Abstract
Alcohol coupling, also known as the Guerbet reaction, is a potentially important process to increase the value of short chain alcohols. Metal oxides, metal phosphates and supported transition metals like copper are some examples of heterogeneous catalysts for the reaction. However, the wide variety of catalyst compositions, reaction conditions, and reactor configurations used to study the reaction complicates a direct comparison of various catalysts. Herein, rates over different catalysts will be compared. One existing gap in the current literature is a thorough comparison of the surface acid and base properties of catalysts employed in the Guerbet coupling of alcohols. In this work, various bifunctional materials were synthesized, characterized, and used in Guerbet alcohol coupling and other probe reactions of acid and base catalysts.
One well-studied reaction catalyzed by acids and bases is transesterification. Two series of Mg:Zr mixed oxides, prepared by either co-precipitation or sol-gel synthesis, were characterized and evaluated in the base-catalyzed transesterification of tributyrin with methanol. A co-precipitated Mg-rich mixed oxide catalyst with Mg:Zr 11:1 was approximately 300% more active than MgO on a surface area basis, whereas pure ZrO2 was inactive for the reaction. To explore the nature of the activity enhancement, samples were characterized by X-ray diffraction, N2 adsorption, CO2 adsorption microcalorimetry and DRIFTS of adsorbed CO2 and CH3OH. Although the sol-gel synthesis method provided better atomic-level mixing of Mg and Zr, the resulting catalysts were not as effective as mixed oxides prepared by co-precipitation. The most active mixed oxide (Mg:Zr 11:1) exhibited a high initial heat of CO2 adsorption and modified modes of methanol adsorption compared to MgO. However, the CO2 adsorption capacity did not correlate to catalyst activity.
To further examine the promotional effect that zirconia has on magnesia, mixed oxides were prepared by co-precipitation under controlled pH conditions or rising pH conditions. The resulting mixed oxides were characterized using NH3 and CO2 adsorption microcalorimetry, X-ray diffraction, and scanning electron microscopy. The samples were also tested as catalysts for transesterification of tributyrin with methanol, coupling of acetone and conversion of ethanol to ethene, ethanal and butanol. Zirconia promoted the activity of MgO for both transesterification and acetone coupling, presumably by exposing new acid-base pairs at the surface. During ethanol conversion, however, zirconia promoted the dehydration reactions. Characterization and reactivity results suggest that a Mg:Zr sample prepared by constant pH precipitation exposes more ZrO2 than a sample prepared by the rising pH method.
To increase our understanding of MgO as a catalyst for ethanol coupling, steady state isotopic transient kinetic analysis was used in conjunction with in-situ IR spectroscopic measurements. The isotopic transient analysis of ethanol coupling to butanol over MgO in a fixed bed reactor at 673 K revealed a surface coverage of adsorbed ethanol equivalent to about 50% of the exposed Mg-O atomic pairs. Diffuse reflectance IR spectroscopy of the ethanol reaction at 673 K confirmed that the surface was populated primarily with adsorbed ethoxide and hydroxide, presumably from the dissociative adsorption of ethanol. The coverage of reactive intermediates leading to butanol was an order of magnitude lower than that of adsorbed ethanol, and about half the surface base sites counted by adsorption of CO2. The intrinsic turnover frequency for the coupling reaction at 673 K determined by isotopic transient analysis was 0.04 s-1, which is independent of any assumptions about the nature of the active sites. Although the ethanol coupling reaction appeared to involve aldol condensation of an aldehyde intermediate, the high coverage of ethanol under steady state conditions apparently inhibited unproductive C-C coupling reactions that deactivate the catalyst at high temperature.
The influence of sodium addition to zirconia on the acid-base properties of the surface and on the catalytic conversion of ethanol and acetone was also investigated. The rates of ethanol dehydration, dehydrogenation and coupling were evaluated in a fixed bed flow reactor operating at temperatures from 613-673 K. The rate of acetone condensation was evaluated in the same reactor operating at 473-573 K. Addition of 1.0 wt.% Na to ZrO2 decreased the rate of ethanol dehydration by more than an order of magnitude, which was consistent with a neutralization of acid sites evaluated by ammonia adsorption microcalorimetry. Addition of 1.0 wt.% Na to ZrO2 also increased the base site density quantified by carbon dioxide adsorption microcalorimetry and the rate of acetone condensation. Although the rate of ethanol coupling was not increased by addition of Na, the overall selectivity of ethanol to butanol was improved over the 1.0 wt.% Na/ZrO2 sample because of the significant inhibition of ethanol dehydration.
