Low-Temperature Steam Reforming of Natural Gas after LPG-Enrichment with MFI Membranes

Low-temperature hydrogen production from natural gas via steam reforming requires novel processing concepts as well as stable catalysts. A process using zeolite membranes of the type MFI (Mobile FIve) was used to enrich natural gas with liquefied petroleum gas (LPG) alkanes (in particular, propane and n-butane), in order to improve the hydrogen production from this mixture at a reduced temperature. For this purpose, a catalyst precursor based on Rh single-sites (1 mol% Rh) on alumina was transformed in situ to a Rh1/Al2O3 catalyst possessing better performance capabilities compared with commercial catalysts. A wet raw natural gas (57.6 vol% CH4) was fully reformed at 650 ◦C, with 1 bar absolute pressure over the Rh1/Al2O3 at a steam to carbon ratio S/C = 4, yielding 74.7% H2. However, at 350 ◦C only 21 vol% H2 was obtained under these conditions. The second mixture, enriched with LPG, was obtained from the raw gas after the membrane process and contained only 25.2 vol% CH4. From this second mixture, 47 vol% H2 was generated at 350 ◦C after steam reforming over the Rh1/Al2O3 catalyst at S/C = 4. At S/C = 1 conversion was suppressed for both gas mixtures. Single alkane reforming of C2–C4 showed different sensitivity for side reactions, e.g., methanation between 350 and 650 ◦C. These results contribute to ongoing research in the field of low-temperature hydrogen release from natural gas alkanes for fuel cell applications as well as for pre-reforming processes.


Introduction
Exploitation of the world's reserves of natural gas has increased tremendously, while the spectrum of utilization has broadened continuously.Methane, the main component in natural gas, is typically combusted for energy generation, since it is difficult to activate this smallest alkane and form more valuable products [1].In terms of direct methane activation, several directions can be pursued which are currently only of academic interest [2][3][4][5][6].Hence, for chemical processing, methane is mainly converted into H 2 /CO mixtures (syngas) which provide key components for the production of chemical products [7][8][9][10], e.g., for the synthesis of ammonia, methanol, and liquid hydrocarbons [11].The steam reforming process of methane is typically performed at high temperatures between 750 and 900 • C. At a pressure of p = 24 bar and a steam to carbon ratio of S/C = 3, nearly 75 vol% H 2 is produced (excluding water) [12].
Wet natural gas, rich in C 2 -C 5 alkanes, or C 2 -C 5 alkane fractions from natural gas conditioning, are valuable sources for syngas production.Depending on the alkane composition and the processing technology (i.e., steam reforming, partial oxidation, autothermal reforming, dry reforming), syngas with different H 2 /CO ratios can be obtained by using suitable catalysts [13].Generally, the highest possible amount of H 2 can be achieved by using steam as a reactant for alkanes, due to the inherent hydrogen content of the water.Industrially, pre-reforming of the higher alkanes in wet natural gas can help to reduce the size of a downstream tubular reformer [9].Moreover, liquid fuels such as butane, alcohols, or diesel are increasingly being considered as easily storable sources of hydrogen which can be applied in fuel cell systems [14][15][16][17][18].Among those, reforming of bioethanol could contribute to sustainable hydrogen production.However, a high temperature of about 700 • C is required to obtain hydrogen yields above 70% [19].
Steam reforming of alkanes requires a significant energy input [20,21].For methane conversion, the required ∆H • is +206 kJ/mol [22], hence, the reaction has to be performed at high temperatures (often higher than 900 • C) to ensure full conversion [23].In order to decrease the required reaction temperature, low pressure and a relatively high S/C ratio are necessary [14].Several attempts have been made in order to lower the required energy input for alkane steam reforming [22].Interestingly, the reforming of higher alkanes can be performed at a lower temperature compared to methane.For instance, complete conversion of n-butane can be achieved at 405 • C over Pt-Ni/δ-Al 2 O 3 [15].Schädel et al. [20] systematically tested an industrial Rh catalyst as a washcoat on cordierite honeycomb monoliths for the steam reforming of methane, ethane, propane, butane, and natural gas.The authors showed that methane requires a much higher temperature for conversion compared to the other alkanes.Consequently, steam reforming of a natural gas with a high liquefied petroleum gas (LPG) fraction would offer lower reaction temperatures.
The enrichment of natural gas with LPG alkanes via MFI (Mobile FIve)-membranes has been reported in the past.Selective gas transport across zeolite membranes can be achieved by either selective adsorption or size exclusion [24].Due to their selective adsorption properties, MFI zeolite membranes are extensively studied for separation of different alkanes [25][26][27][28][29]. From a mechanistic point of view, the higher molecular weight alkanes adsorb preferentially at the membrane surface and thus block the pores for further passage of lighter components.However, a fundamental understanding of transport across the membrane and the accurate analysis of optimal operating parameters are essential for achieving high membrane performances.The separation of natural gas alkanes based on the adsorption behavior of the different alkanes was first demonstrated by Arruebo et al. [30] and detailed parameter studies were performed by our group [31][32][33][34].
Unfortunately, stable catalysts working at lower temperatures in the steam reforming of LPG enriched natural gas are rather scarce.Carbon deposition, the most serious problem affecting the stability of catalysts at low reaction temperatures (400-550 • C), was investigated by Angeli et al. [35] who found significant carbon residues on Ni catalysts, while in the presence of Rh, coke was oxidized.Our initial idea was to reduce the size of the active sites in order reduce coking ability.In terms of methane activation, Bao and co-workers developed an oxygen free route towards C 2 and aromatic products over Fe single-sites [36].Moreover, direct transformation of methane to methanol can be performed over Pd single-sites at a low temperature [37], or over single-site copper species in zeolites [38][39][40][41].Molecularly attached VO x single-sites on silica have been used as catalysts for the selective oxidation of methane towards formaldehyde [42][43][44].However, in terms of steam reforming of methane, Bokhoven and co-workers recently found that the catalytic reaction over single Rh sites on stabilizing supports requires additional nanoparticles to oxidize formed carbon species [45].In the absence of nanoparticles, reaction rates are lowered due to the formation of carbon species which are strongly bound to the surface.
In this work, we started with single Rh site rhodium catalyst precursors in the steam reforming of alkanes and LPG rich natural gas.The single-sites were transformed during the reaction and the in situ formed catalysts containing Rh nanoparticles surprisingly delivered a much better performance compared to commercial catalysts at low temperature.Therefore, a membrane-based pre-enrichment of LPG can be introduced as a feasible concept to obtain an alkane mixture from which a high amount of hydrogen can be produced at temperatures as low as 350 • C.

Catalyst Preparation
Rh1/Al 2 O 3 with 1.0 mol% Rh was prepared by impregnation of Al 2 O 3 .First, 15 g Disperal P2 (Sasol, Brunsbüttel, Germany) were calcined at 800 • C (heating rate 5 • C/min) for 90 min in air.Afterwards, the as-obtained Al 2 O 3 was impregnated with a freshly prepared Rh(NO 3 ) 3 solution.Accordingly, 1.9 g RhCl 3 •4.5H 2 O (Alfa Aesar, Karlsruhe, Germany) were dissolved in 100 mL of water.The solution was heated and 20 mL of aqueous 20 M KOH (Fisher Chemical, Loughborough, UK) were added dropwise under reflux.The precipitated Rh(OH) 3 was centrifuged and washed four times with hot water.The resulting Rh(OH) 3 precipitate was then dissolved in 5 mL of concentrated HNO 3 (Fisher Chemical, Loughborough, UK) and subsequently diluted in 20 mL of deionized water to obtain a Rh(NO 3 ) 3 concentration of about 0.35 mol/L.As a last step, 2.85 mL of this solution was further diluted in 47.2 mL of deionized water and used for the wet impregnation of 10 g of Al 2 O 3 .This slurry was stirred for 1 h, water was subsequently removed using a rotary evaporator, and the resulting catalyst was dried over night at 110 • C. Finally, calcination of the material was performed at 700 • C in air for 60 min (heating rate 5 • C/min).The catalysts were abbreviated as follows: Rh1/Al 2 O 3 for 1.0 mol% Rh on alumina.Several other loadings of Rh on alumina were abbreviated as Rh X /Al 2 O 3 with (x = 0.01, 0,1, and 0.5 in mol% Rh, determined via inductively coupled plasma -optical emission spectroscopy ICP-OES).For comparison, two commercial catalysts consisting of 0.5 % Rh (206172) on alumina and 5% Rh (C-301099-5) on alumina were purchased from Sigma-Aldrich (Steinheim, Germany) and Alfa Aesar (Karlsruhe, Germany), respectively, and tested under comparable conditions.

Catalyst Characterization
The BET (Brunauer-Emmett-Teller) specific surface area of the porous silica and the catalysts were measured by N 2 adsorption using a NOVA 4200e instrument from Quantachrome (Odelzhausen, Germany).As a pre-treatment, samples were outgassed and dried for 2 h at 200 • C at reduced pressure.
A Varian 715-ES ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometer) (Varian Palo Alto, CA, USA) was used for the determination of the elemental composition of the catalysts.Before analysis, the catalysts were completely dissolved in a solution containing 8 mL of aqua regia and 2 mL of hydrofluoric acid.
The H 2 -TPR (temperature programmed reduction with H 2 ) experiments were done as described in the following.160 mg of the respective sample was loaded in a u-shaped quartz reactor and heated from RT (room temperature) to 500 • C at 20 K/min in air, then cooled to RT and flushed with Ar flow (50 mL/min) for 30 min.H 2 -TPR of Rh1/Al 2 O 3 samples were carried out from 0 to 800 • C in a flow of 5% H 2 /Ar (20 mL/min) with a heating rate of 5 K/min.The temperature was held at 800 • C for 1 h.The hydrogen consumption peaks were recorded simultaneously via a thermal conductivity detector (TCD, ChemiSorb 2920-Instrument, Mircomeritics, Norcross, GA, USA).

Membrane Separation
Pressure-stable and defect-free MFI membranes at the inner side of porous alumina tubes (l = 125 mm, d outer = 10 mm, d inner = 8 mm) were prepared via a secondary growth procedure previously reported by our group in Reference [31].The tubes were sealed with glass at both ends and embedded in stainless steel permeation cells with Viton O-rings.LPG enrichment from natural gas using the MFI membranes was performed at p permeate = 0.17 bar, p feed = 7 bar, and T = 75 • C. Compositions of permeate and retentate were analyzed continuously using an online coupled capillary GC HP 6890 from Hewlett Packard (Santa Clara, CA, USA).

Catalytic Steam Reforming Tests
Steam reforming of alkanes, alkane mixtures, and simulated natural gas (before and after membrane enrichment) were performed in a vertical fixed bed plug flow quartz reactor (l = 260 mm, d inner of 8 mm) at 1 bar (detailed composition of the reaction mixtures are given in Table S1 and S2).
If not otherwise stated, 150 mg of catalyst was fixed with quartz wool at the centre of the reactor tube and heated in a furnace at temperatures ranging from 200 to 850 • C. If not otherwise stated, the holding time spent for each temperature set point was about 35 min.Temperature was controlled by two thermocouples (ThermoExpert, Stapelfeld, Germany) at the outer reactor wall and in the middle of the catalyst bed.The gas flow was controlled by mass flow controllers (MKS, Andover, MA, USA).The total gas flow under ambient conditions of 100 cm 3 min −1 consisted of 25 cm 3 min −1 of reactant gas (steam and alkanes) diluted in 75 cm 3 min −1 of nitrogen to reach near isothermal conditions.If not otherwise stated, GHSV (gas hourly space velocity) was about 8000 h −1 .The required water was dosed to a vaporizer using a syringe pump at the front inlet of the reactor.All the transfer lines were heated at 130 • C. At the reactor outlet, water was condensed from the product gas stream in a cold trap at 0 • C. Gas phase products were analyzed by an online-GC 7890A (Agilent, Santa Clara, CA, USA).H 2 , N 2 , and CO were separated with a molsieve column 5Å from Agilent (CP-1306) (Santa Clara, CA, USA) and detected with a TCD.The alkanes and CO 2 were separated with a GS-Q column from Agilent (113-3432) (Santa Clara, CA, USA) and detected with a flame ionization detector (FID, Agilent, Santa Clara, CA, USA) with a methanation unit.For quantification of H 2 , N 2 , CO, CO 2 , and C 1 -C 5 alkanes, an external calibration was done with several test gas mixtures (supplied from Linde Group, Pullach, Germany) and their dilutions in N 2 , as well as by measuring the pure gases.For discussion, H 2 O and N 2 were excluded in the given volumetric gas compositions.

Rh1/Al 2 O 3 Catalyst
Rh single-atoms were deposited on a high surface alumina support of 181 m 2 /g.The BET surface areas and pore volumes of the obtained Rh1/Al 2 O 3 catalyst, given in Table 1, are only less affected after calcination, indicating the integrity of the support material after thermal treatment.The molar fraction of Rh in Rh1/Al 2 O 3 was 1.0 mol%, which is in the range of the targeted value.X-ray diffraction patterns of the catalyst (Figure S1) shows only typical reflections of different alumina phases, namely, γ-, δ-, and θ-Al 2 O 3 (JCPDS 29-0063 [46], PDF 46-1215 [47], and ICSD#082504 [48]).
Table 1.BET (Brunauer-Emmett-Teller) specific surface area (S BET ), total pore volume (V t ), and average pore diameter (D), as well as ICP-OES (inductively coupled plasma -optical emission spectroscopy) data of support and catalysts.the spent Rh1/Al2O3 catalyst, indicating particle formation from Rh single-sites during the steam reforming process.

Samples
H2-TPR was performed with fresh and spent catalysts.The spent catalysts were isolated again after the light-off tests with permeate gas at 850 °C (S/C = 4).As can be concluded from Figure 1c, the absence of typical reduction peaks for particulate RhOx species in the range from 80 to 250 °C [49,50] indicate the nature of the sites in fresh Rh1/Al2O3.Most Rh species in the fresh catalysts are present as single atoms, because the adsorption and dissociation of H2 to H atoms cannot occur over single atoms [51].H2-TPR over spent Rh1/Al2O3 catalysts is in accordance with the TEM results.It is obvious that the single-sites in the fresh catalysts were transformed to nanoparticle species during an initial activation phase.The formed Rh particles in spent Rh1/Al2O3 show an extraordinary reducibility below 50 °C and were the active catalyst species.

Steam Reforming of pure C1-4 Alkanes and Mixtures of C2-4 with Methane over Rh1/Al2O3
Steam reforming of the single alkanes methane, ethane, propane, n-butane, and i-butane were performed over Rh1/Al2O3 using S/C ratio and temperature as a measure for catalyst activity (Figure 2) and selectivity (Figure 3).At a GHSVs of 8000 h −1 , 1 mol% Rh appeared as sufficient loading to reach nearly equilibrium methane conversion [22] (Figure S2).At lower Rh loading, GHSVs have to be reduced to fulfil this task (e.g., Figure S3 and S4 for Rh0.5/Al2O3).In the temperature range 250-400 °C, increasing S/C ratios lead to higher H2 fractions in the product gas and higher alkanes yield higher H2 fractions than methane reforming.Above 500 °C, increasing methane conversion leads to higher H2 fractions due to higher hydrogen/carbon-ratios in such mixtures.Between 550 and 850 °C, the respective H2 fractions remain nearly constant between 70 vol% and 80 vol% at S/C = 4.  Steam reforming of the single alkanes methane, ethane, propane, n-butane, and i-butane were performed over Rh1/Al 2 O 3 using S/C ratio and temperature as a measure for catalyst activity (Figure 2) and selectivity (Figure 3).At a GHSVs of 8000 h −1 , 1 mol% Rh appeared as sufficient loading to reach nearly equilibrium methane conversion [22] (Figure S2).At lower Rh loading, GHSVs have to be reduced to fulfil this task (e.g., Figures S3 and S4 for Rh 0.5 /Al 2 O 3 ).In the temperature range 250-400 • C, increasing S/C ratios lead to higher H 2 fractions in the product gas and higher alkanes yield higher H 2 fractions than methane reforming.Above 500 • C, increasing methane conversion leads to higher H 2 fractions due to higher hydrogen/carbon-ratios in such mixtures.Between 550 and 850 • C, the respective H 2 fractions remain nearly constant between 70 vol% and 80 vol% at S/C = 4.The obtained H2/CO ratios (Figure 2) also depend strongly on the S/C ratio and temperature.The H2/CO ratios from methane steam reforming decrease with increasing temperature and decreasing S/C ratios.A more difficile behavior was found for C2-4 alkanes.For many of those, a maximum H2/CO ratio was observed around 500-600 °C, whereby the values increase with increasing S/C ratios.This region relates to the aforementioned side reactions, namely the water-gas shift reaction as well as the formation of CH4.Concerning the latter, the amount of formed methane from C2-C4 alkanes dependent on the temperature and S/C ratio is depicted in Figure 3.At S/C = 1, the potential to form methane goes along the following order of the alkanes: Ethane~propane > i-butane > n-butane, reaching values up to ~30 vol% methane, and is most pronounced between temperatures of 500 and 600 °C.For higher S/C ratios another trend is observed according to ethane~propane > i-butane~n-butane.The CH4 formation can be suppressed by increasing the amount of steam.Different mechanisms can be responsible for CH4 formation.Schädel et al. [20] observed CH4 formation during the steam reforming of different higher alkanes over Rh catalysts and concluded that CH4 is formed faster from higher hydrocarbons than decomposed, due to its lower reactivity at low temperatures.In addition, CO (and CO2) hydrogenation by formed H2 or the so-called methanation, also contributes 2. H 2 volumetric gas content (water and inert gas excluded) and H 2 /CO ratio in the steam reforming of single alkanes over Rh1/Al 2 O 3 at GHSV (gas hourly space velocity) of 8000 h −1 and different values of temperature and S/C ratios of (a) 1, (b) 2.5, and (c) 4.
The obtained H 2 /CO ratios (Figure 2) also depend strongly on the S/C ratio and temperature.The H 2 /CO ratios from methane steam reforming decrease with increasing temperature and decreasing S/C ratios.A more difficile behavior was found for C 2-4 alkanes.For many of those, a maximum H 2 /CO ratio was observed around 500-600 • C, whereby the values increase with increasing S/C ratios.This region relates to the aforementioned side reactions, namely the water-gas shift reaction as well as the formation of CH 4 .Concerning the latter, the amount of formed methane from C 2 -C 4 alkanes dependent on the temperature and S/C ratio is depicted in Figure 3.At S/C = 1, the potential to form methane goes along the following order of the alkanes: Ethane~propane > i-butane > n-butane, reaching values up to ~30 vol% methane, and is most pronounced between temperatures of 500 and 600 • C. For higher S/C ratios another trend is observed according to ethane~propane > i-butane~n-butane.The CH 4 formation can be suppressed by increasing the amount of steam.Different mechanisms can be responsible for CH 4 formation.Schädel et al. [20] observed CH 4 formation during the steam reforming of different higher alkanes over Rh catalysts and concluded that CH 4 is formed faster from higher hydrocarbons than decomposed, due to its lower reactivity at low temperatures.In addition, CO (and CO 2 ) hydrogenation by formed H 2 or the so-called methanation, also contributes to CH 4 production.Although the hydrogenolysis reaction is discussed as mostly being responsible for CH 4 production [52], methanation can also contribute to a significant loss of CO and consequently very high H 2 /CO ratios.This feature could be suppressed to a certain extend by LPG enrichment as discussed later.As a direct consequence of CH 4 formation from the higher alkanes, two temperature regions can be defined for our approach: (i) before CH 4 formation in a low-temperature region, e.g., up to 350 • C and (ii) a high-temperature region when the formed CH 4 is completely reformed starting at 650 • C.
Processes 2018, 6, x PEER REVIEW 7 of 12 for CH4 production [52], methanation can also contribute to a significant loss of CO and consequently very high H2/CO ratios.This feature could be suppressed to a certain extend by LPG enrichment as discussed later.As a direct consequence of CH4 formation from the higher alkanes, two temperature regions can be defined for our approach: (i) before CH4 formation in a low-temperature region, e.g., up to 350 °C and (ii) a high-temperature region when the formed CH4 is completely reformed starting at 650 °C.Finally, steam reforming tests of binary alkane mixtures with methane, namely, ethane/methane, propane/methane, and n-butane/methane, were performed at 350 °C over Rh1/Al2O3 to demonstrate the impact of higher alkanes on the achievable H2 fractions and H2/CO ratios (Figure S4 and S5).At this temperature, no methane is being formed from the higher alkanes.In general, an increasing content of higher alkanes causes a rise in the respective H2 fractions.In the case of ethane/methane mixtures, for instance, the H2 fractions increase from 5.0 vol% to 8.7 vol% (S/C = 1) when the volume fraction of ethane is increased from 0 to 100%.At the S/C ratio of 2.5, the H2 fraction can be increased from 8.4 vol% to 15.0 vol% and even from 12.6 vol% to 22.6 vol% when the S/C ratio is further increased to 4. However, in the same way, the H2/CO ratios decrease from 3.3 to 2.4 (S/C = 1), 4.6 to 3.9 (S/C = 2.5), and 5.9 to 4.8 (S/C = 4).Lower H2/CO ratios at low methane content are attributed to (i) the reduced hydrogen/carbon-ratios and (ii) high CO selectivity of Rh1/Al2O3.The latter has already been demonstrated for the n-butane steam reforming in low-temperature range over Rh catalysts [53].

Enrichment of LPG from Natural Gas Using MFI-Membranes
Compared to presently used methods of recovering heavier hydrocarbons from methane [54], a membrane process offers a low energy consuming alternative.In this work, a wet natural gas (raw gas) was enriched with propane and n-butane (permeate gas; enriched with LPG) using a pressure stable MFI zeolite membrane.The compositions of the raw gas (sour gas and inert gas depleted, water-free) and permeate gas are depicted in Scheme 1.The subsequent steam reforming was Finally, steam reforming tests of binary alkane mixtures with methane, namely, ethane/methane, propane/methane, and n-butane/methane, were performed at 350 • C over Rh1/Al 2 O 3 to demonstrate the impact of higher alkanes on the achievable H 2 fractions and H 2 /CO ratios (Figures S4 and S5).At this temperature, no methane is being formed from the higher alkanes.In general, an increasing content of higher alkanes causes a rise in the respective H 2 fractions.In the case of ethane/methane mixtures, for instance, the H 2 fractions increase from 5.0 vol% to 8.7 vol% (S/C = 1) when the volume fraction of ethane is increased from 0 to 100%.At the S/C ratio of 2.5, the H 2 fraction can be increased from 8.4 vol% to 15.0 vol% and even from 12.6 vol% to 22.6 vol% when the S/C ratio is further increased to 4. However, in the same way, the H 2 /CO ratios decrease from 3.3 to 2.4 (S/C = 1), 4.6 to 3.9 (S/C = 2.5), and 5.9 to 4.8 (S/C = 4).Lower H 2 /CO ratios at low methane content are attributed to (i) the reduced hydrogen/carbon-ratios and (ii) high CO selectivity of Rh1/Al 2 O 3 .The latter has already been demonstrated for the n-butane steam reforming in low-temperature range over Rh catalysts [53].

Enrichment of LPG from Natural Gas Using MFI-Membranes
Compared to presently used methods of recovering heavier hydrocarbons from methane [54], a membrane process offers a low energy consuming alternative.In this work, a wet natural gas (raw gas) was enriched with propane and n-butane (permeate gas; enriched with LPG) using a pressure stable MFI zeolite membrane.The compositions of the raw gas (sour gas and inert gas depleted, water-free) and permeate gas are depicted in Scheme 1.The subsequent steam reforming was performed in a separate reactor, since both operations require different working temperatures [55] (Scheme 1).
In the membrane process, pores are blocked for methane due to the preferred adsorption of LPG alkanes in the MFI structure and permeation is induced by a present gradient of the chemical potential, preferably by differences between permeate and feed pressure.At p permeate = 0.17 bar and p feed = 7 bar (T = 75 • C), propane and n-butane enrichment of 37.6 vol% and 22.6 vol% in permeate from initial 18.9 vol% and 4.9 vol%, respectively, was obtained.i-butane hardly passes the MFI membrane as it is trapped in the zick-zack-channels of the zeolite [56], which is reflected in a lower amount of this component in the permeate.
Processes 2018, 6, x FOR PEER REVIEW 8 of 12 In the membrane process, pores are blocked for methane due to the preferred adsorption of LPG alkanes in the MFI structure and permeation is induced by a present gradient of the chemical potential, preferably by differences between permeate and feed pressure.At ppermeate = 0.17 bar and pfeed = 7 bar (T = 75 °C), propane and n-butane enrichment of 37.6 vol% and 22.6 vol% in permeate from initial 18.9 vol% and 4.9 vol%, respectively, was obtained.i-butane hardly passes the MFI membrane as it is trapped in the zick-zack-channels of the zeolite [56], which is reflected in a lower amount of this component in the permeate.Scheme 1. Two-step process for the generation of hydrogen from wet natural gas, including membrane-based LPG-enrichment and further steam reforming.

Steam Reforming of Real Natural Gas Mixtures over Rh1/Al2O3
The product gas distributions obtained after steam reforming of the two gas mixtures over Rh1/Al2O3 (S/C = 1 and 4, GHSV~8000 h −1 ) are displayed dependent on temperature in Figure 4.At S/C = 1 the reforming of raw gas starts at 350 °C and full conversion of higher alkanes is achieved at temperatures above 450 °C.With elevation of the reaction temperature, further methane conversion and rising H2 and COx concentrations are observed.Nearly full methane conversion is reached at a temperature of 850 °C.At S/C = 4, complete conversion of higher alkanes and methane occurs at lower temperatures compared to the case of S/C = 1, and the respective H2 concentration exhibits 76 vol% at 550 °C and levels off at higher reaction temperatures.The product gas distribution resulting from the steam reforming of the permeate gas differs positively from that of the raw gas at low temperature, due to the higher LPG fraction in the mixture.Compared to the raw gas reforming, twice the concentration of H2 is achieved from permeate gas reforming at 350 °C and S/C = 4 (H2 concentration of 46.5%).Under such reaction conditions,  The product gas distributions obtained after steam reforming of the two gas mixtures over Rh1/Al 2 O 3 (S/C = 1 and 4, GHSV~8000 h −1 ) are displayed dependent on temperature in Figure 4.At S/C = 1 the reforming of raw gas starts at 350 • C and full conversion of higher alkanes is achieved at temperatures above 450 • C. With elevation of the reaction temperature, further methane conversion and rising H 2 and CO x concentrations are observed.Nearly full methane conversion is reached at a temperature of 850 • C. At S/C = 4, complete conversion of higher alkanes and methane occurs at lower temperatures compared to the case of S/C = 1, and the respective H 2 concentration exhibits 76 vol% at 550 • C and levels off at higher reaction temperatures.
Processes 2018, 6, x FOR PEER REVIEW 8 of 12 In the membrane process, pores are blocked for methane due to the preferred adsorption of LPG alkanes in the MFI structure and permeation is induced by a present gradient of the chemical potential, preferably by differences between permeate and feed pressure.At ppermeate = 0.17 bar and pfeed = 7 bar (T = 75 °C), propane and n-butane enrichment of 37.6 vol% and 22.6 vol% in permeate from initial 18.9 vol% and 4.9 vol%, respectively, was obtained.i-butane hardly passes the MFI membrane as it is trapped in the zick-zack-channels of the zeolite [56], which is reflected in a lower amount of this component in the permeate.Scheme 1. Two-step process for the generation of hydrogen from wet natural gas, including membrane-based LPG-enrichment and further steam reforming.

Steam Reforming of Real Natural Gas Mixtures over Rh1/Al2O3
The product gas distributions obtained after steam reforming of the two gas mixtures over Rh1/Al2O3 (S/C = 1 and 4, GHSV~8000 h −1 ) are displayed dependent on temperature in Figure 4.At S/C = 1 the reforming of raw gas starts at 350 °C and full conversion of higher alkanes is achieved at temperatures above 450 °C.With elevation of the reaction temperature, further methane conversion and rising H2 and COx concentrations are observed.Nearly full methane conversion is reached at a temperature of 850 °C.At S/C = 4, complete conversion of higher alkanes and methane occurs at lower temperatures compared to the case of S/C = 1, and the respective H2 concentration exhibits 76 vol% at 550 °C and levels off at higher reaction temperatures.The product gas distribution resulting from the steam reforming of the permeate gas differs positively from that of the raw gas at low temperature, due to the higher LPG fraction in the mixture.Compared to the raw gas reforming, twice the concentration of H2 is achieved from permeate gas reforming at 350 °C and S/C = 4 (H2 concentration of 46.5%).Under such reaction conditions, The product gas distribution resulting from the steam reforming of the permeate gas differs positively from that of the raw gas at low temperature, due to the higher LPG fraction in the mixture.Compared to the raw gas reforming, twice the concentration of H 2 is achieved from permeate gas reforming at 350 • C and S/C = 4 (H 2 concentration of 46.5%).Under such reaction conditions, Rh1/Al 2 O 3 performs even better than a commercial catalyst consisting of 5% Rh on alumina (Figure S6).At 550 • C higher alkanes are nearly completely converted.
The related H 2 fractions, H 2 /CO ratios, and CO/CO 2 volumetric contents are additionally depicted in Figures S7-S9.For both raw gas as well as permeate gas, the achievable H 2 /CO ratios show a maximum between 500 and 550 • C, and gradually decrease with the elevation of temperatures above 550 • C. H 2 /CO ratios obtained from permeate gas reforming are slightly lower than those from raw gas transformation.At S/C = 4 the CO/CO x -ratio (Figure S9) presents the most significant minimum at 500 • C, leading to H 2 /CO ratios far above 10, which point to the side reactions which were described earlier.
Time-on-stream performance of Rh1/Al 2 O 3 in the steam reforming of LPG-enriched permeate natural gas at 350 • C and 850 • C (S/C ratio = 4) is displayed in Figure 5.The activity of Rh1/Al 2 O 3 increases during the first hour on stream and is nearly stable in the subsequent reaction time.show a maximum between 500 and 550 °C, and gradually decrease with the elevation of temperatures above 550 °C.H2/CO ratios obtained from permeate gas reforming are slightly lower than those from raw gas transformation.At S/C = 4 the CO/COx-ratio (Figure S9) presents the most significant minimum at 500 °C, leading to H2/CO ratios far above 10, which point to the side reactions which were described earlier.Time-on-stream performance of Rh1/Al2O3 in the steam reforming of LPG-enriched permeate natural gas at 350 °C and 850 °C (S/C ratio = 4) is displayed in Figure 5.The activity of Rh1/Al2O3 increases during the first hour on stream and is nearly stable in the subsequent reaction time.This behaviour is not in accordance with general considerations regarding catalyst deactivation [57].Commonly, particle deactivation involves (i) a loss in surface area in the beginning of the reaction, (ii) slowed sintering on stream, and (iii) reaching a stable performance at a certain time.In the present case, the active phase, e.g., formed nanoparticles from Rh single-sites, is being created during the initial stage and seems to possess particularly suitable features for the reaction at low temperatures.At 850 °C, Rh1/Al2O3 shows no deactivation over 40 h on stream, the hydrogen yield remains at ~73 vol%, and the H2/CO ratio was constant at 4.

Conclusions
A highly active Rh catalyst was formed in situ on Al2O3 during the reaction of C1-C5 alkanes with steam.The active phase in Rh1/Al2O3 were Rh nanoparticles in the range 1-3 nm formed from single Rh-atoms.The product spectrum of steam reformed alkanes over Rh1/Al2O3 strongly depends on S/C ratios and temperatures.With this catalyst, an alkane mixture of an LPG-rich natural gas containing 57.8 vol% CH4 can be fully reformed at 650 °C at S/C = 4. Below 650 °C, the higher alkanes showed a positive impact on the hydrogen production from binary mixtures at low temperatures but contributed to methanation.Furthermore, maximum H2/CO ratios (>10) were detected between 350 and 650 °C.Consequently, pre-reforming of LPG-rich natural gas should be performed under very mild conditions, no higher than 350 °C.Therefore, the application of a membrane-based preseparation represents a feasible concept to obtain (i) easily activatable LPG-enriched natural gas for This behaviour is not in accordance with general considerations regarding catalyst deactivation [57].Commonly, particle deactivation involves (i) a loss in surface area in the beginning of the reaction, (ii) slowed sintering on stream, and (iii) reaching a stable performance at a certain time.In the present case, the active phase, e.g., formed nanoparticles from Rh single-sites, is being created during the initial stage and seems to possess particularly suitable features for the reaction at low temperatures.At 850 • C, Rh1/Al 2 O 3 shows no deactivation over 40 h on stream, the hydrogen yield remains at ~73 vol%, and the H 2 /CO ratio was constant at 4.

Conclusions
A highly active Rh catalyst was formed in situ on Al 2 O 3 during the reaction of C 1 -C 5 alkanes with steam.The active phase in Rh1/Al 2 O 3 were Rh nanoparticles in the range 1-3 nm formed from single Rh-atoms.The product spectrum of steam reformed alkanes over Rh1/Al 2 O 3 strongly depends on S/C ratios and temperatures.With this catalyst, an alkane mixture of an LPG-rich natural gas containing 57.8 vol% CH 4 can be fully reformed at 650 • C at S/C = 4. Below 650 • C, the higher alkanes showed a positive impact on the hydrogen production from binary mixtures at low temperatures annular dark-field-scanning transmission electron microscopy (HAADF-STEM) images of fresh and spent catalysts are shown in Figure 1a,b.The spent catalysts were isolated after the light-off test with permeate gas at S/C = 4, running the reaction from 200 to 850 • C. In fresh Rh1/Al 2 O 3 , many visible Rh atoms well-dispersed over the γ-Al 2 O 3 support and islands of single-sites can be detected (bright spots in yellow circles).Rh nanoparticles in the size of 1-3 nm are visible in the spent Rh1/Al 2 O 3 catalyst, indicating particle formation from Rh single-sites during the steam reforming process.H 2 -TPR was performed with fresh and spent catalysts.The spent catalysts were isolated again after the light-off tests with permeate gas at 850 • C (S/C = 4).As can be concluded from Figure 1c, the absence of typical reduction peaks for particulate RhO x species in the range from 80 to 250 • C [49,50] indicate the nature of the sites in fresh Rh1/Al 2 O 3 .Most Rh species in the fresh catalysts are present as single atoms, because the adsorption and dissociation of H 2 to H atoms cannot occur over single atoms [51].H 2 -TPR over spent Rh1/Al 2 O 3 catalysts is in accordance with the TEM results.It is obvious that the single-sites in the fresh catalysts were transformed to nanoparticle species during an initial activation phase.The formed Rh particles in spent Rh1/Al 2 O 3 show an extraordinary reducibility below 50 • C and were the active catalyst species.

Processes 2018, 6 , 12 Figure 2 .
Figure 2. H2 volumetric gas content (water and inert gas excluded) and H2/CO ratio in the steam reforming of single alkanes over Rh1/Al2O3 at GHSV (gas hourly space velocity) of 8000 h −1 and different values of temperature and S/C ratios of (a) 1, (b) 2.5, and (c) 4.

Figure 3 .
Figure 3. Formation of methane during steam reforming of (a) ethane, (b) propane, (c) i-butane and (d) n-butane over Rh1/Al2O3 at various S/C ratios and temperatures at a constant GHSV of ~8000 h −1 .

Figure 3 .
Figure 3. Formation of methane during steam reforming of (a) ethane, (b) propane, (c) i-butane and (d) n-butane over Rh1/Al 2 O 3 at various S/C ratios and temperatures at a constant GHSV of ~8000 h −1 .

Figure 4 .
Figure 4. Volumetric gas contents of raw natural gas and LPG-enriched permeate natural gas from membrane pre-separation and after steam reforming over Rh1/Al2O3 at (a) S/C ratio of 1 and (b) S/C ratio of 4.

Scheme 1 .
Scheme 1. Two-step process for the generation of hydrogen from wet natural gas, including membranebased LPG-enrichment and further steam reforming.

Figure 4 .
Figure 4. Volumetric gas contents of raw natural gas and LPG-enriched permeate natural gas from membrane pre-separation and after steam reforming over Rh1/Al2O3 at (a) S/C ratio of 1 and (b) S/C ratio of 4.

Figure 4 .
Figure 4. Volumetric gas contents of raw natural gas and LPG-enriched permeate natural gas from membrane pre-separation and after steam reforming over Rh1/Al 2 O 3 at (a) S/C ratio of 1 and (b) S/C ratio of 4.

Figure 5 .
Figure 5. Steam reforming of permeate natural gas over Rh1/Al2O3 at S/C ratio of 4 and temperatures of 350 and 850 °C.

Figure 5 .
Figure 5. Steam reforming of permeate natural gas over Rh1/Al 2 O 3 at S/C ratio of 4 and temperatures of 350 and 850 • C.