Butan-1-ol and butan-2-ol dehydration on nitrided aluminophosphates: influence of nitridation on reaction pathways

The dehydration of butan-2-ol and butan-1-ol was studied at atmospheric pressure using an AlPO4 amorphous sample and two nitrided phosphates AlPON (11 and 20 wt.% N), at 200 and 275 °C, respectively. The acid–base properties of AlPON can be tailored by adjusting their nitrogen content, as proved by NH3 chemisorption and SO2 microcalorimetry experiments. In the present work, butan-2-ol and butan-1-ol dehydration reactions were used to characterise the changes induced by nitridation on the acid–base properties of AlPON. The dehydration of butan-2-ol and butan-1-ol over AlPON was studied, because the reactivities of those linear alcohols are not similar. As the basicity of butan-2-ol is higher than that of butan-1-ol it can give olefins as the major product, through an E1 mechanism even on AlPO(N) weak acid sites. Product distribution of 1-butanol dehydration over AlPO(N) rather suggests a bifunctional acid–base E2 mechanism. On AlPON, the rate of butan-2-ol dehydration is proportional to the total number of acid sites detected by measuring the volume of ammonia chemisorbed on the catalyst at 35 °C. On the other hand, the activity of butan-1-ol dehydration passes by a maximum for an intermediate nitrogen content. This indicates that the density of bifunctional acid–base sites is increased at the beginning of nitridation, and there is an optimum for intermediate nitrogen contents. The changes in the nature and the density of surface NHx species during time on stream were also evaluated using DRIFTS and X-ray photoelectron spectroscopy (XPS) spectroscopies.