HIGH-SILICA PD-BASED SMALL PORE ZEOLITE CATALYSTS FOR LOW TEMPERATURE CH ₄ OXIDATION

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under Grant No. DE- AC36-08GO28308 awarded by the Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

[0002] The present invention relates to catalysts for methane (CH ₄ ) remediation and, more specifically, to Pd-based, high silica, small pore zeolite catalysts with improved CH ₄ combustion performance at low temperature under lean-bum conditions and in high water vapor environments.

2. DESCRIPTION OF THE RELATED ART

[0003] Natural gas has garnered attention as a cleaner alternative fuel for vehicles compared to gasoline or diesel. The exhaust of natural gas vehicles (NGVs), however, contains methane (CH ₄ ) which is 25 times more harmful than carbon dioxide (CO2) in global warming potential. The conventional solution for CH ₄ remediation is its catalytic oxidation by using Pd/ALOs and Pd/ZrO2 catalysts. As the NGV exhaust feed is at a low temperature (< 400 °C) and contains high amounts of steam (5-10%), the catalytic oxidation of CH ₄ by Pd/ALCh and Pd/ZrCF catalysts suffers from low conversions and deactivation through Pd sintering. In addition, Pd is susceptible to sintering and hydroxyl accumulation on active PdO sites in the presence of H2O that can lower CH ₄ conversion and reduce catalyst life. As a result, there is a need in the art for a catalyst that can operate in the low temperature and high water vapor environment of NGV exhaust to provide for improved CH ₄ remediation.

BRIEF SUMMARY OF THE INVENTION

[0004] The present invention provides an approach for making Pd-based, high silica (Si/Al molar ratios > 50) small pore zeolite catalysts that have increased durability and improved low temperature CH ₄ oxidation performance (> 90% CH ₄ conversion at temperatures < 400 °C). The zeolite catalyst may be made by forming a zeolite having a silica to aluminum ratio greater than 30 and a pore size of less than 0.5 nm in a reaction vessel, wherein the zeolite has a Chabazite (CHA) topology or a Linde Type A (LTA) topology and then performing ion exchange with a source of palladium to achieve less than 2 weight percent loading of palladium in the zeolite.

[0005] In one aspect, the step of forming a zeolite may comprise positioning a molar gel composition of an amount of water, an amount of sodium hydroxide, an amount of an organic structure directing agent, an amount of aluminum oxide, and an amount of silicon dioxide in the reaction vessel. The step of forming a zeolite may further comprise aging the molar gel composition for a predetermined time period at room temperature. The step of forming a zeolite may further comprise heating the molar gel composition at 160 °C for a second predetermined time period after aging. The step of forming a zeolite may further comprise separating the zeolite from a supernatant by centrifugation after heating the molar gel composition. The step of forming a zeolite may further comprise washing the zeolite and then drying the zeolite after separating the zeolite. The step of forming a zeolite may further comprise calcining the zeolite in a muffle furnace.

[0006] In another aspect, the step of forming a zeolite comprises synthesizing an organic structure directing agent by adding an amount of dimethyl imidazole and an amount of methylbenzyl chloride to an amount of toluene and refluxing at 100 °C for a first predetermined time period under an argon atmosphere. The step of forming a zeolite may further comprise separating the organic structure directing agent from any solution, converting the organic structure directing agent to a hydroxide form. The step of forming a zeolite may further comprise positioning a molar gel composition of an amount of water, an amount of sodium hydroxide, an amount of the organic structure directing agent, an amount of tetramethylammonium hydroxide, an amount of hydrofluoric acid, an amount of aluminum oxide, and an amount of silicon dioxide in the reaction vessel. The step of forming a zeolite may further comprise heating the molar gel composition to 160 °C for a predetermined amount of time while rotating the molar gel composition. The step of forming a zeolite may further comprise separating the zeolite from a supernatant by centrifugation after heating the molar gel composition. The step of forming a zeolite may further comprise washing the zeolite and then drying the zeolite after separating the zeolite.

[0007] In a further aspect, the present invention includes a zeolite formed according to various methods described herein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) [0008] The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which: [0009] FIG. l is a schematic of a microreactor for CH4 temperature programmed oxidation and stability testing of Pd-based, high silica (Si/Al > 50) small pore zeolite catalysts made according to the present invention.

[0010] FIG. 2 is a graph of the T50, 90 temperatures for CPU conversion, corresponding to the temperatures where 50% and 90% of CPU is converted, respectively, under simplified lean conditions over 1 wt.% Pd/ALCL and 1 wt. % Pd/CHA (Si/Al = 15, 33, 53, 137).

[0011] FIG. 3 is a graph of the T50, 90 temperatures for CPU conversion under simplified lean conditions over 1 wt.% Pd/AhCP and 1 wt. % Pd/H-LTA (Si/Al = 1, 22, 31, 39, 52).

[0012] FIG. 4 is a graph of catalyst stability for CPU conversion at 450 °C under the simplified lean conditions over Pd/AhCP and Pd/H-LTA (Si/Al = 52) for 10 h.

[0013] FIG. 5 is a graph of T50, 90 comparison of 2 wt.% Pd/CHA (Si/Al = 156) washcoated mini-core under realistic rich, moderate lean, and lean conditions as fresh, after hydrothermal aging (HTA) at 650 °C for 12 h in 10% H2O and 20% O2 in Ar, and after 20 ppm SO2 poisoning at 300 °C for 5 h.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Referring to the figures, wherein like numeral refer to like parts throughout, the present invention involves the hydrothermal synthesis of high silica (silicon/aluminum molar ratios > 30), small pore (-0.3-0.5 nm pore apertures) zeolites and the use of such zeolites for methane oxidation, such as in NGV exhaust streams. The synthesis employs sodium hydroxide (OH) media (Example 1) and hydrofluoric acid (HF) media (Example 2) that include, but are not limited to, zeolites with Chabazite (CHA) and Linde Type A (LTA) frameworks. After synthesis, Pd is added to the zeolite powder by either ion-exchange or wetimpregnation, up a loading of 1 wt.%, (Example 3) and is evaluated for its CH4 oxidation performance in a microreactor (FIG. 1, Example 4). All zeolite catalysts are named with Si/Al molar ratio in parentheses.

[0015] Referring to FIG. 1, a system 10 for the evaluation of high silica (silicon/aluminum molar ratios > 30), small pore (-0.3-0.5 nm pore apertures) zeolites may include a syringe pump 12 for delivering reactants to a furnace 14. Furnace 14 includes an inlet coupled to a source of argon 16. Furnace 14 includes an outlet coupled to an inlet of a reactor 18. Reactor 18 is further coupled to a source of methane 20, a source of oxygen 22, a source of carbon dioxide 24, a source of carbon monoxide 26, a source of nitric oxide 28, and the source of argon 16. The source of methane 20, source of oxygen 22, source of carbon dioxide 24, source of carbon monoxide 26, source of nitric oxide 28, and source of argon 16 may be premixed together with a premixer 30 and also with the outlet of furnace 14 prior to reactor 18. Reactor 18 includes an outlet coupled to a mass spectrometer 32 and also to a gas analyzer 34. Source of argon 16 may be used to provide dilution to the reactor outlet components provided by gas analyzer 34. Mass spectrometer 32, premixer 30, and gas analyzer may be coupled to an exhaust 36.

[0016] Example 1 - Synthesis of high silica CHA zeolite

[0017] Synthesis of high silica CHA zeolites requires water (H2O), a hydroxide source, an organic structure directing agent (OSD A), an alumina (AI2O3) source, and a silica (SiO2) source. The source for water is ASTM Type I ultra-pure water, hydroxide used is sodium hydroxide (NaOH) pellets from Macron Fine Chemicals (ACS grade), OSDA is N,N,N-trimethyl-l-adamantantylammonium hydroxide (TMAda-OH) (25.3%) from Sachem Zeogen, AI2O3 is aluminum hydroxide (Al(0H)3) from Sigma-Aldrich, and SiO2 is fumed silica (0.007 pm) from Sigma-Aldrich. Molar gel composition of 2200H20: 10NaOH: 10TMAda-OH:xA1203:100Si02 where x is < 1 (Si/Al > 50) is mixed into a PTFE liner that is inserted into a stainless steel autoclave reactor vessel. The synthesis gel is allowed to age for 2 h at room temperature before heating the reactor vessel in a rotary convection oven (60 rpm) at 160 °C for 24 h. The CHA zeolite was separated from the supernatant by centrifugation (4500 rpm, 5 min), and then washed with acetone and ASTM Type I water in alternating wash/centrifuge/decant cycles (2 washes with each solvent, ~15 ml solvent per wash). The solids were dried in air at 100 °C for 24 h, and then calcined in a muffle furnace at 580 °C in 100 cm ³ min' ¹ (gzeoiite)' ¹ dry air with a 1°C min ^-1 ramp for 8 h and a 3 h isothermal step at 150 °C.

[0018] Example 2 - Synthesis of high silica LTA zeolite

[0019] Synthesis of high silica LTA zeolites requires H2O (ASTM Type I ultra-pure water), HF (48%, Sigma Aldrich), AI2O3 (A1(OH)3, Sigma-Aldrich), SiCh (tetraethoxysilane, STREM Chemicals), tetramethylammonium hydroxide (TMAOH 5H2O) (97%, Sigma- Aldrich) and pre-synthesized OSDA-OH. The OSDA-OH was synthesized by adding 11.60 g 1,2-dimethyl imidazole and 15.46 g 4-methylbenzyl chloride to 125 ml toluene and refluxing the reaction mixture at 100 °C for 24 h under Ar atmosphere. The formed material was separated from the mother solution via centrifugation and washed 3 times with 50 ml ethyl acetate followed by drying at 110 °C overnight. The as-synthesized OSDA(Cl) was converted to its hydroxide form (OSDA(OH)) by adding the OSDA(Cl) solution to hydroxide exchange resin until a full exchange was determined by titrating the OSDA(OH) solution with 0.01 M HC1. The obtained OSDA(OH) solution was then concentrated by rotary evaporation at 80 °C to 20-25%. Molar gel composition of 500H ₂ 0:450SDA-

0H:5TMA0H 5H ₂ 0:5HF:xAl ₂ 0 ₃ : 100Si0 ₂ where x is < 1 (Si/Al > 50) is mixed into a PTFE liner that is inserted into a stainless steel autoclave reactor vessel. The vessel is heated to 160 °C for at least 4 days at 60 rpm in a rotary convection oven. LTA zeolites were separated from the supernatant by centrifugation (4500 rpm, 5 min), and then washed with acetone and ASTM Type I water in alternating wash/centrifuge/decant cycles (2 washes with each solvent, ~15 ml solvent per wash). The solids were dried in air at 100 °C for 24 h, and then calcined in a muffle furnace at 580 °C in 100 cm ³ min' ¹ (g zeolite)' ¹ dry air with a 1°C min ^-1 ramp for 8 h and a 3 h isothermal step at 150 °C.

[0020] Example 3 - Preparation of Pd-based zeolite catalysts

[0021] Pd ion-exchange of small pore zeolites was performed using Pd(NO ₃ ) ₂ (Sigma- Aldrich) solution at 80 °C for 24 h to achieve 1-2 wt.% Pd loading. The zeolite catalysts were calcined at 500 °C for 2 h. Calcined catalysts were pressed and sieved to 250- 500 pm pellets.

[0022] Example 4 - Preparation of washcoated Pd-based zeolite mini-cores

[0023] The slurry composition for washcoating cordierite mini-cores (0.5” D x 1” H) with cell per square inch of 400 and wall thickness of 3 mil (1/1000 inch) contained 20 wt.% solid phase and 80 wt.% liquid phase. The solid phase contained 95-100 wt.% catalyst and 0- 5 wt.% P2 Boehmite binder (Sasol). The slurry pH was adjusted between 3-6 using 1 M NH4NO ₃ solution with target washcoat loading was 1 gwashcoat in' ³ . The washcoated monoliths were dried at 110 °C (3 °C min' ¹ ) for 1 h and calcined at 550 °C (3 °C min' ¹ ) for 2 h.

[0024] Example 5 - Evaluation

[0025] Evaluation of Pd-based zeolite catalysts and washcoated mini-cores in microreactor CH4 temperature programmed oxidation experiments over as-synthesized catalysts and washcoated mini-cores were performed in a packed bed reactor, as seen in FIG.

1. The reactor quartz tube was loaded with 100 mg of catalyst pellets, the total flow was maintained to 333 cm ³ min' ¹ (STP), while the weigh-hourly space velocity (WHSV) was kept at 199.8 1 gcat' ¹ h' ¹ . The catalysts were pretreated in 5% O ₂ in Ar at 500 °C (10 °C min' ¹ ) for 20 min and cooled down to 200 °C. Further pretreatment was performed using simplified lean conditions (1500 ppm CH4, 5% O ₂ , 5% H ₂ O) by heating the catalysts to 650 °C (5 °C min' ¹ ) for 1 h and cooled down to 200 °C. For the evaluation of the catalytic performance, the catalysts were heated to 650 °C (5 °C min' ¹ ) under the CH4 oxidation feed. Temperature for 50% and 90% CH4 conversions (Tso,9o, respectively) were determined based on the resulting light-off curves. Stability testing was performed after the pretreatment by holding the catalysts at 450 °C for 10 h. The washcoated mini-cores were evaluated with a flow rate of 163 cm ³ min' ¹ (STP), and gas-hourly hour space velocity (GHSV) of 22,534 h' ¹ . The minicores were pretreated in 20% O2 in Ar at 500 °C for 2 h before each sequential ramp up to 600 °C in the realistic rich, moderate lean, and lean conditions, as set forth in Table 1 below: Table 1 - Emissions from a CNG powered engine under realistic rich, moderate lean, and lean conditions.

Gas Concentrations Rich Moderate Lean Lean

CH ₄ (ppm) 2286 1490 2637

O2 (%) 0.62 3.97 8.38

H2O (%) 10 10 10

CO2 (%) 8.84 7.69 5.65

CO (ppm) 6404 692 752

NO (ppm) 1698 1864 94

[0026] Hydrothermal aging (HTA) on the washcoated mini-cores was performed at 650 °C for 12 h in 10% H2O and 20% 02/Ar followed by evaluation under rich, moderate lean, and lean conditions. After HTA, the mini-core was also subjected to 20 ppm of SO2 poisoning in Ar at 300 °C for 5 h followed by evaluation under rich, moderate lean, and lean conditions.

[0027] Results

[0028] For Pd/CHA zeolite catalysts, Si/Al molar ratios > 53 are required to achieve 90% CH4 conversion temperature (T90) < 400 °C, as seen in FIG. 2. Specifically, Pd/CHA (Si/Al = 137) was able to obtain T90 of 389 °C. For Pd/H-LTA zeolite catalysts, Si/Al molar ratios > 31 are required to achieve T90 < 400 °C, as seen in FIG. 3. Specifically, Pd/H-LTA (Si/Al = 39) and Pd/H-LTA (Si/Al = 52) were able to obtain Tgo’s of 384 °C and 372 °C, respectively. As Si/Al molar ratios increased for both Pd/CHA and Pd/H-LTA zeolite catalysts, the Tgo’s decreased. 100% CH4 conversion at 450 °C was maintained for Pd/H-LTA (Si/Al = 52) compared to 70% conversion over conventional Pd/AhCh over a 10 h time period in 5% H2O, as seen in FIG. 4, highlighting the hydrothermal stability of high silica Pd/H-LTA zeolite catalysts. Further increasing the Si/Al of Pd/CHA to 156 and washcoating on mini-cores shows resistance to HTA and SO2 poisoning with T90 close to 400 °C or below, as seen in FIG. 5.