Heinz Berndt

Zinc promoted H-ZSM-5 catalysts for conversion of propane to aromatics II. Nature of the active sites and their activation

Abstract Zn H - ZSM -5 catalysts, prepared by ion exchange (Zne) and impregnation (Zni) of a parent H-ZSM-5 as well as by physical mixing of ZnO with H-ZSM-5 (Znm) were characterized by TPR H 2 , TPSR CO and TPDA. Their catalytic activity after thermal pretreatment in air or hydrogen was determined in the conversion of propane. Zinc species in Zni and Zne were hardly reducible. TPSR CO revealed a generation of CO2 and H2 explained as a reaction of [Zn(OH)]+ ions with CO. The corresponding TPSR CO profiles indicated two peaks denominated as α- and β-peak and attributed to [Zn(OH)]+ ions located in the channels (α-species) or on the external surface of the zeolite crystals (β-species). Residual ZnO, not reacting with CO was denominated as γ-species, presenting ZnO outside the crystals. Zni and Znm samples could be activated by pretreatment in hydrogen. These samples also exhibited a self activation if they were mildly calcined before the testing. A correlation between the content of α-species and the activity has been demonstrated. TPSR CO has proved to be a suitable new method for the determination of the active portion of zinc in Zn H - ZSM -5 catalysts.

Zinc promoted H-ZSM-5 catalysts for conversion of propane to aromatics I. Acidity and activity

Abstract Zinc promoted H-ZSM-5 catalysts were prepared by ion exchange (Zne) and by impregnation (Zni). The acidity of these samples was characterized by the TPD of ammonia and by FTIR spectroscopy, recording the OH vibration spectra and the spectra of adsorbed pyridine and ammonia. The method of preparation had minor influence on the activity in the conversion of propane over samples containing less than 1.5 wt.-% Zn. However, Zne samples with Zn contents of 1.5 wt.-% and more were much more active than Zni samples. The former displayed an increased number of Lewis sites and a decreased number of Bronsted sites. Otherwise, partial pore blocking with increasing zinc loading seems to limit the promoting effect in case of Zni samples. The results demonstrate that ionic zinc species determine the catalytic properties, acting as strongly acidic Lewis sites.

Oxygen adsorption on Au/Al2O3 catalysts and relation to the catalytic oxidation of ethylene glycol to glycolic acid

Abstract Static oxygen adsorption and hydrogen pulse titration of chemisorbed oxygen were checked and successfully used as tools for characterisation of the dispersity and surface area of Au/Al 2 O 3 catalysts tested in the oxidation of ethylene glycol. Using a stoichiometry of Au s :O=2 a good agreement was obtained between the average gold particle size calculated from oxygen chemisorption results and the particle size preferentially observed by TEM. A correlation of the catalytic activity with the oxygen adsorption of Au/Al 2 O 3 catalysts having different Au dispersity and content showed a steady increase of the rate of the glycolic acid formation with increasing oxygen chemisorption and Au surface area, respectively. No differences in the selectivity of the reaction were observed with low- and high-disperse Au/Al 2 O 3 catalysts, a selectivity to glycolic acid higher than 95% was obtained on all the samples. Therefore, we could clearly demonstrate that the selective oxidation of ethylene glycol to glycolic acid is not a structure-sensitive reaction, i.e. this reaction is not restricted to a defined gold particle size, as for example in the epoxidation of propylene over Au/TiO 2 .

FTIR investigation of surface intermediates formed during the ammoxidation of toluene over vanadyl pyrophosphate

Abstract The mechanism of formation of surface intermediates, appearing during the interaction of vanadyl pyrophosphate (VO) 2 P 2 O 7 with feed components of the toluene ammoxidation was studied by FTIR spectroscopy. The investigation of ammonia adsorption at elevated temperature showed protonated and coordinated ammonia as expected as well as the generation of amido species; all could be so called ‘nitrogen insertion species’ or a source of these at least. The interaction of toluene and possible reaction intermediates such as benzaldehyde and benzylamine with (VO) 2 P 2 O 7 was studied. The investigations revealed that the ammoxidation of toluene probably proceeds via the formation of a benzaldehyde intermediate. Afterwards, benzylimine surface species were probably formed by a N-insertion, using NH + 4 surface species followed by the formation of benzonitrile by subsequent oxidative dehydrogenation. However, no benzamide species were observed. The surface species generated upon adsorption of benzaldehyde were similar to those formed from toluene, indicating the role of the former as intermediate in the nitrile formation path. Otherwise, the adsorption of benzylamine in the presence of oxygen did not lead to the formation of benzonitrile. Therefore, an ammoxidation mechanism of toluene via a benzaldehyde intermediate is preferred and reaction pathways via benzamide or benzylamine as intermediates seem to be improbable.

Redox interaction of ammonia with (VO)2P2O7

The interaction of ammonia with (VO)2P2O7 prepared by calcination of the precursor compound VOHPO4· 0.5H2O under nitrogen has been studied using temperature-programmed desorption of ammonia (TPDA), temperature-programmed reaction spectroscopy (TPRS), and IR and EPR spectroscopy. Mass-spectrometric detection was applied to observe possible ammonia decomposition or oxidation products. The investigation revealed that ammonia is not only adsorbed on but also reacts with (VO)2P2O7 in a redox process generating nitrogen, water and an amorphous VIII-containing compound, the concentration of which could be directly determined by potentiometric titration. The high amount of VIII found pointed towards reduction of VIV not only on the surface but also in deeper layers of the bulk. This was also confirmed by EPR spectroscopy. Furthermore, this reaction results in a change of the Bronsted and Lewis acidity observed by IR spectroscopy. The concentration of the Bronsted-acid OH groups was strongly enhanced by hydrolysis of P—O—P and/or V—O—P links by water formed during the redox reaction. The increased concentration of Lewis sites was caused by the removal of oxygen from surface vanadyl groups, probably creating additional coordinatively unsaturated sites. The influence of the observed redox reaction on the characterization of the acidity and the formation of VPO catalysts in the ammoxidation reaction are discussed.

Study on the nature and the redox properties of cobalt species located in CoAPO molecular sieves

The redox active properties of CoAPO-5 and CoAPO-44 molecular sieves have been investigated by temperature-programmed reduction, oxidation and desorption. Furthermore, the acidities of the ‘reduced’ and ‘oxidized’ forms of the samples were compared by means of temperature-programmed desorption of ammonia and FTIR spectroscopy using pyridine as probe molecule. Additionally, the reactions of ‘oxidized’ samples with gaseous cyclohexanol or 2-propanol were studied by a pulse technique. All these studies were aiming at a critical reinvestigation of the nature of the redox active sites of CoAPO-n materials and at a quantitative determination of the oxidizable/reducible cobalt species. Neither evidence for a valence state change of an essential portion of framework cobalt ions nor for a significant adsorption of oxygen on such sites were obtained. Therefore, the well known colour changes during oxidative or reductive treatment of CoAPO-n does not necessarily indicate redox processes of the framework cobalt as suggested by many researchers. Rather, the observed redox processes could be attributed to reactions of hydrogen, oxygen and alcohols with non-framework cobalt species.

Comparison of the acidic properties of ZSM-5 zeolites isomorphously substituted by Ga, In, B and Fe

Abstract The acidic properties of isomorphously substituted MFI-type zeolites prepared and pretreated in the same way were studied to obtain an exact gradation of the intrinsic acidity caused by the incorporation of Ga, Fe, In and B instead of Al into the framework. The nature, strength and concentration of the acidic sites were studied by temperature-programmed desorption of ammonia (TPDA) and Fourier transform infrared spectroscopy. The assignment of the two peaks observed in the TPDA plots of all zeolite samples required the additional registration of the decomposition plot of the ammonium form in the cases of In-ZSM-5 and B-ZSM-5. A temperature program with two ramps and an intermediate isothermal hold was used to separate the low-temperature peak from the high-temperature peak. In this way, the Bronsted sites could be determined more clearly on samples containing a high portion of non-framework species that inhibit the egress of ammonia from the pores. The nature and the strength of the acidic sites were spectroscopically characterized by recording the stretching vibration of the bridged OH groups of Bronsted sites and the spectra of pyridine adsorbed on Bronsted and Lewis sites. The intrinsic acidity of the Bronsted sites on In-ZSM-5 could be clearly classified to be intermediate between the acid strength of B- and Fe-ZSM-5. A relation could be revealed between the frequencies of the OH-stretching vibration and the temperature of the peak maxima of ammonia desorption from the Bronsted sites of the isomorphously substituted ZSM-5 samples.

Acid properties of ZSM-20-type zeolite

The acid properties of an HZSM-20 Zeolite have been characterized, in comparison to an Hy Zeolite, by IR spectroscopy, temperature-programmed desorption (TPD) of ammonia, time-resolved temperature-programmed FTIR of ammonia desorption and by microcalorimetry. Also, catalytic properties were tested for n-hexane conversion. These studies show that for HZSM-20 there is a shift of the bridging OH band to lower wavenumbers, an increase in the ammounia desorption temperature and a higher initial heat of ammonia chemisorption compared to HY. Thus, HZSM-20 contains medium–strong Bronsted sites, ranging in strength between zeilites HZSM-5 and HY. Obviously, the higher Si: Al ratio of HZSM-20 results in an enhanced number of strong acid sites with no second-neighbour aluminium atoms. This leads to a change in the distribution of Bronsted-acid sites. Superacid Bronsted sites could not be detected. Furthermore, the IR spectra of adsorbed pyridine and ammonia revealed the presence of at least two kinds of Lewis-acid sites. Some Lewis sites of HZSM-20 exhibit a weak acid strength and the strong lewis sites are weaker than those of HY. The different acidic properties of both zeolites are reflected in their catalytic activity and selectivity in n-hexane conversion.

Influence of Brnsted acidity on the redox properties of silver species in zeolite mordenite

The redox behaviour of Ag+ ions exchanged into zeolites is strongly influenced by the presence and concentration of Bronsted acid sites (BS). This is demonstrated by a temperature programmed reduction study of various samples prepared by ion exchange into the Na+ form or by impregnation of the H+ form of mordenite with AgNO3 solution. The Bronsted acidity of the latter was varied by different ways of transforming the NH4+ form to the H+ form. High concentration of BS as well as a high Ag loading favours the reoxidation of A formed in the reduction process. On acidic, Ag-loaded mordenites, a complete reduction of Ag+ occurs only at relatively high temperatures (above 600°C) where the BS are progressively destroyed.

Effect of Si : Al ratio and type of binder on the catalytic properties of HZSM-5 catalysts

Different HZSM-5 catalysts have been prepared by mixing and shaping dealuminated HZSM-5 zeolite samples with aerosil and boehmite as binder components. Catalytic investigations, adsorption measurements, temperature-programmed desorption studies and infrared spectroscopy with probe molecules were carried out to characterize the catalyst properties. The catalysts were tested in the coupled methanol–hydrocarbon cracking (CMHC) reaction with n-butane as hydrocarbon feed. In general, the catalytic data show inhibition of the deactivation resulting in lower coking, increased alkene yields and longer working periods with increasing Si : Al ratio independent of the nature of the binder. Comparing the data obtained from the two series with different types of binder we concluded that boehmite has not only a binder function but also a promoting effect on the catalyst properties. Thus, the catalyst activity was maintained at a more stable level owing to higher cracking activity and reduced rate of dealumination. Temperature-programmed desorption and IR spectroscopy were applied to determine the Bronsted and Lewis acidity of fresh and used samples. It was established that the addition of boehmite causes a partial neutralization of Bronsted sites. Moreover, the acid sites of the admixed boehmite influence considerably the acidic and catalytic properties of the catalyst.