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Applied Catalysis A: General 445– 446 (2012) 35– 41

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General

j ourna l ho me page: www.elsev ier .com/ locate /apcata

utothermal reforming of butanol to butenes in a staged millisecond reactor:ffect of catalysts and isomers

ui Sun, Samuel Blass, Edward Michor, Lanny Schmidt ∗

epartment of Chemical Engineering and Materials Science (CEMS), University of Minnesota, 432 Amundson Hall, 421 Washington Avenue SE, Minneapolis, MN 55455, USA

r t i c l e i n f o

rticle history:eceived 10 May 2012eceived in revised form 27 July 2012ccepted 29 July 2012vailable online 3 August 2012

eywords:iomassutanolutene isomersehydration

somerizationutothermal

a b s t r a c t

Dehydration and isomerization of butanol is studied in an autothermal short contact-time reactor con-taining a 1 wt% Pt stage followed by a zeolite or �-Al2O3 stage downstream to convert butanol into buteneswith up to 95% yield at residence times on the order of 100 ms. CH4 is fed as a sacrificial fuel to the Pt stageand butanol is fed between the stages to avoid undesired oxidation and reforming reactions of butanolover Pt. The energy released by CH4 catalytic partial oxidation drives downstream butanol dehydrationand isomerization.

The effect of catalyst is studied by comparing the performance of HZSM-5, HFER, and �-Al2O3 cat-alysts. Higher yields (20%) of butenes were obtained with �-Al2O3 and HFER than with HZSM-5. Theabsence of Br∅nsted acid sites in �-Al2O3 and the small pore structure of HFER lead to reduced yields oflarge side products such as higher hydrocarbons that promote oligomerization reactions. A 95% buteneyield is obtained with HFER at temperatures ranging from 280–350 ◦C and a 95% yield with �-Al2O3 attemperatures between 320 and 350 ◦C. Only a 75% butene yield was obtained with HZSM-5 at 230 ◦C.

The effect of hydrocarbon structure on product formation is studied by comparing conversions of eachbutanol isomer using a heated tube reactor at temperatures between 200 and 400 ◦C. The reactivityof butanol follows as: t-butanol > 2-butanol > iso-butanol > 1-butanol. trans-2-Butene and cis-2-buteneare primarily formed from linear butanol isomers, while isobutene forms from branched butanol iso-mers. Conversions and product distributions of butanol isomers formed over HZSM-5 in a staged reactorare comparable (<10% difference across all species) with data using a heated tube reactor at similar

temperatures.

We successfully demonstrate an alternative pathway to dehydrate butanol into butenes with anautothermal staged reactor compared to conventional methods for applications in small-scale biomassutilization. The largest advantage of this reactor is the integration of highly exothermic autothermalstage and endothermic alcohol dehydration stage which provides an alternative processing technique to

ature

maintain the bed temper

. Introduction

Significant research has focused on the development of pro-esses to convert biomass to fuels and chemicals with the goal ofecreasing dependence on fossil fuels. The production of butanolrom biomass has recently attracted both industrial and academicnterest [1,2]. Various processes for producing butanol isomersrom biomass are described in the literature. 1-Butanol can be pro-uced by traditional ABE fermentation process with a production

atio of 3:6:1 of acetone:butanol:ethanol. Recently, Liao and co-orkers [3] demonstrated that iso-butanol can be produced by

east or micro-organism from sugar, such as glucose or cellulose. A

∗ Corresponding author. Tel.: +1 612 625 9391; fax: +1 612 626 7246.E-mail addresses: schmi001@umn.edu, schmi001@maroon.tc.umn.edu

L. Schmidt).

926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2012.07.039

.© 2012 Elsevier B.V. All rights reserved.

biosynthetic pathway reported by Atsumi et al. [4] achieved a highyield, high-selectivity production of iso-butanol from glucose.

Butenes are currently priced lower than butanol and thermalconversion results in inevitable process inefficiencies. However,deoxygenation of butanol is investigated as a means of increasingenergy density for potential use as a transportation fuel or chemi-cal precursor in addition to a fuel additive. In addition, dehydrationof butnaol into butenes has a potential application on improvingmarket acceptance of alcohols. In spite of a lot of efforts, alcoholshave rarely gained acceptance beyond their inclusion as blendingcomponents in the fuel. The conversion of butanol into butenesovercomes this obstacle and results in the production of mate-rial that is valuable as fuel intermediate and as petrochemical

intermediate.

Previous studies have focused on the mechanisms of butanoldehydration over solid-acid catalysts [5–7], and skeletal isomer-ization of linear butene into branched butenes [8–11]. Butene can

3 s A: General 445– 446 (2012) 35– 41

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Fig. 1. Simplified schematic of the heated tube reactor (left) and autothermal staged

6 H. Sun et al. / Applied Catalysi

lay a prominent role in the fuel production. The butene isomersbtained from butanol dehydration can be oligomerized to liquiduels for transportation purposes [12–14]. In the alkylation pro-ess, one mole of C4 olefins reacts with one mole of isobutaneo form an isoparaffin which is 4 carbon number heavier [15]. Inddition, Isobutene is also widely used as a precursor for poly-ers and octane-enhancing additives for gasoline [7]. Traditionally,

sobutene is produced via isomerization in a heated tube frominear butene with a 40% yield with residence times up to ∼10 s8,10]. Linear butene is obtained by catalytic cracking of petroleumroducts [7].

Autothermal staged reactors have been studied as a means ofeoxygenating biomass using ethanol as model systems [16]. Ourrevious results demonstrated methanol dehydration to dimethylther (DME) with high yield (∼80%) and ethanol dehydration tothylene with a 95% yield in a staged reactor at a residence timef 100 ms [16,17]. In this work we extend this analysis to butanolecause the feedstock contains a greater number of C C bondshan methanol or ethanol thus enabling it to undergo both dehy-ration and isomerization reactions. Concurrent dehydration and

somerization reactions of butanol isomers to butenes are rarelyeported in the literature. The novelty of the staged reactor liesn the combination of two catalytic functions in a single tube. Thepstream Pt stage functions as an internal heat source by catalyz-

ng partial oxidation reactions of CH4. The energy released heatshe downstream zeolite stage which catalyzes dehydration andsomerization reactions. The autothermal staged reactor is a depar-ure from conventional methods of producing butene from butanol,ecause heat is generated internally rather than being providedxternally. The largest advantage of this reactor is that it pairsxothermic and endothermic dehydration chemistry which pro-ides an alternative processing technique to maintain an adequateed temperature [18].

Dehydration and isomerization in a staged reactor occurs atesidence times of approximately 100 ms which is faster than con-entional processes (residence time is ∼100 times larger [8,10]).mall scale butanol processing in an autothermal staged reactors a high throughput process and is a good candidate for replac-ng conventional fuel and chemical platforms in a biofuel-drivenconomy.

In this work, 1 wt% Pt/�-Al2O3 was used for catalytic partialxidation (CPO) reactions on the first stage and HZSM-5, HFER, or �-l2O3 for butanol dehydration/isomerization on the second stage.he axial temperature gradient along the downstream stage wasesolved to help explain reactor performance. The effect of hydro-arbon structure was investigated by reacting four different butanolsomers into the heated tube reactor. The product selectivities andields obtained from experiments with a heated tube reactor andutothermal reactor containing HZSM-5 catalyst are compared.inally, the results lead to the development of an autothermalteady state operating configuration that permits integration ofPO of CH4 and deoxygenation of biomass derived butanol chem-

stry at high process feed rates.

. Experimental

A schematic of the staged reactor is shown in Fig. 1 (right). Plat-num (1 wt% Pt) was coated on ceramic foam monoliths (99.5%-Al2O3) 18 mm diameter and 10 mm length with 45 pores per

inear inch (ppi) using the incipient wetness method [19–21].ncoated foams were placed at the front- and back-faces of the

atalyst to reduce axial heat loss. All monoliths were wrapped witheramic insulation to eliminate gas bypass. K-type thermocouplesere placed at the center of the monolith to measure catalyst tem-eratures and the reactor was insulated with fiberfrax insulation.

reactor (right) for butanol dehydration and isomerization reactions. The arrow direc-tion indicates the direction of the flow. The dotted lines represent the flow patternof butanol vapor.

Each catalyst was used for at least 20 h with no significant deac-tivation or observed coke formation. All experiments were carriedout at atmospheric pressure.

Commercial catalysts (Zeolyst) NH4-ZSM-5 powder withSi/Al = 25 and NH4-FER powder with Si/Al = 28 were used. The pro-ton form of the zeolite was obtained by calcining the ammoniumform in a furnace from room temperature to 500 ◦C with 1 ◦C/mintemperature ramp and 5 h soak in air. Either HZSM-5, HFER, or �-Al2O3 (0.6 g) were mixed with 2.4 g of quartz and situated betweentwo 80 ppi monoliths in the reactor. The zeolite was regeneratedin a furnace at 600 ◦C with a total flow of 1 standard liter per min(SLPM) of air for 1 h. The upstream and downstream stages wereseparated by 80 mm. A fused silica capillary (Agilent, 0.53 mm ID)was placed in the center of the reactor through a bottom port whichwas then sealed with a septum as a guide for a thermocouple. Theaxial temperature profile was measured by motion of the thermo-couple inside the quartz capillary tube [22,23].

CH4 was fed as a sacrificial fuel to the Pt stage at 0.2 SLPM. TheC/O ratio, defined as the molar feed rate of methane over the twicethe molar feed rate of O2, was changed by varying oxygen flow.Butanol isomers were introduced to the reactor at 0.18 ml/min.Butanol was vaporized using a syringe pump and a resistive heat-ing element and introduced into the reactor halfway between thetwo stages through a 1/8 in. stainless steel tube containing 8 holes(0.016′′ ID) and welded shut at one end as shown in Fig. 1 (right).The holes allowed for more uniform dispersion of butanol vapor inthe reactor. The total flow rate to the reactor was held constant at1.7 SLPM.

Gas samples of products at steady-state were identified and ana-lyzed by a gas chromatograph equipped with thermal conductivityand flame ionization detectors. Response factors and retentiontimes were determined by calibrating known quantities of speciesrelative to N2 which was used as an internal standard. Gas flow rateswere controlled by mass flow controllers (Brooks Instrument). Thecarbon balance is typically closed within ±10%.

The configuration of the heated tube reactor is similar to thestaged reactor but without the Pt stage. A quartz tube (19 mm ID)placed inside a clamshell furnace as shown in Fig. 1(left). The totalflow rate was maintained at 1.7 SLPM with the same amount ofbutanol introduced (0.18 ml/min). Reaction temperature was con-trolled by a temperature controller (Omega, CSC32). For all theexperiments, the carbon selectivity (Sc) is defined in Eq. (1) wherevj is the number of carbon atoms in molecule j, and Fj is the molarflow of species j.

Sc = vjFj∑jvjFj

; j /= fuel (1)

H. Sun et al. / Applied Catalysis A: General 445– 446 (2012) 35– 41 37

Fig. 2. (A) Hydrogen selectivity (SH) and CO selectivity (SCO) and temperature profiles at various C/O ratio in the autothermal reactor. This shows that at higher C/O ratio, ita e profit Tempei

3

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sdptoao[biceri

FlffttHc

ssociates with lower SH and SCO, and also lower TPt and Tzeolite. (B) Axial temperaturhat there is a significant temperature drop (<70 ◦C) at where butanol is introduced.

s HZSM-5 and feed is iso-butanol.

. Results and discussion

.1. Autothermal staged reactor: top stage

The hydrogen selectivity (SH) and CO selectivity (SCO) of the toptage is plotted in Fig. 2A. Fig. 2A also depicts the upstream andownstream stage temperatures (right y-axis). The reaction tem-erature of the downstream zeolite stage is controlled by adjustinghe C/O ratio on the upstream Pt stage. Higher temperatures arebtained by feeding more oxygen into the reactor to increase themount of combustion. The kinetics of catalytic partial oxidationn Pt is too slow to enable the CPO reaction to reach equilibrium24]. The gas samples were taken between two stages before theutanol was introduced. The methane conversion decreases as C/O

s increased. Oxygen conversion is 100% because the kinetics of

ombustion are fast enough in Pt such that all oxygen is consumedven if the reaction is kinetically-limited [25]. The temperatureegime on the Pt stage ranges from 500 ◦C to 825 ◦C as C/O is var-ed from 0.53 to 1.67. This results in zeolite temperatures ranging

ig. 3. C4= yield in autothermal staged reactor at various C/O ratios for three cata-

ysts, (�)HFER catalyst, (�) HZSM-5 catalyst and (�) �-Al2O3 catalyst with i-BuOHeed; (©)HFER catalyst, (�) HZSM-5 catalyst and (�) �-Al2O3 catalyst with n-BuOHeed. Open symbols are C4

= yield with n-butanol feed, while closed symbols arehose with iso-butanol feed. We take the average temperature of the top and bot-om temperature to represent the average temperature (Tavg) of the second stage.ere, the temperature of every data point is the average temperature, and it has aorresponding C/O ratio shown on the top axis.

le at different C/O ratio vs. position in the autothermal reactor. This demonstratedrature is dropping as distance increases further from the top stage. (A), (B), catalyst

from 200 to 355 ◦C. The vertical bar in Fig. 2A indicates that thedifference in top and bottom temperature on the second stage is∼50 ◦C with HZSM-5 catalyst. Despite the 300 ◦C variation in tem-perature of the Pt stage, the zeolite stage temperature gradient isconsistently −3.3 ◦C per mm because the reactor is well insulatedwith ∼0.4 J/cm2 s heat lost as reported in previous results [17].

The axial temperature profile in the reactor shown in Fig. 2Bwith iso-butanol feed is plotted at various C/O ratios. A noticeabletemperature drop (up to 70 ◦C) is apparent at a 55 mm distance fromthe Pt stage which is where butanol is introduced into the reactor.This temperature drop is attributed to heat transfer between thePt stage exhaust at 500 and 700 ◦C to the butanol vapor at 150 ◦C.A steeper drop in temperature is observed at low C/O ratios wherethe back shield monolith temperature can reach up to 700 ◦C. At theC/O = 1.40–1.67, the temperature gradient is constant between thetwo stages while at the C/O = 0.53–1.11, the temperature gradientincreases at 80 mm. At higher temperatures, a greater rate of heattransfer to the butanol vapor occurs. Butanol has to travel fartherbefore radial heat loss becomes significant again. At 80 mm, thetemperature gradient returns to its original value. The temperaturegradient is approximately linear in the zeolite region, suggestingradial heat loss might be dominant and any thermal contributionfrom dehydration/isomerization reaction may be negligible.

3.2. Autothermal staged reactor: effect of catalysts

Figs. 3 and 4 depict C4 olefin yield and butanol conversion over�-Al2O3, HFER and HZSM-5 catalysts in the autothermal stagedreactor. The axial downstream stage temperature is averaged (Tavg).The corresponding C/O ratio is shown on the top axis. As shown inFig. 3, the total C4 olefin yield over HFER and �-Al2O3 increases withTavg and reaches a maximum of 90–95% between 280 and 350 ◦C.Over HZSM-5, the C4 olefin yield goes through a maximum of 75%at 230 ◦C after which it decreases with increasing temperature. Thetemperature at which a maximum C4 olefin yield is obtained overHFER is 40 ◦C lower than yields over �-Al2O3. Over all three cata-lysts studied, the C4 olefin yield from 1-butanol follows the sametrends as those from iso-butanol but requires an additional 20 ◦Cto reach same yields.

Fig. 4 shows 1-butanol and iso-butanol conversion and the selec-tivities of all observed C4 species over each catalyst studied as afunction of Tavg. As seen in Fig. 4A–D, conversion of iso-butanol over

�-Al2O3 reaches 100% at 320 ◦C and over HFER at 280 ◦C, while thetotal C4 selectivity remains constant at 90–95%. On �-Al2O3 andHFER, no isobutene is formed from 1-butanol feed, while 50–60%selectivity of isobutene is formed from iso-butanol over �-Al2O3

38 H. Sun et al. / Applied Catalysis A: General 445– 446 (2012) 35– 41

F anel)c cataly( eeds (

(er35por

B�oso

ig. 4. C4= selectivity, conversions for autothermal reforming of iso-butanol(left ponversion with catalyst �-Al2O3, (C), (D) is with catalyst HFER and (E), (F) is with�), cis-2-C4

= selectivity (�), 1-C4= selectivity (�), conversion with different BuOH f

Fig. 4A) and 40–50% over HFER (Fig. 4C). HZSM-5 has the high-st activity compared to HFER and �-Al2O3 since the conversioneached 100% at 250 ◦C which is lower than 280 ◦C for HFER and20 ◦C for �-Al2O3. However, the total C4 selectivity decreases by0% over HZSM-5 with increasing temperatures as larger com-ounds are formed. The selectivity of isobutene increases by 15%ver HZSM-5 (Fig. 4F), indicating the occurrence of isomerizationeactions.

As evidenced by the conversion of butanol, the presence ofr∅nsted acid sites in the catalysts make zeolites more active than

-Al2O3 which primarily contains Lewis acid sites. The high activityf HZSM-5 limits the range of temperatures over which a high C4electivity can be obtained. A high C4 selectivity is only obtainedver HZSM-5 between 200 and 250 ◦C but it can be reached over

and n-butanol (right-panel) with three catalysts. (A), (B) show C4 selectivity andst HZSM-5; total C4

= selectivity (♦), iso-C4= selectivity (�), trans-2-C4

= selectivity�). X represents conversion here.

the entire range of Tavg studied with HFER and �-Al2O3. The 10-membered ring (MR) HZSM-5 has larger cavities exist in place ofintersection of channels make it more active than HFER which hasonly 10 × 8 MR pores [8]. �-Al2O3 primarily has Lewis acid whichare too weak to catalyze oligomerization and isomerization reac-tions. Therefore C4 olefin yields are higher over HFER and �-Al2O3because C4 olefins are not consumed in oligomerization and iso-merization reactions. In addition, HFER is suggested to be optimalfor isomerization of linear butenes to isobutene due to shape selec-tivity effects [10,26,27]. HFER has a pore size enabling isobutene

diffusion but suppressing dimer and oligomer formation which arebyproducts and cause catalyst deactivation.

While there is precedence for isomerization reactions of C4hydrocarbons over HFER, the conditions used previously are

H. Sun et al. / Applied Catalysis A: General 445– 446 (2012) 35– 41 39

Fig. 5. (A) Conversion of 4 butanol isomers in a heated tube reactor over HZSM-5 at various temperatures. (B) Product yield of heated tube reactor over HZSM-5 catalystwith n-butanol feed at various temperatures, (C) C4 species selectivity for 2-butanol in a heated tube reactor over HZSM-5 catalyst. The C2–C3 and C5–C6+ are not shownh ver HZi

smt1iisdcbnHtb

t∼foosuTcabtir

ere for simplicity. (D) C4 species selectivity for t-butanol in a heated tube reactor osomerization process.

ignificantly different than ours. de Ménorval et al. reported iso-erization of n-butene to isobutene on HFER with residence

imes ranging from 1 min to 4.7 min, and achieving conversions of5.3–43.8% at 350 ◦C, respectively [28]. In Fig. 4C, the C4 selectiv-

ty over HFER remained relatively constant indicating the lack ofsomerization reactions, however, this might simply be due to thehort residence times used (100 ms compared with 1 min used bye Ménorval et al.). In Fig. 4D trans- or cis-isomers shows a slighthanging in selectivity (∼13%), indicating the presence of dou-le bond isomerization between trans- and cis-isomers, but witho isobutene formation. Comparing the performance of HFER andZSM-5, which have the same Si/Al ratio, the extent of isomeriza-

ion over HZSM-5 catalyst is more apparent than on HFER and maye attributed to the larger pore size of HZSM-5.

The deactivation test carried out for three catalysts shows thathe conversion of iso-butanol at 400 ◦C over HZSM-5 decreases by20% over 2 h, due to pore blockage by carbonaceous deposits

ormed over Br∅nsted acid sites. The conversion of iso-butanolver HFER decreased by 10% after 4 h and remained constantver �-Al2O3 for 8 h. The small pore size of HFER and the lack oftrong acid sites on �-Al2O3 minimize the production of side prod-cts such as higher hydrocarbons which deactivate the catalysts.his also demonstrates that HFER and �-Al2O3 are more suitableatalysts for autothermal staged reactors than HZSM-5. These cat-lysts have a slow rate of deactivation and require temperatures

etween 280–350 ◦C to achieve a high C4

= yield. High tempera-ures are not an issue for autothermal staged reactors due to annternal heat source which enables high temperatures to be easilyeached.

SM-5 catalyst. Structure II and IV are corresponding alkoxide structures formed in

3.3. Heated tube reactor: effect of isomers

HZSM-5 is chosen as the catalyst to study the effect of isomerssince it catalyzes the formation of a greater range of products thanHFER or �-Al2O3 enabling us to probe a larger number of reactions.In Fig. 5A, t-butanol has the highest reactivity since the temperatureat which it reaches 98% conversion (200 ◦C) is lower than tem-peratures for other isomers. At 200 ◦C, 2-butanol, iso-butanol, and1-butanol react with 88%, 72%, and 39% conversion respectively.This result is also consistent with the higher activation energies of1-butanol, 2-butanol and iso-butanol dehydration over NaHZSM-5 catalyst ∼135–160 kJ/mol, compared to 80 kJ/mol for t-butanoldehydration at 60–185 ◦C [5,29–31]. A lower activation barrier fort-butanol means a lower temperature is needed to reach 100% con-version.

In Fig. 5B, it can be observed that 1-butanol is mainly trans-formed into linear butenes between 250 and 300 ◦C, while butenesare transformed into higher olefins and paraffins above 300 ◦C. Theyield of C5 and C6+ go through a maximum at 300 ◦C, while theconcentration of propylene and ethylene continually increase withtemperature. This suggests that C5+ products are intermediateswhile C2 and C3 species are final products. The amount of linearbutenes, which are primary products in the dehydration reaction,initially decreases due to oligomerization to higher olefins. Thebutene yield becomes constant at temperatures greater than 300 ◦C.

Product distributions from 2-butanol, t-butanol and iso-butanolshare the same trends as 1-butanol except below 250 ◦C where thetotal C4 yields are different according to butanol isomer reactivity.Dehydration reportedly takes place at temperature less than 150 ◦C

40 H. Sun et al. / Applied Catalysis A: General 445– 446 (2012) 35– 41

Fig. 6. Comparison between autothermal staged reactor (open symbol) and heated tube reactor (closed symbol) in terms of product distribution (Fig. 6A, B) and C4= selectivity

( axis)

s erizat

wa5aw

tbtceasittibts

fpatpT

Fig. 6C, D). Autothermal reaction product selectivity is plotted vs. Tavg (bottom x-implicity. Structure III and I are corresponding alkoxide structures formed in isom

hile skeletal isomerization occurs at temperatures between 300nd 500 ◦C [7] with primarily dimerization mechanism over HZSM-

[27]. In our studies, the selectivity of C3 is larger than that of C5t 300–400 ◦C. We postulate that C5 is the intermediate producthich will be further cracked into C3 and C2.

In Fig. 5C, D, the selectivities of C4 olefins with 2-butanol and-butanol feed are plotted, while those with iso-butanol and 1-utanol feed are plotted in Fig. 6C, D (closed symbol). In Fig. 6D,rans-isomer which has a �Hf = −10.8 kJ/mol is thermodynami-ally preferred over the cis-isomer which has a �Hf = −7.7 kJ/molven though the cis-isomer is more structurally favored on a cat-lyst surface due to reduced steric hindrance [11]. In addition, theelectivity of isobutene increases from 0% to 15% (T = 200–400 ◦C)ndicating the occurrence of isomerization. Fig. 5C shows thathe C4 olefin selectivities from 2-butanol dehydration are similaro 1-butanol dehydration (Fig. 6D). In Fig. 6C, the selectivity ofsobutene decreased from 64% to 20% (T = 200–400 ◦C) with iso-utanol feed. t-Butanol converts to isobutene with greater easehan other butanols, resulting in a high isobutene selectivity (90%electivity at 200 ◦C in Fig. 5D).

The literature suggests that a carbenium-ion transition stateorms over the Br∅nsted acid site in the dehydration/isomerizationrocess [8,32–34]. Willams et al. [5] suggest the presence of an

lkoxide intermediate for butanol dehydration at temperatures lesshan 150 ◦C. In our experiments, dibutyl ether is not observed in theroduct stream due to its instability at high temperature (>200 ◦C).he differences in products formed from each butanol isomer are

and C/O ratio (top x-axis). (C), (D): The C2–C3 and C5–C6+ are not shown here forion process.

attributed to the different intermediate structures that form. Wepostulate that butanol forms an alkoxide intermediate by elimi-nating water. The alkoxide structure then isomerizes to anotheralkoxide structure via a carbenium-ion-like transition state. Thelack of iso-butanol formation from 1-butanol reactions over allcatalysts is evidence of a series reaction in which dehydrationto an alkoxide is followed by isomerization. Branched products(isobutene) form from branched butanol isomers such as t- andiso-butanol while linear products (1-butene, 2-butene) form fromlinear butanol isomers such as 1- and 2-butanol. We postulate thatlinear alkoxide intermediates (structure I in Fig. 6D and II in Fig. 5C)are isomerized into branched products (structure III in Fig. 6C andIV in Fig. 5D) via a methyl-cyclopropyl carbenium-ion-like transi-tion state before desorbing as olefins [32,33]. Branched alkoxidesare more stable than linear alkoxides so they barely isomerize tolinear products.

3.4. Autothermal reactor vs. heated tube reactor and itsimplication

Product selectivities from the autothermal staged reactor exper-iments are plotted against those from the heated tube experimentsin Fig. 6. With an iso-butanol feed (Fig. 6A and C), the selectiv-

ities of all products (C3–C6+, trans- and cis-isomers, iso-C4

= and1-C4

=) from the autothermal experiments are comparable to thosefrom the heated tube experiments. With 1-butanol feed (Fig. 6B),the selectivities of total C4 and C5 are comparable, while the C3

s A: Ge

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(b) C. Williams, M.A. Makarova, L.V. Malysheva, E.A. Paukshtis, E.P. Talsi, J.M.Thomas, K.I. Zamaraev, J. Catal. 127 (1991) 377–392.

H. Sun et al. / Applied Catalysi

electivity is lower in the heated tube experiments by about 8%nd the C6+ selectivity is higher by the same amount. In Fig. 6D, theelectivity of trans- and cis-isomers is ∼8% higher in the autother-al experiments, while 1-butene is lower by the same amount.The differences in product selectivities between the heated

ube and autothermal reactors may be due to the temperatureradients found in the autothermal reactor. The temperatures pre-iously reported here are representative of all temperatures inhe zeolite stage. The product selectivity between the autother-

al and heated tube reactors with iso-butanol and 1-butanol feedre comparable to each other (<10% difference). The differencesan primarily be attributed to the similarity in the dehydra-ion activation energy of iso-butanol and 1-butanol. Zamaraevt al. reported that the activation energy (�H /= ) of iso-butanolo isobutene is 138.1 ± 8.4 kJ/mol [5], while activation energy of-butanol to n-butene is 133.9 ± 8.4 kJ/mol at 105–185 ◦C withaHZSM-5 catalysts [30]. This shows that the dehydration reac-

ion rate of iso-butanol and 1-butanol have similar sensitivity toemperature. Therefore, the deviation from heated tube reactor toutothermal staged reactor is comparable with both iso-butanolnd 1-butanol feed. Additionally, linear butanol isomers undergosomerization reactions while branched isomers do not. Thesesomerization reactions may be more sensitive to temperaturehanges than alcohol dehydration. This increased sensitivity wouldesult in linear butanol isomers displaying a greater difference inroduct selectivities when comparing products formed on a zeo-

ite with a temperature gradient as in the autothermal reactor orithout a gradient as with the heated tube reactor.

The ability to sufficiently preheat the butanol by direct contactith hot exhaust gases implies that it is not necessary to employ

fired heater or a combustion chamber to preheat butanol. Thisould potentially reduce the capital cost of the process.

. Conclusion

Autothermal reforming of butanol isomers were investigatedn a staged reactor with an upstream stage consisting of 1 wt% Ptatalyst on �-Al2O3 and a downstream stage consisting of either-Al2O3, HFER, or HZSM-5 catalysts. The reactivity of the butanol

somers was studied by comparing their conversions over a range ofemperatures. HFER and �-Al2O3 have a 90–95% C4

= yield while theaximum C4

= yield over HZSM-5 is 75% due to its pore structurend acid strength effects. In the heated tube reactor, the reactivityf the butanol isomers is as follows: t-butanol > 2-butanol > iso-utanol > 1-butanol. trans- and cis-isomers are obtained from linearutanol isomers, while isobutene is formed from branched butanol

somers. With HZSM-5, the product selectivities in the autothermaltaged reactor were comparable to those from a heated tube reactorith a difference of <10%. The autothermal staged reactor demon-

trates the integration of exothermic and endothermic chemistry.he direct contact between butanol and hot exhaust gas has theotential applications to replace the fire heater or the combustionhamber, reducing the capital cost of the process.

[[[

neral 445– 446 (2012) 35– 41 41

Acknowledgements

This work was supported by the NSF EFRI (Emerging Frontiersin Research and Innovation) – Hydrocarbons from Biomass (Awardnumber: 0937706). The authors are grateful to Reetam Chakrabartifor helpful discussions.

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