Fischer-Tropsch SSITKA experiments over hcp and fcc Co/Al2O3
Résumé
Introduction
With the need for energy resource diversification, there is an increasing interest in Fischer-Tropsch (FT) synthesis. This process allows syngas (CO+H2) conversion issued from natural gas, coal or biomass into long-chai
n paraffins that can be upgraded into excellent diesel/kerosene fractions. Catalytic properties of cobalt-based FT catalysts strongly depend on their morphological and structural properties [1-2]. Previous in-situ XRD and magnetic studies during the activation showed the possibility to selectively orient the cobalt crystal phase to either mainly face centered cubic (fcc) or mainly hexagonal closed packed (hcp) according to the operating conditions during activation [3]. These catalysts have been tested in Fischer-Tropsch synthesis in order to differentiate activity, selectivity and potential mechanism specificities regarding to each crystal phase orientation as well as the particle size distribution. In order to get a fine description of the catalyst systems, high resolution chromatographic analysis combined with multi-product isotopic transient kinetic experiments have been carried out. In addition to classical data analysis, micro-kinetic modeling using different reaction mechanisms is used to correlate the key kinetic parameters to the catalyst structure.
Experimental
Catalyst testing for FT synthesis was carried out in a quartz plug flow reactor at 1.6 bars, 215-235°C with a H2/CO ratio of 2 and about 300 hours of time on stream (TOS) to account for catalyst ageing. Catalyst preparation and activation (via carbidization/decarbidization) affording mainly hcp and mainly fcc Co° crystal phase orientation have already been described elsewhere. Note that in both cases a mixture of hcp and fcc phases are obtained, but we refer to them as fcc or hcp Co°. The online analysis system for catalyst testing was composed of a cryogenic GC, a GC/MS with 16 storage loops and a MS gas analyzer. The combination of these techniques allowed monitoring product distribution (paraffins, α- and β-olefins and isomers) and determining the isotopic composition of a large range of products during SSITKA experiments (replacement of 12CO by 13CO).
A large product’s isotopic diversity is transiently induced the SSITKA switch of the feed. To face this complexity in the modeling work, an algorithm has been developed to automatically explore all possible paths of the reaction network. It starts from a flexible reaction network based on elementary steps. The code then generates all species involved before and after the 12CO/13CO switch. The fully coupled ordinary differential equation system governed by the considered network is integrated by DVODE while ODRPACK is used for the multi-response adjustment of the kinetic parameters.
Results and Discussion
The hcp Co° oriented catalyst is less active than the fcc Co° oriented catalyst. The methane selectivity is slightly higher for hcp Co° than fcc Co° at low temperature (205°C) whereas hardly any difference between the two samples is exhibited at higher temperature. Analysis of the product distributions reveals that the olefin to paraffin ratio for hcp Co° is lower than fcc Co° at the same time on stream (100h) indicating that hcp Co° have a weaker hydrogenation power. SSITKA experiments were performed over both catalysts. Selected SSITKA responses of 1-hexene are reported in figure 1. After the 12CO/13CO switch, the response curves for fcc Co° are faster than for hcp Co°. This is, the replacement of 12C by 13C in the final products is faster for fcc Co° catalyst.
Based on SSITKA principles [4], it can be deduced that intermediates surface residence times are longer on hcp Co° particles for all intermediates leading to the product distribution than for fcc Co°. This delay over the hcp Co° could be explained by a slow-down of the intrinsic kinetics and/or by the existence of a larger intermediate reservoir taking more time to be replenished.
Figure 1: SSITKA responses of 1-hexene at 220°C, H2/CO ratio of 2 (□,13C6H12 & , ▲ 13C512CH12) after a 12CO/13CO. Open and full symbols refer respectively to hcp and fcc Co0).
SSITKA experiments over fcc and hcp Co° with increasing time on stream show an increase of the surface residence time of intermediate species during the first hundreds hours on stream which could be linked to catalyst ageing.
Since many products share the same intermediates and can be reinserted in the polymerization process, the direct quantification of the reaction rates and reservoirs sizes is not obvious. Micro-kinetic modeling work is in progress in order to shed light on these results.
References
[1]O. Ducreux, B. Rebours, J. Lynch, Oil & Gas Science and Technology – Rev.IFP, 64 (2009)
[2]G. Bezemer, J. Bitter, H. Kuipers, J. Am. Chem. Soc., 128 (2006) 3956
[3]L. Braconnier, E. Landrivon, C. Legens, F. Diehl, Y. Schuurman, Cat. Today, 215 (2013) 18-23
[4]S. Shannon, Jr. J. Goodwin, Chemical Reviews, 95 (1995) 677–695