Reflections on the topic of solar fuels

Abstract

The amount of energy and chemicals extracted to date from sunlight by the solar fuels community is minute in comparison with the corresponding amounts being regularly extracted from non-renewable sources. This brief appraisal discusses why.

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Broader context

This article is an expanded version of the author's introductory remarks for the opening session on Solar Fuels at the ISACS12 meeting in Cambridge, September 2013, on “Challenges in Chemical Renewable Energy”. Its main thrust is to emphasize the enormous gap that now exists between the modest achieved success in generating solar fuels and the massive scale on which both energy and chemical commodities are currently being manufactured from non-renewable (fossil-based sources) using present-day selective catalysts. It emphasises the distance that researchers into solar fuels still have to travel before the rewards, latent in solar radiation, are reaped.

Along with the pressing need for global energy,^1–4 the need for solar fuels is also extremely important and urgent. This state of affairs has arisen not because of the putative diminution in non-renewable (fossil-based) sources. Contrary to many dire predictions made less than a decade ago, it is now apparent that there are adequate world reserves of gas, oil and coal for many years to come. Rather it is the necessity to decrease greatly the quantity of CO₂ that is liberated into the earth's atmosphere. At present this amounts to some 4.2 gigatonnes of CO₂ per annum.⁵

During the course of the next 30 years or so, solar fuel plants will need to be operating on a very large scale, generating not only hydrogen, oxygen, clean water, methane, methanol and other liquid alcohols,^6,7 but also a host of other essential chemicals to sustain civilized life. Such manufacturing plants are needed in addition to the impressive, delocalized, energy-producing units, like the artificial leaves devised by Grätzel,⁸ Nocera,⁹ Barber,¹⁰ Centi,¹¹ Garrone,¹² Sun¹³ and many others.^14,15

To achieve these desirable ends one must embark on vigorous and rigorous quests for active, stable and selective new catalysts. These catalysts should exhibit high turnover numbers and high turnover frequencies. One of the reasons why the Grätzel cell is popular is because it has a turnover number of ca. 10⁸, which means that it has a lifetime of 20 or more years.⁸

The task is by no means straightforward, for what one has to achieve must ultimately replace the processes that currently generate fuels from non-renewable sources. We enumerate below a few key facts pertaining to the fuels for transport and heating as well as numerous other products that are derived from petroleum:

• Petroleum yields fuels for transport and heating: gasoline, diesel, jet fuel, olefins plus a myriad other products.

• La³⁺-exchanged zeolite Y catalysts are used for cracking: these catalysts possess 10¹⁷ to 10¹⁹ active sites per gram and they exhibit turnover frequencies of ca. 0.1–1.0 per second per site.

As well as the fuels that are extracted from petroleum and other non-renewables (fossil based sources) there is also an exceptionally wide range of other products which constitute the materials required to maintain the quality of life which the developed world demands (see Fig. 1).

Fig. 1 Products from petroleum. (This list was compiled by J.D. Keasling.¹⁹)

To illustrate the magnitude of the challenge facing those who seek to replace non-renewable feedstocks with renewable ones we cite, as an example, the situation pertaining to acrylonitrile. At present, the world consumption of this commodity is close to 7 × 10⁹ kg per annum, i.e. ca. 1 kg per person per year. Acrylonitrile is made by ammoxidation using selective, long-lived solid catalysts that are composed of complex mixed-metal oxides, the immediate hydrocarbon precursors being either propylene (obtained from a catalytic cracker of petroleum) or propane, a constituent of natural gas.^16,17

As the rate of growth of the middle classes is expanding much more rapidly than the rate of growth in world population – it doubled to 2 × 10⁹ from 2000 to 2003 – there is a consequential increase in demand for more consumables (and energy).¹⁸

It is when one contemplates the scale and diversity of products enumerated in Fig. 1,¹⁹ that the challenge of exploiting renewable resources, such as biomass^20–23 and solar products^1–4 is seen in its true perspective. For instance, a 75 000 barrel-a-day fluidized catalytic cracker cycles (between the reactor itself and a regenerator) some 5000 tonnes of catalyst per 24 h for two to three years. The catalyst contact time in the reactor is 2 to 4 s at ca. 530 °C; it is regenerated at a temperature of 715 °C. In the Paraguaná Refinery Centre in Falcon, Venezuela, there is a fluidized catalytic cracker (FCC unit) that processes some 940 000 barrels a day of petroleum; and the new South African refinery now being built in the Industrial Development Zone near Port Elizabeth will process 400 000 barrels per day.²⁴ With the recent significant advance made by Garcia-Martinez and co-workers²⁵ in designing highly efficient mesostructured zeolite-Y cracking catalysts, and the dramatic advances made by Italian industrialists²⁶ in the efficiency of cracking and hydrotreating of petroleum, using single-sheet MoS₂ catalysts, the incentives to continue using it as a source of fuel and commodities remain high.

Producing hydrogen on a massive scale would be a significant start, for it has long been recognised that the hydrogen economy^27,28 is an important stepping-stone to sustainable development. Numerous materials can be derived from it by catalytic combination with carbon dioxide,²⁹ such as methanol, which is currently manufactured on a megatonne scale that rivals that of the largest man-made commodity NH₃.

I am grateful to A. Harriman, P.P. Edwards and R. Millini for stimulating discussions.

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