Improved protocol for optimizing Mo/ZSM-5 catalyst for methane aromatization

Highlights

• Development of catalyst with high benzene productivity along with long-term stability is the key for commercial consideration of methane to benzene

• DOE approach for catalyst development is critical to ensure that the potential solution is not only based on the effect of individual factor but also including multiple interactions

• Mo incorporation under basic condition and/or control pre-carburization are vital to form highly efficient methane aromatization catalyst (Mo-carbide species)

Abstract

A systematic DoE approach is developed to optimize Mo-based ZSM-5 catalyst recipe for methane dehydro-aromatization process. Aim is to achieve maximum benzene productivity with improve catalyst stability. Three major factors - Si/Al ratio of ZSM-5, pH of Mo incorporation and catalyst activation along with two different responses - benzene yield and final/peak methane conversion (or in other words the extent of catalyst stability) were considered for constructing the DoE. The benzene yield and final/peak methane conversion vary significantly depending on the catalyst recipe including the catalyst activation protocol/conditions. The combination of Si/Al ratio of 15, pH of Mo incorporation 10 and control catalyst activation by pre-carburization identified through DoE helped to achieve maximum/optimum benzene yield and the catalyst stability. Moderate acidity of zeolite, high Mo dispersion and presence of higher quantity of catalytically active Mo-carbide/oxycarbide species seems to be the reason for high benzene productivity and catalyst stability.

Introduction

Recent discoveries of huge deposits of shale gas re-initiates the interest of using abundant of methane for value-added chemical production. Few indirect routes for converting methane to liquid fuels and valuable chemicals has already been commercialized. However, there is enormous interest to develop direct route for converting methane to chemicals in order to bypass the energy intensive syngas route and to improve carbon footprints [1].

Benzene, the key component of aromatics, is used as raw materials for the production of styrene, phenol and other industrial chemicals. Global benzene demand is anticipated to grow at a rate close to 2.5 % per annum [2]. With the development of shale gas, it is anticipated that the benzene availability may affect due to the transformation of cracker feedstock from liquid to gas. It is, therefore, very important to address the potential scarcity of benzene or benzene derivatives for the aromatics business requirements.

Direct conversion of methane to benzene (MTB) is one of the promising processes for efficient utilization of methane/natural gas/shale gas and also to address benzene gap. MTB reaction proceeds under both oxidative and non-oxidative conditions. In the presence of oxygen, benzene selectivity is found to be extremely low due to the formation of large quantities of unwanted CO and CO₂. Non-oxidative methane dehydro-aromatization over bi-functional Mo/ZSM-5 catalyst considered promising for on-purpose benzene production as shown in Scheme 1 [3], [4], [5], [6], [7], [8]. Mo is mostly responsible for methane activation. ZSM-5 acidity and pore structure facilitates oligomerization/tri-merization and de-hydro cyclization resulting selective formation of benzene. Due to low reactivity of methane and equilibrium limitation, methane to benzene reaction demands higher operating temperature to achieve meaningful conversion, a prime requirement for commercial consideration. However, high temperature expedites coke formation, which not only accelerates catalyst deactivation; but also coking generates more H₂, as a result the equilibrium shifts further towards left. Rapid catalyst deactivation is one of the major bottleneck for industrialization of this process.

Two major approaches have been attempted by researchers to address the coking issue and, thereby, to improve long-term catalyst stability. First one is the modification of catalyst through optimizing acidity of ZSM-5, passivation of external acidity, Mo introduction process, addition of promoter(s), etc. including catalyst activation. The second one is to use of small amount of co-feed(s) like CO, CO₂, H₂, H₂O, O₂, etc. along with methane to control/reduce coking during high temperature methane dehydro-aromatization reaction [5]. However, no systematic investigation for tuning of Mo-based ZSM-5 catalyst recipe to achieve maximize/optimize benzene productivity and minimize catalyst deactivation, the prime requirements for commercial consideration, have been reported so far.

In this work, we attempted systematic DoE approach to optimize catalyst composition considering three major factors - Si/Al ratio of ZSM-5, pH of Mo incorporation and catalyst activation to identify design space for optimizing benzene productivity along with the reduction in catalyst deactivation. Si/Al ratio of ZSM-5, in turn zeolite acidity is very critical for oligomerization/tri-merization of ethylene type intermediate to produce C6-olefins, the precursor for benzene. Basic pH for Mo incorporation helps in forming more monomeric Mo species and thereby increasing formation of catalytically active Mo-carbide/oxy-carbide species. Control pre-carburization is very important to restrict unwanted aromatics/graphitic type coke formation during the transformation of Mo/ZSM-5 to active Mo-carbide(oxy-carbide)/ZSM-5 catalyst. Our approach comprises developing suitable design of experiments and its analysis for identifying optimized catalyst recipe for both improving benzene productivity and catalyst stability.