EP1979089A1

EP1979089A1 - Process for the synthesis of product molecules

Info

Publication number: EP1979089A1
Application number: EP07703853A
Authority: EP
Inventors: Horst Weiss; Gustav Gerber; Patrick Henning NÜRNBERGER; Daniel Stephan Wolpert
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2006-01-16
Filing date: 2007-01-15
Publication date: 2008-10-15
Also published as: US20100173100A1; JP2009525165A; WO2007082861A1; CN101360555A; KR20080091448A

Abstract

The invention relates to a process for the synthesis of product molecules, in which ultrashort laser pulses which have been shaped by means of a pulse shaper are produced and a gas containing starting molecules is directed onto a surface on which the starting molecules are at least partly adsorbed, wherein the shaped ultrashort laser pulses are directed onto the surface in order to control a reaction for the synthesis of the product molecules from starting materials adsorbed on the surface.

Description

Process for the synthesis of product molecules

description

The invention relates to a process for the synthesis of product molecules, with which the course of chemical reactions can be controlled in a targeted manner.

Targeted control of chemical reactions on a molecular level is an old dream of chemistry. The conventional macroscopic control parameters of chemistry such as temperature, concentration or pressure do not allow direct access to the quantum-mechanical reaction process. In recent years, many new findings in the field of quantum control have been gained, using specially designed light fields, which can be used to efficiently and selectively select specific reaction channels. An experimental technique relies on the manipulation of femtosecond laser pulses by means of a pulse shaper. This technique is described, for example, in the review article by T. Brixner, G. Gerber: "Quantum Control of Gas-Phase and Liquid-Phase Femtochemistry", CHEMPHYSCHEM 2003, 4, 418-438. The starting point is a laser pulse of a few femtoseconds duration (usually 10 to 150 fs), which can be generated by a commercial laser system. By means of a so-called pulse shaper, this laser pulse is converted into a "shaped" laser pulse, wherein the shaping relates to the phases and / or amplitudes of the coherent spectral components, which need not be uniform, but can be modulated and are adapted to the reaction sequence to be optimized ,

Such methods have been used mainly successfully for dissociative reactions in the gas phase. The object of the present invention is to avoid the disadvantages of the prior art and in particular to enable a selective formation of molecular bonds with shaped ultrashort laser pulses.

This object is achieved by a method for the synthesis of product molecules, wherein ultrashort laser pulses formed with the aid of a pulse shaper are generated and a gas containing educt molecules is passed onto a surface on which the educt molecules are at least partially adsorbed, wherein the ultrashort formed Laser pulses are directed to the surface to control a reaction course for the synthesis of the product molecules from adsorbed on the surface of educts. The product molecules are molecules whose yield can be specifically influenced by the shaped laser pulses. They are caused by chemical reactions of educts on the surface under the influence of laser pulses.

Ultrashort laser pulses in connection with the method according to the invention are laser pulses whose duration (as short as possible according to the time-bandwidth product) is in the range of one attosecond to 1 nanosecond, preferably in the range of 1 femtosecond to 10 picoseconds, particularly preferably in the range of 5 to 200 femtoseconds. Such ultrashort laser pulses are provided in particular by commercial femtosecond lasers.

Synthesis in connection with the invention means the production of product molecules, in which new molecular bonds are formed and in which the educt molecules or the educts are not only broken down into their constituents. Furthermore, the synthesis of the product molecules in the present invention is preferably carried out by multimolecular reactions, in particular by bimolecular reactions.

According to the invention, a gas containing the educt molecules is passed to the surface on which they are at least partially adsorbed. To control a reaction course for the synthesis of the product molecules from educts adsorbed on the surface, the shaped ultrashort laser pulses are directed onto the surface. The educts adsorbed on the surface can thereby correspond to educt molecules from the gas conducted onto the surface or be smaller (dissociation process) or larger (aggregation) molecules or atoms resulting therefrom.

Preferably, the shaped ultrashort laser pulses for the process of the present invention are shaped so that the reaction yield of the product molecules is maximized. However, the shaped ultrashort laser pulses can also be shaped so that the reaction yield of a product deviating from the product molecules to be synthesized is minimized.

The process according to the invention for the synthesis of product molecules preferably proceeds by passing a continuous mass flow of gas with the educt molecules onto the surface and directing shaped ultrashort laser pulses onto the surface at a specific repetition rate.

The surface serves in the present invention for adsorbing the educts and therefore must have an affinity for the educt molecules in the gas. The surface should at least physisorb the educt molecules or educts and if necessary even chemisorb them. In physisorption, the adsorbed Eduktmo- remain molecules in their original chemical state. In the case of chemisorption, however, the adsorbates form a chemical bond with the surface. The adsorbate can dissociate on the surface. The use of the surface offers, inter alia, the advantage that a relatively high density of the educt substances is achieved. This results in a higher burst rate and higher yields. In the gas phase, the likelihood of multiple particles colliding would be far too low. Furthermore, the surface may act, for example, as a catalyst for the dissociation or aggregation of eu duktmolekülen or the special nature of the (eg metallic) carrier is exploited, as for example in M. Bonn et al.,: "Phonon-Versus Electron-Mediated Desorption and Oxidation of CO on Ru (OOOI) ", SCIENCE 1999, 285, 1042-1045.

The shaped ultrashort laser pulses interact in the method according to the invention with the surface and the adsorbed educts for the synthesis of the product molecules. According to a preferred embodiment of the present invention, in addition to its function as a sorbent, the surface also performs the function of a catalyst which catalyzes the synthesis of the product molecules.

According to a preferred embodiment of the present invention, the pulse former shapes spectral components of laser pulses in terms of intensity (amplitude) and phase. In this case, the unshaped laser pulses, which emits, for example, a commercial femtosecond laser and in which all the spectral components occur at the same time, are converted by the pulse shaper into shaped laser pulses, which can have variably adjustable components of the different spectral colors at different times. By the pulse shaper, the duration of a laser pulse is variable, provided that its spectral width allows the generation of a laser pulse in the desired time range. The relative color components (intensity) and their temporal arrangement (phase) in the shaped ultrashort laser pulse directly influence the yield in the synthesis of the product molecules from educts adsorbed on the surface.

According to an embodiment of the present invention, the pulse shaper forms spectral components of laser pulses with respect to polarization. Thus, another parameter for optimizing the laser pulse shape is given, by which the yield of product molecules can be increased. The pulse shaper controls the polarization state of light on an ultrashort time scale. Such a polarization pulse shaper preferably manipulates the transient intensity, the instantaneous frequency, the degree of ellipticity, and the orientation of the major elliptical axes in each individual ultrashort laser pulse. A preferred embodiment of the method according to the invention is that during the synthesis, an optimization of the shaped ultrashort laser pulses by measuring by means of a detector, which products with what yields in the synthesis of the product molecules, generating a modified pulse shape by means of a computer by an optimization algorithm and Driving the pulse shaper, so that the shaped ultrashort laser pulses are generated with the changed pulse shape takes place. This has the advantage that for the optimization of the product molecule synthesis no prior knowledge about the educt molecules / reactants, the surface or the course of the chemical reaction is required. The computer (for example a personal computer) takes over the control of the pulse shaper and can, for example, optimize the reaction yield. The iterative improvement of the laser pulse form proceeds as follows. Ultrashort laser pulses are directed to the surface to initiate a reaction to synthesize the product molecules. A detector is used to measure which products are produced at what yield. Any suitable detector known to the person skilled in the art and suitable for detecting the products can be used as the detector. Preferably, the detector is a detector based on at least one of the time-resolved detection methods selected from the group of time-of-flight mass spectroscopy, photoelectron spectroscopy, emission spectroscopy, and two-dimensional spectroscopy and four-wave mixing.

The computer processes the information determined by the detector on the yields of the various products. The optimization algorithm generates improved pulse shapes of the laser pulses, which in turn are generated by the pulse shaper and directed to the surface for synthesis of the product molecules. This optimization procedure is repeated until synthesis of the product molecules is optimized, for example, to a maximum of the desired product molecule yield (selectivity) relative to the yield of an undesired product or to a minimum of relative yield (selectivity) of such undesired product.

The optimization algorithm used is preferably an evolutionary algorithm. This computer algorithm is a self-learning process modeled on biological evolution. According to Darwin's principle "Survival of the fittest" (the best survived), laser pulses that particularly fulfill the optimization goal are selected and "propagated" by combination with similarly successful phase settings. Some of the "offspring" produced this way are better suited than their "ancestors" (they are given a higher "fitness"), and they are re-selected for reproduction. If this process of evolution is passed through for enough generations, then finally a laser pulse is found, which produces the optimal reaction result. According to a preferred embodiment of the present invention, the laser pulses are formed by a pulse shaper based on a computer-controlled technique which drives at least one device selected from the group consisting of liquid crystal display, acousto-optic modulator, acousto-optic filter, deformable mirror and Micromirror arrangement comprises. Forming a laser pulse by means of a pulse shaper in terms of spectral intensity and phase, and polarization usually based on the fact that the laser pulse is split (for example by means of a grating or prisms) in its spectral components, from which a parallel beam is generated, as a "Phase shifter" acting causes a transit time difference of the spectral components and the beam is again focused and (for example, by another grid) is superimposed on a shaped laser pulse. For example, at least one liquid crystal display (LCD), an acousto-optical modulator, a deformable mirror or a micromirror arrangement may serve as such a device acting as a "phase shifter," or else an acousto-optic filter, for which, however, no splitting of the spectral components is necessary ,

The manipulation of the pulse shape by means of a liquid crystal display is carried out by applying suitable electrical voltages to the individual LCD pixels, whereby the refractive indices can be chosen such that the different spectral components are delayed relative to one another as they propagate through the LCD.

The acousto-optic filter is based on another method, in which the light does not first have to be split into its spectral components. Rather, the light and an acoustic wave propagate through a special birefringent crystal. Each spectral component may be attached to the acoustic grating at a particular position in the crystal, e.g. be bent from a normal to an extraordinary ray. The resulting pulse therefore consists of the individual color components which have been diffracted at different times, whereby a phase modulation is possible.

According to a preferred embodiment of the present invention, the gas containing the educt molecules is passed onto a surface which is a metallic surface or a metal oxide surface and which preferably at least one element from one of the subgroups MIB to IIB (transition metals), more preferably from one of the groups VIIIB , IB and IIB of the Periodic Table of the Elements. Preferred elements of Groups VIIIB, IB and IIB of the Periodic Table of the Elements are Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, Au and Hg. In the for the inventive Surface treatment method, the metals may also be present as an alloy or according to another embodiment, which is the Gas containing educt molecules is passed onto a surface which is a single-crystal surface of a transition metal, for example Pd (100), Pt (100) or Ag (11 1). According to a further preferred embodiment of the present invention, the surface is a surface of a technical catalyst, for example a technical catalyst, which supports said elements supported or unsupported, pure, as alloys or as oxides.

The gas which is passed onto the surface in the method according to the invention contains one or more types of educt molecules. For example, according to the invention, product molecules can be synthesized by a polymerization reaction from only one kind of starting material molecules. Preferably, it contains a mixture of at least two types of educt molecules, wherein a ratio of mass flows of the at least two types of educt molecules can be regulated by a control device. The ratio of the mass flows, as well as the pulse shape of the shaped laser pulses, has an influence on the yield of the product molecules in carrying out the method according to the invention. A control device can therefore regulate the mass flows of the at least two types of educt molecules in such a way that a certain maximum yield of the product molecules is achieved.

Preferably, in the method according to the invention, a gas is passed to the surface, the at least one kind of Eduktmolekülen selected from the group acetone, acetylene, formic acid, ammonia, arsine, hydrocyanic acid, boron trichloride, boron trifluoride, bromothene, bromomethane, hydrogen bromide, 1.3-butadiene , Butane, 1-butene, 1-butyne, chlorine, chlorodifluoromethane, chloroethene, chloromethane, hydrogen chloride, cis-2-butene, deuterium, dichlorosilane, diethyl ether, difluoromethane, dimethylamine, dimethyl ether, 2,2-dimethylpropane, disilane, nitrous oxide, nitrous oxide , Ethane, ethene, ethylene oxide, fluoromethane, formaldehyde, hexafluoroethane, isobutane, isobutene, carbon dioxide, carbon monoxide, methane, methanol, methylamine, octafluorocyclobutane, octafluoropropane, phosphine, propane, propene, propotine, propylene oxide, oxygen, sulfur dioxide, sulfur felhexafluoride, hydrogen sulfide, silane, silicon tetrafluoride, nitrogen, nitrogen dioxide / dinitrogen tetroxide, nitric oxide, nitrogen trifluoro chloride, tetrafluoromethane, trans-2-butene, trifluoromethane, trimethylamine and hydrogen. The gas particularly preferably contains H ₂ and CO as starting material molecules. The process according to the invention can be used for the synthesis of, for example, formaldehyde or methanol.

According to one embodiment of the present invention, the gas containing the educt molecules is conducted onto the surface in a vacuum chamber. In the vacuum chamber there is preferably a vacuum <10 ^"3 mbar, in particular <10 ^{" 4} mbar, when the gas is supplied to the surface. The present invention is explained below with reference to the drawing.

It shows

Figure 1 shows schematically an arrangement for carrying out the method according to the invention for the synthesis of product molecules and

FIG. 2 shows mass spectra for the following Example 1, in which a gas containing various concentrations of CO and H _{2 was conducted} onto a Pd (100) monocrystal surface and product molecules were synthesized by means of shaped ultrashort laser pulses.

Figure 1 shows schematically an arrangement for carrying out the method according to the invention for the synthesis of product molecules from a gas containing CO and H ₂ on a surface by means of shaped ultrashort laser pulses.

A laser 1 supplies laser pulses 2 whose spectral width enables the generation of laser pulses in the femtosecond range. The laser pulses 2 are optionally amplified (not shown) and strike as unformed laser pulses 3 on a first grid 4, which decomposes the unshaped laser pulses into their spectral components, which meet as divergent beams 6 on a first lens 5. The first lens 5 focuses the divergent beams 6 into a parallel beam 7. A liquid crystal display (LCD) 8 causes a transit time difference of the various spectral components. A second lens 9 focuses the emerging from the liquid crystal display 8 beam 10, which is superimposed by a second grating 1 1 to laser pulses 12. The pulse shaper 14 of this arrangement thus comprises the two gratings 4, 11, the two lenses 5, 9 and the liquid crystal display 8. The ultrashort laser pulses 13 thus formed are directed onto a surface 15 optionally located in a vacuum chamber (not shown) ,

From a CO reservoir 16 and a H ₂ reservoir 17, the educt molecules CO and H _{2 are} mixed in a certain ratio of mass flows. The mixture 18 is passed to the surface 15 via a nozzle 19 and a skimmer 20. From the impact on the surface 15 gas 21 reactants are adsorbed on the surface 15. The shaped laser pulse 13 striking the surface 15 provides the energy to synthesize product molecules 22 from the adsorbed educts. The ionized product molecules 22 are accelerated into a time-of-flight mass spectrometer 23. The ions 27 generate a signal in a detector 24 after times of flight by the time of flight mass spectrometer 23, which depend directly on their mass. These measured signals 25 are transmitted to a computer 28 which is used to optimize the Pulse shape of the shaped laser pulses 13 by means of an optimization algorithm generates a modified pulse shape. The computer 28 then controls the liquid crystal display 8 via the connection 26, so that the pulse shaper 14 forms laser pulses with the generated changed pulse shape, which are used for the synthesis of product molecules on the surface 15.

example 1

A construction as shown in FIG. 1 was used. Laser pulses were amplified by means of a titanium sapphire regenerative femtosecond amplifier, the amplified unshaped laser pulses having a pulse duration of 80 fs, pulse energies of up to 1 mJ at a central wavelength of 800 nm and a repetition rate of 1 kHz. A 128-pixel LCD pulse shaper was used to alter the spectral phase of the laser pulses while leaving their spectrum unchanged. The laser beam (shaped laser pulses) was passed through a lens with a focal length of 40 cm into a main vacuum chamber (pressure without gas flow about 10 ^"6 mbar) onto a Pd (100) single crystal surface (temperature of the crystal 290 K) at an angle of approx The intensity of the laser beam was about 10 ¹² W / cm ^2. The beam was reflected by the surface and left the main vacuum chamber.

The two gases H ₂ and CO were obtained with a purity of at least 99.999% (from Messer-Griesheim, Germany) or at least 99.997% (from Tyczka, Germany). Two mass flow controllers (from Advanced Energy, Germany) dosed the gas volumes that were fed into the system via two gas lines. The two gas lines were combined in front of a nozzle and the gas mixture passed through the nozzle in a secondary vacuum chamber, where it met with a skimmer and was passed from there as a gas stream in the main vacuum chamber. The Pd (10 O) single crystal (diameter 10 mm, thickness 1 mm, from Mateck, Germany) was oriented at an angle of about 5 ° to the molecular stream, so that the gas not only grazed it, but hit it.

Perpendicular to the gas stream and nearly parallel to the surface normal was a time-of-flight mass spectrometer, with an electrode system for accelerating ions formed as the laser beam interacted with the surface and the adsorbed educts.

First results of the method according to the invention are shown in FIG. The ordinate indicates the intensity I of an ion signal measured by means of a detector at the end of the time-of-flight mass spectrometer and the abscissa the time of flight t in μs applied. The results of 6 measurements A to F are shown, in which the respective mass flow of the two CO and H ₂ -enduct molecule flows was selected as follows:

where sccm is standard ccm / min. stands.

In measurement A, only H ₂ and no CO were passed to the surface. No signals can be seen in the time window shown, but three large mass peaks were observed which could be assigned to H ⁺ , H ₂ ⁺ and H ₃ ⁺ .

In measurement B, only CO and no H _{2 were passed} to the surface. The three detected mass peaks are at masses 12, 16 and 28 amu, which can be assigned to C ⁺ , O ⁺ and CO ⁺ .

For the measurements C to F both Eduktmolekülsorten CO and H _{2 of} the surface were supplied as a gas mixture and depending on the ratio of the mass flows, the ions C ⁺ , CH ⁺ , CH ₂ ⁺ , CH ₃ ⁺ , O ⁺ , OH ⁺ , H ₂ O ⁺ , H ₃ O ⁺ , CO ⁺ , HCO ⁺ and H ₂ CO ⁺ detected in different intensities. The water peak (H ₂ O ⁺ ) increases, for example, with increasing H ₂ mass flow, whereby the O ⁺ peak becomes smaller. By optimizing the laser pulse shapes, the yield of certain product molecule forms could be improved (not shown). The demonstration of, for example, CH ₃ ⁺ shows that 3 particles must have met each other and interacted with the surface and the laser pulse. Consequently, a synthesis of product molecules has been successfully achieved by means of the process according to the invention.

Example 2

An optimization of the pulse shape of the shaped ultrashort laser pulses was carried out by means of an evolutionary algorithm. For this purpose, a 1: 1 mixture of the two gases CO and H _{2 was passed} onto the Pd surface. It should be investigated whether it is possible with the help of the evolutionary algorithm to find a pulse shape that influences the formation of C-H bonds. The goal of the optimization experiment was to maximize the formation of CH ⁺ in relation to C ⁺ . A pulse shape of the shaped laser pulses was found by means of the evolutionary algorithm, which achieved an increase in the relative yield of about 50% compared to an unshaped femtosecond laser pulse. At the same time, the CH ₂ ⁺ / C ⁺ ratio was increased by about the same value. A very surprising effect was also the conspicuous reduction of H ₂ O ⁺ formation with the optimized shaped femtosecond laser pulse. By varying the laser intensity, however, no reduction of the H ₂ O ⁺ signal could be achieved relative to the other peaks. From this experimental data, it follows that the determined optimally shaped ultrashort laser pulse causes an increase in the signal of products with CH bonds, whereas the signal of H ₂ O ^{+ is} reduced. The yield of CO ⁺ , HCO ⁺ and H ₂ CO ⁺ relative to C ⁺ was also increased.

Example 3

Under comparable conditions as in Example 2, the H ₂ O ⁺ yield was integrated into the fitness function used by the evolutionary algorithm. The optimization experiment was carried out with the chosen goal of maximizing the ratio CH ⁺ / H ₂ O ⁺ . By optimizing the laser pulse shape using the evolutionary algorithm, the H ₂ O ⁺ peak could be reduced by about 50% relative to the C ⁺ peak. Before optimization, the H ₂ O ⁺ peak was larger than the CH ⁺ peak. This ratio could be reversed by the optimized shaped laser pulse. The CO ⁺ and HCO ⁺ peak intensities were increased whereas CH ⁺ and CH ₂ ⁺ did not change much relative to C ⁺ . In this experiment as well, it was not possible to reduce the H ₂ O ⁺ signal relative to the other peaks by changing the intensity of an unshaped laser pulse.

The results described in these examples clearly demonstrate that an evolutionary algorithm for optimizing the pulse shape of shaped ultrashort laser pulses can be used to control two competing reaction channels in which molecular bonds are formed rather than simply resolved.

Thus, the catalytic synthesis of product molecules can be selectively controlled by the method of the invention. LIST OF REFERENCES

1 laser

2 laser pulses

3 unshaped laser pulses

4 first grid

5 first lens

6 divergent rays

7 parallel beam

8 liquid crystal display

9 second lens

10 beams

1 1 second grid

12 laser pulses

13 shaped laser pulses

14 pulse shaper

15 surface

16 CO reservoir

17 H ₂ reservoir

18 mixture

19 nozzle

20 skimmers

21 gas

22 product molecules

23 time-of-flight mass spectrometer

24 detector

25 signals

26 connection

27 ions

28 computers

Claims

claims

1. A process for the synthesis of product molecules, characterized in that formed by means of a pulse shaper formed ultrashort laser pulses and a gas containing educt molecules is passed to a surface on which the

Eduktmoleküle be at least partially adsorbed, wherein the shaped ultrashort laser pulses are directed to the surface to control a reaction course for the synthesis of the product molecules of adsorbed on the surface of educts.

2. The method according to claim 1, characterized in that the pulse shaper forms spectral components of laser pulses with respect to intensity and phase.

3. The method according to any one of claims 1 or 2, characterized in that the pulse shaper forms spectral components of laser pulses with respect to polarization.

4. The method according to any one of claims 1 to 3, characterized in that during the synthesis, an optimization of the shaped ultrashort laser pulses takes place by measuring by means of a detector, which products with which yields in the synthesis of the product molecules, generating a modified pulse shape means a computer by an optimization algorithm and driving the pulse shaper, so that the shaped ultrashort laser pulses are generated with the changed pulse shape.

5. The method according to any one of claims 1 to 4, characterized in that the laser pulses are formed by a pulse shaper, which is based on a computer-controlled technique, the driving of at least one device selected from the group of liquid crystal display, acousto-optic modulator, acousto optical filter, deformable mirror and micromirror arrangement.

6. The method according to any one of claims 1 to 5, characterized in that the gas is passed onto a surface which is a single crystal surface of a transition metal.

7. The method according to any one of claims 1 to 6, characterized in that a gas is passed to the surface containing one or more types of educt molecules.

8. The method according to any one of claims 1 to 7, characterized in that a gas is passed to the surface containing a mixture of at least two types of reactant molecules, wherein a ratio of mass flows of the at least two types of reactant molecules by a control device regulated is valid.

9. The method according to any one of claims 1 to 8, characterized in that a gas is passed to the surface, the at least one kind of Eduktmolekülen selected from the group acetone, acetylene, formic acid, ammonia, arsine, hydrocyanic acid, boron trichloride, boron trifluoride , Bromothene, bromomethane,

Hydrogen bromide, 1,3-butadiene, butane, 1-butene, 1-butyne, chlorine, chlorodifluoromethane, chloroethene, chloromethane, hydrogen chloride, cis-2-butene, deuterium, dichlorosilane, diethyl ether, difluoromethane, dimethylamine, dimethyl ether, 2,2- Dimethylpropane, disilane, dinitrogen monoxide, nitric oxide, ethane, ethene, ethylene oxide, fluoromethane, formaldehyde, hexafluoroethane, isobutane, isobutene, carbon dioxide, carbon monoxide, methane, methanol, methylamine, octafluorocyclobutane, octafluoropropane, phosphine, propane, propene, propotin, Propylene oxide, oxygen, sulfur dioxide, sulfur hexafluoride, hydrogen sulfide, silane, silicon tetrafluoride, nitrogen, nitrogen dioxide / dinitrogen tetroxide, nitric oxide, nitrogen trifluoride, tetrafluoromethane, trans-2-butene, trifluoromethane, trimethylamine and hydrogen.

10. The method according to any one of claims 1 to 9, characterized in that the gas is passed in a vacuum chamber on the surface.