New Technologies and Alternative Feedstocks in Petrochemistry and Refining DGMK Conference October 9 – 11, 2013, Dresden, Germany

Dry Reforming of Methane with CO2 at Elevated Pressures A. Milanov*, S. Schunk**, E. Schwab*, G. Wasserschaff* *BASF SE, Ludwigshafen, Germany ** hte AG, Heidelberg, Germany

Abstract The indirect conversion of natural gas into higher value chemicals and fuels via syngas is superior with regard to efficiency compared to the currently available direct conversion technologies and remains the industrially preferred route. Typically the syngas production route is generally dictated by the H2/CO ratio requirements of the downstream synthesis process. Processes such as direct DME synthesis, high-temperature Fischer-Tropsch and acetic acid synthesis require CO rich syngas that is not readily accessible by established technologies like steam methane reforming (SMR) and autothermal reforming of methane (ATR). The CO2 reforming of methane, also known as dry reforming (DRM), is an attractive alternative technology for the production of CO-rich syngas. This paper gives an overview of the current joint research activities at BASF and hte AG aiming to develop suitable catalysts for CO2 reforming of methane at elevated pressures with minimized input of process steam. The performance profiles of two newly developed base metal catalysts are presented and discussed. The catalysts exhibit high degrees of methane and CO2 conversion in combination with an extraordinary coking resistance under high severity process conditions.

Introduction The conversion of natural gas into liquid fuels and higher value basic chemicals for the chemical industry remains an emerging challenge to industrial catalysis in the recent years. On the one hand flaring of associated natural gas from oilfields is becoming unacceptable, which increases the need for onsite gas-to-liquid conversion. On the other hand, the recent technological developments in shale gas production over the last decade have a significant impact on the availability and price of natural gas in some parts of the world (Figure 1), thus making it an economically highly attractive feedstock.

DRM

Figure 1. Development of the natural gas Figure 2. H2/CO ratio ranges accessible using price in the US over the last ten years.[11] different syngas production technologies.

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Ever since the pioneering work of Alwin Mittasch at BASF[1] more than 100 years ago, reforming technology proved as the most efficient route for indirect conversion natural gas to higher value products via syngas. The target properties of the syngas are dictated by the requirements of the downstream process, which determines the choice of appropriate reforming technology (Figure 2). In the case of ammonia synthesis, only high purity hydrogen is required. This is provided by steam methane reforming (reaction 1), which produces [2] hydrogen rich syngas with H2/CO ≥ 3 , followed by water gas shift (WGS) reaction (reaction 2) that ensures carbon monoxide conversion and produces additional hydrogen. The synthesis of MeOH, 2-stage DME and the high-temperature Fischer-Tropsch synthesis require H2/CO ratios close to 2. Depending on the plant scale, the syngas could be produced either by SMR or ATR, the latter requiring an energy intense air separation unit.[3]

(1) Steam reforming: CH + H O <=> 3H + CO, = 206 kJ 0 4 2 2 298 (2) WGS reaction: CO + H O <=> H + CO , ∆퐻 = 41 kJ⁄푚표푙 0 2 2 2 298 (3) CO reforming: CH + CO <=> 2H + 2 CO, ∆퐻 = −247 ⁄푚표푙 0 2 4 2 2 DME, acetic acid and oxo-alcohols are attractive molecules that∆퐻 find298 various 푘퐽applications⁄푚표푙 as fuels (DME) and important starting material in the chemical industry. Their synthesis requires a CO-rich syngas with low H2/CO ratio of around 1, which is not accessible by SMR or ATR. The CO2 reforming of methane, also known as dry reforming, is an emerging alternative reforming technology, which produces a CO-rich syngas with the required H2/CO ratio close to one (reaction 3). However, problems associated with severe catalyst coking at elevated pressure and fast deactivation when using industrially feasible base metal catalysts (mainly nickel) have so far limited its commercialisation. The challenges related to catalyst development for DRM have been revealed in several comprehensive reviews.[3,4,5] It has been shown that coking of nickel catalysts can be kinetically inhibited by ensemble control and selective blocking of the step edges of nickel crystallites with sulphur (SPARG process) or gold.[3,6,7,8] The motivation of the present work was to develop coking stable base metal catalysts for the CO2 reforming of methane at elevated pressures. An overview of the developed catalysts and their performance profiles will be presented and discussed in the following.

Catalyst development It is known from literature that platinum group metals (PGM), such as e.g. Rh, Pt and Ru show high activity for DRM and low selectivity for coke formation.[3,4,9,10] The high cost and limited availability of PGM hinders the commercialization of the process. Due to their acceptable turnover rates and lower cost, nickel-based catalysts are considered as the most suitable for industrial applications and were the main focus of this study. The goal of the catalyst development study was the synthesis of active masses with highly dispersed, nano- sized, sintering stable active phase (coking suppression by ensemble control)[3,6,7], which at the same time are capable to provide fast decomposition of the CnHm precursors formed in the gas phase. In order to accelerate catalyst synthesis and testing, design of experiment (DoE) methodology and high-throughput test equipment were used. This approach provides a fast and systematic synthesis and screening of large catalyst libraries. In fact the development of the ammonia catalyst by A. Mittasch can be considered as one of the earliest examples (1909) for successful application of the high-throughput methodology. In the first step of catalyst development, based on the literature data available, the active metals, potential supports and promising promoters (dopants) were chosen. In the second step the DoE strategy was set in a way that the compositional hyperspace is screened in an optimal way (Figure 3).

32 DGMK-Tagungsbericht 2013-2 New Technologies and Alternative Feedstocks in Petrochemistry and Refining

Figure 3. Schematic representation of Figure 4. Six-fold test rig used for catalyst testing the catalyst screening approach. (up to 100ml catalyst, up to 1100°C, 20-40 bar).

The material libraries were prepared by conventional preparation methods used in catalyst production. The reforming performance was tested at elevated pressure (20bar) in a 6-fold parallel test rig equipped with six downstream flow fixed bed reactors (Figure 4). Each reactor was loaded with 90ml fresh catalyst grinded to 0.5-1.0mm fraction. For performance evaluation the catalysts were subjected to a comprehensive screening test program, where feed composition (CH4/CO2 and H2O/CH4 ratios) and process temperature were varied in a way that provided a systematical increase in severity of process conditions (thus increasing the thermodynamic potential for carbon formation). The catalyst performance was evaluated based on the catalyst activity (CH4/CO2 conversions) and the highest severity that allowed stable coke-free operation.

Reforming performance The screened catalyst libraries covered in general a broad activity range. A major part of the catalysts was found to be reasonably active with a significant number approaching the equilibrium conversions under the different process conditions. No clear effect of the carrier and dopants on the activity was observed. In terms of coking stability, however, clear tendencies were observed.

st Figure 5. Reforming performance of the 1 generation catalyst (CH4/CO2=2; H2O/CH4=1, p=20bar).

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The result of the screening phase was the identification of one promising active mass that showed high activity and coke-free operation under severe process conditions (S/C < 1; CH4/CO2 ≤ 2; T = 850 – 950°C). The H2/CO could be adjusted in the range 1.8 – 2.3 by varying on the process conditions. The long term stability (>>100h) was confirmed by a series of durability tests. The reforming performance profile of the catalyst (referred as to 1st generation DRM) in the temperature range 850 – 950°C and S/C = 1, CH4/CO2 = 2 is presented in Figure 5. The CH4 and CO2 conversions are close to equilibrium over the whole range of temperatures. The H2/CO ratio of resulting syngas lies in the range of 2.1 – 2.3, which is favorable for MeOH and low-temperature Fischer-Tropsch synthesis.

The screening of potential coking resistant DRM catalysts was continued taking the results for the first material libraries into account. As a result of these efforts a second prospective active mass was identified (2nd generation catalyst). The catalyst showed good activity and provided coke-free operation at significantly increased severity compared to the 1st generation material. This was confirmed in a series of durability tests under nearly dry process conditions over prolonged period of time. Figure 6 shows exemplary the CH4 and CO2 conversions as well as H2/CO ratio of the product gas obtained at 900°C with feed containing equimolar CH4/CO2 amounts and S/C of 0.5 and 0.38, respectively. For both inlet gas compositions, a nearly equilibrium conversion for CH4 and CO2 are observed. The H2/CO ratio obtained is 1.2 - 1.0, respectively, which corresponds to the optimum ratio for 1-step DME, acetic acid and oxo-alcohol synthesis. Further decrease of S/C ratio is considered as not feasible since it will result in H2/CO less than 1 and will require H2 import if the syngas is to be used for the synthesis of the above mentioned processes.

Figure 6. Reforming performance of the 2nd generation DRM catalyst at 900°C/20bar.

Conclusions

Two complementary catalysts for CO2 reforming of methane at elevated pressure (20bar) and low S/C conditions were identified using a systematic screening approach supported by high-throughput test equipment. The 1st generation catalyst allows coke-free operation at challenging conditions and provides a syngas with H2/CO of 1.8-2.3 depending on the feed composition. The state of the art technology to produce a syngas of this stoichiometry is ATR, which requires pure oxygen from an air separation unit. Using the 2nd generation catalyst, the severity of the process conditions can be further increased. Stable operation

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near equilibrium at S/C as low as 0.38, was successfully demonstrated. The resulting syngas is characterized by a H2/CO ratio of 1.0 – 1.2. Up to date only precious metal based catalysts and sulfur passivated nickel materials (SPARG) could be operated at such severe process conditions. In contrast to these examples our 2nd generation catalyst does not contain precious metals and is free of sulfur.

References

[1] DRP 296866 [2] K. Liu, C. Song, C. Subramani, “Hydrogen and Syngas Production and Purification Technologies”, Wiley-AIChE, Hoboken, New Jersey. [3] J.R. Rostrup-Nielsen, J. Sehested, Adv. Catal., 47, 65 (2002). [4] M.C.J Bradford, M.A. Vannice, Catal. Rev., 41, 1 (1999). [5] Y.H. Hu, E. Ruckenstein, Adv. Catal., 48, 297 (2004). [6] H.S. Bengaard et al., J. Catal., 209, 365 (2002). [7] F. Besenbacher et al., Science, 279, 1913 (1998). [8] N.R. Udengaard et al., Oil&Gas J., 9, 62 (1992). [9] J.R. Rostrup-Nielsen, J-H. Bak Hansen, J. Catal., 144, 38 (1993). [10] J.H. Bitter, K. Seshan, J.A. Lercher, J. Catal., 176, 93 (1998). [11] U.S. Energy information administration (available at http://tonto.eia.gov/dnav/ng/hist/rngwhhda.htm)

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