Acidity modification of MCM-22 for selective p-xylene formation in toluene disproportionation

Studies in Surface Science and Catalysis, volume 158 J. t~ejka, N. 7,ilkov~iand P. Nachtigall (Editors) 9 2005 ElsevierB.V. All rights reserved.

1843

Acidity modification of MCM-22 for selective p-xylene formation in toluene disproportionation V. Mavrodinova a *, M. Popova a, R.M. Mihfilyi b, G. Pfil-Borb61y b and Ch. Minchev a alnstitute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria bChemical Research Center, Institute of Surface Chemistry and Catalysis, Pusztaszeri 6t 59-67, 1025 Budapest, Hungary Improved p-xylene selectivity upon toluene disproportionation has been achieved with MCM-22 zeolite by controllable modification of its acidity through proton replacement with InO + and Cs + counter ions. The former cations were introduced by Reductive Solid-State Ion Exchange (RSSIE) starting from In203. The favorable effect of such modification has been compared with the merely proton replacement with cesium cations accomplished by SolidState Ion Exchange (SSIE). It was found that the Lewis-connected InO + acid sites contribute to the enhanced release of the p-isomer. 1. INTRODUCTION Since its introduction in the early nineties [1 ], MCM-22 zeolite has been widely studied [2-8] and reviewed [9] as catalyst for aromatics transformations. The first results from its structure characterization made by use of catalytic test reactions [2] show an intermediate behavior between medium and large pore zeolites. Later, it appeared that depending on the particular reactant and products involved in the catalytic interaction quite unexpected selectivities were obtained. Higher than the equilibrium concentration of the p-isomers has been observed in toluene alkylation with methanol [2,3] and in the di-alkylated products of benzene alkylation with propylene [4,5] but not in the direct toluene alkylation with propylene [5]. In toluene disproportionation it has been found by Wu et al [6] and Park et al. [7] as well as by Jutto [8] that the fraction of p-xylene in xylene isomers exceeded the equilibrium value. The former authors propose dealumination procedure for additional selectivity improvement in MCM-22 and claim them as most promising catalysts for selective toluene disproportionation if the secondary xylene isomerization is suppressed. In a previous investigation we tried to restrict the accompanying isomerization reaction upon toluene disproportionation over MCM-22 by poisoning the proton sites with alkali metal (Cs +) cations [ 10]. It was not successful since higher p-xylene selectivity was reached only at lower toluene conversions compared to the parent MCM-22 material. It was presumed that introduction of another type of centers neutralizing the isomerization ones but enhancing those responsible for disproport|onation would be more effective. An attempt to check whether Lewis acid InO + cations could be such an alternative ion has been undertaken in the present contribution. Their introduction into Beta and dealuminated or ultrastabilized Y zeolites [11-13] showed an enhancement of the reaction of alkyl transfer in alkyl aromatics

1844 transformations and, in some cases, an increase in the p-selectivity of the di-substituted products. In the present work the effect of modification of the nature and concentration of the acid centers in MCM-22 material by proton replacement with InO + and/or with Cs + ions on the pxylene production upon toluene disproportionation has been compared. 2. E X P E R I M E N T A L

2.1. Catalysts preparation and treatment The initial MCM-22 zeolite was synthesized according to the method used in Ref. [14]. The synthesis mixture (28.34 SIO2:A1203:0.89 Na20:7.3 Na2SO4:14.3 HMI:1185 H20) was crystallized at 418 K for 10 days. The template, i.e. hexamethylene-imine (HMI) was removed in air (853 K for 1.5 h) and a sample with Si/AI=13.1 was obtained. The Cs modified materials were prepared by mixing H-MCM-22 and CsC1 in Cs:A1 molar ratios of 0.15 and 0.30 and designated as 0.15Cs/MCM-22 and 0.30/CsMCM-22. The mixtures were heated up to 823 K (5 K/min) in N2 and kept at this temperature for 2 h according to the procedure for SSIE with Cs of zeolite Beta proposed by us in Ref.[ 15]. The preparation of the In-modified catalyst with In:A1 mol ratio of 0.3, namely 0.3In/MCM-22, is thoroughly described in Refs. [12,13]. In the case of the mixed (In+Cs)/MCM-22 catalyst, 0.15Cs/MCM-22 was first prepared and calcined and then mixed with the corresponding amount of In203 (In:Al=0.3). The respective procedure for thermal treatment of the mixture at 723 K in hydrogen followed by oxidation in air at the same temperature was applied in order to obtain InO § cations together with the Cs § ones. The chemical composition of the catalysts is shown in Table 1. A1 content was determined by AAS after wet digestion with hydrofluoric acid. 2.2. Characterization The crystallinity and phase purity of the parent and ion-exchanged samples were checked by a Philips PW 1810 powder diffractometer, equipped with a graphite monochromator, using CuK, radiation. Textural properties of the studied samples were determined from the N2 adsorption isotherms at 77 K and are given in Table 1. Before N2 adsorption samples were pretreated at 623 K for 10 h in 02 atmosphere. FTIR spectra were measured with a Nicolet Impact 400 spectrometer using the wafer transmission technique and pyridine (Py) as probe molecule. Sample treatments were performed in high vacuum (HV) of about 10-3 Pa for 0.5 h at 723 K. After pretreatment the wafers were contacted in situ with Py at 473 K (5.7 mbar) for 0.5 h and then degassed in HV for 0.5 h at temperatures gradually increased between 373 K and 673 K. 2.3. Catalytic experiments The catalytic experiments were performed in a vapor-phase flow-through microreactor at atmospheric pressure in N2 flow [ 12,13]. The carrier gas passed through a saturator filled with toluene (Ptol=0.9 kPa). The reaction products were analyzed on-line by using GC (HP 5890, Series II) equipped with FID and a 25 m HP-FFAP capillary column. 3. RESULTS AND DISCUSSION The power XRD pattem of the hydrothermally synthesized and calcined MCM-22 sample (Fig. 1, diffractogram A) agreed well with those reported in the patent literature [1]. Upon solid-state ion exchange with indium or cesium the slight intensity decrease of most of the

1845 reflections has to be rather attributed to the increase in the mass absorption coefficients of the samples than to the zeolite lattice damages (Fig. 1, diffractograms B and C). 16 2

1452

0.1

C

J

1690

ii/1441 'x,,,..,a

~1z~55~1

9

1'0

' 2'0 3'0 ' Bragg angle/~

4'0

Fig. 1. X-ray patterns of the parent HMCM-22 (A), 0.3In/MCM-22 (B) and 0.3Cs/MCM-22 (C) samples.

1700

16'00

|

1500

1400

Wavenumber/cm ~ Fig. 2. FTIR spectra of pyridine retained at 373 K on the parent MCM-22 zeolite (A), 0.3Cs/MCM-22 (B), 0.3In/MCM-22 (C) and 0.15Cs+0.3In/MCM-22 (D) samples. After A, B and C further degassing were performed at 473 K (A1,BI,C1), 573 K (A2,C2) and 673 K(A3,C3).

In Fig. 2 the IR spectra of the ring vibration range of pyridine adsorbed on the parent zeolite and the samples ion-exchanged with Cs and/or In were shown. Compared to the parent material (spectrum A) in case of the ion-exchanged samples the intensity of the pyridinium band at 1544 cm -1 decreased (spectra B, C and D) but meanwhile new bands ascribed to the 19b and 8a ring vibration modes of Py coordinatively bounded to Cs + cations (1441 cm -1 and 1590 cm -l, spectra B and D) and to InO + cations (1452 cm l and 1612 cm -i, spectra C-C3, D) appeared in the spectra. These results confirmed that the partial replacement of the bridging OH groups of the initial material (spectra A) occurred upon the SSIE with Cs + (spectra B and D) and during the RSSIE with InO + (spectra C and D). The bands at 1441 and 1590 cm -1 disappear completely upon degassing the Cs containing samples between 373 and 473 K (spectra B and B 1), while the bands at 1452 and 1612 cm -1 were found to decrease in intensity only when the 0.3In/MCM-22 sample was treated in high vacuum at temperatures exceeding 673 K (spectra C-C3). Thus, InO + cations represent Lewis acid sites with much higher acid strength than Cs + cationic species. It is worth noting that the bands at 1455 and 1623 cm -~ reveal the parent MCM-22 zeolite (spectra A-A3) to contain extra-framework aluminum which shows a similar high stability towards evacuation at higher temperatures as InO + cations. According to the N2 adsorption measurements the parent HMCM-22 has a specific surface area typical for this type of zeolite (Table 1). Both the specific surface area and the pore volume were found to decrease with the introduction of the cationic species. This is an indication that the exchanged cations reduced the pore volume accessible for the adsorptive. Among the studied catalysts the sample which contains both indium and cesium cations possesses lowest surface area and pore volume.

1846 Table 1 Chemical composition and textural characteristics of the catalysts Samples

A1

Cs

In

Specific area a Pore volume a

mmol g-lcalc, b HMCM-22 1.33 0.39 0.3 Cs/MCM-22 1.31 0.3In/MCM-22 1.32 0.39 0.39 (0.3In+0.15Cs)/MCM-22 1.31 0.19 a Calculated from the B point of the adsorption isotherms. bRelated to the sample weight after calcination at 1273 K.

m 2 ~'lair.drie d 518 457 487 431

c m 3 ~-lair_dried

0.184 0.162 0.173 0.153

Owing to the reduced proton acidity, a large decrease in the degree of toluene conversion is observed on 0.3Cs/MCM-22 sample and, on the contrary, higher initial conversion is registered for 0.3In/MCM-22 compared to the parent material (Fig. 3).

--I--HMCM-22 ~ 14

9

--A--O.3Cs/MCM-22

oE 12 \ --o-O.31n/MCM-22 ~, u,~ -4)--(0.31 n+O.15Cs)/MCM-22 0~ om~i..l~ CD 8 9 o~ ii~ >~\ o~, m~ o 6 OkOkl~i 4

A "A-A

I--

.

o

.

9 .

.

.

A.

.

A ~ A .

A

.

4:0 go go 16>o15o1 o1 olgo TOS/min

Fig. 3. Toluene conversion in dependence on TOS for the parent and modified catalysts (Tr=573 K, W H S V = 1.2 h-l). This effect of the incorporated InO + cations has been already observed by us for zeolites with different structures [ 11-13]. The strong Lewis-acid InO + sites were found to contribute to the faster alkyl transfer upon the reaction of alkyl aromatics disproportionation. It holds true also for MCM-22 material and proves its validity in comparison with the samples exchanged with Cs cations. Addition of Cs to the InO + containing material (0.3In+0.15Cs/MCM-22) results in substantial activity decrease without stabilization of the activity as it was expected. Replacement of part of the proton sites in all modified catalysts leads to enhanced p-: (o + m)xylene ratio compared to the parent sample. In order to compare correctly the p-xylene selectivity and to establish the primary isomers, different contact times were applied to attain close and low enough initial toluene conversion (about 4 mol %, Fig. 4). As can be seen in the figure, the catalyst exchanged with In provides almost two times higher initial p-xylene formation (Fig. 4B). During the whole experiment and independently on the catalyst deactivation and the diminution in the xylene production, the p-xylene selectivity is increasing slightly (from 58% to 63%) over 0.3In/MCM-22 and

1847 remains higher than that of the parent and Cs-modified (48% - 52%) catalysts. On the contrary, a preferential replacement of the protons from the surface pockets and the supercages with Cs is suggested to take place upon a liquid phase [ 16] or solid-state exchange procedure [10]. It only slightly improves the p-selectivity of this catalyst but stabilize its activity. When the same amount of another type of Lewis-acid sites, i.e. InO + cations is introduced, higher toluene conversion directed to preferential p-xylene formation is observed.

2,0- B

--m--o-x~

--n--l-IlVlC~22 (4.1h-1) -O-0.3irYlVICM-22 (4.2 h-1) -&-0.3Cs/MCM-22 (0.6h-1)

o~ o

E r

O

~> m • l - A - A - - - - - i • , i ~,&

~,&

o. ~ O _ _ _ _ E

~2.

~O~o~

~m------m O--O~o~ O

- o - 0 . 3 ~ 2 2 (4.2h-1),p-xy

20 40 60 80 100 120 140 160 TOS/rnin

-~-o-x~

E

9

t~

\

c:z 1,0>,, x

-i-O3Ds/M3~22 (0.Eh-1),p-xyl

-ZX-rn.x~ -4,-o-x9 --A~~~A~.~

o 0,5"o

o

0

--o-m-xyl

1,5

N

i- 0

-nn-F-IVaVI22(4.lh-1), p-xyl -E]-m.x~

A

\o---o~. 0=~~--Wo -0~0~ 0

o,o

0 20 40 60 80 100 120 140 160 T ( ~ nTin

Fig. 4. Comparison of the yield of xylenes (B) at close initial toluene conversion (A), attained at different WHSV in dependence on TOS (Tr =573 K).

Neutralization of additional portion of active sites induced by the Cs supplement to the Inmodified catalyst (Fig. 5) permits undisturbed release of the primarily formed p-isomer in presence of indium. Compared to the sample exchanged with Cs only the enhanced contact time used in case of the mixed catalyst does not lead to acceleration of the xylenes interconversion. According to Wu et al.[6] the larger (12 MR) surface cups and cages contribute to the reactant disproportionation, since the proton concentration in the smaller (10 MR) channels together with that in the surface pockets determine the extent of xylene isomerization. Considering this, the isomer distribution in our case will be controlled either by the number of protons left in the latter or by the diffusion hindrances imposed by the particular cation. If one approves that the strongest protons are localized in the 12 MR supercage systems [ 17] and on the external cups and are the sites subjected to preferential exchange in accordance with Ref. [ 16] for both cations, the former factor is suppose to lose its significance since same amount of equally localized protons is expected to be replaced by them. Then the spatial constraints posed by these ions should play a decisive role. The bimolecular disproportionation will occur predominantly on the protons left into the inner cavities and the sizes of the 10 MR openings as well as the openings of the 10 MR channels to the exterior will regulate the xylene distribution in dependence of the rate of their diffusion. The transport rate of the p-isomer from these openings narrowed in a greater extent by the bulkier InO + cation should be higher and its preferential formation seems to be a consequence of an enhanced product shape selectivity effect compared to the parent material.

1848 3,0

-m-HIV[lVI-22 (2.lh-1) -~ 12. - o - - C ) . 3 ~ r d ~ ~ (2.4h-1) -~--(0.31n+0.15Cs~22 (1.2h-1) E -~ 10~ O

o~ 2,5 m O

9

E 2,0

; 8~

t-

O

~l~)li-i\m__.m_i~

_~ 1,5 x 6_ 1,0, 0 -- 0,5 >-

m

21 O ~-

--m--HMCM-22 --O--0.31n/MCM-22 --O--(0.3l n+0.15Cs)/MCM-22

0,0

0 0

20 40 60 80 100120140160180 TOCa/rain

1,0-

--m-HMCM-22 -o-0.31n/MCM-22 --O--(0.31n+0.15Cs)/MCM-22

o~

0

20 40 60 80 100 120 140 160 180 TOS/min

3,0

C

o~ 2,5

--B--HMCM-22 --o--0.31n/MCM-22 ~(0.3In+0.15Cs)/MCM-22

9

~2,0 0,5

9

1,5

,m\m~ 9 "(::)

m-m~ m

\O.._

9

,

~

0,0

20 40 60 80 100 120 140 160 180 TOS/min

~ i ~ i ~

\~0,.~O.~

o -o 0,5

i - - m ' - i---------m

0,0 . . . . . . . . . . . . . . . 0

0!~~0 ~i~i~n

1,0

9

0

i

9

20

i

~O----.O~

|

9

40

J

60

9

|

9

|

9

u

,

u

9

!

.

" "~'-'

80 100 120 140 160 180 TOS/min

Fig. 5. Yield of xylene isomers (B-D) at close toluene conversion (A), in dependence on the TOS for the parent and modified catalysts (Tr=573 K) The following explanation of the differences in the behavior of the Cs- and In-containing catalysts could be suggested for the p-xylene selectivity of the studied catalysts (Fig. 6). The 12 MR cages are the only locus of activity for the main disproportionation reaction where 9 9 4x A 9 9

706O

50 > o 40 (1) 30

9

m A

9

HMCM-22 0.08Cs/MCM-22 0.15Cs/MCM-22 0.30Cs/MCM-22 0.301n/MCM-22 (0.3In+0.15Cs)/MCM-22

~

9

o

9

t- 20 (l)

,x 10 0

o ' ~ ' ~,

~

~

1'o

1'2

1'4

Toluene conversion/mol.%

Fig. 6. p-Xylene selectivity in dependence on toluene conversion at Tr=723 K.

1849 actually the xylene isomers are created. The 10 MR channels are much smaller (4.0 x 5.5 A (between layers) and 4.1 x 5.1 A (within layers) [18]) and possess steric hindrances for the bulky reaction intermediate. If the surface pockets are assumed to be practically free from protons, the sites in the 10 MR channels are responsible for the isomerization of the xylene products. These sites provide undesired xylene interconversion especially at longer contact times as it is supposed in case of the Cs-exchanged catalysts. In contrast, in presence of InO + cations preferential formation of p-xylene takes place, accompanied by its fast desorption out of the pores. In addition, these cations reduce not only the number of surface sites like Cs does, but being bulkier also impede, most probably, the access of the xylene product to the 10 MR pores and out of the 10 MR openings of the 12 MR cages thus preventing from their further isomerization. The lower diffusion rates of the larger m- and especially o-isomer calculated by Corma et al. [4] for MCM-22 materials are most probably the reason for substantial concentration of the latter products in the limited space of the 12 MR cages. Their fast accumulation and transformation to high molecular weight carbonaceous deposits is most probably the reason for the fast deactivation observed on the In-containing catalysts and is discussed in another contribution [ 19]. 4. CONCLUSIONS Improved p-xylene selectivity upon toluene disproportionation over MCM-22 catalyst is achieved by replacement of the zeolite protons by InO + cations. These Lewis-connected acid centers provide dual contribution to the enhanced p-selectivity. On the one hand, an accelerated alkyl transfer directed to primary formation of the p-isomer is taking place in their presence. The reduction of the surface proton concentration, on the other hand, inhibits the mand o-xylene diffusion and prevents the consecutive isomerization reaction, thus enhances the preferential p-isomer production. ACKNOWLEDGEMENT Support of this work in the framework of the Hungarian-Bulgarian Inter-Academic Exchange Agreement is gratefully acknowledged. The Hungarian authors also thanks the Hungarian National Scientific Research Foundation (OTKA project No. T 046970) for financial support. REFERENCES

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