Effect of Ti on dibenzothiophene hydrodesulfurization performance over bulk Ni 2 P

Abstract

A series of Ti-incorporated bulk Ni₂P catalysts was prepared by means of temperature-programmed reduction, and the role of metallic Ti on the structure and catalytic activity of the Ni₂P catalysts was studied. For this purpose, bulk Ni₂P catalysts with metal Ti contents of 0.005 wt%, 0.01 wt%, and 0.02 wt% were synthesized. X-ray diffraction, CO uptake, Brunauer–Emmett–Teller measurements, and X-ray photoelectron spectroscopy were utilized to characterize the catalysts. Addition of titanium could increase the surface area and promote the formation of small, highly dispersed Ni₂P particles. The Ti_0.02-Ni₂P system with a Ti molar fraction of 0.02 showed the highest hydrodesulfurization activity of 99.6%, which was an increase of 44% compared with that found for the bulk Ni₂P.

Keywords

hydrodesulfurization dibenzothiophene Ti

Introduction

Hydrodesulfurization (HDS) is a catalytic chemical process most widely used, among all the sulfur removal methods, to remove sulfur atoms from sulfur-containing molecules present in oil feedstocks.¹ It has been realized that current commercial HDS catalysts are insufficient to meet the required standards,^2,3 and according to the restrictions imposed by SO_x regulations and the continuing decline in the quality of petroleum raw materials, HDS has become one of the most important processes in the refining industry.⁴

Recently, the transition metal phosphides, as a new class of materials, have attracted wide attention as a new generation of HDS catalysts for their high HDS activity for fuels^5,6 and high thermal stabilities. The observed activity levels decrease as follows: Ni₂P > WP > MoP > CoP > Fe₂P. Accordingly, Ni₂P has aroused particular attention due to its high activity in the HDS of dibenzothiophene (DBT).⁶

In order to improve the catalytic performance, a third component has been added to the Ni₂P catalyst.⁷ Our previous research showed that yttrium,⁸ zinc, neodymium,⁹ and calcium as elements can increase the surface area of the catalyst, resulting in smaller crystal size and better dispersion of active catalytic particles. It was found that the HDS activity of co-precipitation Ni₂P catalysts can be enhanced by the addition of Ti. Silva-Rodrigo et al.¹⁰ reported that NiMo/MCM-41 catalysts incorporated with Ti display high activity in HDS. Ti species can enhance the thiophene conversion; moreover, it can improve its stability under hydrothermal conditions. Consequently, it is of great interest to investigate the influence of the Ti content on the performance of Ni₂P catalysts for the HDS of refractory sulfur compounds such as DBT. Ti was completely added to the catalyst skeleton to form weak Lewis acid sites, producing a larger pore, mesoporous, structure. The changes to the framework were significantly advantageous in enhancing the catalytic properties of catalysts for transesterification of fuels.¹¹ The increase in Ti content leads to a rise in the amount of framework Ti sites and can thus modify the acidity of the catalysts.

In the present work, a series of Ti-incorporated Ni₂P catalysts was prepared by the temperature-programmed reduction (TPR) method, and the effect of Ti content on the HDS performance of bulk Ni₂P was studied using DBT as the model compound.

Experimental

Preparation of catalysts

The Ni₂P catalysts were prepared by the co-precipitation method according to Song and co-workers^8,9 Briefly, the oxidic precursor of bulk Ni₂P was prepared with excess phosphorus, the mole ratio of nickel and phosphorus being 1.¹² Redundant phosphorus was used to effect the loss of phosphorus during the reduction stage. In a typical preparation, 2.52 g Ni(NO₃)₂·6H₂O and 1.77 g (NH₄)₂HPO₄ were dissolved in deionized water. The mixture was stirred moderately while the water was evaporated to yield a solid product. The resulting solid was heated at 120°C for 12 h and calcined at 500°C for 3 h to obtain the oxidic precursor. The oxidic precursor is denoted as PNi₂P. The different dosages of Ti(OC₄H₉)₄ were dissolved in ethylene glycol (the concentrations being 0.000424, 0.000085, and 0.00017 mol L⁻¹, respectively) and then were impregnated into the PNi₂P to form solid product. Then, the precursors were reduced and passivated using the method described elsewhere.⁹ The corresponding catalysts are denoted as Ti_x-Ni₂P, where the subscript x is the molar fraction of Ti with respect to Ni₂P and has values of 0.005, 0.01 and 0.02. For comparison, a bulk Ni₂P catalyst was prepared following the method described above.

Characterization of catalysts

X-ray diffraction (XRD) was used to analyze the catalyst samples, and analysis was carried out on a D/Max 2200 PC X-ray diffractometer and needs manufacturer using Cu Kα radiation under setting conditions of 40 kV, 30 mA and a scanning range from 10 to 80° at a rate of 10°min^-1.

The typical physicochemical properties of the catalysts were analyzed by the Brunauer–Emmett–Teller (BET) method using Micromeritics adsorption equipment model NOVA 2000e. All samples were outgassed at 473 K until the vacuum pressure was 6 mm Hg. The adsorption isotherms for nitrogen were measured at 77 K.

The X-ray photoelectron spectroscopy (XPS) spectra were acquired using an ESCALAB MKII spectrometer under vacuum. XPS measurements were performed using monochromatic Mg Kα radiation (E = 1253.6 eV) and the spectrometer was equipped with a hemispherical analyzer operating at a fixed pass energy of 40 eV. The recorded photoelectron binding energies were referenced against the C 1s contamination line at 284.8 eV.

Catalytic activity test

The HDS of DBT was measured using a feed consisting of a decalin solution of DBT (1 wt%) in a flowing fixed-bed reactor at high pressure. The conditions of the HDS reaction were 613 K, 3.0 MPa, weight hourly space velocity (WHSV) = 1.5 h⁻¹, and a hydrogen/oil ratio of 700 (V/V). The catalyst was pressed into small pieces, crushed, then and sieved to 30–60 mesh. In the reaction, sampling of liquid products was started 2 h after the steady reaction conditions had been achieved. Liquid samples were collected every hour and a flame ionization detector gas chromatograph with a GC-14C–60 column was used to analyze the samples. According to the ratio of converted DBT to initial DBT, the total conversion was calculated. The turnover frequency (TOF) values of the samples containing nickel phosphide were calculated using equation (1)¹³

TOF = \frac{F}{W} \frac{X}{M}

(1)

where F is the molar rate of DBT fed into the fixed-bed reactor (mol s⁻¹), W is the weight of catalyst (g), X is the conversion of DBT (%), and M is the number of moles of sites loaded on the catalyst which was determined by the CO uptake.

Results and discussion

XRD

XRD patterns of samples with different Ti loadings are shown in Figure 1. For all the samples, the peaks at 2θ = 40.7, 44.6, 47.3, 54.1, and 54.9° are assigned to the Ni₂P phase. For all samples, what can be inferred is that the active phase formed was mainly Ni₂P. Weak peaks of Ni₅P₄ were observed in the bulk Ni₂P sample. However, for samples with Ti, the peaks of Ni₅P₄ disappeared, indicating that the addition of Ti into the bulk Ni₂P could inhibit the formation of the Ni₅P₄ phase. With increasing content of Ti, the Ni₂P peaks of Ti_x-Ni₂P became wider, which indicates that Ti can promote the formation of a smaller Ni₂P active phase. The value of the average crystallite size (D_c) D_c was calculated from Scherrer’s equation and the results are listed in Table 1. As can be seen from Table 1, the D_c values of all the Ti_x-Ni₂P samples were smaller than those found for the bulk Ni₂P. In addition, D_c of the Ti_x-Ni₂P decreased with increase of the Ti loading. This demonstrated that the incorporation of Ti can promote the formation of smaller particles of Ni₂P.

Figure 1.

XRD patterns of bulk Ni₂P and Ti_x-Ni₂P catalysts.

Table 1.

Properties and HDS catalytic performance of the bulk Ni₂P and Ti_x-Ni₂P catalysts.

Sample	S_BET (m² g⁻¹)	V_p (cm³ g⁻¹)	d (nm)	D_c^a (nm)	CO uptake (μmol g⁻¹)	TOF(10⁻³ s⁻¹)	Selectivity (%)		Conversion (%)
Sample	S_BET (m² g⁻¹)	V_p (cm³ g⁻¹)	d (nm)	D_c^a (nm)	CO uptake (μmol g⁻¹)	TOF(10⁻³ s⁻¹)	CHB	BP	1 h	8 h
Ni₂P	6.1	0.025	16.2	55	167	2.7	34	66	19.1	55.2
Ti_0.005-Ni₂P	11.67	0.124	21.1	42	335	5.8	21	79	83.39	98.4
Ti_0.01-Ni₂P	13.85	0.061	27.4	37	377	3.2	28	72	51.53	98.6
Ti_0.02-Ni₂P	16.15	0.095	30.6	35	435	4.5	23	77	84.31	99.6

HDS: hydrodesulfurization; BET: Brunauer–Emmett–Teller; TOF: turnover frequency; CHB: cyclohexyl benzene; BP: biphenyl; S_BET: surface area of the catalysts; V_p: pore volume of the catalysts; d: pore size of the catalysts.

Calculated from D_c = Kλ/β cos(θ) (Scherrer’s equation) based on Ni₂P {1 1 1}.

BET

The textural characterizations of Ni₂P and Ti_x-Ni₂P catalysts are summarized in Table 1. The BET surface area and porosity measurements were performed by N₂-adsorption analysis. The surface area and pore volume of the Ni₂P catalysts were 6.1 m² g⁻¹ and 0.025 cm³ g⁻¹, respectively.¹⁴ As the Ti content increased, the specific surface area of Ti_x-Ni₂P increased, and the pore diameter showed the same trend while the diameter of the particles (D_c) decreased, indicating that Ti metal promoted the formation of smaller and more highly dispersed Ni₂P particles in the catalyst. The surface area of the Ti_0.02-Ni₂P catalyst was the greatest, being almost twice that found for bulk Ni₂P. As for the pore volume, that of the Ti_0.005-Ni₂P catalyst was the greatest, being approximately five times more than that found in bulk Ni₂P.

CO uptake

The CO uptakes for the Ni₂P and Ti_x-Ni₂P catalysts were measured at room temperature, and the results are presented in Table 1 (the CO molecules attached to the phosphorus sites can be ignored¹⁵). As shown in Table 1, the CO uptake of the bulk Ni₂P was 167 μmol g⁻¹. What is notable is that the CO uptakes of the Ti_x-Ni₂P catalysts were markedly higher than the CO uptake of the bulk Ni₂P catalyst. Thus, the CO uptake of Ti_0.02-Ni₂P was 435 μmol g⁻¹ which was approximately three times that of bulk Ni₂P. The values for Ti_0.005-Ni₂P and Ti_0.01-Ni₂P were 335 and 377 μmol g⁻¹, respectively, which showed that incorporating Ti into Ni₂P can improve the dispersion of active Ni₂P particles due to the smaller size (XRD analysis) and higher surface area (BET analysis) of Ti_x-Ni₂P. This will be discussed in detail in the XPS analysis section.

XPS

The XPS spectra of all the catalysts are shown in Figure 2, and the binding energies and the P/Ni atomic ratios are shown in Table 2. As in previous studies,^16,17 for all catalysts, the Ni 2p_3/2 core-level spectrum consists of two components, of which one is assigned to Ni^d+ in the Ni₂P phase and is centered at 851.4–853.5 eV and the second is at 855.4–856.7 eV. As a consequence of superficial passivation, the interaction of Ni²⁺ ions with phosphate ions corresponded to the core-level spectrum, along with broad satellite peaks at almost 6 eV higher than the binding energy of the parent signal,^17,18 and this structured peak is ascribed to a divalent species. Simultaneously, other broad peaks centered at the high binding energy side can be assigned to the Ni 2p signal from oxidized Ni species.¹⁹

Figure 2.

XPS spectra of Ni₂P and Ti_x-Ni₂P catalysts.

Table 2.

Spectral parameters obtained by XPS analysis for bulk Ni₂P and Ti_x-Ni₂P catalysts.

Sample	Binding energy (eV)					Superficial atomic ratio
	Ni 2p_3/2			P 2p_3/2		Superficial atomic ratio
	Ni²⁺	Satellite	Ni^δ+	P⁵⁺	P^δ-	Ti/Ni	P/Ni
Ni₂P	854.7	860.2	850.7	141.4	133.8	–	3.1
Ti_0.005-Ni₂P	854.2	860.0	850.3	141.2	133.1	0.053	1.53
Ti_0.01-Ni₂P	854.4	860.2	850.2	141.1	133.2	0.063	1.65
Ti_0.02-Ni₂P	854.2	860.1	850.2	140.9	133.9	0.071	1.52

XPS: X-ray photoelectron spectroscopy.

As can be seen from Figure 2, the peaks centered at 132.8–133.7 eV can be assigned to P^δ− species in the Ni₂P phase,²⁰ and the peak at 140.3–141.7 eV can be attributed to phosphate (P⁵⁺) arising from superficial oxidation of the Ni₂P particles.^21,22 As can be seen from Figure 2 and Table 2, with Ti species incorporated into bulk Ni₂P, the binding energy of Ni²⁺ and P⁵⁺ shifts slightly to a lower value, indicating that there is an interaction between the Ni₂P particles and the Ti species. In addition, the increase of electron density leads to a slight stabilization of the Ni 3d levels and a small Ni–to–P charge transfer.²³ This special effect is assists the reduction of ${PO}_{3}^{4 -}$ into Ni₂P. Hence, the addition of Ti could promote the formation of the reactive Ni₂P phase.

To be specific, the electrons could be readily transferred and injected from Ti species into the Ni species through the conduction band, because the Ti³⁺ ion is an electron donor and Ni₂P is an excellent electron conductor. The intensity of the peaks located around 853.0 eV (Figure 2) decreases with incorporation of Ti, while the intensity of the peaks located around 857.4 eV increases with incorporation of Ti.²⁴ XPS analysis for the Ni₂P and Ti_x-Ni₂P catalysts was utilized to calculate the surface Ti and phosphorus with Ni atomic ratios, respectively (Table 2). The P/Ni atomic ratios of the Ti_x-Ni₂P samples was lower than that found for bulk Ni₂P, indicating that the addition of Ti to Ni₂P can inhibit the enrichment of phosphorus on the surface of the catalysts. The data obtained demonstrated that there were more Ni sites exposed on the surface of the samples, which was confirmed by the CO uptake analysis (see Table 1, column 6), which may also affect the HDS process of the catalyst.

HDS activity

Generally, two parallel reaction pathways participate in the DBT HDS reaction: (1) direct desulfurization (DDS) and (2) hydrogenation (HYD).⁴ The formation of biphenyl (BP) and a small amount of BP further transformed into cyclohexyl benzene (CHB) occurs in the DDS pathway, and HYD yields mainly CHB. Due to the inappreciable amount of BP transformed into CHB, the DDS selectivity is almost equivalent to the selectivity for BP, and the HYD selectivity could be represented by the CHB selectivity. The conversions of DBT via HDS over Ni₂P and Ti_x-Ni₂P catalysts are shown in Figure 3. The conversion of DBT via the HDS process over the bulk Ni₂P catalyst was only 4.2% after 1 h. The DBT conversion over the Ti_0.005-Ni₂P catalyst was 51.5% after 1 h. As the content of Ti increased, the DBT conversion increased further. For Ti_0.01-Ni₂P and Ti_0.02-Ni₂P catalysts, the DBT conversion was about 83% after 1 h. Over time, the HDS activity of all catalysts conversion rates increased gradually and eventually stabilized. The DBT conversion over bulk Ni₂P catalyst was 55.2% after 8 h, while all the Ti_x-Ni₂P catalysts showed almost 100% conversion of DBT after 8 h, which is approximately two-fold that shown by the bulk Ni₂P. Upon incorporating the Ti into the Ni₂P catalysts, the DBT conversions increased dramatically. Generally, the smaller Ni₂P particles and better dispersion would result in higher HDS activity of the catalyst. The marked increase of HDS activity for the different loadings of Ti _x -Ni₂P catalysts might due to the smaller particles and the more widely dispersed active Ni₂P phase in these catalysts. This observation suggests that a high dispersion of Ni₂P, as can be inferred from Figure 1, can be beneficial in enhancing the HDS activity.^20,25

Figure 3.

HDS activity of the Ni₂P and Ti_x-Ni₂P catalysts. Temperature = 340 °C; pressure = 3.0MPa; H₂/oil ratio = 700 (V/V); WHSV = 1.5 h⁻¹.

HDS selectivity

To investigate further, the effect of Ti on the HDS process, its selectivity data has also been studied. The ratios of DBT to BP and DBT to CHB are given in columns 8 and 9 of Table 1. BP and CHB statistics in the case of in the case of Ni₂P and Ti_x-Ni₂P catalysts are shown in Figure 4.

Figure 4.

HDS selectivity with the Ni₂P and Ti_x-Ni₂P catalysts. Temperature = 340 °C; pressure = 3.0 MPa; H₂/oil ratio = 700 (V/V); WHSV = 1.5 h⁻¹.

For all of the samples, the output of BP is much higher than that of CHB, which means that DBT is primarily removed through the DDS pathway over all the catalysts.²⁶ At the outset, the selectivity for BP and CHB were approximately the same for all the catalysts. However, the selectivity for BP with bulk Ni₂P decreased faster than with Ti_x-Ni₂P with time on stream. Therefore, the selectivity for BP with Ti_x-Ni₂P was higher than what was found with the bulk Ni₂P after 8 h. Among all the samples, Ti_0.005-Ni₂P showed the maximum value, the selectivity for BP being increased approximately 15% compared with bulk Ni₂P, which indicates the DDS route was enhanced by the addition of Ti.²⁷

Conclusion

Ti-modified unsupported Ni₂P catalysts with higher HDS activity have been prepared successfully. XRD analysis indicated that incorporating Ti into the bulk Ni₂P was capable of inhibiting the formation of the Ni₅P₄ phase, thereby forming the more active Ni₂P phase. The crystallite size (D_c) of Ni₂P decreased when the loading of Ti increased, and the D_c of Ti_0.02-Ni₂P catalyst has the lowest value. Besides, the BET analysis results of Ti-incorporated Ni₂P catalyst showed a markedly larger surface area, resulting in the large influence of Ti metal on the surface area of the Ni₂P catalyst and a better dispersion of the active phase.

In addition, the Ti species was enriched on the surface of the promoted nickel phosphide indicating the existence of an electronic effect caused by the surface Ti species, which might result in electron enrichment on the surface Ni and P species. This electron-enriched surface is suggested to be one of the reasons to account for the enhanced HDS activity of Ti_x-Ni₂P. The Ti_x-Ni₂P catalyst exhibited the highest DBT HDS activity of 99.6%, which was an increase of 44% when compared with that found for the bulk Ni₂P.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (21276048) and the Natural Science Foundation of Heilongjiang Province (ZD201201).

Author biographies

Chunbao Han received his BS degree in Chemical Engineering and Technology from Northeast Petroleum University, China. He is currently a Master Degree candidate under Professor Song at College of Chemistry & Chemical Engineering, Northeast Petroleum University, China. Her research interest is hydrogenation theory and catalysts.

Hua Song received her Phd degree in oil and Gas Field Chemical Engineering from Northeast Petroleum University, China. She is currently a Distinguished Professor of Longjiang Scholar and the director of provincial key laboratory of petrochemical engineering at College of Chemistry & Chemical Engineering, Northeast Petroleum University, China. Her laboratory is focused on catalytic materials and catalyst engineering.

Nan Jiang received her MS degree in Chemical Engineering from Clausthal University of Technology, Germany. She is currently a PhD candidate in chemical engineering at College of Chemistry & Chemical Engineering, Northeast Petroleum University, China.

Yanguang Chen received his MS degree in Chemical Technology from China Petroleum University and PhD degree in Chemical Technology from Institute of Process Engineering, Chinese Academy of Sciences. He is currently a Distinguished Professor of Longjiang Scholars at College of Chemistry & Chemical Engineering, Northeast Petroleum University, China. His research is focused on process regulation of catalytic materials and catalytic conversion of fossil energy.

Feng Li received his PhD degree in Chemical Technology from Northeast Petroleum University, China. He is currently a professor in Chemical Engineering at College of Chemistry & Chemical Engineering, Northeast Petroleum University, China. His research is focused on catalytic materials and catalyst engineering.

Tianzhen Hao received his BS degree in Chemical Reaction Engineering, from Northeast Petroleum University, China. He is currently the Chairman of Hebei Fine Technology co. LTD, China. His company is focused on clean oil production technology.

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