Jiri Cejka

Effect of Broensted and Lewis sites in ferrierites on skeletal isomerization of n-butenes

Skeletal isomerization of n-butenes to isobutene at 620 K was followed over NaH-ferrierites (FER) with different concentration of Broensted sites, prepared by NH4+ vs. Na+ ion exchange, and over H-FER containing both, the Broensted and Lewis sites formed by zeolite dehydroxylation at temperatures ranging from 670 to 990 K. Quantitative analysis of IR spectra of OH groups and adsorbed d3-acetonitrile on ferrierite enabled determination of the concentration of Broensted and Lewis sites, and detected also those present in 8-ring ferrierite channels. With H-FER, the yield of isobutene is linearly proportional to the concentration of acidic OH groups. Formation of C3 and C5 olefins, C2–C5 paraffins and higher molecular weight products, resulting from di-, oligomerization of butenes followed by cracking/hydrogen transfer reactions, is enhanced by the presence of Lewis sites, if simultaneously present with the Broensted sites, that is in a substantial change of the selectivity of the reaction. On the other hand, both the conversion and yield of isobutene are nearly constant at decreasing concentrations of OH groups in NaH-FER, and only at low-concentration range of acidic hydroxyls a decrease in conversion was found. This indicates that the exchanged Na+ ions are placed preferably in small ferrierite channels, as also evidenced by 23Na MAS NMR, which size is not large enough to accommodate isobutene molecules. Thus, besides the well-known positive effect of the inner geometry of ferrierite channels on isobutene selectivity, it has been shown that the reaction of n-butenes to isobutene is proportional to the number of hydroxyls located in large channels of ferrierite, and simultaneous presence of Lewis sites enhances di-,oligomerization/cracking reactions leading to higher yields of by-products.

Novel Basic Mesoporous Catalysts for the Friedländer Reaction from 2‐Aminoaryl Ketones: Quinolin‐2(1H)‐ones versus Quinolines

The quinoline ring is present in a number of natural and synthetic products often exhibiting interesting pharmacological activities or physical properties. Different synthetic approaches for the preparation of quinolines have been reported; the Friedl nder reaction (FR) being one of the simplest and most efficient methods. FR is a baseor acid-catalyzed condensation of an aromatic 2-amino-substituted carbonyl compound (aldehyde or ketone) with a carbonyl derivative containing a reactive a-methylene group followed by cyclodehydration (Scheme 1). Generally, the annulation takes place

Palladium Catalysts Supported on Mesoporous Molecular Sieves Bearing Nitrogen Donor Groups: Preparation and Use in Heck and Suzuki CC Bond‐Forming Reactions

A series of supported catalysts is prepared by treatment of SBA-15-type mesoporous molecular sieve bearing [triple chemical bond]SiCH(2)CH(2)CH(2)NHCH(2)CH(2)NEt(2) groups with palladium(II) acetate. These catalysts are studied in Suzuki biaryl couplings and in Heck reactions to establish the influence of metal loading and innocent surface modifications (trimethylsilylation). The Suzuki reaction proceeded efficiently with model and practically relevant substrates; the catalyst performance increasing with an increasing degree of metalation (decreasing N/Pd ratio). Catalyst poisoning tests revealed that the reaction takes place in the liquid phase with the catalyst serving as a reservoir of active metal species and also as a stabilizing support once the reaction is performed. In the Heck reactions, on the other hand, the catalyst performance strongly changed with the reaction temperature and with the N/Pd ratio. The material with the lowest metal loading (0.01 mmol palladium per gram of material, N/Pd ratio ca. 100:1) proved particularly attractive in the Heck coupling, being highly active at elevated temperatures, recyclable, and capable of acting as a bifunctional catalyst (i.e., functioning without any external base.

Thermodynamics of CO2 adsorption on functionalized SBA-15 silica. NLDFT analysis of surface energetic heterogeneity

Siliceous SBA-15 mesoporous molecular sieves were functionalized with different amounts of 3-aminopropyl-trimethoxysilane. To obtain a more detailed insight into the material properties of the prepared samples, their textural parameters were combined with results of thermal analysis. Adsorption isotherms of carbon dioxide on parent and functionalized SBA-15 were measured in the temperature range from 273 to 333 K. From the temperature dependence of CO(2) isotherms the isosteric adsorption heats of CO(2) were determined and discussed. Information about the surface energetic heterogeneity caused by tethered 3-aminopropyl groups were obtained from CO(2) adsorption energy distributions calculated using the theoretical CO(2) adsorption isotherms derived from the non-local density functional theory. The values of isosteric heats and the energy distributions of CO(2) adsorption detect highly energetic sites and enabled quantification of their concentrations.

The role of crystallization parameters for the synthesis of germanosilicate with UTL topology.

The investigation of the critical synthesis parameters of germanosilicate of UTL topology, possessing 14- and 12-rings, has been carried out in detail. (6R,10S)-6,10-Dimethyl-5-azoniaspiro[4.5]decane hydroxide was used as the structure-directing agent (SDA). The kinetics of the synthesis, the role of the Si/Ge ratio in the synthesis mixture, and the effect of the calcination procedure were investigated in relation to the crystallinity and textural properties of the synthesized material. The optimum synthesis time was found to be six days for Si/Ge and (Si+Ge)/SDA molar ratios of 2 and 1.7, respectively. The UTL zeolite crystallizes as small sheets of 10 mum in size. The micropore volume of the best crystals is 0.22 cm(3) g(-1) with a micropore diameter of 1.05 nm, based on DFT and Saito-Foley analyses of adsorption data.

Mutable Lewis and Brønsted acidity of aluminated SBA-15 as revealed by NMR of adsorbed pyridine-15N.

(1)H and variable-temperature (15)N NMR techniques have been used to study the effect of the gradual alumination of SBA-15 on the structure and adsorption properties of this mesoporous material. The interpretation of experimental spectra suggests that aluminum chlorhydrol most effectively reacts with silica surfaces in the confinement of the cavities of rough mesopore walls, instead of forming a homogeneous aluminum film. This first leads to a gradual filling of the cavities and finally results in aluminum islands on the inner surfaces of mesopores. In the sample with a Si/Al atomic ratio of 4.1, up to half of the inner surface area of the mesopores is covered with aluminum. The alumination produces Brønsted acid sites attributed to silanol groups interacting with aluminum but does not affect the proton-donating ability of isolated silanol groups. At high Si/Al ratios, the surface contains only one type of Lewis site attributed to tetracoordinated aluminum. At lower Si/Al ratios, Lewis acid sites with a lower electron-accepting ability appear, as attributed to pentacoordinated aluminum. The numerical values of the surface densities of all chemically active sites have been estimated after annealing at 420 and 700 K. We were surprised to observe that gaseous nitrogen can occupy Lewis acid sites and hinder the interaction of the aluminum with any other electron donor. As a result, aluminated surfaces saturated with nitrogen do not exhibit any Brønsted or Lewis acidity. At room temperature, it takes days before pyridine replaces nitrogen at the Lewis acid sites.

Deactivation and Coking of Hzsm5 Catalysts During Alkylation Reactions

The catalytic methylation of toluene over HZSM5 post-synthesis modified by silylation was studied in order to understand the dependence of the catalysts activity, selectivity and stability upon type and concentration of sorbed surface species. Activity and selectivity are strongly influenced by the concentration of slowly diffusing products (e.g. m-xylene, trimethylbenzenes) in the zeolite pores. While small particle ZSM5 samples did not deactivate, large particle HZSM5 and surface silylated samples decreased in activity with time on stream. Deactivation was attributed to blocking of the catalytically active sites by trimethylbenzene molecules which cannot dealkylate or leave the catalyst under our reaction conditions (570K). The more severe the steric restrictions for the transport of the bulkier products in the pores are, the higher is the p-selectivity of the sample and the stronger is the effect of deactivation due to site blocking.

Catalytic performance of Metal-Organic-Frameworks vs. extra-large pore zeolite UTL in condensation reactions

Catalytic behavior of isomorphously substituted B-, Al-, Ga-, and Fe-containing extra-large pore UTL zeolites was investigated in Knoevenagel condensation involving aldehydes, Pechmann condensation of 1-naphthol with ethylacetoacetate, and Prins reaction of β-pinene with formaldehyde and compared with large-pore aluminosilicate zeolite beta and representative Metal-Organic-Frameworks Cu3(BTC)2 and Fe(BTC). The yield of the target product over the investigated catalysts in Knoevenagel condensation increases in the following sequence: (Al)beta < (Al)UTL < (Ga)UTL < (Fe)UTL < Fe(BTC) < (B)UTL < Cu3(BTC)2 being mainly related to the improving selectivity with decreasing strength of active sites of the individual catalysts. The catalytic performance of Fe(BTC), containing the highest concentration of Lewis acid sites of the appropriate strength is superior over large-pore zeolite (Al)beta and B-, Al-, Ga-, Fe-substituted extra-large pore zeolites UTL in Prins reaction of β-pinene with formaldehyde and Pechmann condensation of 1-naphthol with ethylacetoacetate.

Contribution of framework and extraframework Al and Fe cations in ZSM-5 to disproportionation and C3 alkylation of toluene

Publisher Summary This chapter describes contribution of framework and extra framework A1 and Fe cations in ZSM-5 to disproportionation and C3 alkylation of toluene. Toluene disproportionation and its C3 alkylation with isopropanol have been investigated over series of alumo- and ferrisilicates of ZSM-5 structure. It has been shown that the reaction rate for toluene disproportionation is linearly proportional to the number of both Si-OH-A1 and Si-OH-Fe groups, the latter sites exhibiting a lower activity in agreement with their lower acid site strength. A simultaneous presence of Lewis and Broensted sites does not significantly affect toluene disproportionation, but enhances toluene dealkylation, leading to a higher selectivity to benzene. On the contrary, while the rate of toluene disproportionation is controlled by the molecular sieve intrinsic activity, for toluene C3 alkylation the reaction rate is controlled by the rate of the product desorption, which is assumed to be faster on ferrisilicates, because of a lower acidity of the Si-OH-Fe groups. Therefore, all the ferrisilicates exhibit a higher toluene conversion in the alkylation reaction compared to alumosilicates.

Siting of Co(II), Zn(II) and Cu(i) ions in (A1)MCM-41

Siting of metal ions (Zn2+, Co2+, Cu+) in the cationic sites of (A1)MCM-41 matrix has been investigated employing UV-VIS-NIR DR spectroscopy and UV-VIS emission spectroscopy. Four types of cationic sites were identified in dehydrated (Al)MCM-41. Divalent (Zn2+or Co2+) ions are accommodated only in two sites. Cu+ ions in reduced, Cu2+ ion exchanged (Al)MCM-41, occupy four types of cationic sites. Two sites are accessible for divalent cations, other two only for monovalent cations. Distribution of cations among sites depends on the metal ion loading in molecular sieve.