Origin of Selectivity in the Direct Propylene Epoxidation Reaction Using Molecular Oxygen

Origin of selectivity in the direct propylene epoxidation reaction using molecular oxygen

Janvit Teržan^{a, b}, Petar Djinovic^a, Janez Zavašnik^{c, d}, Iztok Arcon^{b, e}, Gregor Žerjav^a, Matjaž Spreitzer^{b, f} and Albin Pintar^a

^aDepartment for Environmental Sciences and Engineering, National Institute of Chemistry, Slovenia

^bJožef Stefan International Postgraduate School, Slovenia

^cCentre for Electron Microscopy and Microanalysis, Jožef Stefan Institute, Slovenia

^dMax-Planck-Institut für Eisenforschung GmbH, Germany

^eUniversity of Nova Gorica, Slovenia

^fAdvanced Materials Department, Jožef Stefan Institute, Slovenia

Propylene can be catalytically converted into several useful C₃ oxygenates, such as propylene oxide (PO), propanal and acrolein.[1] Propylene oxide is a highly desired chemical as it is used in the production of propylene glycol ethers, propylene glycol and polyether polyols.[2]

In this work,[3] CuO_x/SiO₂ catalysts modified with Na, K and Ca were synthesized and characterized by N₂ physisorption, XRD, TEM-EDS, CO₂-TPD, operando UV/Vis DRS, operando XANES and pyridine DRIFT spectroscopy.

After CuOx deposition, a decrease of about 25 % in specific surface area and pore volume is observed, when compared to pure KIT-6 silica support. The shift in pore size distribution towards lower values indicates that the CuO_x phase lies inside the pores of the silica support. Subsequent modification of the CuO_x catalysts with sodium induced only slight changes in surface area, pore diameter and pore volume. X-ray diffraction patterns at low 2θ angles of pure KIT-6 silica support, 5Cu, 10Cu and 10CuNa catalysts indicates a well-ordered mesoporous structure of the KIT-6 support, which remains unaltered during subsequent copper loading and sodium modifications.

The BF-TEM micrographs show a well-ordered mesoporous structure in both the sodium modified and unmodified sample, which confirm the results of the N₂ physisorption and XRD analyses. Additional morphological properties were probed with the UV/VIS DR analysis. They revealed characteristic charge transfer bands between mononuclear Cu²⁺ and oxygen. A broad but less intense peak corresponding to the d-d transition of Cu²⁺ ions in an octahedral environment, which can be attributed to bulk CuO, was also observed. In all samples a shoulder characteristic of [Cu–O–Cu]_n-type (oligomeric species) was present. Ex-situ and operando UV/VIS DR analyses of the unmodified samples showed a notable absorption red-shift along with the increase of band intensity related to bulk CuO, after the second calcination protocol (to simulate the alkali and earth alkali modification of the samples) and during catalytic conditions. Alkali as well as earth alkali modification of the CuO_x/SiO₂ catalysts noticeably stunted the sintering effect seen in the unmodified samples. Switching to a purely reductive atmosphere results in a progressive increase of light absorption in the whole range of wavelengths for both the modified and the unmodified catalysts. Returning to an oxidative atmosphere causes re-oxidation of the catalyst where the structural changes induced by the reduction are, to a greater extent, reversible in the 5CuNa catalyst, when compared to the unmodified catalyst.

Acid sites are known to negatively affect selectivity for PO, as these strongly electrophilic surface sites can react with the bridging oxygen in PO, resulting in its further oxidation, or surface blocking and deactivation via strong adsorption.[4] The results of pyridine DRIFTS measurements indicate no Brønsted acid sites on the analyzed samples and a negligible change in the Lewis acidity after copper loading. On the other hand, sodium modification decreases the amount of Lewis acid sites for 44 and 29 % in the 5CuNa and 10CuNa catalysts, respectively. The decrease in Lewis acid site abundance is in line with the sodium content, i.e. 0.8 and 0.5 wt. % in 5CuNa and 10CuNa catalysts, respectively.

Abundance and strength of basic sites in the synthesized catalysts was analyzed using CO₂-TPD analysis, which revealed two separate clusters of peaks for all the catalysts: a low-temperature cluster related to weak basic sites (between 0 and 300 °C), and a high-temperature cluster (comprised of two peaks centered at 450 and 600 °C) connected to strongly basic sites. Deconvolution of CO₂-TPD profiles showed that only in case of Na and K modification, the low-temperature cluster consists of three distinct peaks, centered at 60, 94 and 155 °C.

Abundance and binding strength of hydroxyl groups to the catalyst surface was analyzed using H₂O-TPD-MS technique. It can be seen that Cu addition to KIT-6 silica causes an increase in the density of water and hydroxyls. Their number is decreased after Na and K modification. The largest abundance and strength of hydroxyl binding to the catalyst surface (based on the position of the high-temperature desorption peak) is observed after modification with calcium.

Operando XANES analysis revealed that a substantial fraction of Cu²⁺ is transformed to Cu⁺ during the propylene oxidation reaction. The kinetics of oxygen abstraction and replenishment are substantially different, indicative of modified chemistry of the nucleophilic oxygen species, present in 5CuNa catalyst in contrast to others (5Cu and 5CuCa). Using LCF (linear combination fit) analysis of the 5Cu, 5CuNa and 5CuCa catalysts (at 350 °C in O₂/He atmosphere) shows all copper is present as Cu²⁺ and the majority (~90 %) is subnanometer Cu²⁺ clusters. Only ~10 % is present as nanocrystalline CuO.

Over the non-promoted catalysts, the catalytic reaction products are CO₂, CO, water, acrolein and trace amounts of other oxygenates (acetone, propanal, propanol, …), however, the latter present less than 5 % of the C containing products. Regardless of the copper content, strong deactivation is observed over all unmodified catalysts: they lost more than 60 % of initial activity during the 17 h of propylene oxidation reaction, which is likely related to sintering of oligomeric [Cu-O-Cu]_n species. Modification of 5Cu catalysts with sodium and potassium greatly minimized catalyst deactivation: minimal deactivation is observed during 17 h TOS for the 5CuNa catalyst, whereas catalytic activity of 5CuK and 2.5CuNa stabilizes after 10 h following the initial decrease of about 25 %. After Na and K addition, a notable (~10 %) propylene oxide selectivity was also observed, which increased with prolonging TOS. With calcium modification of the 5Cu catalyst, a dramatic increase in the initial catalytic activity is observed (9 and 6.5 % propylene conversion for 5CuCa and 5Cu catalysts, respectively) with trace (below 0.5 %) PO selectivity. Acrolein and CO_x selectivities remain very similar to the unmodified 5Cu catalyst.

Based on the research done, we propose that Cu⁺ is not crucial for PO formation; instead the electropositive Na⁺ and K⁺ decrease the nucleophilic strength of oxygen in CuO_x, making its attack on the allylic hydrogen of propylene less invasive. Alkali (K and Na) modification produces finely dispersed alkali metal cations, associated with the CuO_x phase, resulting in a greatly stabilized morphology and catalytic activity. The effect of calcium on decreasing the nucleophilic character of O species in CuO_x is negated by charge compensation by strongly adsorbed hydroxyl groups. As a result, Ca modification for PO selectivity is inefficient.

Key words: propylene, selective oxidation, epoxidation, alkali doping, nucleophilic oxygen.

References

[1]         D. Torres, N. Lopez, F. Illas, and R. M. Lambert, “Low-basicity oxygen atoms: A key in the search for propylene epoxidation catalysts,” Angew. Chemie - Int. Ed., vol. 46, no. 12, pp. 2055–2058, 2007.

[2]         The Dow Chemical Company, “Propylene oxide,” 2013.

[3]         J. Teržan et al., “Alkali and earth alkali modified CuO x /SiO 2 catalysts for propylene partial oxidation: What determines the selectivity?,” Appl. Catal. B Environ., vol. 237, pp. 214–227, Dec. 2018.

[4]         G. Mul, “Stability and Selectivity of Au/TiO2 and Au/TiO2/SiO2 Catalysts in Propene Epoxidation: An in Situ FT-IR Study,” J. Catal., vol. 201, pp. 128–137, 2001.