Flow reactor mass spectrometry

Ion-trapping and flow reactor mass spectrometry permit gas-phase ions and neutral molecules to be confined and attain equilibrium after a sufficient number of collisions. The equilibrium constant can then be deduced by measurement of gas partial pressures (for Bj and B2) and mass spectrometric ion intensities (for gaseous BiH and B2H ). The resulting AG provides the difference in gas-phase basicity between Bj and B2. If the absolute gas-phase basicity of either base is known, then the GB value of the other can be determined. Proton affinities are subsequently determined via AG = AH — TAS, with the entropy of basicity approximated via quantum chemical approaches. Table 6.6 lists proton affinity and gas-phase basicities for nitrogen bases, with the bases ranked in order of descending gas-phase basicity. The majority of organic bases exhibit GB values between 700 and 1000 kJ/mol. Compilations of PA and GB values are available. " ... 

Over the last 50 years, all three basic elements of the flow-tube mass spectrometry, the ion source, reactor, and detector, have been continnously modified and improved, giving rise to a variety of useful new instmments (see Chaps. 4,5,6 and 8). 

Most ion-molecule techniques study reactivity at pressures below 1000 Pa however, several techniques now exist for studying reactions above this pressure range. These include time-resolved, atmospheric-pressure, mass spectrometry optical spectroscopy in a pulsed discharge ion-mobility spectrometry [108] and the turbulent flow reactor [109]. 

The composition and reactivity of the carbon laid down during the initial stages of the propane dehydrogenation reaction was examined by transient isotope labelling experiments using [2-]3C]-C3HgandC3JHs as tracers in a series of reactions in a pulsed flow microcatalytic reactor. In these experiments alternate series of labelled and unlabelled propane pulses were passed over the catalyst sample and the products analysed by glc and mass spectrometry. 

Analysis obtained by mass spectrometry. The above sample of pyrolysis product was obtained at 835°F, 300 psig, in a continuous-flow coil reactor, residence time 45 min. 

Kinetics can also be studied at surface science conditions. Feed can be leaked at a constant rate into the chamber containing the crystal face, and the gas is removed at a constant rate by the pumps. The composition of the chamber gas can be continuously monitored by mass spectrometry. The pressure in the reaction chamber is low enough to ensure Knudsen flow The gaseous molecules collide almost exclusively with the exposed solid surfaces, and the system behaves as a perfectly mixed flow reactor (CSTR). Experiments in the transient regime with various forcing functions can be performed, and response times can be orders of magnitude smaller than those at atmospheric pressure. The catalytic oxidation of CO on Pt(llO) was one of the first studies of this type (33). 

The common chemical conversion techniques use a flow reactor in which ambient OH reacts with isotopically labeled SO2 or CO to yield observable products (e.g., Felton et al. (1990) used " CO with radioactive " C02 detection, while Eisele and Tanner (1991) used " S02 with H2 " S04 detection by ion-assisted mass spectrometry). 

We have shown that [Ni2+]-OMS-2 and [N 2+]-OMS-1 catalyze the selective conversion of hexane to 1-hexene. Stainless steel flow reactors of 1/4 diameter containing 0.5 g catalyst, charges of 7 g n-hexane in 2 h, 1 atm pressure and temperatures of 500°C are used in these experiments. Both gas chromatography (GC) and mass spectrometry (MS) analyses are done to monitor product distributions. Conversions as high as 60% and selectivities of 90% (to the terminal olefin) have been observed for the OMS-2 system. This may be a consequence of the better shape selectivity of [Ni2+]-OMS-2 (4.6 A tunnel) versus (Ni2+]-OMS-1 (6.9 A). The latter material is not as selective or active. Systems that do not contain N 2+ are totally inactive.91 There is precedence for dehydrogenation activity of these systems since manganese nodules have been reported to be excellent catalysts for dehydrogenation of cyclohexane.63... 

The effect has been studied in a small-scale pilot plant (see detail of the plant in reference [1]). This unit has a 60 cm down-flow fixed bed reactor that operates isothermally. The hydrogen and the hydrocarbon feed were preheated before entering the reactor. After reaction, the liquid product (C5+) was fractionated and analyzed using conventional ASTM method. In addition, a Mass Spectrometry coupled with gas chromatograph (GC MS) was used to measure aromatics, paraffins and naphthenics compounds distribution in the feed and in the products. In addition, a special NMR analysis was performed to determine the PNA. The VGO was desulfurized using commercial catalyst (not described here) and the product characteristics are shown in Table 4 as well as the feed. The MHCK catalysts were tested at 380 and 400 °C, LHSV=0.75 and 100 bar of total pressure, using 800 mVm of H/HC ratio at the inlet of the reactor. The... 

Mass spectrometry. Thermal treatments of the intercalated solids from room temperature to 1000 °C were carried out in quartz reactors under helium, with a flow rate of 3 cc/min. A Hidden Analytical HPR 20 Mass spectrometer (MS) was used for the analysis of the gases evolved upon thermal treatment, in the mass range 1-200 a.m.u. A capillary leak maintained at 170°C was used to divert a fraction of the gas flow to the analysis chamber. 

Although these are the most important considerations in designing a parallel reactor module, another important point is the analysis. While in principle each analytical instrument is suitable and can be connected to the exit of such a multiple-pass reactor, one must ensure that the instrument works with relatively small amounts of sample, as well as low flow rates. For the model reaction under investigation in our reactor, the CO-oxidation, non dispersive IR is used. As C02 concentrations are relatively high under our conditions, the analysis chamber can be kept short, and purge times are therefore also short. Analysis times are around 4 minutes, this being determined mainly by the purge times of the tubes and the sample chamber. However, any analytical techniques, such as mass spectrometry, GC, etc., can in principle be used in connection with this set-up. 

We used a laboratory-made flow reactor. A gaseous mixture (1-10% CO, 1-2% CH4 and/or CsHg, 0.44% NO, 3-16% O2, N2 up to 100%) was allowed to pass through (at 100-750 C) a tube reactor containing the catalysts (flow rate 6-10 -10010 h ). The content of carbon monoxide and hydrocarbons was determined by chromatography using molecular sieve (5 A) columns and aluminum oxide (helium, carrier gas katharometer). Unreacted nitrogen oxide was determined by mass spectrometry. 

Ring closure of nascent CH2=CHCH2CH2CHf within ion-neutral complexes has been studied using a specially designed Electron Bombardment Flow (EBFlow) reactor, schematically drawn in Figure 3. This apparatus has the advantage that the conditions under which ions are formed and react (70 eV electron impact pressure <0.001 mbar) closely parallel those in mass spectrometer sources. The neutral product yields are routinely interpreted with reference to the ionic products observed by the mass spectrometry. Hypotheses based on EBFlow results for ion neutral complexes are further tested by comparison with mass spectrometry. 

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