Continuous 2-Methyl-3-Butyn-2-Ol Selective Hydrogenation on Pd/Γ-Al2o3 As a Green Pathway of Vitamin a Precursor Synthesis

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Article Continuous 2-Methyl-3-butyn-2-ol Selective Hydrogenation on Pd/γ-Al2O3 as a Green Pathway of Vitamin A Precursor Synthesis

Antonio J. Fernández-Ropero 1 , Bartosz Zawadzki 1 , Emil Kowalewski 1, Izabela S. Pieta 1 , Mirosław Krawczyk 1 , Krzysztof Matus 2 , Dmytro Lisovytskiy 1 and Anna Sr˛ebowata´ 1,*

1 Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland; [email protected] (A.J.F.-R.); [email protected] (B.Z.); [email protected] (E.K.); [email protected] (I.S.P.); [email protected] (M.K.); [email protected] (D.L.) 2 Materials Research Laboratory, Silesian University of Technology, Konarskiego 18A, 44-100 Gliwice, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-22-343-3320

Abstract: In this work, the effect of pretreatment conditions (10% H2/Ar ﬂow rate 25 mL/min and ◦ ◦ 400 C, 3 h or 600 C, 17 h) on the catalytic performance of 1 wt.% Pd/γ-Al2O3 has been evaluated for hydrogenation of 2-methyl-3-butyn-2-ol in continuous-ﬂow mode. Two palladium catalysts have been tested under different conditions of pressure and temperature and characterized using ◦ various physicochemical techniques. The catalytic performance of red(400 C)-Pd/γ-Al2O3 and ◦ red(600 C)-Pd/γ-Al2O3 are affected by the coexistence of several related factors like the competition between PdH and PdCx formation during the reaction, structure sensitivity, hydrogen spillover Citation: Fernández-Ropero, A.J.; to the alumina support and presence or absence of Pd–Al species. High-temperature reduction Zawadzki, B.; Kowalewski, E.; Pieta, leads to formation of Pd–Al species in addition to pure Pd. The Pd–Al species which reveal unique I.S.; Krawczyk, M.; Matus, K.; δ− Lisovytskiy, D.; Sr˛ebowata,A.´ electronic properties by decreasing the Pd surface concentration via electron transfer from Pd Continuous 2-Methyl-3-butyn-2-ol to Al, leading to a weaker Pd–Alkyl bonding, additionally assisted by the hydrogen spillover, are Selective Hydrogenation on the sites of improved semi-hydrogenation of 2-methyl-3-butyn-2-ol towards 2-methyl-3-buten-2-ol

Pd/γ-Al2O3 as a Green Pathway of (97%)—an important intermediate for vitamin A synthesis. Vitamin A Precursor Synthesis. Catalysts 2021, 11, 501. https:// Keywords: heterogeneous catalysis; ﬂow chemistry; selective hydrogenation; green chemistry doi.org/10.3390/catal11040501

Academic Editors: Kei Manabe and Nicola Della Ca 1. Introduction Many chemical conversions in the pharmaceutical industry involve stoichiometric Received: 29 January 2021 amounts of reagents and, thus, generate large amounts of waste [1]. However, imple- Accepted: 12 April 2021 mentation of catalytic methods (especially in the continuous-flow mode) in this highly Published: 15 April 2021 competitive industry increases the process efficiency, improves energy efficiency and re- cyclability, reduces costs and improves sustainability. The continuous method allows Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in fine-tuning of the contact time between intermediates and the active catalytic phase, which published maps and institutional affil- may result in high selectivity without using additives or a purification step, and turns into iations. a decrease of the E-factor (kg of generated waste/kg of product) [2,3]. In consequence, the pharmaceutical industry’s application of continuous-flow practices is considered one of the most strategic fields of innovation towards greener manufacturing methods, and complies with twelve principles of green chemistry [2,4]. One of the desired products in the pharmaceutical industry is 2-methyl-3-buten-2-ol Copyright: © 2021 by the authors. (MBE) which is obtained by means of partial reduction of 2-methyl-3-butyn-2-ol (MBY) Licensee MDPI, Basel, Switzerland. This article is an open access article (Scheme1). distributed under the terms and MBE is an important intermediate for the industrial synthesis of vitamins A (Scheme S1)[6] conditions of the Creative Commons and E and are present in a large variety of perfumes as aroma compounds [7]. Attribution (CC BY) license (https:// The current manufacturing processes in batch set-ups of this alcohol avoids the com- creativecommons.org/licenses/by/ plete hydrogenation by using, for instance, Lindlar-type catalyst containing Pd (5 wt.%) 4.0/).

Catalysts 2021, 11, 501. https://doi.org/10.3390/catal11040501 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 501 2 of 19

and Pb (2–3 wt.%) deposited on CaCO3 [8]. However, lead toxicity entails an environmen- tal concern that should not be ignored [9] and the addition of additives to enhance the selectivity results in additional costs due to the necessary separation processes [7]. While it is true that other Pb-free Pd-based catalysts are being used in the synthesis of MBE with Catalysts 2021, 11, x FOR PEER REVIEW 2 of 20 yields of 95–97%, they suffer from high catalyst deactivation due to degradation of the support or sintering of the metal particles [10].

Scheme 1. SchemePathway 1.ofPathway catalytic of2-methyl-3 catalytic-butyn-2-ol 2-methyl-3-butyn-2-ol hydrogenation hydrogenation [5]. [5].

MBE is an importantHydrogenation intermediate of MBY for hasthe beenindustrial investigated synthesis in of continuous vitamins A flow, (Scheme with a good yield, S1) [6] and E bothand are in gaspresent and in liquid a large conditions variety of [5 ,perfumes11–16]. Most as aroma of these compounds studies have [7]. been carried out The currentusing manufacturing Pd-based catalysts, processes where in batch the metal set-ups particles of this werealcohol supported avoids the on com- a large variety of materials. Yields of 89% and 92% were reached in capillary microreactors using bimetallic plete hydrogenation by using, for instance, Lindlar-type catalyst containing Pd (5 wt.%) Pd25Zn75 [11] and pure Pd nanoparticles [12] both supported on TiO2 and using methanol and Pb (2–3 wt.%) deposited on CaCO3 [8]. However, lead toxicity entails an environmen- as the solvent. Non-negligible values were obtained by Kundra et al., in a 3D printed tal concern that should not be ignored [9] and the addition of additives to enhance the catalytic static mixer. In a solution of 30% isopropanol and 70% water, with colloidal selectivity results in additional costs due to the necessary separation processes [7]. While palladium on titanium silicate as a catalyst [13], 96% conversion and 91% alkene selectivity it is true that other Pb-free Pd-based catalysts are being used in the synthesis of MBE with were achieved at a moderate pressure of 4 bars and a relatively high temperature of 100 yields of 95–97%, they suffer from high catalyst deactivation due to degradation of the ◦C. On the other hand, in the same study, the performance was much lower when using support or sintering of the metal particles [10]. Lindlar catalyst. MBE selectivity over 90% was shown in Al O -coated metallic monolithic Hydrogenation of MBY has been investigated in continuous flow, with2 3 a good yield, reactors featuring hybrid hexadecyl(2-hydroxyethyl)dimethyl ammonium dihydrogen- both in gas and liquid conditions [5,11–16]. Most of these studies have been carried out phosphate HHDMA-modified Pd nanoparticles during a three-phase continuous-flow MBY using Pd-based catalysts, where the metal particles were supported on a large variety of hydrogenation in relatively low pressure and temperature ranges (1–8 bar, 30–90 ◦C) [15]. materials. YieldsPromising of 89% resultsand 92% were were also reached obtained in capillary for the isolated microreactors Pd atoms using confined bimetallic into the six-fold Pd25Zn75 [11] and pure Pd nanoparticles [12] both supported on TiO2 and using methanol cavities of mpg-C3N4, in similar conditions. This single-site Pd catalyst showed higher as the solvent.activity Non-negligible than conventional values were heterogeneous obtained by Kundra catalysts et andal., in >90% a 3D ofprinted the selectivity cat- toward alytic static mixer.MBE [In16 ].a solution of 30% isopropanol and 70% water, with colloidal palladium on titanium silicate as a catalyst [13], 96% conversion and 91% alkene selectivity Furthermore, Pd supported on MgO, CeO2, ZnO and Al2O3 and bimetallic Pd–Zn were achieved at a moderate pressure of 4 bars and a relatively high temperature of 100 supported on Al2O3 were investigated for gas-phase 2-methyl-3-butyn-2-ol hydrogena- °C. On the other hand, in the same study, the performance was much lower when using tion [5]. Pd/ZnO and Pd–Zn/Al2O3 exhibited improved selectivity (c.a. 90%) as compared Lindlar catalyst.to the MBE other selectivity catalysts. over The 90% enhancement was shown wasin Al explained2O3-coated by metallic the modification monolithic of electronic reactors featuringproperties hybrid of hybrid Pd by electronhexadecyl(2-hydroxyethyl)dimethyl transfer from Pd to Zn and formationammonium of dihy- Pd–Zn alloy that drogenphosphateblocks HHDMA-modified unselective active sites.Pd nanoparticles during a three-phase continuous- flow MBY hydrogenationThe latter in revealsrelatively the low importance pressure ofand metal-support temperature interactionranges (1–8 andbar, the30– pretreatment 90 °C) [15]. Promisingmethod. Higherresults were activity also of obta Pd–Zn/Alined for2 theO3 asisolated compared Pd atoms to Pd/ZnO confined while into maintaining the six-fold cavitieshigh selectivity of mpg-C3 toN4 MBE, in similar was explained conditions. by This the single-site effect of hydrogen Pd catalyst spillover showed in the case of higher activityalumina than conventional supported catalyst heterogeneous [5]. Moreover, catalysts Pd/Al and2 O>90%3 catalytic of the performanceselectivity to- for the alkane ward MBE [16].isomerization strongly depends on the pretreatment conditions [17–20]. To the best of our Furthermore,knowledge, Pd supported although on Pd/Al MgO,2O 3CeOhas2, been ZnO commonly and Al2O studied3 and bimetallic in the MBY Pd–Zn hydrogenation in supported onflow Al2O mode3 were [ 5investigated,15] and in batch for gas-phase systems [2-methyl-3-butyn-2-ol21], the influence of this hydrogenation phenomenon has not been [5]. Pd/ZnO andyet investigatedPd–Zn/Al2O in3 exhibited the three-phase improved continuous-flow selectivity (c.a. MBY 90%) hydrogenation. as compared to the other catalysts.Based The onenhancement the literature was data, expl theained increasing by the the modification reduction temperature of electronic from 300 ◦C to properties of600 Pd ◦byC generateselectron transfer a considerable from Pd increase to Zn inand the formation overall catalytic of Pd–Zn activity, alloy and,that in particular, blocks unselectivea significant active sites. enhancement of the isomerization selectivity, up to ~90%, in the hydroconver- The lattersion reveals of C6 the alkanes importance [18–20]. of metal-support interaction and the pretreatment method. Higher activity of Pd–Zn/Al2O3 as compared to Pd/ZnO while maintaining high selectivity to MBE was explained by the effect of hydrogen spillover in the case of alumina supported catalyst [5]. Moreover, Pd/Al2O3 catalytic performance for the alkane isomerization strongly depends on the pretreatment conditions [17–20]. To the best of our knowledge, although Pd/Al2O3 has been commonly studied in the MBY hydrogenation in flow mode [5,15] and in batch systems [21], the influence of this phenomenon has not been yet investigated in the three-phase continuous-flow MBY hydrogenation.

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Therefore, we were wondering how the pretreatment conditions affect the catalytic performance of 1 wt.% Pd/γ-Al2O3 in the continuous hydrogenation of 2-methyl-3-butyn- 2-ol. Consequently, the present article shows the results of MBE synthesis as the effect of the continuous partial hydrogenation of MBY in ethanol solution for two Pd/γ-Al2O3 ◦ catalysts reduced in H2/Ar ﬂow under different procedures: 3 h at 400 C (marked as ◦ ◦ ◦ red(400 C)-Pd/γ-Al2O3) and 17 h at 600 C (marked as red(600 C)-Pd/γ-Al2O3). Different performances for both samples are discussed in the context of varying functional structures formed after preheating treatment. Reaction parameters, such as temperature and pressure, were also evaluated. The highly reduced catalyst displayed remarkable selectivity values near room temperature and favourable low-pressure stability during a green pathway of MBE synthesis.

2. Results and Discussion 2.1. Characterization Results ◦ ◦ Two palladium catalysts (red(400 C)-Pd/γ-Al2O3 and red(600 C)-Pd/γ-Al2O3) were thoroughly characterised using different physicochemical methods to diagnose the impact of reduction conditions on their morphology.

2.1.1. Physisorption and Chemisorption Results Nitrogen physisorption experiments were performed to examine the textural properties of the support and the changes after Pd precursor impregnation and calcination. The results of nitrogen adsorption–desorption can be seen in Table1.

Table 1. Textural properties of γ-Al2O3 and calcined PdAl2O3 from N2 physisorption.

Samples BET Speciﬁc Surface Area (m2 g−1) Total Pore Volume (cm3 g−1)

γ-Al2O3 190 0.51 Calcined-Pd/γ-Al2O3 210 0.56

Both materials have similar pore volume, and they have only mesopores in the structure. The slightly higher surface area corresponds to the calcined support with palladium, which probably affects the alumina structure stabilization (Table1). In consequence, it led to more available space for nitrogen adsorption. Pd dispersion and particle size in the reduced samples were determined by chemisorption (Table2). The metal dispersion is higher for catalyst reduced at 400 ◦C, concluding that the higher reduction temperature led to Pd nanoparticle sintering.

Table 2. Palladium particle sizes estimated from CO chemisorption.

Samples Particle Size (nm) * Dispersion (%) ◦ red(400 C)-Pd/γ-Al2O3 10.2 11 ◦ red(600 C)-Pd/γ-Al2O3 24.9 4.5 * Where particles size d = 1.12/D (D is the dispersion estimated from CO chemisorption).

2.1.2. X-ray Diffraction Results Figure1 shows the X-ray diffraction (XRD) patterns for the support and the palladium catalysts. Palladium is clearly detected in the analysed catalysts. The palladium particle ◦ ◦ size was estimated as 25 nm for red(400 C)-Pd/γ-Al2O3 and 30 nm for red 600 C)-Pd/γ- Al2O3. Higher crystallinity of palladium should result in a sharper peak for the sample after the longer reduction time at higher temperature [22]. Additionally, for red(600 ◦C)-Pd/γ- Al2O3, Pd4Al3 and Pd–Al alloy formation could be expected, as it was reported for 10 wt.% Pd/γ-Al2O3 reduced at high temperature [23]. However, the small Pd loading (1 wt.% Pd) makes the formation of Pd–Al species not as invisible in this case as for 10 wt.% Pd/γ-Al2O3 (Figure1). Independent of the reduction conditions, the characteristic reﬂection peak for Pd (1 1 1) appears with a maximum at ~40.15◦ in both catalysts. The difference in positions of Catalysts 2021, 11, x FOR PEER REVIEW 4 of 20

red(600 °C)-Pd/γ-Al2O3, Pd4Al3 and Pd–Al alloy formation could be expected, as it was reported for 10 wt.% Pd/γ-Al2O3 reduced at high temperature [23]. However, the small Pd loading (1 wt.% Pd) makes the formation of Pd–Al species not as invisible in this case as Catalysts 2021, 11, 501 4 of 19 for 10 wt.% Pd/γ-Al2O3 (Figure 1). Independent of the reduction conditions, the characteristic reflection peak for Pd (1 1 1) appears with a maximum at ~40.15° in both catalysts. The difference in positions of the peaks 1 1 1 maximum between red(400 °C)-Pd/γ-Al2O3 ◦ ◦ andthe peaksred(600 1 1 °C)-Pd/ 1 maximumγ-Al2O between3 is not red(400spectacularC)-Pd/ (approximatelyγ-Al2O3 and 0.02°). red(600 In addition,C)-Pd/γ-Al peaks2O3 fromis not the spectacular metallic phase (approximately are overlapped 0.02◦ ).with In addition,the very wide peaks peaks from from the metallic the support. phase Nev- are ertheless,overlapped there with is thea noticeable very wide difference peaks from with the the support. position Nevertheless, of peak 1 1 1 therein both is aPd noticeable catalysts compareddifference withto 39.92° the positionfor pure ofPd peak [24,25]. 1 1 The 1 in shift both of Pd peak catalysts 1 1 1 comparedfrom the position to 39.92 proper◦ for pure for purePd [24 Pd,25 towards]. The shift larger of angles peak 1 suggests 1 1 from the the formation position properof a phase for with pure a Pdsmaller towards lattice larger cell parameterangles suggests (e.g., thePdAl formation alloy) for of both a phase catalysts with ain smaller agreement lattice with cell earlier parameter studies (e.g., and PdAl Ve- gard’salloy) forlaw both [23,26–28]. catalysts in agreement with earlier studies and Vegard’s law [23,26–28].

◦ Figure 1. X-ray diffraction (XRD)(XRD) patternpattern forforγ γ-Al-Al22OO33 andand 11 wt.%wt.% Pd/Pd/γ-Al2O3 reducedreduced at at 400 °CC and 600 °C.◦C.

2.1.3. Transmission Electron Microscopy ResultsResults The morphology morphology and and the the composition composition of of1 wt.% 1 wt.% palladium-loaded palladium-loaded catalysts catalysts were were also also characterised using transmission electron microscopy (TEM). Figure2a–d shows TEM characterised using transmission electron microscopy (TEM). Figure◦ 2a–d shows TEM im- images and palladium nanoparticle size distribution for red(400 C)-Pd/γ-Al2O3 and ages and◦ palladium nanoparticle size distribution for red(400 °C)-Pd/γ-Al2O3 and red(600 C)-Pd/γ-Al2O3. red(600 °C)-Pd/γ-Al2O3. The presented histograms (Figure2c,d) show a relatively uniform distribution of The presented histograms (Figure 2c,d) show a relatively uniform distribution of me- metallic nanoparticles. The shape of all particles is spherical. The average particle size of tallic nanoparticles. The shape of all particles is spherical. The average particle size of the the catalyst reduced at a lower temperature are in the range of 6 nm with similar amounts catalyst reduced at a lower temperature are in the range of 6 nm with similar amounts of of small (1–5 nm) and bigger particles (>5 nm). On the other hand, a broad range of small (1–5 nm) and bigger particles (>5 nm). On the other hand, a broad range of particle particle sizes is contemplated for the Pd loaded γ-alumina catalyst reduced at a higher sizes is contemplated for the Pd loaded γ-alumina ◦catalyst reduced at a higher tempera- temperature. The average particle size for red(600 C)-Pd/γ-Al2O3 is around 9 nm with ture. The average particle size for red(600 °C)-Pd/γ-Al2O3 is around 9 nm with evident evident domination of bigger (>5 nm) Pd particles. domination of bigger (>5 nm) Pd particles. The particle size determined by TEM follows the same trend as CO chemisorption (Table2) and estimation by XRD (Figure1), with smaller particle sizes for red(400 ◦C)- ◦ Pd/γ-Al2O3 and bigger for red(600 C)-Pd/γ-Al2O3. However, a disagreement between these values emerges from different estimation methods. For example, CO chemisorption determines real amounts of the active sites for reagent sorption and TEM allows for identiﬁcation of both accessible and inaccessible particles. Additionally, Figure3, Figures S1 and S2 show scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM images), elemental mapping and energy dispersive X-ray spectroscopy (EDS) analysis for both catalysts. Based on these results, it was possible to determine if the nanocrystals were pure ◦ Pd or Pd–Al species in the case of red(600 C)-Pd/γ-Al2O3 catalysts. Catalysts 2021, 11, 501 5 of 19 Catalysts 2021, 11, x FOR PEER REVIEW 5 of 20

Figure 2. Transmission electron microscopy (TEM) images of: (a) of red(400 ◦C)-Pd/γ-Al O and (b) red(600 ◦C)-Pd/γ- Figure 2. Transmission electron microscopy (TEM) images of: (a) of red(400 °C)-Pd/γ-Al2O3 2and3 (b) red(600 °C)-Pd/γ-Al2O3 ◦ γ ◦ γ Alcatalysts.2O3 catalysts. Pd particle Pd particle size distribution size distribution for: (c) for:red(400 (c) red(400 °C)-Pd/γC)-Pd/-Al2O3 and-Al2 (Od)3 red(600and (d) red(600°C)-Pd/γ-AlC)-Pd/2O3 catalysts.-Al2O3 catalysts.

TheElemental particle mapping size determined shows detectable by TEM differencesfollows the between same trend the intensityas CO chemisorption of aluminum and oxygen in red(400 ◦C)-Pd/γ-Al O and in red(600 ◦C)-Pd/γ-Al O , which could (Table 2) and estimation by XRD (Figure2 3 1), with smaller particle sizes2 for3 red(400 °C)- indicate aluminum and oxygen depletion of the γ-Al2O3 in the case of a highly reduced Pd/γ-Al2O3 and bigger for red(600 °C)-Pd/γ-Al2O3. However, a disagreement between catalyst. Careful EDS analysis (Figures S3–S5) has shown, in the case of red(400 ◦C)- these values emerges from different estimation methods. For example, CO chemisorption Pd/γ-Al O , that the O:Al ratio in atomic % is 2:1 independently on the analysed area determines2 3real amounts of the active sites for reagent sorption and TEM allows for iden- (Figures S3 and S4). On the other hand, red(600 ◦C)-Pd/γ-Al O showed the O:Al ratio tification of both accessible and inaccessible particles. 2 3 of 1.25:1 or even 1:1. This phenomenon revels a deﬁciency of oxygen in the support and Additionally, Figures 3, S1 and S2 show scanning transmission electron microscopy could suggest that a part of Al atoms interacts with Pd. HRTEM measurements provided (STEM), high-resolution transmission electron microscopy (HRTEM images), elemental evident conﬁrmation of our suppositions. Figure3b and Figure S2 show formation of mapping and energy dispersive X-ray spectroscopy (EDS)◦ analysis for both catalysts. Al0.15Pd0.85 phase in addition to pure Pd for red(600 C)-Pd/γ-Al2O3, as opposed to Based on◦ these results, it was possible to determine if the nanocrystals were pure Pd or red(400 C)-Pd/γ-Al2O3 (Figure3a and Figure S1). Pd–Al species in the case of red(600 °C)-Pd/γ-Al2O3 catalysts. Elemental mapping shows detectable differences between the intensity of aluminum and oxygen in red(400 °C)-Pd/γ-Al2O3 and in red(600 °C)-Pd/γ-Al2O3, which could indicate aluminum and oxygen depletion of the γ-Al2O3 in the case of a highly reduced catalyst. Careful EDS analysis (Figures S3–S5) has shown, in the case of red(400 °C)-Pd/γ- Al2O3, that the O:Al ratio in atomic % is 2:1 independently on the analysed area (Figures S3 and S4). On the other hand, red(600 °C)-Pd/γ-Al2O3 showed the O:Al ratio of 1.25:1 or even 1:1. This phenomenon revels a deficiency of oxygen in the support and could suggest that a part of Al atoms interacts with Pd. HRTEM measurements provided evident con- firmation of our suppositions. Figures 3b and S2 show formation of Al0.15Pd0.85 phase in addition to pure Pd for red(600 °C)-Pd/γ-Al2O3, as opposed to red(400 °C)-Pd/γ-Al2O3 (Figures 3a and S1).

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Figure 3. High-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) Figure 3. High-resolution transmission electron◦ microscopy (HRTEM), scanning ◦transmission electron microscopy (STEM) images and elemental mapping of: (a) red(400 C)-Pd/γ-Al2O3 and (b) red(600 C)-Pd/γ-Al2O3 catalysts. images and elemental mapping of: (a) red(400 °C)-Pd/γ-Al2O3 and (b) red(600 °C)-Pd/γ-Al2O3 catalysts. 2.1.4. Temperature Programmed Studies Results 2.1.4. Temperature Programmed Studies Results The temperature-programmed reduction of calcined 1 wt.% Pd/γ-Al2O3 catalyst carriedThe out temperature-programmed in the temperature range reduction from −20 of to calcined 600 ◦C showed 1 wt.% threePd/γ-Al main2O3 temperature-catalyst car- riedprogrammed out in the reduction temperature (TPR) range peaks from (Figure −204 to). In600 accordance °C showed with three literature main temperature- data [ 29,30], programmedone huge peak reduction evident at (TPR) room peaks temperature (Figure is 4). ascribed In accordance to reduction with of literature large PdO data crystallites. [29,30], onewith huge the second peak evident one, at ~130at room◦C, temperature is associated withis ascribed the reduction to reduction of small of large PdO particles,PdO crystal- and lites.the third with one, the second a small one, double at ~130 peak, °C, at is ~340 associated◦C and with above, the with reduction the reduction of small ofPdO small parti- Pd cles,species and interacting the third one, with a alumina small double (Figure peak,4 inset). at Additionally,~340 °C and above, the two with ﬁrst the TPR reduction peaks were of small Pd species interacting with alumina (Figure 4 inset). Additionally, the two first TPR

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peaksdistorted were by distorted a peak associated by a peak with associated decomposition with decomposition of palladium hydride of palladium phase (positivehydride peakphase ~100 (positive◦C). peak ~100 °C).

◦ Figure 4. Temperature programmedprogrammed reduction reduction of of 1 wt.%1 wt.% Pd/ Pd/γ-Alγ-Al2O23Oafter3 after calcination calcination in airin air at 500 at C. 500 °C. Based on TPR results, the low-temperature reduction conditions were estimated. Three hoursBased of the on reduction TPR results, at 400 the◦C seemslow-temperature enough to transformreduction Pdconditions2+ to metallic were form. estimated. ThreeThe hours results of the of temperature-programmedreduction at 400 °C seems hydrideenough decompositionto transform Pd (TPHD)2+ to metallic are shown form. in FigureThe5. Absenceresults of of temperature-programmed multiple and shape-distorted hydride TPHD decomposition peaks suggests (TPHD) a narrow are particle shown insize Figure distribution. 5. Absence However, of multiple the and TPHD shape-distorted maximum appearingTPHD peaks at suggests a higher atemperature narrow par- ◦ ticlecorresponds size distribution. to the sample However, with lower the TPHD particle maximum size (red(400 appearingC)-Pd/ at aγ -Alhigher2O3), temperature contrary to ◦ correspondsexpectations to [31 the]. Similarly, sample with the temperaturelower particle of size palladium (red(400 hydride °C)-Pd/ formationγ-Al2O3), contrary was ~4 toC ◦ ◦ ◦ expectationslower for red(600 [31]. Similarly,C)-Pd/γ -Althe2 temperatureO3 than for red(400of palladiumC)-Pd/ hydrideγ-Al2 Oformation3, 42.1 and was 46.2 ~4 °CC, lowerrespectively. for red(600 Additionally, °C)-Pd/γ this-Al2O shift3 than inhydride for red(400 decomposition °C)-Pd/γ-Al peak2O3, 42.1 is accompanied and 46.2 °C, by re- a spectively.decrease in Additionally, the amount of this released shift hydrogenin hydride (Figure decomposition5), manifested peak as is the accompanied H/Pd = 0.37 by for a ◦ ◦ decreasered(400 C)-Pd/in the amountγ-Al2O 3ofand released H/Pd hydrogen = 0.32 for (Figure red(600 5),C)-Pd/ manifestedγ-Al2O as3. the Our H/Pd TPHD = 0.37 results for are in line with Baker et al. [32], who showed that high-temperature reduction of Pd/Al O red(400 °C)-Pd/γ-Al2O3 and H/Pd = 0.32 for red(600 °C)-Pd/γ-Al2O3. Our TPHD results2 are3 leads to a decrease in adsorbed hydrogen. This phenomenon could indicate that some in line with Baker et al. [32], who showed that high-temperature reduction of Pd/Al2O3 leadsamount to ofa Aldecrease can interact in adsorbed with palladium hydrogen. in theThis Pd phenomenon catalyst reduced could at higherindicate temperature. that some amountIn accordance of Al withcan interact earlier studies with palladium [17,23], high in temperaturethe Pd catalyst reduction reduced of at alumina higher supported tempera- palladiumture. In accordance catalyst leads with to earlier formation studies of Pd–Al[17,23], intermetallic high temperature compounds reduction [23], orof evenalumina Pd– Alsupported alloys as palladium indicated catalyst by Radlik leads et to al. format [17]. Onion theof Pd–Al other intermetallic hand, there arecompounds two possible [23], reasons for TPHD maximum shift to a lower temperature in red(600 ◦C)-Pd/γ-Al O . or even Pd–Al alloys as indicated by Radlik et al. [17]. On the other hand, there are 2two3 Presence of Pd–Al species could increase the enthalpy of hydride formation, thus, the possible reasons for TPHD maximum shift to a lower temperature in red(600 °C)-Pd/γ- heat of hydride formation decreases, as observed for Pd–Ru materials. On the other hand, Al2O3. Presence of Pd–Al species could increase the enthalpy of hydride formation, thus, higher Pd nanoparticles could be sensitive for Pd–Al alloy formation that would block the the heat of hydride formation decreases, as observed for Pd–Ru materials. On the other Pd β-hydride formation, like it was observed for Pd–Au alloys [33,34]. Consequently, only hand, higher Pd nanoparticles could be sensitive for Pd–Al alloy formation that would smaller Pd particles would be sensitive to decomposition of the hydride phase, and hence block the Pd β-hydride formation, like it was observed for Pd–Au alloys [33,34]. Conse- a lower temperature of TPHD maximum would be observed (Figure5). quently, only smaller Pd particles would be sensitive to decomposition of the hydride phase, and hence a lower temperature of TPHD maximum would be observed (Figure 5).

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Figure 5. Temperature-programmed hydride decomposition for red(400 °C)-Pd/◦ γ-Al2O3 and Figure 5. Temperature-programmed hydride decomposition for red(400 C)-Pd/γ-Al2O3 and C- FigureC-red(600 5. Temperature-programmed °C)-Pd/◦ γ-Al2O3. hydride decomposition for red(400 °C)-Pd/γ-Al2O3 and red(600 C)-Pd/γ-Al2O3. C-red(600 °C)-Pd/γ-Al2O3. 2.1.5.2.1.5. X-ray X-ray Photoelectron Photoelectron Spectroscopy Spectroscopy (XPS) (XPS) 2.1.5. X-ray Photoelectron Spectroscopy (XPS) IdentificationIdentiﬁcation of Pd of and Pd and Al metals’ Al metals’ chemical chemical state state was was based based on Pd on Pd3d, 3d,Al 2p Al and 2p and valenceIdentificationvalence band band (not of (not shownPd and shown here) Al here)metals’ photoelectrons photoelectrons chemical (b stateearing (bearing was in mindbased in mind that on thatPdAl 2p3d, Al photoelectrons’ 2pAl photoelectrons’2p and valenceanalysisanalysis band depth (not depth shown is larger is here) larger than photoelectrons than that that of Pd of Pd3d (b earing 3dphotoelectrons)—Figures photoelectrons)—Figures in mind that Al 2p photoelectrons’ 6 and6 and 7.7 And. And the the analysischemicalchemical depth state is state largerof oxygen of oxygenthan and that and carbon of carbon Pd was 3d was photoelectrons)—Figuresbased based on onO 1s O + 1s Pd + Pd3p3/2 3p and3/2 6 and C 1s C7. photoelectron 1sAnd photoelectron the chemicalsignalssignals state (Figures of (Figures oxygen S6–S9). S6–S9). and The carbon Theinvestigation investigationwas based and on and theO 1s theinterpretation + Pd interpretation 3p3/2 and of C the1s of photoelectron theXPS XPS results results were were signalsbased based(Figures in accordance in accordanceS6–S9). Thewith withinvestigation the theliterature literature and data datathe and interpretation and XPS XPS electronic electronic of databasesthe databases XPS results [35–37]. [35 –were37 ]. based in accordance with the literature data and XPS electronic databases [35–37]. (a) (b) (a) (b)

◦ ◦ Figure 6. Pd 3d XPS X-ray photoelectron spectroscopy (XPS) region of: (a) red(400 C)-Pd/γ-Al2O3 and (b) red(600 C)- Figure 6. Pd 3d XPS X-ray photoelectron spectroscopy (XPS) region of: (a) red(400 °C)-Pd/γ-Al2O3 and (b) red(600 °C)- Pd/γ-Al O . FigurePd/ 6.γ-Al Pd2 O3d3. XPS2 3 X-ray photoelectron spectroscopy (XPS) region of: (a) red(400 °C)-Pd/γ-Al2O3 and (b) red(600 °C)- Pd/γ-Al2O3. Photoelectron signals Pd 3p3/2 and Al 2s were also included in the quantitative ◦ surface analysis for both catalysts. The surface of red(400 C)-Pd/γ-Al2O3 contains Pd- ◦ 5.7, Al-61.7, O-16.6 and C-16.0, and the surface of red(600 C)-Pd/γ-Al2O3 contains Pd- 4.5, Al-42.4, O-30.1 and C-23.0 estimated in at. %. It was found that high-temperature reduction caused a signiﬁcant (almost 20% at.) loss of aluminum on the surface with a simultaneous increase (over 13% at.) of oxygen concentration. Deconvolution of the Pd

Catalysts 2021, 11, 501 9 of 19

3d photoelectron signal indicates the presence of two palladium states: Pd (0) and Pd (2+) in both catalysts. However, the characteristic changes of metal–metal (MMI) and/or ◦ metal–support (MSI) (Pd–Al and/or Pd–AlOx) interactions were not found for red(400 C)- ◦ Pd/γ-Al2O3 (Figure6a ). On the other hand, for red 600 C)-Pd/γ-Al2O3, a new state with lower characteristic binding energy was detected for the Pd 3d5/2-Pd 3d3/2 photoelectron doublet. The binding energy was 0.6–0.7 eV lower than that of the metal Pd–Pd bond energy and was related to the strong electron interaction of palladium with the support (SMSI): Pd–AlOx (Figures6b and7). The second Pd 3d 5/2-Pd 3d3/2 photoelectron doublet Catalysts 2021, 11, x FOR PEER REVIEW 9 of 20 with a characteristic binding energy of 335–340.3 eV, identiﬁed in the case of highly reduced catalyst, is the result of the presence of metallic Pd on the surface.

(a) (b)

◦ ◦ Figure 7. Al 2p XPS region of: (a) red(400 C)-Pd/γ-Al2O3 and (b) red(600 C)-Pd/γ-Al2O3. Figure 7. Al 2p XPS region of: (a) red(400 °C)-Pd/γ-Al2O3 and (b) red(600 °C)-Pd/γ-Al2O3. In addition, XPS studies were performed for both catalysts directly after in-situ reduction.Photoelectron It allowed signals us Pd to 3p observe3/2 and Al the 2stransformation were also included of the in the surface quantitative during surface in situ H2 analysisreduction for both processes catalysts. and The to surface compare of thered(400 chemical °C)-Pd/ natureγ-Al and2O3 contains environment Pd-5.7, of Al-61.7, palladium, O-16.6and and aluminum C-16.0, and in the the samples surface afterof red(600 being °C)-Pd/ air exposedγ-Al2O (ex-situ3 contains red) Pd-4.5, and after Al-42.4, in-situ O- H2- 30.1reduction. and C-23.0 Comparison estimated in of at. the %. surface It was composition found that high-temperature for the catalysts after reduction ex-situ andcaused in-situ ◦ a significantactivation (almost showed 20% significant at.) loss differencesof aluminum only on forthered(600 surface withC)-Pd/ a simultaneousγ-Al2O3 (Table increase3). In-situ (overH 213%-reduction at.) of oxygen showed concentration. no characteristic Deco changesnvolution of MMI of the and/or Pd 3d MSIphotoelectron in the obtained signal XPS indicatesspectra. the Moreover, presence quantitativeof two palladium analysis states: showed Pd (0) (partially and Pd oxidised)(2+) in both an Al-enrichedcatalysts. How- surface ever,with the acharacteristic simultaneous changes decrease of in metal–metal the content of(MMI) oxygen, and/or as compared metal–support to the ex-situ(MSI) (Pd– reduced ◦ Al and/orred(600 Pd–AlOC)-Pd/x) γinteractions-Al2O3 sample were and not could found suggest for red(400 partial °C)-Pd/ reversibilityγ-Al2O of3 (Figure Pd–Al species6a). On theformation, other hand, as it wasfor red reported 600 °C)-Pd/ earlierγ-Al [23].2O As3, a a new consequence state with of lower the observed characteristic surface bind- compo- ing sitionenergy change, was detected the palladium’s for the Pd hydrogenolytic 3d5/2-Pd 3d3/2 photoelectron capacity may be doublet. reduced. The The binding slight changeen- ◦ ergyof was XPS 0.6–0.7 surface eV composition lower than forthat red(400 of the metalC)-Pd/ Pd–Pdγ-Al2 Obond3 by energy exposure and to was air couldrelated confirm to ◦ γ the strongthe negligible electron influence interaction of air of palladium on surface composition,with the support opposite (SMSI): to red(600Pd–AlOxC)-Pd/ (Figures-Al 6b2 O3. and 7). The second Pd 3d5/2-Pd 3d3/2 photoelectron doublet with a characteristic binding ◦ energyTable of 3. 335–340.3Pd, Al, O andeV,C identified surface contents in the estimated case of highly in at. % reduced by XPS for catalyst, the two catalysts:is the result red(400 of C)- ◦ the presencePd/γ-Al2 Oof3 andmetallic red(600 Pd onC)-Pd/ the surface.γ-Al2O3, analysed after being air exposed (ex-situ red) and after in-situIn addition, H2-reduction. XPS studies were performed for both catalysts directly after in-situ reduction. It allowed usSamples to observe the transformation Pd of the surface Al during O in situ H2 Cre- duction processes◦ and to compare the chemical nature and environment of palladium, red(400 C)-Pd/γ-Al2O3, ex-situ red. 5.7 61.7 16.6 16.0 and aluminum in◦ the samples after being air exposed (ex-situ red) and after in-situ H2- red(400 C)-Pd/γ-Al2O3, in-situ red. 5.2 59.9 29.2 5.7 reduction. Comparison◦ of the surface composition for the catalysts after ex-situ and in- red(600 C)-Pd/γ-Al2O3, ex-situ red. 4.5 42.4 30.1 23.0 ◦ situ activationred(600 showedC)-Pd/ significantγ-Al2O3, in-situ differences red. only for 7.0 red(600 67.9 °C)-Pd/γ-Al 19.02O3 (Table 6.1 3). In-situ H2-reduction showed no characteristic changes of MMI and/or MSI in the obtained XPS spectra. Moreover, quantitative analysis showed (partially oxidised) an Al-enriched surface with a simultaneous decrease in the content of oxygen, as compared to the ex-situ reduced red(600 °C)-Pd/γ-Al2O3 sample and could suggest partial reversibility of Pd–Al species formation, as it was reported earlier [23]. As a consequence of the observed surface composition change, the palladium’s hydrogenolytic capacity may be reduced. The slight change of XPS surface composition for red(400 °C)-Pd/γ-Al2O3 by exposure to air could confirm the negligible influence of air on surface composition, opposite to red(600 °C)- Pd/γ-Al2O3.

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Table 3. Pd, Al, O and C surface contents estimated in at. % by XPS for the two catalysts: red(400 °C)-Pd/γ-Al2O3 and red(600 °C)-Pd/γ-Al2O3, analysed after being air exposed (ex-situ red) and after in-situ H2-reduction.

Samples Pd Al O C red(400 °C)-Pd/γ-Al2O3, ex-situ red. 5.7 61.7 16.6 16.0

Catalysts 2021, 11, 501 red(400 °C)-Pd/γ-Al2O3, in-situ red. 5.2 59.9 29.2 5.710 of 19 red(600 °C)-Pd/γ-Al2O3, ex-situ red. 4.5 42.4 30.1 23.0 red(600 °C)-Pd/γ-Al2O3, in-situ red. 7.0 67.9 19.0 6.1

2.1.6.2.1.6. NH NH33 Temperature-ProgrammedTemperature-Programmed Desorption Desorption (NH (NH33-TPD)-TPD)

TheThe NH NH33-TPD-TPD profiles proﬁles (Figure (Figure 8)8 )of of the the samples samples show show two two desorption desorption maxima maxima in ac- in cordanceaccordance with with previous previous studies studies [38–40]. [38–40 ].

◦ FigureFigure 8. 8. NHNH33 Temperature-ProgrammedTemperature-Programmed DesorptionDesorption (NH (NH3-TPD)3-TPD) proﬁles profiles of of red(400 red(400C)-Pd/ °C)-Pd/γγ-Al- 2O3 ◦ Aland2O red(6003 and red(600C)-Pd/ °C)-Pd/γ-Al2γO-Al3. 2O3.

TheThe NH 3 desorption peaks cancan bebe classiﬁed classified into into three three types types of of acid acid sites, sites, characterised character- ◦ isedby different by different acidic acidic strength, strength, namely: namely: (i) weak—with (i) weak—with NH3 desorption NH3 desorption maxima maxima at 50–200 at 50–C, ◦ 3 200(ii) medium—with°C, (ii) medium—with NH3 desorption NH3 desorption maxima maxima at 200–400 at 200–400C and °C (iii) and strong—with (iii) strong—with NH ◦ NHdesorption3 desorption maxima maxima above above 400 C.400 The °C. performed The performed deconvolution deconvolution suggests suggests the presence the pres- of encethree of types three of types species of withspecies different with different acidity (P1, acidity P2 and (P1, P3 P2 Figure and P38). Figure They can8). They be assigned can be assignedto weak acidto weak sites, acid characterised sites, characterised by desorption by desorption maxima P1 maxima assigned P1 to physisorbedassigned to phy- NH3 sisorbedand/or NHNH33 and/oradsorbed NH on3 adsorbed weak Lewis on weak acid Lewis sites. acid P2 sites. and P3P2 suggestand P3 suggest the presence the pres- of both medium and strong acid sites, which can be associated with NH absorbed on ence of both medium and strong acid sites, which can be associated with 3NH3 absorbed strong Lewis acid sites and Brønsted acid sites. For both red(400 ◦C)-Pd/γ-Al O and on strong Lewis acid sites and Brønsted acid sites. For both red(400 °C)-Pd/γ-Al22O3 and C-red(600 ◦C)-Pd/γ-Al O samples, the desorption maxima are not shifted, however, the C-red(600 °C)-Pd/γ-Al2O2 3 3samples, the desorption maxima are not shifted, however, the reduction treatment of catalysts inﬂuenced the site population with a given strength. For reduction treatment◦ of catalysts influenced the site population with a given strength.◦ For the red(400 C)-Pd/γ-Al2O3, the P1:P2:P3 ratio is 1:0.62:0.53, while for red(600 C)-Pd/γ- the red(400 °C)-Pd/γ-Al2O3, the P1:P2:P3 ratio is 1:0.62:0.53, while for red(600 °C)-Pd/γ- Al2O3 1:0.85:0.67, suggesting an increase in the number of sites with medium/strong acid Al2O3 1:0.85:0.67, suggesting an increase in the number of sites with medium/strong◦ acid strength with increase of temperature treatment. Moreover, for red(600 C)-Pd/γ-Al2O3 strength with increase of temperature treatment. Moreover, for red(600 °C)-Pd/γ-Al2O3 catalyst, the total amount of acidic sites decreases by ca. 11% along with high reduction catalyst, the total amount of acidic sites decreases by ca. 11% along with high reduction temperature, which is consistent with the previous reports [41]. However, the differences between the amounts and strength of acid sites in both catalysts seem to be too small to be the cause of differences in their catalytic performance.

2.2. Catalytic Results ◦ ◦ Red(400 C)-Pd/γ-Al2O3 and red(600 C)-Pd/γ-Al2O3 catalytic performance was investigated in continuous-ﬂow hydrogenation of 2-methyl-3-butyn-2-ol (MBY) over a wide range of temperatures and pressures (Figures9–13). CatalystsCatalysts 2021, 112021, x, FOR11, 501 PEER REVIEW 12 of 2011 of 19

Catalysts 2021, 11, x FOR PEER REVIEW Figure 9. Effect of reaction conditions on the specific activity (TOF (s−1)) of red(400 ◦C)-Pd/13 of γ20-Al O 2 3 Figure 9. Effect of ◦reaction conditions on the specific activity (TOF (s−1)) of red(400 °C)-Pd/γ-Al2O3 and red(600 C)-Pd/γ-Al2O3 in 2-metyl-3-butyn-2-ol hydrogenation with 0.5 mL/min flow rate of and red(600 °C)-Pd/γ-Al2O3 in 2-metyl-3-butyn-2-ol hydrogenation with 0.5 mL/min flow rate of the reactant and 6 mL/min of the H2 flow. the reactant and 6 mL/min of the H2 flow.

Hence, we concluded that pressure is the crucial parameter that benefits MBY hydrogenation and the changes in conversion with temperature may not follow this trend. Nevertheless, both Pd catalysts show spectacular differences in the products’ distribution (Figures 10 and 11). Although both 2-methyl-3-butyn-2-ol hydrogenation products are desired in the production of pharmaceutical intermediates (MBE) and fine chemicals manufacture (MBA), our research is focused mainly on 2-methyl-3-buten-2-ol production. Based on the literature data [42–44], the selective alkyne to alkene hydrogenation on palladium catalyst is possible only when carbon (from fragmented substrate molecules) occupies interstitial lattice Pd sites and excludes H from populating the subsurface region and hence prevents total hydrogenation of substrate. As it is independent of the temperature and pressure, red(400 °C)-Pd/γ-Al2O3 produces 2-methyl-2-butanol as the main product (Figure 10) in contrast to Pd/γ-Al2O3 reduced at 600 °C (Figure 11), it could be assumed that the rate of C dissolution in red(400 °C)-Pd/γ-Al2O3 is too low to affect MBE selectivity. However, the second significant factor, which should be taken into account, is hydrogen spillover from Pd to the Al2O3 support [42]. It was found that C≡C is only hydrogenated on palladium, whereas hydrogen on the alumina support hydrogenates C=C to the saturated product. The carbonaceous species accumulated on the γ-Al2O3 support act as bridges that accelerate the process and consequently greatly increase the undesirable C=C hydrogenation [42].

◦ Figure 10. Effect of reaction conditions on the conversion and selectivity of red(400 C)-Pd/γ-Al2O3 Figure 10. Effect of reaction conditions on the conversion and selectivity of red(400 °C)-Pd/γ-Al2O3 to 2-methyl-3-buten-2-olto 2-methyl-3-buten-2-ol (MBE) (MBE) and 2-methyl-2-b and 2-methyl-2-butanolutanol (MBA) (MBA) in 2-metyl-3-butyn-2-ol in 2-metyl-3-butyn-2-ol hydro- hydrogena- genationtion with with 0.5 0.5 mL/min mL/min flow ﬂow rate rate of of the the reactant reactant and and 6 6mL/min mL/min of ofthe the H2 H flow.2 ﬂow.

Figure 11. The effect of reaction conditions on the conversion and selectivity of red(600 °C)-Pd/γ- Al2O3 to 2-methyl-3-buten-2-ol and 2-methyl-2-butanol in 2-metyl-3-butyn-2-ol hydrogenation with 0.5 mL/min flow rate of the reactant and 6 mL/min of the H2 flow.

Catalysts 2021, 11, x FOR PEER REVIEW 13 of 20

Figure 10. Effect of reaction conditions on the conversion and selectivity of red(400 °C)-Pd/γ-Al2O3 Catalysts 2021, 11, 501 to 2-methyl-3-buten-2-ol (MBE) and 2-methyl-2-butanol (MBA) in 2-metyl-3-butyn-2-ol hydro- 12 of 19 genation with 0.5 mL/min flow rate of the reactant and 6 mL/min of the H2 flow.

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 20

Figure 11. The effect of reaction conditions on the conversion and selectivity of red(600 ◦C)-Pd/γ- Figure 11. The effect of reaction conditions on the conversion and selectivity of red(600 °C)-Pd/γ- Al O to 2-methyl-3-buten-2-ol and 2-methyl-2-butanol in 2-metyl-3-butyn-2-ol hydrogenation with Al2O3 to2 2-methyl-3-buten-2-ol3 and 2-methyl-2-butanol in 2-metyl-3-butyn-2-ol hydrogenation with 0.50.5 mL/min mL/min flow ﬂow rate rate of of the the reactant reactant and and 6 6mL/min mL/min of of the the H H2 flow.2 ﬂow.

◦ Figure 12. Catalytic behaviour of red(600 C)-Pd/γ-Al2O3 in hydrogenation of 2-methy-3-butyn-2-ol ◦ Figure 12.as aCatalytic function behaviour of time on of stream red(600 at 25°C)-Pd/C andγ-Al 5 bar.2O3 in hydrogenation of 2-methy-3-butyn-2- ol as a function of time on stream at 25 °C and 5 bar.

Figure 13. Catalytic behaviour of red(600 °C)-Pd/γ-Al2O3 in hydrogenation of 2-methy-3-butyn-2- ol as a function of time on stream at 45 °C and 1 bar.

All in all, our studies reveal a substantial modification of the catalyst performance in relation to the reduction treatment. Undoubtedly, high-temperature reduction signifi- cantly promotes the selectivity towards the desired product, minimising over hydrogena-

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 20

Figure 12. Catalytic behaviour of red(600 °C)-Pd/γ-Al2O3 in hydrogenation of 2-methy-3-butyn-2- ol as a function of time on stream at 25 °C and 5 bar. Catalysts 2021, 11, 501 13 of 19

◦ Figure 13. Catalytic behaviour of red(600 C)-Pd/γ-Al2O3 in hydrogenation of 2-methy-3-butyn-2-ol Figure 13. Catalytic behaviour of red(600 °C)-Pd/γ-Al2O3 in hydrogenation of 2-methy-3-butyn-2- as a function of time on stream at 45 ◦C and 1 bar. ol as a function of time on stream at 45 °C and 1 bar.

The differences between two Pd/γ-Al2O3 catalysts after different pretreatment pro- Allcedures in all, couldour studies be explained reveal a insubstantial terms of structuremodification sensitivity of the catalyst (Table2 ,performance Figure2), acidity in of relationdehydroxylated to the reduction alumina treatment. (Section Undoub 2.1.6)tedly, and/or high-temperature formation of Pd–Al reduction species signifi- under high- cantlytemperature promotes the treatment selectivity (Figure towards3). the Due desired to small product, differences minimising in the amountsover hydrogena- and strength ◦ ◦ of acidic sites in red(400 C)-Pd/γ-Al2O3 and red(600 C)-Pd/γ-Al2O3, the acidity seems to not be crucial for explaining differences in catalytic behaviour of the two Pd samples. Therefore, after rejecting a possible catalytic role of the alumina acidity, structure sensitivity of MBE hydrogenation on Pd active sites, the effect of carbon and hydrogen dissolution (β- ◦ PdH and PdCx formation), the potential role of Al–Pd species at red(600 C)-Pd/γ-Al2O3 and hydrogen spillover were considered during interpretation of further results. Figure9 shows the influence of reaction conditions on the specific activity expressed as TOF (s−1) of Pd samples. Generally, an increase of the reaction pressure increases the activity of both Pd catalysts. On the other hand, the increase of temperature at each pressure decreases the activity in line with earlier studies on alkyne hydrogenation. They could be explained by different kinetics of carbon (from fragmented substrate molecules) and hydrogen dissolution during alkyne hydrogenation [42,43]. Due to the fact that C dissolution has to be preceded by MBY fragmentation, hydrogen dissolution occurs more favourably than C dissolution at lower temperatures. Moreover, hydrogen chemisorption on Pd is an exothermic process. It means that it begins less favourably with temperature increase [42,44]. In consequence, higher conversion and slightly lower MBE selectivity were observed at lower temperatures (Figures9–11). The volcanic relationship of activity versus temperature was detected only for the catalyst reduced at the higher temperature (Figure9). However, some activity fluctuations are also known in literature and explained by the coverage fluctuation and/or spatiotemporal heterogeneities, which are intrinsic to the reaction–diffusion problem and not necessarily caused by any dynamical surface effect from the catalyst (e.g., dynamical surface reconstruction) [45,46]. Hence, we concluded that pressure is the crucial parameter that benefits MBY hydrogenation and the changes in conversion with temperature may not follow this trend. Nevertheless, both Pd catalysts show spectacular differences in the products’ distribution (Figures 10 and 11). Although both 2-methyl-3-butyn-2-ol hydrogenation products are desired in the production of pharmaceutical intermediates (MBE) and fine chemicals manufacture (MBA), our research is focused mainly on 2-methyl-3-buten-2-ol produc- Catalysts 2021, 11, 501 14 of 19

tion. Based on the literature data [42–44], the selective alkyne to alkene hydrogenation on palladium catalyst is possible only when carbon (from fragmented substrate molecules) occupies interstitial lattice Pd sites and excludes H from populating the subsurface region and hence prevents total hydrogenation of substrate. As it is independent of the temper- ◦ ature and pressure, red(400 C)-Pd/γ-Al2O3 produces 2-methyl-2-butanol as the main ◦ product (Figure 10) in contrast to Pd/γ-Al2O3 reduced at 600 C (Figure 11), it could be ◦ assumed that the rate of C dissolution in red(400 C)-Pd/γ-Al2O3 is too low to affect MBE selectivity. However, the second significant factor, which should be taken into account, is hydrogen spillover from Pd to the Al2O3 support [42]. It was found that C≡C is only hydrogenated on palladium, whereas hydrogen on the alumina support hydrogenates C=C to the saturated product. The carbonaceous species accumulated on the γ-Al2O3 support act as bridges that accelerate the process and consequently greatly increase the undesirable C=C hydrogenation [42]. Moreover, in accordance with the literature data [47], high selectivity to MBA could suggest that hydrogenation of 2-methyl-3-butyn-2-ol occurs mainly at the edge-active sites, opposite to partial MBY hydrogenation, which occurs preferentially at the plane sites regardless of their crystallographic orientation. This phenomenon confirms structure sensitivity of MBY hydrogenation and it is in line with the results of DFT investigation for MBY hydrogenation [14]. DFT energetic description confirms that C≡C in MBY is activated, by strong interaction energies, preferentially by plane sites with respect to corner/edge atoms. However, further hydrogenation MBE to MBA requires the presence of low coordination sites like edges and corners [14]. Maybe the conclusions go too far, but it seems likely that edge-active sites of Pd are more sensitive to hydrogen subsurface diffusion and dissolution, which strongly enhance hydrogenation of 2-methyl-3-butyn-2-ol toward the fully hydrogenated product ◦ (MBA) [42,48]. Consequently, catalytic performance of red(600 C)-Pd/γ-Al2O3 could be affected by coexistence of three factors: competition between PdH and PdCx formation during the reaction, structure sensitivity, and hydrogen spillover to the alumina support. The desired MBE was formed as a major product of MBY hydrogenation on red(600 ◦C)- Pd/γ-Al2O3 over a wide range of pressures (Figure 11). However, it should be noticed that higher pressures (10 and 20 bar) promote saturated alcohol formation instead of unsaturated MBE, with a maximum selectivity to MBA of 42% at 65 ◦C and 20 bar. This high MBE selectivity indicates a significant role of Al in Pd–Al species in decreasing the Pdδ− surface concentration via electron transfer from Pd to Al and the decreasing amounts of hydrogen in bulk. Consequently, only surface adsorbed hydrogen could favour partial MBY hydrogenation and MBE formation [43]. It is in conformance with the studies concerning MBA gas-phase hydrogenation on Pd–Zn/Al2O3, where the hydrogen spillover increased surface hydrogen content enhanced MBE production over Pd−Zn/Al2O3 in comparison to Pd/ZnO with similar Pd–Zn alloy content [5]. Moreover, following the literature data [17,49], Pd–Al species with modified electronic properties in ◦ comparison to red(400 C)-Pd/γ-Al2O3 could lead to a weaker Pd–Alkyl bonding and hinderance of carbon penetration into the palladium lattice [42,50]. All of these factors contribute to the fact that the hydrogenation of 2-methyl-3-butyl-2-ol on Pd–Al species leads to the formation of partial hydrogenation product—2-methyl-3-buten-2-ol, oppo- ◦ site to red(400 C)-Pd/γ-Al2O3, where the production of the fully hydrogenated product (2-methyl-2-butanol) dominates. Although the highest MBE selectivity was achieved at 45 ◦C and 1 bar, we have determined the optimal pressure and temperature conditions (5 bar, 25 ◦C) for 2-methyl-3- buten-2-ol production, due to a higher TOF value in these conditions (Figure9). ◦ Therefore, the stability tests of red(600 C)-Pd/γ-Al2O3 were performed both at 5 bar, 25 ◦C and at 45 ◦C and 1 bar (Figures 12 and 13). Figures 12 and 13 show changes of conversion and selectivity to the desired product as a function of time of stream. The initial conversion at both conditions was found to be 41% but it decreased to 34% at 5 bar and 25 ◦C and 20% and at 1 bar and 45 ◦C after the first 30 min of reaction, which suggests that the Catalysts 2021, 11, 501 15 of 19

catalyst undergoes some equilibration in the initial phase of hydrogenation. Nevertheless, selectivity of the desired product (2-methyl-3-buten-2-ol) was found to be always 88% and 97%, dependent on the reaction conditions (Figures 12 and 13). All in all, our studies reveal a substantial modiﬁcation of the catalyst performance in relation to the reduction treatment. Undoubtedly, high-temperature reduction signiﬁcantly promotes the selectivity towards the desired product, minimising over hydrogenation. ◦ ◦ Although, comparison of red(400 C)-Pd/γ-Al2O3 and red(600 C)-Pd/γ-Al2O3 activity with literature data [5,8–16] is not easy due to different reaction conditions (e.g., mode, residence time, substrate concentration and H2/substrate ratio), our results seem promising with 97% selectivity to the vitamin A precursor (2-methyl-3-buten-2-ol) obtained at 45 ◦C and 1 bar.

3. Materials and Methods 3.1. Catalysts Preparation The 1 wt.% metal-loaded palladium catalyst was prepared by incipient wetness impregnation. Chlorine-free γ-alumina, Puralox SCCa (supplied by Sasol, Johannesburg, South Africa) (150–200 mesh and 196 m2/g (BET)), was used as a support and aqueous solution of Pd(NO3)2·2H2O (supplied by Chempur, Piekary Sl´ ˛askie,Poland) as a metal precursor. Impregnation was carried out using a rotary beaker with simultaneous heating (with a standard 200 W infrared lamp) for 24 h until complete evaporation of the solvent. Afterwards, the material was calcined at 500 ◦C under flowing air with a 50 cm3/min speed for 3 h and then cooled in the air to room temperature. For the next step, the catalyst was separated into 2 portions. The first portion was reduced with 10% H2/Ar in the flow ◦ reactor at 400 C for 3 h. The second part of the catalyst was exposed to 10% H2/Ar at 600 ◦C for 17 h. The obtained catalysts were labelled sequentially as red(400 ◦C)-Pd/γ- ◦ Al2O3 and red(600 C)-Pd/γ-Al2O3.

3.2. Catalysts Characterization 3.2.1. Physisorption The surface areas and porosities of our materials were measured with ASAP 2020 Chem. Instrument from Micromeritics (Norcross, GA, USA) BET (Brunauer-Emmett-Teller), t-Plot, BJH (Barret-Joyner-Halenda) and HK (Horwath-Kawazoe) methods were used with N2 as the adsorbate. Both γ-Al2O3 and 1 wt.% Pd/γ-Al2O3 after calcination were kept at 300 ◦C for 3 h in vacuum conditions to clean their surfaces before measurement of the adsorption isotherm at −196 ◦C.

3.2.2. Chemisorption Chemisorption measurements of CO using a conventional static method were carried out with an ASAP 2020 Chem. Instrument from Micromeritics (Norcross, GA, USA). ◦ The chemisorption measurements were performed with red(400 C)-Pd/γ-Al2O3 and ◦ red(600 C)-Pd/γ-Al2O3. In accordance with the literature data [51], the surface CO:Pd stoichiometry equals to 1.5:1.

3.2.3. X-ray Diffraction X-ray diffraction (XRD) measurements were done with a Rigaku-Denki instrument (Tokyo, Japan) equipped with Ni-ﬁltered copper lamp Cu Kα. Diffraction proﬁles were scanned in the range of 2θ = 20–90◦, step size of 0.008◦, and counting time 60 s/step.

3.2.4. Temperature Programmed Studies Temperature-programmed reduction (TPR) was run in a glass ﬂow system in an atmo- sphere of a mixture of 10% H2/Ar. A Gow-Mac thermal conductivity detector (Bethlehem, PA, USA) was used. The catalyst precursors were heated with a 10 ◦C/min ramp, started from −20 ◦C. Injections of known amounts of hydrogen into the ﬂow system were provided for calibration. Catalysts 2021, 11, 501 16 of 19

Temperature-programmed hydride decomposition (TPHD) was performed in a glass system equipped with independent gas lines, furnace, temperature controller and TCD detector. Freshly reduced catalyst was cooled down to 0 ◦C and then subjected to a ◦ ◦ temperature ramp (8 C/min) up to 150 C in the gas stream of 10% H2/Ar. The TCD detector allowed monitoring hydrogen concentration in the gas stream, which was related to the formation and decomposition of Pd hydride.

3.2.5. TEM Measurements ◦ Transmission electron microscopy investigations for red(400 C)-Pd/γ-Al2O3 and ◦ red(600 C)-Pd/γ-Al2O3 were performed with a JEOL JEM-100CXII electron microscope (Tokyo, Japan ) operated at an acceleration voltage of 100 keV. At least 200 particles were employed to compose the size distribution histograms. Before TEM measurements, the samples were dispersed in pure alcohol using an ultrasonic cleaner and putting a drop of this suspension onto carbon ﬁlms on copper grids.

3.2.6. XPS Measurements X-ray photoelectron spectroscopic (XPS) measurements were performed using a PHI 5000 VersaProbe (ULVAC-PHI, 2500 Hagisono, Chigasaki, Kanagawa, Japan) spectrometer with monochromatic Al Kα radiation (h = 1486.6 eV) from an X-ray source operating at 100- µm spot size, 25 W and 15 kV. High-resolution (HR) XPS spectra for the samples reduced ex-situ and in-situ by ﬂowing hydrogen (1.5 × 10−5 Torr, 200 ◦C, 1 h) were collected with the hemispherical analyser at the pass energy of 117.4 and the energy step size of 0.1 eV. The X-ray beam was incident at the sample surface at the angle of 45◦ with respect to the surface normal, and the analyser axis was located at 45◦ with respect to the surface. The CasaXPS software (Ver. 2.3.19, Casa Software Ltd., 26 Burford Crescent, Wilmslow, Cheshire, UK) was used to evaluate the XPS data. Deconvolution of all HR XPS spectra was performed using a Shirley background and a Gaussian peak shape with 30% Lorentzian character.

3.2.7. NH3-TPD Measurements

NH3 temperature-programmed desorption (NH3-TPD) was applied to measure the ◦ ◦ acid properties of the red(400 C)-Pd/γ-Al2O3 and red(600 C)-Pd/γ-Al2O3 catalyst samples. The runs were performed in a quartz tube reactor connected to a quadrupole mass spectrometer (HPR 60, Hiden, Warrington, UK), where 50 mg of the catalyst sample was used for each test. Prior to each measurement, the pre-reduced sample was treated with helium at 500 ◦C for 1 h to remove the adsorbed impurities. After cooling to 50 ◦C under a He ﬂow, the sample was exposed to 5% NH3-He mixture (50 mL/min) for 30 min, followed by purging with He for 20 min, and then heated to 500 ◦C by ramping at 10 ◦C/min under He ﬂow.

3.3. Catalytic Tests Hydrogenation of 2-methyl-3-butyn-2-ol (MBY) (98% pure from Sigma-Aldrich, Darm- stadt, Germany) was performed in flow mode in a ThalesNano H-Cube device (Budapest, Hungary). In this system, hydrogen is generated in situ via electrolysis of water, and then dried and mixed with the substrate solution (MBY in ethanol (99.8% pure from Avantor Performance Materials, Gliwice, Poland)). An HPLC pump provides the circulation of the reactants, and their pressure is established by the automatic system conjugated with multiple pressure sensors. The catalyst was placed in a 30 mm long stainless-steel cartridge (CatCart®30) with an inner diameter of 4 mm. Each experiment was performed using 0.1 g of the catalyst and a flow rate of 0.5 mL of solution per min (0.19 mmol MBY/min) and H2 flow 6 mL/min. Different conditions for pressure (1, 3, 5, 10 and 20 bar) and temperature (10, 25, 45 and 65 ◦C) were investigated. Catalysts 2021, 11, 501 17 of 19

Additionally, stability tests were performed at 25 ◦C, 5 bar, and at 45 ◦C, 1 bar with 6 mL/min of hydrogen for 300 min. In this case, samples were taken after 12, 30, 60, 120, 180, 240 and 300 min of the reaction. The obtained samples were analysed in a gas chromatographic system (Bruker 456-GC with FID detector, Billerica, MA, USA) equipped with BP5 column (30 m, 0.25 mm and 0.25 µm). A satisfactory carbon balance (within ~97%) was found for GC analyses. The re- producibility of results was fair—±10% (total activity) or good—±2% (products selectivity).

4. Conclusions

Our results clearly show that 1 wt.% Pd/γ-Al2O3 is active in the continuous hydrogenation of 2-methyl-3-butyn-2-ol toward products of importance in pharmaceutical ◦ and fine-chemical industries. The catalytic performance of red(400 C)-Pd/γ-Al2O3 and ◦ red(600 C)-Pd/γ-Al2O3 are affected by the coexistence of several related factors like competition between PdH and PdCx formation during the reaction, structure sensitivity, hydrogen spillover to the alumina support and the presence or absence of Pd–Al species. Pd–Al surface sites’ presence and the significant enrichment of the surface of the ◦ red(600 C)-Pd/γ-Al2O3 sample with partially oxidised aluminum explained the substantial increase in selectivity to MBE (vitamin A precursor) of this catalyst for the MBY hydrogenation. It is proposed that the Pd–Al species, which reveal unique electronic properties by decreasing the Pdδ− surface concentration via electron transfer from Pd to Al, leading to weaker Pd–Alkyl bonding, additionally assisted by the hydrogen spillover, are the sites of improved semi-hydrogenation. We also demonstrated the general trend in the catalytic performance of 1 wt.% Pd/γ- Al2O3. Pressure is the crucial parameter that benefits MBY hydrogenation and the changes in conversion with temperature may not follow this trend. The increase in conversion leads to decreased selectivity to 2-methyl-3-buten-2-ol at the expense of increasing the selectivity to 2-methyl-2-butanol. Additionally, it was evident that low pressure and temperature values are preferred to synthesise vitamin A precursor. In conclusion, utilising a highly reduced Pd/γ-Al2O3 catalyst can be a powerful ap- proach for the pharmaceutical and the fine chemical industries to enhance the continuous- flow synthesis of 2-methyl-3-buten-2-ol, which can facilitate reaching a more environmen- tally friendly production.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/catal11040501/s1, Scheme S1: Scheme of vitamin A synthesis with 2-methyl-3-buten-2-ol as one ◦ of the substrate, Figure S1: HRTEM images of red(400 C)-Pd/γ-Al2O3, Figure S2: HRTEM images of ◦ ◦ red(600 C)-Pd/γ-Al2O3, Figure S3: SEM-EDS analysis of red(400 C)-Pd/γ-Al2O3, Figure S4: TEM- ◦ ◦ EDS analysis of red(400 C)-Pd/γ-Al2O3, Figure S5: TEM-EDS analysis of red(600 C)-Pd/γ-Al2O3, ◦ Figure S6: O1s + Pd3p3/2 XPS region of red(400 C)-Pd/γ-Al2O3, Figure S7: O1s + Pd3p3/2 XPS ◦ ◦ region of red(600 C)-Pd/γ-Al2O3, Figure S8: C 1s XPS region of red(400 C)-Pd/γ-Al2O3, Figure S9: ◦ C 1s XPS region of red(600 C)-Pd/γ-Al2O3. Author Contributions: Conceptualization, A.S.;´ Methodology, A.S.,´ I.S.P., K.M. and D.L.; Formal Analysis, A.S.;´ Investigation, A.J.F.-R., B.Z., E.K., I.S.P., M.K., K.M. and D.L.; Data Curation, A.S.;´ Writing—original draft preparation, A.J.F.-R., B.Z. and A.S.;´ Writing—review and editing, A.J.F.-R. and A.S.;´ Supervision, A.S.;´ Project Administration, A.S.;´ Funding Acquisition, A.S.´ All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Science Centre in Poland within OPUS 17 Project No. 2019/33/B/ST5/01271. Data Availability Statement: Data is contained within the article or Supplementary Materials. Acknowledgments: This work was partially sponsored by National Science Centre in Poland within projects UMO-2019/33/B/ST5/01271. Conﬂicts of Interest: The authors declare no conﬂict of interest. Catalysts 2021, 11, 501 18 of 19

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