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Mixed-amine Modified SBA-15 as Novel Adsorbent ofCO2 Separation for Biogas UpgradingQuanmin Xue a b & Yingshu Liu ba Department of Chemistry , School of Science, Tianjin University , Tianjin, P.R. Chinab Institute of Gas Separation Engineering, School of Mechanical Engineering, University ofScience and Technology Beijing , Beijing, P.R. ChinaPublished online: 09 Mar 2011.

To cite this article: Quanmin Xue & Yingshu Liu (2011) Mixed-amine Modified SBA-15 as Novel Adsorbent of CO2 Separation forBiogas Upgrading, Separation Science and Technology, 46:4, 679-686, DOI: 10.1080/01496395.2010.517821

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Mixed-amine Modified SBA-15 as Novel Adsorbent of CO2

Separation for Biogas Upgrading

Quanmin Xue1,2 and Yingshu Liu21Department of Chemistry, School of Science, Tianjin University, Tianjin, P.R. China2Institute of Gas Separation Engineering, School of Mechanical Engineering, University of Scienceand Technology Beijing, Beijing, P.R. China

A novel adsorbent of CO2 from biogas was prepared by synthe-sizing and modifying the mesoporous molecular silica of SBA-15with methyl-diethyl-amine (MDEA) and piperazine (PZ). Theadsorbent showed good performance in separating CO2 from biogas.The loaded amines did not change the ordered structure of SBA-15,but enhanced its adsorption of CO2. The adsorbents were character-ized by X-ray powder diffraction (XRD) and N2 adsorption/desorp-tion. With the increase in MDEA loading, the surface area, poresize, and pore volume of the MDEA-loaded SBA-15 decreased.The modification of amines enlarged the difference between theequilibrium adsorption of CO2 and CH4. Quantitatively evaluatedon the basis of the breakthrough curves, the separation factorsbetween CO2 and CH4, was increased more than seven fold due tothe MDEA modification. With mixed-amine (MDEAþPZ) modifi-cation, the separation factors between CO2 and CH4 was furtherimproved. In addition, not only the adsorbent was regenerable bypurging with the purified gas, but also the adsorption performanceis stable in adsorption cycles. Effect of moisture on adsorption ofCO2 is investigated and the results show the increase in the adsorp-tion performance.

Keywords biogas; carbon dioxide; SBA-15; separation

INTRODUCTION

With increasing environmental concerns associated withhigher demand for energy, gas utilities are currently lookingfor clean, natural, and renewable alternatives (1). Biogas hashigh methane (CH4) content and after cleaning and upgrad-ing can be used as the renewable natural gas. However, bio-gas contains only 55–60% of methane with 40–45% carbondioxide (CO2) and various other gases (2,3).

The most important task of methane enrichment in bio-gas is the removal of carbon dioxide, (4). Several typicalprocesses have been developed for this task and some ofthem have already been engineered and implemented intoexisting biogas plants. Absorption process is a dominanttechnology commercially used, as the amine solution has

a higher capacity and selectivity for removing acidic gases.However, there are some major problems in this process(5–7):

1. Large energy consumption,2. Low regenerative rate3. The solvent loss4. Equipment corrosion.

Among these processes, the adsorption process haspromising potential for small to medium-scale operations.Adsorption technology is relatively simple in equipmentand uses less energy in operation. Adsorbent is the keytechnology of an adsorption process. Conventional adsor-bents, such as zeolite, molecular sieves, activated carbons,silica gels, carbon molecular sieves, etc (8–11), were not sat-isfactory for the separation of CO2 from biogas due to theirlow separation performance. After the emergence of novelordered mesoporous silica, mainly MCM and SBA, (12),research on their applications for CO2 capture=separationhas become very active. Harlick et al. grafted Triaminesilane on MCM-41 and achieved a CO2 adsorptioncapacity of 43mg=g (13), Hiyoshi et al. and Huang et algrafted Triamine silane on SBA-15 and MCM-48 respect-ively, reaching a capacity of 34mg=g and 40mg=g (14–15). Franchi et al. reported the loaded triethanolamine onMCM-41 with an enhanced adsorption capacity of55mg=g (16). Similarly, modification of MCM-41 withpolyethylenimine (PEI) exhibited an adsorptive capacityof 246mg=g (17). However, not only grafting but alsoimpregnation methods have problems of long regenerationtime and high regeneration temperature. Therefore, there iscurrently a need for a novel adsorbent for CO2 that is bothhighly adsorptive and can be easily regenerated.

The absorption capability of aqueous MDEA (methyl-diethyl-amine) can be enhanced by blending with piperazine(PZ) (18). Would SBA-15 modified with MDEAþPZenhance its performance of separation of carbon dioxide?

SBA-15 will be used as a preliminary supporter becauseit is stable in basic or hydrothermal condition. Tests of

Received 5 March 2010; accepted 19 August 2010.Address correspondence to Quanmin Xue, Department of

Chemistry, School of Science, Tianjin University, Tianjin, P.R.China. Tel.: þ86 01 62332751. E-mail: xxqm@yahoo.com.cn

Separation Science and Technology, 46: 679–686, 2011

Copyright # Taylor & Francis Group, LLC

ISSN: 0149-6395 print=1520-5754 online

DOI: 10.1080/01496395.2010.517821

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separation and regeneration are studied on the SBA-15modified with MDEA and MDEAþPZ in this article.Adsorption isotherms of CO2 and CH4 on SBA-15 samplesare received with a volumetric method before and afteranime modification. The influence of temperature will alsobe evaluated. Besides, tests on the effect of temperature andstability will be performed on the reversibility of CO2

adsorption on the adsorbent. Finally, the effect of moistureon the adsorption of CO2 is investigated in the adsorptionperformance.

MATERIAL PREPARATION

Synthesis of SBA-15

SBA-15 was synthesized by a direct hydrothermalmethod according to the literature (19,20). Tetraethylorthosilicate (TEOS) was used as the silica source, andPluronic P123 (poly-(ethylene glycol)-block-poly(propyleneglycol)-block-poly(ethylene glycol) was a template. HClwas used to adjust the pH, and distilled water was thesolvent. The specific synthesis procedure is as follows:

1. 4.0 g of P123 was dissolved in 120mL of 2M HClsolution, and then 8.5 g of TEOS was added into thesolution and stirred for 24 h at 313K.

2. The mixture was transferred into a Teflon stainless steelautoclave and got at 393K for another 24 h. calciningfrom filtering the aging solution in flowing air at 1K=min to 773K and keeping it 773K for 6 h.

Modifying SBA-15 with Amines

The amines used for modification of the adsorbent had arelatively high boiling point; MDEA (methyl-diethylamine)and PZ (Piperazine) were selected in experiments. Twoamines were of analytical purity. Acetone of analyticalpurity was used as solvent. Amines were modified onSBA-15 by the impregnation method. Amines were firstdissolved in acetone by mechanically stirring, then theSBA-15 dried at 120�C for 6 h was added into the solutionof amine in acetone by stirring. After stirring and refluxingfor 6 h, the solution was dried at 70�C for 6 h undervacuum (600 mmHg).

The quantity of amine modified on SBA-15 was denotedby the loading ratio, X, defined as the percentage of loadedamine

X ¼ MassðAmineÞMassðSBA� 15Þ�VporeðSBA� 15Þ�DensityðAmineÞ

ð1Þ

X(loading ratio) used for the SBA-15 sample of experi-ments were 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9.

Xmp was the molar ratio of PZ to MDEA in mixedamines when the loading ratio is 0.8.

Characterization of the SBA-15 Samples

Textural property of the SBA-15 before and after modi-fication withMDEA andMDEAþPZ was measured by N2

at 77K. The relative pressure of N2 covered from 10�6 to0.995. The SBA-15 samples were degassed at 473K for12 h before measuring. The specific surface area, pore sizedistribution (PSD), and pore volume were calculated onthe basis of nitrogen adsorption isotherms. The BET sur-face area was evaluated based on the data for relative pres-sures from 0.05 to 0.3. Pore volume was determined basedon the amount adsorbed at relative pressure 0.99, and thePSD was calculated with the BJH method.

XRD measurement was done using Rigaku D=MAX2500=PV diffractometer with Cu-butt. The voltage was40 kV, the electric current was 200mA, the scanning rangeis 0.2� < 20< 4.0�, and the scanning rate was 0.002�=s.

CO2 Adsorption Separation Studies

The Gases

The gas mixture of CO2 (50.2%), and CH4 (49.8%) wereprepared. in the breakthrough experiment, He (carrier)accounts for 50 per cent and the gas mixture (feed gas) is50% by the flow control. Then this gas mixture was used inthe regeneration experiment of the saturated adsorbent. Allgases used in the experiments were of purity above 99.99%and were purchased from the Boc Gas Company, Ltd.

Breakthrough Experiment

The schematic diagram of the breakthrough adsorptionexperiments was shown in Fig. 1. The breakthrough testprinciple was previously described (21,22). The adsorbentwas packed in a stainless steel adsorption column of length300mm and inner diameter of 10mm. The SBA-15 sampleswas filled with column of length 260mm. The column wasimmersed in a water bath of a thermostat. Temperaturewas kept constant within �0.1�C. Two mass-flow control-lers (MFC) from shengye with a 0.1%FS (full-scale) accu-racy, was connected with the gas line and used to controlthe flow rates of gases: one for the gas mixture, anotherfor helium (carrier gas). A back-pressure regulator wasused to control the adsorption pressure. Compositions ofthe outlet gases were analyzed by a quadrapole mass spec-trograph (MS) manufactured by Pfeifer Company fromGermany. Before the breakthrough experiment, theadsorbent was heated up to 350K in helium at a flow of30 cm3=min and kept at this temperature for 0.5 h. Thetemperature was then adjusted to 298K. Breakthroughcurves of a gas mixture were collected at 0.4M Pa. Theflow rate of the stream was kept at 100 cm3=min.

Regeneration Tests

Regeneration tests were carried out on the same setup asshown in Fig. 1. The column was first fed with the mixture

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of CO2 and CH4 at 0.4MPa. The flow rate was kept at100 cm3=min. When the column was saturated with CO2,the column pressure released to the atmosphere, and thenthe column was purged with a stream of pure methane.The flow rate of the purging streamwas kept at 40 cm3=min.

In order to investigate the effect of moisture on the CO2

adsorption, the mixture gas flow saturated with H2O vaporwas introduced at 100 cm3=min. The concentration of thegases in the gas mixture was measured on-line using thequadrapole mass spectrograph (MS).

Calculation of Separation Factor

The separation factor of adsorption was defined as:

aij ¼ðX=yÞiðX=yÞj

ð2Þ

Where x and y was the molar fraction of components i andj in the adsorbed phase and the gas phase at equilibrium.

Since

Xi

xi ¼ 1 ð3Þ

Then

ni ¼ ntxi ð4Þ

Where ni is the adsorbed amount of component i, and nt isthe total moles adsorbed. Therefore,

ninj

¼ xixj

ð5Þ

It yields by substituting Eq. (5) into Eq. (2) that

aij ¼ðx=yÞiðx=yÞj

¼ ðn=yÞiðn=yÞj

ð6Þ

The separation factor was evaluated on the basis of thebreakthrough curves (23). Based on mass balance in theadsorption column, the amount adsorbed for componentg (ng) could be calculated:

Z t

0

uiACyg;idt ¼Z t

0

ueACyg;edtþ eALyg;ip=RT þmng ð7Þ

where ui and ue was the linear speed of flow at the entranceand exit of the adsorption column; yg,i and yg,e was the con-centration of component g in the gas at the entrance andexit of the column; A and L is the section area and lengthof the adsorption column; e was the fractional void of thebed; C was the total concentration of the gas; m was themass of adsorbent.

The value of the separation factor a was then evaluatedbased on Eqs. (6) and (7).

RESULTS AND DISCUSSION

Textural Property of the SBA-15 Material

The nitrogen adsorption isotherms for the SBA-15 sam-ples before and after loading amine at 77K were shown inFig. 2. The pore size distributions (PSD) calculated withthe BJH model from the adsorption data of N2 for thesesamples were shown in Fig. 3. After amine loading, theS-shaped adsorption isotherm was still preserved. As wasshown in Fig. 2, with the increase of the loading ratio,the adsorption amount of nitrogen decreased. Followingthe increase of loading amount, the pore size became smal-ler, but the distribution was still very narrow. The pore sizedecreased from 6.8 nm to 4.8 nm, which could be owing tothe formation of an amine layer in the pores. Detailedchanges in the BET surface area (SBET), pore size (d),and pore volume (Vpore) were shown in Table 1.

The X-ray diffraction patterns of three SBA-15 samplesbefore and after loading amine were displayed in Fig. 4.The XRD reflection peaks were in good agreement withthree characteristic peaks for the 2-D hexagonal structures

FIG. 1. Schematic diagram of the dynamic adsorption experiment.

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of SBA-15 (19) which indicated that the structure ofSBA-15 was preserved. However, the intensity of the dif-fraction patterns of SBA-15 samples after loading aminedecreased, which was caused by the pore filling indicatingthat amine was loaded into the pores of SBA-15.

Adsorption Separation Performance on SBA-15 Modifiedby MDEA

To quantitatively evaluate the adsorption separationperformances of the SBA-15 modified MDEA for CO2=CH4, the breakthrough tests are done and the representa-tive breakthrough curves of CO2=CH4 with the helium

are shown for Rc¼ 0.8 in Fig. 5. The loading ratio of sam-ples changes from 0 to 0.9. The separation factors are cal-culated with Eqs. (3) and (4). All detailed results are shownin Fig. 6.

As shown in Fig. 6, with the increase of the loadingratio, the adsorption amount of CH4 was reduced, but thatof CO2 increased. Therefore, the separation factor of CO2 =CH4 augmented obviously. When the load rate was 0.8, theseparation factor was a maximum. This is because theSBA-15 is a porous material, when too much MDEA isloaded, MDEA is almost full of SBA-15 pores. This willresult in the fact that carbon dioxide cannot smoothly enterinto the pore channels and produce the role of internalMDEA. In turn, it will reduce the adsorption separationperformance.

Adsorption Separation Performance on SBA-15 Modifiedby MDEAþ PZ

In absorption operation, the absorption rate of CO2 canbe remarkably enhanced when PZ was added into aqueous

FIG. 2. Adsorption isotherms of N2 on the SBA-15 samples of different

MDEA-loading ratio at 77K. A: X¼ 0, B: X¼ 0.2, C: X¼ 0.4, D: X¼ 0.6,

E: X¼ 0.8.

FIG. 3. Pore size distribution of SBA-15 samples of different

MDEA-loading ratio. A: X¼ 0, B: X¼ 0.2, C: X¼ 0.4, D: X¼ 0.6, E:

X¼ 0.8.

FIG. 4. XRD pattern of the amine-loading SBA-15 sample. A=SBA-15;

B=SBA-15 modified with MDEA (X¼ 0.6); C=SBA-15 modified with

MDEAþPZ (Xmp¼ 0.167).

TABLE 1Textural properties of SBA-15 samples of different

MDEA-loading ratio

X SBET m2 � g�1 d nm Vpore m3 � g�1

0.0 801 6.8 1.360.2 626 6.4 1.080.4 392 5.6 0.860.6 217 4.9 0.650.8 75 4.8 0.18

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MDEA. When a little PZ was added into the loadedMDEA, whether do the adsorption separation perfor-mances of the SBA-15 modified MDEA for CO2=CH4 bebetter than that of the modified MDEAþPZ?

The test conditions were the same as the SBA-15 modi-fied MDEA. The same gas mixture carried by helium waslet into the adsorbent bed for the collection of break-through curves over the SBA-15 modified PZþMDEA.The results are shown in Fig. 7.

Compared to Fig. 6, the adsorption amount of CH4

almost was no change, but that of CO2 increased consider-ably. Therefore, the separation factors for CO2=CH4 werelarger than that of MDEA modified SBA-15 MDEAþPZ-modification enhanced the selectivity for CO2.

The results could be ascribed by two kinds of reactionmechanisms. With MDEA modified SBA-15, the CO2

chemical adsorption would form the MDEACOO,depicted in Eq. (8) (29).

CO2 þMDEA()MDEACOO ð8Þ

With MDEAþPZ-modification SBA-15, the chemicaladsorption mechanism of CO2 would change. WhenMDEA would react with CO2, the piperazine reacted withCO2 to form PZ(HCOO)2 simultaneously, as follows:

2CO2 þ PZ()PZðHCOOÞ2 ð9Þ

Because Reaction 9 was a more rapid reaction thanreaction 8 and parallel with reaction 8, CO2 could reactwith PZ first. Then CO2 could be transferred throughthe intermediate PZ(HCOO)2 to MDEA, PZ(HCOO)2

FIG. 5. Breakthrough curves of a gas mixture through the SBA-15

column.

FIG. 7. Effect of PZ=MDEA molar ratio on the adsorption separation

of CO2 and CH4.

FIG. 6. Effect of loading ratio of MDEA on the adsorption separation

of CO2 and CH4.

FIG. 8. Effect of different temperatures on the adsorbed amount and the

separation factor.

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could be renewed to PZ by reaction 10. Piperazine actedas an activator.

PZðHCOOÞ2 þ 2MDEA()PZ þ 2MDEACOO ð10Þ

Effect of Temperature

To estimate the influence of temperature change on theseparation factors, breakthrough tests were performed atfour temperatures covering from 10 to 60�C, which coveredthe usual variation range of ambient temperature. Theother experimental conditions are kept the same as for that

at 25�C. The result was shown in Fig. 8. Difference ofadsorption remains obvious. The adsorption amounts ofCO2 are much higher than that of CH4. Its decrease is moreremarkable as the temperature increases, and a decrease inthe separation factor was observed at higher temperatures.However, the separation factor was still high enough at60�C, therefore, the separation performance would not bemuch affected by temperature change.

Regeneration and Stability of the MDEAþ PZ-ModifiedSBA-15 Adsorbent

Application of an adsorbent in industry is decided notonly by the adsorption separation capacity in a specifiedseparation process, but also on how easy is the regener-ation and stable performance of the adsorption operationcycle. To investigate the regeneration and stability of theadsorption separation performance, the operations ofadsorption and regeneration were cycle performed on asingle-fixed column packed with the MDEAþPZ-modifiedSBA-15 with Xpm¼ 0.167.

The CO2 breakthrough times in consecutive 15 cycles areshown in Fig. 9. The first breakthrough time of carbondioxide is 686 s. When the column pressure was releasedto the atmosphere and the bed was purged with a streamof methane at a flow rate of 40 cm3=min. then the break-through test was repeated.

As shown in Fig. 9, although the sorption capacitydecreased, about 82% of the first breakthrough time of car-bon dioxide can be recovered, which was still much large.Although small fluctuation is initially observed, the

FIG. 9. regeneration and stability of the MDEAþPZ-modified SBA-15

adsorbent.

TABLE 2Comparison of CO2 adsorption performance of mixed-amine modified SBA-15 and other adsorbents

supporter AmineAdsorption

temperature=�CAdsorption

capacity=mmol � g�1Regeneration

temperature=�C Test method Ref.

SBA-15 TA 60 0.35 100 Breakthrough curve 14SBA-15 TA 60 1.58 100 Breakthrough curve 24SBA-15 aziridine 25 130 Breakthrough curve 24MCM-41 TRI 25 1.08 100 TGA-MS 25PE-MCM-41 TRI 25 1.51 100 TGA-MS 25PE-MCM-41 TRI 25 2.65 100 TGA-MS 25SBA-16 AEAPS 60 0.727 150 DSC-TGA 26MCM-48 APS 25 1.14 75 TPD-MS 15MCM-41 PEI 75 3.02 75 TGA 27KIT-6 PEI 75 3.07 75 TGA 28MCM-48 PEI 75 2.7 75 TGA 28SBA-15 MDEAþPZ 25 1.36 25 Breakthrough curve This work

TA¼ 3-[2-(2-Aminoethylamino)ethylamino]propyl-trimethoxysilane.TRI¼ 3-[2-(2-Aminoethylamino)ethylamino]propyl-trimethoxysilane.AEAPS=N-(2-aminoethyl)-3-aminopropyltrimethoxysilane.PEI=Polyethylenimine.

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breakthrough time of CO2 shows quite a stable trend ofvariation. The results suggest that the adsorbent still hadgood regenerability and stable sorption performance.

Compared with other adsorbents, the mixed-aminemodified SBA-15 shows better CO2 adsorption perform-ance at ambient temperature. Table 2 shows a comparisonof CO2 adsorption performance of mixed-amine modifiedSBA-15 and other adsorbents. It is noted that the adsorbedamounts of these adsorbents can usually be enhanced afterthe modification by various kinds of amine agents. Underambient conditions, the mixed-amine modified SBA-15could be regenerated by purging or vacuum; other adsor-bents would be regenerated in higher temperature. So,the mixed-amine modified SBA-15 is novel adsorbent ofCO2 separation for biogas upgrading. And as the costs ofcommercial SBA-15 are continuously decreasing, it is poss-ible to utilize these novel materials in the near future.

Effect of Moisture

It is important to investigate the effect of moisture onCO2 adsorption, because the biogas streams always containsome moisture. Figure 10 shows the comparisons of theCO2 breakthrough curve for the mixture gas between theone without moisture and one with moisture. In the pres-ence of moisture, the MDEAþPZ modified SBA-15adsorbent still effectively adsorbs CO2. The breakthroughtime of carbon dioxide is 726 s with dry gas. Comparedwith the adsorption of CO2 without moisture in the samecondition, the breakthrough time is increased by 158 s. Sothe results show that the moisture has a positive effect onthe adsorption of CO2 by the MDEAþPZ modifiedSBA-15 which is in agreement with that reported in Ref.(30–32).

CONCLUSIONS

In this work, MDEAþPZ modified SBA-15 wasdeveloped for efficient removal=separation of CO2 fromCH4 in biogas. This new adsorbent has three advantages:high separation factor, easy regeneration, minor effect oftemperature. PZ added into MDEA improved the adsorp-tion separation of CO2=CH4. The separation factor ofCO2=CH4 was enlarged to as much as more than double.Variation of temperature will have minor affection, sincethe selectivity for CO2 will decrease a little at higher tem-peratures. The amine-loaded adsorbent was regeneratedby purging at ambient temperature. The regeneratedadsorbent maintained good and stable adsorption separ-ation performance. As a result, the MDEAþPZ modifiedSBA-15 can be used to remove CO2 from biogas.

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