A General Method for Aluminum Incorporation into High-Silica Zeolites Prepared in Fluoride Media

The fluoride method of zeolite synthesis yields materials with unique characteristics such as high Si/Al ratio, large crystal size, and hydrophobic properties, and it has been advantageous for the synthesis of new pure-silica or high-silica zeolites. It is often difficult, however, to incorporate aluminumand thus bring about useful catalytic propertiesin materials prepared through the fluorite method. In this report, we show that FAU-type zeolites are an effective source of aluminum to the growing crystals in the high-silica LTA-type zeolite synthesis (ITQ-29). A key advantage of using aluminosilicate zeolite crystals as aluminum source in fluoride media was the high reproducibility and easy control of the Si/Al ratio of the product. The broad applicability of this methodology was demonstrated in the synthesis of several high-silica zeolites in fluoride media: we report the synthesis of aluminosilicate ITQ-12 (ITW-type zeolite). Other more conventional aluminosilicate zeolites (CHA-, *BEA-, and STT-type) were also synthesized by using this methodology. The Si/Al ratio of the final products was controlled by the amount of aluminosilicate zeolite added to the synthesis gel. All the products obtained had the typical features of a fluoride mediated synthesis, and it was found that the thermochemical stability of the aluminum source and seed crystals was an important factor. This simple methodology could be useful for aluminum incorporation into many novel siliceous zeolites, broadening their potential as catalytic materials

Radical Cation Intermediates in Propane Dehydrogenation and Propene Hydrogenation over H‑[Fe] Zeolites

This report investigates the mechanistic relationship between the monomolecular propane dehydrogenation reaction and the reverse reaction, the propene hydrogenation, over H-[Fe]­ZSM-5 catalysts. It is shown that the difference in the apparent activation energies of the forward and reverse reactions is equal to the reaction enthalpy (∼130 kJ mol^–1) and that the rate constants of the reactions have an isokinetic relationship. The ratios of the rate constants of the forward to the reverse reactions are equal to the equilibrium constant (e.g., <i>K</i>_P ≈ 0.033 bar at 773 K) even if the reactions occur separately, away from equilibrium. The results are consistent with the principle of microscopic reversibility and indicate that both forward and reverse reactions are structurally related and proceed through the same elementary steps and reaction intermediates. The pattern of selectivity, the activation energy, and the estimated enthalpy and entropy of formation of the transition states in H-[Fe]­ZSM-5 are very different from the observed values for the isostructural H-[Al]­ZSM-5, indicating that despite their structural similarities the reactions proceed through different mechanisms in each catalyst. Analysis of the energy change along the reaction coordinate, including the reaction enthalpy and the apparent activation energies, suggests that in H-[Fe]­ZSM-5 the reaction proceeds through radical cation-like intermediates. Analysis of a putative reaction mechanism and the energetics of electron transfer in the zeolite channels shows that dehydrogenation of propane is kinetically favored (as observed) over cracking of propane because ethene radical cations are less stable than propene radical cations

Carbon Dioxide and Nitrogen Adsorption on Cation-Exchanged SSZ-13 Zeolites

Samples of high-silica SSZ-13, ion exchanged with protons and alkali-metal cations Li⁺, Na⁺, and K⁺, were investigated using adsorption isotherms of CO₂ and N₂. The results show that Li-, Na-SSZ-13 have excellent CO₂ capacity at ambient temperature and pressure; in general, Li-SSZ-13 shows the highest capacity for N₂, CO₂ particularly in the low-pressure region. The effect of cation type and Si/Al ratio (6 and 12) on the adsorption properties was investigated through analysis of adsorption isotherms and heats of adsorption. The separation of CO₂ in a flue gas mixture was evaluated for these adsorbents in the pressure swing adsorption and vacuum pressure adsorption processes

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

Catalysis of the Diels–Alder Reaction of Furan and Methyl Acrylate in Lewis Acidic Zeolites

Diels–Alder (DA) reactions of furans yield oxanorbornene derivatives which can be converted to a variety of molecules, ranging from molecules of biological interest to naturally occurring organic compounds, and to aromatics via dehydration, a promising alternative for the synthesis of aromatics from renewables. With furan being one of the less reactive dienes, the development of Lewis acidic heterogeneous catalysts, without the shortcomings of the traditional homogeneous catalysts, is critically important. Herein, we use computational chemistry to study the DA reaction of furan and methyl acrylate in three zeotypic Lewis acids, Sn-, Zr-, and Hf-BEA. We find that all three exhibit the same ability to enhance the electrophilic character of the dienophile and promote modest charge transfer from the diene. Despite being moderately Lewis acidic, they still achieve a reduction of about 12.5 kcal/mol in the activation energy relative to the reaction in the absence of catalyst

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

Elucidation of Diels–Alder Reaction Network of 2,5-Dimethylfuran and Ethylene on HY Zeolite Catalyst

The reaction of 2,5-dimethylfuran and ethylene to produce <i>p</i>-xylene represents a potentially important route for the conversion of biomass to high-value organic chemicals. Current preparation methods suffer from low selectivity and produce a number of byproducts. Using modern separation and analytical techniques, the structures of many of the byproducts produced in this reaction when HY zeolite is employed as a catalyst have been identified. From these data, a detailed reaction network is proposed, demonstrating that hydrolysis and electrophilic alkylation reactions compete with the desired Diels–Alder/dehydration sequence. This information will allow the rational identification of more selective catalysts and more selective reaction conditions

H₂ Adsorption on Cu(I)–SSZ-13

We report H₂ adsorption capacities reaching 0.05 wt % at 303 K and at 1 atm H₂ pressure on solid-state CuCl-exchanged [Al]-SSZ-13 and [B]-SSZ-13 zeolites. Differential heat of H₂ adsorption is found in the range between 16 and 48 kJ mol H₂^–1 on Cu­(I)-SSZ-13 at 323 K and isosteric heat of adsorption is found between 20 and 55 kJ mol H₂^–1 on Cu­(I)-[B]-SSZ-13 at temperatures between 293 and 323 K. Strong interactions between H₂ and the copper cations in Cu­(I)-SSZ-13 are evidenced using Rietveld refinements of neutron powder diffraction patterns revealing Cu–deuterium (D₂) distances of 2.3(2) and 2.41(1) Å. A temperature-dependent Cu­(I) migrationaway from the six-membered ring (6MR) and eight-membered ring (8MR)is related to the high adsorption capacities of the samples at 303 K. At 10 K, access to Cu­(I) at 6MR is sterically hindered by framework oxygen atoms (Cu–O_framework distance of 2.196(5) Å), rationalizing the low H₂ adsorption capacities of Cu­(I)-SSZ-13 samples as compared to Na-SSZ-13 at 77 K

H₂ Adsorption on Cu(I)–SSZ-13

We report H₂ adsorption capacities reaching 0.05 wt % at 303 K and at 1 atm H₂ pressure on solid-state CuCl-exchanged [Al]-SSZ-13 and [B]-SSZ-13 zeolites. Differential heat of H₂ adsorption is found in the range between 16 and 48 kJ mol H₂^–1 on Cu­(I)-SSZ-13 at 323 K and isosteric heat of adsorption is found between 20 and 55 kJ mol H₂^–1 on Cu­(I)-[B]-SSZ-13 at temperatures between 293 and 323 K. Strong interactions between H₂ and the copper cations in Cu­(I)-SSZ-13 are evidenced using Rietveld refinements of neutron powder diffraction patterns revealing Cu–deuterium (D₂) distances of 2.3(2) and 2.41(1) Å. A temperature-dependent Cu­(I) migrationaway from the six-membered ring (6MR) and eight-membered ring (8MR)is related to the high adsorption capacities of the samples at 303 K. At 10 K, access to Cu­(I) at 6MR is sterically hindered by framework oxygen atoms (Cu–O_framework distance of 2.196(5) Å), rationalizing the low H₂ adsorption capacities of Cu­(I)-SSZ-13 samples as compared to Na-SSZ-13 at 77 K

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