Investigation of effects of the promoters on performance of Fischer-Tropsch catalysts

With the increase in the cost of petroleum products due to depletion of oil reserves, alternative methods for synthesizing hydrocarbon fuels such as the Fischer–Tropsch (FT) Synthesis have received considerable attention. The FT synthesis is a well-known catalytic process for the conversion of synthesis gas into liquid fuels. The main product of the process is a mixture of hydrocarbons of variable molecular weight. The hydrocarbone product spectrum produced by conventional FT catalyst is very broad and many studies have been carried out with the aim of controlling and limiting the product distribution. The resulting product is threated by various operations such as cracking and isomerization to obtain the desired chemical products such as diesel and gasoline. It is possible to combine these two steps into a single process by using bi-functional catalysts made of zeolite and active catalyst. Zeolites, with their unique channel and por structure properties, have a wide range of applications, especially in the petrochemical industry. Zeolites are good hydrocracking and isomerization catalyst due to their asidic form. Because of this, they are combined with FT catalyst in order to increase the gasoline and diesel fractions in the FT product, so that the downstream upgrading unit might be discarded. . Controling of the product distrubition is typically achieved by modification of the catalyst. Many factors, like support material, promoters and preparation method of catalyst affect the product selectivity. Although some catalytic materials are composed of a single material, most catalysts are composed of three different types of components: 1. active component, 2. a support or carrier and 3. promoters. Active components are responsible for the principal chemical reaction. Selection of the active component is the first step in catalyst design. FT catalysts are typically based on group VIII metals – Fe, Co, Ni and Ru. Industrially, the most preferred catalysts are iron and cobalt catalysts. Cobalt based catalysts mainly yield straight-chain hydrocarbons while iron-based catalysts are more useful when the H2/CO ratio is low (due to the water gas shift (WGS) activity of Fe) or for the production of alkenes, oxygenated products, and branched hydrocarbons, depending upon promoters and process conditions employed. In order to obtain the desired reactions using zeolite catalysts, process temperatures need to be about 300℃. However, at these temperatures, cobalt based bi-functional catalysts have very high methane selectivity. The methane selectivity of iron catalysts at the same temperature level is lower. Furthermore, iron catalysts favor the production of primary olefins, which then can be converted into the gasoline range hydrocarbons via zeolite in the bi-functional catalysts. Hydrocarbons are produced by active metal and chain length of the hydrocarbons are arranged by zeolites with cleavage and isomerization reaction. Promoter is one of the important factor affecting the product selectivity. Besides, enhancing the selectivity towards the desired product, promoters have many other effects such as increasing the reaction rate the surface area, and the mechanical resistance and ensuring the stability of the catalyst. Chemical promoters, such as K, Cu, Ru, Zn, etc., are often added into iron-based catalyst. Especially, Cu and K promoters have been proved to promote the reduction of iron-based catalyst as well as the adsorption and dissociation of CO and play an important role in the FT synthesis performances. In this thesis, promoters were added on the zeolite-supported iron catalyst and their effects on the catalyst activity and selectivity were examined. In this context, the prepared catalysts consisted of three components: the active metal, support material and promoter. In experimental studies, ZSM-5 with very high silica to alumina ratio of 280 was used in the catalyst synthesis due to its low acidity and cation exchange sites. The purpose for choosing this zeolite as a support was to take advantage of its shape selectivity and ability of suppressing the effect of alkali migration on zeolite caution exchange sites. The active metal was iron and promoters were Cu, K, and Mn. Catalysts with single promoter, two and tree promoters were prepared and investigated in order to determine the effects of promoters on the performance of zeolite-iron bi-functional catalyst in FT synthesis. Eight catalysts were synthesized, characterized and tested. The catalysts were synthesized by incipient to wetness impregnation. The synthesized catalyst are designated as FeZ, CuFeZ, KFeZ, MnFeZ, CuKFeZ, CuMnFeZ, KMnFeZ and CuKMnFeZ based on the added promoters. FeZ is the bi-functional "base catalyst" which synthesized from zeolite and iron, others are "catalysts with promoters" (ie: Cu, K, Mn, Cu-K, Cu-Mn, K-Mn and Cu-K-Mn) prepared by adding of promoters to the base catalyst. The catalysts were characterized by using AAS, XRD, SEM, TPR and BET analyses methods. Activity tests of catalysts were carried out in a pressurized fixed bed type reactor. The performances of the catalysts were tested at different temperatures with a fixed H2/CO ratio and operating pressure. The gas and liquid phase products from the FT reactor were analyzed by using gas chromatography method. All catalysts studied were found to be active in FT reactions. The performances of catalysts were compared based on the activity and selectivity. The promoter type was found to have a considerable impact on the activities and selectivities of catalysts. The highest reactant conversions were obtained with potasium (K) promoted catalysts. Base catalyst displayed the highest methane selectivity and with the addition of promoters methane selectiviy was reduced. The selectivity toward gasoline range hydrocarbons (C5–C11) was increased with promoted catalysts. MnFeZ and CuKMnFeZ catalysts displayed the highest selectivity toward gasoline range hydrocarbons. The selectivity toward C5–C11 hydrocarbons of promoted catalysts varied between 48.5% and 69.8%. Results of FT products analysis showed that potasium increased the chain length and shifted product distribution to the heavy hydrocarbon.The highest C12+ selectivity was obtained by KMnFeZ and KFeZ catalysts. In the meantime the highest gas phase olefin selectivity and WGS activity was achieved by potasium promoted catalysts. These results show that, potasium promoter increased the WGS activity and olefin selectivity. In the study, the effects of process parameters such as reaction temperature, gas hourly space velocity (GHSV) and presence of H2S on the catalyst performance were also investigated. Results of experiments carried out with the copper promoted catalyst (CuFeZ) at three different temperatures indicated that the CO and H2 conversions and WGS reaction activity increased with increasing reaction temperature. Opearating temperature also affect the product composition with product distribution shifting to the ligth hydrocarbons as temperature increased. The effect of gas hourly space velocity (GHSV) was investigated for the base catalyst (FeZ). As it might be expected, the conversions decreased as GHSV increased, but the compositions of the FT products seemed not to be affected. Experiment carried out by using 30 ppm H2S containing synthesis gas and a synthesized catalyst (FeZ) showed a relatively limited drop in the catalyst activity in first 100 hours of operation after which activity fastly decreased with time.