An Operando Molecular Spectroscopic and DFT Calculation Investigation of Methane Aromatization By Supported Mo/ZSM-5 Catalysts

An Operando Molecular Spectroscopic and DFT Calculation Investigation of Methane Aromatization by Supported Mo/ZSM-5 Catalysts

The recent discovery of abundant shell gas has renewed interest in conversion of gas to liquids technology (GTL). The GTL technology has the potential to convert a significant percentage of natural gas into liquid petroleum enough to supply the world’s energy needs for the next 25 years. In addition to natural gas, large amounts of methane are available from coal beds and as byproducts of petroleum refining. The relative inertness of CH₄ has limited its application as chemical feedstock in spite of the relative abundance of this inexpensive hydrocarbon.  It has been estimated that the methane byproduct being flared from crude oil fields is equivalent to about 1 million barrels of oil per day. Consequently, a long term research goal is the upgrading of methane into higher molecular weight hydrocarbons that are economically more valuable, more easily transportable or stored, and can be suitable feed stocks for current commercial petrochemical manufacturing processes.

According to the assessment from Honeywell (UOP), the production of synthesis gas (syngas, consisting of a mixture of CO and H₂) is 80% of the cost of the indirect GTL process. Therefore, a more effective process which can avoid the production of syngas is explored. The direct route such as direct non-oxidative conversion of methane to aromatics (benzene and naphthalene) emerges.  The most promising catalyst is the supported Mo/ZSM-5 catalyst for non-oxidative conversion of methane to aromatics.  Supported molybdenum oxide catalysts are widely employed as commercial catalysts for hyrodesulfurization and hydrocracking of heavy fractions of crude oil, olefin metathesis, isomerization, hydrogenation and polymerization of alkenes in the petroleum industry, and partial oxidation of aliphatic alcohols ⁵.  The ZSM-5 is a type of aluminosilicate zeolite patented by Mobil which has a pore opening of about 5.5 Å in size similar to the size of a benzene molecule. It has been widely used in the petroleum industry as a heterogeneous catalyst for hydrocarbon isomerization reactions. The pioneering work of the activity of Mo/ZSM-5 catalysts in dehydro-aromatization of methane was reported by Wang¹ in 1993. Their supported Mo/ZSM-5 catalyst exhibited a high selectivity for methane towards benzene. Many researchers have reported the optimum catalytic activity of 7~ 12% CH₄ conversion and 70~100% selectivity towards benzene over Mo/ZSM-5 catalysts with 2~3 wt% molybdenum loading^1-4.

Supported Mo/ZSM-5 catalysts are able to convert methane to aromatics, but despite intensive studies there is no agreement in the catalysis literature about: the nature of supported Mo species on ZSM-5 (monomer, dimer or cluster) and anchoring site(s) of Mo species (Al-Al, Al-Si or Si-Si). The intense interest in supported molybdena catalysts has motivated numerous molecular structural characterization studies of the surface MoOx species: in situ Raman ^6,7,8, infrared (IR) ^{9, 10},  in situ X-ray diffraction (XRD) ^{9, 10, 11, 12, 13} x-ray absorption spectroscopy (XANES/EXAFS) ¹⁴, temperature programmed desorption (TPD)/temperature programmed reduction (TPR) ¹⁵, X-ray photoelectron spectroscopy (XPS) ¹⁶, and UV-visible diffuse reflectance spectroscopy (DRS) ^{16, 17}. These characterization investigations have significantly contributed to the fundamental understanding of supported molybdena catalysts and impacted both their fundamental research and industrial development efforts.

In situ UV-vis DRS revealed that the initial supported MoO_x are exclusively present as isolated monomeric surface sites on the ZSM-5 support.  Combined in situ Raman spectroscopy and DFT calculations in H₂¹⁸O exchange, dehydrated Raman, and H₂ –TPR experiments, however, revealed the presence of four distinct surface MoO_x structures on the Mo/ZSM-5 support: 1) isolated dioxo (O=)₂MoO₂ surface site over two Brønsted acid sites at low Mo loading and high Al content at 998 cm^-1; 2) isolated dioxo (O=)₂MoO₂ surface site over one Brønsted acid site and an adjacent Si-OH group at increasing Mo loading at 970 cm^-1; 3) isolated dioxo (O=)₂MoO₂ surface site over two Si-OH groups at low Al content at 983 cm^-1; 4) isolated mono-oxo O=MoO₄ surface site over extra framework Al-OH groups at high Al content found at 1026 cm^-1. In situ FT-IR showed that MoO_x prefers to anchor at Brønsted acid sites at low Mo loading, and at Si-OH and external Al-OH groups at higher Mo loading and low Al content. The addition of MoO_x to ZSM-5 also indicates dealumination and forms extra-framework Al₂O₃ NPs. These new studies demonstrate how systematic studies with modern approaches to catalysis research are able to resolve the fundamental issues of complex catalytic systems and establish the relationships between the molecular and electronic structures of the catalytic active sites and the corresponding catalytic activity and selectivity performance.

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