The Catalyst Review September 2020 - 1

Industry Perspectives

The views expressed are those of the individual author and may not reflect those of The Catalyst Review or TCGR

Co-aromatization of an Alkane and Alcohols
There are a host of sources of naphtha (boiling point range ~ 80 °C to 200 °C, carbon number 6 to 10, and containing varying
proportions of paraffins + naphthenes) in a refinery such as straight-run naphtha, hydrotreatment and hydrocracking units, coking
units, vis-breakers, fluidized catalytic cracking units, steam crackers, and so on. In these naphthas, iso-paraffins with high octane
numbers are valuable from a gasoline blending point of view. Straight chain paraffins and naphthenes need further processing to
impart additional value. This is achieved in a naphtha reformer. Straight chain paraffins are converted into aromatics which are
important from an octane number perspective as well as for forming commercially valuable feedstock for petrochemical applications.
This conversion can also be accomplished by another type of reaction, namely aromatization. Aromatization of C3-C7 alkanes has
been extensively reported. Ga and Zn are the chief metals shown to be effective for this reaction. Moreover, conversion of alcohols to
olefins and aromatics typically on ZSM-5 and SAPO-34, respectively, is well established.
Considering aromatization of alkanes and alcohols separately, New Gas Technologies-Synthesis (NGTS) has ingeniously brought out a
process, Methaforming®, which aromatizes alkanes in naphtha feed in the presence of methanol employing zeolite-based catalyst.1
In addition to many operational advantages suggested by NGTS, a pertinent one is from regulations with regard to benzene content
of gasoline and its vapor pressure points of view. Due to the presence of olefins forming from methanol, aromatics generated from
alkanes get alkylated. This feature reduces benzene and olefins as reflected in the final product. They also indicated suitability of FCC
dry gas or other oxygenate, e.g., ethanol as co-feed with naphtha. This suggestion has been worked upon by us.2
Our lab studied alkane-alcohol co-aromatization over parent and dealuminated ZSM-5 catalysts with and without Ga and Zn
impregnation at 500 °C, 30 atm, 4.4 h-1. N-Hexane was used as an alkane representative. Methanol, ethanol, and iso-propanol were
individually and in combination co-fed with n-hexane. In liquid products, only aromatics (in addition to unconverted reactants) were
obtained. Liquid products were about 50 mass% and 70 mass% of feed with parent-and dealuminated- ZSM-5, respectively. Gaseous
products were suggested to be LPG constituents, ethylene, CO, and CO2. In case of parent ZSM-5, pure methanol and ethanol afforded
comparable (about 22 C atom%) yield of aromatics, while this value was 31 for 2-propanol. Interestingly, a mixture of 50 vol% each
of n-hexane (contributing 53.25 % C) and 2-propanol also led to a similar yield. Only 11 C atom% from pure n-hexane converted
into aromatics. Thus, the advantage of co-feeding propanol toward increasing aromatic yield from n-hexane becomes evident.
However, methanol and ethanol did not provide this benefit and, the presence of either of these two alcohols (or their mixture)
with propanol, was detrimental to the advantage brought in by propanol. Moreover, these two alcohols caused faster coking on the
catalyst. Mechanistically, n-hexane dehydrogenation is one of the steps involved in the direction of aromatic formation and alcohol
has been shown to extract hydrogen from n-hexane. This is over and above dehydrogenation caused by metal present on the catalyst.
Hexenes thus formed may or may not cyclize to subsequently form aromatics. Rather, hexenes undergo cracking, products of which
oligomerize and thereafter lead to aromatic formation.
Based on the results for ethanol and methanol, it was concluded that during the reaction the mechanism for alcohol to aromatics
predominates with n-hexane serving as "carbon pool." This was confirmed from the increasing aromatic yields with proportion of
propanol in its mixture with n-hexane. However, higher proportion of n-hexane reduced overall aromatic yields. ZSM-5 dealumination
or impregnation with Zn and Ga did not help increase aromatic yields, though they did reduce coke formation on the catalyst, and
the extent of metal impregnation on soft coke (easier to burn) was high. GC-MS of soft coke dissolved in CH2Cl2 showed almost all
carbonaceous compounds contained oxygen suggesting role of alcohols in formation of coke precursors.
References

1) Sims S, Adeniyi A, Elena L, et al. (2019). "Methaforming: Novel Process for Producing High-Octane Gasoline from Naphtha and
Methanol at Lower CAPEX and OPEX"; 2) Mahale RS and Parikh PA (2020). "Aromatization of n-hexane: Synergism Afforded by C1-C3
Alcohols." Chemical Engineering Science, 217: 115519.
About the Author

Parimal A. Parikh, PhD, received his PhD (Chemical Engineering) from the M.S. University of Baroda, Baroda,
India, by carrying out experimental work at the Research Centre, Indian Petrochemicals Corporation Ltd., Baroda
under supervision of Dr. AB Halgeri. He has guided 30 Master's and 11 PhD students in their work and has
published nearly 100 refereed papers. Parikh successfully completed seven research projects funded by different
Government entities. He is a DAAD fellowship recipient, has been recognized with Gujarat Technological
University Innovation Council Innovation to Impact 2017 Award by the Gujarat Technological University,
Ahmedabad, and is the recipient of the 5th National Award of Department of Chemicals & Petrochemicals, Ministry of Chemicals
& Fertilizers, Government of India for Technology Innovation in Petrochemicals & Downstream Plastic Processing Industry (201415). He has also served as Visiting Professor at the Asian Institute of Technology, Bangkok. Dr. Parikh's current research interests
include zeolite catalysis in refining, petrochemicals and biomass conversion processes, CO2 capture and gas storage in adsorbents.
The Catalyst Review

September 2020

The Catalyst Review September 2020

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