Evaluation of the effect of acid-base properties of Ruthenium on Mn-Al2O3 catalyst for FTS

Abstract

The effect of acid-base properties as a result of calcination temperature of alumina was investigated on manganese modified supports for ruthenium Fischer-Tropsch catalysts. The process of high temperature calcination is one of the methods used to make material with specific low surface areas and a distinct α-alumina phase. Support modification and catalyst preparation were performed by applying the incipient wetness impregnation method and resulted in supports containing 4, 6 and 10 wt% Mn. Selected support samples have in addition been impregnated with 2 wt% Ru. The support modification and catalyst preparation was conducted on calcined Puralox (PuCal) and on uncalcined Puralox (Pu) alumina support. The adjusted Pu and PuCal supports as well as the corresponding catalysts were characterized by N2 adsorption, H2 chemisorption, x-ray diffraction, IPA & CO2 TPD and evaluated in a slurry reactor at 25 bar, 230 ± 0,5 °C and H2/CO = 2,1 for Fischer Tropsch synthesis. It was established that a support calcination temperature of around 1100 °C led to the support material consisting of a mixture of both γ-alumina and ɵ-alumina (PuCal). For the Pu support, only γ-alumina was detected. Elevated calcination temperatures resulted in half the surface areas of the starting material, smaller pore volumes and increased pore diameter. The uncalcined Pu support’s surface area and pore volume was around 162 m2/g and 0.5 cm3/g and the support decreased to 97 m2/g and 0.38 cm3/g respectively indicating a collapse of pore structure and a decreased surface area due to harsh temperature treatment of the support material. A further decrease in surface area was observed upon addition of manganese and the Ru metal to form Mn-modified Ru catalysts on both PuCal and Pu supports. The effect of total acid and base properties of the materials as a function of calcination, Mn addition and subsequently Ru addition was determined using temperature programed desorption (TPD) techniques. Between the Pu and the PuCal systems, the effect Mn per support surface area due to calcination was also determined. Results indicated that Mn made the support more basic, as an effect of increased basicity resulted in a decrease in Brønsted acidity. However the influence of Mn per surface area between the Pu and the PuCal system indicated similar yields of both the basic and acidic sites. The influence of Ru addition on the acid-base properties, shifted the Brønsted acidity peaks to lower temperatures, meaning that Ru occupies the stronger acid sites and on the basicity sites an increase in peak intensity was observed. From Fischer Tropsch synthesis, on Ru based catalysts on Mn modified supports Mn seemed to promoted selectivity towards highly oxygenated products on both systems. With regards to catalysts activity, Mn played a role as a poison on the Pu catalyst system and an activity promoter on the PuCal catalysts system. PuCal/6wt%Mn/Ru was the most active catalyst, with the lowest methane selectivity of all the catalysts prepared. Thus this implies that there should also be an optimum Mn loading, and other surface properties to in order to obtain the desire activity and selectivity