Dealuminated H−Y Zeolites: Influence of the Degree and the Type of

Dec 18, 1999 - ... estimated errors in dealumination degree from each method and is characteristic of the care given to the measurements and the good-...
1 downloads 12 Views 207KB Size
Download PDF
Ind. Eng. Chem. Res. 2000, 39, 307-319

307

Dealuminated H-Y Zeolites: Influence of the Degree and the Type of Dealumination Method on the Structural and Acidic Characteristics of H-Y Zeolites Costas S. Triantafillidis, Athanasios G. Vlessidis, and Nicholaos P. Evmiridis* Department of Chemistry, University of Ioannina, Ioannina 45 110, Greece

Dealuminated zeolite samples prepared by different methods are characterized by different techniques, and the structural and acidic characteristics are investigated in an effort to identify the parameters that decide their structural and acidic characteristics. The investigation gave evidence that the structure breakdown of the parent type-Y zeolite is mainly dependent on the degree of dealumination, but the nature of extraframework aluminum (EFAl) species formed is dependent on the type of dealumination procedure. Monomeric Al species and oligomeric Si,Al species were formed by steam treatment at relatively low and high temperatures, respectively. Aluminosilicate phases of Si/Al ratios 2.0-4.0 and 8.0 were found in SiCl4-treated and ammonium hexafluorosilicate (AHFS)-treated samples, respectively. EFAl species of octahedral and tetrahedral coordination were observed by NMR spectra in steam and SiCl4-treated samples. High degrees of dealumination within each type of treatment resulted in loss of microporosity of the crystalline material, but the increase of mesoporous and/or macroporous structure was dependent on the type of dealumination. The increase of macroporous structure was the result of crystallite agglomerates formed during the procedure. The number of acid sites determined by the ammonia temperature-programmed desorption method, under appropriate experimental conditions, matched with the FAl content of the dealuminated H-Y samples that do not contain a significant amount of EFAl species. The loss of strong acid sites with the degree of dealumination is evidenced to be dependent on the method of dealumination. Dealumination by AHFS up to ca. 50% does not affect the number of strong acid sites considerably, in contrast to the weak/medium acid sites which decrease almost linearly with a FAl decrease. At higher degrees of dealumination achieved by steaming or SiCl4-treatment, both weak/medium and strong acid sites decrease steeply with FAl decrease. There is evidence that the produced EFAl-species by the different methods have different acidic properties and result in low stoichiometries of adsorbing ammonia probe molecules. The EFAl species produced by the SiCl4 method result in more acidic H-Y zeolite catalysts compared to the high-temperature steamed samples. Introduction The dealumination of the zeolitic framework is widely used for the production of high-silica Y-type zeolites. Different types of dealumination procedures have been applied in the past, such as treatment with steam at elevated temperatures,1,2 with SiCl4 vapor at moderate temperatures,2-6 with ammonium hexafluorosilicate (AHFS),7-10 and with chelating agents [i.e., ethylenediaminetetraacetic acid (EDTA)] at relatively low temperatures.11 The effect of the higher framework Si/Al ratio on the acidic and catalytic properties of the H-Y dealuminated zeolites has been found to be influenced by the presence of amorphous phases which are usually formed during dealumination. The amorphous material usually consisted of silica or aluminum species or both, in different coordination and hydration states. It has been reported that the extraframework aluminum (EFAl) species in many cases interact with the framework Al (Bronsted acidity) of the zeolitic structure and increase the activity of the zeolite catalyst,2,3,10,12 but little effort was made to elucidate the microphases formed and their acidic character. Dealumination procedures of the type-Y zeolites has been proposed for modifying the Si/Al ratio * Corresponding author. Tel.: 00-30-651-98404. Fax: 0030-651-44831. E-mail: [email protected].

in an effort of preparing fluid catalytic cracking (FCC) catalysts of special properties. The catalytic behavior of the prepared catalysts for the FCC reaction shows a diversity in selectivity of the products that depends on the extent of dealumination and the procedure of dealumination. More understanding is necessary for the tailor-making of acid sites that perform well at the different catalytic reactions, and this paper is aimed to meet this target by a systematic study of well-defined samples prepared by different dealumination procedures. In this work we modified type-Y zeolites through different dealumination procedures by removing different fractions of framework Al atoms; the samples were then submitted to several characterization procedures, i.e., specific surface area (SSA) and pore-volume measurements, X-ray diffraction (XRD) measurement for structure breakdown, infrared (IR) and XRD measurements for quantitative measurements of the framework aluminum, chemical analysis for the total aluminum content of the dealuminated samples, scanning electron microscopy (SEM) measurements for the morphology of the crystallites, and energy-dispersive spectrometry (EDS) for the homogeneity of the Si/Al content of the crystallites. The dealuminated samples were then titrated with ammonia for the determination of the

10.1021/ie990568k CCC: $19.00 © 2000 American Chemical Society Published on Web 12/18/1999

308

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000

number of acid sites using the ammonia temperatureprogrammed desorption (NH3-TPD) measurements, and the obtained results are discussed in reference to (a) the amount, nature, and coordination of Al atoms in the sample, (b) the structural characteristics of the samples, and (c) the porous structure. Furthermore, the effect of the dealumination procedure on the number of acid sites with different strengths, and the possibility to tailormake different types of acid sites are considered. Experimental Section Samples Preparation. The parent zeolite was either NH4NaY (Linde LZ-Y62; Si/Al ) 2.48 and Na2O ) 2%) or NaY (Linde LZ-Y52; Si/Al ) 2.66 and Na2O ) 9.45%) and was dealuminated by different procedures that have been previously reported. The NH4NaY zeolite was dealuminated by a modified method of AHFS treatment that was introduced by Skeels and Breck.7,8 The amount of (NH4)2SiF6 used in each experiment was changed to achieve different dealumination degrees. AHFS was added dropwise as a 0.1 M solution at a rate of 0.005 mol of AHFS/(mol of Al)‚min to a stirred slurry of zeolite in water buffered at pH ) 5-6 and kept for 3 h at 80 °C. At the end, the hot suspension was filtered and the recovered zeolite was thoroughly washed with hot water. The NH4NaY zeolite was also the parent material for the preparation of the hydrothermally dealuminated samples, as in previous works.1 A flow system which consisted of a cylindrical quartz reactor and a vertical high-temperature furnace was used. The zeolite samples were subjected to a flow of a nitrogen-steam mixture at a flow rate of 30 mL/min and steam partial pressure of 69.9 kPa for 6 h, at two different steaming temperatures, i.e., 600 and 750 °C, to obtain different dealumination degrees. The steamed samples were further treated with an EDTA aqueous solution of 7.5 wt % at 100 °C to remove the EFAl species which were formed during the steaming procedure. The NaY zeolite was treated with SiCl4, according to the method described by Beyer et al.4 The same flow system as in the steaming procedure was used. The zeolite samples were carefully dehydrated before they were exposed to the silicon tetrachloride vapors for 3 h at different temperature, i.e., 250, 350, or 450 °C, to increase the severity of the dealumination process. Afterward, the samples were treated at the same temperature for 3 h under a dry nitrogen flow to remove the AlCl3 which was formed during the reaction. Finally, the zeolite powders were thoroughly washed with water. All of the dealuminated samples were transformed to the ammonium form through ion exchange with a 1 M NH4NO3 solution at 60 °C for 6 h under stirring followed by filtration and washing with water. The zeolite samples after washing were dried at 120 °C in air for 6 h and stored over a saturated MgCl2 solution to equilibrate with water vapor. Composition and Structure Characterization. The chemical analysis procedure of the samples involved a digestion step with 10% HCl,13 to extract Al3+ and Na+ into the acid aqueous solution. The silica was determined gravimetrically from the solid residue, while the aluminum in the solution was determined titrimetrically by EDTA complexation. The sodium content was measured by flame photometry using a Jenway PFP7 flame photometer. The total volatiles (i.e., NH3 and H2O) were

estimated from the weight loss after heating at 550 °C for 2.0 h, and the NH3 content was determined by the Kjeldahl method. The X-ray powder diffraction patterns were taken on a Siemens D-500 automated diffractometer (Cu KR radiation, λ ) 1.5418 Å), in the 2θ range of 5-35° and at a scanning rate of 2°/min. The zeolites were checked for crystallinity by comparing the (533) peak height of the dealuminated samples with that of the parent NaY or NH4NaY zeolites which were considered to be 100% crystalline.4,14,15 The percent relative standard deviation (% RSD) from five replicates of the same sample was found to be 6%. Mid-IR spectra were recorded at 2 cm-1 resolution on a Perkin-Elmer 1650 FT-IR spectrometer using the KBr-pellet technique (1 wt % zeolite in the KBr matrix). High-resolution solid-state MAS NMR spectra were recorded using a Bruker MSL 400 spectrometer, working at 104.26 and 79.49 MHz for 27Al and 29Si nuclei, respectively. 29Si shifts were referenced to tetramethylsilane (TMS) by using an external sample of sodium 2,2-dimethyl-2-silapentane-5-sulfate (SDS) which gave a single peak at 0 ppm as in the case of TMS, while Al(H2O)63+ was used as standard reference for 27Al. SEM pictures were taken with a JEOL scanning microscope (JSM-6300) for all of the samples at different magnification scales to test the morphology of the zeolitic particles, while the bulk Si/Al ratios of different parts of the zeolite solids were measured by EDS analysis. SSA was measured by nitrogen adsorption isothermally at 77 K, using an automatic volumetric sorption analyzer (Autosorb-1, Quantachrome). Prior to the determination of the adsorption isotherms, the samples were evacuated overnight at 450 °C under 1 × 10-6 mbar vacuum. SSA was estimated using the BET method,16 and the results were used for the evaluation of structure breakdown of the dealuminated samples relative to the parent zeolites.12,15 Appropriate analysis of the multipoint BET data was done for the determination of the micropore surface area and the micropore and mesopore volume (t-plot method). Acidity Characterization. TPD of ammonia experiments were performed with a conventional apparatus which consisted of a quartz cylindrical microreactor, a vertical well-controlled high-temperature furnace, and a Shimadzu GC-8A gas chromatograph equipped with a thermal conductivity detector (TCD). A typical 0.2-g zeolite sample (3 mm thick bed) was degassed in a flow system at a 50-60 mL/min He flow rate with a temperature program of 5 °C/min rise up to 420-430 °C; at this temperature the sample was further heated for 3 h. These pretreatment conditions were applied to minimize any dealumination of the samples due to selfsteaming. Adsorption of dry ammonia (Merck; water free) took place at 100 °C, in a static system for 1 h at 1.5 bar ammonia pressure. Stripping was done afterward for 1.5 h at 100 °C under the flow of He. In this way, the weakly and physically adsorbed ammonia was minimized in the zeolite. Desorption of ammonia was done at a rate of 10 °C/min from 100 up to 700 °C under He flow (50 mL/min). The desorbed ammonia was detected on the TCD and recorded; it was then trapped in a 0.01 N HCl aqueous standard solution. The trapped ammonia was estimated by the titrimetric determination of the excess HCl in the standard solution, using a 0.01 N standard NaOH solution.

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 309 Table 1. Physicochemical Characteristics of Samples frequency of unit cell main asym. size, a0 stretch band sample

treatment

(XRD) (Å, (0.01)

NaY NH4NaY NH4Y DY-1 DY-2 DY-3 DY-4 DY-5 DY-6 DY-7 DY-8 DY-9 DY-10 DY-11

parent parent NH4 exchange (NH4)2SiF6 (NH4)2SiF6 (NH4)2SiF6 (NH4)2SiF6 SiCl4 SiCl4 SiCl4 steamed, 600 °C steamed, 750 °C DY-8, EDTA-treated DY-9, EDTA-treated

24.67 24.71 24.72 24.63 24.55 24.48 24.45 24.45 24.30 24.25 24.47 24.34 24.46 24.35

(IR) (cm-1, (2) 1019 1013 1013 1025 1038 1046 1052 1054 1075 1082 1051 1068 1052 1069

aluminum content

silicon content

NFAl/u.c.a

Al/g of sampleb Si/g of sample XRD IR NMR (mg‚atoms/g) ((0.05) NFSi/u.c./ (mg‚atoms/g) ((0.05) ((1) ((1.5) ((1.5) QFAl QTAl QEFAl XRD ((1) QFSi QTSi QEFSi 52.8 57.3 58.5 48.3 39.2 31.2 27.8 27.8 10.8 5.8 30.1 15.3 29.0 16.5

51.5 56.1 56.8 47.6 37.3 31.3 26.4 25.4 8.8 3.8 27.3 14.2 26.5 13.6

57.5 32.8

27.6

3.17 3.51 3.51 3.08 2.57 2.07 1.68 1.63 0.49 0.39 1.88 0.54 1.96 0.70

3.15 3.51 3.63 3.05 2.64 2.21 1.88 2.15 2.24 1.02 3.51 4.23 2.22 1.99

139.2 134.7 133.5 143.7 152.8 160.8 164.2 164.2 181.2 185.7 161.9 176.7 163.0 175.5

0.12 0.07 0.14 0.20 0.52 1.75 0.63 1.63 3.69 0.26 1.29

8.36 8.26 8.01 9.15 10.01 10.69 9.93 9.63 8.17 12.58 10.10 6.22 11.02 7.43

a

b

NFAl/u.c.: number of framework Al atoms per unit cell of crystalline zeolite which is calculated from the XRD, IR, and Refers to grams of hydrated sample, where sample ) crystalline zeolite plus amorphous material.

ResultssObservations and Remarks Effect of the Dealumination Method on the Compositional and Structural Characteristics of the H-Y Zeolites. Framework Dealuminations Formation of Amorphous Phases. The samples prepared by different dealumination procedures were analyzed for the total aluminum content by chemical analysis and for the framework aluminum content (FAl content) by IR and XRD spectra. The number of skeletal Al atoms per unit cell (NFAl/u.c.) is estimated from the value of the unit cell constant, a0 (estimated error ( 0.01 Å, XRD data) with a precision of (1 Al atom/u.c., and from the frequency of the main asymmetric stretch vibration ((2 cm-1, IR data), with a precision of (1.5 Al/u.c., using the equations given in refs 17 and 18, respectively. The amount of EFAl in milligram atoms per gram of hydrated solid and the amount of extraframework silicon (EFSi) milligram atoms per gram of hydrated solid are calculated by eqs 1 and 2, respectively,

QEFAl (mg‚atoms/g*) ) QTAl (mg‚atoms/g*) QFAl (mg‚atoms/g*) × C (1) QEFSi (mg‚atoms/g*) ) QTSi (mg‚atoms/g*) QFSi(mg‚atoms/g*) × C (2) where QFAl (mg‚atoms/g*) ) NFAl/u.c./MW*, QTAl (mg‚ atoms/g*) ) total Al content from chemical analysis, QFSi (mg‚atoms/g*) ) NFSi/u.c./MW*, QTSi (mg‚atoms/g*) ) total Si content from chemical analysis, NFSi/u.c ) 192 - NFAl/u.c., MW* ) weight of the hydrated unit cell, and C ) crystalline fraction of 1-g* solid sample obtained from XRD intensity measurements; * refers to a hydrated solid sample and are given as milligram atoms per gram of hydrated sample in Table 1. The percent dealumination is calculated by the results of NFAl/u.c. of the treated sample in comparison to NFAl/u.c. of the parent zeolite. The structure breakdown was found from the measurements of the relative crystallinity of the samples and from the measurements of the relative loss in SSA. All of the above results are listed in Table 2. The close agreement found for the values of percent dealumination degree between the XRD and IR methods (Table 2) is statistically important to ca. above 25 Al atoms/u.c. based on the percent estimated errors

8.39 8.69 8.62 9.60 10.30 10.87 11.60 11.09 11.82 14.77 9.70 10.73 11.00 12.23 29Si

0.03 0.43 0.61 0.45 0.29 0.18 1.67 1.47 3.65 2.19 4.51 4.80

NMR data.

Table 2. Dealumination and Structure Breakdown dealumination degree (%) sample NaY NH4NaY NH4Y DY-1 DY-2 DY-3 DY-4 DY-5 DY-6 DY-7 DY-8 DY-9 DY-10 DY-11

XRDa

16 32 45 51 47 80 88 47 73 49 71

retained structure (%)

IRb

XRDc

SSAd

15 34 44 53 51 83 93 51 75 53 76

100 100 98 100 100 100 88 87 63 83 94 47 100 58

100 100 94 95 91 87 68 86 82 79 87 49 91 64

a Based on N b FAl/u.c. from XRD data. Based on the NFAl/u.c. from IR data. c Relative crystallinity of the samples using the (533) peak height from XRD spectra and considering NaY or NH4NaY as 100% crystalline. d BET surface area of the samples relative to the area of the parent NaY or NH4NaY.

in dealumination degree from each method and is characteristic of the care given to the measurements and the good-quality instrumentation used. Furthermore, the 29Si and 27Al MAS NMR spectra were obtained only for selected samples, i.e., the parent zeolite NH4Y and samples DY-3 (AHFS) and DY-8 (steamed), and are shown in Figure 1. The following remarks are based on the above experimental data and are related to each dealumination method. (a) AHFS-Treatment Procedure. The values of QTAl and QFAl for each dealuminated product (Table 1) are very close for all of the samples prepared by this method, and thus QEFAl is very low. Additionally, the framework Si/Al ratio (4.9) estimated from the 29Si NMR data, as is discussed later for the sample DY-3 (Table 1), is the same as that found by chemical analysis. The results of Table 1 are in agreement with the results found in the literature,9,10 where it has been reported that dealumination with AHFS does not damage the zeolite structure (up to 50% dealumination) and EFAl is practically absent from such dealuminated samples. However, from the difference in the QFSi and QTSi values, a very small amount of EFSi has been found, which is suddenly increased at high dealumination degree. The small amount of QEFSi in the relatively mildly dealumi-

310

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000

Figure 1. 29Si (left) and 27Al (right) MAS NMR spectra of H-Y zeolite samples: (a and b) parent NH4Y, (c and d) DY-3 (AHFS-treated sample), and (e and f) DY-8 (steamed sample).

nated samples (DY-1-DY-3) is mainly attributed to the presence of silica deposits or other fluorinated Si species, according to previous works.19 The significant loss of crystallinity observed in the most dealuminated sample DY-4 (53% dealumination) is in agreement with the increased QEFSi value obtained from the above calculations and is attributed mainly to the structure breakdown which occurs at relatively high concentrations of AHFS, where the rate of Al removal from the framework is higher than the rate of Si insertion.8,19,20 As a result, some parts of the framework collapse, forming amorphous microphases, consisting mainly of Si. (b) Steam-Treatment Procedure. It is clear that the QTAl values are close to the parent zeolite but very different compared to QFAl (Table 1) in accord with previous works.1,2,10,12 The nature of these species has been a matter of great interest and research among the various workers, because it has been shown to affect significantly the acidic and catalytic properties of zeolite Y.2,6,10,14,21,22 In the present work, the EFAl species are easily removed after appropriate treatment with an EDTA solution without affecting the number of skeletal Al atoms, when the sample is treated hydrothermally at relatively low temperature (600 °C). This is the case with samples DY-8 (steamed) and DY-10 (EDTA-treated DY-8) tested in this study (Table 1). The above results indicate that most of the EFAl produced during steaming at 600 °C was in the form of monomeric cationic species, capable of being extracted by an EDTA solution; this is supported by the zero QEFSi value obtained (Table 1). However, the EDTA extraction of EFAl species is incomplete for the sample DY-9, although the ratio

EDTA/EFAl used for this sample is the same as the ratio applied in the case of DY-8. The high values of QEFAl of DY-9 in comparison with those of DY-11 and the high QEFSi values of both samples give evidence of the existence of monomeric aluminum charged species and polymeric aluminosilicate species in sample DY-9. This suggests that dealumination at high temperatures weakens the bonds of SiO- and releases silicate or aluminosilicate species from the zeolitic framework with probability similar to that of the AlO- species at lower dealumination temperatures; this results in the removal of both charged monomeric Al species and low molecular weight oligomeric aluminosilicate species; this is further supported from 29Si MAS NMR spectra. However, it is possible to suggest that the low extraction of the EFAl species by EDTA (in DY-9) is related to the aluminosilicate oligomers or their further polymerization during the high-temperature treatment as mentioned in previous works.14 (c) SiCl4-Treatment Procedure. The QTAl is different from those of the parent zeolite and, also, different from QFAl, and EFAl species are present in the samples (Table 1) in accord with other works;2-6 the same is true with QTSi and QEFSi, and therefore EFSi species are, also, present. From the data in Table 1, it is shown that the increase in treatment temperature (DY-5, 250 °C, DY6, 350 °C; DY-7, 450 °C) results in a higher degree of framework dealumination, and this is accompanied by an increase in EFSi species formation, suggesting the removal of aluminosilicate species from the zeolitic framework. Although samples DY-5 and DY-7 appear to have a small amount of EFAl species, sample DY-6

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 311

which has been treated in the intermediate temperature of 350 °C has almost 2/3 of its total Al content in extraframework positions and significantly lower XRD crystallinity compared to DY-5 and DY-7. Similar results have been presented by Beyer et al.4 Most of the AlCl3 that is formed during the reaction with SiCl4 is not removed by the subsequent thermal treatment of the samples, thus leading to the deposition of Na(AlCl4) complexes in the zeolite pores. Most of these complexes are removed by washing with water, but some remain in the zeolite, resulting in the formation of oxidic, EFAl species. These species can polymerize with the EFSi species, during thermal pretreatment of the samples before TPD or catalytic tests, thus producing Lewis acid sites.3,5 Microenvironment and Coordination of Framework Si and Al. The MAS NMR data (Figure 1) for the dealuminated type-Y zeolites give detailed information about (a) the nature and coordination of the Al atoms in the various phases of the overall product and (b) the number of FAl that are next neighbors of the framework Si atoms, and these are described below: The presence of a single broad peak at ca. 0 ppm from Al(H2O)63+ in the 27Al spectra of the steamed sample (Figure 1f) gives evidence of the formation of octahedrally coordinated EFAl species, during hydrothermal treatment, which are absent from similar spectra of the parent NH4Y zeolite (Figure 1b) and the AHFS, dealuminated zeolites (Figure 1d); the peak at 57 ppm stands for the tetrahedrally coordinated FAl, but it can also overlap with a low-intensity broad resonance at ca. 3040 ppm, as can be seen in the spectra of the steamed sample DY-8, that is assigned to tetrahedrally coordinated EFAl species.10,23 The 29Si spectrum of the parent zeolite in Figure 1a shows the presence of five components, corresponding to Si surrounded by 4Si, 3Si1Al, 2Si2Al, 1Si3Al, and 4Al.24 Under dealumination with AHFS (DY-3) the peaks associated with Si(4Al and 3Al) disappear, the Si(2Al) decreases significantly, and Si(1Al and 0Al) show a clear increase, especially Si(0Al) (Figure 1c). In DY-8 (Figure 1e), the Si(4Al) peak is also absent but the Si(3Al) peak still exists. Although both of the Si(3Al) and Si(2Al) peaks in the steamed sample are very small, their size is within the sensitivity of the measurement, and this indicates some ordering which needs further investigation. However, the most important point in the above results is the different selectivity of removing framework Al atoms between steaming and the AHFS method. Steaming results in relatively higher amounts of Si(0Al) atoms and retains some Si(3Al) atoms. On the other hand, after dealumination with AHFS, the zeolite framework has relatively higher amounts of Si(1Al) atoms and has no Si(3Al) atoms. The above difference is reflected in our NH3-TPD results where the AHFStreated samples have relatively higher amounts of strong acid sites than the steamed samples. The abovepresented 29Si MAS NMR results are in accord with the results of Lonyi and Lunsford,10 who did an extensive MAS NMR study on AHFS and steam-treated Y zeolites. The framework Si/Al ratio of the three samples tested by NMR is calculated from the intensity (peak area after Gaussian deconvolution of the spectra) of the NMR signal corresponding to a Si(nAl) building unit, using a formula given in related NMR studies.24 The estimated framework Si/Al values 2.34, 4.85, and 5.96 for NH4Y,

DY-3, and DY-8, respectively, correspond to NFAL/u.c values which agree well, within experimental error, with those calculated from XRD or IR data (Table 1). Adsorption and Porosity Characteristics. The adsorption-desorption isotherms of nitrogen are shown in Figure 2 for samples representative of each dealumination method. From these data, the total surface area (using the multipoint and single-point BET methods), the micropore surface area, the micropore and mesopore volume, and the total pore volume have been calculated and are listed in Table 3. The nitrogen adsorption curve of the parent NH4Y zeolite (Figure 2a) is of Langmuir type (type I in the BDDT classification) which is typical for the crystalline microporous materials.16 The different effect of each dealumination method on the porous structure of H-Y zeolites can be clearly seen in the isotherms for the AHFS-treated (Figure 2b), SiCl4-treated (Figure 2c), and hydrothermally (Figure 2d) treated samples. The shape of the adsorption isotherms indicates that the microporous character of the samples is largely retained; however, the loss of micropores is accompanied by the formation of mesopores which give rise to typical adsorption-desorption hysteresis loops. The samples dealuminated by AHFS and SiCl4 have similar types of hysteresis loops, which suggests the formation of bottleneck types of pores. Although the SiCl4-treated sample is almost 90% dealuminated versus 50% of the AHFStreated sample, the hysteresis loop of the latter is much more intense; this difference can be assigned to the different nature of each treatment. On the other hand, the adsorption curve of the steamed sample is typical of microporous materials which exhibit significant mesoporous + macroporous structure because of small agglomerated particles.16 The existence of agglomerates in all of the dealuminated samples has been proven by SEM pictures and is discussed in later paragraphs. Furthermore, the treatment of steamed samples with EDTA does not result in changes of the shape of adsorption-desorption curves (not shown in this paper). From the data in Tables 1 and 3 it is clear that there is no simple relationship between the FAl content and the surface area of the samples. For example the DY-7 sample (SiCl4 treated) is almost 90% dealuminated but has a significantly higher surface area than the DY-9 sample (steamed), which is ∼75% dealuminated. However, within the same dealumination method the decrease of FAl generally is followed by a decrease in the surface area. The removal of the EFAl species from the steamed samples increases the surface area to an extent that is related to the severity of temperature treatment (see samples DY-8-DY-10 and DY-9-DY-11). As a consequence, the micropore volume follows the trend of change of SSA with the extent of dealumination, while the mesopore volume remains rather constant within the samples of the AHFS and the SiCl4 method. A mean value of ca. 0.1 cm3/g for the former method versus 0.09 cm3/g of NH4NaY and a value of ca. 0.065 cm3/g for the latter versus 0.03 cm3/g of NaY have been found. In the case of the steamed samples, the mesopore volume is significantly higher than that of the parent zeolites, and furthermore it increases with the severity of the hydrothermal treatment and the percent of dealumination and amorphous material produced. In addition, it is shown that when the EFAl species are fully removed from almost crystalline samples (DY-8/DY-10) both the micropore volume and the mesopore volume stay con-

312

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000

Figure 2. Nitrogen adsorption/desorption isotherms of H-Y zeolite samples: (a) parent NH4NaY, (b) DY-4 (AHFS-treated sample), (c) DY-7 (SiCl4-treated sample), and (d) DY-9 (steamed sample). Table 3. Surface Area and Porosity Characteristics of the Dealuminated H-Y Zeolites

sample

specific surface areaa (m2/g)

single-point BET areab (m2/g)

total pore volumec (cm3/g)

micro aread (m2/g)

meso + ext areae (m2/g)

micropore volumef (cm3/g)

mesopore + macropore volumeg (cm3/g)

NaY NH4NaY NH4Y DY-1 DY-2 DY-3 DY-4 DY-5 DY-6 DY-7 DY-8 DY-9 DY-10 DY-11

879 924 867 876 842 807 627 757 723 697 808 452 839 595

878 898 841 850 817 785 625 734 703 663 768 442 813 584

0.355 0.416 0.402 0.413 0.397 0.404 0.331 0.334 0.321 0.307 0.422 0.323 0.444 0.499

843 853 810 815 780 732 577 705 665 647 739 388 742 484

36 71 57 61 62 75 50 52 58 50 69 64 97 111

0.327 0.328 0.313 0.314 0.300 0.283 0.232 0.271 0.256 0.244 0.281 0.152 0.286 0.189

0.028 0.088 0.089 0.099 0.097 0.121 0.099 0.063 0.065 0.063 0.141 0.171 0.158 0.310

a

Multipoint BET surface area. b At P/PO ∼ 0.10. c At P/PO ∼ 0.99.

stant, while both are increased when a part of EFAl’s is removed from samples with low crystallinity and large amounts of amorphous material (DY-9/DY-11). Particle Morphology. The particle size of the parent zeolite was in the range of 0.5-2 µm, as can be seen from the SEM picture in Figure 3a. The particles were either in the form of single crystallites or in the form of stacked-together small crystallites; these small crystallites were in the form of irregular polyhedra. After dealumination with each one of the above procedures, and especially after steaming, a significant number of agglomerates, like the one in Figure 3b, appeared in the zeolite samples, with mean dimensions of nearly 1015 µm. The formation of agglomerates could happen through binding of the surface hydroxyl groups (Si-OH) of neighboring crystallites, which can be facilitated at

d-g

t-plot method.

the high temperatures of the hydrothermal treatment or in the presence of a chemical agent, like AHFS. The EDS analysis of the composition of different parts of the zeolite samples showed that in all of the dealuminated samples the bulk Si/Al ratios of (a) the total scanned area of the sample, (b) the above-mentioned agglomerates, and (c) the focused areas with a low number of individual small crystallites were nearly the same. Furthermore, these values were almost the same (within experimental error) as the values estimated from the chemical analysis of the samples. An exception has been found in the case of the SiCl4 samples, where the Si/Al ratio of the agglomerates was considerably higher than the Si/Al ratio of the more separated crystallites or the Si/Al ratio from the chemical analysis.

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 313

the desorbing ammonia, but it is a qualitative indication of how strongly the ammonia molecules are connected to the acid sites. The temperature range for the peak maxima of the weak acid sites has been set between 170 and 190 °C, for the medium strength acid sites between 220 and 270 °C, and for the strong acid sites above 350 °C. A similar analysis of the NH3-TPD chromatograms of AHFS-treated Y zeolites has been reported by Neuber et al.8 The relative area below the deconvoluted peaks has been used for the calculation of the number of acid sites (mmol of NH3/g of hydrated sample) which correspond to sites of weak, medium, and strong acidity, and the results are presented in Table 4. The temperature of the peak maximum of each individual desorption step and the total number of acid sites are also given in Table 4. For the sake of comparison, the content of total Al in milligram atoms of Al per gram of hydrated sample and the content of framework Al in milligram atoms of Al per g of hydrated sample (sample ) crystalline zeolite + amorphous material), minus the content of Na cations that have not been ion-exchanged by NH4 cations (found by chemical analysis), are calculated based on the corresponding QTAl and QFAl values of Table 1 and are included in Table 4. The repeatability of the measurements has been tested by performing five replicates of the TPD measurements for the NH4NaY sample, keeping all of the experimental conditions the same. The resulting mean value (m) was found to be 3.22 mmol of NH3/g of hydrated sample, the standard deviation (s) was 0.04, and for a confidence level of 95%, 3.22 ( 0.05 mmol of NH3/g of hydrated sample was calculated. The relative standard deviation (RSD) was estimated as 1.2%. Figure 3. SEM pictures (a) of individual zeolite crystallites of parent NH4NaY zeolite and (b) of agglomerated zeolitic crystallites after hydrothermal dealumination.

Effect of the Dealumination Method on the Acid Characteristics of the H-Y Zeolites. The amount of ammonia determined by TPD tests of H-Y zeolites, as well as the shape of the TPD curve, has been shown in previous works to depend largely on the experimental conditions, i.e., adsorption temperature, “stripping” temperature and time, desorption heating rate, and carrier gas flow rate.8,25-28 At the present study, adsorption of ammonia took place at 100 °C to get rid of physically adsorbed ammonia, and prior to desorption, dry pure helium was passed over the sample for 1.5 h at the same temperature. This time was sufficient for the TCD signal to be stabilized and reach the baseline in all of the tested samples. Desorbed ammonia was then detected between 100 and 700 °C. The TPD curves of selected samples are given in Figure 4. To follow the changes in the number of acid sites of different strengths found at different desorption temperature regions, the TPD chromatograms have been mathematically deconvoluted into individual desorption peaks of Gaussian shape, as shown in Figure 4. The peaks resulting from the signal processing have been classified and attributed to three types of acid sites with different acidity strengths, i.e., weak, medium, and strong, according to their position on the chromatogram (temperature of the peak maximum). The above characterization of the acid sites does not correspond to a quantitative measure of the actual acidity strength because of the intracrystalline diffusion limitations of

Discussion Identification of the Structural, Phase, and Chemical Characteristics of the Dealuminated Samples. The above investigation on the chemical, structural, and phase characteristics of the dealuminated samples leads to the assessment of the Si and Al compositions of both the framework of crystalline zeolitic material and the extraframework species phase. The results of the Si and Al compositions of the extraframework species phase (as % SiO2 and % Al2O3) are tabulated in Table 5 together with the effective Si/Al ratio and the percent of the total Al content involved in that phase. From the results in Table 5, we derive the following classification of the samples examined in this work. I. Samples with relatively minor EFAl content: NH4Y, DY-1, DY-2, DY-3, DY-4, DY-10, and DY-5. From this group the samples NH4Y, DY-1, DY-2, DY3, and DY-10 are well-crystalline with very little amorphous phase involving silicates or aluminosilicates; a higher extent of breakdown is observed for the samples DY-4 (silica-rich amorphous phase) and DY-5 (EFAlenriched amorphous phase). II. Samples with relatively major EFAl content: DY6, DY-7, DY-8, DY-9, and DY-11. From this group the sample DY-8 shows EDTAsoluble-only EFAl species, the DY-9 shows EDTAsoluble and -insoluble EFAl species, the DY-11 shows EDTA-insoluble-only EFAl species; the samples DY-6 and DY-7 are found to involve the aluminosilicate phase. From DY-11 it is also found that the EDTA-insoluble species are mainly those that have a Si/Al ratio higher

314

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000

Figure 4. Experimental and computer-simulated (s) NH3-TPD curves of H-Y zeolite samples and deconvoluted desorption peaks (- - -), of (a) NH4Y, (b) DY-3, (c) DY-8, and (d) DY-5 (refer to Table 1). Table 4. Aluminum Content and Acidic Characteristics of the Dealuminated H-Y Zeolites aluminum content (mmol/g of sample) sample

QTAla ((0.05)

QFAlb ((0.05)

total

no. of acid sites (desorbed ammonia) (mmol/g of sample)c ((0.05) weak medium strong

NaY NH4NaY NH4Y DY-1 DY-2 DY-3 DY-4 DY-5 DY-6 DY-7 DY-8 DY-9 DY-10 DY-11

0.02 2.84 3.53 2.89 2.51 2.18 1.81 2.02 2.13 0.94 3.30 4.05 1.97 1.80

0.03 2.85 3.41 2.91 2.43 1.95 1.61 1.50 0.37 0.34 1.66 0.36 1.70 0.50

0.81 3.23 3.51 3.23 2.86 2.68 2.28 2.21 1.27 0.30 2.12 0.85 2.05 1.15

0.44 (182)d 0.64 (183) 0.75 (191) 0.70 (189) 0.58 (182) 0.40 (180) 0.23 (175) 0.33 (189) 0.18 (177) 0.04 (171) 0.32 (164) 0.13 (170) 0.31 (172) 0.23 (179)

0.37 (261) 1.10 (262) 1.07 (267) 0.82 (264) 0.74 (252) 0.69 (239) 0.52 (224) 0.53 (233) 0.28 (226) 0.06 (226) 0.64 (217) 0.25 (227) 0.62 (228) 0.25 (243)

1.49 (362) 1.69 (363) 1.71 (354 + 417) 1.54 (351 + 428) 1.59 (361 + 433) 1.53 (356 + 427) 1.35 (358 + 426) 0.81 (378 + 551) 0.25 (353 + 558) 1.16 (388) 0.47 (381 + 586) 1.12 (386) 0.67 (386 + 589)

total acid sites/FAl (mole ratios)

strong/total acid sites (mole ratios)

1.03 1.11 1.18 1.37 1.42 1.47 3.43 1.17 1.27 2.36 1.20 2.30

0.48 0.53 0.54 0.59 0.67 0.61 0.64 0.71 0.55 0.55 0.55 0.58

a The content of total Al is estimated from chemical analysis minus the content of Na cations that have not been ion-exchanged by NH 4 cations. b The content of framework Al (FAl) is estimated from the unit cell size (XRD data, as shown in the text), minus the content of Na cations that have not been ion-exchanged by NH4 cations. c Sample ) crystalline zeolite + amorphous material produced during dealumination. d The numbers in parentheses represent the temperature peak maxima at degrees Celsius of the individual desorption steps.

than 3 (Table 5). Finally from our 27Al MAS NMR data of EFAl species and from NMR data of other research groups, the EFAl species are found of octahedral and of tetrahedral coordination; the Al removed from the framework of DY-8 (in this work) is found to be monomeric Al species and according to NMR data are removed from Si(3Al, 2Al, and 1Al) sites unselectively. However, because the Si/Al ratio in DY-11 is 3.7, we may suggest that aluminosilicate species of 4:1 stoichiometry or greater have been removed from the DY-9 sample zeolitic framework as well. The SiCl4-treated samples have a Si/Al ratio of the extraframework phases that is varied between 2.0 and 4.0, which suggests that there is no particular pattern of zeolitic breakdown during this treatment. Finally the DY-4 sample shows

a breakdown with an 8:1 ratio, suggesting that under conditions of high reactant (SiF62-) concentrations the zeolitic breakdown is facilitated, resulting in the formation of relatively high silica content complexes. Parameters That Affect the Extent of Ammonia Adsorption. The major chemical parameters that are related to acidity in this work are the functional groups that can adsorb ammonia; these may be originated by (a) the involvement of aluminum in the zeolitic structure or the amorphous phase that is produced by the breakdown of the zeolitic structure during the dealumination process and (b) the terminal Si-OH groups which either preexist in the precursor material or are formed during the treatment. The EFAl species are formed during the dealumination process by the re-

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 315 Table 5. Composition of Extraframework Phases

sample NaY NH4NaY NH4Y DY-1 DY-2 DY-3 DY-4 DY-5 DY-6 DY-7 DY-8 DY-9 DY-10 DY-11 a

total Si and Al content of samplesa (%) ((0.3) SiO2 Al2O3 50.4 52.2 51.8 57.7 61.9 65.3 69.7 66.7 71.0 88.7 58.3 64.4 66.1 73.5

amorphous materialb (%)

16.1 17.9 18.5 15.5 13.5 11.3 9.6 11.0 11.4 5.2 17.9 21.6 11.3 10.1

2

12 13 37 17 6 53 42

Si and Al content of the extraframework phases (%) ((0.3) SiO2c Al2O3c Si/Al 1.7 3.7 2.7 1.7 1.1 10.0 8.8 22.0 13.4 27.1 28.8

Al2O3(EFAl)/ Al2O3(TOTAl)d (%)

0.6

5.1

3

0.4 0.7 1.0 2.7 8.9 3.0 8.3 18.8 1.3 6.6

3.9 1.3 8.3 2.8 2.1 3.7

3 6 10 24 78 58 46 87 12 65

1.2 3.7

b

From chemical analysis of the samples. Estimated by the XRD relative crystallinity of the samples (Table 2). c The Si and Al contents of the extraframework phases of the samples have been calculated based on the relative data of Table 1. d Ratio of Al2O3 in extraframework positions (EFAl) over the total Al2O3 found by chemical analysis.

moval of framework Al atom species that are not able to escape the solid phase, and these species may or may not contribute to the ammonia adsorption depending on the coordination of these species, the phase that these are involved in, and their accessibility by the probe molecules. However, because ammonia is a quite small molecule, we would assume inaccessibility of these species only if they are involved inside the hexagonal prism or inside the sodalite cage, assuming the hexagonal windows of the cage are all blocked; the latter, however, is very unlikely. All of the removed FAl atoms stay inside the dealuminated product, in the steam-treated samples, and it is necessary to investigate the nature of these species and their effectiveness in blocking the micropores of zeolite that obstruct the diffusion of ammonia to reach the acid sites. The solubility by EDTA can give evidence of whether these species are monomeric or polymeric, and the Al NMR spectra can identify their coordination. The combination of specific surface area measurements with EDTA treatment of the samples gives evidence of the extent of pore blocking; there is evidence from the SSA data, however, that the monomeric Al species block insignificantly the pore channels but the polymeric Al,Si species block considerably the porous structure. The removed FAl atoms, in SiCl4-treated samples, escape from the solid dealuminated product as volatile AlCl3 or after subsequent washing of the Na(AlCl4) complexes with water. However, some of the EFAl may stay in the solid dealuminated product and during the TPD pretreatment may convert to alumina or aluminosilicate phases in the pore channels; this is revealed from the Si/Al ratio of the EFAl species phase produced in such samples. Finally, the removed FAl atoms in AHFS-treated samples escape the crystalline structure as (NH4)3AlF6 that is soluble in water. However, the weakly acidic solution that is used contributes to the minor breakdown of the structure, which becomes higher at relatively high concentrations of SiF62-, where an amorphous phase, with a Si/Al ratio that is as high as 8.0, is formed. The SSA measurements give evidence of breakdown of the zeolitic structure during the different dealumination methods, and according to the results and the observations in the previous section, it is most unlikely that the adsorption of NH3 is affected seriously by blocking the micropores of the crystalline structure.

Furthermore, the observation that, within the AHFS and the SiCl4 dealumination processes, the percent loss of SSA is not always followed by equal percent loss of the total pore volume suggests that, at least partly, we have a breakdown of the walls of micropores to form mesopores of the same total volume, which assumes that the diameter of the pores is increased. On the contrary we observe that the steam-treated samples have a significant increase of the total pore volume which is related, mainly, to the increase of the macropore volume. However, the blocking of the porous system to NH3 adsorbate is limited mainly at the microporous level by the polymeric Al,Si species as revealed from the SSA data in the pair of DY-9,DY-11 samples. From the above considerations it is realized that the ammonia adsorption process in the different dealuminated samples is mostly affected by (a) the FAl content or the FAl + EFAl content of the catalyst, (b) the nature and the number of the EFAl species as far as their acid property is concerned, (c) the nature of amorphous microphases that are formed and their acidic properties, and (d) the accessibility or blocking of the microporous structure. However, from the obtained surface area measurements, it is realized that there is a blocking effect only in steam-treated highly dealuminated samples (DY-9), and therefore the inaccessibility factor is negligible for most of the samples studied in this work. Dealumination and the Total Number of Acid Sites. From the data of Table 4 it is realized that the total amount of desorbed ammonia depends on the Al content of the sample and the dealumination method. The above observation is illustrated in Figure 5a where the total amount of desorbed NH3 per gram of hydrated sample is plotted against the total Al content per gram of hydrated sample. From Figure 5a it becomes clear that the amount of desorbed ammonia is equivalent to the amount of Al content in NH4Y zeolite and in DY-10 and DY-5 samples; all of these samples have no or a relatively minor quantity of EFAl species. Samples treated by AHFS (DY-1 up to DY-4) show a small increase of desorbed ammonia per unit of total Al content. Samples treated with SiCl4 (DY-7 and DY-6) and samples hydrothermally treated (DY-8 and DY-9) that possess major quantities of EFAl species show a decrease of desorbed ammonia per unit of their total Al content. The increase of the content of EFAl species in the sample increases the deviation from the ideal

316

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000

Figure 5. (a) Relationship between the amount of the total ammonia desorbed (TPD) and the total Al content (chemical analysis). (b) Relationship between the amount of the total ammonia desorbed (TPD) and the framework Al content (XRD). (c) Relationship of the amount of total ammonia desorbed (TPD) from the samples minus their FAl content versus the EFAl content of the samples: (0) parent NH4Y, (O) AHFS-treated samples, (b) SiCl4-treated samples, (9) steamed samples, and (4) steamed and EDTA-treated samples.

stoichiometry, i.e., the one observed in NH4Y zeolite. Therefore, the plot in Figure 5a gives evidence that, in purely crystalline samples, ammonia is adsorbed on acid sites which are generated from Al atoms of the zeolitic framework and each acid site adsorbs one ammonia molecule. The small positive deviation from ideal stoichiometry for samples treated with AHFS may be explained by the presence of acidic Si-OH in “hydroxyl nests” or in amorphous silicate phases, which are formed during the treatment, due to differences in the rates of Al removal and Si insertion.20,27,29 The significantly negative deviation from the ideal stoichiometry

of the hydrothermally treated samples is dependent on the amount of EFAl species formed during the treatment. The EDTA treatment of these samples decreases the deviation to an extent that is related to the extent of extraction of the EFAl species from the crystalline structure. Finally, the negative deviation from ideal stoichiometry, for SiCl4-treated samples that possess EFAl’s, is less compared to that of the hydrothermally treated samples with similar EFAl content, suggesting that the EFAl’s in these samples form a microphase, the nature of which is more capable to adsorb ammonia. An estimate of the extent that the EFAl species can contribute to the adsorption of ammonia can be derived by plotting the desorbed mmoles of total ammonia against the FAl content, as shown in Figure 5b. From Figure 5b it is observed that the data points follow a linear correlation with a slope of 0.96 and an intercept of 0.59. The slope is very close to 1 and corresponds to the ideal stoichiometry for almost all of the samples examined in this work; the significantly higher than zero value of the intercept is the result of acid sites that are not associated with framework Al atoms; for samples DY-6, DY-8, and DY-9 they are due to EFAl species and for AHFS-treated samples due to acidic Si-OH that are formed during the dealumination procedure. The contribution of the EFAl species to the adsorption of ammonia in the different microphases that are involved in the individual samples can be clearly seen by plotting the amount of the ammonia adsorbed in excess of the one justified by the FAl content of the sample versus the EFAl content (Figure 5c). From the plots in Figure 5c, it is shown that the extraframework phases formed by SiCl4, which consist of both Si and Al with a Si/Al ratio 2-3 (as shown in Table 5), are more capable of adsorbing ammonia compared to the phases in the steamed samples, at the same region of the EFAl content. This difference is more pronounced when the steamed samples possess mainly monomeric EFAl species with no EFSi (DY-8 and DY-10) and when a significant amount of oligomeric EFAl species is included in the Si,Al amorphous phases with a relatively low Si/Al ratio of ∼1 (DY-9). The extraction of the greater part of these species from DY-9 by EDTA treatment results in an extraframework phase in DY11, which has a higher Si/Al ratio of ca. 3.5 (close to that of the SiCl4-treated samples) and consists mainly of polymeric Si,Al species. This type of phase is evidenced to be more active in adsorbing ammonia and is more close to the behavior of the EFAl microphases in the SiCl4-treated samples, as shown in Figure 5c. Dealumination and Acid Sites of Different Strength. Plots of the ammonia desorbed per gram of hydrated sample from weak, moderate, and strong acid sites versus the FAl content of the samples (Table 4) are shown in Figure 6a-c. From Figure 6a it is shown that around 21% of the total acid sites of the precursor NH4Y zeolite are of weak strength and the loss of ammonia uptake is proportional to the extent of dealumination but is not dependent on the method of dealumination. The linear equation that best fits the experimental data points give slope and intercept values of 0.20 and 0.04, respectively. The deviation of experimental data points is within the 95% confidence interval, and the intercept is not significantly different from zero. In addition, the data points for each dealumination procedure are scattered randomly around the linear curve, giving evidence that the linear relationship is a

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 317

Figure 6. Relationship between the amount of ammonia desorbed (TPD) from (a) weak, (b) medium, and (c) strong acid sites and the framework Al content (XRD): (0) parent NH4Y, (O) AHFStreated samples, (b) SiCl4-treated samples, (9) steamed samples, and (4) steamed and EDTA-treated samples.

good model equation. From Figure 6b it is shown that the number of moderate strength acid sites is around 30% of the total acid sites in the precursor zeolite; the loss of ammonia uptake on these sites is proportional to the extent of dealumination, and the best-fit straight line that passes through the data points gives an intercept that is significantly higher than the origin of the coordination system. This intercept value may be explained by the existence of acid sites that are not associated with framework Al atoms and are the result of the treatment; however, their number is smaller than 4% of the total acid sites of the precursor zeolite. From Figure 6c it is shown that the number of strong acid sites is around 50% of the total acid sites in the precursor NH4Y zeolite and, generally speaking, the extent of dealumination decreases the uptake of ammonia on the strong acid sites. However, the loss of ammonia uptake along dealumination follows a different trend for each different dealumination procedure. The slope of the best-fit straight line that passes through the samples treated by AHFS is just 0.10, indicating that the number of strong sites in these samples are

not greatly affected by dealumination; the slope of the straight line that passes through the steamed or the SiCl4-treated samples is ∼0.45. On the other hand, there is clear evidence that the number of strong acid sites in all of the treated samples is much higher than the number that is expected from the line that connects the origin of the coordinates with the data point that corresponds to the NH4Y sample. The observed in this study relatively high amounts of strong acid sites left in the samples, after dealumination by AHFS up to ca. 50% (Table 4), are in accord with our 29Si MAS NMR results (as discussed above) and the results of Lonyi and Lunsford,10 regarding the selective character of the AHFS reagent to remove the framework Al atoms that are in Al-rich regions, ca. Si(3 and 4Al), and are associated with acid sites of weak or moderate strength. Some of the remaining FAl atoms are now more isolated and become more acidic (i.e., with higher acidity strength). The compensation of the acidity of these FAl atoms in the weak/medium strength acidity region is probably done by the relatively weak Si-OH sites formed during the treatment with AHFS. According to previous IR studies,4,5 hydroxyl “nests” can also be formed during the SiCl4 treatment, giving rise to nonacidic or weakly acidic Si-OH groups. In addition to the above, a mechanism that involves the reduced affinity of the relative bulky molecules of AHFS with the acid sites of higher strength may be indicative that most of the strong acid sites are in positions which are difficult to be approached, e.g., in the small cages or hexagonal prisms. However, this suggestion needs further investigation. Both of the other two dealumination procedures seem to be less selective in the removal of framework Al atoms; this can be easily justified by the fact that with these methods the samples experience relatively high temperature treatments where the vibrations of the framework elements are more intense and the bonds are relatively weakened. However, the number of strong acid sites in samples treated by these methods are always higher than the number justified by connecting the origin of coordinates with the experimental point corresponding to the precursor NH4Y zeolite. This observation suggests that strong acid sites are existing in phases that involve EFAl species, because the samples produced by these treatments possess EFAl species. It is worth pointing out that the slope of the correlation between the loss of desorbed ammonia and the framework dealumination of hydrothermally treated samples is very close to the slope of the line that connects the origin of the coordinates with the data point of the NH4Y sample. If the EFAl’s had ability similar to that of the adsorption of ammonia with the FAl’s, one would expect that the above slope would decrease with dealumination (i.e., with an increase in the EFAl amount of the samples going from sample DY-8 to DY-9). It can be clearly seen that this in not valid here, perhaps because of limited accessibility of the EFAl’s to NH3 or because of their nature. Additionally, by comparing the intercept of the linear correlation in Figure 5b with the positive deviation of strong acid sites of the dealuminated by hydrothermal treatment samples, we find that there is a fair agreement. The behavior of the SiCl4-treated samples is similar to that of the hydrothermally treated samples, with the exception of the most dealuminated sample DY-7, which has a FAl content similar to that of DY-6 but a considerably lower amount of EFAl. As a

318

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000

Figure 7. Relationship between the ratio strong/total number of acid sites and the framework Al content: (0) parent NH4Y, (O) AHFS-treated samples, (b) SiCl4 treated samples, (9) steamed samples, and (4) steamed and EDTA-treated samples.

result the experimental point of DY-7 is lower that the mean line of the other SiCl4-treated samples and is much closer to the line of the ideal stoichiometry. However, in both dealumination methods, it is observed that the degree of framework dealumination controls the slope of the above-tested relationship between strong sites and FAl content, while at fixed dealumination degree the content and type of EFAl’s control the positive deviation from the ideal stoichiometry. In Figure 7 the strong/total acid sites ratio is plotted against the total framework Al atoms for all of the samples examined in this work. From Figure 7 it is evident that the ratio is increased with the extent of dealumination from the value of 0.5 for the NH4Y sample to the value 0.67 for DY-4 (50% AHFS dealuminated) and further up to 0.7 for DY-7 (90% SiCl4 dealuminated). However, the hydrothermally dealuminated samples exhibit a much lower increase of this ratio which reaches the value of 0.55; this provides evidence that the EFAl species in SiCl4-treated samples exhibit a better capacity for adsorbing ammonia, mainly at strong acid sites, because of either their nature or accessibility. It has to be borne in mind that both accessibility and the nature of EFAl species seem to be the reason for low NH3-adsorption capacity in steamtreated samples; a sample (DY-9) with blocked micropores shows evidently low NH3-adsorption capacity; the same sample when unblocked by EDTA shows better NH3-adsorption capacity, but it is far away from that of the crystalline products and depends on the quantity of their amorphous phase. Conclusions 1. During dealumination several processes take place. The two processes that influence most the chemical and phase composition as well as the structural characteristics of the final product are (a) structure breakdown and (b) formation of EFAl species in monomeric or polymeric structures of aluminates or aluminosilicates. The reaction rates of the individual processes are related to the nature of the dealumination method, and the dealuminated products are mixtures of phases of different compositions that include Al species of different nature and acidic properties; i.e., the AHFS treatment results in a Si-rich amorphous phase at a high dealumination degree with insignificant quantities of EFAl

species at lower dealumination, the steam treatment at relatively low temperatures produces charged monomeric EFAl species, and at relatively high temperatures monomeric Al species + oligomeric aluminosilicate species and the SiCl4 treatment produce generally the aluminosilicate phase 2. The coordination of Al species in crystalline zeolitic structure is tetrahedral, while the coordination of EFAl species is octahedral and tetrahedral. 3. Treatments at relatively high temperatures or in aqueous solutions with acidic species lead to the agglomeration of the crystallites that results in the formation of significant macroporous structure. 4. Treatments that lead to high degrees of dealumination lead to the extended loss of microporous structure. 5. The ammonia probe molecule is adsorbed on Bronsted acid sites of the zeolitic crystalline structure with a stoichiometry of 1:1; the EFAl species adsorb ammonia, but their capacity is considerably less depending on their nature and the microphase they are involved in; i.e., the extraframework phases formed by the SiCl4 method are relatively more acidic than the phases formed by steaming, giving samples with a higher ratio of strong/total acid sites. The monomeric EFAl species formed at low-temperature steaming are even less acidic than the amorphous phases formed at higher steaming temperatures. 6. The loss of weak and/or medium strength acid sites with increasing dealumination is independent of the dealumination method involved, in contrast to the decrease of strong acid sites which are differently affected by each method. In general, the AHFS method is a selective one for preparing H-Y dealuminated zeolites with a high proportion of strong acid sites at relatively low dealumination degrees (ca. up to 50%), while the SiCl4 method results in a higher proportion of strong acid sites, at relatively higher dealumination degrees (ca. higher than 50%). Literature Cited (1) Wang, Q. L.; Giannetto, G.; Torrealba, M.; Perot, G.; Kappenstein, C.; Guisnet, M. Dealumination of Zeolites. II. Kinetic study of the Dealumination by Hydrothermal Treatment of a NH4NaY Zeolite. J. Catal. 1991, 130, 459. (2) Kubelkova, L.; Beran, S.; Malecka, A.; Mastikhin, V. M. Acidity of modified Y zeolites: Effect of nonskeletal Al, formed by hydrothermal treatment, dealumination with SiCl4, and cationic exchange with Al. Zeolites 1989, 9, 12. (3) Kubelkova, L.; Seidl, V.; Novakova, J.; Bednarova, S.; Jiru, P. Properties of Y-type Zeolites with Various Silicon/Aluminium Ratios Obtained by Dealumination with Silicon Tetrachloride. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1367. (4) Beyer, H. K.; Belenykaja, I. M.; Hange, F.; Tielen, M.; Grobet, P. J.; Jacobs, P. A. Preparation of High-silica Faujasites by Treatment with Silicon Tetrachloride. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2889. (5) Anderson, M. W.; Klinowski, J. Zeolites treated with silicon tetrachloride vapour. IV. Acidity. Zeolites 1986, 6, 455. (6) DeCanio, S. J.; Sohn, J. R.; Fritz, P. O.; Lunsford, J. H. Acid Catalysis by Dealuminated Zeolite-Y. I. Methanol Dehydration and Cumene Dealkylation. J. Catal. 1986, 101, 132. (7) Skeels, G. W.; Breck, D. W. Proceedings of the 6th International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butteworths, Guilford, U.K., 1984; p 87. (8) Neuber, M.; Dondur, V.; Karge, H. G.; Pacheco, L.; Ernst, S.; Weitkamp, J. Spectroscopic and Catalytic Characterization of Faujasites Dealuminated via the (NH4)2SiF6 Method. Stud. Surf. Sci. Catal. 1988, 37, 461.

Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 319 (9) Cruz, J. M.; Corma, A.; Fornes, V. Framework and Extraframework Aluminium Distribution in (NH4)2SiF6 Dealuminated Y Zeolites. Appl. Catal. 1989, 50, 287. (10) Lonyi, F.; Lunsford, J. H. The Development of Strong Acidity in Hexafluorosilicate-Modified Y-Type Zeolites. J. Catal. 1992, 136, 566. (11) Kerr, G. T. Chemistry of Crystalline Aluminosilicates. V. Preparation of Aluminium-Deficient Faujasites. J. Phys. Chem., Part V 1968, 72 (7), 2594. (12) Kuehne, M. A.; Babitz, S. M.; Kung, H. H.; Miller, J. T. Effect of framework Al content on HY acidity and cracking activity. Appl. Catal. A 1998, 166, 293. (13) Vogel, A. I., Ed. Textbook of Quantitative Inorganic Analysis, 4th ed.; Longman: London, 1978. (14) Corma, A.; Fornes, V.; Rey, F. Extraction of Extraframework Aluminium in Ultrastable Y Zeolites by (NH4)2SiF6 Treatments. Appl. Catal. 1990, 59, 267. (15) Badran, A. H.; Dwyer, J.; Evmeridis, N. P.; Manford, J. A. Ferric Ion Exchange and Breakdown of Crystalline Structure in Zeolites. Inorg. Chim. Acta 1977, 21, 61. (16) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders & Porous Solids; Academic Press: New York, 1999. (17) Kubelkova, L.; Seidl, V.; Borbely, G.; Beyer, H. K. Correlations between Wavenumbers of Skeletal Vibrations, Unit-cell Size and Molar Fraction of Aluminium of Y Zeolites. J. Chem. Soc., Faraday Trans. 1 1988, 84 (5), 1447. (18) Sohn, J. R.; DeCanio, S. J.; Lunsford, J. H.; O’Donnell, D. J. Determination of framework aluminium content in dealuminated Y-type zeolites: a comparison based on unit cell size and wavenumber of i.r. bands. Zeolites 1986, 6, 225. (19) Matharu, A. P.; Gladden, L. F.; Carr, S. W. Characterization and Catalytic Properties of Dealuminated Zeolite-Y: A comparison of Ammonium Hexafluorosilicate and Hydrothermal Treatments. Stud. Surf. Sci. Catal. 1995, 94, 147. (20) He, Y.; Li, C.; Min, E. A mechanism study of framework Si-Al substitution in Y zeolite during aqueous fluorosilicate treatment. Stud. Surf. Sci. Catal. 1989, 49, 189.

(21) Chen, D.; Sharma, S.; Cardona-Martinez, N.; Dumesic, J. A.; Bell, V. A.; Hodge, G. D.; Madon, R. J. Acidity Studies of Fluid Catalytic Cracking Catalysts by Microcalorimetry and Infrared Spectroscopy. J. Catal. 1992, 136, 392. (22) Biaglow, A. I.; Parrillo, D. J.; Kokotailo, G. T.; Gorte, R. J. A Study of Dealuminated Faujasites. J. Catal. 1994, 148, 213. (23) Sanz, J.; Fornes, V.; Corma, A. Extraframework Aluminium in Steam- and SiCl4-dealuminated Y Zeolite. J. Chem. Soc., Faraday Trans. 1 1988, 84 (9), 3113. (24) Stocker, M. Review on recent NMR results. Stud. Surf. Sci. Catal. 1994, 85, 429. (25) Hidalgo, C. V.; Itoh, H.; Hattori, T.; Niwa, M.; Murakami, Y. Measurement of the Acidity of Various Zeolites by TemperatureProgrammed Desorption of Ammonia. J. Catal. 1984, 85, 362. (26) Karge, H. G.; Jozefowicz, L. C. A comparative study of the acidity of various zeolites using the differential heats of ammonia adsorption as measured by high-vacuum microcalorimetry. Stud. Surf. Sci. Catal. 1994, 84A, 685. (27) Zi, G.; Yi, T.; Yugin, Z. Effect of Dealumination Defects on the Properties of Zeolite Y. Appl. Catal. 1989, 56, 83. (28) Forni, L.; Magni, E.; Ortoleva, E.; Monaci, R.; Solinas, V. Temperature-Programmed Desorption Study of Ammonia Desorption-Diffusion in Molecular Sieves. II. Application to Partially Decationated Y-Zeolites. J. Catal. 1988, 112, 444. (29) Scherzer, J. Octane-Enhancing, Zeolitic FCC Catalysts: Scientific and Technical Aspects. J. Catal. Rev.sSci. Eng. 1989, 31 (3), 215.

Received for review July 26, 1999 Revised manuscript received October 14, 1999 Accepted October 21, 1999 IE990568K