Impact of Water on Condensed Phase Ethanol Guerbet Reactions

Article pubs.acs.org/IECR

Impact of Water on Condensed Phase Ethanol Guerbet Reactions Tyler L. Jordison, Lars Peereboom, and Dennis J. Miller* Department of Chemical Engineering and Materials Science, Michigan State University, 428 S. Shaw Lane, East Lansing, Michigan 48824, United States ABSTRACT: The effect of water on higher alcohol and noncondensable gas formation in condensed-phase ethanol Guerbet chemistry over Ni/La2O3/γ-Al2O3 catalysts is investigated. Addition of 10 wt % water to anhydrous ethanol has a modest effect on conversion rate but significantly reduces both n-butanol and C6+ alcohol yields and increases noncondensable gas yields. Removal of water formed during Guerbet condensation reactions was accomplished by installing a recirculating loop that passed the reacting solution through a bed of 3 Å molecular sieves at low temperature. Removal of reaction water further reduces gas selectivity to less than 10% and increases alcohol selectivity to greater than 75% at 50% ethanol conversion. Water present in reaction is postulated to adsorb on the nickel surface as −OH, increasing C−C bond breakage of the adsorbed acetaldehyde intermediate, and also interact with basic sites responsible for the condensation reaction, weakening their activity. Riittonen et al.10 report formation of H2 and CH4 in a 2:1 molar ratio in condensed phase ethanol reaction over Ni/γAl2O3 at 250 °C and autogenous pressure in a batch reactor. Our prior study12 of condensed phase ethanol Guerbet reactions at 503 K over Ni/La2O3/γ-Al2O3 catalysts gave ∼80% carbon selectivity to higher alcohols, with formation of CH4 and CO2 in molar ratios ranging from 3 to 6 accounting for as much as half of the remaining carbon consumption. We first suspected that low temperature ethanol steam re-forming (reaction 1) and the water gas shift reaction (reaction 2)13,14 were responsible for gas formation, but we observed no CO and little free H2 in the product gas and no consumption of water produced in the Guerbet reactions.

1. INTRODUCTION Condensation reactions of alcohols to form higher alcohols, commonly referred to as Guerbet reactions, are attractive routes to increased biobased ethanol use and to higher value commodity chemicals and biofuels. Selectivities to higher alcohols from ethanol ranging from 40% to 99% have been reported in both vapor phase1−7 and condensed phase8−12 reactions, although the highest selectivities are typically observed only at relatively low ethanol conversion. Side products include aldehydes, ethers, esters such as ethyl acetate, acetals, and noncondensable gases. The effect of water on higher alcohol yields in ethanol-based Guerbet reactions has received little attention, yet it is an important consideration because water is a product of Guerbet reactions and because an azeotropic ethanol−water mixture would be a low-cost feed stock for higher alcohol synthesis. Marcu et al.8,9 first described the impact of water on condensed phase 1-butanol production from ethanol over a Cu−Mg−Al mixed oxide catalyst. They found that 4 wt % water addition to ethanol reduced both ethanol conversion rate and selectivity to 1-butanol; they proposed that water converts strong Lewis base sites (O2−) to weaker Brønsted base (OH−) sites on the metal oxide surface. They also report that removing water from the reaction mixture after partial ethanol conversion by adding MgSO4 restores ethanol conversion rate and 1-butanol selectivity. Riittonen et al.10 added 3 Å molecular sieves to their condensed phase reaction mixture and reported that ethanol conversion increased from 20% to 30% at otherwise identical conditions. Relatively few studies report formation of CH4, CO2, CO, or H2 in ethanol Guerbet reactions; apparently little gas other than ethene is formed in vapor phase conversion over MgO,3 hydroxyapatites,7 mixed metal oxides,1,5 or Ni/γ-Al2O3.2 © XXXX American Chemical Society

ethanol re‐forming:

C2H5OH + H 2O = 2COO + 4H 2 (1)

water gas shift reaction:

CO + H 2O = CO2 + H 2

(2)

It has long been accepted that ethanol dehydrogenation to acetaldehyde is the first step in ethanol Guerbet reaction chemistry.15 acetaldehyde formation:

C2H5OH = CH3CHO + H 2 (3)

Ethanol re-forming studies14,16−18 report that the following reactions are active at Guerbet reaction conditions over nickel catalysts: Received: February 29, 2016 Revised: May 9, 2016 Accepted: May 18, 2016

A

DOI: 10.1021/acs.iecr.6b00700 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research acetaldehyde decomposition:

temperature (230 °C) and high pressures (4.5−14.0 MPa) without otherwise interfering with reaction was constructed. When a desiccant such as a molecular sieve or MgSO4 is placed directly into the reactor, there is the possibility that the desiccant will catalyze side reactions such as dehydration. Further, desiccants work much better at temperatures below 50 °C than at reaction temperatures.19 Therefore, a drying loop with a desiccant bed external to the reactor was designed (Figure 1) to efficiently remove water and avoid side reactions.

CH3CHO = CH4 + CO (4)

CO methanation:

CO + 3H 2 = CH4 + H 2O

(5)

CO methanation:

2CO + 2H 2 = CH4 + CO2

(6)

Reactions 2 and 4−6 above are highly favored thermodynamically at condensed-phase Guerbet reaction temperatures (150− 230 °C),17,18 and CO is a thermodynamically unfavorable product below 250 °C.16,17 The observed ratios of CH4/CO2 of 3:1 to 6:1 suggest that the combination of reactions 3, 4, and 6 constitutes a decomposition pathway of ethanol (reaction 7) that is responsible for the observed gas composition:17,18 2C2H5OH = 3CH4 + CO2

(7)

Even though steam re-forming of methanol is dismissed at our reaction conditions, water remains a primary product and thus an important constituent of Guerbet reaction chemistry. Given that relatively little information exists on the effect of product or added water on Guerbet reaction chemistry, particularly related to noncondensable gas formation, and the unexplored potential to carry out Guerbet reactions on azeotropic ethanol−water mixtures instead of anhydrous ethanol, we carried out the following set of experiments with the goals of further understanding gas formation and improving the yield of higher alcohols in the reaction.

Figure 1. Recirculation loop with drying bed is shown with attachments to a 300 mL Parr reactor. The loop includes a chiller that cools the mixture to ∼40 °C and a heater that reheats it to ∼220 °C.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Ni(NO3)2·6H2O (Reagent grade, Jade Scientific) was used as the Ni precursor, and La(NO3)3·6H2O (99%, Fluka) was used as precursor to the calcined oxide. The support used was 1/16 in. diameter γ-Al2O3 spheres (Strem Chemical). The nickel catalyst modified with La2O3 was prepared in 30 g batches by first depositing La(NO3)3 (0.24 g of La(NO3)3· 6H2O/g of γ-Al2O3) by incipient wetness impregnation of the γ-Al2O3 support (0.7 cm3 solution/g of γ-Al2O3) at 25 °C followed by drying at 130 °C for 18 h and calcining at 600 °C for 20 h in 35 mL/min N2 flow. This ensured there was La2O3 on the γ-Al2O3 surface before the impregnation of the nickel. Nickel was then added by the same incipient wetness technique (0.40 g of Ni(NO3)2·6H2O/g of γ-Al2O3, 0.7 cm3 solution/g of γ-Al2O3) and dried at 130 °C for 18 h. The nickel was reduced at 450 °C and 1 atm in a tubular flow reactor for 20 h in 35 mL (STP) of H2/min. The final catalyst composition, estimated by the quantities of metal salts added via incipient wetness, was 8 wt % Ni/9 wt % La2O3−γ-Al2O3 (8Ni/9La−Al). The same catalyst was used in our earlier work,12 and its properties are given there. 2.2. Catalytic Reactions. All reactions were carried out in a 300 mL Parr autoclave reactor. Anhydrous ethanol (Koptec, 200 proof) with water added in some experiments was used as the initial reactor charge. Typically, an amount of 110 g was placed in the reactor along with catalyst (0.04 g/g of ethanol), and the reactor was sealed, purged with 1 atm nitrogen, and heated to 230 °C. A stir rate of 900 rpm was found to sufficiently suspend the catalyst in the reaction mixture. The autogenous reactor pressure was measured with a head pressure gauge, and liquid samples were taken at specific time intervals during reaction. 2.3. Catalytic Reactions with Water Removal. To test the effect of water removal on rate on selectivity, a system to continuously remove water from the reaction mixture at high

The drying loop has four components: (1) a double pipe heat exchanger−chiller consisting of a 1/8 in. tube inside a 1/4 in. × 10.5 in. length tube, supplied with tap water on the shell side, to reduce the reaction mixture temperature from 230 to 40−50 °C; (2) a self-constructed, magnetically driven reciprocating pump (see details below) to drive flow through the loop; (3) a drying bed consisting of a 1 in. diameter Schedule 80 SS pipe 14 cm in length with volume of 65 cm3, filled with 55 g of 3 Å molecular sieve (2 mm extrudates); (4) a heating section consisting of 1/8 in. tubing wrapped with heating tape to reheat the reaction mixture to reaction temperature before being returned to the reactor. The combined reactor + recirculating loop has a total ethanol capacity of 160 g. The single piston reciprocating pump capable of circulating fluid through the drying loop at reaction pressures was described by Seifried et al.20 It consists entirely of 316 stainless steel tubing and fittings, as shown in Figure 2. The main piston chamber is a 1/2 in. o.d. × 20 cm smooth bore seamless tube; the piston is a 5/16 in. o.d. × 13 cm long magnetic 416 stainless steel precision-ground rod housed inside the chamber. The piston is driven by the push−pull forces of two solenoids, controlled by an in-house built square wave generator. The four check valves on the pump assembly allow liquid to flow in only one direction. For all experiments, the pump was operated at a speed to give a flow rate of about 15 mL/min in the recirculating loop. Several experiments were performed with 2 mm glass beads in the drying bed to ascertain the effect of liquid circulation on reaction without water removal. In all experiments with the recirculating loop, liquid samples were withdrawn from the loop via the needle valve at the pump (Figure 2). 2.4. Analytical Methods. Liquid product samples from reaction were diluted 10-fold in acetonitrile and then analyzed on a Varian 450 gas chromatograph using a 30 m SolGel wax capillary column with FID detection and 100:1 split injection B


Article

Industrial & Engineering Chemistry Research

mole fractions of species i, respectively, at ti. The actual number of moles of water accumulated in the reactor at time ti is given by nH2Oaccum,ti = x H2O(nL + nloop) + yH2O nV

(9)

Initial experiments showed that the actual quantity of water generated (eq 9) was consistently 1.05−1.20 times that predicted from Guerbet condensation reactions alone (eq 8). Ethanol re-forming studies16−18 suggest that at low temperature additional water forms by ethanol reaction with hydrogen via a pathway composed of reactions 3−5 to give 2H 2 + EtOH → 2CH4 + H 2O

Figure 2. Schematic of the recirculating pump, reproduced with permission from Review of Scientif ic Instruments (Seifried et al.).20 Copyright 2009 American Institute of Physics. (a) piston housing, (b) solenoids (S1 and S2), (c) reducing union, (d) union cross, (e) plug, (f) check valve, (g) union-tee, (h) needle valve for sample removal, (i) tubing, (j) piston, and (k) compression springs.

The presence of this reaction is consistent with experimental gas compositions observed in this work, as the actual CH4/CO2 ratios were between 3:1 and 6:1, whereas they should have been exactly 3:1 if only ethanol decomposition (reaction 7) was taking place. The methane product formed in excess of the 3:1 CH4/CO2 ratio was used to estimate additional water generated via reaction 10; this calculation reduced the observed water quantity formed to 1.02−1.1 times that predicted from combined eq 8 and reaction 10, which is within experimental uncertainty. 2.5.2. Modeling of System with Water Removal. Because the continuous removal of water from the reaction mixture affects the calculation of liquid and vapor phase compositions and quantities, the following procedure, which calculates total mass of the reaction mixture and of water present at each time of sampling ti, has been used to account for water removal over the course of reaction. A material balance is first performed on the entire reaction mixture mass including the water removal loop. At the ith sampling point ti, a mass of sample and mass of rinse of the sampling port are taken from the reactor so that the reaction mixture mass (msys,ti) immediately after sampling is msys,ti = msys,ti−1 − msample,ti − mrinse,ti (11)

ratio. The temperature program held for 4 min at 37 °C, ramped to 90 °C at 10 °C/min, held at 90 °C for 3 min, ramped to 150 °C at 10 °C/min, ramped to 230 °C at 30 °C/ min, and then held at 230 °C for 2 min. Response factors were determined by multipoint calibration using standards of known concentration. Gas phase samples were collected at room temperature during depressurization at the end of experiment and analyzed on a Varian 3300 gas chromatograph with 60/80 Carboxen1000 column (15 ft × 1/8 in. SS, 2.1 mm i.d.) and argon carrier gas. The temperature program for gas analysis held at 40 °C for 2.0 min, ramped to 250 °C at 20 °C/min, and held at 250 °C for 5.0 min. A calibration gas mixture containing 2.0 vol % each of CO, CH4, and CO2 in helium, along with 100% CO2 and 100% CH4, was used to develop response factors for the gas analysis. 2.5. Modeling of Reaction System. Guerbet reaction studies in the batch reactor have been modeled using the SR polar equation of state combined with a volume constraint to more closely determine total quantities of each species present in the reactor,7 to improve closure of mass and atom balances, and to more accurately determine product yields. Operating at conditions near the critical temperature of ethanol (241 °C) entails an expanded liquid phase, substantial mass of alcohol in the vapor, strong dependence of reaction pressure on temperature, and significant gas solubility in the liquid, thus necessitating determination of composition and amount of both liquid and vapor phases at reaction conditions in order to reasonably predict ethanol conversion and product selectivities. 2.5.1. Calculation of Actual and Stoichiometric Water Formation. Experiments with water addition to or removal from ethanol were monitored to compare the actual water present with that stoichiometrically formed in reaction. The stoichiometric number of moles of water formed from Guerbet condensation reactions up to any time ti during reaction is calculated as

To account for water removal from the reaction mixture, the mass of reaction mixture in the reactor proper (mR,ti) is estimated by subtracting from the total mass the mass in the drying loop based on the previous (ti−1) loop composition, mR,ti = msys,ti − nloopML,ti − 1

(12)

where ML is the molecular weight of the liquid phase. The SR polar equation of state model is then applied12 to calculate compositions, densities, and molecular weights of the liquid and vapor phases in the reactor, and then the mass of the reactor is combined with the reactor volume constraint to determine the molar quantity of the liquid and vapor phases present in the reactor. mR,ti = nLML + nV MV

(13)

nL n + V ρL ρV

(14)

VR =

nH2Oform,ti = (x BuOH + 2xC6OH + 3xC8OH)(nL + nloop) + (yBuOH + 2yC6OH + 3yC8OH )(nV )

(10)

nV =

(8)

where nL and nV are the number of moles of liquid and vapor phases in the reactor at time ti as determined by the SR polar equation of state, nloop is the number of moles in the liquid drying loop, and the xi and yi are measured liquid and vapor

VR ρL MLρV − mR,,ti ρV ρL ML − ρV MV

(15)

With nV known, nL is found from eq 13 or 14. With nL and nV determined at ti, the number of moles of water formed in the interval ti−1 to ti can be determined from the sum of water produced from eq 8 plus reaction 10. C


Article


Figure 3. (a) Ethanol conversion and (b) water concentration vs reaction time in condensed phase reactions: (■) no loop, EtOH only; (▲) no loop, EtOH with 5 wt % H2O initial; (◆) no loop, EtOH with 10 wt % H2O initial; (●) circulating loop filled with glass beads; (×) circulating loop filled with 3 Å molecular sieves.

nH2O,int.form,ti = nH2O,form,ti − nH2O,form,ti−1

(16)

Convti = 1 − i

x EtOH,ti(nL,ti + nloop) + yEtOH,t nV,ti + ∑0 x EtOH,ti(nsample,ti + n waste,ti)

The number of moles of water accumulated during the interval ti−1 to ti is determined by applying eq 9. nH2O,int.accum,ti = nH2O,accum,ti − nH2O,accum,ti−1

i

n EtOH0

(21) (17)

Once ethanol conversion and all product species are defined at each sampling time ti, carbon, hydrogen, and oxygen atom recoveries relative to initial amounts in the reaction mixture are determined.

The number of moles of water removed (nH2O,int.rem,ti) by molecular sieves over the interval ti−1 to ti is found by subtracting water accumulated from water formed: nH2O,int.rem,ti = nH2O,int.form,ti − nH2O,int.accum,ti

3. RESULTS AND DISCUSSION The effect of water addition and removal on ethanol conversion, higher alcohol selectivity, and gas selectivity in condensed-phase ethanol Guerbet reactions at 230 °C, autogenous pressure, and a catalyst loading of 0.04 g of catalyst/g of initial reactor charge is presented. Three sets of experiments were conducted in the Parr reactor without the recirculation loop: one experiment as a control with pure ethanol as the starting material, and two additional experiments with 5 and 10 wt % water mixed with ethanol. Results of two experiments representing studies with the recirculation loop are reported: a control experiment with glass beads in the packed bed to observe the effect of the recirculation loop on the reaction, and a second with 3 Å molecular sieves in the absorption bed for water removal. In experiments with the recirculation loop, a larger quantity of ethanol (160 g vs 110 g) was used than in experiments with the reactor alone, and the quantity of catalyst used was therefore increased to maintain the same effective loading (0.04 g of cat./g of ethanol). Results of the experiments are given in Figures 3−5. Ethanol conversion rate (Figure 3a) decreases modestly as water concentration increases in reaction; this reduction is likely because of dilution of ethanol by the water present. The pure ethanol conversion rate with and without the recirculation loop is virtually identical, evidence that the recirculation loop does not affect the reaction. Water concentration as a function of time is shown in Figure 3b; using molecular sieves reduces water concentration in the reactor by a factor of 4 over the first 300 min of experiment and by a factor of 2.5 toward the end of experiment as the sieves become saturated. As is true with ethanol conversion, the water concentration in experiments with pure ethanol is essentially identical in the presence and in the absence of the recirculation loop, further evidence that the loop does affect batch reactor performance.

(18)

The mass of water removed over the interval ti−1 to ti (eq 18) is now subtracted from the initial approximation of the reaction mixture mass (eq 11). The SR polar equation of state is reapplied to determine new values of nL and nV, and the quantities of water formation, accumulation, and removal (eqs 16−18) are recalculated. This calculation is repeated until the number of moles of water removed in the interval ti−1 to ti is unchanging. The total quantity of water removed up to time ti over the entire experiment is then determined by summing amounts removed in each interval up to ti: nH2O,tot.rem,ti = nH2O,int.rem,ti + nH2O,int.rem,ti − 1 + ... + nH2O,int.rem,t0

(19)

Selectivity to product “k” is based on total accumulated product formed up to time ti, which must include product removed from the reactor in samples and rinses: Sk , ti = ⎧ ⎨akxk , ti(nL, ti + + nloop) + akyk , t nV, ti + i ⎩ ⎪ ⎪

i

⎫ ⎪

∑ akxk ,ti(nsample,ti + n waste,ti)⎬ ⎭

⎪

0

⎧ ⎨n EtOH0 − x EtOH,ti(nL,ti + nloop) − yEtOH,t nV,ti i ⎩ ⎪ ⎪

i

−

⎫

⎪

∑ xEtOH,ti(nsample,ti + n waste,ti)⎬ ⎭

⎪

0

(20)

where ak is the ratio of the number of carbon atoms in the product k to the number of carbon atoms in ethanol. Ethanol conversion is calculated in a similar manner: D


Article


Figure 4. n-Butanol and C6+ alcohol selectivities (a) vs ethanol conversion and (b) vs water concentration: (■) no loop, EtOH only; (▲) no loop, EtOH with 5 wt % H2O initial; (◆) no loop, EtOH with 10 wt % H2O initial; (●) circulating loop filled with glass beads; (×, +) circulating loop filled with 3 Å molecular sieve. Filled symbols and (×): n-butanol selectivity. Open symbols and (+): C6+ alcohol selectivity.

Figure 5. Gas (CO2 + CH4) selectivity (a) vs ethanol conversion and (b) vs H2O concentration: (■) no loop, EtOH only; (▲) no loop, EtOH with 5 wt % H2O initial; (◆) no loop, EtOH with 10 wt % H2O initial; (●) circulating loop filled with glass beads; (×) circulating loop filled with 3 Å molecular sieves.

Table 1. Atom Balance Closure and Quantity of Water Removed in Condensed Phase Guerbet Reactions at 503 K with Pure Ethanol no loop time (min)

conv (%)

C rec (%)

H rec (%)

0 20 40 60 120 180 240 300 626 1346

2 5 8 9 14 17 20 23 31 48

100 99 99 98 97 98 97 97 96 96

100 99 99 99 98 98 98 98 97 98

circulating loop (glass beads)

O rec (%)

conv (%)

C rec (%)

H rec (%)

100 100 100 100 99 99 99 99 98 98

3 6 8 11 15 18 21 25 33 46

99 99 99 99 98 98 98 98 98 97

99 99 99 99 98 98 98 98 98 98

circulating loop (3 Å molecular sieves)

O rec (%)

conv (%)

C rec (%)

H rec (%)

O rec (%)

H2O removed (g)

99 99 99 99 99 99 99 99 99 99

2 7 10 12 18 22 24 27 39 52

99 98 97 96 96 95 95 95 92 91

99 98 96 95 94 92 93 92 89 87

99 98 96 94 92 89 88 87 82 78

0.88 1.50 2.15 3.44 4.59 4.89 5.37 7.83 9.63 10.0

overall alcohol (n-butanol + C6+) selectivity observed is ∼75% for the experiment with water removal. The effect of water concentration on gas (CO2 + CH4) selectivity is shown in Figure 5. Consistent with the effect on alcohol selectivity, addition of water to ethanol increases selectivity toward undesired gas formation in condensed phase Guerbet reactions (Figure 5a). Likewise, continuous water removal over the course of experiment lowers gas selectivity. Similar to the alcohol selectivities, the selectivity to gases, which varies widely with ethanol conversion at the different reaction conditions, falls onto a common curve when plotted as a function of water content in reaction (Figure 5b). It should be

The n-butanol and higher (C6+) alcohol selectivities are given in Figure 4 as a function of ethanol conversion (Figure 4a) and water content (Figure 4b). Figure 4a shows that the addition of water to ethanol has a deleterious effect on selectivity to nbutanol and C6+ alcohols. Figure 4b shows that the maximum n-butanol and C6+ alcohol selectivity achieved in each reaction falls onto a common curve when plotted against water concentration. The alcohol selectivity first increases for all experiments in the initial stages of reaction regardless of the initial water concentration and reaches a maximum at different values of ethanol conversion but with a common dependence on water concentration as shown in Figure 4b. The highest E


Article


essentially second order in ethanol, the reduction in higher alcohol selectivity is likely because of water diluting reactant ethanol. We also dismiss that the presence of water is affecting the equilibrium distribution of products from reactions 1−7 related to ethanol re-forming, as removal of water from those reactions would reduce H2 formation, whereas H2 concentration in reality is maintained or increases (Table 2) when water is removed. Choong et al.17 propose that water adsorbs onto basic metal oxide surfaces (La2O3) and then dissociatively migrates as −OH groups to the Ni surface. The adsorbed −OH groups interact with adsorbed acetaldehyde (CH3CHO)21 to facilitate C−C bond rupture, thus leading to CH4 and CO2 formation. Further, water may adsorb onto basic support sites responsible for the aldol condensation, either blocking them or weakening them10 and thereby causing extended adsorbed acetaldehyde contact with nickel sites that lead to decomposition. Finally, it is possible that water alters the support structure, such as in irreversibly converting γ-Al2O3 to boehmite, which lacks the catalytic properties of γ-Al2O3.22

noted that the high gas selectivity predicted at low conversion for the pure ethanol run (squares in Figure 5) is likely because of a small temperature excursion in the reactor during the initial stages of reaction. A slight rise in temperature raises the reactor pressure (because of the steep dependence of ethanol vapor pressure on temperature near its critical point of 241 °C) and therefore requires more gases to be present in the SR polar model at the specified reactor temperature in order to satisfy the model constraints. Once reactor temperature returns to the specified value at which the model is applied, gas selectivities return to follow the expected pattern as the reaction progresses. Atom balances were carried out for all reactions using the SR polar EOS and are reported in Table 1. Carbon, hydrogen, and oxygen recoveries were greater than 96% for runs with the reactor alone and with the recirculating loop with glass beads present in the dryer. The carbon recoveries for the run with the drying loop were above 95% up to 5 h of reaction and then fell to 91% at the end of the experiment. Hydrogen and oxygen recoveries were much lower with the 3 Å molecular sieve drying loop present as expected because the values presented in Table 1 do not account for water absorbed. If water absorbed as estimated in eq 19 is included, the H and O recoveries at the end of experiment (t = 1346 min) in Table 1 are 93% and 95%, respectively. The quantity of water removed (10.0 g) during reaction essentially saturated the 55 g of 3 Å molecular sieves present in the drying bed at the circulating loop temperature of 40−50 °C. The specified capacity of 3 Å sieves at 25 °C is 21 wt %,19 which corresponds to 11.5 g; however, this capacity rapidly decreases as temperature rises. The noncondensable product gas composition for each experiment, collected at the end of the reaction period, is given in Table 2 below. Assuming a typical uncertainty in gas

4. CONCLUSIONS The presence of water in condensed phase ethanol Guerbet reactions increases decomposition of ethanol to CH4 and CO2 and reduces selectivity to higher alcohols. When water is continuously removed via a recirculating drying bed, gas selectivity remains below 10% for ethanol conversions up to 50% and higher alcohol selectivity, especially to C6+ alcohols, increases to 75%. It is postulated that water affects the surface of the catalyst and the alumina support, increasing its tendency to break down the intermediate acetaldehyde into product gases.

■

Table 2. Product Gas Composition from Condensed Phase Guerbet Reactions at 503 K starting material ethanol ethanol + 5 wt % water ethanol + 10 wt % water ethanol ethanol

CH4 (vol %)

CO2 (vol %)

H2 (vol %)

no no

75 66

12 17

5.5 5.6

no

65

18

4.2

yes, glass beads yes, 3 Å molecular sieves

68 69

14 13

7.9 11.5

recirculating loop

AUTHOR INFORMATION

Corresponding Author

*Tel: (517) 353-3928. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the U.S. Department of Energy (Award DEFG36-04GO14216) and the National Corn Growers Association for financial support of this work.

■

REFERENCES

(1) Leon, M.; Diaz, E.; Ordonez, S. Ethanol Catalytic Condensation over Mg-Al mixed oxided derived from hydrotalcites. Catal. Today 2011, 164, 436. (2) Yang, K. W.; Jiang, X. Z.; Zhang, W. C. One-Step Synthesis of nButanol from Ethanol Condensation over Alumina-Supported Metal Catalysts. Chin. Chem. Lett. 2004, 15, 1497. (3) Ndou, A. S.; Plint, N.; Coville, N. J. Dimerisation of ethanol to butanol over solid-base catalysts. Appl. Catal., A 2003, 251, 337. (4) Ogo, S.; Onda, A.; Yanagisawa, K. Selective synthesis of 1-butanol from ethanol over strontium phosphate hydroxyapatite catalysts. Appl. Catal., A 2011, 402, 188. (5) Carvalho, D.; de Avillez, R.; Rodrigues, M.; Borges, L.; Appel, L. Mg and Al mixed oxides and the synthesis of n-butanol from ethanol. Appl. Catal., A 2012, 415−416, 96. (6) Kozlowski, J.; Davis, R. Sodium modification of zirconia catalysts for ethanol coupling to 1-butanol. J. Energy Chem. 2013, 22, 58. (7) Tsuchida, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. Direct Synthesis of n-Butanol from Ethanol over Nonstoichiometric Hydroxyapatite. Ind. Eng. Chem. Res. 2006, 45, 8634.

chromatographic analysis of ∼5%, the CO 2 and CH 4 compositions for runs starting with pure ethanol (rows 1, 4, and 5) are essentially the same. Slightly higher CO2 and lower CH4 concentrations are observed when water is initially present (rows 2 and 3). The reason for this shift in composition is not evident at this time. The higher H2 concentration for the experiment with the 3 Å molecular sieves present reflects a nearly constant H2 formation rate but a higher mole fraction at the end of the experiment because of reduced CH4 and CO2 production. The role of water in reducing alcohol selectivity and increasing gas selectivity could arise from several sources. One possibility that is dismissed, based on reaction equilibria calculations, is that water thermodynamically limits ethanol condensation to butryaldehyde or higher alcohol precursors. More likely, because higher alcohol formation reactions are F


Article

Industrial & Engineering Chemistry Research (8) Marcu, I.-C.; Tichit, D.; Fajula, F.; Tanchoux, N. Catalytic valorization of bioethanol over Cu-Mg-Al mixed oxide catalysts. Catal. Today 2009, 147, 231. (9) Marcu, I.-C.; Tanchoux, N.; Fajula, F.; Tichit, D. Catalytic Conversion of Ethanol into Butanol over M-Mg-Al Mixed Oxide Catalysts (M=Pd, Ag, Mn, Fe, Cu, Sm, Yb) Obtained from LDH Precursors. Catal. Lett. 2013, 143, 23. (10) Riittonen, T.; Toukoniitty, E.; Madnani, D. K.; Leino, A.-R.; Kordas, K.; Szabo, M.; Sapi, A.; Arve, K.; Warna, J.; Mikkola, J.-P. OnePot Liquid-Phase Catalytic Conversion of Ethanol to 1-Butanol over Aluminum Oxide - The Effect of the Active Metal on the Selectivity. Catalysts 2012, 2, 68. (11) Chakraborty, S.; Piszel, P. E.; Hayes, C. E.; Baker, R. T.; Jones, W. D. Highly Selective Formation of n-Butanol from Ethanol through the Guerbet Process: A Tandem Catalytic Approach. J. Am. Chem. Soc. 2015, 137, 14264. (12) Jordison, T.; Lira, C. T.; Miller, D. J. Condensed-Phase Ethanol Conversion to Higher Alcohols. Ind. Eng. Chem. Res. 2015, 54, 10991. (13) Sun, J.; Qiu, X.-P.; Wu, F.; Zhu, W. T. H2 from steam reforming of ethanol at low temperature over Ni/Y2O3, Ni/La2O3, and Ni/Al2O3 catalysts for fuel-cell application. Int. J. Hydrogen Energy 2005, 30, 437. (14) Zhang, C.; Li, S.; Wu, G.; Huang, Z.; Han, Z.; Wang, T.; Gong, J. Steam Reforming of Ethanol over Skeletal Ni-based Catalysts: A Temperature Programmed Desorption and Kinetic Study. AIChE J. 2014, 60 (2), 635. (15) Veibel, S.; Nielsen, J. On the Mechanism of the Guerbet Reaction. Tetrahedron 1967, 23, 1723. (16) Birot, A.; Epron, F.; Descorme, C.; Duprez, D. Ethanol steam reforming over Rh/CexZr1‑xO2 catalysts: Impact of the CO-CO2-CH4 interconversion reactions on the H2 production. Appl. Catal., B 2008, 79, 17. (17) Choong, C.; Huang, L.; Zhong, Z.; Lin, J.; Hong, L.; Chen, L. Effect of calcium addition on catalytic ethanol steam reforming of Ni/ Al2O3: II. Acidity/basicity, water adsorption and catalytic activity. Appl. Catal., A 2011, 407, 155. (18) Aupretre, F.; Descorme, C.; Duprez, D.; Casanave, D.; Uzio, D. Ethanol steam reforming over MgxNi1−xAl2O3 spinel oxide-supported Rh catalysts. J. Catal. 2005, 233, 464. (19) https://www.sorbentsystems.com/desiccants_charts.html. (20) Seifried, B.; Temelli, F. Design of a high-pressure circulating pump for viscous liquids. Rev. Sci. Instrum. 2009, 80, 075104. (21) Kratochwil, T.; Wittmann, M.; Kuppers, J. Adsorption of Ethanol on Ni(100) Surfaces. J. Electron Spectrosc. Relat. Phenom. 1993, 64, 609. (22) Ravenelle, R.; Copeland, J.; Kim, W.; Crittenden, J.; Sievers, C. Structural Changes of γ-Al2O3-Supported Catalysts in Hot Liquid Water. ACS Catal. 2011, 1, 552.

G


Impact of Water on Condensed Phase Ethanol Guerbet Reactions

Recommend Documents