Deactivation of External Acid Sites of H-Mordenite by Modification with

Aug 26, 2012 - Australian Institute of Bioengineering and Nanotechnology, The ... S. Al-Deyab , Hoi-Gu Jang , Jong-Ho Kim , Gon Seo , and Ajayan Vinu...
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Deactivation of External Acid Sites of H‑Mordenite by Modification with Lanthanide Oxides for the Isopropylation of Biphenyl and the Cracking of 1,3,5-Triisopropylbenzene and Cumene Chokkalingam Anand,† Ikuyo Toyama,‡ Hiroshi Tamada,‡ Shogo Tawada,‡ Satoshi Noda,‡ Ken-ichi Komura,‡ Yoshihiro Kubota,‡,§ Se Woong Lee,⊥ Sung Jung Cho,⊥ Jong-Ho Kim,⊥ Gon Seo,⊥ Ajayan Vinu,*,† and Yoshihiro Sugi*,†,‡ †

Australian Institute of Bioengineering and Nanotechnology, The University of Queensland, Brisbane 4072 QLD, Australia Department of Materials Science and Technology, Faculty of Engineering, Gifu University, Gifu 501-1193, Japan § Department of Materials Science and Engineering, Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan ⊥ School of Applied Chemical Engineering, Chonnam National University, Gwangju, 500-757, Korea ‡

S Supporting Information *

ABSTRACT: The modification of H-mordenite (MOR) with lanthanide oxides La2O3, CeO2, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 was examined for the deactivation of external acid sites and confirmed in the cracking of 1,3,5-triisopropylbenzene (TIPB) and cumene (IPB) and in the isopropylation of biphenyl (BP). The cracking of TIPB, which cannot enter the pores of MOR, shows that external acid sites were effectively deactivated by the modification of MOR with the lanthanide oxides in small amounts. Only the cracking of IPB over CeO2-modified MOR exhibited excellent catalytic activities, even at a 30 wt % metal loading, whereas the activities of other lanthanide oxide-modified MORs rapidly decreased as the loadings were increased because pore entrances became choked. The isomerization of 4,4′-diisopropylbiphenyl (4,4′-DIPB) during the isopropylation of BP at high temperatures such as 300 °C was also effectively prevented by the modification of MOR with the lanthanide oxides. Particularly, CeO2-modified MOR remained highly active even at a 30 wt % loading. Other lanthanide oxides can deactivate the isomerization of 4,4′-DIPB at 5−10 wt % loadings without significant loss of the activities at 300 °C, while the activity was rapidly lost as the loading amount was increased. The physicochemical properties of lanthanide oxide-modified MORs indicate that the lanthanide oxides modify the surface properties of MOR. The amounts of N2, o-xylene, and NH3 adsorbed on MORs mostly remained high after CeO2 modification; however, they rapidly decreased when loadings of the other oxides increased. These results show that CeO2 remains the open pores at high loadings; however, the other oxides reduce the size of pore entrances as the loading is increased.

1. INTRODUCTION Zeolite-catalyzed shape-selective catalysis is a promising way to synthesize symmetrically substituted isomers for industrial chemicals and advanced materials. Shape-selective catalysis occurs to form the least bulky isomer by steric restriction of reactants, transition intermediates, and/or products with zeolites having confined environments.1−5 Variation of pore entrances, channels, acidic properties, and/or deactivation of external acid sites are important for the enhancement of shapeselective catalysis with zeolites. H-Mordenite (MOR), particularly after dealumination, is highly selective among large-pore zeolites for the formation of the least bulky dialkylated products, for example, 4,4′-diisopropylbiphenyl (4,4′-DIPB) from biphenyl (BP), and 2,6-diisopropylnaphthalene (2,6DIPN) from naphthalene (NP).4−13 However, the selectivities for these isomers decreased at high reaction temperatures, accompanied by increases in 3,4′- and 3,3′-DIPB for BP and in 2,7-DIPN for NP due to the isomerization of 4,4′-DIPB and 2,6-DIPN at external acid sites of MOR.4−7,10,11 Acid sites of zeolites exist mainly inside the pores, whereas some of them are on the external surface. The catalysis on © 2012 American Chemical Society

external acid sites is controlled kinetically or thermodynamically, resulting in the formation of nonregisoselective formation of bulky or stable products. External acid sites sometimes have activities higher than those on internal acid sites, even though their numbers are low. These phenomena indicate that the deactivation of external acid sites is essential for enhancing the selectivities for shape-selective catalysis.14−44 There have been some interesting attempts to prevent nonregioselective reactions at the external acid sites by modification of zeolites for the alkylation of polynuclear aromatic hydrocarbons.16−21 Silanation of MOR enhanced the selectivities for β,β-DIPN in the isopropylation of NP,16 and for 4,4′-DIPB in the isopropylation of BP.17 Kikuchi and Matsuda also reported that phosphorus oxide modification of MOR decreased the isomerization of 4,4′-DIPB in the isopropylation of BP.18 Yashima et al. reported that 2,4Received: Revised: Accepted: Published: 12214

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autoclave with agitation, and temperature and pressure were maintained throughout the reaction. After the autoclave was cooled, products were separated from the catalysts by filtration, and washed with toluene (5−10 mL). The solution (ca. 1.5 mL) taken from combined filtrates was diluted with toluene (1.5−6.0 mL), and an aliquot was analyzed with a gas chromatograph (Shimadzu GC-14C and/or GC-18A) equipped with an Ultra-1 capillary column (25 m × 0.2 mm; film thickness: 0.25 μm, Agilent Technologies, Santa Clara, CA). The products were confirmed by using a Shimadzu gas chromatograph−mass spectrometer GC-MS 5000 with an Ultra-1 capillary column. The yield of every product was calculated on the basis of BP used for the reaction; i.e., the selectivity for each isomer of isopropylbiphenyl (IPBP) and diisopropylbiphenyl (DIPB) is expressed as a percentage of each IPBP and DIPB isomer among the total IPBP and DIPB isomers. 2.4. Characterization of Lanthanide-Modified MOR. XRD was measured using a Shimadzu XRD-6000 diffractometer with Cu Kα radiation (λ = 1.5418 Å). N2 adsorption isotherms were obtained on a Belsorp 28SA (Bel Japan, Osaka, Japan) at 196 °C. SEM images were obtained by using a scanning electron microscope (JSM-7500F, JEOL, Japan) operating at 20 kV. NH3-TPD (temperature-programmed desorption) was measured with a BEL-TPD-66 (Bel Japan). The sample was evacuated at 500 °C for 1 h, and NH3 was adsorbed on the sample at 100 °C for 1 h, followed by further evacuation at 100 °C for 1 h. Then the sample was heated from 100 °C to 710 °C at a rate of 10 °C/min in a helium stream. Adsorption of o-xylene was measured by a gravimetric method using a microbalance at 120 °C after evacuation at 300 °C.

dimethylquinoline improved the selectivity for 2,6-dimethylnaphthalene (2,6-DMN) with iron-substituted ZSM-5 (MFI) in the methylation of 2-methylnaphthalene.20 Song et al. claimed that selectivities for 4,4′-dimethylbiphenyl and 2,6-DMN were improved by the modification of MFI with alkaline earth metal oxides in the methylation of 4-methylbiphenyl and 2methylnaphthalene, respectively.19,21 The attempts to improve para-selectivities were also examined in the alkylation and related reactions of mononuclear aromatics by many research groups.14,15,22−41 Mobil research groups proposed an improvement in the para-selectivities by the modification of MFI with boron, magnesium, and phosphorus oxides.14,15,22−25 Similar results were also reported and confirmed by Yashima et al.28−33 Coke-induced selectivity methods were also proposed by some groups.34−37 Niwa et al. found that the modification of MFI with silicon alkoxide and silane alkoxide involving organic groups improved the shape-selectivity in the methylation of toluene.38−41 These methods also enhanced the paraselectivities by the deactivation of external acid sites as well as by controlling the pore entrances. We have been interested in lanthanide oxides as modifiers of zeolites because lanthanide oxides have relatively large atomic radii and amphoteric characters. We previously proposed the deactivation of external acid sites of CeO2-modified MOR in the isopropylation of NP and BP42,43 and of La2O3- and CeO2modified MFIs in the ethylation of ethylbenzene (EB).44 In this paper, we describe the catalytic behaviors of MORs modified with the lanthanide oxides La2O3, CeO2, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 in the cracking of 1,3,5-triisopropylbenzene (TIPB) and cumene (IPB) and in the isopropylation of BP.

2. EXPERIMENTAL SECTION 2.1. Catalysts. H-Mordenite (MOR; SiO2/A12O3 = 128) was obtained from Tosoh Corporation, Tokyo, Japan. Lanthanide oxide precursors La(NO 3 ) 3 ·6H 2 O, Ce(NO 3 ) 3 ·6H 2 O, Pr(NO 3 ) 3 ·6H 2 O, Sm(NO 3 ) 3 ·6H 2 O, Dy(NO3)3·6H2O, and Yb(NO3)3·6H2O were obtained from Nacalai Tesque, Kyoto, Japan, and used without purification. The lanthanide modification of MOR was carried out by the impregnation from an ethanol solution of the corresponding nitrate, drying at 80 °C, and calcination at 550 °C for 5 h in an air stream. Loading amounts were based on wt % of lanthanide metal on MOR. The resultant lanthanides are oxides, judging from thermal decomposition of the metal nitrates.45 The absence of nitrates was confirmed by XRD patterns after the calcination. 2.2. Cracking of TIPB and IPB. Cracking of TIPB and IPB was carried out using a conventional pulse reactor under N2 flow. Catalyst amount: 20 mg (weight as MOR); pulse size: IPB, 0.2 μL; TIPB, 0.4 μL at 400 °C and under 0.2 MPa of N2 pressure. The samples were injected on the catalyst by using a 5 μL syringe (Hamilton Co., Reno, NV). The products were analyzed with a GC-4C gas chromatograph (Shimadzu Corp. Kyoto, Japan) directly connected to a reactor with a column of 10 wt % Carbowax 6000 on Celite 545 of 2 m length (GL Sciences, Tokyo, Japan) at 80 °C. 2.3. Isopropylation of BP. The isopropylation of BP was carried out in a 100 mL SUS-316 autoclave using propene as an alkylating reagent. Standard conditions were 50 mmol (7.7 g) of BP, 250 mg of catalyst (based on MOR), 0.8 MPa of propene pressure, and 4 h of time at 200−300 °C. The autoclave was purged with nitrogen before heating and heated to reaction temperature. Then propene was supplied to the

3. RESULTS AND DISCUSSION 3.1. Characterization of Lanthanide Oxide-Modified MOR. XRD peaks assigned to CeO2 were observed at their 30 wt % loadings as metal. However, no significant peaks assigned to other lanthanide oxides La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 were found even at high loadings (see Figure S1 in Supporting Information). Figure 1 shows the specific surface areas of lanthanide oxidemodified MOR based on the weight of MOR after the

Figure 1. Specific surface areas of lanthanide oxide-modified MOR based on g-metal over the weight of MOR. Legend (lanthanide oxide): ■: CeO2; □: La2O3; ●: Pr2O3; ○: Sm2O3; ▲: Dy2O3; △: Yb2O3. 12215

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Figure 2. SEM images of CeO2- and La2O3-modified MOR. (a) MOR; (b) CeO2(30 wt %)-modified MOR; (c) La2O3(30 wt %)-modified MOR.

Figure 3. o-Xylene adsorption on CeO2- and La2O3-modified MOR.

Figure 4. NH3-TPD of CeO2- and La2O3-modified MOR. Legends (loading of metal): red line, unmodified; blue line, 5 wt %; light green line, 10 wt %; violet line, 20 wt %; dark green line, 30 wt %.

significant roles for the surface area of modified MOR, judging from the low surface area of the bulk oxides. Note that high surface areas of lanthanide oxide-modified MOR, as measured by N2 adsorption, do not always maintain open pore entrances for the access of bulky organic molecules, such as o-xylene, TIPB, IPB, and BP, to MOR channels. Figure 2 shows SEM images of CeO2- and La2O3-modified MOR at 30 wt % loadings. There were no changes in the morphology of MOR in the images, and the newly forming phase was not observed either. The maps of Ce, Si, and Al of CeO2 (30 wt %)-modified MOR calcined at 550 °C are shown in our previous paper.42 Aluminum and silica can be seen through the CeO2 layer of CeO2 (30 wt %)-modified MOR: cerium oxide covers MOR, keeping the pore entrance open. Figure 3 summarizes rates and amounts of o-xylene adsorption on CeO2- and La2O3-modified MOR based on 1 g of MOR. The initial rates were very rapid for all MORs;

modification (see also Figure S2 in Supporting Information). The surface areas were highly dependent on the states of modified oxides. Interestingly, the surface areas of CeO2modified MOR remained high even at a 30 wt % loading, whereas those of MORs modified with La2O3, Sm2O3, and Dy2O3 gradually decreased although they remained more than 400 m2/g-MOR. The surface areas of MOR modified with Pr2O3 and Yb2O3 were also ca. 400 m2/g for less than 20 wt % loadings; however, they decreased much for a 30% loading. These differences reflect the surface areas of oxides prepared from the corresponding nitrates by calcination at 550 °C. Particularly, CeO2 had high surface area (50 m2/g) compared to the other oxides La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 (less than 2 m2/g). These results should influence on the states of lanthanide oxides of modified MOR. That is, the surface area of CeO2 is involved in the modified MOR in addition to the original surface area of MOR. However, other oxides have no 12216

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Figure 5. Catalytic cracking of TIPB and IPB over lanthanide oxide-modified MOR. Reaction conditions: MOR, 20 mg; TIPB, 0.4 μL or IPB, 0.2 μL; temperature, 450 °C; carrier gas, N2, 0.2 MPa. Legend: see Figure 1.

modified with the other lanthanide oxides Pr2O3, Sm2O3, Dy2O3, and Yb2O3 were very similar to those of La2O3-modified MOR (see Figure S4 in Supporting Information). These lanthanide oxide-modified MORs retained high acid amounts at 5−10 wt % loadings while keeping the pores open, but choking of pore entrances occurred at high loadings. These differences in NH3-TPD based on the types of lanthanide oxide correspond well to the results of adsorption of N2 and oxylene as discussed in the previous section. 3.2. Cracking of TIPB and IPB over Lanthanide OxideModified MOR. The cracking of TIPB occurs only at external acid sites of MOR, and the internal acid sites cannot work as active sites because of the steric limitation for the entrance of TIPB into the channels. However, the cracking of IPB occurs at internal and external acid sites because IPB can enter MOR channels. The differences in the reactivities of the two hydrocarbons among lanthanide-modified MORs indicate whether the catalysis occurs at internal and/or external acid sites. Figure 5 summarizes the cracking of TIPB and IPB over lanthanide oxide-modified MORs. Lanthanide oxides La2O3, CeO2, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 effectively deactivate cracking of TIPB at less than 10 wt % loading. The results show that these lanthanide oxides effectively deactivate external acid sites by modification with a small loading amount. The cracking of IPB was highly dependent on the loading amount as well as the type of oxide, because the activity originated from both internal and external acid sites. CeO2 kept the activities high even at 30 wt % loading, whereas the other oxides rapidly decreased the activities as the loading amount increased. These results indicate that the lanthanide oxides prevent the cracking of IPB by deactivation of external acid sites through control of the pore entrance size. Pore entrances of MOR remained open even at a high loading of CeO2; however, the size of the pore entrances was reduced for the other oxides as the loadings increased, resulting in a decrease in activity for the cracking of IPB. 3.3. Isopropylation of BP over Lanthanide OxideModified MOR. Figure 6 summarizes the effects of loading amount of lanthanide oxides on BP conversion in the isopropylation of BP over unmodified MOR at 300 °C (see also Figure S5 in Supporting Information). BP conversion, an index of the catalytic activity, is highly dependent on the type and loading amount of the oxide. The catalytic activity of CeO2-modified MOR remained high even at a 30 wt % loading.

however, the amounts of saturation varied with the oxides and their loadings. The amounts of o-xylene adsorbed on CeO2modified MOR remained at similar levels irrespective of loadings, even at the 30 wt % loading. These results indicate that o-xylene adsorbs in the MOR channels because the internal surface is much larger than the external surface. However, the amounts of adsorption rapidly decreased as the La2O3 loading was increased. Modification with the other lanthanide oxides Pr2O3, Sm2O3, Dy2O3, and Yb2O3 also resulted in a decrease in the amount of adsorption as the loadings were increased (see Figure S3 in Supporting Information). These results indicate that CeO2 keeps the pore entrances open even at high loadings; however, the other lanthanide oxides La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 reduce the pore entrance as loadings are increased even though they have high surface areas as measured by N2 adsorption. NH3-TPD profiles of CeO2- and La2O3-modified MORs are shown in Figure 4. Two types of peak were observed in the profiles: the peak centered at around 200 °C (l-peak) assigned to physical adsorption, probably on lanthanide oxides, and the peak around 400 °C (h-peak) assigned to the adsorption on protons. The h-peaks originate from the adsorption on acid sites responsible for acid catalysis. The acid strength of MOR was not changed by the modification with CeO2 and La2O3 because peak temperatures of the h-peak, an index of acid strength, were not changed even at high loadings of CeO2 and La2O3. However, the acid amounts were influenced by the modification because the size of h-peak, an index of acid amount, displays different features between CeO2- and La2O3modified MORs. The size of h-peaks remained almost constant for CeO2 even at high loadings, although they decreased compared with unmodified MOR. However, the modification with La2O3 remarkably decreased the size of h-peaks, particularly at high loading. The l-peaks also have different features between CeO2- and La2O3-modified MORs. The size of l-peaks gradually increased for CeO2-modified MOR; however, large l-peaks were observed for La2O3-modified MOR, particularly at high loadings. The differences also suggest a hindrance, caused by the choking of pore entrances, for the access of NH3 to acid sites in the MOR channels. These h- and l-peak results suggest that the NH3 is accessible to MOR channels even at high CeO2 loadings without significant hindrance; however, La2O3 hinders the access of NH3 by choking pore entrances particularly at high loading, resulting in a decrease in acid amounts. NH3-TPD profiles of the MORs 12217

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selectivities for 4,4′-DIPB with high conversion. The modification with other lanthanide oxides also improved the selectivities for 4,4′-DIPB at loadings less than 10 wt % without significant loss of catalytic activities; however, the activities rapidly decreased with a further increase in the loading amounts. The deactivation of external acid sites on CeO2modified MOR was also observed in the isomerization of 2,6DIPN during the isopropylation of NP.42 Figure 8 shows effects of the loading amount of lanthanide oxides on selectivities for 4,4′-DIPB in the isopropylation of BP

Figure 6. Effects of lanthanide oxide modification of MOR on BP conversion in the isopropylation of BP at 300 °C. Reaction conditions: BP, 200 mmol; catalyst, 1.0 g as MOR; propene pressure, 0.8 MPa; period, 4 h. Legend: see Figure 1.

The activity of other lanthanide oxides, La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 remained at a high to moderate level at 5− 10 wt % loadings, while the activities rapidly decreased at the higher loadings. These features of BP conversion are very similar to those for the cracking of IPB (Figure 5). Effects of loading amount of lanthanide oxides on the selectivity for 4,4′-DIPB at 300 °C is shown in Figure 7 (see

Figure 8. Effects of lanthanide oxide modification of MOR on the selectivity for 4,4′-DIPB in the isopropylation of BP at 250 °C. Reaction conditions: BP, 200 mmol; catalyst, 1.0 g as MOR; 0.8 MPa; period, 4 h. Legend: see Figure 1.

at 250 °C. For all MORs including unmodified MOR, the selectivities for 4,4′-DIPB were almost constant at a level of 85−90%, irrespective of the BP conversion. These results indicate that the isomerization of 4,4′-DIPB did not occur significantly at 250 °C although a rapid decrease in the BP conversion as the loadings increased was observed for all MORs. However, CeO2 offered high conversions among the lanthanide oxides (see Figures S5 and S6 in Supporting Information). Figure 9 shows the effects of the loading amount of lanthanide oxide on the yield of 4,4′-DIPB in the isopropylation of BP over lanthanide oxide-modified MORs at 300 °C (see also Figure S7 in Supporting Information). CeO2 modification enhanced the yield of 4,4′-DIPB to 55−65% even at 30 wt % loadings. The modification with other oxides also improved the yield of 4,4′-DIPB up to 10 wt % loadings; however, the yield decreased rapidly as the loading amount increased. These high yields of 4,4′-DIPB were achieved by the prevention of isomerization of 4,4′-DIPB as a result of effective deactivation of the external acid sites by the lanthanide oxides. The results from the isopropylation indicate that modification of MOR with CeO2 retained open pores even at a high loading. The other lanthanide oxides La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 decreased the catalytic activities as the loadings increased. Appropriate loading amounts, up to 10 wt % for these oxides, retain open pores and deactivate external acid sites. However, the size of the pore entrances decreases as the loading of lanthanide oxides increases, and finally, entrance of bulky BP into the pore is hindered. Note that the selectivities for 4,4′-DIPB at 250 °C and 300 °C remained almost constant

Figure 7. Effects of lanthanide oxide modification of MOR on the selectivity for 4,4′-DIPB in the isopropylation of BP at 300 °C. Reaction conditions: see Figure 6 Legends: see Figure 1.

also Figure S6 in Supporting Information). The isopropylation of BP at 300 °C gave only 30% of the selectivity for 4,4′-DIPB although the selectivities were maintained at 80−90% at lower temperatures.5−7,13 The decrease is due to the isomerization of 4,4′-DIPB to thermodynamically more stable 3,4′-DIPB at external acid sites, because the selectivity was around 80−85% in encapsulated products.6,7 Modification with the lanthanide oxide enhanced the selectivities almost up to the level over MOR at 200−250 °C. The deactivation of external acid sites on lanthanide oxide-modified MOR effectively prevented the isomerization of 4,4′-DIPB. Particularly, CeO2 gave high 12218

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Generally, the modification of zeolites by metal oxides influences catalysis in two ways. First, external acid sites are deactivated by the oxides, precluding the catalysis on them. Second, the oxides retard the diffusion of reactants and products into and from the zeolite because the pore entrance size is decreased, resulting in retardation of catalysis and enhancement of the formation of less bulky products. The cracking of bulky TIPB was effectively prevented by modification with lanthanide oxides because the external acid sites were deactivated. However, there are differences in catalytic activities between the lanthanide oxides for the cracking of IPB. The oxides La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 retard the cracking of IPB as the loading amount is increased; however, the modification of MOR with CeO2 retained high activity even at a 30 wt % loading. The modification of MOR with lanthanide oxides also effectively deactivated external acid sites in the isopropylation of BP, precluding the isomerization of 4,4′-DIPB. CeO2modified MOR displayed high activities and selectivities for 4,4′-DIPB, because the pores remained open even at high loadings. Appropriate loading amounts of 5−10 wt % for the oxides La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 effectively deactivate external acid sites while keeping pore entrances open, but the catalytic activity decreased rapidly as the the loadings increased. The results of o-xylene adsorption and NH3TPD suggest that these lanthanide oxides reduce the pore entrance size as the loading is increased and finally choke pore entrances, precluding access of bulky organic molecules into the pore entrances of MOR. Interestingly, the selectivities for 4,4′DIPB remained almost constant after the modification. This means that the catalysis occurred through pores open even at high loadings, and that the controlled pores by “cornice” are not effective for the isopropylation of BP: the isopropylation of BP does not occur at such narrow pores but only at the channels BP can access. Thus, the decreases in catalytic activities are mainly due to the prevention of diffusion of BP and the products into and from MOR channels by the modification with these oxides. We previously described that La2O3 and CeO2 modification of MFI improves the para-selectivities in the ethylation of EB.44 CeO2 modification can improve the para-selectivities by the deactivation of the isomerization of p-diethylbenzene (p-DEB) at external acid sites without a reduction in pore sizes. The para-selectivities were also improved by La2O3 modification due to the deactivation of the isomerization of p-DEB at external acid sites as well as the reduction of pore size. These results show characteristics similar to those from the modification of MOR with La2O3 and CeO2. La2O3 can reduce the pore entrance size of zeolites MFI and MOR, resulting in the retardation of IPB cracking and the enhancement in the selectivity for p-DEB.

Figure 9. Effects of lanthanide oxide modification of MOR on the yield of 4,4′-DIPB in the isopropylation of BP at 300 °C. Reaction conditions: see Figure 6 Legends: see Figure 1.

even at high loadings (Figures 7 and 8). In these cases, the catalysis occurred through the open pores even for high loadings. Thus, the pore entrances controlled by “cornice” are not effective for the isopropylation of BP. BP cannot access such narrow pore entrances: the isopropylation occurs only in pores BP can enter. These results indicate that lanthanide modification with an appropriate loading is an effective method to deactivate external acid sites and to enhance shape-selectivity in zeolite catalysis. 3.4. Lanthanide Oxides on the MOR Surface. A new type of acidity generally appears by mixing metal oxides of different valences.46 However, only typical l- and h-peaks were observed in the NH3-TPD profiles of lanthanide oxide-modified MORs, revealing that no new types of acid were formed between the lanthanide oxides and silica of MOR, and that lanthanide oxides only cover the external surface of MOR and do not interact chemically. Scheme 1 shows the supposed Scheme 1. Suggested Coverage of Lanthanide Oxides on the MOR Surface. A: La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3. B: CeO2

features of CeO2 and the other oxides. Judging from the results of XRD and N2 adsorption, CeO2 is microcrystalline on the external surface of MOR; however, the other oxides La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 are amorphous on the MOR surface. The latter oxides are possibly deposited on the external surface as a dense phase, resulting in the reduction of pore entrance size by formation of “cornice” for highly loaded samples. We previously reported that silica and aluminum could be observed in SEM images of CeO2 (30 wt %)-modified MOR calcined at 550 °C.42 These results reveal that CeO2 is highly dispersed on the MOR surface, retaining open pores with microporosity.

4. CONCLUSION The modification of MOR with lanthanide oxides La2O3, CeO2, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 was examined for the deactivation of the external acid sites and the enhancement of catalysis inside the pores and channels. The deactivation was confirmed in the cracking of TIPB and IPB and in the isopropylation of BP. The cracking of TIPB and IPB showed that the external acid sites were effectively deactivated by the modification of MOR with lanthanide oxides only in small amounts. The catalytic activities for cracking of IPB rapidly decreased as the loading 12219

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amounts increased; however, only CeO2 retained high catalytic activities, even at a 30 wt % loading. The pore entrances remained open for the access of IPB even after the modification with CeO2, whereas other oxides rapidly reduced the size of pore entrances as the loading amounts increased, resulting in a loss of catalytic activities. The modification of MOR with lanthanide oxides effectively prevented the isomerization of 4,4′-DIPB in the isopropylation of BP, which occurred extensively at high temperatures. The isomerization was effectively deactivated by 5−30 wt % loadings of CeO2 without significant loss of activities at 300 °C. The other oxides also prevented the isomerization at 5−10 wt % loadings while activities were maintained; however, catalytic activities decreased for higher loadings. The differences in the results between the lanthanide oxides suggest differences in their physicochemical properties. These oxides are classified as two types on the basis of the state of the external surface. CeO2 is microcrystalline on the external surface, the pore entrances remaining open even at a high loading of 30 wt %. However, La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 cover the external surface as amorphous oxides, and they reduce the size of the pore entrance by the formation of “cornice”, resulting in a loss of activity for the cracking of IPB and for the isopropylation of BP. The catalytic activities in the cracking of IPB and in the isopropylation of BP remained high even for a high loading of CeO2. We believe that the lanthanide modification of zeolites for the deactivation of external acid sites has merit for practical catalytic technology. However, we cannot definitively describe the surface oxide states due to the modification; thus, detailed research on this and other aspects are still underway.

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ASSOCIATED CONTENT

S Supporting Information *

XRD, N2 isotherms, NH 3 -TPD profiles, and o-xylene adsorption, and effects of reaction temperature on the isopropylation of BP over lanthanide-modified MORs are shown in Figures S1−S7. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Y.S.) Tel: +81-47-343-3439. E-mail: [email protected]. (A.V.) Tel: +61733464122. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

A part of this work was financially supported by Grant-in Aid for Scientific Research (B) 16310056, 19061107, and (C) 2151009, the Japan Society for the Promotion of Science (JSPS). A. Vinu thanks the Australian Research Council for the Future Fellowship and the University of Queensland for a startup grant.

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