<i>M</i>_e) of polystyrene (PS) and polybutadiene (PB). A range of intermediate glass transitions was observed, but the inverse-tapered copolymers that satisfied this latter condition were the only copolymers that exhibited a <i>T</i>_g near a composition-weighted average. Solid state NMR reveals dynamic heterogeneity among monomeric components through chain-level identification of relatively large amounts of rigid PB segments and mobile PS chain segments versus that observed in discrete block analogues where essentially all PB segments are mobile and all PS segments are rigid. NMR revealed subtle differences in the temperature-dependent segmental chain dynamics of different inverse-tapered blocks, which were not obvious from the calorimetric studies but which presumably contribute to the longer length scale <i>T</i>_g behavior

C₆₀–Polymer Nanocomposite Networks Enabled by Guest–Host Properties

A modular approach for the synthesis of polymer networks with well-defined node and cross-linking dimensions is described. Each node or tie point in the network is a cyclodextrin molecule, which imparts discrete molecular guest–host capabilities to the network. C₆₀ fullerenes homogeneously intercalate in the network, presumably via van der Waals guest–host interactions with the hydrophobic γ-cyclodextrin cavity, resulting in stable C₆₀-filled polymer networks with improved mechanical properties. Networks prepared with α-cyclodextrin, whose inner cavity is smaller than γ-cyclodextrin, and smaller than the C₆₀ diameter, do not yield materials with stable C₆₀ intercalation. Characterization of the final composites reveals that the cross-linked γ-cyclodextrin-based composites maintain stable C₆₀ concentrations, even after multiple extractions with toluene, which itself is a good solvent for C₆₀. Membranes prepared from the cyclodextrin polymer network, prior to C₆₀ intercalation, should also be useful for C₆₀ extraction from C₆₀–solvent mixtures. The synthetic route we describe here is not limited to C₆₀ and should be generally applicable to a wide variety of guests

Direct Detection of Multiple Acidic Proton Sites in Zeolite HZSM‑5

Direct observation of multiple reactive sites in the zeolite HZSM-5, a member of the MFI family of zeolite structures, contradicts the traditional view of only one type of active protonic species in industrially important zeolites. In addition to the well-known Brönsted acid site proton, two other protonic species undergo room-temperature hydrogen–deuterium exchange with an alkane hydrocarbon reagent, including one zeolite moiety characterized by a broad ¹H chemical shift at ca. 12–15 ppm that is reported here for the first time. Although the ca. 13 ppm chemical shift value is consistent with computational predictions from the literature for a surface-stabilized hydroxonium ion in a zeolite, data suggest that the signal does not arise from hydroxonium species but rather from hydroxyls on extra-lattice aluminol species proximate to Brönsted lattice sites, i.e., a small population of highly deshielded acid sites. Double-resonance experiments show that this species is proximate to Al atoms, similar to the Brönsted acid site proton. These sites can be removed by appropriate postsynthesis chemical treatment, yielding a catalyst with reduced activity for isotopic H/D exchange reactions. Additionally, other extra-lattice aluminum hydroxyl groups previously discussed in the literature but whose protons were considered unreactive are also shown for the first time to react with hydrocarbon probe molecules. Two-dimensional exchange NMR reveals direct proton exchange between the Brönsted site and these two types of extra-lattice Al–OH species, and it also reveals unexpected proton exchange between extra-lattice Al–OH species and an alkane reagent

Zeolite Catalysis: Water Can Dramatically Increase or Suppress Alkane C–H Bond Activation

Zeolite-catalyzed alkane C–H bond activation reactions carried out at room temperature, low pressure, and low reagent loadings demonstrate that water can act either to increase or to suppress the observed reaction rates. Isobutane-d₁₀ undergoes hydrogen/deuterium exchange with the acidic zeolite HZSM-5 at subambient temperatures, as first reported by us (Truitt et al. <i>J. Am. Chem. Soc.</i> <b>2004</b>, 126, 11144 and Truitt et al. <i>J. Am. Chem. Soc.</i> <b>2006</b>, 128, 1847). New experiments demonstrate that the C–H bond activation chemistry is very sensitive to the presence of water. Isobutane reaction rate constants increase by an order of magnitude at water loadings in the range of ≤1 water molecule per catalyst active site relative to the dry catalyst. Conversely, water loadings greater than about 1–3 water molecules per active site retard isobutane reaction. In situ solid-state NMR data show that water molecules and isobutane molecules are simultaneously proximate to the catalyst active site. These results indicate that water can be an active participant in reactions involving hydrophobic molecules in solid acid catalysts, possibly via transition state stabilization, as long as the water concentration is essentially stoichiometric. Such conditions exist in well-known catalytic reactions, e.g., methanol-to-hydrocarbon chemistries, since stoichiometric water is a first-formed byproduct