No abstract

No abstract

Combined operando Raman-GC and MultiTRACK studies provide new insights into the interaction of propane with a V/alumina catalyst. The Raman-GC analysis showed that the catalyst is essentially in the oxidized state during oxidative dehydrogenation reaction conditions, while stable intermediates and/or carbonaceous deposits are not observed on the catalyst surface. In the absence of oxygen, the catalyst is reduced by propane, and two types of carbonaceous deposits can be observed: one with a more aliphatic character at low temperatures, and one with a graphitic character at higher temperatures, of which the particle size increases as a function of increasing temperature. In agreement with the Raman studies, evidence for carbonaceous deposits was also provided by the MultiTRACK experiments. From CO 2 response profiles of the oxidation of these deposits, it was concluded that increasing temperature of operation and increasing propane/oxygen ratio enhance the amount and stability of the surface carbonaceous species formed. Based on the MultiTRACK studies also the participation of two types of oxygen species in the reaction of propane was evident: a highly reactive super-surface oxygen mainly yielding CO 2 , and a catalytic oxygen, associated with the vanadia phase. In MultiTRACK conditions, the extent of participation of vanadia-associated oxygen increases with reaction temperature and/or the propane/oxygen ratio, enhancing the selectivity of the reaction to propene.

No abstract

-Vanadium oxide supported on hydrotalcite-type precursors was studied in the oxidation of isopropanol. Hydrotalcites with different y = Mg/Al ratios were synthesized by the method of coprecipitation nitrates of Mg and Al cations with K 2 CO 3 as precipitant. The decomposition of these hydrotalcite precursors at 450°C yielded homogeneous MgyAlOx mixed oxides that contain the Al +3 cations totally incorporated into the MgO framework. The materials were characterized by chemical analysis, BET superficial area, X-ray diffraction, temperature-programmed reduction (TPR) and the reaction of isopropanol, a probe molecule used to evaluate the acid-base properties. The results of TPR showed that the reducibility of V +5 decreased with the increase in magnesium loading in catalysts. The X-ray diffraction patterns of Al-rich hydrotalcite precursors showed the presence of crystalline phases of brucite and gibbsite. It was shown that chemical composition, texture, acid-base properties of the active sites and also Mg/Al ratio strongly affect the formation of the products in the oxidation of isopropanol. The Al-rich catalysts were much more active than the Mg-rich ones, converting isopropanol mainly to propylene.

The effect of alkali metal addition to alumina on the acid characteristics and on the isopropanol dehydration capacity was analyzed. It was found that the incidence on the catalytic and acid properties of the alkali metal doped alumina depended on the ionic radii of the added element. Thus, K produces a higher poisoning of the acid function than Na and Li. Catalytic results can be interpreted in terms of a modification of the acid sites distribution by the alkali metal addition, the stronger acid sites being mainly affected.

-The influence of molybdenum oxide as a promoter on the V 2 O 5 /Nb 2 O 5 system was investigated. A series of MoO 3 /V 2 O 5 /Nb 2 O 5 catalysts, with MoO 3 loading ranging from 1 to 3 wt% MoO 3 and fixed V 2 O 5 content (21 wt%), were prepared by impregnation of the Nb 2 O 5 support with an aqueous solution of ammonium metavanadate and ammonium molybdate. The acid-base properties of the catalysts were investigated to determine of the selectivity of the isopropanol decomposition reaction. The X-ray diffraction results showed the presence of the β-(Nb,V) 2 O 5 phase. The temperature-programmed reduction profiles showed that the reducibility of vanadium was affected by the presence of molybdenum oxide. Activity results for isopropanol decomposition revealed that the acid-base properties of V 2 O 5 /Nb 2 O 5 catalysts are affected upon incorporation of MoO 3 , specifically for loadings of 3 wt %. For this catalyst composition both propylene and acetone formation rates decreased.

Recebido em 23/2/07; aceito em 14/11/07; publicado na web em 13/8/08 and Al +3 cations with K 2 CO 3 as precipitant. The X-ray diffraction patterns of Al-rich hydrotalcite precursors showed the presence of crystalline phases of brucite and gibbsite. It was shown that chemical composition, texture, acid-base properties of the active sites and also Mg/Al ratio strongly affect the formation of the products in the oxidation of isopropanol. The Al-rich catalysts were much more active than the Mg-rich ones, converting isopropanol mainly to propylene.Keywords: mixed oxide; hydrotalcite; V-Mg-Al catalyst. INTRODUÇÃODesde a década passada tem sido observado um aumento crescente de compostos tipo hidrotalcita como catalisadores. 17 Apesar de serem muito utilizados como catalisadores em diversas reações, os óxidos mistos, derivados das hidrotalcitas, também podem ser usados como suportes de catalisadores devido à sua alta área superficial, pureza química, propriedades ácidas e básicas superficiais e estabilidade estrutural.2 As propriedades estruturais e superficiais dos óxidos mistos Mg-Al dependem da composição química e dos procedimentos de síntese. Estudos sobre as propriedades ácidas e/ou básicas em óxidos metálicos têm sido investigados por meio da reação de decomposição do isopropanol.18 Corma et al. 19 relataram a influência da composição de Mg e Al nos óxidos mistos na reação de desidrogenação do isopropanol e observaram que as amostras com maior teor de magnésio (Mg/Al = 3/1) foram mais seletivas à acetona e esse bom desempenho foi atribuído ao maior número de sítios básicos, devido à presença da fase MgO. Entretanto, o catalisador mais seletivo a propileno foi aquele com maior número de sítios ácidos, produzidos pela maior quantidade de alumínio.Óxidos mistos de vanádio e magnésio, em particular, vanadato de magnésio, são catalisadores muito utilizados na desidrogenação oxidativa de alcanos leves para a produção de olefinas lineares. [20][21][22][23] Os catalisadores são usualmente preparados por impregnação de um suporte contendo magnésio (MgO ou Mg(OH) 2 ) com uma solução precursora contendo vanádio, seguido por calcinação a altas temperaturas ou de uma reação no estado sólido entre magnésio e vanádio. O comportamento catalítico desses óxidos mistos tem sido relacionado com a formação de três tipos de fases cristalinas, tais como ortovanadato de magnésio (Mg 3 V 2 O 8 ), pirovanadato de magnésio (Mg 2 V 2 O 7 ) e metavanadato de magnésio (MgV 2 O 6 ) e também por espécies tetraédricas isoladas (VO 4 ) sobre a superfície de MgO. Além disso, interações entre as fases podem dar origem a catalisadores com mais de uma fase cristalina com desempenho superior àquele com uma única fase.24 Em muitos casos, íons vanádio foram considerados como sítios ativos, porém, sua estrutura e seu estado de valência vêm sendo discutidos amplamente na literatura. [25][26][27][28] Catalisadores de VMgO suportados são uma alternativa aos óxidos mistos de vanadato de magnésio.29 Neste sistema, a natureza do suporte controla as espécies ...