Jana Nováková

Reactivity of surface species on zeolites in methanol conversion

Abstract The surface species formed by the interaction of methanol with HY and HZSM-5 zeolites modified with nonskeletal Al were examined using FT-IR spectroscopy and temperature-programmed desorption accompanied by conversion of preadsorbed methanol with mass spectrometric detection of the products evolved. Physisorbed methanol perturbed by various surface sites, methylcarboxonium ions, skeletal methoxyls , methoxyls on nonskeletal aluminium Al n O(CH 3 ), and methoxyls substituting for silanol groups were found on both zeolite types. Their occurrence depended on the zeolite composition and the reaction temperature. The role of skeletal methoxyls and Al n OCH 3 in the methanol conversion is discussed. In addition to the reaction of preadsorbed methanol the conversion over ZSM-5 catalysts under on-stream conditions was found to be sensitive to the presence of nonskeletal Al n .

Surface reactivity of ZSM-5 zeolites in interaction with ketones at ambient temperature (a FT-i.r. study)

Surface complexes formed at ambient temperature from acetone, acetone- d 6 , diethyl ketone, diacetone alcohol, and mesityl oxide on HZSM-5, dehydroxylated HZSM-5, and NaZSM-5 zeolites were studied using FT i.r. spectroscopy. It was found that the C=O bond in ketones is perturbed far more by the bridging hydroxyls than by electron-accepting sites, Al and Na ions, and that this effect is greater for the more basic diethyl ketone than for acetone. The bridging hydroxyls cause the formation of protonated ketones. In contrast to diethyl ketone, protonated acetone is aldolized by the reaction with the physisorbed molecule to form diacetone alcohol, which is immediately dehydrated to strongly adsorbed mesityl oxide at room temperature. Acetone does not react in the cavities of NaZSM-5, but dehydration of diacetone alcohol occurs readily, most probably due to the effect of Al Lewis sites present in small concentrations.

Primary reaction steps in the methanol-to-olefin transformation on zeolites

The interaction of methanol with HZSM-5 and dealuminated HY zeolites was studied by temperature-programmed desorptionmass spectrometric detection and infrared spectroscopy of surface compounds. The increased amount of preadsorbed methanol andor methanol added during the thermal desorption at 250-400/sup 0/C changed the composition of gaseous products over HZSM-5 from methane and formaldehyde to C/sub 2/..sqrt../sub 5/ aliphatic compounds, mainly ethylene and propylene, while over dealuminated HY only the latter products appeared. The reaction of gaseous methanol with very reactive surface C/sub 1/ species is assumed to be responsible for the formation of the first C-C bonds on both zeolite types

Oxidation of methanol and formaldehyde on Fe2O3 in comparison with MoO3 and a mixed Mo6+Fe3+O catalyst

The oxidation of methanol and formaldehyde was followed on Fe2O3 at 220 °C with and without oxygen in the gaseous phase. The main difference between these two processes is the formation of hydrogen during the reaction without oxygen and the evolution of water in the opposite case. The effect of small amounts of preadsorbed pyridine on the oxidation of methanol was also studied. The inhibition effect of preadsorbed pyridine on methanol oxidation on the mixed Mo6+Fe3+O catalyst together with the infrared spectra of adsorbed pyridine may be explained by a cation-ligand interaction of both reactants with the transitional metal ions of the catalyst. The behavior of the mixed Mo6+Fe3+O catalyst in the oxidation of methanol resembles in many aspects that of MoO3 in an unreduced state.

High nuclearity Pt carbonyls in alkali-metal X zeolites

Diffuse reflectance time-resolved UV–VIS spectroscopy has been used together with FTIR spectroscopy and 13CO–12CO isotopic exchange for the investigation of anionic Pt carbonyl complexes [Pt3(CO)3(µ-CO)3]n2– formed in alkali-metal X zeolites (alkali metal = Li+, Na+, K+ and Cs+) from [Pt(NH3)4]2+. It is shown that, compared with solutions, the zeolite matrix does not alter electronic transitions, while the vibrational frequencies of the CO ligands are appreciably changed. This latter effect is explained by the interaction of linear CO ligands with oxygen atoms of the zeolite lattice [an upward frequency shift of CO stretching vibration ν(CO)1] and the location of the bridging COs in the vicinity of alkali-metal cations [a downward frequency shift of CO stretching vibration ν(CO)b]. The effect of increasing the nuclearity of Chini complexes on the increase of ν(CO)1(at 2000–2100 cm–1) is much higher than the influence of increasing the electropositivity of the alkali-metal cations in the X matrix. Inside all the alkali-metal X zeolites and under all carbonylation conditions used, Pt6 species (n= 2) are formed. The decreasing size and electropositivity of alkali-metal cations in the sequence Cs+ > K+ > Na+ > Li+ assist in stacking of more triangular units and the appearance of the Pt9 and Pt15(n= 3 and 5, respectively) carbonyl complexes.

Reaction of ammonia with surface species formed from acetone on a HZSM-5 zeolite

Temperature-programmed conversion of preadsorbed acetone on HZSM-5 zeolite was studied using the mass spectrometric detection of the gaseous products. The effect of addition of ammonia to the surface complexes formed from acetone at mild temperatures on the composition of the products was investigated. The surface species, both with and without ammonia addition, were also characterized using 13 C cross-polarization magic angle spinning nuclear magnetic resonance. It was found that the added ammonia reacts with the ketonicm group of the surface intermediates and substantially affects the composition of the gaseous products.

Characterization of molybdenum- and/or nickel-containing zeolites, used as catalysts for hydrodesulfurization

Studies were carried out on the effect of nickel and molybdenum on the state of the crystal lattice of Y and ZSM-5 zeolites, on the reducibility of the metals, and on the acidity of the catalysts obtained. Data on the metal-zeolite interaction and on the reducibility of the metals were obtained by temperatureprogrammed reduction. The acidity of the samples was tested toward the reaction of ethylbenzene disproportionation. By infrared spectroscopy and X-ray diffraction, it is proved that the introduction of Mo does not lead to a noticeable breakage of the zeolite crystal structure. Mo reacts with the zeolite lattice by forming at a content of approximately 15% a three-dimensional MoO 3 phase. The simultaneous presence of Ni and Mo causes partial breakage of the zeolite structure. This may be related to the strong interaction of Mo with the zeolite lattice in the case of NiMo zeolites. The high acidity of Ni zeolites facilitates the interaction of Mo with the zeolite

Contribution to the study of methanol oxidation on MoO3

The oxidation of methanol on molybdenum trioxide (with its lattice oxygen only) was investigated. The composition of the gaseous phase analyzed by means of a mass spectrometer changed after the reduction of the oxygen surface layer. A change of the oxidation mechanism due to the lowering of the valency of Mo6+ to Mo4+ is suggested.

A comment on the oxidation of coke deposited on zeolites

Abstract Temperature-programmed oxidation of coke formed during acetone conversion on HZSM-5 and HY zeolites with continuous detection of all the products is described using dioxygen as the oxidation agent. The effects of the following factors were studied: (i) the coke composition, which was changed by pyrolysis prior to oxidation, (ii) the amount of coke formed during acetone conversion on HZSM-5, and (iii) the different reactants, also using diethylketone and isobutene for conversion over HZSM-5. It appears that the zeolite structure is the predominant factor in the oxidation of coke, affecting the oxidation temperature as well as the CO CO 2 ratio in the products.

Catalytic activity of dealuminated Y and HZSM-5 zeolites measured by the temperature-programmed desorption of small amounts of preadsorbed methanol and by the low-pressure flow reaction of methanol

The activity of HZSM-5, NH4Y and Y zeolites, dealuminated using SiCl4, in methanol transformation has been compared and correlated with the acid properties and the Si:Al ratio of the respective zeolites. The temperature-programmed desorption of small amounts of methanol preadsorbed at ambient temperature together with measurements in a flow reactor under low pressure allowed the primary reaction steps to be observed. HZSM-5 and Y zeolites which had been moderately dealuminated showed the highest activity; however, the product distribution was different above 630 K:HZSM-5 gave predominantly methane and formaldehyde (aromatics appearing at higher temperatures), while C3–5 olefins and aromatics were formed above the Y zeolite. HY and highly dealuminated Y zeolite with Si/Al 20 were less active than moderately dealuminated Y and HZSM-5 zeolites. The high activity of the latter zeolites is caused by the action of strong proton-donor sites and electron-acceptor centres (extralattice aluminium in dealuminated zeolites). The difference between HZSM-5 and dealuminated Y zeolites is caused by their different structures, which lead to different reaction mechanisms.For comparison, the temperature-programmed desorption of preadsorbed ethanol was also studied. This resulted predominantly in ethene formation via intramolecular dehydration of ethanol.