716
J. Phys. Chem. B 2000, 104, 716-720
Synthesis and Characterization of Acidic High-Silica Faujasite-Type Gallosilicates Th. L. M. Maesen,*,† J. A. R. van Veen,‡ D. A. Cooper,† I. M. van Vegchel,‡ and J. W. Gosselink§ Zeolyst International, PQ R&D Center, Conshohocken, PennsylVania 19428-2240, Shell International Chemicals, Shell Research & Technology Center, Amsterdam 1031 CM, The Netherlands, and Shell Global Solutions, Shell Research & Technology Center, Amsterdam 1031 CM, The Netherlands ReceiVed: September 10, 1999; In Final Form: NoVember 19, 1999
Ultrastabilization of a (rare-earth-loaded) faujasite (FAU)-type gallosilicate appears to be the only viable route for synthesizing a gallosilicate with an acid site density comparable to that of ultrastable FAU-type aluminosilicates. Neither gallium insertion into a FAU-type silica nor substitution of framework gallium by silicon using ammonium hexafluorosilicate appears feasible. The ultrastable FAU-type gallosilicate shows no sites with enhanced Brønsted acidity. According to the shift observed by IR spectroscopy when CO is adsorbed, the acid site strength of FAU-type gallosilicates is weaker than that of high-silica FAU-type aluminosilicates but stronger than that of FAU-type silicoaluminophosphates. Although the ultrastable FAUtype gallosilicate is (hydro)thermally stable in an oxidizing atmosphere up to 1150 K, it releases its gallium in a reducing atmosphere (starting at 525 K) and loses structural integrity increasingly rapidly above 635 K. This severely limits the potential use of catalysts based on gallosilicate framework structures in hydrocarbon hydroconversion reactions.
Introduction Microporous crystalline aluminosilicates (zeolites) are used extensively as catalysts in the oil refining and (petro)chemical industries.1,2 Refinery processes such as fluid catalytic cracking (FCC) and hydrocracking already consume large quantities of faujasite (FAU)-type3 zeolites,1 while their use in the production of fine chemicals is still growing.2,4 When FAU-type silicoaluminophosphates were discovered5 they raised considerable interest, because they offered the potential advantages of the FAU-type structure coupled with significantly lower intrinsic acidity.6-9 In practice, the use of these silicoaluminophosphates has been hampered by their hydrolytic instability.10 FAU-type gallosilicates ([Ga]-FAU) were synthesized11 at about the same time as the commercially successful FAU-type aluminosilicates.12 Nonetheless, the aluminum-free FAU-type gallosilicates have remained an esoteric material for the last forty years. An occasional publication has addressed mixed FAU-type galloaluminosilicates that combine the acidity of the aluminosilicate with the dehydrogenation properties of intracrystalline gallium compounds.13,14 Any inferences about the catalytic potential of [Ga]-FAU must be based on the results of comparative studies of MFI-type structures. The acid sites in MFI-type gallosilicates appear to be less acidic than those in MFI-type aluminosilicates.1,15 If the acid sites in [Ga]-FAU are also less acidic than those in FAU-type zeolites, [Ga]-FAU should be an attractive candidate for processes requiring a hydrolytically stable, mildly acidic catalyst. An equitable comparison between FAU-type gallosilicates and aluminosilicates requires synthesizing a [Ga]-FAU with an acid site density comparable to that in the conventionally used †
Zeolyst International. Shell International Chemicals. § Shell Global Solutions. ‡
ultrastable FAU-type zeolite, i.e., [Ga]-FAU with no more than 35 Ga atoms (and at least 157 Si atoms) per unit cell. Since there was no known synthesis route for [Ga]-FAU with such a low framework gallium density,11,16-18 we explored various synthesis and modification processes. These have been included in the discussion, because they highlight some salient differences between the chemistry of aluminosilicate and gallosilicate molecular sieves. The final [Ga]-FAU product was evaluated for use in a hydrocracking catalyst. The expectation was that the mild acidity would impede consecutive hydrocracking reactions and so minimize the undesirable gas production, while the FAU-type structure would protect the acid sites and so reduce the formation of coke.1,19,20 Experimental Section Preparation of [Ga]-FAU. A. Insertion of Gallium into Framework Defects. A FAU-type silicate was prepared by leaching the aluminum out of a commercially available FAUtype zeolite (Zeolyst CBV 780). The CBV 780 starting material had a unit cell size (“a0”) of 2.424 nm, as determined by X-ray diffraction (XRD), and a bulk silicon-to-aluminum ratio of 40, as determined to X-ray fluorescence (XRF). The CBV 780 was acid extracted three times by refluxing with concentrated HCl (10 wt % slurry). According to XRD analysis this did not affect its crystallinity. The silicate was then slurried in a solution of NaGaO2 as described in the literature.21 In an alternative synthesis route the framework-aluminumfree FAU-type silicate was impregnated with gallium nitrate (0.14 g/g anhydrous silicate) and calcined at 770 K, for 1 h. The calcined material was treated with dilute KOH solutions of pH close to 7, 8, or 9. In an attempt to make the FAU-type silicate more resistant against treatments with an aqueous base, 0.5 g/g of a 0.01 mol/L
10.1021/jp993209b CCC: $19.00 © 2000 American Chemical Society Published on Web 12/22/1999
Acidic High-Silica Faujasite-Type Gallosilicates ammonium hexafluorosilicate solution was added slowly to a 1 wt % slurry of the silicate.22 B. Replacing Framework Gallium by Silicon. Sodium [Ga]FAU was synthesized using a conventional recipe.18 According to XRF analysis, it had a framework gallium density of 60 gallium atoms per unit cell. According to XRD analysis it had an a0 ) 2.488 nm. It was ammonium-exchanged by slurrying three times for 1 h in 10 mL/g of 0.1 mol/L NH4NO3 at 360 K. To substitute the framework gallium with silicon, 0.5 g/g of a 0.01 mol/L ammonium hexafluorosilicate solution was added slowly to a 1 wt % slurry of the ammonium-exchanged [Ga]FAU.22 In another attempt to replace framework gallium by silicon, the ammonium [Ga]-FAU sample was steamed23 for 1 h at 625 K and at 101 kPa steam. C. Ultrastabilizing [Ga]-FAU. In an attempt to minimize the framework gallium density in [Ga]-FAU from the outset, 18crown-6 ether was added to the synthesis.17 It is our experience that the recipe described in the patent literature17 is only successful if [Ga]-FAU seeds are added. The addition of approximately 4 wt % seeds to the synthesis recipe yielded [Ga]FAU with bulk Si/Ga ) 3.17. After removal of the 18-crown-6 ether by calcination at 770 K for 6 h, the [Ga]-FAU had an a0 ) 2.447 nm, and a BET surface area of 820 m2/g. It was stirred in a solution of ammonium nitrate (20 cm3/g silicate) for 2 h at 355 K; using a 1.25 mol/L ammonium nitrate solution reduced the sodium content to 2.7 wt % ([Ga]-FAU-2.7), and using a 2.1 mol/L solution reduced the sodium content to 1.5 wt % ([Ga]-FAU-1.5). [Ga]-FAU-1.5 was mixed with an aqueous solution of rare-earth elements (0.17 g of lanthanum-Ln nitrate 5248 from Molycorp/Unocal in 5.4 g of water/g of [Ga]-FAU1.5) to yield a slurry. According to XRF analysis, the product (RE-[Ga]-FAU) contained 4.6 wt % rare-earth oxides (viz. 2.7 wt % La2O3, 0.67 wt % CeO2, 0.89 wt % Nd2O3, and 0.31 wt % Pr8O11). Samples of both the ammonium- and the rare-earth-exchanged [Ga]-FAU samples were steamed at various temperatures for 1 h, at 101 kPa steam partial pressure. The [Ga]-FAU-2.7 and [Ga]-FAU-1.5 samples were steamed at 625, 675, or 725 K, and the RE-[Ga]-FAU sample was steamed at 675, 725, or 775 K. After steaming, the residual sodium and extraframework gallium were removed by slurrying (25 mL/g silicate) the samples in a solution of 3.0 mol/L ammonium nitrate and 0.13 mol/L nitric acid and stirring them at 355 K for 2 h. The ammonium-exchanged product obtained from the RE-[Ga]-FAU sample steamed at 725 K will be referred to as US-RE-[Ga]FAU. D. Catalyst Preparation and Testing. A base-metal hydrocracking catalyst incorporating US-RE-[Ga]-FAU was prepared by (1) extruding the US-RE-[Ga]-FAU sample with alumina, employing acetic acid as the peptizing agent, to obtain 1.5 mm cylindrical extrudates, (2) calcining the extrudates at about 825 K for 2 h, and (3) impregnating them with a solution containing nickel nitrate and ammonium metatungstate, followed by calcination at about 775 K. The hydrocracking performance of this catalyst was evaluated using standard microflow equipment, using a feedstock representative for an industrial second-stage hydrocracking feedstock (boiling range 557-845 K, 86.2 wt % C, 13.8 wt % H, 35 ppmw S, 570 K) steam (or ultra-23) stabilization. Ultrastabilization of the ammonium [Ga]-FAU sample containing 60 gallium atoms per unit cell was unsuccessful, because the FAU-type framework collapsed at a temperature below 570 K. This is surprising, for ammonium-exchanged FAU-type aluminosilicates with a comparable fraction of framework silicon atoms do not start to lose their crystallinity before 670 K.26 Thus, FAU-type gallosilicates appear somewhat less thermally stable than FAU-type aluminosilicates. Fortunately, the thermal stability of [Ga]-FAU synthesized with 46 gallium atoms per unit cell was sufficient to allow steam stabilization. The steam stabilization study of [Ga]-FAU containing 46 gallium atoms per unit cell involved [Ga]-FAU samples with different levels of sodium and rare-earth (RE) cations, because these cations are known to affect the steam stabilization of FAUtype aluminosilicates.23 Figures 1 and 2 show how the unit cell size (which is a measure for the framework gallium density16) (Figure 1) and the crystallinity (assessed by surface area (m2/ g), Figure 2) change after 1 h of ultrastabilization at 101 kPa steam at various temperatures. As with FAU-type aluminosilicates,23 a higher temperature and fewer sodium cations yield a lower a0 and a lower surface area. Rare-earth cations stabilize the [Ga]-FAU structure. Thus, the ultrastabilization of the RE[Ga]-FAU sample yields products that combine a lower unit cell size with a higher surface area as compared to the products obtained from [Ga]-FAU-1.5 or [Ga]-FAU-2.7. As a final step for turning the ultrastabilized RE-[Ga]-FAU (US-RE-[Ga]-FAU) and [Ga]-FAU into acidic catalysts, both
Maesen et al.
Figure 2. Variation of surface area (m2/g) with steam temperature for [Ga]-FAU 1.5 (b), [Ga]-FAU 2.7 (9), and RE-[Ga]-FAU (2).
TABLE 1: Steam Temperature (T (K)), and Resultant Unit Cell Size (a0 (nm)), Surface Area (SA (m2/G)), and Framework Gallium Density (N(Ga) (no./unit cell)) As Determined by 29Si MAS NMR precursor
T (K)
a0 (nm)
SA (m2/g)
N(Ga) NMR
[Ga]-FAU 1.5 [Ga]-FAU 1.5 [Ga]-FAU 1.5 [Ga]-FAU 2.7 [Ga]-FAU 2.7 [Ga]-FAU 2.7 RE-[Ga]-FAU RE-[Ga]-FAU RE-[Ga]-FAU
625 675 725 625 675 725 675 725 775
2.459 2.457 2.454 2.462 2.460 2.457 2.460 2.460 2.457
602 597 561 708 712 667 672 686 605
34.8 36.5 33.5 36.2 35.6 33.4 35.0 35.6 31.2
the residual sodium and the amorphous, gallium-containing debris from the ultrastabilization were removed by an acidified ammonium exchange. 71Ga NMR on the ammonium-exchanged US-RE-[Ga]-FAU detected no extraframework gallium, as indicated by the absence of a 71Ga resonance27 at around 0 ppm. The acidified ammonium exchange increased the crystallinity of the 725 K US-RE-[Ga]-FAU to 97% (as determined by XRD) and the surface area to 812 m2/g. Its unit cell size remained at 2.460 nm. Contrary to the US-RE-[Ga]-FAU, the acidified ammonium exchange decreased the crystallinity of the rareearth-free ultrastabilized [Ga]-FAU samples to below 60%. Evaluation. After ammonium exchange and ultrastabilization the acid site density and acid strength of the RE-[Ga]-FAU and [Ga]-FAU samples can be evaluated. A first evaluation of the acid site density can be obtained by assuming that the acid site density approaches the framework gallium density. The framework gallium density can be assessed from (a) the unit cell size16,28 and (b) 29Si MAS NMR (Table 1).18 A linear correlation between unit cell size and N(Ga) can be obtained from literature data16,18,25 on fully hydrated (organic-free) nonsteamed sodium [Ga]-FAU:
N(Ga) ) 957(a0 - 2.4255) This relationship has a correlation coefficient of 0.94. Figure 3 shows an extrapolation of this relationship to the lower unit cell sizes typically obtained after ultrastabilization. The framework gallium densities determined on the ultrastabilized [Ga]FAU samples by 29Si MAS NMR also fit this relationship (Figure 3). The least crystalline ultrastable [Ga]-FAU sample (crystallinity
