Microwave-activated structured reactors to maximize propylene selectivity in the oxidative dehydrogenation of propane

-1- Mic owa e-ac i a ed s uc u ed eac o s o maximize p opylene selec i i y in he oxida i e dehyd ogena ion o p opane Ad ian Rami ez [a, d], Jose L. Hueso [a, b, c]*, Reyes Mallada [a, b, c] and Jesus San ama ia [a, b, c]* aIns i u e o Nanoscience o A agon and Depa men o Chemical and En i onmen al Enginee ing, Uni e si y o Za agoza. C/Ma iano Esquillo s/n, 50018 Za agoza (Spain). bNe wo king Resea ch Cen e on Bioenginee ing, Bioma e ials and Nanomedicine (CIBER-BBN), 28029, Mad id (Spain). cIns i u o de Ciencia de Ma e iales de A agon (ICMA), Consejo Supe io de In es igaciones Cien í icas (CSIC- Uni e sidad de Za agoza) dP esen Add ess: King Abdullah Uni e si y o Science and Technology (KAUST), 23955, Thuwal (Saudi A abia). Abs ac : Mic owa e (MW) hea ing has been applied o inc ease he selec i i y o p opylene in he oxida i e dehyd ogena ion (ODH) o p opane. The p e e en ial hea ing o he solid monoli h (made o SiC, a good mic owa e suscep o ), allows wo king wi h a lowe gas phase empe a u e, educing he o ma ion o undesi ed by-p oduc s in he gas phase ia homogeneous eac ions. Con e sion le els o ~21% and selec i i y o p opylene up o 70% ha e been achie ed wi h MW-hea ed s aigh channel monoli hic eac o s coa ed wi h a VMgO ca alys . These compe i i e alues con as wi h he mo e limi ed pe o mance deli e ed by he same ca aly ic sys em when i is subjec ed o con en ional hea ing in a ixed-bed eac o con igu a ion, he eby co obo a ing he ad an age o wo king unde a signi ican gas-solid empe a u e gap o minimize he ex en o homogeneous eac ions. Keywo ds: mic owa e chemis y • p opylene p oduc ion • silicon ca bide • oxida i e dehyd ogena ion • s uc u ed eac o s -2- 1. In oduc ion The ca aly ic oxida i e dehyd ogena ion (ODH) o alkanes o alkenes o e s an in e es ing al e na i e o he con en ional syn hesis o alkenes in he chemical indus y by di ec dehyd ogena ion [1, 2]. Among all he oxida i e dehyd ogena ions, he ODH o p opane is one o he mos ele an due o he huge impo ance o i s main p oduc , p opylene, he second mos p oduced chemical in e media e in he pe ochemical indus y a e e hylene [3]. T adi ionally, he main eac ion pa hway o p opylene p oduc ion is based on naph a c acking [4]. Ne e heless, o e he cou se o he yea s, his p ocess has been unable o success ully ul ill he inc easing wo ldwide p oduc ion demand and addi ional eac ion ou es ha e been p oposed o o e come he p opylene p oduc ion gap [3]. In his ega d, a la ge ac ion o he p opylene p oduced wo ldwide is p esen ly ob ained h ough he ca aly ic di ec dehyd ogena ion o p opane [5]. The main disad an age o he cu en di ec dehyd ogena ion ou e is ha he eac ion is s ongly endo he mic [6]. In addi ion, coke o ma ion apidly deac i a es he ca alys a he high empe a u es equi ed. The al e na i e oxida i e ou e [7] using molecula oxygen o gi e p opylene and wa e has no been implemen ed ye because o he limi ed selec i i y le els. These low selec i i ies a e a di ec consequence o he compe ing se ies-pa allel pa hway ha leads o he o ma ion o deep oxida ion p oduc s (CO, CO2) as well as he co-gene a ion o ligh e hyd oca bons such as me hane and/o e hylene in he homogeneous gas phase [8, 9]. In his ega d, i would be desi able o ind no el ca alys s capable o inc easing p opylene yields by: (i) minimizing he e ec s o o e -oxida ion [10] and/o (ii) wo king a eac ion empe a u es su icien ly low as o p e en homogeneous compe ing gas phase. The con en ional ca alys s o he ODH o p opane [11-13] a e mos ly based on anadium (VMgO [14-16], V/Al2O3 [17-19], V/SiO2 [20-22], e c.), wo king usually a he highes empe a u e possible, since o a gi en con e sion he selec i i y inc eases wi h empe a u e [23-25]. Howe e , inc easing he empe a u e also leads o he concomi an o ma ion o o he gas-phase by-p oduc s such as e hylene and me hane [1, 9, 26], he eby hinde ing conside ably he p opylene selec i i y. The e o e, hese -3- homogeneous gas phase con ibu ions e ec i ely se an uppe limi in he p opylene yield: as he empe a u e is aised p og essi ely abo e 550 ºC, a s ong dec ease o p opylene selec i i y akes place mainly due o i s he mal c acking in o me hane and e hylene [2, 27]. Two well-di e en ia ed compe ing eac ion pa hways can ake place in he p opane oxida i e p ocess [26]: i) he he e ogeneous ou e in he su ace o he ca alys ollowing a Ma s-Van K e elen mechanism wi h CO and CO2 as undesi ed byp oduc s, and, ii) he homogeneous ou e in he gas phase ha p e e en ially o ms me hane and e hylene (see Scheme 1). So a , mos o he e o s o imp o e p opylene selec i i y ha e been de o ed o he de elopmen o sma e ca alys o mula ions [8, 10]. Howe e , gas phase eac ions a e inhe en o he gas phase chemis y o he p ocess and independen o he ca alys na u e. Fo his eason, he limi a ion associa ed o eac ion empe a u e canno be ci cum en ed. Wi hin his opic, some al e na i es ypically add essed o educe he gas phase ole ha e been al eady explo ed. In 1990 Kung e al. designed a speci ic eac o ha educed he homogeneous con ibu ions by packing mos o he gas phase olume wi h qua z chips, which no only ac ed as a adical quenche bu also educed ee olume a he expense o inc easing bed p essu e [23]. -4- Scheme 1. P oposed ODH p opane eac ion pa hways ha can ake place depending on he ope a ing empe a u e and he hea ing mechanism (con en ional e sus mic owa e-assis ed). MWH acili a es a empe a u e gap be ween he SiC monoli h whe e he VMgO ca alys is deployed and he gas phase. The p e e en ial hea ing enables a di e en empe a u e in he gas phase and p e en undesi ed non-selec i e seconda y eac ions ypically occu ing a highe empe a u es unde con en ional hea ing condi ions (i.e. wi h homogeneous high eac ion empe a u es). Ano he app oach o mi iga e his con ibu ion was p oposed by Sadyko e al. h ough he use o monoli hic suppo s based on P wi h low con ac imes [28]. Cooling he eac o ou le was p oposed by Pa lo a e al. wi h he same goal o minimizing he homogeneous o ma ion o e hane and me hane [29]. Ne e heless, none o hese app oaches was comple ely success ul and nowadays he as majo i y o he expe imen s ne e exceeds 500 ºC as empe a u e h eshold [8, 24] o minimize he gas phase eac ions. -5- In his wo k, we ha e explo ed he use o mic owa e (MW) i adia ion as a ool ha enables wo king wi h lowe gas phase empe a u es. In ou p e ious wo ks we ha e shown ha di ec hea ing o he solid (suppo + ca alys ) unde MW i adia ion ( he gas hea ing is negligible in he MW ield) in monoli h eac o s allows o wo k unde a s able solid-gas empe a u e g adien [30]. Thus, he gas phase empe a u e can be signi ican ly lowe (app ox. 50K) han on o he solid suppo , helping o a oid undesi ed gas phase eac ions. This concep has been p e iously demons a ed in he oxida i e dehyd ogena ion o isobu ane using CO2 ( a he han oxygen) as he oxidan [31]. In ha sys em, he ca alys was almos 100% selec i e and all unselec i e con ibu ions came om he gas phase. In his wo k, we ha e applied he same concep o oxida i e dehyd ogena ion o p opane wi h oxygen as he oxidan , a mo e complex sys em, whe e bo h he ca alys and he gas phase a e able o deli e selec i e and non-selec i e con ibu ions. To demons a e he wide applicabili y o he concep we ha e chosen VMgO, a well known oxida i e dehyd ogena ion ca alys wi h no signi ican MW abso p ion p ope ies a ibu able o he MgO suppo [32]. Selec i e MW hea ing was ins ead ob ained by deploying he ca alys on an excellen MW suscep o , SiC s uc u ed suppo . The e o e, he di ec mic owa e hea ing o he solid phase can be applied o any ca alys , i espec i e o i s MW abso p ion p ope ies. 2. Expe imen al Sec ion 2.1. Chemicals Magnesium Hyd oxide (Mg(OH)2 99%, Ald ich), Ammonium Me a anada e (NH4VO3 99%, Ald ich) and Ammonium Hyd oxide solu ion (NH4OH, Ald ich) we e all used as ecei ed. Silicon ca bide monoli hs wi h s aigh , 1.5 mm channels we e pu chased om IRESA INGENIERIA S.L. and Silicon ca bide oams wi h 30 ppi po osi y we e pu chased om SICAT CATALYSTS INC. 2.2. Cha ac e iza ion echniques Scanning elec on mic oscopy (SEM) analysis was ca ied ou wi h a FEI-Inspec S50 equipmen . X- ay di ac ion pa e ns we e ob ained in a PANaly ical Empy ean equipmen in B agg B en ano con igu a ion using CuK adia ion and equipped wi h a PIXcel1D de ec o . Raman spec a we e -6- eco ded on an Alpha 300 Raman spec ome e o WITec. Exci a ion o he samples was ca ied ou wi h a 633 nm Helium-Neon lase a oom empe a u e. TEM (T ansmission elec on mic oscopy) images we e acqui ed wi h a FEI Tecnai T20 ope a ed a 200 kV and a FEI Tecnai F30 a 300 kV a he LMA (Labo a o io de Mic oscopias A anzadas). Abe a ion co ec ed scanning ansmission elec on mic oscopy images we e acqui ed using a high angle annula da k ield de ec o in a FEI XFEG TITAN elec on mic oscope ope a ed a 300 kV equipped wi h a CETCOR Cs-p obe co ec o om CEOS Company allowing o ming an elec on p obe o 0.08 nm. The geome ic abe a ions o he p obe- o ming sys em we e con olled o allow a beam con e gence o 24.7 m ad hal -angle o be selec ed. Elemen al analysis was ca ied ou wi h EDS (EDAX) de ec o which allows pe o ming EDX expe imen s in scanning mode. The samples we e suspended in doubly-dis illed wa e , unde sonica ion, and 3 d ops o each sample we e ca e ully d opcas ed on o Cu mesh g ids. 2.3. Ca alys p epa a ion and deposi ion The p epa a ion o he VMgO ca alys was done acco ding he wo k o Kung e al. [33, 34]. The p epa a ion o ca alys -coa ed eac o s was done acco ding o ou ecen wo k [31]. B ie ly, he ca alys deposi ion was ca ied ou by con olled imme sion o he s uc u ed suppo s in e hanolic suspensions o ca alys pa icles (0.5 g o ca alys in 30 mL) inside an ul asonic ba h o 15 minu es ollowed by calcina ion o 2 h a 250 ºC. This cycle o imme sion-sonica ion and hea ing was epea ed se e al imes un il he desi ed ca alys loading was achie ed (ca. 50 mg o o al loading co esponding o ~5% loading). Bo h SiC suppo s had less han 20 m2/g o su ace a ea. The syn heses o he nanoma e ials ha e been pe o med by he Pla o m o P oduc ion o Bioma e ials and Nanopa icles o he NANOBIOSIS ICTS, mo e speci ically by he Nanopa icle Syn hesis Uni o he CIBER in BioEnginee ing, Bioma e ials & Nanomedicine (CIBER-BBN). 2.4. Mic owa e hea ing sys em and empe a u e measu emen s Mic owa e-induced hea ing was add essed in a TE510 monomodal ca i y equipped wi h a magne on gene a o ope a ing a 2.45 GHz (maximum powe 300 W). The expe imen al se -up is desc ibed elsewhe e [30, 31] and a de ailed scheme can be ound in he SI. The s uc u ed eac o (15 x -7- 15 mm) was placed inside a qua z ube wi h in e nal diame e 16 mm, in oduced in he mic owa e ca i y, a a nodal posi ion whe e he elec ic ield eaches a maximum. The gas inle (100 mLSTP/min; 10% P opane, 5 % Oxygen, 85% Helium) was in oduced om he bo om pa o he qua z ube. The monoli h/ oam empe a u e was inc eased om 500 o 600 °C in s eps o 25 °C, a ying he mic owa e powe om 50 o 75 W. The low a e was se o ob ain a space eloci y WHSV equals o 2 L min-1 gca - 1. The eac o was p e ea ed unde he eac ion a mosphe e a 550 ºC o 2 hou s. The composi ion o he ou le gas was analysed by online gas ch oma og aphy (Agilen 490 Mic o GC) equipped wi h he mal conduc i i y de ec o s and wo gas sepa a ion columns (a PPQ and a MS5A molecula sie e) using He as ca ie . CO, CH4 and O2 we e analysed on he MS5A while CO2 and he C2-C3 hyd oca bons we e analysed on he PPQ. Hyd ogen o ma ion was no moni o ed du ing he expe imen . Closu e o ca bon balance was be e han 2%. Du ing eac ion, he empe a u e o he ca alys op su ace was measu ed wi h a py ome e loca ed in he uppe pa o he ube and an ex e nal he mog aphic came a ha had a di ec iew o he qua z ube. The ex e nal he mog aphic came a was used as a con ol measu emen o e i y (indi ec ly) ha no signi ican ho -spo s we e o med du ing eac ion. The he mog aphic came a is a NEC In Rec R300 which ope a es o e 8-14 µm and allows empe a u e measu emen be ween -40 ºC and 500 ºC wi h a sensi i i y (NETD) o 0.03 K, a spa ial esolu ion o 1.2 m ad and an accu acy o ±1.0 K. The py ome e is an OPTRIS CL 2MH1 wi h a empe a u e ange 490 o 2000 ºC, a spec al ange o 1.6 µm, a sensi i i y (NETD) o 0.1 K and an accu acy o ±1.0 K. The empe a u es we e con inuously eco ded using comme cial so wa e, a e calib a ing he emissi i y o he su ace as a unc ion o empe a u e [30, 35]. Fo p elimina y expe imen s o compa e empe a u e di e ences be ween solid and gas depending on he hea ing mode, an op ical ibe (Neop ix Fibe Op ic senso , empe a u e ange om −270 o +300 °C, esponse ime o 500 milliseconds, accu acy o ±0.8 K) was placed a 1 mm downs eam o he solid monoli h/ oam o measu e he empe a u e o he exi gas, while he solid empe a u e was measu ed by he py ome e acing he gas exi side o he monoli h/ oam. Howe e , -8- hese measu emen s we e limi ed o a maximum o 300 ºC, due o he empe a u e limi a ions o he op ical ibe . Fo con en ional hea ing, he same qua z ube was placed inside an elec ical o en. In his case empe a u e p o iles we e measu ed wi h he py ome e and a he mocouple loca ed in he cen al posi ion o he s uc u ed eac o , connec ed o a PID con olled elec ical u nace o main ain he desi ed empe a u e. 3. Resul s and Discussion 3.1. P epa a ion o he s uc u ed ca alys s and e alua ion o hei hea ing capabili ies unde mic owa e i adia ion VMgO was selec ed as ODH p opane ca alys and syn hesized acco ding o he wo k by Kung e al. [33, 34] (see also Expe imen al Sec ion o u he de ails). XRD analysis con i med he p esence o wo c ys alline s uc u es co esponding o magnesium oxide (MgO) and magnesium o ho anada e (Mg3(VO4)2), espec i ely (Figu e 1b). This la e s uc u e consis s o chains o edge-sha ing MgO6 uni s linked oge he by isola ed VO4 e ahed a [33]. HAADF-STEM analysis coupled wi h EDX- mapping analysis de ec ed a ai ly homogeneous dis ibu ion o he elemen s, V, Mg and O in he ca alys s nanopa icles (Figu e 1a). A close EDX e alua ion o di e en a eas e ealed di e en V/Mg a ios be ween an isola ed nanopa icle placed in an ou e a ea (Figu e 1c e e ed o A ea 1) and a la ge a ea including se e al nanopa icles (see Figu e 1c e e ed o A ea 2). High-Resolu ion TEM images u he co obo a ed he co-exis ence o bo h c ys alline species, being MgO p e e en ially p esen as an ex e nal laye (Figu e S1). Raman spec oscopy con i med he p esence o he (Mg3(VO4)2) plus a small con ibu ion o nano- s uc u ed anadium oxides a 258 cm-1 (Figu e S2). The SEM-EDX analysis also shows an a e age composi ion con aining ca 37% w . Mg and 28% w . V (Figu e S3). Since he VMgO ca alys does no exhibi a good capabili y o abso b mic owa es, i was deployed on o he su ace o silicon ca bide (SiC) s uc u ed eac o s. The dielec ic p ope ies o SiC make his -9- ma e ial an excellen candida e o MW induced hea ing [36]. In addi ion, SiC emains ema kable ine unde eac ion condi ions (i.e. empe a u es abo e 450 ºC and an oxida i e a mosphe e). Two di e en SiC comme cial monoli hs we e e alua ed as s uc u ed suppo s: α-SiC monoli hs wi h s aigh , 1 mm channels, and β-SiC oams wi h 30 ppi po osi y (see Figu e 1d and 1e). Bo h suppo s had less han 20 m2 g-1 acco ding o N2 iso he m abso p ion measu emen s. The diame e and o al leng h was 12 mm o bo h suppo s. X-Ray Di ac ion and Raman spec oscopy measu emen s con i med he p esence o he SiC phases (ei he in he hexagonal o cubic phase) (see Figu es S4 and S5). One o he mos c i ical challenges when applying mic owa e hea ing o he e ogeneous ca alysis consis s on achie ing a nea ly homogeneous hea ing o he solid o p e en he o ma ion o ho spo s, which usually lowe s p oduc selec i i y o gene a es misleading and di icul o in e p e ca aly ic esul s [35, 37]. Figu e 1. Cha ac e iza ion o he s uc u ed ca alys VMgO@SiC: a) Combined HAADF-STEM-EDX mapping analysis o wo ep esen a i e ees anding VMgO nanopa icles e ealing a pseudo-plana s uc u e wi h a -16- consequence he selec i i y o p opylene, hough be e han unde CH condi ions, is lowe han o he s aigh channel SiC monoli h. Indeed, Figu e 4b shows ha he o ma ion o e hylene and me hane, undesi ed gas phase p oduc s, was mo e a o ed in he oam s uc u ed suppo indica ing a highe a e age gas empe a u e compa ed o he s aigh channel monoli h. To con i m ha hese di e ences we e no caused by he di e en SiC c ys alline phase (β-SiC / α-SiC) o due o in e ac ions ca alys - suppo , we es ed bo h s uc u ed eac o s unde con en ional hea ing (see Figu e S7). We obse ed an analogous pe o mance o bo h suppo s unde con en ional hea ing, he eby disca ding he in luence o he SiC s uc u ed ma e ials on he VMgO ca alys pe o mance unde CH condi ions. Finally, i is wo h men ioning ha he p oduc ion a e o p opylene achie ed in he s aigh monoli h unde MW hea ing using a s anda d V/MgO ca alys eached 3.4 kgC3H6/kgca ·h, abo e ypical alues o he li e a u e [10, 17, 44, 45], including s a e o he a ca alys s (see Figu e 5) and no oo a om he 3.8 kgC3H6/kgca ·h ecen ly epo ed o a sophis ica ed bo on ni ide nano ubes ca alys [10]. These alues a e conside ed close o hose equi ed o comme cial implemen a ion [8]. I is clea ha , o a gi en empe a u e, he in insic pe o mance o he VMgO ca alys used is a om ha o hose made om bo on ni ide nano ubes. The ac ha compa able p oduc i i ies a e ob ained is simply due o he ac ha MW hea ing allows ope a ing wi h a lowe gas phase empe a u e, and he e o e he solid empe a u e can be inc eased o highe alues han o he ca alys s pe o ming unde con en ional hea ing. Fu he mo e, hanks o he empe a u e g adien , he selec i i y o p opylene epo ed in his wo k is he highes among anadium-based ca alys (see Figu e 5b), wi h mo e han 30 poin s o di e ence o simila epo ed yields [46]. -17- Figu e 5. Compa ison o he p opylene p oduc i i y o he VMgO-SiC monoli h unde MW i adia ion and some o he bes ca alys s epo ed in he li e a u e o p opane con e sion alues highe han 10%. Values in pa en hesis ep esen he e e ence and empe a u e o each poin . 4. Conclusions Al hough MW abso p ion by a s anda d VMgO ca alys is low, mic owa e hea ing can be success ully applied o he oxida i e dehyd ogena ion o p opane when he ca alys is deposi ed on a good mic owa e suscep o , such as a SiC s uc u ed monoli h. Thanks o he p e e en ial solid hea ing -18- a o ded by mic owa es, a signi ican solid-gas empe a u e gap is induced. This coole gas phase p e en s he homogeneous o ma ion o e hylene and me hane ha akes place in ensely a gas phase empe a u es abo e 550ºC. As a consequence, highe selec i i y o p opylene is achie ed in compa ison wi h con en ional hea ing condi ions. The SiC monoli h wi h s aigh channels pe o ms be e han he oam-like monoli h due o he less in ense hea ans e in he s aigh channel monoli h ha ansla es in o a highe solid-gas empe a u e g adien . The lowe gas phase empe a u e leads o a consis en inc ease o selec i i y a ound 5 pe cen age poin s h oughou a wide con e sion in e al when compa ed o con en ional hea ing (see Figu e 4). In addi ion, he ca alys empe a u e can be inc eased while s ill main aining a gas phase empe a u e wi hin accep able alues, and he e o e he p oduc i i y ob ained app oaches ha ob ained wi h much mo e e icien ca alys s. The esul s shown he e o p opane oxida i e dehyd ogena ion unde MW hea ing a e, in p inciple, applicable o any ca aly ic sys em wi h undesi ed con ibu ions in he gas phase, allowing o inc ease he selec i i y owa ds he p oduc s o in e es by minimizing hese homogeneous con ibu ions. Acknowledgemen s Financial suppo om he Eu opean Resea ch Council Ad anced G an (HECTOR-267626) and he Regional Go e nmen o A agon (DGA) is g a e ully acknowledged. The CIBER-BBN (ini ia i e unded by he VI Na ional R&D&i Plan 2008-2011, Inicia i a Ingenio 2010, Consolide P og am, CIBER Ac ions and inanced by he Ins i u o de Salud Ca los III wi h assis ance om he Eu opean Regional De elopmen Fund) is g a e ully acknowledged. 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