Chemistry of phosphomolybdate adsorption on alumina surfaces. 2. The molybdate/phosphated alumina and phosphomolybdate/alumina systems

Abstract

The adsorption of ammonium heptamolybdate (AHM) on phosphated alumina and of P2Mo50231 23456~a nd PMo^O^3-on 7-Al203 has been studied by using FTIR, Raman, and 31P solid-state NMR spectroscopies, and TPR. Adsorption of phosphate prior to contacting of 7-Al203 with AHM leads to a deactivation of the alumina surface still remaining. Phosphate reacts with 7-Al203 to form an A!P04-type surface phase, which itself is capable of adsorbing molybdate, with the formation of an irreducible surface molybdophosphate, its adsorption capacity being lower, however, than that of the original alumina. The primary phosphomolybdate adsorption reaction on alumina is shown to be the same as that observed in the case of AHM, viz., a reaction with the basic surface OH groups, leading to the decomposition of the adsorbing species. Thiophene hydrodesulfurization (HDS) activity data of variously prepared Mo(P)/A1203 catalysts are in agreement with the idea that HDS activity increases with increasing Mo reducibility.

On the surface sites of MoP/SiO2 catalyst under sulfiding conditions: IR spectroscopy and catalytic reactivity studies

“…4a). The band at 3665 cm −1 is possibly due to ν(P-OH) according to the study of phosphate on γ -Al 2 O 3 where an IR band at 3680 cm −1 was detected and assigned to a P-OH group of supported phosphate [30]. This assignment is supported by the IR spectrum (Fig.…”

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

“…This is in accordance with literature, in which it is stated that interaction of H x P 2 Mo 5 O 23 (6-x)-with the hydroxyl groups of the Al 2 O 3 supports will result in the disintegration of the complex, when no excess free phosphate is present. 44,46 It is also reported that physisorption of H x P 2 Mo 5 O 23 (6-x)-and Mo 7 O 24 6-can take place on the AlPO 4 layer that has been formed. 46 Impregnation with a Mo-P(1.0) solution results in a reasonably homogeneous distribution of H x P 2 Mo 5 O 23 (6-x)-throughout the support after 90 min.…”

“…44,46 It is also reported that physisorption of H x P 2 Mo 5 O 23 (6-x)-and Mo 7 O 24 6-can take place on the AlPO 4 layer that has been formed. 46 Impregnation with a Mo-P(1.0) solution results in a reasonably homogeneous distribution of H x P 2 Mo 5 O 23 (6-x)-throughout the support after 90 min. Disintegration of the complex (eq 8) is prevented, as enough free phosphate is present.…”

“…However, the reaction between free phosphate and the support is accompanied with an increase in the pH of the impregnation solution inside the pores of the support, as is obvious from eq 7. For this reason, the ν(MoO 2t ) vibration is shifted to lower frequency due to deprotonation of the H x P 2 Mo 5 O 23 (6-x)-complex (Figure 4) 28,[43][44][45][46]51 In this way, discrimination between processes occurring on the outer surface or in the pore system of particles is impossible. As was illustrated in this work, after the impregnation of support bodies, several physicochemical processes play a role that could not be monitored in these studies, due to this lack of spatial resolution.…”

“…It is therefore not surprising that over the years a vast amount of research has been dedicated to the understanding of the interaction of aqueous Mo complexes with Al 2 O 3 surfaces. 28,[37][38][39][40][41][42][43][44][45][46] Raman spectroscopy is a powerful method to monitor the nature of the Mo complexes in both the impregnation solution and in wet, dried, and calcined catalysts and has therefore found extensive application in this field of research. 37,38,[40][41][42][47][48][49][50][51] In this work, the first application of Raman microscopy to monitor the physicochemical processes during the preparation of supported catalysts is presented.…”