High Active Zn / Mg-Modified Ni – P / Al 2 O 3 Catalysts Derived from ZnMgNiAl Layered Double Hydroxides for Hydrodesulfurization of Dibenzothiophene

A series of ZnMgNiAl layered double hydroxides (LDHs) containing 20 wt.% Ni and different Zn/Mg molar ratios were prepared by a coprecipitation method, and then were introduced with H2PO4 via a microwave-hydrothermal method. With the resulting mixtures as the precursors, Zn/Mg-modified ZnMgNi–P/Al2O3 catalysts were prepared. The Zn/Mg molar ratio affected the formation of Ni2P and Ni12P5 in nickel phosphides. The ZnMgNi–P/Al2O3 catalyst with a Zn/Mg molar ratio of 3:1 exhibits the best dibenzothiophene hydrodesulfurization (HDS) activity. Compared with the Ni–P/Al2O3 catalyst prepared from the impregnation method, the ZnMgNi–P/Al2O3 catalyst shows a higher HDS activity (81.6% vs. 54.3%) and promotes the direct desulfurization of dibenzothiophene.


Introduction
Sulfur removal has gained growing attention since the stringent fuel standards have been enacted throughout the world [1,2].Among the existing sulfur removal methods, catalytic hydrodesulfurization (HDS) is a very efficient way to eliminate sulfur from fuel oil [3,4].However, the existing commercial HDS catalysts fail to meet the regulated levels [5].Studies on the hydrotreation properties of metal phosphides show that nickel phosphide is the most promising candidate for the preparation of next-generation HDS catalysts [6][7][8].
Layered double hydroxides (LDHs, M 2+  1−x M 3+ x (OH) 2 ) are lamellarly-mixed hydroxides and a class of anionic clays having a hydrocalcite-like structure, which consist of positively charged mixed metal hydroxide layers and negatively charged interlayer anions [9].LDHs have been demonstrated as effective precursors for the preparation of nickel phosphide catalysts.For instance, a Ni 2 P/Al 2 O 3 catalyst prepared from LDHs shows higher HDS activity than that prepared from the impregnation method, but the Ni loading up to 64.4 wt.% is unfavorable for the dispersion of active components [10].
The addition of other metals to LDH-derived catalysts contributes to HDS.For instance, the incorporation of Zn and Mg improved the catalytic activities for HDS.Chen et al. [11] found that Zn-doped NiAlMoW catalyst prepared from a NiZnAl-layered hydroxide precursor could improve 4,6-dimethyl dibenzothiophene HDS activity.The higher HDS activity is attributed to the promoter effect of Zn, since Zn decreases the interaction between alumina and active components (Ni, Mo, and W) that forms Ni(Zn)-Mo(W)-S active species.CoMgMoAl catalysts drived from CoMgAl-terephthalate LDHs both enhanced the thiophene HDS and cyclohexene hydrogenation activities along with the increasing Mg content [12].Nevertheless, research on development of a Zn/Mg-modified layered precursor for HDS is rare.
In the present work, aseries of Zn x Mg 1−x Ni-P/Al 2 O 3 (x is the Zn/(Zn+Mg) molar fraction) catalysts were prepared by using NH 4 H 2 PO 4 as the phosphorous precursor and Zn x Mg 1−x NiAl LDHs as the nickel precursor.In addition, the effects of Zn/Mg molar ratio on the structure and HDS performance of Zn x Mg 1−x Ni-P/Al 2 O 3 catalyst were investigated.
Catalysts 2017, 7, 202 2 of 9 and W) that forms Ni(Zn)-Mo(W)-S active species.CoMgMoAl catalysts drived from CoMgAl-terephthalate LDHs both enhanced the thiophene HDS and cyclohexene hydrogenation activities along with the increasing Mg content [12].Nevertheless, research on development of a Zn/Mg-modified layered precursor for HDS is rare.
In the present work, aseries of ZnxMg1-xNi-P/Al2O3 (x is the Zn/(Zn+Mg) molar fraction) catalysts were prepared by using NH4H2PO4 as the phosphorous precursor and ZnxMg1-xNiAl LDHs as the nickel precursor.In addition, the effects of Zn/Mg molar ratio on the structure and HDS performance of ZnxMg1-xNi-P/Al2O3 catalyst were investigated.

Characterization of Catalysts
Figure 1 shows the X-ray diffraction (XRD) patterns of ZnxMg1−xNiAl LDHs with different Zn/Mg molar ratios.Clearly, all the ZnxMg1−xNiAl LDHs show typical XRD patterns of LDHs, including a high intensity peak (003) at 2θ = 11.4-11.7°,two weak peaks (006) and (009) at 2θ = 22.9-23.2°and 34.6-34.7°,respectively, and two smaller peaks (110) and (113) of transition metal oxides at 2θ = 60-63°.These results confirm the successful preparation of ZnxMg1-xNiAl LDHs [13].No peaks of impurities were discerned, which indicates the high purity of the products.The structural parameters of ZnxMg1-xNiAl LDHs are listed in Table 1.The lattice parameters a and c are both almost the same among different ZnxMg1-xNiAl LDHs, despite the different Zn/Mg molar ratios.Then, the crystallite sizes at the a-and c-directions were calculated by the Scherrer formula based on the (110) and (003) reflections, respectively.It was found Zn0.75Mg0.25NiAlLDHs had much smaller crystallite size than the other catalysts.The XRD patterns of ZnxMg1-xNi-P/Al2O3 are shown in Figure 2. Clearly, ZnNi-P/Al2O3 shows the peaks at 2θ = 40.7°,44.6°, 47.3°, and 54.1° attributed to Ni2P, and the typical peaks of AlPO4, The structural parameters of Zn x Mg 1−x NiAl LDHs are listed in Table 1.The lattice parameters a and c are both almost the same among different Zn x Mg 1−x NiAl LDHs, despite the different Zn/Mg molar ratios.Then, the crystallite sizes at the aand c-directions were calculated by the Scherrer formula based on the (110) and (003) reflections, respectively.It was found Zn 0.75 Mg 0.25 NiAl LDHs had much smaller crystallite size than the other catalysts.extural characteristics of typical catalysts are listed in Table 2. Clearly, all catalysts almost h me pore volume.The Zn0.75Mg0.25Ni-P/Al2O3has significantly higher specific surface area, cantly smaller pore diameter and narrower pore size distribution (mainly concentrated in ompared with ZnNi-P/Al2O3 and MgNi-P/Al2O3 (Figure 3a).Furthermore, MgNi-P/A s typical IV N2 adsorption isotherms with obvious hysteresis loops at relative press en 0.43 and 0.95 (Figure 3b), which confirms the presence of mesopores.Zn0.75Mg0.25Ni-P/AnNi-P/Al2O3, however, show typical II N2 adsorption isotherms.Textural characteristics of typical catalysts are listed in Table 2. Clearly, all catalysts almost have the same pore volume.The Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 has significantly higher specific surface area, but significantly smaller pore diameter and narrower pore size distribution (mainly concentrated in 2.3 nm) compared with ZnNi-P/Al 2 O 3 and MgNi-P/Al 2 O 3 (Figure 3a).Furthermore, MgNi-P/Al 2 O 3 showes typical IV N 2 adsorption isotherms with obvious hysteresis loops at relative pressures between 0.43 and 0.95 (Figure 3b), which confirms the presence of mesopores.Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 and ZnNi-P/Al 2 O 3 , however, show typical II N 2 adsorption isotherms.Figure 4 shows the X-ray photoelectron spectroscopy (XPS) spectra of Zn x Mg 1−x Ni-P/Al 2 O 3 , and the corresponding binding energies and surface composition are listed in Table 3.For Zn x Mg 1−x Ni-P/Al 2 O 3 , the peaks at 852.2-853.7 and 129.2-129.6 eV are assigned to Ni δ+ (0 < δ < 2) and P δ− (0 < δ < 1) [14] (Figure 4a), respectively.The Ni δ+ has higher binding energy than elemental Ni (852.5-852.9eV), but lower than NiO (853.5-854.1 eV), indicating that the Ni in Ni 2 P bears partial positive charge.The binding energy of P δ− is below the reported value of elemental P (Figure 4b).
In nickel phosphides, because of a covalent bond between the Ni and P atoms and a charge transfer from Ni to P, the electron-deficient Ni formed.Moreover, the peaks at 856.1-857.2and 134.0-134.5 eV are assigned to Ni 2+ and P 5+ [7,15], respectively.In addition, a broad shake-up peak appears at the binding energy of ~5.0 eV, which is higher than that of Ni 2+ [16,17].These peaks can be assigned to its satellite peaks, although they are located close to those of Ni 3+ and nickel oxysulfide [18,19].Moreover, the other broad peaks at higher binding energy are ascribed to the Ni 2p of nickel oxide [20].
showes typical IV N2 adsorption isotherms with obvious hysteresis loops at relative pressures between 0.43 and 0.95 (Figure 3b), which confirms the presence of mesopores.Zn0.75Mg0.25Ni-P/Al2O3and ZnNi-P/Al2O3, however, show typical II N2 adsorption isotherms.Figure 4 shows the X-ray photoelectron spectroscopy (XPS) spectra of ZnxMg1−xNi-P/Al2O3, and the corresponding binding energies and surface composition are listed in Table 3.For ZnxMg1-xNi-P/Al2O3, the peaks at 852.2-853.7 and 129.2-129.6 eV are assigned to Ni δ+ (0 < δ < 2) and P δ− (0 < δ < 1) [14] (Figure 4a), respectively.The Ni δ+ has higher binding energy than elemental Ni (852.5-852.9eV), but lower than NiO (853.5-854.1 eV), indicating that the Ni in Ni2P bears partial positive charge.The binding energy of P δ− is below the reported value of elemental P (Figure 4b).In nickel phosphides, because of a covalent bond between the Ni and P atoms and a charge transfer from Ni to P, the electron-deficient Ni formed.Moreover, the peaks at 856.1-857.2and 134.0-134.5 eV are assigned to Ni 2+ and P 5+ [7,15], respectively.In addition, a broad shake-up peak appears at the binding energy of ~5.0 eV, which is higher than that of Ni 2+ [16,17].These peaks can be assigned to its satellite peaks, although they are located close to those of Ni 3+ and nickel oxysulfide [18,19].Moreover, the other broad peaks at higher binding energy are ascribed to the Ni 2p of nickel oxide [20].For Zn0.75Mg0.25Ni-P/Al2O3, the interaction between the Ni2P particles and the support leads to decrease of the binding energy of Ni δ+ (853.1 eV) [10,21].As reported, the hydrogenation ability of the Ni site reduces with the decrease of electron density [22,23].For MgNi-P/Al2O3, the binding energy of Ni δ+ in Ni12P5 phase declines further (852.2eV).Compared with Zn0.75Mg0.25Ni-P/Al2O3, the binding energy of Ni δ+ in MgNi-P/Al2O3 is further reduced, indicating that less electron density is transferred from Ni to P in Ni12P5 compared with Ni2P.Sawhill et al. [15] also reported that the Ni in Ni12P5 has a higher electronic density than Ni2P.
The superficial atomic ratios of the catalysts were determined by XPS and the results are listed in Table 3.As shown from these results, the P/Ni molar ratio determined from the surface composition is far larger than the stoichiometric ratio of Ni2P or Ni12P5, which confirms the occurrence of surface P enrichment in the catalysts.Nonetheless, the Ni δ+ /ΣNi ratio is lower than 1 for all the catalysts, which indicates the presence of a large proportion of nickel oxide.The phosphorus on the catalyst surfaces essentially exists as PO4 3− .In addition, the P/Ni molar ratio is the lowest in Zn0.75Mg0.25Ni-P/Al2O3,which indicates that more Ni sites on the catalyst surface are exposed under the same Ni loading.For Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 , the interaction between the Ni 2 P particles and the support leads to decrease of the binding energy of Ni δ+ (853.1 eV) [10,21].As reported, the hydrogenation ability of the Ni site reduces with the decrease of electron density [22,23].For MgNi-P/Al 2 O 3 , the binding energy of Ni δ+ in Ni 12 P 5 phase declines further (852.2eV).Compared with Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 , the binding energy of Ni δ+ in MgNi-P/Al 2 O 3 is further reduced, indicating that less electron density is transferred from Ni to P in Ni 12 P 5 compared with Ni 2 P. Sawhill et al. [15] also reported that the Ni in Ni 12 P 5 has a higher electronic density than Ni 2 P.
The superficial atomic ratios of the catalysts were determined by XPS and the results are listed in Table 3.As shown from these results, the P/Ni molar ratio determined from the surface composition is far larger than the stoichiometric ratio of Ni 2 P or Ni 12 P 5 , which confirms the occurrence of surface P enrichment in the catalysts.Nonetheless, the Ni δ+ /ΣNi ratio is lower than 1 for all the catalysts, which indicates the presence of a large proportion of nickel oxide.The phosphorus on the catalyst surfaces essentially exists as PO

Catalytic Activity
Figure 6 shows the HDS of dibenzothiophene (DBT) over ZnxMg1-xNi-P/Al2O3 at varying temperatures.Clearly, the DBT conversion increases with the increase in temperature for all the catalysts (Figure 6a).The DBT conversion is promoted slowly with a further temperature rise above 613 K.Moreover, with the increase of Mg content, the DBT conversion over ZnxMg1-xNi-P/Al2O3 first increases and then decreases compared with ZnNi-P/Al2O3.At the reaction temperature of 653 K, and at the Zn/Mg molar ratio of 3:1 (i.e., x = 0.75), the DBT conversion over Zn0.75Mg0.25Ni-P/Al2O3 is maximized to 81.6%, and under the same conditions the conversion rates of ZnNi-P/Al2O3 and MgNi-P/Al2O3 are 70.6% and 54.6%, respectively.From the perspective of active phase composition, for ZnxMg1−xNi-P/Al2O3, XRD shows the nickel phosphide exists as Ni2P at x ≥ 0.75; the composition at x = 0.5 is mainly Ni2P and a small amount of Ni12P5; the composition at x = 0.25 is mainly Ni12P5 and a small amount of Ni2P; at x = 0, only Ni12P5 exists (Figure 2).For nickel phosphides, Ni2P shows much higher hydrogenation activity than Ni12P5 [6].In addition, the HDS reaction mainly occurs on metal sites.From the perspectives of active phase composition/distribution and active particle size, XPS shows that more nickel sites are exposed on the surfaces of Zn0.75Mg0.25Ni-P/Al2O3compared with ZnNi-P/Al2O3 (Table 3).The particle sizes of Ni2P in Zn0.75Mg0.25Ni-P/Al2O3and Zn0.5Mg0.5Ni-P/Al2O3calculated by the Scherrer formula are 29.4 and 34.8 nm, respectively.The smaller sizes of active Ni2P particles could promote the dispersion of the active Ni2P phase as well as

Catalytic Activity
Figure 6 shows the HDS of dibenzothiophene (DBT) over Zn x Mg 1−x Ni-P/Al 2 O 3 at varying temperatures.Clearly, the DBT conversion increases with the increase in temperature for all the catalysts (Figure 6a).The DBT conversion is promoted slowly with a further temperature rise above 613 K.Moreover, with the increase of Mg content, the DBT conversion over Zn x Mg 1−x Ni-P/Al 2 O 3 first increases and then decreases compared with ZnNi-P/Al 2 O 3 .At the reaction temperature of 653 K, and at the Zn/Mg molar ratio of 3:1 (i.e., x = 0.75), the DBT conversion over Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 is maximized to 81.6%, and under the same conditions the conversion rates of ZnNi-P/Al 2 O 3 and MgNi-P/Al 2 O 3 are 70.6% and 54.6%, respectively.From the perspective of active phase composition, for Zn x Mg 1−x Ni-P/Al 2 O 3 , XRD shows the nickel phosphide exists as Ni 2 P at x ≥ 0.75; the composition at x = 0.5 is mainly Ni 2 P and a small amount of Ni 12 P 5 ; the composition at x = 0.25 is mainly Ni 12 P 5 and a small amount of Ni 2 P; at x = 0, only Ni 12 P 5 exists (Figure 2).For nickel phosphides, Ni 2 P shows much higher hydrogenation activity than Ni 12 P 5 [6].In addition, the HDS reaction mainly occurs on metal sites.From the perspectives of active phase composition/distribution and active particle size, XPS shows that more nickel sites are exposed on the surfaces of Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 compared with ZnNi-P/Al 2 O 3 (Table 3).The particle sizes of Ni 2 P in Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 and Zn 0.5 Mg 0.5 Ni-P/Al 2 O 3 calculated by the Scherrer formula are 29.4 and 34.8 nm, respectively.The smaller sizes of active Ni 2 P particles could promote the dispersion of the active Ni 2 P phase as well as the active specific surface area.Furthermore, compared with ZnNi-P/Al 2 O 3 and MgNi-P/Al 2 O 3 modified by a single metal, the modification by double metals (Zn+Mg) could reduce the mesoporous size and increase the micropore amount of Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 .much higher hydrogenation activity than Ni12P5 [6].In addition, the HDS reaction mainly occurs on metal sites.From the perspectives of active phase composition/distribution and active particle size, XPS shows that more nickel sites are exposed on the surfaces of Zn0.75Mg0.25Ni-P/Al2O3compared with ZnNi-P/Al2O3 (Table 3).The particle sizes of Ni2P in Zn0.75Mg0.25Ni-P/Al2O3and Zn0.5Mg0.5Ni-P/Al2O3calculated by the Scherrer formula are 29.4 and 34.8 nm, respectively.The smaller sizes of active Ni2P particles could promote the dispersion of the active Ni2P phase as well as the active specific surface area.Furthermore, compared with ZnNi-P/Al2O3 and MgNi-P/Al2O3 modified by a single metal, the modification by double metals (Zn+Mg) could reduce the mesoporous size and increase the micropore amount of Zn0.75Mg0.25Ni-P/Al2O3.The reaction scheme for the HDS of DBT is presented in Scheme 1.The HDS of DBT occurs via direct desulfurization (DDS) and hydrogenation (HYD), which mainly form biphenyl (BP) and cyclohexylbenzene (CHB), respectively.The BP formed during DDS would undergo slow The reaction scheme for the HDS of DBT is presented in Scheme 1.The HDS of DBT occurs via direct desulfurization (DDS) and hydrogenation (HYD), which mainly form biphenyl (BP) and cyclohexylbenzene (CHB), respectively.The BP formed during DDS would undergo slow hydrogenation to form CHB, while the intermediates tetrahydrodibenzothiophene (THDBT) and hexahydrodibenzothiophene (HHDBT) formed during HYD were hydrogenated to CHB, which was further hydrogenation to bicyclohexyl (BCH).It was found that the HDS products of DBT on Zn x Mg 1−x Ni-P/Al 2 O 3 are only BP and CHB.Similar product distributions have been reported [24][25][26].As showed in Figure 6b, the selectivity of BP increases and that of CHB decreases with the increase in temperature for all catalysts.The proportions of BP are larger over all the samples, indicating that DBT is mainly desulfurized via the DDS pathway.This conclusion agrees with Song et al. [10]  hydrogenation to form CHB, while the intermediates tetrahydrodibenzothiophene (THDBT) and hexahydrodibenzothiophene (HHDBT) formed during HYD were hydrogenated to CHB, which was further hydrogenation to bicyclohexyl (BCH).It was found that the HDS products of DBT on ZnxMg1-xNi-P/Al2O3 are only BP and CHB.Similar product distributions have been reported [24][25][26].
As showed in Figure 6b, the selectivity of BP increases and that of CHB decreases with the increase in temperature for all catalysts.The proportions of BP are larger over all the samples, indicating that DBT is mainly desulfurized via the DDS pathway.This conclusion agrees with Song et al. [10] who prepared Ni2P/Al2O3 catalysts from Ni-Al-CO3 2− LDHs.Interestingly, the selectivity of BP over Ni12P5 is higher than that of Ni2P for ZnxMg1-xNi-P/Al2O3, which indicates that Ni12P5 is more active in DDS than Ni2P.For comparison, the HDS of DBT over different catalysts is shown in Figure 7.The HDS activities of the catalysts change in the order of Zn0.75Mg0.25Ni-P/Al2O3> Ni-P/Al2O3 > Ni-P/ZnO > Ni-P/MgO.Compared with the widely-studied Ni-P/Al2O3, Zn0.75Mg0.25Ni-P/Al2O3enhances not only HDS activity (from 54.3% to 81.6%), but also the selectivity of BP (from 54.6% to 87.8%) and the DDS pathway.XRD shows that the active phase is Ni2P and not Ni12P5 for both Zn0.75Mg0.25Ni-P/Al2O3and Ni-P/Al2O3 (Figure 2).As reported, Ni2P has two types of sites, including For comparison, the HDS of DBT over different catalysts is shown in Figure7.The HDS activities of the catalysts change in the order of Zn0.75Mg0.25Ni-P/Al2O3> Ni-P/Al2O3 > Ni-P/ZnO > Ni-P/MgO.Compared with the widely-studied Ni-P/Al2O3, Zn0.75Mg0.25Ni-P/Al2O3enhances not only HDS activity (from 54.3% to 81.6%), but also the selectivity of BP (from 54.6% to 87.8%) and the DDS pathway.XRD shows that the active phase is Ni2P and not Ni12P5 for both Zn0.75Mg0.25Ni-P/Al2O3and Ni-P/Al2O3 (Figure 2).As reported, Ni2P has two types of sites, including tetrahedral Ni(1) sites and square pyramidal Ni(2) sites, which are responsible for HDS by the DDS route and desulfurization by the HYD route, respectively [27].Therefore, Zn0.75Mg0.25Ni-P/Al2O3prepared from LDHs enhances the Ni(1) sites compared with Ni-P/Al2O3.The HDS activity of Ni-P/MgO is the lowest, because the preferential reactions between P and Mg inhibits the formation of Ni2P and Ni12P5.

Catalyst Preparation
A series of Zn x Mg 1−x NiAl LDHs with M 2+ /M 3+ molar ratio of 3:1 and different Zn/Mg molar ratios were prepared from coprecipitation under ambient atmosphere.Each time, a mixed solution of Mg(NO 3 ) 2 •6H 2 O, Zn(NO 3 ) 2 •2H 2 O, Ni(NO 3 ) 2 •6H 2 O, and Al(NO 3 ) 3 •9H 2 O was adjusted to pH 10 by adding a NaOH and Na 2 CO 3 aqueous solution dropwise under stirring.The resulting suspension was aged at 333 K for 6 h.The precipitate was filtered and washed several times with deionized water.Then, Zn x Mg 1−x Ni-P/Al 2 O 3 catalysts were prepared by a microwave-hydrothermal treatment and temperature programmed reduction, as described in our previous work [10].Typically, Zn x Mg 1−x NiAl LDHs were impregnated with an ammonium dihydrogenphosphate solution with a Ni/P molar ratio of 1:2 and treated using a microwave-hydrothermal method for 20 min at 363 K under reflux.After drying at 393 K for 12 h, the resulting materials were pressed into discs, crushed, and sieved to particles with 16-30 meshes.After calcination at 773 K for 3 h, the materials were reduced in a H 2 flow (200 mL/min) while the temperature rose to 973 K at a rate of 2 K/min and was then maintained at 973 K for 2 h.Then, the materials were cooled to room temperature in a H 2 flow, and passivated in a 20 mL/min O 2 /N 2 flow (0.5 vol.% O 2 ).

Catalyst Characterization
XRD patterns were measured on a Rigaku D/max-2200 X-ray diffractometer operated at 40 kV and 40 mA using Cu Kα radiation.The textural properties of the catalysts were analyzed by the Brunauer-Emmett-Teller (BET) method using a Tristar II3020 surface area and porosity analyzer.XPS spectra were acquired with a K-Alpha electron spectrometer (Thermofisher Scientific Company, Waltham, MA, USA) using an Al Kα radiation source (1486.6 eV).The binding energy was calibrated by setting the C1s transition at 284.8 eV.

Catalytic Hydrogenation Activity Test
The HDS reaction of DBT was performed on a fixed-bed reactor [10].Prior to the reaction, 0.65 g of a passivated catalyst was activated in H 2 (40 mL/min) at 773 K for 2 h.After activation, the hydrotreation reaction was carried out at 553 K, 3.0 MPa.The liquid reactant, which consisted of a decalin solution of DBT (1 wt.%), was pumped into the reactor.The weight hourly space velocities (WHSV) and H 2 /oil ratio (V/V) were 2.0 h −1 and 500, respectively.Liquid product compositions of the samples collected at a 2-h interval were determined on a GC-14C gas chromatograph equipped with a SE-30 capillary column.

Conclusions
Zn/Mg-modified Zn x Mg 1−x NiAl LDHs with different Zn/Mg molar ratios were prepared.Briefly, H 2 PO 4 2− was introduced by the microwave-hydrothermal method, and Zn x Mg 1−x Ni-P/Al 2 O 3 catalysts were prepared with the mixture as the precursor.In the precursors, under the hydrogenation atmosphere and during temperature programmed reduction, both Zn and Mg reacted with phosphorus substances, which impacted the formation of nickel phosphates.The nickel phosphate mainly existed in the form of Ni 2 P at x ≥ 0.5, but Ni 12 P 5 at x < 0.5.Compared with the single-metal-modified catalysts (Zn or Mg), the Zn/Mg-modified (Zn/Mg molar ratio of 3:1) Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 effectively reduced the pore sizes of catalysts, increased the pore counts, and had the smallest Ni 2 P particles.The catalyst prepared from this method showed the highest dibenzothiophene hydrodesulfurization activity and promoted the direct desulfurization of dibenzothiophene.

Figure 5 .
Figure 5.The XRD patterns of different nickel phosphate catalysts.
who prepared Ni 2 P/Al 2 O 3 catalysts from Ni-Al-CO 3 2− LDHs.Interestingly, the selectivity of BP over Ni 12 P 5 is higher than that of Ni 2 P for Zn x Mg 1−x Ni-P/Al 2 O 3 , which indicates that Ni 12 P 5 is more active in DDS than Ni 2 P. Catalysts 2017, 7, 202 6 of 9

2 S 1 .
Scheme 1. Simplified reaction pathways for the HDS of DBT.

Scheme 1 .
Scheme 1. Simplified reaction pathways for the HDS of DBT.

Table 3 .
Spectral parameters obtained by X-ray photoelectron spectroscopy (XPS) analysis.

Table 3 .
Spectral parameters obtained by X-ray photoelectron spectroscopy (XPS) analysis.
4 3− .In addition, the P/Ni molar ratio is the lowest in Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 , which indicates that more Ni sites on the catalyst surface are exposed under the same Ni loading.XRD of Zn x Mg 1−x Ni-P/Al 2 O 3 (Figure 2) shows that Ni, Zn, and Mg all could react with P, thereby affecting the formation of Ni 2 P and Ni 12 P 5 .Thus, how the single loading of ZnO or MgO would affect the formation of Ni 2 P and Ni 12 P 5 was further studied.The XRD patterns of different nickel phosphate catalysts are shown in Figure 5.For Ni-P/Al 2 O 3 , Ni and P only exist as Ni 2 P, without any other phase, indicating that the active phase is Ni 2 P. For Zn 0.75 Mg 0.25 Ni-P/Al 2 O 3 and Ni-P/ZnO, Ni and P mainly exist as Ni 2 P, accompanied by a small amount of AlPO 4 and Zn 3 (PO 4 ) 2 , respectively.For Ni-P/MgO, Ni, P and Mg exist as MgNiO 2 and Mg 3 (PO 4 ) 2 .
Figure 5.The XRD patterns of different nickel phosphate catalysts.