(689d) Atomically Dispersed Rh-Lewis Acid Sites Pairs for Selective Methanol Carbonylation to Acetic Acid

Ji Qi, Phillip Christopher

Acetic acid (AA) is one of the most important bulk commodity chemicals with a worldwide production of more than 10 million tonnes per year and is used as a precursor for the production of vinyl acetate, acetic anhydride, and acetic esters. The current processes for AA production proceed through methanol carbonylation (known as the Monsanto process and Cativa process) via homogeneous organometallic catalysts in liquid phase batch reactors and require the use of halides to initiate the reaction. While these processes are effective and highly selective the required separation of catalysts and water, and further the use of halides in the catalytic cycle increase process costs.

Many approaches for developing heterogeneous, halide free, gas phase processes for methanol carbonylation to AA have been explored, primarily through the use of Cu doped zeolites. While progress has been made in developed mechanistic insights into this catalytic system, very high CO:methanol feed ratios are need to achieve reasonable selectivity. In this talk, we will discuss the use of paired atomically dispersed Rh – Lewis acidic sites for driving methanol carbonylation in halide free, heterogeneous catalytic processes at stoichiometric methanol:CO ratios. We first demonstrate that AA production in methanol carbonylation requires the use of atomically dispersed Rh and the spatially confined Rh active site minimizes methanol decomposition, which occurs even on small Rh clusters.

In the first approach we explored, atomically dispersed Rh was deposited on ZrO₂ via the strong electrostatic adsorption approach. In addition to the production of AA dimethylether (DME) was observed as a significant by-product due to the inherent acidity of ZrO₂.Na poisoning of the acidic sites on atomically dispersed Rh/ZrO₂ was exploited to block DME formation active site, which enabled promotion of AA selectivity to > 60%.

In another approach we started with a basic support, MgO, deposited atomically dispersed ReO₄ acidic sites, and further deposited atomically dispersed Rh species selectively near ReO₄ units via the strong electrostatic adsorption approach. The Rh and ReO₄ paired sites exhibited perfect AA selectivity, however methanol conversion to CO₂ on MgO basic sites decreased the overall reaction selectivity to AA. Co-flowing CO₂ with CO and methanol in the reaction mixture was used to poison support basic sites and promote AA selectivity to ~90%.

A combination of in-situ IR, temperature programmed desorption and kinetic measurements were used to demonstrate that in these systems a dual site catalytic mechanism exists where methoxy was formed on the acid sites and CO was absorbed on atomically dispersed Rh, and that the CO insertion occurs to form AA on the acid site. This work demonstrates how atomically dispersed Rh, support engineering, and active site pairing can be used to control selectivity in this important process.