From biomass to fuels : hydrotreating of oxygen-containing feeds on a CoMo/Al2O3 hydrodesulfurization catalyst

Abstract

Biomass is a renewable alternative to fossil raw materials in the production of liquid fuels and chemicals. Liquefied biomass contains an abundance of oxygen-containing molecules that need to be removed to improve the stability of the liquids. A hydrotreating process, hydrodeoxygenation (HDO), is used for the purpose. Hydrodeoxygenation is similar to the hydrodesulfurization (HDS) process used in oil refining, relying upon a presulfided CoMo/γ-Al2O3 catalyst. The stability of the sulfided catalyst is critical in HDO because biocrudes usually do not contain the sulfur compounds needed to maintain the sulfidation of the catalyst.

The aim of this work was to examine the role of sulfur in maintaining the activity of the HDO catalyst. Sulfur was introduced as an organic sulfur-containing co-reactant or as a sulfur substituent in an oxygen-containing reactant molecule as a way of simulating mixed feeds composed of biocrudes and conventional crudes, or it was introduced as a low molecular weight sulfiding agent. In addition, the stability of the sulfided catalyst against changes in the feed composition was studied to find out whether the activity of the catalyst could be maintained by carrying out HDO alternately with HDS.

Simultaneous HDO and HDS was studied in a batch reactor with model compounds having a sulfur-containing (mercapto or methylmercapto) and an oxygen-containing (hydroxyl or methoxy) substituent in the same molecule, and with binary mixtures of mono-substituted benzene compounds. In both cases, the reactions of the oxygen-containing substituents were strongly suppressed as long as a sulfur-containing functionality was present. HDS reactions of mercapto and methylmercapto groups were either enhanced or retarded in the presence of oxygen-containing functionality. HDS was enhanced when the oxygen-containing substituent was located in para-position to the sulfur substituent thereby increasing the electronegativity of the sulfur atom and thus facilitating the adsorption of the reactant on the active site of the catalyst. Otherwise, the HDS rate declined due to strong competitive adsorption of the oxygen-containing compounds on the active sites of the catalyst, and due to the formation of less reactive sulfur compounds via methyl transfer from the methoxy groups to sulfur. In conclusion, simultaneous hydrotreating of sulfur- and oxygen-containing feeds leads to strong suppression of oxygen removal reactions and usually also to a decrease in the efficiency of sulfur removal.

The effect of low molecular weight sulfiding agents, H2S and CS2, on HDO of phenol and anisole was studied first in a batch and then in a flow reactor to see whether the addition of sulfiding agents might improve the stability of the presulfided catalyst without decreasing the rate and without affecting the selectivity of HDO. The HDO rate of phenol decreased noticeably in the presence of CS2 in the batch reactor, and the selectivities of the HDO reaction paths were changed: the hydrogenation-hydrogenolysis route was less sensitive to the sulfur compound than was the CArom-O hydrogenolysis path. At higher concentrations of the sulfiding agent, also the hydrogenation route became inhibited. With anisole, there was an increase in the rate of demethylation to phenol, but oxygen removal was virtually unaffected. In the flow reactor studies, the formation of hydrogenated HDO products of phenol remained constant up to the highest concentration of H2S in the feed, but a dramatic decrease in the yield of the aromatic reaction product occurred already at low concentrations of H2S. Selective inhibition of one of the HDO paths confirmed the presence of at least two kinds of active sites on the catalyst. This means that addition of an inhibitor can be used to adjust the product distribution of HDO in process scale. However, the presulfided catalyst deactivated with time on stream also in the presence of sulfiding agents.

Finally, the stability of the presulfided catalyst against changes in the feed composition was studied in a flow reactor. HDO of phenol and HDS of benzothiophene were carried out alternately in periods of four to eight hours. In this way, the deleterious effect of the competition of HDO and HDS was almost totally avoided and the stability of the catalyst during HDO was improved. The lengths of the HDO and HDS periods now need to be optimized.