Selective Alkylation of Benzene by Propane Over Bifunctional Pd-Acid

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Article Selective Alkylation ofof BenzeneBenzene byby PropanePropane overover Bifunctional Pd-AcidPd‐Acid Catalysts

Abdullah Alotaibi, SophieSophie Hodgkiss,Hodgkiss, ElenaElena F.F. Kozhevnikova Kozhevnikova and and Ivan Ivan V. V. Kozhevnikov Kozhevnikov * * ID

Department of Chemistry, UniversityUniversity ofof Liverpool,Liverpool, LiverpoolLiverpool L69 L69 7ZD, 7ZD, UK; UK; [email protected] [email protected] (A.A.); (A.A.); [email protected] (S.H.); [email protected] (E.F.K.) ** Correspondence:Correspondence: [email protected];[email protected]; Tel.:Tel.: +44-151-794-2938 +44‐151‐794‐2938

Received: 12 July 2017; Accepted: 8 August 2017; Published: 11 August 2017

Abstract: TheThe alkylationalkylation of of benzene benzene by by propane propane to yieldto yield isopropylbenzene isopropylbenzene (iPrPh) (iPrPh) was studied was studied using bifunctionalusing bifunctional Pd-acid Pd catalysts,‐acid catalysts, such as such Pd-heteropoly as Pd‐heteropoly acid and acid Pd-zeolite, and Pd in‐zeolite, a fixed in bed a fixed reactor bed at ◦ 300reactorC andat 300 1 bar °C pressure.and 1 bar Keggin-typepressure. Keggin tungstosilicic‐type tungstosilicic acid H4SiW acid12O H404SiW(HSiW)12O40 and (HSiW) zeolite and HZSM-5 zeolite wereHZSM used‐5 were as the used acid as the components acid components in these in catalysts. these catalysts. The reaction The reaction occurred occurred most most efficiently efficiently over 2%Pd/25%HSiW/SiOover 2%Pd/25%HSiW/SiO2, giving2, giving iPrPh iPrPh with with up up to to 88% 88% selectivity. selectivity. The The Pd-HSiW Pd‐HSiW catalyst catalyst waswas more selective than the Pd Pd-HZSM-5;‐HZSM‐5; the latter gave only 11–18% iPrPh selectivity. The reaction proceeded via aa bifunctionalbifunctional mechanism mechanism including including the the dehydrogenation dehydrogenation of propaneof propane to form to form propene propene on Pd on sites, Pd followedsites, followed by the by alkylation the alkylation of benzene of benzene with with the propene the propene on acid on acid sites. sites. The The effect effect of Pdof Pd loading loading in Pd-HSiWin Pd‐HSiW and and Pd-HZSM-5 Pd‐HZSM catalysts‐5 catalysts indicated indicated that that the the first first step step reached reached quasi-equilibrium quasi‐equilibrium at 1.5–2%at 1.5– Pd2% loadingPd loading and and the the second second step step became became rate rate limiting. limiting. The The addition addition of gold of gold to Pd-HSiW to Pd‐HSiW enhanced enhanced the activitythe activity of this of this catalyst, catalyst, although although the Au-HSiWthe Au‐HSiW without without Pd was Pd was inert. inert.

Keywords: alkylationalkylation with with alkanes; alkanes; propane; propane; benzene; benzene; isopropylbenzene; isopropylbenzene; Pd‐heteropoly Pd-heteropoly acid; acid; Pd‐ Pd-zeolitezeolite catalysts catalysts

1. Introduction Acid-catalysedAcid‐catalysed alkylationalkylation of of benzene benzene with with alkenes alkenes is an is established an established commercial commercial process toprocess produce to aproduce wide range a wide of range alkylbenzenes. of alkylbenzenes. The alkylation The alkylation of benzene of benzene with with ethene ethene and and propene propene is used is used to manufactureto manufacture ethylbenzene ethylbenzene (EtPh) (EtPh) and and isopropylbenzene isopropylbenzene (iPrPh), (iPrPh), which which are are thethe intermediatesintermediates in styrene andand phenol phenol production, production, respectively respectively [1 ].[1]. It It has has been been demonstrated demonstrated that that ethene ethene and and propene propene can becan replaced be replaced by abundant by abundant and inexpensiveand inexpensive alkanes, alkanes, ethane ethane and propane, and propane, which which would would lead to lead the to more the cost-effectivemore cost‐effective and environmentally and environmentally friendly friendly production production of these of these commodity commodity chemicals chemicals [2–18 [2–18].]. The alkylationThe alkylation of benzene of benzene with light with alkanes light alkanes occurs inoccurs the gas in phase the gas in the phase presence in the of presence solid bifunctional of solid metal-acidbifunctional catalysts, metal‐acid which catalysts, operate which via a pathway, operate via shown a pathway, in Scheme shown1, including in Scheme the dehydrogenation 1, including the ofdehydrogenation alkane on metal of sites alkane to form on metal alkene sites and to H 2form(step alkene 1) followed and H by2 (step the alkylation 1) followed of benzeneby the alkylation with the alkeneof benzene on acid with sites the (stepalkene 2). on acid sites (step 2).

Metal

C3H8 C3H6 +H2 (1)

H+ +C3H6 (2)

Scheme 1. Alkylation of benzene with propane via a bifunctional pathway.

Catalysts 2017,, 77,, 233;233; doi:doi:10.3390/catal708023310.3390/catal7080233 www.mdpi.com/journal/catalystswww.mdpi.com/journal/catalysts Catalysts 2017, 7, 233 2 of 10

Pt-zeolite bifunctional catalysts have been shown to be the most active for benzene alkylation with ethane [2–17]. The best results have been attained using Pt-modiﬁed HZSM-5 zeolite to obtain >90% selectivity to EtPh at an almost equilibrium benzene conversion of 12% [6]. In contrast, the alkylation of benzene with propane has proved to be more difﬁcult, giving only 32% iPrPh selectivity at 6.6% conversion using Pt/HZSM-5 [13]. The main product was nPrPh (50% selectivity) rather than iPrPh [13], which would be the favourable product from the carbenium ion mechanism. This may be due to the shape selectivity control imposed by the zeolite microporous environment [18]. A much more selective alkylation of benzene by propane (90–93% iPrPh selectivity) has been reported using a mesoporous silica-supported Pt-heteropoly acid catalyst, Pt/H4SiW12O40/SiO2, based on the Keggin-type 12-tungstosilicic acid H4SiW12O40 (HSiW) [18]. Notably, HSiW is superior to its stronger analogue H3PW12O40 in this reaction, probably due to its larger number of proton sites and better resistance to coking [18]. We now investigate the alkylation of benzene by propane using less expensive bifunctional catalysts based on palladium. Similar to platinum, palladium is an active catalyst for alkane dehydrogenation, and is hence capable of promoting the alkylation process. The bifunctional Pd catalysts under study comprise HSiW and HZSM-5 as the acid components. In general, regardless of their formulation, these catalysts are denoted herein as Pd-HSiW and Pd-HZSM-5, respectively. Both supported and physically mixed bifunctional catalysts are used in this work; the supported ones are denoted Pd/HSiW/SiO2 and Pd/HZSM-5 and the mixed ones Pd/C + HSiW/SiO2 and Pd/C + HZSM-5. Also, we looked at the effect of gold additives on catalyst performance by testing the corresponding AuPd-HSiW and AuPd-HZSM-5 bimetallic catalysts. It is demonstrated that in benzene alkylation with propane, the Pd-HSiW and Pd-HZSM-5 catalysts perform similarly to their Pt counterparts, Pt-HSiW and Pt-HZSM-5, as reported previously [18]. The addition of gold to Pd-HSiW is shown to enhance the activity of this catalyst, although the Au-HSiW without Pd is inert.

2. Results and Discussion It is essential to compare the bifunctional Pd catalysts under study with previously reported Pt catalysts [18] under the same reaction conditions. Therefore, the Pd catalysts were tested in benzene alkylation with propane in the same system at 300 ◦C, contact time W/F = 80 g h mol−1 and [C6H6]/[C3H8] = 1:9 mol/mol without dilution of the gas feed with an inert gas, as described previously [18]. In such conditions, the equilibrium conversion of benzene to iPrPh has been reported to be 8.2% [18]. Information about the catalysts studied is given in the Materials and Methods (Section3).

2.1. Alkylation of Benzene over Pd-HZSM-5

Representative results are given in Table1. Monofunctional catalysts HZSM-5 and Pd/C + SiO 2 showed very low activity (entries 1 and 2). In contrast, bifunctional catalysts containing Pd and zeolite HZSM-5, both mixed and supported, were active in benzene alkylation with propane. This conﬁrms the view that the reaction occurs through bifunctional metal-acid catalysis (Scheme1). With mixed catalysts Pd/C + HZSM-5, the benzene conversion increased with Pd loading, levelling off at about 2% Pd loading (Figure1; Table1, entries 3–7). This indicates that the propane dehydrogenation step (1) reaches quasi-equilibrium at ≥ 2% Pd loading, and the alkylation process becomes limited by the acid-catalysed step (2) of the benzene alkylation with propene (Scheme1). With Pd-HZSM-5 catalysts, the selectivity to the desired product iPrPh was 11–18%, the main reaction products being MePh, EtPh, and nPrPh. Similar results have been obtained previously with Pt-HZSM-5 [18]. The predominant formation of nPrPh rather than iPrPh, which would be the favourable product from the carbenium ion mechanism, has been explained [18] by the product shape selectivity control imposed by the HZSM-5 microporous environment. This implies that iPrPh, formed initially, could isomerize on HZSM-5 proton sites to form nPrPh, which possesses higher diffusivity in zeolite micropores. MePh and EtPh, which are thermodynamically more favourable products than iPrPh, probably arise from the cracking of nPrPh and iPrPh on zeolite acid sites [13]. Catalysts 2017, 7, 233 3 of 10

CatalystsSupported 2017, 7, 233 catalyst 2% Pd/HZSM-5 exhibited the best performance, giving 17.7% iPrPh selectivity3 of 9 at 19.5% benzene conversion (3.5% iPrPh yield) (Table1, entry 8). This catalyst also showed good performance stability stability on on stream stream (Figure (Figure 2).2 ).Its Itsplatinum platinum counterpart, counterpart, Pt/HZSM Pt/HZSM-5,‐5, gave similar gave similar results: results:16.4% iPrPh 16.4% selectivity iPrPh selectivity at 19.9% atconversion, 19.9% conversion, i.e., 3.3% i.e.,iPrPh 3.3% yield iPrPh [18]. yield Overall, [18]. in Overall, benzene in alkylation benzene alkylationwith propane, with Pd propane,‐HZSM‐5 Pd-HZSM-5 and Pt‐HZSM and‐5 Pt-HZSM-5catalysts perform catalysts similarly perform regarding similarly the regarding selectivity the to selectivityiPrPh. to iPrPh. It should should be be noted noted that that the the supported supported 2.0%Pd/HZSM 2.0%Pd/HZSM-5‐5 catalyst catalyst is twice is twiceas active as activeas the Pd/C as the + Pd/CHZSM +‐5 HZSM-5 mixed catalyst mixed (2.1% catalyst Pd loading) (2.1% Pd (Table loading) 1, entries (Table 7 1and, entries 8). Since 7 and the distances 8). Since between the distances metal betweenand acid metalsites in and these acid catalysts sites in differ these greatly catalysts (nanometre differ greatly scale (nanometre in the first one scale and in micrometre the ﬁrst one scale and micrometrein the second), scale this in indicates the second), that the this reaction indicates may that be the limited reaction by the may migration be limited of alkene by the intermediates migration of alkenebetween intermediates the metal and between acid sites the within metal andzeolite acid micropores. sites within A zeolite different micropores. picture was A different observed picture in the wascase observedof mesoporous in the Pd case‐HSiW/SiO of mesoporous2 catalysts Pd-HSiW/SiO (see below).2 catalysts (see below).

Table 1.1. AlkylationAlkylation ofof benzenebenzene withwith propanepropane overover Pd-HZSM-5Pd‐HZSM‐5 catalysts.catalysts.11

Aromatic Selectivity (%) 3 2 3 Catalyst Conversion2 (%) Aromatic Selectivity (%) Catalyst Conversion (%) MePh EtPh iPrPh nPrPh C9+ (1) HZSM‐5 0.2 MePh25.5 EtPh 22.8 iPrPh14.2 nPrPh24.6 C9+ 12.9 (2) Pd/C(1) + HZSM-5SiO2 (0.7%Pd) 4 0.20.6 25.588.7 22.80 14.20 24.60 12.911.3 4 4 (3)(2) Pd/C Pd/C + HZSM + SiO2‐5(0.7%Pd) (0.1%Pd) 0.61.1 88.719.3 031.6 015.0 028.9 11.3 5.3 (3)(4) Pd/CPd/C + + HZSM HZSM-5‐5 (0.4%Pd) (0.1%Pd) 44 1.12.5 19.325.2 31.623.9 15.016.4 28.928.2 5.3 6.4 4 (4)(5) Pd/CPd/C + + HZSM HZSM-5‐5 (0.7%Pd) (0.4%Pd) 4 2.53.8 25.227.9 23.920.8 16.414.2 28.225.6 6.4 11.6 4 (5)(6) Pd/CPd/C + + HZSM HZSM-5‐5 (1.4%Pd) (0.7%Pd) 4 3.89.2 27.934.9 20.821.8 14.212.0 25.619.2 11.612.2 4 (6)(7) Pd/CPd/C + + HZSM HZSM-5‐5 (2.1%Pd) (1.4%Pd) 4 9.210.9 34.935.6 21.821.1 12.010.7 19.219.1 12.213.6 4 (7) Pd/C(8) 2.0%Pd/HZSM + HZSM-5 (2.1%Pd)‐5 10.919.5 35.613.0 21.18.4 10.717.7 19.129.9 13.631.1 (8) 2.0%Pd/HZSM-5 19.5 13.0 8.4 17.7 29.9 31.1 1 Reaction conditions: 300 °C, 1 bar pressure, 0.20 g catalyst, inlet molar ratio C6H6/C3H8 = 1:9, 10 mL min−1 flow 1 ◦ −1 Reaction conditions:−1 300 C, 1 bar pressure, 0.20 g catalyst, inlet molar ratio C6H6/C3H−81=2 1:9, 10 mL min flow rate, W/F = 80 g h mol ; catalyst−1 pre‐treatment at 300 °C/1◦ h in H2 flow, 10 mL min−1. 2 Benzene conversion at 3 rate, W/F = 80 g h mol ; catalyst pre-treatment at 300 C/1 h in H2 flow, 10 mL min . Benzene conversion 3 3 4 4 h timeat on 3 h stream. time on stream. SelectivitySelectivity to unidentified to unidentified C9+ alkylbenzenes C9+ alkylbenzenes at 3 h at time 3 h on time stream. on stream. UniformUniform physical physical mixture mixture of 7.1% Pt/C with HZSM-5 or SiO . of 7.1% Pt/C with HZSM‐5 or SiO2. 2

Figure 1.1. EffectEffect of of Pd loading on benzene conversion for benzene alkylation at 300 ◦°CC over (1) 7.1%Pd/C + + HZSM HZSM-5‐5 catalysts catalysts and (2) Pd-HSiWPd‐HSiW includingincluding physicallyphysically mixedmixed catalystscatalysts 7.1%Pd/C7.1%Pd/C +

25%HSiW/SiO22 (solid(solid diamonds) and supportedsupported catalystscatalysts Pd/25%HSiW/SiO Pd/25%HSiW/SiO22 (open(open diamonds) (for reaction conditions,conditions, seesee TablesTables1 1 and and2 ).2).

Figure 2. Time course for benzene alkylation by propane over 2% Pd/HZSM‐5 (Si/Al = 10) (0.20 g

catalyst, 1 bar pressure, inlet molar ratio C6H6/C3H8 = 1:9, 10 mL min−1 flow rate, W/F = 80 g h mol−1; catalyst pre‐treatment at 300 °C/1 h in H2 flow).

Catalysts 2017, 7, 233 3 of 9 performance stability on stream (Figure 2). Its platinum counterpart, Pt/HZSM‐5, gave similar results: 16.4% iPrPh selectivity at 19.9% conversion, i.e., 3.3% iPrPh yield [18]. Overall, in benzene alkylation with propane, Pd‐HZSM‐5 and Pt‐HZSM‐5 catalysts perform similarly regarding the selectivity to iPrPh. It should be noted that the supported 2.0%Pd/HZSM‐5 catalyst is twice as active as the Pd/C + HZSM‐5 mixed catalyst (2.1% Pd loading) (Table 1, entries 7 and 8). Since the distances between metal and acid sites in these catalysts differ greatly (nanometre scale in the first one and micrometre scale in the second), this indicates that the reaction may be limited by the migration of alkene intermediates between the metal and acid sites within zeolite micropores. A different picture was observed in the case of mesoporous Pd‐HSiW/SiO2 catalysts (see below).

Table 1. Alkylation of benzene with propane over Pd‐HZSM‐5 catalysts.1

Aromatic Selectivity (%) 3 Catalyst Conversion 2 (%) MePh EtPh iPrPh nPrPh C9+ (1) HZSM‐5 0.2 25.5 22.8 14.2 24.6 12.9 (2) Pd/C + SiO2 (0.7%Pd) 4 0.6 88.7 0 0 0 11.3 (3) Pd/C + HZSM‐5 (0.1%Pd) 4 1.1 19.3 31.6 15.0 28.9 5.3 (4) Pd/C + HZSM‐5 (0.4%Pd) 4 2.5 25.2 23.9 16.4 28.2 6.4 (5) Pd/C + HZSM‐5 (0.7%Pd) 4 3.8 27.9 20.8 14.2 25.6 11.6 (6) Pd/C + HZSM‐5 (1.4%Pd) 4 9.2 34.9 21.8 12.0 19.2 12.2 (7) Pd/C + HZSM‐5 (2.1%Pd) 4 10.9 35.6 21.1 10.7 19.1 13.6 (8) 2.0%Pd/HZSM‐5 19.5 13.0 8.4 17.7 29.9 31.1

1 Reaction conditions: 300 °C, 1 bar pressure, 0.20 g catalyst, inlet molar ratio C6H6/C3H8 = 1:9, 10 mL min−1 flow rate, W/F = 80 g h mol−1; catalyst pre‐treatment at 300 °C/1 h in H2 flow, 10 mL min−1. 2 Benzene conversion at 3 h time on stream. 3 Selectivity to unidentified C9+ alkylbenzenes at 3 h time on stream. 4 Uniform physical mixture of 7.1% Pt/C with HZSM‐5 or SiO2.

Figure 1. Effect of Pd loading on benzene conversion for benzene alkylation at 300 °C over (1) 7.1%Pd/C + HZSM‐5 catalysts and (2) Pd‐HSiW including physically mixed catalysts 7.1%Pd/C + Catalysts25%HSiW/SiO2017, 7, 233 2 (solid diamonds) and supported catalysts Pd/25%HSiW/SiO2 (open diamonds) (for4 of 10 reaction conditions, see Tables 1 and 2).

Figure 2. Time course forfor benzenebenzene alkylationalkylation byby propanepropane overover 2% 2% Pd/HZSM-5 Pd/HZSM‐5 (Si/Al(Si/Al = = 10) 10) (0.20 g catalyst, 1 bar pressure, inlet molar ratio C6H6/C3H8 = 1:9, 10 mL min−1 flow−1 rate, W/F = 80 g h mol−1; catalyst, 1 bar pressure, inlet molar ratio C6H6/C3H8 = 1:9, 10 mL min ﬂow rate, W/F = 80 g h catalyst−1 pre‐treatment at 300 °C/1 h in◦ H2 flow). mol ; catalyst pre-treatment at 300 C/1 h in H2 ﬂow).

Table 2. Alkylation of benzene with propane over Pd-HSiW catalysts.1

Aromatic Selectivity (%) 3 Catalyst Conversion 2 (%) MePh EtPh iPrPh nPrPh C9+

(1) 25%HSiW/SiO2 0.2 4 (2) Pd/C + HSiW/SiO2 (0.2%Pd) 0.8 0 5.9 44.3 18.8 31.0 4 (3) Pd/C + HSiW/SiO2 (0.4%Pd) 1.4 0 2.7 67.3 10.0 20.0 4 (4) Pd/C + HSiW/SiO2 (0.7%Pd) 2.6 3.6 1.4 69.6 6.4 18.9 4 (5) Pd/C + HSiW/SiO2 (1.4%Pd) 3.7 2.0 2.0 74.2 6.7 15.1 5 (6) Pd/C + HSiW/SiO2 (1.4%Pd) 5.2 4.4 3.1 70.6 15.9 6.0 (7) 1.5% Pd/25%HSiW/SiO2 4.1 0 0.8 86.3 3.0 9.9 6 (8) 2.0% Pd/25%HSiW/SiO2 4.5 0 0.8 85.4 3.6 10.4 (9) 3.0% Pd/25%HSiW/SiO2 4.3 0 0.9 88.1 3.4 7.6 (10) 1% Pt/25%HSiW/SiO2 [18] 4.0 0 0.8 81.6 2.9 14.7 1 ◦ −1 Reaction conditions: 300 C, 1 bar pressure, 0.20 g catalyst, inlet molar ratio C6H6/C3H8 = 1:9, 10 mL min flow −1 ◦ −1 2 rate, W/F = 80 g h mol ; catalyst pre-treatment at 300 C/1 h in H2 flow, 10 mL min . Benzene conversion 3 4 at 3 h time on stream. Selectivity to unidentified C9+ alkylbenzenes at 3 h time on stream. Uniform physical 5 mixture of 7.1% Pt/C with 25% HSiW/SiO2. Two catalyst beds: 7.1% Pd/C (0.04 g, top bed) and 25%HSiW/SiO2 (0.16 g, bottom bed). 6 2.1% coke content in the catalyst after reaction.

2.2. Alkylation of Benzene over Pd-HSiW Silica-supported Pd-HSiW catalysts differ from Pd-HZSM-5 catalysts in two aspects: (i) they are mesoporous rather than microporous, and (ii) HSiW heteropoly acid possesses stronger proton sites than HZSM-5 [18]. Therefore, the Pd-HSiW catalysts can be expected to be much more selective to iPrPh than Pd-HZSM-5, similar to the previously reported Pt-HSiW catalysts [18]. The results for benzene alkylation with propane over Pd-HSiW are given in Table2 for both the mixed Pd/C + 25% HSiW/SiO2 and supported Pd/25% HSiW/SiO2 catalysts. As expected, the 25% HSiW/SiO2 acid catalyst in the absence of Pd was practically inert (entry 1), whereas the bifunctional Pd-HSiW metal-acid catalysts showed good activity in the alkylation reaction. In stark contrast to Pd-HZSM-5, the reaction with the Pd-HSiW catalysts occurred with high selectivity to iPrPh, up to 88.1% (entry 9), as anticipated for the alkylation unaffected by shape selectivity effects. The supported catalysts 1.5–3% Pd/25% HSiW/SiO2 showed the best results to give 85–88% iPrPh selectivity at 4.1–4.5% benzene conversion (up to 3.8% iPrPh yield) (entries 8 and 9). Therefore, with comparable yields, the Pd-HSiW catalysts are much more selective to iPrPh than the Pd-HZSM-5 ones. It can be seen that Pd-HSiW and Pt-HSiW catalysts perform similarly (cf. entries 7 and 10), with the Pt catalysts predictably more active than Pd ones per metal loading. This, however, is compensated for by the lower price (by a factor of ~1.5) of Pd as compared to Pt. Like with Pd-HZSM-5 catalysts, benzene conversion was found to increase with Pd loading in Pd-HSiW catalysts to level off at ≥1.5% Pd loading (Figure1; Table2, entries 2–5). This indicates that Catalysts 2017, 7, 233 5 of 10 the propane dehydrogenation step (1) reached quasi-equilibrium at ca. 1.5% Pd loading. A similar trend has been observed for Pt-HSiW [18]. Similar to the supported Pt-HSiW catalysts [18], the supported Pd-HSiW catalysts suffered from deactivation, as shown in Figure3. This may be explained by catalyst coking (Table2, entry 8). In contrast, the mixed and especially two-bed Pd/C + 25% HSiW/SiO2 catalysts were more stable on stream (Figure4), albeit with a lower iPrPh selectivity of 70–74% (Table2, entries 4–6). A similar performance has also been observed for their Pt counterparts [18]. The high activity of both the mixed and two-bed catalysts indicates no reaction limitation by the migration of alkene intermediates between metal and acid sites within these mesoporous catalysts. This is different from the microporous Pd-HZSM-5 catalysts (see above). The better performance stability of these catalysts compared to the supportedCatalysts 2017 Pd/25%HSiW/SiO, 7, 233 2 catalyst may due to their better resistance to coking [18]. 5 of 9 Catalysts 2017, 7, 233 5 of 9

Figure 3. Time course for benzenebenzene alkylationalkylation by by propane propane over over 2% 2% Pd/25% Pd/25% HSiW/SiOHSiW/SiO2 (0.20(0.20 g g catalyst, catalyst, 22 Figure 3. Time course for benzene alkylation by propane over 2% Pd/25%−1 HSiW/SiO (0.20 g catalyst,−1 300 ◦°C,C, 11 barbar pressure, pressure, inlet inlet molar molar ratio ratio C CH6H/C6/C3HH8 == 1:9,1:9, 1010 mL mL min min−1 ﬂowflow rate,rate,W W//F == 80 gg hh mol mol−1;; 300 °C, 1 bar pressure, inlet molar ratio 6C6H6 6/C33H88 = 1:9, 10 mL min−1 flow rate, W/F = 80 g h mol−1; catalyst pre-treatmentpre‐treatment at 300300 ◦°C/1C/1 h h in in H H2 flow).ﬂow). catalyst pre‐treatment at 300 °C/1 h in H2 2flow).

Figure 4. Time course for benzene alkylation by propane over two‐bed catalyst 7.1% Pd/C (0.04 g, top Figure 4. 4. TimeTime course course for for benzene benzene alkylation alkylation by by propane propane over over two two-bed‐bed catalyst catalyst 7.1% 7.1% Pd/C Pd/C (0.04 (0.04 g, top g, bed) + 25%HSiW/SiO2 (0.16 g, bottom bed) (0.20 g total catalyst weight, 1.4% Pd, 300 °C, 1 bar pressure, 2 ◦ bed)top bed) + 25%HSiW/SiO + 25%HSiW/SiO (0.16 g,(0.16 bottom g, bottom bed) (0.20 bed)−1 g total (0.20 catalyst g total catalystweight, 1.4% weight,−1 Pd, 1.4%300 °C, Pd, 1 bar 300 pressure,C, 1 bar inlet molar ratio C6H6/C3H28 = 1:9, 10 mL min flow rate, W/F = 80 g h mol ; catalyst pre‐treatment at inletpressure, molar inlet ratio molar C6H6/C ratio3H8 C = 1:9,H /C 10 mLH =min 1:9,−1 flow 10 mL rate, min W−/1Fﬂow = 80 rate,g h molW/−1F; catalyst= 80 g h pre mol‐treatment−1; catalyst at 300 °C/1 h in H2 flow). 6 6 3 8 2 ◦ 300pre-treatment °C/1 h in H at flow). 300 C/1 h in H2 ﬂow). 2.3. Effect of Gold Additives 2.3. Effect Effect of of Gold Gold Additives Additives Bimetallic AuPd and AuPt catalysts have attracted much interest because of their enhanced Bimetallic AuPd and AuPt catalysts have attracted much interest because of their enhanced performanceBimetallic compared AuPd and to monometallic AuPt catalysts counterparts have attracted ([19–29] much and interest references because therein), of their especially enhanced in performance compared to monometallic counterparts ([19–29] and references therein), especially in performancehydrogenation compared [29], hydrodeoxygenation to monometallic counterparts [20,26], hydrodesulfurisation ([19–29] and references [27,28], therein), oxidation especially [21–23], in hydrogenation [29], hydrodeoxygenation [20,26], hydrodesulfurisation [27,28], oxidation [21–23], hydrogenationand other reactions [29], (for hydrodeoxygenation review, see [19,24,25]). [20,26 ],For hydrodesulfurisation example, the addition [27 ,of28 ],gold oxidation to the bifunctional [21–23], and and other reactions (for review, see [19,24,25]). For example, the addition of gold to the bifunctional Pt/CsPW catalyst (CsPW = Cs2.5H0.5PW12O40) for the hydrodeoxygenation of 3‐pentanone has been Pt/CsPW catalyst (CsPW = Cs2.5H0.5PW12O40) for the hydrodeoxygenation of 3‐pentanone has been found to increase catalyst activity and catalyst stability to deactivation, whereas the Au itself was found to increase catalyst activity and catalyst stability to deactivation, whereas the Au itself was inert in this reaction [26]. Gold enhancement of catalyst performance is usually attributed to Au inert in this reaction [26]. Gold enhancement of catalyst performance is usually attributed to Au alloying through ensemble (geometric) and ligand (electronic) effects of the constituent elements alloying through ensemble (geometric) and ligand (electronic) effects of the constituent elements [24,25]. Previously reported scanning transmission electron microscopy–energy dispersive X‐ray [24,25]. Previously reported scanning transmission electron microscopy–energy dispersive X‐ray spectroscopy (STEM‐EDX) and X‐ray diffraction (XRD) analyses of AuPt/CsPW catalysts show the spectroscopy (STEM‐EDX) and X‐ray diffraction (XRD) analyses of AuPt/CsPW catalysts show the presence of bimetallic particles with a wide range of Au/Pt atomic ratios in such catalysts [26]. presence of bimetallic particles with a wide range of Au/Pt atomic ratios in such catalysts [26]. Here, we looked at the effect of gold additives on the performance of Pd‐HZSM‐5 and Pd‐HSiW Here, we looked at the effect of gold additives on the performance of Pd‐HZSM‐5 and Pd‐HSiW catalysts in the alkylation of benzene by propane. The results, including benzene conversions and catalysts in the alkylation of benzene by propane. The results, including benzene conversions and product selectivities, are shown in Table S1 in the Supplementary Material. Supported and mixed product selectivities, are shown in Table S1 in the Supplementary Material. Supported and mixed gold catalysts in the absence of Pd (0.5%Au/HZSM‐5, 5% Au/C + HZSM‐5 (2% Au) and 10% Au/C + gold catalysts in the absence of Pd (0.5%Au/HZSM‐5, 5% Au/C + HZSM‐5 (2% Au) and 10% Au/C + HSiW/SiO2 (2% Au)) showed very low activity (0.4–0.5% benzene conversion). No enhancing effect HSiW/SiO2 (2% Au)) showed very low activity (0.4–0.5% benzene conversion). No enhancing effect of gold was found in the case of the mixed Pd/C + HZSM‐5 catalysts. Thus, Pd/C + HZSM‐5 (1.4% Pd) of gold was found in the case of the mixed Pd/C + HZSM‐5 catalysts. Thus, Pd/C + HZSM‐5 (1.4% Pd) and AuPd/C + HZSM‐5 (1.2% Pd, 2% Au) gave similar benzene conversions of 9.2 and 8.8%, and AuPd/C + HZSM‐5 (1.2% Pd, 2% Au) gave similar benzene conversions of 9.2 and 8.8%,

Catalysts 2017, 7, 233 6 of 10 other reactions (for review, see [19,24,25]). For example, the addition of gold to the bifunctional Pt/CsPW catalyst (CsPW = Cs2.5H0.5PW12O40) for the hydrodeoxygenation of 3-pentanone has been found to increase catalyst activity and catalyst stability to deactivation, whereas the Au itself was inert in this reaction [26]. Gold enhancement of catalyst performance is usually attributed to Au alloying through ensemble (geometric) and ligand (electronic) effects of the constituent elements [24,25]. Previously reported scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDX) and X-ray diffraction (XRD) analyses of AuPt/CsPW catalysts show the presence of bimetallic particles with a wide range of Au/Pt atomic ratios in such catalysts [26]. Here, we looked at the effect of gold additives on the performance of Pd-HZSM-5 and Pd-HSiW catalysts in the alkylation of benzene by propane. The results, including benzene conversions and product selectivities, are shown in Table S1 in the Supplementary Material. Supported and mixed gold catalysts in the absence of Pd (0.5%Au/HZSM-5, 5% Au/C + HZSM-5 (2% Au) and 10% Au/C + HSiW/SiO2 (2% Au)) showed very low activity (0.4–0.5% benzene conversion). No enhancing Catalystseffect of 2017 gold, 7, 233 was found in the case of the mixed Pd/C + HZSM-5 catalysts. Thus, Pd/C + HZSM-56 of 9 (1.4% Pd) and AuPd/C + HZSM-5 (1.2% Pd, 2% Au) gave similar benzene conversions of 9.2 and respectively,8.8%, respectively, with the with same the product same product selectivity. selectivity. In contrast, In contrast, the HSiW the‐based HSiW-based mixed catalysts mixed catalystsAuPd/C +AuPd/C HSiW/SiO + HSiW/SiO2 (1.2% Pd, 22.0%(1.2% Au) Pd, showed 2.0% Au) a small showed enhancement a small enhancement of benzene conversion of benzene in conversion comparison in tocomparison Pd/C + HSiW to Pd/C (1.4% + Pd) HSiW (Table (1.4% S1). Pd) Even (Table better S1). activity Even better enhancement activity enhancement was observed was in observed the case of in the casetwo‐ ofbed the catalysts two-bed AuPd/C catalysts + AuPd/C HSiW/SiO + HSiW/SiO2 (1.2% Pd,2 2.0%(1.2% Au) Pd, versus 2.0% Au) Pd/C versus + HSiW/SiO Pd/C + HSiW/SiO2 (1.4% Pd)2 (5.8(1.4% and Pd) 5.2% (5.8 andconversion, 5.2% conversion, respectively, respectively, with product with product selectivity selectivity unchanged) unchanged) (Figure (Figure 5). Given5). Given the differencethe difference in Pd in Pdloading, loading, the the enhancement enhancement of ofbenzene benzene conversion conversion amounts amounts to to about about 20%. 20%. Similar results were were obtained obtained for for the the two two-bed‐bed catalysts catalysts with with lower lower metal metal loadings loadings (Pd/C (Pd/C + HSiW/SiO + HSiW/SiO2 (0.71%2 Pd)(0.71% and Pd) AuPd/C and AuPd/C + HSiW/SiO + HSiW/SiO2 (0.60% Pd,2 (0.60% 1.0% Pd,Au)). 1.0% Au)).

Figure 5. Time course forfor benzenebenzene alkylationalkylation byby propanepropane over over two-bed two‐bed catalyst catalyst 9.9% 9.9% Au/6.0% Au/6.0% Pd/CPd/C (0.04 g, top bed) + 25%HSiW/SiO2 (0.16 g, bottom bed) (0.20 g total catalyst weight, 2% Au, 1.2% Pd, (0.04 g, top bed) + 25%HSiW/SiO2 (0.16 g, bottom bed) (0.20 g total catalyst weight, 2% Au, 1.2% Pd, 300 ◦°C, 1 bar pressure, inlet molar ratio C6H6/C3H8 = 1:9, 10 mL min−−1 flow rate, W/F = 80 g h mol−−1; 300 C, 1 bar pressure, inlet molar ratio C6H6/C3H8 = 1:9, 10 mL min ﬂow rate, W/F = 80 g h mol ; catalyst pre‐treatment at 300 ◦°C/1 h in H2 flow). Open circles show the conversion with Pd/C + 25% catalyst pre-treatment at 300 C/1 h in H2 ﬂow). Open circles show the conversion with Pd/C + 25% HSiW/SiO2 (1.4% Pd) taken from Figure 4 for comparison. HSiW/SiO2 (1.4% Pd) taken from Figure4 for comparison.

Although clearly discernible, this Au enhancement is considerably smaller than that reported Although clearly discernible, this Au enhancement is considerably smaller than that reported previously for the hydrodeoxygenation of octanoic acid (AuPd/SiO2 catalyst) [20] and 3‐pentanone previously for the hydrodeoxygenation of octanoic acid (AuPd/SiO catalyst) [20] and 3-pentanone (AuPt/CsPW catalyst) [26]. This may be explained by the reaction conditions2 of benzene alkylation. (AuPt/CsPW catalyst) [26]. This may be explained by the reaction conditions of benzene alkylation. The relatively small Au effect on the dehydrogenation activity of Pd‐HSiW catalyst can be attributed The relatively small Au effect on the dehydrogenation activity of Pd-HSiW catalyst can be attributed to to the propane dehydrogenation step being close to equilibrium (Figure 1). Therefore, any the propane dehydrogenation step being close to equilibrium (Figure1). Therefore, any enhancement enhancement of Pd activity would not cause a significant increase in benzene conversion. of Pd activity would not cause a signiﬁcant increase in benzene conversion. 3. Materials and Methods

3.1. Chemicals and Catalysts

Heteropoly acid hydrate H4SiW12O40 (HSiW, 99.9%) containing 20–28 H2O molecules per Keggin unit was from Sigma‐Aldrich. The quantity of crystallisation water in HSiW was measured by thermogravimetric analysis (TGA). Zeolite NH4+‐ZSM‐5 (Si/Al = 10, specific surface area SBET = 400 m2 g−1) was from Zeolyst International. It was transformed into the H+ form by calcination in air at 500 °C for 6 h. Carbon‐supported palladium 10%Pd/C was from Johnson Matthey (7.1% Pd content in dried catalyst from inductively coupled plasma atomic emission spectroscopy (ICP‐AES) analysis). Pd(acac)2 and HAuCl4∙3H2O were purchased from Sigma‐Aldrich. Aerosil 300 silica (SBET = 300 m2 g−1) was from Degussa. Propane, N2 and H2 gases (all >99% purity) were from the British Oxygen Company. From GC analysis, the propane contained C3H8 (99.20%), C2H6 (0.76%), C3H6 (0.01%), C4H10 (0.03%), and methane (0.004%). Silica‐supported HSiW catalysts were prepared by the impregnation of Aerosil 300 silica by an aqueous HSiW solution, which was then dried at 150 °C/10−3 kPa for 1.5 h [30,31]. The integrity of the HSiW Keggin structure on the silica surface in fresh and spent catalysts was confirmed by infrared spectroscopy (Figure S1 in the Supporting Material). Pd/HSiW/SiO2 and Pd/HZSM‐5 catalysts were prepared by the impregnation of 25% HSiW/SiO2 or HZSM‐5 with 0.02 M Pd(acac)2 solution in

Catalysts 2017, 7, 233 7 of 10

3. Materials and Methods

3.1. Chemicals and Catalysts

Heteropoly acid hydrate H4SiW12O40 (HSiW, 99.9%) containing 20–28 H2O molecules per Keggin unit was from Sigma-Aldrich. The quantity of crystallisation water in HSiW was measured + by thermogravimetric analysis (TGA). Zeolite NH4 -ZSM-5 (Si/Al = 10, specific surface area 2 −1 + SBET = 400 m g ) was from Zeolyst International. It was transformed into the H form by calcination in air at 500 ◦C for 6 h. Carbon-supported palladium 10%Pd/C was from Johnson Matthey (7.1% Pd content in dried catalyst from inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis). Pd(acac)2 and HAuCl4·3H2O were purchased from Sigma-Aldrich. Aerosil 300 silica 2 −1 (SBET = 300 m g ) was from Degussa. Propane, N2 and H2 gases (all >99% purity) were from the British Oxygen Company. From GC analysis, the propane contained C3H8 (99.20%), C2H6 (0.76%), C3H6 (0.01%), C4H10 (0.03%), and methane (0.004%). Silica-supported HSiW catalysts were prepared by the impregnation of Aerosil 300 silica by an aqueous HSiW solution, which was then dried at 150 ◦C/10−3 kPa for 1.5 h [30,31]. The integrity of the HSiW Keggin structure on the silica surface in fresh and spent catalysts was confirmed by infrared spectroscopy (Figure S1 in the Supporting Material). Pd/HSiW/SiO2 and Pd/HZSM-5 catalysts were prepared by the impregnation of 25% HSiW/SiO2 or HZSM-5 with 0.02 M Pd(acac)2 solution in benzene, followed by the slow rotoevaporation of benzene at room temperature, as described elsewhere [32]. The catalysts were dried under vacuum at 150 ◦C/10−3 kPa, then reduced in an ◦ oven by H2 flow at 250 C for 2 h. The bimetallic 10% Au/6.0% Pd/C catalyst was prepared by the impregnation of 7.1% Pd/C with an aqueous solution of HAuCl4, followed by drying and reduction ◦ by H2 flow at 250 C for 2 h. After reduction, Pd and Au loading in supported catalysts was confirmed by ICP-AES; this was in good agreement with the loading expected from the preparation stoichiometry (e.g., 10% Au/6.4% Pd/C from stoichiometry and 9.9% Au/6.0% Pd/C from ICP-AES analysis). Physical mixtures of 7.1% Pd/C with 25% HSiW/SiO2 or HZSM-5 containing a specified percentage of Pd were prepared by grinding mixtures of these components. Physical mixtures of 10% Au/6.0% Pd/C with 25% HSiW and HZSM-5 were prepared in the same way.

3.2. Techniques

The surface area, pore volume, and pore diameter of catalysts were determined from N2 physisorption at −196 ◦C using a Micromeritics ASAP 2010 analyser. The samples were outgassed at 240 ◦C for 2 h. Nitrogen adsorption isotherms and pore size distribution for Pd-HSiW and Pd-HZSM-5 catalysts are shown in Figures S2–S5. ICP-AES elemental analysis was performed on a Spectro Ciros emission spectrometer. Powder X-ray diffraction (XRD) patterns of catalysts were recorded on a PANalytical Xpert diffractometer using CuKα radiation (λ = 1.542 Å) and attributed using the JCPDS database. Diffuse reﬂectance infrared Fourier transform (DRIFT) spectra of catalysts were recorded on a Nicolet Nexus FTIR spectrometer using powdered samples of catalyst mixtures with KBr. Average metal particle size, d, in supported catalysts was estimated from XRD using the Scherrer equation. Metal dispersion, D, deﬁned as the metal (M) fraction at the surface, D = Ms/Mtotal, was calculated from the empirical equation D = 0.9/d (nm) [33]. XRD patterns of supported Pd and Au catalysts are shown in Figures S6–S8 in the Supplementary Material. Table3 shows the texture and metal dispersion of the catalysts studied. The acid strength of heteropoly acids, including HSiW, has been characterised previously using ammonia adsorption microcalorimetry [18,31]. Catalysts 2017, 7, 233 8 of 10

Table 3. Catalyst characterisation.

1 2 −1 2 3 −1 3 4 5 Catalyst SBET (m g ) Pore Volume (cm g ) Pore Diameter (Å) d (nm) D

25% HSiW/SiO2 215 0.95 171 2% Pd/25% 189 0.75 159 5.0 0.18 HSiW/SiO2 HZSM-5 301 0.22 31 2% Pd/HZSM-5 262 0.20 31 10 0.090 7.1% Pd/C 11 0.082 10% Au/C 89 0.010 13 (Pd) 0.069 (Pd) 9.9% Au/6.0% Pd/C 62 (Au) 0.015 (Au) 1 BET surface area. 2 Single point pore volume. 3 Average BET pore diameter. 4 Metal particle size from XRD. 5 Metal dispersion calculated from the empirical equation D = 0.9/d (nm).

3.3. Catalyst Testing ◦ The gas-phase alkylation of benzene by propane was carried out at 300 C and an inlet C6H6/C3H8 molar ratio of 1:9 under ambient pressure using a quartz fixed bed reactor (9.0 mm i.d.) with online GC analysis (Varian 3800 gas chromatograph with a 30 m × 0.32 mm × 0.5 µm Zebron ZB-WAX capillary column and a flame ionisation detector), as described elsewhere [18]. The reactor was charged with 0.20 g catalyst powder with a particle size of 45–180 µm. The gas feed containing propane (90 kPa) and benzene (10 kPa) entered the reactor from the top with a 10 mL min−1 flow rate (space time W/F = 80 g h mol−1, where W is the catalyst mass and F is the benzene molar flow rate). Prior to −1 reaction, the catalysts were pre-treated in situ in H2 flow (10 mL min ) for 1 h at reaction temperature. The selectivity to alkylbenzenes was defined as moles of alkylbenzene formed per one mole of benzene converted and quoted in mole %. The mean absolute percentage error in product selectivity and conversion was ≤10% and the carbon balance was maintained within 95%.

4. Conclusions It has been demonstrated that bifunctional Pd-acid catalysts (Pd-heteropoly acid and Pd-zeolite) are active in the alkylation of benzene by propane to yield isopropylbenzene (iPrPh). Keggin-type tungstosilicic acid H4SiW12O40 (HSiW) and zeolite HZSM-5 were used as the acid components in these catalysts. The mesoporous catalyst Pd/HSiW/SiO2 was much more selective to iPrPh (up to 88%) than the microporous catalyst Pd/HZSM-5, which gave only 11–18% iPrPh selectivity. The reaction proceeded via the bifunctional reaction pathway, including the Pd-catalysed dehydrogenation of propane to propene followed by the acid-catalysed alkylation of benzene with the propene thus formed. Evidence has been provided that the ﬁrst step reached quasi-equilibrium at 1.5–2% Pd loading, and the second step became rate limiting. The addition of gold to Pd-HSiW has been found to slightly enhance the activity of this catalyst.

Supplementary Materials: The following are available online at www.mdpi.com/2073-4344/7/8/233/s1, Figure S1: FTIR spectra of catalysts, Figures S2–S5: Catalyst texture, Figures S6–S8: XRD pattern of catalysts, Table S1: Alkylation over PdAu catalysts. Acknowledgments: We thank Shaqra University, Shaqra, Saudi Arabia for PhD studentship (A. Alotaibi). Author Contributions: I.K. and E.K. conceived and designed the experiments; A.A. and S.H. performed the experiments; I.K. wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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