Insights into Ethanol Coupling over Hydroxyapatite using Modulation Excitation Operando Infrared Spectroscopy
The coupling of biomass derived ethanol to n butanol is a topic of contemporary interest. Indeed, n butanol can not only be used as a higher energy density fuel additive, it is also a key component in perfumes and serves as a solvent for paints and dyes. Hydroxyapatite (HAP) emerged in the literature as a promising catalysts for this transformation, with n butanol selectivity reaching ~75 at 10 ethanol conversion. However, the molecular level mechanism for this reaction is still unclear and several mechanistic questions remain unanswered. Here, we use diffuse reflectance infrared Fourier Transform spectroscopy, coupled with mass spectrometry following a modulation excitation approach (ME DRIFTS MS) that enables us to better understand the dynamic processes involved. Our approach allows for a vibrational characterization of the active surface species and the formulation of a consistent mechanism. Based on our experimental observations, Ca2+/OH can be put forward as the main active site for the aldol condensation. POH/OH acid base pair is proposed as the active site for the Meerwein Ponndorf Verley (MPV) direct hydrogen transfer of the aldol condensation product, crotonaldehyde.