Sep-2024

Investigating methanol dehydration over beta zeolite catalyst (RI 2024)

Production of fuels and chemicals with a lower carbon footprint is emerging to reduce the environmental impact of petroleum-derived fuels and chemicals

Arundhathi Racha, Chanchal Samanta, Mahesh W Kasture, Rakesh Vankayala and Chiranjeevi Thota
Bharat Petroleum Corporation Ltd, Corporate R&D centre

Article Summary

Production of fuels and chemicals with a lower carbon footprint is emerging to reduce the environmental impact of petroleum-derived fuels and chemicals. Dimethyl ether (DME) stands out as a viable option because of its favourable combustion characteristics, emitting lower levels of nitrogen oxides (NOx) and particulate matter (PM) compared to conventional fossil fuels. For instance, DME can be blended with liquefied petroleum gas (LPG) or used directly in diesel engines, making it an attractive alternative in both transportation and domestic applications. DME has the potential to reduce emissions by up to 85% compared to fossil fuels.¹

DME can be produced from dehydration of renewable methanol, which in turn can be produced from biomass or hydrogenation of captured CO₂.²,³ However, methanol dehydration is not straightforward, and depending on the catalyst system, methanol conversion can lead to the production of different kinds of products, such as olefins, gasoline, aromatics or DME. Some alternative on-purpose production routes, such as methanol-to-gasoline (MTG), methanol-to-olefins (MTO), and methanol-to-aromatics (MTA), are commercially developed and under various stages of development.⁴

Selective production of DME from methanol necessitates the selection of active and selective catalysts that would be non-selective towards undesired side reactions. With this objective, at Bharat Petroleum (BPCL) R&D studies have been conducted to identify a suitable catalyst system for selective conversion to methanol to DME.

In India, the urgency to enhance energy security is paramount, given its heavy reliance on imported fossil fuels. The Indian government is actively promoting the production of methanol and DME from domestic resources, such as coal and biomass, to reduce import dependency and foster self-reliance.

This initiative aligns with India’s commitment to the Paris Agreement and its goal of achieving carbon neutrality by 2070.⁵ The Methanol Economy Research Programme (MERP) highlights the potential of methanol and DME to mitigate rising import costs and improve energy security, as they can be produced from locally available resources, including high-ash coal and captured CO₂ from industrial processes.

The production of DME and light hydrocarbons (olefins) from methanol is crucial for several reasons. Firstly, these compounds can be synthesised from renewable feedstocks, contributing to a circular economy and promoting sustainable development. Secondly, the ability to convert methanol into DME and light hydrocarbons provides flexibility in meeting the diverse needs of the petrochemical industry, which is essential for economic growth. For example, ethylene, propylene, and butylene are key components in the manufacture of plastics and synthetic rubber.

The Indian government has initiated projects to establish methanol and DME production facilities, recognising their potential to support domestic energy needs and reduce reliance on imports.

The National Coal Gasification Mission aims to gasify substantial amounts of coal by 2030, facilitating the production of syngas, which can be converted into methanol and subsequently into DME and light hydrocarbons.

This strategic approach not only addresses energy security but also contributes to pollution reduction, as DME combustion results in lower emissions compared to traditional fossil fuels.

Production routes for DME and light hydrocarbons
The production of DME and light hydrocarbons (C₂ to C₄) can be achieved through various routes, primarily categorised into direct and indirect methods (Figure 1). These processes utilise different feedstocks, including natural gas, coal, biomass, and organic waste, to produce these valuable compounds.

Main production routes for DME
1. Indirect route: This is the conventional method for DME production, which involves two main steps:
* Methanol synthesis: Synthesis gas (syngas), composed mainly of carbon monoxide (CO) and hydrogen (H₂), is produced from feedstocks such as natural gas or coal. The syngas is then converted into methanol through catalytic reactions.
* Dehydration of methanol: The produced methanol is subsequently dehydrated to form DME. This step is typically conducted over solid acid catalysts, such as zeolites, which facilitate the removal of water and promote the formation of DME. The overall reactions can be summarised as follows:
2CH₃OH _CH₃OCH₃ + H₂O

2. Direct route: In this method, DME is synthesised directly from syngas in a single reactor, which simplifies the process and can enhance efficiency. This approach allows for the simultaneous production of methanol and DME, potentially reducing capital and operational costs compared to the indirect method. The direct synthesis of DME from syngas is an area of active research, particularly for improving yield and reaction conditions.

Main production routes for light hydrocarbon production
There are two main routes for producing light hydrocarbons (C₂ to C₄) from methanol (Figure 2):
1. Methanol to olefins (MTO) process:
* Methanol is first synthesised from syngas (CO + H₂), which can be derived from natural gas, coal, biomass, or other carbonaceous feedstocks.
* The methanol is then converted to light hydrocarbons (ethylene, propylene) over zeolite catalysts like SAPO-34 or ZSM-5 at elevated temperatures (400-500°C).
* The light olefins can be further processed into gasoline-range hydrocarbons or other valuable chemicals.
2. Methanol dehydration to DME followed by DME conversion:
* Methanol is dehydrated to DME over solid acid catalysts like zeolites or alumina at 250-400°C.
* The DME is then converted to light olefins and paraffins in a second reactor using the same zeolite catalysts as in the MTO process.
* The light hydrocarbons can be separated and further processed as needed.
Key considerations in these processes include:
* Catalyst design and optimisation to tune product selectivity towards desired light hydrocarbons.
* Efficient heat integration and process configuration to improve energy efficiency and economics.
* Effective separation and purification of the light hydrocarbon products.
These routes allow the production of valuable light hydrocarbons from methanol, which itself can be derived from abundant and renewable feedstocks like natural gas, coal, and biomass. Integrating methanol aromatisation with light hydrocarbon aromatisation can further increase the yield of BTX aromatics (benzene, toluene, xylenes) from the light hydrocarbons.

By converting methanol to DME and to light hydrocarbons, the methanol economy concept can be expanded to produce a wider range of chemicals and fuels beyond just methanol itself. This flexibility is important for adapting to changing market demands and optimising the overall process economics.

Major players like Linde AG, SABIC, and Methanex Corporation are leveraging advanced technologies and catalysts.

BPCL’s Corporate Research & Develop-ment Centre has developed beta zeolite catalysts using a hydrothermal crystallisation method for the production of DME and light hydrocarbons from methanol.⁶

The beta zeolite molecular sieve with a SiO₂/Al₂O₃ molar ratio of 28.5 was synthesised using the hydrothermal crystallisation method and subsequently examined for its performance in methanol dehydration reactions.

This micro-mesoporous beta zeolite exhibited catalytic activity over a temperature range of 280-450°C, with DME emerging as the predominant product across all tested temperatures. Notably, the catalyst achieved a maximum selectivity for DME of 47.9% at 300°C, along with a methanol turnover frequency (TOFMeOH) of 741.9 h−1, indicating a highly efficient conversion process under optimal conditions.

As the reaction temperature increased, an enhanced fraction of strong acid sites on the zeolite, promoted higher hydrocarbon formation through the olefin-based cycle. The reaction environment significantly influenced the crystallinity, porosity, and acidity of the beta zeolite; amorphous carbon deposition was observed, leading to a partial loss of crystallinity.

Additionally, a pore-broadening phenomenon occurred at elevated temperatures, reflecting structural changes within the zeolite framework. Regeneration cycle tests confirmed stable catalytic activity throughout a 280-hour time-on-stream period, underscoring its robustness and effectiveness for continuous operation in methanol dehydration reactions.

This short article originally appeared in the 2024 Refining India Newspaper, which you can VIEW HERE