V. Penchev

Effect of the rehydration on the acidity and catalytic activity of SAPO molecular sieves

The structural changes occurring in the template-free SAPO-5 and SAPO-34 upon the rehydration are reversible and do not affect their acidity and catalytic activity. The rehydrated SAPO-37 loses irreversibly a considerable part of its initial crystallinity, surface area, acidity and catalytic activity. Nevertheless the remaining acid centers are stable against dehydrationrehydration cycles.

Thermochemical and acidic properties of the zeolites offretite, omega and ZSM—5

Abstract The thermochemical and acidic properties of offretite, omega and ZSM—5 have been compared by thermal and X-ray analysis, infrared spectroscopy and temperatureprogrammed desorption of ammonia. It has been established that the temperature range of the decomposition of the organic cation depends on the zeolite structure, and is wider in the case of offretite. The decomposition of the NH 4 -forms of the three types of zeolites proceeds at higher temperatures than in the case of NH 4 -Y and also for the decomposition of the organic cation. The appearance of a high-temperature part in the t.p.d. curves of the samples is attributed to their stronger acidic properties after deammoniation. With respect to the strength of ammonia adsorption at 573 K, the decationized forms of the zeolites showed the sequence omega ∼ offretite>ZSM—5.

Toluene conversion on the zeolites offretite, omega and ZSM-5

Abstract The conversion of toluene and the accompanying formation of coke has been studied on the H-forms of the zeolites offretite, omega and Z5M-5. A qualitative correlation between the initial activity, the acidity and coke formation could be established. The p-selectivity of the ZSM-5 sample remains constant over a long time on stream. The selectivity of omega, and especially that of offretite increases with decreasing activity. As an explanation for these effects an increase of the diffusion paths by a closure of the pore entrances for the ZSM-5 and the interior of the pores of the omega and the offretite is discussed together with transition state selectivity. The results are further compared with (a) the selectivities of a number of other zeolites and (b) the results of theoretical calculations by Wei8,9.

Thermal decomposition of tripropylamine as a template in MeAPO-5 molecular sieves

The thermal decomposition of tripropylamine as a template occluded in the pores of MeAPO-5 aluminophosphate molecular sieves was investigated by means of TG, DTG and DTA. The effects of the incorporation of metal ions [Be, Mg, Ca, Ni, Fe(II), Fe(III) and Zn] in the crystal framework on the complete removal of template molecules and on the thermal stability of the samples were considered.ZusammenfassungMittels TG, DTG und DTA wurde die thermische Zersetzung von Tripropylamin, eingeschlossen in den Poren von MeAPO-5 Aluminophosphat-Molekularsieben untersucht. Es wurde weiterhin der Einfluß der Einlagerung von Metallionen [Be, Mg, Ca, Ni, Fe(II), Fe(III) und Zn] im Kristallgitter auf die vollkommene Abgabe der eingeschlossenen Moleküle und auf die thermische Stabilität der Proben untersucht.

Factors influencing the conversions of alkylaromatic hydrocarbons on high-silica zeolites: Part II. Presence of extralattice Al

Abstract The effect of different amounts of dislodged Al present in hydrothermally treated Y-type zeolites has been studied in the conversions of toluene, m -xylene, and alkylation of benzene with ethylene. T.p.d. of NH 3 is used to explain changes in the catalytic activity and modification of the strength and number of acid sites occurring in relation to the extralattice Al content. No indications for direct participation of the nonframework Al in the alkylaromatic conversions have been observed.

Thermal decomposition of organic templates in silicoaluminophosphate molecular sieves with various structures

The decomposition of organic templates in molecular sieves SAPO-5, SAPO-11, SAPO-20, SAPO-34 and SAPO-37 was investigated by TG, DTG and DTA. In the case of SAPO-S, the decompositions of various organic templates were studied. The effects of the type of crystal structure, the pore size and the channel system on the complete removal of the organic material were considered.ZusammenfassungMittels TG, DTG und DTA wurde die Zersetzung von organischen Modelle in SAPO-5, SAPO-11, SAPO-20, SAPO-34 und SAPO-37 Molekularsteben untersucht. Im Falle von SAPO-5 wurde die Zersetzung verschiedener organischer Modelle untersucht. Weiterhin wurde auf den Einflu\ von Kristallstruktur, Porengrö\e und Kanalsystem auf das vollstÄndige Austragen des organischen Materiales eingegangen.

Factors influencing the conversions of alkylaromatic hydrocarbons on high-silica zeolites: Part I. Structure and density of the active centers

Abstract High-silica faujasites prepared through hydrothermal treatment are compared with HZSM-5-type catalysts with almost the same Si Al lattice ratios in the conversions of toluene, m -xylene, and alkylation of benzene with ethylene. Disproportionation of m -xylene proceeds more intensively on Y-type zeolites than on ZSM-5 materials having practically equal or even higher active center concentration. The results confirm the predominant importance of the structural characteristics over the density factor for determining the differences in the disproportionation ability of these two types of zeolite catalysts with respect to alkylaromatics.

On The Nature of the Catalytic Activity of SAPO-5

Abstract An attempt to estimate the nature of the catalytic activity of SAPO-5 was made by means of the “inner standard” approach. SAPO-5 materials were compared with model mixtures of HY introduced into an AlPO-5 matrix as well as with HZSM-5 and highly dealuminated HY. The selectivity patterns in m-xylene conversion support the assumption of a relatively homogeneous Si-incorporation. The generated active centers are situated far from each other as in the case of the highly dealuminated faujasite.

Effect of the Introduction of Ni(Ii) - on the Catalytic Properties of Sapo-5 Molecular Sieves

Abstract The addition of different Ni compounds (NiO, Ni acetate) during the hydrothermal synthesis of SAPO leads to the formation of Ni-modified SAPO-5 materials with enhanced catalytic acid activity in m-xylene isomerization and toluene disproportionation. Both pure SAPO-5 and NiO-impregnated SAPO-5 samples are used as references. The hydrogen treatment of the NiO-containing materials results in the appearance of a Ni metal phase possessing a strongly expressed hydrogenolytic activity in a hydrogen stream. Physico-chemical characterization reveals the presence of extra-lattice Ni in the form of NiO, Ni metal and other Ni-containing species depending on the method of preparation and the thermal treatment conditions of the samples.

Crystallization of Porous Aluminophosphates and Metal Substitutions

Abstract Crystallization phenomena of a series of the molecular sieves MeAPO 4 -5 and MeASPO-5 have been studied under the influence of different Mecomponents. As Me components Be, Mg, Zn, Ni and Fe have been used. These components have been offered in some excess to study the extent of the incorporation which was determined by SEM-EDAX at the product crystals. For Be and Mg almost no influence on the kinetics of crystallization can be observed compared with a pure ALPO 4 -5 batch. Mg enters the structure with about 0.12 MgO for one Al 2 O 3 P 2 O 5 unit. A batch with Zn shows a reduced crystallinity and a cocrystallization of an unknown phosphate. The ZnO content of the crystals shows about the same molar ratio as the MgO. Extended experiments with NiO show that the Ni is not incorporated into ALPO 4 -5 and SAPO-5 crystals. In the presence of NiO pure ALPO 4 -5 and SAPO-5 crystallize which undergo recrystallization to a cristobalite like phase. Fe is incorporated into the crystals for both valency states taking up about 0,1 Fe 2 O 3 for one Al 2 O 3 P 2 O 5 unit in the structure. Generally the presence of an excess of the Me-ions favours some recrystallization to denser phosphate phases or to cristobalite or tridymite.