Highly Active Catalysts for the Dehydration of Isopropanol

catalysts

Article Highly Active Catalysts for the Dehydration of Isopropanol

Martyna Murat * , ZdenˇekTišler , Josef Šimek and José M. Hidalgo-Herrador Unipetrol VýzkumnˇeVzdˇelávací Centrum, a.s., 400 01 Ústí nad Labem, Revoluˇcní 1521/84, Czech Republic; [email protected] (Z.T.); [email protected] (J.Š.); [email protected] (J.M.H.-H.) * Correspondence: [email protected]; Tel.: +420-471-122-235

Received: 28 May 2020; Accepted: 24 June 2020; Published: 26 June 2020

Abstract: Due to the high costs and low selectivity associated with the production of propylene, new routes for its synthesis are being sought. Dehydration has been widely investigated in this field, but, thus far, no study has produced efficient results for isopropanol. Vanadium-zirconia catalysts have been shown to be effective for the dehydration of ethanol. Therefore, we investigated the activity of such catalysts in the dehydration of isopropanol. The catalysts were synthetized on a SBA-15 base, supplemented with zirconia or combined zirconia and vanadium. Tests were conducted in a continuous flow reactor at 150–300 ◦C. Samples were analyzed using a gas chromatograph. The most active catalyst showed 96% conversion with 100% selectivity to propylene. XRD, SEM and Raman spectroscopy analyses revealed that as the vanadium content increases, the pore size of the catalyst decreases and both isopropanol conversion and propylene selectivity are reduced. Thus, without the addition of vanadium, the Zr-SBA-15 catalyst appears to be suitable for the dehydration of isopropanol to propylene.

Keywords: isopropanol; propylene; dehydration; catalysis; SBA-15; vanadium; zirconia

1. Introduction Propylene is the second most-used raw material in plastics processing and the global demand for it is expected to reach 140 MMT in 2020 [1]. It is utilized in the production of chemicals with a wide range of applications, including polypropylene, phenol, propylene glycol and aldehydes. The biggest producers of propylene are refineries, where it is produced by the steam cracking or fluid catalytic cracking (FCC) of higher hydrocarbons [2]. However, these processes are dependent on crude oil, are costly and produce propylene with low selectivity. Consequently, researchers are looking for ways of obtaining good yields of highly selective propylene at a lower cost and with green chemistry being a hot and necessary topic, in an environmentally friendly way. With green chemistry being necessary and very popular nowadays, isopropyl alcohol that can be produced by bacteria is considered a good potential precursor for propylene production by selective dehydration [3]. Unfortunately, there has been no satisfying research in this field [4]. The dehydration of isopropanol presents several environmental and strategic advantages over classical steam cracking. The development of cost-effective, environmentally friendly propanol fermentation processes, which are now being developed, may not only satisfy the direct demand for isopropanol but also its production as a precursor of propylene [2,5]. Products from dehydration depend on the given reaction conditions and chosen catalyst, which in this particular case, are the metal particle size and composition of the carrier [6]. Specifically, supported vanadium-based catalysts have been found to be active in promoting the dehydration of ethanol to ethylene [7]. These catalysts resulted in being selective to the dehydration reaction to ethylene, when mainly vanadium oxide monomeric species were present on a SBA-15 support [8].

Catalysts 2020, 10, 719; doi:10.3390/catal10060719 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 719 2 of 11

Another important metal used for the dehydration of isopropanol is zirconium oxide, which increases the acidity of the reaction; however, it has not previously been set together with SBA-15 [9,10]. These promising results, together with the fact that isopropanol and ethanol have similar properties, have led us to examine the activity of V/Zr-SBA-15 catalysts in isopropanol dehydration. The SBA-15 materials included two diﬀerent concentrations of zirconium in their structure and vanadium on their surface. We show that some of the tested catalysts appear to be suitable for the isopropanol dehydration reaction, achieving high activity in the conversion of isopropanol to propylene.

2. Results and Discussion

2.1. Characterization of the Catalysts Zirconium-loaded zeolites, as well as the base materials studied in this work, are mesoporous solids. The goal of the catalysts’ synthesis was to obtain zeolites containing 3% or 5% of zirconium and additionally 1% of vanadium. The results of the ICP analysis conﬁrmed that the content of zirconium and vanadium in the samples was similar to those assumed during the synthesis of these catalysts. The compositions of the support and ﬁnal catalysts are shown in Table1.

Table 1. Chemical Zr and V (wt %) composition by ICP-OES of all the synthesized materials.

Assumed Chemical Conﬁrmed Chemical Catalyst Composition (wt %) Composition (wt %) Zr V Zr V SBA-15 0.00 0.00 0.00 0.00 Zr-SBA-15 I 3.00 0.00 2.95 0.00 Zr-SBA-15 II 5.00 0.00 5.49 0.00 1%V/Zr-SBA-15 I 3.00 1.00 2.81 0.88 1%V/Zr-SBA-15 II 5.00 1.00 5.02 0.80

The textural properties of all the solids were determined by mercury porosimetry and the speciﬁc surface area by nitrogen physisorption (Table2). All materials were porous, having a large total 2 1 intrusion volume and a high BET speciﬁc surface area (550–710 m g− ). The majority of the intrusion volume was attributable to macropores, typically 70% to 80% and a smaller proportion of 20% to 30% for mesopores of 3 to 50 nm.

Table 2. Textural properties of fresh support and catalysts.

Intrusion Volume (mL/g) Median Pore Sample SBET (m2/g) Total Mesopores (3–50 nm) Diameter (nm) SBA-15 6.12 0.77 4.5 743.1 Zr-SBA-15 I 5.78 0.99 6.4 787.4 Zr-SBA-15 II 4.77 0.99 7.6 645.3 1%V/Zr-SBA-15 I 2.73 0.79 5.8 721.1 1%V/Zr-SBA-15 II 3.29 0.88 6.3 632.9

By introducing zircon into the structure, the mesopore volume increased by 30% and the median pore diameter increased from 4.5 to 6.4 nm for 3% of Zr and to 7.6 nm for the carrier with 5% of Zr (Table2, Figure1). This is the result of interactions occurring during the milling process that are used to produce catalysts, such as the dehydroxylation of zeolite, the reaction between external silanol groups and the zirconium salt precursor and ultimately the partial blocking of mesopores in zeolite by ZrO2 particles. Catalysts 2020, 10, 719 3 of 11

Figure 1. Mercury porosimetry analyses. Pore diameter and pore volume with a given diameter for the fresh catalysts.

In the case of adding vanadium by impregnation, the volume of pores, their diameter and the speciﬁc surface area of SBET decreased. The incorporation of Zr and V into the support reduces its surface area by approximately 30% (Table2). We think that this is due to the formation of VO x layers in the pores and surface of the support, as well as the formation of small fragments resulting from the impregnation and subsequent heat treatment of the catalysts. In addition, the volume and diameter of the mesopores decreased by 10% to 20% with impregnation; lower volumes were observed for the 1%V/Zr-SBA-15 II catalyst. The presence of macropores, along with a shorter length of mesoporous channels, improved the throughput of the reactants to active centers and the removal of products from the catalyst. In the case of the used catalysts (Table3), the results of the texture analysis showed that the surface area of the catalysts dropped by about 10% after tests.

Table 3. Textural properties of used supports and catalysts.

Intrusion Volume (mL/g) Median Pore 2 Sample Macropores SBET (m /g) Mesopores Diameter Total (50–12,000 (3–50 nm) (nm) nm) Zr-SBA-15 I 2.41 0.81 0.85 6.8 708.2 Zr-SBA-15 II 2.55 0.89 0.94 6.4 626.1 1%V/Zr-SBA-15 I 1.98 0.68 0.68 5.8 640.7 1%V/Zr-SBA-15 II 1.89 0.73 0.52 5.3 564.5

The mesoporous structure of the SBA-15 was confirmed by X-ray diffraction (XRD) at low angles along with scanning electron microscopy (SEM). The obtained diffractograms (Figure2) showed characteristic diffraction lines (100), (110) and (200), which were less visible in vanadium catalysts. Catalysts 2020, 10, 719 4 of 11 Catalysts 2020, 10, x FOR PEER REVIEW 4 of 11

(a)

(b)

Figure 2. SmallFigure angle2. Small XRD angle di XRDffraction diffraction pattern pattern of of Zr-SBA-15 Zr‐SBA‐15 supports and and vanadium vanadium catalysts: catalysts: fresh fresh (a) (a) and used (b). and used (b). The mesoporous SBA‐15 structure in the vanadium catalysts was also confirmed by SEM. The The mesoporousSBA‐15 and Zr‐SBA SBA-15‐15 supports structure (Figure in 3) the showed vanadium a well‐developed catalysts mesoporous was also structure confirmed very by SEM. The SBA-15similar and to Zr-SBA-15 the vanadium supports catalysts (Figure (Figure 4).3 The) showed ordered aSBA well-developed‐15 structure with mesoporouscylindrical hexagonal structure very similar to thepores vanadium can be clearly catalysts distinguished (Figure in4 ).the The SEM ordered images SBA-15of all samples. structure A greater with share cylindrical of small hexagonal pores can be clearly distinguished in the SEM images of all samples. A greater share of small fragments produced during the impregnation and heat treatment of the catalysts was found. Embedding zircon into the structure caused the formation of particles of different thicknesses in the direction of the longitudinal axis of the pores. While the SBA-15 particles presented a thickness of 250–350 nm, Zr-SBA-15 I had a thickness of 150–200 nm and Zr-SBA-15 II was slightly thinner at about 130 nm. The shorter channel length was beneficial due to the diffusion of reactants into the pores and products from the pores, as measured in the resulting tests. Catalysts 2020, 10, x FOR PEER REVIEW 5 of 11 Catalysts 2020, 10, x FOR PEER REVIEW 5 of 11 fragments produced during the impregnation and heat treatment of the catalysts was found. Embeddingfragments produced zircon into during the structure the impregnation caused the formationand heat oftreatment particles ofof differentthe catalysts thicknesses was found. in the Embeddingdirection of zirconthe longitudinal into the structure axis of thecaused pores. the While formation the SBA of particles‐15 particles of different presented thicknesses a thickness in the of direction250–350 nm, of the Zr ‐longitudinalSBA‐15 I had axis a thickness of the pores. of 150–200 While thenm SBAand ‐15Zr ‐particlesSBA‐15 II presented was slightly a thickness thinner ofat about250–350 130 nm, nm. Zr The‐SBA shorter‐15 I had channel a thickness length ofwas 150–200 beneficial nm dueand toZr ‐theSBA diffusion‐15 II was of slightlyreactants thinner into the at Catalystsaboutpores and1302020 products,nm.10, 719 The shorterfrom the channel pores, aslength measured was beneficial in the resulting due to tests. the diffusion of reactants into5 of the 11 pores and products from the pores, as measured in the resulting tests.

(a) (b) (c) (a) (b) (c) Figure 3. SEM images of SBA‐15 (a), Zr‐SBA‐15 I (b) and Zr‐SBA‐15 II (c) catalysts (all images × Figure100,000).Figure 3. SEM imagesimages of of SBA-15 SBA‐15 (a ),(a Zr-SBA-15), Zr‐SBA I‐15 (b )I and (b) Zr-SBA-15and Zr‐SBA II (‐c15) catalysts II (c) catalysts (all images (all images100,000). × × 100,000).

(a) (b) (a) (b) Figure 4.4. SEMSEM images images of of 1%V 1%V/Zr/Zr-SBA-15‐SBA‐ I15 (a )I and (a) 1%Vand /Zr-SBA-151%V/Zr‐SBA II (‐b15) catalysts II (b) catalysts (all images (all images100,000). × × Figure100,000). 4. SEM images of 1%V/Zr‐SBA‐15 I (a) and 1%V/Zr‐SBA‐15 II (b) catalysts (all images × The100,000). elemental analysis results suggest that the presence of carbon deposits in mesopores may be theThe cause elemental of their analysis decreasing results volume. suggest Mercury that the porosimetry presence of showed carbon a deposits similar tendency in mesopores (Figure may5), showingbe theThe cause aelemental decrease of their inanalysis decreasing the volume results volume. of suggest the mesoporesMercury that the porosimetry in presence the order ofshowed of carbon 10%–15%. a depositssimilar However, tendency in mesopores XRD (Figure analysis may 5), wasshowingbe the not cause able a decrease toof showtheir in decreasing changes the volume in volume. the of mesoporous the mesoporesMercury structure porosimetry in the order of the showed of used 10%–15%. catalysts a similar However, (Figure tendency2). XRD The (Figure analysis Raman 5), spectroscopyshowingwas not able a decrease to also show did in changes notthe detectvolume in the the of mesoporous carbon the mesopores deposits, structure in which the orderof should the ofused 10%–15%. be catalysts visible However, as (Figure a characteristic 2). XRD The analysis Raman band 1 aroundspectroscopywas not 1580able to cm also show− did[11 changes ].not As detect the in results thethe mesoporous carbon of the deposits, elemental structure which analysis of should the show,used be catalysts visible the amount as (Figure a characteristic of carbon 2). The used Raman band in thearoundspectroscopy catalysts 1580 is cmalso not−1 did[11]. large, not As typicallydetect the results the about carbon of the 1%. deposits, elemental The used which analysis Zr-SBA-15 should show, be I and visiblethe 1%V amount as/Zr-SBA-15 a characteristic of carbon I catalysts used band in containedaroundthe catalysts 1580 1.08% iscm not−1 and [11]. large, 1.12 As typically wtthe % results of carbon,about of the 1%. respectively, elemental The used analysis Zr while‐SBA the show,‐15 Zr-SBA-15I and the 1%V/Zramount II and‐ SBAof Vcarbon‐/15Zr-SBA-15 I catalysts used in II catalyststhecontained catalysts contained 1.08% is not and lesslarge, 1.12 carbon typicallywt % (0.66% of carbon,about and 1%. 0.83%, respectively, The respectively). used Zrwhile‐SBA Carbonthe‐15 ZrI and‐SBA was 1%V/Zr‐ present15 II and‐SBA in V/Zr lower‐15 ‐ISBA amountscatalysts‐15 II forcatalystscontained catalysts contained 1.08% with and higher less 1.12 carbon acidity. wt % (0.66% of carbon, and 0.83%, respectively, respectively). while Carbonthe Zr‐ SBAwas‐ present15 II and in V/Zrlower‐SBA amounts‐15 II forcatalysts catalysts contained with higher less carbon acidity. (0.66% and 0.83%, respectively). Carbon was present in lower amounts for catalysts with higher acidity. CatalystsCatalysts 20202020,, 1010,, x 719 FOR PEER REVIEW 6 6of of 11 11

Figure 5. Mercury porosimetry analyses. Pore diameter and pore volume with a given diameter for theFigure used 5. catalysts.Mercury porosimetry analyses. Pore diameter and pore volume with a given diameter for the used catalysts. The number of acidic centers and their strength, was determined by the temperature‐ The number of acidic centers and their strength, was determined by the temperature-programmable programmable desorption of ammonia (NH3‐TPD, Figure 6, Table 4). The SBA‐15 solid contained desorption of ammonia (NH -TPD, Figure6, Table4). The SBA-15 solid contained mainly weak acidic mainly weak acidic centers. 3The addition of zirconium to the structure resulted in an increment in centers. The addition of zirconium to the structure resulted in an increment in the number of strong the number of strong acidic centers. This proportion increased with the increasing content of acidic centers. This proportion increased with the increasing content of zirconium in the structure. zirconium in the structure. Although the addition of vanadium caused an increase in active centers, Although the addition of vanadium caused an increase in active centers, it did not improve the catalyst it did not improve the catalyst activity toward propylene production. This is probably due to the redoxactivity VOx toward activity propylene proposed production. by Beck et Thisal. [12]. is probably due to the redox VOx activity proposed by Beck et al. [12].

FigureFigure 6. 6. TemperatureTemperature-programmable‐programmable desorption desorption of of ammonia ammonia (NH3 (NH3-TPD)‐TPD) profiles proﬁles of catalysts.

Due to the low content of vanadium in the catalysts, the changes in the temperature‐ programmed reduction (Figure 7) were not significant between the two vanadium catalysts. There was only a slight shift of the reduction peak at 462 °C for the V/Zr‐SBA‐15 I catalyst and at 470 °C for the V/Zr‐SBA‐15 II catalyst. Catalysts 2020, 10, 719 7 of 11

Table 4. Properties of acid sites determined by NH3-TPD. LT—low temperature sites; HT—high temperature sites.

cSUM Tmax1 cLT Population Tmax2 cHT Population Sample (µmol/g) (◦C) (µmol/g) LT (%) (◦C) (µmol/g) HT (%) SBA-15 1169 114 1118 95.6 311 52 4.4 Zr-SBA-15 I 1051 113 608 57.8 220 443 42.2 Zr-SBA-15 II 1092 112 509 46.6 227 584 53.4 Catalysts 2020, 10, x FOR PEER REVIEW 7 of 11 1%V/Zr-SBA-15 I 1148 113 521 45.4 242 627 54.6 1%V/Zr-SBA-15 II 1408 114 625 44.4 247 784 55.6

Due to the low content of vanadium in the catalysts, the changes in the temperature-programmed reduction (Figure7) were not signiﬁcant between the two vanadium catalysts. There was only a slight shift of the reduction peak at 462 ◦C for the V/Zr-SBA-15 I catalyst and at 470 ◦C for the V/Zr-SBA-15 II catalyst. Catalysts 2020, 10, x FOR PEER REVIEW 7 of 11

Figure 7. H2‐TPR patterns for all catalysts.

Table 4. Properties of acid sites determined by NH3‐TPD. LT—low temperature sites; HT—high temperature sites.

cSUM Tmax cLT Population Tmax cHT Population Sample (μmol/g) 1 (°C) (μmol/g) LT (%) 2 (°C) (μmol/g) HT (%) SBA‐15 1169 114 1118 95.6 311 52 4.4 Zr‐SBA‐15 I 1051 113 608 57.8 220 443 42.2

Zr‐SBA‐15 II 1092Figure 7.112H2-TPR 509 patterns for46.6 all catalysts.227 584 53.4 1%V/Zr‐SBA‐15 I 1148 Figure113 7. H2‐TPR521 patterns for 45.4all catalysts. 242 627 54.6 1%V/Zr‐SBA‐15 II 1408 114 625 44.4 247 784 55.6 2.2. Catalytic TestsTable 4. Properties of acid sites determined by NH3‐TPD. LT—low temperature sites; HT—high temperature sites. For all2.2. catalytic Catalytic Tests tests, the conversion of isopropanol increased with increasing temperature. cSUM Tmax cLT Population Tmax cHT Population Three samplesForSample were all catalytic taken tests, at each the conversion temperature, of isopropanol at an hourly increased interval, with increasing for analysis temperature. with gaseous (μmol/g) 1 (°C) (μmol/g) LT (%) 2 (°C) (μmol/g) HT (%) chromatographyThree samples and theirwere taken results at each were temperature, almost the at same.an hourly A interval, high increment for analysis in with the gaseous conversion was chromatographySBA‐15 and their1169 results 114were almost1118 the same.95.6 A high increment311 in52 the conversion4.4 was Zr‐SBA‐15 I 1051 113 608 57.8 220 443 42.2 found in allfound catalysts in all catalysts in the in range the range of 200–250 of 200–250◦ C°C (Figures(Figures 88 and and 9).9 ). Zr‐SBA‐15 II 1092 112 509 46.6 227 584 53.4 1%V/Zr‐SBA‐15 I 1148 113 521 45.4 242 627 54.6 1%V/Zr‐SBA‐15 II 1408 114 625 44.4 247 784 55.6

2.2. Catalytic Tests For all catalytic tests, the conversion of isopropanol increased with increasing temperature. Three samples were taken at each temperature, at an hourly interval, for analysis with gaseous chromatography and their results were almost the same. A high increment in the conversion was found in all catalysts in the range of 200–250 °C (Figures 8 and 9).

(a) (b)

Figure 8. Isopropanol conversion and selectivity to propene and acetone for the Zr‐SBA‐15 I (a) and Figure 8. IsopropanolZr‐SBA‐15 II (b) conversion catalysts. and selectivity to propene and acetone for the Zr-SBA-15 I (a) and Zr-SBA-15 II (b) catalysts.

(a) (b)

Figure 8. Isopropanol conversion and selectivity to propene and acetone for the Zr‐SBA‐15 I (a) and Zr‐SBA‐15 II (b) catalysts. Catalysts 2020, 10, 719 8 of 11 Catalysts 2020, 10, x FOR PEER REVIEW 8 of 11

(a) (b)

Figure 9. IsopropanolIsopropanol conversion conversion and and selectivity selectivity to to propene propene and and acetone acetone for for the the 1%V/Zr 1%V/‐Zr-SBA-15SBA‐15 I (a I) (anda) and 1%V/Zr 1%V/‐Zr-SBA-15SBA‐15 II (b II) ( catalysts.b) catalysts.

The exceptionexception waswas SBA-15, SBA‐15, for for which which the the conversion conversion was was very very low low and and reached reached only only 5.65% 5.65% at the at highestthe highest point. point. In the In case the ofcase the of Zr-SBA-15 the Zr‐SBA II material,‐15 II material, the conversion the conversion was 96 wtwas %. 96 The wt main%. The reaction main productreaction wasproduct propylene. was propylene. Despite a Despite variable a conversion variable conversion initially, the initially, selectivity the toselectivity propylene to was propylene almost maintainedwas almost maintained at 100 wt %. at The 100 highest wt %. The selectivity highest was selectivity noted for was the noted reaction for the catalyzed reaction by catalyzed Zr-SBA-15 by I andZr‐SBA Zr-SBA-15‐15 I and II. Zr‐SBA‐15 II. However, the use of vanadium supported on the Zr-SBA-15Zr‐SBA‐15 materials resulted in an increase in the productionproduction ofof side side product product (acetone (acetone and and propyl propyl ether). ether). The The lowest lowest conversions, conversions, selectivity selectivity and and the largestthe largest amount amount of side of side products products were were observed observed in the in 1%Vthe 1%V/Zr/Zr-SBA-15‐SBA II‐15 catalyzed II catalyzed reaction. reaction. The reaction reaction achieved achieved a total a total conversion conversion of 96% of 96% with witha selectivity a selectivity to propylene to propylene of 100%. of Similar 100%. Similarresults were results presented were presented by Bedia et by al., Bedia obtaining et al., a obtaining conversion a of conversion 95% at full of selectivity 95% at full to propylene, selectivity toalso propylene, at 275 °C [13]. also However, at 275 ◦C[ in13 his]. However,research he in used his researchstrong H he3PO used4 acid strong and used H3PO 11,0004 acid times and lower used −1 1 11,000WHSV times (0.001 lower h ), which WHSV reduces (0.001 hthe− ), amount which reducesof product the obtained amount ofand product reduces obtained the industrial and reduces utility theof this industrial path of utility propylene of this synthesis. path of propylene In turn, Larmier synthesis. et Inal. turn, decided Larmier to lower et al. the decided temperature to lower and the −1 1 temperatureincrease WHSV and to increase 2.8 h , which WHSV resulted to 2.8 h −in, an which increased resulted conversion in an increased (40%) while conversion maintaining (40%) while100% maintainingselectivity [14]. 100% In selectivityresearch conducted [14]. In research on γ‐Al conducted2O3 catalysts, on γ they-Al2 Ofocused3 catalysts, mainly they on focused studying mainly the onthermodynamics studying the thermodynamicsof the reaction and of concluded the reaction that and the key concluded effect on that selectivity the key is e temperature,ffect on selectivity while iscatalytic temperature, activity while is determined catalytic activityby the morphology is determined of the by thecatalyst. morphology This was of partly the catalyst. confirmed This in wasour partlyresearch, confirmed as we also in noticed our research, the influence as we of also catalyst noticed geometry the influence on the ofavailability catalyst geometryof active centers; on the availabilityhowever, regardless of active centers;of the catalyst, however, the regardless conversion of also the catalyst,increased the with conversion increasing also temperature. increased with Foo −1 1 increasinget al. increased temperature. the temperature Foo et al. range increased (286–320 the temperature °C) and WHSV range (0.95 (286–320 h ); using◦C) and perovskite WHSV (0.95catalysts, h− ); usingthe substrate perovskite was catalysts, mostly dehydrogenated, the substrate was over mostly dehydrated dehydrogenated, and as a result over dehydrated the main reaction and as product a result thewas main acetone reaction [9]. Although product wasBlanco acetone‐Bonilla [9]. et Although al. obtained Blanco-Bonilla a high selectivity et al. obtained(92%) for a a high higher selectivity WHSV −1 1 (92%)(23.6 h for), it a resulted higher WHSV in poor (23.6 conversion h− ), it (15%) resulted [10]. in For poor the first conversion time, Zr (15%)‐SBA‐15 [10 materials]. For the were first tested time, Zr-SBA-15in isopropanol materials dehydration, were tested resulting in isopropanol in the highest dehydration, conversion resulting found in thethat highest we are conversion aware of foundin the thatliterature we are of aware heterogeneous of in the literature catalysis. of In heterogeneous addition, due catalysis. to the use In addition, of a much due bigger to the usestainless of a much‐steel biggerreactor, stainless-steel one gram of catalyst reactor, onewas gramused compared of catalyst to was the used few compared milligrams to used the few by other milligrams authors used and by a otherpotential authors industrial and a application potential industrial was found. application was found.

3. Materials and Methods

3.1. Preparation of the Catalysts SBA-15SBA‐15 mesoporous mesoporous material material was was synthesized synthesized according according to the method to the described method by described Zukal et byal. ® Zukal[11]. Amphiphilic et al. [11]. triblock Amphiphilic copolymer triblock Pluronic copolymer® P123 (Sigma Pluronic‐Aldrich,P123 Schnelldorf, (Sigma-Aldrich, Germany), Schnelldorf, which served as a structure model, was immersed in the acidic reaction medium obtained using an aqueous solution of hydrochloric acid (35% by weight, Lach‐Ner s.r.o., Neratovice, Czech Republic). Silica was Catalysts 2020, 10, 719 9 of 11

Germany), which served as a structure model, was immersed in the acidic reaction medium obtained using an aqueous solution of hydrochloric acid (35% by weight, Lach-Ner s.r.o., Neratovice, Czech Republic). Silica was provided by the addition of tetraethyl orthosilicate (TEOS, Sigma-Aldrich, Schnelldorf, Germany). A reaction mixture with a molar ratio of 1 TEOS:0.017 P123:6.1 HCl:197 H2O was prepared at 35 ◦C and after 24 h heated at 95 ◦C for 66 h under static conditions. The product was obtained by filtration, washing with distilled water and drying at 80 ◦C overnight. To remove 1 the template, the calcination was carried out in air at 540 ◦C for 8 h (ramp temperature 1 ◦C min− ). The SBA-15 was enhanced by supplementation with zirconia (Zr-SBA-15) or with zirconia and vanadium (1%V/Zr-SBA-15). The zirconia catalysts were prepared in two variants, varying by the percentage of the added zirconia enrichment (ZrOCl2, Lach-Ner ). The zirconium was added in the appropriate amount for obtaining two catalysts containing 3 wt % and 6 wt % of zirconium, respectively, using the method developed by Zukal et al. [15]. To obtain the vanadium catalysts, the support was impregnated with aqueous solution with a molar ratio of 1 H2O2:4 H2O and the appropriate amount of ammonium metavanadate (Sigma-Aldrich, Schnelldorf, Germany) for a final catalyst containing 1 wt % of vanadium. The final products were obtained by drying overnight in a stream of air at 120 ◦C and calcined in air flow at 450 ◦C for 6 h. Five catalysts were synthesized, one pure SBA-15 solid, two Zr-SBA-15 named Zr-SBA-15 I (3 wt % of Zr) and Zr-SBA-15 II (5 wt % of Zr) and two V(1 wt %)/Zr-SBA-15 named 1%V/Zr-SBA-15 I and 1%V/Zr-SBA-15 II, which contained the Zr-SBA-15 I and Zr-SBA-15 II supports, respectively.

3.2. Characterization of the Catalysts The composition of the catalysts was verified by using an ICP-OES Agilent 725 (Agilent Technologies Inc., Santa Clara, USA). Before analysis, 500 mg of the sample was dissolved in 10 mL of H2SO4 (1:1). The undissolved residue from acid leaching was filtered with a paper filter, burned and silicon oxide was removed by HF. Residues from HF leaching were melted with KHSO4, dissolved in water and added to filtrate. Finally, the sample solution was introduced into a volumetric flask and measured. XRD was used to determine the crystallographic structure of the catalysts. The samples were analyzed using a D8 Advance Eco (Bruker, Billerica, USA), applying Cu Kα radiation (λ = 1.5406 Å), 0.5 s step time and 0.02◦ step size. Data collected in the 2θ range from 0.5◦ to 70◦ were analyzed using Diffrac.Eva V 4.1.1 (Bruker, Billerica, USA) software with the Powder Diffraction File database (PDF 4 + 2018, International Centre for Diffraction Data). To assess the morphology of the catalysts, a scanning electron microscope (JEOL JSM-7500F) with cold emission to the cathode field at 1 kV in GB high mode was used. The specific surface area (BET) of the solids was determined using N2 adsorption/desorption at 196 C with an Autosorb iQ (Quantachrome Instruments, Boynton Beach, USA). The solids were − ◦ dried before each analysis in a glass cell at 200 ◦C under vacuum for 16 h. Mercury porosimetry measurements were performed on a Micromeritics AutoPore IV 9510. All samples were dried before the analysis in a glass cell at 200 ◦C (under vacuum for 16 h). NH3 temperature-programmed desorption (TPD) was used to characterize the acid-base properties of the catalysts. An Autochem 2950 HP (Micromeritics Instrument Corporation, Norcross, USA) 1 was used. For each analysis, 100 mg of sample was pretreated in He to 500 ◦C (10 ◦C min− ). In this 1 case, the sample was cooled to 50 ◦C before being saturated with ammonia (25 mL min− of 10 vol% 1 NH3/He for 30 min). Then, helium was used (25 mL min− ) to remove physically/weakly adsorbed ammonia with a constant baseline (60 min). After that, the temperature was increased to 500 ◦C 1 (15 ◦C min− ). The NH3 concentration was monitored using a TCD detector. The vanadium species reducibility was determined by H2-TPR (AutoChem 2950 HP, Micromeritics Instrument Corporation, Norcross, USA). A total of 100 mg of the sample (50 mg for V/Zr-SBA-15) was treated in oxygen flow (300 ◦C (2 h)) before measurement. The reduction test was performed Catalysts 2020, 10, 719 10 of 11

1 at 40–700 ◦C (10 ◦C min− ) under hydrogen (5 vol % H2 in Ar). The hydrogen concentration was monitored using a TCD detector. The carbon content was determined by elemental analysis of the solid powder using a Flash2000 (Thermo Fisher Scientiﬁc, Waltham, USA). The sample (2.5 mg) was mixed with oxidant (V2O5) and burned in oxygen ﬂow at 950 ◦C. The resulting gas was reduced to CO, separated with a chromatic column and detected by the TCD detector. The measurement was performed three times to assure its repeatability. The carbon content was also determined by Raman spectral analysis. For this analysis, a DXR-Smart Raman (Thermo Fisher Scientific, Waltham, USA) with an intelligent exciting laser (Thermo Fisher Scientific, Waltham, USA) was used, using a laser wavelength of 780 nm. Powdered samples were thermally activated 1 in a vacuum and placed in a Pyrex cuvette. Spectra analysis was performed at a resolution of 2 cm− ; 400 scans were made for each sample and for each scan, the integration time was 5 s.

3.3. Catalytic Tests A continuous flow reactor with a set of ten thermocouples in the catalytic bed was used. The inner diameter of the reactor was 17 mm. The net diameter of the catalyst bed was 10.7 mm and the total volume was 60 mL. A minimal amount of quartz wool was placed at the bottom of the reactor. Then, a mixture (60 mL) of catalyst (1 g) and SiC (enough to fill the 60 mL of the total final volume) were 1 introduced into the reactor. The catalyst was activated at 300 ◦C under nitrogen atmosphere (50 NL h− ) 1 for 2 h. The reaction was carried out with a total helium gas flow rate of 50 NL h− and an isopropanol 1 feed flow rate of 11 g h− . Under these conditions, the reaction was performed at different temperatures (150, 175, 200, 225, 250, 275 and 300 ◦C). At each temperature, the test was performed for 3 h and three samples were obtained (each one after 1 h). A sampling system of the reactor consisted of two collectors. A liquid product was cooled by water at room temperature (20 ◦C) in the first collector and at 0 ◦C in the second collector. Liquid samples were immediately analyzed by gas chromatograph (GC) in the laboratory. An online GC-FID equipped with a HP-Pona column (50 m 0.25 mm, He as × the carrier gas) was used. The general conditions in the GC were the same as those used for ASTM D6730 for a GC, with the oven temperature being maintained at 100 ◦C for the duration of the analysis and the products of the isopropanol dehydration being measured instead of gasoline (ASTM D6730 is orientated to gasoline analyses). In addition, after the second collector, the gaseous flow was collected and analyzed by GC (Refinery Gas Analysis (RGA), Agilent Technologies Inc., Santa Clara, USA). Calibration was carried out for the quantitative analyses.

4. Conclusions Four zeolite-based catalysts were synthesized in this work. Catalysts were prepared by incorporating 3% or 5% by weight of zirconium and 1% by weight of vanadium into the structure of SBA-15. The materials were then characterized by ICP, SEM, XRD, Hg porosimeters, SBET, TPD and TPR. The incorporation of Zr nanoparticles (in the form of ZrO2) leads to a decrease in the specific surface area of the catalyst, changes its geometry and affects the acidic center distribution. This study provides evidence that the addition of vanadium on the SBA-15 surface or zirconium to the mesoporous SBA-15 structure changes the geometry of the catalyst, affecting the acidic center distribution, which in turn has an influence on the catalyst’s activity. The catalyst without vanadium but with added zirconium (Zr-SBA-15 II) proved to be an effective material for the dehydration of isopropanol, showing the best results in comparison to existing studies and ensuring a selectivity to propylene of 100% at 96% conversion.

Author Contributions: Conceptualization, M.M., Z.T., J.Š. and J.M.H.-H.; methodology, M.M. and Z.T.; software, J.Š.; validation, M.M., Z.T., J.Š. and J.M.H.-H.; formal analysis, M.M., Z.T., J.Š. and J.M.H.-H.; investigation, M.M. and Z.T.; resources, M.M., Z.T., J.Š. and J.M.H.-H.; data curation, M.M., Z.T., J.Š. and J.M.H.-H.; writing—original draft preparation, M.M. and Z.T.; writing—review and editing, J.Š. and J.M.H.-H.; visualization, M.M. and Z.T.; Catalysts 2020, 10, 719 11 of 11 supervision, J.M.H.-H.; project administration, J.Š.; funding acquisition, J.Š. All authors have read and agreed to the published version of the manuscript. Funding: This publication is a result of the Development of the UniCRE Centre project (LO1606), which has been financially supported by the Ministry of Education, Youth and Sports of the Czech Republic (MEYS) under the National Sustainability Program I. The result was achieved using the infrastructure of the Efficient Use of Energy Resources Using Catalytic Processes project (LM2015039), which has been financially supported by MEYS within the targeted support of large infrastructures. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

References

1. Al-Douri, A.; Sengupta, D.; El-Halwagi, M.M. Shale gas monetization—A review of downstream processing to chemicals and fuels. J. Nat. Gas Sci. Eng. 2017, 45, 436–455. [CrossRef] 2. Corma, A.; Corresa, E.; Mathieu, Y.; Sauvanaud, L.; Al-Bogami, S.; Al-Ghrami, M.S.; Bourane, A. Crude oil to chemicals: Light oleﬁns from crude oil. Catal. Sci. Technol. 2017, 7, 12–46. [CrossRef] 3. Akah, A.; Al-Ghrami, M. Maximizing propylene production via FCC technology. Appl. Petrochem. Res. 2015, 5, 377–392. [CrossRef] 4. Walther, T.; François, J.M. Microbial production of propanol. Biotechnol. Adv. 2016, 34, 984–996. [CrossRef] [PubMed] 5. Frank, B.; Dinse, A.; Ovsitser, O.; Kondratenko, E.V.; Schomäcker, R. Mass and heat transfer eﬀects on the

oxidative dehydrogenation of propane (ODP) over a low loaded VOx/Al2O3 catalyst. Appl. Catal. A Gen. 2007, 323, 66–76. [CrossRef] 6. Chokkalingam, S.; Viswanathan, B.; Varadarajan, T.K. Decomposition of isopropyl alcohol on g-phase bismuth molybdate catalyst. Indian J. Chem. 1984, 23, 143–144. 7. Ciˇcmanec,P.;ˇ Raabová, K.; Hidalgo, J.M.; Kubiˇcka,D.; Bulánek, R. Conversion of ethanol to acetaldehyde

over VOX-SiO2 catalysts: The eﬀects of support texture and vanadium speciation. React. Kinet. Mech. Catal. 2017, 121, 353–369. [CrossRef] 8. Ciˇcmanec,P.;ˇ Ganjkhanlou, Y.; Kotera, J.; Hidalgo, J.M.; Tišler, Z.; Bulánek, R. The eﬀect of vanadium content

and speciation on the activity of VOx/ZrO2 catalysts in the conversion of ethanol to acetaldehyde. Appl. Catal. A Gen. 2018, 564, 208–217. [CrossRef] 9. Foo, G.S.; Polo-Garzon, F.; Fung, V.; Jiang, D.; Overbury, S.H.; Wu, Z. Acid–base reactivity of perovskite catalysts probed via conversion of 2-propanol over titanates and zirconates. ACS Catal. 2017, 7, 4423–4434. [CrossRef] 10. Blanco-Bonilla, F.; Lopez-Pedrajas, S.; Luna, D.; Marinas, J.M.; Bautista, F.M. Vanadium oxides supported on amorphous aluminum phosphate: Structural and chemical characterization and catalytic performance in the 2-propanol reaction. J. Mol. Catal. A Chem. 2016, 416, 105–116. [CrossRef] 11. Zukal, A.; Siklová, H.; Cejka, J. Grafting of alumina on SBA-15: Effect of surface roughness. Lagmuir 2008, 24, 9837–9842. [CrossRef][PubMed] 12. Beck, B.; Harth, M.; Hamilton, N.G.; Carrero, C.; Uhlrich, J.J.; Trunschke, A.; Shaikhutdinov, S.; Schubert, H.; Freund, H.J.; Schlögl, R.; et al. Partial oxidation of ethanol on vanadia catalysts on supporting oxides with different redox properties compared to propane. J. Catal. 2012, 296, 120–131. [CrossRef] 13. Bedia, J.; Ruiz-Rosas, R.; Rodríguez-Mirasol, J.; Cordero, T. A kinetic study of 2-propanol dehydration on carbon acid catalysts. J. Catal. 2010, 271, 33–42. [CrossRef] 14. Larmier, K.; Chizallet, C.; Cadran, N.; Maury, S.; Abboud, J.; Lamic-Humblot, A.F.; Marceau, E.; Lauron-Pernot, H. Mechanistic investigation of isopropanol conversion on alumina catalysts: Location of active sites for Alkene/Ether production. ACS Catal. 2015, 5, 4423–4437. [CrossRef] 15. Zukal, A.; Pastva, J.; Cejka,ˇ J. MgO-modified mesoporous silicas impregnated by potassium carbonate for carbon dioxide adsorption. Microporous Mesoporous Mater. 2013, 167, 44–50. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).