TAP-2 reactor

Transient experiments can provide more information than steady-state experiments and are particularly useful in unraveling complex reaction networks [52]. Wagner and Hauffe [53] introduced the concept of transient response methods in the 1930s. In the 1960s, the first fundamental transport kinetics model for a pulse reactor was developed [54]. Since then, a large number of groups have made contributions to theoretical and experimental developments of transient methods, available in different review papers [52,55-64]. 

TAP experiments are performed under vacuum conditions with very small pulsed quantities of reactants. The time resolution of the TAP experiments is on the order of submilliseconds, about two orders of magnitude better than flow experiments. Real catalyst samples can be used and this is what sets it apart from traditional surface science experiments. In the TAP-2 reactor, narrow gas pulses of reactants are introduced in a microreactor (typically 25.4 mm in length and 4 mm in diameter, but different reactors have been developed), which is evacuated continuously. The responses of these pulses as a function of time are detected by a quadrupole mass spectrometer located directly at the reactor outlet The shape of the response reflects diffusion, adsorption, desorption, and reaction. Details of the TAP setup can be found in Ref. [25]. A large number of different reactions have been studied in the TAP reactor [65], but only a few involving perovsldtes as catalysts [66,67]. Numerous studies deal with the modeling of TAP experiments [27,68-71]. 

The TAP reactor is very well suited for kinetic studies. At low pressure, all transport of gas-phase species is by (Knudsen) diffusion, thus ruling out any external mass transfer limitations. The diffusion as a random movement also eliminates all radial concentration gradients. Very low amounts of reactants are pulsed into the reactor, which are on the order of a few nanomoles. Thus, the amount of heat generated is very small even in the case of strongly exo- or endothermic reactions. Therefore, the reactor is operated isothermally and no heat transfer limitations occur. Concentration profiles inside the pores for transient experiments might arise even in the absence of chemical reaction. If significant diffusion of reactants and products inside the catalyst pores occurs, it will be revealed by the transient response and then needs to be addressed correctly by a modeling approach. This is often the case for microporous materials [26,27,72]. 

Case Study TAP Experiments for Ammonia Oxidation over LaCoOs 

Pulses of N2O over LaCoOs were rapidly converted into nitrogen. No significant amount of oxygen was detected. The decomposition of NO over LaCoOs is slower than N2O. Conversions of only 2% were observed for NO pulses over LaCoOs compared with 96% for N2O. 

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The two BCs of the TAP reactor model (1) the reactor inlet BC of the idealization of the pulse input to tiie delta function and (2) the assumption of an infinitely large pumping speed at the reactor outlet BC, are discussed. Gleaves et al. [1] first gave a TAP reactor model for extracting rate parameters, which was extended by Zou et al. [6] and Constales et al. [7]. The reactor equation used here is an equivalent form fi om Wang et al. [8] that is written to be also applicable to reactors with a variable cross-sectional area and diffusivity. The reactor model is based on Knudsen flow in a tube, and the reactor equation is the diffusion equation ... 

Curve fitting using a delta function for the pulse input for a TAP reactor should be limited to the latter % part of the response curve for curves of FWHM < 3 times pulse width, while for curves with FWHM > 4 times pulse width, it is a fair approximation fijr most of the curve. The assumption of a zero concentration at the reactor outlet is not good evrai for a pumping speed of 1,500 Is and broad response curves with FWHM > 1000 ms. 

In summary, the results from the fixed bed reactor study provided evidence as to the effect of Au and KOAc on the performance of the catalyst, though, these experiments did not give any information on the perturbation of the reaction pathways with the addition of Au and KOAc. For this type of information, additional experiments were performed using the TAP reactor with 1,2 C-labeled ethylene used as an isotopic tracer of the kinetics. 

Figure 1. Key elements of the TAP reactor (A) and high pressure fixed bed reactor (B) experimental systems. The TAP reactor schematic shows the heated valve manifold and reactor with the elevated pressure attachment located in the main high vacuum chamber. The fixed bed reactor shows the feed system, liquid vaporizer, oxygen disperser, reactor, and waste recovery system.

Figure 2. Reactant product response curves for a pump-probe pulse sequence in the TAP reactor.

One of the challenges to correctly interpreting the TAP reactor data in this particular plication is the discrimination between the mass spectra data for the various components. The assignments used here for the results included in this paper are listed in Table 2. 

The first set of results from the TAP reactor, as shown in Figure 3, shows the 1,2 C vinyl acetate (MW=88) response curve for the four catalyst samples. The transient response suggests that KOAc dramatically accelerated the desorption of the vinyl acetate off the surface of the catalyst (peak maximum at 7.5 seconds without KOAc, 5.5 seconds with KOAc on average). In addition, Au enhanced the desorption rate of the vinyl acetate, but to a much lesser extent. It is also seen that Au improved the production rate of the catalyst (peak areas Pd-Au > Pd and Pd-Au w/KOAc > Pd w/KOAc). 

The total amount of vinyl acetate produced per pulse agreed with the VAM STY data obtained from the fixed bed reactor with the exception that the Pd-Au catalyst produced more vinyl acetate in the TAP reactor than the Pd-Au w/KOAc catalyst. This can be attributed to the Pd-Au catalyst s VAM production rate being limited by the desoiption of VAM when operated at elevated pressures and with a... 

Bowker, 1993]. This is supported by the TAP reactor experiments since the maximum of the CO response curve occurs between 5.5-6.0 seconds, while the CO2 evolution occurs earlier between 4.5-5.0 seconds. It is also seen that both KOAc and Au enhance the formation of CO. This agrees with the earlier observations with the CO response curves, and supports the hypothesis that the secondary COj peak occurs from the conversion of CO to CO. ... 

The effect of the catalyst composition upon the catalyst activity, selectivity, and reaction pathways was examined using a conventional high pressure fixed reactor and a TAP reactor. Particular emphasis was placed upon the effect of Au and KOAc on the acceleration or impedance of the pathways associated with vinyl acetate synthesis. A summary of the key findings is given below ... 

The authors would like to acknowledge the contributions of several individuals for their insight and hard work in achieving the data included in this report J. Scott McCracken (TAP reactor), Kevin S. Slusser (fixed bed reactor), and Tom Borecki (catalysis synthesis). The authors would also like to thank DuPont s vinyl acetate business and manufacturing teams for allowing this work to be published. 

Transient reactors, such as pulse (chromatographic) reactors, temporary analysis of products (TAP) reactors, multitrack reactors, and temperature-programmed reactors have been developed mainly to study gas-solid (catalyst) reactions. These are rather sophisticated techniques used to study mechanisms of catalytic processes at the molecular level in great detail. Since this is rarely done in the development of processes for the manufacture of fine chemicals and pharmaceuticals, these reactors are not discussed further. The interested reader is referred to works by Anderson and Pratt (1985) and Kapteijn and Moulijn (1997). 

Temporal analysis of products (TAP) reactor systems enable fast transient experiments in the millisecond time regime and include mass spectrometer sampling ability. In a typical TAP experiment, sharp pulses shorter than 2 milliseconds, e.g. a Dirac Pulse, are used to study reactions of a catalyst in its working state and elucidate information on surface reactions. The TAP set-up uses quadrupole mass spectrometers without a separation capillary to provide fast quantitative analysis of the effluent. TAP experiments are considered the link between high vacuum molecular beam investigations and atmospheric pressure packed bed kinetic studies. The TAP reactor was developed by John T. Gleaves and co-workers at Monsanto in the mid 1980 s. The first version had the entire system under vacuum conditions and a schematic is shown in Fig. 3. The first review of TAP reactors systems was published in 1988. 

Figure 1. Schematic of modified TAP reactor system with movable high pressure assembly disengaged for vacuum operation.

Micro reactors operated in the pulsed mode were introduced by Kokes et al. in 1955 [91], but have been intensively used only in the last 10 years. Such transient studies to obtain insight into reaction mechanisms were undertaken by Cleaves et al. with the temporal analysis of products (TAP) reactor 1997 [100], They observed rate coefficients of elementary reaction steps such as adsorption and desorption by applying pulses of reactants to a catalytic micro reactor combined with a quadrupole mass spectrometer. 

There have been two recent kinetic studies of the dismutation behaviour of CHC12F, CHC1F2 and other members of the CHC13 F series, in one case over activated y-alumina under conventional flow conditions [105] in the other, over activated chromia using plug-flow and temporal analysis of products (TAP) reactors... 

A consecutive reaction mechanism was also proposed by Cleaves and Centi [61]. This was based on experimental work to back up the theoretical calculations of Schiott and Jorgensen. Although the proposed intermediates are not detected under reaction conditions they have been seen with fuel-rich gas feeds and under temporal conditions. Using a TAP reactor, the products are detected in the order butane —> butene butadiene furan. However, these conditions differ signifi-... 

This may, in part, be due to this reaction being the only industrial large scale functionalization of an alkane currently in operation, and knowledge gleaned from vanadium phosphate systems can provide valuable information that can be applied to different alkane activation reactions. To this end a number of the publications have focused and continue to focus on a fundamental understanding of the catalyst active surface and the active site. In recent years, transient techniques such as the use of TAP reactors and in situ characterization studies have advanced the under-... 

Butene, cis/trans-2-butenes, butadiene and furan have been detected in the oxidation of n-butane on the VPO catalysts under very unusual conditions, such as under oxygen deficiency at high n-butane concentration and very short contact times [9], or in high vacuum in a temporal analysis of products (TAP) reactor [70]. 

Macroscopic Methods Sorption Rate (12) IR spectroscopy (16, 17) Frequency response (20) Chromatography (12) ZLC (13) Differential adsorption bed (21) TAP reactor (22) Imaging (23) Membrane permeation (14,15) Effectiveness factor (18, 19) Tracer methods (12)... 

Keipert, O.P. Baers, M., Determination of the intraciystalline diffusion coefficients of alkanes in H-ZSM-5 zeolite by a transient technique using the temporal-analysis-of-products (TAP) reactor. Chem. Eng. Sci. S3 (1998) pp. 3623-3634. 

Fig. 2. Response of the TAP reactor to an inlet pulse of a gas that is irreversibly adsorbed (or reacted) with a dimensionless rate constant k. Tp is the dimensionless time, and F p is the dimensionless flow rate. The model takes into account the number of molecules in the pulse A p A, the effective Knudsen diffusion coefficient DeA, the number of surface sites, and the dimensions of the reactor (after 55). A, = 0, standard diffusion curve B, = 3 C, U = 10.

Although the TAP reactor equations can be solved analytically for some simple systems, more complicated reation networks require numerical solutions. Cleaves et al (35) show that useful relations can be found between quantities such as conversion and residence time and the moments of the response of the TAP reactor, analytically obtained from the solutions of the linear system described previously. 

For the TAP reactor, these same effects of pressure also exist so that it is not possible to extrapolate the kinetics of certain reactions at low pressure to conditions that would exist at high pressure. In the TAP-2 system (36) there is a second reactor that can operate at higher pressure, but of course the essence of the TAP reactor is that it makes it possible to deal with catalyst particles, not well-defined surfaces, and at low pressure so as to increase the possibility of measuring fast kinetics. Operated at atmospheric or higher pressures, the results available from the TAP reactor are not fundamentally different from those produced by other transient methods already discussed. 

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