Effect of Atmosphere Calcination and Temperature on the Hydrogenolysis Activity and Selectivity of Copper-Zinc Catalysts

catalysts

Article Effect of Atmosphere Calcination and Temperature on the Hydrogenolysis Activity and Selectivity of Copper-Zinc Catalysts

Oleg Kikhtyanin 1, Violetta Pospelova 2, Jaroslav Aubrecht 2, Miloslav Lhotka 3 and David Kubiˇcka 1,2,* 1 Technopark Kralupy, VŠCHT Praha, Žižkova 7, 278 01 Kralupy nad Vltavou, Czech Republic; [email protected] 2 Department of Petroleum Technology and Alternative Fuels, VŠCHT Praha, Technická 5, 166 28 Praha 6—Dejvice, Czech Republic; [email protected] (V.P.); [email protected] (J.A.) 3 Department of Inorganic Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Praha 6—Dejvice, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.:+420-220-446-106

Received: 23 September 2018; Accepted: 7 October 2018; Published: 11 October 2018

Abstract: A series of CuZn catalysts with a Cu/Zn ratio of 1.6 was prepared by the calcination of a single precursor, CuZn-P consisting of an equimolar mixture of aurichalcite and zincian malachite, in three different calcination atmospheres (air, nitrogen, and hydrogen) at three temperatures (220, ◦ 350, and 500 C). All catalysts were characterized by XRD and N2-physisorption to assess their phase composition, crystallite sizes and textural properties and tested in dimethyl adipate (DMA) ◦ hydrogenolysis in a batch reactor at 220 C and 10 MPa H2. The XRD examination of these catalysts proved that both parameters, calcination temperature and atmosphere, affected the resulting phase composition of the catalysts as well as their crystallite sizes. In an oxidizing atmosphere, CuO and ZnO in intimate contact prevailed whereas in inert or reducing atmosphere both oxides were accompanied by Cu2O and Cu. The crystallite size of Cu2O and Cu was larger than the size of CuO and ZnO thus indicating a less intimate contact between the Cu-phases and ZnO in catalysts calcined in nitrogen and hydrogen. Catalysts prepared by calcination at 220 ◦C and CuZn catalyst calcined in the air at 350 ◦C significantly outperformed the other catalysts in DMA hydrogenolysis with a 59–78% conversion due to the small crystallite size and intimate contact between the CuO and ZnO phases prior to catalyst reduction. Despite the low DMA conversion (<30%), transesterification products were the main reaction products with overall selectivities of >80% over the catalysts calcined in nitrogen or hydrogen at least at 350 ◦C. The obvious change in the preferred reaction pathway because of the atmosphere calcination and temperature shows that there are different active sites responsible for hydrogenolysis and transesterification and that their relative distribution has changed.

Keywords: hydrogenolysis; transesteriﬁcation; CuZn catalysts; calcination atmosphere; calcination temperature

1. Introduction Over the past few decades, increasing attention has been paid to green chemistry principles to achieve a higher sustainability of chemical technologies. One of these principles is based on the application of highly efficient catalysts in industrial processes. However, their efficiency alone does not guarantee the overall technological sustainability as some catalyst productions use or produce harmful materials. This is the case of the traditional, highly efficient Adkins catalysts containing chromium that

Catalysts 2018, 8, 446; doi:10.3390/catal8100446 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 446 2 of 16 haveCatalysts been 2018 used, 8, xfor FOR the PEER industrial REVIEW hydrogenolysis of esters to alcohols since the 1950s [1–3]. Recently,2 of 16 copper-zinc catalysts have been shown to possess significant catalytic activity in the hydrogenolysis of chromium that have been used for the industrial hydrogenolysis of esters to alcohols since the 1950s biomass-derived esters [4–6] and could thus replace copper-chromium catalysts that do not meet the [1–3]. Recently, copper-zinc catalysts have been shown to possess significant catalytic activity in the highest environmental standards. hydrogenolysis of biomass-derived esters [4–6] and could thus replace copper-chromium catalysts thatCopper-zinc do not meetcatalysts the highest belong environmental among widely standards. studied catalysts due to a significant interest in the methanolCopper-zinc synthesis catalysts from synthesis belong gasamong [7–9 widely], as well studied as by catalysts the hydrogenation due to a significant of carbon interest dioxide in [ 10the,11 ]. Frommethanol a structural synthesis point from of view, synthesis several gas studies [7–9], have as well concluded as by the that hydrogenation zinc oxide supports of carbon the adsorptiondioxide of[10,11]. hydrogen From on a the structural copper point surface of andview, ensures several thestudies stabilization have concluded of the copperthat zinc dispersion oxide supports [7,12]. the ZnO hasadsorption also been of suggested hydrogen to on act the as copper a spacer surface between and copper ensures particles the stabilization supporting of the high copper copper dispersion dispersion and[7,12]. thus ZnO maximizing has also thebeen specific suggested surface to act of copperas a spacer [7,8, 13between]. This iscopper of importance particles supporting as Cu is the high active componentcopper dispersion responsible and forthus the maximizing hydrogenation the specific and hydrogenolysis surface of copper activity [7,8,13]. of This catalysts. is of importance Nonetheless, theas exact Cu is nature the active of the component Cu active sites responsible is still a matterfor the ofhydrogenation discussion even and in hydrogenolysis the case of the mostactivity studied of methanolcatalysts. synthesis Nonetheless, [8] and the exact it is not natu yetre of sufficiently the Cu active described sites is still in hydrogenolysis a matter of discussion catalysts. even Recently, in the thecase formation of the ofmost transesterification studied methanol products synthesis during [8] theand hydrogenolysis it is not yet sufficiently of dimethyl described adipate over in a CuZnAlhydrogenolysis catalyst has catalysts. been reported Recently, [6] andthe aformation reaction scheme of transesterification has been proposed products (Scheme during1). However, the a clearhydrogenolysis understanding of dimethyl of the transesterification adipate over a CuZnAl activity catalyst is still lacking. has been Moreover, reported due[6] and to the a reaction sensitivity ofscheme the copper-based has been proposed catalysts (Scheme to thermal 1). However, treatment a at clear elevated understanding temperatures, of the the transesterification optimization and stabilizationactivity is still of the lacking. distribution Moreover, of Cu due active to the sites se isnsitivity challenging of the and copper-based studies dealing catalysts with to the thermal effect of calcinationtreatment conditions at elevated aretemperatures, rather scarce the[ optimization14]. and stabilization of the distribution of Cu active sites is challenging and studies dealing with the effect of calcination conditions are rather scarce [14]. Therefore, the present work is focused on a systematic study of the effect of three different Therefore, the present work is focused on a systematic study of the effect of three different calcination gas atmospheres, i.e., air, nitrogen, and hydrogen, on the structure and catalytic calcination gas atmospheres, i.e., air, nitrogen, and hydrogen, on the structure and catalytic performance of catalysts obtained by the calcination of a precipitated CuZn catalyst precursor at performance of catalysts obtained by the calcination of a precipitated CuZn catalyst precursor at three three different temperatures (220, 350, and 500 ◦C) in these three calcination atmospheres. Particular different temperatures (220, 350, and 500 °C) in these three calcination atmospheres. Particular attentionattention is is paid paid to to the the effect effect of of the the calcinationcalcination on the relative relative activity activity of of th theseese CuZn CuZn catalysts catalysts in inthe the hydrogenolysishydrogenolysis and and transesterification transesterification reactionsreactions in in order order to to contribute contribute to to a abetter better understanding understanding of of bothboth catalytic catalytic functions. functions.

SchemeScheme 1. 1. TheThe proposed reaction reaction network network explaining explaining the theformation formation of the of transesterification the transesteriﬁcation byby-productsproducts during during the the hydrogenolysis hydrogenolysis of ofdimethyl dimethyl adipate adipate [6]. [DMA6]. DMA is dimethyl is dimethyl adipate, adipate, 1C6MEol 1C6MEol is is methyl 6-hydroxyhexanoate, HDOL HDOL is is hexane-1,6-diol, hexane-1,6-diol, 2C 2C6diME6diME is is6-methoxy-6-oxohexyl 6-methoxy-6-oxohexyl methyl methyl adipate,adipate, 2C 2C6MEol6MEol is is 6-hydroxyhexyl 6-hydroxyhexyl methylmethyl adipate, 2C 2C66dioldiol is is 6-hydroxyhexyl 6-hydroxyhexyl 6-hydroxyhexanoate, 6-hydroxyhexanoate, 3C3C6diME6diME is is bis(6-methoxy-6-oxohexyl) bis(6-methoxy-6-oxohexyl) adipate,adipate, 3C 66MEolMEol is is 6-hydroxyhexyl 6-hydroxyhexyl (6-methoxy-6-oxohexyl) (6-methoxy-6-oxohexyl) adipate,adipate, and and 3C 3C6diol6diol is is bis(6-hydroxyhexyl) bis(6-hydroxyhexyl) adipate.

Catalysts 2018, 8, 446 3 of 16 Catalysts 2018, 8, x FOR PEER REVIEW 3 of 16

2. Results and Discussion 2. Results and Discussion

2.1. Catalyst Characterization The Cu/ZnCu/Zn atomic ratioratio inin the the synthesized synthesized catalyst catalyst precursor precursor determined determined by XRFby XRF was was 1.6, i.e.,1.6, veryi.e., veryclose close to that to expectedthat expected from from the chemical the chemical composition composition of the of solution the solution of copper of copper and zinc and nitrateszinc nitrates used usedin the in precipitation the precipitation (1.5). Moreover,(1.5). Moreover, it was assumedit was assumed that all catalystthat all samplescatalyst samples prepared prepared by different by differentthermal treatmentsthermal treatments had the same had the chemical same chemical composition. composition. The phase phase composition composition of of the the catalyst catalyst precursor, precursor, as aswell well as that as thatof the of final the finalcatalysts, catalysts, was wasinvestigated investigated by XRD. by XRD. The XRD The XRDpattern pattern of the of as-p therepared as-prepared CuZn CuZn precursor precursor (Figure (Figure 1) revealed1) revealed that this sample was a mixture of two phases, zincian malachite ((Cu0.8 Zn0.2)2(OH)2CO3; Ref. Code No. 01- that this sample was a mixture of two phases, zincian malachite ((Cu0.8 Zn0.2)2(OH)2CO3; Ref. Code 079-7851) and aurichalcite (Cu2Zn3(CO3)2(OH)6; Ref. Code No. 04-010-3227), found in almost equal No. 01-079-7851) and aurichalcite (Cu2Zn3(CO3)2(OH)6; Ref. Code No. 04-010-3227), found in almost amounts.equal amounts. The XRD patterns of the catalysts prepared by calcination in static air at the three calcination temperatures (220, 350, and 500 °C)◦C) are compared with the XRD pattern of the catalyst precursor in Figure 11.. WhenWhen thethe catalystcatalyst precursorprecursor waswas treatedtreated atat 220220 °C,◦C, the phase composition did did not not change, change, although thethe intensityintensity of of the the XRD XRD reflexes reflexes decreased decrease slightly.d slightly. This canThis be can attributed be attributed to the goodto the thermal good thermalstability stability of zincian of malachite zincian malachite and aurichalcite. and aurichalcite. It was reported It was that reported zincian that malachite zincian and malachite aurichalcite and aurichalciteare stable up are to 270stable◦C andup to 360 270◦C, °C correspondingly and 360 °C, correspondingly [14,15]. When the [14,15]. calcination When temperature the calcination was temperatureincreased to 350was◦ C,increased the zincian to 350 malachite °C, the and zincian aurichalcite malachite crystalline and aurichalcite domains presentcrystalline in thedomains CuZn presentcatalyst in precursor the CuZn decomposed catalyst precursor completely decompos and theed CuO completely and ZnO and crystalline the CuO particles and ZnO were crystalline formed. Aparticles further were increase formed. in the A calcinationfurther increase temperature in the calcination to 500 ◦C temperature did not change to 500 the °C phase did compositionnot change the of phasethe catalyst composition anymore, of i.e.,the thecatalyst CuZn-500AS anymore, sample i.e., the consisted CuZn-500AS exclusively sample of aconsisted mixture of exclusively CuO and ZnO.of a mixtureHowever, of theCuO intensity and ZnO. of theHowever, XRD reflexes the intensity notably of increased, the XRD reflexes suggesting notabl a growthy increased, of the suggesting crystallites asize growth of both of CuOthe crystallites and ZnO. size of both CuO and ZnO.

CuO CuZn-P ZnO CuZn-220AS 240000 ZnO CuZn-350AS 220000 CuZn-500AS ZnO 200000 ZnO CuO CuO 180000

160000

140000

120000 a.u. 100000

80000

60000

40000

20000

0 20 40 60 80 2 Theta, Degrees Figure 1. The XRD patterns of the the catalysts catalysts prepared prepared by the treatment of the CuZn precursor in static air at 220, 350, and 500 °C.◦C.

The XRD patterns of the catalysts catalysts prepared prepared by calcination calcination in in nitrogen nitrogen again again at at temperatures temperatures 220, 220, ◦ 350, and 500 500 °CC were were compared compared with with the the XRD XRD pattern pattern of of the the catalyst catalyst precursor precursor in inFigure Figure 2.2 Unlike. Unlike in ◦ thein the case case of oftreatment treatment in instatic static air air at at 220 220 °C,C, the the thermal thermal treatment treatment of of the the CuZn-P CuZn-P in in a nitrogen ◦ atmosphere at 220 °CC (underflow (underﬂow conditions) resulted in in a change in the phase composition of the prepared catalyst sample (CuZn-220N). Although th thee reflexes reﬂexes of zincian malachite and aurichalcite are still present (Figure 2), they are accompanied by the reflexes of CuO and ZnO. This evidences that

Catalysts 2018, 8, 446 4 of 16 are still present (Figure2), they are accompanied by the reflexes of CuO and ZnO. This evidences that the thermal decomposition of the initial crystalline phases has started under these calcination conditions. This difference can be explained by the different experimental setups, in particular, by the flow conditions or the partial local overheating of the precursor sample inside the autoclave (the sample is in contact with the autoclave wall while the thermocouple measuring the temperature is not). CatalystsAn increase 2018, 8 in, x theFOR treatmentPEER REVIEW temperature to 350 ◦C resulted in a total disappearance of the reflexes4 of 16 of zincian malachite and aurichalcite accompanied by an increase in the intensity of the reflexes of CuO the thermal decomposition of the initial crystalline phases has started under these calcination and ZnO. Additionally, the reflexes belonging to Cu2O and Cu were identified in the XRD pattern conditions. This difference can be explained by the different experimental setups, in particular, by of CuZn-350N as well (Figure2). Because of the significant overlapping of the XRD reflexes of the the flow conditions or the partial local overheating of the precursor sample inside the autoclave (the different phases, only several characteristic reflexes are shown in Figure2. sample is in contact with the autoclave wall while the thermocouple measuring the temperature is not). An increase in the treatment temperature to 350 °C resulted in a total disappearance of the 2Cu2CO3(OH)2 → 4CuO +2CO2 + 2H2O (1) reflexes of zincian malachite and aurichalcite accompanied by an increase in the intensity of the reflexes of CuO and ZnO. Additionally, the2CO reflexes2 → 2CO belonging + O2 to Cu2O and Cu were identified in the(2) XRD pattern of CuZn-350N as well (Figure 2). Because of the significant overlapping of the XRD 4CuO + 2CO → 2Cu O + 2CO (3) reflexes of the different phases, only several characteristic2 reflexes2 are shown in Figure 2.

300000 CuOCuO 280000 CuZn-P ZnO Cu O CuZn-220N 260000 2 ZnO ZnO Cu ZnO CuZn-350N 240000 CuZn-500N 220000 200000 CuO 180000 160000 140000 a.u. 120000 100000 80000 60000 40000 20000 0 20 30 40 50 60 70 80 2 Theta, Degrees FigureFigure 2. 2. TheThe XRD XRD patterns patterns of of catalysts catalysts prepared prepared by by th thee treatment treatment of of the the CuZn CuZn precursor precursor in in flowing ﬂowing ◦ nitrogennitrogen at at 220, 220, 350, 350, and 500 °C.C.

The exclusive formation of CuO resulting from the calcination of malachite in air (Equation (1)) 2 3 2 → 2 2 and the possible formation of Cu2Cu2O duringCO (OH) the calcination 4CuO +2CO of malachite + 2H O in an inert atmosphere was(1) reported previously [15]. It was explained by CO2 decomposition to CO and subsequent reduction of CuO by CO (Equations (2) and (3)) [16]. The2CO decomposition2 → 2CO + O of2 both zincian malachite, and aurichalcite(2) in nitrogen, by analogy with malachite, can be expected to yield Cu2O in addition to CuO. If CuO 4CuO + 2CO → 2Cu2O + 2CO2 (3) is reduced by CO originating from CO2 to Cu2O, the consecutive reduction step, i.e., the reduction ◦ of CuThe2O exclusive to Cu, is plausibleformation as of well.CuO Aresulting further from increase the calcination in the treatment of malachite temperature in air (Equation to 500 C (1)) did andnot changethe possible the phase formation composition of Cu2O of during the catalyst the calcination anymore, of but malachite the intensity in an of inert reflexes atmosphere of all phases was reportedincreased previously substantially [15]. due It was to the explained increased by crystalliteCO2 decomposition size of all to phases. CO and With subsequent a closer inspectionreduction of of CuOFigure by2, itCO can (Equations be established (2) that and the (3)) intensity [16]. ofThe the decomposition reflexes corresponding of both to zincian Cu 2O andmalachite, Cu increased and aurichalcitemore than those in nitrogen, belonging by analogy to CuO andwith ZnO malachite, (Figure can2). be expected to yield Cu2O in addition to CuO. ◦ If CuOFinally, is reduced the catalyst by CO precursor originating was from also CO treated2 to Cu in2O, flowing the consecutive hydrogen atreduction 220, 350, step, and i.e., 500 theC. reductionThe resulting of Cu XRD2O to patterns Cu, is plausible are shown as well. in Figure A further3 together increase with in the treatment XRD pattern temperature of the catalyst to 500 °precursor.С did not Inchange contrast the tophase thetreatment composition in air of andthe cataly nitrogenst anymore, at 220 ◦C, but there the are intensity no reflexes of reflexes of the initialof all phaseshydroxycarbonate increased substantially phases in the due XRD to pattern the increase of the CuZn-220H,d crystallite i.e., size a catalystof all phases. obtained With by treatmenta closer inspection of Figure 2, it can be established that the intensity of the reflexes corresponding to Cu2O and Cu increased more than those belonging to CuO and ZnO (Figure 2). Finally, the catalyst precursor was also treated in flowing hydrogen at 220, 350, and 500 °C. The resulting XRD patterns are shown in Figure 3 together with the XRD pattern of the catalyst precursor. In contrast to the treatment in air and nitrogen at 220 °C, there are no reflexes of the initial hydroxycarbonate phases in the XRD pattern of the CuZn-220H, i.e., a catalyst obtained by treatment in a hydrogen atmosphere at 220 °C. Instead, low-intensive reflexes of ZnO, CuO, Cu2O, and Cu appeared (Figure 3). As the treatment temperature increased from 220 °C to 500 °C, the intensity of

Catalysts 2018, 8, 446 5 of 16

◦ Catalystsin a hydrogen 2018, 8, x FOR atmosphere PEER REVIEW at 220 C. Instead, low-intensive reflexes of ZnO, CuO, Cu2O, and5 of Cu 16 appeared (Figure3). As the treatment temperature increased from 220 ◦C to 500 ◦C, the intensity of thethe reflexes reflexes of of the the oxidic oxidic Cu Cu phases phases declined declined while while that that of of the the metallic metallic copper copper substantially substantially increased increased (Figure(Figure 33).). Simultaneously,Simultaneously, the the intensity intensity of of the the reflexes reflexes of of ZnO ZnO also also increased. increased. This This can can be be attributed attributed to tothe the increase increase in thein the crystallite crystallite size size of ZnO of ZnO and and Cu (discussedCu (discussed below) below) and toand the to gradual the gradual reduction reduction of the ofoxidic the oxidic Cu phases Cu phases to metallic to metallic Cu. Cu.

Cu CuZn-P CuZn-220H CuZn-350H 400000 CuZn-500H

Cu 300000 ZnO ZnO ZnO ZnO a.u. 200000 ZnO CuO CuO

100000

0 20 40 60 80 2 Theta, Degrees FigureFigure 3. TheThe XRD XRD patterns patterns of of catalysts catalysts prepared prepared by by th thee treatment treatment of of the the CuZn CuZn precursor precursor in in flowing ﬂowing ◦ hydrogenhydrogen at 220, 350, and 500 °C.C.

◦ TheThe XRD XRD results demonstrate that that 220 °CC is is a sufficient sufﬁcient temperature to completely decompose hydroxycarbonateshydroxycarbonates (zincian (zincian malachite malachite and and aurichalci aurichalcite)te) in in a hydrogen a hydrogen atmosphere, atmosphere, but but not not in air in airor nitrogenor nitrogen atmospheres. atmospheres. The The lowered lowered thermal thermal stabil stabilityity of of both both hydroxycarbonate hydroxycarbonate phases phases may may be be a a consequenceconsequence of of a a direct reduction of the hydroxycarbonatehydroxycarbonate Cu speciesspecies inin zincianzincian malachitemalachite and/orand/or aurichalciteaurichalcite structures structures according according to to the the proposed proposed overall overall reaction reaction (Equation (Equation (4)). (4)). This This conclusion conclusion is furtheris further corroborated corroborated by by the the observed observed partial partial de destructionstruction of of the the hydroxycarbonate hydroxycarbonate phase phase under under ◦ nitrogennitrogen at 220220 °C.C. TheThe partialpartial destruction destruction is is caused caused by by the the transformation transformation of theof the inert inert atmosphere atmosphere into intoa partially a partially reducing reducing atmosphere atmosphere due todue the toformation the formation of CO of duringCO during the treatment. the treatment.

2(Cu2(Cu0.8Zn0.80.2Zn)20.2CO)2CO3(OH)3(OH)2 +2 + 3.2 3.2 H H2 →2 →3.2Cu 3.2Cu +0.8ZnO +0.8ZnO+CO + CO22 ++ 5.2H 5.2H2O2O (4) (4) Nevertheless, Figure 3 also evidences that CuZn-220H still contained CuO in its composition Nevertheless, Figure3 also evidences that CuZn-220H still contained CuO in its composition after the calcination step (it can be considered as a reduction step at the same time), which is another after the calcination step (it can be considered as a reduction step at the same time), which is another distinctive feature of the experiment performed in hydrogen at 220 °C. This is rather unexpected since distinctive feature of the experiment performed in hydrogen at 220 ◦C. This is rather unexpected the reduction of CuO to metallic copper in CuO/γ-Al2O3 [17] or in CuO/ZnO [11] occurred at since the reduction of CuO to metallic copper in CuO/γ-Al O [17] or in CuO/ZnO [11] occurred at temperatures around 200 °C. On the other hand, the reduction2 3of Cu2+ in bulky CuO was reported to temperatures around 200 ◦C. On the other hand, the reduction of Cu2+ in bulky CuO was reported occur at 300 °C [11]. It was proposed that the low-temperature reduction of CuO to metallic Cu took to occur at 300 ◦C[11]. It was proposed that the low-temperature reduction of CuO to metallic Cu place for highly dispersed copper oxide species while the bulky CuO was reduced at higher took place for highly dispersed copper oxide species while the bulky CuO was reduced at higher temperatures [15,17]. The presence of oxidic Cu species in CuZn-220H allows for the assumption that temperatures [15,17]. The presence of oxidic Cu species in CuZn-220H allows for the assumption that the treatment conditions considerably influenced the properties (oxidation state, dispersion, etc.) of the treatment conditions considerably inﬂuenced the properties (oxidation state, dispersion, etc.) of the Cu atoms in the resulting catalyst sample. Indeed, dedicated experiments showed (Figure A1) the Cu atoms in the resulting catalyst sample. Indeed, dedicated experiments showed (Figure A1) that that the treatment of the CuZn-350AS sample (which is a mixture of ZnO and CuO phases; see Figure the treatment of the CuZn-350AS sample (which is a mixture of ZnO and CuO phases; see Figure1) in 1) in a hydrogen atmosphere at 220 °C resulted in the total reduction of copper oxide to metallic a hydrogen atmosphere at 220 ◦C resulted in the total reduction of copper oxide to metallic copper. copper. Moreover, the exclusive presence of the reflexes of metallic copper, i.e., without any oxidic Moreover, the exclusive presence of the reﬂexes of metallic copper, i.e., without any oxidic phases phases detected suggests that these metallic domains are not susceptible to an easy re-oxidation in the air. In contrast, the treatment of CuZn-P firstly in the air at 220 °C (yielding the CuZn-220AS sample) and then in hydrogen at the same temperature (Figure A1) resulted in the decomposition of zincian malachite and aurichalcite structures. The XRD pattern of CuZn-220AS-220H contained

Catalysts 2018, 8, 446 6 of 16 detected suggests that these metallic domains are not susceptible to an easy re-oxidation in the air. In contrast, the treatment of CuZn-P firstly in the air at 220 ◦C (yielding the CuZn-220AS sample) and then in hydrogen at the same temperature (Figure A1) resulted in the decomposition of zincian malachite and aurichalcite structures. The XRD pattern of CuZn-220AS-220H contained reflexes of CuO together with reflexes of metallic Cu at very low intensities. Thus, the existence of oxidic copper domains in the sample after a reductive treatment at 220 ◦C is undoubtedly a consequence of the sample preparation under specific conditions and their sequence. The collected XRD patterns allowed for the calculation of the crystallite size of all phases detected in the catalysts using the Scherrer equation [18]. The sizes of the crystallites of zinc and copper compounds were calculated from the width of the corresponding diffraction lines that did not overlap with the lines belonging to other phases. As expected, the crystallite size depended on the pretreatment temperature and, more interestingly, on the calcination atmosphere (Table1).

Table 1. The crystallite size of ZnO, CuO, Cu2O, and Cu present in the catalysts prepared by calcination in different atmospheres at different temperatures.

Particle Size, nm (2 Theta, Degrees) Sample ZnO (31.7) CuO (38.7) Cu2O (42.3) Cu (43.3) 220AS ND 1 ND ND ND 220AS-220H 2 7.3 7.3 8.7 10.2 350AS 5.1 7.7 ND ND 350AS-220H 2 12.5 ND ND 18.1 500AS 15.8 11.9 ND ND 220N 5.9 6.5 7.9 11.5 350N 11 10.3 21 22.2 500N 33.9 25.5 36 46.1 220H 9.8 9.2 11.2 18.1 350H 20 16.7 ND 23.4 500H 34.7 ND ND 33.2 1 ND stands for not detected; 2 After the reduction at 220 ◦C.

Due to the increasing pretreatment temperature in static air from 350 ◦C to 500 ◦C, the size of the ZnO and CuO crystallites increased from 5.1 nm and 7.7 nm to 15.8 and 11.9 nm, correspondingly. When the pretreatment was performed in a non-oxidizing atmosphere, either in nitrogen or in hydrogen, the size of the produced oxidic particles was larger compared to the corresponding experiments in static air (Table1). Moreover, at a given temperature, the size of ZnO crystallites was larger in the catalysts prepared by calcination in nitrogen or hydrogen in comparison with those prepared in air (Table1). This allows for the suggestion of a mutual stabilization of the CuO and ZnO domains if these are in intimate contact with each other. This conclusion is further supported by the larger size of the Cu crystallites obtained by direct reduction (calcination in hydrogen) than those obtained by calcination in air followed by reduction at 220 ◦C (Table1). In addition, the size of metallic copper increased in comparison with the size of its oxidic precursors which indicates copper sintering during the reduction step. It is interesting to note that (i) the catalysts reduced after previous calcination in air, (ii) catalysts obtained by calcination in nitrogen (copper formed due to reduction by in-situ formed CO) and (iii) catalysts prepared by calcination in hydrogen (i.e., by direct reduction) exhibit a common linear dependency between the CuO crystallite size and the resulting Cu crystallite size (Figure4). This might indicate that Cu is formed during the calcination of the hydroxycarbonate precursors either via CuO or that the formation of both crystallites is related in another way. Catalysts 2018, 8, 446 7 of 16 Catalysts 2018, 8, x FOR PEER REVIEW 7 of 16

10 treated samples, nm treated The size of Cu crystallites in of Cu crystallites The size 0 0 5 10 15 20 25 30 The size of CuO crystallites in treated samples, nm

Figure 4. A relationship between the CuO and Cu crystallites size in catalyst samples prepared by Figure 4. A relationship between the CuO and Cu crystallites size in catalyst samples prepared by calcination in air (), nitrogen (∆) and hydrogen ( ) at different temperatures. calcination in air (□), nitrogen (∆) and hydrogen (○) at different temperatures. # The calcination atmosphere and temperature signiﬁcantly affected the textural properties of the ﬁnalThe catalysts. calcination The results atmosphere are summarized and temperature in Table signific2. Amongantly the affected samples the calcined textural in properties air, CuZn-350AS of the finalpossessed catalysts. the best The textural results propertiesare summarized with respect in Table to the 2. BETAmong surface, the thesamples external calcined surface, in pore air, volume,CuZn- 350ASand average possessed nanoparticle the best textural size. The properties CuZn-220AS with hadrespect only to slightly the BET worse surface, parameters the external compared surface, to poreCuZn-350AS volume, despiteand average their substantiallynanoparticle size. different The phaseCuZn-220AS composition. had only This slightly could beworse attributed parameters to the comparedfact that CuZn-220AS to CuZn-350AS was despite a mixture their of hydroxycarbonatessubstantially different rather phase than composition. of oxides. TheThis increase could be in attributedthe calcination to the temperature fact that CuZn-220AS to 500 ◦C resultedwas a mixt in aure dramatic of hydroxycarbonates decrease in the rather textural than properties of oxides. of TheCuZn-500AS. increase in On the the calcination other hand, temperature the trend in to the 500 change °С resulted in all characteristics in a dramatic observeddecrease forin the the textural samples propertiesprepared inof nitrogenCuZn-500AS. and hydrogenOn the other is morehand, unambiguous. the trend in the The change BET in surface, all characteristics the external observed surface, forand the the samples pore volume prepared substantially in nitrogen decreased and hydrog while theen averageis more nanoparticle unambiguous. size, The accordingly, BET surface, increased the externalwith the surface, increasing and calcination the pore volume temperature. substantia It shouldlly decreased also be while noted the that average the samples nanoparticle prepared size, in accordingly,hydrogen had increased worse adsorption with the increasing characteristics, calcination except temperature. for the micropore It should surface, also compared be noted withthat the the samplessimilar samplesprepared prepared in hydrogen in nitrogen. had Besides,worse adsorpti Table2 evidenceson characteristics, that the poreexcept diameter for the of differentlymicropore surface,prepared compared catalysts with was approximatelythe similar samples the same prepared in the in range nitrogen. of 13–39 Besides, nm and Table showed 2 evidences no dependence that the poreon either diameter the atmosphere of differently or prepared calcination catalysts temperature. was approximately the same in the range of 13–39 nm and showed no dependence on either the atmosphere or calcination temperature. Table 2. The textural properties of the CuZn catalysts prepared by the calcination of the CuZn precursor Tablein air, nitrogen,2. The textural and hydrogen properties at 220,of the 350, CuZn and 500 catalyst◦C. s prepared by the calcination of the CuZn precursor in air, nitrogen, and hydrogen at 220, 350, and 500 °C. t-Plot Surface, m2·g−1 Average BET Surface, 2 −1 BJH Pore BJH Pore Sample BET2 −1 t-Plot Surface, m ∙g 3 −1 NanoparticleAverage m ·g Micropore External BJHVolume, Pore cm ·g BJHDiameter, Pore nm Sample Surface, NanoparticleSize, nm Micropore External Volume, cm3∙g−1 Diameter, nm CuZn-220ASm2∙g− 491 5.1 43.9 0.342 27Size, 122 nm CuZn-350AS 62 1.3 61.1 0.463 25 96 CuZn-220AS 49 5.1 43.9 0.342 27 122 CuZn-500AS 17 0.1 16.5 0.084 23 364 CuZn-350ASCuZn-220N 62 65 1.3 0 61.1 78.3 0.463 0.4 25 21 96 89 CuZn-500ASCuZn-350N 17 32 0.1 4.7 16.5 27.6 0.084 0.274 23 39 364 186 CuZn-220NCuZn-500N 65 8 0 0.8 78.3 7.5 0.4 0.021 21 13 89722 CuZn-220H 26 3.1 22.7 0.194 31 233 CuZn-350N 32 4.7 27.6 0.274 39 186 CuZn-350H 14 0.6 13.5 0.089 27 423 CuZn-500NCuZn-500H 8 NM 1 0.8 7.5 0.021 13 722 CuZn-220H 26 3.1 1 NM stands 22.7 for not measured. 0.194 31 233 CuZn-350H 14 0.6 13.5 0.089 27 423 CuZn-500H NM 1 1 NM stands for not measured.

Catalysts 2018, 8, x FOR PEER REVIEW 8 of 16

2.2. Activity and Selectivity of the Catalysts Figure 5 depicts the dependency of the DMA conversion on the pretreatment conditions (i.e., different calcination atmosphere and temperature) applied to the synthesized CuZn-P precursor. CuZn-220AS prepared by a mild heating in static air possessed a reasonable activity in the reaction Catalystswith 2018DMA;, 8, 446the DMA conversion reached 64% after 2 h. When the calcination temperature was8 of 16 increased to 350 °C, the catalytic performance of the catalyst further improved; the DMA conversion amounted to 78% after 2 h (Figure 5). The observed increase is in agreement with the improvement 2.2. Activity and Selectivity of the Catalysts in the textural properties of CuZn-220AS and CuZn-350 AS (Table 2). Additionally, the phase compositionFigure5 depicts of the calcined the dependency catalysts after of reduction the DMA has conversion to be considered on the as pretreatmentwell. The catalyst conditions CuZn- (i.e.,220AS, different if exposed calcination to hydrogen atmosphere at 220 and°C for temperature) 2 h, still contained applied Cu to in the the synthesizedoxidic state (Figure CuZn-P 4, precursor.sample CuZn-220ASCuZn-220AS-220H). prepared On by athe mild other heating hand, in the static XRD air pattern possessed of this a reasonable catalyst after activity the inreaction the reaction (will be with DMA;discussed the DMA further) conversion revealed reached the presence 64% afterof only 2 h. meta Whenllic Cu the clusters. calcination This temperature suggests the was occurrence increased of to 350a ◦subsequentC, the catalytic reduction performance of the oxidic of the Cu catalyst species further during improved;the reaction. the In DMA contrast, conversion the catalyst amounted CuZn- to 78%350AS after exclusively 2 h (Figure contained5). The observed metallic increaseCu clusters is inafte agreementr the reduction with step, the improvement so no additional in theactivation textural propertiesduring the of CuZn-220ASreaction was necessary. and CuZn-350 This ASdifference (Table2 in). Additionally,the state of Cu the in phase the catalysts, composition although of the calcinedinexplicable catalysts at the after moment, reduction could has explain to be consideredthe difference as well.in the Thecatalytic catalyst performance CuZn-220AS, of CuZn-220AS if exposed to and CuZn-350AS together with their different textural properties. A further increase in the calcination hydrogen at 220 ◦C for 2 h, still contained Cu in the oxidic state (Figure4, sample CuZn-220AS-220H). temperature in air to 500 °C resulted in a decrease in the activity of the catalyst CuZn-500AS with the On the other hand, the XRD pattern of this catalyst after the reaction (will be discussed further) DMA conversion of 43% after 2 h (Figure 5). revealed the presence of only metallic Cu clusters. This suggests the occurrence of a subsequent The observed decrease in conversion can be attributed to the increase in the size of the copper reduction of the oxidic Cu species during the reaction. In contrast, the catalyst CuZn-350AS exclusively crystallites determined from the XRD patterns (Table 1). The increase in the calcination temperature containedcaused an metallic increase Cu in clusters the size after of copper the reduction crystallites step, leading so no to additional a decrease activation in the number during of theavailable reaction wascatalytically necessary. active This difference centers. A in similar the state dependence of Cu in the of catalysts, the catalytic although performance inexplicable of Cu-containing at the moment, couldcatalysts explain on thethe differenceparticle size in of the copper catalytic being performance influenced ofby CuZn-220AS the calcination and temperature CuZn-350AS was together also withreported their differentfor the hydrogenolysis textural properties. of ethyl A acetate further to increase ethanol in [4]. the In calcination contrast, Peng temperature Yuan et al. in air[5] to ◦ 500observedC resulted an increase in a decrease in the DMA in the conversion activity of over the a catalyst Cu–Zn–Al CuZn-500AS catalyst as withthe calcination the DMA temperature conversion of 43%was after increased 2 h (Figure from5 400). °C to 500 °C.

FigureFigure 5. 5.The The DMA DMA conversion conversionobserved observed onon the CuZn ca catalyststalysts in in dependence dependence on on the the temperature temperature of of ◦ theirtheir treatment treatment in in air air (black (black line), line), N N2 2(red (red line),line), andand HH22 (blue(blue line). line). Samples Samples calcined calcined at at T T= =220 220 °C (C(○), ), ◦ ◦ 350350C( °C (),□), 500 500 °CC( (∆∆).). #

TheThe observed catalysts decrease prepared in by conversion calcination can in be nitr attributedogen and to hydrogen the increase also in theshowed size ofa thetrend copper of crystallitesdecreasing determined the DMA conversion from the XRD with patterns the increasing (Table1 ).calcination The increase temperature. in the calcination For CuZn temperature catalysts causedprepared an increase by calcination in the sizein nitrogen, of copper the crystallites DMA dropped leading significantly to a decrease (from in the 59 numberto 3%) when of available the catalyticallycalcination activetemperature centers. was A increas similared dependencefrom 220 °C to of 500 the °C catalytic (Figure performance5). The effect of of the Cu-containing hydrogen catalysts on the particle size of copper being inﬂuenced by the calcination temperature was also reported for the hydrogenolysis of ethyl acetate to ethanol [4]. In contrast, Peng Yuan et al. [5] observed an increase in the DMA conversion over a Cu–Zn–Al catalyst as the calcination temperature was increased from 400 ◦C to 500 ◦C. The catalysts prepared by calcination in nitrogen and hydrogen also showed a trend of decreasing the DMA conversion with the increasing calcination temperature. For CuZn catalysts prepared by calcination in nitrogen, the DMA dropped signiﬁcantly (from 59 to 3%) when the calcination Catalysts 2018, 8, 446 9 of 16 temperature was increased from 220 ◦C to 500 ◦C (Figure5). The effect of the hydrogen calcination atmosphereCatalysts was 2018 even, 8, x FOR more PEER pronounced. REVIEW The DMA conversion decreased from 63% to9 of a16 negligible 1% when the calcination temperature was increased from 220 ◦C to 500 ◦C. The physicochemical calcination atmosphere was even more pronounced. The DMA conversion decreased from 63% to a characterizationnegligible of 1% the when catalysts the calcination allows thetemperature attributing was increased of this behaviorfrom 220 °C to theto 500 increase °C. The in the Cu crystallitephysicochemical size with increasing characterization calcination of the temperature catalysts allows and the to theattributing worse texturalof this behavior properties to the of catalysts prepared byincrease calcination in the Cu atcrystallite high temperatures. size with increasing The calcination dependence temperature of the and conversion to the worse on textural crystallite size is obviousproperties when the of DMAcatalysts conversion prepared by is calcinatio plottedn asat ahigh function temperatures. of CuO The or dependence Cu crystallites of the (Figures6 conversion on crystallite size is obvious when the DMA conversion is plotted as a function of CuO and A2). or Cu crystallites (Figures 6 and A2).

90 80 70 60 50 40 30 20 DMA Conversion, % DMA Conversion, 10 0 0 5 10 15 20 25 30 The size of CuO crystallites in treated samples, nm

Figure 6. AFigure correlation 6. A correlation between between the the DMA DMA conversion conversion (aft (afterer 120 120 min. min. of the of reaction) the reaction) and the size and of the size of CuO crystallites in the prepared CuZn catalysts. □: calcined in air, ∆: calcined in nitrogen, ○: calcined CuO crystallites in the prepared CuZn catalysts. : calcined in air, ∆: calcined in nitrogen, : calcined in hydrogen. in hydrogen. # The observed correlation between the catalytic performance of CuZn catalysts prepared in the The observedpresent study correlation and the size between of the Cu thedomains catalytic can be performanceexpected since the of performance CuZn catalysts of the Cu- prepared in the presentcontaining study andcatalysts the in size different of the reactions Cu domains has been repeatedly can be expectedshown to depend since on the copper performance particle of the Cu-containingsize [5,11,17]. catalysts Nevertheless, in different the reactions treatment hasof the been starting repeatedly precursor shownin air, on to the depend one hand, on and copper in particle nitrogen or hydrogen, on the other hand, introduces some unique (previously not described) features size [5,11,17to ].the Nevertheless, performance of the CuZn treatment hydrogenolysis of the startingcatalyst. Regardless precursor of inthe air, atmosphere on the oneused, hand, all and in nitrogen orcatalysts hydrogen, which on were the thermally other hand, treated introduces at 220 °C exhibited some uniqueapproximately (previously the same not catalytic described) activity.features to the performanceThe DMA of conversion the CuZn was hydrogenolysis in the range 59–64% catalyst. (Fig Regardlessure 5) despite of their the phase atmosphere composition used, after all catalysts which werecalcination thermally differing treated significantly at 220 ◦ C(Table exhibited 1). Moreov approximatelyer, the comparison the sameof catalytic catalytic results activity. with the The DMA physicochemical properties of the prepared catalysts exhibits no clear correlation between the conversioncatalytic was inperformance the range of the 59–64% catalysts (Figure and their5) textural despite properties. their phase composition after calcination differing significantlyOn the other (Table hand,1). Moreover,catalytic results the comparisonsuggest that the of catalytichigh DMA results conversion with over the catalysts physicochemical propertiesprepared of the prepared at 220 °C is catalystsobserved provided exhibits that no the clear samples correlation possess small-size between copper the catalyticcrystallites performance(Table of the catalysts1). An and increase their in textural the calcination properties. temperature to 350 °C resulted in an increase in the DMA conversion to 78% for CuZn-350AS while the activity of CuZn-350N and CuZn350H markedly decreased. On theAccording other hand, to XRD, catalytic CuZn-350AS results represents suggest a mixture that the of high ZnO DMAand CuO conversion whereas the over other catalysts samples prepared ◦ at 220 Ccontain is observed notable amounts provided of Cu that2O and the Cu samples besides CuO possess and ZnO. small-size The crystallite copper size of crystallitesCu2O and Cu (Table1). An increaseis significantly in the calcination larger than temperature that of CuO and to 350 ZnO◦ Cand resulted they are inplausibly an increase presentin as theseparate DMA phases conversion to 78% for CuZn-350ASthat are not in while intimate the contact activity with of the CuZn-350N ZnO phase. This and difference CuZn350H in the markedly phase composition decreased. of the According samples allows for the suggestion that an intimate contact between the CuO and ZnO crystallites to XRD, CuZn-350ASbecause of the representsthermal treatment a mixture could be of an ZnO important and CuO factor whereas that stabilizes the otherthe small samples size of the contain Cu notable amounts ofdomains. Cu2O Consequently, and Cu besides a high CuO activity and of ZnO. CuZn The catalysts crystallite is achieved. size When of Cu calcining2O and in Cu air, is the significantly larger thanincrease that ofin the CuO calcination and ZnO temperature and they from are 220 plausibly°C to 350 °C presentresulted in as a separatecomplete destruction phases thatof are not in intimatethe contact zincian malachite with the and ZnO aurichalcite phase. structures. This difference This led toin an theincreased phase number composition of CuO and of ZnO the samples allows for the suggestion that an intimate contact between the CuO and ZnO crystallites because of the thermal treatment could be an important factor that stabilizes the small size of the Cu domains. Consequently, a high activity of CuZn catalysts is achieved. When calcining in air, the increase in the calcination temperature from 220 ◦C to 350 ◦C resulted in a complete destruction of the zincian malachite and aurichalcite structures. This led to an increased number of CuO and ZnO crystallites, thus, promoting the probability of close contact between the two phases. Additionally, the very small Catalysts 2018, 8, 446 10 of 16 Catalysts 2018, 8, x FOR PEER REVIEW 10 of 16 sizecrystallites, of ZnO crystallites thus, promoting (Table the1) could probability also contribute of close contact to the between high dispersion the two phases. of the CuOAdditionally, domains. the very small size of ZnO crystallites (Table 1) could also contribute to the high dispersion of the In contrast, the presence of the Cu2O and Cu crystallites in samples prepared in N2 and H2 reduced the probabilityCuO domains. of the In necessary contrast, interaction the presence between of the Cu CuO2O andand ZnO.Cu crystallites As a result, in samples the number prepared of potentially in N2 activeand sitesH2 reduced formed the on probability the interface of the between necessary the interaction CuO and ZnObetween domains CuO and after ZnO. the reductionAs a result, differs the number of potentially active sites formed on the interface between the CuO and ZnO domains after for catalysts prepared by the calcination in different atmospheres. Additionally, the existence of the reduction differs for catalysts prepared by the calcination in different atmospheres. Additionally, the said interfaces contributes to the stabilization of the Cu crystallites in ZnO surroundings. As a the existence of the said interfaces contributes to the stabilization of the Cu crystallites in ZnO result, CuZn-350AS has a higher BET surface area, external surface, and pore volume with lower surroundings. As a result, CuZn-350AS has a higher BET surface area, external surface, and pore crystallites size values than CuZn-350N and CuZn-350H. As can be deduced from Table1, the absence volume with lower crystallites size values than CuZn-350N and CuZn-350H. As can be deduced from ofTable intimate 1, the CuO-ZnO absence of contact intimate resulted CuO-ZnO in acontact more facileresulted sintering in a more of facile both sintering the Cu and of both Zn domainsthe Cu atand elevated Zn domains temperatures. at elevated Consequently, temperatures. the Conseq DMAuently, conversion the DMA over conversion CuZn-350AS over was CuZn-350AS the highest amongwas the all highest catalysts among (78%), all while catalysts the DMA(78%), conversion while the DMA observed conversion over CuZn-350N observed over and CuZn-350N CuZn-350H and was muchCuZn-350H lower, i.e., was 32% much and lower, 17%, i.e., respectively. 32% and 17%, The respectively. significantly The lower significantly conversion lower over conversion CuZn-350H over in comparisonCuZn-350H with in CuZn-350Ncomparison with is the Cu consequenceZn-350N is ofthe a consequence more extensive of reductiona more extensive of CuZn-350H reduction during of calcinationCuZn-350H and, during hence, calcination the lower and, probability hence, the to stabilizelower probability the CuO to domains stabilize by the intimate CuO domains contact by with ZnO.intimate Consequently, contact with the ZnO. CuO Consequently, and ZnO crystallites the CuO areand approximately ZnO crystallites 17–20 are approximately nm large in CuZn-350H17–20 nm whereaslarge in only CuZn-350H 10–11 nm whereas in CuZn-350N only 10–11 (Table nm 1in). CuZn-350N (Table 1). AsAs reported reported previously previously [6[6],], CuZnCuZn catalystscatalysts catalyze not not only only hydrogenolysis, hydrogenolysis, but but higher- higher- molecular-weightmolecular-weight products products originating originating from from transesterification transesterification reactions reactions between between DMA, 1,6-hexanediolDMA, 1,6- (1,6-HDO),hexanediol and (1,6-HDO), partially hydrogenolyzedand partially hydrogen DMA (1HMEol)olyzed DMA are also(1HMEol) formed. are These also transesterificationformed. These transesterification products consist of products with two C6 backbones in their molecules (denoted products consist of products with two C6 backbones in their molecules (denoted H2) or with three C6 backbonesH2) or with in theirthree moleculesC6 backbones (denoted in their H3).molecules Thus, (denoted the selectivity H3). Thus, to and the yieldsselectivity of different to and yields products of allowdifferent for obtaining products additionalallow for obtaining important additional information important on the performanceinformation on of the catalysts performance particularly of catalysts particularly with respect to the origins of the transesterification activity of the CuZn with respect to the origins of the transesterification activity of the CuZn catalysts. Figure7 depicts the catalysts. Figure 7 depicts the selectivity towards reaction products formed by DMA hydrogenolysis selectivity towards reaction products formed by DMA hydrogenolysis over catalysts prepared by the over catalysts prepared by the thermal activation of CuZn-P in air. Regardless, the calcination thermal activation of CuZn-P in air. Regardless, the calcination temperature selectivity to the reaction temperature selectivity to the reaction products observed over the three CuZn-TAS catalysts followed products observed over the three CuZn-TAS catalysts followed the same trend as a function of DMA the same trend as a function of DMA conversion. To maintain readability of the graph, the T conversion.selectivities To over maintain all CuZn- readabilityTAS catalysts of the graph, were thenot selectivitiesdistinguished over according all CuZn- to ASthe catalysts calcination were nottemperature distinguished (Figure according 7). to the calcination temperature (Figure7).

FigureFigure 7. 7.The The change change of of selectivity selectivityto tothe the reactionreaction productsproducts in in dependence dependence on on the the DMA DMA conversion conversion observedobserved over over the the catalysts catalysts prepared prepared byby thethe calcination of of the the prec precursor,ursor, CuZn-P, CuZn-P, in inthe the air air at at220, 220, 350,350, and and 500 500◦ °C.C. 1HMEol1HMEol is is methyl methyl 6-hydroxyhexanoa 6-hydroxyhexanoate;te; 1,6-HDO 1,6-HDO is hexane-1,6-diol; is hexane-1,6-diol; H2 is the H2 sum is the sumof of6-methoxy-6-oxohexyl 6-methoxy-6-oxohexyl methyl methyl adipate, adipate, 6-hydr 6-hydroxyhexyloxyhexyl methyl methyl adipate, adipate, and and6-hydroxyhexyl 6-hydroxyhexyl 6- 6-hydroxyhexanoate;hydroxyhexanoate; and and H3 H3 is isthe the sum sum of ofbis(6-me bis(6-methoxy-6-oxohexyl)thoxy-6-oxohexyl) adipate, adipate, 6-hydroxyhexyl 6-hydroxyhexyl (6- (6-methoxy-6-oxohexyl)methoxy-6-oxohexyl) adipate, adipate, an andd bis(6-hydroxyhexyl) bis(6-hydroxyhexyl) adipate. adipate.

Catalysts 2018, 8, 446 11 of 16

The composition of the main reaction products and their tendency to change with the increasing DMA conversion is consistent with the results from our previous work [6]. 1HMEol is evidently a primary product formed by the hydrogenolysis of only one ester group of the two present in the DMA. Its selectivity approached 100% at DMA conversion close to zero. Subsequently, 1HMEol can be transformed either by the hydrogenolysis of the second ester group yielding 1,6-HDO or by participating in transesterification reactions yielding bulky H2 and H3 molecules (for structures and chemical names, see Scheme1). The hydrogenolysis of the second ester group in the DMA, i.e., the ester group in 1HMEol, also proceeded very fast on the catalysts prepared by calcination in air. Consequently, the selectivity to 1,6-HDO reached 20% at a DMA conversion as low as 12%. However, once 1HMEol and 1,6-HDO were formed, the transesterification reactions involving DMA, 1HMEol, and/or 1,6-HDO became possible and resulted in a formation of bulky compounds with either two or three “hexane backbones” in their molecule, denoted as H2 and H3 (for structures, see Scheme1). During the reaction, the formation of the heavy H2 and H3 compounds became prevalent. Therefore, at DMA conversion of about 80%, the selectivity to H2 and H3 attained 44% and 21%, respectively. In addition, other reaction products, including cyclic ones (due to intramolecular transesterification), have also been identified among the reaction products. Their amount was small and they were excluded from further consideration. It can be inferred that the transesterification reactions are preferred to the second ester group hydrogenolysis under the studied reaction conditions as the selectivity to 1,6-HDO increased at a much more moderate rate with the increasing DMA conversion than the selectivity to the H2 and H3 products. It has been reported that the hydrogenolysis of both ester functions in the DMA molecule does not occur at the same rate [5,19]. Hence, a ratio between the selectivities to HDO and the sum of the H2 and H3 products can help in evaluating the contribution of different reaction routes to the total composition of the reaction products. The ratio SHDO/SH2+H3 observed for CuZn catalysts calcined in air varied only slightly for the three calcination temperatures; it was in the range of 0.61–0.94 at a DMA conversion ≈10%. This shows that the ratio between the active sites responsible for hydrogenolysis and transesterification, respectively, is approximately the same in these catalysts and does not change significantly due to different calcination temperature in air. In the case of catalysts prepared by calcination in nitrogen and hydrogen at 220 ◦C (CuZn-220N and CuZn-220H, respectively), the maximum yield of 1HMEol reached ca. 10–12% at a DMA conversion of ca. 40%. The same maximum yield of 1HMEol was also obtained for all three catalysts prepared by calcination in air (Figure8(1A,2A,3A)), suggesting that all catalysts prepared by calcination at 220 ◦C have a similar distribution of the Cu active sites regardless of the calcination atmosphere. Consequently, the SHDO/SH2+H3 calculated for CuZn-220N and CuZn-220H at DMA conversion ≈10% was 0.77 and 0.7, correspondingly, which is within the range of values observed for the catalysts prepared by the calcination in air. Similar to the air-calcined CuZn catalysts, the selectivity to H2 and H3 increased markedly with the increase in the DMA conversion for CuZn-220N and CuZn-220H catalysts, implying the secondary nature of H2 and H3 formation. A different selectivity character was observed in the case of samples prepared by the calcination of the CuZn-P precursor at 350 and 500 ◦C in nitrogen or hydrogen. The yields of the 1HMEol were minor—the maximum yield of 1HMEol was achieved over CuZn-350N and amounted to 2% at a DMA conversion of 30%. This is significantly less than the samples calcined at 220 ◦C in nitrogen, hydrogen, or in air (ca. 10–12% yield at a 30% DMA conversion). The 1,6-HDO yields were virtually 0%, which is again dramatically lower than the other catalysts that yielded 4–6% HDO at comparable conversions (Figure8(1B,2B,3B)). Surprisingly, the esters (H2 and H3 products) requiring 1HMEol and/or HDO in addition to DMA were the prevailing reaction products. At a given conversion, their yield significantly exceeded the yield of esters formed over the CuZn catalysts calcined at 220 ◦C (in all atmospheres) and all catalysts calcined in air (i.e., calcined at 220, 350, or 500 ◦C); see Figure8(1C,2C,3C,1D,2D,3D). It can be inferred that the calcination temperature in a non-oxidizing atmosphere had a severe impact on the hydrogenolysis activity of the CuZn catalysts, as the SHDO/SH2+H3 dropped to values as low Catalysts 2018, 8, 446 12 of 16

Catalysts 2018, 8, x FOR PEER REVIEW ◦ 12 of 16 as 0.002 for catalysts calcined at 350 C in N2 or H2. The results, thus, provide strong evidence that the hydrogenolysis of the methyl ester function and the transesterification reaction leading to the transesterification reaction leading to the formation of bulky molecules, H2 and H3, occur on different formation of bulky molecules, H2 and H3, occur on different active sites. Obviously, both the calcination active sites. Obviously, both the calcination atmosphere and temperature strongly affect the atmosphere and temperature strongly affect the performance of the CuZn catalysts. The identification performance of the CuZn catalysts. The identification (and possibly also quantification) of the active (and possibly also quantification) of the active sites, however, remains a task for forthcoming studies. sites, however, remains a task for forthcoming studies.

FigureFigure 8.8. The change of the yield of reaction products (A:: 1HMEol,1HMEol, B:: HDO,HDO, C:: H2,H2, D:: H3)H3) withwith aa dependencedependence onon thethe DMADMA conversion conversion observed observed over over the the samples samples prepared prepared by by the the thermal thermal treatment treatment of CuZn-Pof CuZn-P in airin air (1), (1 nitrogen), nitrogen (2) ( and2) and hydrogen hydrogen (3). (3).

3. Materials and Methods 3. Materials and Methods A CuZn precursor with the copper/zinc molar ratio of 1.6 was prepared by a co-precipitation A CuZn precursor with the copper/zinc molar ratio of 1.6 was prepared by a co-precipitation method based on the neutralization of a starting acidic solution with a basic precipitating agent. method based on the neutralization of a starting acidic solution with a basic precipitating agent. For For the synthesis of the precursor, a glass beaker was ﬁlled with 200 mL of distilled water and heated the synthesis◦ of the precursor, a glass beaker was filled with 200 mL of distilled water and heated to to T = 60 C under stirring at 600 RPM. An aqueous solution (0.5 M) containing Cu(NO3)2·3H2O T = 60 °C under stirring at 600 RPM. An aqueous solution (0.5 M) containing Cu(NO3)2·3H2O (99.0%, (99.0%, Penta, s.r.o., Prague, Czech Republic), Zn(NO3)2·6H2O (99.6%, Lach:Ner, s.r.o., Neratovice, Penta, s.r.o., Prague, Czech Republic), Zn(NO3)2·6H2O (99.6%, Lach:Ner, s.r.o., Neratovice, Czech Republic), and an aqueous solution (0.5 M) of Na2CO3 (99.4%, Lach:Ner, s.r.o., Neratovice, Czech Republic) used as the precipitant were dosed simultaneously dropwise to keep the pH value in the beaker in the range of 7 ± 0.1. The temperature and stirring rate were kept constant at 60 °C and 600

Catalysts 2018, 8, 446 13 of 16

Czech Republic), and an aqueous solution (0.5 M) of Na2CO3 (99.4%, Lach:Ner, s.r.o., Neratovice, Czech Republic) used as the precipitant were dosed simultaneously dropwise to keep the pH value in the beaker in the range of 7 ± 0.1. The temperature and stirring rate were kept constant at 60 ◦C and 600 RPM, correspondingly, during the precipitation. The resulting suspension was aged under stirring for 90 min which caused an increase in the pH value to 7.6–7.7. The prepared precipitate was then filtered using a vacuum pump, washed with plenty of distilled water, and finally dried at 70 ◦C overnight. To prepare a series of catalysts for the investigations, the starting CuZn precursor denoted as CuZn-P (fine powder, 1 g used for each experiment) was thermally treated either in an oven in static open air or in a Parr autoclave in a flow of either nitrogen (99.99% SIAD Czech, s.r.o., Prague, Czech Republic) or hydrogen (99.9% SIAD Czech, s.r.o, Prague, Czech Republic). The thermal activation of the precursor in each of the gas atmospheres was performed at 220, 350, and 500 ◦C for 2 h. The prepared samples were denoted as CuZn-TAS, CuZn-TN, and CuZn-TH, where T stands for the temperature of the heat treatment (calcination) while AS, N, and H stand for static air, flowing nitrogen and hydrogen used as a gas atmosphere, respectively. For example, CuZn-350AS denotes a catalyst prepared by the calcination of the precursor, CuZn-P, at 350 ◦C in an air atmosphere. The copper and zinc content in the as-prepared CuZn-P sample were determined by XRF using a spectrometer ARL 9400 XP equipped with a rhodium lamp (Thermo ARL, Ecublens, Switzerland). The phase composition of the CuZn catalysts and the size of the crystallites of the relevant phases present in the catalysts were determined by X-Ray diffraction using a diffractometer PANanalytical X’Pert3 Powder and Cu Kα radiation (PANanalytical, Eindhoven, The Netherlands). The XRD patterns were recorded in a range of 2θ = 5–90◦. The particle sizes were calculated using Scherrer’s equation. Reflections at 2θ = 31.8◦, 36.2◦, and 38.6◦ were used for the particle size calculations of the ZnO, Cu, and CuO crystallites, respectively. Equilibrium adsorption isotherms of nitrogen were measured at 77 K using a static volumetric adsorption system (TriFlex analyzer, Micromeritics, Norcross, GA, USA). The samples were degassed at 473 K (12 h) prior to an N2 adsorption analysis, in order to obtain a clean surface. The adsorption isotherms were fitted using the Brunauer-Emmett-Teller (BET) method for the specific surface area, the t-plot method for the external specific surface area, and the BJH method for the distribution of mesopores. The hydrogenolysis of dimethyl adipate (>99%, Sigma-Aldrich, s.r.o., Prague, Czech Republic) was carried out in a Parr SS autoclave with a reactor volume of 300 mL using the prepared CuZn catalysts. Prior to the catalytic tests, all catalysts (1 g) were reduced in situ in the autoclave at 220 ◦C using flowing hydrogen (H2 99.9%, SIAD Czech, s.r.o., Prague, Czech Republic) at atmospheric pressure for 1 h. After the catalyst reduction, the temperature in the autoclave was decreased to below 100 ◦C and the reactor was loaded with 50 mL of dimethyl adipate (DMA) by a peristaltic pump (i.e., the autoclave with the reduced catalyst was not opened to avoid reoxidation of the catalyst). After that, the temperature in the autoclave was gradually increased to T = 210 ◦C. Then, the hydrogen pressure was increased to 10 MPa and the catalytic experiment was initiated by starting the stirring of the reaction mixture (DMA with a catalyst) at 600 RPM. Liquid reaction products were periodically withdrawn from the autoclave, centrifuged, and analyzed by a GC (Agilent 6890N, HPST, s.r.o., Prague, Czech Republic) equipped with a FID and non-polar column (ULTRA 1, 0.32 mm internal diameter, 20 m length). The DMA conversion was calculated according to Equation (5). Due to the absence of cracking reactions, the C6 backbone present in the DMA molecule, as well as in the reaction products, was used to calculate the product selectivity (Equation (6)). Methanol (also a reaction product) was excluded from these calculations.

nDMA,i − nDMA,t conversionDMA = (5) nDMA,i where nDMA is the number of DMA moles in the reaction mixture, where i stands for the initial number of DMA moles and t stands for the number of DMA moles at the sampling time. Catalysts 2018, 8, 446 14 of 16

nx,t − nx,i Sx,t = (6) ∑ nproducts 1C6 backbone + 2 ∑ nproducts 2C6 backbones + 3 ∑ nproducts 3C6 backbones where Sx,t is the selectivity to product x at time t of the experiment, nx is the number of moles of product x in the starting/initial mixture (index i) or at the time of sampling (index t), nproducts yC6 backbone refers to the number of product moles having one (y = 1), two (y = 2) or three (y = 3) C6 backbones in their molecules.

4. Conclusions The results from the present study show that starting from the same CuZn hydroxycarbonate precursor, it is possible to prepare samples greatly varying in physicochemical properties and catalytic performance. This can be achieved by the thermal treatment of the precursor using different atmospheres (air, nitrogen, and hydrogen) and calcination temperatures (220, 350, and 500 ◦C). Both calcination parameters affected the resulting phase composition of the catalysts as well as their crystallite size. In an oxidizing atmosphere, a mixture of CuO and ZnO was produced with increasing crystallite size as the calcination temperature grew. In contrast, in inert or reducing atmospheres, both oxides were accompanied by Cu2O and Cu. The crystallite sizes of Cu2O and Cu were larger than the sizes of CuO and ZnO, thus indicating a less intimate contact between the Cu-phases and ZnO in catalysts calcined in nitrogen and hydrogen. All three catalysts calcined in different atmospheres at T = 220 ◦C demonstrated good activity, resulting in a DMA conversion of 59–63%. The increase in the calcination temperature to 350 ◦C resulted in a growth of the DMA conversion to 78%, observed for the sample prepared in the oxidizing atmosphere. This is explained by the small crystallite size and intimate contact between the CuO and ZnO phases prior to the catalyst reduction. In contrast, the DMA conversion notably decreased over the samples prepared by the calcination in nitrogen or hydrogen at higher temperatures. This is explained by the decreased number of potentially active sites formed on the interfaces between the CuO and ZnO domains after the reduction and the lower stabilization of Cu crystallites in the ZnO surrounding. As a result, the DMA conversion decreased to ◦ a negligible 1% observed for samples calcined in N2 and H2 at 500 C. Calcination parameters used for the catalyst preparation greatly influenced not only DMA conversion but also product selectivity. The reaction over the air-calcined samples yielded 1,6-hexanediol with selectivity exceeding 20% at DMA conversion >10% independent of the calcination temperature. In contrast, the catalysts calcined in nitrogen or hydrogen at 350 and 500 ◦C exhibited selectivity to 1,6-hexanediol below 2% at DMA conversion ranging from 1% to 30%. Transesterification products (demoted H2 and H3) were the main reaction products with overall selectivities of >80% in these cases. The observed change in the catalytic performance of samples prepared by calcination in N2 and H2 allowed for suggesting that different active sites are responsible for the hydrogenolysis and transesterification reaction pathways and that the relative distribution of these sites has changed in dependence on the calcination procedure applied to the starting CuZn precursor.

Author Contributions: Conceptualization, Methodology and Writing—original draft preparation, O.K. and D.K.; Investigation—experiments and analyses, O.K, J.A., V.P. and M.L.; Writing—review and editing, J.A., V.P. and D.K. Funding: This research was funded by the Czech Science Foundation (project No. GA17-05704S). Acknowledgments: This work was realized within the Operational Programme Prague—Competitiveness (CZ.2.16/3.1.00/24501) and “National Program of Sustainability” (NPU I LO1613) MSMT-43760/2015. Conﬂicts of Interest: The authors declare no conﬂict of interest. Catalysts 2018, 8, x FOR PEER REVIEW 15 of 16 Catalysts 2018, 8, 446 15 of 16 CatalystsAppendix 2018 ,A 8, x FOR PEER REVIEW 15 of 16

Appendix A

300000 Cu CuZn-220AS-220H CuZn-350AS-220H 300000 Cu CuZn-220AS-220H 250000 CuZn-350AS-220H

250000 ZnO ZnO CuO Cu 200000 ZnO ZnO ZnO CuO Cu 200000 ZnO 150000 a.u. 150000

a.u. 100000

100000 50000

50000 0 20 40 60 80 0 2 Theta, degrees 20 40 60 80 2 Theta, degrees Figure A1. The XRD patterns of catalyst samples prepared by the treatment of the CuZn precursor in static air at 220 and 350 ◦°CC followed by the treatment in a hydrogen at T = 220 °C.◦C. Figure A1. The XRD patterns of catalyst samples prepared by the treatment of the CuZn precursor in static air at 220 and 350 °C followed by the treatment in a hydrogen at T = 220 °C. 100 10090 9080 8070 7060 6050 5040 4030 20 DMA Conversion, % DMA Conversion, 30 2010 DMA Conversion, % DMA Conversion, 100 0 0 1020304050 0The size 1020304050 of Cu crystallites in treated samples, nm

The size of Cu crystallites in treated samples, nm Figure A2. AA correlation between the the DMA conversion an andd the size of the Cu crystallites in the prepared CuZnCuZn catalysts.catalysts. □:: calcined calcined in in air, air, ∆:: calcined calcined in in nitrogen, nitrogen, ○: :calcined calcined in in hydrogen. hydrogen. Figure A2. A correlation between the DMA conversion and the size of the Cu crystallites in the # ○ Referencesprepared CuZn catalysts. □: calcined in air, ∆: calcined in nitrogen, : calcined in hydrogen.

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