The Effect of Y Zeolites with Different Pores on Tetralin Hydrocracking for the Production of High-Value Benzene, Toluene, Ethylbenzene and Xylene Products

: A series of Y zeolites with different pore properties was prepared as a support for hydrocracking catalysts for the production of BTEX (benzene, toluene, ethyl-benzene, and xylene) from tetralin. Some important characterizations, including N 2 adsorption–desorption, NH 3 -TPD, Py-IR, and HRTEM, were applied to obtain the properties of different catalysts. Meanwhile, the tetralin hydrocracking performances of those catalysts were investigated on a high-pressure ﬁxed-bed microreactor. The results showed that Si/Al ratio is the core property of zeolites and that the increase in the V micro /V meso of zeolites could facilitate the formation of BTEX products by hydro-cracking tetralin. The method of hydrocracking tetralin was proposed. It was also found that the hydrogenation–cracking path was controlled by aromatic saturation thermodynamics, and strong acidity aided the backward shift of equilibrium temperature.


Introduction
With the continuous implementation of the carbon-neutrality policy and the development trend in energy transformation, the petrochemical industry has undergone great changes in terms of product structure adjustment and the nature of the products produced. From a long-term perspective, the demand for transport fuel will gradually decrease. In addition, the severity of the use of crude oil continues to worsen, which is a greater challenge for the petroleum processing and refining industry.
Light cycle oil (LCO), with a boiling point similar to that of diesel, is derived from fluid catalytic cracking, which has high aromatic, sulfur, and nitrogen contents, as well as a low cetane index [1,2]. There is a large gap between the nature of LCO and the China national VI diesel standard. If LCO is used to produce the blending component of clean diesel fuel, it will consume a large amount of hydrogen and cause high CO 2 emissions, which means it does not meet the requirements of environmental protection policies, the process is not economical, and the product has low commercial value. Therefore, many researchers use LCO, which is rich in aromatic hydrocarbons, to produce high-octane gasoline and light aromatic BTEX (benzene, toluene, ethyl-benzene, and xylene) components through selective hydrocracking technology [3,4]. This process can effectively reduce hydrogen consumption and greatly improve the commercial value of the product. To date, many institutions have developed a number of technologies using this principle, such as FD2G, RLG, and LCO-X.
According to previously reported research, most of the poly-aromatics in LCO are di-aromatic hydrocarbons. Researchers have investigated the reaction regularity of model many ways to adjust the channel structure of a catalyst carrier. However, the topological structure and specific characteristics of different carriers vary greatly, and the influence of other properties, especially acid properties, cannot be explained when exploring the influence of catalyst pore properties. As reported, Y zeolite, which is the main carrier of industrial hydrocracking catalysts has a suitable pore structure, adjustable acidity, and favorable stability. In order to reduce the influence of the acidic properties of a carrier, Y zeolite was modified using appropriate methods, and two series of Y zeolites with similar acid properties but significantly different pore structures were prepared, and the hydrocracking catalyst was prepared using these Y zeolites as carriers. The influence of the pore properties of Y zeolite on the selective hydrocracking of tetralin to produce light aromatics was investigated. The influence of Y zeolite's pore characteristics on the key conversion steps of tetralin was clarified, which played a guiding role in the development of hydrocracking catalysts.

Characterization of Zeolites and Catalysts
Y zeolite has a different order of Si-O-Al or Si-O-Si linkages in the framework, and the molar Si/Al ratio of zeolite result in variations in the acidity of the Si(OH)Al group [21]. The acid properties of Y zeolites are mainly related to the content and distribution of aluminum atoms. Moreover, the amount of aluminum in the framework affects the unit cell constant. In this work, a series of zeolite Y was divided into two parts according to the value of the Si/Al ratio characterized by the XRF and the unit cell constant characterized by the XRD, as shown in Table 1 and Figure 1.  The key factor determining both the unit cell constant and acidity is the Si/Al ratio of zeolites. It can be seen from Table 1 that the Si/Al ratio and the unit cell constants of Y1-Y4 zeolites were very similar, and the Si/Al ratio and the unit cell constants of Y5-Y8 zeolites were also relatively similar. This means that the acidities of the samples may have been similar in each group. From Figure 1, it can be seen that each group of samples showed a typical Y zeolite crystal structure, indicating that the framework structure of all the samples was not destroyed. NH3-TPD was used to estimate the acid site strength distribution using a single temperature programmed desorption method; desorption peaks at less than 200 °C, 200-350 °C and higher than 350 °C represented the acidic sites with The key factor determining both the unit cell constant and acidity is the Si/Al ratio of zeolites. It can be seen from Table 1 that the Si/Al ratio and the unit cell constants of Y1-Y4 zeolites were very similar, and the Si/Al ratio and the unit cell constants of Y5-Y8 zeolites were also relatively similar. This means that the acidities of the samples may have been similar in each group. From Figure 1, it can be seen that each group of samples showed a typical Y zeolite crystal structure, indicating that the framework structure of all the samples was not destroyed. NH 3 -TPD was used to estimate the acid site strength distribution using a single temperature programmed desorption method; desorption peaks at less than 200 • C, 200-350 • C and higher than 350 • C represented the acidic sites with weak strength, medium strength, and high strength [22]. The results are shown in Figure 2. weak strength, medium strength, and high strength [22]. The results are shown in 2. In order to further analyze the difference in the acid strength of each samp profiles in Figure 2 were treated according to the highest degree of fitting, and t amount was calculated according to the peak area and the desorption ammonia ti results are shown in Table 2.

Sample
Total Acid Amount /(μmol· g −1 ) Weak Strength /(μmol· g −1 ) Medium Strength /(μmol· g −1 ) Strong Stre /(μmol· g  Y1  1400  280  230  890  Y2  1620  330  230  1060  Y3  1470  300  190  980  Y4  1500  300  220  980  Y5  1860  520  810  530  Y6  1950  500  780  670  Y7  1840  470  650  720  Y8  1920  450  870  600 It can be seen from Table 2 that the total acid amounts of the samples could divided into two groups according to the unit cell constant, and the acidities of within each group were very similar. The total acid amounts in theY5, Y6, Y7 and Y ples were higher than those in the Y1, Y2, Y3, and Y4 samples. However, the amo strong acid in Y5, Y6, Y7, and Y8 were lower than those in the Y1, Y2, Y3, and Y4 sa a similar trend was shown in the Si/Al ratios. Therefore, it can be concluded that ratio of Si and Al can facilitate more strong acid sites but fewer total acid sites. The standard deviation of most samples was less than 10%. In terms of the d strengths of acidity, Y1, Y2, Y3, and Y4 were classified as strong acid zeolites (SA Y5, Y6, Y7, and Y8 could be defined as medium acid zeolites (MAZ). In a com between SAZ and MAZ, it was indicated that the medium acid sites may be con from strong ones in the preparation of zeolite dealumination to reduce the unit c stant. The IR spectra of pyridine adsorbed onto zeolite at various temperatures is used to identify the Brϕnsted acid sites and Lewis acid sites with different strengt the results are shown in Table 3. In order to further analyze the difference in the acid strength of each sample, the profiles in Figure 2 were treated according to the highest degree of fitting, and the acid amount was calculated according to the peak area and the desorption ammonia titer. The results are shown in Table 2. It can be seen from Table 2 that the total acid amounts of the samples could also be divided into two groups according to the unit cell constant, and the acidities of zeolites within each group were very similar. The total acid amounts in theY5, Y6, Y7 and Y8 samples were higher than those in the Y1, Y2, Y3, and Y4 samples. However, the amounts of strong acid in Y5, Y6, Y7, and Y8 were lower than those in the Y1, Y2, Y3, and Y4 samples; a similar trend was shown in the Si/Al ratios. Therefore, it can be concluded that a high ratio of Si and Al can facilitate more strong acid sites but fewer total acid sites.
The standard deviation of most samples was less than 10%. In terms of the different strengths of acidity, Y1, Y2, Y3, and Y4 were classified as strong acid zeolites (SAZ), and Y5, Y6, Y7, and Y8 could be defined as medium acid zeolites (MAZ). In a comparison between SAZ and MAZ, it was indicated that the medium acid sites may be converted from strong ones in the preparation of zeolite dealumination to reduce the unit cell constant. The IR spectra of pyridine adsorbed onto zeolite at various temperatures is usually used to identify the Brφnsted acid sites and Lewis acid sites with different strengths [23]; the results are shown in Table 3.  Table 3 suggests that the zeolites could still be divided into two parts according to the Brφnsted acid sites and Lewis acid sites; the difference in the acid amount in the SAZ series was small after Y1 was removed. The MAZ produced similar results when Y7 was removed. Compared with MAZ, the SAZ had more Lewis acid and less Brφnsted acid, which was quite different to the NH 3 -TPD results. The differences within each group were also higher than those observed in the NH 3 -TPD results. This can be attributed to the different absorbed molecular sizes of ammonia and pyridine. Through external and internal diffusion, the probe molecule could reach distinct active sites for absorption; therefore, the difference was usually due to the porous property of zeolites. The isotherms of nitrogen physisorption and the pore size distribution of the Y zeolites are presented in Figures 3 and 4.   Table 3 suggests that the zeolites could still be divided into two parts according to the Brϕnsted acid sites and Lewis acid sites; the difference in the acid amount in the SAZ series was small after Y1 was removed. The MAZ produced similar results when Y7 was removed. Compared with MAZ, the SAZ had more Lewis acid and less Brϕnsted acid which was quite different to the NH3-TPD results. The differences within each group were also higher than those observed in the NH3-TPD results. This can be attributed to the dif ferent absorbed molecular sizes of ammonia and pyridine. Through external and interna diffusion, the probe molecule could reach distinct active sites for absorption; therefore the difference was usually due to the porous property of zeolites. The isotherms of nitro gen physisorption and the pore size distribution of the Y zeolites are presented in Figures  3 and 4.  It can be clearly seen that the N2 adsorption-desorption isotherms of all the zeolites were type IV with a typical H2 type hysteresis loop, indicating that these zeolites possess microporous and mesoporous structures [24]. The porous properties are shown in Table  4.   Table 3 suggests that the zeolites could still be divided into two parts according to the Brϕnsted acid sites and Lewis acid sites; the difference in the acid amount in the SAZ series was small after Y1 was removed. The MAZ produced similar results when Y7 wa removed. Compared with MAZ, the SAZ had more Lewis acid and less Brϕnsted acid which was quite different to the NH3-TPD results. The differences within each group were also higher than those observed in the NH3-TPD results. This can be attributed to the dif ferent absorbed molecular sizes of ammonia and pyridine. Through external and interna diffusion, the probe molecule could reach distinct active sites for absorption; therefore the difference was usually due to the porous property of zeolites. The isotherms of nitro gen physisorption and the pore size distribution of the Y zeolites are presented in Figure  3    It can be clearly seen that the N2 adsorption-desorption isotherms of all the zeolite were type IV with a typical H2 type hysteresis loop, indicating that these zeolites posses microporous and mesoporous structures [24]. The porous properties are shown in Table  4. It can be clearly seen that the N 2 adsorption-desorption isotherms of all the zeolites were type IV with a typical H 2 type hysteresis loop, indicating that these zeolites possess microporous and mesoporous structures [24]. The porous properties are shown in Table 4.

Sample
S BET / (m 2 ·g −1 ) The results from Table 4 show that, in SAZ, the S micro increased while the S meso changed randomly; in the MAZ series, the opposite was observed. In addition, the Y8 zeolite showed a significantly high value of V micro /V meso , which could also be verified from the super flat hysteresis loop in Figure 4b.
Due to the high surface tension in the capillary condensation process, transition metal compounds are usually located on the outer surfaces of catalyst microparticles during wet impregnation preparation of the catalyst and further form layered sulfides in the sulfidation process. The sizes of bulk particles from active metal sulfides are usually used to identify the hydrogenation activity of catalysts [25]; here, the catalysts in the sulfide form were investigated using the HRTEM method. Figure 5 shows microphotographs of the catalysts, and the average particle length (L) and stacking number (N) were calculated, as shown in Table 5.
Due to the high surface tension in the capillary condensation process, transition metal compounds are usually located on the outer surfaces of catalyst microparticles during wet impregnation preparation of the catalyst and further form layered sulfides in the sulfidation process. The sizes of bulk particles from active metal sulfides are usually used to identify the hydrogenation activity of catalysts [25]; here, the catalysts in the sulfide form were investigated using the HRTEM method. Figure 5 shows microphotographs of the catalysts, and the average particle length ( ̅ ) and stacking number ( ̅ ) were calculated, as shown in Table 5.  The particle length and stacking number values were close to the results found in common hydrocracking catalysts [26]. The size of bulk particles from the Y7 sample was slightly smaller than that of Y3; this can be explained by the fact that there were stronger Brϕnsted sites, which led to strong metal-support interaction, which ensured it was easy to retain the single-layer structure during the sulfidation and reaction procedure.

Hydrocracking of Tetralin
Tetralin hydrocracking over catalysts takes place through a complex reaction scheme, as shown in Figure 6, which is related to this research [7,27].  The particle length and stacking number values were close to the results found in common hydrocracking catalysts [26]. The size of bulk particles from the Y7 sample was slightly smaller than that of Y3; this can be explained by the fact that there were stronger Brφnsted sites, which led to strong metal-support interaction, which ensured it was easy to retain the single-layer structure during the sulfidation and reaction procedure.

Hydrocracking of Tetralin
Tetralin hydrocracking over catalysts takes place through a complex reaction scheme, as shown in Figure 6, which is related to this research [7,27].  The hydrocracking of tetralin is based on the mechanism of a carbonium reaction, including hydrogenation, isomerization, β cracking, and dehydrogenation. In addition to hydrogenation, tetralin can undergo naphthenic ring isomerization or dehydrogenation; these schemes are differentiated in the first step in the reaction of tetralin. In path A and B, the naphthenic ring opens, followed by cracking reactions to form BTEX or single naphthenic compounds. To facilitate the analysis of the results of the catalytic tests, the identified compounds were assembled in different groups, as indicated in Table 6. Table 6. Compounds assembled in different groups.

Group
Compounds HEAVY naphthalene, alkyl-naphthalene ARO-ISO methyl-indan ARO-RO butyl-benzene, methylpropenyl-benzene ARO-C benzene, toluene, xylenes, ethyl-benzene H decalin HYD-RO butyl-cyclohexane, methyl-butylcyclopentane HYD-C methyl-cyclopentane, ethyl-cyclohexane In order to reasonably explain the influence of the pore properties of Y zeolite on the hydrocracking process of tetralin, the conversion rates, yields of products and selectivity are defined by the following: where and represent the content of tetralin in the material and the product, respectively.  The hydrocracking of tetralin is based on the mechanism of a carbonium reaction, including hydrogenation, isomerization, β cracking, and dehydrogenation. In addition to hydrogenation, tetralin can undergo naphthenic ring isomerization or dehydrogenation; these schemes are differentiated in the first step in the reaction of tetralin. In path A and B, the naphthenic ring opens, followed by cracking reactions to form BTEX or single naphthenic compounds. To facilitate the analysis of the results of the catalytic tests, the identified compounds were assembled in different groups, as indicated in Table 6. In order to reasonably explain the influence of the pore properties of Y zeolite on the hydrocracking process of tetralin, the conversion rates, yields of products and selectivity are defined by the following: where ω i and ω f represent the content of tetralin in the material and the product, respectively.
Catalysts 2022, 12, 848 8 of 14 Tetralin hydrocracking conversion over different catalysts is shown in Figure 7a,b. It can be seen that the conversion of tetralin obviously increased with the increase in temperature, and the conversion of the SAZ group was significantly higher than that of the MAZ group at the same temperature. This was mainly due to the high Si/Al ratio in the SAZ group, which had higher strong acid strength. Sato et al. [7] also found that a higher ratio of strong acid sites contributes to the improvement in the conversion depth of tetralin. In the SAZ group, the conversion of tetralin was mainly proportional to S BET and V total . In the MAZ group, it can be seen that CAT5 displayed higher conversion than the others, and this was mainly due to the high S BET and V total of Y5. Moreover, it can be seen that the conversion curve converged in the high reaction temperature for CAT6, CAT7 and CAT8; this indicated that the reaction barrier was overcome by the diffusion improvement [28].
Tetralin hydrocracking conversion over different catalysts is shown in Figure 7a,b. It can be seen that the conversion of tetralin obviously increased with the increase in temperature, and the conversion of the SAZ group was significantly higher than that of the MAZ group at the same temperature. This was mainly due to the high Si/Al ratio in the SAZ group, which had higher strong acid strength. Sato et al. [7] also found that a higher ratio of strong acid sites contributes to the improvement in the conversion depth of tetralin. In the SAZ group, the conversion of tetralin was mainly proportional to SBET and Vtotal. In the MAZ group, it can be seen that CAT5 displayed higher conversion than the others, and this was mainly due to the high SBET and Vtotal of Y5. Moreover, it can be seen that the conversion curve converged in the high reaction temperature for CAT6, CAT7 and CAT8; this indicated that the reaction barrier was overcome by the diffusion improvement [28].  value of the SAZ group was higher than that of the MAZ group. It can be seen that a higher temperature and more strong acid sites can facilitate the occurrence of the iso-cracking reaction. Strong acid strength is more conducive to the occurrence of the iso-cracking reaction pathway, and the SAZ group with a higher Si/Al ratio performed better. From Figure 8a, it is obvious that the − value of CAT4 was lower than that of the others, and the − values of CAT1, CAT2, and CAT3 were similar. This may have been related to pore size distribution. It can be seen from Figure 4 that Y1, Y2, and Y3 had concentrated pore size distributions which were all concentrated from 3.5 nm to 4.0 nm. The pore size distribution of Y4 was relatively dispersed, which was not conducive to the diffusion reactants. From Figure 8a, it can be seen that the  Figure 8 shows the relationship between Y iso−cracking and temperature over different catalysts. When the reaction temperature was 330 • C and 350 • C, the Y iso−cracking values of the SAZ and MAZ group were similar. When the reaction temperature rose to 370 • C and 390 • C, the Y iso−cracking value of the SAZ group was higher than that of the MAZ group. It can be seen that a higher temperature and more strong acid sites can facilitate the occurrence of the iso-cracking reaction. Strong acid strength is more conducive to the occurrence of the iso-cracking reaction pathway, and the SAZ group with a higher Si/Al ratio performed better. From Figure 8a, it is obvious that the Y iso−cracking value of CAT4 was lower than that of the others, and the Y iso−cracking values of CAT1, CAT2, and CAT3 were similar. This may have been related to pore size distribution. It can be seen from Figure 4 that Y1, Y2, and Y3 had concentrated pore size distributions which were all concentrated from 3.5 nm to 4.0 nm. The pore size distribution of Y4 was relatively dispersed, which was not conducive to the diffusion reactants. From Figure 8a, it can be seen that the Y iso−cracking values of CAT6, CAT7, and CAT8 were more similar. In terms of the pore distribution, Y7 and Y8 had similar pore concentration characteristics; their Y iso−cracking value was consistent at 390 • C. Figure 9 shows a comparison of the yields of BTEX in the SAZ group and MAZ group; it can be seen that the yields the two groups were similar. Generally, there are two main reasons for this. One reason is that in terms of acidity, ring-opening and sidechainbreaking reactions in the iso-cracking pathway do not require many strong acid sites, and more medium acid sites are favorable for the occurrence of these two ideal reactions. This is easily illustrated by the Si/Al ratio of the zeolites: the MAZ group had a lower Si/Al ratio and more moderate acid strength and B-acid content, which are very favorable characteristics for both reaction paths. The other reason is the effect of pore properties. From Table 4, it can be seen that the V total and V meso of the SAZ group were larger than those of the MAZ group, and the V micro of the MAZ group was larger than that of the SAZ group. Trends in the yields of BTEX in the SAZ group and MAZ group were similar to the yields of iso-cracking. However, the MAZ group could produce semblable yields of BTEX, which means that the presence of more micropores is beneficial to the ring-opening and sidechain-breaking reactions.
Catalysts 2022, 12, x FOR PEER REVIEW 9 of 14 − values of CAT6, CAT7, and CAT8 were more similar. In terms of the pore distribution, Y7 and Y8 had similar pore concentration characteristics; their − value was consistent at 390 °C.  Figure 9 shows a comparison of the yields of BTEX in the SAZ group and MAZ group; it can be seen that the yields the two groups were similar. Generally, there are two main reasons for this. One reason is that in terms of acidity, ring-opening and sidechainbreaking reactions in the iso-cracking pathway do not require many strong acid sites, and more medium acid sites are favorable for the occurrence of these two ideal reactions. This is easily illustrated by the Si/Al ratio of the zeolites: the MAZ group had a lower Si/Al ratio and more moderate acid strength and B-acid content, which are very favorable characteristics for both reaction paths. The other reason is the effect of pore properties. From Table 4, it can be seen that the Vtotal and Vmeso of the SAZ group were larger than those of the MAZ group, and the Vmicro of the MAZ group was larger than that of the SAZ group. Trends in the yields of BTEX in the SAZ group and MAZ group were similar to the yields of iso-cracking. However, the MAZ group could produce semblable yields of BTEX, which means that the presence of more micropores is beneficial to the ring-opening and sidechain-breaking reactions.   Figure 9 shows a comparison of the yields of BTEX in the SAZ group and MAZ group; it can be seen that the yields the two groups were similar. Generally, there are two main reasons for this. One reason is that in terms of acidity, ring-opening and sidechainbreaking reactions in the iso-cracking pathway do not require many strong acid sites, and more medium acid sites are favorable for the occurrence of these two ideal reactions. This is easily illustrated by the Si/Al ratio of the zeolites: the MAZ group had a lower Si/Al ratio and more moderate acid strength and B-acid content, which are very favorable characteristics for both reaction paths. The other reason is the effect of pore properties. From Table 4, it can be seen that the Vtotal and Vmeso of the SAZ group were larger than those of the MAZ group, and the Vmicro of the MAZ group was larger than that of the SAZ group. Trends in the yields of BTEX in the SAZ group and MAZ group were similar to the yields of iso-cracking. However, the MAZ group could produce semblable yields of BTEX, which means that the presence of more micropores is beneficial to the ring-opening and sidechain-breaking reactions. The hydro-cracking reaction pathway is an important component of tetralin conversion. Figure 10 shows Y hydro−cracking over different catalysts. The Y hydro−cracking of all the catalysts increased first and then decreased. This inflection point is mainly related to the thermodynamics of aromatics saturation [29]. It can be seen that the SAZ group had the highest Y hydro−cracking value at 370 • C, and most catalysts in the MAZ group had the highest Y hydro−cracking value at 350 • C. This shows that the acid properties of zeolite directly affect the hydro-cracking pathway performance of tetralin. Strong acid sites in zeolites help to delay the aromatics saturation point; therefore, a suitable Si/Al ratio is necessary to reduce the selectivity of the hydro-cracking reaction pathway of aromatics, and this feature can be effectively used to design the development of related catalysts.
The dehydrogenation reaction is the main source of heavy aromatics. From Figure 11, it can be seen that Y HEAVY is proportional to the reaction temperature. This is because the dehydrogenation reaction is endothermic, and raising the temperature favors dehydrogenation. Moreover, the Y HEAVY of the SAZ group was larger than that of the MAZ group. This is mainly because the SAZ group had a higher Si/Al ratio and higher acid strength, which provided suitable conditions for the dehydrogenation reaction. Gutiérrez et al. [30] investigated the role of acid sites in LCO hydrogenation and coking deactivation; it was found that strong acid sites are considered to be the main coking sites of LCO, and there is a linear relationship between the amount of coking and the number of strong acid sites. In the SAZ group, their V meso was larger than that of the MAZ group, and more mesoporous pores provided a reaction space for dehydrogenation. Y2 and Y4 had lower V micro /V meso than Y1 and Y3; this was consistent with the higher yield of CAT2 and CAT4.
Catalysts 2022, 12, x FOR PEER REVIEW 10 of 14 zeolites help to delay the aromatics saturation point; therefore, a suitable Si/Al ratio is necessary to reduce the selectivity of the hydro-cracking reaction pathway of aromatics, and this feature can be effectively used to design the development of related catalysts. The dehydrogenation reaction is the main source of heavy aromatics. From Figure  11, it can be seen that is proportional to the reaction temperature. This is because the dehydrogenation reaction is endothermic, and raising the temperature favors dehydrogenation. Moreover, the of the SAZ group was larger than that of the MAZ group. This is mainly because the SAZ group had a higher Si/Al ratio and higher acid strength, which provided suitable conditions for the dehydrogenation reaction. Gutiérrez et al. [30] investigated the role of acid sites in LCO hydrogenation and coking deactivation; it was found that strong acid sites are considered to be the main coking sites of LCO, and there is a linear relationship between the amount of coking and the number of strong acid sites. In the SAZ group, their Vmeso was larger than that of the MAZ group, and more mesoporous pores provided a reaction space for dehydrogenation. Y2 and Y4 had lower Vmicro/Vmeso than Y1 and Y3; this was consistent with the higher yield of CAT2 and CAT4. By comparing the conversion rate of tetralin and the yield of product, it was found that the size of the micropores and mesopores was the main influencing factor in zeolites with similar acid properties. The presence of more micropores in zeolites is favorable to the formation of BTX products, and the presence of more mesopores is favorable to the dehydrogenation reaction. Therefore, the relationship between microporous and mesoporous molecular sieves should be adjusted to a reasonable range.
In order to investigate the effect of the microporous and mesoporous relationship on reaction selectivity, the selectivity at the same conversion rate was calculated. Due to the high activity of the SAZ series, the selectivity at a 60% conversion rate was calculated and  The dehydrogenation reaction is the main source of heavy aromatics. From Figure  11, it can be seen that is proportional to the reaction temperature. This is because the dehydrogenation reaction is endothermic, and raising the temperature favors dehydrogenation. Moreover, the of the SAZ group was larger than that of the MAZ group. This is mainly because the SAZ group had a higher Si/Al ratio and higher acid strength, which provided suitable conditions for the dehydrogenation reaction. Gutiérrez et al. [30] investigated the role of acid sites in LCO hydrogenation and coking deactivation; it was found that strong acid sites are considered to be the main coking sites of LCO, and there is a linear relationship between the amount of coking and the number of strong acid sites. In the SAZ group, their Vmeso was larger than that of the MAZ group, and more mesoporous pores provided a reaction space for dehydrogenation. Y2 and Y4 had lower Vmicro/Vmeso than Y1 and Y3; this was consistent with the higher yield of CAT2 and CAT4. By comparing the conversion rate of tetralin and the yield of product, it was found that the size of the micropores and mesopores was the main influencing factor in zeolites with similar acid properties. The presence of more micropores in zeolites is favorable to the formation of BTX products, and the presence of more mesopores is favorable to the dehydrogenation reaction. Therefore, the relationship between microporous and mesoporous molecular sieves should be adjusted to a reasonable range.
In order to investigate the effect of the microporous and mesoporous relationship on reaction selectivity, the selectivity at the same conversion rate was calculated. Due to the high activity of the SAZ series, the selectivity at a 60% conversion rate was calculated and By comparing the conversion rate of tetralin and the yield of product, it was found that the size of the micropores and mesopores was the main influencing factor in zeolites with similar acid properties. The presence of more micropores in zeolites is favorable to the formation of BTX products, and the presence of more mesopores is favorable to the dehydrogenation reaction. Therefore, the relationship between microporous and mesoporous molecular sieves should be adjusted to a reasonable range.
In order to investigate the effect of the microporous and mesoporous relationship on reaction selectivity, the selectivity at the same conversion rate was calculated. Due to the high activity of the SAZ series, the selectivity at a 60% conversion rate was calculated and correlated with the pore properties. The conversion of the MAZ series was low even at a high reaction temperature, so 45% conversion was used as the benchmark; the results are shown in Figures 12 and 13.
From the results shown in Figures 12 and 13, it can be seen that although acid properties display a certain influence, the selectivity of the catalyst and the ratio of V micro /V meso still correspond well. With the increase in V micro /V meso , the dehydrogenation selectivity increased and the isomerization cracking selectivity decreased. The isomerization selectivity of the MAZ molecular sieve reached a constant value faster. The selectivity of ring-opening and selectivity of sidechain-breaking decreased almost synchronously. correlated with the pore properties. The conversion of the MAZ series was low even at a high reaction temperature, so 45% conversion was used as the benchmark; the results are shown in Figures 12 and 13.  From the results shown in Figures 12 and 13, it can be seen that although acid properties display a certain influence, the selectivity of the catalyst and the ratio of Vmicro/Vmeso still correspond well. With the increase in Vmicro/Vmeso, the dehydrogenation selectivity increased and the isomerization cracking selectivity decreased. The isomerization selectivity of the MAZ molecular sieve reached a constant value faster. The selectivity of ring-opening and selectivity of sidechain-breaking decreased almost synchronously.

Preparation of Catalysts
The catalysts were prepared according to the method of an industrial catalyst, except the type of zeolites, the composition of the support and the preparation method were consistent. The carrier was extruded in the proportion of Y zeolite (65 wt%) and alumina (35 correlated with the pore properties. The conversion of the MAZ series was low even at a high reaction temperature, so 45% conversion was used as the benchmark; the results are shown in Figures 12 and 13.  From the results shown in Figures 12 and 13, it can be seen that although acid properties display a certain influence, the selectivity of the catalyst and the ratio of Vmicro/Vmeso still correspond well. With the increase in Vmicro/Vmeso, the dehydrogenation selectivity increased and the isomerization cracking selectivity decreased. The isomerization selectivity of the MAZ molecular sieve reached a constant value faster. The selectivity of ring-opening and selectivity of sidechain-breaking decreased almost synchronously.

Preparation of Catalysts
The catalysts were prepared according to the method of an industrial catalyst, except the type of zeolites, the composition of the support and the preparation method were consistent. The carrier was extruded in the proportion of Y zeolite (65 wt%) and alumina (35

Preparation of Catalysts
The catalysts were prepared according to the method of an industrial catalyst, except the type of zeolites, the composition of the support and the preparation method were consistent. The carrier was extruded in the proportion of Y zeolite (65 wt%) and alumina (35 wt%); then, it was dried at 393 k for 180 min and calcinated at 823 K for 180 min. The metal components in the catalyst were NiMo bimetals which were loaded via impregnation; then, they were dried at 393 k for 180 min and calcinated at 723 K for 120 min. The corresponding catalysts prepared using the zeolites were labeled CAT1, CAT2, CAT3, CAT4, CAT5, CAT6, CAT7, and CAT8.
The composition of zeolites was quantified using X-ray fluorescence (XRF) conducted on a PANalytical AxiosMAX analyzer.
The acidity was determined via the IR spectroscopy of adsorbed pyridine (Py-IR) by using a Nicolet Magna-IR 560 ESP spectrophotometer (USA) with a resolution of 1 cm −1 . Purified pyridine gas flowed through the catalyst at room temperature for 40 min. The IR spectra were recorded and degassed at the temperatures of 250 • C and 350 • C, respectively. The amount of acid was calculated by the following formula [23]: C (B-sites) = 1.88 × IA(B) × R 2 /W C (L-sites) = 1.42 × IA(L) × R 2 /W in which C is the concentration (mmol/g catalyst); IA(B,L) is the integrated absorbance of the B or L band (cm −1 ); R is the radius of the catalyst disk (cm); and W is the weight of disk (mg). NH 3 -TPD analysis was conducted using a BEL-CAT instrument [14]. The morphology of the sulfide phase was characterized using a transmission electron microscope, Tecnai G 2 F20S-TWIN [15].
Prior to the TEM analysis, the oxide samples were sulfided. After sulfidation, the temperature was decreased to room temperature under a N 2 atmosphere. Then, sulfide catalysts were suspended in cycloheptane and deposited on a carbon-coated copper grid. The WS 2 slab size distribution was determined by measuring about 700-800 particles through a reference. The average particle length (L,nm) and stacking number (N) were calculated according to the first moment of the distribution; the calculation equations are shown as follows: where L i represents the length of a single MoS 2 wafer, N i represents the stacked layers of a single MoS 2 wafer, and n represents the number of MoS 2 wafers.

Hydrocracking Reaction
The tests were conducted in a continuously flowing tubular fixed-bed reactor loaded with 1.0 g of catalyst (~0.25 mm) diluted with the same quantity of quartz particles. Before the reaction, the catalyst was treated with sulfidation feedstock (5 wt% CS 2 /cyclohexane solution) at 0.4 mL/min in the presence of 360 mL/min of hydrogen at 4.0 MPa of total pressure. Sulfidation was carried out at 633 K for 2 h, and then the feed was switched to tetralin. The reaction temperature varied from 603 K to 663 K, and the liquid flow rate was 0.1 mL/min. The reaction products were analyzed on an Agilent 6890 GC using a PONA column (50 m × 0.2 mm × 0.5 µm) and an FID detector.

Conclusions
The Si/Al ratio is the core property of zeolites, and the acid properties of the zeolites is based on this ratio. Therefore, two groups of molecular sieves were selected from multiple samples. The characterization results showed that in each group of zeolites, the Si/Al ratios were relatively similar, and the acid amounts characterized by NH 3 -TPD were similar, while the distribution of different types acid sites characterized by Py-IR were different; meanwhile, their pore properties varied greatly. The sizes of the metal particles in the sulfidation catalysts changed slightly. The evaluation results from tetralin hydrocracking showed that increasing the proportion of V micro /V meso in zeolites was conducive for the production of BTEX. The hydrogenation-cracking reaction is controlled by aromatic saturation thermodynamics, and strong acidity is beneficial to the backward shift of equilibrium temperature.