*3.3. Influence of CO2 on the Alkylation of Toluene Side-Chain*

The dehydrogenation of ethylbenzene produces the most styrene using the Friedel-Crafts alkylation reaction [87]. However, the ethylbenzene dehydrogenation method has some limitations such as catalyst deactivation and, high energy consumption [88,89]. Alkylation of the toluene side-chain is a promising alternative process that uses basic catalysts for the formation of styrene in the existence of CO2. Another process was reported by Sindorenko et al. [90] utilizing K<sup>+</sup> and Rb<sup>+</sup> ion transposing Faujasite supported catalysts in 1967. However, the catalytic conversion of toluene and styrene monomer (SM) selectivity was low (Table 10) [91]. The side-chain alkylation is primarily carried out on solid base catalysts [92–96]. Toluene side-chain alkylation with methanol enhanced by the promotional use of alkali metal oxides. Greater catalyst acidity accelerates methanol dehydration, [97] while low concentrations of alkali metal ions prevent the decomposition of formaldehyde produced from methanol [98]. Thus, catalysts for this reaction must be optimized for their acidity and basicity [99]. Generally, catalyst sites for the side-chain alkylation are limited to alkali metal-altered zeolites [100]. One reliable, widely studied catalyst is the cesium ion-exchanged or Ce2O-impregnated zeolite-X. The advantages of a MgO-supported mesoporous catalyst for this reaction has also been reported by Park el al. [101]. Hattori et al. observed that the impregnation of Cs2O in ion-exchanged zeolite-X results in high conversion of toluene, owing to the strongly basic sites [102]. Carbon dioxide has been under consideration as a renewable, low-cost, safe, and environmentally beneficial feedstock in current years. CO2 utilization is difficult for commercial applications, owing to its high thermal stability as well as the solid oxidation phase [103]. Hence, remarkable research efforts are being directed to detect innovative technologies for the utilization of CO2. Toluene side-chain alkylation was performed to assess the efficacy of the catalytic approach with methanol over cesium-supported catalysts. Toluene and methanol conversion over the Cs-X and Cs-modified zeolites in the presence of He and CO2 are shown in Table 10. In these reactions, styrene and ethylbenzene were formed as main products. Other side-chain alkylated components, including cumin and α-methyl styrene, as well as other xylenes, tri-methylbenzene, and benzene were identified as by-products. When the catalytic reaction was carried out in the existence of CO2, methanol and toluene conversion increased. Though the styrene selectivity decreased, there was a significant increase in the conversion as well as product selectivity in the presence of He and CO2 streams. TG/DTA analysis of the used Cs-X catalyst in the presence of CO2 and He streams is shown in Figure 6. In the range of 25–200 ◦C, weight loss occurred owing to the desorption of adsorbed water [94]. The continued weight loss in the 200–450 ◦C region occurred due to the deposition of coke on the surface of the catalyst. Relatively high quantities of coke were deposited on the Cs-X catalyst in the existence of the CO2. This suggests greater deactivation of the catalyst in the presence of carbon dioxide owing to coke deposition [89]. Still the Cesium-supported catalysts performed better in the presence of CO2 than under He in terms of toluene and methanol conversion. CO2 acted as a significant performance in hydrogen skulking and enhanced the reaction

rate in the decisive route. Additionally, CO2 increases alkylation to produce cumin and α-methyl styrene, which are side-chain alkylation products. Further, the increased toluene conversion enhances the aromatic yields.

**Table 10.** Performance of CO2 in the toluene side-chain alkylation (Reproduced with permission from [91]; copyright (2018), Elsevier).


Reaction conditions: WHSV = 2.1 h<sup>−</sup>1, Reaction temperature = 425 ◦C, Toluene/MeOH molar ratio = 2, SM = Styrene Monomer and other byproducts = Cumene, Xylenes, TMB and Benzene.

**Figure 6.** TG/DTA results obtained for used Cs-X catalyst in the presence of CO2 and He streams (Redrawn with permission from [91]; copyright (2018), Elsevier).
