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Ind. Eng. Chem. Res. 2008, 47, 9288–9296
FCC Catalysts: Cu(II)-Exchanged USY-Type. Stability, Dealumination, and Acid Sites after Thermal and Hydrothermal Treatment before and after Vanadium Impregnation Vassilis A. Tsiatouras and Nicholaos P. Evmiridis* Laboratory of Analytical Chemistry, Department of Chemistry, UniVersity of Ioannina, Ioannina 45 110, Greece
NaH-USY, H-USY, and Cu(II)-exchanged NaH-USY samples prepared were studied for ion-exchange mass balance, framework aluminum microenvironments (FAl-MEs), and number and strength of acid sites (AS) generated by the increase of the Cu(II)-exchange after thermal treatment (500 °C) and lost under hydrothermal treatment conditions in the presence and in the absence of V species. From the data obtained, conclusions were drawn that (a) the nature and number of Cu(II) species exchanged enter FAl-MEs which can compensate their charge, (b) the strength and number of generated AS in CuNaH-USY samples after thermal treatment is related to the nature and number of Cu(II) species accommodated in the corresponding FAl-MEs, (c) the AS lost during the hydrothermal treatment depend on the nature of counterion involved, and (d) in the presence of V species, an extra loss, restricted to strong AS, takes place. Differences of lost AS and structure-stability during hydrothermal treatment in the presence and absence of V species give evidence of V catalytic action and support the implication of the nature of counterion species for the degree of structure breakdown. These findings lead to the proposed reaction pathway for the chemistry of V detrimental effect. 1. Introduction It is widely known that transition metal ions exchange for Na+ in type-X and Y zeolite structures and under thermal treatments (a) form complexes with zeolitic framework oxygen atoms, (b) release hydronium ions from H2O ligands, and (c) their hydroxy complexes suffer condensation.1–8 In addition, transition metal ions able to undergo tetrahedral or tetragonal coordination are favorably accommodated in the interstitial zeolite positions. Apart from the coordination convenience of such transition metal ions in the zeolite structure, they undergo redox reactions9,10 and may provide Lewis acidity1,2 that may act in synergy with Bronsted acidity. NaH-USY has type-Y zeolite structure and is a precursor of the active catalytic component used in industrial cracking catalyst particles of the fluidized bed reactor (FCC reactor). The preparation of the H-USY is obtained through ion exchange of NH4+ ions for Na+, followed by release of NH3 through heating. Alternatively, the ion-exchange of transition metal ions for Na+ of NaH-USY, can, also, be used for the preparation of H-USY zeolite. The advantage of H-USY zeolite prepared by this method is that it includes strongly dispersed transition metal species in its structure and that the material apart from acid-basic sites has redox sites as well. Exchanged samples with several transition metals were prepared in our laboratory and tested with a microactivity test, MAT, apparatus in a joint project with the Research Institute of Chemical Processes of Greece. CuH-USY sample was one of the prepared samples with good performance (gasoline yield, research octane number (RON)) in the cracking process; thus attention was focused on Cu(II)-exchanged Y-type zeolites that possess redox properties with unique catalytic activities.10–12 The effect of nickel and vanadium on the FCC catalyst has been well-known since the last half of the 1960s. Hydrotreating heavier crude distillation fractionssrich in Ni, V, and N impuritiessfor better gasoline operational yields,13–18 was found * To whom correspondence should be addressed. E-mail: nevmirid@ cc.uoi.gr.
to be not successful enough to remove completely Ni and V; these impurities deposit and deactivate the active catalytic component (H-USY or REY)19 of the industrial catalyst particle. Additives such as tetraethyl lead, used in the past to bring the RON to the specified value for the smooth operation of internal combustion motors, and the partial desulfurization process commonly used were changed lately, when Environmental Protection Agency issued regulations imposing low limits for sulfur content of gasoline and elimination of tetraethyl lead addition in fuels. H-ZSM-5 catalyst was then used as an additive in the matrix of industrial catalyst particle20–22 to increase the RON of produced gasoline, but this took place on the expense of gasoline operational yield. FCC feedstocks with increased percentages of resid, containing relatively high content of Ni and V, are processed today23 to cope with the increasing market demand; therefore, more effective methods are needed for mitigation of Ni and V detrimental effect on FCC catalyst. Research studies on distribution of Ni in used FCC catalyst particle showed that it was mostly accumulated in a thin layer close to the outer surface; thus with short contact times and with proper design of the catalyst-matrix porous structure, the active component of the catalyst can be protected. On the contrary, V penetrates deep into the catalyst matrix and is diffused in the whole internal surface of the catalyst particle homogeneously contributing to the break down of the zeolitic crystalline structure24,25 of the active component. Initially, additives for trapping and/or passivating metal poisons were included26–31 in the catalyst particle matrix, to protect the active catalytic component from the detrimental action of Ni and V. Later, several studies were made to investigate the mechanism of the detrimental action of V species on H-USY catalytic action by following different routes of research. A recently published report27 investigates the ability of Ce3+, incorporated in the H-USY zeolite by separate procedures, i.e. impregnation, precipitation, and ion exchange, to interact with V species and EFAl species. The data obtained in the above report support that Ce-V-EFAl multicomponent
10.1021/ie8008605 CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9289
interaction is responsible for the mitigation of V effect on H-USY zeolite. The challenge for preparation of multicomponent H-USY zeolite is, therefore, more than ever demanded today, for the study of interaction forces between V species and other multivalent elements with the aim to overcome the V detrimental effect. However, more investigations are needed for the better understanding of the chemistry of V species incorporated in the active catalyst component of the FCC catalyst particle and the establishment of a detailed reaction pathway, in order to develop and design a multicomponent H-USY zeolite that is highly resistant to V detrimental effect and possess the properties of increased thermal stability and activity for high gasoline and high RON values. In this report, the number and nature of ion-exchanged Cu(II) species for Na(I) ions in USY zeolite pores is established through a mass balance of the ion-exchange process. Furthermore, the number and type of FAl-MEs available in USY crystalline phase is calculated. On the basis of the assumption that Cu(II) species that entered the pores are directed to the number and nature of FAl-MEs that are able to compensate their charge, it was possible to position them in the specific types of FAl-MEs of the NaH-USY framework. In addition, a study of the number and strength of acid sites (a) generated after TPDthermal treatment and (b) lost by hydrothermal treatment in the presence and absence of vanadium was made to investigate the effect of treatments on the chemistry of the Cu(II) and V species involved in the solid phase. From the data obtained after each progressive treatment, it was easy to associate the strength of generated or lost acid sites to a specific type of FAl-ME, and therefore, besides the changes in the number of specific strength acid sites, it was possible to predict structural changes from acid site strength variations. Finally, the obtained differences in acid sites data between samples in the presence and absence of V species revealed the nature of acid sites that V species prefer to attack and their position in the framework of H-USY; thus, a pathway of chemical reactions that can explain V action during steam treatment in H-USY structure was made partly evident and partly indicative. 2. Experimental Details 2.1. Materials. NaH-USY zeolite was obtained from the Davinson Division of W.R. Grace. Cu(NO3)2 · 3H2O, 35% w/w HCl acid, NH4Cl, and pellets of NaOH were analytical grade reagents and were purchased from Merck. Dry ammonia gas, (Merck, H2O free) and He (Air Liquide) were of high purity (99.999%), and the air was dry. Vanadyl naphthenate oxide (2.8-3.2% vanadium) was obtained from ALFA. Unless otherwise mentioned, distilled water was used throughout all this work, and the samples obtained by ion exchange were washed with water, dried in air, and equilibrated with water vapor in a desiccator with MgCl2 solution. 2.2. Preparation of H-USY. Ammonium ions were ionexchanged for Na(I) ions in NaH-USY type zeolite dispersed in 0.1 M solution of NH4Cl. The NH4H-USY obtained was calcined in vertical quartz tube,32 heated at a rate of 5 °C/min up to 540 °C, and kept at 540 °C for 4 h in an air stream. 2.3. Preparation of Cu(II)-Exchanged-USY Samples. Cu(II) ions were ion-exchanged for Na(I) ions in NaH-USY zeolite with appropriate aqueous volumes of 0.1 M Cu(NO3)2 · 3H2O solution. During the ion-exchange process, the concentration of Cu(II) ion in the solution was monitored by the UV spectrophotometer. The pH of the suspended solids
aqueous solution had a value in the range of 4.0 to 5.0 for all the samples of Cu-exchanged NaH-USY prepared in this work. The prepared samples were designated as xx% CuNaH-USY, where xx% is the percentage of the prospective ion-exchange of Cu(II) ions. 2.4. Impregnation Treatment. Impregnation of NaH-USY, H-USY, and CuNaH-USY zeolite samples with vanadium was made by vanadyl naphthenate oxide solution in toluene through the method reported by Mitchell33 as modified by Tsiatouras et al.32 The used amount of vanadyl naphthenate oxide corresponded to 0.1% and 0.4% w/w vanadium in dry zeolite for all prepared samples. The solvent was evaporated in a rotaryevaporator under vacuum. The resulting product was calcined in a vertical quartz tube, heated at a rate of 5 °C/min up to 540 °C, and remained for 4 h in an air stream to burn the organic matter. The prepared samples are designated NaH-USY-V1, H-USY-V1, 60% CuNaH-USY-V1, and NaH-USY-V4, H-USYV4, 60% CuNaH-USY-V4 for vanadium concentrations of 0.1% and 0.4% w/w, respectively. 2.5. Temperature Programmed Desorption of Ammonia Experiment (NH3-TPD). NH3-TPD experiments were performed with a conventional apparatus22 and the following procedure. Sorption of dry ammonia took place at 100 °C in a static system for 45 min at 1.5 bar ammonia vapor pressure. Afterward, a stripping was done to minimize the weakly and physically adsorbed ammonia for 90 min at 100 °C under flow of He at the same flow rate. Desorption of ammonia was done by programmed temperature increase at a rate of 10 °C/min from 100 up to 700 °C under He flow (50 mL/min). The desorbed ammonia was detected with a TCD detector, recorded, and driven and trapped in an excess of 0.0100 N HCl aqueous standard solution. The total desorbed NH3 was estimated by titrimetric determination of the excess of HCl acid solution with standard 0.0100 N NaOH. 2.6. Hydrothermal Treatment. Samples of NaH-USY, H-USY, and 60%CuNaH-USY type zeolite prepared by Cu(II)exchange for Na(I), of the NaH-USY from Grace, before and after impregnation treatment with vanadyl naphthenate oxide, were subjected to hydrothermal treatment as follows. A flow system consisted of a cylindrical quartz reactor and a vertical high temperature furnace was used. The hydrothermal treatment procedure of samples was as follows: 10.00 g of the prepared samples were sandwiched between layers of silica wool and fine glass beads in the reactor tube and subjected to a flow of a high purity N2-steam mixture at a flow rate of 30 mL/min under ambient temperature for a few minutes, followed by a step of a continuous increase of temperature at a rate of 5 °C/ min until 770 °C and then a step of staying at a constant 760 °C temperature for 6 h with a steam partial pressure of 97.7 kPa. The samples were then cooled to 120 °C under the flow of N2-steam mixture and then were dried under a flow of dry air at 120 °C for 6 h. 2.7. Elemental Analysis. Digestion of solid samples was obtained by the conventional method of mixed acids H2SO4-HF (1:10) in Pt crucibles. The residue in the crucibles was then dissolved, diluted with water, and transferred to a 0.50 L flask. The sodium was determined with flame photometry (Jenway PFP7) and copper with flame atomic absorption spectroscopy (Shimadzu 6800 AAS). 2.8. Powder X-ray Diffraction (P-XRD). P-XRD patterns of the samples were obtained with a Siemens D-500 Diffractometer with Cu KR radiation 1.540 552 Å within the range of 5 to 40° 2θ values and at a scanning rate of 0.2° 2θ/min. The relative crystallinity (RC) between the parent and treated samples
9290 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 1. Structural Characteristics of the Samples rel. unit cell crystallinity parameter % ao ( 0.01 A
samples NaH-USY H-USY 10% CuNaH-USY 22% CuNaH-USY 38% CuNaH-USY 60% CuNaH-USY
100.0 87.0 93.6 84.2 89.4 91.2
24.55 24.48 24.54 24.52 24.53 24.52
Table 4. Distribution of Acid Sites between the Different Strength Classes in the Unit Cell of the NaH-USY Zeolite (atoms/uc) in Accordance with TPD Graphs
NFAl ( 0.05 atoms/uc
sample
33.40 (0.0% deal) 25.90 (22.5% deal) 32.34 (3.2% deal)) 30.20(9.6% deal) 31.27 (6.3% deal) 30.20 (9.6% deal)
NaH-USY 10% CuNaH-USY 22% CuNaH-USY 38% CuNaH-USY 60% CuNaH-USY
Table 2. Cu(II)-Exchange from Elemental Analysis Data and Total Acid Sites from NH3-TPD Experiment from Acid-Base Titrimetric Measurements total acid QCu(II) QNa QFAl sites (mmol/ entered remaining QNa (mmol/g; dry) g; dry) (mmol/g; dry) (mmol/g; dry)
samples NaH-USY H-USY 10% CuNaH-USY 22% CuNaH-USY 38% CuNaH-USY 60% CuNaH-USY
1.62 2.05 1.88 1.77 2.28 2.61
1.05 0.07 0.68 0.40 0.32 0.08
0.15 0.38 0.465 0.74
1.58 2.04 1.68 1.43 1.78 1.94
samples NaHUSY 10% CuNaHUSY 22% CuNaHUSY 38% CuNaHUSY 60% CuNaHUSY
NCu(II) NNa(I) NCu2+species NCu(OH)+species NH+species entered removed entered entered entered (atoms/uc) (atoms/uc) (species/uc) (species/uc) (species/uc) 0.00
0.00
0.00
0.00
0.00
1.85
4.60
1.80
0.00
1.00
4.70
8.10
3.48
1.10
0.00
5.70
9.10
3.50
2.20
0.00
9.10
12.00
3.00
6.00
0.00
was based on the ratio of the sum of the heights of the relatively higher peaks in the range of 2θ between 15 and 34°. The unit cell size (UCS) parameter, ao, of the various samples was calculated by ASTM method D3942. The pure silicon element was used as a calibration standard material. The number of aluminum atoms per unit cell (NFAl) was calculated by eq 1 NFAl ) 107.1(ao - 24.238)
(1)
34
as reported by Sohn et al. The amount of framework Al (QFAl) in millimole per gram of dehydrated solid32 is calculated by eq 2
QFAl )
{
[( ) ( )] NFAl %RC × UCW 100 h 1100
[(
)]
}
× 1000
(2)
Where, UCW ) weight of hydrated unit cell, %RC ) percent relative crystallinity, and h ) humidity. 3. Results 3.1. P-XRD Data. Table 1 gives data of the percent crystallinity (with reference to NaH-USY), calculated ao values, the number of framework aluminum atoms per unit cell, NFAl (calculated by eq 1), and percent dealumination for the samples of NaH-USY, H-USY, and CuNaH-USY prepared in this work using P-XRD patterns of diffraction lines. With the exception of the 22% CuNaH-USY samplesthat shows a minor loss of crystallinitysall other samples show insignificant loss of crystallinity, and apart from the H-USY sample, all other samples have NFAl between 30 and 33.
12.0 12.0 10.2 12.0 12.0
7.0 11.6 10.5 12.1 7.0
0.6 1.3 8.0
0.5 1.0 0.5
1.7 3.5
19.0 23.6 21.8 28.1 31.0
a Strong acid sites with peak maxima at 340, 430, and 510 °C, respectively.
Table 5. Number of Na(I)-Exchanged and Acid Sites Formed in Moles per Unit Cell (uc) from Cu(II)-Exchange samples HNa-USY 10% CuNaH-USY 22% CuNaH-USY 38% CuNaH-USY 60% CuNaH-USY
Table 3. Ion Exchange Data in Atoms Per Unit Cell
NWAS NSAS340a NSAS430a NSAS510a NMAS NTAS per uc per uc per uc per uc per uc per uc
NNa(I)(removed) NCu-SAS(formed) NWAS lost NTAS lost (Na+/uc) (H+/uc) (H+/uc) (H+/uc) 0.0 4.6 8.1 9.1 12.0
0.0 4.6 4.6 9.1 12.0
0.0 0.0 1.8 0.0 0.0
0.0 0.0 5.3 0.0 0.0
3.2. Ion-Exchange Data and Analysis. Table 2 shows the Cu(II) and Na(I) content in millimoles per gram of the prepared samples (columns 3 and 4); it, also, gives the total acid sites (column 2), as obtained by acid-base titrimetry of NH3 desorbed from the TPD experiment and QFAl - QNa(I) (column 5). The data of Cu(II) and Na(I) content (Table 2) are expressed in Table 3 (columns 2 and 3) as atoms per unit cell, NCu(II) (entered) and NNa(I) (removed). Aqueous complex Cu2+ and Cu(OH)+ species are exchanged for Na+ during Cu(II)-exchange of Y-type zeolite;35 thus, from eqs 3 and 4, 2x + y ) NNa(I) (removed)
(3)
x + y ) NCu(II) (entered)
(4)
x ) NCu2+species(entered), and y ) NCu(OH)+species(entered). The number of copper ions per unit cell entered as [Cu(H2O)6]2+ and as [Cu(OH)(H2O)5]+ is calculated and is given in columns 4 and 5 of Table 3, for each of the prepared samples. In addition, column 6 shows the NH+ exchanged at low percent of Cu(II) exchange to satisfy the ion-exchange mass balance requirements. 3.3. NH3-TPD Data and Correlation with Exchanged Cu(II) Species. The distribution of total acid sites per unit cell among sites of different strength, found from desorption peaks of NH3-TPD graphs is given in Table 4 for the samples examined in this work. From data in Table 4, the NaH-USY sample has seven strong acid sites (SAS) with a desorption peak maximum at 340 °C, SAS340. In addition, there are twelve weak acid sites, WAS, which are, conventionally, attributed to nonisolated FAl atoms. Comparing the data of Cu(II) ion-exchanged samples in Table 4 with the ones of NaH-USY, the following observations are made: 1. The number of WAS found in NaH-USY sample is not affected by the number of Cu(II) ionic species entered in the zeolitic phase through ion-exchange; these sites are kept constant at the value found in NaH-USY sample, unless there is partial structure breakdown (loss of two WAS in 22% CuNaH-USY sample). 2. The number of SAS generated from Cu(II)-exchange, NCuSAS, are mainly found at desorption peak maxima of 340 and 430 °C. 3. The NCu-SAS formed is increased with increase of percentage of Cu(II) ion-exchange and especially the ones desorbed at the peak with maximum at 430 °C, NSAS430.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9291 Table 6. Distribution of Cu(II)-Exchanged Species between the Crystalline Zeolitic Phase and the Amorphous Phase samples
NCu-SAS(formed) (ions/uc)
total NCu(II)(entered) in solid (ions/uc)
NCu2+species(entered) in crystalline phase (ions/uc)
NCu(OH)+species + NH+(entered) in crystalline phase (ions/uc)
NCu2+species dropped off crystalline phase (ions/uc)
NaH-USY 10% CuNaH-USY 22% CuNaH-USY 38% CuNaH-USY 60% CuNaH-USY
0.0 4.6 4.6 9.1 12.0
0.0 1.9 4.6 5.7 9.0
0.0 1.9 1.75 3.5 3.0
0.0 NH + )1.0 NCu(OH)+)1.1 NCu(OH)+)2.2 NCu(OH)+)6.0
0.0 0.0 1.75 0.0 0.0
4. Medium strength acid sites, MAS, have appeared when Cu(OH)+ species have entered the zeolitic structure and are increased with increase of Cu(OH)+ species. An estimate of the extra framework aluminum, EFAl, atoms (FAl atoms that drop off the crystalline phase), formed during TPD experiment, is obtained from the difference between NNa(I)(removed) (Table 3) and NCu-SAS(formed) (Table 5), through eq 5 NNa(I)(amorphous) ) NNa(I)(removed) - NCu-SAS(formed) (5) where, NCu-SAS(formed) are excess acid sites formed through Cu(II) ion-exchange i.e. NCu-SAS(formed) ) NSAS340 + NSAS430 + NSAS510 + NMAS - 7.0 Table 5 summarizes the results from eq 5. From data in Table 5 it is evidenced that the 22% CuNaH-USY sample looses 3.5 Na(I) per unit cell in the amorphous phase, suggesting the formation of some kind of NaAlSinO2+2n species. On the contrary, no loss of Na(I) ions in the amorphous phase is observed for the other CuNaH-USY samples. In addition, the number of Cu2+ species that remains in the crystalline phase after TPD-thermal treatment is calculated from eq 6 while the ones that drop off are calculated from eq 7, and the results are given in Table 6. NCu2+species(zeolite) ) [NCu-SAS(formed) - (NCu(OH)+species + NH+)]/2 (6)
giso ∼ 2.19 due to [Cu(H2O)6]2+ species, and the other is anisotropic with g| ∼ 2.34, g⊥ ∼ 2.07, and A| ∼ 14.8 mT, due to Cu(II) species attached to framework oxygen atoms with large distortion from octahedral or pseudopyramidal coordination.6,36–38 Furthermore, a broad signal of low intensity was observed at g ∼ 4.25 that may be assigned to distorted tetrahedral coordination6 perhaps in amorphous phase.36 In addition, the spectrum at 77 K was totally anisotropic with unresolved superfine structure similar to the one found in the work of Soria et al.36 in their Figure 5. The features of the EPR spectrum, therefore, gave an indication of Cu(II) species involved in the amorphous phase. In the light of the data in Table 4 concerning the number and strength of acid sites formed in respect to the number and nature of Cu(II) species exchanged, the following is concluded: (a) Acid sites contributing to desorption peaks with maxima at 340 and 430 °C are sites associated with Cu2+ and Cu(OH)+ species exchanged, respectively. These acid sites are formed during dehydration by TPD-thermal treatment under He flow, when aqueous Cu(II) complex species are moving closer to the framework oxygen atoms, according to ion exchange chemical eqs 8 and 10, and consecutive zeolite complexation to framework oxygen atoms followed by hydronium ion release according to chemical eqs 9 and 11: [Cu(H2O)6]2+ + Cu(II)-exchange
(O)2zeol(O-Na+)2 98 (O)2zeol(O-)2 · · ·
and if NCu(II)(entered) > (NCu2+species(zeolite) + NCu(OH)+species) NCu2+species(amorphous) ) NCu(II)(entered) (NCu2+species(zeolite) + NCu(OH)+species) (7) From Table 6, it is clear that the 22% CuNaH-USY sample has lost 1.75 Cu2+ complex species accompanied by equivalent loss of FAl atoms resulting to the loss of 3.5 acid sites; no other sample gives clear evidence for such a loss. Therefore, for this particular sample we have a loss of 7.0 FAl atoms that have dropped off the crystalline phase through dealumination at positions associated with Cu2+ species and Na+ counterions and two more associated with protons at WAS; thus, the total loss of FAl atoms of this sample is nine. It is not very clear why this sample shows such an increased structure breakdown. Perhaps the Na(I)/Cu(II) ratio has a major effect; the 22% CuNaH-USY sample has a ratio of about 1 (Cu(II) atoms entered per unit cell are about equal to Na(I) remaining in zeolite structure and are ∼5 per unit cell). The migration of Cu2+ species during TPD-thermal treatment, toward the sodalite cages, where Na(I) ions are accommodated, generates electrostatic repulsion forces between the positive charges giving rise to increased stress on the zeolitic framework through the formed complexes of Cu(II) species with the framework oxygen atoms; this stress is more intense when the Na(I)/Cu(II) ratio gets closer to 1. The electron paramagnetic resonance (EPR) spectrum of this sample was, then, obtained to make clear whether Cu(II) species were involved in the amorphous phase. The EPR spectrum of this sample taken at room temperature shows at least two overlapping signals; the one is isotropic with
[Cu(H2O)6]2++2Na+ (8) thermal treatment
(O)2zeol(O-)2 · · · [Cu(H2O)6]2+ 98 (H+O-)2zeol(O)2Cu(OH)2 + 4H2O (9) [Cu(H2O)5(OH)]+ + Cu(II)-exchange
(O)2zeol(O-Na+) 98 (O)2zeol(O-) · · · [Cu(OH)(H2O)5]+ + Na+ (10) thermal treatment
(O)2zeol(O-) · · · [Cu(OH)(H2O)5]+ 98 (H+O-)zeol(O)2Cu(OH)2 + 4H2O (11) For the sake of economy in typing space, the complexes [(H+O-)2zeol(O)2Cu(OH)2)] and [(H+O-)zeol(O)2Cu (OH)2] will be designated as (H2zeol)Cu(OH)2) and as (Hzeol)Cu(OH)2, respectively. (b) Acid sites contributing to medium strength are somehow related to the number of [Cu(H2O)5(OH)]+ species that enter in counterion positions and complexed to framework oxygen atoms through chemical eq 11. The predicted excess of acid sites per unit cell for each CuNaH-USY sample formed through Cu(II)-exchange for Na(I) in NaH-USY sample is calculated, on the basis of the number and nature of Cu(II) species exchanged, and is given in Table
9292 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 7. Comparison of Expected Strong Acid Sites and Found from NH3-TPD Dataa ion-exchange (ions per uc) sample 10% 22% 38% 60%
Cu-USY Cu-USY Cu-USY Cu-USY
experimentally found NETAS (per uc)
dropped off (atoms per uc)
H+
[Cu2+ sp]
[Cu (OH)]+
expected NCu-SAS
NSAS340
NSAS430+
NMAS
Cu2+
Na+/H+
NFAl
1.0 0.0 0.0 0.0
1.8 3.5 3.5 3.0
0.0 1.1 2.1 6.0
4.6 8.1 9.1 12.0
4.6 3.5 5.1 0.0
0.0 1.1 2.3 8.5
0.0 0.0 1.7 3.5
0.0 1.75 0.0 0.0
0.0 3.5/2 0.0 0.0
32.0 23.0 32.0 31.0
a [Cu2+sp] ) [Cu(H2O)6]2+. [Cu(OH)]+ ) [Cu(OH)(H2O)5]+. Cu2+ and Na+ ) species of Cu or Na dropped off the zeolite structure. SAS ) strong acid sites. MAS ) acid sites of medium strength.
Table 8. TPD Results of NaH-USY, H-USY, and CuNaH-USY Zeolite Sample under Different Treatments sample
NWAS/uc
NSAS340/uc
NaH-USY NaH-USYa NaH-USY-V1a H-USY H-USYa H-USY-V1a 60% CuNaH-USY 60% CuNaH-USY-V1 60% CuNaH-USYa 60% CuNaH-USY-V1a
12.0 7.0 7.0 10.8 3.2 3.2 12.0 10.0 2.0 7.3
7.0 7.0 4.5 13.1 2.2 2.2 7.0 7.0 0.8 7.0
a
NSAS430/uc
8.0 8.0 0.05 2.2
NSAS510/uc
0.5 0.5 0.4 1.2
NMAS/uc
3.5 2.5 0.3
crystallinity (%)
NFal/uc
1000 91 72.5 87 84.5 77.4 94
32.2 28.9 26.0 28.1 12.0 9.8 32.2
10 81
3.3 21.9
Steam treated samples.
7. The predicted NCu-SAS in Table 7 are in full agreement with excess total acid sites, ETAS, found from NH3-TPD experimental data; adding the sumsnumber of acid sites found in NaH-USY plus the number of remaining Na(I) ions in crystalline structuresto the excess acid sites, the number of FAl atoms per unit cell of each CuNaH-USY sample is obtained. 3.4. Steam-Treatment Data and Remarks. The number per unit cell of WAS, MAS, and different strength SAS found by TPD experiment for NaH-USY, H-USY, and 60% CuNaH-USY samples after the steam treatment in the absence and presence of 0.1% w/w vanadium are tabulated in Table 8 together with the percent crystallinity and the number of FAl atoms per unit cell. Similar data are given for the 60% CuNaH-USY-V1 sample (0.1% V-impregnated followed by combustion of the organic material) in Table 8.
Figure 1.
29
Si-MAS NMR spectrum of NaH-USY zeolite.
Data in Table 8 give evidence that hydrothermally induced dealumination is mainly followed by loss of acid sites and in some cases with structure breakdown. Furthermore, the extent of structure breakdown depends on the counterion and the type of acid strength site lost depends on the presence of vanadium, as follows: (a) NaH-USY sample, in the absence of vanadium, induces a relatively small scale dealuminution followed by decrease of WAS only, accompanied by small scale structure break down. However, steam treatment in the presence of V (0,1% w/w) results in partial dealumination with equal loss of SAS340, in excess to the loss of WAS observed in its absence, suggesting that vanadium attacks the SAS selectively; in addition a minor structure breakdown is observed. (b) H-USY sample in the absence and in the presence of vanadium (0.1% w/w) results in almost complete loss of WAS and SAS340, followed
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9293
by loss of almost equal numbers of FAl atoms, but with little loss of crystallinity. (c) 60% CuNaH-USY sample in the absence of vanadium, suffers, almost, complete structure breakdown loosing nearly all FAl atoms and all SAS and WAS. In contrast, the crystalline structure is retained and a loss of 4.5 WAS is observed, when the hydrothermal treatment takes place in the presence of 0.1% w/w vanadium. In addition, nine FAl atoms with equal numbers of acid sites (SAS430+ and MAS) are lost; these acid sites are associated with Cux clusters (see section 4.2) formed in this sample. On the other hand, 60% CuNaHUSY-V1 not steam-treated sample (Table 8) has shown no loss of SAS and only partial loss of MAS due to V impregnation and calcinations with air. 3.5. 29Si-MAS NMR Data and Si(nAl) FAl-MEs. The areas of 29Si-MAS NMR lines (Figure 1) of NaH-USY sample are found to be 0.25, 1.00, 4.75, and 4.00 cm2 for Si(3Al), Si(2Al), Si(1Al), and Si(0Al), respectively. The number per unit cell of the Si atoms involved in each of the Si(nAl) lines in the 29SiMAS NMR spectrum is calculated by the use of eq 12 NSi(nAl) )
ISi(nAl) 4
∑I
× 160
(12)
Si(nAl)
n)0
where I ) the line area of each specific Si(nAl), and is found to be Si(3Al) ) 4.0, Si(2Al) ) 16.0, Si(1Al) ) 76.0, and Si(0Al) ) 64.0 On the basis of the above results, the number of Si(nAl) FAlMEs can be calculated as shown in the Supporting Information, and from those calculations, it is evident that the most probable distribution model of the FAl-MEs in the unit cell is either typeIIA, type-IB, or type-IIB since these types give a calculated number of Si(1Al) atoms that is very close to the one found experimentally for the Si(1Al)-line of 29Si-MAS NMR spectrum. A more realistic suggestion would be to propose a blendedcombination model of all the three. However, the data of Cu(II) ion exchange support a blend-combination model of IIA and IB. With the acceptance of this equally shared IIA/IB blend combination model, the number of the Si(nAl) FAl-MEs per unit cell is averaged as follows: Si(3Al) FAl-MEs ) 4.0, Si(2Al) FAl-MEs ) 7.0, Si (1Al) FAl-MEs ) 6.0 Since each unit cell includes eight sodalite cages, it is expected that each sodalite cage should have on the average four FAl atoms, and from the above equally shared IIA/IB blend combination model of FAl-MEs, it is estimated that either the four sodalite cages will have the type-IIA combination and the other four will have type-IB combination of FAl-MEs or the FAl-MEs of the model with four FAl atoms per sodalite cage will be distributed in a random manner among the eight sodalite cages. 4. Discussion 4.1. Distribution of Cu(II) Species among FAl-MEs after the Cu(II)-Exchange Process. Data of Cu(II) ionexchange give evidence that a maximum of 3.5 aqueous Cu2+ complex species and of 6.0 aqueous Cu(OH)+ complex species can enter the unit cell by removing seven and six Na(I) ions, respectively. On the basis of the charge compensation point of view, it is reasonable to associate the positioning of aqueous Cu2+ complex species and the aqueous Cu(OH)+ complex
species with Si(2Al) FAl-MEs and Si(1Al) FAl-MEs, respectively. A conclusion is, then, reached that the nature and number of the species exchanged depends on the type and number of the Si(nAl) FAl-MEs available in NaH-USY sample. 4.2. FAl-MEs and Acid Sites of NaH-USY. Data of the TPD-experiment reveal that NaH-USY sample has 12 WAS and 7 SAS340 per unit cell, the H-USY sample has 13 SAS340 and 11 WAS, and no other sample has more than 12 WAS or more than 14 SAS340; this is a strong evidence to conclude that WAS are formed in the four Si(3Al) FAl-MEs and the SAS340 are formed in the seven Si(2Al) FAl-MEs; thus, the NaH-USY sample has Si(3Al) FAl-MEs fully occupied by WAS, Si(1Al) FAl-MEs fully occupied by Na(I) ions, and the Si(2Al) FAlMEs only half-occupied by Na(I) ions and the other half by SAS340. A conclusion is, then, reached that the type and number of Si(nAl) FAl-MEs that are available in the zeolitic structure determines the strength and number of the acid sites formed. The rule holds well with NaH-USY, H-USY, and 10% and 22% CuNaH-USY samples. In 22% CuNaH-USY sample about 1.0 Cu(OH)+ species is exchanged and about 1.0 SAS430 is formed; in 38% CuNaH-USY sample about 2.0 Cu(OH)+ species are exchanged and about 2.0 SAS430 are formed; thus it is concluded that Si(1Al) FAl-MEs provide SAS430. MAS are, also, formed in 38% and 60% CuNaH-USY samples; these are produced at the expense of SAS340, thus giving evidence that the conformation of some Si(2Al) FAl-MEs has been modified resulting from the interaction of the H2zeolCu(OH)2-complex with the HzeolCu(OH)2-complex through the hydroxyl groups to form Cux clusters,6 followed by distortion or defect formation of the zeolite framework. With this assumption, the formation of one Cu2 cluster between H2zeolCu(OH)2 and HzeolCu(OH)2complexes may be the reason for ∼2.0 MAS formation at the expense of 2.0 SAS340, for the 38% CuNaH-USY sample. On the other hand, the formation of (a) two Cu3 clustersswhere each one consists of one H2zeolCu(OH)2-complex O-bridged with two HzeolCu(OH)2-complexessthat forms 6.0 SAS430+ and 2.0 MAS, (b) one Cu2 clustersconsisting from one H2zeolCu(OH)2-complex O-bridged with one [HzeolCu(OH)2] complexsthat forms 1.0 SAS430+ and 2.0 MAS, and (c) one isolated [HzeolCu(OH)2] complex that forms 1.0 SAS430 may be the reason that the 60% CuNaH-USY sample has formed ∼8 SAS430+ and ∼4 MAS (Table 7). 4.3. Arbitrary Model of FAl-MEs Distribution of the NaH-USY Unit Cell. In the light of the above conclusions, it is possible to formulate a final model (shown in Figure 2) of FAl-MEs distribution in the NaH-USY type zeolite unit cell as follows: 1. A sodalite cage with one Si(2Al)-ME type A (covered by Na+) and two Si(1Al)-MEs (covered by Na+). 2. A sodalite cage with one Al in Si(3Al)-ME type II (covered by H+), one Si(2Al)-ME type A (covered by Na+), and one Si(1Al)-ME (covered by Na+). 3. A sodalite cage with one Si(3Al)-ME type I (covered by H+) and one Si(1Al)-ME (covered by Na+). 4. A sodalite cage with two Al in Si(3Al)-ME type II (covered by H+) and one Si(2Al)-ME type A (covered by H+). 5. A sodalite cage with one Si(2Al)-ME type A (covered by Na+) and two Si(1Al)-MEs (covered by Na+). 6. A sodalite cage with one Al in Si(3Al)-ME type II (covered by H+), one Si(2Al)-ME type A (covered by H+), and one Al in Si(2Al)-ME type B (covered by Na+). 7. A sodalite cage with one Si(3Al)-ME type I (covered by H+) and one Al in Si(2Al)-ME type B (covered by H+).
9294 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 H2O-vapor, T
4[tSiOH]zeol+SiO2 98 [(SiO)3 t Si s O s Si t ]zeol+ 2H2O (14)
Figure 2. Proposed model of FAl-MEs distribution in NaH-USY unit cell.
8. A sodalite cage with two Al in Si(3Al)-ME type II (covered by H+) and one Si(2Al)-ME type A (covered by H+). 4.4. Reaction Pathways of Dealumination during Steam Treatment. Zeolitic structures under conditions of hydrothermal treatment (750 °C, under N2 stream with water vapor) acquire weak bonding between the elements of crystal lattice, especially the ones of FAl atoms, which depending on their FAl-ME-type may form structure defects or be converted to hydroxides and drop off the structure as EFAl species through the action of water vapor,3 leading to dealumination through the following reactions,
The framework repair process is kinetically slower than dealumination and, because of this, in many cases leads to structure breakdown. Furthermore, steam treatment in the presence of other element metal oxide species or phases may induce hydroxylation/condensation reactions to form mixed oxides that contribute to interstice species or to amorphous phases. 4.5. Studies on Vanadium Action during Steam Treatment. Much work has been done to elucidate the mechanism of V-induced breakdown during steam treatment, but still is a matter of controversy. Wormsbecher et al.28 have suggested the formation of H3VO4, through steam/air treatment during regeneration step of FFC reactor procedure, which, being a strong acid, can extract FAl atoms. Evidence for this mechanism was the presence of vanadate phases after hydrothermal treatment; however, it has been reported that such phases are not stable above 650-700 °C.29 Anderson et al.30 suggested the formation of H4V2O7 based on experimental results with vanadium scavengers that form Mg2V2O7. Pine31 showed that vanadium acts as catalyst for deactivation of FCC catalyst during the steam treated reaction. Validation of the catalytic action of the V species in the steam treatment process is examined, in this work, by using a tiny vanadium quantity (0.1% w/w) impregnated in NaH-USY, H-USY, and 60% CuNaH-USY samples. 4.6. Vanadium Catalytic Action at Very Low Levels and Proposed Reaction Pathway. The presence of vanadium during hydrothermal treatment, provides the following: (a) Selectivity for partial elimination of SAS340 (case of NaH-USY) and of SAS430+ and MAS (case of the 60% CuNaH-USY sample); no such selectivity is observed for H-USY sample, since nearly all SAS340 are eliminated even without the presence of vanadium. (b) Structure stability (similar to that of H-USY) is observed for the 60% CuNaH-USY, in contrast to complete structure breakdown in its absence. (c) The acid sites eliminated from the 60% CuNaH-USY sample are those associated with Cux clusters. The selectivity of vanadium to attack the strong acid sites can be explained as follows: The formed V2O5 by the combustion of organic material in air of vanadyl-naphthenate oxide is converted to vanadic acid molecules during hydrothermal treatment by the action of H2O molecules, which in the acidic environment of USY phase takes the form of VO2+ (see ref 40) by the following reactions. V2O5 + hydrothermal treatment
acidic environment
3H2O 98 2H3VO4 98 2VO2+OH- +
H2O-vapor, T
[(SiO)3 t Al s O s Si t ]zeol-H+ 98 [(SiO)3 t Al]zeol + [tSiOH]zeol (13a) H2O-vapor, T
[(SiO)3 t Al s O s Si t ]zeol-H+ 98 4[tSiOH]zeol + AlO(OH)·H2O (13b) Reaction 13a proceeds without dealumination but with a decrease of acid sites and formation of defects in the crystalline structure. Reaction 13b proceeds with decrease of FAl-atoms and corresponding loss of acid sites. Replacement of FAl-atoms by Si is then taking place according to the following reaction
2H2O (15) Vanadic acid and Cu(II) species in USY-type of zeolite may, then, undergo the following reactions, hydrothermal treatment
Al(OH)3 + H3VO4 98 AlVO4 + 3H2O (16) hydrothermal treatment
CuO + SiO2 98 CuSiO3
(17)
Such chemistry leads to dealumination followed by breakdown of the framework that would be proportional to the quantity of vanadium involved. Evidence was found that
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vanadate phases such as SmVO4, LaVO4, and AlVO4 are formed during hydrothermal treatment of REY catalysts and is reported by several authors.30,25,41–44 Such evidence supports the argument that dealumination proceeds through the formation of AlVO4 according reactions 13, 15, and 16. However, it is not possible to explain the loss of so many acid sites (∼8 per unit cell) by the above reactions from such a small quantity of vanadium (∼0.25 V species per unit cell) unless we adopt the V catalytic action concept, proposed by Pine.31 It may, therefore, be suggested that the reactions (eqs 18-20) take place, under hydrothermal treatment conditions, with the catalytic action of V species, as follows: H2O-vapor, T
[(SiO)3 t Al s O s Si t ]zeol-H+ + VO2+OH- 98
Acknowledgment The authors thank Dr. J. Sanakis for assistance in recording ESR spectra of CuH-USY samples. We thank, also, the General Secretary of Research and Technology for funding our project on “Interaction between vanadium and transition metal ions in zeolite-structure catalysts of hydrocarbon pyrolysis” the results of which are presented in this report. Nomenclature
[(SiO)3 t Al s O s Si t ]zeol-VO2+ + H2O (18) [(SiO)3 t Al s O s Si t ]zeol-VO2+ + H2O-vapor, T
4H2O 98 Al(OH)3 + 4[tSi s OH]zeol + VO2+OH-(19) olation/oxolation
nAl(OH)3 98 condensed species
5. Dealumination/repair rate controls the extent of structure breakdown, and this ratio becomes high at relatively high V content. 6. A reaction pathway based on the catalytic action of V species for the selective attack on strong acid sites under hydrothermal treatment conditions is proposed.
(20)
The above, suggested mechanism, demonstrates how VO2+ species act catalytically to forward its detrimental action on the strong acid sites. The terminating step of this mechanism will then be reaction 16, since zeolite phase is not acidic enough if strong acid sites are eliminated and, thus, no VO2+ species are formed through reaction 15 from H3VO4. The overall process is controlled by kinetic regime, and the structure stability of the 60% CuNaHUSY sample may be explained from the slow dealumination rate due to slow diffusion of the VO2+ species from one acid site to another in the supercage of USY-framework between progressive catalytic cycles; perhaps through V interactions with Cu species and/or EFAl phases.27 In addition, the Cux cluster species are destabilized during steam treatment and drop off the framework, leaving a crystalline phase similar to H-USY, which is evidenced from the similarly high stability of CuH-USY structure under conditions of steam treatment. At this point, we have to mention that samples NaH-USY, H-USY, and 60% CuNaH-USY impregnated with 0.4% w/w vanadium were found to be dealuminated with, almost, complete structure breakdown after steam treatment, giving evidence that the rate of competition between dealumination and structure repair processes is critical for controlling the V detrimental effect on crystalline zeolite structure at the relatively high levels of V. 5. Conclusions From the data presented so far it is concluded that 1. The number and type of Si(nAl) FAl-MEs determine (a) the number and type of Cu(II) species that will be ion-exchanged for Na(I) in USY-type zeolite structure and (b) the number and strength of acid sites that will be formed by thermal treatment through dehydration/complexation processes. 2. Dense populations of Cu(II) species inside the USYstructure form Cux cluster species that affect SAS340 of the Si(2Al) FAl-MEs and give SAS430+ and MAS. 3. Steam treatment in the presence of vanadium species looses SAS, selectively, and is followed by corresponding loss of FAl atoms; however, no structure breakdown is observed at impurity levels of vanadium. 4. Vanadium under steam treatment conditions acts as a catalyst for removal of strong acid sites and consequently the dealumination process.
Na-USY ) sodium ultra stable type-Y zeolite structure H-USY ) ion-exchanged Na-USY with NH4Cl, thermally treated to remove NH3 and leave protons in place NaH-USY ) partly replaced by protons Na-USY zeolite CuNaH-USY ) partly ion-exchanged NaH-USY by Cu(II) ions for Na(I) ions Na-ZSM-5 ) sodium type-MFI zeolite structure H-ZSM-5 ) treated Na-ZSM-5 to replace Na(I) ions with protons MAT ) microactivity test REY ) rare earth type-Y zeolite structure RON ) research octane number TCD ) thermal conductivity detector TPD ) temperature programmed desorption P-XRD ) powder X-ray diffraction pattern UCS) unit cell size %RC ) percent relative crystallinity MAS NMR) magic angle spin nuclear magnetic resonance spectroscopy NFAl) number of framework Al atoms per unit cell QCu(II) ) quantity of Cu(II)-exchanged in millimole per gram of USY material QNa ) quantity of Na(I)-exchanged in millimole per gram of USY material QFAl ) framework Al atoms in millimole per gram of USY material UCW ) unit cell weight of hydrated USY material H ) humidity Si(nAl) ) framework Si atom with n FAl atoms FAl-MEs ) Si microenvironments in the framework with various numbers of FAl. NNa(I)(removed) ) number of Na(I) ions removed per unit cell from USY during ion-exchange NNa(I)(amorphous) ) number of Na(I) ions per unit cell dropped off the NaH-USY unit cell NCu-SAS(formed) ) number of acid sites formed by Cu(II) ions exchanged (in addition to the ones found in NaH-USY material) NETAS ) number of excess (compared to those possessed by NaHUSY) total acid sites per unit cell generated by Cu(II) ions exchanged for Na(I) during TPD-thermal treatment in zeolitic structure NWAS ) number of weak acid sites per unit cell NMAS ) number of medium strength acid sites per unit cell NSASxxx ) number of strong acid sites per unit cell desorbed within the TPD peak with its maximum at xxx temperature NCu2+specxies(zeolite) ) number of Cu(II) atoms per unit cell entered in zeolitic structure as aqueous Cu2+ complex species NCu(OH)+specxies(zeolite) ) number of Cu(II) atoms per unit cell entered in zeolitic structure as [Cu(OH)(H2O)5]+ aqueous complex species NCu2+specxies(amorphous) ) number of Cu(II) atoms per unit cell dropped in intersticial parts of the zeolitic lattice during TPDthermal treatment
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ReceiVed for reView May 30, 2008 ReVised manuscript receiVed September 25, 2008 Accepted October 16, 2008 IE8008605
