J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2837-2844 Acid Properties of a ZSM-20-type Zeolite Hendrik Kosslick,* Heinz Berndt, Hoang D. Lanh, Andreas Martin, Hans Miessner and Vu Anh Tuan Institute fur Ange wandte Chemie Berlin-Adlershot (ACA), 0-12484 Berlin, Rudo wer Chaussee 5, Germany JochenJanchen Schuit Institute of Catalysis, TU Eindhoven, Postbus 513,5600 MB Eindhoven, The Netherlands The acid properties of an HZSM-20 zeolite have been characterized, in comparison to an HY zeolite, by IR spectroscopy, temperature-programmed desorption (TPD) of ammonia, time-resolved temperature-programmed FTIR of ammonia desorption and by microcalorimetry. Also, catalytic properties were tested for n-hexane con- version. These studies show that for HZSM-20 there is a shift of the bridging OH band to lower wavenumbers, an increase in the ammonia desorption temperature and a higher initial heat of ammonia chemisorption compared to HY.Thus, HZSM-20 contains medium-strong Brsnsted sites, ranging in strength between zeolites HZSM-5 and HY. Obviously, the higher Si : Al ratio of HZSM-20 results in an enhanced number of strong acid sites with no second-neighbour aluminium atoms. This leads to a change in the distribution of Brsnsted-acid sites. Superacid Brsnsted sites could not be detected. Furthermore, the IR spectra of adsorbed pyridine and ammonia revealed the presence of at least two kinds of Lewis-acid sites. Some Lewis sites of HZSM-20 exhibit a weak acid strength and the strong Lewis sites are weaker than those of HY.The different acidic properties of both zeolites are reflected in their catalytic activity and selectivity in n-hexane conversion. The synthesis of ZSM-20 was reported in 1976,' but this zeolite type has become the subject of intensive studies only recently because it has potential applications in acid-catalysed reactions of hydrocarbons, such as the FCC process.2-8 The structure of ZSM-20 is related to faujasite-type zeolites (FAU).' ZSM-20 is now accepted to be an intergrowth of blocks of FAU and of EMT framework topology. EMT differs from FAU in its stacking." Additionally, differences in the T-0 bond length and T-O-T angles have been sug- gested.'' Until now, little information has been available concern- ing the acidic properties of zeolite HZSM-20 and related material^.'^-'^ Very strong Brsnsted-acid sites are proposed to be the origin of the high activity of this material. Superacid Brsnsted sites could originate from the zeolite structure or from an interaction between Lewis sites (cationic non-framework aluminium species) and Brsnsted sites.' 3*1 It has also been suggested that the high catalytic activity arises from a cooperative action of Brsnsted and Lewis sites.16 Recently, we reported the synthesis, dealumination, physi- cochemical and catalytic properties of a ZSM-20-type zeolite.' 7,18 This material exhibited a distinctly higher cata- lytic activity in the isomerization of rn-xylene and the conver- sion of ethylbenzene compared to the HY zeolite usually used. The reason for this difference is still sought.The aim of this study was to examine the nature, strength and concentration of acid sites of this material using methods such as FTIR spectrocopy using probe molecules, TPD of ammonia and microcalorimetry (differential heat of ammonia chemisorption) in order to clarify the origin of the strong acidity. Experimental Materials Zeolite ZSM-20 was synthesized under hydrothermal con-ditions at 373 K over 14 days from a reaction gel of the following composition: 1.25 Na,O * 1 A1203-30.2 SiO, 26.4a TPAOH 268 H,O. The synthesis product was withdrawn, washed repeatedly with distilled water until neutral and then dried at 393 K. The sample was then ion-exchanged with a 0.5 mol dmP3 solution of NH4N03 at 333 K (twice for 3 h).92% ion-exchange was reached, as determined by AAS." Subsequently, the sample was calcined in air at 773 K for 2 h under shallow-bed conditions (heating rate /3 = 10 K min-') to generate the protonic form. The crystallinity of the product was characterized by XRD to be a well crystallized HZSM-20 zeolite, as described in the literat~re.'~-~, The resulting Si :A1 ratio was cu. 4.3, as determined by 29Si MAS NMR.18 The HY sample (for comparison) was prepared from an industrial NaY zeolite (supplied by CKB Bitterfeld). The latter was treated in the same way as described for the HZSM-20 sample. 85% ion exchange was reached, which is typical.23 The Si : A1 ratio was cu. 3." Highly dealuminated samples of HZSM-20 and HY were included in the investigations for the special IR studies described below.The dealumination procedure of the samples has been described previously.' 8924 The samples were impreg- nated with an ethanolic solution of RhCl,, dried at 393 K in air and calcined subsequently in air at 573 K. The resulting solids contain 1 wt.% Rh. Characterization Methods The IR spectroscopic investigations of the samples were carried out on a Bruker IFS 66 FTIR spectrometer using self- supporting wafers of diameter 20 mm and weight 30 mg. All wafers were pretreated by heating (p = 10 K min-') to 673 K for 1 h under vacuum Pa). The OH vibrational spectra were recorded in the range 3400-3800 cm-'. Additionally, the IR spectra of adsorbed pyridine were recorded in the 1300-1700 cm-' region to discriminate between Brsnsted sites and Lewis sites.Also, the thermal desorption of pyridine was observed by IR spectroscopy to differentiate the strength of different acid sites.25 The CO stretching vibration of rhodium(1) dicarbonyl ions [Rh'(CO),] located on the Brsnsted sites of the dealuminat- + ed and Rh-loaded samples was used as a sensitive probe for the characterization of the strength of Brsnsted site^.^^,^^ CO (1.33 kPa) was adsorbed at 423 K and the spectra were recorded at room temperature (rt) after evacuation Pa). The TPDA experiments were carried out in a conventional flow system with thermal conductivity detection. Pressed zeolite samples of 150 mg were placed in a U-tube reactor. The samples were calcined for 1 h by heating them to 773 K (/3 = 10 K min-') under flowing helium.After cooling the samples to 373 K they were saturated with ammonia by passing a helium flow (2.5 dm3 h-') containing 2.7 vol.% ammonia until no more ammonia was adsorbed (checked by a thermoconductivity cell). Thereafter physically adsorbed ammonia was carefully removed by passing a dry helium flow through the reactor (2.5 dm3 h-' for 2 h) at the same tem- perature. Then the TPDA profiles were recorded whilst increasing the temperature to 823 K (/?= 10 K min-'). The final temperature was held for 45 min to complete the removal of ammonia from the zeolite. The reproducibility was better than 3%.The FTIR-NH,TPD measurements were carried out on a BIORAD FTS 60A spectrometer at a resolution of 2 cm-', accumulating 256 scans. The adsorption of ammonia was performed after an activation of the self-supporting wafers up to 873 K in vacuum Torr). For quantitative evalu- ations the spectra were normalized using the Si-0 overtone as an internal standard. The differential heats of ammonia chemisorption were measured at 423 K using a Calvet-type microcalorimeter (Setaram) which was connected to a standard adsorption apparatus. The samples (900 mg) were calcined at 673 K for ca. 15 h under vacuum (p < 1 mPa). The approach of the adsorption equilibrium was controlled by pressure measure- ment and followed by the thermokinetic curve.n-Hexane conversion over HZSM-20 and HY was carried out in a flow reactor under atmospheric pressure at 773 K with a contact time of W/F= 0-24 g h mol-'. The catalyst was activated in the reactor at 773 K in a dry flow of nitro- gen for 2 h. The reaction products were analysed by gas chro- matography. An alumina column was used for the analysis of cracking products and a Beton 34 column for the analysis of aroma tics. Results and Discussion The OH vibrational spectra of zeolite HZSM-20 and HY are shown in Fig. 1. The appearances of the OH vibrational spectra of HZSM-20 and HY nearly coincide. The spectrum of HZSM-20 reveals three main absorption bands at 3735, 3632 and 3552 cm-'. By analogy with faujasites they are assigned to the vibrations of silanol groups and of the Brsnsted-acid bridging OH groups in the large cavities and in the hexagonal prisms (high-frequency and low-frequency band), respectively.* 7-29 In comparison to zeolite HY the high-frequency band of HZSM-20 is shifted by 8 cm-' to lower wavenumber, indicating a higher acid strength of Brsnsted sites in HZSM-20.In contrast, the low-frequency band is shifted by 6 cm-' to higher wavenumber, indicating a decreased acid strength of weaker acid sites. In conclusion, the Brsnsted sites of HZSM-20 seem to be more differen- tiated with respect to strength in comparison to HY. Similar changes were observed in the OH vibrational spectra of dea- luminated HY zeolites,24 suggesting that these changes are probably due to the increased Si :A1 ratio of HZSM-20 in comparison to HY.30 The IR spectra of pyridine adsorbed on HZSM-20 and HY at 453 K are shown in Fig.2. Two absorption bands were observed, belonging to vibrations of pyridine located on dif- ferent acid sites. The band at 1540 cm-' is assigned to the pyridinium ion formed by the interaction of pyridine and the protons of Brsnsted sites (BS band). The band at cu. 1445 cm-' is assigned to pyridine coordinatively bound to Lewis J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3632 1 3552 al C* s s 3700 3500 waven umber/cm -' Fig. 1 IR spectra of dehydrated HZSM-20 and HY samples in the OH stretching region, recorded at rt 1540 I al Cco e 2n co HY I 1445 HZSM-20 1 I I 1600 1500 1400 wavenumber/cm-' Fig.2 IR spectra of HZSM-20 and HY after pyridine adsorption at rt and partial desorption at 453 K under vacuum, recorded at rt J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 sites (LS band).3' Furthermore, the LS band of HZSM-20 is resolved into two components with maxima occurring at 1445 and 1440 cm-'. This observation points to the presence of at least two types of Lewis sites.'* It was proposed that these bands correspond to chemisorbed pyridine and labile bound species, re~pectively.~~ The thermal desorption of pyridine was followed by IR spectroscopy in order to differentiate between the strength of the different kinds of acid sites in HZSM-20 (Fig. 3). The adsorption on Brarnsted sites is very strong, as shown by the high thermal stability of the pyridinium ion up to 613 K.In contrast, the intensity of the LS band decreases rapidly with increasing temperature. At rt the LS band is dominated by the absorption band at 1440 cm-', but after heating the sample this band disappears indicating a low acid strength of the corresponding Lewis sites. At 453 K the LS band is resolved into two components with maxima arising at 1450 and 1440 cm-'. After additional temperature increase the LS band at 1450 cm-' still remains in the spectrum. This illus- trates the high strength of at least one kind of Lewis site. Interestingly, the decrease in the relative intensity of the BS band with increasing temperature is lower for HZSM-20 than for HY (Table 1).This may indicate stronger Br~rnsted sites in HZSM-20 than in HY. In contrast, the opposite is found for the intensity of the LS band, showing a stronger decrease for HZSM-20. Fig. 4 shows the TPDA profiles of HZSM-20 and HY. It is known that the TPDA profiles of HY zeolite are hardly res~lved.~~*~~The desorption from weak and strong acid sites are not well separated because the desorption of ammonia Table 1 Relative intensity of the BS and LS band in the IR spec-trum of pyridine adsorbed on HZSM-20 and HY as a function of the desorption temperature desorption BS band temperature/K HZSM-20 HY 453 K 100 100 523 K 95 96 613 K 86 76 LS band HZSM-20 HY ~~ 100 100 58 78 49 64 shape. It is resolved into two maxima at 533 (low-tem-perature peak) and 683 K (high-temperature peak).The change in the concentration of acid sites in faujasite-type zeo- lites can be determined by TPDA.23 The ratio of the adsorp- tion peak areas of HZSM-20 and HY amounts to 0.62 +_ 0.03 demonstrating a relatively lower concentration of acid sites in HZSM-20 than in HY. Temperature-programmed FTIR spectroscopic studies on HZSM-20 reveal that the main part of ammonia desorbed up to 723 K comes from the decompo- sition of ammonium ions located on Brarnsted sites (see below). The higher resolution of the TPDA profile of HZSM-20 than that of HY indicates a different distribution of the strength of the Brarnsted sites in HZSM-20.The FTIR difference spectra of ammonia adsorbed on zeolite HY and HZSM-20 are shown in Fig. 5. In the N-H stretching range broad but less well resolved bands appear between 2400 and 3400 cm-'. Below 1800 cm-' resolved N-H bending modes of ammonia adsorbed on dif- ferent Brarnsted and Lewis sites appear. This spectral range is from acid sites with different strengths are superimpo~ed.~~ dominated by an absorption band at ca. 1450 cm-',which is The TPDA profile of HZSM-20 shows a somewhat different usually asigned to ammonium ions formed by the proto- LS I Q1 C Le n8 (I) i&I L I JLl 1 I I I 1600 1400 wavenumber/cm -' Fig. 3 IR spectra of HZSM-20 after (a) pyridine adsorption at rt, partial desorption at (b) 453 K, (c) 523 K and (d) 613 K under vacuum, recorded at rt nation of ammonia molecules located at Brsnsted-acid sites.The sharp band at cu. 1620 cm-' and the band at 1300 cm-' are assigned to bending modes of ammonia coordinatively bound to Lewis From the similarity of the IR spectra of adsorbed ammonia on HY and HZSM-20 it is concluded that, generally, both zeolites contain the same types of acid sites. The intensities of the bending modes of ammonium ions are higher for HY than for HZSM-20. This is in qualitative agreement with the concentration differences found by conventional ammonia TPD as well as with the different A1 framework content of the zeolites, i.e. the lower Si : A1 ratio of HY. I 533 n c.-C .-5 c n U I 683 HZSM-20 473 573 673 773 TIK Fig.4 TPDA profiles of the HZSM-20 and HY samples (heating rate fi = 10 K min-') J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 .o 0.8 0.6 0.4 0.2 0.0 -0.2 b) -0.4 I ~IIIIII~I~I 4000 3600 3200 2800 2400 2000 1600 wavenumber/cm-' Fig. 5 FTIR spectra of HZSM-20 (a)and HY (b)after adsorption of NH, at 373 K on the activated sample and subsequent evacuation at 373 K (top spectrum) and at increasing temperatures To analyse the behaviour of ammonia adsorbed on differ- ent sites at increasing desorption temperature, time-resolved IR spectroscopy was applied.24 The spectra shown in Fig. 5 were recorded continuously during the TPD run, using the spectra of the initial zeolite without NH, recorded during the same temperature ramp as the reference.With increasing temperature, the intensities of the bands due to adsorbed ammonia decrease and, simultaneously, the hydroxy vibra- tion bands reappear. At higher temperatures a band at 1300-1335 cm-' becomes visible in the spectra of both the types of zeolites. The corresponding NH, species remain on the sample even at 773 K, so we assign them to NH, bound to strong Lewis sites. A more quantitative picture can be obtained from the determination of desorption absorbances. Calculating the dif- ferences of the NH, and NH4+ absorbances in subsequent spectra (A, -AT+AT) we obtained difference spectra showing the desorption of NH, from the different adsorption sites in the corresponding temperature interval (T + AT).Owing to the different desorption behaviour of ammonia located at various sites, it is possible to obtain a better resolution of different absorption bands. As a result, from the band at 1440-1450 cm-' a distinct shoulder at 1490 cm-' could be resolved, which is assigned to weakly bound NH4+ (Fig. 6). Integrating the desorption absorbances we can obtain the relative amount of ammonia desorbed from the correspond- ing adsorption site. Even taking into account that the absorption coefficients of ammonia adsorbed on different sites may vary, these relative amounts of desorbed ammonia are now directly comparable with the conventional NH, TPD experiments, with the advantage that we can now calcu- late the desorption profiles for the different adsorbed species independently.8 1.0 C e SI% 0.5 0 0 lu e 5:a C .-E. 0.05 gU -0.10 I (I~ 2( 0 1800 1600 1400 wavenum ber/cm -' Fig. 6 FTIR spectra of NH,-loaded HY (A) and corresponding desorption absorbances (B): (a) 423-436 K, (b) 573-586 K, (c) 677-686 K FTIR-NH, TPD profiles obtained for HY in the range of N-H bending vibrations are shown in Fig. 7. The main part of ammonia desorbed from Bransted sites belongs to the 1450 cm-' absorption band. The desorption of ammonia characterized by the 1450 cm-band occurs in a broad tem- perature interval between 423 and 675 K. The desorption maximum is reached at CQ. 533 K. The desorption of NH, 10 a 0000 Cm $6 i2 zlm C ?JO\ .-t. g4 v) U 2 0 373 473 573 673 773 TIK Fig.7 FTIR-TPD curves of ammonia desorbed from different sites of HY: A, 1675 cm-'; 0,1620 cm-'; V, 1490 m-'; 0,1450 cm-'; 0,1380 cm-'; +, 1330-1300 cm-' '.\ J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 species with absorption bands at 1675 and 1490 cm-' pro-ceeds even at low temperatures (below 473 K), suggesting the assignment of these bands to weakly bound species. A com-parison with the behaviour of the hydroxy bands during the TPD run provides further arguments for assignment of the NH, bands. Fig. 8 shows the evaluation of the hydroxy bands. Additionally, a parallel is observed between the disap- pearance of these absorption bands and the reappearance of the high-frequency hydroxy stretching vibration band at low temperature, where 1450 cm-' ammonia is hardly desorbed (Fig.8). This parallel supports the assignment of the 1675 and 1490 cm-' bands to weakly bound ammonium ions, which interact with Bronsted sites in the large cavities. Above 423 K the reappearance of the high- and low-frequency bands of structural hydroxy groups correlates strongly with the disap- pearance of the 1450 cm-' band. Above 475 K distinct differ- ences in the reappearance of high- and low-frequency bands are not found. Ammonia bound to Lewis-acid sites, which is characterized by an absorption band at CQ. 1620 cm-' is desorbed in a low-temperature interval of 423-523 K, indicating the low acid strength of these sites.Additionally, a relatively large fraction of ammonia (absorption band at 1300-1330 cm-') is bound to strong Lewis sites. It is desorbed at high tem-perature between 573 and 773 K. Even at 773 K some ammonia remains at these sites. For HZSM-20 a similar FTIR-NH, TPD pattern was found (Fig. 9). Again the NH, adsorption from Brsnsted sites dominates, belonging to the curve of the 1440 cm-I band. Ammonia desorption occurred over the same tem-perature interval as found for HY. However, the temperature of maximum desorption was ca. 20 K 'higher, indicating a mean stronger Brsnsted acidity of HZSM-20, but it was ca. 60 K lower than the corresponding temperature observed on the strongly acidic HZSM-5 zeolite (Fig.10). Also, weakly bound ammonium ions characterized by absorption bands at 1675 and 1490 cm-' are present and are desorbed between 373 and 473 K. HZSM-20 also contains weak Lewis sites that give rise to an absorption band at 1620 cm-' and desorb ammonia at low temperature. The maximum temperature of 10 8 a C 46 sn m C 0.-4-e42 -0 2 0 373 473 573 673 773 TIK Fig. 8 FTIR-TPD curves of the reappearance of OH groups during ammonia desorption from HY: A, 1675-1490 cm- '; 0,1620 cm- '; V, 1450-1380 cm-'; V,3560 cm-'; 0,3650 cm-'; A, 3745 cm-' lo r------a c z6 s % C .-4-E4 s -0 2 0 373 473 573 673 773 T/K Fig. 9 FTIR-TPD curves of ammonia desorbed from different sites of HZSM-20: A, 1685 an-'; 0,1620 cm-';V, 1490 cm-'; 0,1440 cm-'; 0,1330-1300 cm-' ammonia desorption from strong Lewis sites, represented by the absorption band at 1300-1330 cm-', is slightly below 673 K. Hence, strong Lewis sites in HZSM-20 exhibit a lower acid strength than the corresponding sites in HY.From the TPD curves it is apparent that the amount of ammonia desorbed from strong Lewis sites is lower in HZSM-20 than in HY. In summary, the results of time-resolved FTIR-NH, TPD experiments reveal that Brsnsted sites in HZSM-20 exhibit a mean stronger acid strength than those of HY. Both zeolites under study contain weak and strong Lewis sites, but the concentration of Lewis sites in HZSM-20 is lower than in HY.Additionally, the acid strength of strong Lewis sites is lower in HZSM-20. These findings indicate distinct differ- I I 373 473 573 673 773 TIK Fig. 10 FTIR-TPD curves of ammonia desorbed from Brsnsted sites of HZSM-5 (band at 1470 cm-'): A, NH, form; 0,activated NH, form; V, H-form 2842 c 1601140 I I I 0 1 2 3 [ammonia]/mmol g-l Fig. 11 Differential heat curve of ammonia chemisorption from HZSM-20 (0)and HY (0)measured at 423 K ences in the nature, concentration and strength of the acid sites between the zeolites. Microcalorimetric determination of the differential heat of chemisorption of ammonia (Q) on the zeolites as a function of the sorbed amount is a further suitable method for the char- acterization of the acid properties of zeolite^.^*-^' The initial heat of ammonia chemisorption (Qo) is assumed to be a measure of the acid strength of Brsnsted sites. The heat curve of ammonia chemisorption on HZSM-20 (Fig.11) is similar to that of dealuminated HY.23 The initial heat of ammonia chemisorption on HZSM-20 amounts to ca. 130 kJ mol-'. The comparison of Qo measured on different zeolites shows that the acid strength of Brsnsted sites of HZSM-20 is higher than that of HY, but lower than that of HZSM-5. The sites may be classified as medium-strong acidic sites (Table 2). The heats of chemisorption of ammonia on strong acid sites of zeolites usually exceed 80 kJ mol-'. Sorption heats below 55 kJ mol-' are expected to belong to physisorbed ammonia. Ca.1.2 mmol g-' of adsorbed ammonia on HZSM-20 exhibits a Q value of 80 kJ mol-', i.e. it is located on strong Brsnsted sites. From this, a portion of 0.5 mmol g- 'possesses a higher acid strength than that of HY. An additional amount of 2 mmol g-' ammonia exceeds a heat of 55 kJ mol-'. Hence, besides strongly bound ammonia (Q > 80 kJ mol- ') a large fraction of the ammonia is only weakly bound (80 > Q/ kJ mol-' > 55). The total amount of chemisorbed ammonia exceeding a heat of 55 kJ mol-' amounts to 3.2 mmol g-'. This value is in agreement with the Si: A1 ratio of 4.3 as determined by 29Si MAS NMR. An exact calculation of the total concentration of Brsnsted sites is, however, difficult due to the tailing of the heat curves and the continuous transition from chemisorption to physisorption.The observed differences in the initial heats of ammonia chemisorption on several zeolites correlate well with the shifts of the wavenumbers of the high-frequency hydroxy bands (Table 2). Despite the discussion on the physical nature of Table 2 Initial heats of ammonia chemisorption (Q,,),wavenumbers of the high-frequency hydroxy bands, and maximum temperatures of ammonia desorption from Brernsted sites (Tmax)of different zeolites ~ ~~~~~~ Tm*x/Kwavenumber zeolite Q&J mol-' /cm-' TPDA FTIR-TPD HY 125-1 10 3644 523 543 HZSM-20 135-125 3632 533, 683 563 HSAPO-5 145-135 3625 - - HZSM-5 150-140 3610 730 630 HMOR 160-140 3605 823 - J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 this shift it seems that it is at least a heuristic indicator of acidity changes . 2-44 The acidity of Brsnsted-acid sites of HZSM-20 is found to be medium-strong again. Differences in the acid strength of Brransted sites could arise from structural effects, i.e. different bond lengths and angles in the framework42 as well as different 'cage' effect^.^' In order to check for possible structural effects, HZSM-20 and HY were highly dealuminated to observe the acid strength of Brsnsted sites located on isolated framework alu- minium atoms (Alf). Moreover, the dealuminated samples were carefully extracted in order to avoid any influence of non-framework aluminium species on the remaining Brsnsted sites.The acid properties were characterized by FTIR spectroscopy in the spectral range of the CO stretching vibration of rhodium(1) dicarbonyl ions [Rh*(CO),] as a + sensitive probe for the strength of Brsnsted The frequencies of the carbonyl stretching vibrations are the same for both types of zeolite, indicating a similar strength of the isolated Brsnsted sites, as shown in Fig. 12. Structural effects influencing the acid strength of Brsnsted sites in HZSM-20 could not be detected. However, a structural effect cannot be fully excluded due to the unknown distribution of rhodium ions over the different cation sites in the structure of HZSM-20. Theoretical calculations suggested that the acid strength of Brsnsted sites is related to the aluminium distribution of the zeolite framework.Only Brsnsted-acid -Si-(OH)-Al-framework groups with no second-neighbour Al, in the oxygen four-rings are responsible for the strong Brransted acidit~.'~*~~.~'They are located in the large cavities and are easily accessible for reacting molecules. The numbers of alu- minium atoms with 0, 1, 2 and 3 Al, as second neighbours in 2053 2118 I 1 I 21 25 2050 1975 wavenumber/cm-l Fig. 12 Comparison of IR spectra in the CO stretching region of rhodium-loaded (1 wt.%) highly dealuminated HZSM-20 (a) and HY (b) (Si : A1 > 100) after CO adsorption (1.33 kPa), recorded at rt J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the oxygen four-rings have been calculated by Beagley et With increasing Si :A1 the number of aluminium atoms with 0 Al, neighbour peaks with increasing Si :A1 ratio at 32 Al, per unit cell.The HZSM-20 zeolite investigated contains ca. 40 Al, per unit cell. At this Al, concentration most of the oxygen four-rings contain an isolated Al, atom. In contrast, zeolite HY has a much higher Al, concentration (>50 Al, per unit cell). Therefore, the number of aluminium atoms with no second-neighbour Al, atom in the oxygen four-rings of HZSM-20 is considerably higher than that of HY. This could explain the higher Brransted-acid strength of zeolite HZSM-20 in comparison with zeolite HY. The samples were tested in n-hexane conversion in order to evaluate the influence of the different acidity of HZSM-20 and HY on their catalytic properties. The n-hexane conver- sion as a function of contact time (W/F)over HZSM-20 and HY is shown in Fig.13. Note that HZSM-20 is more active than HY. Increasing the contact time from 4 to 24 g h mol-' the n-hexane conversion on both zeolites is increased, but over HZSM-20 a distinctly higher degree of conversion is found. This is in qualitative agreement with the enhancement of the Brernsted acidity of HZSM-20, but this alone cannot fully explain the observed substantially high catalytic activity. The contribution of very strong Brransted sites and/or Lewis sites should be considered, but the characterization of acidity presented here gives no evidence for the presence of very strong or 'superacid' Brransted sites in the HZSM-20 sample under study. However, it cannot be excluded that the high catalytic activity found for this material might be a result of the cooperative action of an enhanced number of isolated strong Brransted sites with Lewis sites as proposed by Zho- bolenko et Interestingly, the distribution of the reaction products of n-hexane conversion over HZSM-20 and HY is very different (Fig.14). Over HY deep cracking of n-hexane is observed. The main reaction products are propene, methane and ethane. In contrast, over HZSM-20 deep cracking is dimin- ished and a relatively high amount of aromatics is formed. Generally, the increased Brernsted acidity in faujasite-type zeolites should lead to deeper cracking and only to a low increase in the formation of aromatic^.'^ Hence, it is con- cluded that the increased acid strength of Brransted sites alone is unlikely to be responsible for the different distribu- tion of reaction products found over the two types of zeolite.This indirectly points to the contribution of Lewis sites to the catalytic properties of zeolite HZSM-20. Differences in the number and strength of Lewis sites in HZSM-20 and HY (W/F)/gh mol-I Fig. 13 n-Hexane conversion over HZSM-20 (a) and HY (6) as a function of contact time 2843 50 1 40 30 YO 20 10 n c.1 c2 c3 C3 aromatics -HZSM-20 HY Fig. 14 Distribution of reaction products in n-hexane conversion over HZSM-20 and HY were found.In summary, the catalytic activity and selectivity in n-hexane conversion confirm the differences found in the number and strength of Brsnsted- and Lewis-acid sites between HZSM-20 and HY. Conclusions Zeolite HZSM-20 displays a medium-strong Brernsted acidity, ranging in its strength between HY and HZSM-5. This classification is deduced from the observed initial heat of ammonia chemisorption and from the wavenumber of the high-frequency band of Brernsted hydroxy groups in the large cavities. Owing to the higher Si :A1 ratio of zeolite HZSM-20 in comparison to zeolite HY, the number of strong Brsnsted sites with no second-neighbour aluminium atoms is increased, leading to a change in the distribution of acid sites of different strength.This is reflected in the shift of the bridg- ing OH band to lower wavenumber (3632 cm-'), the higher temperature of NH, desorption from Brransted sites and the higher heat of ammonia chemisorption (ca. 130 kJ mol-') by zeolite HZSM-20. The heat curve of ammonia chemisorption reveals that ca. one-third of the ammonia molecules located at Brransted sites are strongly bound (Q > 80 kJ mol-I). Surprisingly, the main part is weakly bound with chemisorption heats ranging from 80 to 55 kJ mol- '. This observation is confirmed by the high- wavenumber shift of the low-frequency band in the OH spec- trum of HZSM-20 compared to HY. Besides Brernsted sites, Lewis sites were also found. Two kinds of Lewis site of different strength seem to exist.In the IR spectrum of adsorbed pyridine on HZSM-20 two charac- teristic bands at 1450 and 1440 cm-' appear. They are assigned to the interaction of pyridine with stronger and weaker Lewis sites. Additionally, the strong Lewis sites in HZSM-20 are weaker than the strong sites in HY, as shown by FTIR-NH, TPD. In summary, it is concluded that the increased acidity of zeolite HZSM-20 is due to a change in the distribution of the strength of acid sites, where the number of strong Brransted sites with high acid strength is significantly increased. There- fore, in comparison to zeolite HY the intrinsic acidity of HZSM-20 is improved but the number of Brsnsted-acid sites is decreased. The strength of isolated strong acid sites is similar in both types of zeolites.Superacid sites could not be detected. The moderately increased acidity cannot explain the observed high catalytic acitivity of this material alone, because mildly dealuminated HY samples have a similar strength of Brernsted sites but not such a high activity.'l Thus, it is tentatively concluded that the origin of the high 2844 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 catalytic activity of HZSM-20 and the difference in the dis- 24 H. Miessner, H. Kosslick, U. Lohse, B. Parlitz and V. A. Tuan, tribution of reaction products could be a cooperative action J. Phys. Chem., 1993,97,9741. of Brernsted and Lewis-acid sites as proposed by Beyerlein16 25 A. Martin, U. Wolf, H. Berndt and B. Liicke, Zeolites, 1993, 13, 309.and Zh~bolenko.~~ 26 I.Burkhardt, E. Jahn and H. 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