Exploiting the Role of Coke in Catalytic Transformations

Global population growth over recent decades has played a crucial role in the increase in demand for hydrocarbon derivatives. The alkylation of aromatics with olefins over zeolite catalysts is applied extensively in the chemical industry, particularly in the production of detergents. However, during this catalytic transformation, carbonaceous residues are formed and accumulate in and/or on the zeolite pores. This complex carbonaceous product is called ‘coke’ and is the primary reason for catalyst deactivation. Several economic problems occur as a result of coke formation because it is costly to replace or regenerate the zeolite catalyst due to the need to shut down the process which results in the loss of time and money. In addition to the predominate role of coke as a deactivating agent, number of recent studies have focused on the positive role of coke in enhancing catalytic performance in reactions such as alkylation and isomerisation. The overall aim of this thesis is to understand the role of coke that is formed during the catalytic alkylation of toluene with 1-heptene through studying the influence of controlled pre-coking modifications on 2- heptyltoluene selectivity and investigating the kinetics of toluene alkylation with 1- heptene over HY5.1 (SiO2/Al2O3 mole ratio: 5.1:1).
Several approaches have been investigated with the aim of limiting coke formation through the modification of a range of zeolites. These include the formation mesopores; reduction in acidity; covering the external acid sites with bulky molecules; and controlled pre-coking of active sites. Desilication causes the formation of mesopores in the catalyst structure; these are larger than micropores, and hence are able to collect more coke. This contributed to enhancing the selectivity to 2- heptyltoluene from ~33 % to ~39 %. The reaction over dealuminated HY30 (SiO2/Al2O3 mole ratio is 30:1) illustrated an improvement in 2-heptyltoluene selectivity from ~31 % to ~36 % with slightly decreased carbon deposits. Toluene alkylation over silylated HY5.1 and HY30 showed a significant enhancement in 2- heptyltoluene selectivity from ~27 % to ~34 % for HY5.1 and from ~31 % to ~35 % for HY30 concomitant with a reasonable reduction in the percentage of coke.
Toluene and 1-heptene are employed as model coke pre-cursors for toluene alkylation with 1-heptene. The results obtained from thermal characterisation techniques showed that the structure of carbonaceous deposits that are formed from toluene pre-coking are graphitic-like. Toluene pre-coked HY zeolite showed enhanced selectivity to 2-heptyltoluene (from ~26 % to ~33 % and from ~33 % to ~39 % for HY5.1 and HY30 respectively) with a significant decrease in the amount of coke formed.
A kinetic study investigated 1-heptene isomerisation and toluene alkylation over fresh HY5.1 zeolite. This revealed that the activation energies of the alkylation reaction step 25-30 kJ mol-1 are higher than those of the isomerisation steps 15- 20 kJ mol-1. Moreover, the higher reaction temperature 90 ºC and higher contact time 7.04 g min mol-1 resulted in a slight decrease in the amount of coke alongside increasing in 1-heptene conversion from ~60 % to ~98 % and an increase in selectivity to 2-heptyltoluene from ~12 % to ~28 %.
In summary, pre-coking treatment could be considered for implementation in industrial operations because it is a useful method to enhance the selectivity and yield of monoheptyltoluene through toluene alkylation with 1-heptene and also because it decreases the amount of coke deposited during the reaction.