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Somebody I know insists that the use of a catalyst increases the atom economy. They did chemistry at school and were told that a catalyst increases the atom economy. He pointed me to several past exams where this was written as a valid answer.

However, I am convinced that this is false. After all, atom economy is:

$$\text{atom economy} = \frac{\text{molecular mass of desired product}}{\text{molecular mass of all reactants}} \times 100\%$$

I feel this has nothing to do with a catalyst.

Am I missing something?

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There are a couple of ways to think about this. First, considering your atom economy equation. Typically a catalyst is used in place of a stoichiometric reagent. A catalyst is (strictly speaking) not a reagent, so its effect on the atom economy is neutral. The alternative stoichiometric reagent is a reactant, so it will reduce the atom economy. This leads to the conclusion that a catalytic method offers a better atom economy than an alternative, non-catalytic method.

From a slightly different point of view, atom economy is one aspect of measuring the amount of waste that comes out of a reaction. A more relevant quantity is really the ratio of the mass of the desired product(s) to the total mass of all of the reaction inputs (including reactants, catalysts, solvents, purification materials, etc). Since catalysts can be used in smaller amounts than stoichiometric reagents, there will physically be less mass put into the process, which increases that ratio.

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It's a bit of a blanket assumption as processes exist where the catalyst is over-stoichiometric. This would knock the atom economy argument on the head. Also one could argue that the catalytic reaction is no different as without a catalyst, the reaction still occurs albeit more slowly, all other things being equal. It's a completely reaction specific consideration. Technically the catalyst mass shouldn't be included in the calculation. Atom economy is a bit of a misleading measure, mass intensity is better.

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using catalysts mean lower temperatures and pressures can be used. so energy saved, meaning less co2 is released, and fossil fuel reserves are preserved. catalysts can also reduce waste by allowing a different reaction to be used with a better atom economy.

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    $\begingroup$ Huh, this is a bit going far. $\endgroup$
    – M.A.R.
    Feb 23, 2016 at 11:00
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    $\begingroup$ Most of this is irrelevant to the question. $\endgroup$
    – bon
    Feb 23, 2016 at 12:35
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    $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. For more information in general have a look at the help center. At the moment this reads more like a comment than an actual answer - could you elaborate a little more. With a bit more rep, you will be able to post comments on any question/answer. $\endgroup$
    – Martin - マーチン ♦
    Feb 23, 2016 at 13:06
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Catalysts do have an effect on the atom economy, since catalysts have act sides, they eliminate side products( waste), and they are not considered in the equation for atom economy, thus they will increase the atom economy.

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    $\begingroup$ That's wrong on so many levels. A catalyst might reduce side reactions, but he doesn't have to. It might even give you the possibility to run a reaction with worse atom economy (but cheaper or faster than alternatives). I don't know what "act sides" should be and google doesn't find anything either. And strictly speaking if you don't recover the catalyst it is lost and decreases atom economy. $\endgroup$
    – DSVA
    Apr 23, 2017 at 12:55

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