autothermal catalytic partial oxidation of bio-oil … catalytic partial oxidation of bio-oil...

Autothermal Catalytic Partial Oxidation of Bio-Oil FunctionalGroups: Esters and Acids

David C. Rennard, Paul J. Dauenhauer, Sarah A. Tupy, and Lanny D. Schmidt*

Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 421 WashingtonAVenue SE, Minneapolis, Minnesota 55455

ReceiVed September 22, 2007. ReVised Manuscript ReceiVed December 10, 2007

The catalytic conversion of esters and carboxylic acids into synthesis gas or chemicals could permit optimalutilization of organic intermediates such as pyrolysis bio-oils. To examine the ester moiety, two esters, ethyllactate and ethyl propionate, and two acids, lactic acid and propionic acid, were reformed by catalytic partialoxidation. Autothermal reforming was examined over platinum and rhodium based catalysts supported onalumina foam monoliths at a contact time on the order of 10 ms. Conversions >98% were observed for allfour fuels. The addition of cerium or lanthanum was found to increase the selectivity to synthesis gas. Athigher fuel to oxygen ratios, nonequilibrium species such as ethylene and acetaldehyde were observed. Ethylpropionate produced twice as much ethylene as ethyl lactate but very little acetaldehyde. The additional hydroxylgroup in ethyl lactate produced acetaldehyde such that the ratio of ethylene to acetaldehyde was ∼1:1. Theresults provide evidence that the homogeneous decomposition of esters to intermediate acids can contribute tothe overall reforming process. Products are highly tunable between synthesis gas and olefins by varying thefuel-to-oxygen ratio and the catalyst.

1. Introduction

Concerns about energy security and CO2 emissions havemotivated research on the best methods of utilizing biomass.One method, fast pyrolysis, rapidly heats solid particles ofbiomass to form a complex liquid mixture called “bio-oil”.1 Thisliquid provides an energy-dense biomass intermediate that couldbecome a major biorefinery feedstock analogous to crude oil.The next processing step would be to upgrade bio-oil to a streamof building-block chemicals (such as ethylene) or synthesis gas(H2 + CO) for the production of synthetic fuels such as ethanol,alkanes, or dimethyl ether.

Technologies for refining crude oil can be applied in abiorefinery such as conversion of bio-oil to synthesis gas orchemicals by catalytic partial oxidation.2–4 However, bio-oilsproduced by fast pyrolysis can be considerably more complexthan crude oil as they can contain up to 400 different organiccompounds with various oxygenated functional groups.5 There-fore, an understanding of the catalytic reforming of bio-oils mustbe based on simple, representative feedstocks. Previous researchhas examined the catalytic reforming of many of the important

functional groups occurring in bio-oil including alcohols,6,7

polyols,8 ethers,9 and oils and C6 sugars.10 Biodiesel, a methylester of a long fatty acid, produces high selectivity to olefins,including olefinic esters.11 The ester and acid functional groupscomprise a considerable fraction of bio-oil for which significantexperimental study remains.12

This paper examines the catalytic partial oxidation (hereafterreferred to as CPOx) of two ethyl esters, ethyl propionate (Figure2A) and ethyl lactate (Figure 2B), and two acids, propionic acid(Figure 2C) and lactic acid (Figure 2D). These esters and acidsdiffer only by a hydroxyl group (-OH) on the R′ carbonate carbon(Figure 2) but provide sufficiently similar functionality to revealdetails of the complex chemistry defining oxygenate reformation.All of these compounds can be reformed to synthesis gas withoutsignificant heat generation as indicated by the partial oxidation ofethyl lactate,

C5H10O3 + O2 f 5CO + 5H2 ∆H° ∼ 80 kJ/mol

(1)

* To whom correspondence should be addressed. Telephone: +1 612625 9391. Fax: +1 612 626 7246. E-mail: [email protected].

(1) Bridgwater, A. V.; Meier, D.; Radlein, D. An overview of fastpyrolysis of biomass. Org. Geochem. 1999, 30, 1479–1493.

(2) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R.Steam reforming of model compounds and fast pyrolysis bio-oil onsupported noble metal catalysts. Appl. Catal., B 2005, 61, 130–139.

(3) van Rossum, G.; Kersten, S. R. A.; van Swaaij, W. P. M. Catalyticand non catalytic gasification of pyrolysis oil. Ind. Eng. Chem. Res. 2007,46 (12), 3959–3967.

(4) Dauenhauer, P. J.; Dreyer, B. J.; Degenstein, N. J.; Schmidt, L. D.Millisecond reforming of solid biomass for sustainable fuels. Angew. Chem.,Int. Ed. 2007, 46, 5864–5867.

(5) Huber, G. W.; Corma, A. Synergies between bio- and oil refineriesfor the production of fuels from biomass. Angew. Chem, Int. Ed. 2007, 38,7320–7338.

(6) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E.Renewable hydrogen from ethanol by autothermal reforming. Science 2004,303, 993–997.

(7) Wanat, E. C.; Suman, B.; Schmidt, L. D. Partial oxidation of alcoholsto produce hydrogen and chemicals in millisecond-contact time reactors.J. Catal. 2005, 235, 18–25.

(8) Dauenhauer, P. J.; Salge, J. R.; Schmidt, L. D. Renewable hydrogenby autothermal steam reforming of volatile carbohydrates. J. Catal. 2006,244, 238–247.

(9) Wang, S.; Ishihara, T.; Takita, Y. Partial oxidation of dimethyl etherover various supported metal catalysts. Appl. Catal., A 2002, 228, 167–176.

(10) Salge, J. R.; Dreyer, B. J.; Dauenhauer, P. J.; Schmidt, L. D.Renewable hydrogen from nonvolatile fuels by reactive flash volatalization.Science 2006, 314, 801–804.

(11) Subramanian, R.; Schmidt, L. D. Renewable olefins from biodieselby autothermal reforming. Angew. Chem., Int. Ed. 2005, 44, 302–305.

(12) Siplia, K.; Kuoppala, E.; Fagernas, L.; Oasmaa, A. Characterizationof biomass-based flash pyrolysis oils. Biomass Bioenergy 1998, 14, 103–113.

Energy & Fuels 2008, 22, 1318–13271318

10.1021/ef700571a CCC: $40.75 2008 American Chemical SocietyPublished on Web 02/02/2008

which is slightly endothermic. The energy required to maintaina high temperature autothermal operation comes from theproduction of some complete combustion products, H2O andCO2, as indicated by the combustion of ethyl lactate,

C5H10O3 + 6O2 f 5CO2 + 5H2O

∆H° ∼ - 2550 kJ/mol (2)

This exothermic chemistry maintains operation at temperaturesgreater than 600 °C, permitting complete conversion to equi-librium products in ∼10 ms.

At the high temperatures of autothermal operation, thecontribution of gas-phase chemistry cannot be neglected.Evidence for significant gas-phase chemistry under CPOxconditions has been observed by increasing the fuel-to-oxygenratio, such that nonequilibrium intermediates resulting fromhomogeneous chemistry exit the reactor before being completelyreformed on the catalyst surface.13 Under these conditions, theslight variation between fuel molecules provides details of theorigin of each intermediate, as well as the overall mechanism.For example, ethyl esters have been shown to undergo gas phasedecomposition to acids and olefins.14 The thermal cracking ofethyl propionate could produce the intermediate, propionic acid,which could then undergo additional decomposition to ethylene,

C5H10O2 f CH3CH2COOH + C2H4 f

2C2H4 + CO + H2O (3)

While this decomposition indicates decarbonylation (-CO) ofthe acid intermediate, decarboxylation (-CO2) may compete.In this case, CO2 and H2 would replace CO and H2O in theequation above.

The addition of a hydroxyl group to the R′-propionate carbonshould result in different chemical intermediates, indicative ofthe overall decomposition mechanism. Thermal decompositionof ethyl lactate could proceed through a lactic acid intermediate,

C5H10O3 f C2H4 + CH3CHOHCOOH f

C2H4 + CO + C2H4O + H2O (4)

which results in acetaldehyde replacing an ethylene as ahomogeneous product.

These mechanisms were explored with experiments performedon Rh-based catalysts with the addition of Ce or La, which havebeen shown to increase selectivity to synthesis gas and reduceside products.13,15 Pt catalysts, which have been shown to beselective to olefins,16 have been considered for comparison.Additionally, the thermal decomposition of the selected esterson blank supports has been examined to provide evidence forthe mechanism of gas-phase ester decomposition.

2. Experimental Section

Catalysts were prepared by coating noble metals on cylindricalceramic foam monoliths (92% R-Al2O3, 8% SiO2). The foammonoliths measured 17 mm in diameter and 10 mm in length andweighed ∼2.2 g before metal loading. Monolith foam density was

80 pores per inch (PPI) in all cases except for propionic acid onPt, which used 45 PPI foams.

Four different metal loadings were examined: rhodium (Rh 5wt %), rhodium and cerium (Rh-Ce 2.5 wt % each), rhodium andlanthanum (Rh-La 2.5 wt % each), and platinum (Pt 5 wt %). Allcatalysts were applied using the incipient wetness impregnationtechnique: Rh was deposited in solutions of Rh(NO3)3 salt in water,Ce as Ce(NO3)3 ·6H2O, La as La(NO3)3 ·6H2O, and Pt as PtH2Cl6.The foams were then allowed to dry in air for 6 h. Rhodium coatedfoams were calcined in a furnace for 6 h in air at 600 °C. To preventloss of Pt loading, Pt catalysts were reduced under N2 and H2 at600 °C for 6 h.

All fuels examined are liquid at room temperature and haveboiling points between 95 and 160 °C, so they can be vaporizedupstream of the catalyst. The experimental setup for the CPOx ofliquid fuels with similar characteristics has been described previ-ously.6 Considered fuels included ethyl lactate (>99%, NatureWorks), ethyl propionate (>99%, Acros), propionic acid (>99%,Avocado), and lactic acid (∼65% in water, measured experimentallyby closing the carbon balance at low C/O ratio).

A diagram of the reactor is shown in Figure 1. Each fuel wasplaced in a fuel tank pressurized with a blanket of N2 to 20 psi.The fuels were sprayed through an automotive fuel injector into aquartz tube 19 mm in inner diameter wrapped in insulation. Thefuel injector (accurate within (3%) allowed the fuels to be sprayedin small droplets which facilitated mixing and vaporization. Thewalls of the reactor were heated externally by wrapped heating tape.A chromel-alumel thermocouple hereafter referred to as “upstream”was fed through a port 4 cm above the catalyst. This port alsoenabled upstream samples to be taken to monitor flow rates. A blankmonolith wrapped with ceramic paper to hold it in place was placedjust above the upstream thermocouple to promote mixing. Theupstream temperature was maintained near the boiling point of theinjected fuel.

N2 and O2 were independently metered by mass flow controllers(accurate to within (5%). This gas mixture at air stoichiometrywas fed through a side port near the fuel injector. The air andvaporized fuel flowed at a rate of 4 SLPM (standard liters perminute, where standard refers to ideal gases measured at theinjection site, 18 °C, 1 atm). This mixture passed over three foammonoliths: a catalyst placed between two heat shields to minimizeheat loss, one upstream, which also promoted mixing, and onedownstream of the catalyst coated foam monolith. A groove waschiseled in the top face of the downstream heat shield to accom-modate a second thermocouple which measured temperatures onthe back face of the catalyst (“backface”). This thermocouple wasbent to the side of the backface heat shield and trailed downstreamwhere it wound out through a septum at the bottom of the reactor.All three foam monoliths were wrapped in ceramic paper to secure

(13) Salge, J. R.; Deluga, G. A.; Schmidt, L. D. Catalytic partialoxidation of ethanol over noble metal catalysts. J. Catal. 2005, 235, 69–78.

(14) Patai, S. The Chemistry of Acid DeriVatiVes Part 2; John Wileyand Sons: New York, 1979; Supplement B-2.

(15) Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.;Grey, C. P.; Murrell, A. J.; Vernon, P. D. F. Selective oxidation of methaneto synthesis gas using transition metal catalysts. Nature 1990, 344, 319–321.

(16) Bodke, A. S.; Oschki, D. A.; Schmidt, L. D.; Ranzi, E. Highselectivities to ethylene by partial oxidation of ethane. Science 1999, 285,712–715.

Figure 1. Diagram of reactor setup. Fuel and air are fed into the top ofa quartz reactor and vaporized by a preheat section. The reaction occurson a noble metal catalyst that is thermally insulated. Product samplesare collected downstream at the bottom of the reactor.

Autothermal Catalytic Partial Oxidation of Bio-Oil Energy & Fuels, Vol. 22, No. 2, 2008 1319

their position, provide extra insulation, and ensure all flow wasdirected through the foam monoliths.

A port in the reactor positioned 4.0 cm below the backface heatshield and sealed with an NMR septum allowed gas phase samplesof the product stream to be obtained by a gastight syringe. Productgases then exited through another port into a heated exhaust pipeand incinerated in a bunsen burner.

The samples were analyzed by gas chromatography. All productsand fuels were directly observed except for water, which wasderived by balancing oxygen species: oxygen atoms observed inthe exit stream were compared with those known to be enteringthe reactor, and lost oxygen was attributed to water. Balances onatomic carbon typically closed within 10% and balances on atomichydrogen typically closed to within 5%. For fuel rich experimentaltrials at C/O > 1.4, carbon balances exceeded 10% error, likely asa result of condensation of reactor effluent within the samplingsystem.

Experimental runs for the esters were at least 15 h, during whichtime the reactor experienced several start-ups and shut-downs.Temperatures ranged from 600 to 1200 °C, limited in the coldregime by poor conversion and in the hot regime by the possibilityof sintering the metal loaded on the monolith.

For experiments over blank alumina foams, the same reactorsetup was used except that the catalyst was replaced with anuncoated foam monolith. In addition, heating tape and insulationwere replaced with a temperature controlled insulated heating coil.

No O2 was present for these experiments. Because of the longerheating element, contact times for the blank foams were τ ∼ 0.5 s.

Data is presented in terms of conversion and atomic productselectivities for carbon and hydrogen species. Points are averagesof several runs, typically at least three per data point per catalyst.Conversion, X, is defined as the percentage of molar flow of fuel(Ffuel) reformed into products. This value is calculated by subtractingthe ratio of observed fuel inflow to observed fuel outflow from100%. Products containing carbon are described by carbon selectiv-ity to the species i, Sc(i). This selectivity is defined as the ratio ofthe carbons in species i to the total carbon in the converted fuel.

SC(i))CiFi

Cfuel(Ffuelin -Ffuel

out )(5)

Products containing no carbon (H2 and H2O) are analogouslydescribed by hydrogen selectivity to j, SH(j).

SH(j))HjFj

Hfuel(Ffuelin -Ffuel

out )(6)

where Hj indicates the number of hydrogen atoms in species j.Averages at each C/O value, defined as the ratio of carbon in

fuel to oxygen in air, are represented graphically. Deviations fromthese averages were pooled among all C/O values to calculate errorbars which represent one standard deviation. Equilibrium data was

Figure 2. Conversion (X) and backface temperatures observed during the CPOx at various C/O of ethyl propionate (panel A), ethyl lactate (B),propionic acid (C), and lactic acid (D). The bottom left corner of each panel depicts the compound. Lactic acid has the same structure as propionicacid with an additional hydroxyl group (OH) on the R′ carbon. Rh (open triangle up), Rh-Ce (open square), Rh-La (black circle), and Pt (opendiamond) show conversion (left axis); Rh (black triangle up), Rh-Ce (black square), Rh-La (open circle), and Pt (gray diamond) show backfacetemperatures (right axis). Contact times: τ ∼ 10 ms. Pt temperatures are lower than the Rh-based catalysts except at C/O < 1.3 for ethyl lactate.Temperatures rise at lower C/O ratios as a result of more exothermic oxidative chemistry. Conversion is >98% for C/O < 1.7 and >80% at all C/Oratios.

1320 Energy & Fuels, Vol. 22, No. 2, 2008 Rennard et al.

calculated using the software package ChemKin 3.7. Heat transferto the reactor was ignored and the pressure set to 1 atm to modeladiabatic conditions. Inputs included fuel, N2, O2, and input(upstream) temperature, which was taken as an average of allupstream temperatures for a given C/O value. Equilibrium productsconsidered were CO, CO2, H2, H2O, CH4, O2, and N2. ChemKincalculated equilibrium products and quantities are depicted in thedata.

3. Results

Conversion at C/O < 1.7 for both esters and acids was >98%and >80% at all C/O ratios (Figure 2). For C/O < 1.1, anautothermal reaction took place at temperatures just above 1100°C. For ethyl lactate at C/O < 1.2, Pt exhibited the highesttemperatures of the four catalysts, while it had the lowesttemperatures of all four catalysts for ethyl propionate at all C/Oratios, reaching 1000 °C in the most oxygen rich regime.Temperatures dropped by 100 °C between 1.1 < C/O < 1.4.Temperatures as low as 650 °C at C/O ) 2.5 were observed.For ethyl lactate over a Pt catalyst, temperatures were 200 °Clower. Steady state was obtained for these values, but any higherC/O extinguished the reaction.

Generally, the reforming of esters proceeded to near equi-librium syngas for C/O < 1.2. Selectivity to olefins increased

to ∼30% for C/O (>1.5). The production of equilibriumproducts (C1 species, H2, and H2O) and olefins will be discussedseparately.

Equilibrium Products from Esters: CO, CO2, H2, H2O,CH4. For C/O < 1.1, the selectivity to CO was ∼80%, whichis near equilibrium, for both esters on all Rh catalysts (Figure3A,B). The selectivity to H2 was ∼60%, which is within 10%of equilibrium under the same conditions, with Rh-Ce exhibit-ing the highest selectivities to CO and H2 (Figure 3C,D). Ptexhibited lower selectivities by up to 20% for CO and as muchas 35% for H2. Syngas production declined at higher C/O forall four catalysts. At C/O > 2.0, the selectivity to CO was aslow as ∼20%; the selectivity to H2 was as low as 10% on allfour catalysts for both fuels.

Combustion products did not decline with an increase in C/Oas rapidly as syngas products. CO2 declined gradually from 20%at C/O ) 1.1 to 10% at C/O > 2 (Figure 4A,B). The selectivityto H2O remained relatively consistent at 40% for both esters atall C/O (Figure 4C,D).

The selectivity to methane is shown in Figure 4E,F. Methaneproduction reaches a maximum of 5% for ethyl propionate atC/O ) 1.2 and 10% for ethyl lactate at C/O ) 1.5.

Non-Equilibrium Products: Ethylene, Acetylene, Acet-aldehyde. At C/O > 1.3, Pt produced the highest selectivity to

Figure 3. Selectivities to syngas products from ethyl propionate (left column) and ethyl lactate (right column) over four catalysts: Rh (blacktriangle up), Rh-Ce (open square), Rh-La (open circle), and Pt (gray diamond). Carbon selectivity to CO (Sc(CO)) is depicted in panels A andB; hydrogen selectivity to H2 (SH(H2)), in panels C and D. For C/O < 1.1, Rh-Ce and Rh-La exhibit near-equilibrium selectivities to CO and H2.At higher C/O > 1.2, selectivity to syngas products decreases away from equilibrium. Pt exhibits consistently lower selectivity to syngas productsthan Rh-based catalysts.


olefins from all fuels, though Rh-La exhibited slightly higherselectivity to ethylene for ethyl lactate with C/O > 1.7 (Figure5A,B). Ethylene derived from ethyl propionate reaches 35%selectivity on Pt. Ethylene derived from ethyl lactate reaches22%. At C/O < 1.0, selectivity to ethylene from ethyl propionateis as low as 5% on Rh-La. Selectivity to ethylene from ethyllactate under the same conditions is <1%.

The selectivity to acetylene reached 8% from ethyl propionateand 2% from ethyl lactate at the high temperatures correspond-ing to C/O < 1.4. Pt exhibited the highest selectivity to acetylenefrom both fuels.

The selectivity to acetaldehyde did not exceed 2% at any C/Ofrom ethyl propionate on any of the four catalysts. By contrast,the selectivity to acetaldehyde at C/O > 2.0 was >20% on all

Figure 4. CPOx of ethyl propionate (left column) and ethyl lactate (right column) selectivities to combustion products over Rh (black triangleup), Rh-Ce (open square), Rh-La (open circle), and Pt (gray diamond). Panel A shows carbon selectivity to CO2 from ethyl propionate.Panel B shows carbon selectivity to CO2 from ethyl lactate. Panels C and D show hydrogen selectivities to H2 from ethyl propionate andethyl lactate, respectively. Pt exhibits the highest selectivities of all catalysts to combustion products for C/O < 1.3 on both fuels. Panels Eand F depict carbon selectivity to CH4 from ethyl propionate and ethyl lactate, respectively. The selectivity to CH4 peaks at 10% at C/O ∼1.4for both fuels.


four catalysts for ethyl lactate. Rh-La exhibited the highestselectivity, nearly 30% at C/O ) 2.5.

Temperature Controlled Uncoated Foam Experiments. Toexamine the role of homogeneous chemistry, the catalyst wasreplaced with an uncoated foam monolith. To provide heat, theseexperiments were carried out in a temperature controlled furnacebetween 600 and 950 °C in an N2 atmosphere without O2. Thedata points in Figure 6 each represent a single run rather thanaverages of several runs as in the case of the autothermalcatalytic experiments. Because of the length of the furnace, thecontact time was τ ∼ 0.5 s, which is an order of magnitudegreater than the contact time for the CPOx.

The conversion of ethyl lactate was >97% for all temper-atures. For ethyl propionate, the conversion was >94% fortemperatures > 670 °C. The conversion dropped to 69% at600 °C.

The selectivities to most observed speciessC1 and C2

products, H2 and H2Osfrom the temperature controlled experi-ments are detailed in Figure 6A,B. From ethyl propionate, theselectivity to ethylene peaked at 62%, while 10% selectivity toacetylene was observed. No acetaldehyde was observed. Incontrast, for ethyl lactate, the selectivity to acetaldehyde peakedat 40% and steadily dropped with an increase in temperature,

and the selectivity to ethylene was approximately 30% through-out the temperature range.

The selectivity to CO from ethyl propionate increased from 5%at 600 °C to 20% at 750 °C. For ethyl lactate, CO increased linearlyfrom 22% at 600 °C to 40% at 925 °C. This linear increase hasapproximately the same slope as that for methane, which increasedfrom 3% to 15%. Ethyl propionate exhibited a 10% selectivity topropionic acid. No lactic acid was observed from the reforming ofethyl lactate, but ethanol was observed at temperatures <750 °C.CO2 for both experiments was <10% at all temperatures. Verylittle CO2 was observed from ethyl lactate.

Propionic and Lactic Acids. Autothermal reforming ofpropionic acid and lactic acid was explored to examine the roleof acids in the partial oxidation of esters and, more generally,in bio-oils. Propionic acid was reformed over Rh-Ce and Ptcatalysts (Figure 7A,B) and lactic acid over Pt (Figure 7C).

The noteworthy result of these experiments is that propionicacid exhibited less than 1% selectivity to acetaldehyde on bothcatalysts, while lactic acid exhibited high selectivity (up to 35%)to acetaldehyde. In addition, lactic acid produced <1% selectiv-ity to ethylene, while propionic acid showed a selectivity toethylene of about 20% (see Figure 7) and selectivity to acetyleneof ∼7%. Selectivity to CO2 from lactic acid is higher than for

Figure 5. Nonequilibrium products produced from the CPOx of ethyl propionate and ethyl lactate. All products are shown as carbon selectivities;ethyl propionate results are depicted in the left two panels and ethyl lactate results in the right two panels for all four catalysts Rh (black triangleup), Rh-Ce (open square), Rh-La (open circle), and Pt (gray diamond, note the scales for the y axes). Panel A shows selectivity to ethylene (C2H4)and acetylene (C2H2) by reforming of ethyl propionate, and panel B shows selectivity to these products from ethyl lactate. Panels C and D depictselectivity to acetaldehyde (C2H4O) from ethyl propionate and ethyl lactate. Ethyl propionate produces nearly twice as much ethylene as ethyllactate (panels A and B). However, ethyl lactate produces acetaldehyde and ethylene in close to a 1:1 ratio (panels B and D), while ethyl propionateshows negligible selectivity to acetaldehyde (panel C).


propionic acid at a corresponding C/O, consistent with the higherinternal oxygen content of lactic acid. Both acids showselectivity to CO2 > 30% for C/O < 1.3 and slightly less athigher C/O.

4. Discussion

At low C/O, all four fuels produced high selectivity tosynthesis gas. Conversely, at high C/O, selectivity to nonequi-librium products such as ethylene and acetaldehyde increasedsignificantly. The product selectivity is highly tunable betweengasification products and nonequilibrium olefinic products.

Equilibrium Products and Surface Chemistry. As shownin Figure 2, conversion of all these fuels was >98% at all C/Oratios examined. This is likely because ethyl esters readilythermally decompose in the gas phase even before they interactwith the catalyst. The resulting acids react readily on thecatalytic surface because of the ease with which the acidichydrogen is cleaved.17 Consistent with >98% conversion,autothermal reforming temperatures for oxygenates tend to bevery hot (>800 °C) for C/O < 1.3 (Figure 2).

The data in Figure 3 suggest that both esters reform toequilibrium products similar to other small oxygenated fuelssuch as ethanol or glycerol,8,13 especially at C/O < 1.3, whereselectivity to synthesis gas is very near to equilibrium for Rhbased catalysts. The esters examined in this study proceededto just below equilibrium at these C/O values. Adding awashcoat to the catalyst has been shown to increase theselectivity to syngas products,18 though no washcoat wasapplied for these experiments. With the addition of awashcoat, it is likely that the CPOx of esters would proceedto equilibrium for C/O < 1.1.

Temperature programmed desorption (TPD) studies of esterson Ni(111) by Zahidi et al. suggest that esters adsorb prefer-entially via the carbonyl group in the η1 conformation and thatthey decompose via decarbonylation (lose CO) on the metallicsurface.19 However, Figure 4A,B shows a 20% selectivity toCO2 at C/O < 1.3, which is characteristic of a surfacemechanism involving decarboxylation (lose CO2) rather than

(17) Mavrikakis, M.; Barteau, M. A. Oxygenate reaction pathways ontransition metal surfaces. J. Mol. Catal. A: Chem. 1998, 131, 135–147.

(18) Bodke, A. S.; Bharadwaj, S. S.; Schmidt, L. D. The effect of ceramicsupports on partial oxidation of hydrocarbons over noble metal coatedmonoliths. J. Catal. 1998, 179, 138–149.

(19) Zahidi, E.; Castonguay, M.; McBreen, P. RAIRS and TPD studyof methyl formate, ethyl formate, and methyl acetate on Ni(111). J. Am.Chem. Soc. 1994, 116, 5847–5856.

Figure 6. Blank foam, temperature controlled experiments for ethyl propionate (left) and ethyl lactate (right) were performed in an N2 environment.Shown are the most observed species: ethylene (asterisk), acetaldehyde (cross), CO (black square), ethanol (black diamond), CH4 (black triangleup), propionic acid (open circle), H2O (gray circle), H2 (gray diamond), C2H2 (plus sign), and CO2 (black circle). The selectivity to CO2 is <10%,indicating that gas phase decarboxylation is unlikely, while the selectivity to CO is near 20%, which suggests gas phase decarbonylation. For ethylpropionate, lower selectivity to CO is accompanied by propionic acid which did not decarbonylate, perhaps because of the low temperatures. Forethyl lactate, selectivity >20% to CO accompanies an increase in methane and a decrease in acetaldehyde, suggesting the decomposition of theacetaldehyde.


decarbonylation. This disparity is likely due to the differencein operating conditions.

The conditions of the TPD studies are lower operationaltemperatures (∼20 °C) than those observed in autothermal CPOx(∼800 °C) and were studied over different catalysts (Ni(111)rather than Rh or Pt). These reaction conditions are very differentfrom those in the experiments described in this paper. Mostimportantly, ethyl esters undergo significant thermal decomposi-tion in the gas phase, which can change the reactant prior toadsorbing on the catalytic surface. In contrast, in TPD experi-ments, the esters are adsorbed at low temperatures and do notthermally decompose. While TPD studies can lend insight intohow the esters may adsorb onto the catalytic surface, they donot indicate the gas phase decomposition that can occur underCPOx conditions. The observations described in this paperindicate that the main CPOx decomposition pathway might bethe gas phase thermal decomposition of the ester and thecatalytic decomposition of the remaining products.

If esters decompose thermally before the catalytic reaction,then a significant fraction likely adsorbs as acid by cleavingthe acidic hydrogen to form a surface carboxylate.17 Thecarboxylate could form CO2 which then desorbs. This leavesthe rest of the molecule to react in a fashion similar to otherfuels such as ethanol, by severing C-C or C-O bonds while

reacting with other surface species, depending on the noblemetal.17 Syngas, methane, and combustion products desorb.Additionally, water-gas shift is likely present under theseoperating conditions providing an alternative surface mechanismto adjust the ratio of CO to CO2.20

The selectivity to CO2 is relatively constant at ∼20% for bothfuels (Figure 4A,B), revealing evidence of the ester reformingmechanism. For the five-carbon esters studied in this paper, 20%selectivity to CO2 for most C/O ratios suggests a surfacemechanism involving decarboxylation (lose CO2) of the esterlinkage. TPD experiments indicate that esters prefer to decar-bonylate (lose CO) on noble metal catalysts19 while acidsdecarboxylate via adsorbed carbonate species under similarconditions.17,21 Because one in five atoms in the fuel form CO2

at C/O < 1.3, the surface mechanism may proceed through anacid intermediate that decarboxylates. In the acid reformingexperiments, selectivity to CO2 was about >33% (Figure 7),

(20) Wheeler, C.; Jhalani, A.; Klein, E. J.; Tummala, S.; Schmidt, L. D.The water-gas-shift reaction at short contact times. J. Catal. 2004, 223,191–199.

(21) Davis, J. L.; Barteau, M. A. Reactions of carboxylic acids on thePd(111)-(2x2)O surface: multiple roles of surface oxygen atoms. Surf. Sci.1991, 256, 50–56.

Figure 7. CPOx of propionic acid over Rh-Ce (panel A) and Pt (panel B) and of lactic acid over Pt (panel C): hydrogen selectivity (SH to H2O(gray circle), H2 (gray triangle up)), and carbon selectivity (SC to CO (black diamond), CO2 (open square), C2H4 (asterisk), C2H2 (black square),CH4 (black circle)), the most observed species. A 33% selectivity to CO2 from propionic acid suggests decarboxylation of the acid; this selectivitydecreases with an increase in C/O implying that gas phase decarbonylation competes with surface decarboxylation. Lactic acid has a slightly higherselectivity to CO2 because of its greater internal C/O. The selectivity to acetaldehyde is <1% from propionic acid but reaches 35% from lactic acid;the selectivity to ethylene reaches 20% from propionic acid but <1% from lactic acid.


consistent with a surface mechanism involving decarboxylationof the three-carbon acids.

At C/O > 1.5, the selectivity to CO2 decreases for both estersand both acids (Figures 4 and 7). This phenomenon is ac-companied by increases in olefins and aldehydes, probablyproducts of gas-phase chemistry. Thus, with an increase in C/O,surface chemistry decreases in favor of gas phase chemistry andsurface decarboxylation decreases in favor of a gas phasedecarbonylation mechanism.

Nonequilibrium Products and Homogeneous Chemi-stry. Homogeneous chemistry has been used to explain theproduction of olefins from the CPOx of alkanes.22 This isconsistent with the high selectivity to ethylene and acetaldehydeobserved in the blank foam temperature controlled experimentswith ethyl propionate and ethyl lactate as fuels, suggesting thathomogeneous chemistry could also be responsible for olefinformation from oxygenates. Furthermore, esters are known todecompose homogenously to olefins and acids.14

The results in Figure 5 show that small ethyl esters producehigh selectivity to olefins as compared to other small oxygenatedfuels. It has been previously observed that esters thermallydecompose to an acid and an olefin.23 This reaction has beenestablished to proceed through a unimolecular six-memberedtransition state shown in Figure 8.14,24 The decomposition

proceeds through an acid intermediate. However, almost no acidwas observed as a result of the reforming of ethyl esters, thoughselectivity to ethylene was observed for both esters.

The amount of ethylene differed considerably between thetwo esters as shown in Figure 5 and summarized in Figure 9.The selectivity to ethylene (and acetylene, which can be acracking product of ethylene) from ethyl propionate is nearlytwice that of ethylene from ethyl lactate. By contrast, ethyllactate forms acetaldehyde and ethylene in a nearly 1:1 ratiowhile ethyl propionate shows almost no selectivity to acetal-dehyde (Figure 9). These observations are best explained bycontinued cracking of propionic acid and lactic acid into ethyleneand acetaldehyde, respectively. These products were alsoobserved in experiments performed with the acids directly. Inthis way, the ethyl portion of both esters can form ethylene whilethe acid portion decomposes to ethylene or acetaldehyde,depending on the type of acid.

The thermal decomposition of acids in this manner has beenobserved previously. Propionic acid has been shown to decom-pose in the gas phase to ethylene, CO, and H2O.25 Similarly,lactic acid is known to thermally decarbonylate and dehydrateto acetaldehyde.26 These results suggest a possible mechanismwhereby esters (at least ethyl esters which have an availablehydrogen on the �-alcohol carbon) decompose in a two-stepprocess. In the first step, the ester forms an acid and an olefinvia a semiconcerted six-membered ring. In the second step, theacid further decarbonylates and dehydrates to a resultingchemical: ethylene in the case of propionic acid and acetalde-hyde in the case of lactic acid. Because these processes produceCO rather than CO2 from the ester linkage, a net decrease inthe 20% selectivity to CO2 is observed with an increase in gasphase chemistry. Furthermore, the dehydration associated withthis reaction explains the high selectivity to water observed inFigure 4C,D.(22) Panuccio, G. J.; Williams, K. A.; Schmidt, L. D. Contributions of

heterogenous and homogeneous chemistry in the catalytic partial oxidationof octane isomers and mixtures on rhodium coated foams. Chem. Eng. Sci.2006, 61, 4207–4219.

(23) Bilger, E. M.; Hibbert, H. Mechanism of organic reactions. IV.Pyrolysis of esters and acetals. J. Am. Chem. Soc. 1936, 58, 823–826.

(24) Arnold, R. T.; Smith, G. G.; Dodson, R. M. Mechanism of thepyrolysis of esters. J. Org. Chem. 1950, 15, 1256–1260.

(25) Doolan, K. R.; Mackie, J. C.; Reid, C. R. High temperature kineticsof the thermal decomposition of the lower alkanoic acids. Int. J. Chem.Kinet. 1986, 18 (5), 575–596.

(26) Chuchani, G.; Martin, I.; Rotinov, A.; Dominguez, R. M. Elimina-tion kinetics and mechanism of primary, secondary and tertiary R-hydroxy-carboxylic acids in the gas phase. J. Phys. Org. Chem. 1993, 6, 54–58.

Figure 8. Possible mechanism of the decomposition of ethyl lactateand ethyl propionate to olefins via the acid. First, the esters break intoethylene and an acid via a six-membered transition state. The acid thendecarbonylates (perhaps through a free radical process24) to produceeither acetaldehyde in the case of lactic acid or ethylene in the case ofpropionic acid.

Figure 9. Ethyl propionate exhibits twice the selectivity to ethylene(black square) as ethyl lactate, but the selectivity of ethyl lactate toacetaldehyde (gray circle) and ethylene (black circle) is 1:1 whereasethyl propionate exhibits negligible acetaldehyde (gray square). Thisdifference is attributed to the hydroxyl group on ethyl lactate. The acidportion of the propionate decomposes to ethylene while the acid portionof the lactate decomposes to acetaldehyde. The alcohol portion of bothesters forms an ethylene molecule.


Gas phase decarbonylation is supported by the blank foamdata (Figure 6). Thermally cracking ethyl lactate and ethylpropionate over uncoated alumina foams exhibited very littleCO2 but ∼20% selectivity to CO. Ethyl propionate did produce<20% selectivity to CO at low temperatures (Figure 6A), butit also exhibited 10% selectivity to propionic acid which hadnot decarbonylated. By contrast, at high temperatures, ethyllactate exhibited >20% selectivity to CO (Figure 6B). This isaccompanied by an apparent loss of acetaldehyde and anincrease in methane, which suggests that acetaldehyde may bedecomposing to methane and CO, a known thermal decomposi-tion pathway.27

High selectivity to olefins (>40%) in the uncoated foamexperiments (Figure 6) suggests that thermal decomposition toolefins is present in the homogeneous phase of the CPOx andthat by tuning the C/O ratio it is possible to eliminate olefinproduction or to achieve high selectivity to predictable olefinsdepending on the structure of the ester. On the basis of thestoichiometric thermal decomposition shown in eq 3, themaximum selectivity to ethylene and acetylene from ethylpropionate is 80%. Similarly, the maximum selectivity to eachethylene and acetaldehyde from ethyl lactate is 40%.

While intact ester linkages were observed in the products ofbiodiesel reforming,11 no ester groups were observed amongthe CPOx products of the light ethyl esters in these experiments.However, a few differences between biodiesel and these lightesters could explain the disparity. Biodiesel is a methyl esterthat cannot undergo thermal decomposition via the six-membered transition state described in Figure 8, whereas ethylesters readily undergo this thermal decomposition. Furthermore,biodiesel contains long olefinic fatty chains that have resonancestabilization for the cleavage of C-C bonds and the resultingradicals. Neither propionate nor lactate contain such stabilizingchains, so conversion of the ester by dehydrogenation orcleavage of the carbon chain is not favored for light ethyl esters.Finally, in the biodiesel experiments, no olefinic esters wereobserved with fatty chains shorter than six carbons. By contrast,ethyl propionate and ethyl lactate contain three carbons in the

acid chainsalready too short to be observed as the stableproducts of biodiesel CPOx.

5. Conclusions

Light esters such as ethyl lactate and ethyl propionate canbe reformed by autothermal catalytic partial oxidation atmillisecond contact times over Rh and Pt catalysts. Productscan be easily tuned to equilibrium synthesis gas or olefins byadjusting the C/O ratio. Rhodium based catalysts show higherselectivity to synthesis gas while Pt catalysts show higherselectivity for olefins and aldehydes. Reforming occurs at hightemperatures with nearly complete conversion over a broadrange of C/O ratios. Esters produce high selectivity to synthesisgas at C/O < 1.3 with few other products as predicted byequilibrium. Small organic acids such as propionic and lacticacids were also capable of autothermal reforming at these C/Oratios. These results suggest that bio-oils and more complexbiomass will be capable of autothermal CPOx to synthesis gasunder similar conditions.

Esters appear to decompose homogeneously to form an acidand an olefin. The acid may then continue to decarbonylate,thus forming another olefin or aldehyde, or it can react on thesurface to form combustion products or synthesis gas. The typesof chemicals formed by the thermal decomposition of ethylesters depend strongly on the structure of the original molecule.Ethyl lactate differs from ethyl propionate by the presence of ahydroxyl group; this hydroxyl group on the acid portion of themolecule produces acetaldehyde instead of ethylene. The ethylportion of both esters forms another ethylene molecule. It islikely that other esters with an accessible hydrogen on the�-alcohol carbon would produce similar selectivity to olefins.In this manner, a high yield of predictable olefins can beobtained from renewable feeds.

Acknowledgment. Funding for this research was provided by3M through a fellowship for D.C.R., the Department of Energy(DOE), and the Initiative for Renewable Energy and the Environ-ment (IREE) at the University of Minnesota.

EF700571A

(27) Hinshelwood, C. N.; Hutchinson, W. K. A comparison betweenunimolecular and bimolecular gaseous reactions. The thermal decompositionof gaseous acetaldehyde. Proc. R. Soc., Ser. A 1926, 111, 380–385.


autothermal catalytic partial oxidation of bio-oil … catalytic partial oxidation of bio-oil...

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