  Controlling the temperature distribution inside catalytic fixed bed reactors
is crucial for yield optimization and process stability. Yet, in situ
temperature measurements with spatial and temporal resolution are still
challenging. In this work, we perform temperature measurements in a cylindrical
fixed bed reactor by combining the capabilities of real-time magnetic resonance
imaging (MRI) with the temperature-dependent proton resonance frequency (PRF)
shift of water. Three-dimensional (3D) temperature maps are acquired while
heating the bed from room temperature to 60~$^{\circ}$C using hot air. The
obtained results show a clear temperature gradient along the axial and radial
dimensions and agree with optical temperature probe measurements with an
average error of $\pm$ 1.5~$^{\circ}$C. We believe that the MR thermometry
methodology presented here opens new perspectives for the fundamental study of
mass and heat transfer in gas-solid fixed beds and in the future might be
extended to the study of reactive gas-solid systems.
