*5.2. Reactions of Ethanol Transformation into Syngas*

For steady-state of catalysts based on oxides with high oxygen mobility and reactivity in ethanol reforming reactions pulse studies revealed that ethanol conversion and products selectivities in pulses containing reaction mixture or only ethanol are practically the same [103,104,121]. This proves realization of step-wise redox mechanism of these reactions. FTIRS studies combined with SSITKA identified main intermediates of ethanol conversion into syngas, such as ethoxy species and acetaldehyde and estimated rate constants of their transformation, while acetates were shown to be spectators. The rate-limiting step is the cleavage of C–C bond in ethoxy species fixed on rare-earth or transition metal cations due to incorporation of terminal oxygen species located on neighboring Me sites as supported by DFT calculations for the (001) face of MnCr2O4 spinel doped by Ru [104]. Kinetic scheme of the reaction of ethanol partial oxidation used in analysis of SSITKA data is as follows:


where [ZO] and [Z] correspond to oxidized and reduced site of catalysts surface, respectively, without any differentiation of the nature of active sites related to different transition metal cations (Mn, Cr) and metal atoms (Ni, Ru) at this level of analysis similar to that for catalysts based on bulk MnCr2O4 [104].

#### **6. Development of Structured Catalysts**

For design of such catalysts substrates made of metals, alloys and cermets were used [72,73,77,82,84,86,87,90,91,98,117]. Detonation spraying [138] was applied to cover thin foils, microchannel plates or gauzes by protective nonporous layer of alumina or zirconia. Known procedures of stacking and winding were applied to produce monolithic substrates from these constituents. Foam substrates comprised of NiAl alloys, ceramics, etc. were prepared via polyurethane foam duplication followed by required mechanical and thermal treatment [73,86]. These substrates were covered by catalytic layers using suspensions of active components in isopropanol. Structured catalysts were tested in pilot reactors in processes of natural gas, liquid fuels and biofuels reforming/autothermal reforming under realistic conditions. For best catalysts on substrates with a high thermal conductivity a high syngas yield was provided even at short contact times due to more uniform temperature profile along the catalysts' length [73,82,84]. In the autothermal reforming mode it helps to transfer heat generated in the inlet part of monolith due to exothermal combustion of a part of fuel into the following parts where O2 in the gas phase is absent, so endothermic reactions of steam and dry reforming occur. For strongly endothermic reaction of MDR this positive effect of a high thermal conductivity of substrate is demonstrated by the identical temperature dependence of methane conversion in the layer of catalyst fraction for diluted feed and for the stack of microchannel plates with the same active component even in more concentrated feed (Figure 5b). In dry reforming of real natural gas (NG) containing up to 6% of C2–C4 alkanes also high and stable performance of structured catalyst was demonstrated for concentrated feed (Figure 6).

**Figure 6.** Temperature dependence of CH4 conversion and CO/H2 concentrations in NG dry reforming on the package of 5 Fechraloy microchannel plates with 1% NiO+1% Ru /SmPrCeZrO active component. Feed 50% CO2 +40% NG +N2, contact time 0.1 s.

This advantage of structured catalysts on heat-conducting metal/cermet substrates allowed to carry out efficient transformation of a lot of fuels including gasoline and diesel into syngas via partial oxidation and steam/autothermal reforming [82–87,117,139–142]. Reforming of biofuels such as acetone, ethyl acetate and glycerol is known to be accompanied by gas-phase reactions yielding ethylene, which is easily transformed into coke on catalysts. However, structured catalysts with nanocomposite active components demonstrated high and stable performance in the autothermal reforming of such biofuels as ethanol, acetone, ethyl acetate (Figure 7) and glycerol (Figure 8) with fuels content up to 25% and O2 content in the mixture with steam up to 20% [73,82,86,87,117]. Note that Figures 7 and 8 show a high yield of syngas achieved at very short contact times, which is provided by a high efficiency of active components supported on mesoporous MgAl2O4 or Mg-doped –γ-Al2O3. Even anisole (content in the feed up to 10%), sunflower oil (content up to 0.7%) [82,117] and turpentine oil (commercial bio-oil mainly containing C10H16 and C10H18O components, such as α-pinen, α-terpineol, etc., content in the feed up to 6% [90]) (Figure 9) were successfully converted into syngas in the autothermal reforming on developed structured catalysts. Another important problem in design of efficient syngas generators, especially for the small-scale application, is related to the heat management, since endothermic reactions of fuels steam and dry reforming require extensive preheat of inlet streams. This can be dealt with by using the exit stream for preheating the inlet feed in specially designed heat exchangers conjugated with catalytic reactors. Another aspect of energy efficiency problem solution is bound with possibility to conjugate exothermal partial oxidation of methane into syngas with endothermal processes of biofuels steam/dry reforming. These problems were solved in design of a radial-type reactor equipped with the heat exchanger described in detail in our previous work [82,117]. Here a cylindrical stack of catalytic microchannel washers was wrapped by gauze sheets with supported active components as well as with microspherical catalysts loaded between gauzes. The feed enters the central part of the stack of washers and flows in the radial direction. The reformed gas is collected into a plenum around the catalyst arrangement and exited from a single pipe, while inlet feed is heated by passing through the heat exchanger situated around the reactor as an outer shell. Such design made it possible to efficiently carry out partial oxidation of a mixture of natural gas and liquid fuel (ethanol, ethyl acetate, turpentine oil) to syngas at high flow rates (up to 40,000 h<sup>−</sup>1) with preheating the mixture up to 50–100 ◦C at the reactor inlet (Figures 10–12).

**Figure 7.** Temperature dependence of product concentrations in ATR of ethylacetate over the monolithic honeycomb catalyst comprised of FeCrAl gauzes loaded with NiRu/CeZrPrO on mesoporous MgAl2O4. Feed 30% EtAc + 60% H2O + 8% O2, N2—balance, contact time 0.1 s.

**Figure 8.** Temperature dependence of products concentration in the process of glycerol autothermal reforming on microchannel CrAlO substrate with supported (Ni + Ru)/MnCr2O4/10% MgO–γ-Al2O3 active component. Feed 15% O2 + 22% glycerol + 22% H2O+N2, contact time 0.06 s.

**Figure 9.** Dependence of products concentrations on oxygen content in the feed in the process of turpentine oil autothermal reforming on 3.4% LaNiPt/La–Ce–Zr–O/Fechraloy thin foil honeycomb at 800 ◦C. Feed 7% turpentine + 40% H2O+O2+N2, contact time 0.5 s.

**Figure 10.** Dependence of products concentration on ethanol content in feed 24% NG + 16–25% O2 + N2 + EtOH in the process of autothermal reforming in the radial reactor with the heat exchanger. Flow rate 2.5 m3/h, LaNi(Pt)O3/La0.1Ce0.45Zr0.45O2 active component.

**Figure 11.** Dependence of products concentration on ethyl acetate (**a**) or O2 (**b**) content in the autothermal reforming of the mixture of natural gas and ethyl acetate in radial reactor with the heat exchanger and LaNi(Pt)O3/La0.1Ce0.45Zr0.45O2 active component. Feed 20% NG + 18% O2 +N2 +EtAc (**a**) or 10% NG + O2 + N2 + 15% EtAc (**b**), flow rate 2.5 m3/h.

**Figure 12.** Dependence of products concentration on turpentine content in feed 20% NG +21% O2 +N2 +turpentine oil in the process of autothermal reforming in the radial reactor with the heat exchanger. Flow rate 2.5 m3/h, LaNi(Pt)O3/La0.1Ce0.45Zr0.45O2 active component.

#### **7. Conclusions**

Pechini method and synthesis in supercritical alcohols allowed to provide uniformity of the spatial distribution of elements in nanodomains of fluorite, perovskite and spinel oxides required for controlling their oxygen mobility and reactivity. Promoting these oxides and their nanocomposites of optimized composition by platinum group metals and nickel allowed to create effective and coking-resistant catalysts for biogas and biofuels transformation into syngas. Due to developed metal-support interface and strong metal– support interactions bifunctional scheme of reaction mechanism is realized with activation of oxidants (O2, CO2, H2O) on the oxide support sites, while fuel molecules are activated on metal centers. Fast diffusion of surface oxygen species to the metal–oxide interface provides conjugation of these steps resulting in efficient syngas generation and coking suppression. Preparation of mesoporous supports comprised of MgAl2O4 or Mg-doped alumina with enhanced basicity allowed to design core-shell systems with a high working area of supported catalytic nanocomposite layers, increase their activity, thermal and coking stability and decrease content of rare-earth elements. Structured heat-conducting substrates loaded with optimized active components provide efficient heat- and masstransfer in reactors for biofuels transformation into syngas. Pilot reactors with internal heat exchangers permit efficient operation in the autothermal mode on the mixture of natural gas, air and real biofuels such as ethanol, glycerol and turpentine oil with the inlet temperature 50–100◦C and GHSV up to 40,000 h<sup>−</sup>1.

**Author Contributions:** Writing—original draft preparation, V.S., N.E., M.S. and N.M.; writing review and editing, V.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** Design and studies of structured catalysts were carried out in the framework of the budget project of the Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences (project AAAA-A21-121011390007-7). The authors acknowledge support from the Russian Science Foundation according to project no. 18-73-10167 for the synthesis of catalysts in supercritical environment fluids and their characterization.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** Authors are glad to acknowledge efficient international collaboration in frames of INTAS and FP7 Projects OCMOL and BIOGO in this area of research.

**Conflicts of Interest:** The authors declare no conflict of interest.
