Adsorptive Removal Of Nitrogen-containing Compounds Research Articles

Hydroxyl modified nano calcium phosphate, a novel adsorbent for denitrogenation of fuel: Synthesis, characterization and adsorptive performance for quinoline

Adsorptive removal of nitrogen-containing compounds from fuels still faced the challenge of efficient and low-cost adsorbents exploitation. In this study, a novel material hydroxyl modified nano calcium phosphate was hydrothermally synthesized using Polyethylene glycol-4000 (PEG-4000) as template, and used as an efficient and low-cost adsorbent for quinoline removal from model fuel. The hydroxyl modified nano calcium phosphate obtained an excellent morphology of fine nanoparticles by regulating the Ca/P mol ratio and the PEG-4000 amount during synthesis, and thus obtained the intergranular pores. Meanwhile, a number of hydroxyl groups were introduced into the nano calcium phosphate to obtain more solid acidity. The adsorption efficiency of quinoline over the hydroxyl modified nano calcium phosphate was significantly enhanced, being attributed to the improvements in morphology, structure and solid acidity properties. 0.1 g of an optimal adsorbent achieved 94 % removal of quinoline with 70 ppm of initial N concentration in 20 mL model fuel within 10 min. Mechanism study proposed that the adsorption process occurred on both the outer surface and the inner pores of hydroxyl calcium phosphate, and thus the rich hydroxyl groups and pores significantly increased the Brønsted acid sites for the combination of H+ and N in quinoline. The hydroxyl calcium phosphate was easy to be regenerated and exhibited good recyclability for quinoline adsorption. The adsorption thermodynamics and kinetics research essentially presented the feature of the present adsorptive denitrogenation system. The newly developed adsorbent exhibited the advantages of easily modified structure, chemical stability and harmlessness for further industrial applications.

Adjusting surface acidity of hollow mesoporous carbon nanospheres for enhanced adsorptive denitrogenation of fuels

Mesoporous carbons are widely utilized as adsorbents for adsorptive removal of nitrogen-containing compounds in fuel. Herein, hollow mesoporous carbon nanospheres (HMCNs) were successfully synthesized via a facile silica-assisted sol–gel strategy and further oxidized by different acids (HNO3, H2SO4, and (NH4)2S2O8) to adjust their surface chemistry. The HMCNs oxidized with (NH4)2S2O8 contain the most oxygen-containing groups and exhibit the strongest acidity, resulting in high adsorption capacities towards quinoline (QUI, 29.78 mgN·g−1) and indole (IND, 26.59 mgN·g−1) in model fuel. The superior performance is attributed to the acid-base interaction and hydrogen bonding interaction between the adsorbent and QUI/IND molecules. Remarkably, the adsorption capacity towards nitrogen-containing compounds in real diesel is as high as 19.19 mgN·g−1 at 50 °C. Our work has paved the way to the rational design and synthesis of high-performance adsorbents for the removal of nitrogen-containing compounds in real diesel on a large scale.

Phytic acid-encapsulated MIL-101(Cr): Remarkable adsorbent for the removal of both neutral indole and basic quinoline from model liquid fuel

Phytic acid (PA) encapsulated metal-organic framework (MIL-101(Cr) or Cr-benzenedicarboxylate) was prepared for the first time; and applied in the adsorptive removal of nitrogen-containing compounds (NCCs) from liquid model fuel. The modified MIL-101(Cr)s resulted in very promising maximum adsorption capacities (Q0) for both the neutral indole (IND, Q0 = 543 mg/g) and basic quinoline (QUI, Q0 = 549 mg/g) compared to the pristine MIL-101(Cr) (IND; Q0 = 416 mg/g; QUI, Q0 = 409 mg/g). Or, the Qo values of PA(3)@MIL-101(Cr) for the adsorptions of IND and QUI stand at the second and fifth positions, respectively, among any previously reported results. Moreover, the new adsorbent showed around 8 times adsorption capacity to that of conventional activated carbon. PA encapsulation onto MIL-101(Cr) resulted in 86% and 91% increased adsorption (based on unit surface area of adsorbent) of IND and QUI, respectively. Moreover, PA(3)@MIL-101(Cr) showed more effective/selective adsorption than pristine MIL-101(Cr) towards the NCCs especially when toluene (as aromatics) was added as co-solvent. The extraordinary adsorptions could be described by the hydrogen bonding and the acid-base interactions between the NCCs and the PA functionalities of the adsorbents. Moreover, the PA@MIL-101(Cr)s could be regenerated via a simple manner and reused in adsorptions up to several cycles. Therefore, the novel PA@MOF materials could be suggested as a type of very efficient adsorbents for the adsorptive removal of both neutral and basic NCCs from liquid model fuel.

Adsorptive removal of nitrogen-containing compounds from fuel over hierarchical porous aluminosilicates synthesized by kinetic regulation method

Novel hierarchical Al-KIL-2 aluminosilicates with bimodal mesoporous distribution were successfully synthesized via a feasible and cost-effective kinetic regulation method. The molar ratio of Si/Al, the kinetics of Al-Si polyanions growth and its following oriented-attachment on KIL-2 matrix were regulated aiming at various porous and acidic properties. The hierarchical Al-KIL-2 samples were characterized, and their adsorption capacities were tested by adsorptive denitrogenation for model fuels containing pyridine and quinoline. It was revealed that the branching epitaxial growth behavior originating from Al-zoning on KIL-2 matrix contributed to fabrication of the hierarchical porous structures. In addition, introduction of Al endowed KIL-2 with rich Lewis acid sites for the adsorption of basic contaminants. Hierarchical Al-KIL-2 obtained considerable adsorption efficiencies of pyridine and quinoline, the regularities of which relied on the appropriate kinetic regulation conditions during materials synthesis. The adsorption isotherms and adsorption thermodynamics research essentially presented the feature of this adsorptive denitrogenation system over hierarchical Al-KIL-2 for its potential industrial applications.

Remarkable adsorptive removal of nitrogen-containing compounds from hydrotreated fuel by molecularly imprinted poly-2-(1H-imidazol-2-yl)-4-phenol nanofibers.

Molecularly imprinted polymer (MIP) nanofibers were prepared by the electrospinning of poly 2-(1H-imidazol-2-yl)-4-phenol (PIMH) in the presence of various nitrogen containing compounds (N-compounds). Molecularly imprinted polymer nanofibers show selectivity for various target model nitrogen-containing compounds with adsorption capacities of 11.7 ± 0.9 mg g−1, 11.9 ± 0.8 mg g−1 and 11.3 ± 1.1 mg g−1 for quinoline, pyrimidine and carbazole, respectively. Molecular modelling based upon density functional theory (DFT) indicated that hydrogen bond interactions may take place between the lone-pair nitrogen atom of model compounds (quinoline and pyrimidine) and the –OH and –NH groups of the PIMH nanofibers. The adsorption mode followed the Freundlich (multi-layered) adsorption isotherm, which indicated possible nitrogen–nitrogen compound interactions. Molecularly imprinted polymer nanofibers show potential for the removal of nitrogen-containing compounds in fuel.

Adsorptive removal of indole and quinoline from model fuel using adenine-grafted metal-organic frameworks

A highly porous metal-organic framework (MOF), MIL-101, was modified for the first time with the nucleobase adenine (Ade) by grafting onto the MOF. The Ade-grafted MOF, Ade-MIL-101, was further protonated to obtain P-Ade-MIL-101, and these MOFs were utilized to remove nitrogen-containing compounds (NCCs) (such as indole (IND) and quinoline (QUI)) from a model fuel by adsorption. These functionalized MOFs exhibited remarkable adsorption performance for NCCs compared with that shown by commercial activated carbon (AC) and pristine MIL-101, even though the porosities of the functionalized-MOFs were lower than that of pristine MIL-101. P-Ade-MIL-101 has 12.0 and 10.8 times capacity to that of AC for IND and QUI adsorption, respectively; its adsorption performance was competitive with that of other reported adsorbents. The remarkable adsorption of IND and QUI by Ade-MIL-101 was attributed to H-bonding. H-bonding combined with cation-π interactions was proposed as the mechanism for the removal of IND by P-Ade-MIL-101, whereas acid-base interactions were thought to be responsible for QUI adsorption by P-Ade-MIL-101. Moreover, P-Ade-MIL-101 can be regenerated without any severe degradation and used for successive adsorptions. Therefore, P-Ade-MIL-101 was recommended as an effective adsorbent for fuel purification by adsorptive removal of NCCs.

Adsorptive denitrogenation of model fuel by functionalized UiO-66 with acidic and basic moieties

Metal-organic frameworks (MOFs) are usually neutral in nature, even though they can be modified by introducing acidic or basic sites for various applications. However, MOFs with both acidic and basic sites are less common and have not been extensively studied. Here, for the first time, we have applied a MOF (UiO-66) containing both basic and acidic surface groups to the adsorptive removal of nitrogen-containing compounds (NCCs) from a model fuel. In order to introduce both acidic and basic sites in a MOF, aminated UiO-66 (UiO-66-NH2) was treated with 1,4-butanesultone to form UiO-66 functionalized with –NH– and –SO3H groups (UiO-66-NH-SO3H). The UiO-66-NH-SO3H MOF, bearing dual functional groups, exhibited much better performance in the removal of indole (IND) compared with the pristine (UiO-66), acidic (UiO-66-SO3H), and basic (UiO-66-NH2) MOFs. The maximum adsorption capacities of UiO-66-NH-SO3H for IND were 1.63 and 2.22 times to that of UiO-66 based on unit weight and surface area of MOFs, respectively. The superior performance of UiO-66-NH-SO3H (among all UiO-66 MOFs investigated) was attributed to the higher availability of H-bond acceptor sites in this material, compared with the other MOFs. Even though UiO-66-NH-SO3H showed lower adsorbed amounts of basic NCCs (such as quinoline, QUI), it exhibited improved adsorption performances at low (<600ppm) QUI concentrations (relevant for industrial applications), due to acid-base interactions. The UiO-66-NH-SO3H adsorbent can be reused several times by washing it with common solvents, such as ethanol. Therefore, MOFs functionalized with both acidic and basic sites, such as UiO-66-NH-SO3H investigated here, might be used in various applications, including adsorption and separation.

Adsorptive removal of nitrogen-containing compounds from a model fuel using a metal–organic framework having a free carboxylic acid group

Adsorptive removal of nitrogen-containing compounds from a model fuel was carried out by adsorbing indole (IND) and quinolone (QUI) over metal–organic frameworks (MOFs). UiO-66s, prototypes of MOFs, were applied as adsorbents with or without functionalization of free carboxylic groups. Even though the porosity of the functionalized UiO-66 (UiO-66-COOH) was lower than that of the pristine UiO-66, its adsorption capacity for neutral IND improved remarkably (103% and 28% based on surface area and weight, respectively) with functionalization. The enhanced performances of UiO-66-COOH could be explained by the formation of H-bond interactions between O (of –COOH) and H (of IND), which was supported by the differences in the adsorption of pyrrole vs. methylpyrrole. Curiously, the adsorbed amount (based on weight) of basic QUI decreased after the introduction of free –COOH on UiO-66, even though favorable acid–base interactions were expected. However, the amount of adsorbed QUI increased by 41% based on surface area. The adsorptive performances of UiO-66s for pyridine were very similar to those for QUI. Therefore, UiO-66-COOH might be also beneficial for the adsorption of basic NCCs, probably because of the existence of acid–base interactions (between N of NCCs and H of –COOH). However, the effect of free COOH on the adsorption of basic NCCs is generally lower than that of neutral NCCs having hydrogen atoms capable of acting as hydrogen bond donors. Based on the reusability results and the improved performances for IND and pyrrole adsorption, MOFs having free –COOH can be suggested as potential adsorbents to remove neutral NCCs, which are not easy to remove because of their lack of functionality.

Remarkable adsorptive removal of nitrogen-containing compounds from a model fuel by a graphene oxide/MIL-101 composite through a combined effect of improved porosity and hydrogen bonding

A composite was prepared by combining a highly porous metal-organic framework (MOF), MIL-101 (Cr-benzenedicarboxylate), and graphene oxide (GnO). The porosity of the composite increased appreciably by the addition of GnO up to a specific amount in the MOF, though further increases in the quantity of GnO was detrimental to porosity. The improved porosity of the GnO/MIL-101 composite was utilized for adsorptive denitrogenation (ADN) of a model fuel where indole (IND) and quinoline (QUI) were used as nitrogen-containing compounds (NCCs). It was found that both IND and QUI showed improved adsorption on the composite compared with pristine MIL-101 or GnO due to the improved porosity of the composite. Interestingly, the improvement in adsorption of IND was much higher than the quantity estimated for the porosity. Importantly, GnO/MIL-101 showed the highest adsorption capacities for NCCs. Irrespective of the studied solvents and co-presence of IND and QUI, the composite adsorbent performed ADN most effectively. This remarkable improvement is explained by the additional mechanism of hydrogen bonding between the surface functional groups of GnO and the hydrogen attached to the nitrogen atom of IND. This hydrogen bonding mechanism is also supported by the results of the adsorption of pyrrole and methylpyrrole. On the other hand, QUI does not show hydrogen-bonding capability, and therefore, its enhanced adsorption originates from only the increased porosity of the adsorbents.

Graphite Oxide/Metal–Organic Framework (MIL-101): Remarkable Performance in the Adsorptive Denitrogenation of Model Fuels

A highly porous metal-organic framework (MOF), MIL-101 (Cr-benzenedicarboxylate), was synthesized in the presence of graphite oxide (GO) to produce GO/MIL-101 composites. The porosity of the composites increased remarkably in the presence of a small amount of GO (<0.5% of MIL-101); however, further increases in GO reduced the porosity. GO also accelerated the synthesis of the MIL-101. The composites (GO/MIL-101) were used, for the first time, in liquid-phase adsorptions. The adsorptive removal of nitrogen-containing compounds (NCCs) and sulfur-containing compounds (SCCs) from model fuels demonstrated the potential applications of the composites in adsorptions, and the adsorption capacity was dependent on the surface area and pore volume of the composites. Most importantly, the GO/MIL-101 composite has the highest adsorption capacity for NCCs among reported adsorbents so far, partly because of the increased porosity of the composite. Finally, the results suggest that GO could be used in the synthesis of highly porous MOF composites, and the obtained materials could be used in various adsorptions in both liquid and gas/vapor phase (such as H2, CH4, and CO2 storage) adsorptions, because of the high porosity and functional GO.

Adsorptive removal of nitrogen-containing compounds from fuel by metal-organic frameworks

The adsorptive denitrogenation from fuels over three metal-organic frameworks (MIL-96(Al), MIL-53(Al) and MIL-101(Cr)) was studied by batch adsorption experiments. Four nitrogen-containing compounds (NCCs) pyridine, pyrrole, quinoline and indole were used as model NCCs in fuels to study the adsorption mechanism. The physicochemical properties of the adsorbents were characterized by XRD, N2 physical adsorption, FT-IR spectrum and Hammett indicator method. The metal-organic frameworks (MOFs), especially the MIL-101(Cr) containing Lewis acid sites as well as high specific surface area, can adsorb large quantities of NCCs from fuels. In addition, the adsorptive capacity over MIL-101(Cr) will be different for NCCs with different basicity. The stronger basicity of the NCC is, the more it can be absorbed over MIL-101(Cr). Furthermore, pore size and shape also affect the adsorption capacity for a given adsorbate, which can be proved by the adsorption over MIL-53(Al) and MIL-96(Al). The pseudo-second-order kinetic model and Langmuir equation can be used to describe kinetics and thermodynamics of the adsorption process, respectively. Finally, the regeneration of the used adsorbent has been conducted successfully by just washing it with ethanol.

Adsorptive denitrogenation of model fuels with porous metal-organic frameworks (MOFs): Effect of acidity and basicity of MOFs

To understand the effect of the acidity or basicity of porous metal-organic frameworks (MOFs) on the adsorptive removal of nitrogen-containing compounds (NCCs), an MOF (MIL-100(Cr)) was modified to impart acidity or basicity onto the MOFs. The modification was done by grafting ethylenediamine and aminomethanesulfonic acid onto coordinatively unsaturated sites of the MOF, MIL-100(Cr). The adsorptive removal of a basic quinoline or benzothiophene can be improved noticeably, especially at low concentrations, with the introduction of an acidic site; however, a basic MOF causes a severe decrease in the adsorptive performance for a basic adsorbate such as quinoline. The effect of the interaction of the base–base on adsorption was more severe or detrimental for a hard base quinoline than for a soft base benzothiophene. Functionalized MOFs show a slightly decreased adsorption for a neutral adsorbate such as indole probably because of the decreased porosity of the MOFs compared with the virgin MOF without functionalization. Moreover, a functionalized MOF (with SO3H group) can be used several times after simple washing with acetone. From the present research, it may be concluded that acid–base interactions between NCCs and MOFs will lead to favorable adsorptive removal of NCCs. However, for the adsorptive removal of a neutral adsorbate such as indole, another adsorption mechanism (such as π-complexation or hydrogen-bonding) is needed for high uptake and efficient removal.

Adsorptive Removal of Nitrogen-Containing Compounds from Fuel

The feasibility of using microporous and mesoporous materials as adsorbents to remove nitrogen-containing compounds (NCCs) in fuel, such as pyridine, quinoline, pyrrole, and indole, was explored. A mesoporous molecular sieve Ti−HMS was chosen as the optimum adsorbent for the adsorptive denitrogenation (ADN) of fuel. Effect of adsorption time, temperature, and ratio of adsorbent to oil (A/O) on the removal of NCCs was investigated. Meanwhile, the adsorption isotherms and adsorption thermodynamics were studied. The thermodynamic functions show that the adsorption of NCCs on Ti−HMS is an exothermic, irregularity decreased, spontaneous physical process. Furthermore, the adsorption process primarily occurs in the pores of Ti−HMS, which is confirmed by comparing the adsorptive denitrogenation performance of Ti−HMS with uncalcined Ti−HMS.