Understanding the active sites of Mg-Al oxide catalysts in the formation of aromatic platform chemicals from acetaldehyde

Abstract

The aromatization of acetaldehyde derived from bio-based feedstocks offers a sustainable route to obtaining valuable BTX (benzene, toluene, and xylenes)-like platform chemicals traditionally produced from petroleum naphtha. The acid-base properties of the catalyst must be carefully designed in order to selectively promote the multiple condensation-cyclodehydration mechanisms, yet it is not clear what the active sites actually are. To investigate this, we built a microreactor/flow system with flame ionization detector (FID)-quantification sensitivities and residence times ([tau]) on the orders of 10 ppm and 1 ms, respectively. Pure and Al-substituted (molar Mg/Al = 1, 2, and 3) MgOs were synthesized using a co-precipitation-calcination technique. The physicochemical properties of the catalysts were studied using X-ray diffraction (XRD), ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and N₂ adsorption-desorption experiments at 77 K. The acid-base character of the MgO-based materials was analyzed using a homemade temperature programmed desorption-mass spectrometry (TPD-MS) system as well as diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments. Benzene activity on these catalysts at the reaction conditions investigated (250 °C, 10 ms [tau]) is low and is attributed to strong basic sites (low-coordinated surface O₂⁻). Two isomers (ortho- and para-) of tolualdehyde, a drop-in replacement for xylene, were found to form via the self-condensation of the acetaldehyde dimer. Condensation experiments, in-situ acid-base titration studies, and CO2 DRIFTS comparisons on the spent oxides revealed that the same active site, a medium strength M-O type basic site in specific coordination, is responsible for the formation of both isomers. The amount of Al incorporation did not alter the condensation activity, despite changing the densities of acid and base site types. However, we found that, compared to pure MgO, the Al-substituted samples shifted the selectivity towards the significantly more valuable para- isomer. This is likely due to an increase in the kinetic favorability of one of the para-forming routes, induced by the carbonyl oxygen interactions with acid sites that arise from surface-exposed Al³⁺ , cations. These results can inform next generation catalyst design for improved selectivity and active site stability with time on stream.