The encapsulation of metal clusters (Pt, Ru, Rh) within MFI was achieved by exchanging cationic metal precursors into a parent zeolite (BEA, FAU), reducing them with H2 to form metal clusters, and ...transforming these zeolites into daughter structures of higher framework density (MFI) under hydrothermal conditions. These transformations required MFI seeds or organic templates for FAU parent zeolites, but not for BEA, and occurred with the retention of encapsulated clusters. Clusters uniform in size (1.3–1.7 nm) and exposing clean and accessible surfaces formed in BEA and FAU zeolites; their size remained essentially unchanged upon transformation into MFI. Encapsulation selectivities, determined from the relative hydrogenation rates of small (toluene) and large (alkyl arenes) molecules and defined as the ratio of the surface areas of all the clusters in the sample to that of external clusters, were very high (8.1–40.9) for both parent and daughter zeolites. Encapsulation into MFI via direct hydrothermal syntheses was unsuccessful because metal precursors precipitated prematurely at the pH and temperatures required for MFI synthesis. Delayed introduction of metal precursors and F– (instead of OH–) as the mineralizing agent in hydrothermal syntheses increased encapsulation selectivities, but they remained lower than those achieved via interzeolite transformations. These interconversions provide a general and robust strategy for encapsulation of metals when precursors can be introduced via exchange into a zeolite that can be transformed into target daughter zeolites with higher framework densities, whether spontaneously or by using seeds or structure-directing agents (SDA).
We report synthetic protocols and guiding principles inspired by mechanistic considerations for the synthesis of crystalline microporous solids via interzeolite transformations that avoid direct ...intervention by organic structure-directing agents. These protocols are specifically implemented to synthesize high-silica MFI (ZSM-5), CHA (chabazite), STF (SSZ-35), and MTW (ZSM-12) zeolites from FAU (faujasite) or BEA (beta) parent materials. These transformations succeed when they lead to daughter structures with higher framework densities, and their nucleation and growth become possible by the presence of seeds or of structural building units common to the parent and target structures, leading, in the latter case, to spontaneous transformations by choosing appropriate synthesis conditions. These protocols allow the synthesis of high-silica frameworks without the use of organic templates otherwise required. The NaOH/SiO2 ratio and Al content in reagents are used to enforce synchronization between the swelling and local restructuring within parent zeolite domains with the spalling of fragments or building units from seeds of the target structure. Seed-mediated interconversions preserve the habit and volume of the parent crystals because of the incipient nucleation of the target structure at the outer regions of the parent domains. The pseudomorphic nature of these transformations requires the concurrent nucleation of mesopores within daughter zeolite crystals because their framework density is larger than that for the parent zeolites. The approach and evidence described shows, for the first time, that a broad range of zeolites rich in silica, and thus more useful as catalysts, can be made without the organic templates originally used to discover them.
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•A broadly applicable procedure to encapsulate Au clusters within zeolites.•Nanometer-size monodisperse Au clusters thermally stable against sintering.•Size-selective catalysis and ...resistance to sulfur poisoning through zeolite encapsulation.
This study describes successful strategies and guiding principles for the synthesis of small and monodisperse Au clusters protected against coalescence and poisoning by their uniform dispersion throughout the void space of LTA and MFI zeolites. These protocols involve hydrothermal zeolite crystallization around Au3+ precursors stabilized by mercaptosilane ligands, which prevent their premature reduction and enforce connectivity with incipient crystalline frameworks. The confining nanometer scale voids restrict cluster mobility during thermal treatment and allow the selection of reactants, products, and transition states and the exclusion of organosulfur poisons in catalytic applications based on molecular size. UV–visible spectra show that Au3+ forms Au0 clusters in O2 or H2 in a narrow temperature range that sets the dynamics of nucleation and growth and thus cluster size. Reduction protocols that maintain stable temperatures at the lower end of this range lead to small clusters uniform in size (LTA: 1.3nm, MFI: 2.0nm; 1.06–1.09 dispersity indices) with clean and accessible surfaces, as shown by their infrared spectra upon chemisorption of CO. Their unprecedented size and monodispersity are retained during oxidative treatments (773–823K) that sinter Au clusters on mesoporous supports. Oxidative dehydrogenation rates of small (ethanol) and large (isobutanol) alkanols and the poisoning of unprotected clusters by organosulfur titrants show that >90% of the Au surfaces reside within intracrystalline LTA and MFI voids. Their very different structures, compositions, and synthesis protocols suggest that these encapsulation strategies can be adapted readily to other zeolite frameworks with apertures too small for post-synthesis exchange of Au precursors. This study illustrates how confinement favors small, uniquely stable, and monodisperse clusters, even for Au, a metal prone to cluster growth at conditions often required for its catalytic use.
The synthesis protocols for encapsulation of metal clusters reported here expand the diversity in catalytic chemistries made possible by the ability of microporous solids to select reactants, ...transition states, and products on the basis of their molecular size. We report a synthesis strategy for the encapsulation of noble metals and their oxides within SOD (Sodalite, 0.28 nm × 0.28 nm), GIS (Gismondine, 0.45 nm × 0.31 nm), and ANA (Analcime, 0.42 nm × 0.16 nm) zeolites. Encapsulation was achieved via direct hydrothermal synthesis for SOD and GIS using metal precursors stabilized by ammonia or organic amine ligands, which prevent their decomposition or precipitation as colloidal hydroxides at the conditions of hydrothermal synthesis (<380 K) and favor interactions between metal precursors and incipient aluminosilicate nuclei during self-assembly of microporous frameworks. The synthesis of ANA requires higher crystallization temperatures (∼415 K) and high pH (>12), thereby causing precipitation of even ligand-stabilized metal precursors as hydroxides. As a result, encapsulation was achieved by the recrystallization of metal clusters containing GIS into ANA, which retained these metal clusters within voids throughout the GIS–ANA transformation.
A synthetic strategy is reported here for the selective containment of oxide nanoparticles of base metals within zeolitic voids of molecular dimensions. The technique, though generally applicable, is ...specifically illustrated to encapsulate Ni, Co, and Fe oxides within LTA, MFI, and FAU zeolites through hydrothermal framework crystallization in the presence of ligand-protected metal cations. Such ligands contain bidentate amine groups that preclude the precipitation of metal precursors in alkaline synthesis gels, and alkoxysilane moieties that form covalent linkages with nucleating zeolite precursors to enforce metal uptake into crystallized solids. These ligands are removed by subsequent oxidative treatments that nucleate oxide nanoparticles without structural degradation of the zeolites. The clusters are small (<2.5 nm) and uniformly distributed in size, reflecting their constrained growth within zeolite crystals. In contrast with exchange strategies for encapsulation, which lead to grafted cations and dense metal aluminosilicates, these methods form oxide nanoparticles, evident from infrared spectra of samples exposed to CO. Oxide nanoparticles undergo more facile redox cycles than grafted cations or dense aluminosilicates, thus rendering oxide domains more effective oxidation catalysts. The dynamics and stoichiometry of nanoparticle reduction in H2 confirmed the presence of NiO, Co3O4, and Fe2O3 clusters and their more facile reducibility relative to metal aluminosilicates. Ethanol oxidation rates on these clusters were essentially unaffected by exposure to bulky thiol poisons that titrate metal oxide surfaces, reflecting the selective placement of the oxide nanoparticles within the confines of microporous voids, where they are protected from contact by large molecules. These synthetic strategies and guiding principles circumvent long-standing hurdles in the selective encapsulation of base metals, and provide enabling routes for the synthesis of the many metal-zeolite systems that confront similar hurdles.
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•Synthesis method to encapsulate Ni, Co, and Fe oxide nanoparticles within zeolites.•Highly dispersed, uniformly sized nanoparticles within LTA, MFI, and FAU frameworks.•Alternative to ion-exchange methods for base metal encapsulation within zeolites.•Size-selective and sulfur-tolerant catalysis with metal oxides; ethanol oxidation.
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•Intrapore (not only pore-mouth) H+ are active sites for n-alkane hydroisomerization.•Diffusion-enhanced secondary reactions increase product 2-methylalkane ...selectivities.•Methylalkane selectivities on 10-MR zeolites reflect diffusivity ratios.•Single-proton events in cage-window 3-D 10-MR zeolites enhance secondary β-scission.•Absence of undulations in MTT and TON (1-D 10-MR) lead to low β-scission selectivity.
Diffusional hurdles enhance secondary interconversions of monomethyl isomers formed as primary products in bifunctional metal-acid isomerization of n-alkanes. One-dimensional medium-pore zeolites preferentially place methyl branches near the end of the isoalkane chains in what has been described as “pore mouth catalysis”; they also lead to low β-scission selectivities compared with three-dimensional frameworks similar in void size. n-Heptane and n-hexane reactions on acid forms of aluminosilicates of different channel size and connectivity (Al-MCM-41, FAU, SFH, BEA, MFI, MEL, SVR, TON, MTT; as mixtures with a Pt function) and crystallite size and proton density show that intracrystalline alkene concentration gradients are more consequential for larger and more branched molecules, as a result of their higher reactivity and lower diffusivity, and that the products form using all intracrystalline protons instead of only those near pore mouths. For both C6 and C7 reactants, 2-methyl to 3-methyl isoalkane ratios are those expected from equilibrium on large-pore zeolites (FAU, SFH, BEA). In contrast, these isomer ratios reflect the preferential sieving of the faster diffusing 2-methyl isomers from isoalkene products present as equilibrated mixtures within crystallites on medium-pore (10-MR) zeolites. These sieving effects are most evident for one-dimensional 10-MR frameworks because their void structures lead to stronger concentration gradients and more selective sieving of the faster diffusing isoalkenes. These effects are weaker on three-dimensional 10-MR structures because of less severe diffusional constraints, but the intersecting channels in these frameworks create local cage-like structures that favor β-scission through the preferential retention of dimethylpentenes, which act as the sole precursors to scission products. One-dimensional structures lack such channel undulations and cage-like intersections; as a result, one-dimensional medium-pore TON and MTT frameworks lead to low β-scission selectivities. This work demonstrates the essential, yet often overlooked, coupling between reactive and diffusive properties of zeolite materials in determining reactivity and selectivity in practice.
Crystalline silicates with a given structure but different framework heteroatoms (e.g., Al, Ga, Fe, B in MFI) provide similar confining voids but sites with different acid strength. Their known ...structure allows systematic mechanistic inquiries into the role of acid strength on reactivity through theory (DFT) and experiments, as illustrated here for isomerization and β-scission of linear and branched alkanes. Stronger acids lead to higher turnover rates for all reactants because of their more stable conjugate anions at ion-pair transition states. As acid strength decreases, β-scission transition states become preferentially stabilized over those for isomerization because of the differences in charge distributions at their carbocations, leading to higher scission selectivities on weaker acids; these findings contradict prevailing paradigms based on observations that reflect the higher proton reactivity in stronger acids, which, in turn, leads to the diffusion-enhanced secondary β-scission of primary isomer products. The small voids in zeotypes lead to high reactivity through transition state stabilization by confinement but also hinder diffusion, leading primary isomers to undergo secondary reactions before egressing into the extracrystalline fluid phase. These diffusional effects lead to the observed high selectivities for β-scission on stronger acids, as shown by reaction-transport formalisms underpinned by experiments that systematically vary intracrystalline proton densities through the gradual desorption of preadsorbed NH3 titrants during catalysis. These strategies allow intrinsic selectivities (single sojourn at an acid site) to be assessed separately from ubiquitous effects of diffusion-enhanced interconversions. Single-sojourn selectivities are similar on mesoporous and large-pore aluminosilicates (Al-MCM-41, FAU, BEA), reflecting confinement effects that influence isomerization and β-scission transition state carbocations to the same extent. In contrast, single-sojourn selectivities on medium-pore three-dimensional aluminosilicates (SVR, MFI, MEL) are influenced by secondary reactions even as intracrystalline proton densities decrease to very low values, because such reactions are enhanced by diffusional constraints even within a single cage, as a result of the undulating motifs prevalent in these frameworks.
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•CH3OH dehydration rate constants are measured on MFI samples with Al, Ga, Fe and B heteroatoms.•Rate constants decrease exponentially with increasing deprotonation energies ...(DPE).•Polyoxometalates and MFI rate constants are influenced similarly by changes in DPE.•Al density does not influence the acid strength of Al-MFI samples.•High Al densities create H+ in the more confining channels of Al-MFI.
The effects of heteroatom identity (Al3+, Ga3+, Fe3+, or B3+), concentration and location on catalysis by MFI zeolites are examined and interpreted mechanistically using methanol dehydration rate constants and density functional theory estimates of acid strength (deprotonation energies, DPE). In doing so, we shed light on the concomitant effects of confinement and acid strength on catalytic reactivity. Rate constants (per H+ from pyridine titrations during catalysis) in the first-order and zero-order kinetic regimes decreased exponentially as the DPE of MFI with different heteroatoms increased. These trends reflect a decrease in the stability of ion-pair transition states relative to the relevant precursors (H-bonded methanol and methanol dimers, respectively, for these two regimes) with decreasing acid strength and resemble those in mesoporous solid acids (e.g., polyoxometalates). Confinement effects, weaker in mesoporous solids, give larger rate constants on MFI than on POM clusters with similar DPE. Such reactivity enhancements reflect the effects of MFI voids that solvate transition states preferentially over smaller precursors via van der Waals interactions with the confining voids. Both dehydration rate constants on MFI with 0.7–2.4 H+ per unit cell volume (5.2nm3) are independent of Al or H+ densities, indicating that neither H+ location nor acid strength depend on acid site concentration. Higher site densities (3.6 H+ per unit cell) lead to larger first-order rate constants, but do not influence their zero-order analogs. These data reflect, and in turn provide evidence for, the initial siting of H+ in less constrained channel intersections and their ultimate placement within the more solvating environments of the channels themselves. Thus, the higher reactivity of Al-rich samples, often attributed to the stronger acid sites, arises instead from the ubiquitous role of zeolites as inorganic solvents for the relevant transition state, taken together with H+ siting that depends on Al density. We find that heteroatom composition, but not Al density, influences acid strength in MFI, consistent with experiment and theoretical estimates of DPE, and that methanol dehydration rate constants, properly interpreted, provide relevant insights into the combined effects of acid strength and confinement on the reactivity of solid Brønsted acids.
The methanol-to-olefins (MTO) process produces high-value-added light olefins from nonpetroleum sources. Acidic zeotypes containing cages bounded by 8-ring (small-pore) windows can effectively ...catalyze the MTO reaction, since their cages can accommodate the necessary aromatic intermediates that produce the light olefin products that escape. While progress on the mechanisms of the MTO reaction continues, zeotype structure–reaction property relationships have yet to be elucidated. Here, we report MTO reaction results from various small-pore, cage-containing silicoaluminophosphate/metalloaluminophosphates (SAPO/MAPOs) and zeolites under the same reaction conditions. The MTO behaviors of microporous materials having the following topologies are investigated: LEV, ERI, CHA, AFX, SFW, AEI, DDR, RTH, ITE, SAV, LTA, RHO, KFI, and UFI. The previous observation that light olefin product distributions from a series of small-pore, cage-containing zeolites can be classified into four structural categories is further supported by the results shown here from zeolite structures not investigated in the previous study and SAPO and MAPO materials with isostructural frameworks to all the zeolites. Additionally, these data reveal that light olefin product distributions are very similar over a given topology independent of framework composition. To develop a structure–property relationship between the framework topology and the MTO light olefin product distribution, the concept of the cage-defining ring size is introduced. The cage-defining ring size is defined as the minimum number of tetrahedral atoms of the ring encircling the center of the framework cages in the molecular sieve topology. It is shown that the cage-defining ring size correlates with MTO light olefin product distribution.
The catalytic diversity of microporous aluminosilicates reflects their unique ability to confine transition states within intracrystalline voids of molecular dimensions and the number (but not the ...strength) of the protons that act as Brønsted acids. First-order rate constants for CH3OH conversion to dimethyl ether (DME) reflect the energy of transition states relative to those for gaseous and H-bonded CH3OH molecules; on zeolites, these constants depend exponentially on n-hexane physisorption energies for different void size and shape and proton location, indicating that van der Waals stabilization of transition states causes their different reactivity, without concomitant effects of void structure or proton location on acid strength. The dispersive contribution to adsorption enthalpies of DME, a proxy in shape and size for relevant transition states, was calculated using density functional theory and Lennard-Jones interactions on FAU, SFH, BEA, MOR, MTW, MFI, and MTT zeolites and averaged over all proton locations; first-order rate constants also depended exponentially on these enthalpies. In contrast, zero-order rate constants, which reflect the stability of transition states relative to protonated CH3OH dimers similar in size, depended weakly on dispersive stabilization, whether measured from experiment or simulations, because dispersive forces influence species similar in size to the same extent. These results, taken together, demonstrate the preeminent effects of confinement on zeolite reactivity and the manner by which the local voids around protons held within diverse intracrystalline environments give rise to the unique behaviors that have made zeolites ubiquitous in the practice of catalysis. Enthalpic stabilization of relevant transition states prevail over entropic losses caused by confinement at low temperatures in a manner reminiscent of how catalytic pockets and solvents do so in catalysis by molecules or enzymes.
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