Mass normalized current mAmg

E.S. Smotkin B. Gurau

Fig. 32.21. Anode polarization curves (V vs. RHE) of NaBH4 reduced catalysts from ''zoom screens.''

Fig. 32.22. Symyx Technologies electrode array consisting of 64 individually addressable electrodes prepared on an insulating 7.5-cm quartz substrate using lithographic techniques.

of bifunctional oxygen reduction/water oxidation electrocatalysts for regenerative fuel cells [181].

Scientists at Symyx Technologies have developed another combinatorial approach for the rapid synthesis and screening of fuel cell electrocatalysts. Libraries of alloy materials are synthesized directly onto an 8 x 8 electrode array (Fig. 32.22) which consists of 64 individually addressable electrodes prepared on an insulating 7.5-cm quartz substrate using lithographic techniques. The electrode material can be made from gold or titanium. Each electrode is approximately 1.0 mm diameter and electrical contact is made with each electrode via a contact pad on the edge of the wafer. Electrocatalysts were synthesized by a combination of magnetron sputtering or by parallel electroplating. The electrode array was then sealed to a Teflon cylinder that was filled with aqueous electrolyte. All 64 electrodes were therefore exposed to the same electrolyte solution. A common reference electrode and working electrode was immersed in the cell. The array was interfaced to a 64-channel potentiostat via a PCB interface and the electrocatalytic performance of the resulting materials were tested by parallel monitoring of the current-voltage time behavior of each individual electrocatalyst.

The authors tested a number of catalytic concepts for the anodic electrooxida-tion of methanol (DMFC) as well as the cathodic electroreduction of oxygen in aqueous acidic electrolytes [192]. The authors demonstrated that for known systems the electrochemical activity as measured directly on the thin-film samples on the addressable electrochemical array correlated with the activity of known powder samples.

Combinatorial Electrosynthesis

Ward and coworkers applied two different combinatorial approaches to study the electrochemical reduction of 1,4-benzoquinone to hydroquinone at organosulfur-modified gold electrodes [193]. The authors prepared an array of physically but not electrically isolated gold electrodes on a glass substrate. Monolayers of organo-sulfur reagents were prepared on selected electrodes on the 4 x 7 array by dispensing aliquots of either hexanethiol or hexadecanethiol dissolved in ethanol at the respective electrodes. The array was subsequently rinsed and housed in a thin-layer electrochemical cell. The electrolyte contained 1,4-benzoquinone dissolved in 0.5 M KOH acidified to pH 3.0 and also a fluorescent dye, fluorescein, which fluoresces green at pH > 6. A potential of —0.1 V (vs. Ag/AgCl) was applied to the array and the thin-layer cell illuminated with an ultraviolet lamp. Green fluorescence was observed in those electrode regions that had not been modified with an organosulfur reagent as well as on electrode regions that had been modified with a monolayer of hexanethiol. Benzoquinone is reduced by two electrons at this potential with the consumption of two protons. It is the decrease in protons at the electrodes where benzoquinone is reduced that causes the local pH to increase. No fluorescence was observed at the hexadecanethiol-modified electrodes.

In a second approach, the authors created an electrochemically addressable electrode array from which it was possible to measure directly the electrochemical activity by monitoring the current-voltage behavior at each individual electrode. An 8 x 8 gold electrode array was created on an insulating thermally oxidized silicon substrate ensuring physical and electrical isolation of the individual electrodes using standard lithographic techniques. Electrical contact was made via standard 64-pin connectors with a single channel potentiostat and multiplexer. Selected electrodes were subsequently modified with hexanethiol, hexadecanethiol, or dodeca-nethiol monolayers through a series of adsorption and electrochemical desorption steps. An electrochemical cell was created by attaching the electrode array to a Del-rin cylinder with a Delrin cap configured for a reference and counter electrode. Electrolyte containing benzoquinone and KOH acidified to pH 3 was added to the cell, exposing all 64 electrodes to the electrolyte. In a serial manner, the electrochemical reduction of benzoquinone was studied at each electrode by cyclic voltam-metry in which the potential of each electrode was cycled and the current measured. The authors proposed that this direct measurement of the electrochemical activity was more sensitive than the fluorescent screen in quantifying the benzoquinone reduction at the modified electrodes with the results indicating that the activity in the library increases in the order of hexadecanethiol < dodecanethiol < hexanethiol < gold, illustrating suppression of current with increasing alkanethiol chain length. The authors propose that the fluorescent screen allows for measurement of active zones while electrochemical screens can be used to discriminate smaller differences in activity.

Yudin and coworkers demonstrated how combinatorial electrochemistry could be used in the electrosynthesis of small organic molecules [194a]. The authors de veloped what they term a spatially addressable electrolysis platform (SAEP). Each electrochemical cell in the 4 x 4 array was equipped with a stainless-steel cathode and a graphite rod anode. The cathodes were welded directly onto a stainless-steel support that provided a common terminal for the current source. The authors explored the anodic oxidation of carbamates, amides, and sulfonamides leading to libraries of a-alkoxycarbamates, a-alkoxyamides, and a-alkoxysulfonamides, respectively. The authors have also demonstrated the intramolecular cyclization of hy-droxyamides yielding heterobicyclic compounds as well as the generation of vicinal diamines by the reductive hydrocoupling of aldimines.

In another application, Yudin and coworkers generated libraries of catalytic materials on electrode surfaces by the copolymerization of bithiophene and pyrrole-containing TEMPO (2,2,6,6-tetramethilpiperidin-1-yloxy) catalysts [194b,c]. Diversity was created by electrochemical copolymerization and by creating surfaces with different ratios of bithiophene/pyrrole. These catalyst films were utilized in the electrochemical oxidation of primary alcohols to aldehydes, where cyclic voltamme-try was used to screen the catalytic activities of the modified electrodes.

Another example of a high-throughput screen for catalyst activity comes from Hillier and coworkers, who applied a scanning electrochemical microscope to characterize the hydrogen oxidation reaction on a polycrystalline platinum surface [195]. This technique utilizes tip-sample feedback and works reliably for determination of the kinetics of the reaction over a large range of substrate potentials from the hydrogen adsorption region to the platinum oxidation region.

Hillier explored this technique further and directly demonstrated the measurement of the rate constant for hydrogen oxidation and performed reactivity mapping of heterogeneous electrodes consisting of catalytic and noncatalytic domains.

32.10

Novel High-throughput Screening Tools

The process of discovering new catalysts or materials from a large pool of potential candidates requires a reliable and robust screening process. Traditionally, this process presents itself as a bottleneck in the discovery effort, especially if large libraries of potential catalysts or materials have already been synthesized or prepared. The first step in combinatorial catalysis, like all materials design, involves the identification of a specific chemical transformation of interest. A collection of potential catalysts, prepared from a set of chemically diverse ligand sets and metal precursors, are combined in a parallel or combinatorial fashion and screened for activity in a high-throughput primary screen. Although the large volume of potential catalysts examined in a primary screen typically allows only relative activity to be established, the goal at this stage of discovery is to identify promising lead catalysts worthy of further investigation and follow-up. Lead materials are then further examined in a high-throughput secondary screen designed to screen for specific trends in the physical and chemical properties in greater detail and typically at a slightly lower throughput than the primary screen. The information obtained from the second ary screen is then used to prepare additional generations of catalysts that can be optimized into a superior catalyst worthy of commercialization.

Typically, the information necessary to classify new materials cannot be obtained from a single piece of characterization equipment. Therefore, a series of high-throughput screening tools is employed at various stages in the combinatorial process. While a large number of automated commercial systems exist for high-throughput analysis of microliter quantity samples for medical and pharmaceutical applications, the vast majority of the screening tools necessary for advanced materials research are custom-made instruments. Conventional analytical tools such as mass spectrometry, gas [196] and liquid chromatography, electrophoresis [197, 198], Raman [199] and nuclear magnetic resonance spectroscopy, X-ray fluorescence microprobe [182], and X-ray diffraction [200] have been automated and redesigned for rapid serial measurements of hundreds of samples per day [201]. A number of recent reviews of combinatorial catalysis discuss high-throughput screening techniques [20c,d,h, 151, 202-204]. Several examples of novel high-throughput screening systems are discussed below.

32.10.1

Infrared Screening Tools

Infrared detection has become an extremely popular technique for the analysis of combinatorial libraries because it is generally noncontact, nondestructive, and amenable to very high-throughput screening. Use of the infrared can be divided into two general categories: thermal imaging (thermography) and infrared spectros-copy.

32.10.1.1 Infrared Thermography

One of the first truly parallel high-throughput screens utilized infrared thermog-raphy to identify catalytic activity. Infrared radiation is emitted by all materials above 0 K according to the Stefan-Boltzmann Law, W = esT4, where W is the emitted power density, e the emissivity (e = 1 for a blackbody), s is the Stefan-Boltzmann constant, and T is the temperature. Modern infrared cameras utilize a photovoltaic focal plane array (FPA) detector made of InSb, HgCdTe, or PtSi to convert infrared radiation into digital images. It is important to note that the infrared cameras measure emitted or reflected intensity, not temperature, and a calibration must be performed to relate the intensity images to temperature.

Pawlicki and Schmitz first reported using infrared thermography to monitor the dynamics of reactions on solid surfaces in 1987 [205], and Sermon and coworkers applied this technology to the analysis of temperature profiles of exothermic reactions on silicon oxide-supported platinum catalysts [206]. Time-resolved infrared thermographic detection and infrared emission analysis of temperature profiles enable virtually any reaction to be monitored in a truly parallel fashion [63, 207].

As the field of combinatorial materials science began to heat up in the late 1990s, several groups applied infrared thermography to the search for new catalysts. Will-son and coworkers employed infrared thermographic imaging to identify possible

Fig. 32.23. Schematic of a catalytic reactor for infrared thermography.

formulations of heterogeneous catalysts for the oxidation of hydrogen to water [164]. The catalyst entities on the array were prepared in a conventional manner by impregnating g-aluminum oxide pellets as a catalyst support with aqueous stock solutions of 16 metal salt precursors. After reduction to zero-valency metals by exposure to pure hydrogen, the pellets were manually placed in an aluminum reactor and assayed in a parallel manner in a spatially addressable format. The reactor, shown schematically in Fig. 32.23, was equipped with an infrared imaging camera and devices to control reaction conditions such as gas flow, heating rates, and data collection/analysis. After calibrating the radiation intensity with respect to temperature and equilibrating the reactor at 35 °C with hydrogen, a gas stream containing 5 vol% of oxygen was introduced into the hydrogen feed stream. Hydrogen oxidation activity was found for pure Pd, Pt, and Ir within 10 s of oxygen introduction. Heating the reaction chamber up to 300 °C "ignited" the Rh-loaded pellet at approximately 82 °C and the activity was measured with the camera as an increase in catalyst temperature (relative to the background) while continuously heating the reaction chamber. Although basically a proof of concept, the experimental procedure proved useful for parallel screening of new catalyst formulations and evaluating operational issues such as catalyst lifetime, resistance to poisoning, and re-generability [208].

Direct thermal imaging of combinatorial libraries for activity is very useful as a qualitative or semiquantitative tool for determining whether catalytic activity is present in a particular material. True quantitative determination of the temperature profile for a given library of materials is complicated by the difference in emissivity between members of the library. The emissivity of an opaque material is related to the reflectivity (lower reflectivity means higher emissivity), which is dif ferent for most elements in a catalyst library. Furthermore, emissivity must be calibrated as a function of wavelength and temperature. The absolute reflectivity and emissivity characteristics of a new material are not known for most high-throughput experiments, which means that the proportionality factor used to calculate the temperature profile from the emitted infrared radiation is generally not available.

Emissivity can be accounted for (to some degree) by the use of linear corrections to the detector response, and subtraction of a reference image typically taken at the beginning when the experiment is cold. These corrections minimize emissivity effects for systems where the emissivity varies slowly with temperature, such as metals and metal alloys.

In 1998, Maier and coworkers applied emissivity-corrected infrared thermog-raphy to detect activity in heterogeneously catalyzed gas-phase reactions on a model library [165]. The researchers chose transition metal-impregnated, amorphous mi-croporous mixed oxide (AMM) supports, a class of materials previously shown to possess unusual properties as bulk catalysts or as catalytic membranes in selective oxidation, hydrocracking, hydrogenation, etherification, and esterification reactions [209-211]. The catalytic hydrogenation of hexyne and the oxidation of isooctane and toluene were chosen as their test reactions. With automated sol-gel procedures, less then 200 mg of each catalyst was deposited on a low-reflection slate substrate followed by controlled drying, calcination, and reduction to afford a catalyst array.

A high-sensitivity infrared camera with a platinum silicide (PtSi) FPA was used to monitor the heat evolution upon catalytic conversion. The same catalyst array was screened in a customized parallel reactor and on a catalyst microgram scale under appropriate conditions for three test reactions. Several effective catalysts for each of the targeted conversions were identified.

Further extensions of the technology came in 1998 when Reetz and coworkers stressed the general usefulness of infrared thermography to the time-resolved screening of liquid-phase catalytic reactions, such as enantioselective hydrolytic ring opening of epoxides to nonracemic diols and a lipase-catalyzed acetylation of a secondary alcohol [207a]. The experiment utilized a modified microtiter plate consisting of a commercial Eppendorf Thermomixer with the top replaced by an aluminum plate in which holes were drilled then filled with 8 x 32 mm glass vials. As a model reaction, Reetz chose the enantioselective lipase-catalyzed acylation of (R)-, (S)-, and rac-1-phenylethanol with vinylacetate. The reaction was followed with the infrared camera, periodically acquiring 250 images of the library, which were then averaged and visually inspected. To demonstrate the screening capability of the infrared camera under homogeneous conditions, the activity and selectivity of three metal catalysts were tested thermographically in the hydrolysis of epichlo-rhydrin. Finally, relative substrate activity was screened studying the hydrolysis of three different chiral epoxides with the cobalt catalyst that was found to be most active in the previous screening of epichlorhydrin hydrolysis. In all cases, relative trends in the activity and selectivity of the catalytic reactions were reproduced from the available literature.

The same group used IR thermographic screening for thermoneutral or endo-thermic transformations, in this case ring-closing olefin metathesis [207b]. Four different ruthenium-based olefin metathesis catalysts were screened for four different types of ring-closing metathesis reactions. Highest catalyst activity is identified by heat uptake from the surroundings, as monitored by the appearance of ''cold spots.'' The heat of vaporization of one of the reaction products (ethylene or propylene) plays a crucial role in this process.

Infrared thermography holds promise for more extensive application of this technology in homogeneous, heterogeneous, organic, and inorganic catalysis research. However, thermal imaging does not resolve the product composition of the catalyst, an important limitation in chemical catalysis.

32.10.1.2 High-throughput Infrared Spectroscopy

Spectroscopic techniques are extremely popular for the analysis of combinatorial libraries because of their speed, nondestructive nature, and relative ease of use. In addition, infrared spectroscopy is very useful for identifying structural properties of organic solid-phase-supported combinatorial libraries. Indeed, many commercial Fourier transform infrared (FTIR) spectrometers have been equipped with an automated attachment to allow multiple compounds to be screened in rapid succession.

Single-bead FTIR microspectroscopy has been utilized by chemists for quite some time to analyze spectral information of products on solid supports. Extension of this technique to combinatorial chemistry in the form of a scanned high-throughput screen has been reported by Jung and coworkers [212]. In Jung's method, polymer-bound resin beads modified through combinatorial synthesis were withdrawn from the reaction vessel and embedded in a KBr window. The KBr window was then placed on the automated x/ y stage of an infrared microscope and the spectra mapped. Data were presented as a map of the infrared absorption as a function of position across the KBr plate. Direct identification of resin-bound molecules is possible by superposition of maps taken at different absorption wave-numbers. Jung and coworkers mapped approximately 300 different resin beads at a rate of approximately 5 h per wavenumber map.

While extremely useful, scanning spectroscopic measurements have a number of limitations stemming from the relatively slow speed of the x/ y scanning methodology. As combinatorial libraries increase in size, the time necessary to screen libraries in a serial manner becomes cumbersome; the time being dependent on the number of scans averaged, the wavenumber resolution, and the number of elements in the library. Incorporation of the microscope and x/ y scanning stage into a chemical reaction chamber in which the pressure and temperature can be varied is also complicated and expensive. Finally, the time needed to scan from point to point virtually eliminates the ability to derive kinetic information from the measurement.

True parallel spectroscopic measurements rely on the ability of the analytical technique to simultaneously collect information from multiple samples. Parallel measurements offer the throughput and temporal resolution necessary to measure the kinetics of the large number of catalytic reactions possible in combinatorial

Fig. 32.24. Optical layout for an imaging FTIR spectrometer. The light source is composed of an infrared source (S), a KBr beam splitter (BS), a moving mirror (M1), and a stationary mirror (M2). The optical set-up is composed of a bandpass filter (F), a KBr diffuser (D), CaF2 plano-convex lens (L1), reactor (R), and CaF2 biconvex lens (L2). The infrared camera with Hg/Cd/Te detector array (FPA) acts as the detector.

Fig. 32.24. Optical layout for an imaging FTIR spectrometer. The light source is composed of an infrared source (S), a KBr beam splitter (BS), a moving mirror (M1), and a stationary mirror (M2). The optical set-up is composed of a bandpass filter (F), a KBr diffuser (D), CaF2 plano-convex lens (L1), reactor (R), and CaF2 biconvex lens (L2). The infrared camera with Hg/Cd/Te detector array (FPA) acts as the detector.

studies. Lauterbach and coworkers have constructed an imaging FTIR capable of collecting spatially resolved spectral data for 4096 samples in the 1360-2720 cm-1 spectral range (8 cm-1 resolution) in less than 20 s (Fig. 32.24) [213-215]. Depending on the optics used, the field of view can be varied from an area of a few hundred square microns to several square centimeters. The system, shown schematically in Fig. 32.24, consists of an FTIR spectrometer, infrared optics, and a 64 x 64 element HgCdTe (MCT) infrared camera. A detailed description of the instrument can be found in elsewhere [215].

Imaging FTIR spectroscopy was utilized by Lauterbach and coworkers to investigate the effects of adsorbed CO on both Cu-ZSM5 zeolite and silica-supported Pt/ SiO2 catalysts as a function of different process conditions [213a, 214]. CO was preadsorbed onto the catalysts and the temperature increased at a rate of 8 K min-1 in flowing oxygen. Spectral images collected during the heating ramp identified distinct spectral bands assigned to CO adsorbed on the Cu+ oxidation state (2157 cm-1) and to CO adsorbed on the Cu+ oxidation state with a water molecule in the coordination sphere of the Cu+ ion (2139 cm-1). As the temperature increased and the water desorbed, a decrease in the band at 2139 cm-1 was observed. The entire heating process took 11 min and 13 spectral images (4096 spectra per image) were acquired at a spectral resolution of 4 cm-1. Following a chemical process in a high-throughput manner demonstrates the ability of imaging techniques to monitor chemical reactions in situ. Imaging allows kinetic information and spectral data on reaction components to be gained from a combinatorial high-throughput screen.

The same group also applied the parallel FTIR analysis to determine conversion during temperature-programmed complete oxidation of propene in the presence of platinum group metals, a reaction that is important for the automotive three-way catalyst [213b].

32.10.2

Optical High-throughput Screening Techniques

The effort to create parallel assays for solution-phase catalysis has led to the development of a number of optical techniques. Many techniques include probe mole cules that change color or fluoresce with catalytic behavior. Additional techniques incorporate ultraviolet or infrared lasers to ionize or heat catalytic products, followed by detection with electrodes or probe lasers. Finally, circular dichroism has been developed to study the effects of chirality on catalysis.

32.10.2.1 Colorimetric Assays

Visual detection remains one of the simplest and most practical methods for measuring catalyst activity. While direct assays such as thermal imaging are fast and generic to most chemical processes, they lack chemical selectivity and are unable to identify reaction products. One approach, used extensively in enzymatic assays, is to use indirect detection of a molecular probe that fluoresces upon detection of a desired reaction product.

Crabtree and coworkers have developed reactive dyes that photobleach as a chemical reaction occurs [216, 217]. Designed for hydrosilation reactions, the probe must not have any interfering reactive groups, a criteria met by using a ferrocenyl group as an electron donor and a pyridinium group as an acceptor (Scheme 32.36); a benzylic tail was added to make the dye more soluble. When the reactive functionality (C=C or C=N bond) is saturated upon reaction with a catalytic species, the electronic overlap between the donor and acceptor groups is diminished, giving rise to a loss of the parent dye color. The dye color as a function of time was recorded, with a hit being indicated by the dye bleaching in a reaction well.

Scheme 32.36. Reactive dyes as a method for rapid screening of homogeneous catalysts.

Crabtree and coworkers assayed a 60-well discovery library of hydrosilation catalysts in parallel using a digital camera. Of the 12 catalysts examined by the authors, Wilkinson's catalyst, which is a known hydrosilation catalyst, was among the most active of the catalysts screened. However, a palladacyclic Heck reaction catalyst was also quite active, a compound not previously reported as a hydrosilation catalyst. A limitation of the screen is that the dye substrate is a nonstandard alkene with a higher reactivity than conventional substrates and has a strong tendency to give competitive hydrogenation instead of hydrosilation. Despite this, relative trends in activity seem to be well represented.

Hartwig and coworkers utilized resin beads tagged with fluorophors in the investigation of coupling reactions [218]. Their method allows for visual screening of

BPh4

BPh4

a large set of parallel chemical reactions in which two molecules are bound by co-valent interactions. In their study, one substrate (a) is attached to a dye molecule and the other (b) to a solid support. After a successful coupling reaction, the solid supports and the dye molecule would be bound together by a covalent interaction between the two substrates (Scheme 32.37). Analysis under UV illumination following filtering of the reagents allowed the substrate combinations that are capable of covalent coupling to be identified. In their investigation, an acrylate containing a tethered coumarin was reacted with an aryl halide supported on a crosslinked polystyrene bead. Comparison of the results from the fluorescence assay with results from a standard GC analysis showed that the fluorescence assay accurately represented the trends for the Heck coupling of aryl bromides and chlorides. Two ligands identified in the assay, tri-(tert-butyl)phosphine and di-(tert-butylphosphino)ferro-cene, were shown to be the most active systems for the olefination of unactivated aryl bromides, and di-(tert-butylphosphino)ferrocene the most efficient for olefina-tion of unactivated aryl chlorides.

Scheme 32.37. Visual assay for coupling reactions.

Hartwig and coworkers then focused on the development of solution-phase assays for homogeneous catalysis that are based on fluorescent resonance energy transfer (FRET) between a substrate with a tethered fluorophor and a second molecule that is attached to a solid support [219]. FRET occurs when two fluorophors within close proximity (20-80 A) interact such that the emission band of one molecule overlaps the excitation band of a second molecule. Excitation of the higher energy fluorophor (donor), followed by resonant energy transfer to the lower energy fluorophor (acceptor), leads to a quenching of the fluorescent intensity. With a constant total concentration of free and bound FRET pairs, the emission at the donor molecule wavelength is inversely proportional to the mole fraction of paired molecules. Standard plate readers can be used to track the covalent bond formation fluorometrically. In the investigation of Heck reactions, Hartwig and co-workers chose fluorophores that contained functionalities that are compatible with most cross-coupling chemistries. A dansyl fluorophore was tethered to a styrenyl group, and an azodye quencher tethered to an aryl bromide. Upon covalent linking, the danzyl group was quenched by the diazo compound (Scheme 32.38). The results of their investigation show that the FRET method is significantly faster than standard HPLC techniques, while only one in ten cases showed a yield by

HPLC that was more than 10% different from the FRET result. It was suggested that the FRET pair developed for this study should have application to many different reactions, such as aryl halide amination, aryl halide etherification, carbonyl a-arylation, Suzuki coupling, and Hiyama coupling with silanes.

Strong Fluorescence

Catalyst

Catalyst

Weak Fluorescence Scheme 32.38. Assay for homogeneous catalysis based on fluorescent resonance energy transfer (FRET).

Copeland and Miller developed a similar method for the study of acetic acid evolution that employs aminomethylanthracenes as pH-sensitive fluorophors. Neutral aminomethylanthracenes undergo photoinduced electron transfer (PET) and fluoresce when protonated (Fig. 32.25a) [220]. Attachment of the aminomethylan-thracenes to partially derivatized resin beads, followed by attachment of peptide catalysts, created a collection of resin beads related to the structure shown in Fig. 32.25b [221]. Beads functionalized with the most active catalysts appeared brightest as a given acylation reaction proceeded. Furthermore, the beads maintained their relative intensities when examined in separate vessels or pooled together as catalyst mixtures.

Copeland and Miller have applied the same aminomethylanthracenes as pH sensors in solution-phase catalyst libraries as well [220]. In this investigation, seven unique catalysts (Fig. 32.26) were deposited into a standard 96-well plate at three a)

No Fluorescence

Intense Fluorescence b)

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