Du Xiaohui, Gao Xionghou, Liu Pusheng

(1. Lanzhou Petrochemical Research Center, Petrochemical Research Institute, PetroChina, Lanzhou 730060;2. Petrochemical Research Institute, PetroChina, Beijing 102206)

Abstract: The use of lanthanum phosphate as a vanadium trap for preventing destruction of USY zeolite was studied. The effect of deposited vanadium on the hydrothermal destruction of zeolite was investigated by the solid-state NMR technique.LaPO4 species can inhibit the zeolite framework structure from being collapsed by vanadium after steaming treatment.The EPR results show the oxidation-reduction reaction in LaPO4 and V2O5 system and inhibition of zeolite destruction by V5+. The catalysts prepared from USY and LaPO-USY zeolites were also tested in the catalytic reactions of heavy oil. The assessment results indicated that the USY modified with LaPO4 could bring about remarkably high dehydrogenation ability.

Key words: FCC; USY zeolite; vanadium; lanthanum phosphate; hydrothermal stability

1 Introduction

Poisoning and deactivation of the catalyst in fluid catalytic cracking (FCC) by vanadium contained in the oil feedstock is one of the most vexing problems faced by operators of oil refineries. Vanadium is deposited in the form of [VO]2+cations on the catalyst surface as a part of large organic molecules, while coke is deposited during the catalytic cracking process[1]. Coke is burnt off during regeneration, but vanadium cannot be removed from the solid. Instead, it is oxidized to V5+as mobile species that moves from the equilibrium catalyst to the freshly added particles[2]. In the presence of water, vanadic acid forms inside the zeolite. Since vanadic acid is a strong acid, it can destroy the zeolite by hydrolyzing the SiO2/Al2O3framework[3].

Rare earth oxides are commonly used as vanadium traps to increase the zeolite stability. The ability of rareearth elements to form vanadate species is probably responsible for keeping some of the vanadium species in the +5 oxidation state[4]. However, the chemical environment of rare earth element can affect its vanadium trap capability[5]. Rare earth-containing zeolites are destabilized owing to the disappearance of La-O-La stabilizing bridges in the sodalite cages,because vanadium reacts with lanthanum in LaY zeolite to formIn this case, when rare earth cations are located in the small cages, the thermal and hydrothermal stability of zeolite is reinforced, but its vanadium resistance is impaired. According to the patent research, the phosphorous compounds which afford the best results for the direct application of immobilizing the vanadium in the catalytic cracking catalyst, are those ones wherein the phosphorus is encountered in its pentavalent condition[7]. However, the research on vanadium tolerance by using phosphorous compound is rare and the mechanism has not yet been proposed in the literature.

2 Experimental

The reference USY zeolite in this study was a sample obtained from an oxalic acid treatment of a commercial USY zeolite. The purpose of the acid treatment was to remove a certain amount of extra framework aluminum(EFAl). LaPO4was introduced into the USY zeolite via the precipitation method using LaCl3and (NH4)3PO4aqueous solution (with a (NH4)3PO4/LaCl3molar ratio of 1.2) at 363 K. The sample was dried at 363 K overnight and calcined at 823 K for 2 h. Vanadium was introduced by incipient wetness impregnation method from an aqueous solution of ammonium metavanadate, and the samples were dried at 363 K overnight and then calcined at 823 K for 2 h.

The sample of LaPO4·V2O5was a ground mixture of lanthanum phosphate and vanadium pentoxide with an(La)/n(V) ratio of 5. Similarly, La2O3·V2O5was also a ground mixture of lanthanum oxide and vanadium pentoxide. The mixture samples thereby prepared were subjected to hydrothermal treatment at 873 K for 0-6 h.The Brunauer-Emmett-Teller (BET) surface area of samples that had been treated under vacuum at 473 K for 8 h was determined in a Micromeritics ASAP3000 apparatus by nitrogen adsorption at 77 K. Transmission electron microscopy (TEM) analysis was carried out to study the fine morphology of rare earth ions dispersed in catalysts by using a FEI TECNAI G2 microscope operated at 200 kV. Solid-state27Al NMR analysis was conducted by a Bruker Avance III WB 400 spectrometer at 79.4 MHz. Electron paramagnetic resonance (EPR)measurement was carried out in an X-band Bruker ESP 380E spectrometer operating with a microwave frequency of 9.5 GHz.

Catalytic testing was carried out at 530 °C on an advanced cracking evaluation (ACE) unit (KTI, model R+MM).The catalyst loading and the feedstock input were 9 g and 1.80 g, respectively. The gaseous and liquid effluents were determined by gas chromatography (HP 6890). The compositions of produced gasoline were analyzed by gas chromatography (Varian CP-3380). The amount of coke deposited on the catalyst was assessed by burning the sample in a carbon analyzer (Servomex 01440D). The properties of the feedstock are shown in Table 1.

Hydrothermal treatment: The sample was hydrothermally deactivated in a laboratory furnace at 800 °C for 4 h in an 100% steam environment.

3 Results and Discussion

Table 2 shows the samples with their corresponding BET surface areas and microporous volumes. The BET surface areas and microporous volumes of LaPO-USY were slightly decreased compared to those of USY zeolite. It is possible that some zeolite pores were blocked by theprecipitation method and the quality of the zeolite has been reduced.The sample was characterized by TEM to investigate the crystallinity and morphology of the zeolite as well as to discern the presence of LaPO4-containing phases. Figure 1 corresponds to the sample LaPO-USY in which the LaPO4phase is distributed homogeneously throughout the zeolite. This fact suggests that the LaPO4phase is well dispersed on the external surface of the zeolite.Lanthanum orthophosphate LaPO4is a very stable compound with a high melting point[8-9]and does not affect the properties of catalysts.To confirm that the structural changes had occurred,zeolite samples were measured with27Al MAS NMR to determine the Al coordination[10-14]. Figure 2 shows the27Al MAS NMR spectra of USY, V-USY and V-LaPO-USY zeolite samples, which were treated under hydrothermal conditions along with the starting material,a fresh batch of USY zeolite. In the case of zeolite, it is well known that a peak at about 60 and one at about 0 should be assigned to the 4-coordinated Al and the 6-coordinated Al[15], respectively. The spectra of the fresh USY showed that only the signal of the tetrahedrally coordinated framework aluminium at a chemical shift of 60 was identified. After the heat treatment in 100% steam,the spectra of samples showed a significantly different change. The peak assigned to the 4-coordinated Al shifted about 30 to a high magnetic field, which was typical of the distorted tetrahedral species or the five-coordinated Al, and the peak at around 30 broadened significantly. Compared with USY and V-LaPO-USY samples, the peak intensity at 0 increased remarkably in V-USY sample. This result suggests that a number of the 6-coordinated Al species,which generally corresponded to EFAl, were present in V-USY sample. Therefore, it is concluded that LaPO4species can inhibit the zeolite framework structure from being collapsed by vanadium under the steaming condition.To study the reducibility of the vanadium species and to understand the reaction mechanism of LaPO4and V2O5,several mixture samples were characterized before and after a reduction treatment by EPR, and this technique could easily allow the characterization of V4+species. The presence of V4+was demonstrated only after reduction.Figure 3 shows the EPR spectra of LaPO4·V2O5and La2O3·V2O5after the same hydrothermal treatment. The EPR spectrum of LaPO4·V2O5sample shows characteristic lines of the VO2+species. The EPR spectrum of VO2+presents a well hyperfine structure with eight lines in the parallel, which are characteristic of VO2+groups in the penta- and hexa-coordinated environments[16-17].In contrast to LaPO4·V2O5, the hyperfine structure of La2O3·V2O5was not observed because of the generation of LaVO4. The valence states of V in LaPO4and V2O5mixture before and after hydrothermal treatment were investigated by electron paramagnetic resonance spectroscopy. V5+was transformed to V with the low valence state being achieved by the 600 °C hydrothermal treatment, suggesting an oxidation-reduction reaction in LaPO4and V2O5system and inhibition of zeolite destruction by V5+. The vanadium at the tetravalent state does not accelerate zeolite framework hydrolysis[3].

Table 1 Properties of residue oil

Table 2 Characterization of zeolite samples

Figure 1 TEM images of LaPO-USY

Figure 2 27Al MAS NMR spectra of zeolite samples

Figure 3 EPR spectra of LaPO4·V2O5(a) and La2O3·V2O5(b)

In order to further understand the effect of LaPO4introduction on vanadium tolerance of catalyst, the catalytic performance of LaPO-Cat was tested by the ACE equipment. Table 3 shows the comparison of the catalytic performance with that of the catalyst before the vanadium introduction. Compared with the parent catalyst, LaPO-Cat increased the conversion roughly by 5 percentage points,and decreased the bottoms yield by 3 percentage points.

Table 3 ACE evaluation data for catalysts prepared from LaPO- and USY (containing 6 000 μg/g of V)

4 Conclusions

USY zeolites containing LaPO4was prepared by the precipitation method to understand its properties as vanadium traps. In LaPO-USY zeolite, LaPO4and other rare earth species were distributed on the zeolite surface and had slight effects on the properties of the catalyst.

When the zeolite samples of USY and LaPO-USY were impregnated with vanadium compound solution, the LaPO4species could inhibit the destruction of the zeolite framework structure under hydrothermal treatment condition. The EPR results suggest that LaPO4underwent oxidation-reduction reaction in V2O5system and could inhibit zeolite destruction by V5+species. The assessment results indicated that the USY modified with LaPO4could bring about remarkably high dehydrogenation ability owing to its good vanadium resistance.

In summary, vanadium is immobilized by phosphorus compounds in the form of stable V4+compound, and the redox reaction between phosphorus compounds and V2O5has never been reported hitherto. The mechanism is different from that of the reaction between the active alkaline component and acid vanadium oxide, and the negative reaction between the acid oxysulfide in industrial reactors and the alkaline vanadium-trapping substance can be circumvented.