The selectivity of -hexane hydroconversion on MOR-, MAZ-, and FAU-type zeolites

Abstract

Analyses of a series of published n-hexane hydroisomerization product slates suggest that MAZ-type zeolites yield more dimethylbutane and less methylpentane than either FAU-or MOR-type zeolites. Molecular simulations do not corroborate the traditional view that these selectivity differences are specifically related to the MAZ-, FAU-, or MOR-type zeolite topology. A scrutiny of the literature indicates that reported variation in selectivity relates to a variation in the efficiency of the (de)hydrogenation function relative to the acid function. The FAU-type zeolite catalyst had the most efficient hydrogenation function. The efficiency of the hydrogenation function on the MAZ-type zeolite was low enough to significantly enhance the 2,3-dimethylbutane yield relative to the methylpentane yield, but not low enough to decrease the 2,2-dimethylbutane yield. The efficiency of the hydrogenation function on the MOR-type zeolite was low enough to do both. Only at a sufficiently high n-hexane hydroconversion does the catalyst with the most efficient hydrogenation function exhibit the highest dimethylbutane yield. This new perspective on the reported hexane hydroconversion selectivities suggests that a FAU-type zeolite catalyst with a highly efficient hydrogenation function is best suited for n-hexane hydroisomerization. The FAU topology has the highest porosity which should afford the highest activity without impairing selectivity.  2004 Elsevier Inc. All rights reserved.

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…32 When a zeolite facilitates formation of a reaction intermediate, the Brønsted-Evans-Polanyi principle does not necessarily apply, 32 so that formation of a reaction intermediate can be facilitated without affecting the ease of formation of the transition state. 489 channels irrespective of the absence or presence of mass transfer limitations. 381 …”

“…Interestingly, without C 16 one does not observe an optimal selectivity toward dimethylbutane. 489 In contrast to transition-state selectivity, reaction intermediate shape selectivity requires severe mass transfer limitations between gas and adsorbed phases.…”

Towards a molecular understanding of shape selectivity

“…In fact, several shape-selective transformations catalysed by zeolites have been re-analysed using the principles we have outlined above, resulting in the identification of selectivity mechanisms different from those proposed originally [19][20][21][22][23] .…”

Computer Simulations of Shape Selectivity Effects

“…Thus, severe mass transfer limitations between the gas and adsorbed phases are a prerequisite for this type of reaction-intermediate shape selectivity to occur. In the absence of mass transfer limitations, FAU-, MAZ-, and MOR-type zeolites yield a virtually identical C 6 isomer product slate [120]. Whether the optimum branched isomer yield reported for MAZ-and AFI-type zeolites is indeed only a result of reaction-intermediate shape selectivity remains the subject of debate.…”

“…Recently, it was suggested that transitionstate shape selectivity might also contribute [118]. If there is indeed a contribution, it must be small as an optimized MAZ-type zeolite yields the same yield ratio of 2,3-dimethylbutane and n-hexane in the hydrocracking of n-hexadecane as do FAU-or MOR-type zeolites [120]. In addition, it was found that 2,3-dimethylbutane diffuses faster than other hexane isomers in MOR-as compared to FAU-type zeolites [121].…”