Understanding deactivation of catalysts

Ponsubbiah Nadar (PSN), a research scholar with the IITB-Monash Research Academy, has an abiding interest in chemistry – especially the role of catalysts in chemical reactions.

“When understood well and employed correctly, a catalyst is extremely useful,” he explains, “for it is the substance that increases or decreases the rate of a chemical reaction without itself undergoing any permanent chemical change.”

However, even today, with all the advancements in science and technology, catalytic materials are sometimes employed in the synthesis of chemicals without a clear understanding. This is where the challenge lies — to develop a strategy to understand the role of the catalyst.

PSN is therefore working on a project titled, ‘Development of micro-kinetic framework for heterogeneous catalyzed reaction: Methane to Benzene’.

Figure 1: Thermodynamic study for methane to aromatic reaction (excluding coke formation) at temperature ranging from 450 C to 950 C, atm. Pressure (a) Methane equilibrium conversion, (b) product distribution representation at 7000C and (c) selective formation of different hydrocarbon product. Note: H2 is also one among the product, not represented above replotted from (Moghimpour Bijani, Sohrabi, & Sahebdelfar, 2012)

“Studies aimed at converting methane to aromatics are not new,” he admits. “However, there is a less understood side to the process, which is deactivation of the catalyst during reaction condition due to coke formation. Though it is not possible to prevent coke formation in the hydrocarbon process, it can certainly be mitigated, and this can subsequently prevent catalyst deactivation.”

Explaining his work so far, PSN says, “In order to understand the reaction, we conducted reaction studies with catalyst characterization. Our studies highlighted formation of different species of coke. We are currently investigating a different technique for spent catalyst characterization, both qualitatively and quantitatively. We feel that different coke species formed during the reaction may have their individual role in catalyst deactivation, the understanding of which could be a very crucial piece of information. Micro-kinetic modelling is a mathematical approach which is best suited to study such reactions as the model does not consider pseudo steady state assumption of the intermediate / surface species, which is otherwise ignored for simplicity. The strategy we adopted for model development is to divide the complex reaction into a sequence, and the schematic is represented in Figure 2.”

Figure 2: Schematic for different stages in methane to benzene reaction. (a) Fresh catalyst: Mo/ HZSM5 catalyst prepared by wet impregnation method. (b) In presence of CH4, the MoO3 dispersed in the zeolite pore carburizes (c) Carburized Mo, converts the further coming CH4 to oligomer and aromatics (d) formation of coke species takes place inside the pores and over the zeolite surfaces.

The IITB-Monash Research Academy is a collaboration between India and Australia that endeavours to strengthen scientific relationships between the two countries. Graduate research scholars like PSN study for a dually-badged PhD from both IIT Bombay and Monash University, spending time at both institutions to enrich their research experience.

Says Prof Murali Sastry, CEO of the Academy, “Micro-kinetic modelling is emerging as a powerful approach to understand complex catalytic reactions. We hope that models like the one PSN is working on will become a convenient tool to understand and devise methods to improve catalyst selectivity and stability.”

Research scholar: Ponsubbiah Nadar (PSN) , IITB-Monash Research Academy

Project title: Development of micro-kinetic framework for heterogeneous catalyzed reaction: Methane to Benzene

Supervisors: Prof. A K Suresh, Prof. Debabrata Maiti, Prof. Alan Chaffee, Dr. S Sivakumar

Contact details: ponsubbiahnadar@gmail.com

This story was written by Mr Krishna Warrier based on inputs from the research student, his supervisors and IITB-Monash Research Academy. Copyright IITB-Monash Research Academy.