Studies on the Preliminary Cracking of Heavy Oils: The Effect of Matrix

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Studies on the Preliminary Cracking of Heavy Oils: the Effect of Matrix Acidity and a Proposal of a New Reaction Route

Bin Wang, Chaoyi Han, Qiang Zhang, Chunyi Li∗, Chaohe Yang, Honghong Shan State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, PR China

ABSTRACT

Viewpoints on the influence of matrix acidity on the catalytic cracking of heavy oil are still inconsistent hindering the studies of the reaction routes occurred during the matrix-precracking process. In this study, the effect of matrix acidity on heavy oil cracking were systematically studied by preparing alumina and modified alumina with different acidity in a practically meaningful range as the matrix components of FCC catalysts. Results showed that Brönsted sites presented a much higher activity than Lewis sites on the matrix surface. Increasing Brönsted acid strength of matrices improved the activity of catalysts, with the aggravated product distribution, while increasing Lewis acid strength of matrices aggravated the product distribution and decreased the catalyst activity. Interestingly, results also showed that contacting Lewis sites first followed by interacting with Brönsted sites during the matrix-precracking process would facilitate heavy oil cracking more deeply. In addition, a new reaction route was proposed that protolytic cracking route should occur during the matrix-precracking process when cracking heavy oil. Based on this result, one can modify the matrix of the catalyst by introducing Brönsted sites or not to achieve high yield of light olefin or maximize liquid products.

∗

Corresponding author. Tel.: +86 0532 86981862; fax: +86 0532 86981787.

E-mail address: [email protected], [email protected]. 1

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1. INTRODUCTION

Due to the great ability to process very large amounts of heavy oil fractions,1-3 and enough flexibility to direct production preferentially to LPG olefins, gasoline or diesel, with minor modifications of the unit or the operation conditions,4 fluid catalytic cracking (FCC) is still a main conversion unit in oil refinery processes.5-8 Typical FCC catalyst mainly consists of Y-zeolite and matrix. The zeolite component with desired selectivity provides most of the cracking activity while the matrix with large numbers of mesopores (> 20 Å) mainly performs the physical function.9 In recent years, the demand for FCC product, such as LPG olefins and liquid fuels, is growing every year, on the other hand, heavy oil feedstocks are gradually replacing the lighter ones.10 For molecules in heavy oil feedstocks, the critical diameter is ranging from 1.2 to 15 nm,11 while the pore diameter of Y-zeolite, the main active component of FCC catalysts, is only 0.74 nm which is considerably smaller than the critical diameter of the molecules in heavy feedstocts. Even the deformable ability is considered, the size of molecules allowed to penetrate into Y-zeolite pores is less than 1.02 nm.12 It is possible for hydrocarbons with a carbon number less than 20 to enter into the micropores of Y-zeolite, but this process can hardly occur for fractions with boiling point higher than 400 °C. External surface of the zeolite component has a good accessibility, but the surface area is too low and accounts for only about 2 % of the total.13, 14 Post synthesis treatment of Y-zeolite15-19 and/or introducing the mesoporous zeolites20-23 can enlarge the pore size of zeolites, but the former method cannot obviously enhance intracrystalline diffusion in zeolites, as the mesopores created by post synthesis modifications are not connected with each other.24 On the other hand, the acidity and hydrothermal stability of the mesoporous zeolites are less than required for application in the FCC process.25 Therefore, the precracking on the more accessible matrix surface,26

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i.e., large feed molecules are first precracked into moderate ones on the matrix surface and then diffuse into zeolites to be further selectively cracked, is of vital importance to convert heavy oil effectively.27-29 Realizing the significance of the matrix-precracking, Otterstedt and coworkers30 added alumina with large amounts of Lewis sites and no Brönsted sites into matrices to increase the matrix activity, but found that the introduction of the alumina gave rise to an inferior performance than catalysts with an inactive matrix. By contrast, Chen11 and Alerasool9 reported that the Lewis sites on the matrix surface were very important to crack heavy oil effectively. Recently, Feng31 and Xu32 introduced large amounts of Brönsted sites onto the matrix surface and found that the introduction considerably enhanced the conversion of heavy oil and lowered the selectivity to coke as well, whereas Holland and coworkers33 found that increasing Brönsted sites on the matrix surface could indeed enhance the conversion of heavy oil, but would result in a higher selectivity to coke. Summarily, the viewpoints from previous researchers on the effect of the matrix acidity, especially for acid sites with the same acid type, on the catalytic cracking of heavy oil are inconsistent, which requires further work to understand deeply. In addition, it is generally accepted that the catalytic cracking takes place through a classical β-scission cracking mechanism, whereas Corma and coworkers34 found that there are also protolytic cracking occurred on the Brönsted acid sites of Y-zeolite initially based on the researches of Olah35 and Haag36. Of note is that, similar to zeolites, there are also Brönsted acid sites on the matrix surface. Furthermore, as olefins are good proton acceptors, protolytic cracking in catalytic cracking process is kinetically significant only during the initial contact of oil vapor and catalyst, on the other hand, the acid sites that first contact with large feed molecules are just the ones on the more accessible matrix surface.9, 10 Thus, besides the zeolite surface, it is also possible that the protolytic cracking occurs on the matrix surface.

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This study aim to systematically study the influence of matrix acidity on the catalytic cracking of heavy oil and deeply investigate the reaction routes during the matrix-precracking process. In the first part of the work, the effect of the matrix acidity (acid type, acid strength of each type, acid number of weak Brönsted acidity, and the contacting orders of acid sites with different types) on the matrix-precracking and the catalytic cracking of heavy oil was studied. In the second part, it was proposed a new reaction pathway during the matrix-precracking process from the correlation between the matrix acidity and product distribution.

2. EXPERIMENTAL SECTION

2.1. Materials

A commercial alumina (Alumina-1) was purchased from Sinopharm Chemical Reagent Co. Ltd. An alumina (Alumina-2) was prepared by a sol-gel method. Pseudoboehmite powder (Aluminum Corp. of China) was mixed with distilled water in a vessel to obtain a suspended solution. Then HCl solution (36 wt%-38 wt%) was added dropwise under vigorous stirring at 80 °C. The resulting gel was stirred for 2 h, dried at 140 °C for 12 h, and calcined at 700 °C for 2 h in air atmosphere. A series of modified alumina were prepared by modifying Alumina-1. Alumina-1 was dispersed in H3PO4 or H2SO4 solution, and the suspension was stirred vigorously for 20 min. After that, the suspension was filtered, dried at 40 °C for 24 h, and calcined at 600 °C for 2 h in air atmosphere. The as-prepared matrices are denoted as xP/Al and xS/Al, where x represents the concentration of H3PO4 or H2SO4 solution. For example, “0.2P/Al” means that Alumina-1 was modified by 0.2M H3PO4 solution. A series of conventional active matrices (CAM), used as the reference materials for the study of acid strength of the modified alumina, were prepared using the following procedure: a certain amount of kaolin

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(Suzhou Kaolin, China) was added into the pseudoboehmite (Aluminum Corp. of China) sol under stirring condition. The amount of the pseudoboehmite used in these preparations corresponded to 30 wt% alumina in the product. The mixture was then dried at 140 °C for 12 h, and calcined at 700 °C for 2 h in air atmosphere. The obtained matrix was crushed and sieved to 80-180 µm. After that, the matrix particles (MP) were modified in the same manner as the modified alumina above. The as-prepared matrices are denoted as CAM(xP) and CAM(xS), where x represents the concentration of H3PO4 or H2SO4 solution. For example, “CAM(0.5P)” means that the matrix particles (MP) were modified by 0.5M H3PO4 solution. All the matrices was crushed and sieved to 80-180 µm before reaction. A zeolite REUSY (Si/Al = 2.77, Na% = 1.08 wt%, Ce% = 0.6 wt%, La% = 0.3 wt%, Particle size = 3.55 µm) obtained from the Catalyst Factory of PetroChina was treated with 100% steam at 765 °C for 4h (S-REUSY). In order to avoid modification of the matrix acidity during the catalyst preparation process,9, 37 the catalyst used in our experiments were obtained by mixing the S-REUSY (15 wt%) and a matrix physically. The main properties of the matrices and S-REUSY were summarized in Table 1.

2.2. Catalyst Characterization

Nitrogen adsorption-desorption experiments at 77 K were conducted on an autosorb instrument (Quantachrome). The total surface (SBET) was calculated according to the Brunauer-Emmet-Teller (BET) isothermal equation, and the pore size distribution was derived from the desorption branch, using the Barrett-Joyner-Halenda (BJH) method. S or P content left on the modified alumina surface after calcining at 600 °C was determined by X-ray fluorescence (XRF) using an AXIOS-Petro spectrometer. The total acid sites number of catalysts were characterized by ammonia temperature programmed desorption (NH3-TPD). 100 mg of a sample was loaded into the apparatus, pretreated under helium flow at 600 °C for 0.5 h, brought into contact with NH3 after being allowed to cool to 100 °C, and then heated at a 5


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rate of 10 °C/min to 800 °C. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 spectrometer equipped with a MCT liquid nitrogen cooled detector. The spectra of the samples were recorded by accumulating 32 scans at 4 cm-1 resolution. The powder of the catalyst samples was placed in a cell and pretreated for 3 h at 500 °C in a stream of dry air. After cooling to room temperature (20 °C) the background spectrum was recorded, which was always automatically subtracted. Pyridine adsorption was carried out by equilibrating the catalyst for 30 min with the probe molecule vapour at 150 °C. The sample was then evacuated for 30 min at 150 °C and 10-2 Torr, and cooled to room temperature, before recording the spectrum. The desorption of pyridine molecule was successively carried out, by evacuating the sample for 30 min at 350 °C and 10-2 Torr, and cooling to room temperature, to record the spectrum. The amount of pyridine absorbed on acid sites with different acid types of the catalyst was obtained from infrared transmittance spectroscopy.38

2.3. Catalyst Evaluation

Catalytic experiments were carried out in a microactivity test (MAT) unit (fixed bed). The reactions were carried out at 500 °C and with a catalyst time on stream of 60 s, and the catalyst to oil ratio (CTO) was varied in a range of 0.5-5 g/g keeping the amount of catalyst constant (3 g), and changing the amount of oil fed, in order to vary the conversion. Before each experiment, the system was purged with a 30 cm3/min N2 flow for 30 min at the reaction temperature. After reaction, stripping of the catalyst was carried out for 10 min using a N2 flow of 30 cm3/min. During the reaction and stripping steps, the liquid products were collected in a glass receiver dipped in ice bath at the exit of the reactor, meanwhile the gaseous products were collected in a gas burette by water displacement. The closed system allowed mass balance to be made, and only experiments with mass balances of 100 + 5% were considered. 6


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The feed oil is a medium-based FCC feedstock (Table 2). Conversion is defined as the sum of liquid products boiling below 350 °C, dry gas, liquefied petroleum gas (LPG) and coke. The compositions of gas products were analyzed by a Bruker 450 gas chromatograph, and the liquid products collected were analyzed for the simulated distillation by another Bruker 450-GC according to the ASTMD-2887 procedure. The boiling point range of gasoline was defined from IBP to 204 °C; light cycle oil (LCO) 204-350 °C; slurry oil (SO) above 350 °C. The carbonaceous deposit on the spent catalyst was measured by carbon analyzer CS244.

3. RESULTS AND DISCUSSION

3.1. Catalyst Characterization

To identify the acid types and acid strength of these investigated samples, IR and TPD technologies were used together. The results are shown in Figure 1. As seen, only characteristic bands of Lewis acidity (~1445, 1578, 1595 and 1617 cm-1) were observed for Alumina-1 and Alumina-2 (Figure 1a), while the 1545 and 1640 cm-1 bonds assigned to Brönsted acidity appeared for 0.5P/Al and 0.2S/Al (Figure 1c), suggesting that there were only Lewis acidity on the alumina surface, and the modification by H2SO4 and H3PO4 solution could generate Brönsted acid sites on the alumina surface. These results were consistent with the researches of Morales39 and Arata.40 Besides, the bond at ~1445 cm-1 is attributed to the C-H bending of pyridine adsorbed on Lewis acid sites,41 and increasing the acid strength of Lewis sites would lead to a shift of this bond towards higher frequencies, as more electron cloud of the C-H bond was attracted to the bonded Lewis sites.42 As shown in Figure 1a, the C-H bending frequency of Alumina-2 (1448 cm-1) was higher than that of Alumina-1 (1444 cm-1), at the same time, the 1617 cm-1 bond assigned to strong Lewis acidity was obviously stronger for

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Alumina-2 as well, suggesting that the Lewis acidity of Alumina-2 was stronger than that of Alumina-1. This conclusion could be further evidenced by the higher area of the high-temperature peak between 300 and 475 °C of Alumina-2 than that of Alumina-1 shown in NH3-TPD curves (Figure 1b). In addition, as seen in Figure 1c, there was no relative shift for 1444 cm-1 bond between 0.2S/Al and 0.5P/Al, which was the same as Alumina-1. Moreover, for both of the two matrices, the bonds attributed to Lewis acidity almost disappeared after desorption at 350 °C and 10-2 Torr. The above two facts indicated that 0.2S/Al and 0.5P/Al had a close but weak Lewis acidity, which was similar to Alumina-1. By contrast, the 1640 cm-1 bond related to strong Brönsted acid sites was much stronger for 0.2S/Al than that of 0.5P/Al, especially after desorption at 350 °C and 10-2 Torr, suggesting that 0.2S/Al presented a stronger Brönsted acidity than 0.5P/Al did. This conclusion can be further proved from the fact that both the area of high-temperature peak in the range of 300 to 600 °C and the final desorption temperature were higher for 0.2S/Al than those of 0.5P/Al shown in NH3-TPD curves (Figure 1d), combined with the result that the two matrices presented a similar Lewis acidity jointly. Besides, on the condition that the total peak area of each of the 0.5P/Al and 0.2S/Al was similar with that of CAM(0.5P) or CAM(0.2S) generally, the final desorption temperatures of 0.5P/Al and 0.2S/Al were similar with those of these conventional active matrices. This indicated that the acid strength of the modified alumina varied in a practically meaningful range for the matrix component of a FCC catalyst. Table 1 further compares the main physicochemical properties of the investigated samples quantitatively. The textural properties between Alumina-1 and Alumina-2 determined by N2 adsorption-desorption measurement were similar. The surface areas of the modified alumina, especially the ones modified by H3PO4 solution, were slightly lower than that of unmodified alumina (Alumina-1). Nevertheless, the modification had no obvious effect on both surface area and pore size in general, which

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was different from the researches of Gawthrope43 and Araujo et al.44 This was mainly due to the lower concentration of acid solution used in our experiments. Besides that, the method of preparation should be also considered. It was worth to notice that the mean pore size of S-REUSY was relatively large, but because the mesopores created by steaming treatment did not form a connected network,24 the rate of intracystalline diffusion could not be enhanced significantly. With respect to acid number, Alumina-1 presented a similar value with Alumina-2. 0.5P/Al had more Brönsted sites than that of 0.2P/Al, at the same time, 0.5P/Al presented an equal Brönsted acid number with 0.2S/Al. All the three modified alumina had a close number of Lewis sites. In addition, the interactions between the modifiers and the alumina surface generated the Brönsted acid sites. As seen in Table 1, the matrix of 0.5P/Al presented a higher P content than 0.2P/Al did. This further explained the reason why 0.5P/Al had a higher Brönsted acid number compared with 0.2P/Al. Besides, although the S content of 0.2S/Al was sensibly less than the P content of 0.5P/Al, the two matrices, as discussed above, had a similar Brönsted acid number with each other. This indicated that S atoms could generate more Brönsted acid sites on the alumina surface than the same amount of P atoms did. In conclusion, Alumina-1 and Alumina-2 had only Lewis sites, which the acid number was similar, but the acidity of the latter one was stronger. Besides Lewis acid sites with similar acidity to Alumina-1, all the modified alumina also had Brönsted acid sites on the surface. In addition, 0.5P/Al had a similar Brönsted acid strength with 0.2P/Al, while the Brönsted acid number of 0.5P/Al was higher than that of 0.2P/Al. 0.2S/Al and 0.5P/Al had a close number of Brönsted acidity, but the former presented a stronger Brönsted acidity than the latter.

3.2. Effect of Matrix Acidity

Based on the prepared matrices above, in this study, the effect of matrix acidity (acid type, acid 9


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strength of each type, acid number of weak Brönsted acidity, and the contacting orders of acid sites with different types) on the catalytic cracking of heavy oil was further studied, and the activity and overall selectivity of both matrices and corresponding catalysts were compared. The properties of the feed oil are given in Table 2. As seen, the high-density medium-based FCC feedstock contained a considerable amount of aromatics, resins and asphaltenes. To further investigate the molecular structures of the feed oil, GC-MS and 1H-NMR were used together. Figure S1 and Table S1 in the Supporting Information shows the GC-MS analysis results of the saturates and aromatics of the feed oil. As seen, the saturates were mainly composed of a large amount of paraffins ( ~70 wt%) and a small amount of naphthenes, but it was worth to notice that the large hydrocarbons, which had three or more rings per average molecule, accounted for more than 20 wt% of this fraction. At the same time, in the aromatics this ratio was more than 70 wt%. Besides, although GC-MS analysis could accurately characterize the composition and structure of hydrocarbons, it could not be applied to characterize hydrocarbons in heavy fraction, such as the resins and asphaltenes of the feed oil. Therefore, the Brown-Ladner method45 was used to calculate the molecular parameters of the resins and asphaltenes. Total rings per average molecule (RT) of this fraction as well as the molecular weight, elemental analysis and hydrogen distribution, the data needed to calculate RT, were summarized in Table S2 in the Supporting Information. The number of total rings per average molecule of this fraction was larger than five, suggesting that essentially all the molecules in this fraction could not enter into the Y-zeolite pores directly. Results above combined with SARA analysis (Table 2) jointly indicated that the hydrocarbons with three or more rings accounted for more than 40 wt% of the feed oil. According to Nace46 and Magoub et.al.,12 the hydrocarbons with three or more rings could hardly enter into the Y-zeolite pores and preferentially were first cracked on the matrix surface during the catalytic cracking process. More importantly, these large hydrocarbons preferred to associate with other

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hydrocarbons in the feed oil as reactant molecules during the practical catalytic cracking process.47, 48 As a result, a considerable portion of reactant molecules were too large to penetrate into the micropores of Y-zeolite directly. Therefore, this feedstock was representative to investigate the preliminary cracking of heavy oil. To further investigate the effect of the matrix acidity on the catalytic cracking of heavy oil, three indexes were introduced. The first one is the free-radical and protolytic cracking index (FPI), which is defined as the volume ratio of the sum of H2, C1 and C2 yields to the sum of isobutene and isobutene yields {(H2+C1+C2) / (i-C4o + i-C4=)}. With the aim to maximize liquid products, free-radical cracking and protolytic cracking reactions whose characteristic products are H2, C1 and C2 are undesired, while the classical cracking reactions, with isobutane and isobutene being characteristic products, are more beneficial. Therefore, the higher the value means the ratio of undesired reactions is larger. Besides, for the undesired reactions producing dry gas above, C1 and C2 are produced from C-C bond breaking, while H2, with the highest hydrogen content (100%), are produced by C-H breaking. Compared with the formers, the latter is more undesired. Hence, we introduced the dehydrogenation index (DHI) from open literatures,49, 50 defined as the volume ratio of H2 to the sum of C1 and C2 yields {(H2) / (C1+C2)}, to describe the ratio of reactions via C-H bond breaking and reactions via C-C bond breaking during dry gas generation. The higher the ratio value suggests more H2 was produced during the dry gas fraction generation, with more hydrogen atoms remaining in dry gas fraction, rather than in desired products. In addition, large hydrocarbon molecules, which cannot penetrate into the zeolite directly, are expected to remain unconverted in the slurry oil if the catalyst contains an inactive matrix. When matrices are active, they are preferentially cracked into hydrocarbons in the LCO boiling range, which would in turn increase the LCO yield at the expense of slurry oil. Therefore, we introduced LCO/SO from open literatures,9

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defined as the weight ratio of the LCO yield to slurry oil yield, to estimate the performance of matrices for precracking. The higher the value indicates a better precracking function which the matrix presents.

3.2.1. Acid Type and Acid Strength of Each Type

Figure 2 shows the effects of Lewis acid strength of matrices on conversion and overall selectivities for the matrix-precracking of heavy oil. The effects on undesired reactions producing dry gas, dehydrogenation reactions and the matrix-precracking performance during the matrix-precracking process are further shown in Figure 3. As seen in Figure 2, sensible differences were observed in the activity. The matrix of Alumina-2, with stronger Lewis sites than Alumina-1, presented a lower conversion of the feedstock. Differences in selectivity were also remarkable. Because stronger Lewis acid sites, presenting on weaker conjugate bases, made it more difficult for adsorbed carbenium ions to desorb, since that required the abstraction of a proton from the cabenium ion by the base, which would contribute to coke formation,51 the coke selectivity of Alumina-2 was higher than that of Alumina-1. More coke formation, responsible for more sites deactivation and more pores blockage, would weaken the catalytic cracking, but enhanced the thermal cracking, as evidenced by the higher FPI value of Alumina-2 than that of Alumina-1 (Figure 3). Therefore, Alumina-2 presented a higher selectivity to dry gas. Besides, due to the greater ability to abstract hydride ions for stronger Lewis sites, Alumina-2 also gave a higher DHI value than Alumian-1. This indicated that stronger Lewis acidity on the matrix surface resulted in not only more total undesired reactions producing dry gas, but also a higher ratio of dehydrogenation reactions, which made more hydrogen atoms remain in dry gas, rather than desired products. In addition, Alumina-2 presented a similar selectivity to LPG as well as gasoline with Alumina-1. However, its LCO selectivity was lower than that of Alumina-1, which resulted in the lower conversion. This was due to the inferior matrix-precracking 12


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performance of Alumina-2 evidenced by its lower LCO/SO than that of Alumina-1, which indicated that large hydrocarbons prefer to undergo condensation reactions rather than be precracked into moderate ones on stronger Lewis acid sites of matrices. In brief, stronger Lewis acidity on the matrix surface was detrimental for achieving a better activity and selectivity during the matrix-precracking process. The effects of Brönsted acid strength of matrices on the matrix-precracking of heavy oil are shown in Figure 4 and Figure 5. Significant differences were observed in the activity. The conversion was considerably higher for 0.2S/Al, the one with stronger Brönsted acidity, than that of 0.5P/Al, suggesting that stronger Brönsted acidity could increase the matrix activity. This was due to that although stronger Brönsted sites resulted in a longer lifetime of carbenium ions on the matrix surface and thus restricted the process of chain propagation, stronger Brönsted sites facilitated the initial cracking by increasing the capability of the active sites to attack reactant molecules on the one hand, on the other hand, they were easier to interact with olefins produced by initial cracking to generate a large amount of carbenium ions. As a result, the activity of the matrix was enhanced. Of note was that, this variation trend, as discussed above, was contrary to that of stronger Lewis sites on the matrix surface. This was due to that initial cracking was very difficult for Brönsted as well as Lewis sites at energetic aspect,52, 53 but stronger Lewis sites could not interact with olefins produced through initial cracking to generate carbenium ions compared with stronger Brönsted sites. At the same time, stronger Lewis sites restricted the process of chain propagation. Therefore, stronger Lewis sites decreased the matrix activity. Differences in selectivity were also significant. As seen in Figure 5, 0.2S/Al presented a higher value of FPI as well as DHI, indicating that stronger Brönsted acidity on the matrix surface gave rise to more total undesired reactions producing dry gas and a higher ratio of dehydrogenation reactions, which was similar to stronger Lewis acidity. Stronger acid sites, including stronger Brönsted sites, made the carbenium

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ions more difficult to desorb, which facilitated coke formation. More coke formation, responsible for more sites deactivation and more pores blockage, would weaken the catalytic cracking, at the same time, enhance the thermal cracking. Therefore, the selectivity to dry gas and coke were higher for 0.2S/Al than for 0.5P/Al. Besides, 0.2S/Al presented a lower LPG selectivity, but a similar gasoline selectivity compared with 0.5P/Al. In addition, due to the similar precracking performance evidenced by the close value of LCO/SO shown in Figure 5, the selectivity to LCO was similar between the two matrices with different acid strength of Brönsted acidity. According to the results above, strong Brönsted acidity aggravated the product distribution during the matrix precracking process, but enhanced the conversion of heavy oil. However, it needed to be further noted that, the aim of the matrix-precracking was not to crack heavy oil itself but to maximally exert the catalytic function of the zeolite, the one with proper acidity and greater selectivity to liquid products. Considering this, a too high matrix activity probably could not enhance converting a heavy oil effectively, instead, would weaken exerting the zeolite catalytic function, and thereby worsening the overall catalytic performance of catalysts. To further determine the optimal acid type and strength on the matrix surface for the catalytic cracking of heavy oil, the product distribution and selectivity were compared among the four catalysts: Cat.Alumina-1, Cat.Alumina-2, Cat.0.5P/Al and Cat.0.2S/Al, whose matrices were Alumina-1, Alumina-2, 0.5P/Al and 0.2S/Al, respectively. Table 3 compares the activity and product distribution of the four catalysts when cracking heavy oil at the same reaction conditions. As seen, Cat.Alumina-1, with weaker Lewis sites on the matrix surface, presented a higher conversion than Cat.Alumina-2. Besides, either Cat.0.5P/Al or Cat.0.2S/Al presented a much higher conversion than Cat.Alumina-1 as well as Cat.Alumina-2, suggesting that compared with

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Lewis sites, the introduction of Brönsted sites onto the matrix surface could significantly increase the activity of catalysts. Further analysis showed that, compared to Cat.Alumina-1, around 82 % of the additional conversion was used in yielding gasoline fraction for Cat.0.5P/Al, while only around 70 % for Cat.0.2S/Al. This result indicated that a high matrix activity of catalysts indeed contributed to the conversion of heavy oil. Nevertheless, if the matrix activity was excessive, the balance between the matrix activity and the zeolite activity would be broken, which was unbeneficial for the overall catalytic cracking. Figure 6 compares the product selectivity of the four catalysts at a conversion of 75 wt%. Cat.0.5P/Al presented the highest selectivity to liquid products and light oil, with 93.18 % and 83.90 %, respectively, and the lowest selectivity to undesired products (dry gas and coke). The followed was Cat.Alumina-1, the one whose matrix had only weak Lewis sites. By contrast, Cat.0.2S/Al and Cat.Alumina-2, whose acidity on the matrix surface were stronger, were much more selective to dry gas and coke and less selective to liquid products and light oil. Overall, both from catalysts activity and selectivity, the weak Brönsted acidity was most desired for matrices, followed by weak Lewis acidity. Strong Brönsted acidity on the matrix surface could increase the activity, but resulted in a significantly high selectivity to dry gas and coke and a low selectivity to liquid products, while strong Lewis acidity on the matrix surface not only lowered the activity but aggravated the product distribution. These experimental results above well linked up the work performed by previous researchers that gave different conclusions with a similar idea to modify the matrix acidity.

3.2.2. Acid Number of Weak Brönsted Acidity

As discussed above, weak Brönsted acidity was most desired for matrices at acid type and strength aspects. Hence, we further investigated the effects of weak Brönsted acid number of matrices on the catalytic cracking of heavy oil. The results are shown in Figure 7 and 8. The matrices of 0.2P/Al and 15


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0.5P/Al had close physicochemical properties with each other, but 0.5P/Al had more Brönsted sites which were weak (Table 1). As expected, increasing the weak Brönsted sites on the matrix surface could increase the conversion sensibly, while dry gas, coke, and LPG selectivity were relatively constant. Due to the higher activity of matrix-precracking for Cat.0.5P/Al, evidenced by its higher value of LCO/SO than that of Cat.0.2P/Al (Figure 8), Cat.0.5P/Al presented a slightly higher selectivity to LCO. Besides, the improved activity of matrices weakens the relative contribution of zeolites with a high selectivity to gasoline. Therefore, Cat.0.5P/Al gave a slightly lower selectivity to gasoline. The light oil selectivity, however, was similar between the two catalysts. In addition, Cat.0.5P/Al presented a similar value of FPI but a higher value of DHI compared with Cat.0.2P/Al, suggesting that increasing weak Brönsted sites on the matrix surface affected little on the total undesired reactions producing dry gas, but increased the ratio of dehydrogenation reactions. In brief, increasing weak Brönsted sites on the matrix surface enhanced the catalyst activity significantly, at the same time, did not aggravate the product distribution, which was beneficial for converting heavy oil effectively.

3.2.3. Contacting Orders of Lewis and Brönsted sites

According to the results above, Brönsted sites on the matrix surface had a strong influence on the catalytic cracking of heavy oil. Furthermore, although the most of acid sites on the conventional matrix surface are Lewis acid sites, there are a small amount of Brönsted sites on the conventional matrix surface as well. Therefore, it was necessary to investigate the effect of contacting orders of Lewis and Brönsted acidity during the matrix-precraking process on the catalytic cracking of heavy oil. First, we studied the effects on the matrix-precracking process by loading Alumina-1 with only Lewis sites and 0.2S/Al presenting obvious Brönsted acidity in different orders. The results are shown in Figure 9 and 10. For both Lewis and Brönsted sites, the initial cracking is very difficult in energetic aspects,53 but 16


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Brönsted sites are ready to interact with the olefins generated through initial cracking to form large amounts of carbenium ions, and thereby facilitating the catalytic cracking significantly. Therefore, when 0.2S/Al was loaded under Alumina-1, namely, the feed oil first contacted with Lewis sites and then interacted with Brönsted sites, the conversion was considerably higher. Differences in selectivity were also sensible, Alumina-1-on-top experiments presented a lower selectivity to LCO, and a better selectivity to gasoline. This was due to the higher activity of Brönsted sites that would convert more LCO produced on the top layer into gasoline fraction when 0.2S/Al was loaded at bottom. The similar value of LCO/SO indicated that the contacting orders of acid sites with different types affected little on the matrix-precracking ability. In addition, Alumina-1-on-top experiments presented a close value of FPI and a higher value of DHI compared with 0.2S/Al-on-top experiments, suggesting that the contacting orders affected little on the total undesired reactions producing dry gas, but the ratio of dehydrogenation reactions was higher during the matrix-precracking process when the feed oil molecules contacted with the Lewis sites first and then interacted with Brönsted sites on the matrix surface. Due to the large critical diameters, the molecules in FCC feed oils, especially heavy ones, theoretically need to be first precracked on the matrix surface, and then enter into the zeolite pores to be further selectively cracked. From experimental perspective, Roland and coworkers54 loaded zeolites and matrices in different orders and found that when feed oil contacted with the matrix layer first the conversion of vacuum gas oil was higher, which was closer to that of the corresponding real catalyst. This fact further proved that large reactant molecules are preferentially first precracked on the matrix surface, and then enter in to the zeolite pores. Therefore, we loaded a S-REUSY layer (15 wt%) under the matrix layers to simulate the real catalytic cracking process. The results are shown in Figure 11, where the conversion and product distribution (Figure 11a) were obtained at the same reaction conditions, reaction temperature was 500 °C

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and the catalyst to oil ratio was 3, while the selectivity was estimated at a conversion of 70 wt% (Figure 11b). The effects on both the activity and selectivity for overall catalytic cracking of heavy oil were similar to that for the matrix-precracking process. Alumina-1-on-top experiments presented a significantly higher conversion than 0.2S/Al-on-top experiments, and the additional conversion was mainly used to produce gasoline. In addition, Alumina-1-on-top experiments also presented a higher selectivity to gasoline, and a lower selectivity to LCO. Overall, compared with first contacting Brönsted sites on the matrix surfac, first contacting Lewis sites followed by interacting with Brönsted sites during the matrix-precracking process enhanced the conversion of heavy oil significantly and did not aggravate the product distribution, which was beneficial for heavy oil cracking.

3.3. Proposal of a New Reaction Route during Matrix Precracking Process

Interestingly, it was also found from above experiments that when there were Brönsted sites (weak or strong) existing on the matrix surface, the H2 and CH4 selectivity were obviously higher than those of experiments where the matrices had only Lewis sites whether for matrices themselves or corresponding catalysts , as seen in Figure 12. It is generally accepted that dry gas, including H2 and CH4, is produced by thermal cracking. But it has to be noted that, the reaction temperature (500 °C) in the experiments was relatively mild, which did not favor the thermal cracking process. Furthermore, the catalyst-filling weight and textural properties of catalysts for each experiment were almost the same, namely, the factors affecting thermal cracking were nearly the same. Therefore, it was obviously unreasonable to attribute the good correlation between the H2 and CH4 selectivity, and the acid types of matrices to the influence of thermal cracking. 18


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Of note is that, in the 1980’s, based on the researches of Haag and Dessau,36 Corma et al.34 found that besides classical catalytic cracking route, there was also protolytic cracking pathway occurred on the Brönsted sites of Y-zeolite initially,55, 56 and further proposed that H2 and CH4 were the characteristic products for protolytic cracking on the zeolite surface.57, 58 Moreover, despite the relatively large pore size of matrices which was unfavorable to protolytic cracking,55 there are Brönsted acid sites on the matrix surface as well. Most importantly, since olefins that could be generated by catalytic cracking were much better proton acceptors, and their protonation lead to classical cracking, which restricted protolytic cracking significantly, protolytic cracking was kinetically significant only at the initial contact of oil vapor and catalyst, on the other hand, the most probable acid sites that first contacted with large reactant molecules were the ones on the matrix surface due to the significantly better accessibility. Therefore, combined with the results in Figure 12, we proposed that besides on the zeolite surface, there was protolytic cracking route occurred on the matrix surface when cracking heavy oil as well, as illustrated in scheme 1.

4. CONCLUSIONS

Compared with Lewis sites, the introduction of Brönsted sites onto the matrix surface can increase the activity for both matrices and catalysts more significantly. Besides, increasing acid strength of Brönsted acidity on the matrix surface could increase the catalyst activity, but results in a much higher selectivity to dry gas and coke, and a lower selectivity to liquid products. Whereas the increase of Lewis acid strength on the matrix surface not only lowers the catalyst activity but aggravates the product distribution as well. Both from catalysts activity and selectivity for the catalytic cracking of heavy oil, weak Brönsted acidity is most desired for matrices, followed by weak Lewis acidity. Results also showed that increasing the number of weak Brönsted acidity on the matrix surface and/or contacting Lewis sites first followed by interacting with

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Brönsted sites during the matrix-precracking process, would enhance the conversion without affecting the product distribution, which is beneficial to the catalytic cracking of heavy oil. In addition, according to the good correlation between the acid types of matrices and the selectivity to both H2 and CH4, as well as the fact that large reactant molecules are preferentially first precracked on the matrix surface, it is proposed that besides on the zeolite surface, there is protolytic cracking route occurred during the matrix-precracking process when the catalystic cracking of heavy oil as well.

ACKNOWLEDGMENTS

This work was financially supported by “the National Natural Science Foundation for Young Scholars of China” (No. 21406270), “the Fundamental Research Funds for the Central Universities” (No. 15CX06036A) and “the China University of Petroleum for Postgraduate Technology Innovation Project” (No. YCX2015028), “Qingdao People’s Livelihood Project” (No. 13-1-3-126-nsh). We thank Dr. Wang Chengxiu and Dr. Duan Xiaoguang for their help in technical English, and also thank the editor and reviewers for their reviewing and valuable comments leading to a better manuscript.

Supporting Information

GC-MS analysis results of the saturates and aromatics of the feed oil, and properties of the resins and asphaltenes of the feed oil. This information is available free of charge via the Internet at http://pubs.acs.org.

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Tables: Table 1. Characteristics of the samples tested Sample

S-REUSY Alumina-1 Alumina-2 0.2P/Al 0.5P/Al 0.2S/Al

Acidity

BET Specific surface

Mean pore

Pyridine-IR, 10 mol g

NH3-TPD,

P or S Content,

area, m2/g

size, nm

Brönsted

Lewis

10-4mol g -1

wt %

189 175 169 161 152 170

6.5 3.8 4.4 3.8 3.8 3.8

0.10 0.00 0.00 0.05 0.42 0.44

0.54 1.14 1.52 2.05 2.07 2.05

1.1 1.6 2.1 3.3 3.6 3.4

/ / / 0.87 2.09 0.83

-4

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-1

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Table 2. Characteristics of the feed oil density (20 oC), kg/m3 molecular weight, g/mol CCR, wt% Elementary analysis, wt% C H S N SARA analysis, wt% saturates aromatics resins + asphaltenes Distillation curve ASTM D-2887 ( oC)

925 407 0.21 86.23 12.62 0.26 0.19 65.49 20.69 13.82

5%

10%

30%

50%

70%

90%

330

356

408

432

452

486

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Table 3. Comparison of conversion and product distribution in catalysts with different matrices. Operating conditions Cat. temperature, oC Cat / Oil, g/g injection time, s conversion, wt% Product Distribution, wt% dry gas LPG gasoline diesel heavy oil coke light oil yield liquid products yield

Cat.Alumina-1 500 5 60 77.16

Cat.Alumina-2 500 5 60 74.55

Cat.0.5P/Al 500 5 60 85.13

Cat.0.2S/Al 500 5 60 90.71

1.62 7.49 38.21 25.82 22.84 4.01 64.03 71.52

1.70 6.45 39.07 23.09 25.45 4.24 62.15 68.61

1.75 8.64 44.74 25.90 14.87 4.11 70.64 79.28

2.08 9.44 47.71 26.13 9.29 5.36 73.84 83.27

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Figure captions: Figure 1. Acidic properties of the investigated matrices: (a, c) pyridine-IR and (b, d) NH3-TPD. Figure 2. Effect of Lewis acid strength of matrices on the matrix-precracking process. Figure 3. Effect of Lewis acid strength of matrices on the matrix-precracking reactions. Figure 4. Effect of Brönsted acid strength of matrices on the matrix-precracking process. Figure 5. Effect of Brönsted acid strength of matrices on the matrix-precracking reactions. Figure 6. Comparison of product selectivity of catalysts with different matrices. Figure 7. Effect of weak Brönsted acid number of matrices on the catalytic cracking of heavy oil. Figure 8. Effect of weak Brönsted acid number of matrices on the reactions when cracking heavy oil. Figure 9. Effect of contacting orders of Lewis and Brönsted sites over the matrix surface on matrix-precracking process. Figure 10. Effect of contacting orders of Lewis and Brönsted sites over the matrix surface on precracking reactions. Figure 11. Effect of contacting orders of Lewis and Brönsted sites over the matrix surface on the catalytic cracking of heavy oil. Figure 12. Effect of acid types of matrices on producing H2 and CH4. Scheme 1. Protolytic cracking of a large feed molecule during the matrix-precracking process.

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Figure 1. Acidic properties of the investigated matrices: (a, c) pyridine-IR and (b, d) NH3-TPD.

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Figure 2. Effect of Lewis acid strength of matrices on the matrix-precracking process.

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Figure 3. Effect of Lewis acid strength of matrices on the matrix-precracking reactions.

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Figure 4. Effect of Brönsted acid strength of matrices on the matrix-precracking process.

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Figure 5. Effect of Brönsted acid strength of matrices on the matrix-precracking reactions.

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Figure 6. Comparison of product selectivity of catalysts with different matrices.

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Figure 7. Effect of weak Brönsted acid number of matrices on the catalytic cracking of heavy oil.

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Figure 8. Effect of weak Brönsted acid number of matrices on the reactions when cracking heavy oil.

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Figure 9. Effect of contacting orders of Lewis and Brönsted sites over the matrix surface on matrix-precracking process.

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Figure 10. Effect of contacting orders of Lewis and Brönsted sites over the matrix surface on precracking reactions.

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Figure 11. Effect of contacting orders of Lewis and Brönsted sites over the matrix surface on the catalytic cracking of heavy oil.

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Figure 12. Effect of acid types of matrices on producing H2 and CH4.

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Scheme 1. Protolytic cracking of a large feed molecule during the matrix-precracking process

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Studies on the Preliminary Cracking of Heavy Oils: The Effect of Matrix

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