Development of Molybdenum Phosphide Catalysts for Higher Alcohol Synthesis from Syngas by Exploiting Support and Promoter Effects

Abstract

Molybdenum phosphide (MoP) catalysts have recently attracted attention due to their robust methanol synthesis activity from CO/CO2. Synthesis strategies are used to steer MoP selectivity toward higher alcohols by investigating the promotion effects of alkali (K) and CO‐dissociating (Co, Ni) and non‐CO‐dissociating (Pd) metals. A systematic study with transmission electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, and X‐ray absorption spectroscopy (XAS) showed that critical parameters governing the activity of MoP catalysts are P/Mo ratio and K loading, both facilitating MoP formation. The kinetic studies of mesoporous silica‐supported MoP catalysts show a twofold role of K, which also acts as an electronic promoter by increasing the total alcohol selectivity and chain length. Palladium (Pd) increases CO conversion, but decreases alcohol chain length. The use of mesoporous carbon (MC) support has the most significant effect on catalyst performance and yields a KMoP/MC catalyst that ranks among the state‐of‐the‐art in terms of selectivity to higher alcohols.

In Situ Studies of the Formation of MoP Catalysts and Their Structure under Reaction Conditions for Higher Alcohol Synthesis: The Role of Promoters and Mesoporous Supports

“…Regardless of the support and presence of potassium, the XAS studies during the TPR step (Figure ) performed on the three MoP catalytic systems, KMoP/C, KMoP/SBA-15, and MoP/C, provide new insights on the reduction process. It has been noted in the literature that potassium can lower the reduction temperature of metal oxides, an effect we have previously observed on the formation of MoP catalysts as well . In the present study, we observe that the support also plays an important role in the rate of formation of the MoP phase; both the MoP/C and the KMoP/C catalysts show similar behavior, forming a MoP phase at 525 °C, a temperature much lower than that required for the SBA-15-supported catalyst, 675 °C.…”

“…It has been noted in the literature that potassium can lower the reduction temperature of metal oxides, 48 an effect we have previously observed on the formation of MoP catalysts as well. 30 In the present study, we observe that the support also plays an important role in the rate of formation of the MoP phase; both the MoP/C and the KMoP/C catalysts show similar behavior, forming a MoP phase at 525 °C, a temperature much lower than that required for the SBA-15-supported catalyst, 675 °C. In conclusion, the carbon support is strongly promoting the formation of MoP, even in the absence of potassium.…”

“…Potassium, being basic, is known to donate electron density leading to higher alcohol selectivity and suppressing acidic sites that promote the formation of methane and other hydrocarbons. , It is also known to block CO dissociation sites; this lowers catalyst activity but is essential for mitigating the production of hydrocarbons. This allows for CO insertion into intermediates stabilized by the promoter favoring oxygenated products, in agreement with the results we have previously published for MoP systems . The K-promoted samples were far superior in alcohol selectivity and activity than their unpromoted counterparts.…”

“…41 Figure 1 summarizes the performance results from our previous study and demonstrates how promoting MoP supported on carbon (C) with potassium leads to a shift in selectivity from methanol to higher alcohols. 30 Comparing this promoted MoP catalyst system supported on carbon with that on SBA-15, we see that the selectivity toward higher alcohols (29 to 24%) and the CO conversion a (5 to 2%) are both the highest for the carbon-supported MoP. Additionally, it was observed that when the MoP supported on carbon was not promoted with potassium, CO conversion dropped to 0.8% with CO 2 as the major product.…”

“…Carbon weighted alcohol selectivity and CO conversion of KMoP/C, KMoP/SBA-15, and MoP/C. Samples were tested at 270 °C, 60 bar, H 2 /CO = 2, and GHSV = 2600 h –1 …”