Article pubs.acs.org/EF
Cite This: Energy Fuels 2018, 32, 8210−8219
Titanium-Modified TUD‑1 Mesoporous Catalysts for the Hydrotreatment of FCC Diesel Chengkun Xiao,† Zesheng Xia,† Kebin Chi,‡ Aijun Duan,*,† Yuyang Li,† Di Hu,† Chunming Xu,† Zhen Zhao,† Jian Liu,† and Zhiyi Yu† †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, People’s Republic of China Petrochemical Research Institute, PetroChina Company Limited, Beijing, 102206, People’s Republic of China
‡
Energy Fuels 2018.32:8210-8219. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 08/17/18. For personal use only.
S Supporting Information *
ABSTRACT: Titanium-modified pure aluminum-based TUD-1 (TiAT) mesoporous material with a highly active threedimensional was successfully synthesized by using tetraethylene glycol (TEG) as a template through sol−gel method. The series catalysts were characterized by X-ray diffraction (XRD), N2 adsorption−desorption, Raman, pyrolysis-infrared (Py-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) technologies. The characterization results indicated that Ti was incorporated into the TUD-1. In addition, the synthesized materials had large pore size (6.4 nm), high surface area (394.0 m2/ g), and large pore volume (0.83 cm3/g). It was shown that Ti acting as an electronic promoter could promote the formation of Mo7O246− precursors and more type II NiMoS phases. Furthermore, Ti incorporation could bring more Brønsted and Lewis acids into the catalysts, which were beneficial to improve the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) reaction processes. Catalytic performances of diffierent catalysts were estimated in a high-pressure hydrotreating microreactor using FCC diesel as feedstock; moreover, the catalyst of NiMo/TiAT-25 exhibit the highest HDS (97.3%) and (HDN) (99.1%) efficiencies of FCC diesel, respectively, and the highest HDS (97.4%) of DBT, since NiMo/TiAT-25 possessed suitable acidity, pore structure, reducibility, and degree of sulfidation. 1000 m2 g−1), thick mesopore wall, wide pore sizes (2.5−25 nm), and high stability.4−6 TEA and TEG were very inexpensive mesoporous templates that not only can guide the formation of mesostructures but also act as a chelating agent for metal ion (Al, Ti, Fe, and Zr) modifications.3,4 Shan7 reported the synthesis of a very stable mesoporous alumina, which was an amorphous, pure aluminum TUD-1based material, with a relatively high specific surface area (530 m2/g) and adjustable porosity. Simons8 selected four different anionic carrier materials and studied their effects on the asymmetric hydrogenation of dehydrogenated amino acids over noncovalent supported catalysts. The activity evaluation showed that the modified pure aluminum TUD-1 (TUDAl2O3) material, recorded as PWTUD, had the best performance, compared with other materials; furthermore, it demonstrated high stability against leaching in all solvents. Shan et al.9 synthesized Ti-modified TUD-1-based materials via sol−gel methods, which had a 3D interconnected network structure, large specific surface area and favorable aperture, as well as high thermal stability. The results indicated that the catalytic activity of Ti-TUD-1 was 5.6 times higher that of the Ti-MCM-41 catalyst. Amezcua et al.10 prepared a series of Ticontaining mesoporous SBA-16 carriers, and the corresponding series catalysts. The effects of the support preparation method, the characteristics of Ni and Mo surface species, and the TiO2 loading on the titania dispersion were investigated; the catalytic activities of 4,6-dimethyldibenzothiophene HDS
1. INTRODUCTION Catalytic hydrotreating is the most effective technology for desulfurization in industry, and the relative catalysts are the key to promote the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) reactions. However, the traditional hydrotreating catalysts are inferior to realize the deep desulfurization of diesel under the classical operation conditions; hence, it will be significant to develop novel catalysis materials and active catalysts to produce the ultraclean diesel. The HDS reaction requires that the catalysts have an open pore structure to reduce the diffusion limit of the macromolecule reaction and the appropriate acidity to enhance the hydrogenolysis capacity of the sulfide, as well as the suitable carrier and metal interaction force to modify the active metal dispersion and reduction properties. Since the HDS and HDN performances are closely related to the properties of supports, many researchers are interested in the design and application of new materials to be the catalyst supports.1,2 While the refinery focuses more on economical routes to synthesize inexpensive materials, it will be significant to develop novel materials with distinctive advantages of low cost and good physicochemical properties. TUD-1-based newtype mesoporous materials with tailorable pore structure, excellent stability, and low cost display promising applications in FCC diesel hydrotreating and other processes. TUD-1 novel mesoporous material was first synthesized by Jansen in 2001,3 which was an amorphous three-dimensional (3D) wormlike material, and it was prepared through the economical synthesis routes by using triethanolamine (TEA) or tetraethylene glycol (TEG) as a bifunctional template and had the appealing characteristics of high surface area (up to © 2018 American Chemical Society
Received: May 3, 2018 Revised: July 15, 2018 Published: July 17, 2018 8210
DOI: 10.1021/acs.energyfuels.8b01543 Energy Fuels 2018, 32, 8210−8219
Article
Energy & Fuels
the second impregnation step. In addition, the corresponding catalysts were noted as NiMo/TiAT-x (x = 10, 25, 40, 50, and 70). 2.2. Characterization. A powder X-ray diffraction (XRD) method was adopted to characterize the crystal states of each sample. XRD patterns were obtained using a Shimadzu X-6000 X-ray diffractometer in the tube pressure, at an accelerating voltage of 40 kV and current of 30 mA. The samples were scanned from 0.7° to 90° with a step of 0.01° (2θ). Nitrogen sorption isotherms were measured by a Micromeritics TriStar II 2020 porosimetry analyzer. The specific surface area was estimated using the Brunauer−Emmett−Teller (BET) method.15 The pore size distribution was calculated by the Barrett−Joyner−Halenda (BJH) method.16 Raman spectra were measured by the instrument of Renishaw Micro-Raman System 2000, with He/Cd as the light source and a wavelength of 532 nm. The test was operated at room temperature for a time interval of 50 s. Pyrolysis−infrared (Py-IR) characterization was performed on a Digilab FT-IR infrared spectrometer. The sample powder was compressed into a tablet, placed in an in situ pool, and evacuated at 473 K (623 K) for 2 h, then decreased to room temperature and scanned sample. After the adsorption of pyridine for some time, the sample was heated from a temperature of 473 K to a temperature of 623 K, followed by desorption for half an hour; then, it was cooled to room temperature, and the infrared spectrum was recorded. Consequently, the acidities of the samples were measured to analyze the sum of different acid intensities, while the collection of materials at 623 K represented the total medium and strong acids. Lewis and Brønsted acids were characterized by the peaks observed at 1450 and 1540 cm−1, respectively. The amount of acid was calculated by using the following formula:
then were discussed. The results showed that the Ti-SBA-16 catalysts exhibited high HDS activities of 4,6-DMDBT. Cao et al.11 synthesized mesoporous FDU-12 catalysts modified by different amounts of titania. The results corroborated that Ti content had a significant effect on HDS activity of DBT and 4,6-DMDBT. The Mo species in sulfided catalysts had obvious differences and the appropriate amount of Ti could promote the dispersion of Mo and facilitate to form more MoS2 phases. ́ et al.12 prepared a series of Mo or W catalysts Ramirez supported on Al−Ti mixed oxides. They believed that Ti (Ti3+) was an electronic promoter. These electrons could be easily transferred and injected to the Mo 3d conduction band, which caused a weakening of the Mo−S bonds and favored the HDS route of 4,6-DMDBT transformation. Soni et al.13 believed that, in the Ti-modified catalysts, Ti played an important role in improving metal to support interaction. The presence of Ti helps Ni and Mo oxides to present in octahedral position and to reduce at low temperature and sulfided easily to form NiMoS active phase. Therefore, the HDS and HDN activities of the catalyst could be improved. Dalai et al.14 believed that incorporation of Ti results in an increase in the surface acidity and improve metal support interactions, which increased the dispersion of active phases. Therefore, the HDN activity of the catalyst was obtained. The pure aluminum TUD-1 was a good carrier with a relatively high specific surface area, as well as adjustable porosity and aperture. Elemental Ti acts as an electron donor, possibly creating more acidic sites and increasing the dispersion of MoS2, which promotes the HDS and HDN reaction. Therefore, a series of titanium in-situ-modified pure aluminum TUD-1 were synthesized in this research. In addition, the relative catalytic performance was evaluated by the hydrotreating of diesel and DBT to be the feedstock.
C(pyridine on B‐sites) = 1.88 × IA(B) ×
R2 W
C(pyridine on L‐sites) = 1.42 × IA(L) ×
R2 W
where C is the concentration (mmol/g catalyst), IA(B) the integrated absorbance of the B band (cm), IA(L) the integrated absorbance of the L band (cm), R the radius of the catalyst disk (cm), and W the weight of the disk (mg). X-ray photoelectron spectroscopy (XPS) was executed to analyze the Mo species on the samples. In the XPS analysis, Al Kα X-ray was used as the excitation source, and the base vacuum was 3 × 10−9 mbar. The spectra were corrected by electron binding using the C 1s peak (284.8 eV) of polluted carbon. Mo 3d was subjected to XPS analysis in order to obtain the vulcanization degree of the active metals over a catalyst. 2.3. Catalytic Activity Measurement. The HDS and HDN activities were estimated in a high-pressure hydrotreating microreactor using FCC diesel with S and N contents of 1013.8 mg/L and 640.3 mg/L, respectively. First, 2 g of the catalyst was presulfided for 4.2 h with a mixture of H2 and 3 wt % CS2 solution at 593 K and 4 MPa. The HDS experiment was performed under the following conditions: 623 K, 5.0 MPa, a H2:oil ratio of 600, and a weight hourly space velocity (WHSV) of 1.0 h−1. After 9 h of stabilization, the product samples were taken once every 2 h and then the S/N contents were obtained. All the sulfur and nitrogen contents were obtained by using an RPP-2000 SN sulfur and nitrogen analyzer, the deviation of which was within 2 μg mL−1. The HDS and HDN efficiencies were defined as follows:
2. EXPERIMENTAL SECTION 2.1. Material Synthesis. In this research, a series of Ti-modified TUD-1 materials was synthesized through a sol−gel method. A mixture of absolute ethanol and anhydrous isopropanol was used as the solvent and a certain amount of deionized water was used to control the hydrolysis. Aluminum isopropoxide served as the aluminum source and tetrabutyl titanate was a titanium source, while TEG was a template. The specific synthesis steps were as follows. First, 15 g of aluminum isopropoxide was dissolved in a mixture of anhydrous ethanol and anhydrous isopropanol at temperatures of 313−323 K and stirred for a period of time. Then, a certain amount of tetrabutyl titanate was added dropwise into the above system and agitated for 4 h. Subsequently, 15 g of TEG was added to the system. Finally, 2.7 g of water and the remaining alcohol mixture were joined to the above synthesis system. The final suspension was obtained at a molar ratio of 1(Al(i-PrO)3):1(TEG):8(EtOH):6(i-PrOH):2(H2O):x(Ti(n-C4H9O)4). The reactants then were aged at 25 °C for 24 h and dried at 80 °C for 22 h. The xerogel then was thermally treated in the autoclave at 160 °C for 4 h, followed by calcination at 600 °C for 10 h. Ti-containing TUD-1 materials were denoted as TiAT-10, TiAT-25, TiAT-40, TiAT-50, and TiAT70, where the numbers 10, 25, 40, 50, and 70 corresponded to the nominal Al/Ti ratios, respectively. This series of titanium-modified TUD-1 materials was used as supports for the NiMo-active metals. The first step was the impregnation of aqueous solution of ammonium heptamolybdate, followed by an ultrasonic bath for 30 min. Finally, the materials were dried at 80 °C for 4 h and calcined at 550 °C for 6 h. The nickel nitrate was subsequently impregnated in the same procedure. The catalysts had a composition of 15.5 wt % MoO3 and 3.5 wt % NiO in
ij Sf − Sp yz zz × 100 HDS efficiency (%) = jjj j Sf zz k {
ij Nf − Np yz zz × 100 HDN efficiency (%) = jjj j Nf zz k { 8211
DOI: 10.1021/acs.energyfuels.8b01543 Energy Fuels 2018, 32, 8210−8219
Article
Energy & Fuels Here, Sf and Sp were the sulfur concentrations in the feed and product, respectively, and Nf and Np were the nitrogen concentrations in the feed and product, respectively. The HDS activity of DBT (500 ppm) was tested under the following conditions: 613 K, 4 MPa, H2/oil ratio = 200 mL/mL, and WHSV = 20−150 h−1.
3. RESULTS AND DISCUSSION 3.1. Characterization Results of the Materials. 3.1.1. XRD Characterization of Materials. Figure 1A is the
Figure 2. (A) Nitrogen adsorption−desorption isotherms and (B) pore diameter distribution of TiAT-x series materials.
size distributions of AT material and TiAT-x series materials, respectively. Figure 2A shows that AT material and TiAT-x series materials have “type IV” adsorption isotherms, confirming them to be typical mesoporous materials.18,19 Figure 2B demonstrates that the pore sizes of AT material and TiAT-x series materials are mainly ∼10 nm, and the pore sizes increase first with the Ti content enhancing in the material and then subsequently decreasing, which indicate that the introduction of suitable Ti content can increase the average pore sizes of the materials. Table 1 lists the surface area, pore volume, and other parameters of AT material and TiAT-x series materials. From
Figure 1. (A) Low-angle and (B) wide-angle XRD patterns of the TiAT-x series materials.
low-angle XRD patterns, the XRD analysis of the TiAT-x series material and the AT sample exhibit only one diffraction peak at 2θ = 1°, indicating that both the TiAT-x series materials and AT samples have mesoporous structures,6 and the mesoporous structure remains unchanged after titanium modification. As shown in Figure 1B, the characteristic diffraction peaks of anatase appear at 2θ = 25.2°, 36.2°, and 48.1° (JCPDS File No. 21-1272) in the XRD spectra. In the wide-angle XRD spectra of the TiAT-x series, no obvious diffraction peaks of TiO2 phase are observed, which indicate that Ti atoms are incorporated into the material skeleton in TiAT-x samples, or the sizes of TiO2 crystal are very small (
