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Direct Synthesis of Dimethyl Ether from Syngas on Bifunctional Hybrid

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catalysts Article Direct Synthesis of Dimethyl Ether from Syngas on Bifunctional Hybrid Catalysts Based on Supported H3PW12O40 and Cu-ZnO(Al): Effect of Heteropolyacid Loading on Hybrid Structure and Catalytic Activity Elena Millán , Noelia Mota, Rut Guil-López, Bárbara Pawelec , José L. García Fierro and Rufino M. Navarro * Instituto de Catálisis y Petroleoquímica (CSIC), C/Marie Curie 2, Cantoblanco, 28049 Madrid, Spain; [email protected] (E.M.); [email protected] (N.M.); [email protected] (R.G.-L.); [email protected] (B.P.); jlgfi[email protected] (J.L.G.F.) * Correspondence: [email protected] Received: 27 August 2020; Accepted: 15 September 2020; Published: 17 September 2020 Abstract: The performance of bifunctional hybrid catalysts based on phosphotungstic acid (H3PW12O40, HPW) supported on TiO2 combined with Cu-ZnO(Al) catalyst in the direct synthesis of dimethyl ether (DME) from syngas has been investigated. We studied the effect of the HPW loading on TiO2 (from 1.4 to 2.7 monolayers) on the dispersion and acid characteristics of the HPW clusters. When the concentration of the heteropoliacid is slightly higher than the monolayer (1.4 monolayers) the acidity of the clusters is perturbed by the surface of titania, while for concentration higher than 1.7 monolayers results in the formation of three-dimensional HPW nanocrystals with acidity similar to the bulk heteropolyacid. Physical hybridization of supported heteropolyacids with the Cu-ZnO(Al) catalyst modifies both the acid characteristics of the supported heteropolyacids and the copper surface area of the Cu-ZnO(Al) catalyst. Hybridization gives rise to a decrease in the copper surface area and the disappearance of the strong acidic sites typical of HPW nanocrystals, showing all hybrids similar acid sites of weak or medium strength. The activity of the hybrids was tested for direct DME synthesis from syngas at 30 bar and 250 ◦C; only the hybrids with HPW loading higher than 1.4 monolayers showed activity for the direct synthesis of DME, showing that the sample loaded with 2.7 monolayers of heteropolyacid had higher activity than the reference hybrid representative of the most widely applied catalysts based on the combination of Cu-ZnO(Al) with HZSM-5. In spite of the high activity of the hybrids, they show a moderate loss in the DME production with TOS that denotes some kind of deactivation of the acidity function under reaction conditions. Keywords: DME; syngas; copper; zinc; heteropolyacids; titanium oxide 1. Introduction The selective hydrogenation of CO/CO2 into dimethyl ether (DME) is currently one of the most important topics in the research of alternative fuels because DME can be used as a direct and cleaner fuel alternative to conventional diesel [1]. In addition, DME is also an important chemical intermediate for the production of widely used chemicals, such as diethyl sulphate, methyl acetate or olefins [2]. DME is considered an ultra-clean transportation fuel because it has a high cetane number (around 60), low boiling point ( 25 C) and high oxygen content (35 wt %) which allow fast vaporization and − ◦ higher combustion quality (smokeless operation and 90% less NOx emissions) than other alternative CO2-based fuels [2]. DME showed total compliance with the highly strict California ultra-low emission Catalysts 2020, 10, 1071; doi:10.3390/catal10091071 www.mdpi.com/journal/catalysts CatalystsCatalysts 20202020,, 1010,, x 1071 FOR PEER REVIEW 22 of of 23 22 other alternative CO2-based fuels [2]. DME showed total compliance with the highly strict California ultra-lowvehicle (ULEV) emission regulations vehicle for(ULEV) medium-duty regulations vehicles for medium-duty and it has the vehicles highest effiandciency it has of allthe synthetic highest efficiencyliquid fuels of (e.g.,all synthetic F-T diesel, liquid methanol) fuels (e.g., [3]. F-T diesel, methanol) [3]. DMEDME production production is is a a well-established two-step in industrialdustrial process: first first catalytic catalytic hydrogenation hydrogenation ofof syngas syngas into into methanol methanol (CO/CO (CO/CO2 ++ 3H3H2 → CHCH3OHOH + H+2HO) O)and and a subsequent a subsequent catalytic catalytic dehydration dehydration of 2 2 ! 3 2 theof themethanol methanol over over acid acidcatalysts catalysts to produce to produce DME DME(2CH3 (2CHOH →OH CH3OCHCH3 +OCH H2O). +TheH mainO). The problem main 3 ! 3 3 2 ofproblem this two-step of this DME two-step synthesis DME synthesisderives from derives the strong from the thermodynamic strong thermodynamic limitation limitationof the methanol of the synthesismethanol step synthesis that leads step to that low leads gas conversion to low gas pe conversionr pass (15–25%) per pass and (15–25%) therefore and high therefore recirculation high ratiosrecirculation and high ratios capital and and high operating capital and costs. operating To avoid costs. this To limitation, avoid this in limitation, the early 1990s in the researchers early 1990s beganresearchers to study began the todirect study synthesis the direct of synthesisDME in a of pr DMEocess inin awhich process methanol in which synthesis methanol step synthesis is coupled step inis coupledsitu with in the situ dehydration with the dehydration step in a single step inreactor a single (2CO/CO reactor2 (2CO + 6H/2CO → CH+ 6H3OCH3 CH+ 3HOCH2O). The+ 3HdirectO). 2 2 ! 3 3 2 synthesisThe direct of synthesis DME allows of DME increasing allows increasing the level the of level conversion of conversion per perstep, step, up upto to90%, 90%, which which means means significantsignificant savings savings in in capital capital investment investment and and production production costs costs of of DME. DME. In In fact, fact, it it is is estimated that that DMEDME productionproduction costscosts in in direct direct synthesis synthesis are reducedare reduced by 20–30% by 20–30% compared compared to the traditionalto the traditional two-step two-stepprocess. Directprocess. DME Direct synthesis DME requires synthesis highly requires efficient highly hybrid efficient bifunctional hybrid catalytic bifunctional systems catalytic which systemsshould combinewhich should a CO/CO combine2 hydrogenation a CO/CO2 function hydrogenation for methanol function synthesis for methanol and an acidicsynthesis function and foran acidicmethanol function dehydration for methanol (Scheme dehydration1). (Scheme 1). SchemeScheme 1. 1. DirectDirect synthesis synthesis of of dimethyl dimethyl ether ether (DME) (DME) from from syngas syngas on on bifunctional bifunctional catalyst. catalyst. AccordingAccording toto thethe mechanism mechanism operating operating in thein directthe direct synthesis synthesis of DME, of theDME, activity the andactivity efficiency and efficiencyof the hybrid of the bifunctional hybrid bifunctional catalysts depend catalysts on: depen (i) thed development on: (i) the development of effective active of effective sites for active methanol sites forsynthesis, methanol (ii) synthesis, the control (ii) of thethe naturecontrol and of the strength nature of and the acidstrength sites (Brönstedof the acid/Lewis), sites (Brönsted/Lewis), (iii) the control of (iii)the balancethe control between of the methanol balance between synthesis methanol sites and acidsynthesis sites, andsites (iv)and an acid adequate sites, and distance (iv) an between adequate the distancemethanol between synthesis the sites methanol and the synthesis acid dehydration sites and sitesthe acid [4]. Thedehydration state-of the-art sites [4]. in The the developmentstate-of the-art of inthe the hybrids development catalysts of for the the hybrids direct synthesiscatalysts for of DMEthe direct are based synthesis on Cu-ZnO(Al) of DME are catalysts, based on as Cu-ZnO(Al) component catalysts,for methanol as component synthesis, andfor methanol zeolites (mainly synthesis, HZSM-5) and zeolites as acid (mainly component HZSM-5) for methanol as acid dehydrationcomponent fordue methanol to theirhydrophobic dehydration naturedue to their and predominanthydrophobic Brönstednature and acid predominant sites [5]. In Brönsted the hybrid acid bifuctional sites [5]. Incatalysts, the hybrid deff bifuctionalects in the catalysts, Cu particles, deffects changes in the in Cu the particles, morphology changes of in the the Cu morphology particles, dispersion, of the Cu particles,crystallite dispersion, size and interaction crystallite ofsize Zn and atoms interaction with the of metallic Zn atoms Cu particleswith the aremetallic important Cu particles factors thatare importantdefine the activityfactors that for the define methanol the activity synthesis for component the methanol [6–8 synthesis] while the component nature (Brönsted [6–8] while or Lewis), the naturethe acid (Brönsted strenght or (weak Lewis), to medium)the acid strenght as well as(wea thek texturalto medium) properties as well (mesoporosity) as the textural ofproperties the acid (mesoporosity)sites play fundamental of the acid roles sites for play the fundamental methanol dehydration roles for the functionality methanol dehydration [9,10]. Despite functionality the great [9,10].variety Despite of synthesis the methodsgreat variet (physicallyy of synthesis mixture, co-precipitation,methods (physica sonochemicallly mixture, integrated, co-precipitation,::: ) and sonochemicalpromoters or integrated,…) modifiers (MgO, and ZnO, promoters Na, Co, or Fe,modifiers La, Ce, (MgO,::: ) explored ZnO, Na, so Co, far Fe, for La, the Ce,…) preparation explored of soCu-ZnO(Al)-HZSM-5 far for the preparation
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