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 hybrid of Cu-ZnO(Al)-HZSM-5 catalysts, the catalytic hybrid activity catalysts, of these hybridsthe catalytic decrease activity with of time these on hybridsstream owingdecrease to thewith deactivation time on stream by water, owing that to produces the deactivation copper oxidation by water, or sinteringthat produces [4], by copper strong oxidationinteractions or betweensintering the [4], Cu-ZnO(Al) by strong interactions catalyst and between the zeolite, the that Cu-ZnO(Al) produces Cucatalyst migration and the [11 ]zeolite, and by thatcoke produces deposition Cu [12 ].migration Therefore, [11] the eandffectiveness by coke of Cu-ZnO(Al)-HZSM-5deposition [12]. Therefore, catalysts the is stilleffectiveness limited and of it Cu-ZnO(Al)-HZSM-5is necessary considerable catalysts effort inis catalystsstill limited development and it is to necessary improve itsconsiderable level of activity effort and in durability.catalysts developmentAcid materials to improve which its are level highly of activity promising and to durability. avoid the inconveniences associated with the use of HZSM-5Acid zeolitesmaterials are which the heteropolyacids are highly promising (HPAs). to The avoid HPAs the exist inconveniences in different structuresassociated andwith possess the use a ofvery HZSM-5 high Brönsted zeolites acidity, are the close heteropolyacids to the super-acid (HPAs). region, The and HPAs defined exist molecular in different structures. structures Therefore, and possessthey generally a very allowhigh eBrönstedfficient dehydration acidity, close of methanol to the super-acid under milder region, conditions and defined than conventional molecular structures. Therefore, they generally allow efficient dehydration of methanol under milder

Catalysts 2020, 10, 1071 3 of 22

HZSM-5 acid catalysts, offering economic and environmental advantages [13]. The most studied HPAs for the methanol-to-DME process are those having the Keggin structure and the most acidic ones, namely H3PW12O40 and H4SiW12O40. Keggin HPAs consist of cubic crystals of heteropolyanions with n 5+ 4+ formula [XM12O40] − (X is the heteroatom (P , Si ) and M is the so-called addenda atom, typically Mo6+,W6+) and stabilized by protons [14]. The HPAs are structured in three levels: (i) primary structure, corresponding to the heteropolianion itself; (ii) secondary structure including the three-dimensional + organization with H and H2O and, (iii) the tertiary structure including the arrangement of the particles responsible for the surface area of HPA. The location, number and strength of active acid sites for methanol dehydration in HPA are controlled by their structure (primary, secondary and tertiary) which is mainly determined by the water content in the HPA [15,16]. Up to 180 ◦C, six crystallization + water molecules are in each Keggin unit forming a cubic structure in which H5O2 ions link four n [XM12O40] − anions [15]. The flexibility of the lattice of HPAs to absorb polar molecules as methanol and react in the solid bulk (pseudo-liquid behaviour) has as direct consequence the improvement in the activity in the dehydration of methanol over HPAs because the reaction takes place on both surface and bulk acid sites [17]. Therefore, water plays a main role in the acid activity of HPAs because it determines the strenght and accesiblity of the methanol to these acid sites and they could be altered by thermal treatments or by modification of the metal-oxygen clusters in the local structures of the Keggin units [16]. In addition, the HPAs have limited surface area (around 5–10 m2/g) and therefore to improve the availability to their inner protons, the dispersion of HPAs on neutral or acidic porous supports is often investigated (SiO2, ZrO2, Nb2O3, TiO2, boron nitride,...) [18–22]. One of the most studied support in the methanol-to-DME reaction has been TiO2 and the results have shown higher accesibility of supported HPA that means higher conversion of methanol per mass unit than unsupported counterparts [19]. However, remarkable differences in methanol dehydration were observed depending on the HPA loading and the interaction of HPA with the TiO2 support because these interactions change the oxygen-metal bonds in the Keggin units modifiying their acidity strenght and accesibility [19]. Despite the considerable attention paid to the HPAs applied to methanol dehydration for production of DME, their use in hybrid bifuctional catalysts combined with Cu-ZnO(Al) catalysts for direct synthesis of DME from syngas has not yet been explored. The use of HPAs in hybrid catalysts poses new challenges to the heteropolyacids because they will be used under different reaction conditions (T = 225–275 ◦C, P > 30 bar) to that used in the dehydration of methanol (T = 180–200 ◦C, atmospheric pressure). In addition, the possible detrimental interaction of metallic copper with acid sites of HPAs, as previously observed in Cu-ZnO/γ-Al2O3 [23] and Cu-ZnO(Al)/HZSM-5 hybrids [24] could produce partial deactivation of both functionalities. Two main causes for the detrimental interaction of metallic copper with acid sites in hybrid bifuctional catalysts has been reported: (i) Cu sintering [4] or (ii) copper and zinc ion exchange with protons of zeolite [24]. In this scenario, this work was undertaken with the aim to study for the first time the performance of bifunctional hybrids based on phosphotungstic acid (H3PW12O40) supported on TiO2 combined with Cu-ZnO(Al) catalyst in the direct synthesis of DME from syngas. Taking into account that local structure of Keggin units depends on the interaction stablished between the Keggin units with the TiO2 support, in this work we studied specifically the effect of the heteropolyacid loading on the dispersion and acid characteristics of HPAs and its relevance on the hybridization with CZA catalyst applied for the direct synthesis of DME from syngas. Individual and hybrid catalysts were well-characterized (XRD, Raman, DRIFTS, H2-TPR, N2O chemisorption and NH3-TPD) in order to establish the relationships between the properties of individual functionalities (methanol synthesis and methanol dehydration) and their role in the activity of the hybrids in the direct synthesis of DME from syngas. Catalysts 2020, 10, x FOR PEER REVIEW 4 of 23 Catalysts 2020, 10, 1071 4 of 22 2. Results and Discussion

2.2.1. Results Structure and and Discussion Acidity of xHPW/Ti Acid Catalysts 2.1. StructureThe diffraction and Acidity profiles of xHPW of the/Ti AcidxHPW/Ti Catalysts acid catalysts are collected in Figure 1. All catalysts showThe diffraction diffraction peaks profiles (25.3, of the36.9, xHPW 37.7 and/Ti acid 38.5; catalysts 27.4 and are 36.0°) collected characteristic in Figure1 of. All the catalysts anatase show(Joint Committee on Powder Diffraction Standards, JCPDS 21-1272) and rutile (Joint Committee on diffraction peaks (25.3, 36.9, 37.7 and 38.5; 27.4 and 36.0◦) characteristic of the anatase (Joint Committee onPowder Powder Diffraction Diffraction Standards, Standards, JCPDS JCPDS 21-1276) 21-1272) phases and rutile respectively (Joint Committee of the TiO on2 support. Powder In Di ffadditionraction to the diffraction lines of the TiO2 support, the samples also show diffraction lines at 10.2, 14.5, 17.8, Standards, JCPDS 21-1276) phases respectively of the TiO2 support. In addition to the diffraction lines 20.6, 23.1, 25.3, 27.4, 29.3, 31.2, 32.9 and 34.5° associated with crystalline cubic H3PW12O40·6H2O of the TiO2 support, the samples also show diffraction lines at 10.2, 14.5, 17.8, 20.6, 23.1, 25.3, 27.4, 29.3,species 31.2, [15]. 32.9 As and expected, 34.5 associated the inte withnsity crystallineof the diffraction cubic H peaksPW O corresponding6H O species to [ 15these]. As crystalline expected, ◦ 3 12 40· 2 thestructures intensity increases of the diuponffraction increasing peaks the corresponding heteropolyacid to theseloading crystalline (Table 1). structures However increases the increase upon in intensity was not linear with the HPW loading with a marked increase in the crystallinity and increasing the heteropolyacid loading (Table1). However the increase in intensity was not linear domain size of HPW when its content exceeds 1.7 theorethical monolayers (Table 1). In the case of with the HPW loading with a marked increase in the crystallinity and domain size of HPW when its the 1.7HPW/Ti sample, it is also observed a slight peak at 26.2° with some shoulders at 10.6, 15.1 and content exceeds 1.7 theorethical monolayers (Table1). In the case of the 1.7HPW /Ti sample, it is also 21.4° related to small crystals of cubic H3PW12O40·3H2O [25]. The loss of water in H3PW12O40·6H2O observed a slight peak at 26.2◦ with some shoulders at 10.6, 15.1 and 21.4◦ related to small crystals produces a contraction in its structure as is evidenced through the shift of diffraction peaks at higher of cubic H3PW12O40 3H2O[25]. The loss of water in H3PW12O40 6H2O produces a contraction in its angles [25,26]. According· to the literature, dispersed H3PW12O·40·6H2O could be dehydrated with structure as is evidenced through the shift of diffraction peaks at higher angles [25,26]. According to formation of H3PW12O40·3H2O at lower temperature than that observed on the bulk (180 °C) [25] the literature, dispersed H3PW12O40 6H2O could be dehydrated with formation of H3PW12O40 3H2O because the interaction of HPA entities· with the support weakens and facilitates the water loss· from at lower temperature than that observed on the bulk (180 ◦C) [25] because the interaction of HPA the cubic Keggin structures [27]. entities with the support weakens and facilitates the water loss from the cubic Keggin structures [27].

FigureFigure 1.1. XRDXRD patternspatterns ofof xHPWxHPW/Ti/Ti acidacid catalysts:catalysts: 1.4HPW1.4HPW/Ti/Ti (a), 1.7HPW1.7HPW/Ti/Ti (b) and 2.7HPW2.7HPW/Ti/Ti (c)(c) (anatase (), rutile (), H PW O 6H O( ) and H PW O 3H O( )). (anatase (■), rutile (□), H33PW12O4040 6H2O2 (●•) and H3PW3 1212O40 403H2O2 (○)). # Table 1. Relative intensity and particle size of HPW 6H O (from XRD), acid/neutral water (from DRIFTS) · 2 andTable relative 1. Relative intensity intensity of HPW and (from particle Raman) size of of xHPW HPW·6H/Ti acid2O catalysts.(from XRD), acid/neutral water (from DRIFTS) and relative intensity of HPW (from Raman) of xHPW/Ti acid catalysts. HPW 6H2O Acid/Neutral HPW/TiO2 Relative · HPW 6H2O Relative XRD · Acid/NeutralWater Ratio Raman Intensity HPW·6H2O dp (nm) 1 HPW/TiO2 Relative1 Raman Intensity (a.u.) HPW·6H2O (1700Water/1620 Ratio cm ) (990/650 cm ) Relative XRD − Intensity− 1.4HPW/Ti 1.0dp 20.5 (nm) (1700/1620 0.423 0.371 Intensity (a.u.) (990/650 cm−1) 1.7HPW/Ti 1.47 17.6cm 0.403−1) 0.365 1.4HPW/Ti2.7HPW /Ti1.0 4.6 30.6 20.5 0.423 0.784 0.536 0.371 1.7HPW/Ti 1.47 17.6 0.403 0.365 2.7HPW/TiFigure2 shows DRIFT4.6 spectra of the xHPW 30.6 /Ti acid catalysts.0.784 In the region 0.536 of the vibration modes corresponding to the metal-oxygen, the titania support presents an asymmetric contribution 1 centeredFigure on 7782 shows cm− (Ti–O–Ti,DRIFT spectra stretching). of the The xHPW/Ti spectra acid of xHPW catalysts./Ti acid In catalyststhe region show of contributionsthe vibration 1 1 characteristicmodes corresponding of HPW: to at the 1080 metal-oxygen, cm− , corresponding the titania to support P–Oa asymmetrical presents an asymmetric stretching; atcontribution 984 cm− ,

Catalysts 2020, 10, x FOR PEER REVIEW 5 of 23 Catalysts 2020, 10, 1071 5 of 22 centered on 778 cm−1 (Ti–O–Ti, stretching). The spectra of xHPW/Ti acid catalysts show contributions characteristic of HPW: at 1080 cm−1, corresponding1 to P–Oa asymmetrical stretching; at associated to W–Od stretching terminal oxygen; at 888 cm− , ascribed to W–Ob–W corner-shared −1 −1 984 cm , associated to W–Od stretching terminal1 oxygen; at 888 cm , ascribed to W–Ob–W bridged bond and, finally a broad band at 816 cm− , corresponding to W–Oc–W edge-shared bridged corner-shared bridged bond and, finally a broad band at 816 cm−1, corresponding to W–Oc–W bond [28–30]. As the HPW load increases, an increase in the relative intensity of the HPW signals is edge-shared bridged bond [28–30]. As the HPW load increases,1 an increase in the relative intensity of observed in metal-oxygen bonds region (1200–600 cm− ) with respect to the vibration modes of the the HPW signals is observed in metal-oxygen bonds region (1200–600 cm−1) with respect to the titania support. There is no significant displacement or widening of the HPW signals in the xHPW/Ti vibration modes of the titania support. There is no significant displacement or widening of the HPW acid catalysts which indicates that the Keggin structure of HPW has not been significantly distorted signals in the xHPW/Ti acid catalysts which indicates that the Keggin structure of HPW has not been after supporting on TiO2. significantly distorted after supporting on TiO2.

Figure 2. DRIFT spectra at room temperature of TiO2 support (a), bulk HPW (a’) and xHPW/Ti acid Figure 2. DRIFT spectra at room temperature of TiO2 support (a), bulk HPW (a’) and xHPW/Ti acid catalysts: 1.4HPW/Ti (b), 1.7HPW/Ti (c) and 2.7HPW/Ti (d). catalysts: 1.4HPW/Ti (b), 1.7HPW/Ti (c) and 2.7HPW/Ti (d). In the region 3800–2500 cm 1, the titania support (Figure2a) presents a broad band around 3332 In the region 3800–2500 cm−1, the titania support (Figure 2a) presents a broad band around 3332 cm 1 corresponding to physisorbed water molecules and contributions at 3528 and 3638 cm 1 ascribed cm−−1 corresponding to physisorbed water molecules and contributions at 3528 and −3638 cm−1 to isolated hydroxyl groups on TiO2 and dissociatively adsorbed water on TiO2 respectively [31]. ascribed to isolated hydroxyl groups on TiO2 and dissociatively adsorbed water on TiO2 respectively1 The physisorbed water on TiO2 was corroborated by the band of the OH bending at 1636 cm− [32]. [31]. The physisorbed water on TiO2 was corroborated by the band of the OH bending at 1636 cm−1 The xHPW/Ti acid catalysts also show in the 3800–2500 cm 1 region a band centered around 3300 cm 1 [32]. The xHPW/Ti acid catalysts also show in the 3800–2500− cm−1 region a band centered around− which includes physisorbed water and protonated water in the dioxonium ions (H O–H+–H O) [33] −1 2 2 + 3300 cm which includes physisorbed1 water and protonated water in the dioxonium ions (H2O–H – and a separated band at 3638 cm− due to hydroxyl groups on TiO2 or dissociatively adsorbed water H2O) [33] and a separated band at 3638 cm−1 due to hydroxyl groups on TiO2 or dissociatively on TiO2. The relative intensity of this latter band decreases in parallel with the HPW loading which is adsorbed water on TiO2. The relative intensity of this latter band decreases in parallel with the HPW indicative of the decrease of hydroxyl groups of TiO2 covered by HPW units. In the water bending loading which is indicative of the decrease of hydroxyl groups of TiO2 covered by HPW units. In the region, the xHPW/Ti acid catalysts show two bands at 1712 and 1620 cm 1, attributed to the bending − −1 water bending region, the xHPW/Ti+ acid catalysts show two bands at 1712 and 1620 cm , attributed O-H vibration of dioxonium H2O5 and neutral water respectively [34]. The relative intensity of the to the bending O-H vibration of dioxonium H2O5+ and neutral water respectively [34]. The relative acid/neutral water ratio in the xHPW/Ti acid catalysts, calculated from the relative area of the bands intensity of the acid/neutral water ratio in the xHPW/Ti acid catalysts, calculated from the relative at 1712 and 1620 cm 1, is not proportional to the HPW loading (Table1). The samples with 1.4 and area of the bands at− 1712 and 1620 cm−1, is not proportional to the HPW loading (Table 1). The 1.7 monolayers show a similar ratio, while the sample with 2.7 monolayers shows a marked increase in samples with 1.4 and 1.7 monolayers show a similar ratio, while the sample with 2.7 monolayers the relative amount of acid water. shows a marked increase in the relative amount of acid water.

CatalystsCatalysts 20202020, ,1010,, x 1071 FOR PEER REVIEW 66 of of 23 22

The evolution of xHPW/Ti samples upon heating under He flow (25–150-200 and 250 °C) was The evolution of xHPW/Ti samples upon heating under He flow (25–150-200 and 250 C) was followed by DRIFT and the corresponding spectra were collected in Figure 3. The ◦ bands followed by DRIFT and the corresponding spectra were collected in Figure3. The bands corresponding corresponding to neutral water (3638, 3300 and 1620 cm−1) and acidic water (3300 and 1710 cm−1) to neutral water (3638, 3300 and 1620 cm 1) and acidic water (3300 and 1710 cm 1) decreases as decreases as the temperature increase and −disappear at 250 °C while the bands corresponding− to the temperature increase and disappear at 250 C while the bands corresponding to metal-oxygen metal-oxygen bonds of the Keggin structure do not◦ suffer modification. This later fact contrast with bonds of the Keggin structure do not suffer modification. This later fact contrast with the reported the reported splitting of the band at 984 cm−1 (W=Od) for dehydrated bulk heteropolyacids [15,35]. As splitting of the band at 984 cm 1 (W=O ) for dehydrated bulk heteropolyacids [15,35]. As the splitting the splitting is related with the− W=O groupsd which the oxygen participates/non participate in the is related with the W=O groups which the oxygen participates/non participate in the formation of formation of hydrogen bonds with neighboring KU, the absence of splitting in dehydrated xHPW/Ti hydrogen bonds with neighboring KU, the absence of splitting in dehydrated xHPW/Ti may indicate may indicate low participation of the oxygen in the formation of hydrogen bonds between KU units low participation of the oxygen in the formation of hydrogen bonds between KU units when they when they are supported. The temperature at which the acidic water disappears on the xHPW/Ti are supported. The temperature at which the acidic water disappears on the xHPW/Ti acid catalysts acid catalysts is higher than that observed on bulk HPW (results not shown here) in which the acid is higher than that observed on bulk HPW (results not shown here) in which the acid water was water was removed at 150 °C. In addition, the removal of the acidic water on the xHPW/Ti samples removed at 150 C. In addition, the removal of the acidic water on the xHPW/Ti samples depends on depends on its ◦HPW loading being lower when the HPW concentration increases. Therefore, it its HPW loading being lower when the HPW concentration increases. Therefore, it seems to indicate seems to indicate that supported HPW at low loading on TiO2 stabilizes its protonated water that supported HPW at low loading on TiO stabilizes its protonated water molecules. molecules. 2

FigureFigure 3. 3. DRIFTDRIFT spectra spectra of ofxHPW/Ti xHPW/ Tiacid acid catalysts catalysts upon upon heating heating under under He flow He flow (25–150–200 (25–150–200 and 250 and °C):250 1.4HPW/Ti◦C): 1.4HPW (a),/Ti 1.7HPW/Ti (a), 1.7HPW (b)/Ti and (b) and2.7HPW/Ti 2.7HPW (c)./Ti (c).

TheThe structure structure of of the the xHPW/Ti xHPW /acidTi acid catalysts catalysts was wasalso studied also studied by Raman by Raman spectroscopy spectroscopy as shown as 1 inshown Figure in 4. Figure The Raman4. The spectra Raman of spectra all xHPW/Ti of all xHPW samples/Ti show samples bands show at 638, bands 517,at 397 638, cm 517,−1 and 397 a small cm− 1 shoulderand a small at 450 shoulder cm−1 characteristic at 450 cm of− thecharacteristic anatase and of rutile the phases anatase of and the rutileTiO2 support phases respectively of the TiO2 1 [36,37].support The respectively samples also[36, 37show]. The Raman samples bands also at show 216, 236–239, Raman bands990 cm at−1 216,and 236–239,1009 cm− 9901 assigned cm− and to 1 W-O-W1009 cm bending,− assigned W=O tod W-O-Wsymmetric bending, stretch Wand=O W=Od symmetricd asymmetric stretch stretch and respectively W=Od asymmetric [38–40], stretchwhich demonstraterespectively [38the– 40existence], which demonstrateof Keggin structures the existence of HPW of Keggin on the structures TiO2 support. of HPW onThe the Raman TiO2 support. HPW signalsThe Raman become HPW more signals intense become as the more concentration intense as theof HPW concentration in the catalysts of HPW increases in the catalysts (Table 1) increases which confirms,(Table1) whichin agreement confirms, with in agreement the previous with XRD the previous results, XRDthe higher results, development the higher development of crystalline of structurescrystalline of structures HPW in ofthe HPW catalysts in the with catalysts higher with heteropolyacid higher heteropolyacid loading. However loading. some However widening some andwidening shift of and the shift HPW of theRaman HPW bands Raman was bands observed was observedin the sample in the with sample the withlower the HPW lower loading HPW (1.4HPW/Ti)loading (1.4HPW which/Ti) suggests which suggests greater greaterinteractio interactionn between between the oxygen the oxygen atoms atomsof the of HPW the HPW and andthe hydroxylthe hydroxyl groups groups of the of support the support at low at lowheteropolyacid heteropolyacid loading loading [41–43]. [41– 43].

Catalysts 2020, 10, 1071 7 of 22 Catalysts 2020, 10, x FOR PEER REVIEW 7 of 23

FigureFigure 4.4. RamanRaman spectraspectra ofof HPWHPW/Ti/Ti acidacid catalysts:catalysts: 1.4HPW1.4HPW/Ti/Ti (a), (a), 1.7HPW 1.7HPW/Ti/Ti (b) (b) and and 2.7HPW 2.7HPW/Ti/Ti (c). (c).

TheThe acidity acidity of of the the xHPW xHPW/Ti/Ti samples samples was was determined determined by by temperature-programmed temperature-programmed desorption desorption ofof ammoniaammonia whosewhose profilesprofiles areare depicteddepicted inin FigureFigure5 .5. The The TPD TPD profiles profiles show show two two main main regions: regions: low-mediumlow-medium temperature temperature (LT-MT, (LT-MT, ~125–400 ~125–400◦C) °C) and an highd high temperature temperature region region (HT ~550(HT ◦~550C). The °C). peak The 4+ atpeak LT-MT at LT-MT corresponds corresponds to weak-medium to weak-medium acid sites acid associated sites associated with Lewiswith Lewis sites (un-coordinatedsites (un-coordinated Ti ions)Ti4+ ofions) the TiOof 2thesupport TiO2 [44support] while [44] the peakwhile at highthe peak temperature at high corresponds temperature to thecorresponds decomposition to the of (NHdecomposition4)3PW12O40 offormed (NH4 by)3PW reaction12O40 formed of the strong by reaction acid sites of ofthe HPW strong with acid NH sites3 [45 ,of46 ].HPW Integration with NH of 3 the[45,46]. weak-medium Integration acid of sites the (LT-MT weak-medium region, Table acid2) showssites (LT-MT no clear relationshipregion, Table with 2) the shows HPW loading,no clear presentingrelationship the with sample the with HPW 1.7 loading, monolayers presenting the higher the concentration sample with of 1.7 weak-medium monolayers sites the relatedhigher withconcentration the TiO2 support. of weak-medium On the contrary, sites related the intensity with the and TiO desorption2 support. temperature On the contrary, of the the strong intensity acid sitesand relateddesorption with temperature the HPW entities of the (HT, strong Table acid2) increases sites related as the with HPW the load HPW in theentities catalyst (HT, increases. Table 2) Thisincreases shift could as the be HPW consequence load in the of the catalyst higher increases. dispersion This or interactionsshift could ofbe theconsequence HPW with of the the support higher atdispersion lower loading or thatinteractions affect the of decomposition the HPW with of the the ammonium support saltsat formedlower loading by reaction that of NHaffect3 with the thedecomposition supported HPW of the entities ammonium [22]. Assuming salts formed that by the reaction unsupported of NH bulk3 with HPW the supported has three acidic HPW protons entities for[22]. each Assuming Keggin unitthat (KU),the unsupported the number bulk of acidic HPW protons has three in the acidic xHPW protons/Ti catalysts for each has Keggin been estimatedunit (KU), basedthe number on the of relative acidic areaprotons ratio in of the the xHPW/Ti HT peaks cataly (Tablests2 has). Based been estimated on this calculation, based on the relative number area of + acidicratio of protons the HT per peaks KU (Table on the 2). xHPW Based/Ti on varies this calcul with theation, HPW the loadingnumber increasingof acidic protons from 1.26 per HKU/KU on the in + thexHPW/Ti 1.4HPW varies/Ti sample with upthe to HPW 2.37 Hloading/KU, closeincreasing to bulk from HPW, 1.26 in theH+/KU 2.7HPW in the/Ti 1.4HPW/Ti sample. This sample fact is up line to with2.37 previousH+/KU, close works to bulk in the HPW, literature in the [42 2.7HPW/Ti,47] that has sample. shown This the fact loss is of line acidic with protons previous of HPW works by in the the formationliterature of[42,47] strong that H-bonds has shown with available the loss OHof acidic groups protons on the supports.of HPW by the formation of strong H-bonds with available OH groups on the supports. Table 2. Quantitative data from TPD-NH3 profiles of xHPW/Ti acid catalysts.

Area Area H+/KU (LT-MT) (HT) 1.4HPW/Ti 93.25 108.12 1.26 1.7HPW/Ti 177.31 132.65 1.55 2.7HPW/Ti 108.54 202.72 2.37 HPW - 215.15 3.0

Catalysts 2020, 10, x FOR PEER REVIEW 8 of 23

Catalysts 2020, 10, 1071 8 of 22 Catalysts 2020, 10, x FOR PEER REVIEW 8 of 23

Figure 5. TPD-NH3 profiles of bulk HPW (a) and xHPW/Ti acid catalysts: 1.4HPW/Ti (b), 1.7HPW/Ti (c) and 2.7HPW/Ti (d).

Table 2. Quantitative data from TPD-NH3 profiles of xHPW/Ti acid catalysts.

Area Area H+/KU (LT-MT) (HT) 1.4HPW/Ti 93.25 108.12 1.26 1.7HPW/Ti 177.31 132.65 1.55 2.7HPW/Ti 108.54 202.72 2.37 HPW - 215.15 3.0 FigureFigure 5. 5. TPD-NHTPD-NH33 profilesprofiles of of bulk HPW (a) andand xHPWxHPW/Ti/Ti acid acid catalysts: catalysts: 1.4HPW 1.4HPW/Ti/Ti (b), (b), 1.7HPW 1.7HPW/Ti/Ti (c) (c)and and 2.7HPW 2.7HPW/Ti/Ti (d). (d). 2.2. Structure and Acidity of CZA-xHPW/Ti Bifunctional Hybrid Catalysts 2.2. Structure and Acidity of CZA-xHPW/Ti Bifunctional Hybrid Catalysts The morphologicalTable 2. Quantitative characteristics data offrom the TPD-NH CZA-HPW/Ti3 profiles hybrid of xHPW/Ti catalysts acid catalysts.in comparison with the The morphological characteristics of the CZA-HPW/Ti hybrid catalysts in comparison with the CuO/ZnO(Al) catalyst (CZA) were studied byArea SEM (FigureArea 6). TheH+/KU SEM image of the bare CZA CuO/ZnO(Al) catalyst (CZA) were studied by SEM (Figure6). The SEM image of the bare CZA catalyst shows the typical irregular agglomerates(LT-MT) formed(HT) by particles of different size and morphologycatalyst shows observed the typical in this irregular kind of agglomerates catalysts [6]. formed The CZA-HPW/Ti by particles of hybrid different catalysts size and shows morphology similar 1.4HPW/Ti 93.25 108.12 1.26 morphologyobserved in thismaintaining kind of catalyststhe morphological [6]. The CZA-HPW characteri/sticsTi hybrid of the catalysts agglomerates shows previously similar morphology observed 1.7HPW/Ti 177.31 132.65 1.55 inmaintaining the bare CZA. the morphologicalIt is observed that characteristics the small HPW/Ti of the agglomerates particles (arrows previously in Figure observed 6) are inin thesurface bare 2.7HPW/Ti 108.54 202.72 2.37 contactCZA. It with is observed the CZA that agglomerates the small HPW without/Ti particles achiev (arrowsing a homogenous in Figure6) aredistribution in surface and contact covering with the of HPW - 215.15 3.0 theCZA CZA agglomerates particles. without achieving a homogenous distribution and covering of the CZA particles. 2.2. Structure and Acidity of CZA-xHPW/Ti Bifunctional Hybrid Catalysts The morphological characteristics of the CZA-HPW/Ti hybrid catalysts in comparison with the CuO/ZnO(Al) catalyst (CZA) were studied by SEM (Figure 6). The SEM image of the bare CZA catalyst shows the typical irregular agglomerates formed by particles of different size and morphology observed in this kind of catalysts [6]. The CZA-HPW/Ti hybrid catalysts shows similar morphology maintaining the morphological characteristics of the agglomerates previously observed in the bare CZA. It is observed that the small HPW/Ti particles (arrows in Figure 6) are in surface contact with the CZA agglomerates without achieving a homogenous distribution and covering of the CZA particles. Figure 6. SEM images showing the morphology of calcined CZA-xHPW/TiO2 hybrid catalysts: Figure 6. SEM images showing the morphology of calcined CZA-xHPW/TiO2 hybrid catalysts: pristine CZA (a), CZA-1.4HPW/Ti (b), CZA-2.7HPW/Ti (c) (arrows indicate HPW/Ti particles). pristine CZA (a), CZA-1.4HPW/Ti (b), CZA-2.7HPW/Ti (c) (arrows indicate HPW/Ti particles). Figure7 shows the XRD patterns of the CZA-HPW /Ti hybrid catalysts in comparison with

the CuO/ZnO(Al) catalyst (CZA) as reference. The pristine CZA shows broad diffraction peaks corresponding to CuO (JCPDS 1-117) and small peaks of ZnO (JCPDS 43-002) with crystallite size around 4.5 and 5 nm respectively (Table3). The di ffractograms of the CZA-HPW/Ti hybrid catalysts show the diffraction peaks corresponding to CuO and ZnO previously observed in the pristine CZA indicating no noticeable changes in their crystalline size after hybridization. The hybrid catalysts also show the diffraction peaks at 25.2, 37.8, 47.9 and 27.4 assigned to the anatase and rutile phases of ◦ TiO2 as well as the main diffraction peaks assigned to crystalline HPW structures. In the case of the Figure 6. SEM images showing the morphology of calcined CZA-xHPW/TiO2 hybrid catalysts: pristine CZA (a), CZA-1.4HPW/Ti (b), CZA-2.7HPW/Ti (c) (arrows indicate HPW/Ti particles).

Catalysts 2020, 10, x FOR PEER REVIEW 9 of 23

Figure 7 shows the XRD patterns of the CZA-HPW/Ti hybrid catalysts in comparison with the CuO/ZnO(Al) catalyst (CZA) as reference. The pristine CZA shows broad diffraction peaks corresponding to CuO (JCPDS 1-117) and small peaks of ZnO (JCPDS 43-002) with crystallite size around 4.5 and 5 nm respectively (Table 3). The diffractograms of the CZA-HPW/Ti hybrid catalysts show the diffraction peaks corresponding to CuO and ZnO previously observed in the pristine CZA indicating no noticeable changes in their crystalline size after hybridization. The hybrid catalysts alsoCatalysts show2020 the, 10 ,diffraction 1071 peaks at 25.2, 37.8, 47.9 and 27.4° assigned to the anatase and rutile phases9 of 22 of TiO2 as well as the main diffraction peaks assigned to crystalline HPW structures. In the case of the HPW structures, their diffraction peaks are shifted to higher angles wich indicates lattice HPW structures, their diffraction peaks are shifted to higher angles wich indicates lattice contraction contraction associated to dehydratation with formation of the cubic HPW·3H2O phase after associated to dehydratation with formation of the cubic HPW 3H O phase after hybridization [25]. hybridization [25]. The dehydration is complete for the hybrids· with2 1.4 and 1.7 HPW monolayers The dehydration is complete for the hybrids with 1.4 and 1.7 HPW monolayers while in the sample while in the sample with 2.7 monolayers remains a slight contribution of the hydrated HPW·6H2O. with 2.7 monolayers remains a slight contribution of the hydrated HPW 6H O. The intensity and size The intensity and size of the crystalline HPW structures on the hybrids· (Table2 3) increase with the of the crystalline HPW structures on the hybrids (Table3) increase with the HPW loading similarly to HPW loading similarly to that observed in the case of the non-hybridized xHPW/Ti acid catalysts. that observed in the case of the non-hybridized xHPW/Ti acid catalysts. The dehydration on HPW The dehydration on HPW structures after hybridization with CZA could be related with the intimate structures after hybridization with CZA could be related with the intimate contact between them contact between them that allows the migration of water from HPW to CZA taking into account the that allows the migration of water from HPW to CZA taking into account the diffusivity of water on diffusivity of water on TiO2 [48] and the capacity of CZA to be rehydrated by the carbonates retained TiO [48] and the capacity of CZA to be rehydrated by the carbonates retained in its structure [49]. in its2 structure [49].

FigureFigure 7.7.XRD XRD patterns patterns of calcined of calcined CZA-xHPW CZA-xHPW/TiO/TiO2 hybrid catalysts:2 hybrid pristine catalysts: CZA pristine (a), CZA-1.4HPW CZA (a),/Ti CZA-1.4HPW/Ti(b), CZA-1.7HPW (b),/Ti (c),CZA-1.7HPW/Ti and CZA-2.7HPW (c), /Tiand (d) CZA-2.7HPW/Ti (anatase (), Rutile (d) ((anatase), H PW (■),O Rutile6H O( (□),), 3 12 40· 2 • HH3PWPW12OO40 6H3H2O O((●), H),3 ZnOPW12 (O♦40) and3H2O CuO (○), ( ZnO)). (◊) and CuO (◖)). 3 12 40· 2 # G Table 3. Crystallite size (nm from XRD) of Cu, CuO and ZnO in CZA and CZA-xHPW/Ti hybrids

catalysts in calcined and reduced state and specific surface area (N2O chemisorption) of copper in reduced hybrid catalysts.

Calcined Reduced

CuO ZnO HPA 3H2O Cu ZnO Cu Surface Area · 2 dp (nm) dp (nm) dp (nm) dp (nm) dp (nm) (m /gcat) CZA-1.4HPW/Ti 4.5 5.1 11.4 5.1 6.3 28.6 CZA-1.7HPW/Ti 4.4 5.3 4.4 5.4 6.1 35.5 CZA-2.7HPW/Ti 4.8 5.3 4.8 5.7 6.5 39.1 CZA 4.5 4.9 - 5.1 6.7 43.4

The reducibility of the CZA-HPW/Ti hybrid catalysts was studied by temperature-programmed reduction (Figure8). The pristine CZA catalyst exhibits a small broad peak around 155 ◦C followed by a second main reduction peak around 180 ◦C. The first peak is ascribed to the reduction of highly dispersed CuO in undecomposed carbonate species while the second main peak is related with the reduction of CuO nanoparticles interacting with ZnO [50]. The reduction profiles of the CZA-HPW/Ti Catalysts 2020, 10, x FOR PEER REVIEW 10 of 23

Table 3. Crystallite size (nm from XRD) of Cu, CuO and ZnO in CZA and CZA-xHPW/Ti hybrids

catalysts in calcined and reduced state and specific surface area (N2O chemisorption) of copper in reduced hybrid catalysts.

Calcined Reduced Cu Surface CuO ZnO HPA·3H2O Cu ZnO Area dp (nm) dp (nm) dp (nm) dp (nm) dp (nm) (m2/gcat) CZA-1.4HPW/Ti 4.5 5.1 11.4 5.1 6.3 28.6 CZA-1.7HPW/Ti 4.4 5.3 4.4 5.4 6.1 35.5 CZA-2.7HPW/Ti 4.8 5.3 4.8 5.7 6.5 39.1 CZA 4.5 4.9 - 5.1 6.7 43.4

The reducibility of the CZA-HPW/Ti hybrid catalysts was studied by temperature-programmed reduction (Figure 8). The pristine CZA catalyst exhibits a small broad peak around 155 °C followed Catalystsby2020 a second, 10, 1071 main reduction peak around 180 °C. The first peak is ascribed to the reduction of highly10 of 22 dispersed CuO in undecomposed carbonate species while the second main peak is related with the reduction of CuO nanoparticles interacting with ZnO [50]. The reduction profiles of the hybridCZA-HPW/Ti catalysts are hybrid not catalysts significantly are not altered significantly respect altered to therespect pristine to the CZApristine catalyst, CZA catalyst, with thewith main reductionthe main peak reduction around 180peak◦C around attributed, 180 °C as attributed, indicated above,as indicated to the above, reduction to the of reduction CuO nanoparticles of CuO in contactnanoparticles with ZnO. in However, contact with it is observedZnO. However, as the it amount is observed of HPW as the in amount the hybrid of HPW increases in the a hybrid decrease in the reductionincreases a peak decrease at low in the temperature reduction peak and at a lo wideningw temperature of the and main a widening reduction of the peak main with reduction separation of twopeak contributions with separation that of indicates two contributions heterogeneous that indicates reducibility heterogeneous of CuO and reducibility their contacts of CuO with and ZnO. The decreasetheir contacts of the with reduction ZnO. The peak decrease at low oftemperature the reduction in peak the at hybrids low temperature corresponding in the to hybrids the highly dispersedcorresponding CuO in to undecomposed the highly dispersed carbonate CuO speciesin undecomposed could be carbonate related with species the could above be commentedrelated with the above commented migration of water from HPW to CZA that could rehydrate these migration of water from HPW to CZA that could rehydrate these carbonates altering its reducibility [49]. carbonates altering its reducibility [49]. The widening in the main reduction peak is probably The wideningderived from in thethe mainabsence reduction of the small peak peak is probably at 155 °C derivedthat is known from that the absencefacilitates of the the subsequent small peak at 155 ◦Creduction that is knownof the CuO that species facilitates [51]. the subsequent reduction of the CuO species [51].

FigureFigure 8. Temperature-programmed 8. Temperature-programmed reduction reduction (TPR)(TPR) prof profilesiles of of pristine pristine CZA CZA (a) (a)and and CZA-xHPW/Ti CZA-xHPW /Ti hybridhybrid catalysts: catalysts: CZA-1.4HPW CZA-1.4HPW/Ti/Ti (b), (b), CZA-1.7HPW CZA-1.7HPW/Ti/Ti (c) and and CZA-2.7HPW/Ti CZA-2.7HPW/ Ti(d). (d).

XRDXRD profiles profiles of reducedof reduced CZA-HPW CZA-HPW/Ti/Ti hybridhybrid catalystscatalysts are are depicted depicted in inFigure Figure 9. All9. Allreduced reduced hybrid catalysts show diffraction peaks corresponding to reflections of metallic Cu (43.0 and 50.2° hybrid catalysts show diffraction peaks corresponding to reflections of metallic Cu (43.0 and 50.2◦ JCPDS 001-1241) and ZnO phases (31.7, 34.8, 36.3 and 47.0 , JCPDS 075-0576). The size of the crystalline ◦ domains of copper in reduced CZA-HPW/Ti hybrid catalysts was similar to that of the pristine reduced CZA but increases slightly with the HPW loading in the hybrid (Table3). The di ffraction peaks corresponding to the HPW 3H O cubic phase detected on the calcined CZA-xHPW/Ti hybrid catalysts · 2 (Figure6) appears now as small peaks in the reduced catalysts (Figure9) which means the almost total dehydration and amorphization of HPW, which increases with the HPW loading, [25] after the reduction treatment. The Cu surface area, the contacts of Cu nanoparticles with reduced zinc species or defects in copper particles in the CZA-xHPW/Ti hybrid catalysts have been evaluated by N2O chemisorption [52] and the results were collected in Table3. The pristine CZA catalyst exhibits very high specific Cu surface area (43.4 m2/g) in line with the best catalysts published for methanol synthesis. The Cu surface area of the reduced hybrid catalysts decreases respect to the pristine CZA, with the decrease being less significant as the HPW load in the hybrid increases (CZA-1.4HPW/Ti < CZA-1.7HPW/Ti < CZA-2.7HPW/Ti, Table3). Since XRD and TPR data did not show appreciable changes in the average Cu0 particle size or reducibility for the hybrid catalysts with respect to the CZA catalyst, the observed variations in the Cu surface area probably reflect: (i) physical blockage of Cu sites by the xHPW/Ti particles [12] or, (ii) modifications in the Cu-ZnO interactions associated with the migration of Cu2+ or Zn2+ species from the CZA to the acid catalyst which has been identified as main cause of deactivation in of the hybrids formed by Cu-ZnO(Al) and acid catalysts [53–55]. Catalysts 20202020,, 10,, x FOR PEER REVIEWREVIEW 11 of 23

JCPDS 00001-1241)1-1241) and ZnO phases (31.7,(31.7, 34.8, 36.3 andand 47.0°,47.0°, JCPDSJCPDS 07075-0576).5-0576). The size of the crystalcrystallineline domains of copper in reduced CZACZA-HPW/Ti-HPW/Ti hybrid catalysts was similar to that of the pristine reduced CZA but increases slightly with the HPW loading in the hybrid (Table 3). The diffraction peaks corresponding to the HPW·3H2O cubic phase detected on tthehe calcined CZACZA-xHPW/Ti-xHPW/Ti hybrid catalysts (Figure 6) appears now as small peaks in the reduced catalysts Catalysts(Figure2020, 10 9), 1071 which means thethe almost totaltotal dehydrationdehydration and amorphization of HPW, which increases11 of 22 with the HPW loadingloading,, [[25]25] after the reduction treatment.

FigureFigure 9. XRD 9. XRD patterns patterns of reduced of reduced pristine pristine CZA CZA (a) (a) and and reducedreduced CZACZA-xHPW/TiO CZA-xHPW-xHPW/TiO/TiO2 hybrid2 hybrid catalysts: catalysts: CZA-1.4HPWCZACZA-1.4HPW/Ti-r-1.4HPW/Ti/Ti-r (b),-r (b),CZA-1.7HPW CZACZA-1.7HPW/Ti-r-1.7HPW/Ti/Ti-r- (c)r (c) and and CZA-2.7HPW CZACZA-2.7HPW/Ti-r-2.7HPW/Ti/Ti-r-r (d) (d) (anatase (anatase (■), ( Rutile), Rutile (□), (), 00 H PWH3OPW12123HO4040 3HO(2O ),(○ ZnO), ZnO (♦ ()◊ and) and Cu Cu (()))).. 3 12 40· 2 # The acidThe propertiesCu surface ofarea, reduced the contacts 1.4 and of Cu 2.7 nanoparticles CZA-HPW/ Tiwith hybrids reduced were zinc evaluated species or bydefects NH3-TPD in and thecopper results particles were in depicted the CZACZA-xHPW/Ti- inxHPW/Ti Figure hybrid10. Significant catalysts have diff erencesbeen evaluated have been by N2 foundO chemisorption between the [52] and the results were collected in Table 3. The pristine CZA catalyst exhibits very high specific acid properties[52] and the of results the reduced were collected CZA-xHPW in Table/Ti 3. hybridsThe pristine with CZA respect catalyst to exhibits the xHPW very/Ti high acid specific catalysts Cu surface area (43.4 m2/g) in line with the best catalystscatalysts publishedpublished forfor methanolmethanol synthesis. TheThe CuCu before hybridization (Figure5). The reduced CZA-xHPW /Ti hybrids show similar desorption profile surface area of the reduced hybrid catalysts decreases respect to the pristine CZA, with the decrease regardless of the HPW loading with desorption peaks between 200 and 400 C that corresponds to being less significant as the HPW load in the hybrid increases ((CZA-1.4HPW/TiCZA-1.4HPW/Ti◦ < CZACZA-1.7HPW/Ti-1.7HPW/Ti < acid sitesCZACZA-2.7HPW/Ti, of-2.7HPW/Ti, weak or medium Table 3). strength. Since XRD According and TPR to data literature did not studies, show appreciable the acid sites changes of weak in theand /or moderateaverage strength Cu0 particle are the size most or redureducibility desirablecibility for for the DME hybrid selectivity catalysts with since respect the strong to the CZA Brönsted catalyst, acid the sites favourobserved the conversion variations of in DME the Cu to surface hydrocarbons area probably [11,56 reflect:]. The (i) medium-weak physical blockage acid of Cu sites sites observed by the in the reducedxHPW/Ti hybrids particles contrast [[12]12] oror, with, (ii) the modifications very strong in acid the sites CuCu-ZnO-ZnO detected interactions on the xHPW associateassociated/Tid acid with catalysts the migration of Cu2+ or Zn2+2+ species from the CZA to the acid catalyst which has been identified as main beforemigration hybridization of Cu (550–600or Zn species◦C, Figurefrom the5). CZA This to the finding acid catalyst indicates which that has hybridization been identified withas main CZA catalystscause gives of deactivation rise to a decrease in of the in hybrids the acid formed strength by CuCu-ZnO(Al) of-ZnO(Al) the HPW and with acid the catalysts disappearance [53[53–55].–55]. of the strong The acid properties of reduced 1.4 and 2.7 CZA-HPW/Ti hybrids were evaluated by NH3-TPD acidic sitesThe typical acid properties of pristine of HPWreduced units. 1.4 and A similar2.7 CZA- behaviour,HPW/Ti hybrids suppression were evaluated of Brönsted by NH3 acidity-TPD of and the results were depicted in Figure 10. SignificantSignificant differences have been found between thethe acid zeolites after hybridization with Cu-ZnO catalysts, was already observed in the literature [24,57] and properties of the reduced CZACZA-xHPW/Ti-xHPW/Ti hybrids with respect to the xHPW/Ti acid catalysts before attributed to the exchange of protons with Cu2+ and/or Zn2+ cations from Cu-ZnO occurring during hybridization (Figure 5). The reduced CZACZA-xHPW/Ti-xHPW/Ti hybrids show similar desorption profile the hybridregardless preparation. of the HPW In line loading with with this, desorption the partial peakspe exchangeaks between of protons 200 and of 400 heteropolyacids °°CC that corresponds with to metal II III IV III III IV II cationsacid (Cu sites, Al of weak, Sn or, Femedium, Cr strength., Zr , Zn According, ... ) has to literature been reported studies, producing the acid sites Lewis of weak sites aand/ornd/or with the concomitantmoderate reduction strength inare the the amount most desirable of Brönsted for DME sites selectivityse [58lectivity,59]. To since support the strong the inter-cationic Brönsted acid exchange sites betweenfavour Cu2 the+ and conversion/or Zn2+ ofand DME protons to hydrocarbons in CZA-xHPW [11,56[11,56].]./Ti The hybrids, mediummedium-weak they-weak are acid characterized sites observed by in DRIFTS the reduced hybrids contrast with the very strong acid sites detected on the xHPW/Ti1 acid catalysts (Figurereduced 11) because hybrids the contrast structural with modes the very of the strong KU, between acid sites 1150 detected and 700 on the cm − xHPW/Ti, are sensitive acid catalysts to changes in its compositionbefore hybridization [35]. The (55(550–600 DRIFTS0–600 °°C,C, spectra Figure of5). CZA-xHPW This finding/ Ti indicates hybrids that show hybridization the characteristic with CZA bands catalysts gives rise to a decrease in the acid strengthstrength of the1 HPW with the disappearance ofof the of the Keggin structure at around 1080, 983, 886 and 800 cm− already identified in non-hybridized dehydrated xHPW/Ti counterparts (Figure3). However, the hybrids show slight displacements and 1 shoulders at 1053 and 952 cm− that may indicate certain alterations in the structure derived to the change in the composition of the compensating cations, able to interact with terminal and/or bridging oxygens of the Keggin units [59,60]. This result is consistent with the exchange of H+ in HPW by Cu2+/Zn2+ and it may be the cause of the reduction in the amount of Brönsted acidity of xHPW/Ti 2+ 2+ observed by NH3-TPD after the hybridization with CZA. The migration of Cu /Zn species from CZA to xHPW/Ti could proceed through the contact-induced ion exchange mechanism described in literature to explain the exchange of cationic species between zeolites by simple physical contact and mediated by water adsorbed on the pores [61,62]. Catalysts 2020, 10, x FOR PEER REVIEW 12 of 23

strong acidic sites typical of pristine HPW units. A similar behaviour, suppression of Brönsted acidity of zeolites after hybridization with Cu-ZnO catalysts, was already observed in the literature [24,57] and attributed to the exchange of protons with Cu2+ and/or Zn2+ cations from Cu-ZnO occurring during the hybrid preparation. In line with this, the partial exchange of protons of heteropolyacids with metal cations (CuII, AlIII, SnIV, FeIII, CrIII, ZrIV, ZnII,…) has been reported producing Lewis sites with the concomitant reduction in the amount of Brönsted sites [58,59]. To support the inter-cationic exchange between Cu2+ and/or Zn2+ and protons in CZA-xHPW/Ti hybrids, they are characterized by DRIFTS (Figure 11) because the structural modes of the KU, between 1150 and 700 cm−1, are sensitive to changes in its composition [35]. The DRIFTS spectra of CZA-xHPW/Ti hybrids show the characteristic bands of the Keggin structure at around 1080, 983, 886 and 800 cm−1 already identified in non-hybridized dehydrated xHPW/Ti counterparts (Figure 3). However, the hybrids show slight displacements and shoulders at 1053 and 952 cm−1 that may indicate certain alterations in the structure derived to the change in the composition of the compensating cations, able to interact with terminal and/or bridging oxygens of the Keggin units [59,60]. This result is consistent with the exchange of H+ in HPW by Cu2+/Zn2+ and it may be the cause of the reduction in the amount of Brönsted acidity of xHPW/Ti observed by NH3-TPD after the hybridization with CZA. The migration of Cu2+/Zn2+ species from CZA to xHPW/Ti could proceed through the contact-induced ion exchange mechanism described in literature to explain the exchange of Catalystscationic2020 species, 10, 1071 between zeolites by simple physical contact and mediated by water adsorbed 12on of the 22 pores [61,62].

FigureFigure 10.10.TPD-NH TPD-NH33 profilesprofiles ofof reducedreduced CZA-xHPWCZA-xHPW/TiO/TiO22 hybridhybrid catalysts:catalysts: CZA-1.4HPWCZA-1.4HPW/Ti/Ti (a)(a) andand CatalystsCZA-2.7HPWCZA-2.7HPW/Ti 2020, 10, x FOR/Ti PEER (b). (b). REVIEW 13 of 23

FigureFigure 11. 11. DRIFTDRIFT spectra of CZA-xHPWCZA-xHPW/Ti/Ti hybridhybrid catalysts: catalysts: CZA-1.4HPW CZA-1.4HPW/Ti/Ti (a), (a), CZA-1.7HPW CZA-1.7HPW/Ti/Ti (b) (b)and and CZA-2.7HPW CZA-2.7HPW/Ti/Ti (c). (c). 2.3. Activity in Direct Synthesis of DME from Syngas 2.3. Activity in Direct Synthesis of DME from Syngas The activity of the CZA-HPW/Ti hybrid catalysts were evaluated for the direct DME synthesis fom The activity of the CZA-HPW/Ti hybrid catalysts were evaluated for the direct DME synthesis syngas (30 bar and 250 C) and the activity data were collected in Table4 and Figure 12. Activity data fom syngas (30 bar and ◦250 °C) and the activity data were collected in Table 4 and Figure 12. Activity also includes the results for methanol synthesis on the CZA catalyst and the DME synthesis on the data also includes the results for methanol synthesis on the CZA catalyst and the DME synthesis on CZA-HZSM-5 hybrid reference representative of the most widely applied catalysts for direct synthesis the CZA-HZSM-5 hybrid reference representative of the most widely applied catalysts for direct of DME. synthesis of DME.

Figure 12. Methanol and DME time yield (μmol/min/g catalyst) from syngas over CZA-1.4HPW/Ti (□), CZA-1.7HPW/Ti (◊) and CZA-2.7HPW/Ti (●).

Catalysts 2020, 10, x FOR PEER REVIEW 13 of 23

Catalysts 2020, 10, 1071 13 of 22

Table 4. Initial catalytic activity data (CO conversion, selectivity and DME time yield) and deactivation

(decay in umolDME/min gcat) in direct DME synthesis from syngas over reduced CZA-xHPW/Ti hybrid catalysts (Data for methanol synthesis on CZA and DME synthesis on CZA-HZSM-5 catalysts were included as references). Figure 11. DRIFT spectra of CZA-xHPW/Ti hybrid catalysts: CZA-1.4HPW/Ti (a), CZA-1.7HPW/Ti Deactivation (b) and CZA-2.7HPW/Ti (c). Selectivity (%) DME Time Yield CO Conversion Rate DME (%) 2.3. Activity in Direct Synthesis of DME fromMethanol Syngas DME CO2 (µmol/min gcat)(µmol/min gcat) CZA 17.6 98.9 - 1.1 967.3 * 0.22 * The activity of the CZA-HPW/Ti hybrid catalysts were evaluated for the direct DME synthesis HZSM-5-CZA 11.4 6.2 55.7 38.1 229.4 0.24 fomCZA-1.4HPW syngas (30 /barTi and 250 14.5 °C) and the activity 97.4 data 2.5were collected 0.1 in Table 12.3 4 and Figure 0.0612. Activity dataCZA-1.7HPW also includes/Ti the results 10.0 for methanol 12.6 synthesis 51.3 on the 36.1 CZA catalyst 201.9 and the DME synthesis 0.20 on theCZA-2.7HPW CZA-HZSM-5/Ti hybrid 11.0reference representative 6.453.0 of the 40.6most widely 243.0 applied catalysts 0.14 for direct synthesis of DME. * Methanol time yield and deactivation rate (µmol/min gcat).

FigureFigure 12.12. MethanolMethanol and and DME DME time time yield yield (µ (molμmol/min/g/min/g catalyst) catalyst) from from syngas syngas over over CZA-1.4HPW CZA-1.4HPW/Ti/Ti (), CZA-1.7HPW(□), CZA-1.7HPW/Ti/Ti (♦) and (◊) CZA-2.7HPWand CZA-2.7HPW/Ti/Ti ( ). (●). •

The methanol activity on the CZA catalyst is very high (967.3 µmol, CH3OH/min gcat, Table4) and comparable with the best data published in the literature [63]. As expected, the catalytic behaviour of the CZA-xHPW/Ti hybrid catalysts depend on their HPW loading. All CZA-xHPW/Ti hybrids only show methanol and DME as products without formation of hydrocarbons. The absence of hydrocarbons, from DME dehydration, could be justified because the acid sites present on the hybrids were of weak or medium strength (Figure5) and because the formation of hydrocarbons requires higher contact-time than that used in our catalyst test [64]. The overall methanol and DME yield on CZA-HPW/Ti hybrids were in all cases lower than the methanol yield achieved on the CZA reference. This effect was also observed in other works in the literature but without discussing its causes [12,65]. Considering that overall DME production is controlled by the methanol synthesis step, the observed decrase in the overall yield on CZA-xHPW/Ti hybrids can be derived from detrimental effects on the CZA catalyst after hybridization. It is well stablished that the methanol synthesis activity on copper catalysts is directly related with its Cu surface area (reduced zinc species in contact with copper and/or defects in copper particles induced by their contact with Zn) [66]. According to the characterization of the CZA-xHPW/Ti hybrid catalysts, the Cu surface area in the hybrid catalysts, evaluated by N2O chemisorption, shows a reduction in the hybrid catalysts respect to the pristine CZA (Table3) and this fact may be the cause of the observed reduction in the overall yield on CZA-xHPW/Ti hybrid Catalysts 2020, 10, 1071 14 of 22

catalysts respect the pristine CZA. However the sequence in the copper surface area in the hybrids (CZA-1.4HPW/Ti < CZA-1.7HPW/Ti < CZA-2.7HPW/Ti, Table3) does not fit with the overall yield loss on the hybrids, presenting the sample with the smallest Cu surface area (CZA-1.4HPW/Ti) the highest overall yield in the hybrid series. Taking into account the paralellism between the DME production and the overall yield loss on the hybrids, a second cause to explain the overall yield loss may be related with detrimental interactions between CZA and the excess of water from DME production because the kinetics of the methanol formation is very sensitive to the water content [67]. In fact it is observed an increase in the CO2 selectivity associated to the WGS reaction whose contribution increases with increasing water concentration. In addition to the inhibitory effect of the water on the methanol synthesis kinetics, water can also promote the sintering of copper particles [68]. However, Catalysts 2020, 10, x FORthe PEER average REVIEW copper particle size in used CZA-xHPW/Ti hybrids (determined11 of 23 by XRD Figure 13, Table5) compared with the bare CZA demonstrated that excess of water from higher production of DME does JCPDS 001-1241) and ZnO phases (31.7, 34.8, 36.3 and 47.0°, JCPDS 075-0576). The size of the Catalysts 2020, 10, x FOR PEER REVIEW 15 of 23 crystalline domainsnot of significantly copper in reduced affect the CZA particle-HPW/Ti size hybrid as the catalysts Cu growth was similar (8–13%) to wasthat similarof the in all used hybrids. pristine reduced CZA but increases slightly with the HPW loading in the hybrid (Table 3). The diffraction peaks corresponding to the HPW·3H2O cubic phase detected on the calcined CZA-xHPW/Ti hybrid catalysts (Figure 6) appears now as small peaks in the reduced catalysts (Figure 9) which means the almost total dehydration and amorphization of HPW, which increases with the HPW loading, [25] after the reduction treatment.

Figure 13. XRD patterns of used CZA-HPW/TiO2 hybrid catalysts: CZA-1.4HPW/Ti-u (a), Figure 9. XRD patternsFigure of reduced 13. XRDpristine patterns CZA (a) and of usedreduced CZA-HPW CZA-xHPW/TiO/TiO2 2 hybridhybrid catalysts: catalysts: CZA-1.4HPW/Ti-u (a), CZA-1.7HPW/Ti-u (cb) and CZA-2.7HPW/Ti-u (c) (anatase (■), Rutile (□), H3PW12O40 3H2O (○), ZnO CZA-1.4HPW/Ti-r CZA-1.7HPW (b), CZA-1.7HPW/Ti/Ti-u- (cb)r (c) and and CZA-2.7HPW CZA-2.7HPW/Ti/Ti-u-r (d) (c) (anatase (anatase (■ (), ), Rutile Rutile (□), ( ), H3PW12O40 3H2O( ), 0 · 00 H3PW12O40 3H2O (○ZnO),( ◊ZnO) and (♦ ()◊ and)Cu and Cu (Cu)). ( )))).. #

The Cu surface area,TableTable the 5. 5.Crystallitecontacts Crystallite of size Cu size (nmnanoparticles (nm from from XRD) XRD) with of of Cu reducedCu and and ZnO Znzinc inO CZAinspecies CZA and andor CZA-xHPW defects CZA-xHPW/Ti in /Ti hybrids hybrids catalysts catalysts in copper particles in theused inCZA used state.-xHPW/Ti state. hybrid catalysts have been evaluated by N2O chemisorption [52] and the results were collected in Table 3. The pristine CZA catalyst exhibits very high specific Cu ZnO Cu surface area (43.4 m2/g) in line with the best catalysts published for methanolCu synthesis. TheZnO Cu surface area of the reduced hybrid catalysts decreases respect to the pristinedpdp (nm)CZA, (nm) with dp the (nm) decrease dp (nm) being less significant as the HPW load in the hybridCZA-1.4HPW/TiCZA-1.4HPW increases (CZA/Ti -1.4HPW/Ti 5.15.1 < CZA -1.7HPW/Ti 6.3 6.3 < CZA-2.7HPW/Ti, Table 3). Since XRD and TPRCZA-1.7HPW/Ti CZA-1.7HPW data did not/ Ti show appreciable 5.45.4 changes 6.1 in 6.1 the average Cu0 particle size or reducibility for the hybridCZA-2.7HPW/TiCZA-2.7HPW catalysts with/Ti respect 5.7 to 5.7the CZA catalyst, 6.5 6.5 the observed variations in the Cu surface area probably reflect:CZACZA (i) physical blockage 5.15.1 of Cu sites 6.7 by6.7 the xHPW/Ti particles [12] or, (ii) modifications in the Cu-ZnO interactions associated with the migration of Cu2+ or Zn2+ species from the CZA to the acid catalyst which has been identified as main TheThe relationship relationship between between the the rate rate of of the the DME DME production production and and the the acid acid strenght strenght of theof the catalysts catalysts is cause of deactivation in of the hybrids formed by Cu-ZnO(Al) and acid catalysts [53–55]. The acid propertieswellis well documented of documentedreduced 1.4 in heteropolyand in 2.7 heteropoly CZA acid-HPW/Ti catalysts acid hybrids catalysts [21 were]. The evaluated[21]. diff erencesThe by differences NH3 in acidity-TPD in observed acidity onobserved xHPW/ Tion and the results werecatalystsxHPW/Ti depicted (Figure catalysts in Figure5, Table (Figure10. Significant2) dissapear 5, Table differences 2) after dissapear hybridization, have been after found hybridization, showing between allthe showing theacid CZA-xHPW all the CZA-xHPW/Ti/Ti hybrids properties of the reducedhybrids CZA similar-xHPW/Ti acid siteshybrids of withweak respect or medium to the xHPW/Ti strength acid (Figure catalysts 10). before Therefore similar rate of DME hybridization (Figureproduction 5). The per reduced acid siteCZA can-xHPW/Ti be expected hybrids on show the hybrids similar desorptionand the observed profile differences in the DME regardless of the HPWproductivity loading withon thedesorption hybrids peaks (Table between 4) would 200 and be 400 due °C tothat the corresponds different toconcentration of acid sites acid sites of weak accordingor medium tostrength. its different According HPW to content. literature However studies, the it isacid observed sites of weak that thereand/or is not linearity between the moderate strengthHPW are the content most desirablein hybrids for and DME the selectivity production since of the DM strongE (Figure Brönsted 12). acidThe siteslowest loaded hybrid catalyst favour the conversion of DME to hydrocarbons [11,56]. The medium-weak acid sites observed in the (CZA-1.4HPW/Ti) was ineffective, with low DME selectivity (2.5%) while as the HPW loading in reduced hybrids contrast with the very strong acid sites detected on the xHPW/Ti acid catalysts hybrids is increased the selectivity towards DME increases, reaching a maximun selectivity for DME before hybridization (550–600 °C, Figure 5). This finding indicates that hybridization with CZA catalysts gives rise(53.0%) to a decrease for the inhighest the acid HPW strength loaded of the hybrid HPW catalyst with the (CZA-2.7HPW/Ti). disappearance of the The DME production over the most active CZA-2.7HPW/Ti hybrid catalyst (260 μmol/min gcat) is significantly higher than that

achieved on the CZA-HZSM-5 hybrid reference (229 μmol/min gcat) representative of the most widely applied catalysts for direct synthesis of DME. The low effectivity for DME production of the CZA-1.4HPW/Ti sample could be related with the fact that the dehydration of methanol to DME improves in the pseudo-liquid behaviour of the HPW for which it is necessary to develop three-dimensional crystalline structures [69] and as can be seen in Figure 13 they are not formed at this HPW loading. On the contrary, three-dimensional HPW structures are clearly observed in the samples 1.7 and 2.7 CZA-xHPW/Ti [70] in which the pseudo-liquid behaviour could occur

Catalysts 2020, 10, 1071 15 of 22 similar acid sites of weak or medium strength (Figure 10). Therefore similar rate of DME production per acid site can be expected on the hybrids and the observed differences in the DME productivity on the hybrids (Table4) would be due to the di fferent concentration of acid sites according to its different HPW content. However it is observed that there is not linearity between the HPW content in hybrids and the production of DME (Figure 12). The lowest loaded hybrid catalyst (CZA-1.4HPW/Ti) was ineffective, with low DME selectivity (2.5%) while as the HPW loading in hybrids is increased the selectivity towards DME increases, reaching a maximun selectivity for DME (53.0%) for the highest HPW loaded hybrid catalyst (CZA-2.7HPW/Ti). The DME production over the most active CZA-2.7HPW/Ti hybrid catalyst (260 µmol/min gcat) is significantly higher than that achieved on the CZA-HZSM-5 hybrid reference (229 µmol/min gcat) representative of the most widely applied catalysts for direct synthesis of DME. The low effectivity for DME production of the CZA-1.4HPW/Ti sample could be related with the fact that the dehydration of methanol to DME improves in the pseudo-liquid behaviour of the HPW for which it is necessary to develop three-dimensional crystalline structures [69] and as can be seen in Figure 13 they are not formed at this HPW loading. On the contrary, three-dimensional HPW structures are clearly observed in the samples 1.7 and 2.7 CZA-xHPW/Ti [70] in which the pseudo-liquid behaviour could occur improving the methanol access to the acid sites and thus justifying the remarkable increase in DME production observed for these samples. The incomplete conversion of methanol to DME observed on the 1.7CZA-xHPW/Ti hybrid (Figure 12) means that the methanol dehydration reaction is the rate limiting step and therefore the amount of the acid sites in this hybrid is insufficient to convert the methanol formed to DME. On the contrary, the CZA-2.7HPW/Ti hybrid is able to convert most of the methanol formed to DME which indicates that it has a sufficient number of acid sites to transform the methanol formed by the CZA. Therefore for the one step DME synthesis over the CZA-xHPW/Ti hybrids its acidity should be balanced with the CZA catalytic function throught the HPW loading to maximize the selectivity to DME. Finally it is observed a moderate loss in the DME production with TOS (Table4) over the 1.7 and 2.7 CZA-xHPW /Ti hybrids that denotes, particularly in the 1.7CZA-xHPW/Ti sample, some kind of deactivacion in the acid function of the hybrids. However, this deactivation is much less than that observed in the CZA-HZSM-5 hybrid reference. The deactivation of acid catalysts during DME synthesis is well known in the literature and it is mainly related to the blocking of acid sites by carbonaceous species and/or partial exchange of protons by Cu2+/Zn2+ [24,71]. The different deactivation rate of the 1.7 and 2.7 CZA-xHPW/Ti hybrids could be related with differences in its surface/bulk acidity ratio derived from their different three-dimensional development, because the deactivation of the surface acid sites by carbonaceous species is faster than the bulk counterparts [72]. However, further studies will be carried out to corroborate this hipothesis and to establish the precise deactivation mechanism of the acid sites present in the CZA-xHPW/Ti hybrids that have been described for the first time as acive catalysts for the the direct synthesis of DME from syngas.

3. Materials and Methods

3.1. Catalysts Preparation

3.1.1. Cu-ZnO(Al) (CZA) Methanol Synthesis Catalyst The highly active Cu-ZnO(Al) catalyst (CZA) with optimized composition (Cu/Zn/Al = 68/29/3) was prepared following the methodology proposed as benchmark for Cu-based catalysts and described elsewhere [73]. Hydroxycarbonate precursors were synthesized by co-precipitation from Cu, Zn, Al aqueous nitrate solutions (1M Cu(NO3)2 2.7H2O, 99.999%, (Sigma-Aldrich, St. Louis, MO, USA), Zn(NO3)2 6H2O, 99.999%, (Sigma-Aldrich, St. Louis, MO, USA), and Al(NO3)2 9H2O, 99.997% (Sigma-Aldrich, St. Louis, MO, USA) with Na2CO3 1.6M, 99.999%, (Sigma-Aldrich, St. Louis, MO, USA)) under controlled conditions (pH = 6.5, 65 ◦C). The aged precipitates were filtered and washed several times with deionized water and dried in air at 80 ◦C for 12 h and calcined under static air at 340 ◦C for 2 h with a heating rate of 2 ◦C/min. Catalysts 2020, 10, 1071 16 of 22

3.1.2. H3PW12O40/TiO2 Acid Catalysts

A series of supported H3PW12O40 catalysts (HPW/Ti) were prepared by incipient wetness 2 impregnation on TiO2 (P25, 54.5 m /g Alfa Aesar, Ward Hill, MASS, USA). Appropriate amount of H PW O (H PW O xH O (reagent grade, Sigma-Aldrich, St. Louis, MO, USA)) was dissolved 3 12 40 3 12 40· 2 on ethanol and impregnated dropwise on the TiO2 powders. After impregnation, the solids were dried at room temperature for 24 h and thermally treated in an oven using two temperature steps: 50 ◦C (1 ◦C/min, 2 h) and subsequent heating at 150 ◦C (2 ◦C/min, 2 h). Taking into account that local structure of Keggin units depends on the interaction stablished between the HPA with the TiO2 support, we prepared three catalyst samples with H3PW12O40 loading of 26.3, 30.3 and 41.7 wt % that correspond to 1.4, 1.7 and 2.7 theoretical monolayers respectively, assuming a cross sectional area of 1.13 nm2 per Keggin unit [74]. The samples are labelled as xHPW/Ti where x is the theoretical HPAs monolayer (1.4, 1.7 and 2.7).

3.1.3. Bifunctional CZA-xHPW/Ti Hybrid Catalysts The hybrid catalysts were obtained by physical mixing of xHPW/Ti acid catalysts with CZA calcined powders in an agate mortar with milling during 5 min to form the homogenous mixture of the hybrid catalyst. Prior to mixture, the CZA powders were homogenized in ethanol using a high-performance single-stage dispersing machine (Ultra-Turrax T25 IKA, Staufen, Germany, 14,000 rpm during 20 s, 6 times). The xHPW/Ti/CZA hybrid catalysts was prepared in a mass ratio of 1:2 (w/w) to achieve a theoretical H+(HPW)/Cu2+(CZA) molar ratio in the hybrids between 0.015 and 0.023. The final catalysts were obtained from the thermal reduction of hybrids under diluted hydrogen flow (2.2 vol % H2) at 150 ◦C (2 ◦C/min) with a subsequent heating at 200 ◦C (1 ◦C/min) for 2 h. The hybrids were labelled as CZA-xHPW/Ti where x is the theoretical HPAs monolayer (1.4, 1.7 and 2.7). A bifunctional hybrid catalyst based on the mixing of CZA and HZSM-5 in mass ratio of 1:2 (with excess of acid sites), using the same methodology as commented above, was also prepared as reference catalyst representative of the hybrid formulation most widely used in the bibliography [75,76].

3.2. Physicochemical Characterization The crystalline structure of xHPW/Ti, CZA and CZA-xHPW/Ti hybrids (in fresh, reduced and used state) were determined from X-ray powder diffraction (XRD) measured using an X’Pert Pro PANalytical (Malvern, Worcs, UK) diffractometer with nickel-filtered Cu Kα1 radiation (λ = 0.15406 nm, 45 kV, 40 mA). Crystallite sizes of HPW, CuO, ZnO and Cu0 were determined by applying the Debye-Scherrer equation. Structural information of HPW on xHPW/Ti catalysts was studied by Raman and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS-FTIR). Raman spectra of the xHPW/Ti catalysts were recorded in a Raman spectrometer confocal microscope Renishaw (inVia Raman microscope, Glos, UK). Laser beam in red (785 nm) is used as excitation source with the power set at 300 mW. Raman spectra were collected with a 50 objective lens with an acquisition time of 5 s × and 10–15 acquisitions. DRIFTS were measured on an infrared spectrometer with MCT-M detector 1 (JASCO FT/IR-6300, Tokyo, Japan). The DRIFTS spectra were obtained with 200 scans and 4 cm− resolution at different temperature (25, 150, 200 and 250 ◦C) purging the cell with He during the experiments. CZA-xHPW/Ti fresh hybrid catalysts were also analysed by DRIFTS at room temperature. Temperature-programmed reduction (TPR) of CZA catalyst and CZA-xHPW/Ti hybrids were carried out on a semiautomatic PID Eng&Tech (Madrid, Spain) apparatus equipped with a U-shaped quartz reactor and a TC detector. Before reduction the samples (50 mg of pure CZA and 75 mg of hybrids) were treated under He flow (30 mL/min) at 120 ◦C for 20 min to remove water and other adsorbed contaminants. Reduction profiles were obtained by heating the samples in a 10% H2/Ar flow (50 NmL/min) from room temperature to 200 ◦C at a linearly programmed rate of 2 ◦C/min. Catalysts 2020, 10, 1071 17 of 22

The effluent gas was passed through a cold trap to remove water before measuring the amount of hydrogen consumed during reduction by the thermal conductivity detector. Specific copper surface areas in reduced CZA catalyst and CZA-xHPW/Ti hybrids were determined from N2O chemisorption. The reduced samples were treated under diluted N2O/Ar (2 vol %, 30 mL/min) at room temperature for 5 s. After chemisorption the physisorbed N2O was removed with He flow (30 mL/min) for 30 min and subsequently reduced under H2/Ar flow (10 vol % H2, 50 mL/min) at a heating rate of 10 ◦C/min up to 200 ◦C. The hydrogen consumption corresponding to the reduction is equivalent to the amount of consumed N2O during surface chemisorption. The specific copper surface area was calculated considering a molar stoichiometry of Cu(s)/N2O = 2 and an average value of 1.47 1019 copper atoms/m2 for the surface density of copper metal [77]. The values of copper × surface areas in CZA catalyst and CZA-xHPW/Ti hybrids were normalized to the weight of calcined CZA catalysts loaded in the reactor. Total acidity of xHPW/Ti catalysts and CZA-xHPW/Ti hybrids were determined by temperature programmed desorption of ammonia (NH3-TPD) which were obtained in an Micromeritics TPD/TPR apparatus (Norcross, GA, USA) coupled with a quadrupole mass spectrometer (OmniStar from Pfeiffer Vacuum, Asslar, Germany). Before chemisorption of ammonia, the samples (with a loading equivalent to 10 mg of HPW) were pre-treated under He flow (30 mL/min) at 240 ◦C for 20 min and after that, the temperature was lowered to 100 ◦C under the same He flow. Subsequently, a flow of diluted NH3 (5 vol %) were introduced at 100 ◦C until saturation of the sample. After chemisorption, the physisorbed NH3 was removed with He flow (30 mL/min) for 30 min and subsequently the chemisorbed ammonia was desorbed by heating from 100 to 785 ◦C at a heating rate of 10 ◦C/min. The classical detection of NH3 by TCD was not appropriate for the acidity study on heteropolyacids because desorption of ammonia was accompanied by other products formed from the decomposition of ammonia salts of the heteropolyacids [45]. Therefore the effluent gas was analysed by mass spectrometry (m/z = 16, 18) to follow the NH3 desorption profiles.

3.3. DME Synthesis from Syngas Tests Activity tests were performed in a fixed-bed reactor loaded with 0.225 g of hybrid catalysts diluted with SiC (1:3 vol). The hybrids were reduced in situ under hydrogen flow (2.2% H2/Ar, 50 mL/min) using two temperature steps: 150 ◦C (2 ◦C/min) and subsequent heating at 200 ◦C (1 ◦C/min) for 2 h. After reduction, the syngas mixture (75 NmL/min, molar composition: 4.5% CO2, 22.0% CO, 58.8% H2 and 14.7% N2) was fed to the reactor. The catalytic tests were conducted at high GHSV (30 Lsyngas 1 1 gcat− h− ) to achieve low conversions and to exclude the effect of equilibrium and to better visualize the differences in deactivation. Activity was measured at 250 ◦C and 30 bar maintaining the reaction for 6 h after a catalyst stabilisation time under syngas feed of 1h. The reaction gas products were regularly analysed on-line by GC (Varian 450-GC, Bruker, MASS, USA) with TCD equipped with capillary columns connected in series (QS-bond: CO2, CH3OH, DME and H2O and molecular sieve 5A: H2,O2,N2, CO) gas. The carbon balance was in all tests below 3%. Activity data were reported as CO conversion, product selectivity (DME, methanol and CO2) and methanol/DME production rate normalized to the weight of hybrid catalyst loaded in the reactor (µmol CH3OH or DME/gcat min). CO conversion: M M CO(i) − CO( f ) XCOx (%) = 100 MCOx(i) · DME selectivity: MDME SDME (%) = P 100 Mproducts·

CH3OH selectivity: MCH3OH SCH OH (%) = P 100 3 Mproducts· Catalysts 2020, 10, 1071 18 of 22

CO2 selectivity: MCO2 SCO2 (%) = P 100 Mproducts· DME production rate: MDME (µmol) RpDME = mcat (g) t (min) · CH3OH production rate: MCH3OH (µmol) RpCH3OH = mcat (g) t (min) · where M is the molar flow rate of each component.

4. Conclusions

A series of bifunctional hybrid catalysts based on phosphotungstic acid (H3PW12O40) supported on TiO2 combined with Cu-ZnO(Al) catalyst have been prepared, characterised and tested for direct DME synthesis from syngas. The heteropolyacid loading on TiO2 (from 1.4 to 2.7 theoretical monolayers) affects the dispersion and acid characteristics of the heteropolyacid clusters, resulting in the formation of three-dimensional heteropolyacid nanocyrstals with acidity similar to the bulk counterpart when the concentration of the heteropoliacid in the TiO2 support is higher than 1.7 monolayers. Physical hybridization of supported heteropolyacids with the Cu-ZnO(Al) modifies both the acidic characteristics of the supported heteropolyacids and the copper surface area of the Cu-ZnO(Al) catalyst. The hybrids show a decrease in the copper surface area and the disappearance of the strong acidic sites of pristine HPW nanocrystals, showing all hybrids similar acid sites of weak or medium strength. These changes are related with the migration of water and/or Cu2+ or Zn2+ species between the supported HPW and Cu-ZnO(Al) during hybridization. The catalytic behaviour of the CZA-xHPW/Ti hybrid catalysts for the direct DME synthesis fom syngas (30 bar and 250 ◦C) depend on their HPW loading. The overall methanol and DME yield on CZA-HPW/Ti hybrids was in all cases lower than the methanol yield achieved on the CZA reference. Only the hybrids with HPW loading higher than 1.7 monolayers are effective for the direct synthesis of DME, reaching the hybrid with 2.7 monolayers (CZA-2.7HPW/Ti) the maximun selectivity for DME (53.0%). These hybrids present three-dimensional HPW structures in which the pseudo-liquid behaviour could occur improving the methanol access to the acid sites. 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 deactivacion of the acidity function under reaction conditions probably related to the blocking of acid sites by carbonaceous species and/or partial exchange of protons by Cu2+/Zn2+.

Author Contributions: E.M., experiments, investigation, draft preparation; N.M., experiments, investigation, draft preparation; R.G.-L., review; B.P., methodology, review; J.L.G.F., methodology, investigation; R.M.N., conceptualization, methodology, investigation, supervision and writing. All authors have read and agreed to the published version of the manuscript. Funding: The present investigation was performed within the research programs CTQ2016–76505-C3–1 and PID2019–111219RB-100 supported by the Spanish Ministry of Science, Innovation and Universities. The Autonomus Community of Madrid (CAM) is also gratefully acknowledged for funding BIOTRES-CM (P2018/EMT-4344) project. Elena Millán would like to acknowledge the FPI programme from Spanish Ministry of Science, Innovation and Universities for the research grant. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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