Boranes Classification Essay

Higher Boranes

Boron forms a large number of hydries (in addition to dibroane). More than 25 neutral Boranes and a large number of borane anions have been prepared and characterized. We are only dealing with some very simple higher neutral Boranes. These are B4H10, B5H9, B5H11, B6H10 and B10H14. These can be classified into two categories based on their stoichiometry and structure.

1. Nido-Boranes : (BnHn+4)
                                                     B2H6,B5H9,B6H10 and B10H14

The word nido is derived from the Latin word meaning ‘nest’. So they have a nest like open structure.

2. Arachno-boranes : (BnHn+6)
                                                           B4H10,B5H11

The word orachno-means spider’s web. Their structure is open form both ends. In these compounds boron atoms occupy n contiguous corners of an (n + 2) cornered polyhedron.

Nomenclature
Boranes are usually named by indicating the number of B atoms with a Latin prefix and the number of H atoms by an Arabic number in parentheses.

B2H6_____ _ diborane (6)
B4H10_______tetraborane (10)
B5H9___ _____pentaborane (9)
B5H11______ _pentaborane (11)
B6H10______ hexaborane (10)
B10H14_____  decaborane (14)

Preparation   
(1)    Tetraborane (10) (B4H10) is best prepared by keeping B2H6 under pressure at 250 for 10 days.

(2)    (i) pentaborane-9 (B5H9) is prepared by passing a 1:5 mixture of B2H6 and H2 at below one atmospheric pressure through a furnace maintained at 2500 for 3 seconds.

(ii) Alternatively, B5H9 can be prepared by pyrolysing B2H6 for 2.5 days in hot/cold reactor at 1800/-800.

(3)    Pentaborane-11 (B5H11) can be prepared in 70% yield by the reaction of B2H6 and B4H10 in a hot/cold reactor maintained at + 1200/ - 300C.

(4)    Decaborane-14 (B10H14) is prepared by paralysis of B2H6 at 100-2000C in the presence of catalytic amounts of Lewis bases such as demethylether.

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Support materials play not merely the role of carrier for active metal, but also provide a large active surface area and better dispersion of the active phase due to their porous nature. Besides that, a supported catalyst facilitates the diffusion of reactants through the pores to the active phase, which is the major limiting step for the hydrolysis reaction, improves the dissipation of the reaction heat, retards the sintering of the active phase, and increases the poison resistance. In comparison with various metals powder or their salt catalysts, the supported catalysts are highly appreciated in practical applications owing to their easy separation from fuel solution, and consequently, the ready controllability of the hydrolysis reaction and reusability of the catalyst [51,52]. In this section, we briefly survey porous support materials for supporting and immobilizing active species for hydrolysis of ammonia borane.

2.1. Microporous and Mesoporous Inorganic Support Materials

Porous materials are classified as macro-, meso- and microporous depending on the size of the pores, e.g., >50 nm, 50–2 nm and <2 nm, respectively. The use of microporous and mesoporous materials with ordered porous structures as the hosts to encapsulate metal nanoclusters has attracted particular interest in catalysis because the pore size restriction could limit the growth of nanoclusters and lead to an increase in the percentage of the catalytically active surface atoms. The use of nanocluster catalysts in systems with confined void spaces such as inside mesoporous and microporous solids appears to be an efficient way of preventing aggregation [53,54,55].

Co and Co borides/phosphides show attractive catalytic activities [56,57]. The Co–B catalyst is supported on highly ordered mesoporous silica particles prepared by chemical impregnation-reduction method with pore size of 2–10 nm [58]. Co–B nanoparticles on the mesoporous silica are able to block large numbers of mesopores by either completely filling the pores from inside or positioning on the pore face, while Co–B particles supported on non-porous silica are composed of spherical particles in the range of 30–40 nm, all or part of which are present in agglomerated state. In contrast, mesoporous silica particle-supported Co–B affords a large amount of particles (~90%) having a size lower than 10 nm. Co–B nanoparticles are located on the surface of the mesoporous silica particles with some portion of the former particles anchored into the pores. Co–B catalyst supported on mesoporous silica particles was able to produce the expected amount of hydrogen (H2/NH3BH3 = 3.0) by hydrolysis of ammonia borane, while unsupported Co–B catalyst and that which is supported on non-porous silica particles were able to produce only ~85% of H2 yield (H2/NH3BH3 = 2.55). Hydrolysis in the presence of unsupported powder and Co–B catalyst supported on non-porous silica particles is in the first order with respect to concentration of ammonia borane. By comparison, the hydrogen generation data produced by the Co–B catalyst supported on mesoporous silica particles powder was zero order with respect to ammonia borane concentration due to its high effective surface area that permits immediate hydrogen generation through surface reaction. The maximum hydrogen generation rate achieved with mesoporous silica particle-supported Co–B is about 2.5 and 3 times higher than that obtained with non-porous silica particle-supported Co–B and unsupported Co–B powder catalyst.

The dispersion of the catalyst species depends on the types of mesoporous silica [55] (Figure 1). Co–B particles are located inside the pores of SBA-15 silica by keeping the pore structure intact while for MCM-41 and FSM-16 catalyst, particles either completely fill the pores or lie outside on the face of pores. The Co–B particles are well confined in the pores of SBA-15 acquiring the size of pores (~6 nm). Along the channel, the size of Co–B slightly increases to around 10 nm. In case of FSM-16 and MCM-41, the Co–B particles are well dispersed on the surface having broad distribution of size in the range from 3–30 nm, most of which (90%) have a size lower than 15 nm. A particle size greater than the pore size confirms that Co–B particles are located on the surface of the MCM-41 and FSM-16 type silica with some portion of the particle anchored into the pores. During the reduction process by sodium borohydride, the Co–B particles are formed by release of hydrogen gas. Due to the interconnected pore assembly, hydrogen can leave the interior of the SBA-15 easily. In FSM-16 and MCM-41, the pores are not connected and thus, hydrogen can be released only from the pore face which is blocked by the Co–B particles. Thus, due to the pressure exerted by the hydrogen gas, the Co–B particles are pushed out on the external surface of MCM-41 and FSM-16. The hydrogen production data for Co–B supported on all the mesoporous silica materials proves zero order kinetics with respect to concentration of ammonia borane. The maximum hydrogen generation rate achieved by Co–B supported on SBA-15 silica (~1900 mL-H2 g-(Co–B cat.)−1 min−1) is 4.2 and 5.3 times higher than that obtained by non-porous silica supported Co–B (~480 mL-H2 g-(Co–B cat.)−1 min−1) and unsupported Co–B powder catalyst (~360 mL-H2 g-(Co–B cat.)−1 min−1). For mesoporous silica supports, Co–B supported on SBA-15 showed the highest hydrogen generation rate which is about 1.5 times higher than that measured with MCM-41 (~1150 mL-H2 g-(Co–B cat.)−1 min−1) and FSM-16 (~1200 mL-H2 g-(Co–B cat.)−1 min−1). The activation energies of Co–B catalyst supported on SBA-15 (43 kJ mol−1) displays significantly lower energy barriers in comparison to Co–B supported on MCM-41 (51 kJ mol−1) and FSM-16 (58 kJ mol−1).

Figure 1. TEM micrograph of MCM-41 (a); FSM-16 (b); and SBA-15 (c). Reproduced with permission of Ref. [55].

Figure 1. TEM micrograph of MCM-41 (a); FSM-16 (b); and SBA-15 (c). Reproduced with permission of Ref. [55].

Co–B catalysts were also synthesized by pulsed laser deposition (PLD) in form of nanoparticle-assembled films because of easily controllable surface morphology and structure by the use of this method [22,59,60]. Nanoparticle-assembled Co–B thin film on a planar glass substrate was able to produce almost the expected amount of hydrogen (95%) from hydrolysis of ammonia borane with significantly higher rate (about six times) than the same amount of the corresponding Co–B powders [60]. The Co–B nanoparticles were produced during the ablation process on the film surface, with an average size of around ~250 nm and well established spherical form. Highly irregular and porous carbon film was adopted as a support for the Co–B nanoparticles to effectively improve the initial surface area and obtain better dispersion of nanoparticles. The films ranging from diamond-like to highly porous, cluster-assembled structures were deposited using PLD by varying laser parameters [61]. The total amount of hydrogen generated by hydrolysis of ammonia borane using both the Co–B films (H2/NH3BH3 = 2.85) is closer to the quantitative yield expected from the reaction stoichiometry (H2/NH3BH3 = 3.0) than that generated by Co–B powder (H2/NH3BH3= 2.55). The maximum hydrogen generation rate obtained for the carbon-supported Co–B films (~4060 mL-H2 g-cat.−1 min−1) has been found to be significantly higher than that obtained using the unsupported Co–B film (~2400 mL-H2 g-cat.−1 min−1) [62]. Both unsupported and supported Co–B catalyst films synthesized by PLD showed amorphous spherical particle-like morphology (average size ranging between 50 and 300 nm) with some agglomerates. The supported Co–B film had a dendritic microstructure with the Co–B nanoparticles embedded in the porous carbon film with improved dispersion. By comparison, the carbon film deposited at low Ar pressure exhibited a columnar structure with embedded spherical nodules on the surface. By increasing the pressure, dendritic, highly porous microstructure starts to appear with extremely irregular surface features which appear bigger, more loosely packed, and non-spherical with barely any adhesion to the substrate because cluster–cluster collision may also occur. Film-substrate adhesion is slightly poorer than that reached at low Ar pressure. Co–B catalysts supported on the carbon film deposited at low Ar pressures show almost similar catalytic activity as unsupported Co–B film due to the non-porous structure of the carbon films. However, the catalytic activity increases for Co–B catalysts supported on the carbon films deposited at higher Ar pressures. Hydrogen generation rate reached the maximum for the carbon film deposited at low pressure (40 Pa), and then, Co–B catalyst supported on the carbon film deposited at highest pressure of 50 Pa showed drastic decrease in the hydrogen generation rate and was not able to complete the hydrolysis reaction of ammonia borane due to its very weak adhesion with the substrate that leads to slow detachment of the film from the substrate in the reactant solution. The activation energy of the carbon film supported Co–B film is 29 kJ mol−1 which is lower than that obtained with unsupported Co–B film (34 kJ mol−1) and Co–B powder (44 kJ mol−1) [62].

Ni foam-supported amorphous ternary catalysts such as Co–Mo–B and Co–W–B have been prepared by a modified electroless plating method and exhibited enhanced hydrogen generation kinetics [58,63]. In a typical electroless deposition case, hydrogen evolution as a by-product is deliberately inhibited in order to produce a uniform and dense metallic coating by using strong complex agents and stabilizers in the bath solution. In the modified method, the formation of gas by increasing the concentrations of reducing agent and main salts, as well as elimination of the use of stabilizer was favored, and then, the nucleation and deposition rates of the metals are greatly increased, and a large number of hydrogen bubbles are evolved. These hydrogen bubbles function as a dynamic template and metal is chemically deposited and grows within the interstitial spaces between the hydrogen bubbles forming a porous coating of metallic particles on the support. Quaternary cobalt-tungsten-boron-phosphorus porous particles supported on Ni foam (Co–W–B–P/Ni), which are prepared through ultrasonification-assisted electroless deposition route, consisted of interconnected flower-like porous nanospheres (diameters: 200–400 nm) with enhanced contact between the active component and the substrate [64]. The molar ratio of generated hydrogen to the initial ammonia borane was close to 3.0, and the required reaction time with Co–W–B–P/Ni was much shorter than that with Co–W–B/Ni and Co–B/Ni. The catalytic hydrolysis reaction with respect to ammonia borane concentration exhibited quasi first-order character. Hydrogen generation rate in the presence of the most active catalyst was 4000 mL-H2 g-cat.−1 min−1. Moreover, the apparent activation energy for the quaternary catalyst was determined to be 29.0 kJ mol−1. After the 10th usage, the catalyst preserved 68% of its original hydrogen generation rate. The deposited catalyst layer did not peel off from the Ni foam substrate after repeated testing.

The use of zeolites as host materials with confined void spaces for guest metal nanoparticles seems to be one of the up-and-coming strategies to prevent the agglomeration of metal nanoparticles and bulk metal formation. Furthermore, encapsulation of metal nanoparticles within the porous structure of zeolite or between the zeolite-supported layers can help in the kinetic control of catalytic reactions [65].

Ru nanoparticles@ZK-4 were prepared by ion-exchange of Ru3+ ions with the extra framework Na+ cations of ZK-4 zeolite (Na9[(AlO2)9(SiO2)15]·27H2O), LTA type structure (Linde-type A, Figure 2) with highly ordered cavities in 3-D structure, following sodium borohydride reduction of ruthenium(III)-exchanged ZK-4 in water at room temperature [66]. Neither the crystallinity nor the lattice of ZK-4 zeolite is altered by ion exchange. The incorporation of ruthenium(III) ions into ZK-4 zeolite and the reduction of ruthenium(III) ion forming Ru nanoparticles@ZK-4 causes no observable alteration in the framework lattice and no loss in the crystallinity of ZK-4 zeolite. Well dispersed ruthenium metal nanoparticles were present on the external surface of zeolite. The particle size of ruthenium nanoparticles was found to be in the range of 2.0–3.7 nm with a mean diameter of 2.9 nm. On passing from ZK-4 zeolite to ZK-4 zeolite and Ru nanoparticles@ZK-4, a notable decrease in the micropore volume and micropore area are observed, indicating that ruthenium nanoparticles exist not only on the surface but also inside the cavities of ZK-4 zeolite. The initial rates of hydrogen generation from the hydrolysis of ammonia borane are the highest using Ru nanoparticles@ZK-4 containing 1.1 wt. % Ru, mostly on the surface and readily accessible. They provide the complete stoichiometric hydrogen generation ([H2]/[H3NBH3] = 3) at 293 K. The activation energy was 28.2 kJ mol−1 and the turnover frequency (TOF) of Ru nanoparticles@ZK-4 was 5400 mol-H2 mol-Ru−1 h−1. The complete release of hydrogen is achieved in each of the successive catalytic runs (5 runs) without leaching of ruthenium into the reaction solution. The decrease (<15%) in catalytic activity in subsequent runs can be attributed to the passivation of active nanoparticle surfaces by boron products, e.g., metaborate, which decreases accessibility of active sites as previously seen in the case of Rh(0) nanoparticles@zeolite [36] and Cu(0) nanoparticles@zeolite [65].

Figure 2. The framework structure of ZK-4 zeolite with α- and β-cage [66].

Figure 2. The framework structure of ZK-4 zeolite with α- and β-cage [

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