Heterogeneity of OH groups in H-mordenites: Effect of dehydroxylation

Heterogeneity of OH groups in H-mordenites: Effect of dehydroxylation J. Datka, B. Gil, and A. Kubacka Faculty of Chemistry, jagiellonian University, Cracow, Poland Quantitative i.r, studies of ammonia and pyridine sorption in H-mordenite showed that the amount of arensted acid sites (acidic hydroxyls) 6.5 Wlu.c. was close to the theoretical value calculated from the chemical analysis (7.2 W/u.c.). Approximately half ofthis amount (3.0 Wlu.c.) was situated inside the 12-ring channels (the main channels) and half (3.5 W/u.c.) inside the 8-ring channels. Hydroxyls in both kinds of channels were found to be prone to dehydroxylation to the same extent. The number of Lewis acid sites (created by dehydroxylation) detected by pyridine was much lower than detected by ammonia. This observation, together with the fact that in dehydroxylated H-mordenite most of the acidic hydroxyls are inaccessible to pyridine, suggests that dehydroxylation results in a narrowing of pores, making them inaccessible to pyridine (also to other bulky reactant molecules). We studied the acid strength of OH groups by comparing the stretching frequencies and extinction coefficients of free OH bands and in ammonia thermodesorption experiments. We found that in the nondehydroxylated H-mordenite, the acid strength of the OH groups inside the 12-ring channels was higher than inside the 8-ring. Dehydroxylation decreasesthe acid strength of hydroxyls. This concerns both the whole population of OH groups and the population of hydroxyls accessible to pyridine only. These effects are discussed considering the heterogeneity of OH groups in H-mordenite and removal of the most acidic hydroxyls first. © Elsevier Science Inc. 1996 Keywords: Mordenites; acid sites; heterogeneity; i.r. spectroscopy

INTRODUCTION NaH-mordenites have interesting acid and catalytic properties. The acidity of NaH-mordenites was studied by numerous authors usinz i.r. spectroscopy.l"" t.p.d.,10-12 microcalorimetry,!i,13-1S following adsor~ tion isotherms.I'' and e.p.r. studies of NO sorption. 20, 1 The acid properties of NaH-mordenites were modified by varying Na/H exchange degree, dealuminating by acid leaching, and by partial dehydroxylation. The present paper concerns the de hydroxylation of Hmordenite. Acid properties of dehydroxylated mordenites were studied by several authors by i.r. spectroscopy,l,2,5,9 t.p.d.,I0,12 and microcalorimetry.P and many interesting results concerning the heterogeneityI2.1~ and distribution of the strength 13 of acid sites were obtained. Pyridine penetrating only broad channels and ammonia penetrating both broad and narrow channels were used as probe molecules. No study, however, was undertaken using both bases parallel to discriminate the acid sites in the broad and narrow channels and to make possible studying them separately. We undertook Address reprint requests to Prof. Datka at the Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland. Received 1 November 1995; accepted 10 January 1996

quantitative i.r. spectroscopic studies of ammonia and pyridine sorption in the H-mordenite calcined at various temperatures (830-1050 K). The concentration of both Brensted and Lewis acid sites situated inside pores accessible and inaccessible to pyridine (but accessible to ammonia) was determined. The acid strength of Bronsted sites was studied following the frequencies and extinction coefficients of OH bands by i.r. studies of ammonia thermodesorption. Neither concentration nor the strength of acid sites in dehydroxylated mordenites was studied before.

EXPERIMENTAL The parent Na-mordenite Na7.2(Al02h.2(Si02)4o.s] was synthesized in Chemie AG (Bitterfeld-Wolfen Company). An NH 4 form was obtained by ionic exchange in 10% NH 4NO s solution at 355 K for 2 h (repeated fiv~ times). The chemical analysis (AAS) has shown practically 100% degree of exchange. NH 4-mordenite was pressed into thin wafers (5-10 mg em -2) and activated in situ in an i.r, cell in a vacuum (l * 10- 3 torr) at 830,890,950,990, and 1,050 K for 1

h. Ammonia (Linde Carbide) 99.97% (dry) was used without any further purification. Pyridine (POCh, Gliwice), analytical grade, was dried by KOH. The i.r. spec-

Zeolites 17:428-433, 1996

<0 Elsevier Science Inc. 1996 655 Avenue of the Americas, New York, NY 10010

0144-2449196/$15.00 PII 50144-2449(96)00009-7

Heterogeneity of OH groups in H-mordMites: J. Datks et st.

tra were recorded using a Bruker IFS 48 PC spectrometer equipped with an MCT detector.

RESULTS AND DISCUSSION

Hydroxyl groups in dehydroxylated H-mordenite The i.r, spectra of hydroxyl groups in H-mordenite calcined at 830-1,050 Kare presented in Figure 1 (spectra a). The spectra recorded upon the sorption of pyridine excess and the desorption at 570 K are presented in the same figure (spectra b). They represent only hydroxyls inaccessible to pyridine. In a nondehydroxylated zeolite they are localized inside the 8-ring channels. The difference spectra (a - b) representing hydroxyls accessible to pyridine are also presented (spectra c). In the nonhydroxylated mordenite they are localized inside the 12-ring channels. In the nondehydroxylated H-mordenites (calcined at 830 and 890 K) the frequency of hydroxyls in the 12-ring channels is 3,615 cm", in the 8-ring channels 3,584 cm", and a frequency of the whole hydroxyl band is 3,602 cm'". Figure 2 shows the integrated intensities of the hydroxyl bands. Curve a represents all the hydroxyls (in the spectrum of activated zeolite), curve b the hydroxyls inaccessible, and curve c hydroxyls accessible to pyridine (difference of a - b) as function of calcination temperature. Dehydroxylation results in the decrease of the amounts of all of the hydroxyls (both inaccessible and accessible. to pyridine), but this effect is less pron?unced m the cas~ of hydroxyls inaccessible to pyridme. Two explanations can be considered. One of them assumes that hydroxyls inside the 8-ring channels are less prone to dehydroxylation than those in the 12-ring channels, and their contribution increases upon dehydroxylation. Another possible interpretation assume~ ~at the ~2-ring channels are blocked by extrazeolitic matenal produced by dehydroxylation, which makes these hydroxyls inaccessible to bulky molecules. To check whether a preference in removal ordering exists, the spectra of H-mordenite calcined at 890, 950, and 990 K were normalized to the same integrated intensities (Figure 3). The band shape for all of the spectra is the same, indicatin! that the proportion between the 3,615 and 3,584 em bands representing hydroxyls in 12- and 8-ring channels does not change upon dehydroxylation. It means that both kinds of hydroxyls are prone to dehydroxylation to the same extent. It also suggests that the increase of the contribution of hydroxyls inaccessible to pyridine is due to blocking of the 12-ring channels by extrazeolitic material.

Acid sites in dehydroxylated H-mordenites

Infrared studies of ammonia sorption Ammonia reacts with acid sites of Brensted and Lewis

type forming NH 4 +and NHlIL species, and corresponding i.r. bands appear at 1,450 and 1,620 cm'". To determine the concentration of all acid sites (Brensted and Lewis type situated in both kinds of channels in dehydroxylated H-mordenites) small portions of ammonia (penetrating the 8- and 12-ring channels) were

sorbed at 320 K up to neutralization of all acid sites. Acid site concentrations were calculated from the intensities of NH 4+and NH 3L bands and their extinction coefficients (0.147 ± 0.0009 and 0.022 ± 0.0007 em" pmol", respectively) determined in our earlier study'' on the same H-mordenite sample. We observe the decrease in concentration of Brensted acid sites and the increase in concentration of Lewis sites with the calcination temperature (Figure 4, A and B, curves a). A small increase of the concentration of Brensted sites after calcination at 890 K (compared with 830 K) is because the activation of 830 K did not decompose all ammonium ions in the NH 4 form. The maximal concentration of Bronsted sites (observed in H-mordenite activated at 890 K), 6.5 H+ju.c., was close to the theoretical value (7.2 H+/u,c.) calculated from the chemical analysis.

Infrared studies of pyridine sorption Pyridine reacts with Brensted and Lewis acid sites, forming PyH+ and PyL species (the i.r. bands at 1,545 and 1,450 cm'"). To determine the concentration of Brensted and Lewis acid sites accessible to pyridine, the amount of pyridine sufficient to react with all acid sites was sorbed at 420 Kin H-mordenites calcined at various temperatures. Physisorbed pyridine was next removed by desorption at 570 K The amounts of Brensted and Lewis sites accessible to pyridine were calculated from the intensities ofthe bands at 1,545 and 1,450 cm" and their extinction coefficients. The extinction coefficient of the 1,545 cm" band of PyH+ was determined in experiments in which small measured portions of pyridine were sorbed at 420 K in zeolite HY containing only Brensted sites activated at 720 K The linear plots of the intensities of the 1,545 cm- I bands versus the surface concentration of sorbed pyridine were obtained, and the slopes of the lines were taken as the extinction coefficient of the 1,545 cm" band. The average value was 0.078 ± 0.0004 cm 2 pmol'". The extinction coefficient of the 1,450 cm" band of PyL was determined in experiments in which small measured portions of pyridine were sorbed at 420 K in zeolite HY activated at 1,100 K and containing mostly Lewis acid sites and only a few Brensted sites. The linear plot of the intensity of the 1,450 cm" band versus the amount of pyridine reacting with Lewis sites (difference between the amount of pyridine sorbed and the amount of pyridine reacting with Brensted sites) was obtained. The value of the extinction coefficient of the 1,450 em"! PyL band was 0.269 ± 0.01 em" pmol", The concentrations of Brensted acid sites accessible to pyridine decrease with the calcination temperature; the concentrations of the Lewis acid sites increase first (due to condensation of OH groups) and then decrease at higher temperatures because of the collapsing structure (Figure 4, A and B, curves b). The concentrations of acid sites inaccessible to pyridine (difference between the concentration of all acid sites determined by ammonia and sites determined by pyridine) are presented as well (Figure 4, A and B, curves c). As mentioned, in nondehydroxylated H-mordenite (activated at 890 K) the concentration of all Brensted acid sites (6.5 H+lu.c.) is close to the theoretical value Zeolites 17:428-433. 1996 429

Heterogeneity of OH groups in H-mordenites: J. Datka et al.

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CALCINATION TEMPERATURE [K]

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Figure 2 Integrated intensities of OH bands in the spectra of H-mordenites as a function of calcination temperature. e, all hydroxyls Un the spectra of calcined zeolites); b, hydroxyls inaccessible to pyridine (hydroxyls rema ining upon pyridine sorption and desorption at 570 K); C, hydroxyls accessible to pyridine (8 - b).

calcInation temperaturB 950 K 8

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calcInatIon temperature 990 K

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by dehydroxylation. The results obtained with pyridine and ammonia sorption confirm this interpretation, as the concentration of Lewis acid sites determined with pyridine is much lower than determined by ammonia (Figure 4B) . It should be noted that Karge1,2 and Kojima et al.5 reported also that the dehydroxylation dimin ished the concentration of Lewis acid sites accessible to pyridine. Acid strength of hydroxyl groups

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Comparison of the acid strength of OH groups in the 12-ringand 8-ring channels in the nondehydroxyla~ed Himordenite . . .

It was interestmg to check which kind of hydroxyl. III the 12-ring channels or in the 8-ring channels, were more acidic. Makarova and co-workers" suggested that hydroxyls at the 8-ring channels of lower stretching frequency are more acidic. It should, however, be considered that the stretching frequency of hydroxyls in the B-ring channels may be lowered by the intramolecular hydrogen bonding (O-H'" 0) with oxygen across the narrow channels. However, recently Maache et al.,24 based on i.r. studies of CO sorption, concluded that hydroxyls in the 12-ring channels were more acidic. To

Figure 1 Infrared spectra of OH groups in H-mordenites calcined at 830, 890, 950, 990, and 1,050 K. e, spectra of calcined zeolites; b, spectra recorded upon the sorp tion of pyridine and desorpt ion at 570 K; c, difference spectra (a - b).

(7.2 H+/u.c.). The concentration of Brensted acid sites accessible onl y to pyridine is about half this amount (3.0 H +/u.c.). Assuming that 8-ring channels are inaccessible to pyridine, we can say that the rest of the Brensted sites (3.5 H" l u.c.) are localized inside them. Dehydroxylation of H-mordenite changes distinctl y the proportion between the amounts of Brensted sites in pores accessible and inaccessible to pyridine, as one can see from Figure 4A, curves band c, and also Figure2, curves band c. The contribution of the Brensted sites inside pores inaccessible to pyridine increases with the calcination temperature. It can be caused by blocking the l2-ring channels by extrazeolitic material formed

430

Zeolites 17:428-433, 1996

3620

3580 cart

3540

Figure 3 Spectra of OH groups in H-mordenites calcined at 890, 950,990, and 1,050 K normalized to the same band area.

Heterogeneity of OH groups in H-mordBnites: J. Datks et 81.

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CALCINATION TEMPERATURE [K]

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CALCINATION TEMPERATURE [KJ Figure 4 Concentration of 8rlllnsted (A) and Lewis (B) acid sites in H-mor~eni~es as a func~ion of calcination temperature. a, all of the .aCld sites. (.determlned. by ammonia sorption); b, sites accessible to pvrldine (determined by pyridine sorption)' c sites inaccessible to pyridine (a - bl. ' ,

compare the acidity of both kinds of hydroxyls we undertook studies of extinction coefficient of both the l 3,615 and 3,584 cm- bands of hydroxyIs in the 12- and 8-ring channels, knowing that the extinction coefficient of the OH band increases with the acid strength. 22•23 The extinction coefficient of the i.r. band of hydroxyls in 8-ring channels was determined in experiments in which two adsorbates (pyridine and ammonia) were used. The problem of competition of these two bases to acid sites did not occur because pyridine (th e stronger base) was sorbed before ammonia (the weaker base). Three series of experiments were made. 1. To determine the average value of extinction coefficient of all hydroxyls (in the 12- and 8-ring channels) small portions of ammonia were sorbed at 320 K in H-mordenite activated at 890 K. Ammonia reacts with all of the hydroxyls. A linear decrease of the integrated intensity of the 3,602 em"! band versus the intensity of ammonium ions band at 1,450 cm- l was observed. The value of the integrated extinction coefficient of the i.r, band of all hydroxyls was calculated from the slope of th is line and the value of the extinction coefficient of the 1,450 em"! (determined in our earlier study"), The value 2.74 ± 0.01 em prnol'" was obtained. As mentioned, this value characterizes hydroxyls both in the 12- and in 8-ring channels.

2. To determine the extinction coefficient of hydroxyls in the 12-ring channels, small portions of pyridine, reacting only with hydroxyls in the main channels, were sorbed at 420 Kin H-mordenite. A linear decrease of the integrated intensity of the 3,602 ern"! band versus the intensity of pyridinium ions band at 1,545 em? was observed. The value of the integrated extinction coefficient of the i.r. band of hydroxyls in the 12-ring channels was calculated from the slope of this line and the value of the extinction coefficient of the 1,545 ern"! band determined in this study. The value 3.50 ± 0.01 cm pmol'" , characterizing hydroxyls in the main channels only, was obtained. 3. To determine the extinction coefficient of hydroxyls in the 8-ring channels, the hydroxyls in the 12ring channels were first poisoned by pyridine. Pyridine was sorbed in H-mordenite activated at 890 K and next desorbed at 570 K. Only hydroxyls in the 8-ring channels (3,584 cm") survived. Small portions of ammonia were sorbed, and a linear decrease of the integrated intensity of the 3,584 ern"! band versus the in tensity of the 1,450 em -I of NH~ was observed. The value of the integrated extinction coefficient of the i.r, band of the hydroxyl s in the 8-ring channels was calculated from the slope of this line and the value of the extinction coefficient of the 1,450 cm'" band (determined in our earlier studv"). The value 1.55 ± 0.01 ern pmol", characterizing hydroxyls in 8-ring channels, was obtained. The value of the integrated extinction coefficient of hydroxyl groups in the 12-ring channels (3.50 ± 0.01 em prnol'") is d istinctly higher than of hydroxyls in the 8-ring channels (1.55 ± 0.01 ern pmol'"), It may therefore be concluded that hydroxyls in the 12-ring channels are more acidic . As mentioned, the same conclusion was achieved by Maache et al. 24 from i.r. studies of CO sorption in H-mordenite.

Acid strength oj hydroxyl groups in dehydroxylated H-mordenite It can be interesting to study the influence of partial dehydroxylation on the acid strength of OH groups. The information on the acid strength was obtained by the comparison of the frequencies and extinction coefficients of i.r, bands of free hydroxyls and in the studies of ammonia thermodesorption. In the thermodesorption experiments, ammonia was sorbed in Hmordenite up to neutralizing all the acid sites, and next desorbed at 730 K for 30 min . The value A n o! Ao represents what percent of ammonium ions remaining in zeolite upon the desorption was taken as the measure of the strength of the Brensted sites. As ammonia sorption experiments provided information on the "average" acid strength of the whole population of acidic hydroxyls, the pyridine sorption experiments revealed the acid strength of hydroxyIs accessible to pyridine only. The results of our studies on acid strength are presented in Figure 5. All of the data show that partial

Zeolites 17:428-433. 1996

431

Heterogeneity of OH groups in H-mordenites: J. Datka et al.

A

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800

900

1000

1100

CALCINATION TEMPERATURE [K]

plained assuming the heterogeneity of the Si-OH-Al grou~s, 27 and removing the most acidic hydroxyls first. 6 It seems that a similar interpretation may be applied also for H-mordenites. Although our results and the results of Maache et al. 24 evidenced that hydroxyls in the 12-ring channels are more acidic than in the 8-ring channels, the data presented in Figure 3 showed that hydroxyls in both kinds of channels are prone to dehydroxylation to the same extent. It is not excluded that the populations of hydroxyIs in the 8-ring and the 12-ring channels are also heterogeneous, and hydroxyls of various acid strengths are present in each. Heterogeneity of OH groups in H-mordenite may be due to two reasons. 1. There may be hydroxyls of various numbers of AI atoms close to the bridge. According to 29Si MAS n.m.r. results4 ,28 there are Si(OAl), Si(IAl), and small amount of Si(2Al) species, As Si(OAl) cannot create bridging hydroxyls, there may be two kinds of hydroxyls of various acid strength: (SiOhSiOHAI(OSi)3 and a small amount of (SiO)2(AlO)SiOHAl(OSi)3· 2. There are hydroxyls of various bridge geometry. According to XRD results, in mordenite structure there are T -0-T species of bridge angle from 143.2 to 180 As the acid strength of bridging hyd roxyls depends strongly on the bridge angle, there may be hydroxyls of various acid strength for geometric reasons. 0

•

The hydroxyls of various acid strength may be situated in both the 8- and 12-ring channels. It is possible that in each kind of channel the most acidic hydroxyls are removed first by the dehydroxylation, resulting in the decrease of the acid strength of hydroxyls observed in our study.

c 0.9 Q

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ACKNOWLEDGMENfS

O.s....................

......u.............u.............J

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900

1000

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This study was sponsored by Polish Komitet Badan Naukowych Grant 0634/P3/94j7. The sample of Namordenite was supplied by Chemie AG BitterfeldWolfen.

CALCINATION TEMPERATURE [KJ Figure 5 Acid strength of OH groups in H·mordenites as a function of calcination temperature. A, frequency of tha OH band. 8, extinction coefficient of the OH band; B, measured w ith ammonia (concerns all hydroxyls); b, measured with pyrid ine (concerns only hydroxyls accessible to pyridine). C, results of ammonia thermodesorption studies (A 7301Ao).

?ehydroxylation decreases the acid strength of remainmg hydroxyls, The values of the extinction coefficients and A7!l~/ Ao decrease, and the stretching O-H frequency Increases. The decrease of the acid strength concerns both the ~opulation of all hydroxyls (Figure 5, A and B, curve a,' Figure 5G) as well as the population of hydro~yl~ accessible to pyridine only (Figure 5B, curve b). A similar effect was observed in zeolites NaHY (Ref. 25) and NaHZSM-5 (Ref. 26). In the case of zeolite NaHZS~-5: the decrease of the acid strength of hydroxyls remammg upon partial dehydroxylation was ex-

432

Zeolites 17:428-433, 1996

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Heterogeneity of OH groups in H-mordenites: J. Datka et al.

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21 Witzel, F., Karge, H.G. and Gutsze, A. Proceedings of the Ninth International Zeolite Conference Montreal 79921Eds. R. von Balmoos, J.B. Higgins, and M.M.J. Treacy) Am. Chern. soc., Washington, DC, 1992, P 283 22 Jacobs, P.A. Catal. Rev. Sci. Eng. 1982, 24A, 415 23 Datka, J., Geelings, P., Mortier, W. and Jacobs, P.A. J. Phys. Chern. 1985, 89, 3488 24 Maache, M., Janin, A., Lavalley, J.C. and Benazzi, E. Zeolites

1993, 15, 507 25 Datka, J. J. Chern. Soc. Faraday 11981,77,2877 26 Datka, J. and Boezar, M. Zeolites 1991, 11,397 27 Datka, J., Boczar, M. and Rymarowicz, P. J. Catal. 1988, 114, 368 28 Datka, J., to be published

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