Hydrogen Production by Reforming of Sodium Alginate in the Liquid Phase over Pt/C Catalysts

[NNadir]

The paper I'll discuss in this post is this one: Hydrogen Production by Reforming of Sodium Alginate in the Liquid Phase over Pt/C Catalyst (Vinayak N. Kalekar and Prakash D. Vaidya Industrial & Engineering Chemistry Research 2021 60 (27), 9755-9763)

We have, as people paying attention have noted, a real problem with algae overgrowth, much of it connected with run off from agricultural and landscaping fertilizers, as well as climate change.

Whether at sea, as in the case of the now dead offshore Mississippi Delta region, once a rich source of seafood, the Red Tides of Florida, where rotting fish are piling up on the beaches or the the famous cases of microcystin in Lake Erie area which rendered the water supply of major cities toxic, these algae blooms kill by the following mechanism: The photosynthetic algae experience a burst of growth so thick that lower layers of the algae mats lose access to light and die. The dead algae sinks or rots in place. As a result bacteria multiply to feed on the dead algae, consuming oxygen at a level which makes it impossible for large organisms, like fish, or shellfish to survive. Everything then dies, pretty much.

In theory, this process might be stopped by removing the algae before it gets thick enough to kill lower layers, either by filtration, skimming, or similar mechanisms.

If this is done in a timely or continuous fashion, it can actually be a carbon negative approach, in particular when the carbon is recovered and put to use in carbon based materials such as alloys, certain polymers, carbon fibers, nanotubes, etc. Such use may allow for the displacement of the use of dangerous fossil fuels to the extent that they, notably dangerous petroleum and dangerous natural gas, but also albeit more limited, dangerous coal, are used to produce industrial chemicals.

This particular paper describes the utilization of water as an oxidant; however carbon dioxide oxidants may offer certain advantages in situations that I call "the reverse Allam cycle." A problem with biomass reforming is that it is wet, but if the drying is conducted at high temperatures, such as might be produced with nuclear energy, with a mixture of steam and carbon dioxide, "syn gas" - from which any petroleum commodity chemical (or its equivalent) can be made - with adjustable hydrogen to carbon oxide ratios reflecting the ultimate target for synthesis.

From the paper's introductory text:

Macroalgae (or seaweeds), which are large multicellular plants growing in salty or fresh water, represent an attractive renewable energy resource.(1) They are especially promising because they are productive, fast-growing, and easily cultivated and harvested. They are cultivated in many Asian countries for making chemicals, pharmaceuticals, food products, and cosmetics. Even so, they are relatively untapped for making biofuels. Traditional biomass conversion processes such as pyrolysis and gasification are not suited for wet macroalgae, due to high water content and drying energy constraint. Preferably, water should be used as the reaction medium for conversion of such wet biomass. Marine macroalgae are classified as brown, red, or green seaweeds. Although they do not contain lipids or lignin, they are rich in carbohydrates. Alginate is a carbohydrate that is abundantly found in the outer cell wall of brown macroalgae. It comprises mannuronic and guluronic acid units linked by β-1,4-glycosidic bonds. Alginate is used in textile printing, drug delivery system, and food industry. There are a few reports suggesting alginate conversion into organic acids by hydrothermal treatment. For instance, Aida et al.(2) reported the formation of organic acids (lactic, malic, and succinic acids) in the 150–400 °C range. It was found that an increase in temperature favored alginate conversion and acid yields. Jeon et al.(3) found that glycolic acid and furfural were formed in acidic medium, whereas fumaric, lactic, and malic acids were formed in alkaline solutions.

Catalytic aqueous-phase reforming (APR), which reforms biofeeds in the liquid phase,(4) represents a further option for the valorization of seaweeds. Dumesic’s APR process is CO2-neutral because this co-product is utilized in biomass growth. APR produces higher-quality H2 (with less CO) and CO2 from weak solutions of biocarbohydrates in a single reactor at low temperatures near 225 °C. Water is maintained in the liquid state by applying high pressures. The energy constraint is low because the feed is not vaporized. The water gas shift (WGS) reaction is thermodynamically favored under such conditions, and an extra WGS reactor is avoidable. Extra H2 is produced through WGS, and the CO content of the product is lowered...

The authors report the work of others using different approaches. In their work, they use a platinum catalyst supported on carbon. Platinum is an expensive and relatively rare element falling into the critical element category. Notably, they also discuss ruthenium and palladium catalysts. These elements may be more readily available to future generations inasmuch as they are constituents of used nuclear fuel, and thus access to them may not be strictly limited to ores.

From my perspective, a carbon support in a reforming system is less than ideal, and they do report some associated difficulty with catalyst degradation, but anyway, some pictures from the text:



The caption:

Figure 1. Typical product gas profile for a trial over Pt/C (at T = 225 °C, t = 3 h, alginate = 1 wt %, catalyst loading = 2 kg/m3).

Characterization of the catalyst included the use of a scanning electron micscrope:



The caption:

Figure 2. SEM images of the unused Pt/C catalyst.

The catalyst was noted to have significant porosity by use of imaging and BET (Brunauer–Emmett–Teller) nitrogen adsorption device, and the active sites characterized by ammonia desorption experiments.

Two tables from the paper touch on the efficacy of two elements other than platinum:



While the ruthenium and palladium experiments gave lower hydrogen yields, they were explored only at one temperature. Modern materials science gives access to higher temperatures. This would justify exploring the use of more readily available - over many generations - palladium and ruthenium catalysts, albeit on different supports than porous carbon.

As it is, the platinum catalyst loses activity, and various analytical techniques show the changes to unused catalysts by use:



The caption:

Figure 3. XRD patterns of the unused and spent Pt/C catalysts.

IR:



The caption:

Figure 4. FTIR spectra of the unused and spent Pt/C catalysts.

X-ray photoelectron spectroscopy:



The caption:

Figure 5. (a) XPS wide spectra of the unused Pt/C catalyst. (b) Carbon (C 1s) spectra of XPS. (c) Oxygen (O 1s) spectra of XPS. (d) Platinum (Pt 4f) spectra of XPS.

The caption:

Figure 6. TEM image of the unused Pt/C catalyst.

Ammonia desorption (TPD = "temperature programmed desorption) shows the loss of acidic sites on the catalyst upon use:



The caption:

Figure 7. Results of NH3-TPD analysis of the unused and spent Pt/C catalyst.

Finally, reaction pathways leading to the products, mostly by decarboxylation reactions.:



The caption:

Figure 8. Reaction pathway for alginate reforming.

This is just one of many thousands, tens of thousands, of papers along these lines, the reforming of biomass. I mention it only as a signpost along the way for the reversing of climate change, turning a pollutant, algae, into a resource. The added benefit is the recovery of phosphate, which is decidedly not a renewable resource.

The paper's conclusion:

Alginate is a major constituent of the outer cell wall of brown macroalgae. Using this carbohydrate as feedstock, the aqueous-phase reforming (APR) process was investigated using Pt/C catalyst. APR trials were performed in the 200–230 °C range at 2.5 MPa pressure in a stirred batch reactor. The gaseous (H2, CO, CO2, and CH4) and liquid (acetic, propionic, lactic, and succinic acids) reaction products were identified. The effects of reaction variables (such as temperature, reaction time, alginate concentration, and catalyst loading) on the carbon conversion to gas (X), hydrogen selectivity (S-H2), and hydrogen yield (Y-H2) were studied. Using Pt/C catalyst, the carbon-to-gas phase conversion and H2 selectivity after 3 h were 9.7 and 35.5%, respectively. Other catalysts such as Ru/C and Pd/C were less selective to H2. The Pt/C catalyst was comprehensively characterized, and probable alginate conversion pathway was discussed.

A fun little paper, not necessarily the best paper on this subject, but worth reading if one has the chance.

Have a pleasant Friday.

[lordsummerisle]

1. fascinating n/t