The molecular structure and cationic charge density of organic and inorganic structure-directing agents (SDAs) influence the siting and arrangement of Al substituted in zeolite frameworks. Yet, developing robust synthesis−structure relations for MFI zeolites is difficult because of the complexities inherent to its low-symmetry framework (12 unique tetrahedral sites), which generates a large combinatorial space of Al−Al site pairs to exhaustively model by density functional theory (DFT) and quantify by experiment. Here, we develop an experimental protocol to reproducibly quantify Co 2+ -titratable Al−Al site pairs in MFI with saturation uptakes validated by corroborating spectroscopic and cation site balance data. Using tetrapropylammonium (TPA + ) as the sole SDA, MFI zeolites were crystallized with varying Al contents (Si/Al = 37−185; 0.52−2.52 Al per unit cell) within a composition range consistent with charge density mismatch theory and the occlusion of one TPA + per channel intersection with fractions of paired Al (0.0−0.34) that increased with bulk Al content. DFT calculations performed using a 96 T-site MFI unit cell containing either an isolated Al site (all 96 configurations) or various Al−Al site pairs (1773 out of 13 680 total configurations), charge balanced by one or two TPA + , respectively, reveal the dominant influence of electrostatic interactions between the cationic N of TPA + and the anionic lattice charge on Al siting energies. Together with DFT calculations of Co 2+ exchange energies at Al−Al site pairs, theory predicts that two TPA + cations confined within adjacent channel intersections can form many Al−Al site pair ensembles that are Co 2+ -titratable, rationalizing the considerable presence of paired Al sites in MFI samples crystallized using only TPA + . The use of TPA + and Na + as co-SDAs in the synthesis gel, while varying the Na + /TPA + ratio (0−5) at a constant SDA/Al ratio ((TPA + + Na + )/Al = 30), crystallized MFI with a similar bulk Al content (Si/Al ≈ 50) but varying fractions of Al in pairs (0.12−0.44). Separate crystallization experiments performed using charge-neutral organic SDAs, either pentaerythritol or a mixture of 1,4-diazabicyclo[2.2.2]octane and methylamine, together with Na + to compensate for framework Al, crystallized MFI at similar bulk Al content (Si/Al ≈ 50) but with lower fractions of Al in pairs (<0.14). Among MFI samples crystallized with an organic SDA and Na + as a co-SDA, the number of paired Al sites formed generally increased with the co-occluded Na + content on the zeolite, a synthesis−structure relation that resembles our prior observations on CHA zeolites. The combined theoretical and experimental approach used here provides a microscopic model to define and quantify Al−Al site pairs in MFI, which can be adapted to do so for other framework topologies. These findings highlight how such Al siting models can be exercised to quantitatively characterize zeolite materials to develop synthetic strategies that can predictably vary their framework Al arrang...

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This study uses periodic density functional theory (DFT) to determine the reaction mechanism and effects of reactant size for all 20 arene (C6-C12) methylation reactions using CH3OH and CH3OCH3 as methylating agents in H-MFI zeolites. Reactant, product, and transition state structures were manually generated, optimized, and then systematically reoriented and reoptimized to sufficiently sample the potential energy surface and thus identify global minima and the most stable transition states which interconnect them. These systematic reorientations decreased energies by up to 50 kJ mol −1 , demonstrating their necessity when analyzing reaction pathways or adsorptive properties of zeolites. Benzene-DME methylation occurs via sequential pathways, consistent with prior reports, but is limited by surface methylation which is stabilized by co-adsorbed benzene via novel cooperativity between the channels and intersections within MFI. These co-adsorbate assisted surface methylations generally prevail over unassisted routes. Calculated free energy barriers and reaction energies suggest that both the sequential and concerted methylation mechanisms can generally occur, depending on the methylating agent and methylbenzene being reacted-there is no consensus mechanism for these homologous reactions. Intrinsic methylation barriers for step-wise reactions of benzene to hexamethylbenzene remain between 75-137 kJ mol −1 at conditions relevant to methanol-to-hydrocarbon (MTH) reactions where such arene species act as co-catalysts. Intrinsic methylation barriers are similar between CH3OH and CH3OCH3 suggesting that both species are equally capable of interconverting between methylbenzene species. Additionally, these methylation barriers do not systematically increase as the number of methyl-substituents on the arene increases and the formation of higher methylated arenes is thermodynamically favorable. These barriers are significantly lower than those associated with alkene formation during the aromatic cycle, suggesting that aromatic species formed during MTH reactions either egress from the catalyst-depending on that zeolite's pore structure-or become trapped as extensivelysubstituted C10-C12 species which can either isomerize to form olefins or ultimately create polyaromatic species that deactivate MTH catalysts.

This study examines how Brønsted acid strengthsas predicted by dispersion-corrected periodic DFT calculations of deprotonation energy (DPE), dehydrogenation energy (DHE), and NH3 binding energy (NH3 BE)are affected by site proximity in proton-form zeolites and how adsorbates on one acid site alter the strength of nearby acids. Protons can bind to four distinct O atoms around the single crystallographically unique T-site of CHA, and all such locations were examined as bare and NH3-occupied sites. Protons prefer to bind to O1 atoms and orient within the plane of six-membered-ring (6MR) structures of CHA. NH4 + cations show a strong preference for binding in 8MR windows; 6MR structures are too small to solvate them. These preferences govern proximity effects on acid strength, studied here by probing the strength of a Brønsted acid site while a second site is placed in 23 locations separated by 1–3 T-sites. Placing a second acid in the 6MR of CHA decreased DPE and NH3 BE values for the first site by >10 kJ mol–1 because the proton of the second site stabilized the deprotonated site across the 6MR. Acid site-pairs across 8MR structures interact very little when the second acid is bare, as residual protons do not prefer to orient within 8MR. One location of the second acid stabilized the adsorbed proton without stabilizing the deprotonated state, resulting in a significantly weaker acid. All of these effects are altered when the second site is instead occupied by an adsorbed NH3, which acts as a proxy for strongly bound reactive intermediates and cationic transition states. The strength of the first site is significantly weakened (DPE and NH3 BE increases of >20 kJ mol–1) when a second site is NH3-occupied and placed in the 6MR because such structures are too small to effectively solvate NH4 + cations. Acid sites are strengthened, however, when second sites are NH3-occupied and placed across 8MR windows, because they are appropriately sized to solvate the NH4 + cations that simultaneously interact with both deprotonated sites. The alteration of acid strength by acid site proximity therefore depends on the specific arrangement (not merely Al–Al distances), the structural motifs present (such as 6MR structures which allow protons, but not NH4 +, to stabilize proximal conjugate base anions), and the status of proximal sites as vacant or occupied, which determines the distances over which cationic–anionic stabilizations of deprotonated sites can take place.

Zeolite reactivity depends on the solvating environments of their micropores and the proximity of their Brønsted acid sites.T urnover rates (per H +)f or methanol and ethanol dehydration increase with the fraction of H + sites sharing sixmembered rings of chabazite (CHA) zeolites.D ensity functional theory (DFT) shows that activation barriers vary widely with the number and arrangement of Al (1-5 per 36 T-site unit cell), but cannot be described solely by Al-Al distance or density.C ertain Al distributions yield rigid arrangements of anionic charge that stabilizec ationic intermediates and transition states via H-bonding to decrease barriers.T his is ak ey feature of acid catalysis in zeolites olvents,w hichl ack the isotropyo fl iquid solvents.T he sensitivity of polar transition states to specific arrangements of charge in their solvating environments and the ability to position such charges in zeolite lattices with increasing precision herald rich catalytic diversity among zeolites of varying Al arrangement.

Etherification of 5-hydroxymethylfurfural (HMF) and ethanol to yield ethoxymethylfurfural (EMF) is an important reaction for producing diesel blendstocks from biomass resources. This aqueous-phase etherification reaction occurs with higher selectivity on H-BEA (95%) than on Amberlyst-15 (76%), H-FAU (64%), H-MFI (88%), and H-MOR (63%) at high conversions (>60%). This selectivity toward EMF is not driven by transport limitations or Brønsted acid site concentrations; instead, a kinetic preference for etherification on BEA leads to such high yields. Nonidealities associated with the liquid-phase reaction are addressed by using UNIFAC-derived activities rather than concentrations, and reaction kinetics measurements indicate that the reaction proceeds on a surface saturated by ethanol with rates that are first order with respect to HMF activity and negative order with respect to ethanol activity. Gas-phase density functional theory calculations are unified with aqueous liquid-phase kinetics measurements by using partial pressures of a hypothetical gas phase calculated from UNIFAC-derived activities. DFT calculations indicate that etherification occurs in a single concerted steprather than through a two-step sequential pathwayin which HMF is protonated and dehydrated to form a relatively stable methoxyfurfural carbocation (in contrast to ethanol dehydration). This methoxyfurfural carbocation is stabilized by resonance in contrast to ethyl carbocations formed from ethanol protonation and dehydration, and it also leads to the observed high selectivity for cross-etherification (to EMF) vs selfetherification of either HMF or ethanol. This rigorous mechanistic investigation uses gas-phase DFT calculations to offer insights into aqueous-phase catalysis, thereby elucidating an important reaction in biomass upgrading.

Oxide-supported Rh catalysts are important components of commercial three-way catalysts for pollution abatement. Despite their universal application, many mysteries remain about the active structure of Rh on oxide supports as these materials often contain a mixture of nanoparticles and single-atom Rh species on the same support, even after aging. Probe molecule Fourier transform infrared (FTIR) spectroscopy in this work shows that atomically dispersed Rh on γ-Al2O3 prefer to strongly bind CO when exposed to NO and CO mixtures and that light-off of NO reduction occurs at temperatures similar to CO desorption, suggesting that the first and rate-determining step in NO–CO reactions may be the desorption of CO from single-atom Rh dicarbonyl complexes, Rh­(CO)2. Two sets of symmetric and asymmetric stretching frequencies associated with distinct Rh­(CO)2 species are observed in FTIR spectra at 2084/2010 and 2094/2020 cm–1. During temperature ramps, the latter pair of bands at 2094/2020 cm–1 converts to the 2084/2010 cm–1 bands at 463 K before all symmetric and asymmetric bands disappear at 573 K. Bands then appear in the range of 1975–1985 cm–1 associated with Rh monocarbonyl, Rh­(CO), species upon the disappearance of the 2084/2010 cm–1 bands, suggesting that CO desorbs sequentially from Rh­(CO)2 by forming Rh­(CO) intermediates. Combined DFT and FTIR experiments suggest that local OH coverage on the γ-Al2O3 surface distinguishes the two Rh­(CO)2 species: the higher frequency species resides on a less hydroxylated region and migrates to a more hydroxylated region at higher temperatures, causing the CO vibrational frequency to decrease by ∼10 cm–1. CO desorption occurs from this Rh­(CO)2 structure with high local OH coverage, consistent with the DFT predicted trend of CO binding energies. Because of the coincidence of CO desorption with the light-off of NO reduction, local support hydroxylation of atomically dispersed Rh1/γ-Al2O3 catalysts likely affects both the Rh structure after CO desorption and the kinetics of NO reduction, studies of which are enabled by the Rh­(CO)2 model developed here.

This study compares and evaluates multiple orthorhombic silicalite MFI framework structures using periodic density functional theory (DFT) calculations implemented with a wide range of exchange−correlation functionals and dispersion-correction schemes. Optimization of the structure available from the International Zeolite Association (IZA) yields only metastable forms, which restructure to arrangements 18−156 kJ mol −1 lower in energy (55 kJ mol −1 on average) through annealing and adsorption/ desorption processes without altering their connectivity. These restructuring events can occur unintentionally during DFT studies of adsorptive and catalytic properties, leading to very large artifacts in DFT-predicted adsorption, reaction, and activation energies. Pre-annealing the IZA structure prevents restructuring and these artifacts but forms MFI structures which do not conform to the Pnma spacegroup symmetry and have significantly perturbed sinusoidal and straight channel geometries. These issues persist across a wide range of exchange−correlation functionals, including common choices such as the Perdew− Burke−Ernzerhof and Bayesian error estimation functionals, and dispersion-correction schemes such as the D3 method. Direct optimization of structures generated from the work of van Koningsveld et al. and Olson et al., in contrast, yields structures that are extremely similar across all functionals, restructure less often during annealing, and have smaller energy shifts when they do restructure (5 kJ mol −1 , on average). Optimizing the unit cell parameters of these structures without constraining atoms or the unit cell shape also yields more stable structures, though often with unit cell parameters that do not closely match structures found experimentally. Annealing of other commonly studied zeolites (BEA, CHA, and LTA) does not yield structures with energy decreases or structural changes as significant as those for MFI. This study thus illuminates a potential source of significant error for DFT studies of MFI and provides evidence-based solutions for a variety of DFT methods.