catalysts 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.