Fabrication of nickel oxide functionalized zeolite USY composite as a promising adsorbent for CO2 capture

Jipeng Dong,Fei Wang,Guanghui Chen,Shougui Wang,Cailin Ji,Fei Gao,*

1 Shandong Key Laboratory of Multiphase Fluid Reaction and Separation Engineering,College of Chemical Engineering,Qingdao University of Science and Technology,Qingdao 266042,China

2 Department of Chemical Engineering,Qingdao University of Science and Technology,Gaomi 261500,China

Keywords: NiO@USY composites Carbon dioxide Adsorption Separation Adsorption selectivity

ABSTRACT Adsorption process is considered to be the most promising alternative for the CO2 capture to the traditional energy-intensive amine absorption process,and the development of feasible and efficient CO2 adsorbents is still a challenge.In this work,the NiO@USY (ultrastable Y) composites with different NiO loadings were prepared for the CO2 adsorption using Ni(NO3)2 as the precursor.The composites were characterized by X-ray photoelectron spectroscopy,X-ray diffraction,nitrogen adsorption–desorption test,scanning electron microscopy analysis,and thermogravimetric analysis,and were evaluated for the CO2 adsorption capacity,CO2/N2 adsorption selectivity and CO2 cycle adsorption capacity.The characterization results show that after the activation at 423 K,the Ni(NO3)2 species were well dispersed into the surface of zeolite USY,and after the further activation at 823 K,Ni(NO3)2 could be converted into highly dispersed NiO.The adsorption results show that the presence of the active component NiO plays an important role in improving the CO2 adsorption performance,and the NiO@USY composite with a NiO loading of 1.5 mmol?g-1 USY support displays a high adsorption capacity and adsorption selectivity for CO2,and shows a good cycle stability.In addition,the Clausius–Clapeyron equation was used to evaluate the isosteric heat of adsorption of CO2 on the NiO(1.5)@USY composite,and the heat of adsorption was 17.39–38.34 kJ?mol-1.

1.Introduction

CO2is the main source of greenhouse gases,which is nearly higher than 60% of the global total greenhouse gases [1].The flue gas emitted from fossil fuel combustion is the largest CO2source,which is nearly one-third of the total CO2emissions[2].The excessive CO2emission have led to some serious consequences,including the temperature rise,the ozone layer destruction,the sea level change,etc[3].Thus,the development of an effective material and/or process for removing CO2from the flue gas has become an urgent problem for reducing CO2emissions and mitigating the impact of greenhouse gases on the environment and climate.CCUS(carbon capture,utilization and storage) approach has become an attractive and practical technology to reduce CO2emissions,and the carbon capture is an important prerequisite of the CCUS method [4].Therefore,numerous technologies for capturing CO2have been developed,including the absorption using amine solutions [5],the membrane separation [6],the cryogenic distillation[7],and the adsorption using solid adsorbents [8],etc.Currently,the amine solution absorption is a relatively mature technology for separating CO2from flue gases [9,10].However,this method has some disadvantages including the high energy consumption,the cumbersome operation,the serious solvent losses,the equipment corrosion,and the high cost[11,12].Adsorption method will become the most promising alternative to amine adsorption due to its simple process flow,easy operation,wide range of adsorbent sources and low energy consumption for the adsorption/desorption operation [13].And the key for the adsorption method is to select a feasible and efficient adsorbent material.

An ideal CO2adsorbent should have a high adsorption capacity and adsorption selectivity for CO2and a good cycle stability [14].The reported CO2adsorbents are generally divided into physical adsorbents and chemical adsorbents.The physical adsorbents mainly include carbon material adsorbents,molecular sieve adsorbents,and metal organic framework adsorbents,etc[15–18].Molecular sieve materials are considered to be the excellent candidates for the CO2adsorption due to their high specific surface areas and large pore volumes,the uniform pore channel structure,and the high porosity.Among them,zeolite ultrastable Y(USY)materials have been extensively used in the catalysis and separation fields due to its stable crystal structure,excellent surface properties and the low cost [19–21],and display a high CO2adsorption capacity [22].However,the physical interaction of USY with CO2by van der Waals force or electrostatic force makes the adsorbent sensitive to the adsorption temperature and poor adsorption selectivity[23].Therefore,the chemical modification of the porous solid material surfaces has attracted more and more attentions to improve the CO2adsorption performance.

CO2is Lewis acid gas,and can attract electrons of Lewis base.The solid materials can be modified by introducing metal oxides and amine groups to improve the basic properties of the solid adsorbents[24].Amine modified zeolite USY materials show a high adsorption capacity and adsorption selectivity for CO2.However,the stability of these materials is poor,and many scholars are working on solving this problem[25–28].The introduction of alkaline earth metal[29,30]and alkali metal[31]into porous materials also can significantly improve the CO2adsorption performance.Nevertheless,these composite materials always require higher temperatures for the decomposition of carbonates into metal oxides to regenerate the adsorbent [32].Recently,transition metal oxide adsorbents [33–36] for the CO2adsorption have been attracted great attentions.

Transition metal oxides can interact with CO2molecules through π-complexation by forming σ bonds (electrons on the bonding π2porbitals of CO2molecules transfer to the s orbital of transition metal) and feedback π bonds (electrons on the unoccupied d orbital of transition metal transfer to the π2p* orbital of CO2molecules) [34,37,38].This interaction is much stronger than van der Waals force or electrostatic force,so the transition metal oxide based adsorbents can achieve a higher CO2capture performance.In addition,this π-complexation bond can be easily broken by utilizing simple operations of PSA (pressure swing adsorption)and/or TSA (temperature swing adsorption) [39].Boruban and Esenturk [33] introduced sharp peaks and spherical CuO species into zeolite Y,and compared with the zeolite Y support,the CO2adsorption capacity of CuO@Y zeolite increased by 112% and 86% ,respectively.Chanapattharapolet al.[34] loadediron oxide into MCM-41 by an impregnation method,and the CO2adsorption amounts increased significantly after introducing iron oxide.Huonget al.[37] and Sunet al.[40] exchanged transition metal ions to zeolite to improve the CO2adsorption capacity.To the best of our knowledge,none study of transition metal nickel oxide(NiO)-modified zeolite Y for CO2adsorption has been reported.

Herein,transition metal nickel oxide(NiO)was introduced into the porous USY to prepare the NiO@USY composites for the CO2adsorption.The synthesized NiO@USY composites with various NiO loadings were characterized by X-ray photoelectron spectroscopy (XPS),powder X-ray diffraction (XRD),nitrogen adsorption/desorption test at 77 K,scanning electron microscopy (SEM),and thermogravimetric (TG).The CO2adsorption performance including the CO2adsorption capacity and the CO2/N2adsorption selectivity for the synthesized NiO@USY composites were investigated,the reversibility and the isosteric heat of CO2adsorption were estimated.

2.Materials and Methods

2.1.Materials

Ni(NO3)2?6H2O (AR) and zeolite USY were purchased from Chengdu Kelong Chemical Co.,Ltd.(China)and Tianjin Nankai Catalyst Plant (China),respectively.CO2,He and N2gases were supplied from Qingdao Dehai Gas Co.,Ltd.(China).The purities of all the gases are 99.99% or above.

2.2.Composite preparation

Solid-state grinding method is a facile and efficient approach for introducing metal components into the pore channels of the porous supports,and can completely introduce the metal precursor into the support.The NiO@USY composite materials were synthesized by this approach.A specified amounts of powdered zeolite USY and Ni(NO3)2?6H2O powder was thoroughly ground and mixed,and then the mixed powder were conducted the activation process at 423 K for 4 h and 823 K for 4 h in air.The composites with various nickel contents of 1.0,1.2,1.5,1.8 and 2.0 mmol?g-1zeolite USY were prepared.The samples obtained before and after the activation are denoted as Ni(NO3)2(x)@USY and NiO(x)@USY,respectively,wherexrepresents the content of nickel per gram USY in mmol.

2.3.Sample characterizations

The powder X-ray diffraction(XRD)patterns were obtained by a D/max-2500 diffractometer (Rigaku,Japan) using a graphite diffusion Cu Kα source (λ=0.0156 nm).The samples were scanned in range of 2θ=5°–80° with a scanning step of 0.02°.A TriStar 3000 automatic physical adsorption instrument (Micromeritics,USA)was used for the nitrogen physical adsorption test at 77 K,and before the test,the samples were vacuum degassed at 573 K for 4 h.The total pore volumes of the samples were estimated from the adsorption equilibrium isotherms,and their specific surface areas were estimated by using the Brunauer–Emmett–Teller(BET) method.A Sigma-500 scanning electron microscope (SEM,ZEISS,Germany) instrument was used for observing the SEM images to identify the composite morphology.X-ray photoelectron spectroscopy was performed on a ESCALAB 250 XPS equipment(ThermoFisher Scientific,USA) with an Al Kα X-ray radiation source.The thermal stability of the composite was evaluated on a TA-Q500 thermogravimetric (TG,TA,USA) analyzer in the temperature range from room temperature to 1073 K in air.

2.4.Adsorption isotherm tests

CO2and N2adsorption isotherm measurements were performed by a static adsorption device,and the operation and calculation procedures were presented in detail in our previous work[41].The adsorption device is equipped with an adsorption tank,a gas storage tank,a temperature-controlled electric furnace,a circulating water bath,two pressure sensors,and a vacuum pump.Prior to each test,the samples were processed under a vacuum at 473 K for 4 h.After the adsorbent regeneration,the adsorption measurements were carried out in the pressure range of 0–500 kPa at the given temperature kept by the circulating water bath.The pressure of the adsorption tank usually stabilizes in about 10 min during each adsorption point measurement,and an adsorption time of 30 min was adopted to ensure complete equilibrium.For the cyclic stability test,five consecutive adsorption and desorption measurements on the same sample are completed according to the above apparatus and steps.

2.5.Adsorption theories

2.5.1.Adsorption isotherm models

Numerous equations have been developed for fitting the adsorption isotherms,such as Langmuir,Freundlich,Sips,Toth,etc[42].In this work,the Sips equation was employed to correlate the adsorption isotherm,and can be expressed as:

whereqis the adsorption capacity at a specific temperature and pressure (mmol?g-1);qmis the saturated adsorption capacity(mmol?g-1);pis the adsorption equilibrium pressure (kPa);bis the adsorption equilibrium constant(kPa-1);nis the heterogeneity of the system.

2.5.2.Adsorption selectivity

CO2adsorption selectivity is one of the most important indexes to investigate the CO2separation performance of adsorbents for mixed gases [43].The ideal adsorption solution theory (IAST)[44] is the most common model for predicting the multicomponent adsorption from single component adsorption isotherm.The CO2/N2adsorption selectivityfor a mixture of CO2and N2is defined as [45]:

wherexis mole fraction of CO2and N2in the adsorbed phase,yis the mole fraction in the gas phase.

2.5.3.Adsorption isosteric heat

According to the adsorption data measured at different temperatures,the Clausius–Clapeyron equation was used to calculate the CO2adsorption isosteric heat [46]:

whereQstis the adsorption isosteric heat (kJ?mol-1);Ris the ideal gas constant (J?mol-1?K-1);Tis the experiment temperature (K);qais a given CO2loading (mmol?g-1).

3.Results and Discussion

3.1.Composite characterization

XPS analysis was used to investigate the chemical state of Ni elements in the Ni(NO3)2@USY sample before and after the activation,as depicted in Fig.1.Before the activation,in Ni 2p spectrum,two characteristic peaks of Ni 2p1/2and Ni 2p3/2are observed at 875 and 857 eV,accompanied by satellite peaks at 880 and 862 eV,respectively,assigned to the Ni 2p binding energy values of Ni2+species in the Ni(NO3)2@USY sample [47].In addition,the N 1s spectrum show the characteristic peak of oxidized nitrogen at 407 eV [48],attributed to thespecies in Ni(NO3)2@USY sample.After the activation 423 K,the sample shows the similar Ni 2p and N 1s XPS spectra with Ni(NO3)2@USY sample,and the peak positions are also consistent,indicating that the nickel species still exists in the form of nickel nitrate during this dispersion step.After the further activation at 823 K,Ni 2p peaks of Ni2+species are observed,however,no N species were detected in N 1s XPS spectrum,confirming that Ni(NO3)2precursors were decomposed into active component NiO under this activation condition.The XPS results indicate that the NiO@USY composites were successfully prepared by this solid-state method using Ni(NO3)2as precursors.

Fig.1.XPS spectra of Ni 2p (a) and N 1s (b) for Ni(NO3)2 loaded samples before and after activation.

Fig.2.XRD patterns of Ni(NO3)2,USY,Ni(NO3)2 loaded samples before and after activation,and NiO@USY after moisture treatment.

Fig.3.Nitrogen adsorption–desorption isotherms of USY and NiO@USY composites with different nickel loading at 77 K.

Fig.2 shows the XRD patterns of Ni(NO3)2,zeolite USY support,and the samples loaded with Ni(NO3)2before and after the activation.Before the activation,the Ni(NO3)2characteristic diffraction peaks were detected in the Ni(NO3)2@USY sample.For the sample activated at 423 K of higher than the melting point of Ni(NO3)2and lower than the decomposition temperature of Ni(NO3)2,the diffraction peaks of Ni(NO3)2were disappeared,indicating that the molten Ni(NO3)2was dispersed into the zeolite USY pore channels.For the sample further activated at 823 K of higher than the decomposition temperature of nickel nitrate into nickel oxide,no Ni(NO3)2and NiO diffraction peaks were detected,indicating that the NiO generated by the decomposition of Ni(NO3)2was highly dispersed into the USY support pore channels and not accumulated on the surface of the NiO@USY composites,which is conducive to the CO2adsorption.In addition,the XRD pattern of the NiO@USY composite after the moisture treatment was measured to evaluate the moisture susceptibility of the composite.It can be seen that after the moisture treatment,the diffraction peaks of the NiO@USY composites have no obvious change,indicating that this composite has good water tolerance.

The adsorption–desorption isotherms of nitrogen on the pure zeolite USY and the NiO@USY composites with various NiO contents were measured at 77 K to evaluate the textural properties of the samples.As shown in Fig.3,after introducing NiO,the adsorption capacity of N2on the NiO@USY samples decreases gradually with increasing the NiO loading.Table 1 lists total pore volumes and the surface areas of the zeolite USY support and NiO@USY composites.As shown in Table 1,the specific surface areas as well as total pore volumes of the NiO@USY composites decrease gradually with increasing the NiO loading,which is caused by the introduction of NiO materials occupying part of the pore channels of zeolite USY.

Table 1 Textural properties of USY and NiO@USY composites

Fig.4.SEM images of zeolite USY (a,b) and NiO(1.5)@USY composite (c,d).

Fig.5.TG curve of NiO(1.5)@USY composite.

Fig.6.CO2 adsorption isotherms on USY and the NiO@USY adsorbents with various nickel contents at 303 K.

Fig.4 presents the SEM images of the NiO(1.5)@USY composite and the USY support.From Fig.4(a) and (b) in the SEM images of the USY support,some USY particles in different sizes are observed,and abundant pores are observed on the USY particle surfaces,generating a large pore volume and high specific surface area,which is beneficial to the active components NiO loading.From the SEM images of NiO(1.5)@USY composites (Fig.4(c) and (d)),it can be seen that the morphology of the composite have no significant change after the NiO modification.This phenomenon indicates that the NiO(1.5)@USY composite has no active component aggregation on the USY support surface and that NiO species were well dispersed in the pores of the USY supports,which is also confirmed by the result of XRD characterization.In addition,compared with the USY support,the NiO(1.5)@USY composite has fewer internal pores.The result is consistent with the pore volume and ratio of the N2adsorption/desorption test.

Fig.5 depicts the TG curve of NiO(1.5)@USY composite.A obvious mass loss is observed from room temperature to about 473 K,corresponding to the evaporation of physically adsorbed water and crystallization water[49].Moreover,it can be seen that the curve is basically flat when further increasing the temperature to 1073 K,indicating that the NiO(1.5)@USY composite has a good thermal stability.

3.2.Adsorption performance of CO2 on NiO@USY composites

Fig.6 shows the CO2adsorption capacities of pure zeolite USY and NiO@USY composites with various nickel contents at 303 K.It can be seen that the adsorption capacities of CO2on all composite materials are better than that of the pure zeolite USY when introducing NiO,indicating that the introduced NiO played an important role in enhancing the CO2adsorption performance.In addition,the adsorption capacity of CO2on the NiO@USY composites gradually increase with an increase in the NiO loading to 1.5 mmol?g-1USY support,and then drops sharply as the content of NiO continues to increase.The result shows that the excessive CuO species loading will accumulate on the surface of the composites,causing the reduction of effective active sites and the clogging of pores.Therefore,the monolayer loading threshold of NiO on the used zeolite USY is 1.5 mmol?g-1,and the NiO(1.5)@USY composite achieves a higher CO2adsorption amounts of 2.92 mmol?g-1at 500 kPa.

In industrial exhaust gas,in addition to CO2,there are usually many other gases,of which N2is the most common gas.Therefore,the CO2/N2adsorption selectivity was investigated to evaluate the CO2separation performance of the NiO(1.5)@USY composite from the mixed gases.Fig.7(a) presents the adsorption isotherms of pure CO2and N2on NiO(1.5)@USY and the zeolite USY,and IAST method was employed to calculate the CO2/N2adsorption selectivity on the NiO(1.5)@USY composite for a CO2/N2(15:85) mixture,which is close to the actual flue gases from power plant.It can be seen from Fig.7(b) that the adsorption selectivity of CO2/N2on pure zeolite USY is obviously lower than that of NiO(1.5)@USY composites in the entire experimental pressure range,attributed to the fact that the π-complexation interaction of CO2with the NiO species on the adsorbent surfaces is stronger than the physical interaction with N2.Additionally,the CO2/N2adsorption selectivity decreases from 56 to 26 as the pressure of adsorption increases to 500 kPa.The results show that the NiO(1.5)@USY composite have a higher CO2/N2adsorption selectivity,can be used for effectively separating CO2from CO2/N2mixture.

In practical application for the CO2adsorbents,the cycle stability for the CO2adsorption performance is also an important indicator.Therefore,in order to evaluate the cycle stability of the prepared NiO(1.5)@USY composite,adsorption of CO2at 303 K and the desorption vacuumed at 473 K on the NiO(1.5)@USY composite was conducted five times continuously on the same sample.As shown in Fig.8,in the five adsorption/desorption operations,the adsorption capacity of CO2on the NiO(1.5)@USY composite material remained basically unchanged.Therefore,the NiO(1.5)@USY composites has a certain adsorption cycle stability,and can be stably used for the CO2adsorption and separation.In addition,the influence of moisture on the CO2adsorption capacity of NiO(1.5)@USY composite was evaluated by the gravimetric method.The CO2adsorption capacity of the composite is 1.03 mmol?g-1for the pure CO2at 100 kPa,whereas the CO2adsorption capacity is 0.92 mmol?g-1for CO2gas with a moisture content of 5% .This slight decrease in CO2adsorption capacity is related to the hydrophilicity of the adsorbent,i.e.,the hydroxyl groups on the surface of zeolite USY easily adsorb water molecules,limiting the diffusion of CO2into the adsorbent pore channels.

To estimate the CO2adsorption heat on the adsorbent,the CO2adsorption were measured at the temperatures of 303,313,323 and 333 K,the adsorption isotherms are shown in Fig.9(a).The adsorption capacity of CO2on NiO(1.5)@USY sample show a downward trend with the increase of adsorption temperature,suggesting an exothermic process of the CO2adsorption on NiO(1.5)@USY composites.Furthermore,the adsorption isosteric heats for CO2on NiO(1.5)@USY as a function of CO2loadings were calculated according to the Clausius–Clapeyron equation from the fitted CO2adsorption isotherms at different temperatures by the Sips model,and the calculated results are presented in Fig.9(b).The fitted model coefficients are listed in Table 2,and the adsorption data are well fitted by Sips model with all nonlinear regression coefficientsof higher than 0.99.The initial adsorption isosteric heat value is 38.34 kJ?mol-1,indicative of the stronger interaction between the NiO(1.5)@USY composite and CO2molecules.In addition,the adsorption heat values decrease gradually with increasing CO2loading,suggesting that the used USY support show the surface energetic heterogeneity.Overall,the isometric adsorption heat of CO2on NiO(1.5)@USY composite is 17.39–38.34 kJ?mol-1,suggesting that the prepared NiO(1.5)@USY composites will need a lower energy consumption for the regeneration,realizing a moderate operation condition.

Table 2 Sips fitting parameters for CO2 and N2 adsorption on NiO(1.5)@USY and USY with nonlinear regression coefficients

Table 2 Sips fitting parameters for CO2 and N2 adsorption on NiO(1.5)@USY and USY with nonlinear regression coefficients

Fig.7.CO2 and N2 adsorption isotherms of on NiO(1.5)@USY and USY at 303 K (a) and CO2/N2 adsorption selectivity for the CO2/N2 (15:85) mixture (b).

Fig.8.CO2 cyclic adsorption capacity on NiO(1.5)@USY with adsorption at 303 K and desorption at 473 K.

4.Conclusions

In this paper,the NiO@USY adsorbents with different NiO loadings were prepared for the CO2adsorption.The characterization results show that the active component NiO was successfully introduced and highly dispersed into the the zeolite USY pore channels after the activation at high temperatures.The optimized adsorbent NiO@USY has a NiO loading of 1.5 mmol?g-1USY,obtaining a high CO2adsorption capacity of 2.92 mmol?g-1and a high CO2/N2adsorption selectivity of 56.Moreover,the prepared NiO@USY composite shows a good cycle stability and need a lower energy consumption for the regeneration.The excellent adsorption performance make the NiO@USY composite a potential candidate for separating CO2from gas mixtures,and could achieve a mild operating conditions.

Fig.9.CO2 adsorption isotherms at various temperatures (a) and adsorption isosteric heats for CO2 on NiO(1.5)@USY (b).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by the Qingdao Science and Technology Plan Application Foundation Research Project (19-6-2-28-cg).