Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Phosphorous-Modified Beta Zeolite and Its Performance in Vacuum Gas Oil Hydrocracking Activity Cecilia Manrique,*,† Alexander Guzmań ,‡ Roger Solano,† and Adriana Echavarría† †
Grupo Catalizadores y Adsorbentes, Universidad de Antioquia-UdeA, A.A 1226 Medellín, Colombia Instituto Colombiano del PetróleoICP, Ecopetrol S.A., km 7 vía a Piedecuesta, 681011 Piedecuesta, Colombia
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‡
ABSTRACT: The present work reports the synthesis of beta zeolite and its modification with phosphorous in different percentages of P2O5 (1 and 5%) and vacuum gas oil (VGO) hydrocracking (HCK) conversion and yield to naphtha and middle distillates. The solids were characterized by X-ray diffraction, elemental analysis by atomic absorption spectroscopy and inductively coupled plasma atomic emission spectrometry (ICP-AES), N2 adsorption, 27Al magic angle spinning-nuclear magnetic resonance (27Al-MAS-NMR), temperature-programmed desorption of ammonia (NH3-TPD) and isopropylamine (IPam-TPD), pyridine adsorption followed by Fourier transform infrared spectroscopy (Py-FTIR), hydrogen temperatureprogrammed reduction, transmission electron microscopy, and energy-dispersive X-ray (TEM−EDX). The catalysts were tested in a tubular reaction system at 350 °C, 10 342 kPa, H2/feed: 1250 NL/L, LHSV: 1 h−1 with prehydrotreated VGO. Overall activity and middle distillates and naphtha yields were influenced by the phosphorous impregnation on beta zeolite. Between 4 and 6% higher conversion was observed. Textural and acid properties were modified by the phosphorous treatment leading to a total acidity and surface area decrease with phosphorous content. Relative changes in tetrahedral and extra-framework (2) aluminum were followed by 27Al-MAS-NMR. Al(4) T assigned to distorted extra-lattice tetrahedral aluminum and AlO extraframework aluminum increased with phosphorous impregnation. Strong acidity monitored by IR of adsorbed pyridine showed a direct correlation with VGO conversion and naphtha yields.
1. INTRODUCTION Hydrocracking of vacuum gas oils is one of the most important refinery processes, which involves the conversion of high molecular weight hydrocarbons into lighter and valuable products. Beta zeolite has been considered as a suitable catalyst for hydrocracking of vacuum gas oils, owing to its activity and its high thermal stability. Furthermore, phosphorus modification of beta zeolite has proven to facilitate adjustment of acid sites. Acidity properties of a catalyst are considered as a key factor for understanding its catalytic behavior, especially, the presence of Brønsted acid sites and the accessibility of molecules to these active sites. Acid-site variations and textural properties of a vacuum gas oil hydrocracking (VGO) hydrocracking are determinants of its catalytic activity. Generally, modification of these properties in hydrocracking catalysts are carried out through chemical and hydrothermal treatment of the supports,1 or using promoters such as phosphorous (P), fluorine (F), and boron (B) supported on the catalyst.2 The effect of phosphorous on chemistry of metal-oxide surfaces is of particular interest. For instance, acid properties and pore structure of the Y zeolite have been modified with phosphorus in order to improve the yield to middle distillates. This modification allowed a secondary type of mesoporosity on the zeolite, while changing acid-site distribution and strength.3 According to studies reported in the literature, the impregnated phosphorus reacts with the hydroxyl groups of the zeolite, resulting in the formation of Si−O−P bonds. At the same time, phosphorus can cause dealumination and/or react with the extra-framework aluminum species to give rise to the formation of aluminophosphates.4−6 These phosphorous © XXXX American Chemical Society
species facilitate the breaking of C−N bonds of the molecules present in vacuum gas oil, increasing the activity of the catalyst and inhibiting the formation of coke.7 Because of the variety of the acid sites present in the beta zeolite, attributed to its structural defects, this zeolite is a material with interesting characteristics to study the interaction of its acid sites with different phosphorus species. Guan et al.4 studied the interaction of phosphorus molecules (trimethylphosphine and trimethylphosphine oxide) with the acid sites of zeolite beta using 31P MAS NMR reporting the existence of two types of Brønsted and Lewis acid sites (BAS and LAS, respectively). On the other hand, Gu et al.8 reported the modification with orthophosphoric acid in a material with structure *BEA, this modified zeolite presented an increase in Brønsted acid sites with the increase in the amount of phosphorus deposited, while the Lewis acid sites gradually decrease. Despite many studies using modified zeolites for hydrocracking have been conducted, most are based on model molecules instead of using real feedstocks as vacuum gasoil. In addition, the role of phosphorous in the beta zeolite acidity and its evaluation in VGO has not been systematically investigated. The present work reports the synthesis of beta zeolite and its modification with phosphorous in different proportions of P2O5 (1% and 5%) and the hydrocracking conversion and yield to middle distillates of a heavy feedstock. Received: December 27, 2018 Revised: February 21, 2019 Published: March 4, 2019 A
DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
with highest intensity of the peak at a 2θ angle of 22.30° was considered as 100% crystallinity. The nitrogen sorption measurements of zeolites and catalysts were performed with a Micromeritics ASAP 2020 gas sorption system at −196 °C. Prior to the measurements, all samples were degassed under high-vacuum conditions for 8 h at 350 °C. The micropore volume was calculated based on the t-plot method, while the Brunauer−Emmet− Teller (BET) method was applied to determine the apparent specific surface area. The mesopore volume and mesopore size distribution were calculated considering the adsorption branch of the isotherm by the Barrett−Joyner−Halenda (BJH) method. Si, Al, and Na contents were determined by atomic absorption spectroscopy in a Thermo Scientific iCE 3000 Series equipment. All samples were treated with mineral acids (hydrochloric and hydrofluoric acids) to achieve complete dissolution. Interferences associated with each metal were corrected with the nitrous oxide and acetylene flame. The P content of the samples was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) using an ICP Optima 3300 DV PerkinElmer spectrometer. 27 Al MAS NMR spectra in the solid state were recorded on a Bruker MAS equipped with a triple-channel probe using 2.5 mm ZrO2 rotors at room temperature. The rotation speed was 20 KHz in all cases. The 27Al chemical shift was referred to a saturated Al(NO3)3 solution. 31P MAS NMR spectra was acquired in a Bruker AV-400WB spectrometer operating at 161.97 MHz. Acid properties of zeolites were determined by temperatureprogrammed desorption of ammonia (NH3-TPD), isopropylamine (IPam-TPD), and pyridine adsorption followed by Fourier transform infrared spectroscopy (Py-FTIR). NH3-TPD experiments were carried out in a Micromeritics TPD/TPR 2900 equipment with a thermal conductivity detector (TCD). Here, 0.3 ± 0.01 g of the zeolite was placed in a quartz reactor and then heated up to 550 °C with a temperature ramp of 8 °C/min under He flow (50 mL/min) for 1 h. After cooling down to 150 °C, ammonia adsorption took place through small pulses. Full saturation was verified and physically adsorbed ammonia was removed by flushing the sample with He flow for 1 h. Ammonia desorption was performed at a rate of 10 °C/min starting at 150 °C up to 550 °C under He flow (50 mL/min) followed by a 1 h isothermal step. To determine the amount of Brønsted acid sites, temperatureprogrammed desorption of isopropylamine measurements were performed. Experiments were carried out in a microreactor “microactivity effi” supplied by Micromeritics coupled to the mass spectrometer. Here, 0.04 ± 0.01 g of the zeolite was placed in a stainless steel reactor and then heated up to 200 °C with a temperature ramp of 5 °C/min under N2 flow (50 mL/min) for 2 h. After, cooling down to 100 °C, isopropylamine adsorption took place through small pulses. Full saturation was verified and physically adsorbed isopropylamine was removed by flushing the sample with N2 flow for 1 h. Isopropylamine desorption was performed at a rate of 10 °C/min starting at 25 °C up to 550 °C under N2 flow (50 mL/min) followed by a 1 h isothermal step. Py-FTIR spectra of zeolites were recorded in a Thermo-Nicolet Nexus 670 FTIR spectrometer equipped with an MCT cryodetector. About 10 mg of zeolite powder samples were pressed into selfsupported wafer discs of 13 mm diameter, which was placed into a self-made glass cell with CaF2 windows. The sample was heated under vacuum in the IR cell at a ramp of 5 °C/min up to 450 °C and kept at this temperature for 10 h. Subsequently, the cell was cooled down to room temperature and spectra were recorded between 4000 and 1000 cm−1 at 4 cm−1 resolution. Pyridine (Fluka) was then adsorbed on the zeolite at 30 °C until reaching full saturation and later was desorbed at 250 °C, 350 °C, and 400 °C during 1 h prior to recording the spectra. Temperature-programmed reduction of catalysts was performed in an AUTOCHEM II 2920 Automated Catalyst Characterization System (Micromeritics). Each sample (300 mg) was loaded on top of glass wool in a quartz tube inside an electric furnace. All samples were pretreated by heating in a helium atmosphere at 350 °C for 1 h before the TPR tests in order to remove contaminants. After cooling down to room temperature, reductions were carried out using a 50
2. EXPERIMENTAL SECTION 2.1. Modification of Beta Zeolite with Phosphoric Acid. Sodium hydroxide (NaOH, Merck) and sodium aluminate (Al2O3 50−56 wt %, Na2O 40−45 wt %, Sigma-Aldrich) were mixed with deionized water. Subsequently, tetraethylammonium hydroxide solution (TEAOH 35 wt %, Sigma-Aldrich) was added. The source of silica (Ludox 30 wt %; Sigma-Aldrich) was added dropwise to obtain the following molar composition: 2.69Na 2O: Al 2O 3: 50.76SiO2: 12.68TEAOH: 810.24H2O. The obtained mixture was stirred for 3 h with gel formation. The gel was transferred to Teflonlined autoclaves and treated hydrothermally under static condition at 170 °C for 24 h. Then, the autoclaves were quenched and the solid was recovered by filtration and washed with deionized water until the pH of the supernatant was neutral. The final solid was calcined at 600 °C for 6 h under air atmosphere. The heating rate was controlled at 2 °C/min. Subsequently, samples were transformed into the ammonium form by four successive exchanges in 0.2 M NH4NO3 aqueous solution at 50 °C (1 g of zeolite per 50 mL of aqueous solution). Finally, the ammonium form was converted to the protonic form by calcination in air at 600 °C for 2 h with a heating rate of 5 °C/min. Chemical analysis confirmed that most of the sodium cations were exchanged and reduced to less than 0.01 wt %. The synthesized zeolite has been denoted as BEA. Phosphorous was impregnated on the BEA zeolite by incipient wetness with a phosphoric acid (85 w/v %) aqueous solution. The resulting samples were calcined at 550 °C for 2 h under air. The phosphorus contents were 0.5, and 2.5 wt % and the samples were named as P(0.5)−BEA and P(2.5)−BEA, respectively. 2.2. Preparation of the Catalysts. Hydrocracking catalysts extrudates were prepared as follows: the mechanical mixture of zeolite (40 wt %) and alumina binder support (Versal 250 pseudoboehmite alumina, surface area = 320 m2/g) was peptized using 1 wt % HNO3 solution as the peptizing agent. The obtained dough was then extruded into cylindrical-shaped pellets with a diameter of 1.2 mm and length of 4−8 mm. The resulting material was dried at 110 °C for 2 h and calcined at 550 °C for 2 h. Subsequently, the extrudates were sequentially impregnated, with 15 wt % of oxide of molybdenum and 1 wt % of oxide of nickel by incipient wetness with intermediate calcinations at 550 °C for 2 h. Nickel nitrate hexahydrate (99 wt %, Merck) and ammonium heptamolybdate tetrahydrate (99 wt %, Merck) were used as metal precursors. Finally, the catalysts were identified as: NiMo/BEA−Al2 O3; NiMo/P(0.5)−BEA−Al2O3; NiMo/P(2.5)−BEA. 2.3. Hydrocracking Catalytic Tests. Hydrocracking of VGO was carried out in a fixed bed stainless steel tubular reaction system (Parr Instruments). The feedstock consisted of a pretreated Colombian representative VGO. NiMo catalysts (4.2 g) were diluted with inert sand (0.212−0.310 mm size), resulting in a bed volume of about 8 cm.3 Prior to reaction, the system was pressurized to 10 342 kPa with hydrogen and the catalysts were sulfided in situ with a mixture of diesel, 4 wt % of dimethyl disulfide, and 0.5 wt % of aniline at 350 °C for 36 h. A weight hourly space velocity (WHSV) of 1 h−1 was used and maintained for 12 h before VGO injection. HCK tests were carried out at 350 °C, 10 342 kPa, and 1250 NL/L of the H2/feed ratio. These conditions are similar to those used in Colombian industrial hydrocracking units. The WHSV on stream was 2 h−1 during the first 18 h and then it was lowered to 1 h−1. The duration of every run was approximately 90 h. Conversion was referred to the cut 370 °C+ in the feed. Selectivity to middle distillates was referred as the fraction 180−370 °C in products. Products were analyzed by SIMDIS-GC in accordance with the ASTM D7213 standard test method. 2.4. Characterization Methods. Zeolites were characterized by X-ray diffraction (XRD) at room temperature in a PANalytical Empyrean diffractometer with a Cu Kα radiation (λ = 1.5406 Å) operating at 45 kV and 40 mA. Samples were analyzed in the 5°−50° (2θ) range with a step size of 0.04°. Relative crystallinity of the solid was calculated as the sum of the intensities of X-ray peaks at 2θ positions of 21.13° and 22.30°. Relative crystallinity of the sample B
DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels mL/min gas flow of 10% H2/Ar in the temperature range of 25−1000 °C at a heating rate of 10 °C/min. Hydrogen consumption was monitored using a TCD. Surface analysis for the catalyst was carried out using the TEM/ STEM FEI Tecnai G2 F20 microscope operating at 200 kV. Prior to measurements, the samples were suspended in ethanol, sonicated for 30 min, and placed over a carbon-coated holey Cu microgrid.
3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties of Zeolites. XRD diffractograms of the samples with and without phosphorous
Figure 1. XRD patterns of the parent and modified zeolites. Figure 3. 27Al MAS NMR spectra of zeolites in acid form and simulated spectra (red line) using standard deconvolution with symmetric function Gaussian.
increases. This suggests a loss of crystallinity during the impregnation and/or calcination process.9,10 Impregnation with 2.5% phosphorus (P(2.5)−BEA zeolite) decreased crystallinity (about 15%), which may be because of structural defects caused by dealumination during the phosphorus modification process leading to the formation of amorphous aluminophosphates.3 Figure 2 presents the N2 adsorption/desorption isotherms of the samples in their acid form and BJH pore size distributions. The corresponding textural properties are listed in Table 1. The table shows material textural properties decrease with respect to original zeolite in the presence of phosphorus. The percentage of impregnated phosphorus measured by ICP-OES chemical analysis is also shown. The results show that the impregnation method used, allowed a P-loading close to the theoretically expected results. Studies that report zeolite modification with phosphorus,6,11,12 show a decrease in
Figure 2. N2-adsorption/desorption isotherms at −196 °C.
are shown in Figure 1. The materials showed a well-defined diffraction pattern corresponding to the beta zeolite structure. It suggests that modification by H3PO4 solution does not change the phase structure of the zeolite. However, the intensity of the peaks decreases as the amount of phosphorus Table 1. Physicochemical Properties of Zeolite Samples sample
P (wt %)
SBET (m2/g)
Smicro(m2/g)
Smeso(m2/g)
VT (cm3/g)
Vmicro(cm3/g)
Vmeso(cm3/g)
BEA P(0.5)−BEA P(2.5)−BEA
0.00 0.46 2.38
770 739 635
491 451 408
279 288 227
0.44 0.40 0.33
0.20 0.18 0.16
0.24 0.22 0.17
C
DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 2. NMR Parameters from the Deconvolution of 27Al MAS NMR and Relative Content of Al Species in Each Zeolite species
parameter
BEA
P(0.5)−BEA
P(2.5)−BEA
Al(1) T
position (ppm) area (%) position (ppm) area (%) position (ppm) area (%) position (ppm) area (%) position (ppm) area (%) position (ppm) area (%) position (ppm) area (%)
62.1 4.3 58.0 12.1 53.6 47.9 44.0 15.5 0.0 3.9 −14.9 16.3 0.0 0.0 79.8 20.2
61.7 4.1 57.0 16.2 53.2 36.0 40.0 12.9 −0.6 5.7 −9.7 25.1 0.0 0.0 69.3 30.7
0.0 0.0 0.0 0.0 54.1 17.9 39.1 39.8 0.0 0.0 −12.7 22.7 −26.5 19.7 57.7 42.3
Al(2) T Al(3) T Al(4) T Al(1) O Al(2) O Al(3) O AlT total AlO total
as a progressive surface area decrease, most likely caused by zeolite pore blocking with the increase of the phosphorous concentration. 27 Al MAS NMR spectra for the BEA zeolite and those modified with phosphorus are shown in Figure 3, along with the deconvolution results of Table 2 shows the relative area extracted by deconvolution of peaks from 27Al MAS NMR spectrums. As can be observed, BEA and P(0.5)−BEA zeolite spectra show an intense and wide band between 40 and 65 ppm attributed to the presence of tetrahedral aluminum, and by deconvolution of this band, it was possible to obtain four (2) (3) (4) (1) different contributions denoted as Al(1) T , AlT , AlT , AlT . AlT (∼62 ppm), and Al(4) (∼44 ppm) signals are assigned to the T presence of extra-lattice tetrahedral and distorted extra-lattice tetrahedral aluminum species, respectively; these species increase with the increase in the phosphorus percentage. On (3) the other hand, Al(2) T (∼57 ppm) and AlT (∼53 ppm) signals are attributed to the presence of tetrahedral aluminum species incorporated into the lattice.13 27Al MAS NMR spectra show an intensity reduction of these bands as the phosphorus content increases. NMR results also show two bands close to 0 ppm assigned to extra-lattice octahedral aluminum species, (2) denoted as Al(1) O and AlO ; the presence of these species is a consequence of dealumination during the calcination or exchange process.13,14 It is to remark that an increase in phosphorus content causes (2) the disappearance of Al(1) T (∼62 ppm) and AlT (∼57 ppm) signals in the case of the zeolite with the highest phosphorous concentration (P(2.5)−BEA), while the band at ∼ 44 ppm (Al(4) T ) increases significantly compared to the pristine BEA zeolite. Concomitantly, the generation of octahedral aluminum is observed, attributed to the formation of species Al(3) O pentacoordinated aluminum probably because of the dealumination process caused by phosphorus incorporation.1,3 27 Al MAS NMR studies reported in the literature demonstrated the existence of two types of Brønsted acid sites in the beta zeolite13 (Brønsted acid site, less acidic, attributed to the signal at ∼58 ppm Al(2) T and Brønsted acid site, more acidic, assigned to the signal at ∼54 ppm Al(3) T ). Phosphorus incorporation produces a modification in the concentration of these types of aluminum. As can be observed, impregnation of smaller amounts of phosphorus as in zeolite P(0.5)−BEA increases concentration of aluminum species Al(2) T , whereas at higher percentages, these species disappear completely suggesting a correlation between the generation and removal of these type of sites and the phosphorous content. Likewise, aluminum species assigned to the Al(3) T band were progressively decreasing with an increase in the phosphorus amount. 31 P MAS NMR was used for a better understanding of the nature of phosphorus species in the P(2.5)−BEA zeolite. After deconvolution of 31P MAS NMR spectrum (Figure 4), at least three contributions are observed at −16.9, −29.86, and −35.97 ppm, assigned to the presence of extra-lattice aluminum phosphate and polyphosphates partially joined to aluminum, AlPO4.3,15 The high amount of phosphorus impregnated over the zeolite induced the removal of tetrahedral aluminum from the zeolite framework to form octahedral aluminum species as those observed in the 31P MAS NMR (type Aloct−O−P). These results are in line with the results obtained by 27Al MAS NMR, where significant changes in the chemistry environment of aluminum species in this zeolite were evidenced as phosphorous is incorporated.
Figure 4. 31P MAS NMR spectra of zeolite P(2.5)−BEA and simulated spectra (red line) using standard deconvolution with symmetric function Gaussian.
Figure 5. NH3-TPD for zeolites.
textural properties of the material with an increase in phosphorus content. For the modified materials in this work, a similar trend is observed. Pore area and pore volume decrease for zeolites modified with phosphorus is generally attributed to the presence of condensed polyphosphates in micropores and to structural defects caused by impregnation and subsequent calcination processes.3,6 This is observed here D
DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. Acidity Measured by NH3 and Ipam-TPD acidic strength distributionb (mmol NH3/gcat) a
sample
Si/Al
total acid sites (mmol NH3/gcat)
low temperature peak
high temperature peak
H+c (mmol/gcat)
BEA P(0.5)−BEA P(2.5)−BEA
15.6 16.2 16.3
0.910 0.680 0.496
0.451(261 °C) 0.304(274 °C) 0.287(258 °C)
0.459(395 °C) 0.375(413 °C) 0.209(376 °C)
0.560 0.394 0.277
a
NH3 temperature-programmed desorption (TPD). bFrom deconvolution of NH3-TPD profiles. cBrønsted acid site concentration by IPam-TPD.
Table 4. Acidity Measured by Pyridine FT-IR Brønsted acid sites (mmol/g)
Lewis acid sites (mmol/g)
zeolite
totala
strongb
totala
strongb
total acidity mmol Py/g (Brønsted + Lewis)
strong total acidity mmol Py/g (Brønsted + Lewis)
BEA P(0.5)−BEA P(2.5)−BEA
0.145 0.149 0.063
0.055 0.048 0.026
0.093 0.034 0.083
0.012 0.030 0.046
0.245 0.183 0.146
0.064 0.078 0.072
Acid sites after desorption at 250 °C. bAcid sites after desorption at 400 °C.
a
Total acidity of zeolites modified with phosphorus was investigated by ammonia temperature-programmed desorption. Resulting curve profiles are shown in Figure 5. Distribution of acid sites is represented with two well-defined bands: a first one between 200 and 300 °C attributed to weak acidic sites and the second one between 370 and 550 °C attributed to strong acidic sites. As can be seen, the TPD signal intensity changes with phosphorus impregnation. In case of the zeolite with the lower amount of phosphorus, P(0.5)−BEA, an increase in the band signal at high temperature is observed, whereas the band intensity at low temperature decreases. This result is probably related with the relative increase in the hydroxyl groups associated with the tetrahedral aluminum sites Al(2) T for this sample. In both cases, this band is more intense than for the zeolite with no phosphorus or those with higher content. Table 3 shows the Si/Al ratio, concentration of acid sites (weak and strong) obtained by deconvolution of NH3-TPD profiles, and Brønsted acid site concentration obtained by IPam-TPD. Results show a decrease in the total acid site concentration measured by TPD-NH3 and Brønsted acid site concentration measured by isopropylamine adsorption, as the amount of phosphorous is increased. This acidity reduction is attributed to the formation of Al−O−P species during calcination that takes place after the impregnation process.3 Significant differences were observed in the acid distribution between weak and strong acid sites based on desorption temperatures. For instance, the P(0.5)−BEA zeolite shows a higher proportion of strong acid sites in comparison to the P(2.5)−BEA zeolite, indicating that higher phosphorous concentrations modifies the beta zeolite acid site distribution. This is in line with previous reports.3,12 With the purpose to obtain more information on the accessibility of acid sites, changes in acid strength, generation of acid sites (Brønsted and Lewis), and site location (internal or external) of zeolites modified with phosphorus, the Fouriertransform infrared spectroscopy (FTIR) technique by adsorption/desorption of pyridine, was used. Table 4 shows the results obtained with Py-FTIR adsorption/desorption for each zeolite. It is known that Brønsted acid sites are attributed to hydroxyl groups bonded to tetrahedrally coordinated aluminum atoms, which are active sites for a variety of reactions. However, it has been reported that acid properties of the beta
Figure 6. Correlation between the weak and strong Brønsted acid sites, with different tetrahedral aluminum obtained from 27Al MAS (3) NMR spectra: Al(2) T y AlT , respectively.
Figure 7. IR spectra in the region of OH groups stretching vibration of Beta zeolites. E
DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 5. Textural Properties of the Supports and Catalysts support (zeolite/alumina)
catalysts
sample
SBET (m2/g)
Smicro (m2/g)
Smeso (m2/g)
VTotal (cm3/g)
BEA−Al2O3 P(0.5)−BEA−Al2O3 P(2.5)−BEA−Al2O3 NiMo/BEA−Al2O3 NiMo/P(0.5)−BEA−Al2O3 NiMo/P(2.5)−BEA−Al2O3
739 473 285 721 518 321
451 194 64 465 250 143
288 278 221 256 268 178
0.40 0.62 0.45 0.35 0.56 0.40
with the Brønsted acid sites and the stretches associated with Lewis acid sites, respectively. In this study, the P(0.5)−BEA zeolite shows a slight increase in Brønsted acid sites as seen in the Py-FTIR Table 4, and a decrease in total acidity, compared to the pristine BEA zeolite, as seen in TPD-NH3 results. Similar results are reported in the literature for *BEA structure and postsynthesis treatment with orthophosphoric acid,16 and it is explained as a possible incorporation of phosphorus atoms into the zeolite framework through formation of (SiO)xAl(OP)4−x species that are generated by substitution of Si atoms by P; an increase in Brønsted acidity is also favored by incorporation of small amounts of phosphorus. In case of the zeolite impregnated with higher concentration of phosphorous, the P(2.5)−BEA zeolite, a decrease in Brønsted as well as in Lewis acidity was observed, probably because of more phosphorus reacting with OH groups to form Al−O−P bonds, so less OH groups would be available to interact with the pyridine. Zeolite acid properties are directly related to structural hydroxyl groups. Figure 7 shows FT-IR spectra recorded in the OH region for each zeolite. When there is presence of phosphorus, there is an intensity loss in the absorption band at 3610 cm−1, corresponding to OH vibrations in hydroxyl groups −SiOH−Al (Brønsted acid sites),17 specially for P(2.5)−BEA material. In addition, spectra of materials with phosphorus also show a modification in the band located close to 3745 cm−1. With an increase in phosphorus content there is a slight increase in this band’s intensity, mostly for the P(2.5)− BEA zeolite; this band is assigned to terminal hydroxyl group species −SiOH− located in the external surface of the zeolite, formed by tetrahedral aluminum removal that occurs as a consequence of the dealumination process when impregnation with phosphorus takes place.3,17,18 Furthermore, for zeolites with P, a decrease in the bands at 3785 and 3665 cm−1 is observed. These bands are assigned to AlOOH+ species and Al−OH groups located in the external surface, respectively.18,19 The less intense signal observed at 3700 cm−1 is attributed to the presence of P−OH groups and it is more notorious in P(0.5)−BEA and P(2.5)−BEA zeolites. 3.2. Physicochemical Properties of Supports and NiMo Catalysts. Table 5 shows the textural properties of supports and NiMo catalysts. A decrease in the BET area and an increase in the total pore volume in the supports compared to the zeolites, because of the presence of alumina, are observed. Pore volume values obtained for the catalysts show the same trend that the ones reported by zeolites modified with phosphorus, indicating that the interaction between zeolite and Al2O3 is not affected by the presence of phosphorus and its content.20 However, when nickel and molybdenum are impregnated, significant changes in textural properties are observed.21 Figure 8 shows the 27Al MAS NMR spectra for the four (2) prepared catalysts. Three main signals denoted as Al(1) T , AlT ,
Figure 8. 27Al MAS NMR spectra for NiMo catalysts.
Figure 9. TPR profiles of supported NiMo catalysts.
zeolite are also influenced by local defects, which can induce acid strength variations from site to site.4 In order to analyze this effect, the relation between pyridine adsorption/ desorption experiments and aluminum types identified by 27 (3) Al NMR, is explored. Figure 6 shows that Al(2) T and AlT are related with the acid site concentrations determined by pyridine adsorption. On the one hand, there is a close relation between aluminum sites Al(2) T and the weak acidity (calculated with the difference between total and strong Brønsted acidity). In case of aluminum sites, Al(3) T , a relation with the strong acid sites, acid sites that maintain pyridine adsorbed at 400 °C, was observed. Studies on modification of zeolites with phosphorus have reported a decrease in strength and concentration of Brønsted−Lewis acid sites associated with dealumination and the concomitant formation of aluminophosphates.3 Fourier transform infrared spectroscopy of pyridine adsorbed (PyFTIR) on solid acid catalysts allows determination of Brønsted acid sites and Lewis acid sites, through the identification of pyridinium ions, resulting from the interactions of pyridine F
DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 10. TEM−EDX images of the catalyst.
Figure 11. VGO hydrocracking conversion and middle distillate selectivity vs time on stream. (Temperature: 350 °C; WHSV: 1 h−1, 10 MPa).
and Al(1) O are observed. In comparison with those obtained for the phosphorous-modified zeolites (Figure 3), intensity of Al(1) T (2) and Al(1) O signals increases, whereas the AlT signal decreases. Besides, the mixing effect of alumina and zeolite (40:60 ratio), it is important to note the notorious presence of the signal Al(4) T at 40 ppm for the NiMo/P(2.5)−BEA−Al2O3 catalyst, that is attributed to the presence of either pentacoordinated Al in alumina22 or distorted tetrahedral aluminum in the zeolite. Figure 9 shows hydrogen temperature-programmed reduction profiles. In general, for all the catalysts, two reduction events at different temperatures are observed. The first event (between 350 and 580 °C) is associated to the first step of reduction, Mo6+ → Mo4+, of octahedral molybdenum species, and the second event (between 750 and 980 °C) is attributed to complete reduction of strongly bonded molybdenum.23−25 The shoulder observed between 550 and 750 °C is attributed to the presence of reducible intermediate crystalline phases of
Figure 12. Correlation between the conversion and strong total acidity (Brønsted + Lewis by pyridine IR).
Table 6. Catalytic Activity of Supported NiMo Catalysts after 70 h on Stream a
catalyst NiMo/BEA−Al2O3 NiMo/P(0.5)−BEA−Al2O3 NiMo/P(2.5)−BEA−Al2O3
a
b
hydrocracking conversion wt % 16 22 20
middle distillate 55 41 65
a
selectivity wt % c
naphtha 20 24 26
b
yields wt %
middle distillate
c
9 9 13
naphtha 3 5 5
Average values after 70 h on stream. bMiddle distillates: B.P. in the range 180−370 °C. cNaphtha: B.P. in the range IPB to 180 °C.
a
G
DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
the formation of aluminophosphates. On the other hand, acidity results show a clear decrease in Brønsted and Lewis acid sites because of the interaction of phosphorus with OH groups from the zeolite, being lower as phosphorus content increases. Catalysts based on the zeolites obtained were evaluated in vacuum gas oil hydrocracking reaction and results show better cracking activity for the catalysts based on the zeolites that had higher strong acidity (Brønsted + Lewis).
orthorhombic species of MoO3 and Al2(MoO4)3 because of the strong interaction of the Mo species with the carrier and total reduction of Ni2+ species over the carrier.24,25 The TEM−EDX mapping micrographs shown in Figure 10 of the catalysts permitted to determine that the crystalline structure of the zeolite remain after the impregnation process with P, Ni, and Mo. The metal nanoparticles are well dispersed on the external surface of the catalysts and there is no evidence of agglomeration of particles. 3.3. Catalytic Activity of Supported NiMo Catalysts. NiMo catalysts were evaluated in VGO reaction and its catalytic performance is shown in Figure 11. In general, under the experimental conditions used, materials show a stable behavior during the first 55 h of time on stream. However, after this reaction time, a slight loss of activity is observed, which could be attributed to coke deposition over the catalyst’s pores. The conversion of phosphorous-modified zeolites is higher than that found in the unmodified zeolite, being the zeolite with less phosphorus (P(0.5)−BEA) that is more active. As the time on stream increased, conversion differences decreased up to about 5%. Nevertheless, considering the large differences in the middle distillate selectivities for the catalyst with the highest content of phosphorus, it can be concluded that modifications in the beta zeolite with more phosphorus are more beneficial in VGO hydrocracking. In order to correlate catalyst activity with the acidity measurements conducted, various correlations were evaluated. For instance, in Figure 12 is shown that VGO conversion is directly related with the total strong acidity determined by pyridine IR spectra collected at (400 °C), indicating a possible synergistic effect between both types of acids sites, that is, Brønsted and Lewis. Catalysts that contain phosphorus in zeolites show better activity in the reaction than the catalyst prepared with the reference zeolite, which suggests that the presence of Al−O−P species can favor catalytic cracking. These results are in agreement with previous reports where the presence of alumina and aluminophosphates has proved to increase the activity of the catalysts in different processes for the conversion of hydrocarbons.26 Selectivity was inversely proportional to the content of octahedral aluminum species, specifically with Al(1) O aluminum (higher extra-lattice aluminum led to lower selectivity). Extrastructural aluminum species have demonstrated to have a negative effect on the catalysts’ behavior for gasoil cracking because its presence facilitates higher naphtha production.27 Moreover, its presence in the internal and external surface of the zeolite can lead to pore blocking,28 which prevents the access of high-molecular weight molecules like the ones from gasoil to the active sites. Table 6 shows conversion, selectivity, and yield in middle distillates and naphtha after 70 h of reaction. In general, catalysts have higher selectivity and yield in middle distillates than naphtha, which indicates the high capacity of catalysts to obtain hydrocarbons in the desired range.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +57 4 2195667. ORCID
Cecilia Manrique: 0000-0002-7218-5804 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors acknowledge financial support by Universidad de Antioquia, Instituto Colombiano del PetroleoICP, ECOPETROL S.A., and COLCIENCIAS financial support project 1115-669-46635. C.M. acknowledges COLCIENCIAS her doctoral fellowship.
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REFERENCES
(1) Peixoto, D. P. B.; Cabral de Menezes, S. M.; Pais da Silva, M. I. Influence of Different Processes of Dealumination on Acid Properties of an H-Ferrierite Zeolite. Mater. Lett. 2003, 57, 3933−3942. (2) Ramakanta, Q. Journal of Industrial and Engineering Chemistry A Review of Recent Advances in Catalytic Hydrocracking of Heavy Residues. J. Ind. Eng. Chem. 2015, 27, 12. (3) Göhlich, M.; Reschetilowski, W.; Paasch, S. Spectroscopic Study of Phosphorus Modified H-ZSM-5. Microporous Mesoporous Mater. 2011, 142, 178−183. (4) Guan, J.; Li, X.; Yang, G.; Zhang, W.; Liu, X.; Han, X.; Bao, X. Interactions of Phosphorous Molecules with the Acid Sites of H-Beta Zeolite: Insights from Solid-State NMR Techniques and Theoretical Calculations. J. Mol. Catal. A: Chem. 2009, 310, 113−120. (5) Ghiaci, M.; Abbaspur, A.; Arshadi, M.; Aghabarari, B. Internal versus External Surface Active Sites in ZSM-5 ZeolitePart 2: Toluene Alkylation with Methanol and 2-Propanol Catalyzed by Modified and Unmodified H3PO4/ZSM-5. Appl. Catal., A 2007, 316, 32−46. (6) Corma, A.; Mengual, J.; Miguel, P. J. Stabilization of ZSM-5 Zeolite Catalysts for Steam Catalytic Cracking of Naphtha for Production of Propene and Ethene. Appl. Catal., A 2012, 421-422, 121−134. (7) Rayo, P.; Ramírez, J.; Torres-Mancera, P.; Marroquín, G.; Maity, S. K.; Ancheyta, J. Hydrodesulfurization and Hydrocracking of Maya Crude with P-Modified NiMo/Al 2O 3 Catalysts. Fuel 2012, 100, 34−42. (8) Gu, J.-F.; Zhang, X.-J.; Wang, J.-Z.; Xu, J.; Deng, F.; Yuan, Z.-Y. Synthesis of B-Beta Zeolite and Its Modification with Orthophosphoric Acid. Stud. Surf. Sci. Catal. 2008, 174, 209−212. (9) Ma, L.; Jia, I.; Guo, X.; Xiang, L. Catalytic activity of Ag/SBA-15 for low-temperature gas-phase selective oxidation of benzyl alcohol to benzaldehyde. Chinese J. Catal. 2014, 35, 108−119. (10) de Menezes, S. M. C.; Lam, Y. L.; Damodaran, K.; Pruski, M. Modification of H-ZSM-5 Zeolites with Phosphorus. 1. Identification of Aluminum Species by 27Al Solid-State NMR and Characterization of Their Catalytic Properties. Microporous Mesoporous Mater. 2006, 95, 286−295. (11) Jang, H.-G.; Min, H.-K.; Hong, S. B.; Seo, G. Tetramethylbenzenium Radical Cations as Major Active Intermediates of Methanol-to-Olefin Conversions over Phosphorous-Modified HZSM-5 Zeolites. J. Catal. 2013, 299, 240−248.
4. CONCLUSIONS Incorporation of phosphoric acid to the BEA zeolite produced a decrease in the crystalline structure, being more notorious as phosphorus content increases. Likewise, a change in textural properties was evidenced as decrease in the pore volume and surface area. According to 27Al MAS NMR results, an increase in phosphorus content decreased tetrahedral aluminum concentration and increased extra reticular species through H
DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels (12) Epelde, E.; Gayubo, A. G.; Olazar, M.; Bilbao, J.; Aguayo, A. T. Modified HZSM-5 Zeolites for Intensifying Propylene Production in the Transformation of 1-Butene. Chem. Eng. J. 2014, 251, 80−91. (13) Maier, S. M.; Jentys, A.; Lercher, J. A. Steaming of Zeolite BEA and Its Effect on Acidity: A Comparative NMR and IR Spectroscopic Study. J. Phys. Chem. C 2011, 115, 8005−8013. (14) Abraham, A.; Lee, S.-H.; Shin, C.-H.; Bong Hong, S.; Prins, R.; van Bokhoven, J. A. Influence of Framework Silicon to Aluminium Ratio on Aluminium Coordination and Distribution in Zeolite Beta Investigated by 27 Al MAS and 27 Al MQ MAS NMR. Phys. Chem. Chem. Phys. 2004, 6, 3031−3036. (15) Zeng, P.; Liang, Y.; Ji, S.; Shen, B.; Liu, H.; Wang, B.; Zhao, H.; Li, M. Preparation of Phosphorus-Modified PITQ-13 Catalysts and Their Performance in 1-Butene Catalytic Cracking. J. Energy Chem. 2014, 23, 193−200. (16) Gu, J.; Zhang, X.; Wang, J.; Xu, J.; Deng, F. Zeolites and Related Materials: Trends, Targets and Challenges, Proceedings of the 4th International FEZA Conference; Studies in Surface Science and Catalysis; Elsevier, 2008; Vol. 174. (17) Shcherban, N.; Barakov, R.; Yaremov, P.; Solomakha, V.; Gryn, S.; Khaynakova, O.; Ilyin, V. Template Synthesis, Structure, Sorption Properties and Acidity of Micro-Mesoporous Materials Obtained from Sol-Precursor of Zeolite BEA. J. Porous Mater. 2014, 21, 355− 363. (18) Gabrienko, A. A.; Danilova, I. G.; Arzumanov, S. S.; Toktarev, A. V.; Freude, D.; Stepanov, A. G. Strong Acidity of Silanol Groups of Zeolite Beta: Evidence from the Studies by IR Spectroscopy of Adsorbed CO and 1H MAS NMR. Microporous Mesoporous Mater. 2010, 131, 210−216. (19) Trombetta, M.; Busca, G.; Storaro, L.; Lenarda, M.; Casagrande, M.; Zambon, A. Surface Acidity Modifications Induced by Thermal Treatments and Acid Leaching on Microcrystalline HBEA Zeolite. A FTIR, XRD and MAS-NMR Study. Phys. Chem. Chem. Phys. 2000, 2, 3529−3537. (20) Wang, Y.; Shen, B.; Wang, L.; Feng, B.; Li, J.; Guo, Q. Effect of Phosphorus Modified USY on Coupled Hydrogenation and Ring Opening Performance of NiW/USY + Al2O3 Hydro-Upgrading Catalyst. Fuel Process. Technol. 2013, 106, 141−148. (21) Pashigreva, A. V.; Klimov, O. V.; Bukhtiyarova, G. A.; Fedotov, M. A.; Kochubey, D. I.; Chesalov, Y. A.; Zaikovskii, V. I.; Prosvirin, I. P.; Noskov, A. S. The Superior Activity of the CoMo Hydrotreating Catalysts, Prepared Using Citric Acid: What’s the Reason? Stud. Surf. Sci. Catal. 2010, 175, 109−116. (22) Lee, M.-H.; Cheng, C.-F.; Heine, V.; Klinowski, J. Distribution of Tetrahedral and Octahedral A1 Sites in Gamma Alumina. Chem. Phys. Lett. 1997, 265, 673−676. (23) Al-Dalama, K.; Stanislaus, A. Temperature Programmed Reduction of SiO2−Al2O3 Supported Ni, Mo and NiMo Catalysts Prepared with EDTA. Thermochim. Acta 2011, 520, 67−74. (24) Qu, L.; Zhang, W.; Kooyman, P. J.; Prins, R. MAS NMR, TPR, and TEM studies of the interaction of NiMo with alumina and silicaalumina supports. J. Catal. 2003, 215, 7−13. (25) Solís, D.; Agudo, A. L.; Ramírez, J.; Klimova, T. Hydrodesulfurization of Hindered Dibenzothiophenes on Bifunctional NiMo Catalysts Supported on Zeolite−alumina Composites. Catal. Today 2006, 116, 469−477. (26) Pine, L. A. Hydrocarbon Conversion Catalyst Comprising Alumina and Aluminum Phosphate. U.S. Patent 3,904,550 A, 1975. (27) Pellet, R. J.; Blackwell, C. S.; Rabo, J. A. Catalytic cracking studies and characterization of steamed Y and LZ-210 zeolites. J. Catal. 1988, 114, 71−89. (28) Wojciechowski, B. W. The Reaction Mechanism of Catalytic Cracking: Quantifying Activity, Selectivity, and Catalyst Decay. Catal. Rev. 1998, 40, 209−328.
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DOI: 10.1021/acs.energyfuels.8b04485 Energy Fuels XXXX, XXX, XXX−XXX
