A Unified Single-Event Microkinetic Model for Alkane Hydroconversion in Different Aggregation States on Pt/H−USY-Zeolites

Abstract

A single-event microkinetic model for the catalytic hydroconversion of hydrocarbons on Pt/H-US-Y bifunctional zeolite catalysts developed for low-pressure vapor phase conditions was extended to cover high-pressure vapor phase and liquid phase conditions. The effect of the density of the bulk hydrocarbon phase on the physisorption as well as on the protonation steps of the reaction network was accounted for explicitly and can be interpreted in terms of "compression" of the hydrocarbon sorbate inside the zeolite pores and "solvation" of the catalyst framework by the dense bulk hydrocarbon phase. The bulk phase density effect on the physisorbed state is described via a single excess free enthalpy of physisorption. A dense bulk hydrocarbon phase destabilizes the sorbate molecules inside the catalyst pores. An expression of the excess free enthalpy of physisorption involving the fugacity coefficient and a zeolite dependent factor allows description of physisorption data. Typical excess free enthalpy values are in the range 1.5-5.1 kJ mol(-1) increasing with carbon number in the series of C5-C16 alkanes. At high-pressure vapor phase and liquid phase conditions, the excess standard protonation enthalpy is estimated at -7.8 kJ mol(-1) leading to relatively more stable carbenium ions at dense bulk phase conditions. As a result of the excess physisorption and protonation properties, the lightest hydrocarbons in mixtures are more competitive at dense phase conditions and their conversion is enhanced compared to low-density conditions.

n-Hexadecane hydrocracking Single-Event MicroKinetics on Pt/H-beta

“…However, such an isotherm is empirical in nature and the estimated interaction parameters have no explicit physical meaning. In liquid-phase hydrocracking, a similar competitive physisorption step was modelled by quantifying the non-ideal behaviour of the physisorbed phase with excess parameters and the liquid fugacity coefficients of the individual components in the bulk phase [25]. The physical meaning of these excess parameters was clarified via a Born-Haber cycle and could be applicable at dense vapour phase conditions.…”

“…However, solvation effects which typically occur at dense phases and, consequently, at saturation conditions, might be responsible for these enhanced protonation enthalpies. These effects are denoted as 'protonation excess' and were quantified on a similar value of about -7.8 kJ mol -1 for a USY (CBV720, Si/Al = 15) zeolite during liquid-phase hydrocracking [25,26]. Considering the cracked product distribution, the advanced model is able to adequately reproduce the trends in the experimental data as the modelled yields of the 'cracking pairs', i.e., C 3 and C 13 , C 4 and C 12 , etc., exhibit a more pronounced formation of the lightest component, vide Figure 8 for the C 5 -C 11 pair.…”

“…A fundamental Single-Event MicroKinetic (SEMK) methodology has been developed to describe the complex reaction mechanism in gas-phase hydroconversion of n-alkanes over large-pore USY zeolites [20][21][22][23], and has later been extended to deal with extreme shape selectivity [24] and also bulk-phase non-idealities encountered during liquid hydrocracking [25,26]. For other well-known large-pore zeolites, such as mordenite and beta, a fundamental microkinetic model has not yet been constructed.…”

Fischer-Tropsch Waxes UpgradingviaHydrocracking and Selective Hydroisomerization

“…Typically the H 2 /HC pressure ratio was 13 to 375. The models were adapted to liquid phase conditions by considering that the density of the bulk reactant phase affects the physisorption as well as the protonation steps in the reaction network [25]. Under vapor phase reaction conditions, the paraffins react strictly in order of decreasing chain length because of competitive physisorption.…”

“…Next to the change in physisorption competition, phase density has also an impact on the protonation. Carbocations are better stabilized at higher bulk phase density [25]. Denayer et al [27] reported that the rate of n-heptane conversion relative to n-nonane in a binary mixture was enhanced by more than a factor of 2 by changing the phase from vapor to liquid.…”