Ludmila Kubelková

Various types of Ga in MFI metallosilicates: characterization and catalytic activity

Abstract Ga was introduced into MFI structures in three ways: direct synthesis, postsynthetic modification of boralites, and `solid state exchange with HZSM-5. The state of Ga species was characterized by 71 Ga MAS NMR, XRD and TPR; additional information was provided from acidity measurements, which were carried out by FTIR spectroscopy of adsorbed acetonitrile-d3 and by TPD of ammonia. The catalytic activity of samples prepared was checked by the oxidative dehydrogenation and aromatization of ethane. The methods used showed that the direct synthesis leads to the substitution of T atoms by Ga and to a relatively slight decrease in acidity compared to H[Al]ZSM-5. The reduction of Ga starts above 600°C. The postsynthetic modification of boralites yields Ga both in framework and extraframework positions; with increased Ga content an extraphase Ga is formed, which most probably belongs to GaO(OH). Two types of acid centers appear and Ga species are much more readily reducible, but with increasing Ga content its reduced fraction substantially decreases. Ga introduction by `solid state exchange does not change the acidity strength of HZSM-5, the extent of reduction is relatively low but a part of Ga is reduced at ∼350°C. Catalytic activity of the catalysts except the latter sample is low which implies that the presence of strong acid sites together with easily reducible Ga species enhances the oxidative dehydrogenation as well as aromatization of ethane. In some cases, the CO oxidation by 18 O 2 was examined and it was found that the oxygen of the zeolite lattice is incorporated into CO2.

Catalytic conversion of oxygen containing cyclic compounds. Part I. Cyclohexanol conversion over H[Al]ZSM-5 and H[B]ZSM-5

Abstract Cyclohexanol (CHL) conversion was measured under low-pressure on-stream conditions on HZSM-5 zeolites and on H-boralite using small amounts of catalysts. Catalytic runs were followed by TPD of the surface species, which also were studied using FTIR spectroscopy. It was found that the individual reaction steps, low-temperature dehydration (yielding cyclohexene), skeletal isomerization (ring contraction to methylcyclopentene) and reactions involving H-transfer (formation of methylcyclopentane, substituted aromatics and simple olefins) were conditioned by the presence of strong acid sites whose number needed for these reactions increased from the dehydration to the H-transfer reactions. The CHL reaction was compared with the reactions of cyclohexene, and the effect of Lewis acid sites present in some cases together with the Bronsted sites was discussed. Ammonia was found to block the active centers, reduce the isomerization and especially the reactions involving H-transfer. Contrary to ammonia, methanol reacted with the CHL surface complexes yielding more substituted aromatics and more olefins than CHL or methanol alone (and/or simple sum of CHL and methanol products).

Amination of cyclohexanol over metallosilicate-based materials

Abstract Reductive amination of cyclohexanol over acidic and nickel containing metallosilicates was studied at atmospheric pressure and different temperatures. Pure H + as well as Ni 2+ charge-compensating cations were found to be inactive in this reaction. Experiments carried out over Ni-impregnated metallosilicates showed that the catalyst activity and selectivity to cyclohexylamine rose with the content of Ni 0 to 10xa0wt%, and remained practically constant between 10–40xa0wt%. On the contrary, both the conversion and selectivity to cyclohexylamine continued to increase with increasing Ni content over the Ni/SiO 2 catalyst within the same concentration range. The nickel dispersions followed a trend similar to the conversions.

Activation of reactants by hydroxyl groups of solid acids. An FTIR study

The comparison of the FTIR spectra of acetonitrile (AN), pyridine (Py) and acetone (AC)adsorbed at room temperature on the zeolite in sequence HX HY/2.2, HY/2.5, HY/2.9, HM and HZSM-5 was employed for the discussion of the proton transfer between zeolite bridging hydroxyls and adsorbed acetone. It was shown that the absolute values of the shifts, of stretching (v) and bending (δ, γ) vibrations of OH caused by the formation of “neutral” AH...HO(zeol) complex increase with acid strength of hydroxyls while the shifts, of v(NH) for “ion-pair” complex PyH+…−O(zeol) increase with increasing basicity of the skeletal oxygens. FTIR spectra of adsorbed acetone are explained by Fermi resonance interactions of v(OH), 28(OH) and 2γ(OH) of (CH3)2CO…HO(zeol) complex. This complex is abundant in all the zeolites studied and its interaction energy is higher than that of AN…HO(zeol). The hydrogen transfer, leading to aldol condensation, is most probably conditioned by the presence of additional acetone molecules in physisorbed state.

H- and Cu- Forms of MFI Boralites with Enhanced Number of Skeletal Boron Atoms. Synthesis and Properties

Abstract Preparation, physico-chemical properties and catalytic activity in the selective reduction of NO are described for CuH-[B]ZSM-5 metallosilicates with relatively high number of B atoms inside the skeleton and, consequently, unusually small unit cell. Na content is so small that Na+compensation of skeleton charge can be neglected. It is shown that Cu ions are introduced into the boralites directly during the synthesis if tetrapropylammonium TPA+template is replaced by ethylamine which forms complexes with Cu2+. These complexes are located inside the boralite channels together with protonated ethylamine, and on the outer surface of zeolite grains. EAH+is easily decomposed already at 200°C while EA ligands on Cu2+are removed at 350-400°C. The decomposition in oxygen (at 450°C) and the rehydration are accompanied with the migration of Cu2+ions from the outer surface to the interior where they remain in the form of isolated and small oxidic species. Simultaneously, some of the tetrahedral B atoms change the coordination to the trigonal one. The Cu samples prepared by direct synthesis are compared with (EA)-boralites impregnated with Cu2+and with CuH-boralites prepared by ionic exchange. Synthesized Cu boralites were found to be good catalysts for selective reduction of NO by ammonia to N2.

Interaction between CD3OH and a H-mordenite studied by 1H NMR, broad-line at 4 K and MAS at 300 K

The simulation of 4 K 1H broad-line NMR spectra of CD3OH interacting with the acidic OH groups of H-mordenite zeolite shows no CD3OH2+ ions but hydrogen-bonded complexes. For high CD3OH concentrations, clusters of hydrogen-bonded methanol molecules are formed, also hydrogen-bonded to Brønsted acid sites of the zeolite. The 300 K 1H MAS spectra prove that the desorbed H-mordenite sample contains also silanol groups in electrostatic interaction with the framework; these silanols do not interact with methanol under our experimental conditions.

Vibrational spectra of hydrogen-bonded complexes on zeolite surfaces as a benchmark for evaluating performance of ab initio methods. Complex with the pyridinium ion

Abstract Vibrational spectroscopy of molecules hydrogen-bonded to bridging hydroxyls in zeolites is suggested as a promising tool for the direct comparison with gas-phase ab initio results. As an illustration, the FT-IR spectra of pyridine adsorbed on H-Y and H-mordenite zeolites which differ in their acid strength while having the same local structure of the interacting site, are compared with HF 6–31 G calculations of the (zeol)OH⋯pyridine complexes. For comparison purposes, adsorption on weakly acidic silicagel was also studied. Within the double minimum potential for the proton transfer, HF 6–31 G spectra of both the ‘ion-pair’ and ‘neutral’ hydrogen bonded complexes were calculated. HF 6–31 G calculations strongly overestimate observed frequency shifts, but this discrepancy can be successfully corrected through few empirical scale factors. It was shown that within the each stationary state, effects of varying zeolite acid strength on the vibrational spectra can be reproduced by modifying these scale factors. For pyridine adsorbed at HY zeolite, a coexistence of the ‘neutral’ and ‘ion-pair’ types of hydrogen bonds in the sample was observed.

Catalytic conversion of oxygen containing cyclic compounds. Part II. Cyclohexanone conversion on HZSM-5 zeolites

Abstract Low-pressure on-stream conversion of cyclohexanone (CHN) was studied over HZSM-5 zeolites with various Si/ Al ratios (different number of strong acid sites). Each conversion run was followed by thermal decomposition of surface species which also were investigated using FTIR spectroscopy. It was found that, below 300°C, surface complexes consisting predominantly of CHN were held in the zeolites. The release of products, consisting of dehydrated CHN (C 6 H 8 ), substituted aromatics and olefins, began above 250°C. The latter two compounds were formed via intermediates most probably of the aldol type (however, the dimerized methylcyclopentadiene intermediate cannot be excluded). The CHN dehydration to C 6 H 8 predominated at 480°C. The yield of all products increased with the increasing number of zeolite strong acid sites. Ammonia, co-fed or preadsorbed, reacted with CHN to surface imines which hindered all other CHN reactions. The imine was strongly held in zeolites and decomposed at about 450°C to gaseous C 6 H 8 and ammonia. Methanol partially methylated the reaction products but did not affect the main reaction routes, monomolecular dehydration and secondary intermolecular reactions.

Activation of alcohols and ketones by surface hydroxyls of strong solid acids

Preferential formation of “neutral” hydrogen bonded complexes of acetone and methanol on the surface of H-zeolites of various acid strength at room temperature is evidenced using Fourier-transform infrared and 1 H broad-line NMR spectroscopies together with quantumchemical calculations. It is shown that, compared to solutions, strongly acidic surfaces are less able to stabilize protonized “ion-pair” forms of adsorbed reactants if surface coverage is low. At high cavity filling the reaction medium is more similar to that in solutions and reactivity is enhanced.

Cyclohexanol and cyclohexanone reactions on HZSM-5 zeolites

Reactions of cyclohexanol (CHL) and cyclohexanone (CHN) with HZSM-5 zeolites differing in the number of strong acid sites were investigated under low-pressure on-stream conditions with small amounts of catalysts at various temperatures which allowed to come nearer to the primary reaction steps. The surface species formed during the catalytic runs were decomposed by temperature programmed heating. Gaseous products were analysed using a mass spectrometer. Surface species were formed during the catalytic runs below 300°C, most probably from CHL via reactions of cyclohexene (the primary reaction product) and from CHN via dimerized methylcyclopentadiene, product of the CHN dehydratation. Concentration of the surface species increased with increasing number of zeolite strong acid sites and lowered the catalytic activity. The reaction route of CHN changed at higher temperature from the formation and decomposition of condensed surface intermediates to the intramolecular dehydration.