Kinetic Study of Heavy Reformate Conversion to Xylenes over MCM-41 on Zeolite Beta Hierarchical Composite Catalyst

The process for heavy reformate conversion to xylenes involves dealkylation, transalkylation, disproportionation, and other reactions [1]. The zeolite topology as well as the type and strength of acid sites significantly influence the catalytic process. The metal-acid bifunctional catalysts are reported to be suitable for xylene formation. To obtain a combined micro- and meso-porous structure, and to overcome limitations of a conventional large pore zeolite, hierarchical composite catalysts have been developed [2]. This paper reports the kinetic modeling of a catalyst containing 4 wt.% Mo impregnated on a hierarchical composite of MCM-41 on zeolite Beta. Heavy reformate, containing trimethyl benzenes (TMBs), methyl ethyl benzenes (MEBs) as major components, was used for determining the dealkylation-transalkylation performances.

Catalytic experiments for kinetic modeling were carried out in a CREC Riser Simulator (T: 300-400 °C; t: 5-20s). Based on the product analysis of a set of of experiments and on the similar studies by our research group [1-3], a reaction network involving five series-parallel reactions is proposed:

Reaction 1: Dealkylation of MEBs to toluene

Reaction 2: Transalkylation of TMBs and toluene to xylenes

Reaction 3: Disproportionation of TMBs to xylenes and toluene

Reaction 4: Paring reaction of tetramethylbenzenes (TeMBs) to toluene

Reaction 5: Dealkylation of MEBs to xylenes

The intrinsic reaction rates of the individual steps were related to the overall mole balance for the species involved.

Table 1: Estimated Kinetic Parameters
Parameter	Value	Parameter	Value
k1,0	1.31 ± 0.06	E1	29.25 ± 2.43
k2,0	0.03 ± 0.00	E2	16.82 ± 1.56
k3,0	0.06 ± 0.01	E3	11.46 ± 9.14
k4,0	2.71 ± 0.15	E4	46.33 ± 3.83
k5,0	0.98 ± 0.04	E5	19.54 ± 2.41
λ(deactivation)			0.41 ± 0.15
ki: units vary; Ei: kJ/mol

The kinetics parameters (Table 1) were estimated by a least square fitting of the rate equations using the experimental data (120 points). The k₀ values for monomolecular reactions were higher than the values for bimolecular reactions. The lowest activation energy required is for transalkylation of TMB with toluene (11.46 kJ/mol), which compares well with the reported value of 11.2 kJ/mol for zeolite β³. Disproportionation of TMBs require somewhat higher activation energy of 16.8 kJ/mol, which is attributed to involvement of two large molecules in this reaction. However, the value of activation energy was much lower than the reported value 29.9 kJ/mol for zeolite β. The highest apparent activation energy (46.3 kJ/mol) was observed for the paring reaction of TeMBs which are the molecules with largest diameter. The catalyst deactivation, estimated as λ = 0.41, indicates a very low coke formation. In conclusion, the hierarchical pore structured composite catalyst is favorable for the conversion of heavy reformate to xylenes. A series-parallel network of five reactions represents the experimental data adequately.

Literature Cited

1. S. Ali, K. Al-Nawad, C. Ercan, Y. Wang, Parametric Study of Dealkylation-Transalkylation Reactions over Mordenite-Based Bi-Functional Catalysts, Applied Catalysis A: General, 393 (2010) 96-108.
2. S. Ali, F. Almulla, B. Jermy, A. Aitani, R. Abudawoud, M. AlAmer, Z. Qureshi, T. Mohammad, H. Alasiri, Hierarchical composite catalysts of MCM-41 on zeolite Beta for conversion of heavy reformate to xylenes, Journal of Industrial and Engineering Chemistry 98 (2021) 189-199
3. Al-Mubaiyedh, S. Ali, S. Al-Khattaf, Kinetic modeling of heavy reformate conversion into xylenes over mordenite-ZSM5 based catalysts, Chemical Engineering Research and Design, 90 (2012) 1943-1955.

Acknowledgements

The authors acknowledge the support of King Fahd University of Petroleum and Minerals for conducting and for permission to present this work.