4. Current approaches to increasing hydrogen adsorption in MOFs

4. Current approaches to increasing hydrogen adsorption in MOFs

Unfortunately, no MOFs currently satisfy the proposed DoE target under ambient conditions. The porous MOFs adsorb molecular hydrogen physically; therefore, H

₂

adsorption capacity sharply decreases at higher temperatures. The obtained theoretical results suggest the existence of three adsorption modes at 77 K: (1) at low pressure (~1 bar of H

₂

), the

n_a

value is proportional to the value of

Q_st

; (2) at ~30 bar of H

₂

, the

n_a

value is proportional to

A_s

; and (3) at ~120 bar of H

₂

, the

n_a

value is proportional to

V_a

.

4.1. Factor of the surface area and pore volume

A linear relationship between the H

₂

adsorption capacity at 77 K and the

A_s

value has been confirmed in numerous publications [10]. As a result that the

V_a

value is proportional to

A_s

, higher values of both

V_a

and

A_s

are recommended to improve hydrogen adsorption in MOF at 77 K. The adsorption capacity at 77 K and 1 bar of H

₂

is related to the

A_s

values within the range of 100–2000 m

²

·g

^–1

and does not correlate in the case of higher

A_s

values. This may be because the sorbent surface cannot be fully covered by H

₂

molecules at these high

A_s

and low

P

(H

₂

) values. In addition, it is most likely that the H

₂

molecules will preferentially bind on the most thermodynamically favorable sites in MOFs with the largest affinity to hydrogen. In the porous MOFs at a low pressure of 77 K, H

₂

adsorption may be affected by ligand functionalization, catenation, open metal sites, and pore size, whereas high-pressure H

₂

adsorption is directly proportional to the

A_s

value.

4.1.1. Length of organic linkers

For a given crystal structure of MOFs, the

A_s

value can be increased using longer ligands (linkers). For example, BBC (4,4',4"-[benzene-1,3,5-trilytris(benzene-4,1-diyl)]tribenzoate) may be considered to a longer model of BTB (4,4',4"-benzene-1,3,5-triyl-tribenzoate). MOF-200, which comprises the BBC linker, demonstrates an

A_s

value of 4520 m

²

·g

^–1

, and MOF-177 with the BTB linker has an

A_s

value of 4750 m

²

·g

^–1

(both using the BET method) [11]. A thorough comparison of isostructural UiO-66 and UiO-67 was made in [12]. These MOFs are constructed from Zr

₆

O

₄

(OH)

₄

nodes, and organic linkers 1,4-benzene-dicarboxylate and 4,4′-biphenyldicarboxylate for UiO-66 and UiO-67, respectively. Resulting from of the longer linker in the case of UiO-67, the value of

A_s

(Langmuir) was 2483 m

²

·g

^–1

, which is higher than that value of 1281 m

²

·g

^–1

in the case of UiO-66. A similar effect was observed for the

V_a

value (0.85 vs. 0.43 cm

³

·g

^–1

). As a consequence, the hydrogen adsorption capacity measured at 77 K and 38 bar of H

₂

was almost twice as high for UiO-67 (4.6 wt%) as for UiO-66 (2.4 wt%).

The number of aromatic benzene rings may also influence the hydrogen sorption, as determined from the study of IRMOFs. The hydrogen sorption changed within the range of 0.89–1.73 wt% H

₂

[13].

4.1.2. Catenation process

From a thermodynamic perspective, the value of surface energy must be as low as possible to form a compact structure of the material. Therefore, when large pores in an MOF from a long organic linker are anticipated using only the geometrical calculation, an interweaving or interpenetration of the MOF framework may occur in practice. Interweaving and interpenetration are considered to be two types of the catenation process (bonding of atoms of the same element into a chain) where, respectively, a minimal and maximal displacement occurs between the catenated frameworks [14]. It was proposed that catenation of MOFs may be controlled by adding a template during the solvothermal MOF synthesis (e.g., the oxalic acid used as a template promotes a non-catenated MOF framework). Using a brominated and non-brominated ligand, in which a non-catenated and catenated MOF framework was produced, the authors of [15] concluded that catenation may be controlled by the organic linker design. As a result of the synthesis of organic linkers and their organization in the MOF structure depending on the reaction parameters (e.g., the precursor concentration, temperature, and time of stirring), the catenation process in a different form is strongly guided by the MOF synthesis conditions.

It is obvious that catenation directly affects the H

₂

adsorption in MOFs. The copper paddle wheel node and the TATB linker (4,4',4"-s-triazine2,4,6-triyltribenzoate) are the building units for both porous coordination networks (PCNs), catenated PCN-6 and non-catenated PCN-60, and their general formula is Cu

₃

(TATB)

₂

[16]. After sample activation at 323 K, the hydrogen adsorption capacity at 77 K and 1 bar of H

₂

was 1.74 and 1.35 wt% for PCN-6 and PCN-60, whereas after activation at 423 K, these values increased to 1.9 and 1.62 wt%, respectively. The larger improvement in H

₂

adsorption for the non-catenated PCN-60 was explained by the open metal sites, which were blocked in the catenated PCN-6.

4.1.3. Different organic linkers

A combination of two types of organic linker within the same MOF structure could be another approach to increase the pore characteristics of MOFs. For example, the SNU-6 framework was constructed from the BPnDC (4,4'-benzophenone dicarboxylate) ligand and the bpy (4,4'-bipyridine) ligand, together with Cu

²⁺

nodes [17]. This MOF was characterized by large pores (the pore diameter of 18.2 Å), and the

A_s

values obtained by the BET and Langmuir methods were 2590 and 2910 m

²

·g

^–1

, respectively. The hydrogen adsorption capacity at 77 K and 1 bar of H

₂

was relatively low (1.68 wt%); however, at high pressure (77 K and 70 bar of H

₂

), the

n_a

and

n_σ

values were 10.0 and 4.87 wt%, respectively.

Approaches using different organic linkers have been recently developed with a mixed–matrix hybrid strategy to provide a tool for facile characterization of molecular transport in MOFs. For example, in [18], incorporation of the MOF crystals into polymers resulted in hybrid membranes with excellent molecular sieving properties. The improved membrane performance resulted from precise control of the organic linkers in the MOF, which delimited the entrance to the pores.

4.1.4. Flexible organic linkers

Dynamic MOFs can respond to external conditions (e.g.,

T

,

P

, electric or magnetic fields, and chemical insertion) and reversibly change their channels by a large magnitude while maintaining the same or similar topologies. Therefore, this type of MOF is often associated with reversible transformations between the expansion and the contraction states, which are called the “breathing” or “sponge” effect. For 3D dynamic MOFs, three situations have been distinguished [19]. As a result of the interlayer extension and shortening, reversible transformations may occur using suitable flexible pillars (class a), sponge-like dynamic behavior (class b) or interpenetration (class c).

A systemic synthesis and characterization of a series of MOFs with the pillared layer structure was carried out in [20]. The [Zn

₄

(bpta)

₂

(H

₂

O)

₂

] (H

₄

bpta = 1,1’-biphenyl-2,2’,6,6’-tetracarboxylic acid) layers were connected by length-controllable bipyridine pillars. The authors developed a synthetic strategy that allowed systematic variation of the pillar to construct open frameworks with a similar structure. It was concluded that pore design could be adjusted by the selection of pillar ligands, and hence lead to different hydrogen adsorption properties. The results obtained indicated that the activation process of these MOFs could affect the

Q_st

value.

The hydrogen adsorption analysis of three flexible sulfur-containing MOF materials named M-URJC-n (M = Co, Cu, Zn) based on the 5,5’-thiodiisophthalic acid linker (H

₄

TBTC) showed that these compounds display a gate-opening type adsorption mechanism at low pressures, attributed to the flexible nature of the ligand [21]. The hydrogen adsorption capacities of the compounds were not high, with levels of 2.81, 2.21, and 1.99 wt%, for Co-, Cu-, and Zn modification, respectively, at 77 K and up to 18 bar, and 0.12, 0.14, and 0.13 wt%, at 298 K and 170 bar, due to the presence of flexible ligands. These compounds showed an interesting gate-opening type adsorption mechanism. Considering the flexibility and the dynamic nature of the new structures, these compounds are candidates for applications such as hydrogen selective adsorption and gas separation processes, including hydrogen purification in precombustion mixtures of H

₂

/CO

₂ ^[21].

[21].

4.2. Factor of isosteric enthalpy of hydrogen adsorption

Among MOFs with high

A_s

values, some of the MOF examples have high values of hydrogen adsorption capacity but only at cryogenic temperature. In practice, the hydrogen adsorption capacity at moderate temperatures designed by DoE falls to less than 1/10 of the value obtained at 77 K, and the

Q_st

values in most porous MOFs is within the range of 5–10 kJ·mol

^–1

[22]. As a result of physical sorption of H

₂

molecules, van der Waals interaction between H

₂

molecules and the pore surface of MOFs is very weak. To increase the interaction at the ambient temperature, two recommendations have been made: (1) strong adsorption sites incorporated into the pores; and (2) optimization of the internal surface of MOF. In practice, creation of open metal sites, introduction of cations generating a strong electrostatic field within the cavities, doping with metal ions, infiltration of metal nanoparticles, and organic linker functionalization have been used to increase molecular hydrogen affinity to MOFs. It was concluded that MOFs can reach approximately 6 wt% H

₂

when they are characterized by a

Q_st

value of 10–15 kJ·mol

^–1

and a free volume within the range of 1.6–2.4 cm

³

·g

^–1

, or

Q_st

> 20 kJ·mol

^–1

and a free volume smaller than 1.5 cm

³

·g

^–1

. The authors of [23] studied H

₂

adsorption in an MOF at 298 K and 1.5 or 120 bar of H

₂

, and measured deliverable capacity as the difference between the

n_a

value at the high and the low hydrogen pressure. Their GCMC calculations suggested that for the maximum H

₂

delivery at 298 K, the optimal

Q_st

value must be around ~20 kJ·mol

^–1

.

4.2.1. Open metal sites at secondary building units and organic linkers

It is known that metal centers unsaturated in coordination, often called “open” or “accessible” metal sites, may increase the

Q_st

value. In practice, the H

₂

adsorption values measured at 298 K for MOFs with open metal sites are higher than those for MOFs without open metal sites. In addition, this tendency is pronounced for MOFs with a high

A_s

value (>3000 m

²

·g

^–1

). Based on experiments involving the introduction of open metal sites in MOFs, a number of possible approaches have been highlighted: (1) metal nodes with open metal sites; (2) metal clusters coordinating solvent molecules, in which the solvent is removable; (3) organometallic complexes connected with the aromatic organic linkers; (4) metal complexes intercalated through electrostatic forces; and (5) metal cations in the anionic frameworks.

Open metal sites in MOFs may be constructed of metal cluster SBUs coordinating solvent molecules, followed by removal of the solvent using thermal treatment. In this case, the SBUs are very often the bimetallic paddle wheel units of M

₂

(O

₂

CR)

₄

(M=Cu

²⁺

, Zn

²⁺

, Cd

²⁺

), in which solvent molecules are coordinated at the axial sites. The fluorite-like structure of the [Co

^II₄

(μ-OH

₂

)

₄

(MTB)

₂

(H

₂

O)

₄

]

_n

·13nDMF·11nH

₂

O framework (SNU-15) promotes 3D channels in which every Co

²⁺

ion coordinates the aqua ligand, and the Co-Co distances are 3.550 and 3.428 Å. When the coordinated H

₂

O molecules are removed, the vacant coordination sites are created on the Co

²⁺

ions. Due to the values of the Co–Co distance, H

₂

molecules can be bonded in a side-on manner that results in the

Q_st

value of 15 kJ·mol

^-1

at zero coverage [23].

The open metal sites at organic linkers are also a highly popular approach used to increase the

Q_st

value. For example, the structures of MOFs constructed from various types of porphyrin complexes are very well known [24]. They contain open metal sites with respect to the square-planar coordination plane, therefore the porphyrin-based MOFs may be recommended for effective H

₂

adsorption.

4.2.2. Metal ions for an electrostatic field within the cavities

Another approach to increasing the

Q_st

value is to exchange the cations integrated in the anionic framework with metal ions that have higher affinity to H

₂

molecules. For example, in the Mn

₃

[(Mn

₄

Cl)

₃

(BTT)

₈

]

₂

, (BTT = 1,3,5-benzenetristetrazolate) framework, Mn

²⁺

ions can be exchanged with Li

⁺

, Cu

⁺

, Fe

²⁺

, Co

²⁺

, Ni

²⁺

, Cu

²⁺

, or Zn

²⁺

ions [25]. Chemical compositions based on the relative ratio of Mn

⁺

/Mn

²⁺

showed that the total number of extra framework cations is not higher than five. The

Q_st

values, calculated using a virial fit to the H

₂

adsorption isotherms at 77 and 87 K, demonstrate a relatively large variation, but they deviate in almost the same region as those for the Mn

₃

[(Mn

₄

Cl)

₃

(BTT)

₈

]

₂

framework. The values of hydrogen adsorption capacity measured at 77 K and 1.2 bar of H

₂

for the ion-exchange frameworks and the original Mn

₃

[(Mn

₄

Cl)

₃

(BTT)

₈

]

₂

are within 2.00–2.29 wt%.

4.3. Catalytic effect and pore design

Due to their high affinity to hydrogen, the platinum group metals were the first catalysts used in various hydrogenation reactions and processes related to H

₂

molecules. Subsequently, a hydrogen spill-over effect was defined as dissociative chemical sorption (i.e., H

₂

→2H) on the catalyst surface and the later migration of H atoms onto the surface of support. The catalytic effect was also used for MOFs in an attempt to accelerate the hydrogen adsorption process. In addition, the morphology of pores and their extra functionalization were taken into consideration. In the case of microporous MOFs, the morphology of the pores represents their main characteristics, including the geometrical shape (pore width and pore volume) and roughness of the pore walls. A small pore diameter was found to be an important factor for the achievement of a high value of hydrogen adsorption capacity at room temperature.

4.3.1. Incorporated metal nanoparticles

Recently, the idea of the hydrogen spillover effect was transferred to MOFs, and the introduction of several types of metal nanoparticles (NPs) to MOFs was considered. The simplest method involves preparing a coating made of the NP metal on the MOF surface, but without completely wrapping the MOF in a metal thin film. Using a solid grinding approach, Au NPs were attached to the surface of the well-known MOF examples, including MOF-5 [26]. The solution impregnation method was reported to be a convenient tool for preparing metal NPs infiltrated inside MOFs. Typically, the metal precursor solution fills the MOF pores by capillary force, and the chemical reduction (e.g., with H

₂

, NaBH

₄) then produces the NPs settled in the body of the MOF. This method was successfully used to prepare mono- and bimetallic NPs inside MOFs (e.g., Pt NPs ^{[27][28][29][30]} and Pt/Ni NPs ^[31]). In addition, the chemical vapor deposition (CVD) technique was found to be advantageous in the deposition of metal nanoparticles in MOFs because, during the CVD vapor phase, the precursor is loaded in MOFs, and the precursor is then decomposed or reduced to obtain nanoparticles inside the MOF pores.

) then produces the NPs settled in the body of the MOF. This method was successfully used to prepare mono- and bimetallic NPs inside MOFs (e.g., Pt NPs [159-162] and Pt/Ni NPs [162]). In addition, the chemical vapor deposition (CVD) technique was found to be advantageous in the deposition of metal nanoparticles in MOFs because, during the CVD vapor phase, the precursor is loaded in MOFs, and the precursor is then decomposed or reduced to obtain nanoparticles inside the MOF pores.

Palladium was among the first examples of metal NPs inside MOFs. For example, Pd NPs (with an average size of 2.5 nm) were successfully infiltrated in MIL-100(Al) with a high metal content (10 wt%) without degradation of MOF [27]. As a result of Pd impregnation, the reduction in the

n_σ

value at 77 K and 40 bar of H

₂

was directly related to the decrease in the

A_s

and

V_a

values (from 380 to 1200 m

²

·g

^–1

, and from 0.33 to 0.65 cm

³

·g

^–1

, respectively), whereas at 298 K, the

n_σ

value increased, most probably because of the Pd hydride formation. Formation of PdH

_0.6

was experimentally confirmed for the Pd NPs incorporated in MIL-100(Al), although the spill-over effect was also considered.

Platinum, scandium and titanium are other attractive examples of the incorporation of metal NPs in MOFs.

4.3.2. Morphology of pores and their functionalization

The effect of the pore size may be explained by the fact that the small diameter of pores enables energy potentials to overlap between the opposing walls, resulting in higher interaction energy with H

₂

molecules. For example, MOFs of M(HBTC) (4,4'-bpy)·3DMF (M = Ni, Co; HBTC = 1,3,5-benzenetricarboxylic acid; 4,4'-bipy = 4,4'-bipyridine; DMF = N,N'-dimethylformamide) showed H

₂

adsorption of 3.42 and 2.05 wt% at 77 K and 1 bar of H

₂

, and 1.20 and 0.96 wt% at 298 K and 70 bar of H

₂

, respectively [28]. The

A_s

values measured by the BET method were 1590 and 887 m

²

·g

^–1

respectively, but it was noted that these two frameworks had nonlinear rectangular channels with an average size of 7 × 6 Å. Another [Co

₃

(NDC)

₃

(dabco)] (dabco = 1,4-diazabicyclo[2.2.2]octane) framework with a primitive cubic structure demonstrated 0.89 wt% of H

₂

at 298 K and

P

(H

₂

) = 17.2 bar, which was explained by both the

A_s

value (1502 m

²

·g

^-1) and the average pore diameter (4.5 Å) ^[32]]. The four-fold interpenetrating networks based on the [OZn

) and the average pore diameter (4.5 Å) [185]. The four-fold interpenetrating networks based on the [OZn

₄

]

⁶⁺

nodes with 3D channels (<5 Å) might also be considered. These examples indicate that, even with a small

A_s

value, hydrogen adsorption can be significant and, most importantly, controlled by pore size (5–7 Å).

5. Prospective views and outlooks

5. Prospective views and outlooks

MOFs are unique chemical compounds that can be used in numerous scientific applications, including hydrogen adsorption; however, adapting a pure MOF to the DoE target to be used as an effective hydrogen storage material remains a significant challenge. The factors of surface area and pore volume remain the most critical issue, because at 298 K, the

n_a

value correlates mainly with the

V_a

value at low

P

(H

₂

) and the

A_s

value at high

P

(H

₂

). Organic linkers have a direct influence on the pore morphology, and therefore, on the surface area of MOFs, their length, chemical composition, and flexibility may explain the catenation and sponge-like dynamic mechanism in MOFs. These effects may change the hydrogen adsorption in pure MOFs, but their absolute quantity of adsorbed hydrogen does not meet the DoE requirements at 298 K, or at 100 bar of H

₂

. The value of

Q_st

may be considered to be a reasonable indicator of the applicability of pure MOFs as a hydrogen storage material. At 298 K, a tendency exists to form a proportional relationship between the

Q_st

and the

n_σ

values, and using the approach of open metal sites or an electrostatic field, isosteric enthalpy of hydrogen adsorption increases slightly in pure MOFs. Numerous calculations suggest that the value of

Q_st

within the range of 15–25 kJ·mol

^–1

, with a

V_a

value of ~2.5 cm

³

·g

^–1 and porosity of ~85%, would be desirable for an MOF as a successful candidate for hydrogen storage with 9 wt% of H

and porosity of ~85%, would be desirable for an MOF as a successful candidate for hydrogen storage with 9 wt% of H₂.

Nanoscaffolding materials based on MOFs, sometimes generalized to “hybrid” or “functional” composites, may combine properties of their components. In practice, isosteric enthalpy of hydrogen adsorption and hydrogen adsorption capacity has not been fully examined for this kind of material, and is not reproducible for the same examples. The main reasons for these discrepancies may lie in the morphology of pores and their functionalization, which is highly sensitive to the synthesis procedure and post-synthetic modification of MOFs. Another approach of nanoscaffolding materials based on MOFs may be nanoconfinement of metal hydrides inside the frameworks, in which MOFs are considered to be nanoscaffolds rather than H₂ sorbents. The main advantages of nanoconfined hydrides relate to their faster kinetics, higher reversibility, and the change in the mechanism of hydrogen interaction. The additional weight of nanoscaffolds leads to lower gravimetric hydrogen sorption capacity in the composite material.

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Nanoscaffolding materials based on MOFs, sometimes generalized to “hybrid” or “functional” composites, may combine properties of their components. In practice, isosteric enthalpy of hydrogen adsorption and hydrogen adsorption capacity has not been fully examined for this kind of material, and is not reproducible for the same examples. The main reasons for these discrepancies may lie in the morphology of pores and their functionalization, which is highly sensitive to the synthesis procedure and post-synthetic modification of MOFs. Another approach of nanoscaffolding materials based on MOFs may be nanoconfinement of metal hydrides inside the frameworks, in which MOFs are considered to be nanoscaffolds rather than H

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₂ sorbents. The main advantages of nanoconfined hydrides relate to their faster kinetics, higher reversibility, and the change in the mechanism of hydrogen interaction. The additional weight of nanoscaffolds leads to lower gravimetric hydrogen sorption capacity in the composite material.

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