Modification of USY zeolites with malic-nitric acid for hydrocracking

Abstract

The modification of commercial ultra-stable Y zeolite using malic acid (MA) and nitric acid (NA) was investigated. A series of factors including the amount of MA and NA solutions, the pH of the solutions, the treatment time, and the reaction temperature were investigated and optimized. The pore structure, acid properties, and crystal structure of modified USY zeolite were characterized by N2-adsorption, temperature-programmed desorption of ammonia (NH3-TPD), pyridine adsorbed Fourier transform infrared spectroscopy, and X-ray diffraction techniques. The as-obtained sample under the optimum conditions presents an increased secondary pore volume up to 0.202 cm ³ g^-1, which accounts for 45.3 % of the total pore volume, and appropriate acid properties as well as good crystallinity. Furthermore, the USY zeolite modified with different methods was also investigated, indicating that malic-nitric combined acid is an effective modifier for USY zeolite. The modified USY zeolite was used as support to prepare hydrocracking catalysts. The 140-370 °C middle distillate yield of the catalyst is 68.59 %, and middle distillate selectivity can reach up to 81.52 %. Compared with commercial catalyst, the yield and selectivity increased by 8.17 and 5.14 %, respectively.

Appl Petrochem Res (2016) 6:353359 DOI 10.1007/s13203-015-0145-7

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Modication of USY zeolites with malicnitric acid for hydrocracking

Ke Qiao1 Xuejin Li1 Yang Yang1 Fazle Subhan2 Xinmei Liu1

Zifeng Yan1 Wei Xing1 Lihong Qin3 Baoqin Dai3 Zhihua Zhang3

Received: 20 October 2015 / Accepted: 17 December 2015 / Published online: 29 January 2016 The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract The modication of commercial ultra-stable Y zeolite using malic acid (MA) and nitric acid (NA) was investigated. A series of factors including the amount of MA and NA solutions, the pH of the solutions, the treatment time, and the reaction temperature were investigated and optimized. The pore structure, acid properties, and crystal structure of modied USY zeolite were characterized by N2-adsorption, temperature-programmed desorption of ammonia (NH3-TPD), pyridine adsorbed Fourier transform infrared spectroscopy, and X-ray diffraction techniques. The as-obtained sample under the optimum conditions presents an increased secondary pore volume up to 0.202 cm3 g-1, which accounts for 45.3 % of the total pore volume, and appropriate acid properties as well as good crystallinity. Furthermore, the USY zeolite modied with different methods was also investigated, indicating that malicnitric combined acid is an effective modier for USY zeolite. The modied USY zeolite was used as support to prepare hydrocracking catalysts. The 140370 C middle distillate yield of the catalyst is 68.59 %, and middle distillate selectivity can reach up to 81.52 %. Compared with commercial catalyst, the yield and selectivity increased by 8.17 and 5.14 %, respectively.

Keywords USY zeolite Modication Malic acid

Nitric acid

Introduction

With the depletion of petroleum, coal, natural gas, and other fossil fuels, the global energy crisis is increasing rapidly. However, due to rapid population growth, the consumption of fuels, energy, and petrochemical products has been increasing tremendously. Thus, new challenges for the petroleum reneries to upgrade heavy oils and residues are (1) to produce more middle distillate products [13], (2) high-quality transportation fuels, and (3) depletion of crude oil sources [4]. During the past three decades, hydrocracking has gained prominence in light petroleum rening processes [5]. After complete industrialization of light petroleum oil, hydrocracking processes are gradually applied for heavy oil and VR upgradation. However, these processes still needed the development of hydrocracking catalyst with rapid ion/mass transfer channels as well as appropriate acid sites.

Due to the novel pore structure and surface acidy, USY zeolite has been widely used in hydrocracking. However, the performance of commercial USY (CUSY) zeolites is unsatisfactory due to the lack of mesopores and large acidic sites. Cracking is often limited by the diffusion of reactants inside the micropore of zeolite. The mesoporous structure is more suitable for diffusion of reactants. In addition, high acidity can cause coking, which leads to catalyst deactivation. The catalyst deactivation will cause a reduction of product selectivity and quality. Thus, tremendous research about modication of USY zeolite has been reported in literature in the past [610]. Dealumination of framework aluminum is an effective way to improve the Si/Al ratio

& Zifeng Yan [email protected]

1 State Key Laboratory of Heavy Oil Processing, PetroChina Key Laboratory of Catalysis, China University of Petroleum, Qingdao 266580, China

2 Department of Chemistry, Abdul Wali Khan University Mardan, Mardan, K.P.K, Pakistan

3 Daqing Petrochemical Research Center, PetroChina Petrochemical Institute, CNPC, Daqing 163714, China

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and acid property. For this purpose, thermal or hydrothermal treatments [9], acids leaching [10], and chemical treatments with hexauorosilicate or silicon tetrachloride are reported [1114]. Among these methods, hydrothermal treatment is one of the most promising and sole industrialized methods. In this method, high-temperature steam reacts with the framework aluminum to produce Al(OH)x which will remain inside the pores [15]. The modied USY zeolite not only provide mesopores but also appropriate amount of Brnsted acid sites. However, the removed aluminum species will block the pore channels. In addition, the crystallinity reduces rapidly with an increase of temperature. Ludmila Kubelkov et al. [13] modied Y zeolite with SiCl4, which could react with the framework aluminum under high temperature. The skeleton vacancies caused by dealumination can be compensated by extra-framework silica. The modied USY has good acidic properties and high crystallinity, as well as improved hydrothermal stability. But the lack of abundant mesopores limits its applications in hydrocracking. Organic co-ordination reaction is another useful way to remove framework or extra-framework aluminum. The organic complex reacts with aluminum via complexation under a moderate reaction conditions, and no obvious defects are observed in the modied samples [16, 17]. For example, Liu et al. [17] studied the modication of USY zeolite using citric acid in an unbuffered reaction system. The obtained zeolite possessed improved Si/Al ratio, smaller unit cell parameters, and more developed pore structure. Up to date, the aforementioned modication methods still experience a lot of problems and challenges, such as poor hydrothermal stability, less developed mesoporous structure, and harsh operation conditions. Exploring a facile and efcient approach to modify CUSY zeolite is still challenging.

In this work, we reported a combined modication of USY zeolite using malic acid (MA) and nitric acid (NA). MA removes aluminum by coordination reaction to create abundant secondary pores while NA removes the extra-framework alumina. A series of experiments were carried out to investigate the effects of various factors on the USY zeolites. Furthermore, the crystal structure, textural, and acid properties of the modied USY (MNUSY) zeolite were also investigated by XRD, N2 adsorptiondesorption isotherms, Py-FT-IR, and NH3-TPD.

Experimental

Materials

Malic acid (AR), nitric acid (AR), citric acid (AR), and phosphoric acid (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. Commercial USY zeolites were

purchased from Zibo HuaXin catalyst Co. Ltd. All chemicals were used as received.

Sample preparation

Firstly, MA solution and NA solution with different concentrations were prepared. Then, a xed quantity of 4 g CUSY zeolite and different amounts of NA solution were put into a 250-ml three-neck ask. Subsequently, the ask was transferred into a water bath set at a certain temperature. The same volume of MA solution was then added into the ask within 5 min. The modication was completed after a certain time period. Afterwards, the products were ltrated and washed with distilled water. Finally, the product was obtained after drying at 110 C in an oven overnight.

Sample characterization

Nitrogen adsorptiondesorption measurements were performed on a TriStar 3000 analyzer (Micromeritics, USA) to obtain specic surface area and pore structure parameters of the as-prepared samples. The total surface area was calculated using BrunauerEmmettTeller (BET) method. The mesopore surface area, mesopore volumes, and pore size distribution were obtained from the desorption branch by BarretJoynerHalenda (BJH) method. Si/Al ratio, crystal cell parameters, and crystallinity were characterized by X-ray diffraction (XRD) analysis (XPert PRO MPD, Holland). FT-IR spectra of samples with pyridine (py) adsorption were measured on Nicolet 6700, U.S.A. Temperature-programmed desorption of ammonia (NH3-TPD)

was as carried out on CHEMBET-3000 TPR/TPD Chemisorption analyzer (Quantachrome Instrument, U.S.A.). About 100 mg of sample was pretreated at 200 C in nitrogen (20 mL/min) for 30 min in a U-shaped quartz tube. After the sample was cool down to room temperature, ammonia was then injected into the tube. TPD was performed from 50 to 700 C at a heating rate of 15 C/min when physisorbed ammonia was purged with nitrogen.

Evaluation of catalyst

Prior to the hydrocracking test, the modied USY sample (10 wt%) and amorphous silica-alumina (90 wt%) were mixed together using alumina as peptizator, and then the carrier was produced by extrusion machine in the shape of long column. Ni-W as an active component was loaded on carrier by impregnation method. After drying and calci-nation, the catalyst was prepared.

Performance evaluation of the hydrocracking catalyst was carried out on a 200 mL xed-bed single-stage hydrogenation unit using Daqing VGO as feedstock under

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the following conditions: pressure 15.0 MPa, volume space velocity 1.5 h-1, V(H2)/V(oil) = 1250, and reaction temperature 385 C. 100 mL catalyst was loaded. The main properties of Daqing VGO were as follows: Density (20 C), 0.8519 g/cm3, distillate range, 240500 C, S content, 827 lg/g, N content, 986 lg/g.

Results and discussions

Optimization of operation conditions

Mesopores USY are important for fast transmission of reactants and products of hydrocracking when it was used in hydrocracking. Thus, the mesopores of USY should be generated as much as possible after the USY zeolite was treated with MA and NA solutions. However, the crystallinity of USY will be reduced with an increase of mesopores during the dealumination. Therefore, we investigated the optimum operation conditions by a single-factor experiment using mesopore volume and relative crystallinity as a criteria. The modication conditions and results are listed in Table 1. The experimental conditions, such as the amount of MA and NA solutions, pH of the solutions, treatment time, and reaction temperature are investigated.

As shown in Table 1, with the increase of MA and NA solutions content, the secondary pore volume increased while the relative crystallinity reduced. When the amount of MA and NA are less than 60 mL, the secondary pore

volume decreases to some extent, whereas the relative crystallinity reduces to a great extent. Therefore, 80 mL of MA and NA was considered as an appropriate level. During the modication process, most of the extra-framework aluminum and a part of framework aluminum were removed via protons from NA and carboxylate ions from MA, leading to an increase of pore volume and decrease of crystallinity. Then, we investigate the inuence of pH on the modication. It is obvious that the pH of acid solutions has a great inuence on the pore volume and crystallinity.With the decrease of pH, the pore volume increases and the relative crystallinity decreases. The results also indicate that the relative crystallinity decreased with the decrease in pH from 2 to 1.5. To prevent the excessive framework damage and obtain abundant mesopores, pH was adjusted to an optimum level of 1.5. In addition, the treatment time is also a very important factor for the modication of USY zeolite. As listed in Table 1, the mesopore volume increases from 0.153 to 0.202 cm3 g-1 when the reaction time increases from 0 to 4 h. This is due to the fact that the extra-framework and a part of framework aluminum can substantially be removed with the increase of treatment time. However, when the treatment time rises to 8 h, a dramatic decrease of mesopore volume and relative crystallinity is observed. Thus, 4 h is considered as an optimum treatment time. Furthermore, the modications of USY zeolite are also performed at different temperatures. The secondary pore volume of modied USY zeolite increases with the increase of reaction temperature as expected.However, the framework structure of USY zeolite will be

Table 1 The secondary pore volume and relative crystallinity of modied USY zeolite at different operation conditions

Samples Volume of MA (mL)

Volume of NA (mL)

Relative crystallinity (%)

MN-1 20 20 1.5 1.5 4 80 0.189 65.99

MN-2 40 40 1.5 1.5 4 80 0.188 63.86

MN-3 60 60 1.5 1.5 4 80 0.192 59.41

MN-4 80 80 1.5 1.5 4 80 0.202 56.56

MN-5 80 80 7 7 4 80 0.153 60.71

MN-6 80 80 4 4 4 80 0.164 64.14

MN-7 80 80 2 2 4 80 0.190 66.59

MN-8 80 80 1.5 1.5 4 80 0.202 56.56

MN-9 80 80 1.5 1.5 0 80 0.153 60.71

MN-10 80 80 1.5 1.5 2 80 0.175 63.60

MN-11 80 80 1.5 1.5 4 80 0.202 56.97

MN-12 80 80 1.5 1.5 8 80 0.126 48.27

MN-13 80 80 1.5 1.5 4 25 0.153 60.71

MN-14 80 80 1.5 1.5 4 60 0.167 58.01

MN-15 80 80 1.5 1.5 4 80 0.202 56.97 MN-16 80 80 1.5 1.5 4 100 0.151 52.57

A-refers to the pH of MA and NA solutions used in the modication

pHa of

MA

pHa of

NA

Treatment time (h)

Reaction temperature ( C)

Secondary pore volume (cm3/g)

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destroyed heavily at high temperature because of corrosion of H?, leading to a rapid decrease of crystallinity. For this purpose, USY zeolite was modied at 80 C in the present study. According to the results of single-factor experiment, it can be concluded that the modication of USY zeolite has the best performance when the amount of MA, NA, pH of MA, pH of NA, reaction time, and temperature are 80 mL, 80 mL, 1.5, 1.5, 4 h, and 80 C, respectively.

Then, MNUSY was prepared at optimum modication conditions.

Characterization and discussion

Textural properties of modied USY zeolite

N2 adsorptiondesorption isotherms determined at 77 K and the pore size distribution of the prepared samples are shown in Fig. 1. Both CUSY and MNUSY present type IV isotherms with an H2 hysteresis loops in the relative pressure (p/p0) range of 0.431.0, characteristics of developed mesoporous structures. Compared with CUSY,

MNUSY possesses larger N2-adsorption quantity and mesopore volume. As shown in Table 2, MNUSY has a high specic surface area of 613 m2 g-1 and a large mesopore volume of 0.202 m3 g-1. The pore size distribution curves also conrmed the existence of mesopores, centered at 8.0 and 23.0 nm. Nitric acid, as a small molecular inorganic acid, can attract the framework aluminum on the surface of USY zeolite to weaken the framework structure. A part of extra-framework aluminum

can also be removed by NA, leading to an unobstructed pore channel to facilitate the entrance of MA molecular. MA molecular possesses two carboxyls and a hydroxyl which can coordinate with extra-framework aluminum, resulting in abundant of tetrahydroxy vacancies. The micropores generate with the processing of dealumination as shown in Table 2. With further removal of extra-framework aluminum and framework aluminum, microp-ores connect with each other to form mesopores.

Acid characterization of modied USY zeolite

The acidic properties of samples were characterized by Py-FT-IR and NH3-TPD techniques (Fig. 2). As shown in

Fig. 2a, all the samples exhibited three IR bands at 1443, 1545, and 1490 cm-1. The bands of CUSY at 1443 and 1545 cm-1 corresponded to the characteristic of pyridine molecules chemisorbed on Lewis (L) and Brnsted(B) acidic sites, respectively, while the band at 1490 cm-1

can be assigned to both B and L acidic sites [18]. In comparison with CUSY, the bands of MNUSY attributed to L and B acidic sites were shifted to 1445 and 1546 cm-1,

respectively, indicating that the existing modication process can strengthen the acidic sites in the sample. The acidic strength of samples was measured by NH3-TPD. As shown in Fig. 2b, all samples exhibited two ammonia desorption peaks at 200 and 450 C. According to the peak area, the amount of both strong acid and weak acidic sites was decreased. However, the decrease of weak acid was

Fig. 1 a N2 adsorptiondesorption isotherms and b pore size distributions of modied USY zeolite

Table 2 Texture properties of modied USY zeolite

Samples BET surface area (m2/g)

Micropore area(m2/g)

Secondary pore area (m2/g)

Total volume (cm3/g)

Micropore volume (cm3/g)

Secondary pore volume (cm3/g)

CUSY 542 485 85 0.388 0.249 0.153

MNUSY 613 488 136 0.446 0.255 0.202

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Fig. 2 a Pyridine adsorbed FT-IR diffuse reection spectra and b NH3-TPD proles of modied USY zeolite

much higher than medium acid, which will play an important role in improving the yield of middle distillate.

XRD characterization

The wide angle-XRD patterns of samples are shown in Fig. 3. As can be seen from Fig. 3, CUSY exhibited high intensity characteristic diffraction peaks of faujasite framework, revealed the existence of high crystallinity in structure. After modication with MA and NA solutions, the diffraction peak intensity corresponding to FAU structure decreased slightly in intensity. By calculating the diffraction peak height, we can say that the relative crystallinity of USY zeolite was decreased from 60.71 to56.56 % after modication. These results indicate that MANA combined solution has slightly destroyed the crystal structure by creating abundant secondary pores. During the dealumination of MANA solution, most of the extra-framework aluminum in the pore channels was removed, meanwhile the extra-framework silicon can insert into the vacancy of tetrahydroxy. The inserted silicon can

increase the crystallinity to a certain extent. This can be conrmed by the change of framework Si/Al ratio. The Si/ Al ratio increased dramatically from 10.8 (CUSY) to 26.5 (MNUSY) after modication. However, as the dealumi-nation processed, maximum amount of aluminum would be removed by MA and NA, leading to a continuous destruction of crystal structure.

Comparison of modied USY zeolite with different acids

According to the reports in literature [19], citric acid combined with phosphoric acid seems to be an effective modier for USY zeolite. Therefore, we compared the modied samples prepared under optimum conditions with two different methods. From XRD pattern (Fig. 3), it can be seen that MNUSY possessed almost the same diffraction peaks as that of citric acid and phosphoric combined acid-modied USY (CPUSY). As shown in Table 3, these two samples have similar relative crystallinity and framework Si/Al ratio, indicating the comparable dealumination performance of these two different acid systems. As shown in Fig. 4a, MNUSY presents a larger hysteresis loop at a P/P0 ranging from 0.43 to 1.0 due to capillary condensation, which is an indicator of large mesopore volume. This phenomenon can also be observed in the pore size distribution curves (Fig. 4b). MNUSY presents larger pore volume at 8 and 23 nm than CPUSY, suggesting its much more abundant mesopores. The texture properties of modied USY are listed in Table 4. The CPUSY has a

Fig. 3 The XRD pattern of USY zeolites

Table 3 Crystal structural parameters of modied USY

Sample Degree of crystallinity (CRX/%) Si/Al ratio

CUSY 60.71 10.8

MNUSY 56.56 26.5

CPUSY 56.21 25.3

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Fig. 4 a N2 adsorptiondesorption isotherms and b pore size distributions of modied USY zeolite with different acids

Table 4 Texture properties of modied USY zeolite

Sample BET surface area (m2/g)

Micropore area (m2/g)

Secondary pore area (m2/g)

Total volume (cm3/g)

Micropore volume (cm3/g)

Secondary pore volume (cm3/g)

CPUSY 606 519 116 0.432 0.269 0.179

MNUSY 613 489 136 0.446 0.255 0.202

1440 1500 1560

Reflectance(%)

1490 cm

1545 cm

Wavenumber/cm-1

Fig. 5 Pyridine-adsorbed FT-IR diffuse reection spectra of modied USY zeolite

specic surface area of 606 m2 g-1 and secondary pore volume of 0.179 cm3 g-1 while MNUSY presents a higher specic surface area of 613 m2 g-1 and larger secondary pore volume of 0.202 cm3 g-1. Higher number of secondary pores of MNUSY led it to be a better hydrocracking catalyst than CPUSY. The Py-FT-IR was also used to characterize the acid properties of CPUSY and MNUSY. As shown in Fig. 5, they both exhibited characteristic peaks located at 1445, 1490, and 1545 cm-1. The amount ratio of B acid and L acid were calculated by their corresponding peak areas. Its obvious that the B/L of MNUSY

is higher than that of CPUSY, which is protable for hydrocracking reactions.

Evaluation of catalyst

According to the 77.1 % conversion of[350 C feedstock (Table 5), MNUSY supported catalyst-exhibited excellent hydrocracking performance. The 140370 C middle distillate yield of hydrocracking product is 68.6 %. Meanwhile the selectivity to middle distillate can reach up to81.5 %. Compared with CUSY-supported catalyst, the yield and selectivity of MNUSY-supported catalyst increased by 8.2 and 5.1 %, respectively. In addition, MNUSY-supported catalyst also presents higher yield and selectivity than CPUSY-supported catalyst, indicating a better hydrocracking performance of MNUSY than CPUSY. These results demonstrate that combined modication of USY zeolites using malic acid and nitric acid can meet the requirements of productive middle distillate in the industrial unit, which may have potential applications in the commercial methods of Y zeolites modication.

Conclusion

A combined modication of CUSY zeolite using malic acid and nitric acid was successfully developed. The optimum operation conditions were investigated via a single-factor experiment. The modied USY zeolite under

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Appl Petrochem Res (2016) 6:353359 359

Table 5 Hydrocracking performance of catalysts

Products distribution (%) CUSY CPUSY MNUSY

HK-140 C 18.68 16.08 15.54 140370 C 60.42 66.09 68.59

[370 C 20.16 16.85 15.03 Loss 0.74 0.98 0.84

Conversion of [350 C feedstock (%) 75.0 75.9 77.1 Selectivity to middle distillate (%) 76.38 80.45 81.52

Conversion = (1 - [350 C fraction of production/[350 C fraction of feed oil) 9 100 %. Yield = (140370 C middle distillates fraction of production) 9 100 %. Middle distillates selectivity = (140370 C fraction of production/\370 C fraction of production) 9 100 %

the optimum technological conditions presents an enhanced secondary pore volume and appropriate acid distribution as well as good crystallinity. In addition, the USY zeolite modied by malic acid and nitric acid possesses larger secondary pore volume and more appropriate acid properties than the USY modied with citric acid and phosphoric acid. Abundant mesoporous pore structure and appropriate acidic sites make MNUSY an excellent catalyst support in hydrocracking catalysts.

Acknowledgments This work was nancially supported by Petro-China Renery Catalyst Key Project (2010E-1903) and Key Program of NSFC (U1362202).

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

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