Evidence for Diffusion-Controlled Hydrocarbon Selectivities in the Fischer–Tropsch Synthesis Over Cobalt Supported on Ordered Mesoporous Silica

Abstract

A series of four cobalt-based catalysts (two of which promoted with ruthenium) supported on SiO 2 or SBA-15 were prepared and tested in the Fischer-Tropsch synthesis at industrially relevant process conditions (483 K, 20 bar, H 2 /CO ratio = 2.1, pellet size: 53-90 lm). The catalysts were characterized by N 2 -adsorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), H 2 -chemisorption and transmission electron microscopy (TEM). Ru as promoter enhanced the activity but not the selectivity to long-chain hydrocarbons (S C 5þ ). The S C 5þ values of the SBA-supported catalysts were very low, especially at low conversion levels (i.e. low water partial pressure), suggesting that CO diffusion limitation increased the H 2 /CO ratio inside the 1-dimensional (1-D) porous network. A superimposition of the selectivity results on the correlations found in our recent study, derived for Co-based catalysts supported on c-Al 2 O 3 , a-Al 2 O 3 and TiO 2 free from diffusion limitations, was made. While the SiO 2 -supported catalysts with a 3-D porous structure followed the correlations, the SBA-catalysts deviated significantly at low conversions, giving a further indication that the selectivity results of these catalysts were affected by CO diffusion limitations. Hence, it may be concluded that the kinetically significant diffusion distances (i.e. those long enough to cause an intrapore H 2 / CO ratio higher than that of the bulk gas phase) are probably much shorter for 1-D porous networks than for conventional 3-D supports. This is explained by a significantly lower effective diffusivity in 1-D porous networks. The potential of using the correlations between non-ASF distributed hydrocarbons and C 5? , to give insight on the occurrence of diffusion limitations, was confirmed by superimposing data from the literature that were anticipated to be influenced by CO diffusion limitations.

Fischer–Tropsch Synthesis Over Zr-Promoted Co/γ-Al2O3 Catalysts

“…This high catalyst dilution, together with an aluminum jacket placed between the reactor and the oven, allowed for an even temperature profile (210 ± 2 °C) along the catalyst bed. A catalyst particle size in the range of 53-90 μm was used in order to suppress the influence of mass transfer on the catalyst performance [3,8,39,40]. The samples were in situ reduced at 350 °C (heating rate =1 °C/min) for 16 h in pure H 2 flow [gas hourly space velocity =7.5 NL/(h·g catalyst )].…”

Bridging the gap between insightful simplicity and successful complexity: From fundamental studies on model systems to technical catalysts

“…The use of ordered silica mesostructures as carriers has been instrumental to reveal the impact of pore size [104], morphology [105], and connectivity [101,106] on the stability of the supported metal nanoparticles against agglomeration and growth. Analogous studies revealed the influence of the pore dimension on diffusion phenomena during catalyst operation [107,108]. In recent years, high-resolution spectroscopic methods have reached the spatiotemporal resolution required to detect single turnover events, at the nanoscale, during the catalytic action of porous catalysts [84,109].…”

ZSM-5 seed-grafted SBA-15 as a high performance support for cobalt Fischer–Tropsch synthesis catalysts

“…For industry to selectively synthesize a specific type of product is still one of the most important challenges in FTS. 25 Many efforts have been dedicated to investigate the combination of acid catalysts with a conventional FTS catalyst to form a bifunctional catalyst system, in order to increase the product selectivity to a specific range 3-6 . In traditional system, hydrocracking catalysts (acidic zeolites) are usually packed in a 30 separate reactor downstream or a separate layer below the FTS catalyst 7 .…”