Deactivation of a Biomass-Derived Zirconium-Doped Phosphorus-Containing Carbon Catalyst in the Production of Dimethyl Ether from Methanol Dehydration

Abstract

Dehydration of methanol to produce dimethyl ether (DME) was studied at relatively high temperatures (400−600 °C) on biomass-derived phosphorus-containing carbon impregnated with a zirconium salt. Highly thermally stable zirconium phosphate surface groups could be obtained on the final catalyst, which were responsible for the high stability and selectivity to DME of the catalyst at temperatures lower than 400 °C. However, harder operation conditions, closer to those of the industrial process, were evaluated to analyze the changes of the catalyst surface properties with the reaction temperature and the possible causes of deactivation. Thus, high methanol conversion and selectivity to DME were also observed in the temperature range of 400−600 °C, although deactivation was detected. Coke deposition was responsible for a decrease in microporosity and surface concentration of zirconium and phosphorus of the catalyst. Temperature-programmed desorption, 31 P magic angle spinning nuclear magnetic resonance, and X-ray photoelectron spectroscopy results suggest that the Zr−O−P groups from zirconium phosphate species were responsible for the long-term stability of the catalyst and that the C−O−P-type active sites were deactivated very fast. However, coke deposition on Zr− O−P-type active sites caused a slow and irreversible deactivation, while deposited coke on the C−O−P-type active sites was easily eliminated by the oxidative treatment in air. A reaction scheme that accounted for the gas product distribution and the production of coke was proposed. A kinetic model for coke formation as a function of time on stream that successfully represents the experimental results was also propounded, which yielded a value for the activation energy for the production of coke of 124 kJ/mol.

A Kinetic Model Considering Catalyst Deactivation for Methanol-to-Dimethyl Ether on a Biomass-Derived Zr/P-Carbon Catalyst

“…To explain the main product distribution shown in Figure 1, the following reactions were considered: methanol dehydration to DME (Equation ( 4)); methane formation, which takes into account the additional formation of water (Equation ( 5)); CO formation (Equation ( 6)); and coke production (Equation ( 7)) [29].…”

“…In this sense, Palomo et al following a Langmuir-Hinshelwood mechanism, proposed a fourstep mechanism for DME production, in which two methanol molecules were sequentially adsorbed in the active sites, and then reacted with each other to produce DME and adsorbed water, which was desorbed to regenerate the initial active sites [28]. On the other hand, at higher temperatures (between 450 • C and 550 • C), two adsorbed methanol molecules could also react through a six-member ring, producing methane and an intermediate that mainly evolved into coke and water or CO and hydrogen [29].…”

“…However, when oxygen was not present in the reaction medium, conversion decayed to a residual value of around 7% after only 20 min [14]. To overcome this problem, zirconium was added to those P-containing activated carbons, obtaining surface zirconium phosphate species [28,29]. The latter catalyst achieved a stable 50% yield to DME for more than 72 h at 350 • C, without the release of any other byproducts [30].…”

“…C-O-P groups were also present in the catalyst, but those groups were deactivated faster than Zr-O-P ones. Moreover, when the spent catalyst was treated with air at 350 • C for 2 h, only a partial regeneration of the catalyst could be observed, as C-O-P groups were recovered; meanwhile, deactivated Zr-O-P-type active sites remained inactive [29].…”

“…When that Zr-loaded P-containing carbon was submitted to higher temperatures, a different reaction pathway seemed to take place, because some coke formation was observed. This reaction pathway considered the release of methane and the formation of a formaldehyde intermediate, which could yield water and coke, which were irreversibly adsorbed on the active site, or carbon monoxide and hydrogen, regenerating the active site [29].…”