Abstract

The modifications of NaY zeolites with different SiO₂/Al₂O₃ ratio were carried out, the results revealed that modified USY zeolites showed some mesopores, as the SiO₂/Al₂O₃ ratio of USY increased, the weak acidity strength of USY had little change, medium and strong acidity strength of USY decreased, the weak acidity and medium-strong concentration decreased.The selective ring opening evaluation of tetralin revealed that hydro-upgrading catalyst with different USY zeolites had different tetralin conversion, different yield and selectivity of ROP. The catalyst prepared with USY zeolite modified from NaY with the SiO₂/Al₂O₃ ratio of 5.7 showed best hydro-upgrading performance, which had higher tetralin conversion, higher yield and selectivity of ROP. Acidity and Si/Al ratio of USY zeolites both were important factors affecting catalyst performance.

1. Introduction

Diesel is extensively used as a fuel since the diesel engine is 25-40% more fuel-efficient than the gasoline engine ¹. Therefore, LCO (light cycle oil) from FCC units will need to be processed in the refineries to supply the high quality diesel. However, high aromatics content of FCC LCO lowers diesel fuel quality and brings about the formation of undesirable emissions in exhaust gases from diesel engines. Since the gradual change to more stringent environmental regulations, much attention has been paid to the deep reduction of aromatics in diesel. In such a case, there is considerable interest in the new catalyst development for FCC LCO hydro-upgrading ^{2, 3, 4, 5, 6, 7, 8}.

It has been proved that the addition of acidic components into the traditional alumina-supported hydrotreating catalysts can greatly enhance their hydro-upgrading performance. As an important catalytic material, Y zeolite has tunable acidity, well-organized pore structure, larger specific surface area, hydrothermal stability and excellent catalytic performance, it has been widely used in petrochemical industry in recent years ^{9, 10, 11}. But the synthesized NaY zeolite is not acidic, the pore diameter is also smaller, only modified USY zeolites can be used in the catalytic field, Y zeolite modified from different Si/Al ratio starting NaY are not as well-documented.

In this study, we examined the effect of Si/Al ratio of the starting NaY on the USY zeolites, and used different Y type zeolites as cracking component for hydro-upgrading catalysts. Y type zeolites were characterized by various methods, and the selective ring opening evaluation of tetralin on different hydro-upgrading catalysts was examined in detail. The different Si/Al ratio of Y type zeolites may play an important role in dispersion of active phases and catalytic activity of hydro-upgrading catalyst. In this study, we examined the effect of the Si/Al ratio of starting NaY zeolites on USY zeolites properties and hydroaromatization catalyst performance.We found that suitable Si/Al ratio of USY zeolites was very important on the performance of hydro-upgrading catalyst, and laid a solid foundation for the commercial application of Y type zeolites in hydro-upgrading catalysts.

2. Experimental

Five USY samples, USY1, USY2, USY3, USY4 and USY5 were obtained with starting NaY1, NaY2, NaY3, NaY4 and NaY5 zeolites of different SiO₂/Al₂O₃ ratio by the same modified method. The modified procedures are as follows: USY1, USY2 and USY3 were prepared by NH₄⁺ exchange of five NaY zeolites using a 2.0N aqueous solution of NH₄Cl at 80°C for 2 h under agitation, followed by washing until absence of Cl^– and drying at 100°C overnight. Then samples were submitted to a steaming treatment at 600°C for 6 h. Finally, the samples were calcined in the dry air at 550°C for 3 h.

Five hydro-upgrading catalysts using USY zeolites as acidic component, Ni-W as hydrogen component were prepared by pore volume co-impregnation method, respectively. The tetralin selective ring opening evaluation of five hydro-upgrading catalysts were carried out.

The relative crystallinity and unit cell parameters of zeolites were calculated from XRD patterns recorded with a SIMADU XRD6000 diffractometer and CuKa radiation (0.1542 nm, 40 kV and 40 mA). The zeolite powder was packed in the cavity of a XRD sample holder made of glass. Then the holder was kept in a closed vessel containing a saturated CaCl₂·6H₂O solution for 16 h to make the moisture in the sample at constant. The relative crystallinity was estimated by comparing the peak intensities of the modified sample with standard NaY sample. The total intensities of the eight peaks, assigned to (311), (511, 333), (440), (533), (642), (822, 660), (555, 751) and (664) reflections, were used for the comparison according to the following equation:

X_R: the crystallinity of the standard NaY provided by Lanzhou catalyst company of Petrochina.

∑A_i: the sum of eight peaks area of Y zeolites to be determined.

∑A_R: the sum of eight peaks area of standard NaY sample.

The unit cell parameters were determined from the reflection peak of the silicon powder (2θ=28.443º) as an internal standard and calculated from the (555) reflection peak position. The framework Si/Al ratio was obtained from the calculated unit cell parameters by using Breck-Flanigen equation ¹².

Nitrogen adsorption/desorption measurements were performed on an ASAP2405M type surface area and pore structures instrument of American Mike Instrument Company. Before adsorption, the samples were calcined at 823 K for 4 h. Powder samples of 30-40 mg were degassed in a sample preparation station under 473 K and a vacuum of 1.33 × 10⁻³ Pa for 15 h, then switched to the analysis station for adsorption and desorption at 77 K in liquid nitrogen. Surface area was calculated with the multipoint BET equation with linear region in the P/P₀ range of 0.05-0.35. Pore volume was calculated from the maximum adsorption amount of nitrogen at P/P₀ = 0.99.

IR spectra were recorded on an American PerkinElmer Company GS-2000 type FT-IR Instrument. The samples and potassium bromide were mixed evenly at the ratio of 1:150, then fully ground and pressed samples into slices, and the spectrum of potassium bromide was used as reference, scan range was 400–4000cm^-1, the infrared spectra of the samples were recorded.

Na₂O contents were determined using a RIX3000-type X-ray fluorescence spectrocomparator of Rigaku Corporation.

NH₃-TPD spectra were recorded by Autosorb-1-C of Quantachrome Instruments Company. A sample of 0.2 g zeolite was heated at 510°C for 0.5 h under helium flow (30 mL/min). After the temperature was decreased to 100°C, NH₃ was introduced into the sample until saturation. After purging for 0.5 h with helium, the NH₃ was desorbed by increasing the temperature to 650°C at a rate of 20°C/min. The desorpted NH₃ was detected by TCD detector.

The content of Y zeolite non-framework was determined using the equation N_EFAl=1-[(Si/Al)_XRF/(Si/Al)_XRD].

Single-stage, once-through process was adopted in hydro-upgrading catalyst performance evaluation. It was conducted on a 200ml continuous flow fixed-bed hydrogenation test unit. The catalyst were sulfided in situ before evaluation, sulfidation oil was a kind of straight kerosene containing 2 wt% CS₂. Figure 1 showed the schematic diagram of 200ml hydrogenation test unit.

3. Results and Discussion

A combination of ion exchange-calcination-hydrothermal treatment methods was used for five different SiO₂/Al₂O₃ ratio NaY modification research. XRD patterns of five NaY zeolites are shown in Figure 2.The SiO₂/Al₂O₃ ratio, crystallinity and Na₂O content are listed in Table 1.

According to XRD patterns of NaY zeolites, there were no impurity crystals, and the crystalline form was complete.

Table 1 showed that Si/Al ratio of five NaY zeolites gradually increased, relative crystallinity and Na₂O content changed very little.

XRD pattern of five USY zeolites are presented in Figure 3.

Figure 3 revealed that (555) crystal face diffraction peak position of five USY zeolites gradually moved in a higher angle direction. According to calculation method of the unit cell parameter, with the increasement of diffraction peak position,or 2θ angles of (555) crystal face, zeolite unit cell parameter reduced. Since the framework Al–O bonds of Y type zeolite were replaced by Si–O bonds during the modification process, but the bond length (0.163 nm) of Si–O bond is less than the bond length (0.171 nm) of Al–O bond ¹³, therefore led to zeolite unit cell contraction,unit cell parameter reduction.

Na₂O content, framework Si/Al ratio, unit cell parameter, relative crystallinity and Al content of five zeolites are listed in Table 2.

Table 2 summarizes the results that under the same modification conditions, as the SiO₂/Al₂O₃ ratio of NaY increased, the framework Si/Al ratio and relative crystallinity of USY zeolites were increased accordingly, Si/Al ratio of starting NaY had an important influence on the performance of modified zeolite Y. Zeolite Y modified from higher Si/Al ratio of starting NaY also had higher framework Si/Al ratio and relative crystallinity, lower crystal lattice constant and better hydrothermal stability ^{14, 15}.

IR spectra data of five USY zeolites are given Table 3.

The results of Table 3 showed that as the SiO₂/Al₂O₃ ratio of USY increased, the frequencies of asymmetric stretching vibration, symmetric stretching vibration, double ring vibration, T–O bending vibration and pore opening vibration moved in the high wave direction, which revealed that as framework Si/Al ratio of USY zeolites increased, some Al atoms of USY zeolites were replaced by Si atoms, since Al–O bond length was longer than Si–O bond length, so framework vibration peak appeared blue shift.

N₂ adsorption-desorption isotherms, pore size distribution curves and surface area and pore structure data of five USY zeolites are presented in Figure 4, Figure 5 and Table 4, respectively.

Figure 4 indicated that five USY zeolites all belonged to type IV isotherm and H4 hysteresis ring, which was due to the hydroxyl group generation after the removal of some framework Al in the water vapour environment, made zeolites pore channel connection and formed lots of mesopores ¹⁶. Figure 5 revealed that five USY zeolites all generated some mesopores, the most probable pore diameter was 4 nm.

Pore structure data of five USY zeolites indicated that as the SiO₂/Al₂O₃ ratio of USY increased, specific surface area of USY zeolites showed the trend that increased first and then decreased, micropore volume changed a little, mesopore specific surface area showed the trend that increased first and then decreased, micropore volume decreased first and then increased, mesopore volume increased first and then decreased. It can be seen that specific surface area increasement was mainly due to the mesopore increasement. Specific surface area and mesopore volume of USY4 zeolite were both larger,which made macromolecules,such as aromatics in the diesel were more easier to contact the acid center in catalyst, and it spreaded rapidly after the chemical reaction, avoided the occurrence of secondary cracking reactions.

The acidity is one of the most important factors of the catalyst, it directly influences the cracking ability as well as ring opening, and thus hydrodearomatization performance and cetane number. The acidity of the catalyst is mainly originated from the zeolite, the study on the acidity of the zeolite is very important for the development of the catalyst, the NH₃-TPD profiles of five USY zeolites are presented in Figure 6.

According to NH₃-TPD profiles of five USY zeolites, as the SiO₂/Al₂O₃ ratio of USY increased, the weak acidity strength of USY had little change, medium and strong acidity strength of USY decreased, the weak acidity concentration, medium-strong concentration decreased.

Performance results of five Ni-W/USY catalysts are listed in Table 5.

The selective ring opening evaluation of tetralin revealed that hydro-upgrading catalyst with different USY zeolites had different tetralin conversion, different yield and selectivity of ROP. The catalyst prepared with USY4 zeolite showed best hydro-upgrading performance, which had higher tetralin conversion, higher yield and selectivity of ROP. Acidity and Si/Al ratio of USY zeolites both were important factors affecting catalyst performance.

Performance results of five Ni-W/USY catalysts on was FCC diesel hydrogenation listed in Figure 7 The operating condition of performance evaluation: temperature of 360°C, LHSV of 1.0 h^-1, pressure of 8.0 MPa, the ratio of hydrogen to oil of 800:1.

Figure 7 showed saturates, olefins and aromatics contents of FCC diesel and diesel product. NiW-USY4 catalyst has the best hydrogenation performance than other catalysts, the result is consistent with the performance on tetralin hydrogenation. NiW-USY4 hydro-upgrading catalyst can greatly decrease aromatics content of inferior diesel, such as FCC diesel, straight run diesel and coker diesel, thus effectively improving the cetane number of inferior diesel, and the catalyst has a broad commercial application prospect.

4. Conclusions

It was found that Si/Al ratio of starting NaY had an important influence on the performance of modified zeolite Y. Zeolite Y modified from higher Si/Al ratio of starting NaY also had higher framework Si/Al ratio, less non-framework aluminum, better hydrothermal stability. The pore distribution of USY zeolites was very centralized, the most probable pore diameter was 4 nm, and USY zeolites had more mesopores and higher relative crystallinity. As the SiO₂/Al₂O₃ ratio of NaY zeolites increased, the weak acidity strength of USY had little change, the weak acidity and medium-strong acidity concentration, medium and strong acidity strength of USY decreased. The selective ring opening evaluation of tetralin and FCC diesel hydrodearomatization revealed that hydro-upgrading catalyst with different USY zeolites had different tetralin conversion,different yield and selectivity of ROP,and different dehydroaromatization. Acidity and Si/Al ratio of USY zeolites both were important factors affecting catalyst performance.

References