Envisaging the Physicochemical Processes during the Preparation of Supported Catalysts:  Raman Microscopy on the Impregnation of Mo onto Al₂O₃ Extrudates

Abstract

Raman microscopy has been applied to study the preparation of shaped Mo/Al2O3 catalysts. The speciation of different Mo complexes over γ-Al2O3 support bodies was followed in time after pore volume impregnation with aqueous solutions containing different Mo complexes. The addition of NO3 -to the impregnation solutions allows for a quantitative Raman analysis of the distribution of different complexes over the catalyst bodies as this ion can be used as an internal standard. After impregnation with an acidic ammonium heptamolybdate (AHM) solution, the strong interaction between Mo7O24 6-and Al2O3 results in slow transport of this complex through the support and extensive formation of Al(OH)6Mo6O18 3-near the outer surface of the support bodies. This may be prevented by decreasing the interaction between Mo and Al2O3. In this way, transport is facilitated and a homogeneous distribution of Mo is obtained on a reasonable time scale. A decrease in interaction between Mo and Al2O3 can be achieved by using alkaline impregnation solutions or by the addition of complexing agents, such as citrate and phosphate, to the impregnation solution. In general, time-resolved in situ Raman microscopy can be a valuable tool to study the physicochemical processes during the preparation of supported catalysts.

Monitoring the preparation of (Co)Mo/Al2O3 extrudates using spatially resolved spectroscopic techniques

“…This slow transport of Mo-complexes is in line with the observations in the previous paragraph (Fig. 2) 6 ] + is present throughout the support. Clearly, a disintegration of the heteropolyanion has taken place inside the A1 2 O 3 pore system.…”

“…Impregnation with the AHM-solution (without citric acid) at its natural pH of 5 leads to the formation of Al(OH)6Mo 6 Oig 3 " at the external surface of the pellets [12]. Intense peaks at 947, 899, 567 and 351 cm" 1 indicate the presence of a Al(OH) 6 Mo 6 O]g 3 " precipitate, as shown in Fig. 3 [13].…”

“…Probably, a strong Coulomb interaction between the highly negatively charged Mo-complexes and the surface therefore exists and transport of these anions is a slow process [11]. Impregnation with the AHM-solution (without citric acid) at its natural pH of 5 leads to the formation of Al(OH)6Mo 6 Oig 3 " at the external surface of the pellets [12]. Intense peaks at 947, 899, 567 and 351 cm" 1 indicate the presence of a Al(OH) 6 Mo 6 O]g 3 " precipitate, as shown in Fig.…”

“…Although several studies dedicated to the physicochemical processes during the preparation of supported catalyst bodies have revealed the importance of these phenomena, characterization was always carried out after completion of the preparation process. Recently, spatially resolved spectroscopie techniques were developed that allow to monitor the nature and distribution of metal oxide precursors inside supported catalyst bodies, during their preparation [4][5][6]. Cobalt-promoted MoS2 catalyst extrudates are still the main catalysts for the hydrotreatment of fuel streams [7].…”

“…In this paper it will be shown how a multitechnique approach can be applied to monitor the nature and macro-distribution of all components in the impregnation solution, throughout the preparation procedure of these materials. Due to the high Raman cross-section of Mo-O vibrations, Raman microscopy is ideally suited to study the speciation of Mocomplexes inside catalyst bodies [6]. The d-d transitions of Co + -complexes that can be observed by UV-Vis micro spectroscopy yield detailed information on the nature and distribution of the Co-precursor in the extrudates [4,8,9].…”