Bismuth Molybdate Catalysts Prepared by Mild Hydrothermal Synthesis: Influence of pH on the Selective Oxidation of Propylene

Kirsten Schuh,Wolfgang Kleist,Anker Jensen,Pablo Beato,Martin Høj,Vanessa Trouillet,Jan-Dierk Grunwaldt

doi:10.3390/catal5031554

Abstract

A series of bismuth molybdate catalysts with relatively high surface area was prepared via mild hydrothermal synthesis. Variation of the pH value and Bi/Mo ratio during the synthesis allowed tuning of the crystalline Bi-Mo oxide phases, as determined by X-ray diffraction (XRD) and Raman spectroscopy. The pH value during synthesis had a strong influence on the catalytic performance. Synthesis using a Bi/Mo ratio of 1/1 at pH ≥ 6 resulted in γ-Bi2MoO6, which exhibited a better catalytic performance than phase mixtures obtained at lower pH values. However, a significantly lower catalytic activity was observed at pH = 9 due to the low specific surface area. γ-Bi2MoO6 synthesized with Bi/Mo = 1/1 at pH = 6 and 7 exhibited relatively high surface areas and the best catalytic performance. All samples prepared with Bi/Mo = 1/1, except samples synthesized at pH = 1 and 9, showed better catalytic performance than samples synthesized with Bi/Mo = 2/3 at pH = 4 and 9 and γ-Bi2MoO6 synthesized by co-precipitation at pH = 7. At temperatures above 440 °C, the catalytic activity of the hydrothermally synthesized bismuth molybdates started to decrease due to sintering and loss of surface area. These results support that a combination of the required bismuth molybdate phase and a high specific surface area is crucial for a good performance in the selective oxidation of propylene.

Highlights

Since the development of bismuth molybdate catalysts for the oxidation and ammoxidation of propylene to acrolein or acrylonitrile by Sohio in 1959 [1,2], these mixed oxides have received strong attention and their catalytic properties have been studied in considerable detail [3,4,5,6,7,8,9,10,11,12]
We have recently shown that hydrothermally synthesized bismuth molybdates were active catalysts for the oxidation of propylene to acrolein [50] and found that the use of nitric acid led to an improved catalytic performance
The applied Bi/Mo ratio and the pH value had a strong influence on crystalline phases, specific surface area, bulk and surface composition and, the catalytic performance

Summary

Introduction

Since the development of bismuth molybdate catalysts for the oxidation and ammoxidation of propylene to acrolein or acrylonitrile by Sohio in 1959 [1,2], these mixed oxides have received strong attention and their catalytic properties have been studied in considerable detail [3,4,5,6,7,8,9,10,11,12]. To further explore catalysts with higher surface area and altered structure, we have systematically studied the performance of hydrothermally synthesized bismuth molybdates They were prepared under mild hydrothermal conditions by variation of the pH value, which should alter the structure of bismuth molybdates as reported by Li et al [36] To understand the role of the composition and the surface area on the catalytic activity, the resulting product phases, the ratio of bismuth to molybdenum in the bulk and on the surface, as well as the specific surface area have been characterized and correlated to the performance in the selective oxidation of propylene to acrolein. This knowledge may in the future be applied to more complex systems, such as multicomponent molybdenum oxides

Results and Discussion

Catalytic Performance in Propylene Oxidation to Acrolein

Catalyst Preparation

Catalyst Characterization

Catalytic Tests

Conclusions