Continuous-Flow Methyl Methacrylate Synthesis over Gallium-Based Bifunctional Catalysts

This paper reports on a systematic study on the catalytic gas-phase synthesis of methyl methacrylate (MMA) by means of the hydroxymethylation and dehydration of methyl propionate (MP) with formaldehyde (FAL), the latter being produced in situ by methanol (MeOH) dehydrogenation. This represents a promising variant of the current industrial Alpha process in which pure FAL is fed, alongside MP, into a gas-phase fixed-bed reactor filled with a supported Cs2O catalyst. In this way, an alternative process avoiding the need to feed carcinogenic FAL has been developed by feeding MeOH vapors onto a catalyst with dehydrogenating properties. For this purpose, an innovative Ga oxide-based bifunctional catalytic system is herein described for the first time for this peculiar application. Its catalytic performance and its chemical-physical features were investigated and evaluated to explain structure- activity relationships. Side reactions, MP ketonization, and MMA hydrogenation via H-transfer were accelerated in the presence of strong basic sites. The commercial, low-surface-area beta-Ga2O3 catalyst showed a strong dehydrogenating activity and the highest selectivity toward MMA due to its weak basicity. However, deactivation was observed due to (i) the deposition of carbonaceous species onto surface acidic sites, as evidenced by means of X-ray photoelectron spectroscopy (XPS) analysis, and (ii) the loss of specific surface area. On the other hand, Ga3+ could be easily incorporated into MgO by means of a simple co-precipitation technique, leading to Mg/Ga mixed oxides with a high specific surface area. When relatively small amounts of Ga3+ were used (e.g., in a Mg/Ga mixed oxide with the Mg/Ga atomic ratio = 10), the catalyst was much more active in MeOH dehydrogenation with respect to pure MgO. Moreover, the presence of Ga3+ ions reduced the density and strength of the basic sites while increasing the selectivity toward MMA by decreasing the occurrence of the H-transfer and ketonization reactions, which, conversely, readily occurred on highly basic MgO. On the other hand, the Mg/Ga mixed oxide with Mg/Ga = 10 showed a weaker acidity compared to beta-Ga2O3 and better stability due to limited coking. These results have shown both the potential and limitations of this alternative strategy. In particular, a more selective transformation of methanol to MMA needs to be achieved to allow the applicability of the proposed strategy on a bigger scale. Therefore, future efforts will be devoted to catalyst and condition optimization, also with the aim of limiting methanol unselective decomposition reactions.