(582e) Catalytic Upgradation of Biomass-Derived Bio-Oil By C-C Coupling of Phenolic Compounds with Light Oxygenates

Catalytic upgradation of biomass-derived bio-oil by C-C coupling of phenolic compounds with light oxygenates

Gul Afreen, Tanmoy Patra, Ratan Mohan, Sreedevi Upadhyayula*

Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi-110016, India

*corresponding author: sreedevi@chemical.iitd.ac.in

With the rapid depletion of crude petroleum resources, the demand for renewable energy sources has increased immensely. Extensive research in utilization of biomass indicates that it is a promising feedstock for the sustainable production of fuels and chemicals. Pyrolysis of lignocellulosic biomass results in liquid bio-oil which contains a high percentage of light oxygenates like acetic acid, ketones, etc as well as lignin-derived phenolic compounds. Large amount of oxygen present in the bio-oil is removed by performing hydrodeoxygenation, to make it compatible with the crude petroleum. However, during this process small oxygenates are lost in the form of light gases. So to retain the carbon of such small oxygenates as well as to bring the phenolic compound in the fuel range (C10-C13), C-C coupling by alkylation of the phenolic compounds with small oxygenates is thought to be beneficial for the biofuel upgradation, before the hydrodeoxygenation step.¹ The resulting compounds not only fall in the desirable carbon-chain fuel range but also are widely used as chemical intermediates in the synthesis of antioxidants, fragrances, agrochemicals, and fuel additives.^2-4In this context, to represent the phenolic compounds and light oxygenates, the model compounds chosen are m-cresol and isopropanol, respectively. The main product of alkylation of m-cresol with isopropanol is 2-isopropyl-5-methylphenol (IMP), also known as thymol, a precursor of commercially useful pharmaceutical product called menthol. A number of different types of catalysts like mineral acids, Lewis acids, ionic liquids, mesoporous solid superacidic materials, modified zirconia, HY zeolites, Pt metal loaded on Hβ zeolite, iron catalyst containing Cr, Si and K oxides, zinc aluminate spinel, silica-alumina clays, metals loaded on mesoporous materials, γ-alumina, etc. have been reported earlier for the production of thymol by alkylation of m-cresol with isopropanol.^1,2,5-9 However, the drawbacks of these catalysts are that the use of mineral acids and Lewis acids result in equipment corrosion, environmental pollution, low reactant to catalyst ratio and loss of selectivity to the desired product. Homogeneous ionic liquid catalysts have difficulties in product purification and catalyst recovery. Reported mesoporous materials and other solid acidic catalysts have the advantages like easy separation of the products, excellent reusability, etc. However, the use of solid catalysts results in disadvantages like rapid deactivation due to leaching of active metals, pore blocking due to coke formation, etc. Among the reported heterogeneous catalysts, zeolites contain Bronsted acid sites and have high shape selectivity and thermal/hydrothermal stability which can give high conversion of m-cresol to form thymol.¹⁰ Hence, for the large scale production of thymol, the zeolite-based catalysts with good thermal stability, least coking issues, and controlled Bronsted and Lewis acidity is to be developed.

In this study, isopropylation of m-cresol was performed over HY, ZnY, Hbeta, HZSM5 and HMCM22 zeolites as catalysts to ascertain the effect of the nature and strength of superficial acid sites as well as their pore size on the catalytic activity and product distribution. The reaction was conducted using a fixed-bed continuous flow reactor with length to diameter (L/D) ratio of 30. The liquid products were analyzed using a NUCON-GC supplied by AIMIL India Ltd. equipped with a CHROMSORB-WHP (2 m × 3.175 mm × 2 mm) column and flame ionization detector (FID). Conversion of m­-cresol at temperatures between 200 ⁰C and 300 ⁰C is shown in Figure 1. The maximum conversion of m-cresol (86%) was achieved at 275 ⁰C for ZnY catalyst and selectivity to thymol was 41% . The use of HY as a catalyst resulted in 69% conversion of m-cresol at 250 ⁰C, while the conversion of m-cresol was very low for HZSM5, HMCM22 and Hbeta catalysts. The higher conversion of m-cresol over ZnY catalyst can be attributed to the higher Lewis as well as Bronsted acidity of the active metal containing catalyst. This concludes that a strong Lewis plus Bronsted acidity is required for this reaction. The low conversion of m-cresol on HZSM5, HMCM22 and Hbeta catalysts can also be explained due to diffusional constraint. The selectivity to thymol was also analyzed by using these catalysts.

Figure 1: Conversion of m-cresol using different catalysts. (WHSV=2.8 h^-1, m-cresol:IPA mole ratio=1:2, Catalyst loading=1.03 g, P= 1 bar)

The maximum conversion for each catalyst, except ZnY, was obtained at 250 ⁰C, thereafter, it decreases with further increase in temperature due to the deactivation of catalysts at higher temperatures. However, for ZnY the maximum conversion was achieved at 275 ⁰C, which shows that it deactivates comparatively slower than other catalysts. The deactivation studies were performed using characterization tools like NH₃-TPO. The influence of reaction parameters such as reaction temperature, WHSV, mole ratio of reactants and time-on-stream has also been investigated for the best catalyst i.e ZnY. Thus, development of a continuous, environmentally benign, energy efficient and economical process route for the production of alkylated m-cresol for bio-fuel upgradation can be made using metal-containing solid acid catalysts.

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