Research and development of Co and Rh-promoted alkali-modified molybdenum sulfide catalysts for higher alcohols synthesis from synthesis gas

The demand for mixed alcohols has grown since ether compounds were banned as gasoline octane improvers in North America. Molybdenum-based catalysts in sulfide form are an attractive catalyst system for the conversion of synthesis gas to alcohols, due to their excellent resistance to sulfur poisoning and high activity for the water-gas shift reaction. The higher alcohols activity over these catalysts is low, due to the formation of hydrocarbons and CO2. Although a number of catalysts have been developed for this purpose, not any are used commercially at this time. The main objective of this Ph.D. research is to develop a catalyst system that is capable of selectively producing higher alcohols, particularly ethyl alcohols from synthesis gas. In the present series of studies, the investigation of an alkali-promoted trimetallic Co-Rh-Mo catalyst system has led to improvements in product stream composition. The effect of different loadings of active metal (Mo), alkali (K) promoter, and metal promoters (Co and Rh) on higher alcohol synthesis from synthesis gas were investigated using commercially available multi-walled carbon nanotubes (MWCNTs) as the catalyst support. The role of support on higher alcohols synthesis was also studied using different supports, such as ã-Al2O3, activated carbons with different textural characteristics, and MWCNTs. The catalysts were prepared using the incipient wetness impregnation method and extensively characterized in both oxide and sulfide phases using different techniques. Transmission electron microscopy (TEM) results revealed that the metal particles were uniformly distributed inside and outside of the carbon nanotubes, and that metal dispersions were higher on the alkali-promoted trimetallic catalyst supported on MWCNTs. The existence of promoted and un-promoted MoS2 sites was confirmed by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies of adsorbed CO over sulfided catalysts. Temperature programmed reduction (TPR) tests showed that the addition of metal promoters improved the reduction behaviour of the catalysts. XRD patterns showed that alkali-promoted catalysts were less crystalline compared to that of the catalyst not promoted with K. The formation of Co (Rh)-Mo-S species was evident in the XANES spectra of bimetallic and trimetallic alkali-promoted MoS2 catalysts. The activity and selectivity of the catalysts were assessed in a fixed-bed micro-reactor using temperature, pressure, and gas hourly space velocity in the ranges of 275 to 350°C, 800 to 1400 psig (5.52–9.65 Mpa), and 2.4 to 4.2 m3 (STP)/(kg of cat.)/h, respectively. The Ni-promoted catalyst showed higher activity towards the formation of hydrocarbons over that of alcohols. The total alcohols space time yield (STY) and higher alcohols selectivities are significantly higher over the activated carbon-supported catalysts compared to those supported on alumina. With increased content of K, the formation of alcohols increased and hydrocarbons formation rate was suppressed. The total alcohols STY increased with increased Co content over the Co-promoted MoS2-K/MWCNTs catalysts, whereas, the maximum ethyl alcohol and higher alcohols selectivities were observed on the catalyst promoted with 4.5 wt % Co. Over the Rh-promoted MoS2-K/MWCNTs catalysts, the maximum total alcohol yield, ethanol selectivity, and higher alcohols selectivity were observed on the catalyst with 1.5 wt % Rh. The MWCNT-supported alkali-promoted trimetallic catalyst with 9 wt % K, 4.5 wt % Co, 1.5 wt % Rh, and 15 wt % Mo showed the maximum higher alcohols STY and selectivity compared to other catalysts investigated. The textural properties of the support, such as average pore diameter, pore volume and surface area, could significantly influence the extent of reduction, morphology, adsorption and has direct influence on the synthesis of mixed alcohols from synthesis gas. The optimum higher alcohols STY and selectivity were obtained over the Co-Rh-Mo-K/MWCNT catalyst at 330°C, 1320 psi (9.1 Mpa), 3.8 m3 (STP)/(kg of cat./h) using a H2 to CO molar ratio value of 1.25. To predict the reaction rate for higher alcohols synthesis, the power law model was used for the reaction between CO and H2 on the catalyst surface and the data of this study are well fitted by the model. The activation energies of ethanol and higher alcohols obtained over Co-Rh-Mo-K/MWCNTs were low compared to those values reported in the literature. The sulfided alkali-promoted trimetallic Co-Rh-Mo catalyst supported on MWCNTs was stable over a period of 720 h of continuous reaction