Study of Zn and Ga Exchange in H‑[Fe]ZSM‑5 and H‑[B]ZSM‑5 Zeolites

The catalytic properties of H-[Fe]­ZSM-5 and H-[B]­ZSM-5 were explored after addition of Zn or Ga. TPD–TGA of 2-propanamine adsorbed on Zn- and Ga-exchanged H-[Fe]­ZSM-5 showed a decrease in Brønsted-acid site densities and the formation of new dehydrogenation sites, similar to what is observed following exchange in H-[Al]­ZSM-5 and in amorphous silica–alumina. Exchanged Zn cations in [Fe]­ZSM-5 also exhibited Lewis-acid character, as demonstrated by the appearance of a υ­(CN) stretch at 2310 cm^–1 upon adsorption of CD₃CN. By contrast, the sites in H-[B]­ZSM-5 were not capable of protonating 2-propanamine, did not form dehydrogenation sites when Zn or Ga were added, and showed no evidence for sites with Lewis-acid character from the FTIR spectroscopy of CD₃CN. Neither H­(Zn)-[Fe]­ZSM-5 nor H­(Zn)-[B]­ZSM-5 catalyzed reactions of <i>n</i>-hexane at 773 K, but the TPD–TGA of adsorbed propene on H­(Zn)-[Fe]­ZSM-5 showed strong interactions between the Zn and olefins that might be responsible for the dehydrocyclization of light alkanes in Zn-exchanged, Al-containing zeolites

Adsorption of Small Alkanes on ZSM‑5 Zeolites: Influence of Brønsted Sites

The adsorption of a series of small alkanes was studied experimentally on H-ZSM-5 zeolites using calorimetric measurements in order to determine their interactions with the Brønsted sites. Differential heats measured on four ZSM-5 samples with different Si/Al₂ ratio and with different defect concentrations were found to depend strongly on the Brønsted-site density but not on the presence of defects. The interactions for CH₄ with the Brønsted sites were minimal but the effect was significant (up to 11 ± 2 kJ/mol extra heats) for larger alkanes, such as <i>n</i>-C₆H₁₄. The affinity of the alkanes with the Brønsted sites increased with the gas-phase proton affinity of the alkanes and the calculated affinity of the alkanes for the strong acid, fluorosulfonic acid. The extra heats of adsorption in H-ZSM-5 over its siliceous counterparts can therefore be associated with the strength of hydrogen bonding between the adsorbed alkane and the Brønsted sites, which in turn increases with molecular size. Specifically, extra heats were found to vary linearly with acid affinity corrected for dispersion interactions. The comparison of the experimental and computational results, therefore, indicates that the hydrogen bonded interaction theory describes the effect of Brønsted sites for alkane adsorption on zeolites

Endothermic Reforming of <i>n</i>‑Hexane on Metal (Pt, Ga) Containing H‑ZSM‑5 at High Pressures

The supercritical, high-pressure reaction of <i>n</i>-hexane over H-ZSM-5, with and without the addition of Pt and Ga, has been studied for application to endothermic reforming in scramjet engines. The endothermicities of the reactions were determined from the product distributions. For unpromoted H-ZSM-5, the product distribution indicated that the endothermicity is low and decreases with increasing pressure. The addition of Ga to H-ZSM-5 has a relatively small effect on <i>n</i>-hexane conversion but significantly increases the endothermicity of the reaction by increasing the selectivity to form small aromatics. The H­(Ga)-ZSM-5 catalyst showed no deactivation for at least 5 h at 633 K and 137 bar of <i>n</i>-hexane. By contrast, the addition of Pt had a minor effect on both the rate and the reaction endothermicity

Diels–Alder and Dehydration Reactions of Biomass-Derived Furan and Acrylic Acid for the Synthesis of Benzoic Acid

Routes to benzoic acid starting from furanobtained from hemicellulose in high yieldand methyl acrylate are reported. These routes involve Diels–Alder and dehydration reactions of furan and acrylic acid (or methyl acrylate) in a two-step reaction protocol that minimizes side reactions. The Diels–Alder reaction of furan and methyl acrylate (or acrylic acid) was run at 298 K and was catalyzed by Lewis acidic (Hf-, Zr-, and Sn-Beta) zeolite catalysts, and achieving a high turnover frequency (∼2 h^–1) and no side reactions were observed. The oxanorbornene product was dehydrated at low temperatures (298 to 353 K) in mixtures of methanesulfonic acid and acetic anhydride in 96% yield. This is compared to an only 1.7% yield of methyl benzoate obtained for the dehydration of the oxanorbornene in neat methanesulfonic acid. The effect of oxanorbornene concentration and stereochemistry was found not to decrease the yield of aromatics, while dehydration of the carboxylic acid form of the oxanorbornene led to a decrease in selectivity to 43% at complete conversion in mixtures of methanesulfonic acid and acetic anhydride. This reaction sequence could be an important entry point for selectively directing high-yield, hemicellulose-derived furans to aromatic products used in the existing chemical process industry

Probing Lewis Acid Sites in Sn-Beta Zeolite

The adsorption properties of framework Sn sites in a siliceous zeolite beta were examined by comparing the adsorption of acetonitrile, diethyl ether, and 2-methyl-2-propanol on a Sn-Beta zeolite, a purely siliceous Beta zeolite, and a siliceous Beta zeolite with impregnated SnO₂, using temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA). Adsorption stoichiometries close to one molecule per framework Sn site were observed for each of the probe molecules. Although the 1:1 complexes with acetonitrile and diethyl ether decompose reversibly upon mild heating in vacuo, the 1:1 complex formed by 2-methyl-2-propanol underwent dehydration to butene and water over a very narrow temperature range centered at 410 K. FTIR spectra of acetonitrile-<i>d3</i> at a coverage of one molecule per site exhibit a υ­(C–N) stretching frequency at 2312 cm^–1 that is not observed with nonframework Sn, providing a convenient method for characterizing the presence of framework Sn sites. Water interacts strongly enough with the Sn sites to prevent adsorption of acetonitrile

Zn-Promoted H‑ZSM‑5 for Endothermic Reforming of <i>n</i>‑Hexane at High Pressures

The addition of Zn to H-ZSM-5 zeolites was studied for application to endothermic reforming in hypersonic aircraft engines. Temperature-programmed-desorption (TPD)/thermogravimetric-analysis (TGA) measurements with 2-propanamine on two H­(Zn)-ZSM-5 samples showed that at low ion-exchange levels, less than 0.5 Zn/Al, each Zn cation displaces one Brønsted-acid site. Although rates for <i>n</i>-hexane conversion at 633 and 823 K and at a pressure of 137 bar decreased with the loss of Brønsted sites, Zn promotion greatly increased the production of H₂ and the formation of small aromatic molecules. FTIR of adsorbed acetonitrile-<i>d</i>₃ and calorimetric measurements of adsorbed CO at 195 K indicate that the exchanged Zn cations form Lewis-acid centers. A model in which the Zn cations, acting as Lewis-acid centers, polarize intermediates formed at Brønsted sites is presented as a way of understanding the observations

Improved Coking Resistance of “Intelligent” Ni Catalysts Prepared by Atomic Layer Deposition

Conformal CaTiO₃ films were deposited onto MgAl₂O₄ by atomic layer deposition (ALD) and then examined as “intelligent” catalyst supports for Ni in the steam and CO₂ reforming of methane. CaTiO₃ films (1 nm) were characterized by scanning transmission electron microscopy and XRD and shown to be stable to at least 1073 K. Catalysts with 1 and 20 wt % Ni were studied, and it was found that, following calcination at 1073 K, the Ni-CaTiO₃/MgAl₂O₄ catalysts required high-temperature reduction to achieve activities comparable to that of their Ni/MgAl₂O₄ counterparts. However, the Ni-CaTiO₃/MgAl₂O₄ catalysts exhibited dramatically improved tolerance toward carbon-whisker formation. The carbon content on the 1 wt % Ni catalyst on CaTiO₃/MgAl₂O₄ was small even after heating the catalyst in a dry, 10% CH₄–90% He mixture at 1073 K for 12 h. Possible mechanisms for the high carbon tolerance of the perovskite-containing catalysts are discussed

Site-Dependent Lewis Acidity of γ‑Al₂O₃ and Its Impact on Ethanol Dehydration and Etherification

We examine the heterogeneity of the Lewis acidity on the (100) and (110) facets of γ-Al₂O₃ by computing the binding energies of various oxygenates, in addition to the reaction barriers of dehydration and etherification reactions of ethanol. We show that the ethanol dehydration barrier is moderately affected by site heterogeneity (barriers between 1.2 and 1.6 eV); in contrast, a nearly 3-fold change in the ethanol etherification barrier is found among the various Al³⁺ sites. In order to rationalize these results, the <i>s</i>-conduction band mean of the Al³⁺ sites is introduced as a descriptor to characterize the ability to transfer electron charge from the adsorbate to the Lewis acid site. It is shown for the first time that this descriptor quantitatively correlates the oxygenate binding energies and the ethanol dehydration reaction barriers. However, for the ethanol etherification reactions the <i>s</i>-conduction band mean of the Al³⁺ sites describes barriers only qualitatively due to the bimolecular nature of this reaction, which results in a change in the nucleophilicity of the ethoxy species by a nearby adsorbed ethanol. As a result, the strength of the Lewis acid sites is not the only descriptor for etherification chemistry. Hydration of the (110) facet indicates an increase in Lewis acidity strength as described by the <i>s</i>-conduction band mean that results in stronger binding. However, this increase in Lewis acidity results in either a negligible change of the ethanol dehydration reaction barriers on some sites or an increase due to a reduction in the basicity of the adjacent oxygen by the dissociated water. Similarly, ethanol etherification is slowed down by the presence of water due primarily to the change in nucleophilicity of the ethoxy species. Overall, our results clearly indicate that while the binding energy is an excellent descriptor of Lewis acidity strength and dehydration chemistry on the clean alumina surfaces, cooperative phenomena (i.e., modulation of the nucleophilicity of the ethoxy by the nearby oxygen or water and the basicity of oxygen in the presence of water) are key issues that lead to a breakdown in the correlation between Lewis acid strength in terms of the binding energy or the <i>s</i>-conduction band mean and the reaction barriers

Efficient Removal of Organic Ligands from Supported Nanocrystals by Fast Thermal Annealing Enables Catalytic Studies on Well-Defined Active Phases

A simple yet efficient method to remove organic ligands from supported nanocrystals is reported for activating uniform catalysts prepared by colloidal synthesis procedures. The method relies on a fast thermal treatment in which ligands are quickly removed in air, before sintering can cause changes in the size and shape of the supported nanocrystals. A short treatment at high temperatures is found to be sufficient for activating the systems for catalytic reactions. We show that this method is widely applicable to nanostructures of different sizes, shapes, and compositions. Being rapid and effective, this procedure allows the production of monodisperse heterogeneous catalysts for studying a variety of structure–activity relationships. We show here results on methane steam reforming, where the particle size controls the CO/CO₂ ratio on alumina-supported Pd, demonstrating the potential applications of the method in catalysis

Exceptional Thermal Stability of Pd@CeO₂ Core–Shell Catalyst Nanostructures Grafted onto an Oxide Surface

Monolayer films of highly catalytically active Pd@CeO₂ core–shell nanocomposites were grafted onto a planar YSZ(100) (yttria-stabilized zirconia, YSZ) single crystal support that was functionalized with a CVD-deposited layer of triethoxy­(octyl)­silane (TEOOS). The resulting monolayer films were found to exhibit exceptionally high thermal stability compared to bare Pd nanoparticles with the Pd@CeO₂ nanostructures remaining intact and highly dispersed upon calcining in air at temperatures in excess of 1000 K. The CeO₂ shells were also shown to be more easily reduced than bulk CeO₂, which may partially explain their unique activity as oxidation catalysts. The use of both TEOOS and tetradecylphosphonic acid (TDPA) as coupling agents for dispersing Pd@CeO₂ core–shell nanocomposites onto a high surface area γ-Al₂O₃ support is also demonstrated