Abstract
Drawbacks posed by corn-based bioethanol as a gasoline fuel additive have called attention to its catalytic transformation into a higher-value fuel or chemical, such as butanol. The catalytic conversion of bio-derived ethanol to butanol occurs via the so-called Guerbet reaction – a multi-step sequence of reactions that ultimately couples two short-chain alcohols to produce a longer chain saturated alcohol. Recent studies have demonstrated unusually high activity and high butanol selectivity during the Guerbet coupling of ethanol over calcium hydroxyapatite (HAP; Ca10(PO4)6(OH)2) catalysts; however the nature and composition of the active site(s) on these materials have not been clearly defined and therefore a detailed, molecular-scale understanding of the reaction mechanism is lacking.
In this work, hydroxyapatite catalysts of varying chemical compositions (Ca/P = 1.50, 1.66, 1.88) were synthesized via co-precipitation and compared to typical solid base metal oxides (e.g. MgO and CaO). Acid-base surface characterization using adsorption microcalorimetry and IR spectroscopy of various adsorbed molecular probes combined with gas-phase reactivity testing were used to identify key structure-function relationships of the catalytic materials. The results conclusively showed that the excellent performance of stoichiometric HAP is the result of a high surface density of acid-base site pairs of intermediate strength that facilitate all of the steps in the Guerbet sequence.
Additionally, multiproduct steady-state isotopic transient kinetic analysis (SSITKA) of the ethanol coupling reaction was used for the first time over stoichiometric HAP (613 K) and results were compared to those obtained with MgO (653 K). The SSITKA results provided a direct quantification of important intrinsic kinetic parameters of the reaction, i.e. surface concentrations of reaction intermediates, mean surface residence times, and turnover frequencies. Given the generally accepted mechanism for Guerbet coupling that involves aldol condensation of acetaldehyde, the SSITKA results revealed that a greater fraction of the acetaldehyde produced during the reaction proceeds toward coupling products on HAP relative to MgO. The TOF associated with intermediates that form butanol on HAP (0.016 s-1) was lower than that of MgO (0.059 s-1). Therefore, the higher rates of butanol formation observed over HAP compared with MgO is a consequence of a much higher coverage of surface intermediates leading to butanol during the steady-state reaction. This finding is consistent with the considerably higher surface density of appropriate-strength acid-base site pairs measured on the HAP surface compared with MgO.
To advance our scientific understanding of the active site on the surface of HAP required for butanol formation, the ethanol coupling reaction was investigated at 633 K over beta tricalcium phosphate (β-TCP; β-Ca3(PO4)2) and fluorine-substituted hydroxyapatite (FAP; Ca10(PO4)¬6F2) catalysts. Both β-TCP and FAP were catalytically active for C-C bond formation suggesting that the PO43- group is likely the base site in the active acid-base site pair for butanol formation during ethanol coupling over HAP. Water co-feeding experiments over MgO revealed that water irreversibly adsorbed onto Lewis acid-strong base site pairs that were catalytically active for butanol formation whereas weak and reversible interactions were observed between water and the HAP surface.
Finally, the influence of Lewis acidity was explored by comparing the catalytic activity during reactions of ethanol over Mg3(PO4)2, β-Ca3(PO4)2, and Sr3(PO4)2. Of the three phosphates tested, β-Ca3(PO4)2 was the most selective for Guerbet coupling, indicating Ca2+ cations have the appropriate Lewis acid strength for the reaction. Interestingly, exceptional activity and selectivity of Sr3(PO4)2 for ethanol dehydrogenation was also observed.
