CHEMICAL TRANSFORMATIONS: BEYOND THE PALE

by Drs. Viktor BARELKO and Dmitry KIRYUKHIN, RAS Institute of Chemical Physics Problems (Chernogolovka, Moscow Region); Dr. Oleg SAFONOV, RAS Institute of Experimental Mineralogy (Chernogolovka, Moscow Region); Dr. Maxim KUZNETSOV, All-Russia Institute of Civil Defense and Emergency Situations (Federal Ministry for Emergency Situations, Moscow)

Chemical processes taking place in the universe at temperatures close to absolute zero (-273.15 degrees centigrade, or 0 on the Kelvin scale) and such geotectonic phenomena as earthquakes and geochemical conversions of fluids in the earth crust are interconnected, strange as it may seem. To understand all that we should first look into certain physical concepts related to combustion, explosion, detonation and multiple catalysis.

"UNORTHODOX" REACTIONS

Back in the early 1980s we, a group of researchers at the RAS Institute of Chemical Physics Problems (Chernogolovka, Moscow Region), discovered surprising phenomena. Studying radiation and chemical transformations at liquid nitrogen (77 K) and liquid helium (4.2 K) temperatures, Igor Barkalov, Dmitry Kiryukhin, Anatoly Zanin and Viktor Barelko observed reactions proceeding in the solid phase at very high rates characteristic of high-temperature combustion and explosion. Quite uncommon, unorthodox phenomena! Our experiment boiled down to the following. Placed in a vat filled with liquid nitrogen or helium was a solid frozen mixture of methylcyclohexane and chlorine; the medium was exposed to gamma radiation. No signs of chemical reaction! (Under ordinary conditions these two substances interact vigorously at room temperature and produce chlormethylcyclohexane.) Yet mechanical action accompanied by local destruction of the sample initiated a high-rate calorimeter-registered reaction.

It has been proved experimentally that this phenomenon is proper to a wide range of chemical reac-

Cinegram of the wave front propagation in acetaldehyde polymerization at 77 K. Irradiation dose, 300 kGr, propagation rate, 1 cm/s. Ampoule diameter, 8 mm. Traveling wave velocity registered calorimetrically and visually with the use of a high-performance cine camera.

tions, such as polymerization and copolymerization of monomers, chlorination of saturated hydrocarbons as well as hydrobromination of olefins in the crystalline and vitreous state. Local mechanical action in such frozen solid body systems at liquid nitrogen and helium temperatures triggered a reaction in what seemed an inert mixture of reagents, and a spontaneous process of vigorous chemical transformation, which is at variance with the canons of orthodox chemistry stating this is impossible at temperatures close to Kelvin zero.

As many as fifteen hundred articles have been published on the results of this and follow-up experiments, let alone reports and communications at homeland and international conferences. Such ample evidence has prompted us to suggest a novel system of approaches ("new paradigm", we dare say) in the science of chemistry, coexisting in real terms with classical chemistry based on the fundamental concept of the exponential dependence of the chemical transformation rate on temperature (Arrhenius law).

To explain this effect we have made a working hypothesis predicated on the action of a trigger nonlinear mechanism via mechanochemical feedback.

The point is that the destruction of the sample gives rise to surface-active chips responsible for chemical transformation which, in turn, initiates comminution of the solid substance, an activating process that takes the reaction deep into the sample. Thus a self-sustained solid-phase chemical reaction is on.

The proposed approaches related the loss of a cryo-system's stability to the local perturbations and allowed for abnormally fast rates of a chemical process at ul-tralow temperatures. The reaction spreads down into the sample like a traveling wave in high-temperature combustion processes. To register this traveling front of conversion we carried out a series of experiments as local destruction was caused just by turning a rod frozen into the reaction sample. The very first experiments revealed an "autowave" mode of chemical transformation in the frozen solid body of reagents.

To exclude the effect of a heat factor our experiments also involved very small samples where a traveling wave was touched off in capillaries and even in thin films (say, by a needle prick) free of envelopes and immersed in a liquid nitrogen vat. And this proof-of-concept experiment: sluggish loads on the sample (i.e. local plastic deformation) did not initiate a reaction, while a "trigger needle" strike activated a traveling wave hands down.

Our mathematical models of the process made it possible to obtain tentative values of the traveling wave rates that proved to be close to experimental ones (in cm/s). At t° 4-77 K they are commensurate with combustion rates in solid-fuel rocket engines at temperatures of thousands of degrees centigrade (such rates may be even higher).

The following experimental fact is intriguing indeed. The autowave dynamics largely depends on the initiation place-either above (on the free surface) or below (on the test-tube bottom). If on the bottom, the conversion wave rate is higher. For instance, in solid-phase chlorination of hydrocarbons the difference margin was as large as two orders, 10², in m/sec!

This phenomenon may be explained as follows. If the traveling wave is initiated in the lower part of the ampoule (test-tube), the "mechanical feed" of the sample is obstructed by a hard plug composed of reagents that did not enter into the reaction. This boosts the mechanical action of the traveling wave in triggering this reaction in adjacent strata of the stock frozen material and thus speeds up the autowave transformation process. That is, if the process is initiated from the bottom up, the Shockwave mechanism comes into play, and the frozen substance is dispersed throughout the wave front due to the difference of the densities of the stock and the reacted material. Actually this mechanism is closely related to "gas-free" detonation. If this process is initiated from the free surface, the mechanical offloading is facilitated, with the heat factor emerging as

a trigger of the conversion reaction caused by a critical value of the temperature differential throughout the wave front.

Yet ordinary "deflagration" (slow burning) may break into detonation, a transition observed, e.g. in cryochemical experiments of acetaldehyde (CH₃CHO) and hydrogen cyanide copolymerization.

WHAT MODELS DEMONSTRATE

The discovery of violent cryogenic reactions has made it possible to hypothesize about their implication in processes involving evolution of cold matter in the universe and the possible role of cosmochemical transformations at prebiological stages of incipient life in the cold cosmos. A concept based on the extrapolation of combustion (explosion, detonation) on the cosmological chemistry sphere may help unravel many riddles, say, why cold planets of the solar system like Pluto are coated with a crust of methane and ammonia formed here on earth only at very high temperatures and pressures in the presence of catalysts.

The results of research into autowave cryopolimer-ization in the acetaldehyde/hydrogen cyanide system (there is conclusive evidence on the presence of these compounds in outer space) attests directly to the formation of macromolecules at low temperatures and cosmic radiation. Furthermore, this reaction process may be viewed as a basis of the prebiological chemical evolution of matter: such conversions give rise to amino acids and other large molecules in the cold universe. Incidentally, under laboratory conditions gamma radiation models cosmic radiation found to exert a straight effect on macromolecules formative conditions so different in outer space.

And now let's turn to a field other than that...

FROM COSMIC CHEMISTRY ONTO SOLID STATE PHYSICS

This science has important problem areas related to metastable, i.e. thermodynamically nonequilibrium states, and the dynamics of their metamorphosis into steady, stable phases. Here we might as well mention examples from physics of metals (for instance, mar-tensite changes induced in the mode of "explosive" recrystallization in the production technologies of su-perductile alloys and pieces of hardware restoring their original state when heated after plastic deformation); or examples from semiconductor physics (stability of amorphous, vitreous states, stability of semiconductor films, "explosive crystallization" of supercooled liquid media).

We believe that our approach based on the phenomenology of combustion, explosion and detonation could be quite productive. We should mention here that the basic mathematical model of "gas-free" detonation arising with the disintegration of metastable phases was designed with the participation of our French colleague, Alain Poumir (L'Institut des Problemes Non-lineaires, Nice, France). Using this model, we proved that the mechanism of "gas-free" detonation could accelerate the disintegration wave of the metastable state to supersonic speed in response to local mechanical interference

(B. Barelko et al., Izvestia (Proceedings) of the Russian Academy of Sciences, Chemistry, No. 7, 2011).

Now how real is the "gas-free" detonation phenomenon? This puts us in mind of a phenomenon discovered by alchemists five hundred years ago which they dubbed "Batavian lacrimulae (teardrop) explosion", or "explosion of a droplet of Prince Rupert of Bayern": amorphous drops of soda-lime glass hardened in a water bath go off without any gassing (in consequence of frontal explosive crystallization) and turn to dust upon the destruction of their tailings. The supersonic velocities of the destructive detonation waves of such "lacrimulae" measure up to km/sec!

FROM THE EXPLOSION THEORY ON TO GEOTECTONICS

Research scientists of Kemerovo University explored in the early 1990s the detonation process of nitrogen-containing compounds of heavy metals (silver, lead azides, etc.), the classical explosives. A team of experimentalists took a single azide crystal and watched its destruction dynamics by transparency changes. Oscillograph records showed a fast turbidity of the crystal before the gas detonation stage of its chemical decomposition. Studying this effect, the researchers concluded the initial stage was a phasic breakup of the metastable state of the stock sample (crystal); such disintegration proceeded without gassing and at detonation velocities, with the crystal, dispersed, losing its transparence.

Physicists, expert in the area of explosions and explosives, have shown a positive response to our theory on the role of "gas-free" detonation in the dynamics of disintegration of initiating igniters; this finding has become a basis for a new approach to the sensitivity of explosives to friction and shock. Whereas it was believed earlier that a shock causes a local heat buildup initiating a gas-detonation breakup of the charge, the new approach pinpoints the true cause of an explosive process, namely the destruction of a sample giving rise to active reactive chips. Two approaches are thus married: that of the shock action responsible for cryo-chemical conversion, and the conventional detonation model.

Accordingly, an utterly new approach was proposed concerning the mechanisms and dynamics of geotec-tonic events and earthquakes proceeding from the theory of vigorous autowave processes and solid-state phasic transformations. This approach allows for explosive (sic, explosive!) modes of polymorphous conversions of metastable phases in the earth's crust. Such phasic transitions cause substantial changes of rock densities resulting in significant deformations of rock, i.e. tremors. The authors of this concept have described such phenomena mathematically on the basis of the "gas-free" detonation theory.

Now what concerns metastable phasic states in terrestrial rock: these do exist and are rather common. Here are some instances of polymorphic conversions in the earth crust and mantle. Polymorphous modifications of carbon (diamond, blacklead, lonsdaleite, chaoite) and of silica (quartz, cristobalite, tridymite, coesite, stischovite) are ocular examples of phasic transitions in the crust and upper mantle. This is particularly important since the mantle makes up around 83 percent of the globe in volume. Small wonder that the mantle is so inhomogeneous. It carries numerous sections (divides) in-depth both of global and local nature, on whose boundaries seismic waves exhibit the greatest velocity changes, which is consistent with high density gradients. Such are the principal seismic divides 410,520 and 670 km deep. Different petrological models have been suggested for such plutonic geophysical divides of the mantle. Most popular today is the model of a homogeneous "pyrolytic" mantle proposed by Dr. Alfred Ringwood of Australia. In this model the geophysical interfaces evolve as boundaries of phasic isochemical transitions in constituent minerals, chiefly

in olivine, a hard solution of (Mg, Fe)₂ SiO₄ that accounts for 57 percent of the mantle volume. Thus, the 410 km deep boundary is estimated to have a pressure of 14 to 15 gigapascals, where olivine turns into a high-density rhombic phase of the same composition, va-deleite. At the 520 km deep boundary vadeleite passes into a cubic conformation, ringwoodite, at pressure 17 to 18 gigapascals. Yet phasic transitions are also true of such mantle minerals as pyroxenes (Ca, Mg, Fe, Na) (Mg, Fe, Al) Si₂O₆, garnets (Ca, Mg, Fe, Na)₃ (Al, Cr, Si)₂ Si₃ O₁₂, among other minerals.

In the subduction zones where the continents and oceans join up (with the oceanic crust thrusting under the continental one), phasic transitions into SiO₂: quartz-coesite-stischovite are of immense role.

All these phasic transitions are realized via metastable states. And so in order to describe initiation and dynamics conditions relevant to such conversions and see their role in geotectonic events and earthquakes, it will be proper to consider in this context the above conclusions bearing on the nature of vigorous, explosive processes concomitant to the breakup of metastable phases.

According to the now standard assessment criteria, earth tremors may be natural or else of anthropogenic origin, that is caused by human activities. Rated in the natural category are landslides and volcanicity (as even weak underground shocks may get lava to rise during volcanic eruptions). The other, anthropogenic cause is due to nuclear blasts, and any other human-related activities, industrial ones in particular. It would be in place to note here that processes implicated in polymorphous transformations and explosive breakup of metastable rock in the earth crust thus far have not be regarded as a possible cause of earthquakes.

Laboratory models of earthquakes will be essential for a cogent, conclusive scenario of geotectonic disasters orchestrated by the explosive disintegration of metastable phases in the crust. The "Batavian lacrimulae" explosion is one such model. Actually the "lac-rimulae (teardrops)" and most of the crust and mantle rock are of the same silicate origin.

Reusable crystal hot-water bottles on sale in drugstores are a spinoff from phasic transformations in magmatic processes. An overcooled (metastable) salt melt within them may touch off an autowave mode of frontal crystallization through local "ignition" via mechanical (cavitational) action. Hot-water bottle users can watch a traveling wave of explosive crystallization by a changing transparence of the salt melt. Under definite conditions a wave like that may speed up and enter the "gas-free"-wave-of-detonation stage. This poses no danger since the crystallization process will be just faster, no more. Similar processes take place deep under in course of phasic conversions and changing densities of matter; here on the earth surface we can feel tremors now and then as Shockwaves reach the surface, an aftereffect of disintegrating metastable states in rock.

So far neither a well-formulated theory nor the available experimental material can give straight answers about earthquake prediction or prevention. Yet even at this stage our new approaches to the mechanisms implicated in the initiation and propagation of geotectonic phenomena are certainly significant for geophysics and seismology. Our theory may well complement conventional methods of earthquake prediction.

"STEAM EXPLOSION" MECHANISMS

From solid-state explosive phenomena in metastable phases we now turn to an explosive boiling of overheated liquid media. The recent blowup of the Chelyabinsk meteorite was a sad experience to all of us. We have dealt with this matter (Science in Russia, No. 1, 2014) supposing that the real cause resides among the gas detonation formative mechanisms in supersonic Shockwaves, namely in the "steam explosion" mechanisms. We find it makes sense to go into a kindred pro-

cess of volumetric boiling of a body or its part if heated to thousands of degrees centigrade.

The steam explosion mechanism is involved in such volcanic events as phreatic eruptions as magma and its flows contact water-containing fluidal media in the crust or ice caps on volcanic domes. The steam explosion concept may help explain such technogenic disasters as the Chernobyl nuclear blast of 1986 and that at the Sayan-Shushenskoye hydro-power station in 2009.

A steam explosion is among nonlinear processes combining chemical physics phenomena as well as those of combustion, explosion and detonation. We have already looked into interdisciplinary concepts related to cosmochemistry and geotectonics, and explosive modes of metastable solid state disintegration (dealt with in solid-state physics, metal science and "explosive crystallization") of a supercool melt in magmatic processes.

And now yet another, literally global application domain of chemical physics-deep under again!..

EARTH CRUST, A CATALYTIC REACTOR

Catalysis is part and parcel of chemical transformation of plutonic fluids. Geochemistry of fluids holds an important place in the earth sciences. Their role in the chemical and physical evolution of crust and mantle rock is well known. The fluidal phase speeds up greatly chemical reactions in minerals, it stimulates the growth and dissolution of mineral granules and activates melting and polymorphous conversion processes, and thus is conducive to sundry deformation and recrystalliza-tion phenomena in mineral structures. Yet the reverse process-the catalytic action of rock and minerals on chemical reactions within fluids is still a far-out area in the geochemistry of fluidal-mineral interactions.

Catalytic mechanisms stimulating chemical transformations of fluids during their filtration through crust rock strata are playing an important part in plutonic chemistry. First, rock (SiO₂ and Al₂O₃) modified by catalytically active metal components is analogous to regular catalytic systems used in industrial technologies. Second, temperature and pressure conditions of crust and mantle fluids are a good model for industrial uses of catalytic processes. The fluid components (H₂O, CO₂, CO, CH₄, H₂, N₂, SO₂, NH₃) ought to be regarded as a stock material for the catalytic generation of a wide range of products obtained through chemical conversions.

Natural catalytic conversions in the earth crust and those created industrially have much in common. For instance, the abionic processes implicated in the formation of hydrocarbons, namely their synthesis and the synthesis of their oxygen-containing derivatives through H₂O, CO and CO₂ reactions via the Fischer-Tropsch process used in the production of synthetic fuels. The same holds for catalytic processes like catalytic pyrolysis of heavy hydrocarbons (petroleum cuts), catalytic cracking, platforming, reforming and catalyt-ical synthesis of ammonia from H₂ and N₂ known in applied catalysis as Gaber's synthesis.

Experimental studies of the catalytic activity of ser-pentinite, a widespread crust rock, was the first step in our research. Then we studied the steam conversion of methane, the "gas synthesis" reaction (Barelko et al., Doklady of the Russian Academy of Sciences. Vol. 453, No. 4, 2013). CH₄+H₂O fluids are among the commonest liquids in the lithosphere. In its composition (MgO-SiO₂ as a base component doped by the catalytically active Fe, Ni, Cr) and structure (fine-fiber, porous) serpentinite is a close analog of conventional artificial materials used in industrial catalysis.

In our experiments we took a large sample of serpentinite from the Bogorodskoye deposit in the Southern Urals. We crushed its block but did not treat it otherwise. In the course of our experiment we found that the conversion of methane into hydrogen rose with higher temperature and at 825 °C it was equal to 14 percent; the conversion of methane to carbon monoxide (CO) and carbon dioxide (CO₂) at the same temperature amounted to 3 percent for each component; surprisingly, we detected CH₃OH and C₂H₅OH in the conversion products. In the technological process of methane conversion with the use of standard catalysis no oxygen-containing hydrocarbons were formed. There could be more complex hydrocarbons present in the products. But exact quantitative measurements and a balance of the conversion products call for new measuring procedures beyond our scope at this stage of research.

Be that as it may, serpentinite was found to exhibit quite satisfactory catalytic qualities in the process of steam conversion of methane. Although the contact of the fluid-steam flow with serpentinite was very short,

the degree of CH₄ conversion to hydrogen- and oxygen-containing compounds was high enough.

Such conversions of fluids may occur under natural conditions in the course of the fluid phase evolution with diamond pipes (kimberlites) formed thereby. Serpentine (serpentinite rock) is the main component of the kimberlite (blue earth) mass, with the temperatures of kimberlite magma eruption near the terrestrial surface as high as 800-900 °C. Intensive H₂ and CH₄ flows (as much as 105 m³ daily) as well as the presence of hydrocarbons are found in some of Yakutia's diamond pipes, at "Udachnaya" in particular.

Our experiments, as we see it, should stimulate regular research into catalytic characteristics in a wide range of crust rock types relative to diverse chemical conversions of fluids and their components and thus promote the advancement of "catalytic geochemistry", a novel area much in need of further research. For this purpose we should make use of the available theoretical and experimental evidence amassed by basic catalysis since physical chemistry came into being. For instance, studying the mechanisms implicated in what we call "source clustering" observed in mineral deposits we can resort to the "domain instability" idea of catalytic processes, a theory applied to the space time structuring of industrial catalytic reactors. Obviously, the classical geochemical approach of thermodynamic equilibrium should be revised, for chemical processes in the earth crust occur in a nonequilibrium mode.

The material of this article is certainly interdisciplinary. Research carried out at the junction of such different disciplines as chemical physics and geology have broken ground for new, unorthodox approaches towards the nature and mechanisms of cosmochemical and geotectonic processes, polymorphous conversions, and geochemistry of fluids in the crust; this is likewise true of explosive events in meteoritics (science of meteors and meteorites) and volcanology. Multifarious results of this research have become possible thanks to the body of physics of combustion, explosion, detonation and catalytical chemistry applied to geology and the earth sciences at large. The authors are well aware that some of their concepts and statements are open to discussion. That is why chemists, physicists and geologists should join forces in creative cooperation.

The authors wish to thank Dr. Mikhail Drozdov (RAS Institute of New Energy Problems at Chernogolovka, Moscow Region) for assistance in preparing the present article.