Dehydrogenation of Propane over Chromia-Pillared Zirconium Phosphate Catalysts

Abstract

Two series of porous chromia-pillared α-zirconium phosphate materials, prepared using two different methods of colloidization of the initial phase and with variable chromium contents (CrZrP-Xa and CrZrP-Xb) have been tested in the oxidative and nonoxidative dehydrogenation of propane in a flow reactor at atmospheric pressure. All catalysts are highly selective to propene under nonoxidative conditions at 823 K. In both series of catalysts, the initial activity increases with the chromium content, but generally CrZrP-Xb catalysts are more active than those of series CrZrP-Xa, which is in good agreement with their higher chromium contents and greater dispersions. In all cases, deactivation was detected due to coke formed from undesired reactions. When the reactions were carried out under oxidative conditions at 673 K, the activities were enhanced and the observed deactivation was minimum. The activities found vary between 0.47 and 1.31 μmol of propene g-1 s-1 and are maintained after 200 min of reaction. These activity values were also related to the chromium content, being slightly higher for CrZrP-Xb materials. A parallel study to evaluate the influence of acidity in the obtained results has been carried out. The activities found of these catalysts seem to be related to the presence of Cr(III) centers with vacancies in their coordination sphere. These vacancies, in nonoxidative conditions, can activate the reactive molecules originating propene and hydrogen. On the other hand, in an oxidative atmosphere, Cr(III) species can activate oxygen molecules, through an electronic transference process, yielding propene and water.

Characterization of Cr−MCM-41 and Al,Cr−MCM-41 Mesoporous Catalysts for Gas-Phase Oxidative Dehydrogenation of Cyclohexane

“…The Cr 3+ ion, with a t 2g 3 configuration, is able to supply electron density to an empty π u *(2p) orbital of O 2 , provoking its activation. During this process, the oxidation state changes from Cr III to Cr V . , On the basis of an EPR study of the interaction between Cr−MCM-41 and O 2 , Kevan et al concluded that tetrahedral Cr V , formed during the calcination of Cr−MCM-41, coordinates with one molecule of O 2 to form a five-coordinated, square-pyramidal complex. At elevated temperatures, electron transfer occurs between Cr V and O 2 to form Cr VI- O 2 - …”

Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides

“…With respect to coordination, it appears that Cr 3+ sites need to be coordinatively unsaturated to show catalytic activity . Indeed, a semiquantitative relationship has been found between the amount of pseudo-octahedral Cr 3+ formed during reduction and the dehydrogenation activity shown by the catalyst. , Furthermore, it has been observed that isolated Cr 3+ species with two coordinative vacancies are formed on CrO x /Al 2 O 3 catalysts in the presence of an alkane.…”

Activation of Tungsten Oxide for Propane Dehydrogenation and Its High Catalytic Activity and Selectivity

“…In spite of the fact that platinum-and chromium oxidebased catalysts show excellent catalytic performance and they are currently used in industrial processes, there have been attempts to develop new catalysts for propane dehydrogenation due to a high cost of the noble metal Pt and environmental toxicity of Cr 6+ species [10]. It has been reported that in addition to CrO x , various transition metal oxides, including GaO x , VO x , FeO x , InO x , ZrO x and ZnO x , are active for C-H bond activation of propane [1,8,[11][12][13][14][15][16][17][18][19][20]. Previous study of propane dehydrogenation over ZrO x promoted with La 2 O 3 showed that the bulk metal oxide exhibits comparable catalytic activity to that of noble metal Pt [15].…”